Phase Transformation for the Large-Scale Synthesis and Assembly via Welding of Metal Silicide Nanowires for Thermoelectric Applications
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Solid-state thermoelectrics that convert thermal energy into electricity have the potential to increase the efficiencies of existing process and systems (e.g., automobiles). The large-scale deployment of thermoelectrics for terrestrial use requires the following: a) enhancing their efficiencies beyond that are currently possible, and b) their fabrication from non-toxic, inexpensive, earth-abundant elements. Recent studies have determined that nanostructuring of earth-abundant materials, such as magnesium silicide (Mg2Si), is a possible pathway for accomplishing this task. Contextually, the overall aim of this dissertation is to engineer highly efficient Mg2Si nanowire-based thermoelectrics. This was achieved through the design of strategies for (1) the large-scale synthesis of a form of nanostructured Mg2Si, nanowires of Mg2Si, and (2) the interface-engineered assembly of the synthesized nanowires into welded nanowire networks that do not have any insulating MgO at the nanowire interfaces. Together, these strategies are intended to offer the ability to control thermal and electrical transport through Mg2Si. For the large-scale synthesis of Mg2Si nanowires, a phase transformation strategy that converts pre-synthesized silicon nanowires into Mg2Si nanowires was engineered. Experimentation performed indicated that 20 to 300 nm-thick, 5 to 20 ?m-long silicon nanowires obtained by electroless etching can be phase transformed into polycrystalline Mg2Si nanowires by reacting them with magnesium supplied via the vapor phase. To prevent the formation of multiple nuclei within each silicon nanowire during the phase transformation process and the formation of polycrystalline Mg2Si nanowires, a solid-state phase transformation process was engineered. Here, the solid-state reaction of sharp-tipped silicon nanowires with magnesium foils was employed for obtaining single-crystalline Mg2Si nanowires. To assemble the nanowires, the solid-state phase transformation strategy was extended and the phase transformation of silica nanoparticle coated silicon nanowires was employed. This procedure led to the formation of welded Mg2Si nanowire networks, where both the nanowires and the bridges connecting the nanowires were composed of Mg2Si. Thermoelectric performance evaluation of these networks and microcrystalline Mg2Si devices proved our hypothesis and indicated a 2-fold increase in the power factors. The high power factor of 0.972 x 10^-3 Wm^-1K^-2 achieved at 875 K is twice that reported in the literature for undoped Mg2Si.