JP2018168006A - Method for producing conductive silicon carbide-based sintered body and conductive silicon carbide-based sintered body - Google Patents

Abstract

To provide a method for producing a conductive silicon carbide-based sintered body, in which: variation of resistivity associated with oxidization is suppressed; and a ratio of a β-type silicon carbide is made larger to reduce temperature dependency of the resistivity.SOLUTION: A sintered body is formed of a starting material comprising a raw material of a conductive phase, the silicon carbide-based conductive phase, wherein the raw material comprises nitride as a dopant. The sintered body is produced, in which variation of resistivity associated with oxidization is smaller than a sintered body having no high-resistance phase by forming the high-resistance phase on at least an external surface of the sintered body, the high-resistance phase being the silicon carbide-based conductive phase in which nitride density is lower than an average nitride density of the conductive phase. By making the starting material comprise the β-type aggregate, the coarse particles that comprise a β-type conductive silicon carbide and have a particle diameter of 5-50 μm, a ratio of a β-type silicon carbide in the whole sintered body is changed to vary temperature dependency of the resistivity; and the ratio of the β-type silicon carbide in the high-resistance phase is made larger than a sintered body formed of a starting material that does not comprise a β-type aggregate.SELECTED DRAWING: Figure 1

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.

  Silicon carbide has excellent thermal shock resistance due to its low thermal expansion coefficient in addition to its high thermal conductivity, making it suitable as a base for structures used at high temperatures, such as filters, catalyst carriers, and heat exchangers. ing. In addition, high-purity silicon carbide has high electrical resistance and is close to an insulator, but silicon carbide-based ceramics with conductivity can be used as a self-heating structure that is heated to high temperatures by energization. is there. In the past, the present applicant has proposed a method for manufacturing a silicon carbide ceramic sintered body imparted with conductivity by doping nitrogen when reacting silicon carbide from a silicon source and a carbon source. (For example, refer to Patent Document 1).

  Silicon carbide has a problem that it is oxidized when heated to a high temperature in the presence of oxygen. It is said that if the surface of silicon carbide is coated with a silicon dioxide film produced by the oxidation of silicon carbide, further oxidation will be suppressed to some extent, but that is not enough to suppress oxidation. Is the actual situation. Since the phase of silicon dioxide formed on the surface of the sintered body by oxidation has a large electric resistance, the specific resistance value of the conductive silicon carbide sintered body increases as the oxidation proceeds.

  In order to solve this problem, the present applicant has already applied nitrogen in the conductive phase to at least the outer surface of the sintered body of the silicon carbide based ceramic containing a conductive phase that is a phase of silicon carbide containing nitrogen as a dopant. It has been proposed to form a high resistance phase which is a silicon carbide phase having a nitrogen concentration lower than the average concentration (see Patent Document 2). The high resistance phase is a phase having a small contribution to the electric conductivity of the entire sintered body because the number of free electrons is small due to the low concentration of nitrogen and the electric resistance is larger than that of the conductive phase.

  In a sintered body in which such a high resistance phase is formed on the outer surface of the sintered body, the high resistance phase is oxidized when used in an atmosphere in which oxygen is present. When a high-resistance phase, which originally contributes little to the electrical conductivity of the entire sintered body, is oxidized, compared to when the conductive phase, which is a phase that has a large contribution to the electrical conductivity of the entire sintered body, is oxidized. The influence on the electrical conductivity of the entire sintered body is small. In addition, the presence of the high resistance phase on the outer surface of the sintered body makes it difficult for the oxidation reaction to reach the conductive phase having a large contribution to electrical conductivity. Therefore, by forming a high resistance phase on the outer surface of the sintered body, it is possible to suppress a change in specific resistance value when continuously used at a high temperature in an atmosphere in which oxygen is present.

  Further, according to the study by the present applicant, the sintered body containing the conductive phase is heated in a non-oxidizing atmosphere substantially free of nitrogen gas, so that once doped nitrogen is discharged from the sintered body. Thus, it was found that a high resistance phase can be formed, and nitrogen can be efficiently discharged by performing this process at a high temperature of 2100 ° C. to 2300 ° C.

