JP2004226171A - Oxygen concentration detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To activate a solid electrolyte layer at an early stage when starting an engine, etc., for example, by efficiently conducting heat from a heater part to the electrolyte layer. <P>SOLUTION: A detection element 21 for oxygen concentration is made up of the heater part 22, the solid electrolyte layer 27, an inside electrode 28, an outside electrode 29, and the like. Between the heater part 22 and the inside electrode 28, a porous layer 26 is provided having better thermal conductivity than air has and having an internal vent path 26B comprising vacancies of continuous gas bubbles for letting oxygen flow from the position of the inside electrode 28 in the length direction (the arrow A direction) of the heated part 22. Further, on the outer circumferential side of the electrolyte layer 27, an opening part 31 through which a measured gas passes is provided in a position corresponding to the outside electrode 29 and, in portions other than the opening part 31, a dense layer 30 is provided for covering the electrolyte layer 27, the inside electrode 28, and the like, from the outside to inhibit the measured gas from permeating into the interior. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【００９４】
そして、このような酸素濃度の検出素子２１に対する特性評価試験を行った結果、図５中に実線で示す特性線３７のように、検出素子２１の素子温度をエンジン始動後に５秒以内で、例えば６００℃まで昇温できることが確認された。この評価試験は、外気温度が−４０℃で、ヒータ電源３３の電圧を１２Ｖ（ボルト）とした条件下で行われたものである。
【００９５】
これに対し、従来技術の検出素子にあっては、図５中に二点鎖線で示す特性線３８のように、素子温度をエンジン始動後に５秒以内で、例えば５００℃程度までしか昇温することができず、例えば大気室内の空気層により熱伝導性が悪くなっていることが確認された。
【００９６】
また、検出素子２１の耐熱衝撃性についても、図６に示す如くワイブルプロットと呼ばれる評価試験を行った。この評価試験は、ヒータ電源３３の印加により素子温度を可変に調整しつつ、例えば０．００２ＣＣの水滴を検出素子２１の外側面（外側電極２９の近傍）に滴下して、検出素子２１の外側面に割れ（損傷、破損）等が発生する確率を素子割れ率（％）として表したものである。
【００９７】
この結果、本実施の形態による検出素子２１は、図６中に実線で示す特性線３９のように素子温度を、４５０℃以上に上げるまでは素子割れが全く発生せず、例えば４８０℃程度で素子割れの確率が４０％を越え、５００℃で素子割れの確率が１００％に達することが確認された。
【００９８】
なお、この評価試験では、例えば０．００２ＣＣの水滴を検出素子２１の外側面に滴下したが、このような多量の水分が排気ガス中に含まれることは、実際には無いものである。このため、自動車用エンジンの排気管等に酸素センサを取付けた状態で素子割れが発生する可能性は、実際上ほとんど無いと考えられる。
【００９９】
これに対し、従来技術の検出素子にあっては、図６中に二点鎖線で示す特性線４０のように素子温度が、例えば３８０℃程度を越えると素子割れが発生し、４５０℃程度で素子割れの確率が６０％に達し、４６０℃で素子割れの確率が１００％になることが確認された。
【０１００】
また、別の従来技術としてプレート型の検出素子を用いた場合には、図６中に破線で示す特性線４１のように素子温度が、例えば３００℃程度を越えると素子割れが発生し始め、３５０℃程度で素子割れの確率がほぼ１００％になることが確認された。
【０１０１】
従って、本実施の形態によれば、緻密な構造を有する緻密層３０を用いて固体電解質層２７を外側から覆うことにより、外部の水滴等が固体電解質層２７へと浸透するのを抑えることができ、検出素子２１全体の耐熱衝撃性、耐久性、寿命を向上をできる。
【０１０２】
また、外側にエッジ部が形成されるプレート型の素子に比較して、検出素子２１の外形状をエッジ部がない、円形のロッド状に成形することができるので、検出素子２１の熱応力等を十分に小さくすることが可能となり、例えば固体電解質層２７の割れ等を抑えることができる。
【０１０３】
そして、ヒータ部２２をほぼ全周にわたって外側から固体電解質層２７および緻密層３０等で覆うことにより、ヒータ部２２が直接外気と接触するのを抑えて外気温による影響を低減でき、ヒータ部２２の伝熱面積を大きくして該ヒータ部２２からの熱を固体電解質層２７等に効率的に伝えることができる。
【０１０４】
また、本実施の形態では、ヒータ部２２からの熱を多孔体層２６、内側電極２８を介して固体電解質層２７側に効率的に伝熱できると共に、固体電解質層２７には直付け部２７Ａを通じてもヒータ部２２からの熱を直に伝えることができるので、固体電解質層２７等からなる検出素子２１の昇温特性を向上でき、例えばエンジン始動時等に固体電解質層２７を早期に活性化することができる。
【０１０５】
このため、本実施の形態によれば、検出素子２１を短時間で活性化でき、エンジンの始動時でも排気ガス中の酸素濃度等を早期に検出して、燃料噴射量のフィードバック制御を即座に行うことが可能になる。また、当該酸素センサの取付自由度を大きくすることができ、ヒータ部２２の消費電力も低減できる。
【０１０６】
また、検出素子２１の内部に基準となる大気室等を従来技術のように形成して大気を導入する必要がないので、当該検出素子２１の構造を簡略化することができ、製造時の作業性を向上することができる。
【０１０７】
次に、図７および図８は本発明の第２の実施の形態を示し、本実施の形態の特徴は、内側電極を多孔質構造をなす電極として形成する構成したことにある。なお、本実施の形態では前記第１の実施の形態と同一の構成要素に同一の符号を付し、その説明を省略するものとする。
【０１０８】
図中、５１は酸素濃度検出装置の主要部となる酸素濃度の検出素子で、該検出素子５１は、第１の実施の形態で述べた検出素子２１とほぼ同様に、ヒータ部２２、固体電解質層２７、外側電極２９、緻密層３０および保護層３２等により構成されている。しかし、この場合の検出素子５１は、第１の実施の形態で述べた多孔体層２６が廃止され、ヒータ部２２と固体電解質層２７との間には後述の内側電極５２のみが設けられている。
【０１０９】
５２は本実施の形態で採用した内側電極で、該内側電極５２は、第１の実施の形態で述べた内側電極２８と同様に、ヒータ部２２のヒータ被覆層２５と固体電解質層２７との間に位置して固体電解質層２７の内周側に設けられ、ヒータ部２２の基端側に向けてヒータ被覆層２５の軸方向に延びたリード部５２Ａを有している。
【０１１０】
しかし、この場合の内側電極５２は、図８に例示するように後述の貴金属材料５３およびセラミックス粒子５４等からなるペースト状物を、ヒータ被覆層２５の外周側に直接的に曲面印刷することにより、ヒータ被覆層２５（固体電解質層２７）の軸方向に予め決められた長さをもって延びる環状電極として形成されるものである。
【０１１１】
この場合、内側電極５２は、白金等の貴金属材料５３と、セラミックス粒子５４，５４，…と、空孔５５，５５，…とにより構成されている。そして、セラミックス粒子５４は、例えば粒径が０．４〜１．０μｍ程度のジルコニアの粉体と、粒径が０．４〜１．０μｍ程度のアルミナの粉体を混合することにより形成されている。
【０１１２】
また、空孔５５は、例えば粒径が０．４〜１．０μｍ程度のカーボン粉等からなる空孔形成剤を用いて形成され、この空孔形成剤は、検出素子５１の焼成時に焼き飛ばされて内側電極５２内に連続気泡となる空孔５５，５５，…を形成するものである。そして、空孔５５は、セラミックス粒子５４の粒径と同等、または粒径よりも大きい孔径をもって形成されている。
【０１１３】
即ち、内側電極５２は、貴金属材料５３の粉体に、例えば１５〜２０重量％のジルコニア粉、１〜２重量％のアルミナ粉と、１〜３重量％のカーボン粉とを添加して、ペースト状物を調整することにより形成される。そして、内側電極５２は、貴金属材料５３が電極反応を起こすことにより内部で電子を移動させる働きをする。
【０１１４】
また、セラミックス粒子５４は、ジルコニアの粉体とアルミナの粉体とを含有することにより、図８に示すヒータ被覆層２５（多孔質絶縁層）と固体電解質層２７との間で内側電極５２の接合性を高める働きをするものである。
【０１１５】
また、内側電極５２内に形成された連続気泡となる空孔５５は、内側電極５２内で通気路となる酸素収容室を構成し、図７に示す内側電極５２のリード部５２Ａにも空孔５５が延在する。これによって、空孔５５からなる通気路は、内側電極５２内をケーシング１（図１中に例示した絶縁筒体７）内と常時連通させ、固体電解質層２７を通じて内側電極５２側に輸送されてくる酸素を、図７中の矢示Ａ方向に逃散させるガス逃散路を構成するものである。
