CHAPTER FOUR
Energy and Power Materials

CHAPTER SUMMARY

The flow of energy and power forms the lifeblood of military systems. The Panel on Energy and Power Materials was formed to examine advanced materials and processes in this area. Every other panel of the full committee also contributed to identifying challenges to be met in energy and power materials.

DoD needs for energy and power materials are many, among them:

  • Batteries for energy storage, from small portables to large shipboard units,

  • Capacitors for storage and release of pulsed power,

  • Fuel cells for efficient direct conversion of chemical to electrical energy for platform power,

  • Photovoltaics for harvesting energy from the environment,

  • Explosives for enhanced and tailorable lethality, and

  • Microturbines for powering small unmanned aerial vehicles (UAVs).

Because this area is so broad, a comprehensive study was not possible. Instead, the panel identified key materials aspects of each major application and derived broad themes for materials research. Areas identified were those where DoD funding would be needed due to the lack of commercial interest, highlighting differences in commercial and military requirements. Major research themes identified are

  • Nanomaterials science and engineering for control of structure and properties at the nanoscale;



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 55
CHAPTER FOUR Energy and Power Materials CHAPTER SUMMARY The flow of energy and power forms the lifeblood of military systems. The Panel on Energy and Power Materials was formed to examine advanced materials and processes in this area. Every other panel of the full committee also contributed to identifying challenges to be met in energy and power materials. DoD needs for energy and power materials are many, among them: Batteries for energy storage, from small portables to large shipboard units, Capacitors for storage and release of pulsed power, Fuel cells for efficient direct conversion of chemical to electrical energy for platform power, Photovoltaics for harvesting energy from the environment, Explosives for enhanced and tailorable lethality, and Microturbines for powering small unmanned aerial vehicles (UAVs). Because this area is so broad, a comprehensive study was not possible. Instead, the panel identified key materials aspects of each major application and derived broad themes for materials research. Areas identified were those where DoD funding would be needed due to the lack of commercial interest, highlighting differences in commercial and military requirements. Major research themes identified are Nanomaterials science and engineering for control of structure and properties at the nanoscale;

OCR for page 55
Engineering interfaces and surfaces in materials by tailoring material structure to optimize rate and extent of reaction processes; Advanced energy storage and conversion materials; Tools for accelerated, systematic materials discovery, both analytical and experimental, e.g., computational materials science and combinatorial materials science; and Materials as the foundation for systems, based on systems approach to materials in which the entire suite of material properties relevant to a class of applications is identified as early as possible to avoid point solutions. Successful pursuit of these themes will provide numerous benefits to the DoD, including: Reduced development time and cost; Increased energy density in storage devices, with associated weight reduction; Improvements in lethality of munitions, including increases in range and payload; Practical energy-harvesting devices to allow capture and storage of solar radiation in the battle area; and Reduced weight of energy and power systems, which will reduce soldier and system payload. Additional information on DoD needs, research themes, and benefits is provided in the body of this chapter and in Appendix D. INTRODUCTION Energy and power are ubiquitous in DoD platforms. To satisfy the broad-ranging charter given to this committee, a separate Panel on Energy and Power Materials was appointed to treat such subareas as batteries, fuel cells, and energetic materials. Other panels addressed specific energy and power areas that fell within their purviews. As an example, biological materials approaches to power and energy (e.g., ATP analogs) were covered by the Bioinspired and Bioderived Materials Panel (see Chapter 7). Figure 4-1 identifies which panels covered which energy and power materials subareas.

OCR for page 55
FIGURE 4-1 Energy and power materials addressed by panels of the committee.

OCR for page 55
Under energy storage, the Panel on Energy and Power Materials examined materials challenges for electrical, electrochemical, chemical, and magnetic storage of energy. This area included materials for improved batteries and for capacitors, both electrical and electrochemical, as well as chemical energy storage in the form of explosives, propellants, and fuels that could provide a significant advantage to the military in 2020. Explosives and propellants are developed almost solely by government entities or their industrial contractors. While the scientific literature was examined, the information available likely reflects work already known to DoD. In this case, the most important contribution of the panel may have been to identify opportunities that are not being pursued aggressively due to limited budgets and a current focus on immediate needs and near-term payoff. Also, while mechanical energy storage (particularly in flywheels/ compulsators) is important to DoD, the associated issues are largely structural (see the work of the Panel on Structural and Multifunctional Materials reported in Chapter 3). The panel also examined challenges for efficient conversion of energy from one form to another. The most important component in this category, for both small-scale and large-scale energy conversion systems, is the fuel cell. The panel also examined materials for inorganic photovoltaic (PV) devices—organic PV materials were covered by the Functional Organic Materials Panel (see Chapter 6). The Energy and Power Panel also considered the conversion of chemical energy into thrust in very small-scale applications via microturbines, while the Structural Materials Panel addressed materials for larger gas turbine engines. Materials challenges to fielding advanced weapons, i.e., placing energy on-target, were considered by the panel only in the context of energy storage or conversion, because many of the components of these weapons use materials within the purview of other panels. Materials for advanced high-power lasers, for instance, were addressed within the Panel on Electronic and Photonic Materials (see Chapter 5). Similarly, the problems with materials for advanced gun tubes and for electromagnetic launch rails primarily relate to erosion, wear, and structural integrity, again more aligned with the mission of the Panel on Structural Materials. However, the Panel on Energy and Power Materials did consider the development of materials for energy storage at high levels that might accelerate associated applications. The need to dissipate concentrated energy and protect systems against its effects is ubiquitous and was addressed by several panels. Materials

