JP2017178771A - Production method of conductive silicon carbide-based sintered body and conductive silicon carbide-based sintered body - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a production method of a conductive silicon carbide-based sintered body, with which degradation of mechanical strength and thermal shock resistance is suppressed and porosity is increased, and to provide the conductive silicon carbide-based sintered body produced by the production method.SOLUTION: In a production method of a conductive silicon carbide-based sintered body in which a silicon carbide-based conductive phase composed of silicon carbide including a dopant, generated from a silicon source and a carbon source, is made to surround an aggregate and is sintered, by burning a molding molded from a mixed raw material including a silicon carbide-producing raw material including the silicon source and the carbon source and the aggregate, aggregated particles composed of sintered primary particles are used as the aggregate. The conductive silicon carbide-based sintered body obtained by the production method is configured such that the aggregated particles with a primary particle diameter of 0.5 to 5 μm and a secondary particle diameter of 2 to 25 μm are sintered in a state in which they are surrounded by the silicon carbide-based conductive phase composed of particles with particle diameters smaller than the secondary particle diameter of the aggregated particles.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a method for producing a conductive silicon carbide sintered body, and a conductive silicon carbide sintered body produced by the production method.

  Since silicon carbide has a high thermal conductivity and a low coefficient of thermal expansion, it has excellent thermal shock resistance and is therefore suitable as a substrate for filters, catalyst carriers, heat exchangers and the like used at high temperatures. High-purity silicon carbide has high electrical resistance and is close to an insulator, but a silicon carbide sintered body (silicon carbide ceramic sintered body) with conductivity is a self-heating type structure that generates heat when energized. It can be used as a body.

  For example, an electrically conductive silicon carbide sintered body can be used to circulate an exhaust gas containing a volatile organic compound (VOC) in a state where a gas circulated structure is formed and the structure is energized and self-heated. For example, the VOC can be decomposed without the need for external heating with a heater, a burner, or the like (see, for example, Patent Document 1). Also, if the structure of the conductive silicon carbide sintered body is used as the base of the diesel particulate filter (DPF), the filter base is energized and self-heated, so there is no need for external heating by a heater, burner, etc. The filter can be regenerated by burning and removing the particulate matter collected and deposited by the filter (see, for example, Patent Document 2).

  Thus, when the conductive silicon carbide sintered body is used as a self-heating structure, it is desirable to reach the operating temperature in a short time. As a means for raising the temperature of the ceramic sintered body in a short time, it can be conceived to reduce the heat capacity as a high porosity. However, simply increasing the porosity may reduce mechanical strength and thermal shock resistance.

Japanese Patent No. 5142146 Japanese Patent No. 3642836

  Therefore, in view of the above circumstances, the present invention provides a method for producing a conductive silicon carbide sintered body capable of increasing the porosity by suppressing a decrease in mechanical strength and thermal shock resistance, and produced by the production method. It is an object of the present invention to provide a conductive silicon carbide sintered body.

In order to solve the above problems, a method for producing a conductive silicon carbide sintered body according to the present invention (sometimes simply referred to as “manufacturing method”)
“The silicon carbide produced from the silicon source and the carbon source contains a dopant by firing a molded body formed from a mixed raw material comprising a silicon carbide producing raw material containing a silicon source and a carbon source and an aggregate. A method of producing a conductive silicon carbide sintered body in which a silicon carbide conductive phase surrounds and sinters the aggregate;
As the aggregate, aggregated particles in which primary particles are sintered are used.

  When silicon carbide is reacted and generated from a carbon source and a silicon source to obtain a sintered body of silicon carbide (reaction sintering), in addition to the core of reaction generation, in order to ensure the mechanical strength of the sintered body, It is common to mix aggregate with the raw material. Conventionally, coarse particles having a particle diameter of 10 μm to 50 μm have been used as aggregates. On the other hand, in this configuration, instead of coarse particles, aggregated particles in which a large number of primary particles are aggregated by sintering are used as aggregates. As a result, as will be described in detail later, compared to the conventional manufacturing method using coarse particles as aggregate, the conductivity is high, while the mechanical strength is high and the decrease in thermal shock resistance is suppressed. A silicon carbide sintered body can be produced. Therefore, according to the manufacturing method of this configuration, a conductive silicon carbide sintered body having a small heat capacity can be manufactured by suppressing a decrease in mechanical strength and thermal shock resistance and increasing the porosity.

