DE102013217863A1 - Sensor element for detecting at least one property of a sample gas in a sample gas space and method for producing the same - Google Patents

Abstract

A sensor element (10) for detecting at least one property of a measurement gas in a measurement gas space, in particular for detecting a proportion of a gas component in the measurement gas or a temperature of the measurement gas, is proposed. The sensor element (10) comprises at least one solid electrolyte (12) with at least one functional element (14) and a thermal shock protection layer (20). The thermal shock protective layer (20) has a first portion (22) with a closed porosity and a second portion (24) with an open porosity. The first section (22) surrounds a section (16) of the solid electrolyte (12) facing away from the measurement gas, and the second section (24) surrounds a section (18) of the solid electrolyte (12) which can be exposed to the measurement gas. Furthermore, a method is proposed for producing a sensor element (10) for detecting at least one property of a measurement gas in a measurement gas space, in particular for detecting a proportion of a gas component in the measurement gas or a temperature of the measurement gas.

Description

State of the art

A large number of sensor elements and methods for detecting at least one property of a measurement gas in a measurement gas space are known from the prior art. In principle, these can be any physical and / or chemical properties of the measurement gas, one or more properties being able to be detected. The invention will be described below in particular with reference to a qualitative and / or quantitative detection of a portion of a gas component of the measurement gas, in particular with reference to a detection of an oxygen content in the measurement gas. The oxygen content can 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 measuring gas are detectable, such as the temperature.

For example, such sensor elements can be configured as so-called lambda probes, as they are made, for example Konrad Reif (ed.): Sensors in the motor vehicle, 1st edition 2010, pages 160-165 , are known. With broadband lambda probes, in particular with planar broadband lambda probes, it is possible, for example, to determine the oxygen concentration in the exhaust gas over a large range and thus to deduce the air-fuel ratio in the combustion chamber. The air ratio λ describes this air-fuel ratio.

In particular ceramic sensor elements are known from the prior art, which are based on the use of electrolytic properties of certain solids, that is to ion-conducting properties of these solids. In particular, these solids can be ceramic solid electrolytes, such as zirconium dioxide (ZrO ₂ ), in particular yttrium-stabilized zirconium dioxide (YSZ), and scandium-doped zirconium dioxide (ScSZ), the small additions of aluminum oxide (Al ₂ O ₃ ) and / or silicon oxide ( SiO ₂ ).

At such sensor elements increasing functional requirements are made. In particular, a fast operational readiness of lambda sensors after an engine start plays a major role. This is essentially influenced by two aspects. The first aspect relates to a rapid heating of the lambda probe to its operating temperature, which is usually above 600 ° C, which can be achieved by a corresponding design of a heating element or a reduction of the area to be heated. The other aspect relates to the robustness against thermal shock due to water hammer during operation. Said thermal shock is based on the fact that for a certain period of time after engine start, the temperature in the exhaust pipe is below the dew point for water, so that the water vapor formed in the combustion of fuel in the exhaust pipe can condense. This causes the formation of drops of water in the exhaust pipe. The heated ceramic of the lambda probe can be damaged or even destroyed by the impact of water droplets due to thermal stresses or fractures in the sensor ceramic. Therefore, lambda probes have been developed which have a porous ceramic protective layer on their surface, which is also referred to as a thermal shock protection layer or thermal shock protection layer. This protective layer ensures that drops of water impinging on the lambda probe are distributed over a large area and thus the occurring local temperature gradients in the solid electrolyte or the probe ceramic are reduced. This lambda probes tolerate in the heated state so a certain drop size of condensation, without being damaged.

The protective layer is usually applied to the sensor element in an additional method step. Various materials, such as alumina or spinel (MgAl ₂ O ₄ ), and deposition techniques are used. For example, the application of the thermal shock protective layer is carried out in the dipping process, in the spraying process such as, for example, by slip spraying or by spraying. For example, it is known to apply a uniformly thick thermal shock protective layer of porous aluminum oxide by means of atmospheric plasma spraying. With such a thermal coating process, introduced particles are melted and accelerated onto the solid electrolyte surface, so that the thermal shock protective layer is applied to the entire solid electrolyte surface. This reduced in the low temperature range, ie in a temperature range of about 300 ° C to 400 ° C, by their limited permeability to the water access to the solid electrolyte of the sensor element, which is at least partially made of zirconia, and limited in the high temperature range, ie in a temperature range of about 400 ° C to 600 ° C, the cooling via the heat conduction. At higher temperatures, the Leidenfrost effect prevents cooling.

