JP5539815B2 - Porous silicon carbide ceramic sintered body having conductivity - Google Patents

Description

  The present invention relates to an electrically conductive porous silicon carbide ceramic sintered body.

  Since silicon carbide has a high thermal conductivity and a low coefficient of thermal expansion, it has excellent thermal shock resistance. Therefore, a porous silicon carbide based ceramic sintered body is used as a filter substrate used at high temperatures. It is used. High-purity silicon carbide has high electrical resistance and is close to an insulator, but a porous silicon carbide ceramic sintered body with conductivity is used as a self-heating filter base that generates heat when energized. Is possible.

  For example, in a diesel particulate filter (hereinafter sometimes referred to as “DPF”) that collects and removes particulate matter contained in gas discharged from a diesel engine, the collected particulate matter is accumulated to some extent. Thus, a regeneration process for burning the particulate matter is performed. At this time, if the filter base is self-heated by energization and the particulate matter is combusted and removed, unlike the case where the particulate matter is combusted by external heating, a heating device such as a burner or a heater becomes unnecessary. In the case of external heating, the filter base may be melted by local heating, or the filter base may be cracked or cracked by a large temperature gradient. In the case of a self-heating type filter base, There is also an advantage that such a fear is small.

  Here, as silicon carbide to which conductivity is imparted, silicon carbide that is made into a semiconductor by adding a small amount of impurities is known. It is manufactured by a manufacturing method in which at least one additive selected from carbides, nitrides, borides and oxides excluding silicon carbide is added to silicon carbide, and has conductivity and is suitable for collection of particulate matter. Silicon carbide has been proposed as a DPF substrate having open pores (see Patent Document 1).

  However, in the technique of Patent Document 1, it is necessary to add a large amount of an additive such as aluminum nitride, titanium nitride, titanium diboride or the like at 5 to 15 parts by weight with respect to 100 parts by weight of silicon carbide. In addition, in the manufacturing process, in order to knead the raw material silicon carbide powder and the additive with water, it is necessary to coat the additive with a resin having water repellency in advance, which makes the process complicated and laborious. There was a problem that it took.

Moreover, the specific resistance value of the silicon carbide based sintered body produced by Patent Document 1 is as small as 10 ^{−2 to 1} Ω · cm, and it was considered that the application range is limited. This is because the conductive silicon carbide based ceramic sintered body has NTC characteristics in which the electrical resistance decreases as the temperature rises. Therefore, when the specific resistance value is small, the temperature is increased to the target temperature. This is because the current value may be excessive.

  In addition, as a practical silicon carbide based ceramic sintered body, the specific resistance value is required to be controlled in a wider range. This is because, for example, in the case of a DPF, since the size of the DPF is defined by the size and structure of the vehicle body to be attached, the required filter base sizes become extremely various. This is because the specific resistance value required for raising the temperature to a predetermined temperature within a predetermined time at this time also varies.

  Therefore, in view of the above situation, the present invention is a porous silicon carbide ceramic sintered body having conductivity, which can be manufactured by a simple process, and has various specific resistance values within a wide range. An object of the present invention is to provide a silicon carbide ceramic sintered body that can be obtained.

In order to solve the above-described problems, the silicon carbide based ceramic sintered body according to the present invention is “a porous silicon carbide based ceramic sintered body having conductivity, in which nitrogen is present between silicon carbide and between particles. A silicon dioxide layer having an oxygen concentration of 0.4% by mass to 4.8% by mass in the sintered body is formed on the matrix surface where the solid solution silicon carbide is generated, and the specific resistance at room temperature with respect to the oxygen concentration. When the oxygen concentration is 0.4 mass%, the specific resistance value is 5 Ω · cm, and when the oxygen concentration is 4.8 mass%, the specific resistance value is 20 Ω · cm. It is expressed by a linear function .

  The present inventors have found that if the silicon carbide based ceramic sintered body has a silicon dioxide layer on the particle surface, it takes various specific resistance values depending on the amount of silicon dioxide layer formed. It has come. Here, a silicon carbide based ceramic sintered body having a silicon dioxide layer on the particle surface is manufactured by performing an oxidation treatment in which the sintered body is heated in an oxidizing atmosphere at a predetermined heating temperature for a predetermined heating time. Can do. In this manufacturing method, if the oxidizing atmosphere is the same, the specific resistance value can be controlled by changing one or both of the heating temperature and the heating time, as will be described later.

