JP4226176B2 - Gas sensor and gas detection method - Google Patents

Description

【００２０】
実施例１のガスセンサ２と、比較例１,２のガスセンサ３２,４２に対して、陽イオンでの汚染時の特性を評価した。ガスセンサ２,３２,４２のＳnＯ2膜６に、ＮaＣl水溶液を滴下して、ＳnＯ2に対してＮaを１mol％濃度で添加した。乾燥後に、回路電圧ＶCを５Ｖ、負荷抵抗ＲLを１ｋΩ、ヒータ膜１４での消費電力ＰＨを２８０ｍＷとして３日間通電後、４５０℃で、メタン５０００ppm中にて、陽極／陰極間の電圧を０ｖから７Ｖまで０.５Ｖ／minで増加させ、センサ電流を測定した。なお一般に、非オーミック性はセンサ電流を増すと発現し、空気中などでＳnＯ２膜６が高抵抗の場合、検出電圧を大きくしても、非オーミック性は余り表れなかった。
【００２１】
結果を図６〜図１１に示す。図６,図７は比較例２（図５のパターン）での結果で、図８,図９は比較例１（図４のパターン）での結果で、図１０、図１１は実施例１でのガスセンサの結果である。図６〜図１１において、Ｒsはセンサ抵抗で、Ｒoはセンサ電圧Ｖsが１Ｖの時のセンサ抵抗を表し、Ｉsはセンサ電流である。
【００２２】
図６〜図１１から明らかなように、非オーミック性は比較例２＞比較例１＞実施例１の順に減少し、これは接触面積比Ｓ１／Ｓ２が大きく、沿面長比Ｌ１／Ｌ２が大きいほど、非オーミックな特性が生じにくいことを示している。そして接触面積比Ｓ１／Ｓ２を大きく、沿面長比Ｌ１／Ｌ２を大きくすることは、陰極とＳnＯ２膜との接触を改善することで、非オーミック性の原因が陰極とＳnＯ２膜の界面にあることを示している。図６〜図１１の特性は不純物の添加により得られた特性で、実施例１のセンサ２では、人為的に加えたＮaイオンによる汚染への耐久性が高く、これと同様に、ＳnＯ２膜６の製造時や、電極８，１０からの金属イオン、あるいは触媒やバインダーなどからの金属イオン等への耐久性も高い。
【００２３】
【実施例２】
実施例２のガスセンサ６２を図１２に示す。陰極６８を大きなコの字形にして、内側を半円状にし、小さく突き出した陽極７０を取り巻くようにした。このとき沿面長の比Ｌ１／Ｌ２は２.３５、接触面積の比Ｓ１／Ｓ２は６.３５で、陰極６８とＳnＯ２膜６との接触を改善している。ただし図１２のパターンでは、陽極７０をＬ字状にしなかったため、ＳnＯ２膜６とのパターンずれで特性が変化する。
【００２４】
実施例では特定の陰極や陽極のパターンを示したが、これらに限るものではない。この発明では、陰極とＳnＯ２膜との接触面積Ｓ１と陽極とＳnＯ２膜との接触面積Ｓ２との比を選び、陰極側と陽極側との沿面長の比を選ぶことにより、陰極側での非オーミックな特性の発生を抑制できる。このため、不純物に対する耐久性が高いガスセンサが得られる。また陰極と陽極の双方をＬ字状のパターンとして対向させることにより、ＳnＯ２膜と電極とのパターンがずれても、特性への影響を最小にすることができる。
【図面の簡単な説明】
【図１】 実施例１のガスセンサの表面と背面とを示す図
【図２】 実施例１での沿面長Ｌ１,Ｌ２,接触面積Ｓ１,Ｓ２を示す図
【図３】 実施例１のガスセンサの電位分布を模式的に示す図
【図４】 従来例１のガスセンサの電極パターンを示す平面図
【図５】 従来例２のガスセンサの電極パターンを示す平面図
【図６】 従来例２のガスセンサでの電圧／電流特性を示す特性図で、ＶSはセンサに加えた電圧を、Ｉsはセンサ電流を示す
【図７】 従来例２のガスセンサでの電圧／抵抗特性を示す特性図
ＶSはセンサに加えた電圧を、Ｒsはセンサ電圧が１ｖでの抵抗値を基準とするとセンサ抵抗を示す
【図８】 従来例１のガスセンサでの電圧／電流特性を示す特性図
【図９】 従来例１のガスセンサでの電圧／抵抗特性を示す特性図
【図１０】 実施例１のガスセンサでの電圧／電流特性を示す特性図
【図１１】 実施例１のガスセンサでの電圧／抵抗特性を示す特性図
【図１２】 実施例２のガスセンサでの電極パターンを示す平面図
【符号の説明】
２，６２ ガスセンサ
４ 基板
６ ＳnＯ2膜
８，６８ 陰極
８ａ，８ｂ 露出部
９ Ｒ部
１０，７０ 陽極
１２ スルーホール
１４ ヒータ膜
１６ ヒータ電極
１８，２０ 電極パッド
２２ リード
３８，４８ 陰極
５０ 陽極
４５ エッジ
Ｌ１ 陰極沿面長
Ｌ２ 陽極沿面長
Ｓ１ 陰極接触面積
Ｓ２ 陽極接触面積[0001]
[Field of the Invention]
The present invention relates to gas detection using a change in the resistance value of a SnO 2 film.
