Transition of rate-controlling process for reactive diffusion between Ta and bronze in superconductor

Abstract

The reactive diffusion between Ta and a bronze was experimentally examined using sandwich diffusion couples consisting of Ta and a Cu-9.3Sn-0.3Ti alloy. The diffusion couples were isothermally annealed at a temperature of T = 1053 K for various times up to t = 1200 h. During annealing, a layer of Ta9Sn is formed at the interface in the diffusion couple. The mean thickness of the Ta9Sn layer increases in proportion to a power function of the annealing time. The exponent of the power function is equal to unity at t < 167 h but smaller than 0.5 at t > 167 h. This means that the transition of rate-controlling process occurs at t = 167 h. For the reactive diffusion between Ta and the bronze at T = 1053 K, the growth of Ta9Sn is controlled by the interface reaction at t < 167 h but by the volume and boundary diffusion at t > 167 h.