Gas sensor and nitrogen oxide sensor

A gas sensor and nitrogen oxide sensor, which, when fitted to the exhaust system of an internal combustion engine, can suppress the influence of harmful substances contained in a measurement gas and can prevent the reduction in sensitivity over time. A harmful substance-trapping layer is formed at a gas inlet for introducing a to-be-measured gas from an external space into an internal space, and in a buffering space formed between diffusion resistance portions. In a trap-formed portion of a gas passage in which the harmful substance-trapping layer is formed, the measurement gas can pass in an amount of 80% or more of when the harmful substance-trapping layer is not formed in the trap-formed portion. A diffusion resistance is attained in the diffusion resistance portions; a harmful substance is trapped in the harmful substance-trapping layer; and the measurement gas is allowed to flow into a detection electrode side.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a gas sensor for measurement of a particular gas component in a to-be-measured gas and, in particular, to a nitrogen oxide sensor for measurement of NOx as a to-be-measured gas component.

2. Description of the Related Art

Various measurement methods and measurement apparatuses have been developed in order to know the concentration of a particular gas component in a to-be-measured gas. For example, there is known a gas sensor of limit current type which uses an ion-conductive solid electrolyte. Further, as a method for measurement of NOx in a to-be-measured gas (e.g. a combustion gas), there is known a technique which uses a sensor comprising an oxygen ion-conductive solid electrolyte (e.g. zirconia) and a Pt electrode and a Rh electrode both formed on the solid electrolyte and measures an electromotive force between the two electrodes utilizing the reducing ability of Rh for NOx (for example, Patent Literature 1).

In such a sensor, the electromotive force changes greatly for the change in the concentration of oxygen in a combustion gas (a to-be-measured gas) but the change of the electromotive force is small for the change in the concentration of to-be-measured gas component (e.g. NOx); therefore, the measurement result of concentration tends to be influenced by various factors.

When such a gas sensor is fitted to the exhaust system of internal combustion engine (e.g. automotive engine) and the internal combustion engine is operated, there has been a problem that the sensitivity of gas sensor to to-be-measured gas component (e.g. NOx) reduces gradually. The present inventors investigated on the reasons therefor and, as a result, it was found that the harmful substances (e.g. Mg) contained in the exhaust gas of internal combustion engine have a large influence.

The aim of the present invention is to provide a gas sensor and a nitrogen oxide sensor, which, when fitted to, for example, the exhaust system of internal combustion engine, can suppress the influence of harmful substances (e.g. Mg) contained in a to-be-measured gas and can prevent the reduction in sensitivity with the increase in use time.

SUMMARY OF THE INVENTION

In order to achieve the above aim, the present invention provides a gas sensor for measuring the amount of a to-be-measured gas component in a to-be-measured gas present in an external space, which comprises:

a solid electrolyte which is in contact with an external space,

an internal space formed inside the solid electrolyte,

a diffusion resistance portion for introducing a to-be-measured gas from the external space via a gas inlet, at a given diffusion resistance,

a pumping means which has an inner pumping electrode formed inside the internal space and an outer pumping electrode formed outside the internal space and which subjects the oxygen contained in the to-be-measured gas introduced from the external space, to a pumping treatment based on a controlling voltage applied between the two electrodes,

a gas component-measuring means having a detection electrode, which electrode decomposes a to-be-measured gas component contained in the to-be-measured gas after the pumping treatment by the pumping means and measures the to-be-measured gas component contained in the to-be-measured gas, based on the oxygen generated by the decomposition, and

a harmful substance-trapping layer made of a porous material capable of trapping a harmful substance, which is formed apart from and upstream of the detection electrode in a gas passage formed inside the solid electrolyte so as to enable the flowing of the to-be-measured gas.

In order to achieve the above aim, the present invention also provides a nitrogen oxide sensor for measuring the amount of a nitrogen oxide component in a to-be-measured gas present in an external space, which sensor comprises:

a main body made of an oxygen ion-conductive solid electrolyte, which is in contact with the external space,

a first internal space formed in the solid electrolyte, communicating with an external space,

an upstream-side diffusion resistance portion formed as a slit for introducing a to-be-measured gas into the first internal space at a given diffusion resistance,

a pumping means which has a first, inside, pumping electrode formed inside the first internal space and a first, outside, pumping electrode formed outside the first internal space and which subjects the oxygen contained in the to-be-measured gas introduced from the external space, to a pumping treatment based on a controlling voltage applied between the two electrodes, to control the oxygen partial pressure in the first internal space to a given level at which NO is not decomposed substantially,

a second internal space communicating with the first internal space,

a downstream-side diffusion resistance portion formed as a slit for introducing the atmosphere subjected to the pumping treatment in the first internal space, into the second internal space at a given diffusion resistance,

a gas component-measuring means which has a second, inside, pumping electrode formed inside the second internal space and a second, outside, pumping electrode formed outside the second internal space and further has a detection electrode for decomposing the NO contained in the atmosphere introduced from the first internal space, by either of catalysis and electrolysis and measuring the to-be-measured gas component in the to-be-measured gas based on the oxygen generated by the decomposition, and

a harmful substance-trapping layer made of a porous material capable of trapping a harmful substance, which is formed apart from and upstream of the detection electrode in a gas passage formed inside the solid electrolyte so as to enable the flowing of the to-be-measured gas.