  If the material of the agglomerated particles is silicon carbide, in the conductive silicon carbide sintered body to be produced, the thermal expansion coefficient is equal to the silicon carbide conductive phase that surrounds and sinters the agglomerated particles. desirable.

The method for producing a conductive silicon carbide sintered body according to the present invention has the above-described configuration.
“The particle diameter of the primary particles is 0.5 μm to 5 μm”. In addition, “the secondary particle diameter of the aggregated particles is 2 μm to 25 μm”.

  By setting the particle diameter of the primary particles constituting the aggregated particles and the secondary particle diameter of the aggregated particles in the above range, as will be described in detail later, according to a conventional manufacturing method in which coarse particles are used as aggregates. Compared with the manufactured sintered body, a conductive silicon carbide based sintered body having a superiority can be manufactured.

Next, the conductive silicon carbide based sintered body according to the present invention is:
“Agglomerated particles having a primary particle size of 0.5 μm to 5 μm and a secondary particle size of 2 μm to 25 μm,
A silicon carbide conductive phase composed of particles having a particle size smaller than the secondary particle size of the aggregated particles surrounds and sinters.

  This is a conductive silicon carbide sintered body manufactured by the manufacturing method having the above-described configuration. Compared to a conventional conductive silicon carbide sintered body containing coarse particles as an aggregate, the mechanical strength is high and the reduction in thermal shock resistance is suppressed despite the high porosity and small heat capacity.

  As described above, as an effect of the present invention, a method for producing a conductive silicon carbide sintered body capable of suppressing a decrease in mechanical strength and thermal shock resistance and increasing the porosity, and produced by the production method. A conductive silicon carbide sintered body can be provided.

It is an example of secondary particle size distribution of the aggregated particle used as an aggregate of an Example. It is the graph which contrasted the relationship between the particle diameter of an aggregate, and 4 point | piece bending strength with an Example and a comparative example. It is the graph which contrasted the relationship between an apparent porosity and 4-point bending strength by the Example and the comparative example. It is the graph which contrasted the relationship between the numerical value used as a parameter | index of a thermal shock resistance, and an apparent porosity with an Example and a comparative example. It is the graph which compared the relationship between an average pore diameter and an apparent porosity with an Example and a comparative example. It is the graph which contrasted the relationship between the particle diameter of an aggregate, an average pore diameter, and an apparent porosity with an Example and a comparative example, respectively. (A) Aggregated particle | grains which are the aggregate of an Example, (b) Coarse particle | grains which are the aggregate of a comparative example, It is an observation image by each scanning electron microscope. It is an example of the observation image by a scanning electron microscope about (a) fracture surface and (b) polished surface about the sintered compact of an Example.

  Hereinafter, a method for producing a conductive silicon carbide sintered body according to an embodiment of the present invention and a conductive silicon carbide sintered body produced by the production method will be described. The method for producing a conductive silicon carbide sintered body according to the present embodiment includes a silicon source by firing a molded body formed from a mixed raw material including a silicon carbide generating raw material containing a silicon source and a carbon source and an aggregate. And a silicon carbide conductive phase in which silicon carbide generated from a carbon source contains a dopant surrounds and sinters the aggregate, and is a method for producing a conductive silicon carbide sintered body. The agglomerated particles in which the primary particles are sintered are used. In this embodiment, aggregated particles of silicon carbide are used as aggregated particles as aggregates.

  As a silicon source of the silicon carbide generating raw material, silicon compounds such as silicon nitride can be used in addition to silicon (simple substance). On the other hand, carbonaceous materials such as graphite, coal, coke, charcoal, and carbon black can be used as the carbon source of the silicon carbide generating raw material. When the molar ratio of silicon to carbon (Si / C) in the silicon carbide producing raw material is 1, silicon carbide is produced with no stoichiometric excess or deficiency, but Si / C is 0.8 to 1.2. For example, the excess or deficiency of silicon and carbon is small and desirable.

  A method of forming silicon carbide-containing conductive phase by generating silicon carbide containing a dopant from a silicon source and a carbon source, wherein the firing atmosphere is a non-oxidizing atmosphere containing nitrogen gas, and nitrogen in the atmosphere is produced by reaction. A method of doping silicon carbide can be employed. In this case, the non-oxidizing atmosphere containing nitrogen gas can be a 100% nitrogen gas atmosphere or a mixed atmosphere of a rare gas such as argon or helium and nitrogen gas.

