Electrode for gas sensor, and gas sensor

Provided are: an electrode for a gas sensor formed as a porous electrode so as to stably allow reduction in electrode resistance for excellent low-temperature activity; and a gas sensor. The electrode (108, 110) for the gas sensor is adapted for use on a surface of a solid electrolyte body (109), which is predominantly formed of zirconia, and contains particles (2) of a noble metal or an alloy thereof, first ceramic particles (4) of stabilized zirconia or partially stabilized zirconia and second ceramic particles (6) of one or more selected from the group consisting of Al2O3, MgO, La2O3, spinel, zircon, mullite and cordierite, wherein the second ceramic particles are contained in an amount smaller than that of the first ceramic particles.

FIELD OF THE INVENTION

The present invention relates to an electrode for a gas sensor suitably used to detect the concentration of a specific gas component in e.g. a combustion gas or exhaust gas of a combustion device, internal combustion engine etc. The present invention also relates to a gas sensor.

BACKGROUND ART

There is conventionally used a gas sensor for detecting the concentration of a specific component (e.g. oxygen) in an exhaust gas of an internal combustion engine. This gas sensor includes therein a gas sensor element equipped with at least one cell, each cell having a solid electrolyte body of oxygen-ion-conducting partially stabilized zirconia etc. and a pair of electrodes arranged on the solid electrolyte body. There is also known a gas sensor (such as wide range oxygen sensor, NOx sensor etc.) of the type having two or more cells, one of which is configured as an oxygen pumping cell.

It is common practice to use, as the electrodes of the oxygen pumping cell, porous electrodes each formed with a plurality of pores by adding a vanishable solid material (such as theobromine or carbon) into an electrode paste of noble metal particles and ceramic particles and then sintering the resulting paste (see Patent Document 1). The use of such porous electrodes leads to increase in the three-phase interface between the electrodes, the solid electrolyte body and the air (gas under measurement) for improvement of oxygen pumping performance.

For reduction of power consumption, the gas sensor using the solid electrolyte body is required to have low-temperature activity. In the gas sensor having the oxygen pumping cell, it is particularly required to increase the amount of three-phase interface of the electrodes of the oxygen pumping cell, allow reduction of electrode resistance and improvement of oxygen pumping performance and thereby achieve improved low-temperature activity.

PRIOR ART DOCUMENTS

Patent Documents

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

In the case of forming the porous electrodes with the use of the vanishable solid material, however, there are problems such as difficulty of particle size control of the vanishable solid material and variations in the pore size of the formed electrodes. Although the pores are formed in the electrodes due to vanishing of the vanishable solid material at 600 to 800° C., some of the pores are destroyed during the process of temperature rise to a final firing temperature (about 1000 to 1500° C.) so that there occurs variations in the pore size of the finally obtained electrodes. In the case of using theobromine as the vanishable solid material, the theobromine has poor compatibility with solvent and binder so that there also occurs variations in the thickness of the electrodes due to deterioration of leveling (flatness) during the application of the electrode paste. These problems cause variations in the oxygen pumping performance of the electrodes and thereby make it difficult to allow reduction of electrode resistance and achieve improvement of low-temperature activity.

It is accordingly an object of the present invention to provide an electrode for a gas sensor, formed as a porous electrode so as to stably allow reduction of electrode resistance for excellent low-temperature activity. It is also an object of the present invention to provide a gas sensor.

Means for Solving the Problems

In order to solve the above problems, there is provided according to the present invention an electrode for a gas sensor, the gas sensor having a solid electrolyte body predominantly formed of zirconia, the electrode being adapted for use on a surface of the solid electrolyte body, the electrode comprising: particles formed of a noble metal or an alloy thereof; first ceramic particles formed of stabilized zirconia or partially stabilized zirconia; and second ceramic particles formed of one or more selected from the group consisting of Al2O3, MgO, La2O3, spinel, zircon, mullite and cordierite, wherein the second ceramic particles are contained in an amount smaller than that of the first ceramic particles.

In the electrode for the gas sensor, the second ceramic particles are brought into contact with the first ceramic particles and deposited on grain boundaries around the first ceramic particles so as to retard the particle growth of the first ceramic particles during sintering. This makes it less likely that the contact points between the first ceramic particles and the predominant noble metal or noble metal alloy particles will be lost during the sintering. It is therefore possible to effectively allow reduction of electrode resistance without decrease in the number of the pores in the electrode (i.e. without decrease in the three-phase interface of the electrode). As the pores are formed in the electrode with the use of no vanishable solid material, it is possible to stably allow reduction of electrode resistance without variations in the diameter and distribution of the pores in the electrode.

The first ceramic particles and the second ceramic particles do not vanish during the sintering. This also leads to less variations in the diameter and distribution of the pores in the electrode. Furthermore, both of the first ceramic particles and the second ceramic particles have good compatibility with solvent and binder for improved dispersibility. This leads to less variations in the thickness of the electrode by improvement of leveling (flatness) during the application of the electrode paste.

