Search NASA⌕ Search

SEARCH · Search NASA

Results for “Phase Transition”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

An X-ray diffraction study of pressure-induced phase transitions in Bi{sub 2}MoO{sub 6}

Synchrotron based X-ray diffraction through a diamond anvil cell was used to determine the equations of state and pressure-induced phase transitions in Bi{sub 2}MoO{sub 6}. It was observed that Bi{sub 2}MoO{sub 6} undergoes a phase transformation at {approx}6.8 GPa. The high-pressure phase can be indexed to the orthorhombic structure and the transition is reversible on decompression from {approx}47 GPa. The bulk moduli of the low and high-pressure phases were calculated, while holding K Prime =4, to be: K=51{+-}1 GPa and K=141.5 {+-}0.1 GPa, respectively. - Graphical abstract: The material Bi{sub 2}MoO{sub 6} was placed inside a diamond anvil cell and then studied under high pressure at beamline X17C of the National Synchrotron Light source. X-ray diffraction data was analyzed using the Rietveld method. Highlights: Black-Right-Pointing-Pointer A high-pressure study of bismuth molybdate was performed. Black-Right-Pointing-Pointer Pressure-induced phase transitions were observed. Black-Right-Pointing-Pointer The low pressure phase bulk modulus was calculated to be K=51{+-}1 GPa. Black-Right-Pointing-Pointer The high pressure phase bulk modulus was calculated to be B=141.5{+-}0.1 GPa.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Antiferromagnetic and nematic phase transitions in BaF e 2 ( A s 1 - x P x ) 2 studied by ac microcalorimetry and SQUID magnetometry

In this work, we study the antiferromagnetic (AFM) and structural phase transitions in single-crystal BaF e 2 ( A s 1 – x P x ) 2 ( x = 0 , 0.3 ) at temperatures T N and T S , respectively, by high-resolution ac microcalorimetry and SQUID magnetometry. The specific heat measurements of both as-grown and annealed BaF e 2 A s 2 display a sharp peak at the AFM-structural transitions. A kink in the entropy of annealed BaF e 2 A s 2 gives evidence for splitting of the two transitions by approximately 0.5 K. No additional features could be identified in the specific heat of both BaF e 2 A s 2 and BaF e 2 ( A s 0.7 P 0.3 ) 2 in the temperature regions around T * > T S where torque measurements had revealed the “true” nematic phase transition, even though the Ginzburg-Landau model used to fit the magnetic torque data indicates that the expected thermal anomaly should be easily observable with our experimental resolution.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

High-pressure structural phase transitions in chromium-doped BaFe[subscript 2]As[subscript 2]

We report on the results from high pressure x-ray powder diffraction and electrical resistance measurements for hole doped BaFe{sub 2-x}Cr{sub x}As{sub 2} (x = 0, 0.05, 0.15, 0.4, 0.61) up to 81 GPa and down to 10 K using a synchrotron source and diamond anvil cell (DAC). At ambient temperature, an isostructural phase transition from a tetragonal (T) phase (I4/mmm) to a collapsed tetragonal (CT) phase is observed at 17 GPa. This transition is found to be dependent on ambient pressure unit cell volume and is slightly shifted to higher pressure upon increase in the Cr-doping. Unlike BaFe{sub 2}As{sub 2} which superconduct under high pressure, we have not detected any evidence of pressure induced superconductivity in chromium doped samples in the pressure and temperature range of this study. The measured equation of state parameters are presented for both the tetragonal and collapsed tetragonal phases for x = 0.05, 0.15, 0.40 and 0.61.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Femtosecond x-ray diffraction reveals a liquid–liquid phase transition in phase-change materials

In phase-change memory devices, a material is cycled between glassy and crystalline states. The highly temperature-dependent kinetics of its crystallization process enables application in memory technology, but the transition has not been resolved on an atomic scale. Using femtosecond x-ray diffraction and ab initio computer simulations, we determined the time-dependent pair-correlation function of phase-change materials throughout the melt-quenching and crystallization process. We found a liquid–liquid phase transition in the phase-change materials Ag 4 In 3 Sb 67 Te 26 and Ge 15 Sb 85 at 660 and 610 kelvin, respectively. The transition is predominantly caused by the onset of Peierls distortions, the amplitude of which correlates with an increase of the apparent activation energy of diffusivity. This reveals a relationship between atomic structure and kinetics, enabling a systematic optimization of the memory-switching kinetics.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2021

Phase transition mechanism and bandgap engineering of Sb2S3 at gigapascal pressures