  On the other hand, the conductive silicon carbide-based sintered body has NTC characteristics in which the electrical resistance is greatly reduced as the temperature rises, and the temperature dependence of the specific resistance value is high. For this reason, there are problems that the specific resistance value becomes too small at a high temperature, the current value becomes excessive and control becomes difficult, and the sintered body is damaged by overheating due to overcurrent.

  In response to this problem, the present applicant has found that the temperature dependence of the specific resistance value varies depending on the proportion of β-type silicon carbide in the silicon carbide sintered body, and the conductive carbonization having the high resistance phase described above. It has been proposed to reduce the temperature dependence of the specific resistance value by increasing the proportion of β-type silicon carbide in the silicon-based sintered body (see Patent Document 3).

  However, if the process of forming a high resistance phase as described above is performed at a high temperature in order to efficiently discharge nitrogen, a part of β-type silicon carbide is transferred to α-type silicon carbide that is stable at high temperature. That is, in the technique of Patent Document 3, it is desirable that the ratio of β-type silicon carbide is high in order to reduce the temperature dependence of the specific resistance value, but high resistance is required to suppress the change in specific resistance value due to oxidation. When attempting to form a phase, there was a situation in which the proportion of β-type silicon carbide decreased due to the transition to α-type silicon carbide. In other words, in order to reduce the temperature dependence of the specific resistance value, even if an attempt is made to increase the proportion of β-type silicon carbide in the sintered body, in order to obtain an effect of suppressing the change in the specific resistance value accompanying oxidation. The proportion of β-type silicon carbide was limited to some extent.

Japanese Patent No. 3691536 Japanese Patent No. 5723429 JP, 2006-183081, A

  Therefore, in view of the above situation, the present invention suppresses a change in specific resistance value due to oxidation, and can further reduce the temperature dependency of the specific resistance value by increasing the proportion of β-type silicon carbide. An object of the present invention is to provide a method for producing a conductive silicon carbide sintered body and a conductive silicon carbide sintered body produced by the production method.

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”)
“Carbonization in which the concentration of nitrogen is lower than the average concentration of nitrogen in the conductive phase on at least the outer surface of the sintered body formed from the starting material including the raw material of the conductive phase which is a phase of silicon carbide containing nitrogen as a dopant. By forming a high resistance phase that is a phase of silicon, a change in specific resistance value due to oxidation is smaller than that of the sintered body without the high resistance phase, and a sintered body is manufactured.
In the method for producing a conductive silicon carbide sintered body for producing a sintered body having different temperature dependence of specific resistance value, depending on the ratio of β-type silicon carbide in the entire sintered body,
By adding β-type aggregate which is coarse particles having a particle diameter of 5 μm to 50 μm made of conductive β-type silicon carbide to the starting material, the ratio of β-type silicon carbide in the entire sintered body can be changed. In addition, the ratio of β-type silicon carbide in the high resistance phase is increased as compared with a sintered body formed from the starting material not containing the β-type aggregate ”.

  As a result of investigation, even if β-type silicon carbide contained in the sintered body is derived from conductive coarse particles, the temperature dependence of the specific resistance value is changed depending on the proportion of β-type silicon carbide in the sintered body. The larger the proportion of β-type silicon carbide in the coarse particles contained in the starting material, the lower the temperature dependency of the specific resistance value, and the larger the proportion of β-type silicon carbide contained in the starting material. This not only increases the proportion of β-type silicon carbide in the entire silicon carbide of the sintered body, but also makes it difficult for coarse-grained β-type silicon carbide to transfer to α-type silicon carbide, and in the high-resistance phase β-type carbonization. The present inventors have found that the proportion of silicon can be increased and have arrived at the present invention.

  According to the manufacturing method of this configuration, by adjusting the proportion of β-type silicon carbide in the entire silicon carbide of the sintered body by using the β-type aggregate contained in the starting material, the conductivity having different temperature dependence of the specific resistance value. A silicon carbide sintered body can be produced. In addition, the ratio of β-type silicon carbide in the entire silicon carbide of the sintered body can be increased while maintaining the action of effectively suppressing the change in specific resistance value due to oxidation due to the presence of the high resistance phase. The temperature dependence of the resistance value can be greatly reduced.

