Process for producing thin glass film by sol-gel method

Abstract: A thin glass film having a large film thickness is produced by a sol-gel method comprising applying a hydrolyzable solution which contains a metal alkoxide, water, an alcohol and an acid, contacting the applied solution to an atmosphere containing ammonia to polycondensate the metal alkoxide and heating it to form a thin glass film.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an improved process for producing a thin glass film by a sol-gel method. Description of the Related Art A sol-gel method comprises hydrolyzing and poly-condensating a metal alkoxide in a Solution containing the metal alkoxide, water and an alcohol (hereinafter referred to as "hydrolyzable solution") to form a porous solid material and heating the porous material to produce a glass material. Since the sol-gel method produces the glass through the hydrolyzable solution, it is easy to uniformly coat a wide substrate with the glass. Then, the sol-gel method is a promising method for coating the substrate with a glass film having a new function of mechanical properties, chemical protection and optical properties. As described in, for example, "YOGYO KYOKAI-SHI" (Ceramic Society Journal), 90 (6), 328-332 (1952), a general procedure of the sol-gel method comprises mixing ethyl silicate, titanium isopropoxide, ethanol, water and HCl as a catalyst to prepare a hydrolyzable solution (a coating liquid), dipping a substrate in the hydrolyzable solution, lifting up the substrate at a constant rate to form a film of the hydrolyzable solution on the substrate and drying and heating the film to obtain a glass film. US-A-4636440 discloses a process for producing a glass-like film, which comprises applying an aqueous alcoholic solution of a partially hydrolyzable metal alkoxide to a substrate. The solution may include an acid to partially hydrolyze the alkoxide such as hydrochloric acid or nitric acid. After the solution is applied, the substrate is coated with an aqueous compound such as ammonium hydroxide. Hitherto, the sol-gel method can produce a transparent uniform coating film having a thickness of 0.1 to 0.3 micrometers. However, though it is tried to produce a glass film having a thickness larger than 0.3 µmm, the glass film tends to have various defects such as cracks, white turbidity, peeling from the substrate. To increase the film thickness, it is known to add glycerin or ethyl ether as a thickener. By the addition of the thickner, the maximum film thickness reaches about 0.5 µm. SUMMARY OF THE INVENTION One object of the present invention is to provide a process for producing a thin glass film by a sol-gel method. Another object of the present invention is to provide a process for producing a uniform thin glass film by a sol-gel method by which a film thickness is well controlled. According to the present invention, there is provided a process for producing a thin glass film which comprises the steps of: applying to a substrate a hydrolyzable solution which comprises a metal alkoxide, water, an alcohol and an acid, wherein the molar ratio of said alcohol to said metal alkoxide is from 0.5:1 to 5:1, and the molar ratio of water to said alkoxide is from 1:1 to 10:1, contacting said solution with an atmosphere comprising ammonia and an alcohol to polycondensate said metal alkoxide; and heating the polycondensed solution to form a thin glass film. DETAILED DESCRIPTION OF THE INVENTION By the conventional sol-gel method, the coated film shrinks and solidifies during heating. In this step, if the polycondensation has not been sufficiently proceeded, the film cannot stand against a stress caused by shrinkage, in particular when the film thickness is large, and then the film tends to be cracked. According to the present invention, since the polycondensation may be accelerated by contact to ammonia, the film can stand against the stress caused by shrinkage even in case of the thick film, and the transparent uniform glass film is obtained. Probably, ammonia may act as a catalyst of the polycondensation of the metal alkoxide, and its amount can be a catalytic amount. In the hydrolyzable solution to be used in the process of the present invention, contents of the alcohol and water should be low. A molar ratio of the alcohol to the metal alkoxide is from 0.5:1 to 5:1, preferably from 1:1 to 4:1. A molar ratio of water to the metal alkoxide is from 1:1 to 10:1, preferably from 2:1 to 5:1. As the metal alkoxide, any of the conventionally used alkoxides may be used. Preferred examples are methoxide, ethoxide and propoxide of silicon, titanium, germanium, aluminum and baron. Also, as the alcohol, any of the conventionally used ones may be used. Examples are methanol, ethanol, isopropanol, butanol and the like. The acid