The panel met on November 29-30, 2018, at the National Academies of Sciences, Engineering, and Medicine (National Academies) facility in Washington, D.C., to review the In-House Laboratory Independent Research (ILIR) program projects in chemistry conducted in 2018 at the following U.S. Army Research, Development, and Engineering Centers (RDECs): Armament Research, Development, and Engineering Center (ARDEC); Communications–Electronics Research, Development, and Engineering Center (CERDEC); Edgewood Chemical Biological Center (ECBC); and Tank Automotive Research, Development, and Engineering Center (TARDEC). The panel received overview presentations on the ILIR programs at each RDEC and technical presentations describing the projects. During each presentation, the panel engaged in question-and-answer sessions with the presenter. After formulating initial impressions and developing additional questions during its closed-session deliberations, the panel participated in a general discussion with RDEC staff.
Project: Investigations in Glyoxal Condensations
This project focuses on the development of cheaper and more efficient synthetic routes for the production of a specific (classified) caged nitramine. This is a 3-year-long and experimentally based project that started at the beginning of 2018. Caged nitramines, such as 2, 4, 6, 8, 10, 12-hexanitrohexaazaisowurtzitane (HNIW), also known as CL-20, appear to be related to the target compound and are a class of high-energy compounds used extensively in both civil and military applications. The high performance of these compounds is associated with their cage structures. Conventional approaches to the synthesis of HNIW involve the condensation of glyoxal with benzylamine followed by a deprotection step and nitration. Although a viable path for the synthesis of HNIW starting from
glyoxal and benzylamines was first reported in 1990, the associated cost is too high for most applications. On the other hand, incomplete knowledge of the molecular mechanisms that make benzylamines particularly effective in forming isowurzitane cages compared to other aromatic and allylic amines (e.g., furfuryl, allyl, and naphthyl) has hindered the development of alternative synthetic pathways. During the first year of the project, two different approaches to determining alternative reaction pathways for the synthesis of the target compound have been explored. The first approach used kinetics measurements and in situ infrared (IR) spectroscopy to characterize the reaction mechanisms. However, both glyoxal condensations with benzylamine and the following N, N’-dibenzyldiiminoethane trimerization reaction were found to be too fast to be studied through IR spectroscopy.
In the second approach, synthesis of the target compound was attempted starting from the condensation of glyoxal with nitroamide. Two different synthesis conditions employed in the condensation reaction were not successful in providing the desired compound. It was hypothesized that aqueous conditions interfere with the condensation reaction, and future work will focus on investigating suitable anhydrous conditions for the condensation of glyoxal with nitroamide.
Precise characterization of the elementary steps that lead to the formation of caged nitromines is critical to the development of cheap and efficient synthetic routes that would facilitate widespread utilization of these materials, which is particularly relevant to the Army. At the same time, identifying alternative and cost-effective synthetic routes is extremely challenging, requiring fundamental knowledge of the underlying reaction mechanisms, which remain poorly understood. In this context, current research carried out at ARDEC is key to providing fundamental knowledge about the elementary steps involved in the formation of caged nitroamines.
Although at an early stage, the research presented on alternative routes for the synthesis of caged nitroamines was systematic. The possibility of complementing synthetic work with theoretical calculations was discussed during the review, and it became clear that the application of computational tools could help determine reaction pathways and associated energy barriers, which, in turn, could provide fundamental insights into the underlying molecular mechanisms.
Identifying alternative, cost-effective synthetic pathways for the production of caged nitroamines will be particularly beneficial to the Army, enabling widespread use of these materials in emerging technologies, such as amorphous explosives and additive manufacturing. Consequently, it would be important for ARDEC to promote synergistic interactions with complementary activities in these areas, not only within ARDEC but also with other RDECs and Army laboratories. Such interactions could broaden the ARDEC researchers’ skill sets and facilitate the integration of a computational component into the project, which, by complementing the project’s synthetic activities, could provide guidance for future experimental work.
This project is extremely relevant to Army needs. At the same time, identification of new synthetic routes as well as elucidation of associated mechanistic steps are of interest to the broader scientific community working in the area of synthetic chemistry. There seem to be opportunities for interactions within other academic laboratories. Participation with the broader scientific community through conferences, symposia, and workshops as well as research periods in academic settings may also provide ARDEC researchers opportunities to expand their skill sets, which could enable the successful completion of this project.
Recommendation: ARDEC should consider promoting interactions of its researchers with the broader scientific community.
The ability to quantitatively determine reaction pathways represents an important step toward understanding reaction mechanisms, which, in turn, provides molecular-level insights into structural
arrangements and associated energetics that are key to the development of new synthetic routes. The development of successful synthetic routes depends on several factors that are often difficult to disentangle. This poses unique challenges, but it also provides opportunities to develop and characterize new chemistry associated with intermediate steps along a particular reaction pathway.
