High-Temperature Materials Systems: Emerging Applications, Materials, and Science Gaps

Proceedings of a Workshop—in Brief

The U.S. Department of Defense faces a complex array of challenges in its efforts to retain the United States’ position of technological dominance and to protect national security. Several aspects of materials and manufacturing, especially the ability of materials to operate with predictable behaviors in high-temperature environments, pose particular challenges. To illuminate issues surrounding novel high-temperature materials discovery, development, manufacturing, and application, the National Academies of Sciences, Engineering, and Medicine hosted a workshop titled High-Temperature Materials Systems: Emerging Applications, Materials, and Science Gaps on May 10–11, 2022.

The workshop was organized by a planning committee of the National Academies’ Defense Materials, Manufacturing, and Its Infrastructure (DMMI) Standing Committee in order to facilitate a multifaceted, multidisciplinary discussion around the discovery and development of emerging materials that can predictably withstand the extreme challenges of high-temperature environments (>1500°C), as well as what theoretical, scientific, and engineering gaps impede progress. In presentations and moderated discussions attended by more than 180 participants in person and online, speakers from academia, industry, and government shared their experience working with high-temperature materials, examined the industry’s current needs, and explored potential future directions. Of those that self-identified at the start of the workshop, 28% were professors, 9% postdoctoral researchers, 9% graduate students, 19% industry employees, 17% government employees, and 19% nonprofit employees and others.

Haydn N.G. Wadley (Workshop Chair, University of Virginia) opened with an overview of the workshop structure. The first session explored challenges with the design and discovery of materials, including alloys, ceramics, and composites. The second session examined new approaches in manufacturing intended to create scalable, sustainable supply chains for novel materials systems. The third session focused on existing materials testing facilities and the high-temperature testing approaches they support. Last, the workshop’s fourth session explored fundamental gaps such as the thermodynamic properties that must be bridged to accelerate the development and application of novel materials systems. The workshop showed that there is a connection between today’s research needs and the existing materials testing facilities and the testing approaches they support. One of the crosscutting issues in these sessions is workforce development. Today, an aging workforce increases the

urgency to create new pathways for training, collaboration, and retention.

This Proceedings of a Workshop—in Brief provides the rapporteur’s high-level summary of the presentations and discussions. Additional materials from the workshop are available online.¹ This proceedings highlights potential opportunities for action, but these should not be viewed as consensus conclusions or recommendations of the National Academies.

FUTURE NEEDS AND APPLICATIONS FOR HIGH-TEMPERATURE MATERIALS

Kevin Bowcutt (Boeing) gave the first keynote address, focusing on the ways in which hypersonic vehicle design drives material requirements. The design of hypersonic vehicles, from spacecraft to airplanes to missiles, is dictated by key parameters such as speed range, lift-to-drag ratio, dynamic operating pressure, and expected lifespan. These factors in turn dictate the material requirements for such vehicles.^2,3 Additionally, the challenges related to temperature, aerothermal chemistry, and materials increase when designing vehicles that are intended for reuse. The best materials currently available for these applications include titanium, Inconel, ceramic matrix composites (CMCs), ultra-high-temperature ceramics (UHTCs), emerging complex concentrated alloys (CCAs), and high-enthalpy alloys (HEAs). Bowcutt highlighted that each material has different implications for component design and performance, such as the design of leading edges. There are also impacts on other factors, such as heat flow, shock and boundary layer interactions, radiation, and cooling.

Bowcutt stressed that hypersonic vehicle designs must be highly integrated and link key parameters back to the properties of the materials used. He said that Boeing uses multidisciplinary design optimization modeling to determine if a design satisfies material constraints and noted that this process could also be used independently to create new materials. He also stressed that testing is important for all new materials. Because of the limitations of simulations and ground-based tests, flight testing on reusable hypersonic vehicles presents a key opportunity to test and refine new materials.

Bowcutt expanded on key gaps and future needs for high-temperature materials in an open discussion with workshop participants. Tresa Pollock (University of California, Santa Barbara) asked if materials were included in Boeing’s overall design optimization process, and Bowcutt replied that materials are still mostly separate, but emerging tools are making it more feasible to include materials parameters. In response to a question from Brent Carey (Battelle), Bowcutt said that in his opinion new materials are most needed for leading edges, nozzles, and joints. Todd Steyer (Boeing) added that Boeing is also investigating new materials for shock reduction, among many other needs.

