Thermophysical behavior and the effect of the crystal structure on the thermoelectric properties of NaxCoO2

Four different structure types depending on composition and reaction temperature are known of the highly anisotropic non-stoichiometric oxide Na_xCoO₂. Only γ-Na_xCoO₂ (0.55≤x≤0.74) was reported to possess large thermopower and studied in detail. Despite the corresponding stoichiometry of γ-, α´-Na_xCoO₂ and γ-, β-Na_xCoO₂, respectively, no phase transition has been reported for γ-Na_xCoO₂ while investigating its thermoelectric properties. This aspect motivated our study of the thermophysical behavior of γ-Na_xCoO₂ by means of TG/DTA, DSC, XRD and SEM/EDX. No phase transition is detected in air up to 1100°C. In contrast, the decomposition of γ-Na_xCoO₂ under Ar is accompanied by the formation of α-and α´-Na_xCoO₂. Samples with various contents of α-, α´- and γ-Na_xCoO₂ have been prepared by heat treatment of γ-Na_xCoO₂ employing different temperatures and sintering atmospheres, namely Ar, air and O₂. The thermoelectric properties of α- and α´-Na_xCoO₂ were investigated to study the effect of the crystal structure on the thermoelectric properties of Na_xCoO₂.