FR2953293A1 - Sensor element for lambda sensor for determining e.g. oxygen concentration of exhaust gas of internal combustion engine, has end section covered with porous protective layer, where porous layer leaves hole open - Google Patents

Abstract

The element has a ceramic sensor body (10) exposed to measuring gas with a body end section. A hollow space (14) is formed in the end section, and a gas access hole (15) is guided from a body surface to the space at the end section. A diffusion barrier is arranged upstream to the space towards the hole. The barrier is protruded into the hole and is provided with a gas-permeable cover layer (18) on a surface turned towards the body surface. The end section is covered with a porous protective layer (24) in an integrated manner, and the porous layer leaves the hole open. An independent claim is also included for a method for manufacturing a sensor element for a gas sensor for determining physical characteristics e.g. concentration of gas components or temperature, of measuring gas.

Description

FIELD OF THE INVENTION The present invention relates to a sensor element for a gas sensor for determining at least one physical property of a measurement gas, in particular the concentration of a component of the gas or the temperature of the measurement gas. comprising: - a ceramic sensor body of which an end segment of the body is exposed to the measurement gas, - a cavity made in the end segment of the body, - a gas passage in the end segment of the body, ranging from the body surface to the cavity, and - an advanced diffusion barrier in the cavity towards the gas passage opening, this barrier projecting into the gas orifice and the surface of which upper facing the upper surface of the body is provided with a gas impermeable cover layer. The invention also relates to a method for manufacturing a sensor element for a gas sensor for determining at least one physical property of a measurement gas, in particular the concentration of a component of the gas or the temperature of the gas. measuring, said element comprising a sensor body of ceramic material with an end segment exposed to the measuring gas in which are formed a cavity and a gas passage opening from the body surface to the cavity and a diffusion barrier advancing into the gas passage opening with a gas-impermeable cover layer covering the surface facing the surface of the body projecting into the cavity. STATE OF THE ART A known sensor element for a gas sensor for finding a physical characteristic of a measurement gas, in particular for determining the oxygen concentration or the temperature of the exhaust gas of a heat engine (DE 103 45 141 A1) comprises a ceramic material sensor body consisting of solid electrolyte layers with gas-sensitive electrodes in the end segment of the body exposed to the measurement gas as well as an electrical resistance heating device. in the body of

2 sensor up to the end segment of the sensor body, on the connection side with electrical conductor paths to which a connection line to the control unit is connected The outer side of a first electrolyte layer solid which constitutes the upper surface of the sensor body, is provided with a first annular electrode, covered by a porous protective layer. A gas passage is provided in the first electrode; this orifice reaches a cavity formed between the first and the second solid electrolyte layer. The cavity is delimited with respect to the gas passage orifice by a diffusion barrier, of annular shape projecting into the gas orifice and whose upper surface facing towards the first electrolyte layer. solid is covered with a gas permeable layer. The annular cavity delimited by the diffusion barrier constitutes a measurement gas chamber in which there is a second electrode forming an electrochemical pumping cell with the first electrode and a third electrode facing the second electrode and constituting a cell. Electrochemical Nernst with a reference electrode. The reference electrode is installed in a reference channel formed between the first and the second solid electrolyte; this channel receives a reference gas, for example air. Between the second solid electrolyte layer and a third solid electrolyte layer, under the electrode assembly there is a resistance electric heating device integrated in an aluminum oxide insulation layer. For an extremely precise adjustment of diffusion diffusion diffusion resistance which varies due to variations in the fabrication and geometry of the diffusion barrier, the sintered sensor element is set by the electrical heating device by resistance to the operating temperature and is exposed a measurement gas with a defined concentration of oxygen that is measured by the limit current flowing through the pumping cell. The protruding zone of the diffusion barrier and the gas-impermeable layer covering this barrier, the substance of the gas is evolved through the gas orifice.

3 diffusion barrier to reach a set point of the limit current flowing in the electric pumping cell, limit current corresponding to the set value of the oxygen vein passing through the diffusion barrier.

