Sol-gel method for making ultra-low expansion glass

DETAILED DESCRIPTION Silicate solutions useful for the production of silicate glasses by gelation are known in the prior art.
Typically, these are true solutions comprising silicates, although colloidal suspensions of silica or of silicate compounds are also useful.
Since silica itself is essentially insoluble in water, the silica in these solutions and/or suspensions is normally present in the form of a dissolved or suspended alkali metal silicate or organic ammonium silicate such as a quaternary ammonium silicate compound.
In order to maintain the silica in a dissolved or suspended colloidal state in such solutions, the concentration of alkali metal and/or ammonium ion must be sufficient to yield pH values greater than about 10 for ammonium silicates, and greater than about 11 for alkali silicates.
Further, to secure gelation characteristics suitable for providing a firm homogeneous gel, the silica content of these solutions or colloids should be maintained in the range of about 1-12 moles/liter, more preferably 3-12 moles/liter.
As has been noted above, whereas dispersed titania particles or crystals can be added to these silicate suspensions and incorporated into the gel structure as the silica is precipitated therefrom, the resulting gels cannot be consolidated to fully transparent, ultra-low expansion glasses having uniform expansion characteristics throughout the body of the glass.
In order to achieve the requisite homogeneity for low, uniform expansion characteristics, the TiO.
sub.
2 component of the glasses of the present invention is incorporated as colloidal TiO.
sub.
2, most preferably as an aqueous colloidal TiO.
sub.
2 suspension stabilized at alkaline pH and free of precipitated TiO.
sub.
2 particles.
In general, titania sols having pH values in the range of about 9-10 and titania concentrations in the range of 0.
5-3 moles/liter can be used.
The preferred sols are those using quaternary ammonium counter ions.
The avoidance of premature titania precipitation from these sols is achieved by maintaining the pH at high values during the step of blending the titania and silicate components, and by carrying out the blending slowly and with continued stirring