JPH06116054A - Ceramic porous body and its production - Google Patents

Abstract

PURPOSE:To improve thermal shock resistance by etching a ceramic sintered body containing plural idiomorphic sialon crystal grains to make a porous body. CONSTITUTION:A raw material powder is prepared by blending an amorphous SiO2 powder and AlN powder, each of which has <=2.0mum particle diameter, in the mol ratio the former per latter of 0.5-4, adding 1-5wt.% sintering assistant such as Y2O3 and, if necessary, <=50wt.% Si3N4, wet-mixing in an organic solvent and pulverizing. Next, the raw material powder is metal molded, is heated at 5-10 deg.C/min temp. rising rate and sintered in a non-oxidizing atmosphere such as gaseous N2 at 1300-1700 deg.C to obtain the sialon sintered body made of the plural idiomorphic crystal grains of beta type expressed by formula, Si6-zAlzOzN8-z ((z) is <=4) and alpha type expressed by a formula, MxSi12Al12O16N16 (M is Li, Ca, etc., (x) is <=0.4) or the like and having the structure shown with each other in the crystal grain. The sintered body is dipped into NaOH solution or the like to elute and remove an amorphous phase and the ceramic porous body such as the sialon porous body having 10-70 volume % porosity is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ceramic porous body and a method for producing the same, and particularly to heat resistance, a low coefficient of thermal expansion,
It is promising as a carrier for high-temperature catalysts that require mechanical strength and chemical stability. As high-temperature catalysts, automobile exhaust gas purification, catalytic combustion heaters, gas turbine combustion catalysts,
It is particularly suitable for automobile catalysts for manifolds, trap catalysts for diesel, and the like.

Further, it is also suitably used as a membrane (separation membrane, permselective membrane), shape-selective crystal carrier (molecular sieve) and the like.

[0003]

BACKGROUND ART Well known porous ceramics include alumina, silica, alumina silica, titania, cordierite and the like. Generally, these ceramics have good heat resistance up to 700-800 ° C and excellent chemical stability.
It is used as a catalyst carrier in various applications depending on its chemical properties.

Recently, the combustion catalyst carrier has been improved especially from the standpoint of suppressing NO _X or improving energy efficiency. Alumina-coated cordierite honeycomb that has been conventionally used for treating automobile exhaust gas and for catalytic combustion has a limit in heat resistance, and has a drawback that alumina sinters at a high temperature exceeding 1000 ° C and the surface area is reduced. Further, at 1316 ° C or higher, there is also a drawback that alumina and cordierite react with each other to form mullite having a high coefficient of thermal expansion. Furthermore, the maximum operating temperature of cordierite is 13400 because its melting point is 1400 ° C.
The maximum temperature is 00 ° C, and for a high temperature combustion catalyst, a honeycomb structure having heat resistance superior to cordierite and thermal shock resistance equal to or higher than this is required.

The conventional porous ceramics are almost always obtained by controlling the porosity depending on the conditions in the sintering process, and the pores are a repeating pattern of voids and pores connecting them. . And its skeleton is
Since the particles are bonded through the grain boundaries, they are structurally discontinuous and have a weak structure.

On the other hand, in the case of porous glass known as one of the porous ceramics, the manufacturing method by the separated phase of this porous glass is adopted, but the problem that the porous glass also has low heat resistance Had a point.

[0007]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned drawbacks of the prior art and provides a porous ceramic body having heat resistance, chemical stability and mechanical strength, and a method for producing the same.

[0008]

In order to achieve the above object, the present invention has the following constitution. “(1) A ceramic porous body comprising a plurality of automorphic sialon crystal particles and having a structure in which crystals are entangled with each other.

(2) A method of manufacturing a ceramic porous body, which comprises subjecting a ceramic sintered body containing sialon crystal particles to an etching treatment to form a porous body. The sialon crystal particles of the present invention are self-forming crystals generated by the reaction during sintering. These self-made sialon crystal particles have extremely high heat resistance, are chemically stable in a high temperature gas atmosphere, and the sintered body structure obtained is an entangled structure and has mechanically high strength. Have.
The skeleton of the porous ceramics by the conventional sintering method described in the section of the prior art is composed of particles bonded through grain boundaries,
Whereas it is structurally discontinuous and has a fragile structure,
The pore structure of the porous ceramic body of the present invention forms a strong skeleton by uniform pores without constrictions and a three-dimensional entangled structure. Here, in the present invention, having an entangled structure means that two or more sialon crystal particles intersect at at least one point (that is, the case where all are parallel).

