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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 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Impacts of point defects on shallow doping in cubic boron arsenide: A first principles study

Cubic boron arsenide (BAs) stands out as a promising material for advanced electronics, thanks to its exceptional thermal conductivity and ambipolar mobility. However, effective control of p- and n-type doping in BAs poses a significant challenge, mostly as a result of the influence of defects. In the present study, we employed density functional theory (DFT) to explore the impacts of the common point defects and impurities on p-type doping of Be B and Si As , and on n-type doping of Si B and Se As . We found that the most favorable point defects formed by C, O, and Si are C As , O B O As , Si As , C As Si B , and O B Si As , which have formation energies of less than 1.5 eV. While the O impurity detrimentally affects both p- and n-type dopings, C and Si impurities are harmful for n-type dopings, making n-type doping a potential challenge. Interestingly, the antisite defect pair A s B B As benefits both p- and n-type doping. Finally, the doping limitation analysis presented in this study can potentially pave the way for strategic development in the area of BAs-based electronics.

36 MATERIALS SCIENCE↗
DOE OSTI2021

On the correlation between plastic strain and misorientation in polycrystalline body-centered-cubic microstructures with an emphasis on the grain size, loading history, and crystallographic orientation

In this work, the correlation between plastic strain and crystallographic misorientation, grain size, grain orientation, distance from grain boundary, and loading history were investigated experimentally and numerically for body-centered-cubic (BCC) polycrystalline microstructures based on a misorientation deviation (MD) approach. Nine monotonic tensile experiments were performed on two BCC titanium alloys inside a scanning electron microscope (SEM). The influence of reference orientation was explored both at the grain scale and at the mesoscale using electron backscattered diffraction (EBSD). The correlation between global plastic strain and the MD was quantified. The tendency for orientation change was quantified as a function of plastic strain and grain orientation for three crystallographic orientations (i.e., [100], [110], and [111]) with respect to tensile direction. The subpopulation of small grains exhibited a lower level of misorientation dispersion compared with larger grains, and this discrepancy became more pronounced at higher strains. An empirical equation was proposed to estimate the level of misorientation dispersion for individual grains as a function of grain size and global plastic strain level. Two interrupted in-situ SEM experiments were performed, and this resulted in a significantly increased misorientation compared with uninterrupted tests performed to similar plastic strain levels.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Effects of Zr substitution on soot combustion over cubic fluorite-structured nanoceria: Soot-ceria contact and interfacial oxygen evolution

Ceria is widely used as a catalyst for soot combustion, but effects of Zr substitution on the reaction mechanism is ambiguous. The present work elucidates effects of Zr substitution on soot combustion over cubic fluorite-structured nanoceria. The nanostructured CeO 2 , Ce 0.92 Zr 0.08 O 2 , and Ce0.84Zr0.16O2 composed of 5–6 nm crystallites display Tm-CO 2 (the temperature at maximum CO 2 yield) at 383, 355, and 375°C under 10 vol.% O 2 /N 2 , respectively. Additionally, the size of agglomerate decreases from 165.5 to 51.9–57.3 nm, which is beneficial for the soot-ceria contact. Moreover, Zr increases the amount of surface oxygen vacancies, generating more active oxygen (O2- and O-) for soot oxidation. Thus, the activities of Ce 0.92 Zr 0.08 O 2 and Ce 0.84 Zr 0.16 O 2 in soot combustion are better than that of CeO2. Although oxygen vacancies promote the migration of lattice O 2- , the enriched surface Zr also inhibits the mobility of lattice O 2- . Therefore, the Tm-CO 2 of Ce 0.84 Zr 0.16 O 2 is higher than that of Ce 0.92 Zr 0.08 O 2 . Based on reaction kinetic study, soot in direct contact with ceria preferentially decomposes with low activation energy, while the oxidation of isolated soot occurs through diffusion with high activation energy. The obtained findings provide new understanding on the soot combustion over nanoceria.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2020

The lattice parameter – composition relationship of the body centered cubic uranium-niobium alloys

