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

Abstract

Method for producing a sensor element (10) for detecting at least one property of a gas in a gas space, in particular for detecting a gas component in the gas or a temperature of the gas, comprising the steps: - providing at least one functional element (12) which has at least one solid electrolyte body (14) and at least one functional layer (16), in particular an electrode (18), - providing a suspension, in particular an aqueous and / or alcoholic suspension, of particles for a thermal shock protection layer (22), and - applying the suspension in the form of a thermal shock protection layer (22) to the functional element (12) by means of a thermal spraying process; characterized in that the particles of the suspension for the thermal shock protection layer (22) are α-aluminum oxide and the thermal shock protection layer (22) is applied such that at least 50 vol.% α-aluminum oxide is present in the applied thermal shock protection layer (22).

Description

State of the art

[0001] A variety of sensor elements and methods for detecting at least one property of a gas in a measuring gas chamber are known from the prior art. In principle, these can be any physical and / or chemical properties of the gas, with one or more properties being able to be detected. The invention is described below in particular with reference to a qualitative and / or quantitative detection of a gas component of the gas, in particular with reference to a detection of an oxygen content in the gas. The oxygen content can be detected, 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 gas can also be detected.

[0002] Ceramic sensor elements based on the electrolytic properties of certain solids, i.e., the ion-conducting properties of these solids, are known from the prior art. These solids can be ceramic solid electrolytes, such as zirconium dioxide (ZrO2), especially yttrium-stabilized zirconium dioxide (YSZ) and / or scandium-doped zirconium dioxide (ScSZ), which may contain small amounts of aluminum oxide (Al2O3) and / or silicon oxide (SiO2).

[0003] For example, such sensor elements can be designed as so-called lambda sensors, as described, for example, in Konrad Reif (ed.): Sensors in Motor Vehicles, 1st edition 2010, pages 160-165. Broadband lambda sensors, especially planar broadband lambda sensors, can be used to determine the oxygen concentration in the exhaust gas over a wide range, thus providing information about the air-fuel ratio in the combustion chamber. The air / fuel ratio λ describes this air / fuel ratio.

[0004] Increasing functional demands are placed on such sensor elements. In particular, the rapid operational readiness of lambda sensors after an engine start plays a major role. This is essentially influenced by two aspects. The first aspect concerns the rapid heating of the lambda sensor to its operating temperature of above 600 °C, which can be achieved by appropriately designing a heating element or reducing the size of the area to be heated. The other aspect concerns robustness against thermal shock caused by water hammer during operation. This thermal shock is based on the fact that for a certain period of time after the engine start, the temperature in the exhaust pipe is below the dew point for water, so that the water vapor produced during fuel combustion can condense in the exhaust pipe. This leads to the formation of water droplets in the exhaust pipe.The heated ceramic of the lambda sensor can be damaged or even destroyed by water droplets hitting it due to thermal stress or cracks in the sensor ceramic. Therefore, lambda sensors have been developed that feature a porous ceramic protective or covering layer on their surface, also known as a thermal shock protection layer or thermal shock protection (TSP). This protective layer ensures that water droplets striking the lambda sensor are distributed over a large area, thus reducing the local temperature gradients that occur in the solid-state electrolyte or the sensor ceramic. These lambda sensors can therefore tolerate a certain droplet size of condensation in the heated state without being damaged. Below approximately 300°C, the sensor ceramic is thermal shock-resistant due to its high strength.A thermal shock protection layer reduces the water access to the probe ceramic in a temperature range of 300 °C to 450 °C due to its limited permeability and limits the cooling by thermal conduction in a temperature range above 450 °C.

[0005] Typically, the thermal shock protection layer contains aluminum oxide, which is thermally sprayed onto the probe ceramic in powder form. The thickness of a thermal shock protection layer produced in this way is typically approximately 300 µm.

[0006] DE 10 2008 026 101 A1 discloses the production of low-porosity aluminum oxide coatings for electrical insulation using thermal spraying processes. These spraying processes utilize aqueous or alcoholic suspensions of α-aluminum oxide.

