Gas sensor

A gas sensor that is unlikely to have Au evaporation from an external electrode even when used under a high temperature atmosphere is provided. The gas sensor includes a sensor element mainly made of an oxygen-ion conductive solid electrolyte; an external electrode provided on the sensor element and containing a Pt—Au alloy; and an electrode evaporation preventing film provided on the sensor element while being insulated from the sensor element and separated from the external electrode, and made of Au or a Pt—Au alloy having an Au composition ratio not smaller than an Au composition ratio of the Pt—Au alloy contained in the external electrode. A protection cover is provided so that at least part of the sensor element, at which the external electrode and the electrode evaporation preventing film is positioned, is inside the protection cover, and so that a measurement gas is introduced inside the protection cover.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a gas sensor for sensing a predetermined gas component in a measurement gas, and particularly relates to suppression of degradation of an electrode of the gas sensor.

Description of the Background Art

Conventionally widely known gas sensors are configured to sense a predetermined target gas component in a measurement gas, such as an exhaust gas from the engine of an automobile. Among such gas sensors, a gas sensor whose sensing electrode (measurement electrode) for sensing the target gas component includes a Pt—Au alloy have been publicly known (refer to Japanese Patent No. 4914447, Japanese Patent Application Laid-Open No. 2012-211928, and Japanese Patent No. 5883976, for example).

A large number of exhaust gas sensors each comprising a measurement electrode including a Pt—Au alloy have been disclosed.

For example, an exhaust gas from the engine of an automobile reaches at a high temperature of 900° C. approximately. When a conventionally known gas sensor whose sensing electrode formed of a Pt—Au alloy, as disclosed in Japanese Patent No. 4914447, Japanese Patent Application Laid-Open No. 2012-211928, and Japanese Patent No. 5883976, is used in an atmosphere at such a high temperature, there arises a problem that Au evaporates and the sensing electrode becomes Pt-riched because of the high temperature, which is not largely different from the melting point of Au (1064° C.). When the gas sensor is continuously used in such a high temperature atmosphere, its measurement accuracy degrades with time. Thus, each of the gas sensors disclosed in Japanese Patent No. 4914447, Japanese Patent Application Laid-Open No. 2012-211928, and Japanese Patent No. 5883976 has a problem that it has short product lifetime.

SUMMARY

The present invention relates to a gas sensor for sensing a predetermined gas component in a measurement gas, and is particularly directed to suppression of degradation of an electrode of the gas sensor.

According to the present invention, a gas sensor configured to measure a concentration of a predetermined gas component in a measurement gas includes: a sensor element mainly made of an oxygen-ion conductive solid electrolyte; at least one external electrode provided on the sensor element and containing a Pt—Au alloy; an electrode evaporation preventing film provided on the sensor element while being insulated from the sensor element and separated from the at least one external electrode, and made of Au or a Pt—Au alloy having an Au composition ratio not smaller than an Au composition ratio of the Pt—Au alloy contained in the at least one external electrode; and a protection cover provided so that at least part of the sensor element is positioned inside the protection cover and the measurement gas is introduced inside the protection cover, the at least one external electrode and the electrode evaporation preventing film being provided on the at least part of the sensor element.

The present invention achieves a gas sensor in which Au evaporation from a sensing electrode made of a Pt—Au alloy is suppressed so that degradation is unlikely to occur even when the gas sensor is continuously used under a high temperature atmosphere.

Thus, the present invention is intended to provide a gas sensor that is unlikely to have Au evaporation from an external electrode made of a Pt—Au alloy even when used under a high temperature atmosphere.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

<Outline of Gas Sensor>

FIGS. 1A and 1Bare schematic sectional views schematically illustrating a configuration of a gas sensor100according to an exemplary preferred embodiment of the present invention.FIG. 1Ais a vertical sectional view of a sensor element101, which is a main component of the gas sensor100, taken along the longitudinal direction of the sensor element101.FIG. 1Bis a view including a section vertical to the longitudinal direction of the sensor element101at position A-A′ ofFIG. 1A.

The gas sensor100is a so-called mixed-potential gas sensor. Generally speaking, the gas sensor100determines the concentration of a gas component, which is a measurement target, in a measurement gas using a potential difference that occurs between a sensing electrode10, which is an external electrode provided on the surface of the sensor element101mainly made of ceramic that is an oxygen-ion conductive solid electrolyte such as zirconia (ZrO2), and a reference electrode20, which is provided inside the sensor element101, due to a difference in the concentration of the gas component between the portions near the electrodes based on the principle of mixed potential.

More specifically, the gas sensor100preferably determines a target gas component in a measurement gas, where the measurement gas is an exhaust gas present in an exhaust pipe of an internal combustion engine such as a diesel engine or a gasoline engine. As the target gas component, a hydrocarbon (HC) gas, a carbon monoxide (CO) gas, and an ammonia (NH3) gas are exemplified. Preferably, the gas sensor100is configured to excellently sense only a particular target gas component by changing the composition of the sensing electrode10, the structure (for example, the porosity of a protective layer) or a control manner (for example, a temperature control manner) of the gas sensor100and the like.

The sensor element101mainly includes an insulating layer11, an electrode evaporation protective film12, a reference gas introduction layer30, a reference gas introduction space40, and a protective layer50, in addition to the sensing electrode10and the reference electrode20described above.

In the present preferred embodiment, the sensor element101has the structure in which six layers, namely, a first solid electrolyte layer1, a second solid electrolyte layer2, a third solid electrolyte layer3, a fourth solid electrolyte layer4, a fifth solid electrolyte layer5, and a sixth solid electrolyte layer6, each formed of an oxygen-ion conductive solid electrolyte, are laminated in the stated order from the bottom side ofFIGS. 1A and 1B. The sensor element101additionally includes other components mainly between these layers or on an outer peripheral surface of the element. The solid electrolytes constituting these six layers are fully airtight. Such a sensor element101is manufactured by, for example, laminating ceramic green sheets corresponding to the individual layers, which have been subjected to a predetermined process and printing of a circuit pattern, and further, by integrating the laminated layers through firing.

The gas sensor100does not necessarily need to include the sensor element101formed of such a laminated body including the six layers. The sensor element101may be formed as a laminated body having more or fewer layers or may not have a laminated structure.

In the following description, for convenience' sake, the surface located as the upper surface of the sixth solid electrolyte layer6inFIGS. 1A and 1Bis referred to as a front surface Sa of the sensor element101, and the surface located as the lower surface of the first solid electrolyte layer1inFIGS. 1A and 1Bis referred to as a rear surface Sb of the sensor element101. In the determination of the concentration of the target gas component in a measurement gas with the gas sensor100, a predetermined range starting from a distal end E1being one end of the sensor element101, which includes at least the sensing electrode10, is disposed in a measurement gas atmosphere; the other portion including a base end E2opposite to the distal end E1is disposed so as not to be in contact with the measurement gas atmosphere.