OCR for page 55
challenges for effective kinetic energy dissipation (e.g., novel armor) were addressed by the Panel on Energy and Power Materials (transparent armor and body armor) and the Panel on Structural Materials (integrated structural armor and protection of large platforms). In both cases, information on potential performance improvements from combining new materials with new design approaches was difficult to obtain, because that information was often classified. Materials challenges for hardening against other forms of energy (e.g., acoustic, thermal, radar) are discussed in Chapter 3 in the examination of multifunctional materials. DoD dependence on natural energy sources was considered, in relation to DoD platforms rather than fixed installations. Accordingly, less emphasis was placed on sources such as water and wind power. Solar energy was recognized as having some potential for use in energy conversion devices for platforms and individuals. The panel did look at materials that could significantly improve DoD’s ability to harvest energy from alternative sources, thereby improving field power-generating capability while decreasing the logistics burden of supporting expeditionary forces. Potential shifts in the reliance of DoD on natural sources such as fossil fuels were examined only briefly because the need for changes in U.S. energy sources, particularly a shift from fossil fuels to more rapidly renew-able resources, has been well documented. If petroleum-based fuel prices were to accelerate dramatically, a substantial U.S. initiative to find and apply alternative energy sources would likely result, with DoD being one major beneficiary. The potential need by 2020 for fossil fuels, as well as future dependence on nuclear power, are also discussed in this chapter. DOD NEEDS FOR ENERGY AND POWER MATERIALS Virtually every DoD system requires energy and power to function. Attaining higher energy and power levels in a smaller package is a continual goal in development of new materials, manufacturing processes, and design approaches. Examples are Battery materials (small, ultralightweight power cells to large submarine batteries); Fuel cells (direct conversion of chemical to electrical energy across a range of applications); Capacitors (pulsed power for electromagnetic launch of projectiles, aircraft, etc.);

OCR for page 55
Explosives and propellants (insensitive yet possessing higher energy density); Advanced individual body armor (to offset the increased lethality of individual weapons); Novel power system components (for all-electric weapons platforms); and Components for harvesting energy from the environment (e.g., photovoltaics, thermoelectrics). These examples are illustrative only; an exhaustive list of energy and power applications would require enumeration of every DoD system. SPECIFIC AREAS OF OPPORTUNITY In the sections below, DoD needs for energy and power materials are examined in specific categories: Energy storage, Energy conversion, Electric power generation and transmission, Kinetic energy dissipation, and DoD reliance on energy sources. Additional information on materials for specific applications is provided in Appendix D. Energy Storage In any system, the rates of energy storage and consumption, combined with the total energy available, determine how long the system will be effective, i.e., the system run time. From this standpoint, the ideal system would have an infinite reservoir of energy (or infinite rate of energy storage) and a negligible rate of energy consumption. While real systems never approach these limits, there is nevertheless a continuing drive for approaches that can store large amounts of energy in small packages and use it efficiently. In materials terms, advanced energy storage requires materials having high volumetric and gravimetric energy densities. Although storing electrochemical energy in batteries may be most familiar, energy can be stored in many forms: chemical (in fuels and

