JP2001342082A - Method for producing porous body - Google Patents

Abstract

(57) [Problem] To provide a method for easily producing a porous body having excellent mechanical properties while having a large porosity. After a microwave absorbing layer is formed on at least a part of a surface of a ceramic molded body, the ceramic molded body is irradiated with a microwave having a frequency of 1 to 30 GHz. By heating the ceramic molded body 1 through 2, a porous body 7 having a gradient structure with a density such that the density decreases from the surface on which the microwave absorbing layer 2 is formed toward the inside is formed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a filter for dust and the like, a carrier for a catalyst, a fluid permeable member such as a solid electrolyte for a fuel cell, and a substance that enters and exits into pores of a structure such as a living body substitute. The present invention relates to a porous body and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, a porous body made of ceramics (hereinafter, referred to as a ceramic porous body) has excellent stability at high temperatures and excellent corrosion resistance.
It is expected to be applied as a fluid permeable member such as a filter for fluid filtration and a catalyst carrier, and further as an artificial biological member.

[0003] As a method of manufacturing such a ceramic porous body, a cylindrical tube such as a honeycomb is filled with a slurry containing a ceramic raw material to which a foaming agent generating carbon dioxide is added, and this is foamed and fired to form a cylindrical tube. Producing an exhaust gas purifying structure provided with a ceramic porous body has been proposed, for example, in Japanese Patent Publication No. 63-63249.

Further, pressure is applied to a sol containing a metal, which is a ceramic raw material, and the sol is extruded from a nozzle into a fibrous form.
This was deposited on a sheet to produce a three-dimensional network structure, and a ceramic porous body was obtained (see, for example,
0-130002). These fluid-permeable materials require fluid permeability, that is, low pressure loss, to allow fluid to permeate, but ceramic porous bodies having high permeability generally have low strength due to low density. On the other hand, if the density is increased to increase the strength, the transmittance is remarkably reduced, and the characteristics as a fluid permeable member are poor.

To solve such a problem, see, for example,
In Japanese Patent No. 330941, a synthetic resin foam such as urethane foam is immersed in a slurry containing a ceramic raw material powder to form a film on the surface of the foam, which is then baked by hot isostatic pressing (HIP). It is described that a high-strength ceramic porous body made of a ceramic coating can be produced by burning out a synthetic resin foam.

Further, Japanese Patent Publication No. 3-27251 discloses a method of manufacturing a laminated ceramic filter in which a slurry having a small pore diameter and a small porosity is applied to the outer peripheral portion of a cylindrical ceramic filter while rotating the slurry.

[0007]

However, these methods have the problems that the process is complicated and costly, and that it is difficult to apply the method to a fluid permeable member having a large or complicated shape. .

In Japanese Patent Application Laid-Open No. 5-3094091, the matrix cannot be sufficiently improved because the matrix is hollow, and in Japanese Patent Publication No. 3-27251, cracks due to differences in the coefficient of thermal expansion between the layers during firing are disclosed. It was difficult to prevent the generation of such as.

As described above, heretofore, a method for producing a porous body having both high fluid permeation characteristics and high mechanical characteristics and applicable to a large member such as a dust filter by a simple process has not been established so far. Was.

Accordingly, an object of the present invention is to provide a porous body having a high porosity and excellent mechanical properties, and a method for easily producing the porous body.

[0011]

SUMMARY OF THE INVENTION According to the present invention, in order to improve mechanical properties while maintaining a high porosity of a porous body, a portion having a high stress (for example, a cylindrical porous body) generated in the porous body is required. Based on the idea that a gradient structure having a high density compared to a portion having a small stress (for example, the inside in the case of a cylindrical porous body) at the outer periphery in the case of a body is most ideal, This is easily realized by utilizing microwave heating for a porous body having the same.

That is, according to the method for producing a porous body of the present invention, after a microwave absorbing layer is formed on at least a part of the surface of a ceramic molded body, a microwave having a frequency of 1 to 30 GHz is applied to the ceramic molded body. By irradiating and heating the ceramic molded body through the microwave absorbing layer, a porous body having a gradient structure of a density such that the density decreases from the surface on which the microwave absorbing layer is formed toward the inside is formed. It is characterized by the following.