In the gas sensor and the nitrogen oxide sensor both of the present invention, the harmful substance-trapping layer can be formed specifically in a trap-formed portion of the gas passage so that the to-be-measured gas can pass in an amount of 80% or more of when the harmful substance-trapping layer is not formed in the trap-formed portion.

The harmful substance-trapping layer is preferably made of any of alumina, zirconia and magnesium aluminum spinel.

Also, the harmful substance-trapping layer can be formed at a gas inlet through which the to-be-measured gas is introduced from the external space into the internal space. Further, the diffusion resistance portion can be provided at a plurality of locations and the harmful substance-trapping layer can be formed between these diffusion resistance portions.

Or, the diffusion resistance portion can be formed as a slit and the harmful substance-trapping layer can be formed in the gas passage portion formed by the slit of the diffusion resistance portion.

In the above constitution, the harmful substance-trapping layer can be formed on a wall of the gas passage portion. In that case, the harmful substance-trapping layer is preferably made of a porous alumina material having a porosity of 10% to 70%. The harmful substance-trapping layer can have, in a gas-flowing direction, a length of at least two times the thickness of the gas passage portion. Further, the harmful substance-trapping layer can have a thickness of at least 1/10 of the thickness of the gas passage portion.

Meanwhile, the harmful substance-trapping layer can be formed by being filled in the gas passage portion. In that case, the harmful substance-trapping layer is preferably made of a porous alumina material having a porosity of 40% to 80%. Such a harmful substance-trapping layer can trap at least any of Si, P, Zn, Ca and Mg.

Further, the gas component-measuring means can be constituted as a pumping means for measurement which decomposes, by at least either of catalysis and electrolysis, the to-be-measured gas component contained in the to-be-measured gas after the pumping treatment by the pumping means and subjects the oxygen generated by the decomposition, to a pumping treatment, and the to-be-measured gas component in the to-be-measured gas can be measured based on a pumping current which flows in the pumping means for measurement based on the pumping treatment of the pumping means for measurement.

Or, the gas component-measuring means can be constituted as an oxygen partial pressure detection means which decomposes, by catalysis, the to-be-measured gas component contained in the to-be-measured gas after the pumping treatment by the pumping means and generates an electromotive force corresponding to a difference between the oxygen amount generated by the decomposition and the oxygen amount contained in a reference gas, and the to-be-measured gas component in the to-be-measured gas can be measured based on the electromotive force detected by the oxygen partial pressure detection means.

In the gas sensor and the nitrogen oxide sensor both of the present invention, a harmful substance in a to-be-measured gas is trapped by the harmful substance-trapping layer, whereby the reduction in sensitivity to to-be-measured gas component, caused by the harmful substance can be prevented. Therefore, the sensor of the present invention has high durability. Further, the harmful substance-trapping layer traps the harmful substance and, moreover, has a small influence on the flow amount of to-be-measured gas; therefore, the influence of the harmful substance-trapping layer on the diffusion resistance portion formed in the gas passage can be made small.

EXPLANATION OF SYMBOLS

1: gas sensor;5: harmful substance-trapping layer;12a: first substrate layer;12b: second substrate layer;12c: third substrate layer;12d: first solid electrolyte layer;12e: spacer layer;12f: second solid electrolyte layer;14: sensor element;16: space for introduction of reference gas;18: first chamber;20: second chamber;22: gas inlet;23: buffering space;26: first diffusion resistance portion;27: second diffusion resistance portion;28: third diffusion resistance portion;30aand30b: slits;30k: wall between slits;40: inner pumping electrode;42: outer pumping electrode;44: main pumping cell;48: reference electrode;50: oxygen partial pressure detection cell for control;60: detection cell;62: fourth diffusion resistance portion;64; pumping cell for measurement;66: DC electric source;68: ammeter;70: auxiliary pumping electrode;72: auxiliary pumping cell;74: DC electric source;80: heater;82: insulating layer;160: voltmeter;164: oxygen partial pressure detection cell for measurement

DETAILED DESCRIPTION OF THE INVENTION

The gas sensor according to the present invention is described below with reference to the accompanying drawings, on gas sensor embodiments used for measurement of, for example, oxides (e.g. O2, NO, NO2, SO2, CO2and H2O) contained in the exhaust gas of vehicle or in the air, or combustible gases (e.g. CO and CnHm). The present invention is not restricted to the following embodiments and may be subjected to changes, modifications and improvements as long as there is no deviation from the scope of the present invention.

