JPH08217565A - Porous electrically conductive silicon carbide sintered body, its production and use - Google Patents

Abstract

PURPOSE: To ensure superior oxidation resistance for a sintered body by specifying the α-SiC content and using a β-SiC phase as the principal phase of the grain boundary bonding parts of the sintered body. CONSTITUTION: The particle size distribution of SiC powder having 5-50μm average particle diameter is regulated so that the ratio of the particle diameter of 10% cumulative particle size of the distribution to that of 50% cumulative particle size is made >=0.2 and 20-80wt.% of the SiC powder is mixed with 80-20wt.% powdery mixture prepd. by mixing metallic Si powder with a carbonaceous material such as carbon black so that the molar ratio of Si to C is made 1.0-2.0. Water and an org. binder are added to the resultant powdery mixture and they are kneaded and molded. The resultant molded body is heated from 400 deg.C to 1,100-1,800 deg.C at <=600 deg.C/hr heating rate in a nonoxidizing atmosphere under >=0.2atm partial pressure of N0 and it is further heated to a temp. above 1,500 deg.C and above the max. temp. in the former heating process at <=600 deg.C/hr heating rate in a nonoxidizing atmosphere under <0.2atm partial pressure of N2 to obtain the objective porous electrically conductive SiC sintered body having 40-80% porosity, 5-40μm average pore diameter, <=10Ω.cm specific resistance at room temp. and 20-80vol.% α-SiC content. The principal phase of the grain boundary bonding parts of this sintered body is made of β-SiC.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a porous conductive silicon carbide sintered body used for collecting combustible fine particles discharged from a diesel engine or the like, and more particularly to collected combustible fine particles. The present invention relates to a conductive diesel particulate filter that can be regenerated by uniformly and efficiently incinerating by heat generated by energization.

[0002]

2. Description of the Related Art A filter which collects combustible fine particles discharged from a diesel engine or the like uses a cordierite ceramic having a low thermal expansion coefficient, but when a certain amount of fine particles are collected, pressure loss increases. Therefore, it is necessary to incinerate and regenerate regularly. The filter is regenerated by injecting the combustion gas of the burner and incinerating it with the combustion heat, or by incinerating it by heating in combination with a nichrome wire heater or a heating metal layer. However, since these methods heat the filter from the outside, there is a problem that melting loss and thermal stress cracking of the filter occur due to local heat generation and a large temperature gradient as the combustible particles burn.

For this reason, a self-heating type filter, which is capable of uniformly incinerating the collected combustible fine particles, and which is compact and can be attached without largely changing the existing apparatus, has been studied. The filter used in this method is mainly made of conductive ceramics containing silicon carbide, molybdenum silicide, titanium carbide or lanthanum chromite as a main component (Japanese Patent Laid-Open Publication No. Sho.
58-119317, JP-A-2-42112) is disclosed.

However, conductive oxide ceramics such as lanthanum chromite have a problem that thermal stress cracking occurs due to their low heat resistance and high coefficient of thermal expansion. On the other hand, conductive non-oxide ceramics such as molybdenum silicide and titanium carbide have a problem that they easily oxidize and lose conductivity when the porosity and pore diameter are increased in order to impart a filter function. Further, since silicon carbide ceramic is basically an insulator, it is necessary to use T
i, the periodic table IV _a-group element or V, such as Zr, carbides of V _a group element such as Nb, nitrides, added boride, thereby forming a continuous conductive phase in the sintered body Therefore, it is necessary to impart conductivity. However, it is necessary to add a large amount of these electroconductive substances, and there is a problem that the electroconductivity is easily lost in an atmosphere containing oxygen such as the atmosphere due to the addition of a large amount thereof, and the electroconductivity is lost.

[Problems to be Solved by the Present Invention]

The present invention has been made in view of the above circumstances, and improves conductivity and imparts excellent oxidation resistance without adding a conductivity-imparting substance, and has a pore size and an optimum pore size as a filter. An object is to provide a porous conductive silicon carbide sintered body having a porosity, a method for producing the same, and a conductive diesel particulate filter composed of this sintered body.

[0006]

That is, the porous conductive silicon carbide sintered body of the present invention is a sintered body composed of α-type silicon carbide crystal grains and grain boundary bonding portions, and the α-type silicon carbide The content of crystal grains is 20 to 80% by volume, and the main phase of the grain boundary bonding part is β
Type silicon carbide.