  Therefore, the silicon carbide based ceramic sintered body having the above-described configuration has a simple configuration in which an additive such as a nitride or an oxide is not contained in the particle and a silicon dioxide layer is provided only on the particle surface.

  Here, as in the prior art, by adjusting the type and amount of additive added to the raw material powder, an attempt is made to produce silicon carbide ceramic sintered bodies having different specific resistance values. In response to the demand for a silicon carbide ceramic sintered body having a value, raw materials having various kinds of compositions must be prepared, which requires much labor and is inferior in economic efficiency. On the other hand, the silicon carbide based ceramic sintered body of the present invention is additionally subjected to an oxidation treatment as described above for a single sintered body having a specific resistance value manufactured from a raw material having the same composition. It is possible to manufacture easily by the manufacturing method to apply.

  In the present invention having the above-described configuration, by providing a silicon dioxide layer having an oxygen concentration of 0.4% by mass to 4.8% by mass, a filter base that generates heat by energization or a catalyst that activates the catalyst by heating as described later. As a carrier or the like, it has a specific resistance value within a practical range.

  The silicon carbide based ceramic sintered body according to the present invention has the above-described configuration, “formed in a honeycomb structure including a plurality of cells partitioned by partition walls extending in a single direction”. be able to.

  By adopting the honeycomb structure as described above, the fluid flowing through the cells can be efficiently heated by generating heat by energization, and the catalyst carrier that activates the catalyst by heating and the base of the heating device that heats the fluid It becomes a suitable structure. In addition, if the cells are sealed so that the cells sealed at one end and the cells sealed at the other end are alternated, the exhaust gas discharged from the diesel engine is introduced from the open end side of the cell to form a partition wall. The structure is suitable as a base of a self-heating type filter such as a diesel particulate filter (DPF) that passes and supplements and removes particulate matter from the exhaust gas.

  As described above, as an effect of the present invention, a porous silicon carbide ceramic sintered body having conductivity, which can be manufactured by a simple process, has various specific resistance values within a wide range. It is possible to provide a silicon carbide based ceramic sintered body that can be used.

It is a graph which shows the relationship between the conditions (heating temperature, heating time) in an oxidation treatment process, and oxygen concentration. It is a graph which shows the relationship between oxygen concentration and the specific resistance value in room temperature. It is a graph which shows the change of the electrical resistance value at the time of raising temperature. It is a graph which shows the relationship of the specific resistance value with respect to oxygen concentration about the case where temperature differs. It is a graph which shows the relationship between oxygen concentration and three-point bending strength. (A) SEM observation image and EDX image of sample surface of sample 14 and (b) sample surface of control sample. It is the SEM observation image and EDX image of the torn surface of the sample 14.

  Hereinafter, a silicon carbide based ceramic sintered body according to an embodiment of the present invention and a manufacturing method thereof will be described. The silicon carbide based ceramic sintered body of the present embodiment is a porous silicon carbide based ceramic sintered body having conductivity, and has an oxygen concentration of 0.4% by mass to 4.8% by mass on the particle surface. A silicon dioxide layer is formed. Further, the silicon carbide based ceramic sintered body of the present embodiment is formed in a honeycomb structure including a plurality of cells partitioned by partition walls extending in a single direction.

  The silicon carbide ceramic sintered body having the above-described configuration can be manufactured by the following manufacturing method. That is, in the manufacturing method for manufacturing the silicon carbide based ceramic sintered body of the present embodiment (hereinafter, simply referred to as “manufacturing method”), the sintered body is oxidized at a predetermined heating temperature for a predetermined heating time. Silicon carbide ceramics having a specific resistance value by changing the heating temperature and / or the heating time in the oxidation treatment step, comprising an oxidation treatment step of heating under and forming a silicon dioxide layer on the surface of the silicon carbide particles A sintered body is manufactured. In the oxidation treatment step, the specific resistance value is controlled by changing the heating temperature within the range of 900 ° C. to 1300 ° C. and the heating time within the range of 5 hours to 40 hours.

  In the above manufacturing method, the sintered body includes a silicon carbide forming raw material composed of silicon nitride powder and a carbonaceous material and having a silicon to carbon molar ratio of 0.5 to 1.5, and carbonization as an aggregate. What is manufactured through a molding step of molding a mixed raw material containing silicon powder and a firing step of firing the molded body obtained in the molding step in a non-oxidizing atmosphere at 1800 to 2300 ° C. are used.

