Microstructures and critical current densities of melt-processed Bi₂Sr₂CaCu₂Ox superconductor [thesis]

Thomas Lang, Ludwig J. Gauckler, Nicholas D. Spencer
The best method to prepare high-temperature superconducting devices of Bi 2 Sr 2 CaCu 2 O x (Bi-2212), able to carry large transport currents at 77 K is the partial melt-processing. During this heat-treatment the cuprate is melted and subsequently crystallized upon controlled cooling. Almost all involved processing parameters have to be controlled within narrow limits in order to get good superconducting properties in the fully processed sample. This work investigates the influence of the
more » ... sing conditions on the microstructure and the superconducting properties of Bi-2212 thick films and bulk components covering the thickness range from 10 to 1000 µm. In section IV the melting behavior of Bi-2212 in various atmospheres and with silver additions (0 to 16 wt%) is described. Thick films were submitted to an optimized partial melt-processing and the microstructural evolution from the porous green-body to the dense and aligned microstructure in the fully processed samples was studied in section V. The influence of processing parameters on the microstructure and the superconducting properties of fully processed thick films and bulk components was examined in section VI. The melting behavior of Bi-2212 is strongly determined by the oxygen partial pressure of the surrounding atmosphere and the silver content of the ceramic powder. The solidus temperature of the pure Bi-2212 powder is decreased from 893°C in oxygen (pO 2 =1 atm) to 834°C if melted at pO 2 =0.001 atm. The addition of silver to Bi-2212 further decreases T solidus by up to 25 K if melted in oxygen. 2 During thermal decomposition, Bi-2212 releases oxygen into the atmosphere, resulting in a weight loss of the sample. The oxygen loss is strongest at low oxygen partial pressures of the atmosphere and is reduced from 1.6 wt% at pO 2 =0.001 atm to 1.1 wt% in pure oxygen atmosphere. The extra addition of silver further reduces the oxygen loss upon melting by increasing the oxygen solubility of the melt. The corrosion of the silver substrate is reduced because less silver is dissolved. During melt processing the Bi-2212 compound decomposes peritectically forming several solid and liquid phases. For the first time quantitative data showing the phase contents and the grain sizes of all phases during an optimized heat treatment are pre-
doi:10.3929/ethz-a-001793718 fatcat:cva47vlkxzbjlohkjuncowliza