An Oxide Single Crystal with High Thermoelectric Performance in Air

An oxide single-crystalline whisker with high thermoelectric properties at temperatures (T) higher than 600 K in air has been discovered. This whisker is assigned to Ca₂Co₂O₅ phase (abbreviated to Co-225 whiskers) and has a layered structure in which Co–O layers of two different kinds alternate in the direction of the c-axis. Seebeck coefficient of the whiskers is higher than 100 µV·K^-1 at 100 K and increases with temperature up to 210 µV·K^-1. Temperature dependence of electric resistivity shows a semiconducting-like behavior. These results indicate that the electric carriers are transported via hopping conduction. Using thermal conductivity of a Co-225 polycrystalline sample, figure of merit (ZT) of the Co-225 whiskers is estimated 1.2–2.7 at T≥873 K. This compound is characterized with regard to low mobility and high density of carriers, which contradicts the conventional materials with high thermoelectric properties.