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Materials Research
INTRODUCTION
The Panel on Materials Science and Engineering at the Army Research Laboratory (ARL) conducted its review of ARL’s programs in biological and bioinspired materials, energy and power materials, and engineered photonics materials at Adelphi, Maryland, on June 10-12, 2015. This chapter provides an evaluation of that work, recognizing that it represents only a portion of ARL’s Materials Research campaign.
ARL’s Materials Research spans the spectrum of technology maturity and addresses Army applications, working from the state of the art to the art of the possible—25 years into the future—according to the ARL. Materials research efforts and expertise are spread throughout the ARL enterprise. As the ensemble of the materials discipline and capabilities, materials sciences is one of ARL’s primary core technical competencies. The materials sciences work supports the mission of ARL, as the U.S. Army’s corporate laboratory, to provide innovative science, technology, and analyses to enable a full spectrum of operations.
BIOLOGICAL AND BIOINSPIRED MATERIALS
The scientific quality of the work in this area is on par with that of leading federal, university, and industry laboratories, reflects a broad understanding of the underlying science and research being conducted elsewhere, and is recognized as a component of the broader national effort in biomaterials research through its government (e.g., the U.S. Army Edgewood Chemical Biological Center and the U.S. Army Natick Soldier Systems Center) and university (e.g., Institute for Collaborative Biotechnology at the University of California, Santa Barbara) partnerships and collaborations.
This research area has grown substantially over the last 2 years. The knowledgeable leadership direct principally competent, early-career scientists. Relative to the importance of this research, the person-power in this area is considered suboptimal.
The laboratories are generally well equipped to perform the types of studies and analyses required for the biological research. The biological characterization and imaging tools are good, with recent additions of next-generation sequencing and protein synthesis and medium-scale bioreactors with real-time metabolism analysis capabilities. A peptide sequencer would provide important missing capabilities and would help to accelerate research.
Among the excellent research activities of this group, particular promise is shown by the stabilization of proteins against thermal and chemical extremes, using new chemistries and methods to derive antibody-like reagents that improve on antibody properties (specifically, biomolecular recognition and binding characteristics).
Accomplishments and Advances
Biomaterials for Hazardous Materials Detection
The development of synthetic bioreceptors as alternatives to antibodies is being pursued to allow biosensing outside the laboratory and in conditions more representative of those found in the field (e.g., high temperature). Overall, this project, conducted by a relatively new group of researchers, is equivalent to the best work performed elsewhere. The work supports a variety of missions, including water and food defense, individual soldier protection, and collective protection. The concept is to mimic antibody binding with small peptides. An approach screens large libraries of enhancing affinity, selectivity, and other desired features (e.g., serum stability) via an iterative process. The group is exploring a number of different strategies to perform this screening, which is a significant strength of its approach.
To create high-affinity and robust biosensors, independently binding peptides are chemically conjugated using click chemistry to identify bi- or greater ligands. The use of cyclic peptides in place of linear peptides is also being explored as a means to higher-affinity molecules. It is impressive that this technology has allowed rapid (less than 1 week) identification of binding peptides. The group’s demonstration of binding to aluminum alloys provides a practical example of its capabilities. Overall, this very productive group is doing cutting-edge work that complements work ongoing in extramural laboratories.
Given the alternative strategies for achieving similar outcomes (e.g., single-chain thermostable antibodies), more specific performance criteria or target product profiles will be needed for further development of some of these areas.
Biohybrid Materials for Sensing
Bio-nano-hybrid systems are being investigated for their potential applications for in vivo physiological monitoring, nanomedicine, traumatic brain injury (TBI) dosimetry, and other photonics-based sensing. The intent of this research is to understand the interactions taking place at the biomediated, nanocrystalline, photonic or nanophotonic biomaterials interface, and to develop new designs for tailored light or matter interactions that can be applied to Army needs. The examples presented use proteins to stabilize nanoclusters and control photonic materials properties. Protein-nanocrystalline structures have been embedded with neurons to detect primary blast-induced neurotrauma, a potential means to investigate mild TBI. The protein-stabilized nanoclusters (P-NCs) were synthesized in situ in neuronal and nontumorigenic cells—the first demonstration of in situ nanocluster growth in nontumorigenic cell lines.
This project provides a good example of grassroots-driven collaboration with outside laboratories. It is very good fundamental research with potential applications to sensors (ligand recognition) and to cell targeting for drug discovery and development. The research is characterized by good integration of modeling and experimental work across bio-molecular- to cellular-length scales. However, more fundamental work is needed to determine the location of nanoclusters, to determine whether protein(s) stabilize the nanoclusters, and to validate in vitro expressions under high pressure.
The effort to utilize P-NCs for monitoring pressure in TBI appears to yield distinct spectral peak intensity changes. Without concurrent modeling efforts it is not certain whether these intensity differences can be due solely to changes in nanocrystal clusters in proteins or can also be due to other effects. It is therefore unclear whether this research would be better directed toward sensors for TBI or other extreme conditions.
A smaller project is focused on the development of a real-time handheld detector for synthetic cannabinoids based on use of a cannabinoid receptor as a transduction element for detection of contraband material. This is an attractive approach, given the diversity of targets (generated by the illegal synthetic drug community) that can be detected using the functional receptors that trigger downstream
cognitive effects. Further investment in this project is expected to depend on performance parameters that are yet to be determined, including the limits of detection, the dose response across a useful operational range, and the signal to noise performance in the presence of interference.
