【０００１】
【産業上の利用分野】
本発明はセラミックヒータに関するものであり、例えばディーゼルエンジンのセラミックグロープラグに適用される。
【０００２】
【従来の技術】
電気絶縁性のセラミック部材からなる棒状の基体と、基体の一端部に埋設された電気抵抗体からなる発熱体と、基体に埋設されるとともに発熱体の両端に個別に接続された給電用の電極線とを備え、発熱体が、電気絶縁性のセラミック粉末からなる絶縁性粒部と、導電性を有する導電性粒部とが互いに分散する焼結体からなるセラミックヒータが、従来より知られている。
【０００３】
例えば、本出願人の出願になる特開昭６３ー９６８８３号公報は、絶縁性粒部（Ｓｉ３ Ｎ４ ）と導電性粒部（ＭｏＳｉ２ ）からなる混合粉末の粒径を変えることにより発熱体と基体とを構成している。
すなわち、発熱体は大粒径の絶縁性粒部と小粒径の導電性粒部とから構成される。このようにすると、絶縁性粒部のまわりを導電性粒部が包むようにして導電性粒部が連続する組織となり、所望の抵抗率が得られる。一方、基体はほぼ同粒径の絶縁性粒部と導電性粒部とから構成される。このようにすると、導電性粒部が絶縁性粒部により相互に分断され、絶縁性となる。
【０００４】
このように同一材料で発熱体と基体とを構成することにより、最適条件で一体焼結ができ、線膨張率差の減少、発熱体と基体との一体化により信頼性が向上する。
また、本出願人の出願になる特開昭６４ー６１３５６号公報は、絶縁性セラミックからなる母材粒子に、島状に凝集したセラミックからなる添加物粒子を分散させ、かつ、母材粒子が添加物粒子を個別に囲包して、添加物粒子間を分断するセラミック体を提案している。
【０００５】
【発明が解決しようとする課題】
しかしながら、発熱体の特性は抵抗値と抵抗温度係数（常温時の抵抗値と高温時の抵抗値の比）で決まるが、上記した特開昭６３ー９６８８３号公報の構成では、図５に示すように、導電性粒部の混合比が多いと低抵抗値、高抵抗温度係数となり、導電性粒部の混合比が少ないと高抵抗値、低抵抗温度係数となり、実現可能な発熱体の特性の範囲が限定されるという問題があった。
【０００６】
すなわち、絶縁性粒部と導電性粒部の混合比により抵抗値と抵抗温度係数を任意の値に設定することが極めて困難であり、図５に示す線上の特定の抵抗値と抵抗温度係数のヒータしか供給できない。
ここで、導電性粒部の配合量と抵抗値、抵抗温度係数との関係について、以下に説明する。
【０００７】
発熱体の抵抗値は、導電性粒部自体の固有抵抗値と導電性粒部間の接触抵抗値とが多数直並列構成になった合成抵抗値と見做すことができ、導電性粒部の配列状態の影響を大きく受ける。接触抵抗値は、ヒータを焼結後、常温にした時、線膨張係数の大きい導電性粒部（ＭｏＳｉ２ ）と線膨張係数の小さい絶縁性粒部（Ｓｉ３ Ｎ４ ）とよりなる発熱体をミクロ的に見ると、導電性粒部に引っ張り力が作用するので、常温時の接触抵抗値はこの状態での値となる。
【０００８】
ここで、通電により発熱体が発熱するとこの引っ張り力が緩和され、接触性が改善されるので、発熱時の接触抵抗値が減少すると考えられる。絶縁性粒部の粒径が大きい程、絶縁性粒部の量が多い程、常温時の上記引っ張り力が大きくなり、発熱による接触抵抗値の低下度合いも顕著となると推定される。
結局、この種の発熱体の抵抗温度係数は、発熱による固有抵抗値の上昇及び接触抵抗値の低下によって決定されることがわかる。
【０００９】
次に、導電性粒部の量と上記抵抗値の関係を説明する。
導電性粒部の量が増加すると、導電性セラミック体内の導電部の断面積が大きくなり、低抵抗値となると考えられる。また、導電性粒部の量が増加すると、常温時の引っ張り力が小さく、発熱による引っ張り力の緩和も小さく、導電性粒部自体の固有抵抗値の上昇の影響を大きく受け、導電性粒部（ＭｏＳｉ２ ）自体の抵抗温度係数に近づいてゆくと考えられる。
【００１０】
本発明は上記問題点を鑑みたものであり、発熱体の抵抗値及び抵抗温度係数の設定が容易なセラミックヒータを提供することを、その目的としている。
【００１１】
【課題を解決するための手段】
本発明の第１の構成は、大粒径の電気絶縁性セラミック粒体からなる絶縁性大粒部と、前記絶縁性大粒部よりも粒径が小さくかつ線膨張率が大きい導電性粒体からなるとともに前記絶縁性大粒部の間に介在して導電経路を構成する導電性粒部と、前記絶縁性大粒部よりも小粒径の電気絶縁性セラミック粒体からなるとともに前記導電経路の間に介在して前記導電経路の電気抵抗を増大させる絶縁性小粒部とを含む焼結体からなることを特徴とする発熱体である。
【００１２】
本発明の第２の構成は、上記第１の構成に加えて、前記絶縁性小粒部及び前記導電性粒部が、前記絶縁性大粒部の１／５以下の平均粒径を有する点を特徴とする。
本発明の第３の構成は、上記第１〜第３の構成に加えて、前記絶縁性小粒部は、導電性粒部の１／３〜３倍の平均粒径を有する点を特徴とする。
