JP4165982B2

JP4165982B2 - Gas sensor

Info

Publication number: JP4165982B2
Application number: JP2000011479A
Authority: JP
Inventors: 英樹松原; 登松井
Original assignee: NGK Spark Plug Co Ltd
Current assignee: Niterra Co Ltd
Priority date: 2000-01-20
Filing date: 2000-01-20
Publication date: 2008-10-15
Anticipated expiration: 2020-01-20
Also published as: JP2001201483A

Description

【０００１】
【発明の属する技術分野】
本発明はガスセンサに関し、特に自動車、船舶、飛行機等の移動用、産業用の内燃機関の排ガス中、或いはボイラ等の燃焼ガス中のＮＯｘガス濃度、ＨＣガス濃度又はＮＨ₃ガス濃度の測定に用いられるガスセンサに関し、中でもガスセンサの構成要素である測定用ポンプセルの電極成分に関する。
【０００２】
【従来の技術】
従来、被測定ガス雰囲気に含まれる測定対象ガス成分の濃度を測定するガスセンサ、例えばＮＯｘガスセンサでは、かかる被測定ガスにおける酸素濃度が制御されたガスを生成するための第１室と、測定対象ガス成分の濃度、即ちＮＯｘガス濃度を測定するための第２室とを有する限界電流式のものが知られている。このようなＮＯｘガスセンサにあっては、第２室内でＮＯｘガスを解離させるため、第２室内に面するガス濃度測定用電極に所定の電圧を印加している。この電極（第２内部電極）としては、一般的に、Ｐｔ電極が用いられている。一方、特開平９−２８８０８４号公報には、この第２室内に面するＮＯｘガス濃度測定用電極として、ＲｈとジルコニアからなるＲｈサーメット電極が提案されている。
【０００３】
【発明が解決しようとする課題】
しかしながら、上記Ｐｔ電極は、妨害ガス、例えばＨ₂Ｏ、ＣＯを解離させる触媒能力も高いため、これら妨害ガスの影響を受けて、ＮＯｘガス濃度の正確な測定が困難である場合がある。一方、上記特開平９−２８８０８４号公報に提案されたＲｈサーメット電極は、高温領域（例えば、７００〜８００℃）での長期間の使用に伴いＲｈが酸化してしまい、ＮＯｘを解離させる触媒能力が十分に発揮されない場合があるという問題がある。
【０００４】
本発明は、妨害ガスの影響を受け難く、正確な測定対象ガス成分の濃度測定が可能なガスセンサを提供することを目的とする。特に、本発明は、測定対象ガス成分の濃度を測定するための第２室内に面し、低い印加電圧においても測定対象ガス成分に対する触媒能力が高いガス濃度測定用電極を有するガスセンサを提供することを目的とする。
【０００５】
【課題を解決するための手段】
本発明は、第１の視点において、Ｐｔを主たる電極成分とし且つＩｒを含み、さらにこのＩｒが、該Ｐｔ及び該Ｉｒの合量に対して０．２〜５０ｗｔ％含まれるガス濃度測定用電極を提供する。
【０００６】
本発明は、第２の視点において、Ｐｔを主たる電極成分とし且つＲｕを含み、さらにこのＲｕが、該Ｐｔ及び該Ｒｕの合量に対して０．２〜５０ｗｔ％含まれるガス濃度測定用電極を提供する。
【０００７】
前記第１及び第２の視点に係るガス濃度測定用電極は、複数の室（気体流通部）を有する限界電流式のガスセンサに適用される。このガスセンサは、測定対象ガス成分を含む被測定ガスを内部に導き、かかる被測定ガス中の酸素濃度を制御するための第１室と、測定対象ガス成分の濃度を測定するための第２室と、前記第１室と前記第２室とを拡散抵抗をもって連通する連通部と、酸素イオン伝導体上に形成された複数の電極と、を有し、該複数の電極のうち少なくとも一つの電極が該第２室内に面すると共に、前記測定対象ガス成分を解離或いは酸化又は還元させるガス濃度測定電極をなし、前記複数の電極間に電圧が印加されて前記測定対象ガス成分の濃度に応じた電流が発生される。このガスセンサは、測定対象ガスに対する触媒能力が高い電極を有するため、この電極に印加する電圧を低くして、該電極上での妨害ガス（含酸素ガス成分）の解離を抑制することができる。これによって、ＣＯ₂、Ｈ₂Ｏ等の妨害ガスの影響が少なく、精度の高い測定対象ガス成分の濃度測定が可能となる。また、このガスセンサは、第２室内に面するガス濃度測定用電極として、測定対象ガスに対する触媒能力が高い電極を有することにより、該電極上においてより短時間で、測定対象ガス成分が例えばＮＯｘガスの場合にはそのＮＯｘガスが解離され、ＨＣガスやＮＨ₃ガスの場合にはそのＨＣガス又はＮＨ₃ガスが酸化されることになる。この結果、ガスセンサの検出精度や応答性も向上する。
【０００８】
本発明のその他の視点及び特徴は、各請求項に記載のとおりであり、その引用をもってその重複記載を省略する。よって、各請求項の各特徴は、ここに記載されているものとみなされる。なお、従属項はそれぞれ、各独立項に記載された発明の原理に反しない限り、各独立項に適用され得、又従属項は他の従属項に適用され得る。
【０００９】
【発明の実施の形態】
以下、本発明の好ましい実施の形態を説明する。
【００１０】
本発明の好ましい実施の形態によれば、第２室内に面するガス濃度測定用電極において、その電極成分が測定用セルを構成する酸素イオン伝導体成分に担持されている。
