Synthesis of Mg2Si and its use for Silane production
Abstract
The increase in the use of renewable energy sources is crucial to address global warming and its impact on our planet. Solar energy is predicted to experience the most rapid growth of the different sources of renewable energy, including biomass, wind, hydroelectric, and geothermal energy. Si is a crucial material in the production of solar cells, with a requirement of purity of at least 7N, known as Solar Grade Si (SoG-Si). However, an even higher level of purity is required for making monocrystalline Si, which provides solar cells with higher efficiency. Currently, high-purity Si is produced mainly through the “chemical route” which refers to two methods. The first method is the so-called Siemens process based on chlorination of metallurgical grade silicon (MG-Si) to generate gaseous trichlorosilane which is further distilled and decomposed/reduced to Si. The second process converts trichlorosilane to monosilane before decomposition/reduction in a fluidized bed reactor (FBR). The FBR process is predicted to obtain a larger share of the market in the future as it is more energy efficient and has higher productivity than the Siemens process.
This Ph.D. research proposes an alternative method for producing silane gas through magnesiothermic reduction of natural high-purity quartz followed by hydrolysis of Mg2Si to produce silane gas. This alternative method has the potential of high-purity silicon production with a more environmentally friendly process and lower CO2 emissions.
The first part of this Ph.D. research focused on investigating the metallothermic reduction of natural quartz using Mg to produce Mg2Si under various reaction conditions, including quartz types, quartz particle size, Mg/SiO2 mole ratio, temperature, and time. The findings revealed that temperature had a strong influence on the reaction kinetics, while the effect of quartz type and particle size was not significant for the reaction kinetics at higher temperatures. As a diffusion-controlled reaction, the rate of reaction decreased with time, where longer reaction times did not lead to a notable change in product formation. The study also proposed two reaction mechanisms based on the Mg/SiO2 mole ratio and the state of the formed metal phase. To gain further insights into the reaction mechanism, the SiO2/Mg wetting system was investigated at a temperature range of 973 K to 1273 K. The results indicated a strong effect of temperature on wetting behavior, which is also supported by the reaction rate studies.
The second part of this Ph.D. research aimed at the production of silane gas through the hydrolysis of Mg2Si in HCl acid solution under various reaction conditions. The silane gas formation was evaluated using two methods: firstly; reacting silane gas with KOH solution and subsequently measuring the Si concentration of the KOH solution using ICP-MS and secondly; on-line gas analysis using GC-MS. The results indicated the formation of different silane types, including SiH4, Si2H6, Si3H8, Si4H10, and Si5H12, where SiH4 is the main component with a yield of 18% of total silanes measured, i.e SiH4, Si2H6, and Si3H8. The total silane yield measured/calculated was approximately 29 and 32% respectively for the two methods at a temperature of 40 °C and acid concentration of 12%.