Enhancing the Thermoelectric Power Factor of Sr_0.9Nd_0.1Ti_O3 through Control of the Nanostructure and Microstructure

Donor-doped SrTiO₃ ceramics are very promising n-type oxide thermoelectrics. We show that significant improvements in the thermoelectric Power Factor can be achieved by control of the nanostructure and microstructure. Using additions of B₂O₃ and ZrO₂, high density, high quality Sr_0.9Nd_0.1TiO₃ ceramics were synthesised by the mixed oxide route; samples were heat treated in a single step under reducing atmospheres at 1673 K. Synchrotron and electron diffraction studies revealed an I4/mcm tetragonal symmetry for all specimens. Microstructure development depended on the ZrO₂ content; low level additions of ZrO₂ (up to 0.3 wt%) led to a uniform grain size with transformation-induced sub-grain boundaries. HRTEM studies showed high density of dislocations within the grains; the dislocations comprised (100) and (110) edge dislocations with Burger vector of d(₁₀₀) and d(₁₁₀) respectively. Zr doping promoted atomic level homogenization and a uniform distribution of Nd and Sr in the lattice, inducing greatly enhanced carrier mobility. Transport property measurements showed a significant increase in power factor, mainly resulting from the enhanced electrical conductivity while the Seebeck coefficients were unchanged. In optimised samples a power factor of 2.0×10^-3 W m^-1 K^-2 was obtained at 500 K. This is a ~ 30% improvement compared to the highest values reported for SrTiO₃–based ceramics. The highest ZT value for Sr_0.9Nd_0.1TiO₃ was 0.37 at 1015 K. This paper demonstrates the critical importance of controlling the structure at the atomic level and the effectiveness of minor dopants in enhancing the thermoelectric response.