Towards magnesium-silicide-based thermoelectric generators: material optimization, contact development and prototypes
Magnesium silicide based solid solutions Mg2X (X = Si, Ge, Sn) are among the most promising thermoelectric materials for the temperature range of 500 K to 800 K, where a large fraction of the reusable heat is available. Very good thermoelectric properties have been demonstrated repeatedly, especially for the n-type material. This, combined with a high material availability, low cost of raw materials and environmental compatibility, makes these materials suitable for large scale applications.
For p-type solid solutions of Mg2Si and Mg2Sn, we show that the thermoelectric properties can be well understood for the full solid solution range employing a relatively simple single parabolic band model. This allows for a straightforward optimization of the material with respect to carrier concentration and composition. On the other hand, we find clear indications that defects in the material system influence, in particular, the carrier concentration (and hence thermoelectric performance) and that their impact depends on the Si:Sn ratio. In contrast to the Mg2(Si,Sn) system, for p-type Mg2Ge we find thermoelectric transport properties that are atypical for a usual degenerate semiconductor. We show that the observed behavior can be understood in the framework of a multi-valence band model, with the bands having very dissimilar curvatures and shifting against each other with temperature. This leads to a significantly enhanced power factor compared to Mg2Si and Mg2Sn and a thermoelectric figure of merit comparable to the best solid solutions of Mg2Si and Mg2Sn. Alloying of Mg2Ge with Mg2Si and Mg2Sn helps to improve the thermoelectric properties further.
We synthesize functionalized legs of p- and n-type Mg2X using combined sintering of the thermoelectric material and the electrode. The influence of process parameters and material properties (CTE) on the resulting microstructure and the resulting electrical contact resistance are examined. For some of the investigated electrodes (Cu, Ag), we observe a deterioration of the material properties close to the interface for n-type but not for p-type material. We show that this can be understood from the formation energies of intrinsic and extrinsic defects in the material and their dependence on the dominant carrier type. Finally, first results for Mg2X -based thermoelectric generator modules will be presented.