Effect of semiconductor to metal transition on thermoelectric performance in oxide nanocomposites of SrTi_0.85Nb_0.15O₃ with graphene oxide

Donor doped SrTiO₃ based perovskites are considered as very promising candidates for high temperature thermoelectric applications, but the figure of merit (ZT) of these n-type oxide thermoelectrics need to be enhanced for making it commercially viable. However, it is challenging to improve the transport properties of these oxides in order to achieve high ZT. Here we put forward a strategy of boosting the electron transport by inducing semiconductor to metal transition in Nb doped SrTiO₃ (STN) by formation of nanocomposites with graphene oxide (GO). We could enhance electrical conductivity remarkably by gradual incorporation of GO in the matrix of STN, which changed its semiconductor (dσ/dT>0) behavior into metallic (dσ/dT<0) for more than 1 wt% GO content. GO sheets embedded in the grain boundaries of polycrystalline STN facilitated electron transport without increasing much it's thermal conductivity, resulting in the order of magnitude increase in the ratio of electrical conductivity and thermal conductivity in the temperature range from 300 K to 1200 K. Hence, ZT of STN ceramics was raised by 50 times due to GO incorporation. Maximum ZT of 0.5 was attained at 1200K in n-type bulk oxide nanocomposite made of environmentally benign, earth-abundant elements.