Despite the numerous advantages of the prior art methods and devices, these still have room for improvement. Thus, the sensor elements described above tolerate only small amounts of water or liquid.

Disclosure of the invention

A sensor element and method for producing the same are therefore proposed, which at least largely avoid the disadvantages of known sensor elements, in which in particular the robustness to thermal shock can be improved and the impact of larger quantities of liquid than 1 .mu.l, 3 .mu.l or even 10 .mu.l over a wide Minimize temperature range on the sensor element so that there is no damage to the sensor function supporting sensor element structure.

A sensor element according to the invention for detecting at least one property of a measurement gas in a measurement gas space, in particular for detecting a proportion of a gas component in the measurement gas or a temperature of the measurement gas, comprises at least one solid electrolyte having at least one functional element and a thermal shock protection layer. The thermal shock protective layer has a first portion with a closed porosity and a second portion with an open porosity. The first section surrounds a portion of the solid electrolyte remote from the sample gas. The second section surrounds a section of the solid electrolyte which can be exposed to the measurement gas.

The thermal shock protection layer may be at least partially made of an oxide ceramic material. The thermal shock protective layer may be at least partially made of MgAl ₂ O ₄ . The closed porosity of the first section may have pores having an average diameter of from 0.25 μm to 75 μm, preferably from 0.5 μm to 50 μm, and more preferably from 1.0 μm to 10 μm, for example 4 μm. The first portion may have a thickness that is equal to or greater than a thickness of the second portion. The sensor element may be part of a sensor device. The sensor device has a sensor housing and such a sensor element. The sensor element is held by the sensor housing. The sensor element projects out of the sensor housing. The first portion is at least partially disposed within the sensor housing.

• – Bereitstellen mindestens eines Festelektrolyten mit mindestens einem Funktionselement,
• – Aufbringen einer Mischung aus mindestens einem keramischen Material und mindestens einem metallischen Material auf den Festelektrolyten derart, dass ein erster Abschnitt gebildet wird, der den Festelektrolyten in einem dem Messgas abgewandten Abschnitt umgibt,
• – Sintern der auf den Festelektrolyten aufgebrachten Mischung bei einer ersten Temperatur derart, dass in dem ersten Abschnitt eine geschlossene Porosität gebildet wird,
• – Aufbringen der Mischung aus mindestens einem keramischen Material und mindestens einem metallischen Material auf den Festelektrolyten derart, dass ein zweiter Abschnitt gebildet wird, der den Festelektrolyten in einem dem Messgas aussetzbaren Abschnitt umgibt, und
• – Sintern der auf den Festelektrolyten aufgebrachten Mischung bei einer zweiten Temperatur derart, dass in dem zweiten Abschnitt eine offene Porosität gebildet wird.

An inventive method for producing a sensor element for detecting at least one property of a measurement gas in a measurement gas space, in particular for detecting a proportion of a gas component in the measurement gas or a temperature of the measurement gas comprises the following steps, preferably in the order given:

• Providing at least one solid electrolyte having at least one functional element,
• Applying a mixture of at least one ceramic material and at least one metallic material to the solid electrolyte such that a first section is formed which surrounds the solid electrolyte in a section facing away from the measurement gas,
• Sintering the mixture applied to the solid electrolyte at a first temperature such that a closed porosity is formed in the first section,
• - Applying the mixture of at least one ceramic material and at least one metallic material on the solid electrolyte such that a second portion is formed, which surrounds the solid electrolyte in a sample gas exposable portion, and
• Sintering the mixture applied to the solid electrolyte at a second temperature such that an open porosity is formed in the second section.

The second temperature may be lower than the first temperature. The ceramic material may be used in the form of powder for the mixture. The mixture can be prepared as a paste and processed as a paste and contain activated charcoal particles for the second section. Particles of the powder for the ceramic material may have an average diameter of from 0.25 μm to 75 μm, preferably from 0.5 μm to 50 μm, and more preferably from 1 μm to 10 μm, for example 4 μm. The metallic material may be used in the form of powder for the mixture. Particles of the powder for the metallic material may have an average diameter of from 0.5 μm to 250 μm, preferably from 1 μm to 200 μm, and more preferably from 10 μm to 50 μm, for example 30 μm. The ceramic material may be a metal oxide. The ceramic material may have a specific surface area of from 0.5 m ² / g to 2.5 m ² / g, preferably from 0.75 m ² / g to 2.0 m ² / g, and more preferably 1.0 m ² / g to 1.5 m ² / g, for example, 1.2 m ² / g. The specification of the specific surface refers to a property of the ceramic material in its starting form, which is in particular a powder.