  In the manufacturing method of this configuration, the particle size of the coarse particles is 50% in the volume-based cumulative distribution by the laser diffraction / scattering method. The case where the specific resistance value is less than 1000 Ωcm is defined as “conductive”.

Next, the conductive silicon carbide based sintered body according to the present invention is:
“It is a sintered body containing a conductive phase which is a phase of silicon carbide containing nitrogen as a dopant,
At least the outer surface of the sintered body has a high resistance phase that is a silicon carbide phase having a nitrogen concentration lower than the average concentration of nitrogen in the conductive phase;
The conductive phase is made of conductive β-type silicon carbide and contains coarse particles having a particle diameter of 5 μm to 50 μm ”.

  This is the configuration of the conductive silicon carbide sintered body produced by the above production method. Here, since the “conductive phase” contains coarse particles that are conductive β-type silicon carbide, the “average concentration of nitrogen in the conductive phase” is the coarse particles and the conductive material surrounding the coarse particles. It refers to the concentration of nitrogen averaged over the entire silicon carbide phase. Further, the “conductive phase” may include a plurality of conductive phases having different nitrogen concentrations in addition to coarse particles that are conductive β-type silicon carbide. In this case, “the average concentration of nitrogen in the conductive phase” refers to the concentration of nitrogen that is obtained by averaging a plurality of conductive phases and coarse particles that are conductive β-type silicon carbide.

  Furthermore, if the “high resistance phase” is formed on at least the outer surface of the sintered body, a high resistance phase having an average nitrogen concentration lower than that of the conductive phase is formed on a portion that is not the outer surface of the sintered body. You may have. For example, when coarse particles of non-conductive silicon carbide are included inside the sintered body, the phase is a high resistance phase present in a portion that is not the outer surface of the sintered body. “Conductive silicon carbide sintered body” is the same as “conductive silicon carbide ceramic sintered body”.

  As described above, as an effect of the present invention, the electrical conductivity capable of suppressing the change in specific resistance value due to oxidation and further reducing the temperature dependence of the specific resistance value by increasing the proportion of β-type silicon carbide. The manufacturing method of a silicon carbide sintered body and the conductive silicon carbide sintered body manufactured by the manufacturing method can be provided.

It is a graph which shows the relationship between the ratio of (beta) -type silicon carbide, and the temperature dependence of a specific resistance value. It is the figure which added the display which shows the practical range of the temperature dependence of a specific resistance value to the graph of FIG.

  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 of the present embodiment has at least an outer surface of a sintered body formed from a starting material containing a raw material of a conductive phase that is a phase of silicon carbide containing nitrogen as a dopant. A method for producing a conductive silicon carbide-based sintered body that forms a high resistance phase that is a silicon carbide phase having a nitrogen concentration lower than the average concentration of nitrogen in the conductive phase, the conductive material being a conductive β-type A β-type aggregate which is a coarse particle made of silicon carbide and having a particle diameter of 5 μm to 50 μm is contained.

  A sintered body containing a conductive phase that is a phase of silicon carbide containing nitrogen as a dopant is fired, for example, in a molding step of obtaining a molded body from a starting material containing silicon carbide powder and in a non-oxidizing atmosphere containing nitrogen gas It can be obtained through a firing step. In this case, if the firing step is performed under pressure, nitrogen in the atmosphere can be efficiently doped into the sintered body. If fine particles are included in the silicon carbide powder in the starting material, nitrogen in the atmosphere can be efficiently doped when the fine particles are sintered. Since the starting material contains β-type aggregates, which are coarse particles of conductive β-type silicon carbide, the silicon carbide phase doped with nitrogen becomes β-type aggregates as the sintering of the silicon carbide powder proceeds. Sinter to surround. Note that the non-oxidizing atmosphere containing nitrogen gas can be a nitrogen gas 100% atmosphere or a mixed atmosphere of a rare gas such as argon or helium and nitrogen gas.

  As will be described later, in this embodiment, the ratio of β-type silicon carbide in the entire silicon carbide of the sintered body can be adjusted by the ratio of β-type aggregate to be included in the starting material. The phase may not contain β-type silicon carbide.