  Alternatively, silicon nitride can be used as a silicon source, and nitrogen generated by the decomposition can be doped into the silicon carbide produced by the reaction. In this case, the firing atmosphere can be a non-oxidizing atmosphere containing no nitrogen gas in addition to the non-oxidizing atmosphere containing nitrogen gas. Here, the non-oxidizing atmosphere containing no nitrogen gas can be a rare gas atmosphere such as argon or helium, or a vacuum atmosphere.

  Alternatively, it may be a method of adding a compound of nitrogen or aluminum as a dopant to a mixed raw material containing a silicon carbide forming raw material and an aggregate, and firing a molded body formed from the mixed raw material in a non-oxidizing atmosphere. it can.

  The mixed raw material may contain fine particles of silicon carbide in addition to the silicon carbide generating raw material and the aggregate. In this case, if the firing atmosphere is a non-oxidizing atmosphere containing nitrogen gas, or if the silicon source of the silicon carbide generating raw material is silicon nitride, nitrogen or silicon nitride in the atmosphere is used when the fine particles are sintered. Nitrogen generated by decomposition of is doped. That is, in this case, the silicon carbide conductive phase, which is silicon carbide containing a dopant, includes silicon carbide fine particles of silicon carbide in addition to the phase containing silicon dopant produced by reaction from a silicon source and a carbon source. A phase containing a dopant derived from is included. Here, the particle diameter of the fine particles of silicon carbide can be 1/5 to 1/40 of the particle diameter of the aggregate.

  Aggregated particles as an aggregate can be obtained by pulverizing a sintered body of silicon carbide. The pulverization can be performed using a dry crushing granulator, a wet ball mill or a bead mill. The silicon carbide sintered body to be crushed can be a sintered body obtained by reactively sintering the silicon source and the carbon source. In this case, conductivity can be imparted to the aggregated particles as the aggregate by using silicon nitride as the silicon source or by setting the sintering atmosphere to a non-oxidizing atmosphere containing nitrogen gas. Alternatively, a sintered body obtained by sintering fine particles of silicon carbide together with a binder or the like can be used as a silicon carbide sintered body to be pulverized. Also in this case, it is possible to impart conductivity to the aggregated particles as the aggregate by setting the sintering atmosphere to a non-oxidizing atmosphere containing nitrogen gas. The aggregated particles as the aggregate may be classified in a predetermined range by pulverizing the silicon carbide sintered body and then classifying the aggregated particles.

  Silicon carbide sintered body (sintering object to be crushed) used as a raw material for aggregated particles as an aggregate and a firing temperature when a compact formed from a mixed raw material including a silicon carbide generating raw material and an aggregate is fired The firing temperature for obtaining the body can be 1800 ° C. to 2350 ° C. in both cases. If the firing temperature is lower than 1800 ° C., sintering may be insufficient, and if it exceeds 2350 ° C., silicon carbide may sublime.

<Aggregated particles>
A pellet-shaped molded body was molded from a raw material in which silicon nitride was used as a silicon source for reaction generation of silicon carbide, graphite was used as a carbon source, and the molar ratio of silicon to carbon (Si / C) was 1. The molded body was fired in a non-oxidizing atmosphere not containing nitrogen to obtain a silicon carbide sintered body. The obtained silicon carbide sintered body was pulverized with a ball mill. At that time, a plurality of types of agglomerated particles having different secondary particle diameters were obtained by varying the grinding conditions depending on the diameter and amount of the balls. Hereinafter, the aggregated particles as the aggregate may be referred to as “aggregated particle aggregate”.

<Conductive silicon carbide sintered body>
Secondary particles prepared as described above for a silicon carbide forming raw material in which silicon (single) or silicon nitride is used as the silicon source, graphite is used as the carbon source, and the molar ratio of silicon to carbon (Si / C) is 1. One of aggregated aggregates having different diameters and fine particles of silicon carbide were added to obtain a mixed raw material. Binder and water were added to the mixed raw material and mixed to obtain a kneaded product. A honeycomb structure was formed by extrusion molding of the kneaded material. The honeycomb structure had a prismatic shape of 36 mm × 36 mm × 100 mm, a cell density of 300 cells / inch ² , and a partition wall thickness of 10 mil (about 0.25 mm). After this molded body was dried and degreased, it was fired in an argon atmosphere containing nitrogen to prepare samples S1 to S10 which are conductive silicon carbide sintered bodies of the examples.