[0007] Despite the numerous advantages of the prior art processes for producing sensor elements for lambda sensors, these still have potential for improvement. For example, the layers sprayed using powder place a very high load on the lambda sensor ceramic and the thermomechanical strength is significantly reduced. Furthermore, the sprayed layers are relatively inhomogeneous, which means that the layers must be sprayed thicker than necessary to achieve sufficient thermal shock stability. A higher thickness increases the heat capacity of the sensor element to such an extent that the rapid switching on of the sensor element, the so-called Fast Light-Off (FLO), is noticeably impaired. The achieved porosity of the layers produced by thermal spraying processes using powders is borderline with regard to sensor function. Furthermore, the connection andIn many cases, the adhesion of the thermal shock protection layer to the sensor element is insufficient. Furthermore, the spraying process using an aqueous or alcoholic suspension typically results in a maximum porosity of 19%, which may be insufficient for proper sensor function.

[0008] Further sensor elements and methods for their production are known from DE 11 2006 001 721 T5, DE 100 08 595 A1, JP 2007-033374 A1 and JP 2010-164591 A1. Disclosure of the invention

[0009] Therefore, a method for producing a sensor element for detecting at least one property of a gas in a gas space and a sensor element that can be produced by this method are proposed, which at least largely avoid the disadvantages of known methods and sensor elements and in which the thermal shock protection layer can be made thinner, more homogeneous, more porous and with better adhesion compared to the prior art.

[0010] The procedure comprises the following steps, preferably in the order mentioned: - Providing at least one functional element which has at least one solid electrolyte body and at least one functional layer, in particular an electrode, - Providing a suspension, in particular an aqueous and / or alcoholic suspension of particles for a thermal shock protection layer, - Applying the suspension in the form of a thermal shock protection layer to the functional element by means of a thermal spraying process.

[0011] The thermal spraying process can be carried out such that the thermal shock protection layer has a porosity of 25% to 75%, preferably of 30% to 70%, and more preferably of 30% to 60%. The particles of the suspension for the thermal shock protection layer can contain metal or metal compounds, in particular one or more metal compounds selected from the group consisting of aluminum oxide, zirconium dioxide, and titanium dioxide. The particles of the suspension for the thermal shock protection layer can be α-aluminum oxide, and the thermal shock protection layer can be applied such that at least 50 vol.% α-aluminum oxide, preferably at least 60 vol.% α-aluminum oxide, and more preferably at least 70 vol.% α-aluminum oxide, is present in the applied thermal shock protection layer.The suspension can be prepared using a powder containing α-alumina, the powder having a purity of at least 98%, preferably at least 99%, and more preferably at least 99.5%. The suspension can have a solids content of at least 1% by volume, preferably at least 5% by volume, and more preferably at least 10% by volume. The particles can have a diameter d. 50(50% quantile of the particle size distribution) of 10 nm to 20 µm, preferably of 15 nm to 15 µm, and more preferably of 20 nm to 10 µm. The thermal shock protection layer can be applied such that it has a thickness of 40 µm to 550 µm, preferably of 45 µm to 525 µm, and more preferably of 50 µm to 500 µm. The thermal shock protection layer can be applied such that it has a surface roughness parameter Sa of not more than 2.5 µm, preferably not more than 2 µm, and more preferably not more than 1.8 µm. The suspension can be thermally sprayed such that the temperature of the functional element is not more than 400°C, preferably not more than 350°C, and even more preferably not more than 300°C. The functional element can be cooled during the application of the thermal shock protection layer. The thermal spraying process can be selected from the group consisting of plasma spraying, laser spraying, and high-velocity flame spraying.

[0012] The sensor element can be designed, for example, as a finger probe or as a planar probe, in particular as a planar lambda probe, i.e., for example, as a lambda probe with a layered structure. For example, step probes and / or broadband lambda sensors can be implemented.