  More specifically, in the molding step, a molded body is molded from a silicon carbide production raw material made of silicon nitride powder and a carbonaceous material, and a mixed raw material of silicon carbide powder as an aggregate. The molding method is not particularly limited, and for example, extrusion molding, dry pressure molding, and cast molding can be used.

  A sintered body obtained by firing the obtained molded body is a silicon carbide ceramic produced by so-called reactive sintering. Here, the silicon source of silicon carbide to be generated by reaction is “silicon nitride powder”, and the carbon source is “carbonaceous material”. Therefore, in terms of stoichiometry, when the molar ratio of silicon and carbon (Si / C) is 1, silicon carbide is generated without excess or deficiency. Here, if Si / C is smaller than 0.5, the remaining carbon content is too much, which causes coarse atmospheric pores and inhibits the growth of the generated silicon carbide particles. On the other hand, when Si / C is larger than 1.5, the reaction generation amount of silicon carbide is small, and the reaction sintering becomes insufficient. In addition, if Si / C is 0.8-1.2, there are few excess or deficiencies of silicon and carbon, and it is more desirable.

  As the carbonaceous material, graphite, coal, coke, charcoal, carbon black and the like can be used. In addition, when using a silicon carbide ceramic sintered body as a filter base, if a carbonaceous substance having an average particle diameter of 10 μm to 50 μm is used, pores having a size suitable as a filter base for the disappearance trace Is desirable. In addition, by using a carbonaceous material having a relatively large particle size of 10 μm to 50 μm in average particle size, the carbon carbide generation reaction was delayed compared to the case where the carbon source was fine particles, but the carbon carbide was generated. Since the particle growth is fast enough to form a neck of silicon carbide, and a strong neck can be formed at an early stage, a high-strength porous sintered body can be obtained.

  If the proportion of the silicon carbide powder as an aggregate in the mixed raw material is too small, the strength of the resulting sintered body tends to be low, and if it is too large, the proportion of the silicon carbide forming raw material is reduced accordingly, and reaction sintering May become insufficient. Therefore, the ratio of the silicon carbide powder as the aggregate in the mixed raw material is desirably 60 to 95% by mass.

  Here, extrusion molding is exemplified as a molding method, and additives such as an organic binder such as methylcellulose are added to the above mixed raw material, and the mixture is mixed and kneaded to form a kneaded product. Thus, a formed article having a honeycomb structure is obtained.

  You may perform the drying process which dries a molded object after this shaping | molding process. Such a drying process can be performed by air drying in a temperature-controlled humidity control tank, external heating drying, internal heating drying by microwave irradiation, or the like. When the silicon carbide based ceramic sintered body is used as the DPF substrate, one end of the cell is formed so that the cells opened in one direction and the cells opened in the other direction alternate in the honeycomb structure formed body. A sealing step for sealing is provided. This sealing step can be provided between the molding step and the drying step or after the drying step.

  In the firing step, the heating furnace is set to a non-oxidizing atmosphere, the temperature is raised at a speed that does not give a thermal shock to the molded body, and the temperature is maintained at a predetermined firing temperature of 1800 to 2300 ° C. for a certain period of time. Here, if the firing temperature is low, reactive sintering may be insufficient, and if it exceeds 2350 ° C, the generated silicon carbide may be sublimated. A sintered body having sufficient mechanical strength can be obtained within the firing time.

  The non-oxidizing atmosphere can be an inert gas atmosphere such as argon or helium, or a vacuum atmosphere. Furthermore, although baking time is based also on the size of a molded object, it can be made into 10 minutes-3 hours, for example. In this firing step, silicon nitride as a silicon source and a carbonaceous material as a carbon source react to form silicon carbide, and reactive sintering is performed so as to surround the silicon carbide as an aggregate.

  At the same time, pores are formed in the disappearance trace of the carbonaceous material used in the reaction for generating silicon carbide. Further, silicon carbide particles grow to such an extent that necks can be formed, and necks formed between the particles further grow. Here, since a carbonaceous material having a large average particle diameter of 10 μm to 50 μm is used, pores formed in the disappearance trace are not blocked by silicon carbide particle growth and neck growth, and are suitable as a filter substrate. Pores of different sizes are formed.

