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CHAPTER EIGHT Integration of Research Opportunities INTRODUCTION The Committee on Materials Research for Defense After Next examined a broad range of materials research areas, including such diverse areas as bioderived materials for wound healing, high-temperature structural materials for advanced jet engines, and materials for advanced explosives and propellants. Despite this diversity, the recommendations resulting from this effort can be summarized into four principal themes: Design of materials, devices, and systems assisted by computation and phenomenological models of materials and materials behavior; Convergence, combination, and integration of biological, organic, semiconductor/photonic, and structural materials; Discovery and characterization of new materials with unique or substantially improved (50 percent) properties; and New strategies for synthesis, manufacture, inspection, and maintenance of materials and systems. These themes are explored in detail in the sections that follow. DESIGN OF MATERIALS, DEVICES, AND SYSTEMS ASSISTED BY COMPUTATION AND PHENOMENOLOGICAL MODELS OF MATERIALS AND MATERIALS BEHAVIOR Throughout history, humans have continually sought to improve their existence by finding novel approaches to the
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discovery and application of useful materials. Early man refined processes for melting and shaping metals and their alloys through exhaustive trial-and-error approaches extending over millennia, giving rise to the Bronze and Iron Ages. Such trial-and-error processes have continued into modern times. The protracted search for an acceptable electric light bulb material culminated in the tungsten filament and contributed a new phrase to the English language: “Edisonian approach.” The pace of materials discovery continues to quicken. Today’s materials scientists have at their disposal increasingly powerful tools. Foremost among them is the breathtaking improvement in computational power, leading to the ability to predict certain structures from first principles. Indeed, in some cases, such as development of insensitive energetic materials, computational approaches are leading experiments in new materials synthesis and are reducing the time required to discover and apply new materials. Coupled with the use of phenomenological models to help identify and predict the characteristics and behavior of potentially revolutionary materials, advanced computational approaches offer a rapid and powerful means for discovery. RECOMMENDATION 2. THE DEPARTMENT OF DEFENSE SHOULD MAKE RESEARCH INVESTMENTS IN THE DESIGN OF MATERIALS, DEVICES, AND SYSTEMS ASSISTED BY COMPUTATION AND PHENOMENOLOGICAL MODELS OF MATERIALS AND MATERIALS BEHAVIOR. The potential benefits from such investments for DOD could be staggering; a few of the opportunities and applications discussed in this report are outlined below: In advanced structural composites strength, toughness, durability, and damage tolerance are highly dependent on the particle-matrix or fiber-matrix interface. Reliable tailoring of composite characteristics to specific applications requires detailed understanding of these interfaces, which will come only after there are models for interface structures, properties, and thermodynamic stability. Success would lead to reduced need to overdesign the structural components of military systems, with a concomitant reduction in weight and energy consumption. Advanced photonic devices such as optical analog/digital (A/D) converters promise processing speeds significantly faster than their electronic counterparts, enabling advanced optical networking and communi-
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cations. This capability will require materials with 500 times higher electro-optical coefficients than current materials like LiNbO3. Such properties can probably be achieved only in artificially structured materials that are unlikely to be found “by accident.” Enhanced theory and modeling will be essential in guiding experimental efforts if these promising compositions and structures are to be identified and applied. Protection of eyes and sensors against battlefield lasers is important across the DoD spectrum. This problem has been exacerbated by the advent of tunable lasers, making multiwavelength protection a necessity. Use of a priori quantum chemical models combined with phenomenological models in the design of advanced materials has led to increased third-order nonlinear susceptibilities and two-photon absorption cross sections. Enhanced modeling would assist in the discovery of new families of optical materials for protecting eyes and sensors from hostile or accidental laser beam impingement over wide ranges of frequency and flux intensities. Lightweight mobile power is particularly important for the Army and the Marine Corps. Advanced computational techniques combined with phenomenological models should assist in identifying macromolecules for advanced thermoelectric materials that possess higher figures of merit. DoD could then take advantage of waste heat to generate mobile power and could also miniaturize electronic and EO components through use of positive thermal management protocols. Cost of ownership of military vehicles over their lifetime is very important to DoD. Prediction and extension of component life would remarkably reduce this cost but will be possible only as computational models are created for the aggregate properties of reliability, durability, and affordability. There is a veritable cornucopia of inorganic materials that are promising for future DoD applications, ranging from lean alloys for structural use to wide bandgap semiconductors for high-power electronic applications. Determining which are most appropriate to DoD needs depends on understanding the mechanisms by which trace elements and individual defects affect key materials properties. Such understanding will come only from the use of appropriate computational models. The computational power required to support all these needs was unimagined a generation ago, particularly the massively parallel architectures of today’s most powerful computers. The concept of massive parallelism also applies on the experimental front. For example, combinatorial synthesis (the automated parallel examination of many potential solutions
