JP3953175B2 - Nitrogen oxide concentration measuring apparatus and nitrogen oxide concentration measuring method - Google Patents

Description

【００３４】
（センサー素子の製造）
【００３５】
（測定実験１・素子単体のガス選択性）
被計測ガスとして酸素濃度を１１％、水分率を９．５％に調整したガスをベースとし、前記ベースガス中に検出対象ガスとしてＮＯ、Ｈ₂、ＣＯを混合したガスを準備した。
ガスセンサー素子を、３５０℃に加熱保持し、前述の被計測ガスと接触させて抵抗値の変化を測定した。この実験においてはガス検出部には触媒層は設けなかった。
（測定結果）
表１に対応する各実施例の素子の測定結果を、表２に示した。
表記に当たっては、ベースガス中における抵抗値（Ｒｂａｓｅ）を（Ω）単位で示すともに、ＮＯ ５００ｐｐｍ、Ｈ₂ ５００ｐｐｍ、ＣＯ ５００ｐｐｍにする感度を示した。ここで、感度とは、Ｒｇ（被計測ガス中の抵抗値）／Ｒｂａｓｅ（ベースガス中の抵抗値）と定義している。すなわち、感度＝１とは、感度がないこと意味しており、感度が１から離れるほど、感度が大きいことを意味している。
表２の結果より、金属元素換算で、ビスマスを所定量以上（７５、９０ある いは９５ａｔ％以上）含む材料からなるガス検出部を備えた素子にあっては、ＮＯをＨ₂、ＣＯに対して選択的に検出できることが判る。
【００３６】
【表２】

【００３７】
表２の結果より、無添加のＢｉ₂Ｏ₃より感度が小さくなるものについても、ｐ型半導体性を示すものに関して、選択性は維持された。
ここで、ｐ型伝導性であるか否かを容易に判断する手法は、以下のとおりであ。
１ 酸素分圧が高くなると、抵抗値が小さくなる。
２ 可燃性ガスに対して生じるごくわずかな抵抗値の変動が、抵抗値が増加する向である。
材料がｎ型伝導性であれば、上記１の要件、２の要件の変化は、全く逆となるさらに、酸素イオン伝導性である場合は、著しい化学量論比の変動が無い場合は、原理的に、１の要件に変動はなく、２の要件に関しては、感度を有しない。
本願において、ｐ型伝導性を示す材料に関して、主に注目している理由は以下のような背景があるためである。
即ち、無添加のＢｉ₂Ｏ₃は４００℃以下でＮＯの特異な吸着現象に基づく、選択的なセンサー特性を発揮できるが、これを小型化、薄膜化するには、その抵抗値の高さ（窒素酸化物に感応して変化する抵抗値の変化量ではなく、抵抗の全体）が問題となり、素子の抵抗値が高くなりがちである。したがって、素子の比抗値を下げる検討が必要となる。この効果を検証するための一手法として、様々な添加物の効果を鋭意調査した。
Ｂｉ₂Ｏ₃素子の比抵抗を下げる方法としては、大きく分け、Ｂｉ₂Ｏ₃素子への固溶の効果によるものと、感度に影響を与えない導電性第２相を加えるものの２つに分けられる。
前者においては、導電機構で分類すると、酸素イオン伝導、ｐ型伝導性、ｎ型伝導性のいずれか一つ、もしくは複数を向上させることができるが、中でもｐ型伝導性を顕著に増大させるのが好ましい態様である。
一方、後者の感度に影響を与えない導電性第２相を加える場合においては、結果的に、ｐ型伝導性が最も支配的であるものについて有効であり、この場合、有効なＮＯセンシング特性を維持できることが判明した。
即ち、３価未満の原子価を取りえる金属元素を添加したＢｉ₂Ｏ₃は、金属元素が、そのＢｉ₂Ｏ₃の結晶格子に固溶する原子価制御効果によって、ｐ型伝導性がより向上し（抵抗値が減少し）、低温域での検知が容易になる。
この内、実施例５、１３については、抵抗値の減少（原子価制御の効果）が顕著でない。これは、添加した金属元素が、Ｂｉ₂Ｏ₃に充分に固溶せず、析出しているためである。
実施例１５、１６のような３価以上の原価を取りえる金属元素については、結晶粒子の粒界に感度に影響を与えない析出（導電性第２相）を形成できるため、抵抗値を下げ選択性を維持できる。
３価以上の原子価を取りえる元素としてはほかに、ＶやＭｏがあるが、これらの元素を添加したものは、全てｎ型伝導性が顕著となる。
ここで、Ｂｉ₂Ｏ₃のベース材に対して添加によりＮＯの感度を大きく増大させるものとしては、Ｎｉ，Ｃｕ等を挙げることができることが判る。
【００３８】
（好ましいセンサー素子材料の詳細特性）
１ Ｂｉ₂Ｏ₃
（Ｂｉ₂Ｏ₃の選択性）
被計測ガスとして、酸素濃度を１１％、水分率を９．５％に調整したガスをベースとし、前記ベースガス中に検出対象ガスとしてＮＯ、Ｈ₂、ＣＯ、ＣＨ₄を最大３０００ｐｐｍの濃度に混入したガスを準備した。また、ＮＯ₂は最高３００ｐｐｍ、ＣＯ₂ は５％、７％濃度について測定を行った。
ガスセンサー素子を、３２５〜３５０℃に加熱保持し、前述の被計測ガスと接触軸は検出対象ガスの濃度を１０００ｐｐｍ単位で表示し、縦軸は抵抗値をΩ単位て表示した。図３、４に測定の結果を示した。本願のセンサにあっては、ＮＯ選択的に検知できることが判る。ＣＯ₂はスケールの関係上図にはプロットされていないが、ベースガスに接触させた場合とほぼ同じ抵抗値を示し、併存して窒素酸化物の検出に影響しないことが分かった。
（Ｂｉ₂Ｏ₃の回復性の評価）
センサーは、検出すべき成分の濃度がゼロになったときは元の抵抗値に復帰しなければならない。ＮＯ濃度が５００ｐｐｍ、２５０ｐｐｍ、１００ｐｐｍ、５０ｐｐｍのガスに、この順に接触させた場合の応答性を測定した。測定結果を図５に示した。（５００ｐｐｍと２５０ｐｐｍの間には、この２種の濃度のガスとの接触の間に接触させたベースガスによる谷部認められる。）
最終的にベースガスと接触させると元の抵抗値に回復し、センサーとして必要な回復性を有していることが分かる。
（Ｂｉ₂Ｏ₃の動作温度の範囲）
この例に於ける動作温度と感度〔Ｒｇ（被計測ガスに対する抵抗値）／Ｒｂａｓｅ（ベースガスに対する抵抗値）〕との関係を図６に示した。
同図において横軸は温度（℃）であり、縦軸は、上記した感度である。
さらに、被計測ガスの濃度は、ＮＯに関して２５０ｐｐｍ、Ｈ₂に関して１０００ｐｐｍ、ＣＯに関して１０００ｐｐｍとした。従って、同図においては、異種の被計測ガスにおいて同等な感度を示す場合にあっても、同一濃度の場合は、ＮＯ選択的に検出できる状態である。結果、ＮＯの選択検知にあたっては、２５０〜４００℃の温度範囲が好ましいことが判る。ここで、この温度域は、電子（ホール伝導性が顕著に表れる温度域である。
従来、Ｂｉ₂Ｏ₃をガスセンサ用の素材として用いる場合は、その原理は、固体電解質型に分類されるセンシング方式であり、イオン導電性が十分に起こるだけの温度に加熱されることが大前提であり、少なくとも４００℃より高い温度に加熱されることが必要であった。これに対して、本発明に於ける使用領域はｐ型伝導領域であり、４００℃以下の低温で、ＮＯ感度がＣＯ感度を大きく上回る特異な新現象を発したことに基づく。
図６に示されるように、４００℃以下の温度領域において、２５０ｐｐｍのＮＯ感度は１０００ｐｐｍのＣＯ感度を遙かに上回る。これが４００℃になると、ＮＯの感度が低下すると同時に、還元性ガス感度が大きくなる。４００℃以下の温度領域において、ＮＯの選択的吸着現象が起こっていると考えられる。２００℃から２５０℃の温度範囲では、上記とほぼ同様な傾向を維持した。２００℃未満の低温では、感度が大きくなるが、応答性、特に回復性が劣化する傾向がある。ただし、応答性、回復性は許容できる範囲であり、使用できる。
実用上、センサー素子は２００℃〜３５０℃に加熱されることが好ましい。
【００３９】
２ Ｂｉ₂Ｏ₃・ＮｉＯ系
（Ｂｉ₂Ｏ₃・ＮｉＯの選択性）
この系のものは、先に表２に示す実施例１１に示すように、ガス選択性が非常高い。そこで、Ｎｉを添加物をして含むことが好ましいことが判るが、この添加量（Ｂｉに対する割合）に関して検討をおこなった結果を以下に説明する。
この系の材料に於ける表１に対応する素子の製造条件を表３に、表２に対応すＮｉの添加量を変化させた場合の感度特性を表４に示した。
【００４０】
【表３】

