Anisotropy of Microstructure and Its Influence on Thermoelectricity: The Case of Cu₂Te-Sb₂Te₃ Eutectic

Abstract

Using a set of controlled in situ grown lamellar composites of (Cu2Te)62.02–(Sb2Te3)37.98, we report a remarkable variation of transport properties of thermoelectricity not only as a function of microstructural length scale but also as a function of direction-dependent arrangement of the phases and hence their interfaces. A quantitative evaluation of the microstructure along the transverse and the longitudinal directions of growth, imposed by the temperature gradient and growth rate in a unidirectional solidification setup, has been carried out. The microstructure is quantified through image analysis using fast Fourier transforms as well as a cluster base connectivity model and is further correlated with the thermoelectric transport properties. A marked anisotropy of properties as a function of measurement direction in the microstructural landscape could be observed. A maximum power factor of ∼1.4 mW m⁻¹ K⁻² and a figure of merit of 0.29 could be obtained at 580 K along the transverse direction for the sample with the characteristic microstructural length scale of 2.41 μm. This has an implication in engineering a thermoelectric device in terms of engineering power factor and output power density. For a ΔT of 250 K, we report a difference of 0.4 W cm⁻² in output power density between the transverse and the longitudinal directions that have an identical microstructural length scale of 2.41 μm.