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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 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Investigating the Role π -Rich Solvents Play in the Growth of Cesium Lead Bromide Nanocrystals

In this report, the role that a high-boiling-point solvent type plays on the nucleation and growth, morphology, and crystal-phase transformation of cesium lead bromide nanocrystals (CsPbBr 3 ) is studied. The CsPbBr 3 products were compared between a one-pot growth mechanism at room temperature (RT) versus a hot-injection mechanism (HI) control using dibenzyl ether (DBE), diphenyl ether (DPE), dioctyl ether (DOE), or 1- octadecene (ODE). The coordination between these solvents and the PbBr 2 salt precursors resulted in different plumbate [PbSBr n ] 2−n precursors being formed. The S-to-Pb 2+ coordination within [PbSBr n ] 2−n was probed by UV−vis and solvent-phase 207 Pb NMR, both of which showed considerable coordination between [PbSBr n ] 2−n and the π-rich DBE and DPE, whose reactivity affected CsPbBr 3 growth. The effect was more pronounced for CsPbBr 3 prepared via RT, where the morphology was tunable, with π-rich solvents producing thin rod-like CsPbB r3 with a blue emission, compared to the green-emitting thicker platelets formed via HI. While XRD showed crystalline products for both RT and HI, with orthorhombic and cubic forms, respectively, the RT products had considerable surface defects, as was indicated by lower quantum yields, and to understand this the photoluminescent lifetimes were measured by time-correlated single photon counting.

207Pb NMR↗
DOE OSTI2017

Structure transformation of Ti films deposited on SiC single crystal substrates

Highlights: • In this paper, an interesting intermediate state of the fcc-hcp transformation in Ti films was observed. • We figured this intermediate structure out by XRD and HRTEM. • The formation of this intermediate state and the mechanism of thickness-dependent fcc-hcp transformation were discussed. - Abstract: This paper reports the structure transformation of Ti films deposited on SiC(0001) single crystal substrates using DC magnetron sputtering. Film thickness, sputter power and temperature of deposition were changed to investigate structure transformation of Ti films. The structure characterization of Ti film was performed by means of X-Ray Diffraction (XRD) and High-Resolution Transmission Electron Microscope (HRTEM). The results showed that the Ti film grew epitaxially with a face centered cubic (fcc) structure even the thickness is up to about 50 nm. High temperature and low sputter power are propitious to the formation of fcc-Ti. An interesting intermediate state of the fcc-hcp transformation was observed. This intermediate structure could help us to understand the mechanism of thickness-dependent fcc-hcp transformation in Ti thin films.

36 MATERIALS SCIENCE↗
DOE OSTI2018

The Evolution of Second-Phase Particles in 6111 Aluminum Alloy Processed by Hot and Cold Rolling

The evolution of coarse Al{sub 9.9}Fe{sub 2.65}Ni{sub 1.45} phase, spherical Al{sub 12}(Mn,Fe){sub 3}Si phase and rod-like Q phase in a 6111 aluminum alloy during hot and cold rolling deformation processes was systematically investigated in this work. The results showed that the coarse Al{sub 9.9}Fe{sub 2.65}Ni{sub 1.45} particles are mainly distributed at the grain boundaries, accompanied by the co-formation of Al{sub 12}(Fe,Mn){sub 3}Si phase and Mg{sub 2}Si phase, while the spherical Al{sub 12}(Mn,Fe){sub 3}Si particles are mainly distributed in the grain interiors. Hot rolling has little effects on the size and distribution of both phases, but cold deformation can severely decrease the size of the particles by breaking the particles into small pieces. In addition, the temperature of 450 °C is not high enough for the dissolution of Q phase in the Al matrix, but the Q particles can be broken into small pieces due to the stress concentration during both hot and cold rolling deformation. In addition, the influences of phase evolution, dislocations and recrystallization on the mechanical properties evolution were also discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Size-dependent structural phase transitions and their correlation with photoluminescence and optical absorption behavior of annealed Zn0.45Cd0.55S quantum dots

