Silicon tetrafluoride

Occurrence

Volcanic plumes contain significant amounts of silicon tetrafluoride. Production can reach several tonnes per day.{{cite journal | doi-access =free}} Some amounts are also emitted from spontaneous coal fires. The silicon tetrafluoride is partly hydrolysed and forms hexafluorosilicic acid.

Preparation

is a by-product of the production of phosphate fertilizers wet process production, resulting from the attack of HF (derived from fluorapatite protonolysis) on silicates, which are present as impurities in the phosphate rocks. The hydrofluoric acid and silicon dioxide (SiO2) react to produce hexafluorosilicic acid: : 6 HF + SiO2 → H2SiF6 + 2 H2O

In the laboratory, the compound is prepared by heating barium hexafluorosilicate (Ba[SiF6]) above 300 C whereupon the solid releases volatile , leaving a residue of .

:

Alternatively, sodium hexafluorosilicate () may also be thermally decomposed at 400 C—600 C (optionally in inert nitrogen gas atmosphere)

:

Uses

This volatile compound finds limited use in microelectronics and organic synthesis.

It is also used in production of fluorosilicic acid (see above).

Staying in the 1980s, as part of the Low-Cost Solar Array Project by Jet Propulsion Laboratory, it was investigated as a potentially cheap feedstock for polycrystalline silicon production in fluidized bed reactors. Few methods using it for the said production process were patented.

The Ethyl Corporation process

In 80s the Ethyl Corporation came up with a process that uses hexafluorosilicic acid and sodium aluminium hydride (NaAlH4) (or other alkali metal hydride) to produce silane (SiH4).

Safety

In 2001 it was listed by New Jersey authorities as a hazardous substance that is corrosive and may severely irritate or even burn skin and eyes. It is fatal if inhaled.

References

References

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2. (1994). "CRC Handbook of Thermophysical and Themochemical Data". CRC Press.
3. {{IDLH. fluoride. Fluorides (as F)
4. (November 2001). "Hazardous Substance Fact Sheet". New Jersey Department of Health and Senior services.
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6. (1953). "Inorganic Syntheses".
7. "Patent Silicon tetrafluoride generation".
8. Shimizu, M. "Silicon(IV) Fluoride" Encyclopedia of Reagents for Organic Synthesis, 2001 John Wiley & Sons. {{doi. 10.1002/047084289X.rs011
9. Callaghan, William T.. (1981). "Photovoltaic Solar Energy Conference". Springer Netherlands.
10. (1982-09-01). "Low-temperature preparation of polycrystalline silicon from silicon tetrachloride". Materials Letters.
11. "Method for the production of polycrystalline silicon".
12. (August 11, 2015). "The Ethyl Corporation Process: Silane and Fluidised Bed Reactor". Everything about solar energy | Solar energy, helio systems, solar panels, collectors, equipment for water heating and space heating using solar energy and everything about solar energy..
13. (April 9, 2018). "SAFETY DATA SHEET: Silicon Tetrafluoride". [[Airgas]].