WO2015028268A1 - Sensor element for determining at least one property of a measuring gas in a measuring chamber - Google Patents

Abstract

The invention relates to a sensor element (10) for detecting at least one property of a measuring gas in a measuring gas chamber, in particular for detecting a proportion of the gas component in the measuring gas or a temperature of the measuring gas. Said sensor element (10) comprises at least one solid electrolyte layer (12), at least one functional element and a heating element (16) for generating heat. Said heating element (16) is made at least partially from a material which comprises at least aluminum oxide. One proportion of the aluminum oxide is 1.5 wt.-% - 8.5 wt.- %, preferably 2.0 wt.-% - 8.0 wt.-% and more preferably 3.0 wt.-% - 6.0 wt. %.

Description

 description

title

 Sensor element for detecting at least one property of a Messqases in a sample gas space

State of the art

A large number of sensor elements and methods for detecting at least one property of a measurement gas in a measurement gas space are known from the prior art. In principle, these may be any desired physical and / or chemical properties of the measurement gas, one or more of these

 Properties can be detected. The invention will be described below in particular with reference to a qualitative and / or quantitative detection of a portion of a gas component of the measurement gas, in particular with reference to a detection of an oxygen content in the measurement gas. The oxygen content can

for example, in the form of a partial pressure and / or in the form of a percentage. Alternatively or additionally, however, other properties of the

Measuring gas detected, such as other exhaust gas constituents, in particular water, nitrogen oxides, hydrocarbons, etc., or the temperature. For example, such sensor elements may be configured as so-called lambda probes, as they are known, for example, from Konrad Reif (ed.): Sensors in

Motor vehicle, 1. Edition 2010, pp. 160-165. With broadband lambda probes, in particular with planar broadband lambda probes, for example, the

Determined oxygen concentration in the exhaust gas in a wide range and thus be closed to the air-fuel ratio in the combustion chamber. The air ratio λ describes this air-fuel ratio.

Such sensor elements usually have a heating element. The heating element can be integrated in the sensor element. The sensor elements are heated by the heating element after engine start within a few seconds to an operating temperature of about 700 ° C to 800 'Ό. The time to reach the Operating temperature, ie the so-called "light off", depends heavily on that in the

Heating element converted heating power from. Moreover, the light off decreases the more local the heating energy is introduced near the Nernst electrodes, since here the

Temperature determination via internal resistance measurement takes place.

Despite the numerous advantages of the sensor elements known from the prior art, these still contain room for improvement. Thus, the maximum heating power that can be introduced in the heating element is limited, among other things, by two factors: the maximum current level of the output stage in the engine control unit and the maximum permissible temperature in the heating area of the heating element without material damage. From the maximum permissible current level of the output stage, a minimum heating resistance of approx. 2.5 Ω is derived. The desire for a fast light off requires that the heating loop be designed as short as possible and heated as locally as possible Nernstelektroden. Using the known platinum-containing pastes of series production results from the combination of the two above-mentioned requirements, d. H. a resistance <2.5 Ω and a shortest possible length, very small cross sections of the heating loop. This limits the heat transfer surface of the heating loop. This can lead to a build-up of heat and the maximum permissible temperature in the platinum of the heating element can be exceeded. The heater can burn up locally, the track can be interrupted and thus lose the function of conduction.

DISCLOSURE OF THE INVENTION Therefore, a sensor element is proposed for detecting at least one property of a measurement gas in a measurement gas space which at least largely avoids the disadvantages of known sensor elements and in which, in particular, the

Heat transfer surface is significantly increased for a given total resistance or given length of the heating region of the heating element.

The sensor element according to the invention for detecting at least one property of a measurement gas in a measurement gas space, in particular for detecting a proportion of a gas component in the measurement gas or a temperature of the measurement gas, comprises at least one solid electrolyte layer, at least one functional element and a

Heating element for generating heat. The heating element is at least partially made of a material comprising at least alumina (Al ₂ O ₃ ). A share of Alumina is from 1.5% to 8.5%, preferably from 2.0% to 8.0%, and more preferably from 3.0% to 6, 0 wt .-%, for example, 5 wt .-%.

The heating element may have a resistivity of from 0.21 Q ^* mm ² / m to 0.70 Q ^* mm ² / m, preferably from 0.24 Q ^* mm ² / m to 0.50 Q ^* mm ² / m and even more more preferably from 0.30 Q ^* mm ² / m to 0.40 Q ^* mm ² / m, for example 0.34 Q ^* mm ² / m to 0.38 Q ^* mm ² / m, in particular 0.36 Q ^* mm ² / m.

Resistance values and specific resistances refer to this

Registration unless otherwise noted at a temperature of 20 ° C.

The material of the heating element may further comprise platinum. A proportion of the platinum may be from 92% to 97% by weight and preferably from 93% to 96% by weight,

for example 94% by weight. The functional element may be a first electrode. The solid electrolyte layer, the first electrode and the heating element form one

 Layer structure. The solid electrolyte layer has an end face facing the measurement gas space and at least two arranged parallel to the layer structure

Side surfaces on. The heating element has a heating area and at least one

Supply line on. The heating element is arranged in the layer structure such that the heating region at least in the vicinity of the side surfaces of the solid electrolyte layer

is arranged. The first electrode is arranged in the layer structure such that the first electrode overlaps at least partially with the heating region when viewed in a direction parallel to the layer structure. The first electrode may be arranged in the layer structure such that at least 60% of a surface of the first electrode facing the heating element overlaps with the heating region in a direction parallel to the layer structure. The first electrode may be arranged at least in the vicinity of the side surfaces of the solid electrolyte layer. The heating element may be arranged in the layer structure such that the heating region is arranged in the vicinity of the end face facing the measuring gas chamber. The sensor element further comprises at least one second electrode, wherein the first electrode, the second electrode and the solid electrolyte layer form a pumping cell, wherein the first electrode is arranged in the interior of the layer structure and the second electrode is disposed on a side exposed to the measurement gas space outside of the layer structure. The first electrode may be at least partially annular

be educated. The first electrode may be formed, for example, as a ring segment be. The first electrode may alternatively be formed as a ring electrode. The

