EP0647319A1

EP0647319A1 - Sensor unit for determining the gas component concentration

Info

Publication number: EP0647319A1
Application number: EP94912450A
Authority: EP
Inventors: Karl-Hermann Friese; Werner Gruenwald; Roland Stahl; Claudio De La Prieta; Gerhard Schneider; Harald Neumann
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1993-04-23
Filing date: 1994-04-13
Publication date: 1995-04-12
Also published as: DE4313251A1; US5545301A; DE4313251C2; WO1994025864A1; JPH07508353A

Abstract

The invention relates to a sensor unit (21) to determine the concentration of a component of a gas mixture, with a diffusion channel (12) leading into a measuring gas chamber (18) and bordering an electrode chamber (24, 25). The dimensions (w) of the diffusion channel of the gas chamber (18) of the sensor (21) are each a multiple of the mean free path. The electrode chamber (25) used is connected to the diffusion channel (12). Supersaturation of the electrode system (13, 15) through the design with additional supports is prevented.

Description

Sensor element for determining the gas component concentration

State of the art

The invention is based on a sensor with a diffusion channel according to the preamble of the main claim, for example for exhaust gas measurement of internal combustion engines in various embodiments, most often as a laminar or finger probe, which with different cavity systems d. H. Sample gas spaces are executed.

From EP-A 0188900 a generic sensor is known, in which the design of the measuring gas space with its diffusion channel is determined by a relationship of distances that have been empirically determined for different types. This relationship was constant regardless of the design of the planar probe and defined distances between the exhaust gas chamber and the electrodes and their geometry.

The mathematical relationship m - 1> 5 w, or in the limit case m - 1 = 5 w, of EP-A 0 188 900 with 1 as the distance between the inlet of the measuring gas space and a subsequent point of the first electrode relative to the inlet, m as the distance between the inlet of the measurement gas space and a subsequent point of the third electrode relative to the inlet, and w as the distance between the first and third electrodes which are exposed to the exhaust gas relate to a planar arrangement which has been constructed in a laminar manner. This formula loses its validity for dimensions that correspond to atomic orders of magnitude and would lead to liquefaction of the gas due to wall reactions and adsorption for very small dimensions (Lin Zhang and Nigel A. Seaton, Prediction of the effective diffusity in Pore Networks close to the percolation threshold , AIChE Journal H »1992, 1816 - 1824). Furthermore, because of the planar electrode arrangements, a measurement can predominantly only be carried out with homogeneous electrical fields.

A further restriction is given for gas flows from the measuring gas space into the disk-shaped diffusion channel through the right-angled deflection of diffusing gas components, even in the case where w already assumes the multiple dimension of the diameter of the gas component.

Such a sensor element is constructed from a base body made of z. B. zirconia ceramic, which serves as a solid electrolyte. A laminar design of the sensor is advantageous for economic reasons, but this is not the only design.

Examples of exhaust gas probes are EGOS (exhaust gas oxygen sensor), HEGOS (heated EGOS), PEGOS (proportional EGOS), UEGOS (universal EGOS) and TF-HEGOS (thin film HEGOS). Measurements are made in the electrochemical sensor element, including in a limit current probe, with at least two electrodes, one of which can come into contact with the reference gas, the other with the exhaust gas.

The gas component to be measured comes completely into contact with the porous electrode. In the event of the appearance of saturation on the electrode surface due to complete coverage with one or more gas components, this can lead to poisoning or overloading of the contact surface. Such sensors work according to the polarographic measuring principle. A constant electrode voltage is applied between an anode and a cathode and a diffusion limit current is measured. However, the sensor could also use a different electrochemical measuring principle, e.g. B. the potentiometric measuring principle.

The diffusion limit current, for example in the case of a limit current probe, is determined by ions after having overcome a diffusion barrier of the component of the measurement gas to be measured, the charges of which cause the current. The design of the sample gas space, in particular the diffusion channel in front of the electrodes, determines the diffusion resistance for the sample gas and influences the gradient of the concentration of the sample gas component to be measured. This affects the control position of the sensor.

