2
Materials Research
The Panel on Materials Science and Engineering at the Army Research Laboratory (ARL) conducted its review of ARL’s programs in biological and bioinspired materials, energy and power materials, and engineered photonics materials at Adelphi, Maryland, on June 10-12, 2015, and its review of ARL’s programs in high strain rate and ballistic materials, structural materials, and electronic materials at Aberdeen Proving Ground, Maryland, on May 24-26, 2016. This chapter provides an evaluation of that work, recognizing that it represents only a portion of ARL’s Materials Research Campaign.
ARL’s materials research spans the spectrum of technology maturity and addresses Army applications, working from the state of the art to the art of the possible—25 years into the future—according to ARL. Materials research efforts and expertise are spread throughout the ARL enterprise. As the ensemble of the materials discipline and capabilities, materials sciences is one of ARL’s primary core technical competencies. The materials sciences work supports the mission of ARL, as the U.S. Army’s corporate laboratory, to provide innovative science, technology, and analyses to enable a full spectrum of operations.
The Army’s mission is fundamentally intertwined with its ability to produce new and improved materials with a combination of properties that are more often than not unique to the requirements of the warfighter. As such, materials science is one of ARL’s core technical competencies, with the Materials Research Campaign spread throughout the ARL enterprise.
BIOLOGICAL AND BIOINSPIRED MATERIALS
The scientific quality of the work in this area is on par with that of leading federal, university, and industry laboratories, reflects a broad understanding of the underlying science and research being conducted elsewhere, and is recognized as a component of the broader national effort in biomaterials research through its government (e.g., the U.S. Army Edgewood Chemical Biological Center and the
U.S. Army Natick Soldier Systems Center) and university (e.g., Institute for Collaborative Biotechnology at the University of California, Santa Barbara) partnerships and collaborations.
This research area has grown substantially over the last 2 years. The knowledgeable leadership direct principally competent, early-career scientists. Relative to the importance of this research, the person-power in this area is considered suboptimal.
The laboratories are generally well equipped to perform the types of studies and analyses required for the biological research. The biological characterization and imaging tools are good, with recent additions of next-generation sequencing and protein synthesis and medium-scale bioreactors with real-time metabolism analysis capabilities. A peptide sequencer would provide important missing capabilities and would help to accelerate research.
Among the excellent research activities of this group, particular promise is shown by the stabilization of proteins against thermal and chemical extremes, using new chemistries and methods to derive antibody-like reagents that improve on antibody properties (specifically, biomolecular recognition and binding characteristics).
Accomplishments and Advances
Biomaterials for Hazardous Materials Detection
The development of synthetic bioreceptors as alternatives to antibodies is being pursued to allow biosensing outside the laboratory and in conditions more representative of those found in the field (e.g., high temperature). Overall, this project, conducted by a relatively new group of researchers, is equivalent to the best work performed elsewhere. The work supports a variety of missions, including water and food defense, individual soldier protection, and collective protection. The concept is to mimic antibody binding with small peptides. An approach screens large libraries of enhancing affinity, selectivity, and other desired features (e.g., serum stability) via an iterative process. The group is exploring a number of different strategies to perform this screening, which is a significant strength of its approach.
To create high-affinity and robust biosensors, independently binding peptides are chemically conjugated using click chemistry to identify bi- or greater ligands. The use of cyclic peptides in place of linear peptides is also being explored as a means to higher-affinity molecules. It is impressive that this technology has allowed rapid (less than 1 week) identification of binding peptides. The group’s demonstration of binding to aluminum alloys provides a practical example of its capabilities. Overall, this very productive group is doing cutting-edge work that complements work ongoing in extramural laboratories.
Given the alternative strategies for achieving similar outcomes (e.g., single-chain thermostable antibodies), more specific performance criteria or target product profiles will be needed for further development of some of these areas.
Biohybrid Materials for Sensing
Bio-nano-hybrid systems are being investigated for their potential applications for in vivo physiological monitoring, nanomedicine, traumatic brain injury (TBI) dosimetry, and other photonics-based sensing. The intent of this research is to understand the interactions taking place at the biomediated, nanocrystalline, photonic or nanophotonic biomaterials interface, and to develop new designs for tailored light or matter interactions that can be applied to Army needs. The examples presented use proteins to stabilize nanoclusters and control photonic materials properties. Protein-nanocrystalline structures have been embedded with neurons to detect primary blast-induced neurotrauma, a potential means to inves-
tigate mild TBI. The protein-stabilized nanoclusters (P-NCs) were synthesized in situ in neuronal and nontumorigenic cells—the first demonstration of in situ nanocluster growth in nontumorigenic cell lines.
This project provides a good example of grassroots-driven collaboration with outside laboratories. It is very good fundamental research with potential applications to sensors (ligand recognition) and to cell targeting for drug discovery and development. The research is characterized by good integration of modeling and experimental work across bio-molecular- to cellular-length scales. However, more fundamental work is needed to determine the location of nanoclusters, to determine whether protein(s) stabilize the nanoclusters, and to validate in vitro expressions under high pressure.
The effort to utilize P-NCs for monitoring pressure in TBI appears to yield distinct spectral peak intensity changes. Without concurrent modeling efforts it is not certain whether these intensity differences can be due solely to changes in nanocrystal clusters in proteins or can also be due to other effects. It is therefore unclear whether this research would be better directed toward sensors for TBI or other extreme conditions.
A smaller project is focused on the development of a real-time handheld detector for synthetic cannabinoids based on use of a cannabinoid receptor as a transduction element for detection of contraband material. This is an attractive approach, given the diversity of targets (generated by the illegal synthetic drug community) that can be detected using the functional receptors that trigger downstream cognitive effects. Further investment in this project is expected to depend on performance parameters that are yet to be determined, including the limits of detection, the dose response across a useful operational range, and the signal to noise performance in the presence of interference.
Bioinspired and Biomimetic Materials for Protection
This effort addresses a number of related topics intended to improve the performance of polymers in areas relevant to the Army mission, as well as the use of polymers in studies of TBI. ARL’s biobased polymer program has been used to produce transitioned biorubber toughening agents, reactive diluents, monomers for polyamides, biobased bisphenol A analogues, and multiphenolic monomers. One program was directed toward developing high-performance biobased polymers for Army applications. The goal of the program is to utilize renewable lignin-based resources to create molecules for the production of high-performance polymers. Successes to date include synthesis of monomers of diepoxy and demonstration of polymers with very high glass transition temperatures. The associated challenges include development of scalable chemistries and structure-property-toxicity capabilities that would allow for transition of the technology to industrial partners.
Another project focused on improving the properties of polymers by incorporating reversible cross-links to enhance toughness. The goal of a third project is to develop high-temperature adhesives that are inspired by the extraordinary properties of spider silk and muscle titin, specifically by incorporating reversible cross-links whose breakage can allow the unfolding of polymer domains. While inspired by natural polymers that derive their mechanical properties from hydrogen bonding, this project focuses on the use of reversible metal bonds as high-temperature tougheners. The emphasis in both projects on developing a mechanistic understanding is a strength, because so much of the other work on these topics is empirical. The emphasis on high-temperature performance differs from the focus in most other laboratories that work on these topics. It is unclear, however, whether simple insertion of metallic elements will achieve the strength and toughness levels of the natural polymers, but new insights are likely, and the coordination with modeling is laudable.
A small project presented in this area addresses the extremely important problem of TBI. The experimental setup devised is fairly simple: A small (2 g) explosive charge is detonated close to a tank
that contains neuron cells. This approach to load cells is novel compared to other TBI studies using cell cultures and may yield new insights. Although this is an exciting project, it needs to be part of a much larger and broader program studying TBI; it could be connected to efforts taking place at other Department of Defense (DOD) laboratories. This project needs to also consider more interpretable dynamic loading of neuron cells. Test configuration could allow simulation of pressure waves so that any observed changes in the neuron cells can be related to a known pressure history.
Bioconversion, Biosourced Energy
This is an appropriately focused long-term effort to address Army-specific needs for dealing with food and water wastes. It connects well with other Army entities and is appropriately resourced in terms of both equipment and competent personnel.
A positive characteristic of the program is its university outreach and collaborations, intended to draw in expertise and technologies. These relationships may be leveraged or enhanced through the developing ARL open campus initiative. A more formal connection with the Army Medical Command, particularly in the areas of wastes and health, will be important.
To achieve the programs’ goals, it may be worthwhile to put more emphasis on high-throughput approaches to empirically screening large numbers of communities; this could allow more rapid identification of desired bacterial communities. It might also prove useful to examine lessons that may be learned from the limitations of previously fielded systems.
Opportunities and Challenges
Because biology is a growth area, ARL has an opportunity to identify and recruit a critical mass of biologists, including microbiologists and polymer/organic chemists, looking well into the future to create an integrated community of researchers. The process of recruiting and retaining talent could encourage better articulation of the expectations and career paths that lead from postdoctoral researchers who are contractors to scientists who are government employees, and to develop an effective mentorship program emphasizing professional development and job satisfaction.
ARL needs to reexamine its polymer-related work to assure the closest possible relations between the researchers at its Adelphi and Aberdeen locations.
ENERGY AND POWER MATERIALS
Accomplishments and Advances
The quality of the research projects, the staff, and the facilities is comparable to high-quality research laboratories elsewhere in industrial and academic environments. Where there are gaps in the technical skills or methods needed for a project, the ARL staff demonstrate mature experience and judgment in seeking out high-quality collaboration with other non-ARL researchers within and beyond the Army research enterprise.
The early-career researchers are strong and have excellent skills, which likely reflect good mentorship by senior personnel. Importantly, the research staff are enthusiastic and throughout the review demonstrated a clear focus on Army needs, an appreciation for the importance of moving basic research to technology to impact, and skill in selecting research methods and tools involving experiment, theory, and simulation.
The portfolio of research projects reviewed included an appropriate balance of high-risk, long-term-impact projects along with mid-term and short-term projects. There was a broad, deep coverage of different devices, different fuels, and different applications covering a wide range of size and time scales.
There are continuing improvements in research quality, staff hiring in both postdoctoral and permanent positions, and collaborative activity. As part of these improvements, ARL has expanded its modeling capabilities. The current in-house capability for carrying out high-level simulation and modeling activities is of high quality and moving in the right direction.
Advanced Energy Storage: Advanced Battery Chemistry
Though the advanced battery effort at ARL is small relative to similarly focused programs at other federal laboratories (e.g., Department of Energy laboratories), it is internationally recognized for its high scientific quality and long history of productivity and innovation.
The research includes significant elements of experimental and computational numerical modeling work. The laboratory equipment for experimental work is excellent, spanning an impressive range of capabilities from materials synthesis and characterization, to electrode and cell fabrication. Computational efforts in the battery area are good but appear to be relatively recent. They could possibly benefit from additional resources and emphasis.
The team has excellent qualifications that are well matched to their research challenges. In addition, program participants have an excellent understanding of research conducted elsewhere and are well aware of critical research issues and advances from around the world. The ARL team has formed a local Center for Batteries in Extreme Environments, which provides a good model for interaction of ARL staff with non-ARL scientists.
The projects in this area also are synergistic with one another. The team thinks hard about transition pathways to scale-up, manufacture, and commercialization and seems well positioned to make decisions and negotiate arrangements to transition ideas to the field.
The team has begun generating a database of properties on electrolytes, including interfacial reactivity. A pathway may exist for organizing these data in a manner similar to that being pursued for other battery materials in the materials genome initiative. Productive work may come from the team’s interactions with that initiative focused on electrolytes, possibly including the effect of additives on interfacial reactivity.
Advanced Energy Storage: Structural Batteries Using Additive Manufacturing
The researchers are successfully developing techniques for fabricating multifunctional battery materials using additive manufacturing (AM). The lattice structures constructed using AM have favorable mechanical properties and controllable surface area per unit volume, which permits tailoring and optimization of electrochemical performance. With respect to weight reduction for batteries and capacitors, the AM method has clear advantages over earlier methods. The measured elastic properties and electrical performances of the fabricated materials agree well with the finite-element modeling performed as part of the project.
The techniques and materials are promising, and the scientific quality of the project is comparable to quality at leading research institutions. A more comprehensive modeling and simulation component addressing chemistry and physics, in addition to mechanics, might be desirable for understanding effects such as the influence of porosity on performance of gels. The project has a good balance of theory and experimentation. It would be good for the team to consider the previous work on nanotrusses done
at the Naval Research Laboratory to see if there is anything in this work that might be applicable. To help design for sufficient mechanical durability and reliability of materials, it might be worthwhile to investigate the strength and failure properties of the fabricated materials as well as their elastic moduli. It may also be desirable to consider the practical issues associated with scaling up the laboratory AM process to full-size batteries. This project has significant potential for innovative discovery.
