An Experimental Investigation of the U-Mo-C Ternary Diagram

The isothermal sections of the U-Mo-C ternary system have been established at 1000°C and 1400°C, using powder X-ray diffraction, scanning electron microscopy coupled with energy dispersive X-ray analysis for the quantification of U and Mo and differential thermal analysis. The main differences between the two sections are the appearance of liquid phase at about 1230°C, due to the peritectic decomposition of γ-UMo, and the peritectoid decompositions of MoC and β’’ Mo2C. No other transformation was detected in this temperature range, especially one involving the two only ternary phases found, UMoC2 and U2Mo2C3.

Real-time Investigations on the Formation of CuIn(S,Se)2 while annealing precursors with varying sulfur content

CIS based chalcopyrite absorber materials are usually substituted in the cation and anion lattice to yield mixed pentanary crystals with the general composition Cu(In,Ga)(Se,S)2 to achieve an optimised adaptation of the semiconductor bandgap to the terrestrial solar spectrum. Real-time investigations during the annealing of stacked elemental layers (SEL) of sputtered metals Cu and In and evaporated chalcogens S and Se with varying ratios were performed by angle-dispersive time-resolved XRD (X-ray diffraction) measurements. After qualitative phase analysis the measured powder diagrams were quantitatively analysed by the Rietveld method, the phases formed determined and their reaction kinetics obtained. Ternary indium and copper sulfoselenides form by the sulfoselenisation of the intermetallic alloy yielding different educts for the chalcopyrite formation with varying sulfur content. For S/(S+Se) ≥ 0.5 the formation of the chalcopyrite CuIn(S,Se)2 is similar to the crystallisation path of CuInS2. With increasing amount of selenium (S/(S+Se) = 0.25) different ternary sulfoselenides contribute to the semiconductor formation. For small amounts of sulfur, i.e. S/(S+Se) ≤ 0.1, the chalcopyrite crystallisation proceeds comparable to the one observed for sulfur-free Cu-In-Se precursors. The formation of CuIn(S,Se)2 is accelerated and proceeds mainly after the peritectic decomposition of Cu(S,Se) to Cu2(S,Se). The sulfur content determines the crystallisation temperature of the semiconductor because Cu(S,Se) decomposes at higher temperatures with increasing sulfur. Upon heating S ↔ Se exchange reactions take place in the Cu-S-Se and Cu-In-S-Se system.

The Effects of BaF2 Addition on Melt-Processed YBa2Cu3O7-x Superconductors

The effects of BaF2 on thermal behavior and microstructure of melt-processed YBCO superconductors have been investigated. The differential thermal analysis (DTA) results indicated that the peritectic decomposition temperature of YBCO precursor powder was lowered when BaF2 was added (from 1020 to 976). The peritectic decomposition temperature was lowest at the content of 5 wt.% BaF2, and the variation of melting temperature was not significant above 5 wt.% BaF2. The microstructures of the doped samples have been observed by scanning electron microscopy (SEM). The results showed that the of BaF2 microstructure .

Growth morphology of large YBCO grains fabricated by seeded peritectic solidification: (I) The seeding process

The growth of large grain YBa2Cu3O7−δ (YBCO) by peritectic solidification in the presence of a (Sm,Y)Ba2Cu3O7−δ seed is characterized by the initial seeding process, development of a facet plane around the seed, and finally by continuous nonlocal growth away from the seed. A detailed investigation of the seeding process using electron microscopy, electron probe microanalysis, and thermal analysis techniques is reported here as the first in a series of studies of these key growth features. Results show that the seed partially melts below its nominal melting temperature due to a distribution of yttrium cations across the seed/YBCO interface. The formation of a Sm/YBa2Cu3O7−δ solid solution, which occurs via a reaction between (Sm,Y)2Ba2CuO5 and liquid state Ba3Cu5O8, has been observed across this interface at temperatures below the peritectic temperature Tp of the seed. The temperature window available for melting the YBCO phase while avoiding full peritectic decomposition of the (Sm,Y)Ba2Cu3O7−δ seed is maximized for seeds of high Sm content and thickness in excess of 0.2 mm. Finally, the dwell time at temperatures above Tp should be as short as possible if the integrity of the seed is to be maintained throughout the YBCO growth process.

Precipitate size refinement by CeO2 and Y2BaCuO5 additions in directionally solidified YBa2Cu3O7

Directional solidification of YBa2Cu3O7 has been carried out through a Bridgman technique, and the influence of Y2BaCuO5 and CeO2 additives on the size of Y2BaCuO5 precipitates has been investigated. It is demonstrated in this work that the most efficient procedure to reduce the size of the Y2BaCuO5 precipitates is to increase the concentration of nucleation centers present in the peritectic decomposition of YBa2Cu3O7−x. A small concentration (0.3−1 wt. %) of CeO2 has a strong influence on the solidification process and on the size of Y2BaCuO5 precipitates. It is shown that when CeO2 is added, further refinement of the size of precipitates results from the formation of nanometric Y2O3 particles which further enhance the multinucleation effect. We have also observed that coarsening effects are avoided with CeO2 additives.

Preparation of large single grains of the quasicrystalline icosahedral Al–Cu–Fe ψ phase

A cyclic heat-treatment process was used to prepare single grains of the quasicrystalline icosahedral phase, ψ–Al65Cu23Fe12. Alloys of appropriate composition are melted and chill cast into copper molds. Multiple cyclic heat treatments at successively higher temperatures below 860 °C, the peritectic decomposition temperature of the quasicrystal phase, are used to enhance the growth of the ψ phase. Single grains up to 10 mm × 5 mm × 5 mm have been prepared.

Formation of BaCeO3 and its influence on microstructure of sintered/melt-textured Y-Ba-Cu-O oxides with CeO2 addition

The formation of BaCeO3 and its effects on microstructure were studied in sintered/melt-textured Y-Ba-Cu-O oxides containing 5 wt. % CeO2 and various amounts of Y2Ba1Cu1O5. The added CeO2 was converted to fine particles of BaCeO3 near 930 °C, which is the conventional sintering temperature for Y-Ba–Cu-O. Y2Ba1Cu1O5 and CuO are formed as by-products of the reaction between CeO2 and Y1Ba2Cu3O7−y phase. The CeO2 addition reduced the particle size of Y2Ba1Cu1O5 which was trapped in the Y1Ba2Cu3O7−y matrix after the melt-texture growth. During the peritectic decomposition stage of Y1Ba2Cu3O7−y phase into Y2Ba1Cu1O5 and liquid phase, the morphology of the decomposed Y2Ba1Cu1O5 was changed from a blocky shape in the undoped sample to an acicular shape of high anisotropy in the CeO2-added sample. The formation of the highly anisotropic Y2Ba1Cu1O5 particles appears to be responsible for the refinement of Y2Ba1Cu1O5 particle after the melt-texture processing.