银合金护套Bi-2212导线中导线长度相关的膨胀、细丝去致密化和随后的临界电流密度降低的证据
Evidence for length-dependent wire expansion, filament dedensification and consequent degradation of critical current density in Ag-alloy sheathed Bi-2212 wires
摘要：
众所周知，较长的Bi-2212导体比较短的导体具有显著较低的临界电流密度（Jc），并且最近已经清楚的是，这种降低的主要原因是在热处理期间产生的内部气体压力，该内部气体压力使线直径膨胀并且使Bi-2212细丝去致密化。在这里，我们报告的5至240厘米长度的国家的最先进的，商业银合金护套Bi-2212线完全和部分热处理后的长度依赖性膨胀。沿着金属丝长度的详细图像分析显示，金属丝直径随着距离末端的距离而增加，较长的样品通常显示出由内部气体压力引起的明显损坏和泄漏。通过比较在略低于熔点的温度下进行的热处理和通常的熔化工艺，可以清楚地看出，熔化对形成高内压至关重要。远离端部的Jc的衰减与局部线直径增加直接相关，这降低了局部Bi-2212细丝质量密度并降低了Jc，通常远超过50%。很明显，控制内部气体压力对于获得这些非常有前途的圆线的完全Jc是至关重要的，并且Bi-2212线的非常可变的性质是由于该内部气体压力迄今为止尚未得到良好控制的事实。
英文摘要：
It is well known that longer Bi-2212 conductors have significantly lower critical current density (Jc) than shorter ones, and recently it has become clear that a major cause of this reduction is internal gas pressure generated during heat treatment, which expands the wire diameter and dedensifies the Bi-2212 filaments. Here we report on the length-dependent expansion of 5 to 240 cm lengths of state-of-the-art, commercial Ag alloy-sheathed Bi-2212 wire after full and some partial heat treatments. Detailed image analysis along the wire length shows that the wire diameter increases with distance from the ends, longer samples often showing evident damage and leaks provoked by the internal gas pressure. Comparison of heat treatments carried out just below the melting point and with the usual melt process makes it clear that melting is crucial to developing high internal pressure. The decay of Jc away from the ends is directly correlated to the local wire diameter increase, which decreases the local Bi-2212 filament mass density and lowers Jc, often by well over 50%. It is clear that control of the internal gas pressure is crucial to attaining the full Jc of these very promising round wires and that the very variable properties of Bi-2212 wires are due to the fact that this internal gas pressure has so far not been well controlled.
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