JPH0297472A - Conductive silicon carbide sintered porous body and production thereof - Google Patents

Abstract

PURPOSE:To obtain a conductive silicon carbide sintered porous body which is low in specific resistance and excellent in self-heat releasing properties by adopting a specified sintering stage. CONSTITUTION:This conductive silicon carbide sintered porous body has 10<-1>-10<3>OMEGA.cm specific resistance. Further the silicon carbide sintered porous body is formed by the following five stages. (1) A first stage for obtaining a mixture by utilizing silicon carbide powder as a starting raw material and adding a crystal growing auxiliary in accordance with necessity. (2) A second stage for obtaining a green molded body by adding a binder for molding to this mixture and molding the mixture into a prescribed shape. (3) A third stage for obtaining a primary sintered body by introducing this green molded body into a heat resistant vessel and performing primary sintering at 1500-1900 deg.C while interrupting infiltration of the outside air. (4) A fourth stage for obtaining a secondary sintered body by heating the primary sintered body at 500-900 deg.C in the oxidizing atmosphere to perform decarbonizing treatment and thereafter introducing it into the heat resistant vessel and performing secondary sintering at 2000-2400 deg.C while interrupting infiltration of the outside air. (5) A fifth stage for obtaining a tertiary sintered body by performing tertiary sintering for the secondary sintered body at 1700-2300 deg.C in the reduced pressure of 200-10<-3>Torr.

Description

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

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

However, if the filter is used for a long period of time, it has the drawback that fine carbon particles accumulate and cause clogging, resulting in pressure loss.

To overcome this drawback, methods have been proposed, such as a burner combustion method in which fuel and air are periodically blown into the particulate carbon collection part of the filter to forcefully heat it, and methods in which an electric heater is installed in front of the filter to heat and burn the particulate carbon. .

However, in the burner method, since it is difficult to control the combustion temperature, there are problems such as filter melting and cracking due to abnormally high temperatures. Furthermore, in the heater method, combustion remains in the vicinity of the heater, making it impossible to burn the entire filter, and it is also difficult to attach electrical resistance wires to the entire filter. especially,
The problem with filters made of materials with high thermal conductivity such as silicon carbide is that heat applied locally spreads throughout the body, making it difficult to reach the ignition temperature of 500 to 600 degrees Celsius, which is the ignition temperature of fine carbon particles, resulting in no combustion. There is.

On the other hand, if the filter is self-heating, the entire body can be heated uniformly without the above-mentioned problems, and the combustion can be completed in a short time6. , devices that can self-heat by direct energization are being considered.

However, conventional filters made of silicon carbide have a specific resistance of 10s to 1010s as a current carrying resistance characteristic.
The resistance was as high as Ω·cot, and it was virtually impossible to conduct electricity, making it impossible to obtain a self-heating effect.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems and provide a conductive sintered porous silicon carbide body having low resistivity and excellent self-heating properties, and a method for manufacturing the same.

[Means for Solving the Problems] As a result of intensive research to achieve -F distribution, the present inventors found that the reason for the high specific resistance is that the surface of the silicon carbide powder, which is the starting material, is extremely active. Therefore, oxygen from the oxide film formed on the surface and oxygen in the atmosphere remain (solid solution) in the grain boundaries and vicinity of the silicon carbide crystals during sintering. I thought. Therefore,
After sintering, we thought that if the oxygen was removed by further high-temperature treatment under reduced pressure (vacuum), we could obtain a sintered silicon carbide porous body with low resistivity and excellent conductivity. The invention was completed.

That is, the silicon carbide sintered porous body of the present invention is characterized by having a specific resistance of 10<-1> to 10<3 >[Omega].cm.

The specific resistance when used as a resistance heating element is 10-1 to 1.
The range of 03Ω・cm is preferable, and the specific resistance is 10−1Ω・cm.
If it is lower than C11, it is necessary to consume a large amount of current to obtain a certain amount of heat, which is not economical, and if the resistivity exceeds 103Ω・Cff1, it will be necessary to consume a large amount of current to obtain a certain amount of heat. It is necessary to apply voltage. Normal automotive power supply 12V or 24V, industrial power supply 100V or 20V
Since 0V is used, the specific resistance is 10-1.
~10 3 Ω·cm is preferable.

The specific resistance is as low as 10-1 to 103Ω・cm (the reason is
By performing the tertiary sintering treatment under the conditions described below, the oxygen remaining at and near the grain boundaries of silicon carbide crystals is removed.
For example, (S i C+ 02→Si
O↑10C0T) or (S i C+ S i O2
It is considered that the oxygen content is reduced to 1% by weight or less and the specific resistance is lowered.

In addition, due to this treatment, silicon carbide in the surface portion is simultaneously thermally decomposed (S i C-4S i T 10c) and a layer containing a large amount of carbon is formed in the surface portion, which lowers the specific resistance ( This is considered to be a factor.

