攙銀或攙金 YBa2Cu4O8 高溫超導體複材電子傳導與超導性之研究

Abstract

本研究重點在探討釔系: 釔-124 高溫超導體分別與金屬銀和金 金所形成 的複合材料之製程及電子傳導; 反磁性;微結構和臨界電流密度等特性分 析.我們已對一系列金屬體積分率(Pm)由0.0 到1.00之複材電阻率作量測. 其電阻率可利用 Halbritter et al.所提出的模型加以描述之.另外當銀 體積分率大於0.8和金體積分率大於0.7時,複材不再有超導現象.在金屬/ 超導體複材的電子傳導行為可用二元組成的三維滲透模型加以描述.其結 果為以銀或金為母體之複材的滲透起始值出現在P(Ag)為20%和P(Au) 為35%,而相對應地以超導體為母體的滲透起始值為80%和65%.另外我們也 探討由釘扎效應以增加臨界電流密度的現象.並利用Bean model估算出兩 複材的Jc值. 在電子微探分析中,我們亦觀察到在超導體內形成金屬 cluster的能力將與實驗值是否偏離理論值有絕對的關係. This research attempted to investigate the materials preparation,electrical transport,magnetic properties,micro- structure and critical current density (Jc) of the silver- and gold- Y-124 superconductor composite system. The observed electrical resistivities of the silver(Ag)- and gold(Au)- YBa2 Cu4O8 composite systems with volume fraction of metal,Pm ranging from 0.0 to 1.00. The electrical resis- tivities can be interpreted in terms of a model for granular superconductor proposed by Halbritter et al. For samples with P(Ag). gtoreq.0.8 and P(Au).gtoreq.0.7, respectively, no zero- resistivity state is observed. The electrical transport behavior in both metal/superconductor composites can be des- cribed by a two-component three-dimensional(3D)percolation model. The 3D percolation thresholds (Pc) for the Ag and Au matrices were determined to be at 20% and 35%,and consequently the percolation thresholds for the YBa2Cu4O8 superconductor are 80% and 65%, respectively. Furthermore,we have also inves- tigated the enhancement of Jc presumably due to flux-pinning effect and estimated the intragrain Jc in the two metal/super- conductor systems by employing Bean model. The electron probe microanalysis(EPMA) carried out on the composite samples indicated clustering of metal grains in the superconductor matrices which may be related to the deviation of the observed percolation threshold in both systems from the ideal values.