In such a sensor element whose body is heated in operation for example up to 750 ° C, it can have a thermal shock during operation if cold water droplets, driven by the exhaust gas arrive on the ceramic body of the sensor. The thermal gradient thus produced in the sensor body results in considerable thermal stresses in the sensor body triggering the formation of cracks in the ceramic, which in the long run results in malfunctions or total failure of the element. sensor. DESCRIPTION AND ADVANTAGES OF THE INVENTION The sensor element of the type defined above characterized according to the invention in that the end segment of the body is coated in a totally uniform manner with a porous protective layer which allows clearing at least the gas passage opening.

The sensor element according to the invention has the advantage of protecting the end segment of the body against the arrival of drops of cold water entrained by the measurement gas and which could trigger a thermal shock in the body of the body. hot sensor. Due to the porosity and the material property of the porous protective layer, the water droplets arriving at the end segment are distributed finely in the protective layer and vaporise which avoids too great a thermal gradient at the level of the porous protective layer. the ceramic material of the sensor body which could produce high local tensile stresses and consequently the formation of cracks in the sensor body. In addition, the damage to the sensor element during assembly is considerably reduced. The remaining clearance around the gas passage in the protective layer ensures that the gas passage hole remains clear of the material forming the protective layer and that the diffusion resistance, calibrated of the diffusion barrier, comes into contact. protruding into the gas orifice

4 is not modified so that the accuracy of the measurement and operation of the sensor element will not be impaired by the application of the porous protective layer. According to an advantageous development of the sensor element, the porous protective layer is made of aluminum oxide. A protective layer of this material has a high thermal conductivity; it is robust against thermal shock and can easily adapt to porosity requirements. The method according to the invention for producing such a sensor element has the advantage, thanks to the atmospheric plasma spraying method (also known as the APS method), of applying the protective layer on the upper surface of the sensor body in a very precise manner. level of the end segment of the sensor body exposed to the measurement gas and releasing very reliably the area surrounding the gas passage. According to an advantageous embodiment of the method, a covering on the passage of the gas for plasma spraying prevents the pulverized material from entering the gas orifice. This process is very economical from the point of view of manufacture but requires a substantially larger clearance around the gas orifice and for which the ceramic sensor body is not protected. However, this can be compensated by a suitable mounting position of the end segment of the body in a protective tube with gas passage holes, the relative position of the end segment of the body being chosen as a protective tube so that the Measuring gas stream that arrives through the gas outlet ports does not fall directly on the release of the protective layer. According to an advantageous development of the method, a strictly delimited and strongly grouped plasma beam is generated and directed on the sensor element, the plasma beam being guided on the surface of the sensor body at its end segment. . Such a closely grouped plasma beam makes it possible to effect a coating of the surface of the end segment of the body at the level of the gas passage orifice, in a very precise manner and makes it possible to drive the plasma beam very close to the gas passage without risk that the pulverized material arrives in the passage opening. The clustering of the plasma beam with a very strict limit is realized in different ways as will be developed hereinafter. According to an advantageous development of the invention, before subjecting the sensor element to the APS process, the gas passage orifice is filled and, if appropriate, an edge zone surrounding the gas passage orifice on the surface of the body with an organic filler, for example fuel wax and after the APS process the sensor element is subjected to a heating method for burning the filler. This combustion can be done in an oven but also by applying a heating voltage to the resistance heater integrated in the sensor element. By removing the filler material by burning it, the pulverized material that covers the filler will no longer cling and can be removed without difficulty to relieve the gas orifice. Drawings The present invention will be described hereinafter with the aid of exemplary embodiments shown in the accompanying drawings in which: - Figure 1 is a section of a gas sensor element cut along the lines II of Figure 2 FIG. 2 shows a detail in top view of the sensor element with an end segment of the sensor element body surrounded by a protective layer; FIG. 3 is a view similar to that of FIG. FIG. 4 is a diagram illustrating a first method of applying the protective layer to the sensor element; FIG. 5 is a diagram illustrating a second method; FIG. for applying the protective layer; FIG. 6 is a diagram illustrating a third method for applying the protective layer,

