EP1438572A2

EP1438572A2 - Gas sensor

Info

Publication number: EP1438572A2
Application number: EP02781128A
Authority: EP
Inventors: Heiner Scheer; Carsten Springhorn; Hans-Joerg Renz; Frank Haag; Walter Strassner; Lothar Diehl; Thomas Moser; Stefan Rodewald; Marten Mamey; Juergen Karle
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2001-10-17
Filing date: 2002-10-04
Publication date: 2004-07-21
Also published as: DE10151328B4; JP2005506548A; WO2003036281A3; DE10151328A1; WO2003036281A2; JP4532113B2; US7445699B2; US20050034986A1

Abstract

A gas sensor for the determination of at least one physical parameter of a gas, in particular the exhaust gases of an internal combustion engine, is disclosed, comprising a sensor element (10, 110) with an electrochemical cell. The electrochemical cell comprises a first solid electrolyte body (21, 121), on which a first electrode (41, 141, 241) and a second electrode (42, 142) are mounted. The first and the second electrodes (41, 141, 241, 42, 142) are electrically connected through the first solid electrolyte body (21, 121). The first electrode (41, 141, 241) is in contact with the gas. The surface of the first electrode (41, 141, 241) is smaller than the surface of the second electrode (42, 142).

Description

gas sensor

State of the art

The invention relates to a gas sensor according to the preamble of claim 1.

Such a gas sensor is described for example in DE 199 41 051 AI for use in exhaust gas analysis of internal combustion engines. The gas sensor is used to control the air / fuel ratio of combustion mixtures in motor vehicle engines and contains a sensor element in which a concentration cell (Nernst cell) is combined with an electrochemical pump cell.

The concentration cell of the sensor element has a measuring electrode arranged in a measuring gas area and a reference electrode arranged in a reference gas area. The two electrodes are applied to a solid electrolyte body and electrically connected via the solid electrolyte body. The measuring gas area in which the measuring electrode is arranged is connected to the exhaust gas outside the sensor element via a diffusion barrier and a gas access hole. The reference gas region is in a reference atmosphere via an opening on the side of the sensor element facing away from the measurement gas region Connection. The measuring gas area and the reference gas area lie in the same layer plane of the sensor element constructed as a layer system and are separated by a gas-tight separating body. With different oxygen partial pressures in the measuring gas range and reference gas range, a so-called Nernst voltage is formed between the measuring electrode and the reference electrode. With constant oxygen partial pressure in the reference gas space, the oxygen partial pressure in the sample gas range can be determined from the Nernst voltage.

The pump cell of the sensor element has an annular outer pump electrode arranged on an outer surface of the sensor element and exposed to the exhaust gas, and an likewise ring-shaped inner pump electrode arranged on the solid electrolyte body in the measuring gas region. The inner pump electrode can coincide with the measuring electrode of the Nernst cell or be electrically connected to it. The outer pump electrode has a larger outer radius and a smaller inner radius than the inner pump electrode, so that the area of the outer pump electrode is larger than the area of the inner pump electrode. The electrodes are electrically connected via leads to contact surfaces arranged on the side of the sensor element facing away from the electrodes. The feed lines of the electrodes, in particular the feed line of the outer pump electrode, are electrically insulated from the solid electrolyte body by an insulation layer.

