JP3865498B2 - Limit current type oxygen sensor and oxygen detection method - Google Patents

Description

【００１１】
さらに、この混合導電体は、単一の材料から構成されている必要はなく、酸素拡散律速層の表裏面間に亙って、酸化物イオンと電子とが同時に伝導できる回路があればよく、例えば、酸化物イオンの伝導径路を形成する材料と、電子の伝導径路とが別材料からなる径路として構成されていてもよい。このような材料は、Ｙなどを添加した酸化ジルコニアや酸化セシウム等の酸素イオン導電体と、ＰｔやＡｕ等の電子伝導物質の複合体等を挙げることができる。
さらに、この材料内に、混合導電性を阻害しない材料が含まれていても、本願の目的を達成できる。即ち、例えば、焼結助剤として普通用いられる、アルミナシリカ、酸化マグネシウム、酸化ナトリウム、酸化カルシウム等が単独、もしくは混合状態で混在していてもよい。アルミナシリカ、酸化マグネシウム、酸化ナトリウムの場合は、ガラス状として結晶界面に晶出させて、混在させることができる。
緻密質酸素イオン透過層１は、基本的に、センサ検出部４０において汲み出される酸素ガスの排出速度より少ない量の酸素ガスを供給するように構成される必要がある。この構造を実現するにあっては、緻密質酸素イオン透過層１の混合導電体のイオン輸率は低く、イオン導電率がセンサ検出部４０の固体電解質層２よりも小さい物質を用いるか、イオン導電率がほぼ等しい緻密式酸素イオン透過層１では、層の厚みを厚くするか、有効面積を小さくするなどが考えられる。
緻密質酸素イオン透過層１の厚みは、ガスの通り抜けない緻密なバリヤーとなること及び工業的に使用に耐えうるという実用的意義から１〜５０００μｍが望ましい。また、緻密質酸素イオン透過層１の製造方法は、特に制限されない。一般には、蒸着法、スパッタリング法、粉末焼結法、レーザーアブレーション法等による方法が採用される。
【００１２】
センサ検出部４０の固体電解質層２の厚みは１０〜５０００μｍが望ましい。
この固体電解質層２の両面に形成される電極４，４４は、同種または異種の導電体で構成される。好ましい材質例は、白金、白金パラジウム、銀、ロジウム等の貴金属類及びそれらの酸化物、
【００１３】
【化３】

【００１４】
のペロブスカイト型酸化物において、Ａサイト金属元素が、Ｌａ，Ｙ，Ｎｄ，Ｐｒ，Ｇｄ，Ｓｃ，Ｃｅ，Ｙｂ，Ｃａ，Ｓｒ，Ｂａからなる群からなる少なくとも１種以上の元素が、Ｂサイト金属元素が、Ｃｏ，Ｍｎ，Ｆｅ，Ｎｉ，Ｔｉ，Ｃｒからなる群から少なくとも１種以上の元素が含まれるペロブスカイト型酸化物から伝導性酸化物、上記貴金属と金属酸化物を混合したサーメットが挙げられる。
電極層の形成方法は、特に限定されるものではなく、公知の方法が制限なく採用される。代表的な方法を例示すれば、スクリーン印刷法、真空蒸着法、化学メッキ法、イオンプレーティング法、スパッタリング法、ＣＶＤ法等が挙げられる。
【００１５】
また、センサの作動する温度が高温である場合には、必要に応じてセンサを加熱すればよい。係るガスセンサの加熱には、センサ外部の熱源からの放熱によっても良いし、あらかじめヒーターをセンサに装着して、ヒーターからの伝熱や放熱を利用してもよい。センサへのヒーターの装着位置は、センサの作動を妨げない位置であれば特に限定されない。
【００１６】
本発明の実施形態は、図３に示すように、イオン化電極４に混合導電体を用いて、酸素拡散機能を備えた固体電解質センサとするものである。この場合、本発明は、酸化物イオン導電性の固体電解質層２の両面に別個の電極４、４４を有して構成され、限界電流方式で酸素の検出をおこなう固体電解質センサは、以下のように構成される。
即ち、前記固体電解質センサの検出部４０に備えられる一方の電極であるイオン化電極４が、酸化物イオンと電子を同時に伝導できる混合導電体から形成される酸素拡散律速層１とされ、この酸素拡散律速層１の単位時間当りの酸化物イオン透過量が、作動条件下に於ける前記検出部４０に備えられる固体電解質層２の単位時間当りのイオン透過量よりも小さく構成され、限界電流値を電極４、４４により測定可能に構成される（以下、本構成を構成（ W ）とする。）。
この場合、酸素拡散律速機能を有するイオン化電極全体の表裏面間（被測定ガスが存する外側空間から固体電解質表面間）を介して単位時間当たりに透過する酸化物イオン量が、固体電解質層単独で、全体として単位時間当りに透過する酸化物イオン量より小さければよい。
この構成（ W ）にあっては、イオン化電極４に、先に説明した酸素拡散律速層の有する機能を備えるのであるが、センサは、先に説明した酸素拡散律速層１（イオン化電極４）と固体電解質層との間に於けるイオン透過量（能力）の関係から、後者の酸素ポンプ機能により、酸素拡散律速層１（イオン化電極４）が拡散律速能を発揮し、限界電流方式の酸素濃度検出をおこなうことができる。ここで、この酸素拡散律速層１（イオン化電極４）に使用される材料としては、これまで説明してきた材料を、ほぼ、そのまま使用できるが、電極として働く状態においてその電気抵抗値が小さいほうが好ましいことから、この材料の比抵抗は、１０Ω・ｃｍ以下とされることが好ましい。
この構造（構成（ W ））の限界電流式酸素センサにあっては、イオン化電極が酸素拡散律速層としても働くため、構造が非常にシンプルになり、性能が安定し、非常に使用がってがよい。