【０１１６】
そして、内側電極５２の空孔５５は、固体電解質層２７と貴金属材料５３と空孔５５との３相界面（電極反応点）を増やし、通気路として機能することにより内側電極５２内の酸素を濃度、圧力勾配に従って拡散させる働きをするものである。なお、固体電解質層２７と内側電極５２との境界面側には、図８に示すように複数の凹空間５６，５６，…が形成され、この凹空間５６と固体電解質層２７と貴金属材料５３との間にも３相界面が形成される。
【０１１７】
かくして、このように構成される本実施の形態でも、ヒータ部２２からの熱を内側電極５２を介して固体電解質層２７側に効率的に伝熱できると共に、固体電解質層２７には直付け部２７Ａを通じてもヒータ部２２からの熱を直に伝えることができ、第１の実施の形態とほぼ同様の作用効果を得ることができる。
【０１１８】
そして、このような酸素濃度の検出素子２１に対しても、第１の実施の形態と同様の特性評価試験を行った。この結果、図５中に一点鎖線で示す特性線５７のように、検出素子２１の素子温度をエンジン始動後に４秒程度で、例えば６００℃まで昇温できることが確認された。
【０１１９】
また、本実施の形態では、ヒータ部２２のヒータ被覆層２５と固体電解質層２７との間に設ける内側電極５２を、図８に示す如く白金等の貴金属材料５３、セラミックス粒子５４および連続気泡となる空孔５５により構成しているので、貴金属材料５３が電極反応を起こすことにより内部で電子を移動させ、連続気泡となる空孔５５を内側電極５２内での酸素収容室として活かすことができる。
【０１２０】
これにより、固体電解質層２７、貴金属材料５３および空孔５５（凹空間５６）からなる３相界面で電極反応を生起させ、電子の移動を促進できると共に、内側電極５２内の酸素を濃度、圧力勾配に従って速く拡散させることができる。また、内側電極５２内の空孔５５を、セラミックス粒子５４の粒径よりも大きい孔径に形成することにより、空孔５５内での酸素収容量を増やすことができ、内側電極５２内での酸素の拡散を円滑に行うことができる。
【０１２１】
また、セラミックス粒子５４の構成材料として、ヒータ被覆層２５、固体電解質層２７と同一の材料を用いることにより、素子の焼成時には内側電極５２をヒータ被覆層２５と固体電解質層２７との間で両者に対して強く接合できる。そして、前記３相界面（電極反応点）を多数、均一に分布させ、内側電極５２と外側電極２９との対向面全体で電極反応を均一に生じさせることができる。
【０１２２】
そして、空孔５５内で酸素を拡散できるため、電極反応、温度変化で生じる酸素の濃度、圧力差を減らすことができ、前記３相界面（電極反応点）での酸素の滞留、欠乏を防ぐことができる。これにより、センサの内部抵抗を下げることが可能となり、検出素子５１の低温活性化を促進することができる。
【０１２３】
さらに、検出素子５１の固体電解質層２７等に対して昇温、降温等を繰返すような耐久試験を行った場合でも、固体電解質層２７と内側電極５２との３相界面において剥離等が生じるのを抑えることができ、これによる内部抵抗の増大等も防止することができる。
【０１２４】
次に、図９および図１０は本発明の第３の実施の形態を示し、本実施の形態の特徴は、内側電極の一部を構成するセラミックス粒子をペロブスカイト型酸化物により構成したことにある。なお、本実施の形態では前記第２の実施の形態と同一の構成要素に同一の符号を付し、その説明を省略するものとする。
【０１２５】
図中、６１はヒータ被覆層２５と固体電解質層２７との間に設けられた本実施の形態で用いる内側電極で、該内側電極６１は、第２の実施の形態で述べた内側電極５２とほぼ同様に、白金等の貴金属材料６２と、セラミックス粒子６３，６３，…と、空孔６４，６４，…とにより構成されている。
【０１２６】
また、内側電極６１と固体電解質層２７との境界面側には、第２の実施の形態で述べた凹空間５６と同様に、複数の凹空間６５，６５，…が形成されている。しかし、本実施の形態による内側電極６１は、セラミックス粒子６３が後述のペロブスカイト型酸化物からなる混合電導体により構成されている点で第２の実施の形態とは異なるものである。
【０１２７】
即ち、プロブスカイト型酸化物は、例えば（Ｌａ_１−ｘＳｒ_ｘＣｏＯ_３ ）系の酸化物、または（Ｌａ_１−ｘＳｒ_ｘＭｎＯ_３ ）系の酸化物からなり、混合電導体であるセラミックス粒子６３を構成しているものである。
【０１２８】
そして、混合電導体であるセラミックス粒子６３は、図１０に示す如く固体電解質層２７との２相界面で酸素イオン（Ｏ^２−）を伝導させ、セラミックス粒子６３と空孔６４との２相界面で、前述の化１、化２式による電極反応を図１０中に示す如く生起させるものである。
【０１２９】
また、図１０に示すように固体電解質層２７と貴金属材料６２と空孔６４との３相界面Ｆ１ ，Ｆ２ 、固体電解質層２７と貴金属材料６２と凹空間６５との３相界面Ｆ３ ，Ｆ４ 等でも電極反応が起こる。そして、空孔６４は通気路（酸素収容室）として機能することにより、内側電極６１内の酸素を濃度、圧力勾配に従って拡散させるものである。
【０１３０】
この場合、内側電極６１内では、図１０中の左側部位（素子の先端部側）が右側部位（素子の基端部側）よりも酸素濃度が高くなり、酸素（Ｏ_２ ）、イオン等は図１０中の矢示ｂ方向に濃度勾配等に従って移動するものである。
【０１３１】
かくして、このように構成される本実施の形態でも、前記第２の実施の形態とほぼ同様の作用効果を得ることができる。特に本実施の形態では、内側電極６１のセラミックス粒子６３をペロブスカイト型酸化物等の混合電導体で構成することにより、前述した３相界面（電極反応点）を多数、均一に分布させることができ、内側電極６１の電極面全体で電極反応を効率的に起こすことができる。
【０１３２】
また、内側電極６１内での酸素濃度勾配に対して、空孔６４によるガス拡散に混合電導体（セラミックス粒子６３）内のイオン伝導が加わり、図１０中の矢示ｂ方向における酸素ガスの移動速度を加速することができる。
【０１３３】
この結果、素子の内部抵抗を下げることができ、低温活性等を向上できると共に、例えば固体電解質層２７と内側電極６１との間の剥離を長期にわたって抑えることができ、酸素センサとしての信頼性を高めることができる。
【０１３４】
なお、前記各実施の形態では、内側電極２８（５２，６１）と外側電極２９との間に外部から直流電源３４によるポンプ電圧Ｖｐ を印加する場合を例に挙げて説明した。しかし、本発明はこれに限るものではなく、例えば内側電極２８（５２，６１）側を基準となる酸素濃度に通気路２８Ｂ（空孔５５，６４）を介して設定し、固体電解質層を挟む内側電極と外側電極との間に外部から電圧を印加することなく、被測定ガス（排気ガス）中の酸素濃度に対応した起電力を発生させる構成とした酸素センサ（酸素濃度検出装置）に適用してもよい。
【０１３５】
次に、上記各実施の形態から把握し得る請求項以外の技術的思想について、以下にその効果と共に記載する。
【０１３６】
（１）．請求項１，２または３に記載の酸素濃度検出装置において、前記内側電極と外側電極との間には外部から電圧を印加し、前記内側電極側には前記通気路からなる疑似基準の酸素収容室を設ける構成とした酸素濃度検出装置。
【０１３７】
この場合には、電気化学的な接触分解反応により外側電極側で発生した酸素イオンを、固体電解質層中の酸素欠陥を介して外側電極から内側電極に向けて輸送できるため、内側電極側においては通気路からなる酸素収容室に酸素を吸入、流通させ、外側電極（外部の被測定ガス）側よりも酸素分圧が高い疑似基準の酸素濃度を生じさせることができる。
【０１３８】
（２）．請求項２または３に記載の酸素濃度検出装置において、前記空孔形成剤により内側電極中に形成される空孔は、前記セラミックス粒子の粒径以上となる孔径を有してなる酸素濃度検出装置。これにより、空孔内での酸素収容量を増やすことができ、内側電極内での酸素の拡散を円滑に行うことができる。
【０１３９】
（３）．請求項２または３に記載の酸素濃度検出装置において、前記セラミックス粒子は、少なくとも一部を前記固体電解質層の構成材料と同一の材料により構成してなる酸素濃度検出装置。これにより、検出素子の焼成時には内側電極をヒータ部と固体電解質層との間で両者に対して強く接合することができる。
【図面の簡単な説明】
【図１】本発明の第１の実施の形態による検出素子が設けられた酸素センサを示す縦断面図である。
【図２】図１に示す検出素子の拡大縦断面図である。
【図３】検出素子を図２中の矢示 ＩＩＩ−ＩＩＩ 方向からみた横断面図である。
【図４】ヒータコアの外周にヒータパターン、ヒータ被覆層、多孔体層、内側電極、固体電解質層、外側電極、緻密層および保護層等を形成する工程を示す斜視図である。
【図５】検出素子の昇温特性を示す特性線図である。
【図６】水滴による検出素子の素子割れ率を示す特性線図である。
【図７】第２の実施の形態による酸素濃度の検出素子を示す半断面図である。
【図８】第２の実施の形態による内側電極の内部構造を示す拡大断面図である。
【図９】第３の実施の形態による検出素子の内側電極等を示す図８と同様位置での拡大断面図である。
【図１０】図９中のａ部を拡大して示す要部断面図である。
【符号の説明】
１ ケーシング
１１，１２ リード線
１３，１４ コンタクトプレート
２１，５１ 酸素濃度の検出素子
２２ ヒータ部
２３ ヒータコア
２４ ヒータパターン
２４Ａ，２８Ａ，２９Ａ リード部
２５ ヒータ被覆層
２６ 多孔体層
２６Ａ 引出し部
２６Ｂ 通気路
２７ 固体電解質層
２８ 内側電極
２９ 外側電極
３０ 緻密層
３１ 開口部
３２ 保護層
３３ ヒータ電源
３４ 直流電源
３５ 差動増幅器
３６ 出力側端子
５２，６１ 内側電極（多孔質構造の電極）
５３，６２ 貴金属材料
５４ セラミックス粒子
５５，６４ 空孔（通気路）
５６，６５ 凹空間
６３ セラミックス粒子（混合電導体）
Ｖｓ 出力電圧（酸素濃度の検出信号）[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an oxygen concentration detection device suitably used for detecting an air-fuel ratio (mixing ratio of fuel and intake air) of an automobile engine or the like as oxygen concentration in exhaust gas.