Alkaline Fuel Cells: Optimizing Structure and Chemistry of Ion-Containing Polymers for Charge Transport
This project focuses on the development of alkaline fuel cells. Conventional approaches rely on a liquid KOH electrolyte as a means of OH− exchange. This electrolyte is problematic because it is a liquid and can be poisoned with CO2 owing to the formation of K2CO3. The goal of this project is to circumvent these problems using a polymer electrolyte. In particular, a mixture of dicyclopentadiene and CO is used to create a bicontinuous microstructure intended to maintain high OH− conductivity and strong mechanical properties. The OH− conductivities achieved were the best reported, though the mechanical behavior was not adequate. By increasing polymer molecular weight and cross-linking, strength and toughness were increased nearly twofold, but this is still far less than competing materials. It is unclear whether this mechanical behavior would be acceptable. The researchers were able to enhance material behavior by optimizing microstructure, which was in turn achieved by increasing the connectivity of the hydrophilic domains.
This research is Army-relevant, reflects a correct understanding of the literature, makes use of state-of-the-art facilities, and is of high quality, comparable to similar university and industrial efforts. However, the scope of the effort requires expansion if the work is to have a substantial impact within this community. Additionally, being more engaged with this community—e.g., the multiuniversity research initiative at the Colorado School of Mines—would help this work proceed by enhancing critical decision making—for example, in materials selection. This project would be strengthened significantly by the addition of a modeling component. There are many existing methods and codes that could be helpful; for example, the group at the National Renewable Energy Laboratory is modeling this sort of system.
The publication record of the researchers is prolific: Seven papers have been published, one is under review, and four more are in preparation.
Alternative Energy Photovoltaics
This project utilizes quantum dot nanomaterials for photovoltaic conversion and focuses on enhanced light absorption and minimizing reflective losses. This nanomaterial approach eliminates the need for a traditional tracking system and, if successful, would significantly impact a number of Army applications requiring flexible and efficient power.
The researchers have established productive collaborations with the communities at the University of Texas, the State University of New York, Microlink Devices, and the University of Michigan—all characterized by a good mix of experiment, theory, and simulation. The researchers demonstrate a broad understanding of the related science as exemplified by their efforts to modify the wetting layer thickness to increase electronic capture; they have achieved photovoltaic (PV) efficiency 6 percent above the record for GaN.
Alternative Energy: Highly Mismatched Alloys
This project develops material to split water by using sunlight as the energy source. This research is high risk but potentially offers a very high payoff. The idea, based on results appearing in the literature, is to replace some N with Sb in GaN to form GaNxSb1-x. It was predicted that by adding Sb, the bandgap could be lowered to about 2.2 eV, producing an efficient light absorber. These alloys, highly mismatched in size or electronegativity, have never before been synthesized. The group has significant experience with Group V alloys and apparently a unique synthesis capability.
The experimental results shown verify the bandgap crossing model (developed at Lawrence Berkeley National Laboratory [LBNL]) up to x = 0.22. Materials produced remain crystalline. It appears that very small amounts of Sb lower the bandgap significantly. The principal investigator did not understand why the model predicted such behavior. An understanding of the controlling physics needs to be developed.
Attempts could be made to fabricate a device, though this necessitates doping these materials. Doping can now be done for GaN, but it is not clear what effect the Sb will have on this process.
Good collaborations with LBNL and with the University of Strathclyde and the University of Nottingham (both in the United Kingdom) are ongoing. Only theory is done at LBNL. There is also significant competition from the National Renewable Energy Laboratory and the University of North Carolina; these groups are focused on different materials. This project could be aided by more modeling. There have been five publications by this group in the last year.
Alkaline Fuel Cells
The principal objective of this research is to create anion exchange membrane/proton exchange membrane stacks, eliminating the need to transport water throughout the cell and potentially reducing the mass and footprint of the device. The principal investigators have pulled together an excellent team, including a group at Georgia Institute of Technology.
The principal investigators were aware of many of the relevant issues for successfully constructing such a cell, including issues regarding delamination. This work has a robust modeling component. Overall, this research is innovative and promising.
Alloy Type Anodes for Lithium-Ion Batteries
Lithium (Li)-ion battery performance and weight reduction may be improved by using silicon (Si) anodes to increase the capacity for Li storage. This project addresses an important practical difficulty with Si anodes, which is their tendency to experience mechanical failure after a small number of electrical discharge and recharge cycles. The principal investigator has performed careful in situ measurements of this effect using an atomic force microscopy technique. The implementation of these experimental techniques is the main achievement of the work so far. The principal investigator is also working on coatings for anodes to reduce cracking, with encouraging results. The project also supports collaborations with the University of Utah to use molecular dynamics (MD) simulations to study the electrical and mechanical processes involved. MD seems to be a very promising method for understanding the fundamental aspects of the cracking problem. In particular, analysis might help to reveal why thin coatings apparently reduce damage in spite of the very large linear strains to which the coatings are subjected. The project would benefit from a closer working relationship between the experimental and computational team members.
Beta(photo)voltaics
The project is intended to develop a long-lived (25-year goal) power source using beta and alpha energy conversion in wide bandgap (WBG) semiconductor materials and phosphors. The approach uses a beta emitter (tritium) to produce electricity. Current designs do not produce enough power to be useful for the Army, and so the isotope power source is coupled with a Li-ion battery to take care of power demands during higher current demands such as during signal generation. The isotope power source is used as a trickle charger for the Li-ion battery. Electrochemical capacitors were used, but there was high leakage current.
This project started as an engineering problem. Isotope power sources have been used for years in weapons applications. Since the Army has access to isotope materials, it made sense to utilize this approach for power production. To test the concept, an isotope power source was fabricated that generated 100 µW.
The innovative concept applies to the investigation of three-dimensional (3D) interaction space in WBG materials and phosphors to increase energy conversion and efficiency. The project is well thought out, and it has a high probability of success. The principal investigator is performing mechanical cross-section simulations to aid in design, so the mix of theory, experimentation, and computation is sufficient. The qualifications of the researcher and the facilities appear to be compatible with this research challenge. If successful, this could open a myriad of small power source applications for the Army.
Carbon Formation During Catalytic Oxidation of Hydrocarbon and JP-8 Fuel
The use of logistics fuel for compact, heat-driven electric power generation is compromised because sulfur impurities poison the catalytic activity of microcombustors. In this project, a materials-by-design approach is being used to identify promising combustion catalysts, which are investigated with experimental and computational methods. In situ spectroscopy is incorporated with short contact-time reactors to identify surface species during catalytic combustion of prototype fuel, while simultaneously monitoring poisoning. These data, used in conjunction with a microscopic reaction diffusion model of surface events during combustion, clarify the effect of sulfur. It has been recognized that sulfur enhances carbon formation on platinum (Pt) but not on rhodium (Rh). The project promises to accelerate microcombustor catalytic design through reactive flow modeling. This is good scientific work linked with sound engineering methods for scale-up and extension to logistic fuels.
Critical Solvation Issues in Lithium-Ion Batteries
This poster describes part of the excellent battery program that focuses on Li salt solvation in organic carbonate and water solvents. The objective is to better understand fundamental electrolyte interface properties. The work showed preferential solvation of Li by ethylene carbonate (EC) in EC/dimethyl carbonate mixtures, which is relevant to solid-electrolyte interphase formation at carbon anodes. New water-in-salt electrolytes having less than 20 weight percent water in Li bis-trifluoromethanesulfonimide were prepared, and early-stage results are quite promising. This is an example of an emerging class of electrolytes called deep eutectics. Water in this electrolyte is thought to have very different properties from conventional water because such a large fraction of these water molecules are contained in solvation shells. Understanding of interface passivation could help in electrolyte material choice. The work is led by an energetic PI and is synergistic with the overall thrust of the ARL battery effort. Its high scientific quality is demonstrated through publication in quality journals.
High-Voltage Li-Ion Electrodes and Electrolytes
This project involves work on olivine LiMPO4-type cathodes, where M is Co, Mn, and Ni substituting for the usual Fe. These three metals have more positive redox potentials, so batteries using these materials have higher voltages. LiCoPO4 has a high potential but usually exhibits significant capacity fade upon cycling. The ARL team found that mixing some Fe with the Co results in much less capacity fade, with minimal loss of overall capacity. A mechanistic understanding of the diminished capacity fade is being pursued. The work is of high quality and fits well with the significant worldwide effort to identify new battery cathodes to enable higher energy density batteries.
Isomeric Materials Research
This project addresses the use of nuclear transitions for energy-on-demand. The concept is to convert a long-lived, excited nuclear state to a short-lived ground state by excitation by photons or neutrons. The main scientific content is nuclear physics, in contrast to the mainstream atom/electron/photon-centered work in the ARL Materials Research Campaign. The work requires investigation of level diagrams for candidate nuclei. Because of the complexity of the few-body problem for large nuclei, the nuclei cannot be modeled to sufficient accuracy but have to be measured. The investigators combine information from the literature with their measurements. The conversion concept has been demonstrated for a silver isotope. The work is of high technical quality and is published in the appropriate journals. This is a long-term, high-risk approach with regard to practical applications, with many questions to be answered, including how to produce the long-lived excited state in sufficient quantities, but it is worth pursuing. If successful, impact could be high.
Lattice Conductivity of Dense Ta-Doped Li7La3Zr2O12
LLZO is a candidate for a solid electrolyte in Li batteries. The goal of this study is to enhance Li conductivity via doping Ta for Zr. Using this approach, the investigators were able to reproduce a similar study by Goodenough and suggested that other enhanced results in the literature were likely due to other defects. This is a very good addition to the literature, where reproducibility of key results is often lacking. Furthermore, this work had a strong density functional theory component that was performed at the Naval Research Laboratory (NRL). The work would be outstanding if there were researchers within ARL who could complement the efforts of NRL.
Mathematical Modeling and Lifetime Extension of Thermal Batteries
Thin-film thermal batteries could provide improved reliability and performance over present designs in munitions. This project is successfully developing a comprehensive analysis tool that models the thermal energy balance, gas generation, and electrical performance of thin-film thermal batteries. So far, it is mainly the thermal problem that has been addressed. The ARL thermal model has been integrated into Sandia National Laboratories’ Sierra finite element code. The method that ARL is developing could be combined with a mathematical optimization tool, providing a direct and systematic way to improve battery design. There was no mention of validation of the model, suggesting that this is not a central focus. The principal investigator did not seem to understand the model being used. Therefore, in addition to adding the multi-physics capabilities that are planned, it would be desirable to obtain experimental data that would be needed to validate the submodels (e.g., thermal, gas transport, electrical, chemical) as the code grows in size and complexity.
Pyroelectric Materials for Energy Applications
Led by a competent postdoctoral researcher, this project draws on a 2006 paper1 reporting a giant electrocaloric effect in perovskite oxide PZT (metallic oxide based piezoelectric material) to propose and prototype a device to convert heat/infrared photons to electrical energy. The concept is to run a heat engine loop between the low polarization branch of the polarization versus field (P-E) hysteresis loop at high temperature and the high polarization branch at low temperature. There is much room for optimization by choice of materials and by controlling the quality of the thin film, with leakage current being of particular concern. The collaboration with the University of California at Berkeley will help with the latter. The long-term application will be the remote supply of energy by an infrared laser and could be used, for example, to recharge drones in flight. The work is of high technical quality, and the project leader communicates well with the broader community. As the work progresses, more modeling could be integrated into the project.
Understanding C-C Bond Breakage on Plasmonic Nanostructures
This proof-of-concept project is directed toward developing catalyst structures for breaking the C-C bonds associated with high energy density logistic fuels (e.g., ethanol) using light-harvesting nanoscale arrays formed by localized surface plasmon resonance. This is a unique fabrication approach based on a good concept for photo-reformation of logistic fuel. The project represents a high-risk endeavor. The first steps toward forming such structures have been made with use of the Specialty Electronic Materials and Sensors Cleanroom facility. Initial work includes modeling the plasmonic aspects of the structure. Additional modeling worked is planned to take account of the immersed, reactive, electrochemical environment. Experimental characterization of the photo-induced reactions is planned, using well-established electrochemical and surface science methods. Desorption mass spectroscopy electrophotometry has been reported by others and may be considered for this project. At this point, there are no experimental data on the structure, and the current understanding of the reaction mechanism is speculative. It would be helpful at this point to create a device and test it out. It is not yet clear whether this fabrication method can be implemented at large scale.
Grain-Boundary Engineering of Ion-Conducting Ceramics
This project examines two approaches to reducing ion transfer resistance at grain boundaries in the fast lithium-ion-conducting ceramic Li3xLa2/3-xTiO3, x = 0.11. Previous work showed that this solid-state electrolyte material had good conductivity. The idea is to change grain boundary (GB) properties to improve GB conductivity. Grain boundary modification was achieved by simple lithium-ion exchange from solution, and by silica coating the starting particles with SiO2 using magnetron sputtering. Both approaches provided modest increases in conductance, both at GBs and in the bulk. A study on variations in bulk ionic conductivity with thermally induced changes in crystal structure was also pursued. The mechanism by which surface coatings change GB conductance is not fully understood; more structural characterization work—for example, by transmission electron microscopy (TEM)—is planned to help address this point. It was also found that different processing conditions could change crystal structure and conductivity. The principal investigator, a postdoctoral researcher with ceramics experience relevant
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1 A.S. Mischenko, Q. Zhang, J.F. Scott, R.W. Whatmore, and N.D. Mathur, Giant electrocaloric effect in thin-film PbZr0.95Ti0.05O3, Science 311:1270, 2006.
to Li battery technology, needs to learn more about the battery aspects of the work to become fully integrated into the overall effort. Still, this is excellent work for a relatively new employee. The work is of high quality and contributes to a growing body of knowledge regarding use of ceramic materials to replace liquid or polymer electrolytes in Li batteries.