  Samples S1 to S10 consist of mixed raw materials, that is, the materials and particle sizes of silicon sources (S1 to S5 and S10 are silicon, and S6 to S9 are silicon nitride), carbon source materials and particle sizes, and fine particles. Conductive silicon carbide having various pore diameters by changing the composition of the mixed raw material and the particle diameter of the aggregated particle aggregate, which are common in the material, the particle diameter, and the aggregated particle aggregate material This is intended to obtain a sintered body. Table 1 shows the average particle diameter of the aggregated particle aggregate and the composition of the mixed raw material for the samples S1 to S10.

  As comparative examples, samples R1 to R6 using coarse particles of silicon carbide as an aggregate were prepared as in the conventional method. Samples R1 to R6 consist of raw materials constituting the mixed raw material, that is, materials and particle diameters of silicon sources (R1 to R4 are silicon nitride, R5 and R6 are silicon), carbon source materials and particle diameters, and fine particles The material and the particle diameter are the same as those in the examples, and by varying the composition of the mixed raw material and the particle diameter of coarse particles as aggregate (hereinafter sometimes referred to as “coarse particle aggregate”), various It is intended to obtain an electrically conductive silicon carbide sintered body having a pore diameter of. About samples R1-R6, the average particle diameter of a coarse particle aggregate and the composition of a mixed raw material are shown according to Table 1.

  Here, the secondary particle diameter of the aggregated particle aggregates of the samples S1 to S10 and the particle diameter of the coarse particle aggregates of the samples R1 to R6 are both laser diffraction / scattering particle size distribution measuring devices (Microtrack It is a volume-based median diameter in a particle size distribution measured by Bell Co., Ltd. (MT3000II). According to the measured particle size distribution, any aggregated aggregate is composed of aggregated particles with a secondary particle size of 2 μm to 25 μm and accounts for 95% by volume or more, and aggregated particles with a secondary particle size of 3 μm to 25 μm as a whole. Accounted for 80% by volume or more. As an example, the secondary particle size distribution of the aggregated particle aggregate used in Examples S2 to S4 and S6 is shown in FIG.

  Moreover, when the aggregated particle aggregate used for each sample of an Example was observed with the scanning electron microscope, the particle diameter of the primary particle which forms the aggregated particle in any sample is 0.5 micrometer-5 micrometers. there were. As an example, a scanning electron microscope observation image of the aggregated particle aggregate used in Examples S2 to S4 and S6 is shown in FIG. Moreover, the scanning electron microscope observation image of the coarse particle aggregate used for Comparative Examples R2 to R4 and R6 is shown in FIG. 7B for comparison.

  The apparent porosity, 4-point bending strength, and Young's modulus of each of the conductive silicon carbide sintered bodies of the samples S1 to S10 of the example and the samples R1 to R6 of the comparative example were measured by the following methods. The results are shown in Table 2.

Average pore diameter: It was determined as a volume-based median diameter from a pore diameter distribution measured by a mercury intrusion method using a mercury porosimeter (manufactured by Micromeritics, Autopore IV9500).
Apparent porosity: In measuring the average pore diameter, it was calculated from the volume of mercury pressed into the sample and the sample volume.
Four-point bending strength: A sample having a honeycomb structure was cut into a size of 5 cells × 4 cells × 40 mm to obtain a test piece. Based on JIS R1601, the measurement was performed under the conditions of a distance between lower fulcrums of 30 mm, a distance between upper fulcrums of 10 mm, and a crosshead speed of 0.5 mm / min.
Young's modulus: Calculated from load point displacement in a four-point bending strength test in accordance with JIS R1602.

