JPH0737323B2 - Method for drying ceramic precursor gel - Google Patents

Description

Detailed Description of the Invention

[0001]

INDUSTRIAL APPLICABILITY The present invention can provide large-scale silica glass of high purity and high homogeneity, which is used for applications such as optical fibers and optical glass, at low cost by firing it, and special heat insulating material, The present invention relates to a method for producing a porous ceramic precursor which can be applied to various ceramic carriers, filters and the like.

[0002]

2. Description of the Related Art A wet gel obtained by subjecting a mixture containing a metal compound such as an alkoxide to hydrolysis and polycondensation reaction is dried under normal pressure to form a porous gel, which is then calcined. Many attempts have been made to produce high-purity glass and ceramic materials (for example, with respect to silica glass, JP-A-61-106428).
Japanese Patent Publication No. 55-167143, "Better C"
eramics Through Che-mistry "Vol.32, P47-48). In particular, JP-A-61-106428 discloses a sol solution in which a metal alkoxide is diluted with an organic solvent and water containing ammonia is added to cause hydrolysis. After the production of the above, the solution is aged, then the aged solution is gelled, and then dried to form a porous gel body, and the porous gel body is subjected to a high temperature treatment to obtain a vitrified glass. A method of producing a porous gel via such a sol and then firing it to obtain glass and ceramics is called a sol-gel method, which has a higher quality than conventional melting methods and powder mixing methods. It has the advantages that glass and ceramics of (high purity and high homogeneity) can be obtained, and that they can be manufactured at relatively low cost.
(For example, when compared with high-purity silica glass for optical fiber preforms produced by a vapor phase method).

Further, the gel is dried by using an autoclave, by subjecting the gel to a temperature and pressure which exceed the critical point of the solvent of the solution remaining therein, and then removing the solvent and residual water present in the gel. , A method for obtaining porous silica has been proposed (for example, JP-A-57-209).
817 and JP-A-60-141627).

[0004]

However, a porous gel prepared by an atmospheric pressure drying method and a glass prepared by firing the same,
In the production of ceramics, there are the following problems.

When the wet gel is dried, cracks are generated as the solvent evaporates. Since the gel is dense and has a small pore size, residual organic components, OH groups, etc. are difficult to separate during the firing process, and these gases are confined inside the pores,
Cracks, breaks, and foaming easily occur.

These problems are solved by using an autoclave and drying the gel under the supercritical conditions of the organic solvent itself, but when the gel to be treated becomes large, the probability of cracking increases and the monolithic There is a limit in obtaining a large dry gel. Further, since the critical point of the organic solvent is generally a high temperature of 200 ° C. or higher, the structural change or deterioration of the gel due to heating may be a problem.

The present invention aims to solve these problems and provide a large-sized, crack-free, ceramic precursor.

[0008]

According to the present invention, a ceramic composition containing a metal compound is made into a homogeneous sol, which is then made into a wet ceramic precursor gel, and the pores and surroundings in the gel are filled with an organic solvent. Then, a method of drying the gel by extracting and removing the organic solvent in the gel with the extraction solvent under supercritical conditions in the extraction solvent-organic solvent mixture system, the temperature and pressure specified below and Under the conditions, that is, above the critical temperature of the extraction solvent used and from now on 3
Extraction is started at a temperature not exceeding 0 ° C and at a pressure not lower than the critical pressure in the extraction solvent-organic solvent mixed system at a set temperature and not higher than 50 kg / cm ² , and the gel content in the gel Is a method for drying a ceramic precursor gel, which comprises extracting and removing 80% or more of an organic solvent present in 1.

In the present invention, a wet ceramic precursor gel (hereinafter referred to as "wet gel") has a specific low temperature and low pressure in respective ranges (T: unit ° C), and the critical temperature of the extraction solvent is Tc. Then, T is defined in the range of Tc ≦ T ≦ Tc + 30, and the pressure (P: unit kg / cm ² ) is critical in the mixed system of the extraction solvent and the organic solvent filling the gel pores at the set temperature. If pressure is Pc, P is Pc
Supercritical conditions defined within the range of ≦ P ≦ Pc + 50 (however,
Including the critical point. ), The extraction solvent is circulated to extract and remove most (80% or more) of the organic solvent in the gel, so that the dried gel of the integrated ceramic precursor without cracks and cracks is finally obtained. It is obtained, and good ceramics can be provided by heating this.