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with criteria for identification of best options built in) is inspired by the deeply parallel discovery processes found in living systems. Examples of applications of combinatorial synthesis range from drug discovery to identification of high-temperature superconductors. Real-time application of combinatorial synthesis could be the basis for identifying agents to counter chemical or biological agents. CONVERGENCE, COMBINATION, AND INTEGRATION OF BIOLOGICAL, ORGANIC, SEMICONDUCTOR/PHOTONIC, AND STRUCTURAL MATERIALS Convergence, combination, and integration are major themes for addressing future defense needs. History has shown that major advances usually occur at intersections or points of convergence between disparate areas. This is true in both broad fields of endeavor (e.g., chemistry, physics, biology) or more specific areas (e.g., lithography, microsystems). RECOMMENDATION 3. THE DEPARTMENT OF DEFENSE SHOULD MAKE RESEARCH INVESTMENTS THAT PROMOTE CONVERGENCE, COMBINATION, AND INTEGRATION OF BIOLOGICAL, ORGANIC, SEMICONDUCTOR, PHOTONIC, AND STRUCTURAL MATERIALS. Examples of the most significant opportunities for investments that support the future needs of the DoD are provided below. Convergence The convergence of biology with traditional materials science with the application of biological principles to the design of structural materials and microsystems is an area particularly ripe for revolutionary advances in materials for DoD. The concept of bioinspired processing applies lessons from biology to creation of synthetic materials. This approach is ideally suited for the design and fabrication of nanostructured organic and organic/inorganic composites. Materials like bone, teeth, and shells are simultaneously hard, strong, and tough and have unique hierarchical structural motifs originating at the nanometer scale. Mimicking such designs should lead to very strong, tough materials usable in, e.g., lightweight armor for both warfighters and vehicles. It could also be used for mechanical system components.
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Examining the lessons of biology allows us to envision microsystems based on entirely different architectures than are used today. While today’s microsystems use components like gears whose features are essentially scaled down from macroscopic dimensions, future microsystems may be built from components that are either inspired by the ways that biological systems work or that exploit actual biological materials. DoD should actively seek first to identify and then to exploit such points of convergence between materials science and biology. Combination Many major advances over the history of materials resulted from synergistic combinations of materials that yield property levels that cannot be obtained in a single material. Once it was recognized that the presence of other atoms could lead to improved performance, the use of metals in ancient times quickly gave rise to intentional alloying for structural applications. Similarly, bricks made from mud and straw proved more durable than mud alone. In the latter half of the 20th century, advanced composites incorporating polymers, metals, and ceramics reinforced with other materials in fiber, whisker, platelet, and other geometries gave the structural designer new possibilities. Because many material combinations have already been explored, the probability of obtaining substantial improvements in properties through empirical combination of different materials and geometries has been decreasing. However, a resurgence is under way due to exploitation of a new variable: size scale. In particular, there is the potential for major improvements in performance from intimate and controlled mixing of materials at the nanoscale. This area provides fertile ground for significant advancements in many material properties of importance to DoD. While the practice of combining materials, each adding its own unique characteristics and functionality, has been widely explored in structural materials, exploration of combinations for other applications is only beginning. An example is the use of high-yield photon absorbers with materials that are highly efficient charge carriers, vastly improving solar-energy harvesting compared with a single material. Material combinations in nontraditional areas provide a compelling strategy in the search for next-generation functional materials. Combining organic and inorganic materials into composites to provide new ionically conducting pathways in polymer battery systems, modifying
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the current mechanism responsible for ionic mobility, offers DoD many potential benefits. Nanostructured clays in polyethylene oxide (PEO) and nanoparticulate oxides in PEO have both yielded improved conductivity and transference number. Composites can alter the mechanism of ionic transport, suggesting the possibility of high conductivity, with ionic transport number of the “working ion” (e.g., Li+) near unity. Integration Integration is a third important subtheme. Whereas combination connotes the union of two or more materials to achieve a characteristic or property level not possible using a single material, integration is the purposeful, seamless union of separate functions in a material, device, or structure that contributes to true multifunctionality. Function can be integrated at different size scales; multiple functions can be integrated in several materials or in a single material. At the macroscale, a traditional structural composite contains more than one material, but it is a combination that has only one primary function, i.e., to carry loads and moments. A more advanced, integrated composite might, in addition to providing structural capability, be able to absorb radar wavelengths or to effect