Sensor element may further comprise at least a third electrode. The first electrode, the third electrode and the solid electrolyte layer may form a Nernst cell. The third electrode may be arranged in the layer structure such that the Nernst cell can be heated by the heating region. The third electrode can partly overlap with the heating area in a direction parallel to the layer structure. The heating region may include at least a portion extending from at least one of the side surfaces toward the third electrode. The sensor element may extend in a longitudinal direction in the measuring gas space. The heating area may be designed to be in

 Longitudinal direction seen at the height of the third electrode a smaller

Cross-sectional area than at the level of the first electrode. The sensor element may extend in a longitudinal direction in the measuring gas space. In

Seen longitudinal direction, the third electrode can be arranged closer to the end face than the first electrode. The longitudinal direction can be perpendicular to the end face of the solid electrolyte layer. The sensor element can furthermore have a gas access path. The first electrode can be acted on by means of the Gaszutrittswegs with the sample gas. The first electrode may be located between the end face of the solid electrolyte layer and the gas access path. The first electrode and / or the third electrode may be formed substantially rectangular in a view parallel to the layer structure. Depending on how the sensor works, additional electrodes may be included.

In the context of the present invention, a solid electrolyte layer is a body or article having electrolytic properties, that is, ion-conducting

 Properties to understand, for example, oxygen ion-conductive properties. In particular, it may be a ceramic solid electrolyte. This also includes the raw material of a solid electrolyte and therefore the formation as a so-called green or brown, which become a solid electrolyte after sintering. In particular, the solid electrolyte may be in the form of a solid electrolyte layer or of several

Be formed solid electrolyte layers. For example, the solid electrolyte layer may contain yttrium stabilized zirconia and / or scandium stabilized zirconia. In the context of the present invention, a layer is to be understood as a uniform mass in a planar extent at a certain height, which lies above, below or between other elements. In the context of the present invention, a layer structure is generally to be understood as meaning an element which has at least two layers and / or layer planes arranged one above the other. The layers can be distinguished by the production of the layer structure distinguishable and / or made of different materials and / or starting materials. In particular, the layer structure can be designed completely or partially as a ceramic layer structure. The direction of the

Layer structure determined perpendicular to the respective layers or

Layer planes.

In the context of the present invention, a specific resistance means the specific electrical resistance of a component. The specific one

 Resistance is a temperature-dependent material constant. In the context of the present invention, the specific resistance is given predominantly for a heating element. Since the heating element is designed as an electrical resistance path or electrical conductor, the specific resistance is mainly for calculating the electrical

Resistance of the electrical conductor used. The derived SI unit is Ω ^* m, which results from the dimension-related reduction of Ω ^* m ² / m. Since the heating element has a comparatively small cross-sectional area, in the context of the present invention the specific resistance is given as Ω ^* mm ² / m. The reciprocal of the resistivity is the specific electrical conductivity. In the context of the present invention, the specific resistance of the heating element relates in particular to a sintered state of the heating element, unless stated otherwise. The heating element is usually by means of a paste on the

Solid electrolyte layer applied and has a much higher resistance or is much higher impedance than the sintered heating element. If reference is made in the context of the present invention to a specific resistance of the paste, this relates essentially to the solids content of the paste. Since only the organic constituents of the paste disappear during sintering, the composition of the sintered heating element is corresponding to the composition of the solids content of the paste. An electrode in the context of the present invention is generally to be understood as an element which is able to contact the solid electrolyte layer in such a way that a current can be maintained through the solid electrolyte layer and the electrode. Accordingly, the electrode may comprise an element on which the ions can be incorporated into the solid electrolyte layer and / or removed from the solid electrolyte layer. The electrode is located at a position of the solid electrolyte layer at which the ion conduction takes place during operation or a certain minimum value of the ionic conductivity of the solid electrolyte layer is present. Typically, these include

Electrodes a noble metal electrode, which may be applied, for example as a metal-ceramic electrode on the solid electrolyte layer or otherwise with the

 Solid electrolyte layer may be in communication. Typical electrode materials are platinum cermet electrodes. However, other precious metals, such as gold or palladium, are in principle applicable. The actual electrode can be distinguished from its supply line in that it has a larger cross section than the supply line. The feed line is located at a location of the solid electrolyte layer at which no or only a very small ion conduction takes place during operation.

In the context of the present invention, a heating element is to be understood as meaning an element which serves to heat the solid electrolyte layer and the electrodes to at least their functional temperature and preferably to their operating temperature. The functional temperature is the temperature at which the solid electrolyte layer becomes conductive to ions and is about 350 ° C. Of this, the operating temperature is to be distinguished, which is the temperature at which the sensor element is usually operated and which is higher than the operating temperature. The operating temperature may be, for example, from 600 ° C to 950 ° C.

The heating element may comprise a heating area and at least one feed track. In the context of the present invention, a heating region is to be understood as meaning that region of the heating element which overlaps the first electrode in the layer structure along a direction perpendicular to the surface of the sensor element.

Usually, the heating area heats up more during operation than the

Lead path. The heating area and / or the supply track are formed, for example, as an electrical resistance path and heat up by applying an electrical voltage. The heating element may for example be made of a platinum cermet. The greater heating of the heating area compared to the

Resistance path can be realized, for example, that the heating area has a higher electrical resistance than the supply track. This can be realized by selecting the material and / or a smaller cross-sectional area than the supply track. In the context of the present invention, an arrangement of a component or functional element at least in the vicinity of the side surfaces means an arrangement of the component in which the component or functional element is arranged as close as technically possible to the side surfaces. The component is preferably in a

certain maximum distance from the side surfaces arranged. The distance is determined perpendicular to one of the side surfaces starting from the side surface up to a side surface facing the component or functional element. The maximum distance from the side surface is from 0 μηι to 100 μηι, for example, 10 μηι, and preferably 0 μηι. In the context of the present invention, an arrangement of the heating region at least in the vicinity of the side surfaces of the solid electrolyte layer is to be understood as an arrangement of the heating region in which the heating region is arranged as close as technically possible to the side surfaces. Typically, such an arrangement is technically limited by the provision of a so-called sealing frame made of an electrically insulating material, such as alumina or zirconia, containing the

Heater and the heating area against the Au ßenumgebung the sensor element shields. The maximum distance from the side surface is from 0 μηι to 100 μηι, for example, 10 μηι, and preferably 0 μηι. In the context of the present invention, an arrangement of the first electrode at least in the vicinity of the side surfaces of the solid electrolyte layer is understood to mean an arrangement of the first electrode in which the first electrode is arranged as close as technically possible to the side surfaces. The maximum distance from the side surface is from 0 μηι to 100 μηι, for example, 10 μηι, and preferably 0 μηι.