  Alternatively, a sintered body containing a conductive phase that is a phase of silicon carbide containing nitrogen as a dopant is obtained by converting a molded body formed from a starting material containing a reaction raw material consisting of a silicon source and a carbon source into a non-oxidized material containing nitrogen gas. It can obtain by passing through the reaction baking process of making silicon carbide react by baking in a neutral atmosphere. In this case, the β-type aggregate contained in the starting material serves as a nucleus of silicon carbide produced by reaction. Here, as the “silicon source” in the reaction product raw material, silicon nitride or silicon (simple substance) can be used. On the other hand, as the “carbon source”, carbonaceous materials such as graphite, coal, coke, charcoal, and carbon black can be used. Stoichiometrically, silicon carbide is generated without excess or deficiency when the molar ratio of silicon and carbon (Si / C) is 1, but if Si / C is 0.8 to 1.2, silicon and carbon It is desirable that there is little excess or deficiency.

  When silicon nitride is used as the silicon source, the nitrogen generated by the decomposition of silicon nitride accompanying the reaction generation of silicon carbide is also doped into the silicon carbide generated by the reaction, so the concentration of nitrogen in the conductive phase should be increased. The electrical conductivity of the conductive phase can be further increased. As a result, by forming a high resistance phase on at least the outer surface of the sintered body, the specific resistance value of the entire sintered body may increase even if the volume that can contribute to electrical conductivity in the sintered body decreases. Can be reduced.

  Alternatively, when silicon nitride is used as the silicon source, only nitrogen generated by decomposition of silicon nitride can be used as a dopant, and the atmosphere in the reaction firing step can be a non-oxidizing atmosphere that does not contain nitrogen gas. The non-oxidizing atmosphere containing no nitrogen gas can be a rare gas atmosphere such as argon or helium, or a vacuum atmosphere.

  When silicon carbide is generated from a silicon source and a carbon source, past studies have shown that the proportion of β-type silicon carbide in the reaction-generated silicon carbide can be changed by the firing temperature in the reaction firing step. As the firing temperature in the firing step increases, the proportion of α-type silicon carbide that is stable at high temperatures increases and the proportion of β-type silicon carbide decreases. However, in the present embodiment, the ratio of β-type silicon carbide in the entire silicon carbide of the sintered body can be adjusted by the ratio of β-type aggregate to be included in the starting material, and therefore β is added to the phase of silicon carbide generated in the reaction firing step. The reaction baking process can be performed at a high temperature (2000 ° C. to 2200 ° C.) at which the α-type is stable without intending to form type silicon carbide.

  In the plurality of production methods exemplified above, the starting material can contain aggregate particles made of other materials in addition to the β-type aggregate. As aggregate particles made of other materials, coarse particles of α-type silicon carbide or coarse particles made of ceramics other than silicon carbide can be used. Further, the aggregate particles made of other materials may be conductive or non-conductive.

  The high resistance phase is formed by a high resistance phase forming step in which a sintered body including a conductive phase which is a silicon carbide phase containing nitrogen as a dopant is heated in a non-oxidizing atmosphere substantially free of nitrogen gas. Can do. Through such a process, once doped nitrogen is discharged from the sintered body, and a silicon carbide phase having a low nitrogen concentration is formed on the outer surface of the sintered body. Here, the “non-oxidizing atmosphere substantially free of nitrogen gas” can be a rare gas atmosphere such as argon or helium. In this case, the concentration of nitrogen gas in the atmosphere is ideally zero, but the nitrogen gas concentration is acceptable if it is less than 5000 ppm, more preferably less than 500 ppm. Alternatively, the “non-oxidizing atmosphere substantially free of nitrogen gas” may be a vacuum atmosphere. By performing the high resistance phase forming step at a high temperature of 2100 ° C. to 2300 ° C., nitrogen can be efficiently discharged.

  A firing step or reaction firing step for obtaining a sintered body containing a conductive phase, and a high resistance phase forming step for heating the sintered body containing a conductive phase in a non-oxidizing atmosphere substantially free of nitrogen gas, It can carry out continuously using the continuous baking furnace which bakes, conveying a to-be-fired body. For example, the atmosphere on the upstream side in the conveying direction in the continuous firing furnace is a non-oxidizing atmosphere containing nitrogen gas to obtain a sintered body having a conductive phase doped with nitrogen, and the atmosphere on the downstream side in the conveying direction is substantially Thus, a non-oxidizing atmosphere containing no nitrogen gas can be formed, and a high resistance phase can be formed on the outer surface of the sintered body.