Bioinspired and Biomimetic Materials for Protection
This effort addresses a number of related topics intended to improve the performance of polymers in areas relevant to the Army mission, as well as the use of polymers in studies of TBI. ARL’s biobased polymer program has been used to produce transitioned biorubber toughening agents, reactive diluents, monomers for polyamides, biobased bisphenol A analogues, and multiphenolic monomers. One program was directed toward developing high-performance biobased polymers for Army applications. The goal of the program is to utilize renewable lignin-based resources to create molecules for the production of high-performance polymers. Successes to date include synthesis of monomers of diepoxy and demonstration of polymers with very high glass transition temperatures. The associated challenges include development of scalable chemistries and structure-property-toxicity capabilities that would allow for transition of the technology to industrial partners.
Another project focused on improving the properties of polymers by incorporating reversible cross-links to enhance toughness. The goal of a third project is to develop high-temperature adhesives that are inspired by the extraordinary properties of spider silk and muscle titin, specifically by incorporating reversible cross-links whose breakage can allow the unfolding of polymer domains. While inspired by natural polymers that derive their mechanical properties from hydrogen bonding, this project focuses on the use of reversible metal bonds as high-temperature tougheners. The emphasis in both projects on developing a mechanistic understanding is a strength, because so much of the other work on these topics is empirical. The emphasis on high-temperature performance differs from the focus in most other laboratories that work on these topics. It is unclear, however, whether simple insertion of metallic elements will achieve the strength and toughness levels of the natural polymers, but new insights are likely, and the coordination with modeling is laudable.
A small project presented in this area addresses the extremely important problem of TBI. The experimental setup devised is fairly simple: A small (2 g) explosive charge is detonated close to a tank that contains neuron cells. This approach to load cells is novel compared to other TBI studies using cell cultures and may yield new insights. Although this is an exciting project, it needs to be part of a much larger and broader program studying TBI; it could be connected to efforts taking place at other Department of Defense (DOD) laboratories. This project needs to also consider more interpretable dynamic loading of neuron cells. Test configuration could allow simulation of pressure waves so that any observed changes in the neuron cells can be related to a known pressure history.
Bioconversion, Biosourced Energy
This is an appropriately focused long-term effort to address Army-specific needs for dealing with food and water wastes. It connects well with other Army entities and is appropriately resourced in terms of both equipment and competent personnel.
A positive characteristic of the program is its university outreach and collaborations, intended to draw in expertise and technologies. These relationships may be leveraged or enhanced through the developing ARL open campus initiative. A more formal connection with the Army Medical Command, particularly in the areas of wastes and health, will be important.
To achieve the programs’ goals, it may be worthwhile to put more emphasis on high-throughput approaches to empirically screening large numbers of communities; this could allow more rapid identification of desired bacterial communities. It might also prove useful to examine lessons that may be learned from the limitations of previously fielded systems.
Opportunities and Challenges
Because biology is a growth area, ARL has an opportunity to identify and recruit a critical mass of biologists, including microbiologists and polymer/organic chemists, looking well into the future to create an integrated community of researchers. The process of recruiting and retaining talent could encourage better articulation of the expectations and career paths that lead from postdoctoral researchers who are contractors to scientists who are government employees, and to develop an effective mentorship program emphasizing professional development and job satisfaction.
ARL needs to reexamine its polymer-related work to assure the closest possible relations between the researchers at its Adelphi and Aberdeen locations.
ENERGY AND POWER MATERIALS
Accomplishments and Advances
The quality of the research projects, the staff, and the facilities is comparable to high-quality research laboratories elsewhere in industrial and academic environments. Where there are gaps in the technical skills or methods needed for a project, the ARL staff demonstrate mature experience and judgment in seeking out high-quality collaboration with other non-ARL researchers within and beyond the Army research enterprise.
The early-career researchers are strong and have excellent skills, which likely reflect good mentorship by senior personnel. Importantly, the research staff are enthusiastic and throughout the review demonstrated a clear focus on Army needs, an appreciation for the importance of moving basic research to technology to impact, and skill in selecting research methods and tools involving experiment, theory, and simulation.
The portfolio of research projects reviewed included an appropriate balance of high-risk, long-term-impact projects along with mid-term and short-term projects. There was a broad, deep coverage of different devices, different fuels, and different applications covering a wide range of size and time scales.
There are continuing improvements in research quality, staff hiring in both postdoctoral and permanent positions, and collaborative activity. As part of these improvements, ARL has expanded its modeling capabilities. The current in-house capability for carrying out high-level simulation and modeling activities is of high quality and moving in the right direction.
Advanced Energy Storage: Advanced Battery Chemistry
Though the advanced battery effort at ARL is small relative to similarly focused programs at other federal laboratories (e.g., Department of Energy laboratories), it is internationally recognized for its high scientific quality and long history of productivity and innovation.
The research includes significant elements of experimental and computational numerical modeling work. The laboratory equipment for experimental work is excellent, spanning an impressive range of capabilities from materials synthesis and characterization, to electrode and cell fabrication. Computational efforts in the battery area are good but appear to be relatively recent. They could possibly benefit from additional resources and emphasis.
The team has excellent qualifications that are well matched to their research challenges. In addition, program participants have an excellent understanding of research conducted elsewhere and are well aware of critical research issues and advances from around the world. The ARL team has formed a local Center for Batteries in Extreme Environments, which provides a good model for interaction of ARL staff with non-ARL scientists.