【００１７】
【実施例】
以下に図２に示した限界電流式酸素センサの形態例を図面に基づいて説明する。
なお、ガスセンサの限界電流特性の測定は、センサを検知するガス雰囲気内に置き、センサ部の電極４、４４に直流電圧を印加した際の電流計７により測定し、レコーダーに記録した。印加する電圧は、０〜０．０５Ｖ／ｍｉｎの速度で連続的に変化させ、その際に流れる電流の変化を測定した。この結果より、それぞれ検知する酸素濃度において共通に限界電流が得られる電圧を、センサ素子の最適印加電圧とした。
図２に示した固体電解質層２の両面に電極４，４４を形成した検出部４０とイオン化電極４を被うように通気性絶縁性多孔質体９を取り付け、さらにこの絶縁性多孔質体９を被うように緻密質酸素イオン透過層１を取り付けることにより、図２に示す限界電流式酸素センサを構成した。
これを５００℃に加熱した電気炉内にセットし、一定温度に加熱した。電極４，４４に直流電圧６を印加し、その際に流れる電流値を測定するために電流計７を電源に直列に接続した。
【００１８】
上記限界電流式酸素センサにおいて、固体電解質層２には、酸素イオン導電体である酸化イットリウムを８ｍｏｌ％固溶させた安定化ジルコニアの１０ｍｍ角で厚さ０．５ｍｍの緻密な板状の固体電解質焼結体を用いた。
電極４，４４には、白金ペーストをスクリーン印刷して９００℃で焼き付けたものを用いた。イオン化電極４上に形成した絶縁性多孔質体９には酸化アルミニウムを用い、レーザーアブレーション法により通気性を有する多孔質に形成したものを用いた。さらに、緻密質酸素イオン透過層１には、Ｔｂ₂Ｏ_3.5 を４０ｍｏｌ固溶させた酸化ジルコニウムを用い、レーザーアブレーション法により、５０μｍの厚みで緻密に形成した。
【００１９】
このターゲットとして使用したＴｂ₂Ｏ_3.5 を４０ｍｏｌ％固溶させた酸化ジルコニウムは、以下のように作成した。
所定量のＴｂ₂Ｏ_3.5 とＺｒＯ₂を秤りとり、ジルコニアボールを用いたポットミル内にいれ、更に、溶媒としてエチルアルコールを粉体１００ｇ当たり５０ｍｌ入れて、１８時間混合した後、エバポレーターにて、エチルアルコール脱媒し、得られた粉末をペレット状に成形し、１４００℃で１０時間空気中で焼成した。得られたペレットを、乳鉢で粗粉砕した後、ポットミルでエタノールを溶媒として更に１８時間粉砕し、エチルアルコールをエバポレータで除去して原料粉末とした。得られた原料粉末の粒径は、０．１〜５μｍであった。この粉末を５×５×３ｍｍの板状に１Ｔｏｎ／ｃｍ²の圧力でプレス成形した後、１５００℃で１０時間空気中で焼成することにより、レーザーアブレーション用の原料ターゲットとした。この場合、シリカ等の助剤を必要に応じて添加してもよい。
【００２０】
図７は酸素濃度の異なる被測定ガス中に酸素センサを置き、電極４，４４間に印加する電圧を変化させたときの電流値を、印加電圧に対してプロットしたものである。図７において電圧が変化しても電流が変化しないで横軸に対してほぼ平行な変化を示す部分が認められ、各酸素濃度において限界電流値が観測できた。
【００２１】
図８は、図７より各酸素濃度において共通に限界電流が得られる電圧７００ｍＶにおける限界電流値と酸素濃度の関係を示したものである。縦軸に限界電流値、横軸に酸素濃度の関数−ｌｎ（１−Ｐｏ₂／Ｐ）（Ｐは被測定ガスの全圧、Ｐｏ₂は酸素分圧を示す）をとってプロットした。限界電流と酸素濃度の関数の間には良好な直線関係が得られ、広い濃度範囲に亘って良好に酸素濃度を検出できることが分かった。
【００２２】
図９は、図２に示す限界電流式酸素センサと従来のピンホール型限界電流式酸素センサをＳＯｘ，ＮＯｘを１０００ｐｐｍ含む被測定ガス中で５０００時間連続で暴露しながら酸素濃度を測定したときの限界電流値の初期値からの変化を限界電流値（Ｉ）と初期限界電流値（Ｉ₀ ）の比Ｉ／Ｉ₀ の形で示した。
ピンホール型の限界電流式酸素センサ（図４に示す構成となる）は１０００時間を経過したころから急激にＩ／Ｉ₀ が低下した。これは、限界電流式酸素センサのピンホールからＮＯｘ，ＳＯｘが拡散して酸素検出用センサセルの電極材料を腐食し、酸素のイオン化に対する活性が低下したために限界電流値が低下したものである。ところが図２に示す限界電流式酸素センサの、５０００時間後の限界電流値は、初期値に対して３％以内の変化で良く一致し、殆ど劣化しておらず、図２に示すセンサの長寿命化を実現できた。
さらに、図６に示す検出ガス制御スクリーンを備えた従来型の限界電流式酸素センサにおいては、その劣化の程度は低いものの、ピンホール型限界電流式酸素センサより早期に劣化が発生する傾向を示した。
なお、図３に示す本発明の上記構造（ W ）を有する限界電流式酸素センサにおいては、本実施例で示される絶縁性多孔質体９は必要なく、また、電極４と緻密質酸素イオン透過層１とは、兼用される。
【００２３】
【発明の効果】
以上の説明により理解されるように、本発明の限界電流式酸素センサは、酸化物イオン−電子混合導電体からなる緻密質酸素イオンの透過層により、被測定混合ガス中の検出ガスを混合導電体表面で生成するイオンとして混合導電体を通して移動させて、センサ検出部への検出する酸素ガスの移動を制限する方式により、従来の限界電流式酸素センサに比べて特性のばらつきが少なく、細孔の目詰まりや電極の腐食劣化によるセンサ特性の劣化が少なく長時間に亘って精度良く酸素濃度を測定することが可能である。
【図面の簡単な説明】
