Scott T. Misture

STABLE TI-BASED QUASICRYSTAL OFFERS PROSPECT FOR IMPROVED HYDROGEN STORAGE

The desorption of hydrogen from a novel material, a Ti45Zr38Ni17‐H quasicrystal, was observed using high‐temperature powder x‐ray diffraction, demonstrating the potential utility of Ti‐based quasicrystals in place of crystalline or amorphous hydrides for hydrogen storage applications. The maximum observed change in hydrogen concentration was from 61 at. %, corresponding to a hydrogen‐to‐metal ratio (H/M) of 1.54, at 91 °C to less than 2.5 at. % (H/M=0.025) at 620 °C. The onset temperature of desorption is below 350 °C. Surface oxidation was found to promote the formation of crystalline hydride phases. Highly oxidized samples transformed to a mixture of the C14 Laves and C15 Laves crystalline hydrides, and the Ti2Ni phase. When the oxidation was less severe, a reversible transformation between the quasicrystal and crystalline hydride phases was clearly observed, demonstrating the stability of the Ti45Zr38Ni17 quasicrystal at very low hydrogen concentrations, and temperatures as high as 661 °C. This is the ...

In-situ diffraction study of Ba2In2O5

Abstract In-situ neutron and X-ray diffraction experiments were used to determine the crystal structure of Ba 2 In 2 O 5 as related to temperature. Ba 2 In 2 O 5 has a Brownmillerite-type crystal structure from room temperature to 900 °C, consisting of an orthorhombic unit cell with lattice parameters 6.0864(3)×16.7903(7)×5.9697(3) A and Icmm (74) space group symmetry. At 900 °C, oxygen vacancies begin to disorder. By 925 °C, barium indate crystallizes in a tetragonal, 6.0348(4)×17.0688(22) A, unit cell with I4cm (108) space group symmetry. This oxygen vacancy order–disorder transition is associated with an evolution to fast oxide-ion conduction. At 1040 °C, Ba 2 In 2 O 5 becomes a cubic oxygen-deficient perovskite, 4.2743(1) A, in the Pm3m space group. At 1080 °C, Ba 2 In 2 O 5 begins to decompose.

The high-temperature reactions of YBa2Cu3O7-δ

Abstract Real-time observations of the YBa 2 Cu 3 O 7− δ (123) melting sequence in air and oxygen were made using high-temperature XRD techniques and high-temperature optical microscopy. Results obtained from these analyses show a surprising surface peritectic reaction before the melting peritectic reported from quench studies. The present data indicate the probable existence of Y 2 Cu 2 O 5 between about 700 and 940°C, in air, but not oxygen, due to reaction between the 123 and CO 2 . The first liquid phase reaction beginning at about 940°C, is a slow surface reaction producing BaCuO 2 . This reaction has been misinterpreted as an impurity effect by most other workers. BaCuO 2 continues to grow in concentration until about 1020°C where it and the remaining 123 rapidly disappear during the peritectic reaction forming Y 2 BaCuO 5 (211) and liquid. At approximately 1150°C, Y 2 O 3 begins to form from the 211 phase. The kinetics of this reaction are much slower than the 1020°C peritectic melting.

Direct observation of textured YBa2Cu3O7−δ crystal growth from the melt

Abstract The behavior and kinetics of textured growth of the YBa 2 Cu 3 O 7−δ (123) compound from Y 2 BaCuO S (211) + liquid have been investigated by hot-stage optical microscopy and high-temperature x-ray diffraction methods. The formation of the 123 is found to begin in the liquid of the peritectic melt, not nucleated from 211 crystals as is widely assumed. The quantity of the liquid phase strongly influences the crystal growth and final morphology of the 123. Dynamic studies show that the optimum temperature for 00 l orientation of 123 is 940°C on an MgO substrate. The growth rate of 123 from the melt is rapid; the rate constant was determined to be 0.526×10 −2 S −1 .

High temperature x-ray and calorimetric studies of phase transformations in quasicrystalline Ti–Zr–Ni alloys

We present the first high temperature x-ray diffraction (HTXRD) studies of in situ quasicrystal-crystal and crystal-crystal transformations in Ti–Zr–Ni alloys. Together with differential scanning calorimetry studies, these x-ray measurements indicate three separate paths for the Ti–Zr–Ni quasicrystal-crystal transformation: single exothermic, single endothermic, or multiple endothermic. The mode of transformation depends on the alloy composition and the level of environmental oxygen. The crystalline products include the Ti 2 Ni, MgZn 2 Laves, α−(Ti, Zr), and β−(Ti, Zr) phases. In the absence of oxygen, the endothermic transformation of the quasicrystal demonstrates that it is the lowest free energy (stable) phase at the Ti 53 Zr 27 Ni 20 composition. Oxygen stabilizes the Ti 2 Ni phase, eliminating both the quasicrystal and the MgZn 2 Laves phase, at partial pressures as low as a few hundred ppm.

