JPH0581523B2 - - Google Patents

Description

[Detailed description of the invention]

(Technical field) The present invention relates to a method for producing high-purity cordierite powder, and in particular, the present invention relates to a method for producing high-purity cordierite powder, and in particular, a cordierite powder that can produce a pure cordierite sintered body without cracks and having excellent properties such as strength with good reproducibility. The present invention relates to a method for advantageously producing yerite powder. (Background technology) Cordierite porcelain has traditionally had a composition centered around cordierite (2MgO・2Al ₂ O ₃・5SiO ₂ ), and has a low coefficient of thermal expansion and high thermal shock resistance. It has been used as engineering ceramics, such as honeycomb materials and thermal shock-resistant materials, and because its dielectric constant is second only to quartz, it is also expected to be used as a low dielectric constant multilayer substrate material. By the way, such cordierite ceramics are usually made from a mixture of naturally occurring talc and clay, and alumina (Al ₂ O ₃ ) is added to this raw material in order to approximate the composition of cordierite. It is also manufactured by adding feldspar to improve sinterability and firing it, but it is difficult to avoid contamination with impurities, and due to the addition of third components etc. Undesirable effects have been brought about on the properties of the resulting porcelain. For this reason, it has been considered to obtain cordierite porcelain using synthesized pure cordierite raw materials, but pure cordierite raw materials are difficult to sinter, so it is necessary to change the composition or add Although they were made denser by adding materials, these measures were not appropriate for use as high-performance materials. For this reason, the applicants of this application first filed a patent application filed in 1983-
No. 115964, we revealed a method for manufacturing high-purity, high-strength cordierite sintered bodies using high-purity raw materials (cordierite powder) synthesized by an alkoxide partial hydrolysis method. Highly purified cordierite powder obtained by such alkoxide partial hydrolysis method has high activity, so
It has the characteristic of being densified at a much lower temperature than conventional porcelain. For example, cordierite porcelain can be sufficiently densified by calcining it at around 800℃ and firing it at around 1300℃. (sintered body) can be obtained. However, as a result of different studies by the present inventors, the high-purity cordierite raw material obtained by such alkoxide partial hydrolysis method has extremely high activity and densifies at low temperatures, but The sintered body is prone to cracks,
It has also become clear that there is an inherent problem in the reproducibility of crystal properties. this is,
Organic matter and moisture adsorbed on the surface of fine powder
This is thought to be because grains rapidly separate at around 800°C, and grains grow rapidly accordingly. For example, at a calcination temperature of 780°C, although it becomes dense, the resulting sintered body cracks, while the calcination temperature Temperature: It does not become densified at 830℃. In short, the present inventors have made the following findings as a result of intensive studies on the production of high-purity cordierite raw materials by the alkoxide partial hydrolysis method proposed previously. (a) In other words, the powder calcined at 780°C is an ultrafine particle with a specific surface area of several hundred m ² /g, and a considerable amount of organic matter and moisture remains on the particle surface, so the resulting sintered Both the shrinkage rate and weight loss of the sintered body are large, grain growth is suppressed, and the sintered body is well densified, but cracks appear in the sintered body. (b) In addition, in the case of powder calcined at 830℃, grain growth and netting between grains are remarkable due to rapid desorption of adsorbate, and therefore the shrinkage rate and weight loss of the obtained sintered body are both low. Small, no cracks, but no densification. (c) When calcined at 800°C, the specific surface area value varies widely from 80 m ² /g to 250 m ² /g, and therefore the shrinkage rate, weight loss rate, and relative density of the sintered body cannot be reproduced. If it has poor properties and a large specific surface area, cracks may occur. (Problem to be solved) Under these circumstances, the present invention aims to improve the high purity cordierite raw material powder obtained by the alkoxide partial hydrolysis method proposed above, and to produce a product using such raw material powder. The objective is to suppress the occurrence of cracks in the sintered body and improve the reproducibility of its sintering properties. (Solution Means) In order to solve this problem, the present invention has the following features:
(a) A step of reacting magnesium alkoxide and aluminum alkoxide in an alcohol solvent to prepare a double alkoxide solution, and (b) adding hydrochloric acid to an alcohol solution containing silicon alkoxide at a concentration of 10 ^-2 to 1 mol/. 10 ^-1 to ^{10 -3} mol/
(c) mixing the double alkoxide solution obtained in the above step and the partial hydrolysis solution; (d) producing an amorphous powder by adding a large amount of water to a mixed solution to carry out hydrolysis and polymerization reactions and drying the resulting gel having a cordierite composition; (e) primary calcination of the dried cordierite composition powder of high quality at a temperature of 780°C to 810°C; The gist thereof is a method for producing high-purity cordierite powder, which is characterized by comprising a step of secondary calcination at a temperature to obtain a powder with a specific surface area of 25 to 95 m ² /g. be. (Function) In short, cordierite fine powder synthesized by the alkoxide hydrolysis method has high purity and high activity, but it adsorbs a considerable amount of organic matter and water, and therefore the calcination temperature is higher than 800℃. If it's low,
Although the particles are fine and well densified, the detachment of adsorbed substances is not sufficient, and as a result, a large amount of adsorbed substances are detached during the sintering process, which tends to cause cracks. On the other hand, if the calcination temperature is higher than 800°C, grain growth and netting between grains will occur at the same time as the adsorbate is desorbed, making the sintered body porous and not densified. Therefore, in order to obtain a dense and crack-free sintered body, it is necessary to produce a calcined powder that is fine-grained and contains fewer adsorbed substances. Furthermore, powders whose calcining temperature is 800°C have large variations in specific surface area, which may cause cracks in the sintered body. This is because the synthesized fine powder is
Since adsorbates are rapidly desorbed at around 800℃, slight differences in hydrolysis conditions and slight temperature ranges in calcination temperature can affect the amount of adsorption and specific surface area, resulting in variations. Therefore, it is necessary to produce powder with good reproducibility of sintering properties. The present invention was completed based on the discovery that all of these problems could be solved by secondary calcination of powder calcined at around 800°C at around 830°C. be. i.e. 800℃
By calcining in the vicinity and separating a considerable amount of adsorbed matter from the cordierite raw material powder in advance, netting between the grains of the raw material powder can be suppressed even if the temperature is further increased to around 830℃. It was completed after gaining the knowledge that (Specific Configuration) By the way, according to the method for producing high-purity cordierite powder according to the present invention, the step (a) of preparing a double alkoxide solution, and the partial hydrolysis solution in which silicon alkoxide is partially hydrolyzed. Step (b) of preparing and manufacturing amorphous powder
(c) is carried out in the same manner as the alkoxide partial hydrolysis method disclosed in the previous Japanese Patent Application No. 115964/1983. That is, in order to produce fine powder with a pure and homogeneous cordierite composition, an alcoholic solution of magnesium and aluminum alkoxide,
