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One of the most important properties of fused quartz is its extremely low coefficient of expansion: 5.5 x 10-7 mm °C (20-320°C). Its coefficient is 1/34 that of copper and only 1/7 of borosilicate glass. This makes the material particularly useful for optical flats, mirrors, furnace windows and critical optical applications which require minimum sensitivity to thermal changes.
A related property is its unusually high thermal shock resistance. For example, thin sections can be heated rapidly to above 1500°C and then plunged into water without cracking.
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Annealing of Fused Quartz
When quartz is flame worked, the glass worker may induce thermal stress in the piece. As in metals and other vitreous (glassy) materials, this thermal stress is relieved by annealing. The principles of annealing is simple, but can easily be misunderstood resulting in possible breakage of parts during use. Before you can understand the principles of annealing, you need to understand the some common terms used to describe the thermal properties of glass. Details for the principles of Annealing Quartz are covered in the Annealing of Fused Quartz PDF to the right.
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Empirical Annealing Rates of Fused Quartz
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Cooling From Two Sides:
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Cooling From One Side:
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The residual stress or design, depending on the application, may be in the range of 1.7 x 107 to 20.4 x 107 Pa (25 to 300 psi).
As a general rule, is is possible to cool up to 100°C/hour for sections less than 25 mm thick.
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Effects Of Temperature
Fused quartz is a solid material at room temperature, but at high temperatures, it behaves like all glasses. It does not experience a distinct melting point as crystalline materials do, but softens over a fairly broad temperature range. This transition from a solid to a plastic-like behavior, called the transformation range, is distinguished by a continuous change in viscosity with temperature.
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Viscosity
Viscosity is the measure of the resistance to flow of a material when exposed to a 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/cm2 sec.
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, the range resulting from differing conditions of measurement.
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Devitrification
Devitrification and particle generation are limiting factors in the high temperature performance of fused quartz. Devitrification is a two step process of nucleation and growth. In general, the devitrification rate of fused quartz is slow for two reasons: the nucleation of the cristobalite phase is possible only at the free surface, and the growth rate of the crystalline phase is low.
Nucleation in fused quartz materials is generally initiated by surface contamination from alkali elements and other metals. This heterogeneous nucleation is slower in non stoichiometric fused quartz, such as Momentive Performance Materials quartz, than in stoichiometric quartz materials.
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Cristobalite Growth
The growth rate of cristobalite from the nucleation site depends on certain environmental factors and material characteristics. Temperature and quartz viscosity are the most significant factors, but oxygen and water vapor partial pressures also impact the crystal growth rate. Consequently, the rate of devitrification of fused quartz increases with increasing hydroxyl (OH)- content, decreasing viscosity and increasing temperature. High viscosity, low hydroxyl fused quartz materials produced by Momentive Performance Materials, therefore, provide an advantage in devitrification resistance.
The phase transformation to Beta-cristobalite generally does not occur below 1000°C. This transformation can be detrimental to the structural integrity of fused quartz if it is thermally cycled through the crystallographic inversion temperature range (250 °C). This inversion is accompanied by a large change in density and can result in spalling and possible mechanical failure.
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An Advantage
In certain applications, devitrification can be put to the user's advantage since the cristobalite tends to inhibit sag of the fused quartz.
For example, if a diffusion furnace tube is to be used at high temperatures for extended periods of time, and is not subject to thermal cycling below the beta to alpha cristobalite transformation, rotation procedures have been found to be beneficial.
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Contamination
Contamination in almost any form is detrimental. Alkaline solutions, salts, or vapors are particularly deleterious. Handling of fused quartz with the 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 water marks.
Surface contamination affects devitrification in two ways. First, the contaminant promotes nucleation of the cristobalite. Second, it acts as a flux to enhance the cristobalite to beta (high) tridymite transformation.
Under some conditions, the tridymite devitrification will grow deeply and rapidly into the interior of the fused quartz.
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 + 1/2 O2. Consequently, when flame-working fused quartz, there is a band of haze or smoke which forms just outside the intensely heated region. This haze presumably forms because the SiO recombines with oxygen from the air (and perhaps water) and condenses as extremely small particles of amorphous SiO2. The haze can be removed from the surface by a gentle heating 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.
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Resistance To Sag
The most significant chemical factor effecting the sag resistance of fused quartz is the hydroxyl (OH)- content. Momentive Performance Materials controls the (OH)- content in its quartz to meet the specific needs of its customers.
To maximize the performance of tubes used in high temperature semiconductor processes, it is important to understand the impact of changes in diameter and wall thickness.
In one study using GE 214 LD fused quartz tubing, it was found that the sag rate decreases as the wall thickness of the tube is increased. Generally, as the wall thickness doubles, the sag rate decreases by a factor of approximately 3.
Also, it was shown that with a fixed wall thickness, the sag rate decreases as the tube diameter decreases.
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Back to Properties
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Momentive Performance Materials
