JP6291446B2 - Method for producing conductive silicon carbide sintered body - Google Patents
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- 229910010271 silicon carbide Inorganic materials 0.000 title claims description 234
- HBMJWWWQQXIZIP-UHFFFAOYSA-N silicon carbide Chemical compound [Si+]#[C-] HBMJWWWQQXIZIP-UHFFFAOYSA-N 0.000 title claims description 221
- 238000004519 manufacturing process Methods 0.000 title claims description 26
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 claims description 118
- 229910052757 nitrogen Inorganic materials 0.000 claims description 47
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- 229910052710 silicon Inorganic materials 0.000 claims description 30
- 239000010703 silicon Substances 0.000 claims description 30
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 claims description 29
- 229910001873 dinitrogen Inorganic materials 0.000 claims description 24
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims description 22
- 230000001590 oxidative effect Effects 0.000 claims description 21
- 229910052799 carbon Inorganic materials 0.000 claims description 18
- 229910052581 Si3N4 Inorganic materials 0.000 claims description 15
- HQVNEWCFYHHQES-UHFFFAOYSA-N silicon nitride Chemical compound N12[Si]34N5[Si]62N3[Si]51N64 HQVNEWCFYHHQES-UHFFFAOYSA-N 0.000 claims description 15
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- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 description 12
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- 239000000377 silicon dioxide Substances 0.000 description 6
- 239000007789 gas Substances 0.000 description 5
- 238000002441 X-ray diffraction Methods 0.000 description 4
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- 238000007561 laser diffraction method Methods 0.000 description 4
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Description
本発明は、導電性炭化珪素質焼結体の製造方法に関するものである。 The present invention relates to the production how the conductive silicon carbide sintered body.
炭化珪素は、熱伝導率が高いことに加えて熱膨張率が小さいことから耐熱衝撃性に優れるため、高温下で使用されるフィルタ、触媒担体、熱交換体などの基体として適している。また、高純度の炭化珪素は電気抵抗が高く絶縁体に近いが、導電性が付与された炭化珪素質セラミックスは、通電により発熱させる自己発熱型の構造体として使用することが可能である。本出願人は過去に、炭化珪素を珪素源及び炭素源から反応生成させる際に窒素をドープすることにより、導電性が付与された炭化珪素質セラミックス焼結体を製造する方法を提案している（例えば、特許文献１参照）。 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. Further, high-purity silicon carbide has high electrical resistance and is close to an insulator, but silicon carbide-based ceramics imparted with conductivity can be used as a self-heating structure that generates heat when energized. 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).
炭化珪素は、酸素の存在下で高温に加熱されると、酸化してしまうという問題がある。炭化珪素の酸化により生成した二酸化珪素の皮膜で炭化珪素の表面が被覆されると、それ以上の酸化がある程度は抑制されると言われているが、それでは酸化の抑制としては不十分であるのが実情である。本出願人が過去に詳細に検討しているように、導電性炭化珪素質セラミックスの表面に二酸化珪素が生成すると、比抵抗値が変化する（例えば、特許文献２参照）。酸化によって焼結体の表面に形成される二酸化珪素の相は電気抵抗が大きいため、酸化の進行に伴って、炭化珪素質セラミックス焼結体の比抵抗値が増大する。そこで、高温下での使用を継続しても、比抵抗値が一定に保持される導電性炭化珪素質セラミックス焼結体が要請されていた。 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. As the present applicant has studied in detail in the past, when silicon dioxide is generated on the surface of the conductive silicon carbide ceramic, the specific resistance value changes (for example, see Patent Document 2). 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 silicon carbide based ceramic sintered body increases with the progress of oxidation. Therefore, there has been a demand for a conductive silicon carbide based ceramic sintered body that maintains a specific resistance value constant even if the use at a high temperature is continued.
一方、導電性炭化珪素質セラミックス焼結体は、温度の上昇に伴って電気抵抗が大きく低下するＮＴＣ特性を有し、比抵抗値の温度依存性が高い。そのため、高温下で比抵抗値が小さくなり過ぎ、電流値が過大となって制御が困難となったり、過電流による過熱により焼結体が損傷したりする問題があり、比抵抗値の温度依存性が低減された導電性炭化珪素質セラミックス焼結体が要請されていた。 On the other hand, the conductive silicon carbide based ceramic sintered body has NTC characteristics in which the electrical resistance greatly decreases with an increase in temperature, and the temperature dependence of the specific resistance value is high. Therefore, there is a problem that the specific resistance value becomes too small at high temperature, the current value becomes excessive and difficult to control, and the sintered body is damaged by overheating due to overcurrent. There has been a demand for a conductive silicon carbide ceramic sintered body having reduced properties.
そこで、本発明は、上記の実情に鑑み、酸化による比抵抗値の変化が抑制されていると共に、比抵抗値の温度依存性が低減されている導電性炭化珪素質焼結体を、製造することができる導電性炭化珪素質焼結体の製造方法の提供を、課題とするものである。 Therefore, in view of the above circumstances, the present invention manufactures a conductive silicon carbide sintered body in which the change in specific resistance value due to oxidation is suppressed and the temperature dependence of the specific resistance value is reduced. providing manufacturing how the conductive silicon carbide sintered body which can, it is an issue.
上記の課題を解決するため、本発明にかかる導電性炭化珪素質焼結体の製造方法（単に、「製造方法」と称することがある）は、
「ドーパントとして窒素を含む炭化珪素の相である導電性相を含む焼結体において、少なくとも前記導電性相の外側に、前記導電性相における窒素の平均濃度より窒素の濃度が低い炭化珪素の相である高抵抗相を形成することにより、酸化に伴う比抵抗値の変化が前記高抵抗相のない焼結体に比べて小さい焼結体を製造すると共に、
焼結体におけるβ型炭化珪素の割合により、比抵抗値の温度依存性を異ならせた焼結体を製造する」ものである。
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”)
“In a sintered body containing a conductive phase that is a phase of silicon carbide containing nitrogen as a dopant, a silicon carbide phase having a nitrogen concentration lower than the average concentration of nitrogen in the conductive phase at least outside the conductive phase. By forming a high resistance phase that is, a change in specific resistance value accompanying oxidation is smaller than that of the sintered body without the high resistance phase,
A sintered body in which the temperature dependence of the specific resistance value is varied depending on the ratio of β-type silicon carbide in the sintered body is manufactured.
高純度の炭化珪素は電気絶縁体に近いが、ドーパントとして窒素を含むことによりｎ型半導体となる。このような炭化珪素の焼結体を酸素が存在する雰囲気で高温にて使用すると、炭化珪素の酸化によって焼結体の表面に二酸化珪素の相が生成する。炭化珪素の酸化により同一モル数の二酸化珪素が生成すると質量が増加するため、図５（ａ）に示すように、高温下での使用時間に伴う質量増加から、炭化珪素の酸化が進行していることが分かる。二酸化珪素の相は電気抵抗が大きいため、図５（ｂ）に示すように、酸化の進行に伴い焼結体全体の比抵抗値が増大してしまう。なお、図５は、従来の炭化珪素焼結体Ａ，Ｂを、空気雰囲気で１０００℃の温度で加熱した酸化試験の結果であり、図５（ａ）の質量増加率（％）は、酸化試験に供する前の初期質量からの質量増加分の初期質量に対する割合であり、図５（ｂ）の比抵抗値変化率（％）は、酸化試験に供する前の初期の比抵抗値を１００％とした値である。また、炭化珪素焼結体Ａ，Ｂは、何れも珪素源である窒化珪素と炭素源である炭素質物質からなる炭化珪素生成原料に、骨材としての炭化珪素を混合した混合原料から製造した焼結体であり、原料組成及び原料粉末の粒度が相違している。 High-purity silicon carbide is close to an electrical insulator, but becomes an n-type semiconductor by containing nitrogen as a dopant. When such a silicon carbide sintered body is used at a high temperature in an oxygen-containing atmosphere, a silicon dioxide phase is generated on the surface of the sintered body due to oxidation of silicon carbide. Since the mass increases when silicon dioxide having the same number of moles is generated by the oxidation of silicon carbide, as shown in FIG. 5 (a), the oxidation of silicon carbide proceeds from the increase in mass with the use time at high temperature. I understand that. Since the phase of silicon dioxide has a large electric resistance, as shown in FIG. 5B, the specific resistance value of the entire sintered body increases with the progress of oxidation. FIG. 5 shows the results of an oxidation test in which conventional silicon carbide sintered bodies A and B were heated in an air atmosphere at a temperature of 1000 ° C. The mass increase rate (%) in FIG. The ratio of the increase in mass from the initial mass before being subjected to the test to the initial mass, and the specific resistance value change rate (%) in FIG. 5B is 100% of the initial specific resistance value before being subjected to the oxidation test. It is a value. Silicon carbide sintered bodies A and B were both manufactured from a mixed raw material in which silicon carbide as an aggregate was mixed with a silicon carbide forming raw material made of silicon nitride as a silicon source and a carbonaceous material as a carbon source. It is a sintered body, and the raw material composition and the particle size of the raw material powder are different.
