JP3601079B2

JP3601079B2 - Ceramic heater

Info

Publication number: JP3601079B2
Application number: JP20023694A
Authority: JP
Inventors: 敦倉野; 郁也安藤
Original assignee: Denso Corp
Current assignee: Denso Corp
Priority date: 1994-08-02
Filing date: 1994-08-02
Publication date: 2004-12-15
Anticipated expiration: 2019-12-15
Also published as: JPH0845648A

Description

【０００１】
【産業上の利用分野】
本発明はセラミックヒータに関し、特に、ディーゼルエンジンのセラミックグロープラグ等に好適に使用されるセラミックヒータに関するものである。
【０００２】
【従来の技術】
ディーゼルエンジンの始動を補助するために、燃焼室内にセラミックグロープラグを配置し、発熱部に通電加熱して燃料の着火、燃焼を促すことが行なわれている。この発熱部を構成するセラミックヒータとしては、従来より種々のものが提案されており、例えば、特開昭６２−１４０３８６号公報には、サイアロン焼結体中に導電性の窒化チタンを分散させた複合焼結体を用いたヒータが開示されている。
【０００３】
また、絶縁性セラミックよりなる支持体の先端に、導電性セラミックよりなる発熱体を設けたセラミックヒータが知られ、例えば、窒化珪素、酸化アルミニウム等よりなる棒状の絶縁性セラミックの先端に、窒化珪素と珪化モリブデンの混合体等よりなるＵ字状の導電性セラミックを設けた構成のものがある。
【０００４】
ところが、上記従来のセラミックヒータは、支持体と発熱体の熱膨張係数差から、急速な昇温、または冷却により両者の間に熱応力が発生し、接合部を破損するおそれがあった。そこで本出願人は、先に、支持体と発熱体とをいずれも導電性の珪化モリブデン（ＭｏＳｉ_２）と絶縁性の窒化珪素（Ｓｉ_３Ｎ_４）の混合体で構成し、支持体においては導電性のＭｏＳｉ_２粒子がこれを包む絶縁性のＳｉ_３Ｎ_４粒子により互いに分断されることにより絶縁性を示し、発熱体においては、互いに連続する導電性のＭｏＳｉ_２粒子で絶縁性のＳｉ_３Ｎ_４粒子を包むことにより導電性を示すようになしたセラミックヒータを提案した（特開昭６３−９６８８３号公報）。具体的には、支持体と発熱体の基本成分をいずれも７０Ｓｉ_３Ｎ_４−３０ＭｏＳｉ_２（重量％）とし、これらの総量に対し焼結助剤として、例えば酸化イットリウム（Ｙ_２Ｏ_３）を７重量％、酸化アルミニウム（Ａｌ_２Ｏ_３）を３重量％添加しており、支持体と発熱体を同一組成とすることで、熱応力が大幅に緩和される。
【０００５】
【発明が解決しようとする課題】
ところで、近年、セラミックヒータの速熱性をさらに向上し、エンジン始動までの待ち時間を短縮しようとする要求が高まっており、これに伴いヒータの温度を従来の１０００℃〜１１００℃から１２００℃〜１３００℃にする必要が生じている。
【０００６】
しかしながら、最高温度を１３００℃に設定した上記構成のセラミックヒータをエンジンで使用したところ、次のような問題が生じた。その１つは、長時間の使用で抵抗値が上昇することで、飽和温度が低くなってエンジンの始動性が悪くなる。もう１つは、セラミックヒータの、燃料が直撃する部分にクラックが発生する場合があることで、このクラックが進展すると、ヒータの一部がエンジン内に脱落し、エンジンが損傷するおそれがある。
【０００７】
これらの原因につき調査した結果、以下のことが判明した。先ず、抵抗値上昇については、長時間使用して抵抗値が上昇したヒータを軸方向に切断し、発熱体およびその近傍を調査した結果、発熱体のＵ字状に屈曲する先端部の、正または負電極よりの端部で発熱体が最高温度になること、このうち負電極側の最高温度部にイットリウム（Ｙ）が集中し、逆に正電極側にはＹがほとんど存在せず、モリブデン（Ｍｏ）の酸化物が多くなっていることがわかった。また、発熱体の両最高温度部間に位置する支持体においても、同様に、負電極側でＹが多く、次第にＹが減少して正電極側ではＹがほとんど存在していなかった。
【０００８】
この現象は、通電により生じる電界の作用により、高温部のＹ_２Ｏ_３が分解されてＹが負電極側に移動し、正電極側では残った酸素がＭｏＳｉ_２を酸化して、ＭｏＳｉ_２よりなる電流パスが細化または断線したことを意味する。特に、１３００℃という高温状態では、上記のＹの移動が起こりやすくなって、これが抵抗値上昇の原因になったものと考えられる。
【０００９】
一方、クラックの発生については、未使用のヒータを軸方向に切断し、詳細に観察すると、発熱体にポアが認められた。上記構成のセラミックヒータは熱応力が小さくなるように支持体と発熱体とを同一組成としているが、それぞれに絶縁性または導電性という相反する特性を付与するため、セラミック粒子の粒径を変えており、発熱体では、平均粒径１３μｍのＳｉ_３Ｎ_４と０．９μｍのＭｏＳｉ_２を、支持体では平均粒径０．６μｍのＳｉ_３Ｎ_４と平均粒径０．９μｍのＭｏＳｉ_２を用いている。従って、セラミック粒子の粒径の差により支持体と発熱体の最適焼結条件にずれを生じ、発熱体が焼結不足になってポアが生じたものと考えられる。そして、１３００℃という新たな温度条件において、このポアがクラックを誘発する要因となり、表面と内部および軸方向での温度差や噴霧燃料による冷却等により熱応力が作用し、耐熱衝撃性を低下させたものと思われる。
【００１０】
本発明は、上記実情に鑑みなされたものであり、１３００℃以上の高温条件下でも抵抗値の変化が小さく、かつクラックの生じない耐熱衝撃性に優れたセラミックヒータを提供することを目的とするものである。
【００１１】
【課題を解決するための手段】
本発明の構成を図１で説明すると、セラミックヒータ１は、電気絶縁性の支持体３と、その先端部に一体に形成される導電性の発熱体２を備え、上記支持体３および上記発熱体２はいずれも導電性セラミックと絶縁性セラミックの混合焼結体よりなる。上記支持体３は、導電性セラミック粒子がこれを包む絶縁性セラミック粒子により互いに分断されることにより絶縁性を示し、上記発熱体２は、互いに連続する導電性セラミック粒子で絶縁性セラミック粒子を包むことにより導電性を示している。そして、上記発熱体２は、焼結助剤として少なくとも酸化アルミニウム（Ａｌ_２Ｏ_３）を、上記支持体３は、焼結助剤として少なくとも希土類元素の酸化物およびＡｌ_２Ｏ_３を含有し、かつ上記発熱体２のＡｌ_２Ｏ_３添加量を上記支持体３のＡｌ_２Ｏ_３添加量よりも多くしてある（請求項１）。
