Silicon dioxide , also known as silica (from Latin silex ), is a silicon oxide with the chemical formula SiO 2 , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the main constituent of sand. Silica is one of the family's most complex and most abundant materials, which exists as a mixture of several minerals and as a synthetic product. Important examples include fused quartz, smoked silica, silica gel, and aerogel. It is used in structural materials, microelectronics, and as components in the food and pharmaceutical industries.
Inhalation of finely divided silica crystals is toxic and may cause silicosis, bronchitis, lung cancer, and systemic autoimmune diseases, such as lupus and rheumatoid arthritis.
Video Silicon dioxide
Structure
In most silicates, silicon atoms show tetrahedral coordination, with four oxygen atoms surrounding the central Si atom. The most common examples are seen in quartz polymorphs.
For example, in a "-quartz" unit cell, the center of the tetrahedron divides all four of its O atoms, two side-centered tetrahedra, sharing two angles of the O atoms, and four centered tetrahedra sharing only one of them. O atoms with SiO 4 other tetrahedra. This leaves the net average of 12 of the 24 total vertices for the part of the seven SiO 4 tetrahedra which are considered part of the unit cell for silica (see Unit 3-D Cells).
SiO 2 has a number of different crystalline forms (polymorphs) in addition to amorphous forms. With the exception of stishovite and fibrous silica, all crystalline forms involve the tetrahedral SiO 4 units that are connected together by shared nodes in different settings. The length of silicone-oxygen bonding varies between different crystalline forms; for example in? -quartz bond length is 161 pm, whereas in? -tridymite is in the range 154-171 pm. The Si-O-Si angle also varies between the low value of 140Ã, Â ° in? -tridymite, up to 180Ã, Â ° at? -tridymite. In? -quartz, the Si-O-Si angle is 144 Â °.
The fibrous silica has a structure similar to SiS 2 with the edge-sharing SiO 4 chain of tetrahedra. Stishovite, a higher form of pressure, by contrast, has a rutile-like structure in which silicon is 6-coordinate. The stishovite density was 4.287 g/cm 3 , which compared with quartz, the most dense of the low-pressure form, having a density of 2,648 g/cm 3 . The difference in density may be attributed to increased coordination as the six shortest Si-O bond lengths in the stishovite (four Si-O bonds of 176 pm and the other two of 181 pm) greater than the Si-O bond length (161 pm) at? -quartz. Co-ordination changes increase the Si-O bond ionisity. More importantly, any deviation from these standard parameters is a difference or variation of the microstructure, which represents the approach to amorphous, vitreous, or glass solids.
The only stable form under normal conditions is alpha quartz, where crystalline silicon dioxide is usually found. In nature, dirt in crystal? -quartz can cause color (see list). High-temperature minerals, cristobalite and tridim, have lower density and refractive indices than quartz. Because the composition is identical, the reasons for nonconformity should be in increasing distance in high temperature minerals. As usual with many substances, the higher the temperature, the farther apart the atoms, due to the increased energy of vibration.
The transformation from quartz to beta-quartz takes place suddenly at 573 ° C. Since the transformation is accompanied by significant volume changes, it can easily lead to the breaking of ceramic or stone over this temperature limit.
High pressure minerals, seifertite, stishovite, and coesite, have higher density and refraction index than quartz. This may be due to the intense compression of atoms occurring during its formation, resulting in a more viscous structure.
Silica Faujasite is another form of crystalline silica. It is obtained by zeolite dealumination â € <â €
Molten silica exhibits some strange physical characteristics similar to those observed in liquid water: negative temperature expansion, maximum density at ~ 5000 ° C, and minimum heat capacity. Its density decreased from 2.08 g/cm 3 at 1950 Â ° C to 2.03 g/cm 3 at 2200 ° C.
Molecular SiO 2 with a linear structure produced when the molecular silicon monoxide, SiO, is condensed in a helium-cooled argon matrix together with the oxygen atom produced by microwave release. The silicon dioxide dimer, (SiO 2 ) 2 has been prepared by reacting O 2 with an isolated matrix of silicon monoxide dimer, (Si 2 O 2 ). In silicon dioxide dimerik there are two oxygen atoms that bridge between silicon atoms with the angle of Si-O-Si 94 Â ° and length of bond 164,6 pm and length of terminal of Si-O is 150,2 pm. The Si-O bond length is 148.3 pm, which compares with length of 161 pm at? -quartz. The bond energy is estimated at 621.7 kJ/mol.