このように酸化に伴い比抵抗値が増大してしまう従来の炭化珪素焼結体とは異なり、本製造方法により製造される導電性炭化珪素質焼結体は、少なくとも導電性相の外側に、ドーパントとして窒素を含む炭化珪素の相であり、導電性相より窒素の平均濃度が低い炭化珪素の相を有している。この相は、窒素の濃度が低いことにより自由電子の数が少なく、導電性相より電気抵抗が大きいため、本発明では「高抵抗相」と称している。このように、元々電気抵抗が大きい相は、焼結体全体の電気伝導性に対する寄与度が小さい。 Unlike the conventional silicon carbide sintered body in which the specific resistance value increases with oxidation in this way, the conductive silicon carbide sintered body produced by this production method is at least outside the conductive phase, It is a silicon carbide phase containing nitrogen as a dopant, and has a silicon carbide phase having an average nitrogen concentration lower than that of the conductive phase. This phase is referred to as a “high resistance phase” in the present invention 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. As described above, the phase having a large electrical resistance originally has a small contribution to the electrical conductivity of the entire sintered body.
導電性相の外側に高抵抗相が形成されている構成の焼結体では、酸素が存在する雰囲気で使用されたときに、酸化されるのは高抵抗相である。元々焼結体全体の電気伝導性に対する寄与度が小さい高抵抗相が酸化された場合は、焼結体全体の電気伝導性に対する寄与度が大きい相、すなわち導電性相が酸化された場合に比べ、焼結体全体の電気伝導性に及ぼす影響は小さい。加えて、導電性相の外側に高抵抗相が存在することにより、電気伝導性への寄与度の大きい導電性相まで、酸化反応が及びにくい。従って、本製造方法により製造される導電性炭化珪素質焼結体は、酸素の存在する雰囲気で高温で継続して使用されても、比抵抗値が変化しにくい。酸化に伴う比抵抗値の増大を抑制しようとする場合、炭化珪素の酸化を抑制する手段を採用しようとするのが、当業者の通常の考え方である。これに対し、本構成の製造方法によれば、これと全く異なるアプローチによって、酸化に伴う比抵抗値の変化が抑制された導電性炭化珪素質焼結体を製造することができる。 In a sintered body having a structure in which a high resistance phase is formed outside the conductive phase, the high resistance phase is oxidized when used in an atmosphere in which oxygen is present. When the high resistance phase, which originally contributed little to the electrical conductivity of the entire sintered body, was oxidized, compared to the phase where the contribution to the electrical conductivity of the entire sintered body was large, that is, the conductive phase was oxidized. The influence on the electrical conductivity of the entire sintered body is small. In addition, the presence of the high resistance phase outside the conductive phase makes it difficult for the oxidation reaction to reach the conductive phase having a large contribution to electrical conductivity. Therefore, even if the conductive silicon carbide sintered body produced by this production method is continuously used at a high temperature in an atmosphere in which oxygen is present, the specific resistance value hardly changes. When it is intended to suppress an increase in specific resistance value due to oxidation, it is a general idea of those skilled in the art to employ means for suppressing the oxidation of silicon carbide. On the other hand, according to the manufacturing method of this configuration, a conductive silicon carbide sintered body in which a change in specific resistance value due to oxidation is suppressed can be manufactured by a completely different approach.
加えて、検討の結果、詳細は後述するように、焼結体におけるβ型炭化珪素の割合を増加させることにより、比抵抗値の温度依存性を低減させることができることが見出された。従って、本製造方法によれば、高抵抗相の存在によって酸化に伴う比抵抗値の変化が抑制されていると共に、β型炭化珪素の割合によって比抵抗値の温度依存性が調整された導電性炭化珪素質焼結体を、製造することができる。なお、導電性相は高抵抗相に比べて焼結体の比抵抗値に対する寄与度が大きいため、導電性相におけるβ型炭化珪素の割合が、より重要である。 In addition, as a result of the study, it was found that the temperature dependence of the specific resistance value can be reduced by increasing the proportion of β-type silicon carbide in the sintered body, as will be described in detail later. Therefore, according to the present manufacturing method, the change in specific resistance value due to oxidation is suppressed by the presence of the high resistance phase, and the temperature dependency of the specific resistance value is adjusted by the ratio of β-type silicon carbide. A silicon carbide based sintered body can be produced. Since the conductive phase contributes more to the specific resistance value of the sintered body than the high resistance phase, the proportion of β-type silicon carbide in the conductive phase is more important.
なお、本製造方法により製造される導電性炭化珪素質焼結体は、少なくとも導電性相の外側に高抵抗相が形成されているものであれば、導電性相の外側ではない部分に導電性相より窒素の平均濃度が低い高抵抗の相を有していても構わない。例えば、導電性相の内部に骨材粒子を含む場合に、骨材粒子が非導電性であれば、この骨材粒子の相は導電性相の外側ではない部分に存在する高抵抗の相である。また、導電性相も単相である必要はなく、例えば、窒素の濃度の異なる複数の導電性相を有していてもよい。複数の導電性相を有する場合、「導電性相における窒素の平均濃度」は、複数の導電性相を総合して平均した窒素の濃度を指すものとする。なお、「導電性炭化珪素質焼結体」は、「導電性炭化珪素質セラミックス焼結体」と同意として使用している。また、ここでは、比抵抗値が１０００Ωｃｍ以上の場合に非導電性と称して、導電性と区別する。 In addition, the conductive silicon carbide sintered body manufactured by this manufacturing method has a conductive property in a portion that is not outside the conductive phase as long as a high resistance phase is formed at least outside the conductive phase. You may have a high resistance phase in which the average concentration of nitrogen is lower than the phase. For example, when aggregate particles are included in the conductive phase, if the aggregate particles are non-conductive, the aggregate particle phase is a high-resistance phase present in a portion not outside the conductive phase. is there. Also, the conductive phase need not be a single phase, and for example, it may have a plurality of conductive phases with different concentrations of nitrogen. In the case of having a plurality of conductive phases, the “average concentration of nitrogen in the conductive phase” refers to the concentration of nitrogen obtained by averaging the plurality of conductive phases. Note that “conductive silicon carbide sintered body” is used as an agreement with “conductive silicon carbide ceramic sintered body”. Further, here, when the specific resistance value is 1000 Ωcm or more, it is referred to as non-conductive and distinguished from conductive.
本発明にかかる導電性炭化珪素質焼結体の製造方法は、上記構成に加え、
「前記導電性相の外側の前記高抵抗相は、前記導電性相を含む焼結体を、実質的に窒素ガスを含まない非酸化性雰囲気で加熱する高抵抗相形成工程により形成する」ものとすることができる。
In addition to the above configuration, the method for producing a conductive silicon carbide sintered body according to the present invention includes:
“The high resistance phase outside the conductive phase is formed by a high resistance phase forming step of heating the sintered body containing the conductive phase in a non-oxidizing atmosphere substantially free of nitrogen gas”. It can be.
