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Thermal Properties
 
 

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.

 

Strain Point**(StP)

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.

 

Annealing Point**(AP)

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.

 

Softening Point**(SP)

The 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

 

Transformation Temperature (Tg) and Yield Point (At)

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. The yield point is shown for the temperature when deformation starts because of the measured pressure (load) exerted on the specimen at the time of measuring expansion, as shown in Figure 1.


Fig.1
Transformation Temperature (Tg) and Yield Point (At)
 

Linear Coefficient of Thermal Expansion* (α)

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.

 

Thermal Conductivity (k)

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 0.599W/(m.K) which is that of S-PHM52. 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%.

 
* indicates measurement by the methods stipulated by Japan Optical Glass Manufacturers' Association
** indicates a test by methods stipulated by ASTM
***indicates a test by methods stipulated by International Organization for Standardization (ISO)

 
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