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TEM Characterization of Microstructure and Chemistry in Magnesium Diboride Superconductor

机译:二硼化镁超导体微观结构和化学的TEm表征

摘要

MgB2 is a superconductor with promising properties, but to meet applicationrequirements, the properties of MgB2 have to be improved through microstructuremodifications: chemical doping, introduction of precipitates, and atomic-scale control ofdefects. The small length scale of these modifications means that structure and chemistrycharacterization using the transmission electron microscope (TEM) are required to betterunderstand the correlation between MgB2?s superconducting properties and itsmicrostructure.We have used TEM-based techniques to characterize microstructures of pure MgB2 bulks, SiC-doped MgB2 tapes, pure and C-doped MgB2 thin films prepared by hybrid physical-chemical vapor deposition (HPCVD) method with different carbon precursors,degraded pure HPCVD thin films, and O-doped MgB2 thin films prepared by molecular beam epitaxiy. Characterization results and effects of microstructure on superconducting properties are discussed for each material.We have found that in pure MgB2 the main contaminating current-blocking phases are MgO grains and amorphous MgO + boron + BxOy phases. Both second phase nanosegregations and grain boundaries are effective pinning defects in MgB2, which can enhance Jc(H) at high fields. However, second phases also act as current-blocking phasesthat reduce the measured Jc, which suggests that MgB2 samples with small grain size andwithout any boundary second phases are desirable to achieve higher Jc.Extensive studies have been made to understand the Hc2 enhancement mechanisms in MgB2. Both carbon dopant and grain boundaries in MgB2 can cause more electron scattering that increases Hc2. Degradation in MgB2 leads to the formation of theamorphous boundary phase which also enhances Hc2 by introducing electron scattering. The over 60 T Hc2 ||(0 K) obtained in C-doped HPCVD MgB2 films is caused by the twoband nature and strong ?-band scattering of MgB2. We believe that the strain fields generated from c-axis tilt boundaries and coherent MgO nano-platelets are responsiblefor the anomalous ?-band scattering.
机译:MgB2是具有良好性能的超导体,但要满足应用要求,必须通过微结构修改来改善MgB2的性能:化学掺杂,沉淀物的引入以及缺陷的原子级控制。这些修饰的长度小,意味着需要使用透射电子显微镜(TEM)进行结构和化学表征,以更好地了解MgB2的超导性能与其微观结构之间的关系。掺杂SiC的MgB2胶带,通过混合化学化学气相沉积(HPCVD)方法和不同的碳前体制备的纯C掺杂MgB2薄膜,降解的纯HPCVD薄膜以及通过分子束外延制备的O掺杂MgB2薄膜。讨论了每种材料的表征结果和微观结构对超导性能的影响。我们发现,在纯MgB2中,主要的污染电流阻挡相为MgO晶粒和非晶MgO +硼+ BxOy相。第二相纳米偏析和晶界都是MgB2中的有效钉扎缺陷,可在高电场下增强Jc(H)。然而,第二相也可以作为电流阻挡相来降低测得的Jc,这表明具有较小晶粒尺寸且没有任何边界第二相的MgB2样品是获得更高Jc的理想方法。 。 MgB2中的碳掺杂剂和晶界均可引起更多的电子散射,从而增加Hc2。 MgB2中的降解会导致非晶边界相的形成,该非晶相也会通过引入电子散射而增强Hc2。在C掺杂的HPCVD MgB2薄膜中获得的超过60 T Hc2 ||(0 K)的原因是MgB2的双带性质和强的α带散射。我们认为,由c轴倾斜边界和相干的MgO纳米片产生的应变场是造成异常α波段散射的原因。

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  • 作者

    Zhu Ye;

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  • 年度 2008
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  • 原文格式 PDF
  • 正文语种 {"code":"en","name":"English","id":9}
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