While the main focus of the project is on producing the target caged nitroamine compound, attention to mechanistic details and a deeper understanding of the causes that led to the two approaches explored in the first year of the project that were failing to achieve the target compound would be key to guiding future studies. Combining synthetic and analytical efforts with computer modeling could provide molecular-level insights into structures and energetics of intermediate and transition states and would help characterize reaction pathways.
Recommendation: ARDEC should consider combining synthetic and analytical efforts with computer modeling, which would help characterize reaction pathways for its project on investigations in glyoxal condensations by providing molecular-level insights into structures and energetics of intermediate and transition states.
ARDEC Crosscutting Findings
ILIR projects play a key role in advancing fundamental knowledge that can then be transferred to applied research. They also form and train a unique type of researcher, with specialized expertise and skills at the intersection of different fields (e.g., chemistry, physics, chemical engineering, and mechanical engineering). Every year, approximately 40 projects, including new proposals and renewals, are submitted for evaluation, and 9-10 of them are selected for funding (usually for 2 years) by an independent panel, based on ARDEC’s needs and priorities.
To facilitate the formation and training of specialized researchers, which requires 7-10 years of training after a bachelor’s degree, ARDEC is planning to establish an Armament Graduate School, which will be an accredited institution located at Picatinny Arsenal that will offer both civilian and military engineers and scientists M.S. and Ph.D. degrees in specialized fields unique to the Army. The establishment of the Armament Graduate School is an effective approach to address issues related to the training of specialized researchers and workforce attrition. The proposed pilot program in Armament Engineering will play a key role in the assessment of the validity and feasibility of the Armament Graduate School.
Project: Enhanced Electrochemical Couple Through sp3d2 Hybrid Covalent Bond Stabilization
This project aims to enhance the electrochemical performance of MnO2-based cathode materials of lithium-ion (Li-ion) batteries through the application of the sp3d2 hybrid covalent bond theory. The idea is to dope the material with sulfur in order to provide the d-orbitals necessary to stabilize the crystalline structure of the octahedral configuration of the cathode materials. It is hypothesized that the sulfur-doped λ-MnO2 catcode material, through the establishment of the sp3d2 hybrid covalent bond, would prevent or reduce manganese dissociation into the electrolyte, which is one of the main drawbacks in MnO2-based cathode materials. Preventing or reducing manganese dissociation would lead to an improvement of the cycle life and stability of the Li-ion batteries. The research project includes the formulation, synthesis, and characterization of the sulfur-doped λ-MnO2 materials as a potentially robust cathode for use in lithium or Li-ion electrochemical cells for military mission duty.
Li-ion batteries can be utilized in a variety of military applications, especially for applications in harsh environments, such as the extreme pulse power environments, which are challenging in terms of both load and temperature. In addition to overcoming the current electrochemical energy storage limits of the Li-ion batteries (for the ever-increasing demand in consumer electronics, electric vehicles, and grid-scale storage), high-energy-density, safe, and long cycle-life Li-ion batteries are highly desirable for military applications. In this regard, the research project aligns well with the overall goals of CERDEC. The successful implementation will allow for more robust mission duty cycles of the constructed batteries. The project idea is clear and promising, and the theoretical basis of this research is solid. The objectives seem reachable within the time frame. This is a relatively new project, just started in 2018, although the fundamental theory has been in place for many years. The preliminary results have demonstrated the technical feasibility of the sulfur doping approach, although it might be too early to draw a solid conclusion. The research team has tremendous experience and expertise and can access all the necessary facilities to carry out the research project.
Tackling the long cycle life and stability of rechargeable batteries is a fundamental challenge in the scientific community. How to precisely control the doping level of the sulfur element (and/or other elemental doping) could be challenging for this specific project. It has already been evident from the preliminary results that the stability of the cathode material does depend on the dopant concentrations. At the moment, it is unclear what approaches the CERDEC research team will take to control the doping level of the sulfur element. The research team could seek assistance from a computational perspective to tackle the doping level issues, such as determining the best doping level and how to achieve it from experiments. The computational findings might lay the foundation for such an approach. Because of the basic and fundamental nature of the research presented, publication of the research findings in scientific journals would have a broader impact.
Recommendation: CERDEC should consider exploring simulation approaches that could guide ongoing experimental efforts aimed at optimizing the mechanisms that can prevent manganese dissociation through hybrid covalent bond stabilization.
Recommendation: CERDEC should consider publishing its research findings in peer-reviewed journals to maximize the impact of the research carried out in its laboratories.
