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At least 271 records · Page 15

All sources NASA NTRS DOE OSTI

DOE OSTI2020

Highly-Accessible Catalysts for Durable High-Power Performance (Final Technical Report)

Reduction of costly Pt usage in proton exchange membrane fuel cell (PEMFC) electrodes is one of the major challenges towards development and commercialization of fuel cell electric vehicles (FCEV). However, reducing the Pt loading results in increasing local mass transport fluxes, and eventual performance loss, especially at high power. In this project, a team of 4 universities, 2 companies, and a national lab came together to develop fuel cell catalysts and electrodes with high performance and durability. The were several key concepts addressed in this project. The first concept is to develop a carbon support that possess internal porosity which are believed to enhance kinetic activity of the catalysts, and yet the pore morphology allows for good reactant transport. The second concept is to improve the Pt-electrolyte interface in hope to reduce the local transport resistance. This was done by using new ionomers and ionic liquids. The last concept involves improving the catalyst stability by introducing Pt alloy catalyst with ordered intermetallic phase. The collaborative platform provided by DOE funding enabled development of a highly active and durable catalyst with performance that exceeds previous catalysts and meets the DOE targets for light duty vehicle application.

08 HYDROGEN↗

DOE OSTI2023

Experiments and Simulations to Describe Alkalinity Release from Particle-Containing Oil-in-Water Emulsions and Particle Suspensions

Among the most common amendments added to groundwater during site remediation are compounds used to adjust or maintain the pH. This research describes an approach to encapsulate mineral particles (MgO and CaCO 3 ) within oil droplets suspended within an aqueous phase for the purpose of delivery to the subsurface environment. A series of batch experiments was combined with mathematical modeling to illustrate the encapsulation and understand the influence of particle encapsulation on rates and extents of alkalinity release. The encapsulation of the alkalinity-releasing particles results in slower rates of amendment release as compared to rates obtained using suspensions of bare mineral particles, allowing for the possibility of control as a function of the pH. The results indicate that the alkalinity release from particle suspensions followed a mineral dissolution mechanism that could not explain the rate of the alkalinity release of the encapsulated particles. The reduction in mineral dissolution rates observed with the encapsulated particles was found to result from a mass transfer limitation. This limitation was well described using a linear driving force expression to account for the resistance to mass transfer at the oil–water interface.

EVO↗

NASA NTRS1986

Flat-plate solar array project. Volume 7: Module encapsulation

The objective of the Encapsulation Task was to develop, demonstrate, and qualify photovoltaic (PV) module encapsulation systems that would provide 20 year (later decreased to 30 year) life expectancies in terrestrial environments, and which would be compatible with the cost and performance goals of the Flat-Plate Solar Array (FSA) Project. The scope of the Encapsulation Task included the identification, development, and evaluation of material systems and configurations required to support and protect the optically and electrically active solar cell circuit components in the PV module operating environment. Encapsulation material technologies summarized include the development of low cost ultraviolet protection techniques, stable low cost pottants, soiling resistant coatings, electrical isolation criteria, processes for optimum interface bonding, and analytical and experimental tools for evaluating the long term durability and structural adequacy of encapsulated modules. Field testing, accelerated stress testing, and design studies have demonstrated that encapsulation materials, processes, and configurations are available that meet the FSA cost and performance goals.

Cuddihy, E.↗

NASA NTRS1988

Electronic states of semiconductor-metal-semiconductor quantum-well structures

Quantum-size effects are calculated in thin layered semiconductor-metal-semiconductor structures using an ideal free-electron model for the metal layer. The results suggest new quantum-well structures having device applications. Structures with sufficiently high-quality interfaces should exhibit effects such as negative differential resistance due to tunneling between allowed states. Similarly, optical detection by intersubband absorption may be possible. Ultrathin metal layers are predicted to behave as high-density dopant sheets.

Huberman, M. L.↗

NASA NTRS1989

CMCs for the long run

The structural and environmental requirements for ceramic matrix composites (CMCs) are most demanding in aerospace applications involving human transport; this need for extreme reliability has led to intensive investigations of criteria for high CMC integrity. The ideal properties of a fiber-reinforced CMC have been found to depend on the use of high aspect ratio fibers capable of maximum strength retention during fabrication, uniform fiber infiltration by the matrix material, high matrix oxidation resistance in elevated temperature applications, and a fiber/matrix interface that accommodates debonding in the presence of matrix cracks.