When asked for details on heat exchangers for active cooling, Bowcutt said that existing manufacturing techniques are not well suited to building effective heat exchangers, but that he expects additive manufacturing (AM) to become standard in hypersonic vehicle production because AM is better suited to challenges like a heat exchanger’s complex internal channels. Danny Drury (Colorado School of Mines) asked if Boeing was considering high-temperature electronic systems within vehicles. Bowcutt said yes and added that the most promising design is a “nervous system” of high-temperature electronics that can detect and repair any degradation.

Andrew Detor (GE Research) asked about the use of rockets versus turbines for reusable vehicles. Bowcutt replied that despite rockets’ inefficiencies and fuel needs, if they can be made more reliable, they have more reuse potential and thus a stronger business edge. Hypersonic

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vehicles and reusable launch vehicles share challenges, the most important being landing reliability. Bowcutt also acknowledged that skin friction has significant effects on drag and noted that Boeing is investigating what impact thermal effects and vehicle emissions could have on aerothermal chemistry. The catalytic effects he mentioned are mostly seen at extremely high speeds; at lower speeds, there is less disassociation. Last, he noted that Boeing is pursuing artificial intelligence (AI) and machine learning (ML) approaches in several areas, including in the design of new alloys and efforts to overcome the lack of affordable carbon-carbon structures and other CMCs.

DESIGN CHALLENGES IN HIGH-TEMPERATURE MATERIALS SYSTEMS

There are very few materials that can function in ultrahigh temperatures. The workshop’s first session focused on opportunities to overcome challenges in designing high-temperature materials systems.

Pollock discussed the search for refractory, or multi-principal element alloys, such as CCAs and HEAs, that can perform beyond 1500°C. Methods for development and evaluation of all alloys have been revolutionized through innovations in design tool suites, combinatorial approaches, AI and ML techniques, rapid characterization and testing methodologies, and AM capabilities. Despite these advancements, Pollock said that her team has yet to identify a refractory alloy that can perform beyond 1200°C.^4,5

Nevertheless, Pollock said that refractory alloys, especially those that can be 3D-printed, represent a largely unexplored space with great promise. She outlined the need for research and investments to overcome a multitude of challenges. These include moving from small-scale discovery to large-scale processing and issues with solidification, heat transfer, aerodynamics, oxidation, and coatings. Furthermore, there is the difficulty and expense of making and testing alloy powders as well as the lack of high-temperature data, sequential learning approaches, and instrumentation to control melting temperatures. In addition, she pointed to infrastructure challenges and barriers to systems integration, such as siloed software and codes, and suggested a need for targeted workforce education to increase collaborative development and sharing of tools.

William Fahrenholtz (Missouri University of Science and Technology) discussed opportunities and challenges with UHTCs. Boride and carbide ceramics, first synthesized in the 1800s, have for decades been evaluated for hypersonic vehicle design criteria, such as heat flux, surface temperatures above 2000°C, recombination of dissociative gases, thermal shock, oxidation, and aerodynamic load. Tests of UHTCs have demonstrated strong covalent bonding, hardness, high melting temperatures, and high thermal and electrical conductivities.⁶ New ceramic composites could transform hypersonic design if they can be identified and tested, he said, noting that hierarchical designs, multiscale materials, fiber-reinforced composites, and AM present promising opportunities in this space.

Fahrenholtz explained that designing with UHTCs requires more and more robust experimental data about a material’s intrinsic behavior, shape, density, and chemical stability before AI and ML techniques can be applied. Researchers face a variety of challenges in testing such materials, including a lack of standards, a dearth of tests that can measure beyond 1600°C, the difficulty of eliminating testing interactions, as well as the cost and other barriers to accessing relevant environmental testing capabilities.⁷ Key research needs include a better understanding of high synthesis temperatures and

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high-temperature material properties along with thermal insulators, and kinetics, ductility, and emissivity.⁸ Last, he noted that a culture of “one experiment, one publication” discourages deeper insight into materials’ intrinsic behaviors and can lead to misinterpretation of data.

Matthew Begley (University of California, Santa Barbara) stated that materials systems design is critical to overall design integration. While design, development, and failure mechanisms of coatings are well understood, the geometries, properties, and structures of emerging materials are not. In addition, ultra-high temperatures can cause unpredicted cracking, oxidation, and creep. To address these issues, he suggested that advanced multi-system integrated modeling and simulation tools are needed to develop new coating materials and to evaluate their printability, durability, ductility, potential downstream damage, thermophysical properties, and cooling and failure mechanisms.

