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Materials Sciences: Biomaterials, Energy Materials and Devices, and Photonic Materials and Devices

INTRODUCTION

The Panel on Materials Science and Engineering at the Army Research Laboratory conducted its review at Adelphi, Maryland, on June 11-13, 2013. This chapter provides an evaluation of that work, recognizing that it represents only a portion of ARL’s materials sciences core technology competency portfolio. The review addressed the areas of biomaterials, energy materials and devices, and photonic materials and devices.

ARL’s materials sciences span the spectrum of technology maturity and address Army applications, working from the state of the art to the art of the possible, according to the ARL. Materials research efforts and expertise are spread throughout the ARL enterprise. As the ensemble of the materials discipline and capabilities, the area of materials sciences is one of ARL’s primary core technical competencies. In the larger context, the mission of ARL, as the U.S. Army’s corporate laboratory, is to provide innovative science, technology, and analyses to enable a full spectrum of operations.

The criteria for the assessment are the technical merits of the work; facilities and equipment; research talents; underlying broad-based sciences; and the desired impact and the balance of theory, computation, and experimentation. They include as well alternative directions or approaches to catapult a project toward its promised accomplishments.

In this chapter, ARLTAB’s carries out its assessment by category of material and by project and goes on to offer general observations, propose future thrusts, and make suggestions.

Overall, the researchers and the management are of high caliber and deserve kudos. Researchers appeared ebullient and passionate about their work. ARL’s work in preparing for the review was superb. The ARL Director’s webinar overview and the read-ahead materials, presentation viewgraphs, poster materials, and laboratory tours greatly facilitated the review process. It was highly valuable to have an interactive session with the ARL Director to become acquainted firsthand with the ARL’s mission and goals. This is also the first review involving the new director of ARL’s Sensors and Electron Devices Directorate, whose vision and plans were presented in an energizing fashion.

As for institutional aspiration, there is no shortage of challenges and opportunities. One of the larger questions being asked is, How can ARL be meritoriously unique as a research laboratory that is nourished by an innovative culture? In expanding innovation, discovering new science and new technology is as rewarding as crafting new uses of existing technologies to develop advanced Army products.

Striking a balance between the projects that tackle known unknowns, driven by application and innovation on demand, and the projects that explore unknown unknowns to achieve high-risk and high-reward outcomes needs to be an ongoing effort.

Another opportunity is presented by asking how the distance between the science and the Army end-use applications can be narrowed. The ARL scientists maintain knowledge of Army applications through direct exposure with the end-users in the field. To enhance human capital, nurturing a working environment that offers positive energy, organizational stability, and high retention rate is essential. Establishing a



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6 Materials Sciences: Biomaterials, Energy Materials and Devices, and Photonic Materials and Devices INTRODUCTION The Panel on Materials Science and Engineering at the Army Research Laboratory conducted its review at Adelphi, Maryland, on June 11-13, 2013. This chapter provides an evaluation of that work, recognizing that it represents only a portion of ARL’s materials sciences core technology competency portfolio. The review addressed the areas of biomaterials, energy materials and devices, and photonic materials and devices. ARL’s materials sciences span the spectrum of technology maturity and address Army applications, working from the state of the art to the art of the possible, according to the ARL. Materials research efforts and expertise are spread throughout the ARL enterprise. As the ensemble of the materials discipline and capabilities, the area of materials sciences is one of ARL’s primary core technical competencies. In the larger context, the mission of ARL, as the U.S. Army’s corporate laboratory, is to provide innovative science, technology, and analyses to enable a full spectrum of operations. The criteria for the assessment are the technical merits of the work; facilities and equipment; research talents; underlying broad-based sciences; and the desired impact and the balance of theory, computation, and experimentation. They include as well alternative directions or approaches to catapult a project toward its promised accomplishments. In this chapter, ARLTAB’s carries out its assessment by category of material and by project and goes on to offer general observations, propose future thrusts, and make suggestions. Overall, the researchers and the management are of high caliber and deserve kudos. Researchers appeared ebullient and passionate about their work. ARL’s work in preparing for the review was superb. The ARL Director’s webinar overview and the read-ahead materials, presentation viewgraphs, poster materials, and laboratory tours greatly facilitated the review process. It was highly valuable to have an interactive session with the ARL Director to become acquainted firsthand with the ARL’s mission and goals. This is also the first review involving the new director of ARL’s Sensors and Electron Devices Directorate, whose vision and plans were presented in an energizing fashion. As for institutional aspiration, there is no shortage of challenges and opportunities. One of the larger questions being asked is, How can ARL be meritoriously unique as a research laboratory that is nourished by an innovative culture? In expanding innovation, discovering new science and new technology is as rewarding as crafting new uses of existing technologies to develop advanced Army products. Striking a balance between the projects that tackle known unknowns, driven by application and innovation on demand, and the projects that explore unknown unknowns to achieve high-risk and high-reward outcomes needs to be an ongoing effort. Another opportunity is presented by asking how the distance between the science and the Army end- use applications can be narrowed. The ARL scientists maintain knowledge of Army applications through direct exposure with the end-users in the field. To enhance human capital, nurturing a working environment that offers positive energy, organizational stability, and high retention rate is essential. Establishing a 58