The projects in this area also are synergistic with one another. The team thinks hard about transition pathways to scale-up, manufacture, and commercialization and seems well positioned to make decisions and negotiate arrangements to transition ideas to the field.
The team has begun generating a database of properties on electrolytes, including interfacial reactivity. A pathway may exist for organizing these data in a manner similar to that being pursued for other battery materials in the materials genome initiative. Productive work may come from the team’s interactions with that initiative focused on electrolytes, possibly including the effect of additives on interfacial reactivity.
Advanced Energy Storage: Structural Batteries Using Additive Manufacturing
The researchers are successfully developing techniques for fabricating multifunctional battery materials using additive manufacturing (AM). The lattice structures constructed using AM have favorable mechanical properties and controllable surface area per unit volume, which permits tailoring and optimization of electrochemical performance. With respect to weight reduction for batteries and capacitors, the AM method has clear advantages over earlier methods. The measured elastic properties and electrical performances of the fabricated materials agree well with the finite-element modeling performed as part of the project.
The techniques and materials are promising, and the scientific quality of the project is comparable to quality at leading research institutions. A more comprehensive modeling and simulation component addressing chemistry and physics, in addition to mechanics, might be desirable for understanding effects such as the influence of porosity on performance of gels. The project has a good balance of theory and experimentation. It would be good for the team to consider the previous work on nanotrusses done at the Naval Research Laboratory to see if there is anything in this work that might be applicable. To help design for sufficient mechanical durability and reliability of materials, it might be worthwhile to investigate the strength and failure properties of the fabricated materials as well as their elastic moduli. It may also be desirable to consider the practical issues associated with scaling up the laboratory AM process to full-size batteries. This project has significant potential for innovative discovery.
Alkaline Fuel Cells: Optimizing Structure and Chemistry of Ion-Containing Polymers for Charge Transport
This project focuses on the development of alkaline fuel cells. Conventional approaches rely on a liquid KOH electrolyte as a means of OH− exchange. This electrolyte is problematic because it is a liquid and can be poisoned with CO2 owing to the formation of K2CO3. The goal of this project is to circumvent these problems using a polymer electrolyte. In particular, a mixture of dicyclopentadiene and CO is used to create a bicontinuous microstructure intended to maintain high OH− conductivity and strong mechanical properties. The OH− conductivities achieved were the best reported, though the mechanical behavior was not adequate. By increasing polymer molecular weight and cross-linking, strength and toughness were increased nearly twofold, but this is still far less than competing materials. It is unclear whether this mechanical behavior would be acceptable. The researchers were able to enhance material behavior by optimizing microstructure, which was in turn achieved by increasing the connectivity of the hydrophilic domains.
This research is Army-relevant, reflects a correct understanding of the literature, makes use of state-of-the-art facilities, and is of high quality, comparable to similar university and industrial efforts. However, the scope of the effort requires expansion if the work is to have a substantial impact within this community. Additionally, being more engaged with this community—e.g., the multiuniversity research initiative at the Colorado School of Mines—would help this work proceed by enhancing critical decision making—for example, in materials selection. This project would be strengthened significantly by the
addition of a modeling component. There are many existing methods and codes that could be helpful; for example, the group at the National Renewable Energy Laboratory is modeling this sort of system.
The publication record of the researchers is prolific: Seven papers have been published, one is under review, and four more are in preparation.
Alternative Energy Photovoltaics
This project utilizes quantum dot nanomaterials for photovoltaic conversion and focuses on enhanced light absorption and minimizing reflective losses. This nanomaterial approach eliminates the need for a traditional tracking system and, if successful, would significantly impact a number of Army applications requiring flexible and efficient power.
The researchers have established productive collaborations with the communities at the University of Texas, the State University of New York, Microlink Devices, and the University of Michigan—all characterized by a good mix of experiment, theory, and simulation. The researchers demonstrate a broad understanding of the related science as exemplified by their efforts to modify the wetting layer thickness to increase electronic capture; they have achieved photovoltaic (PV) efficiency 6 percent above the record for GaN.
Alternative Energy: Highly Mismatched Alloys
This project develops material to split water by using sunlight as the energy source. This research is high risk but potentially offers a very high payoff. The idea, based on results appearing in the literature, is to replace some N with Sb in GaN to form GaNxSb1-x. It was predicted that by adding Sb, the bandgap could be lowered to about 2.2 eV, producing an efficient light absorber. These alloys, highly mismatched in size or electronegativity, have never before been synthesized. The group has significant experience with Group V alloys and apparently a unique synthesis capability.
The experimental results shown verify the bandgap crossing model (developed at Lawrence Berkeley National Laboratory [LBNL]) up to x = 0.22. Materials produced remain crystalline. It appears that very small amounts of Sb lower the bandgap significantly. The principal investigator did not understand why the model predicted such behavior. An understanding of the controlling physics needs to be developed.
Attempts could be made to fabricate a device, though this necessitates doping these materials. Doping can now be done for GaN, but it is not clear what effect the Sb will have on this process.
Good collaborations with LBNL and with the University of Strathclyde and the University of Nottingham (both in the United Kingdom) are ongoing. Only theory is done at LBNL. There is also significant competition from the National Renewable Energy Laboratory and the University of North Carolina; these groups are focused on different materials. This project could be aided by more modeling. There have been five publications by this group in the last year.
Alkaline Fuel Cells
The principal objective of this research is to create anion exchange membrane/proton exchange membrane stacks, eliminating the need to transport water throughout the cell and potentially reducing the mass and footprint of the device. The principal investigators have pulled together an excellent team, including a group at Georgia Institute of Technology.