【００１３】
本発明の第４の構成は、上記第１〜第４の構成に加えて、前記絶縁性小粒部が、導電性粒部の５〜２００体積％だけ混合される点を特徴とする。
本発明の第５の構成は、上記第１〜第５の構成に加えて、前記絶縁性大粒部及び絶縁性小粒部が同一素材からなる点を特徴とする。
本発明の第６の構成は、上記第６の構成に加えて、前記導電性粒部及び前記絶縁性小粒部と同一素材の導電性粒部及び絶縁性粒部からなる基体と、前記基体に埋設される上記第１〜第５の構成の前記発熱体と、前記基体に埋設されるとともに前記発熱体の端部に接続される高融点金属部材からなる電極線とを備えるセラミックヒータである。
【００１４】
本発明の第７の構成は、前記基体の線膨張係数をＡ／℃、前記発熱体２の線膨張係数をＢ／℃、前記電極線の線膨張係数をＣ／℃としたとき、Ａ≦Ｂで、かつ、０≦（Ｃ−Ａ）≦１．５×１０−６の関係を満足する点を特徴とする。
【００１５】
【作用及び発明の効果】
本発明の発熱体の第１の構成において、導電性粒部は電気抵抗体としての導電経路を構成する。絶縁性大粒部は電気絶縁性であり、導電性粒部に対するその相対的な増加は上記導電経路の抵抗値の増加を招く。また、絶縁性大粒部は導電性粒部よりも大粒径で線膨張率が小さいので、導電経路の抵抗値のうちの接触抵抗値は発熱とともに減少する。このため、導電性粒部に対するその相対的な増加は抵抗温度係数を負方向にシフトさせる。
【００１６】
特に本発明では、絶縁性大粒部より小粒径の絶縁性小粒部が導電性粒部間すなわち導電経路に分散される。
このようにすれば、導電性粒部に対する絶縁性小粒部の相対的な増加により電気経路の電気抵抗値を大幅に変化させることができる。一方、絶縁性小粒部は小粒径であり、この絶縁性小粒部に対してそれに隣接する導電性粒部に引っ張り応力を付与して絶縁性大粒部と同様に導電経路の抵抗温度係数を負方向にシフトさせるものの、その影響の度合いは電気経路の電気抵抗値の変化の度合いに比べて格段に小さい。
【００１７】
結局、導電性粒部を基準として絶縁性大粒部の増減により抵抗温度係数の調整を実施し、更に絶縁性小粒部の増減により抵抗値の調整を実施することができ、発熱体の抵抗温度係数及び抵抗値の調整の自由度が従来より格段に増大する。
図５にこの発熱体のモデル図を示す。
１１３は導電性粒部、１１４は絶縁性大粒部、１１５は絶縁性小粒部である。
【００１８】
導電性粒部１１３が絶縁性小粒部１１５を囲包し、導電性粒部１１３及び絶縁性小粒部１１５が絶縁性大粒部１１４を囲包している。
絶縁性大粒部の量で導電性粒部の引っ張り力を変化させ、抵抗温度係数を設定し、絶縁性小粒部の量で、導電経路の断面積を変化させ、抵抗値の設定することができることがわかる。
【００１９】
本発明の第２構成によれば更に、上記効果を一層良好とすることができる。
すなわち、絶縁性小粒部の平均粒径が上記範囲以上であると、絶縁性小粒部が導電性粒よりなる導電経路内に分散しないため、狙いの抵抗値と抵抗温度係数の関係が得られない。また、導電性粒部の平均粒径が上記範囲以上であると、導電性粒部が絶縁性大粒部を囲包しにくくなるため、導電経路が形成されないという不具合が生じる。
【００２０】
本発明の第３の構成によれば更に、上記効果を一層良好とすることができる。
すなわち、絶縁性小粒部の平均粒径が上記範囲未満であれば絶縁性小粒部に導電性粒部が囲まれるような状態となり、導電経路が断たれてしまうという不具合が生じ、逆に上記範囲を超過すれば絶縁性小粒部が導電経路内に分散しにくくなるため、狙いの抵抗値と抵抗温度係数の関係が得られないという不具合が生じる。
【００２１】
本発明の第４の構成によれば更に、上記効果を一層良好とすることができる。
すなわち、絶縁性小粒部の体積が上記範囲以上であると、絶縁性小粒部が多すぎるため、導電経路が断たれてしまうという不具合が生じる。
本発明の第５の構成によれば、発熱体の接合性が向上するため、信頼性の向上が図れるという作用効果を奏することができる。
【００２２】
本発明の第６の構成によれば、この発熱体は、この発熱体の絶縁性小粒部及び導電性粒部と同一素材の絶縁性粒部及び導電性粒部からなり、絶縁性小粒部が導電経路を遮断する基体に埋設される。このようにすれば、発熱体と基体との接合性が向上する他、線膨張差により発生する熱応力の低減が可能という効果も奏することができる。
【００２３】
結局、以上の構成により、抵抗値と抵抗温度係数を広範囲に調節できる。
【００２４】
【実施例】
以下、本発明を具体的実施例により説明する。
図１は本発明のセラミックヒータ１の実施例を示した断面図であり、図２はその要部の拡大図を示す。
このセラミックヒータ１は、円棒状の基体３と、基体３の先端部に埋設されるＵ字状の発熱体２と、基体３の基端部及び中央部に埋設される断面円形の電極線４、５とからなる。
