JPH02192464A - Production of sintered porous material of silicon carbide - Google Patents

Abstract

PURPOSE:To increase porosity, uniformalize crystal size and pore size, and increase an effective surface area in contact with fluid by molding a mixture of a SiC powder and molding binder and subjecting to primary sintering, de- carbon treating and secondary sintering. CONSTITUTION:A molding binder (e.g. methyl cellulose) is added to a mixture of a SiC powder containing >=70wt.% beta-type SiC and a crystal growth auxiliary (e.g. Al) as necessary and molded to a fixed shape to obtain a raw molded material. Next, the raw molded material is dewaxed by heating by 700-800 deg.C for 1-2 hour in an inert gas atmosphere and sintering at 1500-1900 deg.C for 0.5-2 hour to obtain a first sintered material. Then, said sintered material is subjected to de-carbon treatment by heating at 400-900 deg.C for 1-4 hour, thus to secondary sintering by heating at 2000-2400 deg.C for 2-6 hour to afford aimed SiC sintered porous material having 20-95vol% porosity and 5-50mum pore size.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for manufacturing a sintered silicon carbide porous body, and more specifically, to internal combustion engines such as automobiles and generators, industrial furnaces in the ceramic industry, metal industry, etc. The present invention relates to a method for producing a porous sintered silicon carbide body used as a filter to collect particulate carbon contained in stray gas from.

[Prior art and problems to be solved by the invention] Conventionally,
In order to capture and remove particulate carbon contained in wandering gas,
Many attempts have been made to use a ceramic sintered porous body as a filter in the exhaust path.

For example, honeycomb-shaped materials whose main components are cordierite or silicon carbide are often used.

However, when cordierite is the main component, the ceramic particles in the honeycomb-shaped sintered porous partition walls tend to be oriented in the extrusion direction during extrusion molding, making it difficult for fluid to pass through the partition walls, resulting in pressure loss. The problem is that it becomes large. In addition, since the ceramic particles are plate-shaped and have a relatively smooth surface, the surface area that comes into contact with the fluid is small.
There is a problem in that heat transfer between partition walls cannot be carried out efficiently.

On the other hand, those whose main component is silicon carbide have a higher melting point than those whose main component is cordierite, and can withstand high-temperature heating for filter reuse. However, since the proportion of pores in the partition walls is relatively small at 30 to 40%, the ventilation resistance is large and the effective surface area for contact with fluids is small (for applications such as catalyst carriers and filters). The problem is that there are many things that are not suitable.

In addition, silicon carbide as a main component is easily affected by sintering conditions and auxiliary agents, and this effect becomes especially large as the shape becomes larger, resulting in a lack of uniformity in crystal shape and porosity. There is.

The present invention solves the above problems, has high porosity, uniform crystal size and pore size, has a large effective surface area for contact with fluid (exhaust gas), and efficiently collects and removes particulate carbon from fluid. The purpose of the present invention is to provide a method for manufacturing a sintered silicon carbide porous body that can be used to produce a silicon carbide sintered porous body.

[Means for Solving the Problems] The method for producing a sintered silicon carbide porous body of the present invention includes a first step of using silicon carbide powder as a starting material and adding a crystal growth aid if necessary to obtain a mixture; A second step of adding a molding binder to obtain a green body molded into a predetermined shape: A first step in which the green body is primarily sintered within a temperature range of 1500 to 1900°C after degreasing in an inert gas atmosphere. 3
Step: The primary sintered body is placed in an inert gas atmosphere,
The fourth step is secondary sintering within a temperature range of 2000 to 2400°C.
Step: In the method for producing a sintered silicon carbide porous body, the primary sintered body obtained in the third step is heated at a temperature of 400 to 900°C in an oxidizing atmosphere before the secondary sintering in the fourth step. It is characterized by performing decarburization treatment by heating within a range.

First, in the first step, it is preferable to use β-type silicon carbide powder as a starting material. The reason for this is that β-type silicon carbide crystals are low-temperature stable crystals that are synthesized at relatively low temperatures, and during sintering, the − part of the β-type silicon carbide crystal is synthesized at a relatively low temperature.
This is because it easily undergoes a phase transition to a high-temperature stable α-type crystal such as the R-type and forms a plate-like crystal. This is also because it has excellent crystal growth properties. In particular, by using a starting material containing 60% by weight or more of β-type silicon carbide, the sintered porous body targeted by the present invention can be suitably produced. Among these, it is advantageous to use a starting material containing 70% by weight or more of β-type silicon carbide.

