Tunable β-FeSi2 – Si1-yGey nanocomposites by a novel React/Transform Spark Plasma Sintering approach for thermoelectric applications

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We developed a novel React/Transform Spark Plasma Sintering approach for nanocomposite synthesis, providing accelerated phase transformation and selective alloying. We synthesized compositionally tunable SiGe alloy particles, embedded within a β-FeSi2 matrix, both with nanoscale grain size. This is achieved simply by adding a few atomic percent of Ge into pre-cast α-FeSi2+δ via light ball-milling, followed by two-step spark plasma sintering. This processing approach is much more efficient than conventional β-FeSi2 synthesis methods, due to significantly accelerated eutectoid decomposition, α-FeSi2+δ → β-FeSi2 + Si, followed by a facile Ge incorporation into eutectoid Si, without mechanical alloying and post-annealing. Increasing the Ge content of the nanocomposite drives significant reductions in the overall thermal conductivity. We analyzed the temperature dependence of thermal conductivity of the nanocomposite system by including phonon-phonon scattering, alloying scattering (in SiGe) and boundary scattering (size effect) in each phase, and then average over the two phases. The modeling indicates that the reduction of κL(T) results from both the nanostructuring and Ge alloying achieved by the processing route. Our approach offers a promising means for synthesizing self-assembled nanocomposite materials for thermoelectric applications.