Dicarlo, James A.↗

NASA NTRS1995

Forced three-dimensional magnetic reconnection due to linkage of magnetic flux tubes

During periods of southward interplanetary magnetic field (IMF) orientation the magnetic field geometry at the dayside magnetopause is susceptible to magnetic reconnection. It has been suggested that reconnection may occur in a localized manner at several patches on the magnetopause. A major problem with this picture is the interaction of magnetic flux ropes which are generated by different reconnection processes. An individual flux rope is bent elbowlike where it intersects the magnetopause and the magnetic field changes from magnetospheric to interplanetary magnetic field orientation. Multiple patches of reconnection can lead to the formation of interlinked magnetic flux tubes. Although the corresponding flux is connected to the IMF the northward and southward connected branches are hooked into each other and cannot develop independently. We have studied this problem in the framework of three-dimensional magnetohydrodynamic simulations. The results indicate that a singular current sheet forms at the interface of two interlinked flux tubes if no resistivity is present in the simulation. This current sheet is strongly tilted compared to the original current sheet. In the presence of resistivity the interaction of the two flux tubes forces a fast reconnection process which generates helically twisted closed magnetospheric flux. This linkage induced reconnection generates a boundary layer with layers of open and closed magnetospheric flux and may account for the brightening of auroral arcs poleward of the boundary between open and closed magnetic flux.

Otto, A.↗

NASA NTRS2013

Nickel-Graphite Composite Compliant Interface and/or Hot Shoe Material

Next-generation high-temperature thermoelectric-power-generating devices will employ segmented architectures and will have to reliably withstand thermally induced mechanical stresses produced during component fabrication, device assembly, and operation. Thermoelectric materials have typically poor mechanical strength, exhibit brittle behavior, and possess a wide range of coefficient of thermal expansion (CTE) values. As a result, the direct bonding at elevated temperatures of these materials to each other to produce segmented leg components is difficult, and often results in localized microcracking at interfaces and mec hanical failure due to the stresses that arise from the CTE mismatch between the various materials. Even in the absence of full mechanical failure, degraded interfaces can lead to increased electrical and thermal resistances, which adversely impact conversion efficiency and power output. The proposed solution is the insertion of a mechanically compliant layer, with high electrical and thermal conductivity, between the low- and high-temperature segments to relieve thermomechanical stresses during device fabrication and operation. This composite material can be used as a stress-relieving layer between the thermoelectric segments and/or between a thermoelectric segment and a hot- or cold-side interconnect material. The material also can be used as a compliant hot shoe. Nickel-coated graphite powders were hot-pressed to form a nickel-graphite composite material. A freestanding thermoelectric segmented leg was fabricated by brazing the compliant pad layer between the high-temperature p- Zintl and low-temperature p-SKD TE segments using Cu-Ag braze foils. The segmented leg stack was heated in vacuum under a compressive load to achieve bonding. The novelty of the innovation is the use of composite material that re duces the thermomechanical stresses en - countered in the construction of high-efficiency, high-temperature therm - o-electric devices. The compliant pad enables the bonding of dissimilar thermoelectric materials while maintaining the desired electrical and thermal properties essential for efficient device operation. The modulus, CTE, electrical, and thermal conductances of the composite can be controlled by varying the ratio of nickel to graphite.

Firdosy, Samad A.↗

DOE OSTI2024

Elucidating the corrosion mechanism of commercial Ni-based Superalloys in UCl3 containing-chloride Molten Salt Systems

Molten salt reactors (MSRs) have gained renewed interest, providing several advantages over their predecessors, including the capability to consume spent fuels, enhancing the environmental sustainability of the uranium fuel cycle. For example, molten chloride fast reactors (MCFRs) can reach criticality with molten chloride spent fuel containing high concentrations of impurities, such as actinide products like uranium chloride (UCl3). However, the redox potential of chloride molten salt fuels may change in the presence of these impurities, dictating their corrosivity and in turn the corrosion performance of structural components, such as those constructed from nickel (Ni)-based alloys. The purpose of this investigation is to assess the extent of corrosion of Ni-based alloy, Inconel 617, when exposed to UCl3-LiCl-KCl eutectic salt. Inconel 617 one of only six structural materials that are fully qualified by the American Society for Mechanical Engineers (ASME) Boiler and Pressure Vessel Code for high-temperature nuclear reactor components, making it a technologically mature material to consider for constructing MCFRs. Inconel 617 specimens were submerged in a static LiCl-KCl-UCl3 eutectic salt mixture heated at 700 C for 1000 h under an inert atmosphere. Upon completion, the extent of corrosion was analyzed through a multi-modal characterization approach spanning the engineering to nanoscale, employing computed tomography, focused-ion beam, and transmission electron microscopy techniques. Results from this investigation will enhance our understanding of property-to-performance relationships of candidate structural materials for MSRs with respect to corrosion resistance and interactions between the salt and alloy interface.

