The Panel on Materials Science and Engineering at the Army Research Laboratory (ARL) conducted its 2013 review at Adelphi, Maryland, on June 11-13, 2013. The 2013 review addressed the areas of biomaterials, energy materials and devices, and photonic materials and devices. The same panel conducted its 2014 review of the ARL facility in Aberdeen, Maryland, on June 3-5, 2014. The 2014 review addressed the areas of electronic materials and devices and structural materials, including materials in extreme environments and multiscale modeling.
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.
Overall, the researchers and the management are of high caliber and deserve kudos. Researchers appeared passionate about their work. ARL’s work in preparing for the review was superb. The ARL director’s webinar overview and 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 (SEDD), whose vision and plans were presented in an energizing fashion.
The criteria for ARLTAB’s assessment focus on 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 achieve a project’s promise.
In this chapter, ARLTAB’s findings are set forth in several sections that provide evaluations of groups, projects, and future thrusts and offer general observations and suggestions.
It is gratifying that some postdoctoral researchers have joined ARL as full-time employees after completing their fellowships, an indication that laboratory management is providing an attractive environment for early-career researchers. The ARL director conveyed openness to suggestions, and the SEDD director is well versed in the research at SEDD and in emerging technologies for materials.
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 would 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 deserves attention.
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. The Research@ARL monograph series on energy and energetics1 and the materials modeling at multiple scales 2 are commendable. When it comes to publications, “merit” vs. “quantity” is another judgment to be made.
Most of the projects presented are excellent and have a pervasive impact. The scientific soundness and the use of 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 bright, early-career professionals. There appears to be good diversity with respect to gender and ethnicity. The project portfolio fits well with both global thrusts and the national agenda, with research projects falling at the intersections of biotechnology, nanotechnology, advanced materials, energy, and the environment.
As for institutional aspirations, there is no shortage of challenges and opportunities. One of the larger questions being asked is: How can ARL be a unique 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 their knowledge of Army applications through direct exposure with the end-users in the field. To enhance human capital, nurturing a work environment that offers positive energy, organizational stability, and a high retention rate is essential. Establishing a 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.
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1 Army Research Laboratory, 2012, Research@ARL: Energy & Energetics, June, http://www.arl.army.mil/www/pages/172/docs/Research_at_ARL_2012_s.pdf.
2 Army Research Laboratory, 2014, Research@ARL: Materials Modeling at Multiple Scales, July, http://www.arl.army.mil/www/pages/172/docs/research-at-arl.vol3-issue2.pdf.
Researchers need to consistently analyze data and contemplate theories that are behind the observed physical phenomena, test data, and modeling systems to effectively design the path forward for each project. Given the many exciting experimental and computationally derived results that were reported during this review, such efforts will further optimize the progress of research. As a first step toward this goal, data analysis needs to be highlighted in the research efforts. This does not necessarily mean the use of advanced computation tools, but rather the incorporation of even simple mathematical analysis to further uncover trends and correlations in data and deep diving into plausible fundamental theories. This will help to advance the materials by design and by demand paradigms.
To further document the competitive posture of the ARL research programs vis-à-vis those of comparable organizations, formal metrics are needed to enable comparison of ARL research activities with those of other government-owned research laboratories in the United States and overseas.
Working toward making the Army the best Army for 2035 and the ARL toward becoming a top choice for researchers to pursue careers, one priority will be further elevating the ARL’s national and international stature. This stature will rely on performing first-rate research, solid productivity, ability to attract and retain the best and brightest talent, effective communication with the scientific community and dissemination of ARL research findings to that community, and, eventually, the transfer of knowledge to Army applications. The review materials and presentations did not clearly explain what ARL has accomplished in the way of providing better armaments.
Collaborative efforts have been demonstrated both across ARL and with external entities. 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 the efficiency of this collaboration to deliver better focus, quality, and selection of projects. Internal collaboration across the divisions and directorates is as beneficial as extramural collaboration. ARL’s move toward the use of science and technology (S&T) campaign plans can enhance such collaborations.3 Also, research output could be enhanced by the periodic evaluation of project feasibility and milestones.
Biomaterials, Energy Materials and Devices, and Photonic Materials and Devices
Overall, the researchers and the management are of high caliber and deserve kudos. Researchers appeared passionate about their work. Most of the projects presented are excellent and are having a pervasive impact. Scientific soundness and the use of the fundamental sciences are outstanding. 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 done topnotch research by developing iterative and integrated multiscale computational biology capabilities. It 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 provide a periodic power boost is another research project important to the Army.
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3 Army Research Laboratory, 2014, Army Research Laboratory. S&T Campaign Plans. 2015-2035, Adelphi, Md.: Army Research Laboratory, http://www.arl.army.mil/www/default.cfm?page=2401.
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 and stability demonstrated, this could represent a significant breakthrough. For lightweight, quiet, efficient, and reliable power sources that 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 tested in an unmanned aerial vehicle, representing a welcome 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 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 the development of QWIP technology, ARL can leverage its 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 tools. The research involves the growth of defect-free unstrained and unrelaxed InAsSb material on binary substrates such as GaSb, InSb, or InAs. 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 terahertz and nanonuclear magnetic resonance (nano-NMR), time-resolved ultraviolet (UV) materials growth and characterization, and a clean room fuel-cell laboratory, all supported by trained and knowledgeable personnel. However, synergistic capabilities could be boosted through the tie-in of facilities across division branches, as well as through collaborations with the targeted external facilities.
The sections that follow summarize the comments and suggestions for three groups of projects: biomaterials, photonic materials and devices, and energy materials and devices.
Electronic Materials and Devices, and Structural Materials
Research projects under this review span three families of materials—metal, ceramics, and polymers—plus another category, composite materials. The overall portfolio comprises short-term and long-term projects. A relatively large proportion of the work reviewed consists of high-risk, high-impact projects. The research scope covers experimental, computational, and modeling projects. In the laboratory’s physical facilities, state-of-the-art equipment and instruments are available to perform quality research
work, and there are excellent material characterization capabilities and exceptional mechanical testing equipment.
In the area of electronic materials and devices, the research presented throughout the review is of high scientific and technical merit and evinces a great deal of innovation. High-profile research aligned with ARL’s mission needs is evident. Staff members are working on forefront research projects that could potentially yield breakthroughs, and the work is directed toward the emerging needs of the future (2035) Army. For instance, the low-dimensionality—two-dimensional (2D)— materials program is particularly impressive, covering fundamental aspects of synthesis, characterization, device design, and manufacturing. Tuning 2D materials at the atomic scale opens enormous opportunities to design electronic properties for innovative applications through controlling surface conditions, defects, and the interfaces with other 2D materials. This is a potentially high-impact area. The piezoelectric materials program is exceptionally mature, so it is now transitioning to the implementation of unique actuators, microelectromechanical systems (MEMS), robots, and further miniaturization of low-power relays.
