Effect of heat treatment on diffusion of oxygen in silver-sheathed oxide superconductors

Abstract

The diffusion of oxygen through the sheath of silver-encapsulated oxide superconductors during the course of heat treatment is modelled. Although the emphasis is on silver-sheathed Bi 2Sr 2CaCu 2O 8+ x conductors, the model is applicable to other silversheathed oxide superconductors. Numerical results show that processing in air at atmospheric pressure results in a loss of oxygen in the conductor following heat treatment. Most oxygen is depleted before the typical maximum processing temperature of 870–900°C is reached. The amount of depletion depends on the ambient pressure and composition. Subatmospheric ambient air pressure results in a net loss in oxygen following heat treatment, whereas high ambient pressure results in a net gain in the oxygen. Increasing the partial pressure of oxygen in the ambient decreases the net loss of oxygen to the extent that an oxygen rich ambient will result in a net gain in the oxygen content within the conductor. The thickness of the silver sheath is relatively insensitive to the variation of the oxygen content since oxygen diffuses readily through the silver sheath. The recovery of oxygen by diffusion from the ambient air into the conductor occurs during the cool-down process and is generally small. Recovery of oxygen is large with an oxygen-rich ambient. The diffusion of oxygen and the consequent buildup of pressure of trapped gases within the conductor are also dependent on the heat treatment (temperature-time). This can influence the overall quality of the conductor for the given heat treatment. The numerical results suggest that processing in air can result in insufficient oxygen during the crucial part of the heat treatment, wherein oxygen is essential for forming the proper superconducting structure. Although qualitative interpretations are presented, this model can be used to gain understanding of the process parameters and aid in their optimization for long silversheathed conductor development.