AMORPHOUS SILICA

Amorphous Silica is used as a dehumidifying desiccant, a dehydrating agent, a moisture barrier, and an air humidity regulator.
Amorphous Silica is also used for the drying of gases.
Amorphous Silica is also used as a catalyst and a cutting body of a catalyst, a reinforcing agent for silicone rubber, and a sizing agent used in the textile industry.

CAS Number: 7631-86-9 
EC Number: 231-545-4
E Number: E551 (acidity regulators, ...)
MDL Number: MFCD00011232
Molecular Formula: O2Si
Molar Mass: 60.08 g/mol

SYNONYMS:
Silicon dioxide, Quartz, Silica, Silicic oxide, Silicon(IV) oxide, Crystalline silica, Pure Silica, Silicea, Silica sand, Silica, Quartz, Aerosil, acticel, cab-o-sil, Quartz sand, Fumed silica, cab-o-sperse, Silica, fused, MATTING AGENT, flatting agent, Silicon dioxide, flatting agents, Amorphous silica, Silica, vitreous, delustering agent, Quartz-beta (SiO2) (9CI)

 
Amorphous Silica, also known as silica, is an oxide of silicon with the chemical formula SiO2, commonly found in nature as quartz.
In many parts of the world, Amorphous Silica is the major constituent of sand.
Amorphous 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.
Amorphous Silica  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.

Amorphous Silica is a common fundamental constituent of glass.
Amorphous silica (SiO2⋅nH2O) is also referred to as amorphous opal (opal-A).
Amorphous silica forms in a variety of settings as dissolved silica molecules (SiO2 (aq)) combine (i.e., polymerize) and precipitate.

During diagenesis, amorphous silica precipitates can gradually transform into more stable silica polymorphs and ultimately the mineral quartz.
Dissolved silica necessary to form amorphous silica originates from dissolution of silicate minerals where those minerals are undersaturated.
If those solutions eventually become supersaturated with respect to amorphous silica, microorganisms can help precipitation occur by providing nucleation surfaces.

As an example, numerous studies have investigated potential roles of microorganisms in the formation of siliceous hot-spring deposits known as sinters.
In these systems, high temperature water dissolves silicate minerals in the subsurface.

Following discharge at the surface, the hot spring water cools, decreasing the amount of dissolved silica that it can hold and ultimately causing amorphous silica to become supersaturated.
Thus, abiotic processes drive precipitation of amorphous silica from hot springs.

However, laboratory and field studies indicate that biomass generated by hot spring microorganisms helps nucleate amorphous silica precipitates and also affects the structure of the sinter deposits.
Similar roles of microorganisms have also been observed in non-hydrothermal settings.

In addition to this passive role, amorphous silica precipitation can also be biologically controlled.
Some microorganisms actively take up dissolved silica and use it to form structures composed of amorphous silica.
Examples include diatoms and radiolarians as well as some bacteria and fungi.

Diatoms are a group of algae that live in fresh and marine systems and silicify their cell walls to form structures known as frustules.
Diatoms are considered to be the most important silicifiers in modern marine environments.
Diatoms lower silica concentrations in shallow ocean water and transport it along with other nutrients into the deep ocean as they settle.

Diatom growth also influences the flux of silica from terrestrial environments to the oceans.
In the Amazon River estuary, for example, Milliman and Boyle (1975) estimated that diatoms remove 25% of the dissolved silica from the river water, which is then deposited landward into the river system rather than into the adjacent ocean sediment.

Silica supersaturation in the external environment is not necessary for frustule formation because diatoms can induce supersaturation internally.
They do this by actively pumping dissolved silica from the external environment into a membrane bound vesicle where amorphous silica can become supersaturated and precipitate.

Amorphous Silica, also known as pyrogenic silica, is prepared by burning SiCl4 in an oxygen-rich hydrogen flame to produce a "smoke" of SiO2.
SiCl4+2H2+O2⟶SiO2+4HCl
Amorphous Silica 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.

Amorphous Silica is an ultrafine powder collected as a by-product of the silicon and ferrosilicon alloy production.
Amorphous Silica consists of amorphous (non-crystalline) spherical particles with an average particle diameter of 150 nm, without the branching of the pyrogenic product.

