Structural use About 95% of the commercial use of silicon dioxide (sand) is in the construction industry, e.g. in the production of concrete (
Portland cement concrete). The high melting point of silica enables it to be used in such applications such as iron casting; modern sand casting sometimes uses other minerals for other reasons. Crystalline silica is used in
hydraulic fracturing of formations which contain
tight oil and
shale gas.
Precursor to glass and silicon Silica is the primary ingredient in the production of most
glass. As other minerals are melted with silica, the principle of
freezing point depression lowers the melting point of the mixture and increases fluidity. The
glass transition temperature of pure SiO2 is about 1475 K. When molten silicon dioxide SiO2 is rapidly cooled, it does not crystallize, but solidifies as a glass. Because of this, most
ceramic glazes have silica as the main ingredient. The structural geometry of silicon and oxygen in glass is similar to that in quartz and most other crystalline forms of silicon and oxygen, with silicon surrounded by regular tetrahedra of oxygen centres. The difference between the glass and crystalline forms arises from the connectivity of the tetrahedral units: Although there is no long-range periodicity in the glassy network, ordering remains at length scales well beyond the SiO bond length. One example of this ordering is the preference to form rings of 6-tetrahedra. The majority of
optical fibers for
telecommunications are also made from silica. It is a primary raw material for many ceramics such as
earthenware,
stoneware, and
porcelain. Silicon dioxide is used to produce elemental
silicon. The process involves
carbothermic reduction in an
electric arc furnace: :SiO2 + 2 C -> Si + 2 CO
Fumed silica Fumed silica, also known as pyrogenic silica, is prepared by burning
SiCl4 in an oxygen-rich hydrogen flame to produce a "smoke" of SiO2. :SiCl4 + 2 H2 + O2 -> SiO2 + 4 HCl It can also be produced by vaporizing quartz sand in a 3000 °C electric arc. Both processes result in microscopic droplets of
amorphous silica fused into branched, chainlike, three-dimensional secondary particles which then agglomerate into tertiary particles, a white powder with extremely low bulk density (0.03-0.15 g/cm3) and thus high surface area. The particles act as a
thixotropic thickening agent, or as an anti-caking agent, and can be treated to make them hydrophilic or hydrophobic for either water or organic liquid applications.
Silica fume is an ultrafine powder collected as a by-product of the silicon and
ferrosilicon alloy production. It consists of amorphous (non-crystalline) spherical particles with an average particle diameter of 150 nm, without the branching of the pyrogenic product. The main use is as
pozzolanic material for high performance concrete. Fumed silica nanoparticles can be successfully used as an anti-aging agent in asphalt binders.
Food, cosmetic, and pharmaceutical applications Silica, either colloidal, precipitated, or pyrogenic fumed, is a common additive in food production. It is used primarily as a flow or anti-
caking agent in powdered foods such as spices and non-dairy coffee creamer, or powders to be formed into pharmaceutical tablets. and natural absorbency.
Diatomaceous earth, a mined product, has been used in food and cosmetics for centuries. It consists of the silica shells of microscopic
diatoms; in a less processed form it was sold as
tooth powder. Manufactured or mined
hydrated silica is used as the hard abrasive in
toothpaste.
Semiconductors Silicon dioxide is widely used in the semiconductor technology: • for the primary passivation (directly on the semiconductor surface), • as an original
gate dielectric in
MOS technology. Today when scaling (dimension of the gate length of the MOS transistor) has progressed below 10 nm, silicon dioxide has been replaced by other
dielectric materials like
hafnium oxide or similar with higher dielectric constant compared to silicon dioxide, • as a dielectric layer between metal (wiring) layers (sometimes up to 8–10) connecting elements and • as a second passivation layer (for protecting semiconductor elements and the metallization layers) typically today layered with some other dielectrics like
silicon nitride. Because silicon dioxide is a native oxide of silicon it is more widely used compared to other semiconductors like
gallium arsenide or
indium phosphide. Silicon dioxide could be grown on a silicon
semiconductor surface. Silicon oxide layers could protect silicon surfaces during
diffusion processes, and could be used for diffusion masking.
Surface passivation is the process by which a semiconductor surface is rendered inert, and does not change semiconductor properties as a result of interaction with air or other materials in contact with the surface or edge of the crystal. The formation of a
thermally grown silicon dioxide layer greatly reduces the concentration of
electronic states at the silicon surface. The process of silicon surface passivation by
thermal oxidation (silicon dioxide) is critical to the
semiconductor industry. It is commonly used to manufacture
metal–oxide–semiconductor field-effect transistors (MOSFETs) and silicon
integrated circuit chips (with the
planar process). Silica is used in the
extraction of DNA and
RNA due to its ability to bind to the nucleic acids under the presence of
chaotropes.
Silica aerogel was used in the
Stardust spacecraft to collect extraterrestrial particles. Pure silica (silicon dioxide), when cooled as fused quartz into a glass with no true melting point, can be used as a glass fibre for fibreglass, fiberglass contains silica, which is a key component in its composition, typically making up about 65% of the material. Silica is combined with other compounds to create the glass fibers used in fiberglass products. ==Production==