  Further, in the firing step, simultaneously with reactive sintering of silicon carbide and formation of pores, nitrogen generated by decomposition of silicon nitride is dissolved in silicon carbide, and if the purity is high, silicon carbide which is an insulator is n-type. Become a semiconductor. At this time, it is considered that nitrogen is mainly dissolved in silicon carbide as an aggregate, in silicon carbide newly generated around it by reaction sintering, and in a neck portion that grows between silicon carbide particles. After holding at a predetermined firing temperature for a predetermined time, the temperature is lowered at a speed that does not give a thermal shock.

  An oxidation treatment process is performed about the sintered compact obtained through the baking process. In this oxidation treatment step, the sintered body is accommodated in a heating furnace having an oxidizing atmosphere containing oxygen gas such as an air atmosphere and held at a predetermined heating temperature for a predetermined heating time. Here, the heating temperature and the heating time are adjusted according to the specific resistance value required depending on the use and size of the silicon carbide ceramic sintered body, the heating temperature is in the range of 900 ° C. to 1300 ° C., and the heating time is Change in the range of 5 to 40 hours. Here, when the heating temperature is lower than 900 ° C., the oxidation reaction hardly occurs and the silicon dioxide layer is hardly formed. On the other hand, when the heating temperature is higher than 1300 ° C., the oxidation reaction proceeds rapidly, making it difficult to control the specific resistance value. If the oxidation time is shorter than 5 hours, it is difficult to form a silicon dioxide layer sufficient to affect the specific resistance value. On the other hand, if the oxidation time is longer than 40 hours, the production efficiency of the silicon carbide ceramic sintered body is low and the economy is inferior, which is not practical.

In such an oxidation treatment step, silicon carbide reacts with oxygen on the surface of the silicon carbide particles to form a silicon dioxide layer as follows. As a result, it is considered that the conductive path in the conductive silicon carbide is limited by the silicon dioxide layer, and the specific resistance value increases.
SiC + 2O ₂ → SiO ₂ + CO ₂

  In addition, before the oxidation treatment step, a decarburization step can be provided for the purpose of burning and removing carbonaceous substances that may remain without being used in the silicon carbide formation reaction in the firing step. This decarburization process can be performed by holding at 650 ° C. to 850 ° C. for about 1 hour to 3 hours in an oxidizing atmosphere. If the temperature and the holding time are about this level, silicon carbide is hardly oxidized in the decarburization step.

  Next, specific examples will be described. To the mixed raw material having the composition shown in Table 1, methyl cellulose as a binder, oleic acid and a polyoxyalkylene compound (manufactured by NOF Corporation, Unilube (registered trademark)) as a lubricant are added, and kneaded by adding water and kneading. Obtained. This kneaded product was extrusion-molded to produce a formed body having a honeycomb structure having a size of 35 mm × 35 mm × height of 150 mm, a cell density of 300 cpsi, and a partition wall thickness of 0.25 mm (molding step).

  After drying the obtained molded body (drying step), it was subjected to reactive sintering by firing at 2100 ° C. for 10 minutes in a non-oxidizing atmosphere to obtain a honeycomb-structured sintered body (porous silicon carbide having conductivity) Sintered ceramic) was obtained (firing step). Then, it heated at 850 degreeC in the air atmosphere for 1 hour, and the carbonaceous substance which may remain | survive as an unreacted part was removed (decarburization process).

About the sintered compact sample 1-sample 14 which passed through said process, the oxidation process was performed in the air atmosphere with the heating temperature and the heating time which are shown in Table 2 (oxidation process process). For each sample, the mass was measured before and after the oxidation treatment, and the mass of the sintered body ((mass of the sintered body after the oxidation treatment) − (mass of the sintered body before the oxidation treatment)) increased by the oxidation treatment, The oxygen concentration (mass%) increased by the oxidation treatment was calculated as follows.
Oxygen concentration increased by oxidation treatment (mass%) = (mass of oxygen increased by oxidation treatment) / (mass of sintered body after oxidation treatment) × 100
Here, (mass of oxygen increased by oxidation treatment) = (mass of sintered body increased by oxidation treatment) × (molecular weight of oxygen / ((molecular weight of silicon dioxide) − (molecular weight of silicon carbide))

Further, the oxygen concentration in the sintered body not subjected to the oxidation treatment was measured in accordance with the oxygen determination method (inert gas melting-infrared absorption method) of JIS R1616 and found to be 0.3% by mass. Therefore, the oxygen concentration (mass%) in the sintered body after the oxidation treatment was calculated as follows.
Oxygen concentration (mass%) in the sintered body after oxidation treatment = ((mass of oxygen increased by oxidation treatment) + (mass of sintered body before oxidation treatment) × 0.3 / 100) / (after oxidation treatment) Mass of sintered body) × 100
The “sintered body after oxidation treatment” in the examples corresponds to “a silicon carbide ceramic sintered body in which a silicon dioxide layer is formed on the particle surface” of the present invention.