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comprehensive reward system is another challenge and opportunity. Several ideas that may be considered are awards (monetary and nonmonetary), internal recognition, external peer review, research freedom, laboratory- wide recognition of stature (e.g., fellowships), and dual-track career advancement. All the projects reviewed are engaged in collaborative efforts to various degrees; this is commendable. The next level of excellence can be achieved by improving efficiency of collaboration to deliver better focus, quality, and selection of projects. Internal collaboration across the divisions and directorates is as beneficial as extramural collaboration. In today’s fast-moving technological landscape, additional opportunity is presented by the challenge of effectively utilizing commercial technologies, particularly as they pertain to wearability, mobility, and connectivity, which are critical to the well-being of the soldier. A systematic, structured effort to scout technologies from the private sector to complement in-house projects will be highly rewarding. As technology marches on at an unprecedented pace, it is important that new approaches to shorten the research cycle from science to useful product always be on ARL’s radar. A concerted effort to understand future needs and craft projects relevant to them is the ultimate challenge and opportunity. To this end, the materials genome initiative is one frontier that could usefully be paid attention to. Branding and marketing represent an additional opportunity worthy of consideration so that good work is not kept secret. Measures of success for the research work—for example, invention disclosures and patents, citations, and publications—need to be tracked. When it comes to publications, value versus volume is another judgment to be made. Most of projects presented are excellent and exerting pervasive impact. The scientific soundness and the use of the fundamental sciences are outstanding. It is commendable that the ARL materials science talent pool has a good mix, ranging from experienced, savvy scientists and engineers to the bright, early-career professionals. There appears to be good diversity with respect to gender and ethnic groups. The project portfolio fits well with both global thrusts and the national agenda, with research projects falling at the intersection of the pillar technologies of biotechnology, nanotechnology, advanced materials, energy, and the environment. To further enhance the research output, project feasibility and milestones call for critical periodic evaluation. Some of the projects in the portfolio are particularly impressive. The biomaterials group is making particularly noteworthy progress, following the ARLTAB’s previous suggestions to recruit a new branch chief and to begin to establish a long-term program in biotechnology. The project on synthetic biomolecular materials is especially significant for the Army. It addresses needs in situation awareness and force protection in such areas as on-demand production of biomolecular sensing materials in response to new and emerging hazardous threat materials; functional biomolecular materials that are stable in austere environments; persistent surveillance; and ubiquitous sensing. The project has already shown success by developing iterative and integrated multiscale computational biology capabilities—this is topnotch research. The project has also demonstrated, for the first time, rapid development of peptides as synthetic alternatives to antibody-based bioreceptors, which are difficult to produce and maintain in the field. The use of biogenerated fuel to drive a fuel cell and generate a periodic power boost is another research project important to the Army. ARL is a technology-driven and warfighter-focused institution; developing technologies to deliver ubiquitous power and energy for warfighters is a compelling mission. The project on hydrogen production from water by photosystem for use as fuel in energy conversion devices offers promise. The project on nonnoble metal catalysts for alkaline fuel cells studies the catalysts supported on graphene. Impressive power density (300 mWcm−2 at 60°C) was demonstrated using a Pt-free cathode with an anode of standard carbon- supported Pt. If the performance can be improved further and stability demonstrated, this could represent a significant breakthrough. For lightweight, quiet, efficient, and reliable power sources that would enhance soldier combat capability, the project on fuel cells for military applications tests and evaluates commercial technologies—namely, direct methanol fuel cells and solid oxide fuel cell (SOFC) systems. Fuel cells reduce weight and decrease the logistic burden associated with batteries. The 300 W SOFC systems, operated on propane, can be thermally cycled more than 40 times between room temperature and 800°C without significant degradation and can be heated to 800°C in less than 10 minutes. The system was successfully 59

