Among many thermoelectric materials, oxide-based materials draw significant interest due to their environmental compatibility. In particular, layered cobaltite, Na0.75CoO2, shows a large thermoelectric power factor parallel to the layers. However,...

Layered cobalt oxides, AxCoO2 (A = Li, Na, Ca, and Sr), are attracting attention as thermoelectric materials showing large thermoelectric figure of merit ZT = S2σTκ−1 (S: thermopower, σ: electrical conductivity, T: absolute temperature, κ: thermal conductivity) at higher temperatures. Due to the layered structure, AxCoO2 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 AxCoO2 is shown systematically with the effect of Ax on κ. AxCoO2 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 Ax though κ⊥ is insensitive to it; anisotropy of κ is controlled by substituting heavier Ax ions. The present result provides new material design concept for good thermoelectric materials.

Layered compounds often show the anisotropic transport properties of both electron and phonon; the electrical/thermal conductivity along with the layer is high and the electrical/thermal conductivity perpendicular to the layer is low. For example, layered Na x CoO2, which is known as a promising candidate for oxide thermoelectric material [1], exhibits a rather large thermoelectric power factor along with the CoO2 layer, whereas it shows low electrical conductivity in perpendicular to the CoO2 layer. In addition, the thermal conductivity is large along with the CoO2 layer, whereas it is small in perpendicular to the CoO2 layer. Therefore, the anisotropic electron/phonon transport properties need to be suppressed to improve the thermoelectric figure of merit of Na x CoO2. Here we demonstrate that the anisotropy in thermal conductivity κ of Ax CoO2 (A = Li, Na, Ca, and Sr) can be suppressed by increasing the weight of Ax . We fabricated Ax CoO2 epitaxial films with two different crystallographic orientations with respect to the surface normal to the substrates by the Reactive Solid-Phase Epitaxy method [2,3]. We measured the electrical conductivity σ and thermopower S along with the substrate surface whereas we measured the κ in normal to the substrate surface. We extracted the κ parallel to the layers (κ p) from the comparison of two different crystallographic orientations [4]. The κ p was 3-6 times larger than that perpendicular to the layers (κ v). The κ p decreased with increasing the atomic mass of Ax though κ v was insensitive to it; anisotropy of κ was controlled by substituting heavier Ax -ions [5]. The results of this study will be of great value in understanding fundamental heat transport properties of layered structures as well as developing Ax CoO2 based thermoelectric materials. In addition, the analysis of the anisotropic thermal conductivity used in this study can be applied to other layered materials. [1] I. Terasaki et al., Phys. Rev. B 56, R12685(R) (1997). [2] H. Ohta et al., Cryst. Growth Des. 5, 25 (2005). [3] K. Sugiura, H. Ohta et al., Appl. Phys. Lett. 88, 082109 (2006). [4] K. Sugiura, H. Ohta et al., Appl. Phys. Lett. 94, 152105 (2009). [5] H.J. Cho, H. Ohta et al., Adv. Mater. Interfaces 7, 1901816 (2019). Figure 1

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