The sensing electrode10is an electrode for sensing a measurement gas. The sensing electrode10is formed as a porous cermet electrode made of Pt containing a predetermined ratio of Au, namely, Pt—Au alloy and zirconia. The sensing electrode10is provided in a substantially rectangular shape in plan view to have a thickness of 5 μm or more and 30 μm or less, at a position closer to the distal end E1that is one end in the longitudinal direction of the sensor element101on the front surface Sa of the sensor element101.

The gas sensor100is placed such that, in its use, at least the portion in which the sensing electrode10is provided is exposed to a measurement gas. In more detail, in the gas sensor100, while a protection cover102, which is not illustrated inFIGS. 1A and 1B(refer toFIGS. 2A and 2B), surrounds the sensor element101, the protection cover102is provided with gas introduction holes (a distal end gas introduction hole H1and a side gas introduction hole H2(refer toFIGS. 2A and 2B)) for allowing the measurement gas to flow into and out of the protection cover102, so that the sensing electrode10contacts the measurement gas in the protection cover102.

The catalytic activity of the sensing electrode10against a target gas component is inactivated in a predetermined concentration range by suitably determining the composition of the Pt—Au alloy being its constituent material. That is, the decomposition reaction of the target gas component is prevented or reduced in the sensing electrode10. Thus, in the gas sensor100, the potential of the sensing electrode10selectively varies with respect to (has correlation with) the target gas component in the predetermined concentration range in accordance with the concentration thereof. In other words, the sensing electrode10is provided so as to have high concentration dependence of the potential for the target gas component in the predetermined concentration range while having low concentration dependence of the potential for other components of the measurement gas.

More specifically, in the sensor element101of the gas sensor according to the present preferred embodiment, with an Au abundance ratio on the surfaces of Pt—Au alloy particles included in the sensing electrode10being suitably determined, the sensing electrode10is provided so that the potential thereof has noticeable dependence on the concentration of the target gas component in a predetermined concentration range.

In this specification, the Au abundance ratio means an area ratio of a portion covered with Au to a portion at which Pt is exposed in the surface of noble metal particles included in the sensing electrode10. In this specification, the Au abundance ratio is calculated from an expression shown below using Au and Pt detection values in an Auger spectrum obtained by performing Auger electron spectroscopy (AES) analysis on the surface of the noble metal particles.
Au abundance ratio=Au detection value/Pt detection value  (1)

The Au abundance ratio is one when the area of the portion at which Pt is exposed and the area of the portion covered with Au are equal to each other.

The Au abundance ratio can also be calculated using a relative sensitivity coefficient method from a peak intensity of a peak detected for Au and Pt, which is obtained by subjecting the surface of the noble metal particles to X-ray photoelectron spectroscopy (XPS) analysis. The value of the Au abundance ratio obtained by this method can be considered to be substantially the same as the value of the Au abundance ratio calculated based on the result of AES analysis.

The reference electrode20is an electrode having a substantially rectangular shape in plan view, which is provided inside the sensor element101and serves as a reference in the determination of the concentration of the measurement gas. The reference electrode20is provided as a porous cermet electrode of Pt and zirconia.

It suffices that the reference electrode20has a porosity of 10% or more and 30% or less and a thickness of 5 μm or more and 15 μm or less. The plane size of the reference electrode20may be smaller than that of the sensing electrode10as illustrated inFIGS. 1A and 1B, or may be equal to that of the sensing electrode10.

The insulating layer11and the electrode evaporation preventing film12are laminated in this order on the surface Sa of the sensor element101.

The insulating layer11is an underlayer of the electrode evaporation preventing film12, which is formed of alumina and provided to electrically insulate the electrode evaporation preventing film12from any other part of the sensor element101. It is not preferable to provide the electrode evaporation preventing film12in contact with the sixth solid electrolyte layer6without the insulating layer11because the electrode evaporation preventing film12would act as an electrode same as the sensing electrode10. In addition, when being in contact with the sensing electrode10, the electrode evaporation preventing film12would be an electrode integrated with the sensing electrode10. To avoid this, the insulating layer11(and the electrode evaporation preventing film12) is provided separately from the sensing electrode10. The insulating layer11may have a thickness of 10 μm or more and 35 μm or less.

The electrode evaporation preventing film12is provided to prevent Au evaporation from the sensing electrode10. The electrode evaporation preventing film12is formed of Au or a Pt—Au alloy that is Au-riched (has a large Au composition ratio) same as or more than the Pt—Au alloy constituting the sensing electrode10, and is formed on the insulating layer11. The electrode evaporation preventing film12has a plane shape substantially identical to that of the insulating layer11and a thickness of 5 μm or more and 30 μm or less.

Preferably, the Pt—Au alloy constituting the electrode evaporation preventing film12is more Au-riched than the Pt—Au alloy constituting in the sensing electrode10by 20 wt % or higher. In this case, Au evaporation from the sensing electrode10is more reliably prevented. The Au composition ratio of the Pt—Au alloy constituting the electrode evaporation preventing film12, which is hereinafter simply referred to as an Au composition of the electrode evaporation preventing film12, has an upper limit of 100 wt % when the electrode evaporation preventing film12is made of Au. Thus, in the present preferred embodiment, the description that the electrode evaporation preventing film12is formed of a Pt—Au alloy includes the case in which the electrode evaporation preventing film12is formed of Au only.

When the sensing electrode10and the electrode evaporation preventing film12are formed through screen printing and then integral firing (co-firing) of the solid electrolyte layers and the electrodes as described later, at least the Au composition of the electrode evaporation preventing film12is preferably 60 wt % or lower. When the Au composition is excessively large, the sensing electrode10and the electrode evaporation preventing film12melt during the firing because the melting point (1064° C.) of Au is lower than a firing temperature, which is not preferable. Since the electrode evaporation preventing film12has a composition same as that of the sensing electrode10or a composition more Au-riched than that of the sensing electrode10by 20 wt % or higher, the sensing electrode10is excellently formed when the composition of the electrode evaporation preventing film12is in a range of excellent formation.

InFIG. 1A, the electrode evaporation preventing film12(and the insulating layer11) is provided at a position closer to the leading end part E1by a predetermined distance than the sensing electrode10in an element longitudinal direction that is the horizontal direction inFIG. 1A, but this configuration is merely exemplary.

The electrode evaporation preventing film12will be described later in detail.

