Synthesis of silicon nanoparticles via hot-wall reactor for high temperature thermoelectric applications

Abstract

Thermoelectric materials allow direct solid-state energy conversion between thermal and electrical energy. This enables applications of these materials in power generation, waste heat recovery and environmentally-friendly refrigeration, and may contribute to a solution to the present day energy crisis and environmental issues. The efficiency of these materials is defined by the dimensionless figure of merit ( ZT ) which is directly proportional to the square of the Seebeck coefficient ( S ), the electrical conductivity (σ), and the absolute temperature ( T ) and inversely proportional to the thermal conductivity (κ) of the material. Present day bulk thermoelectric materials used for commercial applications have relatively low efficiency ( ZT ∼1), limiting the applications of these materials to a very small segment of market where simplicity and reliability is more important than cost and efficiency. However, recent studies on thermoelectric materials have shown that efficiency of these materials can be significantly enhanced by reducing the size of these materials to the nanometer length scale. This increase in the efficiency is observed mainly because of reduced thermal conductivity of the nanostructured materials. In this thesis, thermoelectric materials are reviewed with the ultimate goal of producing bulk SiGe nanocomposites for high temperature thermoelectric applications. To produce large quantities of nanoparticles, an aerosol-based hot-wall reactor was designed and developed in the laboratory. For initial proof-of-concept, gram-scale production of silicon nanoparticles with high crystallinity and narrow size distribution was demonstrated. The reactor produces Si nanoparticles with an average size of 13 nm and with an average production rate of 1000 mg/hr. By design the reactor is made with the flexibility to produce SiGe nanocomposites in a single step or in two steps (by producing Si and Ge nanoparticles separately and then mixing them). An additional line for dopant can also be added with slight modification, so as to produce doped SiGe nanocomposites in one step. The material properties were characterized at different temperatures and pressure, and the optimized process conditions were explored experimentally. In order to understand complete process of device fabrication from the nanopowders, solid dense samples were prepared by spark plasma sintering. The governing operating parameters of spark plasma sintering process were optimized in order to achieve ∼100 % theoretical density with minimum grain growth. The dense solid samples were also further processed by polishing, grinding and cutting to the sizes defined by thermoelectric property measurement equipment. In addition, the photoluminescence of silicon quantum dots produced by HF/HNO 3 etching of the silicon nanoparticles generated in the hot-wall reactor was studied to attain a better understanding of the other possible applications of these bulk nanoparticles. After a few experiments, nearly the full spectrum of visible photoluminescence was obtained from the particles etched with different etching time and acid mixture composition. In parallel to the synthesizing Si nanoparticles by hot-wall reactor, particles were also produced using laser pyrolysis (developed by our lab previously) and a comparative study between the two is presented.