「実質的に窒素ガスを含まない非酸化性雰囲気」は、アルゴンやヘリウム等の希ガス雰囲気とすることができる。この場合、雰囲気中の窒素ガスの濃度は理想的にはゼロであるが、窒素ガスの濃度は５０００ｐｐｍ未満であれば許容され、より好ましくは５００ｐｐｍ未満である。或いは、「実質的に窒素ガスを含まない非酸化性雰囲気」は、真空雰囲気とすることもできる。 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.
本発明者らは、ドーパントとして窒素を含む炭化珪素の相である導電性相を含む炭化珪素質セラミックスの焼結体を、実質的に窒素ガスを含まない非酸化性雰囲気で加熱することにより、一旦はドープされた窒素が焼結体から排出され、導電性相の外側に、窒素の濃度が低い炭化珪素の相が形成されることを見出した。この高抵抗相形成工程は、通常の焼成設備で行うことができ、特殊な設備や工程を要しないため、非常に簡易に高抵抗相を形成することができる。 The present inventors heated a sintered body of a silicon carbide based ceramic containing a conductive phase that is a phase of silicon carbide containing nitrogen as a dopant in a non-oxidizing atmosphere substantially containing no nitrogen gas, It has been found that once doped nitrogen is discharged from the sintered body, a silicon carbide phase having a low nitrogen concentration is formed outside the conductive phase. This high resistance phase forming step can be performed with ordinary firing equipment and does not require any special equipment or process, so that the high resistance phase can be formed very easily.
本発明にかかる導電性炭化珪素質焼結体の製造方法は、上記構成に加え、
「前記導電性相を含む焼結体は、珪素源及び炭素源から炭化珪素を反応生成させる反応焼成工程を経て得るものであり、
焼結体におけるβ型炭化珪素の割合は、前記反応焼成工程の焼成温度によって変化させる」ものとすることができる。
In addition to the above configuration, the method for producing a conductive silicon carbide sintered body according to the present invention includes:
“The sintered body containing the conductive phase is obtained through a reaction firing step in which silicon carbide is reacted and generated from a silicon source and a carbon source.
The ratio of β-type silicon carbide in the sintered body can be changed according to the firing temperature in the reaction firing step ”.
詳細は後述するように、炭化珪素におけるβ型炭化珪素の割合を変化させる方法として、本発明者らは複数の方法を想到しているが、その中でも、珪素源及び炭素源から炭化珪素を反応生成させる際の焼成温度によってβ型炭化珪素の割合を調整する方法は、極めて簡易である。 As will be described in detail later, the present inventors have conceived a plurality of methods as a method for changing the proportion of β-type silicon carbide in silicon carbide. Among them, silicon carbide is reacted from a silicon source and a carbon source. The method of adjusting the proportion of β-type silicon carbide depending on the firing temperature at the time of generation is very simple.
本発明にかかる導電性炭化珪素質焼結体の製造方法は、上記構成に加え、
「前記高抵抗相形成工程では、β型炭化珪素の一部をα型炭化珪素に転移させて前記高抵抗相を形成する」ものとすることができる。
In addition to the above configuration, the method for producing a conductive silicon carbide sintered body according to the present invention includes:
“In the high resistance phase forming step, a part of β-type silicon carbide is transferred to α-type silicon carbide to form the high resistance phase”.
検討の結果、詳細は後述するように、炭化珪素におけるβ型炭化珪素の割合が高いほど、比抵抗値の温度依存性が低下することが分かった。つまり、比抵抗値の温度依存性を低下させるという目的においては、導電性炭化珪素質焼結体におけるβ型炭化珪素の割合は高いほど望ましい。そして、酸化に伴う比抵抗値の増大を抑制する目的で高抵抗相を形成するのに際し、窒素がドープされたβ型炭化珪素の焼結体から、結晶構造をβ型炭化珪素に維持したまま窒素を排出させることも可能である。しかしながら、本構成では、高抵抗相を形成するのに際し、β型炭化珪素の一部をα型炭化珪素に転移させる、すなわち、導電性相の外側においてβ型炭化珪素の一部をα型炭化珪素に転移させると共に窒素を排出させて高抵抗相とする手段を採用している。高抵抗相は導電性相に比べて比抵抗値に及ぼす寄与度が小さいため、比抵抗値の温度依存性に対する寄与度も小さく、β型炭化珪素の割合を高める意義が小さいからである。そして、α型炭化珪素はβ型炭化珪素より高温で安定であるため、β型炭化珪素の一部をα型炭化珪素に転移させる高抵抗相形成工程は、β型炭化珪素のまま窒素を排出させる場合より、高温で行うことができる。 As a result of the study, as will be described in detail later, it has been found that the higher the proportion of β-type silicon carbide in silicon carbide, the lower the temperature dependence of the specific resistance value. That is, for the purpose of reducing the temperature dependence of the specific resistance value, it is desirable that the proportion of β-type silicon carbide in the conductive silicon carbide sintered body is higher. And, in forming a high resistance phase for the purpose of suppressing an increase in specific resistance value due to oxidation, a crystal structure is maintained in β-type silicon carbide from a β-type silicon carbide sintered body doped with nitrogen. Nitrogen can also be discharged. However, in this configuration, when forming the high resistance phase, a part of β-type silicon carbide is transferred to α-type silicon carbide, that is, a part of β-type silicon carbide is α-type carbonized outside the conductive phase. A means for transferring to silicon and exhausting nitrogen to form a high resistance phase is employed. This is because the high resistance phase has a smaller contribution to the specific resistance value than the conductive phase, so the contribution to the temperature dependence of the specific resistance value is also small, and the significance of increasing the proportion of β-type silicon carbide is small. Since α-type silicon carbide is more stable at higher temperatures than β-type silicon carbide, the high resistance phase forming step of transferring a part of β-type silicon carbide to α-type silicon carbide exhausts nitrogen while maintaining β-type silicon carbide. It can be performed at a higher temperature than in the case of making it.
従って、本構成の製造方法によれば、実質的に窒素を含まない非酸化性雰囲気で加熱する高抵抗相形成工程を、より高温で行うことが可能となるため、効率的に窒素を排出して、簡易かつ短時間で高抵抗相を形成することができる。なお、炭化珪素は２３５０℃を超えると昇華するため、高温で行う高抵抗相形成工程の加熱温度は、２１００℃〜２３００℃とすることができる。 Therefore, according to the manufacturing method of this configuration, the high resistance phase forming step of heating in a non-oxidizing atmosphere substantially not containing nitrogen can be performed at a higher temperature, so that nitrogen is efficiently discharged. Thus, a high resistance phase can be formed easily and in a short time. In addition, since silicon carbide sublimates when it exceeds 2350 degreeC, the heating temperature of the high resistance phase formation process performed at high temperature can be 2100 degreeC-2300 degreeC.
次に、本発明にかかる導電性炭化珪素質焼結体の製造方法により製造される導電性炭化珪素質焼結体は、
「ドーパントとして窒素を含む炭化珪素の相である導電性相を含む焼結体であり、
少なくとも前記導電性相の外側に、前記導電性相における窒素の平均濃度より窒素の濃度が低い炭化珪素の相である高抵抗相が形成されており、
炭化珪素におけるβ型炭化珪素の割合は、前記高抵抗相より前記導電性相の方が大きい」ものである。
Next, the conductive silicon carbide sintered body produced by the method for producing a conductive silicon carbide 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,
A high resistance phase, which is a silicon carbide phase having a nitrogen concentration lower than the average concentration of nitrogen in the conductive phase, is formed at least outside the conductive phase,
The ratio of β-type silicon carbide in silicon carbide is larger in the conductive phase than in the high resistance phase ”.