As the self-supporting sialon crystal particles of the present invention, columnar particles are easily obtained, which is preferable for forming an entangled structure thereof. Further, the ceramic porous body of the present invention,
Although it is composed of a plurality of automorphic sialon crystal particles, the sialon crystal particles may be of the same kind or of two or more kinds.

The porosity of the porous material of the present invention is not particularly limited and can be controlled according to the purpose. However, since a functional catalyst carrier having a sufficiently large surface area and a small pressure loss can be obtained, It is preferably 10 to 70% by volume, more preferably 30 to 60% by volume.

In the method for producing a ceramic porous body of the present invention, a ceramic composition raw material containing sialon is
A sintering reaction is performed at a temperature above the sialon generation temperature, and then a heat treatment reaction is performed if necessary to generate a separated phase of the sialon crystal particles, and then a chemical etching treatment is performed to remove the phases other than the sialon crystal particles. This makes it possible to obtain a ceramic porous body having microscopic pores. The manufacturing method will be described in detail below. (1) Preparation Step of Mixed Powder The ceramic porous body of the present invention uses, for example, 3SiO ₂ + 3AlN → Si ₃ by using SiO ₂ and AlN as starting materials in order to generate sialon crystal particles.
As shown in Al ₃ O ₆ N ₃ , a sialon forming reaction can occur during sintering.

Si as a raw material in the powder mixing process
It is also possible to prepare a mixed powder containing Si ₃ N ₄ powder and a small amount of Y ₂ O ₃ powder in addition to O ₂ powder and AlN powder. That is, when the produced sialon is designed as a grain boundary phase component of Si ₃ N ₄ and / or Si ₂ ON ₂ , Si ₃ N ₄ powder is added and Y is added as a sintering aid to promote densification. ₂ O ₃ is added. SiO ₂ / A
In order to carry out the sintering with diffusion of the element in the 1N system more rapidly and more uniformly, it is preferable that the particle diameters are all 2.0 μm or less.

The SiO ₂ powder is preferably amorphous, and the mixing ratio of the ceramic powder to be adjusted is preferably 0.5 to 4 in terms of the molar ratio of SiO _{2 to} AlN powder. Amorphous SiO ₂ is preferably used because it is relatively easy to control the crystallinity by heat treatment because most of the produced sialon crystal particles are amorphous. When Si ₃ N ₄ is added, it is preferably 50% by weight or less. The reason for this is that Si ₃ N ₄ is a crystal phase whose composition is fixed from the beginning and is considered to be a parameter that is not related to the sialon crystal particle separation phase, and therefore contributes little to the control of porosity. Therefore, if it exceeds 50% by weight, only a porous body having a small porosity can be produced. Further, it is preferable to add Y ₂ O ₃ powder in an amount of 1 to 5% by weight as a sintering aid because the sinterability is remarkably improved and the densification is promoted.

The mixing operation may be dry or wet, but it is preferable to use a wet mixing method in order to disperse the mixed powder more uniformly. For example, an organic dispersion medium such as isopropyl alcohol, ethyl alcohol, ethylene glycol, dimethylsulfoxide is added to the mixed powder, and the mixture is thoroughly mixed and pulverized with an attrition mill, ball mill or the like.

As a result, secondary agglomeration is well diffused, and a mixed powder in which primary particles are extremely uniformly dispersed is obtained.
After mixing and pulverizing, it is dried under reduced pressure by a rotary evaporator or the like. The evaporator can prevent segregation due to the difference in specific gravity that tends to occur in natural drying or constant temperature drying. (2) Molding Step and Sintering Step In the present invention, the mixed powder prepared as described above is then sintered, and there are two methods.

