Effect of grain boundary & interface modification on thermoelectric properties at high temperature

Thermoelectric (TE) power generation is considered to be one most promising emerging clean energy technologies for harvesting electricity from heat without producing any direct emission of greenhouse gases. The conversion efficiency of a TE material is quantified by a dimensionless quantity called figure of merit, ZT = α2Tσ/κ. A large ZT value corresponds to a TE material with high conversion efficiency, meaning that the TE material is characterized by high Seebeck coefficient (α), high electrical conductivity (σ) and low thermal conductivity (κ). Owing to the tight interrelation between these three physical properties only very few materials currently satisfy these conditions. New approaches suggested for further understanding the role of surfaces and interfaces on transport properties open new ways to design materials with enhanced properties and new functionalities. Additionally, the need for fundamental understanding of material properties is as equally important as the ability to develop scalable and inexpensive manufacturing process. It is known that the electrical transport and thermoelectric properties along grain boundaries and interfaces can be improved or depressed, even by several orders of magnitudes. Therefore, microstructural design particularly of interfaces can be crucial for improving the thermoelectric functionality. From this point of view, in this thesis electric transport properties, Seebeck coefficient and thermal conductivity of oxynitride (SiAlON), carbide (SiC) and oxide (SrTiO3, La2CuO4, LaNiO3) based materials have been investigated by modifying interfaces at the micro, nano and atomic scale. It is found that segregated network approach can improve ZT unusually, La decoration tailored the electrical conductivity whilst multilayer approach is useful for tuning transport properties.