これは、上記の製造方法のうち、「高抵抗相形成工程では、β型炭化珪素の一部をα型炭化珪素に転移させて前記高抵抗相を形成する」製造方法によって製造された導電性炭化珪素質焼結体の構成である。この製造方法では、導電性相の外側においてβ型炭化珪素の一部がα型炭化珪素に転移して高抵抗相となるため、高抵抗相はα型炭化珪素の割合が大きな相であり、炭化珪素におけるβ型炭化珪素の割合は高抵抗相より導電性相の方が大きなものとなる。つまり、本構成は、高抵抗相形成工程を高温で行うことにより、効率的に窒素を排出し、簡易かつ短時間で高抵抗相を形成することができる製造方法によって、製造される導電性炭化珪素質焼結体の構成である。 This is because of the conductivity produced by the above-mentioned production method, “in the high resistance phase forming step, a part of β-type silicon carbide is transferred to α-type silicon carbide to form the high resistance phase”. This is a structure of a silicon carbide sintered body. In this manufacturing method, since a part of β-type silicon carbide is transferred to α-type silicon carbide outside the conductive phase to become a high-resistance phase, the high-resistance phase is a phase with a large proportion of α-type silicon carbide, The proportion of β-type silicon carbide in silicon carbide is larger in the conductive phase than in the high resistance phase. In other words, this configuration is a conductive carbonization produced by a production method that can efficiently discharge nitrogen and form a high resistance phase in a short time by performing the high resistance phase formation step at a high temperature. This is a structure of a silicon-based sintered body.
以上のように、本発明の効果として、酸化による比抵抗値の変化が抑制されていると共に、比抵抗値の温度依存性が低減されている導電性炭化珪素質焼結体を、製造することができる導電性炭化珪素質焼結体の製造方法を、提供することができる。 As described above, as an effect of the present invention, a conductive silicon carbide sintered body in which the change in specific resistance value due to oxidation is suppressed and the temperature dependence of the specific resistance value is reduced is manufactured. manufacturing how the conductive silicon carbide sintered body which can, can be provided.
以下、本発明の一実施形態である導電性炭化珪素質焼結体の製造方法、及び、該製造方法により製造される導電性炭化珪素質焼結体について説明する。本実施形態の導電性炭化珪素質焼結体の製造方法は、ドーパントとして窒素を含む炭化珪素の相である導電性相を含む焼結体を、実質的に窒素ガスを含まない非酸化性雰囲気で加熱する高抵抗相形成工程により、導電性相における窒素の平均濃度より窒素の濃度が低い炭化珪素の相である高抵抗相を導電性相の外側に形成することにより、酸化に伴う比抵抗値の変化が高抵抗相のない焼結体に比べて小さい焼結体を製造すると共に、焼結体におけるβ型炭化珪素の割合により、比抵抗値の温度依存性を異ならせた焼結体を製造するものである。 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. In the method for producing a conductive silicon carbide sintered body according to the present embodiment, a sintered body containing a conductive phase that is a phase of silicon carbide containing nitrogen as a dopant is used in a non-oxidizing atmosphere substantially free of nitrogen gas. By forming a high resistance phase, which is a silicon carbide phase having a nitrogen concentration lower than the average concentration of nitrogen in the conductive phase, by forming a high resistance phase outside the conductive phase by a high resistance phase forming step heated at Sintered body in which the change in value is smaller than that of a sintered body without a high resistance phase, and the temperature dependence of the specific resistance value is varied depending on the proportion of β-type silicon carbide in the sintered body Is to be manufactured.
ドーパントとして窒素を含む炭化珪素の相である導電性相を含む焼結体は、例えば、炭化珪素粉末を原料として成形体を得る成形工程と、窒素ガスを含む非酸化性雰囲気で焼成する焼成工程を経ることにより得ることができる。この場合、加圧下で焼成工程を行えば、雰囲気中の窒素を効率良く焼結体中にドープすることができる。また、原料の炭化珪素粉末に微細粒子を含めれば、微細粒子が焼結する際に、雰囲気中の窒素を効率良くドープすることができる。なお、窒素ガスを含む非酸化性雰囲気は、窒素ガス１００％雰囲気、アルゴンやヘリウム等の希ガスと窒素ガスとの混合雰囲気とすることができる。なお、成形工程は、ハニカム構造の成形体を成形する工程とすることができる。
The sintered body containing a conductive phase that is a phase of silicon carbide containing nitrogen as a dopant includes, for example, a forming step of obtaining a formed body using silicon carbide powder as a raw material, and a firing step of firing in a non-oxidizing atmosphere containing nitrogen gas It can obtain by going through. 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 raw material silicon carbide powder, nitrogen in the atmosphere can be efficiently doped when the fine particles are sintered. Note that the non-oxidizing atmosphere containing nitrogen gas can be a
或いは、ドーパントとして窒素を含む炭化珪素の相である導電性相を含む焼結体は、珪素源及び炭素源を含む原料で成形した成形体を、窒素ガスを含む非酸化性雰囲気で焼成することにより炭化珪素を反応生成させる反応焼成工程を経ることにより、得ることができる。ここで、「珪素源」としては、窒化珪素や珪素（単体）を使用可能である。「炭素源」としては、黒鉛、石炭、コークス、木炭、カーボンブラックなどの炭素質物質を使用可能である。化学量論的には珪素及び炭素のモル比（Ｓｉ／Ｃ）が１のときに過不足なく炭化珪素が生成するが、Ｓｉ／Ｃが０．８〜１．２であれば、珪素及び炭素の過剰分または不足分が少なく、望ましい。 Or the sintered compact containing the electroconductive phase which is a phase of silicon carbide containing nitrogen as a dopant is fired in a non-oxidizing atmosphere containing nitrogen gas formed from a raw material containing a silicon source and a carbon source. It can be obtained through a reaction baking step in which silicon carbide is produced by reaction. Here, as the “silicon source”, silicon nitride or silicon (simple substance) can be used. 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.
珪素源として窒化珪素を使用する場合は、炭化珪素の反応生成に伴い窒化珪素の分解により発生した窒素も、反応生成する炭化珪素にドープされるため、導電性相の窒素の濃度を大きなものとし、導電性相の電気伝導性をより高めることができる。これにより、少なくとも導電性相の外側に高抵抗相を形成することにより、焼結体において電気伝導性に寄与できる体積が減少しても、焼結体全体としての比抵抗値が増大するおそれを低減することができる。 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 at least outside the conductive phase, 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.
或いは、珪素源として窒化珪素を使用した場合は、窒化珪素の分解により発生する窒素のみをドーパントとし、反応焼成工程における雰囲気は窒素ガスを含まない非酸化性雰囲気とすることができる。窒素ガスを含まない非酸化性雰囲気は、アルゴンやヘリウム等の希ガス雰囲気、真空雰囲気とすることができる。 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.
また、上記の複数の製造方法において、成形体を得る原料には、骨材となる粒子を含有させることができる。骨材となる粒子は、炭化珪素であってもその他の材料であってもよく、導電性であっても非導電性であってもよい。骨材粒子が炭化珪素である場合、骨材粒子はドーパントとして窒素を含むものであっても含まないものであっても良いが、窒素の濃度が導電性相における平均濃度より低い場合、骨材粒子の相は「導電性相の外側にはない高抵抗相」に相当する。 Further, in the above-described plurality of production methods, the raw material for obtaining the molded body can contain particles that become aggregates. The particles used as the aggregate may be silicon carbide or other materials, and may be conductive or non-conductive. When the aggregate particles are silicon carbide, the aggregate particles may or may not contain nitrogen as a dopant, but if the concentration of nitrogen is lower than the average concentration in the conductive phase, the aggregate The phase of the particles corresponds to “a high resistance phase not outside the conductive phase”.
導電性相を含む焼結体を得る焼成工程と、導電性相を含む焼結体を実質的に窒素ガスを含まない非酸化性雰囲気で加熱する高抵抗相形成工程とは、被焼成体を搬送しながら焼成する連続焼成炉を使用して連続的に行うことができる。すなわち、連続焼成炉における搬送方向の上流側の雰囲気を窒素ガスを含む非酸化性雰囲気として、窒素がドープされた導電性相を有する焼結体を得ると共に、搬送方向の下流側の雰囲気を実質的に窒素ガスを含まない非酸化性雰囲気とし、導電性相の外側に高抵抗相を形成することができる。 A 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 are: It can carry out continuously using the continuous baking furnace baked while conveying. That is, the atmosphere on the upstream side in the transport 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 transport direction is substantially In particular, a non-oxidizing atmosphere containing no nitrogen gas can be formed, and a high resistance phase can be formed outside the conductive phase.