contained in the hydrolyzable solution accelerates the hydrolysis of the metal alkoxide. Specific examples of the acid are hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid. The acid is used in an amount such as to adjust pH of the solution to, for example, 1 (one) to 6. The hydrolyzable solution may contain a surfactant (e.g. higher alcohols such as polyethylene glycol) so as to improve adhesion of the solution to the substrate and to stably form the thin glass film. To the hydrolyzable solution, a rare earth element or its compound, or an optical non-linear material may be added. Examples of the rare earth metal or its compound are Er, Nd or their salts. By the addition of these elements or compounds, the formed glass film can be used to generate a laser beam. Examples of the optically non-linear material are organic materials such as p-NA and MNA, or semiconductive particles such as CdS, CuCl, PbS and the like. By the addition of the optical non-linear material, the formed glass film has the optical non-linearity. In the process of the present invention, the hydrolyzable solution applied on the substrate is subjected to the polycondensation reaction while contacting it to the atmosphere comprising ammonia and an alcohol (e.g. methanol, ethanol, isopropanol). Though a temperature and pressure may be room temperature and atmospheric pressure, higher temperature and higher pressure will be more effective. When the polycondensation is carried out at an elevated temperature, the reaction is accelerated so that the film can stand against the stress caused by shrinkage. However, a temperature higher than 300°C will evaporate the solvent vigorously, so that the film shrinks before the polycondensation proceeds sufficiently and is cracked. The higher pressure also effectively accelerates the polycondensation and prevents the evaporation of the solvent. Up to 0.51 MPa (5 Atm.), the effect increases, but at higher pressure, the increase of the effect is not remarkable. In addition, the too high pressure makes an apparatus large and complicate. Then, the polycondensation is preferably carried out at a temperature from room temperature to 300°C under pressure from atmospheric pressure to 0.51 MPa (5 Atm). Other steps of the process of the present invention can be carried out under the same conditions as those in the conventional sol-gel method. For example, the hydrolyzable solution may be applied to the substrate by dip coating or spin coating. The substrate may be any one of the conventionally used material, for example, glass, metals or plastics. In the dip coating, the substrate is dipped in the hydrolyzable solution and lifted up gradually to form a thin film of the hydrolyzable solution on the substrate surface. In the spin coating, the substrate is set in a sample holder which can be rotated at a high speed and the hydrolyzable solution is dropped on the substrate rotating at 500 to 5000 rpm to form a uniform thin film of the hydrolyzable solution on the substrate surface. Preferably, a viscosity of the hydrolyzable solution is increased before the application onto the substrate. In one preferred embodiment, the hydrolyzable solution is aged to adjust its viscosity at room temperature in a range 0.01 - 1 Pa.s (from 0.1 to 10 poises). By the increase of the viscosity of the hydrolyzable solution, it is easy to control a film thickness of the hydrolyzable solution on the substrate. When the viscosity is less than 0.01 Pa.s (0.1 poise), the film thickness cannot be increased. When the viscosity is larger than 1 Pa.s (10 poises), the film surface has unevenness and the finally formed glass film is not uniform. The viscosity can be changed by changing the aging time. In another preferred embodiment, the hydrolyzable solution is cooled to a temperature of -150°C to +10°C and then applied on the substrate which is cooled to the same temperature as the hydrolyzable solution or lower. When the cooling temperature is lower than -150°C, the viscosity becomes too large to form a uniform film. When the substrate temperature is higher than the cooled hydrolyzable solution, the temperature of the applied solution is raised and the viscosity is decreased. In a further preferred embodiment, the hydrolyzable solution is subjected to reduced pressure, for example, from 1.33 kPa to 66.7 kPa (from 10 to 500 Torr.) to evaporate volatile components such as the alcohol. By the increase of the viscosity of the hydrolyzable solution, the film thickness of the hydrolyzable solution applied on the substrate can be increased. Thereby, the cracking of the formed film due to shrinkage can be prevented during the subsequent heating of the film. PREFERRED EMBODIMENTS OF THE INVENTION The present invention will be illustrated by the following Examples, which do not