And, the production method of the present invention is
5-50 μm, cumulative particle size distribution particle size 10% diameter (D ₁₀ ) and 50%
Diameter (D ₅₀₎ of the particle size ratio (D ₁₀ / D ₅₀₎ of 0.2 or more silicon carbide powder
After forming 20 to 80% by weight and 80 to 20% by weight of a mixed powder of metallic silicon and a carbon material having a molar ratio of metallic silicon to carbon (metallic silicon / carbon) of 1.0 to 2.0 into a molded body, the molding is carried out. It is characterized in that the body is heated in an atmosphere containing nitrogen gas to be nitrided and then carbonized. At this time, the molded body is nitrided to generate silicon nitride, and then carbonized to form the reaction-produced β-type silicon carbide as a grain boundary phase main phase of the α-type silicon carbide. It is possible to improve the conductivity without adding Al, increase the grain boundary bonding force, and densify the grain boundary phase to improve the oxidation resistance.

A more preferable manufacturing method is characterized in that the step of heating the above-mentioned molded body in an atmosphere containing nitrogen gas for nitriding and then carbonizing comprises the following steps (1) and (2). Is. (1) After heating from a temperature of 400 ° C to an arbitrary temperature of 1100 ° C to 1800 ° C at a temperature rising rate of 600 ° C / hr or less in a non-oxidizing atmosphere with a N ₂ partial pressure of 0.2 atm or more ,
(2) In non-oxidizing atmosphere with N ₂ partial pressure of less than 0.2 atm
1500 ℃ or more at the heating rate of 600 ℃ / hr or less and (1)
Heat above the maximum temperature of.

Further, a porous conductive silicon carbide sintered body having a desired pore size and porosity can be manufactured by appropriately setting the particle size and the compounding amount of the α-type silicon carbide powder as a starting material. You can

The present invention will be described in more detail below.

The porous conductive silicon carbide sintered body of the present invention comprises:
For example, it can be obtained by nitriding a mixed raw material composed of metallic silicon powder, carbonaceous material and α-type silicon carbide powder, and further carbonizing the produced silicon nitride. By nitriding metallic silicon in the raw material and further carbonizing it, an impurity conductive phase is formed at the grain boundary portion, and a sintered body having a specific resistance of 10 Ω · cm or less is easily obtained. Further, it is also possible to mix silicon nitride formed by previously nitriding, without using the metallic silicon powder. The silicon carbide sintered body thus obtained is a silicon carbide sintered body composed of α-type silicon carbide and a grain boundary part that binds the α-type silicon carbide, and the α-type silicon carbide content is 20 to 80% by volume. Since the main phase of the grain boundary portion is composed of β-type silicon carbide, the grain boundary phase is densified as compared with a normal silicon carbide powder sintered body, so that the specific resistance is small and excellent. Has oxidation resistance. In addition, the specific resistance is 10Ω ・ cm or less.
If it is higher than cm, it is usually installed when conducting electric heating as a conductive diesel particulate filter.
With a battery capacity of V, it becomes difficult to heat the collected particulates to a temperature at which they can be incinerated.

The content of α-type silicon carbide in the sintered body of the present invention is
It is in the range of 20 to 80% by volume, but if it is less than 20% by volume, the mechanical strength is reduced, and if it exceeds 80% by volume, β constituting the grain boundary phase is formed.
Since the amount of type silicon carbide is small, the specific resistance is high and the oxidation resistance is reduced. The changes in the specific resistance and oxidation resistance of the grain boundary phase are due to the following reasons. That is, β-type silicon carbide forming the grain boundary phase is generated via silicon nitride, and can secure a large amount of solid solution of nitrogen and has a low specific resistance. This is because when the amount of β-type silicon carbide forming the composition is small, the specific resistance becomes high, and the denseness of the grain boundary joint portion is lowered, so that the grain boundary oxidation easily progresses and the oxidation resistance is lowered.

On the other hand, the pore characteristic of the porous conductive silicon carbide sintered body of the present invention is 5 μm in the range of the average pore diameter of 5-40 μm.
When it is smaller than m, the combustible fine particles are significantly clogged and the pressure loss increases in a short time. Also, the pore size is 40 μ
When it exceeds m, the collection efficiency is lowered and the filter function is lowered. Further, the porosity is 40% or more, preferably 50-80%
In the range of, if it is lower than 40%, the pressure loss is high, and if it exceeds 80%, the mechanical strength decreases.

As a manufacturing method for obtaining the above-mentioned porous conductive silicon carbide sintered body, the average particle size and particle size distribution of the α-type silicon carbide powder as a starting material are important and affect the pore characteristics of the sintered body. To do. That is, the average particle size of α-type silicon carbide, which is a starting material, is 5 to 50 μm, and the cumulative particle size of the particle size distribution is 10 μm.
The particle size ratio (D ₁₀ / D ₅₀ ) of% diameter and 50% diameter must be 0.2 or more. That is, when the average particle size is 5μm smaller, or the particle size ratio of the particle size distribution (D ₁₀ / D _{5 0)} is of less than 0.2, the pore size of the sintered body is reduced. On the other hand, the average particle size is 5
If it exceeds 0 μm, the mechanical strength of the sintered body decreases. As the metal silicon powder, ordinary industrial metal silicon is sufficient, and the average particle size is preferably 100 μm or less in view of moldability and reactivity with carbonaceous substances. As the carbonaceous substance, fine particle solid carbon powders such as carbon black and acetylene black, as well as organic resins such as phenol, furan and polyimide, which are thermally decomposed to become carbon, can be used.

