Silica (SiO2) occurs commonly in nature as sandstone, silica sand or quartzite. It is the starting material for the production of silicate glasses and ceramics. Silica is one of the most abundant oxide materials in the earth's crust. It can exist in an amorphous form (vitreous silica) or in a variety of crystalline forms. Often it will occur as a non-crystalline oxidation product on the surface of silicon or silicon compounds.
Crystalline Forms of Silica
There are three crystalline forms of silica; quartz, tridymite, cristobalite and there are two variations of each of these (high and low.)
A high purity grade of silica, fused silica (which is around 99.4-99.9% SiO2) is produced by carbon arc, plasma arc, gas fired continual extrusion or carbon electrode fusion. Fused is primarily used in the electronics industry (around 70% of the total market) where its good dielectric and insulating properties are exploited; though it may also be used as a refractory material or in investment casting.
Silica is a group IV metal oxide, which has good abrasion resistance, electrical insulation and high thermal stability. It is insoluble in all acids with the exception of hydrogen fluoride (HF).
Table 1. Physical, mechanical, thermal and electrical properties of quartz and fused silica.
|Thermal conductivity (Wm-1 K)
|Thermal expansion coeff. (10-6 K-1)
|Tensile strength (MPa)
|Compressive strength (MPa)
|Fracture toughness (MPa)
|Melting point (°C)
|Modulus of elasticity (GPa)
|Thermal shock resistance
|Permittivity (ε') *
|Tan (δ x 104) *
|Loss factor (ε'') *
|Dielectric field strength (kV/mm) *
|Resistivity (Ωm) *
* Dielectric Properties at 1 MHz 25 °C
Table 2. Differences between the different crystal structures of silica.
||Thermal expansion (10-6 K-1)
Silica is a fairly widely used ceramic material both as a precursor to the fabrication of other ceramic products and as a material on its own.
Most silicon is produced as a ferroalloy either ferrosilicon or silicon manganese, which is used exclusively in steel making. Silicon, as ferroalloy, is the most important deoxidiser in steel making. Semiconductor silicon is made mostly by reacting powdered crude metal with a gaseous mixture of hydrogen and hydrogen chloride in a fluidised bed. The main product SiHCl3 is fractionally distilled then reduced by hydrogen and is then deposited on a pre silicon filament, which is heated to about 1150 °C. Further purification may be done by zone refining if required. Dopants are generally added subsequent to crystal growth.
Feedstock for silicon production >98.5-99% SiO2, <0.1%Fe2O3 and <0.15% Al2O3.
Fused silica has extremely good dielectric and insulating properties. For these reasons it is used as an inert, low expansion filler material for epoxy resins in electronic circuits.
The electrical conductivity of the semiconductors is not as high as that of metals; nevertheless, they have unique electrical characteristics that render them especially useful. The electrical properties of these materials are incredibly sensitive to the presence of impurities. Silicon is an intrinsic semiconductor, which means that its electrical behaviour is based on its inherent electronic structure. Silicon semiconductors are used in integrated circuits applications.
Piezoelectricity is a property of a material where polarisation is induced and an electric field is established across a specimen by the application of an external force. Reversing the direction of the external force (i.e. tension to compression) reverses the direction of the electric field. Quartz has this property as it has a complicated crystal structure with a low degree of symmetry.
Piezoelectric materials such as quartz are used in transducers such as phonograph pickups, ultrasonic generators, buzzers, alarms, strain gauges and igniters in cookers.
Silica has considerably low thermal expansion, a fairly high melting point and is resistant to creep making it a good refractory material. It tends to be used in acid environments if used on its own or used as a starting material for the synthesis of other refractory products. Due to the fact that silica is insoluble in the majority of acids, it is used as a refractory material in acidic environments. Silica is classified as an acid refractory as it behaves like an acid at high temperatures reacting with bases. The majority of its applications are in the glass industry.
Silica is an important material for use as a refractory in the production of glass. There are essentially two types of silica commonly used in glass refractories. The first is crystallised silica, which is composed of pure crystalline quartz. This material is crushed, graded and pressed into bricks. Upon heating up to high temperatures the quartz will transform to tridymite and cristobalite. In bricks tridymite tends to be the most favourable of all crystalline forms of silica, as it has a smooth, predictable and low thermal expansion up to 600 °C. Once beyond this temperature its thermal expansion is practically zero. Bricks of this type are used in the melter crown of the glass furnace as they provide good resistance to creep and good mechanical strength at the operating temperature.
Also in glass furnaces where the glass is in direct contact with the refractory, fused silica is used because of its good resistance to thermal shock and there is no risk of contamination of the glass.
Production of Refractory Materials
Silica sand can be reacted with Bayer alumina to form synthetic mullite refractory bricks, which are used in furnace linings. Mullite imparts good refractoriness and has excellent creep resistance. Mullite can be used to line blast, copper roasting and sections of blast furnaces. The high shock resistant refractory material silicon carbide is produced by the reaction of silica sand with coke. Silica is also used in the production of AZS by fusion casting, which used the glass industry.
There are numerous types of sands that exist which have various applications. The principal mineral phase in them all is quartz. They are classified on the level of other oxides present and their respective particle size distributions. Depending on the nature of the application sand is subjected to various treatments to produce the desired grade.
Ceramic grade sand is less than 75μm and has a silica content above 97.5%, impurities include <0.55% Al2O3 and <0.2 Fe2O3. It is used in the production of glazes and ceramic materials.
Foundry sand is produced at particle size of less than 75μm. It has a silica content of 98% with limits placed upon on the amounts of magnesia (MgO) and lime (CaO) present. In additional to this a refractory grade is produced for the manufacture of refractory materials which can be slightly lower in purity (>95%SiO2).
Flux sand for iron and steel making >90%SiO2.
Consolidated sandstone is fairly resilient which favours its use in the construction industry. Sandstone is both crushed and graded for use as an aggregate or alternatively if it is aesthetically attractive it may be cut into slabs for building (dimension stone.) The use of silica aggregate is by far it biggest use in the construction industry.
Silica is used as a raw material feedstock for the production of glass where it is mixed with lime and soda to produce domestic glass for windows, bottles, jars, light bulbs and plate glass. Alternatively it may be mixed with boron oxide and soda to produce thermal shock resistant glass for cooking such as Pyrex. High-grade fused silica may be used on its own for the production of glass where a high thermal stability and shock resistance is required such as in the windows of the NASA Shuttle.
The majority of domestic glass tends to be made via the float glass method whereby a ribbon of molten glass is fed across a bath of liquid tin in a controlled atmosphere. The Pilkington Brothers in the UK first developed this process for glass production.
Fused silica is used in the refractory casting slurry, where it coats a wax replica of the component to be cast. The refractory stucco is fired and molten metal is poured into the mould. Finally, the fused silica shell is knocked out. Fused silica makes this knock out stage easier than other refractories such as zirconia and aluminosilicates.
The grindability of silica to specific particle size distributions facilitates its use as a filler material to bulk out products. Silica is commonly used as a filler in paints, plastics, rubber, adhesives, putty and sealants.
Silica fume or microsilica is a by-product of producing silicon metal or ferrosilicon alloys. One of the most beneficial uses for silica fume is in concrete. Due to its chemical and physical properties, it is a very reactive pozzolan. Concrete that contains silica fume can have very high strength and be very durable. Silica fume consists of amorphous silicon dioxide. Each individual particle of silica fume is very small in the order of 1/100th the size of a cement particle.
Silica fume is also used as a filler material in refractory concretes, where its function is to improve the particle packing of the product increasing strength and reducing porosity.
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