Underscoring the need to test emerging materials systems at ultra-high temperatures—as opposed to individual materials design and experimentation—Begley emphasized that more research is also needed to advance rapid-throughput analysis tools for AM processes and lattice topology optimizations, especially for damage prediction. He added that materials research for bone implants, another porous architected material that is 3D-printable and reliable, can also be a model for hypersonic material needs.⁹

Panel Discussion on Design Challenges

In the presentations and ensuing discussion, the importance of collaborative and integrative materials system design emerged as a major theme. Jeffrey Williams (GE Aviation) emphasized the need for teams of materials experts, designers, and engineers to collaborate and optimize from the start to ensure that high-temperature materials development is integrated into the overall system design. He said that a focus on just one requirement is no longer relevant. Instead, multiphysics, multiple thermophysical properties, stability, and damage mechanisms must all be considered, ideally for several candidate materials simultaneously.

Michael Maloney (Pratt & Whitney) and David Van Wie (Johns Hopkins University) built on this point, noting that the issue is not a question of whether structures should be designed for existing materials, nor whether novel materials should be sought to realize ideal structures. Rather a collaborative team must first arrive at an understanding of the conditions, temperatures, and environments that materials will face to enable integration. Clearly articulating these requirements can help to inform directions and allay doubts. Olivier Sudre (Pratt & Whitney) compared vehicle design to a puzzle, where each piece is essential to the whole.

Participants also raised a range of critical issues and gaps. Maloney highlighted inevitable coating loss and oxidation, which varies by temperature, as critical issues for vehicle design.¹⁰ Van Wie added that energy dissipation, thermal balance, and thermal protection pose enormous materials and design challenges, especially at extreme velocities. Sudre emphasized the need for tools to design and evaluate smaller components’ failure mechanisms, as well as research into water interactions with calcium-magnesium alumino silicate and high-temperature fibers. Sudre and Williams both cited the need for a stronger workforce and a more collaborative culture. In addition, as Fahrenholtz noted, hypersonic materials testing is so complex that ground testing is often insufficient, although Williams pointed out that one-off testing for specific temperatures can be useful for extracting performance data. Van Wie suggested that in the absence of realistic ground testing, adding simulations that account for the impacts of stress can be useful.

Last, participants explored future research directions for advancing the design of high-temperature materials

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systems. Sudre said that widely shared software analysis tools would improve systems integration and suggested that academic–industry tension could be reduced by a consortium working on mutually beneficial discovery and testing infrastructure. Noting that coating systems are not homogeneous, as they are often assumed to be, Van Wie said that integrative design across systems could improve the performance of coatings’ outermost layer. Other participants suggested investigating several issues, such as ablative metals for leading edges, carbon fiber properties, novel and compatible interfacial bond coatings, gradient materials, closed-loop cooling systems, interdigital structures, and approaches that account for overall structural issues at ultra-high temperatures.

MANUFACTURING CHALLENGES IN HIGH-TEMPERATURE MATERIALS SYSTEMS

To enable critical applications, materials must not only perform well but must also be feasible to fabricate and to integrate into component designs. The workshop’s second session focused on manufacturing issues for high-temperature materials systems.

In the quest to develop refractory alloys that are viable beyond 1500°C, Noah Philips (ATI) noted that the raw materials are expensive, but the real cost is the complex, costly, and labor-intensive processes required to manufacture these materials. Expensive ultra-high-temperature testing environments, homogeneous melting, and contamination and defect control pose significant barriers to the manufacture of high-temperature, crack-resistant alloys. He also noted, however, that none of these key requirements are typically included in materials design specifications.

Philips suggested that this process could be radically simplified by adopting AM techniques, especially for producing powders and pursuing alloys that have critically important reduced strength at lower temperatures, and by developing AI/ML–based design tools to improve oxidation resistance and strength.¹¹ However, there are still knowledge gaps in areas such as atomization physics, fluid dynamics, gas–metal interactions, melting technology, oxidation of coatings, thermal roll-off, and ductility. In addition, he asserted that an upgraded supply chain for these materials will require substantial national investment in collaborative, collective facilities with equipment that can deliver larger scale, lower cost, and faster access to high-quality alloy powders with properties far beyond today’s materials.

Frédéric Monteverde (National Research Council of Italy) stated that it is possible to find materials that survive repeat operation and behave predictably at ultra-high temperatures. In fact, a consortium recently created a nozzle insert that is the largest ultra-high-temperature ceramic matrix composite (UHTCMC) piece in Europe. However, challenges remain, including size, cost-effectiveness, financial scalability, and reliability.¹² If these challenges are overcome, he suggested that UHTCMCs may usher in Frank Zok’s “new epoch” of materials systems that meet mechanical, thermal, reliability, size, and cost requisites.¹³ Looking forward, it will be necessary to enhance design, performance, and durability optimization and develop predictive tools to understand failure mechanisms. UHTCMCs with self-repairing abilities, he noted, could also significantly expand a component’s reliability, cost, and lifespan.