  Further, a silver paste was baked to a width of 1 cm at a distance of 10 cm in the major axis direction at both ends of each sample after the oxidation treatment to form an electrode, and the electrical resistance between the two electrodes was measured with a tester to compare the ratio at room temperature. The resistance value was determined.

  For each sample, the mass of the sintered body before and after the oxidation treatment, the oxygen concentration (mass%) increased by the oxidation treatment, the oxygen concentration (mass%) in the sintered body after the oxidation treatment, and the resistivity at room temperature (Ω) Cm) are summarized in Table 2.

  Based on the above results, a graph in which the oxygen concentration (mass%) increased by the oxidation treatment is plotted with respect to the heating temperature in the oxidation treatment step is shown in FIG. In FIG. 1, the second Y axis is the oxygen concentration (mass%) in the sintered body after the oxidation treatment. Below, when it is not necessary to distinguish the oxygen concentration (mass%) increased by the oxidation treatment and the oxygen concentration (mass%) in the sintered body after the oxidation treatment, they may be simply referred to as “oxygen concentration”. . Further, the sintered body after the oxidation treatment corresponds to “a silicon carbide ceramic sintered body on which a silicon dioxide layer is formed” of the present invention.

  As is apparent from FIG. 1, the oxygen concentration increases as the heating temperature is higher at both the heating time of 5 hours and 40 hours, and the increase curve draws a curve that curves smoothly and convex downward. . In addition, when the heating time was 40 hours, the oxygen concentration was higher than when the heating time was 5 hours even when the heating temperature was the same, and the difference became larger as the heating temperature was higher. Also, from these results, in the case of a heating time between 5 hours and 40 hours, heating is performed so that a similar curve is drawn between the curve with a heating time of 5 hours and the curve with a heating time of 40 hours. It was thought that oxygen concentration increased with temperature.

  In the oxidation treatment step, the oxygen concentration is increased in the range of 0.1% by mass to 4.5% by mass by changing the heating temperature from 900 ° C. to 1300 ° C. and the heating time from 5 hours to 40 hours. The oxygen concentration in the sintered body after the oxidation treatment can be adjusted in the range of 0.4% by mass to 4.8% by mass. In this case, the oxygen concentration can be changed by changing the heating temperature with a constant heating time, by changing the heating time with a constant heating temperature, or by changing both the heating time and the heating temperature. It was thought that it could be adjusted to a desired value within the above range.

  Next, FIG. 2 shows a graph in which the specific resistance value (Ω · cm) at room temperature is plotted against the oxygen concentration (mass%) increased by the oxidation treatment. In FIG. 2, the second X axis is the oxygen concentration (mass%) in the sintered body after the oxidation treatment. As is clear from FIG. 2, the specific resistance value at room temperature increased in a straight line as the oxygen concentration increased. The specific resistance value in the above oxygen concentration range is in the range of 5 Ω · cm to 20 Ω · cm, which is a practical value as a silicon carbide based ceramic sintered body used for a filter base that generates heat when energized. .

  From the above results, by changing the oxygen concentration in the sintered body, the specific resistance value at room temperature can be changed as a linear function of the oxygen concentration. The oxygen concentration in the sintered body is determined by the heating temperature and the oxidation treatment step. / Or can be adjusted by the heating time. That is, the specific resistance value of the silicon carbide based ceramic sintered body is obtained by setting the oxygen concentration in the sintered body after the oxidation treatment in the range of 0.4 mass% to 4.8 mass% by setting the conditions of the oxidation treatment process. Can be controlled within a wide range of 5 Ω · cm to 20 Ω · cm.