Project: Improving Li-ion Cathode Materials Through Surface Modification and Enhanced Electrode Deposition
The goal of this research is to improve the specific capacity and cycle life of Li-ion batteries by exploring a new cathode material, which is based on particles with a core-shell structure, while excluding n-methylpyrrolidone (NMP), a commonly used but very expensive solvent and a highly toxic pollutant. The research is well aligned with the Army’s mission; it could potentially afford these batteries to reach higher energy densities and longer run times while facilitating manufacturability, which thereby will shorten the time for acquisition.
This is a very ambitious research project. Even addressing one of the challenges—either the improvement of specific capacity and cycle life or elimination of NMP in the fabrication process—would have major implications for the Li-ion battery industry. To this end, it would be beneficial if the principal investigator were to prioritize the efforts and focus on one of the two research thrusts. This project offers a range of opportunities in materials chemistry with respect to the development of advanced cathode materials for Li-ion batteries, including a good understanding of the nucleation and growth mechanisms involved in the formation of a conformal coating of a LiMn2O4 (LMO) shell on LiNixMnyCo1-x-yO2
(NMC) core; a deep understanding of the limitations arising from the difference in lattice expansion and contraction for the core and shell during charging and charging cycles; and elucidation of the correlation between the cathode performance and the shell’s uniformity and thickness. How to precisely control the stoichiometry of the core and shell materials during the synthesis is also a fundamental issue of key importance. In addition to experimental studies, all these processes can be simulated using computational models for the achievement of a deeper understanding. A good understanding of these fundamental issues could eventually lead to the design and rational synthesis of advanced cathode materials with the desired performance.
Recommendation: CERDEC should consider integrating ongoing experimental efforts with computer modeling, which would help gain the fundamental knowledge needed for the rational design of advanced cathode materials.
Unfortunately, the research has not been entirely focused on the aforementioned fundamental issues. For example, very little attention has been paid to the fundamental aspects involved in the formation of the proposed core–shell particles. The overall attention has been directed to the electrochemical performance of the poorly controlled particles, making the project look more like practical engineering research than basic research in materials chemistry. CERDEC needs to consider giving additional attention to the fundamental aspects associated with the current project on Li-ion cathode materials.
In addition, most of the presentation slides lacked a detailed description of the experimental studies and observations. The principal investigator also needs to interact more intensively with the battery-related research groups at the Army Research Laboratory (ARL), as well as the general community of materials research outside of the Department of Defense (DoD) laboratories.
Recommendation: CERDEC should encourage its principal investigators to interact more intensively with the battery-related research groups at the Army Research Laboratory and with the broader community of materials research outside of the Department of Defense laboratories.
CERDEC Crosscutting Findings
In two CERDEC projects, Enhanced Electrochemical Couple Through sp3d2 Hybrid Covalent Bond Stabilization and Improving Li-ion Cathode Materials Through Surface Modification and Enhanced Electrode Deposition, there appears to be a preference for filing patents and presenting at conferences as primary methods of disseminating findings. Publishing the research findings in scientific journals would have a broader impact.
Recommendation: CERDEC should aim for effective dissemination of its findings through the timely submission of manuscripts to peer-reviewed journals in an effort to enhance its engagement with the broader scientific community and evaluate its research.
Project: Characterization of Aerosol Particle Charge and the Impact of a High Degree of Charge on the Particle’s Physical and Chemical Properties
This project builds on the hypothesis that the presence of a net charge on an aerosol particle gives rise to specific vibrational features that can be detected using infrared (IR) spectroscopy. This is a 3-year-long,
experimentally based project that started in October 2016 and focuses primarily on silicon dioxide (SiO2) and titanium dioxide (TiO2) particles. Charge distributions were characterized for particles with sizes between ~10 nm and ~2.5 μm using a high-flow dual-channel differential mobility analyzer (HDDMA) recently developed by collaborators at Clarkson University. Results presented for SiO2 particles show that the largest fraction of SiO2 particles with a diameter of ~200 nm are singly and doubly charged, although particles with net charges up to +30 are detected. By integrating mass spectrometry with IR, SiO2, and TiO2 particles were further characterized, and the results were compared with those obtained from analogous measurements carried out for corresponding powder samples. Notable differences were found between IR spectra measured for SiO2 and TiO2 particles.
Interestingly, comparisons between IR spectra measured for TiO2 aerosol particles and powder samples show distinct differences that can be attributed to the presence of charges on the particle surface, which are generated during the aerosolization process. Further analysis of gas adsorption, using oxygen (O2), hydrogen (H2), and carbon monoxide (CO) gases, on TiO2 aerosol particles shows that the presence of these surface charges significantly affects the reactivity of the particles. Future studies will focus on characterizing charge distributions on TiO2 aerosol particles and the nature of the interactions that modulate the adsorption of CO molecules on TiO2 aerosol particles. These additional studies will be key to the development of a molecular-level understanding of the physicochemical properties of charged aerosols. One manuscript is currently waiting for clearance from internal review for submission to the Journal of Physical Chemistry Letters.
