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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 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Phase Stability in the Fe-Rich Fe-Cr-Ni-Zr Alloys

Knowledge on phase stability in Fe-rich Fe-Cr-Ni-Zr alloys is needed for the development of Laves-phase strengthened Fe-Cr-Ni-Zr ferritic alloys which have shown promising applications as new cladding materials of nuclear reactors due to enhanced high temperature strength, and resistance to creep and irradiation hardening. Phase stability in four Fe-rich Fe-Cr-Ni-Zr alloys was carefully investigated using scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffraction techniques. The samples were arc-melted and heat treated at 700 °C for 336 hours and 1000 °C for 1275 hours. The experimental results showed extensive solubility of Ni in the intermetallic phases Fe 23 Zr 6 and Fe 2 Zr_C15. Nickel stabilizes the Laves Fe 2 Zr_C15 structure more than the C36 and C14 structures. In addition to Fe 23 Zr 6 and Fe 2 Zr_C15, Ni 7 Zr 2 was found to be stable in samples with higher Ni content and lower annealing temperature. The body-centered-cubic, Fe 2 Zr_C15 and Fe 23 Z 6 coexist in all samples regardless of composition and temperature. Detailed analysis suggested that the coexistence of these three phases is not a result of thermodynamic equilibrium, but a result of the competing formation kinetics of Fe 2 Zr_C15 and Fe 23 Z 6 .

36 MATERIALS SCIENCE↗
DOE OSTI2015

Grain size dependent phase stabilities and presence of a monoclinic (Pm) phase in the morphotropic phase boundary region of (1−x)Bi(Mg{sub 1/2}Ti{sub 1/2})O{sub 3}-xPbTiO{sub 3} piezoceramics

Results of the room temperature structural studies on (1−x)Bi(Mg{sub 1/2}Ti{sub 1/2})O{sub 3}-xPbTiO{sub 3} ceramics using Rietveld analysis of the powder x-ray diffraction data in the composition range 0.28 ≤ x ≤ 0.45 are presented. The morphotropic phase boundary region exhibits coexistence of monoclinic (space group Pm) and tetragonal (space group P4 mm) phases in the composition range 0.33 ≤ x ≤ 0.40. The structure is nearly single phase monoclinic (space group Pm) in the composition range 0.28 ≤ x ≤ 0.32. The structure for the compositions with x ≥ 0.45 is found to be predominantly tetragonal with space group P4 mm. Rietveld refinement of the structure rules out the coexistence of rhombohedral and tetragonal phases in the morphotropic phase boundary region reported by earlier authors. The Rietveld structure analysis for the sample x = .35 calcined at various temperatures reveals that phase fraction of the coexisting phases in the morphotropic phase boundary region varies with grain size. The structural parameters of the two coexisting phases also change slightly with changing grain size.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Solid phase stability of molybdenum under compression: Sound velocity measurements and first-principles calculations

The high-pressure solid phase stability of molybdenum (Mo) has been the center of a long-standing controversy on its high-pressure melting. In this work, experimental and theoretical researches have been conducted to check its solid phase stability under compression. First, we performed sound velocity measurements from 38 to 160 GPa using the two-stage light gas gun and explosive loading in backward- and forward-impact geometries, along with the high-precision velocity interferometry. From the sound velocities, we found no solid-solid phase transition in Mo before shock melting, which does not support the previous solid-solid phase transition conclusion inferred from the sharp drops of the longitudinal sound velocity [Hixson et al., Phys. Rev. Lett. 62, 637 (1989)]. Then, we searched its structures globally using the multi-algorithm collaborative crystal structure prediction technique combined with the density functional theory. By comparing the enthalpies of body centered cubic structure with those of the metastable structures, we found that bcc is the most stable structure in the range of 0–300 GPa. The present theoretical results together with previous ones greatly support our experimental conclusions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

(Y0.5In0.5)Ba(Co,Zn)4O7 cathodes with superior high-temperature phase stability for solid oxide fuel cells

