JPH08217568A - Production of porous electrically conductive silicon carbide sintered body - Google Patents

Abstract

PURPOSE: To ensure superior oxidation resistance by molding a powdery mixture of specified SiC powder with Si3 N4 powder and a carbonaceous material and firing the resultant molded body in a nonoxidizing atmosphere not contg. N2 . CONSTITUTION: The particle size distribution of SiC powder having 1-50μm average particle diameter is regulated so that the ratio (D10 /D50 ) of the particle diameter of 10% cumulative particle size of the distribution to that of 50% cumulative particle size is made >=0.2 and the SiC powder is mixed with a powdery mixture prepd. by mixing Si3 N4 with a carbonaceous material such as carbon black so that the molar ratio of metallic Si forming the Si3 N4 to carbon is made 1.0-2.0. Proper amts. of water and an org. binder such as PVA are added to the resultant powdery mixture contg. 20-70wt.% SiC and they are kneaded and molded in a prescribed shape. The resultant molded body is dried, dewaxed by heating and fired at 1,600-2,300 deg.C in a nonoxidizing atmosphere not contg. gaseous N2 to obtain the objective porous electrically conductive SiC sintered body having about 1-40μm pore diameter and about >=40% porosity.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a porous conductive silicon carbide sintered body used for a filter for collecting combustible fine particles discharged from a diesel engine or the like.

[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.

Therefore, a self-heating type filter, which burns the collected combustible fine particles uniformly, 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) are 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 is increased to impart a filter function. Furthermore, since high-purity silicon carbide ceramics are basically insulators, in order to obtain the desired conductivity, T
i, the periodic table IV _a-group element or V, such as Zr, carbides of V _a group element such as Nb, nitrides, borides added,
It is necessary to impart conductivity by forming a continuous conductive phase in the sintered body. 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. It improves the conductivity of silicon carbide without adding a conductivity-imparting substance and imparts excellent oxidation resistance.
The object is to obtain a porous conductive silicon carbide sintered body having an optimum pore size and porosity as a filter.

[0006]

That is, according to the method for producing a porous conductive silicon carbide sintered body of the present invention, a molded body made of silicon carbide, silicon nitride and a carbonaceous material is used in a non-oxidizing atmosphere containing no nitrogen. It is characterized by being fired at. Furthermore, the feature of the production method of the present invention is that the average particle size is 1 to 50 μm, the particle size ratio (D ₁₀ / D ₅₀ ) of the cumulative particle size 10% size and 50% size of the particle size distribution is
A mixed powder of silicon carbide powder of 0.2 or more, silicon nitride powder and a carbonaceous substance, wherein the content of silicon carbide powder is
20 to 70% by weight, and a mixed powder having a molar ratio of silicon metal constituting silicon nitride to carbon in the range of 1.0 to 2.0 is molded, and the mixture is heated to 1600 to 1600 in a non-oxidizing atmosphere containing no nitrogen.
It is characterized by firing at a temperature of 2300 ° C.

The present invention will be described in more detail below. In the production method of the present invention, a porous conductive silicon carbide sintered body is obtained by firing a molded body made of silicon carbide, silicon nitride and a carbonaceous material in a non-oxidizing atmosphere containing no nitrogen. A molded body made of silicon nitride and a carbonaceous material may be a mixed raw material made of silicon carbide powder, silicon nitride powder and a carbonaceous material, or a mixture made of silicon carbide powder, metallic silicon and a carbonaceous material. It may be a molded body made of silicon carbide, silicon nitride and a carbonaceous material obtained by molding and nitriding the raw material.

By the carbonization reaction of the molded product thus obtained, the conductivity is improved without adding a conductive substance, and excellent oxidation resistance can be imparted. In addition, a porous conductive silicon carbide sintered body having a desired porosity and pore diameter can be manufactured by appropriately setting the compounding amount and particle diameter of the silicon carbide powder.

In the method for producing a porous conductive silicon carbide sintered body of the present invention, a molded body made of silicon carbide, silicon nitride and a carbonaceous material is fired in a nitrogen-free non-oxidizing atmosphere because the reason is as follows. Nitrogen element generated by decomposition of silicon nitride is contained as an impurity in β-type silicon carbide or silicon carbide added in advance as a reaction to improve conductivity, and the nitrogen element contained here is a grain boundary phase or a grain boundary phase. In some cases, it may be present as a solid solution or alone, but it may also be present as a compound by the reaction with silicon carbide.

When a molded body made of silicon carbide, silicon nitride and a carbonaceous material is obtained by molding a mixed raw material made of silicon carbide powder, silicon nitride powder and a carbonaceous material, the blending of these raw material powders includes the inclusion of silicon carbide powder. The amount is 20-70% by weight,
The molar ratio of silicon metal constituting silicon nitride to carbon is in the range of 1.0 to 2.0. When the content of silicon carbide is less than 20% by weight, the mechanical strength is lowered, and when it exceeds 70% by weight, the specific resistance is increased because the amount of β-type silicon carbide containing the nitrogen element produced by the conversion of silicon nitride is small. Further, the molar ratio of silicon metal constituting silicon nitride to carbon is
If it is less than 1.0, carbon remains in the sintered body and inhibits the sinterability, so that the grain boundary phase is easily oxidized. On the other hand, the molar ratio is
If it exceeds 2.0, the amount of metallic silicon produced by the decomposition of silicon nitride is large and the oxidation resistance and mechanical strength are lowered.