  Next, FIG. 3 shows the result of measuring the change in specific resistance value with the temperature rise for samples 5, 13, and 14 having different oxygen concentrations. The measurement was performed by energizing each sample using the two electrodes described above, and measuring the voltage and current when the indicated temperature of the thermocouple reached a predetermined temperature. From FIG. 3, the specific resistance value decreased for all the samples so as to draw a curve that smoothly curved and convex downward with increasing temperature. Further, the degree of decrease in the specific resistance value was greater as the oxidation concentration was higher. That is, the difference in specific resistance value between samples decreases with increasing temperature, but it can be seen that there is a difference in specific resistance value between samples until the temperature reaches nearly 400 ° C. In general, when a silicon carbide ceramic sintered body is used as a heating element of a volatile organic compound (VOC) decomposition apparatus or a catalyst carrier for activating a catalyst by heating, the sintered body is heated to 200 to 300 ° C. Accordingly, the silicon carbide based ceramic sintered body according to the present embodiment, which is manufactured by the above-described manufacturing method for forming a silicon dioxide layer by oxidation treatment and has various specific resistance values up to about 400 ° C., is used in such applications. It is practical as a silicon carbide ceramic sintered body.

  In addition, when a silicon carbide ceramic sintered body is used as a DPF substrate, it is usually necessary to heat up to 500 to 700 ° C. during the regeneration treatment for removing the deposited particulate matter by combustion. If it is possible, the heating temperature can be lowered to about 400-500 ° C. Therefore, the silicon carbide based ceramic sintered body of the present embodiment, which is manufactured by the above-described manufacturing method of forming a silicon dioxide layer by oxidation treatment and has various specific resistance values up to about 400 ° C., is self-heating. It is also useful as a silicon carbide ceramic sintered body as a base of a type DPF.

  Specifically, the silicon carbide ceramic sintered body with controlled conductivity can be manufactured as follows. First, in the measurement results described above with reference to FIG. 3, an approximate expression of a curve indicating a change in specific resistance value accompanying a temperature rise is obtained for each sample having a different oxygen concentration. The obtained approximate expression is shown in the graph of FIG. Using this approximate expression, the specific resistance value at a certain temperature can be calculated. Then, by calculating the specific resistance value at the same temperature for samples having different oxygen concentrations, the relationship of the specific resistance value to the oxygen concentration at a certain temperature can be known as shown in FIG.

  Therefore, for example, when it is desired to manufacture a conductive silicon carbide based porous material having a specific resistance value of 0.3 Ω · cm at a temperature of 100 ° C., as shown by a one-dot chain line in FIG. Is increased by 2.5 mass%, and the oxygen concentration of the sintered body after the oxidation treatment is 2.8 mass%. And the oxidation process conditions required for that can be read from the graph which shows the relationship between the conditions (heating temperature, heating time) in the oxidation process illustrated in FIG. 1, and oxygen concentration. That is, as shown by a one-dot chain line from FIG. 1, the heating time is required to increase the oxidation concentration by 2.5% by mass by the oxidation treatment so that the oxygen concentration of the sintered body after the oxidation treatment is 2.8% by mass. When the heating time is 40 hours, the heating temperature may be 1100 ° C., and when the heating time is 5 hours, the heating temperature may be 1300 ° C.

  In general, it is said that a ceramic heating element that generates heat by resistance has a use limit when the resistance value reaches four times the initial value. In the case of silicon carbide, the use temperature range in the exemplified application has NTC characteristics in which the electrical resistance decreases as the temperature rises as described above, and thus can be used up to a higher resistance value. However, with use, the oxidation reaction gradually proceeds to the inside of the particles, and the specific resistance value increases. Therefore, when the main purpose is to extend the period of use of the silicon carbide ceramic sintered body, it is desirable to keep the oxygen concentration in the initial state before use to some extent, for example, the initial value of the specific resistance value at room temperature. However, it is set as the range which does not exceed 4 times the specific resistance value in room temperature of the sintered compact which does not oxidize. The specific resistance value at room temperature of the sintered body produced by the same method except that the oxidation treatment of the sample of this example was not performed was about 4 Ω · cm on average. Therefore, when the main purpose is to extend the period of use of the silicon carbide ceramic sintered body, the initial value of the specific resistance value at room temperature of the silicon carbide ceramic sintered body should be in a range not exceeding 16 Ω · cm. desirable. In order to set the specific resistance value within this range, the oxygen concentration increased by the oxidation treatment is set not to exceed 3.4 mass%, and the final oxygen concentration in the silicon carbide based ceramic sintered body is set to 3.7. It can be read from FIG. 2 (two-dot chain line) that the mass range is not exceeded.

