Linear thermal expansion coefficient refers to the elongation of the unit length of glass when heated per 1℃ in a certain temperature range. It is measured with thermal dilatometer by quartz comparison method. After obtaining the relationship curve between temperature and sample elongation from room temperature to softening temperature, we can get transition and sag temperatures through graphing method. In a certain temperature range, after every 1℃ increase in temperature, the elongation of the unit length of sample indicates the mean linear expansion coefficient in this temperature range.
Test equipment: NETZSCH expansion coefficient tester (DIL402 SELECT)
Temperature range: -150℃~1000℃
Measurement accuracy: better than ±1%.
Sample requirement: No visible stripes, bubbles, stones or other impurity with naked eye, stress birefringence up to class 1.
The mean thermal expansion coefficients from -50℃~80℃ and from 100 ℃ to300 ℃ (10-7/K) are given in the handbook (see figure 2).
Heat an annealed sample at a certain rate, we can obtain the thermal expansion curve between temperature and sample elongation, as shown in figure 2. The transformation temperature Tg is defined as the intersection of two lines that are extrapolated from the low and high temperature sides respectively.
The sag temperature Ts is defined as the temperature at which annealed sample stops expanding when being heated, as shown in figure 2. The peak temperature on the expansion curve is sag temperature.
The strain point refers to the temperature at which the glass viscosity is 1014.5 dpa·s. It is also called the lower limit temperature at which inner stress in the glass can be eliminated within several hours.
The annealing point refers to the temperature at which the glass viscosity is T1013 dPa·s. It is also called the upper limit temperature at which inner stress in the glass can be eliminated within several minutes.