Naturally Nanostructured Thermoelectric Oxides

Abstract

We have given an outline of our previous and on-going works related to naturally nanostructured thermoelectric oxides. One example is that buildup of different “nano-blocks” is effective to produce noble thermoelectric property as the intrinsic structure of thermoelectric materials. ${{\mathrm{Co}}_{3}}{{\mathrm{Co}}_{4}}{{\mathrm{O}}_{9}}$ and ${{\mathrm{Bi}}_{2}}{{\mathrm{Sr}}_{2}}{{\mathrm{Co}}_{2}}{{\mathrm{O}}_{9}}$ have intriguing crystal structure consisting of two blocks, such as ${\mathrm{CoO}}_{2}$ sub-lattice, and ${{\mathrm{Ca}}_{2}}{{\mathrm{CoO}}_{3}}$ and ${{\mathrm{Bi}}_{2}}{{\mathrm{Sr}}_{2}}{{\mathrm{O}}_{4}}$ sub-lattice, respectively. Once these two blocks are built up as “nano-blocks” in about 1.1–1.5 nm scale corresponding to the c-cell parameter, high dimensionless figure of merit ZT of ca. 1.1 at 973 K emerged in a single crystal of ${{\mathrm{Co}}_{3}}{{\mathrm{Co}}_{4}}{{\mathrm{O}}_{9}}$ and ${{\mathrm{Bi}}_{2}}{{\mathrm{Sr}}_{2}}{{\mathrm{Co}}_{2}}{{\mathrm{O}}_{9}}$ by harmony of the function of each part. However, power density of thermoelectric generation is 1–10 W/cm$^{3}$ against device volume, so that bulk devices are indispensable for thermoelectric power application to generate kW or higher class output. The next challenge is to produce naturally nanostructure-controlled bulk oxides. ${{\mathrm{ZnMnGaO}}_{4}}$ and ${{\mathrm{Co}}_{1.5}}{{\mathrm{Mn}}_{1.5}}{{\mathrm{O}}_{4}}$ were taken as a model case for constructing a self-assembled nanostructure. By utilization of spinodal decomposition, these oxides have naturally formed characteristics nanostructures, such as nanochckerboard and twin-related domains, leading to low thermal conductivity of these oxides. This result would open up a possibility for realization of bulk oxides with high thermoelectric properties by nanostructure production via self organization.