【００４１】
【表４】

【００４２】
表４に示すように、ＮｉＯ／Ｂｉ₂Ｏ₃の割合に関しては、ＮＯ感度の大きさ点で、特に０．０３／０．９７〜０．２／０．８の範囲が好ましいことが判る。 Ｎｉ／Ｂｉ＝１／１（Ｂｉが金属元素換算で５０ａｔ％）においても、ＮＯ選択的であるが、Ｎｉ／Ｂｉ＝１／１を越え、析出したＮｉＯが多量になると、ＯやＨ₂の感度を生じ選択性が低下する。この点は、別途、確認できた。
さて、最も選択性が表れるＮｉＯ／Ｂｉ₂Ｏ₃＝５／９５のものに関して、図６に対応するセンサの温度特性を求めた。結果を図７に示した。但し、ＮＯ濃度は、２５０と５００ｐｐｍとした。結果、ＮＯの選択検知にあたっては、２５０〜４００℃の温度範囲が好ましいことが判る。ただし、室温以上で使用できることを別途確認した。
この材料の場合は、無添加のＢｉ₂Ｏ₃ よりも、高温になった場合のＮＯ感度低下が少ないため、より高温での動作を可能にする。
【００４３】
（測定実験２・酸素分圧の影響）
ＮＯ濃度を０、５００ｐｐｍと２水準、また酸素濃度を１％、１０％、２０％にそれぞれ設定した被計測ガスを作成し、センサーを３００〜３５０℃に加熱、維持し酸素センサーを作動させない状態、及び酸素センサーを作動させて測定室内の酸素濃度を１％（１００００ｐｐｍ）に調節した状態にて、それぞれ測定を行った。
結果を、表１に対応して、表５、６に示した。ここで、表５の結果が酸素ポンプを作動させない非作動時の抵抗値の変動を示し、表６の結果が酸素ポンプを作動させた作動時にの抵抗値の変動を示している。これらの表において、(Bi₂O₃)_0.5(NiO)_0.5のものを実施例１７、(Bi₂O₃)_0.33(NiO)_0.66のものを実施例１８と称している。さらに、動作温度の表示にあたっては、スペースの関係から、動作温度を１０で除算した値で示した。例えば、Bi₂O₃ の場合は、その動作温度は、３５０℃である。
【００４４】
【表５】

【００４５】
【表６】

【００４６】
表５の結果から、非動作時は、酸素濃度の変動に対して、ベース抵抗値、ＮＯ導入時の抵抗値も変動するため、酸素濃度が広範囲（１〜２０％）に変動する場合は、大きな誤差を発生しやすいことが判る。
一方、表６の結果から、酸素ポンプより、酸素濃度を一定となるように制御すると、外気の酸素濃度（１〜２０％）が変化したとしても、抵抗値の変動が殆ど起こすことがないことが判る。
この結果より、酸素ポンプを作動させると被計測気体中の酸素濃度の変動の影響を受けることなく、精度の高いＮＯの測定が行えることが明らかである。
【００４７】
（測定実験３）
本発明の窒素酸化物濃度測定装置において、ガス検出部として使用するビスマス酸化物半導体は、元来ＮＯに対して選択性が高い化合物であるが、ＮＯ₂ に対する感度特性はＮＯに対するそれとは異なるために、総窒素酸化物濃度を画一的に検出することは困難である。しかし、酸素センサーを作動させて酸素濃度を一定に保持すると、ＮＯ₂ は平衡反応である下記の（化１）に従って、ＮＯ対ＮＯ₂ の比率が固定され、総窒素酸化物濃度として測定できるものと考えられる。
【００４８】
【化１】

