An experimental investigation into the joining of bulk high temperature superconductors.

Abstract

Current melt texturing processes can only produce high-quality High Temperature Superconducting (HTSC) domains of a few centimetres in size. Increasing the size of the domain by a joining technique is investigated. The parent HTSC melt textured domains of YBa2CU307-li (Y 123), were used in this research to investigate the joining technique. The solder powders used to form the seam were HTSC YbBa2Cu307-li (YbI23), which has a peritectic temperature of approximately 925 cC, Yb2BatCu\07_o (Yb211), Silver Oxide (Ag20) and Y123. A total of 8 different superconducting 'solders' were manufactured using these powders. Microstructural analyses using a scanning electron microscope (SEM) and an optical microscope were conducted on each sample. It was found that mechanisms occurring during the growth of the seam affect the performance of the join. During the growth, three interfaces are formed. The interface between the parent and the growth front produced an excellent join whereas most of the microstructural defects were present at the intersection of the two growth fronts also known as the impingement boundary. The effect of the addition of Yb211 and silver particles to the join were investigated. It was found that although Yb211 particles improve the flux pinning and viscosity of the solder and the silver improves the intergranular critical current density (Jc) [1], they also reduce the effective cross-sectional area of the joined sample. Thus, the amount of non-superconducting particles added needs to be optimised. Transport measurements were taken to evaluate the current carrying capacity of each sample. It was found that Sample 2-A (Yb123 + 5%Ag20) has the highest Jc of approximately 142 A/cm 2 at 80 K while Sample 3-A (Y123 + 5%Ag20) has the poorest Jc of about 37 A/cm2 at 80 K. Resistivity vs. Temperature graphs show that all joined samples were superconducting implying that the joining process was successful in establishing a superconducting join. Magnetic field maps of the parent and joined samples were captured using a Hall Probe. It was found that the joined sample could trap almost 95 % of the field that could be trapped by the parent. From these results, it was concluded that joining HTSC bulk pieces is possible using an external 'soldering' agent.