Opportunities and Challenges
Questions remain as to whether ARL was mobilizing aggressively enough to capitalize on both internal advances and external advances made by the broader community—for example, whether the recent world-leading results on enhancement in quantum well infrared photodetector (QWIP) efficiencies are being translated into capability demonstrators for manufacturers and customers. Similarly, ARL may not be working to leverage external advances in silicon photonics, especially with regard to heterogeneous materials. However, in both cases, these concerns were partially allayed by discussions with staff and management regarding the status of some programs related to these technologies. For the QWIP work, for example, ARL has hired an external business consultant and is working with NASA and others on technology transfer for the QWIP breakthroughs. Nonetheless, there remains more opportunity for ARL to capitalize on its internal and external advances.
The enormous potential impact of the photonics work could have been presented more vigorously and compellingly. One way of doing so could be to augment an individual photonics presentation with an explicit description of the broader potential impact if it succeeds. Army goals were noted, but they often comprised immediate technical targets as opposed to what the ultimate impact could be for a more comprehensive field of science or for broader Army applications.
The presentation on structural batteries using additive manufacturing has significant potential associated with its innovative approach. The project combines novel fabrication methods with insight into selection of compatible multifunctional elements that combine structural components with energy storage components. Experimental work is carried out concurrent with modeling studies that guide system design choices. The external collaborations are facilitated by a flexible methodology that provides easy incorporation of next-generation subcomponent materials as they are developed. However, the effort needs to grow across a wider range of projects, with a focus on identifying appropriate modeling methods and on closing the experiment–theory–simulation loop. Increased interaction with the significant computational resources of ARL could help bridge the gap until additional capacity is available within the Materials Research Campaign. At present, first-principles computational modeling is growing, mainly through collaboration with recognized experts elsewhere, guided by very capable but limited-in-number experienced internal research staff.
In comparison to the expansion in first-principles modeling, engineering models are underutilized, perhaps because in-house expertise in this facet of modeling is limited. Engineering models are typically developed at the outset from a simple set of input parameters or components that, together with the model, predict system behavior. These components are improved as empirical knowledge of the system’s behavior increases. Routine methods are now available to identify the most sensitive components for which improved fundamental knowledge is needed, to provide uncertainty quantification, and to guide system-level optimization during scale-up or scale-down beyond experimental regimes. The combination of an appropriate engineering modeling effort with the intuitive understanding of experimentalists is a highly effective engineering approach and needs to be targeted as a growth area.
In some energy and power applications, such as Li-ion batteries and fuel cells, there is a broad, vigorous, fast-moving, worldwide research effort directed toward identifying fundamental scientific issues and developing novel materials and entire systems. Accordingly, the narrowly focused ARL projects
need to pick the right niche in order to have impact. The knowledge necessary to define the goals of such projects depends critically on tracking research advances elsewhere. Because postdoctoral and other early-career permanent staff researchers benefit from exposure to research activities beyond ARL, it is critically important to promote and expand active mentoring by senior staff.
ENGINEERED PHOTONICS MATERIALS
The quality of the work in photonics materials is comparable to that found at most research universities. This is an impressive accomplishment in light of the inherently wide scope of the technical program, which is essential to addressing diverse current and future Army needs. The quality of the work presented reflects a high level of technical competence and professionalism on the part of the researchers and management.
The portfolio of the engineered photonics materials group shows a good balance of high-risk, longer-term work with nearer-term customer-driven solutions or incremental, critical technology refinement. This well-balanced portfolio is supported by a strong materials capability in staff expertise and laboratory or clean room infrastructure. Investments are impressive for computational modeling and simulation that ARL has successfully implemented to complement its strengths and core competencies in materials synthesis and characterization, as well as device work. All of these facilities and capabilities are being leveraged into compelling device and application-driven work, especially in ultraviolet (UV) materials, infrared (IR) devices, and the device physics in both areas. In addition to technical diversity, there is workforce diversity.
Accomplishments and Advances
Alternative Energy: Photovoltaics
This project involves work to improve performance of low-concentration photovoltaic cells targeting robust, lightweight power for soldiers in theater. The technical focus is developing solutions using III-V quantum-dot materials to extend performance into the longer wave regions of the solar spectrum, and to improve efficiency by minimizing recombination.
Solar PV is one of the important pathways to reducing the weight of power solutions in theater. The experimental work showed solid progress, reflecting the strong competence of the team, which evinced expertise that includes epitaxy and sophisticated quantum dot engineering, polyethylene terephthalate (PET) moth eye surfaces, and intentionally induced morphological features on III-V layers for enhanced photon capture. There appeared to be extensive collaborations with researchers outside ARL.
The wetting layer state-engineering designs might benefit from more direct experimental verification of their efficacy in reducing recombination in the dots. There was a lack of clarity on the trade-offs between the high concentration (30 to 100 times what is typically seen when realizing the benefits of advanced materials) and the low concentration (less than 4 times what is typically required in nontracking applications). More clarity is needed on the system-level incremental cost of multijunction cells with significantly higher efficiencies relative to single-junction material solutions such as GaAs, which is still very high ($40,000 per square meter), or the quantum dot approach pursued in this work. Additional questions include comparisons with spectral splitting, which was examined in the DARPA-sponsored very-high-efficiency solar cell program.
Biophotonics
Progress was reported on protein-wrapped fluorescent metal nanoparticles, motivated by their potential use as neuronal pressure sensors. The long-term goal is to develop a fundamental mechanistic understanding of mild traumatic brain injury onset and development.
The fundamental work on the biomediated synthesis of atomic nanoclusters was compelling, and the fact that the proteins retain their native functionality after synthesis has tremendous potential. For example, the resulting nanoparticles may be noncytotoxic, and it may be possible to direct them to specific locations within a cell. These nanoparticle building blocks are anticipated to provide unique opportunities based on their interesting optical and physical properties. An example given was fluorescence change with pressure seen for one protein but not a different protein, an indication that interesting protein science may be enabled by this system.
There is some concern regarding the specific proposed application for these particles for understanding shock waves in tissue. The fluorescence changes with pressure were small (20 percent over 400 MPa for one system and about 6 percent over 600 kPa for a different system). In real tissue, these small changes over less than 1 ms, from a single or a few particles, will be very hard to observe. What is needed is a deeper physical analysis of the full system, including the signal-to-noise ratio in realistic shock wave and illumination conditions, and what is anticipated at a single neuron level. Also needed is a comparison with other potential techniques, such as Forster resonance energy transfer and plasmonic particles, in the context of nanoscale pressure sensors.
This ambitious work offers strong opportunities for discovery; it is a high-risk early-stage effort in ARL’s expanding biophotonics effort.
Modeling and Analysis of Ultraviolet-Light-Emitting-Diode Materials
The objective of this project is to use many-body theory to model lifetime in III-nitride structures, including free carrier and exciton effects, polarization fields, and density-dependent screening of Coulomb interaction and polarization fields. This is one of the projects indicative of ARL’s investments in more comprehensive modeling to support its strong core materials capabilities and competencies.
This a very challenging problem, and the principal investigator is making good progress in describing radiative lifetime, including many-body effects such as phase-phase filling, screening, and quasi-particle renormalization. However, nonradiative processes were not described at the same level of theory. Semiempirical, nonradiative models using activation energy were shown not to fit experimental data well, but improved fits were achieved with a combination of a fixed temperature-independent component plus an activation-energy component.
The development of first-principles-based and self-consistent predictive capabilities to describe carrier lifetime in III-nitride structures, including both radiative and nonradiative processes, is not easy. However, the principal investigator presented a scientific strategy to make progress toward addressing this challenge. The strategy calls for alloy fluctuations, a many-band description of the electronic wave function, the use of nonparabolic bands, and the inclusion of nonradiative recombination processes. This is a project of high technical merit and of potentially high impact in support of the Army’s mission.
Ultraviolet Avalanche Photodetector Research
This work entailed the compelling development of models and experimental devices and materials to evaluate the efficacy of novel solutions for improved single-photonic avalanche detectors in the UV as replacements for photomultiplier tubes.
The principal concept is to use GaN and AlGaN epitaxial layers to address the reduction in quantum efficiencies that stems from the use of semitransparent metal electrodes on current SiC devices. Self-assembled monolayer structures were introduced to either isolate the SiC to a multiplication layer or to just use the AlGaN as a transparent contact layer to keep the SiC away from surface so as to avoid surface recombination.
This work is promising and has high-quality external partnerships. It has mainly involved epitaxy development and Si diffusion studies, and the transitioning of these to device results in avalanche operation is awaited.
Short-Wavelength Infrared Device Modeling and Optimization
This project is directed at the development of a comprehensive model that combines the finite-difference, time-domain electromagnetics of nanostructured surfaces with finite-element modeling, drift-diffusion transport to understand and optimize device designs and material structures. The model is comprehensive in that it included material, electronic, optical, and especially nanostructured geometric properties that strongly impact the electromagnetics. The integrated software suite allowed analysis of very complicated multipixel arrays, and the principal investigator showed how more simplistic models would not properly capture major performance factors. One example was that the performance of nanostructured cones could be estimated reasonably well with effective medium models at longer wavelength, but at shorter wavelengths complex scattering among the cones dominated the performance. The model was shown to be useful in assessing pixel cross talk in arrays, as well as heterostructure design and junction location for optimization of collection efficiency while minimizing generation-recombination (GR) dark current.
This is an excellent project directed toward an important topic in terms of the needs of both the Army and the broader technical community.
Diode-Pumped Tm/Ho Composite Fiber 2.1 µm Single-Mode Laser
The goal of this research is to provide a simpler and more compact 2.1 µm thulium (Tm)/holmium (Ho) source capable of achieving 100 W power in eye-safe lasers for situation awareness, monitoring, and tracking illumination and, perhaps, frequency conversion to directional IR countermeasures.
Early work has been conducted on an innovative concept to make a dual-core fiber laser that would support thulium lasing at 1,950 nm in a multimode core that would, in turn, pump a Ho single-mode core at 2.1 µm. This design is intended to achieve two excitations in the Tm with a single optical pump in the 800 nm range. This is an interesting concept, but it is too early to expect definitive evaluation of the potential.
This effort may now be positioned to benefit from a stronger modeling component to resolve the impact of saturation on spatial mode competition and laser performance. Suitable baseline modeling capabilities are readily available in the literature, and in conjunction with a more deliberate experimental plan, the modeling may be useful for isolating critical performance trade-offs.
The principal investigator is engaged in a valuable external partnership with strong competence in these fiber materials.
Thermal Property Engineering: Exploiting the Properties of Ceramic
This project consists of preliminary work on improving mid-IR lasers by increasing the effective thermal conductivity of the gain media, using nanoscale composite MgO (high thermal conductivity) with Er:Y2O3 (the gain media). This work addresses many scientific and engineering challenges, including the achievable effective thermal conductivity of the composite, which may be limited by phonon scattering, and the achievable volume fraction of gain media needed to be competitive with current solutions.
This work has high potential, and it may benefit from some early modeling to determine the property bounds and trade-offs. The team could also be more vigilant in reaching out to others, including the Air Force Research Laboratory, to evaluate similar work.
Photoacoustic Spectroscopy for Hazard Detection
This project involves work on an elegant and simple device approach for detecting trace elements. While many optical detection techniques are available, these are usually large and contain many precision optical elements. The detection technique proposed is small, robust, and potentially inexpensive, if applications supporting high-volume laser production are realized.
Engaging more broadly with the outside community would be beneficial, including with vendors of existing optical sensors and comparative testing on species of current interest, perhaps in the context of ARL’s open campus initiative. In addition to offering a potential for more pervasive use, this will better ensure that this transitions into a product useful to the Army.
Understanding Inkjet Printed Standards for Optical Measurements
This work involves a system based on the well-tested use of inkjet printing. Although ARL has used only a single print head, the researchers have been able to print on many materials (e.g., rubber, metal, and wood) with contaminants included. The system can be used to understand how the samples age, and the flexibility of patterning and reproducibility of the technique were shown to be useful in capturing the unexpected impact of real-life variations of species on surfaces in the field. This is important work that continues to be funded by customers.
Single-Beam Femtosecond Multiplex CARS
This work illustrates the outstanding evolution of research aimed at using a collinear approach to coherent anti-Stokes Raman spectroscopy (CARS) for trace gas detection. These studies focused initially on pulse characterization but transitioned to the examination of mathematical methods and algorithms for extracting the desired spectral signal from broadband background spectra. The principal investigator was able to demonstrate strong signal-to-noise ratio improvements that substantially enhance the efficacy of the CARS approach.
Photon Trap for Infrared Detection
This project involved the expanded modeling and experiments on the microresonator enhancement presented 2 years earlier. The work showed that very small variations in microresonator dimensional control had strong impact on both the peak efficiencies and the bandwidth of the enhancement. The
results were encouraging, indicating that design regimes existed where very high efficiency could be supported over a band that was easily large enough for many Army applications.
This important advance may not be receiving sufficient resources to move quickly to highly optimized commercial technology. Also, ARL’s studies of the dynamic behavior of materials are likely to be advanced by improvements in infrared detection at modest elevated temperatures.
