利用鎂熱還原法合成矽奈米材料於能源之研究

Abstract

Silicon is a widely used material for a variety of applications. Despite its popularity, there is still a need to look for a cheaper, cleaner and easier to way to obtain silicon in its various forms. In this study, a metallothermic reduction process was used as a means of converting silicon from a low-cost and abundant source, silicon dioxide. The reaction pathway was optimized by monitoring the process in-situ and observing the various effects of the variable parameters (e.g. temperature, amount of reactants,reaction environment and time) ex-situ. Using various analytical tools, a better understanding of the reaction process is highlighted. The optimized parameters of the magnesiothermic reduction allowed us to synthesize silicon nanoparticles (Si NPs) and silicon thin films for energy applications. We found that there is a transition temperature in which above that temperature some of the produced silicon will consequently react with magnesium to form magnesium silicide. Therefore, adjustment should be made on the reaction process accordingly. Since most of the low-cost processes known today require the integration of Si NPs in a solution-based system, a systematic approach was undertaken to comprehend its interaction towards various common solvents. Consequently, a solvent exchange process was proposed to improve the interaction of the silicon nanoparticles in poor solvent environment. Using N-methylpyrrolidone to disperse the silicon nanoparticles and by solvent exchange process method, we were able to improve the dispersion and stability of Si NPs in H2O. This enabled us to improve performances in lithium battery systems that utilize a nanocomposite material based on silicon nanoparticles. We have obtained specific capacity of 1264 mAh/g with retention of about 70% after 30 cycles. Finally, the optimized magnesiothermic reduction reaction was applied to produce silicon from rice husk, a waste by-product in rice milling. Rice husk was first converted to silicon dioxide, which was in turn converted to silicon and subsequently tested for its potential application in lithium ion battery. We have achieved a specific capacity of 1000 mAh/g at applied current density of 1000 mA/g. The silicon nanoparticles and silicon thin films from the magnesiothermic reduction reaction were also tried in various energy-related applications such as solar cells and thermoelectric systems. Initial thermoelectric data have shown that our thin films can achieve Seebeck coefficient of 700 μV/K at 500C.