Don Lipkin (GE Research) described a variety of opportunities to improve access to “critical” materials, which he defined as those needed for industry success or national security, without necessarily purchasing them or producing them from scratch. After examining its own element needs and sourcing, GE developed strategies to replace, reuse, recycle, and reduce waste for every element the company uses.¹⁴ For example, he explained that GE researchers used integrated computational materials

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engineering (ICME) tools to create a replacement alloy when tantalum became too expensive. Other opportunities for increasing access to raw materials include processing innovations to increase efficiency and reduce costs, which can also allow for increased demand, and specialized supply chain infrastructure to access elements and better prepare for inevitable disruptions and shortages.¹⁵ He concluded by observing that as technologies evolve, critical materials components will also evolve, and both materials scientists and supply chain experts must anticipate and respond quickly to these changes.

Panel Discussion on Manufacturing Technology Gaps and Supply Chain Challenges

“Critical materials,” as defined by Rod Eggert (Colorado School of Mines) are those that are hard to obtain but needed for important situations, and whose catalog is constantly evolving. Each material has unique challenges, but they all face short- and long-term supply chain issues, such as a lack of diversity or transparency, impacts from geopolitical events, fluctuating prices, fragmentation, delays, and competing demands. To reduce supply chain risks, Eggert said that materials designers from all sectors should constantly assess the landscape to identify opportunities for collaborative research and strategic alliances. He speculated that there may also be renewed interest in national industrial policies to encourage innovation and development.

As several speakers noted, the materials manufacturing process poses several key challenges. Carolina Tallon Galdeano (Virginia Tech) suggested that aligning fundamental research with industry efforts can push technological innovation and create mutually beneficial manufacturing improvements, enabling research findings to be translated into industrial production of larger pieces of materials with fewer defects. In particular, Galdeano said that university researchers struggle to make safe, high-quality, versatile, cost-effective UHTCs because they are difficult to densify and require microstructural control and reliability at large, complex geometries. This control is challenging because interparticle size, shape, and distribution in the raw materials determine properties and performance. Shaping routes also affect performance and can constrain or enable new macrostructures. Solving these issues, she noted, would allow novel geometries, manufacturing techniques, and components to emerge.

Vasisht Venkatesh (Pratt & Whitney) cited ICME as an important tool to create a more integrated, holistic materials design process because it optimizes the properties and manufacturing requirements of a system’s various components, examining every portion of the design cycle via interconnected, verified, and validated modeling. He explained that while there are challenges with not just the supply chain but also with regard to intellectual property, as well as testing, manufacturing, scaling, data uncertainty, usability, and interoperability, ICME can enable materials to be used to their fullest advantages. In addition, he asserted that using ICME tools collaboratively with suppliers could add value to less commonly used raw materials and incentivize diversification. Eggert pointed out that when the market is too small for an element, suppliers are reluctant to invest in infrastructure to obtain it, and designers are reluctant to experiment with it.

Several participants shared new ideas and approaches. Many agreed with Philips’s suggestion of national investments to create a collaborative facility of shared integrated tools and equipment. Creating large quantities of alloy powders requires more investment and real estate than most researchers have access to. David Smathers (H.C. Starck/Materion) and Pollock added that such a facility could also help bolster the industrial and academic workforce, which Smathers and Venkatesh pointed out was aging.

Smathers suggested that one opportunity lies in processing large batch “master alloys” that can be individualized for a specific endpoint, although crafting such an alloy with common non-rare elements could pose significant challenges, depending on the material performance requirements. He also suggested better accessibility and quality of alloy databases to enable mixing, heat treatment, or temperature behavior modeling.

One participant expressed support for sourcing every element domestically, which Eggert characterized as

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possible but difficult and risky. Participants also expressed support for exploring opportunities in space mining; using wrought processes and AM instead of casting for better strength and ductility; creating complex, integrated, accessible tools to design specific solutions; and creating refractory alloys with intrinsic oxidation resistance.