Examples of crystal growth aids include aluminum, boron, iron, carbon, and the like.

Among the above-mentioned substances, aluminum, boron, and iron have the function of accelerating the growth rate of crystal grains of silicon carbide. Therefore, in areas where these substances exist, a large number of plate-like crystal nuclei are generated. As a result of the development of plate-like crystals in each part, the size of the plate-like crystals that are formed is limited, so the areas where many of these substances exist have fine three-dimensional structures. It can have a mesh structure.

On the other hand, carbon has the function of slowing down the growth rate of silicon carbide crystal grains, contrary to the above-mentioned substances. Therefore, nucleation of plate crystals is suppressed at locations where these substances exist, so the number of plate crystals formed becomes relatively small. As a result, each plate-shaped crystal grows relatively large, so that a large three-dimensional network structure can be formed in areas where many of these substances are present.

Next, in the second step, a molding binder such as methyl cellulose, polyvinyl alcohol, or water glass is added to the mixture obtained in the first step, and the mixture is formed into a predetermined shape 1 by extrusion molding, sheet molding, press molding, etc. For example, it is a formed body formed into a honeycomb shape.

In the third step, first, the formed body obtained in the second step is degreased by heating at 700 to 800° C. for 1 to 2 hours in an inert gas atmosphere. After that, it was heated to 0 within the temperature range of 1500 to 1900°C under the same inert gas atmosphere.
．． Sinter for 5 to 2 hours to obtain a primary sintered body.

The reason why the primary sintering is performed at a lower temperature than the temperature of the secondary sintering treatment described later is to facilitate processing or surface treatment of the porous body. Further, the reason why the sintering temperature is set in the range of 1500 to 1900°C is to obtain strength that is easy to process into the porous body, and to set the temperature at a temperature that does not affect the carbon in the formed body.

For example, when forming by cutting or filling after coating or continuous forming, processing and processing are difficult in the formed form, and
If post-processing is required, if post-processing is attempted after secondary sintering, the ceramic material is hard and difficult to perform machining, making it uneconomical in terms of cost.

In the present invention, in the fourth step, before performing the secondary sintering treatment, the secondary sintered body is heated to a temperature of 40 to 9
The biggest feature is that the decarburization treatment is performed by heating within the temperature range of 00°C for 1 to 4 hours. This is done in order to create a porous body with uniform crystal size and pore size as well as high porosity.
This is a treatment performed to remove carbon remaining after the secondary sintering.6 In other words, as mentioned above, the carbon remaining after the secondary sintering acts as a crystal growth inhibitor during the secondary sintering, creating a uniform atmosphere. It becomes difficult to form a porous body with a high pore size and high porosity.

In particular, in a porous body having a large shape, for example, an outer diameter of 50 mm or more, the residual carbon concentration differs from part to part, causing a difference in crystal growth rate, making it difficult to obtain uniform and large pore diameters.

Therefore, the manufacturing method of the present invention attempts to manufacture a sintered porous body having uniform crystal diameter and pore diameter throughout the porous body by performing this decarburization treatment. If the decarburization temperature is lower than 400° C., the decarburization rate is slow and decarburization becomes substantially difficult. Moreover, if the temperature exceeds 900°C, there is a possibility that the silicon carbide of Madfuchs will be oxidized together with the carbon.

Preferably it is in the range of 500 to 750°C. Generally, air or oxygen can be used as the oxidizing atmosphere, but an atmosphere containing CO□ gas allows mild oxidation to proceed and suppresses oxidation of silicon carbide in the matrix.

By performing such a treatment, the average pore diameter of the silicon carbide sintered porous body produced by the present invention can be in the range of 5 to 50 μm. If it is less than 5 μm, the pores formed by the silicon carbide crystals will be smaller than the volume occupied by the crystals, making it difficult to have high porosity and large pore diameter. In addition, ventilation resistance also increases. on the other hand,
If it exceeds 50 μm, the strength of the bonded portion of the plate crystals will be low, and the strength of the sintered porous body itself will be extremely low, making it difficult to maintain the shape of the sintered porous body.