36 MATERIALS SCIENCE↗

NASA NTRSDate not supplied

Understanding Thermal Transport in Polymer - Silver Nanowire Composites

Understanding thermal transport across filler-polymer interfaces and filler-filler contacts within polymer composites is of great importance for better thermal design of the composites that are widely used in high-performance heat exchangers, energy storage devices, and flexible electronics. Over the past several decades, considerable progress has been made in improving the thermal conductivity of polymer composites, but several key questions concerning the influence of interfacial thermal resistance, or Kapitza resistance, still remain. Firstly, the thermal properties of these composites are highly dependent on thermal transport through the filler network and its contacts. For metallic nanofillers, the thermal conductivity is often estimated using the Wiedemann-Franz law based on electrical conductivity; however, it remains a question whether the Wiedemann-Franz law still holds at nanoscale contacts. Through investigation of silver nanowires of varying sizes, we were able to demonstrate that the Lorenz number for silver nanowire increases with decreasing nanowire diameter. Examination of the corresponding electrical and thermal conductivities indicate that these changes are due to that the relative contribution of phonons becomes more significant as a result of elastic stiffening. Furthermore, we show that for silver nanowires, the contact thermal resistance is ~8 times lower than that of multi-walled carbon nanotube (MWCNT) of similar diameters. Additionally, through systematic studies of electrospun polymer-silver nanowire composite nanofibers, we investigated the impact of interface morphology on the thermal conductivity enhancement of the composite system and probed the value of the Kapitza resistance for individual polymer-filler interfaces. For polymer nanofibers containing continuous, single silver nanowires, the thermal conductivity increases linearly with increasing volume fraction of silver, which is consistent with the prediction of percolation theory for samples above the percolation limit. By comparing this linear trend to the measured thermal conductivities of composite nanofibers with more complex structures, we were able to determine the resistance associated with any additional polymer-filler boundaries. In doing so we find that the thermal boundary resistance for polyvinylpyrrolidone (PVP)–silver interfaces is significantly lower than that of comparable polymer-MWCNT composite systems. Together our studies provide new insights into thermal transport in polymer nanocomposites and should help facilitate the design of high performance polymeric thermal interface material.

Matthew L. Fitzgerald↗

DOE OSTI2021

A Power-Law Decrease in Interfacial Resistance Between Li 7 La 3 Zr 2 O 12 and Lithium Metal After Removing Stack Pressure

The high interfacial resistance between solid electrolytes and lithium metal is a hurdle to developing all solid-state batteries. External pressure applied on the lithium and solid electrolyte interface prior to electrochemical cycling is known to effectively lower the interfacial resistance. Here we report that the interfacial resistance between Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 (LLZTO) and lithium metal decreases over time even after removing the external pressure. The irreversible decrease of interfacial resistance can be understood by a gradual reduction of the total energy of the system, including strain energy and interfacial energy. Under external pressure exceeding ~25 MPa, however, lithium can be squeezed into LLZTO, fracturing the ceramic solid electrolyte. As a result, these observations can help improve the understanding of lithium metal creep and the interactions between garnet-type solid electrolytes and lithium metal.

25 ENERGY STORAGE↗

DOE OSTI2024

High-temperature quantum valley Hall effect with quantized resistance and a topological switch

Edge states of a topological insulator can be used to explore fundamental science emerging at the interface of low dimensionality and topology. Achieving a robust conductance quantization, however, has proven challenging for helical edge states. In this work, we show wide resistance plateaus in kink states—a manifestation of the quantum valley Hall effect in Bernal bilayer graphene—quantized to the predicted value at zero magnetic field. The plateau resistance has a very weak temperature dependence up to 50 kelvin and is flat within a dc bias window of tens of millivolts. We demonstrate the electrical operation of a topology-controlled switch with an on/off ratio of 200. These results demonstrate the robustness and tunability of the kink states and its promise in constructing electron quantum optics devices.

