Enhancement of the Seebeck coefficient in touching M/Bi–Te/M (M = Cu and Ni) composites

The resultant Seebeck coefficient α of the touching p- and n-type M/Bi–Te/M (M = Cu and Ni) composites was measured as a function of z at a scan step of 0.5 mm using thermocouples set at three different intervals of s = 4, 6.5 and 8 mm, where s is the interval between two probes and z is the distance from the center of Bi–Te compound to the middle of two thermocouples. Bi–Te compounds have a thickness of t _Bi–Te = 6 mm but the thickness t _M of both end metals sandwiching their compounds was varied from 0.5 mm to 6 mm. The composites were compacted tightly at a force of about 10 N by a ratchet. When two probes are placed on both end metals, the resultant α was significantly enhanced and exhibited a tendency to increase as s approaches t _Bi–Te, like the welded composites. The enhancement in α is attributed to the contribution from the barrier thermo-emf generated near the interface. When the thickness t ₀ of metal outside two probes set at s = 6.5 mm was increased from 0.25 mm to 5.75 mm, the averaged α for M = Cu and Ni was increased by 3.8% in the p-type composite, while reversely it was decreased by 4.8% in the n-type one. It was first observed that t ₀ also has a significant influence on the resultant α. The maximum α of the p- and n-type Ni/Bi–Te/Ni composites then reached great values of 264 μV/K at t _M = 6 mm (corresponding to t ₀ = 5.75 mm) and −280 μV/K at t _M = 1.2 mm (corresponding to t ₀ = 0.95 mm), respectively, which are 29% and 23% larger in absolute value than their intrinsic α values. These maximum α were barely changed with time. It was thus found that the barrier thermo-emf is generated steadily even in touching composites and the resultant α is highly sensitive to the position of leads connected to the metal electrode of a thermoelement.