Development of Thermoelectric Materials Based on NaTaO3 - Composite Ceramics

Abstract

This chapter describes the development of novel thermoelectric materials for hightemperature applications like gas burners, combustion engines, nuclear fuel, or furnaces. The goal of this development is to recycle waste heat for energy harvesting in order to contribute in saving the environment. The research results are described in the following sub-chapters in four different sections. After a general review about perovskites and NaTaO3 in section 2, ab-initio-simulations of the Seebeck coefficient are described in section 3. The Seebeck coefficient strongly depends on the effective mass and carrier concentration. The electronic band-structure calculations showed a large electron effective mass for NaTaO3. Heavily doping changes NaTaO3’s bandstructure in a similar way as the well-known thermoelectric material Nb-doped SrTiO3. Hence, NaTaO3, which is stable up 2083 K and which is known as a material with excellent photo-catalytic properties, was chosen as a candidate for thermoelectric materials. Section 4 describes the finding of suitable doping elements by sintering NaTaO3 with different raw materials. While both, pure NaTaO3 and NaTaO3 sintered with Fe2O3, are almost insulators, it was discovered that sintering with metallic iron increases both, electric conductivity and Seebeck coefficient. Microstructural characterization by SEM and XRD measurements showed that a NaTaO3-Fe2O3 composite material is formed. The amount of Fe solved in the NaTaO3 lattice is much higher when the starting materials consist of Fe instead of Fe2O3. Addition of several metals like Mn, Cr, Ti, Ni, Cu, Mo, W, Fe, and Ag were tested, but only the later two elements lead to remarkable electric conductivity observed above 773 K. Section 5 describes the measurement of thermoelectric properties such as Seebeck-voltage at a large temperature gradient, a method which is close to applications, but not yet commonly used, because the thermoelectric theory is based on small temperature gradients. Thermal conductivity is not measured, but only estimated. The doping is achieved by sintering metallic iron or silver together with NaTaO3. The results are high negative Seebeck voltages up to -320 mV at a temperature difference of 700 K, as well as high closed-circuit currents up to -250 A for Fe-doping and positive values for Ag-doping. Besides reporting previous results, several new findings are described here for the first time. Composites with Cu yield 1