local color change where damage has occurred (“bruising”) to alert maintenance personnel. The integration of multiple functions at the macroscale, the microscale, and the nanoscale will likely be characteristic of the DoD systems of 2020. As an example, an almost limitless number of organic composites can be made by combining matrix resins that have different bonding capabilities (e.g., trifunctionality, tetrafunctionality) and functional groups that have multifunctional complementary phases. This may lead to structural materials that can also absorb radar energy, conduct electricity, serve as large area sensors, or perform other useful functions. Another example is monolithically integrated microsystems on a warfighter’s clothing or gear that would sense exposure to a chemical or biological agent, diagnose the type and degree of exposure, and then fabricate and administer an antidote. This might require integration of both materials and functions, e.g., semiconductors (for logic), optical materials (for sensing), organic or biological materials (as molecular recognition sites for chemical or biological agents or for another analytical technique), and organic or biological materials (for fabricating the antidote).
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A limiting factor in the use of biological molecules or cells as the active component of sensing or diagnostic devices is preservation of the biological function and stabilization of biological activity in a non-biological environment for extended periods of time. Packaging strategies will be needed to assure continued activity of the biological components of such devices. Integration of function at the nanoscale in organic materials for computational devices has been of interest for some time. Most, if not all, of the individual molecular-scale building blocks that are required to create electronic circuits have recently been demonstrated, although reproducibility has been problematic. The next step is to create economically viable and highly reliable electronic circuits that are equivalent or superior to silicon technology but operate on a molecular scale. A number of concepts and designs on how to build basic molecular-scale electronic adders or logic circuits have been put forward. To make these a reality, advanced computational design and then synthesis of durable molecular organic, polymer, and hybrid building blocks will be necessary, coupled with advanced fabrication, connection, and assembly technologies. A key aspect of materials science that must be considered in both combination and integration of multiple materials is the importance of preserving material performance at interfaces. Nowhere is this more clearly illustrated than in the advanced electronic and photonic devices that could revolutionize computer, optical, and microsystem architectures. Successful realization of quantum electronic devices will live or die with our ability to control atomic-level composition and atomic placement in immediate proximity to interfaces, and to control overall device dimensions with the same degree of accuracy. The semiconductor industry already accomplishes this routinely in one dimension, but future devices will require the same degree of control in the other two dimensions as well. Devices that operate at wavelengths of interest and use photonic bandgap engineering will likely not be as sensitive to atomic-level accuracy, but the materials that need to be integrated will probably be more incompatible chemically. This challenge is likely to be equally as great as the challenges of quantum electronic devices. Processing and characterization are integral to success in any merging of materials or functions. For example, 21st-century devices may require the union of rigid inorganic materials that have hitherto required high processing temperatures with synthetic or even biological macromolecular
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structures that have drastically different material characteristics. Processing such hybrid structures presents a tremendous challenge. The interfaces between materials must be clearly understood because functionality is often derived from the materials properties very close to an interface. As device and system dimensions shrink, interfaces become even more dominant because an increasing proportion of atoms or molecules find themselves at one interface or another. Advances in methods to characterize interfaces, both during processing and after deployment, coupled with predictive models that enable interface property optimization, will increase in importance. Solving this type of problem will necessitate addressing all four of the research opportunity themes identified by the committee and discussed in this chapter. DISCOVERY AND CHARACTERIZATION OF NEW MATERIALS WITH UNIQUE OR SUBSTANTIALLY IMPROVED PROPERTIES DoD systems of 2020 would benefit significantly from the discovery, production, and application of materials that have either unique properties or properties that considerably exceed those of today’s materials. DoD weapon systems and platforms must be lethal and sustainable, and they must enhance the survivability of the user. These requirements continue to drive new materials discovery. Reducing volume, mass, or both while enhancing functionality are key system drivers. Munitions must be more compact; power and energy sources must have higher volumetric and mass densities; armor must be lighter while providing equivalent or enhanced protection; platform structure should be lighter but remain strong so that payload can be increased; and electronic and optical communication systems must be smaller and lighter but have higher capability and bandwidth. All these are tangible system requirements that depend on the discovery and evolution of substantially improved new materials in subsystems, device components, and subcomponents. RECOMMENDATION 4. THE DEPARTMENT OF DEFENSE SHOULD MAKE RESEARCH INVESTMENTS THAT PROMOTE DISCOVERY AND CHARACTERIZATION OF NEW MATERIALS WITH UNIQUE OR SUBSTANTIALLY IMPROVED PROPERTIES (BY 50 PERCENT OVER CURRENT PROPERTIES).
