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5 Nanomixtures and Nanocomposites INTRODUCTION AND CURRENT RESEARCH ACTIVITIES Broadly speaking, energetic nanomixtures and composites consist of a support matrix containing submicron particles of metals, metal oxides, and/or organic and inorganic energetic materials. Defense-related applications of nanomixtures and nanocomposites that hold near-term and midterm promise include their use to enhance the performance of conventional explosives, propellants, and pyrotechnics in terms of stability, energy release, and mechanical properties. Nanomixtures and nanocomposites overlap other technology areas addressed in this report, especially that of reactive materials, and possibly thermobarics. Federal support for nanotechnologies of about $0.5 billion was provided in FY 2002; of that amount, about 32 percent was earmarked for DoD applications.) A small fraction of this DoD support (about 5 percent, or $8 million) is designated for the nanotechnology of energetic materials.2 Three DOE laboratories (Lawrence Livermore National Laboratory, Los Alamos National Laboratory, and Sandia National Laboratoriesy, the DoD laboratories and centers (mainly ARDEC, Army Research Laboratory fARLl, NSWC, the Naval Air Warfare Center tNAWCI, and Eglin Air Force Base), DARPA, and DTRA are each sponsoring or conducting modest-sized research activities devoted to the preparation, characterization, and application of nanometric energetic materials in service-specific settings. The Army Research Office sponsors the Defense University Initiative in Nanotechnology (DURINT), which is centered at the University of Minnesota. The DURINT program represents a substantial investment in enlisting the academic community to help gain a fundamental understanding of the formation, processing, chemistry, and modeling of energetic nanoparticles and their formulations. Most of the briefings to the committee about nanotechnologies were mostly at the program level as opposed to the bench level, since most research in this area is still in the discovery phase. The range of ideas presented to the committee was large in relation to the David Mann. 2002. Presentation to 38th JAN NAF Combustion Subcommittee meeting, Destin, Fla., April 8-10. 2 R.Doherty,OS&T/IH. 2002. Presentationto the committee. June6. 24
NANOMIXTURES AND NANOCOMPOSITES tangible successes and quantitative test results available to date. Thus, the practicality of these materials or processes remains to be determined. The materials and processes discussed in the presentations to the committee were primarily in the following areas: Nanometric fuels, which mainly involve the use of aluminum powder sized at less than 50 nm, although other reactive metals and alloys were not excluded; Passivation and coating of nanometric metal particles to eliminate, or at least reduce, the rate of aging due to reaction with air and moisture; · Sol-gel processing as a strategy for synthesizing and/or incorporating nanodimensional organic and inorganic materials in a supporting gel; and · The use of carbon nanotubes and nanoporous materials as a support or container for energetic materials and reactive gases. 25 Arguments in favor of the use of nanoscale energetic materials and fuels rely primarily on gains resulting from altered chemical kinetics rather than thermodynamics. For example, 1-nary aluminum reacting with oxygen releases only 1.04 times as much energy as ultrafine aluminum (the micron particle-size range).3 On the other hand, the rate of energy release of the latter is potentially faster because the balance of the rate-controlling factors shifts as the particle size is reduced. More specifically, the rate of a combustion or explosion process is controlled by the balance of the generally slower mass transport rates and faster chemical reaction rates. This fact frequently makes mass transport a controlling energy release process in conventional munitions and propellants in which micron and larger particle sizes of energetic materials are used. By contrast, the high surface area (104 to 106 times that of the particles used in traditional formulations) and the short diffusion length of nanoscale particles are expected to enhance the role of chemical kinetics. Consequently, an unprecedented degree of control of the energy release rate may be possible by varying the composition on the nanodimensional scale. The burning rate might be accelerated, the delivered specific impulse could be increased by improved combustion efficiency, and the detonation might achieve greater ideality. Companies such as Technanogy, Nanotechnology, Argonide, and Aveka have begun to provide nanoscale metals in response to commercial applications of nanoscale reactive metals beyond their use in military systems. It is hoped, therefore, that this activity will provide additional research and development beyond that supported by the the Department of Defense, as well a stable source of materials. TRANSITION CHALLENGES The well-touted advantages of nanodimensional fuels and oxidizers appear against the backdrop of several disadvantages. In particular, the problem of aging of formulations containing nanoparticles is aggravated by the high surface area and the resulting higher reactivity of these particles. Oxygen and moisture cause a metal oxide to form on the particle surface, which leads to a loss of energy, lower reactivity, and added dead weight. Processing difficulties have been observed in the experimental development of nanoscale composites because the high surface area leads to problems such as drastically increased viscosity of formulations. Many military energetic material applications require a high degree of quality control of the particle size, surface characteristics, and physical and chemical properties. Meeting these requirements has yet to be demonstrated by nanomaterial manufacturers. 3 Michael Zachariah, Universityof Minnesota. 2001. Presentationto the committee. December 13.
