STUDY ON ELECTRICAL DISSIPATION OF HIGH TEMPERATURE SUPERCONDUCTOR WHISKERS

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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.