(Y0.5In0.5)BaCo4-xZnxO7 (1.0 x 2.0) oxides crystallizing in a trigonal P31c structure have been synthesized and explored as cathode materials for solid oxide fuel cells (SOFC). At a given Zn content, the (Y0.5In0.5)BaCo4-xZnxO7 sample with 50 % Y and 50 % In exhibits much improved phase stability at intermediate temperatures (600 - 800 oC) compared to the samples with 100 % Y or In. However, the substitution of Zn for Co in (Y0.5In0.5)Ba(Co4-xZnx)O7 (1.0 x 2.0) decreases the amount of oxygen loss on heating, total electrical conductivity, and cathode performance in SOFC while providing good long-term phase stability at high temperatures. Among the various chemical compositions investigated in the (Y0.5In0.5)Ba(Co4-xZnx)O7 system, the (Y0.5In0.5)BaCo3ZnO7 sample offers a combination of good electrochemical performance and low thermal expansion coefficient (TEC) while maintaining superior phase stability at 600 800 oC for 100 h. Fuel cell performances of the (Y0.5In0.5)Ba(Co3Zn)O7 + Ce0.8Gd0.2O1.9 (GDC) (50 : 50 wt. %) composite cathodes collected with anode-supported single cell reveal a maximum power density value of 521 mW cm-2 at 700 oC.

cathodes↗
DOE OSTI2024

Predictions of Boron Phase Stability Using an Efficient Bayesian Machine Learning Interatomic Potential

Thermodynamic phase stability of three elemental boron allotropes, i.e., α-B, β-B, and γ-B, was investigated using a Bayesian interatomic potential trained via a sparse Gaussian process (SGP). SGP potentials trained with datasets from on-the-fly active learning achieve quantum mechanical level accuracy when employed in molecular dynamics simulations to predict wide-ranging thermodynamic, structural, and vibrational properties. The simulated phase diagram (500~1400 K and 0~16 GPa) agrees with experimental measurements. The SGP-based MD simulations also successfully predicted that the B13 defect is critical in stabilizing β-B below 700 K. At higher temperatures, the entropy becomes the dominant factor, making β-B the more stable phase over α-B. Furthermore, this Letter demonstrates that SGP potentials based on a training set consisting of defect-free-only systems could make correct predictions of defect-related phenomena in solid-state crystals, paving the path to investigate crystal phase stability and transitions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Phase stability, structures and properties of the (Bi{sub 2}){sub m} (Bi{sub 2}Te{sub 3}){sub n} natural superlattices

The phase stability of the (Bi{sub 2}){sub m} (Bi{sub 2}Te{sub 3}){sub n} natural superlattices has been investigated through the low temperature solid state synthesis of a number of new binary Bi{sub x}Te{sub 1-x} compositions. Powder X-ray diffraction revealed that an infinitely adaptive series forms for 0.44{<=}x{<=}0.70, while an unusual 2-phase region with continuously changing compositions is observed for 0.41{<=}x{<=}0.43. For x>0.70, mixtures of elemental Bi and an almost constant composition (Bi{sub 2}){sub m} (Bi{sub 2}Te{sub 3}){sub n} phase are observed. Rietveld analysis of synchrotron X-ray powder diffraction data collected on Bi{sub 2}Te (m=2, n=1) revealed substantial interchange of Bi and Te between Bi{sub 2} and Bi{sub 2}Te{sub 3} blocks, demonstrating that the block compositions are variable. All investigated phase pure compositions are degenerate semiconductors with low residual resistivity ratios and moderate positive magnetoresistances (R/R{sub 0}=1.05 in 9 T). The maximum Seebeck coefficient is +80 {mu}V K{sup -1} for x=0.63, leading to an estimated thermoelectric figure of merit, zT=0.2 at 250 K. - Graphical abstract: An infinite number of possible (Bi{sub 2}){sub m} (Bi{sub 2}Te{sub 3}){sub n} structures exist for Bi{sub x}Te{sub 1-x} compositions between x=0.44 and x=0.70. Compositions near x=2/3 are promising p-type thermoelectrics. Highlights: Black-Right-Pointing-Pointer Phase Stability of the (Bi{sub 2}){sub m}{center_dot}(Bi{sub 2}Te{sub 3}){sub n} Natural Superlattices. Black-Right-Pointing-Pointer Thermoelectric Properties in the Bi{sub x}Te{sub 1-x} Phase Diagram. Black-Right-Pointing-Pointer Crystal Structure of Bi{sub 2}Te from Synchrotron X-ray Powder Diffraction.