By applying a pump voltage between the outer pump electrode and the inner pump electrode, the pump cell pumps oxygen ions via the solid electrolyte body from the sample gas area into the exhaust gas or vice versa from the exhaust gas into the sample gas area. The pump voltage is regulated by an external circuit so that a Nernst voltage of approximately 450 mV is present between the electrodes of the Nernst cell, which means an oxygen partial pressure in the measuring gas range of lambda = l corresponds to (stoichiometric air-fuel ratio). Accordingly, in the case of a lean exhaust gas (lambda> 1), oxygen is pumped out of the measurement gas area, the pump current flowing in the pump cell being limited by the diffusion flow of the oxygen molecules flowing through the diffusion barrier into the measurement gas area. When the exhaust gas is rich (lambda <1), oxygen is pumped into the sample gas area, and the pump current flowing in the pump cell is limited by the diffusion flow of the gas molecules flowing through the diffusion barrier and consuming oxygen in the sample gas area (the oxygen pumped into the sample gas area reacts with the oxygen there consuming gas molecules). The diffusion flow is proportional to the oxygen concentration of the exhaust gas in the case of lean exhaust gas and to the concentration of the oxygen-consuming gas molecules in the case of rich exhaust gas. In this way, the oxygen partial pressure of the exhaust gas or the partial pressure of the oxygen-consuming gas molecules can be determined from the pump current.

From DE 199 60 329 AI a gas sensor with a similar sensor element is known. In contrast to the sensor element described in DE 199 41 051 AI, the measuring gas area and the reference gas area are arranged in different layer levels. The areas of the outer pump electrode and the inner pump electrode are the same.

With such sensor elements, it is disadvantageous that when the direction of the pump current changes during operation of the

Gas sensor, for example, when there is a change from lean to rich exhaust gas, an overshoot or a counter-oscillation is caused in the probe signal. This so-called λ = 1 ripple affects the evaluation of the probe signal. Advantages of the invention

The gas sensor according to the invention has the advantage that the λ = 1 ripple is greatly reduced or avoided entirely.

The sensor element contains an electrochemical cell, which has a first electrode (outer pump electrode) arranged on an outer surface of the sensor element facing the gas and a second electrode (inner pump electrode, measuring electrode) arranged in a measuring gas region, as well as a solid electrolyte body arranged between the two electrodes and electrically connecting them. The first electrode is directly exposed to the exhaust gas, the oxygen partial pressure of which is subject to major changes. If the exhaust gas is lean, that is to say oxygen-rich, the solid electrolyte also has a high proportion of oxygen in the region of the first electrode. Since the oxygen in the solid electrolyte is in the form of ions, a large amount of charge is formed in the region of the first electrode when the exhaust gas is lean. Accordingly, there is a small amount of charge in the region of the first electrode in the case of rich, low-oxygen exhaust gas. The second electrode, however, is exposed to a largely constant oxygen partial pressure, since an oxygen partial pressure of λ = 1 is set in the measuring gas range.

It has been shown that the amount of charge which forms in the case of lean exhaust gas leads to the λ = 1 ripple in the region of the first electrode when the pump voltage is reversed. Therefore, to reduce the amount of charge on the first electrode, the area of the first electrode is reduced. Since the amount of charge on the second electrode is subject to smaller fluctuations, the area of the second electrode can be larger than the area of the first electrode without the λ = 1 ripple being increased thereby. The measures specified in the dependent claims allow advantageous refinements and developments of the gas sensor specified in the independent claim.

The first and second electrodes are advantageously designed such that, in addition to a reduction in the λ = 1 ripple, there is also a sufficiently low resistance between the first and second electrodes. With a low resistance, a comparatively low pump voltage is sufficient to produce a pump current which is sufficient for the regulation to λ = 1. Since a larger electrode area means a lower resistance, the second electrode is therefore designed to be significantly larger than the first electrode. If the area of the first electrode is 0.06 to 0.6 times the area of the second electrode, the λ = 1 ripple is particularly effectively reduced if the resistance between the first and second electrodes is sufficiently low. The annular first electrode advantageously has an outer radius in the range from 1.1 to 1.7 mm, preferably 1.4 mm, and an inner radius from 0.3 to 0.9 mm, preferably 0.6 mm. The outer radius of the annular second electrode is in the range from 1.7 to 2.1 mm, in particular at 1.9 mm, and the inner radius in the range from 0.8 to 1.2 mm, preferably at 1.0 mm.