After mixing a partially hydrolyzed silicon alkoxide with an alcohol solution, sufficient water is added to perform hydrolysis and polymerization, and the resulting gel is dried to produce the desired fine powder. That's how you do it. In order to obtain such a fine powder with a homogeneous cordierite composition, first, an alcoholic solution containing silicon alkoxide at a concentration of 10 ^-2 to 1 mol/h is prepared, while hydrochloric acid is added as a hydrolysis accelerator. is present at a concentration of 10 ^-1 to 10 ^-3 mol/, preferably by heating within a temperature range from room temperature to the boiling point of the solvent, and by allowing the reaction to proceed in an inert gas or atmospheric atmosphere. , an alcohol solution of magnesium and aluminum alkoxides prepared separately after partially hydrolyzing the silicon alkoxide, in other words, a double alkoxide solution obtained by reacting magnesium alkoxide and aluminum alkoxide in an alcohol solvent. After adding and mixing, water was added to the mixed solution to form a gel-like precipitate, and this precipitate gel was dried to obtain the desired fine powder. In addition, during partial hydrolysis, if the concentration of silicon alkoxide and the concentration of hydrochloric acid in the solution are outside the above conditions, a single-phase calcined product of α-cordierite will not be obtained, and a sintered body will not be obtained. The strength also decreases. Furthermore, when three types of alkoxide solutions are mixed at the same time, the difference in the hydrolysis rate between silicon alkoxide and other alkoxides causes non-uniformity of the powder composition. When a partially hydrolyzed solution of If the concentration is too low, the yield will decrease. In addition, if the concentration of hydrochloric acid added is too high, the hydrolysis rate becomes too high and the polymerization reaction is accelerated, causing problems such as non-uniformity.On the other hand, if the concentration of added hydrochloric acid is too low, problems such as insufficient hydrolysis occur. cause Furthermore, in the present invention, various known alkoxides may be used as magnesium or aluminum alkoxides used to prepare the double alkoxide solution, and silicon alkoxides that are partially hydrolyzed in advance. However, in general, aliphatic alcohols such as methanol, ethanol, propanol, isopropanol, butanol, etc., particularly alkoxides into which lower aliphatic alcohol groups are introduced, are preferably used. Further, as the alcohol for dissolving these alkoxides, aliphatic alcohols such as methanol, ethanol, propanol, butanol, and especially lower aliphatic alcohols are preferably used. Next, since the powder gel thus obtained (dried cordierite composition powder) contains organic substances such as water and alcohol as adsorbents, as described above, it is suitable for the purpose. In order to obtain a cordierite sintered body, it is necessary to calcinate and remove them before sintering, but the inventors' research has shown that water, organic matter, etc. In order to remove the powder gel without crystallizing it and obtain cordierite raw material powder with good reproducibility of sintering characteristics and less likely to cause cracks, the following two-stage calcination operation is performed. Recruitment proved to be effective. That is, in the present invention, the powder gel is first subjected to primary calcination at a temperature of 780°C to 810°C. Note that this primary calcination is usually carried out for at least 3 hours or more to ensure that the adsorbed substances adsorbed to the powder are sufficiently removed. Next, this primarily calcined cordierite amorphous powder is further successively or once cooled to room temperature, crushed if necessary, and then crystallized again to a temperature of 820°C to 840°C. It is subjected to secondary calcination at a high temperature to form a powder having a specific surface area of 25 to 95 m ² /g, preferably 35 to 85 m ² /g. Moreover, this secondary calcination operation is generally performed at least once.
This will take more than a long time. By combining such primary and secondary calcination operations to produce a powder with a specific surface area of 25 to 95 m ² /g, a cordier with good reproducibility of sintering characteristics and no cracks can be produced. A light sintered body can be advantageously obtained. In addition, in such a two-stage calcination operation, if each calcination temperature deviates from the range specified above, it will not be possible to effectively remove organic matter, moisture, etc. adsorbed on the surface of the fine powder, and Therefore, the effect of combining the two-stage calcination operation cannot be fully exhibited, making it difficult to fully achieve the object of the present invention. The high-purity cordierite powder according to the present invention obtained by such a two-stage calcination operation is processed according to a known method according to the desired shape of the sintered body. After forming into the desired shape,
The formed body is then subjected to an operation of sintering, but in the present invention, the cordierite sintered body thus obtained is free from cracks and has excellent reproducibility. Good sintering characteristics can be obtained. (Examples) Below, some examples of the present invention will be shown to clarify the present invention more specifically. Needless to say, it is not subject to any restrictions. In addition to the following examples and the above-mentioned specific description, the present invention includes various changes, modifications, and changes based on the knowledge of those skilled in the art, as long as they do not depart from the spirit of the present invention. It should be understood that improvements and the like may be made. Example 1 Ethyl silicate: Si was used as the reaction raw materials silico alkoxide, magnesium alkoxide, and aluminum alkoxide, respectively.
( _OC2H5 ) _{4 ,} magnesium ethoxide: Mg
(OC ₂ H ₅ ) ₂ and aluminum isopropoxide:
Al[OCH(CH ₃ ) ₂ ] ₃ was used. Among these alkoxides, partial hydrolysis of ethyl silicate is
5 moles of such ethyl silicate were dissolved in 3 ethanol, 0.5 moles of hydrochloric acid and 5 moles of water were added, and 24
This was done by refluxing for an hour. On the other hand, a double alkoxide solution was prepared by dissolving 2 moles of magnesium ethoxide and 4 moles of aluminum isopropoxide in 4 parts of n-butanol and reacting them under reflux for 24 hours. Next, the two solutions thus obtained, namely the partially hydrolyzed ethyl silicate solution and the double alkoxide solution, were mixed to give the theoretical composition of cordierite, and the mixed solution was heated under reflux. Below, approximately 10 times the equivalent amount of water was added to carry out hydrolysis. Then, after reflux aging and drying,
Cordierite composition powder, which is amorphous ultrafine particles of several tens of angstroms, was obtained. Next, the obtained cordierite composition powder was used as a raw material fine powder in the atmosphere at a temperature of 780°C.
After performing primary calcination for 12 hours, it was cooled to room temperature, and then the temperature was raised to perform secondary calcination at a temperature of 830° C. for 3 hours in the atmosphere. The BET non-surface area after primary calcination was 384 m ² /g, and the BET specific surface area after secondary calcination decreased to 58.3 m ² /g. Furthermore, after grinding the amorphous fine powder obtained in this way, 100 kg/cm ² was obtained without adding a binder.
After molding with a pressure of 3 ton/cm ² and cold isostatic pressing (CIP molding) with a pressure of 3 ton/cm 2 , the resulting molded body was fired at a temperature of 1300°C for 2 hours. A cordierite sintered body was obtained. In addition, heating was performed at a rate of 5°C/min to 800°C.
After holding for a period of time, the temperature was raised to 1300°C at a heating rate of 1.5°C/min, held at that temperature for 2 hours, and then allowed to cool in the furnace. When the density of the obtained sintered body was measured by the Archimedes method, the relative density was 94.2%, the shrinkage rate was 17.1%, and the weight loss rate was 3.7%. Moreover, no cracks were observed in the obtained sintered body. On the other hand, for comparison, using the cordierite raw material fine powder synthesized above,
When a single calcination operation was performed at 780℃ for 12 hours or 830℃ for 3 hours, in the case of the former method,
An amorphous fine powder with a non-surface area of 384 m ² /g was obtained, and by the latter method a specific surface area of 27.2
An amorphous fine powder of m ² /g was obtained. Furthermore, using these two types of fine powders obtained, sintered bodies were produced in the same manner as above, and the result was 780°C x 12
A sintered body with a relative density of 99.5% was obtained from the fine powder subjected to the calcination operation for several hours, but cracks were observed in the sintered body. Furthermore,
The sintered body obtained from the fine powder subjected to the calcination operation at 830° C. for 3 hours had a relative density of 78.6% and was not densified. In addition, after performing the two-stage calcination operation according to the present invention or the one-stage calcination operation as a comparative example, the operation for producing a sintered body in the same manner as above was performed multiple times (n = 10 ) The characteristics of each calcined powder and 1300°C sintered body were repeatedly evaluated, and the results are shown in Table 1 below.