[0002]
[Prior art]
Generally, in a vehicle engine or the like, for example, an oxygen concentration detection device (oxygen sensor) is provided in the middle of an exhaust pipe, and the oxygen sensor detects oxygen concentration and the like contained in exhaust gas (for example, Patent Document 1).
[0003]
[Patent Document 1]
JP-A-7-27737
[0004]
An oxygen sensor according to the related art of this type includes, for example, a heater portion formed in an elongated rod shape, and an oxygen ion conductive solid electrolyte layer provided on an outer peripheral side of the heater portion and activated by heat from the heater portion. An annular air chamber provided between the solid electrolyte layer and the heater portion, through which oxygen in the air flows in and out, and an inner electrode provided on the inner peripheral side of the solid electrolyte layer facing the air chamber, An outer electrode and the like are provided on the outer peripheral side of the solid electrolyte layer and sandwich the solid electrolyte layer between the inner electrode and the inner electrode.
[0005]
Then, when an oxygen concentration difference occurs between a gas to be measured such as exhaust gas flowing outside the outer electrode and an atmosphere chamber facing the inner electrode, the oxygen sensor generates an electromotive force corresponding to the difference in concentration. Since it is generated between the inner electrode and the outer electrode sandwiching the solid electrolyte layer, a detection signal of the oxygen concentration is output to the outside according to the electromotive force.
[0006]
[Problems to be solved by the invention]
By the way, the oxygen sensor according to the prior art described above has a structure in which an annular air chamber is interposed between the heater section and the solid electrolyte layer. Therefore, when the heat from the heater section is transmitted to the solid electrolyte layer, the heat is generated by the air chamber. There is a problem that conductivity is deteriorated and it is difficult to activate the solid electrolyte layer early, for example, when starting the engine.
[0007]
Further, in the case of the prior art, since the outer peripheral side of the solid electrolyte layer is merely covered with a protective layer or the like, for example, moisture, oil, and the like contained in the gas to be measured such as exhaust gas become droplets. When the droplets adhere to the outer peripheral side of the sensor, the droplets may penetrate the protective layer and reach the solid electrolyte layer.
[0008]
Therefore, the solid electrolyte layer has a large temperature difference between the position where the droplets are attached and the other position, which may cause damage, breakage, etc. in the solid electrolyte layer. There is a problem that performance cannot be improved.
[0009]
SUMMARY OF THE INVENTION The present invention has been made in view of the above-described problems of the related art, and an object of the present invention is to efficiently transfer heat from a heater section to a solid electrolyte layer and activate the solid electrolyte layer early, for example, when starting an engine. It is an object of the present invention to provide an oxygen concentration detecting device which can be made into an oxygen concentration.
[0010]
Another object of the present invention is to provide an oxygen concentration detecting device that can improve thermal shock resistance by covering the solid electrolyte layer with a dense layer from the outside, and can improve the durability and life of the entire detecting element. To provide.
[0011]
[Means for Solving the Problems]
In order to solve the above-mentioned problem, the oxygen concentration detecting device according to the first aspect of the present invention allows oxygen to flow between the heater section and the inner electrode in the length direction of the heater section from the position of the inner electrode. Therefore, a porous layer made of a porous ceramic material and having an air passage made of open pores is provided inside, and the gas to be measured permeates to the outer peripheral side of the solid electrolyte layer at a position corresponding to the outer electrode. In a portion other than the opening, a dense layer is provided to cover the solid electrolyte layer and the inner electrode from the outside and to prevent the gas to be measured from entering the inside.
[0012]
With this configuration, a porous layer made of a porous ceramic material having a higher thermal conductivity than air can be formed between the heater section and the inner electrode (solid electrolyte layer). Can be efficiently transferred to the solid electrolyte layer and the like. Therefore, it is possible to improve the temperature rising characteristics of the detection element including the solid electrolyte layer and the like, and to activate the solid electrolyte layer early, for example, at the time of starting the engine. In addition, by covering the solid electrolyte layer from the outside, the dense layer can suppress the penetration of external water droplets and the like into the solid electrolyte layer, thereby improving the thermal shock resistance, durability, and life of the entire detection element. it can.
[0013]
Further, the oxygen concentration detecting device according to the second aspect of the present invention is an apparatus for detecting oxygen concentration, in which a material obtained by mixing a noble metal, ceramic particles, and a pore-forming agent for flowing oxygen from the position of the inner electrode in the length direction of the heater section is used. The solid electrolyte layer is formed by using an electrode having a porous structure having an air passage formed of open cells, and the gas to be measured is transmitted to the outer peripheral side of the solid electrolyte layer at a position corresponding to the outer electrode. An opening is provided, and a portion other than the opening is provided with a dense layer that covers the solid electrolyte layer and the inner electrode from the outside and suppresses entry of the gas to be measured into the inside.
[0014]
With this configuration, the inner electrode interposed between the heater and the solid electrolyte layer can be formed using a material (noble metal, ceramic particles, or the like) having a higher thermal conductivity than air. By eliminating the need for the porous layer used in the present invention, the structure can be simplified, and the heat from the heater can be efficiently transferred to the solid electrolyte layer and the like. Therefore, the temperature rise characteristics of the detection element can be improved, and the solid electrolyte layer can be activated early, for example, when starting the engine. Further, the dense layer can suppress penetration of external water droplets and the like into the solid electrolyte layer, and can improve thermal shock resistance, durability, and life of the entire detection element.
[0015]
In particular, the inner electrode in this case functions so that the noble metal as the material causes an electrode reaction to move electrons inside, and the ceramic particles enhance the bondability between the heater section and the solid electrolyte layer. A pore-forming agent made of, for example, carbon powder can be burned out during element baking to form pores that become continuous cells in the inner electrode, and these pores can be used as oxygen storage chambers that serve as ventilation paths. Thereby, the three-phase interface (electrode reaction point) can be increased, and oxygen in the inner electrode can be diffused according to the concentration and the pressure gradient.
[0016]
Further, in the invention of claim 3, the ceramic particles serving as the material of the inner electrode are made of a mixed conductor such as a perovskite-type oxide which can conduct oxygen ions together with electrons. Thereby, in addition to having substantially the same operation and effect as the invention of claim 2, oxygen ions can be conducted at the two-phase interface between the solid electrolyte layer and the mixed conductor (perovskite oxide), and An electrode reaction can take place at the two-phase interface between the pores.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an example in which the oxygen concentration detecting device according to the embodiment of the present invention is applied to an oxygen sensor attached to an exhaust pipe side of an automobile engine will be described in detail with reference to the accompanying drawings.
[0018]
Here, FIGS. 1 to 6 show a first embodiment of the present invention. In the drawing, reference numeral 1 denotes a casing of an oxygen sensor. The casing 1 has a stepped cylindrical holder 2 having an external thread 2A as an attachment portion formed on an outer periphery on one side (tip side) in the axial direction; And a bottomed cylindrical cap 3 integrally fixed to the other side (base end side) in the axial direction, and coaxially disposed in the cap 3 and positioned between a seal cap 10 and a holder 2 described later. And the guide cylinder 4 provided.
[0019]
The holder 2, the cap 3, and the guide cylinder 4, which are components of the casing 1, are formed using a metal material such as stainless steel. In the casing 1, an external thread portion 2 </ b> A of the holder 2 is screwed to the exhaust pipe in order to mount an oxygen concentration detecting element 21, which will be described later, in a protruding state, for example, in an exhaust pipe (not shown) of an automobile engine. Things.
[0020]
Reference numeral 5 denotes an insulating support provided in the holder 2 of the casing 1 via a metal seal ring 6. The insulating support 5 is made of, for example, aluminum oxide (Al).₂O₃  ) Are formed in a cylindrical shape, and a detection element 21 is fixed to the inner peripheral side thereof using an inorganic adhesive or the like. The insulating support 5 positions the detection element 21 in the casing 1 and holds the detection element 21 in an electrically and thermally insulated state.