【００１２】
また、上記発熱体２は、焼結助剤として、Ａｌ_２Ｏ_３に加え希土類元素の酸化物を含有していてもよく、この場合は、上記支持体３の希土類元素の酸化物の添加量を上記発熱体２のそれより多く、上記発熱体２のＡｌ_２Ｏ_３添加量を上記支持体３のそれよりも多くする（請求項３）。
【００１３】
上記発熱体２において、導電性セラミックおよび絶縁性セラミックの総量に対するＡｌ_２Ｏ_３添加量は０．１〜１５重量％とすることが好ましい（請求項２）。また、希土類元素の酸化物を含有する場合の、導電性セラミックおよび絶縁性セラミックの総量に対する添加量は０．１〜１９．５重量％とし、かつこれら焼結助剤の添加総量を２５重量％以下とするのがよい（請求項４）。
【００１４】
上記支持体３においては、導電性セラミックおよび絶縁性セラミックの総量に対する希土類元素の酸化物の添加量を０．１〜２０重量％、Ａｌ_２Ｏ_３の添加量を０．１〜１４．５重量％とし、これら焼結助剤の添加総量を２５重量％以下とすることが望ましい（請求項５）。
【００１５】
上記支持体３の希土類元素の酸化物の添加量は、上記発熱体２より０．５重量％以上多くし、上記発熱体２のＡｌ_２Ｏ_３添加量は、上記支持体３より０．５重量％以上多くするのがよい（請求項６）。
【００１６】
上記希土類の酸化物としては、酸化イットリウム（Ｙ_２Ｏ_３）、または酸化イッテルビウム（Ｙｂ_２Ｏ_３）等が挙げられる（請求項７）。上記絶縁性セラミックとしては、窒化珪素が好適に使用される。また、上記導電性セラミックとしては、金属の炭化物、珪化物、窒化物、またはホウ化物が使用でき、これらより選ばれた少なくとも１種を使用すればよい（請求項８）。
【００１７】
【作用】
発熱体２は、Ｕ字状に屈曲する先端部の、正または負電極よりの端部（図１のａまたはｂの部分）で最高温度になる。本発明のセラミックヒータ１は、支持体３の希土類元素の酸化物、例えばＹ_２Ｏ_３添加量が、発熱体２のＹ_２Ｏ_３添加量より多いので、発熱体２の上記ａまたはｂで示される部分の間にＹ_２Ｏ_３の多い支持体３が存在することになる（図１のｃの部分）。通電時、ヒーター材に電界が作用すると、発熱体２の負電極側の最高温度部へ向けてＹの移動が生じるが、まず、その近傍のＹ_２Ｏ_３の多い支持体３からＹが移動し、Ｙが移動した部位には、その周囲の支持体３からさらにＹが移動する。この時、発熱体２の正電極側の最高温度部の周囲には、Ｙ_２Ｏ_３の多い支持体３が存在するため、発熱体２中のＹの移動は起こりにくい。そして、支持体３中のＹがある程度移動し、発熱体２のＹ_２Ｏ_３量とほぼ同量となるまで、発熱体２の正電極側におけるＹの移動は起こらず、抵抗値の上昇が抑制される。
【００１８】
また、Ａｌ_２Ｏ_３は低融点で焼結を促進する作用を有する。本発明のセラミックヒータ１は、このＡｌ_２Ｏ_３の発熱体２への添加量が支持体３より多いので、発熱体２の焼結が促進され、焼結不足が解消される。従って、焼結体中のポアが大幅に減少し、クラックの発生を防止する。
【００１９】
【実施例】
以下、本発明を実施例により説明する。
図２には本発明を適用したディーゼルエンジンのグロープラグを示す。６は両端開口の筒状金属ハウジングであり、その下端開口内に筒状金属部材６１が固着してある。上記ハウジング６内には下方より本発明のセラミックヒータ１が挿通され、その中間部を、上記筒状金属部材６１に嵌合固定してある。ハウジング６の中央部外周には取付けネジ６２が形成してあり、グロープラグは該取付けネジ６２にて図略のエンジンに取付けられる。
【００２０】
上記セラミックヒータ１の上端部には、電源接続用の金属キャップ７が固定してあり、金属キャップ７は、上記ハウジング６の上半部内に挿通された金属製の中軸８と、金属線７１により接続されている。上記ハウジング６上端部の、中軸８周りにはガラスシール９が配設され、さらに上方より絶縁ブッシュ１０が嵌着されて、上記中軸８を電気的に絶縁している。中軸８の基端部には、図略の電源に接続される雄ネジ８１が形成してあり、上記絶縁ブッシュ１０は上記雄ネジ８１に螺着されたナット８２にて固定される。
【００２１】
図１には上記セラミックヒータ１の詳細を示す。図において、セラミックヒータ１は、断面円形の棒状体である支持体３と、支持体３の先端部内に埋設される断面Ｕ字形の発熱体２からなる。発熱体２の一端には電極線４の先端４１が埋設され、電極線４の他端は支持体３の基端部へ延びて支持体３の外周表面に露出する端子部４２を形成している。また発熱体２の他端には電極線５の先端５１が埋設され、電極線５の他端は支持体３の中間部で支持体３の外周表面に露出する端子部５２を形成している。なお、上記電極線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。
【００２２】
上記電極線４、５の端子部４２、５２が露出する支持体３の外周表面にはニッケルメッキが施されている。しかして、セラミックヒータ１をハウジング６内に挿通すると（図２）、上記支持体３は、このニッケルメッキ層を介して上記筒状金属部材６１内周面にロウ付けされる。上記筒状金属部材６１はセラミックヒータ１を保持するとともに、上記電極線５の端子部５２と電気的に接続される。一方、支持体３の基端部に露出する上記電極線４の端子部４２は、上記金属キャップ７の内周面にロウ付けされ、金属線７１より中軸８を経て電源に接続している。かくして図示しない電源より、中軸８、金属線７１、金属キャップ７、電極線４、発熱体２、電極線５、筒状部材６１、ハウジング６を経て、図示しないエンジンブロックへ通電が可能となる。
【００２３】
セラミックヒータ１の上記支持体３は、導電性セラミックであるＭｏＳｉ_２と、絶縁性セラミックであるＳｉ_３Ｎ_４を基本成分とし、焼結助剤としてＹ_２Ｏ_３およびＡｌ_２Ｏ_３を添加したセラミック焼結体よりなる。また、Ｙ_２Ｏ_３に代えて他の希土類元素の酸化物、例えばＹｂ_２Ｏ_３等を用いてもよく、その一種またはそれ以上を使用すればよい。そして、Ｓｉ_３Ｎ_４の粒径を、ＭｏＳｉ_２と同じかやや小さくすることにより、導電性のＭｏＳｉ_２粒子が絶縁性のＳｉ_３Ｎ_４粒子で囲まれて分断された組織となり、絶縁性を発現する。具体的には、例えば、平均粒径０．９μｍのＭｏＳｉ_２と、平均粒径０．６μｍのＳｉ_３Ｎ_４を用いることができる。