或いは、バッチ炉で成形体を焼成しながら、炉内に導入するガスを窒素ガスを含む非酸化性のガスから実質的に窒素ガスを含まない非酸化性のガスに切り替えることにより、導電性相を含む焼結体を得る焼成工程と導電性相の外側に高抵抗相を形成する高抵抗相形成工程とを、連続的に行うことができる。 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. The firing process for obtaining a sintered body including the high resistance phase forming process for forming the high resistance phase outside the conductive phase can be performed continuously.
また或いは、バッチ炉に成形体を収容して窒素ガスを含む非酸化性雰囲気で焼成し、導電性相を含む焼結体を得る焼成工程の後で、バッチ炉に焼結体を収容して実質的に窒素ガスを含まない非酸化性雰囲気で加熱する高抵抗相形成工程を、不連続に行うことができる。 Alternatively, after the firing step of storing the compact in a batch furnace and firing in a non-oxidizing atmosphere containing nitrogen gas to obtain a sintered body including a conductive phase, the sintered body is accommodated in the batch furnace. The high resistance phase forming step of heating in a non-oxidizing atmosphere substantially free of nitrogen gas can be performed discontinuously.
焼結体におけるβ型炭化珪素の割合は、珪素源及び炭素源から炭化珪素を生成させる反応焼成工程の焼成温度によって、変化させることができる。或いは、原料としてβ型炭化珪素の粉末を使用して成形体を成形し、これを焼成する際の温度によって、β型からα型に転移させる炭化珪素の量を変化させることにより、導電性相を含む焼結体の炭化珪素におけるβ型炭化珪素の割合を変化させることができる。また或いは、高抵抗相を形成させる高抵抗相形成工程の温度によって、β型からα型に転移させる炭化珪素の量を変化させることにより、高抵抗相を有する焼結体の炭化珪素におけるβ型炭化珪素の割合を変化させることができる。 The ratio of β-type silicon carbide in the sintered body can be changed by the firing temperature in the reaction firing step of generating silicon carbide from the silicon source and the carbon source. Alternatively, by forming a compact using a β-type silicon carbide powder as a raw material and changing the amount of silicon carbide transferred from the β-type to the α-type according to the temperature at which the powder is fired, the conductive phase is changed. The ratio of β-type silicon carbide in the silicon carbide of the sintered body containing can be changed. Alternatively, the β type in the silicon carbide of the sintered body having the high resistance phase is changed by changing the amount of silicon carbide to be transferred from the β type to the α type according to the temperature of the high resistance phase forming step for forming the high resistance phase. The proportion of silicon carbide can be changed.
炭化珪素を反応生成させる珪素源として窒化珪素を、炭素源としてグラファイトを使用し、珪素及び炭素のモル比（Ｓｉ／Ｃ）を１とした反応生成原料に、骨材としての粗大粒子を混合した混合原料から成形体を成形し（成形工程）、所定の温度で４時間焼成し、窒素がドープされた炭化珪素の相である導電性相を含む焼結体を得た（反応焼成工程）。骨材としては、窒素などはドープされていない非導電性のα型炭化珪素であり、レーザ回折法により測定された粒子径が約２０μｍの粗大粒子を使用した。 Silicon nitride is used as a silicon source for reaction generation of silicon carbide, graphite is used as a carbon source, and coarse particles as an aggregate are mixed with a reaction raw material in which the molar ratio of silicon and carbon (Si / C) is 1. A molded body was molded from the mixed raw material (molding process) and fired at a predetermined temperature for 4 hours to obtain a sintered body containing a conductive phase which is a silicon carbide phase doped with nitrogen (reaction firing process). As the aggregate, non-conductive α-type silicon carbide not doped with nitrogen or the like was used, and coarse particles having a particle diameter measured by a laser diffraction method of about 20 μm were used.
得られた焼結体を十分に粉砕してＸ線回折パターンを測定し、α型炭化珪素のピークとβ型炭化珪素のピークから、リートベルト法によりα型炭化珪素とβ型炭化珪素の比を求めた。ここで、結晶構造３Ｃのピークをβ型炭化珪素のピークとし、６Ｈ、１５Ｒ、４Ｈなど、３Ｃ以外の結晶構造の炭化珪素のピークをα型炭化珪素のピークとして解析した。珪素源である窒化珪素と骨材としてのα型炭化珪素の割合（質量比）が異なる複数の試料Ｓ１〜Ｓ４について、反応焼成工程の焼成温度と、焼結体の炭化珪素全体におけるα型炭化珪素とβ型炭化珪素の比の測定結果とを、表１に示す。 The obtained sintered body was sufficiently pulverized and the X-ray diffraction pattern was measured. From the α-type silicon carbide peak and the β-type silicon carbide peak, the ratio of α-type silicon carbide to β-type silicon carbide was determined by the Rietveld method. Asked. Here, the peak of the crystal structure 3C was analyzed as a β-type silicon carbide peak, and the peak of silicon carbide having a crystal structure other than 3C such as 6H, 15R, and 4H was analyzed as the peak of α-type silicon carbide. For a plurality of samples S1 to S4 having different proportions (mass ratio) of silicon nitride as a silicon source and α-type silicon carbide as an aggregate, the firing temperature in the reaction firing step and the α-type carbonization in the entire silicon carbide of the sintered body Table 1 shows the measurement results of the ratio of silicon to β-type silicon carbide.
骨材としてのα型炭化珪素の粗大粒子は、反応焼成後もそのままα型炭化珪素として存在すると考えられる。珪素源としての窒化珪素と炭素源としてのグラファイトから反応生成する炭化珪素が、仮に全てβ型炭化珪素であると仮定すると、骨材としてのα型炭化珪素と珪素源としての窒化珪素との質量比が１：１のとき、α型炭化珪素とβ型炭化珪素との比（α−ＳｉＣ：β−ＳｉＣ）は「５４：４６」である。反応焼成工程における焼成温度が１７００℃と低温である試料Ｓ１は、α−ＳｉＣ：β−ＳｉＣの比がこの計算値にほぼ等しく、珪素源としての窒化珪素と炭素源としてのグラファイトから反応生成した炭化珪素のほぼ全量が、β型炭化珪素であると考えられた。同様に、珪素源と炭素源とから反応生成する炭化珪素が全てβ型炭化珪素であると仮定すると、骨材としてのα型炭化珪素と珪素源としての窒化珪素との質量比が１：２のとき、α型炭化珪素とβ型炭化珪素との比（α−ＳｉＣ：β−ＳｉＣ）は「３７：６３」である。反応焼成工程における焼成温度が１７００℃と低温である試料Ｓ３は、α−ＳｉＣ：β−ＳｉＣの比がこの計算値にほぼ等しく、珪素源と炭素源とから反応生成した炭化珪素のほぼ全量が、β型炭化珪素であると考えられた。 The coarse particles of α-type silicon carbide as the aggregate are considered to exist as α-type silicon carbide as it is even after the reaction firing. Assuming that all silicon carbide produced by reaction from silicon nitride as the silicon source and graphite as the carbon source is β-type silicon carbide, the mass of α-type silicon carbide as the aggregate and silicon nitride as the silicon source When the ratio is 1: 1, the ratio of α-type silicon carbide to β-type silicon carbide (α-SiC: β-SiC) is “54:46”. Sample S1, which has a low firing temperature of 1700 ° C. in the reaction firing step, has a ratio of α-SiC: β-SiC substantially equal to this calculated value, and is produced by reaction from silicon nitride as the silicon source and graphite as the carbon source. Almost all of the silicon carbide was considered to be β-type silicon carbide. Similarly, assuming that all silicon carbide produced by reaction from a silicon source and a carbon source is β-type silicon carbide, the mass ratio of α-type silicon carbide as an aggregate to silicon nitride as a silicon source is 1: 2. In this case, the ratio of α-type silicon carbide to β-type silicon carbide (α-SiC: β-SiC) is “37:63”. In the sample S3 where the firing temperature in the reaction firing step is as low as 1700 ° C., the ratio of α-SiC: β-SiC is almost equal to this calculated value, and almost the total amount of silicon carbide produced by reaction from the silicon source and the carbon source is It was considered to be β-type silicon carbide.