【００２５】
基体３は、Ｓｉ３ Ｎ４ からなる絶縁性セラミック粉末に、ＭｏＳｉ２ からなる導電体セラミック粉末を少量分散させた断面円形の絶縁性セラミック焼結体からなる。
発熱体２は、ＭｏＳｉ２ からなる導電体セラミック粉末と、Ｓｉ３ Ｎ４ からなる絶縁性セラミック粉末とを含む断面円形の導電性セラミック焼結体である。
【００２６】
電極線４、５の基端部は基体１の外周に露出して部分円筒面４ｃ，５ｃとなっており、それらの先端部４ａ，５ａは発熱体２の両端面から発熱体２の内部に埋設されている。電極線４、５は、タングステン、モリブデン等の高融点金属またはその合金からなるが、ここでは断面円形のタングステン線とされている。部分円筒面４ｃ，５ｃの曲率半径は基体３の半径に等しくされている。
【００２７】
本実施例のセラミックヒータ１をディーゼルエンジンのグロープラグに採用した例を図３に示す。
電極線４、５の部分円筒面４ｃ，５ｃが露出する基体３の側面にはニッケルメッキが施されている。金属の中空パイプ６がこのニッケルメッキ層を介して基体３の中央部に嵌着、ロウ付けされており、中空パイプ６はセラミックヒータ１を保持するとともに、電極線４の部分円筒面４ｃと電気的に接続されている。 中空パイプ６の外周には図示しないエンジンへの取付けネジ１０ａを有する両端開口筒状の金属ハウジング１０の先端部が嵌着、ロウ付けされている。
【００２８】
一方、基体３の基端部に導出された電極線５の部分円筒面５ｃには、金属キャップ７がロウ付けされており、金属キャップ７には金属線８の一端が溶接され、金属線８の他端は中軸９の先端に溶接されている。中軸９の基端部に形成された雄ネジ部９１は図示しない電源に接続されている。中軸９はハウジング１０内に嵌入されており、中軸９はハウジング１０からガラスシール１１および絶縁ブッシュ１２により電気的に絶縁され、これらガラスシール１１および絶縁ブッシュ１２は雄ネジ部９１に螺着されたナット１３により固定されている。
【００２９】
このような構成とすることにより、不図示の電源から中軸９、金属線８、金属キャップ７、電極５、発熱体２、電極４、中空パイプ６、ハウジング１０を介して図示しないエンジンブロックへ電流が通電可能となっている。
セラミックヒータ１の製造方法について説明すると、電極線４、５を金型内の所定位置にセットした状態で発熱体２を射出成形し、次にこの電極線４、５及び発熱体２の一体物を金型内の所定位置にセットした状態で基体３を射出成形した成形品をホットプレスにより焼結させる。しかるのち、外周を研削して形成する。
【００３０】
基体３の拡大模式断面図（モデル図）を図４に示す。
基体３は、導電性粒部１１１としての平均粒径１μｍのＭｏＳｉ２ と絶縁性粒部１１２としての平均粒径１μｍのＳｉ３ Ｎ４ との混合粉末に、添加物としてのＹ２ Ｏ３ 、Ａｌ２ Ｏ３ をＭｏＳｉ２ 及びＳｉ３ Ｎ４ の総質量に対して各々３質量％、５質量％添加した混合物の焼結体であり、ＭｏＳｉ２ 粒子がＳｉ３ Ｎ４ 粒子に取り囲まれた組織になっている。
【００３１】
発熱体２の拡大模式断面図（モデル図）を図５に示す。
発熱体２は、導電性粒部１１３としての平均粒径１μｍのＭｏＳｉ２ と第１の絶縁性粒部（本発明でいう絶縁性大粒部）１１４としての平均粒径１７μｍのＳｉ３ Ｎ４ と第２の絶縁性粒部（本発明でいう絶縁性小粒部）１１５としての平均粒径１μｍのＳｉ３ Ｎ４ との混合粉末に、添加物としてのＹ２ Ｏ３ 、Ａｌ２ Ｏ３ をＭｏＳｉ２ とＳｉ３ Ｎ４ の総質量に対して各々３質量％、５質量％添加した混合物の焼結体であり、第２の絶縁性粒部１１５が導電性粒部１１３により囲包され、第１の絶縁性粒部１１４が第２の絶縁性粒部１１５及び導電性粒部１１３により囲包された構造となっている。
【００３２】
上記特開昭６３ー９６８８３号公報の発熱体の導電性粒部の重量比率と抵抗値及び抵抗温度係数との関係を図６に示す。ここで、抵抗温度係数はこの発熱体を２０℃から９００℃まで発熱した時の抵抗値の変化の比で表してある。材料は図４のモデル図に示したものである。図６から、ＭｏＳｉ２ の配合量を増加すると抵抗値は低下し、抵抗温度係数は増加することがわかる。例えば、４０ｗｔ％のＭｏＳｉ２ は、抵抗値が０．１Ωで、抵抗温度係数が４．０であり、この発熱体の要求仕様として、抵抗値が１．０Ω、抵抗温度係数が４．０といったヒータは導電性粒部の材質を変更しなければ製造することができないことが分かる。
【００３３】
次に、本実施例の発熱体２について説明する。材料は図５のモデル図に示したもので、導電性粒部１１３（ＭｏＳｉ２ ）の配合量を４０ｗｔ％一定とし、残りの６０ｗｔを、第１の絶縁性粒部１１４と第２の絶縁性粒部１１５との配合量（ｗｔ％）を種々変化させて製造した発熱体２の抵抗値と抵抗温度係数の関係を図７に示す。
【００３４】
