Silicon dioxide
The chemical compound silicon dioxide, also known as silica or silox (from the Latin "silex"), is an oxide of silicon, chemical formula , and has been known for its hardness since the 9th century[Lynn Townsend White, Jr. (Spring, 1961). "Eilmer of Malmesbury, an Eleventh Century Aviator: A Case Study of Technological Innovation, Its Context and Tradition", Technology and Culture 2 (2), pp. 97-111 ] .
. Silica is most commonly found in nature as sand or quartz, as well as in the cell walls of diatoms. It is a principal component of most types of glass and substances such as concrete.
Silica is the most abundant mineral in the earth's crust.
Manufactured forms
Silica is manufactured in several forms including:
 glass (a colorless, high-purity form is called fused silica)
synthetic amorphous silica, silica gel
pyrogenic silica (sometimes called fumed silica or silica fume)
It is used in the production of various products.
Inexpensive soda-lime glass is the most common and typically found in drinking glasses, bottles, and windows.
A raw material for many whiteware ceramics such as earthenware, stoneware and porcelain.
A raw material for the production of Portland cement.
A food additive, primarily as a flow agent in powdered foods, or to absorb water (see the ingredients list for).
The natural ("native") oxide coating that grows on silicon is hugely beneficial in microelectronics. It is a superior electric insulator, with high chemical stability. In electrical applications, it can protect the silicon, store charge, block current, and even act as a controlled pathway to allow small currents to flow through a device. At room temperature, however, it grows extremely slowly, and so to manufacture such oxide layers, the traditional method has been heating of silicon in high-temperature furnaces within an oxygen ambient (thermal oxidation).
Raw material for aerogel in the Stardust spacecraft
Used in the extraction of DNA and RNA due to its ability to bind to the nucleic acids under the presence of chaotropes.
Added to medicinal anti-foaming agent, like Simethicone, in a small proportion to enhance defoaming activity.
As hydrated silica in toothpaste (abrasive to fight away plaque.)
As a pesticide as Diatomaceous Earth.
As a high-temperature thermal protection fabric (http://www.firesleeveandtape.com/silica-high-temperature-fabric.html)
Inhalation health effects
Inhaling powder of any kind (other than prescribed medicines) is not recommended. When considering potential inhalation effects of silica, it is important to understand whether the silica is crystalline (such as quartz) or amorphous (non-crystalline). See the structure and properties section below for more information on which forms are crystalline and non-crystalline.
Inhaling finely divided crystalline silica dust in very small quantities (OSHA allows 0.1mg/m3) over time can lead to silicosis, bronchitis or (much more rarely) cancer, as the dust becomes lodged in the lungs and continuously irritates them, reducing lung capacities (silica does not dissolve over time). This effect can be an occupational hazard for people working with sandblasting equipment, products that contain powdered crystalline silica, and so on. Children, asthmatics of any age, allergy sufferers and the elderly (all of whom have reduced lung capacity) can be affected in much shorter periods of time.
In studies across Europe (Britain/France) amorphous silica has been found to be biologically inert when ingested and inhaled. Warheit ET Al found during that their bio assays that any minimal damage was reversible. In the USA the Food and Drug Administration has found it to be exempt from normal process and it is allowed in food and medicine.
Other health effects
In respects other than inhalation, pure silicon dioxide is inert and harmless.
Pure silicon dioxide produces no fumes and is insoluble in vivo. It is indigestible, with zero nutritional value and zero toxicity. When silica is ingested orally, it passes unchanged through the gastrointestinal tract, exiting in the feces, leaving no trace behind. Small pieces of silicon dioxide are equally harmless, as long as they are not large enough to mechanically obstruct the GI tract, or jagged enough to lacerate its lining. (Eating glass and other unpure silica is not recommended, however.)
Because some silicas take on water, extended exposure may cause local drying of the skin or other tissue.
Chemistry
Silicon dioxide is formed when silicon is exposed to oxygen (or air). A very thin layer (approximately 1 nm or 10 Å) of so-called 'native oxide' is formed on the surface when silicon is exposed to air under ambient conditions.
Higher temperatures and alternate environments are used to grow well-controlled layers of silicon dioxide on silicon, for example at temperatures of 600 -1200 °C so-called "dry" or "wet" oxidation using O2 or H2O respectively.[Sunggyu Lee (2006) Encyclopedia of chemical processing , CRC Press, ISBN 0824755634] The thickness of the layer of silicon replaced by the dioxide is 44% of the thickness of the silicon dioxide layer produced.
Alternative methods used to deposit a layer of SiO2 include:[Robert Doering, Yoshio Nishi (2007), Handbook of Semiconductor Manufacturing Technology, CRC Press, ISBN 1574446754]
Low temperature oxidation (LTO) of silane
SiH4 + 2O2? SiO2 + 2H2O (at 400-450 °C)
Decomposition of tetraethyl orthosilicate (TEOS) at 680 – 730°C
Si(OC2H5)4? SiO2 + H2O + 2C2H4
Plasma enhanced chemical vapour deposition using TEOS at approximately 400°C
Si(OC2H5)4 + 12O2? SiO2 + 10H2O + 8CO2
Pyrogenic silica (sometimes called fumed silica or silica fume), which is a very fine particulate form of silicon dioxide, is prepared by burning SiCl4 in an oxygen rich hydrocarbon flame to produce a "smoke" of SiO2:
SiCl4 + 2H2 + O2? SiO2 + 4HCl
Amorphous silica, silica gel, is produced by the acidification of solutions of sodium silicate to produce a gelatinous precipitate that is then washed and then dehydrated to produce colorless microporous silica.
