JP4389890B2 - Silica gel and its production method, synthetic quartz glass powder and its production method, and synthetic quartz glass molded body and its production method - Google Patents

Description

  The present invention relates to a silica gel useful as a raw material for high-purity and high-quality synthetic quartz and a production method thereof, a synthetic quartz glass powder and a production method thereof, and a synthetic quartz glass molded body and a production method thereof.

  In recent years, quartz glass products used in the optical communication field, the semiconductor industry, and the like have been subjected to very strict management regarding purity. Such high-purity quartz glass mainly consists of (i) a method using raw natural sand powder (so-called sand) obtained by pulverizing natural quartz as a raw material, and (ii) higher purity. In the oxyhydrogen flame method using a mass of fumes obtained by adhering and growing fumes generated by decomposition of silicon tetrachloride in an oxyhydrogen flame, and (iii) organosilicon compounds such as silicon alkoxide Is produced by a so-called sol-gel method or the like using a gel obtained as a raw material.

  However, each of these manufacturing methods has advantages and disadvantages. (I) When natural quartz is used as a raw material, metal elements such as aluminum and iron are essentially contained in the quartz particles, and even if purification such as acid washing is repeated, these should be purified to 10 ppb or less. It is difficult. In addition, although (ii) silicon tetrachloride oxyhydrogen flame method can achieve high purity, there is a difficulty in cost commensurate with industry, and mass production has not been achieved. On the other hand, although (iii) the sol-gel method can be mass-produced, the raw materials, intermediates and products are destined to come into contact with the production equipment, and impurities due to equipment contact are likely to be mixed. In addition, quartz glass obtained by the sol-gel method contains fine bubbles that are generally derived from silanol groups, which is particularly problematic in the semiconductor industry such as photomasks and single crystal pulling crucibles, optical fibers, etc. Even in the optical communication field, there may be a problem during fiber drawing or use.

  The inventors of the present invention have intensively studied to solve the above-mentioned problems in the sol-gel method, and surprisingly, the bubbles in the quartz glass body obtained are derived from silanol groups in the gel, and abnormal particles mixed in the gel. We found that there was something to come from. That is, according to the study by the present inventors, a scale composed of a hard and dense silica precursor is generated and adhered to the inner wall surface of a device such as a reaction vessel, a dryer or a pipe used in the sol-gel method. Is mixed into the silica gel obtained by the sol-gel reaction. Particles composed of such scales are apparently indistinguishable from other silica gels, but unlike normal silica gels, they are hard and dense, so that even after subsequent drying and firing, carbon derived from organic groups does not easily escape, It remains in the synthetic quartz glass powder obtained by baking the gel. Since these carbon components are combusted to generate gas when the synthetic quartz glass powder is melt processed to form a quartz glass body, the resulting quartz glass body contains bubbles.

  As a result of further earnest studies based on the above findings, the present inventors have managed and reduced the amount of abnormal particles composed of scales adhering to the inner wall surface of the device, thereby reducing the target high quality. The inventors have found that synthetic quartz powder can be obtained and have reached the present invention. That is, the object of the present invention is to maintain the purity and quality of the synthetic quartz glass powder obtained by the sol-gel method extremely high, to obtain a quartz glass body without foaming, and to obtain a silica gel suitable for these purposes. .

The gist of the present invention is a method for producing silica gel by performing a sol-gel reaction that hydrolyzes alkoxysilane in a reaction vessel using at least a reaction vessel equipped with the reaction vessel, A method for producing silica gel, wherein the inner wall portion in contact with the silica gel is previously alkali washed at 50 ° C. or higher for 30 minutes or longer to reduce the scale layer attached to the inner wall portion to 2 mm or less, and then the sol-gel reaction is performed. Exist.

  According to the present invention, it is possible to obtain a high-quality synthetic quartz powder that has few impurities and does not generate bubbles when melted.

  Hereinafter, the present invention will be described in detail. First, the silica gel of the present invention is obtained by a sol-gel method. Here, the sol-gel method generally includes (1) a colloidal dispersion method in which a silica sol formed by dispersing fumed deer in water is gelled, and (2) a silica compound obtained by hydrolyzing a silicon compound such as alkoxysilane. Among them, the hydrolysis method is preferable from the viewpoints of physical properties, yield, ease of operation, and the like of the gel obtained. Examples of the raw silicon compound include silicon salts, oxides, and alkoxides. Among these, the use of alkoxides, that is, alkoxysilanes, is preferable in terms of physical properties of the gel obtained in addition to preventing impurities.