Understanding how charges at the surface or in the bulk of aerosol particles alter the particles’ properties is particularly challenging and relevant to the Army. The challenges arise from the lack of specific experimental techniques and theoretical models that have the resolution, accuracy, and predictive power necessary for characterizing the physicochemical properties of aerosol particles at the molecular level. The relevance to the Army derives from the acquisition of fundamental knowledge that is key to the development of new and effective defense solutions that can keep the warfighter safe from chemical and biological threats.
A molecular view of aerosol chemistry is extremely complex, with several aspects related to particle formation and growth, collision and coagulation behavior, and heterogeneous and multiphase processes on aerosol particles remaining poorly understood. The current project that is carried out at ECBC is of particular significance because, despite the great effort that has been devoted to the characterization of neutral aerosols, how the presence of charges in the bulk or at the surface of the particles affects particle properties and reactivity is still an open question. This research provides several opportunities for cross-pollination, exchange of ideas, and knowledge sharing with other scientific communities interested in different areas of aerosol chemistry (e.g., cloud formation and public health) as well as heterogeneous and multiphase chemistry.
The quality of the research presented on SiO2 and TiO2 was high, with significant data having already been collected and analyzed. The possibility of using surface-sensitive, nonlinear spectroscopic techniques (e.g., sum-frequency generation spectroscopy and second harmonic scattering) was discussed. It was realized that, although the application of these techniques to aerosols is in its infancy, these measurements would provide molecular insights into the interfacial properties of the particles, which is key to understanding the nature of interactions between gas molecules and aerosol surfaces, as well as the mechanisms that regulate heterogeneous and multiphase processes on aerosol surfaces. If a molecular-level characterization of the physicochemical properties and reactivity of charged aerosols is achieved, it will have important repercussions on several research areas, representing an important step toward a microscopic understanding of aerosol chemistry. Consequently, it would make sense to promote interactions with other activities in these areas, within ECBC and the other Army RDECs and with the broader scientific community beyond Clarkson University.
This project is relevant to specific Army needs and also of significant interest to the broader scientific community. Beyond the existing collaboration with Clarkson University, there seem to be opportunities for synergistic interactions through participation and research presentations at conferences, symposia, and workshops, as well as research periods with other Army laboratories. Interfacing with researchers interested in different aspects of aerosol chemistry could advance the current understanding of both physicochemical properties and the behavior of aerosol particles at the molecular level.
Recommendation: ECBC should continue to take advantage of its current collaboration with Clarkson University and also consider promoting interactions of its researchers with the broader scientific community through participation and research presentations at conferences, symposia, and workshops, as well as research periods in academic and other Army laboratories.
Data acquisition and analysis during the first 2 years of the project Characterization of Aerosol Particle Charge and the Impact of a High Degree of Charge on the Particle’s Physical and Chemical Properties have already provided interesting results, leading to a manuscript that at the time of this review was ready for submission, pending obtaining clearance from internal review. Further analysis is under way. Efficient and timely publication of the research findings in scientific journals is key to knowledge sharing and provides a fundamental step toward establishing interactions with other researchers working in the broad area of aerosol chemistry.
Recommendation: ECBC should continue developing effective dissemination of its findings on the characterization of aerosol particle charge and the impact of a high degree of charge on the particle’s physical and chemical properties through timely submission of manuscripts to peer-reviewed journals as part of its engagement with the broader scientific community.
Aerosol chemistry is complex, depending on the physical composition, heterogeneity, and phase behavior of the particles; thermodynamic conditions (e.g., temperature and relative humidity); and interactions with the surrounding gas phase. While this complexity is challenging to disentangle, it also provides opportunities for the development and application of new analytical and modeling tools. The ability to determine particle size and charge distributions is an important step toward understanding the behavior of aerosols at the single-particle level. This provides new opportunities for additional and complementary analyses. ECBC researchers could consider exploring the possibility of using surface-specific nonlinear vibrational spectroscopic techniques to characterize the interfacial properties of charged aerosol particles, which is key to understanding the role played by surface charges in mediating adsorption and reactive processes.
Recommendation: ECBC should consider exploring the possibility of using surface-specific nonlinear vibrational spectroscopic techniques to characterize the interfacial properties of charged aerosol particles, which is key to understanding the role played by surface charges in mediating adsorption and reactive processes.
Combining experimental measurements on aerosol particles with computer simulations (e.g., molecular dynamics and Monte Carlo simulations) would provide molecular-level insights into both the interfacial properties of charged aerosol particles and molecular mechanisms of gas adsorption through complementing mobility and spectroscopic data.