図７から、第２の絶縁性粒部１１５の配合量が０ｗｔ％の時は図６のＭｏＳｉ２ が４０ｗｔ％の値を示すが、第１の絶縁性粒部１１４の配合量を減少させ、第２の絶縁性粒部１１５の配合量を増加させてゆくと抵抗値は高くなり、抵抗温度係数は若干減少する。そして、種々のＭｏＳｉ２ の配合量にて第２の絶縁性粒部１１５の配合量を変えることにより、この関係が成り立つことを確認した。また、前述の抵抗値が１．０Ωで、抵抗温度係数が４．０の発熱体２は、ＭｏＳｉ２ が４５ｗｔ％、第１の絶縁性粒部１１４が２０ｗｔ％、第２の絶縁性粒部１１５が３５ｗｔ％で製造することができることも確認できた。
【００３５】
以上、導電性粒部１１３が平均粒径１μｍのＭｏＳｉ２ 、第１の絶縁性粒部１１４が平均粒径１７μｍのＳｉ３ Ｎ４ 、第２の絶縁性粒部１１５が平均粒径１μｍのＳｉ３ Ｎ４ の例で述べたが、次に、粒径による本実施例の効果の変化を試験した結果を表１にて示す。試験は導電性粒部１１３と第１の絶縁性粒部１１４と第２の絶縁性粒部１１５との平均粒径を変化させて、図７に示す本実施例の効果が認められるか否かの確認をした。
【００３６】
【表１】
【００３７】
表１から、導電性粒部１１３の平均粒径が１μｍにおいて、第１の絶縁性粒部１１４の平均粒径が１７μｍの場合、第２の絶縁性粒部１１５の平均粒径は９μｍ以下で本実施例の効果が認められ、第１の絶縁性粒部１１４の平均粒径が９μｍの場合、第２の絶縁性粒部１１５の平均粒径は５μｍ以下で本実施例の効果が認められた。
【００３８】
以上の結果から、第２の絶縁性粒部１１５は第１の絶縁性粒部１１４のほぼ１／２以下の粒径ならば、本発明の効果が認められることが分かる。また、導電性粒部１１３の平均粒径を変化させた試験結果も示してあるが、本発明の効果は導電性粒部１１３の平均粒径の影響を受けないことも分かる。
次に、導電性粒部１１３、第１の絶縁性粒部１１４、第２の絶縁性粒部１１５について、種々の材料で試験したが、導電性粒部１１３は導電性を有するならばＭｏＳｉ２ 以外の部材でも本実施例の効果が得られ、第１の絶縁性粒部１１４と第２の絶縁性粒部１１５は絶縁性を有するならばＳｉ３ Ｎ４ 以外の部材でも本実施例の効果が得られることも確認できた。
【００３９】
さらに、第１の絶縁性粒部１１４と第２の絶縁性粒部１１５とは異なる素材を用いても同様の効果が得られた。
但し、上記素材選定に際し、要求されるヒータ寿命に対しては、以下の留意が必要である。発熱体２は通電により高温になるため、基体３と導電性粒部１３と電極線４、５との間の線膨張係数の差により発生する大きな繰り返し熱応力のため、クラックが発生する場合がある。即ち、基体３と発熱体２との線膨張係数の差が大きいと、基体３または発熱体にクラックが発生し、基体３と電極線４、５の線膨張係数の差が大きいと、基体３にクラックが発生する。これらのクラックは、長時間の使用において進展し、ヒータとしての機能を果たさなくなる。
【００４０】
そこで、基体３と発熱体２と電極線４、５との線膨張係数を変化させて、クラックの発生状況を試験した結果、基体３の線膨張係数をＡ／℃、発熱体２の線膨張係数をＢ／℃、電極線４、５の線膨張係数をＣ／℃としたとき、Ａ≦Ｂで、かつ、０≦（Ｃ−Ａ）≦１．５×１０−６の関係を満足する必要があることも分かった。
【００４１】
以上、発熱体２として、導電性セラミック体の例について述べたが、基体３のグリーンシートを作成し、そのうえに発熱体２を構成する印刷パターンを形成し、さらこのグリーンシートに電極線４、５を配して、前記のグリーンシートを適当量積層したのちに焼成し、研削してセラミックヒータを形成する場合においても、上述の効果と同様の効果が得られることは、言うまでもない。
【図面の簡単な説明】
【図１】本発明のセラミックヒータの一実施例を示す断面図である。
【図２】図１のヒータの要部拡大断面図である。
【図３】図１のセラミックヒータをセラミックグロープラグに採用した実施例を示した断面図である。
【図４】基体の一実施例を示すモデル図である。
【図５】発熱体の一実施例を示すモデル図である。
【図６】特開昭６３ー９６８８３号公報の発熱体の導電性粒部の配合比率と抵抗値、抵抗温度係数の関係を示す図である。
【図７】本実施例の発熱体の配合比率と抵抗値、抵抗温度係数の関係を示す図である。
【符号の説明】
１ セラミックグロープラグ
２ 発熱体
３ 基体
４ 電極線
５ 電極線
１１３ 導電性粒部
１１４ 第１の絶縁性粒部（絶縁性大粒部）
１１５ 第２の絶縁性粒部（絶縁性小粒部）[0001]
[Industrial applications]
The present invention relates to a ceramic heater, and is applied to, for example, a ceramic glow plug of a diesel engine.