[Table] As is clear from the results in Table 1, cracks appear in the sintered body obtained from powder calcined only at 780°C, and cracks appear in the sintered body obtained from powder calcined only at 830°C. However, the powder obtained by primary calcination at 780°C and then secondary calcination at 830°C according to the present invention shows no cracking and no densification. We were able to produce a sintered body that was Example 2 Using the cordierite raw material fine powder synthesized in Example 1, it was first calcined at 800°C for 12 hours, and then heated to 830°C for 3 hours at that temperature. Secondary calcination was performed for a time. After primary calcination
The BET non-surface area was 174 m ² /g, and the BET specific surface area after secondary calcination was 73.5 m ² /g. Then, after pulverizing the amorphous fine powder thus obtained, the powder was pulverized to 100 kg/cm ² without adding a binder.
It was molded using a mold and further CIP molded at a pressure of 3 ton/cm ² . The obtained molded body was then fired at a temperature of 1300° C. for 2 hours. In addition, the heating operation is performed at 5℃/
The temperature was raised to 800°C at a rate of 1.5°C/minute, held at that temperature for 1 hour, then raised to 1300°C at a rate of 1.5°C/minute, held for 2 hours, and then allowed to cool in the furnace. After cooling, the density was measured by the Archimedes method, and the relative density was 96.9%, the shrinkage rate was 17.2%, and the weight loss rate was 3.6%.
Note that no cracks were observed in the obtained sintered body. On the other hand, when similar cordierite raw material powder was calcined at 800°C for 12 hours,
A specific surface area of 174 m ² /g was obtained, and the shrinkage rate of the sintered body obtained using this calcined powder was 20.4.
%, weight reduction rate was 2.9%, and relative density was 95.4%. Furthermore, it was observed that the sintered body contained microcracks. In addition, after adding the two-stage calcination operation according to the present invention and the one-stage calcination operation as a comparative example, the experiment to obtain a sintered body was repeated multiple times (n = 10), and the obtained results were summarized. , shown in Table 2 below.