The reference gas introduction layer30is a layer made of porous alumina, which is provided inside the sensor element101to cover the reference electrode20. The reference gas introduction space40is an internal space provided near the base end E2of the sensor element101. Air (oxygen), serving as a reference gas in the determination of the concentration of the target gas component, is externally introduced into the reference gas introduction space40.

The reference gas introduction space40and the reference gas introduction layer30are in communication with each other, and accordingly, in the use of the gas sensor100, the surrounding of the reference electrode20is always filled with air (oxygen) through the reference gas introduction space40and the reference gas introduction layer30. During the use of the gas sensor100, thus, the reference electrode20always has a constant potential.

The reference gas introduction space40and the reference gas introduction layer30are provided so as not to come into contact with a measurement gas owing to their surrounding solid electrolytes. This prevents the reference electrode20from coming into contact with the measurement gas even when the sensing electrode10is exposed to the measurement gas.

In the case illustrated inFIG. 1A, the reference gas introduction space40is provided in such a manner that part of the fifth solid electrolyte layer5is in communication with the exterior on the base end E2of the sensor element101. The reference gas introduction layer30is provided so as to extend in the longitudinal direction of the sensor element101between the fifth solid electrolyte layer5and the sixth solid electrolyte layer6.

The protective layer50is a porous layer made of alumina being an insulating material, which is provided so as to cover at least the sensing electrode10on the front surface Sa of the sensor element101. The protective layer50is provided as an electrode protective layer that prevents or reduces the degradation of the sensing electrode10due to continuous exposure to a measurement gas during the use of the gas sensor100. In the case illustrated inFIG. 1A, the protective layer50is provided so as to cover not only the sensing electrode10but also the electrode evaporation preventing film12(and the insulating layer11).

The protective layer50may be provided so as to have a thickness of 10 μm to 50 μm, and may have a pore size of 1 μm or less. The porosity of the protective layer50is preferably 5% or more and 40% or less. It is not preferable that the porosity is less than 5% because the measurement gas does not preferably arrive at the sensing electrode10, and thus the responsiveness of the gas sensor100deteriorates. Such a porosity is not preferable also from the viewpoint that a formation of an Au saturated vapor field, as be described later, due to Au evaporation from the electrode evaporation preventing film12is prevented. It is not preferable that the porosity is more than 40% because a poisoning substance easily sticks to the sensing electrode10and the electrode evaporation preventing film12, and thus a function of protecting the sensing electrode10and the electrode evaporation preventing film12cannot be sufficiently performed.

When the gas sensor100is used as an ammonia sensor, the protective layer50having the porosity of 40% or less also exhibits such an effect that influences of interference from other gas components can be suppressed, as described later.

In the present preferred embodiment, the porosity is evaluated by analyzing an enlarged cross-sectional SEM image (secondary electron image) (by referencing descriptions in Nobuyasu Mizutani et. al, “Ceramic Processing” (GIHODO SHUPPAN Co., Ltd.)).

As illustrated inFIG. 1B, the gas sensor100is equipped with a potentiometer60capable of measuring a potential difference between the sensing electrode10and the reference electrode20. AlthoughFIG. 1Bschematically illustrates wiring of the sensing electrode10, the reference electrode20, and the potentiometer60, in an actual sensor element101, connection terminals (not shown) are provided correspondingly to the respective electrodes on the front surface Sa or the rear surface Sb on the base end E2side, and wiring patterns (not shown), which connect the respective electrodes and their corresponding connection terminals, are formed on the front surface Sa and inside the element. The sensing electrode10and the reference electrode20are electrically connected with the potentiometer60through the wiring patterns and the connection terminals. Hereinafter, a potential difference between the sensing electrode10and the reference electrode20, which is measured by the potentiometer60, is also referred to as a sensor output.

The sensor element101further includes a heater part70, which performs temperature control of heating the sensor element101and maintaining the temperature of the sensor element101, to enhance the oxygen ion conductivity of the solid electrolyte. The heater part70includes a heater electrode71, a heater72, a through hole73, a heater insulating layer74, and a pressure diffusion hole75.

The heater electrode71is an electrode formed while being in contact with the rear surface Sb of the sensor element101(inFIGS. 1A and 1B, the lower surface of the first solid electrolyte layer1). The heater part70is electrically connected with an external power supply80, so that it can be powered from the external power supply80through the heater electrode71.

The heater72is an electric resistor provided inside the sensor element101. The heater72is connected with the heater electrode71through the through hole73and generates heat by being powered externally via the heater electrode71to heat the solid electrolytes forming the sensor element101and maintain their temperatures.

In the case illustrated inFIGS. 1A and 1B, the heater72is buried while being vertically sandwiched between the second solid electrolyte layer2and the third solid electrolyte layer3so as to extend from the base end E2to the position below the sensing electrode10near the distal end E1. The value of a voltage applied to the heater72by the external power source80is appropriately controlled by control means (not shown) to flow a heater current according to a desired temperature, thereby enabling the adjustment of the entire sensor element101to the temperature at which the solid electrolytes are activated.

The heater insulating layer74is an insulating layer formed of an insulator such as alumina on the upper and lower surfaces of the heater72. The heater insulating layer74is formed for electrical insulation between the second solid electrolyte layer2and the heater72and for electrical insulation between the third solid electrolyte layer3and the heater72.

The pressure diffusion hole75is a part provided to penetrate the third solid electrolyte layer3and the fourth solid electrolyte layer4and to be in communication with the reference gas introduction space40, and is formed to mitigate an internal pressure rise associated with a temperature rise in the heater insulating layer74.

In the determination of the concentration of the target gas component in a measurement gas using the gas sensor100having such a configuration, as described above, air (oxygen) is supplied to the reference gas introduction space40, with the sensor element101in only a predetermined range, which starts from the distal end E1and includes at least the sensing electrode10, being disposed in a space inside the protection cover102containing a measurement gas, and with the sensor element101on the base end E2being apart from the space. The heater72heats the sensor element101to a predetermined temperature of 400° C. or higher and 800° C. or lower, which is set in accordance with the kind of the target gas component. The temperature of the sensor element101being heated by the heater72is also referred to as an element control temperature. In this preferred embodiment, the element control temperature is evaluated from the surface temperature of the sensing electrode10. The surface temperature of the sensing electrode10can be evaluated by infrared thermography.

In a state described above, a potential difference occurs between the sensing electrode10exposed to the measurement gas and the reference electrode20exposed to the air. As described above, however, the potential of the reference electrode20disposed in the air (having a constant oxygen concentration) atmosphere is maintained at a constant potential, whereas the potential of the sensing electrode10selectively has a dependence on concentration for the target gas component in the measurement gas. The potential difference (sensor output) is thus substantially a value according to the composition of the measurement gas present around the sensing electrode10. Therefore, a certain functional relationship (referred to as sensitivity characteristics) holds between the concentration of the target gas component and the sensor output. In the description below, such sensitivity characteristics may also be referred to as, for example, sensitivity characteristics for the sensing electrode10.