【００１１】
本発明の好ましい実施の形態においては、測定対象ガス成分を解離或いは酸化又は還元させるために、第２室内に面するガス濃度測定用電極の少なくとも一部が、ＰｔとＩｒの合量に対して、Ｉｒを２〜２０ｗｔ％含む。
【００１２】
本発明の好ましい実施の形態においては、測定対象ガス成分を解離或いは酸化又は還元させるために、第２室内に面するガス濃度測定用電極の少なくとも一部が、ＰｔとＲｕの合量に対して、Ｒｕを１〜１２ｗｔ％含む。
【００１３】
本発明の好ましい実施の形態においては、測定対象ガス成分を解離或いは酸化又は還元させるために、第２室内に面するガス濃度測定用電極の少なくとも一部が、ＰｔとＩｒとＲｕの合量に対して、Ｉｒを２〜２０ｗｔ％及びＲｕを１〜１２ｗｔ％含む。
【００１４】
Ｐｔに対してＩｒ及びＲｕを添加する場合、これらの添加量が規定量より少ない場合には、Ｐｔが粒成長して三相界面（気相、電極成分（触媒相）及び酸素イオン伝導相の界面）を形成する面積が減少し、測定対象ガス成分を解離或いは酸化又は還元する触媒能力が低下する。反対に、これらの添加量が規定量より多い場合には、Ｉｒ及びＲｕが粒成長して三相界面を形成する面積が減少し、測定対象ガス成分を解離或いは酸化又は還元する触媒能力が低下する。また、Ｉｒ、Ｒｕが添加されることにより、ガス濃度測定用電極の高温領域における耐酸化性が向上する。
【００１５】
本発明の好ましい実施の形態においては、第２室内に面するガス濃度測定用電極の一部、例えば、第２室内に直接的に面する外周（外側）部分のみを、ＩｒないしＲｕを含むよう形成する。また、この電極には、本発明の効果が得られる限りにおいて、他の金属元素や酸素イオン伝導体と同質の成分を添加することができる。
【００１６】
本発明の好ましい実施の形態によれば、本発明の効果が得られる限りにおいて、第２室内に面するガス濃度測定用電極に、Ｐｔ、Ｉｒ及びＲｕ以外の電極成分、例えば、その他の卑金属ないし貴金属成分を所定量単独又は複合添加することを許容する。
【００１７】
本発明の好ましい実施の形態に係るガスセンサは、第１拡散抵抗部に通じる第１室と、酸素イオン伝導体上に第１室内外にそれぞれ面して形成された内部電極及び外部電極を備え、酸素を該第１室内外間で汲み出し入れする酸素ポンプセルと、第１室内に面して酸素イオン伝導体上に形成された酸素濃度検知電極と、該第１室で酸素濃度が制御されたガスが前記連通部に配設された第２拡散抵抗部を通じて導入される第２室と、酸素イオン伝導体上に形成された複数の電極を含み該複数の電極のうち少なくとも一つの電極が第２室内に面するガス濃度測定用電極（第２内部電極）である測定対象ガス成分濃度の測定用セルと、を有し、酸素濃度検知電極の電位に基づいて酸素ポンプセルによる酸素の汲み出し入れ量が制御され、又測定用セルの複数の電極間に電圧が印加されて測定対象ガス成分の濃度に応じた電流が発生される。さらに好ましくは、第１室において酸素ポンプセルにより、第２室に拡散するガス中の酸素濃度を可及的に低く制御する。これによって、測定用セル（第２酸素ポンプセル）に流れる酸素イオン伝導に起因する酸素ポンプ電流量が、第２室に拡散する酸素濃度の影響を受け難くされて、ガス濃度をよりよく反映するようになる。
【００１８】
例えば、本発明によるガスセンサをＮＯｘガス濃度測定に用いる場合、触媒能力が高いＩｒ又は／及びＲｕが添加されたＰｔ電極上でＮＯｘ、特にＮＯの解離が促進され、より低い印加電圧で短時間に、ＮＯｘが解離して生じた酸素イオンを酸素イオン伝導体を通じて汲み出すことができる。この結果、ＮＯｘガス濃度測定の検出精度や応答性が向上する。また、ＨＣガス濃度測定に用いる場合には、触媒能力が高いＩｒ又は／及びＲｕが添加されたＰｔ電極上でＨＣガス成分の酸化が促進され、効率よくＨＣガスが酸化されて酸素が消費されるため、その酸素の消費量から求められるＨＣガス濃度測定の検出精度や応答性が向上する。
【００１９】
本発明の好ましい実施の形態においては、酸素濃度検出電極と酸素イオン伝導体を介して配設される酸素濃度基準電極の電位を安定化させるため、酸素濃度基準電極と第２室に面するガス濃度測定用電極の間を、拡散抵抗をもって連通する。これによって、第２室に面するＮＯｘガス濃度測定用電極上でＮＯｘガスが解離されて生じた酸素イオン由来のＯ₂ガスが酸素濃度基準電極に向かって拡散していく。かくして、酸素濃度基準電極は酸素濃度が高められた自己生成基準極となる。また、好ましくは、酸素濃度基準電極に微小電流を供給することによって、酸素濃度基準電極を酸素濃度が高められた自己生成基準極とする。
【００２０】
また、好ましくは、上記第１及び第２室（気体流通部）をセラミック等の多孔体、微細な貫通孔又は細かいスリット（すきま状にする）から構成する。
【００２１】
本発明によるガスセンサ（素子）は、公知のグリーンシートの積層焼成によるガスセンサの製造技術に基づき、次のように作製することができる。すなわち、酸素イオン伝導性を有するＺｒＯ₂を主体とするグリーンシート単体、酸素ポンプセルの内部電極等となる多孔質な電極を形成するためＰｔ等の電極成分を含むペースト、第１拡散抵抗部（拡散孔）を形成するためのアルミナペースト、層間を絶縁するため或いは処理室を画成するための緻密層を形成するためのアルミナペースト、並びに処理室を画成するためのカーボンペーストなどがそれぞれ所定位置にスクリーン印刷法を用いて塗布されたＺｒＯ₂を主体とするグリーンシート、及び第２拡散抵抗部（拡散孔）を形成するための多孔質セラミックス体（例えばアルミナ）が埋め込まれたＺｒＯ₂を主体とするグリーンシート等を積層し、乾燥し、焼成する。なお、ここでは安定化剤（例えば、ＣａＯやＭｇＯ）を含有するＺｒＯ₂を用いることが好ましい。
