High Figure of Merit in Nanostructured n-Type KPbmSbTem+2 Thermoelectric Materials

We demonstrate that the KPbmSbTe2+m system (PLAT-m for tellurium, antimony, lead potassium, m = 19-21) of materials exhibits high thermoelectric performance. Samples with compositions K1-xPbm+delta Sb1+gamma Tem+2 were prepared using several combinations of x, delta, gamma and m and their thermoelectric properties were investigated in the temperature range of 300 - 800 K. All K1-xPbm+delta Sb1+gamma Tem+2 samples exhibited n-type conduction over the measured temperature range. Their lattice thermal conductivities were found to be significantly reduced when compared to PbTe and even AgPbmSbTem+2. For example, for K0.95Pb20Sb1.2Te22 a lattice thermal conductivity as low as 0.4 W/(m.K) was estimated at 650 K (based on a Lorenz number of 1.25 x 10(-8) W.Omega/K-2). High resolution transmission electron microscopy on several samples revealed a widely dispersed nanoscale particle with varying size and shape endotaxially embedded inside a PbTe-rIch matrix which is believed to be responsible for the reduced lattice thermal conductivity of K1-xPbm+delta Sb1+gamma Tem+2 materials. Because of their small size, the nanoinclusions are coherent with the matrix and therefore do not markedly degrade the electrical conductivity of the materials. As a result, very high figures of merit are achieved at high temperature for several compositions. For K0.95Pb20Sb1.2Te22, a maximum figure of merit ZT similar to 1.6 was obtained around 750 K. This Value is similar to that of n-type LAST-18 and is two times larger than that of the-state-of-the-art n-type PbTe.