[0013] A basic idea of the invention is to apply a thermal shock protection layer to a functional element using a suspension and a thermal spraying process, wherein the particle size and the temperature of the particles during thermal spraying influence the porosity of the applied thermal shock protection layer. Therefore, the porosity can be influenced by changing these parameters. For example, smaller particle diameters can make the layer thickness more uniform or more homogeneous and thinner. This also allows the voids in the layer to be distributed more finely and more voids to be introduced into the layer, thus increasing the porosity. For example, a lower temperature during spraying prevents the particles from melting as strongly, meaning they do not flow into one another as strongly and voids can remain between them.The manufacturing method according to the invention makes it possible to create a sensor element in which the probe function and the accuracy of the probe signal are significantly improved compared to conventionally sprayed thermal shock protection layers.

[0014] For the purposes of the present invention, a solid electrolyte body is understood to mean a body, in particular a sintered body, with electrolytic properties, i.e., ion-conducting properties. In particular, it can be a ceramic solid electrolyte.

[0015] In the context of the present invention, a layer is understood to mean a uniform mass in planar extension at a certain height, which lies above, below or between other elements.

[0016] In the context of the present invention, a functional layer is understood to mean an element selected from the group consisting of: electrode, conductor track, diffusion barrier, diffusion gap, reference gas channel, heating element, Nernst cell, and oxygen pump cell. In particular, this refers to those elements that fulfill the essential chemical and / or physical and / or electrical and / or electrochemical functions of a lambda sensor.

[0017] In the context of the present invention, a thermal shock protection layer is understood to be a layer designed to reduce the local temperature gradients occurring in the functional element or the sensor ceramic, for example, by distributing water droplets impinging on the lambda sensor over a large area. The layer can be made of a ceramic material, contain metal or metal compounds, and can be porous.

[0018] In the context of the present invention, a suspension is understood to mean a heterogeneous mixture of substances consisting of a liquid and finely distributed solids which are suspended in the liquid and kept in suspension.

[0019] For the purposes of the present invention, a thermal spraying process refers to all surface coating processes in which additional materials, known as spray additives, are melted, partially melted, or heated inside or outside a spray gun, accelerated in a stream, such as a gas stream, in the form of spray particles or agglomerates, and projected onto the surface of the component to be coated. The component surface is not melted and is only subjected to minimal thermal stress. A layer forms because, depending on the process and material, the spray particles flatten to a greater or lesser extent upon impact with the component surface, adhere primarily through mechanical bonding, and build up the spray layer layer by layer.The resulting coating properties are significantly influenced by the temperature and velocity of the spray particles at the time they impact the surface to be coated. The surface condition, such as purity, activation, and temperature, also significantly influences quality characteristics such as adhesion strength. Energy sources for melting or melting the spray filler material include electric arc (arc spraying), plasma jet (plasma spraying), fuel-oxygen flame or fuel-oxygen high-velocity flame (conventional and high-velocity flame spraying), fast, preheated gases (cold gas spraying), and laser beams (laser beam spraying).

[0020] Plasma spraying in the context of the present invention is understood to be a thermal spraying process using a plasma jet as the energy carrier for melting or melting the spray filler material. In a plasma torch, an anode and up to three cathodes are usually separated by a narrow gap. A direct current creates an arc between the anode and cathode. The gas or gas mixture flowing through the plasma torch is passed through the arc and is ionized in the process. The dissociation or subsequent ionization creates an electrically conductive gas made up of positive ions and electrons, heated to a temperature of up to 20,000 K. In the context of the present invention, a suspension is injected into this generated plasma jet, the solids of which have a particle size orhave a particle diameter of 10 nm to 20 µm, preferably 15 nm to 15 µm, and more preferably 20 nm to 10 µm, and which are melted by the high plasma temperature. Due to the water or alcohol component of the suspension, energy is withdrawn from the thermal process in the form of the evaporation enthalpy of the liquid, so that the particles impose less thermal stress on the sensor element when they hit it. The particles are cooled accordingly, so that the temperature of the particles at the time of injection into the plasma jet is no more than 400 °C, preferably no more than 350 °C, and more preferably no more than 300 °C. The plasma stream then entrains the molten particles and hurls them onto the workpiece to be coated, which is a functional element within the scope of the present invention.The gas molecules return to a stable state very quickly, and the plasma temperature drops again after a short distance. Plasma coating is carried out in a normal atmosphere, an inert atmosphere under a protective gas such as argon, in a vacuum, or even underwater. The speed, temperature, and composition of the plasma gas are important for the coating quality. The gases used are argon, nitrogen, hydrogen, or helium.