Further, the structure of the sintered porous body of the present invention is preferably a honeycomb structure because even if the porous body has fine pores, there is little pressure loss and it is compact.

Next, a manufacturing method of the present invention will be explained.

The method for producing a conductive sintered porous silicon carbide body of the present invention includes using silicon carbide powder as a starting material and adding a crystal growth aid as necessary to obtain a mixture.The first step is to obtain a mixture by adding a molding binder to the mixture. A second step of obtaining a green body formed into a predetermined shape; the green body is inserted into a heat-resistant container to block outside air from entering, and primary sintering is performed within a temperature range of 1500 to 1900°C. Third step of obtaining a sintered body: After decarburizing the primary sintered body by heating it in an oxidizing atmosphere within a temperature range of 500 to 900°C, it is inserted into a heat-resistant container and exposed to outside air. 4th step of obtaining a secondary sintered body by performing secondary sintering within a temperature range of 2000 to 2400°C while blocking the , a fifth step of performing tertiary sintering within a temperature range of 1700 to 2300°C to obtain a tertiary sintered body.

First, in the first step, it is preferable to use β-type silicon carbide powder as the starting material.The reason is that β-type silicon carbide crystals are low-temperature stable crystals synthesized at relatively low temperatures, and are This is because it also has excellent growth properties of crystals. 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 them, 70% by weight
It is advantageous to use starting materials containing the above β-type silicon carbide.

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

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 shaped into a predetermined shape by extrusion molding, sheet molding, press molding, etc. For example, a honeycomb shaped product is obtained.

Next, in the third step, the formed body obtained in the second step is sealed in a heat-resistant container, and while blocking the intrusion of outside air,
A primary sintered body is obtained by sintering within a temperature range of 500 to 1900'c.

In the third step, the primary sintering is performed at a relatively low temperature in order to facilitate processing, surface treatment, etc.

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 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, it is difficult to process and process the formed form, and when post-processing it, after secondary sintering. When processed, the ceramic material is hard and difficult to machine, making it uneconomical in terms of cost.

In addition, when performing the second sintering, it may be carried out after degreasing the formed body. This degreasing is a process performed in advance to remove organic matter mixed into the formed body, and degreasing is performed. In such cases, it is better to avoid rapid heating and sintering and to gradually eliminate the organic matter.

In the fourth step, first, the primary sintered body is heated in an oxidizing atmosphere within a temperature range of 500 to 900°C for 1 to 4 hours to perform decarburization treatment. This is a process performed to remove carbon remaining in the secondary sintered body.

Next, the decarburized primary sintered body is sealed in a heat-resistant container, and heated to 2000 to 2400 ml while blocking the intrusion of outside air.
The setting treatment is carried out for 1 to 2 hours within the temperature range of .degree.

In the case of a honeycomb structure, after the honeycomb-shaped primary sintered body is decarburized, a sealing material made of, for example, the same material as the formed body is usually placed at the end of a predetermined through hole. After filling, a secondary sintering process will be performed.

By enclosing it in a heat-resistant container and sintering it while blocking the intrusion of outside air, adjacent silicon carbide crystals can be fused together and the growth of plate-shaped silicon carbide crystals can be promoted. Therefore, the sintered porous body can be made into a three-dimensional network structure in which plate crystals intertwine in a complicated state, and as a result, the effective surface area that comes into contact with the fluid (exhaust gas) increases, and the collection efficiency of fine carbon particles increases. can be increased.

In addition, in the case of a honeycomb structure, it is easier to incorporate the flow from the axial direction of the honeycomb into the partition walls, and since the fluid flow generated on the partition wall surface becomes turbulent, it is possible to achieve uniformity by diffusion, stirring, etc. within the flow. The heat transfer, chemical reaction, mass transfer, etc. that occur on the surface of the partition wall can be effectively carried out.

Next, the thus obtained secondary sintered body is further tertiary sintered in a fifth step. By performing such treatment, a sintered porous body with low resistivity can be obtained.

Sintering is performed under reduced pressure (in vacuum) of 200 to 10-” torr.
, it is preferable to carry out at a temperature range of 1700 to 2300°C.

The reason for using it under reduced pressure (vacuum) of 200 to 10 torr is that if it exceeds 200 torr, the
For example, in the temperature range of °C.

This is because the reaction equilibrium of SIC+SiO2;=2SiO+CO is not reached and one reaction does not substantially proceed.On the other hand, if the temperature is lower than 10-"torr, the above reaction proceeds satisfactorily in the above temperature range, but the apparatus becomes complicated. This is because the cost increases, which is undesirable.

The reason why the above temperature is set in the temperature range of 1700 to 2300°C is that if it is lower than 1700°C, the reaction equilibrium constant and reaction rate are low and it takes a long time and sufficient oxygen removal becomes impossible. If the temperature exceeds .degree. C., the reaction rate will be fast, but it will be difficult to control and will have an adverse effect on the SiC matrix. More preferably, it is in the range of 1850 to 2150'C.

Note that the sintering time is preferably 1 to 6 hours.