The principal investigators were aware of many of the relevant issues for successfully constructing such a cell, including issues regarding delamination. This work has a robust modeling component. Overall, this research is innovative and promising.
Alloy Type Anodes for Lithium-Ion Batteries
Lithium (Li)-ion battery performance and weight reduction may be improved by using silicon (Si) anodes to increase the capacity for Li storage. This project addresses an important practical difficulty with Si anodes, which is their tendency to experience mechanical failure after a small number of electrical discharge and recharge cycles. The principal investigator has performed careful in situ measurements of this effect using an atomic force microscopy technique. The implementation of these experimental techniques is the main achievement of the work so far. The principal investigator is also working on coatings for anodes to reduce cracking, with encouraging results. The project also supports collaborations with the University of Utah to use molecular dynamics (MD) simulations to study the electrical and mechanical processes involved. MD seems to be a very promising method for understanding the fundamental aspects of the cracking problem. In particular, analysis might help to reveal why thin coatings apparently reduce damage in spite of the very large linear strains to which the coatings are subjected. The project would benefit from a closer working relationship between the experimental and computational team members.
Beta(photo)voltaics
The project is intended to develop a long-lived (25-year goal) power source using beta and alpha energy conversion in wide bandgap (WBG) semiconductor materials and phosphors. The approach uses a beta emitter (tritium) to produce electricity. Current designs do not produce enough power to be useful for the Army, and so the isotope power source is coupled with a Li-ion battery to take care of power demands during higher current demands such as during signal generation. The isotope power source is used as a trickle charger for the Li-ion battery. Electrochemical capacitors were used, but there was high leakage current.
This project started as an engineering problem. Isotope power sources have been used for years in weapons applications. Since the Army has access to isotope materials, it made sense to utilize this approach for power production. To test the concept, an isotope power source was fabricated that generated 100 µW.
The innovative concept applies to the investigation of three-dimensional (3D) interaction space in WBG materials and phosphors to increase energy conversion and efficiency. The project is well thought out, and it has a high probability of success. The principal investigator is performing mechanical cross-section simulations to aid in design, so the mix of theory, experimentation, and computation is sufficient. The qualifications of the researcher and the facilities appear to be compatible with this research challenge. If successful, this could open a myriad of small power source applications for the Army.
Carbon Formation During Catalytic Oxidation of Hydrocarbon and JP-8 Fuel
The use of logistics fuel for compact, heat-driven electric power generation is compromised because sulfur impurities poison the catalytic activity of microcombustors. In this project, a materials-by-design approach is being used to identify promising combustion catalysts, which are investigated with experimental and computational methods. In situ spectroscopy is incorporated with short contact-time reactors to identify surface species during catalytic combustion of prototype fuel, while simultaneously monitoring poisoning. These data, used in conjunction with a microscopic reaction diffusion model of surface events during combustion, clarify the effect of sulfur. It has been recognized that sulfur enhances carbon formation on platinum (Pt) but not on rhodium (Rh). The project promises to accelerate microcombustor catalytic design through reactive flow modeling. This is good scientific work linked with sound engineering methods for scale-up and extension to logistic fuels.
Critical Solvation Issues in Lithium-Ion Batteries
This poster describes part of the excellent battery program that focuses on Li salt solvation in organic carbonate and water solvents. The objective is to better understand fundamental electrolyte interface properties. The work showed preferential solvation of Li by ethylene carbonate (EC) in EC/dimethyl carbonate mixtures, which is relevant to solid-electrolyte interphase formation at carbon anodes. New water-in-salt electrolytes having less than 20 weight percent water in Li bistrifluoromethanesulfonimide were prepared, and early-stage results are quite promising. This is an example of an emerging class of electrolytes called deep eutectics. Water in this electrolyte is thought to have very different properties from conventional water because such a large fraction of these water molecules are contained in solvation shells. Understanding of interface passivation could help in electrolyte material choice. The work is led by an energetic PI and is synergistic with the overall thrust of the ARL battery effort. Its high scientific quality is demonstrated through publication in quality journals.
High-Voltage Li-Ion Electrodes and Electrolytes
This project involves work on olivine LiMPO4-type cathodes, where M is Co, Mn, and Ni substituting for the usual Fe. These three metals have more positive redox potentials, so batteries using these materials have higher voltages. LiCoPO4 has a high potential but usually exhibits significant capacity fade upon cycling. The ARL team found that mixing some Fe with the Co results in much less capacity fade, with minimal loss of overall capacity. A mechanistic understanding of the diminished capacity fade is being pursued. The work is of high quality and fits well with the significant worldwide effort to identify new battery cathodes to enable higher energy density batteries.
Isomeric Materials Research
This project addresses the use of nuclear transitions for energy-on-demand. The concept is to convert a long-lived, excited nuclear state to a short-lived ground state by excitation by photons or neutrons. The main scientific content is nuclear physics, in contrast to the mainstream atom/electron/photon-centered work in the ARL materials campaign. The work requires investigation of level diagrams for candidate nuclei. Because of the complexity of the few-body problem for large nuclei, the nuclei cannot be modeled to sufficient accuracy but have to be measured. The investigators combine information from the literature with their measurements. The conversion concept has been demonstrated for a silver isotope. The work is of high technical quality and is published in the appropriate journals. This is a long-term, high-risk approach with regard to practical applications, with many questions to be answered, including how to produce the long-lived excited state in sufficient quantities, but it is worth pursuing. If successful, impact could be high.