In the electrode for the gas sensor, it is preferable that a ratio of the amount of the second ceramic particles to the amount of the first ceramic particles is greater than or equal to 0.1 volume % and less than 50 volume % in order to more effectively allow reduction of electrode resistance without causing deterioration in the adhesion of the electrode.

It is more preferable that, in the electrode for the gas sensor, the ratio of the amount of the second ceramic particles to the amount of the first ceramic particles is greater than or equal to 3 volume % and less than 40 volume % in order to particularly effectively allow reduction of electrode resistance.

It is preferable that an average sintered grain size of the second ceramic particles is 0.1 to 1 time that of the first ceramic particles.

It is preferable that the first ceramic particles are formed of partially stabilized zirconia.

There is also provided according to the present invention a gas sensor comprising: a solid electrolyte body; and a pair of electrodes arranged on the solid electrolyte body, wherein the above-mentioned electrode for the gas sensor is used as each of the pair of electrodes.

There is further provided according to the present invention a gas sensor comprising at least: an oxygen pumping cell having a first solid electrolyte body and a pair of oxygen pumping electrodes arranged on a surface of the first solid electrolyte body; and a detecting cell having a second solid electrolyte body and a pair of detecting electrodes arranged on the second solid electrolyte body, wherein the above-mentioned electrode for the gas sensor is used as each of the pair of oxygen pumping electrodes or each of the pair of detecting cells.

In particular, it is possible to effectively allow reduction of electrode resistance and improvement of oxygen pumping performance and achieve further improved low-temperature activity when the above-mentioned electrode for the gas sensor is used as the oxygen pumping cell.

It is preferable that the electrode for the gas sensor has a thickness of 20 μm or larger for more effective reduction of electrode resistance.

Effects of the Invention

It is possible according to the present invention to stably allow reduction of electrode resistance and achieve improvement of low-temperature activity.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1is a section view of a gas sensor (oxygen sensor)1, taken in a longitudinal direction (in the direction of an axis L) thereof, according to a first embodiment of the present invention.FIG. 2is a perspective exploded view of a detection element unit300and a heater unit200of the gas sensor1.FIG. 3is an enlarged section view of the detection element unit300, taken in a direction perpendicular to the axis L.

As shown inFIG. 1, the gas sensor1includes a gas sensor element100in which the heater unit200is laminated on the detection element unit300, a metal shell30holding therein the gas sensor element100and the like and a protector24attached to a front end portion of the metal shell30. The gas sensor element100is arranged so as to extend in the direction of the axis L.

The heater unit200has first and second substrates101and103predominantly formed of alumina and a heating member102predominantly formed of platinum and sandwiched between the first and second substrates101and103as shown inFIG. 2. The heating member102has a heating portion102alocated on a front end side thereof and a pair of heater lead portions102bextending from the heating portion102ain a longitudinal direction of the first substrate101. Heater-side through holes101aare formed with conductors in the first substrate101. Terminal ends of the heater leads102bare electrically connected to heater-side pads120via the conductors of the heater-side through hole101a, respectively. The laminate of the first and second substrates101and102herein corresponds to an insulating ceramic body.

The detection element unit300has an oxygen concentration detecting cell130and an oxygen pumping cell140. The oxygen concentration detecting cell130has a first solid electrolyte body105and first and second electrodes104and106formed on respective opposite surfaces of the first solid electrolyte body105. The first electrode104includes a first electrode portion104aand a first lead portion104bextending from the first electrode104ain a longitudinal direction of the first solid electrolyte body105. The second electrode106includes a second electrode portion106aand a second lead portion106bextending from the second electrode portion106ain the longitudinal direction of the first solid electrolyte body105.

A first through hole105a, a second through hole107a, a fourth through hole109aand a sixth through hole111aare formed with respective conductors in the first solid electrolyte body105, the after-mentioned insulation layer107, the after-mentioned second solid electrolyte body109and the after-mentioned protection layer111. A terminal end of the first lead portion104bis electrically connected to a detection-element-side pad121through the conductors of the first, second, fourth and sixth through holes105a,107a,109aand111a. A third through hole107b, a fifth through hole109band a seventh through hole111bare formed with respective conductors in the after-mentioned insulation layer107, the after-mentioned second solid electrolyte body109and the after-mentioned protection layer111. A terminal end of the second lead portion106bis electrically connected to another detection-element-side pad121through the conductors of the third, fifth and seventh through holes107b,109band111b.

The oxygen pumping cell140has the solid electrolyte body109and third and fourth electrodes108and110formed on respective opposite surfaces of the solid electrolyte body109. The third electrode108includes a third electrode portion108aand a third lead portion108bextending from the third electrode portion108in a longitudinal direction of the second solid electrolyte body109. The forth electrode110includes a fourth electrode portion110aand a fourth lead portion110bextending from the fourth electrode portion110ain the longitudinal direction of the second solid electrolyte body109.