[0002]
[Prior art]
A rod-shaped base made of an electrically insulating ceramic member, a heating element made of an electric resistor embedded at one end of the base, and a power supply electrode embedded in the base and individually connected to both ends of the heating element Conventionally, a ceramic heater comprising a wire and a heating element formed of a sintered body in which insulating particles made of electrically insulating ceramic powder and conductive particles having conductivity are mutually dispersed has been known. I have.
[0003]
For example, Japanese Patent Application Laid-Open No. 63-96883 filed by the present applicant discloses that heat is generated by changing the particle size of a mixed powder composed of insulating particles (Si 3 N 4 ) and conductive particles (MoSi 2 ). It constitutes the body and the substrate.
That is, the heating element is composed of large-diameter insulating particles and small-particle conductive particles. With this configuration, the conductive particles are formed into a continuous structure such that the conductive particles wrap around the insulating particles, and a desired resistivity can be obtained. On the other hand, the base is composed of insulating particles and conductive particles having substantially the same particle size. In this case, the conductive particles are separated from each other by the insulating particles, and become insulative.
[0004]
By composing the heating element and the base with the same material in this manner, integral sintering can be performed under optimum conditions, the difference in linear expansion coefficient is reduced, and the reliability is improved by integrating the heating element and the base.
Japanese Patent Application Laid-Open No. 64-61356, filed by the present applicant, discloses a method in which additive particles made of an island-like aggregated ceramic are dispersed in base material particles made of an insulating ceramic, and A ceramic body which encloses additive particles individually and divides between additive particles has been proposed.
[0005]
[Problems to be solved by the invention]
However, the characteristics of the heating element are determined by the resistance value and the temperature coefficient of resistance (the ratio of the resistance value at normal temperature to the resistance value at high temperature). However, in the configuration of JP-A-63-96883 described above, the characteristics shown in FIG. As shown, if the mixing ratio of the conductive particles is high, the resistance value and the temperature coefficient of resistance are high, and if the mixing ratio of the conductive particles is low, the resistance value and the temperature coefficient of the resistance are low. Is limited.
[0006]
That is, it is extremely difficult to set the resistance value and the resistance temperature coefficient to arbitrary values depending on the mixing ratio of the insulating particles and the conductive particles, and the specific resistance and the resistance temperature coefficient on the line shown in FIG. Only heaters can be supplied.
Here, the relationship between the compounding amount of the conductive particles, the resistance value, and the temperature coefficient of resistance will be described below.
[0007]
The resistance value of the heating element can be regarded as a combined resistance value in which the specific resistance value of the conductive particle portion itself and the contact resistance value between the conductive particle portions are in a series-parallel configuration. Is greatly affected by the arrangement state of. The contact resistance value is such that, when the heater is heated to room temperature after sintering, the heating element is composed of a conductive particle part (MoSi 2 ) having a large linear expansion coefficient and an insulating particle part (Si 3 N 4 ) having a small linear expansion coefficient. When viewed microscopically, since a tensile force acts on the conductive particles, the contact resistance at room temperature is the value in this state.
[0008]
Here, when the heating element generates heat by energization, the tensile force is reduced and the contact property is improved, so that it is considered that the contact resistance value during the heat generation decreases. It is presumed that the larger the particle size of the insulating particles and the larger the amount of the insulating particles, the greater the above-mentioned tensile force at normal temperature, and the more the degree of decrease in the contact resistance value due to heat generation becomes remarkable.
After all, it can be seen that the temperature coefficient of resistance of this type of heating element is determined by an increase in the specific resistance value and a decrease in the contact resistance value due to heat generation.
[0009]
Next, the relationship between the amount of the conductive particles and the resistance value will be described.
It is considered that when the amount of the conductive particles increases, the cross-sectional area of the conductive portion in the conductive ceramic body increases, resulting in a low resistance value. Also, when the amount of the conductive particles increases, the tensile force at normal temperature is small, the relaxation of the tensile force due to heat generation is small, and the specific resistance of the conductive particles themselves is greatly affected by the increase in the specific resistance. It is considered that the temperature coefficient of resistance of (MoSi 2 ) itself approaches.
[0010]
The present invention has been made in view of the above problems, and has as its object to provide a ceramic heater in which the resistance value and the resistance temperature coefficient of a heating element can be easily set.
[0011]
[Means for Solving the Problems]
A first configuration of the present invention comprises an insulating large particle portion made of an electrically insulating ceramic particle having a large particle size, and a conductive particle having a smaller particle size and a larger linear expansion coefficient than the insulating large particle portion. And a conductive grain portion that constitutes a conductive path interposed between the large insulating particles and an electrically insulating ceramic particle having a smaller particle size than the large insulating particle portion and interposed between the conductive paths. A heat-generating element comprising: a sintered body including: an insulating small-grain portion for increasing an electric resistance of the conductive path.