The gas sensor1according to the embodiment 1-1 has, as shown inFIG. 1, a sensor element14constituted by six (for example), laminated, solid electrolyte layers12ato12fmade of a ceramic using an oxygen-conductive solid electrolyte such as ZrO2or the like.

In the sensor element14, the first to third layers from below are a first substrate12a, a second substrate layer12band a third substrate layer12c, respectively; the fourth and sixth layers from below are a first solid electrolyte layer12dand a second solid electrolyte layer12f; and the fifth layer from below is a spacer layer12e. Between the third substrate layer12cand the spacer layer12e, a space16for introduction of reference gas, into which a reference gas (e.g. air) used as a reference in oxide measurement is to be introduced, is surrounded and formed by the lower surface of the spacer layer12e, the upper surface of the third substrate layer12cand the side of the first solid electrolyte layer12d.

Between the lower surface of the second solid electrolyte layer12fand the upper surface of the first solid electrolyte layer12dare formed a first chamber18which is a first internal space for adjusting the oxygen partial pressure in a to-be-measured gas, and a second chamber20which is a second internal space for finely adjusting the oxygen partial pressure in the to-be-measured gas and further measuring the oxide (e.g. nitrogen oxygen NOx) in the to-be-measured gas. A gas inlet22formed at the front end of the sensor element14and the first chamber18communicate with each other via a first diffusion resistance portion26, a buffering space23and a second diffusion resistance portion27; and the first chamber18and the second chamber20communicate with each other via a third diffusion resistance portion28. A passage of to-be-measured gas extending from the gas inlet22to the second chamber20is called a gas passage.

Here, the first diffusion resistance portion26and the second diffusion resistance portion27are an upstream-side diffusion resistance portion; and the third diffusion resistance portion28is a downstream-side diffusion resistance portion to the to-be-measured gas introduced into the first chamber18and gives a given diffusion resistance to the to-be-measured gas introduced into the second chamber20. As shown inFIG. 2which is an A-A sectional view ofFIG. 1, the first diffusion resistance portion26is formed by two oblong slits30aand30b. Specifically explaining, the first diffusion resistance portion26is constituted by a slit30awhich is formed by an oblong opening provided in contact with the lower surface of the second solid electrolyte layer12fso as to extend to the buffering space23at the same opening width, and a slit30bwhich is formed by an oblong opening provided in contact with the upper surface of the first solid electrolyte layer12dso as to extend to the buffering space23at the same opening width.

In the embodiment 1-1, the slits30aand30bhave about the same sectional shape of, for example, 10 μm or less in vertical direction length and about 2.5 mm in lateral direction length.

The second diffusion resistance portion27and the third diffusion resistance portion28are each formed as well by two oblong slits30aand30bhaving the same sectional shape as of the first diffusion resistance portion26.

Between the first diffusion resistance portion26and the second diffusion resistance portion27is formed the buffering space23by being surrounded by the lower surface of the second solid electrolyte layer12fand the upper surface of the first solid electrolyte layer12d. Owing to the pulsation of an exhaust gas in an external space, oxygen suddenly enters the sensor element through the gas inlet. This oxygen from the external space passes through the first diffusion resistance portion26and enters the buffering space23. The sudden change of oxygen concentration caused by the pulsation of exhaust gas is cancelled by the buffering space23and the influence of the pulsation of exhaust gas in treatment space becomes substantially negligible.

The atmosphere in the first chamber18is introduced into the second chamber20through the third diffusion resistance portion28at a given diffusion resistance.

As shown inFIG. 1,FIG. 3(a) andFIG. 3(b), in a gas passage formed in the solid electrolyte and apart from and upstream of a detection electrode60is formed harmful substance-trapping layers5made of a porous material capable of trapping a harmful substance. Specifically explaining, In the embodiment 1-1 shown inFIG. 1, the harmful substance-trapping layers5are formed at the gas inlet22for introduction of to-be-measured gas from external space into internal space and also in the buffering space23formed between the diffusion resistance portions26and27. These harmful substance-trapping layers5are formed on the walls forming the gas passage, specifically on the lower surface of the second solid electrolyte layer12fand on the upper surface of the first solid electrolyte layer12d.