In the context of the present invention, a solid electrolyte is to be understood as meaning a body or article having electrolytic properties, that is to say having ion-conducting properties. In particular, it may be a ceramic solid electrolyte. This also includes the raw material of a solid electrolyte, i. H. Training as a so-called greenling or brownling, which only after sintering to a solid electrolyte.

Under a functional element is in the context of the present invention, an element to which is selected from the group consisting of: electrode, trace, diffusion barrier, diffusion gap, reference gas channel, heating element, Nernst cell and pump cell. In particular, these include those elements which fulfill the essential chemical and / or physical and / or electrical and / or electrochemical functions of a lambda probe.

In the context of the present invention, a thermal shock protection layer is to be understood as meaning a layer which is suitable for distributing water droplets impinging on the sensor element over a large area and thus reducing the local temperature gradients occurring in the solid electrolyte, thus protecting the sensor element against water hammer. Under a layer is to be understood as a uniform mass in areal extent of a certain height, which may be on, over or between other components.

For the purposes of the present invention, porosity means the ratio of void volume to total volume of a substance or mixture of substances as a dimensionless measured variable. This measure can be specified in particular in percent. Under an open porosity here is the proportion of the void volume of those cavities to understand the total void volume, which communicate with each other and with the ambient air. Under a closed porosity is the proportion of the void volume of those voids on the total void volume to understand that are not related to each other or with the ambient air.

A basic idea of the present invention is to specify a sensor element packaging which has a foam-like structure which is produced from a starting mixture of ceramic powder particles and metal powder particles and can be produced by a heat treatment of the starting mixture. In a first process step, in a first section that is not exposed to the measurement gas, closed porosity is achieved by the formation of oxide shells around the metal powder particles and molten metal within the oxide sheaths and by the leakage of the molten metal into the ceramic material. In a second method step, an open porosity arises at a lower temperature than in the first method step in a second section, which is located in the region of the sensor element facing the measurement gas.

Accordingly, in the invention, a sensor element is provided in which in a measurement gas exposable or facing portion of the thermal shock protective layer, a porous shaped body is created, which surrounds the sensor element in the region of the functional element. This shaped body is thus designed to be open-porous, at least in the region of the at least one functional element, in order to ensure gas access to the functional element, ie. H. in the area of the functional element, a percolation threshold of the thermal shock protective layer is exceeded.

For example, aluminum oxide, spinel, yttrium-stabilized zirconium dioxide or forsterite are suitable as the material for the molding or the thermal shock protection layer. Percolation describes the formation of contiguous areas (clusters) by randomly occupying structures (lattices). In point percolation, lattice points are occupied with a certain probability, in edge percolation occupied points are connected to each other. As the likelihood that one field of the grid is occupied, larger clusters form.

The occupation probability is defined as the value at which at least one cluster reaches a size in which it extends through the entire system, for example has an extension on a two-dimensional grid from the right to the left or from the upper to the lower side. It is said that the cluster percolates through the system. This value of the occupancy probability is the so-called percolation threshold. In the example mentioned, therefore, the percolation threshold describes the percolation probability with which at least one cavity extending through the thermal shock protection layer is formed or several interconnected cavities are formed so that the gas moves from a side of the thermal shock protection layer facing away from the functional element to the side facing the functional element the thermal shock protection layer can get.

As an example of the materials for the starting mixture of the above-mentioned foamy structure, there may be exemplified a mixture of an aluminum-magnesium alloy such as AlMg5 containing 5% by weight of magnesium with a ceramic powder, especially a metal oxide powder such as magnesium oxide , be used. In detail, for example, 25.00 g of this AlMg5 powder having an average particle size of 1 .mu.m to 200 .mu.m, preferably 10 .mu.m to 50 .mu.m, for example 30 .mu.m, with a standard deviation of about 10 .mu.m, with 15.66 g ceramic magnesium oxide powder an average particle size of 0.5 .mu.m to 50 .mu.m, for example 4 .mu.m, and a specific surface area of, for example, 1.2 ^m.sup.2 / g. This mixture is based on the fact that in a complete oxidation of the aluminum-magnesium alloy in the context of subsequent reaction sintering forms a spinel of the type MgAl ₂ O ₄ with the magnesium oxide.