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tested in an unmanned aerial vehicle. This represents an upward potential for Army applications. In the area of photonic materials and devices, the accomplishments of the project on electromagnetic modeling of quantum well infrared photodetectors (QWIPs) are laudable. The model described explains the quantum efficiency (QE) of all existing detector structures, including the most advanced optical effects, and expresses the detector QE in terms of the material’s absorption coefficient and the volumetric integral of vertical electric field. Because affordable, high-speed, high-resolution, long-wavelength infrared (IR) cameras are critically important to the Army’s night vision, large-area surveillance, and navigation in degraded vision environments, the success of this project is of enormous value. As a leader in QWIP technology, ARL can leverage this achievement to develop advanced technology and to strategically brand ARL’s leadership. Another high-impact project is developing a low-cost, III-V, direct-bandgap long-wavelength infrared (LWIR) detector for night vision technology. LWIR detection is a niche Army technology requiring dedicated equipment and highly specialized skills and tool sets. The research involves the growth of defect-free unstrained and unrelaxed InAsSb material on binary substrates such as GaSb, InSb, or InA. This detector is expected to be a disruptive technology for the LWIR field and to potentially replace the costly II-VI based technologies. As for laboratory physical facilities, state-of-the-art equipment and instruments are available to perform quality research work, and there are many material characterization capabilities—for example, ultrafast THz and nano-nuclear magnetic resonance (nano-NMR), time-resolved ultraviolet (UV) materials growth and characterization, and a clean room fuel-cell laboratory, which are all supported by trained and knowledgeable personnel. However, synergistic capabilities can be further harvested through the tie-in of facilities across division branches, as well as through collaborations with the targeted external facilities. The sections below summarize the comments and suggestions for three groups of projects: biomaterials, photonic materials and devices, and energy materials and devices. ACCOMPLISHMENTS AND ADVANCEMENTS Biomaterials Synthetic Biomolecular Materials The investigators have successfully demonstrated, for the first time, rapid development (in less than a week) of peptides as synthetic alternatives to antibody-based bioreceptors, which are difficult to produce and maintain in the field. This project, which used combinatorial chemistry and genetically modified E. coli to find peptides capable of strong binding to inorganic materials, is excellent research. For example, finding 15- unit amino acid sequences that bind strongly to metals and other targets compounds will facilitate development of biosensor technology. This research addresses an important need in situation awareness and force protection such as on-demand production of biomolecular sensing materials in response to new and emerging hazardous threat materials. The collaboration with the Institute for Collaborative Biotechnologies at the University of California at Santa Barbara seems to be productive; extended visits by staff members leading to joint publications are encouraged. Tissue Scaffolding This is an excellent effort and certainly is of benefit to the Army because it is aimed at designing nanofibrous scaffolds that enable wound healing and regeneration of damaged nerve tissues. These scaffolds will be used for three-dimensional (3D) cell culture platforms for the study of blast-induced traumatic brain injury (TBI) at the cellular level. The tissue engineering research involves a scaffold for neural cells, and the work shows the cells aligned if the fibers are oriented, a feature well known in the field. The work is being carried out expertly; the gradient approach to place adhesive protein in the scaffold deserves further study. 60

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By far the most interesting application was that the scaffolds were used to culture neural cells, which could be exposed to a simulated blast or shock wave followed by analysis of changes in the metabolic characteristics of the cells. This research could lead to unique treatments to heal nerve injuries as well as to get a better fundamental understanding of neural damage from ballistic blast in order to design better protective equipment. Synthetic Biology Using Quorum Sensing Work on harnessing the natural bacterial communication system involving the communication molecules AI-2 is relevant to the Army needs in areas such as bioterrorism, warfare, food and water safety, medical applications, and fuel integrity. The investigators use synthetic biology to engineer a bacterial sensor and to rewire bacterial communication machinery in order to develop a robust and sensitive biosensing system. Additional theoretical studies in gene circuits would help to further develop advanced and/or improved systems for a wider range of applications. Nanocellulose Nanofibrils of cellulose (NC) is an area of research relevant to Army needs because it has the potential for developing materials that exhibit high strength and modulus (comparable to Kevlar), controllable adhesion and dispersion properties, and nanoscale-enabled transparency. This NC product could be an excellent logistical material that is sustainable, inexpensive, bioderived, biocompatible, and eco-friendly. Theoretical investigations (at ARL or in conjunction with outside academic collaborators) into the structure and physicochemical processes involved in the development of NC are encouraged. This work is interesting but does not seem to have breakthrough potential. The work on improving impact strength, the ability to reinforce polymer matrices to yield transparent composites, and the synthesis of bio-based polymers is of high quality, but it is not many steps beyond the current state of the art in composite technology. Also, the type of research involved in this project involves mainly chemistry and is only marginally biological (and thus not directly related to biotechnology). Effects of Atmospheric Environmental Conditions on Bio-aerosol Properties This project develops a system built and tested to measure the effects of atmospheric environmental conditions, including gases, sunlight, and humidity on the UV-laser-induced fluorescence spectra and viability of bio-aerosols. This project is clearly critical to developing useful tools for soldiers on the ground, who face serious environmental threats. However, it appears to be focused on large-scale measurement capabilities that were not distinguished from other efforts in environmental monitoring and to follow rather than lead efforts that address the need for monitoring highly mobile individual soldiers. The results to date are interesting and informative, and the fluorescence data are useful. It would be useful to consider adding other spectroscopic techniques, such as Raman scattering, to obtain complementary spectral information. Theoretical studies on the photochemical pathways and fate of the photoproducts would also help to provide a fundamental basis for the project. Atmospheric photochemistry is well known, and the study of simple bio- aerosols needs to be expanded to include the study of more complex systems. The project on bio-aerosol chemistry involved interesting, high-quality research, and it is important to tie it in with other work on modeling toxic plume development due to industrial accidents or to a bioterror event. Collaboration with scientists at other laboratories and universities is strongly encouraged. 61

