Synthesis and high temperature thermoelectric properties of Ca3.0−x−yNdxNayCo4O9+δ

New polycrystalline Ca3.0−x−yNdxNayCo4O9+δ (x=0.0x=0.0, 0.1, 0.2, 0.3; y=0.0y=0.0, 0.1) samples were prepared using a sol–gel method followed by hot-pressing sintering technique. Powder X-ray diffraction analysis showed that all the samples were single phase. The high-temperature (350 to 1073 K) thermoelectric properties of the samples were investigated. The substitution of Nd3+ for Ca2.9Na0.1Co4O9+δ resulted in the decrease of the electrical conductivity, thermal conductivity and the increase of the Seebeck coefficient. The electrical conduction was dominated by the small polaron hopping conduction mechanism in the temperature region above 550 K. Compared with Nd or Na single substitution, Nd and Na double substitution can further reduce thermal conductivity of Ca3Co4O9+δ. The value of ZT reaches 0.29 for Ca2.6Nd0.3Na0.1Co4O9+δ at 1073 K.