The extraction solvent referred to here is a substance used for extracting the organic solvent in the wet gel from the gel, and does not need to be a liquid at room temperature and pressure, and has a specific critical temperature and critical temperature. It can be any fluid that has a pressure and can function under critical and supercritical conditions. The extraction solvent may be a single substance or a mixture of a plurality of extraction solvents.

Usually, in the pores of the synthesized wet gel, aqueous and organic solvents used for dissolving and mixing the ceramic composition, modifiers added for controlling the reaction of the compound and various additions. When a substance or a reaction product, for example, a metal alkoxide is used, it is filled with alcohol, water and the like produced by the hydrolysis and polycondensation reaction thereof. On the other hand, it is preferable that the solvent to be extracted by the extraction solvent is mainly a limited organic solvent such as alcohol. Therefore, in order to perform drying with the extraction solvent, it is necessary to fill the pores of the wet gel with these desired organic solvents. As a means for this, it is conceivable to study the formation conditions of the wet gel in advance and satisfy the conditions together with the formation of the wet gel, but more realistically, the formed wet gel is dipped in a desired organic solvent and wet. The solvent substitution method of substituting the organic solvent in the gel with the organic solvent is most suitable for easily and surely setting the supercritical conditions in the extraction solvent-organic solvent mixture system.

Specific examples of the preferred extraction solvent, extraction solvent-organic solvent mixture system used in the present invention and specific conditions thereof are as follows. For example, when CO ₂ is used as the extraction solvent, the criticality of CO ₂ Since the temperature is 31 ° C., the range of the temperature T is defined as 31 ≦ T ≦ 61. Further, when Freon 22 (CHClF ₂ ) is used as the extraction solvent, the critical temperature is 96 ° C., so that 96 ≦ T ≦ 126 is specified.

As the extraction solvent-organic solvent mixture system, CO ₂ -alcohol system is preferable, for example, CO ₂
-In the case of an ethanol system, when T is 40 ° C, the critical pressure Pc is 80 kg / cm ² , so the specified P
From the above formula, the range is 80 ≦ P ≦ 130. Similarly, when T is 50 ° C., Pc is 95 kg / cm ² ,
The range is 95 ≦ P ≦ 145, and when T is 60 ° C., the range is 110 ≦ P ≦ 160. However, the above critical pressure is not strict and allows a slight variation depending on other conditions.

In the present invention, as a method of using CO ₂ as an extraction solvent, a suitable method for drying a wet gel is exemplified as follows. A wet gel is produced from the ceramic composition, and the gel containing the organic solvent substituted with the desired solvent such as the alcohol is subjected to CO ₂ conversion through solvent substitution.
In pressure, warmed, and then the supercritical fluid most C in the vessel to remove CO ₂ and organic solvent out of the system while the CO ₂ adding these CO ₂ + organic solvent mixture system as the supercritical fluid
The removal of the organic solvent is continued until it becomes O ₂ , and then pressure reduction,
The temperature is lowered to obtain a dry gel.

In the present invention, a dried ceramic precursor gel is prepared by extracting most (80%) or more of the organic solvent in the wet gel under the above-mentioned conditions. It also has the effect of preventing gel damage, structural change, non-uniformity of multi-component gel, etc. Conversely, while the gel is immersed in the organic solvent,
Since the temperature and the pressure greatly affect the damage of the wet gel and the like, it is preferable to appropriately select the optimum conditions depending on the type of the gel and the like.

The ceramic composition used in the present invention comprises a metal element compound containing a basic metal element constituting the ceramic. The metal element compound includes any substance for forming a sol, and thus a wet gel. For example, a hydrolyzable metal alkoxide or a hydrolyzable oligomer thereof, such as Si, is used.
(OR) ₄ , Zr (OR) ₄ , Nd (OR) ₃ , Al
(OR) ₃ , PO (OR) ₃ , NaOR, Mg (OR)
₂ , KOR, Ca (OR) ₂ , Ti (OR) ₄ , Sr
(OR) ₂ , Y (OR) ₃ , Ba (OR) _{2 and the} like (R is an alkyl group such as methyl and ethyl), inorganic compounds such as metal salts (for example, halides such as NaCl, MgBr ₂ and TiCl ₄ ; NaNO ₃ , K ₂ SO ₄ , Ca (NO ₃ ) ₂
Nitrates, sulfates, etc.), organometallic compounds other than alkoxides (eg, CH ₃ COONa, (CH ₃ COO))
₂ Acetates such as Ca, (COONa) ₂ , (COOK) ₂
Oxalates, etc., complexes with chelate compounds such as EDTA, NTA, etc., and in some cases finely reactive metals and oxide fine powders (eg CaO, TiO ₂ , SiO ₂ ,
P ₂ O ₅ , ZrO _2, etc.) can be used together.