In the context of the present invention, a partial overlap in a direction parallel to the layer structure is understood to mean an arrangement in which the first element projects with at least 30% of its surface facing the heating element when the first electrode projects onto the heating element in the direction of the heating element Layer structure is located on the heating area. In the context of the present invention, a functional element is to be understood as meaning an element which is selected from the group consisting of: electrode, heating element, pumping cell, Nernst cell, diffusion barrier. In particular, functional elements are those elements of a sensor element that are intended for chemical, physical, electrical, electrophysical, and / or electrochemical

Functions are important.

In the context of the present invention, a thickness of a component or an element is to be understood as meaning a dimension in the direction of the layer structure and thus perpendicular to the individual layer planes of the layer structure.

In the context of the present invention, a gas-access hole is to be understood as meaning a channel-shaped opening with an arbitrary cross-section, such as rectangular, square or circular, which is suitable for transferring a sample gas from the sample gas space into an actual measurement space arranged in the interior of the sensor element or the solid electrolyte layer to let in.

In the context of the present invention, an end face facing the measuring gas space is to be understood as that area of a solid electrolyte layer which is arranged furthest within the measuring gas space. The position of the end face is seen in an extension direction of the sensor element, so that the end face is that surface of the sensor element which, viewed in the extension direction, is arranged furthest within the measurement gas space. For example, the sensor element defines a longitudinal extension direction, which is a direction in which the sensor element extends into the measurement gas space, wherein the end face extends perpendicular to this extension direction. Since the extension direction is usually parallel to the largest surface of the sensor element, this is also called

Longitudinal direction referred to. An arrangement of an electrochemical cell between the end face and the gas access hole is understood to mean an arrangement that is inverted with a gas access hole compared with conventional sensor elements. In conventional

Sensor elements is the gas inlet hole between the end face and the electrochemical cell. According to the invention, it is provided that the gas inlet hole is displaced in the direction of the terminal contact electrodes, so that the electrochemical cell is located between the end face and the gas inlet hole, ie the gas inlet hole is located farther from the end surface than the gas inlet hole

electrochemical cell. The arrangement is in the extension direction of the

Sensor element seen. The invention is described below in particular with reference to the so-called

Broadband lambda probes described. In such broadband lambda probes, the amount of oxygen or fatty gas diffusing into the measuring cavity is measured either by means of a limiting current or by means of the pumping current necessary for controlling the cavity concentration to λ = 1. The flowing measuring current is proportional to the oxygen or fat gas content in the exhaust gas.

A basic idea of the present invention is to adapt the geometry of the heating element and of the measuring cell of the sensor element to one another in such a way that a local or a localization of the sensor element is necessary

Heating the measuring cell while minimizing the tensile stresses is possible and thus the heating rate can be increased. This can be achieved by an arrangement of the heating element in the vicinity of the side surfaces of the solid electrolyte layer. In this case, a partial overlap of the first electrode and the heating region of the heating element is also proposed. This can be achieved by arranging the first electrode at the heating region of the heating element or, conversely, the heating region of the heating element at the first electrode, so that overall the first electrode is arranged in the hotspot of the heating element. This type of arrangement is to be understood in a projection plane in a direction parallel to the layer structure. The maximum tensile stresses typically occur at the side edges or

Side surfaces of the sensor element at the level of a hotspot generated by the heating element, since the tensile stresses are proportional to a temperature difference between the temperature of the hotspot and the temperature at the side surface. By a targeted heating of the side surfaces or the side edges therefore a possibility is proposed according to the invention, with which the tensile stresses can be minimized.

The functional elements to be heated quickly to the usual operating temperatures of 600 ° to 850 ° C. are the pumping cell, the Nernst cell and the diffusion barrier. The pump cell has the highest temperature requirement. To a sufficiently high

To achieve oxygen ion conductivity and sufficient dynamics, namely

Minimum temperatures of about 700 'Ό required. The Nernst voltage for measurement or regulation of the oxygen partial pressure in the cavity is temperature-dependent. Temperatures above 350 'Ό are required to measure the Nernst voltage. The Nernst cell also has a temperature measuring function or

Temperature control function. The diffusion barrier is of secondary importance. In principle, no high temperature is required for gas diffusion. However, diffusion through the diffusion barrier is temperature dependent. In order to deliver as accurate a measurement signal as possible, the temperature of the three mentioned functional elements, i. H. Pump cell, Nernst cell and diffusion barrier to keep constant during operation.

Conventional measuring cell arrangements in conventional sensor elements disregard the above-mentioned aspect in that in order to minimize the tensile stresses, the sensor element side edge has to be heated up quickly. Pump cell, Nernst cell and diffusion barrier are usually arranged in the middle of the sensor element and not adapted to the specific geometry of the heating element. In the context of the present invention, the temperature distribution during the

Heating process by appropriate design of the heating element with respect to geometry and material adjusted so that the thermo-mechanical stresses are minimal. For this purpose, the energy must be introduced from the outer edge of the sensor element in order to avoid tensile stresses on the side edges. This allows a fast heating of the sensor element. The measuring cell geometry is adapted to the existing temperature distribution. The pump cell is placed in the hottest area, so that the inner pumping and Nernst electrode comes to lie over a large area at the hottest point of the heating element. The Nernst cell and the diffusion barrier with lower temperature requirements can be arranged in colder or slower heated areas of the sensor element.