[0002]
[Prior art]
The inventors have found that in a gas sensor using a SnO 2 film, the sensor characteristics change due to the intrusion of impurities such as Na ions, and non-ohmic voltage / current characteristics appear. The inventor examined the pattern of the anode and the cathode with respect to the SnO 2 film, and found a pattern in which non-ohmic characteristics are difficult to appear.
[0003]
[Problems of the Invention]
An object of the present invention is to provide a SnO2-based gas sensor that hardly causes non-ohmic characteristics even when it is contaminated by impurities or the like (claims 1 to 4).
[0004]
[Structure of the invention]
In the gas sensor in which the SnO2 film, the cathode and the anode are provided on the insulating substrate, the ratio S1 / S2 between the contact area S1 between the cathode and the SnO2 film and the contact area S2 between the anode and the SnO2 film is obtained. 3.5 or more, preferably 4 or more, more preferably 5 or more, and this ratio is, for example, 100 or less, preferably 30 or less. The ratio L1 / L2 between the creeping length L1 of the cathode end along the current path from the SnO2 film to the cathode and the creeping length L2 of the anode end along the current path from the anode to the SnO2 film is 1.5 or more. The ratio is, for example, 20 or less, preferably 10 or less.
[0005]
Preferably, the cathode and the anode are both formed in an L-shape, one side outside the L-shape of the anode is opposed to one side inside the L-shape of the cathode, and two sides outside the L-shape of the cathode Is exposed from the SnO 2 film. Particularly preferably, an R portion is provided at the apex inside the L-shape of the cathode so that the current density in the vicinity of the apex is made uniform.
[0006]
The present invention also provides a gas detection method in which an SnO2 film, a cathode, and an anode are provided on an insulating substrate, and gas is detected from a current flowing from the anode to the cathode.
The ratio S1 / S2 between the contact area S1 between the cathode and the SnO2 film and the contact area S2 between the anode and the SnO2 film is 3.5 or more, and the creeping length L1 of the cathode end along the current path from the SnO2 film to the cathode And the ratio L1 / L2 of the creeping length L2 at the end of the anode along the current path from the anode to the SnO2 film is 1.5 or more, so that the current density is increased on the anode side and lowered on the cathode side. The generation of the non-ohmic characteristic of the current with respect to the voltage between the anode and the cathode is suppressed.
[0007]
[Operation and effect of the invention]
In the present invention, the contact area ratio S1 / S2 is set to 3.5 or more so that the contact area from the SnO 2 film to the cathode is sufficiently larger than the contact area from the anode to the SnO 2 film. Further, by setting the creepage length ratio L1 / L2 to 1.5 or more, the current density from the SnO 2 film to the cathode is made smaller than the current density from the anode to the SnO 2 film. As a result, the current density is high on the anode side and low on the cathode side, and generation of non-ohmic characteristics can be suppressed.
[0008]
For this reason, even when the gas sensor is contaminated with impurities, the appearance of non-ohmic characteristics can be delayed, and the voltage applied to the gas sensor can be increased to facilitate handling.
[0009]
Here, when both the cathode / anode are L-shaped, the outer side of the anode L-shape is opposed to the inner side of the cathode, and the two outer sides of the cathode L-shape are exposed from the SnO 2 film. Even when the contact area S1 is increased and the pattern of the cathode or anode and the SnO 2 film is shifted, the influence on the characteristics can be minimized.
