Effect of the thickness of Bi–Te compound and Cu electrode on the resultant Seebeck coefficient in touching Cu/Bi–Te/Cu composites

The resultant Seebeck coefficient a of the touching p- and n-type Cu/Bi–Te/Cu composites with different thicknesses of tBi–Te and tCu was measured as a function of t, where tBi–Te was varied from 0.1 to 2.0 mm, tCu from 0.3 to 4.0 mm and t is the lapse time after imposing the voltage. The temperature difference DT is produced by imposing a constant voltage of 1.70 V on two Peltier modules connected in series. The resultant a of composites was calculated from the relation a = DV/DT, where DV and DT were measured with two probes placed on both end coppers. DV decreases abruptly with an increase of t below t = 5 min, while above t = 7 min, it tends to saturate to a constant value. The resultant a and saturated DV vary significantly with changes in tCu and tBi–Te. When a composite has a combination of tCu = 1.0 mm and tBi–Te=0.1 mm, the generating powers DW (=(DV)2/4R) estimated using the saturated DV and calculated electrical resistance R for the p- and n-type composites have great local maximum values which are 4–5 times as large as those obtained for the conventional combination of tBi- Te = 2.0 mm and tCu = 0.3 mm. It is surprising that the generating power DW is enhanced significantly by sandwiching a very thin Bi–Te material between two thick coppers, unlike the conventional composition of thermoelectric modules. On the other hand, when a composite has a combination of tBi–Te = 0.1 mm and tCu = 0.3 mm, the resultant a of the p- and n-type composites exhibited great values of 711 and –755 lV/ K, respectively, so that the maximum resultant ZT of the p- and n-type composites reached extremely large values of 8.81 and 5.99 at 298 K. However, the resultant ZT decreases rapidly with an increase of tCu or tBi–Te. The resultant ZT is thus found to be enhanced significantly not only in superlattice systems but also in macroscopic composites. The present enhancement in ZT is attributed to the large barrier thermo-emf generated in the Bi–Te region shallower than 50 lm from the boundary