In a modification of the invention, the first and the second electrodes are elliptical in shape and have an elliptical recess, the ratio of the main axis to the secondary axis being in the range from 2: 1 to 1.1: 1, preferably 1.5: 1. In the case of sensor elements provided with a heater, a temperature distribution is formed in which areas with the same in the large areas of the sensor element, for example on the outer surface on which the first electrode is applied Temperature are elliptical. Therefore, an elliptical shape of the electrodes ensures that the temperature differences in different areas of the electrode surface are reduced. 5

The first and second electrodes advantageously contain a recess in which there is a gas inlet opening, through which the gas can reach the measuring gas area. The sensor element also has a reference gas range,

.0 the one reference air with a sufficiently constant

Contains partial pressure of oxygen. A third electrode is arranged in the reference gas area. The reference gas region is advantageously provided in the layer plane of the measuring gas region.

.5

In the case of the invention described here, the electrode is understood to mean that region of the conductor track which is applied to a solid electrolyte body and which is in direct contact with the solid electrolyte body and is therefore electrical

! 0 is connected to the solid electrolyte body. On the other hand, the area of the conductor track that is from

Solid electrolyte body is electrically insulated, referred to as the lead of the electrode. The conductor track is therefore in the areas in which it is directly on the solid electrolyte body

25 is applied and in which it makes a contribution to the measurement signal due to its electrochemical properties, as an electrode and in the areas in which it is electrically insulated from the solid electrolyte body and does not or only makes a small contribution to the measurement signal as

SO called lead to the electrode.

In a further development of the invention, the shortest distance between the first electrode and a third electrode arranged in the reference gas region is significantly larger than the distance between the first and the second electrode, which is the layer thickness of the first Solid electrolyte body corresponds. Increasing the distance also increases the resistance between the first and third electrodes, as a result of which the coupling of the first electrode to the third electrode and thus the λ = 1 ripple is further reduced. For this purpose, for example, the feed line of the first electrode is arranged at least in regions in the section which is formed by the vertical projection of the second electrode onto the large area of the first electrode. This means that the conductor track of the first electrode has a partial area which lies in the area of the projection of the second electrode on the large area of the first electrode and in which the conductor track of the first electrode is electrically insulated from the first solid electrolyte body by insulation. In the case of a sensor element in which the measurement gas area and the reference gas area are arranged in the same layer plane, the insulated partial area is advantageously provided on or adjacent to the side of the first electrode facing the reference gas area.

drawing

Embodiments of the invention are shown in the drawing and explained in more detail in the following description. 1 shows a section along the

Longitudinal axis of a sensor element of a first embodiment of a gas sensor according to the invention, Figure 2 shows a section perpendicular to the longitudinal axis of the sensor element of a second embodiment of the gas sensor according to the invention, Figure 3 and Figure 4 is a plan view of the sensor element of the first and second embodiment of the invention, Figure 5 shows a section along the Longitudinal axis of the sensor element of a third embodiment of the gas sensor according to the invention along the line V - V in Figure 6 and Figure 6 is a plan view on the sensor element of the third embodiment of the invention.

5 Description of the exemplary embodiments

FIG. 1 shows, as the first exemplary embodiment of the invention, a sensor element 10 of a gas sensor designated as a broadband lambda probe. The sensor element 10 is as

0 Layer system constructed and contains a first, a second and a third solid electrolyte body 21, 22, 23. In the first solid electrolyte body 21, a gas access opening 36 is introduced. Between the first and the second solid electrolyte body there is a measuring gas area 31,

5 a reference gas region 32, a separating body 33, a

Diffusion barrier 34 and a sealing frame 35 are arranged. In the middle of the flat, hollow-cylindrical measuring gas region 31, the likewise hollow-cylindrical diffusion barrier 34 is arranged, in the middle of which the gas inlet opening 36

! 0 opens. The measuring gas can pass through the gas inlet opening 36 via the diffusion barrier 34 into the measuring gas region 31. The separating body 33 forms a gas-tight barrier between the measuring gas region 31 and the reference gas region 32. The channel-shaped reference gas region 32 contains a porous one

15 material and is connected to a reference atmosphere on the side of the sensor element 10 facing away from the measuring range. Sample gas area 31 and reference gas area 32 are laterally surrounded by a sealing frame 35.