[0021]
Reference numerals 7 and 8 denote insulating cylinders disposed in the guide cylinder 4 of the casing 1. The insulating cylinders 7 and 8 are formed in a cylindrical shape from a ceramic material such as aluminum oxide (hereinafter, referred to as alumina). The contact plates 13, 14 and the like described below are held in an insulated state with respect to the casing 1.
[0022]
Reference numeral 9 denotes a spring serving as an elastic member disposed in the casing 1 and disposed between the insulating support 5 and the insulating cylinder 7, and the spring 9 always directs the insulating support 5 toward the holder 2 side. This is to prevent the vibration and the shock acting on the casing 1 from being transmitted to the detecting element 21 directly.
[0023]
Reference numeral 10 denotes a seal cap in which the base end side of the cap 3 is closed. The seal cap 10 is formed of a heat-resistant resin material such as polytetrafluoroethylene (PTFE) into a stepped cylindrical shape. The insulating cylinders 7, 8 and the like are positioned via a spring 9.
[0024]
Also, lead wires 11, 11,... For detection and lead wires 12, 12 (only one is shown) for heaters are inserted through the seal cap 10. These lead wires 11 and 12 are individually connected to the detection contact plates 13, 13,... And the heater contact plates 14, 14 in the insulating cylinder 8.
[0025]
Reference numeral 15 denotes a protector provided on the holder 2 of the casing 1, and the protector 15 is formed in a closed cylindrical shape using, for example, a metal plate having high heat resistance. The protector 15 has a base end attached to the holder 2 so as to cover a distal end portion of a detection element 21 to be described later from the outside, and a distal end (lid) side is provided to protrude from the holder 2 in the axial direction.
[0026]
Further, a plurality of windows 15A, 15A,... Which allow the exhaust gas to flow are formed on the cylindrical portion side of the protector 15. These windows 15 </ b> A guide the exhaust gas flowing in the exhaust pipe to the vicinity of the distal end side of the detection element 21.
[0027]
Next, reference numeral 21 denotes an oxygen concentration detecting element which is a main part of the oxygen concentration detecting device. The detecting element 21 is mounted in the holder 2 of the casing 1 via the insulating support 5, and the tip side is connected to the holder 2 by the shaft. Projecting in the direction. The detection element 21 includes a heater 22, a porous layer 26, a solid electrolyte layer 27, electrodes 28 and 29, a dense layer 30, and a protective layer 32, which will be described later, as shown in FIGS.
[0028]
Reference numeral 22 denotes a heater portion which is a mandrel portion formed in an elongated rod shape. As shown in FIGS. 2 to 4, the heater portion 22 is formed in a solid or hollow small-diameter rod shape by a ceramic material such as alumina. A heater core 23, a heater pattern 24 and an insulating heater coating layer 25.
[0029]
As shown in FIG. 4, the heater pattern 24 is formed on the outer peripheral surface of the heater core 23 by using means such as curved surface printing, and a pair of leads 24A, 24A extending from the distal end side to the proximal end side of the heater core 23. have. The heater coating layer 25 is formed by printing a ceramic material such as alumina on the outer peripheral side of the heater core 23 in a thick film so as to protect the heater pattern 24 together with the lead portion 24A from the radial outside. .
[0030]
Here, the heater core 23 is formed, for example, as a circular rod having an outer dimension of about 3 to 3.5 mm and a length of about 40 to 60 mm by injection molding or extrusion molding of a ceramic material such as alumina. . The heater pattern 24 is made of a heat-generating conductive material such as platinum mixed with, for example, about 5% by weight of alumina, and each lead portion 24A is provided at the base end side of the heater core 23 with each of the heater contacts illustrated in FIG. Connected to plate 14.
[0031]
The heater pattern 24 is supplied with power from a heater power supply 33 to be described later via each of the heater lead wires 12, each of the contact plates 14, and each of the lead portions 24A. This is for causing the part 22 to generate heat.
[0032]
Reference numeral 26 denotes a porous layer provided on the heater coating layer 25 of the heater section 22. The porous layer 26 is provided on the outer periphery of the heater coating layer 25 before printing an inner electrode 28 described later as shown in FIG. It is formed by performing curved surface printing on the side, and is disposed in a state sandwiched between the heater coating layer 25 and the inner electrode 28.
[0033]
That is, the porous layer 26 is made of, for example, alumina and zirconia (Zr 2 O 3).₂  ) Is added in an amount of about 3 to 20% by weight of a carbon powder (average particle diameter of 0.4 to 4 μm) or the like as a pore-forming agent to the composite oxide powder (average particle diameter of 0.4 to 1 μm). The paste-like material is prepared by printing the paste-like material on the outer peripheral side of the heater coating layer 25 in a thick film.
[0034]
In this case, as shown in FIG. 4, the porous material layer 26 is formed to have an L-shape as a whole with an area equal to or slightly larger than that of the inner electrode 28, and a portion corresponding to a lead portion 28A described later. Are elongated drawers 26A and extend in the longitudinal direction of the heater coating layer 25. The carbon powder and the like are burned off as a pore-forming agent when the entire detection element 21 is fired, whereby the porous layer 26 is provided with an air passage 26B formed of open pores therein. 26A.
[0035]
The porous layer 26 has a thickness of, for example, about 10 to 20 μm after sintering, and the internal ventilation path 26B is provided near the inner peripheral side of the inner electrode 28 in the casing 1 (for example, as shown in FIG. 1). The gas is always communicated within the insulating cylinder 7), thereby forming a gas escape path for allowing oxygen transported to the inner electrode 28 side through the solid electrolyte layer 27 described later to escape in the direction of arrow A in FIG. Things.
[0036]
Further, since the porous layer 26 is formed of a porous ceramic material such as alumina, as shown in Table 1 below, for example, the heat is about 1000 times higher than that of air (atmosphere chamber described in the related art). It has conductivity and can satisfactorily transfer heat from the heater section 22 to the solid electrolyte layer 27 side.
[0037]
Reference numeral 27 denotes an oxygen ion conductive solid electrolyte layer provided on the outer peripheral side of the heater coating layer 25 of the heater section 22. The solid electrolyte layer 27 is made of, for example, 95% mol of zirconia (Zr 2 O 3).₂  ), 5% mol of yttria (Y₂  O₃  ) Is mixed to prepare a paste made of so-called yttria-stabilized zirconia (YSZ), and then the paste is printed on the outer peripheral side of the heater coating layer 25 as a thick film as shown in FIG. It is formed in an annular shape.
[0038]
The solid electrolyte layer 27 has a thickness of, for example, about 30 to 50 μm after firing, and generates an electromotive force between an inner electrode 28 and an outer electrode 29, which will be described later, according to a difference in surrounding oxygen concentration. The oxygen ions at this time are transported between the electrodes 28 and 29.
[0039]
Further, as shown in FIG. 4, the solid electrolyte layer 27 is smaller than the porous material layer 26 and the inner electrode 28 so as to completely surround the porous material layer 26 and the inner electrode 28 from the outside except for a lead portion 28A described later. It is formed with a large area, and is printed on the outer peripheral surface of the heater coating layer 25 by a curved surface. The solid electrolyte layer 27 has a directly attached portion 27A and the like that directly contact the outer peripheral surface of the heater coating layer 25 as shown in FIG.
[0040]
Reference numeral 28 denotes an inner electrode located between the heater coating layer 25 of the heater section 22 and the solid electrolyte layer 27 and provided on the inner peripheral side of the solid electrolyte layer 27. As shown in FIG. Before the solid electrolyte layer 27 is printed on a curved surface, the solid electrolyte layer 27 is formed on the outer peripheral side of the heater coating layer 25 from above the porous material layer 26 using a means such as curved surface printing.
[0041]
That is, the inner electrode 28 is formed by forming a conductive paste made of, for example, platinum or the like into a substantially L-shaped print pattern as shown in FIG. It is formed by printing on the upper side. The inner electrode 28 has an elongated lead portion 28A corresponding to the lead portion 26A of the porous body layer 26, and the lead portion 28A is placed on the base end side of the heater portion 22 so as to overlap the lead portion 26A. The heater coating layer 25 extends in the axial direction.
[0042]
Further, since the inner electrode 28 is formed of a conductive material such as platinum, as shown in Table 1 below, for example, the thermal conductivity is about 3000 times higher than that of air (atmosphere chamber described in the related art). And the heat from the heater section 22 can be satisfactorily transferred to the solid electrolyte layer 27 via the porous material layer 26.
[0043]
Reference numeral 29 denotes an outer electrode provided on the outer peripheral side of the solid electrolyte layer 27. The outer electrode 29 is formed of a conductive paste made of, for example, platinum or the like in a printed pattern as shown in FIG. The solid electrolyte layer 27 is formed between the inner electrode 28 and the inner electrode 28 as shown in FIG.
[0044]
Further, the outer electrode 29 has a lead portion 29A shown in FIG. 4, and the lead portion 29A extends toward the base end side of the heater portion 22 similarly to the lead portion 28A of the inner electrode 28. These lead portions 28A and 29A are connected to the respective contact plates 13 and lead wires 11 on the base end side of the detection element 21 shown in FIG. As shown in FIG. 2, a pump voltage Vp is applied from a DC power supply 34 described later.