By way of example only and not exhaustively, some are now possible

Exemplary embodiments explained briefly. An annular or omega-shaped heater is the ideal geometry to place the conductor tracks of the heating element, which bring the energy into the sensor element, as far as possible to the edges of the sensor element. In this way, the sensor element is heated exclusively from the outside inwards, so that the

thermomechanical tensile stresses are minimal. Likewise, the "omega" geometry of the heating element is ideally adaptable to an annular pump electrode, which proves the temperature distribution in the sensor element when it is ready for operation an optimally short heat flow from the heating element to the pumping electrode is ensured. This causes the fastest possible heating of the pump electrode.

Alternatively, the electrode may be formed as a ring segment. The heating element according to the embodiment described above is optimally adapted to the frontal and lateral geometry of the inner pumping electrode. Only the contact-side area is not optimally heated. As a possible alternative this could be

Electrode sector omitted so that there is an open electrode ring and the reference electrode could be positioned closer to the forehead. This would have the advantage that the Nernst cell is also moved further into the hot area. The latter opens the possibility that for the release of the sensor element in the vehicle not only the

Internal resistance of the pumping cell, but also the Nernst cell can be used.

Another possible embodiment is an annular heating element with additional heating of the Nernst cell. If one wishes to retain the full electrode ring of the inner pumping electrode and simultaneously heat the Nernst cell rapidly with an omega-shaped heating element, then this can be supplemented by the first embodiment described by a tapering of the heating area at the height of the

Nernst cell be heated up faster. As a further alternative, the heating area at the level of the Nernst cell could be laid more strongly in the middle of the sensor element. This would have the advantage that the Nernst cell is heated faster. The latter opens the possibility that not only the internal resistance of the pump cell, but also the Nernst cell can be used for the release of the probe in the vehicle. Another possible embodiment is a linear design of the sensor element with inverted arrangement of the diffusion barrier and the pumping electrode. For a

Sensor element with a linear arrangement of diffusion barrier and pump electrode, ie linear design compared to the previously considered radial design with annular pump electrode and diffusion barrier, can increase the thermo-mechanical robustness during rapid heating by the pump electrode is placed directly on the front edge. This is followed by the connection contact side diffusion barrier and gas access. In comparison to the conventional structure of the linear design thus results in an inverted arrangement of pumping electrode and diffusion barrier. The advantage of this arrangement is that the sensor element can be heated by an omega-shaped heating element from the outside, which ensures a high thermo-mechanical robustness, and nevertheless the pump electrode is heated directly, which ensures a fast fast light-off. In addition, the area enclosed by the electrode is due to the linear

Messzellenaufbaus compared to the annular structure, i. Radial design, downsized. This design combines the advantages of linear and radial design and continues the idea of designing the pump electrode as a ring segment.

Another possible embodiment is a sandwich construction of pumping cell and Nernst cell. The classical structure of a sensor element of a broadband lambda probe provides for an arrangement of the inner pumping and Nernst electrode and the reference electrode on one level. By a sandwich-like construction, in which the reference electrode between the inner pumping and Nernstelektrode and the externa ßeren pumping electrode is arranged, the reference electrode direction end edge can be moved into the hotspot and so a faster internal resistance control readiness so temperature control readiness can be achieved. Because the Nernst cell with the help of

Internal resistance measurement serves as a thermometer of the sensor element, it can be ensured by this arrangement that the temperature is measured in the hottest area, and thus prevent overheating of the sensor element. This would allow the measurement of the temperature to be maintained via the Nernst cell to release the probe instead of using the internal resistance signal of the pumping cell. In the area between the inner pumping and Nernst electrode and externa ßere pumping electrode, the reference channel must be narrow so that the electrodes are not isolated from each other.

Another possible embodiment is a sensor element with lateral or front gas inlet. The embodiment described above with the inverted arrangement of the diffusion barrier and the pumping electrode and the classical

Linear design can also be realized with lateral or frontal gas access. This has the advantage that at the same time the Wasserschlagrobustheit can be increased. In this case, if an open gas access is avoided by pulling the porous diffusion barrier to the outside, the thermomechanical robustness of the

Sensor element can be increased.

A basic idea of the invention is an increase of the scaffold portion (Al ₂ O ₃ ) from 1 wt.%, As it is present in pastes for conventional sensor elements, to 5 wt.%. The heat transfer surface is thereby significantly increased at a given total resistance or given length of the meander of the heating element. This leads to a lowering of the maximum temperatures in Heizmäander and thereby to a longer life in Heizerdauerglühen and Heizerdauerschalt. BRIEF DESCRIPTION OF THE DRAWINGS Further optional details and features of the invention will become apparent from the following description of preferred embodiments, which are shown schematically in the figures.

Show it:

Figure 1 is a sectional view of a sensor element according to a first

 Embodiment of the invention, a distribution of the temperature in the sensor element according to the first embodiment during operation, a distribution of the tensile stress in the sensor element according to the first embodiment during operation, a sectional view of a sensor element according to a second embodiment of the invention, a sectional view of a sensor element according to a third

 Embodiment of the invention, a distribution of temperature in the sensor element according to the third embodiment during operation, a distribution of tensile stress in the sensor element according to the third embodiment during operation, a sectional view of a sensor element according to a fourth embodiment of the invention, Figure 9 is a sectional view of a Sensor element according to a fifth

Embodiment of the invention, Figure 10 is a sectional view of a sensor element according to a sixth

 Embodiment of the invention and Figure 1 1 is a sectional view of the sensor element according to the sixth

 Embodiment.

Embodiments of the invention

FIG. 1 shows a sectional view of a sensor element 10 according to a first embodiment

Embodiment of the invention. The cut runs parallel to one of the largest

Surfaces of the sensor element 10 and a longitudinal direction of the

Sensor element 10. The sensor element 10 shown in Figure 1 can be used to detect physical and / or chemical properties of a sample gas, wherein one or more properties can be detected. The invention will be described below in particular with reference to a qualitative and / or quantitative detection of a gas component of the measurement gas, in particular with reference to a detection of an oxygen content in the measurement gas. The oxygen content can be detected, for example, in the form of a partial pressure and / or in the form of a percentage. Basically, however, are also other types of

Gas components detectable, such as nitrogen oxides, hydrocarbons and / or hydrogen. Alternatively or additionally, however, other properties of the measuring gas can also be detected. The invention can be used in particular in the field of motor vehicle technology, so that the measuring gas chamber can be, in particular, an exhaust gas tract of an internal combustion engine, with the measuring gas in particular being an exhaust gas.