The mixing ratio of these starting materials is 20 to 80% by weight of α-type silicon carbide powder and 80% mixed powder of metallic silicon powder and carbonaceous material.
~ 20% by weight. At this time, the mixing ratio of the metallic silicon powder and the carbonaceous material is such that the molar ratio of metallic silicon to carbon is 1.0.
It should be in the range of ~ 2.0. If this molar ratio is less than 1.0, carbon remains in the sintered body and inhibits the sintering of reaction-produced βSiC. On the other hand, if it is more than 2.0, a large amount of metallic silicon remains and the mechanical strength and oxidation resistance are reduced.

As a method of mixing these raw material powders, any mixing method such as dry type and wet type can be applied as long as they can be uniformly mixed. The mixed raw material can be molded by adding an organic binder such as methyl cellulose or polyvinyl alcohol, press molding, extrusion molding, injection molding, or by adjusting a slurry and pouring into a container having a desired shape for solidification.

Next, the method of firing the obtained molded body is as follows: (1) Non-oxidation with a N ₂ partial pressure of 0.2 atm or more from a temperature of at most 400 ° C. to an arbitrary temperature of 1100 ° C. to 1800 ° C. In a neutral atmosphere, heat at a heating rate of 600 ° C / hr or less. The molded body is degreased as necessary depending on the type of binder,
In some cases, it is preferable to perform degreasing in an oxidizing atmosphere. At a temperature of at least 400 ° C, it is necessary to heat in a non-oxidizing atmosphere having a N ₂ partial pressure of at least 0.2 atm, but at a temperature of less than 400 ° C It need not be in a non-oxidizing atmosphere. When the N ₂ partial pressure is less than 0.2 atm, nitriding of metallic silicon is insufficient and the specific resistance of the sintered body obtained by carbonization becomes high. Similarly, if the heating rate exceeds 600 ° C / hr, nitriding is insufficient and the specific resistance increases. Further, if the temperature of the nitriding reaction is less than 1100 ° C, the nitriding reaction does not occur, and if it exceeds 1800 ° C, the generated silicon nitride is converted into silicon carbide, and the sintered body obtained under such conditions has a pore diameter of It is small and the oxidation resistance is reduced.

Next, the formed body of silicon nitride was subjected to (2) N ₂ partial pressure in a non-oxidizing atmosphere of less than 0.2 atm.
Heat to a temperature of 1500 ℃ or more and the maximum temperature of (1) or more at a heating rate of 00 ℃ / hr or less. The temperature is preferably 1800 ° C or higher, more preferably 1900 ° C or higher. N ₂ partial pressure
The non-oxidizing atmosphere of less than 0.2 atm may be vacuum or an atmosphere of a mixed gas of nitrogen and argon, carbon monoxide, ammonia, methane, hydrogen or the like. When the temperature of this step is 1500 ° C. or lower than the maximum temperature of the step (1), carbonization is insufficient, silicon nitride remains, and the specific resistance increases.

The conductive silicon carbide sintered body of the present invention is used as a porous conductive silicon carbide sintered body having a porosity of 40% or more, an average pore diameter of 5 to 40 μm, and a room temperature specific resistance of 10 Ω · cm or less. Since it is a united body, it is optimal as a conductive filter with added heater performance capable of collecting and burning combustible fine particles discharged from a diesel engine. On the other hand, these characteristics are suitable for a duct heater and a heater for a hot air generator used as a heat source for a large-sized dryer, from the viewpoint of increasing the heating area and increasing the thermal efficiency. Further, it can be sufficiently used as a heater used for ordinary heating equipment, cooking equipment, drying equipment, firing furnace and the like.

[0020]

EXAMPLES The present invention will be described more specifically with reference to Examples and Comparative Examples.

(Examples 1 to 7 and Comparative Examples 1 to 10) As starting materials, silicon carbide powder having a particle size (average particle size, particle size distribution particle size ratio) shown in Table 1, industrial metal silicon powder and carbon black. Raw material in which the mixed powder of
To 100 parts by weight, 20 parts by weight of water and 8.0 parts by weight of methylcellulose as a binder were further added, mixed for 10 minutes with a Henschel mixer, and then using a kneader type kneader.
Kneaded for 30 minutes. The obtained kneaded product was sized 50 mm in outer diameter, 2.5 mm in cell size and 0.5 in rib pressure using a high pressure vacuum extruder.
mm honeycomb was extruded at a forming pressure of 60 kg / cm ² . The obtained molded product is dried and then in a nitrogen atmosphere at 450 ° C for 1 hr.
After degreasing, it was nitrided under the firing conditions in the nitriding reaction column shown in Table 2 and then carbonized under the firing conditions in the carbonization reaction column.