[0045]
Reference numeral 30 denotes a dense layer provided so as to cover the solid electrolyte layer 27 together with the outer electrode 29 from the outside. The dense layer 30 is formed, for example, by adding alumina powder to silicon oxide (Si 2 O 3).₂4) is added to prepare a paste-like material, and the paste-like material is formed by thick-film printing from the outside of the solid electrolyte layer 27 along the length direction of the heater coating layer 25. As shown, it extends from the distal side to the proximal side of the heater coating layer 25.
[0046]
The dense layer 30 has a dense structure when the powder to be the paste-like material has a small average particle size (for example, about 0.3 to 0.5 μm), and serves as a gas to be measured. It has a function of blocking and suppressing exhaust gas (including moisture and oil) from entering the inside.
[0047]
Therefore, the porous layer 26, the solid electrolyte layer 27, and the inner electrode 28 are almost completely surrounded by the dense layer 30 and are kept in a state of being isolated from external exhaust gas (including moisture and oil). . Thereby, the solid electrolyte layer 27 can be reduced in thickness to, for example, about 30 to 50 μm (the conventional product has a thickness of, for example, about 50 to 100 μm), and the thermal shock resistance against moisture and the like can be improved.
[0048]
Also, the outer electrode 29 is covered with the dense layer 30 as shown in FIGS. 2 and 3 except for the position of the opening 31 described later. Contact is interrupted. Thereby, the location of the exhaust gas entering and contacting the outer electrode 29 is limited to the range of the opening 31.
[0049]
Reference numeral 31 denotes an opening provided in the dense layer 30. The opening 31 cuts a part of the dense layer 30 located near the front end side of the heater section 22 into, for example, a rectangular shape as shown in FIGS. It is formed by chipping. The opening 31 partially exposes the outer electrode 29 from the dense layer 30 and allows external exhaust gas to enter and contact the outer electrode 29 via a protective layer 32 described later.
[0050]
Reference numeral 32 denotes a protective layer provided on the outer peripheral side of the dense layer 30 so as to cover the opening 31 from the outside. The protective layer 32 is formed using a porous material such as alumina having a relatively high porosity. The paste-like material made of these powders is printed on the outer peripheral side of the dense layer 30 by a curved surface.
[0051]
The protective layer 32 has a function of covering the outer electrode 29 exposed to the outside through the opening 31 of the dense layer 30 from the outside, and protecting the outer electrode 29 and the like from external dust and the like. In addition, a part of the exhaust gas (measured gas) flowing around the protective layer 32 passes through the protective layer 32 having a high porosity and flows from the position of the opening 31 toward the outer electrode 29 in FIG. It invades in the direction of arrow B.
[0052]
Next, reference numeral 33 denotes a heater power supply provided outside the casing 1, and the heater power supply 33 is connected to the heater pattern 24 via the lead wires 12 and the like as shown in FIG. The heater power supply 33 applies a voltage to the heater pattern 24 of the heater section 22 to cause the heater section 22 to generate heat at a temperature before and after 600 to 800 ° C., for example.
[0053]
Reference numeral 34 denotes a DC power supply as a voltage applying means provided outside the casing 1. The DC power supply 34 is connected to the inner electrode 28 via the resistor R 0, the lead wire 11, and the like, as shown in FIG. Is connected to ground via a lead wire 11 or the like. The DC power supply 34 applies a pump voltage Vp between the electrodes 28 and 29.
[0054]
Numeral 35 denotes a differential amplifier provided outside the casing 1 and constituting an oxygen concentration detecting circuit. The differential amplifier 35 has a non-inverting input terminal connected to the ground and an inverting input terminal as shown in FIG. Is connected between the resistor R0 and the inner electrode 28 via the lead wire 11 or the like. The differential amplifier 35 outputs an output voltage Vs as an oxygen concentration detection signal from the output terminal 36.
[0055]
Here, in the solid electrolyte layer 27 of the detection element 21, when a concentration difference occurs between the oxygen concentration △ Pex in the exhaust gas flowing around the protective layer 32 and the like and the oxygen concentration △ Pa in the porous layer 26 side. , An electromotive force E is generated between the inner electrode 28 and the outer electrode 29 by the following reaction formulas (1) to (4).
[0056]
(Equation 1)
E = − (R × T / 4 × F) × ln (△ Pex / △ Pa)
Where R: gas constant (8.3145 J / K · mol)
T: Absolute temperature
F: Faraday constant (9.68453 × 10⁴  C / mol)
[0057]
If the internal resistance of the solid electrolyte layer 25 is Rs, the pump current Ip is supplied between the inner electrode 28 and the outer electrode 29 by the DC voltage 34 from the pump voltage Vp. The voltage Vs is output from the output terminal 36 of the differential amplifier 35.
[0058]
(Equation 2)
Vs = E + (Rs × Ip)
[0059]
The relationship between the output voltage Vs and the pump voltage Vp is represented by the following equation (3).
[0060]
(Equation 3)
Vp = Vs + (R0 × Ip)
[0061]
The oxygen sensor that is the oxygen concentration detecting device according to the present embodiment has the above-described configuration. Next, a method for manufacturing the detecting element 21 will be described with reference to FIGS.
[0062]
First, when the heater section 22 is manufactured, the heater core 23 is formed as a solid circular rod from a ceramic material such as alumina as shown in FIG. 4, and the heater core 23 is temporarily fired in this state.
[0063]
Next, in a pattern printing step, a heater pattern 24 made of a heat-generating conductive material such as platinum is attached to the outer peripheral surface of the heater core 23 while a support shaft such as a chuck is engaged with both ends of the heater core 23 and the heater core 23 is rotated. Print on a curved surface. Further, each lead portion 24A of the heater pattern 24 is integrally formed by printing so as to extend toward the base end side of the heater core 23.
[0064]
Next, in the step of forming the heater coating layer 25, the heater coating layer 25 is formed by printing a paste-like material made of, for example, alumina or the like on the curved surface so as to cover the heater pattern 24 from the outside in the radial direction. As a result, the heater section 22 including the heater core 23, the heater pattern 24, and the heater coating layer 25 is formed.
[0065]
Next, in the step of forming the porous layer 26, a paste-like substance composed of the above-described composite oxide powder and a pore-forming agent is printed on a curved surface so as to be applied to the outer peripheral surface of the heater coating layer 25. The body layer 26 is formed. At this time, the drawn portion 26A of the porous layer 26 is formed by printing so as to extend to the base end side of the heater coating layer 25.
[0066]
In the next step of forming the inner electrode 28, the above-described conductive paste is printed on the outer peripheral surface of the heater coating layer 25 by a curved surface so as to overlap the porous layer 26 from above, thereby forming the inner electrode 28. At this time, the lead portion 28A of the inner electrode 28 is formed by printing so as to overlap the lead portion 26A of the porous layer 26 and extend to the base end side of the heater coating layer 25.
[0067]
In the next step of forming the solid electrolyte layer 27, the oxygen ion-conductive solid electrolyte layer 27 is formed by performing a curved surface printing so that a paste-like material made of, for example, zirconia and yttria is applied to the outer peripheral surface of the heater coating layer 25. The porous layer 26 and the inner electrode 28 are almost completely covered from the outside by the solid electrolyte layer 27.
[0068]
Then, in the next step of forming the outer electrode 29, the outer electrode 29 is formed outside the solid electrolyte layer 27 by performing a curved surface printing of a conductive paste made of platinum or the like on the outer peripheral surface of the solid electrolyte layer 27. Further, it is formed by printing so that the lead portion 29A of the outer electrode 29 is extended from the lead portion 28A of the inner electrode 28 to the base end side of the heater coating layer 25 in a state where it is separated.
[0069]
Next, in the step of forming the dense layer 30, the dense layer 30 is formed on the outer peripheral side of the solid electrolyte layer 27 and the outer peripheral side of the heater coating layer 25 by, for example, performing a curved surface printing of a paste-like material made of alumina and silicon oxide. . An opening 31 is formed in the dense layer 30 at a position corresponding to the outer electrode 29.
[0070]
In the subsequent step of forming the protective layer 32, a paste-like substance made of, for example, alumina or the like is printed on the outer peripheral side of the dense layer 30 so as to cover the outer electrode 29 shown in FIGS. To form the protective layer 32.
[0071]
Then, in the next firing step, a detection including the heater core 23, the heater pattern 24, the heater coating layer 25, the porous body layer 26, the solid electrolyte layers 27, the electrodes 28 and 29, the dense layer 30, and the protective layer 32 formed as described above. The molded product of the element 21 is fired at a high temperature to sinter them integrally.
[0072]
Thus, after manufacturing the detecting element 21 by the above-described steps, the detecting element 21 is housed in the casing 1 of the oxygen sensor as shown in FIG. 1, and the lead portions 24A, 28A, 29A are connected to the respective contact plates 13, 14 is abutted with a spring property, and these are electrically connected to complete the oxygen sensor.
[0073]
Next, the operation of detecting the oxygen concentration by the oxygen sensor will be described. First, the casing 1 of the oxygen sensor is screwed to an exhaust pipe or the like of the vehicle via the external thread 2A of the holder 2, and the detection element 21 Is fixed in a state where the tip side of the is projected into the exhaust pipe.