The sensor element 10 comprises at least one solid electrolyte layer 12 with at least one functional element 13, which in this embodiment is a first electrode 14, and a heating element 16. The heating element 16 is at least partially made of a material comprising at least alumina (Al ₂ 0 ₃ ) , The proportion of the alumina based on the solids of the material of the heating element 16 is from 1, 5 wt .-% to 8.5 wt .-%, preferably from 2.0 wt .-% to 8.0 wt .-% and still

more preferably from 3.0% to 6.0% by weight, for example 5% by weight: The heating element 16 has a resistivity of from 0.21 Q ^* mm ² / m to 0.70 Q ^* mm ² / m, preferably from 0.24 Q ^* mm ² / m to 0.50 Q ^* mm ² / m, and more preferably from 0.30 Q ^* mm ² / m to 0.40 Q ^* mm ² / m. In particular, the resistivity of the heating element may be from 0.34 Q ^* mm ² / m to 0.38 Q ^* mm ² / m, for example 0.36 Q ^* mm ² / m. The material of the heating element 16 further comprises platinum. A proportion of the platinum may be from 92% to 97% by weight, and preferably from 93% to 96% by weight, for example 94% by weight. Furthermore, the material of the heating element may comprise 16% by weight rhodium.

The solid electrolyte layer 12, the first electrode 14 and the heating element 16 form a layer structure 18, the orientation of which is indicated by an arrow. The view of FIG. 1 is a view in the direction of the layer structure 18, so that the layer structure 18 is oriented in the plane of the drawing or out of the plane of the drawing. The first electrode 14 is projected in the view of Figure 1 parallel to the layer structure 18 on the solid electrolyte layer 12 and the heating element 16. The first electrode 14 is at least partially annular. In the first embodiment, the first electrode 14 is formed as a ring electrode. The first electrode 14 is disposed inside the layer structure 18. The solid electrolyte layer 12 has a the

Measuring gas chamber facing end face 20 and at least two parallel to the

Layer structure 18 arranged side surfaces 22, 24.

The heating element 16 has a heating area 26 and at least one feed track 28. The heating element 16 is designed as a resistance path. At this

Embodiment, the heating element 16 therefore has two supply tracks 28. The heating element 16 is arranged in the layer structure 18 such that the heating region 26 is arranged at least in the vicinity of the side surfaces 22, 24 of the solid electrolyte layer 12. For example, only a so-called sealing frame 29 is arranged between the heating region 26 and the side surfaces 22, 24 and optionally also the end surface 20, so that the heating region 26 is arranged with the smallest possible distance to the side surfaces 22, 24 and optionally also to the end surface 20. The first electrode 14 is arranged in the layer structure 18 such that the first electrode 14, viewed in a direction parallel to the layer structure 18, at least partially overlaps the heating region 26. For example, in the embodiment shown in FIG. 1, the first electrode 14 overlaps the heating region 26 by at least 60%. In other words, if the first electrode 14 is projected onto the heating element 16 parallel to the layer structure, at least 60% of the area of the first electrode 14 facing the heating element 16 is located on the heating area 26 and is congruent. Furthermore, the heating element 16 is arranged in the layer structure 18 such that the heating area 26 additionally in the vicinity of the measuring gas chamber facing end surface 20 is arranged. The first electrode 14 is also disposed in the vicinity of at least the side surfaces 22, 24.

The sensor element 10 furthermore has at least one second electrode 30, which can not be seen in the view of FIG. 1, but can be seen, for example, from FIG. 9. The second electrode 30 is arranged on an outer surface or surface of the layer structure 18 which can be exposed to the measuring gas space. The first electrode 14, the second

Electrode 30 and the solid electrolyte layer 14 between them form a pumping cell. The second electrode 30 is thus an external pumping electrode. The sensor element 10 furthermore has at least one third electrode 32. The first electrode 14, the third

Electrode 32 and the solid electrolyte layer 14 between them form a Nernst cell. The third electrode 32 is thus a reference electrode. The first electrode 14 is an inner pumping and Nernst electrode, i. a combination of inner pumping electrode and Nernst electrode.

FIG. 2 shows the distribution of the temperature of the sensor element 10 according to the first embodiment during operation, i. With the heating element 16 switched on. As can be seen from FIG. 2, there is a clearly recognizable region 34 in which the temperature is highest and which extends parallel to the heating region 26. On the other hand, the temperature increases toward the third electrode 32 and the

 Supply lines 28 of the heating element 16 from. Furthermore, it can be seen from FIG. 2 that the first electrode 14 is arranged in this hottest region 34 and is thus located in a so-called hotspot. FIG. 3 shows the distribution of the tensile stress during operation of the sensor element according to the first embodiment. As can be seen in FIG. 3, in regions 35 in the vicinity of the side surfaces 22, 24 and the end surface 20, the tensile stress in the solid electrolyte layer 12 is higher than in regions 36 in the vicinity of the third electrode 32 and the supply paths 28 of the heating element 16, however, these are

Tensile stresses in the areas 35 compared to the conventional ones

Tensile stresses occurring sensor elements minimal or significantly lower.

Accordingly, when heating the sensor element 10, the tensile stresses in the critical regions of the side surfaces 22, 24 and the end face 20 are significantly minimized. As a concrete embodiment of the heating element 16, for example, the above-mentioned material is used in the form of a paste. The paste is on the Solid electrolyte layer 12 is applied and the sensor element 10 after application or printing of the further functional elements 13 as the electrode 14 is sintered. Since the preparation is known per se, it is not discussed in more detail, but referred to the above-described prior art. With such a paste, for example, a heating area 26 with a resistance of 1.67 Ω and a length of 30.9 mm in a sintered state has a cross-sectional area of 0.0063 to 0.0067 mm ² . A strip width of the heating region 26 is, for example, 535 μηι to 566 μηι. These dimensions result in a length of 37.3 mm and a thickness of 16.5 μηι in an unsintered state. In this case, a sintering shrinkage of about 20% based on the length to take into account, ie the heating element 16 shrinks during sintering, which is why the length in the sintered state is shorter than in the unsintered state.