【００２２】
なお、第２室内に面するガス濃度測定用電極を形成するための原料ペーストは、Ｐｔ粉末、Ｉｒ粉末ないしＲｕ粉末、及び適量の有機溶剤、場合によってはＺｒＯ₂粉末を混合し、分散させ、これに有機バインダーを有機溶剤に溶解させたものを添加し、さらに粘度調整剤を添加し、混合することにより作製できる。或いは、ＩｒないしＲｕを含む合金粉末を用いて、乾式混合、分散、湿式混合を経て原料ペーストを作製することもできる。
【００２３】
【実施例】
以上説明した本発明の好ましい実施の形態をさらに明確化するために、以下図面を参照して、本発明の一実施例を説明する。なお、本実施例では、ガスセンサの内でＮＯｘガス濃度の測定を行うＮＯｘガスセンサについて説明する。
【００２４】
図１は、本発明の一実施例に係るＮＯｘガスセンサ（ＮＯｘガスセンサ素子）の説明図であり、このガスセンサ素子の先端部を長手方向かつ積層方向に沿って切断した断面を示している。図１を参照すると、このガスセンサ素子は、酸素イオン伝導性の固体電解質（例えば、ＺｒＯ₂）を主体とする第１層１〜第５層５が順に積層されて構成されている。なお、各層１〜５間には絶縁層が形成されている。第２層２及び第４層４は、酸素イオン伝導性の固体電解質層に代えて絶縁層（例えば緻密なアルミナ絶縁層）とすることができる。さらに、第２層２と同層に、第１層１、第２層２及び第３層３に囲まれて、測定対象ガス成分を含む被測定ガス雰囲気と画されてなる第１室７が画成されている。第４層４と同層に、第３層３、第４層４及び第５層５に囲まれて第２室９が画成されている。第２層２の両側面の一部には、第１拡散抵抗部６が形成されている。第１室７の一端側は、第１拡散抵抗部６を通じて被測定ガス雰囲気に連通している。第３層３中には、第２拡散抵抗部８（拡散抵抗をもつ連通部のこと）が形成されている。第２拡散抵抗部８の一端側は、第１室７の他端側であって第１拡散抵抗部６から離間した位置に開口している。第２拡散抵抗部８の他端側は、第２室９に開口している。これによって、第１室７と第２室９は拡散抵抗をもつ連通部（ここでは拡散抵抗部８のこと）を介して互いに連通している。
【００２５】
酸素イオン伝導性の固体電解質層である第１層１の被測定ガス雰囲気に面する一面上には第１外部電極１０、その第１室７に面する他面上には第１内部電極１１が形成されている。但し、第１内部電極１１の長さは、第１外部電極１０の長さより短くされ、第１拡散抵抗部６近傍から第２拡散抵抗部８の開口直上直前まで延在し、同開口直上までは形成されていない。第１酸素ポンプセルは、第１層１、第１外部電極１０及び第１内部電極１１から構成される。
【００２６】
酸素イオン伝導性の固体電解質層である第３層３の第１室７に面する一面上には、酸素濃度検知電極１２が第２拡散抵抗部８の開口近傍に形成されている。第３層３の他面上（第３層３、第４層４間）には、酸素濃度基準電極１３が形成されている。酸素濃度測定セルは、第３層３、酸素濃度検知電極１２及び酸素濃度基準電極１３から構成される。また、酸素濃度基準電極１３の電位を安定化させるため、酸素濃度基準１３と後述の第２外部電極１５の間は、第３拡散抵抗部１６を通じて連通されている。
【００２７】
酸素イオン伝導性の固体電解質層である第５層５の一面上、第２室９内に面してガス濃度測定用電極（以下「第２内部電極」という）１４が形成され、第２室９外（第４層４、第５層５間）にはガス濃度測定用外部電極（以下「第２外部電極」という）１５が形成されている。第２酸素ポンプセル、すなわち測定対象ガス成分（ＮＯｘガス）濃度の測定用セルは、第５層５、第２内部電極１４及び第２外部電極１５から構成される。
【００２８】
次に、このＮＯｘガスセンサ（素子）に適用される制御回路について説明する。
【００２９】
第１外部電極１０と第１内部電極１１間には、電源及び第１電流計が互いに直列に接続されている。電源が第１外部電極１０と第１内部電極１１間に印加する第１印加電圧Ｖｐ１は、酸素濃度検知電極１２と酸素濃度基準電極１３の電位差が一定となるよう可変に制御される。第１電流計は、第１外部電極１０と第１内部電極１１間に流れる第１酸素ポンプ電流Ｉｐ１を検出する。第１酸素ポンプ電流Ｉｐ１の大きさは基本的に被測定ガス中の酸素濃度に比例する。一方、第２内部電極１４と第２外部電極１５間には定電圧源及び第２電流計が接続されている。第２電流計は、第２内部電極１４と第２外部電極１５間に流れる第２酸素ポンプ電流Ｉｐ２を検出する。第２酸素ポンプ電流Ｉｐ２の大きさは基本的に被測定ガス中の測定対象ガス成分（ＮＯｘガス）の濃度に比例している。
【００３０】
なお、ガスセンサ素子を適温で動作させるために、素子の上層及び／又は下層には、ヒータが設置ないし貼着される（図１参照）。
【００３１】
［測定例］
図１に示したような構造を有し、第２内部電極の電極成分（表１参照）が互いに異なるＮＯｘガスセンサ（試料Ｎｏ．１〜１１）を作製した。試料Ｎｏ．２〜６に係るＮＯｘガスセンサにおいては、ＰｔとＩｒの合量（100wt%）にＩｒ粉末を下記の表１に示す重量で添加したペーストを用いて第２内部電極を作製した。試料Ｎｏ．７〜１１に係るＮＯｘガスセンサにおいては、ＰｔとＲｕの合量（100wt%）にＲｕ粉末を下記の表１に示す重量で添加したペーストをそれぞれ用いて第２内部電極を作製した。なお、試料Ｎｏ．１に係るＮＯｘガスセンサ（比較例）においては、Ｉｒ及びＲｕ粉末が添加されていないＰｔペーストを用いて第２内部電極を作製した。
