Enhanced Ca3Co4O9 thermoelectric transport properties through Gd doping based on spin entropy and size effect

Optimising thermoelectric transport properties by controlling carrier concentration and grain size through doping ions is essential for the design of thermoelectric materials. The extra spin entropy can be increased by modulating the Co4+/Co3+ ratio of the sample by doping ions at the Co site to increase the Seebeck coefficient and minimise the competition between the Seebeck coefficient and the electrical conductivity. We have systematically investigated the thermoelectric properties of Gd-doped Ca3Co4−xGdxO9 ceramic materials. The XRD and SEM exhibit that the sample is a single phase; gradually, the grain size became smaller as the amount of Gd doping increased. The XPS and Raman tests determined that Gd doping occurs at the Co site in the [CoO2] layer. Gd doping drives up the sample carrier concentration and increases the conductivity. Concurrently, the spin entropy modulation by Gd doping increased the Seebeck coefficient. The Gd doping introduces point defects while allowing a reduction in grain size, reducing the thermal conductivity by 14%. The ZT of Ca3Co3.94Gd0.06O9 was the highest with the interaction of electrical and thermal properties, reaching 0.13 at 773 K, an improvement of approximately 44%.