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Materials Research

The Panel on Materials Science and Engineering at the Army Research Laboratory (ARL) conducted its review of ARL’s programs in energy-efficient electronics and photonics, materials for soldier and platform power systems, and quantum sciences at Adelphi, Maryland, on June 14-16, 2017, and its review of ARL’s programs in adaptive and responsive materials, agile expedient manufacturing, and high-rate materials and mechanisms at Aberdeen, Maryland, on June 5-7, 2018. This chapter provides an evaluation of that work.

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—“25 years out”—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 discover, innovate, and transition science and technology to ensure dominant strategic land power.

ENERGY-EFFICIENT ELECTRONICS AND PHOTONICS

The research into energy-efficient electronics and photonics is intended to address the size, weight, power, cost, and time (SWAPCT) of soldier technologies on the battlefield. The impact of advances utilizing optical equivalents, efficiencies realized through new radio frequency (RF) waveform and encoding strategies, and efficiencies for directed-energy applications are envisioned as being significant and important targets. Examples include escalation of electronic warfare technologies down to the individual unit and soldier in a continually contested RF environment, exacerbating current power challenges even more.

Accomplishments and Advancements

ARL has made excellent progress in enhancing its linkages, both throughout the Department of Defense (DoD) community with the Air Force Research Laboratory (AFRL), Naval Research Laboratory (NRL), and Defense Advanced Research Projects Agency (DARPA), and with Trusted Access Program Office semiconductor fabrication operations with the Joint Technology Office in high-energy lasers, and certainly with the academic and industry community through extensive research and development (R&D) collaborations. Many of these linkages have been enhanced by the Open Campus Initiative and have had visible, positive impact on the quality and potential for impact of the work program at ARL.

Posters were of high quality overall, on contemporary research topics, and with presenters who were generally highly accomplished and qualified in their research area. Several of the posters provided descriptions of centers (modeling and nanofabrication), which provide collaborative opportunities to drive quality of work and accelerate impact in the community at large. These centers benefit from critical inputs from ARL both in facilities and in unique cutting-edge Army technology challenges, and are expected to become valuable elements of the Open Campus Initiative.

Circuit Design

This is an exciting program with aggressive and bold goals squarely focused on critical soldier needs. This serves as an excellent example of how effectively a well-defined application driver can illustrate ARL’s strong potential for high-impact contributions. The ARL team is comprised of talented and well-informed staff, firmly plugged into evolving external commercial and government program capabilities. ARL has also identified, and is working with, the best partners in the field.

This team has developed and executed effective strategies both to access state-of-the-art commercial technology and to manage DoD organizational challenges associated with integrating security solutions as noted in prior years. These strategies now include actively working pathways to leverage truly cutting-edge technologies, including multiple DARPA programs such as the Diverse Accessible Heterogeneous Integration (DAHI) program to integrate complementary metal-oxide-semiconductor (CMOS) with GaN RF front-end chiplets, as well as with digital hardware implementation of next-generation soldier radio.

Ultraviolet Sources

Ultraviolet (UV) epitaxy and device development for efficient emitters and high-performance detectors has been a long-standing program at ARL with a record of strong accomplishments. This technology supports a range of identified objectives for soldier battlefield technology, with broader applications in agent detection; purification and decontamination; low size, weight, and power (SWAP) atomic clocks; and potential uses in quantum memory.

This project presented ARL advances in understanding physical processes in III-Nitride materials that affect UV emitter performance, with challenges including injection efficiency, poor carrier transport across hetero-interfaces, clarifying the relative impact of tunneling, scattering, polarization fields, and various contributions to carrier localization. The work is well supported by epitaxial growth, device processing, materials, and device modeling.

The ARL team has demonstrated creative application of the laboratory’s strong ultrafast spectroscopy epitaxial growth capabilities, utilizing specially designed structures incorporating sequential quantum wells with different emission wavelengths to capture time of flight in transport in pin configurations. The program has also advanced the development of materials diagnostics relevant to understanding the

unique transport and structural issues encountered in higher power AlGaN UV emitters engineered for 10 kV-level e-beam pumping. Time-resolved photoluminescence generally had good agreement with modeling on short time scales, but for longer time scales the model does not properly account for localization and the resulting strong temperature dependence. As a result, these new diagnostic techniques are revealing valuable insights into nanoscale compositional fluctuations and local monolayer quantum well thickness fluctuations that can contribute to recombination dynamics through localization.

Efficient High-Energy Lasers

Directed energy weapons using lasers are of great interest to the Army, but their size, weight, power, and cost (SWAPC) remains too high for the Army platforms. ARL has begun a program to develop a single double-clad crystalline core/crystalline clad fiber to replace the 58 glass fiber lasers at 1 kW each that Lockheed Martin Corporation utilized in a recent 58 kW laser demonstration. The crystalline fiber lasers have the potential for 10 times or more of the power output of glass fibers.

This is a creative program already demonstrating near-term success, with rapid and significant progress against numerically well-defined goals. The work has been effectively managed and executed using a combination of internal and outside resources combined with novel fabrication processes. A very high optical conversion of 78 percent from pump diode lasers to fiber laser output has already been demonstrated.

Diamond Power Radio Frequency Electronics

This new effort at ARL (fiscal year [FY] 2016 start) aims to use Raman, scanning electron microscopy (SEM), digital image correlation (DIC), X-ray photoelectron spectroscopy, and ultra-high-vacuum Kelvin probe to investigate optimal surface preparation and different transfer dopants. The team has defined quantitative metrics for the transistor performance it hopes to demonstrate (15 W/mm and frequency [f_T] and maximum oscillation frequency [f_max] above 100 GHz). ARL has made good progress in studying the hydrogenation properties of the diamond surface. A first-pass transistor has been demonstrated with reasonable current density (90 mA/mm) and with f_T and f_max around 10 GHz. This is an area of high potential impact to several DoD mission-critical systems. It leverages ARL’s strong metrology and material processing expertise, and there is high likelihood for device-level improvement and improved understanding of the surface transfer doping diamond mechanism.

Broadband Doherty Building Block

Power amplifiers (PAs) are needed to amplify signals for transmission in RF systems ranging from communications to radar and electronic warfare (EW). For many of these applications, it is critical to have power amplifier high-energy efficiency (PAE) over a wide range of input/output (I/O) powers. Unfortunately, most PAs suffer from dramatic reductions in efficiency under output power back-off (OPBO) conditions—when transmitting below peak output power. Doherty amplifiers seek to use a transmission line network to transform the load impedance to a variable-impedance seen at the output of the power transistor so as to draw more power from the amplifiers at low-RF output powers. Such an impedance transforming network has an intrinsic trade-off between PAE under OPBO conditions and signal bandwidth. ARL has demonstrated an asymmetric transformation network that improves the operating bandwidth (1.6-3.4 GHz) while limiting the reduction in PAE at OPBO. This work represents a solid—although incremental—engineering effort. The ARL effort has attracted DoD customer interest in further development and customization of this PA architecture.

Semiconductor Research Nanofabrication Center

The Semiconductor Research Nanofabrication Center (SRNC) comprises a broad and well-equipped 15,000-square-foot class 100/10 facility to allow both internal and external users to build a variety of needed prototypes and support research demonstrations. With extensive epitaxy, deposition, lithography, processing, and characterization tools, the SRNC facility not only allows rapid progress on in-house projects but also enhances collaboration with outside researchers, which helps to facilitate the Open Campus Initiative. One of the difficulties of operating such a facility is maintenance, and ARL has implemented small fees charged to most outside users to support this, as well as staffing the facility with professionals to maintain and help the scientists use the equipment.

Integrating Energetic Materials On-Chip

The team has developed a unique technology that uses porous silicon (which can be formed directly on-chip), as the basis of a highly energetic material. In brief, the porous silicon is first formed via an electrochemical etch, and then in-filled with a strong oxidizer. Upon triggering, the oxidizer and silicon react explosively. The effort at ARL started 10 years ago, and it is now supported by both ARL and customer funds. This is a project with clear Army relevance, and the team has a number of publications and patents demonstrating its leadership in this area. The project has matured to where it currently is more a 6.2 effort than a 6.1 effort, although there are still 6.1 aspects such as development of new fabrication methodologies. This is a clear example of the kind of project ARL needs to support.

Nano-Engineered Polymer Dielectrics

The investigators propose to use electron traps to improve the breakdown strength of polymers. Initial results indicate an increase in breakdown strength of 5 to 20 percent, but the physics behind the improvement is not understood.

Two-Dimensional Flexible Electronics

While not addressed in the poster, a key motivation for this work is that emerging two-dimensional (2D) materials, such as MoS₂, may be more amenable to enabling low-cost, scalable printing technology solutions for flexible electronics. These materials may also lend themselves to high-frequency, high-performance requirements. Work at this time on this early stage project is making credible progress on understanding the basic characteristics of interfaces on plastic films, comparisons to structures on SiO₂, and development and integration of low-resistance contacting structures into basic devices that can scale to complex circuits.

Modeling the Modulation Transfer Function in Infrared Focal Plane Arrays

This work is a direct example of the modeling strengths resident in ARL’s new Center for Semiconductor Modeling (CSM)—here applied to capture the impact of lateral carrier diffusion on the modulation transfer of HgCdTe and nBn focal plane arrays. Combining finite-difference time-domain electromagnetics with finite element method (FEM) carrier dynamic modeling, the project has demonstrated the ability to create accurate three-dimensional (3D) models for realistic pixel structures that would enable evaluation of design variants for ARL and its industry partners. The project will benefit from continued strengthening of U.S. Army Communications-Electronics Research, Development and Engineering Center’s Night Vision and Electronic Sensors Directorate, and industry partner engagement.

Ultraviolet Avalanche Photodetector Research

This study aims to improve SiC-based single-photon counting avalanche photodetectors in SiC, AlGaN/SiC, and InGaN/SiC for Army-relevant deep-UV and near-UV wavelength regimes not well addressed by commercial technologies that often require bulky photomultiplier tubes. The ARL effort spans from basic epitaxy techniques demonstrating 2D epitaxy of GaN on SiC using monolayer AlN buffers to high-gain (M > 1,000) SiC avalanche photodiodes with peak response at 226 nm to optimized quenching circuits for system applications. This is a well-connected effort with globally leading researchers, and the materials growth and device work are state of the art.

Efficient Erbium-Based Mid-Infrared Laser System

This project has demonstrated substantial enhancements in mid-infrared (mid-IR) laser efficiencies using a novel combination of cascade lasing and follow-on optical amplification using the higher energy 1.6 µm photon from the Er ion cascade laser to pump a Cr:ZnSe gain medium for the lower energy 2.7 µm mid-IR photon from the cascade laser. Making efficient use of the original 1 µm pump offers promise for substantial improvements in SWAP. Significant work lies ahead in addressing system complexity and assessing efficacy and power scaling qualities.

Materials Development for Piezo Microelectromechanical Systems

This program has developed a combination of basic, mission, and customer work to advance the development of piezo microelectromechanical system (MEMS) devices for RF systems and sensors across a variety of applications. The device work is of high quality, both in design and processing.

Miniaturized Shape Memory Alloy Microelectromechanical Systems Actuators

This program aims to assess the scaling potential of shape memory alloy (SMA) for MEMS actuators down to thin-film device dimensions. Reversible SMA effects were observed down to 100 nm thickness, and SMA actuation at 2 kHz for millions of cycles has been demonstrated. The basic concepts appear viable.

