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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,063 records · Page 59
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Size-dependent second-order-like phase transitions in Fe nanocluster melting from low-temperature structural isomerization

Here, in this work, the melting phase transitions of Fe n nanoclusters with 10 ≤ n ≤ 100 atoms are investigated using classical many-body molecular dynamics simulations. For many cluster sizes, surface melting occurs at much lower temperatures than core melting. Surface and core melting points and energetic melting points (temperatures of maximum heat capacity, C v ) are calculated for all cluster sizes. Melting properties are found to be strong functions of cluster structure. Cluster sizes with closed-shell structures always have first-order-like phase transitions. Almost one-third of cluster sizes in the analyzed range exhibit second-order-like phase transitions due to the presence of multiple structural configurations close in energy. 1-shell clusters with one to a few more atoms than a neighboring closed-shell structure have very low surface melting points and very high energetic melting points compared to their closed-shell counterparts. In clusters above 50 atoms with certain core structures, melting of the surface before the core was observed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Low temperature, ambient pressure electrochemical ammonia synthesis in alkaline media — Mechanistic studies and catalyst design. Final Report

Ammonia synthesis via the Haber-Bosch process is a pillar of modern agriculture, which converts the abundant but inert dinitrogen in the atmosphere to nitrogen-based fertilizers. Despite more than a century of optimization, the Haber-Bosch process remains energy intensive and reliant on fossil fuels and produces large amounts of CO 2 . Distributed and modular ammonia synthesis via the electrochemical nitrogen reduction reaction (ENRR) at or close to ambient conditions, powered by renewable electricity is an attractive alternative because it allows as needed production of ammonia, and in turn fertilizers. However, selective ENRR catalysts remain lacking. In this project, we systematically investigated two categories of catalytic materials, i.e., precious metals and transition metal nitrides, in the ENRR. Precious metals exhibit low activity and selectivity in the ENRR, which is in agreement with recent computational studies. Transition metal nitrides, especially VN, has been demonstrated as active and selective ENRR catalysts. We developed the first quantitative method to determine the amount of ammonia produced in the ENRR on N-containing containing catalysts in the membrane electrode assembly configuration. An initial ammonia production rate of 6.6 × 10 -10 mol s -1 mg -1 and a Faradaic efficiency of 6.0% at -0.1 V were reached on VN. After the initial deactivation, VN was shown to be stable in the ENRR for 116 hours with a steady state ammonia production rate of 1.1 × 10 -10 mol s -1 cm -2 . A vanadium oxynitride species was identified as the active phase in the ENRR via a combination of operando and ex situ characterization techniques. In addition, we established that the ENRR proceeded on transition metal nitrides via the Mars van Krevelen mechanism and determined the initial and steady state active site densities by developing a quantitative isotopic exchange method.

42 ENGINEERING↗
NASA NTRS2017

Passive Gas-Gap Heat Switches for Use in Low-Temperature Cryogenic Systems

We present the current state of development in passive gas-gap heat switches. This type of switch does not require a separate heater to activate heat transfer but, instead, relies upon the warming of one end due to an intrinsic step in a thermodynamic cycle to raise a getter above a threshold temperature. Above this temperature sequestered gas is released to couple both sides of the switch. This enhances the thermodynamic efficiency of the system and reduces the complexity of the control system. Various gas mixtures and getter configurations will be presented.

Heat Switches↗
DOE OSTI2026

Proton Conducting Silicon Oxide Membranes as a Fluorine Free Alternative to Nafion for Low Temperature Water Electrolysis

Driven by environmental and health concerns related to per- and polyfluoroalkyl substances (PFAS), there has been growing interest in developing fluorine-free proton (H + ) exchange membrane (PEM) materials for fuel cells and water electrolyzers. In this study, we present a side-by-side comparison of the key transport properties of submicron thick, PFAS-free amorphous silicon dioxide (SiO 2 ) membranes to Nafion, a fluorinated polymer electrolyte membrane that represents the industry standard for PEM fuel cells and electrolyzers. Here, measurements of proton (H + ) conductivity (σ H + ), hydrogen (H 2 ) permeability (P H 2 ), and electrical resistivity (ρ e – ) were conducted using model thin films comprised of SiO 2 membranes deposited by atomic layer deposition (ALD). Although the H + conductivity of the SiO 2 membranes is 2–3 orders of magnitude lower than Nafion, the addition of phosphorus dopants (PO x ) improves H + conductivity such that the area specific membrane resistance of thin (<50 nm) PO x -doped SiO 2 membranes is more than an order of magnitude lower than Nafion-117. Importantly, the safe operation of such nanoscale membranes within a PEM electrolyzer is feasible thanks to the low H 2 permeability of dense SiO 2 -based membranes, which are predicted to limit H 2 crossover rates to acceptable levels for pressures up to ≈ 100 bar. As a proof-of-principle demonstration, a chip-scale water electrolyzer based on 100 nm thick PO x -SiO 2 membrane is shown to achieve a current density of 2 A cm –2 at a potential of 2.5 V. If this technology can be successfully scaled up, H + conducting oxide membranes offer an attractive PFAS-free alternative to Nafion for efficient and durable water electrolysis and fuel cell technologies.

