PROCESS FOR THE PREPARATION OF SILICA GLASS POWDERS

Abstract: Silica glass powders are prepared by mixing a silicic acid ester, water in an amount equal to or larger than 0.5 times of gram equivalent of an alkoxy group of the silicic acid ester and siliceous particles and dispersing the siliceous particles which have particle sizes of larger than 400 µ m in an amount of less than 10% by weight and particle sizes of less than 20 µ m in an amount of less than 50% by weight and which are dispersed in an amount equal to or larger than 10% by weight of the weight of silica prepared by roasting the silicic acid ester in the same amount as used for the preparation of the mixture, gelling the dispersion to give a gel, drying the gel, and sinterring the dried gel. The silica glass powders of high purity is suitable for varioususes such as raw materials for silica glass products for semiconductor industry and for multicomponent glass for optical communication.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a process for the preparation of silica glass powder and, more particularly, to a process for the preparation of silica glass powder by a simple operation and with high efficiency, which requires no mechanical pulverizing step which leads to contamination by impurities. The process is able to provide highly purified silica glass powder capable of being used for various purposes, such as a raw material of silica glass products for the semiconductor industry and multicomponent glass in the field of optical communication. 2. Description of Related Art Recently, there have been increasing demands to purify silica glass powder to be used, for example, as a raw material for silica glass product for the semiconductor industry and multicomponent glass in the field of optical communication. At present, there has been a demand to provide a silicon single crystal of extremely high purity as integration of semiconductor elements developes to a greater extent. As a result highly purified crucibles for preparing such highly purified silicon single crystals have also been demanded. Heretofore, the crucibles used for the preparation of silicon single crystals for use in a semiconductor have been prepared by melting finely pulverized powder of natural quartz and sinterring it in a carbon arc furnace. The finely pulverized powders of natural quartz, however, suffer from contamination by impurities such as aluminium resulting from alumina ball mills used in the pulverizing step, in addition to impurities contained in the natural quartz itself. The crucibles usually contain impurities such as Al in amounts as high as 10 ppm, Fe, Ti, Zr, Na, Ca and K in amounts as high as several ppm, as well as B, Cu and Ni in ammounts of several 10th ppm. If these crucibles are used to prepare silicon single crystals, the impurities contained therein may be transferred to the silicon single crystal, thereby reducing the high purity of the silicon single crystal. As a result, a silicon single crystal of an extremely high purity can not be produced. Further, a transparent silica glass tube of high purity has been demanded for use as a furnace core tube and so on in the thermal processing step of semiconductor wafers. The transparent silica glass tubes have conventionally been prepared from a natural rock crystal containing minute quantity of impurities such as aluminium, alkali metal, etc., so that the transparent silica glass tubes resulting therefrom may also contain such impurities. Hence, conventional silica glass tubes suffer from the problems of heat transformation and lack of transparency due to contamination from these impurities. It is generally known to the art that heat transformation and lack of transparency in silica glass material may occur to a remarkable extent in instances where an alkali metal is present as an impurity. In this respect, high purity silica glass powder is required as the raw material. Mechanical pulverization of natural quartz or synthetic quartz for the preparation of powder of the raw material has the problem that the yield of the powder it produces having a particle size of 75µ m to 300µ m is as low as 50% or lower, while the rest is fine powder having particle sizes of 75 µ m or smaller. SUMMARY OF THE INVENTION The present invention has the object of providing a process for the preparation of silica glass powder of high purity with high efficiency, which can solve the problems encountered when using conventional processes and satisfy the demands of industry. As a result of an extensive search for a solution to the above problems, it has been found that silica gel powder can readily be prepared during drying a gel obtained by dispersing a particular amount of siliceous particles with a particular range of particle sizes in a mixture of a silicic acid ester with a particular amount of water and gelling the resulting dispersion. Highly pure powder of synthetic silica glass