Challenges and Opportunities

Many posters presented only scientific or technological objectives without clearly articulating the key value proposition from an Army perspective, and some junior-level presenters did not seem to know or understand the nature or future potential of the customer funding. However, upon questioning, most presenters were able to address relevant applications issues and customer value proposition well. In relatively few cases, the ARL work was neither fundamentally state of the art nor strongly application focused, reducing potential for high impact. Management guidance and vigilance to address the risk/impact balance is important.

Circuit Design

Further discussions in the breakout sessions made it clear that the effort at ARL is quite deep and comprehensive, including all the circuit design using the various platform development kits. The

aggressive design for a digitally sampled passive mixer front end, developed in the context of highly effective academic collaborations, serves as a great target for DAHI technology. Both the 2.5D field programmable gate array (FPGA) integration and 3D DARPA chip stacking provide pathways to scaling to higher CMOS nodes in the future. This is a well-conceived program with performance targets that cannot be readily achieved in CMOS due to need for high Internet protocol 3 (IP3) intercept.

Ultraviolet Sources

Introductory comments illustrating the breadth and history of ARL work in this area would have been helpful to provide a deeper context for this work, and more discussion of how the impact of the work is supported through device modeling and design would be useful to see.

Efficient High-Energy Lasers

Since the number of pump laser diodes for the glass fiber and double-clad single crystal fiber may be comparable for both approaches, it will be prudent to develop a detailed SWAPC comparison between these two approaches to quantify the benefits, including trade-offs in required pump-module size, number, and cost; size and cost of multiplexers; and cost and number of wavelength multiplexed sources that can be supported in the achievable gain bandwidths. Such an analysis can help to quantify the expected laser system improvement and better inform the direction for the research.

Diamond Power Radio Frequency Electronics

Wide-bandgap (WBG) semiconductors have been important for both high-power and high-frequency applications from radar to power electronics. While GaN and SiC electronics have already had significant commercial and military impact, diamond is potentially the highest performance WBG material. Until recently, it has been difficult to effectively dope diamond so as to generate the mobile electrical charge needed for electronics. Recently, surface transfer doping—where a transfer dopant is introduced as a thin film on the surface rather than impurity atoms distributed within the bulk—has been demonstrated as an effective doping mechanism by a group in Israel. There remain several open questions regarding the surface and interfacial properties required for optimal doping.

Nonlinear Plasmonic Transport and Plasmonic Metamaterials

At high mobility, two-dimensional (2D) electron gases in semiconductors can exhibit plasma hydrodynamic behavior—most notably the existence of a high-frequency plasma instability. ARL has an exploratory effort that is considering potential applications of these plasma hydrodynamic phenomena. The researchers are solving the coupled balance equations (semiconductor analogue of the Navier-Stokes equations). The group is exploring a wide range of different phenomena from noise, to the boundaries between various transport regimes, to interactions with terahertz electromagnetic waves. As this is a small exploratory project, the group would benefit greatly from validating its model against existing experimental data in the literature, and using its model to explore a single promising application (for example, tunable terahertz detection or efficient terahertz generation) to design and rigorously model a proposed device design. With sufficient benchmarking against existing experiments and a compelling design, this work is likely to draw interest from several DoD customers. This is an off-center project with a potential for useful insight and real application.

Center for Semiconductor Modeling of Materials and Devices

A collaborative project stemming from ARL’s Open Campus Initiative, the Center for Semiconductor Modeling of Materials and Devices aims to leverage shared advances in concepts and implementation of multiscale modeling methodologies together with ARL’s powerful high-performance computing center. In addition to bringing together community scientific and numerical modeling expertise from academic, government, and industry leaders, the Center for Semiconductor Modeling of Materials and Devices will benefit from a host of highly relevant Army materials and device problems of interest. Workshops are already under way with diverse participation and encouraging collaborations developing. Focus will be required to achieve outcomes greater than the sum of the parts.

Metallocene Membranes

The team is investigating metallocene-based polymer membrane systems with suggested Army applications including coatings, thin-film electronics, sensors, and self-healing structures. The specific scientific or technological objectives for this project are rather unclear, and the poster did not articulate why metallocene membranes will offer advantages over other membrane systems. While there are interesting graphics in the poster illustrating how metallocene membranes could form into supramolecular structures with interesting morphologies, so far it is not clear that such supramolecular structures will form. The investigators need to define the key scientific or technological objectives, and then focus their efforts on meeting those objectives.

Nano-Engineered Polymer Dielectrics

The investigators plan a program of work to better understand if the improvement in breakdown is due to the electron traps, and also to start forming their own polymer dielectric films so that they can better control the dopant concentration and distribution in the polymer film. The investigators are rightly concerned, however, that making their own films will be challenging, and industry collaborations with experienced commercial polymer vendors may help address this. Also, the team would benefit from clear performance metrics and associated impact to define success and guide future investments and benchmarks for this applied research project.

Ultraviolet Avalanche Photodetector Research

The reseachers are addressing some important technical issues, but concerns remain about the influence of misfit stresses on device behavior.

Materials Development for Piezo Microelectromechanical Systems

This program could benefit from a stronger materials effort. There was no information on how the investigators will evaluate the basic mechanisms of irradiation damage in lead zirconate titanate (PZT) films, and the need for deeper capabilities in the structural evaluation of materials appears to be of paramount importance at ARL.

Miniaturized Shape Memory Alloy Microelectromechanical Systems Actuators

Gaining a full understanding of the NiTi material and possible phase transformations is still at an early stage. ARL may not have in-house capability to address all the technical issues in doing this, suggesting that external collaborations may be beneficial here.

MATERIALS FOR SOLDIER AND PLATFORM POWER SYSTEMS

The research programs of materials for soldier and platform power systems (MS&PP) are motivated by soldier battlefield power needs both currently and in the future. The research supports the tactical unit energy independence (TUEI) essential research area (ERA), with focus on unburdening the soldier by making power lightweight, providing power on-site, and diminishing power needs, all essential enabling factors in supporting soldier welfare and effectiveness.

Accomplishments and Advancements

The MS&PP portfolio presented for review included a large emphasis on electrical energy storage and conversion—for example, with batteries, fuel cells and capacitors, pyroelectric and radioisotope power sources, and others. Battery work, particularly focused on novel electrolytes, is a long-standing, high-quality research topic at ARL and is showing substantial returns. The work on high-voltage aqueous electrolytes could be revolutionary for battery technology for the Army and elsewhere. Work on radioisotope-based power sources is also noteworthy, having progressed very rapidly from concept to implementation. This work has significant upward potential. Fuel-cell and other electrochemical energy conversion work is of high quality, but it could benefit from finer definition of how it is distinguished from other work being pursued worldwide. Combustion catalysis work has clear relevance to Army needs and has potential to reduce mass and volume of energy carried into the field.

Work on pyroelectric materials for energy harvesting has progressed rapidly in a short time, particularly on the modeling front, for which the team is praised. Work on phase-change materials for thermal management is of high quality, with excellent synergy between experimental and modeling work, and is foundational to leveraging the rapid developments in WBG power semiconductors. The work is meeting the unique army needs for heat dissipation and shielding and is showing promise for use in pulsed power and directed energy applications.

Alkaline and Bipolar Membranes for Fuel Cell Systems

Army work on alkaline exchange membrane (AEM) fuel cells has adapted to include a major focus on bipolar hybrid membranes with an alkaline AEM bonded to an acidic proton exchange membrane (PEM). The overall goal is to support Army power needs in the field using transportable energy-dense fuels that are liquids or solids—for example, alcohols, or other energy-rich materials.

The team has done excellent work modeling continuum behavior associated with ion and water transport in membranes by using an overall cell voltage balance with membrane transport, pH variation, and simple reaction kinetics. These serve to address mechanical stresses that are caused by water generation at the AEM/PEM interface. The mechanical modeling of stresses at the interface is excellent and important. Overall, the integration with outside groups working on AEM devices is improved, with connections to Andrew Herring at CSM and Paul Kohl at Georgia Tech being particularly noteworthy.

Long-Life Isotope Power Sources

Beta-photovoltaics potentially offer new power cell design alternatives and performance regimes that may be advantageous for soldier applications relative to more thoroughly studied beta-voltaic power sources. ARL’s experimentally demonstrated power levels appear to be approaching useful performance targets even at an early stage, and the program is benefiting from quantitative 10 mW power generation as an intermediate goal.

In Situ Spectroscopic Study of Catalytic Oxidation of Propane on Platinum

The goal of this work, to reduce the weight/volume of logistics fuel and combustor needed to provide energy for Army missions, is well aligned with the ARL mission. Evidence of platinum oxide formation during catalytic fuel combustion is reported. The team shows broad understanding of the key issues in its research and has produced several high-quality publications. The team has appropriate equipment and facilities and is using appropriate models. The research includes both experimental and computational modeling aspects; the density functional theory (DFT) modeling with Delaware is a strength.

Aqueous Batteries and Beyond

The aqueous battery research on Li-ion batteries at ARL is of very high scientific quality, and is receiving international recognition for its highly innovative discoveries, carried out by a very small group. This is an “especially promising project” that merits strong internal support and that will attract strong external recognition. The development of the WiSE/polymer gel is an “outside the box” approach that could revolutionize the component materials and fabrication of future Li-ion batteries. The participants have a deep understanding of related work going on elsewhere, and they are in touch with the best battery research laboratories throughout the world. They are keenly aware of international research advances where there is currently very high quality work being reported elsewhere at a high rate of discovery. The ARL group has identified a niche based on clearly stated and justified Army needs and is pursuing it with high effectiveness. Its work applies broadly to the Li-ion battery field and is therefore likely to have a very high impact. The effort to date has generated the clear need to pursue applications at the 6.2 level, as well as additional fundamental studies at the 6.1 level.

The unique strength of the group is its combination of physical intuition (which guides it in exploring innovative materials for lithium battery systems) and computational abilities (which provide the means of exploring material behavior at the atomistic scale). The development of the WiSE/polymer gel that enables the presence of small amounts of water that enables higher voltage operation without associated stability aspects is the key discovery. The concept has been demonstrated in the laboratory in coin-size cells that have operated up to 4,000 cycles without degradation. The mix of theory, computation, and experiment is exemplary, and deserves recognition as a model for assembling multidisciplinary teams that explore with skill, and initiate research and engineering connections within the laboratory as needed to make rapid advances.

The laboratory equipment is excellent, and includes synthesis and characterization methods, small-scale cell fabrication and testing equipment, and high-performance computing. The research team has an excellent portfolio of skills, qualifications, and, especially, attitudes toward the different types of problems that arise, which span from fundamental science to engineering design and development.

Pyroelectric Energy Conversion

This project focused on development of pyroelectric PZT, PLZT, and HfZrO₂ materials, and has a balanced combination of experimental and modeling work, with modeling being completed with the University of Connecticut. The background and relevance were well presented with a good “survey of the landscape” of how the work relates to other research outside ARL. The goal is to increase the efficiency of conversion of heat to electricity. The team consists of a mixture of ARL investigators, students, and outside collaborators. The experimental characterization of the materials being done at ARL is commendable. Three journal papers have been published in high-impact journals, and the team appears to be heading in the direction of further unique results. The lead on this project appears to have a clear vision of where this project is going, and it has potential paths forward for future efforts.