[0012]
The second configuration of the present invention is characterized in that, in addition to the first configuration, the insulating small particles and the conductive particles have an average particle size of 1/5 or less of the insulating large particles. And
A third structure of the present invention is characterized in that, in addition to the first to third structures, the insulating small particles have an average particle diameter that is 1/3 to 3 times that of the conductive particles. .
[0013]
A fourth configuration of the present invention is characterized in that, in addition to the above-described first to fourth configurations, the insulating small particles are mixed by 5 to 200% by volume of the conductive particles.
A fifth configuration of the present invention is characterized in that, in addition to the above-described first to fifth configurations, the large insulating portion and the small insulating portion are made of the same material.
According to a sixth aspect of the present invention, in addition to the sixth aspect, a base made of a conductive grain part and an insulating grain part of the same material as the conductive grain part and the insulating small grain part; A ceramic heater comprising: the heating element according to any one of the first to fifth configurations to be embedded; and an electrode wire made of a high melting point metal member embedded in the base and connected to an end of the heating element.
[0014]
In a seventh configuration of the present invention, when the linear expansion coefficient of the base is A / ° C., the linear expansion coefficient of the heating element 2 is B / ° C., and the linear expansion coefficient of the electrode wire is C / ° C., A ≦ B, and satisfying a relationship of 0 ≦ ( CA ) ≦ 1.5 × 10 −6 .
[0015]
[Action and effect of the invention]
In the first configuration of the heating element of the present invention, the conductive particles form a conductive path as an electric resistor. The insulating large particles are electrically insulating, and their relative increase with respect to the conductive particles results in an increase in the resistance of the conductive path. Further, since the large insulating particles have a larger particle diameter and a smaller linear expansion coefficient than the conductive particles, the contact resistance value among the resistance values of the conductive paths decreases with heat generation. Thus, its relative increase with respect to the conductive grains causes the temperature coefficient of resistance to shift in the negative direction.
[0016]
In particular, in the present invention, the small insulating particles having a smaller particle size than the large insulating particles are dispersed between the conductive particles, that is, in the conductive paths.
With this configuration, the electric resistance value of the electric path can be largely changed by the relative increase of the small insulating particles relative to the conductive particles. On the other hand, the insulating small-grain portion has a small particle size, and a tensile stress is applied to the conductive small-grain portion adjacent to the small insulating-grain portion so that the temperature coefficient of resistance of the conductive path is negative similarly to the large insulating-grain portion. Although it is shifted in the direction, the degree of the influence is much smaller than the degree of change in the electric resistance value of the electric path.
[0017]
Eventually, the resistance temperature coefficient can be adjusted by increasing or decreasing the insulating large particles based on the conductive particles, and the resistance can be adjusted by increasing or decreasing the insulating small particles. In addition, the degree of freedom in adjusting the resistance value is significantly increased as compared with the related art.
FIG. 5 shows a model diagram of this heating element.
113 is a conductive grain portion, 114 is an insulating large grain portion, and 115 is an insulating small grain portion.
[0018]
The conductive particles 113 surround the small insulating particles 115, and the conductive particles 113 and the small insulating particles 115 surround the large insulating particles 114.
The resistance of the conductive particles can be changed by changing the tensile force of the conductive particles by the amount of the insulating large particles, and the cross-sectional area of the conductive path can be changed by setting the amount of the insulating small particles. I understand.
[0019]
According to the second configuration of the present invention, the above effects can be further improved.
That is, if the average particle size of the insulating small particles is not less than the above range, the insulating small particles do not disperse in the conductive path made of the conductive particles, so that the relationship between the target resistance value and the resistance temperature coefficient cannot be obtained. . Further, when the average particle size of the conductive particles is more than the above range, the conductive particles hardly surround the large insulating particles, so that a problem that a conductive path is not formed occurs.
[0020]
According to the third configuration of the present invention, the above effects can be further improved.
That is, if the average particle size of the insulating small particles is less than the above range, the conductive small particles will be surrounded by the conductive small particles, causing a problem that the conductive path is cut off. Is exceeded, it becomes difficult for the insulating small particles to disperse in the conductive path, so that a problem arises in that the relationship between the target resistance value and the resistance temperature coefficient cannot be obtained.
[0021]
According to the fourth configuration of the present invention, the above effects can be further improved.
In other words, when the volume of the insulating small particles is more than the above range, there is a problem that the conductive path is cut off because the insulating small particles are too large.
According to the fifth configuration of the present invention, since the bonding property of the heating element is improved, it is possible to achieve an operational effect that reliability can be improved.
[0022]
According to the sixth configuration of the present invention, the heating element is composed of the insulating particles and the conductive particles of the same material as the insulating small particles and the conductive particles of the heating element. It is embedded in a substrate that blocks the conductive path. With this configuration, it is possible to improve the bonding property between the heating element and the base and also to reduce the thermal stress generated due to the difference in linear expansion.
[0023]
After all, with the above configuration, the resistance value and the resistance temperature coefficient can be adjusted over a wide range.
[0024]
【Example】
Hereinafter, the present invention will be described with reference to specific examples.
FIG. 1 is a sectional view showing an embodiment of a ceramic heater 1 of the present invention, and FIG. 2 is an enlarged view of a main part thereof.
The ceramic heater 1 includes a rod-shaped base 3, a U-shaped heating element 2 buried at the tip of the base 3, and a circular electrode wire 4 buried at the base and center of the base 3. , And 5.