  The hydrolysis method will be specifically described. For example, an alkoxysilane and high-pure water are charged in an equivalent amount to 10 times the equivalent amount, and the hydrolysis and condensation reaction is allowed to proceed with standing or stirring to obtain silica gel. Thereafter, this can be pulverized and / or dried to obtain a dry gel powder. The alkoxysilane used may be any alkoxysilane that undergoes a hydrolysis reaction, but tetramethoxysilane, tetraethoxysilane, or the like that does not have an alkyl group directly bonded to a silicon atom is the property of the gel. To preferred. As a method for promoting the reaction, an acid or an alkali catalyst may be used. In this case, a catalyst containing no metal is preferable, and generally an organic acid, aqueous ammonia, or the like is good. The silica gel thus obtained is usually a wet gel containing several tens of percent of water, and if a sufficiently refined raw material is used, the content of metal impurities including alkali metals such as Na and K can be suppressed to a very low level. It becomes a precursor of high-purity synthetic quartz glass suitable for optical communication applications such as optical fibers. The wet gel is dried in advance or pulverized as it is to adjust to an arbitrary particle size. Since the particle size distribution in the gel dominates the particle size distribution of the synthetic quartz glass powder obtained by firing the gel, the particle size distribution of the target synthetic quartz glass powder is expected, and the shrinkage of the particles due to drying and firing is taken into account. What is necessary is just to determine the optimal particle size of silica gel.

  Usually, a wet gel of 1000 microns or less, preferably 900 microns or less, is heated at 100 ° C or more to remove moisture and organic components such as alcohol produced by hydrolysis reaction, and the moisture content is 30% by weight or less. The dry gel may preferably be 20% by weight or less, more preferably about 1 to 10% by weight. If the particle size distribution of the dry gel is set to about 100 to 500 microns by pre-heating or classification after heating, the particle size distribution of the synthetic quartz glass powder obtained by firing this is easily controlled within the desired range. can do.

  Here, the silica gel of the present invention is obtained by the sol-gel method as described above, and the number of abnormal particles described below is 200/10 g or less, preferably 100/10 g or less. And Here, abnormal particles are apparently indistinguishable from silica gel particles obtained by a normal sol-gel reaction, but the scale adhered to the inner wall of the reactor used for the sol-gel reaction and hard and densely solidified is normal. It is peeled and mixed by contact with a product of a sol-gel reaction. The abnormal particles formed by the separation of the scales are mainly composed of siloxane bonds (Si-O-Si) similar to silica gel, which is a product of normal sol-gel reaction. Many organic groups derived from the raw material silicon compound, such as groups and alkoxy groups, remain as a large amount of carbon components even after firing, and are solidified to such an extent that it is difficult to remove them. In addition, the abnormal particles often contain metal components constituting the inner wall of the apparatus, and cause the incorporation of metal impurities into the resulting synthetic quartz glass powder and, consequently, the quartz glass body obtained by melting it. There is a fear. Since such abnormal particles contain an abnormally large amount of carbon and cannot be easily removed, they can be easily detected and measured as black spots with the naked eye by heating at 800 ° C. for 15 minutes under a nitrogen gas flow. In order to control the number of such abnormal particles, these abnormal particles mixed in the gel may be removed, but it is desirable to prevent the contamination in the gel preparation process in advance. To that end, a reactor for a sol-gel reaction, that is, a reactor having an inner wall portion in contact with a liquid or gel, such as a reaction vessel, piping, pulverizer, dryer, etc. Is easy to use. Keeping the scale layer below the peeling limit thickness means that the thickness is within a range where the peeling and dropping of the scale can be substantially ignored, and is usually 2 mm or less. Therefore, when the thickness becomes 2 mm or more, the inner wall portion is cleaned. This cleaning method is not particularly limited as long as the scale layer can be kept within the peeling limit thickness, and is performed by, for example, alkali cleaning. In order to wash efficiently, alkaline water (caustic soda is generally used) is heated to 50 ° C. or higher, preferably 70 ° C. or higher. The washing time is 30 minutes to 10 hours, preferably 1 to 4 hours. After alkali washing, washing is performed several times to several tens of times with pure water. In addition to alkali cleaning, friend washing with silica gel slurry is also effective. In addition, since the scale layer is useful as a protective layer for the inner wall base material if the thickness is within the separation limit thickness, it is desirable to hold the scale layer for about 10 μm or more.

  The silica gel of the present invention thus obtained is further baked to remove residual carbon and silanol, and closed to obtain a synthetic quartz glass powder. Desirable synthetic quartz glass powder usually exhibits a particle size distribution of 75 to 700 microns, preferably 75 to 400 microns (at least 90 wt% or more is included in the particle size distribution), and has less than 5 ppm carbon and 50 ppm silanol or less. By firing the silica gel of the present invention in which the number of abnormal particles is controlled, a high-quality synthetic quartz glass powder having such desirable range of physical properties and no foaming when melted can be obtained. Further, the synthetic quartz glass powder obtained by the present invention has a very small impurity content of other elements, for example, Fe and Cr are 1 ppm or less, and Na and K are 100 ppb or less. This is thought to be due to the prevention of contamination from the wall surface material due to scale contamination and dropout.