In the first embodiment, each of the third and fourth electrodes108and110of the oxygen pumping cell140corresponds to “an electrode for a gas sensor” as set forth in the scope of claims. As the third and fourth electrodes108and110are used in the oxygen pumping cell140, each of the third and fourth electrodes108and110also corresponds to “an oxygen pumping electrode” as set forth in the scope of claims. As a matter of course, each of the first and second electrodes104and106can be configured as “an electrode for a gas sensor” as set forth in the scope of claims.

A terminal end of the third lead portion108bis electrically connected to the detection-element-side pad121through the conductors of the fifth and seventh through holes109band111b. An eighth through hole111cis formed with a conductor in the protection layer111. A terminal end of the fourth lead portion110bis electrically connected to another detection-element-side pad121through the conductor of the eighth through hole111c. Herein, the second and third lead portions106band108bare set to the same potential.

Each of the first and second solid electrolyte bodies105and109is in the form of a sintered body of partially stabilized zirconia material in which yttria (Y2O3) or calcia (CaO) is added as a stabilizer to zirconia (ZrO2). As the stabilizer added to zirconia (ZrO2), there can be used not only the above mentioned oxide but also Yb2O3, Sc2O3, Gd2O3or Nd2O3. The first and second solid electrolyte bodies105and109may each alternatively be in the form of a sintered body of completely stabilized zirconia material in which the occurrence of modification of zirconia is completely retarded by increasing the amount of the stabilizer added.

Each of the heating member102, the first and second electrodes104and106, the heat-side pad120and the detection-element-side pad121are formed of a platinum-group element. As the platinum-group element, Pt, Rh or Pd is preferred. These elements can be used solely or in combination of two or more thereof.

The compositions of the third and fourth electrodes108and110will be explained later.

It is preferable that the heating member102, the first and second electrodes104and106, the heat-side pad120and the detection-element-side pad121are each predominantly formed of Pt in view of heat resistance and oxidation resistance. It is also preferable that each of the heating member102, the first and second electrodes104and106, the heat-side pad120and the detection-element-side pad121contains a ceramic component in addition to the predominant platinum-group element component. This ceramic component is preferably of the same as the material of the laminated-side structural part (e.g. the predominant component of the first, second solid electrolyte body105,109) in view of adhesion.

The insulation layer107is arranged between the oxygen pumping cell140and the oxygen concentration detecting cell130. The insulation layer107includes an insulating portion114and a diffusion limiting portion115. A hollow measurement chamber107cis defined in the insulating portion114of the insulation layer107, at a position corresponding to the second and third electrode portions106aand108a, so as to be in communication with the outside in a width direction of the insulation layer107. In this communication part, the diffusion limiting portion115is situated so as to allow gas diffusion between the outside and the measurement chamber107cunder predetermined diffusion-limited conditions.

There is no particular limitation on the material of the insulating portion114as long as the insulating portion114is in the form of a sintered ceramic body. As the material of the insulating portion114, there can be used an oxide ceramic material such as alumina or mullite.

The diffusion limiting portion115is in the form of a porous body of alumina to limit the flow of gas under measurement into the measurement chamber107c.

The protection layer111is arranged on the surface of the second solid electrolyte body109such that the fourth electrode110is sandwiched between the second solid electrolyte body109and the protection layer111. The protection layer111includes a porous electrode protecting portion113acovering the fourth electrode portion110aand protecting the fourth electrode portion110afrom poisoning and a reinforcing portion112covering and protecting the fourth lead portion110b.

In the first embodiment, the gas sensor element100is configured as an oxygen sensor element to adjust the direction and magnitude of current flow between the electrodes of the oxygen pumping cell140in such a manner as to control the voltage (electromotive force) between the electrodes of the oxygen concentration detecting cell130to a predetermined value (e.g. 450 mV), and then, detect the concentration of oxygen in the gas under measurement linearly according to the current flow of the oxygen pumping cell140.

As shown inFIGS. 3 and 4, the entire circumference of the front end part of the gas sensor element100(the laminate of the detection element unit300and the heater unit200) is covered with a porous protection layer20(inner and outer porous layers21and23).FIG. 3is an enlarged section view of the front end part of the gas sensor element100.FIG. 4is a section view of the gas sensor element100with the inner and outer porous layers21and23, taken in a direction perpendicular to the direction of the axis L.

The inner porous layer21is set higher in porosity than the outer porous layer23. There is a gas-permeable three-dimensional network structure defined by pores of the diffusion limiting portion115and the inner and outer porous layers21and23.