The harmful substance trapped by the harmful substance-trapping layers5refer to a substance contained in, for example, the exhaust gas of internal combustion engine, which has an influence on the reduction in the sensitivity of the gas sensor1. It is, for example, Si, P, Zn, Ca or Mg. The harmful substance-trapping layers5can effectively remove, in particular, Mg (which has an influence on the reduction in sensitivity) from a to-be-measured gas.

As shown inFIG. 3(a), the thickness (t1+t2) of each harmful substance-trapping layer5is preferably at least 1/10 of the thickness (space thickness) t of the gas passage because such a thickness makes large the volume of the layer5which functions for the trapping of harmful substance (e.g. Mg). Incidentally, the space thickness t is a length between the upper surface of the first solid electrolyte layer12dand the lower surface of the second solid electrolyte layer12f. By employing such a constitution, the influence on the diffusion resistance is suppressed and yet the harmful substance such as Si, P, Zn, Ca, Mg or the like can be trapped.

As shown inFIG. 3(b), the length of each harmful substance-trapping layer5in gas-flowing direction is made at least two times the thickness t of gas passage because the possibility of contact of harmful substance (e.g. Mg) with harmful substance-trapping layer5becomes high. By employing such a constitution, the influence on the diffusion resistance is suppressed and yet the harmful substance can be trapped. That is, by employing a constitution shown inFIG. 3(a) andFIG. 3(b), the to-be-measured gas can pass through a trap-formed portion of gas passage in which the harmful substance-trapping layer5is formed, in an amount of 80% or more of when the harmful substance-trapping layer5is not formed in the trap-formed region; a diffusion resistance can be attained in the diffusion resistance portion; and the harmful substance can be trapped in the harmful substance-trapping layer and the to-be-measured gas can be sent to a detection electrode60side. By thus constituting each harmful substance-trapping layer5, the flow amount of to-be-measured gas in the trap-formed portion in which the harmful substance-trapping layer5is formed, is secured; thereby, the harmful substance can be trapped efficiently by the harmful substance-trapping layer5. As a result, the present gas sensor, when fitted to, for example, the exhaust system of internal combustion engine and used, can suppress the influence of harmful substances contained in a to-be-measured gas and can prevent the reduction in sensitivity with the increase in use time.

Each harmful substance-trapping layer5is preferably made of any of alumina (Al2O3), zirconia (ZrO2) and magnesium aluminum spinel (MgAl2O4). Desirably, the harmful substance-trapping layer5formed on the wall of gas passage is made of a porous alumina material having a porosity of 10% to 70%. More desirably, it is made of a porous alumina material having a porosity of 15% to 30%.

Back inFIG. 1, an inner pumping electrode40made of a flat, nearly rectangular, porous cermet electrode (e.g. a cermet electrode of Pt.ZrO2containing 1% of Au) is formed on the whole portion of the lower surface of the second solid electrolyte layer12f, forming the first chamber18; an outer pumping electrode42is formed on the portion of the upper surface of the second solid electrolyte layer12f, corresponding to the inner pumping electrode40; and an electrochemical pumping cell, that is, a main pumping cell (a pumping means)44is constituted by the inner pumping electrode40, the outer pumping electrode42and the portion of second solid electrolyte layer12fsandwiched by the two electrodes40and42.

Between the inner pumping electrode40and the outer pumping electrode42, of the main pumping cell44is applied a required control voltage (a pumping voltage) Vp1by an external variable electric source46, to pass a pumping current Ip1to a positive direction or a negative direction between the outer pumping electrode42and the inner pumping electrode40; thereby, the oxygen in the atmosphere inside the first chamber18can be pumped out into an external space, or the oxygen in an external space can be pumped into the first chamber18.

A reference electrode48is formed at a position opposing the detection electrode60, which is sandwiched by the lower surface of the first solid electrolyte layer12dand the third substrate layer12c; and an electrochemical sensor cell, i.e. an oxygen partial pressure detection cell50for control is constituted by the inner pumping electrode40, the reference electrode48, the second solid electrolyte layer12f, the spacer layer12eand the first solid electrolyte layer12d.

The oxygen partial pressure detection cell50for control can detect the oxygen partial pressure of the atmosphere in the first chamber18by the electromotive force V1generated between the inner pumping electrode40and the reference electrode48, based on the difference in oxygen concentration between the atmosphere of the first chamber18and the reference gas (air) in the space16for introduction of reference gas.

The oxygen partial pressure detected is used for feed back control of variable electric source46. Specifically explaining, the pumping operation of main pumping cell44is controlled through a feed back control system52for main pump so that the oxygen partial pressure of the atmosphere in the first chamber18becomes a given value which is sufficiently low to conduct the control of oxygen partial pressure in the next second chamber20.