OCR for page 55
explosives); mechanical (flywheels); electrical (capacitors); thermal (solids, liquids, or gases); magnetic (superconducting circuits); and potential (reservoirs). This report focuses on energy storage in the forms most traditionally associated with DoD platforms and systems. The future of materials science as it relates to energy storage systems is exciting. For electrochemical systems, advances in nanomaterials can yield ionically conducting electrolytes that can provide both high power and high safety. New discoveries promise high-energy electrode materials and more stable electrolytes that will greatly increase the energy content of current batteries for applications ranging from the dismounted soldier to submarines. Tailored interfaces will increase the lifetime of electrochemical power systems and make them capable of power pulses well beyond what can be delivered today, including in radio burst communication from ground to satellite. Similarly, electrical energy can be stored in dielectric capacitors for use in pulsed power systems for high-energy lasers, railguns, and other advanced weapons. Materials that are extremely mass-efficient in storing chemical energy are important for DoD. These “energetic materials” will enable the lighter, more lethal force structure envisioned by the Army beginning now and continuing through 2020.1 A driving goal for much of the DoD-wide system transformation is a shift to smaller platforms with increased lethality.2 Higher-energy-density explosives and propellants combined with precision targeting systems translate into smaller warheads, enhanced penetration, longer range, and reduced ammunition logistics support.3 In addition, novel propellant materials may be able to reduce launch signature, thus increasing survivability by reducing detectability.4 This class of materials includes chemical fuels for platform mobility, which now account for some 70 percent of war tonnage shipped to combat locations 1   Andrews, M., “Army Vision and S&T: Accelerating the Pace of Transformation,” paper presented to the Committee on Materials Research for Defense After Next, National Research Council, Washington, DC, February 15, 2000. 2   Ullrich, G.W., “Advanced Energetic Materials: Introduction and Overview,” paper presented to the Committee on Advanced Energetic Materials and Manufacturing Technologies, National Research Council, Washington, DC, July 31, 2001. 3   Lannon, J., “U.S. Army Energetic Materials,” paper presented to the Committee on Advanced Energetic Materials and Manufacturing Technologies, National Research Council, Washington, DC, July 31, 2001. 4   Finger, M., “Energetics Survey,” paper presented to the Committee on Advanced Energetic Materials and Manufacturing Technologies, National Research Council, Washington, DC, July 31, 2001.

OCR for page 55
(Potomac Institute, 2000) and which are absolutely essential for a wide range of platforms. In addition, energy stored in magnetic fields can reduce the effects of short-term power disruptions. Novel superconducting materials that may increase storage efficiency and decrease the size and complexity of the storage system are discussed later in this chapter. What energy storage technology will best meet DoD needs? A single answer is not possible, because all power sources have their “sweet spots” (areas of best application), and this panel envisions that a range of energy storage (and conversion) technologies will be important. For example, low-power needs (usually for electronic devices) are optimally met by storage in batteries. Other applications require other technology choices, from storing energy in a highly concentrated, liquid fuel form for use in internal combustion engines to storage in capacitors for high-power directed energy weapons. The National Academies have published two studies of the use of power sources for defense applications, Energy Efficient Technologies for the Dismounted Soldier (NRC, 1997) and Reducing the Logistics Burden for the Army After Next (NRC, 1999). Both highlight the importance of developing advanced technology for power sources, including batteries, for DoD applications. This study builds on these documents. The sections that follow discuss novel materials and processes for use in storing energy. Additional information is provided in Appendix D. Electrochemical Energy Storage Background: Batteries and Electrochemical Capacitors Batteries and electrochemical capacitors are energy storage devices that convert chemical into electrical energy and are particularly suited to provide the energy to power electrical devices. They cannot be easily replaced, and are very likely to be an essential element in any future military power applications. Their simplicity (no moving parts), reliability, and wide power capability make them attractive as stand-alone power systems or as enabling elements in hybrid power system configurations. Additionally, batteries are the best option for stealthy operation. They produce power with no noise, no heat signature, no intake of oxygen, and no exhaust gas. Special Operations Forces will always need better batteries. Moreover, while there are more efficient ways to store energy, few other devices can match the attributes of batteries and electrochemical capacitors. Of particular importance is the fact that very few other power sources have a wide enough dynamic range to efficiently deliver power to

OCR for page 55
follow a dynamic load over three to four orders of magnitude. Several types of batteries have this feature, which is of particular importance in very high power applications and in hybrid power sources (ones that have a tailored power source with a high energy component and a high power component designed to work together for optimum performance). Batteries are electrochemical cells that have two electrodes, an anode and a cathode. During discharge, the anode is oxidized and the cathode is reduced. The electrolyte (solid or liquid) is ionically conductive, allowing ions to be transported between the electrodes. In these systems, the electrons are driven through an external circuit to power electronic devices. Batteries can be disposable (a primary cell) or reusable (a rechargeable or secondary cell). Electrochemical capacitors also have two electrodes separated by a separator having an ionically conductive electrolyte, but their energy can be composed of double layer capacitance (DLC) or it can be stored in Faradaic processes (much like a battery). Either way, they are rechargeable devices designed to have a very high cycle life. Like conventional capacitors, electrochemical capacitors have higher power and lower energy than rechargeable batteries. Figure 4-2 demonstrates the comparison. DoD system energy and energy storage requirements vary enormously, ranging from tiny batteries to power man-portable communication systems to huge systems to power submarines. Diversity in capacity, physical size, weight and shape, drain rate capability, thermal performance, cycle life, shelf life, and cost make it impossible to define a single optimal battery. The need for a systems approach to finding new materials to match power source to application for DoD will be a continuing theme of this chapter. Both batteries and electrochemical capacitors can deliver energy over a wide power range, making them well suited to hybrid power systems. A timely example is the hybrid electric vehicle (HEV), which contains both an internal combustion engine (ICE) and a battery pack. The ICE can operate at peak efficiency to charge the battery pack, which in turn provides power for propulsion. Properly designed, one can have the benefits of both systems—the high energy content of a liquid fuel and the high power of a battery. Military-Unique Requirements There are specific military requirements that limit the use of commercial off-the-shelf (COTS) power sources, among them differences in required power and energy levels, in temperature operating range, in the need for shock resistance, in the requirement

