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Occurrence, Preparation and Properties of Silicon

Silicon is the most abundant element in the earth's crust after oxygen (Si=27.2%, O=45.5%). It does not occur freely in nature. Due to its great affinity for oxygen, it occurs either as oxide (silica, SiO2) or as silicates like feldspar, kaolinite, mica, etc., in rocks and clays.

Silicon (96-99%) is produced by the reduction of sand with coke in an electric arc furnace.
SiO2 + 2C Si + 2CO

Sand is kept in excess to prevent the accumulation of silicon carbide (SiC).
2SiC + SiO2 3 Si + 2CO

Silicon, thus obtained, is used in the manufacture of its alloys and silicon polymers. These polymers containing a Si—O—Si framework are water-repellent and heat-resistant and find many applications.

Very pure silicon for semiconductor applications is obtained by reduction of pure SiCl4 or SiHCl3 with pure Mg or Zn. Thermal decomposition of SiI4 with dihydrogen is also done. Recently, high-purity silicon for solar cells has been produced by the reduction of Na2SiF6 with metallic sodium.

Silicon is a hard solid with a shining metallic luster. It melts at 1793 K and boils at about 3550 K. It reacts with fluorine at room temperature to form silicon tetrafluoride, SiF4. With other elements, it reacts at elevated temperatures.

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