【図１】 限界電流式酸素センサの基本構成図
【図２】 限界電流式酸素センサの形態を示す図
【図３】 本発明の限界電流式酸素センサの基本構成図
【図４】 従来の限界電流式酸素センサの概念図
【図５】 従来の限界電流式酸素センサの概念図
【図６】 従来の限界電流式酸素センサの概念図
【図７】 本願の限界電流式酸素センサの異なる酸素濃度における印加電圧と電流の関係
を示す図
【図８】 本願の限界電流式酸素センサの酸素分圧の関数と限界電流の関係を示す図
【図９】 本発明の限界電流式酸素センサとピンホール型限界電流式酸素センサの長寿命
特性の差を示す図
【符号の説明】
１ 緻密質酸素イオン透過層（酸素拡散律速層）
２ 固体電解質層
３ 封止材
４，４４ 電極
５ リード線
６ 直流電源
７ 電流計
８ 間隙部
９ 通気性を有する絶縁性多孔質体
１０ セラミックスキャップ
１１ ガス拡散孔（キャピラリー）
１２ 多孔質セラミックス（ガス拡散層）
１３ ケーシング １４ イオン透過層（固体電解質層）[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a technique for detecting the concentration of oxygen gas present in, for example, exhaust gas. More specifically, the present invention relates to a conventionally known solid electrolyte gas sensor, which limits oxygen gas detection to its application. In addition to the current type oxygen sensor, the present invention also relates to such a limiting current type oxygen detection method.
[0002]
[Prior art]
Limit current type oxygen sensors have been known in the past, and have higher detection accuracy in a high concentration range than a concentration cell type oxygen sensor, and do not require a reference gas or solid standard substance, and can be miniaturized. Advantages such as the ability to selectively measure free oxygen gas are known.
A conventional limiting current type oxygen sensor has a structure in which electrodes are arranged on both sides of an oxide ion conductive solid electrolyte layer, and is supplied to one electrode (electrode for ionizing oxygen gas) of the sensor. to order to control the supply amount of the gas to be measured (a plurality of kinds of gases are mixed gas containing oxygen), a ceramic having pores (capillary) 11 for a gas diffusion the ionization electrode as shown in FIG. 4 or covered with a cap 10, or a structure for clothing the ionization electrode with the porous ceramic 12 as shown in FIG. 5 were mainly.
In these limiting current oxygen sensors, the gas to be measured is supplied to the ionization electrode through the pores 11 of the ceramic cap 10 or the pores of the porous ceramic 12. Therefore, the oxygen gas in the mixed gas to be measured is ionized by the electrode reaction to become oxide ions, and reaches the opposite electrode through the solid electrolyte layer. On this electrode, oxide ions emit electrons, turn into oxygen, and are gasified and released to the outside air.