Abstract Earth-abundant antimony trisulfide (Sb 2 S 3 ), or simply antimonite, is a promising material for capturing natural energies like solar power and heat flux. The layered structure, held up by weak van-der Waals forces, induces anisotropic behaviors in carrier transportation and thermal expansion. Here, we used stress as mechanical stimuli to destabilize the layered structure and observed the structural phase transition to a three-dimensional (3D) structure. We combined in situ x-ray diffraction (XRD), Raman spectroscopy, ultraviolet-visible spectroscopy, and first-principles calculations to study the evolution of structure and bandgap width up to 20.1 GPa. The optical band gap energy of Sb 2 S 3 followed a two-step hierarchical sequence at approximately 4 and 11 GPa. We also revealed that the first step of change is mainly caused by the redistribution of band states near the conduction band maximum. The second transition is controlled by an isostructural phase transition, with collapsed layers and the formation of a higher coordinated bulky structure. The band gap reduced from 1.73 eV at ambient to 0.68 eV at 15 GPa, making it a promising thermoelectric material under high pressure.

Cui, Zhongxun (ORCID:0000000275044177)↗
DOE OSTI2016

Phase transitions and magnetocaloric and transport properties in off-stoichiometric GdNi{sub 2}Mn{sub x}

The structural, magnetic, magnetocaloric, transport, and magnetoresistance properties of the rare-earth intermetallic compounds GdNi{sub 2}Mn{sub x} (0.5 ≤ x ≤ 1.5) have been studied. The compounds with x = 0.5 and 0.6 crystallize in the cubic MgCu{sub 2} type phase, whereas samples with x ≥ 0.8 form a mixed MgCu{sub 2} and rhombohedral PuNi{sub 3} phase. A second order magnetic phase transition from a ferromagnetic to paramagnetic state was observed near the Curie temperature (T{sub C}). The GdNi{sub 2}Mn{sub x} (0.5 ≤ x ≤ 1.5) compounds order in a ferrimagnetic structure in the ground state. The largest observed values of magnetic entropy changes (at T{sub C} for ΔH = 5T) were 3.9, 3.5, and 3.1 J/kg K for x = 0.5, 0.6, and 0.8, respectively. The respective relative values of the cooling power were 395, 483, and 220 J/kg. These values are greater than some well-known prototype magnetocaloric materials such as Gd (400 J/kg) and Gd{sub 5}Si{sub 2}Ge{sub 2} (240 J/kg). Analysis of the resistivity data showed a T{sup 2} dependence at low temperatures, suggesting strong electron-phonon interactions, whereas at higher temperatures s-d scattering was dominated by the electron-phonon contribution, resulting in a slow increase in resistivity. Magnetoresistance values of ∼−1.1% were found for x = 0.5 near T{sub C}, and −7% for x = 1.5 near T = 80 K.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Highly ordered graphite (HOPG) to hexagonal diamond (lonsdaleite) phase transition observed on picosecond time scales using ultrafast x-ray diffraction

Here, the response of rapidly compressed highly oriented pyrolytic graphite (HOPG) normal to its basal plane was investigated at a pressure of ~80 GPa. Ultrafast x-ray diffraction using ~100 fs pulses at the Materials Under Extreme Conditions sector of the Linac Coherent Light Source was used to probe the changes in crystal structure resulting from picosecond timescale compression at laser drive energies ranging from 2.5 to 250 mJ. A phase transformation from HOPG to a highly textured hexagonal diamond structure is observed at the highest energy, followed by relaxation to a still highly oriented, but distorted graphite structure following release. We observe the formation of a highly oriented lonsdaleite within 20 ps, subsequent to compression. This suggests that a diffusionless martensitic mechanism may play a fundamental role in phase transition, as speculated in an early work on this system, and more recent static studies of diamonds formed in impact events.

58 GEOSCIENCES↗
DOE OSTI2019

Phase transition and energy storage behavior of antiferroelectric PLZT thin films epitaxially deposited on SRO buffered STO single crystal substrates [Phase transition and energy storage behavior of antiferroelectric PLZT thin films epitaxially deposited on SRO buffered STO single crystal substrates]

Thin films of 300-nm-thick (Pb 0.98 , La 0.02 )(Zr 0.95 , Ti 0.05 )O 3 (PLZT) were epitaxially deposited on (100), (110), and (111) SrTiO3 single crystal substrates by pulsed laser deposition. X-ray diffraction line and reciprocal space mapping (RSM) scans were used to determine the crystal structure. Tetragonal ((001) PLZT) and monoclinic MA ((O 1 1 ) and (111) PLZT) structures were found, which influenced the stored energy density. Electric field induced antiferroelectric to ferroelectric (AFE—>FE) phase transitions were found to have a large reversible energy density of up to 30J/cm 3 . With increasing temperature, an AFE to relaxor ferroelectric (AFE—>RFE) transition was found. The RFE phase exhibited lower energy loss, and an improved energy storage efficiency. Here, the results are discussed from the perspective of crystal structure, dielectric phase transitions, and energy storage characteristics.

