October 1, 2022


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Five questions and answers about silicon

Five questions and answers about silicon

Silicon is used, among other things, in solar cells, cell phones, and computers. It is one of the important semiconductors of great importance for electronic production. Chip shortages – that is, a shortage of semiconductors – in the past year have to do with a shortage of silicon microchips.

But how is silicone made? We ask Associate Professor Jafar Safarian in the Department of Materials Technology at NTNU.

1. What is silicone?

– Silicon (Si) is a chemical element in Group 14 in the periodic table. It is the second most abundant element in the Earth’s crust after oxygen – about 28 percent by mass – and is found in nature mostly in the form of oxide (quartz) and silicate. More than 90 percent of the Earth’s crust is made up of silicate minerals. Silicon behaves like a metal when melted, but it has no conductivity in solid form and is classified as a metal. A brittle hard crystalline material with a blue-gray metallic luster. The melting and boiling points of silicon are 1414 °C and 3265 °C, respectively.

2. What is silicone used for?

The main challenge in silicon production is environmental and emissions issues, says Associate Professor Jafar Safarian in NTNU’s Department of Materials Technology. Photo: private

Silicon is in many products such as semiconductors, the alloying element in metal products and silicon compounds, says Savarian.

– 15 percent of world production of metallic silicon, with a purity of 96-99 percent of silicon, is refined to semiconductor purity; Usually 99.9999999%. High purity is very important for silicon solar cells. The main use of mass is as an alloying element in metal products. Ferrosilicon – iron / silicon alloy – used for steel and cast iron products. Metallic silicon is also used in alloys, such as commercial aluminum alloys. Compounds such as silicones, carbides and oxides have different applications. Silicon carbide (SiC) as ceramic or cutting material and molybdenum sinter (MoSi .)2) as a nutrient in ovens and in microelectronics as a contact material, he explains.

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3. How is silicone produced?

Silicon is extracted from quartz (SiO .).2). The current technology is a high-temperature carbon heat reduction process in which quartz particles are heated, melted, and reduced by solid carbon – coal, metallurgical coke, wood – in submerged electric arc furnace (SAF). Process Chemical Reaction: SiO2 + 2 C → Si + 2 CO silicon carbide (SiC) is formed in the upper parts of the furnace, and some silicon oxide reduction can be done by the following reaction: 2 SiC + SiO2 → 3 Si + 2 CO The produced silicon is melted and drained from the SAF, while the CO and SiO gases leave the furnace. SAF technology is used with quartz and iron ore as raw materials for fused ferrosilicon alloy with 80 percent Si and 20 percent Fe. SisAl – patented by NTNU – is a CO2– Free process in beta domain testing. In this technology, aluminum from secondary sources reduces silicon oxide instead of carbon: 3 SiO2 + 4 Al → 3 Si + 2 Al2a3

4. What are the biggest challenges in silicon production?

The main challenge is environmental issues and emissions. Reducing carbohydrates in SAF gives approx. 7.5 kg of carbon dioxide2 per kilo of silicone product. The combustion of furnace exhaust gas using air produces nitrogen oxides. 10-13 grams of nitrogen oxides are produced per kilo of magnesium silicide (MG-Si), Saffarian says.

Reducing NOx emissions presents technical and financial challenges. The MG-Si in SAF gives off large amounts of SiO in the furnace exhaust gas and undesirable silicon loss despite valuable microsilica production. The yield of silicon in the furnace is practically 80-90 percent. Since silicon metal is more valuable than microsilicon and the dust in the exhaust gas faces special challenges, increasing silicon production in the furnace is interesting. The production of MG-Si in carbon reduction has a high energy consumption of 11-13 kW/kg Si.

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5. What should future research focus on?

– Reducing carbon dioxide2– NOx emissions are a top priority. Decarbonization of the metals industry to reach the targets set in international agreements for 2050. Existing technologies must be improved in order to significantly reduce or eliminate CO22– emissions. The electrical energy used in this process can produce indirect carbon dioxide2– emissions, so lower energy consumption for the process is important, he points out:

– To develop new alternative sustainable processes, of which SisAl is a good example with significant CO2 reduction2– Emissions, no NOx emissions, low energy consumption and high yield of silicon. Recycling of end-of-life (EOL) products that use photovoltaic panels. Research into product component separation, high purity solar silicon recycling and technology development.

The article was first published in TU Magazine No. 6/2022