【００３２】
これらのＮＯｘガスセンサに妨害ガス成分を含む被測定ガス（１）或いは（２）を投入し、測定用セルの両電極間（第２内部電極と第２外部電極間）に印加する電圧（第２印加電圧）Ｖｐ２を変えて、測定用セルに流れる第２酸素ポンプ電流Ｉｐ２を測定した。下記に測定条件を示し、表１に測定結果を示す。なお、表１中、Ｉｐ２［Ｏ₂］は、被測定ガス（１）投入時、測定用セルに流れたＯ₂に対するＩｐ２の限界電流を表し、Ｉｐ２［ＮＯ］は被測定ガス（２）投入時、測定用セルに流れたＮＯに対するＩｐ２の限界電流を表す。
【００３３】
［測定条件］
［被測定ガス（１）の組成７５０ｐｐｍ、ＣＯ₂：１０％、Ｈ₂Ｏ：１０％、Ｎ₂：ｂａｌ、
被測定ガス組成（２）の組成ＮＯ：１５００ｐｐｍ、ＣＯ₂：１０％、Ｈ₂Ｏ：１０％、Ｎ₂：ｂａｌ、
被測定ガス温度６８０℃、
測定用セル（第２の酸素ポンプセル）への印加電圧４５０［ｍＶ］。
【００３４】
【表１】

（＊）試料Ｎｏ．１は比較例
【００３５】
ところで、Ｏ₂は酸素２原子を有するのに対してＮＯは酸素１原子を有するから、理想的には、図２に示したＯ₂＝７５０ｐｐｍ時のＩｐ２限界電流値と、図３に示したＮＯ＝１５００ｐｐｍ時のＩｐ２限界電流値は一致する。つまり、ＮＯｘガス濃度を酸素イオン伝導によって流れる電流に基づいて測定するためには、Ｏ₂＝７５０ｐｐｍ時の限界電流値に対するＮＯ＝１５００ｐｐｍ時の限界電流値が「１」に近い方が好ましい。
【００３６】
表１を参照すると、Ｉｒ及びＲｕが添加されていない試料Ｎｏ．１のＮＯｘガスセンサ（比較例）の「Ｉｐ２［ＮＯ］／Ｉｐ２［Ｏ₂］」が最も小さくなっている。よって、ＩｒないしＲｕの添加によって、「Ｉｐ２［ＮＯ］／Ｉｐ２［Ｏ₂］」が「１」に近づき、低い印加電圧でもＮＯｘが十分に解離されることが分かる。
【００３７】
さらに、試料Ｎｏ．５に係るＮＯｘガスセンサ（実施例）のＯ₂及びＮＯに対する限界電流特性を、試料Ｎｏ．１に係るＮＯｘガスセンサ（比較例）の同特性と対比して説明する。図２には、試料Ｎｏ．１及び５のＮＯｘガスセンサについて、被測定ガスがＮＯｘガスを含まない場合のＶｐ２とＩｐ２の関係、特に酸素に対する測定用セルの限界電流特性を示す。一方、図３には、両センサについて、被測定ガスがＮＯｘガスを含む場合のＶｐ２とＩｐ２の関係、特にＮＯガスに対する測定用セルの限界電流特性を示す。
【００３８】
図２及び図３を参照すると、試料Ｎｏ．５のＮＯｘガスセンサ（実施例）によれば、特に、第２印加電圧Ｖｐ２が低い範囲で、図２に示したＯ₂によるＩｐ２と図３に示したＮＯによるＩｐ２の差が小さく、ＮＯが第２室において十分に解離されていることが分かる。このように第２印加電圧Ｖｐ２を低く設定できることにより、第２内部電極上における妨害ガス、例えばＣＯ、Ｈ₂Ｏ等の解離が抑制される。この結果、ＮＯｘガス濃度をより正確に測定することが可能となる。
【００３９】
【発明の効果】
本発明によれば、妨害ガスの影響を受け難く、正確な測定対象ガス成分の濃度測定が可能なガスセンサが提供される。また、本発明によれば、低い印加電圧においても測定対象ガス成分に対する触媒能力が高いガス濃度測定用電極を有するガスセンサが提供される。
【図面の簡単な説明】
【図１】本発明の一実施例に係るＮＯｘガスセンサの説明図である。
【図２】試料Ｎｏ．１及び５に係るＮＯｘガスセンサにおいて、Ｏ₂に対する測定用セルの限界電流特性を示すグラフである。
【図３】試料Ｎｏ．１及び５に係るＮＯｘガスセンサにおいて、ＮＯに対する測定用セルの限界電流特性を示すグラフである。
【符号の説明】
１，…，５第１層，…，第５層
６第１拡散抵抗部
７第１室
８第２拡散抵抗部
９第２室
１０第１外部電極（酸素濃度制御ガス生成用の外部電極）
１１第１内部電極（第１室内に面する酸素濃度制御ガス生成用の内部電極）
１２酸素濃度検知電極
１３酸素濃度基準電極
１４第２内部電極（第２室内に面するガス濃度測定用電極）
１５第２外部電極（ガス濃度測定用外部電極）
１６第３拡散抵抗部[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a gas sensor, and particularly used for measuring NOx gas concentration, HC gas concentration, or NH ₃ gas concentration in exhaust gas of an internal combustion engine for industrial use, or in combustion gas of a boiler or the like for movement of automobiles, ships, airplanes, etc. In particular, the present invention relates to an electrode component of a pump cell for measurement which is a constituent element of the gas sensor.