そして、表１から分かるように、反応焼成工程における焼成温度が試料Ｓ１より高い試料Ｓ２では、α型炭化珪素の割合が試料Ｓ１より大きくなっており、同じく反応焼成工程における焼成温度が試料Ｓ３より高い試料Ｓ４では、α型炭化珪素の割合が試料Ｓ３より大きくなっている。これらの結果から、反応焼成工程における焼成温度が高くなると、反応生成したβ型炭化珪素の一部が、高温で安定なα型に転移すると考えられた。すなわち、珪素源と炭素源とから炭化珪素を反応生成させる反応焼成工程における焼成温度によって、得られる焼結体の炭化珪素全体におけるβ型炭化珪素の割合を、変化させることができると考えられた。 As can be seen from Table 1, in the sample S2 in which the firing temperature in the reaction firing step is higher than that in the sample S1, the proportion of α-type silicon carbide is larger than that in the sample S1, and the firing temperature in the reaction firing step is similarly higher than that in the sample S3. In the high sample S4, the proportion of α-type silicon carbide is larger than that in the sample S3. From these results, it was considered that when the firing temperature in the reaction firing step is increased, a part of the β-type silicon carbide produced by the reaction is transferred to the α-type which is stable at a high temperature. That is, it was considered that the ratio of β-type silicon carbide in the entire silicon carbide of the obtained sintered body could be changed by the firing temperature in the reaction firing step in which silicon carbide was produced by reacting silicon source and carbon source. .
次に、珪素源である窒化珪素と骨材としてのα型炭化珪素の割合（質量比）、反応焼成工程における焼成温度、実質的に窒素ガスを含まない非酸化性雰囲気で加熱する高抵抗相形成工程における加熱温度のうち、少なくとも一つを異ならせた試料Ｓ１１〜Ｓ１７の試験片（サイズ、４．５ｍｍ×４．５ｍｍ×４０ｍｍ）について、ＪＩＳ Ｒ１６５０−２に準拠して、比抵抗値を四端子法で測定した。温度５００℃における比抵抗値ρＴｈ（Ω・ｃｍ）を常温における比抵抗値ρＴｎ（Ω・ｃｍ）で除した値「ρＴｈ／ρＴｎ」を、表２に示す。この「ρＴｈ／ρＴｎ」は、比抵抗値の温度依存性の指標であり、値が大きいほど比抵抗値の温度依存性が低いことを示している。 Next, the ratio (mass ratio) of silicon nitride as the silicon source and α-type silicon carbide as the aggregate, the firing temperature in the reactive firing step, and the high resistance phase heated in a non-oxidizing atmosphere substantially free of nitrogen gas Regarding the test pieces (size, 4.5 mm × 4.5 mm × 40 mm) of samples S11 to S17 in which at least one of the heating temperatures in the forming process is different, the specific resistance value is determined in accordance with JIS R1650-2. Measured by the four probe method. Table 2 shows values “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. This “ρ Th / ρ Tn ” is an index of the temperature dependence of the specific resistance value, and indicates that the larger the value, the lower the temperature dependence of the specific resistance value.
また、各試料について、上記と同様にＸ線回折パターンから、リートベルト法によりα型炭化珪素とβ型炭化珪素との比（α−ＳｉＣ：β−ＳｉＣ）を求めた。α型炭化珪素とβ型炭化珪素との比としては、比抵抗値を測定した後の試験片を小片（サイズ、４．５ｍｍ×２ｍｍ×５ｍｍ）に加工し、未加工の表面にＸ線を照射して測定したＸ線回折パターンから「α−ＳｉＣ：β−ＳｉＣ（焼結体表面）」を求め、レーザ回折法により測定される粒子径が２０μｍとなるまで焼結体を乳鉢で粉砕した粉末について測定したＸ線回折パターンから「α−ＳｉＣ：β−ＳｉＣ（粉砕物）」を求めた。測定結果を表２にあわせて示す。 For each sample, the ratio of α-type silicon carbide to β-type silicon carbide (α-SiC: β-SiC) was determined from the X-ray diffraction pattern in the same manner as described above by the Rietveld method. As a ratio of α-type silicon carbide and β-type silicon carbide, the test piece after measuring the specific resistance value is processed into small pieces (size, 4.5 mm × 2 mm × 5 mm), and X-rays are applied to the unprocessed surface. “Α-SiC: β-SiC (sintered body surface)” was obtained from the X-ray diffraction pattern measured by irradiation, and the sintered body was pulverized in a mortar until the particle diameter measured by the laser diffraction method became 20 μm. “Α-SiC: β-SiC (ground product)” was determined from the X-ray diffraction pattern measured for the powder. The measurement results are also shown in Table 2.
対比のために、反応焼成工程の後、高抵抗相を形成する高抵抗相形成工程を行わなかった比較例の試料Ｒ１，Ｒ２についても、上記と同様に、「α−ＳｉＣ：β−ＳｉＣ（焼結体表面）」、「α−ＳｉＣ：β−ＳｉＣ（粉砕物）」、及び「ρＴｈ／ρＴｎ」を求めた。その結果を表２にあわせて示す。 For comparison, the samples R1 and R2 of the comparative example that did not perform the high resistance phase forming step for forming the high resistance phase after the reaction firing step were also “α-SiC: β-SiC ( The surface of the sintered body), “α-SiC: β-SiC (pulverized product)”, and “ρ Th / ρ Tn ” were obtained. The results are also shown in Table 2.
表２から分かるように、高抵抗相を形成する高抵抗相形成工程を行った試料Ｓ１１〜Ｓ１７は、高抵抗相形成工程を行わなかった試料Ｒ１，Ｒ２に比べて、焼結体表面においてα型炭化珪素の割合が高く、９０％以上がα型炭化珪素である。そして、試料Ｓ１１〜Ｓ１７について、「α−ＳｉＣ：β−ＳｉＣ」を焼結体表面と焼結体粉砕物とで比較すると、粉砕物の方がβ型炭化珪素の割合が大きくなっている。これらのことから、高抵抗相形成工程を２１００℃以上の温度で行うことにより焼結体における外層（外表面に近い層）に形成された高抵抗相は、反応焼成工程により生成したβ型炭化珪素がα型に転移した相であり、高抵抗相形成工程を経た焼結体においては、炭化珪素の殆どがα型炭化珪素である高抵抗相の内側に、β型炭化珪素が存在しているということができる。換言すれば、高抵抗相形成工程を経た焼結体では、β型炭化珪素が多く存在する導電性相の外側に、殆どがα型炭化珪素である高抵抗相が形成されているということができる。例えば、試料Ｒ１と試料Ｓ１３とを対比すると、反応焼成工程を経て生成されたβ型炭化珪素が高抵抗相形成工程でα型に転移することにより、炭化珪素全体におけるβ型炭化珪素の割合が１６％減少し、その分だけα型炭化珪素が増加していると共に、α型炭化珪素の殆どが焼結体表面の近くに存在していることが分かる。 As can be seen from Table 2, the samples S11 to S17 that have undergone the high resistance phase forming step for forming the high resistance phase have α on the sintered body surface as compared to the samples R1 and R2 that have not undergone the high resistance phase formation step. The proportion of type silicon carbide is high, and 90% or more is α-type silicon carbide. And about sample S11-S17, when "(alpha) -SiC: (beta) -SiC" is compared with the sintered compact surface and sintered compact ground material, the ratio of beta type silicon carbide is larger in the ground material. From these facts, the high resistance phase formed in the outer layer (layer close to the outer surface) of the sintered body by performing the high resistance phase forming step at a temperature of 2100 ° C. or higher is the β-type carbonization generated by the reaction firing step. In a sintered body that has been transformed into α-type silicon and has undergone a high-resistance phase forming step, β-type silicon carbide exists inside the high-resistance phase in which most silicon carbide is α-type silicon carbide. It can be said that In other words, in the sintered body that has undergone the high-resistance phase forming step, a high-resistance phase that is mostly α-type silicon carbide is formed outside the conductive phase in which a large amount of β-type silicon carbide exists. it can. For example, when the sample R1 and the sample S13 are compared, the ratio of β-type silicon carbide in the entire silicon carbide is increased by the β-type silicon carbide generated through the reaction firing step being transferred to the α-type in the high resistance phase forming step. It can be seen that the amount of α-type silicon carbide is increased by 16%, and the amount of α-type silicon carbide is mostly present near the surface of the sintered body.