Such a foam-like structure essentially has the properties of an oxide-ceramic material having an oxide-ceramic matrix with pores embedded therein and optionally metallic structural areas, the locations and sizes of the pores being largely defined by the locations of the metal powder particles previously present there. This material thus has a largely closed porosity in the first section due to the high reaction temperature and a largely open, gas-permeable porosity in the second section due to the lower reaction sintering temperature. Mechanical stresses resulting, for example, from the temperature change and drops of water occurring during operation, d. H. The so-called thermal shock, can be reduced by the elasticity and the drop absorption capacity of the foam-like structure. This reduces the force effects and stresses on the embedded sensor element.

Brief description of the drawings

Further optional details and features of the invention will become apparent from the following description of preferred embodiments, which are shown schematically in the figure. It shows:

1 a longitudinal sectional view of a sensor element according to the invention

Embodiments of the invention

1 shows a longitudinal sectional view of a sensor element according to the invention 10 , This in 1 illustrated sensor element 10 can be used to detect physical and / or chemical properties of a sample gas, wherein one or more properties can be detected. The invention will be described below in particular with reference to a qualitative and / or quantitative detection of a gas component of the measurement gas, in particular with reference to a detection of an oxygen content in the measurement gas. The oxygen content can be detected, for example, in the form of a partial pressure and / or in the form of a percentage. In principle, however, other types of gas components are also detectable, for example nitrogen oxides, hydrocarbons and / or hydrogen. Alternatively or additionally, however, other properties of the measuring gas are detectable, such as the temperature. The invention can be used in particular in the field of motor vehicle technology, so that the measuring gas chamber can be, in particular, an exhaust gas tract of an internal combustion engine and, in the case of the measuring gas, in particular an exhaust gas.

The sensor element 10 as an exemplary component of a planar lambda probe has a solid electrolyte 12 on. The solid electrolyte 12 can be composed of several solid electrolyte layers or comprise several solid electrolyte layers. In the solid electrolyte 12 it may in particular be a ceramic solid electrolyte 12 such as zirconia, especially yttria stabilized zirconia and scandium doped zirconia which may contain minor additions of alumina and / or silica. The solid electrolyte 12 has at least one functional element 14 on. In the embodiment shown, the solid electrolyte 12 For example, a first electrode, a second electrode and a heating element, which are not shown in detail. However, the first electrode is on a surface of the solid electrolyte 12 arranged. The second electrode is inside the solid electrolyte 12 arranged. The heating element is also inside the solid electrolyte 12 arranged. The first electrode, the second electrode and that part of the solid electrolyte 12 between form, for example, a so-called Nernst cell, which is heated by the heating element. The solid electrolyte 12 has a section facing away from the sample gas 16 and a section that can be exposed or exposed to the measuring gas 18 as described in more detail below.

The sensor element 10 further has a thermal shock protection layer 20 on. The thermal shock protection layer 20 has a first section 22 with a closed porosity and a second section 24 with an open porosity. The first paragraph 22 surrounds the section facing away from the sample gas 16 of the solid electrolyte 12 , The second section 24 surrounds the section which can be exposed or exposed to the measuring gas 18 of the solid electrolyte 12 , For example, the sensor element 10 Part of a sensor device 26 , The sensor device 26 has a sensor housing 28 on. The sensor element 10 is from the sensor housing 28 held. The sensor element 10 partially protrudes from the sensor housing 28 towards the sample gas chamber. The section facing away from the sample gas space 16 of the solid electrolyte 12 is at least partially within the sensor housing 28 arranged and exposable to the sample gas space section 18 of the solid electrolyte 12 protrudes from the sensor housing 28 out. The first section is corresponding 22 the thermal shock protection layer 20 at least partially within the sensor housing 28 arranged, whereas the second section 24 the thermal shock protection layer 20 outside the sensor housing 28 is arranged. The section that can be exposed to the sample gas chamber 18 of solid electrolyte 12 can from a protective tube 30 be surrounded, which allows a gas access into its interior, but the sensor element protects against the impact of larger particles.