This feed back control system52constitutes a circuit which feed back controls the pumping voltage Vp1between the outer pumping electrode42and the inner pumping electrode40so that the difference between the potential of the inner pumping electrode40and the potential of the reference electrode48(i.e. detection voltage V1) becomes a given voltage level. In this case, the inner pumping electrode40is earthed.

Therefore, the main pumping cell44pumps out or pumps in the oxygen of the to-be-measured gas introduced into the first chamber18, by an amount corresponding to the pumping voltage Vp1. By repeating a series of operations, the oxygen concentration in the first chamber18is feed back controlled to a desired level. In this state, the pumping current Ip1flowing between the outer pumping electrode42and the inner pumping electrode40indicates a difference between the oxygen concentration in the to-be-measured gas and the controlled oxygen concentration in the first chamber18and can be used for measurement of the oxygen concentration in the to-be-measured gas.

Incidentally, the porous cermet electrode constituting each of the inner pumping electrode40and the outer pumping electrode42is constituted by a metal (e.g. Pt) and a ceramic (e.g. ZrO2). However, the inner pumping electrode40which is provided in the first chamber18and which comes into contact with the to-be-measured gas, needs to be made of a material which is low in reduction ability for NO component of to-be-measured gas or has no such ability, and is preferably constituted, for example, by a compound of perovskite structure (e.g. La3CuO4), a cermet between a metal of low catalytic activity (e.g. Au) and a ceramic, or a cermet between a metal of low catalytic activity (e.g. Au), a Pt group metal and a ceramic. When an alloy between Au and a Pt group metal is used as the electrode material, the addition amount of Au is preferably 0.03 to 35 vol. % of the total metal components.

In the gas sensor1according to the embodiment 1-1, the detection electrode60made of a flat, nearly rectangular, porous cermet electrode is formed on the portion of the upper surface of the first solid electrolyte layer12d, which forms the upper surface of the second chamber20and which is apart from the third diffusion resistance portion28. So as to cover this detection electrode60, there is formed an alumina film which constitutes a fourth diffusion resistance portion62and which is a porous protective layer for detection electrode. An electrochemical pumping cell, i.e. a pumping cell64for measurement is constituted by the detection electrode60, the reference electrode48and the first solid electrolyte layer12d.

The detection electrode60is constituted by a porous cermet composed of a metal capable of reducing NOx (a to-be-measured gas component) and zirconia (a ceramic), whereby it functions as a NOx reduction catalyst which reduces NOx present in the atmosphere of second chamber20. In addition, the detection electrode60can pump out the oxygen in the atmosphere of second chamber20, into the space16for introduction of reference gas, by applying a given voltage Vp2between the detection electrode60and the reference electrode48via a DC electric source66. The pumping current Ip2which flows based on the pumping action of the pumping cell64for measurement, can be detected by an ammeter68.

The given-voltage (DC) electric source66can apply such a voltage that can give a limit current to the pumping of the oxygen generated during the decomposition in the pumping cell64for measurement, in the NOx flow restricted by the fourth diffusion resistance portion62.

Meanwhile, an auxiliary pumping electrode70made of a flat, nearly rectangular, porous cermet electrode (for example, a cermet electrode of Pt.ZrO2containing 1% of Au) is formed on the whole portion of the lower surface of the second solid electrolyte layer12f, forming the second chamber20; and an auxiliary electrochemical pumping cell, i.e. an auxiliary pumping cell72is constituted by the auxiliary pumping electrode70, the second solid electrolyte layer12f, the spacer layer12e, the first solid electrolyte layer12dand the reference electrode48.

The auxiliary pumping electrode70, like the inner pumping electrode40of the main pumping cell44, is made of a material which is low in reduction ability for NO component of to-be-measured gas or has no such ability, and is preferably constituted, for example, by a compound of perovskite structure (e.g. La3CuO4), a cermet between a metal of low catalytic activity (e.g. Au) and a ceramic, or a cermet between a metal of low catalytic activity (e.g. Au), a Pt group metal and a ceramic. When an alloy between Au and a Pt group metal is used as the electrode material, the addition amount of Au is preferably 0.03 to 35 vol. % of the total metal components.

The oxygen in the atmosphere of the second chamber20can be pumped out into the space16for introduction of reference gas by applying a desired given voltage Vp3between the auxiliary pumping electrode70and the reference electrode48both of the auxiliary pumping cell72, using an external DC electric source74.

Thereby, the oxygen partial pressure in the atmosphere of the second chamber20is controlled to a low level in which there is substantially no reduction or decomposition of to-be-measured gas component (NOx) and which gives substantially no influence on the measurement of the amount of target component. In this case, the change in oxygen amount introduced into second chamber20is made far smaller than the change in to-be-measured gas, by the action of the main pumping cell44in the first chamber18, whereby the oxygen partial pressure in the second chamber20is controlled precisely at a given level.

