Classifications

• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B19/00—Other methods of shaping glass
  • C03B19/12—Other methods of shaping glass by liquid-phase reaction processes

Definitions

• This invention relates generally to sol-gel processes for making articles of silica glass and, more particularly, to sol-gel processes that utilize both an alkoxide precursor and a suspension of colloidal particles.
• Sol-gel processes are useful for making articles of synthetic silica glass having shapes close to that desired for the final product. This reduces costs associated with machining and polishing the articles.
• sol-gel processes involve steps of (1) synthesizing a liquid silica solution, or sol, typically by hydrolyzing an alkoxide precursor or stabilizing a colloidal silica solution, (2) condensing the sol to yield a wet gel, (3) aging the wet gel to strengthen it, (4) drying the wet gel to produce a dry gel, and (5) sintering the dry gel to yield a dense glass article.
• the gels yielded by this sol gel route typically are quite porous, with large pore (void) volumes and relatively low volume loading of silica. Because the porous gels are later sintered into dense glass articles, the large void volumes bring about a large amount of shrinkage during the sintering step and thus yield silica articles of reduced size. It therefore follows that reducing the void volume will increase the amount of sintered silica glass that can be obtained from a single processing cycle and thus reduce manufacturing costs. Increasing the loading of silica in the sol is one way to achieve this.
• silica sols Three distinct processes have been used in the past to produce silica sols.
• One such process uses pure alkoxide precursors, but it typically yields a silica loading of only about 17 to 20%.
• a second such process uses colloidal silica in lieu of pure alkoxides. This latter process yields higher silica loading, but it requires stabilizing agents to be incorporated into the glass matrix. This can add to the cost of the process and can degrade the quality of the resultant glass.
• a third process used in the past to produce silica sols uses a combination of an alkoxide and colloidal silica.
• a hybrid colloidal silica/alkoxide sol is produced by adding fumed silica powder to an already hydrolyzed alkoxide of silica.
• the hydrolyzed alkoxide is produced initially by mixing together water and silica alkoxides, e.g., tetra-ethoxysilane (TEOS), which are initially immiscible but which dissolve into each other when an alcohol reaction product is generated in sufficient quantity.
• TEOS tetra-ethoxysilane
• hydrolysis is initiated and carried out by adding acidic water to an alkoxide and stirring the resulting mixture.
• the fumed silica powder e.g., Aerosil OX-50, is added and the solution is violently stirred.
• the addition of a large amount of fumed silica powder to the hydrolyzed alkoxide solution typically yields a non-homogeneous sol incorporating large particles of aggregated silica powder and TEOS. These particles can weaken the resulting gel, which can lead to cracking during the drying stage.
• This problem can be overcome by adding the fumed silica powder only with continuous ultra-sonication and by centrifuging the solution to remove aggregated particles, but this can add to the cost of the process.
• photoblanks typically include substrates formed of high-quality synthetic silica coated with chromium and photoresist.
• the substrates typically have been made using a traditional soot deposition and densification process, in which a large cylindrical boule of very high quality silica is first grown and then core drilled to form a smaller square shape, most commonly 15 cm x 15 cm in size. Individual blanks then are sliced from the square core and polished.
• Synthetic glass suitable for use as photoblank substrates also must avoid warpage during the final sintering step.
• This sintering step typically causes shrinkage on of the order of 50%, on a linear scale, and warpage can occur when objects of high aspect ratios, i.e., length.thickness, undergo such large shrinkages.
• Photoblanks typically can have high aspect ratios of about 25.
• the present invention resides in an improved sol-gel process for producing a synthetic silica glass article, in which a sol is formed having a high loading of silica without the need for a stabilizing agent and without the need for significant additional processing equipment or steps such as continuous ultra- sonication, centrifugation, or violent mixing of the sol. More particularly, the process includes forming a sol by introducing an aqueous colloidal suspension into an organic silicon alkoxide solution and then allowing the organic silicon alkoxide to hydrolyze into a sol containing fine aggregates of silica particles. The sol then undergoes gelation, to form a wet gel, which is in turn dried and sintered to produce a dense glass article.
• the suspended particles are broken down by chemical reaction.
• the agglomeration of the colloidal particles into particulates of excessive size e.g., greater than about 10 microns, is avoided.