ここで、粉砕の程度の異なる焼結体粉砕物について「α−ＳｉＣ：β−ＳｉＣ」を測定すると、図４に示すように、レーザ回折法による粒子径が３０μｍとなるまでは、粉砕が進むほどα型炭化珪素の割合が減少し、これに伴いβ型炭化珪素の割合が増加するが、粉砕によって３０μｍより粒子径が小さくなると、α型炭化珪素及びβ型炭化珪素の割合は変化しない。このことから、レーザ回折法による粒子径が２０μｍとなるまで焼結体を粉砕した粉砕物について測定した「α−ＳｉＣ：β−ＳｉＣ（粉砕物）」は、高抵抗相形成工程を経た焼結体の炭化珪素全体におけるα型炭化珪素とβ型炭化珪素物との比として、考えることができる。なお、図４は試料Ｓ１１についての測定結果を例示しているが、他の試料についても同様である。 Here, when “α-SiC: β-SiC” is measured for the pulverized sintered bodies having different degrees of pulverization, as shown in FIG. 4, the pulverization proceeds until the particle diameter by the laser diffraction method reaches 30 μm. As the proportion of α-type silicon carbide decreases and the proportion of β-type silicon carbide increases accordingly, the proportion of α-type silicon carbide and β-type silicon carbide does not change when the particle diameter becomes smaller than 30 μm by grinding. Therefore, “α-SiC: β-SiC (pulverized product)” measured for the pulverized product obtained by pulverizing the sintered body until the particle diameter by laser diffraction method becomes 20 μm is sintered through the high resistance phase forming step. It can be considered as the ratio of α-type silicon carbide and β-type silicon carbide in the entire body silicon carbide. FIG. 4 illustrates the measurement result for the sample S11, but the same applies to other samples.
このように「α−ＳｉＣ：β−ＳｉＣ（粉砕物）」は、高抵抗相形成工程を経た焼結体の炭化珪素全体におけるα型炭化珪素とβ型炭化珪素物との比であると考えると、他の条件が同じであれば、反応焼成工程における焼成温度が低いほど焼結体におけるβ型炭化珪素の割合を大きくすることができ（例えば、試料Ｓ１２と試料Ｓ１３との対比から）、高抵抗相形成工程における加熱温度が高いほど、多くの割合のβ型炭化珪素がα型に転移すると言うことができる（例えば、試料Ｓ１１と試料Ｓ１２との対比から）。 Thus, “α-SiC: β-SiC (pulverized product)” is considered to be the ratio of α-type silicon carbide to β-type silicon carbide in the entire silicon carbide of the sintered body that has undergone the high resistance phase forming step. If the other conditions are the same, the lower the firing temperature in the reaction firing step, the larger the proportion of β-type silicon carbide in the sintered body (for example, from the comparison between sample S12 and sample S13), It can be said that as the heating temperature in the high resistance phase forming step is higher, a larger proportion of β-type silicon carbide is transferred to α-type (for example, from the comparison between sample S11 and sample S12).
なお、ここでは骨材としてα型炭化珪素を使用しているが、骨材は炭化珪素でなくてもよいため、各試料の焼結体（試料Ｓ１１〜Ｓ１７については高抵抗相形成工程を経た焼結体、試料Ｒ１，Ｒ２については反応焼成工程を経た焼結体）について、骨材に由来するα型炭化珪素と反応生成した炭化珪素（骨材に由来しないα型炭化珪素とβ型炭化珪素）とを区別すると、表３のようになる。 Here, α-type silicon carbide is used as the aggregate. However, since the aggregate need not be silicon carbide, the sintered body of each sample (the samples S11 to S17 have undergone a high resistance phase forming step). Sintered bodies, sintered bodies that have undergone a reaction firing process for samples R1 and R2, and silicon carbide produced by reaction with α-type silicon carbide derived from the aggregate (α-type silicon carbide and β-type carbonized not derived from the aggregate) Table 3 is distinguished from silicon.
高抵抗相形成工程を経た焼結体におけるβ型炭化珪素物の割合（質量％）に対して、温度５００℃における比抵抗値ρＴｈ（Ω・ｃｍ）を常温における比抵抗値ρＴｎ（Ω・ｃｍ）で除した値「ρＴｈ／ρＴｎ」をプロットすると、図１のようになる。この図１から、焼結体におけるβ型炭化珪素物の割合が大きいほど「ρＴｈ／ρＴｎ」は大きくなっており、ほぼ線形の関係にあることが分かる。これにより、焼結体におけるβ型炭化珪素物の割合によって、比抵抗値の温度依存性を変化させることができ、β型炭化珪素物の割合を大きくするほど、比抵抗値の温度依存性を低下させることができることが判明した。 The specific resistance value ρ Th (Ω · cm) at a temperature of 500 ° C. is changed to the specific resistance value ρ Tn (Ω The value “ρ Th / ρ Tn ” divided by cm) is plotted as shown in FIG. From FIG. 1, it can be seen that “ρ Th / ρ Tn ” increases as the proportion of β-type silicon carbide in the sintered body increases, indicating a substantially linear relationship. Thereby, the temperature dependence of the specific resistance value can be changed depending on the ratio of the β-type silicon carbide in the sintered body, and the temperature dependence of the specific resistance value increases as the ratio of the β-type silicon carbide increases. It has been found that it can be reduced.
出願人の経験から、導電性炭化珪素質焼結体の一般的な用途において、「ρＴｈ／ρＴｎ」が０．１より小さくなると、電流値の制御が困難となることが分かっている。そこで、図１における線形近似曲線から、「ρＴｈ／ρＴｎ」が０．１のときβ型炭化珪素物の割合を読み取ると１４質量％である。従って、導電性炭化珪素質焼結体におけるβ型炭化珪素の割合を１４質量％以上とすることにより、比抵抗値の温度依存性を実用的な範囲とすることができる。 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, from the linear approximation curve in FIG. 1, when “ρ Th / ρ Tn ” is 0.1, the proportion of β-type silicon carbide is 14 mass%. Therefore, by setting the ratio of β-type silicon carbide in the conductive silicon carbide sintered body to 14% by mass or more, the temperature dependency of the specific resistance value can be within a practical range.
加えて、試料Ｓ１１〜Ｓ１７及び試料Ｒ１，Ｒ２について、酸化に伴う比抵抗値の変化を「耐酸化性」として評価した。各試料について空気雰囲気で１０００℃の温度で加熱する酸化試験を行い、所定の時間間隔で上記と同様の方法で比抵抗値を測定し、酸化試験に供する前の初期の比抵抗値を１００％とした比抵抗値変化率（％）を求めた。各試料について、酸化時間に対する比抵抗値変化率を表４に示すと共に、酸化時間１２８時間後の比抵抗値変化率が１１０％未満の場合を、耐酸化性が良好である（酸化に伴う比抵抗値の変化が小さい）として「○」と評価し、酸化時間１２８時間後の比抵抗値変化率が１１０％以上の場合を、耐酸化性が不良であるとして「×」と評価した。 In addition, with respect to Samples S11 to S17 and Samples R1 and R2, 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, the specific resistance value change rate with respect to the oxidation time is shown in Table 4, and when the specific resistance value change rate after the oxidation time 128 hours is less than 110%, the oxidation resistance is good (the ratio due to oxidation). The resistance value change was small) and evaluated as “◯”, and when the specific resistance value change rate after the oxidation time of 128 hours was 110% or more, the oxidation resistance was evaluated as “x”.
