Carbon has been a standard doping agent in MgB₂ for long time. It, however, has also participated in a non‐uniform distribution of the constituents throughout the bulk MgB₂. To address this issue, carbon encapsulated boron (CEB) was used instead of the manually added mixture of boron and carbon. The previous studies confirmed that only low concentrations of carbon in CEB were effective for synthesis of a high‐performance bulk MgB₂. Here, a further step in optimization carbon content in CEB is reported. Carbon content in CEB varied as 1, 1.1, 1.35, 1.5, and 1.9 wt%. X‐ray diffraction (XRD) results depict a slight shift in peaks corresponding to a–b plane, indicating carbon substitution into the lattice. High superconducting critical current density in self‐field, such as 660, 550, and 435 kA cm⁻², was observed in the samples with 1.5 wt% CEB at 10, 15, and 20 K, respectively. In addition, J_c of 75 kA cm⁻² at 2 T and 20 K was observed in the 1.5 wt% CEB sample, which is thrice the value observed in the pure sample, with a minute tradeoff in T_c (around 37.5 K). Scanning electron microscope (SEM) images reveal that small particles of size ranging from 50 to 200 nm contribute to J_c improvement. Energy‐dispersive X‐ray (EDX) results show carbon uniformly distributed throughout the bulk.

中文翻译：

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高性能大块烧结MgB2碳掺杂硼掺杂的优化。

长期以来，碳一直是MgB _2中的标准掺杂剂。但是，它也参与了整个MgB ₂的成分的不均匀分布。为了解决此问题，使用了碳封装的硼（CEB）代替了人工添加的硼和碳的混合物。先前的研究证实，CEB中只有低浓度的碳才能有效合成高性能的块状MgB ₂。在此，报告了优化CEB中碳含量的进一步步骤。CEB中的碳含量变化为1、1.1、1.35、1.5和1.9 wt％。X射线衍射（XRD）结果表明对应于a – b的峰发生了轻微的偏移平面，表明碳被取代为晶格。在CEB浓度为1.5 wt％的样品中，分别在10 K，15 K和20 K下观察到自电场中的超导临界电流密度很高，例如660、550和435 kA cm ^-2。此外，在1.5 wt％的CEB样品中，在2 T和20 K下观察到的J _c为75 kA cm ^-2，是纯样品中观察到的值的三倍，而T _c却有微小的折衷（约37.5 K） 。扫描电子显微镜（SEM）图像显示，粒径在50至200 nm之间的小颗粒有助于改善J _c。能量色散X射线（EDX）结果表明，碳均匀分布在整个主体中。