Here, the oxygen discharge rate at the detection part of the sensor is made larger than the supply rate of the detection gas supplied by the diffusion of the gas from the pores 11 of the ceramic cap 10 or the pores of the porous ceramics 12, thereby limiting the limit. Although the electric current is generated, the supply of oxygen gas to be detected to the sensor detection unit is limited by the subtle change in the diffusion resistance in the pores 11 (capillaries) in the ceramic cap 10 or the porous ceramics 12. The current value changes and tends to vary.
Actually, the gas to be measured contains suspended solids and mist, and these suspended solids and mist adhere to the pores, increasing the diffusion resistance and deviating the measured value. Also, if corrosive gas such as SOx or NOx contained in the gas mixture to be measured diffuses in the pores and adheres to the ionization electrode of the detection part of the oxygen sensor, the activity of the electrode decreases, and the oxygen to be detected is ionized. And the limit current value becomes low, and a change with time occurs.
[0003]
To solve these problems, instead of the gas diffusion barrier layer made of ceramic cap or a porous ceramic having pores, a casing 13 for holding a solid electrolyte sensor is provided, as shown in FIG. 6, the oxide A structure in which a porous conductor is attached as electrodes 15 on both surfaces of an ion conductive solid electrolyte layer film 14, and the two electrodes 15 are electrically connected so as to form a potential required for oxidation or reduction of oxygen gas detected. A limiting current type oxygen sensor using a detection gas control screen 70 is developed.
[0004]
[Problems to be solved by the invention]
However, in this sensor, it is possible to prevent the corrosion of the electrode of the sensor detection unit due to the corrosive gas contained in the gas to be measured and the change in the gas diffusion resistance of the gas diffusion rate limiting layer due to the suspended solid or mist in the gas to be measured. Although the ionization electrode of the newly provided detection gas control screen 70 is corroded by the corrosive gas and the ionization activity is deteriorated, the permeation amount of oxide ions that permeate the detection gas control screen 70 changes with time. As a result, the limit current value to be measured also changes, and there is a problem that the measurement accuracy deteriorates.
[0005]
Therefore, in view of the above-mentioned drawbacks, the object of the present invention is to provide an oxygen concentration (or oxygen partial pressure) in the gas to be measured over a long period of time even when floating gas, mist, or corrosive gas is present in the gas to be measured. Is to provide a limiting current type oxygen sensor capable of accurately measuring the oxygen concentration, and to obtain such an oxygen detection method.
[0006]
[Means for Solving the Problems]
To achieve this object, the limiting current oxygen sensor of the present invention is a solid electrolyte sensor (conventional type) having an oxide ion conductive solid electrolyte layer and electrodes separately provided on both sides of this layer. And the oxygen diffusion rate-determining layer unique to the present application. Here, the oxygen diffusion limiting layer is for limiting the amount of oxygen supplied to the ionization electrode of the solid electrolyte sensor. However, in the present application, the ionization electrode also serves as an oxygen diffusion rate limiting layer, and also exhibits a function of current detection while exhibiting an oxygen diffusion rate limiting function.
That is, the limiting current type oxygen sensor is configured to have separate electrodes on both surfaces of the oxide ion conductive solid electrolyte layer, and includes a solid electrolyte sensor that detects oxygen, and the detection unit of the solid electrolyte sensor includes while the electrode is a ionisation electrode provided are also acts as an oxygen diffusion barrier, together with the oxygen diffusion barrier is formed from a mixed conductor oxide ions and electrons can be simultaneously conducting, the unit of the oxygen diffusion barrier The oxide ion permeation amount per time is configured to be smaller than the ion permeation amount per unit time of the solid electrolyte layer provided in the detection unit under the operating conditions, and the limit current value of the solid electrolyte sensor is determined by the electrode. It is configured to be measurable.
[0007]
In the limiting current type oxygen sensor having this configuration, the oxygen gas in the gas to be measured is ionized on the surface of the mixed conductor (the surface on the side in contact with the gas to be measured), and the ionized oxide ions and electrons are mixed in the mixed conductor. , And oxide gas or oxygen gas from which these ions have released electrons is supplied to the detection unit of the solid electrolyte sensor.
Here, this supply sets the oxide ion permeation amount per unit time of the oxygen diffusion rate controlling layer to be smaller than the ion permeation amount per unit time of the solid electrolyte layer alone provided in the detection section under the operating conditions. As a result, the pumping speed of the solid electrolyte sensor having an oxygen pump function is continuously generated by being faster than the oxygen gas supply speed of the oxygen diffusion rate controlling layer. Therefore, the oxygen concentration in the gas to be measured can be measured by measuring the limit current value in the solid electrolyte sensor.
In this case, the amount of oxide ions that permeate per unit time through the front and back surfaces of the entire oxygen diffusion rate limiting layer may be smaller than the amount of oxide ions that permeate per unit time as a whole by the solid electrolyte layer alone.