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Examples of the most significant opportunities for such investments that support the future needs of the DoD are provided below. Design and fabrication of tunable materials with low-loss dielectric and piezoelectric properties would enable low-loss phase shifters that could operate at the high frequencies required for IR countermeasure systems. Improved organic photovoltaic materials that have higher efficiencies could permit localized power harvesting and generation, providing increased battlefield mobility at relatively low cost. High-quantum-efficiency electroluminescent materials, assuming 50 percent improvement over today’s levels, will permit organic light-emitting materials to compete effectively as efficient large-area solid-state light sources and in flat-panel displays. This would have far-reaching benefits for DoD in terms of savings in weight and cost and increased battlefield ruggedness. Energetic materials also offer the potential for substantially improved (greater than 50 percent) properties. For example, polyatomic nitrogen compounds could yield propellants with greatly enhanced specific impulse, and could also be used in enhanced explosives. Novel salts of polyatomic nitrogen species might be able to provide a 5-6 times mass reduction in energetic material for the same payload mass in missiles, allowing for huge performance increases. While highly nonlinear effects in traditional bulk materials are unlikely, nonlinear optical materials offer the potential for substantially improved optical limiting capabilities. Strong confinement of electrons in organic materials or quantum dots may lead to enhanced nonlinear optical coefficients. Nanostructured material approaches could provide wholly new routes to solving the considerable problem of protecting eyes and sensors from laser radiation at a variety of frequencies. Computational and combinatorial approaches may accelerate the pace of developing such materials. Novel materials with unique properties could also lead to tunable IR detectors capable of operating across the 1- to 15-micron region. Carbon nanotubes, whose semiconducting bandgaps can range from less than 100 meV to several hundred meV, offer a new approach to tuned materials; the ability to control bandgap by controlling tube diameter and chirality of nanotubes in a well-aligned film offers tantalizing possibilities. Binding site-receptor interactions in biology offer a strategy for the discovery of useful active agents for interdicting unknown toxins and pathogens while identifying known pathogens. For example, antibodies active against an unknown agent may be identified with recombinant DNA
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techniques while an antibody known to bind to a specific pathogen may be used as the active site of a sensor for that pathogen. Techniques for remote detection of biological agents and pollutants continue to interest DoD. Bacteria, viruses, and many bacterial toxins, as well as all known nutrient media for biological agents, have strong UV fluorescence spectra that can serve as indicators for remote biodetection. Efficient UV laser and detector media are largely materials-limited. GaN-based wide-bandgap semiconductors are promising for blue and UV lasers, but because a fundamental understanding of these materials is lacking, how they respond to DoD needs is not established. Membrane research will continue to be a fertile area for DoD over the next two decades. Membranes are ubiquitous in numerous applications, among them chemical-biological defense, water purification, and energy technology (e.g., fuel cells). The fundamental science of modeling, synthesis, and processing of membrane materials could lead to smart membrane materials that could report on the local environment and change properties as needed, as well as repair their own defects. Other advances could enable energy-efficient water cleaning and ion-transporting membranes for high-performance power generation. NEW STRATEGIES FOR PROCESSING, MANUFACTURE, INSPECTION, AND MAINTENANCE OF MATERIALS AND SYSTEMS The themes already discussed describe an approach to the discovery of novel materials, combinations of materials, and integrated functionalities. Discovery of their structures and potential properties is an important step toward realizing their potential for new and far-ranging DoD capabilities. However, this is only the starting point for introducing materials into systems. Materials must be processed and characterized successfully in the laboratory on a small scale, and then practical and robust methods for scale-up to sufficient quantities must be applied. Techniques must be available for fabricating and assembling the materials into prescribed geometries, and for quality assurance to make certain they meet requirements. Finally, there must be techniques for inspecting them in the field to identify problems, repair them, and assure the adequacy of repair. The panel reports have suggested a number of areas that could help assure the successful transition of new materials into defense applications.