【００４９】
実施例にて得られたガス検出部を使用し、ＮＯ₂ とＮＯが併せて存在する被計測ガスについて、上記の原理のとおりに総窒素酸化物の濃度が測定できるかどうかについて評価を行った。実験は、ＮＯｘ全体として、０、５０、１００、１５０、３００ｐｐｍ含有する気体において、ＮＯ₂ を、その濃度割合（ＮＯ₂ 量／（ＮＯ量＋ＮＯ₂ 量））％が０、２０、４０、５０となるものとし、酸素ポンプを作動させなかった条件と酸素ポンプを作動させ、１％とした条件にて測定を行った。
結果を、作動されない場合に関して図８に、酸素ポンプを作動させる場合に関して図９に、示した。
【００５０】
結果、酸素ポンプを作動させなかった場合には、ＮＯ₂ の割合の変化に対して、抵抗値の変化が大きい。
しかし、酸素ポンプを作動させて酸素濃度を固定させた場合は、ＮＯ₂ の割合の変化に対して、抵抗値の変化が小さかった。
従って、本願構成を採用して、酸素ポンプを作動させることにより、化１の反応を平衡させて、ＮＯ対ＮＯ₂の比率が一定とできるため、全窒素酸化物濃度を正しく測定できることが判明した。
ここで、酸素濃度の設定範囲としては、５００ｐｐｍ以上、２５％以下程度がよい。前記範囲より酸素濃度が低いと、ビスマス酸化物の還元を招来しやすい。前記範囲より高いと、ＮＯ₂の含有率が高くなり出力が低下する。
【００５１】
（測定実験４）
酸素ポンプを作動させる場合と、作動させない場合に対応した、所謂、妨害ガスの影響を調べた。検討にあたっては、妨害ガスとして、ＣＯ、ＣＨ₄の影響を調べた。
本願のＢｉ₂Ｏ₃・ＮｉＯ系である実施例１１の結果を図１０・１１に、示した。図１０が酸素ポンプを働かせない場合の結果に、図１１が酸素ポンプを働かせた場合の結果に対応している。これらのグラフ及び、図１２〜１５に示すグラフにおいて、横軸はガス濃度であり、縦軸は、個々のガスに感応した場合のセンサ素子の抵抗値を示している。
結果、図１０と図１１との比較において、これまで説明してきたように酸素ポンプを働かせることが好ましいことが判るが、さらに、図１１に示すように、本願が対象とする素子材料の場合は、少なくとも妨害ガスに対して、その抵抗値変化がほとんど発生していず、ＮＯに対して、特異的に反応していることが判る。
この視点から図１１、１３、１５の結果を比較すると、本願の材料が、従来型のＳｎＯ₂、及びＴｉＯ₂に対して、優位であることが明確である。
【図面の簡単な説明】
【図１】本発明の窒素酸化物濃度測定装置のセンサー部の断面構造であって、標準酸素濃度気体室を設けたものを例示した図
【図２】本発明の窒素酸化物濃度測定装置の別の実施形態のセンサー部の断面構造を示した図
【図３】実施例１のビスマス含有酸化物をガス検出部とするセンサー素子を使用した場合の窒素酸化物に対する選択性を示したグラフ
【図４】実施例１のビスマス含有酸化物をガス検出部とするセンサー素子を使用した場合の窒素酸化物に対する選択性を示したグラフ
【図５】実施例１のビスマス含有酸化物をガス検出部とするセンサー素子を使用した場合の、抵抗値の濃度依存性を示すグラフ
【図６】実施例１のビスマス含有酸化物をガス検出部とするセンサー素子を使用した場合の、感度の温度依存性を示すグラフ
【図７】ＮｉＯ／Ｂｉ₂Ｏ₃＝５／９５のビスマス含有酸化物をガス検出部とするセンサー素子を使用した場合の、感度の温度依存性を示すグラフ
【図８】ＮｉＯ／Ｂｉ₂Ｏ₃＝１／１のビスマス含有酸化物をガス検出部とするセンサー素子を使用し、ＮＯに対するＮＯ₂濃度をかえた場合で、酸素ポンプ非作動時の、抵抗値変化を測定した結果を示したグラフ
【図９】ＮｉＯ／Ｂｉ₂Ｏ₃＝１／１のビスマス含有酸化物をガス検出部とするセンサー素子を使用し、ＮＯに対するＮＯ₂濃度をかえた場合で、酸素ポンプを作動させて測定室内の酸素濃度を１％に調節した場合の、抵抗値変化を測定した結果を示したグラフ
【図１０】ＮｉＯ／Ｂｉ₂Ｏ₃＝１０／９０のビスマス含有酸化物をガス検出部とするセンサー素子を使用した場合の、酸素ポンプを非作動として測定室内の酸素濃度を調節しない場合の、複数種のガスに対する、ガス濃度と抵抗値変化との関係を示す図
【図１１】ＮｉＯ／Ｂｉ₂Ｏ₃＝１０／９０のビスマス含有酸化物をガス検出部とするセンサー素子を使用した場合の、酸素ポンプを作動して測定室内の酸素濃度を１％に調節する場合の、複数種のガスに対する、ガス濃度と抵抗値変化との関係を示す図
【図１２】ＳｎＯ₂をガス検出部とするセンサー素子を使用した場合の、酸素ポンプを非作動として測定室内の酸素濃度を調節しない場合の、複数種のガスに対する、ガス濃度と抵抗値変化との関係を示す図
【図１３】ＳｎＯ₂をガス検出部とするセンサー素子を使用した場合の、酸素ポンプを作動して測定室内の酸素濃度を１％に調節する場合の、複数種のガスに対する、ガス濃度と抵抗値変化との関係を示す図
【図１４】ＴｉＯ₂をガス検出部とするセンサー素子を使用した場合の、酸素ポンプを非作動として測定室内の酸素濃度を調節しない場合の、複数種のガスに対する、ガス濃度と抵抗値変化との関係を示す図
【図１５】ＴｉＯ₂をガス検出部とするセンサー素子を使用した場合の、酸素ポンプを作動して測定室内の酸素濃度を１％に調節する場合の、複数種のガスに対する、ガス濃度と抵抗値変化との関係を示す図
【符号の説明】
１ 筐体
３ ガス拡散制限手段
５ 測定室
７ ガス検出部[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a measuring apparatus for measuring the concentration of nitrogen oxides in gas, particularly in combustion exhaust gas, and to a method for measuring such nitrogen oxide concentrations.
[0002]
[Prior art]
Reduction of nitrogen oxides, which are the main cause of air pollution, is an important matter for environmental measures. Measures to suppress the generation of nitrogen oxides in various combustion engines such as internal combustion engines, boilers, and incinerators. is needed. In taking this measure, a sensor for measuring the concentration of generated nitrogen oxide with high accuracy is required.
Conventionally, CLD, NDIR, etc. are used for the measurement of the nitrogen oxide concentration. However, when these apparatuses are used, there is a problem that the apparatus is large and expensive as well known, and complicated maintenance is required.
[0003]
In recent years, tin oxide (SnO) which is a semiconductive oxide₂ ), Titanium oxide (TiO₂ ) And nitrogen oxide sensors are being studied, and it is becoming possible to design a relatively compact measuring device. However, SnO₂ The semiconductor sensor using the above, etc., measures the concentration by measuring the conductivity, and has the advantage that it can be a sensor having a relatively simple structure as a whole. In particular, it is strongly influenced by the oxygen concentration present in the combustion exhaust gas.
Therefore, such SnO₂ TiO₂ A technique described in Japanese Patent Application Laid-Open No. 8-122287 has been proposed in order to use a sensor element using a semiconductor such as, etc. and to avoid a decrease in measurement accuracy due to fluctuations in oxygen concentration.
[0004]
[Problems to be solved by the invention]
According to the technique described in the above-mentioned Japanese Patent Application Laid-Open No. 8-122287, it is considered that the influence of fluctuations in oxygen concentration can be avoided. However, SnO used as a sensor constituent material in the prior art₂ TiO₂ Such oxide semiconductors are sensitive to various gases other than nitrogen oxides (NOx), and are particularly present in combustion exhaust gas, and the concentration thereof varies depending on the combustion state, such as carbon monoxide (CO), hydrogen (H₂ ), Methane (CH_Four) Etc. also strongly affect the measured values, and there are still problems in selectivity and measurement accuracy.
12 and 13 show SnO₂ NO, CH when adopting_FourThe change state of the resistance value output when the gas concentrations of CO and CO are changed is shown. FIG. 12 shows an output state when oxygen concentration control is not performed, and FIG. 13 shows an output state when oxygen concentration control is performed.
Similarly, FIGS. 14 and 15 show TiO.₂ The results corresponding to FIGS. 12 and 13 in the case of adopting the above are shown. Also in this case, FIG. 14 shows an output state when the oxygen concentration control is not performed, and FIG. 15 shows an output state when the oxygen concentration control is performed. From these results, as described above, even when oxygen concentration control is performed (results of FIGS. 13 and 15), these materials are CO, CH_FourIt can be seen that the influence of these gases is also liable to affect the measured values, and there are still problems in selectivity and measurement accuracy.
Although mentioned later, an example of the result of this application corresponding to these FIGS. 12-15 is shown by FIG. As can be seen by referring to FIG. 11, the present application is sensitive to NO, but CO, CH_FourYou can realize a situation that is not sensitive to.
[0005]
An object of the present invention is a simple configuration, is compact, has high measurement accuracy, and is present in the gas to be measured.₂ , CO, H₂ , CH_FourIs less affected by fluctuations in the concentration of interfering gas components such as NO and NO₂ In addition, the present invention provides a nitrogen oxide concentration measuring apparatus that can measure the nitrogen oxide concentration and obtain a measurement method that can measure the nitrogen oxide concentration satisfactorily in such a state.
[0006]
[Means for Solving the Problems]
  The nitrogen oxide concentration measuring apparatus according to the present invention has an oxygen concentration control means for controlling at least the oxygen concentration in the measurement chamber to a predetermined concentration in a housing having a measurement chamber provided with a sensor element for measuring the nitrogen oxide concentration. And a nitrogen oxide concentration measuring apparatus for measuring a nitrogen oxide concentration in a measurement gas comprising gas diffusion limiting means for connecting a measurement gas inside and outside the measurement chamber, wherein the sensor element is a metal Metal oxide containing 50at% or more of bismuth in terms of elementThe nitrogen oxide concentration is measured as a resistance value generated in the sensor element.It is characterized by doing.
