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Autor     Wikipedia
Titel    Silicon dioxide 2011
Jahr    2011
URL    http://en.wikipedia.org/w/index.php?title=Silicon_dioxide&oldid=452689759

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The chemical compound silicon dioxide, also known as silica (from the Latin silex), is an oxide of silicon with the chemical formula SiO2. It has been known for its hardness since antiquity. Silica is most commonly found in nature as sand or quartz, as well as in the cell walls of diatoms [113, 117]. [...] Silica is manufactured in several forms including fused quartz, crystal, fumed silica (or pyrogenic silica, trademarked Aerosil or Cab-O-Sil), colloidal silica, silica gel, and aerogel.

[113] R.K. Iler The chemistry of silica. New York: Wiley, 1979.

[117] Lynn Townsend White, Jr. (1961). "Eilmer of Malmesbury, an Eleventh Century Aviator: A Case Study of Technological Innovation, Its Context and Tradition". Technology and Culture (Society for the History of Technology) 2 (2): 97–111. doi:10.2307/3101411.

The chemical compound silicon dioxide, also known as silica (from the Latin silex), is an oxide of silicon with the chemical formula SiO2. It has been known for its hardness since antiquity. Silica is most commonly found in nature as sand or quartz, as well as in the cell walls of diatoms.[1][2] Silica is manufactured in several forms including fused quartz, crystal, fumed silica (or pyrogenic silica, trademarked Aerosil or Cab-O-Sil), colloidal silica, silica gel, and aerogel.

1. Iler, R.K. (1979). The Chemistry of Silica. Plenum Press. ISBN 047102404X.

2. Lynn Townsend White, Jr. (1961). "Eilmer of Malmesbury, an Eleventh Century Aviator: A Case Study of Technological Innovation, Its Context and Tradition".

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In the vast majority of silicates, the Si atom shows tetrahedral coordination, with 4 oxygen atoms surrounding a central Si atom. The most common example is seen in the quartz crystalline form of silica SiO2. In each of the thermodynamically most stable crystalline forms of silica, on average, all 4 of the vertices (or oxygen atoms) of the SiO4 tetrahedron are shared with others, yielding the net chemical formula: SiO2

Mrs 033a diss.png

Figure 3.2 Tetrahedral structural unit of silica (SiO4), the basic building block of the most ideal glass former

The amorphous structure of glassy silica (SiO2) is in two-dimensions. No long-range order is present; however there is local ordering with respect to tetrahedral arrangement of oxygen (O) atoms around the silicon (Si) atoms. Note that a fourth oxygen atom is bonded to each silicon atom, either behind the plane of the screen or in front of it; these atoms are omitted for clarity.

Mrs 033a source.png

Tetrahedral structural unit of silica (SiO4), the basic building block of the most ideal glass former.

In the vast majority of silicates, the Si atom shows tetrahedral coordination, with 4 oxygen atoms surrounding a central Si atom. The most common example is seen in the quartz crystalline form of silica SiO2. In each of the most thermodynamically stable crystalline forms of silica, on average, all 4 of the vertices (or oxygen atoms) of the SiO4 tetrahedra are shared with others, yielding the net chemical formula: SiO2.

[...]

The amorphous structure of glassy silica (SiO2) in two-dimensions. No long-range order is present; however there is local ordering with respect to the tetrahedral arrangement of oxygen (O) atoms around the silicon (Si) atoms. Note that a fourth oxygen atom is bonded to each silicon atom, either behind the plane of the screen or in front of it; these atoms are omitted for clarity.

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Figure 3.3 The amorphous structure of glassy silica (SiO2) in two-dimensions. Note that a fourth oxygen atom is bonded to each silicon atom, either behind the plane of the screen or in front of it; these atoms are omitted for clarity

For example, in the unit cell of alpha-quartz, the central tetrahedron shares all 4 of its corner O atoms, the 2 face-centered tetrahedra share 2 of their corner O atoms, and the 4 edge-centered tetrahedra share just one of their O atoms with other SiO4 terahedra. This leaves a net average of 12 out of 24 total vertices for that portion of the 7 SiO4 tetrahedra which are considered to be a part of the unit cell for silica (see 3-D Unit Cell). SiO2 has a number of distinct crystalline forms (polymorphs) 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 α-quartz the bond length is 161 pm, whereas in α-tridymite it is in the range 154-171 pm. The Si-O-Si angle also varies between low values of 140° in α-tridymite, up to 180° in β-tridymite. In α-quartz the Si-O-Si angle is 144° [118].