• the aqueous colloidal suspension includes fumed silica powder, e.g., Aerosil OX-50 or Aerosil 200.
• Aerosil OX-50 its mole ratio to water preferably is in the range of about 1:4 to 1:8, and most preferably about 1:5.
• the sol has a silica loading of greater than about 20%, and preferably in the range of about 34 to 40%.
• the organic silicon alkoxide solution can take the form of a tetra-alkoxy-silane, preferably tetraethoxysilane, tetramethoxysilane, tetrapropoxysilane, and mono- and bi-substituents of such silanes.
• the mole ratio of the silicon alkoxide to water in the sol preferably is in the range of about 1:4 to 1:20, and most preferably about 1:6 to 1:10.
• the aqueous colloidal suspension can incorporate titania, zirconia, erbia, alumina, or combinations thereof, or it can include colloidal metal particles or colloidal particles of glass and/or metal having an outer coating of gold, silver, rhodia, platinum, or combinations thereof.
• the silica is initially purified before mixing with the organic silicon alkoxide solution. This purification ensures that the silica photoblank substrate exhibits good optical transmission at a wavelength of 200 nm. Purification can be accomplished by heating the fumed silica powder in a cUorine-containing atmosphere and then rehydrating the chlorine-treated fumed silica powder.
• the cUorine-containing atmosphere can be selected from the group consisting of chlorine gas (Cl 2 ) and thionyl chloride (SOCl 2 ).
• the step of sintering includes placing a flat weight on the substrate, to prevent warping.
• the flat weight which can be formed of a material that includes silicon carbide and/or boron nitride, preferably is initially passivated in a cMo_rine-containing atmosphere, to minhnize the transfer of impurities to the photoblank substrate.
• Figure 1 shows the optical transmission of a photoblank substrate made by the process defined in Example 8, below, as compared to the optical transmissions of two industry standard photoblank glasses, Dynasil 1100 and Suprasil 2. DETAILED DESCRIPTION OF THE PREFERRED PROCESSES
• the preferred process of the invention efficiently produces high quality silica glass articles of increased size, without the added expense and inconvenience of requiring special stabilizing agents and/or additional processing equipment or steps in the sol synthesis stage.
• high loadings of colloidal particles e.g., fumed silica powder
• an acidic water medium to produce an aqueous slurry.
• This slurry then is added to a silicon alkoxide solution and the mixture is slowly stirred together, during which time the silicon alkoxide hydrolyzes and the slurry is broken down by chemical reaction. This produces a hydrolyzed sol incorporating a suspension of very fine aggregates of colloidal particles.
• the process of the invention can achieve substantially higher loadings by weight over what can be achieved using a standard pure alkoxide process.
• a sol that is homogeneous is considered necessary for the production of a gel having the desired strength and uniformity.
• Experimental data indicates that the maximum particle size that can be tolerated in such a sol is about 10 microns.
• the process of the invention produces a sol having this characteristic, with reduced expense and process complexity.
• the colloidal particles take the form of Aerosil OX-50, a common fumed silica powder.
• sols having silica loadings in the range of 34 to 40% can be achieved. This is roughly double what can be achieved using a standard pure alkoxide process.
• the mole ratio of the OX-50 powder to water in the aqueous colloidal suspension is preferably in the range of about 1:4 to about 1:8, and most preferably about 1:5.
• the organic silicon alkoxide solution preferably comprises a tetra-alkoxy-silane such as tetraethoxysilane, tetramethoxysilane, tetrapropoxysilane, and mono- and bi- substituents of such silanes.
• the mole ratio of the tetra-alkoxy-silane to water in the sol preferably is in the range of about 1:4 to about 1:20, and most preferably in the range of about 1 :6 to about 1:10.
• the aqueous colloidal suspension can incorporate other forms of fumed silica (e.g., Aerosil 200), titania, zirconia, erbia, alumina, or combinations thereof, or alternatively it can incorporate colloidal metal particles or colloidal particles of glass and/or metal having an outer coating of gold, silver, rhodia, platinum, or combinations thereof.
• fumed silica e.g., Aerosil 200
• titania e.g., titania, zirconia, erbia, alumina, or combinations thereof
• colloidal metal particles or colloidal particles of glass and/or metal having an outer coating of gold, silver, rhodia, platinum, or combinations thereof e.g., Aerosil 200
• Aerosil OX-50 silica powder was slowly added to about 108 grams of acidified de-ionized water at a pH of 2.0, to form a viscous paste.
• the paste's OX-50:H 2 O mole ratio was about 1:5.
• the acid was selected from hydrochloric acid (HCl), nitric acid (HN0 3 ), acetic acid (CH 3 COOH), sulfuric acid (H 2 S0 4 ), and combinations thereof.
• About 208 grams of tetra-ethoxysilane (TEOS) was added to this paste, to produce a two-phase mixture having a TEOS:H 2 0 mole ratio of about 1:6.