In the following, the term measuring gas space will also include the diffusion channel and the electrode space, if these are not specifically mentioned. In a diffusion channel, the part of the measuring gas space, for. B. is an exhaust gas probe, the gas mixture to be measured flows into the probe via the sample gas chamber supplied from the outside. The measuring gas space should include every gas space that can house the gas component of the sensor to be measured.

The electrode space is the space that lies between the electrodes and contains the gas. It connects to the diffusion channel and is at least flooded by the gas component to be measured.

Statements and realizations that would make it possible to design a measuring gas space for an exhaust gas probe deviating from planar structures have so far largely been lacking. A disadvantage of known sensors having a diffusion channel tunnel is that the output of the signals remains sensitive to temperature and pressure or at least does not work without interfering dependencies. In conventional designs with small ones

Diffusion channel tunnel dimensions or filled tunnels of the diffusion channel are known as mixed diffusion from Knudsen and gas phase diffusion, which can be the reason for the pressure dependence of the probe signals.

From DE-PS 37 28 289 it is known to carry out diffusion channels either through filler material for Knudsen diffusion or without filler material for gas phase diffusion and to design series and / or parallel connections with the filler materials, this requiring several manufacturing steps and a spread of the physical and chemical Properties of the sensor copies.

Advantages of the invention

Sensor elements with diffusion channels, which are designed according to the invention, have sufficient free path lengths for the measuring gas component. This means that gas phase diffusion essentially occurs for the measuring gas, without impacts against the wall or chemical reactions which could falsify this. The diffusion coefficient of the sample gas component is then inversely proportional to the pressure in the sample gas space. Flow problems and tendencies to wall reactions of the gas component to be measured due to collisions of the molecules are largely avoided.

Converted to a working temperature T of 800 degrees Celsius and p = 1 bar, an average free path length of about 0.3 microns follows. From this, the minimum dimension of the diffusion channel is determined to be 30 micrometers, which corresponds to 100 times the mean free path of the oxygen anion. For functional reasons, a probe is limited in terms of its volume of the sample gas space and the diffusion channel for its inner channel system. The minimum expansion of the measuring gas space at any point in the interior of the sensor is known.

Furthermore, taking into account the minimal dimensions of the diffusion channel and measuring gas space gains advantages through a greater freedom of choice of the electrode geometry, for example in the electrode space. Additional components, for example support elements or channel expansion systems of the electrode space, can be attached at a suitable location.

drawing

Advantageous embodiments of the diffusion channel are shown in the drawings. FIG. 1 shows a cross section through the measuring gas space with a diffusion channel for a sensor, FIG. 2 shows a cross section through a sensor that was constructed from a pump and measuring cell, FIG. 3 shows a part of the measuring gas space in a cylindrical shape, to which a diffusion channel belongs vertically 4 shows a cross section through the measurement gas space of a sensor of FIG. 3 along the axis AB with the diffusion channel not widened, FIG. 5 shows a cross section through the measurement gas space of a sensor of FIG. 3 along the axis AB with the diffusion channel widened in a fan shape, FIG. 6 shows the hatched area The area of the electrode coating using the example of a partially coated circular sector, ie a ring electrode coating, FIG. 7 shows, based on a cross section for the angle β = 60 degrees, the installation of additional components or components (pillars) in the electrode space, FIG. 8 shows a pump current-pump voltage diagram to represent the pressure Dependency of a sensor, Figure 9 shows the middle free path length for a selection of different gases depending on the gas pressure of a measuring gas and the channel dimensions resulting at 20 degrees Celsius, Figure 10 shows the operation of a non-regulated heating element of an exhaust gas probe and the pressure-sensitive position of the working point of the exhaust gas probe.