OCR for page 55
FIGURE 4-2 Ragone plot comparing nominal performance of batteries, electrochemical capacitors, and dielectric capacitors. SOURCE: Reprinted from Christen and Carlen (2000), Copyright 2000, with permission from Elsevier Science.

OCR for page 55
for longevity, and in reliability. Except in HEVs, high-power batteries are generally not needed in the commercial marketplace, and even the HEV is not in a performance realm that can meet future DoD requirements. Operation in a broader temperature range and under more rigorous environmental extremes will be necessary. The military has a greater need for increased shelf life and lower self-discharge rates than typical commercial batteries offer. Furthermore, commercial devices are not held to the same reliability standards as those required by the military. The reliability driver for commercial power sources is to minimize warranty expenses; the reliability driver for future DoD systems is mission success and soldiers’ lives. Such differences are typified in Figure D-2 on naval batteries (see Appendix D). Materials R&D will be necessary to help fill these gaps. Research Challenges and Materials Opportunities To translate technology needs into materials needs, one must map the system requirements to the materials domain. From a materials perspective, battery performance is dictated by materials selection and stability (Salkind, 1998; Doughty, 1996). Intrinsic properties of the active electrode materials determine the cell potential, capacity, and energy density. The stability of interfaces between reactive materials dictates calendar life and cycle life (in rechargeable systems); safety depends on the stability of materials. Because battery performance depends heavily on tailored stable interfaces, it is here that solutions will be found. Nanomaterials Nanostructured materials are of wide interest in research programs today. They hold the promise of tailored and engineered materials that were inconceivable a few years ago. Continuing these advances will provide important benefits for power sources if they can be reliably produced and stabilized so that their nanostructure (and associated properties) remain relatively constant over extended use. Specific areas that will benefit are tailored electrode materials and electrolytes. New processing methods for advanced micro- and nanostructures is one fertile area. While high-power electrodes require high-surface-area materials, processing often does not allow the technology to take full advantage of the material. Nanostructured materials combined with a one-step deposition process or a self-assembly approach to fabricating the electrode may overcome these limitations. Tantalizing initial steps are

OCR for page 55
Kinetic Energy Dissipation and Protection This section briefly summarizes advanced lightweight armor materials (e.g., body armor) and transparent materials for dissipating kinetic energy. Other areas of energy dissipation (laser, structural armor, etc.) are discussed elsewhere in this report. The most important asset and most complex system on the battlefield is the individual soldier. Protecting soldiers from fragments and small-arms kinetic energy (KE) threats is perhaps the single most important energy dissipation issue for DoD. With the trend toward a smaller but more capable force, even a relatively small number of casualties may severely degrade mission effectiveness. Because major improvements in armor materials and designs may be classified, highly promising existing and developmental materials and designs may have been omitted in this study, which is limited to unclassified sources. Lightweight Armor Three classes of nonstructural armor were considered by the panel: soft (fabric) personnel armor capable of defeating fragments and low KE ball ammunition (presently typified by Kevlar fabric); hard-faced composite light personnel armor for defeating high energy ball and armor piercing (AP) rounds (presently typified by ceramic tiles backed by layers of Kevlar); and transparent armor for face, riot shields, and lightweight vehicle windshields (currently typified by ballistic glass/polymer multilayers). These engineered systems require trade-offs among such factors as penetration resistance, weight, bulk, deflection, multihit capability, flexibility, comfort, and field durability. Although this deals with materials properties, ballistic performance is in fact a systems property. Projected increases in the ballistic resistance of materials, while important, are unlikely in and of themselves to make dramatic increases (i.e., >100 percent) in ballistic efficiency for all classes of personnel armor. Rather, such materials performance increases will enable improved or innovative designs, which will synergistically leverage the materials property gains to yield dramatic armor systems improvements. The next section summarizes principal conclusions. Potential lightweight armor goals between now and 2020 are discussed in Appendix D. Soft Fabric—Fragmentation Protection Of the classes of armor considered in this assessment, soft, textile-fiber-based armor for fragment and

