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 t(Bi-Te) and t(Cu) was measured as a function of t, where t(Bi-Te) was varied from 0.1 to 2.0 mm, t(Cu) from 0.3 to 4.0 mm and t is the lapse time after imposing the voltage. The temperature difference Delta T is produced by imposing a constant voltage of 1.70 V on two Peltier modules connected in series. The resultant alpha of composites was calculated from the relation alpha = Delta V/Delta T, where Delta V and Delta T were measured with two probes placed on both end coppers. Delta V 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 alpha and saturated Delta V vary significantly with changes in t(Cu) and t(Bi-Te). When a composite has a combination of t(Cu) = 1.0 min and t(Bi-Te) =0.1 mm, the generating powers Delta W (=(Delta V)(2)/4R) estimated using the saturated Delta V 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 t(Bi-Te) = 2.0 min and t(Cu) = 0.3 mm. It is surprising that the generating, power Delta W 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 t(Bi-Te) = 0.1 mm and t(Cu) = 0.3 mm, the resultant alpha of the p- and n-type composites exhibited great values of 711 and -755 mu V/ 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 t(Cu) or t(Bi-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 mu m from the boundary.