Accordingly, in the gas sensor1according to the embodiment 1-1 having the above constitution, the to-be-measured gas whose oxygen partial pressure has been controlled in the second chamber20, is introduced into the detection electrode60.

Further, in the gas sensor1according to the embodiment 1-1, as shown inFIG. 1, a heater80which generates a heat when electrified from outside, is buried in a state that the heater80is sandwiched from above and below by the second substrate layer12band the third substrate layer12c. The heater80is provided to enhance the conductivity of oxygen ion; and, on the upper and lower surfaces of the heater80, an insulating layer (e.g. alumina)82is formed in order to obtain electrical insulation between the second substrate layer12band the third substrate layer12c.

The heater80is provided so as to correspond to the whole portion extending from the first chamber18to the second chamber20. Thereby, the first chamber18and the second chamber20are heated to respective given temperatures and, further, the main pumping cell44, the oxygen partial pressure detection cell50for control and the pumping cell64for measurement are heated to respective given temperatures and maintained at the temperatures.

Next, description is made on the operation of the gas sensor1according to the embodiment 1-1. First, the front end side of the gas sensor1is provided in an external space, whereby a to-be-measured gas is introduced into the first chamber18via the first diffusion resistance portion26(slits30aand30b) and the second diffusion resistance portion27at given diffusion resistances. The to-be-measured gas introduced into the first chamber18undergoes the pumping action of oxygen, caused by applying a given pumping voltage Vp1between the outer pumping electrode42and the inner pumping electrode40both constituting the main pumping cell44; the oxygen partial pressure of the to-be-measured gas introduced is controlled at a given level, for example, 10−7atm. This control is conducted using the feed back control system52.

Incidentally, the first diffusion resistance portion26and the second diffusion resistance portion27, when the pumping voltage Pp1is applied to the main pumping cell44, reduce the amount of the oxygen in the to-be-measured gas, which flows into the measurement space (the first chamber18), and suppress the current flowing in the main pumping cell44.

Also in the first chamber18, even when it is heated by an external to-be-measured gas or further by the heater80, there is formed such a state of oxygen partial pressure that the NOx in the atmosphere is not reduced by the inner pumping electrode40(there takes place no reaction of, for example, NO→½N2+½O2). The reason is that the reduction of NOx in the to-be-measured gas (the atmosphere) makes impossible the exact measurement of NOx in the next second chamber20. Therefore, in the first chamber18, it is necessary to form a state that NOx is not reduced by the component participating in the reduction of NOx (here, the metal component of the inner pimping electrode40). Such a state can be achieved specifically by using, in the inner pumping electrode40, a material low in reduction ability for NOx, for example, an alloy of Au and Pt, as mentioned above.

The gas in the first chamber18is introduced into the second chamber20through the third diffusion resistance portion28at a given diffusion resistance. The gas introduced into the second chamber20undergoes the oxygen-pumping action caused by applying a voltage Vp3between the auxiliary pumping electrode70and the reference electrode48both constituting the auxiliary pumping cell72, whereby the oxygen partial pressure of the gas is finely adjusted to become a given low level.

The third diffusion resistance portion28, like the first diffusion resistance portion26and the second diffusion resistance portion27, when the pumping voltage Vp3is applied to the auxiliary pumping cell72, reduces the amount of the oxygen in the to-be-measured gas, which flows into the measurement space (the second chamber20), and suppresses the current Ip3flowing in the auxiliary pumping cell72.

The to-be-measured gas whose oxygen partial pressure has been controlled in the second chamber20, as described above, is introduced into the detection electrode60through the fourth diffusion resistance portion62at a given diffusion resistance.

When the oxygen partial pressure of the atmosphere in the first chamber18is controlled to a low level which gives substantially no influence on the measurement of NOx, by operating the main pumping cell44, in other words, the pumping voltage Vp1of variable electric source46is adjusted, using the feed back control system52, so that the voltage V1detected by the oxygen partial pressure detection cell50for control becomes a given level and when the oxygen concentration in the to-be-measured gas changes largely in a range of, for example, 0 to 20%, the oxygen partial pressure of the atmosphere in the second chamber20and the oxygen partial pressure of the atmosphere around the detection electrode60ordinarily change slightly. This is considered to be because, when the oxygen concentration in to-be-measured gas becomes high, an oxygen concentration distribution appears in the width direction and thickness direction of the first chamber18and it changes correspondingly to the change of the oxygen concentration in to-be-measured gas.

