Introduction
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula SiO2, commonly found in nature as quartz.[5][6] In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant families of materials, existing as a compound of several minerals and as a synthetic product.
Examples include fused quartz, fumed silica, opal, and aerogels. It is used in structural materials, microelectronics, and as components in the food and pharmaceutical industries. All forms are white or colorless, although impure samples can be colored.Silicon dioxide is a common fundamental constituent of glass. SiO2 is an oxide of silicon with a chemical name silicon dioxide. It is also called Silica or Kalii bromidum or Silicic oxide or silicic acid. It is widely found in nature as quartz.
It is obtained as a transparent to grey, in its crystalline or amorphous powdered form. It is odourless and tasteless compound.
Silicon is a significant element that is essential for several physiological and metabolic processes in plants. Silicon is widely regarded as the predominant semiconductor material due to its versatile applications in various electrical devices such as transistors, solar cells, integrated circuits, and others.
1. Structure
In the majority of silicon dioxides, the silicon atom shows tetrahedral coordination, with four oxygen atoms surrounding a central Si atom (see 3-D Unit Cell). Thus, SiO2 forms 3-dimensional network solids in which each silicon atom is covalently bonded in a tetrahedral manner to 4 oxygen atoms.[8][9] In contrast, CO2 is a linear molecule. The starkly different structures of the dioxides of carbon and silicon are a manifestation of the double bond rule.[10]
Based on the crystal structural differences, silicon dioxide can be divided into two categories: crystalline and non-crystalline (amorphous). In crystalline form, this substance can be found naturally occurring as quartz, tridymite (high-temperature form), cristobalite (high-temperature form), stishovite (high-pressure form), and coesite (high-pressure form). On the other hand, amorphous silica can be found in nature as opal and diatomaceous earth. Quartz glass is a form of intermediate state between these structures
1. Polymorphism
Alpha quartz is the most stable form of solid SiO2 at room temperature. The high-temperature minerals, cristobalite and tridymite, have both lower densities and indices of refraction than quartz. The transformation from α-quartz to beta-quartz takes place abruptly at 573 °C. Since the transformation is accompanied by a significant change in volume, it can easily induce fracturing of ceramics or rocks passing through this temperature limit.[13]
The high-pressure minerals, seifertite, stishovite, and coesite, though, have higher densities and indices of refraction than quartz.[14] Stishovite has a rutile-like structure where silicon is 6-coordinate. The density of stishovite is 4.287 g/cm3, which compares to α-quartz, the densest of the low-pressure forms, which has a density of 2.648 g/cm3.[15]
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 α-quartz.[16] The change in the coordination increases the ionicity of the Si–O bond.[17]
2. Molten SiO2
Molten silica exhibits several peculiar physical characteristics that are similar to those observed in liquid water: negative temperature expansion, density maximum at temperatures ~5000 °C, and a heat capacity minimum.[19] Its density decreases from 2.08 g/cm3 at 1950 °C to 2.03 g/cm3 at 2200 °C
3. Molecular SiO2
The molecular SiO2 has a linear structure like CO2. It has been produced by combining silicon monoxide (SiO) with oxygen in an argon matrix. The dimeric silicon dioxide, (SiO2)2 has been obtained 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 150.2 pm. The Si–O bond length is 148.3 pm, which compares with the length of 161 pm in α-quartz. The bond energy is estimated at 621.7 kJ/mol.
2. Uses
1. 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).[22]
Certain deposits of silica sand, with desirable particle size and shape and desirable clay and other mineral content, were important for sand casting of metallic products.[33]
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.[
2. 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.[35] When molten silicon dioxide SiO2 is rapidly cooled, it does not crystallize, but solidifies as a glass.[36] Because of this, most ceramic glazes have silica as the main ingredient.[37]
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.[38]
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:[39]
- 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.[15]
SiCl4+2H2+O2⟶SiO2+4HCl
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.[40] 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.
4. 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.[40] It can adsorb water in hygroscopic applications. Colloidal silica is used as a fining agent for wine, beer, and juice, with the E number reference E551.[22]
In cosmetics, silica is useful for its light-diffusing properties[42] and natural absorbency.[43]
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”.[44][45] Manufactured or mined hydrated silica is used as the hard abrasive in toothpaste.
More SiO2Uses (Silicon dioxide)
- Silicon dioxide is used in the construction industry to produce concrete.
- In its crystalline form it is used in hydraulic fracturing.