Referring back toFIG. 1, the metal shell30is formed of SUS430and has a male thread portion31for mounting the gas sensor into an exhaust pipe and a hexagonal portion32for engaging with a mounting tool at the time of mounting of the gas sensor. The metal shell31also has a shell-side step portion33protruding radially inwardly so as to support thereon a metal holder34for holding the gas sensor element100. A ceramic holder35and a talc material36are placed in this order from the front end side in the metal holder34. The talc material36includes a first talc material37situated within the metal holder34and a second talc material38situated over a rear end of the metal holder34. The gas sensor element100is fixed to the metal holder34by filling and compressing the first talc material37in the metal holder34. Further, the sealing between an outer surface of the gas sensor element100and an inner surface of the metal shell30is secured by filling and compressing the second talc material38in the metal shell30. A sleeve39of alumina is placed on a rear end of the second talc material38. This sleeve38has a multi-diameter cylindrical shape with an axial hole39ain the direction of the axis so that the gas sensor element100is inserted through the axial hole39a. A rear end crimp portion30aof the metal shell30is bent inwardly so as to push the sleeve39toward the front via a stainless ring member40.

The protector24is formed of a metal material with a plurality of gas introduction holes24aand welded to an outer circumference of the front end portion of the metal shell30so as to cover the front end part of the gas sensor element100protruding from the front end of the metal shell30. This protector24has a double-layer structure that consists of a bottomed cylindrical-shaped outer protector member41of uniform outer diameter and a bottomed cylindrical-shaped inner protector member42formed with a front end portion42band a rear end portion42aof larger outer diameter than the front end portion42b.

An outer tube25of SUS430 is fitted at a front end side thereof around a rear end side of the metal shell30by laser welding an enlarged-diameter front end portion25aof the outer tube25to the metal shell30. A separator50is arranged in a rear end side of the outer tube25. A holding member51is arranged in a space between the outer tube25and the separator50and secured with the outer tube25and the separator50by engaging the holding member51with the after-mentioned protruding portion50aof the separator50and crimping the outer tube25.

Through holes50bare formed in the separator50from the front end side to the rear end side so that leads11to15for the detection element unit300and the heater unit200are inserted through the respective through holes50b. (In the drawings, the leads24and15are not shown.) Connection terminals16are accommodated in the through holes50bfor connection to the leads11to15with the detection-element-side pads121of the detection element unit300and the heater-side pads120of the heater unit200. The leads11to15are connected to connectors (not shown) outside of the gas sensor for input and output of electric signals between the leads11to15and the external device such as ECU through the connectors. Although not specifically shown in the drawings, each of the leads11to15has a lead wire covered with an insulating coating of resin.

A substantially column-shaped rubber cap52is situated in a rear end of the separator50so as to close a rear end opening25bof the outer tube25. The rubber cap52is fixed to the outer tube25by crimping an outer circumference of the outer tube25radially inwardly with the rubber cap52fitted in the rear end of the outer tube25. Through holes52aare also formed in the rubber cap52from the front end side to the rear end side so that the leads11to15are inserted through the respective through holes52a.

The characteristic configuration of the electrode for the gas sensor according to the present invention (in the first embodiment, the third and fourth electrodes108and110) will be explained below with reference toFIGS. 5 and 6.

As mentioned above, porous electrodes each formed with a plurality of pores by adding a vanishable solid material (such as theobromine or carbon) to an electrode paste and sintering the resulting paste are commonly used as electrodes for a gas sensor. There is however a problem that it is difficult to allow reduction of electrode resistance due to variations in the diameter and distribution of the pores in the porous electrodes.

In the case of sintering an electrode paste containing noble metal particles2of noble metal or alloy thereof and first ceramic particles4of solid electrolyte with no distinguishable solid material as shown inFIG. 5, for example, two first ceramic particles4are brought into contact with each other and, at the same time, brought into contact with five noble metal particles2(seeFIG. 5(a)) in the early stage of sintering. There thus occur total seven contact points C1to C7between the two first ceramic particles4and the five noble metal particles2. These contact points C1to C7constitute a three-phase interface. With the progress of sintering, however, the first ceramic particles4are bonded to each other and grown to a coarse particle so that some of the contact points C1to C7are lost (that is, the contact points are reduced to four (C1, C2, C4and C7)) (seeFIG. 5(b)). In this way, the particle growth of the first ceramic particles4proceeds without the use of the vanishable solid material so as to cause decrease in the number of the pores and, by extension, decrease in the three-phase interface and thereby fail to allow reduction of electrode resistance.

In the first embodiment, second ceramic particles6are added to the electrode paste. During the sintering of the electrode paste, the second ceramic particles6are brought into contact with the first ceramic particles4and deposited on grain boundaries around the first ceramic particles4so as to retard the particle growth of the first ceramic particles4as shown inFIG. 6. This makes it less likely that the contact points (C1 to C7) between the first ceramic particles4and the noble metal (or noble metal alloy) particles2will be lost during the sintering. It is therefore possible to effectively allow reduction of electrode resistance without decrease in the number of the pores (i.e. without decrease in the three-phase interface). As the pores are formed with the use of no vanishable solid material, it is possible to stably allow reduction of electrode resistance without variations in the diameter and distribution of the pores.