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

Phase Stability of Yttrium Titanium Oxide

This proposed study aims at elevating Idaho National Laboratory (INL)’s portfolio in nuclear material research on two major fronts: 1) expanding INL’s material characterization capabilities to provide holistic microstructural and microchemical characterization ranging from atomic scale to millimeter scale, and 2) filling the knowledge gap on the effect of microstructural evolution on material phase stability at extreme conditions resembling those inside nuclear reactors, using Yttrium-Titanium-Oxide (YTO) as a model material. Previous research indicated the kinetics of phase transition of YTO materials varies between bulk samples and powder samples, suggesting that microstructural characteristics such as defect, micro-strain, and crystallite size could play a significant role in phase transition. The effect of crystallite size on the thermodynamics of phase transition has been well documented for other ceramic materials at nanometer crystallite size, with potential impacts from surface and strain energy and defects. In this proposed study, we will perform microstructural (micro-strain, crystallite size, defects, phase) and microchemical (elemental mapping) analyses through in-situ high temperature measurements using X-ray diffraction (XRD; at millimeter scale for powder and bulk samples) and transmission electron microscopy (TEM, at nanometer scale and atomic scale for powder and thin foil samples), to provide a comprehensive view of the effect of microstructural and microchemical evolution on phase transition and phase stability of YTO materials. These findings will also establish a roadmap and provide new insights on the phase stability of other materials that are critical to nuclear energy and other renewable energy infrastructure. Besides advancing our understanding of this fundamental material science question, this project will also enable the development of state-of-the-art material characterization capabilities at INL/MFC for microstructural and microchemical analysis at high temperature conditions. The new collaboration between the High-Resolution Materials Characterization group at INL and the Electron Microscopy group at the University of Florida (UF) in this project will open the possibilities for more fruitful and synergetic cutting-edge nuclear material research in the future.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Crystal structure, chemical expansion and phase stability of HoMnO{sub 3} at high temperature

Anisotropic thermal and chemical expansion of hexagonal HoMnO{sub 3} was investigated by high temperature X-ray diffraction in inert (N{sub 2}) and oxidizing (air) atmospheres up to 1623 K. A second order structural phase transition directly from P6{sub 3}cm to P6{sub 3}/mmc was found at 1298{+-}4 K in N{sub 2} atmosphere, and 1318{+-}4 K in air. For the low temperature polymorph P6{sub 3}cm the contraction of the c-axis was more rapid in inert than in oxidizing atmosphere. The c-axis of the P6{sub 3}/mmc polymorph of HoMnO{sub 3} displayed anomalously high expansion above 1400 K, which is discussed in relation to chemical expansion caused by point defects. The a-axis expanded stronger in inert than oxidizing atmosphere. Anisotropic chemical and thermal expansion of the P6{sub 3}cm phase of YMnO{sub 3} in N{sub 2}, air and O{sub 2} atmospheres was found to be qualitatively similar to that of HoMnO{sub 3}. Decomposition of hexagonal HoMnO{sub 3} by two different processes occurs in oxidizing atmosphere above {approx}1200 K followed by nucleation and growth of the perovskite polymorph of HoMnO{sub 3}. A rapid, reconstructive transition from the perovskite back to the hexagonal polymorph was observed in situ at 1623 K upon reduction of the partial pressure of oxygen. A phase stability diagram of the hexagonal and orthorhombic polymorphs is proposed. Finally, distinctly non-linear electrical conductivity was observed for both HoMnO{sub 3} and YMnO{sub 3} in oxidizing atmosphere between 555 and 630 K, and shown to be associated with excess oxygen. - Graphical abstract: Chemical expansion of hexagonal HoMnO{sub 3} is observed during HTXRD in different pO{sub 2}. Oxidizing atmosphere favors the competing perovskite polymorph. Electrical conductivity anomalies related to excess oxygen are found at 550-630 K. Highlights: Black-Right-Pointing-Pointer Thermal evolution of crystal structure of HoMnO{sub 3} studied up to 1623 K in air and N{sub 2}. Black-Right-Pointing-Pointer Anisotropic chemical expansion of HoMnO{sub 3} and YMnO{sub 3} in N{sub 2}, air and O{sub 2}. Black-Right-Pointing-Pointer Hexagonal phase destabilized with respect to perovskite in oxidizing atmosphere. Black-Right-Pointing-Pointer Crystal structure and phase stability discussed in terms of point defect chemistry. Black-Right-Pointing-Pointer Electrical conductivity anomalies associated with excess oxygen at 550-630 K.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Phase Stability in Calcia‐Doped Zirconia Nanocrystals