In the present invention, a ceramic composition containing at least Si (OR) ₄ is selected so that the ceramic precursor is a silica-based glass precursor, and the ceramic composition is hydrolyzed and wet. It is particularly preferable that the silica-based glass precursor gel is selected, alcohol is selected as the main solvent that fills the pores in the gel, and carbon dioxide is selected as the extraction solvent used for drying the gel.

The metal alkoxide can be used by reacting it with an arbitrary modifier, and for example, it can be reacted with an organic compound having active hydrogen to control hydrolysis, that is, to adjust a wet gel. . Such organic compounds include alkanolamines such as monoethanolamine, mono-n-propanolamine, monoiso-propanolamine, diethanolamine, diiso-propanolamine, triethanolamine, triiso-propanolamine. , Β-keto acid ester, for example, ethyl acetoacetate, methyl acetoacetate, ethyl malonate,
Examples include β-diketone compounds such as diethyl malonate, such as acetylacetone.

The ceramic composition optionally comprises a base catalyst such as ammonia, pyridine, piperidine,
It can be hydrolyzed in the presence of piperazine. The amount of water necessary for hydrolysis may be set appropriately according to the composition of the ceramic composition such as metal alkoxide.

[0020]

In the present invention, under supercritical conditions defined in the temperature range of Tc ≦ T ≦ Tc + 30 and the pressure range of Pc ≦ P ≦ Pc + 50, the wet gel is dried, that is, supercritically dried, so that the temperature rises and the pressure rises. Since the density change of the solvent that fills the gel pores is small and the stress applied to the gel is reduced during extraction, the occurrence of cracks and cracks is suppressed, and it is easy to obtain an integrated dry gel of the ceramic precursor gel. By doing so, good quality ceramics can be obtained.

[0021]

EXAMPLES Example 1 70 g of tetramethylorthosilicate Si (OCH ₃ ) _{4 was} mixed with 70 g of methanol and 3 g of water.
After adding 4 g, stirring and reacting, the mixture was kept at 60 ° C. for 3 days to obtain a wet gel. After sufficiently replacing the solvent with methanol, the gel was immersed in methanol in a high-pressure container. In this state, gradually pressurize with carbon dioxide, heat with an exterior heater, and reach 40 ° C and 110 kg / cm ² ,
Carbon dioxide was caused to flow at a rate of 20 to 30 L (liter) / minute, and the solvent methanol was extracted and removed. Then, the isothermal decompression of methanol was extracted and removed. Then, isothermal depressurization was performed, and after cooling, the gel was taken out. The gel was free from cracks and cracks, and an integrated dry gel body was obtained.

Example 2 0.6 g of neodymium triethoxide was added to 100 mL of 2-ethoxyethanol,
20 mL of ethyl acetoacetate was mixed with this, and heated and refluxed at 80 ° C. to obtain a yellow green transparent solution. To this solution, 106 mL of silicon tetramethoxide, 106 mL of methanol, and 12 mL of water were added, and after stirring at room temperature for 2 hours, 25 g of an average pentamer oligomer of silicon ethoxide, 30 mL of ethanol, 7 mL of water, and 0.3 mL of piperidine were added, The mixture was stirred and reacted at room temperature to form a gel. The obtained wet gel solution was sufficiently exchanged with ethanol, and the solution was immersed in ethanol in a high-pressure container and sealed. In this state, gradually pressurize with carbon dioxide, heat with an exterior heater, hold at 45 ° C, 120 kg / cm ² for about 1 hour,
Carbon dioxide was caused to flow at a rate of 20 to 30 L / min, and most of the solvent ethanol was extracted and removed. After that, the temperature is further raised,
After pressurizing and flowing carbon dioxide at 80 ° C. under 160 kg / cm ² , the remaining solvent was extracted and removed, isothermal decompression was performed, and the gel was taken out after cooling. The gel was free from cracks and cracks, and an integrated dry gel body was obtained. This is an oxygen atmosphere up to 800 ℃ 1250 ℃
By firing in a helium atmosphere, a neodymium-containing silica glass applicable to a laser glass containing 0.6 mol% of Nd ₂ O ₃ was obtained.

