Anisotropic Heat Conduction in Ion‐Substituted Layered Cobalt Oxides

Layered cobalt oxides, A x CoO 2 (A = Li, Na, Ca, and Sr), are attracting attention as thermoelectric materials showing large thermoelectric figure of merit ZT = S 2 σTκ −1 ( S : thermopower, σ: electrical conductivity, T : absolute temperature, κ: thermal conductivity) at higher temperatures. Due to the layered structure, A x CoO 2 shows strong anisotropy in the thermoelectric properties; both S and σ are large along the layer though systematic study in κ is not reported thus far. Here, the anisotropy in κ of A x CoO 2 is shown systematically with the effect of A x on κ. A x CoO 2 epitaxial films are fabricated with two different crystallographic orientations with respect to the surface normal to the substrates and the κ is measured in normal to the substrate. The κ parallel to the layers ( κ || ) is 3–6 times larger than that perpendicular to the layers (κ ⊥ ). The κ || decreases with increasing the atomic mass of A x though κ ⊥ is insensitive to it; anisotropy of κ is controlled by substituting heavier A x ions. The present result provides new material design concept for good thermoelectric materials.