About quartz glass, material properties

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The production method of fused quartz glass can be:

electrically fused, flame fused, plasma fused and electric-arc fused. The raw material can be natural quartz sand SiO2 and synthetic chemical-silicon tetrachloride(SiCl4). The combination of production methods and raw material composes the main 6 types of our quartz glass: EN (electrically fused natural quartz) series; FN (flame fused natural quartz) series; PN (plasma fused natural quartz) series; FS (flame fused synthetic quartz) series; PS (plasma fused synthetic quartz) series; AN (electric-arc fused natural quartz) series.

Electrically Fused Quartz Glass

Electric fusion is the most commonly used melting process for manufacturing quartz glass. This process uses resistance Heaters to melt natural quartz sand into quartz glass tubes, rods, blocks etc.. There are two methods of electric fusion, continuous fusion and batch fusion. In the continuous process, quartz sand is continuously poured into the upper part of a tungsten crucible surrounded by electric heating elements. The inside crucible is maintained in a neutral or slightly reduced atmosphere that keeps silica from reacting with the refractory metal while the silica is melted inside. The melted quartz exits at the forming part at the bottom of crucible which is shaped into tube, rod and so on. The OH can be reduced by annealing. In the batch fusion process, several tons of raw material are placed inside a refractory vacuum chamber containing graphite heating elements to produce tubes, rods and so on. The OH is usually below 2ppm. The low OH makes electrically fused quartz glass has a high infrared transmission. The electrically fused quartz glass usually has certain bubbles and drawing lines on the glass surface.

Flame Fused Quartz Glass

Flame fusion is a 2-step process. The first process is the natural quartz sand or synthetic SiC14 is melted in hydrogen/oxygen (H2/O2) flame into a solid round ingot. The second process is to further shape the ingot into various shape and dimension. Flame fused quartz glass contains a abundant amount of hydroxyl (OH) as a result of the direct contact between the H2/O2 and the silica raw material. This OH content can not be reduced by annealing and its presence lowers the viscosity and infrared transmission. The flame fused quartz has no drawing lines and very low bubble content.

Plasma Fused Quartz Glass

Plasma fusion is a 2-step process. The first process is to melt the natural quartz sand or synthetic SiCl4 in a plasma flame into a solid round ingot. The second process is to further shape the ingot into various shapes and dimensions. Plasma fused quartz has low OH usually below 5ppm, so it has good infrared transmission. The plasma fused quartz has very low bubble content and has no drawing line.

Electric-arc Fused Opaque Crucibles

Electric-arc fusion is the commonly used process for manufacturing crucibles which are used to grow monocrystalline silicon ingots.

Thermal properties

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 3 blocks of stainless steel, borosilicate glass and quartz ware 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.

Technical properties Electrically Fused Quartz Flame Fused Quartz Fused Silica
Thermal data (°C) Softening temperature
Annealing temperature
Strain temperature
Max. working temperature continuous
Max. working temperature short-term
1710
1220
1125
1100
1300
1660
1160
1070
1110
1250
1600
1100
1000
950
1200
Mean specific heat
(J/kg · K)
0 …100 °C
0 …500 °C
0 …900 °C
772
964
1052
772
964
1052
772
964
1052
Heat conductivity
(W/m · K)
20 °C
100 °C
200 °C
300 °C
400 °C
950 °C
1.38
1.47
1.55
1.67
1.84
2.68
1.38
1.47
1.55
1.67
1.84
2.68
1.38
1.47
1.55
1.67
1.84
2.68
Mean expansion
coefficient (K–1)
0 …100 °C
0 …200 °C
0 …300 °C
0 …600 °C
0 …900 °C
– 50 …0 °C
5.1 × 10 –7
5.8 × 10 -7
5.9 × 10 -7
5.4 × 10 -7
4.8 × 10 -7
2.7 × 10 -7
5.1 × 10 -7
5.8 × 10 -7
5.9 × 10 -7
5.4 × 10 -7
4.8 × 10 -7
2.7 × 10 -7
5.1 × 10 –7
5.8 × 10 -7
5.9 × 10 -7
5.4 × 10 -7
4.8 × 10 -7
2.7 × 10 -7

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 chemistry and atomic structure as is 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 control 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.