Calcia‐doped zirconia exhibits all of the polymorphism seen in the yttria‐doped zirconia ceramics, but can be produced at lowered costs and in greater abundance due to the accessibility of Calcium precursors in comparison to Yttrium. Although with great challenges, there exists an opportunity to replace yttria with calcia in applications such as ionic conductors where phase stability is critical. There is a dearth of surface characterization to enable design and prediction of the polymorphism in nanoparticulate calcia–zirconia. With recent advances in water adsorption microcalorimetry, one can accurately probe surface energies of the four zirconia polymorphs: monoclinic, tetragonal, cubic, and amorphous. The surface energies can then be coupled with bulk enthalpies extracted from oxide melt drop solution calorimetry to create a nanocrystalline phase stability diagram similar to its bulk counterpart. We report here the surface and bulk thermodynamic data on polymorphs of calcia–zirconia with composition ranging from 0 to 20 mol% calcia and use it to build a nanophase diagram for this system. The effect of the humidity in the phase stability diagram trends is also addressed and demonstrated to minimize the effect of the surface energies in the overall polymorphism trends.

Drazin, John W.↗
DOE OSTI2016

Unexpected role of linker position on ammonium gemini surfactant lyotropic gyroid phase stability

Arising from the water-driven self-assembly of amphiphiles over generally narrow temperature and composition phase windows, aqueous lyotropic liquid crystal (LLC) network phases are useful in applications as therapeutic delivery vehicles and templates for mesoporous material syntheses. While a clear set of amphiphile design rules that enables access to these intricate three-dimensional structures has yet to emerge, recent work indicates that bis(ammonium), bis(phosphonium), and dicarboxylate gemini (“twin tail”) surfactants enable enhanced access to LLC network phases such as the double gyroid (G). In order to better understand the scope of this amphiphile design strategy, we investigated the synthesis and aqueous LLC self-assembly behaviors of a homologous series of quaternary gemini bis(ammonium) dichloride surfactants, in which we varied the position of the hydrophobic linker that connects the constituent single tail surfactants. These experiments show that the position of the linker directly impacts the maximum counterion–headgroup hydration capacity and the extent of counterion–headgroup association, all of which contribute to the aqueous lyotropic double gyroid network phase stability. Thus, judicious selection of the linker position in ionic gemini surfactants provides a new molecular design tool for manipulating LLC network phase stability.