According to this configuration, although the ceramic molded body has a low microwave absorptivity, the microwave is absorbed by the microwave absorbing layer, and as a result, the ceramic molded body is heated and sintering of the molded body proceeds. And, since the inside is porous, the fluid permeability is excellent, and the density of the outer peripheral portion is higher than that of the inside, so that a porous body with improved mechanical properties can be realized.

In particular, it is preferable that the microwave absorbing layer is a tape-shaped molded body and is provided so as to cover at least a part of the ceramic molded body. Thereby, the microwave absorbing layer can be easily formed.

Further, it is preferable that the ceramic molded body has a cylindrical shape and the microwave absorbing layer is provided on an outer peripheral portion. This makes it possible to obtain a porous body having a high density and a high mechanical strength on the outside as compared to the inside of the cylindrical porous body, which is hard to break and easy to handle.

Further, it is preferable to remove the microwave absorbing layer after firing. By removing the microwave absorbing layer, the distribution of pores in the porous body can be easily controlled.

Preferably, the microwave absorbing layer is at least one of aluminum nitride, silicon carbide and zirconia. These are suitable for the microwave absorption layer because of their high dielectric loss factor, and a tape molded product is easily obtained.

It is preferable that the ceramic compact is at least one selected from silicon nitride, alumina, aluminate, aluminum borate, mullite, lanthanum chromite, cordierite, and magnesia. These have a low dielectric loss factor and a low microwave absorptivity, so that the internal temperature can be lowered and a gradient material is easily obtained.

The porous body of the present invention has a porosity of 20%.
Of the outer periphery of the perforated portion, is provided with a high-density layer, the relative density d _s of the dense layer is preferably higher than 5% with respect to relative density d _p of the porous portion.

With this configuration, the density of the inside of the porous body is lower than that of the side on which the microwave absorbing layer is formed, the resistance of the fluid is small, and the porous body easily flows. Because of the high density and the inclined structure with relatively high mechanical characteristics, the inside is porous, has excellent fluid permeability, and the density of the microwave absorbing layer formation side is higher than the inside. Therefore, the density of the surface on the side where the microwave absorbing layer is formed is high, and the surface functions as a reinforcing layer. As a result, high mechanical properties can be realized as the porous body as a whole.

It is preferable that the high-density layer has a relative density of 90% or less. As a result, the porosity can be kept large, and for example, the pressure loss of the filter can be reduced.

Further, it is preferable to use as a main component at least one selected from silicon nitride, alumina, aluminate, aluminum borate, mullite, lanthanum chromite, cordierite and magnesia, whereby an inclined structure can be easily obtained.

Furthermore, the ceramic filter of the present invention is a filter for separating a specific component from a gas or a liquid, which is provided with the porous body of the present invention as a constituent member of the filter, and is excellent in fluid permeation performance. The porous body of the present invention can be suitably used as a member of a fluid filter.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION In the method for producing a porous body according to the present invention, the ceramic molded body is partially heated to a high temperature by using a microwave absorbing layer having a higher microwave absorbing power than the ceramic molded body. The structure is formed. For this purpose, it is important that the microwave absorbing layer mainly absorbs microwaves to generate heat, and the ceramic molded body does not generate heat due to microwaves. That is,
In firing, the temperature of the ceramic molded body is not uniform, and it is required to generate a temperature distribution.

Therefore, according to the present invention, FIG.
As shown in (1), a microwave absorbing layer 2 is provided on at least a part of the surface of the ceramic molded body 1, and a temperature distribution is applied to the ceramic molded body 1 through the microwave absorbing layer 2 that has absorbed microwaves and generated heat. By firing the ceramic molded body 1 while causing the above, a porous body 7 in which the density of the high-density layer 5 is higher than that of the porous portion 6 can be easily obtained as shown in FIG. .

That is, the surface of the ceramic molded body 1 on the side of the microwave absorbing layer 2 has a high temperature due to a large amount of heat obtained from the microwave absorbing layer 2, and the surface of the ceramic molded body 1 distant from the microwave absorbing layer 2. The interior has a lower heat due to less heat available, resulting in a higher surface density than the interior and a graded structure.

In particular, as shown in FIG. 2A, it is desirable that the ceramic molded body 11 has a tubular shape and a microwave absorber 12 is provided on the outer periphery thereof. Thereby, as shown in FIG. 2B, it is possible to easily obtain a tubular ceramic porous body 17 in which the density of the outer peripheral portion 15 is higher than that of the inner portion 16. Although the center of the tube is hollow, it may have a cylindrical shape with no space.