Figure 7 is illustrative of a fourth method of applying the protective layer. DESCRIPTION OF EMBODIMENTS OF THE INVENTION The sensor element shown in sectional view in FIG. 1 and in plan view in part in FIG. 2 serves to determine at least one physical property of a measurement gas in particular. the concentration of a gas component or the temperature of the measurement gas; this sensor element serves for example lambda probe to determine the oxygen concentration in the exhaust gas of a heat engine (or internal combustion engine). The sensor element comprises a sensor body 10 of ceramic material composed of solid electrolyte layers 11, 12, 13; in the exemplary embodiment, the sensor body is in the form of a bar with a rectangular section. The sensor body 10 gas-tightly housed in a not shown sensor housing, comes with an end segment 101 (Fig. 2) exposed to the measurement gas, exiting the sensor housing; this projecting portion is surrounded by a not shown protective tube and provided with gas passage orifices; the tube is attached to the sensor housing. Between a first solid electrolyte layer 11, external and a second solid electrolyte layer 12 in the end segment 101 of the body, there is a cavity 14 and a central gas passage 15 opening into the cavity 14. The cavity 14 made with an annular shape in this example is delimited externally by a sealing frame 16 installed between the first solid electrolyte layer 11 and the second solid electrolyte layer 12; internally the cavity is delimited by an annular diffusion barrier 17, made of a porous material. The diffusion barrier 17 projects into the gas passage 15 and is covered on the upper surface facing the first solid electrolyte layer 11 or the upper surface of the sensor body 10 by a watertight layer 18. gas. The cavity 14 receives an annular inner electrode 19 and a Nernst electrode 20, opposite it. In accordance with the inner electrode 19, on the outer side of the first solid electrolyte layer 11 there is an annular outer electrode 21 surrounding the orifice

This outer electrode forms an electrochemical pumping cell with the inner electrode 19 and the solid electrolyte which is in the gap. The Nernst electrode forms an electrochemical Nernst cell with a reference electrode, not shown here, provided in a reference channel formed between the first and the second solid electrolyte layer 11, 12 and exposed to a reference gas, for example air and solid electrolyte in the meantime. Under the electrode assembly described above by way of example, between the second solid electrolyte layer 12 and the third solid electrolyte layer 13 there is a resistance electric heating device 22; this heating device is integrated in an electrical insulation 23 for example aluminum oxide. All the electrodes 19, 20, 21, the reference electrode, not shown, and the resistance heating device 22 are connected to branch contacts by unrepresented conductive paths passing through the sensor body 10; these branching contacts are provided at the other end segment of the sensor body and are connected by a contact device via the branch line to a control apparatus. The upper surface of the sensor body 10 is covered at the end segment 101 of the body, exposed to the measurement gas, in a completely uniform manner, i.e. all over the surface and completely, with a protective layer porous for example aluminum oxide; however, this layer leaves open the gas passage 15 and optionally a small upper surface area surrounding the gas passage 15. The porous protective layer 24 prevents finely distributed water droplets finely distributed in the measuring gas and which have a significantly lower temperature than for example 750 ° C of the heated sensor body 10, do not arrive directly on the ceramic sensor body 10 and produce high-level thermal gradients , unacceptable when arriving on the ceramic; these gradients could produce in the sensor body 10 considerable and long-term cracks in the ceramic.

The porosity and the high thermal conductivity of the protective layer 24 cause the drops of water arriving on the protective layer 24 to be distributed finely in the protective layer 24 and are very largely vaporized in the protective layer 24 by not reaching the protective layer 24. the ceramic material to produce high tensile stresses that could destroy the sensor body 10 by developing cracks. The cavity 25 of the protective layer 24 at the gas passage 15 is circular in the embodiment of Figures 1 and 2; in the case of the example of Figure 3 this clearance is rectangular; the clearance 25 extends over the entire width of the sensor body 10. The clearance 25 in the protective layer 24 ensures that the operating parameters of the sensor element such as for example the accuracy of these characteristic lines is not influenced. by the protective layer 24 and remain the same whether or not the sensor element has a protective layer 24. For manufacturing the sensor element, the solid electrolyte layers 11 are laminated one on the other, 12, 13 provided with the electrodes 19-21 and the resistance heating device 22 integrated in the insulation 23 and then sintered. The radial width of the diffusion barrier 17 is chosen to be large enough so that the diffusion resistance of the diffusion barrier 17 is greater than the set value to be respected. During a subsequent calibration process performed on the sintered sensor element, the sensor element is heated by the resistance heater 22 to the operating temperature and exposed to a sample gas and a concentration. defined in oxygen. The oxygen which passes through the cavity 14 is pumped outwards by the pumping cell and the corresponding limit current is measured. At the same time material is removed from the diffusion barrier 17 until a target value is reached for the limit current which corresponds to a set value of the oxygen passing through the diffusion barrier 17. The subsequent coating of the 101 end of the body with the porous protective layer 24 is by the method of plasma atmospheric spraying (also called APS process) by proceeding