First, the mixed powder is processed by a dry hydrostatic molding method,
This is a method of molding into a desired shape using a die molding method, a wet slip casting method, an injection molding method or the like, and then sintering the molded body under pressure or without pressure.

The other is a method of pressure sintering using a hot pressing method, a hot isostatic pressing (HIP) method or the like without molding the mixed powder. In either case, sintering is performed in a non-oxidizing atmosphere, for example, an inert gas atmosphere such as nitrogen gas or argon gas, a reducing gas atmosphere such as carbon monoxide gas or hydrogen gas, or a reduced pressure of 0.1 Torr or less. It is preferably performed in an atmosphere. The sintering temperature is the mixing ratio of the adjusted ceramic powder, the average particle size and the above-mentioned third value.
The temperature is preferably in the range of 1300 to 1700 ° C., though it depends on the presence or absence of addition of the components, the addition amount, and the sintering conditions.

In the sintering, it is preferable that the temperature is raised to the sintering temperature at a rate of 5 to 10 ° C./min, and the temperature is maintained for a desired period of time so that the temperature cannot be distributed. In the case of sintering with pressurization, the pressurization may be performed before the temperature is raised, may be gradually increased according to the temperature increase rate, or may not be applied at all during the temperature increase. The pressure may be applied when the sintering temperature is reached.

The composition and process conditions described above were examined, and the sialon crystal grains in the sintered body structure were mainly composed of Si ₃
Β-sialon represented by Si _6-z Al _z O _z N _8-z (z is a natural number of 4 or less) such as Al ₃ O ₃ N ₅ ;
M _x Si ₁₂ Al ₁₂ O ₁₆ N formed as a solid solution with a specific metal
₁₆ (x is an integer of 0.4 or less) represented by α-sialon, and other sialon compounds (Si ₂ Al) that do not conform to the above general formula and are made of silicon aluminum oxynitride.
₃ O ₇ N, Si ₆ Al ₁₀ O ₂₁ N ₄ , Si ₃ Al ₃ O
_{3 + 1.5X} N _5-X , Si ₃ Al _2.67 N ₄ O ₄ , Si ₃ Al ₇
O ₃ N ₉ , Si ₁₂ Al ₁₈ O ₃₉ N ₈ , Al ₆ Si ₆ N ₈ O
₉ , Si ₃ Al ₃ O ₃ N ₅ , Si ₆ Al ₁₀ O ₂₁ N ₄ , S
i ₂ Al ₃ O ₇ N, etc.), Al ₃ O ₃ N, Al ₂ SiO
Porous ceramics having a structure in which one or more compounds out of ₅ , ₅ , Al ₆ Si ₂ O ₁₃ , Al ₂ O ₃ , Si ₂ ON ₂ and Si ₃ N ₄ are mixed at an atomic level. The body is synthesized.

The obtained sintered body structure can be crystallized by X-ray diffraction, Raman spectroscopy, analytical electron microscope, electron beam diffraction and the like, and is composed of a part of crystalline part and most of amorphous part. Sialon crystal particles are obtained. The amorphous portion at this stage is usually 40 to 70% by volume.

A dense sialon crystal grain sintered body can be obtained by the above-mentioned sintering process, and the amorphous part in the sintered body is crystallized by a cooling process during sintering or a heat treatment after sintering. Add the step to do.

The crystallization at this time is preferably performed at 1200 to 1650 ° C. for 30 minutes to 48 hours in the air or in an inert gas atmosphere or under a reduced pressure of 0.1 Torr or less.

When performing sintering involving reaction using SiO ₂ and AlN as raw materials, the following method is particularly preferable.

First, it is desirable that the rate of temperature rise during sintering is slowly in the range of 5 to 10 ° C./min in consideration of the softening point and melting point of glass (glass has a melting point higher than that of ceramics). Since it is low, when heated rapidly, a large temperature difference occurs between the surface and the inside, and the bubbles inside remain without being removed to the outside,
The sintered body may become porous).

A specific example of a method for producing a sialon sintered body by changing the raw material composition ratio is shown below.

First, the raw material composition ratio of SiO ₂ / AlN is set to 0.
The following is an example when 5 to 1.5 are set.