Highlights: • Thermal Induced Structural Phase transition in nanostructured Zn{sub 0.45}Cd{sub 0.55}S. • Low temperature structural phase transition from cubic to hexagonal structure. • Thermal annealing effect on FTIR and Raman Spectra. • Thermal annealing effect on optical absorption of Zn{sub 0.45}Cd{sub 0.55}S nanoparticles. • Excitation-wavelength dependent photoluminescence emission spectra. - Abstract: In this paper, we investigate the effect of thermally induced structural phase transitions on the photoluminescence (PL) and optical absorption behaviour of Zn{sub 0.45}Cd{sub 0.55}S nanoparticles (NPs). Analysis of X-ray diffraction (XRD) patterns and high-resolution electron microscope (HRTEM) images reveal that the as-synthesized sample possesses zinc-blende-type cubic structure. In addition, at annealing temperature (T{sub a}) 400 °C, the cubic structure transforms completely into the wurtzite-type hexagonal structure. Furthermore, the second phase transition of the as-synthesized sample has observed at 700 °C, where the cubic structure has transformed into mixed polycrystalline phases of hexagonal ZnO, cubic CdO, monoclinic CdSO{sub 3}, and orthorhombic ZnSO{sub 4} structures. These new phases have also confirmed from the analysis of Raman and FTIR spectra. Analysis of UV–visible optical absorption spectra demonstrates that Increasing T{sub a} results in the decrease of optical band gap due the improvement in crystallinity accompanied by the increase in the particle size. The PL emission bands at an excitation energy of 3.818 eV exhibit redshift and a decrease in the intensity with increasing T{sub a} up to 500 °C. Meanwhile, further increase in T{sub a} up to 700 °C results in the enhancement of green emission intensity. On the other hand, PL emission spectra at 3.354 eV and T{sub a} 700 °C, reveal a dramatic increase in the emission intensity nearly by one-order of magnitude with respect to its value of the as-synthesized sample. This behaviour is ascribed to the incorporation of oxygen-related defects via thermal annealing in air, which act as additive radiative centers. Also, we have interpreted the observed spectral blue shift of PL emission spectrum with increasing excitation energy.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

Structure and phase formation behavior and dielectric and magnetic properties of lead iron tantalate-lead zirconate titanate multiferroic ceramics

Highlights: • The multiferroic ceramics consisted of PFT and PZT. • Crystal structure changed from cubic to mixedcubic and tetragonal with increasing PZT content. • Dielectric showed the samples underwent a typical relaxor ferroelectric behavior. • Magnetic properties showed very interesting behavior with square saturated magnetic hysteresis loops. - Abstract: Multiferroic (1 − x)Pb(Fe{sub 0.5}Ta{sub 0.5})O{sub 3}–xPb(Zr{sub 0.53}Ti{sub 0.47})O{sub 3} (or PFT–PZT) ceramics were synthesized by solid-state reaction method. The crystal structure and phase formation of the ceramics were examined by X-ray diffraction (XRD). The local structure surrounding Fe and Ti absorbing atoms was investigated by synchrotron X-ray Absorption Near-Edge Structure (XANES) measurement. Dielectric properties were studied as a function of frequency and temperature using a LCR meter. A vibrating sample magnetometer (VSM) was used to determine the magnetic hysteresis loops. XRD study indicated that the crystal structure of the sample changed from pure cubic to mixed cubic and tetragonal with increasing PZT content. XANES measurements showed that the local structure surrounding Fe and Ti ions was similar. Dielectric study showed that the samples underwent a typical relaxor ferroelectric behavior while the magnetic properties showed very interesting behavior with square saturated magnetic hysteresis loops.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Evolution of structure in Na{sub 0.5}Bi{sub 0.5}TiO{sub 3} single crystals with BaTiO{sub 3}

The structural, dielectric, and piezoelectric properties of Na{sub 0.5}Bi{sub 0.5}TiO{sub 3}-x mol. %BaTiO{sub 3} (NBT-x%BT) crystals have been investigated. The dielectric and piezoelectric properties of NBT-x%BT were enhanced near x = 5–7. High resolution synchrotron x-ray powder diffraction studies revealed the presence of a phase boundary between monoclinic (Cc) and tetragonal (P4bm) phases near x = 5–7, where the dielectric and piezoelectric properties were enhanced.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Pressure dependence of band-gap and phase transitions in bulk CuX (X = Cl, Br, I)

