The Siemens process is usually used for the production of pure (or almost pure) silicon ready for use in the electronic field and especially in the production of photovoltaic cells. Silicon is the second most common component of the earth's crust; production follows the following work phases:
1) In the extraction mines, SiO2 (quartz) sand is precipitated;
2) By heating the quartz in high-temperature furnaces at very high temperatures and combining it with carbon, it is possible to obtain metallurgical silicon + carbon monoxide: SiO2 + 2C ---- & gt; M-Si + 2CO
This kind of silicon has a purity of about 98% and therefore requires a further processing phase that takes the name of Siemens process;
3) The Siemens process consists in reacting metallurgical silicon (M-Si) with a hydrochloric acid solution (3HCl): M-Si + 3HCl ---- & gt; SiHCl3 + H2
The new compound obtained SiHCl3 takes the name of silane trichloride which appears as a form of gaseous silicon; this is condensed and subsequently distilled thoroughly by going back to the previous chemical reaction: SiHCl3 + H2 ---- & gt; Si + 3HCl
The polycrystalline silicon now obtained will have a much higher purity than 99.9%.
More in detail:
The Siemens process is a process used for the purification of multicrystalline silicon, which allows to obtain electronic grade silicon. The technology is based on the chemical vapor deposition CVD technique and takes its name from the company that developed this method. In the Siemens reactors, where the purification process takes place, containment bells are placed on a platform. Within these bells are placed several silicon bars of high purity of a thickness of about 10 mm. These bars are connected two to two at the top end and are placed vertically within the bell. To start the process the bars are electrically heated between 1100 and 1200 ° C. Subsequently a mixture of purified silicone trichloride is transported via hydrogen in the reactor where it is decomposed when coming into contact with the heated silicon bars. As a result, polycrystalline silicon is deposited on the bars, increasing the thickness. When the bars are raised to the desired diameter the procedure is stopped, the bars are disconnected and the process is ready to start by introducing new silicon bars (cold) of about 1 cm thick.
At the end of the process, polycrystalline silicon bars of up to 20 cm in diameter are obtained, with a degree of impurity of about 0.2 parts per billion.
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