[Example 1 The silicon carbide fine powder used as the starting material in the example consisted of 80% by weight of β-type crystals. This starting material contains 0.01% of B as an impurity. C is 0.5. Aff is 0.
01. N is 0.2. Fe is 0.08 atom m? , traces of other elements are present, and the total amount of these impurities is 0.
It was 81 parts by atomic weight. Further, the average particle size of this starting material was 0.3 um, and the specific surface area was 18.7 rn''/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 extruded into a shape with a diameter of 130 mm and a length of 120 m.
A honeycomb-like formed body was obtained, in which the thickness of the partition walls of the through-holes was 0.3 mm and the number of through-holes was 200 per square inch.

This formed body was heated to 750° C. at a heating rate of 0.5° C./min in an Ar gas atmosphere and held at the maximum temperature for 1 hour to oxidize and remove the binder, completing degreasing.

Thereafter, this compact was placed in a 20% graphite crucible and primary sintered in an Ar gas atmosphere of 1 atm. For sintering, the temperature was raised to 1700°C at a rate of 2°C/min and held at the maximum temperature for 1 hour.

Next, one end of the through hole is filled with a sealing material in every row and column, and the other end of the through hole that is not filled with the sealing material is also filled with the same (filling with the sealing material). .

Next, this sealed primary sintered body was subjected to a decarburization treatment in which it was heated to 700° C. in an oxidizing atmosphere at a temperature increase rate of 1′C/min.

After that, the primary sintered body subjected to this decarburization treatment was
% graphite crucible and subjected to secondary sintering treatment in an Ar gas atmosphere of 1 atm. Sintering at 2150°C at 1.5°C/min
The temperature was raised to .degree. C. and maintained at the maximum temperature for 4 hours.

After that, after cooling, it was taken out from the graphite crucible and heated to 0.4 torr.
Tertiary sintering treatment was performed under reduced pressure of r.

For tertiary sintering, the pressure was reduced to 0.4 torr using a vacuum pump, and then the temperature was raised to 1900 °C at a rate of 10 °C/min, and the maximum temperature was 1.
Holds time.

The obtained honeycomb-shaped sintered silicon carbide porous body had a root-like crystal structure, an open pore diameter of 23 μm, and an open porosity of 50% by volume.

In addition, the free carbon in this sintered porous body is 4% by weight,
The oxygen content was 0.8% by weight.

As a result, the specific resistance was 3.0 Ω·cm.

This was the same as the comparison example, except that only the secondary sintering was performed and the tertiary sintering process was not performed to form a honeycomb-shaped sintered porous body of silicon carbide.

The obtained sintered porous body had a plate-like crystal structure, an open pore diameter of 20 μm, and an open porosity of 52% by volume.

Furthermore, the free carbon in this sintered porous body was 0.2% by weight, and the oxygen content was O.1% by weight.

As a result, the specific resistance was 5XlO'Ω·cm.

Comparative Example 2 This was the same as in the example, but the sintering temperature in the tertiary sintering treatment was 2400° C. to obtain a honeycomb-shaped sintered porous body of silicon carbide.

The obtained sintered porous body had a plate-like crystal structure, an open pore diameter of 30 um, and an open porosity of 60% by volume.

Further, the oxygen content in this sintered porous body was 0.2% by weight.

As a result, the specific resistance was 0.3 Ωcm, and although it had electrical conductivity, it lacked the strength as a honeycomb-shaped sintered porous body (
It collapsed when removed from the furnace.

[Effects of the Invention] Since the conductive silicon carbide sintered porous body of the present invention has a low specific resistance, it can generate heat even at a low voltage and can efficiently burn and remove particulate carbon in exhaust gas. Therefore, it is useful as a filter for various combustion equipment including diesel engines.

Claims (4)

[Claims] (1) A conductive silicon carbide sintered porous body characterized by having a specific resistance of 10^-^1 to 10^3 Ω·cm. (2) The conductive silicon carbide sintered porous body according to claim 1, wherein the porous body has a honeycomb structure. (3) The conductive silicon carbide sintered porous body according to claim 1 or 2, wherein the porous body has an oxygen content of 1% by weight or less. (4) First step using silicon carbide powder as a starting material and adding a crystal growth aid if necessary to obtain a mixture; Second step adding a molding binder to the mixture to obtain a product molded into a predetermined shape. A third step of obtaining a primary sintered body by inserting the formed body into a heat-resistant container to block outside air from entering and performing primary sintering within a temperature range of 1500 to 1900°C; The body is heated to 500-900℃ under an oxidizing atmosphere.
After decarburizing by heating within the temperature range of 2,000 to 2,000
Fourth step of performing secondary sintering within a temperature range of 400°C to obtain a secondary sintered body;
A method for manufacturing a conductive sintered porous silicon carbide body, comprising: a fifth step of performing tertiary sintering under a reduced pressure of 1,700 to 2,300° C. to obtain a tertiary sintered body.