Bi-2212基高温超导材料新工艺探索及超声场作用机理研究

Bi-2212 based high temperature superconductors, which can be made into round wires with isotropic cross section, exhibit very high current capacity under high magnetic field and low temperature. Therefore it has been considered to be a promising candidate for the fabrication of insert coils for high field magnets. Based on the analysis of traditional powder in tube process, the existing problems such as "sausage" effect, broken of filaments, bubbles and residual secondary phases are the main causes of the decreasing transport properties in Bi-2212 wires. Therefore the innovative optimization of PIT process is necessary for the further development of Bi-2212 HTS materials. In our project, we will develop the new techniques, including the ultrasonic powder filling system combining with vacuum pumping process, ultrasonic drawing system and ultrasonic field assisted partial melt sintering system. With the systematical optimization of the crucial processing parameters, the Bi-2212 long wires with high filament density, high texture degree and low secondary phase content can be obtained, and the intergrain connection and flux pinning properties can be optimized. Based on the experimental data, the relationship between these new techniques and microstructures as well as the current capacity of the wires can be established, and the influence mechanism of ultrasonic field can be analyzed. Meanwhile，with the adoption of computational simulation, the random packing theory of plate-like powders, the drawing mechanism of composite wires under ultrasonic field and the phase transition and solidification mechanism of oxides under ultrasonic field would be discussed. The research of our project will not only provide long Bi-2212 superconducting wires with high current capacity, but also will build both the experimental and theory basis for the further application of ultrasonic machining in our country.

Bi-2212基高温超导材料具有优异的低温高场性能并可制备成圆线，因此具有高场内插磁体应用的潜力。为改进传统粉末装管工艺存在的问题，将分别对装管、拉拔和部分熔化热处理等重点工艺环节进行优化，开发出适合于Bi-2212长线制备的超声装管工艺、超声拉拔工艺和超声辅助部分熔化热处理工艺，通过对各环节重点工艺参数的优化和组合，获得具有高芯丝密度、高织构度、低第二相含量的Bi-2212长线，并对其晶间连接性和涡旋钉扎性能进行表征和分析，建立工艺与微结构和超导性能之间的相互关系，探讨超声场对最终线材性能的影响机理。同时在基础研究方面，将结合计算机模拟对片状颗粒堆积理论、复合线材超声拉拔机理进行系统的阐述，并对超声场中复杂氧化物的相变凝固机理进行研究。本项目的研究不仅有望获得具有高载流性能的Bi-2212超导长线，并且可为拓展我国在超声加工领域的研究提供理论和实验依据。

Bi-2212基高温超导材料具有优异的低温高场载流性能，并且作为目前唯一一种可以制备成具有各向同性截面的高温超导材料，在高场磁体和大电流应用领域都有非常明确的应用前景。本项目通过对目前Bi-2212线材制备的传统粉末装管工艺进行分析，创新性的提出在装管工艺、拉拔工艺以及热处理工艺中加载超声波，开发出了Bi-2212线材的超声拉拔和超声热处理工艺，并在此基础上，对各个工艺过程中的关键参数进行了系统性的优化，获得了材料载流性能的提升。主要研究内容包括：1. 前驱体粉末颗粒度对装管密度和最终超导线性能的影响；2. 通过有限元模拟，对超声波加载所引起的体系摩擦力等方面的变化进行了理论计算；3. 在实际操作过程中，研究了复合线材的本征力学性能和拉拔力随超声振动频率的变化规律，并对拉拔后圆线的微结构和性能进行了分析；4. 采用超声热处理工艺对Bi-2212圆线进行了烧结，系统分析了熔化和冷却阶段内超声振动对Bi-2212微结构和载流性能的影响机理；5. 优化了高压热处理工艺，获得了载流性能的显著提高。本项目获得的最高临界电流在4.2 K自场条件下达到1070 A，对应于临界电流密度达到11000 Acm-2量级，圆满完成了课题指标。