In the actual determination of the concentration of the target gas component, in advance, a plurality of different mixed gases, each of which has a known concentration of the target gas component, are used as the measurement gas, and the sensitivity characteristics are experimentally identified by performing a measurement on the sensor output for each measurement gas. In the actual use of the gas sensor100, accordingly, an operation processor (not shown) converts the sensor output, which varies from moment to moment in accordance with the concentration of the target gas component in a measurement gas, into the concentration of the target gas component based on the sensitivity characteristics. The concentration of the target gas component in the measurement gas can thus be determined almost in real time.

<Suppression of Au Evaporation by Electrode Evaporation Preventing Film>

FIGS. 2A and 2Bare each diagrams for description of suppression of Au evaporation from the sensing electrode10, which is achieved by the electrode evaporation preventing film12included in the sensor element101.FIG. 2Ais a sectional pattern diagram in the case that the gas sensor100including no electrode evaporation preventing film12in the sensor element101is placed in a measurement gas atmosphere at high temperature (for example, 900° C. approximately).FIG. 2Bis a sectional pattern diagram in the case that the gas sensor100according to the present preferred embodiment including the electrode evaporation preventing film12in the sensor element101is placed under the same condition. However, for simplification of description, the protective layer50is omitted in the illustrations. In each of the cases, the sensor element101itself is heated to the element control temperature of 400° C. or higher and 800° C. or lower by the heater72provided inside.

As illustrated inFIGS. 2A and 2B, in the gas sensor100, the sensor element101is surrounded by the tubular (for example, cylindrical) protection cover102. InFIGS. 2A and 2B, the protection cover102is provided with the distal end gas introduction hole H1at a distal end part thereof, which is positioned further on the left side of the leading end part E1of the sensor element101inFIGS. 2A and 2B, and the side gas introduction holes H2at side thereof, which is positioned above and below the sensor element101. These gas introduction holes allow a measurement gas G to flow into and out of the protection cover102therethrough.

The left side of the protection cover102is open for simplification of illustration inFIGS. 2A and 2B, but in reality, the protection cover102has a bottom, and the distal end gas introduction hole H1is provided at part of the bottom. The side gas introduction holes H2are typically provided at a plurality of places equally spaced from each other in the circumferential direction of the protection cover102. In addition, the side gas introduction holes H2may be provided at multiple stages in the longitudinal direction of the protection cover102(the horizontal direction inFIGS. 2A and 2B).

In the case that the sensor element101includes no electrode evaporation preventing film12as illustrated inFIG. 2A, Au evaporation indicated by arrow AR1occurs from the sensing electrode10contacting the measurement gas G at high temperature, and an Au saturated vapor field is formed near the sensing electrode10, whereas the composition of the sensing electrode10becomes Pt-riched. However, since the measurement gas G constantly flows into the protection cover102and continuously replaces an atmosphere inside the protection cover102little by little, the Au evaporation continuously occurs. Thus, the Au evaporation is not suppressed by formation of the Au saturated vapor field.

In the gas sensor100according to the present preferred embodiment illustrated inFIG. 2B, the measurement gas G at high temperature contacts not only the sensing electrode10but also the electrode evaporation preventing film12. As described above, the electrode evaporation preventing film12is formed of the Pt—Au alloy having a composition same as or more Au-riched than that of the sensing electrode10. Thus, in the gas sensor100, Au evaporation occurs also from the electrode evaporation preventing film12as indicated by arrow AR2. The evaporation from the electrode evaporation preventing film12is more dominant than Au evaporation from the sensing electrode10indicated by arrow AR3as the electrode evaporation preventing film12is more Au-riched. In the gas sensor100, too, an Au saturated vapor field is formed near the sensing electrode10due to the Au evaporation from the electrode evaporation preventing film12and the sensing electrode10. However, unlike the case illustrated inFIG. 2A, Au evaporating from the electrode evaporation preventing film12contributes to formation of the Au saturated vapor field, and thus the Au evaporation from the sensing electrode10is suppressed as compared to the case illustrated inFIG. 2A. In other words, the Au evaporation from the sensing electrode10is suppressed by formation of the Au saturated vapor field through the Au evaporation from the electrode evaporation preventing film12. In particular, as the electrode evaporation preventing film12is more Au-riched, the effect of the suppression is more significant.

The effect of the suppression of Au evaporation is checked by, for example, temporarily measuring the sensor output. Specifically, if Au evaporation is excellently suppressed, no temporal variation occurs to the sensor output as long as the concentration of the target gas component in the measurement gas is constant. However, if the Au evaporation proceeds, the sensor output decreases with time. Alternatively, whether the effect is achieved may be determined by performing an accelerated degradation test to check the effect and comparing the sensitivity characteristic before and after the test.

In the case that the sensor element101is provided with no protective layer50, the effect can be checked by evaluating the Au abundance ratio at the surface of the sensing electrode10by a surface analysis method such as XPS.

Although the protective layer50is not illustrated inFIGS. 2A and 2B, also in the configuration in which the protective layer50is provided, the Au evaporation from the sensing electrode10and formation of the Au saturated vapor field occur if the sensor element101includes no electrode evaporation preventing film12, but the Au evaporation from the sensing electrode10is suppressed if the sensor element101includes the electrode evaporation preventing film12since the Au evaporation from the electrode evaporation preventing film12contributes to formation of the Au saturated vapor field, as described above. In the case that the protective layer50is provided, the Au saturated vapor field is formed from the inside of pores in the protective layer50to the outside of the protective layer50.

The above describes the mechanism of suppression of the Au evaporation from the sensing electrode10in the gas sensor100according to the present preferred embodiment, which is achieved by the sensor element101including the electrode evaporation preventing film12.

<Variation of Electrode Evaporation Preventing Film>

FIGS. 3A, 3B, 3C, 3D, and 3Eare diagrams exemplarily illustrating variations of the size, shape, and disposition of the electrode evaporation preventing film12in the sensor element101. More specifically,FIGS. 3A, 3B, 3C, 3D, and 3Eare top views of the sensor element101, illustrating variations of the disposition of the electrode evaporation preventing film12when the sensing electrode10is disposed at the same position. The protective layer50is omitted in the illustrations. In any of these cases, the insulating layer11having a plane shape same as that of the electrode evaporation preventing film12is provided directly below the electrode evaporation preventing film12.

