Achieving high figure-of-merit in Nb-doped Na${}_{0.74}$CoO${}_2$ compound at high temperature region

Abstract

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).