[0010]
【Example】
1 and 2 show the structure of the gas sensor 2 of the first embodiment. The upper part of FIG. 1 shows the electrode pattern on the front surface of the gas sensor 2, and the lower part of FIG. 4 is a substrate of alumina or the like, 6 is a SnO2 film, alumina or the like may be mixed, and a noble metal catalyst such as Pd is added to SnO2. The SnO2 film 6 has a thickness of 40 [mu] m here, but may be a thin film, for example, about 0.1 [mu] m to 500 [mu] m.
[0011]
8 is a cathode, 10 is an anode, and is formed on the surface of the substrate 4 by, for example, Au paste printing. The film thickness is 4 μm for both the cathode 8 and the anode 10, but a thin film may be used. The material of the cathode 8 and the anode 10 is not limited to Au, and a noble metal such as Pt, Rh, Ir, Pd or a noble metal alloy is preferable. A through hole 12 connects the cathode 8 and the anode 10 on the front surface of the substrate 4 to the electrode pads 20 and 20 on the back surface. A heater film 14 such as RuO 2 is provided on the back surface of the gas sensor 2 and connected to the electrode pad 18 through the heater electrode 16, and the lead 20 is connected to the electrode pads 18 and 20. The SnO 2 film 6 is heated by the heater film 14 and a voltage is applied from the anode 10 to the cathode 8 to detect combustible gas, CO, and the like. In addition, the heating temperature and heating pattern of the gas sensor 2 are arbitrary.
[0012]
FIG. 2 shows the contact areas S1 and S2 (indicated by hatching) and the creepage lengths L1 and L2 (indicated by bold lines) between the SnO2 film 6 and the cathode or anode, and the contact area S1 is the contact between the cathode 8 and the SnO2 film 6. In terms of area, the contact area S2 is the contact area between the anode 10 and the SnO2 film 6, and the ratio S1 / S2 is set to 3.5 or more, preferably 4 or more, and most preferably 5 or more. The ratio S1 / S2 is, for example, 100 or less, preferably 30 or less, from the viewpoint of ease of pattern formation. The creeping lengths L1 and L2 mean lengths at which the ends of the cathode 8 and the anode 10 cross the current path between the cathode 8 and the anode 10, and are portions covered with the SnO2 film 6 at the ends of the cathode 8 and the anode 10. However, it does not include those that deviate from the current path, nor does it include a portion that protrudes from the range facing the end of the counterpart electrode. The ratio L1 / L2 between the creeping length L1 of the cathode 8 and the creeping length L2 of the anode 10 is 1.5 or more, and is, for example, 20 or less, preferably 10 or less from the viewpoint of ease of pattern formation.
[0013]
The large contact area S1 on the cathode 8 side and the long creepage length L1 mean that the contact area between the cathode 8 and the SnO 2 film 6 is large and the current density at the end is small, and the cathode / SnO 2 film This is a condition for preventing the occurrence of non-ohmic characteristics at the interface. When the contact area S2 on the anode 10 side is narrow and the creeping length L2 is short, the contact area between the anode 10 and the SnO2 film 6 is narrow, the current density at the end on the anode side is large, and the voltage near the anode 10 is large. It means that the descent is large. The large contact area ratio S1 / S2 and the large creepage length ratio L1 / L2 mean that the current density near the boundary between the anode 10 and the SnO2 film 6 is large and the current density near the cathode 8 is small. Therefore, it means that polarization is hardly generated in the vicinity of the cathode 8 and non-ohmic characteristics are hardly generated.
[0014]
The cathode 8 preferably has no edge at the portion along the creepage length. For example, the R portion 9 is provided inside the L-shaped cathode 8 to eliminate the edge. In order to reduce the influence even when the area of the cathode 8 is increased and the pattern of the SnO 2 film 6 and the cathode 8 is shifted, the shape of the cathode 8 is made L-shaped and the cathode is exposed outside the SnO 2 film 6. It is preferable to provide the portions 8a and 8b on the two outer sides of the L-shape. Preferably, the pattern of the anode 10 is also L-shaped so that the influence on the characteristics is small even when the pattern of the anode 10 and the SnO 2 film 6 is shifted. In addition, the outer side of the L-shape of the anode 10 is opposed to the inner side of the L-shape of the cathode 8.