- Used in the production of glass.
- Used as a Sedative.
- Used in the production of elemental silicon.
- Used as an anti-caking agent in powdered foods like spices.
- Used as a fining agent in juice, beer, and wine.
- Used pharmaceuticals for making tablets.
- Used in toothpaste to remove tooth plaque.
3. Chemical reactions
Silicon dioxide is a relatively inert material (hence its widespread occurrence as a mineral). Silica is often used as inert containers for chemical reactions. At high temperatures, it is converted to silicon by reduction with carbon.
Fluorine reacts with silicon dioxide to form SiF4 and O2 whereas the other halogen gases (Cl2, Br2, I2) are unreactive.[15]
Most forms of silicon dioxide are attacked (“etched”) by hydrofluoric acid (HF) to produce hexafluorosilicic acid:[12]
SiO2 + 6 HF → H2SiF6 + 2 H2O
Stishovite does not react to HF to any significant degree.[65] HF is used to remove or pattern silicon dioxide in the semiconductor industry.
4. Health effects
Silica ingested orally is essentially nontoxic, with an LD50 of 5000 mg/kg (5 g/kg).[22] A 2008 study following subjects for 15 years found that higher levels of silica in water appeared to decrease the risk of dementia. An increase of 10 mg/day of silica in drinking water was associated with a reduced risk of dementia of 11%.[70]
Inhaling finely divided crystalline silica dust can lead to silicosis, bronchitis, or lung cancer, as the dust becomes lodged in the lungs and continuously irritates the tissue, reducing lung capacities.[71] When fine silica particles are inhaled in large enough quantities (such as through occupational exposure), it increases the risk of systemic autoimmune diseases such as lupus[72] and rheumatoid arthritis compared to expected rates in the general population.[73]
Health hazards
Silica when ingested orally is non-toxic. As per a study conducted in the year 2008, found that the higher the levels of silica in water, the risk of dementia decreased. Therefore, the dose was increased to 10 mg/day of silica in drinking water as the risk of dementia decreased .
When finely divided crystalline silica dust is inhaled, it can lead to bronchitis, lung cancer, or silicosis, due to the lodging of dust in the lungs. When fine silica particles are inhaled in large enough quantities, it increases the risk of rheumatoid arthritis and lupus.
5. Manufactured forms
Silica is manufactured in several forms including:
- glass (a colorless, high-purity form is called fused silica)
- synthetic amorphous silica
- silica gel (used e.g. as desiccants in new clothes and leather goods)
- 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 siliconis hugely beneficial in microelectronics. It is a superior electric insulator, possessing 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 on silicon, the traditional method has been the deliberate 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.)
6. 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.
Silicon dioxide has covalent bonding and forms a network structure (also known as lattice or continuous).
Silicon dioxide is attacked by hydrofluoric acid (HF). HF is used to remove or pattern silicon dioxide in the semiconductor industry.
7. Successful Esthetic and Cosmetic Dentistry for the Modern Dental Practice
Steven R. Jefferies MS, DDS, in Dental Clinics of North America, 2007
Silicon dioxide is used primarily as a polishing agent in bonded abrasive rubber or elastomeric finishing and polishing devices. In dental devices, it is primarily used in rubber or elastomeric cups and points for finishing and polishing. This abrasive is used in the initial finishers and second grit polishers in the Astropol Finishing and Polishing System (Ivoclar North America, Amherst, New York)
8. Silane adhesion mechanism in dental applications and surface treatments: A review
Jukka Pekka Matinlinna, … James Kit Hon Tsoi, in Dental Materials, 2018
Introduction: silicon
Silicon (Si) belongs to the main Group 14 (formerly IVA) with carbon (C), germanium (Ge), tin (Sn), and lead (Pb) on the periodic table and it has four valence electrons. There is another four valent group of elements, Group 4 (IVB), transition metals such as titanium (Ti), zirconium (Zr), hafnium (Hf), and rutherfordium (Rf) also with four valence electrons for the elements.
The difference between the two groups is a partial filling electron configuration of 3d–5d orbitals in Group 14 4. The empty 3d orbital of Si makes it chemically different from other Group 14 elements, in terms of structure, reactivity and thereby physical and chemical properties.
Carbon (6C) is a typical non-metal and silicon (14Si) is a metalloid, semi-metal (a semiconductor). The metallic character increases down the group with tin and lead as the metals. Carbon occurs as graphite and diamond (allotropes), silicon is found as silica (silicon dioxide, SiO2) and numerous silicates and alumino-silicates in earth.