[0074]
When the exhaust gas flowing through the exhaust pipe by the operation of the engine is introduced around the detecting element 21 through the protector 15, a part of the exhaust gas passes through the protective layer 32 in the direction of arrow B in FIG. Then, it reaches the surface of the outer electrode 29 via the opening 31 of the dense layer 30.
[0075]
In this state, when power is supplied from the heater power supply 33 to the heater pattern 24 and the entire detection element 21 is heated by the heater section 22, the solid electrolyte layer 27 is activated. Then, a pump voltage Vp from a DC power supply 34 is applied between the inner electrode 28 and the outer electrode 29 opposed to each other with the solid electrolyte layer 27 interposed therebetween.
[0076]
As a result, the solid electrolyte layer 27 constitutes an oxygen pump together with the inner electrode 28 and the outer electrode 29, and the exhaust gas is provided between the inner electrode 28 and the outer electrode 29 by the following reaction formulas 1 and 2 described below. An electromotive force corresponding to the oxygen concentration in the medium is generated as shown by the above equations (1) to (3). The output terminal 36 of the differential amplifier 35 outputs an output voltage Vs as a detection signal of the oxygen concentration.
[0077]
That is, when the air-fuel ratio of the engine is a lean air-fuel ratio that is higher than the stoichiometric air-fuel ratio, oxygen remains in the exhaust gas flowing around the protective layer 32 without being burned by the lean mixture in the combustion chamber. Therefore, an electrochemical catalytic cracking reaction is carried out at the cathode-side outer electrode 29 by the pump current Ip (pump voltage Vp from the DC power supply 34) flowing between the inner electrode 28 and the outer electrode 29 according to the following chemical formula 1. Then, electrons are given to oxygen remaining in the exhaust gas to generate oxygen ions.
[0078]
Embedded image
O₂  + 4e → 2O^2-
Where O₂  : Oxygen molecule
e: Electronic
O^2-: Oxygen ion
[0079]
Then, oxygen ions at this time are transported from the cathode-side outer electrode 29 to the anode-side inner electrode 28 via oxygen defects in the solid electrolyte layer 27. Therefore, in the inner electrode 28 on the anode side, an electrochemical catalytic decomposition reaction according to the following chemical formula 2 is performed, and at this time, oxygen ions are decomposed into oxygen and electrons.
[0080]
Embedded image
2O^2-  → O₂  + 4e
[0081]
As a result, in the anode-side inner electrode 28, oxygen is sucked into the ventilation passage 26B (oxygen storage chamber) in the porous layer 26 laminated inside, and the outer electrode 29 (the external An oxygen partial pressure (pseudo-reference oxygen concentration) higher than the (exhaust gas) side occurs. At this time, an electromotive force is generated between the electrodes 28 and 29 based on the difference in oxygen partial pressure between the electrodes 28 and 29 according to the equation (1).
[0082]
However, the air passage 26 </ b> B in the porous layer 26 always allows the vicinity of the inner periphery of the inner electrode 28 to communicate with the casing 1 (for example, the insulating cylinder 7 shown in FIG. 1), and the inner electrode 28 through the solid electrolyte layer 27. A gas escape path for allowing oxygen transported to the 28 side to escape in the direction of arrow A in FIG. 2 is configured.
[0083]
For this reason, the oxygen concentration on the inner electrode 28 side is always maintained at the atmospheric pressure state, and in the state of the lean air-fuel ratio, a large difference in the oxygen concentration between the inner and outer electrodes 28 and 29 may not occur. Instead, the output voltage Vs from the output terminal 36 is output as a voltage value lower than the reference voltage.
[0084]
Next, when the air-fuel ratio of the engine is a rich air-fuel ratio that is smaller than the stoichiometric air-fuel ratio, no oxygen remains in the exhaust gas flowing around the protective layer 32 due to the rich mixture in the combustion chamber, and for example, carbon dioxide ( CO₂  ), Moisture (H₂  O) and the like are contained in the exhaust gas.
[0085]
For this reason, due to the pump current Ip flowing between the inner electrode 28 and the outer electrode 29, an electrochemical catalytic decomposition reaction is carried out on the cathode-side outer electrode 29 according to the following chemical formulas 3 and 4, for example, in the exhaust gas. The remaining carbon dioxide and water molecules are given electrons to generate oxygen ions, carbon monoxide and hydrogen.
[0086]
Embedded image
2CO₂  + 4e → 2O^2-  + 2CO
However, CO₂  : Carbon dioxide molecule
e: Electronic
O^2-: Oxygen ion
CO: carbon monoxide molecule
[0087]
Embedded image
2H₂  O + 4e → 2O^2-  + 2H₂
Where H₂  O: water molecule
H₂  ： Hydrogen molecule
[0088]
Then, oxygen ions at this time are transported from the cathode-side outer electrode 29 to the anode-side inner electrode 28 via oxygen defects in the solid electrolyte layer 27. For this reason, in the inner electrode 28 on the anode side, an electrochemical catalytic decomposition reaction according to the above formula (2) is performed, and at this time, oxygen ions are decomposed into oxygen and electrons.
[0089]
As a result, in the anode 28 on the anode side, oxygen is sucked into the ventilation passage 26B in the porous layer 26, and the oxygen content is much higher in the ventilation passage 26B than in the outer electrode 29 (external exhaust gas). Pressure develops. As a result, in the state of the rich air-fuel ratio, a very large oxygen concentration difference occurs between the inner and outer electrodes 28 and 29, and the output voltage Vs from the output terminal 36 becomes a voltage value higher than the reference voltage. Is output.
[0090]
By the way, in such an oxygen concentration detecting element 21, unless the solid electrolyte layer 27 can be activated early at the time of starting the engine, the air-fuel ratio control can be stably performed when the engine is idling or the like. Can not. Further, for example, when water, oil, or the like contained in the exhaust gas becomes droplets and adheres to the outer peripheral side of the detection element 21, a large temperature difference occurs between the location where the droplets adhere and other locations. As a result, there is a possibility that the solid electrolyte layer 27 may be damaged or broken.
[0091]
Therefore, according to the present embodiment, as shown in Table 1 below, between the heater portion 22 and the inner electrode 28, the porous layer 26 made of a porous ceramic material such as alumina having a higher thermal conductivity than air is used. And the solid electrolyte layer 27 is formed to have a larger area than the porous material layer 26 and the inner electrode 28. As shown in FIG. Have. The solid electrolyte layer 27 is almost completely covered by the dense layer 30 from the outside.
[0092]
Thereby, the heat from the heater section 22 can be efficiently transferred to the solid electrolyte layer 27 through the porous layer 26 and the inner electrode 28, and the heater section 22 can be also connected to the solid electrolyte layer 27 through the direct attachment section 27A. Heat can be transmitted directly. In addition, the dense layer 30 covers the solid electrolyte layer 27 from the outside, so that external water droplets and the like can be prevented from penetrating into the solid electrolyte layer 27 side.
[0093]
[Table 1]

[0094]
Then, as a result of performing a characteristic evaluation test on the detection element 21 having such an oxygen concentration, as shown by a characteristic line 37 indicated by a solid line in FIG. 5, the element temperature of the detection element 21 was set within 5 seconds after the engine was started, for example. It was confirmed that the temperature could be raised to 600 ° C. This evaluation test was performed under the conditions where the outside air temperature was -40 ° C. and the voltage of the heater power supply 33 was 12 V (volt).
[0095]
On the other hand, in the case of the detection element of the prior art, as shown by a characteristic line 38 shown by a two-dot chain line in FIG. 5, the element temperature is raised only to about 500 ° C. within 5 seconds after the start of the engine. It was confirmed that thermal conductivity was deteriorated due to, for example, an air layer in the atmosphere chamber.
[0096]
In addition, as for the thermal shock resistance of the detection element 21, an evaluation test called a Weibull plot was performed as shown in FIG. In this evaluation test, for example, a water droplet of 0.002 CC was dropped on the outer surface (near the outer electrode 29) of the detection element 21 while variably adjusting the element temperature by applying the heater power supply 33, The probability of occurrence of cracks (damage, breakage) on the side surface is expressed as an element cracking rate (%).
[0097]
As a result, in the detecting element 21 according to the present embodiment, no element cracking occurs at all until the element temperature is increased to 450 ° C. or higher, as indicated by a characteristic line 39 indicated by a solid line in FIG. It was confirmed that the probability of element cracking exceeded 40%, and the probability of element cracking at 500 ° C. reached 100%.
[0098]
In this evaluation test, for example, a water droplet of 0.002 CC was dropped on the outer surface of the detection element 21, but such a large amount of moisture is not actually contained in the exhaust gas. Therefore, it is considered that there is practically no possibility that an element crack occurs when the oxygen sensor is attached to an exhaust pipe of an automobile engine.
[0099]
On the other hand, in the detection element of the prior art, when the element temperature exceeds about 380 ° C., for example, as indicated by a characteristic line 40 shown by a two-dot chain line in FIG. It was confirmed that the probability of element cracking reached 60%, and the probability of element cracking at 460 ° C. reached 100%.
[0100]
Further, when a plate-type detection element is used as another conventional technique, when the element temperature exceeds about 300 ° C., for example, as indicated by a characteristic line 41 indicated by a broken line in FIG. It was confirmed that at about 350 ° C., the probability of element cracking became almost 100%.