Lattice Conductivity of Dense Ta-Doped Li7La3Zr2O12
LLZO is a candidate for a solid electrolyte in Li batteries. The goal of this study is to enhance Li conductivity via doping Ta for Zr. Using this approach, the investigators were able to reproduce a similar study by Goodenough and suggested that other enhanced results in the literature were likely due to other defects. This is a very good addition to the literature, where reproducibility of key results is often lacking. Furthermore, this work had a strong density functional theory component that was performed at the Naval Research Laboratory (NRL). The work would be outstanding if there were researchers within ARL who could complement the efforts of NRL.
Mathematical Modeling and Lifetime Extension of Thermal Batteries
Thin-film thermal batteries could provide improved reliability and performance over present designs in munitions. This project is successfully developing a comprehensive analysis tool that models the thermal energy balance, gas generation, and electrical performance of thin-film thermal batteries. So far, it is mainly the thermal problem that has been addressed. The ARL thermal model has been integrated into Sandia National Laboratories’ Sierra finite element code. The method that ARL is developing could be combined with a mathematical optimization tool, providing a direct and systematic way to improve battery design. There was no mention of validation of the model, suggesting that this is not a central focus. The principal investigator did not seem to understand the model being used. Therefore, in addition to adding the multi-physics capabilities that are planned, it would be desirable to obtain experimental data that would be needed to validate the submodels (e.g., thermal, gas transport, electrical, chemical) as the code grows in size and complexity.
Pyroelectric Materials for Energy Applications
Led by a competent postdoctoral researcher, this project draws on a 2006 paper1 reporting a giant electrocaloric effect in perovskite oxide PZT (metallic oxide based piezoelectric material) to propose and prototype a device to convert heat/infrared photons to electrical energy. The concept is to run a heat engine loop between the low polarization branch of the polarization versus field (P-E) hysteresis loop at high temperature and the high polarization branch at low temperature. There is much room for optimization by choice of materials and by controlling the quality of the thin film, with leakage current being of particular concern. The collaboration with the University of California at Berkeley will help with the latter. The long-term application will be the remote supply of energy by an infrared laser and could be used, for example, to recharge drones in flight. The work is of high technical quality, and the project leader communicates well with the broader community. As the work progresses, more modeling could be integrated into the project.
Understanding C-C Bond Breakage on Plasmonic Nanostructures
This proof-of-concept project is directed toward developing catalyst structures for breaking the CC bonds associated with high energy density logistic fuels (e.g., ethanol) using light-harvesting nanoscale arrays formed by localized surface plasmon resonance. This is a unique fabrication approach based on a good concept for photo-reformation of logistic fuel. The project represents a high-risk endeavor. The first steps toward forming such structures have been made with use of the Specialty Electronic Materials and Sensors Cleanroom facility. Initial work includes modeling the plasmonic aspects of the structure. Additional modeling worked is planned to take account of the immersed, reactive, electrochemical environment. Experimental characterization of the photo-induced reactions is planned, using well-established electrochemical and surface science methods. Desorption mass spectroscopy electrophotometry has been reported by others and may be considered for this project. At this point, there are no experimental data on the structure, and the current understanding of the reaction mechanism is speculative. It would be helpful at this point to create a device and test it out. It is not yet clear whether this fabrication method can be implemented at large scale.
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1 A.S. Mischenko, Q. Zhang, J.F. Scott, R.W. Whatmore, and N.D. Mathur, Giant electrocaloric effect in thin-film PbZr0.95Ti0.05O3, Science 311:1270, 2006.
Grain-Boundary Engineering of Ion-Conducting Ceramics
This project examines two approaches to reducing ion transfer resistance at grain boundaries in the fast lithium-ion-conducting ceramic Li3xLa2/3-xTiO3, x = 0.11. Previous work showed that this solid-state electrolyte material had good conductivity. The idea is to change grain boundary (GB) properties to improve GB conductivity. Grain boundary modification was achieved by simple lithium-ion exchange from solution, and by silica coating the starting particles with SiO2 using magnetron sputtering. Both approaches provided modest increases in conductance, both at GBs and in the bulk. A study on variations in bulk ionic conductivity with thermally induced changes in crystal structure was also pursued. The mechanism by which surface coatings change GB conductance is not fully understood; more structural characterization work—for example, by transmission electron microscopy (TEM)—is planned to help address this point. It was also found that different processing conditions could change crystal structure and conductivity. The principal investigator, a postdoctoral researcher with ceramics experience relevant to Li battery technology, needs to learn more about the battery aspects of the work to become fully integrated into the overall effort. Still, this is excellent work for a relatively new employee. The work is of high quality and contributes to a growing body of knowledge regarding use of ceramic materials to replace liquid or polymer electrolytes in Li batteries.
Opportunities and Challenges
Questions remain as to whether ARL was mobilizing aggressively enough to capitalize on both internal advances and external advances made by the broader community—for example, whether the recent world-leading results on enhancement in quantum well infrared photodetector (QWIP) efficiencies are being translated into capability demonstrators for manufacturers and customers. Similarly, ARL may not be working to leverage external advances in silicon photonics, especially with regard to heterogeneous materials. However, in both cases, these concerns were partially allayed by discussions with staff and management regarding the status of some programs related to these technologies. For the QWIP work, for example, ARL has hired an external business consultant and is working with NASA and others on technology transfer for the QWIP breakthroughs. Nonetheless, there remains more opportunity for ARL to capitalize on its internal and external advances.