Maps Silicon dioxide
Natural events
Geology
Silica with the chemical formula SiO 2 is most often found in nature as quartz SiO 4 , which comprises more than 10% by mass of the earth's crust. In many parts of the world, silica is the main constituent of sand.
Biology
Although it is difficult to dissolve, silica occurs in many plants. Plant materials with high phytolith silica content seem important to grazing animals, ranging from chewing insects to nails. Silica accelerates tooth wear, and high levels of silica in plants that are often eaten by herbivores may have evolved as a defense mechanism against predation.
Silica is also a major component of rice husk ash, which is used, for example, in filtration and cement manufacture.
For more than a billion years, silicification in and by cells has become common in the biological world. In the modern world it occurs in bacteria, single-celled organisms, plants, and animals (invertebrates and vertebrates). Prominent examples include:
- Test or frustration (ie shell) diatoms, Radiolaria and testate amoebae.
- Silica phytoliths in cells of many plants, including Equisetaceae, almost all grasses, and various dicotyledons.
- Spicules form the skeleton of many sponges.
Crystal minerals formed in physiological environments often exhibit exceptional physical properties (eg, strength, hardness, fracture toughness) and tend to form hierarchical structures that show the order of microstructures over multiple scales. The minerals are crystallized from environments that are undersaturated with respect to silicon, and under conditions of neutral pH and low temperature (0-40Ã, Â ° C).
Mineral formation can occur either inside the cell wall of an organism (such as with phytoliths), or outside the cell wall, as is usually the case with the test. Specific biochemical reactions exist for mineral deposition. Such reactions include those involving lipids, proteins, and carbohydrates.
It is not clear how silica is important in animal nutrition. This field of research is challenging because silica is ubiquitous and is in many cases only dissolved in trace amounts. All that does happen in a living body, leaving us with a difficult problem to make proper silica-free control for research purposes. This makes it difficult to ascertain when the silica has a beneficial effect on the operation, and when its presence is accidental, or even dangerous. The current consensus is that it is clearly important in the growth, strength, and management of many connective tissues. This is true not only for hard connective tissues such as bones and teeth but also possibly in the biochemistry of structures containing subcellular enzymes as well.
Usage
Structural use
It is estimated that 95% of the silicon dioxide produced is consumed in the construction industry, for example for Portland cement production.
Silica, in the form of sand is used as the main ingredient in sand casting for the manufacture of metal components in engineering and other applications. The high silica melting point allows it to be used in the application.
Crystalline silica is used in hydraulic forming fractures that contain tighter oils and flake gas.
Precursors for glass and silicon
Silica is the main ingredient in the production of most glass. The pure SiO transition temperature temperature 2 is about 1475 K. When the liquid silicon dioxide SiO 2 is cooled rapidly, it does not crystallize, but solidifies as a glass.
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 from the center of oxygen. The difference between glass and crystal forms arises from tetrahedral unit connectivity: Although there is no long periodicity range in fixed glass ordering networks on long scales far beyond the SiO bond length. One example of this booking is the preference for forming a 6-tetrahedra ring.
Smoky Silica
Smoky silica also known as pyrogenic silica is a fine particle or silicon dioxide colloidal form. It is prepared by burning SiCl 4 in a hydrogen-rich hydrogen fire to produce "smoke" from SiO 2 .
- SiCl 4 2 H 2 O 2 -> SiO 2 4 HCl.
The majority of optical fibers for telecommunications are also made of silica. It is the main raw material for many ceramics such as pottery, stoneware, and porcelain.
Silicon dioxide is used to produce elemental silicon. This process involves carbothermic reduction in electric arc furnaces:
- SiO 2 2 C -> Si 2 CO
Food and pharmaceutical apps
Silica is a common additive in food production, where it is used primarily as a flow agent in powdered food, or to absorb water in hygroscopic applications. It is used as an anti-caking agent in food powders such as spices and non-dairy creamer coffee. This is the main component of diatomaceous earth. Colloidal silica is also used as a wine agent, beer, and fining juice.
In pharmaceutical products, silica powder helps to flow when the tablet is formed.
Personal care
In cosmetics, it is useful for the nature of light-spreading and natural absorption.
Hydrated silica is used in toothpaste as a hard abrasive to remove dental plaque.
More
Hydrophobic silica is used as a foam prevention component.
In its capacity as a refractory, it is useful in the form of fiber as a high temperature thermal protection cloth.
It is used as a heat enhancing compound in the heat pump heat pump industry.
Silica is used in DNA and RNA extraction because of its ability to bind nucleic acids under the presence of chaotropes.