In order to solve the problems of the conventional limiting current type oxygen sensor, the present invention controls a diffusion rate or amount of oxygen gas by a ceramic cap having a pore (capillary) or porous ceramics, or the like, or Instead of limiting oxygen by moving oxygen through the solid electrolyte through a control screen using an ion conductive solid electrolyte layer, oxygen is diffused in a layer that does not allow gas to pass through a mixed conductor that can conduct oxide ions and electrons simultaneously. Use as a rate-limiting layer, ionize the oxygen gas to be detected on the surface of the mixed conductor, and move oxygen ions to the detection part of the solid electrolyte gas sensor using the oxygen concentration difference between the two sides of the mixed conductor as the driving force. Is used.
When this configuration is adopted, the oxygen gas in the gas to be measured can be supplied to the sensor detection unit without being moved through the pores by diffusion, so that the problem of floating solids and mist can be solved. The electrode of the sensor detection unit is not corroded by corrosive gas and the sensor characteristics are not deteriorated, and a sensor with little variation in measured value due to long-term use can be obtained.
In addition, since a mixed conductor that is not corroded by corrosive gas can be used for the oxygen diffusion rate-determining layer, the electrode 15 of the detection gas control screen that ionizes the oxygen gas and controls its supply amount is not required, and the corrosion is prevented. It is possible to provide a stable oxygen diffusion rate-determining layer that is not deteriorated by a property gas, and a sensor with little variation in measured values due to long-term use can be obtained.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
First, a limiting current type oxygen sensor will be described with reference to FIG.
As can be seen from FIG. 1, the limiting current type oxygen sensor includes a detection unit of a solid electrolyte sensor configured by interposing a solid electrolyte layer 2 between electrodes 4 and 44, a mixed conductor of oxide ions and electrons (of this) A dense oxygen ion transmission layer 1 (an example is a mixed conductive oxide) (this layer is the oxygen diffusion-controlling layer in the present application) is provided.
The mechanism of the detection unit 40 of the solid electrolyte sensor is the same as the detection unit of a conventionally known limiting current oxygen sensor. That is, the solid electrolyte layer 2 is interposed between the electrodes 4 and 44, and the power source 6 and the ammeter 7 are connected in series with the electrodes 4 and 44 by the lead wire 5 to form a circuit. The electrode 4 is a so-called ionization electrode, and the electrode 44 is referred to as an electron emission electrode in the present application. The dense oxygen ion permeable layer 1 and the sensor detection unit 40 are hermetically sealed by the sealing material 3 at the periphery. The material of the sealing material 3 is not particularly limited as long as the material is impermeable to oxygen gas. Examples of suitable materials include glass, inorganic cement, low melting point ceramics and the like. Among these materials, it is desirable to use a material having a thermal expansion coefficient equal to or slightly smaller than the dense oxygen ion permeable layer 1 made of a mixed conductor and the solid electrolyte layer of the detection unit 40 of the sensor. The sealing material to be used is desirably used as a powder having a particle size of 10 μm or less in view of bondability.
The limiting current type oxygen sensor shown in FIG. 2 has electrodes 4 and 44 attached to the center of both surfaces of a plate-like solid electrolyte layer 2 that is an oxygen ion conductor, and breathable insulation so as to be in contact with and cover the oxygen ionization electrode 4. The porous porous body 9 is attached, and a dense oxygen ion permeable layer 1 of a mixed conductor is attached so as to be in contact with the porous body 9. In the case of this sensor, the sensor can be easily downsized by thinning each layer.
[0009]
In the present invention, a mixed conductor of oxide ions and electrons is used for the dense oxygen ion permeable layer 1, and this oxide ion permeable layer is a mixed conductive material through which the target oxide ions pass. It is sufficient that the oxide ion transport number is 90% or less under the use conditions, and conventionally known ones can be used. The oxide ion transport number is preferably 50% or less. If the oxide ion transport number of oxygen is 90% or more, the current that flows because oxygen ions move in the mixed conductor is reduced, the amount of diffused oxygen is reduced, and the limit current value of the sensor obtained when detecting the oxygen concentration And the sensitivity of the sensor to changes in oxygen concentration is reduced, and the oxygen concentration cannot be detected accurately over a wide concentration range. On the other hand, if it is 50% or less, it can effectively cope with a wide concentration range of oxygen.
[0010]
[Chemical 2]

[0011]
Furthermore, this mixed conductor does not need to be composed of a single material, and only needs to have a circuit capable of conducting oxide ions and electrons simultaneously across the front and back surfaces of the oxygen diffusion-controlling layer. For example, the material forming the conduction path of oxide ions and the conduction path of electrons may be configured as a path made of different materials. Examples of such a material include a composite of an oxygen ion conductor such as zirconia oxide or cesium oxide to which Y or the like is added and an electron conductive substance such as Pt or Au.
Furthermore, even if a material that does not inhibit the mixed conductivity is included in this material, the object of the present application can be achieved. That is, for example, alumina silica, magnesium oxide, sodium oxide, calcium oxide and the like, which are usually used as sintering aids, may be present alone or in a mixed state. In the case of alumina silica, magnesium oxide, and sodium oxide, they can be crystallized at the crystal interface as glassy and mixed.
