Suppression of Anisotropic Heat Conduction in Ion-Substituted Layered Cobalt Oxides

Abstract

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