Amorphous silica, a noncrystalline form of SiO2, traditionally called “silica glass” or “vitreous silica,” is important in electronics and photonics because of its excellent electrical and optical properties.
This chapter uses these different names interchangeably to represent all noncrystalline Amorphous Silica.

Amorphous Silica's score is higher if used in product forms that are inhalable (e.g., sprays, powders) because of respiratory concerns.
The score is lower if used in product forms that have lower risk of inhalation (e.g., solids, liquids).
Amorphous Silica's score is higher if used in products that may not meet industry safety guidelines or U.S. and international government requirements.

The score will be lower if used in products that meet these safety guidelines and requirements.
Silica is the most common constituent of sand.
Fine silica, typically used in industrial applications and inhaled by workers, is associated with a wide range of disorders, particularly affecting the lungs and respiratory system.

In cosmetics for skin use, regular sand presents little, if any risk to people.
In contrast, for products that might be inhaled (such as a facial powder), silica particles are finely ground down and may be associated with respiratory toxicity.

Amorphous silica (SiO2) is an inorganic material commonly used in semiconductor circuits to isolate different conducting regions.
Due to its mechanical resistance, high dielectric strength, and selectivity for chemical modification, amorphous silica has also become a key material in microelectronics and chromatography.

Because of its unique properties, Amorphous Silica is quintessential for a broad range of applications: chips, optical fibers, and telescope glasses are manufacture on silica.
Furthermore, molecular biologists employ Amorphous Silica in resins and optical beads to study the biomacromolecules.

In recent years, the synergy between molecular biology and nanotechnology has opened up opportunities for many applications that involve macromolecules and Amorphous Silica, such as nanoelectronics, self-assembly of nanostructures, microfluidics, DNA microarray technology and nanopore sensors.

The picture on the left side shows one of such nanodevices, a MOS nanopore manufactured on a poly-Silicon-Amorphous Silica-Silicon membrane and a single-stranded DNA molecule (blue) translocating through it.
Amorphous Silica has been proposed that nanopores could be used to sequence DNA with single base resolution, leading to a fast and cheap DNA sequencing technology, which promises to have a enormous impact in life sciences and personal medicine.

Therefore, an atomic level understanding of the interactions between biomolecules and Amorphous Silica is now central for further development of bionanotechnology applications.
Currently, no experimental technique is yet sensitive enough to resolve atomic-scale dynamics at the amorphous interface.

Molecular dynamics simulations can be tailored to study those systems, becoming unique imaging tools.
However, modeling systems that combine amorphous silica and biomolecules imposes a variety of challenges to modelers.

Until recently, computer simulations of biomacromolecules and inorganic materials, such as amorphous silica and DNA, have evolved independently from each other, and joining the expertise from both areas is a formidable task.

USES and APPLICATIONS of AMORPHOUS SILICA:
Amorphous Silica is used as a dehumidifying desiccant, a dehydrating agent, a moisture barrier, and an air humidity regulator.
Amorphous Silica is also used for the drying of gases.
Amorphous Silica is also used as a catalyst and a cutting body of a catalyst, a reinforcing agent for silicone rubber, and a sizing agent used in the textile industry.

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 micro porous silica
The Rice Hull contain 18-20% high purity amorphous Silica

One pound of Rice Hulls contains same energy as one pound of Coal.
Colloidal silica is composed of the same substance (silicon dioxide) as quartz sand but it has an amorphous (random) rather than a crystalline structure
Amorphous Silica functions as a high temperature binder in refractories and ceramics.

Amorphous Silica is used as a binder in the investment casting process (lost wax process).
Amorphous Silica polishes silicon and semiconductor wafers to a mirror-like finish.
Amorphous Silica helps to clarify and improve the quality of wines, juices and beer

Amorphous Silica is open-pit mined in the United States and used to produce cristobalite
Amorphous silica is generally less toxic than crystalline silica.
Since Amorphous Silica has greater water solubility, it is cleared more rapidly from the body.