[0002]
[Prior art]
Conventionally, in a gas sensor that measures the concentration of a measurement target gas component contained in a measurement target gas atmosphere, for example, a NOx gas sensor, a first chamber for generating a gas with a controlled oxygen concentration in the measurement target gas, and a measurement target gas A limiting current type having a second chamber for measuring the concentration of the component, that is, the NOx gas concentration is known. In such a NOx gas sensor, in order to dissociate NOx gas in the second chamber, a predetermined voltage is applied to the gas concentration measuring electrode facing the second chamber. As this electrode (second internal electrode), a Pt electrode is generally used. On the other hand, Japanese Patent Laid-Open No. 9-288084 proposes an Rh cermet electrode made of Rh and zirconia as an NOx gas concentration measuring electrode facing the second chamber.
[0003]
[Problems to be solved by the invention]
However, since the Pt electrode has a high catalytic ability to dissociate interfering gases such as H ₂ O and CO, it is sometimes difficult to accurately measure the NOx gas concentration due to the influence of these interfering gases. On the other hand, the Rh cermet electrode proposed in Japanese Patent Laid-Open No. 9-288084 has a catalytic ability to dissociate NOx because Rh is oxidized with long-term use in a high temperature region (for example, 700 to 800 ° C.). There is a problem that may not be fully demonstrated.
[0004]
It is an object of the present invention to provide a gas sensor that is not easily affected by an interfering gas and that can accurately measure the concentration of a gas component to be measured. In particular, the present invention provides a gas sensor having a gas concentration measurement electrode that faces the second chamber for measuring the concentration of the gas component to be measured and has a high catalytic ability for the gas component to be measured even at a low applied voltage. With the goal.
[0005]
[Means for Solving the Problems]
In the first aspect, the present invention provides a gas concentration measuring electrode containing Pt as a main electrode component and containing Ir, and further containing 0.2 to 50 wt% of Ir with respect to the total amount of Pt and Ir. I will provide a.
[0006]
In the second aspect, the present invention provides a gas concentration measurement electrode comprising Pt as a main electrode component and containing Ru, and further containing Ru in an amount of 0.2 to 50 wt% with respect to the total amount of Pt and Ru. I will provide a.
[0007]
The gas concentration measurement electrodes according to the first and second viewpoints are applied to a limiting current type gas sensor having a plurality of chambers (gas flow portions). The gas sensor includes a first chamber for guiding a gas to be measured containing a gas component to be measured inside, and a second chamber for measuring the concentration of the gas component to be measured, and a first chamber for controlling the oxygen concentration in the gas to be measured. A communication portion that communicates the first chamber and the second chamber with diffusion resistance, and a plurality of electrodes formed on the oxygen ion conductor, and at least one of the plurality of electrodes Facing the second chamber and forming a gas concentration measurement electrode for dissociating, oxidizing or reducing the gas component to be measured, and a voltage is applied between the plurality of electrodes in accordance with the concentration of the gas component to be measured. A current is generated. Since this gas sensor has an electrode having a high catalytic ability for the gas to be measured, the voltage applied to this electrode can be lowered to suppress the dissociation of interfering gas (oxygen-containing gas component) on the electrode. As a result, the influence of the interference gas such as CO ₂ and H ₂ O is small, and the concentration of the measurement target gas component can be measured with high accuracy. Further, this gas sensor has an electrode having a high catalytic ability for the gas to be measured as a gas concentration measuring electrode facing the second chamber, so that the gas component to be measured can be, for example, NOx gas in a shorter time on the electrode. in the case of the dissociated its NOx gas, in the case of HC gas and NH ₃ gas will be the HC gas or NH ₃ gas is oxidized. As a result, the detection accuracy and responsiveness of the gas sensor are also improved.
[0008]
The other viewpoints and features of the present invention are as described in the respective claims, and duplication of the description is omitted with reference thereto. Thus, each feature of each claim is considered to be described herein. Each dependent claim can be applied to each independent claim as long as it does not violate the principle of the invention described in each independent claim, and a dependent claim can be applied to other dependent claims.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, preferred embodiments of the present invention will be described.
[0010]
According to a preferred embodiment of the present invention, in the gas concentration measurement electrode facing the second chamber, the electrode component is supported on the oxygen ion conductor component constituting the measurement cell.
[0011]
In a preferred embodiment of the present invention, in order to dissociate, oxidize, or reduce the gas component to be measured, at least a part of the gas concentration measurement electrode facing the second chamber is in proportion to the total amount of Pt and Ir. , Ir is contained in an amount of 2 to 20 wt%.
[0012]
In a preferred embodiment of the present invention, in order to dissociate, oxidize, or reduce the gas component to be measured, at least a part of the gas concentration measurement electrode facing the second chamber is in proportion to the total amount of Pt and Ru. , Ru is contained in an amount of 1 to 12 wt%.
[0013]
In a preferred embodiment of the present invention, in order to dissociate, oxidize, or reduce the gas component to be measured, at least a part of the gas concentration measurement electrode facing the second chamber has a total amount of Pt, Ir, and Ru. On the other hand, it contains 2 to 20 wt% of Ir and 1 to 12 wt% of Ru.
[0014]
When Ir and Ru are added to Pt, if these addition amounts are less than the specified amount, Pt grains grow and the three-phase interface (gas phase, electrode component (catalyst phase) and oxygen ion conduction phase The area for forming the interface is reduced, and the catalytic ability to dissociate, oxidize or reduce the gas component to be measured is reduced. On the contrary, when these addition amounts are larger than the specified amount, the area where Ir and Ru grow and form a three-phase interface decreases, and the catalytic ability to dissociate, oxidize or reduce the gas component to be measured decreases. To do. Further, the addition of Ir and Ru improves the oxidation resistance in the high temperature region of the gas concentration measuring electrode.
[0015]
In a preferred embodiment of the present invention, only a part of the gas concentration measurement electrode facing the second chamber, for example, the outer (outer) portion directly facing the second chamber contains Ir or Ru. Form. Moreover, as long as the effect of this invention is acquired, the same quality component as another metal element and an oxygen ion conductor can be added to this electrode.