A known material composition for conventional sensor elements and in particular for conventional heating elements comprise, for example, 98% by weight of platinum, 1% by weight of aluminum oxide and 1% by weight of rhodium. This results in a resistivity of 0.16 Q ^* mm ² / m to 0.20 Q ^* mm ² / m. In a sintered state results for a heating region 26 with a resistance of 1.67 Ω and a length of 30.9 mm, a cross-sectional area of 0.0030 to 0.0037 mm ² . A conductor track width of such a heating region is, for example, 252 μηι to 315 μηι. These dimensions result in a length of 37.3 mm and a thickness of 16.5 μηι in an unsintered state.

Another known material composition for conventional sensor elements and in particular for conventional heating elements comprises, for example, 91% by weight of platinum and 9% by weight of aluminum oxide. This results in a resistivity of 0.87 Q ^* mm ² / m to 0.91 Q ^* mm ² / m. In a sintered state, the cross-sectional area for a heating zone having a resistance of 1.67 Ω and a length of 30.9 mm is 0.0161 to 0.0169 mm ² . A track width for such a heating range is from 1368 to 1431 μηι. These dimensions result in a length of 37.3 mm and a thickness of 16.5 μηι in an unsintered state. In particular, in the last-described composition with 9 wt .-% alumina, there is the disadvantage that the resistivity very much on the sintering time, the temperature, the grain size of the alumina, etc. depends. This results from strongly fluctuating percolation of the platinum grains. In such a material, the scaffold portion of the alumina is increased so much that the properties of the material are not very robust. Due to the material composition of the heating element according to the invention, however, the heat transfer surface is significantly increased for a given total resistance or given length of the heating region 26. This leads to a lowering of the

Maximum temperatures in the heating area and thereby to a prolonged life in Heizerdauerglühen and Heizerdauerschaltschalten.

FIG. 4 shows a sectional view of a sensor element 10 according to a second

Embodiment of the invention. Hereinafter, only the differences from the first embodiment will be described and the same components are the same

Provided with reference numerals. The section of the view of FIG. 4 is analogous to the section of the view of FIG. 1. As can be seen from the illustration of FIG. 4, the first electrode 14 is designed as a ring segment or an open ring electrode. The third electrode 32 is arranged so as to partially overlap with the heating region 26 as viewed in a direction parallel to the layer structure 18 and closer to the first electrode 14 as compared with the first embodiment. As a result, the third electrode 32, i. the reference electrode, laid in the hottest area 34.

Consequently, the Nernst cell is also in the hottest region 34. This opens up the possibility that not only the internal resistance of the pump cell, but also the Nernst cell can be used for the release of the sensor element 10 by a motor control, not shown, of the motor vehicle.

FIG. 5 shows a sectional view of a sensor element 10 according to a third

Embodiment. In the following, only the differences from the previous embodiments will be described and the same components will be given the same reference numerals. The section of the view of FIG. 5 is analogous to the section of the view of FIG. 1. The sensor element 10 extends in a longitudinal direction 38 in the measuring gas space. The longitudinal extension direction 38 is perpendicular to the end face 20 of the solid electrolyte layer 12. The first electrode 14 is as in the first

Embodiment designed as a ring electrode. The third electrode 32 does not overlap with the heating area in a direction parallel to the layer structure 18. In the third embodiment shown in FIG. 5, the heating region 26 is designed so that it has a smaller cross-sectional area at the level of the third electrode 32 than at the level of the first electrode 14, viewed in the direction of longitudinal extension 38. In other words, the electrical resistance path of the heating region 26 of FIG Heating element 16 at the level of the third electrode 32 has a smaller cross-sectional area than at the level of the first electrode 14. The cross-sectional area of the resistance path of the heating region 26 of the heating element 16 thus increases from the third electrode 32 in the direction of the end face 20 toward. The increase in cross-sectional area may be uniform or nonuniform, such as stepped. A smaller cross-sectional area causes greater electrical resistance compared to a larger one

Cross-sectional area, so that in the region of the smaller cross-sectional area greater heating than in the region of the larger cross-sectional area occurs. This has the advantage that the Nernst cell is heated faster. The latter opens the possibility that not only the internal resistance of the pump cell, but also the Nernst cell can be used for the release of the sensor element 10 in the vehicle.

FIG. 6 shows the distribution of the temperature of the sensor element 10 according to the third embodiment during operation. As can be seen from FIG. 6, the hottest region 34, which extends parallel to the heating region 26, is again formed by the heating region 26. By contrast, the temperature decreases in the direction of the third electrode 32 and the supply tracks 28 of the heating element 16. In this hottest region 34, the first electrode 14 is arranged so that it is located in a hotspot of the heating element 16.

FIG. 7 shows the distribution of the tensile stress of the sensor element 10 according to the third embodiment during operation. As can be seen from FIG. 7, FIG

Regions 35 in the vicinity of the side surfaces 22, 24 and the end face 20 the

Tensile stress in the solid electrolyte layer 12 is higher than in areas 36 in the vicinity of the third electrode 32 and the supply tracks 28 of the heating element 16, however, these tensile stresses in the areas 35 compared to the tensile stresses occurring in conventional sensor elements are minimal or significantly lower.

 Accordingly, when heating the sensor element 10, the tensile stresses in the critical regions of the side surfaces 22, 24 and the end face 20 are significantly minimized.