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Energy Materials and Devices Li 7 La 3 Zr 2 O 12 Electrolytes Samples of high quality were fabricated by hot-pressing. They were evaluated in electrochemical cells by cycling. While the current passed at room temperature was modest (0.01 mAcm−2), the cycling performance was stable. Some electrochemical impedance spectroscopy (EIS) was also presented. Future work is planned in evaluating electrode impedance. The refractory nature of the electrolyte is likely to facilitate the design and construction of Li batteries with a broad range of operating temperatures, similar to NaS batteries but with higher voltage and specific energy. Options also exist for further improving Li ion conductivity by doping with other ions. Materials for Advanced Battery Chemistry In collaboration with the University of Maryland at College Park, electrolytes were formulated for Li/S, Na, Mg, and conversion reaction materials. In Situ Investigation into High-Capacity Alloy-Type Li-Ion Battery Anodes The objective of this work was to systematically investigate by means of atomic force microscopy (AFM) the volume changes occurring during charge and discharge using samples of controlled geometry (size- controlled Si islands formed by microfabrication). The AFM was able to capture the morphological evolution of the islands during electrochemical cycling and showed that islands as small as 100 nm in height and diameter readily suffer irreversible mechanical degradation. It is not clear, however, how to avoid this type of degradation. Nevertheless, the approach used to systematically investigate degradation is good. Well-defined geometry is likely to allow the development of theoretical models describing state of charge and the associated volume changes. Atomic Force Microscopy for In Situ Analysis of Li-Ion Battery Materials The formation of a thin, contiguous solid electrolyte interface (SEI) layer is important for the satisfactory operation of Li-ion batteries. It is important that it be thin and be a reasonable Li-ion conductor but a poor electronic conductor. The layer is typically a few nanometers to ~100 nm thick. This layer is difficult to characterize. In the present work AFM was used to image the SEI layer. This appears to be a very fruitful way of investigating the dynamics of SEI layer formation. It would add considerable value if these studies are supplemented with electrochemical tests, such as EIS. Oxidation Stability of Electrolytes from Density Function Theory Calculations Stability is given in terms of voltage. Work shows that density function theory (DFT) can be used to identify the correct reaction mechanisms (and products). Calculations are in good agreement with the experimental results. This appears to be a very in-depth study. Two or three papers have been published to date. 62

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Prototyping of 5 V Li-ion batteries This work concerns the possible development of a high-voltage cathode for Li-ion batteries. LiFePO 4 , LiCoPO 4 , LiMnPO 4 , and LiNiPO 4 were investigated using first-principles calculations. Through these calculations, LiFePO 4 and LiCoPO 4 were identified as good candidates. From the standpoint of thermodynamics, LiCoPO 4 is determined to be the best (highest voltage), while LiFePO 4 is shown to be more stable. The present work showed that a cathode containing both Co and Fe exhibited high voltage and was also stable. It appears that by suitably tailoring composition, it may be possible to achieve both good performance and good stability. Liquid Electrolyte Li-S Battery This is a good problem-solving project based on classic electrochemical methods to reveal qualitative mechanisms during discharge. Awareness of the effect of these materials on the entire system is an important strength of this work. There are several recent publications in high-quality technical journals. Developing Next-Generation Thermal Batteries The approach of this project consists of forming multilayer laminates of two metals, Ni and Al. The individual layers are a few tens to a few hundreds of nanometers thick. The relative thicknesses are selected based on the final intermetallic composition desired, which in turn is based on the heat released during the thermite reaction. Very thin layers allow attainment of very high velocities of reaction front travel (10 ms-1). Ignition is achieved at one end. This is a very specialized topic but of direct relevance to Army operations. Nonnoble Metal Catalysts for Alkaline Fuel Cells These catalysts are supported on graphene. By suitable thermal treatment, the catalyst is pyrolyzed to change its chemistry. Impressive power density (300 mWcm−2 at 60°C) was demonstrated using a Pt-free cathode. The anode was standard carbon-supported Pt. The work is very impressive. If the performance can be improved further and stability can be demonstrated, this could represent a significant breakthrough. Fuel Cells for Military Applications The 300 W SOFC system operated on propane and could be thermally cycled more than 40 times between room temperature and 800°C without significant degradation. The system could be heated to 800°C in less than 10 minutes. Also, the system was successfully tested in an unmanned aerial vehicle. This represents a significant achievement for the SOFC systems and utility for military applications. Palladium Membranes for Purification of Reformer Gases Thin (500 nm) Pd membranes were fabricated by supporting them on lithographically patterned 15 µ thick Ni substrate having 15 µ hexagonal holes. A pressure differential of 20 psi could be maintained without rupturing the membrane. The permeation rates were comparable to or better than rates for other Pd membranes supported on other substrates (alumina, stainless steel). The reformate contained CO and H 2 S as 63