9 by atmospheric plasma spraying of the material of the protective layer on the surface of the end segment 101 of the body, disengaging the area of the gas passage 15. In a first method shown in FIG. an ungrouped plasma beam 26, with a protective layer material on the end segment 101 of the body; the plasma beam 26 and the sensor body 10 are moved relative to each other so that the entire surface of the end segment 101 of the body is successively touched by the plasma beam 26. A cover 27 is connected integrally with the sensor body 10, directly in front of the gas passage opening 15 so that the gas passage opening 15 can not be touched by the plasma beam 26. According to the embodiment of the covering means 27 as a circular disk or rectangular, the cavity formed in the sputtered protective layer 26 also extends over an upper surface area around the gas outlet port 15 (Fig. 2) or a rectangular shape (Fig. 3). The relative movement between the sensor body 10 and the plasma beam 26 can be obtained by an axial displacement movement and a rotational movement of the sensor body 10 or the plasma beam 26. According to a variant of the method, a diaphragm to strongly group the plasma beam 26 so that it arrives with a strictly limited beam point on the sensor body 10. The diaphragm is installed directly in the plasma beam 26 and in the simplest case it can be act of a mechanical diaphragm. According to the method sketched in FIG. 5, the diaphragm is produced by means of a clipping system 28 in which, before reaching the sensor body 10, the plasma beam 26 is passed through two conical nozzles 29 , 30 successive. The first nozzle 29 is called the sampling cone; it comprises a flat metal cone having at the top of the cone a bore of about one millimeter in diameter. The second nozzle 30, also called a clipping cone, has a deep cone with a smaller diameter hole in the apex. Between the two conical nozzles 29, there is a pressure difference of about 103 mbar. The plasma beam 26 '

10 strongly grouped out of the clipping system 28 has a strictly limited edge area and passes on the end segment 101 of the body. The strict delineation of the plasma spot arriving at the end segment 101 of the body greatly reduces the upper surface area cleared around the gas passage 15 by spraying the surface of the end segment. 101 of the body with the material of the covering layer, without the risk that the pulverized material arrives in the gas passage opening 15. The sensor body 10 or the bundled plasma beam 26 'are in this case also moved relative to each other so that all areas of the surface of the end segment 101 of the body are scanned by the plasma beam 26 '. The process sketched in FIGS. 6 and 7 for plasma spraying the material of the protective layer consists in producing the diaphragm for the plasma beam 26 by an ion optic 31. In this ion optic 31, the plasma beam 26 is passed through an electric field generated between electrically charged plates 32, 32. In the method sketched in FIG. 6, the uncharged particles of the plasma beam 26 are separated in an electric field and the charged particles or ions are directed as a grouped particle beam 26 "on the end segment of the sensor body. 10 according to the method sketched in FIG. 7, the particles with a negative or positive charge with respect to the plasma beam 26 are separated by means of the electric field between the plates 32 and 33. and directing the beam as a sharply bundled particle beam 26 on the sensor body 10. In the method shown in FIG. 7, the negatively charged particles are removed from the plasma beam 26 by eliminating them. To better group the particle beam 26 ", it is passed through a metal tube 34 electrically charged, the electric charge of the metal tube 34 is opposite the charge of the particle stream 26". In the embodiment shown, the metal tube 34 is thus positively charged so that the negative charge particles are grouped together in the middle of the metal tube 34.

It is also possible to use at the same time an ion optic 31 and the clipping system 28. To drive the plasma beam 26 "coming out of the clipping system 28 then through the ion optics 31 before the beam does not arrive on the end segment 101 of the sensor body 10. According to a variant of the method, the gas outlet orifice 15 is filled with an organic filling material, for example with wax, and the coating is exposed. sensor element thus prepared with its end segment 101 to the APS method, the resulting plasma spraying corresponds to the representation of FIG. 4 with the covering 27 removed, and after spraying the protective layer 24, the element of FIG. sensor at a heating operation for burning the organic filling material, the gas passage opening 15 is thus completely cleared and the gas inlet in the cavity 14 is not impeded by the material of the neck. The combustion of the organic filler may be in an oven but also to heat the resistance heater 22 of the sensor 10.