[0021] In the context of the present invention, laser spraying is understood to mean a thermal spraying process using a laser beam as the energy carrier for melting or melting the spray filler material. During laser spraying, the spray filler in the form of a suspension with the above-mentioned particle size for the solids is introduced via a nozzle into the laser beam focused on the workpiece and is projected onto the workpiece surface with the aid of a gas. By means of laser radiation, both the particles and a minimal portion of the component surface are melted and the supplied particle is bonded to the component material. Downstream of the focusing optics, the gas escapes with the laser beam, usually argon, which on the one hand prevents the oxidation of the melt and on the other hand transports the spray filler. It thus acts as both a protective gas and a carrier gas.

[0022] In the context of the present invention, high-velocity flame spraying is understood to mean a thermal spraying process in which continuous fuel combustion takes place under high pressure within a water- or air-cooled combustion chamber. Combustion gases such as propane, ethylene, propene, butane, acetylene, hydrogen, liquid fuels such as diesel, kerosene, and combinations thereof are used as fuels. The oxidizing agent is usually oxygen, but air can also be used. This process is also known by the process name HVAF, derived from high-velocity air fuel. The high pressure of the burning fuel-oxygen mixture generated in the combustion chamber and the expansion nozzle, which is usually arranged downstream, generate the necessary high velocity of the gas jet. The spray materials are fed axially into the combustion chamber or radially in the area of the expansion nozzle.This accelerates the spray particles to high speeds, resulting in spray coatings with excellent adhesion properties. Due to the controllable and just sufficient heat input, the spray material undergoes only minimal metallurgical changes during the spraying process.

[0023] In the context of the present invention, porosity is understood as the ratio of void volume to total volume of a substance or mixture of substances as a dimensionless measurement. In particular, open porosity is understood to be the proportion of the void volume of those voids in the total volume that are connected to one another and to the environment, such as the gas space. In particular, the aim of the present invention is for the porosity of the thermal shock protection layer to exceed the percolation threshold. Percolation describes the formation of connected regions (clusters) with random occupation of structures (lattices). In point percolation, lattice points are occupied with a certain probability; in edge percolation, occupied points are connected to one another. As the probability that a field of the lattice is occupied increases, larger clusters form.The occupancy probability is defined as the value at which at least one cluster reaches a size that extends through the entire system, for example, an extension on a two-dimensional lattice from the right to the left or from the top to the bottom. One says: The cluster percolates through the system. This value of the occupancy probability is the so-called percolation threshold. In the example mentioned, the percolation threshold therefore describes the percolation probability with which at least one cavity extending through the carrier or several interconnected cavities are formed, so that the gas can pass from a side of the functional element facing away from the functional layer to a side of the functional element facing the functional layer.

[0024] In the context of the present invention, the surface roughness parameter Sa is understood to be a parameter perpendicular to a surface that relates to the amplitudes of roughness elevations. In particular, Sa is the arithmetic mean roughness value of a surface, i.e., the arithmetic mean of absolute values of the surface deviation within a reference surface. In contrast to a surface parameter measured only along a line, the surface roughness parameter Sa can be used to determine the three-dimensional structure of a surface.

[0025] Diameter refers to the average particle size. In particular, d 50 that 50% of the particles are smaller than the specified value.

[0026] The manufacturing method according to the invention makes it possible to produce a sensor element which has a finer porosity distribution due to the conveying of smaller particles. In particular, a more homogeneous layer thickness can be achieved, i.e. a layer with less roughness or less layer thickness variation, so that the thermal shock protection layer can be reduced in thickness or strength and made thinner overall while maintaining the same thermal robustness. The heat capacity of the thermal shock protection layer can be reduced and the time until operational readiness (fast light-off) of the sensor remains stable or can be reduced. In particular, a time until operational readiness of less than 5 seconds can be achieved. Furthermore, a greater variation in layer thickness is possible, whereby the permissible amount of water can be further increased. The porosity can also be increased.Due to the more homogeneous layer thicknesses and the higher porosity, faster sensor function is achieved, especially with regard to dynamic specifications. The thermal stress on the sensor element during the coating process is also reduced due to the removal of the evaporation enthalpy of the liquid from the suspension, and the thermomechanical strength of the sensor element is increased, leading to an extended service life of the lambda sensor. Due to smaller coating particles, both the kinetic and thermal energy per particle are reduced. Compared to powder-sprayed coatings, the existing adhesive layer system allows for better adhesion of the particles to the surface and the side edge of the sensor element.