Description of the embodiments

example 1

In FIG. 1, a diffusion channel 12 of minimum dimension w = 30 micrometers built on a substrate 11 is provided for an oxygen gas component measurement with an electrode pair of a porous, non-hardened cathode 13 and a porous anode 15 with a porous intermediate layer 14. The porous anode 15 is covered with a cover layer 16. The minimum dimension is indicated by a drawn ball with the diameter w, which can be moved freely. The structure is built with the dimension w = 30 micrometers from one with ϊ _? Stabilized zirconium dioxide ceramic for the porous cover layer 16 and the porous intermediate layer 14 and an aluminum oxide ceramic for the substrate layers 11). The porous electrodes 15 and 13 are made of platinum or a platinum alloy. For the example of this diffusion channel 12, a ratio of the height w, which contains the minimum dimension of the channel, and the length L was set to w: L = 3:80.

In the case of a larger dimension w> 30 microns may filler material 17, for example, ^A l porous be ₃ ° **** introduced ⁿ ^ ^s diffusion channel. It is conceivable that the measurement gas 20 flows into the diffusion channel 12 of the measurement gas chamber 18 from several sides. In the direction of the third dimension, the exhaust gas probe 21 can also be in a non-planar form be executed. Due to tests with more complex sensors, the w: L ratio could also be maintained for other sensors.

It would be possible to temper the electrodes. For example, the cathode 13 can be coated with a porous protective layer or a plurality of porous protective layers in order to prevent the effects of corrosion and poisoning of its surface, as well as the removal of its material, or to avoid damage and / or impairments caused by components of the measurement gas 20. A diffusion channel bent at its ends could be of more fluid design.

Example 2

FIG. 2 shows a largely pressure-independent exhaust gas probe 21, consisting of a measuring cell 22 and a pump cell 23. Not all electrodes were tempered, e.g. B. the cathode 13 and the first electrode 24 are exposed.

If measuring gas 20 enters the electrode space 24, 31 via the diffusion channel 12, it comes into contact with the first electrode 24 of the measuring cell 22, the potential of which changes relative to the second electrode 31. The first electrode 24 and the second electrode 31 are separated by means of an intermediate layer 32 which conducts oxygen ions. The potential difference is detected and used to regulate the potential between cathode 13 and anode 15 of the pump cell 23. The oxygen concentration to be measured is measured after connecting the electrodes to an electrical network. The measuring principle is described in more detail, for example, in EP-A 0 194 082. The electrodes of the measuring cell 22 could also be arranged centrally below the electrodes of the pump cell 23. The number of diffusion channels 12, their topography and their cross-sectional dimensions relative to one another are decisive for their position. Furthermore, a distance is conceivable for the exhaust gas, the oxygen gas concentration of which is measured at several points in order to achieve better regulation of the inflowing gases, that is to say a supply regulation via several electrode pairs used for the measurement.

Example 3

In FIG. 3, an electrode space 25 is spread out like a fan. The measuring gas 20 flows into this electrode space 25 via a cylindrical measuring gas space 18, which can also be a partial volume of the measuring gas space, via the diffusion channel 12. The electrode surfaces 26 and 27 are well suited as a covered anode surface 26 and a covered cathode surface 27 for contacting the electrodes. The electrodes are designed as sector-shaped sections, as shown in FIG. 4. The opening angle β is 60 degrees.

By contacting the electrodes outside the measuring gas space 18, the manufacturing possibilities of the electrode contacts are expanded. Within the covered area, very thin layers of different material thickness and different local composition for contacting can be realized with great freedom for the choice of material and thus good electrical and mechanical contact stability and freedom from noise. The active electrode area and a contact-dispensing means for coupling the leads to the electrode to an external electrical network lie in separate areas, which is why a double function at a geometrical location does not lead to compromises for the electrode and forcing contact. The covered electrode surfaces 26, 27 directly adjoin the uncovered active electrode surfaces 28, 29, as shown in FIG. 6. It would be possible to choose the opening angle on one or both sides differently for the covered and effective electrode surface, so that there are interruptions in the conductor track or are knotted.

