Niobium doped silica titania glass and method of preparation

A method for making a silica-titania-niobia glass having a composition comprising niobia in an amount in the range of 0.005 wt. % to 1.2 wt. %, titania in an amount in the range of 5 wt. % to 10 wt. %, and the remainder of the glass is silica, the method comprising the steps of:

• obtaining a silica-titania soot have a titania content in the range of 5-10 wt., %
• preparing an aqueous hydroxide solution containing a selected amount of a niobium compound and adjusting the pH to above 12, and adding the silica-titania soot to the solution, the amount of the niobium compound in the solution being such that a silica-titania-niobia glass formed therefrom consists essentially of 0.005 wt. % to 1.2 wt. % niobia, 5 wt. % to 10 wt. % titania, and the remainder of the glass is silica,
• mixing the resulting silica-titania soot containing solution for a time of 14-24 hours to form a stable sol, then reducing the pH to less than 10 by the addition of a time dependent buffering, gelling agent to form a gel, and degassing to remove entrapped air;
• transferring the degassed sol to a partially-closed container and aging the sol 0.2 to 48 hours at a relative humidity of >75% to form a gel;
• opening the container and allowing the gel to air dry for a time in the range of 4-8 days or until the weight of the gel is reduced to less than 70% of the weight of the starting gel;
• drying the gel in an oven by slowly heating it to a temperature 100-150° C. for a time in the range of 20-30 hours to form a dried gel material;
• slowly heating the dried gel material in a furnace to a temperature of 700-800° C. over a time in the range of 70 hours to 100 hours to slowly remove the organic binders present in the dried material without damaging the dried gel material and thereby forming a silica-titania-niobia glass precursor;
• consolidating silica-titania-niobia glass precursor into a glass by heating the precursor to a selected temperature in the range of 1600° C. to 1700° C. in flowing helium over a time in the range of 4 hours to 24 hours, with the provision that the heating be at a rate exceeding 8° C./minute above 1535° C. to avoid crystallization in the glass;
• and
• cooling the consolidated glass from the consolidation temperature to room temperature.