The thermal shock protection layer 20 may be at least partially made of an oxide ceramic material. The thermal shock protection layer 20 For example, it can be made at least partially from spinel (MgAl ₂ O ₄ ). The closed porosity of the first section 22 has pores having an average diameter of from 0.25 μm to 75 μm, preferably from 0.5 μm to 50 μm, more preferably from 1.0 μm to 10 μm, for example 4 μm. The first paragraph 22 may have a thickness equal to or greater than a thickness of the second portion 24 is. Thickness is the thickness of the material. For example, it is possible to use the solid electrolyte 12 comparatively very thin, in particular 0.5 to 0.33 the thickness of conventional solid electrolytes, and this by the thermal shock protective layer 20 Thicken so that the sensor element 10 becomes mechanically stable, in particular receives increased fatigue strength. So far, the thickness of the functional ceramic films, ie the solid electrolyte 12 with the functional element 14 , Also due to the fatigue strength of the cuboid sensor element 10 limited to the bottom.

Such a sensor element 10 can be prepared, for example, as described below. First, the solid electrolyte 12 with the functional element 14 provided. For example, the solid electrolyte becomes 12 made of a plurality of solid electrolyte layers, which are printed with the above-mentioned functional element in a conventional manner, ie with the first electrode, the second electrode and the heating element. Known techniques are, for example, the so-called film technology or multi-layer technology. Such a design of a planar sensor element 10 is sufficiently known from the above-mentioned prior art, so that it is not discussed further.

At least one ceramic material and at least one metallic material are mixed. The ceramic material may be used in the form of powder for the mixture. The metallic material may also be used in the form of powder for the mixture. Particles of the powder for the ceramic material may have an average diameter of from 0.25 μm to 75 μm, preferably from 0.5 μm to 50 μm, and more preferably from 1 μm to 10 μm, for example 4 μm. Particles of the powder for the metallic material may have an average diameter of from 0.5 μm to 250 μm, preferably from 1 μm to 200 μm, and more preferably from 10 μm to 50 μm, for example 30 μm. The ceramic material may be a metal oxide. The ceramic material may have a specific surface area of from 0.5 m ² / g to 2.5 m ² / g, preferably from 0.75 m ² / g to 2.0 m ² / g, and more preferably 1.0 m ² / g to 1.5 m ² / g, for example, 1.2 m ² / g.

As a concrete embodiment, for example, a mixture of an aluminum-magnesium alloy with a content of 5 wt .-% magnesium with a ceramic powder, in particular a metal oxide powder such as magnesium oxide, is used. Specifically, 25.00 g of powder of the aluminum-magnesium alloy having an average particle size between 1 .mu.m and 200 .mu.m, preferably 10 .mu.m to 50 .mu.m, for example 30 .mu.m, with a standard deviation of about 10 .mu.m, and 15, 66 g powder of magnesium oxide having an average particle size of 0.5 .mu.m to 50 .mu.m, for example 4 .mu.m, and a specific surface area of, for example, 1.2 m ² / g. This mixture is based on the fact that in a complete oxidation of the aluminum-magnesium alloy in the reaction sintering described below forms a spinel of the type MgAl2O4 with the magnesium oxide.

On the solid electrolyte 12 the mixture of the ceramic material and the metallic material is applied such that the first section 22 is formed, the solid electrolyte 12 in the section facing away from the sample gas 16 surrounds. The application can be carried out, for example, by means of a dipping method or by immersion in the pasty prepared material mixture or by screen printing of such a paste or by means of atmospheric plasma spraying. Then it's on the solid electrolyte 12 applied mixture sintered at a first temperature such that in the first section 22 a closed porosity is formed. The sintering may, for example, at a first temperature of 1350 ° C to 1550 ° C, in particular at 1385 ° C, take place, the temperature is kept constant for example 5.5 hours. The formation of the closed porosity occurs through the formation of oxide sheaths around the metal powder particles and molten metal within the oxide sheaths and through the leakage of the molten metal into the ceramic material.