[0007]
  One of the features of the nitrogen oxide concentration measuring apparatus of the present invention is that a metal oxide containing bismuth of 50 at% or more in terms of metal element is used as a gas detection part of the sensor element.Measure the nitrogen oxide concentration as a resistance value generated in the sensor element as a gas detectorThere is in point to do. Here, in terms of metal element, it means that only the metal element is focused (for example, in the case of an oxide, the amount of oxygen is not taken into account), and the amount of bismuth (Bi) and other metal elements is viewed in element units. It is. That is, if the Bi metal element amount is Amol and the metal element amount other than Bi is Bmol, it is defined as A / (A + B) × 100%.
  As will be described later, such a bismuth-containing oxide material is a material excellent in selectivity as compared with a conventional tin oxide semiconductor with respect to detection of nitrogen oxides.
  Another feature of the nitrogen oxide concentration measuring apparatus according to the present invention is that a means for controlling the oxygen concentration in the measurement chamber containing the sensor element is provided. The oxygen concentration control and the bismuth-containing oxide of the present application are provided. By using the material as a gas detector, a nitrogen oxide concentration measuring device having higher selectivity than a semiconductor sensor such as tin oxide can be obtained.
[0008]
It is preferable that the gas detection unit according to claim 1 includes a metal element capable of taking a valence less than trivalent and one or more elements selected from In and Sn as additives other than bismuth. .
[0009]
As such a bismuth-containing oxide material, in particular, an additive other than bismuth is selected from Ca, Sr, Ba, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, In, and Sn. The above elements are preferable.
By including such an element, the resistance value of the element can be reduced, and a resistance value change in a low temperature region can be easily detected. Furthermore, this is because the response and recovery characteristics in a low temperature range can be improved, which is preferable as a nitrogen oxide concentration measuring apparatus.
[0010]
In the nitrogen oxide concentration measurement apparatus of the present invention, the oxygen concentration control means is an oxygen pump in which a pair of electrodes are provided on an oxygen ion conductive solid electrolyte and oxygen is moved between the measurement chamber and the measurement chamber. preferable.
Such an oxygen pump has a simple configuration, and the oxygen concentration in the measurement chamber can be easily controlled by controlling the voltage applied to the electrodes.
[0011]
The measurement chamber further includes an oxygen sensor, and the oxygen pump controls the voltage applied to the pair of electrodes based on a signal from the oxygen sensor, thereby controlling the oxygen concentration in the measurement chamber. A configuration is preferred.
[0012]
The above-described oxygen pump is provided with a pair of electrodes on both surfaces of an oxygen ion conductive solid electrolyte, one of the electrodes facing the measurement chamber, and the other surface is a gas having a standard oxygen concentration. It is a particularly preferable embodiment that the contact is made.
Such a configuration is the simplest electrode configuration. Moreover, as a result of suppressing the fluctuation of the oxygen concentration in the measurement chamber due to the fluctuation of the oxygen concentration in the gas to be measured outside the measurement chamber, the nitrogen oxide concentration measurement of the present invention is possible. The measurement accuracy of the apparatus can be greatly increased.
[0013]
The method for measuring the nitrogen oxide concentration according to the present invention includes a step of making the gas composition in the measurement chamber correspond to the composition of the gas to be measured by diffusion through the gas diffusion limiting means, and the oxygen concentration for controlling the oxygen concentration in the measurement chamber The nitrogen oxide concentration in the measurement chamber is measured as a resistance value generated in a control element and a sensor element that is provided in the measurement chamber and uses a metal oxide containing bismuth of 50 at% or more in terms of metal element as a gas detection unit. It is characterized by including a process.
[0014]
By adjusting the oxygen concentration in the measurement chamber and using the bismuth-containing oxide material as a gas detection unit, measurement with high selectivity and accuracy can be performed.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
(1) Configuration of sensor unit
FIG. 1 shows an example of a nitrogen oxide concentration measuring apparatus according to the present invention, centering on a sensor portion. The measurement chamber 5 is formed by the housing 1, and the sensor element constituted by the gas detection unit 7 and the measurement electrode 9 is accommodated in the measurement chamber 5, and the inside of the casing constituent material in the vicinity of the sensor element. A heater 11 is embedded in the sensor element for heating the sensor element.
[0016]
The casing constituent material is not particularly limited as long as it does not affect the measurement value, but a ceramic material such as alumina or zirconia is a preferable material in terms of heat resistance. In particular, zirconia is an oxygen ion conductive solid electrolyte, and when a housing is made using this, there is an advantage that the entire sensor can be simply constructed because the portion becomes an oxygen pump simply by providing an electrode.
[0017]
A diffusion restriction hole 3 (an example of a gas diffusion restriction unit) is provided in a part of the casing, and the composition of the measurement target gas is directly reflected in the measurement chamber 5 by diffusion through the diffusion restriction hole 3. . One or more diffusion restriction holes may be provided. Furthermore, since such gas diffusion limiting means may basically be any means that can limit the amount of gas flow through this means, it can also be configured as a portion made of a porous material having continuous air holes.
[0018]
The correspondence between the composition of the gas in the measurement chamber 5 via the diffusion limiting hole 3 and the composition of the gas to be measured outside the measurement chamber is most preferably 1: 1, but if the ratio is constant and does not vary, the measurement is performed. There is no hindrance to the measurement of gaseous nitrogen oxide concentration.
[0019]
A part of the casing constituent material includes an oxygen pump formed as an oxygen ion conductive solid electrolyte 13 provided with an electrode 14 and an oxygen sensor portion formed as an oxygen ion conductive solid electrolyte 15 provided with an electrode 16. Is provided.
[0020]
