1. Hydroxyl defects in quartz glass

The structural defects of quartz glass are defects in its tetrahedral network structure of SiO4, including oxygen defects caused by oxygen displacement and Si-O bond breakage defects. In the high-temperature melting of quartz glass, quartz glass and water can react to form a hydroxyl group in the quartz glass. Hydroxyl is the most prevalent structural defect in various microscopic defects of quartz glass. The preparation of quartz glass is usually carried out in an atmosphere full of H2, which provides the main heat for the entire high-temperature melting process. Moreover, as the protective gas of the entire heating system, H2 provides a safe and stable environmental atmosphere for the system. Since the H in the hydroxyl group of quartz glass mainly comes from H2, so the presence of hydroxyl defects in quartz glass is inevitable.

2. Effect of hydroxyl defects on properties of quartz glass

(1) Effect on chemical properties of quartz glass

Hydroxyl defects in quartz glass are formed by the breakage of Si-O bonds. This means that the presence of hydroxyl in quartz glass can open the Si-O bond and thereby reduce the chemical stability of quartz glass. 

(2) Effect on thermal properties of quartz glass

The viscosity of quartz glass is a characteristic of its thermal properties, which are closely related to temperature. The presence of hydroxyl defects in quartz glass causes the Si-O bond to break, thereby reducing the viscosity of quartz glass. The effect of the hydroxyl defects on the viscosity of quartz glass is more obvious at lower temperatures.

(3) Effect on optical properties of quartz glass

Impurities and structural defects in quartz glass can affect its ability to absorb various wavelengths of light, thereby influencing the spectral transmittance of the quartz glass.

(4) Effect on mechanical properties of quartz glass

Impurities and hydroxyl groups present in quartz glass can affect its density. The higher the content of such impurities and hydroxyl, the lower the density of quartz glass.

3. How to reduce the effect of hydroxyl on quartz glass?

(1) Calcinating quartz glass in air is an effective process of removing the hydroxyl groups formed by hydrogen. This is mainly due to the presence of water in air, and the equipment used for dehydroxylation is typically tunnel kilns, push plate kilns and box-type electric furnaces.

(2) The addition of high-purity nitrogen to the dehydroxylation process is an effective method of removing all hydroxyl groups formed from hydrogen, as well as a small amount of hydroxyl groups formed from water. Due to the stability of the hydroxy groups formed by water, the diffusion rate of water in quartz glass is slow. This results in a prolonged calcination time for removal of the hydroxy groups. The equipment required for this process is a push-plate kiln or a box-type electric furnace. It is imperative that the furnace door is closed while ensuring the air intake and outlet remain open during the calcination process. In addition, the process of dehydroxylation requires constant feeding of high-purity nitrogen to ensure the water content in the furnace atmosphere remains at a negligible level (1×10-6 ~ 2×10-6).

(3) Vacuum dehydroxylation is a process that is carried out in a vacuum electric furnace with a vacuum level of 10-4 Pa. The furnace is designed to have an atmosphere with almost no water content, which creates ideal conditions for dehydroxylation and increases the rate of hydroxyl diffusion.