Enhancement in thermoelectric power factor of M/T/N and M/Bi/M/Bi/M (M and N = Cu or Ni) composite devices welded with T = Bi or Bi0.88Sb0.12 alloy

Abstract

The resultant thermoelectric power factors P of M/T/N and M/Bi/M/Bi/M (M and N = Cu or Ni) devices welded with T = Bi or Bi0.88Sb0.12 alloy were measured at 298 K and compared with P values calculated as a function of x by treating these devices as an electrical and thermal circuit, where x is the ratio of thickness of Bi or Bi-Sb alloy to the interval between two thermocouples. Consequently, the Seebeck coefficients α of M/T/N devices were enhanced significantly in the middle range from x = 0.3 to 0.8, and the observed P values have a local maximum at small x below x = 0.35. The x-dependence of P values of M/T/N devices was found to be explained well by the simple model proposed here when an enhancement factor in α and some reduction in thermal conductivity κ of pure metal were taken into the calculation. As a whole, however, the observed P values of double-layered M/Bi/M/Bi/M devices are lower than those of single-layered M/Bi/M, so that the macroscopic multi-layering of thermoelectric materials has no effect on the enhancement in P. The maximum P (= 22.1 mW/K2m) of Cu/Bi-Sb/Cu is 1.5 times larger than that of Cu/Bi/Cu and reached 3.6 times as large as the mean value of 5.7 and 6.6 mW/K2m of the high-performance bulk p- and n-type bismuth-telluride compounds.