Recommendation: ECBC should combine experimental measurements with computer simulations (e.g., molecular dynamics and Monte Carlo simulations) in order to progress their molecular-level insights into both the interfacial properties of charged aerosol particles and molecular mechanisms of gas adsorption.
Project: Diesel Engine Heat Transfer Model Development at Military-Relevant Conditions
This project focuses on understanding the fundamentals of in-cylinder heat transfer and the extent to which it can be reduced in diesel engines through the use of thermal barrier coatings on pistons and other in-cylinder surfaces. The program is experimentally based and has acquired an array of piston temperature measurements on four different pistons, with different thermal barrier coating thicknesses and surface finishes, for a range of engine operating conditions. The measurements are used along with thermodynamic and inverse conduction analyses to determine spatially and temporally resolved heat transfer during the closed cycle portion of the engine operation. From these measurements, convective heat transfer correlations will be developed that are more applicable to the unique operating conditions to which Army vehicles are subjected. Having this understanding, along with the resultant convective heat transfer correlations, will enable more extensive use of computer simulations in the design and development of future military engines and in determining real-world performance.
This research is challenging and extremely relevant to the Army. The Army faces unique design requirements because it needs to protect its vehicles from incoming projectiles; specifically, radiators need to be protected with ballistic grills. Ballistic grills add weight to the vehicle and are somewhat self-defeating in that their presence impedes the airflow to the radiator. For the Army, the smaller the radiator, the better—or, more specifically, the less energy that needs to be rejected via a coolant system, the better. Using materials as coatings on the pistons and other combustion chamber surfaces that have either low thermal conductivity or low thermal diffusivity is an approach that could reduce the required coolant flow. However, these coatings need to be able to survive in the engine, and they may cause unwanted changes to the engine’s operation. These are the technical issues that motivate this research.
This research is being conducted at the TARDEC laboratory in Warren, Michigan. It appears that the facilities are very good. The engine laboratory and supporting physical plant, instrumentation, and data analysis system appear to be state of the art. The researchers are augmenting their analysis with a research grade, industry standard, three-dimensional (3D) Computational Fluid Dynamic (CFD) analysis program. The CFD analysis was instrumental in explaining their experimental results, which showed that a smoothed surface on the thermal barrier coating resulted in increased indicated closed cycle efficiency. (The smoother surface did not reduce the wall jet velocity or impede fluid mixing like the rougher surface did.)
Understanding in-cylinder heat transfer has always been of significant interest to the civilian mobility industry. There is work in industry and work being supported by the Department of Energy that is pursuing material development for thermal barrier coatings and studying variations in engine operating characteristics as a result of their use. However, engine operating conditions for which these efforts are targeted are not as extreme as those of interest to the Army. Even though the overall descriptions of the various research efforts are similar, the operating domains of interest are different—TARDEC efforts are complementary and synergistic, not duplicative. Consequently, there are opportunities for technical interaction and information exchange between this Army project and work taking place directed at civilian applications. TARDEC is well situated for such exchanges.
The quality of the research presented on diesel engine heat transfer was high, with significant accomplishments having already been made. Both the quantity and quality of the engine temperature data and heat flux assessments are impressive. The investigators are using CFD analysis to promote a deeper understanding of the fundamental phenomena affected by changing the test conditions, such as the effect of smoothing the surface of the thermal barrier coating. Like most experimental efforts, some of the data seem confounding—for example, the observation that for the same engine operating conditions the metal substrate temperature for the thermal barrier coated–pistons increased relative to the non-coated piston. Hopefully this will be understood with further analysis. As the researchers pursue further analysis, it will be important to assess whether observed changes could be an artifact of how the experiment was run. For example, the mass flow rate of air was fixed as a boundary condition for the experimental setup. In real life, the engine’s boundary conditions are the temperature and pressure of the ambient air. The engine then establishes the mass flow based on these boundary conditions and the instantaneous condition of each of its components and subsystems. Could a difference between how the engine is running in the laboratory versus how it would be running in a real-world application result in the effects of the thermal barrier coating being different? The data analysis is subtle; however, it seems that the researchers are mindful of this and are capable of such analyses.
The remaining work for this research will be the computational corroboration of the experimental results with system-level modeling through the development of improved correlations for the convective heat transfer coefficient. Success in this final task will allow engine system performance to be assessed computationally for environmental conditions that are outside of what the commercial sector would see but relevant to the Army.
The possibility of using these data as a foundation for a conjugate heat transfer (CHT) analysis was discussed. In such an analysis, the in-cylinder conditions predicted with CFD are iteratively coupled with a finite element (FE) program of the engine structure—for example, the piston. Using the measured data of spatially resolved temperatures in the piston, conduction heat transfer equations are imposed through the piston. Thermodynamic analysis, coupled with the CFD results for the combustion chamber gases, gives the heat transfer to the piston. By iterating on imposed temperature distributions through the piston, while keeping the actual temperature data as anchors, a temperature distribution through the piston can be calculated that yields a conductive heat transfer through the piston that matches the heat transfer known from the coupled CFD–thermodynamic analysis. This will yield temperature distributions through the piston or other engine components, which can be used for thermal analysis of those components. The CFD program that the TARDEC researchers are using has this capability, and the researchers indicated this is on their wish list of things that could be done as follow-on research.