The enormous potential impact of the photonics work could have been presented more vigorously and compellingly. One way of doing so could be to augment an individual photonics presentation with an explicit description of the broader potential impact if it succeeds. Army goals were noted, but they often comprised immediate technical targets as opposed to what the ultimate impact could be for a more comprehensive field of science or for broader Army applications.
The presentation on structural batteries using additive manufacturing has significant potential associated with its innovative approach. The project combines novel fabrication methods with insight into selection of compatible multifunctional elements that combine structural components with energy storage components. Experimental work is carried out concurrent with modeling studies that guide system design choices. The external collaborations are facilitated by a flexible methodology that provides easy incorporation of next-generation subcomponent materials as they are developed. However, the effort needs to grow across a wider range of projects, with a focus on identifying appropriate modeling methods and on closing the experiment–theory–simulation loop. Increased interaction with the significant computational resources of ARL could help bridge the gap until additional capacity is available within the Materials Research campaign. At present, first-principles computational modeling is growing, mainly through collaboration with recognized experts elsewhere, guided by very capable but limited-in-number experienced internal research staff.
In comparison to the expansion in first-principles modeling, engineering models are underutilized, perhaps because in-house expertise in this facet of modeling is limited. Engineering models are typically developed at the outset from a simple set of input parameters or components that, together
with the model, predict system behavior. These components are improved as empirical knowledge of the system’s behavior increases. Routine methods are now available to identify the most sensitive components for which improved fundamental knowledge is needed, to provide uncertainty quantification, and to guide system-level optimization during scale-up or scale-down beyond experimental regimes. The combination of an appropriate engineering modeling effort with the intuitive understanding of experimentalists is a highly effective engineering approach and needs to be targeted as a growth area.
In some energy and power applications, such as Li-ion batteries and fuel cells, there is a broad, vigorous, fast-moving, worldwide research effort directed toward identifying fundamental scientific issues and developing novel materials and entire systems. Accordingly, the narrowly focused ARL projects need to pick the right niche in order to have impact. The knowledge necessary to define the goals of such projects depends critically on tracking research advances elsewhere. Because postdoctoral and other early-career permanent staff researchers benefit from exposure to research activities beyond ARL, it is critically important to promote and expand active mentoring by senior staff.
ENGINEERED PHOTONICS MATERIALS
The quality of the work in photonics materials is comparable to that found at most research universities. This is an impressive accomplishment in light of the inherently wide scope of the technical program, which is essential to addressing diverse current and future Army needs. The quality of the work presented reflects a high level of technical competence and professionalism on the part of the researchers and management.
The portfolio of the engineered photonics materials group shows a good balance of high-risk, longer-term work with nearer-term customer-driven solutions or incremental, critical technology refinement. This well-balanced portfolio is supported by a strong materials capability in staff expertise and laboratory or clean room infrastructure. Investments are impressive for computational modeling and simulation that ARL has successfully implemented to complement its strengths and core competencies in materials synthesis and characterization, as well as device work. All of these facilities and capabilities are being leveraged into compelling device and application-driven work, especially in ultraviolet (UV) materials, infrared (IR) devices, and the device physics in both areas. In addition to technical diversity, there is workforce diversity.
Accomplishments and Advances
Alternative Energy: Photovoltaics
This project involves work to improve performance of low-concentration photovoltaic cells targeting robust, lightweight power for soldiers in theater. The technical focus is developing solutions using III-V quantum-dot materials to extend performance into the longer wave regions of the solar spectrum, and to improve efficiency by minimizing recombination.
Solar PV is one of the important pathways to reducing the weight of power solutions in theater. The experimental work showed solid progress, reflecting the strong competence of the team, which evinced expertise that includes epitaxy and sophisticated quantum dot engineering, polyethylene terephthalate (PET) moth eye surfaces, and intentionally induced morphological features on III-V layers for enhanced photon capture. There appeared to be extensive collaborations with researchers outside ARL.
The wetting layer state-engineering designs might benefit from more direct experimental verification of their efficacy in reducing recombination in the dots. There was a lack of clarity on the trade-offs between the high concentration (30 to 100 times what is typically seen when realizing the benefits of advanced materials) and the low concentration (less than 4 times what is typically required in
nontracking applications). More clarity is needed on the system-level incremental cost of multijunction cells with significantly higher efficiencies relative to single-junction material solutions such as GaAs, which is still very high ($40,000 per square meter), or the quantum dot approach pursued in this work. Additional questions include comparisons with spectral splitting, which was examined in the DARPA-sponsored very-high-efficiency solar cell program.
Biophotonics
Progress was reported on protein-wrapped fluorescent metal nanoparticles, motivated by their potential use as neuronal pressure sensors. The long-term goal is to develop a fundamental mechanistic understanding of mild traumatic brain injury onset and development.
The fundamental work on the biomediated synthesis of atomic nanoclusters was compelling, and the fact that the proteins retain their native functionality after synthesis has tremendous potential. For example, the resulting nanoparticles may be noncytotoxic, and it may be possible to direct them to specific locations within a cell. These nanoparticle building blocks are anticipated to provide unique opportunities based on their interesting optical and physical properties. An example given was fluorescence change with pressure seen for one protein but not a different protein, an indication that interesting protein science may be enabled by this system.