Aerogel-based silica is used in the Stardust spacecraft to collect spacecraft particles.
Pure silica (silicon dioxide), when cooled as a quartz melt into a glass without a true melting point, can be used as a glass fiber for fiberglass.
Production
Silicon dioxide is mostly obtained from mining including sand mining and quartz purification. Quartz is suitable for many purposes, while chemical processing is required to make the product more pure or more appropriate (eg more reactive or fine-grained).
Silica fume
Silica fume is obtained as a by-product of heat processes such as the production of ferro-silicon. It is purer than smoky silica and should not be equated with that product. The production process, particle characteristics and the application of smoky silica are all different from silica fume.
Pre-precipitated Silica
The precipitated silica or amorphous silica is produced by acidification of the sodium silicate solution. The precipitate of gelatin or silica gel, first washed and then dehydrated to produce a colorless microporous silica. The ideal equation involving trisilicate and sulfuric acid is shown: 3 < 4 > <3> 2 4
About one billion kilograms/year (1999) silica is produced in this way, especially for use for tin polymer composites and shoe soles.
On microchip
The thin film of silica grows spontaneously on silicon wafers through thermal oxidation, producing a very shallow layer of about 1 nm or 10 ÃÆ'... of what is called the original oxide. Higher temperatures and alternative environments are used to grow a silicon-controlled silicon dioxide layer, for example at temperatures between 600 and 1200 ° C, using so-called dry or wet oxidation with O 2
- Si O 2 -> SiO 2
atau H 2 O, masing-masing.
- Si 2Â H 2 O -> SiO 2 2 H 2
The depth of the silicon layer that replaces the dioxide is 44% of the depth of the resulting silicon dioxide layer.
The original oxide layer is useful in microelectronics, where it acts as an electrical insulator with high chemical stability. It can protect silicon, store charge, block current, and even act as a controlled line to limit current flow.
Laboratory or special method
From silicate ester
Many routes for silicon dioxide start with silicate esters, most known as tetraethyl ortosilicate (TEOS). Simply warm up TEOS at 680-730Ã, Ã, Â ° C gives dioxide:
- 4 -> SiO 2 2 O (C 2 H 5 ) 2
Demikian pula TEOS membakar sekitar 400 ° C:
-
- Si (OC 2 H 5 ) 4 12 O 2 -> SiO < sub> 2 10 H 2 O 8 CO 2
TEOS undergoes hydrolysis through a process called sol-gel. The course of the reaction and the nature of the product is influenced by the catalyst, but the ideal equation is:
-
- Si (OC 2 H 5 ) 4 2 H 2 O -> SiO < sub> 2 4 HOCH 2 CH 3
Other methods
Because it is very stable, silicon dioxide emerges from many methods. Conceptually simple, but small practical value, silane combustion gives silicon dioxide. This reaction is analogous to the burning of methane:
-
- SiH 4 2 O 2 -> SiO 2 2 H 2 O.
However, chemical vapor deposition of silicon dioxide to the crystal surface of silane has been used using nitrogen as carrier gas at 200-500 o C.
Chemical reactions
Silica is converted into silicon by carbon reduction.
Fluorine reacts with silicon dioxide to form SiF 4 and O 2 while other halogen gases (Cl 2 , Br 2 , I 2 ) is basically not reactive.
Silicon dioxide is invaded by fluoride acid (HF) to produce hexafluorosilicic acid:
HF is used to remove or silicon dioxide patterns in the semiconductor industry.
Silicon dioxide acts as a Lux-Flood acid, capable of reacting with a base under certain conditions. Since it does not contain hydrogen, it can not act as BrÃÆ'¸nsted-Lowry acid. Although not soluble in water, some strong bases will react with glass and should be stored in plastic bottles as a result.
Silicon dioxide is soluble in the hot concentrate alkali or molten hydroxide, as described in this ideal equation: 2 2 2 2 2 2 NaOH -> Na dd>
Silicon dioxide neutralizes the base metal oxide (eg sodium oxide, potassium oxide, lead (II) oxide, zinc oxide, or oxide mixture, forming silicates and glass as Si-O-Si bonds in successively broken silica). For example sodium oxide and SiO 2 reaction can produce sodium orthosilicate, sodium silicate, and glass, depending on the proportion of reactants:
- 2 Na 2 O SiO 2 -> Na 4 SiO 4 ; 2 O SiO 2 -> Na 2 SiO 3 ;
- (0.25-0.8) Na 2 O SiO 2 -> glass.
Examples of such glasses have a commercial meaning, e.g. soda-lime glass, borosilicate glass, lead glass. In these glasses, silica is called the former tissue or the previous lattice. The reaction is also used in a blast furnace to remove sand impurities in the ore by neutralizing with calcium oxide, forming a calcium silicate slag.
Silicon dioxide reacts in heat reflux under dinitrogen with ethylene glycol and alkali metal base to produce highly reactive pentakoordinate silicate, which provides access to a wide range of new silicon compounds. Silicates are essentially insoluble in all polar solvents except methanol.
Silicon dioxide reacts with elemental silicon at high temperatures to produce SiO:
- SiO 2 Si -> 2 SiO
Water solubility
The solubility of silicon dioxide in water relies heavily on its crystalline form and is three-four times higher for silica than quartz; as a function of temperature, it reaches about 340 ° C. This property is used to grow single quartz crystals in a hydrothermal process in which natural quartz is dissolved in super hot water in a cooler pressure vessel at the top. Crystals of 0.5-1 kg can grow over a period of 1-2 months. These crystals are a very pure quartz source for use in electronic applications.
Health effects
Orally-digested silica is essentially non-toxic, with LD <50 5000Ã, μg/kg (5 g/kg). A 2008 study that followed subjects for 15 years found that higher levels of silica in water appeared to lower the risk of dementia. An increase of 10 mg/day of silica in drinking water was associated with a 11% reduction in dementia risk.
Inhaling dust of finely divided silica crystals can cause silicosis, bronchitis, or lung cancer, since dust is lodged in the lungs and continues to irritate the tissues, reducing lung capacity. When fine silica particles are inhaled in considerable amounts (such as through occupational exposure), it increases the risk of systemic autoimmune diseases such as lupus and rheumatoid arthritis compared to levels expected in the general population.
Occupational hazards
Silica is a work hazard for people who do sandblasting, or work with products containing powdered crystalline silica. Amorphous silica, such as smoky silica, can cause irreversible lung damage in some cases, but is not associated with the development of silicosis. Children, people with asthma of all ages, those with allergies, and the elderly (all of whom have reduced lung capacity) may be affected in less time.
Silica crystals are a work hazard for those who work with stone countertops, because the cutting and installation of countertops creates a large amount of air silica. Crystal crystals used in hydraulic fractures present a health hazard to workers.
Pathophysiology
In the body, crystalline silica particles are insoluble during clinically relevant periods. Silica crystals in the lungs can activate NLRP3 inflammation in macrophages and dendritic cells and thus produce interleukin production, a cytokine that is highly pro-inflammatory in the immune system.
Rule
Regulations that limit the exposure of silica 'with respect to the danger of silicosis' determine that they are concerned only with silica, which is crystalline and dust.
In 2013, the US Occupational Safety and Health Administration reduced exposure limits to 50 Âμg/m 3 air. Before 2013, it has allowed 100 Ã,Âμg/m 3 and in construction workers even 250 Âμg/m 3 . In 2013, OSHA also requires a "green settlement" of foamed wells to reduce crystalline silica exposure in addition to limiting exposure limits.
Crystal shapes
SiO 2 , more than almost any material, exists in many crystals. These forms are called polymorphs.
See also
- Mesoporous silica
- Silicon carbide
References
External links
- Tridymite, International Chemical Safety Card 0807
- Quartz, International Chemical Safety Card 0808
- Cristobalite, International Chemical Safety Card 0809
- amorphous, NIOSH Pocket Guide to Chemical Hazards
- crystals, such as inhaled dust, NIOSH Pocket Guide to Chemical Hazards
- The formation of silicon oxide coatings in the semiconductor industry. The LPCVD and PECVD methods are compared. Prevention of stress.
- Quartz SiO 2 piezoelectric property
- Silica (SiO 2 ) and Water
- Epidemiological evidence about the carcinogenicity of silica: factors in scientific appraisal by C. Soutar and others. Research Report of Vocational Research Institute TM/97/09
- Scientific opinion about the health effect of air silica by A Pilkington and others. Research Report of Vocational Research Institute TM/95/08
- The effects of silica toxicity by A Seaton and others. Research Report of Vocational Research Institute TM/87/13
- The precipitated silica structure
Media yang berhubungan dengan Silicon dioksida di Wikimedia Commons
- Â Chisholm, Hugh, ed. (1911). "Silica". EncyclopÃÆ'¦dia Britannica (edisi 11). Cambridge University Press Â
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