[0015]
FIG. 3 schematically shows the potential distribution between the cathode 8 and the anode 10. As will be described later, non-ohmic characteristics occur at the interface between the SnO 2 film and the cathode 8, the potential changes linearly inside the SnO 2 film 6, and the interface between the anode 10 and the SnO 2 film 6 contacts non-ohmically. Although there is a possibility, it is thought that the degree is small. It is considered that the interface between the cathode 8 and the SnO 2 film 6 has non-ohmic characteristics, and when the current density passing through this interface is high, non-ohmic characteristics are exhibited. Possible causes of the non-ohmic characteristics at the interface between the cathode 8 and the SnO 2 film are that impurities cations contained in the SnO 2 film 6 and the like are concentrated on the interface of the cathode 8 to cause contamination with impurities in use, the SnO 2 film It may be caused by impurities of itself, impurities such as addition of a catalyst or binder for sintering, or impurities entering through a filter for surface coating.
[0016]
[Test Example 1]
The gas sensor 2 of Example 1, the gas sensor 32 of FIG. 4 (Comparative Example 1), and the gas sensor 42 of FIG. 5 (Comparative Example 2) were manufactured. The heater film 16, electrode pads 18, 20 and the like were formed on the back surface of the alumina substrate, and Au paste was printed on the surface of the substrate and baked to form Au cathodes 8 and anodes 10 having a thickness of 4 μm. Next, SnO 2 (1 wt% Pd added to SnO 2) was printed and baked at 700 ° C. to form a SnO 2 film 6 having a thickness of 40 μm. Thereafter, the substrate was divided into a square shape with a side of 1.5 mm. In the sensor of Example 1, the contact area S1 is 0.432 mm2, the contact area S2 is 0.0825 mm2, the creepage length L1 is 1.07 mm, the creepage length L2 is about 0.6 mm, and the distance between the cathode 8 and the anode 10 is 0.5. 3 mm.
[0017]
When the gas sensor 32 of Comparative Example 1 is shown in FIG. 4, since 38 is a cathode and there is no exposed portion 8a, 8b in FIG. 1, the contact area S1 between the cathode 8 and the SnO 2 film 6 is narrow. When the gas sensor 42 of Comparative Example 2 is shown in FIG. 5, the cathode 48 has an edge 45, and the contact area S <b> 2 between the anode 50 and the SnO 2 film is larger than the sensors 2 and 32 of Example 1 and Comparative Example 1. The creeping length L1 of the cathode 48 in Comparative Example 2 is larger than the sensors 2 and 32 of Example 1 and Comparative Example 1, but the creeping length L2 of the anode 50 is larger than the sensors 2 and 32 of Example 1 and Comparative Example 1. Is even bigger. The manufacturing conditions of the sensors 32 and 42 of Comparative Example 1 and Comparative Example 2 are the same as those of Example 1 except for the electrode pattern.
[0018]
Table 1 shows the creepage length ratio L1 / L2 and the contact area ratio S1 / S2 for Example 1, Comparative Example 1, Comparative Example 2, and the like.
[0019]
[Table 1]

[0020]
The characteristics at the time of contamination with positive ions were evaluated for the gas sensor 2 of Example 1 and the gas sensors 32 and 42 of Comparative Examples 1 and 2. A NaCl aqueous solution was dropped on the SnO2 film 6 of the gas sensors 2, 32, and 42, and Na was added at a concentration of 1 mol% with respect to SnO2. After drying, the circuit voltage VC is 5 V, the load resistance RL is 1 kΩ, the power consumption PH in the heater film 14 is 280 mW, the current is applied for 3 days, and the voltage between the anode and the cathode is changed from 0 v in 5000 ppm of methane at 450 ° C. The sensor current was measured by increasing the voltage to 7 V at 0.5 V / min. In general, the non-ohmic property appears when the sensor current is increased. When the SnO 2 film 6 has a high resistance in air or the like, the non-ohmic property does not appear much even if the detection voltage is increased.
[0021]
The results are shown in FIGS. 6 and 7 show the results of Comparative Example 2 (pattern of FIG. 5), FIGS. 8 and 9 show the results of Comparative Example 1 (pattern of FIG. 4), and FIGS. 10 and 11 show the results of Example 1. It is a result of the gas sensor. 6 to 11, Rs is a sensor resistance, Ro is a sensor resistance when the sensor voltage Vs is 1 V, and Is is a sensor current.
[0022]
As apparent from FIGS. 6 to 11, the non-ohmic property decreases in the order of Comparative Example 2> Comparative Example 1> Example 1, which has a large contact area ratio S1 / S2 and a large creepage length ratio L1 / L2. It shows that non-ohmic characteristics are less likely to occur. Increasing the contact area ratio S1 / S2 and increasing the creepage length ratio L1 / L2 improves the contact between the cathode and the SnO2 film, and causes non-ohmicity at the interface between the cathode and the SnO2 film. Is shown. The characteristics shown in FIGS. 6 to 11 are obtained by the addition of impurities, and the sensor 2 of Example 1 has high durability against contamination caused by artificially added Na ions. Similarly, the SnO 2 film 6 has the characteristics shown in FIGS. In addition, durability against metal ions from the electrodes 8, 10 or metal ions from a catalyst or a binder is also high.