There is some concern regarding the specific proposed application for these particles for understanding shock waves in tissue. The fluorescence changes with pressure were small (20 percent over 400 MPa for one system and about 6 percent over 600 kPa for a different system). In real tissue, these small changes over less than 1 ms, from a single or a few particles, will be very hard to observe. What is needed is a deeper physical analysis of the full system, including the signal-to-noise ratio in realistic shock wave and illumination conditions, and what is anticipated at a single neuron level. Also needed is a comparison with other potential techniques, such as Forster resonance energy transfer and plasmonic particles, in the context of nanoscale pressure sensors.
This ambitious work offers strong opportunities for discovery; it is a high-risk early-stage effort in ARL’s expanding biophotonics effort.
Modeling and Analysis of Ultraviolet-Light-Emitting-Diode Materials
The objective of this project is to use many-body theory to model lifetime in III-nitride structures, including free carrier and exciton effects, polarization fields, and density-dependent screening of Coulomb interaction and polarization fields. This is one of the projects indicative of ARL’s investments in more comprehensive modeling to support its strong core materials capabilities and competencies.
This a very challenging problem, and the principal investigator is making good progress in describing radiative lifetime, including many-body effects such as phase-phase filling, screening, and quasi-particle renormalization. However, nonradiative processes were not described at the same level of theory. Semiempirical, nonradiative models using activation energy were shown not to fit experimental data well, but improved fits were achieved with a combination of a fixed temperature-independent component plus an activation-energy component.
The development of first-principles-based and self-consistent predictive capabilities to describe carrier lifetime in III-nitride structures, including both radiative and nonradiative processes, is not easy. However, the principal investigator presented a scientific strategy to make progress toward addressing this challenge. The strategy calls for alloy fluctuations, a many-band description of the electronic wave function, the use of nonparabolic bands, and the inclusion of nonradiative recombination processes. This is a project of high technical merit and of potentially high impact in support of the Army’s mission.
Ultraviolet Avalanche Photodetector Research
This work entailed the compelling development of models and experimental devices and materials to evaluate the efficacy of novel solutions for improved single-photonic avalanche detectors in the UV as replacements for photomultiplier tubes.
The principal concept is to use GaN and AlGaN epitaxial layers to address the reduction in quantum efficiencies that stems from the use of semitransparent metal electrodes on current SiC devices. Self-assembled monolayer structures were introduced to either isolate the SiC to a multiplication layer or to just use the AlGaN as a transparent contact layer to keep the SiC away from surface so as to avoid surface recombination.
This work is promising and has high-quality external partnerships. It has mainly involved epitaxy development and Si diffusion studies, and the transitioning of these to device results in avalanche operation is awaited.
Short-Wavelength Infrared Device Modeling and Optimization
This project is directed at the development of a comprehensive model that combines the finite-difference, time-domain electromagnetics of nanostructured surfaces with finite-element modeling, drift-diffusion transport to understand and optimize device designs and material structures. The model is comprehensive in that it included material, electronic, optical, and especially nanostructured geometric properties that strongly impact the electromagnetics. The integrated software suite allowed analysis of very complicated multipixel arrays, and the principal investigator showed how more simplistic models would not properly capture major performance factors. One example was that the performance of nanostructured cones could be estimated reasonably well with effective medium models at longer wavelength, but at shorter wavelengths complex scattering among the cones dominated the performance. The model was shown to be useful in assessing pixel cross talk in arrays, as well as heterostructure design and junction location for optimization of collection efficiency while minimizing generation-recombination (GR) dark current.
This is an excellent project directed toward an important topic in terms of the needs of both the Army and the broader technical community.
Diode-Pumped Tm/Ho Composite Fiber 2.1 µm Single-Mode Laser
The goal of this research is to provide a simpler and more compact 2.1 µm thulium (Tm)/holmium (Ho) source capable of achieving 100 W power in eye-safe lasers for situation awareness, monitoring, and tracking illumination and, perhaps, frequency conversion to directional IR countermeasures.
Early work has been conducted on an innovative concept to make a dual-core fiber laser that would support thulium lasing at 1,950 nm in a multimode core that would, in turn, pump a Ho single-mode core at 2.1 µm. This design is intended to achieve two excitations in the Tm with a single optical pump in the 800 nm range. This is an interesting concept, but it is too early to expect definitive evaluation of the potential.
This effort may now be positioned to benefit from a stronger modeling component to resolve the impact of saturation on spatial mode competition and laser performance. Suitable baseline modeling capabilities are readily available in the literature, and in conjunction with a more deliberate experimental plan, the modeling may be useful for isolating critical performance trade-offs.
The principal investigator is engaged in a valuable external partnership with strong competence in these fiber materials.
Thermal Property Engineering: Exploiting the Properties of Ceramic
This project consists of preliminary work on improving mid-IR lasers by increasing the effective thermal conductivity of the gain media, using nanoscale composite MgO (high thermal conductivity) with Er:Y2O3 (the gain media). This work addresses many scientific and engineering challenges, including the achievable effective thermal conductivity of the composite, which may be limited by phonon scattering, and the achievable volume fraction of gain media needed to be competitive with current solutions.
This work has high potential, and it may benefit from some early modeling to determine the property bounds and trade-offs. The team could also be more vigilant in reaching out to others, including the Air Force Research Laboratory, to evaluate similar work.
Photoacoustic Spectroscopy for Hazard Detection
This project involves work on an elegant and simple device approach for detecting trace elements. While many optical detection techniques are available, these are usually large and contain many precision optical elements. The detection technique proposed is small, robust, and potentially inexpensive, if applications supporting high-volume laser production are realized.