Therefore, it is preferable to use at least one selected from the group consisting of silicon nitride, alumina, aluminate, aluminum borate, mullite, lanthanum chromite, cordierite, and magnesia, which are materials that hardly absorb microwaves.

Further, since the energy for absorbing microwaves depends on the dielectric loss, the dielectric loss of the molded product is 0.00
A value of 1 or more, especially 0.01 or more, is preferable in that the microwave absorption efficiency is significantly increased and the energy efficiency is increased. Specifically, as a material used for the microwave absorbing layer, it is preferable to use at least one of aluminum nitride, silicon carbide, and zirconia which can absorb microwaves and directly generate heat and have a high melting point.

In addition to these materials, depending on the frequency of the microwave, a substance having a large dielectric loss such as a sintering aid can be contained in the microwave absorbing layer. In addition, since the dielectric constant also affects microwave absorption, there is no problem as a material for the microwave absorbing layer as long as a material having a large dielectric constant does not adversely affect strength and thermal conductivity.

The microwave absorbing layer can be formed by coating, spraying, coating with a solution such as a sol-gel method, CVD, PVD, or the like, in addition to laminating a tape-shaped molded body. It is preferable to laminate a tape-shaped molded body because it can be formed easily and easily. This tape is formed by a method such as a doctor blade and can be wound around a ceramic molded body.

Further, the microwave absorbing layer is preferably removed after firing. Therefore, it is desirable to select a material that does not densify the microwave absorbing layer at the firing temperature of the ceramic molded body.

Here, the microwave is selectively absorbed by the microwave absorbing layer to generate heat, and is used for heating and firing the ceramic molded body. Therefore, the frequency of the microwave needs to be 1 GHz to 30 GHz at which the microwave absorbing layer efficiently absorbs, and by using this frequency, the microwave absorbing layer can be heated at a relatively low cost. Further, in consideration of microwave absorption efficiency and uniform heating, it is particularly preferable to use a frequency of 2 to 28 GHz.

In the heating by microwave, the particles in the microwave absorbing layer may be directly heated and the temperature may be increased monotonously. However, since the temperature is easily increased rapidly, the reaction accompanying sintering may be insufficient. is there. For example, in the case of silicon nitride, in some cases, silicon is used as a raw material.
Therefore, if the heating rate is too high, the nitridation reaction may not proceed sufficiently, and as a result, there is a risk that a desired porous body cannot be obtained.

Therefore, in the method for producing a porous body according to the present invention, in the first step, in order to promote a chemical reaction, microwaves are irradiated at a low output, and for example, the temperature of the compact is lower than the maximum firing temperature. It is possible to maintain the temperature and maintain the temperature for a certain period of time until the reaction is completed, then raise the microwave output to increase the temperature, and perform the calcination in the second step.

Further, in order to further promote the chemical reaction, it is possible to increase the pressure of the atmospheric gas or to perform the reaction under reduced pressure. For example, in order to accelerate the nitridation reaction of silicon, nitrogen can be used as an atmospheric gas and the nitrogen pressure in the firing furnace can be set to 2 atm or more.

The firing temperature depends on the type of ceramic used and the type and amount of the sintering aid, and the temperature of the compact is 8
It can be selected in the range of 00 ° C to 2200 ° C. The firing time also depends on the type and amount of the sintering aid, but is not less than 5 minutes, especially 1
The time is preferably at least 5 minutes, more preferably at least 30 minutes, and most preferably at least 45 minutes.

As a specific example, when a porous body of silicon nitride is produced using silicon powder, in order to accelerate the nitridation reaction of silicon, nitrogen gas is used as an atmospheric gas in the first step at 1300 ° C. for one hour. The temperature may be maintained, then the temperature may be increased, and firing may be performed at 1500 ° C. for 30 minutes.

The ceramic molded body used in the present invention is not particularly limited in purity or average particle diameter.
In order to prevent partial oversintering and control the void diameter due to the characteristics of the porous body, the purity must be 97% or more, especially 98% or more.
More preferably, it is 99% or more. For the same reason, the average particle size of the ceramic powder in the ceramic molded body is preferably 20 μm or less, particularly preferably 5 μm or less, and more preferably 2 μm or less.