It is more preferable that the average pore diameter is in the range of 25 to 30 μm.

Next, the primary sintered body subjected to the decarburization treatment is subjected to a secondary sintering treatment in an inert gas atmosphere within a temperature range of 2000 to 2400° C. for 2 to 6 hours.

In the case of a honeycomb structure, after the honeycomb-shaped formed body is formed or after decarburization treatment, a seal made of the same material as the formed body is usually placed at the end of a predetermined through hole. The material will be filled with a sintering agent and the sintering process will be performed.

By sintering in an inert gas atmosphere while blocking the intrusion of outside air, adjacent silicon carbide crystals are fused together,
Moreover, the growth of plate-like crystals of silicon carbide can be promoted. Therefore, the sintered porous body can have a three-dimensional network structure in which plate crystals are entangled in a complicated state. As a result, the effective surface area that comes into contact with the fluid (exhaust gas) becomes larger, and the efficiency of collecting particulate carbon can be increased.

In addition, in the case of a honeycomb structure, the flow from the axial direction of the honeycomb structure is easily taken into the partition walls.

Furthermore, since the fluid flow generated on the partition wall surface becomes turbulent, uniformity through diffusion, stirring, etc. within the flow is promoted, and heat transfer, chemical reactions, mass transfer, etc. that occur on the partition wall surface are carried out effectively. Become.

In the present invention, when performing the sintering treatment in each of the above steps, the porous body may be removed from the container and sintered while changing the atmosphere and temperature each time the sintering conditions such as the atmosphere and sintering temperature are changed. In addition, continuous sintering treatment may be performed in which the atmosphere and temperature are changed while the porous body is kept in the same container, instead of being taken out to the outside each time.In addition, when performing continuous sintering, desorption performed in an oxidizing atmosphere may be used. It is necessary to use a container made of silicon carbide, titanium carbide, boron carbide, etc. so that the container will not burn even after charcoal treatment.

The silicon carbide sintered porous body thus obtained has sufficient strength while having a high porosity (20 to 95% by volume) and a large pore diameter (5 to 50 μm) throughout the porous body. In addition, the particulate carbon collecting parts such as the partition walls of the honeycomb-shaped sintered porous body have a three-dimensional network structure composed of plate-shaped crystals of silicon carbide, and can efficiently collect and remove particulate carbon. Can be done.

In addition, the sintered porous body obtained in this way is further
Under reduced pressure of 200-1O-3 torr, 1700-230
The sintering treatment may be carried out within a temperature range of 0°C.

When such a treatment is performed, a sintered porous body having a low specific resistance of 10 −1 to 10 −3 Ω·Cm can be obtained.

Therefore, when heating the filter for reuse, the entire filter can be uniformly heated by direct current application and at low voltage.

[Example] Example 1 The silicon carbide fine powder used as a starting material was one in which 80% by weight consisted of β-type crystals. This starting material contains 0.01% of B as an impurity. C is 0, 5. An is 0.
01. N is 0.2. It contained 0.08 atomic weight part of Fe and trace amounts of other elements, and the total amount of these impurities was 0.81 atomic weight part. Moreover, the average particle diameter of this starting material was 0.3 um, and the specific surface area was 1B, 7rn'/g.

To this starting material were added 10 parts by weight of methylcellulose as a molding binder and 20 parts by weight of water. This was kneaded and extrusion molded to create a mold with a diameter of 140nv+ and a length of 140nm.
++us, the thickness of the partition walls of the through-holes was 0.3n+m, and a honeycomb-like product shape was obtained with 200 through-holes per square inch.

This formed body was placed in a graphite crucible, heated to 750''C at a rate of 0.5°C/min in an Ar gas atmosphere, and degreased at the maximum temperature for 1 hour.

Thereafter, this compact was placed in a graphite crucible with a porosity of 20%, and primary sintered in an Ar gas atmosphere at 1 atm.

For sintering, the temperature was increased to 1700°C at 2°C/min and held at i&high temperature for 1 hour.

Next, the secondary sintered body is taken out from the graphite crucible, and a sealing material having the same composition as the formed body is filled in one end of the through hole in every vertical and horizontal direction.