In this paper, in-situ time-of-flight neutron diffraction experiments were performed on uranium-niobium alloy with 6 wt% Nb to study the lattice parameter-composition relationship for the body centered cubic (bcc) alloys. Based on lattice-parameter measurements of Nb-supersaturated γs phase over the range of 450 – 790 °C, an improved Vegard's-type relationship was established for determination of Nb concentrations at elevated temperatures. Neutron diffraction data were also collected as a function of aging time over the 450 - 600 °C range when γs phase decomposed isothermally into the mixture of orthorhombic α-U and Nb-rich bcc phase, as well as at room-temperature on the alloys aged ex-situ at 500 °C up to five years. From these measurements, the average Nb concentrations in the bcc phase were determined based on the Rietveld refinements of weight fraction and mass conservation relations (lever rule). Over the 15at% – 78at% range of Nb concentrations that correspond to different experimental aging times, the lattice parameters at constant temperatures exhibit a nonlinear S-shaped variation with Nb concentration, and the associated excess volumes of mixing can be described by a subregular solution model of the Redlich–Kister type of polynomial. Over the full range of composition the S-shaped deviation from Vegard's law can be modeled using a combination of an elastic continuum model and a perturbation to the radii of the solute atoms in the solvent, suggesting that electronic interactions between solute and solvent atoms could play an important role in the compositional dependence of lattice parameter for the γ-phase U-Nb alloys. While Vegard's law is a straightforward and reasonably good approximation for the bcc solid solutions in the U-Nb system, the Nb concentrations determined from the weight fraction refinements of diffraction data provide internally consistent, mass-conserving estimates of solute redistribution for the monotectoid reaction.

36 MATERIALS SCIENCE↗
DOE OSTI2026

A simple Rice-Ashby ductile–brittle transition temperature (DBTT) model based on dislocation mobility for body-centered cubic complex concentrated alloys

A simple Rice-Ashby type model for ductile–brittle transition temperature (DBTT) of body-centered cubic (bcc) complex concentrated alloys (structures) is presented. The effect of accumulation of dislocation density on DBTT is also analyzed. The model results are compared with experimental yield stress vs. temperature data for four complex concentrated alloys: Nb 45 Ta 25 Ti 15 Hf 15 (NTTH), MoNbTaW, HfNbTaTiZr, NbTiZr and two pure bcc metals, Fe and W. It is shown that the DBTT behavior of these alloys and pure metals are in agreement with the simple ductility model presented in this manuscript. The DBTT model presented in this manuscript along with yield strength models for bcc complex concentrated alloys described in the literature should serve as a useful guide for designing such alloys with good high temperature strength and significant room temperature ductility.

Crack tip processes↗
DOE OSTI2024

Prediction of ambient pressure superconductivity in cubic ternary hydrides with MH 6 octahedra

Exploring high-temperature superconducting (high-T c ) material at ambient pressure holds immense significance for physics, chemistry, and materials science. In this study, we perform a high-throughput screening of strong electron-phonon interactions in X 2 MH 6 compounds (X = Li, Na, Mg, Al, K, Ca, Ga, Rb, Sr, and In; M are 3d, 4d, and 5d transition metals). These compounds have a cubic structure featuring an MH 6 octahedron motif. Our screening calculations suggest that 26 compounds exhibit dynamic stability and strong electron-phonon coupling. Among them, Mg 2 RhH 6 , Mg 2 IrH 6 , Al 2 MnH 6 , and Li 2 CuH 6 show promising energetic stability and T c of more than 50 K at ambient pressure. This study underscores promising high-T c compounds at ambient pressure with distinctive MH 6 motifs.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Phonon lifetimes and mode softening in cubic Cs 2 AgBiBr 6