The produced sialon crystal particles include Si ₃
The general formula such as Al ₃ O ₃ N ₅ is Si _6-z Al _z O _z N
_In addition to β-sialon represented by _8-z (z represents a natural number of 4 or less), Si ₃ Al ₇ O ₃ N ₉ , Si ₁₂ Al ₁₈
Sialon with a large amount of solid solution of alumina such as O ₃₉ N ₈ is easily generated.

When the raw material composition SiO ₂ / AlN ratio is extremely smaller than 1, the amount of SiO ₂ is too small and the synthesis reaction of sialon is difficult to occur, and most of it tends to remain AlN.

The raw material composition of SiO ₂ / AlN is 1.5
The following is an example of setting ~ 4.

The produced sialon crystal particles include Si ₃
Al ₃ O _{3 + 1.5X} N _5-X , Si ₁₂ Al ₁₈ O ₃₉ N ₈ and S
i ₂ Al ₃ O ₇ N or the like is generated.

Within this range, Y ₂ O ₃ , Ln ₂
By adding a small amount of a lanthanum group element oxide such as O ₃ or an alkaline earth element oxide such as MgO or CaO, Mx Si ₁₂ Al ₁₂ which forms a solid solution with these metal elements
It is also possible to synthesize α-sialon represented by O ₁₆ N ₁₆ (x is an integer of 0.4 or less). Particularly when the composition ratio of SiO ₂ is large, α-sialon is preferentially deposited by adding Y ₂ O ₃ . In addition, SiO ₂ / AlN
Becomes extremely large, the amount of residual SiO ₂ tends to be too large, and it tends to be difficult to obtain a sintered body having a sialon composition. Furthermore, amorphous and crystallized silica (SiO ₂ ) is present at the grain boundaries, and Y ₂ O ₃ added as a sintering aid forms garnets at the grain boundaries. Similar effects as a sintering aid are observed not only in Y ₂ O ₃ but also in rare earth element oxides such as CeO ₂ and lanthanum, and alkaline earth element oxides such as MgO and CaO.

Regarding the sintered body thus obtained,
By performing an etching treatment to elute and remove the phases other than the sialon crystal particles, a microscopic ceramic porous body is obtained. The etching treatment in this case includes an alkali solution treatment, an acid solution treatment, and the like, and refers to a step of eluting and removing the separated phase that has precipitated by immersing in an alkali solution, an acid solution, or the like. As the alkaline solution treatment, for example, when a strong alkaline solution such as a sodium hydroxide solution or a potassium hydroxide solution is used, the treatment can be performed in a relatively short time, which is about 1 to 2 hours. Further, when the treatment is performed by immersing the above strong alkali in a melt heated to the melting point, the phase separation can be eluted and removed in a shorter time (several minutes). As the strong acid solution, hot concentrated sulfuric acid, nitric acid, aqua regia, hydrofluoric acid, etc. are used.

As raw material powders, Si ₃ N ₄ , SiO ₂ ,
When a mixed powder of AlN and Y ₂ O ₃ is used, during the sintering and the heat treatment process, for example, 3SiO ₂ + 3AlN → Si ₃ Al _{3 is} formed between SiO ₂ and AlN.
A reaction of O ₃ N ₃ occurs to produce sialon crystal particles.

As described above, in the sialon porous material of the present invention, the dense sialon ceramics obtained in the previous step has a structure in which crystalline and amorphous parts are entangled with each other, A desired porous structure is obtained by removing the amorphous portion. Since the sizes of crystalline phase and amorphous phase (separated phase) are determined by thermodynamic factors, it is possible to control the temperature and time of heat treatment that causes phase separation if the composition is kept constant. As a porous sialon ceramic having various sizes of pores can be designed. The size of the crystal particles is, for example, at a sintering temperature of 1500 ° C. and a columnar minor axis diameter of 0.
It is 1 to 0.2 μm and the major axis diameter is 2 to 4 μm. Also 1600
At ℃, minor axis diameter is 0.3-0.4 μm, major axis diameter is 5-6 μm.
Becomes

In the case of compounding silicon nitride, sialon is formed in the gap between silicon nitride and silicon nitride to form a grain boundary. Si ₃ N ₄ surface is Si ₂ ON ₂ and / or Si
Since it consists of O ₂ and easily reacts with AlN, it is considered that the sialon and Si ₃ N ₄ have good compatibility and form a grain boundary phase continuously. In addition, a part of SiO ₂ and Al
_It may become a glass phase containing ₂ O ₃ as a main component.