[0101]
Therefore, according to the present embodiment, by covering the solid electrolyte layer 27 from the outside with the dense layer 30 having a dense structure, it is possible to suppress the penetration of external water droplets and the like into the solid electrolyte layer 27. Thus, the thermal shock resistance, durability, and life of the entire detection element 21 can be improved.
[0102]
In addition, since the outer shape of the detection element 21 can be formed into a circular rod shape having no edge part compared to a plate-type element having an edge part formed on the outside, the thermal stress of the detection element 21 can be reduced. Can be sufficiently reduced, and for example, cracking of the solid electrolyte layer 27 can be suppressed.
[0103]
By covering the heater section 22 with the solid electrolyte layer 27 and the dense layer 30 from the outside over almost the entire circumference, it is possible to suppress the heater section 22 from directly contacting the outside air and reduce the influence of the outside air temperature. , The heat from the heater section 22 can be efficiently transmitted to the solid electrolyte layer 27 and the like.
[0104]
Further, in the present embodiment, the heat from the heater section 22 can be efficiently transferred to the solid electrolyte layer 27 side via the porous layer 26 and the inner electrode 28, and the solid electrolyte layer 27 has a direct attachment section 27A. The heat from the heater section 22 can also be transmitted directly through the heater, so that the temperature-raising characteristics of the detection element 21 composed of the solid electrolyte layer 27 and the like can be improved. For example, the solid electrolyte layer 27 can be quickly activated when the engine is started. can do.
[0105]
Therefore, according to the present embodiment, the detection element 21 can be activated in a short time, and even when the engine is started, the oxygen concentration and the like in the exhaust gas are detected early, and the feedback control of the fuel injection amount is immediately performed. It is possible to do. In addition, the degree of freedom in mounting the oxygen sensor can be increased, and the power consumption of the heater section 22 can be reduced.
[0106]
In addition, since it is not necessary to form a reference atmosphere chamber or the like inside the detection element 21 and introduce the atmosphere as in the related art, the structure of the detection element 21 can be simplified, and the work at the time of manufacturing can be simplified. Performance can be improved.
[0107]
Next, FIGS. 7 and 8 show a second embodiment of the present invention, which is characterized in that the inner electrode is formed as an electrode having a porous structure. In the present embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, and description thereof will be omitted.
[0108]
In the figure, reference numeral 51 denotes an oxygen concentration detecting element which is a main part of the oxygen concentration detecting device. The detecting element 51 is substantially the same as the detecting element 21 described in the first embodiment. It comprises a layer 27, an outer electrode 29, a dense layer 30, a protective layer 32 and the like. However, in this case, in the detection element 51, the porous layer 26 described in the first embodiment is omitted, and only an inner electrode 52 described later is provided between the heater section 22 and the solid electrolyte layer 27. I have.
[0109]
Reference numeral 52 denotes an inner electrode used in the present embodiment. The inner electrode 52 is formed by the heater coating layer 25 and the solid electrolyte layer 27 of the heater section 22 similarly to the inner electrode 28 described in the first embodiment. It has a lead portion 52 </ b> A that is provided on the inner peripheral side of the solid electrolyte layer 27 and extends in the axial direction of the heater coating layer 25 toward the base end side of the heater portion 22.
[0110]
However, in this case, the inner electrode 52 is formed by directly printing a paste made of a noble metal material 53 and ceramic particles 54 described later on the outer peripheral side of the heater coating layer 25 as illustrated in FIG. Are formed as annular electrodes extending with a predetermined length in the axial direction of the heater coating layer 25 (solid electrolyte layer 27).
[0111]
In this case, the inner electrode 52 is composed of a noble metal material 53 such as platinum, ceramic particles 54, 54,..., And holes 55, 55,. The ceramic particles 54 are formed, for example, by mixing zirconia powder having a particle size of about 0.4 to 1.0 μm and alumina powder having a particle size of about 0.4 to 1.0 μm. I have.
[0112]
The pores 55 are formed by using a pore forming agent made of, for example, carbon powder having a particle size of about 0.4 to 1.0 μm, and this pore forming agent is burned off when the detection element 51 is fired. Are formed in the inner electrode 52 so as to become open cells. The pores 55 are formed with a pore diameter equal to or larger than the particle diameter of the ceramic particles 54.
[0113]
That is, the inner electrode 52 is formed by adding, for example, 15 to 20% by weight of zirconia powder, 1 to 2% by weight of alumina powder, and 1 to 3% by weight of carbon powder to a powder of the noble metal material 53, It is formed by adjusting the shape. Then, the inner electrode 52 has a function of moving electrons inside the noble metal material 53 by causing an electrode reaction.
[0114]
The ceramic particles 54 contain zirconia powder and alumina powder, so that the inner electrode 52 between the heater coating layer 25 (porous insulating layer) and the solid electrolyte layer 27 shown in FIG. It functions to enhance the bonding property.
[0115]
In addition, the pores 55 which are formed as open cells formed in the inner electrode 52 constitute an oxygen storage chamber which becomes an air passage in the inner electrode 52, and the pores 55 are also formed in the lead portions 52A of the inner electrode 52 shown in FIG. 55 extends. As a result, the air passage formed by the holes 55 allows the inside of the inner electrode 52 to always communicate with the inside of the casing 1 (the insulating cylinder 7 illustrated in FIG. 1), and is transported to the inner electrode 52 side through the solid electrolyte layer 27. This constitutes a gas escape path for allowing incoming oxygen to escape in the direction of arrow A in FIG.
[0116]
The pores 55 of the inner electrode 52 increase the three-phase interface (electrode reaction point) between the solid electrolyte layer 27, the noble metal material 53, and the pores 55, and function as an air passage to reduce oxygen in the inner electrode 52. It functions to diffuse according to the concentration and pressure gradient. A plurality of concave spaces 56 are formed on the boundary surface between the solid electrolyte layer 27 and the inner electrode 52 as shown in FIG. 8, and the concave spaces 56, the solid electrolyte layer 27, and the noble metal material 53 are formed. Also forms a three-phase interface.
[0117]
Thus, also in the present embodiment configured as above, the heat from the heater section 22 can be efficiently transferred to the solid electrolyte layer 27 side through the inner electrode 52, and the solid electrolyte layer 27 The heat from the heater section 22 can also be directly transmitted through 27A, and substantially the same operation and effect as those of the first embodiment can be obtained.
[0118]
Then, a characteristic evaluation test similar to that of the first embodiment was performed on the detection element 21 having such an oxygen concentration. As a result, it was confirmed that the element temperature of the detection element 21 can be raised to, for example, 600 ° C. in about 4 seconds after the engine is started, as indicated by a characteristic line 57 shown by a dashed line in FIG.
[0119]
Further, in the present embodiment, the inner electrode 52 provided between the heater coating layer 25 and the solid electrolyte layer 27 of the heater section 22 is made of a noble metal material 53 such as platinum, ceramic particles 54 and open cells as shown in FIG. Since the precious metal material 53 causes an electrode reaction, electrons can be moved inside by the pores 55, and the pores 55 serving as open cells can be utilized as an oxygen storage chamber in the inner electrode 52. .
[0120]
Thereby, an electrode reaction is caused at the three-phase interface including the solid electrolyte layer 27, the noble metal material 53, and the holes 55 (concave spaces 56), and the movement of electrons can be promoted. It can be quickly diffused according to the gradient. Further, by forming the holes 55 in the inner electrode 52 to have a diameter larger than the particle diameter of the ceramic particles 54, the amount of oxygen contained in the holes 55 can be increased, and the oxygen in the inner electrode 52 can be increased. Can be smoothly diffused.
[0121]
In addition, by using the same material as that of the heater coating layer 25 and the solid electrolyte layer 27 as a constituent material of the ceramic particles 54, the inner electrode 52 is moved between the heater coating layer 25 and the solid electrolyte layer 27 when the element is fired. Can be strongly bonded to In addition, a large number of the three-phase interfaces (electrode reaction points) can be uniformly distributed, and the electrode reaction can be uniformly generated on the entire opposing surface of the inner electrode 52 and the outer electrode 29.
[0122]
Since oxygen can be diffused in the holes 55, the concentration of oxygen and the pressure difference caused by the electrode reaction and the temperature change can be reduced, and the stagnation and depletion of oxygen at the three-phase interface (electrode reaction point) can be prevented. be able to. Thereby, the internal resistance of the sensor can be reduced, and the low-temperature activation of the detection element 51 can be promoted.
[0123]
Furthermore, even when a durability test is performed on the solid electrolyte layer 27 and the like of the detection element 51 by repeatedly raising and lowering the temperature, separation or the like occurs at the three-phase interface between the solid electrolyte layer 27 and the inner electrode 52. Can be suppressed, and an increase in internal resistance and the like due to this can be prevented.
[0124]
Next, FIGS. 9 and 10 show a third embodiment of the present invention. The feature of this embodiment is that ceramic particles constituting a part of the inner electrode are made of perovskite oxide. . In this embodiment, the same components as those in the second embodiment are denoted by the same reference numerals, and the description thereof will be omitted.