The first ceramic particles4and the second ceramic particles6do not vanish during the sintering. This also leads to less variations in the diameter and distribution of the pores. Furthermore, both of the first ceramic particles4and the second ceramic particles6have good compatibility with solvent and binder for improved dispersibility. This leads to less variations in the thickness of the electrode by improvement of leveling (flatness) during the application of the electrode paste.

As the first ceramic particles4are formed of oxygen-ion-conducting ceramic material such as stabilized zirconia or partially stabilized zirconia, the above effect can be obtained without decrease in the number of the pores by retarding the particle growth of the first ceramic particles4. The first ceramic particles4, if formed of full zirconia (zirconia only), do not show oxygen ion conductivity. In such a case, the resulting electrode do not perform oxygen pumping function even though the pores (three-phase interface) are formed in the electrode. There is no contribution to reduction of electrode resistance even though the particle growth of those ceramic particles is retarded. For this reason, the first ceramic particles4are formed of stabilized zirconia or partially stabilized zirconia in the present invention.

In the gas sensor1, the oxygen pumping cell140pumps oxygen in and out from the measurement chamber107C. It is thus possible to achieve high oxygen pumping performance by reduction of electrode resistance for further improvement of low-temperature activity in the case of forming the third and fourth electrodes108and110of the oxygen pumping cell140from the electrode paste containing the second ceramic particles.

It is herein assumed that, due to a large difference in ion radius between the first ceramic particles4and the second ceramic particles6, the second ceramic particles6are difficult to dissolve in the first ceramic particles4and thus are deposited on the grain boundaries around the first ceramic particles4.

There can be used Au, Ag, Pt, Pd, Rh, Ir, Ru or Os as the noble metal of the predominant noble metal particles2in the electrode for the gas sensor. There can be used an alloy of one or more of the above noble metal elements as the noble metal alloy of the predominant noble metal particles2in the electrode for the gas sensor. Among others, Pt, Pd, Rh, Ir, Ru or Ag is preferred as the noble metal; and an alloy of one or more elements selected from the group consisting of Pt, Pd, Rh, Ir, Ru and Ag is preferred as the noble metal alloy. Specific examples of the noble metal alloy are Pt—Pd alloy, Pt—Rh alloy, Pt—Pd—Rh alloy, Pt—Ru alloy, Pt—Ru—Ir alloy, Pt—Au alloy and Pt—Ag alloy.

The first ceramic particles4are preferably formed of the same material as that of the first, second solid electrolyte body105,109, i.e., partially stabilized zirconia in which Y2O3, CaO, Yb2O3, Sc2O3, Gd2O3or Nd2O3is added as a stabilizer to zirconia (ZrO2). It is alternatively feasible to use, as the material of the first ceramic particles4, completely stabilized zirconia in which the occurrence of modification of zirconia is completely retarded by increasing the amount of the stabilizer added.

The second ceramic particles6are formed of one or more selected from the group consisting of Al2O3, MgO, La2O3, spinel, zircon, mullite and cordierite. Among others, alumina ceramic such as Al2O3is preferred as the material of the second ceramic particles6in view of differences in ion radius and crystal structure to the predominant component, i.e., zirconia of the solid electrolyte body.

It is preferable that, in the electrode for the gas sensor, the ratio of the amount of the second ceramic particles6contained to the amount of the first ceramic particles4contained is greater than or equal to 0.1 volume % and less than 50 volume %. If the ratio of the amount of the second ceramic particles6is less than 0.1 volume %, the particle growth of the first ceramic particles4may not be sufficiently retarded during the sintering of the electrode so as to cause decrease in the number of the pores and, by extension, decrease in the three-phase interface and result in increase of electrode resistance. If the ratio of the amount of the second ceramic particles6exceeds 50 volume %, the particle growth of the first ceramic particles4may be excessively retarded so as to cause deterioration in the adhesion of the electrode due to poor bonding between the first ceramic particles4.

It is more preferable that the ratio of the amount of the second ceramic particles6contained to the amount of the first ceramic particles4contained is greater than or equal to 3 volume % and less than 40 volume %.

The ratio (volume %) of the amount of the second ceramic particles6to the amount of the first ceramic particles4can be determined as, in a cross-sectional SEM image of the electrode for the gas sensor, a ratio of the cross-sectional area of the second ceramic particles6to the cross-sectional area of the first ceramic particles4. As the particle growth retarding effect of the second ceramic particles6on the first ceramic particles4depends on the volume ratio of the second ceramic particles6relative to the first ceramic particles4, the above ratio value “volume %” can suitably be used as an index of the particle growth retarding effect. The ratio of the amount of the second ceramic particles6to the amount of the first ceramic particle4, if given in units of mass %, is difficult to reflect the particle growth retarding effect in the case where there is a large difference in density between the first ceramic particles4and the second ceramic particles6.

Next, a gas sensor (NOx sensor) according to a second embodiment of the present invention will be described below with reference toFIG. 7. The gas sensor according to the second embodiment is structurally similar to the gas sensor according to the first embodiment, except for the configuration of a gas sensor element100C. Thus, a description and illustrations of the other structural parts such as metal shell for holding the gas sensor element100C will be omitted herefrom.

