Reactive diffusion between Pd and Sn at solid-state temperatures

The kinetics of the reactive diffusion between Pd and Sn at solid-state temperatures was experimentally examined using Sn/Pd/Sn diffusion couples prepared by a diffusion bonding technique. The diffusion couples were isothermally annealed at temperatures of T = 433, 453 and 473 K for various times up to 300 h in an oil bath with silicone oil. After annealing, PdSn4, PdSn3 and PdSn2 compound layers were observed at T = 433 K, but only PdSn4 and PdSn3 layers were recognized at T = 453 and 473 K. The total thickness l of the Pd-Sn compound layers is mathematically described as a power function of the annealing time t as follows: l = k(t/t0)n, where t0 is unit time, 1 s. The diameter d of the columnar grain in the compound layers is also expressed as a power function of the annealing time t: d = kd(t/t0)p. At T = 433-473 K, n is close to 0.5, and p takes values of 0.14-0.17. Such values of p indicate that the grain growth occurs at certain rates in the compound layers. Nevertheless, the total thickness l is almost proportional to the square root of the annealing time t. Thus, it is concluded that the volume diffusion is the rate-controlling process of the reactive diffusion between Pd and Sn.