Doping effects on coarsening of Y 2 BaCuO 5 phase in liquid
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The coarsening rate of Y 2 BaCu0 5 (211) phase in liquid was investigated by observations of samples which were held at 1070 °C for different holding times. The 211 phase formed rod-like shapes, and the diameters increased with the increase in the holding time. The average diameter increased in proportion to tV3 (t, holding time). This means that coarsening can be explained by the modified Ostwald ripening theory. Platinum-doping suppressed the coarsening rate and alumina-doping promoted it. However, both coarsening rates were also proportional to t113. The changes in the rate can be represented by the changes of the DLT value (DL, diffusivity in liquid and T, Gibbs-Thomson coefficient) using the Ostwald ripening theory; i.e., the value is made lower by platinum-doping and higher by alumina-doping.
I. INTRODUCTION
II. EXPERIMENTAL
Since the discovery of the high Tc oxide superconductors such as YBCO and BSCCO, many processes have been tried in order to obtain a higher criticalcurrent density (Jc). On bulk materials, melt-processing is suitable for this purpose because of high density and the strong linkage of superconducting phases. Actually, high Jc values have been reported in samples prepared by several melt-processings, including QMG 1 and MPMG 2 methods. They succeeded in introducing pinning centers, which are fine 211 phase particles, in the YBa 2 Cu 3 0 ) , (123) phase. Recently, it was clarified that the fine 211 phase particles can be obtained by the effects of platinum3^5 which was mingled from a platinum crucible during the first melting procedure of both methods. Ogawa et al. suggested that the fine 211 particles are caused by the roll of the platinum in liquid as a nucleation site for the 211 phase. However, the exact mechanism for refinement has not been confirmed yet. The understanding of coarsening behavior of the 211 phase is extremely important for obtaining the finer 211 phase particles, since fine dispersion of nonsuperconducting phase particles could enhance pinning, resulting in the achievement of high Jc.
The raw materials of Y2O3, BaCO 3 , and CuO were mixed in the ratio of Y : B a : C u = 1.2:2.1:3.1 and preheated twice in air at 900 °C for 12 h. The calcined powder was pressed into pellets (20 mm in diameter and 5 mm in height) and placed on a MgO single crystal plate, which is reacted little with the sample. The pellets were inserted into the furnace heated up to 1070 °C and held for 3 X 1 0 3 - l X 106 s and then quenched. In order to investigate the effect of platinum- and aluminadopings, 0.1, 0.3, and 0.5 wt. % of Pt and A12O3 were independently mixed into the calcined powder. The same procedure was carried out as mentioned above. The conditions used in this study are shown in Table I. The microstructures were observed by EPMA after polishing. Additionally, three-dimensional observation of the 211 phase was carried out in the following manner. The quenched sample was crushed and placed on a filter paper. The acid water was flown through the paper, and
On the other hand, an alumina
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