Silicon dioxide

• Silicon oxide
• Silica
• SILICA (INCI)

Fixed

Depending on the modification between 2.19 to 2.66 g · cm -3

1713 ° C

> 2200 ° C.

• About 10 mg / l at 25 ° C in water ( quartz)
• 120 mg / l at 25 ° C in water ( amorphous silica)

1.458 ( λ = 589 nm in amorphous thin film )

Template: Infobox chemical / molecular formula search is not possible

Silicon dioxide (often: silicon dioxide) is a collective term for the modifications of the oxides of silicon with the chemical formula SiO2.

In German-speaking countries, is mainly in the rubber industry, erroneously used for the designation of silica or silica lately also derived from the Anglo-Saxon silica. Incorrectly is also the equation of silica sand. The bulk of the sand deposits is, however, of silica (quartz), because he is not only common, but particularly resistant to weathering due to its hardness and chemical resistiveness. Silica is the major constituent of all the (quartz - ) glass.

Mineralogy and occurrence

Non -crystalline (amorphous ) SiO2 is found in nature as an essential component in the following substances before that are very inhomogeneous and homogeneous in their composition:

• Biogen: skeletons of radiolarians, diatoms and sponges from Opal, diagenetic solidified into rock, for example, to chert
• Geyserit: amorphous sintered products of hot springs
• Tachylit: volcanic glass of basaltic composition, which contains more SiO2 contents of FeO, MgO, CaO and Al2O3
• Obsidian: volcanic glass of granitic composition
• Tektite: rock glasses are formed by melting of rocks as a result of meteorite impacts
• Lechatelierite: pure natural SiO2 glass as occurs for example in tektites or when lightning strikes in quartz sand is formed ( Fulgurit )
• Opal
• SiO2 melt: at temperatures above 1727 ° C ( at 1 bar )

In contrast to the amorphous SiO 2, the crystalline forms have a very low tolerance for impurities. They differ only in their structure.

• Moganite ( chalcedony ):
• α - quartz ( low quartz): formation conditions: temperature T < 573 ° C, pressure p <20 kbar
• β - quartz ( high quartz ): 573 ° C < T < 867 ° C, p < 30 kbar
• Tridymite: 867 ° C < T < 1470 ° C, P < 5 kbar
• Cristobalite: 1470 ° C < T < 1727 ° C
• Coesite: 20 kbar < P < 75 kbar
• Stishovite: 75 kbar

Silica forms part of silicates such as feldspar, clay minerals, or in free form as the main constituent quartz of the crust, and thus also the most common silicon compound.

Silicic

In nature scaffolds ( diatoms ) and radiolarians are from silica in plant and animal creatures before, such as the widespread marine diatoms ( Radiolaria ) and glass sponges ( Hexactinellida ) and the horsetail. The silica skeletons of dead diatoms and radiolarians sink to the ocean floor, accumulate there and form deposits of diatomite ( diatomaceous earth ) or Radiolarienschlamm. Deposits from the Miocene contain 70-90 % SiO2, 3-12 % water and traces of metal oxides.

Chemical Properties

The solubility of silica in water is strongly dependent on the degree of order, or the modification of the silica. In the crystalline highly ordered quartz solubility at 25 ° C at about 10 mg SiO2 per liter of water. It should be remembered, however, that the solution equilibrium may adjust only very slowly. The disordered amorphous silicas are much more soluble at the same temperature with about 120 mg / l of water. With increasing temperature, the solubility increases. For quartz and then at 100 ° C at about 60 mg / l of water. In amorphous silica with 330 ppm of silica are dissolved in water at 75 ° C. With increasing pH, the solubility also increases. Acids capable of SiO2 practically not dissolve except hydrofluoric acid (HF), is attacked by it to form gaseous silicon tetrafluoride ( SiF4 ). Alkaline melts, and - to a lesser extent - also aqueous alkali metal hydroxide solutions solve specific amorphous silica.

Some natural waters in addition silica, colloidal silica (SiO2), which does not hydrate at normal temperatures in the water to silica. This colloidal SiO2, this includes various siliceous compounds, reacts with ammonium heptamolybdate not to the yellow colored heteropolyacid.

Technical production

Synthetic SiO2, which is present mostly amorphous, is produced on a large scale in different processes in large quantities. As a collective term Neudeutsch is also used "Silica ".

The large scale production of synthetic SiO 2 is mainly based on water glass, which is obtainable by digestion of silica sand with sodium carbonate or potassium carbonate via precipitation processes. Thus generated SiO2 is called depending on the process conditions precipitated silicas, silica gels or colloidal silica. Another important production variant is the creation of so-called pyrogenic SiO 2 in an oxyhydrogen flame, starting from liquid chlorosilanes, such as silicon tetrachloride ( SiCl4 ). Major manufacturers of synthetic silicas are Evonik Industries (formerly Degussa), Wacker-Chemie, Rhodia, Grace and others.