Nafion↗
DOE OSTI2024

Catalyst-Specific Accelerated Stress Tests in Proton Exchange Membrane Low-Temperature Electrolysis for Intermittent Operation

Device-level stress tests are developed that focus on anode catalyst layer degradation and future anticipated operating conditions, including intermittent load and reduced platinum group metal content. Square-wave cycles with an upper load limit of 2.5 A cm -2 are utilized to screen commercial iridium (Ir) materials. Performance losses are primarily due to decreasing kinetics and are accompanied by catalyst migration into the membrane, worsening catalyst/ionomer integration, and weakening of the catalyst/membrane interface. For ruthenium-containing catalysts, the in situ performances are higher but durabilities lower than Ir baselines, and any performance advantage is lost within the test. Increased loss is likely due to the higher dissolution rate; microscopy confirmed greater degrees of ruthenium migration. For Ir metal or mixed oxides, ex situ activity improvements generally did not translate to in situ performance. The durability, however, is significantly lower and the loss rate increased from 3 (oxide) to 9 (metal) μ V cycle -1 . These results are consistent with historical findings in literature, rationalize the continued use of iridium oxide as a baseline catalyst, and demonstrate that traditional catalyst development approaches may not improve device-level durability when focused on low-cost applications. A shift in focus may therefore be more effective at improving catalyst utilization and lessening load requirements.

08 HYDROGEN↗
DOE OSTI2023

Low-temperature trapping of N 2 reduction reaction intermediates in nitrogenase MoFe protein–CdS quantum dot complexes

The biological reduction of N 2 to ammonia requires the ATP-dependent, sequential delivery of electrons from the Fe protein to the MoFe protein of nitrogenase. It has been demonstrated that CdS nanocrystals can replace the Fe protein to deliver photoexcited electrons to the MoFe protein. Herein, light-activated electron delivery within the CdS:MoFe protein complex was achieved in the frozen state, revealing that all the electron paramagnetic resonance (EPR) active E-state intermediates in the catalytic cycle can be trapped and characterized by EPR spectroscopy. Prior to illumination, the CdS:MoFe protein complex EPR spectrum was composed of a S = 3/2 rhombic signal (g = 4.33, 3.63, and 2.01) consistent with the FeMo-cofactor in the resting state, E 0 . Illumination for sequential 1-h periods at 233 K under 1 atm of N 2 led to a cumulative attenuation of E 0 by 75%. This coincided with the appearance of S = 3/2 and S = 1/2 signals assigned to two-electron (E 2 ) and four-electron (E 4 ) reduced states of the FeMo-cofactor, together with additional S = 1/2 signals consistent with the formation of E 6 and E 8 states. Simulations of EPR spectra allowed quantification of the different E-state populations, along with mapping of these populations onto the Lowe–Thorneley kinetic scheme. The outcome of this work demonstrates that the photochemical delivery of electrons to the MoFe protein can be used to populate all of the EPR active E-state intermediates of the nitrogenase MoFe protein cycle.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Community Resilience through Low-Temperature Geothermal Reservoir Thermal Energy Storage

Submitted data include simulations related to underground thermal battery (UTB) simulations described in Modeling and efficiency study of large scale underground thermal battery deployment, presented at GRC, October 2021. The UTB is comprised of a tank of water, a helical heat exchanger in the center of tank and connected to a water source heat pump, and a phase change material (PCM). Compared to a conventional VBGHE, the UTB is designed to be installed at a much shallower depth, therefore, with a cheaper cost. In addition, the GSHP efficiency is improved due to natural convection of water and additional load capacity provided by PCM. The goal of this study is to explore factors that may affect the efficiency of large-scale UTB deployment. The simulations found in this submission relate to the report on UTB deployment.