Phase-Change Materials for Electronics Thermal Management

This project examines metal-based phase-change materials for transient thermal management in power semiconductor devices, to avoid overheating and possibly to include thermal signature reduction. The focus on new WBG power devices is essential to capitalize on the heavy investments the DoD has provided over the last several decades, and to help direct further device developments toward Army needs. The combined experimental/modeling approach is excellent and is lending insight into how new thermal management materials could be used in devices. The speaker was a very energetic researcher who understood the project goals quite well. The work was based on a well-cited body of knowledge, and had good collaboration with five outside academic and federal laboratory groups. The speaker felt that current materials were sufficient to satisfy project goals and new material development was not necessary for this project. The team is well qualified, with broad understanding of the issues and tradeoffs involved in the research. The experimental equipment, facilities, and access to modeling expertise have been used with skill to fabricate chips for experimental investigations, and to predict fast-transient behavior associated with reliability studies.

Modeling High-Voltage Wide Bandgap Insulated Gate Bipolar Transistors

This work is leading and essential. The work uses advanced modeling at the device structures level to predict charge distribution during steady state and, particularly, transient operation, and is well supported with experimental verification, requiring capabilities nearly unique to ARL. Partnerships with the device manufactures—for example, Wolfspeed—are well developed, and the work is well grounded on cited works. The research team is well qualified, and research results are well documented through publications.

Modeling of High-Voltage Wide Bandgap Gate Turn-Off Thyristor

The DoD has invested heavily in WBG power semiconductor devices for over two decades. The opportunities for high voltage, high power with SiC is well recognized. However, partnerships with device manufacturers, such as Wolfspeed, and further investment are needed to evolve very high power devices. The gate turn-off thyristor is being evaluated as one possible device. The device is targeted for high pulse power discharge applications, with focus on “turn-on” characteristics. Advanced modeling is developed for such transient characterization, and is impressively verified through difficult testing for operation into destruction. Weaknesses in device packaging for pulse power have been identified. Typi-

cal thyristors for high power use “press-pack” technology rather than wire bonds, as can be concluded by testing to date. A greater focus on packaging may be appropriate now that a datum is established.

The work is very advanced and essential with high returns in areas such as directed energy, with possible dual-use application in terrestrial power systems. The team is strong and takes advantage of unique pulsed-power capabilities of ARL, combined with semiconductor modeling. Because of the complexity of the device physics, the mix of simulation and experimentation is suitable and expected.

Additive Manufacturing for High-Temperature, High-Voltage Power Packaging

This work is exploratory in developing new thermal structures that can be created by additive manufacturing (AM). This gives ARL a beginning in the AM technology crossed-coupled with enabling new thermal management technology to be used in power electronics. The work applies sprayed copper for electrical and thermal conduction. The project applied current manufacturing, measurement, and modeling techniques, which were not innovative, but were appropriate. The project was clear in stating and accomplishing its goals and moving the technology forward.

(Al)GaN High-Power Electronic Devices

This device program targets vertical devices on native AlN substrates, with early work assessing commercially supplied AlGaN films and planar processed devices. Atomic force microscopy and X-ray diffraction with reciprocal strain mapping are employed for morphology and crystalline quality. The device effort is adequate and in a preliminary stage.

Infrared Plasmonic Effects on Electrochemistry

This project proposes use of IR irradiation of catalytic metal electrodes to cause removal of poisons—for example, chemisorbed CO—to produce higher activity catalysts. Preliminary IR spectro-electrochemical studies on formic acid oxidation at platinum revealed the due pathway reaction mechanisms in that at a low electrode potential. The reaction proceeds primarily via adsorbed formate as the reaction intermediates, while at a high potential, the reaction proceeds through adsorbed CO, and studies on the hydrogen evolution reaction (HER) reveal the effect of CO poisoning on the reaction rate. A broad understanding of electrocatalysis and the importance of eliminating poisoning is demonstrated. The project includes a good mix of theory and experiment, with the theory coming via collaboration with Boston University. The team is well matched to the challenge of studying IR irradiation effects on electrode reactions.

Radioisotope Power Sources

This poster describes work on the use of tritium sources in beta-voltaic power sources, in which beta emission from tritium excites electron-hole pairs in semiconductor p/n junctions located near the source, producing a steady-state electrical current across a potential difference. The project started as a recent seedling and has produced a 100 µW source, with plans to create a 10 mW source in the near future. The latter could provide sufficient output to power a small remote sensor for the lifetime of the radioisotope source, which for tritium is approximately a 12-year half-life. Work in progress seeks to improve output using microstructured columnar p/n junctions to produce and capture more e/h pairs per beta particle. The project is unique and seems well matched to Army needs. It includes a healthy balance

of theory/computation and experiment. Connections with prospective private-sector partners are in place that could produce field-deployable power sources in a reasonable time frame.

Electrolytes for Next-Generation Lithium Batteries

The search for improved electrolytes for Li-ion batteries is based on expanding the voltage window of operation, with special attention to oxidative stability of electrolytes at cathodes, while achieving temperature and safety requirements. This area of research is broadly pursued worldwide, and in a wide variety of defense and commercial entities. In this small 6.1 project, ARL is pursuing a niche for which it has the clearly relevant skill and background knowledge. The team has a clear understanding of Army needs vis-à-vis other major players with strong programs adjacent to the ARL research space; these include Navy, SAFT, and auto manufacturers.

The unique concepts, published elsewhere but reinterpreted for this purpose, include new solvents and additives that influence the solid-electrolyte interphase film at the anode as well as the cathode. Electrolytes prepared from organic sulfone solvents containing lithium fluorosulfonimide salts are of particular interest, but reference is made to nitrile and fluorinated solvents with and without additives, and solvents having high salt concentration. These directions are all sensible and worth exploring. The critically important laboratory equipment and facilities for carrying out this work are in place. The skill set of the personnel is well matched to the project. The team has many excellent collaborative arrangements in place, including with multiple Department of Energy (DOE) laboratories and some private-sector partners. This is excellent research that needs to continue.

There is a good match of experimental work with computational work based on molecular orbital theory to explore electrolyte stability. Patents are emerging for the sulfone electrolyte chemistry.

Li-Ion Hybrid Capacitors for Embedded Power

Innovation of new supercapacitor configurations for rechargeable high-temperature and long-life power applications are being investigated by combining the best components of two systems from existing high-energy and high-power systems. The work is supported by industrial collaboration for cell fabrication and by university collaboration for analysis tools.

The scientific and technological quality of this small-scale project is high, and discussion during poster presentations indicated a clear understanding of the background literature and underlying principles. The collaborative group has the equipment and research facilities needed for this work.

The scientific focus is on interface stability and electrolyte breakdown. These issues will be addressed based on foundation work published at ARL and elsewhere in a two-prong approach. One prong involves the use of improved electrolyte additives to reduce oxidation of the activated carbon electrode. Industrial partners will assist in procuring additive materials and establishing purity metrics. The second prong consists of creating a hybrid approach by combining Li-doped hard carbon electrodes with porous activated carbon electrodes. Experimental data based on these concepts have demonstrated improved performance for a range of additives and electrode structures, and have achieved performance metrics that approach the design goals required for the application.

Center for Research in Extreme Batteries

The scientific quality of the battery group at ARL is very high and is internationally recognized. The Center for Research in Extreme Batteries (CREB) provides a venue for a new level of collaboration

that will address gaps that are not addressed elsewhere in military/government/commercial sectors. The focus on “extreme” batteries is central to Army demands for long life, high power, wide operating temperature, and low maintenance. The growth of the battery field is very rapid worldwide, and the demand for experienced battery researchers far exceeds the number currently in the field. The CREB concept serves both to advance understanding of batteries in extreme environments as well as to promote through collaboration the expansion of skilled researchers in the field. The underlying science and engineering skills elsewhere have been recognized and are embedded in the initial steps of constructing the CREB. The range of equipment associated with the collaborators will enable a wide range of initial projects.

Mechanisms for Isomer Energy Release: Rhenium Campaign and Nuclear Excitation by Electron Capture

This project examined a very clever energy storage and release idea via stimulated nuclear excitation (nuclear excitation by electron capture) to produce metastable nuclear excited states that could subsequently be made to release their energy upon a second stimulation. This highly unusual process has the potential to allow for release of large amounts of stored energy on demand, which could be of significant benefit to the Army. The project is in very early stages with many aspects as yet unsolved, but it provides the potential for high payoff. The team is knowledgeable and demonstrates deep understanding. The project appears to be adequately resourced, although facilities were not seen and some, possibly much, of the experimental work was done off site. It is premature to suggest allocating significant new resources to the project, but it does provide a high-risk high-payoff route to energy storage that could be of high value to the Army.

This project has already produced some high-quality (very publishable) data on excited nuclear states and has demonstrated the existence of an excited long-lived state. Support of a small number of such outside-the-box projects is encouraged.

Design and Development of a Combustion-Based Portable Power Platform

This project demonstrates and examines a technology involving catalytic combustion of complex fuel—for example, JP-8 or jet propellant 8 (fuel)—for maximizing thermal-to-energy conversion. It uses mostly existing materials, computational resources, and approaches to guide small-scale research reactors toward thermal-to-energy conversion at high efficiency. The research team is well qualified with adequate resources. The project includes a good balance of theory and experiments. The project had well-defined goals of increasing energy density for dismounted soldiers. These goals were met by the small-scale research reactors developed by the team.

The experimental combustion reactor is configured to facilitate comparison between physical experiments and numerical computations. The fluid dynamics are modeled with an object-oriented finite volume approach that can be modified by code users. The parameters associated with about 2,000 reactions are available. Results to date serve to demonstrate that numerical experiments can be performed and compared with observations from physical experiments. The combined approach is structured with the goal that it may be modified as new experimental results become available. The results to date have identified the need for better high-temperature emissivity data. Future studies with this approach can be used to guide scale-up and associated choice of materials and manufacturing methods, to optimize flow geometry and catalytic performance to achieve reduced heat loss and improved efficiency.

The plan is to create an open-source code so that others beyond the creator can make adjustments. With these improved design tools and insights, the project may be ready for transition to a technology development team.

Challenges and Opportunities

The review of the combustion catalysis work was too simplified to show the strong impact, and would benefit from substantial clarification of the linkages to other areas being pursued—for example, JP-8 production and use. A clearer link between catalyzed combustion and electrical energy conversion—for example, using thermoelectric materials—would help one see more clearly the system-level benefits of this approach to providing soldiers with power.

A suggestion is that future presentations provide modestly more detail in how the subsequent selected talks link with the broader division activities and that they identify research expectations.

Alkaline and Bipolar Membranes for Fuel-Cell Systems

The bipolar membrane configuration is widely known in electrolysis cells, but less studied for galvanic cells such as fuel cells. It offers the advantage of acidic operation on one side and alkaline operation on the other side, which can allow for separate optimization of catalysts for anode and cathode, and for alternative water management schemes. A key limitation of such cells is the AEM/PEM interface, which can be mechanically unstable and could provide undesired high interfacial impedance.