Taking into account that projects in structural materials are at different stages of research and development, the research, overall, is of high quality. The maturity of the work in synthesis and processing is commendable. Modeling has been nicely integrated into many experimental studies such as Mg alloy development and field-assisted processing. The project on boron oxide ceramics stood out among many excellent presentations. The mechanical press, capable of applying up to 700,000 lb pressure in developing lightweight armor, is exceptionally enabling equipment. The experimental work conducted in cold spraying, Mg processing, and nanocrystalline metals is excellent, and the researchers have access to centralized characterization facilities. Researchers have made inroads in publishing their results (such as in nanocrystalline metals) in the archival literature. However, the utility of this technology for making scaled-up parts is still a matter of conjecture.
Another area that may need attention is powder technology, which appears to be a crosscutting technology at ARL, crucially applicable to multiple projects. Whether there is a need for in-house capability and expertise in this area calls for thorough consideration.
In the context of the focus on correct modeling at specific scales and physics, notable results include the atomistic modeling, the piezoelectric materials, and the computational fluid dynamics (CFD) modeling for cold spray deposition. The modeling is well integrated into an overall fabrication and testing program, enabling the selection of fabrication techniques (e.g., Mg alloy development) or device manufacture (e.g. piezoelectric MEMS). There is a developing effort in linking models at various scales and expressing different physical phenomena, which is producing good results, especially in the transition from smaller to larger scales (e.g., brittle material modeling or polymer coarse graining).
Many projects have an overarching theme of interfacial behavior across different classes of materials, such as the nanocrystalline alloy stability, grain boundary engineering of ceramics, and the role of adhesives. This provides a unique opportunity to explore crosscutting themes and activities of research that enhance the role of computational methods of mesoscale modeling, stochastics, optimization, and informatics (e.g., statistical learning and data mining), as well as experimental methods such as advanced microscopy and microstructural characterization studies.
ACCOMPLISHMENTS AND ADVANCEMENTS
Biomaterials
Synthetic Biomolecular Materials
ARL 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 target 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 publication 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. 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 blasts 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 develop better 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. The work is interesting but does not seem to have breakthrough potential. While 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, 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 UV-laser-induced fluorescence spectra and the viability of bio-aerosols. This project is 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.
Energy Materials and Devices
Li7La3Zr2O12 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 atomic force microscopy 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. The work shows that density functional 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. Few papers have been published to date.
Prototyping of 5 V Li-ion batteries
This work concerns the possible development of a high-voltage cathode for Li-ion batteries. LiFePO4, LiCoPO4, LiMnPO4, and LiNiPO4 were investigated using first-principles calculations. Through these calculations, LiFePO4 and LiCoPO4 were identified as good candidates. From the standpoint of thermodynamics, LiCoPO4 is determined to be the best (highest voltage), while LiFePO4 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 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. The SOFC system showed 4 times improvement in endurance over SOA battery. 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 H2S as 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 are excellent and make 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 qualitatively. 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 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; it requires a general knowledge of flow regimes and associated heat transfer correlations. 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 the modeling 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 Board noted in its 2011-2012 assessment of the ARL: “Resonator quantum-well infrared photodetectors (QWIPs) continue to be a crown jewel among the achievements of SEDD [ARL Sensors and Electron Devices Directorate], which has been able to incorporate advanced optical concepts into the detector design. SEDD has developed a better understanding of new near-field optical phenomena that has enabled the design of a QWIP structure with quantum efficiencies nearly 70 percent, doubling the previous record of 35 percent for corrugated-QWIPs. Research aimed at creating extremely low-noise.”
The achievements of this project are remarkable. Twenty-five years after the invention of 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.
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 its
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 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 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 either 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 whether there is going to be a serious effort in terahertz applications at ARL or whether this work is considered to be a basic science investigation into possible new metamaterials.
Terahertz Probe of Nitride Semiconductor Opto-electronic Materials and Devices
This project described a remarkable collection of scientific results on nitride device and materials characterization and overall was a very promising exploration of new capabilities. The presentation on the project amounted to a catalog of noteworthy achievements but would have been more informative if it had discussed future directions and identified those with the greatest potential. This was a good example of high-risk work, and while it may not become a mainstream characterization tool, it might provide a 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. From an applications perspective, this is high-risk work.
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.
Electronic Materials and Devices
Low-Dimensional 2D-Atomic Layer Materials
The projects studying 2D atomic-layer materials are on the forefront of materials research; they are paving the way for future Army electronics applications, including low-power electronics, transparent electronics, and flexible and conformal electronics. One project, initiated in fiscal year 2013 (FY13), uses advanced Raman spectroscopy and offers an alternative means of understanding material properties by mapping the Raman spectrum of molecular-level photon and electron integrations. Another project, also initiated in FY13, is studying the electrical performance of 2D atomic-layer material. These new projects complement and support the 2D field effect transistor (FET) interface study, which delivered outstanding results and produced a Best Paper award at the 2012 International Electron Devices Meeting of the Institute of Electrical and Electronics Engineers. The project on emerging radio frequency (RF) electronic devices examines 2D materials for RF applications and van der Waals solids; and a novel device is under construction after having completed material exploration.
ARL work in 2D materials research seems to be well integrated within the overall Department of Defense (DoD) effort. The fields of 2D materials research and device physics offer several opportunities for collaboration and outreach activities with academia and with the scientific community at large, and ARL has been proactively forging partnerships with leading universities such as the Massachusetts Institute of Technology, Columbia University, and the University of Texas at Austin.
The project on low-dimensionality electronic materials and devices is timely and well recognized within the scientific community. In particular, the projects involving 2D materials, such as graphene on hexagonal-boron nitride (h-BN) and transition metals dichalcogenides (TMDs), exemplify the very productive integration of compelling fundamental science, mission-driven research, and university outreach opportunities. The program elements related to materials synthesis and device characterization appear to be progressing successfully; they make use of innovative investigative capabilities, such as Raman spectroscopy, and advanced growth capabilities, such as metal organic chemical vapor deposition (MOCVD) and atomic layer deposition (ALD).
This work also makes innovative use of Raman spectroscopy as a sensitive probe of graphene-based structures. The investigators identified a new Raman peak for graphene on h-BN. The other aspect of this work was studying trilayer graphene. One stacking is insulating while the others are metallic. Interestingly, the investigators demonstrated that they could drag the boundary between different stackings using a scanning tunneling microscope (STM). This is excellent basic science that could lead to novel applications, and the effort could serve as a model for other projects. This work also demonstrated the development of an excellent Raman scattering measurement system with high spatial resolution that has enabled the excellent characterization of both graphene on h-BN and molybdenum disulfide (MoS2) on polymers. The research team also discovered that not only does the stacking order (A-B-A-B vs. A-B-C-A-B-C) in multilayer graphene determine its optical and electronic properties but also that this stacking and phase can be reversibly switched and moved by applying an external bias field. This finding has already resulted in the publication of several high-visibility articles and is well coupled to the applied project on performance of 2D electronic materials.
In the project on understanding the electrical performance of stacked 2D atomic layered materials, the ARL researchers were integrated within a larger team of university collaborators, but they have clearly carved out their own research topics. A variety of noteworthy scientific advances were made during the course of this project. The investigators on this project were able to fabricate functional logic circuits based on chemical vapor deposition (CVD)-grown MoS2. This was achieved both on silicon dioxide
(SiO2) and on a flexible substrate. Analyses were performed to understand the performance limitations due to fabrication flaws. Other outstanding basic science issues were addressed in this context. Specifically, the investigators characterized the changes in conductivity due to grain boundaries. Additionally, they discovered that MoS2 is contracted by 1.3 percent when free-standing than when deposited on a substrate.