FIGS. 3A, 3B, and 3Cillustrate cases in which the electrode evaporation preventing film12having a rectangular shape in plan view is provided closer to the distal end part E1in the element longitudinal direction than the sensing electrode10and has an area that is half, equal, and twice, respectively, the area of the sensing electrode10. However, the electrode evaporation preventing film12is disposed at the same distance (in-plane distance) from the sensing electrode10.FIG. 3Bcorresponds to the case exemplarily illustrated inFIGS. 1A and 1B. The in-plane distance means a shortest distance between the electrode evaporation preventing film12and the sensing electrode10in the plane of the sensor element101. In other words, the in-plane distance is not the distance between the gravity centers of the electrode evaporation preventing film12and the sensing electrode10.

FIG. 3Dillustrates a case in which the electrode evaporation preventing film12surrounds the sensing electrode10in plan view. In the case illustrated inFIG. 3D, the in-plane distance between the electrode evaporation preventing film12and the sensing electrode10, and the area ratio of the electrode evaporation preventing film12relative to the sensing electrode10are both smaller than those in the cases illustrated inFIGS. 3A, 3B, and 3C.

In the case illustrated inFIG. 3E, the electrode evaporation preventing film12having a rectangular shape in plan view and having an area same as that in the case illustrated inFIG. 3Bis disposed on a side opposite to the distal end part E1across the sensing electrode10(that is, a side closer to the base end part E2) in the element longitudinal direction, and the in-plane distance between the electrode evaporation preventing film12and the sensing electrode10is larger than that in the cases illustrated inFIGS. 3A, 3B, and 3C.

As illustrated inFIGS. 3A, 3B, 3C, 3D, and 3E, the area and shape of the electrode evaporation preventing film12, the area ratio of the electrode evaporation preventing film12relative to the sensing electrode10, and the in-plane distance between the electrode evaporation preventing film12and the sensing electrode10may differ and are not limited as long as the Au evaporation from the sensing electrode10is excellently suppressed. Although not illustrated, since the area of the sensing electrode10may differ in accordance with, for example, a characteristic required for the gas sensor100, the area of the electrode evaporation preventing film12may be set accordingly as appropriate.

In any of the cases illustrated inFIGS. 3A, 3B, 3C, 3D, and 3E, the sensing electrode10and the electrode evaporation preventing film12are covered by the protective layer50. The sensing electrode10and the electrode evaporation preventing film12may be covered by different protective layers50, in the case that they are separated from each other with a relatively large in-plane distance therebetween as in the case illustrated inFIG. 3E, for example.

FIGS. 4, 5A, and 5Bare diagrams exemplarily illustrating other different variations of the electrode evaporation preventing film12. InFIGS. 5A and 5B, the insulating layer11is not illustrated. In reality, the insulating layer11is interposed between the sensor element101and the electrode evaporation preventing film12.

In the sensor element101whose vertical cross-sectional view in the longitudinal direction is illustrated inFIG. 4, the electrode evaporation preventing film12is provided on the protective layer50covering the sensing electrode10to protect the sensing electrode10. Thus, the in-plane distance between the electrode evaporation preventing film12and the sensing electrode10is zero in this case. The shortest distance between the electrode evaporation preventing film12and the sensing electrode10is equal to the thickness of the protective layer50.

Also in the case illustrated inFIG. 4, the Au evaporation from the electrode evaporation preventing film12is more likely to occur than the Au evaporation from the sensing electrode10, and the Au saturated vapor field is formed around the sensing electrode10mainly due to the Au evaporation from the electrode evaporation preventing film12. The Au evaporation from the sensing electrode10is suppressed accordingly. The protective layer50, which is made of alumina as an insulation material, serves as the insulating layer11. In other words, the insulating layer11does not need to be formed in the case illustrated inFIG. 4.

Meanwhile, in the sensor element101whose vertical cross-sectional view in the longitudinal direction is illustrated inFIG. 5A, the electrode evaporation preventing film12is formed on the back surface Sb of the sensor element101. Such a disposition is applicable. Also in this case, the area and shape of the electrode evaporation preventing film12, the area ratio of the electrode evaporation preventing film12relative to the sensing electrode10, and the in-plane distance between the electrode evaporation preventing film12and the sensing electrode10may be set as appropriate as long as the Au evaporation from the sensing electrode10is excellently suppressed by formation of the Au saturated vapor field mainly due to the Au evaporation from the electrode evaporation preventing film12. Although no protective layer50is illustrated inFIG. 5A, the electrode evaporation preventing film12and the sensing electrode10may be covered by different protective layers50in this case.

Alternatively, as illustrated inFIG. 5B, the electrode evaporation preventing film12and the sensing electrode10may be collectively covered by a distal end protective film51provided at the distal end part E1of the sensor element101.

InFIGS. 1A and 1B, the sensing electrode10and the electrode evaporation preventing film12as external electrodes are provided on the surface Sa of the gas sensor100, which is the upper surface of the sixth solid electrolyte layer6. Alternatively, when an external electrode made of a Pt—Au alloy is provided on the back surface Sb or a side surface of the gas sensor100, the electrode evaporation preventing film12may be provided in a corresponding manner to the external electrode.

<Process of Manufacturing Sensor Element>

The following describes a process of manufacturing the sensor element101using the configuration exemplarily illustrated inFIGS. 1A and 1Bas an example. Generally speaking, the sensor element101as illustrated inFIGS. 1A and 1Bis manufactured by forming a laminated body formed of green sheets containing an oxygen-ion conductive solid electrolyte such as zirconia as a ceramic component and by cutting and firing the laminated body. The oxygen-ion conductive solid electrolyte may be, for example, yttrium partially stabilized zirconia (YSZ).

FIG. 6is a flowchart illustrating the process of manufacturing the sensor element101. In the manufacture of the sensor elements101A to101C, first, blank sheets (not shown) that are green sheets having no pattern formed thereon are prepared (step S1). Specifically, six blank sheets corresponding to the first to sixth solid electrolyte layers1to6are prepared. A plurality of sheet holes used for positioning in printing and lamination are provided in the blank sheets. Such sheet holes are formed in advance through, for example, punching by a punching machine. For a green sheet whose corresponding layer forms an internal space, a penetration corresponding to the internal space is also provided in advance through, for example, punching as described above. All the blank sheets corresponding to the respective layers of the sensor element101need not to have the same thickness.

After the preparation of the blank sheets corresponding to the respective layers, pattern printing and drying are performed to form various patterns on the individual blank sheets (step S2). Specifically, electrode patterns for the sensing electrode10, the reference electrode20, and the like, patterns for the insulating layer11and the electrode evaporation preventing film12, the reference gas introduction layer30, patterns for the heater72and the heater insulating layer74, and internal wiring (not shown) are formed. In this case, the patterns for the insulating layer11and the electrode evaporation preventing film12may be set in accordance with the various kinds of dispositions exemplarily illustrated inFIGS. 3A, 3B, 3C, 3D, 3E, 4, 5A, and 5B.