Ultrafast Spectroscopic Noninvasive Probe of Vertical Carrier Transport in Heterostructure Devices
This work involved pump-probe studies of ultrafast carrier dynamics and charge transport in heterostructures, with the ability to interrogate charge-generated terahertz field profiles in materials prepared for device structures. This research represents a valuable investment in advanced characterization, and the quality of both the topics and investigators is excellent. In addition to being of immediate value to materials and device researchers, the projects are conducive to quality papers and conference presentations of broad interest to the technical community.
Tunable Solid-State Quantum Memory Using Rare-Earth-Ion-Doped Crystal, Nd3+:GaN
This project involved high-risk, early work aimed at using GaN as a host material for the neodymium ion (Nd3+) in quantum memory research. The objective of this project is to perform photon echo experiments to provide an estimate of the memory storage time and capacity in cryogenically cooled Nd3+:GaN crystals. The plan is to ultimately fabricate GaN polar heterostructures from which to design a quantum memory device with multimode capacity. Although this project is in its early stages, this work makes strategic use of ARL’s strong GaN materials and molecular beam epitaxy (MBE) growth capabilities to gain a competitive position in a field that is drawing worldwide attention. Moreover, the ARL team has a strong track record of published contributions in this field.
Opportunities and Challenges
The consistent development and extension of modeling to broader sets of problems and applications is an opportunity area. One prototype project is short-wavelength IR device modeling and optimization. This research illustrates ARL’s expanded efforts to provide critical modeling support in areas where there is high investment in underlying materials and device technologies.
The important software tool set coming from this research is not only essential for designers, but it may also provide critically sensitive parameters that could be used in process control for commercial partners and suppliers of imaging solutions to the Army, which necessitates engaging with the manufacturers. The project’s principal investigator has started this engagement.
HIGH STRAIN RATE AND BALLISTICS MATERIALS
The overall impression of the materials in extreme dynamic environments program and the high strain rate and ballistics materials research at ARL was positive. The projects showed an excellent degree of integration between materials science fundamentals and applications, combining simulations and experiments aimed at developing structure-property correlations with advanced processing and fabrication approaches.
ARL is also establishing itself as a world leader by building novel capabilities, including, for example, extensive facilities for metals, polymers, and composites processing. The miniaturized Hopkinson bar and multiscale, rate-dependent mechanical testing equipment along with microscale sample preparation set-up for investigating polymers, metals, ceramics, fibers, and threads, are unique facilities. The commitment by ARL to take advantage of the dynamic sector facilities at the Advanced Photon Source is noteworthy. In situ measurements performed using these facilities will provide the needed fundamental knowledge for developing and validating computational models for improved understanding of high-strain-rate effects. Throughout the high-strain-rate and ballistic materials efforts, there is substantial growth in the use of computation and modeling and its integration with experimentation. Continued advances in this area are needed, adding, wherever possible, physics-based analysis.
Accomplishments and Advances
Materials in Extreme Dynamic Environments
This program is aimed at establishing the capabilities to design materials for use in specific extreme dynamic environments. It has considerable synergy with ARL’s mission and is having a significant impact on internal and external collaborations, with about 54 faculty and senior scientists from various universities collaborating with about 35 ARL scientists. The collaboration has obvious advantages and is producing good fundamental research as evidenced by about 94 jointly authored publications. It is also promoting internal collaboration between different subfields and disciplines with expertise in computations, experiments, and manufacturing—pushing the envelope further in terms of the research conducted. The choice of simple metallic (Mg), ceramic (boron carbide, B4C), polymers (ultrahigh-molecular-weight polyethylene [UHMWPE]), and composites (S2 glass/epoxy) is ideal for a materials by design approach. The “see it, understand it, control it, design it” paradigm is healthy and keeps the researchers focused on this holistic approach to the research. Additionally, interactions with the Defence Science and Technology Laboratory in the United Kingdom and the establishment of the Mach conference2 are laudable efforts. The level of the research at ARL is benefitting significantly from these interactions. ARL is providing the necessary leadership to ensure this collaboration.
Methodology for Scale-Bridging in Multiscale Modeling of Materials
This program concerns the development of practical multiscale modeling strategies for applications relevant to the simulations of structural and energetic materials. The emphasis is on the use of small-scale models within elements, in effect, as constitutive models. The details of the small-scale models are invisible to the continuum-scale finite element code. The use of surrogate subscale models is a key innovation. This is a good idea because it allows subscale results to be recycled to future time steps, avoiding the need to repeat small-scale calculations. The effectiveness of the method results from the fact that the subscale model is applied to only a small fraction of continuum elements at any given time. This results in a significant reduction in computational time. Much of the development has focused on the computational infrastructure to make this strategy effective. The infrastructure adaptively assigns processors to the calculations in the two levels. The software tools make efficient use of many processors in a medium-sized application.
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2 The Mach Conference showcases the state of the art of multiscale research in materials, with an emphasis on advancing the fundamental science and engineering of materials and structures in extreme environments.
Polymer Modeling Research
This program is an excellent example of coordination between various topics aimed at microscale experimental investigation of UHMWPE fibers, including processing of fibers and films, and their effects on ballistic performance. The modeling examines the three-dimensional network of polymer chains as a double network, at an atomistic level, allowing for development of simulations of mechanisms associated with polyethylene. One of the keys to understanding this system has been the development of the reactive potentials used for modeling UHMWPE. The use of quantum mechanical data to develop this potential has been shown to be fast and accurate.
Graphene and Two-Dimensional Polymers
This program is enabling the design and exploration of properties of new materials—for example, different variants of graphene, or graphylene—that may lead to better armor. The possibility of developing a two-dimensional (2D) structure from polyethylene to increase ballistic performance is indeed an out-of-the-box thinking. Graphylene, an example of a proposed 2D polymer, consists of carbon rings connected by polyethylene chains. Density functional theory (DFT) and MD modeling of this polymer suggests that the carbon rings make it compliant and ductile while also providing the stiffness, strength, and fracture toughness needed for increased ballistic performance, in particular for body armor applications. This project, which is also leveraging an Army Research Office-funded multidisciplinary university research initiative program on 2D polymer synthesis, is an example of successfully using computational tools to design and develop new materials.
Grain Boundary Modeling and Simulation for Lightweight Protective Materials
This program included a fundamental study on B4C that addressed the important issues at the root of its poor ballistic performance. The computational work, which can also be extended to other ceramic systems, is leading to an improved understanding of the amorphization process that occurs under the combined effects of pressure and shear. The potentials developed by Goddard and others were compared, through the virtual diffraction patterns generated, to experimental ones by Anselmi-Tamburini. The overall goal was to investigate the improvement of toughness mediated by engineered grain boundaries. The work represents an excellent fundamental effort.
Synthesis and Multiscale Rate-Dependent Response of Fibers as a Function of Microstructure
This project represents an impressive array of experiments used to establish the fundamental deformation mechanisms in polyethylene (PE) and UHMWPE. The strain-rate range investigated up to approximately 106 s–1 is impressive and is being enabled with the use of a miniature tensile Hopkinson bar. The gripping of the specimens is especially crucial and seems to have been successfully accomplished. Diagnostics, including digital image correlation (DIC) and X-ray measurement techniques, are revealing details about the internal process of failure within fibers that are apparently a first in the polymer fiber community. Future small-angle X-ray scattering characterization is planned, and fibril-level testing will be conducted to establish the effects of the spatial scale. The tensile strength of UHMWPE fibers is very remarkable: 3 GPa. The strain-rate sensitivity is surprisingly low and attributed to viscoelastic effects. The project is providing valuable data about the high-rate deformation and failure of polymer fibers that will be of lasting value to the armor material design community. The project is
well aligned with other projects on polymer armor materials. Multiaxial loading is important in armor materials because the shear strength can depend on other stress components and the hydrostatic pressure. The future work involving atomic force microscopy (AFM) in real time is well thought of and at the frontier of knowledge.
The project is testing micron sized fibers to assess multi-axial stress states. Handling such small samples is no trivial task, leave alone getting high quality data on the material. An impressive array of experiments is being used to establish the fundamental deformation mechanisms in UHMWPE. Multiaxial loading is being initiated, with compression on the fibers superimposed along with tensile loading. The discussion of setting up a fiber and film facility, with future plans of adding additional diagnostics such as dynamic X-ray system is indeed exciting and fascinating.
Grain Boundaries and Interfaces
This project addresses the inelastic deformation due to contact loading of B4C. Both material and model development efforts were supported by transmission electron microscopy (TEM) examination of sections subjected to inelastic deformation, which consisted of a Knoop hardness indentation. Increase in the load resulted in increasing planar defects, amorphous bands, and micro- and macro-cracking. Most amorphous bands followed the maximum shear stress trajectories.
Modeling and Performance of UHMWPE
This project is intended to develop an understanding of the connection between processing of composites with UHMWPE fibers and their ballistic performance. The researchers identified inter-laminar shear strength as a key material property and developed a simple but effective test to measure this property in a laminate. DIC diagnostics are used to visualize the deformation and crack growth at the interface. Finite element modeling is used to help understand the progression of failure. The main process variable being studied is fabrication pressure. Higher pressure decreases the inter-laminar shear strength. The researchers measured changes in the morphology of the ply interfaces that apparently help to explain this effect.
Multiscale Material Characterization, Modeling, and Experimental Methods for Fibers and Fabrics in Soft Armor Protection Systems
This project is developing and applying testing techniques to observe the microscale processes that occur in the interior of UHMWPE fibers during mechanical loading. ARL researchers have developed a novel sample preparation technique that helps to reveal the mechanics of fibrils within a fiber. Microscopic imaging techniques, including AFM, show the evolution of structures and defects as a fiber is strained. Force-displacement measurements are correlated with these microscale events. The techniques and data being developed in this project significantly advance the state of knowledge of the behavior of fibers beyond what is available in the literature. This project is providing unique insights into the materials science underlying the failure process in fibers. The data collected and the new techniques developed will be of great value in the improvement of fiber materials and processes. The data will also be useful to computational model developers.
Depleted Uranium Replacement
This project builds on the on-going search for new penetrator materials. Currently, tungsten-based alloys are used, but the performance is inferior to that of depleted uranium (DU). One important feature of DU penetrators is the formation of adiabatic shear bands, leading to a reduction in the diameter of the hole and increased penetration depth. The current research on nanocrystalline tungsten powders and their consolidation by pressure-less sintering seems to be producing materials with desirable properties with the ability to shear localize.
Optimized Tungsten Carbide Materials for Improved Lethality
This project uses nanocrystalline iron to bind tungsten carbide (WC) powders that show excellent properties. This binder replacement is motivated by the toxicity of the current cobalt binders. Pressure-less sintering is used to produce the necessary homogeneous microstructure with the desired hardness and toughness properties. The promising feature of this project is the repeatability of the process, which is the first step necessary for further studies prior to transition. There is a close collaboration with the lethality group and the U.S. Army Armament Research, Development and Engineering Center (ARDEC) for further evaluations. The fracture toughness of the WC-nanocrystalline iron is equivalent to that of the conventional WC: about 12 MPa.m1/2.
Exploiting Oxide Dispersion Strengthening in Ferritic Alloys for Lethality Applications
This project is using the scale-up capabilities available at ARL for high-energy ball milling of powders and equal channel angular pressing processing to synthesize and fabricate oxide-dispersion strengthened ferritic alloys. The bulk material fabricated with a microstructure consisting of nano- to microscale grains with larger-sized intermetallic precipitates and zirconium oxide dispersed particles, demonstrates substantially high room-temperature compressive strengths of the order of 1.2 GPa for those consolidated at 700°C, and 2.4 GPa for those at 1000°C. Although the compressive strength at elevated temperature is lower than that at room temperature, the work illustrates the control of interstitial elements picked up during powder processing.
Superhard Diamond-Diamond Composites Made via Laser Chemical Vapor Infiltration
This project combines selective area laser sintering and localized laser chemical vapor deposition with motion control and real-time temperature feedback loop to fabricate bulk and complex shapes from diamond-diamond powder composites. The technique uses conventional additive methods to first create a preform starting with diamond powders, and next follows that with chemical vapor deposition of diamond coatings on the particles in the preform to grow and form the fully dense diamond-diamond composite. The approach combines the benefit of both processes, which individually are unable to produce diamond powder compacts in bulk and complex forms. The approach also provides the versatility to employ various ceramics to create the matrix (binder) phase for the diamond preforms. The acquired components for building the reactor for the deposition system have been assembled, and the control program is being developed. Once the system is built and the control program is optimized, it will be used to identify the sintering conditions for fabrication of the diamond preforms and the desired deposition chemistries to achieve the growth conditions needed to make bulk composites with different reinforcement and matrix phases. This is a good example of a project in which an ARL postdoctoral
researcher is building on the technology developed as part of their Ph.D. dissertation and extending it to a new capability at ARL for making diamond-based composites for potential armor applications.