The project team has been working its current focus for about a year, and they have demonstrated not only simple back-gated transistors but also front-gated devices that require far more sophistication to process. They have also enabled demonstration of an inverter, a ring oscillator, Negated AND or NOT AND (NAND) and Negated OR (NOR) gates, and a static random-access memory (SRAM) cell. The research team went through three iterations of MoS2 growth on different polymer substrates that are optically transparent and completely flexible and that make the MoS2 now possibly the best 2D layered material approach for low-power electronics. This project has moved from being one among many to being one of the leaders in 2D electronics, showing very impressive results in a very short time and advancing the state of the art in materials and in the application of 2D materials for electronics.
The project on analysis of 2D FET interfaces to optimize efficiency and speed led to a breakthrough in the field—namely, the discovery that by using a 2D layer a few atoms thick from source to drain (S-D), transistors can be turned off when the S-D is very small. This cannot be done if bulk material is used for the S-D, as is usually done. This approach leads to a higher density of transistors on the chip and to better efficiency and speed: As such, it represents a new type of transistor that promises future advances.
On-Chip Energetics: Porous Silicon
This is a mission- and customer-driven project, studying how porous morphology drives reaction and the effects of morphological parameters on the desired results. The project is engaged in a potentially enabling technology: the generation of thermal energy in modular initiators by using porous materials (e.g., silicon). The reaction rates can range from slow burns to the upper limit of acoustic velocity of the porous material.
Successfully delivering thermal energy on demand would be an important and very valuable achievement for the Army.
Emerging Technology for Power-Efficient Electronics
This mission-driven project represents important basic research that could enable mixed signal operation (RF and digital circuits on same chip) on gallium nitride (GaN). This approach would reduce power consumption, size, and weight, and could also enable a system on a chip (SOC). This modeling and calculation effort could be integrated with other experimental and fabrication efforts.
Piezoelectric Materials for Frequency-Agile Radio Frequency MEMS Front Ends
ARL is a world leader in the piezoMEMS field, providing state-of-the-art materials, fabrication, modeling and simulation, and device prototyping.
This mission- and customer-driven project has been ongoing for several years. It is an enabling MEMS technology applicable to switches, resonators, transformers, tunable filters, phase shifters, and tunable passives for secure communication and phased-array radar systems, as well as for improvised explosive device (IED) detection.
The project uses the state-of-the-art piezoelectric materials processing and device fabrication capabilities, in conjunction with modeling and simulation, to develop high-performance, frequency-agile RF MEMS components—common, low cost, reconfigurable hardware for tactical radios, IED detection, and missile systems.
The project investigator has forged extensive collaborative ties with other agencies and academia and has disseminated research results through technical publications and patents.
Magnetic Metamaterials: New Concept, Modeling, and Applications to Low-Profile Wideband Antenna Design
Significant progress has been made in magnetic metamaterials. Accomplishments include the development of very low profile (3.3 in.), wideband ultrahigh frequency (UHF) (250-505 MHz) magnetic metamaterial antennas, which can replace the large, very visible whip antenna now used on Army vehicles; low-profile, wideband UHF (300-1200 MHz) electromagnetic bandgap (EBG)-backed antennas; and collapsible, umbrella-shaped spiral antennas for satellite communications with simple feed design.
Structural Materials
Silicon Carbide–Aluminum Metal Matrix Composites
Work on SiC-reinforced aluminum matrix composites shows the successful implementation of the more highly loaded composites accomplished by rolling. There is good characterization (in situ scanning electron microscope [SEM] with crack initiation and propagation) and finite element modeling (FEM).
Cobalt-Free Tungsten Carbide
This new project to develop environmentally sustainable (“green”) materials for armor-piercing (AP) projectiles has identified viable materials and has been performed in collaboration with industry. Concerns about potential carcinogenic properties as well as any cleanup requirements of Co-containing tungsten carbide (WC), utilized in the cores of AP rounds, has motivated research into the development of binders for WC that do not contain Co. Research at ARL is exploring routes for the fabrication of non-Co-containing binders; each of the routes is being explored with the objective of benchmarking it against the performance of current WC-Co cores.
Boron Suboxide Ceramics
Because of its higher hardness and comparable elastic modulus, boron suboxide (B6O) is a potential replacement for boron carbide (B4C). This experimental program is geared toward processing of B6O and is closely coupled with a computational program. One of the advantages of processing B6O rather than B4C is the lower temperature required for hot pressing, which reduces the overall production cost.
Ion-Containing Polymers
Research in this project uses reaction-induced phase separation to synthesize membranes having co-continuous hydrophilic and cross-linked hydrophobic domains. The work has resulted in the synthesis of membranes that have high charge transport, improved mechanical properties, and water vapor transport.
The researchers have determined key attributes of alkaline stable cations (acidic protons, aliphatic cations) and have identified cations with over 1,000-hr half lives at 88oC in 1 M sodium hydroxide solution. The work also disproved steric hindrance as a most effective strategy for stabilization.
Nanocrystalline Iron and Other Metal Alloys
The team has developed a high-quality program for thermodynamically stabilizing the grain size of nanocrystalline metals. The underlying concept is to create close to equilibrium grain boundaries (GBs) by lowering the GB energy through the addition of a second element. This concept was proposed years ago in Germany but had not been realized until the recent ARL work. Production of engineering materials has placed this project in the lead of this internationally competitive field. The project uses simple models to screen candidate alloy materials. After selection, the second element is introduced by ball milling. In-house ball-milling facilities appear to be sufficient for this project. This process is a practical method for large-scale production. This research project has developed an alloying strategy for the control of grain size and achieved significant improvements in properties vis-à-vis state-of-the-art nanocrystalline alloys.
Magnesium Alloys
This project has demonstrated significant progress toward the goal of developing Mg alloys that possess increased strength and fracture toughness for armor applications. This research represents a well-planned program combining modeling, process development, process scale-up, and characterization efforts. The utilization of viscoplastic self-consistent (VPSC) modeling to guide the equal channel angular pressing (ECAP) design and process route optimization is particularly praiseworthy. Ties to university researchers, including Johns Hopkins University and Monash University, are well focused in the area of understanding the microstructure/property relations in Mg alloys, such as AMX602, that have improved properties but do not contain rare earths. The project’s approach of utilizing ECAP processing to refine the grain structure appears fruitful and is supported by the significant grain size reductions realized so far.
Cold Spraying
Cold spraying of metals is a technology that was invented in the mid-1980s in the former Soviet Union. It is a versatile process for applying metal coatings to substrate surfaces. Typically, fine particles (1-50 µm in diameter) are accelerated in a supersonic gas stream to velocities up to 1,000 m/sec. Upon impact with the substrate, plastic deformation of the particles yields a dense deposit that strongly adheres to the substrate. The process has been used most successfully to deposit aluminum, copper, and titanium on a wide range of substrates. Using robotic control, uniform deposits can be made on flat or profiled substrates. The technology has already found application in the repair of machined parts. ARL’s cold spray research has advanced from a laboratory-scale demonstration to a pilot-scale operation. Beyond that, the ability to fabricate shaped pieces by combining cold spraying and additive manufacturing technologies has been demonstrated. For example, a cone-shaped Al alloy article with excellent surface finish has been fabricated—something that no other laboratory has been able to do. This is an impressive achievement and opens exciting opportunities for in-field refurbishment of worn-out parts and even the replication of components or parts that have failed in service.