[0016]
According to a preferred embodiment of the present invention, an electrode component other than Pt, Ir, and Ru, for example, other base metals or the like, is used for the gas concentration measurement electrode facing the second chamber as long as the effects of the present invention are obtained. It is allowed to add a predetermined amount of the noble metal component alone or in combination.
[0017]
A gas sensor according to a preferred embodiment of the present invention includes a first chamber that communicates with the first diffusion resistance portion, and an internal electrode and an external electrode that are formed on the oxygen ion conductor so as to face the outside of the first chamber, respectively. An oxygen pump cell for pumping oxygen in and out of the first chamber, an oxygen concentration detection electrode formed on the oxygen ion conductor facing the first chamber, and a gas whose oxygen concentration is controlled in the first chamber A second chamber introduced through a second diffusion resistor disposed in the communicating portion; and a plurality of electrodes formed on the oxygen ion conductor, wherein at least one of the plurality of electrodes is in the second chamber A gas concentration measuring electrode (second internal electrode) facing to the measuring cell for measuring the concentration of the gas component to be measured, and the amount of oxygen pumped in by the oxygen pump cell is controlled based on the potential of the oxygen concentration detecting electrode Measurement cell Current voltage between the plurality of electrodes depending on the concentration of the measurement target gas component is applied is generated. More preferably, the oxygen concentration in the gas diffusing into the second chamber is controlled as low as possible by the oxygen pump cell in the first chamber. As a result, the amount of oxygen pump current caused by the conduction of oxygen ions flowing through the measurement cell (second oxygen pump cell) is made less affected by the oxygen concentration diffusing into the second chamber, so that the gas concentration is better reflected. become.
[0018]
For example, when the gas sensor according to the present invention is used for NOx gas concentration measurement, dissociation of NOx, particularly NO, is promoted on a Pt electrode to which Ir or / and Ru having a high catalytic ability is added, and at a lower applied voltage in a short time. , Oxygen ions generated by dissociation of NOx can be pumped out through the oxygen ion conductor. As a result, the detection accuracy and responsiveness of NOx gas concentration measurement are improved. Also, when used for HC gas concentration measurement, oxidation of HC gas components is promoted on a Pt electrode to which Ir or / and Ru having high catalytic ability is added, and HC gas is efficiently oxidized and oxygen is consumed. Therefore, the detection accuracy and responsiveness of the HC gas concentration measurement obtained from the oxygen consumption are improved.
[0019]
In a preferred embodiment of the present invention, the gas facing the oxygen concentration reference electrode and the second chamber is used to stabilize the potential of the oxygen concentration reference electrode disposed via the oxygen concentration detection electrode and the oxygen ion conductor. The concentration measurement electrodes communicate with each other with diffusion resistance. Thereby, O ₂ gas derived from oxygen ions generated by dissociation of NOx gas on the NOx gas concentration measurement electrode facing the second chamber diffuses toward the oxygen concentration reference electrode. Thus, the oxygen concentration reference electrode becomes a self-generated reference electrode with an increased oxygen concentration. Preferably, the oxygen concentration reference electrode is a self-generated reference electrode with an increased oxygen concentration by supplying a small current to the oxygen concentration reference electrode.
[0020]
Preferably, the first and second chambers (gas flow portions) are composed of a porous body such as ceramic, a fine through-hole, or a fine slit (a gap).
[0021]
The gas sensor (element) according to the present invention can be manufactured as follows based on a known technology for manufacturing a gas sensor by laminating and firing green sheets. That is, a paste containing an electrode component such as Pt, a first diffusion resistance portion (diffusion) in order to form a green sheet consisting mainly of ZrO ₂ having oxygen ion conductivity, a porous electrode which is an internal electrode of an oxygen pump cell, etc. Alumina paste for forming holes), alumina paste for forming a dense layer for insulating the layers or for defining the processing chamber, and carbon paste for defining the processing chamber, etc. are in predetermined positions, respectively. Mainly composed of ZrO ₂ embedded with a green sheet mainly composed of ZrO ₂ coated by screen printing and a porous ceramic body (for example, alumina) for forming the second diffusion resistance portion (diffusion hole). A green sheet or the like is laminated, dried, and fired. Here, it is preferable to use ZrO ₂ containing a stabilizer (for example, CaO or MgO).
[0022]
Note that the raw material paste for forming the gas concentration measurement electrode facing the second chamber is a mixture of Pt powder, Ir powder or Ru powder, and an appropriate amount of organic solvent, in some cases ZrO ₂ powder, dispersed, This can be prepared by adding an organic binder dissolved in an organic solvent, adding a viscosity modifier, and mixing them. Alternatively, an alloy powder containing Ir or Ru can be used to produce a raw material paste through dry mixing, dispersion, and wet mixing.
[0023]
【Example】
In order to further clarify the preferred embodiment of the present invention described above, an embodiment of the present invention will be described below with reference to the drawings. In the present embodiment, a NOx gas sensor that measures the NOx gas concentration in the gas sensor will be described.
[0024]
FIG. 1 is an explanatory diagram of a NOx gas sensor (NOx gas sensor element) according to an embodiment of the present invention, and shows a cross section obtained by cutting the tip of the gas sensor element along the longitudinal direction and the stacking direction. Referring to FIG. 1, this gas sensor element is configured by sequentially laminating a first layer 1 to a fifth layer 5 mainly composed of an oxygen ion conductive solid electrolyte (for example, ZrO ₂ ). An insulating layer is formed between each layer 1-5. The second layer 2 and the fourth layer 4 may be insulating layers (for example, dense alumina insulating layers) instead of the oxygen ion conductive solid electrolyte layer. Further, in the same layer as the second layer 2, a first chamber 7 surrounded by the first layer 1, the second layer 2, and the third layer 3 and defined as a measurement gas atmosphere containing a measurement target gas component is provided. It is defined. A second chamber 9 is defined in the same layer as the fourth layer 4 and surrounded by the third layer 3, the fourth layer 4, and the fifth layer 5. First diffused resistor portions 6 are formed on part of both side surfaces of the second layer 2. One end side of the first chamber 7 communicates with the measurement gas atmosphere through the first diffusion resistance unit 6. In the third layer 3, a second diffusion resistance portion 8 (a communication portion having a diffusion resistance) is formed. One end side of the second diffusion resistance portion 8 is open at a position on the other end side of the first chamber 7 and spaced from the first diffusion resistance portion 6. The other end side of the second diffusion resistance unit 8 opens into the second chamber 9. As a result, the first chamber 7 and the second chamber 9 communicate with each other via a communication portion having diffusion resistance (here, the diffusion resistance portion 8).