in a pulverized form can readily be obtained by baking the resulting silica gel. The present invention has been completed on the basis of this finding. In order to achieve the above object, the present invention provides a process for the preparation of silica glass powder comprising a step of preparing a dispersion of siliceous particles in a silicic acid ester and water in an amount equal to or larger than 0.5 times of the gram equivalent of the alkoxy group of the silicic acid, wherein said siliceous particles contain particles having particle sizes of larger than 400µ m in an amount of less than 10% by weight and particles having particle sizes of less than 20µ m in an amount of less than 50% by weight and wherein said siliceous particles are dispersed in an amount equal to or larger than 10% by weight of the weight of silica prepared by roasting the silicic acid ester in an amount which is the same as used for the preparation of the mixture; the step of gelling the dispersion to give a gel; the step of drying the gel; and the step of baking the dried gel to produce the silica glass powder. DESCRIPTION OF THE PREFERRED EMBODIMENTS (1) Kinds of silicic acid esters The silicic acid ester may include, for example, methyl silicate, ethyl silicate, propyl silicate or any other silicic acid ester which is capable of forming a solution with water in the presence of an acid or an alkali. In particular, it is preferable to use a silicic acid ester whose alkoxy group has from one to three carbon atoms because such a silicic acid ester can be rapidly hydrolyzed. (2) Amounts of water, etc. In the process according to the present invention, water may be used in an amount which is equal to or greater than 0.5 times, preferably ranging from 2 to 10 times, more preferably ranging from 2 to 5 times, of the gram equivalent of the alkoxy group in the silicic acid ester. If the amount of water is smaller than 0.5 times the gram equivalent of the alkoxy group in the silicic acid ester, hydrolysis does not proceed sufficiently resulting in the remaining alkoxy groups being carbonated, when sinterred, causing the product to turn black. It is preferred to use water which has been purified to a sufficient extent. In the process according to the present invention, a hydrolytic catalyst may be used together with water. The hydrolytic catalyst may include, for example, an acid or an alkali, although there is no restriction imposed upon its kind as long as it can adjust the pH of the mixture of the silicic acid ester with water. The acid may preferably include, for example, nitric acid, oxalic acid and acetic acid. The alkali may preferably include, for example, ammonia, triethylamine and ethylene diamine. The pH of the mixture is preferably in take range of from about 3 to 10. This allows the hydrolysis of the silicic acid ester to proceed rapidly when the catalyst is used, although no limit is placed on the pH of the mixture. (3) Siliceous particles The siliceous particles to be dispersed in the mixture of the silicic acid ester with water contain particles having particle sizes greater than 400 µ m in an amount of less than 10% by weight and having particle sizes smaller than 20 µ m in an amount of less than 50% by weight; preferably having particle sizes greater than 300 µ m in an amount of 10% by weight or smaller and having particle sizes smaller than 20 µ m in an amount of less than 30% by weight. If the siliceous particles having particle sizes greater than 400µ m are used in an amount which is equal to or greater than 10% by weight or the siliceous particles having particle sizes smaller than 20µ m are used in an amount equal to or greater than 50% by weight, particles having particle sizes larger than 300 µ m may be increased, in particular whilst the yield of particles having particle sizes in the range from 75µ m to 300µ m may be decreased. This is undesirable because it is particles having a size within this range which are preferred as the raw material powder for forming quartz crucibles. The amount of the siliceous particles in 10% by weight or greater, preferably 20% by weight or greater, with respect to the weight of silica formed when the silicic acid ester is roasted in the same amount as the silicic acid ester used for the mixture with water. If the amount of siliceous particles is less than 10% by weight with respect to the weight of silica formed when the silicic acid ester is roasted in the same amount as the silicic acid ester used for the mixture with water, the yield of silica glass powder may be reduced remarkably. The siliceous particles to be used may include, for example, silica gel, synthetic silica powders, natural quartz flour and so on. In particular, in order to provide silica glass powder of high purity, it is preferred to re-use particles having particle sizes of 100µ m or larger after classification from the synthetic silica glass powder synthesized by the