Thermophysical behavior and the effect of the crystal structure on the thermoelectric properties of Na/sub x/CoO/sub 2/

Four different structure types depending on composition and reaction temperature are known of the highly anisotropic non-stoichiometric oxide Na/sub x/CoO/sub 2/. Only /spl gamma/-Na/sub x/CoO/sub 2/ (0.55/spl les/x/spl les/0.74) was reported to possess large thermopower and studied in detail. Despite the corresponding stoichiometry of /spl gamma/-, /spl alpha/'-Na/sub x/CoO/sub 2/ and /spl gamma/-, /spl beta/-Na/sub x/CoO/sub 2/, respectively, no phase transition has been reported for /spl gamma/-Na/sub x/CoO/sub 2/ while investigating its thermoelectric properties. This aspect motivated our study of the thermophysical behavior of /spl gamma/-Na/sub x/CoO/sub 2/ by means of TG/DTA, DSC, XRD and SEM/EDX. No phase transition is detected in air up to 1100/spl deg/C. In contrast, the decomposition of /spl gamma/-Na/sub x/CoO/sub 2/ under Ar is accompanied by the formation of /spl alpha/-and /spl alpha/'-Na/sub x/CoO/sub 2/. Samples with various contents of /spl alpha/-, /spl alpha/'- and /spl gamma/-Na/sub x/CoO/sub 2/ have been prepared by heat treatment of /spl gamma/-Na/sub x/CoO/sub 2/ employing different temperatures and sintering atmospheres, namely Ar, air and O/sub 2/. The thermoelectric properties of /spl alpha/- and /spl alpha/'-Na/sub x/CoO/sub 2/ were investigated to study the effect of the crystal structure on the thermoelectric properties of Na/sub x/CoO/sub 2/.