PLASMA TESTING OF HIGH-TEMPERATURE MATERIALS

Douglas Fletcher (University of Vermont [UVM]) delivered the workshop’s second keynote address, focused on testing approaches to advance high-temperature materials development. UVM’s 30 kW Inductively Coupled Plasma (ICP) Torch Facility has unique capabilities for evaluating candidate thermal protection materials for sustained hypersonic flight by providing quantitative measurements—which cannot be achieved through arc-jet testing—that are critical to understanding how the plasma produced by a vehicle’s gas–material surface interactions affects material properties.

Fletcher explained that the ICP Torch Facility enables researchers to investigate materials’ flexible gas composition and laminar and turbulent flow capability with extensive optical access for simultaneous laser and emission spectroscopy, and with the long-duration test times and steady conditions needed to evaluate energy accommodation in the gas phase just above a material’s surface. This approach was proven with simple plasmas for single reactions before increasing complexity to determine materials’ surface catalysis, carbon nitridation, and carbon oxidation rates.

While no test can replicate the full hypersonic environment, Fletcher continued, the ICP Torch Facility improves on the pass/fail binary test through its capacity to measure static pressure, specific enthalpy, the stagnation streamline, the velocity gradient, and reaction rates at the surface. In addition, ICP testing can address and improve the environmental definition in relation to material performance. Researchers have shown that ICP and arc-jet tests’ flux agreements can be related and that nitrous oxide is dominant at the boundary layer, which increases confidence in extrapolation to flight performance.¹⁶

The ICP Torch Facility is open for stand-alone testing mechanisms, and Fletcher said that developing materials screening tests is an area ripe for future collaborations. ICP testing is well suited for a variety of critical applications, including the testing of UHTCs, not only because arc-jet testing is inadequate to understand gas–surface interactions but also because recent research has revealed direct evidence of energy being added to the surface via recombination.¹⁷ ICP testing remains a complicated endeavor, but it may be possible to simplify the process if materials designers can control surface emissivity and eliminate oxide formation or design a material that changes temperature through transpiration cooling or oxide ejection, Fletcher noted.

TESTING FOR EXTREME ENVIRONMENTS

By their nature, extreme environments are difficult to replicate in the laboratory, making it challenging to fully evaluate how materials systems will perform in the field under extreme conditions. In the workshop’s third session, participants examined gaps and opportunities in testing approaches.

Mark Opeka (Southern Research) described his organization’s efforts to improve understanding of materials performance at extreme temperatures, along with goals to increase materials development and testing capabilities. Southern Research is equipped to characterize materials for hypersonic vehicles through mechanical and subscale structural testing, thermal properties and analysis, and complete ground testing. To inform materials exploration, Opeka pointed to a need for more data on material properties, particularly about tensile stress, thermal stress, thermal expansion, and thermal conductivity up to 2750°C. He also added that noncontact strain measurements in multiple gas environments with consistent heater compositions and hardware would help

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future development. To advance materials design, he said that oxidation modeling, real-time emittance data, and testing for oxidation at higher temperatures are needed and suggested that low-cost arc heaters would improve static oxidation measures for kinetics modeling. Last, Opeka stressed the need for considerable investment in materials property databases to improve knowledge about novel, untested materials, adding that increased support from the University Consortium for Applied Hypersonics (UCAH) and the U.S. Department of Defense could be helpful toward these goals.

Frank Zok (University of California, Santa Barbara) explained his team’s efforts to develop a quantitative understanding of the topology and evolution of the high-temperature thermomechanical behavior of woven CMCs in order to fill any size interstice with solid ceramic to create a fully dense material. The researchers paired precursor impregnation and pyrolysis with high-temperature in situ X-ray diffraction measurements to determine the taxonomy and temporal hierarchy of cracks and correlate that with the size of their matrix pockets. This led to a quantitative understanding of crack topology, temperature, and evolution that can be used to design novel materials systems capable of controlling cracks.^18,19

Zok also discussed high-temperature digital image correlation (DIC), a technique for measuring material contour, deformation, vibration, and strain. Despite challenges related to imaging, image stability, and surface conditioning,²⁰ he posited that improved DIC techniques can be used to develop faithful virtual test specimens that enable a better understanding of materials characterization and advance CMC processing and thermomechanical performance. He stressed the need to pursue new approaches to making CMCs and bolster pathways for training the future workforce in the field.

David Marshall (University of Colorado Boulder) discussed methods to better understand materials’ properties in order to inform architectures for improved functionality. Accurately modeling a material’s mechanical properties, Marshall explained, requires high-temperature measurements with attention to the material’s constitutive properties, conventional macro properties, and most importantly, how it will be used—all of which varies locally. These insights can in turn make it possible to design material architectures for function in addition to structural needs.