OCR for page 55
low-velocity ball rounds is the best understood from a theoretical basis. Cunniff (1999a) found that, given projected improvements in fiber properties, reductions in areal densities of at least 50 percent for a given threat level are possible by 2020. This increased performance may entail excessive behind-armor deflection, necessitating redesign of the entire soldier protection system to compensate. Introduction of new fibers could provide an impetus if a clear “requirements pull” should justify the cost of new fiber development. Some fiber families (e.g., PIPD) employ rigid rods within the chain combined with strong interchain hydrogen bonding. Continuous fibers based on single-walled carbon nanotubes could be potentially significant if processing can control structure and limit defects. Hybrid yarns employing fibers having complementary characteristics could also provide synergy. Hard-Faced Protection Against Ball and AP Rounds While the conventional wisdom in hard-faced armor systems is that the thickness of the hard face must be near that of the projectile diameter, recent work on the defeat of ball ammunition has challenged this rule of thumb. Cunniff (1999b) has shown that judicious design can achieve equal protection with less damage by using thinner face plates of the same ceramic. There may also be benefits derivable from engineered nanomaterials for protection against such threats. Withers has tested nanomaterial configurations against ball projectiles and shown potential benefits.15 The wide range of possible nanomaterials and configurations makes this a fertile ground for examination; the challenge is to narrow the range appropriately in order to limit the testing required. Computational modeling could enhance understanding of the behavior of such materials, but a real payoff could be realized if an accurate, small-scale lab test could be developed to predict and supplement results of full-scale configuration testing. Ceramic-faced armor for personnel protection against AP projectiles dates from the early 1960s with the development of boron carbide/GRP lightweight composite armor (Viechnicki et al., 1991). In general, the material must be harder than the projectile and should have as high an impedance (l=√ρE ) as possible. The most efficient hard-face materials tend to be the oxides, carbides, or nitrides of Al, Si, or B. Currently, B4C 15   Withers, J.C., chief executive officer, Mer Corp., Tucson, AZ, private communication, September 26, 2001.

OCR for page 55
provides the most mass-efficient hard-face material with a density of ~2.5 g/cc. There are presently no known materials that have significantly higher impedance combined with a lower density; the properties of some armor ceramics are shown in Table 4-3. In the panel’s view, only incremental improvement (<30-40 percent), can be expected in the ballistic resistance of rigid and hard-faced armor systems based on straight materials improvements. Similarly, incremental improvements in design are not likely to yield performance increments of the order of 100 percent. What is required are totally new approaches in armor design that lead to new mechanisms for defeating the more severe small arms threats, such as 0.30 cal AP. Possibilities include projectile tipping or rotating, momentum trapping, and confinement. Transparent Protection Currently, there are three main developmental transparent armor ceramics: AlON (aluminum oxynitride), spinel (magnesium aluminate), and sapphire (single-crystal aluminum oxide). Sapphire is currently the most industrially available of these materials, and AlON appears to be the most ballistically promising. However, all three materials have a density in the range of 3.6 to 4 g/cc. Even though these materials perform significantly better than glass/polycarbonate laminates, they still have areal densities nearly double those of ceramic-faced opaque armors. It is therefore critical to try to synthesize new transparent ceramics that have densities close to that of boron carbide (2.5 g/cc). Armor Materials Research Needs: Summary Progress in fibers will lead to sizable improvements in soft armor only if DoD takes the initiative to fund new fiber development. Improvements in ballistic performance of ceramics for hard-facing of lightweight armor will most likely be incre- TABLE 4-3 Properties of Armor Ceramics Material Density (g/cc) Hardness (GPa) Young’s Modulus (GPa) AlN 3.2 14 280 Al2O3 (90 percent) 3.6 20 275 B4C 2.5 30 445 SiC (sintered) 3.2 27.5 390