26 ADVANCED ENERGETIC MATERIALS In most of the DOE and DoD laboratories mentioned above, the majority of work on nanoscale energetic materials is directed at nanoscale aluminum fuel. The expectation is that performance enhancements will be achieved by the increased rate and magnitude of the energy release step. A lesser amount of work has been conducted on other reactive metals, such as magnesium and aluminum/magnesium. An illustration of the type of work being conducted is a substantial program underway at Eglin Air Force Base to incorporate nanoscale Al into formulations. This effort is being bolstered by attempts to passivate the aluminum particles by coating them with a polymer.4 Eglin Air Force Base has proposed an evaluation protocol for nanostructured materials, but it has been acknowledged that an understanding at the fundamental physical chemistry level will be needed to exploit these materials fully. In particular, the initiation and growth mechanisms, the solid-liquid-gas phase chemistry, and the detailed kinetics of nanoscale energetic materials remain unknown. The DURINT program is attempting to provide some insight into these areas. In general, however, the needed level of characterization of nanomixtures and nanocomposites is very difficult to achieve and may not be available any time soon. A second major area of effort is being pursued at LLNL, where the incorporation of metal oxides (e.g., ferric oxide) with nanoscale aluminum into polymer support is being sought using sol-gel technology to produce a xerogel.5 The reaction of aluminum with ferric oxide produces a thermite reaction. The polymer binder and/or added gas-generating materials are able to produce a working fluid when this is desired. The incorporation of nanodimensional organic energetic materials (e.g., RDX and pentaerythritol tetranitrate) and inorganic oxidizers (e.~., ammonium perchlorate) into silicate Rels or resorcinol-formaldeLyde ~ ~ it, . . . · · . (RF) gels by the use of sol-gel methods was initially pursued at LLNL, with tantalizing results. Recently this direction has also become a thrust area in the DURINT program, where an attempt is being made to desensitize impact-sensitive materials such as CL-20 by incorporating them on the nanoscale into RF and nitrocellulose gels. The Army laboratories at ARDEC and ARL have plans to investigate the possible applications and gains that nanoscience might provide for gun propellants and explosives.6 The possibilities include the use of functionalized carbon nanotubes and nanoporous materials to store hydrogen and oxygen separately, as is envisioned in fuel cell technology, to create an explosive. The energy released in the explosion was estimated to be twice that of HMX. The functionalized carbon nanotube could possibly be filled with nanodimensional energetic material, which is easily and reliably ignited with a plasma source. Such materials might have application in the Electrothermal Chemical (ETC) gun propellant program. It should be noted that efforts in these areas are preliminary basic research programs; transition to fielded systems would be many years away. MANUFACTURING AND TRANSITION BARRIERS In all of the cases discussed above, a central issue in the use of nanotechnologies is the capability of the manufacturing complex to convert the most promising advances into mass production. The procedures for making and formulating nanomaterials of the future are quite different from those for conventional munitions. One problem that nanoenergetic developers may encounter is the challenge of making valid comparisons of these new materials and conventional materials. This barrier probably cannot be surmounted without 4 W.H. Wilson, AFRL/Eglin. 2001. Presentation to the committee. July 31. 5 A xerogel is a solid material with an open porous framework; xerogels have very high interior surface area and a density between 25 and 75 percent of bulk properties. J.A. Lannon, ARDEC/Picatinny. 2001. Presentation to the committee. July 31.
NANOMIXTURES AND NANOCOMPOSITES side-by-side fundamental studies of the physical and chemical properties of the two types of materials. FINDINGS AND RECOMMENDATIONS Findings 27 The major findings of the committee on the topic of nanomixtures and nanocomposites are as follows: · Research on nanotechnology and nanomaterials for military energetic material applications is essentially still in the discovery phase; The range of ideas presented to the committee regarding nanomixtures and nanocomposites was large in relation to the tangible successes and quantitative test results available to date; The practicality of these materials or processes remains to be determined; and Suppliers of nanomaterials have yet to attain reproducible product properties. The combination of nanotechnologies and nanocomposite materials offers potentially high payoff at moderate to low risk. As a class, these materials offer high energy density andin nanomaterial formthe capability for high power. This combination appears to generate a different category of explosion, and it does not typically generate gas, thus providing no efficient working fluid. It remains to be demonstrated whether nanomaterials can be combined with gas generators to make them extremely powerful propellants and explosives. The approach of making composites of various oxidizers and fuels, especially at the nanometer-length scale, appears very powerful in that it might allow formulations scientists and engineers to overcome the diffusion-limited problems of reactivity that have hampered previous efforts. The approach also allows tailoring the sensitivity and performance attributes for increased effectiveness. Applications of nanoscale metal particles into formulations will rest heavily on achieving the long-term passivation of the particle surface without losing the benefits of the faster reaction rate. Recommendations Research efforts focused on nanotechnologies destined for energetic applications have a modest but justifiable level of support. The current research and development program in nanomaterials and nanocomposites is a diverse collection of activities. The Advanced Energetics Initiative should develop a national strategy for focusing the effort on these interesting materials. The committee recommends that this strategy include the following: A suite of experiments and test criteria should be developed so that the properties of these various nanomaterials and procedures can be directly compared with those of conventional materials. Higher emphasis should be placed on performance testing of nanoaluminum formulations. Standard characterization techniques should be developed and implemented for nanomixtures and nanocomposites. The passivation process should be studied to ensure the stability of these composite materials.