EXAMPLES Next, although an Example demonstrates this invention still in detail, this invention is not limited to a following example, unless the summary is exceeded.
[Example 1]
A jacket heating type SUS304 reactor was charged with tetramethoxysilane and 5 equivalents of water, and reacted at 65 ° C. for 30 minutes to obtain a wet gel. Next, the wet gel was pulverized with a SUS304 corn mill type pulverizer, charged with the pulverized wet gel obtained from a SUS304 conical dryer, dried at 140 ° C. for 3 hours while rolling, and then the gel was dried in the conical dryer. It was washed with water for 4 hours and dried to obtain a dry gel having a water content of about 10% by weight.

  The dry gel thus obtained was classified to obtain a particle size distribution of 100 to 500 microns (95% by weight or more before classification). 10g of the dry gel after classification is accurately weighed, heated in an electric furnace under nitrogen gas flow at 800 ° C for 15 minutes, taken out from the electric furnace, and visually measured for the number of sunspot particles. When the number of particles was examined, it was 10. Moreover, the dry gel after the classification was baked by raising the temperature from room temperature to 1200 ° C. over 4 hours in a rotary furnace under air circulation to obtain synthetic quartz glass powder. When 50 g of the obtained synthetic quartz glass powder was visually observed, the number of black spot particles was one. This synthetic quartz glass powder was melted at 1700 ° C. using a known Bernoulli melting apparatus to obtain 10 g of a quartz glass ingot. As a result of visual observation, no bubbles were found in the ingot, and the product was of good quality.

[Example 2]
In the same manner as in Example 1, the operations of reaction, pulverization, drying and classification were repeated 2 to 50 times. In 10 g of the dry gel after classification, the number of abnormal particles detected as black spots by heating at 800 ° C. as in Example 1 was 50 for the 10th, 45 for the 20th, 55 for the 30th, and 60 for the 40th. The 50th time was 70 pieces. When the amount of metal impurities in the dry gel after classification obtained by the tenth operation was measured, Na and K were each 10 ppb or less by atomic absorption method, and Cr and Fe were 10 ppb each by ICP (high frequency induction plasma method). Hereinafter, it was 0.03 ppm. Further, after the 50th operation, the thickness of the scale layer on the inner wall of the conical dryer was measured and found to be 1 mm.

  When these gels were baked in the same manner as in Example 1, the number of black spot particles in the obtained synthetic quartz glass powder was in the range of 1 to 3 in 50 g. Furthermore, when these synthetic quartz glass powders were melted in the same manner as in Example 1 to obtain 10 g of ingots, none of the bubbles were found and good quality products were obtained. Further, the reaction, pulverization, drying and classification were repeated up to 100 times in the same manner as in Example 1, and then the entire apparatus was washed with caustic soda (0.1% solution) at 80 ° C. for 3 hours while evacuating. The scale of the inner wall of the apparatus was removed. When the reaction, pulverization, drying, and classification were performed for the first time after alkali washing, that is, 101 times in total, the number of abnormal particles detected as black spots by heating at 800 ° C. as in Example 1 was 2 in 10 g of the gel. It was. The dry gel after classification was also fired in the same manner as in Example 1. As a result, the obtained synthetic quartz glass powder had no black spot particles in 50 g. Further, when this synthetic quartz glass powder was melted in the same manner as in Example 1 to obtain a 10 g ingot, it was of a high quality without bubbles.

[Comparative Example 1]
The reaction, pulverization, drying and classification operations were repeated 100 times in the same manner as in Example 1. In 10 g of the dry gel after classification obtained at the 100th time, the number of abnormal particles detected as black spots by heating at 800 ° C. as in Example 1 was 255. When this dry gel was baked in the same manner as in Example 1, there were 20 black spot particles in 50 g of the obtained synthetic quartz glass powder. Furthermore, when this synthetic quartz glass powder was melted in the same manner as in Example 1, the obtained 10 g of ingot was found to have 20 bubbles by visual observation and was not resistant to use as a quartz glass molded body. . After the 99th operation, the thickness of the scale layer on the inner wall of the conical dryer exceeded 2 mm.

Claims (3)

A method for producing silica gel by performing a sol-gel reaction in which at least a reaction vessel equipped with a reaction vessel is used to hydrolyze alkoxysilane in the reaction vessel,
The inner wall portion in contact with the liquid in the reaction vessel or the silica gel is previously washed with alkali at 50 ° C. or higher for 30 minutes or longer to reduce the scale layer adhering to the inner wall portion to 2 mm or less, and then the sol-gel reaction is performed. A method for producing silica gel. The method for producing silica gel according to claim 1, wherein a jacket heating type reactor is used as the reaction vessel. The method for producing silica gel according to claim 1 or 2, wherein the inner wall of the reaction vessel comprises a metal component .