Effect of nanostructure on thermoelectric properties of La_(0.7)Sr_(0.3)MnO₃ in 300-600 K range

In oxide materials, nanostructuring effect has been found very promising approach for the enhancement of \textit{figure-of-merit}, \textit{ZT}. In the present work, we have synthesized La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) compound using sol-gel method and samples of crystallite size of 34, 41, and 49 nm were obtained by giving different heat treatment. Seebeck coefficient ($\alpha$), electrical resistivity ($\rho$), and thermal conductivity ($\kappa$) measurements were carried out in 300-600 K temperature range. The systematic change in the values of $\alpha$ from $\sim$ -19 $\mu$V/K to $\sim$ -24 $\mu$V/K and drastic reduction in the values of $\kappa$ from $\sim$0.88 W/mK to $\sim$0.23 W/mK are observed as crystallite size is reduced from 49 nm to 34 nm at $\sim$600 K. Also, fall in the values of $\rho$ in the paramagnetic (PM) insulator phase (400-600 K) are effectively responsible for the increasing trend in the values of \textit{ZT} at high temperature. For the crystallite size of 41 nm, value of \textit{ZT} at 600 K was found to be $\sim$0.017, which can be further increased up to $\sim$0.045 around 650 K temperature. The predicted value of \textit{ZT} suggests that LSMO can be suitable oxide material for thermoelectric applications at high temperature.