STUDY ON ELECTRICAL DISSIPATION OF HIGH TEMPERATURE SUPERCONDUCTOR WHISKERS

通过在氧气流中在880度下对熔融淬火的Bi系非晶块体热处理3天，可以生长几毫米长的超导体晶须。通过透射电子显微镜观察发现，这些晶须是低温相，其摩尔比为Bi：Sr：Ca：Cu=2：2：1：2，并沿钙钛矿晶胞的沿着a方向生长。高温相不能检测到的X-射线衍射图案上生长的晶须与2223的标称起始组合物的散装。在晶须的平坦表面上经常观察到尺寸为几微米的台阶和岛状物，其形状向顶部逐渐变窄。晶须的电阻随着温度的降低而减小，通常在两个台阶处降至零。这两种电阻下降的临界起始温度分别为107 K和75 K，表明晶须由（2223）和（2212）两相组成。低温相和高温相之间的差异是半晶胞中的CuO 2层的数量，已知前者为2，后者为3。从晶体学角度考虑，这两种结构可能共存于晶须中，这涉及单晶中的一些缺陷。构建了一种配备最高范围温度控制器（Oxford：ITC 503）的新低温恒温器系统（Oxford：DN 1710）。改进了电性能测试仪器，并对大块超导体的电流-电压特性进行了初步实验。

Superconductor whiskers of several millimeters in length can be grown by heat treatment at 880 degrees of melt-quenched Bi-system amorphous bulk for 3 days in an oxygen gas stream. These whiskers were identified to be the low temperature phase with a typical mole-ratio : Bi : Sr : Ca : Cu=2 : 2 : 1 : 2, and found to grow along a-direction of the perovskite unit cell from the crystallographic observation using transmission electron microscope. The high temperature phase cannot be detected in the X-ray diffraction patterns of whiskers grown on the bulk with a nominal starting composition of 2223. Steps and also islands with a size of a few microns were frequently observed on a flat surface of the whiskers, the shape of those becoming narrower toward the top.Electrical resistance of whiskers decreases with decreasing the temperature, and drops to zero with two steps in general. The critical onset temperatures of these two resistance drops were 107K and 75K.This suggests that one whisker is composed of two phases of (2223) and (2212). The difference between the low- and the high-temperature phases is the number of CuO2 layrs in a half unit cell, as is known to be 2 in the former and 3 in the latter. From crystallographic consideration the both structures may coexist in a whisker, which involves some imperfections in a single crystal.A new cryostat system (Oxford : DN1710) equipped with the top range temperature controller (Oxford : ITC503) was constructed. The instruments to measure the electrical properties were improved, and preliminary experiments on current-voltage characteristics of bulk superconductors were carried out.