In a system not using Si ₃ N ₄ , the formed sialon is the main component, which is composed of the above-mentioned automorphic crystalline portion and amorphous portion, but SiO ₂ and Al ₂ O ₃ are partially contained as the main components. There is also a case where it becomes an amorphous phase.

As described above, the ceramic porous body obtained by the method of the present invention has excellent strength and has an average coefficient of thermal expansion of 2.0 × 10 ⁻⁶ / K or less in the temperature range from room temperature to 800 ° C. Have.

As an application field utilizing the characteristics of these materials, automobile exhaust gas purification, catalytic combustion heater, gas turbine combustion catalyst, manifold automobile which are required to have heat resistance, low thermal expansion coefficient, mechanical strength and chemical stability. There is a honeycomb carrier for high temperature catalysts such as catalysts and trap catalysts for diesel.

Further, sialon works by the silanol group existing on the surface of the sialon, which facilitates the surface design by surface chemical modification, and makes it possible to hydrophobize or introduce a charged group.

[0041]

【Example】

Example 1 Amorphous SiO ₂ powder having an average particle diameter of 1.2 μm and AlN powder having an average particle diameter of 0.3 μm were prepared in a molar ratio of 1: 1 and further 1 weight of Y ₂ O ₃ was added. % Added. The prepared powder was pulverized and mixed in ethanol for 12 hours using a ball mill having Si ₃ N ₄ balls as a pulverizing medium, and dried to obtain a raw material powder.

Next, the above powder was press-molded with a die at 1 ton / cm ^{2, and it} was heated at 1500 ° C. for 2 minutes in a N ₂ gas stream of 3 liters per minute.
Sintered at normal pressure for an hour. When the obtained sintered body was identified by X-ray diffraction, crystalline Si ₂ Al ₃ O ₇ N and Si ₁₂ Al ₁₈ O ₃₉ N ₈ were confirmed. As a result of electron diffraction using an analytical electron microscope, it was found that 60% by volume of the whole was an amorphous phase. The microstructure is a mosaic structure in which columnar crystal phases are entangled with each other, and has a complicated structure in which an amorphous phase is present in a gap. As a result of Raman spectroscopic analysis, the amorphous phase in this case was found to be Sialon or Y ₂ Si ₂ O ₇ having a small amount of solid solution of alumina and low crystallinity.

The bending strength of this sintered body is 37 kgf / m on average.
It had a m ² , a Vickers hardness of 1280 and a fracture toughness of 2.4 MPa · m ^1/2 . As a result of thermal expansion measurement, the average value from 20 ° C to 30 ° C was 1.4 × 10 ^-6 / K.

Further, this sintered body was immersed in sodium hydroxide (NaOH) melted at 320 ° C. to elute and remove the amorphous phase to obtain a sialon porous body. The photograph is shown in FIG. The room temperature strength of the obtained porous body is 12 kgf / mm ² , 1
The strength at 360 ° C is 8 kgf / mm ² , and the coefficient of thermal expansion is 1.
It was 5 × 10 ⁻⁶ / k (average value at 20 to 800 ° C.). Example 2 20% by weight of Si ₃ N ₄ powder having an average particle size of 0.3 μm,
80% by weight of amorphous SiO ₂ powder having an average particle size of 1.2 μm and AlN powder having an average particle size of 0.3 μm were mixed with SiO 2
2 against ₂ molar ratio: adjusted to be 1, the average particle size such that more and 1 wt% of the total 0.2 [mu] m of Y ₂
O ₃ powder is added and blended. This prepared powder was pulverized and mixed in ethanol for 12 hours using a ball mill using Si ₃ N ₄ balls as a pulverizing medium, and then dried to obtain a raw material powder.

