Sintering process and nonstoichiometry of NaCo2O4 layered thermoelectric oxide

Sintering process of layered oxide NaCo₂O₄, which has been the best p-type thermoelectric oxide candidate to date since its debut in the world of thermoelectrics in 1997, was investigated in terms of sodium nonstoichiometry occurring during solid state reaction between Na₂CO₃ and Co₃O₄ for the synthesis of the oxide. Thermal gravimetry (TG) and differential thermal analysis (DTA) measurements revealed that solid-gas equilibrium plays an important role on phase formation process and on controlling chemical composition of the oxide, Partial pressure of oxygen and carbon dioxide in the reaction atmosphere was revealed to be crucial for controlling vaporization of Na, as well as for promotion of phase formation reaction of NaCo₂O₄, In particular, whereas the CO₂ atmosphere effectively suppressed abrupt dissociation reaction of Na₂CO₃, low oxygen partial pressure accelerated vaporization of Na at lower temperature. Interestingly, high partial pressure Of CO₂ appeared to hinder phase formation reaction of NaCo₂O₄ even at the same oxygen partial pressure as that under the aerobic conditions.