[Table] As is clear from the results in Table 2, 800℃
The reproducibility of the specific surface area of the calcined powder is poor, and cracks are observed in the sintered body obtained from the calcined powder with a large specific surface area. In this way, those that only undergo primary calcination at 800℃ are
Although the variation in the characteristic values is large, it is recognized that by performing secondary calcination according to the present invention, the variation in the specific surface area is reduced and the reproducibility of the characteristic value is improved. (Effects of the Invention) As is clear from the above explanation, according to the method of the present invention, cordierite porcelain which is much purer than cordierite porcelain using conventional natural raw materials can be produced at a relatively low temperature. It is possible to advantageously obtain a fine powder having a cordierite composition, which can improve the reproducibility of its sintering properties, and also effectively reduce the occurrence of cracks in the sintered body obtained from it. It is possible to do so.

Claims (1)

[Claims] 1. Reacting magnesium alkoxide and aluminum alkoxide in an alcohol solvent,
A step of preparing a double alkoxide solution, and adding hydrochloric acid at a concentration of 10 ^-1 to 10 ^-3 in an alcoholic solution containing silicon alkoxide at a concentration of 10 ^-2 to 1 mol/.
A step of preparing a partially hydrolyzed solution by partially hydrolyzing the silicon alkoxide by adding the silicon alkoxide at a concentration of mol/molar, and mixing the double alkoxide solution obtained in the above step with the partially hydrolyzed solution. A process for producing an amorphous powder by adding a large amount of water to a solution to perform hydrolysis and polymerization reactions, and drying the resulting gel having a cordierite composition; A step of primary calcination of the yellowite composition powder at a temperature of 780°C to 810°C, and a second calcination of the primarily calcined cordierite composition powder at a temperature of 820°C to 840°C. hand,
A method for producing high-purity cordierite powder, comprising the step of obtaining powder having a specific surface area of 25 to 95 m ² /g.