  Alternatively, while the compact is fired in a batch furnace, the conductive phase is switched from a non-oxidizing gas containing nitrogen gas to a non-oxidizing gas containing substantially no nitrogen gas. It is possible to continuously perform a firing step or a reaction firing step for obtaining a sintered body including a high resistance phase forming step for forming a high resistance phase on the outer surface of the sintered body. Alternatively, the compact is placed in a batch furnace and then fired in a non-oxidizing atmosphere containing nitrogen gas to obtain a sintered body containing a conductive phase. The high-resistance phase forming step of containing the heat and heating in a non-oxidizing atmosphere substantially free of nitrogen gas can be performed discontinuously.

  In the present embodiment, the proportion of β-type silicon carbide in the entire silicon carbide of the sintered body is changed mainly depending on the proportion of β-type aggregate contained in the starting material. Since the β-type aggregate, which is a coarse particle, hardly transfers to α-type silicon carbide in the firing step or the reaction firing step or in the high resistance phase forming step, most of the β-type aggregates contained in the starting material are It remains in the final sintered body as it is. Therefore, there is an advantage that the ratio of β-type silicon carbide in the entire sintered body can be easily adjusted to an “intended numerical value” as compared with the case where the ratio of β-type silicon carbide is adjusted by the temperature of the reaction firing step.

  In addition, since it is possible to increase the proportion of β-type silicon carbide in the entire silicon carbide of the sintered body by increasing the proportion of β-type aggregate to be included in the starting material, a manufacturing method for reacting and generating silicon carbide is provided. Even when it is employed, the reaction firing step can be performed at a high temperature without intending to produce β-type silicon carbide in the silicon carbide phase to be produced, and thus there is an advantage that the efficiency of the reaction firing step is good.

  When β-type silicon carbide contained in the starting material is not coarse particles, it is considered that transition to α-type silicon carbide is likely to occur in the firing step, reaction firing step, or high resistance phase forming step.

  The conductive silicon carbide sintered body manufactured by the above manufacturing method is a sintered body including a conductive phase that is a phase of silicon carbide containing nitrogen as a dopant, and at least the outer surface of the sintered body is electrically conductive. Having a high resistance phase, which is a silicon carbide phase having a nitrogen concentration lower than the average concentration of nitrogen in the sex phase, and the conductive phase is made of conductive β-type silicon carbide and is coarse with a particle size of 5 μm to 50 μm Contains particles.

  Coarse particles as an aggregate are mixed with a reaction raw material in which silicon nitride is used as a silicon source for reacting silicon carbide, graphite is used as a carbon source, and a molar ratio of silicon and carbon (Si / C) is 1. The starting material was prepared. Aggregates include β-type aggregates, which are coarse particles of conductive β-type silicon carbide (sintered body) doped with nitrogen, and non-conductive α-type silicon carbide (sintered) that is not doped with nitrogen or the like. (Coated) coarse particles (hereinafter sometimes referred to as “α-type aggregate”) were mixed at different ratios. Both the β-type aggregate and the α-type aggregate had a particle diameter of about 15 μm, which is a 50% diameter in a volume-based cumulative distribution by laser diffraction / scattering method.

  Samples S21 to S28 in which the proportion of β-type aggregate in the aggregate is varied in the range of 100% to 20%, and sample R29 in which the aggregate is composed of only α-type aggregate are respectively the aggregate and silicon source in the starting material. The mass ratio was set to 3: 2. Here, the α type aggregate used for each sample is the same. The β-type aggregates used for the samples S22 to S28 are the same, and are obtained by pulverizing a reaction sintered body of β-type silicon carbide doped with nitrogen, but the β-type aggregates used for the sample S21 are commercially available. It is a product.

  For each sample, an organic binder and water were added to the starting materials, and the kneaded kneaded product was extrusion molded to have a size of 36 mm × 36 mm × length of 100 mm, a cell density of 300 cpsi, and a partition wall thickness of 10 mil (about 0.25 mm). A formed article having a honeycomb structure was produced (forming process).