The dense oxygen ion permeable layer 1 basically needs to be configured to supply an amount of oxygen gas that is smaller than the discharge rate of the oxygen gas pumped out by the sensor detection unit 40. In realizing this structure, the ion transport number of the mixed conductor of the dense oxygen ion permeable layer 1 is low and a substance having an ionic conductivity smaller than that of the solid electrolyte layer 2 of the sensor detection unit 40 is used. In the dense oxygen ion permeable layer 1 having substantially the same electrical conductivity, it is conceivable to increase the thickness of the layer or reduce the effective area.
The thickness of the dense oxygen ion permeable layer 1 is desirably 1 to 5000 μm from the practical significance that it becomes a dense barrier that does not allow gas to pass through and can be used industrially. Moreover, the manufacturing method of the dense oxygen ion permeable layer 1 is not particularly limited. In general, a vapor deposition method, a sputtering method, a powder sintering method, a laser ablation method, or the like is employed.
[0012]
As for the thickness of the solid electrolyte layer 2 of the sensor detection part 40, 10-5000 micrometers is desirable.
The electrodes 4 and 44 formed on both surfaces of the solid electrolyte layer 2 are made of the same or different kinds of conductors. Examples of preferable materials include noble metals such as platinum, platinum palladium, silver and rhodium and oxides thereof.
[0013]
[Chemical 3]

[0014]
In the perovskite oxide, the A-site metal element is at least one element selected from the group consisting of La, Y, Nd, Pr, Gd, Sc, Ce, Yb, Ca, Sr, and Ba. Examples include perovskite oxides containing at least one element from the group consisting of Co, Mn, Fe, Ni, Ti, and Cr, conductive oxides, and cermets in which the above precious metals and metal oxides are mixed. .
The formation method of an electrode layer is not specifically limited, A well-known method is employ | adopted without a restriction | limiting. Examples of typical methods include screen printing, vacuum deposition, chemical plating, ion plating, sputtering, and CVD.
[0015]
Moreover, what is necessary is just to heat a sensor as needed, when the temperature which the sensor act | operates is high temperature. The gas sensor may be heated by heat radiation from a heat source outside the sensor, or a heater may be attached to the sensor in advance to use heat transfer or heat radiation from the heater. The mounting position of the heater on the sensor is not particularly limited as long as it does not hinder the operation of the sensor.
[0016]
As shown in FIG. 3, the embodiment of the present invention uses a mixed conductor for the ionization electrode 4 to provide a solid electrolyte sensor having an oxygen diffusion function. In this case, according to the present invention, a solid electrolyte sensor that includes separate electrodes 4 and 44 on both surfaces of the oxide ion conductive solid electrolyte layer 2 and detects oxygen by a limiting current method is as follows. Configured.
That is, the ionization electrode 4 which is one of the electrodes provided in the detection unit 40 of the solid electrolyte sensor is an oxygen diffusion rate controlling layer 1 formed of a mixed conductor capable of conducting oxide ions and electrons simultaneously. The oxide ion permeation amount per unit time of the rate limiting layer 1 is configured to be smaller than the ion permeation amount per unit time of the solid electrolyte layer 2 provided in the detection unit 40 under operating conditions, and the limiting current value is set. The electrodes 4 and 44 are configured to be measurable (hereinafter, this configuration is referred to as a configuration ( W )).
In this case, the amount of oxide ions per unit time passing between the front and back surfaces of the entire ionization electrode having an oxygen diffusion rate limiting function (between the outer space where the gas to be measured exists and the surface of the solid electrolyte) is determined by the solid electrolyte layer alone. As a whole, it should be smaller than the amount of oxide ions permeating per unit time.
In this configuration ( W ) , the ionization electrode 4 is provided with the function of the oxygen diffusion rate-determining layer described above, but the sensor includes the oxygen diffusion rate-limiting layer 1 (ionization electrode 4) described above. Due to the relationship between the amount of ion permeation (capacity) between the solid electrolyte layer and the oxygen pump function of the latter, the oxygen diffusion rate limiting layer 1 (ionization electrode 4) exhibits the diffusion rate limiting capability, and the oxygen concentration of the limiting current method is used. Detection can be performed. Here, as the material used for the oxygen diffusion-controlling layer 1 (ionization electrode 4), the materials described so far can be used almost as they are, but it is preferable that the electric resistance value is small in a state of acting as an electrode. Therefore, the specific resistance of this material is preferably 10 Ω · cm or less.
In the limiting current type oxygen sensor of this structure (configuration ( W )) , since the ionization electrode also functions as an oxygen diffusion rate controlling layer, the structure becomes very simple, the performance is stable, and it is very used. Is good.
[0017]
【Example】
An embodiment of the limiting current type oxygen sensor shown in FIG. 2 will be described below with reference to the drawings.
