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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 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Improved phase stability of formamidinium lead triiodide perovskite by strain relaxation

Though formamidinium lead triiodide (FAPbI 3 ) possesses a suitable band gap and good thermal stability, the phase transition from the pure black perovskite phase (α-phase) to the undesirable yellow nonperovskite polymorph (δ-phase) at room temperature, especially under humid air, hinders its practical application. Here, we investigate the intrinsic instability mechanism of the α-phase at ambient temperature and demonstrate the existence of an anisotropic strained lattice in the (111) plane that drives phase transformation into the δ-phase. Methylammonium bromide (MABr) alloying (or FAPbI 3 -MABr) was found to cause lattice contraction, thereby balancing the lattice strain. This led to dramatic improvement in the stability of α-FAPbI 3 . As a result, solar cells fabricated using FAPbI 3 -MABr demonstrated significantly enhanced stability under the humid air.

14 SOLAR ENERGY↗
DOE OSTI2014

Phase stability limit of c-BN under hydrostatic and non-hydrostatic pressure conditions

Phase stability limit of cubic boron nitride (c-BN) has been investigated by the crystal structure search technique. It indicated that this limit is ∼1000 GPa at hydrostatic pressure condition. Above this pressure, c-BN turns into a metastable phase with respect to rocksalt type boron nitride (rs-BN). However, rs-BN cannot be retained at 0 GPa owing to its instability at pressure below 250 GPa. For non-hydrostatic pressure conditions, the phase stability limit of c-BN is substantially lower than that under hydrostatic pressure conditions and it is also dramatically different for other pressure mode.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Low-Density Parity-Check Stabilizer Codes as Gapped Quantum Phases: Stability under Graph-Local Perturbations

We generalize the proof of stability of topological order, due to Bravyi, Hastings, and Michalakis, to stabilizer Hamiltonians corresponding to low-density parity-check (LDPC) codes without the restriction of geometric locality in Euclidean space. We consider Hamiltonians 𝐻 0 defined by ⟦𝑁,𝐾,𝑑⟧ LDPC codes, which obey certain topological quantum order conditions: (i) code distance 𝑑 ≥ 𝑐⁢log (𝑁), implying local indistinguishability of ground states, and (ii) a mild condition on local and global compatibility of ground states—these include good quantum LDPC codes and the toric code on a hyperbolic lattice, among others. We consider stability under weak perturbations that are quasilocal on the interaction graph defined by 𝐻 0 and that can be represented as sums of bounded-norm terms. As long as the local perturbation strength is smaller than a finite constant, we show that the perturbed Hamiltonian has well-defined spectral bands originating from the 𝑂⁡(1) smallest eigenvalues of 𝐻 0 . The band originating from the smallest eigenvalue has 2 𝐾 states, is separated from the rest of the spectrum by a finite energy gap, and has exponentially narrow bandwidth 𝛿 =𝐶⁢𝑁⁢𝑒 −Θ⁡(𝑑) , which is tighter than the best-known bounds even in the Euclidean case. We also obtain that the new ground-state subspace is related to the initial-code subspace by a quasilocal unitary, allowing one to relate their physical properties. Our proof uses an iterative procedure that performs successive rotations to eliminate non-frustration-free terms in the Hamiltonian. Our results extend to quantum Hamiltonians built from classical LDPC codes, which give rise to stable symmetry-breaking phases. These results show that LDPC codes very generally define stable gapped quantum phases, even in the non-Euclidean setting, initiating a systematic study of such phases of matter.

mathematical physics↗
DOE OSTI2014

Phase stabilized homodyne of infrared scattering type scanning near-field optical microscopy

Scattering type scanning near-field optical microscopy (s-SNOM) allows sub diffraction limited spatial resolution. Interferometric homodyne detection in s-SNOM can amplify the signal and extract vibrational responses based on sample absorption. A stable reference phase is required for a high quality homodyne-detected near-field signal. This work presents the development of a phase stabilization mechanism for s-SNOM to provide stable homodyne conditions. The phase stability is found to be better than 0.05 rad for the mid infrared light source. Phase stabilization results in improved near field images and vibrational spectroscopies. Spatial inhomogeneities of the boron nitride nanotubes are measured and compared.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Understanding phase stability of Al-Co-Cr-Fe-Ni high entropy alloys

The concept of high entropy alloy (HEA) opens a vast unexplored composition range for alloy design. As a well-studied system, Al-Co-Cr-Fe-Ni has attracted tremendous amount of attention to develop new-generation low-density structural materials for automobile and aerospace applications. In spite of intensive investigations in the past few years, the phase stability within this HEA system is still poorly understood and needs to be clarified, which poses obstacles to the discovery of promising Al-Co-Cr-Fe-Ni HEAs. In the present work, the CALPHAD approach is employed to understand the phase stability and explore the phase transformation within the Al-Co-Cr-Fe-Ni system. As a result, the phase-stability mapping coupled with density contours is then constructed within the composition - temperature space, which provides useful guidelines for the design of low-density Al-Co-Cr-Fe-Ni HEAs with desirable properties.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Solid phase stability of a double-minimum interaction potential system

