Basic study on the behavior of impurities in silicon produced with carbothermic reduction of silica

• Project/Area Number: 11450281
• Research Category: Grant-in-Aid for Scientific Research (B)
• Allocation Type: Single-year Grants
• Section: 一般
• Research Field: Metal making engineering
• Research Institution: Tohoku University
• Principal Investigator: HIRASAWA Masahiro Tohoku University, Institute of Multidisciprinary Research for Advanced Materials, Professor, 多元物質科学研究所, 教授 (90126897)
• Project Period (FY): 1999 – 2001
• Project Status: Completed (Fiscal Year 2001)
• Budget Amount: ¥14,600,000 (Direct Cost: ¥14,600,000); Fiscal Year 2001: ¥1,100,000 (Direct Cost: ¥1,100,000); Fiscal Year 2000: ¥1,800,000 (Direct Cost: ¥1,800,000); Fiscal Year 1999: ¥11,700,000 (Direct Cost: ¥11,700,000)
• Keywords: suboxide / SiO / silicon / nano particle / nano fiber / coal ash / volume reducing / materials recycling / 凝縮 / 高温反応 / 蒸発 / 速度論 / 一酸化ケイ素 / ホウ素

Research Abstract

In the present report, the author investigates the behavior of impurity elements in the carbotrhermic reduction of silica and other oxide containing silica like coal ash. The present work is aimed at the basic study for the mass production of silicon and the materials recycling in coal ashes. The followings are the results of the present study In the case of carbothermic reduction of molten B_2O_3-SiO_2, B evaporates from the melt simultaneously with SiO, while, in the case of the reduction of molten K_2O-SiO_2, K evaporation is more favorably than SiO.
Carbothermic reduction of model coal ash is also investigated. Microscopic observation of the condensate of SiO shows that the condensate consists basically of particulate and fibrous materials, which indicates that the condensation of SiO takes place in the gas phase to form particles and, simultaneously, gaseous SiO also deposits on the particulate SiO attached to the crucible wall to enhance the epitaxial growth. In the carbothermic r eduction of model coal ash containing P as an impurity at 1800 °C, SiO recovered at low temperature zone is free from the contamination of P because of complete evaporation of P from the melt and SiO. On the other hand, in the case of B in the model coal ash, the content of B in SiO is about 50% lower than that of the initial model ash.
More close observation of the morphology of SiO condensed from gas phase reveals that there are three forms in the condensed SiO, i.e. fiber of nano-meter size, nano-particle, and, chain-like agglegate of nano-particles. The nano-particles have an average composition of SiO_x with x = 1.2 to 1.5. The nano-fibers and nano-chains have average composition of SiO_x with x = 1.7 to 1.9. The nano-particulate SiO is collected and treated with HF solution to dissolve SiO_2 in the material. The residue after the treatment is investigated with XRD. The results of XRD shows that the residue consists of metal Si with small amount of SiO. This result indicates that the SiO obtained in the present study separates into SiO_2 and Si according to disproportionation reaction, and thus we can obtain metal Si simply by the treatment of SiO with HF solution.