  For each sample, the compact was fired at a temperature of 2100 ° C. in a non-oxidizing atmosphere containing nitrogen gas to produce silicon carbide by reaction, and a fired product containing a conductive phase that is a silicon carbide phase doped with nitrogen. A bonded body was obtained (reaction baking step). Thereafter, the sintered body was heated at a temperature of 2200 ° C. in a non-oxidizing atmosphere substantially containing no nitrogen gas (high resistance phase forming step).

About the sintered compact which passed through the high resistance phase formation process, based on JISR1650-2, the specific resistance value was measured by the four terminal method. A value “ρ _Th / ρ _Tn ” obtained by dividing the specific resistance value ρ _Th (Ω · cm) at a temperature of 500 ° C. by the specific resistance value ρ _Tn (Ω · cm) at room temperature was calculated. This “ρ _Th / ρ _Tn ” is an index of the temperature dependence of the specific resistance value, and indicates that the larger the numerical value, the lower the temperature dependence of the specific resistance value.

  Further, the test piece after measuring the specific resistance value is processed into a small piece (size 4.5 mm × 2 mm × 5 mm), and an unprocessed surface (corresponding to the outer surface of the sintered body) is irradiated with X-rays. From the measured X-ray diffraction pattern, the ratio “α-SiC: β-SiC (sintered body surface)” of α-type silicon carbide and β-type silicon carbide on the surface of the sintered body was determined. The ratio of α-type silicon carbide and β-type silicon carbide was determined by the Rietveld method from the peak of α-type silicon carbide and the peak of β-type silicon carbide in the X-ray diffraction pattern. Of the silicon carbide peaks in the X-ray diffraction pattern, the peak of the crystal structure 3C is the β-type silicon carbide peak, and the silicon carbide peaks of crystal structures other than 3C such as 6H, 15R, and 4H are the peaks of the α-type silicon carbide. As analyzed. “Α-SiC: β-SiC (sintered body surface)” can be considered as the ratio of α-type silicon carbide to β-type silicon carbide in the high resistance phase formed on the outer surface of the sintered body.

  Further, from the X-ray diffraction pattern measured for the powder obtained by pulverizing the sintered body with a mortar until the particle diameter measured by the laser diffraction / scattering method becomes 15 μm, the ratio of α-type silicon carbide to β-type silicon carbide in the pulverized product “Α-SiC: β-SiC (ground product)” was determined. When “α-SiC: β-SiC” is measured for the pulverized sintered bodies having different pulverization levels, the ratio changes with the progress of pulverization, but when the particle diameter by the laser diffraction / scattering method reaches 15 μm, Even if pulverization is further performed, the ratio of α-type silicon carbide and β-type silicon carbide is constant. From this, “α-SiC: β-SiC (crushed material)” measured for the pulverized material obtained by pulverizing the sintered body until the particle diameter by laser diffraction / scattering method reaches 15 μm was subjected to a high resistance phase forming step. It can be considered that it is the ratio of α-type silicon carbide and β-type silicon carbide in the entire silicon carbide of the sintered body.

For samples S21 to S28 and sample R29, “α-SiC: β-SiC (sintered body surface)” and “α-SiC: β-SiC (pulverized product)” are used as an index of the temperature dependence of the specific resistance value. It is shown in Table 1 together with certain “ρ _Th / ρ _Tn ”.

  From Table 1, the higher the proportion of β-type aggregate in the aggregate, the higher the proportion of β-type silicon carbide in the entire silicon carbide of the sintered body, and the higher the proportion of β-type silicon carbide in the high resistance phase. You can see that In samples S21 and S22 in which 100% of the aggregate is β-type aggregate, the ratio of β-type silicon carbide in the high resistance phase is a high value of about 40%. This is because, in the past examination by the applicant (see Patent Document 3), in any of the samples in which the proportion of β-type silicon carbide in the entire silicon carbide of the sintered body is different, α-type silicon carbide is used in the high resistance phase. This is different from the result in which the ratio exceeded 90%.