NOMENCLATURE 10 11, 12, 13 14 15 16 17 18 19 20 21 22 23 24 25 26 26 '26 ", 26" 27 28 29, 30 31 32 34 101Storage body solid electrolyte layer cavity gas orifice Sealing frame diffusion barrier gastight layer internal electrode Nernst electrode outer electrode resistance electrical insulation protective layer porous cavity plasma beam plasma beam bundle of particles grouped covering clipping system conical nozzle optical ionic plate metal tube segment extrémité30

Claims (1)

CLAIMS1 °) Sensor element for a gas sensor for determining at least one physical property of a measurement gas, in particular the concentration of a component of the gas or the temperature of the measurement gas, comprising: a sensor body ( 10) of ceramic having an end segment (101) exposed to the measurement gas, - a cavity (14) formed in the end segment (101), - a gas passage (15) in the segment end (101), from the body surface to the cavity (14), and - a diffusion barrier (17) advanced in the cavity (14) towards the gas passage (15), this a barrier projecting into the gas passage (15) and whose upper surface facing the upper surface of the body is provided with a gas-impermeable cover layer (18), said sensor element characterized in that the end segment (101) is coated in a totally uniform manner with a layer porous sensor (24) which allows at least the gas orifice (15) to be clear. 2 °) sensor element according to claim 1, characterized in that the porous protective layer (24) is aluminum oxide. 3 °) A method of manufacturing a sensor element for a gas sensor for determining at least one physical property of a measurement gas, in particular the concentration of a component of the gas or the temperature of the measurement gas, element having a sensor body (10) of ceramic material with an end segment (101) exposed to the measuring gas in which a cavity (14) and a gas passage (15) extending from the surface are made from the body to the cavity (14) and a diffusion barrier (17) advancing in the gas passage (15) with a gas-impermeable covering layer (18) covering the surface facing the surface of the body protruding into the cavity (14), characterized in that by atmospheric spraying of the plasma is applied a porous protective layer (24) on the surface of the end segment (101) of the body sparing at least the orifice of gas passage (15). 4) Method according to claim 3, characterized in that for the plasma spraying is applied a coating on the gas passage (15). Method according to Claim 3, characterized in that a highly grouped plasma beam (26 ') is generated and directed onto the surface of the sensor body (10) at the end segment ( 101) and it is passed on this one. Method according to claim 5, characterized in that before reaching the surface of the body the plasma beam (26) directed on the end segment (101) of the body is passed through the diaphragm. mechanical. Method according to claim 5, characterized in that prior to the arrival at the surface of the bodies the plasma beam (26) directed on the end segment (101) of the body passes through a clipping system ( 28) formed of two conical nozzles (29, 30) installed one behind the other, the front conical nozzle (29) corresponding to a flat cone with a hole at the top of the cone and the rear conical nozzle (30) 30 in downstream of the forward conical nozzle (29) into the plasma beam (26) having a deeper cone with a hole in the cone apex and a drilling diameter smaller than the diameter of the bore of the front cone nozzle (30) and applies a pressure drop between the conical nozzles (29, 30). Method according to claim 5 or 7, characterized in that before reaching the surface of the body the plasma beam (26) directed to the end segment (101) of the body is passed through a electric field of an ion optics (31). 9) A method according to claim 8, characterized in that the charged particles of the plasma beam (26) are separated by means of the electric field and the uncharged particles are directed in the form of a combined flow of particles (26 ") on the end segment (101) of the body 10 °) Method according to claim 8, characterized in that the electric field is used to separate the particles from the plasma beam (26). negative or positive charge and directed as a grouped particle stream (26 "') on the end segment (101) of the body and preferably before reaching the surface of the body passed through the grouped flow of particles (26 "') in a metal tube (34) having an electric charge opposite to the charge of the particles (11) Method according to claim 3, characterized in that prior to plasma spraying is performed the gas passage (15) with a filling material o The fuel element is preferably a fuel for wax and the sensor element provided with the protective layer (24) is subjected to a heating process resulting in the combustion of the filler. 30