The project has not pursued much experimental work addressing the structure or properties of the various components that compose the AEM/PEM interface, which is critical to the operation of bipolar cells. More work is needed here focused on this interface, with attention to including quantitative metrics and accelerated testing protocols.

Project work in this area is somewhat hampered by a lack of clarity on choice of fuel. Fuel-cell materials development work follows quite different paths for different fuels and different scales of operation. For example, it is not clear that developing fuel cells using hydrogen as fuel will be helpful for creating fuel cells using methanol as fuel. This project has pursued little work on methanol crossover, which is a deficiency, since crossover is perhaps the leading cause of inefficiency in direct methanol fuel cells. The project could be managed more easily if there was clarity on the fuel being targeted.

The team would benefit from a clear description of their “niche”—for example, how they differ from the many other groups worldwide that are working on AEM-based electrochemical devices. The team’s niche may be the bipolar membrane approach, and if this is the case, they could work toward demonstrating clear advantages of that configuration relative to conventional PEM or AEM approaches, especially for Army applications. Doing so would establish ARL as a significant player in this field.

Long-Life Isotope Power Sources

More effort toward detailed analysis of phosphor photon yield optimization and photon collection efficiency will be essential to understanding and achieving ultimate performance potential.

In Situ Spectroscopic Study of Catalytic Oxidation of Propane on Platinum

It was not clear whether the platinum oxide formation would be a problem at the temperatures at which the cell would operate. Potential problems with sulfur in logistics fuel are likely and were not

addressed. A specific examination of sulfur effects that are likely to be important in catalytic combustion technologies is needed.

Aqueous Batteries and Beyond

Continued fundamental work is justified for the search for alternative salts, for high-throughput computational work to identify regimes of stability. The concept could be applied to other metal systems, such as sodium, that would reduce cost. Continued applied work is needed to scale up the cells, and to explore new methods of fabrication with the participation of industrial partners. The computational platform could be expanded so that others can use it and modify it for numerical studies on additional design and engineering. Important aspects of this research include high-speed computations suitable for optimization, identification of the most sensitive parameters, uncertainty quantification (UQ), and tracking of errors bars that arise from experimental as well as numerical sources associated with the prediction of behavior.

Phase Change Materials for Electronics Thermal Management

A suggestion on further direction is to provide more guidance to material formulators in addressing higher temperature capabilities, and to provide a benchmark with non-phase-change materials. This project crosscuts many technologies, such as electronic packaging, electronic materials, power electronics, and thermal systems, which is a technical strength. However, outside recognition of the work will be difficult since it does not align with a singular identifiable research area, such as batteries. The general difficulty in recognition of multidisciplinary results also applies to ARL’s broad spectrum of contributions. A suggestion is that ARL consider hosting a yearly “summit” that provides an open forum for display of its research endeavors, and this may tie well with an “open campus” open house. This also aligns with the reinitiation of the Interagency Advanced Power Group.

Modeling High-Voltage Wide Bandgap Insulated Gate Bipolar Transistors

The DoD has invested heavily in WBG power semiconductor devices for over two decades. The opportunities for high voltage with SiC are well recognized. Commercial market forces are driving lower voltage—for example, 600 V to 1700 V—WBG devices, particularly metal-oxide semiconductor field-effect transistors (MOSFETs). However, DoD investment is needed in continued manufacturing development of very high voltage, high-power insulated gate bipolar transistor devices, which are recognized to best perform over MOSFETs in the 20 kV to 27 kV range. The Army has a need for high-voltage devices for directed energy and advanced sensing.

Additive Manufacturing for High-Temperature, High-Voltage Power Packaging

A key technology development with new materials and processes is AM. Application of AM to power electronics is in its infancy and is an area that needs aggressive pursuit. A growing proportion of electrical power used in the battlefield is processed through power electronics. Concurrently, AM has been identified by the Army as a critical need for use in “manufacturing and remanufacturing at point of need” in the battlefield. As such, developing AM processes in creating power electronics modules provides an early technology development to support a critical Army need. A suggestion going forward is to also consider long-term integration of sprayed ceramic into such structures as a replacement to ceramic plate to further enhance thermal performance.

(Al)GaN High-Power Electronic Devices

The material effort needs to be strengthened and the coupling between device and materials teams could be stronger. This will become very important when devices may not work as designed, and the team will benefit from deeper familiarity with the published literature in this field.

Infrared Plasmonic Effects on Electrochemistry

The anticipated effects of IR irradiation on electrocatalytic activity are still somewhat speculative, since IR plasmonic experiments have not yet been done on catalytic electrodes but are planned following fabrication of IR-plasmonic-active electrodes by microfabrication. The team might benefit from collaboration with condensed-matter physicists, who could provide a more detailed description of the effects expected from IR irradiation of electrodes. It is not fully clear which groups if any the team is collaborating with at ARL on this project. The team would benefit from a close collaboration with other electrochemistry experts at ARL focusing on electrocatalysis for energy conversion.

Electrolytes for Next-Generation Lithium Batteries

Consideration of a wider range of additives, electrolyte molecules, and temperature is needed to guide discovery of more stable multiple salt/solvent systems, and to understand how anions serve to disrupt water and shield electrodes from degradation. Such exploratory studies also represent a large computational demand that exceeds the availability of current personnel within the Materials Research Campaign. While there are interactions with computing personnel elsewhere at Adelphi and Aberdeen, the direct day-to-day contact between experimentalists and computational people could be improved.

Li-Ion Hybrid Capacitors for Embedded Power

Work on improved lifetime is a clear focus for future work. Understanding of the role of additives at the carbon-electrolyte interface in the activated carbon electrode is clearly warranted, although that represents a difficult experimental challenge owing to the hidden, difficult-to-access nature of the interface. Auxiliary experiments in a simpler system than the entire system may be considered in order to develop improved intuition to guide system improvements in the components of the system. The molecular structure of additives at the solid-electrolyte interface represents an enormous field of research with applications in many disciplines. The likelihood of drawing significant inspiration from seemingly distant applications is therefore high.

Center for Research in Extreme Batteries

CREB is meant to bring together industry leaders that will contribute to the research work of the Center via membership fees to the Center. There are other large-scale collaborations in the battery field—for example, supported by other government (DoD, DOE, National Science Foundation [NSF], Joint Center for Energy Storage Research) and commercial collaborations—from which examples of excellent management structure and practice could be beneficial to the emerging CREB program. The linkages created by CREB are on the critical path for achievements that would not be possible otherwise.

The communication between collaborators is critically important in broadly configured programs such as this. Once the initial stages of the launch are completed, it will therefore be important to review

the organizational structure to identify opportunities for sustained communication among partners. For example, the role of postdoctoral researchers is often critical to facile flow of information between collaborating units through seminars on common scientific interests and training sessions on shared equipment.

The speed of reaching cross-institutional agreements is important in such cooperative research and development agreement (CRADA)-like interactions. For the CREB, the headquarters resides in the University of Maryland, but it took a very long time to get contracts in place. Meanwhile, the focus on industry may move on, and opportunities are thus lost. The sharing of experience among early adapters of such programs represents a critical path issue.

Gas Removal and Mathematical Modeling for Operating Thermal Batteries

This project is long-standing and requires some new concepts in order to move it forward. The problem is that the investment in this project continues not to resolve the issue of hydrogen removal from batteries. Apparently, the problem is a result of very complex chemistry that is difficult to untangle, and the investigator mentioned having a difficult time finding collaborators that can support the effort. It is perhaps time to reconsider where this effort is going.

QUANTUM SCIENCES

Quantum sciences is a new program area of high scientific quality and is well aligned with the long-term goals of ARL’s mission to provide the Army of the future with clear tactical advantage. It is anticipated that quantum sciences will provide game-changing capabilities for command, control, communications, computers, intelligence, surveillance, and reconnaissance (C4ISR) for the Army of the future.

Accomplishments and Advancements

In just a few years, the quantum sciences program has attracted outstanding investigators, driven in part by their membership with the University of Maryland and the National Institute of Standards and Technology (NIST) in the Joint Quantum Institute. ARL has done an outstanding job of bringing in a strong team of a well-established, mid-career leader with extensive knowledge of, service laboratory experience with, and connections to military systems and needs, and a second, well-established academic who is a recognized quantum sciences leader. In the past 2 years under their leadership, an excellent research team of six experimentalists and one theoretician has been established.

In addition, ARL has built impressive research facilities with a focus on three areas—well-known rubidium atom traps to provide an experimental facility to investigate new ideas in quantum information, entangled states and their deterministic generation, detection, and coupling; investigation and characterization of related systems of single atoms in a solid-state host that could be operational at room temperature and be far more easily field deployable; and investigation of newly discovered native defect systems in WBG semiconductors: n-vacancy in diamond and the C-Si vacancy pair in SiC.

These are attractive candidate systems because they have in-house materials growth facilities to produce host materials with unique doping and layered structures and they are photonically active in the wavelength region of long-distance communications fibers. ARL has built up excellent characterization facilities to determine coherence times and manipulation of quantum states in these candidate materials systems.

The quantum sciences program is an outstanding example of vision for a future Army need—defining specific areas that are Army unique, are not well covered by universities or national laboratories, and are hiring recognized leadership and creating a well-funded, exciting program that has attracted an outstanding group of young scientists and postdoctoral researchers.

Hybrid Quantum Systems for Networking and Sensing

This presentation provided an excellent description of specific projects on developing a quantum repeater and ultrasensitive quantum sensors. The focus is on novel materials for photonic-based sources and detectors. The laboratory for experimental work is impressive and provides excellent facilities for growth of novel materials and experimental characterization of new quantum defect materials and quantum sensors. The nitrogen defect in diamond is the most widely studied solid-state quantum structure, but it is difficult to scale up. ARL is establishing in-house materials growth facilities for the new V_Si-V_C divacancy in SiC. This is a particularly attractive system, as both single-isotope Si and C are reasonably available, and this could produce a quantum state with much longer lifetime and possibly greater ability to set and read the state variable. This is an area in which ARL is truly leading the field and, if successful, would be a real breakthrough for a broad range of quantum-based sensors and communications.

Divacancy Modeling in Silicon Carbide

This is a strong theoretical project on basic study of the electronic structure of both the ground and excited states of the V_Si-V_C divacancy in SiC. ARL has excellent supercomputer resources for this effort. In addition, ARL is establishing hot wall chemical vapor deposition growth facilities in which to prepare unique SiC materials and has optical characterization facilities to thoroughly characterize this quantum system.

Rare-Earth Solids for Long-Lived Quantum Memories

The principal investigator (PI) and postdoctoral researcher have set up a very impressive experimental characterization facility to investigate different rare-earth atoms incorporated into a variety of solid-state hosts for both quantum memories and quantum information processing. Current focus is developing optical addressing and readout of known rare-earth crystal systems with plans to expand search for doped materials with longer spin-state coherence times. The researchers are also well coupled into the theoretical effort in identifying quantum states with better external perturbations and coupled interactions in the excitation in an ensemble system with disorder.

Entanglement Swapping of Two Arbitrarily Degraded Entangled States

The goal of this effort is to characterize the reach, rates, and entanglement quality of entanglement distribution networks in order to design optimal quantum networks for different functionalities. This is a project of high scientific quality and is well aligned with the long-term goals of ARL’s quantum science program to create, manipulate, and use quantum entanglement over long distances for quantum networks. The effort has resulted in publications in the peer-reviewed literature and in conference proceedings. The scientific strategy for future work has been clearly articulated.

