Hydroxyl content in quartz glass

The purities of fused quartz and fused silica are outstandingly high.  The most common impurities are metals (such as Al, Na and Fe among others), water (present as OH groups) and chlorine.  OH content influences the physical properties like attenuation and viscosity. General, high OH contents means lower use temperature.  Electrically fused quartz has the lowest hydroxyl content (< 1 – 30 ppm) since it is normally made in vacuum or a dry atmosphere. Hydroxyl content in this range is not fixed in the glass structure. It can go up or down depending on the thermal treatment and amount of moisture to which the quartz glass is exposed at elevated temperature. Flame fused quartz has significantly more hydroxyl (150 – 200 ppm) since fusion occurs in a hydrogen/ oxygen flame. Due to the production method synthetic fused silica has similar high OH contents of up to 1000 ppm.
 
Dehydroxylation
According to the findings of hydroxyl in quartz glass, quartz glass can be put into two categories, one category is the production of quartz glass oxidizing atmosphere; another kind is the quartz glass melting reduction atmosphere. The former hydroxyl with heating method is difficult to remove, the latter is easy to take off.
 
Oxidizing atmosphere, the production of quartz glass:
(1) synthetic quartz glass, four chlorinated silicon pyrolysis in an oxyhydrogen flame, hydroxyl content about 1000--2000ppm.
(2) gas refining quartz glass: oxyhydrogen melting quartz powder, 100-200ppm hydroxyl content
(3) plasma method for quartz glass: plasma flame fused quartz powder, radical content 20-30ppm.
(4) fused silica: melting quartz powder, 300-500ppm hydroxyl content.
This kind of quartz glass hydroxyl by heat treatment is difficult to take off, has been heated to 1350 degrees, the crystallization temperature of above, can obviously hydroxyl removal.
 
Quartz glass melting reduction atmosphere:
Quartz glass melting under hydrogen atmosphere, 100-200ppm hydroxyl content; heating in 900 degrees above, can take off most of the hydroxyl. In the few quartz glass melting helium or hydroxyl content in vacuum (5ppm).