Example 3 A solution of 11 g of zirconium n-butoxide and 0.8 g of lanthanum n-butoxide dissolved in 15 g of ethanol and 3 g of acetylacetone was sufficiently reacted at 80 ° C. for 1 hour under reflux. Water on this 2.5
A solution obtained by diluting 4 g of ethanol with 4 g of ethanol was added, and further a solution of 4.3 g of piperidine diluted with 4 g of ethanol was added to cause gelation. After sufficiently replacing the solvent of this wet gel with ethanol, the gel was immersed in ethanol in a high-pressure container.
In this state, gradually pressurize with carbon dioxide, heat with an air bath, and reach 40 ° C and 110 kg / cm ² ,
Carbon dioxide was caused to flow at a rate of 5 to 10 L / min to extract and remove ethanol as a solvent. After that, perform isothermal decompression,
After cooling, the gel was taken out. The gel was free from cracks and cracks, and an integrated zirconia-lanthanum dry gel body was obtained. The dried gel was calcined in an oxygen atmosphere at 1000 ° C. for 5 hours to obtain a ZrO ₂ —La ₂ O ₃ based porous body applicable to a catalyst carrier having a specific surface area of 20 m ² / g.

(Comparative Example 1) The wet gels listed in Examples 1 and 2 were treated in the same manner at 45 ° C and 200 kg / c.
As a result of increasing the pressure to m ² and performing a solvent extraction operation, cracks were generated in the obtained dried gel body, and it was difficult to perform integrated drying.

(Comparative Example 2) The silica-neodymium wet gel mentioned in Example 3 was treated in the same manner at 150 ° C and 20 ° C.
As a result of heating to 0 kg / cm ² and increasing the pressure to perform a solvent extraction operation, a transparent homogeneous glass body could not be obtained even when the obtained dried gel was fired under the same conditions as in Example 2.

[0026]

The present invention has the following effects. (1) Conventionally, in the case of a gel hydrolyzed with an acidic catalyst, shrink cracking or the like may occur due to a structural change when supercritical drying is performed at a high temperature. However, the present invention prevents the shrink crack by the low temperature treatment. it can. (2) Conventionally, in the case of supercritical drying of a silica-other component type gel, troubles such as alteration of additive components occur at high temperature,
Further, under high temperature and high pressure, the gel structure may change, and the structure of the dry gel may be different from that of the wet gel. However, according to the present invention, this is avoided, and the designed wet gel is used. A homogeneous silica-based dry gel having a structure close to that of can be obtained.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location C04B 38/00 304 B

Claims (2)

[Claims] 1. A ceramic composition containing a metal compound is made into a homogeneous sol, which is then made into a wet ceramic precursor gel.
After filling the pores and surroundings in the gel with an organic solvent,
Extraction solvent-a method of drying the gel by extracting and removing the organic solvent in the gel with the extraction solvent under supercritical conditions in an organic solvent mixed system, under the temperature and pressure conditions specified below and That is, the temperature above the critical temperature of the extraction solvent used and not exceeding 30 ℃
Extraction solvent in in set temperature - above the critical pressure of an organic solvent mixture system, and now begin extraction at a pressure not exceeding 50kg / cm ^2, 80% or more present in the said gel in the conditions A method for drying a ceramic precursor gel, which comprises extracting and removing the organic solvent according to 1. 2. A silica-based glass precursor which is moist by hydrolyzing the ceramic composition by selecting a ceramic composition containing at least a silicon alkoxide so that the ceramic precursor is a silica-based glass precursor. The method for producing a ceramic precursor gel according to claim 1, wherein alcohol is selected as a main solvent for forming a gel and filling pores in the gel, and carbon dioxide is selected as an extraction solvent used for drying the gel.