In the example of FIG. 1, a standard oxygen concentration gas chamber 19 is formed adjacent to the measurement chamber 5, a standard oxygen concentration gas is supplied through the supply port 21, and the oxygen in the gas in the standard oxygen concentration gas chamber 19 is supplied. The concentration is always maintained at a constant concentration.
Further, the measurement chamber 5 and the standard oxygen concentration gas chamber 19 are formed through a common casing constituent material constituting the partition wall portion 23, and the above-described oxygen pump is provided in the partition wall portion 23 so that oxygen is ionized. By conducting between the measurement chamber 5 and the standard oxygen concentration gas chamber 19, the oxygen concentration in the measurement chamber 5 can be maintained constant, and a significant improvement in measurement accuracy can be achieved.
[0021]
In this case, by providing the standard oxygen concentration gas chamber 19 as described above, the influence of fluctuations in the oxygen concentration outside the measurement chamber can be reduced. In particular, when oxygen is supplied from the standard oxygen concentration gas chamber 19 to the measurement chamber, there is no fluctuation in the oxygen concentration of the supply source, which is effective. Furthermore, the oxygen concentration can be measured by providing an electrode 16 and measuring an electrical signal generated between the two electrodes based on the difference in oxygen concentration between the measurement chamber 5 and the standard oxygen concentration gas chamber 19, and a simple oxygen sensor. Can be configured.
[0022]
The oxygen pump is configured by providing the electrode 14 on the oxygen ion conductive solid electrolyte 13 as described above. As a preferred example, as shown in FIG. 1, one surface is provided in the measurement chamber 5 and the other surface is provided outside the measurement chamber, preferably the standard oxygen concentration gas chamber 19, and the electrodes are preferably provided. Directly provided on each side.
[0023]
The operation of the oxygen pump is as follows.
When a voltage is applied with the electrode provided on the oxygen ion conductive solid electrolyte 13 in contact with a gas containing oxygen as the positive electrode and the other electrode as the negative electrode, oxygen in the gas is ionized, and the oxygen ion conductive solid electrolyte 13 is applied. It moves from the negative electrode to the positive electrode through and is released as oxygen molecules at the positive electrode. Therefore, by controlling the voltage applied to the electrode 14, oxygen in the measurement chamber can be increased or decreased freely.
[0024]
In FIG. 2, a standard oxygen concentration gas chamber is not provided, and one surface of the oxygen pump is in contact with outside air outside the measurement chamber, and an oxygen sensor 17 based on a principle different from that shown in FIG. The structure of the sensor is shown. The oxygen concentration signal from the oxygen sensor 17 is sent to the oxygen concentration control device, the voltage applied to the electrode 14 is controlled based on this signal, the movement of oxygen is controlled, and the oxygen concentration in the measurement chamber is maintained at a predetermined set value. Is done.
[0025]
(2) About the gas detector
The gas detection part constituting the sensor element used in the nitrogen oxide concentration measuring apparatus of the present invention may be either a thin film formed on a substrate or a sintered method. Examples of the thin film forming method include sputtering, vacuum deposition, laser ablation, and CVD.
Both are methods that can increase the specific surface area of contact with the combustion exhaust gas that is the gas to be measured, and are preferable as the gas detection unit.
[0026]
When the gas detection part is formed by a sintering method, it is preferable to use a binder material that does not affect the detection sensitivity as one component of the material of the gas detection part and to sinter it. Such binder materials include alumina (Al₂O_Three), Silica (SiO₂ ) Etc. can be illustrated.
By using the binder, the physical strength of the gas detection unit is improved, and effects such as fewer failures can be obtained.
[0027]
It is also a preferred aspect that a catalyst layer is further provided in the gas detection unit.
By providing such a catalyst layer, the selectivity of the gas sensor element can be further enhanced. The bismuth-containing oxide material, which is the material used for the detection part of the present invention, is CO, H₂ , CH_FourAlthough the influence of such is small, it cannot be said that there is nothing. This catalyst is slightly present in combustion exhaust gas etc.₂ , CH_FourIt has the effect | action which oxidizes the component which may affect the sensitivity of a sensor etc., and has the effect | action which contributes to the improvement of selectivity and a measurement precision as a result.
[0028]
As the material constituting the catalyst layer used for the gas detector, a noble metal catalyst such as platinum (Pt) or palladium (Pd) can be used, and it may be attached to the surface of the gas detector. When manufacturing by a sintering method, you may make it adhere by mixing with raw material powder, a paste, etc., and sintering.
[0029]
As the electrode material of the gas sensor element of the present invention, commonly used noble metal materials such as gold, silver and platinum can be used. The electrode is attached to a gas detector formed of the above-described complex oxide semiconductor by a conventional technique.
A sensor element is formed by attaching an electrode to the gas detector.
[0030]
In the nitrogen oxide concentration measuring apparatus of the present invention, the gas detector is preferably heated to 200 to 400 ° C., particularly preferably in the range of 250 to 350 ° C. In this range, sensitivity to nitrogen oxides is high, sensitivity to other gas species is low, and particularly excellent selectivity can be obtained. Furthermore, the response is somewhat slow even in the range of 200 ° C. or lower and normal temperature or higher, but it can be used because it has sensitivity.
[0031]
(3) Manufacturing of sensor unit
The manufacturing method will be described by taking the sensor of FIG. 1 as an example. The housing is fired using a zirconia as a material, with the measurement chamber and one surface of the standard oxygen concentration gas chamber as an opening, with a heater embedded in the partition wall 23. And create.
A sensor can be obtained by installing a separately prepared sensor element, mounting an electrode constituting an oxygen pump and an electrode constituting an oxygen sensor, and then sealing the opening.
[0032]
【Example】
Examples of the present invention will be described below.
(Create gas detector)
In the present embodiment, an example of manufacturing a gas detector by a sintering method will be described.
The raw material powder was weighed and mixed so as to have the composition described in the composition column of Table 1, calcined as necessary, pressed and molded, and then fired at a predetermined temperature to create a gas detector. .
[0033]
[Table 1]