Exploring Unique Electronic States at Topological Insulator-Superconductor Interfaces

This project examined PbSnTe Josephson junctions as semiconductors and topological materials. PbSnTe exhibited proximity-induced superconductivity for the first time, although this effect has been seen in other materials. The team demonstrated broad understanding with skills well matched to the project challenges. The project is mostly experimental with little theory or modeling, and could possibly benefit from some application of theory. Proximity effects on electronic behavior may have application in quantum computing and related devices, so this work provides materials support for the discovery-based effort on quantum devices. Most likely a high-impact publication will result from this research.

This is definitely a hot field, with a recent high-profile announcement of discovery of the first Majorana Fermion. There are good potential Army applications if ARL can demonstrate the novel quasi-particle physics that shows Qubit immunity to decoherence in such a quantum state. ARL has a good theoretical effort and a unique molecular beam expitaxy (MBE) growth facility to prepare near-perfect planar superconductor/topological material interfaces required to observe a Majorana Fermion, and has observed the first proximity-induced superconductivity in a superconductor/PbSnTe topological material interface. This group has made three presentations at recent high-profile meetings, and with future publications of their recent work and unique materials, growth capabilities could become among the most highly recognized research groups in this field.

Challenges and Opportunities

Among the applications addressed by the quantum science program is absolutely secure communications, a major challenge for a highly mobile and ever-changing battle scene where the opponent very likely will be able to receive one’s communications and thus unbreakable encryption is essential. The ARL team has made a significant effort on different approaches for a “quantum repeater” to extend the range over which secure communications can be assured. The quantum sciences team is also working on the full range of required components—quantum memories and single- and entangled-photon sources and detectors, with a balanced approach looking at both ensemble and single-atom phenomena, and within each of these, well-studied neutral atom and trapped ions to enable investigation of new, Army-specific applications; and in parallel, the investigation of new materials that could provide far more easily field deployable communications and sensor systems that are well beyond basic scientific knowledge and laboratory demonstrations to enable their incorporation into Army applications. A second important area is ultrasensitive gravitational and magnetic gradient sensors. There is a good balance of theory and experimental work with investigation into quantum oscillators with entangled sensors versus independent atom oscillators that could significantly change the scaling of sensors and improve sensitivity for quantum metrology.

There are three areas in which quantum sciences and entanglement can offer tactical advantages for the Army—transfer of quantum information over long distances in a quantum network utilizing both fiber and free space communications; precise timing, position, and navigation systems; and ultrasensitive gravitational and magnetic field gradient sensors. These are areas in which a small team at ARL can make significant and scientifically leading contributions specific to the Army needs. This would avoid relying on the much greater push of major university and commercial research organizations in quantum computing.

Quantum Networks: Many-Body Physics, Emergent Phenomena, and Applications of Distributed Entanglement

This presentation described a theoretical project on quantum oscillators and basic condensed matter physics and noise limits in quantum systems and particularly their application to entangled quantum sensors. Quantum systems based upon neutral atoms, trapped ions, and tunneling are coupled over distances of a micron or less. The theoretical modeling of entangled quantum oscillators over greater distances will be required to achieve useful sensors based upon entangled states.

Divacancy Modeling in Silicon Carbide

This is an exciting new system that could be far more scalable and field deployable than other solid-state quantum systems, but it is highly dependent upon the energy difference of ground and excited states and their immunity from outside perturbations and access for writing and reading entangled states. It is critical to understand these basic properties of the system before investing too much experimentally.

Entanglement Swapping of Two Arbitrarily Degraded Entangled States

The development of robust quantum networks is challenging because nearly perfect entanglement is required for many quantum networking functionalities. To address this challenge, the investigators’ team has developed models and theories for the analysis of entanglement decoherence—both in transmission and in networking operations—and established tools to implement engineering trade-offs. The result of these analyses provides a comprehensive method for analyzing entanglement swapping in quantum networks.

ADAPTIVE AND RESPONSIVE MATERIALS

Adaptive and responsive materials is an emerging and promising crosscutting ARL program. The targeted applications span a wide range of Army needs such as novel alloys for ballistics, formability of lightweight alloys, new coatings, multifuel capable hybrid propulsion, and tribology for future rotorcraft propulsion. The program has two thrusts: energy coupled to matter (ECM) and materials for soldier augmentation.

The overall objective of the ECM program is to expand the processing parameter space to enable discovery and processing of metals, ceramics, and polymers with controlled microstructures not accessible through conventional processing. A focus of ECM, ECM metals, is to couple external fields to materials processing and thereby tailor microstructures to improve robustness and formability of metals. The program consists of a strong mesoscale modeling component used to discover new microstructures, coupled with experimental input and validation.

The overall objective of the soldier augmentation program is to improve protection, increase lethality, and reduce the burden on the soldier. The technical goal of the program is to bridge the gap between the current state of the art and the needs of the army in the field through new materials and tools to augment the capabilities of the unmounted soldier.

Accomplishments and Advancements

Both thrust areas are making excellent progress in addressing critical mission needs. The ECM has begun with the strong fundamental science research necessary to develop solutions for in-use Army applications, and the soldier augmentation program has shown its mission relevance.

The ECM team is highly qualified for both simulation and experimental work. ARL is well equipped for external field studies and applications and has highly knowledgeable and well-trained staff. It is commendable that the present equipment continues the tradition of custom-designed field-assisted machines for initial basic research and future scale-up using a 50-ton press.

Researchers of the ECM are making progress with three computational approaches, which demonstrated dynamic external fields applications for novel microstructures (e. g., texture, phase and grain-size evolution and distribution; new nonequilibrium phase formation; and grain boundary engineering). The first experimental validation of simulation in magnetic field electrodeposition of copper demonstrated the formation of an unconventional, nonequilibrium copper hydride.

Under the soldier augmentation thrust, three significant accomplishments are noted: progress in the development of the third arm prototypes, demonstration and characterization of rate-activated tether (RAT) prototype materials based on shear-thickening fluids, and the initiative to develop complex actuator fibers through a combination of printing and drawing.

Energy Coupled to Matter

Energy Coupled to Matter: Metals

This project focused on improving an industrially relevant process—electrodeposition—by applying external magnetic fields. One unique microstructure was demonstrated in electrodeposited Cu, which formed an unexpected copper hydride phase. Other significant results were the changes in thickness and morphology of copper deposition and increase of twin fraction with magnetic field. The work would be strengthened by understanding microstructural results and linking the modeling guided experiments to specific Army applications.

Scientific Understanding of Loss-of-Lubrication Mechanism in Rotorcraft Applications

This project was focused on examining tribology within the engines of vertical lift machinery, such as helicopters. The objective is to enable these machines to function for 30 minutes after the loss of oil lubricants. The work includes fundamental and applied aspects and is of high quality. It would be enhanced through the incorporation of computation and data informatics.

ECM Ceramics

The objective of this work was to enable field-assisted processing of ceramics to produce microstructures not accessible through conventional processing. This work is of high technical quality. The possibilities of stabilization of new material phases and densification at reduced temperatures have demonstrated an exciting potential advantage offered by these processing capabilities for specific Army needs.

Soldier Augmentation

Nanogalvanic Aluminum Alloys for Instantaneous Hydrogen Generation for Soldier Power

This work uses aluminum nanograin micron-size particles to generate hydrogen through a water-splitting hydrolysis process. The project is of high technical merit with direct applications for point-of-need power generation. This is a project at a high technology readiness level (TRL) and the proof-of-principle results show great progress for this concept. Applications to generate hydrogen for large-scale power generation are planned for demonstration in the field. This project is well aligned with the strategic priorities of ARL.

3D Hybrid Electronics Programs

The objective of this project is to develop electronics and sensor technologies integrated with 3D printed (plastic) structures using AM processes. This is a project of high TRL, and the team has successfully demonstrated the integration of electronic devices and sensors with 3D printed structures.

Embedded Self-Sensing Composite Materials for Army Vehicle Platforms

This project was focused on developing multilayered systems of carbon nanotubes, MXenes, and other 2D materials to enable the production of next-generation sensors. The effort is in the preliminary stages and is promising. The ARL staff are collaborating with top researchers at academic institutions.

Size Effects on the Apparent Thermal Conductivity of Metal and Amorphous Metal Oxide Thin Films

The objectives of this work are to develop next generation of smaller and lighter long-wavelength infrared (LWIR) detectors by gaining insight into thermal properties of metal and amorphous thin films. The project outlined the successful development of time-domain thermo-reflectance technique at ARL to characterize dependence of thermal conductivity on thickness and density in ceramic and metal thin films down to 1 nm. Ru metal was demonstrated to be the next suitable material in LWIR for its combination of thermal conductivity and susceptibility to size effects. Further development of detectors will be done by an industry partner. The basic research work would have been strengthened by addressing the understanding of thermal properties at small scales.

Defect Chemistry and Strain Engineering Control for PiezoMEMS Filter Performance

This project is centered on enabling low-power RF devices by studying and processing active and adaptive piezoelectric components. The group achieved good results with new PZT thin film that reached about halfway toward the piezoelectric targeted metric. Close collaboration with universities is a plus. The work can be strengthened by a better understanding of materials structure and performance.

Challenges and Opportunities

The ECM metals team is taking the appropriate steps toward demonstrating the applicability of external fields to create and manipulate microstructures. Initial computational results show the merit of field application in microstructural modifications; a clear unified scientific strategy toward processing-microstructure-property coupling is still to be developed. The challenge is to articulate the vision and strategy of the scientific approach. There is opportunity to better design the computationally guided microstructural targets with emphasis on guiding hypotheses for final desired properties specific for Army applications.

Once the proof-of-principles concepts have been demonstrated and applications have been chosen, the opportunity is to develop appropriate scale-up strategies. A major challenge will be in scaling the external field processing to large-scale components and structures.

Development of a roadmap to process these unique microstructures by field processing can provide a basis to better define strategic priorities within the ECM program.

Opportunity to impact the field hinges on focusing on the identification of materials whose processing is uniquely enabled by these new capabilities and systematically organizing the data into a state-of-the-art database, as well as publishing in relevant and high-impact publications and conference presentations.

Adaptive and responsive materials for soldier augmentation has been largely focused on the bio-mechanical aspects of the wearable devices. The effort is challenged by the fact that less time has been spent on targeting unique materials that need to be developed to enable the applications.

One opportunity is to enhance the third arm by utilizing living hinges that more closely mimic living joints and move away from clunky, heavy joints. There is also an opportunity to more generally move away from discrete devices and move into distributed items such as complex functional fibers that function as actuators similar to muscles in wearable robotics that would enable more complex movements.

In addition, there are opportunities to integrate RATs into clothing and equipment to enhance soldier protection as well as enhance the safety of Air Force pilots who are violently ejected from aircraft.

There is a valuable opportunity for synergy within the adaptive and responsive materials program for the development of enhanced composite materials. These materials can incorporate sensors to function more like living muscles by drawing on the work in the vehicle applications. Examples include incorporating sensors composed of materials that change properties under stress and developing innovative techniques for making composites.

