Devitrification of quartz glass

Devitrification (also called crystallization) is an inherent defect of quartz glass. From a thermodynamic point of view, the internal energy of quartz glass is higher than that of crystalline quartz, belonging to a metastable state that is thermodynamically unstable. When the temperature is higher than 1000℃, the SiO2 molecules accelerate their vibrations, and after a relatively long period of rearrangement and orientation, crystallization is formed. Devitrification is expressed by the crystal nucleus growth rate. The crystallization rate of opaque quartz glass reaches its maximum value at 1520℃, and that of transparent quartz glass at 1620℃.

Crystallization mainly occurs on the surface, followed by internal defects, because these areas are prone to contamination, causing the local accumulation of impurity ions, especially alkali ions (such as K, Na, Li, Ca, Mg, etc.). Entering the network, these ions reduce viscosity and accelerate devitrification.

Because the thermal expansion coefficient and specific gravity of quartz glass are similar to those of the crystallization product β-quartz, even though the crystallization area continuously expands during continuous high-temperature use, the volume change is not significant, and it can still be used satisfactorily. At this time, it can also reduce the plastic deformation of the glass and improve its refractoriness. When the crystallization product cools to 800℃, a fine network of cracks appears. Continuing to cool to 200-275℃, a structural change occurs from the high-temperature type to the low-temperature type of quartz (i.e., β-quartz → α-quartz), accompanied by volume shrinkage. If the crystallization layer is very deep, the quartz glass will also crack. Since crystallization often occurs in places with impurities, the surface state before high-temperature use and the surrounding refractory materials and atmosphere are very important. Precautions for using quartz glass will be introduced later.