[0025]
The first external electrode 10 is formed on one surface of the first layer 1 that is an oxygen ion conductive solid electrolyte layer facing the measurement gas atmosphere, and the first internal electrode 11 is formed on the other surface facing the first chamber 7. Is formed. However, the length of the first internal electrode 11 is shorter than the length of the first external electrode 10, extends from the vicinity of the first diffusion resistance portion 6 to immediately before the opening of the second diffusion resistance portion 8, and immediately above the opening. Is not formed. The first oxygen pump cell includes a first layer 1, a first external electrode 10, and a first internal electrode 11.
[0026]
An oxygen concentration detection electrode 12 is formed in the vicinity of the opening of the second diffusion resistance portion 8 on one surface of the third layer 3 which is an oxygen ion conductive solid electrolyte layer facing the first chamber 7. An oxygen concentration reference electrode 13 is formed on the other surface of the third layer 3 (between the third layer 3 and the fourth layer 4). The oxygen concentration measurement cell includes the third layer 3, the oxygen concentration detection electrode 12, and the oxygen concentration reference electrode 13. Further, in order to stabilize the potential of the oxygen concentration reference electrode 13, the oxygen concentration reference 13 and a second external electrode 15 to be described later are communicated with each other through the third diffusion resistance portion 16.
[0027]
A gas concentration measurement electrode (hereinafter referred to as “second internal electrode”) 14 is formed on one surface of the fifth layer 5, which is an oxygen ion conductive solid electrolyte layer, facing the inside of the second chamber 9. An external electrode for gas concentration measurement (hereinafter referred to as “second external electrode”) 15 is formed outside 9 (between the fourth layer 4 and the fifth layer 5). The second oxygen pump cell, that is, the measurement cell for the measurement target gas component (NOx gas) concentration is composed of the fifth layer 5, the second internal electrode 14, and the second external electrode 15.
[0028]
Next, a control circuit applied to this NOx gas sensor (element) will be described.
[0029]
A power supply and a first ammeter are connected in series between the first external electrode 10 and the first internal electrode 11. The first applied voltage Vp1 applied by the power source between the first external electrode 10 and the first internal electrode 11 is variably controlled so that the potential difference between the oxygen concentration detection electrode 12 and the oxygen concentration reference electrode 13 is constant. The first ammeter detects a first oxygen pump current Ip1 flowing between the first external electrode 10 and the first internal electrode 11. The magnitude of the first oxygen pump current Ip1 is basically proportional to the oxygen concentration in the gas to be measured. On the other hand, a constant voltage source and a second ammeter are connected between the second internal electrode 14 and the second external electrode 15. The second ammeter detects a second oxygen pump current Ip2 flowing between the second internal electrode 14 and the second external electrode 15. The magnitude of the second oxygen pump current Ip2 is basically proportional to the concentration of the measurement target gas component (NOx gas) in the gas to be measured.
[0030]
In order to operate the gas sensor element at an appropriate temperature, a heater is installed or adhered to the upper layer and / or the lower layer of the element (see FIG. 1).
[0031]
[Measurement example]
NOx gas sensors (sample Nos. 1 to 11) having the structure shown in FIG. 1 and having different electrode components (see Table 1) of the second internal electrode were produced. Sample No. In the NOx gas sensors according to 2 to 6, a second internal electrode was prepared using a paste in which Ir powder was added to the total amount (100 wt%) of Pt and Ir at a weight shown in Table 1 below. Sample No. In the NOx gas sensors according to 7 to 11, second internal electrodes were prepared using pastes obtained by adding Ru powder to the total amount (100 wt%) of Pt and Ru at the weight shown in Table 1 below. Sample No. In the NOx gas sensor according to No. 1 (comparative example), a second internal electrode was produced using a Pt paste to which no Ir and Ru powder was added.
[0032]
A gas to be measured (1) or (2) containing a disturbing gas component is put into these NOx gas sensors, and a voltage (second) applied between both electrodes (between the second internal electrode and the second external electrode) of the measurement cell. The second oxygen pump current Ip2 flowing through the measurement cell was measured by changing the applied voltage Vp2. The measurement conditions are shown below, and Table 1 shows the measurement results. In Table 1, Ip2 [O ₂ ] represents the limit current of Ip2 with respect to O ₂ flowing into the measurement cell when the gas to be measured (1) is charged, and Ip2 [NO] is the gas to be measured (2) charged. Time, it represents the limit current of Ip2 with respect to NO flowing into the measurement cell.
[0033]
[Measurement condition]
[Composition of gas to be measured (1) 750 ppm, CO ₂ : 10%, H ₂ O: 10%, N ₂ : bal,
Composition of measured gas composition (2) NO: 1500 ppm, CO ₂ : 10%, H ₂ O: 10%, N ₂ : bal,
Measured gas temperature 680 ° C,
An applied voltage of 450 [mV] to the measurement cell (second oxygen pump cell).
[0034]
[Table 1]

(*) Sample No. 1 is a comparative example.
By the way, since O ₂ has ₂ oxygen atoms and NO has 1 oxygen atom, ideally, the Ip2 limit current value at O ₂ = 750 ppm shown in FIG. 2 and FIG. The Ip2 limit current values when NO = 1500 ppm coincide. That is, in order to measure the NOx gas concentration based on the current flowing through oxygen ion conduction, the limit current value at NO = 1500 ppm with respect to the limit current value at O ₂ = 750 ppm is preferably close to “1”.