A circular disk coating in a corresponding modification according to FIG. 6 is conceivable.

Example 4

FIG. 4 shows a section A-B, in which the effective electrode surface 28 directly adjoins the edge of the rectangular, columnar diffusion channel 12. The opening angle ß is 45 degrees.

Example 5

The exhaust gas probe according to FIG. 5 and FIG. 6 is designed as in example 3 and FIG. 3, but the electrodes are designed in a ring. FIG. 5 shows a section A-B, in which the effective electrode surface 29 does not directly adjoin the edge of the rectangular, column-shaped diffusion channel 12 but only comprises one ring section. In this case, the diffusion channel 12 is also expanded in a fan shape.

The opening angle β can be chosen up to 90 degrees for those in FIGS. 4 and 5, depending on how many electrode spaces are connected to one another via a plurality of diffusion channels. It would be conceivable to design the anode surface like the cathode surface. Another geometry of the electric field for measurement with non-homogeneous fields can also be produced. Example 6

FIG. 7 shows an example of a cross section through the diffusion and electrode space of an exhaust gas probe with an internal structure. The column-shaped pillars, that is, cavity supports 30 serve to regulate the flow of measurement gas to the electrode surface. The aperture function of the columns protects the electrodes from oversaturation and contamination. Furthermore, the structure becomes mechanically more stable and easier to shrink.

Example 7

It is also possible to construct an exhaust gas probe from a short-circuit cell, pump cell and measuring cell, as can be seen from Examples 1 and 2. The dimensions of the examples mentioned according to the invention are retained. A measuring gas space with three diffusion channels inclined at 60 degrees to one another can also form a star with adjoining electrode spaces.

Using the relationship 1 = (1. T. P) / (T. P) at a known temperature T, known pressures and p, and known mean free path length 1 for the known temperature T, the mean free path length of the gas component to be measured can be calculated (Source: KG Müller, vacuum engineering calculation bases, Verlag Chemie, Weinheim 1961, pages 15, 16).

For e.g. B. air, a gas mixture of essentially 0, N, CO and noble gases, or pure oxygen 0 or pure nitrogen, this value is p = 1 bar and 20 degrees Celsius, with an average free path of the order of 0.08 micrometers. This means that the minimum diameter of the diffusion channel is 8 micrometers for an ambient temperature of 20 degrees Celsius. Converted to a working temperature T of 800 degrees Celsius and P = 1 bar, an average free path length of about 0.3 microns follows. This determines the minimum dimension of the diffusion channel to 30 microns at 800 degrees Celsius.

For a dimension of the diffusion channel of 30 micrometers at a measuring temperature in operation, in the area from the ambient temperature to 800 degrees Celsius, the oxygen content is measured at a pressure between 0.1 and 10 bar for the examples.

A measurement with two sensors of the same type differing in dimension w (FIGS. 1, 2), with an electrode design as shown in FIG. 5, was carried out with w = 5 and w = 30 micrometers for the lateral ones

Cross-sectional dimensions of the diameter of the diffusion channel. A limit current measurement for each example of both limit current sensors showed a pressure dependency of 8 percent / bar at w = 30 micrometers compared to 40 percent / bar at w ___ 5 micrometers for the measured limit currents, as shown in FIG. 8 for a = 1.5 bar / 1.09 mA, b = 1.25 bar / 1.07 mA and c = 1.0 bar / 1.05 mA in the voltage interval.