Mechanics and Performance of Unidirectional Film Laminates for Ballistic Protection
This project is investigating the effect of processing conditions for laminates made of UHMWPE films. Film laminates behave differently from the more conventional UHMWPE fiber-reinforced laminates. The film laminates do not need a separate matrix material like epoxy. However, their mechanical properties are sensitive to processing conditions, especially temperature. The film plies bind to each other just below the melt temperature. The properties of the film laminates are unknown, compared to the fiber laminates. However, it has been discovered that under suitable processing history, the film laminates have excellent mechanical strength, even without the inter-laminar matrix material. The project is also seeking out lower-cost solutions to fabrication. It is an important project to the ARL mission of developing effective and lightweight armor materials. Important results are being obtained that will aid the application of film composites for land forces.
Modeling and Characterization of Two-Dimensional Polymer Ultra-Membranes
This project is using atomistic modeling to demonstrate the possibility of a new class of a graphene/ polyethylene 2D material, termed graphylene or GrE-2, with enhanced fracture toughness while maintaining twice the stiffness and nine times the strength of Kevlar. The MD simulations considering two-carbon long ethylene chains connecting the benzyne rings in the polymer framework, illustrate that the energy release rate for crack propagation in graphelyne is twice that for graphene due to effects of dissipative processes such as delocalized failure and crack branching. The simulations were extended to predict the response of ensembles of discrete platelets of 2D materials and to demonstrate superior mechanical performance through careful design of inter-layer interactions. This is the first such study predicting the design and mechanical behavior of 2D materials aimed at guiding the synthesis of novel polymers. These efforts have yet to be extended to the prediction of the performance under high-rate impact conditions relevant for applications in body armor or to the synthesis of such a material.
Peridynamics Modeling of Projectile Impact and Penetration in Vehicular Glass
This project is a novel computational simulation method that is intended to predict the impact resistance of glass armor materials. The peridynamic method is compatible with the physical nature of the problem because it allows for discontinuous deformations, since it does not rely on differential equations. The peridynamic equations do not require smoothness of the displacement field. The method therefore has inherent advantages over traditional approaches to fracture modeling that rely on finite elements to approximate the partial differential equations of the standard theory of solid mechanics. The researchers have developed a peridynamic solver that couples with a finite difference solver, allowing the peridynamic portion of the mesh to adaptively follow the growing crack tips. This innovation allows for greater computational efficiency and reduced wave dispersion in the coupled model. The project team is acquiring an ISRA optical instrument that can measure flaw distributions in glass samples prior to impact. This will allow measurements of flaw distributions to be initialized into the computational model, thus providing more meaningful validation of fracture and fragmentation than was previously possible. The principal investigator (PI) collaborates with other staff members at ARL who fabricate glass with specific compositions, permitting the modeling effort to be coordinated with materials pro-
cessing. The modeling technique and validation data being developed in this project will provide valuable capabilities that are unique to ARL and directly align with ARL’s mission to design and improve armor materials. ARL is establishing itself as a widely respected center of research on the peridynamic theory and computational methods. The project also contributes to ARL’s leadership status in the field of transparent armor impact mechanics and materials science.
Understanding the Role of Glass Composition on Properties and Performance
There were several impressive posters reporting on new measurement techniques and diagnostics capabilities. For example, the exciting and novel development of a miniaturized Hopkinson bar (also known as a Kolsky bar), is used for characterization of polymers, metals, ceramics, fibers, threads, and fibrils, at high strain rates. This will help with the development of new models or validation of models at strain rates higher than those that are currently accessible. The development is novel and a breakthrough in the ability to understand materials at the micro scale. Coupled with time-resolved diagnostics such as DIC, dynamic X-ray technology, framing cameras, and so on, this capability has the potential of making ARL a leading research institution in investigating material behavior and properties measurements at high strain rates. The projects that highlight these capabilities are described below.
Advanced Experimental Techniques
This project focuses on sample preparation and fabrication processes for characterizing materials at high-strain-rate loading. Using lasers, mini samples with dimensions of about 500 µm can be made. This technique was developed at University of California, Santa Barbara. Large numbers of specimens can be produced for quasi-static and dynamic testing.
High-Strain-Rate Deformation Mechanisms of Polymers
This project uses new capability to characterize polymers such as polycarbonate and polymethyl methacrylate over a broad range of strain rates as accessed by the mini Hopkinson bar. A range of capabilities was used to establish the response of bulk polymers (primarily polycarbonate and acrylic) over a broad dynamic strain rate regime. Infrared detectors measure the increase in temperature due to deformation, which is especially important in polymers where the glass transition temperature is not far from room temperature.
Rate Dependent Mechanical Response of Polymer Networks
This project is exploring another application of the mini Hopkinson bar for testing of epoxy-based polymers. It involves an integrated modeling and simulation approach to identify possible chain mechanisms to explain the improved strength performance observed in high-strain-rate measurements on epoxy.
Opportunities and Challenges
Methodology for Scale-Bridging in Multiscale Modeling of Materials
In its present state, the multiscale computational method apparently has been applied only to subscale models that essentially provide an equation of state to the continuum finite element model. While the
equation-of-state work represents a significant achievement and promise for this technique, the incorporation of other aspects of material response, such as evaluation of the full stress tensor, which can be challenging, has not yet been addressed. Realistic subscale models for materials involve the evolution of defects that lead to phenomena such as work hardening, thermal softening, shear localization, dynamic fracture and spallation, and so on, which remain to be incorporated. The example provided in a presentation to the panel, a Taylor test, is an oversimplified case. The incorporation of physics is an essential part of predictive codes and cannot be ignored. In the case of explosives and propellants, chemical reactions, deflagration, and detonation have to be incorporated. Close work with Betsy Rice, a globally recognized leader in this field, is highly recommended. The stated goal of connecting quantum mechanics, MD, dissipative particle dynamics (DPD), and continuum models will require the ARL team to investigate multiscale strategies and go beyond the scale-coupling focus and software aspects of multiscale computational modeling. Furthermore, although these aspects present significant challenges that the ARL team is addressing very well, the physical models underlying the subscale computations are worthy of careful research and cannot be treated as a black box. For example, it cannot be assumed that data estimation from DFT subscale models will work the same as with MD or DPD. Collaborations with researchers at the California Institute of Technology (e.g., with Michael Ortiz and William A. Goddard) and other institutions, as well as at the Department of Energy (DOE) laboratories, may be worthwhile to seek paths for the incorporation of the physics in the subscale models and their implementation into the multiscale infrastructure.
Polymer Modeling Research
The methods used in this program can be applied to link continuum models to better understand mechanisms in more bridged structures. Understanding of the mechanisms of these materials and the interface interactions has the potential to lead to improved polymeric armor with better ballistic performance.
Graphene and 2D Polymers
The goal of computational materials design is still largely unrealized in the larger multiscale modeling community. If the ARL group can succeed, the payoff would be very high. Currently, materials such as Kevlar and boron carbide are used for body armor applications. The use of materials such as graphylene would have the potential of a significant reduction in body-armor weight. The ARL effort in this area appears promising, but the activity is largely academic if the material modeled cannot be synthesized. This program needs to include a strong synthesis component.
Composite Materials in High-Strain-Rate Environments
This program is intended to evaluate a set of continuum-level material properties for composites on the basis of computational modeling at smaller-length scales, with emphasis on the use of modeling to optimize the materials composition. The prediction of the composite response from the properties of its building blocks (constituent materials and interfaces) is an exceptionally difficult challenge that has frustrated researchers over the years, partially because of the multiplicity of length and time scales whose mutual interaction affects the progression of material failure. The ARL team has access to new computational tools that help to address the multiscale aspects of the challenge. The divergence in time scales from picoseconds up to milliseconds presents major difficulties. On the other hand, the objec-
tive of understanding and optimizing the properties of individual constituent materials and interfaces appears more tractable. For example, the use of network models for epoxy to improve its properties seems promising. Similarly, the use of MD to understand and improve polymer fiber performance is a reasonable objective. The use of mesoscale models to reproduce and predict the failure modes in woven composites is a sensible plan. The progress composite damage model method for homogenizing and scaling up mesoscale damage simulations also makes sense as part of the overall analysis. The traction versus displacement data that were shown for epoxy-glass interfaces appear to reveal that such interfaces fail under tension at around 5 GPa, which seems high and far exceeds the strength of any epoxy. In summary, the prediction of a full set of LS-DYNA MAT1623 parameters for a real composite solely on the basis of MD and mesoscale computations is perhaps not within reach. The challenges in doing so are enormous, and the investigators need to be objective about the prospects for really accomplishing this. Nevertheless, the computational mechanics community learns a lot by trying to address such challenges. Furthermore, the individual component models for constituent materials and interfaces have value in themselves and may lead to improvements in real materials by providing fundamental understanding. It is good that the modeling team has experience with the practical aspects of composite materials science and processing. It would be helpful to include team members who can synthesize the new materials to confirm the results of the computations in the laboratory, if this is not already being done.
Physical Response of Boron Carbide Under Extreme Conditions
This project involves connecting the quantum-level response to macroscale impact, combining characterization approaches and penetration studies to demonstrate improved performance. B4C is a material of interest to the Army because it is used in body armor, due to its relative low density and high strength. However, unlike other ceramics, there are two distinct features that are observed experimentally. One is the loss of strength of the material under uniaxial-strain shock or dynamic loading conditions, and the other is the observation of localized amorphous phase observed in recovered specimens under bullet or ballistic impact. There has been extensive discussion of strength loss of B4C leading to a lower than anticipated ballistic response of the material. There has been speculation that these two events are related. This has led to a detailed study identifying other physical mechanisms for the observed strength loss. Mechanisms such as amorphization, friction, localized melting, oxidation, and others that were proposed to the panel as reasons leading to strength loss were not conclusive. The technical aspects presented in trying to identify mechanisms for strength loss were quite detailed but not successful. What leads to amorphization is still not clear, nor is it confirmed that it is the primary mechanism responsible for strength loss. B4C is in extensive use and, as a practical matter, it is not clear if significant timely and substantial improvement in performance can be made. However, as a research exercise, it may be necessary to tie loose ends to make an impact on the design and performance of similar future ceramics when deployed.
Grain-Boundary Modeling and Simulation for Lightweight Protective Materials
The researchers work collaboratively with experimentalists in an effort to verify their calculations and thereby generate a better understanding of the effects observed in B4C and other ceramics.
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3 LS-DYNA is a finite element code and MAT162 is a material model for use in LS-DYNA that may be used to simulate the onset and progression of damage.
Grain Boundaries and Interfaces
More work is needed to understand the morphology that has been observed to-date and to relate the results to the dynamics of the system. The methodology could also be used to examine other systems. While the TEM examination of indented samples is being used, one of the major challenges will be the need for operators knowledgeable with the techniques and the scientific information that can be extracted. Sample preparation and interrogation require a level of understanding unlike other techniques used for characterization.
Atomistic and Mesoscale Modeling of Grain Boundaries
This project involves DFT and MD work aimed at explaining how grain boundaries can be engineered to increase the toughness of B4C. The role of intergranular films is still not being evaluated, but it is a goal. The role of amorphization of B4C and its relationship with failure is being evaluated. Failure is another instance of critical phenomena related to approaching a percolation threshold. This is a fundamental research project, and applications are still far away. Nevertheless, the fundamental knowledge gained justifies this effort, especially in view of the fact that the methodology can be applied to other ceramic systems, such as boron oxide.
Modeling and Performance of UHMWPE
Surprisingly, increasing the inter-laminar shear strength of materials has a detrimental effect on ballistic performance. It would be interesting to apply computational modeling to understand why this trend is observed. The project will help to standardize the processing of UHMWPE materials by identifying an optimal set of conditions for ballistic resistance. This in turn will help improve the uniformity and quality of armor materials.
Depleted Uranium (DU) Replacement
Nanocrystalline tungsten still lacks tensile ductility, but processing improvements could lead to improved performance. The research being performed will identify the causes of strain localization and determine critical mechanisms of plastic flow at various strain rates. The project is challenging and is the basis of the doctoral dissertation of an ARL researcher at Johns Hopkins University under the advice of Kevin Hemker. This is a good example of an internal ARL project being used to develop the professional growth and career for existing early-career, bright, and talented staff. This project needs to incorporate wires (W or steel) to increase the tensile strength of the penetrators and their survival during launch.
Optimized Tungsten Carbide Materials for Improved Lethality
This is a perfect example of how ARL researchers are finding applications for powder metallurgy in which a fine-grained, pressure-less sintering process appears to have a high payoff. Current scale-up efforts and ballistic testing will determine whether this new material can be incorporated into weapons.
Exploiting Oxide Dispersion Strengthening in Ferritic Alloys for Lethality Applications
This work demonstrates the potential scale-up possible with the capabilities available at ARL guided by in-depth processing-structure-property correlation studies. Developing dispersion-strengthening analytical models would be helpful in determining the role of dispersoid particles on high-strain-rate properties, as oxide dispersion-strengthened materials typically show sustained strengthening at high temperatures due to the stability of the oxide dispersoids.
Mechanics and Performance of Unidirectional Film Laminates for Ballistic Protection
After further progress is made on the optimal processing conditions, it might be helpful to include a continuum-level modeling component to the project. This might give insight into the thermal and mechanical history in the interior of a laminate during processing and help reveal hard-to-measure effects such as thermal residual stress.