Further, in the method for producing a porous body of the present invention, it is preferable that the amount of the sintering aid is not more than a certain amount in order to prevent oversintering. The sintering aid is generally used as a well-known technique to promote densification, and its amount depends on the ceramics forming the molded body, but the amount is 10% by weight or less, particularly 5% by weight or less, and furthermore It is preferably at most 2% by weight. Further, in order for the sintering aid to be uniformly dispersed in the compact, the average particle diameter of the sintering aid is preferably 3.0 μm or less, particularly 2.0 μm or less, and more preferably 1 μm or less.

Next, a method for producing a porous body made of ceramics (hereinafter referred to as porous ceramics) will be described as a specific example using the above-described production method.

First, a raw material mainly containing at least one of silicon nitride, alumina, aluminate, aluminum borate, mullite, lanthanum chromite, cordierite, and magnesia is prepared. The purity of these raw materials is 99
% Or more, and the average particle size is 5 μm or less.

According to the present invention, the ceramic material used for the porous ceramic has a relatively small dielectric loss factor and a low microwave absorptivity, so that it is difficult for the microwave to generate heat.

These raw materials are granulated and formed by a known technique. For example, a desired molded body can be obtained by pulverizing with a ball mill, spray-drying, and molding by rubber press, mold molding, extrusion molding, or the like.

Next, a microwave absorbing tape is prepared.
As the microwave absorbing material, it is necessary that the dielectric loss factor is larger in the firing temperature range than the porous ceramics, and further, there is no evaporation or decomposition in the firing atmosphere,
Those that are chemically stable are preferred. Aluminum nitride, zirconia, silicon carbide and the like are suitable as microwave absorbing materials as materials having a relatively high dielectric loss factor from room temperature to high temperature and having a high melting point.

These microwave absorbing materials are formed into a tape shape by using a known technique such as a doctor blade. The thickness of the microwave absorbing material is preferably 10 μm or more, more preferably 100 μm, and even more preferably 500 μm.
m or more. If the thickness is less than 10 μm, the microwave absorbing effect cannot be obtained.

According to the present invention, the microwave absorbing layer formed into a tape shape is laminated and adhered so as to cover at least a part of the ceramic molded body. At this time, it is preferable that the microwave absorbing layer is formed on the surface of the ceramic molded body where the stress load is increased during use. The portion where the stress load is high may be strictly determined by FEM analysis or the like, but in the case of a simple shape, for example, a flat plate, as shown in FIG. Can be provided with a microwave absorbing layer 2. In the case of a cylindrical shape, for example, as shown in FIG. 2A, a microwave absorbing layer 12 is provided on the outer periphery of a ceramic molded body 11, and firing can be performed by microwave heating.

The method of bonding the tape is not particularly limited. For example, the tape can be uniformly bonded with an adhesive such as a resin. In order to improve microwave absorption, the clearance between the molded article and the tape is preferably as small as possible, and it is particularly preferable that the tape be in close contact with the tape.

It is necessary to degrease the ceramic molded body in advance. The method of degreasing depends on the type of binder added to the molded body, but it is sufficient to heat the molded body at an appropriate temperature in an oxidizing atmosphere, in a vacuum or in an inert atmosphere for several hours.

Then, the molded body is heated and sintered by using a microwave heating and sintering furnace for ceramics having a magnetron, a gyrotron, and a klystron which oscillate at a frequency of 1 to 30 GHz in an oscillation tube.

At this time, it is preferable that the molded body is placed in a heat insulating material such as an alumina porous body, porous silicon nitride, or mullite and heated. The firing temperature varies depending on the material, but is 100 to 200 ° C. in comparison with the firing temperature in a normal electric furnace.
Baking at low temperature, holding time may be performed for 5 to 120 minutes,
Under such conditions, a porous body having a porosity of 20% or more can be obtained. If the porosity is less than 20%, the fluid permeability is poor and cannot be used as a filter or the like.

In this firing step, the microwave absorbing material has a higher microwave absorption rate than the porous molded body, and thus the temperature of the material itself becomes higher, and the density near the surface of the ceramic molded body on the side where the microwave absorbing layer is formed is densified. Promote. But,
In order to prevent the molded article from cracking due to rapid temperature rise, it is desirable to reduce the rate of temperature rise near the final holding temperature. For example, in the case of firing at 1500 ° C., at 1200 ° C. or more, the rate may be about 5 ° C./min. If the rate of temperature rise is high, the rate of temperature rise of the microwave-absorbing material will increase, causing thermal runaway and the danger of breakage of the compact.