[118] A.F. Holleman, E. Wiberg, Inorganic Chemistry, San Diego: Academic Press, (2001)

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The amorphous structure of glassy silica (SiO2) in two-dimensions.[...] Note that a fourth oxygen atom is bonded to each silicon atom, either behind the plane of the screen or in front of it; these atoms are omitted for clarity.

[...]

For example, in the unit cell of α-quartz, the central tetrahedron shares all 4 of its corner O atoms, the 2 face-centered tetrahedra share 2 of their corner O atoms, and the 4 edge-centered terahedra share just one of their O atoms with other SiO4 tetrahedra. This leaves a net average of 12 out of 24 total vertices for that portion of the 7 SiO4 tetrahedra which are considered to be a part of the unit cell for silica (see 3-D Unit Cell).

SiO2 has a number of distinct crystalline forms (polymorphs) 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 α-quartz the bond length is 161 pm, whereas in α-tridymite it is in the range 154-171 pm. The Si-O-Si angle also varies between a low value of 140° in α-tridymite, up to 180° in β-tridymite. In α-quartz the Si-O-Si angle is 144°.[5]


5. Holleman, A. F.; Wiberg, E. (2001), Inorganic Chemistry, San Diego: Academic Press, ISBN 0-12-352651-5

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3.4.2. Chemistry of silica

Silicon dioxide is formed when silicon is exposed to oxygen (or air). A very shallow 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 alternative environments are used to grow well-controlled layers of silicon dioxide on silicon, for example at temperatures between 600 and 1200 °C, using so-called dry or wet oxidation with O2 or H2O, respectively [119]. The depth of the layer of silicon replaced by the dioxide is 44% of the depth of the silicon dioxide layer produced [119].

Alternative methods used to deposit a layer of SiO2 include [120]

  • Low temperature oxidation (400–450 °C) of silane

SiH4 + 2 O2 → SiO2 + 2 H2O.

  • Decomposition of tetraethyl orthosilicate (TEOS) at 680–730 °C

Si(OC2H5)4 → SiO2 + 2 H2O + 4 C2H4.

  • Plasma enhanced chemical vapor deposition using TEOS at about 400 °C

Si(OC2H5)4 + 12 O2 → SiO2 + 10 H2O + 8 CO2.

  • polycondensation of tetraethyl orthosilicate (TEOS) at below 100 °C using amino acid as catalyst.[121]

[119] L. Sunggyu, Encyclopedia of chemical processing. CRC Press. (2006)

[120] R. Doering, Y. Nishi (2007). Handbook of Semiconductor Manufacturing Technolog, Marcel Dekker, New York

[121] A.B.D. Nandiyanto; S.-G Kim; F. Iskandar; and K. Okuyama (2009), Microporous and Mesoporous Materials 120 (3): 447–453

Chemistry

Silicon dioxide is formed when silicon is exposed to oxygen (or air). A very shallow 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 alternative environments are used to grow well-controlled layers of silicon dioxide on silicon, for example at temperatures between 600 and 1200 °C, using so-called dry or wet oxidation with O2 or H2O, respectively.[28] The depth of the layer of silicon replaced by the dioxide is 44% of the depth of the silicon dioxide layer produced.[28]

Alternative methods used to deposit a layer of SiO2 include[29]

  • Low temperature oxidation (400–450 °C) of silane SiH4 + 2 O2 → SiO2 + 2 H2O.
  • Decomposition of tetraethyl orthosilicate (TEOS) at 680–730 °C Si(OC2H5)4 → SiO2 + 2 H2O + 4 C2H4.
  • Plasma enhanced chemical vapor deposition using TEOS at about 400 °C Si(OC2H5)4 + 12 O2 → SiO2 + 10 H2O + 8 CO2.
  • Polymerization of tetraethyl orthosilicate (TEOS) at below 100 °C using amino acid as catalyst.[30]

[28] Sunggyu Lee (2006). Encyclopedia of chemical processing. CRC Press. ISBN 0824755634.

[29] Robert Doering, Yoshio Nishi (2007). Handbook of Semiconductor Manufacturing Technology. CRC Press. ISBN 1574446754.

[30] A.B.D. Nandiyanto; S.-G Kim; F. Iskandar; and K. Okuyama (2009). "Synthesis of Silica Nanoparticles with Nanometer-Size Controllable Mesopores and Outer Diameters". Microporous and Mesoporous Materials 120 (3): 447–453. doi:10.1016/j.micromeso.2008.12.019. Innovation, Its Context and Tradition".

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