  The results of measuring the three-point bending strength for Samples 1 to 4, 7 to 10 and 12 are shown in FIG. Here, for the measurement of the three-point bending strength, the above-mentioned sintered body (size 35 mm × 35 mm × height 150 mm, cell density 300 cpsi, partition wall thickness 0.25 mm molded body fired at 2100 ° C. for 10 minutes. ) Was used as one specimen without cutting. Moreover, the measuring method was based on JISR1601, and measured at room temperature on the conditions of the distance between fulcrums of 10 cm, and the crosshead speed of 1 mm / min. As can be seen from FIG. 5, the effect of increasing the three-point bending strength can be incidentally obtained by increasing the oxygen concentration for the purpose of controlling the specific resistance value.

  Next, with respect to the sample 14 (heating temperature 1200 ° C., heating time 40 hours), observation (SEM observation) with a scanning electron microscope (manufactured by JEOL Ltd., JXA-840 type), and energy dispersive X-ray analyzer The results of elemental analysis (EDX analysis) by JEOL Ltd. (JED-2200 type) are shown in FIGS. 6 and 7 in comparison with a control sample that is not subjected to oxidation treatment. Here, regarding elemental analysis, FIG. 6 shows the results for oxygen, and FIG. 7 shows the results for silicon and oxygen.

  When the SEM observation image and EDX image of the sample surface are compared between the sample 14 (FIG. 6 (a)) and the control sample (FIG. 6 (b)), oxygen is distributed in the sample 14 in accordance with the particle shape. It is clear that there is no oxygen element distribution in the EDX image of the control sample. From this, it can be seen that the silicon dioxide layer is formed through the oxidation treatment step.

  Further, an SEM observation image and an EDX image of the fracture surface of the sample 14 are shown in FIG. From FIG. 7, it is clear that oxygen does not exist inside the particles appearing on the fracture surface, and oxygen is distributed only along the surface of the particles. Therefore, it was confirmed that the silicon dioxide layer was formed on the surface of the silicon carbide particles by the oxidation treatment.

  As described above, in the silicon carbide based ceramic sintered body of the present embodiment, the silicon dioxide layer is formed on the surface of the silicon carbide particles, and there are various types according to the amount (oxygen concentration) of the silicon dioxide layer. Take the resistivity value. In addition, by changing the amount of the silicon dioxide layer in the range of 0.4 mass% to 4.8 mass% as the oxygen concentration, the specific resistance value at room temperature can be controlled in a wide range of 5 to 20 Ω · cm. It is.

  In the present embodiment, as a sintered body (sintered body before oxidation treatment) for forming the silicon dioxide layer, a silicon carbide sintered body obtained by reactive sintering with a carbon source using silicon nitride as a nitrogen source Is used. Therefore, in order to impart conductivity to the silicon carbide, no additive is added to silicon carbide. In addition to having a simple configuration, a specific resistance value can be obtained by providing a silicon dioxide layer only on the particle surface. Since the structure can be changed after the fact, it can be manufactured by an extremely simple manufacturing method.

  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, the conductive porous silicon carbide ceramic sintered body of the present invention can be used as a base of a self-heating DPF that purifies gas discharged from a diesel engine, as well as other internal combustion engines and steam. A self-heating filter base used for purifying exhaust gas in a turbine, a catalyst carrier that activates a catalyst by heating, or a heating device that heats a fluid flowing through cells or continuous pores in a honeycomb structure It can be used as a substrate.

Japanese Patent No. 3431670

Claims (2)

A porous silicon carbide based ceramic sintered body having conductivity,
A silicon dioxide layer having an oxygen concentration of 0.4 mass% to 4.8 mass% in the sintered body is formed around the silicon carbide and on the matrix surface where silicon carbide in which nitrogen is dissolved between particles is formed. When the oxygen concentration is 0.4 mass%, the specific resistance value is 5 Ω · cm, and the oxygen concentration is 4.8 mass%. The silicon carbide based ceramic sintered body is characterized by being expressed by a linear function having a specific resistance value of 20 Ω · cm . The silicon carbide based ceramic sintered body according to claim 1, wherein the sintered body is made of a honeycomb structure including a plurality of cells partitioned by partition walls extending in a single direction. .

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