Then on the solid electrolyte 12 the mixture of the ceramic material and the metallic material on the solid electrolyte 12 so applied that the second section 24 is formed, the solid electrolyte 12 in the section which can be exposed to the measuring gas 18 surrounds. Then it's on the solid electrolyte 12 applied mixture sintered at a second temperature such that in the second section 24 an open porosity is formed. Activated charcoal particles contained in the mixture assist the production of Porosity. The second temperature may be lower than the first temperature. The second temperature may, for example, be 1200 ° C. to 1325 ° C., in particular 1285 ° C., the temperature being kept constant for, for example, 5.5 hours.

In this way, a thermal shock protection layer is formed 20 in the form of a foamy structure. Such a foam-like structure essentially has the properties of an oxide-ceramic material having an oxide-ceramic matrix with pores embedded therein and optionally metallic structural areas, the locations and sizes of the pores being largely defined by the locations of the metal powder particles previously present there. This material thus has a largely closed porosity in the first section 22 due to the high reaction temperature and a largely open, gas-permeable porosity in the second section 24 due to the lower reaction sintering temperature. Mechanical stresses resulting, for example, from the temperature change and drops of water which occur during operation, ie the so-called thermal shock, can be reduced by the elasticity and drop absorption capacity of the foam-like structure. Thus, the force effects and stresses on the embedded solid electrolyte 12 reduced.

The sensor element according to the invention 10 Alternatively, it can be prepared as follows. The method described below constitutes a modification of the method for producing porous materials of DE 20 2012 005 818 U1 In a first process step, nanoparticles of the above-described mixture of at least one ceramic material and at least one metallic material are introduced into the pores of a porous three-dimensional matrix material ("exotic template"), wherein the porous three-dimensional matrix material is particulate, in particular granular, preferably spherical activated carbon is used. The matrix material specifies the pores to be formed. In a second process step, the matrix material loaded with the nanoparticles is deposited on the solid electrolyte 12 so applied that the first section 22 and the second section 24 be formed, with the first section 22 the section facing away from the sample gas 16 of the solid electrolyte 12 surrounds and the second section 24 the section that can be exposed to the sample gas 18 of the solid electrolyte 12 surrounds. In other words, one portion of the closed porosity matrix material and another portion of the open porosity matrix material are provided and deposited on the solid electrolyte 12 applied in the manner described above. In a third process step, the solid electrolyte 12 with the applied first section 22 and the applied section 24 a thermal treatment for the purpose of at least partial, preferably for purposes of at least substantially complete removal of the matrix material subjected. This is a thermal shock protection layer 20 formed at the first section 22 a closed porosity and the second section 24 has an open porosity.

As is apparent from the above description, the production of the thermal shock protective layer takes place 20 by means of a so-called template method (template synthesis). In a so-called template synthesis, a chemical template organizes a collection of atoms, ions, or molecules with respect to one or more geometric locations in order to achieve a specific crosslinking of these atoms, ions or molecules. In general terms, the template serves as a pre-orientation of the components to be linked. Depending on whether the components to be linked are introduced into the template or the templates are surrounded by the components to be linked, a distinction is made between so-called exotic templates on the one hand and endotemplates on the other. In the context of template synthesis, the dimensions and geometry of the template are ideally transferred to the resulting product in the sense of a negative impression.

The use of activated charcoal as an exotemplate in particular has many advantages: the activated carbon used as an exotemplate can easily be produced by prior art processes and is also commercially available on a large scale; Commercially available activated carbon, as used according to the invention as an exotemplate, is sold, for example, by Blücher GmbH, Erkrath, Germany, and by Adsor-Tech GmbH, Premnitz, Germany.

Activated carbon has the further advantage that in the process step of the thermal treatment a complete removal, in particular via combustion or pyrolysis, is possible, so that ultimately self-supporting porous materials are obtained in particulate form. Furthermore, activated carbon has the advantage that in addition to the particle size and the inner pore system is adjustable or controllable and thus can be tailored for the application in question; In this way, the pore system of the resulting mixed metal oxides can be readily adjusted or controlled and thus tailored, since the activated carbon used as an exotic template acts as a template for the resulting end products. As a result, the porosity in the first section can be achieved by means of the activated carbon 22 and the second section 24 pretend. In other words, depending on the desired porosity Activated carbon shaped accordingly, so in the first section 22 a closed porosity arises and in the second section 24 an open porosity arises. The advantage here is that the mass of activated carbon and introduced into their pores mixture of at least one ceramic material and at least one metallic material in a single step on the solid electrolyte 12 can be applied. Furthermore, the used granular, in particular spherical activated carbon has the advantage that it is mechanically strong and, moreover, transmits the spherical morphology also on the resulting end products.