[0025]
The base 3 is made of an insulating ceramic sintered body having a circular cross section in which a small amount of a conductive ceramic powder made of MoSi 2 is dispersed in an insulating ceramic powder made of Si 3 N 4 .
Heating element 2 is a conductive ceramic sintered body having a circular cross section including conductive ceramic powder made of MoSi 2 and insulating ceramic powder made of Si 3 N 4 .
[0026]
The base ends of the electrode wires 4 and 5 are exposed to the outer periphery of the base 1 to form partial cylindrical surfaces 4c and 5c, and the front ends 4a and 5a extend from both end surfaces of the heating element 2 to the inside of the heating element 2. It is buried. The electrode wires 4 and 5 are made of a refractory metal such as tungsten or molybdenum, or an alloy thereof, and here are tungsten wires having a circular cross section. The radii of curvature of the partial cylindrical surfaces 4c and 5c are made equal to the radius of the base 3.
[0027]
FIG. 3 shows an example in which the ceramic heater 1 of the present embodiment is used for a glow plug of a diesel engine.
The side surfaces of the base 3 where the partial cylindrical surfaces 4c and 5c of the electrode wires 4 and 5 are exposed are plated with nickel. A metal hollow pipe 6 is fitted and brazed to the central portion of the base 3 via the nickel plating layer. The hollow pipe 6 holds the ceramic heater 1 and electrically connects with the partial cylindrical surface 4c of the electrode wire 4. Connected. The distal end of a cylindrical metal housing 10 having both ends open and having a screw 10a for mounting to an engine (not shown) is fitted and brazed to the outer periphery of the hollow pipe 6.
[0028]
On the other hand, a metal cap 7 is brazed to the partial cylindrical surface 5c of the electrode wire 5 led out to the base end of the base 3, and one end of the metal wire 8 is welded to the metal cap 7, Is welded to the tip of the center shaft 9. A male screw portion 91 formed at the base end of the center shaft 9 is connected to a power source (not shown). The center shaft 9 is fitted in a housing 10, and the center shaft 9 is electrically insulated from the housing 10 by a glass seal 11 and an insulating bush 12, and the glass seal 11 and the insulating bush 12 are screwed to a male screw portion 91. It is fixed by a nut 13.
[0029]
With such a configuration, current is supplied from a power source (not shown) to the engine block (not shown) via the center shaft 9, the metal wire 8, the metal cap 7, the electrode 5, the heating element 2, the electrode 4, the hollow pipe 6, and the housing 10. Can be energized.
The method of manufacturing the ceramic heater 1 will be described. The heating element 2 is injection-molded in a state where the electrode wires 4 and 5 are set at predetermined positions in a mold, and then the electrode wire 4 and 5 and the heating element 2 are integrally formed. Is set at a predetermined position in a mold, and a molded product obtained by injection molding the base 3 is sintered by hot pressing. Thereafter, the outer periphery is formed by grinding.
[0030]
FIG. 4 shows an enlarged schematic cross-sectional view (model diagram) of the base 3.
The base 3 is composed of a mixed powder of MoSi 2 having an average particle size of 1 μm as the conductive particle portion 111 and Si 3 N 4 having an average particle size of 1 μm as the insulating particle portion 112, Y 2 O 3 as an additive, This is a sintered body of a mixture in which Al 2 O 3 is added at 3% by mass and 5% by mass with respect to the total mass of MoSi 2 and Si 3 N 4 , respectively, in which MoSi 2 particles are surrounded by Si 3 N 4 particles. It has become.
[0031]
FIG. 5 shows an enlarged schematic cross-sectional view (model diagram) of the heating element 2.
The heating element 2 includes MoSi 2 having an average particle size of 1 μm as the conductive particle portion 113 and Si 3 N 4 having an average particle size of 17 μm as the first insulating particle portion (insulating large particle portion in the present invention) 114. To a mixed powder of Si 3 N 4 having an average particle size of 1 μm as a second insulating particle portion (insulating small particle portion in the present invention) 115, Y 2 O 3 and Al 2 O 3 as additives are added to MoSi. 2 and Si 3 N 4 are a sintered body of a mixture in which 3% by mass and 5% by mass are added to the total mass, respectively, wherein the second insulating particles 115 are surrounded by the conductive particles 113, The structure is such that one insulating grain portion 114 is surrounded by the second insulating grain portion 115 and the conductive grain portion 113.
[0032]
FIG. 6 shows the relationship between the weight ratio of the conductive particles of the heating element of JP-A-63-96883 and the resistance value and the temperature coefficient of resistance. Here, the temperature coefficient of resistance is represented by a ratio of a change in resistance value when the heating element generates heat from 20 ° C. to 900 ° C. The materials are shown in the model diagram of FIG. FIG. 6 shows that the resistance value decreases and the temperature coefficient of resistance increases as the amount of MoSi 2 increases. For example, MoSi 2 of 40 wt% has a resistance value of 0.1Ω and a temperature coefficient of resistance of 4.0, and the required specifications of the heating element include a resistance value of 1.0Ω and a temperature coefficient of resistance of 4.0. It can be seen that the heater cannot be manufactured without changing the material of the conductive particles.