Understanding the Role of Glass Composition on Properties and Performance
This project is an experimental investigation to develop improved glasses as a transparent armor for lighter, durable protection systems. Commercial soda lime and borosilicate glasses currently deployed are not optimum armor materials, and without an identified consumer need for improved glasses, it falls to ARL to develop these materials using an in-house glass processing facility. The focus is to understand the role of Na-B-Al-Si in the casting of glass. The goal is to tie in and identify the role of composition to ballistic performance. As a part of this effort, the researcher is using finite difference peridynamics to model penetration events. It is not clear, however, if there is a modeling effort that correlates the composition of added impurities that would result in identifying the necessary dynamic properties that lead to optimum performance. It would be a benefit to the glass manufacturing program if such a modeling program existed.
STRUCTURAL MATERIALS
Within the structural materials portfolio, there are projects tackling tough, challenging problems in collaboration with ARDEC and other Army users to understand the importance and impact of the work to the success of the mission.
The research approach of coupling modeling with experimentation was evident across many of the efforts reviewed. Of note are programs designed not only to support specific and narrowly focused materials development efforts but also those that will produce tools that may be used more broadly. An example of one such effort is a program directed toward grain boundary modeling of ceramics for light-weight protective materials. A suite of tools is being developed to permit simulation of grain boundary structure and properties under high-rate loading conditions. Although these tools are being used to investigate grain boundary structure and properties of boron-based lightweight ceramics, these same tools will be applicable to the study of grain boundary interfacial relationships across all ceramic materials.
The potential wide applicability for these modeling and simulation techniques has provided the motivation for sparking university collaboration and cooperative research and development agreements (CRADAs) with industry. It is projects like this one that will provide ARL with the capabilities it needs to respond rapidly to future threats.
Accomplishments and Advances
Grain Boundary Modeling and Simulation for Lightweight Protective Materials
This program is focused on simulation of grain boundary structure and properties of boron-based lightweight ceramics—B6O and B4C. The key technical challenge is to discover and exploit grain boundary interfacial relationships at the nanoscale. This can inform experimental efforts to manipulate interfacial properties to optimize lightweight protective armor materials. A suite of tools is being developed to permit simulation of grain boundary structure and properties under high-rate loading conditions.
The research is complicated by the triclinic crystal structure and the existence of many boron-based ceramic polytopes. The tools developed include code to generate initial (unrelaxed) structures based on the 5 degrees of freedom of a grain boundary and simulation of diffraction patterns of polycrystalline material. Simulations of grain boundary relaxed structures and properties are planned after interatomic potentials for boron-based ceramics are tested and refined. Tools are also under development to derive grain boundary mobility and fracture strength once reliable potentials are available.
This is an excellent beginning to model and ultimately improve the properties of grain boundaries and hence polycrystalline boron-based ceramics. These tools may be extended to other crystalline material systems. The effort has collaborations with a large number of universities and is developing CRADAs with various industrial companies.
Nanostructured Metallic Materials
An important nearer-term potential application is a shaped charge liner strong enough to contain a hyper-velocity jet and sustain severe strain rates of 106 to 107 at 200 GPa pressure. It is known that a shaped-charge jet length improves at smaller grain sizes, but no data exists for performance below 10 micron grain size; it is possible that nano-grained copper may provide substantial improvement. It is hypothesized that the Cu-Be materials can be replaced by nanocrystalline Cu-Ta using a nonequilibrium processing method if the material can be effectively produced in bulk form; it is important to address this challenge. Small batches of Cu-10Ta have demonstrated high grain stability, since the Ta particles do not dissolve in the copper and effectively control grain growth. In addition, the material has high ductility, low creep rate, and exhibits superplastic behavior, making it a potential candidate material for a shaped charge liner.
This program is in its third year and is meeting the objectives. Future work will include experimentation with a Ni-Y system for higher-temperature applications.
Next Generation Lightweight Armor Ceramics
This is a follow on project to previous work on ceramic armor materials. Previous work resulted in transition of boron ceramic armor from companies such as CoorsTek. The objective for this new project is to develop new ceramics and geometric structures that deliver improved armor protection against small-arms fire with less weight. Desired weight reduction is on the order of 20 to 30 percent, and it is unlikely that existing standard boron or silicon carbide ceramics will achieve this objective. The hypothesis of the project is to use new, very hard ceramics. Initially, B6O and AlMgB14 are being studied. Both of these ceramics have better hardness properties than existing boron and silicon carbide ceramics. In order to use these new materials, the project has been divided into three tasks:
- Scalable synthesis and processing of the materials. Powders for many of the desired ceramics are not available, prohibitively expensive, or of questionable quality. Sintering of ARL synthesized powder is being studied to assess sintering properties that lead to economic sintering of test coupons. These coupons are being ballistic tested at the subscale.
- Characterization of the novel heterogeneous microstructures that occur in these ceramics. Understanding the properties of these unique heterogeneous microstructures is not well established. Microstructure models and resulting properties are being correlated to ballistic testing and damage mitigation.
- Development of processing science and exploitation of heterogeneous microstructure for improved armor performance. This task is just starting. Exploration of previous related theory, process development, and modeling is under way. Methods for heterogeneous layering and feature creation for better protection is also being studied.
The project is aligned with Army needs, and team members are coordinating with Program Executive Office Soldier and the industrial community. The project appears to be on track with superhard ceramic powders being produced at ARL in the quantity required for sintering experiments. Sintering samples and small-scale plates have been produced. These samples are being examined for microstructure and unique microstructures have been identified.
Grain/Interface Boundaries in Boron Carbide Due to Contact Loading
This project addresses the formation of regions in B4C that are interpreted as amorphous bands. The amorphous bands are generated with Knoop microhardness indents and are analyzed in cross-sectional TEM analysis. The main project objective is to assist in interpreting the indentation size effect. This effect describes the rapid increase in hardness with decreasing indentation load. In ceramic materials, this effect is attributed to the formation of cracks and bands such as the presumably amorphous bands observed in the B4C indentation. The project has yielded TEM images that are used to rationalize earlier results and parallel efforts by other ARL personnel.
The TEM images show that microcracks develop from the linear bands. However, the link to the overarching objective of explaining the indentation size effect is still in question.
Three-Dimensional Through-Thickness Reinforcement Composite Armor Materials for Advanced Vehicle Protection
This is a mature project in its sixth year. The effort involves development of a complex model at the material level that is able to predict the fracture and energy dissipation of a very complex arrangement of materials, including ceramics and woven fibers. By looking at experimental impact data, the PI has quantified the crack initiation and propagation in the ceramic as well as at the multiple material interfaces. The model has been used to design armor that performs as well as conventional steel armor at half the weight. A cost/material/fabrication model has been developed that can be used to select an optimum design at the system level. There is one application in production and a number of others under evaluation.
The project team has excellent knowledge of the technical challenges involved, and even though the cost model was not developed in the most scientific way, all of the relevant issues have been addressed in an appropriate manner. The results met the project objectives.
Multi-Functional Multi-Material Topology Optimization for Lightweight Unmanned Systems
This is a computationally intense design-oriented effort with the potential to disrupt maneuverability and protection capabilities in unmanned systems. This agility goal is motivated by the Army’s identification of remote and autonomous systems (RAS) as a vital effort to maintain operational advantage and overmatch on the battlefield. In order to lighten the burden of the soldier, augment protection, increase mobility, and enable joint combined arms maneuvers, advanced topology optimization methodologies for the design of multifunctional, multimaterial components are formulated and implemented to enable and fully exploit unmanned systems.
The work has demonstrated the use of computational algorithms for designing some model actuators. A demonstration platform was developed by combining materials with different moduli to expand the design window for actuator systems.
Defect Engineering of Nanocrystalline Alloys from Powered Precursors
This project is intended to develop bulk, thermally stable nanocrystalline materials with tailored properties for future force and ballistic applications.
Cu-10T, an alloy powder, is prepared by ball milling and consolidated by equiangular extrusion, which produces a solid solution of Cu and Ta, despite being completely insoluble. Upon heating, TEM investigation showed nanometer-sized Ta particles that pin the fine 150 nm grained Cu structure. Heat treatment up to 1000°C showed very limited grain and particle coarsening. The Ta particles appear to have an amorphous core, but further research is necessary for confirmation. Atom probe tomography is planned to determine the extent of carbon and oxygen contamination of the interphase boundaries. Collaboration with the Mishin research group at George Mason University is providing theoretical support of the coarsening processes through MD simulations. The simulations and TEM observations show only Ta particle coarsening at grain boundaries.
This is excellent research that combines theory, experiments, and analysis. Work remains to understand mechanical properties at elevated temperatures.
High-Performance Powder Metallurgy Titanium
This project has the objective of producing low-cost (estimate $1 to $5 per pound for unalloyed powder) powder that can be consolidated into various Army applications by additive manufacturing or metal-injection molding processes with properties equivalent to those of wrought titanium. The effort employs a unique process of hydrogen sintering and phase-transformation process developed at the University of Utah. Variations of this process can use blended elemental powder, titanium sponge, fines, and turnings to make alloyed and unalloyed powder, achieving densities of 99 percent as sintered, and approach 100 percent density with gas isostatic forging technology. The material produced through this process may be strong and tough enough for armor applications and components for a machine gun, among others. It has not been demonstrated for aerospace or high-temperature applications.
This is a new fiscal year (FY) 2016 effort with great potential. If this effort is able to demonstrate feasibility, further development of the process will be required for scale up. Work so far meets the program objective, and the path forward has been delineated. Work scope includes identifying the most promising Army applications so that demonstration parts can be produced. This approach will allow the Army to make an informed decision regarding the scale-up of this technology.
Corrosion Science
This research effort is intended to develop an understanding of fundamental corrosion mechanisms of metallic materials used by the Army and then apply this knowledge to develop suitable corrosion-mitigation strategies. It is currently focusing on magnesium alloys corrosion in aqueous environments—including understanding of corrosion initiation sites, influence of alloy chemistry, trace element effects, and so on. The research employs state-of-the-art instruments, such as an electrochemical atomic force microscope.
The effort is in its fourth year and has produced a predictive atomic level model (based on density function theory) of magnesium corrosion. Various mitigation strategies suggested by the model are being tested.
Extreme Mechanical Response of Electro-Chemical Fabricated Nano-Crystalline Metals
This is an early research effort focused on grain refinement to the nanometer scale through a systematic approach of electrochemical fabrication, unique high-rate testing, and mechanistic crystal plasticity modeling. This research is an excellent demonstration of the approach to tightly couple modeling and experimental work. This project will develop an understanding of the mechanical response of grain refinement in three common crystal structures at high-deformation rates.
The success in developing small-scale fabrication and testing capabilities will chart new territory in the understanding of grain refinement and strain-rate design space. This project leverages work from the advanced experimental techniques project and greatly reduces the laser setup time for sample preparation from days to hours. While miniaturization of the split Hopkinson pressure bar test for very high strain-rate testing is not new, the testing of strain rates up to 106 s-1 is novel. The microscale sample preparation and miniaturization of the Hopkinson test demonstrates the resourcefulness of the researchers and collaboration across projects. This work is foundational, and the researchers are encouraged to continue to look at synergies in other research areas, such as the enhancement of corrosion-resistant materials.
Functionally Graded Protective Coatings by Materials by Design
This project addresses an urgent Army need. Sand is readily ingested into jet engines where it melts in the hot section of the engine and adheres to component surfaces, rapidly degrading material performance. Large particles can be removed by various techniques, but small particles remain.
Superalloy components have ceramic coatings that act as thermal barriers. Modifying the coating (either the entire coating or a surface layer) could change the ability of the molten sand to physically penetrate the coating, forestalling degradation. Another possibility is that some kind of mechanical cycling could fracture and force off the sand layer after it re-solidifies.
The project includes spray synthesis of different ceramic coatings, followed by characterization and testing to simulate field conditions. The project is currently focusing on developing a basic understanding of the interaction of the molten silica with the surface of the coating. Incomplete information is available on basic properties of proprietary coatings. The project is at a relatively early stage; for example, no work has yet been done on graded coatings. Publications show excellent teaming on the project.
Interfacial Load Transfer in Ring-Opening Metathesis Polymer-Based Composites
These composites are considered candidate resins for ballistic resin-fiber composites. A drawback of this new class of polymers is the surface chemistry that hinders, if not prevents, bonding with fibers. The current research is an integrated modeling and experimentation project that addresses several aspects, including the interface engineering to achieve tailoring resin-fiber bond strength, the synthesis and scale-up of ring-opening metathesis polymer-based composites (ROMPs), and mechanical testing of the composites. Most research on resin-fiber composites is driven by aerospace applications and, therefore, does not consider high-strain-rate effects. A particular focus for the Army is therefore to develop capabilities to control the fiber-resin bond strength and to tailor the composites to varying ballistic conditions.
The approach to achieve ROMP-fiber interfaces suitable for high-strain-rate deformation on the experimental side is based on film and coupling agent chemistry experiments. Modeling efforts involve mostly MD simulations that are used to determine constitutive models for the interface that are then used for finite element method (FEM) simulations of the composite mechanical behavior. The predictions are evaluated with mechanical shear tests and sample geometry. A test was developed to facilitate comparison with the modeling predictions.
