Development of new-type cobalt oxide thermoelectric materials

Single phase cobalt oxide thermoelectric materials, A_xCoO₂ with A=Na,Sr, Ca and Ba, [A´₂CoO₃]_z[CoO₂] with A´=Ca, Sr(349-phase), z=0.62 and [Ca₂(Co_1-yCu_y)₂O₄]_z[CoO₂] with y=0.33 are synthesized successfully by the ion-exchange method or solid state reaction method. Seebeck coefficient, electric resistivity and magnetization of these oxides are measured in the temperature range from 4 K to 300K. These cobalt oxides show p-type conductivity and relatively high positive Seebeck coefficient, i.e., about 120 μV/K or above at 300 K. The electric resistivity of sintered samples has been measured at the level of 10 mΩcm at 300 K. New nomenclature, the naming system, of modulated layered cobalt oxide thermoelectric materials, e.g., the latter two ones, is proposed according to the numbers of stacking cation layers, namely Co-121 for [Ca₂CoO₃]_0.62[CoO₂], Co(Cu)-221 for [Ca₂(Co_1-yCu_y)₂O₄]_z [CoO₂] and Bi-221 for [Bi_0.87SrO₂]₂[CoO₂]_1.82, respectively. The top letter is the cation(s) sitting in the rocksalt layer(s). The 1^st, 2^nd and 3^rd digits indicate the numbers of the rocksalt -type metal - oxygen layer(s), alkali earth metal - oxygen layer(s) and conducting triangle cobalt oxygen layer(s) in a unit cell, respectively.