Metal halide perovskites (MHPs) have emerged as noteworthy candidates for photovoltaic applications in recent years. Their high power-conversion efficiency is largely attributed to the structure-function relationship, which is not well understood. In this study, we examine the low-frequency phonons of the double perovskite Cs 2 AgBiBr 6 using neutron inelastic scattering. We find that the acoustic phonon lifetimes decrease from 16 to 3 ps along Γ to X, which is indicative of significant anharmonicity that contributes to the ultralow thermal conductivity. Additionally, we observe a linear temperature dependence of the square of the zone-center soft optical phonon energy, which is consistent with a weakly first-order displacive cubic-tetragonal structural phase transition. These results provide a deeper understanding of the lattice dynamics and phase transitions in MHPs as well as the effects of anharmonicity by comparison with prototypical hybrid organic-inorganic MHPs, (CH 3 NH 3 ) PbX 3 (X = Br, I).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Generation of electric-field stabilized zirconium monoxide secondary phase within cubic zirconia

Flash sintering of cubic yttria-stabilized zirconia has been studied via in situ energy dispersive diffraction. A secondary rocksalt-phase zirconium monoxide was observed emerging from the primary fluorite-phase zirconium dioxide, previously only observed as a thin interlayer at metal-oxide interfaces, upon application of a direct current electric field. The average secondary phase intensity rose over extended flow of direct current through the ceramic, faded upon removal of the field and disappeared entirely upon application of an alternating current electric field. The results indicate that the sustained application of a direct current electric field can result in significant electrochemical reduction of zirconia under high current density conditions. Poor sinterability and cracking under direct current is apparent and potential mechanisms discussed therein.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Lithium thiostannate spinels: air-stable cubic semiconductors.

Lithium chalcogenide materials have been studied less than other alkali metal analogs and are of interest as ion conductors and semiconductors capable of thermal neutron detection. Herein, we describe the structural, thermal, magnetic, electronic, and ionic properties of a new family of cubic lithium thiospinels Li2MSn3S8 (M = Mg, Fe, Mn, Ni, Co) and Li1.66CuSn3.33S8. All compounds crystallize in the Fd (3) over barm space group and exhibit site occupancy disorder with Sn and M on the octahedral site for M = Mg, Fe, Mn, Ni, Co. The Li1.66CuSn3.33S8 compound also exhibits occupancy disorder on the tetrahedral sites between Li and Cu, as well as on the octahedral sites between Li and Sn. We demonstrate the ability to tune the physical properties of the compounds by changing their composition. By tuning the identity of the transition metal in the structure, the optical band gaps can vary between 0.91(2) and 2.19(2) eV, and electrical resistivities between 2.8 x 10(3) Omega-cm and 2.2 x 10(7)Omega-cm can be achieved. Density functional theory calculations for Li2MgSn3S8 and Li1.66CuSn3.33S8 suggest that both materials have indirect band gaps and significantly dispersive conduction bands. A bandwidth dispersion of 0.75 and 0.70 eV along the Gamma -> M direction in the conduction band are predicted for Li2MgSn3S8 and Li1.66CuSn3.33S8, respectively, with an electron effective mass of 0.49 and 0.55 m(e) at the Gamma point. The hole effective mass in the valence band is calculated to be 1.25 me at the M point and 0.75 me at the L point for Li2MgSn3S8 and Li1.66CuSn3.33S8, respectively. The ionic/electronic conductivity of each compound was determined using AC electrochemical impedance spectroscopy.

Quintero, Michael A.↗
DOE OSTI2023

Novel 3D Cubic Topology in Hybrid Lead Halides with a Symmetric Aromatic Triammonium Exhibiting Water Stability