Opportunities and Challenges
Materials for Robotic Augmented Soldier Protection
This program is a systems engineering challenge that presents opportunities for the synergy of materials, design, and manufacturing. It addresses the need for an overall strategy for robotics in the battlefield and in particular for the role that materials will have in the different robotic missions. At this stage in the project, from a materials-driven perspective, the optimum suite of materials for different robotic missions has yet to be identified. Starting with an overall robotics strategy, a comprehensive materials science strategy could be developed. As an example, this project could be quite ambitious with the identification of materials needed by unmanned aerial vehicles to protect soldiers against various threats (ballistics, radio frequency [RF] bombardment, electronic warfare) and micro air vehicles to operate in different environments. It could include disruptive materials needed for energy absorption and utilization of incoming threats (ballistics, shrapnel, RF) for perimeter robots or soldier-assigned robots; materials needed for robots to survive extreme environments (e.g., fire, sand, corrosive, radioactive, volcanic ash) to perform dangerous missions, including in situ repair and maintenance in hazardous environments and rescue operations.
This project has the potential to be a strategic campaign with a focus on the triad—materials, design, and manufacturing—approach to multiscale, multimaterial, multidisciplinary, and multifunctional robotics strategy. Research and collaboration with the Defense Advanced Research Projects Agency’s (DARPA’s) robotics initiative and SPARC (the partnership for robotics in Europe) and understanding of the competitive landscape is needed if this project is to succeed.
Nanostructured Metallic Materials
This program is intended to develop stable bulk nanocrystalline metallic alloys from powder precursors for lethality applications. Nanostructured metallic materials can be unstable, making the retention of high strength and exotic properties challenging. Low thermal stability also constrains processing techniques, as well as service temperatures. Nanocrystalline metals usually have low ductility but high strength, making
their application as a structural material difficult, requiring methods to control nano-grain size to achieve desired strength levels while retaining required ductility and fracture toughness properties.
Potential applications of these materials include advanced personal and vehicle armor, warheads and kinetic energy penetrators, and advanced structural alloys for weapon and surveillance platforms.
The program will continue to develop the foundational science of grain boundary solute engineering in nano-duplex structures, devise engineering methods to control deformation at the nanoscale, advance processing techniques capable of producing suitable bulk material, and pursue the design and development of components that utilize these nanostructured materials. It needs to be noted that related work for producing bulk nanocrystalline material is ongoing in Australia, the European Union, Russia, and China.
Grain/Interface Boundaries in Boron Carbide Due to Contact Loading
This project is part of a larger effort to understand the deformation behavior of B4C under high-strain-rate conditions. The combination of Knoop hardness indentation and TEM cross-sectional analysis is used to reveal cracks, linear amorphous bands, and underlying microstructures. Additional work will be necessary to establish a relation between the linear bands, cracks, and the indentation size effect. Suggested additional experiments include annealing experiments that could be used to crystallize the amorphous bands and corroborate the amorphous nature of the bands. Instrumented indentation experiments could be used in conjunction with stress analysis to compare the experimental observations with predictions of maximum shear stress patterns in the cross-sectional images.
Multi-Functional Multi-Material Topology Optimization for Lightweight Unmanned Systems
Numerous potential applications have been identified, including structural radomes and structural capacitors, but much more work is needed before these applications are realized. A major gap in the research is to include microstructural sensitive properties into the topology optimization algorithms. Added expertise or collaboration with materials scientists would help.
Corrosion Science
As this effort moves forward, there will be a need to consider how the mitigation would be applied to Army hardware. For example, ion implantation does not perceptually change a surface, so detecting if an area has been adequately treated is challenging, and components with large surface areas may not be able to use this process. This research effort could consider applying a protective coating using cold spray methods.
Extreme Mechanical Response of Electro-Chemical Fabricated NanoCrystalline Metals
A follow-on project to investigate the differences in properties at the component level due to the influence of factors (such as geometrical features, volume, size, heat treatment, surface finish, and so on), compared to the specimen properties, would enhance the computational framework to validate the models of component-level structures and provide guidance on material-design-manufacturing choices. Researchers are encouraged to stay connected with the National Nanotechnology Initiative and the Institute for Soldier Nanotechnologies. To ensure that ARL continues to be at the forefront of this research, it is highly encouraged that researchers participate in nanotechnology-related conferences, such as the SPIE (the International Society for Optics and Photonics) nanoscience and engineering conference and the Materials Research Society conference on nanoscale materials.
Interfacial Load Transfer in Ring-Opening Metathesis Polymer-Based Composites
This effort began in FY2016 and ends in FY2018. The metrics for the project involve demonstrating the ability to synthesize and control interface bonding agents and to develop validated predictive MD and FEM models. So far, a bonding agent has been found that can be tailored to achieve different bonding strengths between fiber and resin. A conventional fiber pullout test could yield data that are more suitable for the project than the lap shear test that represents the composite as a whole, as opposed to the fiber-resin interface.
The project has clearly met a main objective—to identify a resin-fiber bonding agent that can be adjusted for different interface strengths. But at the same time it is not clear who and how this agent was identified: Did the input come from elsewhere? Was the bonding agent found empirically? Some of the references by the PI on the project seem to have been published in 2015, before the start of the program. The other project objectives have not yet been met.
NanoMicro Particle Integration for Composites
There is still much work to be done to meet the objective of developing computational algorithms to design game-changing, lightweight, multifunctional, multimaterial components produced via additive manufacturing. Numerous potential applications have been identified, including structural radomes and structural capacitors, but much more work is needed before these applications are realized. The project is not yet at the point of incorporating materials properties and processing into the topology optimization algorithms, and including microstructural sensitive properties in the algorithms is a major unrealized opportunity. Broadening the project scope to take advantage of materials design strategies and adding expertise or collaboration with materials scientists would help.
ELECTRONIC MATERIALS
Overall, the quality of the electronic materials applied research and development (R&D) efforts is outstanding with well-supported, long-range projects that are maturing and moving into manufacturing (MANTECH programs), balanced by new, advanced research initiatives.
The R&D efforts undertaken in response to the dramatic change in the battlefield environment—transformed from a relatively small number of large, well-supported divisions to smaller distributed groups—are impressive. This transformation challenges the Army to provide the infrastructure for supplies and, particularly, equipment repair. Many of the ARL projects reviewed are concerned with facilitating this transition. An exciting example of one of these efforts is the application of basic metallurgical science to provide field-deployable, individual custom repairs to significantly damaged vehicles—helicopters, armored vehicles, and so on.
Accomplishments and Advances
Vertical 2D-3D Semiconductor Heterostructures
This program captures one of ARL’s more aggressive and admirable efforts to map basic research in advanced 2D electronic materials into Army needs. The underlying concept of this newly funded project is to harness vertical transport through 3D-2D-3D stacks, as opposed to more conventional in-plane strategies, to achieve bipolar-like transistors that may open up new performance regimes. Specifically,
the project includes both experimental and theoretical efforts to optimize epitaxial growth, electronic interface functionality, and transport characteristics of 2D MoS2 on bulk GaN with an eye toward applications for high-power, high-speed RF transistors.
This work is at an early stage, but the PI and the team have set out a promising multifaceted program that is well positioned to contribute to the basic understanding of the efficacy of fabricating and harnessing vertical transport in these structures. The initial epitaxy has illustrated islands of single-crystal MoS2 films that have shown promising characteristics, including 20 times photoluminescence enhancement relative to exfoliated films. Experimental work has also included conductive AFM and Kelvin probe measurements to capture contact resistances, tip/MoS2 Schottky barrier height, and work functions for the epitaxially prepared materials. While efforts to date have been all n-type, the team is moving to p-type GaN to enable evaluation of expected diode behavior.
Energy Efficient Soldier Radios
This program is directed to the critical need to improve power efficiencies in mobile, secure radio communications to relieve the battery weight burdens on the soldier, which can be as large as 60 percent of a soldier’s load. Improvements of 10 times would reduce this load by half. The presentation to the panel demonstrated a strong grasp of the landscape and technology options, ranging from custom integrated circuits (ICs), field-programmable gate arrays (FPGAs), and graphics processing units (GPUs) to simply updating designs to more current Si design nodes and III-V technology. Radio range requirements, combined with the digital processing demands encountered in a contested-spectrum environment, present unique challenges for soldier radios relative to commercial telecommunications. In addition to targeting nonconventional mixer-first architectures, this effort is harnessing state-of-the-art 3D architectures available through DARPA’s diverse accessible heterogeneous integration (DAHI) project to optimally utilize III-V’s for RF power and front-end, with Si for custom FPGAs and dynamic random-access memory for digital and baseband to substantially reduce power.
The project is targeting compelling Army needs and is leveraging current technology advances. The ARL team is working effectively in an industry and academic collaboration that is proving fruitful. Beyond just updating the older soldier technology to current low-power Si, modeling suggests that receiver power can be reduced to as little as 29 to 54 percent of any previously reported designs.
Porous Silicon for On-Chip Energetic Materials
This is an innovative, well-conceived project directed at a clear and unique Army need: Equipment left behind, or provided to a former ally that is now an adversary, is being used by the highly distributed terrorist groups that have appeared in the past decade. Being able to remotely disable any weapon, vehicle, land mine, etc. is a crucial requirement for new equipment. ARL appears to be breaking new ground in this area and needs to ensure that the other DOD laboratories are aware of this basic technology.
Creating a Soft and Stretchable Power System
This project addresses the trade-off between performance and stretchability in power conversion. Inductors have been chosen for study because they are components in wireless power systems, and most features that make an inductor good also make it rigid (thick metal cross sections, parallel traces, magnetic cores). This research involves creating a stretchable magnetic core inductor. The project fits well with ARL campaign plans, including plans to integrate multimodal harvesters with small-scale power
management units for low- or no-power sensors and wearable electronics as well as plans to exploit new materials and transduction phenomena for new power conversion topologies. The project appears to be well motivated by future Army needs in wearable electronics.
ARL staff believes that they are at the forefront in area of stretchable power; whereas others are more focused on sensing, displays, etc. ARL noted its strengths in magnetic materials and modeling and design, which is supported by its excellent publication record. The focus is on stretchable magnetic materials (liquid metal inductors and magnetic particles in elastomers) for power components and on modeling and design; this appears to be well differentiated from research at other institutions. Devices have been fabricated and tested, and the publication record for the project is extensive. There is an excellent vision for a range of future applications.
Wide Bandgap Materials and Devices
This project is a new (1 year), fundamental program motivated by the need for more efficient high-power RF sources than are currently available in SiC devices. The work is at a materials and properties level, including performance diminishing factors, such as defects. The researchers on this effort have a good range of materials capabilities and characterization tools available to pursue the development of AlGaN materials and RF devices and good connections to startup companies for collaboration and potential manufacturing, if successful several years down the road. The project is well supported in the materials and devices area but might be aided by adding a staff member with a strong background in the areas of high-power inverters, drives, and hybrid vehicles.
Improved Voltage Control and Stability of SiC Power MOSFETs
This project is a noteworthy example of ARL work maturing into materials processing and devices that can strongly impact Army needs. The effort has discovered a defect source of instability that was completely unknown to the larger technical community and has led to new testing standards for SiC metal-oxide semiconductor field-effect transistors (MOSFETs). Working collaboratively with academic and industry partners, the team has been able to model the physics of voltage instabilities in gate dielectrics for SiC power MOSFETs and promote design and process modifications to address the problem in a commercial implementation. ARL needs to be sure to include examples such as this in future visits because it gives the panel concrete, unambiguous examples of impact. It is also a remarkable illustration of the effectiveness of geographically and institutionally distributed teams in solving design and process challenges in manufacture.
SiC Avalanche Diode
This project addresses an Army need for higher performance protection of electronics (e.g., 600 V vehicle buss) and is leveraging industrial collaboration with Cree, Inc.
High-voltage avalanche breakdown diodes (ABDs) in SiC were shown experimentally to provide improved inductive surge suppression performance relative to commercially available silicon transient voltage-suppression devices. Follow-on work is addressing packaging and higher energy dissipation designs. Beyond publications, the research conducted in this project has successfully established a baseline proof-of-concept to support a commercialization decision by Cree, Inc., to provide SiC ABDs for application in Army kV-level surge environments.
Validated Numerical Model of 4H-SiC Insulated Gate Bipolar Transistor
This project is motivated by high-voltage Army applications (20 to 27 kV) such as fusing, tamper response, and thermal batteries. Because of the higher voltages, special challenges and differentiation include corona effects not seen at lower commercial voltages. Good modeling work is coupled to a foundry that supplies functional chips for high-power switching and systems incorporation. The models have been validated with experiments and predict, for example, temperature profiles in the materials.
Hybrid 3-D Additive Electronics
This is an exciting new capability with wide-ranging applications, including prototyping of new devices and structures and the ability to produce replacement parts in the field. Specifically, this project is investigating the rapid automated manufacture of 3D-additive-printed structures with integrated sensor and other electronic packaging. It is supported by a unique high-precision 3D-additive-process machine that performs extrusion printing, film printing, sintering, and pick and place robotics. The work is well motivated by the Army’s need for in-field fabrication and repair, cost reduction (e.g., 3D printed components compared to high-cost machined components), and sensors and other structures on textiles (e.g., for human monitoring). The unique ARL-developed system was made possible by great deal of sound, basic science.