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contaminants. The results are impressive. It is probably possible to use porous Ni foils (made by consolidating Ni powder). This may decrease the cost and may also improve strength, allowing for larger pressure differentials. The use of Pd membranes for extracting hydrogen from reformer gas is investigated with the goal of reducing cost. The quality of the experimental approach is outstanding and provides a well-characterized system that can be used for fundamental studies as well as sustained engineering development. The researchers’ understanding of the underlying issues is excellent, as are the qualifications of the personnel for making advances in both science and engineering. The experimental approach leads the project at present, although modeling aspects could be brought to bear for predicting hydrogen transfer rates and strain based on pressure differences, among other design issues. It may be important to extend knowledge of mechanisms associated with Pd crystal size, diffusion path through crystals versus grain boundaries, lifetime issues that might be associated with crystal ripening, and alloying effects with the support structure that may lead to degradation on the reformer gas side. There is an extensive literature on Pd membranes and their use in exploring reaction mechanisms associated with surface hydrogen. The present project could be expanded significantly to applications well beyond the one used here for purification. Thermophotovoltaic Energy Conversion Director’s Strategic Initiative (DSI) This is an ambitious project that integrates multiple components, each of which is well-studied in the classic literature and each of which is capable of improvement. The effort benefits from vigorous pursuit with excellent insights based on knowledge of the basic phenomena that are involved. The experimental design and fabrication is excellent and makes possible, in principle, a wide variety of system parameters (for example, geometry, flow rates, emitting surfaces, and spectra filters). The reaction engineering of the combustion is treated quantitatively, while the remaining components are at present characterized as qualitative. There is a good mix of experiment and early-stage modeling, and there is significant potential for improved quantitative modeling and optimization. Exploiting Drop Resonance for Improved Condensation In this project, experiments are carried out on vertical copper surfaces. The quality of the experimental arrangement is good, and the data measurement methods are reliable. There is an extensive literature on heat transfer in dropwise condensation, as well as on the effect of surface movement on droplets, that has yet to be brought into the project. The effect of vertical height is also important, because droplets that detach from the upper region will clear a path as they detach and sweep up additional droplets on their way down the surface. Copper forms an oxide surface, which could represent a significant variable that needs to be characterized. Similarly, the use of surfactants is critical, because they influence contact angle and propensity of the droplets to shear. While the project is still in the early stages of development, it would benefit from more detailed consideration of the underlying physical phenomena and selection of well-characterized materials for assembling the experiment. There is great potential for significant improvements in modeling and engineering correlations for predicting heat transfer rates. Phase Change Thermal Buffering for Army Systems The use of latent heat from phase changes to reduce cooling demand during transient heat loads is investigated in this project. This is an excellent project with an experienced investigator. It benefits from deep understanding of fundamentals across a very wide range of materials and phase-change applications, many of which are described in the literature. Numerical simulations of transient effects play a useful role in modeling transient melting fronts. The project provides a path forward for identifying improved materials with 64

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characteristics suitable for Army applications. Integrated Thermal Solutions for Electronics Systems In this project configurations for improved heat transfer for various types of electronic packages are investigated by means of simulation models. Chip configurations include 3D chip stacks and power chip stacking. Various levels of sophistication of modeling methods using commercial packages were developed in order to identify the most numerically efficient methods. The quality of the work and the understanding of the underlying physical and numerical methods are excellent. There is an excellent match with practical aspects of reducing the work to practice that involves knowledge of the application as well as the heat transfer fluids and materials. While the Army may have unique applications, it is likely that there are approaches reported in the literature that can be brought to bear, provided that there is in-house expertise of the quality represented in this project. Advanced Thermal Interphase Testing and Development This laboratory tour demonstrated that experimental methods are being designed to accurately measure temperature drops (ΔTs) that occur across interfaces. In many energy systems when multiple interfaces exist, it is important to take into account such ΔTs, because they dictate the overall heat rejection rates. Modified Model for Improved Flow Regime Prediction in Internally Grooved Tubes Heat transfer during two-phase flow in small-diameter grooved tubes is investigated in this project. This is an important project that has a wide range of specific applications, requiring that the development of general knowledge of flow regimes and associated heat transfer correlations be obtained. The experimental arrangement is excellent, as are the qualifications of the personnel for experimental work. There is an extensive literature on two-phase flow in large pipeline systems as well as small-scale refrigeration systems, from which additional ideas for experimental design and analysis could be extracted. In some flow regimes associated with two-phase flow in large pipelines, the sophistication of the computational modeling has improved to the extent that it has replaced experimental measurements. While the knowledge of underlying behavior in small-scale systems is presently at an early stage of development, the path forward could include a quantitative modeling component that might grow in time. Photonic Materials and Devices Electromagnetic Modeling of Resonant Quantum-Well Infrared Photodetector Structures The achievements of this project are remarkable. Twenty-five years after the invention of quantum- well infrared photodetectors (QWIPs), the work described amounts to a rebirth of the field. This work also comprises one of the most convincing applications of plasmonic enhancement for real, deployable technologies that have been demonstrated. QWIPs clearly have an exciting potential to impact the Army mission, with competitive sensitivity but based on mature materials technology that can provide high uniformity and high pixel counts; they also offer the potential for low-cost foundry manufacturing. 65

