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| 3. Thermal Properties
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Thermal properties are essential to processing optical
glass for annealing, heat treatment and coating. We have listed the
strain point, annealing point, softening point, transformation point,
yield point and thermal conductivity. The linear coefficient of thermal
expansion is given for two temperature ranges.
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3.1 Strain Point StP
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The strain point
corresponds to the lowest temperature in the annealing range at
which viscous flow of glass will not occur. Viscosity of the glass
is 1014.5 dPa s { poise } at this temperature. The strain
point is measured by the Fiber EIongation Method prescribed in
JIS-R3103 and ASTM-C336.
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3.2 Annealing Point (AP)
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The annealing point corresponds to the
maximum temperature in the annealing range at which the internal
strain of glass will be substantially eliminated. Viscosity of the
glass is 1013 dPa · s { poise } at this temperature. The
annealing point is measured by the Fiber EIongation Method
prescribed in ASTM-C336.
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3.3 Softening Point (SP)
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T he softening point is the temperature at which
glass deforms under its own weight. Viscosity of the glass is
107.65 dPa s { poise } at this temperature. The softening
point is measured by the Fiber EIongation Method prescribed in
JIS-R3104 and ASTM-C338. 6
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3.4 Transformation Temperature
(Tg) and Yield Point (At) |
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The transformation region is that temperature
range in which a glass gradually transforms from its solid state
into a "plastic" state. This region of transformation is defined as
the transformation temperature (Tg). The transformation temperature
can be determined from the thermal expansion curve (Fig. 1).
Viscosity coefficient at this temperature is approximately
1013 poise. Yield point (At) is the deformation point
temperature on the thermal expansion curve, or the point at which
elongation becomes zero.
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Fig1 |
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3.5 Linear Coefficient of
Thermal Expansion (a) |
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The thermal expansion curve is obtained by
measuring a well-annealed glass sample of 4 mm diameter by 50 mm
long heated at a rate of 2 °C /min in the low temperature range and
at a rate of 4 °C /min in the high temperature range. From the
temperature and elongation of the sample glass, the mean linear
coefficient of thermal expansion between -30 °C to + 70 °C and + 100
°C to + 300 °C respectively up to 10-7 /K is determined
and is given in the catalog.
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3.6 Thermal Conductivity (k)
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| The thermal conductivity of
most optical glasses at room temperature is located between
1.126W/(m.K) which is that of S-BSL 7 and 0.546W/(m.K) which is that
of PBH71. The thermal conductivity is measured in accordance with
methods prescribed in JIS-R2618. The thermal conductivity of glass
at a temperature of 35 °C is listed in the catalog. Accuracy is ±5%.
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