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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 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Inverted metamorphic GaInAs solar cell grown by dynamic hydride vapor phase epitaxy

We present an inverted metamorphic rear heterojunction ~1.0 eV GaInAs solar cell deposited by dynamic hydride vapor phase epitaxy (D-HVPE) with high growth rate. This device uses a Ga 1-x In x P compositionally graded buffer (CGB) to bridge the lattice constant gap between the GaAs substrate and the Ga0.71In0.29As emitter layer. High-resolution x-ray diffraction and transmission electron microscopy confirm that the Ga 0.71 In 0.29 As emitter is grown lattice-matched to the in-plane lattice constant of the CGB with minimal generation of defects at the GaInAs/GaInP interface. The device contains a threading dislocation density of 2.3 × 10 6 cm -2 , a level that enables high-performance minority carrier devices and is comparable to previously demonstrated GaInP CGBs grown by D-HVPE. The device exhibits an open-circuit voltage of 0.589 V under a one-sun AM1.5G illumination condition and a bandgap-voltage offset of 0.407 V, indicating metamorphic epitaxial performance nearly equal to state-of-the-art devices. We analyze the dark current of the device and determine that reducing recombination in the depletion region, which can be achieved by reducing the threading dislocation density and optimizing the device doping density, will improve the device performance. Furthermore, the CGB and device layers, comprising ~8 µm of thickness, are grown in under 10 min, highlighting the ability of D-HVPE to produce high-quality metamorphic devices of all types with the potential for dramatically higher throughput compared to present technology.

14 SOLAR ENERGY↗
DOE OSTI2022

Guiding Self-Assembly of Active Colloids by Temporal Modulation of Activity

Self-organization phenomena in ensembles of self-propelled particles open pathways to the synthesis of new dynamic states not accessible by traditional equilibrium processes. The challenge is to develop a set of principles that facilitate the control and manipulation of emergent active states. In this study, we report that dielectric rolling colloids energized by a pulsating electric field self-organize into alternating square lattices with a lattice constant controlled by the parameters of the field. We combine experiments and simulations to examine spatiotemporal properties of the emergent collective patterns and investigate the underlying dynamics of the self-organization.We reveal the resistance of the dynamic lattices to compression and expansion stresses leading to a hysteretic behavior of the lattice constant. The general mechanism of pattern synthesis and control in active ensembles via temporal modulation of activity can be applied to other active colloidal systems.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Ground State Properties of the Wide Band Gap Semiconductor Beryllium Sulfide (BeS)

We report the results from self-consistent calculations of electronic, transport, and bulk properties of beryllium sulfide (BeS) in the zinc-blende phase, and employed an ab-initio local density approximation (LDA) potential and the linear combination of atomic orbitals (LCAO). We obtained the ground state properties of zb-BeS with the Bagayoko, Zhao, and Williams (BZW) computational method, as enhanced by Ekuma and Franklin (BZW-EF). Our findings include the electronic energy bands, the total (DOS) and partial (pDOS) densities of states, electron and hole effective masses, the equilibrium lattice constant, and the bulk modulus. The calculated band structure clearly shows that zb-BeS has an indirect energy band gap of 5.436 eV, from Γ to a point between Γ and X, for an experimental lattice constant of 4.863 Å. This is in excellent agreement with the experiment, unlike the findings of more than 15 previous density functional theory (DFT) calculations that did not perform the generalized minimization of the energy functional, required by the second DFT theorem, which is inherent to the implementation of our BZW-EF method.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Suppression of ferromagnetism governed by a critical lattice parameter in CeTiGe 3 with hydrostatic pressure or V substitution

