Glass manufacturing – Processes – With shaping of particulate material and subsequent fusing...
Reexamination Certificate
2001-02-28
2004-09-28
Chin, Peter (Department: 1731)
Glass manufacturing
Processes
With shaping of particulate material and subsequent fusing...
C065S017600
Reexamination Certificate
active
06796143
ABSTRACT:
The invention relates to a process for producing SiO
2
—TiO
2
glasses having a low coefficient of thermal expansion.
Known SiO
2
—TiO
2
glass-forming systems are distinguished by their good thermal properties, their high refractive index and their low coefficients of thermal expansion. In comparison with silica glass, this system has good resistance to alkali vapors at relatively high temperatures. This property finds application, for example, in the coating of lamps (E. B. Yoldas, Method of conforming clear vitreous gel of silica-titania material. U.S. Pat. No. 4,278,632, Westinghouse Electric Corp., USA, 8.2.1980).
A further important property of the SiO
2
—TiO
2
system is the low coefficient of thermal expansion. This is of significance for the production of optical components in which the dimensional accuracy over a wide range of temperature plays a prominent role (D. R. Shoup, Sol-gel method for making ultra-low expansion glass. U.S. Pat. No. 4,786,618, Corning Glass Works (USA), 29.05.1998).
On account of the great commercial interest in these systems, processes are being sought that guarantee rapid and cost-effective production.
One known possibility for producing binary TiO
2
—SiO
2
glasses is offered by melting. However, the melting-temperature is very high at 1,700° C., and phase separation and devitrification occur very readily during cooling of the glass melt (Z. Deng, E. Breval and C. G. Pantano, Colloidal sol/gel processing of ultra-low expansion TiO
2
/SiO
2
glass. J. Non-Cryst. Solids 100 (1988) 364-370).
The phase diagram shows that the crystalline form of TiO
2
possesses low solubility in SiO
2
even at elevated temperatures (
FIG. 1
) (E. M. Levin, C. R. Robbin and H. F. McMurdie, SiO
2
—TiO
2
phase diagram, in: Phase Diagrams for Ceramists, M. K. Reser, Editor (1956), The American Ceramic Society: Columbus, Ohio (USA), p 1).
A known alternative to fusing of the initial components is offered by flame hydrolysis. Initial investigations in this field were carried out by Nordberg. In U.S. Pat. No. 2,326,059, dating from 1943, he describes the production of TiO
2
—SiO
2
glass by a process of flame oxidation of a mixture consisting of TiCl
4
and SiCl
4
. In this process a glass is formed having a TiO
2
content of 5-11 wt. % and a coefficient of thermal expansion below that of silica glass. He ascribes this behavior to the direct interchange of SiO
2
with TiO
2
(M. E. Nordberg, Glass having an expansion lower than that of silica. U.S. Pat. No. 2,326,059, Corning Glass Works, New York).
Further investigations in this field were carried out by P. C. Schultz. For the process of flame hydrolysis he used a small two-burner furnace in order to produce small pear-shaped mouldings in a manner similar to the Nordberg patent. These glass mouldings are precipitated on a vessel at 1,700° C. In this process it was possible for SiO
2
—TiO
2
glasses to be produced having a TiO
2
content of up to 16.5 wt. %. Glasses with a content of titanium dioxide between 12 and 17 wt. % are metastable and show structural alterations that have an effect on the coefficient of thermal expansion. Larger contents of titanium dioxide cause phase separation or crystallization. Starting from 18.5 wt. % TiO
2
the glass was semitranslucently white and at 19.4 wt. % it was white and opaque. By means of X-ray diffraction it was possible for small quantities of rutile and anatase to be detected. The limit for vitrification via flame hydrolysis lies between 16.5 and 18.5 wt. % TiO
2
.
Moreover, Schultz ascertained that between 0 and 10 wt. % TiO
2
and in the temperature-range between 25 and 700° C. the coefficient of thermal expansion becomes negative with increasing content of titanium oxide (P. C. Shultz, Binary Titania-Silica Glasses Containing 10 to 20 wt. % TiO2. J. Am. Ceram. Soc. 59 (1976) 214-219).