The limit current characteristics of the gas sensor were measured in the gas atmosphere for detecting the sensor, measured by the ammeter 7 when a DC voltage was applied to the electrodes 4 and 44 of the sensor unit, and recorded on the recorder. The applied voltage was continuously changed at a speed of 0 to 0.05 V / min, and the change in the current flowing at that time was measured. Based on this result, the voltage at which the limit current is commonly obtained for each detected oxygen concentration was determined as the optimum applied voltage of the sensor element.
Mounting breathable insulating porous body 9 so as to cover the detection section 40 and the ionization electrode 4 having electrodes formed 4, 44 to the surfaces of the solid electrolyte layer 2 shown in FIG. 2, further insulating porous body 9 The limiting current type oxygen sensor shown in FIG. 2 was constructed by attaching the dense oxygen ion permeable layer 1 so as to cover.
This was set in an electric furnace heated to 500 ° C. and heated to a constant temperature. A DC voltage 6 was applied to the electrodes 4 and 44, and an ammeter 7 was connected in series with the power source in order to measure the value of the current flowing at that time.
[0018]
In the limiting current type oxygen sensor, the solid electrolyte layer 2 is a dense plate-shaped solid electrolyte of 10 mm square and 0.5 mm thick of stabilized zirconia in which 8 mol% of yttrium oxide, which is an oxygen ion conductor, is dissolved. A sintered body was used.
As the electrodes 4 and 44, a platinum paste screen-printed and baked at 900 ° C. was used. The insulating porous body 9 formed on the ionization electrode 4 was made of aluminum oxide and made porous with air permeability by a laser ablation method. Further, the dense oxygen ion permeable layer 1 was densely formed with a thickness of 50 μm by a laser ablation method using zirconium oxide in which 40 mol of Tb ₂ O _3.5 was dissolved.
[0019]
Zirconium oxide in which 40 mol% of Tb ₂ O _3.5 used as the target was dissolved was prepared as follows.
A predetermined amount of Tb ₂ O _3.5 and ZrO ₂ were weighed and placed in a pot mill using zirconia balls. Further, 50 ml of ethyl alcohol was added as a solvent per 100 g of powder and mixed for 18 hours. Ethyl alcohol was removed, and the resulting powder was formed into pellets and fired at 1400 ° C. for 10 hours in air. The obtained pellets were coarsely pulverized in a mortar and then further pulverized in a pot mill using ethanol as a solvent for 18 hours, and ethyl alcohol was removed by an evaporator to obtain a raw material powder. The particle size of the obtained raw material powder was 0.1 to 5 μm. This powder was press-molded into a 5 × 5 × 3 mm plate at a pressure of 1 Ton / cm ² and then fired in air at 1500 ° C. for 10 hours to obtain a raw material target for laser ablation. In this case, an auxiliary such as silica may be added as necessary.
[0020]
FIG. 7 plots the current value when the voltage applied between the electrodes 4 and 44 is changed with respect to the applied voltage when oxygen sensors are placed in the gases to be measured having different oxygen concentrations. In FIG. 7 , even when the voltage is changed, the current does not change, and a portion showing a change almost parallel to the horizontal axis is recognized, and the limit current value can be observed at each oxygen concentration.
[0021]
Figure 8 is a graph showing the relationship between the limiting current value and the oxygen concentration in the voltage 700mV the limiting current is obtained common to the oxygen concentration than FIG. The vertical axis represents the limiting current value, and the horizontal axis represents the oxygen concentration function −ln (1-Po ₂ / P) (P represents the total pressure of the gas to be measured and Po ₂ represents the oxygen partial pressure). It was found that a good linear relationship was obtained between the limiting current and the oxygen concentration function, and the oxygen concentration could be detected well over a wide concentration range.
[0022]
FIG. 9 shows the oxygen concentration measured when the limiting current type oxygen sensor shown in FIG. 2 and the conventional pinhole type limiting current type oxygen sensor are exposed continuously in a gas to be measured containing 1000 ppm of SOx and NOx for 5000 hours. The change of the limit current value from the initial value is shown in the form of the ratio I / I ₀ between the limit current value (I) and the initial limit current value (I ₀ ).
In the pinhole type limiting current type oxygen sensor (having the configuration shown in FIG. 4 ), the I / I ₀ suddenly decreased after 1000 hours. This is because NOx and SOx diffuse from the pinhole of the limiting current type oxygen sensor to corrode the electrode material of the sensor cell for oxygen detection, and the activity against ionization of oxygen decreases, so that the limiting current value decreases. However, the limit current value after 5000 hours of the limit current type oxygen sensor shown in FIG. 2 agrees well with a change within 3% with respect to the initial value, hardly deteriorates, and the length of the sensor shown in FIG. Realized long life.
Furthermore, the conventional limit current oxygen sensor having the detection gas control screen shown in FIG. 6 shows a tendency to deteriorate earlier than the pinhole type limit current oxygen sensor although the degree of deterioration is low. It was.
In the limiting current type oxygen sensor having the above-described structure ( W ) of the present invention shown in FIG. 3, the insulating porous body 9 shown in this embodiment is not necessary, and the electrode 4 and dense oxygen ion permeation are not required. The layer 1 is also used.