We combine structural and magnetic measurements to compare the different magnetic phase diagrams between the pressure and substitution studies in CeTiGe 3 . We report on the structural, magnetic, and electrical transport properties of single crystals of CeTi 1–x V x Ge 3 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.9, and 1), and of polycrystalline samples (x = 0.5, 0.6, 0.7, 0.8), as well as structural properties of CeTiGe 3 under pressure up to 9 GPa. The ferromagnetic ordering in CeTiGe 3 is suppressed with V doping in CeTi 1–x V x Ge 3 , and suggests a possible ferromagnetic quantum critical point near x = 0.45. We perform a detailed crystalline electric field (CEF) analysis, and the magnetic susceptibility data in pure CeTiGe 3 and CeVGe 3 can be well explained by the CEF model. The proposed CEF energy levels suggest that there is a gradual change of the ground state from |±5/2 in CeTiGe 3 to |±1/2 in CeVGe 3 , and a suppression of CEF splitting energies near the quantum critical region. When hydrostatic pressure is used instead of chemical substitution, the quantum critical point is avoided by the appearance of magnetic phases above around 4.1 GPa. In the substitution study, the ferromagnetic and antiferromagnetic regions are well separated, whereas they touch in the pressure study. We observe a different trend in the temperature dependence of the resistivity maximum in both studies, suggesting that the CEF splitting energy is suppressed by V substitution but enhanced by pressure. Here, we also observe different responses in lattice constants between the two studies, highlighting the fact that substitution effects cannot be reduced to chemical pressure effects only. Nevertheless, when the magnetic phase diagrams of both hydrostatic pressure and substitution are compared, we find a common critical lattice constant c = 5.78 Å where the ferromagnetic ordering is suppressed in both studies.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Unraveling the Early-Stage Ordering of Krypton Solid Bubbles in Molybdenum: A Multimodal Study

Self-organization of defects such as fission gas bubbles in materials can lead to high inventory capacity for fission gas storage and help mitigate swelling caused by fission gases in nuclear fuel materials under radiation in nuclear reactors. Here, we report the physical mechanism of self-organization of krypton (Kr) gas bubbles in molybdenum (Mo) under ion implantation. The ion fluence and temperature-dependent formation of Kr solid bubble superlattice (SBS) in Mo were investigated by using both synchrotron-based small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Early stage self-organization of gas bubbles is observed at a fluence of 2.5 × 10 16 ions/cm 2 at temperatures of 300–400 °C. The bubble lattice constant increases with increasing implantation temperature from 300 to 400 °C. Both experiments and atomic kinetic Monte Carlo modeling indicate that the Kr solid bubbles are weakly ordered in comparison to previously studied helium (He) gas bubble superlattice (GBS) while the lattice constant are relatively smaller for Kr SBS compared to that of He GBS. The irradiation conditions suggest that spinodal decomposition, which is a form of phase separation, probably precedes gas bubble ordering in Mo. Altogether, our work sheds light on the formation mechanism of noble gas superlattice toward the development of radiation-tolerant materials which are important for the design of advanced nuclear reactors.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Polyhedral Distortions and Unusual Magnetic Order in Spinel FeMn 2 O 4

Spinel compounds AB 2 X 4 consist of both tetrahedral (AX 4 ) and octahedral (BX 6) environments with the former forming a diamond lattice and the latter a geometrically frustrated pyrochlore lattice. Exploring the fascinating physical properties and their correlations with structural features is critical in understanding these materials. FeMn 2 O 4 has been reported to exhibit one structural transition and two successive magnetic transitions. In this work, we report the polyhedral distortions and their correlations to the structural and two magnetic transitions in FeMn 2 O 4 by employing the high-resolution neutron powder diffraction. The cation distribution is found to be ($Mn_{0.9}^{2+}Fe_{0.1}^{3+}$)$_{A}$($Mn^{3+}Fe_{0.9}^{3+}Mn_{0.1}^{2+}$)$_{B}$O 4 . While large trigonal distortion is found even in the high-temperature cubic phase, the first-order cubic-tetragonal structural transition associated with the elongation of both tetrahedra and octahedra with shared oxygen atoms along the c axis occurs at T S ≈ 750 K, driven by the Jahn–Teller effect of the orbital active B-site Mn 3+ cation. Strong magnetoelastic coupling is unveiled at T N1 ≈ 400 K as manifested by the appearance of Néel-type collinear ferrimagnetic order, an anomaly in both tetrahedral and octahedral distortions, as well as an anomalous decrease of the lattice constants c and a weak anomaly of a. Upon cooling to T N2 ≈ 65 K, it evolves to a noncollinear ferrimagnetic order accompanied by the different moments at the split magnetic sites B1 and B2. Only one-half of the B-site Mn 3+ /Fe 3+ spins, i.e., the B2-site spins in the pyrochlore lattice, are canted, which is a unique magnetic order among spinels. The canting angle between A-site and B2-site moments is ~25°, but the B1-site moment stays antiparallel to the A-site moment even at 10 K. This noncollinear order is accompanied by a modification of the O–B–O bond angles in the octahedra without significant change in lattice constants or tetrahedral/octahedral distortion parameters, indicating a distinct magnetoelastic coupling. We demonstrate distinct roles of the A-site and B-site magnetic cations in the structural and magnetic properties of FeMn 2 O 4 . Our study indicates that FeMn 2 O 4 is a wonderful platform to unveil interesting magnetic order and to investigate their correlations with polyhedral distortions and lattice.