[0034]
(Manufacture of sensor elements)
[0035]
(Measurement experiment 1-Gas selectivity of a single element)
The gas to be measured is based on a gas whose oxygen concentration is adjusted to 11% and the moisture content is adjusted to 9.5%, and NO, H as detection target gases in the base gas.₂A gas mixed with CO was prepared.
The gas sensor element was heated and held at 350 ° C., and contacted with the above-described measurement gas, and a change in resistance value was measured. In this experiment, no catalyst layer was provided in the gas detection section.
(Measurement result)
Table 2 shows the measurement results of the elements of the examples corresponding to Table 1.
In the notation, the resistance value (Rbase) in the base gas is shown in units of (Ω) and NO 500 ppm, H₂  The sensitivity was set to 500 ppm and CO 500 ppm. Here, the sensitivity is defined as Rg (resistance value in the gas to be measured) / Rbase (resistance value in the base gas). That is, sensitivity = 1 means that there is no sensitivity, and that the sensitivity is higher as the sensitivity is farther from 1.
From the results shown in Table 2, NO is H in an element having a gas detection part made of a material containing bismuth or more in a predetermined amount (75, 90 or 95 at% or more) in terms of metal element.₂, It can be detected selectively with respect to CO.
[0036]
[Table 2]

[0037]
From the results in Table 2, no additive Bi₂O_ThreeEven for those with lower sensitivity, selectivity was maintained for those showing p-type semiconductor properties.
Here, the method for easily judging whether or not the p-type conductivity is as follows.
1 When the oxygen partial pressure increases, the resistance value decreases.
2 A slight change in resistance value that occurs with respect to the combustible gas tends to increase the resistance value.
If the material is n-type conductivity, the changes in requirements 1 and 2 above are completely opposite. Further, if the material is oxygen ion conductive, the principle can be used when there is no significant stoichiometric ratio variation. Thus, the requirement 1 does not vary, and the requirement 2 does not have sensitivity.
In the present application, the reason for mainly focusing on the material exhibiting p-type conductivity is because of the following background.
That is, additive-free Bi₂O_ThreeCan exhibit selective sensor characteristics based on the unique adsorption phenomenon of NO at 400 ° C or lower, but in order to make it smaller and thinner, its resistance value (changes in response to nitrogen oxides) The overall resistance (not the amount of change in resistance value) is a problem, and the resistance value of the element tends to be high. Therefore, it is necessary to consider reducing the resistivity of the element. As one method for verifying this effect, the effects of various additives were intensively investigated.
Bi₂O_ThreeThe methods for lowering the specific resistance of the element are roughly classified as Bi₂O_ThreeThere are two types, one based on the effect of solid solution on the element and the other added with a conductive second phase that does not affect the sensitivity.
In the former, when classified according to the conduction mechanism, one or more of oxygen ion conductivity, p-type conductivity, and n-type conductivity can be improved, but among them, the p-type conductivity is remarkably increased. Is a preferred embodiment.
On the other hand, in the case of adding a conductive second phase that does not affect the latter sensitivity, as a result, it is effective for the case where p-type conductivity is the most dominant. In this case, effective NO sensing characteristics are obtained. It was found that it can be maintained.
That is, Bi to which a metal element capable of taking a valence of less than 3 is added.₂O_ThreeThe metal element is Bi₂O_ThreeDue to the valence control effect of solid solution in the crystal lattice, p-type conductivity is further improved (resistance value is decreased), and detection at a low temperature range is facilitated.
Among these, in Examples 5 and 13, the decrease in resistance value (effect of valence control) is not remarkable. This is because the added metal element is Bi₂O_ThreeThis is because the solid solution is not sufficiently dissolved.
For metal elements that can have a trivalent or higher cost as in Examples 15 and 16, precipitation (conductive second phase) that does not affect the sensitivity at the grain boundaries of crystal grains can be formed, so the resistance value is lowered. Selectivity can be maintained.
Other elements that can have a valence of 3 or more include V and Mo. However, all of those elements to which these elements are added exhibit remarkable n-type conductivity.
Where Bi₂O_ThreeIt can be seen that Ni, Cu, and the like can be mentioned as those that greatly increase the sensitivity of NO when added to the base material.
[0038]
(Detailed characteristics of preferred sensor element materials)
1 Bi₂O_Three
(Bi₂O_ThreeSelectivity)
As a measurement gas, a gas whose oxygen concentration is adjusted to 11% and moisture content is adjusted to 9.5% is used as a base, and NO, H as detection target gases in the base gas.₂, CO, CH_FourWas prepared in a concentration of up to 3000 ppm. NO₂Is up to 300ppm CO₂ Were measured for 5% and 7% concentrations.
The gas sensor element was heated and maintained at 325 to 350 ° C., the above-mentioned measurement target gas and the contact axis displayed the concentration of the detection target gas in units of 1000 ppm, and the vertical axis displayed the resistance value in units of Ω. 3 and 4 show the measurement results. It can be seen that the sensor of the present application can detect NO selectively. CO₂Although not plotted in the figure because of the scale, it was found that the resistance value was almost the same as that in contact with the base gas, and it did not affect the detection of nitrogen oxides.
(Bi₂O_ThreeEvaluation of recoverability of
The sensor must return to its original resistance value when the concentration of the component to be detected becomes zero. Responsiveness was measured when contacting NO gas of 500 ppm, 250 ppm, 100 ppm, and 50 ppm in this order. The measurement results are shown in FIG. (Between 500 ppm and 250 ppm, valleys are observed due to the base gas brought into contact between the two concentrations of gas.)
When it finally comes into contact with the base gas, the original resistance value is restored, indicating that the sensor has the necessary recovery properties.
(Bi₂O_ThreeOperating temperature range)
FIG. 6 shows the relationship between the operating temperature and sensitivity [Rg (resistance value against the gas to be measured) / Rbase (resistance value against the base gas)] in this example.
In the figure, the horizontal axis is temperature (° C.), and the vertical axis is the sensitivity described above.
Furthermore, the concentration of the gas to be measured is 250 ppm for NO, H₂1000 ppm and CO 1000 ppm. Therefore, in the same figure, even when different sensitivities of gases to be measured show the same sensitivity, it is in a state where NO can be selectively detected at the same concentration. As a result, it can be seen that a temperature range of 250 to 400 ° C. is preferable for NO selection detection. Here, this temperature range is a temperature range in which electron (hole conductivity appears remarkably).
Conventionally, Bi₂O_ThreeIs used as a material for a gas sensor, the principle is a sensing method classified as a solid electrolyte type, which is premised on being heated to a temperature at which ion conductivity sufficiently occurs, and at least 400 ° C. It was necessary to be heated to a higher temperature. On the other hand, the use region in the present invention is a p-type conduction region, which is based on the occurrence of a unique new phenomenon in which the NO sensitivity greatly exceeds the CO sensitivity at a low temperature of 400 ° C. or lower.
As shown in FIG. 6, in the temperature region of 400 ° C. or lower, the NO sensitivity of 250 ppm far exceeds the CO sensitivity of 1000 ppm. When this is 400 ° C., the sensitivity of NO decreases and at the same time, the reducing gas sensitivity increases. It is considered that a selective adsorption phenomenon of NO occurs in a temperature range of 400 ° C. or lower. In the temperature range of 200 ° C. to 250 ° C., the same tendency as above was maintained. At a low temperature of less than 200 ° C., the sensitivity increases, but the response, particularly the recoverability, tends to deteriorate. However, responsiveness and recoverability are in an acceptable range and can be used.
Practically, the sensor element is preferably heated to 200 ° C to 350 ° C.
[0039]
2 Bi₂O_Three・ NiO system
(Bi₂O_Three・ Selectivity of NiO
This system has a very high gas selectivity as shown in Example 11 shown in Table 2 above. Then, it turns out that it is preferable to contain Ni as an additive, However, The result of having examined regarding this addition amount (ratio with respect to Bi) is demonstrated below.
Table 3 shows the manufacturing conditions of the element corresponding to Table 1 in this system material, and Table 4 shows the sensitivity characteristics when the amount of Ni added corresponding to Table 2 is changed.
[0040]
[Table 3]

