Devitrification (also known as devitrification) 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 cristobalite, which is a thermodynamically unstable metastable state when the temperature is higher than 1000 °C. At the same time, the vibration of the SiO2 molecules is accelerated, and the crystals are formed after a long period of rearrangement and orientation. The devitrification is expressed by the growth rate of the nucleus. The crystallization rate of the opaque quartz glass at 1520 ° C and the transparent quartz glass at 1620 ° C respectively reaches a larger value.
The crystallization is mainly on the surface, followed by the internal defects, because these places are easily contaminated, causing local accumulation of impurity ions, especially alkali ions (such as K, Na, Li, Ca, Mg, etc.) entering the network and causing viscosity. Decrease, causing devitrification to accelerate.
Since the thermal expansion coefficient and specific gravity of quartz glass are similar to those of the crystallized product β-cristobalite, when it is continuously used at high temperature, although the crystallization zone is continuously expanded, the volume change is not obvious, and it can be satisfactorily continued to be used. Reduce the plastic deformation of the glass and improve the refractoriness. When the crystallization product is cooled to 800 ° C, a fine crack network occurs. When cooling is continued to 200-275 ° C, the structural change of cristobalite from high temperature type to low temperature type (ie β-cristobalite → a-cristobalite) occurs, accompanied by volume fusion, if the devitrification layer is deep, then Quartz glass also ruptured. Since crystallization is often found in places with impurities, the surface state before high temperature use and surrounding refractories and atmospheres are very important.
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