The gas sensor element (NOx sensor element)100C is formed into an elongated plate shape and has a laminated structure in which insulators180and185of alumina etc. are laminated between respective adjacent ones of three plate-shaped solid electrolyte bodies109C,105C and151. This laminated structure constitutes a detection element unit300C. A heater unit200is arranged on an outer side of the solid electrolyte body151(opposite from the solid electrolyte body105C inFIG. 1) and includes sheet-like insulation layers103C and101C formed predominantly of alumina and laminated to each other and a heater pattern102C formed predominantly of platinum and embedded between the insulation layers103C and101C.

Each of the solid electrolyte bodies109C,105C and151is in the form of a solid electrolyte body of partially stabilized zirconia (YSZ) and shows oxygen ion conductivity.

The detection element unit300C is equipped with a first pumping cell (Ip1 cell)140C, an oxygen concentration detecting cell (Vs cell)130C and a second pumping cell (Ip2 cell)150.

The first pumping cell140C has the second solid electrolyte body109C and third and fourth electrodes108C and110C formed on respective opposite surfaces of the second solid electrolyte body109C. A porous protection layer114of ceramic material is formed on a surface of the fourth electrode110C so as to protect the fourth electrode110C from deterioration by exposure to poisoning gas component (reducing atmosphere) of exhaust gas.

The first pumping cell140C performs the same function as that of the oxygen pumping cell140so as to pump oxygen in and out (so called “oxygen pumping”) between the after-mentioned first measurement chamber107C2and the outside. In the second embodiment, each of the third and fourth electrodes108C and110C thus corresponds to “an electrode for a gas sensor” as set forth in the scope of claims.

The oxygen concentration detecting cell130C has the first solid electrolyte body105C and first and second electrodes104C and106C formed on respective opposite surfaces of the first solid electrolyte body105C. The above-mentioned first measurement chamber107C2and the after-mentioned reference oxygen chamber170are separated by the solid electrolyte body105C. The oxygen concentration detecting cell130generates an electromotive force according to a difference in oxygen partial pressure between these chambers107C2and170.

The first measurement chamber107C2is defined as a small hollow space between the solid electrolyte bodies109C and105C. The second and third electrodes106C and108C are placed in the first measurement chamber107C2. It is herein noted that the first measurement chamber107C2is the small space to which the gas under measurement is first introduced from the outside within the gas sensor element100C.

A porous first diffusion limiting portion115C is situated in a front end side of the first measurement chamber107C2of the gas sensor element100C and lies between the first measurement chamber107C2and the outside so as to limit the flow of the gas under measurement into the first measurement chamber107C2.

A second diffusion limiting portion117is situated in a rear end side of the first measurement chamber107C2of the gas sensor element100C, as a partition between the first measurement chamber107C2and an opening181to the after-mentioned second measurement chamber160, so as to limit the diffusion of the gas.

The second pumping cell150has the third solid electrolyte body151and fifth and sixth electrodes152and153formed on respective opposite surfaces of the third solid electrolyte body151. The third solid electrolyte body151faces the solid electrolyte body105C so as to sandwich the insulator185therebetween. The insulator185is not arranged in a space between the solid electrolyte bodies151and105C in which the fifth electrode152is located. This independent space is defined as the reference oxygen chamber170. The first electrode104C of the oxygen concentration detecting cell130C is also located in the reference oxygen chamber170. The reference oxygen chamber170is filled with a porous ceramic material. Further, the insulator185is not arranged in a space between the solid electrolyte bodies151and105C in which the sixth electrode156is located. This independent small hollow space is defined as the second measurement chamber160. Openings125and141are formed in the solid electrolyte body105C and the insulator180, respectively, so as to be in communication with the second measurement chamber160. The first measurement chamber107C2and the opening181are connected to each other via the second gas diffusion layer117as mentioned above.

The reference oxygen chamber170and the second measurement chamber160are separated by the insulator185. The second pumping cell150pumps oxygen in and out between these chambers170and160.

Further, the entire circumference of the front end part of the gas sensor element100C (the laminate of the detection element unit300C and the heater unit200C) is covered with a porous protection layer20C (inner and outer porous layers21C and23C).

In the second embodiment, the third and fourth electrodes108C and110C are each formed as the electrode for the gas sensor by adding the second ceramic particles6to the electrode paste and sintering the resulting electrode paste so as to retard the particle growth of the first ceramic particles4. This makes it less likely that the contact points between the first and second ceramic particles4and6and the noble metal (or noble metal alloy) particles2will be lost during the sintering. It is therefore possible to allow reduction of electrode resistance without decrease in the number of the pores (i.e. without decrease in the three-phase interface). As the pores are formed without the use of the vanishable solid material, it is possible to stably allow reduction of electrode resistance without variations in the diameter and distribution of the pores.

The NOx concentration detection operation of the NOx sensor element100C will be next briefly explained below.