Usually a phase transition, in theoretical studies, is explored or verified by studying the total energy as a function of the volume considering various plausible phases. The intersection point, if any, of the free energy vs. volume curves for the different phases is then the indicator of the phase transition(s). The question is, can the theoretical study of a single phase alone indicate a phase transition? i.e. can we look beyond the phase under consideration through such a study? Using density-functional theory, we report a novel approach to suggest phase transition(s) through theoretical study of a single phase. Copper halides have been engaged for this study. These are direct band-gap semiconductors, with zinc blende structure at ambient conditions, and are reported to exhibit many phase transitions. We show that the study of volume dependence of energy band-gap in a single phase facilitates looking beyond the phase under consideration. This, when translated to pressures, reflects the phase transition pressures for CuX (X = Cl, Br, I) with an encouraging accuracy. This work thus offers a simple, yet reliable, approach based on electronic structure calculations to investigate new semiconducting materials for phase changes under pressure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Structural properties of rare earth chalcogenides

The pressure induced NaCl (B1) to CsCl (B2) structural phase transition of rare earth mono-chalcogenide (PuTe) has been investigated in this paper. A modified interaction potential model (MIPM) (including the covalency effect) has been developed. Phase transition pressures are associated with a sudden collapse in volume. The phase transition pressures and associated volume collapses obtained from present potential model show a generally good agreement with available experimental data than others.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Pressure and temperature induced elastic properties of Am and Cf monobismuthides

The pressure and temperature dependent mechanical properties as melting temperature, hardness and brittle nature of XBi (X = Am and Cf) are studied. The rare earth actinides pnictides showed a structural phase transition (B1–B2) at a transition pressure (P{sub T}) of 14.3 GPa (AmBi) and 10.8 GPa (CfBi). Pressure dependence of melting temperature (T{sub m}) discerns an increase inferring the hardening or stiffening of the lattice as a consequence of bond compression and bond strengthening. Suppressed T{sub M} as functions of temperature infers the weakening of the lattice results in bond weakening in XBi (X = Am, Cf). Vickers Hardness (H{sub V}), Poisson’s and Pugh ratio of XBi (X = Am and Cf) demonstrates that XBi (X = Am and Cf) is mechanically stiffened, thermally softened and brittle on applied pressure and temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Structural phase transition, electronic structure and optical properties of half Heusler alloys LiBeZ (Z = As, Sb)

Ab initio calculations are performed to investigate the structural stability, electronic structure, mechanical properties and optical properties of half Heusler alloys (LiBeAs and LiBeSb) for three different phases of zinc blende crystal structure. Among the considered phases, α- phase is found to be the most stable phase for these alloys at normal pressure. A pressure induced structural phase transition from α-phase to β- phase is observed for LiBeAs. The electronic structure reveals that these alloys are semiconductors. The optical properties confirm that these alloys are semiconductor in nature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Structural transitions in alumina nanoparticles by heat treatment

γ-alumina nanoparticles were annealed sequentially at 800°C, 950°C and 1100°C and structural transitions as a function of heat treatment were studied by X-ray diffraction (XRD), Differential Scanning Calorimetry (DSC) and {sup 27}Al Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR) methods.. XRD studies found that γ-Al{sub 2}O{sub 3} is stable upto a temperature of at least 950°C and transforms to the thermodynamically stable α-phase after annealing at 1100°C. MAS-NMR revealed that γ-alumina contains AlO{sub 4} and AlO{sub 6} structural units in the ratio 1: 2, while α-phase contains only AlO{sub 6} units. DSC confirmed that γ → α transition initiates at 1060°C.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

Raman spectroscopic investigations of tetragonal to cubic transition in BaTiO{sub 3} films grown on LaAlO{sub 3} substrate

BaTiO{sub 3} [BTO] films have been grown on LaAlO{sub 3} (LAO) substrate using pulse laser deposition technique. The film is found highly oriented, possess 400 nm thickness and single phase in nature. However, the out-of-plane lattice parameter of the film obtained from the x-ray diffraction was found to be lower when compared to bulk compound indicating towards strain reversal. The Raman spectra recorded at room temperature showed that the film is tetragonal with C4v symmetry. Raman spectra collected at high temperatures for the film and the bulk compound showed that the tetragonal to cubic transition temperature in film is enhanced by 100 K.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Effect of pressure on the band structure of BC{sub 3}

Density functional theory (DFT) calculations were carried out to study the effect of pressure on the band structure of two dimensional BC{sub 3} sheet. BC{sub 3} is a semiconductor at ambient conditions having a band gap of ~0.3 eV. Electronic structure calculations are carried out on BC{sub 3} at pressures of 5, 20, 50 and 100 GPa. The system shows a semiconductor – metal transition by the application of pressure without any structural transition.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