At this time, the researchers are realizing the first fruits of their data analysis, and so there was no mention during the review of any submissions of manuscripts for publication. The work is generating results that are worthy of publication, and this needs to be one of the goals of their program.
This project is relevant to specific Army needs and also of interest to researchers in the civilian mobility sector. TARDEC’s location in the metropolitan Detroit area puts the researchers in close physical proximity to many mobility original equipment manufacturers (OEMs); Tier 1, 2, and 3 suppliers; and technical consulting firms. It is also close to multiple universities with strong research programs. Interactions with the Ford Motor Company were briefly described during the review, as well as interactions with local universities—university colleagues visited TARDEC, and TARDEC researchers spent time at the university laboratories as part of their advanced degree programs. The Automotive Research Center (ARC) is an example of this synergy in action. There appears to be the opportunity of synergistic interactions with the larger technical community to advance the
understanding and potential of thermal barrier coatings on internal engine surfaces. It appears that the TARDEC researchers are engaged in such interactions.
Recommendation: TARDEC should continue to take advantage of its proximity to the many stakeholders in the mobility arena to enhance the interactions of its researchers with the broader academic and industrial technical community to advance the understanding and potential of thermal barrier coatings on internal engine surfaces.
If the research team were to successfully develop thermal barrier coatings, such developments will also be closely linked to advancements in materials and lubrication capabilities. The team could promote greater interactions between the activities in these areas, within ILIR and elsewhere within TARDEC and other Army RDECs.
Recommendation: TARDEC should consider looking for opportunities to showcase its research efforts in diesel engine heat transfer model development for military-relevant conditions to the broader Army Research, Development, and Engineering Center laboratories.
The data analysis is facilitating a deeper fundamental understanding of the phenomena governing the heat transfer processes during the closed cycle portion of the engine operation. There is still significant analysis to do. In light of the complexity of the heat transfer processes in engines and the possibilities of unrecognized feedback between changes in in-cylinder conditions and changes in engine performance, the researchers need to assess whether control conditions in the laboratory that are different from what the engine would experience in the real world are having an unexpected impact on the data being measured. Extending the analysis to include conjugate heat transfer analysis would also be worthwhile.
Recommendation: TARDEC should consider controlling the conditions in the laboratory that are different from what the engine would experience in real-world environments, because these environments may have an unexpected influence on data being measured. This analysis should also include the conjugate heat transfer analysis.
Early data analysis is yielding interesting results, and further analysis is under way. Publication of the findings and analysis in scientific journals would be a constructive addition to the fundamental understanding of heat transfer in engines and the impact of thermal barrier coatings.
Recommendation: TARDEC should seek to publish its important findings on diesel engine heat transfer model development for military-relevant conditions as soon as is practical after completing the initial phase of this work.
Project: Modeling Tribofilm Formation of Chemically Inert Nanoparticle and Oil Dispersions
This project considers the effect of metal oxide nanoparticle additives in Poly-α-Olefin (PAO) lubrication oils on the tribological properties and wear characteristics of the oils. The lubricant is thought to perform these functions by forming a stable interfacial film between surfaces, which improves their load-carrying capacities and reduces friction. The degree to which any lubricant reduces wear is known to depend on a wide variety of factors, including surface roughness, contact geometry, load, sliding
speed, and interface chemistry.1 The scientific questions that motivate the present study date back to at least 2006,2 when it was shown that at relatively small concentrations metal oxide nanostructures, including zirconium dioxide (ZrO2), provide measurable and significant benefits as antiwear additives for polyolefin lubricating oils. The prior literature suggests that these benefits accrue over time and are a result of a nanoparticle-rich tribofilm that concentrates in the wear track and mediates physical contacts between metal components.
Fundamental science questions related to the film formation process, including the roles played by nanoparticle dispersion in the base oil, surface chemistry, dispersion stability, and mass transport under the high interfacial loads in the wear track are of fundamental interest. Related questions concerning what role the specific chemistry of the nanoparticles, base oil, and lubricated components play in the improvements achieved are of contemporary practical interest for developing broader design rules for nanoparticle and base oil selection for lubricating parts of arbitrary chemistry (e.g., ferrous metals, aluminum, and plastics).
