Synthesis and Characterization of New Inorganic Thermoelectric Materials

Doctoral thesis, 2012

With thermoelectric materials it is possible to convert a difference in temperature into electrical energy. This can be utilized for a more efficient use of available energy, eg. by converting waste heat from a combustion process. Based on recent theoretical predictions, there is a renewed interest to find and develop new and more efficient thermoelectric materials, improving this potential even further. To do this the thermoelectric properties, thermopower, electrical and thermal conductivity, must be optimized. There are a number of possible ways to achieve this, among them nanostructuring to reduce thermal conductivity, doping to improve electrical properties and crystal structure optimization to potentially improve all properties. For this, it is essential to have the appropriate tools for accurate characterization of these material properties. Thermoelectric properties may be determined using different analytic techniques and it is therefore important that these are compared to assess their level of agreement. Synthesis and characterization of new thermoelectric materials have been the aim of this thesis. Work to improve the thermoelectric properties of Ba8Ga16Ge30 and Mg2Si through substitution and nanoinclusions have been performed as well as studies of the thermal stability of p-type Ba8Ga16Ge30. It is shown that p-type Ba8Ga16Ge30 is not stable above 400°C, in fact it is possible to change the doping from p- to n-type through thermal treatment of the material. Substituting Ge with Zn in n-type Ba8Ga16Ge30 results in slight improvement of ZT in the low to intermediate temperature range, which likely could be further improved with optimization of the system. With TiO2 nanoparticles added to Ba8Ga16Ge30 some improvements of ZT were recorded and a complex effect on the charge carrier concentration in the material was observed. The thermoelectric properties of Mg2Si were improved after addition of TiO2 nanoparticles, even though TiO2 was completely reduced or reacted to metallic Ti and TiSi2. The charge carrier concentration of the titanium-containing Mg2Si material was improved, likely through doping of the material. The Laser Flash (LFA) and Transient Plane Source (TPS) techniques for thermal conductivity measurements have been evaluated and compared. The methods were concluded to be comparable, with the TPS technique requiring larger sample specimens for materials with high thermal conductivity and the LFA method sometimes suffers from with compromised surface coating and only indirectly measuring thermal conductivity.