Controlling independently the electric and thermal properties by shrinking the particle size down to nanosize in quasi-one-dimensional Ca3Co2O6 and Sr6Co5O15

We report on the preparation of conventional and nanosized crystals of the quasi-one-dimensional compounds, Ca3Co2O6 and Sr6Co5O15, which are potential candidates for thermoelectric materials at higher temperatures; phonon scattering in nanosized crystals reduces the thermal conductivity. Simultaneous modification of the particle size and the oxygen content by milling processes is found to be critical not only for reducing the thermal conductivity, but also for improving an electrical conductivity, especially in the vicinity of compositions fulfilling the condition of a band insulator where one can control the sign of the thermoelectric power.