[0033]
Next, the heating element 2 of the present embodiment will be described. The material is shown in the model diagram of FIG. 5, and the blending amount of the conductive particles 113 (MoSi 2 ) is fixed at 40 wt%, and the remaining 60 wts are used for the first insulating particles 114 and the second insulating particles. FIG. 7 shows the relationship between the resistance value and the resistance temperature coefficient of the heating element 2 manufactured by changing the blending amount (wt%) with the grain portion 115 variously.
[0034]
From FIG. 7, when the blending amount of the second insulating granule 115 is 0 wt%, the MoSi 2 of FIG. 6 shows a value of 40 wt%, but the blending amount of the first insulating granule 114 is reduced. As the blending amount of the second insulating particles 115 increases, the resistance value increases and the temperature coefficient of resistance decreases slightly. Then, it was confirmed that this relationship was established by changing the compounding amount of the second insulating particles 115 with various compounding amounts of MoSi 2 . The heating element 2 having a resistance value of 1.0Ω and a temperature coefficient of resistance of 4.0 has MoSi 2 of 45 wt%, the first insulating particles 114 of 20 wt%, and the second insulating particles 114. It was also confirmed that 115 could be produced at 35 wt%.
[0035]
As described above, the conductive particles 113 are MoSi 2 having an average particle diameter of 1 μm, the first insulating particles 114 are Si 3 N 4 having an average particle diameter of 17 μm, and the second insulating particles 115 are Si having an average particle diameter of 1 μm. has been described in example 3 N 4, then, shows the results of testing the change of the effect of the present embodiment according to the particle size in Table 1. The test was performed by changing the average particle size of the conductive particles 113, the first insulating particles 114, and the second insulating particles 115 to determine whether the effect of the present embodiment shown in FIG. Was confirmed.
[0036]
[Table 1]
[0037]
From Table 1, when the average particle size of the conductive particles 113 is 1 μm and the average particle size of the first insulating particles 114 is 17 μm, the average particle size of the second insulating particles 115 is 9 μm or less. The effect of the present embodiment is recognized. When the average particle diameter of the first insulating particles 114 is 9 μm, the effect of the present embodiment is recognized when the average particle diameter of the second insulating particles 115 is 5 μm or less. Was.
[0038]
From the above results, it can be seen that the effect of the present invention can be recognized if the second insulating particles 115 have a particle size of about 以下 or less of the first insulating particles 114. In addition, although the test results in which the average particle size of the conductive particles 113 is changed are shown, it is also understood that the effect of the present invention is not affected by the average particle size of the conductive particles 113.
Next, the conductive particles 113, the first insulating particles 114, and the second insulating particles 115 were tested using various materials. If the conductive particles 113 have conductivity, MoSi 2 The effects of the present embodiment can be obtained with members other than those described above. If the first insulating particles 114 and the second insulating particles 115 have insulating properties, members other than Si 3 N 4 can be used. Was also obtained.
[0039]
Further, the same effect was obtained even if different materials were used for the first insulating particles 114 and the second insulating particles 115.
However, the following considerations are required for the required heater life when selecting the above materials. Since the heating element 2 is heated to a high temperature by energization, cracks may occur due to a large repetitive thermal stress generated due to a difference in linear expansion coefficient between the base 3, the conductive particles 13, and the electrode wires 4, 5. is there. That is, if the difference in the linear expansion coefficient between the base 3 and the heating element 2 is large, cracks occur in the base 3 or the heating element, and if the difference in the linear expansion coefficient between the base 3 and the electrode wires 4 and 5 is large, the base 3 Cracks occur. These cracks develop over a long period of use and no longer function as a heater.
[0040]
Thus, as a result of changing the linear expansion coefficient of the base 3, the heating element 2, and the electrode wires 4 and 5 to test the occurrence of cracks, the linear expansion coefficient of the base 3 was A / ° C. When the coefficient is B / ° C. and the linear expansion coefficients of the electrode wires 4 and 5 are C / ° C., A ≦ B and 0 ≦ ( CA ) ≦ 1.5 × 10 −6 are satisfied. I knew it was necessary.
[0041]
As described above, the example of the conductive ceramic body as the heating element 2 has been described. However, a green sheet of the base 3 is formed, a printing pattern constituting the heating element 2 is formed thereon, and the electrode lines 4 and 5 are further formed on the green sheet. It is needless to say that the same effect as described above can be obtained also in the case where a ceramic heater is formed by laminating the above green sheets in an appropriate amount and then firing and grinding the green sheets.
[Brief description of the drawings]
FIG. 1 is a sectional view showing an embodiment of a ceramic heater according to the present invention.
FIG. 2 is an enlarged sectional view of a main part of the heater of FIG.
FIG. 3 is a sectional view showing an embodiment in which the ceramic heater of FIG. 1 is used for a ceramic glow plug.
FIG. 4 is a model diagram showing an example of a base.
FIG. 5 is a model diagram showing one embodiment of a heating element.
FIG. 6 is a diagram showing the relationship among the mixing ratio of conductive particles of a heating element, resistance value, and resistance temperature coefficient in JP-A-63-96883.
FIG. 7 is a diagram showing the relationship between the mixing ratio of the heating element, the resistance value, and the temperature coefficient of resistance in the present example.
[Explanation of symbols]
REFERENCE SIGNS LIST 1 ceramic glow plug 2 heating element 3 base 4 electrode wire 5 electrode wire 113 conductive grain portion 114 first insulating grain portion (insulating large grain portion)
115 2nd insulating grain part (insulating small grain part)