【００２４】
上記発熱体２は、ＭｏＳｉ_２等の導電性セラミックと、Ｓｉ_３Ｎ_４等の絶縁性セラミックを基本成分とし、焼結助剤として少なくともＡｌ_２Ｏ_３を添加したセラミック焼結体よりなる。焼結助剤として、さらにＹ_２Ｏ_３またはＹｂ_２Ｏ_３等の希土類元素の酸化物を添加してもよい。そして、Ｓｉ_３Ｎ_４の粒径をＭｏＳｉ_２より大きくすることにより、絶縁性のＳｉ_３Ｎ_４粒子が、互いに連続する導電性のＭｏＳｉ_２粒子で包まれた組織となり、導電性を発現する。具体的には、例えば、平均粒径０．９μｍのＭｏＳｉ_２と、平均粒径１３μｍのＳｉ_３Ｎ_４を用いることができる。
【００２５】
上記発熱体２または支持体３における導電性セラミックとしては、上記したＭｏＳｉ_２以外の金属の炭化物、珪化物、窒化物、またはホウ化物を用いてもよく、これらの少なくとも一種を使用する。導電性セラミックと絶縁性セラミックの配合割合は、例えば１０〜４０：９０〜６０（重量％）とすればよく、発熱体２、支持体３で同一またはそれに近い配合割合とすれば熱膨張係数等の差が小さくなるのでより好ましい。焼結助剤としては、Ｙ_２Ｏ_３、Ｙｂ_２Ｏ_３、またはＡｌ_２Ｏ_３以外の他の焼結助剤を少量添加することも可能である。
【００２６】
本発明では、上記支持体３の希土類元素の酸化物の添加量を、上記発熱体２よりも多くすること、および上記発熱体２のＡｌ_２Ｏ_３の添加量を上記支持体３のＡｌ_２Ｏ_３の添加量より多くすることを必須要件とする。具体的には、例えば、Ｓｉ_３Ｎ_４とＭｏＳｉ_２の総量に対する希土類元素の酸化物の添加量を、支持体３は７重量％、発熱体２は３重量％とすることで、長時間使用による抵抗値の上昇を防止できる。また、Ａｌ_２Ｏ_３の添加量を、支持体３は３重量％、発熱体２は７重量％とすることで、クラックの発生が抑制できる。
【００２７】
次に、この希土類元素の酸化物とＡｌ_２Ｏ_３の添加量の違いによる効果を確認するための試験を行なった。
（１）まず、以下のようにして試験用の試料を作成した。支持体と発熱体の基本成分をいずれも７０Ｓｉ_３Ｎ_４−３０ＭｏＳｉ_２（重量％）とし、支持体は平均粒径が０．９μｍのＭｏＳｉ_２と平均粒径が０．６μｍのＳｉ_３Ｎ_４を、発熱体には平均粒径が０．９μｍのＭｏＳｉ_２と平均粒径が１３μｍのＳｉ_３Ｎ_４を使用した。Ｓｉ_３Ｎ_４とＭｏＳｉ_２の総量に対する焼結助剤の添加量を、発熱体はＹ_２Ｏ_３３重量％、Ａｌ_２Ｏ_３７重量％とし、支持体はＹ_２Ｏ_３７重量％、Ａｌ_２Ｏ_３３重量％として、上記図１の構成のセラミックヒータを作成した（試料Ｎｏ１）。次いで、Ｙ_２Ｏ_３とＡｌ_２Ｏ_３の添加量を、表１のように変更し、支持体のＡｌ_２Ｏ_３添加量が発熱体より多いもの、発熱体のＹ_２Ｏ_３添加量が支持体より多いもの、これらを組合せたものについてそれぞれ試料を作成して試料Ｎｏ２〜４とした。さらに、発熱体、支持体ともにＹ_２Ｏ_３７重量％、Ａｌ_２Ｏ_３３重量％とした従来の組成によるヒータを作成し、比較用の試料Ｎｏ５とした。
【００２８】
焼成は、アルゴンガス雰囲気下、１気圧で、圧力は５００Ｋｇｆ／ｃｍ^２とし、焼結助剤量の変更による焼成条件の変化に対しては、各々の最適な焼成条件となるように、焼成温度を１５６０℃〜１８５０℃の範囲内で変えて行なった（以下に説明する試料は全て、各々の最適な焼成条件となるように、焼成温度を適宜変更して作成した）。
【００２９】
上記試料Ｎｏ１〜５のセラミックヒータを用いて上記図２に示したグロープラグを作成し、以下の試験を行なった。まず、通電の繰り返しによる抵抗値変化を調べるため、通電１分、非通電１分の繰り返しを１サイクルとした冷熱試験を行なった。このときのヒータ温度は初期に通電時の発熱による飽和温度を１３００℃にし、非通電時はファンでヒータを１００℃以下に冷却した。評価は各試料につき４本づつ同様の試験を実施して、そのうちの１本が抵抗値上昇により通電時のヒータ飽和温度が１００℃低下して１２００℃になったサイクル数を寿命サイクルとした。表１に結果を示す。
【００３０】
次に、クラックの発生に関し、水中スポーリング試験を行なった。これは、グロープラグに通電し、所定の飽和温度に発熱させた後、２０℃の水中に金属パイプから突出しているヒータ先端部を浸漬させ、表面に発生するクラックの有無を調査する試験で、具体的には、飽和温度が５００℃で水中スポーリング試験を行ない、クラックが発生していなければ、飽和温度を１００℃上げ、６００℃として水中スポーリング試験を行なった。このようにして１３００℃まで、もしくはクラックが発生するまで、１００℃づつ温度を上げて評価した。評価は各試料について４本づつ同様の試験を実施し、結果を表１に併記した。
【００３１】
表１より、冷熱試験については、発熱体のＹ_２Ｏ_３添加量（３重量％）を支持体の添加量（７重量％）より少なくした試料Ｎｏ１、２が、従来組成の試料Ｎｏ５に比べ、寿命が向上している。また、水中スポーリング試験については、発熱体のＡｌ_２Ｏ_３添加量（７重量％）を支持体の添加量（３重量％）より多くした試料Ｎｏ１、３が試料Ｎｏ５に比べ改良されていることがわかる。この結果、通電の繰り返しによる抵抗値変化とクラックの発生に対して共に効果があるのは、試料Ｎｏ１の場合、つまり、発熱体のＹ_２Ｏ_３添加量が支持体より少なく、かつ発熱体のＡｌ_２Ｏ_３添加量が支持体より多い場合であることがわかる。
【００３２】
（２）次に、この関係が添加量の多少にかかわらず成り立つかどうかを調べた。表２に示すように、試料Ｎｏ６〜９では、Ａｌ_２Ｏ_３添加量を一定（発熱体：７重量％、支持体：３重量％）とし、Ｙ_２Ｏ_３添加量を、支持体が発熱体より常に多くなるように保ったまま増減した。試料Ｎｏ１０〜１３では、Ｙ_２Ｏ_３添加量を一定（発熱体：３重量％、支持体：７重量％）とし、Ａｌ_２Ｏ_３添加量を発熱体が常に多くなるように保ったまま増減した。上記（１）と同様にして試料を作成し、評価を行なった。結果を表２に併記する。
【００３３】
表２の結果を従来組成の試料Ｎｏ５（表１）と比較すると、全試料で冷熱試験結果が向上しており、クラックの発生も見られない。なお、寿命サイクルは市場での信頼性を考慮すると１００００サイクル以上であることがより好ましく、特に、支持体のＹ_２Ｏ_３添加量、または発熱体のＡｌ_２Ｏ_３添加量が比較的少ない試料Ｎｏ６〜８、試料Ｎｏ１０〜１２で１２０００〜１５０００サイクルと高く、良好な結果が得られた。支持体のＹ_２Ｏ_３添加量を２５重量％とした試料Ｎｏ９と、発熱体のＡｌ_２Ｏ_３添加量を２０重量％とした試料Ｎｏ１３では冷熱試験の寿命サイクルがやや低い値となった。