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

Using resonant energy X-ray diffraction to extract chemical order parameters in ternary semiconductors

II–IV–V 2 materials, ternary analogs to III–V materials, are emerging for their potential applications in devices such as LEDs and solar cells. Controlling cation ordering in II–IV–V 2 materials offers the potential to tune properties at nearly fixed compositions and lattice parameters. While tuning properties at a fixed lattice constant through ordering has the potential to be a powerful tool used in device fabrication, cation ordering also creates challenges with characterization and quantification of ordering. In this work, we investigate two different methods to quantify cation ordering in ZnGeP 2 thin films: a stretching parameter calculated from lattice constants , and an order parameter determined from the cation site occupancies ( S ). We use high resolution X-ray diffraction (HRXRD) to determine and resonant energy X-ray diffraction (REXD) to extract S . REXD is critical to distinguish between elements with similar Z -number ( e.g. Zn and Ge). We found that samples with a corresponding to the ordered chalcopyrite structure had only partially ordered S values. The optical absorption onset for these films occurred at lower energy than expected for fully ordered ZnGeP 2 , indicating that S is a more accurate descriptor of cation order than the stretching parameter. Since disorder is complex and can occur on many length scales, metrics for quantifying disorder should be chosen that most accurately reflect the physical properties of interest.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Assessing the Performance of Exchange‐Correlation Density Functionals in Describing the Iron‐Catalyzed Ammonia Synthesis System

Density functional theory (DFT) has been widely employed for elucidating mechanistic aspects of heterogeneous catalysis. However, the accuracy of DFT calculations relies heavily on selecting an exchange-correlation (XC) functional that correctly describes the electronic structure of materials involved in the reactions. This study assesses the accuracy of several XC density functionals for modeling the iron-catalyzed ammonia synthesis system. In the assessment of functional accuracy, experimental references are compared to DFT-calculated values for the formation energy of gas-phase ammonia and nitrogen, bulk Fe/Fe-nitride (γ′-Fe 4 N) lattice constants and cohesive/formation energies, and nitrogen and ammonia binding energies on Fe(100), Fe(111), Fe(110), and γ′-Fe 4 N(111). It is observed that the experimental value for each of these descriptors is accurately modeled by at least one functional. RPBE alone provides reliable estimates for both the lattice constant and cohesive energy of Fe and γ′-Fe 4 N. Temperature-programmed desorption experiments led to estimates for N and NH 3 adsorption across several Fe-based facets that are best captured by RPBE. These results highlight the importance of choosing an appropriate XC functional that accurately describes experimental systems and offer insights into effectively modeling the interactions between nitrogen and ammonia on Fe-based surfaces.

adsorption↗
DOE OSTI2022

Nonvanishing quadrature derivatives in the analytical gradients of density functional energies in crystals and helices

It is shown that the quadrature derivatives in some analytical gradients of energies evaluated with a multi-centre radial-angular grid do not vanish even in the limit of an infinitely dense grid, causing severe errors when neglected. The gradients in question are those with respect to a lattice constant of a crystal or to the helical angle of a chain with screw axis symmetry. Furthermore, this is in contrast with the quadrature derivatives in atomic gradients, which can be made arbitrarily small by grid extension. The disparate behaviour is traced to whether the grid points depend on the coordinate with respect to which the derivative of energy is taken. Whereas the nonvanishing quadrature derivative in the lattice-constant gradient is identified as the surface integral arising from an expanding integration domain, the analytical origin of the nonvanishing quadrature derivative in the helical-angle gradient remains unknown.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2020

Disorder-Tunable ZnGeP2 for Epitaxial Top Cells on Si

There has been a longstanding search for top cell materials for Si-based tandems. ZnGeP 2 is one material that could fit this need. It is lattice matched to Si and has the potential for tuning its band gap at fixed lattice constant via cation ordering. In this study, we investigate the effects of growth and annealing conditions on the structure of ZnGeP 2 thin films. Films were deposited amorphous and then annealed ex-situ. Using low anneal temperatures or short anneal times, we were able to kinetically trap the disordered phase. We also found composition to play a role in the degree of ordering in our films. Our findings support the hypothesis that ZnGeP 2 could be implemented as a material with tunable properties at fixed lattice constant through cation ordering.