However, in the gas sensor1according to the embodiment 1-1, the auxiliary pumping cell72is provided so that the oxygen partial pressure of the atmosphere in the second chamber20becomes a given low level always; therefore, even if the oxygen partial pressure of the atmosphere introduced from the first chamber18into the second chamber20has changed correspondingly to the oxygen concentration in to-be-measured gas, the oxygen partial pressure of the atmosphere in the second chamber20can be set at a given low level always by the pumping action of the auxiliary pumping cell72and, as a result, can be controlled to a low level which gives substantially no influence on the measurement of NOx.

The NOx of the to-be-measured gas introduced into the detection electrode60is reduced or decomposed around the detection electrode60, and a reaction of, for example, NO→½N2+½O2takes place. In this case, between the detection electrode60and the reference electrode48both constituting the pumping cell64for measurement, a given voltage Vp2[for example, 430 mV (700° C.)] is applied in a direction in which oxygen is pumped out from the second chamber20into a reference gas-introducing space16side.

Therefore, the pumping current Ip2flowing in the pumping cell64for measurement becomes a value which is proportional to the sum of the oxygen concentration in the atmosphere introduced into the second chamber20, that is, the oxygen concentration in the second chamber20and the oxygen concentration generated by the reduction or decomposition of NOx by the detection electrode60.

In this case, since the oxygen concentration in the atmosphere in the second chamber20is controlled at a given level by the auxiliary pumping cell72, the pumping current Ip2flowing in the pumping cell64for measurement is proportional to the concentration of NOx. Further, since this NOx concentration corresponds to the diffusion amount of NOx restricted by the fourth diffusion resistance portion62, the NOx concentration can be measured accurately from the pumping cell64for measurement through the ammeter68.

Thus, the pumping current Ip2in the pumping cell64for measurement indicates mostly the amount of NOx reduction or decomposition and accordingly does not depend upon the oxygen concentration in to-be-measured gas.

As described above, in the gas sensor1according to the embodiment 1-1, the harmful substance-trapping layers5made of a porous material are formed apart from and upstream of the detection electrode60, in the passage of to-be-measured gas inside the solid electrode, whereby a harmful substance can be trapped by the harmful substance-trapping layers and a to-be-measured gas can be sent to a detection electrode60side. Therefore, the present gas sensor1, when fitted to, for example, the exhaust system of internal combustion engine and used, can suppress the influence of harmful substance contained in to-be-measured gas and can prevent the reduction in sensitivity with the increase in use time.

A gas sensor according to the embodiment 1-2 of the present invention is shown inFIG. 4. In the embodiment 1-2, a first diffusion resistance portion26, a second diffusion resistance portion27and a third diffusion resistance portion28are formed as slits, and harmful substance-trapping layers are formed in the gas passage portions formed by the slits30aand30bof the diffusion resistance portions26,27and28.

InFIG. 5(a) andFIG. 5(b) are shown partially enlarged views for explaining the harmful substance-trapping layers5ofFIG. 4. As shown inFIG. 5(a), the harmful substance-trapping layers5are formed on the walls forming the gas passage portions, specifically on the lower surface of the second solid electrolyte layer12fand on the upper surface of the first solid electrolyte layer12d. Or, as shown inFIG. 5(b), the harmful substance-trapping layers5may be formed on the upper surface and lower surface of the wall30kbetween slits sandwiched by the slits30aand30b. In both cases, each gas passage portion has therein a harmful substance-trapping layer5so that a space remains.

In the present embodiment 1-2, each harmful substance-trapping layer5is preferably made of any of alumina, ZrO2and magnesium aluminum spinel. Desirably, the harmful substance-trapping layer5formed so that a space remains in each slit, is made of a porous alumina material having a porosity of 10% to 70%. More desirably, it is made of a porous alumina material having a porosity of 15% to 30%.

An embodiment 1-3 is shown inFIG. 6. In the embodiment shown inFIG. 6, harmful substance-trapping layers5are formed in a gas inlet22through which a to-be-measured gas is introduced from an external space into an internal space, and in a buffering space23formed between diffusion resistance portions26and27. The harmful substance-trapping layers5formed in respective locations are formed by being filled in a gas passage.

The harmful substance-trapping layers5formed by being filled in a gas passage are desirably formed of a porous alumina material having a porosity of 40% to 80%, in order to secure the flow of to-be-measured gas. By forming the harmful substance-trapping layers5in this way, the flow amount of to-be-measured gas can be secured, a harmful substance can be trapped, and the to-be-measured gas can be sent to a detection electrode60side. Thus, the present embodiment 1-3, when fitted to, for example, the exhaust system of internal combustion engine and used, can suppress the influence of the harmful substance contained in the to-be-measured gas and can prevent the reduction in sensitivity with the increase in use time.