FIG. 8 shows a sectional view of a sensor element 10 according to a fourth

Embodiment of the present invention. Below are only the

 Differences from the previous embodiments described and the same components are provided with the same reference numerals. The section of the view of FIG. 8 is analogous to the section of the view of FIG. 1. The sensor element 10 according to the fourth embodiment is an alternative to the sensor element 10 according to the third embodiment. The heating area 26 is designed to have at least one

Section 40 extending from at least one of the side surfaces 22, 24 in the direction extends to the third electrode 32. In the embodiment of FIG. 8, two sections 40 are provided, which are symmetrical with respect to the longitudinal extension direction 38 and are partially overlapped by the third electrode 32. In the fourth embodiment is thus compared to the third

Embodiment of the heating region 26 at the height of the Nernst cell laid more in the center of the sensor element 10. A greater length of the resistance path causes a greater electrical resistance compared to a shorter length of the

Resistance path, so that at the greater length more heating than at the shorter length occurs This has the advantage that the Nernst cell is heated faster. The latter opens the possibility that not only the internal resistance of the pump cell, but also the Nernst cell can be used for the release of the sensor element 10 in the vehicle.

FIG. 9 shows a sectional view of a sensor element 10 according to a fifth

Embodiment. The section runs perpendicular to one of the largest surfaces of the sensor element 10 and a longitudinal extension direction of the sensor element 10.

In the following, only the differences from the previous embodiments will be described and the same components will be given the same reference numerals. The sensor element 10 has a gas inlet path 42. The gas access path 42 has a gas access hole 44 that extends into the interior of the layer structure 18 from an outer surface or surface 46 of the solid electrolyte layer 12 on which the second electrode 30 is disposed. In the solid electrolyte layer 12, an electrode cavity 48 may be provided adjacent to and connected to the gas inlet hole 44. The electrode cavity 48 is formed, for example cuboid. Of the

 Electrode cavity 48 is part of the gas inlet path 42 and may communicate with the sample gas space via the gas inlet hole 44. For example, the gas inlet hole 44 extends as a cylindrical blind hole perpendicular to the surface 46 of the solid electrolyte layer 12 into the interior of the layer structure 18. Specifically, the electrode cavity 48 is bounded from the solid electrolyte layer 12 on the solid electrolyte layer 12 with respect to the view of FIG. 9 and toward the gas access hole 44 open. Between the gas inlet hole 44 and the electrode cavity 48, a channel 50 is arranged, which is also part of the Gaszutrittswegs 42. In this channel 50 is a

Diffusion barrier 52 is arranged, which a subsequent flow of gas from the

Sample gas space in the electrode cavity 48 is reduced or even prevented and only allows diffusion. About this diffusion barrier 52 can be Set the limiting current of a pumping cell 54. The pumping cell 54 includes those on the

Surface 46 disposed second electrode 30, the first electrode 14 which is disposed in the electrode cavity 48, and the region of the solid electrolyte layer 12 between these two electrodes. The above-mentioned limiting current thus represents a current flow between the second electrode 30 and the first electrode 14 via the solid electrolyte layer 12. In the extension of the extension direction of the

Gas access hole 44, the heating element 16 is arranged in the layer structure 18.

Furthermore, the layer structure 18 comprises the third electrode 32, which is arranged as a pumped reference in a so-called reference gas channel 56. The reference gas channel 56 is not a macroscopic reference gas channel but a pumped reference, i. H. an artificial reference. The first electrode 14, the third electrode 32 and the part of the solid electrolyte layer 12 between these two electrodes form a Nernst cell 58. By means of the pumping cell 54, for example, a pumping current through the pumping cell can be set such that in the electrode cavity 48 the condition λ = 1 or another known composition prevails. This composition, in turn, is detected by the Nernst cell 58 by applying a Nernst voltage between the first

Electrode 14 and the third electrode 32 is measured. As in the reference gas channel 56 or on the third electrode 32, which serves as a reference electrode, a

Oxygen excess prevails, based on the measured voltage on the

Composition in the electrode cavity 48 are closed.

As shown in FIG. 9, unlike the prior art, the gas introduction hole 44 is located between the pumping cell 54 and the Nernst cell 58. Specifically, the first electrode 14 is located closer to the end surface 20 than the gas access path 42 an inverted arrangement of

Gas access path 42 are spoken. Since the third electrode 32 and the fourth

Electrode 36, which form the pumping electrodes, and the end edges, which form the transition from the end face 19 into the adjacent surfaces of the solid electrolyte layer 14, such as the upper side 23, are close to each other in this design is, moreover, the thermal mass, the must be heated by the heating element 38 at a fast-light-off, low. The substantial portion of the heat generated by the heating element 16 is thus introduced in an area which, viewed in a direction parallel to the layer structure 18, overlaps with the pumping cell 54. The gas inlet hole 44 is outside this range, as can be clearly seen in FIG. In other words, the hotspot of the heating element 16 is in the region of the pumping cell 54 disposed and a second hot spot, which heats the gas inlet hole 44 in conventional sensor elements, eliminates or coincides with the first hotspot.

FIG. 10 shows a sectional view of a sensor element 10 according to a sixth embodiment. The section runs perpendicular to one of the largest surfaces of the sensor element 10 and a longitudinal extension direction of the sensor element 10.

In the following, only the differences from the previous embodiments will be described and the same components will be given the same reference numerals. The sixth embodiment is an alternative to the sensor element 10 according to the fifth embodiment. In the sensor element 10 according to the sixth

Embodiment, a sandwich arrangement of the pumping cell 54 and the Nernst cell 58 is realized, as will be described in more detail below. In this case, the third electrode 32 is closer to the end face 20 than the first electrode 14. The third electrode 32 is also located in a direction parallel to a direction of the layer structure 18 above the first electrode 14. More specifically, the third electrode 32 is between the first electrode 14 and second electrode 30 are arranged in a direction parallel to a direction of the layer structure 18. Figure 1 1 shows a sectional view of the sensor element 10 according to the sixth

 Embodiment. The section is parallel to one of the largest surfaces of the sensor element 10 and a longitudinal extension direction of the sensor element 10. As can be seen in FIG. 11, the first electrode 14 and the third electrode 32 are seen as rectangular electrodes in a view parallel to the layer structure 18 educated. In comparison with a conventional construction of a sensor element in the form of a broadband lambda probe, the sixth embodiment of FIG. 11 does not provide for arrangement of the first electrode 14 and the third electrode 32 on one plane, but on different planes. For simplicity only, the projection of the first electrode 14 onto the plane of the third electrode 32 is shown.