[0125]
In the figure, reference numeral 61 denotes an inner electrode provided between the heater coating layer 25 and the solid electrolyte layer 27 and used in the present embodiment, and the inner electrode 61 is provided between the inner electrode 52 and the inner electrode 52 described in the second embodiment. In substantially the same manner, it is composed of a noble metal material 62 such as platinum, ceramic particles 63, 63,..., And holes 64, 64,.
[0126]
Further, a plurality of concave spaces 65 are formed on the boundary surface side between the inner electrode 61 and the solid electrolyte layer 27, similarly to the concave space 56 described in the second embodiment. However, the inner electrode 61 according to the present embodiment is different from the second embodiment in that the ceramic particles 63 are composed of a mixed conductor made of a perovskite oxide described later.
[0127]
That is, the prosbite oxide is, for example, (La_1-xSr_xCoO₃  ) -Based oxide or (La_1-xSr_xMnO₃  ) -Based oxides and constitute the ceramic particles 63 which are mixed conductors.
[0128]
Then, as shown in FIG. 10, the ceramic particles 63, which are mixed conductors, form oxygen ions (O 2) at the two-phase interface with the solid electrolyte layer 27.^2-Is conducted at the two-phase interface between the ceramic particles 63 and the pores 64 to cause the electrode reaction according to the above-mentioned chemical formulas 1 and 2 as shown in FIG.
[0129]
Also, as shown in FIG. 10, three-phase interfaces F1 and F2 between the solid electrolyte layer 27 and the noble metal material 62 and the holes 64, three-phase interfaces F3 and F4 between the solid electrolyte layer 27 and the noble metal material 62 and the concave space 65, and the like. However, an electrode reaction occurs. The holes 64 function as a ventilation path (oxygen storage chamber) to diffuse oxygen in the inner electrode 61 in accordance with the concentration and the pressure gradient.
[0130]
In this case, in the inner electrode 61, the oxygen concentration is higher at the left portion (at the tip end of the element) in FIG. 10 than at the right portion (at the base end of the element) in FIG.₂  ), Ions and the like move in the direction of arrow b in FIG. 10 according to the concentration gradient and the like.
[0131]
Thus, also in the present embodiment configured as described above, it is possible to obtain substantially the same operation and effect as in the second embodiment. In particular, in the present embodiment, by forming the ceramic particles 63 of the inner electrode 61 with a mixed conductor such as a perovskite oxide, a large number of the three-phase interfaces (electrode reaction points) can be uniformly distributed. The electrode reaction can be efficiently caused on the entire electrode surface of the inner electrode 61.
[0132]
Further, the ion diffusion in the mixed conductor (ceramic particles 63) is added to the gas diffusion by the holes 64 with respect to the oxygen concentration gradient in the inner electrode 61, and the oxygen gas moves in the direction indicated by the arrow b in FIG. Speed can be accelerated.
[0133]
As a result, the internal resistance of the element can be reduced, the low-temperature activity and the like can be improved, and for example, the separation between the solid electrolyte layer 27 and the inner electrode 61 can be suppressed for a long time, and the reliability as an oxygen sensor can be reduced. Can be enhanced.
[0134]
In each of the above embodiments, the case where the pump voltage Vp from the DC power supply 34 is externally applied between the inner electrode 28 (52, 61) and the outer electrode 29 has been described. However, the present invention is not limited to this. For example, the inside electrode 28 (52, 61) side is set to the reference oxygen concentration through the ventilation path 28B (holes 55, 64) to sandwich the solid electrolyte layer. Applied to an oxygen sensor (oxygen concentration detector) that generates an electromotive force corresponding to the oxygen concentration in the gas to be measured (exhaust gas) without applying an external voltage between the inner electrode and the outer electrode May be.
[0135]
Next, technical ideas other than the claims that can be grasped from the above embodiments will be described below together with their effects.
[0136]
(1). 4. The oxygen concentration detecting device according to claim 1, wherein a voltage is externally applied between the inner electrode and the outer electrode, and a pseudo-reference oxygen storage comprising the ventilation path is provided on the inner electrode side. An oxygen concentration detecting device having a chamber.
[0137]
In this case, oxygen ions generated on the outer electrode side by the electrochemical catalytic decomposition reaction can be transported from the outer electrode to the inner electrode via oxygen defects in the solid electrolyte layer. Oxygen is sucked in and circulated through the oxygen storage chamber formed by the ventilation path, and a pseudo standard oxygen concentration having a higher oxygen partial pressure than the outer electrode (external gas to be measured) side can be generated.
[0138]
(2). 4. The oxygen concentration detecting device according to claim 2, wherein the pores formed in the inner electrode by the pore forming agent have a pore diameter equal to or larger than the diameter of the ceramic particles. . Thereby, the oxygen storage amount in the pores can be increased, and the diffusion of oxygen in the inner electrode can be performed smoothly.
[0139]
(3). The oxygen concentration detecting device according to claim 2 or 3, wherein at least a part of the ceramic particles is made of the same material as a constituent material of the solid electrolyte layer. Thereby, when the detection element is fired, the inner electrode can be strongly bonded to both the heater portion and the solid electrolyte layer.
[Brief description of the drawings]
FIG. 1 is a longitudinal sectional view showing an oxygen sensor provided with a detection element according to a first embodiment of the present invention.
FIG. 2 is an enlarged vertical sectional view of the detection element shown in FIG.
FIG. 3 is a cross-sectional view of the detection element viewed from a direction indicated by arrows III-III in FIG.
FIG. 4 is a perspective view showing a step of forming a heater pattern, a heater coating layer, a porous layer, an inner electrode, a solid electrolyte layer, an outer electrode, a dense layer, a protective layer, and the like on the outer periphery of the heater core.
FIG. 5 is a characteristic diagram showing a temperature rise characteristic of a detection element.
FIG. 6 is a characteristic diagram showing an element cracking rate of a detection element due to a water droplet.
FIG. 7 is a half sectional view showing an oxygen concentration detecting element according to a second embodiment.
FIG. 8 is an enlarged sectional view showing an internal structure of an inner electrode according to a second embodiment.
FIG. 9 is an enlarged cross-sectional view at the same position as FIG. 8 showing an inner electrode and the like of a detection element according to a third embodiment.
FIG. 10 is an enlarged cross-sectional view of an essential part of a part a in FIG. 9;
[Explanation of symbols]
1 casing
11,12 Lead wire
13,14 Contact plate
21,51 Oxygen concentration detection element
22 heater section
23 heater core
24 Heater pattern
24A, 28A, 29A Lead
25 Heater coating layer
26 Porous material layer
26A drawer
26B ventilation path
27 Solid electrolyte layer
28 inner electrode
29 Outer electrode
30 dense layer
31 Opening
32 protective layer
33 heater power supply
34 DC power supply
35 differential amplifier
36 Output terminal
52, 61 inner electrode (porous structure electrode)
53,62 precious metal materials
54 ceramic particles
55,64 air holes (air passages)
56,65 concave space
63 ceramic particles (mixed conductor)
Vs output voltage (oxygen concentration detection signal)

Claims (3)

A rod-shaped heater portion that generates heat by applying an electric current from the outside, a solid electrolyte layer provided on the outer peripheral side of the heater portion and activated by heat from the heater portion, and the heater portion and the solid electrolyte Between the inner electrode provided on the inner peripheral side of the solid electrolyte layer and an outer electrode provided on the outer peripheral side of the solid electrolyte layer and sandwiching the solid electrolyte layer between the inner electrode and the inner electrode. Oxygen concentration detection device
An air passage formed between the heater portion and the inner electrode, made of a porous ceramic material for flowing oxygen from the position of the inner electrode in the length direction of the heater portion, and having pores of open cells therein. Providing a porous body layer having
On the outer peripheral side of the solid electrolyte layer, an opening through which the gas to be measured is transmitted is provided at a position corresponding to the outer electrode, and in portions other than the opening, the solid electrolyte layer and the inner electrode are covered from the outside. An oxygen concentration detection device having a structure in which a dense layer is provided to prevent a measurement gas from entering the inside. A rod-shaped heater portion that generates heat by applying an electric current from the outside, a solid electrolyte layer provided on the outer peripheral side of the heater portion and activated by heat from the heater portion, and the heater portion and the solid electrolyte Between the inner electrode provided on the inner peripheral side of the solid electrolyte layer and an outer electrode provided on the outer peripheral side of the solid electrolyte layer and sandwiching the solid electrolyte layer between the inner electrode and the inner electrode. Oxygen concentration detection device
The inner electrode is formed using a material obtained by mixing a noble metal, ceramic particles, and a pore-forming agent in order to allow oxygen to flow from the position of the inner electrode in the length direction of the heater portion, and has pores of open cells inside. Composed of a porous electrode having a ventilation path consisting of
On the outer peripheral side of the solid electrolyte layer, an opening through which the gas to be measured is transmitted is provided at a position corresponding to the outer electrode, and in portions other than the opening, the solid electrolyte layer and the inner electrode are covered from the outside. An oxygen concentration detection device having a structure in which a dense layer is provided to prevent a measurement gas from entering the inside. 3. The oxygen concentration detecting device according to claim 2, wherein the ceramic particles serving as the material of the inner electrode are made of a mixed conductor such as a perovskite oxide capable of conducting oxygen ions together with electrons.

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