Tetrafluorosilane

Structural formula
General
Surname	Tetrafluorosilane
other names	Silicon (IV) fluoride Silicon tetrafluoride
Molecular formula	SiF 4
Brief description	colorless gas with a pungent odor
External identifiers / databases
CAS number 7783-61-1 EC number 232-015-5 ECHA InfoCard 100.029.104 PubChem 24556 Wikidata Q413140
properties
Molar mass	104.1 g mol −1
Physical state	gaseous
density	4.372 kg m −3 (15 ° C)
Sublimation point	-95.2 ° C
solubility	hydrolyzed in water
safety instructions
GHS labeling of hazardous substances danger H and P phrases H: 330-314-280 EUH: 071 P: 260-280-304 + 340-303 + 361 + 353-305 + 351 + 338-315-405-403
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Tetrafluorosilane (SiF ₄ ), a silicon compound from the group of silicon tetrahalides . It is a colorless, toxic, non-flammable gas. In the semiconductor industry , silicon tetrafluoride is formed when silicon dioxide layers are removed from wafers using hydrofluoric acid .

Manufacturing

Procedure 1

It is made from calcium fluoride and silicon dioxide (sand) with concentrated sulfuric acid . The following reactions take place: Conversion of fluorite to hydrogen fluoride

${\displaystyle \mathrm {H_{2}SO_{4}+CaF_{2}\rightarrow 2\;HF+CaSO_{4}} }$

The hydrogen fluoride then reacts with the SiO ₂

${\displaystyle \mathrm {4\;HF+SiO_{2}\rightarrow SiF_{4}+2\;H_{2}O} }$

If there is an excess of acid, the hydrolysis of SiF _{4 is} prevented.

The SiF ₄ obtained in this way , however, is only 80 to 90% pure. It contains oxygen-containing products such as F ₃ Si – O – SiF ₃ or F ₃ Si – O – SiF ₂ –O – SiF ₃ . When SiF _{4 is transported} in pipelines, these molecules break down into SiF ₄ and SiO ₂ . The SiO ₂ clogs the pipes. The chemical industry had developed the method of transporting SiF ₄ in pipelines instead of the more dangerous hydrogen fluoride in order to supply more distant production sites with fluorides by pipeline . However, because of the problems described, this method was not widely used.

Procedure 2

When heating Bariumfluorosilikat generated SiF ₄ , it can also ammonium hexafluorosilicate or hexafluorosilicic even be used:

${\displaystyle \mathrm {BaSiF_{6}\rightarrow BaF_{2}+SiF_{4}\ } }$

${\displaystyle \mathrm {H_{2}SiF_{6}\rightarrow 2\ HF+SiF_{4}} }$

Procedure 3

On a laboratory scale, tetrafluorosilane can be produced by reacting tetrachlorosilane with calcium fluoride at 450 to 500 ° C:

${\displaystyle \mathrm {SiCl_{4}+2CaF_{2}\rightarrow SiF_{4}+\ 2\ CaCl_{2}} }$

properties

Silicon tetrafluoride has a pungent odor and is heavily fuming when wet, the smoke consisting of fluosilicic acid and silicon dioxide. When it cools down, it changes from the gaseous phase directly to the solid phase through resublimation . The gas can be liquefied from a pressure of 50 atm. Its critical temperature is −14.15 ° C, its critical pressure is 37.2 bar, its triple point temperature is −86.8 ° C and its triple point pressure is 2.240 bar.

Individual evidence

1. ↑ ^{a b c d e f g h} Entry on silicon tetrafluoride in the GESTIS material database of the IFA , accessed on February 1, 2016(JavaScript required) .
2. ^ W. Legat, dissertation 1978, Giessen.
3. ↑ Entry on ammonium hexafluorosilicate in the GESTIS substance database of the IFA , accessed on January 8, 2018(JavaScript required) .
4. ↑ G. Brauer (Ed.): Handbook of Preparative Inorganic Chemistry 2nd ed., Vol. 1, Academic Press 1963, pp. 212-213.
5. ↑ HP Boehm: A simple method for the preparation of silicon tetrafluoride . In: Journal of Inorganic and General Chemistry . tape 365 , no. 3-4 , April 1969, pp. 176-179 , doi : 10.1002 / zaac.19693650311 .

Web links

Commons : Silicon Tetrafluoride  - Collection of Images, Videos and Audio Files

• International Chemical Safety Card (ICSC) for tetrafluorosilanes at the National Institute for Occupational Safety and Health (NIOSH).