New route of phase transition for enhanced TCO property of ZnO: A first-principles study

The ZnO is one of the best transparent conducting oxide (TCO) materials for optoelectronic properties. Primarilly three phases of ZnO namely wurtzite, rocksalt and zincblend along with other prominent phases under different pressure and with different phase transformation routes are reported. The route of phase transformation may differ and depends on the external parameters such as pressure. The conventional route for the phase transformation of wurtzite (B4) phase into CsCl type (B2) phase via rocksalt phase (B1) is estimated at the relatively high pressure value of the 256 GPa. In present paper, we propose a new route of transformation of B4 to B2 phase of ZnO via β-BeO type phase at relatively low pressure value of 43 GPa. In present study, the phase transformation analysis and the electronic structure analysis for different phases are carried out by using density functional theory (DFT). It is also observed that B2 phase have high dispersive nature of bottom of conduction band and consequently high mobility of charge carriers. It indicates the possibility of B2 phase may have better TCO properties among other phases of ZnO.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Colossal thermoelectric power in charge-ordered Li-doped La{sub 0.75}Li{sub 0.25}MnO{sub 3} manganite system

A detail study of transport and magnetic properties of La{sub 1-x}Li{sub x}MnO{sub 3+δ} (0.05 ≤ x ≤ 0.3) system synthesized by wet-chemical mixing route has been done. The room temperature x-ray powder diffraction (XRD) data show single phase behavior of all samples except x = 0.3. Rietveld refinement of XRD data shows structural transition from rhombohedral (R3-C) to orthorhombic (Pnma) symmetry occurs at the Li-doping level x > 0.2 with both the lattice parameter and unit-cell volume decrease with increase of ‘x’. All the samples show ferromagnetic (FM) behavior while metallic behavior are shown by the samples up to Li-concentration x = 0.2. With further Li doping i.e. for x = 0.25, the sample shows insulating behavior accompanied by charge-order transition around T ~ 225 K. Metallic part of the resistivity data of the samples is best fitted with an expression ρ(T) = ρ{sub 0} + ρ{sub 4.5}T{sup 4.5} + C/ sinh{sup 2}(hv{sub s}/2k{sub B}T) containing small-polaron contribution (last term). Most interesting finding in the present study is the observation of large anomalous decrease in thermoelectric power (S) below 100 K shown by the sample with x = 0.25. Probable mechanisms responsible for the observed colossal thermoelectric power have been discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Solvothermal synthesis of Fe{sub 7}C{sub 3} and Fe{sub 3}C nanostructures with phase and morphology control

A phase transition, from orthorhombic Fe{sub 3}C to hexagonal Fe{sub 7}C{sub 3}, was observed using a wet synthesis mediated by hexadecyltrimethylammonium chloride (CTAC). In this study, CTAC has been shown to control carbide phase, morphology, and size of the iron carbide nanostructures. Fe{sub 7}C{sub 3} hexagonal prisms were formed with an average diameter of 960 nm, the thickness of 150 nm, and Fe{sub 3}C nanostructures with an approximate size of 50 nm. Magnetic studies show ferromagnetic behavior with M{sub s} of 126 emu/g, and H{sub c} of 170 Oe with respect to Fe{sub 7}C{sub 3} and 95 emu/g and 590 Oe with respect to Fe{sub 3}C. The thermal studies using high temperature x-ray diffraction show stability of Fe{sub 7}C{sub 3} up to 500 °C. Upon slow cooling, the Fe{sub 7}C{sub 3} phase is recovered with an intermediate oxide phase occurring around 300 °C. This study has demonstrated a simple route in synthesizing iron carbides for an in depth magnetic study and crystal phase transition study of Fe{sub 7}C{sub 3} at elevated temperatures.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Gas–crystal phase transition in a 2D dipolar exciton system

A system of dipolar excitons at temperatures exceeding the expected Bose–Einstein condensation temperature is considered. It is shown that a first-order phase transition with the formation of a phase close to the crystal of such excitons is possible at such temperatures. The phase diagram in the range of low concentrations and temperatures is constructed. The effect of this transition on the luminescence spectrum of the system is analyzed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