Widespread application of thermoelectric devices for waste heat recovery requires low-cost high-performance materials. The currently available n-type thermoelectric materials are limited either by their low efficiencies or by being based on expensive, scarce or toxic elements. Here we report a low-cost n-type material, Te-doped Mg3Sb1.5Bi0.5, that exhibits a very high figure of merit zT ranging from 0.56 to 1.65 at 300−725 K. Using combined theoretical prediction and experimental validation, we show that the high thermoelectric performance originates from the significantly enhanced power factor because of the multi-valley band behaviour dominated by a unique near-edge conduction band with a sixfold valley degeneracy. This makes Te-doped Mg3Sb1.5Bi0.5 a promising candidate for the low- and intermediate-temperature thermoelectric applications.

The effects of Cd substitution in M x Zn4−x Sb3 on the high-temperature thermal stability, low-temperature phase transitions and thermoelectric properties have been studied on three samples with a substitution degree of 0.1, 1, and 2 at % Cd (x = 0.004, 0.04, 0.08). The high-temperature thermal stability in atmospheric air of a 1% substituted sample is compared with an unsubstituted Zn4Sb3 sample. Multitemperature synchrotron powder diffraction data reveals that while only ∼42 wt % of the original Zn4Sb3 phase remains in the unsubstituted sample after three heating cycles to 625 K, 78 wt % is preserved in the Cd-substituted sample. Thus, Cd-substitution provides a significant improvement of the thermal stability of Zn4Sb3. Multitemperature synchrotron powder diffraction data measured between 90 and 300 K reveal that Cd substitution has a suppressing effect on the α′−α−β phase transitions. With increasing substitution, there is also a significant change in the individual Zn site occupancies. Differential scanning calorimetry shows an apparent correlation between Cd content and phase transition temperature. Thermoelectric properties have been measured from 2 to 400 K for all samples, and although some physical properties are significantly affected by doping, no immediate improvement of ZT was achieved.

The effects of nano-sized TiO2 and ZnO ceramic inclusions on the high temperature stability of Zn4Sb3 have been studied using multi-temperature synchrotron powder X-ray diffraction. Samples with 9 nm TiO2 nanoinclusions exhibit remarkable stability after three heating cycles to 625 K.

Zn4Sb3 is among the cheapest high performance thermoelectric materials, and it is made of relatively nontoxic elements. Strong activities are aimed at developing commercial power generation modules based on Zn4Sb3 making it vital to develop fast reliable synthesis processes for high-quality material. Here direct synthesis and compaction of homogeneous phase-pure thermoelectric Zn4Sb3 by spark plasma sintering (SPS) has been developed. Compared with the traditional quench and press method, the complexity and process time of the new method is very significantly reduced (order of magnitude), making large-scale production feasible. A composition gradient is observed in the pellet along the axis of applied pressure and current. The homogeneity of the pressed pellets is studied as a function of the SPS parameters: sintering time, applied current, sintering temperature and applied pressure, and the mechanism behind the formation of the gradient is discussed. The key finding is that pure and homogeneous Zn4Sb3 pellets can be produced by adding an extra layer of elemental Zn foil to compensate the Zn migration.