Microstructure investigations of Yb- and Bi-doped Mg2Si prepared from metal hydrides for thermoelectric applications

Within the field of thermoelectric materials for energy conversion magnesium silicide, Mg2Si, is an outstanding candidate due to its low density, abundant constituents and low toxicity. However electronic and thermal tuning of the material is a required necessity to improve its Figure of Merit, zT. Doping of Yb via reactive YbH2 into the structure is performed with the goal of reducing the thermal conductivity. Hydrogen is released as a by-product at high temperatures allowing for facile incorporation of Yb into the structure. We report on the properties of Yb- and Bi-doped Mg2Si prepared with MgH2 and YbH2 with the focus on the synthetic conditions, and samples’ microstructure, investigated by various electron microscopy techniques. Yb is found in the form of both Yb3Si5 inclusions and Yb dopant segregated at the grain boundary substituting for Mg. The addition of 1at% Yb concentration reduced the thermal conductivity, providing a value of 30mW/cmK at 800K. In order to adjust carrier concentration, the sample is additionally doped with Bi. The impact of the microstructure on the transport properties of the obtained material is studied. Idealy, the reduction of the thermal conductivity is achieved by doping with Yb and the electronic transport is adjusted by doping with Bi. Large grain microstructure facilitates the electronic transport. However, the synthetic conditions that provide the optimized microstructure for electrical transport do not facilitate the additional Yb dopant incorporation. Therefore, the Yb and Bi containing sample with the optimized microstructure provides a zT=0.46 at 800K. 1% or less addition of YbH2 to Mg2Si significantly reduces the thermal conductivity of the material. Yb replaces some Mg in Mg2Si and the remainder is distributed as Yb3Sb5 in the Yb-doped Mg2Si matrix. Correlation between the observed grain size and transport show the difficulty in simultaneously reducing the thermal conductivity with optimal electronic properties. [Display omitted]