The limitations of three-dimensional (3D) perovskites are related to their narrow structural tunability of the organic cations and their moisture sensitivity. Herein, we report a new family of 3D cubic hybrid metal halides (T-Et 6 ) 3 Pb 11 X 31 (X = I, Br), where T is 1,3,5-tris-(4-aminophenyl)benzene. The materials are synthesized through an in situ N-alkylation of T and an efficient one-step solvothermal reaction containing ethanol, initiating a tunable synthetic avenue for the acquisition of structurally complex hybrid halides with luminophores. (T-Et 6 ) 3 Pb 11 X 31 consist of an unprecedented Ia3̅ framework of [Pb 11 X 31 ] 9– one-dimensional (1D) chains embedded with (T-Et 6 ) 3+ cations, affording an overall 3D topology. Here, the constituent [Pb 11 X 31 ] 9– chains include exclusively octahedral lead halide units with clusters of face- and edge-sharing connectivity, giving rise to weak broad emission centered at ~660 nm observed at 78 K. (T-Et 6 ) 3 Pb 11 I 31 demonstrates water stability for at least 7 days. Synthesis through ambient pressure results in tunable structural variations of zero-dimensional (0D) structures rendering T 7 Pb 3 Br 27 ·DMF and T 2 Sn 3 Br 18 ·4H 2 O·0.5Br 2 , both of which feature blue PL emission at room temperature.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Influence of the Cubic Sublattice on Magnetic Coupling between the Tetrahedral Sites of Garnet

Here, we present a study on the nuclear and magnetic structures of two iron-based garnets with magnetic cations isolated on tetrahedral sites. Ca 2 YZr 2 Fe 3 O 12 and Ca 2 LaZr 2 Fe 3 O 12 offer an interesting comparison for examining the effect of increasing cation size within the diamagnetic backbone of the garnet crystal structure, and how such changes affect the magnetic order. Despite both systems exhibiting well-pronounced magnetic transitions at low temperatures, we also find evidence for diffuse magnetic scattering due to a competition between the nearest-neighbor, next nearest-neighbor, and so on, within the tetrahedral sites. This competition results in a complex noncollinear magnetic structure on the tetrahedral sublattice creating a mixture of ferro- and antiferromagnetic interactions above the long-range ordering temperature near 20 K and suggests that the cubic site of the garnet plays a significant role in mediating the superexchange interactions between tetrahedral cations.

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

Synthesis of an Isostructural Series of 12-Coordinate Lanthanide Nitrate Hybrid Double Perovskites with Cubic Symmetry

In efforts to study the periodic chemical properties of the rare earth elements and their structural chemistry, a hybrid double perovskite phase A 2 B'BX 6 with the formula ((CH 3 ) 4 N) 2 KLn(NO 3 ) 6 (Ln = La–Lu, Y ex. Pm) was synthesized that crystallizes in the cubic space group, $Fm$$\bar{3}$$m$. This series was obtained via evaporative crystallization from a mixture of Ln(NO 3 ) 3 , KNO 3 , and (CH 3 ) 4 N·NO 3 in a 1:1:2 ratio from either H 2 O or 4.0 M HNO 3 . In this double perovskite structure, the B site containing the lanthanide ion is coordinated by six bidentate nitrate ligands, with the distal N$=$O oxygen atoms coordinating the potassium on the B' site in an octahedral six-coordinate environment. The two-remaining charge-compensating (CH 3 ) 4 N + cations occupy the interstitial voids in the lattice on the A site. This periodic series was characterized via single-crystal X-ray diffraction, powder X-ray diffraction, IR, and Raman spectroscopy. Emission spectra of the Eu complex indicate a phase transition to trigonal symmetry upon cooling. In conclusion, this series is unique as it represents a rare isostructural series spanning the entirety of the rare earth elements excluding promethium with homoleptic 12-coordinate rare earth metal ions.

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

Transforming Thermal Expansion from Positive to Negative: The Case of Cubic Magnetic Compounds of (Zr,Nb)Fe 2

Negative thermal expansion (NTE) is an intriguing property for not only fundamental studies but also technological applications. However, few NTE materials are available compared with the huge amount of positive thermal expansion materials. The discovery of new NTE materials remains challenging. In this paper, we report a chemical modification strategy to transform thermal expansion from positive to negative in cubic magnetic compounds of (Zr,Nb)Fe 2 by tuning the magnetic exchange interaction. Furthermore, an isotropic zero thermal expansion can be established in Zr 0.8 Nb 0.2 Fe 2 (α 1 = 1.4 x 10 -6 K -1 , 3-470 K) over a broad temperature range that is even wider than that of the prototype Invar alloy of Fe 0.64 Ni 0.36 . The NTE of (Zr,Nb)Fe 2 is originated from the weakened magnetic exchange interaction and the increased d electrons of Fe by the Nb chemical substitution, so that the magnetovolume effect overwhelms the contribution of anharmonic lattice vibration.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Engineering Active Surface Oxygen Sites of Cubic Perovskite Cobalt Oxides toward Catalytic Oxidation Reactions

