Increase of the thermoelectric power factor in Cu ∕ Bi ∕ Cu , Ni ∕ Bi ∕ Ni , and Cu ∕ Bi ∕ Ni composite materials

The resultant thermoelectric power factors P of three types of Cu ∕ Bi ∕ Cu , Ni ∕ Bi ∕ Ni , and Cu ∕ Bi ∕ Ni composite materials welded with Bi were measured at 298 K as a function of relative thickness of Bi and compared with P values calculated by treating these devices as an electrical and thermal circuit. It was first demonstrated experimentally that the observed P values of composite devices have a local maximum at an optimum volume fraction (corresponding to the thickness) of Bi, as predicted theoretically. The maximum P values of composite materials were several times higher than those of pure Ni and Bi and were about 100 times larger than that of pure Cu. The dependence of P on the thickness of Bi was found to be explained roughly by introducing the modified thermal conductivity and an enhancement factor in the Seebeck coefficient to our simple model in which a device was treated as an electrical and thermal circuit.