Title of Invention

"A PROCESS FOR THE PREPARATION OF SILICA GLASS"

Abstract

A process for the preparation of silica glasses which comprises, mixing a silicon alkoxide with acetic acid at a temperature in the range of 15 - 25°C, adding water solvent azeotrope to the said mixture under stirring at a temperature in the range of 20 - 40°C for a period in the range of 5-25 hours to obtain a sol, maintaining the sol so obtained at a temperature in the range of 40-65°C to get monolithic gel, drying the resultant gel by heating to a temperature in the range of 180-250°C, heating the dried gel in an inert atmosphere at a temperature in the range of 500±50°C till completionofcarbonization of unhydrolsed alkoxide groups, cooling to room temperature in an inert atmosphere, reheating to a temperature of at least 1000°C in an oxidizing atmosphere, and finally cooling to room temperature to obtain silica glass.

Full Text

This invention relates to a process for the preparation of silica glasses. This invention particularly relates to a process using sol gel technology with water-solvent azeotropes. The sol gel technology has generated a wide spread interest due to inherent advantages of the method including : low processing temperature, high purity of the processed glass. These advantages make it a suitable method for the preparation of basic materials for fabrication of optical fibres, optical components and other engineering products. It is also suitable in semiconductor industry, electronics, scientific and laboratory uses. A Japanese patent (JP 59116135)describes a method in which additional silica powder is introduced in the sol gel mixture before gelling. Preparation of silica gel monolith using ethylenediamine as an additive is also reported(JP 63307126) Drying Control Chemical Additives (DCCA) are described as potential chemicals to avoid the crack formation. Currently for maintaining the monolithicity of the gels, DCCA are being successfully used. However, removal of DCCA, having high boiling point, at the gel to glass conversion stage leads to crack formation after its degradation. The gels obtained using DCCA have low density, high silanol content as well as carbon residue caused by the cracking of organic groups. Another method reported inEuropean patent EP 366197ywhich is being successfully used is the hypercritical drying using autoclave. Removal of water and solvent from the pores of an alcogel by heating the gel in an autoclave beyond the critical temperature and pressure of the liquid is called hypercritical drying. Hypercritical drying for an alcogel generally require a temperature of 300°C and a pressure of about 20 M Pa of an autoclave. The gels prepared using this technique have large pore size distribution and high hydroxyl content. The main object of the present invention is to provide a process for the preparation of silica glasses which obviates the drawbacks of the hitherto known processes. Another object of the present invention is to provide a process using the water-solvent azeotropes Yet another object of the present invention is to provide a process for the preparation of monolithic silica gels and glasses by hydrolytic polycondensation of silicon alkoxides. Still another object of the present invention is to provide a process for the preparation of monolithic silica gels and glasses wherein water which is responsible for stress generation and consequently crack formation during drying and which also plays a fundamental role in bloating of the gels at higher temperatures i.e. > 1000°C, is removed by azeotrope formation. Another object of the present invention is to provide a process for the preparation of monolithic silica gels and glasses which have high diametral compressive strength and high flexural strength. The process of the present invention particularly involves preparation of monolithic silica gels and glasses by sol gel technique using water-solvent azeotropes. Accordingly the present invention provides a process for the preparation of silica glasses which comprises, mixing a silicon alkoxide with acetic' acid at a temperature in the range of 15 - 25 °C, adding water solvent azeotrope to the said mixture under stirring at a temperature in the range of 20 - 40 °C for a period in the range of 5-25 hours to obtain a sol, maintaining the sol so obtained at a temperature in the range of 40-65 °C to get monolithic gel, drying the resultant gel by heating to a temperature in the range of 180-250°C, heating the dried gel in an inert atmosphere at a temperature in the range of 500±50°C till completion of carbonization of unhydrolysed alkoxide groups, cooling to room temperature in the inert atmosphere, reheating to a temperature of atleast 1000°C in an oxidising atmosphere, and finally cooling to room temperature to obtain silica glass. In an embodiment of the present invention the silicon alkoxide used may be such as tetramethoxysilane, tetraethyoxysilane. In another embodiment of the present invention the solvent used in the water-solvent azeotrope may be such as acetonitrile, 2 methoxyethanol, 1,2 dimethoxyethane. In yet another embodiment of the present invention the alkoxide to water molar ratio may be in the range of 1:2 to 1:4. In still another embodiment of the present invention acetic acid used may be in a molar ratio in the range of 0.04 - 0.06 with respect to silicon alcoxide. In yet another embodiment of the present invention the inert atmosphere may be provided by using an inert gas such as Nitrogen, Argon, Helium. In another embodiment of the present invention oxidising atmosphere may be provided by using air, Oxygen gas. Water-solvent azeotrope is added to silicon alkoxide premixed with acetic acid while stirring preferabily magnetically to obtain homogenous solution of oligomers for a period not exceeding twenty five hours at a temperature in the range of 20-40°C. The 'sol' thus obtained is maintained at a temperature in the range of 40-65°C for gelling and ageing. The resultant gel is heated to a temperature in the range of 180-250°C for drying. The dried gel is heated in an inert atmosphere to a temperature in the range of 500±50°C and cooled in inert atmosphere to room temperature and further heated to 1000°C in an oxidising atmosphere to get silica glass. The following examples are given by way of illustration and should not be construed to limit the scope of the present invention. Example 1 0.2 mole tetraethoxysilane was mixed with 0.01 mole of acetic acid at 20°C. To this 100 ml of water-acetonitrile azeotrope was added drop by drop while stirring at a