Further, in order to facilitate obtaining a gradient structure of density, a part of the ceramic molded body may be cooled in order to promote formation of a temperature distribution of the ceramic molded body. For example, in FIG. 1A, the opposite side of the ceramic molded body 1 from the microwave absorber 2 can be cooled. For cooling, a gas can be supplied or a water-cooled cooling plate can be provided near the ceramic molded body.

The density of the ceramic molded body (high-density layer) at the portion where the microwave absorbing material is stuck must be higher than the density of the inside (porous portion) by 5% or more in relative density. Since this density difference is determined by the type and thickness of the microwave absorbing material, it is necessary to optimize the material, thickness, or firing conditions to obtain an arbitrary density difference.

If the difference in density is less than 5%, the layer cannot function as a reinforcing layer and cannot withstand mechanical impacts or the like, so that it is easily broken. In particular, 7% or more is preferable for achieving high strength. The density of the high-density layer needs to be 90% or less in relative density. If it exceeds 90%, even if the thickness of the high-density layer is small, the pressure loss becomes extremely large, and the characteristics as a filter deteriorate.

The microwave absorbing layer adhering to the obtained porous ceramic is easily removed by sandblasting or the like. For example, the ceramic molded body 1 provided with the microwave absorbing layer 2 of FIG. 1A is fired by using microwave heating, and the microwave absorbing layer 2 is removed after firing, thereby obtaining the structure of FIG. A porous body having an inclined structure can be obtained. That is, the dense high-density layer 5 having few pores is formed on the outer periphery of the porous portion 6 containing many pores. The porous portion 6 has a porosity of 20% or more, and the relative density d _s of the high-density layer 5 is higher than the relative density d _p of the porous portion 6 by 5% or more.

As described above, by using the method for producing a porous body of the present invention, a porous ceramic having high strength and excellent permeability can be easily obtained.

Further, the porous body of the present invention has a high strength at a portion where a large stress is applied, such as an outer periphery in a cylindrical shape, and has excellent transmission performance, and can be suitably used as a ceramic filter. For example, in a cylindrical or rectangular pipe-shaped filter, the internal porous portion is used as a main flow path for gas or liquid, and the high-density layer on the side where the microwave absorbing layer is formed has a specific component, for example, fine dust or solution in exhaust gas. The impurities and the like in the inside can be separated.

[0059]

EXAMPLES Raw materials, organic binders, dispersants, and
Water was mixed and pulverized by a ball mill, and the obtained slurry was dried and granulated by spray drying. Φ granulated powder
Use a 60mm mold so that the shape is 20mm thick.
It was formed by a shaft press and dried in an inert atmosphere at 500 ° C. for 3 hours to obtain a formed body. In addition, as a microwave absorbing material, aluminum nitride, zirconia in which 3 mol% of yttria was dissolved in solid solution, and silicon carbide were rotationally mixed together with an organic solvent and an organic binder in a ball mill to prepare a tape slurry. The slurry is prepared by the doctor blade method.
A tape was formed at a thickness of 00 μm, and dried at 80 ° C. in the atmosphere to obtain a tape.

Each of the molded body obtained by attaching a tape to the upper surface of the molded body using an organic adhesive and the one not attached is placed in a microwave firing furnace and / or a resistance heating furnace.
It was fired under the conditions shown in Table 1. The temperature of microwave heating was measured by bringing a platinum thermocouple B thermocouple in the atmosphere into contact with a molybdenum-sheathed W-Re thermocouple in an inert atmosphere directly on the sample. 8 GHz, output 5 kW and 2.45 GHz, output 5
Different waveguides were used in combination with a kW magnetron, 6 GHz, 10 kW klystron, 28 GHz, 10 kW gyrotron. The sample N
o. No. 1 was fired using an electric furnace as a comparative example.

The microwave absorbing layer on the surface of the obtained porous body was removed by sandblasting, and every 1 mm from the surface,
A test piece of 1 × 4 × 35 mm was cut out as a reinforcing portion, an intermediate portion, and a porous portion, respectively. The density and porosity of the test pieces at these three sites were measured by the Archimedes method, and the relative density was calculated using the obtained density value and the theoretical density. In addition, the test piece of the reinforcing portion was set so that the surface on the side of the microwave absorbing layer became a tensile surface and JISR160
A three-point bending strength test was performed according to 1.

