An important consideration for today’s users of fused quartz is the availability of technical product support. Momentive Performance Materials Quartz backs its products with fully-equipped analytical and development laboratories and a staff of materials and fusion experts available to support customer requirements. State-of-the-art analytical equipment assures optimal production quality and also enables certification and subsequent verification of Momentive Performance Materials Quartz product compliance with stringent industry standards.
Physical properties and other information shown on this site were developed from a number of sources, including Momentive’s technical laboratories, textbooks and technical publications.
While Momentive believes that this information is accurate, it is not an exhaustive review of the subjects covered and, accordingly, Momentive makes no warranty as to the accuracy or completeness of the information. Customers are advised to check references to ensure that the product is suitable for the customer’s particular use of requirements.
Cleaning of Fused Quartz
For applications in which cleanliness is important, Momentive Performance Materials recommends the following:
The product, particularly tubing, should be washed in deionized or distilled water with a degreasing agent added to the water.
Fused quartz should then be placed in a 7% (maximum) solution of ammonium bi-flouride for no more than ten minutes, or a 10 vol % (maximum) solution of hydrofluoric acid for no more than five minutes.
Etching of the surface will remove a small amount of fused quartz material as well as any surface contaminants.
To avoid water spotting, which may attract dirt and cause devitrification upon subsequent heating, fused quartz should be rinsed several times in deionized or distilled water and dried rapidly.
Use of clean, cotton gloves at all times is essential to reduce possibility of contamination.
Annealing of Fused Quartz
Fused quartz, like other vitreous (glassy) materials, may have stress after thermal treatment. To avoid this stress, the glass must be cooled properly. The principles of annealing are:
Elevate the temperature of the glass to point where the stress is relieved.
Hold at this temperature until the entire body of the glass reaches temperature equilibrium.
Cool the glass slowly to a temperature where the glass is rigid.
Empirical Annealing Rates of Fused Quartz
Cooking From One Side:
Rate °C/minute = 2.867×10-2 x residual stress, Pa
(2 x thickness, mm)2
Rate °C/minute = 2.867×10-2 x residual stress, Pa
The residual stress or design, depending on the application, may be in the range of 1.7 x 105 to 20.4 x 105 Pa (25 to 300 psi). As a general rule, it is possible to cool up to 100°C/hour for sections less than 25mm thick.
Handling of Fused Quartz
Like any material that is expected to provide a design life at high temperatures, fused quartz demands some care in handling and use to achieve maximum performance from the product.
- Space permitting, fused quartz should be stored in its original shipping container
- If that is not practical, at least the wrapping should be retained
- In the case of tubing, the end coverings should be kept in place until the product is used. This protects the ends from chipping and keeps out dirt and moisture that may compromise the purity and performance of the tubing
- Contamination in almost any form is detrimental
- Alkaline solutions, salts or vapors are particularly deleterious
- Handling with bare hands deposits sufficient alkali from perspiration to leave clearly defined fingerprints upon devitrification
- Drops of water allowed to stand on the surface will collect enough contamination from the air to promote devitrified spots and watermarks.
- Surface contamination promotes nucleation of the cristobalite
Rotation Procedures For Fused Quartz Furnace Tubes
The following procedure has been used to create an even layer of crystobalite on diffusion tubes in order to increase resistance to devitrification.
- Place tube in furnace at 1200°C, and rotate it 90° every two hours for the first 30 hours
- If the working schedule does not permit adherence to this procedure, place the tube in a furnace at 1200°C and rotate it 90° every two hours for the first 8 hours, then reset the furnace to operating temperature.
- This procedure helps minimize sag at high temperature to increase the lifetime of the diffusion tube, assuming sufficient mechanical support is designed into the furnace.
- Fused quartz made from natural raw material solarizes or discolors upon prolonged irradiation by high energy radiation (such as short UV, X-rays, gamma rays and neutrons)
- Resistance to this type of solarization increases with the purity of fused quartz
- Synthetic fused silica is highly resistant to solarization
- Solarization in fused quartz can be thermally bleached by heating it to high temperatures
- Heating fused quartz to elevated temperatures (ca. 2000°C) causes the SiO2 to undergo dissociation or sublimation.
- This is generally considered to be: SiO2 -> SiO + ½ O2.
- When flame-working fused quartz, there is a band of haze or smoke which forms just outside the intensely heated region. The haze presumably forms because the SiO recombines with oxygen from the air (and perhaps water) and condenses as extremely small particles of amorphous SiO2.
- Haze can be removed from the surface by heating gently in the oxy-hydrogen flame.
- The dissociation is greatly enhanced when the heating of fused quartz is carried out in reducing conditions. For example, the proximity or contact with graphite during heating will cause rapid dissociation of the SiO2.
Viscosity is the measure of the resistance to flow of a material when exposed to shear stress. Since the range in “flowability” is extremely wide, the viscosity scale is generally expressed logarithmically. Common glass terms for expressing viscosity include strain point, annealing point, and softening point, which are defined as:
Strain Point: The temperature at which the internal stress is substantially relieved in four hours. This corresponds to a viscosity of 1014.5 poise, where poise = dynes/cm2sec.
Annealing Point: The temperature at which the internal stress is substantially relieved in 15 minutes, a viscosity of 1013.2 poise.
Softening Point: The temperature at which glass will deform under its own weight, a viscosity of approximately 107.6 poise. The softening point of fused quartz has been variously reported from 1500°C to 1670°C, resulting from differing conditions of measurement.