【００３４】
Ｙ_２Ｏ_３およびＡｌ_２Ｏ_３の増量は、マトリックスを構成するＭｏＳｉ_２とＳｉ_３Ｎ_４の粒界のガラス層が増えることを意味する。試料Ｎｏ９は冷熱試験の温度条件で粒界のガラスが支持体表面へ溶出したものであり、試料Ｎｏ１３は粒界のガラス層が多いためＭｏＳｉ_２の導電パスが不安定になり、冷熱の繰り返しにより抵抗値が上昇したものと考えられる。
【００３５】
以上より、支持体については、Ｙ_２Ｏ_３添加量を２０重量％以内で発熱体より多くし、発熱体については、Ａｌ_２Ｏ_３添加量を１５重量％以内で支持体より添加量を多くすることがより望ましいといえる。
【００３６】
（３）次にＹ_２Ｏ_３とＡｌ_２Ｏ_３の添加量がある範囲を越えると、冷熱試験での寿命が短くなるという（２）の結果に基づき、添加総量の最適範囲を確認する試験を行なった。Ｙ_２Ｏ_３とＡｌ_２Ｏ_３の添加量を表３のように変更した以外は上記（１）と同様にして試料Ｎｏ１４〜１７を作成し、評価を行なった。結果をガラス溶出の有無とともに表３に併記する。
【００３７】
表３に明らかなように、いずれの試料も上記試料Ｎｏ５より冷熱試験における寿命サイクルが向上している。特に、発熱体および支持体のいずれも添加総量を２５重量％とした試料Ｎｏ１６、１７はガラスの溶出がなく、良好な結果が得られた。発熱体のＹ_２Ｏ_３とＡｌ_２Ｏ_３の添加総量を３０重量％とした試料Ｎｏ１４、１５はガラスの溶出が見られ、試料Ｎｏ１４は、寿命サイクルがやや短い。以上より、好ましくは、Ｙ_２Ｏ_３とＡｌ_２Ｏ_３の添加総量を発熱体、支持体ともに２５重量％以下とするのがよい。
【００３８】
（４）さらに、発熱体、支持体におけるＹ_２Ｏ_３またはＡｌ_２Ｏ_３の添加量の差が寿命サイクル、クラックの発生にどのように影響するかを確認するための試験を行なった。表４に示すように、Ａｌ_２Ｏ_３添加量を一定（発熱体：７重量％、支持体：３重量％）とし、Ｙ_２Ｏ_３添加量を変更して試料Ｎｏ１８〜２０を作成した。また、Ｙ_２Ｏ_３添加量を一定（発熱体：３重量％、支持体：７重量％）とし、Ａｌ_２Ｏ_３添加量を表のように変更して試料Ｎｏ２１〜２３を作成した。それぞれの試料につき、上記（１）と同様の試験を行ない、結果を表４に併記した。
【００３９】
表４の結果を上記試料Ｎｏ５と比較すると、冷熱試験における寿命サイクルはいずれも向上しており、発熱体より支持体のＹ_２Ｏ_３添加量が多ければ抵抗値変化に対し効果があることがわかる。特に、Ｙ_２Ｏ_３添加量の差が０．５重量％以上ある試料では寿命サイクルが１００００サイクルを越えており、より高い効果が得られる。また、クラックの発生に関しては、Ａｌ_２Ｏ_３添加量の差を０．５重量％以上とした試料Ｎｏ２１、２２ではクラックが全く発生しなかった。このように、Ｙ_２Ｏ_３、Ａｌ_２Ｏ_３ともに発熱体、支持体における添加量の差を０．５重量％以上とすることが好ましく、より高い効果が得られる。
【００４０】
（５）次に、発熱体のＹ_２Ｏ_３添加量、支持体のＡｌ_２Ｏ_３添加量の下限を確認するための試験を行なった。表４に示すように、Ａｌ_２Ｏ_３添加量を一定（発熱体：７重量％、支持体：３重量％）とし、Ｙ_２Ｏ_３添加量を表のように変更して試料Ｎｏ２４〜２６を作成した。また、Ｙ_２Ｏ_３の添加量を一定（発熱体：３重量％、支持体：７重量％）とし、Ａｌ_２Ｏ_３添加量を表のように変更して試料Ｎｏ２７〜２９を作成した。それぞれの試料につき、上記（１）と同様の試験を行ない、結果を表５に併記した。
【００４１】
表５の結果を上記試料Ｎｏ５と比較すると、冷熱試験における寿命サイクルは、発熱体のＹ_２Ｏ_３添加量を０重量％とした試料Ｎｏ２４においても他の試料と同等の効果が見られた。また、水中スポーリング試験に関しては、支持体のＡｌ_２Ｏ_３添加量が０重量％の試料Ｎｏ２７ではクラックの発生が見られるが、Ａｌ_２Ｏ_３を０．１重量％添加した試料Ｎｏ２８ではクラックが発生しなかった。従って、発熱体には必ずしもＹ_２Ｏ_３を添加する必要はなく、支持体においてはＡｌ_２Ｏ_３を０．１重量％以上添加することが望ましいことがわかる。
【００４２】
（６）なお、上記実施例では導電性セラミックとして、ＭｏＳｉ_２の場合について述べたが、導電性セラミックを他の金属の炭化物、窒化物、ホウ化物としてもよく、同様の効果が得られる。これを確認するため、表６に示すように、導電性セラミックをＷＣ、ＴａＣ、ＴｉＮ、ＺｒＢ_２に変更し、それぞれについて、Ｙ_２Ｏ_３、Ａｌ_２Ｏ_３の添加量を上記試料Ｎｏ１、試料Ｎｏ５と同じにした試料を作成して（試料Ｎｏ３０〜３６）、上記（１）と同様の試験を行なった。結果を表６に併記する。表中、試料Ｎｏ３０、３２、３４、３６が試料Ｎｏ１と、試料Ｎｏ３１、３３、３５、３７が試料Ｎｏ５と助剤の添加量を同じにした例である。表に明らかなように、導電性セラミックの種類を変更した場合においても、支持体のＹ_２Ｏ_３添加量を発熱体より多く、発熱体のＡｌ_２Ｏ_３添加量を支持体より多くすることで、寿命サイクルが大きく改善され、クラックの発生が抑制されていることがわかる。
【００４３】
さらに、焼結助剤として、Ｙ_２Ｏ_３と同様の汎用性のあるＹｂ_２Ｏ_３を用いた場合の効果を調べた。上記（１）の試料Ｎｏ１〜５のＹ_２Ｏ_３の代わりにＹｂ_２Ｏ_３を用いた以外は同様にして５種類の試料を作成し（試料Ｎｏ３８〜４２）、試験を行なった。結果を表７に示す。表に明らかなように、Ｙｂ_２Ｏ_３を用いた場合でも、寿命サイクル、クラックの発生に対し同様の効果が得られることがわかる。
【００４４】
以上の結果に明らかなように、発熱体、支持体ともに、焼結助剤として希土類元素の酸化物とＡｌ_２Ｏ_３を添加し、かつ支持体の希土類元素の酸化物の添加量を発熱体より多くし、発熱体のＡｌ_２Ｏ_３量を支持体より多くすることにより、通電の繰り返しによる抵抗値変化と、クラックの発生に対し効果があることがわかる。さらに、発熱体において、導電性セラミックおよび絶縁性セラミックの総量に対し、希土類元素の酸化物の添加量を０〜１９．５重量％、Ａｌ_２Ｏ_３の添加量を０．１〜１５重量％とし、かつこれら焼結助剤の添加総量が２５重量％以下とすること、支持体においては、導電性セラミックおよび絶縁性セラミックの総量に対し、希土類元素の酸化物の添加量を０．１〜２０重量％、Ａｌ_２Ｏ_３の添加量を０．１〜１４．５重量％とし、かつこれら焼結助剤の添加総量を２５重量％以下とすることでより大きな効果が得られる。
【００４５】
【表１】