Further, the porous conductive silicon carbide sintered body of the present invention has a pore size of 1 to 40 μm, a porosity of 40% or more, and a pore size of 5 to 40 μm when applied as a diesel particulate filter. The porosity is more preferably in the range of 50 to 80%. The reason for this is that if the pore size is smaller than 5.0 μm, the combustible fine particles will be clogged significantly and the pressure loss will increase in a short time. ,Also,
This is because if the porosity is lower than 50%, the pressure loss increases, and if it exceeds 80%, the mechanical strength decreases.

The pore characteristics in the method for producing a conductive porous silicon carbide sintered body of the present invention depend on the average particle size and particle size distribution of silicon carbide powder as a starting material, and in order to obtain a desired pore size. Average particle size of 1 to 50 μm, cumulative particle size distribution
It is necessary to use a powder having a particle size ratio (D ₁₀ / D ₅₀ ) of 10% diameter to 50% diameter of 0.2 or more. On the other hand, the average particle size of the silicon nitride powder is preferably 100 μm or less from the viewpoint of moldability and reactivity with the carbonaceous substance. Further, as the carbonaceous substance serving as the C source, fine particle solid carbon powder such as carbon black or acetylene black, or an organic resin such as phenol, furan, or polyimide which becomes carbon by thermal decomposition can be used.

As a method for molding the mixed raw material, an organic binder such as methyl cellulose or polyvinyl alcohol is added, press molding, extrusion molding, injection molding, or slurry is prepared and poured into a container having a desired shape and solidified. it can.

Next, the method of firing the molded body made of silicon carbide, silicon nitride and a carbonaceous material is performed at a firing temperature of 1600 to 2300 ° C. in a non-oxidizing atmosphere containing no nitrogen. Examples of the non-oxidizing atmosphere containing no nitrogen include vacuum, argon, carbon monoxide, ammonia, methane, hydrogen, or a mixed gas atmosphere of two or more of these. Conditions such as firing temperature and atmosphere are necessary for converting silicon nitride into β-type silicon carbide and further improving oxidation resistance. At temperatures lower than 1600 ° C, carbonization reaction of silicon nitride is insufficient. Since silicon nitride remains in the sintered body, the specific resistance becomes high. Further, in an atmosphere containing nitrogen, grain growth of silicon carbide generated by carbonization of silicon nitride is hindered, so that the crystal grains are small and the oxidation resistance is lowered. The preferred firing temperature is
1800 to 2300 ° C, more preferably 1900 to 2300 ° C,
A more preferable atmosphere is vacuum.

[0015]

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

(Examples 1 to 5 and Comparative Examples 1 to 9) A silicon carbide powder having a particle size (average particle size, particle size distribution particle size ratio) shown in Table 1 as a starting material and a molar ratio of metallic silicon to carbon were used. With respect to 100 parts by weight of raw materials in which the mixed powder of the adjusted silicon nitride powder and carbon black was mixed in the ratio shown in Table 1,
25 parts by weight of water and 10 parts by weight of methyl cellulose as a binder were added, mixed with a Henschel mixer for 10 minutes, and then kneaded with a kneader type kneader for 30 minutes. Using a high-pressure vacuum extruder, the kneaded product was used to form a honeycomb with an outer diameter of □ 50 mm, a cell size of 2.5 mm, and a rib pressure of 0.5 mm at a pressure of 60 kg.
Extrusion was performed at / cm ² . The molded body thus obtained was dried, degreased at 450 ° C. for 1 hr in a non-oxidizing atmosphere, and then fired under the firing conditions shown in Table 2.

The following properties of the obtained sintered body were measured and shown in Table 3. (1) Porosity: Archimedes method. (2) Average pore size: mercury porosimetry. (3) Room temperature resistivity: Honeycomb structure was cut into □ 10 × 50mmL, silver electrode was formed and measured by 4-probe method. (4) Oxidation resistance: Ratio after treatment at 1000 ℃ for 100 hours in air. The resistance was measured. (5) Mechanical strength: The honeycomb structure was cut into □ 10 × 10 mmL and the compressive strength in the extrusion direction was measured.

[0018]

[Table 1]

[0019]

[Table 2]

As is clear from Tables 1 and 2, Examples 1 to 1
The electrically conductive porous silicon carbide sintered body obtained in No. 5 has suitable porosity and average pore diameter, has low room temperature specific resistance and excellent conductivity, and has excellent compressive strength and oxidation resistance. It has excellent characteristics as a conductive diesel particulate filter and the like.

[0021]

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, in particular, 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 It is most suitable as a duct heater and a heater for a hot air generator used as a heat source for a large dryer from the viewpoint of greatly increasing the heat efficiency. Further, it is also suitable as a heater used for heating equipment, cooking equipment, drying equipment, firing furnaces and the like.

Claims (2)

[Claims] 1. A method for producing a porous conductive silicon carbide sintered body, which comprises firing a molded body made of silicon carbide, silicon nitride and a carbonaceous material in a non-oxidizing atmosphere containing no nitrogen. 2. A silicon carbide powder, a silicon nitride powder and a carbonaceous material having an average particle size of 1 to 50 μm and a particle size ratio (D ₁₀ / D ₅₀ ) of cumulative particle size of 10% to 50% of particle size distribution of 0.2 or more. A mixed powder of substances, wherein the content of silicon carbide powder is 20 to 70% by weight.
And forming a mixed powder in which the molar ratio of metallic silicon constituting silicon nitride to carbon is in the range of 1.0 to 2.0,
A method for producing a porous conductive silicon carbide sintered body, which comprises firing at a temperature of 1600 to 2300 ° C. in a non-oxidizing atmosphere containing no nitrogen.