First, the oxygen pumping cell140C pumps oxygen in and out between the first measurement chamber107C2and the outside in such a manner that the potential difference between the electrodes104C and106C becomes constant at around 425 mV.

After the oxygen concentration of the exhaust gas in the first measurement chamber107C2is adjusted as mentioned above, the exhaust gas is introduced from the first measurement chamber107C2to the second measurement chamber160through the second gas diffusion layer117. Then, NOx in the exhaust gas is brought into contact with the sixth electrode153within the second measurement chamber160and decomposed (reduced) to N2and O2by the catalytic action of the sixth electrode153. The thus-generated oxygen is converted to oxygen ions upon receipt of electrodes from the sixth electrode153. These oxygen ions flow through the third solid electrolyte body151and move to the fifth electrode152. At this time, the remaining unpumped oxygen in the first measurement chamber107C2is moved to the reference oxygen chamber170by the Ip2 cell150in the same manner as above. Consequently, there occur a flow of current through the Ip2 cell150, which includes current derived from the NOx and current derived from the remaining oxygen.

The current derived from the remaining oxygen can be regarded as substantially constant because the concentration of the remaining unpumped oxygen in the first measurement chamber107C2has been adjusted to a predetermined value as mentioned above. The current derived from the remaining oxygen has less influence on the current flowing through the Ip2 cell150than variations in the current derived from the NOx. The current flowing through the Ip2 cell150is thus proportional to the NOx concentration.

The present invention is not limited to the above embodiments. The present invention can be applied to any electrodes for gas sensors using solid electrolyte bodies. In the present invention, it is feasible to use the electrode for the gas sensor in any type of gas sensor. The use of the electrode is not limited to the above-embodied oxygen sensor (oxygen sensor element) and NOx sensor (NOx sensor element). Various modifications and equivalents are possible as long as they fall within the scope of the invention. For example, the electrode can be used in a HC sensor (HC sensor element) for detecting the concentration of HC. Moreover, the use of the electrode is not limited to the above-embodied two-cell-type gas sensor. The electrode can also be used in one-cell-type gas sensor.

EXAMPLES

A gas sensor element of Example 1 will be explained below.

Samples of the plate-shaped gas sensor element (wide-range air/fuel ratio sensor element)100shown inFIGS. 1 to 4were produced. Herein, each of the third and fourth electrodes108and110of the oxygen pumping cell140was configured as “an electrode for a gas sensor”.

An electrode paste was first prepared by mixing Pt particles, first and second ceramic particles of the composition shown in TABLE 1, a binder (ethyl cellulose) and a solvent (butyl carbitol) together.

The average grain size of the sintered particles of each type shown in TABLE 1 was determined from a cross-sectional SEM image of the electrode for the gas sensor. More specifically, a cross section of the electrode was observed by a SEM with a magnification of the order of 3500 times. The particles of each type were sketched in the SEM image. The total cross-sectional area of the particles of each type was obtained by analysis of the SEM image. Then, the area ((SA) per particle of each type) was calculated by dividing the total cross-sectional area of the particles of each type by the number of the particles of each type. The diameter of the circle equivalent to the area was determined as the average grain size. These parameters can be represented by the following expressions (1) and (2).
Area per particle of each type (SA)=Total area of particles of each type/Number of particles of each type  (1)
Average grain size of particles of each type (DA)=2×√(SG/π)  (2)

In TABLE 1, the content (wt %) of the first ceramic particles refers to the amount of the first ceramic particles contained relative to the amount of the Pt particles; and the content (vol %) of the second ceramic particles refers to the amount of the second ceramic particles relative to the amount of the first ceramic particles.

The third and fourth electrodes108and110were each produced by applying the electrode paste to appropriated areas of the respective opposite surfaces of the second solid electrolyte body109, drying the applied electrode paste and sintering the dried electrode paste at a predetermined temperature (1000° C. or higher)

The gas sensor element100was obtained by forming the other structural parts as appropriate.

The gas sensor element100was tested for the oxygen pumping performance, by mounting the gas sensor element100to the gas sensor1, setting the temperature of the oxygen pumping cell140to 700° C. and measuring a Cole-Cole Plot of the electrode resistance between the third and fourth electrodes108and110(electrode area: 5 mm2). The conditions of measurement of the electrode resistance were as follows: application voltage: 100 mV; and frequency: 0.1 Hz to 100,000 Hz. The rate of reduction of the electrode resistance determined from the Cole-Cole Plot relative to that of Comparative Example 1 was determined. The oxygen pumping performance was evaluated as: “Δ” when the rate of reduction of the electrode resistance was less than 10% (including the case where the electrode resistance was increased relative to that of Comparative Example 1); “◯” when the rate of reduction of the electrode resistance was greater than or equal to 10% and less than 20%; and “⊚” when the rate of reduction of the electrode resistance was greater than or equal to 20%.