[0027] By providing an increased α-aluminum oxide content of more than 50 vol.% after coating according to the invention, the aging of the thermal shock protection layer is minimized, ie the porosity after aging increases slightly, thereby limiting the static and dynamic specifications of the customer's technical specifications. Short description of the drawings

[0028] Further optional details and features of the invention will become apparent from the following description of preferred embodiments, which are schematically illustrated in the figures. They show: Fig. 1 a plan view of a sensor element according to the invention, Fig. 2 is a diagram showing the result of a measurement of a pumping current at the thermal shock protection layer of a conventionally sprayed thermal shock protection layer and a sprayed thermal shock protection layer according to the invention, Fig. 3 is a diagram showing the result of a surface roughness measurement of a conventionally sprayed thermal shock protection layer and a thermal shock protection layer sprayed according to the invention, measured on the side of the electrode and the heating element, Fig. 4 an enlarged cross-sectional view of a section of a sensor element produced according to the invention, and Fig. 5 is a diagram showing, by way of example, the result of an X-ray diffraction (XRD) measurement of thermal shock protection layers sprayed in different ways and of a powder used. Embodiments of the invention

[0029] Fig. 1 is a plan view of a sensor element 10 according to the invention. Fig. The sensor element 10 shown in Figure 1 can be used to detect physical and / or chemical properties of a gas in a measuring gas space, wherein one or more properties can be detected. The invention is described below in particular with reference to a qualitative and / or quantitative detection of a gas component of the gas, in particular with reference to a detection of an oxygen content in the gas. The oxygen content can be detected, for example, in the form of a partial pressure and / or in the form of a percentage. In principle, however, other types of gas components can also be detected, for example nitrogen oxides, hydrocarbons and / or hydrogen. Alternatively or additionally, however, other properties of the gas can also be detected.The invention can be used in particular in the field of automotive engineering, so that the measuring gas chamber can in particular be an exhaust tract of an internal combustion engine and the gas can in particular be exhaust gas.

[0030] The sensor element 10, as an exemplary component of a planar lambda probe, has a functional element 12 that has a solid electrolyte body 14 and at least one functional layer 16. The solid electrolyte body 12 can, in particular, be a ceramic solid electrolyte body that forms the sensor ceramic of the sensor element 10. The functional element 12 can, for example, have one or more functional layers 16 that can be integrated into it or attached to it. As an example, an electrode 18 and its terminals 20 are shown. However, it is explicitly emphasized that a heating element can also be provided to heat the functional element 12 to its operating temperature. Furthermore, the sensor element 10 has a thermal shock protection layer 22 that can partially or completely cover the functional layers 16. For illustration purposes, the thermal shock protection layer 22 is shown only partially covering the electrode 18.However, complete covering of the entire surface of the functional element 12 is expressly possible.

[0031] The thermal shock protection layer 22 can be a porous ceramic material containing metal or metal compounds. The metal compounds can in particular be selected from the group consisting of aluminum oxide, zirconium dioxide, and titanium dioxide. In particular, the thermal shock protection layer 22 can contain α-aluminum oxide at at least 50 vol. %, preferably at least 60 vol. %, and more preferably at least 70 vol. %, such as, for example, 82 vol. %. The thermal shock protection layer 22 can have a thickness of 40 µm to 550 µm, preferably 45 µm to 525 µm, and more preferably 50 µm to 500 µm, such as, for example, 200 µm. The thermal shock protection layer 22 may have a porosity of 25% to 75%, preferably 30% to 70%, and more preferably 30% to 60%, such as 45%.The thermal shock protection layer 22 may have a surface roughness characteristic Sa of not more than 2.5 µm, preferably not more than 2 µm and more preferably not more than 1.8 µm, such as 1.5 µm.

