Deformation of silver-sheathed oxide superconducting tapes by trapped gases

A simple model was developed to describe the deformation in long silver-sheathed oxide superconducting tapes caused by the thermal expansion of air trapped within the conductor and by carbon dioxide released by the decomposition of SrCO₃ in the precursor powder during heat treatment. The analysis presented can be extended to include other carbon-containing compounds, impurities and gases released during the annealing process. The model assumes a zero-strain deformation process of the thin silver sheath in response to the gas pressure developed within the conductor due to heating. Results show that swelling (uniform deformation along the conductor length) increases linearly with temperature. For a carbon-free tape, swelling increases with decreasing precursor powder packing density. The reverse is true for tapes that contain carbon which is distributed uniformly within the tape. A carbon content of ≈0.001% by weight produces swelling similar to that produced by air. Increasing carbon content increases swelling. Carbon content in excess of 0.5% by weight will deform the conductor to the extent that elastic/plastic deformation can occur. Discrete SrCO₃ particles dispersed among the precursor powder produce bubbles (localized deformation). The size and concentration of these bubbles depend on the SrCO₃ particle size and concentration. For a given carbon content, small SrCO₃ particles form a large number of bubbles, which merge together resulting in swelling along the conductor length. Large SrCO₃ particles, on the other hand, form only a few discrete bubbles whose size increases with particle size.