15 GEOTHERMAL ENERGY↗
DOE OSTI2023

Octahedral Distortion and Excitonic Behavior of Cs 3 Bi 2 Br 9 Halide Perovskite at Low Temperature

The metal halide ionic octahedron, represented as [MX 6 ] $n$- (M = metal cation, X = halide anion), serves as the basic structural unit in halide perovskites and plays a crucial role in determining their optoelectronic and chemical properties. Thus, it is possible to correlate the responses of metal halide perovskites to various environmental stimuli with the dynamic behaviors of the [MX 6 ] $n$- octahedra. In this study, with the temperature-dependent single-crystal X ray diffraction (SCXRD) measurements on Cs 3 Bi 2 Br 9 2D halide perovskites, we can identify two classes of distortions through the lowering of temperature: intraoctahedral distortion, which is the off-centering of Bi 3+ cation within a [BiBr 6 ] 3– octahedron due to the Bi 3+ 6s 2 lone pair electrons, and interoctahedral distortion, which is the collective misalignments among the [BiBr 6 ] 3– building blocks. Free exciton (FE) and self-trapped exciton (STE) models are used to study the relationship between the distortion of octahedra in Cs 3 Bi 2 Br 9 and the corresponding changes in its optoelectronic properties, which transform from dominating blue emission above 100 K to red emission at 4 K. In conclusion, this work provides new insights into the excitonic behaviors of perovskites and suggests a possibility that we can design and rationalize the optical properties of halide perovskites by regulating the environmental stimuli based on the knowledge of behaviors of the individual [MX 6 ] $n$- building blocks.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Low-Temperature Gold Deposition Improves CdTe Back Contacts

Improved back contacts can benefit CdTe photovoltaics (PV). In this work, Cd(Se,Te) PV absorbers are cooled during Au evaporation to thermally quench a chemical reaction occurring between gold and CdTe and the generation of a reaction product that lowers device efficiency. Reducing substrate temperature enhances PV power conversion efficiency via open-circuit voltage and fill factor increases. X-ray photoelectron spectroscopy (XPS) reveals that lower temperature also reduces chemical perturbations of the CdTe, potentially linking back contact formation to a CdTe degradation product that hinders PV performance. Comparing reaction enthalpy and substrate heating energy shows that back contact formation by sputtering elemental metals onto ZnTe may exhibit a degradation pathway analogous to that of CdTe/Au reported here. Degradation-diminishing contact formation processes are therefore of general interest for optoelectronic devices, and the reduced substrate temperature in this study is one example.

14 SOLAR ENERGY↗
DOE OSTI2025

Revealing a Pathway for Low-Temperature Recrystallization in Germanium

Thermally activated annealing in semiconductors faces inherent limitations, such as dopant diffusion. Here, a nonthermal pathway is demonstrated for a complete structural restoration in predamaged germanium via ionization-induced recovery. By combining experiments and modeling, this study reveals that the energy transfer of only 2.4 keV nm −1 from incident ions to target electrons can effectively annihilate pre-existing defects and restore the original crystalline structure at room temperature. Moreover, it is revealed that the irradiation-induced crystalline-to-amorphous (c/a) transformation in Ge is reversible, a phenomenon previously considered unattainable without additional thermal energy imposed during irradiation. For partially damaged Ge, the overall damage fraction decreases exponentially with increasing fluence. Surprisingly, the recovery process in preamorphized Ge starts with defect recovery outside the amorphous layer and a shrinkage of the amorphous thickness. After this initial stage, the remaining damage decreases slowly with increasing fluence, but full restoration of the pristine state is not achieved. These differences in recovery are interpreted in the framework of structural differences in the initial defective layers that affect recovery kinetics. This study provides new insights on reversing the c/a transformation in Ge using highly-ionizing irradiation and has broad implications across materials science, radiation damage mitigation, and fabrication of Ge-based devices.

athermal recovery↗
DOE OSTI2023

Size‐Dependent Dispersion of Rhodium Clusters into Isolated Single Atoms at Low Temperature and the Consequences for CO Oxidation Activity

Abstract Understanding the dynamic structural evolution of supported metal clusters under reaction conditions is crucial to develop structure reactivity relations. Here, we followed the structure of different size Rh clusters supported on Al 2 O 3 using in situ/operando spectroscopy and ex situ aberration‐corrected electron microscopy. We report a dynamic evolution of rhodium clusters into thermally stable isolated single atoms upon exposure to oxygen and during CO oxidation. Rh clusters partially disperse into single atoms at room temperature and the extent of dispersion increases as the Rh size decreases and as the reaction temperature increases. A strong correlation is found between the extent of dispersion and the CO oxidation kinetics. More importantly, dispersing Rh clusters into single atoms increases the activity at room temperature by more than two orders of magnitude due to the much lower activation energy on single atoms (40 vs. 130 kJ/mol). This work demonstrates that the structure and reactivity of small Rh clusters are very sensitive to the reaction environment.