By the synthesis or production of the porous materials in the spatially limited pores of the activated carbon-exotic template also small crystallites of the mixture of at least one ceramic material and at least one metallic material are stabilized. In this way, the dimensions and geometry of the Exotemplat-pore system can be transferred to the resulting end product in the sense of a negative impression. In addition, since the activated carbon-based exotemplate has a defined particle morphology, in particular a defined particle size, this remains even after removal of the activated carbon-based exotemplate.

The nanoparticles can be used in the form of a dispersion or a colloidal solution, in particular in the form of a sol. In other words, the nanoparticles in the form of a dispersion or a colloidal solution, in particular a sol, are injected into the pores of the matrix material, i. the activated carbon, introduced. In this case, the dispersion or colloidal solution of the nanoparticles used may in principle be water-based or else be organically based, preferably aqueous-based.

In general, the nanoparticles, in particular the dispersions or colloidal solutions, preferably sols, of the relevant nanoparticles are obtained by precipitation, in which case in particular solutions or dispersions of salts or other compounds, such as, for example, alkoxides of the respective metals are used with subsequent oxidation can be. Usually, the preparation of the nanoparticles, in particular the dispersions and colloidal solutions of the nanoparticles, takes place in situ.

In general, the method step of introducing the nanoparticle-containing dispersion or colloidal solution, in particular of the sol, into the pores of the matrix material (exotic template) is preceded by a drying process step before the thermal treatment is carried out. This drying step serves in particular for the removal of the liquid medium of the dispersion or colloidal solution containing the nanoparticles, in particular of the sol.

In order to enable introduction of the nanoparticles into the pore system of the exotic template, the particle sizes of the nanoparticles are smaller than the diameters of the pores of the matrix material. It is particularly preferred if the ratio of the diameters of the pores of the matrix material to the particle sizes of the nanoparticles is> 1: 1 (eg at least 1.05: 1), in particular at least 1.25: 1, preferably at least 1.5: 1, preferably at least 1.75: 1, more preferably at least 2, most preferably at least 5, even more preferably at least 10, even more preferably at least 20, even more preferably at least 50. According to the invention, it is preferred if at least 50%, in particular at least 70%, preferably at least 90%, particularly preferably at least 99% and more, very particularly preferably 100%, of the nanoparticles used fulfill this requirement; Nevertheless, if a proportion of the nanoparticles as previously specified does not fulfill this requirement, this is not detrimental since this part of the nanoparticles can then be attached to the matrix material and, after thermal treatment of the matrix material, form an outer shell layer in the end product.

In general, the nanoparticles of the respective metal oxides to be introduced have particle sizes in the range from 0.1 nm to 2,000 nm, in particular 0.5 nm to 1,000 nm, preferably 0.75 nm to 750 nm, particularly preferably 1 nm to 500 nm, very particularly preferably 1 nm to 250 nm, more preferably 2 nm to 100 nm, on. In particular, the relevant nanoparticles have average particle sizes (D50) in the range from 0.1 nm to 500 nm, in particular 0.2 nm to 100 nm, preferably 0.5 nm to 50 nm, particularly preferably 1 nm to 25 nm.

In summary, a matrix material is used that already has the pores in the desired shape, ie, a closed porosity portion and an open porosity portion. In the respective pores, the above-described mixture of at least one ceramic material and at least one metallic material is introduced. Subsequently, the matrix material containing the mixture is applied to the solid electrolyte 12 applied such that a portion of the matrix material with the mixture and the closed porosity facing away from the sample gas section 16 of the solid electrolyte 12 surrounds and a portion of the matrix material with the mixture and the open porosity the exposable to the sample gas section 18 of the solid electrolyte 12 surrounds. The following is a thermal treatment performed, leaving a thermal shock protective layer 20 is formed, at the first section 22 the section facing away from the sample gas 16 of the solid electrolyte 12 surrounds and the second section 24 the section that can be exposed to the sample gas 18 of the solid electrolyte 12 surrounds. This method has the advantage that the first section 22 and the second section 24 can be formed together and at the same time.