The research takes advantage of a mini traction apparatus, which combines conventional ball-on-flat tribology measurements with in-situ optical interferometry to simultaneously characterize the mechanical properties of nanoparticle-filled PAO oils and to quantify physical features, including the thickness and width, of the lubricant film accumulated in the wear track. The nanoparticle-filled PAO materials used in the study were formulated by a commercial entity and used without additional treatment. The principal finding of the study is that, consistent with earlier literature reports,3 sterically stabilized ZrO2 particle additives with average sizes around 11 nm concentrate in the wear track to form a robust film on ferrous metal surfaces, which protects the surfaces against wear. An additional notable finding is that only a fraction of the film is removed when the material is exposed to the unfilled base oil, suggesting that a permanent boundary lubricant coating may be formed. It is hypothesized that mechanical sintering of the ZrO2 nanoparticles under the unique localized thermal and loading conditions in the wear track are crucial to the formation of robust tribofilms.
To understand the coating formation mechanism, the film formation kinetics are studied using time-resolved experiments across PAO lubricants with different ZrO2 particle contents and under variable temperature and normal load conditions. Kinetic parameters deduced from the results are fitted using Arrhenius and stress-activated diffusion models to extract preliminary information about energy barriers for the film formation process and to provide information about the stress activation volume. Preliminary efforts have also been made to characterize the chemistry of the tribofilm using energy dispersive spectroscopy (EDS) performed in tandem with a scanning electron microscope (SEM) analysis.
A final component of the study used a simple two-state, transition state theoretical analysis to fit the time-dependent tribofilm data and found that a four-parameter model describes the experimental results quite well. Attempts to extend the study to other nanoparticle and substrate chemistries are at an earlier state of maturity, but they are important for evaluating the broader relevance of the observations from this study to lubricant design.
1 See D. Kim and L. Archer, 2011, Nanoscale organic–inorganic hybrid lubricants, Langmuir 27(6):3083–3094.
2 See A. Hernández-Battez, J.E. Fernandez Rico, A. Navas Arias, J.L. Viesca, R. Chou Rodriguez, and J.M. Diaz Fernandez, 2006, The tribological behavior of ZnO nanoparticles to an additive to PAO6, Wear 261(3-4):256–263; A. Hernández-Battez, R. González Rodriguez, and J.L. Viesca, 2008, CuO, ZrO2, and ZnO nanoparticles as antiwear additive in oil lubricants, Wear 265(3-4):422–428.
3 See, for example, A. Hernández-Battez, R. González Rodriguez, and J.L. Viesca, 2008, CuO, ZrO2, and ZnO nanoparticles; S. Ma, S. Zheng, D. Cao, and H. Guo, 2010, Anti-wear and friction performance of ZrO2 nanoparticles as lubricant additive, Particuology 8(5):468–472.
The project is aligned well with the broader TARDEC program mission because it addresses a problem of obvious relevance to fleet readiness both inside and outside the Army. It is a strength that it does so in the context of state-of-the art nanoparticle-based lubricant designs, which introduce multiple important surface science and transport questions related to how tribofilms form, how they evolve, what is the chemistry through which they consolidate and sinter, and ultimately what are the physicochemical processes responsible for their durability. If one adds to this the spectrum of substrate chemistries and libraries of available nanoparticle physical and chemical characteristics that might be employed as testbeds to evaluate broader impacts of the findings, there are opportunities here for far reaching basic science work that has practical impacts over a range of applications. Considering the scale and scope of the opportunities noted, a potential weakness is that the project is insufficiently resourced—from the perspective of personnel allocation—to achieve the sorts of impacts that are possible.
TARDEC Crosscutting Findings
In general, there are two equally important approaches to improving the quality of the research and raising the technical aptitude of TARDEC’s researchers: (1) draw proposals from a larger pool of applicants, thereby increasing the competition; and (2) enhance the mentoring of the researchers who have been awarded research funding. With the programmatic constraint of doing the work in-house, the project proposals are naturally limited to be only from staff members. The number of proposals from staff members at TARDEC has been steadily growing with time. This is a very positive result—still, one can ask, are there things that can be done to expand participation by the staff, or continued engagement of the staff members whose proposals were not chosen, by keeping them informed of the research that is taking place?
Because of time constraints during the review, only high-level overviews of the ILIR program were shown at the meeting. Therefore, it was hard to glean an intimate look at the day-to-day workings of the program. TARDEC uses in-house scientific technical experts and the Office of the Chief Scientist in a panel evaluation of its proposals. It was not made clear whether there is an opportunity to include external technical experts in the proposal review process. The researchers meet with their supervisors mid-term during their work; it was not made clear whether mentoring occurs during these meetings. Having a mechanism for the researchers to describe and discuss their research with their peers more frequently than at mid-term meetings would help them mature more quickly into scientific researchers. Overall it was not made clear whether there are mechanisms by which the researchers can receive feedback on both their work and their thinking processes throughout their projects—in addition to what they receive at the end, when their final reports are submitted and critiqued by internal and external reviewers. It was also not made clear whether final reports are prepared and critiqued with an eye toward publication in the reviewed scientific literature and whether there are opportunities for the researchers to present their work in poster sessions to their immediate colleagues, colleagues at other Army RDECs, relevant DoD meetings, or to the broader technical community—for example, at the annual ARC review meeting.