10 A first electrode 41 (outer pump electrode) is arranged on an outer surface of the first solid electrolyte body 21 and is covered by a porous protective layer 45. On the large surface of the first solid electrolyte body 21 opposite the outer surface, a second 15 electrode 42 (measuring electrode, inner pump electrode) is provided in the measuring gas region 31. In the reference gas region 32 there is the second in the layer plane Electrode 42 a third electrode 43 (reference electrode) is provided. The first electrode 41 forms, together with the second electrode 42, a pump cell which pumps oxygen into or out of the measurement gas region 31 by means of external wiring. The pump voltage applied to the pump cell by the external circuitry is regulated in such a way that a predetermined oxygen partial pressure is present in the measuring gas region 31. An oxygen partial pressure of λ = 1 is preferably adjusted, that is to say the oxygen partial pressure in the measurement gas region 31 corresponds to the stoichiometric air / fuel ratio.

The oxygen partial pressure present in the measuring gas region 31 is determined by a Nernst cell which is formed by the second electrode 42 and the third electrode 43. The Nernst cell is used to measure a Nernst voltage caused by different oxygen partial pressures in the measuring gas region 31 and in the reference gas region 32, which - as described above - is used to regulate the pump voltage. In an alternative embodiment, not shown, the electrode belonging to the Nernst cell in the measurement gas region 31 and / or the electrode belonging to the Nernst cell in the reference gas region 32 can be applied to the second solid electrolyte body 22. Furthermore, in addition to the second and third electrodes 42, 43 applied to the first solid electrolyte body 21, at least one further electrode belonging to the Nernst cell can be arranged in the measurement gas region 31 and / or in the reference gas region 32 on the second solid electrolyte body 22.

A heater 37 is arranged between the second and the third solid electrolyte bodies 22, 23 and is electrically insulated from the surrounding solid electrolyte bodies 22, 23 by a heater insulation 38. FIG. 2 shows a second exemplary embodiment of the invention, which differs from the first exemplary embodiment in that the measuring gas region and reference gas region are not arranged in the same layer plane but in different layer planes of the sensor element 110. The sensor element 110 has a first, a second, a third and a fourth solid electrolyte body 121, 122, 123, 124. A measuring gas region 131, a diffusion barrier 134 and a sealing frame 135 are arranged between the first and the second solid electrolyte bodies 121, 122. The exhaust gas passes through a gas access opening 136 introduced into the first solid electrolyte body 121 and via the diffusion barrier 134 into the measuring gas region 131. A reference gas region 132 is introduced into the third solid electrolyte body 123. A heater 137, which is embedded in a heater insulation 138, is provided between the third and fourth solid electrolyte bodies 123, 124.

A first electrode 141, which is covered by a porous protective layer 145, is applied to the outer surface of the first solid electrolyte body 121. In the measuring gas region 131, a second electrode 142 is arranged on the first solid electrolyte body 121 and a third electrode 143 on the second solid electrolyte body. A fourth electrode 144 is provided on the second solid electrolyte body 122 in the reference gas region 132. The first and second electrodes 141, 142 form a pump cell with the first solid electrolyte body 121, the third and fourth electrodes 143, 144 form a Nernst cell with the second solid electrolyte body 122. The functioning of these electrochemical cells corresponds to that of the first exemplary embodiment.

