Hydroxyl Groups in Quartz Tubes

Hydroxyl dissolved in quartz glass is known as hydroxyl. Hydroxyl is the main impurity in quartz glass, and the primary factors affecting its content are raw materials, processes, and manufacturing methods. As the hydroxyl content in quartz glass varies, so does the glass’s performance. Increasing hydroxyl content leads to a decrease in viscosity, density, and refractive index, and an increase in infrared absorption and expansion coefficient.

Based on the behavior of hydroxyl in quartz glass, it can be divided into two categories: quartz glass produced in an oxidizing atmosphere and quartz glass melted in a reducing atmosphere. The hydroxyl in the former is difficult to remove by heating, while it is easier to remove in the latter.

Quartz glass produced in an oxidizing atmosphere includes:

1. Synthetic Quartz Glass: Silicon tetrachloride is thermally decomposed in a hydro-oxygen flame, with hydroxyl content around 1000-2000 ppm.
2. Gas-Refined Quartz Glass: Quartz powder melted in a hydro-oxygen flame, with hydroxyl content of 100-200 ppm.
3. Plasma Quartz Glass: Quartz powder melted in a plasma flame, with hydroxyl content of 20-30 ppm.
4. Fused Quartz: Quartz powder melted in an air atmosphere, with hydroxyl content of 300-500 ppm.

This type of quartz glass’s hydroxyl is difficult to remove by heat treatment and only visibly dehydroxylates above the crystallization temperature at around 1350 degrees Celsius.

In a hydrogen atmosphere, the hydroxyl content is 100-200 ppm; heating above 900 degrees Celsius can remove most of the hydroxyl. In helium or vacuum conditions, the hydroxyl content is very low (below 5 ppm).

The hydroxyl in quartz glass melted in a hydrogen atmosphere is related to the following factors:

1. Related to Raw Material Properties

a. Dissolved water; b. Crystalline water; c. Interstitial water; b. Surface adsorbed water; e. Gas-liquid inclusions;

2. Related to Raw Material Impurity Content and Type

a. Alkali metal oxides; b. Alkaline earth metal oxides; c. Rare earth oxides;

3. Related to Melting Condition

Temperature; Time; Atmosphere;

4. Related to Dehydroxylation Conditions

Environment, vacuum level; Time; Temperature;

Re-melting quartz glass produced in an oxidizing atmosphere in a hydrogen atmosphere shows no change in the hydroxyl absorption peak at 2.73 microns; this indicates that the melting temperature is not the cause of the difference in dehydroxylation performance between the two types of glass.

Powdered quartz glass melted in an oxidizing atmosphere (particle size 0.2—0.05mm) shows a significant change in the hydroxyl peak at 2.73 microns after re-melting in a hydrogen atmosphere, making it easier to remove hydroxyl and its performance similar to that of quartz glass melted in a hydrogen atmosphere. This demonstrates that particle size is an important factor in dehydroxylation differences.

Low Hydroxyl Quartz Glass Powder re-melted in a hydrogen atmosphere increases hydroxyl from 3 ppm to 100 ppm, indicating that a hydrogen atmosphere can increase hydroxyl content in quartz glass. Low hydroxyl quartz glass blocks re-melted in a hydrogen atmosphere show virtually no change in hydroxyl content (3 ppm), indicating that the interaction between hydrogen and quartz glass begins at the surface (melting time about 30 minutes).

A formula by GE: C = 910/T * LOG10(Ta/Tb) mm-1

• C: Hydroxyl content (C, ppm)
• T: Thickness (mm)
• Ta: Transmittance at 2600 nanometer wavelength
• Tb: Transmittance at 2730 nanometer wavelength

Chinese national standard formula: C = 96.5/d * LG10(Ia/I) mm-1

• C: Hydroxyl content (ppm)
• d: Thickness (cm)
• Ia: Distance from 2730 nm baseline to zero line (mm)
• I: Distance from 2730 nm absorption peak to zero line (mm)

At Global Quartz Tube, we specialize in producing high-quality quartz tubes with precise control over hydroxyl content to meet diverse industry needs. For more information on our products and customization options, visit our website at www.globalquartztube.com or contact us via email at contact@globalquartztube.com.