Silicon Dioxide - Chemistry

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. The depth of the layer of silicon replaced by the dioxide is 44% of the depth of the silicon dioxide layer produced.

Alternative methods used to deposit a layer of SiO₂ include

• 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.

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 + 2 H₂ + O₂ → SiO₂ + 4 HCl.

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.

The solubility of silicon dioxide in water strongly depends on its crystalline form and is 3–4 times higher for silica than quartz; as a function of temperature, it peaks at about 340 °C. 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. These crystals are a source 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 + 6 HF → H₂SiF₆ + 2 H₂O.

HF is used to remove or pattern silicon dioxide in the semiconductor industry.

Silicon dioxide dissolves in hot concentrated alkali or fused hydroxide:

SiO₂ + 2 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, dependent on the proportions of reactants:

2 Na₂O + SiO₂ → Na₄SiO₄;

Na₂O + SiO₂ → Na₂SiO₃;

(0.25–0.8)Na₂O + SiO₂ → glass.

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 → 2 SiO (gas).