The electronic configuration of C is 1s22s22p2 and for Si 1s22s22p63s23p23d0. They have, in some aspect, similar unique properties that distinguish them from the other element group members. Both C and Si have catenation property which decreases dramatically down the group. Carbon can form “endlessly” long chains with other carbon
atoms (CC)n. Silicon can also bond to other silicon atoms (SiSi)n but with farshorter chain length (only up to 2–3 units) and such compounds are, in contrary, unstable.
9. The emergence of metal oxide nanoparticles (NPs) as a phytomedicine: A two-facet role in plant growth, nano-toxicity and anti-phyto-microbial activity
Rahul Bhattacharjee, … Jarosław Proćków, in Biomedicine & Pharmacotherapy, 2022
Silicon oxide
Silicon dioxide has application in the construction industry to produce concrete, however, its crystalline form is widely used in hydraulic fracturing. The added surface area of SiO2 at the nanoscale would make its antimicrobial activity more important [83]. Si NPs, according to Cousins et al., prevented bacteria from adhering to oral biofilms [84]. Si NPs combined with other biocidal metals like Ag are major targets for substantial research in recent years.
The antimicrobial activity of Novel Ag-Si nanocomposite was studied by Egger et al. Their findings showed that the nanocomposite has superior antibacterial properties to conventional materials like silver zeolite and silver nitrate against a variety of microbes [85]. In a similar investigation, Mukha et al. synthesized Ag/SiO2 and Au/SiO2 nanostructures to exhibit antibacterial properties of these materials [86]. The scientists proposed that these nanocomposites could be utilized for applications in medicine and pharmaceuticals, as well as water purification [86].
According to several studies, Si nanowires can interact with bacteria and living cells, preventing the differentiation of cells, adhesion of cells, and their proliferation. Ag NPs-Si nanowires depicted antibacterial efficacy which was studied by Lee et al. Their findings showed that these nanostructures had strong antibacterial properties [87,88]. Fellahi et al., prepared Si nanowire substrates coated with Cu or Ag NPs and assessed their antibacterial activity.
The authors claim that the produced NPs demonstrated strong antibacterial properties against E. coli [43,87]. Silicon oxide has shown promising results as an antimicrobial agent. Further investigation and research are required to explore its utility as an anti-plant pathogen agent.
Question
1. What are the uses of silicon dioxide?
Approximately 95 per cent of the industrial usage of silicon dioxide (sand) exists in the building industry, e.g. for concrete production (Portland cement concrete). Silica, in the form of sand, is used as the key ingredient for the manufacture of metallic components in engineering and other applications of sand casting. The relatively high melting point of silica allows for its use in these applications.
2. How is silicon dioxide produced?
Mostly, silicon dioxide is obtained via mining activities including sand extraction, and quartz purification. Quartz is suitable for many purposes, whereas chemical processing is needed to render a more suitable product (e.g. more reactive or fine-grained) purer or otherwise. Silica fume is derived from hot processes such as the processing of ferrosilicon as a by-product.
3. Is silicon dioxide toxic?
When orally consumed, silica is essentially non-toxic. However, inhaling finely divided silica crystalline dust may contribute to silicosis, bronchitis, or lung cancer, as the dust becomes trapped in the lungs and constantly irritates the tissue, decreasing lung capacity.
4. What is the function of silicon dioxide?
Silicon dioxide is also added to many foods and supplements. As a food additive, it serves as an anticaking agent to avoid clumping. In supplements, it’s used to prevent the various powdered ingredients from sticking together.
5. Is SiO2 linear or tetrahedral?
SiO2 has a linear shape with the two oxygens connected to the Si by double bonds. There are no lone pairs on the Si. This would be an example of sp hybridisation. In polymer form SiO2 is tetrahedral.
6. How much silicon dioxide is safe?
The FDA and other governing agencies worldwide have set standards for the amount of silicon dioxide that is safe to include in foods and other products. Based on animal studies, some researchers feel that humans may safely consume as much as 1,500 mg per day.
7. Is silica safe for human consumption?
The upper safe limit has been reported as 700–1,750 mg a day. As silica is water-soluble, excess is simply passed out by the body in your urine, meaning it’s unlikely to cause side effects if you take too much. Silica should be avoided by the following people: children – horsetail contains traces of nicotine