Silica – Silicon Dioxide (SiO2)

This article was updated on the 11th September 2019.

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Chemical Formula

SiO2

Background

Silica 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 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 and cristobalite) and each has two variations (high and low).

Fused Silica

Fused silica is a high purity grade, containing 99.4-99.9% SiO2. It is produced by carbon arc, plasma arc, gas-fired continual extrusion or carbon electrode fusion. Fused silica is primarily used in the electronics industry where its dielectric and insulating properties are exploited. Fused silica can also be used as a refractory material or in investment casting, and it is used to make windows for manned spacecraft due to its strength.

Key Properties

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).

Fused silica and quartz are both high-performance materials used in many industries due to their low coefficient of expansion, high-temperature resistance and optical transmission properties.

Table 1. Physical, mechanical, thermal and electrical properties of quartz and fused silica.

Material

Quartz

Fused silica

Density (g/cm3)

2.65

2.2

Thermal conductivity (Wm-1 K)

1.3

1.4

Thermal expansion coeff. (10-6 K-1)

12.3

0.4

Tensile strength (MPa)

55

110

Compressive strength (MPa)

2070

690-1380

Poisson's ratio

0.17

0.165

Fracture toughness (MPa)

-

0.79

Melting point (°C)

1830

1830

Modulus of elasticity (GPa)

70

73

Thermal shock resistance

Excellent

Excellent

Permittivity (ε') *

3.8-5.4

3.8

Tan (δ x 104) *

3

Loss factor (ε'') *

0.0015

Dielectric field strength (kV/mm) *

15.0-25.0

15.0-40.0

Resistivity (Ωm) *

1012-1016

>1018

* Dielectric Properties at 1 MHz 25°C

Table 2. Differences between the different crystal structures of silica.

Crystal Structure

Density (g/cm3)

Thermal expansion (10-6 K-1)

Quartz

2.65

12.3

Tridymite

2.3

21

Cristobalite

2.2

10.3

Applications

Silica is a fairly widely used ceramic material in a number of industries, including the fabrication of other ceramic products.

Silicon Production

Most silicon is produced as a ferroalloy, either ferrosilicon or silicon manganese, which is used exclusively in the manufacture of steel. Silicon as a ferroalloy is the most important deoxidizer in steel manufacturing.  Semiconductor silicon is made mostly by reacting powdered crude metal with a gaseous mixture of hydrogen and hydrogen chloride in a fluidized bed. The main product SiHCl3 is fractionally distilled then reduced by hydrogen. This reduction is then deposited on a silicon filament, which is heated to about 1150°C. Further purification may be achieved by zone refining, if required. Dopants are generally added subsequent to crystal growth.

Feedstock for silicon production is >98.5-99% SiO2, <0.1%Fe2O3 and <0.15% Al2O3.

Electronics Industry

  • Circuit boards

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.

  • Semiconductors

Semiconductors are materials with electrical conductivity between insulating and conductive, making them especially useful when applied as the diodes and transistors which are building blocks of the information age. The electrical properties of these materials are incredibly sensitive to the presence of impurities. Silicon is an intrinsic semiconductor, which means that its electrical behavior is based on its inherent electronic structure. Silicon semiconductors are used in the integrated circuits which give computing ability to cell phones, laptops and other smart technology.

  • Piezoelectrics

Piezoelectricity is a property of a material where polarization 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, which has a complicated crystal structure with a low degree of symmetry, has this property.

Piezoelectric materials like quartz are used in transducers such as phonograph pickups, ultrasonic generators, buzzers, alarms, strain gauges and igniters in cookers.

Refractory Materials

Silica has a considerably low thermal expansion, a fairly high melting point and is resistant to creep. These properties make it a good refractory material. On its own, it is typically used in acid environments due to its insolubility in most acids. Silica is classified as an acid refractory as it behaves like an acid at high temperatures by reacting with bases. It is also a common starting material for the synthesis of other refractory products. The majority of refractory applications for silica are in the glass industry.

  • Glass Refractories

Silica is an important refractory material in the production of glass. Crystallized silica, which is composed of pure crystalline quartz, is one commonly used silica type for glass refractories. This material is crushed, graded and pressed into bricks. Upon heating up to high temperatures the quartz will transform to tridymite and cristobalite. Tridymite bricks tend to be the most favorable of all crystalline forms of silica, as they have a smooth, predictable and low thermal expansion up to 600°C. Once beyond this temperature, 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.

Fused silica is also used in glass furnaces where the glass is in direct contact with the refractory. This is because of its good resistance to thermal shock and the fact that it poses no risk of contamination to the glass.

  • Production of Refractory Materials

Silica sand can be reacted with aluminum during production using the Bayer process 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.

Sand

There are numerous types of sand which have various applications. The principal mineral phase in them all is quartz. They are classified by 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 Sand

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

Foundry sand is produced at a 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 addition to this, a refractory grade is produced for the manufacture of refractory materials which can be slightly lower in purity (>95%SiO2).

  • Flux Sand

Flux sand for iron and steel making is >90%SiO2.

Building Materials

Consolidated sandstone is fairly resilient which favors its use in the construction industry. Sandstone is crushed and graded for use as an aggregate, or if it is aesthetically attractive it may be cut into slabs for building (dimension stone). The use of silica aggregate is by far silica’s biggest use in the construction industry.

Glass Production

Silica is used as a raw material feedstock for the production of glass. 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 glasses for cooking such as Pyrex. High-grade fused silica may be used on its own for the production of glass where high thermal stability and shock resistance is required (as in manned spacecraft).

The majority of domestic glass tends to be made using the float glass method. Here, a ribbon of molten glass is fed across a bath of liquid tin in a controlled atmosphere. The Pilkington Brothers first developed this process for glass production in the 1950s in Lancashire, UK.

Investment Casting

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 mold. Finally, the fused silica shell is knocked out. Fused silica makes this knocking out stage easier than other refractories such as zirconia and aluminosilicates.

Filler Material

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

Silica fume, or micro-silica, consists of amorphous silicon dioxide. It is a byproduct of silicon metal or ferrosilicon alloy production. 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. The size of each particle of silica fume is approximately 1% 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.

Source: Lucideon
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