Albrahim, Malik A.↗
DOE OSTI2023

Size‐Dependent Dispersion of Rhodium Clusters into Isolated Single Atoms at Low Temperature and the Consequences for CO Oxidation Activity

Abstract Understanding the dynamic structural evolution of supported metal clusters under reaction conditions is crucial to develop structure reactivity relations. Here, we followed the structure of different size Rh clusters supported on Al 2 O 3 using in situ/operando spectroscopy and ex situ aberration‐corrected electron microscopy. We report a dynamic evolution of rhodium clusters into thermally stable isolated single atoms upon exposure to oxygen and during CO oxidation. Rh clusters partially disperse into single atoms at room temperature and the extent of dispersion increases as the Rh size decreases and as the reaction temperature increases. A strong correlation is found between the extent of dispersion and the CO oxidation kinetics. More importantly, dispersing Rh clusters into single atoms increases the activity at room temperature by more than two orders of magnitude due to the much lower activation energy on single atoms (40 vs. 130 kJ/mol). This work demonstrates that the structure and reactivity of small Rh clusters are very sensitive to the reaction environment.

Albrahim, Malik A.↗
DOE OSTI2025

Rapid and low‐temperature synthesis of MoAlB MAB phase by using stress‐induced Al‐particles

Abstract Currently, MoAlB particles are produced at 1000°C and higher temperatures. In this manuscript, we report the rapid synthesis of MoAlB by altering the stress of Al particles to affect Al reactivity. The Al particles were purposefully stress‐altered to reduce their threshold for reaction. The stress‐altering process consisted of annealing followed by quenching of the aluminum powder. The combined process caused a reaction at a lower temperature between Mo, B, and Al to form MoAlB. The stress‐altering of Al particles (i.e., Al [TT‐treated]) was effective in producing >95 wt.% MoAlB after reaction at only 700°C for 10 min, a significant reduction in temperature. Based on these results, we can further project that this process can be used for manufacturing Al‐based compositions at significantly lower temperatures.

Dey, Maharshi [Department of Mechanical Engineerin↗
DOE OSTI2023

Photophysics of Intrinsic Single‐Photon Emitters in Silicon Nitride at Low Temperatures

A robust process for fabricating intrinsic single-photon emitters in silicon nitride is recently established. These emitters show promise for quantum applications due to room-temperature operation and monolithic integration with technologically mature silicon nitride photonics platforms. Here, the fundamental photophysical properties of these emitters are probed through measurements of optical transition wavelengths, linewidths, and photon antibunching as a function of temperature from 4.2 to 300 K. Important insight into the potential for lifetime-limited linewidths is provided through measurements of inhomogeneous and temperature-dependent broadening of the zero-phonon lines. At 4.2 K, spectral diffusion is found to be the main broadening mechanism, while spectroscopy time series reveal zero-phonon lines with instrument-limited linewidths.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2020

Comparison Between the f-Electron and Conduction-Electron Density of States in the Falicov-Kimball Model at Low Temperature

The spinless Falicov-Kimball model is one of the simplest models of many-body physics. While the conduction-electron density of states is temperature independent in the normal state, the f -electron density of states is strongly temperature dependent—it has an orthogonality catastrophe singularity in the metallic phase and is gapped in the insulating phase. The question we address here is whether the spectral gap is the same for both electron species as T → 0. Finally, we find strong evidence to indicate that the answer is affirmative.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Tension-compression asymmetry in superelasticity of SrNi 2 P 2 single crystals and the influence of low temperatures

ThCr 2 Si 2 -type intermetallic compounds are known to exhibit superelasticity associated with structural transitions through lattice collapse and expansion. These transitions occur via the formation and breaking of Si-type bonds, respectively, under uniaxial loading along the [0 0 1] direction. Unlike most ThCr 2 Si 2 -type intermetallic compounds, which have either an uncollapsed tetragonal structure or a collapsed tetragonal structure, SrNi 2 P 2 possesses a third type of collapsed structured: a one-third orthorhombic structure, for which one expects the occurrence of unique structural transitions and superelastic behavior. In this study, uniaxial compression and tension tests were conducted on micron-sized SrNi 2 P 2 single crystalline columns at room temperature, 200 K, and 100 K, to investigate the influence of loading direction and temperature on the superelasticity of SrNi 2 P 2 . Experimental data and density functional theory calculations revealed the presence of tension-compression asymmetry in the structural transitions and superelasticity, as well as an asymmetry in their temperature dependence, due to the opposite superelastic process associated with compression (forming P-P bonds) and tension (breaking P-P bonds). Additionally, following thermodynamics, the observations suggest that this asymmetric superelasticity could lead to an opposite elastocaloric effect between compression and tension, which could be beneficial potentially in obtaining large temperature changes compared to conventional superelastic solids that show the same elastocaloric effect regardless of loading direction. Furthermore, these results provide an important fundamental insight into the structural transitions, superelasticity processes, and potential elastocaloric effects in SrNi 2 P 2 .

36 MATERIALS SCIENCE↗