Each pattern is printed by applying a paste for pattern formation, prepared in accordance with the characteristic required for each formation target, to the blank sheet by a known screen printing technique. Any known drying means is available for drying after printing.

After the pattern printing, printing and drying of a bonding paste are performed to laminate and bond the green sheets corresponding to the respective layers (step S3). Any known screen printing technique is available for printing of a bonding paste, and any known drying means is available for drying after printing.

Subsequently, crimping is performed in which the adhesive-applied green sheets are laminated in a predetermined order, and the laminated green sheets are crimped on the predetermined temperature and pressure conditions, to thereby form a laminated body (step S4). Specifically, green sheets that are lamination targets are laminated while being positioned at the sheet holes to be held in a predetermined lamination jig (not shown), and the green sheets together with the lamination jig are heated and pressurized by a lamination machine such as a known hydraulic pressing machine. The pressure, temperature, and time for heating and pressurizing depend on a lamination machine to be used, whose conditions may be set appropriately for good lamination.

After the laminated body has been obtained as described above, subsequently, a plurality of parts of the laminated body are cut out as individual units (referred to as element bodies) of the sensor element101(step S5). The cut out element bodies are fired under predetermined conditions, thereby producing the sensor element101as described above (step S6). In other words, the sensor element101is produced by co-firing the solid electrolyte layers and the electrodes. The firing temperature is preferably 1200° C. or higher and 1500° C. or lower (for example, 1400° C.). The integral firing performed in such a manner provides satisfactory adhesion strength to the respective electrodes of the sensor element101.

The resultant sensor element101is housed in a predetermined housing and incorporated into main bodies (not shown) of the gas sensor100. Subsequently, the protection cover102and any other component are attached, thereby to obtain the gas sensor100.

As described above, the present preferred embodiment achieves a gas sensor in which Au evaporation from a sensing electrode made of a Pt—Au alloy is suppressed so that degradation is unlikely to occur even when the gas sensor is continuously used under a high temperature atmosphere.

As described above, in the case that the electrode evaporation preventing film12made of a Pt—Au alloy is formed through screen printing and co-firing, the Au composition is preferably 60 wt % or lower due to the melting point of Au. However, if the electrode evaporation preventing film12is formed by other methods, the electrode evaporation preventing film12having an Au composition exceeding 60 wt % can be formed. Specifically, a method of manufacturing a laminated body and further the fired body therefrom without formation of the electrode evaporation preventing film12, and then forming the electrode evaporation preventing film12with respect to the fired body is considered. For example, so-called secondary firing that is a method of forming the pattern of the electrode evaporation preventing film by screen printing and then performing firing again or a method of forming the electrode evaporation preventing film12by plating may be used.

In the above preferred embodiment, suppression of Au evaporation from an electrode provided on the surface of a sensor element in a mixed-potential type gas sensor is described as an example. However, the suppression of Au evaporation from an electrode by providing an electrode evaporation preventing film is applicable to a gas sensor of other type such as a limiting current type gas sensor (for example, an NOx sensor).

In the above-described preferred embodiment, the gas sensor100includes only the single sensing electrode10as an external electrode made of a Pt—Au alloy. However, the gas sensor100may include a plurality of such external electrodes. In this case, the electrode evaporation preventing film12may be provided individually for each external electrode, or the single electrode evaporation preventing film12may be provided to suppress Au evaporation from the external electrodes.

EXAMPLES

Fabrication of the sensor element101was tried by forming the electrode evaporation preventing film12through screen printing and co-firing. Specifically, fabrication of a total of 15 of different sensor elements101(No. 1 to 15) was tried. In the sensor elements101, the electrode evaporation preventing film12was disposed as in the cases illustrated inFIGS. 3A, 3B, and 3Cwith the Au composition ratio of the electrode evaporation preventing film12being set to five levels of 100 wt %, 60 wt %, 50 wt %, 30 wt %, and 10 wt %, and the area of the electrode evaporation preventing film12being set to three levels of 3.9 mm2, 7.8 mm2, and 15.6 mm2. Formation of the protective layer50was omitted.

Each sensor element101had a length (size in the horizontal direction inFIG. 1A) of 97.5 mm, a width (size in the horizontal direction inFIG. 1B) of 4.25 mm, and a thickness (size in the vertical direction inFIGS. 1A and 1B) of 1.2 mm.

The sensing electrode10in each sensor element101had an Au composition of 10 wt % in a Pt—Au alloy and had a rectangular shape with an area of 7.8 mm2in plan view. The sensing electrode10was disposed at such a position that the center of gravity thereof is positioned at 8.0 mm from the distal end part E1. The distance between (in-plane distance) the electrode evaporation preventing film12and the sensing electrode10was constant at 0.46 mm between all sensor elements101. The firing temperature for obtaining a fired body was 1400° C.

Table 1 lists, for each of the 15 sensor elements101(No. 1 to 15), main formation conditions of the electrode evaporation preventing film (simply referred to as an evaporation preventing film in Table 1 and the following tables), the ratio (“evaporation preventing film/sensing electrode area” ratio) of the area of the evaporation preventing film relative to the area of the sensing electrode, the number of a drawing illustrating exemplary arrangement of the sensing electrode and the electrode evaporation preventing film in the sensor element, and success or failure (success: circle, failure: cross) of co-firing. The success or failure of co-firing was determined by observing a section of each sensor element101obtained by the firing. Specifically, in the case that Au in the electrode evaporation preventing film12did not reach at the solid electrolyte through the penetration into the insulating layer11, formation conditions of the sensor element101were determined to be conditions with which the electrode evaporation preventing film12can be formed by co-firing.

As indicated in Table 1, the formation of the electrode evaporation preventing film12by co-firing was not successful for the sensor elements101of No. 1 to 3 in which the Au composition of the electrode evaporation preventing film12ratio was 100 wt %, but the electrode evaporation preventing film12was successfully formed in the sensor elements101of No. 4 to 15 having an Au composition ratio of 60 wt % or lower.

The gas sensor100was assembled by using each of the sensor elements101of No. 4 to 15 in which it was confirmed that the electrode evaporation preventing film12was successfully formed in Example 1. The protection cover102has an inner diameter of 7.5 mm. An accelerated degradation test and evaluation of the sensitivity characteristics before and after the test were performed for each gas sensor100thus fabricated, and the level (evaporation prevention level) of Au evaporation suppress was evaluated by comparing those two sensitivity characteristics. As a comparative example, a gas sensor including no electrode evaporation preventing film12was fabricated, and the accelerated degradation test and evaluation of the evaporation prevention degree were performed.