  This is because, in past studies, the ratio of β-type silicon carbide in the entire silicon carbide of the sintered body is adjusted by the ratio of β-type silicon carbide in the reaction-generated silicon carbide. Since silicon is easily α-ized in the high-resistance phase forming step, the β-type aggregate contained in the starting material in this embodiment is coarse particles of a sintered body and is not easily α-ized in the high-resistance phase forming step. It was considered. Actually, the ratio of the β-type silicon carbide derived from the aggregate in the entire silicon carbide of the sintered body is the ratio of the β-type silicon carbide in the entire silicon carbide of the sintered body (the β-type silicon carbide in the pulverized sintered body). In comparison with the ratio), it was considered that the sample S21 had the same degree within the measurement error range, and the β-type silicon carbide derived from the aggregate remained almost as it was. In other samples S22 to S28, since the latter is a little smaller, it is considered that a part of the β-type silicon carbide derived from the aggregate is α-ized, but the β-type silicon carbide has a considerable proportion. It turns out that it remains.

FIG. 1 shows the specific resistance value ρ _Th (Ω · cm) at a temperature of 500 ° C. as a specific resistance value ρ at room temperature with respect to the ratio of β-type silicon carbide in the entire silicon carbide of the sintered body that has undergone the high resistance phase forming step. _The graph which plotted the value "(rho) _Th / (rho) _Tn " divided _{| segmented by Tn} ((omega _| ohm) * cm) is shown. From FIG. 1, it can be seen that “ρ _Th / ρ _Tn ” increases as the proportion of β-type silicon carbide in the entire silicon carbide of the sintered body increases, indicating a substantially linear relationship. This is the same result as in the case where β-type silicon carbide is formed in the phase of silicon carbide by reaction generation in the applicant's past examination (see Patent Document 3). Therefore, even if the β-type silicon carbide contained in the sintered body is derived from conductive coarse particles, the temperature of the specific resistance value depends on the proportion of β-type silicon carbide in the entire silicon carbide of the sintered body. It was confirmed that the dependence can be changed, and that the temperature dependence of the specific resistance value can be reduced as the proportion of β-type silicon carbide is increased. In this embodiment, the proportion of β-type silicon carbide in the entire silicon carbide of the sintered body can be increased to at least 66%, and “ρ _Th / ρ _Tn ” at that time is a large value of about 0.4. there were.

From the applicant's experience, it has been found that, in a general application of a conductive silicon carbide sintered body, when “ρ _Th / ρ _Tn ” is smaller than 0.1, it is difficult to control the current value. Therefore, when the ratio of β-type silicon carbide when “ρ _Th / ρ _Tn ” is 0.1 is read from the linear approximation curve in the graph of FIG. 1, it is about 15% as shown in FIG. Therefore, by setting the ratio of β-type silicon carbide in the entire silicon carbide of the conductive silicon carbide sintered body to 15% or more, the temperature dependence of the specific resistance value can be within a practical range. In addition, this result is the result when β-type silicon carbide is generated in the silicon carbide phase by reaction generation in the applicant's past examination (see Patent Document 3) (β-type in the entire silicon carbide of the sintered body). The result was almost the same as the ratio of silicon carbide of 14% or more.

  Furthermore, with respect to Samples S21 to S28 and Sample R29, the change in specific resistance value accompanying oxidation was evaluated as “oxidation resistance”. Each sample is subjected to an oxidation test in which it is heated in an air atmosphere at a temperature of 1000 ° C., and a specific resistance value is measured at a predetermined time interval by the same method as described above. The specific resistance value change rate (%) was determined. For each sample, when the specific resistance value change rate after the oxidation time of 128 hours is less than 110%, it is evaluated as “◯” as having good oxidation resistance (small change in specific resistance value accompanying oxidation) When the specific resistance change rate after the oxidation time of 128 hours was 110% or more, the oxidation resistance was evaluated as “x” because the oxidation resistance was poor. The results are also shown in Table 1.

  When heated for 128 hours at a temperature of 1000 ° C. in an air atmosphere, the oxidation of the silicon carbide sintered body proceeds considerably. Nevertheless, the specific resistance value of each sample hardly changed even after the oxidation time of 128 hours (the specific resistance value change rate was close to 100%), and the oxidation resistance was good. From this, it can be seen that in any sample, the change in specific resistance value due to oxidation is effectively suppressed due to the presence of the high resistance phase.