14 SOLAR ENERGY↗
DOE OSTI2023

Near surface structure of ultrathin epitaxial Ru films on graphene/amorphous SiO 2 revealed by azimuthal RHEED

Ru has been considered as an alternative metallic candidate for future local interconnects. The 2D reciprocal space map constructed from the azimuthal reflection high-energy electron diffraction patterns reveals that ultrathin Ru(0001) is epitaxially grown on transferred graphene on amorphous SiO 2 through quasi-van der Waals interaction. The in-plane and out-of-plane lattice constants are measured from streaks’ separation and intensity modulations along streaks, respectively. Weak and broad rings indicate that a low density of nanoscale polycrystals exist on the surface. In this work, The intensities of 00 and non-00 diffraction spots vs. azimuthal angles in the 2D map show a few degrees out-of-plane and in-plane angular misorientations among grains, respectively. As the film thickness decreases these angular misorientations increase. Transmission electron microscopy carried out in this study also provides precise values of lattice constant and sub-grain sizes in the films. These findings show that ultrathin Ru film is epitaxial but not exactly single crystalline.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Graded Buffer Bragg Reflectors with High Reflectivity and Transparency for Metamorphic Optoelectronics

A graded buffer Bragg reflector (GBBR) is a bifunctional device component that provides the reflection of a distributed Bragg reflector and the adjustable lattice constant of a compositionally graded buffer. Prior work showed that these properties can be combined with low threading dislocation density and high reflectivity. Here, we design and demonstrate complex GBBRs for specific metamorphic solar cell applications. We design buffers that provide (1) high reflectivity over a narrow bandwidth, for quantum well solar cells, (2) reflectivity over a wide bandwidth, for optically thin solar cells, and (3) low sidelobe reflection, for multijunction devices that demand low out-of-band reflection. Apodized and triple GBBRs are demonstrated, and transparency is always considered, requiring designs with carefully engineered material combinations. A GBBR with a reflection of 99% is demonstrated, as well as a triple GBBR that has over 80% reflection for 100?nm of the spectrum around 800?nm. We also analyze potential deviations in a baseline GBBR from that of a perfect DBR using transmission electron microscopy to analyze imperfections in the material and modeling to analyze the impact of imperfect refractive index data for lattice-mismatched AlGaInAs. Minimal crosshatch roughness and unintentional thickness variation occur throughout the buffer, which likely influences reflection slightly. Small deviations between the calculated AlGaInAs and utilized AlGaAs refractive index exist, giving methods for future GBBR improvement. While the GBBR designs are intended for metamorphic solar cells, the broad and high reflection may also be useful for other optoelectronic devices such as light-emitting diodes or lasers.

41 EE - Solar Energy Technologies Office (EE-4S)↗
DOE OSTI2020

Rethinking the magnetic properties of lepidocrocite: A density functional theory and cluster expansion study

The iron oxyhydroxide lepidocrocite (g-FeOOH) is an abundant mineral critical to a number of chemical and technological applications. Of particular interest is the ground state and nite temperature magnetic order, and the subsequent impact this has upon crystal properties. The magnetic properties, investigated in this work are governed primarily through superexchange interactions, and have been calculated using density functional theory and cluster expansion methods. Quantication of these exchange terms has facilitated the determination of the ground state magneto-crystalline structure and subsequent calculation of its lattice constants, elastic moduli, cohesive enthalpy, and electronic density of states. Further, using a collinear magnetic conguration model, the magnetic heat capacity versus temperature has been studied and the Neel temperature obtained.