By the sandwich structure shown in FIGS. 10 and 11, in which the third electrode 32 is disposed between the first electrode 14 and the second electrode 30, the third electrode 32 can be moved toward the end face 20 in the hot spot of the heating region 26 and so faster

Resistance measurement and temperature control readiness can be achieved. Because the

Nernstzelle 58 serves as a thermometer of the sensor element 10 can, through this Arrangement can be ensured that the temperature is measured in the hottest region 34 of the heating region 26 and thus overheating of the sensor element 10 can be prevented. In this case, the temperature measurement can be maintained via the Nernstzelle 58 instead of using the resistance signal of the pumping cell 54. In the area between the first electrode 14 and the second electrode 30, the

Reference gas channel 56 narrow so that the first electrode 14, the second electrode 30 and the third electrode 32 are not isolated from each other.

As further possible embodiments, all the sensor elements 10 shown above can be realized with a lateral or end gas access path 42. This has the advantage that at the same time the Wasserschlagrobustheit can be increased. In this case, if an open gas access is avoided by pulling the porous diffusion barrier 52 outwards, the thermomechanical robustness of the

Sensor element 10 can be increased.

Claims

Claims 1. Sensor element (10) for detecting at least one property of a sample gas in a sample gas space, in particular for detecting a portion of a  Gas component in the measurement gas or a temperature of the measurement gas comprising at least one solid electrolyte layer (12), at least one functional element and a heating element (16) for generating heat, wherein the heating element (16) is at least partially made of a material comprising at least alumina wherein a proportion of the alumina of from 1, 5 wt .-% to 8.5 wt .-%, preferably from 2.0 wt .-% to 8.0 wt .-% and more preferably of 3.0 wt. % to 6.0 wt%. Second sensor element (10) according to the preceding claim, wherein the heating element (16) has a resistivity of 0.21 Q ^* mm ² / m to 0.70 Q ^* mm ² / m, preferably of 0.24 Q ^* mm ² / m to 0.50 Q ^* mm ² / m, and more preferably from 0.30 Q ^* mm ² / m to 0.40 Q ^* mm ² / m. 3. sensor element (10) according to any one of the preceding claims, wherein the material of the heating element further comprises platinum, wherein a proportion of the platinum of 92 wt .-% to 97 wt .-% and preferably from 93 wt .-% to 96 wt. -% is. 4. Sensor element (10) according to one of the preceding claims, wherein the Functional element (13) is a first electrode (14), wherein the solid electrolyte layer (12), the first electrode (14) and the heating element (16) form a layer structure (18), wherein the solid electrolyte layer (12) facing the measuring gas chamber end face ( 20) and at least two parallel to the layer structure (18) arranged side surfaces (22, 24), wherein the heating element (16) has a heating region (26) and at least one supply line (28), wherein the heating element (16) so in the Layer structure (18) is arranged, that the heating region (26) at least in the vicinity of the side surfaces (22, 24) of the solid electrolyte layer (12) is arranged, wherein the first electrode (14) is arranged in the layer structure (18) the first electrode (14) at least partially overlaps the heating region (26) when viewed in a direction parallel to the layer structure (18). 5. sensor element (10) according to the preceding claim, wherein the first electrode (14) is arranged in the layer structure (18) that at least 60% of the heating element (16) facing surface of the first electrode (14) in a direction parallel overlapped with the heating area (26) seen to the layer structure (18). 6. Sensor element (10) according to one of the two preceding claims, wherein the first electrode (14) at least in the vicinity of the side surfaces (22, 24) of the  Solid electrolyte layer (12) is arranged. 7. Sensor element (10) according to one of the three preceding claims, wherein the Heating element (16) is arranged in the layer structure (18), that the heating region (26) in the vicinity of the measuring gas chamber facing end face (20) is arranged. 8. Sensor element (10) according to one of the four preceding claims, wherein the sensor element (10) further comprises at least one second electrode (30), wherein the first electrode (14), the second electrode (30) and the solid electrolyte layer (12) Pump cell (54) form, wherein the first electrode (14) inside the  Layer structure (18) is arranged and the second electrode (30) on a the measuring gas space exposable outer side (46) of the layer structure (18) is arranged. 9. Sensor element (10) according to the preceding claim, wherein the first electrode (14) is at least partially annular. 10. Sensor element (10) according to one of the six preceding claims, wherein the sensor element (10) further comprises at least one third electrode (32), wherein the first electrode (14), the third electrode (32) and the solid electrolyte layer (12) Nernst cell (58) form, wherein the third electrode (32) is arranged in the layer structure (18), that the Nernstzelle (58) from the heating area (26) is heated. 1 1. Sensor element (10) according to the preceding claim, wherein the third electrode (32) in a direction parallel to the layer structure (18) partially overlaps with the heating region (26). The sensor element (10) of claim 8, wherein the heating region (26) has at least a portion (40) extending from at least one of the side surfaces (22, 24) toward the third electrode (32). 3. Sensor element (10) according to one of the three preceding claims, wherein the sensor element (10) extends in a longitudinal direction (38) in the sample gas space, wherein the heating region (26) is formed so that it  Longitudinal extension direction (38) seen at the level of the third electrode (32) has a smaller cross-sectional area than at the level of the first electrode (14). 4. Sensor element (10) according to claim 8, wherein the sensor element (10) extends in a longitudinal direction of extension (38) in the measuring gas space, wherein in  Longitudinal extension direction (38) seen the third electrode (32) closer to the end face (20) is arranged than the first electrode (14). 5. Sensor element (10) according to one of the ten preceding claims, wherein the sensor element (10) further comprises a Gaszutrittsweg (42), wherein the first electrode (14) by means of the Gaszutrittswegs (42) is acted upon with the measurement gas, wherein the first Electrode (14) between the end face (20) of the  Solid electrolyte layer (12) and the Gaszutrittsweg (42) is located.