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

Phase stability and microstructure of neutron-irradiated substoichiometric yttrium dihydrides

The impact of the neutron-displacement damage on phase stability and microstructure of substoichiometric yttrium dihydrides (YH x , x <2) were investigated to assess their use as solid moderator in high-temperature nuclear reactors. YH x specimens were, thus, subjected to neutron irradiations in the range of 0.1–2 displacements per yttrium atom (dpa-Y) in the temperature range of 536–878°C at the Oak Ridge National Laboratory's (ORNL's) High Flux Isotope Reactor (HFIR). YH x specimens were initially prepared at stoichiometry (H/Y) ratios of 1.69 and 1.83. HFIR-irradiated specimens were characterized by variety of techniques to investigate H retention characteristics including dimensional analysis, optical microscopy, scanning electron microscopy electron back scatter diffraction (EBSD), transmission electron microscopy, thermal desorption spectroscopy (TDS), and high-energy x-ray diffraction (HE-XRD) characterizations. Overall, YH x exhibited notable structural and phase stability under short-term neutron-irradiation, except for the samples with significant silicon carbide (SiC) interaction at high doses and temperatures. Basic dimensional and mass measurements were misleading for accurate assessment of H retention, as confirmed by EBSD phase maps, XRD line profiles, and TDS signals. Thus, it was discussed that a robust H retention metric is needed to assess irradiated hydrides. Further, nanoscale cavities were observed as a result of the neutron irradiation in all samples. Although no clear impact of dose and irradiation temperature was determined, the initial H/Y ratio had an impact on the cavity number density where low H/Y specimens had high-resistance to cavity formation. The Y-vacancy cluster formation at the collision stage of the displacement cascade and their stabilization by H were considered to be the likely underlying mechanisms for the observed cavity microstructure.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2025

Interplay between electron localization, magnetic order, and Jahn-Teller distortion dictates LiMnO2 phase stability

The development of manganese (Mn)-rich cathodes for Li-ion batteries promises to alleviate potential supply chain bottlenecks in battery manufacturing. Fundamental challenges in Mn-rich cathodes arise from phenomena such as structural changes due to cooperative Jahn-Teller (JT) distortions of in octahedral environments, Mn migration, and phase transformations to spinel-like order, all of which affect the electrochemical performance. These physically complex phenomena motivate an re-examination of the Li-Mn-O rock-salt space, with a focus on the thermodynamics of the prototypical, polymorphs. It is found that the generalized gradient approximation (GGA-PBEsol) and meta-GGA ( ) density functionals with empirically fitted on-site Hubbard corrections yield spurious stable phases for , such as predicting a phase with -like order ( ) to be the ground state instead of the orthorhombic (Pmmn) phase, which is the experimentally known ground state. Accounting for antiferromagnetic order in each structure is shown to have a substantial effect on the total energies and resulting phase stability. By using hybrid-GGA (HSE06) and GGA with self-consistent Hubbard parameters (on-site and inter-site ) calculated from linear response theory, the experimentally observed phase stability trends are recovered. The calculated on-site between Mn- states in the experimentally observed orthorhombic, layered, and spinel phases are significantly smaller than in and disordered layered structures, by within GGA. The smaller values of are shown to be correlated with a collinear ordering of JT distortions, in which all orbitals are oriented in the same direction. This cooperative JT effect can lead to greater electron delocalization from Mn along the states due to increased Mn-O covalency, which contributes to the greater electronic stability compared to the phases with noncollinear JT arrangements. The structures with collinear ordering of JT distortions also generate greater vibrational entropy, which helps stabilize these phases at high temperature. These phases are shown to be strongly insulating with large calculated band gaps , which are computed using HSE06 and .

Kam, Ronald L↗
DOE OSTI2024

Cluster expansion by transfer learning for phase stability predictions

Recent progress towards universal machine-learned interatomic potentials holds considerable promise for materials discovery. Yet the accuracy of these potentials for predicting phase stability may still be limited. In contrast, cluster expansions provide accurate phase stability predictions but are computationally demanding to parameterize from first principles, especially for structures of low dimension or with a large number of components, such as interfaces or multimetal catalysts. We overcome this trade-off via transfer learning. Using Bayesian inference, we incorporate prior statistical knowledge from machine-learned and physics-based potentials, enabling us to sample the most informative configurations and to efficiently fit first-principles cluster expansions. Furthermore, this algorithm is tested on Pt:Ni, showing robust convergence of the mixing energies as a function of sample size with reduced statistical fluctuations.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Computational investigation of the phase stability and the electronic properties for Gd-doped HfO{sub 2}