[0036]
Referring to Table 1, Sample No. without Ir and Ru added. “Ip2 [NO] / Ip2 [O ₂ ]” of No. 1 NOx gas sensor (comparative example) is the smallest. Therefore, it can be seen that by adding Ir or Ru, “Ip2 [NO] / Ip2 [O ₂ ]” approaches “1”, and NOx is sufficiently dissociated even at a low applied voltage.
[0037]
Furthermore, sample no. The limiting current characteristics for O ₂ and NO of the NOx gas sensor (Example) according to FIG. 1 will be described in comparison with the same characteristics of the NOx gas sensor (comparative example). In FIG. The NOx gas sensors 1 and 5 show the relationship between Vp2 and Ip2 when the gas to be measured does not contain NOx gas, particularly the limiting current characteristics of the measuring cell for oxygen. On the other hand, FIG. 3 shows the relationship between Vp2 and Ip2 when the gas to be measured contains NOx gas, particularly the limiting current characteristics of the measurement cell for NO gas, for both sensors.
[0038]
Referring to FIG. 2 and FIG. 5, the difference between Ip2 due to O ₂ shown in FIG. 2 and Ip2 due to NO shown in FIG. 3 is small, especially when the second applied voltage Vp2 is low. It can be seen that the two chambers are sufficiently dissociated. Since the second applied voltage Vp2 can be set low as described above, dissociation of interfering gases such as CO and H ₂ O on the second internal electrode is suppressed. As a result, the NOx gas concentration can be measured more accurately.
[0039]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, the gas sensor which is hard to receive to the influence of interference gas and can perform the density | concentration measurement of the measurement object gas component correctly is provided. In addition, according to the present invention, there is provided a gas sensor having a gas concentration measurement electrode having a high catalytic ability for a measurement target gas component even at a low applied voltage.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram of a NOx gas sensor according to an embodiment of the present invention.
FIG. 6 is a graph showing limit current characteristics of a measurement cell with respect to O ₂ in the NOx gas sensors according to 1 and 5;
FIG. 6 is a graph showing limit current characteristics of a measurement cell with respect to NO in the NOx gas sensors according to 1 and 5;
[Explanation of symbols]
1, ..., 5 1st layer, ..., 5th layer 6 1st diffusion resistance part 7 1st chamber 8 2nd diffusion resistance part 9 2nd chamber 10 1st external electrode (external electrode for oxygen concentration control gas production | generation)
11 First internal electrode (internal electrode for oxygen concentration control gas generation facing the first chamber)
12 Oxygen concentration detection electrode 13 Oxygen concentration reference electrode 14 Second internal electrode (gas concentration measuring electrode facing the second chamber)
15 Second external electrode (external electrode for gas concentration measurement)
16 Third diffusion resistance section

Claims

A first chamber for guiding a gas to be measured containing a gas component to be measured, and controlling an oxygen concentration in the gas to be measured; a second chamber for measuring the concentration of the gas component to be measured; A communication portion that communicates the first chamber and the second chamber with diffusion resistance ; and a plurality of electrodes formed on the oxygen ion conductor, wherein at least one of the plurality of electrodes is the second electrode. with facing chamber, the measurement target gas components without dissociation or oxidation or gas concentration measuring electrode to be reduced, the second facing chamber measurement target gas component concentration the gas concentration measurement electrodes for measuring the At least a part of Pt is the main electrode component and contains Ir. Further, Ir is contained in an amount of 0.2 to 50 wt% with respect to the total amount of Pt and Ir, and a voltage is applied between the plurality of electrodes. Concentration of the gas component to be measured Gas sensor and a current is generated in accordance with the.

2. The gas sensor according to claim 1, wherein the gas concentration measurement electrode facing the second chamber contains 2 to 20 wt% of Ir with respect to a total amount of the Pt and the Ir.

A first chamber for guiding a gas to be measured containing a gas component to be measured, and controlling an oxygen concentration in the gas to be measured; a second chamber for measuring the concentration of the gas component to be measured; A communication portion that communicates the first chamber and the second chamber with diffusion resistance ; and a plurality of electrodes formed on the oxygen ion conductor, wherein at least one of the plurality of electrodes is the second electrode. with facing chamber, the measurement target gas components without dissociation or oxidation or gas concentration measuring electrode to be reduced, the second facing chamber measurement target gas component concentration the gas concentration measurement electrodes for measuring the At least a part of Pt is the main electrode component and contains Ru, and further, this Ru is contained in an amount of 0.2 to 50 wt% with respect to the total amount of Pt and Ru, and a voltage is applied between the plurality of electrodes. Concentration of the gas component to be measured Gas sensor and a current is generated in accordance with the.

The gas sensor according to claim 3, wherein the gas concentration measurement electrode facing the second chamber contains 1 to 12 wt% of Ru with respect to the total amount of Pt and Ru.

A first diffusion resistance portion that communicates with a measurement gas atmosphere containing a measurement target gas component; the first chamber that communicates with the first diffusion resistance portion; and the oxygen ion conductor formed on the first chamber so as to face the first diffusion resistance portion. An oxygen pump cell having an internal electrode and an external electrode formed on the oxygen ion conductor so as to face the outside of the first chamber, and pumping oxygen in and out of the first chamber; and oxygen ions facing the first chamber An oxygen concentration detection electrode formed on the conductor; a second chamber into which a gas whose oxygen concentration is controlled in the first chamber is introduced through a second diffusion resistance portion disposed in the communication portion; A gas concentration measuring cell that includes a plurality of electrodes formed on the ion conductor, and at least one of the plurality of electrodes is the gas concentration measuring electrode facing the second chamber; The oxygen based on the potential of the oxygen concentration detection electrode The pumping amount or pumping amount of oxygen by the pump cell is controlled, and a current is generated according to the concentration of the gas component to be measured by applying a voltage between the plurality of electrodes of the measuring cell. The gas sensor according to any one of claims 1 to 4, wherein the gas sensor is characterized in that: