Faceted Peritecric Growth Mechanism of Superconductive RE-Oxides

为了阐明定向凝固YBCO超导氧化物的凝固机理，对123(YBa2Cu3O；7-X&gt；)123(YBa2Cu3O&lt；7-X&gt；)小平面晶体从液态211相中的凝固过程进行了数值和分析研究。首先，为了阐明生长条件和形貌对123晶体微观结构的影响，通过对123晶体的刻面生长、211颗粒在液体中的熔化和溶质在液体中的扩散等一系列原始过程的评估，对123晶体的刻面包晶生长进行了二维数值模拟。用有限差分法计算了123生长过程中溶液中的溶质分布，并根据实验得到的211粒子在YBCO溶液中的对数正态分布计算了123小面晶体中残留的211粒子和液池的分布。在不同的生长模式和条件下，计算结果与实验结果符合得很好。其次，对区熔和定向凝固的YBCO中的柱状晶向等轴晶的转变进行了解析研究。柱状-等轴转变的临界条件(生长速率(R)与温度梯度(G)的关系)是通过将柱状生长的过冷度(ΔT_c)和新晶体形核和生长的过冷度(ΔT_t)等同于固相分数fS(=10^&lt；-10&gt；，或0.01)来计算的。为了评估Δ的TT，根据TTT曲线确定CCT曲线，该曲线是通过应用改进的约翰逊-Mehl-Avrami方程，在有效样本量中评估成核的孵化时间(TINC)而获得的。在较宽的温度梯度(G)范围内，计算结果与实验结果吻合较好。

To clarify the solidification mechanism of unidirectionally solidified superconductive YBCO oxides, numerical and analytical studies were performed on solidification process of faceted 123 (YBa_2Cu_3O_<7-X>) crystals from liquid + 211 phases. Firstly, to clarify the effects of growth conditions and morphologies on the microstructures of 123 crystals, two-dimensional numerical simulation of faceted peritectic growth of 123 crystal was made by evaluating a series of primitive processes ; the faceted growth of 123 crystal, the melting of 211 particles in the liquid and the solute diffusion in the liquid. Solute distributions in the liquid during the 123 growth were calculated by FDM, and the distributions of residual 211 particles and liquid pools in the faceted 123 crystals were calculated from the experimentally obtained log-normal distributions of 211 particles in the liquid of YBCO.The calculated results well agreed with the experimental ones for various growth modes and conditions. Secondary, the transition of macrostructures from columnar to equiaxed 123 crystals in zone-melted or unidirectionally solidified YBCO was also studied analytically. Critical conditions for columnar-equiaxed transition (relations of growth rate (R) and temperature gradient (G) ) were calculated by equating the undercooling(ΔTc) for the columnar growth of faceted 123 cells and that (ΔTt) for the nucleation and growth of new crystals to a fraction of solid : fs (=10^<-10>, or 0.01). To assess the ΔTt, the CCT-curve was determined from TTT-curve, which was obtained by applying a modified Johnson-Mehl-Avrami equation, evaluating the incubation time (tinc) for nucleation in the effective sample volume. Calculated results agreed well with experimental results also for wide ranges of temperature gradient (G).