We study phase stability of a system with double-minimum interaction potential in a wide range of parameters by a thermodynamic perturbation theory. The present double-minimum potential is the Lennard-Jones-Gauss potential, which has a Gaussian pocket as well as a standard Lennard-Jones minimum. As a function of the depth and position of the Gaussian pocket in the potential, we determine the coexistence pressure of crystals (fcc and bcc). We show that the fcc crystallizes even at zero pressure when the position of the Gaussian pocket is coincident with the first or third nearest neighbor site of the fcc crystal. The bcc crystal is more stable than the fcc crystal when the position of the Gaussian pocket is coincident with the second nearest neighbor sites of the bcc crystal. The stable crystal structure is determined by the position of the Gaussian pocket. These results show that we can control the stability of the solid phase by tuning the potential function.

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

Variation of γ′ Formers and Refractory Elements for Enhanced Creep resistance and Phase Stability of HAYNES® 282® Alloy

Modifications to the chemistry of alloy 282 were performed to improve the alloy’s resistance to long term creep deformation as well as phase stability. Alloys were prepared using vacuum induction melting, computationally optimized homogenization heat treatment and hot working. Phase stability studies were carried out for up to 5,000 hours at 800°C and 900°C while creep testing was performed at conditions leading to lives past 7,000 hours for temperatures ranging from 740°C to 900°C. The formation of σ and μ phases were reduced in the modified alloy. Atom probe tomography (APT) was performed on the specimens from the phase stability study to investigate changes in the elemental partitioning to the γ and γ′ phases. Post deformation microstructures were analyzed using EBSD and TEM to study the influence of the detrimental phases on damage accumulation during creep.

Detrois, Martin↗
DOE OSTI2014

Initial phase zone for phase locking to the resonance, using “main condition” of phase stability in DC-biased single-sided multipactor

In the present work, the concept of accurate phase stability is employed to study a DC-biased single-sided multipactor. A “main condition” of phase stability was introduced in our previous studies of two-sided multipactors [M. Mostajeran, J. Instrum. 8, P04024 (2013); M. Mostajeran and M. Lamehi Rachti, Nucl. Instrum. Methods Phys. Res. A 615, 1–5 (2010)]. Using the same condition and assuming zero initial velocity for the secondary electrons, a regime of multipactors outside the resonance zones is found. The theoretical results are then verified by numerical simulation.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Subtlety of TiO 2 phase stability: Reliability of the density functional theory predictions and persistence of the self-interaction error

TiO 2 is an important material with broad applications that can exist in different phases with dramatically different properties. Theoretical prediction of their polymorph energetics is therefore critical for the material design and for identifying thermodynamically accessible structures. Determining TiO 2 relative phase stabilities remains challenging for first-principles methods, and density functional theory is the only approach available for studying phase stabilities at finite temperatures with acceptable computational efficiency. Here, we show that density functional theory using the recently developed efficient strongly constrained and appropriately normed (SCAN) [Sun et al., Phys. Rev. Lett. 115, 036402 (2015)] exchange-correlation functional for the first time predicts the phase stability in qualitative agreement with the experimental results at realistic conditions. Further analysis shows that the self-interaction error intrinsic in the density functional persists in the stability prediction. By correcting the self-interaction error through an empirical approach, SCAN predicts the relative stability as well as defect properties in excellent agreement with the experimental results.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Phase stability, physical properties and strengthening mechanisms of concentrated solid solution alloys

Here, we review recent research developments in a special class of multicomponent concentrated solid solution alloys (CSAs) – of which the recently discovered high entropy alloys (HEAs) are exemplars – that offer a new paradigm for the development of next generation structural materials. This review focuses on the role of inherent extreme chemical complexity on the phase stability, electronic, transport, and mechanical properties of this remarkable class of disordered solid solution alloys. Both experimental observations and theoretical models indicate that the phase stability of HEAs goes beyond the original conjecture that these alloys are stabilized by configurational/mixing entropy; rather, it results from competition between the homogeneously disordered phase and phase separation/intermetallic compound formation. Although the number of single-phase HEAs with equiatomic composition is limited, those that do exist often exhibit remarkable electronic, magnetic, transport, and mechanical properties. For the mechanical response, we discuss the solution strengthening mechanism which governs the strength and deformation behaviors of the CSAs, as well as the increasing evidence that low stacking fault energies (deformation twinning) plays an important role in the low temperature strength and ductility of CrMnFeCoNi related alloys. We also review the current understanding of the role of the number and type of alloy elements in determining the electronic, magnetic, and transport properties, in particular the dominant role of magnetic interactions in the properties of 3d-transition metal based alloys. Finally, we emphasize that, despite rapid progress in characterization and understanding of the phase stability and physical/mechanical responses of CSAs, there remain significant challenges to fully exploring the new paradigm that these alloys represent.

36 MATERIALS SCIENCE↗
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 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 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 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.↗