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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.

 

Storage

  • 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

 

Rotation Procedures For Fused Quartz Furnace Tubes

  • Place tube in furnace at 1200 °C, and rotate it 90 degrees 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 degrees 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. 

 

Solarization

  • 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

 

Surface Cleanliness

  • 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

 

Dissociation

  • 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.