[0041]
[Table 4]

[0042]
As shown in Table 4, NiO / Bi₂O_ThreeAs for the ratio, it can be seen that the range of 0.03 / 0.97 to 0.2 / 0.8 is particularly preferable in terms of the magnitude of NO sensitivity. Even when Ni / Bi = 1/1 (Bi is 50 at% in terms of metal element), NO is selective. However, when Ni / Bi = 1/1 and a large amount of NiO is precipitated, O and H₂And the selectivity is reduced. This point was confirmed separately.
Now, the most selective NiO / Bi₂O_Three= 5/95, the temperature characteristics of the sensor corresponding to FIG. 6 were obtained. The results are shown in FIG. However, the NO concentrations were 250 and 500 ppm. As a result, it can be seen that a temperature range of 250 to 400 ° C. is preferable for NO selection detection. However, it was separately confirmed that it can be used at room temperature or higher.
In the case of this material, additive-free Bi₂O_Three Since the NO sensitivity is less decreased when the temperature is higher, operation at a higher temperature is possible.
[0043]
(Measurement experiment 2-Effect of oxygen partial pressure)
Measurement gas with NO concentration set to 0, 500ppm and 2 levels, oxygen concentration set to 1%, 10%, 20%, respectively, is created and the sensor is heated and maintained at 300-350 ° C, and the oxygen sensor is not activated The oxygen sensor was operated and the oxygen concentration in the measurement chamber was adjusted to 1% (10000 ppm), and the measurement was performed.
The results are shown in Tables 5 and 6 corresponding to Table 1. Here, the result of Table 5 shows the fluctuation of the resistance value when the oxygen pump is not operated, and the result of Table 6 shows the fluctuation of the resistance value when the oxygen pump is operated. In these tables, (Bi₂O_Three)_0.5(NiO)_0.5Of Example 17 (Bi₂O_Three)_0.33(NiO)_0.66This is referred to as Example 18. Further, the display of the operating temperature is indicated by a value obtained by dividing the operating temperature by 10 because of space. For example, Bi₂O_Three In this case, the operating temperature is 350 ° C.
[0044]
[Table 5]

[0045]
[Table 6]

[0046]
From the results of Table 5, when not operating, the base resistance value and the resistance value at the time of introducing NO also change with respect to the oxygen concentration change, so when the oxygen concentration fluctuates over a wide range (1 to 20%), It turns out that it is easy to generate a big error.
On the other hand, from the results in Table 6, when the oxygen concentration is controlled to be constant by the oxygen pump, even if the oxygen concentration (1 to 20%) of the outside air changes, the resistance value hardly fluctuates. I understand.
From this result, it is clear that when the oxygen pump is operated, it is possible to measure NO with high accuracy without being affected by fluctuations in the oxygen concentration in the gas to be measured.
[0047]
(Measurement experiment 3)
In the nitrogen oxide concentration measuring apparatus of the present invention, the bismuth oxide semiconductor used as the gas detection unit is originally a compound having high selectivity with respect to NO, but NO.₂ Since the sensitivity characteristic for is different from that for NO, it is difficult to uniformly detect the total nitrogen oxide concentration. However, if the oxygen concentration is kept constant by operating the oxygen sensor, NO₂ Is an equilibrium reaction according to the following (Chemical Formula 1): NO vs. NO₂ The ratio is fixed and can be measured as the total nitrogen oxide concentration.
[0048]
[Chemical 1]