Rare earth doping is an important approach to improve the desired properties of high-k gate dielectric oxides. We have carried out a comprehensive theoretical investigation on the phase stability, band gap, formation of oxygen vacancies, and dielectric properties for the Gd-doped HfO{sub 2}. Our calculated results indicate that the tetragonal phase is more stable than the monoclinic phase when the Gd doping concentration is greater than 15.5%, which is in a good agreement with the experimental observations. The dopant's geometric effect is mainly responsible for the phase stability. The Gd doping enlarges the band gap of the material. The dielectric constant for the Gd-doped HfO{sub 2} is in the range of 20–30 that is suitable for high-k dielectric applications. The neutral oxygen vacancy formation energy is 3.2 eV lower in the doped material than in pure HfO{sub 2}. We explain the experimental observation on the decrease of photoluminescence intensities in the Gd-doped HfO{sub 2} according to forming the dopant-oxygen vacancy complexes.

36 MATERIALS SCIENCE↗
DOE OSTI2013

MD simulations of phase stability of PuGa alloys: Effects of primary radiation defects and helium bubbles

We present classical molecular dynamics (MD) with Modified Embedded Atom Model (MEAM) simulations to investigate the role of primary radiation defects and radiogenic helium as factors affecting the phase stability of PuGa alloys in cooling–heating cycles at ambient pressure. The models of PuGa alloys equilibrated at ambient conditions were subjected to cooling–heating cycles in which they were initially cooled down to 100 K and then heated up to 500 K at ambient pressure. The rate of temperature change in the cycles was 10 K/ns. The simulations showed that the initial FCC phase of PuGa alloys undergo polymorphous transition in cooling to a lower symmetry α'-phase. All the alloys undergo direct and reverse polymorphous transitions in the cooling–heating cycles. The alloys containing vacancies shift in both transitions to lower temperatures relative to the defect-free alloys. The radiogenic helium has much less effect on the phase stability compared to that of primary radiation defects (in spite of the fact that helium concentration is twice of that for the primary radiation defects). Lastly, this computational result agrees with experimental data on unconventional stabilization mechanism of PuGa alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2023

VTAnDeM: A python toolkit for simultaneously visualizing phase stability, defect energetics, and carrier concentrations of materials

Phase stability, defect formation energies, and carrier concentrations are closely interrelated features of semiconductors. Due to their joint dependence on the multidimensional chemical potential space, it is challenging to quantitatively establish patterns between these quantities in a given semiconductor, especially when the semiconductor is comprised of multiple elements. To enable synchronous visualization and analysis of these complementary material properties and their interdependence, we developed the Visualization Toolkit for Analyzing Defects in Materials (VTAnDeM). This python-based toolkit allows users to interactively explore how defect formation energies and carrier concentrations vary across the composition and chemical potential spaces of multicomponent semiconductors. Here, we illustrate the computational workflow that employs VTAnDeM as a post-processing tool for first-principles calculations and describe the data organization and theory underlying the visualization scheme. Furthermore, we believe that this software will serve as a useful tool for simultaneously visualizing the often complex and non-intuitive chemical potential – defect – carrier concentration phase space of semiconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Phase stability of cubic pyrochlore rare earth tantalate pinning additives in YBa2Cu3O7- superconductor

Phase stability of cubic-pyrochlore-structured RE3TaO7 (RTO, where RE = rare earth elements) pinning additives in YBa2Cu3O7- (YBCO) superconductor and the pinning properties influenced by RTO addition into YBCO films were investigated. RTO completely reacts with YBCO and is converted to cubic-double-perovskite-structured Ba2RETaO6 (BRETO), a more thermodynamically stable tantalate phase within YBCO. In RTO-doped YBCO films, BRETO self-assembled nanocolumnns align along the c-axis of the film and play a major role in the improvement of flux pinning and Jc performance over wide magnetic field and angular ranges.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