図５を用いて上述したように、空気雰囲気において１０００℃の温度で１２８時間加熱すると、炭化珪素焼結体は酸化がかなり進行する。それにも関わらず、導電性相の外側に高抵抗相を形成した試料Ｓ１１〜Ｓ１７は、表４から分かるように、何れも酸化時間１２８時間後であっても比抵抗値は殆ど変化していない（比抵抗値変化率は１００％に近い）。これに対し、高抵抗相を形成していない試料Ｒ１，Ｒ２は、酸化時間の経過に伴い比抵抗値が増加し続けている。従って、高抵抗相の存在により、酸化に伴う比抵抗値の変化が有効に抑制されていると考えられた。試料Ｓ１１、Ｒ１、Ｒ２について、酸化時間に対する比抵抗値変化率をグラフ化して図２に示す。 As described above with reference to FIG. 5, when the silicon carbide sintered body is heated at a temperature of 1000 ° C. for 128 hours in an air atmosphere, the oxidation of the silicon carbide sintered body proceeds considerably. Nevertheless, as can be seen from Table 4, the specific resistance values of the samples S11 to S17 in which the high resistance phase is formed outside the conductive phase hardly change even after the oxidation time of 128 hours. (The resistivity change rate is close to 100%). On the other hand, the specific resistance values of the samples R1 and R2 that do not form the high resistance phase continue to increase as the oxidation time elapses. Therefore, it was considered that the change in specific resistance value accompanying oxidation was effectively suppressed due to the presence of the high resistance phase. For the samples S11, R1, and R2, the specific resistance value change rate with respect to the oxidation time is graphed and shown in FIG.
また、焼結体におけるβ型炭化珪素の割合と比抵抗値の温度依存性の関係とを示すために上記の説明で用いた図１について、耐酸化性が良好である試料と不良である試料とをマーカーで識別した場合の図を、図３に示す。本実施例のように、高抵抗相を形成する際に窒素を効率良く排出させるために、高抵抗相形成工程を２１００℃以上の高温で行う場合は、導電性相の外側のβ型炭化珪素がα型に転移して高抵抗相となるため、その分だけβ型炭化珪素の割合が減少する。このような場合、比抵抗値の温度依存性を実用的な範囲とし、且つ、高抵抗相の存在により酸化に伴う比抵抗値の変化を十分に抑制するためには、図３に示すように、導電性炭化珪素質焼結体におけるβ型炭化珪素の割合を、１４質量％〜３４質量％とすることが望ましい。 Also, in FIG. 1 used in the above description to show the ratio of the β-type silicon carbide in the sintered body and the temperature dependence relationship of the specific resistance value, a sample with good oxidation resistance and a sample with poor quality FIG. 3 shows a diagram when and are identified by a marker. When the high resistance phase forming step is performed at a high temperature of 2100 ° C. or higher in order to efficiently discharge nitrogen when forming the high resistance phase as in this embodiment, β-type silicon carbide outside the conductive phase is used. Transitions to α-type to form a high resistance phase, and the proportion of β-type silicon carbide decreases accordingly. In such a case, in order to make the temperature dependency of the specific resistance value within a practical range and sufficiently suppress the change in the specific resistance value due to oxidation due to the presence of the high resistance phase, as shown in FIG. It is desirable that the proportion of β-type silicon carbide in the conductive silicon carbide sintered body is 14% by mass to 34% by mass.
以上のように、導電性相を含む焼結体を、実質的に窒素ガスを含まない非酸化性雰囲気で加熱することにより、導電性相における平均濃度より窒素の濃度が低い高抵抗相を導電性相の外側に形成することにより、酸化に伴う比抵抗値の変化が抑制された導電性炭化珪素質焼結体を製造することができる。加えて、導電性炭化珪素質焼結体におけるβ型炭化珪素の割合を変化させることにより、比抵抗値の温度依存性の異なる導電性炭化珪素質焼結体を製造することができる。 As described above, by heating a sintered body containing a conductive phase in a non-oxidizing atmosphere substantially free of nitrogen gas, a high resistance phase having a lower concentration of nitrogen than the average concentration in the conductive phase is conducted. By forming on the outer side of the sex phase, it is possible to produce a conductive silicon carbide sintered body in which a change in specific resistance value due to oxidation is suppressed. In addition, by changing the ratio of β-type silicon carbide in the conductive silicon carbide-based sintered body, conductive silicon carbide-based sintered bodies having different specific resistance values depending on temperature can be manufactured.
以上、本発明について好適な実施形態を挙げて説明したが、本発明は上記の実施形態に限定されるものではなく、以下に示すように、本発明の要旨を逸脱しない範囲において、種々の改良及び設計の変更が可能である。 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.
例えば、反応生成する炭化珪素の核となる骨材として、α型炭化珪素の粗大粒子を使用する場合を実施例として例示した。このα型炭化珪素は窒素などがドープされていない非導電性の粒子であったが、これに限定されず、骨材として導電性の材料を使用することができる。これにより、内部に骨材が含まれる導電性相の電気伝導性を、より高めることができる。 For example, the case where coarse particles of α-type silicon carbide are used as an aggregate serving as a nucleus of silicon carbide generated by reaction is illustrated as an example. The α-type silicon carbide is a non-conductive particle which is not doped with nitrogen or the like, but is not limited thereto, and a conductive material can be used as an aggregate. Thereby, the electrical conductivity of the electroconductive phase in which an aggregate is contained can be improved more.
Claims (2)
該反応焼成工程を経た焼結体を、実質的に窒素ガスを含まない非酸化性雰囲気で加熱し、ドープされた窒素の一部を排出させることにより、少なくとも前記導電性相の外側に、前記導電性相における窒素の平均濃度より窒素の濃度が低い炭化珪素の相である高抵抗相を形成する高抵抗相形成工程と、を具備し、
酸化に伴う比抵抗値の変化が前記高抵抗相のない焼結体に比べて小さい焼結体を製造すると共に、
前記高抵抗相形成工程を経た焼結体におけるβ型炭化珪素の割合により、比抵抗値の温度依存性を異ならせた焼結体を製造する
ことを特徴とする導電性炭化珪素質焼結体の製造方法。 A reaction firing step of obtaining a sintered body containing a conductive phase that is a phase of silicon carbide containing nitrogen as a dopant by reacting silicon carbide from silicon nitride as a silicon source and a carbon source ;
The sintered body that has undergone the reaction firing step is heated in a non-oxidizing atmosphere that does not substantially contain nitrogen gas, and a portion of the doped nitrogen is discharged, so that at least outside the conductive phase, A high resistance phase forming step of forming a high resistance phase that is a silicon carbide phase having a nitrogen concentration lower than the average nitrogen concentration in the conductive phase ,
While producing a sintered body in which the change in specific resistance value due to oxidation is smaller than the sintered body without the high resistance phase,
A conductive silicon carbide sintered body characterized by producing a sintered body having a temperature dependency of a specific resistance value different depending on a ratio of β-type silicon carbide in the sintered body subjected to the high resistance phase forming step. Manufacturing method.
前記高抵抗相形成工程では、前記反応焼成工程で生成したβ型炭化珪素の一部をα型炭化珪素に転移させて前記高抵抗相を形成することにより、
前記高抵抗相形成工程を経た焼結体におけるβ型炭化珪素の割合を、１４質量％〜３４質量％とする
ことを特徴とする請求項１に記載の導電性炭化珪素質焼結体の製造方法。 The reaction firing step is a step of obtaining a sintered body containing β-type silicon carbide from a mixed raw material containing α-type silicon carbide as an aggregate in addition to silicon nitride and a carbon source as a silicon source,
In the high resistance phase forming step, a part of β-type silicon carbide generated in the reaction firing step is transferred to α-type silicon carbide to form the high resistance phase,
The conductive silicon carbide based firing according to claim 1, wherein a ratio of β-type silicon carbide in the sintered body that has undergone the high resistance phase forming step is set to 14 mass% to 34 mass%. A method for producing a knot.
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