Processing of Bi-2212 and Nb$_3$Sn studied in situ by high energy synchrotron diffraction and micro-tomography

Next generation superconducting wires have been studied to obtain more information on the evolution of phase growth, crystallite size and strain state during wire processing. The high energy scattering beam line ID15 at the European Synchrotron Radiation Facility provides a very high flux of high energy photons for very fast in situ X-ray diffraction and micro-tomography studies of Bi-2212/Ag and Nb$_3$S/Cu wire samples. The typical wire processing conditions could be imitated in the X-ray transparent furnace at ID15 for diffraction and tomography studies. Efficient data analysis is mandatory in order to handle the very fast data acquisition rate. For this purpose an Excel-VBA based program was developed that allows a semi-automated fitting and tracking of peaks with pre-set constraints. With this method, more than one thousand diffraction patterns have been analysed to extract d-spacing, peak intensity and peak width values. X ray absorption micro tomograms were recorded simultaneously with the X-ray diffraction patterns during Bi-2212 processing in order to monitor microstructural changes. The influence of temperatures up to 900 °C and oxygen partial pressure on the Ag–Bi–Sr–Ca–Cu–O system in Bi-2212/Ag wires has been studied in situ. Inert gas and oxygen partial pressures of P$_{O2}$ = 0.21 and 21 bar have been used as process gas. Without oxygen in the process gas, no Bi-2212 reformation is possible after the liquid state. Too much oxygen restrains the Bi 2212 crystal stability, making it melt already at 650 °C and preventing Bi-2212 recrystallisation during cool-down. The Bi-2212 filament size has no significant influence on the Bi-2212 decomposition and formation temperatures, but it strongly influences the Bi-2212 texture formation, as seen from the Bi-2212 diffraction peak intensity ratios. The Nb$_3$S formation in Powder-In-Tube and Restacked-Rod-Process type wires has been compared by X-ray diffraction measurements during identical wire heat treatments.