25 ENERGY STORAGE↗
DOE OSTI2024

La 0.6 Sr 0.4 CoO 3−δ Films Under Deoxygenation: Magnetic And Electronic Transitions Are Apart from The Structural Phase Transition

Abstract Topotactic phase transitions induced by changes in the oxygen vacancy concentration can largely alter the physical properties of complex oxides, including electronic and magnetic phases, while maintaining the structural integrity of the crystal lattice. An oxygen‐vacancy‐induced topotactic phase transition from perovskite (PV) to brownmillerite (BM) is achieved in epitaxial La 0.6 Sr 0.4 CoO 3−δ (LSCO) thin films. Two novel intermediate states with different oxygen content are identified by X‐ray diffraction, which involves a single‐phase reduced PV state and a mixed state of co‐existing PV and BM. The combination of depth‐sensitive polarized neutron reflectometry (PNR) and Rutherford backscattering (RBS) allows a quantitative determination of magnetization and the mean oxygen content in all states, revealing a continuous transition from La 0.6 Sr 0.4 CoO 2.97 to La 0.6 Sr 0.4 CoO 2.5 . BM formation is observed for an LSCO layer with an oxygen content of 2.67, while the magnetic and electronic transition already occurs for a layer with a higher oxygen content of 2.77 (and above) and in the absence of a BM signature. These results demonstrate that the physics of electronic metal‐to‐insulator transition (MIT), magnetic ferromagnet‐to‐non‐ferromagnet transition (FM‐to‐non‐FM), and structural PV‐to‐BM phase transition should be considered within the framework of separate but interrelated processes.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Structural crossover in a supercooled metallic liquid and the link to a liquid-to-liquid phase transition

Time-resolved synchrotron measurements were carried out to capture the structure evolution of an electrostatically levitated metallic-glass-forming liquid during free cooling. The experimental data shows a crossover in the liquid structure at ~1000 K, about 115 K below the melting temperature and 150 K above the crystallization temperature. The structure change is characterized by a dramatic growth in the extended-range order below the crossover temperature. Molecular dynamics simulations have identified that the growth of the extended-range order was due to an increased correlation between solute atoms. These results provide structural evidence for a liquid-to-liquid-phase-transition in the supercooled metallic liquid.

Glass transitions↗
DOE OSTI2018

Effects of non-hydrostaticity and grain size on the pressure-induced phase transition of the CoCrFeMnNi high-entropy alloy

Recently, an irreversible polymorphic transition from face-centered cubic to hexagonal close-packing was surprisingly observed under high pressure in the prototype CoCrFeMnNi high-entropy alloys (HEAs) by various research groups. This unexpected phase transition brings new insights into the stability of HEAs, and its irreversibility stimulates exploration for new HEAs via high-pressure compression synthesis. However, the onset pressure for the phase transition was reported to fluctuate over a vast range from ~7 to above 49 GPa in the reported experiments. The reason for this inconsistency remains unclear and puzzles the HEA community. To address this problem, this work systematically investigates the effects of non-hydrostaticity and grain size. Our results demonstrate that larger deviatoric stress induced by the non-hydrostaticity of the pressure medium and larger grain size of the initial sample can both promote a phase transition and, therefore, considerably depress the onset pressure.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Structural and magnetic phase transitions near optimal superconductivity in BaFe 2 (As 1-x P x ) 2

In this study, we use nuclear magnetic resonance (NMR), high-resolution x-ray and neutron scattering to study structural and magnetic phase transitions in phosphorus-doped BaFe 2 (As 1-x P x ) 2 . Thus, previous transport, NMR, specific heat, and magnetic penetration depth measurements have provided compelling evidence for the presence of a quantum critical point (QCP) near optimal superconductivity at x = 0.3. However, we show that the tetragonal-to-orthorhombic structural (T s ) and paramagnetic to antiferromagnetic (AF, T N ) transitions in BaFe 2 (As 1-x P x ) 2 are always coupled and approach to T N ≈ T s ≥ T c (≈ 29 K) for x = 0.29 before vanishing abruptly for x ≥ 0.3. These results suggest that AF order in BaFe 2 (As 1-x P x ) 2 disappears in a weakly first order fashion near optimal superconductivity, much like the electron-doped iron pnictides with an avoided QCP.

36 MATERIALS SCIENCE↗