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RECOMMENDATION 5. THE DEPARTMENT OF DEFENSE SHOULD MAKE INVESTMENTS IN RESEARCH LEADING TO NEW STRATEGIES FOR THE PROCESSING, MANUFACTURE, INSPECTION, AND MAINTENANCE OF MATERIALS AND SYSTEMS. Examples of the most significant opportunities for such investments to respond to future needs of DoD are provided below. The capability for continuous characterization of materials while they are in service could provide significant benefits to DoD. Military systems require predictable high-level performance in a range of adverse environments. In-service characterization would allow for continuous monitoring of the health of systems and would provide essential information on how key properties change as a result of service-induced damage. One of the key enablers for this capability would be smaller, more sensitive NDI/NDE sensors that can be attached to or included in structural materials and the structures themselves when they are manufactured. Such sensors could allow for continuous monitoring and real-time recovery of information from multifunctional material systems for use in evaluating safety, need for replacement or maintenance, or remaining life. Reducing the large amounts of data collected by such sensors into much smaller amounts of useful information will be a challenge. Repair, maintenance, and life extension of military systems is an important current and future issue for DoD. Materials that self-repair or heal themselves could pay major dividends, at least for localized damage if there were a practical means for incorporating self-repair mechanisms in them. The feasibility of restoring the strength of matrix materials using embedded chemicals has already been demonstrated. However, other approaches, if fully realized, might lead to a wide array of self-healing polymers, composites, and adhesives for structures. A critical aspect to be addressed is decreasing the time required for healing response so that it is rapid enough to be useful. Purity will be increasingly important for multifunctional, organic, electro-optically active materials capable of high performance and reliability. This will require routine, affordable synthetic processing strategies that reduce chemical and morphological defects to levels that are currently at best barely achievable. The future of macromolecular materials in high-performance DoD applications depends on processing of high-purity, well-defined, reproducible structures that are affordable. Likewise, fielding new
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materials with extreme properties will require emphasizing, first, understanding their characteristics and, then, controlling them through fabrication. This emphasis will allow for production of components and subsystems for frequency-agile communication and radar applications that take advantage of the field-variable properties of ferroelectrics, ferrites, and other new materials. Similarly, accelerated DoD systems application of solar-energy-harvesting devices would benefit from low-cost manufacturing of materials and devices that provide efficient photon collection and rapid charge conduction, and that can be applied over large areas. As noted in Chapter 6, organic photovoltaics are one possible solution that also allows use of flexible substrates. If such approaches are to become widely applied in military systems, a cost-effective way to make photovoltaic cells in large quantities is essential.Today’s laboratory-scale techniques for generating promising material configurations must give way to truly manufacturable approaches. The promise of nanomaterials for a wide variety of DoD applications has been discussed throughout this report. Today, such materials are barely more than a laboratory curiosity. Usually, only excruciatingly small quantities of material can be produced and production is generally measured in hours or days. If nanomaterials are to become widely applied, it will be essential to address the challenge of scaling up their production to macroscopic quantities while retaining the performance properties of the small samples currently available. Some properties of nanomaterials also need to be better understood so that they can be engineered for specific applications. Biomimetic and bioinspired manufacturing approaches have considerable potential for meeting future defense needs. Much current research, particularly in biotechnology, has been oriented toward pharmaceutical and medical products; biomanufacturing approaches can also apply to the nonmedical arena. For example, biocatalysis is an exceptionally efficient manufacturing process that can accelerate reactions by up to 13 orders of magnitude; it has exquisite specificity for starting materials and products (no byproducts) at room temperature and atmospheric pressure. The activity of enzymes can be controlled over several orders of magnitude by binding specific effector molecules. Selective activation of enzymes could effect specific chemical conversions in materials for munitions, therapeutic agents, odorants, and other uses. Indeed, harnessing such approaches would be revolutionary. Materials are rarely applied in isolation; usually, a system of materials must work together seamlessly. As an example, a fuel cell contains such