Further, an embodiment 1-4 is shown inFIG. 7. In the embodiment 1-4, a first diffusion resistance portion26, a second diffusion resistance portion27and a third diffusion resistance portion28are formed as slits, and harmful substance-trapping layers5are formed by being filled in gas passage portions formed by the slits of diffusion resistance portions. As shown inFIG. 7, each harmful substance-trapping layer5is formed on the walls forming each gas passage portion in each slit; specifically between the lower surface of a second solid electrolyte layer12fand the upper surface of a wall30kbetween slits, or between the upper surface of a first solid electrolyte layer12dand the lower surface of a wall30kbetween slits. In this case, each harmful substance-trapping layer5is formed in each gas passage portion so that no space remains in the gas passage portion.

The harmful substance-trapping layers5formed so that no space remains, are desirably formed by a porous alumina material having a porosity of 40% to 80%, in order to secure the flow of to-be-measured gas. By forming the harmful substance-trapping layers5in this way, the flow amount of to-be-measured gas can be secured, a harmful substance can be trapped, and the to-be-measured gas can be sent to a detection electrode60side. Thus, the present embodiment 1-4, when fitted to, for example, the exhaust system of internal combustion engine and used, can suppress the influence of the harmful substance contained in the to-be-measured gas and can prevent the reduction in sensitivity with the increase in use time.

Next, description is made on a gas sensor1according to the embodiment 2 of the present invention, with reference toFIG. 8. Incidentally, the sensor components corresponding to those ofFIG. 1are given the same signals, and repeated explanation of these signals is not made.

As shown inFIG. 8, a gas sensor1according to the embodiment 2 has about the same constitution as the gas sensor1(seeFIG. 1) according to the embodiment 1-1, but is different in that an oxygen partial pressure detection cell164for measurement is provided in place of the pumping cell64for measurement.

The oxygen partial pressure detection cell164for measurement is constituted by a detection electrode60formed on the portion of the upper surface of a first solid electrolyte layer12d, forming a second chamber20, a reference electrode48formed on the lower surface of the first solid electrolyte layer12d, and the first solid electrolyte layer12dsandwiched by the two electrodes60and48.

In this case, between the detection electrode60and the reference electrode48, of the oxygen partial pressure detection cell164for measurement is generated an electromotive force (an electromotive force of oxygen concentration cell) V2which corresponds to the difference in oxygen concentration between the atmosphere around the detection electrode60and the atmosphere around the reference electrode48.

Therefore, by measuring the electromotive force (voltage) V2generated between the detection electrode60and the reference electrode48by a voltmeter160, an oxygen partial pressure around the detection electrode60, in other words, an oxygen partial pressure specified by the oxygen generated by reduction or decomposition of to-be-measured gas component (NOx) can be detected as a voltage V2.

By employing such a constitution, the flow amount of to-be-measured gas can be secured, a harmful substance can be trapped, and the to-be-measured gas can be sent to a detection electrode60side. Thus, the present embodiment 2, when fitted to, for example, the exhaust system of internal combustion engine and used, can suppress the influence of the harmful substance contained in the to-be-measured gas and can prevent the reduction in sensitivity with the increase in use time.

EXAMPLES

In order to confirm the effect of the above-described sensor of the present invention, the present sensor and a conventional sensor provided with no harmful substance-trapping layer5were examined for the change in NOx sensitivity with the increase in use time. The test results of the present invention and the conventional technique are shown inFIG. 9.

As shown inFIG. 9, in the present invention, harmful substance is trapped by the harmful substance-trapping layer5, whereby the influence of harmful substance contained in to-be-measured gas can be suppressed and the reduction in sensitivity with the increase in use time can be prevented.

In the gas sensors1(including modification cases) according to the embodiments 1 and 2, oxygen and NOx were selected as to-be-measured gas components; however, these gas sensors can be effectively used as well to the measurement of bound oxygen-containing gas components (e.g. H2O and CO2) other than NOx, which is influenced by the oxygen present in to-be-measured gas.

The gas sensors can also be used, for example, in a gas sensor in which the O2generated by electrolysis of CO2or H2O is pumped out by an oxygen pump, or in a gas sensor in which the H2generated by electrolysis of H2O is subjected to a pumping treatment using a proton-conductive solid electrolyte.

In the above, cases having first to fourth diffusion resistance portions were described. However, the number of diffusion resistance portions formed is not restricted thereto. Cases having diffusion resistance portions as slits were also described. The present invention is applicable also to a case having diffusion resistance portions other than slits, made of, for example, a porous material.

INDUSTRIAL APPLICABILITY

The gas sensor and nitrogen oxide sensor of the present invention can be used as a sensor fitted to, for example, the exhaust system of internal combustion engine.