Energy Coupled to Matter

ECM Ceramics

The effort would be strengthened through the integration of simulation and experiment. This project comprises processing, characterization, and modeling and offers a valuable opportunity to align with state-of-the-art standards in data management for materials design from an early stage of the project. The project researchers can draw on ARL work on data science and database structures and techniques at nearby institutions, including NIST.

Simulation of Field Responsive Polymers

This project was focused on the atomic-scale and coarse-grained modeling of polymers. The effort included methodology development and the application of the methods to specific problems associated with polymer structure-property relationships. Connections to experimental work were not apparent from the poster, which would have been strengthened by more fully presenting this connection.

Soldier Augmentation

3D Hybrid Electronics Programs

The technical challenges associated with this project include the lithographic deposition of conducting metal lines on AM 3D-printed support structures for specialized applications involving high-g environments. Future directions could build on these initial integration successes and open the design space toward increasing levels of complexity regarding to the functionality of electronic and sensing devices.

AGILE EXPEDIENT MANUFACTURING

To win in the deep future operational environment, the Army will need to be more adaptive, more expeditionary, and have a near-zero logistic demand. The objective of this key campaign initiative (KCI) is to develop novel adaptive and rapid manufacturing technologies to enable deployable materials-on-demand capability. The ability to expedite fabrication and part certification in the field will enable operational readiness concomitant with the capability to counter new threats with point-of-use solutions. This is expected to dramatically reduce the logistic tail and mitigate uncertainties for expeditionary maneuver. The materials-on-demand capability enabled through discovery, innovation, and transition of synthesis, processing, fabrication, and manufacturing science methodologies will lead the way toward a new paradigm in flexible, rapid, low-rate production for the factory of the future. Science and innovation in synthesis from reclaimed, renewable, and indigenous resourcing will enable cost reduction and the capability to mobilize this manufacturing technology with minimal materials burden to support expeditionary operations on location and in time.

The overriding goal of the Agile Expedient Manufacturing KCI is to enable adaptive, rapid, and low-cost production of consumable parts that are easily qualified for service through development of novel synthesis and processing capabilities. Manufacturing capabilities developed through this effort are expected to enable 3D additive approaches that facilitate, for example, real-time alloying by design simultaneous with near-net-shape fabrication, which shorten times from ingot-to-components.

Accomplishments and Advancements

Several projects reviewed represented pioneering research that have wide-ranging implications for science and innovation in synthesis. Examples include making batteries safe and flexible with water; studying microbial reactors (transforming indigenous feed stocks to functional materials, and understanding and controlling living/inorganic interfaces to enable reconfigurable switchable materials); molecular optimization for optically adaptive materials; graphene for reduced electromagnetic interference applications in flexible electronics; and covetics (nanocarbon metal composites).

The unique projects being developed in the area of agile platforms that are mobile and that have the capability to address the challenges future army deployments will face are commendable. Significant advancements in this area enable rapid manufacturing capabilities with point-of-use materials. The work focusing on novel feed stocks from reclaimed materials for additive manufacturing of polymers and the reclaimed metals and indigenous materials for manufacturing at the point-of-need are examples. This is expected to dramatically reduce the logistic tail and mitigate uncertainties for expeditionary maneuver.

The noticeable increase in partnerships with academia, other government laboratories, and industry is very encouraging. This enables leveraging capabilities and shared resources within and outside ARL, with many open campus collaborations and multiservice community involvement. A collaborative work environment such as this is needed to advance further and faster. It is clear that there is a supportive and collaborative work environment. There is an appropriate division between fundamental research and applied research, as well as engineering design projects. Decisions on funding the different project levels seem to work well and are connected to well-defined needs. The facilities capabilities are excellent, especially in the area of polymer synthesis and processing.

Additive Manufacturing

The area of AM is rapidly evolving and involves understanding and integrating knowledge across materials, design, and manufacturing. ARL has an impressive set of many different AM machines and capabilities. Notable achievements include recognition in Manufacturing Science Magazine for cold spray addressing hexavalent chrome replacements.

Agile Additive Manufacturing for Polymer Materials

The polymer materials effort in Agile AM includes several approaches, including extrusion, photo curing, and powder bed fusion. There is good balance and breadth in this very important, fast-moving area. ARL’s efforts on polymeric materials with good processability for high-rate deformation or high-impact situations are unique and at the forefront of research. The elements of molecular simulation, continuum models, synthesis, characterization, and processing have been woven into a tightly integrated organization that has already produced impressive achievements. Comprehensive efforts of this nature, which include synthesis and mechanical studies, are rare, and found at only a few laboratories around the world. Examples include the design and creation of thermosets with mechanical properties that are vastly superior to those of existing materials. These materials could find their way into production thanks to the strategies adopted by ARL researchers (e.g., in-house synthesis, interpretation of experiments based on molecular models, and production of large quantities for meaningful testing in relevant situations). Another achievement is the development of more forward-looking design of materials that incorporate required mechanical attributes and light-active moieties that can impart local heating or plastification on demand. The more recent efforts to formulate and characterize hybrid materials that incorporate inorganic or metallic elements into polymers are equally promising and ambitious. Much of the success of these efforts can be attributed to the leadership of the group, whose vision is apparent in key aspects of the overall polymer research portfolio, including recruiting the necessary expertise, acquiring the necessary equipment and laboratories, and, importantly, identifying key areas of opportunity for ARL. The work on the deformation rate dependence of polymer networks is also remarkable. The researchers are asking the right questions and have a nice combination of very basic work and are yet able to tie to V50 ballistic limit testing and the practical world of the warfighter. The topology optimization aspect of computer-aided design appears strong and well connected to the field. Similarly, work with RATs has

a great combination of basic polymer rheology and physics relating to very applied, meaningful Army needs. Simply put, the polymer program is second to none in these areas.

High-Strength Steels

Significant advancements have been made in the area of high-strength steels. One example is addressing the erosion issue of the impeller blade on the Abrams tank. Excellent use of advanced energy materials in advancing propellant design and manufacturing to enable better propulsion metrics is notable. Future research will use purposefully designed surface-to-volume ratio using particle sizes and void inclusion and even gradients in composition versus radius and position.

Challenges and Opportunities

As anticipated, there are many opportunities for further improvement in additive manufacturing research. These include intelligent design of components and systems, control and correction during build and nondestructive evaluation component certification, and development of a master AM database. A comprehensive understanding of battlefield environment and conditions is required to develop highly mission-oriented, mobile, agile platforms. ARL’s ability to continually talk to the customer in the field (soldiers, contractors) about what breaks and what wears out, and to identify long lead time critical parts is a big advantage in bringing AM to fruition and value to the Army. This information and feedback can guide researchers on how to best decide what tools to provide to “make it yourself,” and help researchers to focus and demonstrate early success and build up customer buy-in. There has been some initial success with the U.S. Marine Corps operators, demonstrating use of recycled plastic feedstock with 3D printers.

In order to help rapidly bring AM to the field, advances in system and component design for recycling that supports the cradle-to-cradle concept is a great opportunity. This is especially so with equipment and soldier kit battlefield issues and the ability to leverage scrap (e.g., pallets, water bottles, and damaged vehicles). This also drives demand for intelligent design that involves materials and manufacturing processes and an understanding of the morphologies and chemistries that will work with the different additive manufacturing processes that can be theater-deployed. In addition to evolving current manufacturing processes, AM can be applied to the ever-increasing number of older legacy systems and components that fail and will no longer be stocked by original equipment manufacturers. AM will become ever more impactful as equipment ages.

The detection of manufacturing flaws (these range from voids in metals to unreacted components in polymers) and control and correction during build processes is an area of opportunity to be developed. This will be enabled with near-term capabilities provided by machine enhancements that open up the machine process parameters. The work on understanding the impact of scan strategies is an important element in the broader understanding of variations that influence material and structural properties.

It was evident in the results presented on dynamic polymers that reaching very high chemical conversion in photo-curable materials will be key to achieving properties. In situ monitoring of as many relevant parameters as practicable is important. Everything from fidelity in the part shape versus design shape to the extent of cure in every part location (x, y, z, t) is valuable. Industry will also want this capability, but it may be that for Army needs the level of tolerance can be sometimes lower and still be good enough. An effort to establish the minimum level of quality that satisfies the need under critical time limits for a given item needs to be undertaken.

ARL did not have a strategic approach and an overall roadmap in the pursuit of various facets of additive manufacturing research. There will always be new machines developed, and to accelerate the learning curve a deep understanding of the influence of key process parameters and their impact on material and structural properties is needed. While there was evidence of research into the influence of build orientation and scan patterns, the confluence of parameters such as laser power, chamber pressure and temperature, laser spot diameter size, and any other parameters that can be controlled before and after obtaining full access would be of benefit. The methodical approaches in design of experiments to generate empirical data would support the development of a comprehensive integrated computational materials engineering (ICME) approach to understand process-structures-properties-performance of a material. The participation in the NIST workshop on additive manufacturing and through consortiums and other networks will be important to understand the state of the art for modeling and determine in the overall landscape, what ARL will specialize in and in which areas to engage in partnerships.

The ARCAM electron beam melting machine is an exciting addition to the capabilities at ARL. It will be important to work with others in this space, including Professor J.-P. Kreuth from the University of Belgium at Leuwen and the Walter Reed facility, to accelerate the learning curve.

As AM continually finds successes, there will be generation of large amounts of valuable data. The ARL’s materials selection and analysis tool (MSAT) currently under development seems the right place for consolidating materials databases, manufacturing process parameters, and component pedigree in order to capture and retrieve valuable research data across projects for all ARL research areas. Awareness of and the ability to leverage central professional librarian resources to support literature research and analysis and provide context for the particular data sets will help researchers to rapidly gather the latest information in critical areas of research and link these sources to additional informational sources and people relevant to the respective projects.

In addition to showing how areal density for ceramic armor for a specific V50 was decreasing over the past decades, it would be useful to illustrate the specific conceptual advances that led to the overall decrease in areal density versus time and to highlight current new concepts being modeled, tested, and developed.

The ceramics effort needs some fresh ideas along with new applications. Some of the work is incremental. Moving the development of armor materials forward requires a new paradigm. This is an opportunity for ARL, since it is the one facility in the United States that is specifically tasked with this. Perhaps what is required is connecting the modeling to the experiments in a more comprehensive way. Some of this connectivity appears to be occurring, but improvements in this area could be made.

For some projects, there appears to be a disconnect between the applied scientists and engineers and the scientists working on fundamental research. Efforts need to be made to connect all steps of the research and development enterprise. Even very applied projects could try to connect to the science being developed at ARL. Some applied projects could improve by not just focusing on solving an immediate problem but also by developing a hypothesis-driven effort. There are ways of doing this, requiring system-level thinking. This can be accomplished by connecting the fundamental and applied researchers in a more cohesive way.

Improvements to aging ARL laboratory facilities and building infrastructure are much needed. Remodeling and build-out need to be more efficient. It is unacceptable to have disruptions such as roof leaks and lack of climate control within the laboratories. Research time is lost, and as sensitive equipment needs to be recalibrated, the actual data measured can be corrupted. If such big picture items are addressed, many other more minor problems will be resolved, enhancing progress on projects.