Tungsten Alloys
Since the Gulf War demonstrated that cleanup of depleted uranium after warfare is both hazardous and costly, research into the development of alternatives to projectiles made of depleted uranium, an area of focused research at ARL and other Army laboratories for over three decades, continues. The current focus on ultrafine-grained W or W alloys is showing some progress. The coupling of experimental process development efforts and DFT computational modeling addressing approaches to increase the ductility of W by examining W-rhenium analogs, including W-zirconium and W-titanium, is noteworthy.
Energy Coupled to Matter
This is a large-scope and long-term project involving a variety of external process parameters that could affect and change the characteristics and properties of materials by modifying their microstructure under a set of target conditions. The project focuses mostly on innovative synthesis, processing, and manufacturing processes under extreme conditions, including high electric and magnetic field environments. The project’s general strategy is to use extreme environments to tailor the microstructure of materials during processing in order to develop materials by design and on demand with superior and/or optimized properties. Current examples include electric-field-assisted sintering (EFAS), microwave processing, and flash sintering, as well as the rapidly evolving field of additive manufacturing. This project is currently in the exploratory stages of development and execution; while promising, it appears somewhat unfocused and would greatly benefit from a clearly articulated scientific strategy and scientific roadmap. As the fundamental principles and design rules governing materials processes in extreme E&M environments are being established on a scientific basis, it is expected that the definition of such a strategy will improve and be brought into a sharper focus. This project would also benefit from a better integration of in situ diagnostics and characterization, as well as modeling and simulation.
This is a relatively new program (1 year) that has produced few results, but strong staff leadership and good vision can enable the project to collaborate with strong external U.S. government and university programs to define the best facilities for ARL to develop for investigating high magnetic and electric fields, microwave, acoustics, light, temperature, pressure to control phase chemistry, and nanostructuring of materials. ARL is also focused on developing in situ characterization tools, which are essential for understanding how to create novel materials by applying new high-energy sources during synthesis.
Electric-Field-Assisted Sintering
EFAS has reached its highest level of maturity at ARL, where an integrated pilot-scale unit for the production and consolidation of nanopowders of metals and ceramics has become operational. However, much remains to be done to establish sintering mechanisms. In particular, the very high heating rates achievable in this process introduce complex thermal-electrical gradients, which result in gross heterogeneities in the sintered structures that strongly influence properties and performance of the final products.
The investigators have developed a coupled thermoelectric-sintering modeling framework for two EFAS machines (vacuum chamber with furnace and open air). They have also instrumented an open-air EFAS machine to ensure repeatability and record temperature(s), current, and ram displacement during a run for high-fidelity modeling. Their immediate effort is to explore the effect of electric current on diffusivity and grain growth in light metals and ceramics.
Another project is exploring the impact of EFAS in a single-step processing operation for producing bulk materials from powders. In this process the powder is heated by the application of electric current
under applied pressure. This process has demonstrated success in consolidating bulk nanocrystalline and other traditional materials that are difficult to sinter, such as ceramics and refractory metals.
Encapsulation Technology
This project presented an effort to combine metal matrix composites with monolithic ceramic tiles and structural steels to achieve engineered armor packages possessing improved ballistic performance. Improved specific strength and modulus in hybridized materials systems was realized. This project has established the basis for designing large-scale protective systems for land-based vehicles. The investigators have developed prototype designs that show much promise.
Composite Adhesive Design
This project is establishing the foundations for a data-driven framework for designing materials for adhesives. The investigators have managed the difficult and arduous task of developing a large and vetted data set for engineering properties where the provenance is well defined and formatted into a 100 percent digital retention framework.
Grain Boundaries in Ceramics
This program is supported by a complementary ab initio atomic simulation effort. The work to date has been focused on synthesis and exploring different processing routes such as colloidal processing and sputter deposition for depositing amorphous silica and boron suboxide onto the surfaces of boron carbide powder particles. They have successfully used hot-pressing and spark plasma sintering methods to successfully consolidate boron carbide powders to high densities. To date an exploratory microstructural characterization effort has been initiated using transmission electron microscopy (TEM).
Piezoelectric Materials
The research targets understanding of the materials structure that leads to the intriguing properties of piezoelectric materials starting from first principles. Currently the project is at the level of ascertaining the effects of grain geometry on continuum properties.
Silicon Carbide
The research currently concentrates on molecular dynamic simulations of SiC systems with new force laws that provide better results on prediction of shock response and sintering.
Boron-Based Ceramics
The modeling work in this project is part of close collaboration with the processing and synthesis group; these two groups have collaborated well in initiating a project to develop boron-based ceramics. Even though their entry into a well-researched area, especially B4C, is only recent, significant progress has been made is setting up the mechanism for understanding the behavior of these very complex materials. These materials could have a large impact on the development of armor owing to their high hardness
and low density if the issue of low fracture toughness can be resolved. Even though these materials have been studied for a long time, the defects in them are not well understood. The opportunity for materials design is enormous here. Expertise in this area appears to be available at ARL. Although not as far along as the nanocrystalline metals project, some samples have been made and characterization started. At this point the micrographs show silica at the boundaries, but no mechanical measurements have been made. It is too early to assess the impact this work will have, but it is an exciting research area being pursued by an appropriate team of highly qualified scientists, who have done a nice job of calculating elastic moduli and solution energies for a number of impurities.
Multiscale Modeling of Polymers
This FY14 project is aimed at the development and validation of multiscale models to elucidate the effects of the fibrillated microstructure of ultrahigh molecular weight poly-ethylene (UHMWPE) fibers on the unique properties that govern their exceptional ballistic-resistant characteristics. The project developed a computational framework for fibril-to-fiber length scales and subsequently made preliminary correlations between the modeling and fiber twist-tension experimental data on Dyneema. The researchers’ pursuit of the advances of multiscale modeling of collagen and Kevlar fibers with various approaches that do not provide a rigorous treatment of fibril-to-fibril interactions to the modeling of UHMWPE is commendable.
Atomistic Modeling of Polymers
In this research program polymers are modeled by molecular dynamics. In order to simulate the polymer chains, the coarse graining method is implemented. Reactive force fields were added. This approach is state of the art in atomistic/molecular modeling and was applied to polyurethane urea, polymer composites, and the design of new composite materials. For example, SiO2 nanoparticles were added to polymers, and their effect on the mechanical properties was established.
ARL Enterprise for Multiscale Research in Materials
There is a broad joint ARL-funded program involving Johns Hopkins University (for experiments and magnesium), California Institute of Technology (for modeling), Drexel University (for polymers), the University of Delaware (for composites), and Rutgers University (for ceramics). ARL is in position to capitalize on the strengths at these universities. Processing, where ARL is very strong, is primarily conducted in-house. The materials under investigation are divided into four groups: metals (including Mg by ECAP, nanocrystalline metals via powder and consolidation, and cold spraying technology); ceramics (boron carbide and boron suboxide); composites; and polymers (UHMWPEs).
