Synthesis and thermal transport of eco-friendly Fe-Si-Ge alloys with eutectic/eutectoid microstructure

The beta-FeSi2/Si semiconducting nanocomposite is a promising thermoelectric system with eco-friendly materials. We show. that small quantities of Ge can enhance the thermoelectric properties and widen the design space, if Ge content and spatial disposition can be controlled. We investigated the use of solidification combined with solid-state transformations to reduce the thermal conductivity via hierarchical modification of microstructure. Solidification of Fe28Si68Ge4 alloys leads to eutectic lamellar microstructure comprised of hyperstoichiometric (alpha-FeSi2+delta phase and diamond cubic The eutectic lengthscales can be varied over two orders of magnitude depending on solidification rate. Subsequent aging of the eutectic produces eutectoid decomposition, alpha-FeSi2 -> beta-FeSi2 + Si1-yGey, where the additional diamond cubic product is interleaved with the eutectic lamellae. By controlling both the frequency of beta-FeSi2/diamond cubic heterointerfaces, as well as the degree of Ge segregation into the eutectoid microconstituent, the thermal conductivity of the composite was varied from 22.8 W M-1 K-1 down to 8.3 W m(-1) K-1. We analyze the thermal conductivity in terms of a series thermal resistance model, including thermal boundary conductances at the heterointerfaces, and show that the thermal boundary conductance is reduced by at least an order of magnitude when the diamond cubic micro constituent is enriched from 0 to 30 at% Ge. Avenues for additional microstructural improvements towards thermoelectric applications are discussed. (C) 2017 Elsevier B.V. All rights reserved.