Engaging more broadly with the outside community would be beneficial, including with vendors of existing optical sensors and comparative testing on species of current interest, perhaps in the context of ARL’s open campus initiative. In addition to offering a potential for more pervasive use, this will better ensure that this transitions into a product useful to the Army.
Understanding Inkjet Printed Standards for Optical Measurements
This work involves a system based on the well-tested use of inkjet printing. Although ARL has used only a single print head, the researchers have been able to print on many materials (e.g., rubber, metal, and wood) with contaminants included. The system can be used to understand how the samples age, and the flexibility of patterning and reproducibility of the technique were shown to be useful in capturing the unexpected impact of real-life variations of species on surfaces in the field. This is important work that continues to be funded by customers.
Single-Beam Femtosecond Multiplex CARS
This work illustrates the outstanding evolution of research aimed at using a collinear approach to coherent anti-Stokes Raman spectroscopy (CARS) for trace gas detection. These studies focused initially on pulse characterization but transitioned to the examination of mathematical methods and algorithms for extracting the desired spectral signal from broadband background spectra. The principal investigator was able to demonstrate strong signal-to-noise ratio improvements that substantially enhance the efficacy of the CARS approach.
Photon Trap for Infrared Detection
This project involved the expanded modeling and experiments on the microresonator enhancement presented two years earlier. The work showed that very small variations in microresonator dimensional control had strong impact on both the peak efficiencies and the bandwidth of the enhancement. The results were encouraging, indicating that design regimes existed where very high efficiency could be supported over a band that was easily large enough for many Army applications.
This important advance may not be receiving sufficient resources to move quickly to highly optimized commercial technology. Also, ARL’s studies of the dynamic behavior of materials are likely to be advanced by improvements in infrared detection at modest elevated temperatures.
Ultrafast Spectroscopic Noninvasive Probe of Vertical Carrier Transport in Heterostructure Devices
This work involved pump-probe studies of ultrafast carrier dynamics and charge transport in heterostructures, with the ability to interrogate charge-generated terahertz field profiles in materials prepared for device structures. This research represents a valuable investment in advanced characterization, and the quality of both the topics and investigators is excellent. In addition to being of immediate value to materials and device researchers, the projects are conducive to quality papers and conference presentations of broad interest to the technical community.
Tunable Solid-State Quantum Memory Using Rare-Earth-Ion-Doped Crystal, Nd3+:GaN
This project involved high-risk, early work aimed at using GaN as a host material for the neodymium ion (Nd3+) in quantum memory research. The objective of this project is to perform photon echo experiments to provide an estimate of the memory storage time and capacity in cryogenically cooled Nd3+:GaN crystals. The plan is to ultimately fabricate GaN polar heterostructures from which to design a quantum memory device with multimode capacity. Although this project is in its early stages, this work makes strategic use of ARL’s strong GaN materials and molecular beam epitaxy (MBE) growth capabilities to gain a competitive position in a field that is drawing worldwide attention. Moreover, the ARL team has a strong track record of published contributions in this field.
Opportunities and Challenges
The consistent development and extension of modeling to broader sets of problems and applications is an opportunity area. One prototype project is short-wavelength IR device modeling and optimization. This research illustrates ARL’s expanded efforts to provide critical modeling support in areas where there is high investment in underlying materials and device technologies.
The important software tool set coming from this research is not only essential for designers, but it may also provide critically sensitive parameters that could be used in process control for commercial partners and suppliers of imaging solutions to the Army, which necessitates engaging with the manufacturers. The project’s principal investigator has started this engagement.
OVERALL QUALITY OF THE WORK
Overall, the researchers and the management are of high caliber. Most of the projects presented are excellent and have a pervasive potential impact. The scientific soundness and the use of fundamental sciences are outstanding. 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.
ARL is making progress in its quest to become a premier research institution in the area of materials science. Several 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. 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.
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 open campus initiative can enhance collaborations.
Advances in biomaterials are essential for the application of biology to detection and sensing. The fledgling field of bioinspired and biomimetic materials will be an important source of inspiration and insight for the future materials scientist. This relatively immature ARL thrust is growing rapidly and shows tremendous potential. Because biology is a growth area, ARL has an opportunity to identify and recruit a critical mass of microbiologists and polymer/organic chemists and needs to be looking well into the future to create an integrated community of researchers.
Developing and improving energy storage devices and batteries will be essential if the future warfighter is to gain an advantage from the increasing availability of relevant technology. The same advances will also find applications across a wide nonmilitary spectrum. ARL’s research in this crucial arena is broad, covering different devices, fuels, and applications across a wide range of time and size scales. ARL needs to move aggressively to capitalize on internal and external advances in the energy and power arena. For example, the world-leading results on enhancement in QWIP efficiencies need to be translated into capability demonstrators for manufacturers and customers. ARL needs to work more aggressively to leverage external advances in silicon photonics, especially with regard to heterogeneous materials.
Engineered photonic materials are necessary for sensors, energy generation, and improvements to device performance—all essential to the future warfighter. The portfolio of the engineered photonics materials group shows a good balance of high-risk, longer-term work with nearer-term, customer-driven solutions or incremental but critical technology refinement. ARL needs to continue on its course to broaden modeling in support of a larger number of problems and applications. As a prototype for this expansion, ARL needs to look to its short-wavelength IR device modeling and optimization. The software tool set coming from this research is essential for designers, and it may also provide critically sensitive parameters for potential use in process control for commercial partners and suppliers of imaging solutions to the Army, which necessitates engaging with the manufacturers.