FIG. 3 shows the arrangement of the first electrode 41, 141 and the second electrode 42, 142 on the first solid electrolyte body 21, 121 in a first embodiment of the first and second embodiments. To simplify the drawing, the porous protective layer 45, 145 has been omitted. The first electrode 41, 141 is arranged in a ring around the gas inlet opening 36, 136. The inner radius of the first electrode 41, 141 is 0.6 mm, the outer radius is 1.4 mm. A supply line 41a, 141a connects to the first electrode 41, 141 and leads to a contact surface (not shown) on the side of the sensor element 10, 110 facing away from the electrodes. The first electrode is over the contact surface

41, 141 connected to an evaluation circuit arranged outside the gas sensor. The feed line 41a, 141a to the first electrode 41, 141 is electrically insulated from the first solid electrolyte body 21, 121 by an insulation layer 47 147. The insulation layer 47, 147 follows the circular outer contour of the first electrode 41, 141 in the transition region between the first electrode 41, 141 and the lead 41a, 141a to the first electrode 41, 141.

The second electrode 42, 142 (shown in dashed lines in FIG. 3) is also arranged in a ring around the gas inlet opening 36, 136. Their inside diameter is 10 mm, their outside diameter is 20 mm. The area of the first electrode 41, 141 is thus approximately half the area of the second electrode 42, 142. The second electrode

42, 142 and the further electrodes, like the first electrode, are electrically contacted by a feed line (not shown).

FIG. 4 shows a second embodiment of the first and second exemplary embodiments. To simplify the drawing, the porous protective layer 45, 145 and the insulating layer 47, 147 have been omitted. In the second embodiment, the first electrode 41, 141 is elliptically shaped and has an elliptical recess in which the gas inlet opening 36, 136 is arranged. The ratio of the main axis to the secondary axis of both the outer and the inner boundary of the first electrode 41, 141 is 1.5: 1. The second electrode (not shown) is elliptically shaped like the first electrode 41, 141, the area of the second electrode being twice as large as the area of the first electrode 41, 141. The main axes of the two ellipses of the inner and outer boundaries of the first electrodes 41, 141 are parallel to the longitudinal axis of sensor element 10, 110.

FIG. 5 and FIG. 6 show a third exemplary embodiment of the invention, which differs from the first exemplary embodiment in the design of the first electrode 241, the lead 241a to the first electrode 241, the insulation layer 247 and the porous protective layer 245. The further elements of the sensor element of the third exemplary embodiment were designated with the same reference numerals as in the first exemplary embodiment shown in FIG.

> 0

In the third exemplary embodiment, the first conductor track (ie the first electrode 241 and the lead 241a to the first electrode 241) and the second conductor track (ie the second electrode 42 and the one not shown)

! 5 supply line to the second electrode 42) at least in the area of the measuring gas area 31 of the sensor element 10 of the same shape. The projection of the ring-shaped section of the second conductor track, ie essentially the electrode 42, onto the outer surface of the first corresponds

SO solid electrolyte body 21 just the shape of the first

Trace in this area. The lead 241a of the first electrode 241 is electrically insulated from the first solid electrolyte body 21 by the insulation layer 247. Insulation layer 247 also extends into one

S5 isolated portion 250 of the projection of the second electrode 242 onto the outer surface of the first Solid electrolyte body 21. The insulated partial region 250 borders on the side of the first electrode 241 facing the reference gas region 32 and the third electrode 43. The insulation layer 247 consists essentially of aluminum oxide.

Embodiments of the third exemplary embodiment are conceivable in which the first conductor track and the second conductor track are not of the same shape even in the measuring range of the sensor element 10. In particular, the first electrode 241 can be configured smaller than the second electrode 242, that is to say, for example, have a smaller outer radius or a smaller inner and outer radius or a larger inner radius than the second electrode 242.

The invention is not limited to the exemplary embodiments described and can also be applied to sensor elements with a different structure in which malfunctions occur due to a high amount of charge in the area of an electrode applied to an outer surface of the sensor element.