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elements as electrodes, membrane, electrolyte, and catalyst. Optimal application of new materials approaches (e.g., nanostructured electrodes, polymeric membranes) to this complex electrochemical system will require methods for characterizing the structure and stability of the individual materials as well as their interaction within the system as a function of such factors as time, temperature, and local chemical environment. Accomplishing this in the laboratory is only the first step. Cost-effective, reliable synthesis and manufacturing of the individual components must be coupled with appropriate assembly and quality assurance techniques, followed by testing in military environments. There are many steps between laboratory demonstration of a single material and successful introduction of a system of materials into a military application where problems can arise. The committee strongly recommends that a life-cycle view be taken as soon as possible in the materials design process so that questions about application-relevant characteristics can be formulated and answered early. This will help systems designers to avoid individual materials or materials combinations that are unsuitable and that could lead to major difficulties in a system or subsystem. Short term, this will clearly require spending time and other resources to resolve uncertainties about materials options; long term, this approach should reduce the life-cycle cost of maintenance and repair. CONCLUSIONS Materials science and engineering is entering a renaissance due to the advent of enhanced tools for discovering, processing, structuring, combining, and characterizing matter in wholly new ways and at ever-decreasing scales. This has profound implications for DoD. Today’s DoD requires technology that yields high performance and is reliable under extremely adverse conditions. Tomorrow’s DoD will rely on tremendous advances in technology. Intelligence will be gathered using large numbers of unmanned autonomous vehicles and sensors that are fully networked, allowing the battlespace to be monitored from different vantage points—undersea, on the ground, in the air, and from low Earth orbit. Battles will be fought using more lethal weapon systems that have tailorable effects and will require fewer but more highly skilled and better-equipped soldiers, sailors, and aviators. Transformation of the way the United States chooses to fight will eliminate heavy, slower systems and place a premium on speed and tactics, as well as communication. This will
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necessitate major improvements to weapon platforms that increase mobility and enhance survivability. Except for software, most of these advances for 2020 will be enabled by improvements in materials, processing methods, and associated design techniques. To achieve these capabilities, DoD should emphasize the four overriding themes described in this chapter. First, computational and phenomenological models of materials and their behavior are essential to the design of both novel materials themselves and the devices and systems incorporating them. Second, convergence, combination, and integration are essential. DoD should seek out the intersections of major bodies of knowledge, for it is here that major advances disrupting the status quo are often found. Of particular interest is the intersection of biology and materials science. Combination of materials in many forms, taking advantage of new approaches to specifically tailor structure at different length scales, should be pursued. Integration of biological, organic, semiconductor, and structural materials should be examined as a way to obtain true multifunctionality at both the macroscale and the microscale. Third, DoD should seek to discover and characterize materials that have either unique or substantially improved properties. The numerous possibilities identified in the reports of the panels span the gamut of DoD applications and involve a broad range of materials. Foremost among these is the exploitation of our increasing ability to control the structure of materials at the nanoscale. Fourth, new strategies for the synthesis, manufacture, inspection, and maintenance of materials and systems are essential. Materials science and engineering may be entering a renaissance, but its potential for the DoD will be realized only if exciting small-scale advances in the laboratory can be translated into practical large-scale production processes and systems. Traditional stovepipe approaches to management of materials and processes within the materiel life cycle will result in delayed or missed opportunities for transition into fielded military hardware. Overcoming these historical barriers will require a sustained, concerted effort by DoD and the industrial managers responsible for advancing materials throughout the life cycle. This shift in approach will undoubtedly require a painful cultural change. However, without it the pace of transition to new materials in DoD platforms and weapons systems will remain unacceptably slow, representing lost opportunities to make rapid, dramatic, and far-reaching changes to our military systems that meet constantly evolving and increasingly more sophisticated threats to the well-being of the nation.
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