Effects observed after inhalation of amorphous silica were milder than those observed for crystalline silica
Amorphous silica may be added to food as an anti-caking or anti-foaming agent or to pharmaceuticals as an excipient
Fused amorphous silica or quartz (minimum 99.8% SiO2) in the form of ingots, rods, tubes and powder are required for the chemical and electronic industries

The production of fumed amorphous silica from the hydrolysis of silicon tetrachloride2 in a flame of hydrogen and oxygen is for use in rubbers; as a thickening agent in inks, paints, cosmetics, etc.; for use in polyester; and in specialty coatings such as powder coatings
The Amorphous Silica appears as white free flowing powder.

It is highly efficient anti-blocking aid for plastic films.
Amorphous Silica also provides slip improvement.
Due to Amorphous Silica's high purity and low hardness its abrasiveness is negligible.

It's no sedimentation, Amorphous Silica is easy adding and well-dispersed features make this product enjoy the application fields like: Food packing films, Furniture coatings, Decorative paint, Other general industrial coatings
Amorphous Silica is used a masking film and a protective layer for impurity diffusion in transistors and integrated circuits.

Amorphous Silica is used as a filler used in epoxy casting, optical fibers, coatings and other fields.
Amorphous Silica can also be used in the manufacture of glass, emission spectrum analysis reagent, and the control of antimony concentration in the production of antimony in solid state circuit.

Hydrophobic silica is used as a defoamer component.
In its capacity as a refractory, it is useful in fiber form as a high-temperature thermal protection fabric.
Amorphous 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.
Amorphous Silica, when cooled as fused quartz into a glass with no true melting point, can be used as a glass fibre for fibreglass.

The main use of Amorphous Silica is as pozzolanic material for high performance concrete.
Amorphous Silica nanoparticles can be successfully used as an anti-aging agent in asphalt binders.

-Laboratory or special methods
From organosilicon compounds
Many routes to Amorphous Silica start with an organosilicon compound, e.g., HMDSO, TEOS.
Synthesis of silica is illustrated below using tetraethyl orthosilicate (TEOS).

Simply heating TEOS at 680–730 °C results in the oxide:
Si(OC2H5)4⟶SiO2+2O(C2H5)2
Similarly TEOS combusts around 400 °C:
Si(OC2H5)4+12O2⟶SiO2+10H2O+8CO2

TEOS undergoes hydrolysis via the so-called sol-gel process.
The course of the reaction and nature of the product are affected by catalysts, but the idealized equation is:
Si(OC2H5)4+2H2O⟶SiO2+4HOCH2CH3

-Food Additive
Amorphous Silica is a light powder that is often used as a food additive as an anti-caking agent.
An anti-caking agent, Amorphous Silica, is something that could be added to a powder to prevent the development of lumps and caking.
Amorphous Silica is versatile and can also be used as a defoaming agent which reduces the formation of foam and bubbles in industrially processed liquids.

Amorphous Silica can also help control the viscosity or how thick a liquid is and prevent that liquid from settling.
Another one of the Amorphous Silica’s properties is its ability to act as an emulsifying agent.

An emulsifier allows two different substances that wouldn’t normally mix, stay in a homozygous mixture, or where they are uniform and not separating.
Amorphous Silica Gel is also used in other applications due to its useful properties.

These include Amorphous Silica's ability to absorb water and filter water.
Amorphous Silica’s ability to absorb moisture out of the air prompted it to be used as a desiccant in those little bags commonly found in products to protect against excess moisture.

Amorphous Silica pulls water out of the air and stores it protecting the product.
Amorphous Silica can also act partly as a water filter with its unique structure having the ability to absorb some minerals dissolved in water.

Amorphous silica can be used as an excipient, such as viscosifier, suspending agent, tablet disintegrating agent, adsorbent dispersing agent as liquid in powders.
Pharmaceutical excipients, or pharmaceutical auxiliaries, refer to other chemical substances used in the pharmaceutical process other than pharmaceutical ingredients.

Pharmaceutical excipients generally refer to inactive ingredients in pharmaceutical preparations, which can improve the stability, solubility and processability of pharmaceutical preparations.
Pharmaceutical excipients also affect the absorption, distribution, metabolism, and elimination (ADME) processes of co-administered drugs.