process according to the present invention . (4) Procedures for dispersing siliceous particles and drying the gel: In the process according to the present invention, the siliceous particles may be dispersed and gelled according to one of the following manners. As the first manner, the siliceous particles are dispersed and gelled by adding the siliceous particles to the mixture of silicic acid ester with water in a rotary drier, for example in a rotary evaporator, and then by heating the resulting mixture preferably at a temperature ranging from 20 to 60°C. As the second manner, the siliceous particles are dispersed by adding water to the mixture of silicic acid ester and siliceous particles or silicic acid ester to the mixture of water and siliceous particles in a ratary drier and then by heating the resulting mixture as in the above first manner. In both of the above manners, the siliceous particles have particle sizes of greater than 400 µ m in an amount less than 10% by weight and particle sizes of smaller than 20 µ m in an amount less than 50% by weight and are added in an amount of 10% by weight of greater with respect to the weight of silica formed when the silicic acid ester is roasted in the same amount as the silicic acid ester used for the mixture. It is important that the siliceous particles are dispersed well in the mixture of the silicic acid ester and water. If the extent of dispersion is insufficient, the distribution of particle sizes of the resulting silica glass powders is not steep. In the process according to the present invention, the gel formed is dried at a temperature usually higher than 100 °C. The gel is dried, for example, with a drier set to a predetermined temperature. (5) Sinterring temperature and time period: The sinterring temperature may be above 1,000 °C like the sinterring temperature in a so-called sol-gel method. The sinterring time period may be usually 1 hour or longer. (6) Miscellaneous: The synthetic silica glass powder as prepared hereinabove may be in a pulverized shape and may be used conveniently as a raw material for silica glass products for use in the semiconductor industry, particularly quartz crucibles, and as a raw material for multicomponent glass in the field of optical communication because of their high purity. The present invention will now be described by way of variety of examples and comparative examples. Example 1: A three-necked glass flask having stirring blades covered with Teflon was charged with 3,044 grams (methoxy group; 80 gram equivalent) of methyl silicate. 4,320 grams (240 gram equivalent) of ion-exchanged water was gradually added into the flask at 20°C over a 2 hour period. During its addition hydrolysis proceeded. Thereafter, the reaction mixture was transferred to a rotary evaporator and the evaporator then was charged with 240 grams of silica glass powder (having an average particle size of approximately 60 µ m) having 3% by weight of particle sizes exceeding 150 µ m and 25% by weight of particle sizes smaller than 20 µ m. It was found that, when 3,044 grams of methyl silicate was baked, silica was obtained in the yield of 1,202 grams. The rotary evaporator was then rotated at 40 °C, dispersing the silica glass powder well to give a gel which, in turn, was dried at 150°C and reduced pressure of 13.3 kPa (100 mmHg). The dried gel was then pulverized during drying to yield a powder having a maximum particle size of 350 µ m and an average particle size of 214 µ m. The gel in a pulverized form was transferred to a silica glass crucible and the gel was sinterred at 1,050°C for 4 hours, yielding a synthetic silica powder which, in turn, was filtered to collect particles having particle sizes ranging from 75 µ m to 300µ m, which are suitable for formation of quartz crucibles. It was also found that the synthetic silica powder was obtained in a yield of 74% based on silica obtained from methyl silicate used for hydrolysis. The X-ray analysis of the silica glass powder revealed no peak corresponding to crystal such as rock crystal and the pattern was broad like usual silica glass. Hence, it was confirmed that the powder was silica glass. The amount of impurities was observed for natural products and powders of this example by luminescent spectrometry analysis. The results are shown in the following TABLE: Example 2: A dried gel was obtained in the same manner as in Example 1, except that 4,166 grams of ethyl silicate (ethoxy group, 80 gram equivalent) was used instead of 3,044 grams of methyl silicate. The dried gel was found to have a maximum particle size of 350 µ m and an average particle size of 232 µ m. The gel in a powdery form was then sinterred in an electric furnace in the same manner as in Example 1, thereby resulting in silica glass powder in a pulverized form. The silica glass powder was filtered, thereby collecting synthetic glass powder having particle sizes in the range from 75 µ m to 300µ m in the yield of 79% based on silica formed from ethyl silicate used for hydrolysis. Example 3: A dried gel was obtained in the same manner as in Example 1, except for the use of ion-exchanged water whose pH was adjusted to 3 with nitric acid. The dried gel was found to have a maximum particle size of 350 µ m and an average particle size of 216µ m. The gel in a powdery form was then sinterred in an electric furnace in the same manner as in Example 1, producing silica glass powder in a pulverized form. The silica glass powder was then filtered, thereby collecting synthetic glass powders having particle sizes in the range from 75 µ m to 300 µ m in the yield of 76% based on silica formed from ethyl silicate used for hydrolysis. Comparative Example 1: A dried gel was obtained in the same manner as in Example 1, with the exception that silica glass powders were not added to a hydrolyzed solution. The dried gel was found to be in a granular form, not in a powdery form, having an average particle size as large as 3 mm. The gel in a powdery form was then sinterred in an electric furnace in the same manner as in Example 1, producing silica glass powder in a pulverized form. The silica glass powder was filtered, thereby collecting synthetic silica glass powders having particle sizes in the range from 75 µ m to 300 µ m only in a yield of 5% based on silica formed from ethyl silicate used for hydrolysis. Comparative Example 2: In Comparative Example 1, the gel in the granular form prepared by sintering was pulverized using an alumina ball mill and balls, thereby yielding silica glass powder having an average particle size of 205µ m. The silica glass powder was then filtered to collect particles having particle sizes ranging from 75 µ m to 300µ m. The yield of silica glass powder was 39% based on silica formed from methyl silicate used for hydrolysis. It was further found that aluminium was contained in the amount of 32 ppm as an impurity. As have been described hereinabove, the process according to the present invention presents the advantages that contamination of impurities can be prevented because no mechanical pulverizing step is required; silica glass powders of high purity can be prepared by a simple operation; a distribution of particle sizes of the powders formed is steep; and the yield is high. Hence, the process according to the present invention is an industrially useful one for the preparation of silica glass powders.

Claims: A process for the preparation of silica glass powder comprising: preparing a dispersion of siliceous particles in a silicic acid ester and water in an amount equal to or larger than 0.5 times of gram equivalent of an alkoxy group of the silicic acid, wherein said siliceous particles contain particles having particle sizes of larger than 400µ m in an amount of less than 10% by weight and particles having particle sizes of less than 20µ m in an amount of less than 50% by weight and wherein said siliceous particles are dispersed in an amount equal to or larger than 10% by weight of the weight of silica prepared by roasting the silicic acid ester in an amount which is the same as used for the preparation of the dispersion; gelling the dispersion to give a gel; drying the gel; and baking the dried gel to produce the silica glass powder A process as claimed in claim 1, wherein the silicic acid ester has an alkoxy group having from one to three carbon atoms. A process as claimed in claim 1, wherein the silicic acid ester is methyl silicate, ethyl silicate or propyl silicate. A process as claimed in claim 1, wherein the mixture contains a hydrolytic catalyst. A process as claimed in claim 4, wherein the hydrolytic catalyst is an acid or an alkali. A process as claimed in claim 5, wherein the acid is nitric acid, oxalic acid or acetic acid. A process as claimed in claim 4, wherein the alkali is ammonia, triethylamine or ethylene diamine. A process as claimed in claim 1, wherein the siliceous particles are formed of silica gel, synethetic silica powder, natural quartz flour or synthetic quartz glass powder. A process as claimed in claim 1, wherein the siliceous particles contain less than 10% by weight of particles having particle sizes exceeding 300 µ m and 30% by weight of particles having particle sizes less than 20µ m. A process as claimed in claim 1, wherein said siliceous particles are dispersed in an amount equal to or larger than 20% by weight of the weight of silica prepared by roasting the silicic acid ester in an amount which is the same as used for the preparation of the mixture. A process as claimed in claim 1, wherein said dispersion is heated at a temperature ranging from 20 °C to 60°C . A process as claimed in claim 1, wherein the gel is dried at a temperature above 100°C.

Inventors: ASAMI, MUNEO, YASUEDA, MINORU
Applicants: CHISSO CORPORATION
Application year: 1990
Main classification: C01B33
IPC codes: C03C12, C03B8, C03B20, C01B33, C03C1