Properties of fused silica

At first glance, quartz glass appears very simple, both chemically and structurally, since it is made from a single oxide component (silicon dioxide – SiO₂).

Chemical structure

Silica, as it is also known, is found throughout the earth’s crust. However, only a small fraction has sufficient purity (> 99.98 %) to be suitable as raw material for quartz glass. Sand at the beach is also SiO₂, but isn’t suitable for use in the semiconductor industry.

Structure of quartz and fused silica

In the quartz glass structure all atoms are bonded to at least two others. The strength of the silicon-oxygen (Si-O) chemical bond gives quartz glass high temperature stability and chemical resistance. But the structure is also rather open with wide spaces (interstices) between the structural units. This accounts for higher gas permeability and a much lower thermal expansion coefficient for quartz glass relative to other materials.

Synthetic silica produced by flame hydrolysis of silicon tetrachloride can have high (> 1000 ppm) or very low hydroxyl content depending on whether a hot chlorination step is employed to remove it. The hydroxyl level can be so high because the silica particles that result from the hydrolysis reaction are extremely fine and therefore have tremendous surface area capable of absorbing moisture present in the flame.

The main attributes of electrically fused materials are the low hydroxyl content and reduced devitrification rates. The low hydroxyl content increases infrared transparency and viscosity. The higher viscosity results in an increased maximum use temperature as well as helping to inhibit devitrification. Devitrification is also restrained by the neutral/ reducing atmosphere used during melting. This causes the material to be slightly oxygen deficient, which helps to restrain devitrification.

The high resistance of quartz glass to elements and compounds is another advantage for high-end applications. Fused quartz is outstandingly resistant to water, salt solutions and acids. Only hydrofluoric and phosphoric acid can attack it. Metals which are free from oxide, with the exception of alkalis and alkaline-earths, do not react with fused quartz or fused silica.

Quartz glass is sensitive to all alkali and alkaline-earth compounds because even slight traces hasten devitrification at high temperatures. It is always advisable to remove fingerprints, which contain traces of alkalis, from quartz glass with alcohol before heating.

One of the most attractive features of quartz glass is its very low thermal coefficient of expansion (CTE). The average CTE value for quartz glass at about 5.0 × 10 ^-7/ °C is many times lower than that of other common materials. To put this in perspective, imagine if 1 m³ blocks of stainless steel, borosilicate glass and quartz were placed in a furnace and heated by 500 °C. The volume of the stainless steel block would increase by more than 28 liters and that of the borosilicate block by 5 liters. The quartz block would expand by less than one liter. Such low expansion makes it possible for the material to withstand very severe thermal shock.

It is possible to rapidly quench thin particles of quartz glass from over 1000 °C by plunging them into cold water without breakage. However, it is important to realize that the thermal shock resistance depends on factors other than CTE such as surface condition (which defines strength) and geometry. The various types of fused silica and fused quartz have nearly identical CTE’s and thus can be joined together with no added risk of thermally induced breakage.

Mechanical properties, strength and reliability

The theoretical tensile strength of silica glass is greater than 1 million psi. Unfortunately, the strength observed in practice is always far below this value. The reason is that the practical strength of glass is extrinsically determined rather than being solely a result of an intrinsic property like density. It is the surface quality in combination with design considerations and chemical effects of the atmosphere (water vapor in particular) that ultimately determine the strength and reliability of a finished piece of quartz glass. Because of stress concentration on surface flaws, failure most always occurs in tension rather than compression.

In other words: „reliablility depends on the chance“.

This could also be stated as the probability that the piece will experience a mechanical stress greater than the strength of any existing flaws. As a result of this dependence on probability, reliability decreases as the size of the glass article increases. Similarly, if the number of pieces in service increases, so does the chance of experience a failure.

Surface condition is very important. For example, machined surfaces tend to be weaker than fire polished ones. Also, older surfaces are usually weaker than younger ones due to exposure to dust, moisture or general wear and tear. These factors have to be considered thoroughly when comparing the strengths of different “brands” of quartz glass.

This is because these tests in reality often turn out to be just comparisons of surface quality resulting from sample preparation, small differences in which easily overwhelm any differences in intrinsic strength.

Controlled heat management and sustained high temperatures is crucial in many industrial processes, especially in semiconductor industries.

Fused silica is a good electrical insulator, retaining high resistivity at elevated temperatures and excellent high-frequency characteristics. The large band gap inherent in the electronic structure of the silicon-oxygen bond results in electrical conduction being limited to current carried by mobile ionic impurities. Since the level of these impurities is very low, the electrical resistivity is correspondingly high.

Since ionic conduction is related to the diffusion coefficient of the ionic carriers, the resistivity also has a strong exponential temperature dependence. Hence, unlike typical conductors such as metals, the resistivity decreases with increasing temperature.

The dielectric constant of quartz glass has a value of about 4 which is significantly lower than that of other glasses. This value changes little over a wide range of frequencies. The reason for the low dielectric constant is, once again, the lack of highly charged mobile ions but it also results from the stiffness of the silicon-oxygen network which imparts a very low polarizability to the structure.

Because it has very low absorption down into the vacuum ultraviolet spectral range (the cutoff is at about 160 nm for a 1 mm thickness), synthetic fused silica is used for optical lenses in high-energy laser applications and as envelope material for ultraviolet light sources such as excimer or deuterium lamps. Depending on the exact experimental conditions, such as the wavelength, energy density and peak intensities in pulsed laser applications, various kinds of damage can occur to the glass.

At very high laser intensities, photoionization and plasma generation can take place locally at certain positions in the glass. This mechanical damage typically appears at the front or rear surface of the optical element. A related type of mechanical damage which can occur is the generation of fine microchannels within the glass along the propagation direction of a laser beam.

In addition to these visible damage phenomena, a more subtle damage mechanism can take place as defect centers (sometime called color centers) are generated in a photochemical process under irradiation of the glass. These centers cause absorption at characteristic wavelengths. Examples are the E’ center with an absorption maximum at 215 nm and the non-bridging oxygen hole (NBOH) center at 265 nm. The NBOH hole also emits red fluorescence at about 650 nm when excited in its absorption band. These defects also interact with dissolved hydrogen in the glass. Hydrogen can passivate E’ centers to create SiH groups, and NBOH centers to create SiOH groups, thus mitigating the transmission loss at the absorption wavelengths of these defects. Therefore, the hydrogen concentration is often controlled precisely during production processes, and measured by Raman spectroscopy in the analytical lab.

A third type of damage which can take place is a change in the refractive index of the silica glass due to restructuring of the glass network under irradiation. The refractive index can either increase (called compaction), or it can decrease (rarefaction), depending on the silica glass type and the irradiation conditions.