By means of the two methods described above, it is possible to use the solid electrolyte 12 comparatively very thin, in particular 0.5 to 0.33 the thickness of conventional solid electrolytes, and this by the thermal shock protective layer 20 Thicken so that the sensor element 10 becomes mechanically stable, in particular receives increased fatigue strength.

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• DE 202012005818 U1 [0034]

Cited non-patent literature

• Konrad Reif (ed.): Sensors in the motor vehicle, 1st edition 2010, pages 160-165 [0002]

Claims (14)

Sensor element ( 10 ) for detecting at least one property of a measurement gas in a measurement gas space, in particular for detecting a proportion of a gas component in the measurement gas or a temperature of the measurement gas comprising at least one solid electrolyte ( 12 ) with at least one functional element ( 14 ) and a thermal shock protection layer ( 20 ), wherein the thermal shock protective layer ( 20 ) a first section ( 22 ) with a closed porosity and a second section ( 24 ) having an open porosity, the first section ( 22 ) a section facing away from the sample gas ( 16 ) of the solid electrolyte ( 12 ) and the second section ( 24 ) a section which can be exposed to the sample gas ( 18 ) of the solid electrolyte ( 12 ) surrounds. Sensor element ( 10 ) according to the preceding claim, wherein the thermal shock protective layer ( 20 ) is at least partially made of an oxide ceramic material. Sensor element ( 10 ) according to one of the preceding claims, wherein the thermal shock protective layer ( 20 ) is at least partially made of MgAl ₂ O ₄ . Sensor element ( 10 ) according to one of the preceding claims, wherein the closed porosity of the first section pores having an average diameter of 0.25 microns to 75 microns, preferably from 0.5 microns to 50 microns and more preferably from 1.0 microns to 10 microns. Sensor element ( 10 ) according to one of the preceding claims, wherein the first section ( 22 ) has a thickness which is equal to or greater than a thickness of the second portion ( 24 ). Sensor device ( 26 ) with a sensor housing ( 28 ) and a sensor element ( 10 ) according to one of the preceding claims, wherein the sensor element ( 10 ) from the sensor housing ( 28 ) and out of the sensor housing ( 28 ), the first section ( 22 ) at least partially within the sensor housing ( 28 ) is arranged. Method for producing a sensor element ( 10 ) for detecting at least one property of a measurement gas in a measurement gas space, in particular for detecting a proportion of a gas component in the measurement gas or a temperature of the measurement gas, comprising the steps of: providing at least one solid electrolyte 12 ) with at least one functional element ( 14 ), - applying a mixture of at least one ceramic material and at least one metallic material to the solid electrolyte ( 12 ) such that a first section ( 22 ), which is the solid electrolyte ( 12 ) in a section facing away from the sample gas ( 16 ), sintering the on the solid electrolyte ( 12 ) applied at a first temperature such that in the first section ( 22 ) a closed porosity is formed, - application of the mixture of at least one ceramic material and at least one metallic material to the solid electrolyte ( 12 ) such that a second section ( 24 ), which is the solid electrolyte ( 12 ) in a section which can be exposed to the measuring gas ( 18 ), and - sintering the on the solid electrolyte ( 12 ) at a second temperature such that in the second section ( 24 ) an open porosity is formed.  A method according to the preceding claim, wherein the second temperature is lower than the first temperature. Method according to one of the two preceding claims, wherein the ceramic material is used in the form of powder for the mixture. Method according to one of the three preceding claims, wherein the mixture is prepared as a paste and processed as a paste and thereby for the second section ( 24 ) Contains activated carbon particles.  A method according to claim 9, wherein particles of the powder for the ceramic material have an average diameter of from 0.25 μm to 75 μm, preferably from 0.5 μm to 50 μm and more preferably from 1 μm to 10 μm.  Method according to one of the five preceding claims, wherein the metallic material is used in the form of powder for the mixture.  Process according to the preceding claim, wherein particles of the powder for the metallic material have an average diameter of 0.5 μm to 250 μm, preferably of 1 μm to 200 μm and more preferably of 10 μm to 50 μm.  Method according to one of the seven preceding claims, wherein the ceramic material is a metal oxide. Method according to one of the seven preceding claims, wherein the ceramic material has a specific surface area of 0.5 m ² / g to 2.5 m ² / g, preferably from 0.75 m ² / g to 2.0 m ² / g and more preferably from 1.0 m ² / g to 1.5 m ² / g.

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