Dependence of the resultant Seebeck coefficient on the thickness of Bi0.88Sb0.12 alloy in welded M/Bi–Sb/M (M = Cu and Ni) composites

The local Seebeck coefficient αL and the resultant Seebeck coefficient αR of M/Bi0.88Sb0.12/M (M = Cu and Ni) composites with different thicknesses tBi–Sb of Bi–Sb alloy were measured as functions of z and T, where T is the absolute temperature, z is the distance from a center of Bi–Sb alloy to the middle point of two probes and αL and αR were measured using two probes separated by s=1.0 mm and s=tBi−Sb+0.1 mm, respectively. As a result, αL was enhanced extremely at the position of 0.2–0.3 mm away from the interfaces, while the local temperature along a composite varies linearly with changes in z within Bi–Sb alloy. The local maximum of αR at 344 K appeared at tBi−Sb≈0.9 mm, so that it is expected to increase up to −167 μV/K at tBi−Sb=0.87 mm from the expression fitted well to the experimental data, which is 2.1 times as large as the intrinsic α at 344 K of Bi–Sb alloy. Such a local enhancement in αL would probably be caused by a temperature gradient across the depletion layer formed at the interface. The thermoelectric figure of merit ZT of a composite with an optimum tBi−Sb of 0.87 mm is expected to reach the large value of 0.98 at 344 K, which corresponds to 4.5 times as high a value as ZT=0.22 at 344 K of Bi–Sb alloy. It is thus considered that the increase in αL at the interface is available as a useful mean of further increase in ZT of thermoelectric devices.