  In the applicant's past examination (see Patent Document 3), when the sintered body is subjected to a high resistance phase forming process for suppressing the change in specific resistance value due to oxidation due to the presence of a high resistance phase, a reaction is generated. Even if an attempt is made to increase the proportion of β-type silicon carbide due to the effect of reducing the temperature dependence of the specific resistance value, a part of β-type silicon carbide in the silicon carbide phase that has been transformed into α-type was there. Specifically, in order to achieve harmony between the action of effectively suppressing the change in specific resistance value due to oxidation and the action of reducing the temperature dependence of the specific resistance value, β-type in the entire silicon carbide of the sintered body The proportion of silicon carbide had an upper limit within the range of 34% to 39%. On the other hand, in this embodiment in which the proportion of β-type silicon carbide in the entire silicon carbide of the sintered body is adjusted with coarse particles of conductive β-type silicon carbide, the change in specific resistance value accompanying oxidation is effectively suppressed. However, the proportion of β-type silicon carbide can be increased to at least 66%.

  As described above, according to the manufacturing method of this example, by adjusting the ratio of β-type silicon carbide in the entire silicon carbide of the sintered body by the β-type aggregate to be included in the starting material, the specific resistance value Conductive silicon carbide sintered bodies with different temperature dependence can be manufactured, and the presence of a high resistance phase effectively suppresses the change in specific resistance value due to oxidation while carbonizing the sintered body. The temperature dependence of the specific resistance value can be greatly reduced by increasing the proportion of β-type silicon carbide in the entire silicon.

  In addition, since the coarse-grained β-type silicon carbide is difficult to transfer to α-type silicon carbide in the high resistance phase forming step even in the reaction firing step, the β-type aggregate contained in the starting material becomes the final sintered body. It remains easy to remain in β type. In addition, the reaction firing step is performed at a high temperature of 2100 ° C., and almost no β-type silicon carbide is generated in the phase of silicon carbide generated by reaction. Therefore, the ratio of β-type silicon carbide in the entire silicon carbide of the sintered body can be easily adjusted to “intended numerical value” by the ratio of β-type aggregate to be included in the starting material. In addition, the reaction baking process can be performed at a high temperature without intending the formation of β-type silicon carbide in the silicon carbide phase that is generated by the reaction, and the high resistance phase forming process without concern about the alpha formation in the high resistance phase forming process Therefore, the intended conductive silicon carbide sintered body can be efficiently produced.

  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 embodiment, in order to change the proportion of β-type aggregate in the aggregate contained in the starting material, α-type aggregate (non-conductive) is used as another aggregate to be mixed with β-type aggregate. The case of using is illustrated. It is not limited to this, The coarse particle which consists of materials other than silicon carbide can be used as another aggregate.

Claims (2)

Silicon carbide having a nitrogen concentration lower than the average concentration of nitrogen in the conductive phase on at least the outer surface of a sintered body formed from a starting material including a conductive phase material that is a phase of silicon carbide containing nitrogen as a dopant By forming a high resistance phase that is a phase of the above, a sintered body in which a change in specific resistance value due to oxidation is smaller than that of the sintered body without the high resistance phase is manufactured,
In the method for producing a conductive silicon carbide sintered body for producing a sintered body having different temperature dependence of specific resistance value, depending on the ratio of β-type silicon carbide in the entire sintered body,
By adding β-type aggregate which is coarse particles having a particle diameter of 5 μm to 50 μm made of conductive β-type silicon carbide to the starting material, the ratio of β-type silicon carbide in the entire sintered body can be changed. And a ratio of β-type silicon carbide in the high resistance phase as compared with a sintered body formed from the starting material not containing the β-type aggregate. Manufacturing method. It is a sintered body containing a conductive phase that is a phase of silicon carbide containing nitrogen as a dopant,
At least the outer surface of the sintered body has a high resistance phase that is a silicon carbide phase having a nitrogen concentration lower than the average concentration of nitrogen in the conductive phase;
The conductive phase is made of conductive β-type silicon carbide and contains coarse particles having a particle diameter of 5 μm to 50 μm.

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