• the silica loading of the sol was about 34% silica by weight.
• the sol could be further concentrated, using an evaporating device, to achieve 50% by weight of silica (a volume reduction of about 36%, carried out at 60° C and reduced pressure). This mixture did not settle when centrifuged at 3000g for 20 minutes. The mixture is flowable for casting and is free of any stabilizing agents.
• Aerosil OX-50 powder was slowly added to 180 grams of deionized water at a pH of 2.0, to form a viscous paste.
• the paste's Si:H 2 0 mole ratio was about 1:5.
• About 208 grams of TEOS was added to this paste, providing a two-phase mixture having a TEOS:H 2 0 mole ratio of about 1:10.
• This mixture was then slowly mixed together and, after about 90 minutes, a white, single-phase liquid was produced.
• This single-phase liquid was ultra-sonicated for 5 minutes and then centrifuged for 30 minutes at 3000g. No settling was observed, and the sol flowed smoothly through a filter paper of 10-micron mesh size.
• Aerosil OX-50 powder was slowly added to about 180 grams of deionized water at a pH of 2.0, to form a viscous paste.
• the paste's Si:H 2 0 mole ratio was about 1:10.
• About 208 grams of TEOS was added to this paste, providing a two-phase mixture having a TEOS:H 2 0 mole ratio of about 1 : 10.
• This mixture was then slowly mixed together and, after about 90 minutes, a white, single-phase liquid was produced.
• This single-phase liquid was ultra-sonicated for 5 minutes and then centrifuged for 30 minutes at 3000g. Some settling was observed, and the mixture failed to flow smoothly through a filter paper of 10- micron mesh size. This mesh filter screened out about 5 to 25% by mass of the silica.
• the silica loading of the mixture typically was in the range of about 18.5 to 22.0% silica by weight.
• the median particle size as observed by diluting the sieved sol in alcohol and measuring with a Horiba L-900 laser particle size analyzer, was 25.5 microns (see Table 1). This is substantially larger than the desired median particle size of less than about 10 microns. This example reveals the detrimental effect of reducing the mole ratio of Aerosil OX-50 silica powder to water to a value less than about 1:8.
• Aerosil OX-50 powder was slowly added to about 450 grams of deionized water at a pH of 2.0, to form a viscous paste.
• the paste's Si:H 2 0 mole ratio was about 1:5.
• About 208 grams of TEOS was added to this paste, providing a two-phase mixture having a TEOS:H 2 0 mole ratio of about 1:25.
• This mixture was then slowly mixed together and, after about 90 minutes, a white, single-phase liquid was produced.
• This single-phase liquid was ultra- sonicated for 5 minutes and then centrifuged for 30 minutes at 3000g. Some settling was observed, and the liquid failed to flow smoothly through a filter paper of 10 microns mesh size.
• the median particle size as observed by diluting the sieved sol in alcohol and measuring with a Horiba L-900 laser particle size analyzer, was 35 microns (see Table 1). This is substantially larger than the desired median particle size of less than about 10 microns. This example reveals the detrimental effect of having a TEOS:water mole ratio of less than 1:20.
• Aerosil OX-50 powder was slowly added to about 108 grams of deionized water at a pH of 2..0, to form a viscous paste.
• the paste's Si:H 2 0 mole ratio was about 1:5.
• About 208 grams of TEOS was added to this paste, providing a two-phase mixture having a TEOS:H 2 0 mole ratio of about 1 :6.
• This mixture was then slowly mixed together and, after about 90 minutes, a white, single-phase liquid was produced.
• This single-phase liquid was ultra-sonicated for 5 minutes and then centrifuged for 30 minutes at 3000g. The liquid flowed smoothly through a filter paper of 10-micron mesh size.
• the sol's silica loading was about 34% silica by weight.
• the mixture was further concentrated by evaporating alcohol under reduced pressure at about 60° C.
• About 116 grams of ethanol was evaporated to provide a final silica loading of 48.5% by mass.
• This resultant sol smoothly flowed through filter paper with a 10-micron mesh size.
• This example shows that higher silica loadings, up to about 50% by mass, can be prepared by evaporation of the chemically mixed sol.
• Aerosil OX-50 powder was slowly added to about 108 grams of de-ionized water at a pH of 2.0, to form a viscous paste.
• the paste's Si:H 2 0 mole ratio was about 1:5.
• the paste was ultra-sonicated for about 5 minutes and then diluted in alcohol and slowly stirred.
• the median particle size of this solution was then analyzed using the Horiba L900 particle size analyzer and observed to be 40 microns (see Table 1).
• the agglomerate size of the Aerosil OX-50 powder in an aqueous solution was significantly larger than the acceptable size of about 10 microns.
• Comparison with Examples 2, 3 and 6 indicates than the hydrolysis reaction of these previous Examples is effective in reducing the agglomerate size of the OX-50 particles to an acceptable level, i.e., less than 10 microns in size.
