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

We demonstrate that the KPb m SbTe 2+ m system (PLAT- m for tellurium, antimony, lead potassium, m = 19−21) of materials exhibits high thermoelectric performance. Samples with compositions K 1− x Pb m +δ Sb 1+γ Te m+2 were prepared using several combinations of x, δ, γ and m and their thermoelectric properties were investigated in the temperature range of 300 − 800 K. All K 1− x Pb m +δ Sb 1+γ Te m +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 AgPb m SbTe m +2 . For example, for K 0.95 Pb 20 Sb 1.2 Te 22 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 × 10 −8 W·Ω/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 K 1− x Pb m +δ Sb 1+γ Te m +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 K 0.95 Pb 20 Sb 1.2 Te 22, a maximum figure of merit ZT ∼ 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.