【００４６】
【表２】

【００４７】
【表３】

【００４８】
【表４】

【００４９】
【表５】

【００５０】
【表６】

【００５１】
【表７】

【００５２】
【発明の効果】
このように、本発明によれば、高温で使用しても抵抗値の変化が小さく、かつ、クラックの発生しない耐熱衝撃性に優れたセラミックヒータが得られる。従って、グロープラグ等に適用されてその信頼性を大きく向上することができる。
【図面の簡単な説明】
【図１】本発明の一実施例を示すセラミックヒータの断面図である。
【図２】本発明のセラミックヒータを適用したグロープラグの全体断面図である。
【符号の説明】
１セラミックヒ−タ
２発熱体
３支持体
４、５電極線[0001]
[Industrial applications]
The present invention relates to a ceramic heater, and more particularly to a ceramic heater suitably used for a ceramic glow plug of a diesel engine.
[0002]
[Prior art]
In order to assist the start of a diesel engine, a ceramic glow plug is disposed in a combustion chamber, and a heating section is energized and heated to promote ignition and combustion of fuel. Various types of ceramic heaters have been proposed as the ceramic heater constituting the heat generating portion. For example, Japanese Patent Application Laid-Open No. 62-140386 discloses that a conductive titanium nitride is dispersed in a sialon sintered body. A heater using a composite sintered body is disclosed.
[0003]
Further, a ceramic heater in which a heating element made of a conductive ceramic is provided at the tip of a support made of an insulating ceramic is known. For example, silicon nitride, silicon oxide is attached to the tip of a rod-shaped insulating ceramic made of aluminum oxide, or the like. There is a configuration provided with a U-shaped conductive ceramic made of a mixture of manganese and molybdenum silicide.
[0004]
However, in the above-described conventional ceramic heater, a thermal stress is generated between the support and the heating element due to a rapid temperature rise or cooling due to a difference in thermal expansion coefficient between the support and the heating element, and there is a possibility that the joint may be damaged. Therefore, the present applicant has previously constructed both the support and the heating element with a mixture of conductive molybdenum silicide (MoSi ₂ ) and insulating silicon nitride (Si ₃ N ₄ ). by conductive MoSi ₂ particles are separated from each other by insulating the Si ₃ N ₄ particles enveloping it shows insulating properties in the heating element, Si ₃ insulating a conductive MoSi ₂ particles succeed one another It proposed a ceramic heater None to exhibit conductivity by wrapping the N ₄ particles (JP 63-96883 JP). More specifically, the basic components of the support and the heating element are both 70Si ₃ N ₄ -30MoSi ₂ (% by weight), and for the total amount thereof, for example, yttrium oxide (Y ₂ O ₃ ) is used as a sintering aid. 7% by weight and 3% by weight of aluminum oxide (Al ₂ O ₃ ) are added, and by using the same composition for the support and the heating element, thermal stress is greatly reduced.
[0005]
[Problems to be solved by the invention]
By the way, in recent years, there has been an increasing demand for further improving the quick heating property of the ceramic heater and shortening the waiting time until the engine is started. Accordingly, the temperature of the heater has been increased from the conventional 1000 ° C. to 1100 ° C. to 1200 ° C. to 1300 ° C. ° C.
[0006]
However, when the ceramic heater having the above-described configuration in which the maximum temperature was set to 1300 ° C. was used in an engine, the following problem occurred. One of them is that the resistance value increases after long-term use, which lowers the saturation temperature and deteriorates the startability of the engine. The other is that a crack may be generated in a portion of the ceramic heater where fuel directly hits, and when the crack progresses, a part of the heater may fall into the engine and damage the engine.
[0007]
As a result of investigating these causes, the following was found. First, regarding the increase in the resistance value, the heater whose resistance value has been increased for a long time was cut in the axial direction, and the heating element and its vicinity were examined. Or, the heating element has the highest temperature at the end from the negative electrode. Among these, yttrium (Y) concentrates on the highest temperature part on the negative electrode side, and conversely, there is almost no Y on the positive electrode side, and molybdenum It was found that the oxide of (Mo) was increased. Similarly, in the support located between the highest temperature portions of the heating element, the amount of Y was similarly large on the negative electrode side, gradually decreased, and almost no Y was present on the positive electrode side.
[0008]
This phenomenon is due to the action of an electric field caused by energization, Y ₂ O ₃ of high-temperature portion is decomposed and moved Y is the negative electrode side and oxygen remaining in the positive electrode side is oxidized to MoSi _2, from MoSi ₂ Means that the current path is thinned or disconnected. In particular, in a high temperature state of 1300 ° C., the above-mentioned movement of Y is likely to occur, which is considered to have caused a rise in the resistance value.
[0009]
On the other hand, regarding the generation of cracks, when an unused heater was cut in the axial direction and observed in detail, pores were observed in the heating element. In the ceramic heater having the above configuration, the support and the heating element have the same composition so that the thermal stress is reduced.However, in order to impart contradictory properties of insulating or conductive to each, the particle size of the ceramic particles is changed. In the heating element, Si ₃ N ₄ having an average particle diameter of 13 μm and MoSi ₂ having an average particle diameter of 0.9 μm are used. As the support, Si ₃ N ₄ having an average particle diameter of 0.6 μm and MoSi ₂ having an average particle diameter of 0.9 μm are used. ing. Therefore, it is considered that the optimum sintering conditions of the support and the heating element were shifted due to the difference in the particle diameter of the ceramic particles, and the heating element was insufficiently sintered and pores were generated. Under a new temperature condition of 1300 ° C., these pores cause cracks, and thermal stress acts due to a temperature difference between the surface and the inside and in the axial direction, cooling by the spray fuel, etc., and the thermal shock resistance is reduced. It seems to have been.
[0010]
The present invention has been made in view of the above circumstances, and has as its object to provide a ceramic heater having a small resistance change even under a high temperature condition of 1300 ° C. or more and having excellent thermal shock resistance without cracks. Things.
[0011]
[Means for Solving the Problems]
The configuration of the present invention will be described with reference to FIG. 1. A ceramic heater 1 includes an electrically insulating support 3 and a conductive heating element 2 integrally formed at a tip end thereof. Each of the bodies 2 is made of a mixed sintered body of a conductive ceramic and an insulating ceramic. The support 3 shows insulation by the conductive ceramic particles being separated from each other by insulating ceramic particles surrounding the conductive ceramic particles, and the heating element 2 wraps the insulating ceramic particles with the conductive ceramic particles continuous with each other. This indicates conductivity. The heating element 2 contains at least aluminum oxide (Al ₂ O ₃ ) as a sintering aid, and the support 3 contains at least a rare earth element oxide and Al ₂ O ₃ as a sintering aid. and the _Al 2 _{O 3} addition amount of the heating body 2 are then larger than _Al 2 _{O 3} addition amount of the support 3 (claim 1).
[0012]
Further, the heating element 2 may contain an oxide of a rare earth element in addition to Al ₂ O ₃ as a sintering aid. In this case, the amount of the oxide of the rare earth element of the support 3 is added. Is larger than that of the heating element 2, and the amount of Al ₂ O ₃ added to the heating element 2 is larger than that of the support 3 (Claim 3).
[0013]
In the heating element 2, the amount of Al ₂ O ₃ added to the total amount of the conductive ceramic and the insulating ceramic is preferably 0.1 to 15% by weight (Claim 2). Further, when the oxide of the rare earth element is contained, the amount of addition to the total amount of the conductive ceramic and the insulating ceramic is 0.1 to 19.5% by weight, and the total amount of addition of these sintering aids is 25% by weight. It is preferable to set the following (claim 4).
[0014]
In the support 3, the amount of the rare earth element oxide added is 0.1 to 20% by weight and the amount of Al ₂ O ₃ is 0.1 to 14.5% by weight based on the total amount of the conductive ceramic and the insulating ceramic. %, And the total added amount of these sintering aids is desirably 25% by weight or less (claim 5).
[0015]
The amount of the rare earth element oxide added to the support 3 is 0.5% by weight or more than that of the heating element 2, and the amount of Al ₂ O ₃ added to the heating element 2 is 0.5% more than that of the support 3. It is preferable to increase the amount by weight or more (claim 6).
[0016]
Examples of the rare earth oxide include yttrium oxide (Y ₂ O ₃ ) and ytterbium oxide (Yb ₂ O ₃ ). Silicon nitride is preferably used as the insulating ceramic. Further, as the conductive ceramic, metal carbide, silicide, nitride, or boride can be used, and at least one selected from these may be used (claim 8).
[0017]
[Action]
The heating element 2 has the highest temperature at the end (the portion a or b in FIG. 1) of the tip portion bent in a U-shape from the positive or negative electrode. In the ceramic heater 1 of the present invention, the amount of the oxide of the rare earth element, for example, Y ₂ O ₃ added to the support 3 is larger than the amount of Y ₂ O ₃ added to the heating element 2. There will be a support 3 rich in Y ₂ O ₃ between the parts shown (part c in FIG. 1). When an electric field acts on the heater material during energization, Y moves toward the highest temperature portion on the negative electrode side of the heating element 2, but first, Y moves from the vicinity of the support _{3 having} a large amount of Y ₂ O _3. Then, to the site where Y has moved, Y further moves from the surrounding support 3. At this time, around the maximum temperature portion of the positive electrode side of the heat generating element 2, since Y ₂ O ₃ of high carrier 3 is present, the less likely the movement of Y in the heat generating element 2. Until Y in the support 3 moves to some extent and becomes almost equal to the amount of Y ₂ O ₃ of the heating element 2, Y does not move on the positive electrode side of the heating element 2, and the resistance value increases. Be suppressed.
[0018]
Further, Al ₂ O ₃ has a function of promoting sintering at a low melting point. In the ceramic heater 1 according to the present invention, since the amount of Al ₂ O ₃ added to the heating element 2 is larger than that of the support 3, sintering of the heating element 2 is promoted, and insufficient sintering is eliminated. Therefore, pores in the sintered body are greatly reduced, and the occurrence of cracks is prevented.
[0019]
【Example】
Hereinafter, the present invention will be described with reference to examples.
FIG. 2 shows a glow plug of a diesel engine to which the present invention is applied. Reference numeral 6 denotes a cylindrical metal housing having openings at both ends, and a cylindrical metal member 61 is fixed in the lower end opening. The ceramic heater 1 of the present invention is inserted into the housing 6 from below, and an intermediate portion thereof is fitted and fixed to the cylindrical metal member 61. A mounting screw 62 is formed on the outer periphery of the central portion of the housing 6, and the glow plug is mounted on an engine (not shown) by the mounting screw 62.
[0020]
A metal cap 7 for power connection is fixed to the upper end of the ceramic heater 1. The metal cap 7 is connected to a metal center shaft 8 inserted into the upper half of the housing 6 and a metal wire 71. It is connected. A glass seal 9 is disposed around the center shaft 8 at the upper end of the housing 6, and an insulating bush 10 is fitted from above to further insulate the center shaft 8 electrically. A male screw 81 connected to a power supply (not shown) is formed at the base end of the center shaft 8, and the insulating bush 10 is fixed by a nut 82 screwed to the male screw 81.
[0021]
FIG. 1 shows details of the ceramic heater 1. In the figure, a ceramic heater 1 includes a support 3 which is a rod-shaped body having a circular cross section, and a heating element 2 having a U-shaped cross section which is embedded in the tip of the support 3. A distal end 41 of the electrode wire 4 is embedded at one end of the heating element 2, and the other end of the electrode wire 4 extends to a base end of the support 3 to form a terminal portion 42 exposed on the outer peripheral surface of the support 3. I have. Further, the other end of the heating element 2 is embedded with the tip 51 of the electrode wire 5, and the other end of the electrode wire 5 forms a terminal portion 52 which is exposed at the outer peripheral surface of the support 3 in the middle of the support 3. . The