Science & Technology - Other Topics↗

DOE OSTI2024

Constrained patterning of orientated metal chalcogenide nanowires and their growth mechanism

One-dimensional metallic transition-metal chalcogenide nanowires (TMC-NWs) hold promise for interconnecting devices built on two-dimensional (2D) transition-metal dichalcogenides, but only isotropic growth has so far been demonstrated. Here we show the direct patterning of highly oriented Mo 6 Te 6 NWs in 2D molybdenum ditelluride (MoTe 2 ) using graphite as confined encapsulation layers under external stimuli. The atomic structural transition is studied through in-situ electrical biasing the fabricated heterostructure in a scanning transmission electron microscope. Atomic resolution high-angle annular dark-field STEM images reveal that the conversion of Mo 6 Te 6 NWs from MoTe 2 occurs only along specific directions. Combined with first-principles calculations, we attribute the oriented growth to the local Joule-heating induced by electrical bias near the interface of the graphite-MoTe 2 heterostructure and the confinement effect generated by graphite. Using the same strategy, we fabricate oriented NWs confined in graphite as lateral contact electrodes in the 2H-MoTe 2 FET, achieving a low Schottky barrier of 11.5 meV, and low contact resistance of 43.7 Ω µm at the metal-NW interface. Our work introduces possible approaches to fabricate oriented NWs for interconnections in flexible 2D nanoelectronics through direct metal phase patterning.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Thickness-Independent Vibrational Thermal Conductance across Confined Solid-Solution Thin Films

In this study, we experimentally show that the thermal conductance across confined solid-solution crystalline thin films between parent materials does not necessarily lead to an increase in thermal resistances across the thin-film geometries with increasing film thicknesses, which is counterintuitive to the notion that adding a material serves to increase the total thermal resistance. Confined thin epitaxial Ca 0.5 Sr 0.5 TiO 3 solid-solution films with systematically varying thicknesses in between two parent perovskite materials of calcium titanate and (001)-oriented strontium titanate are grown, and thermoreflectance techniques are used to accurately measure the thermal boundary conductance across the confined solid-solution films, showing that the thermal resistance does not substantially increase with the addition of solid-solution films with increasing thicknesses from ~1 to ~10 nm. Contrary to the macroscopic understanding of thermal transport where adding more material along the heat propagation direction leads to larger thermal resistances, our results potentially offer experimental support to the computationally predicted concept of vibrational matching across interfaces. This concept is based on the fact that a better match in the available heat-carrying vibrations due to an interfacial layer can lead to lower thermal boundary resistances, thus leading to an enhancement in thermal boundary conductance across interfaces driven by the addition of a thin “vibrational bridge” layer between two solids.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Avoiding CO 2 Improves Thermal Stability at the Interface of Li 7 La 3 Zr 2 O 12 Electrolyte with Layered Oxide Cathodes

Abstract Solid‐state batteries promise higher energy densities and better safety than Li‐ion batteries with liquid electrolytes. However, the interface between solid electrolyte and cathode is unstable at the elevated temperatures that are needed while sintering to achieve good bonding between ceramic components. Here, the hypothesis is that, the gas environment, especially the presence of CO 2, is critical in determining the stability of the solid electrolyte–cathode interface. The effect of gas species on the interface is systematically probed, by a using Li 7 La 3 Zr 2 O 12 (LLZO) solid electrolyte with a thin film LiNi 0.6 Mn 0.2 Co 0.2 O 2 cathode as a model system to enable interface sensitivity. Detrimental phases formed at the interface and their onset conditions are identified by X‐ray absorption spectroscopy, X‐ray diffraction, and Gibbs free energy analysis. As a result, removing CO 2 and minimizing H 2 O(g) during sintering is necessary to obtain good contact at the LLZO|cathode interface without forming secondary phases. Sintering in O 2 is ideal, yielding excellent chemical stability and low interfacial resistance. Secondary phases also do not form in N 2, but oxygen loss occurs at elevated temperatures. The interfacial resistance obtained upon sintering in pure O 2 is comparable to the lowest values at LLZO interfaces with protective coatings, but here without the need for interface coatings.