limit the scope of the present invention. Example 1 In a beaker, silicon ethoxide (50 ml), ethanol (20 ml), water (20 ml), 12 N hydrochloric acid (0.6 ml) and polyethylene glycol (4 g) were charged and stirred for about 30 minutes to obtain a hydrolyzable solution, which was aged at room temperature for 24 hours. As a substrate, a soda lime silica glass plate (a width of 40 mm, a length of 50 mm and a thickness of 1 mm) was cleaned with acetone. Then, on the substrate, the aged hydrolyzable solution was spin coated. Before the coating, the hydrolyzable solution had a viscosity of 0.4 Pa.s (4 poises). Then, the substrate carrying the film of the hydrolyzable solution was kept in an atmosphere of a mixture of ammonia and ethanol (a volume ratio of 1 : 1) at 60°C for about one hour, and then dried at 100°C in air for 24 hours followed by heating at 500°C in air to obtain a solidified transparent glass film having a thickness of 1.7 µm. Example 2 In the same manner as in Example 1 except that the hydrolyzable solution was cooled to -50°C or -100°C and the substrate was cooled to -50°C or -100°C, the hydrolyzable solution was spin coated on the substrate, dried and heated to obtain a transparent glass thin film having a thickness of 1.2 µm Comparative Example 1 In the same manner as in Example 2 except that the hydrolyzable solution and the substrate were both cooled to -180°C, a glass thin film was formed but had unevenness on the film surface. Example 3 In a beaker, silicon ethoxide (50 ml), titanium isopropoxide (1 ml), ethanol (10 ml) and 1 N hydrochloric acid (20 ml) were charged and stirred for about 30 minutes to obtain a hydrolyzable solution, which was kept under pressure of 1.33 kPa (10 Torr.) for 15 minutes to concentrate the solution. Then, in the same manner as in Example 1, the concentrated hydrolyzable solution was spin coated on the substrate, dried and heated to obtain a solidified transparent glass film having a thickness of 1.5 µm. Example 4 In a beaker, silicon ethoxide (50 ml), ethanol (10 ml), 1 N hydrochloric acid (20 ml) and erbium chloride (900 mg) were charged and stirred for about 30 minutes to obtain a hydrolyzable solution. Then, a quartz substrate was dipped in the hydrolyzable solution and lifted up at a constant rate to coat the substrate with the hydrolyzable solution. The substrate having the coated hydrolyzable solution film was treated in the same manner as in Example 1 to obtain a Er-doped transparent glass film having a thickness of 1 µm.

Claims: A process for producing a thin glass film which comprises the steps of: applying to a substrate a hydrolyzable solution which comprises a metal alkoxide, water, an alcohol and an acid, wherein the molar ratio of said alcohol to said metal alkoxide is from 0.5:1 to 5:1, and the molar ratio of water to said alkoxide is from 1:1 to 10:1, contacting said solution with an atmosphere to polycondensate said metal alkoxide; and heating the polycondensed solution to form a thin glass film, characterised in that the atmosphere for polycondensating the metal alkoxide comprises ammonia and an alcohol. A process according to claim 1, wherein said hydrolyzable solution is kept standing before application to adjust its viscosity to 0.01-1 Pa.s (0.1 - 10 poises). A process according to claim 1, wherein said hydrolyzable solution is cooled to a temperature of -150° to +10°C and then applied on said substrate cooled to the same temperature as that of said hydrolyzable solution or lower. A process according to claim 1, wherein said hydrolyzable solution is subjected to reduced pressure to concentrate said solution before application. A process according to claim 1, wherein said applied hydrolyzable solution is polycondensated at a temperature of room temperature to 300°C. The process according to claim 1, wherein said applied hydrolyzable solution is polycondensated under pressure of from atmospheric pressure to 0.51 MPa (5 Atm). The process according to claim 1, wherein said metal alkoxide is at least one selected from the group consisting of methoxide, ethoxide and propoxide of silicon, titanium, germanium, aluminum and boron. The process according to claim 1, wherein said hydrolyzable solution contains at least one compound of a lanthanide. The process according to claim 1, wherein said hydrolyzable solution contains at least one optically non-linear material selected from optically non-linear organic material or semiconductive particles.

Inventors: HOSHINO SUMIO, ITO MASUMI, KANAMORI HIROO, HOSHINO, SUMIO, ITO, MASUMI, KANAMORI, HIROO, HOSHINO, SUMIO, C/O YOKOHAMA WORKS, ITO, MASUMI, C/O YOKOHAMA WORKS, KANAMORI, HIROO, C/O YOKOHAMA WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LIMITED
Application year: 1991
Main classification: C03C17
IPC codes: C03C17, C03B8, C03C1, C23C18