HIGH-RATE MATERIALS AND MECHANISMS

High-rate materials and mechanisms play a key role in addressing both current and future challenges associated with the development of new Army platforms and capabilities, including the next-generation combat vehicle, long-range precision fire, and soldier lethality. As a consequence, ARL has struck a balance between use-inspired and curiosity-driven research and development activities. These include near-term projects to develop various forms of lightweight and low-cost ceramic, metal, polymer, and composite armor; and far-term investigations toward a fundamental understanding of ballistic impact—exposing materials to unique and extreme conditions of loading states with material deformation and failure mechanisms influenced by effects of pressure, stress states (tri-axiality), and strain rate.

Four broad areas under the high-rate materials and mechanisms effort were reviewed: ceramics for soldier protection: high-rate modeling and materials development, lightweight metallic materials for ground vehicle applications, computer-aided design and fabrication of cross-linked polymer networks, and simulation-based fabrication of composites for advanced soldier head protection

Accomplishments and Advancements

ARL has made significant advances in developing manufacturing capabilities for different materials, in meeting design requirements for resistance against ballistic impact (e.g., in ceramics and metals), and in synthesizing new formulations based on insights obtained through modeling by correlating structural changes during newly developed test methods (e.g., in polymers and composites). Advances in processing of ceramics for body armor (made at ARL) have shown significant reduction in areal density, with progression in materials from the well-studied B₄C, to higher hardness B₆O, and hybrids based on lessons from bio-enabled materials. In the case of polymers, unique capabilities for modeling, synthesis, and characterization have enabled the fabrication of molecular blends in forms suitable for use as soft replacements for armor backing and hard and rigid transparent armor. The capabilities developed and expertise available is ripe for discovery of new forms of molecular compounds. Similarly, processing coupled with modeling of composite fabrication process from individual layers has enabled understanding of helmet design through improvements in processing methodologies.

Ceramics for Soldier Protection: High-Rate Modeling and Materials Development

ARL researchers are persuing three strategies for developing higher performance and lighter weight armor ceramics: discovering and developing new materials, creating and designing optimal microstructures and composite designs, and identifying and mitigating materials variability. ARL researchers have shown an excellent understanding of processing-performance needs and relationship between materials and system performance. Current efforts on next-generation lightweight armor are focused on exploring novel materials through grain boundary engineering, new material combinations including ceramic blends and hybrid structures, and innovative designs created through additive processes. Ceramics research activities related to the materials in extreme dynamic environments (MEDE) program are providing important understanding of the influence of microstructure—in particular, the role of grain boundaries—on high-rate performance.

Improved powder processing techniques have enabled innovations in nanostructure stability in metals and alloys, providing novel performance over extremes of strain rates applicable during creep at rates of 10^-6 s^-1 to dynamic loading at rates of 10⁶ s^-1 for vehicle protection applications.

Important partnerships exist with end users, industries, academia, and other ARL units and national laboratories. ARL is well positioned to provide leadership and serve as the authority in ceramic armor materials development and design.

Lightweight Metallic Materials for Ground Vehicle Applications

Lightweight metals and alloys are of major interest for ground vehicle applications, with high strength combined with toughness and reparability being major concerns. Significant efforts are focused on investigating new processing approaches for retaining microstructural stability needed for the desired dynamic strength-ductility combination. The materials include Fe-Mn-Al alloys with density 10 percent less than that of steels; powder metallurgy-based processes combining additive and hydrogen sintering to produce Ti-6Al-4V alloys with tunable mechanical properties; and nanocrystalline Cu-Ta and FeNiZr alloys made by powder processing, which exceed traditional strength-ductility trade-offs.

The investigations of FeMnAl low-density armor steel, a derivative of Hadfield steel, is showing impressive strengthening and toughening behaviors. The approach being pursued needs to continue in order to fully develop the process-structure-property-performance understanding necessary for overcoming the challenges of scale-up and industrial production.

Near-net shape processing of Ti-6Al-4V alloys for applications relevant to Army needs requires improved cost-to-performance ratio. Current efforts under way at ARL combining hydrogen sintering and heat treatment processes are enabling fabrication of Ti-6Al-4V with reduced porosity, refined microstructure, and minimized anisotropy. The path forward combining low-cost additive manufacturing employing a non-laser-based approach with the novel sintering treatments needs to continue.

The powder processing involving Cu-Ta is a mature effort initiated almost 8 years ago. Its objective is to retain nanoscale structure in cryogenic powder-processed materials. Although the mechanisms of deformation remain to be understood and need to be further investigated, this effort has produced materials exhibiting unique properties over a wide range of strain rates, from those characteristic of creep conditions to the dynamic (ballistic) behavior. Particularly interesting is their flow strength exhibiting no strain-rate dependence over the range of 10^-4 to 10⁵ per second. The microstructures presented with no microstructural evolution and without deformation substructures upon impact at pressures of about 15 GPa. The basic research performed by the metals group at ARL in this area is a great example of combined scientific impact, which has resulted in 86 peer-reviewed publications (including two papers in Nature) and licensing of nine patents.

Computation-Aided Design and Fabrication of Cross-Linked Polymer Networks

ARL has world-class capabilities and outstanding expertise for formulation, synthesis, and processing of polymers in form and scale relevant for scientific studies for various applications—for example, armor, structural composites, intelligent systems, and additive-mobile manufacturing. The work presented demonstrated an outstanding example of computation-aided design of new formulations for soft-to-rigid polymers using modifications in molecular weight and network structure.

The work on high-performance films of ensembles of hydrogen-bonded “graphamid” 2D polymers for lightweight soft armor applications was impressive. This new 2D polymer closely resembles Kevlar (a very robust material) in chemical composition, but it is mechanically more advantageous by virtue of its 2D structure. It also has strong intermolecular interactions leading to high strength and high fracture toughness. It represents an excellent example of a material synthesized using quantum (density functional theory) simulations with classical atomistic (molecular dynamics) and continuum (bulk) material

models. Equally impressive was the effort presented in the project on computational modeling of polymer networks to help determine the influence of nano- to meso-scale mechanisms associated with chemistry, monomer, and chain-level features of the polymer on the macro-scale mechanical response. The path forward includes characterization of energy dissipation mechanisms over a wide range of strain rates.

The development of new soft polymers as drop-in replacements for armor backing, simulating human tissue, is a remarkable success. In comparison to currently used ballistic gel and clay, which suffer from extensive variability and aging, the tissue simulant materials developed at ARL provide improved accuracy and reproducibility during assessment and certification of body armor. The gel used as an ordnance gelatin is a reversibly cross-linked tri-block polymer, while the clay material is a soft composite of silicone-based polymer filled with silica and starch. Both materials match the quasi-static and ballistic properties of currently used systems, with better environmental stability, processing variability, and low cost.

Developments in polymers also extend to those with energetic characteristics, as well as rigid polymers used as transparent armor. In-depth studies targeting the influence of the glass transition temperature and segmental relaxations on the ballistic performance have allowed synthesis of new resins for epoxies, as well as novel polymers exhibiting improved ballistic performance. The V₅₀ and KE₅₀ tests remain as a standard for determining the ballistic performance of armor materials. It is uncertain how these terminal ballistic tests can provide an understanding of the failure and damage mechanisms necessary for design of the next generation polymer systems.

Simulation-Based Fabrication of Composites for Advanced Soldier Head Protection

Fiber-reinforced polymer composites are of increasing importance for soldier head protection. The challenges lie both in the fabrication process with problems associated with wrinkling and degradation of filler plies, as well as in ballistic performance with the need to resist penetration while permitting shear deflection. Recent work on design and manufacture of helmets aided by model-informed and experimentally validated computations has provided valuable insight into the understanding of defects caused by forming of flat laminates made from ultrahigh molecular weight polyethylene (UHMWPE). The in-plane shear (picture frame) testing of UHMWPE composites extended to the new interlaminar shear test method used for model validation has helped to identify mechanisms governing formation of manufacturing defects with potential for significantly improved ballistic performance.

Ongoing efforts on advanced impact-resistant composites for use as armor and structural materials in autonomous systems focus on development of new resins, understanding of resin-fiber interface chemistry and characteristics, and atomic-scale structure characterization of individual fibers. Use of novel chemistries, control of film former reactivity, and multiscale modeling linking molecular dynamics and finite element modeling of high-rate mechanical behavior have demonstrated the ability to influence interface adhesion and sizing formulations with tailored reactivity and potential for technology transition for fabrication of autonomous vehicle parts. Characterization of the multiscale behavior of fibers and textiles has involved a novel approach to probe the internal structure of fibers. This involves starting with generation of a cleanly sheared fiber sample with notches made using a focused ion beam. The sheared UHMWPE fiber viewed under an atomic force microscope (AFM) reveals the internal structure with subnanometer resolution. Fibers are investigated using this method across a set of processing conditions and are correlated with simulation-based predictions of structural characteristics and linked to their mechanical behavior. The tools developed have the potential to help accelerate the understanding of processing on structure and performance of next-generation fibers and accelerate their commercial deployment.

Challenges and Opportunities

The absence of a complete understanding of the failure mechanism(s) provides a challenge for developing and designing. Achieving significant progress to meet this challenge depends on many factors: understanding at a fundamental level the mechanisms that govern dynamic, high-rate failure and how the various deformation processes relate to material composition and microstructure; developing computational capabilities for inverse design of microstructure and composite structure; discovering how rationally designed microstructures can be controlled and produced via novel processing methods; harnessing and analyzing manufacturing, microstructure characterization, and property data obtained from time-resolved measurements to identify critical correlations in the processing-structure-properties-performance paradigm; and inspiring innovative approaches to armor design and development. Ceramics research activities related to the MEDE program are providing important understanding of the influence of microstructure—in particular, the role of grain boundaries—on high-rate performance. However, the reliance on terminal ballistic testing may not be able to provide direct correlation with microstructure effects. Testing under conditions of controlled stress and strain states and rates is needed for better understanding of the effects of microstructure on failure mechanisms.

There is an opportunity and need to take a deep dive into the understanding of deformation behavior of Cu-Ta and Cu-Zr-Ni nanostructured materials with regard to the stability of their nanoscale structure, defect evolution at high strain rates (>10⁶ s^-1), strengthening mechanisms, constitutive modeling, and equations of state. Researchers could consider characterizations employing atom probe tomography and transmission electron microscopy. This deeper dive will help develop and extend the understanding of process-structure-property-performance correlations for discovery and design of low-density alloys for use in ground vehicle armor. This exciting work justifies an infusion of resources to achieve its full potential for basic and applied research.

Much of the high-strain-rate work reviewed focused on postmortem characterization of the evolution of microstructure of the deformed state of the material, in addition to measurements of the flow stress at strain rates up to 10⁵ per second using traditional tensile testing, Hopkinson bar, and Taylor impact experiments. Each of these test methods subjects materials to different stress and strain states. Use of instrumented Taylor impact (uniaxial stress) and plate-on-plate impact (uniaxial strain) tests employing time-resolved diagnostics are needed to generate data that can be used with computations to enable a better understanding of the deformation and failure processes, as well as to develop constitutive strength models that can be used to design systems for armor for ground vehicles.

