Development of Large Scale YBa2Cu3O7-x Superconductor with Plastic Forming

Abstract

As superconducting oxides with a critical temperature above 77 K were discovered in 1986[1], subsequent developments and research into superconducting materials such as YBa2Cu3O7-x and Bi2Sr2CaCu2Ox have been widely carried out. Depending on the application, superconducting materials have been prepared as pellets, wires, or films. In order to synthesize products, numerous approaches have been tried using a variety of synthesis technologies, such as melt-powder-melt growth (MPMG)[2], and quench and melt growth (QMG)[3]. In order to produce a large bulk sample with these methods, the equipment must be remodeled to fit the sample size. This limits the size of the samples that can be produced. For thin films, several methods have been used such as the sol–gel method[4], metalorganic chemical vapor deposition (MOCVD)[5], pulsed laser deposition[6]. However, it is difficult to produce thick and large samples with various three-dimensional structures by these methods. Large bulk superconducting materials for making magnetic shielding for use in magnetic resonance imaging (MRI) cannot be produced by the above techniques. Therefore, a technique for easily and inexpensively making a large bulk superconductor is highly desired. Plastic forming is very simple method and has been widely used for producing ceramics. However, there have been few papers about the preparation of bulk superconductor with using plastic forming. In the plastic forming, the slurry is prepared with kneading the mixture of ceramic powder, binder, dispersant and solvent. Polyvinyl alcohol (PVA), poly ethylene glycol, clay, sodium silicate and alumina colloid are used as the organic or inorganic binder. The addition of several % these binders to the slurry improves the forming performance of the green body and the homogenization properties of the sintered sample. Surfactants or sodium diphosphate are used as organic or inorganic dispersant. After firing, the organic binder and dispersant above described may be left in the sample as some kind of impurity. It is also known that the residual carbon and other impurities exist along the grain boundary and influence the electrical and mechanical properties. It was reported that the residual carbon decreased the superconducting properties, especially the critical current density [7]. Therefore, the plastic forming has not been used to make the bulk superconductor sample, because the used slurry includes many impurities.