electrode wires

4 and 5 are made of a refractory metal such as tungsten or molybdenum or an alloy thereof.
[0022]
The outer peripheral surface of the support 3 where the

terminal portions

42 and 52 of the

electrode wires

4 and 5 are exposed is plated with nickel. When the ceramic heater 1 is inserted into the housing 6 (FIG. 2), the support 3 is brazed to the inner peripheral surface of the tubular metal member 61 via the nickel plating layer. The cylindrical metal member 61 holds the ceramic heater 1 and is electrically connected to the terminal 52 of the electrode wire 5. On the other hand, the terminal portion 42 of the electrode wire 4 exposed at the base end of the support 3 is brazed to the inner peripheral surface of the metal cap 7, and is connected to the power supply via the metal shaft 71 and the center shaft 8. Thus, power can be supplied to an engine block (not shown) from the power supply (not shown) through the center shaft 8, the metal wire 71, the metal cap 7, the electrode wire 4, the heating element 2, the electrode wire 5, the tubular member 61, and the housing 6.
[0023]
The support 3 of the ceramic heater 1 was made of MoSi _{2 as} a conductive ceramic and Si ₃ N ₄ as an insulating ceramic as basic components, and Y ₂ O ₃ and Al ₂ O ₃ were added as sintering aids. It consists of a ceramic sintered body. Further, an oxide of another rare earth element, for example, Yb ₂ O ₃ or the like may be used instead of Y ₂ O _3, and one or more of them may be used. Then, the particle size the _Si 3 _{N 4,} by the same or slightly smaller and MoSi _2, conductive MoSi ₂ particles becomes tissue divided surrounded by insulating the _Si 3 _{N 4} particles, the insulating Express. Specifically, for example, MoSi ₂ having an average particle size of 0.9 μm and Si ₃ N ₄ having an average particle size of 0.6 μm can be used.
[0024]
The heating element 2 is made of a ceramic sintered body having a conductive ceramic such as MoSi ₂ and an insulating ceramic such as Si ₃ N ₄ as basic components and adding at least Al ₂ O ₃ as a sintering aid. As a sintering aid, an oxide of a rare earth element such as Y ₂ O ₃ or Yb ₂ O ₃ may be further added. Then, by making the particle size of Si ₃ N ₄ larger than that of MoSi ₂ , the insulating Si ₃ N ₄ particles become a structure wrapped with conductive MoSi ₂ particles that are continuous with each other, and exhibit conductivity. Specifically, for example, MoSi ₂ having an average particle size of 0.9 μm and Si ₃ N ₄ having an average particle size of 13 μm can be used.
[0025]
As the conductive ceramic in the heating element 2 or the support 3, carbide, silicide, nitride, or boride of a metal other than MoSi ₂ described above may be used, and at least one of them is used. The mixing ratio of the conductive ceramic and the insulating ceramic may be, for example, 10 to 40: 90 to 60 (% by weight), and if the mixing ratio of the heating element 2 and the support 3 is the same or close thereto, the coefficient of thermal expansion, etc. Is more preferable because the difference between the two becomes smaller. As the sintering aid, a small amount of a sintering aid other than Y ₂ O ₃ , Yb ₂ O ₃ , or Al ₂ O ₃ can be added.
[0026]
In the present invention, the addition of the oxide of a rare earth element of the support 3, the heating body 2 more than, and Al ₂ of the support 3 the addition amount of Al ₂ O ₃ of the heat generating element 2 It is an essential requirement that the amount be larger than the added amount of O ₃ . Specifically, for example, the amount of the oxide of the rare earth element added to the total amount of Si ₃ N ₄ and MoSi ₂ is set to 7% by weight for the

support

3 and 3% by weight for the heating element 2 so as to be used for a long time. The resistance value can be prevented from increasing. Further, by setting the addition amount of Al ₂ O ₃ to 3% by weight of the