Claims

Expectations

1. Gas sensor, preferably for detecting at least one physical quantity of a gas, in particular exhaust gases from an internal combustion engine, with a sensor element

(10, 110), which has a first solid electrolyte body (21, 121), on which a first electrode (41, 141, 241) and a second electrode (42, 142) are applied, the first and second electrodes (41 , 141, 241, 42, 142) are electrically connected by the first solid electrolyte body (21, 121), and wherein the first electrode (41, 141, 241) is in contact with the gas, characterized in that the surface of the first electrode (41, 141, 241) is smaller than the area of the second electrode (42, 142).

2. Gas sensor according to claim 1, characterized in that the area of the first electrode (41, 141, 241) is at most 60 percent, in particular between 5 and 30 percent of the area of the second electrode (42, 142).

3. Gas sensor according to claim 1 or 2, characterized in that the first electrode (41, 141, 241) is arranged on a surface of the sensor element (10, 110) facing the gas, that the second electrode (42, 142) in one in the sensor element (10, 110) introduced measuring gas area (31, 131) is arranged, and that the first solid electrolyte body (21, 121) has a gas access opening (36, 136) through which the gas can reach the measuring gas region (31, 131).

5 4. Gas sensor according to claim 3, characterized in that the first and / or the second electrode (41, 141, 241, 42, 142) has a recess in which the gas inlet opening (36, 136) is arranged.

.0 5. Gas sensor according to one of the preceding claims, characterized in that the at least partially annular first electrode (41, 141) has an outer radius in the range from 1.0 to 1.7 mm, in particular 1.2 mm, and an inner radius in Range from 0.3 to 1.3 mm,

L5 in particular 1.0 mm, and that the annular second electrode (42, 142) has an outer radius in the range from 1.7 to 2.1 mm, in particular 1.9 mm, and an inner radius in the range from 0.8 to 1.2 mm, in particular 1.0 mm. 0

6. Gas sensor according to one of claims 1 to 4, characterized in that the first and / or the second electrode (41, 141, 241, 42, 142) is elliptical with an elliptical recess, the

! 5 Ratio of major axis to minor axis of the ellipses in the

Range from 2: 1 to 1.1: 1, in particular 1.5: 1, and the main axis is a parallel to the longitudinal axis of the sensor element (10, 110).

SO 7. Gas sensor according to one of claims 3 to 6, characterized in that the first electrode (41, 141, 241) extends to the edge of the gas inlet opening (36, 136).

S5 8. Gas sensor according to one of the preceding claims, characterized in that the measuring gas area (31) and a reference gas region (32) and / or an electrode arranged in the measurement gas region (31), in particular the second electrode (42), and an electrode (43) arranged in the reference gas region (32) lie in the same layer plane of the sensor element (10).

9. Gas sensor according to one of the preceding claims, characterized in that the electrochemical cell is a pump cell and / or that the sensor element (10, 100) as a further electrochemical cell contains a Nernst cell which has at least one electrode (42, 143) arranged in the measuring gas region ), in particular the second electrode (42), and a further electrode (43, 144) arranged in a reference gas region, the second and the further electrode (42, 143; 43, 144) being electrical by a solid electrolyte (21, 122) are connected.

10. Gas sensor according to claim 1, characterized in that in the region of the vertical projection of the second electrode (42) on the layer plane of the first electrode (241) an insulated partial region (250) is provided, in which one of the first electrode (241) and a supply line (241a) to the first electrode (241) having a conductor track is electrically insulated from the first solid electrolyte body (21) by an insulation layer (247).

11. Gas sensor according to claim 10, characterized in that the second electrode (42) is arranged in a measuring gas region (31) introduced into the sensor element (10), that a reference gas region (32) is provided in the layer plane of the measuring gas region (31), and that the insulated partial region (250) is provided adjacent to the side of the first electrode (241) facing the reference gas region (32).

2. Gas sensor according to one of the preceding claims, characterized in that a third electrode (43) is arranged in a reference gas region (32), and that the shortest distance between the first electrode (41, 241) and the third electrode (43) at least is 50 percent greater than the layer thickness of the first solid electrolyte body (21).