The accelerated degradation test was performed in a manner that each gas sensor100was attached to an exhaust pipe of a gasoline engine (displacement: 1.8 L), and the gasoline engine was continuously operated for 60 hours under a periodic operating condition in a cycle of 30 minutes. The parameter λ was set to one. The element control temperature of the gas sensor100was set to 500° C.

FIG. 7is a diagram illustrating temporal change of an exhaust gas temperature in one cycle of operation of the gasoline engine used in the accelerated degradation test. As understood fromFIG. 7, the exhaust gas temperature periodically changed in the range of 400° C. to 850° C.

The evaluation of the sensitivity characteristic was performed with conditions below by using a plurality of kinds of model gasses containing a C2H4gas having a known concentration as the target gas component (including a case with 0 ppm).

The evaporation prevention level was evaluated by determining whether the ratio of the sensor output after the accelerated degradation test relative to the sensor output before the accelerated degradation test was (A) 90% or higher, (B) 70% or higher (lower than 90%), or (C) lower than 70% when the concentration of C2H4was 300 ppm.

To check the validity of the evaporation prevention level, a surface analysis by XPS was performed on the sensing electrode10and the electrode evaporation preventing film12(not in the comparative example) before and after the test for each of the gas sensors100of No. 4 to 6 and 13 to 15 and the gas sensor according to the comparative example to evaluate an Au ratio (Au/Au+Pt, unit: at %) in the Pt—Au alloy at each surface.

FIGS. 8 to 11are diagrams illustrating the sensitivity characteristics obtained before and after the accelerated degradation test for the gas sensors100of No. 4 to 15 according to the present example and the gas sensor according to the comparative example. The sensitivity characteristics of the comparative example are illustrated in all ofFIGS. 8 to 11.

Table 2 lists, for each of a total of 12 of the gas sensors100(No. 4 to 15) according to the present example and the gas sensor according to the comparative example, main formation conditions of the evaporation preventing film and the “evaporation preventing film/sensing electrode area” ratio (same as those listed in Table 1 except for the comparative example), a result of determination of the evaporation prevention level, and the Au ratios of the sensing electrode10and the electrode evaporation preventing film12before and after the test. In Table 2, the “Evaporation Prevention Level Determination” column has, based on the above-described determination, a double circle for (A), a circle for (B), and a cross for (C).

As for the comparative example, firstly, as understood fromFIGS. 8 to 11, the sensitivity characteristic before the test was substantially equivalent to those of the gas sensors100of No. 4 to 15, but no sensitivity characteristic was obtained after the test. Accordingly, the gas sensor100according to the comparative example was determined to have an evaporation prevention level of lower than 70% as indicated in Table 2.

However, as understood fromFIGS. 8 to 10, the gas sensors100of No. 4 to 12 in which the electrode evaporation preventing film12had an Au composition of 30 wt % or higher, which is larger than the Au composition of 10 wt % of the sensing electrode10by 20 wt % or more, had almost no difference in the sensitivity characteristic before and after the accelerated degradation test. As indicated in Table 2, these gas sensors100were each determined to have an evaporation prevention level of 90% or higher.

For the gas sensors100of No. 13 to 15 illustrated inFIG. 11in which the electrode evaporation preventing film12had an Au composition same as that of the sensing electrode10, the sensitivity characteristic after the test degraded somehow as compared to the sensitivity characteristic before the test. However, as indicated in Table 2, these gas sensors100were each determined to have an evaporation prevention level of 70% or higher.

As for the Au ratios of the sensing electrode10and the electrode evaporation preventing film12, the Au ratio of the surface of the sensing electrode10had no change before and after the test for the gas sensors100of No. 4 to 6, which were determined to have an evaporation prevention level of 90% or higher. In other words, the Au evaporation from the sensing electrode10was completely suppressed. Only the Au ratio at the surface of the electrode evaporation preventing film12was smaller after the test for any of the gas sensors100of No. 4 to 6.

On the other hand, for the gas sensors100of No. 13 to 15, the Au ratios at the surfaces of the sensing electrode10and the electrode evaporation preventing film12were smaller after the test. For the comparative example, the evaluation of the Au ratio at the surface of the sensing electrode10was tried only after the test, but no Au was detected.

Consideration on these Au ratio evaluation results and the above-described results of determination of the evaporation prevention level indicates that disposition of the electrode evaporation preventing film12having a composition at least same as or more Au-riched than that of the sensing electrode10facilitates suppression of the Au evaporation from the sensing electrode10. In addition, there is a correspondence relation between the magnitude of the evaporation prevention level and change in the Au ratio of the sensing electrode10. This indicates that it is appropriate to evaluate the degree of Au evaporation suppression based on the evaporation prevention level.

It is also indicated that, when having an area half of that of the sensing electrode10, the electrode evaporation preventing film12sufficiently provides the Au evaporation suppression effect.

The four gas sensors100(No. 16 to 19) were fabricated as in Examples 1 and 2 except that the sensing electrode10and the electrode evaporation preventing film12in each sensor element101are arranged differently from those in Example 2. For each gas sensor100thus fabricated, the evaporation prevention level was evaluated by performing the accelerated degradation test and the evaluation of the sensitivity characteristic before and after the test, as in Example 2.

Specifically, the two gas sensors100(No. 16 and 17) were fabricated. In these two gas sensors100, as in the gas sensor100of No. 8 in Examples 1 and 2, the sensing electrode10having an Au composition of 10 wt % and the electrode evaporation preventing film12having an Au composition of 50 wt % were provided in rectangular shapes with the same area in plan view as illustrated inFIG. 3B, and had an in-plane distance of 0.46 mm therebetween, which is same as that of the gas sensor100of No. 8, and the area was 0.4 mm2and 3 mm2, respectively, which are smaller than that of the gas sensor100of No. 8.

In addition, the two gas sensors100(No. 18 and 19) were fabricated. In these two gas sensors100, the sensing electrode10and the electrode evaporation preventing film12had Au compositions same as those of the gas sensors of No. 8, 16, and 17, the area was 3 mm2, which is same as that of the gas sensor100of No. 17, the sensing electrode10was provided in a rectangular shape in plan view, and the electrode evaporation preventing film12was disposed as illustrated inFIGS. 3D and 3E. The in-plane distance of the sensing electrode10and the electrode evaporation preventing film12was 0.15 mm and 4.5 mm in the gas sensors100of No. 18 and 19, respectively. In the gas sensor100of No. 19, the electrode evaporation preventing film12was disposed at a threshold disposition position at the base end E2, where it is difficult to further separate the electrode evaporation preventing film12from the sensing electrode10because of contact with any other member due to the structure of the gas sensor100.

