High figure of merit in nanostructured n-type KPb_mSbTe _m+2 thermoelectric materials

We demonstrate that the KPb_mSbTe_2+m system (PLAT-m for tellurium, antimony, lead potassium, m = 19-21) of materials exhibits high thermoelectric performance. Samples with compositions K_1-xPb _m+δSb_1-γTe_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-xPb_m+δSb_1+γTe_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_mSbTe_m+2 For example, for K_0.95Pb₂₀Sb_1.2Te₂₂ 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, Ω/K²). 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+δTe_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_0.95Pb₂₀Sb _1.2Te₂₂, 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.