[0032] The sensor element 10 can be manufactured in particular as follows. First, a functional element 12 with the above-mentioned solid electrolyte body 14 and the at least one functional layer 16 in a sintered state is provided in a known manner. The production of such a functional element 12 is known per se, so its production will not be discussed in detail. Examples of production include the thick-film process, the thin-film process, and the film lamination process. Furthermore, a suspension with particles containing α-aluminum oxide is provided. This can be done using a powder containing α-aluminum oxide with a purity of at least 98%, preferably at least 99%, and more preferably at least 99.5%, such as 99.8%, which is introduced into an aqueous or alcoholic solution, for example with at least 1% by volume, preferably at least 5% by volume.-% and more preferably at least 10 vol.%, such as 14 vol.%. The particles of the suspension have a diameter d. 50from 10 nm to 20 µm, preferably from 15 nm to 15 µm and even more preferably from 20 nm to 10 µm, such as 1.6 µm. The suspension is then applied to the functional element 12 using a thermal spraying process, for example in layer form, where the solids of the suspension form a thermal shock protection layer 22. The thermal shock protection layer 22 can, for example, be thicker on the side edges of the functional element 12 than on the surfaces, since these are more sensitive to water hammer. Conventional processes, such as immersion bathing, generally do not allow any variation in layer thickness. With immersion bathing, the adhesion of the thermal shock protection layer is also lower than with the thermal spraying process present here. The thermal spraying process can, for example, be a plasma spraying process in which the suspension is injected axially or radially into a nozzle (not shown) of the spraying device.Due to the water or alcohol content, heat is extracted from the particles in the form of vaporization enthalpy of the water or alcohol, so that the functional element reaches a temperature of maximum 400°C, preferably maximum 350°C, and even more preferably 300°C. The temperature of the plasma is adjusted so that the plasma is still stable and does not collapse, but the thermal stress on the particles is not too great. The temperature of the plasma can be, for example, 10,000 K. The functional element 12 can be additionally cooled to reduce the thermal stress. Due to the heating in the plasma, the particles assume a dough-like or viscous consistency and are then sprayed onto the surface of the functional element 12. The spraying accelerates the particles onto the surface of the functional element 12 and, due to their small particle size, adhere to it.They then finally cool down on the functional element 10 and solidify. This also creates the aforementioned porosity in the thermal shock protection layer 22. The porosity can be determined, for example, by quantitative microstructure analysis of micrographs. The aforementioned proportion of α-aluminum oxide can also be achieved in this way, so that the thermal shock protection layer 22 is highly resistant to water hammer. The proportion of α-aluminum oxide can be determined, for example, by X-ray diffraction analysis. Other thermal spraying processes, such as laser spraying and high-velocity flame spraying, can also be used for production.

[0033] The spraying can take place at a spray angle to the (longitudinal) axis of the functional element of at least 30°, preferably at least 40° and more preferably at least 50°, for example at a spray angle of 60°. The distance between the nozzle and the sensor element can be at least 15 mm, preferably at least 20 mm and more preferably at least 25 mm, for example 30 mm. The suspension flow rate can be at least 10 g / min, preferably at least 15 g / min and more preferably at least 20 g / min, for example 25 g / min. The plasma current can be at least 300 A, preferably at least 350 A and more preferably at least 400 A, for example 450 A. The plasma gas volume flow can be at least 20 slpm (standard liters per minute), preferably at least 25 slpm and more preferably at least 30 slpm, for example 35 slpm.