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III-V Materials for Infrared The investigations into narrow direct bandgap III-V materials constitute one of the best projects in the Electro-Optics and Photonics Division. As a whole, this work could radically alter the applications and deployment of mid-IR detectors and focal plane arrays, with performance equal to or better than HgCdTe, because the energy band properties are likely to be more favorable to reduced dark current. Surface recombination and passivation are easier for InAs than for GaAs or InP, so this might provide additional performance advantages. The presentation of this project was particularly outstanding, with the research very clearly put in context and the ARL contribution compellingly told. This included the history, the probable source of historical scientific confusion in the literature, and the role of compositional ordering. This team deserves commendation for leveraging the Small Business Technology Transfer (STTR) resources to further their Auger characterization capabilities, and the team is looking forward to carrier lifetime and preliminary device results in the coming year. This work is an outstanding example of the ways in which ARL makes unique contributions. The project’s success reflects a combination of insight, effective modeling, motivation, and carefully selecting the right epitaxy technology to realize a real breakthrough in technology for Army applications. Metamaterials and Metastructures Director’s Strategic Initiative This research activity illustrated two applications of novel integrated photonics. In the work on slow- light waveguides, the innovative designs required very challenging fabrication techniques, and the results were quite impressive. Nonetheless, performance targets were not articulated clearly enough to assess the promise or efficacy of chip-scale slow-light devices. Additionally, the fact that the results differed by a factor of 10 from the model suggests an opportunity for discovery, or that discrepancies need to be more seriously addressed. In the work on tunable metamaterials with active adjustable bias structure, insufficient information on performance targets and results was made available to permit adequate assessment, and it is not clear if there is going to be a serious effort in terahertz (THz) applications at ARL or if this work is considered to be a basic science investigation into possible new metamaterials. Terahertz Probe of Nitride Semiconductor Optoelectronic Materials and Devices This project described a remarkable collection of scientific results on nitride device and materials characterization, and overall it comprised a very promising exploration of new capabilities. The presentation on the project provided a catalog of excellent achievements, but the presentation would have been more informative if it had provided more specific discussion of future directions and the most promising potential for impact. This was a good example of high-risk work, and while it may not become a mainstream characterization tool, it could provide unique capability that is not possible with other techniques. Mid-Infrared Solid-State Laser Materials This different method for high-power mid-IR lasers is intriguing. In particular, the elimination of thermal problems through 1.6 µ lasing is an interesting possibility. However, net quantum gain appears to be dependent on an efficient and complex combination of lasers and an optical parametric oscillator that has not yet been demonstrated. This is high-risk work from an applications perspective. 66

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Research Presented in Posters The work on HgCdSe materials development for GaSb substrates appears to still be limited by Se purity. The atom optics is an outstanding example of interesting but high-risk and potentially high-impact work by a strong team making steady progress in technology platform development. The heteroepitaxy of GaN on SiC appears to have demonstrated good absorption with good low-noise avalanche characteristics, claiming 75 percent QE at 240 nm; a U.S. patent has been issued. The swept frequency and other beam- combining achievements are very promising and appear to be unique and important contributions to the field. The opto-electronic oscillator (OEO) work is very strong, and the team has unraveled some interesting new acoustic effects not typically apparent in telecom applications owing to the very low frequency offsets. OPPORTUNITIES AND CHALLENGES Biomaterials In previous years there was a clear lack of focus and of critical mass in the Biotechnology branch. This might be addressed with the hiring of a new branch chief. One of the important elements of developing collaborations is a leader who is well recognized in the scientific community. Therefore, in addition to the new branch chief, ARL will also need a scientifically recognized leader who can provide the scientific vision, direction, and connections to lead this program in the right direction for ARL. The Biotechnology branch will need more resources and personnel to reach critical mass. It is still a small group, which will need some time to grow and find its way. Internally, the branch could reach out to personnel inside ARL and develop joint biorelated projects, grants, and other activities. Also, it would be helpful to hire additional mid-career scientists if funding is available. In the meantime, the branch can take advantage of other Department of Defense (DoD ) programs, such as the Institute for Collaborative Biotechnologies and the soldier nanotechnology programs. New interactions with other Army-supported institutes, such as the Institute for Soldier Nanotechnology at MIT, are also encouraged, and the new cooperative research and development agreement involving Johns Hopkins University, the Center for Innovative Technology, the University of Delaware, and Penn State University could be initiated. Since the Biotechnology program is relatively new, it is premature to evaluate it against the other more mature programs (e.g., the Photonics program). It appears that the Biotechnology branch still needs a defined vision and differentiation from other laboratories and universities. Then, over time, it can become like the Photonics programs and department. The three focus areas discussed in the Biotechnology branch’s presentation (biotools, biotargets, and human factors) are excellent topics that reflect a long-term, wide- ranging vision; however, these topics are still too broadly defined and will need to be further narrowed in order to benefit from current expertise at ARL. The main focus seems to be on biotools, but this was not spelled out in the general presentation. The four thrust areas mentioned in the presentations—bioinspired materials for energy, detection, and force protection; bioinspired materials for robust networks; bioinspired materials for structural awareness and evolving threats; and bioinspired materials for cognitive nanoscience and transformational medicine—are quite ambitious for a relatively small biotechnology program. Since these thrusts require a much stronger level of research commitment and resources, a further downselection and narrowing of programs that better suit the resources and capabilities of ARL is required. The Biotechnology program is not only newer but also much smaller and has fewer financial resources to build upon than its counterpart branches at ARL; however, it could make much better effort to connect to and leverage some of the strongest programs in ARL, in particular the Photonics program. The new Biotechnology branch has now a unique opportunity to refine its vision and focus it on collaboration. The Biotechnology branch could beneficially also seek more collaboration with outside groups to define a unique place for itself with relevance to Army needs and where its researchers become leaders, not just followers. The group needs to seek new exciting areas that have direct relevance to the mission of the Army. This could be stimulated by initiating and hosting a series of quarterly seminars with the participation 67