Also in this fabrication of the gas sensors100of No. 16 to 19, the electrode evaporation preventing film12was successfully formed through screen printing and co-firing.

FIG. 12is a diagram illustrating the sensitivity characteristics obtained before and after the accelerated degradation test for the four gas sensors100(No. 16 to 19) according to the present example and the gas sensor100of No. 8 for comparison.

Table 3 lists, for the gas sensors100(No. 16 to 19) according to the present example and the gas sensor100of No. 8 for comparison, the areas of the sensing electrode10and the electrode evaporation preventing film12, the in-plane distance therebetween, the number of a drawing illustrating exemplary arrangement of the sensing electrode and the electrode evaporation preventing film, and a result of determination of the evaporation prevention level.

As understood fromFIG. 12, the sensitivity characteristic had almost no difference before and after the accelerated degradation test for all gas sensors100including the gas sensor100of No. 8. Accordingly, as indicated in Table 3, any of the gas sensors100was determined to have an evaporation prevention level of 90% or higher.

This result indicates that the Au evaporation can be suppressed when the electrode evaporation preventing film12having an area in accordance with that of the sensing electrode10is provided, and also even when the electrode evaporation preventing film12has different shapes and is disposed at different positions. According to a result on the gas sensor100of No. 19, in particular, it is understood that, when the electrode evaporation preventing film12is provided, the Au evaporation suppression effect can be obtained up to at least a position separated from the sensing electrode10by 4.5 mm.

The gas sensor100was fabricated as in Examples 1 to 3 except that the protective layer50was provided to the sensor element101and the electrode evaporation preventing film12was disposed in a different relation with the protective layer50. For each gas sensor100thus fabricated, the evaporation prevention level was evaluated by performing the accelerated degradation test and the evaluation of the sensitivity characteristic before and after the test, as in Examples 2 and 3.

Specifically, the sensing electrode10and the electrode evaporation preventing film12had Au compositions and an area same as those of the gas sensor100of No. 8 in Examples 1 and 2. Specifically, the Au compositions, the area, and the in-plane distance were 10 wt %, 50 wt %, 7.8 mm2, and 0.46 mm, respectively.

The protective layer50was formed with the porosity being set to three levels of 12%, 20%, 40% and the thickness being set to two levels of 15 μm and 30 μm. The electrode evaporation preventing film12was disposed in two manners. In one of the dispositions, as exemplarily illustrated inFIG. 4, the electrode evaporation preventing film12was exposed on the protective layer50formed to cover the sensing electrode10. In the other disposition, as exemplarily illustrated inFIG. 3B, the electrode evaporation preventing film12was disposed similarly to the gas sensor100of No. 8 and covered by the protective layer50together with the sensing electrode10as illustrated inFIGS. 1A and 1B. In the former disposition, the sensing electrode10and the electrode evaporation preventing film12were provided above and below the protective layer50inFIG. 4but had an in-plane distance of 0 mm. In the latter disposition, the in-plane distance was 4.6 mm, which was same as that in the gas sensor of No. 8.

In the above-described manner, the 12 gas sensors100(No. 20 to 31) were fabricated. Also in this fabrication of the gas sensors100of No. 20 to 31, the electrode evaporation preventing film12was successfully formed through screen printing and co-firing.

FIGS. 13 to 15are diagrams illustrating the sensitivity characteristics obtained before and after the accelerated degradation test for the gas sensors100of No. 20 to 31 according to the present example and the gas sensor100of No. 8 for comparison. The sensitivity characteristics for the gas sensor100of No. 8 are illustrated in all ofFIGS. 13 to 15.

Table 4 lists, for the gas sensors100(No. 20 to 31) according to the present example and the gas sensor100of No. 8 for comparison, main formation conditions (porosity and thickness) of the protective layer, disposition of the electrode evaporation preventing film12, the in-plane distance of the sensing electrode10and the electrode evaporation preventing film12, and a result of determination of the evaporation prevention level.

TABLE 4IN-PLANEDISTANCEBETWEEN SENSINGELECTRODE ANDEVAPORATIONPROTECTIVE LAYEREVAPORATIONPREVENTIONSENSORPOROSITYTHICKNESSDISPOSITION OF EVAPORATIONPREVENTING FILMLEVELNO.(%)(μm)PREVENTING FILM(mm)DETERMINATION201215EXPOSED ON PROTECTIVE LAYER0⊚2130EXPOSED ON PROTECTIVE LAYER0⊚2215COVERED BY PROTECTIVE LAYER0.46⊚2330COVERED BY PROTECTIVE LAYER0.46⊚242015EXPOSED ON PROTECTIVE LAYER0⊚2530EXPOSED ON PROTECTIVE LAYER0⊚2615COVERED BY PROTECTIVE LAYER0.46⊚2730COVERED BY PROTECTIVE LAYER0.46⊚284015EXPOSED ON PROTECTIVE LAYER0⊚2930EXPOSED ON PROTECTIVE LAYER0⊚3015COVERED BY PROTECTIVE LAYER0.46⊚3130COVERED BY PROTECTIVE LAYER0.46⊚8NO PROTECTIVE LAYEREXPOSED0.46⊚

As understood fromFIGS. 13 to 15, the sensitivity characteristic had almost no difference before and after the accelerated degradation test for any of the gas sensors100including the gas sensor100of No. 8. Thus, as indicated in Table 4, any of the gas sensors100was determined to have an evaporation prevention level of 90% or higher.

However, there was a difference in the sensitivity characteristics among these gas sensors. The gas sensors of No. 28 to 31 illustrated inFIG. 15in which the protective layer50had a porosity of 40% had sensitivity characteristics similarly to that of the gas sensor100of No. 8 including no protective layer50. The gas sensors of No. 20 to 23 illustrated inFIG. 13in which the protective layer50had a porosity of 12% and the gas sensors100of No. 24 to 27 illustrated inFIG. 14in which the protective layer50had a porosity of 20% had slightly degraded sensitivity characteristics as compared to that of the gas sensor100of No. 8. In more detail, the sensitivity characteristics obtained for the gas sensors100(No. 20 and 24), in which the protective layer50had a thickness of 15 μm and the electrode evaporation preventing film12was provided on the protective layer50, were closer to those for the gas sensor100of No. 8 than those of the other gas sensors. Like the case illustrated inFIG. 15, more excellent sensitivity characteristic is obtained for a larger porosity of the protective layer50if the other conditions are same.

The above-described results indicate that, as long as formation conditions of the protective layer50and disposition of the electrode evaporation preventing film12are appropriate, the protective layer50does not hinder the Au evaporation suppression effect, and thus the formation conditions of the protective layer50can be set in accordance with a desired sensitivity characteristic.