[0034] Fig. Figure 2 shows a bar chart indicating the result of a measurement of a pumping current Iporo in mA on the thermal shock protection layer of a conventionally sprayed thermal shock protection layer, ie, using a powder by means of so-called atmospheric plasma spraying, and a thermal shock protection layer 22 sprayed according to the invention according to the method described above. Reference numeral 24 denotes the bar for the conventionally sprayed thermal shock protection layer, and reference numeral 26 denotes the bar for the thermal shock protection layer 22 sprayed according to the invention. As shown in Fig. As shown in Figure 2, the pumping current Iporo is higher for the thermal shock protection layer 22 sprayed according to the invention compared to a conventionally sprayed thermal shock protection layer, indicating increased percolating open porosity. Thus, the pumping current Iporo for the conventionally sprayed thermal shock protection layer is approximately 4.5 mA, whereas the pumping current Iporo for the thermal shock protection layer 22 sprayed according to the invention is approximately 6.3 mA.

[0035] Fig. 3 shows a bar chart indicating the result of a surface roughness measurement of a conventionally sprayed thermal shock protection layer and a thermal shock protection layer 22 sprayed according to the invention. More precisely, the surface roughness parameter Sa is given in mm. Reference numeral 28A denotes the bar for a conventionally sprayed thermal shock protection layer measured on the side of the functional element closest to an electrode, and reference numeral 28B denotes the bar for a conventionally sprayed thermal shock protection layer measured on the side of the functional element closest to a heating element.Analogously, reference numeral 30A designates the bar for a thermal shock protection layer 22 sprayed according to the invention, measured on the side of the functional element 12 to which an electrode 18 is located closest, and reference numeral 28B designates the bar for a thermal shock protection layer 22 sprayed according to the invention, measured on the side of the functional element 12 to which a heating element is located closest. Fig. Figure 3 shows that the method according to the invention results in a smoother surface for the thermal shock protection layer 22 sprayed according to the invention, which is evident from the surface roughness value Sa, which for the conventional method is approximately 0.0060 mm at 28A and approximately 0.0068 mm at 28B. For the thermal shock protection layer 22 according to the method according to the invention, the surface roughness value Sa is approximately 0.0017 mm at 30A and approximately 0.0015 mm at 30B. This shows that with the method according to the invention, a surface roughness value Sa of no more than 2.5 µm, preferably no more than 2 µm, and even more preferably no more than 1.8 µm, can be achieved. Fig. Figure 3 also shows the significantly increased homogeneity of the thermal shock protection layer 22 compared to the conventionally manufactured thermal shock protection layer. The surface roughness can be determined, for example, using optical measurement methods, such as scattered light measurement.

[0036] Fig. 4 shows an enlarged cross-sectional view of a section of a sensor element 10 produced according to the invention. Fig. 4 shows a significantly smoother surface of the thermal shock protection layer 22 in the cross-section shown, even with the same porosity as a conventionally sprayed thermal shock protection layer. This confirms the results from the Fig. 2 and Fig. 3 confirmed.

[0037] Fig. Figure 5 shows a diagram illustrating, by way of example, the result of an X-ray diffraction (XRD) measurement of thermal shock protection layers sprayed in different ways and of a powder used. The diffraction angle 2Θ is indicated in degrees on the X-axis. The counting rate in counting steps or counting pulses per second is plotted linearly on the Y-axis. Each division of the Y-axis represents 100 counting steps or counting pulses per second. Reference numeral 32 indicates the result of the X-ray diffraction measurement for an aluminum oxide-containing powder with a purity of at least 98%, which can be used to produce the above suspensions for the production process according to the invention.Reference numeral 34 indicates the result of the X-ray diffraction measurement of a thermal shock protection layer 22 that was sprayed using an alcoholic suspension, as described above, and can be used for the manufacturing method according to the invention. Reference numeral 36 indicates the result of the X-ray diffraction measurement of a thermal shock protection layer 22 that was sprayed using an aqueous suspension, as described above, and can be used for the manufacturing method according to the invention. Reference numeral 38 indicates the result of the X-ray diffraction measurement of a thermal shock protection layer conventionally sprayed using atmospheric plasma spraying. Reference numerals 40 indicate peaks for the α-phase of the aluminum oxide in the respective thermal shock protection layers and the above powder. Reference numerals 42 indicate the γ-phase of the aluminum oxide in the respective thermal shock protection layers and the above powder.