Further, “4-point bending strength / Young's modulus” (σ / E) was calculated as an index of thermal shock resistance. In the thermal shock fracture resistance R expressed by the following equation, the coefficient of thermal expansion, which is a coefficient inherent to the material, and the Poisson ratio, which is regarded as a coefficient inherent to the material within the elastic limit together with the Young's modulus, are taken as constants. It is what I handled. Therefore, it can be evaluated that the greater the value (σ / E), the higher the thermal shock resistance. Table 2 shows the calculation result of the value (σ / E) for each sample.
Thermal shock fracture resistance R = σ (1-ν) / αE
Where σ: stress
ν: Poisson's ratio
α: Thermal expansion coefficient
E: Young's modulus

  As can be seen from Table 2, samples having various apparent porosities in the range of about 40% to 60% were produced for both the examples and the comparative examples.

  Here, Example S4 and Comparative Example R5, and Example S10 and Comparative Example R6 differ only in whether the aggregate is aggregated particles or coarse particles, and all other conditions, that is, the average of the aggregates. Particle size, aggregate content, fine particle material / particle size / content rate, silicon source material / particle size / content rate, and carbon source material / particle size / content rate are the same combination . FIG. 2 shows the relationship between the particle size of the aggregate (secondary particle size in the case of aggregated particle aggregate) and the 4-point bending strength for each of these combinations. From FIG. 2, it can be seen that the example in which the aggregated particles are aggregates shows high mechanical strength even when the particle diameter of the aggregate is approximately the same as the comparative example in which coarse particles are aggregated.

  The relationship between the apparent porosity and the 4-point bending strength including other samples is shown in FIG. As is apparent from FIG. 3, the four-point bending strength is convex downward as the apparent porosity increases in both the example using the aggregated particles as the aggregate and the comparative example using the coarse particles as the aggregate. It decreases to draw a curved line. And the Example which uses an aggregated particle as an aggregate has shown the tendency for the apparent porosity of the sample which shows the same 4 point | piece bending strength compared with the comparative example which uses a coarse particle as an aggregate. In other words, the example in which the aggregated particles are aggregates shows high mechanical strength even at a high porosity as compared with the comparative example in which coarse particles are aggregated. Such a difference between the example and the comparative example was significant until the apparent porosity reached 52%. When the apparent porosity exceeded 52%, no significant difference was observed in the 4-point bending strength between the example and the comparative example.

  FIG. 4 shows the relationship between the 4-point bending strength / Young's modulus (σ / E), which is an index of thermal shock resistance, and the apparent porosity. In both the examples and the comparative examples, the thermal shock resistance slightly decreases with an increase in the apparent porosity, but the examples and the comparative examples are along a single straight line. From this, it can be seen that even if the apparent porosity is increased by making the aggregate into aggregated particles, the thermal shock resistance is not lowered as compared with the case where the aggregate is coarse particles.

  Thus, by using aggregates as aggregated particles, mechanical strength is high even when the apparent porosity is high, and a decrease in thermal shock resistance is suppressed as compared with the case where aggregates are coarse particles. This is because the number of necks (neck between primary particles, neck between aggregate and surrounding particles) increases because the aggregate itself is an aggregated particle in which a large number of primary particles are aggregated by sintering. It is done.

  Further, since the aggregated particles that are aggregates have small voids between the primary particles, high porosity can be realized even if the secondary particle size of the aggregates that are aggregated particles is small. Even when the porosity is the same as that of the case, the gap between the aggregate particles is small compared to the case where the aggregate is coarse particles, and the gap between the aggregate and the surrounding silicon carbide particles is also small. Therefore, when the aggregate is aggregated particles, the average pore diameter when the porosity is about the same as when the aggregate is coarse particles is also considered to be one of the reasons described above. FIG. 5 shows a graph in which the apparent porosity is plotted against the average pore diameter for each sample of the example and the comparative example. From FIG. 5, it is clear that the average pore diameter in the case of showing the same apparent porosity is smaller in the example in which the aggregate is aggregated particles than in the comparative example in which the aggregate is coarse particles.

  Moreover, only about whether the aggregate is an aggregated particle or a coarse particle and the other combinations are the same, the above combination, Example S4 and Comparative Example R5, and Example S10 and Comparative Example R6 FIG. 6 is a graph in which the average pore diameter and the apparent porosity are plotted with respect to the particle diameter of the aggregate (secondary particle diameter in the case of the aggregated particle aggregate). FIG. 6 also shows that in the example in which the aggregate is aggregated particles, the average porosity is small but the apparent porosity is larger than in the comparative example in which the aggregate is coarse particles.

