Method for producing iron-doped silica glass
A method for producing a blank of iron-doped silica glass with high silicic acid content for use as heat protection glass is provided. The method involves: (a) producing an iron-doped SiO 2 soot body which contains iron in a first oxidation state Fe 3+ by flame hydrolysis of a silicon-containing and an iron-containing starting substance, (b) drying the soot body to obtain a mean hydroxyl group content of less than 10 ppm by weight, and (c) vitrifying the soot body under a reducing atmosphere that is suitable for at least partially reducing the iron from the first oxidation state Fe 3+ to a second, lower oxidation state Fe 2+ . A blank is obtained having an iron content between 0.1 and 1% by weight which exhibits an internal transmission of at most 40% in the infrared wavelength range and an internal transmission of at least 85% in the visible spectral range.
1. A method for producing a blank of iron-doped glass having a high silicic acid content, an iron content of between 0.1% by wt. and 1% by wt., an internal transmission of not more than 40% in the infrared wavelength range of 800 nm to 2000 nm, and an internal transmission of at least 85% in the visible spectral range with wavelengths of 400 nm to 800 nm at a sample thickness of 1 mm, the method comprising:
(a) producing an iron-doped SiO 2 soot body comprising iron in a first oxidation state Fe 3+ by flame hydrolysis of a silicon-containing and an iron-containing precursor substance,
(b) drying the soot body to obtain a mean hydroxyl group content of less than 10 wt. ppm, and
(c) vitrifying the soot body in an atmosphere having a reducing effect to form the blank of iron-doped glass having a high silicic acid content, wherein at least a part of the iron is reduced from the first oxidation state Fe 3+ to a second lower oxidation state Fe 2+ .
2. The method according to claim 1 , wherein the atmosphere having a reducing effect contains hydrogen.
3. The method according to claim 2 , wherein the atmosphere having a reducing effect comprises 1 mbar to 100 mbar hydrogen in helium.
4. The method according to claim 1 , wherein the atmosphere having a reducing effect has a pressure below atmospheric pressure.
5. The method according to claim 1 , wherein step (c) comprises isothermally heating the soot body in a furnace to a vitrification temperature and exposing the soot body to the atmosphere having a reducing effect for a treatment period of 3 to 15 hours.
6. The method according to claim 5 , wherein after step (c), at least 97% of the iron in the blank is in oxidation state Fe 2+ .
7. The method according to claim 1 , further comprising after vitrification of the soot body according to step (c), annealing the blank at a temperature range of 300° C. to 1000° C. in an atmosphere containing hydrogen at a pressure of 15 mbar to 30 bar.
8. The method according to claim 1 , further comprising prior to vitrification according to step (c), performing a conditioning treatment of the soot body in an atmosphere having a reducing effect at a temperature of 800° C. to 1100° C.
9. The method according to claim 1 , wherein step (b) is performed at a temperature of 800° C. to 1100° C. in vacuum at a pressure of less than 10 mbar.
10. The method according to claim 1 , wherein step (b) is performed at a temperature of 800° C. to 1100° C. an atmosphere containing carbon monoxide at a pressure of 5 mbar to 100 mbar.
11. The method according to claim 1 , wherein halogen-free organic compounds are used as the iron- and silicon-containing precursor substances.
12. The method according to claim 11 , wherein octamethylcyclotetrasiloxane (OMCTS) is used as the silicon-containing precursor substance.
13. The method according to claim 11 , wherein iron pentacarbonyl (Fe(CO) 5 ) is used as the iron-containing precursor substance.
14. The method according to claim 11 , wherein ferrocene (C 10 H 10 Fe) is used as the iron-containing precursor substance.