The materials in extreme dynamic environments program is in place, with collaborations occurring at the experimental (processing, testing, characterization) and computational levels. Not much effort is being devoted to analytical approaches. The Mach Conference on materials in extreme environments, organized by Johns Hopkins University, showed that the participating groups are actively engaged and generating new results.
Biomaterials
The hiring of a new branch chief might improve the focus and critical mass in the Biotechnology branch. One of the important elements of developing a collaboration is having 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 that 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 more mid-career scientists if funding is available. In the meantime, the branch can take advantage of other Army 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 program. 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 for bioinspired materials mentioned in the presentations—energy, detection, and force protection; robust networks; structural awareness and evolving threats; and cognitive nanoscience and transformational medicine—are 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 in the rest of 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 benefit from greater 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 branch needs to seek new areas that have direct relevance to the mission of the Army. This search could be stimulated by initiating and hosting quarterly seminars with the participation 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 insights that are relevant 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 under way.
DFT calculations have already been useful in understanding the stability of electrolytes, 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 the 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.
Considerable work on DFT is under way in other parts of ARL. The work would be improved by integrating the disparate efforts or by hiring researchers with expertise relevant to individual areas.
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 the mechanisms associated with Pd crystal size; diffusion paths through crystals versus diffusion paths through 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 could strengthen this area, specifically in catalyst design. Long-term stability of these catalysts is a challenge.
Photonic Materials and Devices
A substantial fraction of the work in the photonics portfolio can be characterized as high risk in the sense that it appears to be quite speculative or to have a long-term horizon. One good example of strong work in this category is the work on cold-atom optics, but there are many other examples as well.
There are fewer projects that exhibit high risk by addressing 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 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 many cases, the photonics work at ARL seemed to evince an aversion to risk and a low tolerance for failure. In these cases, 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 it may be more symptomatic of a predominantly top-down project selection. Other peer laboratories may have a more bottom-up approach, with decisions on entirely new approaches being made at the investigator level.
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 to articulate capabilities, accomplishments, and impact. The breadth of work is commendable, but 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 technology fielded by the Army 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 the world’s number one spot 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.
Electronic Materials and Devices
Low-Dimensional 2D-Atomic Layer Materials
In the work on low-dimensionality electronic materials and devices, most of the applications of 2D electronics will require improvements in materials and a large scale-up of materials size, uniformity, and quality to achieve an integration level of at least millions of devices operating at speeds up to 10 GHz to address any meaningful applications. This would represent a game-changing opportunity. This fundamental project, focused on atomic layer characterization, is critically important if such progress is to be realized.
The investigators have positioned themselves at the interface between materials science and condensed matter physics. There are many new monolayers being isolated as the overall field matures, and the investigators have already taken an appropriate interest in phosphorene. One of the challenges is that this field moves with blazing speed. Given the limited manpower on this project, careful decisions need to be made to effectively balance new exploration, device optimization, and collaborations.
The project on understanding the electrical performance of stacked 2D atomic layered materials was initiated in 2008 to study the potential of carbon nanotubes (CNTs) for low-power electronics. At that time, there were significant challenges to first separate the metallic and semiconducting CNTs and then fabricate more than the most primitive transistors, so in 2011 ARL switched to graphene. While ARL fabricated back-gated graphene field-effect transistors (FETs), this was not a leading technical effort, so in 2013 the project switched to growing MoS2 2D materials and fabricating impressive transistors and simple but important integrated circuits (ICs). This is an interesting success that showed the evolution of materials and an approach that reflects well on both the research staff and management. There are excellent opportunities for incorporating this technology into very lightweight, low-power wearable electronics and displays. As in the case of low-dimensionality electronic materials and devices, most of
such applications will require relatively high-performance electronics, so further device optimization and scale-up from the relatively simple circuits already demonstrated to large-scale integration of at least millions of devices operating at speeds up to 10 GHz are necessary.
On-Chip Energetics: Porous Silicon
The role of integrating additive manufacturing, which can be valuable to the project, is not clear. The outcome of the project is expected to deliver thermal energy on-demand systems for the Army and for other applications. This requires the successful execution of experimentation—for example, in reaction control.
Emerging Technology for Power-Efficient Electronics
This effort appears to be a good niche area of research for ARL to pursue in the context of GaN technology, where much related, but not exactly the same, work is already going on around the world. The Defense Advanced Research Projects Agency (DARPA) Compound Semiconductor Materials on Silicon (COSMOS) program would be achieving similar goals by allowing mixed material on the same substrate. It would be beneficial for ARL to explore how its project relates to, and is synergistic with, DARPA’s COSMOS program.
Piezoelectric Materials for Frequency-Agile Radio Frequency MEMS Front Ends
Several opportunities exist in the RF MEMS area, and ARL is in a good position to move aggressively into this field and to consolidate its leadership.
Magnetic Metamaterials—New Concept, Modeling, and Applications to Low Profile Wideband Antenna Design
This research effort uses engineered materials (metamaterials) and electromagnetic surfaces to achieve low-profile designs for Army antennas. Metamaterials can step up the radiated power of an antenna, enabling miniature antennas. Several opportunities exist and ARL is in a good position to move aggressively into this field.
Structural Materials
Silicon Carbide–Aluminum Metal Matrix Composites
The challenge for this project is to address critical science and engineering issues that are not addressed by the vast body of literature that already exists in the field of particulate metal matrix composites. This was one of the few posters that explicitly identified the need for a formal feedback mechanism and for optimization to develop robust modeling. This reinforces the need for optimization methods to be included in the overall materials research program.
Cobalt-Free Tungsten Carbide
Addressing environmental and safety concerns while maintaining performance requirements for non-cobalt-containing WC cores for inclusion in AP rounds is technically challenging. The manufacturing approach being pursued appears to have been carefully considered, in particular the potential for a nano-iron binder, which appears to be very promising and presents some unique opportunities to remove Co from the WC binder phase.
Boron Suboxide Ceramics
There are considerable challenges in the scale-up of the processing of these B6O powders. This experimental effort, combined with the computational DFT work and the mechanism-based research being conducted by other groups at ARL, constitutes a worthwhile and serious pursuit. The integration of the three efforts represents an opportunity for success in this arena.
Ion-Containing Polymers
To address oxidative, thermal, and humidity cycling concerns, the investigators have proposed incorporating compounds 10 and 14 (or compounds like them) into ion exchange membranes and fuel cells and investigating chemical stability in the actual fuel cell environment. The challenge here will involve establishing a link between chemistry, processing, and microstructure.