FIG. 9 contains examples of channel design for the gases a H, b air, c CO, and shows the mean free path length on the vertical axes on the left and the channel dimensions w on the right. The horizontal axis corresponds to the gas pressure. For carbon dioxide there is a diffusion channel dimension of w = 22 micrometers and analogously for hydrogen w = 80 micrometers. An example of the pressure insensitivity of a probe, e.g. B. the probe of Figure 2 is shown in Figure 10. If a heating element is attached to the lower substrate of the exhaust gas probe, the probe can be heated. FIG. 10 illustrates for two exhaust gas probes A and B designed in this way, with electrode spaces corresponding to FIG. 4 and FIG. 5, the ratio of the limit currents with different temperature stability, which is set by the heating power.

For two different probes, the electrode spaces corresponding to those in FIGS. 4 and 5, respectively, are recorded in FIG. 10, the ratio of the limit currents of these probes at a gas pressure in the vicinity of 1 bar, or an increased gas pressure of 2 bar. Hatched areas denote variable temperatures for the gas, light areas denote constant temperatures. The dashed lines indicate no overlap of the measuring fields for constant and changeable temperatures in the case of the probe of FIG. 4 and therefore a somewhat higher temperature sensitivity of this layout for measuring the exhaust gas of the oxygen concentration. A limit current ratio of i (2bar) / i (lbar) = 1 is defined as the operating point.

5 proved to be a better embodiment compared to the embodiment shown in FIG. The height of the rectangles in FIG. 10 covers the manufacturing spread of various specimens of the probe type produced according to the invention.

The best embodiment of a probe is in FIG. 10 for probe B, in the case of small rectangular areas of FIG. 10 which are symmetrical about the limit current ratio at a setpoint of 1 for the limit current ratio at different pressures. The parameter d is a measure of the missing overlap of the working point of the exhaust gas probes A or B). The invention relates generally to sensors in which a reaction electrode is preceded by a diffusion barrier. A technique for producing an exhaust gas probe is screen pressing e.g. B. a crackable organic form-forming layer, the form-forming agent, or a body part of this material on a substrate 11 or generally on another layer. Taking into account the shrinkage of the ceramic used, the cavity of the measuring gas chamber 18 is determined in shape and dimensions, that is to say also the w / L ratio. This screen-printed layer forms, for example, the diffusion channel volume. After all the layers constituting the sensor have been laminated together, the measuring gas space 18 is later decomposed, evaporated or burned out during sintering. The superimposition can include, for example, ceramic, matching, electrode, catalyst, line, cover or ceramic layers of the exhaust gas probe and can optionally be carried out by machine. The ceramic layers are often between 0.3 and 2 millimeters thick.

A manufacturing process by sintering the diffusion barrier requires the minimum height of the channel of 30 micrometers. The shrinkage for a 20 volume percent shrinkage when using theobromine as the shape-forming material is then chosen to be 42 micrometers and the laminar structure is sintered at at least 1000 degrees Celsius. ZrO with 4 mol percent Y.0 was used as the ceramic

** ^* £ t _* -_5 selected.

The electrodes 28/29 for the cathode and / or the corresponding anode of the sensor preferably consist of a metal from the platinum group, in particular platinum, or from alloys from the platinum group or alloys from metals from the platinum group with other substances, as is described, inter alia, in DE PS 41 00 106 is described. If necessary, you will receive a ceramic yttrium-stabilized zirconium oxide support framework material, for example in the form of a YSZ powder, with a volume fraction of preferably about 40 volume percent. They are porous and as thin as possible. They preferably have a thickness of 8 to 15 micrometers. The conductor tracks belonging to the electrodes preferably also consist of platinum or a platinum alloy of the type described. They can also be produced from a paste based on a precious metal cermet.

The solid electrolyte layer (intermediate layer) 14 or 26 consists of one of the known, for the production of divalent negative

Oxygen conducting solid electrolytes used oxides, such as in particular ZrO ", CeO .., HfO" and ThO "with a content

2 2 2 2 on divalent alkaline earth oxides and / or trivalent oxides of rare earths. Typically, the layer can be about 80 to 97 mole percent ZrO, CeO, HfO or ThO and 3 to 20

Mole percent consist of MgO, CaO, SrO and / or rare earth oxides and / or in particular Y, 0. The layer advantageously consists of ZrO stabilized with ϊ.0. Sc 0 can be used as a full or partial replacement for Y-0. The thickness of the layer can advantageously be 10 to 200 micrometers, in particular 15 to 50 micrometers.