Unraveling the role of surface oxygen sites in transition metal oxides during catalytic reactions has always been the focus of environmental and energy chemistry research. For this work, active surface oxygen sites of cubic perovskite cobalt oxide were engineered to comprehend their crucial role and catalytic mechanism at the molecular level. By removing those inert Sr/La–O termination layers, active oxygen sites were exposed on the Co terminated surface of Sr 0.6 La 0.4 CoO 3–δ that furnished the dominant catalytic process of CO oxidation via the Mars–van Krevelen (MvK) mechanism. The fabrication of five-coordinate cobalt ions and the enhanced covalency of Co–O bonds not only optimize the surface electronic structure of Co 3d–O 2p, but also supply active surface oxygen sites, which effectively oxidizes CO to CO 2 with a significantly improved oxidation performance and stability as evidenced by soft/hard XAS, XPS, and O 2 -TPD. Furthermore, online isotopic 18 O 2 mass spectrometry, in situ DRIFTS, and theoretical simulation demonstrate that the activity of surface oxygen sites enhances the kinetics of the MvK reaction, while unsaturated coordination sites from five-coordinate cobalt ions primarily contribute to the activated oxygen molecules and the stable catalytic cycle. The results reported here provide a deep insight into the comprehension of the relationships among active oxygen sites, surface electronic structure, and the reaction mechanism of transition metal oxides necessary for catalytic oxidation reactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Real-Time KMC Simulation of Vacancy-Mediated Intermixing in Au@Ag Octahedral Core–Cubic Shell Nanocrystals with Ab Initio-Guided Kinetics

Utilization of core–shell rather than monometallic nanocrystals (NCs) facilitates fine-tuning of NC properties for applications. However, compositional evolution via intermixing can degrade these properties prompting recent experimental studies. We develop an atomistic-level stochastic model for vacancy-mediated intermixing exploiting a formalism which allows incorporation at an ab initio density functional theory level of not just the thermodynamics of vacancy formation, but also relevant diffusion barriers for a vast number of possible local environments (in the core and in the shell, at the interface, and in the intermixed phase). This facilitates a predictive treatment and comprehensive understanding of intermixing on the relevant time scale (e.g., 10 1 –10 3 s). In contrast, previous modeling at the atomistic level utilized only unrealistic generic prescriptions of barriers or employed simplified continuum treatments. For Au@Ag octahedral core–cubic shell NCs, our modeling not only captures the experimentally observed rate or time scale for intermixing of ~100 s at 450 °C for 60 nm NCs, but also elucidates the underlying rate controlling processes and the effective intermixing barrier.

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

High Thermoelectric Performance in the New Cubic Semiconductor AgSnSbSe 3 by High-Entropy Engineering