-Semiconductors
Amorphous Silica 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, Amorphous Silica has been replaced by other dielectric materials like hafnium oxide or similar with higher dielectric constant compared to Amorphous Silica, 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 Amorphous Silica is a native oxide of silicon it is more widely used compared to other semiconductors like gallium arsenide or indium phosphide.
Amorphous Silica could be grown on a silicon semiconductor surface.

Amorphous Silica 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 Amorphous Silica layer greatly reduces the concentration of electronic states at the silicon surface.
Amorphous Silica films preserve the electrical characteristics of p–n junctions and prevent these electrical characteristics from deteriorating by the gaseous ambient environment.

Amorphous Silica layers could be used to electrically stabilize silicon surfaces.
The surface passivation process is an important method of semiconductor device fabrication that involves coating a silicon wafer with an insulating layer of silicon oxide so that electricity could reliably penetrate to the conducting silicon below.

Growing a layer of Amorphous Silica on top of a silicon wafer enables it to overcome the surface states that otherwise prevent electricity from reaching the semiconducting layer.
The process of silicon surface passivation by thermal oxidation (Amorphous Silica) is critical to the semiconductor industry.

Amorphous Silica is commonly used to manufacture metal–oxide–semiconductor field-effect transistors (MOSFETs) and silicon integrated circuit chips (with the planar process).

-Structural use of Amorphous Silica:
About 95% of the commercial use of Amorphous Silica (sand) is in the construction industry, e.g. in the production of concrete (Portland cement concrete).

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.

The high melting point of silica enables Amorphous Silica 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:
Amorphous 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 Amorphous Silica is about 1475 K.
When molten Amorphous Silica 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.
Amorphous Silica is a primary raw material for many ceramics such as earthenware, stoneware, and porcelain.

Amorphous Silica is used to produce elemental silicon.
The process involves carbothermic reduction in an electric arc furnace:
SiO2+2C⟶Si+2CO

-Food, cosmetic, and pharmaceutical applications of Amorphous Silica:
Amorphous Silica, either colloidal, precipitated, or pyrogenic fumed, is a common additive in food production.
Amorphous Silica 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.

Amorphous Silica 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.
In cosmetics, Amorphous Silica is useful for its light-diffusing properties and natural absorbency.

Diatomaceous earth, a mined product, Amorphous Silica has been used in food and cosmetics for centuries.
Amorphous Silica 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.

PHYSICAL AND CHEMICAL PROPERTIES of AMORPHOUS SILICA:
*Glass-like or translucent coarse particles.
*The specific surface area is 450m2/g or more, and is a highly active, renewable, microporous structure and high thermal stability material.
*Amorphous Silica has a strong adsorption capacity for liquid and gas phase substances.
*The hardness was slightly softer than that of glass.
*In addition to hydrofluoric acid and strong alkali, insoluble in other chemical solvents.

STRUCTURE of AMORPHOUS SILICA:
In the majority of Amorphous Silicas, 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.

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.
Based on the crystal structural differences, Amorphous Silica can be divided into two categories: crystalline and non-crystalline (amorphous).

In crystalline form, Amorphous Silica 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.
All of these distinct crystalline forms always have the same local structure around Si and O.
In α-quartz the Si–O 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°

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.
The high-pressure minerals, seifertite, stishovite, and coesite, though, have higher densities and indices of refraction than quartz.

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.

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.
The change in the coordination increases the ionicity of the Si–O bond.

Faujasite silica, another polymorph, is obtained by the dealumination of a low-sodium, ultra-stable Y zeolite with combined acid and thermal treatment.

The resulting product contains over 99% silica, and has high crystallinity and specific surface area (over 800 m2/g).
Faujasite-silica has very high thermal and acid stability.
For example, it maintains a high degree of long-range molecular order or crystallinity even after boiling in concentrated hydrochloric acid.

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.
Its density decreases from 2.08 g/cm3 at 1950 °C to 2.03 g/cm3 at 2200 °C.

MOLECULAR SIO2
The molecular SiO2 has a linear structure like CO2.
Amorphous Silica has been produced by combining silicon monoxide (SiO) with oxygen in an argon matrix.

The dimeric Amorphous Silica, (SiO2)2 has been obtained by reacting O2 with matrix isolated dimeric silicon monoxide, (Si2O2).
In dimeric Amorphous Silica 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.