While high-temperature, in situ X-ray micro-CT and scanning electron microscope imaging each have technical limitations, Marshall said that combining these two techniques delivers extensive quantitative data of bridged cracks in ceramic composites.²¹ Because the necessary image analysis is time-consuming, his team is applying deep learning methods, which he said are efficient, but should be made more easily accessible to clarify distinctions in the material’s 3D structure and enable the measurements needed for accurate mechanical modeling.²²

Panel Discussion on Testing High-Temperature Materials Systems

Participants discussed challenges and opportunities across many stages of materials testing. Noting that current testing methods are difficult, expensive, and not widely available and require material maturity, David Poerschke (University of Minnesota) said that smaller materials companies often struggle to have their innovations tested. Processing can also introduce defects that affect testing. Poerschke said that there is a need for

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better pathways to enable new materials to become test-ready as well as methods to extract sufficient datasets for modeling, and advanced characterization and AI tools to study cracks or failures in those datasets to identify new test spaces and bridge the modeling-testing gap.

Carey suggested that such tests could also help bridge the gap between focused experiments and large-scale testing, and Rodney Bowersox (Texas A&M University) added that more low-cost, early-exploration, hypersonic testing facilities should be built. Detor agreed, adding that easier, cheaper tests would encourage designers to experiment with new materials, whereas the existing path discourages designers from taking risks by increasing the costs of failure. Any such testing facility, Fletcher noted, should provide a complete profile of the environment to which a material will be exposed. Several participants also suggested using sounding rocket launches to enable more frequent, more accurate, cheaper, and faster testing.

In addition, Detor advocated that the materials community begin a substantial, well-funded, well-defined, collective, and coordinated comparison of new materials tests on benchmarked designs to create a quick and accurate multiphysics testing structure and reinvigorate materials design practices. Such larger-scale, lower-cost comparison tests, he explained, can help understanding of the relatedness and differences of materials in advance of more expensive testing, but at issue is who will fund and conduct them. While academic laboratories often lack funding or proper security clearances, in government laboratories clear goals must be identified before funding is granted. Collaborations could provide a possible route forward. Bowersox suggested approaching NASA and the Office of the Under Secretary of Defense, and Craig Robinson (NASA) agreed that a joint research–military collaboration could design custom tests. NASA’s High Heat Flux Laser Facility is affordable and accessible, Robinson continued, and will soon be able to test materials above 2000°C.

While communication between materials scientists and test engineers is improving, participants also cited a need for more collaboration. Opeka noted that systems design is so segregated that basic information, such as vehicle trajectory, is not fully understood by the entire team, making materials development very difficult. Data restrictions, especially those in place for security reasons, present another constraint. He suggested that this barrier could be eliminated in some cases by categorizing data as Controlled Unclassified Information (CUI), enabling academic researchers to better navigate the intersections between research and national security. Poerschke noted that research restrictions also hinder workforce development, although Bowersox countered that the Joint Hypersonics Transition Office (JHTO) student association was created to address that issue. Bowersox also noted that one of UCAH’s main goals is to share information, but acknowledged that it is limited to universities compliant with the strict CUI stipulations. He added that JHTO is creating a reference vehicle for researchers and has made representative trajectories easy to find.

In exploring a vision for a novel testing sequence, several participants raised the need for standards. Participants agreed that designing new tests would first require high enthalpy testing standards for data comparison. Several organizations, such as the American Society of Mechanical Engineers, ASTM International, JHTO, the Arnold Engineering Development Complex, UCAH, NASA, and original equipment manufacturers, could collaborate on developing such standards. Opeka noted that there are also no standards for leading-edge materials, and despite testing, material–vehicle integration is not guaranteed.

CRITICAL RESEARCH NEEDS TO ENABLE PROGRESS IN HIGH-TEMPERATURE SYSTEMS

For the workshop’s fourth and final session, participants explored needs, gaps, and opportunities to advance high-temperature systems research and development. Issues include corrosion at elevated temperatures and the fact that at high temperatures water vapor reacts with oxides on structural alloys either as a coating or an electrolyte, or an interconnect in a fuel cell to form volatile hydroxides and oxyhydroxides.