[0023]
[Example 2]
The gas sensor 62 of Example 2 is shown in FIG. The cathode 68 was made into a large U shape, the inside was semicircular, and the anode 70 that protruded small was surrounded. At this time, the creepage length ratio L1 / L2 is 2.35, and the contact area ratio S1 / S2 is 6.35, which improves the contact between the cathode 68 and the SnO 2 film 6. However, in the pattern of FIG. 12, since the anode 70 is not L-shaped, the characteristics change due to the pattern deviation from the SnO 2 film 6.
[0024]
Although the specific cathode and anode patterns are shown in the embodiments, the present invention is not limited to these. In the present invention, the ratio of the contact area S1 between the cathode and the SnO 2 film and the contact area S2 between the anode and the SnO 2 film is selected, and the ratio of the creeping length between the cathode side and the anode side is selected, so that Generation of ohmic characteristics can be suppressed. For this reason, a gas sensor having high durability against impurities can be obtained. Further, by making both the cathode and the anode face each other as an L-shaped pattern, even if the SnO 2 film and the electrode are misaligned, the influence on the characteristics can be minimized.
[Brief description of the drawings]
FIG. 1 is a diagram showing a front surface and a back surface of a gas sensor according to a first embodiment. FIG. 2 is a diagram showing creepage lengths L1 and L2 and contact areas S1 and S2 in the first embodiment. FIG. 4 is a plan view showing an electrode pattern of a gas sensor of Conventional Example 1. FIG. 5 is a plan view showing an electrode pattern of a gas sensor of Conventional Example 2. FIG. FIG. 7 is a characteristic diagram showing the voltage / current characteristics of the gas sensor, VS is the voltage applied to the sensor, Is is the sensor current. FIG. 8 is a characteristic diagram showing voltage / current characteristics of the gas sensor of Conventional Example 1. FIG. 9 is a gas sensor of Conventional Example 1. Characteristic diagram showing the voltage / resistance characteristics in FIG. 10 [Example] FIG. 11 is a characteristic diagram showing voltage / current characteristics of the gas sensor of Example 1. FIG. 12 is a plan view showing electrode patterns of the gas sensor of Example 2. Explanation of]
2,62 Gas sensor 4 Substrate 6 SnO2 film 8, 68 Cathode 8a, 8b Exposed part 9 R part 10, 70 Anode 12 Through hole 14 Heater film 16 Heater electrode 18, 20 Electrode pad 22 Lead 38, 48 Cathode 50 Anode 45 Edge L1 Cathode creepage length L2 Anode creepage length S1 Cathode contact area S2 Anode contact area

Claims (4)

In the gas sensor in which the SnO2 film, the cathode and the anode are provided on the insulating substrate, the ratio S1 / S2 between the contact area S1 between the cathode and the SnO2 film and the contact area S2 between the anode and the SnO2 film is set to 3.5 or more, and SnO2 The ratio L1 / L2 between the creeping length L1 of the cathode end along the current path from the film to the cathode and the creeping length L2 of the anode end along the current path from the anode to the SnO2 film is 1.5 or more. A gas sensor characterized by that. The gas sensor according to claim 1, wherein the ratio S1 / S2 of the contact area is 4 or more. The cathode and the anode are both formed in an L shape, one side outside the L shape of the anode is opposed to one side inside the L shape of the cathode, and two sides outside the L shape of the cathode are 2. The gas sensor according to claim 1, wherein the gas sensor is exposed from the SnO2 film. In a gas detection method in which an SnO 2 film, a cathode and an anode are provided on an insulating substrate, and gas is detected from a current flowing from the anode to the cathode,
The ratio S1 / S2 between the contact area S1 between the cathode and the SnO2 film and the contact area S2 between the anode and the SnO2 film is 3.5 or more, and the creeping length L1 of the cathode end along the current path from the SnO2 film to the cathode And the ratio L1 / L2 of the creeping length L2 at the end of the anode along the current path from the anode to the SnO2 film is 1.5 or more, so that the current density is increased on the anode side and lowered on the cathode side. A gas detection method characterized by suppressing generation of a non-ohmic characteristic of the current with respect to a voltage between an anode and a cathode.

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