Faceted Peritecric Growth Mechanism of Superconductive RE-Oxides

为了阐明单向凝固超导YBCO氧化物的凝固机理，对液相+ 211相中多面123 （YBa_2Cu_3O_<7-X>）晶体的凝固过程进行了数值和分析研究。首先，为了明确生长条件和形貌对123晶体微观结构的影响，通过评估一系列原始过程，对123晶体的面形包晶生长进行了二维数值模拟；123晶体的多面生长、211粒子在液体中的熔化和溶质在液体中的扩散。利用FDM计算了123生长过程中液体中的溶质分布，并根据实验得到的YBCO液体中211颗粒的对数正态分布计算了面形123晶体中残余211颗粒和液池的分布。在各种生长方式和条件下，计算结果与实验结果吻合较好。其次，分析研究了区熔和单向凝固YBCO中123晶从柱状到等轴的宏观组织转变。柱状-等轴转变的临界条件(生长速率（R)和温度梯度(G)的关系）通过将面形123细胞柱状生长的过冷（ΔTc）和新晶体成核和生长的过冷（ΔTt）等效为固体的一部分：fs（=10^<-10>，或0.01）来计算。为了评估ΔTt，利用修正的Johnson-Mehl-Avrami方程得到的ttt曲线，确定cct曲线，评估有效样品体积内成核的孵育时间（tinc）。在较宽的温度梯度(G)范围内，计算结果与实验结果吻合良好。

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).