Next, the above powder was sintered at 1550 ° C. for 2 hours at normal pressure in a N ₂ gas flow of 3 liters per minute. The pressure applied at this time was 20 MPa. The obtained sintered body was heat-treated at 1400 ° C for 6 hours in order to crystallize sialon.

When the obtained sintered body was identified by X-ray diffraction, it was found to be crystalline Si ₃ N ₄ , Si ₂ ON ₂ and S.
i ₁₂ Al ₁₈ O ₃₉ N ₈ was confirmed. As a result of electron diffraction using an analytical electron microscope, it was found that 50% by volume of the whole was an amorphous phase. The microstructure is a mosaic structure in which columnar crystal phases are entangled with each other, and has a complicated structure in which an amorphous phase is present in a gap. As a result of Raman spectroscopic analysis, the amorphous phase in this case was found to be Sialon or Y ₂ Si ₂ O ₇ having a small amount of solid solution of alumina and low crystallinity.

The bending strength of the obtained sintered body is 95 kgf / average.
mm ² , Vickers hardness of 1850, and fracture toughness of 5.9 MPa · m ^1/2 . As a result of thermal expansion measurement, the average value from 20 ° C to 30 ° C was 0.7 × 10 ^-6 / K.

Furthermore, from this sintered body, 10 × 10 × 1 (mm)
The test piece was cut out and immersed in hot concentrated sulfuric acid (H ₂ SO ₄ ) at 160 ° C. for about 1 hour to elute and remove the amorphous phase to obtain a sialon porous body.

The room temperature strength of the obtained porous body is 18 kgf / m.
The strength at m ² and 1350 ° C. was 15 kgf / mm ² , and the thermal expansion coefficient was 1.8 × 10 ⁻⁶ / k (average value at 20 to 800 ° C.).

[0050]

The ceramic porous body obtained according to the present invention is a material having excellent heat resistance and an extremely small coefficient of thermal expansion, and thus has extremely excellent thermal shock resistance. Furthermore, it also has mechanical strength.

[Brief description of drawings]

FIG. 1 is a photograph showing the crystal structure of a sialon porous body obtained according to Example 1 of the present invention.

Claims (8)

[Claims] 1. A ceramic porous body comprising a plurality of automorphic sialon crystal particles, having a structure in which crystals are entangled with each other. 2. The ceramic porous body according to claim 1, wherein the crystal grains are columnar crystals. 3. The ceramic porous body according to claim 1, wherein the porosity is in the range of 10% by volume or more and 70% by volume or less. 4. The sialon crystal particles are mainly β type sialon system represented by the general formula Si _6-z Al _z O _z N _8-z (z is a natural number of 4 or less). The ceramic porous body according to any one of claims 1 to 3, which is characterized in that. 5. The sialon crystal particles are mainly composed of the general formula M _x Si ₁₂ Al ₁₂ O ₁₆ N ₁₆ (M is Li, Y, Ca, M).
5. An α-sialon system selected from g and Ln (lanthanide metals excluding La and Ce, and x is an integer of 0.4 or less), and is an α-sialon system. The ceramic porous body according to item 1. 6. The sialon crystal particles are mainly composed of Si.
₂ Al ₃ O ₇ N, Si ₆ Al ₁₀ O ₂₁ N ₄ , Si ₃ Al ₃
O _{3 + 1.5X} N _5-X , Si ₃ Al _2.67 N ₄ O ₄ , Si ₃ Al
₇ O ₃ N ₉ , Si ₁₂ Al ₁₈ O ₃₉ N ₈ , Al ₆ Si ₆ N ₈
O ₉ , Si ₃ Al ₃ O ₃ N ₅ , Si ₆ Al ₁₀ O ₂₁ N ₄ ,
Si ₂ Al ₃ O ₇ N, ceramics porous body according to any one of claims 1 to 5, characterized in that it consists of Si ₂ ON ₂ and Si ₃ N ₄ 2 or more compounds selected from. 7. A method for producing a porous ceramic body, which comprises subjecting a sintered ceramic body containing a plurality of automorphic sialon crystal particles to an etching treatment to form a porous body. 8. The method for producing a ceramic porous body according to claim 7, wherein the porosity is in the range of 10% by volume to 70% by volume.