SiGe and Mg₂Si Nanostructured Fibers as Thermoelectric Materials for Energy Harvesting

The enormous increase in energy demand for expanding the internet of things (IoT) to create more sustainable cities and communities has led to a much-needed boost in energy sources for low-power microelectronics. Thermoelectric (TE) materials present a versatile and environmentally beneficial option to harvest energy from ambient waste heat to power this transition. Moreover, thanks to nanostructurization, commonly poor TE materials (i.e., silicon) have remarkably enhanced their TE properties; this is the case for nanowires and Si-based fabrics made of nanotubes. Nonetheless, the scalability of Si-based nanostructured materials remains a challenge.<br/>The present work studied Silicon-alloys (SiGe and silicides) nanotube-based fibers due to their integrability in the electronics industry and the abundance and low extraction environmental impact of the herein-used materials. Most importantly, the fabrication of TE nanotubes was up-scaled to large areas. The nanotube-based fabric was fabricated using an electrospun fiber template and chemical vapor deposition (CVD) in a bottom-up approach, allowing the tailoring of the orientation and thickness of nanotubes and the final fabric.<br/>Different strategies to improve Silicon-alloys performance as<br/>TE materials were explored in order to optimize their <i>Figure of Merit, zT = S²σ/κ</i>. The thermal conductivity, κ, was tuned by rapid thermal treatments (RTP) techniques and studied employing Laser Flash Measurements; the Seebeck coefficient, S, was modified by varying the ratio between silicon and the alloying elements and characterized using Seebeck-coefficient equipment. We have also studied the change of the electrical conductivity, σ, by controlling the material’s doping and the thickness of the nanostructures. Likewise, a complete morphological and compositional characterization was performed using electron microscopy techniques, EDX, and XRD.<br/>Thus, the suitability of electrospinning and CVD to fabricate large-area TE materials was successfully accessed. Furthermore, the TE properties were investigated, and steps to optimal morphology and compositional conditions of the Si-alloy materials were found.