Nathan Jacobson (NASA) discussed the use of vapor pressure measurements to improve understanding of thermodynamic material properties. Vapor pressure is

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a limiting factor in extreme environments because the generation of volatile species like hydrocarbons, metal hydroxides, and oxyhydroxides can cause material loss.²³ After years of both experimental and computational research, Jacobson’s team has populated a database of computational thermochemistry codes for metal oxyhydroxide candidate coatings. These vapor pressure measurements illuminate the thermodynamic processes in a solid, enable structural inferences, and determine material loss and recession rates. His team also found that silicon-based ceramics and composites are especially useful as coatings because their low activity can be a measure of reactivity.²⁴

Jacobson said that future progress in this area will require more thermodynamic data in order to improve computational fluid dynamics modeling, make more robust vapor pressure measurements, improve collaboration between modelers and experimentalists, and encourage more user-friendly instrumentation, especially for computational chemistry. He stressed that a robust pipeline of qualified personnel will be needed to perform this work and explore unstudied systems.

Alexandra Navrotsky (Arizona State University) described her research using reaction calorimetry (the study of chemical reactions under controlled conditions) to measure a material’s heat effects to derive enthalpy and entropy for use in phase diagram calculations. In situ calorimetric measurements at ultra-high temperatures represent an effective methodology to accurately capture material transition phases, and Navrotsky said that aerodynamic levitation is especially useful to avoid unwanted reactions, to measure melting points, and to make samples and materials.²⁵ Calorimetry, combined with computational modeling, she explained, has rejuvenated the field of thermodynamics. Further improvements can enhance measurements of thermodynamic and structural data, transition enthalpies, and transition entropy of new materials, which she demonstrated in pyrochlores.²⁶ These approaches are particularly promising for creating better materials for space exploration and for aiding the study of exoplanetary materials. Navrotsky noted that Arizona State University is also opening an ultra-high-pressure laboratory to investigate this largely under-studied area.

Mark Asta (University of California, Berkeley, and Lawrence Berkeley National Laboratory) shared his experience of finding a rhenium substitute using electronic structure and thermodynamic criteria to illustrate the opportunities to design lower cost, more accessible, and reusable high-temperature alloys. Rhenium is scarce, expensive, and oxidizes at relatively low temperatures, even when coated with iridium. After experiments integrated with computational techniques provided a better understanding of rhenium’s mechanisms and properties, his team created design criteria to identify a candidate alloy that mimics the one containing rhenium and whose properties could be modeled and measured.^27,28 The researchers are also exploring redesigning the iridium coating to better align with the new alloy’s properties.

Although the process required patience and trade-offs, Asta noted that this work illustrates how techniques including high-throughput experimentation and automated computation are increasing the speed of metal-coating co-design. He said that integrating high-quality, high-temperature thermodynamic data into a collaborative platform for experimentalists and computationalists to use together would help to accelerate future progress.

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Panel Discussion on Science Gaps for Implementation of High-Temperature Materials Systems

In the final discussion session, participants described various gaps and barriers that are impeding the implementation of new high-temperature materials and offered suggestions to overcome them.

Anton van der Ven (University of California, Santa Barbara) cited the fact that many material phases are dynamically unstable at lower temperatures, which makes high-temperature modeling difficult.²⁹ This instability, widespread in hypersonic candidate materials, points to fundamental questions about diffusion mechanisms, the structure and dynamics of dislocations, and the nature of vacancies, interfaces, and grain boundaries that present such significant theoretical and experimental challenges that new physics discoveries may be required to overcome them. He also suggested that more knowledge could be gleaned by using transmission electron microscopy or other in situ measurements that can be used at high temperature, developing new modeling or simulation techniques to investigate structural defects, and studying high-temperature properties with molecular dynamics.

Describing his efforts to test high-temperature levitation techniques for oxide and non-oxide materials up to 4000°C, Scott McCormack (University of California, Davis) said that the existing equipment works well, but needs slight upgrades to improve the melting point, in situ oxidation, emissivity measurements, and diffusion couple experiments above 3000°C. Overcoming these gaps will require more ML tools and keeping an open mind about previously unknown mechanisms, he said.

David Clarke (Harvard University) stressed that researchers should aim to manipulate the design of materials for a particular function, such as high-temperature thermal conductivity properties, from the start. He also noted that AM could be used to create structures that predictably redistribute heat better than a monolithic or an isotropic material, and Edwin Thomas (Texas A&M University) suggested that 2D layering could be a promising method for this.

Participants also discussed the future of the workforce. Daniel Marren (Scientific Research Corporation) described his work with UCAH to attract and retain employees who can transition basic research into applied technology through industry or government partnerships with academia. Because of the sensitive nature of many applications in this space, he noted that UCAH membership is mainly limited to U.S. citizens. UCAH also fosters relationships between systems designers and materials developers, a need mentioned several times during the workshop, by pairing research solicitations with government agencies to identify systems applications. Last, UCAH sponsors large, multidisciplinary challenges to encourage application-focused collaborations between experts in industry and academia.

