A Review of Synthesis Techniques for Thermoelectric Materials: From Bulk Processing and Nanostructuring to Thin-Film...

Thermoelectric (TE) materials offer a direct and sustainable means of converting heat into electricity, enabling applications ranging from industrial waste-heat recovery to solid-state cooling and self-powered microdevices. The performance of TE systems is critically influenced by the synthesis techniques employed, which determine phase purity, compositional uniformity, microstructural features, and transport behavior. This review comprehensively analyses synthesis approaches across multiple material scales, bulk, nanostructured, and thin-film, highlighting how processing-structure-property correlations govern TE efficiency. Bulk synthesis routes such as arc melting, levitation melting, melt spinning, zone melting and self-propagating high-temperature synthesis (SHS) are discussed with emphasis on their control of grain growth, defect formation, and compositional homogeneity. Nanostructure-oriented methods, including high-energy ball milling, hydrothermal/solvothermal synthesis, coprecipitation, sol-gel processing, spark plasma sintering (SPS), and hot extrusion (HE), are evaluated for their ability to enhance phonon scattering and tailor carrier concentration through controlled grain refinement and defect engineering. Thin-film deposition techniques such as physical vapor deposition (PVD), chemical vapor deposition (CVD), and wet-chemical methods are further reviewed for their precision in thickness control, crystallographic orientation, and interface stability, which are crucial for device-level integration. By linking processing strategies to TE performance, this review highlights hybrid synthesis, interface engineering, and eco-friendly materials as critical avenues for developing efficient and scalable TE technologies.