  As described above, according to the manufacturing method of the present embodiment, the aggregated particles are used as the aggregate, and the porosity is higher than that in the conventional manufacturing method in which coarse particles are used as the aggregate. A conductive silicon carbide sintered body having high mechanical strength and the same thermal shock resistance can be manufactured. Therefore, a conductive silicon carbide sintered body having a small heat capacity can be manufactured by increasing the porosity while suppressing a decrease in mechanical strength and thermal shock resistance.

  In addition, the aggregate has been non-conductive in the past where coarse particles were used as the aggregate. In contrast, in the manufacturing method of the present embodiment, aggregated particles as an aggregate are produced by reaction from a silicon source and a carbon source, and silicon nitride is used as the silicon source. Are doped to form conductive aggregated particles. Accordingly, in addition to the silicon carbide conductive phase that is sintered so as to surround the aggregate, the aggregate also has conductivity, and the conductivity of the entire conductive silicon carbide sintered body can be increased. .

  And by the manufacturing method of the said embodiment, the carbonization which consists of a particle | grain with a primary particle diameter of 0.5 micrometer-5 micrometers and a secondary particle diameter of 2 micrometers-25 micrometers from a particle | grain with a particle diameter smaller than the secondary particle diameter of aggregated particle A conductive silicon carbide sintered body having a structure in which the silicon conductive phase is surrounded and sintered is manufactured. As an example, for the conductive silicon carbide sintered body of Example S2, a fracture surface and an image observed by a scanning electron microscope of the polished surface are shown in FIGS. 8 (a) and 8 (b), respectively. By contrast with FIG. 7 (a), it can be seen that aggregated particles of 2 μm to 25 μm considered to be aggregated particle aggregate (secondary particle diameter) are surrounded by small particles of 0.2 μm to less than 2 μm and sintered. Observed.

  In the present embodiment, the use of aggregated particles in which the aggregated particles having a secondary particle diameter of 2 μm to 25 μm occupy 95% by volume or more is used as the aggregated particle aggregate. Although a ligated body has been obtained, such a particle diameter range is suitable as the particle diameter of the aggregated aggregate that exhibits the intended effect as described above. If the secondary particle diameter of the aggregated particles is smaller than 2 μm, it is considered that the role as an aggregate cannot be sufficiently exhibited. On the other hand, if the secondary particle diameter exceeds 25 μm, the average pore diameter increases as estimated from the results of Example S9, and coarse particles are used as aggregates as estimated from the comparison with Comparative Example R4. It is considered that the mechanical strength with respect to the apparent porosity is almost the same as that in the case, and the advantage of using aggregated particles as an aggregate cannot be obtained sufficiently.

  The present invention has been described with reference to the preferred embodiments. However, the present invention is not limited to the above-described embodiments, and various improvements can be made without departing from the scope of the present invention as described below. And design changes are possible.

  For example, in the above, the case where the aggregated particles used as the aggregate are produced by reacting and sintering the silicon source and the carbon source and pulverizing the obtained sintered body is illustrated. However, the present invention is not limited thereto, and a coarse particle aggregate that is a nucleus of reaction generation is added to a silicon source and a carbon source for generating silicon carbide, and sintered. After the obtained sintered body is pulverized, it is coarsened by classification. By removing the particles, aggregated particles can be produced as aggregates.

Claims (4)

Carbonization of silicon carbide produced from the silicon source and the carbon source by containing a dopant by firing a molded body formed from a mixed raw material comprising a silicon carbide production raw material containing a silicon source and a carbon source and an aggregate A method for producing a conductive silicon carbide sintered body in which a silicon conductive phase surrounds and sinters the aggregate,
An aggregated particle in which primary particles are sintered is used as the aggregate. 2. The method for producing a conductive silicon carbide sintered body according to claim 1, wherein a particle diameter of the primary particles is 0.5 μm to 5 μm. The method for producing a conductive silicon carbide-based sintered body according to claim 1 or 2, wherein the aggregated particles have a secondary particle diameter of 2 to 25 µm. Aggregated particles having a primary particle size of 0.5 μm to 5 μm and a secondary particle size of 2 μm to 25 μm,
A conductive silicon carbide sintered body characterized in that a silicon carbide conductive phase composed of particles having a particle diameter smaller than the secondary particle diameter of the aggregated particles is surrounded and sintered.