Linear, Efficient 216 GHz InP Transmitter
This project is directed toward a specific Army need based on InP technology—a novel mixer-less transmitter design in high-speed InP circuits that enables higher precision in amplitude in phase to achieve complex constellations despite underlying strong nonlinearities of components. The research team achieved a record 100 mW transmitter power with a spectral efficiency 16-QAM (or quadrature amplitude modulation) format at 216 GHz. This work demonstrates impressive sophistication in RF design at ARL that leverages both strong corporate collaborations for component support and prior ARL work to achieve outstanding results captured in patent applications, publications, and conferences. The devices and amplifiers are all made by outside foundries with good industrial collaboration.
Enhanced RF Device Performance Using Metaferrites
This is an outstanding project motivated by Army communication needs (high-performance, low-profile antennas for vehicles and aircraft; tailored bands). This project provides another powerful illustration of transitioning basic research into novel but practical solutions to address Army needs. Using a novel metamaterials approach in a highly sophisticated layered medium, the international team was able to realize reductions in the required stand-off from a vehicle surface by 85 percent relative to commercial solutions, while also improving the signal-to-noise ratio. Development proceeded from recognition of opportunity of novel materials, to initial evaluation, to refinement, to prototype antenna for vehicle (2″ height versus 14″ height, higher bandwidth, lower losses, etc.). The effort is integrated with other ARL capabilities in RF system design and is now transitioning to industry through the MANTECH program for use by U.S. Special Operations Command to reduce cost. However, there will still be a significant role for ARL to ensure reliable knowledge transfer of the very sophisticated, many-layer structure that needs to be uniform over a quite large area. This project is an example of a successful transition of 6.1/6.2 mission funding.
PiezoMEMS
This project is well motivated by Army communication needs (high performance mechanical filtering in radios including reduced losses and noise) and potential applications for gyroscopes.
The project leverages ARL’s leadership in PiezoMEMS research and is well integrated with other ARL capabilities, including fabrication and RF system design and coupled into commercial designs for Army use, such as the common sensor radio. The team is commended for outstanding collaborations across the academic community, engagement in a strong portfolio of industry CRADAs, and commercial licensing of technology. This is also one of ARL’s most prolific efforts with 55 publications and presentations in the past 2 years alone. The underlying materials capability can now support sophisticated filter architectures, putting continued pressure on the development of highly accurate modeling.
Opportunities and Challenges
Vertical 2D-3D Semiconductor Heterostructures
There is a significant effort in modeling in this program—which is absolutely critical, although the impact is less clear—because the efficacy of efforts to model vertical transport at van der Waals interfaces using the various available codes, especially in the context of doped materials, has not yet been well developed. The focus is entirely on vertical transport, but lateral transport and novel planar devices might also benefit from this effort.
This innovative program raises many good questions and opportunities for scientific discovery, and also includes valuable collaborative elements with the National Institute of Standards and Technology, Pennsylvania State University, and the University of Arizona that may help to illustrate the impact of ARL’s efforts to achieve a more open, collaborative national engagement. ARL needs to give this high-risk program ample time to build a foundational understanding of the technological potential of 2D/3D solutions to address challenging Army needs.
Energy Efficient Soldier Radios
This project spawned extensive discussion of the impact of digital security implementations that have difficulty keeping up with commercial advances, owing to long defense procurement cycles, and the potential that adversaries may achieve an advantage in some circumstances using commercial off-the-shelf (COTS) telecommunications solutions with lower size, weight, power, and cost. With commercial smartphones having a 2-year design cycle, in about a decade, a soldier could be stuck with a 3-generations-old phone compared to the weight, power, and performance capabilities of the newest-generation phones. The Army is uniquely sensitive to this problem, both from a cost and weight perspective, and ARL needs to work closely with the security agencies to quantify these risks and find ways to dramatically accelerate deployments using current commercial design nodes plus FPGAs that might be added to customize and provide security codes that could be changed on a very frequent basis and provide adequate security to commercial designs.
Porous Silicon for On-Chip Energetic Materials
This technology hinges on the ability to integrate fuses, a small destructive charge, with silicon ICs. One application for these ICs is already in transition, a thermal battery supported by a MANTECH pro-
gram. The researchers have done an excellent job of understanding the system and mechanisms from an analytical and an experimental perspective, which is especially true for the important roles of hydrogen and unreacted amorphous silicon. The investigators need to look for a more environmentally friendly oxidizer and establish its stability with age as well as continue both experimental and analytic studies of the mechanisms and morphology of void formation and oxidizer penetration in the manufacturing process.
Hybrid 3D Additive Electronics
The ability to do 3D microscopy using computerized tomography (CT) scanning is intriguing, and the researchers need to reach out within ARL to see where else it might be useful. One application that comes to mind is the analysis of potential counterfeit electronic parts, which is an expanding problem as IC manufacture has moved mostly to China and obsolescent parts are difficult to find.
The work now is more focused on technological testing and developmental trial on application to Army field-deployed systems. Immediate challenges include integrating G codes for 3D printing assembly robotics that are generated by different software packages; vendors are expected to eventually provide integrated software. The presenter to the panel was a relatively new postdoctoral researcher; hence, the project is only in its very early stage. While it was not clear what the future scientific dimensions of the project will be or what was the anticipated research timeline, it was clear that the project will be relevant to several research thrusts in areas such as alternative energy, novel sensing technologies, lightweighting, and lower-cost manufacture and supply.
Diamond: Novel Material for RF Electronics
This is an excellent research topic because there are major unanswered challenges. This effort addresses an Army need and is supported by good industrial collaboration. Success will definitely be well into the future.
Pristine Layered Materials-Complex Crystalline Structures
This is a long-range effort investigating van der Waals–bonded crystalline layers separated by self-assembled monolayers, with the goal of improving transport and possible foundation for multilevel logic through tunneling between levels created in the ultrathin, multilayer structure. This is a high-risk project with uncertain payoff and demands that evaluation criteria be established by which to assess the continuation of the project.
Although motivated as a device performance investigation, this effort is in fact basic materials processing research focused on deposition and microscopic evaluation of novel, complex, 2D-layered structures. At this early stage, this is understandable, but the team needs to nevertheless select some prototype device architectures that will help to drive the materials configurations under study toward the most potentially useful targets. Overall, this project is at too early a stage and too undefined to assess potential for impact.
OVERALL QUALITY OF THE WORK
Overall, the researchers and the management in ARL’s Materials Research Campaign are of high caliber. ARL has utilized short-term (5 years) hiring options effectively to increase the number of early--
career staff and postdoctoral researchers who are bringing new ideas, technologies, and enthusiasm to the organization. The level of dedication and enthusiasm was notable and contributed to the creative environment important to scientific and engineering productivity. ARL is to be complimented for following through to establish distinguished postdoctoral fellowships, as recommended by the ARLTAB 2 years ago and reiterated last year. This will undoubtedly attract more top early-career talent.
Most of the projects presented are excellent and have a pervasive potential impact—both short and intermediate term—as well as providing capabilities that will enable the swift response to unanticipated future needs. The scientific soundness and the use of fundamental sciences are outstanding. The project portfolio fits well with both global thrusts and the national agenda, with research projects falling at the intersections of biotechnology, nanotechnology, advanced materials, energy, and the environment.
ARL is making progress in its quest to become a premier research institution in the area of materials science. Several postdoctoral researchers have joined ARL as full-time employees after completing their fellowships, an indication that laboratory management is providing an attractive environment for early-career researchers. It is commendable that the ARL materials science talent pool has a good mix that ranges from experienced, savvy scientists and engineers to bright early-career professionals. There appears to be good diversity with respect to gender and ethnicity.
Collaborative efforts have been demonstrated both across ARL and with external entities. All the projects reviewed are engaged in collaborative efforts to various degrees; this is commendable. The next level of excellence can be achieved by improving the efficiency of this collaboration to deliver better focus, quality, and selection of projects. Internal collaboration across the divisions and directorates is as beneficial as extramural collaboration. ARL’s open campus initiative can enhance collaborations.
Advances in biomaterials are essential for applying biology to detection and sensing. The fledgling field of bioinspired and biomimetic materials will be an important source of inspiration and insight for the future materials scientist. This relatively immature ARL thrust is growing rapidly and shows tremendous potential. Because biology is a growth area, ARL has an opportunity to identify and recruit a critical mass of microbiologists and polymer/organic chemists and needs to be looking well into the future to create an integrated community of researchers.
Developing and improving energy storage devices and batteries will be essential if the future warfighter is to gain an advantage from the increasing availability of relevant technology. The same advances will also find applications across a wide nonmilitary spectrum. ARL’s research in this crucial arena is broad, covering different devices, fuels, and applications across a wide range of time and size scales. ARL needs to move aggressively to capitalize on internal and external advances in the energy and power arena. For example, the world-leading results on enhancement in QWIP efficiencies need to be translated into capability demonstrators for manufacturers and customers. ARL needs to work more aggressively to leverage external advances in silicon photonics, especially with regard to heterogeneous materials.
Engineered photonic materials are necessary for sensors, energy generation, and improvements to device performance—all essential to the future warfighter. The portfolio of the engineered photonics materials group shows a good balance of high-risk, longer-term work with nearer-term, customer-driven solutions or incremental but critical technology refinement. ARL needs to continue on its course to broaden modeling in support of a larger number of problems and applications. As a prototype for this expansion, ARL needs to look to its short-wavelength infrared device modeling and optimization. The software tool set coming from this research is essential for designers, and it may also provide critically sensitive parameters for potential use in process control for commercial partners and suppliers of imaging solutions to the Army, which necessitates engaging with the manufacturers.
Other methods of high-strain-rate deformation and fracture and shock wave physics experiments that are beyond just ballistic testing need to be explored either through collaborations or development
of in-house capabilities. Such experiments are essential in helping to probe material-level mechanisms that can be used for validation of models. Some shock physics experimental capabilities already exist at ARL. It would be valuable to integrate these capabilities as part of the high-strain-rate efforts, while also leveraging the dynamic sector facilities at the Advanced Photon Source. ARL is leading efforts in miniaturized Hopkinson bar experimentation to obtain high-strain-rate material properties. Strategically, technology transfer of this capability could be pursued at various institutions and laboratories in the technical community to help foster novel ideas, and ultimately make this a viable and significant technology nationwide.
With regards to multiscale computations, it would be necessary to integrate physical models in conjunction with the framework being developed, since it may not be a trivial task. There may be opportunities for collaborations in multiscale computations with other laboratories as well as in other fields. Just like the open campus concept is enhancing collaborations and bringing new ideas, embedding ARL researchers at other university and national laboratory campuses, particularly in the area of high-strain-rate deformation, will be equally beneficial. The work on polymers seems to integrate aspects of modeling and simulations, with characterization and testing, and synthesis and fabrication with the unique facilities available. Similar integration was not obvious for the work on metals and ceramics. The research on glass fabrication can be further enhanced by understanding the role of impurities (boron, sodium, etc.) at a molecular level. This is crucial and would promote glass fabrication research using computational material design concepts to obtain an optimum material with the desired (dielectric, optical, and strength) properties for applications other than just for ballistics.
CONCLUSIONS AND RECOMMENDATION
ARL’s Materials Research Campaign has two mission elements: (1) to respond to existing and anticipated threats and capitalize on recognized opportunities to protect and enable the modern warfighter and (2) to develop the necessary knowledge base, tools, and capabilities that will allow ARL to respond rapidly to unanticipated threats and opportunities—those of 2035.
Every program reviewed can be described in a space of these two mission elements. For example, the scale-bridging and multiscale modeling of the materials program is intended to develop tools and capabilities that will enable materials design at some point in the future—the first mission element. On the other hand, the energy-efficient soldier radios program is directed toward a specific goal—the second mission element. While one mission element or the other dominates these two efforts, most programs have some component of each. For instance, grain boundary modeling and simulation for lightweight protective materials is a fundamental study of B4C that addressed the important issues at the root of its poor ballistic performance but at the same time is developing tools that may be usefully applied to the study of other ceramic materials.
The ARLTAB was confronted with the task of assessing programs monolithically, which in general involved determining how successful or how likely the program is to meet it materials development goals. One can imagine that the goals are being met but that opportunities are missed to enhance or broaden the tools or knowledge base, or that the tools available are simply not found to be applicable to a specific problem, although they might have potential elsewhere. In essence, ARL is constructing a number of subsystems that may be integrated and employed to respond rapidly to the unanticipated, but it is for the most part evaluating the performance of the these subsystems within the context of a full system.
It would be useful to consider the product of the Materials Research Campaign at the directorate level as new materials subsystems are created and an understanding of their utility is gained, while the
product at the campaign level becomes the expertise to assemble these into an efficient materials development system as well as the ability to determine what new subsystems are required.
Recommendation. ARL should create a structure that places all of its Materials Research Campaign programs within the context of a materials development system. On any project, a researcher should be able to clearly explain how their research fits within this system. ARL should develop criteria that allows for the assessment of individual subsystems of the program (to determining if a subsystem allows ARL to respond to unanticipated threats and opportunities in a rapid and efficient manner) as well as the overall performance of the system.