OCR for page 55
mental (20 to 40 percent range). For opaque lightweight armor, the most promising approach to attain major gains in performance (~100 percent) may be by innovative design. Significant increases in transparent armor performance can be made by implementing systems based on the current generation of developmental materials (i.e., AlON and sapphire), but further advances in the performance of ceramic-faced transparent armors will require new transparent ceramics with a density of less than 3 g/cc (with other properties equal to or better than AlON). Finally, the panel emphasizes that nanomaterials technology should be explored as a potential “paradigm breaker” in light-armor technology. DoD Reliance on Energy Sources National energy dependence, particularly with respect to reliance on energy imports, has been the subject of considerable policy discussion for many years. Though this is not a materials development issue per se, the committee agreed that a brief discussion of overall DoD energy usage, dependence on various sources, and issues related to DoD energy dependence through 2020 was desirable. The committee’s conclusion was that, although new approaches to energy harvesting (e.g., photovoltaics) will be combined with increasing use of fuel cells, existing infrastructure makes it likely that current fuels (e.g., JP-8, diesel) and nuclear power (for the Navy) will still make up the bulk of energy sources used by DoD platforms of 2020. Supporting information is given in Appendix D. RESEARCH AND DEVELOPMENT PRIORITIES The energy and power area spans numerous applications, device types, materials families, and subspecialties of expertise. Nevertheless, the materials R&D priorities that would most benefit future DoD systems can be articulated in a short list, given below. Nanomaterials Science and Engineering A common theme throughout the report of the Panel on Energy and Power Materials is the potential contribution of materials either containing nanoscale components or having structure control at the nanometer scale.

OCR for page 55
Materials exhibit wholly different properties at the nanoscale, which often translate into radically different macroscopic behavior. Nanostructuring offers a powerful lens through which to both reexamine the vast array of existing materials and to approach new materials. Nanomaterials science and engineering affect virtually every energy and power area. For batteries, nanocomposite electrolytes could lead to entirely new mechanisms of ionic transport with possibility for very high conductivities. In dielectric capacitors, copolymers with structured nanodomains have the potential that may greatly increase dielectric constants. In energetic materials, nanoscale aluminum powders are already used to tailor burn rate in propellants; engineering of energetic material nanocomposites by controlling such factors as nanoparticle size and size distribution, composition, and morphology may lead to major improvements in these materials. New synthetic methodologies that allow for architectural control of fuel cell electrodes at the nanoscale could enhance reaction kinetics by dramatically increasing electrode surface area and restricting reactions to confined regions. The use of nanoscale and nanostructured materials spans the entire range of metals, ceramics, polymers, and composites of interest to DoD, and the paybacks are potentially huge. The challenge is to produce nanostructures that have useful material properties in a controlled and reliable manner, while assuring that the structures remain stable over extended use. Accordingly, DoD should continue to support efforts to develop new ways to structure materials and to combine them with other materials at that nanoscale. Engineered Interfaces and Surfaces in Materials The importance of engineered interfaces and surfaces in materials arose continually, either directly or indirectly, as the Energy and Power Materials Panel identified materials challenges. Electrochemical capacitors will have improved power density if the interfaces on which charge is stored are tailored to provide electrochemical stability as well as optimum charge storage. Similarly, lithium batteries with interfaces tailored to eliminate pyrophoric dendrites could lead to 2-3 times power density improvements. Advanced approaches for increasing the surface area of intermixed components in energetic materials may be able to significantly raise the energy/mass ratio, leading to smaller projectiles of similar or

OCR for page 55
greater capability. MEMS devices may be dramatically improved by an enhanced understanding of reactions occurring at surfaces between dissimilar materials at elevated temperatures. DoD should place renewed emphasis on interfaces and surfaces, either separately or under a nanomaterials initiative. Advanced Energy Storage and Conversion Materials New materials will be essential in meeting the energy and power needs of DoD in 2020. The largest subarea that can potentially benefit is materials for electrochemical energy storage and conversion for use in batteries, electrochemical capacitors, fuel cells, and fuel reforming. As an example, high-surface-area battery electrodes made using a one-step process may allow full exploitation of the material’s possibilities. Synthesis of new solid-state electrolytes (e.g., perovskite or fluorite oxide conductor) could produce revolutionary advances for the DoD of 2020. For instance, a zero methanol crossover electrolyte would reduce the water requirements for a direct methanol fuel cell by a factor of four while at least doubling the power density, thus greatly reducing the burden of water carried by a dismounted soldier or supporting robot. New electrocatalysts for reforming diesel fuel could have a huge DoD payoff. Similarly, diesel fuel anode catalysts should receive considerable attention, because the civilian sector has little interest in this area, while it is crucial to DoD for reforming logistic fuels. The panel also suggests that novel energetic materials, including propellants and explosives, continue to be a fruitful area for research for future defense needs. Some of the key subareas of this important category have been highlighted in this chapter. Tools for Accelerated, Systematic Materials Discovery Considerable time may separate the discovery and the introduction of a new material, depending on the application area and the forces driving the new material. Structural materials have required on average approximately 20 years for introduction; energetic materials have often taken considerably longer. Traditionally, the development of new materials has relied on the intuition, ingenuity, and knowledge base of experienced