[0023]
【The invention's effect】
As can be understood from the above description, the limiting current type oxygen sensor of the present invention is configured such that a detection gas in a mixed gas to be measured is mixed and conductive by a dense oxygen ion transmission layer made of a mixed oxide ion-electron conductor. By moving the mixed gas through the mixed conductor as ions generated on the body surface and limiting the movement of oxygen gas to be detected to the sensor detector, there is less variation in characteristics compared to conventional limiting current oxygen sensors, and pores Therefore, it is possible to measure the oxygen concentration with high accuracy over a long period of time with little deterioration of sensor characteristics due to clogging of the electrodes and corrosion deterioration of the electrodes.
[Brief description of the drawings]
[1] Basic configuration diagram Figure 2 of the limiting current type oxygen sensor limiting current type oxygen basic configuration diagram of a limiting current type oxygen sensor of Figure 3 the present invention showing the embodiment of a sensor 4 shows a conventional limit Schematic diagram of current-type oxygen sensor [Fig. 5 ] Schematic diagram of conventional limit-current-type oxygen sensor [Fig. 6 ] Schematic diagram of conventional limit-current-type oxygen sensor [Fig. 7 ] Different oxygen concentrations of the limit-current-type oxygen sensor of the present application limiting current type oxygen sensor and the pinhole of the applied voltage Figure 8 showing the relationship between the current application in Fig. 9 present invention showing the relationship between the function and the limiting current of the limiting current type oxygen sensor of the oxygen partial pressure in Diagram showing the difference in long-life characteristics of type limiting current oxygen sensors 【Explanation of symbols】
1 Dense oxygen ion permeation layer (oxygen diffusion controlled layer)
DESCRIPTION OF SYMBOLS 2 Solid electrolyte layer 3 Sealing material 4,44 Electrode 5 Lead wire 6 DC power supply 7 Ammeter 8 Gap part 9 Breathable insulating porous body 10 Ceramic cap 11 Gas diffusion hole (capillary)
12 Porous ceramics (gas diffusion layer)
13 Casing 14 Ion permeation layer (solid electrolyte layer)

Claims (5)

A solid electrolyte sensor configured to have separate electrodes on both surfaces of an oxide ion conductive solid electrolyte layer and detect oxygen by a limiting current method,
The ionization electrode, which is one of the electrodes provided in the detection unit of the solid electrolyte sensor, is an oxygen diffusion rate limiting layer formed of a mixed conductor capable of conducting oxide ions and electrons simultaneously, and is a unit of the oxygen diffusion rate limiting layer The oxide ion permeation amount per time is configured to be smaller than the ion permeation amount per unit time of the solid electrolyte layer provided in the detection unit under operating conditions,
A limiting current type oxygen sensor configured such that a limiting current value can be measured by the electrode.   The limiting current oxygen sensor according to claim 1, wherein the mixed conductor has an oxide ion transport number of 90% or less. The mixed conductor is

The A-site metal element is a perovskite type oxide represented by the formula consisting of at least one element selected from the group consisting of La, Y, Nd, Gd, Yb, Dy, Sc, Sr, Ba, Ca, Ce, Zr, The B-site metal element is at least one or more selected from the group consisting of Co, Mn, Fe, Cr, Ni, Cu, Ti, Ce, Zr, V, Pb, La, Y, Nd, Gd, Yb, Dy, Sc, and In. Oxides of the elements
Or an oxide of a fluorite structure represented by the general formula (MO ₂ ) _1-X (N ₂ O ₃ ) _X , wherein the M element is composed of at least one of Zr or Ce, and the N element is at least Tb, The limiting current type oxygen sensor according to claim 1 or 2, which is an oxide composed of one or more of Pr, Er, Y, and Fe. A limit current for detecting an oxygen concentration of a gas to be measured by measuring a limit current value flowing between the ionization electrode and the electron emission electrode using the limit current oxygen sensor according to any one of claims 1 to 3. Formula oxygen detection method. The ionization electrode provided in the detection part of the solid electrolyte sensor that detects oxygen is an oxygen diffusion rate-determining layer formed from a mixed conductor capable of conducting oxide ions and electrons simultaneously,
In the operating state in which oxygen is moved from the ionization electrode of the solid electrolyte sensor to the electron emission electrode by the oxygen pump function of the solid electrolyte sensor and released to the outside.
Oxygen gas in the gas to be measured is ionized on the surface of the oxygen diffusion rate limiting layer serving as the ionization electrode that is in contact with the gas to be measured, and the oxide diffusion is controlled by the oxygen diffusion rate limiting layer in the solid electrolyte of the solid electrolyte sensor. The surface of the solid electrolyte sensor is moved to the surface where the oxygen diffusion rate-determining layer and the gas to be measured are in contact with each other, and oxygen is guided to the solid electrolyte sensor. A limit current type oxygen detection method for measuring a limit current value flowing between discharge electrodes and detecting an oxygen concentration of the gas to be measured.

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