Nanocrystalline Iron and Other Metal Alloys
The research group identified an opportunity: the ability to create bulk parts and materials. With the promise of enhanced properties and advancing manufacturing, there is a strong need for characterizing and modeling the underlying mechanisms to fully exploit this opportunity. The team has assembled a wide array of tools, from microscopy to first-principles calculations, to support this work, and the next step is to build up and enhance the integrative science that links the modeling and characterization to achieve a predictive, multiscale materials by design approach. The synthesized material is characterized using a number of techniques, including TEM. Apparently there are no technically qualified scientists at ARL to perform TEM measurements, so an outside source is used. It would be extremely useful for this project and many other materials science projects at ARL to have a 3D atom probe.
Magnesium Alloys
Before they can be used in armor applications, Mg alloys will have to be made strong enough and with good enough fracture toughness in large plate form to supplant the Al alloys currently in service in vehicle platforms. Attainment of this goal would mean a huge opportunity for Mg in the lightweighting of armor and vehicles. Once Mg alloys that meet the strength and toughness requirements have been developed, the challenges of corrosion, erosion, and fatigue will have to be tackled, so that the alloys can be widely applied for vehicular armor. ARL’s planned path forward—the installation of a crucible furnace and rolling mill facilities to support Mg alloy development—is appropriate.
Cold Spraying
ARL’s cold spray research has advanced from a laboratory-scale demonstration to a pilot-scale operation. Beyond that, the ability to fabricate shaped pieces by combining cold spraying and additive manufacturing technologies has been demonstrated. Rapid implementation in the field of this new integrated spray-deposition process is warranted. The impressive progress in cold spray technology is leading to applications such as the repair of Mg helicopter components and the reactive munition (shaped charges) that can be deployed in the BattleAx system. The Mg ECAP program has potential, but it is doubtful that the increase in strength derived from the reduction in grain size below 1 µm will be significant and will justify the process, which has an inherent difficulty and is very energy-intensive. The nanocrystalline research is very contemporary and an outgrowth of the doctoral work of some participants. The challenge is to find applications for this processing methodology.
Tungsten Alloys
Increasing the inherent low fracture toughness of W and its lack of shear-banding driven rod resharpening behavior remain a grand challenge. Exploitation of modeling approaches to understand the atomic bonding and operative mechanisms underpinning these weaknesses in W is a promising modeling and experimental manufacturing process development activity. The researchers need to consider the applicability of super-hard-faced WC/Co alloys as inserts in kinetic-energy projectiles. These materials are available commercially in various sizes, shapes, and forms. A typical super-hard facing consists of a high fraction of diamond or cubic boron nitride particles in a ductile metal matrix, thereby imparting fracture toughness (bend strength) to the graded-composite structure.
Energy Coupled to Matter
Owing to the nature and the scope of the project, one of challenges is the planning vis-à-vis the target goals—for example, which material to study, which field-assisted parameters to study, and what are the determining factors, milestones, and success criteria.
The researchers need to continue investigation and development of high magnetic and electric fields, microwaves, acoustics, light, temperature, pressure to control phase chemistry, and nano-structuring of materials. The research team also needs to pursue the development and implementation of in situ characterization tools.
The investigators have correctly noted that a key challenge is the need to develop a fundamental understanding of mechanisms that influence field–material interactions. The approach is to implement a complex array of modeling tools, including thermodynamic modeling, DFT modeling, and phase diagram simulations. Further thought needs to be given to how the results of such modeling will be integrated.
Electric-Field-Assisted Sintering
This research is aimed at understanding and control of sintering kinetics to better address challenges in the reproducible fabrication of high-quality parts, particularly those of complex geometries. Without a near-net-shape capability, the applicability of this new technology will be seriously limited. In today’s industry, it is not sufficient to make preforms, and then to resort to post-thermomechanical treatments to produce the desired shapes.
Encapsulation Technology
Development of improved (in performance and cost) kinetic energy (KE) armors requires the concurrent development of the materials of construction, design of the structure, and the processes utilized to fabricate the armor system in addition to the development of predictive computational modeling to support armor design and validation via ballistic testing. From a basic science perspective, this project is a good example of hierarchical design of hybrid materials, where the fundamental principles of micro-mechanics, shape, and form at an engineering scale and basic materials properties are optimized with respect to ease of processing and manufacturing. There exists an opportunity to develop this project as a template for other multiscale design projects.
Composite Adhesive Design
This program could be readily expanded by exploring a probabilistic relationship between failure modes and loading history. This could help to address the lack of understanding of the physical phenomena at multiple scales that govern high-stress and high-strain-rate material performance, resulting from the paucity of validated linkages between experimental and computational research tools at critical length and time scales.
Grain Boundaries in Ceramics
Boron carbide is an immensely complex material, with unmeasurable (and probably variable) stoichiometry, unknown defect structure, unknown impurity content, uncharacterized grain boundary structure, and a tendency to amorphize or melt. The greatest long-term challenge in the program is to find the few keys to performance enhancement—if they exist. In the short term, both the B3C and the B3C-SiO2 materials require substantial microstructural-scale analysis, including electron backscatter diffraction (EBSD) to uncover texture and grain boundary crystallography, energy dispersive x-ray spectroscopy (EDS) for identification of phases, TEM for grain boundary segregant structures, and—ideally—atom probe, also for grain boundary segregants. Some of these can be performed in-house, including on the new scanning electron microscope-focused ion beam (SEM-FIB) system. Others will require external collaboration. The researchers indicated that they aim to explore the science that enables the engineering of grain and interphase boundaries in low-density boron icosahedra–based ceramics like boron carbide and boron suboxide for improved fracture resistance. To meet this challenge, a detailed microstructural program where microscopy and spectroscopy are used as research tools and not just for routine characterization will be key. Although prior work in the field of grain boundaries in other ceramic systems such as nitrides was acknowledged, to ensure that this project has the maximum impact for both the Army mission as well as advancing the field scientifically, there needs to be a more refined set of objectives as to what aspects of the microstructure, especially in terms of grain boundaries, will be explored. Topics to consider include but are not limited to the impact of processing chemistry on intergranular film thickness, wetting and evolution of grain boundary geometry and dihedral angles, and chemical characterization of bonding states at grain boundaries via electron energy loss spectroscopy. When linked to property measurements such as high-rate deformation, this project could have an important impact on many basic science studies.
Boron-Based Ceramics
Because this study requires an extensive survey of large systems, computational resources are a limiting factor. While computer time has not been an issue to date (the investigator has used high-performance computing [HPC] resources within the DoD laboratories), the decision to use a genetic algorithm optimization scheme in the next stages of the project will tax current central processing unit (CPU) cycle availability. Management needs to look at a variety of options inside and outside ARL, including dedicating appropriate resources to this program. Computational throughput has also been limited by a shortage of software licenses. Apparently, a cutback in the number or type of DFT software licenses has limited the number of computational replicants that can run simultaneously. This will become critical as the next stage of the project commences. ARL needs to investigate solutions, including purchasing more licenses, negotiating the license agreement to allow multiple replicants per license, or considering open source software such as Abinit (with the understanding that this will require some retrenchment).
This project has a high degree of risk, because the small computational cell size used in ab initio methods may not be able to capture the complex and potentially nonperiodic phenomena in this large unit cell material. High risk is appropriate for a basic research project, but realistically evaluating the potential for this approach to solve the problems of interest needs to be a component of the project. The basic concept is to weaken the GBs to improve fracture toughness. The DFT calculations are reasonably well focused and use service-oriented architecture techniques. There appears to be an issue with respect to use of the Air Force computer system, pertaining to the Vienna Ab-initio Simulation Package (VASP) and/or CASTEP4 licenses. This issue needs to be resolved.