speed of 1000 rpm preferably magnetically. The temperature of this homogenous solution was raised to 40°C and again stirred. The total period of stirring was done for twenty five hours. The sol thus obtained was maintained at a temperature of 50°C for gelling and aging. The resultant gel was heated to a temperature of 200°C for drying. The dried gel was heated in nitrogen atmosphere to a temperature of 520°C and cooled to room temperature and further heated to 1000°C in an air atmosphere to get silica glass. The resultant monolithic gel was subjected to surface analysis studies and density measurement. The surface area of the gel was found to be 225 m2/g. The gel was found to have the pores in the range of 10 A to 60 A out of which 80 % of the pores are in the range of 20 to 40 A. Density of the gel is found to be 1.34 g/cc. Density and surface area of the glass is found to be 2.22 g/cc and 15 m2/g at 1000°C. The diametral compressive strength and fexural strength of silica glass obtained by this method are 16 M Pa and 108 M Pa respectively. Example 2 0.2 mole tetraethoxysilane was mixed with 0.01 mole of acetic acid at 20°C. To this 100 ml of water-1,2 dimethoxy ethane azeotrope was added drop by drop while stirring at a speed of 1000 rpm preferably magnetically. The temperature of this homogenous solution was raised to 40°C and again stirred. The stirring was done for twenty hours. The sol thus obtained was maintained at a temperature range of 65°C for gelling and aging. The resultant gel was heated to a temperature of 200°C for drying. The dried gel was heated in nitrogen atmosphere to a temperature of 550°C and cooled to room temperature and further heated to 1000°C in an oxygen atmosphere to get silica glass. The resultant monolithic gel was subjected to surface analysis studies and density measurement. The surface area of the gel is found to be 240 m2/g. The gel is found to have the pores in the range of 10 A to 100 A out of which 40 % of the pores are in the range of 20 to 40 A and 10 % are in the range of 50 A to 70 A. Density of the gel is found to be 1.36 g/cc. Density and surface area of the glass is found to be 2.17 g/cc and 36 m2/g at 1000°C. The diametral compressive strength and fexural strength of silica glass obtained by this method are 12 M Pa and 85 M Pa respectively. The main advantages of the present invention are: 1. A crack free monolitic silica can be prepared. 2. The monoliths can be used for engineering applications. 3. The sol gel technique enables to prepare the polymeric network at room temperature. 4. The material prepared is in a pure form. We Claim: 1. A process for the preparation of silica glasses which comprises, mixing a silicon alkoxide with acetic acid in a molar ratio in the range of 0.04- 0.06 at a temperature in the range of 15 - 25°C, adding water solvent azeotrope of the kind such as herein described, to the said mixture under stirring at a temperature in the range of 20 - 40°C for a period in the range of 5-25 hours to obtain a sol, maintaining the sol so obtained at a temperature in the range of 40-65°C to get monolithic gel, drying the resultant gel by heating to a temperature in the range of 180-250°C, heating the dried gel in an inert atmosphere at a temperature in the range of 500±50°C till completion o carbonization of unhydrolsed alkoxide groups, cooling to room temperature in an inert atmosphere, reheating to a temperature of at least 1000°C in an oxidizing atmosphere, and finally cooling to room temperature to obtain silica glass. 2. A process as claimed in claim 1 wherein the silicon alkoxide used is selected from tetramethoxysilance or tetrethoxysilane. 3. A process as claimed in claims 1 and 2 wherein the solvent used in the water- solvent azeotrope is selected from acetonitrile, 2 methoxyethanol, 1,2 dimethoxyethane. 4. A process as claimed in claims 1-3 wherein the inert atmosphere is provided by using an inert gas such as Nitrogen, Argon, Helium 5. A process as claimed in claims 1-4 wherein oxidizing atmosphere is provided by using air or Oxygen gas. 6. A process for the preparation of silica glasses substantially as herein described with reference to the examples. Yet another object of the present invention is to provide a process for the preparation of monolithic silica gels and glasses by hydrolytic polycfondensation of silicon alkoxides. Still another object of the present invention is to provide a process for the preparation of monolithic silica gels and glasses wherein water which is responsible for stress generation and consequently crack formation during drying and which also plays a fundamental role in bloating of the gels at higher temperatures i.e. > 1000°C, is removed by azeotrope formation. Another object of the present invention is to provide a process for the preparation of monolithic silica gels and glasses which have high diametral compressive strength and high flexural strength. The process of the present invention particularly involves preparation of monolithic silica gels and glasses by sol gel technique using water- solvent azeotropes. Accordingly the present invention provides a process for the preparation of silica glasses which comprises, mixing a silicon alkoxide with acetic acid in a molar ratio in the range of 0.04- 0.06 at a temperature in the range of 15 - 25°C, adding water solvent azeotrope of the kind such as herein described, to the said mixture under stirring at a temperature in the range of 20 - 40°C for a period in the range of 5-25 hours to obtain a sol, maintaining the sol so obtained at a temperature in the range of 40-65°C to get monolithic gel, drying the resultant gel by heating to a temperature in the range of 180-250°C, heating the dried gel in an inert atmosphere at a temperature in the range of 500±50°C till completion o carbonization of unhydrolsed alkoxide groups, cooling to room temperature in an inert atmosphere, reheating to a temperature of at least 1000°C in an oxidizing atmosphere, and finally cooling to room temperature to obtain silica glass. In an embodiment of the present invention the silicon alkoxide used may be such as tetramethoxysilane, tetraethyoxysilane. In another embodiment of the present invention the solvent used in the water-solvent azeotrope may be such as acetonitrile, 2 methoxyethanol, 1,2 dimethoxyethane.

Documents:

3076-del-1998-abstract.pdf

3076-del-1998-claims.pdf

3076-del-1998-correspondence-others.pdf

3076-del-1998-correspondence-po.pdf

3076-del-1998-description (complete).pdf

3076-del-1998-form-1.pdf

3076-del-1998-form-19.pdf

3076-del-1998-form-2.pdf