The following properties of the obtained sintered body were measured and are shown in Table 3. (1) Porosity: Archimedes method. (2) Average pore size: mercury porosimetry. (3) Identification of silicon carbide crystal phase: X-ray diffraction was performed and calculated by the following formula. α-type silicon carbide content (volume%) = 100− {β-type silicon carbide content (volume%)} where β-type silicon carbide content (volume%) = 100 / (1+
a + b) a = 4.571Ia / (100−2.721Ia−0.665Ib) b = 2.531Ib / (100−2.721Ia−0.665Ib) where Ia is the peak intensity at CuKα 2θ of 34.3 ° and Ib is the peak at 34.9 °. Strength, CuKα
This is a relative value when the peak intensity at 2θ = 36.5 ° is 100. (4) Room temperature resistivity: Honeycomb structure was cut into □ 10 × 50 mm L, silver electrodes were formed and measured by the 4-terminal method. (5) Oxidation resistance: After treatment in air at a temperature of 1000 ° C for 100 hours. The specific resistance was measured. (6) Mechanical strength: The honeycomb structure was cut into □ 10 × 10 mm L, and the compressive strength in the extrusion direction was measured.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

[Table 3]

As is clear from Tables 1 to 3, Examples 1 to 1
The conductive porous silicon carbide sintered body obtained in No. 7 has suitable porosity and average pore diameter, shows low conductivity at room temperature and excellent conductivity, and has excellent compressive strength and oxidation resistance. Indicated. The properties of these sintered bodies as a conductive diesel particulate filter, that is, the collection efficiency, the evaluation results of the heat removal performance of the collected particulates are good, and they have excellent properties as a diesel particulate filter. there were.

[0027]

EFFECT OF THE INVENTION According to the method for producing a conductive silicon carbide sintered body of the present invention, the conductivity and the oxidation resistance are excellent without adding a conductivity-imparting material, and the optimum porosity and pore diameter as a filter are obtained. There is provided a porous conductive silicon carbide sintered body having: In addition, the application of the porous conductive silicon carbide sintered body of the present invention, particularly, as a conductive diesel particulate filter having a heater performance of collecting and burning incineration of fine particles discharged from a diesel engine, further, a heating area From the viewpoint that the heat efficiency is greatly increased, it is also suitable as a duct heater, a heater for a hot air generator used as a heat source for large dryers, and a heater used for heating equipment, cooking equipment, drying equipment, baking furnaces, etc. There is.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yasuo Imamura 1 Shinkai-cho, Omuta-shi, Fukuoka Electric Chemical Industry Co., Ltd. Omuta factory

Claims (4)

[Claims] 1. A sintered body composed of α-type silicon carbide and a grain boundary bonding part, wherein the content of the α-type silicon carbide is 20 to 80% by volume, and the main phase of the grain boundary bonding part is β type. A porous conductive silicon carbide sintered body characterized by being composed of silicon carbide. Wherein the average particle diameter. 5 to 50 [mu] m, the particle size ratio (D ₁₀ / D ₅₀₎ of the cumulative particle size of 10% the diameter of the particle size distribution (D ₁₀₎ and 50% diameter (D ₅₀₎
Of 20 to 80% by weight of silicon carbide powder of 0.2 or more and the molar ratio of metal silicon to carbon (metal silicon / carbon) is 1.0 to
Mixed powder of metallic silicon powder and carbon material that is 2.0 80 ~ 2
A method for producing a porous conductive silicon carbide sintered body, which comprises: blending 0% by weight to form a formed body, heating the formed body in an atmosphere containing nitrogen gas, nitriding it, and then carbonizing. 3. The molded body is (1) 600 ° C./hr at a temperature of 400 ° C. to an arbitrary temperature of 1100 ° C. to 1800 ° C. in a non-oxidizing atmosphere having a N ₂ partial pressure of 0.2 atm or more. After heating at the following heating rate, (2) at a heating rate of 600 ° C / hr or less in a non-oxidizing atmosphere with a N ₂ partial pressure of less than 0.2 atm 1
The method for producing a porous conductive silicon carbide sintered body according to claim 2, wherein the heating is performed at a temperature of 500 ° C or higher and a temperature of (1) or higher. 4. A porosity of 40% to 80% and an average pore diameter of 5 to 40.
A conductive diesel particulate filter comprising the porous conductive silicon carbide sintered body according to claim 1, having a specific resistance of 10 Ω · cm or less.