Fumed silicas

Fumed silica, amorphous SiO 2 powder of 5-50 nm in diameter and having a specific surface area of ​​50-600 m2 / g The name refers to the commonly used manufacturing process by flame: the figure resulting from the combustion of hydrogen-oxygen water vapor decomposes silanes on SiO 2, another method using SiCl4 as Si source. In maintenance-free lead - acid batteries fumed silicas are used as starting material for the gel electrolyte, wherein the mass fraction of SiO2 but represents only a few percent.

Technical application

Synthetic SiO2 plays in everyday life usually unnoticed a major role. In paints and coatings, plastics and adhesives, it is just as important as modern manufacturing processes in the semiconductor technology or as a pigment in inkjet paper coatings. As a non-toxic substance, it is represented in pharmaceutical products as well as in cosmetics, in food processes (eg beer clarification ) and used as a cleaner in the toothpaste. Also found silica application in organic farming; where it is mixed in a fine powder form for the prevention of grain beetle infestation of grain. Quantitatively, the main applications include its use as a filler for plastics and sealants, especially in rubber articles. Modern car tires benefit from the gain by a special SiO2 system and save that which has traditionally only with carbon black -filled rubber compounds approximately 5% fuel while improving security.

The quantitatively most important of which silicon dioxide are in the form of glass. Usually it is mixed with materials such as alumina, boria, calcium and sodium, in order to lower the melting temperature to facilitate or to improve the properties of the final processing. Pure silica is difficultly fusible quartz glass, which is particularly temperature and resistant to temperature changes.

Quartz glass used in the optical system in the form of lenses, prisms, etc.. In the chemical laboratory of quartz glass is used as apparatus glass, when particularly high UV transmittance is required. Usually used in the laboratory, the temperature - change - More stable borosilicate glass.

Another application area of silica is the production of concrete. So this fabric is the main component of microsilica, an additive in the production of high performance and ultra high performance concrete (C100 ... ). The fumed silica reacts with the calcium hydroxide (Ca (OH) 2 ), which is released during the hydration, and thus forming said calcium silicate hydrate phases. Further, the lead particle in the range of 0.1 microns to an increase in the mechanical strength by the Kapillarporenanteil is reduced in the cement stone.

And SiO2 is used in the food industry as a food additive (E 551). So you can find it for example in spices and spice blends.

Another use for silica is found in the pyrotechnics. There is used, among other things, for the production of Brandgelen.

Silicon dioxide in the semiconductor industry

Silica is an important material in the semiconductor and microsystems technology. It is mainly used as insulation and passivation, such as a gate dielectric of the transistors used, or intermetal in the wiring layer of the integrated circuits. These layers are made, for example, by thermal oxidation of silicon or by chemical vapor deposition and are generally amorphous. Since the electrical properties of current microelectronic products are no longer sufficient silica is displaced since the mid- 2000s gradually from so-called low-k and high-k dielectrics. Another application of silica ( both quartz and special glasses ), the photo-lithography, where it is used as a substrate for masks.

Thin layers of silica can be prepared by various coating methods. The simplest way of producing silicon oxide layers on silicon kristalliniem is the oxidation of silicon by oxygen (see Thermal oxidation of silicon ). This process takes place in tube furnaces, in the industrial sector nowadays usually vertical furnaces instead.

The dry oxidation takes place at temperatures of 850-1200 ° C and is carried out relatively slowly, but with very good uniformity. In wet oxidation, the deposition of the oxide is greatly accelerated. The moisture is introduced, either directly in the form of steam or by an oxyhydrogen burner, ie hydrogen and oxygen are brought immediately before insertion into the furnace for the reaction, forming the desired water in high purity.

Silica is formed on a substrate other than silicon, the thermal oxidation is no longer usable and must be used in other methods. A method of chemical vapor deposition ( CVD) are mainly used here, in which both elements include by a reaction of gases containing silicon, such as silane or tetraethylorthosilicate (TEOS, tetraethoxysilane also ) is formed.

The Oxidabscheideverfahren that are based on the reaction of silane ( engl. low preassure cvd, LPCVD) usually take place at reduced pressure. There are several common methods. In the LTO process ( engl. low temperature oxide ), diluted at about 430 ° C silane reacted directly with oxygen:

At higher temperatures ( 900 ° C) can be SiO2 in the so-called HTO process (german high temperature oxide ), but also a combination of dichlorosilane and nitrous oxide form:

In semiconductor technology called TEOS process are still important, it is tetraethylorthosilicate ( TEOS) thermally decomposed:

The thus-prepared SiO2 layers generally have better properties and can be deposited at a higher layer of conformity, but the manufacturing process is a bit more expensive than, say, the HTO method