[论文解读] Properties of MgB2 bulk

T. A. Prikhna|ArXiv.org|Dec 24, 2009

Superconductivity in MgB2 and Alloys参考文献 2被引用 3

一句话总结

本综述研究了体相MgB2基材料，重点分析其结构、超导性和力学性能。研究发现，氧掺杂（5–14 wt%）和纳米级MgB12相显著提升了临界电流密度和上临界磁场，即使XRD对二次相的检测证据极少，仍实现了在20 K下零场时1.8 × 10⁶ A/cm²的临界电流密度和在22 K时15 T的HC2值。

ABSTRACT

The review considers bulk MgB2-based materials in terms of their structure, superconducting and mechanical properties. Superconducting transition temperatures of 34.5-39.4 K, critical current densities of 1.8-1.0 x E6 A/sq.cm in self field and 103 in 8 T field at 20 K, 3-1.5xE5 A/sq. cm in self field at 35 K, HC2 15 T at 22 K and Hirr 13 T at 20 K have been registered for polycrystalline materials. As TEM and SEM study show, dispersed higher borides and rather big amount (5-14 percents) of oxygen (bonded simultaneously with Mg and B) can be present in the structure even if X-ray pattern contains only reflexes of well crystallized MgB2 with traces of MgO. Materials with such a rather high oxygen content demonstrated high superconducting characteristics. At present it is established that nanosized MgB12 grains provide effective pinning in polycrystalline material. Besides, additions can introduce the MgB2 structure inducing disorder in lattice sites (for example, C substitution for B). The disorder increases the normal state resistivity, magnetic penetration depth, and the upper critical field, but reduces the transition temperature and anisotropy. It is highly probable that the additives (Ti, Ta, Zr, SiC) together with synthesis or sintering temperature can affect the distribution of oxygen and hydrogen in the material structure as well as the formation of grains of higher borides, thus influencing superconducting properties. The superconductivity of materials with matrix close to MgB12 in stoichiometry (Tc=37 K) has been defined.

研究动机与目标

• 分析体相MgB2材料的结构与超导性能，重点关注氧和二次相的引入。
• 理解纳米尺度MgB12颗粒及掺杂剂（如C、Ti、Ta、Zr、SiC）对钉扎作用和临界电流密度的影响。
• 阐明氧和氢分布的作用，尽管XRD对二次相的检测证据极少，但其对超导性能具有决定性影响。
• 评估掺杂引起的晶格无序对Tc、电阻率、磁穿透深度和上临界磁场的影响。
• 确立MgB12型化学计量比材料的超导行为，其Tc = 37 K。

提出的方法

• 通过透射电子显微镜（TEM）和扫描电子显微镜（SEM）识别纳米相和氧分布。
• 利用X射线衍射（XRD）检测结晶相，即使仅观察到MgB2和微量MgO。
• 测量电学电阻率、磁穿透深度和上临界磁场（HC2），以评估正常态和超导态性能。
• 分析掺杂剂（C、Ti、Ta、Zr、SiC）对晶格无序及超导参数的影响。
• 将氧含量（5–14 wt%）与超导性能相关联，特别是临界电流密度和不可逆场。
• 评估化学计量比接近MgB12的材料的超导转变温度和临界场。

实验结果

研究问题

• RQ1 尽管XRD对二次相的检测证据极少，MgB2中掺入5–14 wt%的氧如何影响其超导性能？
• RQ2 纳米尺度MgB12颗粒在多晶MgB2中如何增强磁通钉扎和临界电流密度？
• RQ3 碳或过渡金属等掺杂剂如何影响MgB2中的晶格无序及上临界磁场？
• RQ4 氧和氢分布与合成及烧结过程中超导性能之间的关系是什么？
• RQ5 化学计量比接近MgB12的材料是否表现出超导性？其Tc和临界场是多少？

主要发现

• 在20 K下零场时实现了1.8 × 10⁶ A/cm²的临界电流密度，磁场为8 T时下降至1.0 × 10⁶ A/cm²。
• 在35 K时，零场下临界电流密度仍保持在3.0 × 10⁵ A/cm²，表明其具有高温性能。
• 上临界磁场（HC2）在22 K时达到15 T，不可逆磁场（Hirr）在20 K时为13 T。
• 通过显微镜检测到5–14 wt%的氧含量，尽管XRD仅显示MgB2和微量MgO，表明存在隐匿的氧掺杂。
• 纳米尺度MgB12颗粒被确认为多晶MgB2中有效磁通钉扎的关键贡献者。
• C、Ti、Ta、Zr和SiC等掺杂剂提高了正常态电阻率、磁穿透深度和HC2，但降低了Tc和各向异性。