Thermoelectric power generation is considered as one of the most impactful non-conventional energy generation method. The major reason behind this is mainly no carbon footprints and an amazing effort to convert the low grade heat energy into useful electrical energy. Mg2Si based materials are proven materials for n-leg of TE power generation modules due to their high ZT coupled with low cost. n-type Mg2Si based thermoelectric materials are reported to show a figure of merit as high as 1.5 but their p-type counterparts are really inferior on basis of power generation as even the theoretical predictions show the maximum figure of merit for these thermoelectric materials can reach only up to 0.8. Now a days n-type materials are combined with Manganese Silicide to produce thermoelectric generators but often failed to meet the expectation because of highly mismatching thermal expansion co-efficient. So an efficient p- type Mg2Si based materials are highly sought as they are cost effective also.
In our experiment attempt was made to increase the ZT of p-type Mg2Si using an alloying approach. Elemental Pb was chosen as a substituent for Si site in order to enhance the phonon scattering due to mass disorder. This is expected to lower the thermal conductivity and thereby enhance ZT of the material. Compositions of Mg2.1(Si1-x-ySnxPby) were melt synthesized and various room temperature and high temperature thermoelectric properties were studied. Mg alongside with Si, Sn granules, and Li2CO3 were taken inside the crucible for melting. After melting the resultant material was crushed followed by sintering to obtain dense compacts. The optimization process involved several attempts with different relative ratios of Mg: Li. The optimized base composition was found to be Mg2.1Li0.03Si0.4Sn0.6. It was also observed that the melting temperature influenced the Li loss and the optimum temperature was found to be 9000C.
After optimization of the process parameters, the final compositions with Pb substitution were prepared (compositions given in the Figure caption). The temperature trend of electrical conductivity (s) measurements indicate typical behavior of heavy doped semiconductors. It is also observed that for 5% Pb substitution, the absolute values of s are much different from the other samples which is unusual (replacement of Si by Pb is an isovalent substitution). The lattice thermal conductivity (kL) data shows a drastic reduction in the absolute values with 5% Pb followed by an increase in 10% Pb sample. The overall effect in an enhancement in ZT at high temperatures for 5% Pb sample (0.37 at 700 K compared to 0.25 for the Pb free composition).
The results highlight the beneficial effect of Pb substitution in p-type Mg2.1Si0.4Sn0.6 when added in small amounts. Further investigation is required to understand the cause which might lie in the microstructural changes induced by Pb substitution.