Structures and anisotropic thermal expansion of the α , β , γ , and δ polymorphs of Y 2 Si 2 O 7

The α , β , γ , and δ polymorphs of Y 2 Si 2 O 7 were synthesized using sol-gel and solid-state methods. The structures of the α and γ polymorphs were determined by identification of isostructural rare-earth disilicates, and the structures were refined using Rietveld analysis of X-ray powder diffraction data. The α polymorph crystallizes in space group P 1 , with a =6.5872(6) A, b =6.6387(7) A, c =12.032(1) A, α =94.501(7)°, β =90.984(8)°, γ =91.771(7)°, and volume=524.16(9) A 3 . The γ form is described by space group P 2 1 / c , a =4.68824(5) A, b =10.84072(9) A, c =5.58219(6) A, and γ =96.0325(3)°. The anisotropic thermal expansion of each phase was measured using high temperature diffraction up to 1200 or 1400 °C, depending on the stability of the polymorph. The thermal expansion is highly anisotropic for all polymorphs, with the low-expansion direction normal to the long axis of the corner-shared SiO 4 tetrahedra.

Effect of undercooling temperature on the solidification kinetics and morphology of Y-Ba-Cu-O during melt texturing

Abstract The phase content of a series of YBa 2 Cu 3 O 7-δ (123) bars was determined as a function of the holding time during the post-melting solidification process at different undercooling temperatures. Quantitative XRD analysis of these samples shows that 123 recrystallization occurs in three distinct stages after cooling from the melting temperature. The first stage is an incubation period during which no 123 forms, followed by a rapid liquid-assisted reaction stage, then finally a slow solid-state sintering stage. Nucleation of 123 from the partial melt which contains 211+BaCuO 2 + liquid is a low-probability process which results in only a limited number of nuclei forming during the texturing process. The undercooling temperature is a critical parameter which controls the final morphology of 123 in the case of non-directional solidification. Large amounts of undercooling results in a microstructure characterized by randomly-oriented dendritic needles or plates, while a low degree of undercooling yields large, blocky domains of oriented 123.

Phase formation and growth mechanisms in Bi2Sr2CaCu2O8 glass ceramics

Abstract Devitrification of Bi 2 Sr 2 CaCu 2 O 8 (Bi-2212) glasses, including phase formation sequences and mechanisms, has been investigated using both thermal analysis and high-temperature X-ray diffraction (HTXRD). Differential scanning calorimetry (DSC) was used to determine the activation energy for crystallization to be 395 kJ/mol, and in addition showed that the crystallization of these glasses occurs via a bulk mechanism. Differential thermal analysis (DTA) and simultaneous differential thermal and thermogravimetric analysis (STA), in conjunction with HTXRD, were used to establish the phase evolution of both bulk and powdered glass samples heated under O 2 and N 2 atmospheres. The results clearly indicate that Bi 2 Sr 2 Cu 1 O 6 (Bi-2201) crystallizes directly from the glass first, and that Bi-2212 forms at temperatures above ~800°C by the diffusion of Ca, Cu and O into the Bi-2201 lattices.

In situ high-temperature X-ray diffraction characterization of silver sulfide, Ag2S

Silver sulfide, Ag 2 S, is most commonly known as the tarnish that forms on silver surfaces due to the exposure of silver to hydrogen sulfide. The mineral acanthite is a monoclinic crystalline form of Ag 2 S that is stable to 176°C. Upon heating above 176°C, there is a phase conversion to a body-centered cubic (bcc) form referred to as argentite. Further heating above 586°C results in conversion of the bcc phase to a face-centered cubic (fcc) phase polymorph. Both high-temperature cubic phases are solid-state silver ion conductors. In situ high-temperature X-ray diffraction was used to better understand the polymorphs of Ag 2 S on heating. The existing powder diffraction file (PDF) entries for the high-temperature fcc polymorph are of questionable reliability, prompting a full Rietveld structure refinement of the bcc and fcc polymorphs. Rietveld analysis was useful to show that the silver atoms are largely disordered and can only be described by unreasonably large isotropic displacement parameters or split site models.

Chemical synthesis of the high-pressure cubic-spinel phase of ZnIn2S4

Chemical reactions conducted in solution are known to generate solid precursors containing molecular units that help in the formation of high-temperature phases. The structural units are created by controlling the molecular environments in solution, and as a result, phases that normally form and are stable at high temperatures can be synthesized at low or moderately elevated temperatures. However, the application of chemical approaches for synthesizing phases that normally form at high pressure are relatively unknown. In this work, a simple room-temperature aqueous chemical precipitation route has been used to synthesize the high-pressure cubic spinel modification of ZnIn2S4. A solution coordination model (SCM) has been proposed to explain the formation of the high-pressure phase. The crystallinity, phase purity and phase transformation characteristics of the cubic phase have been studied using X-ray diffraction (XRD) including Rietveld refinement, transmission electron microscopy (TEM), and Auger electron microscopy (AEM). Results of these studies are discussed in the light of a proposed solution coordination model (SCM).