Multiscale Modeling of Polymers
Development of predictive models of the mechanical behavior of UHMWPE will afford improved opportunity to maximize UHMWPE fiber use in armor applications. A challenge in modeling subfiber mechanics is to gain an understanding of the defects within UHMWPE fibers, their length scale, and the linkages between manufacturing and defect distribution and their effects on subfiber and fiber mechanical behavior. Researchers need to continue addressing the gap between molecular dynamics modeling and fiber behavior.
Atomistic Modeling of Polymers
Shock wave propagation through these polymers was investigated. This modeling approach needs to be coupled to experiments in order to advance the field.
ARL Enterprise for Multiscale Research in Materials
The success of this broad collaboration hinges on the ability and desire of participating scientists to physically visit the institutions and have extended (2 weeks or more) visits that are periodically repeated. This is more easily accomplished at the graduate student, postdoctoral, and junior scientist levels. If such a program of visits is not established, the universities will pursue their different and diverging paths with a significant diminution of results. The danger (for each institution) of conducting incremental work
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4 CASTEP is a commercial and academic software package that uses DFT with a plane wave basis set to calculate the electronic properties of crystalline solids, surfaces, molecules, liquids, and amorphous materials from first principles.
and repeating work that has been done before is real, and the challenge to the participating researchers is to seek new areas.
OVERALL TECHNICAL QUALITY OF THE WORK
Knowledge of materials is the foundation of designing, constructing, and manufacturing useful products that meet target purposes in functional as well as aesthetic qualities and characteristics. Superior knowledge spurs superior products. This knowledge must be continually built and augmented through creative, innovative, and ingenious studies.
In general, ARL’s materials science research, being dedicated to address the Army needs, includes those from the state of the art to the art of the possible. Materials research efforts and expertise are spread throughout the ARL enterprise. It is a crucial core competency and a crosscutting discipline.
To achieve an optimal impact, ARL has tried harder to strike a balance between projects that tackle known unknowns, driven by application and innovation on demand, and projects that explore unknown unknowns to achieve high-risk and high-reward outcomes. The technical merits of research projects and the caliber of researchers have also demonstrated upward momentum.
Several leading and high-impact scientific studies as outlined in this report show tremendous promise and potential breakthroughs for future applications to benefit, protect, facilitate, and empower soldiers of the world’s unrivaled armed force, the U.S. Army. The following are outstanding and exceptional areas in materials sciences.
Critically important to the Army’s night vision, large-area surveillance, and navigation in degraded vision environments, the electromagnetic modeling of quantum-well infrared photodetectors (QWIPs) work is exceptionally valuable. Affordable, high-speed, high-resolution, long-wavelength infrared cameras will be one of the fruits of this project. ARL is the world leader in QWIP technology.
To support development of lightweight, quiet, efficient, and reliable power sources for Army applications to enhance soldier combat capability, the project on fuel cells for military applications tests and evaluates commercially made technologies—namely, direct methanol fuel cell (DMFC) and SOFC systems. The technology reduces weight and decreases the logistic burden associated with batteries. This represents an upward potential for Army applications and an outstanding value.
The work on synthetic biomolecular materials is highly significant for the Army. The project has already shown success by developing iterative and integrated multiscale computational biology capabilities for in silico studies and studies on the evolution of material interfaces. This is innovative and ingenious work.
To explore potential breakthroughs that could meet the emerging needs of the 2035 Army, the low-dimensionality (2D) materials program covers fundamental aspects of synthesis, characterization, device design, and manufacturing. Tuning 2D materials at the atomic scale opens enormous opportunities to design electronic properties. This is a potentially high-impact area.
To develop lightweight armor, ARL’s mechanical press capability of up to 700,000 lb of pressure is unique. This is exceptionally enabling test equipment that facilitates materials discovery and development.
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 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 and synthetic biology could be an exciting area if it can include protein engineering, synthetic biomaterials, and extremophiles. The Biotechnology branch could closely couple with the well-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 would be very useful. The ARL has outstanding laboratory facilities and equipment, and the experimental research there 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 and theoretical elements of the program. However, if this branch begins to conduct research at the interface of the biological and physical worlds, it would discover many opportunities for the integration 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 update on the status of this topic would have been useful, especially given the significant focus on it over 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 lower-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, ARL’s technical work is outstanding, demonstrating both breadth and depth. The researchers showed clear and deep understanding of their fields and they evinced understanding of their particular work in both its applications context and its place on the world research stage.
Electronic Materials and Devices
Low-Dimensional 2D-Atomic Layer Materials
The overall technical quality of the work presented in this cluster of projects is outstanding. The research project on low-dimensionality electronic materials and devices demonstrated outstanding technical achievements in characterization of 2D layered electronic materials and the discovery of external bias change of the stacking structure in multilayered graphene on h-BN. There are a good number of highly visible publications on this work.
The project on understanding the electrical performance of stacked 2D atomic layered materials is an example of the high quality of the ARL research portfolio in 2D layered electronic materials and devices. In 2011, the investigators switched from studying carbon nanotubes to graphene. In 2013, the investigators recognized the importance and relevance of MoS2 and embarked on this journey. Since then the investigators have contributed significantly to the scientific literature and made substantial progress on building functional devices. This research is a success, as evidenced by the 12 publications that were produced in the course of this work, 6 of which featured an ARL researcher as either the first or the last author. The technical quality of the work is consistently very good, from the foundation of understanding and developing the growth of MoS2, to device modeling and fabrication, to realizing small demonstration circuits required for logic operation. This project is well supported and coupled to the scientific fundamentals project on low-dimensionality electronic materials and devices. The project exhibits high-quality scanning probe microscopy (SPM) and Raman characterization of the MoS2 materials, theory work on 2D materials, and the effects of surface and edge states on mobility.
On-Chip Energetics: Porous Silicon
The projects were found to be of high quality. An additional merit of the project is its use of the ARL clean-room facility, a valuable ARL capital investment.
Emerging Technology for Power-Efficient Electronics
The investigator is extremely competent and is capable of advancing the state of the art in this promising research area.
Piezoelectric Materials for Frequency-Agile Radio Frequency MEMS Front Ends
The project is of very high technical merit. The leader of the effort has been working on MEMS at ARL for more than 15 years, is very capable and knowledgeable about the technology, and is a leading expert in the field.
Magnetic Metamaterials: New Concept, Modeling, and Applications to Low-Profile Wideband Antenna Design
The projects collectively are of very high technical merit. Prompt success of the effort can offer immediate benefits to the Army by reducing the profile of antennas mounted on Army vehicles, helping to limit their visibility.
Structural Materials
Silicon Carbide–Aluminum Metal Matrix Composites
The technical quality of this work is good, with complementary experimental, characterization, and modeling (FEM) components.
Cobalt-Free Tungsten Carbide
The green WC project entails manufacturing process development, materials characterization, and performance optimization. The effort appears well planned and technically focused, and the project is on track to produce meaningful binder replacement options.