We investigate the structural and physical properties of the AgSn m SbSe m+2 system with m = 1-20 (i.e., SnSe matrix and ~5-50% AgSbSe 2 ) from atomic, nano, and macro length scales. We find the 50:50 composition, with m = 1 (i.e., AgSnSbSe 3 ), forms a stable cation-disordered cubic rock-salt p-type semiconductor with a special multi-peak electronic valence band structure. AgSnSbSe 3 has an intrinsically low lattice thermal conductivity of ~0.47 W m -1 K -1 at 673 K owing to the synergy of cation disorder, phonon anharmonicity, low phonon velocity, and low-frequency optical modes. Furthermore, Te alloying on Se sites creates a quinary high-entropy NaCl-type solid solution AgSnSbSe 3-x Te x with randomly disordered cations and anions. The extra point defects and lattice dislocations lead to glass-like lattice thermal conductivities of ~0.32 W m -1 K -1 at 723 K and higher hole carrier concentration than AgSnSbSe 3 . Concurrently, the Te alloying promotes greater convergence of the multiple valence band maxima in AgSnSbSe 1.5 Te 1.5 , the composition with the highest configurational entropy. Facilitated by these favorable modifications, we achieve a high average power factor of ~9.54 μW cm -1 K -2 (400-773 K), a peak thermoelectric figure of merit ZT of 1.14 at 723 K, and a high average ZT of ~1.0 over a wide temperature range of 400-773 K in AgSnSbSe 1.5 Te 1.5 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Tailoring Interface Energies via Phosphonic Acids to Grow and Stabilize Cubic FAPbI 3 Deposited by Thermal Evaporation

Coevaporation of formamidinium lead iodide (FAPbI 3 ) is a promising route for the fabrication of highly efficient and scalable optoelectronic devices, such as perovskite solar cells. However, it poses experimental challenges in achieving stoichiometric FAPbI 3 films with a cubic structure (α-FAPbI 3 ). In this work, we show that undesired hexagonal phases of both PbI 2 and FAPbI 3 form during thermal evaporation, including the well-known 2H-FAPbI 3 , which are detrimental for optoelectronic performance. We demonstrate the growth of α-FAPbI 3 at room temperature via thermal evaporation by depositing phosphonic acids (PAc) on substrates and subsequently coevaporating PbI 2 and formamidinium iodide. We use density-functional theory to develop a theoretical model to understand the relative growth energetics of the α and 2H phases of FAPbI 3 for different molecular interactions. Experiments and theory show that the presence of PAc molecules stabilizes the formation of α-FAPbI 3 in thin films when excess molecules are available to migrate during growth. This migration of molecules facilitates the continued presence of adsorbed organic precursors at the free surface throughout the evaporation, which lowers the growth energy of the α-FAPbI 3 phase. Our theoretical analyses of PAc molecule–molecule interactions show that ligands can form hydrogen bonding to reduce the migration rate of the molecules through the deposited film, limiting the effects on the crystal structure stabilization. Our results also show that the phase stabilization with molecules that migrate is long-lasting and resistant to moist air. These findings enable reliable formation and processing of α-FAPbI 3 films via vapor deposition.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Correlated Anion Disorder in Heteroanionic Cubic TiOF 2

Resolving anion configurations in heteroanionic materials is crucial for understanding and controlling their properties. For anion-disordered oxyfluorides, conventional Bragg diffraction cannot fully resolve the anionic structure, necessitating alternative structure determination methods. We have investigated the anionic structure of anion-disordered cubic (ReO 3 -type) TiOF 2 using X-ray pair distribution function (PDF), 19 F MAS NMR analysis, density functional theory (DFT), cluster expansion modeling, and genetic-algorithm structure prediction. Our computational data predict short-range anion ordering in TiOF 2 , characterized by predominant cis-[O 2 F 4 ] titanium coordination, resulting in correlated anion disorder at longer ranges. To validate our predictions, we generated partially disordered supercells using genetic-algorithm structure prediction and computed simulated X-ray PDF data and 19 F MAS NMR spectra, which we compared directly to experimental data. To construct our simulated 19 F NMR spectra, we derived new transformation functions for mapping calculated magnetic shieldings to predicted magnetic chemical shifts in titanium (oxy)fluorides, obtained by fitting DFT-calculated magnetic shieldings to previously published experimental chemical shift data for TiF 4 . We find good agreement between our simulated and experimental data, which supports our computationally predicted structural model and demonstrates the effectiveness of complementary experimental and computational techniques in resolving anionic structure in anion-disordered oxyfluorides. From additional DFT calculations, we predict that increasing anion disorder makes lithium intercalation more favorable by, on average, up to 2 eV, highlighting the significant effect of variations in short-range order on the intercalation properties of anion-disordered materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