OCR for page 55
individuals who can extrapolate the properties and structures of existing materials to those of new materials. To be exploited, materials must first be discovered. To speed the discovery of new materials, the panel emphasizes, tools for accelerated systematic materials discovery and application should be employed to the maximum extent possible. These tools include (1) computational materials science, to assist in analytical design of promising new materials; and (2) combinatorial materials science, to speed the experimental discovery of new materials. The latter is particularly important in those cases where there are few signposts to promising materials, such as fuel cell electrocatalysts, or where it appears desirable to map out possible limits on the material properties of a family of compounds. DoD is already using these tools effectively. This suggestion emphasizes the importance of expanding their use in materials discovery and application, to hasten the discovery and exploitation of novel materials. Materials as the Foundation for Systems Materials are the basic ingredients of all DoD hardware systems. They (together with the design) affect system cost throughout the system life cycle in terms of the costs for raw materials, processing and fabrication, assembly, NDI/NDE, repair and maintenance, and disposal. It is axiomatic that much of the cost associated with a system is locked in very early in system development by the materials and design decisions made at that time. It is also clear that many materials scientists often become so strongly associated with a material or family of materials that they become advocates for that material. Similarly, materials and design engineers often become wedded to particular approaches. During the investigations conducted by the Energy and Power Panel, it was not uncommon to encounter strong advocates for particular materials. Typically, one or more key properties of a material, which happened to be optimized and which made the material a candidate for an application, were cited as evidence for the material’s value in that application. In reality, however, every material has strengths and weaknesses. Virtually every application places multiple requirements upon a given material, making the selection of material a complex tradeoff among different, often conflicting, factors.

OCR for page 55
Similarly, the cost savings for using a given material in a component were sometimes provided as a “point comparison” with another material, without regard to the contribution of that component to total system cost. Such comparisons provide little real insight. For example, a system (e.g., a fuel cell) is a collection of interdependent components (e.g., anode, cathode, electrolyte), and the introduction of one material into a system has consequences for every other material used in that system in terms of such issues as reactivity, thermal stability, or need for compatibility coatings. Systems can be quite complex. They require a broad view, while materials research requires that the materials scientist focus largely on one or only a few aspects of material structure or behavior. Therefore, it is only natural that advocacy occurs. However, DoD materials science and engineering managers should emphasize a systems-based approach in which all material properties relevant to a class of applications are identified as early as possible, and in which point solutions are avoided. In addition to challenging point solution advocacy wherever it occurs, this would entail evaluating materials in a subscale device (or minisystem configuration) as early as possible in the research stage. This should help identify material strengths and weaknesses and should enable decisions to be made as early as possible about the potential effectiveness or limitations of a material in a class of applications. REFERENCES AFRL (Air Force Research Laboratory). 2001. Scientists are developing next-generation microelectromechanical systems from silicon carbide. P. 29 in AFRL Technology Horizons. Ohio: AFRL, Wright Patterson Air Force Base. Bates, J.B. 2000. Thin film lithium and Li ion batteries. Solid State Ionics 135: 33. Bronstein, L.M., C. Joo, R. Karlinsey, A. Ryder, and J.W. Zwanziger. 2001. Nanostructured inorganic-organic composites as a basis for solid polymer electrolytes with enhanced properties. Chemistry of Materials 13(10):3678-3684. Cava, R., F. DiSalvo, L. Brus, K. Dunbar, C. Gorman, S. Haile, L. Interrante, J. Musfeldt, A. Navrotsky, R. Nuzzo, W. Picket, A. Stacey, and A. Wilkinson. 2001. Future Directions in Solid State Chemistry, Report of NSF-Sponsored Workshop, October, UC-Davis. Arlington, VA: National Science Foundation. Christen, T., and M.W. Carlen. 2000. Theory of ragone plots, J. Power Sources 91:210-216. Clelland, I., R. Price, and J. Sarjeant. 2000. Advances in capacitor technology for modern power electronics. Pp. 145-148 in Proceedings of the 24th International Power Modulator Symposium, June, Norfolk, VA. Piscataway, NJ: IEEE. Croce, F., G.B. Appetecchi, L. Persi, and B. Scrosati. 1998. Nanocomposite polymer electrolyte for lithium batteries. Nature 394(6692):456.