OTHER METHODS of AMORPHOUS SILICA:
Being highly stable, Amorphous Silica arises from many methods.
Conceptually simple, but of little practical value, combustion of silane gives Amorphous Silica.

This reaction is analogous to the combustion of methane:
SiH4+2O2⟶SiO2+2H2O
However the chemical vapor deposition of Amorphous Silica onto crystal surface from silane had been used using nitrogen as a carrier gas at 200–500 °C.

CHEMICAL REACTIONS of AMORPHOUS SILICA:
Amorphous Silica is a relatively inert material (hence its widespread occurrence as a mineral).
Amorphous Silica is often used as inert containers for chemical reactions.
At high temperatures, Amorphous Silica is converted to silicon by reduction with carbon.

Fluorine reacts with Amorphous Silica to form SiF4 and O2 whereas the other halogen gases (Cl2, Br2, I2) are unreactive.
Most forms of Amorphous Silica are attacked ("etched") by hydrofluoric acid (HF) to produce hexafluorosilicic acid:
SiO2 + 6 HF → H2SiF6 + 2 H2O
Stishovite does not react to HF to any significant degree.

HF is used to remove or pattern Amorphous Silica in the semiconductor industry.
Amorphous Silica acts as a Lux–Flood acid, being able to react with bases under certain conditions.
As Amorphous Silica does not contain any hydrogen, non-hydrated silica cannot directly act as a Brønsted–Lowry acid.

While Amorphous Silica is only poorly soluble in water at low or neutral pH (typically, 2 × 10−4 M for quartz up to 10−3 M for cryptocrystalline chalcedony), strong bases react with glass and easily dissolve it.
Therefore, strong bases have to be stored in plastic bottles to avoid jamming the bottle cap, to preserve the integrity of the recipient, and to avoid undesirable contamination by silicate anions.

Amorphous Silica dissolves in hot concentrated alkali or fused hydroxide, as described in this idealized equation:
SiO2+2NaOH⟶Na2SiO3+H2O
Amorphous Silica will neutralise 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 Amorphous Silica can produce sodium orthosilicate, sodium silicate, and glasses, dependent on the proportions of reactants:

2Na2O+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, Amorphous Silica is termed the network former or lattice former.
The reaction is also used in blast furnaces to remove sand impurities in the ore by neutralisation with calcium oxide, forming calcium silicate slag.

Amorphous Silica reacts in heated reflux under dinitrogen with ethylene glycol and an alkali metal base to produce highly reactive, pentacoordinate silicates which provide access to a wide variety of new silicon compounds.

The silicates are essentially insoluble in all polar solvent except methanol.
Amorphous Silica reacts with elemental silicon at high temperatures to produce SiO:
SiO2+Si⟶2SiO

WATER SOLUBILITY of AMORPHOUS SILICA:
The solubility of Amorphous Silica in water strongly depends on its crystalline form and is three to four times higher for amorphous silica than quartz; as a function of temperature, it peaks around 340 °C (644 °F).

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 that is cooler at the top.
Crystals of 0.5–1 kg can be grown for 1–2 months.

These crystals are a source of very pure quartz for use in electronic applications.
Above the critical temperature of water 647.096 K (373.946 °C; 705.103 °F) and a pressure of 22.064 megapascals (3,200.1 psi) or higher, water is a supercritical fluid and solubility is once again higher than at lower temperatures.

NATURAL OCCURRENCE of AMORPHOUS SILICA:
Amorphous Silica is most commonly encountered in nature as quartz, which comprises more than 10% by mass of the Earth's crust.
Quartz is the only polymorph of silica stable at the Earth's surface.

Metastable occurrences of the high-pressure forms coesite and stishovite have been found around impact structures and associated with eclogites formed during ultra-high-pressure metamorphism.

The high-temperature forms of tridymite and cristobalite are known from silica-rich volcanic rocks.
In many parts of the world, silica is the major constituent of sand.

BIOLOGY of AMORPHOUS SILICA:
Even though Amorphous Silica is poorly soluble, silica occurs in many plants such as rice.
Plant materials with high silica phytolith content appear to be of importance to grazing animals, from chewing insects to ungulates.