support

3 and 7% by weight of the heating element 2, generation of cracks can be suppressed.
[0027]
Next, a test was performed to confirm the effect of the difference in the amount of the rare earth element oxide and Al ₂ O ₃ added.
(1) First, a test sample was prepared as follows. The basic components of the support and the heating element were both 70Si ₃ N ₄ -30MoSi ₂ (wt%), and the support was MoSi ₂ having an average particle size of 0.9 μm and Si ₃ N ₄ having an average particle size of 0.6 μm. MoSi ₂ having an average particle diameter of 0.9 μm and Si ₃ N ₄ having an average particle diameter of 13 μm were used for the heating element. Si of ₃ N ₄ and the addition amount of sintering aid to the total amount of MoSi _2, heating element _Y 2 _{O 3} 3% by weight, and _Al 2 _{O 3} 7 wt%, the support is _Y 2 _{O 3} 7% by weight, A ceramic heater having the configuration shown in FIG. 1 was prepared with 3% by weight of Al ₂ O ₃ (sample No. 1). _Then, Y the amount of 2 _{O 3} and _Al 2 _{O 3,} and changed as shown in Table 1, those _Al 2 _{O 3} addition amount of the support is larger than the heating element, _Y 2 _{O 3} addition amount of the heating element Samples were prepared for each of more than the support and those obtained by combining these, and Sample Nos. 2 to 4 were obtained. Further, a heater having a conventional composition in which both the heating element and the support were 7% by weight of Y ₂ O _{3 and 3} % by weight of Al ₂ O ₃ was prepared, and a sample No. 5 for comparison was prepared.
[0028]
The firing was performed under an argon gas atmosphere at 1 atm and a pressure of 500 kgf / cm ^2, and the firing temperature was adjusted so that the optimum firing conditions would be obtained with respect to changes in firing conditions due to changes in the amount of the sintering aid. Was carried out within the range of 1560 ° C. to 1850 ° C. (all the samples described below were prepared by appropriately changing the firing temperature so as to obtain the optimum firing conditions).
[0029]
The glow plug shown in FIG. 2 was prepared using the ceramic heaters of Sample Nos. 1 to 5, and the following tests were performed. First, in order to examine a change in resistance value due to repetition of energization, a cooling / heating test was performed in which repetition of 1 minute of energization and 1 minute of non-energization were defined as one cycle. At this time, the heater temperature was initially set to a saturation temperature of 1300 ° C. due to heat generation during energization, and the heater was cooled to 100 ° C. or less by a fan when no current was applied. For the evaluation, the same test was performed on four samples for each sample, and the life cycle was defined as the number of cycles in which one of the samples decreased the heater saturation temperature during energization by 100 ° C. to 1200 ° C. due to an increase in the resistance value. Table 1 shows the results.
[0030]
Next, an underwater spalling test was performed for the occurrence of cracks. This is a test to energize the glow plug and generate heat to a predetermined saturation temperature, then immerse the heater tip protruding from the metal pipe in water at 20 ° C, and investigate the presence or absence of cracks generated on the surface. Specifically, an underwater spalling test was performed at a saturation temperature of 500 ° C., and if no crack was generated, the underwater spalling test was performed at a saturation temperature of 100 ° C. and 600 ° C. In this way, the temperature was increased by 100 ° C. until 1300 ° C. or cracks occurred, and the evaluation was performed. For the evaluation, the same test was performed for each of the four samples, and the results are shown in Table 1.
[0031]
From Table 1, in the thermal test, Sample Nos. 1 and _{2 in} which the amount of Y ₂ O ₃ added to the heating element (3% by weight) was smaller than the amount of the support added (7% by weight) were compared with Sample No. 5 of the conventional composition. , Life is improved. In the underwater spalling test, Samples Nos. 1 and 3, in which the amount of Al ₂ O ₃ added to the heating element (7% by weight) was larger than the amount of support added (3% by weight), were improved as compared with Sample No. 5. You can see that. As a result, both the resistance value change and the occurrence of cracks due to the repetition of energization are effective in the case of sample No. 1, that is, the amount of Y ₂ O ₃ added to the heating element is smaller than that of the support, and It can be seen that this is the case where the added amount of Al ₂ O ₃ is larger than that of the support.
[0032]
(2) Next, it was examined whether this relationship holds regardless of the amount of addition. As shown in Table 2, in Samples Nos. 6 to 9, the addition amount of Al ₂ O ₃ was constant (heating element: 7% by weight, support: 3% by weight), and the addition amount of Y ₂ O ₃ was determined. Increased or decreased while always keeping more than the body. In Samples Nos. 10 to 13, the addition amount of Y ₂ O ₃ was fixed (heating element: 3% by weight, support: 7% by weight), and the addition amount of Al ₂ O ₃ was increased and decreased while the heating element was always increased. did. A sample was prepared and evaluated in the same manner as in the above (1). The results are also shown in Table 2.
[0033]
Comparing the results in Table 2 with Sample No. 5 (Table 1) of the conventional composition, the results of the thermal test were improved for all the samples, and no cracks were observed. The life cycle is more preferably 10,000 cycles or more in consideration of the reliability in the market. In particular, a sample in which the amount of Y ₂ O ₃ added to the support or the amount of Al ₂ O ₃ added to the heating element is relatively small. Nos. 6 to 8 and samples Nos. 10 to 12 were as high as 12000 to 15000 cycles, and good results were obtained. In Sample No. 9 in which the amount of Y ₂ O ₃ added to the support was 25% by weight and in Sample No. 13 in which the amount of Al ₂ O ₃ added to the heating element was 20% by weight, the life cycle of the thermal test was slightly lower.
[0034]
The increase in the amount of Y ₂ O ₃ and Al ₂ O ₃ means that the glass layer at the grain boundaries of MoSi ₂ and Si ₃ N ₄ constituting the matrix increases. In sample No. 9, the glass at the grain boundary was eluted to the surface of the support under the temperature condition of the cold test, and in sample No. 13, the conductive path of MoSi ₂ became unstable due to the large number of glass layers at the grain boundary, and the repeated cooling and heating It is considered that the resistance value increased.
[0035]
As described above, for the support, the addition amount of Y ₂ O ₃ is set to be larger than that of the heating element within 20% by weight, and for the heating element, the addition amount of Al ₂ O ₃ is set to be larger than that of the support within 15% by weight. It is more desirable to do so.
[0036]
(3) Next, a test for confirming the optimum range of the total amount of addition based on the result of (2) that when the addition amounts of Y ₂ O ₃ and Al ₂ O ₃ exceed a certain range, the service life in the thermal test becomes short. Was performed. Sample Nos. 14 to 17 were prepared and evaluated in the same manner as in (1) above, except that the addition amounts of Y ₂ O ₃ and Al ₂ O ₃ were changed as shown in Table 3. The results are shown in Table 3 together with the presence or absence of glass elution.
[0037]
As is clear from Table 3, all of the samples have improved life cycles in the thermal test compared to Sample No. 5 described above. In particular, Sample Nos. 16 and 17, in which the total amount of both the heating element and the support was 25% by weight, had no glass elution, and good results were obtained. Samples Nos. 14 and 15 in which the total amount of the heating elements Y ₂ O ₃ and Al ₂ O ₃ were 30% by weight showed glass elution, and Sample No. 14 had a slightly shorter life cycle. From the above, it is preferable that the total amount of Y ₂ O ₃ and Al ₂ O ₃ added be 25 wt% or less for both the heating element and the support.
[0038]
(4) Further, a test was conducted to confirm how the difference in the amount of Y ₂ O ₃ or Al ₂ O ₃ added between the heating element and the support affects the life cycle and the occurrence of cracks. As shown in Table 4, samples Nos. 18 to 20 were prepared with the amount of Al ₂ O ₃ added constant (heating element: 7% by weight, support: 3% by weight) and the amount of Y ₂ O ₃ added changed. Samples Nos. 21 to 23 were prepared with the addition amount of Y ₂ O ₃ being constant (heating element: 3% by weight, support: 7% by weight) and the addition amount of Al ₂ O ₃ changed as shown in the table. The same test as in the above (1) was performed for each sample, and the results are shown in Table 4.
[0039]
Comparing the results in Table 4 with Sample No. 5 above, the life cycles in the thermal test were all improved, indicating that the larger the amount of Y ₂ O ₃ added to the support than the heating element, the more effective the resistance value change. Understand. In particular, in the sample in which the difference in the amount of added Y ₂ O ₃ is 0.5% by weight or more, the life cycle exceeds 10,000 cycles, and a higher effect can be obtained. Regarding the occurrence of cracks, cracks did not occur at all in Sample Nos. 21 and 22 in which the difference in the amount of Al ₂ O ₃ added was 0.5% by weight or more. As described above, it is preferable that the difference between the amounts of Y ₂ O ₃ and Al ₂ O ₃ added in the heating element and the support is 0.5% by weight or more, and a higher effect can be obtained.
[0040]
(5) Next, a test was performed to confirm the lower limit of the added amount of Y ₂ O ₃ of the heating element and the added amount of Al ₂ O ₃ of the support. As shown in Table 4, the amount of Al ₂ O ₃ added was fixed (heating element: 7% by weight, support: 3% by weight), and the amount of Y ₂ O ₃ added was changed as shown in the table, and Sample Nos. 24-26. It was created. Samples Nos. 27 to 29 were prepared with the addition amount of Y ₂ O ₃ being constant (heating element: 3% by weight, support: 7% by weight) and the addition amount of Al ₂ O ₃ changed as shown in the table. The same test as in the above (1) was performed for each sample, and the results are shown in Table 5.
[0041]
Comparing the results in Table 5 with Sample No. 5 above, the same life cycle as in the other samples was observed in Sample No. 24 in which the amount of Y ₂ O ₃ added to the heating element was 0% by weight in the thermal cycle test. Regarding the underwater spalling test, cracks were observed in sample No. 27 in which the amount of Al ₂ O ₃ added to the support was 0% by weight, but cracks were observed in sample No. 28 in which 0.1% by weight of Al ₂ O ₃ was added. Did not occur. Therefore, it is understood that it is not necessary to add Y ₂ O ₃ to the heating element, and it is desirable to add Al ₂ O ₃ to the support at 0.1% by weight or more.
[0042]
(6) In the above embodiment, the case where MoSi ₂ is used as the conductive ceramic has been described. However, the conductive ceramic may be a carbide, nitride, or boride of another metal, and the same effect can be obtained. To confirm this, as shown in Table 6, the conductive ceramic WC, TaC, TiN, and change the ZrB _2, _each, Y ₂ O 3, _Al 2 amount of the sample No1 of _{O 3,} the sample Samples the same as No. 5 were prepared (samples Nos. 30 to 36), and the same test as in the above (1) was performed. The results are also shown in Table 6. In the table, Samples Nos. 30, 32, 34, and 36 are examples in which the additive amounts of the sample No. 1 and Sample Nos. 31, 33, 35, and 37 are the same as those of Sample No. 5 in the additive amount. As is clear from the table, even when the type of the conductive ceramic is changed, the amount of Y ₂ O ₃ added to the support is larger than that of the heating element, and the amount of Al ₂ O ₃ added to the heating element is larger than that of the support. It can be seen that the life cycle was greatly improved, and the occurrence of cracks was suppressed.
[0043]
Further, the effect of using Yb ₂ O ₃ having the same general versatility as Y ₂ O ₃ as a sintering aid was examined. Five kinds of samples were prepared in the same manner except that Yb ₂ O ₃ was used instead of Y ₂ O ₃ of samples Nos. 1 to 5 in (1) above (samples Nos. 38 to 42) and tested. Table 7 shows the results. As is clear from the table, even when Yb ₂ O ₃ is used, the same effect can be obtained on the life cycle and crack generation.
[0044]
As is clear from the above results, for both the heating element and the support, a rare earth element oxide and Al ₂ O ₃ were added as sintering aids, and the amount of the rare earth element oxide added to the support was adjusted to the heating element. It can be seen that increasing the number of heating elements and increasing the amount of Al ₂ O ₃ of the heating element as compared with the support has an effect on the change in resistance value due to repetition of energization and the occurrence of cracks. Further, in the heating element, the addition amount of the rare earth element oxide is 0 to 19.5% by weight and the addition amount of Al ₂ O ₃ is 0.1 to 15% by weight based on the total amount of the conductive ceramic and the insulating ceramic. And the total amount of these sintering additives is 25% by weight or less. In the support, the amount of the oxide of the rare earth element is 0.1 to 10% of the total amount of the conductive ceramic and the insulating ceramic. Greater effects can be obtained by setting the addition amount of Al ₂ O ₃ to 0.1 to 14.5% by weight and the total addition amount of these sintering aids to 25% by weight or less.
[0045]
[Table 1]