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of leaders in the field. Of particular interest is the work being conducted at MIT on bioinspired batteries using phages and carbon nanotubes and work at Cambridge University and Imperial College London that is subjecting pig tissues and cells to impulse loading to determine the effect of shock waves. Such work could provide insight that has relevance to the mission of ARL. Energy Materials and Devices Increasing the use of atomistic modeling and mesoscopic (continuum) modeling represents a great opportunity to complement many of the outstanding experiments that are being performed. The use of DFT calculations has already been useful in understanding the stability of the electrolytes, the SEI layer formation, and the development of high-voltage cathodes. In addition to DFT calculations, there is also an opportunity to conduct mesoscopic modeling of entire electrochemical systems. For instance, charge-discharge cycles of a Li-ion battery or analysis of performance characteristics of a fuel cell require solving a number of coupled equations. A comprehensive approach to system-level modeling will likely lead to new insights, which will help identify fundamental materials-related issues that in turn require further investigations. These areas offer excellent high-risk, high-reward options. In the DFT area, considerable work is under way in other areas of ARL. It would be of value if the management of ARL could facilitate integration of these efforts. If this is not possible, perhaps ARL needs to consider hiring researchers with expertise in the computational area. Pd membrane is another example of an opportunity. The use of Pd membranes for extracting hydrogen from reformer gas is being investigated with the goal of reducing cost. The experimental approach leads the project at present. Modeling aspects could be brought to bear for predicting hydrogen transfer rates and strain based on pressure differences, among other design issues. It may be important to extend knowledge of mechanisms associated with Pd crystal size; diffusion path through crystals versus grain boundaries; lifetime issues such as might be associated with crystal ripening; and alloying effects with the support structure that may lead to degradation on the reformer gas side. The present project could be expanded significantly to other applications well beyond the one used here for purification. Multiple functionality of a structure appears to be another exciting area of research. The concept is to design a structural component (e.g., the wing of an aircraft) to do multiple functions. The main function is mechanical (support structure), meaning that strength, toughness, and modulus are the important properties. By designing composites with interspersed electrodes, one can also store energy as capacitors or supercapacitors. Nonnoble metal catalysts for alkaline fuel cells also present opportunities and challenges. The objective is to synthesize organic-based catalysts containing Fe for oxygen reduction reactions. These catalysts are supported on graphene. By suitable thermal treatment, the catalyst is pyrolyzed to change its chemistry. The anode was standard carbon-supported Pt. If the performance can be improved further and stability can be demonstrated, this could represent a significant breakthrough. A computational component can further strengthen this area, specifically in catalyst design. Long-term stability of these catalysts is a challenge. Photonic Materials and Devices The mapping onto Army needs of some elements of the photonics portfolio appears to be quite speculative or to have a long-term horizon, and in this sense a substantial fraction of the work can be characterized as high risk. A good example of strong work in this category is the work on cold-atom optics, but there are many other examples as well. The number of projects that are high risk in a broader context is smaller. Projects in this category would address the following questions: Will a broad investigation into a new area of study yield anything interesting? Will an unproven concept demonstrate any useful level of efficacy, independent of specific Army 68