Further, the sample is processed to a thickness of 50 mmφ × 10 mm and arranged inside a cylindrical housing so that the inside of the housing is divided into two parts by the sample, and air is flowed at a flow rate of 10 m / s from the opening side of one of the housings. The pressure loss when passing through the sample was measured by a pressure difference meter. The results are shown in Table 1.

[0063]

[Table 1]

Sample No. of the present invention 3-6,8-14,1
6 to 22 are Si ₃ N ₄ , Al ₂ O ₃ , La ₂ O ₃ .11Al
ZrO ₂ , ZrO ₂ , a ceramic molded body selected from ₂ O ₃ (aluminate), 2B ₂ O _3.9 · Al ₂ O ₃ (aluminum borate), mullite, LaCrO ₃ (lanthanum chromite), cordierite and MgO One type of microwave absorbing layer selected from AlN and SiC is provided, and the average porosity is at least 20% by microwave heating, the relative density difference between the reinforcing portion and the porous portion is at least 5%, and high strength and low strength are obtained. Pressure loss was indicated.

On the other hand, the sample No. which was not provided with the microwave absorbing layer and irradiated with an electric furnace or microwave at a high output was outside the scope of the present invention. 1, 2, and 16 were lower in strength than the case where the microwave absorbing layer was provided in each material.

The sample No. having a frequency as small as 0.8 GHz and out of the range of the present invention was used. In No. 7, the relative density difference between the reinforcing portion and the porous portion was less than 5%, and the strength was small.

[0067]

According to the present invention, the microwave absorbing layer provided on the surface of the ceramic molded body is heated to form an inclined structure. As a result, a portion having a high porosity and a strength having a higher density than the portion have a higher porosity. A high-density layer is formed, and a porous body having excellent mechanical properties and excellent fluid permeability can be realized.

[Brief description of the drawings]

FIG. 1 shows a porous body of the present invention, in which (a) is a schematic cross-sectional view of a plate-shaped ceramic molded body, and (b) is a schematic cross-sectional view of the porous body.

FIGS. 2A and 2B show a porous body of the present invention, wherein FIG. 2A is a schematic sectional view of a tubular ceramic molded body, and FIG. 2B is a schematic sectional view of the porous body.

[Explanation of symbols]

 1, 11: Ceramic molded body 2, 12: Microwave absorption layer 3, 13: Microwave 5: High density layer 6: Porous part 7, 17: Porous body 15 ... Outer circumference 16 ... Inner

Claims (10)

[Claims] After a microwave absorbing layer is formed on at least a part of a surface of a ceramic molded body, the ceramic molded body is irradiated with microwaves having a frequency of 1 to 30 GHz, and the microwave is applied through the microwave absorbing layer. A method for producing a porous body, comprising: forming a porous body having a gradient structure of a density such that the density decreases from the surface on which the microwave absorbing layer is formed to the inside by heating the ceramic molded body. . 2. The porous body according to claim 1, wherein said microwave absorbing layer is a tape-shaped molded body and is provided so as to cover at least a part of said ceramic molded body. Manufacturing method. 3. The method for producing a porous body according to claim 1, wherein the ceramic molded body has a cylindrical shape, and the microwave absorbing layer is provided on an outer peripheral portion. 4. The method for producing a porous body according to claim 1, wherein said microwave absorbing layer is removed after firing. 5. The method according to claim 1, wherein the microwave absorbing layer is at least one of aluminum nitride, silicon carbide, and zirconia. 6. The ceramic molded body according to claim 1, wherein the ceramic molded body is at least one selected from silicon nitride, alumina, aluminate, aluminum borate, mullite, lanthanum chromite, cordierite and magnesia.
6. The method for producing a porous body according to any one of Items 1 to 5. 7. A high-density layer is provided on an outer periphery of a porous portion having a porosity of 20% or more, and a relative density d _s of the high-density layer is:
A porous body characterized by being higher by 5% or more with respect to the relative density d _{p of the} porous portion. 8. The porous body according to claim 7, wherein the relative density of the high-density layer is 90% or less. 9. The method according to claim 7, wherein at least one selected from silicon nitride, alumina, aluminate, aluminum borate, mullite, lanthanum chromite, cordierite and magnesia is mainly used. The porous body as described in the above. 10. A filter for separating a specific component from a gas or a liquid, comprising a porous body according to any one of claims 7 to 9.