Engineering thermoelectric and mechanical properties by nanoporosity in calcium cobaltate films from reactions of Ca(OH)2/Co3O4 multilayers

Abstract

Controlling nanoporosity to favorably alter multiple properties in layered crystalline inorganic thin films is a challenge. Here, we demonstrate that the thermoelectric and mechanical properties of Ca₃Co₄O₉ films can be engineered through nanoporosity control by annealing multiple Ca(OH)₂/Co₃O₄ reactant bilayers with characteristic bilayer thicknesses (b_t). Our results show that doubling b_t, e.g., from 12 to 26 nm, more than triples the average pore size from ∼120 nm to ∼400 nm and increases the pore fraction from 3% to 17.1%. The higher porosity film exhibits not only a 50% higher electrical conductivity of σ ∼ 90 S cm⁻¹ and a high Seebeck coefficient of α ∼ 135 μV K⁻¹, but also a thermal conductivity as low as κ ∼ 0.87 W m⁻¹ K⁻¹. The nanoporous Ca₃Co₄O₉ films exhibit greater mechanical compliance and resilience to bending than the bulk. These results indicate that annealing reactant multilayers with controlled thicknesses is an attractive way to engineer nanoporosity and realize mechanically flexible oxide-based thermoelectric materials.