[0046]
[Table 2]

[0047]
[Table 3]

[0048]
[Table 4]

[0049]
[Table 5]

[0050]
[Table 6]

[0051]
[Table 7]

[0052]
【The invention's effect】
As described above, according to the present invention, it is possible to obtain a ceramic heater which has a small change in resistance value even when used at a high temperature and has excellent thermal shock resistance without cracks. Therefore, the present invention is applied to a glow plug or the like, and its reliability can be greatly improved.
[Brief description of the drawings]
FIG. 1 is a sectional view of a ceramic heater showing one embodiment of the present invention.
FIG. 2 is an overall sectional view of a glow plug to which the ceramic heater of the present invention is applied.
[Explanation of symbols]
REFERENCE SIGNS LIST 1 ceramic heater 2 heating element 3

support

4, 5 electrode wire

Claims

An electrically insulating support, comprising a conductive heating element integrally formed at the tip thereof, wherein the support and the heating element are each formed of a mixed sintered body of conductive ceramic and insulating ceramic, The support has an insulating property in that the conductive ceramic particles are separated from each other by insulating ceramic particles surrounding the conductive ceramic particles, and the heating element is formed by wrapping the insulating ceramic particles with the conductive ceramic particles continuous with each other. In a ceramic heater exhibiting conductivity, at least aluminum oxide is added to the heating element as a sintering aid, and at least an oxide of a rare earth element and aluminum oxide are added to the support as a sintering aid. A ceramic characterized in that the amount of aluminum oxide added to the heating element is greater than the amount of aluminum oxide added to the support. Heater.

2. The ceramic heater according to claim 1, wherein the amount of aluminum oxide in the heating element is 0.1 to 15% by weight based on the total amount of the conductive ceramic and the insulating ceramic.

An electrically insulating support, comprising a conductive heating element integrally formed at the tip thereof, wherein the support and the heating element are each formed of a mixed sintered body of conductive ceramic and insulating ceramic, The support has an insulating property in that the conductive ceramic particles are separated from each other by insulating ceramic particles surrounding the conductive ceramic particles, and the heating element is formed by wrapping the insulating ceramic particles with the conductive ceramic particles continuous with each other. In a ceramic heater having conductivity, at least an oxide of a rare earth element and aluminum oxide are added as sintering aids to the support and the heating element, and the amount of the oxide of the rare earth element of the support is added. Is larger than the amount of the rare earth element oxide added to the heating element, and the amount of the aluminum oxide added to the heating element is Ceramic heater, characterized in that the more than additive amount.

In the heating element, the addition amount of the rare earth element oxide is 0.1 to 19.5% by weight, and the addition amount of aluminum oxide is 0.1 to 15% by weight based on the total amount of the conductive ceramic and the insulating ceramic. 4. The ceramic heater according to claim 3, wherein the total amount of the rare earth oxide and the aluminum oxide is 25% by weight or less.

In the above support, the addition amount of the rare earth element oxide is 0.1 to 20% by weight, and the addition amount of aluminum oxide is 0.1 to 14.5% by weight based on the total amount of the conductive ceramic and the insulating ceramic; 5. The ceramic heater according to claim 1, wherein the total amount of the rare earth element oxide and aluminum oxide added is 25% by weight or less.

The amount of the rare earth element oxide added to the support was increased by 0.5% by weight or more than the heating element, and the amount of aluminum oxide added to the heating element was increased by 0.5% by weight or more than the support. The ceramic heater according to any one of claims 1 to 5, wherein:

7. The ceramic heater according to claim 1, wherein the oxide of the rare earth element is yttrium oxide or ytterbium oxide.

8. The ceramic heater according to claim 1, wherein the insulating ceramic is silicon nitride, and the conductive ceramic is at least one selected from metal carbides, silicides, nitrides, and borides.