Pope, Daniel J.↗
DOE OSTI2021

Raman signatures from age-dating PuO 2 since last calcination

The self-irradiation of PuO 2 has been shown to affect the attributes of fifteen vibrational bands in the Raman spectrum. The vibrational bands correspond to the Raman allowed vibrational mode T 2g , overtones, defects, and bands from electronic origin. Defects in the crystal lattice from low-calcination temperature and alpha decay origins are responsible for the changes in the band attributes (band position, full-width half maximum (FWHM), intensity). Crystal lattice defects are also responsible for the growth of the defect bands. The temporal behavior of the vibrational bands can be used to estimate the age of the material since last calcination. Laser annealing was used to reset the material to full crystallinity prior to the aging study. 240 PuO 2 , with an alpha decay rate of 3.67 greater than the decay rate of 239 PuO 2 , was primarily used to assess the alpha decay-induced damage within 3 years. The aging study focused on the time-dependent properties of the T 2g band and the growth of defect bands. The FWHM of the T 2g band and the ratio of the areas of the defect bands/T 2g band were measured through a time period in excess of 3 years. The data from this study were found very close to the data describing the temporal evolution of the lattice constant measured with the X-ray diffraction (XRD) technique. The local physical properties of the material measured with Raman spectroscopy and the lattice parameter constant measured with XRD indicates a strong correlation between the two techniques. Although both techniques provide similar age-dating information, in contrast to the few milligram quantities required for XRD, Raman spectroscopy requires a 10 µm diameter PuO 2 particle to provide an estimate of the material age since last calcination.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2020

M06-SX screened-exchange density functional for chemistry and solid-state physics

Significance Density functionals with Hartree–Fock exchange have been widely used for a wide range of chemical applications, but the nonlocal character of exchange makes long-range exchange computationally expensive for solid-state calculations with periodic boundary conditions, and full exchange is nonphysical for condensed-phase systems. Here, we present a screened-exchange (SX) density functional, M06-SX, that is especially designed to have good accuracy for both solid-state physics and chemical applications with less computational cost than full Hartree–Fock exchange. The M06-SX functional gives accuracy comparable to functionals with full Hartree–Fock exchange for predicting chemical properties of molecules, while also being practical with good accuracy for plane wave calculations on band gaps and lattice constants of solids.

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

The In Situ Observation of Phase Transformations During Intercritical Annealing of a Medium Manganese Advanced High Strength Steel by High Energy X-Ray Diffraction

Microstructural changes during thermal processing of a medium manganese steel containing (in wt%) 0.19C and 4.39 Mn were evaluated in situ with a high energy X-ray diffraction system (HEXRD). Samples with an initial fully martensitic microstructure were heated to intercritical annealing (IA) temperatures of 600 or 650°C, held for 30 min, and cooled to room temperature. Diffraction data were analyzed to determine the variations in austenite and ferrite phase fractions and phase lattice constants throughout the ICA cycles. On heating, the 2 vol. pct of austenite present in the starting microstructure decomposed, and cementite precipitation then occurred. During isothermal holding, the austenite fraction increased, up to 20% for the sample annealed at 650°C. The measured austenite fractions were less than those calculated by Thermo-Calc for equilibrium conditions, indicating that the 30-min hold time was insufficient to achieve near-equilibrium conditions. Observed changes in lattice parameters during isothermal holding were interpreted to reflect composition changes due to redistribution of the C and Mn between austenite and ferrite. The results are discussed in relation to the potential for controlling austenite stability during ambient temperature deformation.

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

Ultrathin epitaxial NbN superconducting films with high upper critical field grown at low temperature

Ultrathin (5–50 nm) epitaxial superconducting niobium nitride (NbN) films were grown on AlN-buffered c-plane Al 2 O 3 by an industrial scale physical vapor deposition technique at 400°C. Both X-ray diffraction and scanning electron microscopy analysis show high crystallinity of the (111)-oriented NbN films, with a narrow full-width-at-half-maximum of the rocking curve down to 0.030°. The lattice constant decreases with decreasing NbN layer thickness, suggesting lattice strain for films with thicknesses below 20 nm. The superconducting transition temperature, the transition width, the upper critical field, the irreversibility line, and the coherence length are closely correlated to the film thickness. This work realized high quality ultrathin epitaxial NbN films by an industry-scale PVD technology at low substrate temperature, which opens up new opportunities for quantum devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