Mechanical Data Electrically
Fused Quartz
Flame
Fused Quartz
Fused Silica
Density (g/cm2) 2.203 2.203 2.201
Mohs hardness 5.5 … 6.5 5.5 … 6.5 5.5 … 6.5
Micro hardness (N/mm2) 8600 … 9800 8600 … 9800 8600 … 9800
Knoop hardness (N/mm2) 5800 … 6100 5800 … 6100 5800… 6200
Modulus of elasticity at 20 °C (N/mm2) 7.25 × 104 7.25 × 104 7.25 x 104
Modulus of torsion (N/mm2) 3.0 × 104 3.1 × 104 3.0 x 104
Poisson’s ratio 0.17 0.17 0.17
Compressive strength (approx.) (N/mm2) 1150 1150 1150

Electrical properties

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

Fused silica is a good electrical insulator, retaining high resistivity at elevated temperatures and has 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.

Parameter Electrically
Fused Quartz
Flame
Fused Quartz
Fused Silica
Electrical resistivity in Ω × m 20 °C 1018 1018 1016
400 °C 1010 1010 1010
800 °C 6.3 × 106 6.3 × 106 6.3 × 106
1200 °C 1.3 × 105 1.3 × 105 1.3 × 105
Dielectric strength in KV/mm 20 °C 25 …40 25 …40 25 …40
(sample thickness ≥ 5 mm) 500 °C 4 …5 4 …5 4 …5
Dielectric loss angle (tg δ) 1 kHZ 5.0 × 10 –4 5.0 × 10 –4 5.0 × 10 –4
1 MHz 1.0 × 10 –4 1.0 × 10 –4 1.0 × 10 –4
30 GHz 4.0 × 10 –4 4.0 × 10 –4 4.0 × 10 –4
Dielectric constant (ε) 20 °C: 0 …106 Hz 3.70 3.70 3.70
23 °C: 9 × 108 Hz 3.77 3.77 3.77
23 °C: 3 × 1010 Hz 3.81 3.81 3.81

Chemical purity

Purity is crucial for most industrial applications and processes. Fused silica has an outstanding high purity and therefore is an indispensable material in the fabrication of high-tech products.
Despite existing at very low levels, contaminants have subtle yet significant effects. Purity is mostly determined by the raw material, the manufacturing method and subsequent handling procedures. Special precautions must be taken at all stages of manufacture to maintain high purity. Additionally, Heraeus has different purification steps to improve the quality of the quartz sand as raw material even further.

The most common impurities are metals (such as Al, Na and Fe among others), water (present as OH groups) and chlorine. These contaminants not only affect the viscosity, optical absorption and electrical properties of the quartz glass. They can also influence the properties of material processed in contact with the quartz glass during the final use application.

The purities of fused quartz and fused silica are outstandingly high. Synthetic fused silica from Heraeus contains total metallic contamination below 1ppm. For fused quartz the amount is approximately 20 ppm and consists primarily of Al2O3 with much smaller amounts of alkalis, Fe2O3, TiO2, MgO and ZrO2.

OH content
In addition to metallic impurities, fused quartz and fused silica also contain water present as OH units. OH content influences the physical properties like attenuation and viscosity. General, high OH contents means lower use temperature. Typical values are given in the table. Electrically fused quartz has the lowest hydroxyl content (< 1 – 30 ppm) since it is normally made in vacuum or a dry atmosphere. Hydroxyl content in this range is not fixed in the glass structure. It can go up or down depending on the thermal treatment and amount of moisture to which the quartz glass is exposed at elevated temperature. Flame fused quartz has significantly more hydroxyl (150 – 200 ppm) since fusion occurs in a hydrogen/oxygen flame. Due to the production method synthetic fused silica has similar high OH contents of up to 1000 ppm.

Chemical elements composition (ppm) example of our EN quality

SiO2 Al Fe Ca Mg Ti Ni Mn Cu B Co K Na Li OH
99,98% 12,25 0.10 0,38 0,05 1.30 0,03 0,04 0,02 0,09 0,01 0,33 0,78 0,63 210

Handling – Cleaning – Storage of Fused Quartz

Careful and proper cleaning and treatment of quartz glass products are important factors in preserving the properties of quartz glass and extending its life. Quartz glass is very sensitive to alkali and alkaline earth compounds, since even the smallest traces accelerate the devitrification (recrystallization) at high temperatures. Therefore, quartz glass should in principle be touched only with gloves. Fingerprints and the associated alkali traces should be removed before use with cleaning alcohol.

For easy cleaning before and after use, we recommend:

1. Clean quartz glass products with non-alkaline detergents and / or isopropyl alcohol.

2. Rinse with deionized (distilled) water

3. Dry in a clean environment, then the fastest possible packaging or processing.

For persistent contaminants we offer acid cleaning as a service depending on the type and extent of contamination.

Quartz sand

Quartz sand