The same goes for the other end of the through-hole that is not filled with the sealing material (the sealing material is filled with it. After that, the temperature is set at 1°C in an oxidizing atmosphere.
Decarburization treatment was carried out by heating to 700° C. at a heating rate of /min.

After that, this decarburized primary sintered body is reused with a porosity of 2.
It was placed in a 0% graphite crucible and subjected to secondary sintering in an Ar gas atmosphere of 1 atm. For sintering, the temperature was raised to 2200°C at a rate of 1.5°C/min and held at the maximum temperature for 4 hours.

The obtained honeycomb-shaped sintered silicon carbide porous body had a plate-like crystal structure, and the average aspect ratio of the plate-like crystals was 1.3, the open pore diameter was 30 μm, and the open porosity was 48% by volume.

Example 2 In this example, while the formed bodies were kept in the same crucible,
This was done using a continuous sintering process that changes the atmosphere and temperature.

The silicon carbide fine powder used as a starting material consisted of 80% by weight β-type crystals.

This starting material contains 0.01°C and 0.01°C of B as impurities.
5. , l is 0.01. N is 0.2. Fe is 0.0
8 parts by atomic weight, trace amounts of other elements were included, and the total amount of these impurities was 0.81 parts by atomic weight. Moreover, the average particle size of this starting material is 0.3 gm, and the specific surface area is 18.
It was 7rn”/g.

To this starting material were added 10 parts by weight of methylcellulose as a molding binder and 20 parts by weight of water. This was mixed and extruded to a diameter of 140 m and a length of 140 m.
(2) A honeycomb-shaped product was obtained, with a partition wall having 0 through-holes having a thickness of 0.5 mm and having 150 through-holes per square inch. One end of the through-hole of this formed body is filled with a sealing material having the same composition as that of the formed body in every vertical and horizontal direction, and the other end of the through-hole is not filled with the sealing material. was also filled with sealing material.

Thereafter, this formed body was placed in a SiC crucible with a porosity of 2%, and the temperature was raised to 700° C. at a rate of 0.5° C./min in an Ar gas atmosphere of 1 atm, and degreasing was performed at the maximum temperature for 1 hour.

Subsequently, the temperature was raised to 1900°C at a rate of 2°C/min and held at the maximum temperature for 1 hour to perform primary sintering.

Next, the inside of the SiC crucible was cooled for 30 minutes at a cooling rate of 5°C/min.
After cooling to 0° C., the atmosphere inside the crucible was made into an oxidizing atmosphere.

Thereafter, a decarburization treatment was performed in which the material was heated to 700° C. in an oxidizing atmosphere at a temperature increase rate of 1° C./min.

Next, secondary sintering was performed using an Ar gas atmosphere in the crucible. For sintering, the temperature was raised to 2200°C at a rate of 2°C/min and held at the maximum temperature for 4 hours.

The obtained honeycomb-shaped sintered silicon carbide porous body has a plate-like crystal structure, and the average aspect ratio of the plate-like crystals is 1.3. The open pore diameter was 32 μm and the open porosity was 49% by volume.

Comparative Example Same as Example 1 except that decarburization treatment was not performed.
A honeycomb-shaped sintered silicon carbide porous body was manufactured.

The obtained sintered porous body had a plate-like crystal structure, and the average aspect ratio of the plate-like crystals was 1.3, the open pore diameter was 2 μm, and the open porosity was 46% by volume.

[Effects of the Invention] According to the method for producing a sintered silicon carbide porous body of the present invention, by performing decarburization treatment, the porous body has a uniformly high porosity and large pore diameter, and can trap fine carbon. A silicon carbide sintered porous body with high collection efficiency and high strength can be manufactured.

Claims (1)

[Claims] The first step is to obtain a mixture using silicon carbide powder as a starting material and adding a crystal growth aid if necessary; a molding binder is added to the mixture to obtain a product molded into a predetermined shape. Second
Step: A third step of primary sintering the formed body under an inert gas atmosphere within a temperature range of 1500 to 1900°C after degreasing; The primary sintered body is sintered under an inert gas atmosphere at
A fourth step of performing secondary sintering within a temperature range of 0 to 2400°C; A method for producing a porous sintered silicon carbide body, which comprises decarburizing a primary sintered body by heating it within a temperature range of 400 to 900°C in an oxidizing atmosphere.