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performance goals? The payoff for such basic work would be in finding applications for the Army at some point in the future that cannot be well-predicted today. In this context, ARL seems to have adopted a “fast follower” approach, whereby most of the work comprises investigations into whether external scientific advances can be successfully mapped onto the Army mission. This is entirely consistent with elements of ARL’s definition of innovation, but this may be more symptomatic of a predominantly top-down project selection. Other peer laboratories may have a more bottom-up approach with investigator-level risk decisions regarding entirely new approaches. The photonics work at ARL seemed to evince an aversion to risk and a low tolerance for failure. ARL could improve the marketing of its work in the photonics area. While the story of ARL photonics can be compelling, the overviews and core competencies lists appeared to catalog activities rather than articulate capabilities, accomplishments, and impact. While the breadth of work is commendable, ARL needs to find ways to tell the world where it is truly outstanding, how it wants to be measured, and the context of its work. In many presentations of the photonics work, the connectivity to the Army-fielded technology was not strongly articulated; it would help to highlight the success stories. There seemed to be a culture of soft-selling evident in the photonics work. For example, the recent game-changing advances in QWIP technology have catapulted ARL to a position of unambiguously being the best in the world for this technology. Presentations could include charts to show the competition, and this work could be very successfully highlighted in the international scientific community. The recent advances in narrow bandgap III-V work are also highly provocative and could be very successfully promoted in the outside technical community to raise the prestige of ARL. Businesses understand this kind of promotional activity, and ARL could emulate their approach. OVERALL TECHNICAL QUALITY OF THE WORK Biomaterials The technical quality of the research and personnel in the biotechnology branch seems highly variable and ranges from good to excellent. The researchers are very knowledgeable and capable, and they have a very good understanding of the relevance of their research to Army missions. The scientists and engineers in the biomaterials area are remarkably enthusiastic and effective. Their research productivity is more than adequate in general and excellent in several areas such as applied molecular biology and biosensor development. The photonics programs are more mature and have good projects and personnel, with a couple of projects being world-leading efforts; the biotechnology effort is newer and as such needs careful nurturing. The overall quality of the research in the biotechnology program has greatly improved over the past year and is excellent in several areas. These include the excellent research on synthetic biomolecular materials; the use of modified bacteria to study the feasibility of converting food waste to butanol; and the search for extremophile dry-tolerant, thermostable proteins for sensing applications. Several studies involving applied molecular biology are well chosen and have great potential. Some projects reflect good underlying fundamental science; others do not involve basic understanding. If the research is conducted at ARL with outside help, systems biology could be an exciting area if it can include protein engineering, synthetic biomaterials, and extremophiles. The Biotechnology branch could closely couple with the strongly established photonics groups, which have outstanding laboratory resources and equipment. Further collaboration with outside researchers is also encouraged. For some projects, the addition of theoretical studies to accompany the experimental work will be very useful. The ARL has outstanding laboratory facilities and equipment, and the experimental research at ARL reflects the excellent work of ARL staff and efficient application of these outstanding resources. The Biotechnology branch is also well equipped to do classical molecular biology and biochemical research. The nascent systems biology and some of the computational modeling of protein structures represent some basic computational 69

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and theoretical elements of the program. However, if this branch begins to conduct research at the interface of biological and physical worlds, it would discover many opportunities for the synthesis of theoretical, computational, and experimental observations and results. Further collaboration needs to be pursued with other research institutions, including universities, to add some theoretical studies and obtain improved fundamental understanding of the processes being investigated. Collaboration with organizations where Army-sponsored research is conducted (for example, MIT, University of California at Santa Barbara, Johns Hopkins University, and the Center for Innovative Technologies) is encouraged. These collaborations would energize the talented researchers and provide a good opportunity to conduct collaborative research and write joint papers, which would enhance the reputation of ARL in the global scientific community. Energy Materials and Devices The scope of the research was impressive, covering a broad spectrum of materials physics relevant to Army technologies. In general, the quality of the research reviewed was high. The quality of the scientific content of the presentations and the overall selection of topics were impressive. However, some of the specific projects presented in previous years were not presented in sufficient depth during the current review. An example is the power platform for flapping wings (microautonomous systems). Some presentation on this topic detailing its status would have been useful, especially given the significant focus on it the last couple of years. Overall, in the materials sciences discipline, the quality of the scientific research is excellent, reflecting a broad understanding of the underlying science. There was more effort devoted to understanding and monitoring global research activities than in previous years. Increased collaboration with external organizations within the DoD community, in the industry, and in the academic community is also notable. The question on the mix of low-risk and high-risk research to reach an optimal balance continues to be a discussion item. Nonetheless, the project portfolio does indicate a wide array of work embracing high- risk and lesser-risk research. Photonic Materials and Devices The science and technology in this area are strong, with some examples of world-class, breakthrough work. As exhibited in the posters and presentations, technical work is outstanding, demonstrating both the breadth and depth of work at ARL. The researchers showed clear and deep understanding of their fields, and they evinced understanding of their work both in the context of its applications potential as well as on the world research stage. 70