[0049]
Using the gas detector obtained in the example, NO₂ For the gas to be measured in which NO and NO exist together, it was evaluated whether the concentration of total nitrogen oxides could be measured according to the above principle. The experiment was conducted for NOx as a whole in gases containing 0, 50, 100, 150, and 300 ppm.₂ The concentration ratio (NO₂ Amount / (NO amount + NO₂ The amount))% was assumed to be 0, 20, 40, 50, and the measurement was performed under the condition that the oxygen pump was not operated and the condition that the oxygen pump was operated and 1%.
The results are shown in FIG. 8 when not activated and in FIG. 9 when the oxygen pump is activated.
[0050]
As a result, if the oxygen pump was not activated, NO₂ The change in the resistance value is large with respect to the change in the ratio.
However, when the oxygen concentration is fixed by operating the oxygen pump, NO₂ The change in resistance value was small with respect to the change in the ratio.
Therefore, by adopting the configuration of the present application and operating the oxygen pump, the reaction of chemical formula 1 is balanced, and NO vs. NO₂It has been found that the total nitrogen oxide concentration can be measured correctly because the ratio of can be kept constant.
Here, the setting range of the oxygen concentration is preferably about 500 ppm or more and about 25% or less. If the oxygen concentration is lower than the above range, reduction of the bismuth oxide is likely to occur. Above that range, NO₂The content rate of becomes higher and the output decreases.
[0051]
(Measurement experiment 4)
The influence of so-called interference gas corresponding to the case where the oxygen pump was operated and the case where the oxygen pump was not operated was examined. In the examination, CO, CH_FourThe influence of was investigated.
Bi of this application₂O_Three-The result of Example 11 which is NiO type | system | group was shown to FIG. FIG. 10 corresponds to the result when the oxygen pump is not operated, and FIG. 11 corresponds to the result when the oxygen pump is operated. In these graphs and the graphs shown in FIGS. 12 to 15, the horizontal axis represents the gas concentration, and the vertical axis represents the resistance value of the sensor element when sensitive to each gas.
As a result, in the comparison between FIG. 10 and FIG. 11, it can be seen that it is preferable to operate the oxygen pump as described above, but, as shown in FIG. It can be seen that the resistance value hardly changes at least with respect to the interfering gas and reacts specifically with NO.
From this point of view, when comparing the results of FIGS. 11, 13, and 15, the material of the present application is a conventional SnO.₂And TiO₂It is clear that this is an advantage.
[Brief description of the drawings]
FIG. 1 is a cross-sectional structure of a sensor unit of a nitrogen oxide concentration measuring apparatus according to the present invention, which is provided with a standard oxygen concentration gas chamber.
FIG. 2 is a diagram showing a cross-sectional structure of a sensor unit according to another embodiment of the nitrogen oxide concentration measuring apparatus of the present invention.
FIG. 3 is a graph showing selectivity with respect to nitrogen oxide when a sensor element using the bismuth-containing oxide of Example 1 as a gas detection unit is used.
FIG. 4 is a graph showing selectivity with respect to nitrogen oxide when a sensor element using the bismuth-containing oxide of Example 1 as a gas detection unit is used.
FIG. 5 is a graph showing the concentration dependency of the resistance value when the sensor element using the bismuth-containing oxide of Example 1 as a gas detection unit is used.
6 is a graph showing the temperature dependence of sensitivity when a sensor element using the bismuth-containing oxide of Example 1 as a gas detection unit is used. FIG.
FIG. 7: NiO / Bi₂O_Three= Graph showing the temperature dependence of sensitivity when a sensor element using a bismuth-containing oxide of 5/95 as a gas detection part is used
FIG. 8: NiO / Bi₂O_Three= A sensor element using a bismuth-containing oxide of 1/1 as a gas detection part is used, and NO with respect to NO.₂Graph showing the results of measuring the change in resistance when the oxygen pump is not operating when the concentration is changed
FIG. 9: NiO / Bi₂O_Three= A sensor element using a bismuth-containing oxide of 1/1 as a gas detection part is used, and NO with respect to NO.₂A graph showing the result of measuring the change in resistance when the oxygen concentration is changed and the oxygen concentration in the measurement chamber is adjusted to 1% by operating the oxygen pump.
FIG. 10: NiO / Bi₂O_Three= Gas concentration and resistance for multiple types of gases when a sensor element using a bismuth-containing oxide of 10/90 as a gas detector is used and the oxygen pump is not operated and the oxygen concentration in the measurement chamber is not adjusted Diagram showing the relationship with value change
FIG. 11: NiO / Bi₂O_ThreeGas for a plurality of types of gases when the oxygen pump is operated and the oxygen concentration in the measurement chamber is adjusted to 1% when a sensor element using a bismuth-containing oxide of 10/90 as a gas detector is used. Diagram showing the relationship between concentration and resistance value change
FIG. 12 SnO₂The figure which shows the relationship between gas concentration and resistance value change with respect to multiple types of gas when the oxygen pump is not operated and the oxygen concentration in the measurement chamber is not adjusted when the sensor element having the gas detection unit is used.
FIG. 13: SnO₂The relationship between the gas concentration and the change in resistance value for multiple types of gases when the oxygen sensor is used to adjust the oxygen concentration in the measurement chamber to 1% when the sensor element is used as the gas detector. Illustration
FIG. 14 TiO₂The figure which shows the relationship between gas concentration and resistance value change with respect to multiple types of gas when the oxygen pump is not operated and the oxygen concentration in the measurement chamber is not adjusted when the sensor element having the gas detection unit is used.
FIG. 15 TiO₂The relationship between the gas concentration and the change in resistance value for multiple types of gases when the oxygen sensor is used to adjust the oxygen concentration in the measurement chamber to 1% when the sensor element is used as the gas detector. Illustration
[Explanation of symbols]
1 housing
3 Gas diffusion limiting means
5 Measurement room
7 Gas detector

Claims (8)

A housing having a measurement chamber provided with a sensor element for measuring the nitrogen oxide concentration inside, an oxygen concentration control means for controlling at least the oxygen concentration in the measurement chamber to a predetermined concentration, and measured objects outside the measurement chamber and the measurement chamber A nitrogen oxide concentration measuring apparatus for measuring a nitrogen oxide concentration in a measurement gas provided with a gas diffusion limiting means for connecting a gas, wherein the sensor element is a metal containing bismuth of 50 at% or more in terms of a metal element A nitrogen oxide concentration measuring apparatus , wherein an oxide is used as a gas detection unit, and the nitrogen oxide concentration is measured as a resistance value generated in the sensor element .   2. The nitrogen oxide concentration according to claim 1, wherein the gas detection unit includes a metal element capable of taking a valence of less than 3 as an additive other than bismuth, and one or more elements selected from In and Sn. measuring device.   The gas detection unit includes one or more elements selected from Ca, Sr, Ba, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, In, and Sn as additives other than bismuth. The nitrogen oxide concentration measuring apparatus according to claim 2.   The oxygen concentration control means is an oxygen pump configured to include a pair of electrodes provided on an oxygen ion conductive solid electrolyte, and to move oxygen between the measurement chamber and the outside of the measurement chamber. The nitrogen oxide concentration measuring apparatus according to 1.   The oxygen chamber is further provided with an oxygen sensor, and the oxygen pump controls an oxygen concentration in the measurement chamber by controlling a voltage applied to the pair of electrodes based on a signal from the oxygen sensor. The nitrogen oxide concentration measuring apparatus according to 1.   6. The nitrogen oxide concentration measuring apparatus according to claim 5, wherein the oxygen pump is configured by providing a pair of electrodes on both surfaces of the oxygen ion conductive solid electrolyte, and one of the electrodes faces a measurement chamber.   The oxygen pump is provided with a pair of electrodes on both surfaces of the oxygen ion conductive solid electrolyte, one surface facing the measurement chamber, and the other surface is a gas having a constant oxygen concentration. The nitrogen oxide concentration measuring apparatus according to claim 6, which is in contact with the above. A method for measuring the concentration of nitrogen oxides in a gas to be measured,
A step of setting the gas composition of the measurement chamber to a composition corresponding to the composition of the gas to be measured by limited diffusion through the gas diffusion limiting means, an oxygen concentration control step of controlling the oxygen concentration in the measurement chamber, and the measurement chamber. A nitrogen oxide concentration measuring method including a step of measuring a nitrogen oxide concentration in the measurement chamber as a resistance value generated in a sensor element using a metal oxide containing 50 at% or more of bismuth in terms of a metal element as a gas detection unit.

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