Silica accelerates tooth wear, and high levels of silica in plants frequently eaten by insects may have developed as a defense mechanism against predation.
Silica is also the primary component of rice husk ash, which is used, for example, in filtration and as supplementary cementitious material (SCM) in cement and concrete manufacturing.

Silicification in and by cells has been common in the biological world and it occurs in bacteria, protists, plants, and animals (invertebrates and vertebrates).
Prominent examples include:

Tests or frustules (i.e. shells) of diatoms, Radiolaria, and testate amoebae.
Silica phytoliths in the cells of many plants including Equisetaceae, many grasses, and a wide range of dicotyledons.
The spicules forming the skeleton of many sponges.

NATURE of AMORPHOUS SILICA:
Amorphous Silica is a colorless transparent crystal or amorphous powder and is tasteless.
The melting point of Amorphous Silica is 1710 ° C. (cristobalite),1670 ° C. (titrimetric quartz), and the boiling point is 2230 ° C.

Amorphous Silica is almost insoluble in water, common acid, can be dissolved in hydrofluoric acid to generate silicon fluoride gas, slowly with the heat of concentrated phosphoric acid.

Amorphous Silica can function with a base.
The physical and chemical properties of Amorphous Silica are stable, easy to form, inert, the melt is layered, and the expansion coefficient is small when heated.

PREPARATION METHOD of AMORPHOUS SILICA:
using tetraethyl orthosilicate as a raw material, tetraethyl orthosilicate was first subjected to high-efficiency rectification, and a fraction at 160-168 ° C. Was collected.

Then, the tetraethyl orthosilicate and ammonia are mixed and heated in a certain proportion, and the reaction mixture is sufficiently stirred, and the reactant will change from turbid to viscous until the solution is boiled, the stirring is stopped, and the solution is left to settle, after centrifugation, Amorphous Silica was dried at low temperature for a certain period of time, then burned at 900 ° C. 
And cooled naturally.

STANDARD of AMORPHOUS SILICA:
This strain reacts sodium silicate with acid (such as hydrochloric acid, sulfuric acid, phosphoric acid, etc.) or with salt (such as ammonium chloride, ammonium sulfate, ammonium bicarbonate, etc.), silicic acid precipitates (I. E., hydrated silica) are produced by washing with water, removing impurities, and drying.
The content & 0 2 shall not be less than 99.0% based on the calculation of the ignition product.

TRAIT of AMORPHOUS SILICA:
Amorphous Silica is a white loose powder; Odorless, tasteless.
Amorphous Silica is insoluble in water, dissolved in hot sodium hydroxide solution, insoluble in dilute hydrochloric acid.

PRODUCTION of AMORPHOUS SILICA:
Amorphous Silica is mostly obtained by mining, including sand mining and purification of quartz.
Quartz is suitable for many purposes, while chemical processing is required to make a purer or otherwise more suitable (e.g. more reactive or fine-grained) product.

*Precipitated silica
Precipitated silica or amorphous silica is produced by the acidification of solutions of sodium silicate.
The gelatinous precipitate or silica gel, is first washed and then dehydrated to produce colorless microporous silica.
The idealized equation involving a trisilicate and sulfuric acid is:

Na2Si3O7+H2SO4⟶3SiO2+Na2SO4+H2O
Approximately one billion kilograms/year (1999) of silica were produced in this manner, mainly for use for polymer composites – tires and shoe soles.

*On microchips
Thin films of silica grow spontaneously on silicon wafers via thermal oxidation, producing a very shallow layer of about 1 nm or 10 Å of so-called native oxide.
Higher temperatures and alternative environments are used to grow well-controlled layers of Amorphous Silica on silicon, for example at temperatures between 600 and 1200 °C, using so-called dry oxidation with O2

Si+O2⟶SiO2
or wet oxidation with H2O.

Si+2H2O⟶SiO2+2H2
The native oxide layer is beneficial in microelectronics, where it acts as electric insulator with high chemical stability.
It can protect the silicon, store charge, block current, and even act as a controlled pathway to limit current flow.

PHYSICAL and CHEMICAL PROPERTIES of AMORPHOUS SILICA:
Chemical Formula: SiO₂
Molar Mass: 60.08 g/mol
Appearance: Transparent or white; colorless, odorless solid
Density: 2.648 g/cm³ (α-quartz), 2.196 g/cm³ (amorphous), 2.2 g/cm³
Melting Point: 1,713 °C (3,115 °F; 1,986 K) (amorphous): 4.88 ; 1,610 °C
Boiling Point: 2,950 °C (5,340 °F; 3,220 K); 2,230 °C
Magnetic Susceptibility (χ): −29.6·10⁻⁶ cm³/mol
Thermal Conductivity: 12 W/(m·K) (‖ c-axis), 6.8 W/(m·K) (⊥ c-axis), 
1.4 W/(m·K) (amorphous): 12.213 

Refractive Index (nD): 1.544 (o), 1.553 (e): 4.143 
Physical State: Solid
Color: White
Odor: No data available
Melting Point/Freezing Point: No data available
Initial Boiling Point and Boiling Range: 2,230 °C
Flammability (solid, gas): The product is not flammable.
Upper/Lower Flammability or Explosive Limits: No data available

Flash Point: Not applicable
Autoignition Temperature: No data available
Decomposition Temperature: No data available
pH: No data available
Viscosity:
– Viscosity, kinematic: No data available
– Viscosity, dynamic: No data available

Water Solubility: Insoluble; no data available
Partition Coefficient (n-octanol/water): No data available
Vapor Pressure: No data available
Density: No data available
Relative Density: No data available
Relative Vapor Density: No data available
Particle Characteristics: No data available
Explosive Properties: No data available

Oxidizing Properties: None
Other Safety Information:
– Bulk Density: ca. 200–800 kg/m³
– Particle Size: < 2 mm
CAS Numbers: 14808-60-7, 112945-52-5, 60676-86-0, 7631-86-9, 99439-28-8
EINECS Numbers: 215-683-2, 238-878-4, 262-373-8, 231-545-4
InChI: InChI=1/O2Si/c1-3-2
Molecular Formula: O₂Si
Storage Condition: Room temperature
MDL Number: MFCD00011232
Compound Canonicalized: Yes

FIRST AID MEASURES of AMORPHOUS SILICA:
-Description of first-aid measures
*General advice:
Show this material safety data sheet to the doctor in attendance.
*If inhaled:
After inhalation: 
Fresh air.
*In case of skin contact: 
Take off immediately all contaminated clothing. 
Rinse skin with
water/ shower.
*In case of eye contact:
After eye contact: 
Rinse out with plenty of water. 
Call in ophthalmologist. 
Remove contact lenses.
*If swallowed:
After swallowing: 
Immediately make victim drink water (two glasses at most). 
Consult a physician.
-Indication of any immediate medical attention and special treatment needed.
No data available

ACCIDENTAL RELEASE MEASURES of AMORPHOUS SILICA:
-Environmental precautions:
Do not let product enter drains.
-Methods and materials for containment and cleaning up:
Cover drains. 
Collect, bind, and pump off spills. 
Observe possible material restrictions. 
Take up dry. 
Dispose of properly. 
Clean up affected area.

FIRE FIGHTING MEASURES of AMORPHOUS SILICA:
-Extinguishing media:
*Suitable extinguishing media:
Carbon dioxide (CO2) 
Foam 
Dry powder
*Unsuitable extinguishing media:
For this substance/mixture no limitations of extinguishing agents are given.
-Further information:
Prevent fire extinguishing water from contaminating surface water or the ground water system.

EXPOSURE CONTROLS/PERSONAL PROTECTION of AMORPHOUS SILICA:
-Control parameters:
--Ingredients with workplace control parameters:
-Exposure controls:
--Personal protective equipment:
*Eye/face protection:
Use equipment for eye protection. 
Safety glasses
*Body Protection:
protective clothing
*Respiratory protection:
Recommended Filter type: Filter A 
-Control of environmental exposure:
Do not let product enter drains.

HANDLING and STORAGE of AMORPHOUS SILICA:
-Conditions for safe storage, including any incompatibilities:
*Storage conditions:
Tightly closed. 
Dry.

 
STABILITY and REACTIVITY of AMORPHOUS SILICA:
-Chemical stability:
The product is chemically stable under standard ambient conditions (room temperature).
-Possibility of hazardous reactions:
No data available