Promotion of phase formation and introduction of pinning centers in Bi based high Tc superconductors, and their applications to tapes and bulks

铋氧化物超导体(Bi2212.Bi2223)是最具实用价值的候选方案。在实际应用中，需要高的临界电流密度J_c和低的交流损耗。我们发现在BiO_2超导体中加入大量的V可以在不降低T_c的情况下极大地促进Bi2223的形成。添加的V以片状的Sr6V2O11(SVO)粒子形式存在，对产物的形成起到了重要的促进作用。以掺V的Bi2223粉末为原料，采用管内粉末法制备了单丝和多丝银套带材。当V含量达到0.15时，Jc值随V含量的增加而增加，对于多丝带材，这一趋势变得越来越明显。此外，我们还制备了银套Bi2223多丝带，每个丝的内外都有一层SVO势垒。SVO阻挡层对带材的临界电流密度Jc没有不良影响，交流损耗明显降低。在每个细丝内外都有障碍物的磁带中，JC明显增加。因此，SVO将是降低交流损耗势垒的重要候选材料之一。为了获得高的晶体取向，Bi2212块材和厚带材已经采用了磁熔融处理或在强磁场中生长晶体。到目前为止，还没有为它们开发出任何技术。磁场倾向于使Bi2212晶粒沿c轴平行取向，织构程度随磁场的增大而显着增加。因此，Bi2212块材和厚带材的Jc值有了明显的提高。这表明，基质金属氧化物是一种制备高取向、高Jc值的Bi2212块材和厚带的很有前途的方法。

Bi oxide superconductors (Bi2212. Bi2223) are the most important candidates for practical use. For practical applications high critical current density Jc and low a.c.losses will be required.we found that a considerable amount of V added into Bi oxide superconductors greatly enhances the Bi2223 formation without any degradation of Tc. The added V exists as Sr6V2O11 (SVO) plate-like particles, which play an important role to promote the formation. Using V doped Bi2223 powder, we prepared Ag-sheathed tapes with monofilament and multifilaments by the powder-in-tube method. Jc values increase with increasing the V content up to 0.15 and this tendency becomes more and more conspicuous for multifilametary tapes. Furthermore, we prepared Ag-sheathed Bi2223 multifilamentary tapes with a layer of SVO barrier outside and inside each filament. The SVO barrier layers have no bad effects on the critical current density Jc of the tapes and decrease a.c.losses obviously. Jc increases obviously in the tapes with barriers both inside and outside each filament. Therefore, the SVO will be one of the important candidates for barriers to reduce a.c.losses.Magnetic melt processing (MMP) or crystal growth in high magnetic field have been applied to Bi2212 bulks and thick tapes in order to achieve high crystal orientation. For them no techniques have been developed so far. Magnetic field tends to orient Bi2212 grains with the c-axis parallel to it and the degree of texture increases notably with increasing the magnetic field. Therefore, the Jc values of Bi2212 bulks and thick tapes are improved obviously. This shows that MMP is a promising method to prepare Bi2212 bulks and thick tapes with high grain orientation and high Jc values.