The diffusion channel can have a filling made of coarsely porous sintering material, for example based on Al 0 or ZrO,

• * & <b if this is not only appropriate for gas phase diffusion of the sample gas for all volume ranges of the sample gas space of the sensor.

Thermal soot powder, graphite, plastics, for example based on polyurethane, salts, for example ammonium carbonate, and other organic substances, such as theobromine, were used as pore-forming agents or shape-forming agents for the design of the measuring gas space 18 and / or the diffusion channel 12 and / or the electrode space 24, 25 and indanthrene blue are used. The choice of room shapes expands considerably. Unsupported structures also change their shape at sintering temperatures that are above a threshold temperature of around 300 degrees Celsius, which, for example when using theobromine, results in structural deformations because theobromine has already been completely removed from the cavity structure. A solidification, that is to say a shape that is true to shape, can only be obtained at temperatures above about 700 degrees Celsius. Further advantages arise when different shape-forming agents are used together, in that the volumes of individual cavities which are burned out are produced by joining, for example gluing together individual shape-forming partial volumes. Spacers can also be made from glass ceramic. Example 7 was made using picein.

Claims

Expectations

1. Sensor element for determining the concentration of a component or chemically related components of a gas mixture, in particular on the basis of an electrochemical measuring method working sensor, preferably for exhaust gas probes of internal combustion engines, with a diffusion channel which leads into a measuring gas space and adjoins an electrode space, the Measuring gas space has at least the dimensions of the diffusion channel, characterized in that the dimensions of the diffusion channel (12) in each dimension of a section have at least a multiple of the mean free path length of the component of the measuring gas (20) to be measured.

2. Sensor element according to claim 1, characterized in that it is constructed from a short-circuit cell known per se, a pump cell (23) and a measuring cell (22) or at least two of these cells, which by means of diffusion channels (12) with the measuring gas space (18) are connected.

3. Sensor element according to claim 1, characterized in that a diffusion channel (12) with a height to length ratio of w: L = 2-4: 70-90 adhering to the minimum dimension of the mean free path dimension of the gas component to be measured connects.

4. Sensor element according to claim 1, characterized in that the dimensions of a cross section of the diffusion channel (12) have at least ten times, preferably a hundred times the value of the mean free path of the gas component to be measured.

5. Sensor element according to claim 1, characterized in that the diffusion channel (12) has more than one inflow (20, b) and more than one outflow (19) for the measuring gas (20).

6. Sensor element according to claim 1, characterized in that the measuring gas space (18) and the diffusion channel (12) at fixed minimum dimensions (w), expressed in multiples of the mean free path length of the measuring gas component, have any spatial shape.

7. Sensor element according to claim 1, characterized in that in the case of a measuring method with reference gases, these also have a suitable measuring gas space (18) or suitable measuring gas spaces (18) of the pressure-insensitive construction.

8. Sensor element according to one of the preceding claims, characterized in that the diffusion channel (12) in one or more dimensions funnel-shaped by the electrode space (25) is expanded. 1 1

9. Sensor element according to claim 1, characterized in that the measuring gas space (18) with diffusion channel (12) and electrode space (25) is produced in screen printing technology and by applying a shape-forming agent.

10. Sensor element according to claim 1, characterized in that the electrodes of the measuring cell (22) are circular or circular sector-shaped.

11. Sensor element according to one of the preceding claims, with a shape-forming means for the design of the measuring gas space and / or the diffusion channel and or the electrode space, characterized in that different shape-forming means are used for different areas (18; 12; 24, 25) and different Areas are put together, in particular glued.