The TARDEC project could benefit from more interactions with other RDECs and Army laboratories. For the first TARDEC project, Diesel Engine Heat Transfer Model Development at Military-Relevant Conditions, if the development of thermal barrier coatings in engines is successful, it will also be closely linked to advancements in materials and lubrication capabilities. Consequently, it would make sense to promote interactions between the activities in these areas, especially the ILIR taking place elsewhere within TARDEC and the other RDECs. This would also facilitate the cross-pollination of ideas from the different areas within TARDEC as well as across the other RDECs.
The second project, on Modeling Tribofilm Formation of Chemically Inert Nanoparticle and Oil Dispersions, also appears to be done in isolation from similar efforts in Army laboratories
(and, unlike the first project, which is working with outside researchers, also in the broader community of researchers) that are also working toward defining working principles for nanoparticle-reinforced lubricants. This isolation is problematic because it limits access to more sophisticated instrumentation, computer simulation methods, and materials preparation methodologies that could accelerate progress toward a mechanistic understanding. It also prevents opportunities for continuous assessments of the work through external peer assessments that would improve its quality and clarify its focus.
It is important for the Army to have high-caliber researchers who can bring critical scientific thinking to technical challenges that are unique to the Army. This crosscutting aspect of the Army RDEC programs was emphasized by many of the presenters: a focus on high-quality research that is recognized by the scientific community, as well as the cultivation of a cadre of high-caliber scientific staff with backgrounds appropriate to address problems unique to the Army.
Overall, internal research programs such as ILIR are a beneficial activity for the Army and can serve the purpose of developing the Army’s technical staff by promoting high-quality scientific research with a focus on technical challenges that are unique to the Army. This is a need and a challenge for all of the RDECs. Since basic research focuses on fundamentals, there is an opportunity to enhance the benefits of internal research projects by engaging the expertise and insight for all of the RDECs in a collaborative way. It seems that the ILIR program is providing benefits; it could be made even better.
The following findings apply across more than one RDEC: (1) ARDEC, CERDEC, and TARDEC could have greater engagement with other RDECs and Army laboratories; (2) ARDEC, CERDEC, ECBC, and TARDEC could benefit from adopting uniform criteria for the evaluation of ILIR projects; (3) ARDEC, CERDEC, ECBC, and TARDEC could benefit from the adoption of a structured process for mentoring and active engagement of the ILIR researchers; (4) ARDEC, CERDEC, ECBC, and TARDEC could expand their relationships with the scientific community at large as a means of developing greater cross-pollination that would enhance the work of individual projects; (5) CERDEC, ECBC, and TARDEC could utilize publishing in peer-reviewed journals as a means of greater engagement with and oversight by the scientific community; and (6) ARDEC, CERDEC, ECBC, and TARDEC could increase the visibility of their research activities through the organization of workshops and symposia open to external subject matter experts as well as through broader advertisement of the Science, Mathematics and Research for Transformation (SMART) Scholarship program.
Recommendation: ARDEC, CERDEC, and TARDEC should consider enacting greater interactions with other RDECs and Army laboratories in an effort to enhance their projects.
Recommendation: ARDEC, CERDEC, ECBC, and TARDEC should adopt uniform processes for the evaluation of ILIR projects, including the standardization of key criteria (e.g., number of internal and external reviewers, scoring schemes, and type of feedback).
Recommendation: ARDEC, CERDEC, ECBC, and TARDEC should adopt a structured process for mentoring and active engagement with researchers involved in the ILIR projects, which could include periodic meetings between ILIR researchers and senior scientists and external subject matter experts, poster sessions as part of the annual review, and regular external reviews of each RDEC’s portfolio.
Recommendation: To advance greater scientific understandings to enhance their projects, ARDEC, CERDEC, ECBC, and TARDEC should promote interactions of their researchers with the broader scientific community.
Recommendation: To enhance their engagement with the broader scientific community and evaluate their research, CERDEC, ECBC, and TARDEC should aim at effective dissemination of their findings through the timely submission of manuscripts to peer-reviewed journals and should adopt criteria to assess productivity.
Recommendation: To increase visibility of their research activities, ARDEC, CERDEC, ECBC, and TARDEC should consider organizing workshops and symposia open to external subject matter experts and should consider broader advertisement of the Science, Mathematics and Research for Transformation (SMART) Scholarship program.