OCR for page 55
Cunniff, P.M. 1999a. Dimensionless parameters for optimization of textile-based body armor systems. Proceedings of the 18th International Symposium on Ballistics, November, San Antonio, TX. Lancaster, PA: Technomic. Cunniff, P.M. 1999b. Assessment of small arms (ball round) body armor systems. Proceedings of the 18th International Symposium on Ballistics, November, San Antonio, TX. Lancaster, PA: Technomic. DoD (Department of Defense). 1999. Militarily Critical Technologies (MCT) Part I: Weapons System Technologies. Fort Belvoir, VA: Defense Technical Information Center. Dong, W., D.R. Rolison, and B. Dunn. 2000. Electrochemical properties of high surface area vanadium oxide aerogels. Electrochemical and Solid State Letters 3(10):457. Doughty D.H. 1996. Materials development for lithium-ion batteries. Pp. 102-107 in Electric and Hybrid Vehicle Technology ’96: The International Review of Electric and Hybrid Vehicle Design and Development, G. Lindsay, ed. Dorking, Surrey, UK: UK & International Press. Hamlen, R.P. 1995. Metal/Air batteries. Pp. 38-1 to 38-45 in Handbook of Batteries, 2nd Edition, David Linden, ed. New York: McGraw-Hill, Inc. Kornmann, X., L.A. Berglund, J. Sterte, and E.P. Giannelis. 1998. Nanocomposites based on montmorillonite and unsaturated polyester. Polymer Engineering and Science 38(8):1351-1358. McNaught, A.D., and A. Wilkinson. 1997. IUPAC (International Union of Pure and Applied Chemistry) Compendium of Chemical Terminology, 2nd Edition. P. 54:1545. Oxford, UK: Blackwell Science. Messersmith, Phillip B., and Emmanuel P. Giannellis. 1995. Synthesis and barrier properties of poly(e-caprolactone)-layered silicate nanocomposites. Journal of Polymer Science: Part A: Polymer Chemistry 33:1047-1057. National Research Council. 1997. Energy Efficient Technologies for the Dismounted Soldier. Washington, DC: National Academy Press. Available online at <www.nap.edu/catalog/5905.html>. Accessed October 15, 2001. National Research Council. 1999. Reducing the Logistics Burden for the Army After Next: Doing More with Less. Washington, DC: National Academy Press. Available online at <www.nap.edu/catalog/6402.html>. Accessed January 15, 2002. Pang, S.C., and M.A. Anderson. 2000. Novel electrode materials for ultracapacitors: Structure and electrochemical properties of sol-gel derived manganese dioxide thin films . P. 415 in Proceedings of New Materials for Batteries and Fuel Cells, San Francisco, CA, D.H. Doughty, L.F. Nazar, M. Arakawa, H.P. Brack, and K. Naoi, eds. Warrendale, PA: Materials Research Society. Poizot, P., S. Laruelle, S. Grugeon, L. Dupont, B. Beaudoin, and J.M. Tarascon. 2000. Electrochemical Reactivity and Reversibility of Cobalt Oxides Towards Lithium. Comptes Rendus de l’Académie des Sciences, Series II, p. 681. Paris: Elsevier. Potomac Institute. 2000. Conference Summary Report of Out of the Box and Into the Future: A Dialogue Between Warfighters and Scientists on Far-Future Warfare. Arlington, VA: Potomac Institute. Available online at <http://www.potomacinstitute.org/pubs/otb_summary.pdf>. Accessed December 15, 2001. Riley, M.W., P.S. Fedkiw, and S.A. Khan. 2000. Nanocomposite based electrolyte for lithiumion batteries. P. 137 in Proceedings of New Materials for Batteries and Fuel Cells, San Francisco, CA, D.H. Doughty, L.F. Nazar, M. Arakawa, H.P. Brack, and K. Naoi, eds. Warrendale, PA: Materials Research Society.

OCR for page 55
Salkind, A.J. 1998. Advances in battery technologies and markets—Materials science aspects. P. 1 in Proceedings of Materials for Electrochemical Energy Storage and Conversion II— Batteries, Capacitors and Fuel Cells, Boston, MA, D.S. Ginley, D.H. Doughty, B. Scrosati, T. Takamura, and Z.M.J. Zhang, eds. Warrendale, PA: Materials Research Society. Sata, N., K. Eberman, K. Eberl, and J. Maier. 2000. Mesoscopic fast ion conduction in nanometre-scale planar heterostructures. Nature 408(6815):946. Schlapbach, L., and A. Zuttel. 2001. Hydrogen-storage materials for mobile applications. Nature 414(6861):353. Singh, D., R. Houriet, R. Vacassy, H. Hofmann, V. Craciun, and R.K. Singh. 2000. Pulsed laser deposition and characterization of LiMn2O4 thin films for application in Li ion rechargeable battery systems. P. 83 in Proceedings of New Materials for Batteries and Fuel Cells, San Francisco, CA, D.H. Doughty, L.F. Nazar, M. Arakawa, H.P. Brack, and K. Naoi, eds. Warrendale, PA: Materials Research Society. Viechnicki, D.J., M.J. Slavin, and M.I. Kliman. 1991. Development and current status of armor ceramics. Ceramic Bulletin 70(6):1035-1039.

OCR for page 55
This page in the original is blank.