25 ENERGY STORAGE↗

DOE OSTI2026

Assess The Water Resistance And Thermal Performance Of Pre-flashing Methods When Adding Continuous Insulation During Re-siding (AIRS)

Retrofitting existing buildings by adding continuous insulation during re-siding projects has become a popular method for enhancing energy efficiency, especially given the aging building stock and stricter energy codes. When properly integrated with existing window systems, continuous insulation can significantly improve thermal performance, but it also presents challenges related to water resistance and building envelope integrity. Research indicates that the interface between windows and wall assemblies is critical as improper installation or sealing can lead to water intrusion, materials deterioration, and energy loss. Moreover, studies show that fully integrating the windows with the insulation layer can reduce window heat loss by up to 40%, emphasizing the importance of optimizing window placement and sealing during retrofitting to ensure both energy efficiency and structural performance. In this study, we conducted experimental tests to evaluate the water penetration and thermal performance of two window types: an aluminum window with a 2-inch installation fin and a wood window, representing typical mid-20th century designs. Using the Heat, Air, and Humidity (HAM) chamber at Oak Ridge National Laboratory (ORNL), we assessed bulk water penetration and performed COMSOL analysis for thermal flux and examined the effectiveness of different pre-flashing methods, i.e., standard self-adhered flashing tape and high-performance flashing tape with low-expansion foam, to enhance water resistance. The results indicate that applying proper flashing techniques and moisture management strategies can mitigate these risks, improving the overall performance and durability of retrofitted buildings. Additionally, proper sealing during retrofitting is essential, as improper installation can lead to moisture issues that compromise both energy efficiency and structural integrity.

Shen, Zhenglai [ORNL]↗

DOE OSTI2021

Bedrock architecture, soil texture, and hyporheic zone characterization combining electrical resistivity and induced polarization imaging

The shallow (<15 m depth) subsurface environment of a short reach along a bedrock stream was investigated with electrical resistivity and induced polarization (IP) to map details of bedrock and soil sediments. The bedrock is mostly comprised of limestone, and is generally resistive compared to the overlying soil. The soil-bedrock interface was determined through a trial and error approach using a sharp boundary feature in the inversion model. The inferred bedrock surface determined from the inversions exhibited undulating patterns with troughs and ridges. A near continuous trough ran alongside the stream within the floodplain, and is suggestive of a paleochannel among other interpretations of this feature. The structure of the electrical resistivity above the bedrock showed small-scale elongated features. The chargeability from the IP method showed larger scale features. High values of chargeability were associated with the sediments in the floodplain, and low values were associated with bedrock, stream, and soil on the elevated banks above the stream. If the chargeability is associated with membrane polarization characteristic of clayey soils, then IP seems to highlight the mere existence of clay, while the resistivity may be more discernable of the relative proportion of clay. Vegetation differences may also explain the chargeability distribution, where parts of the survey with high chargeability had dense pine with no understory making the soils more organically rich.

54 ENVIRONMENTAL SCIENCES↗

NASA NTRS1999

Key Durability Issues with Mullite-Based Environmental Barrier Coatings for Si-Based Ceramics

Plasma-sprayed mullite (3Al2O3 central dot 2SiO2) and mullite/yttria-stabilized-zirconia (YSZ) dual layer coatings have been developed to protect silicon-based ceramics from environmental attack. Mullite-based coating systems show excellent durability in air. However, in combustion environments, corrosive species such as molten salt or water vapor penetrate through cracks in the coating and attack the Si-based ceramics along the interface, Thus modification of the coating system for enhanced crack-resistance is necessary for long-term durability in combustion environments. Other key durability issues include interfacial contamination and coating/substrate bonding. Interfacial contamination leads to enhanced oxidation and interfacial pore formation, while weak coating/substrate bonding leads to rapid attack of the interface by corrosive species, both of which can cause premature failure of the coating. Interfacial contamination can be minimized by limiting impurities in coating and substrate materials. The interface may be modified to improve the coating/substrate bond.

Lee, Kang N.↗

DOE OSTI2021

Differences in c-Si Solar Cell Metallization and Susceptibility to Series Resistance Degradation

In this case study, we investigate a degradation mode occurring at the cell level in fielded multi-Si modules. Affected cells in the module show a progressive, series-resistance-related power degradation observed in module- and cell-level IV curves along with EL and PL imaging at the module, cell, and cell core sample scale. SEM and elemental analysis via EDS reveal a difference in the oxides in the silver paste used in screen printing of the finger contacts. This suggests that the cells were screen printed with different silver paste compositions and possibly firing conditions. One of these combinations leads to degradation of the contact at the interface between the cell and contact finger, causing severe series resistance across the cell that continues to progress over time.

acetic acid↗