Boron Suboxide Ceramics
The program is of high quality, with beneficial collaborations with Rutgers University (for synthesis) and Johns Hopkins University (for TEM). High-quality TEM is an essential component of this program.
Ion-Containing Polymers
This work shows promise in advancing important engineering problems. To meet the challenges and opportunities, this project can build on a multiscale paradigm both computationally and experimentally. The work would be strengthened by also considering a modeling/theoretical foundation when choosing compounds to explore experimentally.
Nanocrystalline Iron and Other Metal Alloys
This group has disseminated its technical results by publishing in a large number of high-quality publications. Now that a number of material combinations have been identified, it is a natural extension to utilize semiempirical potentials to calculate the stability and structure of GBs in specific materials systems. This work would help to define process parameters and quantify the reliability of these novel materials. In-house expertise for this extension is available at ARL. This project is one of the best materi-
als research projects presented during this review. The project shows a good application of fundamental science to critical manufacturing problems.
Magnesium Alloys
This project has made excellent progress over the past 3-4 years, in particular as demonstrated by its 12 in. × 12 in. × 1 in. processed plates.
Cold Spraying
This work had been going on at ARL for a number of years. Its results are significant, and the research is leading to patentable technology and could lead to industrial applications. Two applications are of special interest:
- For reactive munitions, ARL has been able to codeposit Ni and Al powders in a shaped charge. These powders can react exothermically but are prevented from doing so by the ambient temperature of the process. However, the formation of a jet could initiate the reaction, increasing the energy of the warhead. Similarly, shells made with Ni, Al, and a third mystery component could react after fragmentation or upon impacting the target, increasing the lethality of the weapon.
- For helicopter gearbox repairs, a single gearbox casing costs around $800,000, and so it is advantageous to repair these magnesium alloy components, which can be done by cold spraying. This technique is applicable to the more than 4,000 helicopters in service for the U.S. defense.
Tungsten Alloys
Replacement of depleted uranium (DU) remains a complex materials science challenge strongly driven by political and environmental cleanup realities. The current ultra-fine-grained W and W alloy development efforts, including powder purity improvement and powder consolidation methods, appear warranted and are worthy of further research. Excellent progress has already been made in this challenging materials development area. In particular, the embedded WC/Co insert appears to be a solution to the oblique-angle impact problem. Building on this achievement, there is the prospect of further improvements in performance by the use of inserts of super-hard-faced WC/Co alloys.
Electric-Field-Assisted Sintering
A computational modeling approach to EFAS is being developed to understand and control sintering parameters to achieve reproducible structures and properties for complex parts. The model is helpful in guiding EFAS development, particularly in addressing challenges associated with process scale-up and optimization, leading eventually to full-scale manufacturing. This research is well targeted and likely to yield results of major technical importance.
Energy Coupled to Matter
The team is excellent and exhibited good judgment in defining projects. The project evinces a close connection of computational modeling to couple to and define an optimal approach for the proposed experimental work.
This program has provided critical capabilities for ARL. While it is in its early stages, it would be useful to develop projects that distinguish it from other groups here and abroad. To enhance the quality of the program it has to be competitive with other laboratories. For instance, some thought needs to be given to where ARL can carve out a research niche for itself—say, in developing new materials or in exploring fundamental physics that no other groups are exploring.
Encapsulation Technology
The combination of purposefully selected structural materials in combination with metal matrix composites and monolithic ceramics to achieve lightweight, multi-hit-capable armor systems is a very fruitful line of research. The work has progressed well; it now needs to move to the next phase, where the design is guided more strongly by analytical and stochastic-type modeling. Adding a theoretical component to the design would also help to ensure the long-term success and impact of the project.
Composite Adhesive Design
This project is at its early stages, and the investigator has established a program that can serve as a template for a data-driven approach to materials design. The project has the ability to uncover unexpected new correlations in structure–processing–property relationships. If the project moves to the next stage, from building a data repository to incorporating analytical tools to interpret that data for models, then the project can significantly accelerate the transition from experimental validation and data management to materials design. This project is one of the few, if any, that explicitly call for building stochastic/statistical and informatics approaches to materials design.
Nanocrystalline Alloy Microstructures
This group has also disseminated its technical results in a large number of high-quality publications. Now that a number of material combinations have been identified, it is a natural extension to utilize semiempirical potentials to calculate the stability and structure of GBs in specific materials systems. This work would help to define process parameters and quantify the reliability of these novel materials. In-house expertise for this extension is available at ARL.
Grain Boundaries in Ceramics
This project is exploring the fundamental issue of GBs in engineering ceramics that goes far beyond the specific chemistry of the material system. The work is off to a good start and is addressing the critical issues of materials synthesis and processing and ensuring that the provenance of the materials is well established before embarking on a detailed mechanical and microstructural characterization of interphase boundaries in boron-icosahedra-based ceramics. The overall technical quality is strong, and the problem is compelling and has significant growth potential. In contrast to the modeling component of this effort, the experimental component is not discovery-directed basic research but, rather, science-guided development, and there is a clear line of sight from the technology to the customer. Thus, this project has an appropriate balance between basic research and development.
Piezoelectric Materials
A good foundation has been established for continuing work on the hard problem of linkage to atomistic descriptions.
Silicon Carbide
The effort has led to validation of single crack propagation and bifurcation, and it sets a good foundation for analysis of more complex crack propagation scenarios.
Boron-Based Ceramics
The overall technical quality of this work is strong. It is very positive to see ARL adding to its modeling portfolio at this length scale. This is an area that can grow substantially. Missing at this time are the calculations of intrinsic defects, although this information may be available in the literature. Interaction of impurities with intrinsic defects may be critical in these materials and is an effect well suited to investigation by DFT. The research team has a good background in materials science as well as DFT expertise. This is a combination rarely found and is a strong advantage for this project. GBs and amorphization will be difficult to study with DFT, owing to their complexity. More empirical methods, calibrated with DFT, can be used to examine these complex structures. This project has taken up the great challenge of predicting structure and properties of bulk boron suboxide (BxO) and BxO grain boundaries. Accomplishments include construction of the BxO simulation cell (168 atoms), development of a GB simulation cell (~500 atoms), a survey of bulk dopants in BxO, and calculation of elastic constants in pure and doped BxO. The results have been analyzed in depth, and a journal paper is in progress. This work has a line of sight to the effort to apply grain boundary engineering to BxO, but it is better categorized as basic research. As such, the contributions of this relatively new project are substantial and strong.
Multiscale Modeling of Polymers
This new project represents a well-conceived multiscale project displaying strong initial progress. Continuing leverage of experimental and modeling insights and techniques from medical research on collagen is expected to prove beneficial.
Atomistic Modeling of Polymers
Seventeen peer-reviewed publications resulted from this work, a significant accomplishment by the project’s enthusiastic early-career participants.
ARL Enterprise for Multiscale Research in Materials
The technical quality is high, but the challenges are daunting as well. Although a significant component of computational modeling is present for the four classes of materials under study (metals, ceramics, polymers, and composites), there is limited evidence of true multiscale modeling. This is a challenge that is recognized by ARL and that will require significant effort going forward.