[0038] Based on the count rate of the diagram of the Fig. 5, the phase composition of the material can be deduced. Curve 38 shows a composition consisting primarily of γ-phase (thermal shock protection layer from atmospheric plasma spraying). Curves 34 and 36, in contrast (thermal shock protection layers from suspension plasma spraying), contain only small amounts of γ-phase and primarily α-phase. This is advantageous because the α-phase is the thermodynamically stable phase, whereas the γ-phase can undergo phase transformation into the α-phase at elevated ambient temperatures (> 850 °C), thus potentially changing the coating properties.

[0039] It is explicitly emphasized that all features disclosed in the description and / or the claims are to be considered separate and independent of each other for the purpose of the original disclosure as well as for the purpose of limiting the claimed invention, regardless of the feature combinations in the embodiments and / or the claims. It is explicitly stated that all range specifications or specifications of groups of units disclose every possible intermediate value or subgroup of units for the purpose of the original disclosure as well as for the purpose of limiting the claimed invention, in particular also as a limit of a range specification.

Claims

[1] Method for producing a sensor element (10) for detecting at least one property of a gas in a gas space, in particular for detecting a gas component in the gas or a temperature of the gas, comprising the steps: - providing at least one functional element (12) which has at least one solid electrolyte body (14) and at least one functional layer (16), in particular an electrode (18), - providing a suspension, in particular an aqueous and / or alcoholic suspension, of particles for a thermal shock protection layer (22), and - applying the suspension in the form of a thermal shock protection layer (22) to the functional element (12) by means of a thermal spraying process; characterized bythat the particles of the suspension for the thermal shock protection layer (22) are α-aluminum oxide and the thermal shock protection layer (22) is applied in such a way that at least 50 vol.% α-aluminum oxide is present in the applied thermal shock protection layer (22). [2] Method according to the preceding claim, wherein the thermal spraying process is carried out such that the thermal shock protection layer (22) has a porosity of 25% to 75%, preferably of 30% to 70% and more preferably of 30% to 60%. [3] Method according to one of the preceding claims, wherein the particles of the suspension for the thermal shock protection layer (22) contain metal or metal compounds, in particular metal compounds selected from the group consisting of aluminum oxide, zirconium dioxide and titanium dioxide. [4] Method according to one of the preceding claims, wherein the thermal shock protection layer (22) is applied such that at least 60 vol.% α-aluminum oxide, preferably at least 70 vol.% α-aluminum oxide, is present in the applied thermal shock protection layer (22). [5] A process according to the preceding claim, wherein the suspension is prepared using a powder containing α-alumina, the powder having a purity of at least 98%, preferably at least 99%, and more preferably at least 99.5%. [6] Process according to one of the preceding claims, wherein the suspension has a solids content of at least 1 vol.%, preferably at least 5 vol.% and more preferably at least 10 vol.%. [7] Method according to one of the preceding claims, wherein the particles have a diameter of 10 nm to 20 µm, preferably 15 nm to 15 µm and more preferably 20 nm to 10 µm. [8] Method according to one of the preceding claims, wherein the thermal shock protection layer (22) is applied such that it has a thickness of 40 µm to 550 µm, preferably of 45 µm to 525 µm and more preferably of 50 µm to 500 µm. [9] Method according to one of the preceding claims, wherein the thermal shock protection layer (22) is applied such that it has a surface roughness characteristic (Sa) of not more than 2.5 µm, preferably not more than 2 µm and more preferably not more than 1.8 µm. [10] Method according to one of the preceding claims, wherein the suspension is thermally sprayed such that the temperature of the functional element (12) is not more than 400 °C, preferably not more than 350 °C and even more preferably not more than 300 °C. [11] Method according to one of the preceding claims, wherein the functional element (12) is cooled during the application of the thermal shock protection layer (22). [12] A method according to any one of the preceding claims, wherein the thermal spraying method is selected from the group consisting of plasma spraying, laser spraying and high-velocity flame spraying. [13] Sensor element (10) for detecting at least one property of a gas in a gas space, in particular for detecting a gas component in the gas or a temperature of the gas, wherein the sensor element (10) can be produced by a method according to one of the preceding claims.

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