Further known investigations on this subject were carried out by J. E. Maxon. His patent U.S. Pat. No. 5,970,751, dating from 1999, is concerned with the production of silicate glass with admixture of titanium dioxide by flame hydrolysis a mixture consisting of SiO
2
precursor and TiO
2
precursor.
Relatively pure metal oxides can be produced by thermal decomposition of the precursors and precipitation of the resulting oxides. The precursors can be converted into vapor form or supplied to the flame in finely divided form in gas (for example, N
2
). In addition to titanium chloride or silicon chloride, octamethyl cyclotetrasiloxanes (OMCTS), titanium alkoxides or titanium isopropoxides are employed as precursors. In the case where use is made of these chlorine-free precursors, deposits may occur in the pipelines that transport the mixture of the two precursors. This results in undesirable variations in the chemical composition of the powders, and ultimately it becomes necessary to close the apparatus down for the purpose of cleaning. The invention that is set forth in the patent in question, U.S. Pat. No. 5,970,751, is intended to lessen these deposits; as a result, the running-time until the next cleaning is to be extended. In addition, the quality of the glass is to be improved.
The process due to Maxon arose in the course of efforts to convert the process from chlorides to more environmentally friendly precursor material (OMCTS and titanium isopropoxide). Alkoxides of transition metals are known for their sensitivity to light and moisture. Metal alkoxides are oxidized by moisture to give hydroxides and the oxides of the corresponding metal. OMCTS has proved to be a source of the moisture. Care needs to be taken to ensure that the water content lies below 2 ppm, in order that the white precipitate can be avoided. It has likewise been shown that the temperature of the pipelines through which the precursor mixtures flow has to be controlled; if it is too low, the constituents can be precipitated by condensation (E. J. Maxon, Fused SiO
2
—TiO
2
glass. U.S. Pat. No. 5,970,751, Corning INC (USA), 22.9.1998).
Another known possibility for producing clear titanium-oxide/silicate glass with 7.4 wt. % titanium dioxide has been offered by Corning for as long as 20 years with the CVD process. This is a gas-phase deposition process. The temperatures lie between 200 and 2,000° C. Depending on the manner of the supply of energy, one speaks of thermal, plasma-activated, photon-activated or laser-activated gas-phase deposition. With this method the production of a clear glass with 16 wt. % TiO
2
is possible, but the rate of deposition is relatively low. In order to obtain glass with various shapes, machining is necessary (W. T. Minehan, G. L. Messing and C. G. Pantano, Titania-silica glasses prepared by sintering alkoxide derived spherical colloids. J. Non-Cryst. Solids 108 (1989) 163-168).
The most frequently employed method for producing SiO
2
—TiO
2
glasses is the sol-gel process. In many cases this is a matter of the hydrolysis and condensation of organometallic compounds. In his U.S. Pat. No. 4,278,632, Yoldas showed a method with the aid of which clear vitreous material can be produced that mainly contains silicon dioxide and titanium dioxide, whereby the content of titanium dioxide can amount to up to 40 wt. % without the constituents having to be melted.
For the production of these glasses a clear organic solution of partially hydrolysed alkoxides is produced separately from one component of the binary system. The other component of the binary system is added to this solution in the form of alkoxides or a clear organic, partially hydrolysed solution of the alkoxides. Water is additionally added to the resulting reaction solution, in order to complete the hydrolysis of the silicon and titanium alkoxides. After the hydrolysis, the material is dried and then heated to temperatures from 400° C. to 600° C., in order to remove the residual organic constituents and to produce a clear binary material. With this method a very active glass powder can also be produced which after the shaping is sintered at temperatures of 1,200° C. for 2 hours.
Known results were achieved by the Japanese researchers T
Clasen Rolf
Deller Klaus
Mangold Helmut
Schneider Gerrit
Wagner Christine
Chin Peter
Degussa - AG
Lopez Carlos
Smith , Gambrell & Russell, LLP
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