Marren added that fostering a robust student-employee pipeline is an important priority for the field. UCAH’s advisory boards support employees at every career phase, including with retraining, certification, and help in obtaining advanced degrees. Thomas suggested that more widespread sharing of the challenges and achievements noted in this workshop, as well as a modernized curriculum, could inspire more students to pursue materials design. Marren agreed, noting that UCAH encourages schools to partner with local companies to meet workforce and mentoring needs.

REFLECTIONS

Taken together, the workshop presentations and discussions provided a picture of the current state of the science on high-temperature materials systems as a multidimensional, multidisciplinary field with important areas of progress but with many remaining gaps. Some areas with gaps that require future research include plasma physics, oxidation, emittance, aerothermal interactions, and vapor-solid phases, among others. While there are a near-infinite number of conceivable materials combinations, designers lack quick and low-cost means

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to understand, measure, and test their behavior at high temperatures. In addition, several participants stressed that a more complete and fundamental understanding of materials systems is essential for seamless integration into overall system design.

Several participants suggested that novel automated and computational tools—ideally easy to use and widely accessible—are needed to further accelerate materials discovery and testing. Participants also identified a need for a greater variety of testing methods, new testing standards, and expanded testing facilities. There is a clear link between what is needed and what current facilities can and cannot supply. Another critical need underlying all materials research and development is growing the workforce to explore new material spaces, bridge knowledge gaps, and collect high-quality data for modeling and material properties databases.

These needs highlight the importance of multidisciplinary collaboration and investment not only across technology readiness levels but also across academia, industry, government, suppliers, and materials manufacturers to encourage experimentation and create affordable, reliable, scalable, collaborative access to materials, and the means to test them. As Wadley noted in his concluding remarks, the workshop discussions demonstrated that there remains more to learn about how materials behave at high temperatures: “In these two days, we’ve explored all of these various aspects, and it is a complex challenge the community confronts.”

WORKSHOP PLANNING COMMITTEE MEMBERS Carlos Levi (Chair), University of California, Santa Barbara; Brent Carey, Battelle; Dianne Chong (NAE), retired, Boeing Research and Technology; Katherine T. Faber, California Institute of Technology; David B. Marshall (NAE), University of Colorado Boulder; Lourdes Salamanca-Riba, University of Maryland; Subhash Singhal (NAE), retired, Pacific Northwest National Laboratory; Susan Sinnott, Pennsylvania State University; Edwin L. Thomas (NAE), Texas A&M University, College Station; Haydn N.G. Wadley, University of Virginia.

STAFF Erik B. Svedberg, Scholar and Study Director; Michelle Schwalbe, Director, National Materials and Manufacturing Board, and Director, Board on Mathematical Sciences and Analytics; Neeraj Gorkhaly, Associate Program Officer; Amisha Jinandra, Research Associate; Joseph Palmer, Senior Project Assistant.

DISCLAIMER This Proceedings of a Workshop—in Brief was prepared by Anne Johnson as a factual summary of what occurred at the workshop. The statements made are those of the rapporteur or individual workshop participants and do not necessarily represent the views of all workshop participants; the planning committee; or the National Academies of Sciences, Engineering, and Medicine.

REVIEWERS To ensure that it meets institutional standards for quality and objectivity, this Proceedings of a Workshop—in Brief was reviewed by Kevin Hemker, Johns Hopkins University; Wenxiao Pan, University of Wisconsin–Madison; Elizabeth Rasmussen, National Institute of Standards and Technology; Kathleen Sevener, University of Michigan; and Mark Weaver, University of Alabama. Katiria Ortiz, National Academies of Sciences, Engineering, and Medicine, served as the review coordinator.

SPONSOR This project was supported by Contract W911NF-10-C-0098 with the U.S. Department of Defense.

SUGGESTED CITATION National Academies of Sciences, Engineering, and Medicine. 2023. High-Temperature Materials Systems: Emerging Applications, Materials, and Science Gaps: Proceedings of a Workshop—in Brief. Washington, DC: The National Academies Press. https://doi.org/10.17226/26849.

For additional information regarding the workshop, visit https://www.nationalacademies.org/our-work/high-temperature-materials-systems-emerging-applications-materials-and-science-gaps-a-workshop.

Division on Engineering and Physical Sciences

Copyright 2023 by the National Academy of Sciences. All rights reserved.