Achieving high figure-of-merit in Nb-doped Na$_{0.74}$CoO$_{2}$ compound at high temperature region
We report the thermoelectric (TE) properties of Na$_{0.74}$Co$_{0.95}$Nb$_{0.05}$O$_{2}$ in the temperature range $300-1200$ K, as a potential candidate for p-type thermoelectric material. The experimental values of Seebeck coefficient (S) are $ \sim $ $82-121$ $ \mu $V/K measured in the temperature range $300-620$ K. The positive values of S in the entire temperature range indicates p-type behaviour of the compound. At 300 K the experimental value of thermal conductivity ($ \kappa $) is $ \sim $ 1.88 W/m-K that increases up to $ \sim $ 420 K, then decreases till 620 K with corresponding value $ \sim $ 1.86 W/m-K. To understand the experimentally observed transport properties, we have calculated S and $\rho$ of this compound. Then, based on theoretical understanding, we have estimated \textit{figure-of-merit} (ZT) up to 1200 K by using calculated S and $\rho$ values with extrapolated experimental $\kappa$. The value of ZT is found to be $\sim$ 0.03 at 300 K, whereas, the highest value is observed as $\sim$ 1.7 at 1200 K. Finally, we have calculated the efficiency ($\eta$) by keeping the cold end temperature (T$_{c}$) fixed at 500 K and varying hot end temperature (T$_{h}$) from 500 to 1200 K, respectively. The maximum value of $\eta$ is found to be $\sim$ 8 %, when T$_{c}$ and T$_{h}$ are fixed at 500 and 1200 K, respectively. This result suggests that Na$_{0.74}$Co$_{0.95}$Nb$_{0.05}$O$_{2}$ compound can be used as a p-leg for making high temperature TE generator (TEG).