The effort in modeling of the preforming and consolidation process accounting for the material properties and relevant tooling and processing parameters has allowed identification of the need for increasing consolidation pressure to mitigate wrinkling. The characterization of in-plane interlaminar shear behavior of UHMWPE using a modified test and data reduction method has allowed process-property mapping resulting in improvements in mechanical properties, which can significantly help with the ballistic performance. Use of analytical modeling to complement simulation-based modeling is needed, and would provide an additional level of validation. If these two modeling approaches, usually based on different assumptions, result in close calculated data, then there is a good reason to believe that the obtained results are both accurate and trustworthy. While ballistic testing is still pending, it would be good to identify instrumented dynamic testing methods that further aid in validation of models at different length scales in the path forward, which otherwise appears well planned with modeling and characterization coupled via integrated ICME design approach. There also exists a lot of work on helmet design for nonmilitary applications. While certain conditions may be different, the lessons learned may provide valuable information on new materials and cost-effective manufacturing processes.

The excellent computer modeling of cross-linked polymers provides an opportunity for a deep dive into the understanding of deformation and failure mechanisms of polymers at high rates, through correlated in-depth microstructural studies, time-resolved measurements of relevant properties, and computational modeling across length scales. There is also room for extending efforts to discovery of new polymers and design of novel composites.

OVERALL QUALITY OF THE WORK

In the energy-efficient electronics and photonics area, the impact of advances utilizing optical equivalents, efficiencies realized through RF new waveform and encoding strategies, and efficiencies for directed-energy applications are significant and important targets.

The quality of research in the MS&PP program is high and is showing continual improvement. ARL is among the country’s top-tier research organizations in the areas addressed by this program. Its research portfolio includes a mixture of world-leading, established, innovative projects and recently initiated programs anticipating scientific trends. Project investigators were generally clear in presenting the linkage of their research to Army needs along with connections to relevant research communities both within and outside the DoD community.

Quantum sciences is a new program area of high scientific quality and well aligned with the long-term goals of ARL’s mission to provide the Army of the future with clear tactical advantage. It is anticipated that quantum sciences will provide game-changing capabilities for C4ISR for the Army of the future. It is critical that ARL maintain and expand this research effort.

The ECM is a revolutionary research program incorporating computational materials science and engineering and avant-garde materials processing. The computational-experimental approach is exciting and can produce unique results for the Army. The program is of high quality in both simulations and experiments.

The soldier augmentation program is a key research program for the Army. While it has overlap with developments outside the DoD, such as better helmets for football players, better goggles for lacrosse players, and wearable robotics for assembly line workers carrying out repetitive tasks, it is largely unique to the ARL. The overall quality of this program is high.

The ARL’s efforts on polymeric materials with good processability for high-rate deformation or high-impact situations are unique and at the forefront of research. The elements of molecular simulation, continuum models, synthesis, characterization, and processing have been woven into a tightly integrated organization that has already produced impressive achievements. Comprehensive efforts of this nature, which range from synthesis to mechanical studies, are rare, and are found at only a few laboratories around the world.

The high strain rate research is characterized by excellent talented staff at ARL. It was impressive to see the scale of processing capabilities across all material forms including ceramics, metals, polymers, and composite materials. The MEDE collaborative effort combining materials-by-design process and mechanism-based approach focusing on magnesium was exceptional and could serve as a model exploration for all alloy systems.

The overall quality of the work across the program areas reviewed is high and in some cases preeminent; and the researchers and the management are of high caliber and deserve recognition. Researchers appeared passionate about their work. ARL’s work in preparing for the review was superb. The ARL read-ahead materials, presentation viewgraphs, poster materials, and laboratory tours facilitated the review process.

Most of the projects presented are excellent and have an actual or a 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.

Presentations were of high quality, and most presentations addressed motivations, context, and specific performance targets. However, many posters and some presentations addressed only scientific objectives, and objectives were not always measurable—that is, the focus seemed to be on how to understand a phenomenon or problem, rather than a clear quantitative projection of how project outcome would contribute to goals. In prior years, this issue has been raised (for example, in context of the Heilmeier catechism), resulting in presentation formats that explicitly capture these features in subsequent years, but this is not uniformly retained year to year.

It would be useful if the description and presentation of ARL programs captured up front, clearly and explicitly, exactly how the Army need differs from the trajectory of commercial off-the-shelf (COTS) private sector technology, and how the ARL program is structured to address this difference. Future descriptions and presentations could include the intended pipeline, specify exactly where it departs from and leverages COTS to achieve targets, and plans for transition to technology development to customer or vendor programs. It is often hard to put things into context without this information.

It is refreshing to see outstanding early-career professionals coming into the workforce and giving excellent presentations. ARL does not seem to be suffering from any aging workforce challenges in the Materials Research Campaign.

CONCLUSIONS AND RECOMMENDATIONS

ARL’s Open Campus Initiative is having significant impact on the culture of ARL. While initially conceived as a way to bring outside participation into ARL resources, equally, if not more, important is the complementary aspect of promoting contact between ARL researchers and the external scientific community, which is providing ARL researchers perspective on their work. Many of these external collaborations were substantive, project-based, and valuable. Overall, the Open Campus Initiative is having positive impact on collaborations and is helpful in recruiting, both from the point of view of outreach and visibility and in the working culture of the laboratory.

ARL has demonstrated the ability to rapidly ramp new project areas (batteries and quantum sciences as examples). ARL has also been doing outstanding hiring, and it is exciting to see ARL attracting its share of the nation’s high-caliber talent. These are some of the facets of ARL’s notable advancement toward becoming a premier scientific laboratory. In this process, it is appropriate and meritorious for ARL to devote additional resources to high-risk, potentially high-reward areas. It is important to note, however, that high-risk work in the basic materials and device sciences will still benefit strongly from parallel high-risk research on applications, and vice versa. This is distinct from a premature push of high-quality discovery into application; instead, the application work is of sufficiently high risk to be rightfully characterized as basic research as well.

There are national models of how successful in-house basic science, materials, and device work can synergistically couple with in-house systems and applications research—the two linked to each other in a tight feedback loop to drive rapid advancement, solving problems that the rest of the world has not yet become aware of. As a relevant example, the global ramp of funded activity in quantum sciences is driven in part by the prospective transition from science to application development.

Internal Army awards seem to be well targeted, but management needs to also focus attention to external award opportunities, including aspiring for membership in the National Academy of Engineering (NAE). More broadly, there is still strong opportunity to increase outreach and external visibility—ARL

is not as well known as it could be, which impacts perceptions and recruitment. ARL could include a more compelling and useful web presence for the technical community.

Quantum Sciences

Quantum sciences is a new program area of high scientific quality and well aligned with the long-term goals of ARL’s mission to provide the Army of the future with clear tactical advantage. It is anticipated that quantum sciences will provide game-changing capabilities for C4ISR for the Army of the future. It is critical that ARL maintain and expand this research effort.

Adaptive and Responsive Materials

The emerging ECM program has the promise of a great future to ensure the Army’s technological superiority. ARL has demonstrated the ability to speedily develop an integrated computational-experimental program to become a leading laboratory in advanced field processing of materials. It behooves the ARL management to continuously support and expand this effort and to define this program.

Based on novel strong modeling-experimental capabilities and on past pioneering experience in field-assisted processing, ARL may become the leader in the related processing science to ensure the fast succession from materials discoveries to their production. This will have a high impact on perceptions and recruitment in the technical community.

The soldier augmentation program is very focused on enhancing the capabilities of the unmounted soldier in the field. Its focus is short term, and the program is very applied. It could be further strengthened by augmenting current short-term goals with long-term objectives to develop new materials to enhance future capabilities.

ARL has significant capabilities in synergistic modeling and experimental endeavors, and this approach can be followed to develop future materials for soldier augmentation.

Agile Expedient Manufacturing

The polymer program has excellent leadership and presents a comprehensive and impressive research effort covering all aspects from molecular design to synthesis, with the ability to process into various forms and then to characterize the morphology and the physical properties relevant to Army mission-oriented products.

The agile mobile platform is very timely and fits well with the Army’s 2050 mission. In general, the deep dives where it was possible to show the origin of the problem, the approaches, and how eventually success was reached are impressive.

It was impressive to meet some of the many student interns and postdoctoral researchers during the laboratory tours. The open campus concept is greatly helping with collaborative research as well as workforce development.

High-Rate Materials and Mechanisms

There is excellent talented staff at ARL. It is impressive to see the scale of processing capabilities across all material forms including ceramics, metals, polymers, and composite materials.

ARL has made significant investments in multiscale modeling efforts, particularly via the MEDE program. The combined computational and experimental work for evaluating the mechanical response of individual grain boundaries enabling grain boundary engineering for improved ballistic performance of ceramics, and similar examples with other material forms, are the types of effort needed for discovery of next-generation ceramics, metals, polymers, and composites for soldier protection. The ability to inverse design materials for ballistic performance, however, remains elusive, largely because it is such

a complex problem, and fundamental degradation mechanisms and relevant fundamental property measurements are not fully available to guide the models. While unique progress has been made at ARL in peridynamic modeling of brittle materials (glass), modeling efforts integrating material microstructure and heterogeneity effects on dynamic fracture, particularly pertinent to ceramics, metals, hybrids, and composites, are lagging.

ARL has made recent significant investments in analytical tools for characterizing microstructure (atom-probe tomography, electron microscopy, focused ion beam milling, and sample preparation), which are critical for materials investigations. Several poster presentations focusing on collaborative research as part of the MEDE program showed excellent examples of the usefulness of such efforts. In-depth microstructure characterization is helping define the origins of localized deformation and damage features in dynamically impacted boron carbide, magnesium, polymers, and fibers. Such infrastructure investments need to continue, although there is a need to add staff expertise to infuse state-of-the-art developments in quantitative microscopy and four-dimensional (4D) microscopy techniques into the broader research and development efforts. Since high-rate failure is extremely microstructure sensitive, it is important that ARL adopt state-of-the-art practices in microstructure quantification that can be input directly into performance simulation models or serve as the basis of creation of realistic simulated microstructures. Moreover, there is an opportunity to push the broader scientific community on dynamic high-rate characterization, some of which is being done by ARL using the dynamic compression sector; this needs to be emphasized and leveraged fully. The approach involving high-throughput experimentation for rapid characterization and discovery of armor ceramics is in the right direction. It was impressive to see this effort merging combinatorial high-volume processing, high-throughput microstructure characterization, and intelligent ballistic testing. The path forward could include integrating quantitative microscopy, time-resolved diagnostics, and computations with high-throughput experimentation.

The effort on identifying sources of variability (e.g., in SiC armor plates described in one of the posters) is recognized as an important example of how data analytics can help home in on key processing-structure-performance correlations. However, the in-house armor development program has not yet vigorously embraced the opportunities afforded by data analytics, machine learning, and advanced statistical models. Integrated efforts in fully instrumented processing, quantitative microstructure characterization, and instrumented time-resolved testing, along with advanced data management and data informatics approaches, can yield important insight into critical correlations governing ballistic performance. This would help guide fundamental studies aimed at elucidating underlying dynamic deformation and failure mechanisms and their relationship to material structure, chemistry, and microstructure. Data analytics is not an end unto itself; rather it can inform the focus of fundamental science.

ARL researchers could explore development and utilization of new test methods employing novel diagnostic approaches and utilizing embedded sensors to understand the failure and damage mechanisms with desired spatial and temporal resolution. These efforts require closer involvement of computational staff—for example, embedded in materials teams.