Enhanced current transport at grain boundaries in high-Tc superconductors
Enhanced current transport at grain boundaries in high-Tc superconductors
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DOI：
10.1038/nature03644
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发表时间：
2005-05-26
期刊：
影响因子：
64.8
通讯作者：
Pennycook, SJ
中科院分区：
文献类型：
--
作者：
Klie, RF;Buban, JP;Pennycook, SJ
高转变温度（高 T-c）超导体的大规模应用（例如在超导电缆中的使用）受到多晶材料（唯一实用的选择）支持比单晶低得多的电流密度这一事实的阻碍（1-6）。如果取向差角超过2度至7度，则穿过晶界的超导临界电流密度(J(c))呈指数下降。颗粒纹理减少了平均定向误差角，但问题仍然存在(7,8)。添加杂质（例如 YBa2Cu3O7-δ 中的 Ca；YBCO）会导致 J(c) 增加（参考文献 9、10），这通常归因于 Ca2+ 取代 Y3+ 引入的过量空穴（参考文献 11）。然而，晶界和钙掺杂作用的综合物理模型仍然难以捉摸。在这里，我们报告了原子尺度的计算、成像和光谱，证明在多晶 YBCO 中，高应变晶界区域含有过量的 O 空位，从而降低了局部空穴浓度。 Ca 杂质确实替代了 Y，但在压缩和拉伸下的晶界区域，它们也替代了 Ba 和 Cu，从而减轻了应变并抑制了 O 空位的形成。我们的结果表明，对于增强 J(c)，离子半径比电子价更重要。
Large-scale applications of high-transition-temperature (high-T-c) superconductors, such as their use in superconducting cables, are impeded by the fact that polycrystalline materials ( the only practical option) support significantly lower current densities than single crystals(1-6). The superconducting critical current density (J(c)) across a grain boundary drops exponentially if the misorientation angle exceeds 2 degrees- 7 degrees. Grain texturing reduces the average misorientation angle, but problems persist(7,8). Adding impurities ( such as Ca in YBa2Cu3O7-delta; YBCO) leads to increased J(c) (refs 9, 10), which is generally attributed to excess holes introduced by Ca2+ substituting for Y3+ (ref. 11). However, a comprehensive physical model for the role of grain boundaries and Ca doping has remained elusive. Here we report calculations, imaging and spectroscopy at the atomic scale that demonstrate that in poly-crystalline YBCO, highly strained grain-boundary regions contain excess O vacancies, which reduce the local hole concentration. The Ca impurities indeed substitute for Y, but in grain-boundary regions under compression and tension they also replace Ba and Cu, relieving strain and suppressing O-vacancy formation. Our results demonstrate that the ionic radii are more important than their electronic valences for enhancing J(c).