Further, the adhesion of the electrode was tested by subjecting the gas sensor1to 30,000 cycles of on-off operations between room temperature and 800° C. under a normal control state of the gas sensor1. After the above cycle test, the pumping voltage (Vp) between the third and fourth electrodes108and110was measured. The rate of increase of the pumping voltage (Vp) relative to that of Comparative Example 1 was determined. The adhesion of the electrode was evaluated as: “Δ” when the rate of increase of the pumping voltage (Vp) 5% or more; and “◯” when the rate of increase of the pumping voltage (Vp) less than 5%.

The same evaluation tests as in Example 1 were also performed in Examples 2 to 23. The evaluation results are shown in TABLE 1.

As shown in TABLE 1, it was possible to obtain not only high oxygen pumping performance under low-temperature conditions (700° C.), i.e., excellent low-temperature activity but also good electrode adhesion in each of Examples 5 to 23 where the electrodes were formed by sintering the electrode paste of the noble metal particles and the first and second ceramic particles and used for the oxygen pumping cell.

On the other hand, the oxygen pumping performance was low under low-temperature conditions (700° C.), that is, the low-temperature activity was poor in Comparative Example 1 where the electrode was formed without the use of the second ceramic particles.

Further, the oxygen pumping performance was low under low-temperature conditions (700° C.), that is, the low-temperature activity was poor in Example 3 where the ratio of the amount of the second ceramic particles to the amount of the first ceramic particles was 0.08 vol %.

In Example 4 where the ratio of the amount of the second ceramic particles to the amount of the first ceramic particles was 53 vol %, the electrode adhesion was poor even though the oxygen pumping performance was high under low-temperature conditions (700° C.). The reason for such poor electrode adhesion is assumed that the bonding between the first ceramic particles was poor due to excessive retardation of the sintering of the first ceramic particles.

It has been shown by these results that it is preferable that the ratio of the amount of the second ceramic particles to the amount of the first ceramic particles is greater than or equal to 1 vol % and less than 50 vol %.

The oxygen pumping performance was higher in Example 21 where the ratio of the amount of the second ceramic particles to the amount of the first ceramic particles was 3 vol % than in Example 6 where the ratio of the amount of the second ceramic particles to the amount of the first ceramic particles was 1 vol %.

The oxygen pumping performance was higher in Example 22 where the ratio of the amount of the second ceramic particles to the amount of the first ceramic particles was 40 vol % than in Example 10 where the ratio of the amount of the second ceramic particles to the amount of the first ceramic particles was 50 vol %.

It has been shown by these results that it is more preferable that the ratio of the amount of the second ceramic particles to the amount of the first ceramic particles is greater than or equal to 3 vol % and less than 40 vol %.

In Example 1 where: the average sintered grain size of the first ceramic particles was 0.8 μm; and the average sintered grain size of the second ceramic particles was 0.07 the oxygen pumping performance was low under low-temperature conditions (700° C.), that is, the low-temperature activity was poor.

In Example 2 where: the average sintered grain size of the first ceramic particles was 0.8 μm; and the average sintered grain size of the second ceramic particle was 0.85 the electrode adhesion was poor even though the oxygen pumping performance was high under low-temperature conditions (700° C.). The reason for such poor electrode adhesion is assumed that the bonding between the first ceramic particles was poor due to excessive retardation of the sintering of the first ceramic particles.

It has been shown by these results that it is preferable that the average sintered grain size of the second ceramic particles is 0.1 to 1 time that of the first ceramic particles.

Further, the oxygen pumping performance was higher under low-temperature conditions (700° C.) in Example 6 where: the ratio of the amount of the second ceramic particles to the amount of the first ceramic particles was 1 vol %; and the thickness of the electrode was 12 μm than in Examples 23 where: the ratio of the amount of the second ceramic particles to the amount of the first ceramic particles was 1 vol %; and the thickness of the electrode was 21 μm.

It has been shown by these results that it is preferable that the thickness of the electrode is 20 μm or larger.

As the materials of the second solid electrolyte body109and the third and fourth electrodes108and110of the oxygen pumping cell140, stabilized zirconia was used in Examples 1 to 7 and 9 to 23; and partially stabilized zirconia was used in Example 8.

FIGS. 8 and 9are scanning electron microscope (SEM) images showing the cross section C of the third electrode108of Example 5 and Comparative Example 1, respectively. Each of the SEM images ofFIGS. 8 and 9is a secondary electron image (composition image) where the grey and white areas of the cross section C correspond to the first ceramic particles and the noble metal (Pt) particles, respectively. In each image, the area above the cross section C corresponds to the solid electrolyte body.

As the grain size of the grey area is smaller in Example 5 than in Comparative Example, it can be confirmed that the particle growth of the first ceramic particles was effectively retarded in Example 5.

DESCRIPTION OF REFERENCE NUMERALS

1: Gas sensor2: Noble metal particle or noble metal alloy particle4: First ceramic particle6: Second ceramic particle108,110,108C,110C: Electrode for gas sensor109,109C: Solid electrolyte body