1. The core purpose of quartz material annealing

Quartz annealing is a vital process that eliminates internal defects and stabilizes the performance of materials through controlled heating, holding and cooling. Its core functions are threefold.

(1) Relieving of internal stress

The process of fusion forming and cold working (cutting/grinding) is associated with the generation of thermal and mechanical stresses. Annealing is a process that releases stress through the slow rearrangement of atoms, thereby preventing stress cracking during subsequent use.

(2) Stabilize microstructure

Annealing has been shown to reduce defects such as pores and microcracks in quartz glass, optimize the Si-O network structure, and enhance the high-temperature resistance, light transmittance and chemical stability of the material.

(3) Performance adjustment

In the case of doped quartz, the distribution state of doped ions can be controlled through the process of annealing. For transparent quartz (such as the JGS series), it is possible to enhance its infrared/ultraviolet transmittance.

2. The reasons for crystallization after annealing

During the annealing process, the crystallization is the result of a number of factors, including thermodynamic trends, kinetic conditions, and nucleation sites, and it is also related to the annealing process parameters.

(1) Thermodynamic drive

Quartz glass is metastable, with an internal energy higher than that of crystalline SiO2 (α-quartz, cristobalite). The high temperature during the annealing process provides the necessary energy for the transition between the glass phase and the crystal phase, which is a spontaneous trend.

(2) Dynamic condition matching

If the annealing temperature is within the crystallization-sensitive range (1100–1400°C), this will have a significant impact on the viscosity of quartz glass, enhancing the diffusion ability of Si and O atoms. This is key to meeting the kinetic requirements for crystal nucleus formation and grain growth. Maintaining high temperatures for extended periods will exacerbate the formation of crystallization.

(3) Nucleation site induction

Bubbles, cracks, dislocations within the material, and impurities on the surface (e.g. Na+, K+, Al3+) and the interface with the annealing fixture will generate numerous heterogeneous nucleation sites. This will significantly reduce the activation energy for crystallization, thereby establishing the starting point for crystallization.

(4) Improper process parameters

A number of factors may induce crystallization: namely, a too-fast heating rate; remaining in the sensitive zone for too long during the cooling stage; and an oxidizing annealing atmosphere (which accelerates surface crystallization).