Thermoelectric performance of n-type Mg2Ge

Abstract

Magnesium-based thermoelectric materials (Mg2X, X = Si, Ge, Sn) have received considerable attention due to their availability, low toxicity, and reasonably good thermoelectric performance. The synthesis of these materials with high purity is challenging, however, due to the reactive nature and high vapour pressure of magnesium. In the current study, high purity single phase n-type Mg2Ge has been fabricated through a one-step reaction of MgH2 and elemental Ge, using spark plasma sintering (SPS) to reduce the formation of magnesium oxides due to the liberation of hydrogen. We have found that Bi has a very limited solubility in Mg2Ge and results in the precipitation of Mg2Bi3. Bismuth doping increases the electrical conductivity of Mg2Ge up to its solubility limit, beyond which the variation is minimal. The main improvement in the thermoelectric performance is originated from the significant phonon scattering achieved by the Mg2Bi3 precipitates located mainly at grain boundaries. This reduces the lattice thermal conductivity by ~50% and increases the maximum zT for n-type Mg2Ge to 0.32, compared to previously reported maximum value of 0.2 for Sb-doped Mg2Ge.

Effect of the Fabrication Technique on the Thermoelectric Performance of Mg-Based Compounds—A Case Study of n-Type Mg2Ge

“…We have also detected that these precipitates in the Bi-doped samples are synthesized by a one-step SPS process. 14 The Sb-doped sample is free of precipitates (Figure 2b).…”

“…% in our previous report, based on the one-step SPS synthesis of Mg 2 Ge. 14 The latter method comprises a single step at a lower temperature that relies on the rapid reaction of Ge and MgH 2 upon hydrogen liberation, with an extremely short time for homogenization. The reaction of Mg with Bi also appears to have been preferred in the one-step SPS synthesis, reducing the amount of Bi available for doping.…”

“…The bipolar effect is observed at temperatures above ∼650 K in the Seebeck coefficient plot (Figure 3a), similar to our previous study. 14 This effect is not visible on the thermal conductivity plot as a function of temperature (Figure 5a) because of the negligible bipolar component of the thermal conductivity 29 (κ b ) (Figure S1 of the Supporting Information).…”

“…The transport properties of these samples are compared with those of previous studies, 10,13 including our recent report on n-type Mg 2 Ge samples, fabricated via one-step SPS of elemental Ge and Bi/Sb with MgH 2 . 14 A maximum zT of ∼0.4 at 725 K is achieved for Mg 2 Ge 0.98 Bi 0.02 , an improvement over the zT of 0.2 for the Sb-doped Mg 2 Ge 13 because of a significant decrease in the thermal conductivity, while obtaining a similar power factor (PF). Our work underscores the impact that different synthesis techniques have on the properties of Mg-based thermoelectrics and the difficulty in comparing these materials when they are synthesized by different methods.…”

Recent progress in magnesium-based thermoelectric materials

“…Mechanical alloying reduces the Mg losses to practically zero, due to it being conducted at low temperature and in Ar atmosphere 53 . This process is often used as a mixing and homogenization step, before reaction through other methods 56,95,99 , boron nitride 32 , and graphite 60 . Magnesium-based thermoelectric materials are also reported to have been synthesized through less common techniques such as melt spinning 112 , where the molten materials are injected onto a .…”