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THERMAL PROPERTIES
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.
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.
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.
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 GE quartz, than in stoichiometric quartz
materials.
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 GE Quartz, 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.
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.
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.
Resistance To Sag
The most significant chemical factor effecting the sag resistance of fused quartz
is the hydroxyl (OH)- content. GE 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
214LD 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.
Cristobalite Thickness/Time Chart
Diffusion Tubing, Collapse vs. Time For Tube ID Chart
Diffusion Tubing, Collapse vs. Time For Wall Thickness Chart