Quartz exhibits a maximum solubility in water at around 340 °C [The solubility of amorphous silica in water at high temperatures and high pressures Fournier R.O , Rowe J.J., American Mineralogist (1977), 62, 1052-1056]. This property is used to grow single crystals of quartz in a hydrothermal process where natural quartz is dissolved in superheated water in a pressure vessel which is cooler at the top. Crystals of 0.5 -1 kg can be grown over a period of 1-2 months.[Egon Wiberg, Arnold Frederick Holleman (2001) Inorganic Chemistry, Elsevier ISBN 0123526515] these crystals are a sourcer of very pure quartz for use in electronic applications.
Fluorine reacts with silicon dioxide to form SiF4 and O2 whereas the other halogen gases (Cl2, Br2, I2) react much less readily.
Silicon dioxide is attacked by hydrofluoric acid (HF) to produce "hexafluorosilicic acid":
SiO2 + 6HF? H2SiF6 + 2H2O
HF is used to remove or pattern silicon dioxide in the semiconductor industry.
Silicon dioxide dissolves in hot concentrated alkali or fused hydroxide (e.g):
SiO2 + NaOH? Na2SiO3 + H2O
Silicon dioxide reacts with basic metal oxides (e.g. sodium oxide, potassium oxide, lead(II) oxide, zinc oxide
or mixtures of oxides forming silicates and glasses as the Si-O-Si bonds in silica are broken successively.
As an example the reaction of sodium oxide and SiO2 can produce sodium orthosilicate, sodium silicate and glasses, depending on the proportions of reactants:
2Na2O + SiO2? Na4SiO4
Na2O + SiO2? Na2SiO3
(0.25 - 0.8)Na2O + SiO2? glasses
Examples of such glasses have commercial significance e.g. soda lime glass,borosilicate glass, lead glass. In these glasses silica is termed the network former or lattice former.
With silicon at high temperatures gaseous SiO is produced:
SiO2 + Si? 2SiO (gas)
Structure and properties
SiO2 has a number of distinct crystalline forms in addition to amorphous forms. With the exception of stishovite and fibrous silica, all of the crystalline forms involve tetrahedral SiO4 units linked together by shared vertices in different arrangements. Silicon-oxygen bond lengths vary between the different crystal forms, for example in a-quartz the bond length is 161 pm, whereas in a-tridymite it is in the range 154-171 pm.[. The Si-O-Si angle also varies between a low value of 140° in a-tridymite, up to 180° in ß-tridymite. In a-quartz the Si-O-Si angle is 144°.]
Fibrous sulfur has a structure similar to that of SiS2 with chains of edge-sharing SiO4 tetrahedra.
Stishovite, the highest pressure form, in contrast has a rutile like structure where silicon is 6 coordinate. The density of stishovite is 4.287 g/cm3, which compares to a-quartz, the densest of the low pressure forms, which has a density of 2.648 g/cm3.[ The difference in density can be ascribed to the increase in coordination as the six shortest Si-O bond lengths in stishovite (four Si-O bond lengths of 176 pm and two others of 181 pm) are greater than the Si-O bond length (161 pm) in a-quartz.][ Wells A.F. (1984) Structural Inorganic Chemistry 5th edition Oxford Science Publications ISBN 0-19-855370-6 ] The change in the coordination increases the ionicity of the Si-O bond.[Electron-density distribution in stishovite, SiO2: a new high-energy synchrotron-radiation study Kirfel A., Krane H.-G., Blaha P., Schwarz K., Lippmann T., Acta Cryst. (2001). A57, 663-677 ]
Note that the only stable form under normal conditions is a-quartz and this is the form in which crystalline silicon dioxide is usually encountered.[ In nature impurities in crystalline a-quartz can give rise to colours][ (see quartz for a list).]
Molecular forms of silicon dioxide
When molecular silicon monoxide, SiO, is condensed in an argon matrix cooled with helium along with oxygen atoms generated by microwave discharge, molecular SiO2 is produced which has a linear structure.[ Peter Jutzi, Ulrich Schubert, (2003), Silicon chemistry: from the atom to extended systems, Wiley-VCH ISBN 3527306471] The Si-O bond length is 148.3 pm which compares with the length of 161 pm in a-quartz. The bond energy is estimated at 621.7 kJ/mol.
Dimeric silicon dioxide, (SiO2)2 has been prepared by reacting O2 with matrix isolated dimeric silicon monoxide, (Si2O2).[ In dimeric silicon dioxide there are two oxygen atoms bridging between the silicon atoms with an Si-O-Si angle of 94° and bond length of 164.6 pm and the terminal Si-O bond length is 148.2pm.]
References
R. K. Iler, The Chemistry of Silica (ISBN 0-471-02404-X)
See also
Amorphous carbonia
Fused silica
Silicon carbide
Mesoporous silica
External links
(Tridymite)
(Quartz)
(Cristobalite)
NIOSH Pocket Guide to Chemical Hazards (amorphous)
NIOSH Pocket Guide to Chemical Hazards (crystalline, as respirable dust)
Quartz SiO2 piezolelctric properties
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