• the process of the invention is used to make a synthetic silica photoblank substrate.
• the silica powder must first be purified, to remove metal particle impurities that are considered to reduce the optical transmission of the glass at the short (200 nm) wavelengths of interest. This purification is accomplished by heating the powder to 1000°C for 5 hours in a synthetic silica furnace, after which the heated powder is exposed for 1 hour to a gas mixture containing chlorine and nitrogen, or containing thionyl chloride
• the sol then is cast into a square-shaped graphite mold having a size of about 27 cm x 27 cm.
• the graphite mold with the cast sol is introduced into an autoclave, and the sol gels within about 2 hours after which the autoclave is sealed. The gel remains within the autoclave for 20 additional hours, to complete the gelation step.
• the gel is topped off with a liquid composed of 88% ethanol and 12% deionized water and the topped-off gel then is aged.
• the topping off liquid is pumped into the autoclave to fill the autoclave chamber, and the chamber is then immediately drained, leaving the gel inside the mold topped off with the liquid.
• the temperature of the autoclave then is ramped up to 60 °C, over a span of 6 hours, and held at this temperature for 42 hours.
• the topping-off liquid including a portion that remains on the floor of the autoclave chamber following its drainage from the chamber, is in an amount sufficient to maintain a saturation pressure of about 450 mm Hg at 60 °C. This completes the in-situ aging step.
• the remaining topping liquid is drained and pure isopropanol is pumped into the autoclave at 450 mm Hg, while the temperature is maintained at 60°C.
• About 1.5 liters of isopropanol are added for an autoclave volume of about 20 liters.
• the temperature of the autoclave then is ramped to 215° C, over a span of 20 hours, and in turn to 240 °C, over a span of 22 hours.
• the autoclave's pressure is allowed to equilibrate (typically at about 42 atmospheres) at 240 °C for 1 hour.
• the pressure inside the autoclave then is slowly released over a span of 5 hours, at a rate of about 100 mm Hg/min., while maintaining the temperature at 240 °C.
• the autoclave then is cooled to room temperature, and the mold is removed. After drying is complete, a crack-free monolithic aerogel is removed from the mold.
• the size of the gel after drying is about 25 mm x 25 mm x 2.5 mm.
• This monolithic gel then is sintered between two silicon carbide (SiC) plates according the sequence set forth in Table 2.
• SiC silicon carbide
• the two SiC plates are passivated by subjecting them to a chlorine- containing atmosphere at 900 °C for 5 hours.
• Suitable passivating atmosphere include chlorine gas (Cl 2 ) or thionyl chloride (SOCl 2 ).
• This sintering step yields a flat glass substrate, of size 15 cm x l5 cm x l.5 cm.
• the flat glass substrate then is subjected to a high-temperature treatment, to dissolve inclusions.
• a high-temperature treatment can be accomplished by heating the substrate to 1800°C and mamta ⁇ iing that temperature for 12 minutes.
• This provides an inclusion-free glass substrate having an optical transmission comparable to the best UV-grade synthetic silica commercially available, i.e., Suprasil ® , available from Heraeus Quarzschmelze G.m.b.H. Corporation, and Dynasil ® , available from Dynasil Corporation of America.
• Figure 1 depicts the optical transmission of a glass substrate produced by the process of Example 8, as compared with the optical transmission of Suprasil and Dynasil substrates.
• Glass substrates made in accordance with the invention meet the Semiconductor Equipment Manufacturing Industry (SEMI) standards for transmission in the UV region ( ⁇ 200 nm).
• SEMI Semiconductor Equipment Manufacturing Industry
• the present invention provides an improved sol-gel process for producing a synthetic silica glass article, in which a sol is formed having a silica loading as high as 34 to 40%.
• This high loading is achieved by introducing an aqueous colloidal suspension into a silicon alkoxide solution and the mixture is slowly stirred together, during which time the mixture hydrolyzes and the colloidal suspension is broken down by chemical reaction.
• the need for a stabilizing agent and/or for continuous ultra-sonicating, centrifugation, or violently sthring of the sol is eliminated.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Manufacturing & Machinery (AREA)
• Materials Engineering (AREA)
• Organic Chemistry (AREA)
• Glass Melting And Manufacturing (AREA)
• Glass Compositions (AREA)
• Silicon Compounds (AREA)

Applications Claiming Priority (5)

Publications (1)

Family

ID=26874724

Family Applications (1)

Country Status (4)

Families Citing this family (5)

• 2001
  • 2001-01-23 JP JP2001553236A patent/JP2003520181A/ja active Pending
  • 2001-01-23 EP EP01903279A patent/EP1250295A1/de not_active Withdrawn
  • 2001-01-23 AU AU31118/01A patent/AU3111801A/en not_active Abandoned
  • 2001-01-23 CA CA002398876A patent/CA2398876A1/en not_active Abandoned

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events