SENSOR ELEMENT FOR A GARASS SENSOR

Abstract

Sensor element (10) for a gas sensor (100), comprising: an element base (1) which is a ceramic structure comprising a sensing part for sensing a gas component to be measured; and a protective layer (2) at the front end, which is a porous layer arranged around an outer edge of the element base (1) in a predetermined area of ​​an end section of the element base (1) on one side of the detection part and comprising: a first protective layer (21) at the front end, which is arranged on at least two main surfaces of the element base (1); a second protective layer (22) at the front end, arranged to cover the end section and four side surfaces of the element base (1), including the two main surfaces on which the first protective layer (21) is arranged at the front end; and a third protective layer (23) at the front end, arranged to cover the second protective layer (22) at the front end, wherein the first protective layer (21) at the front end has a porosity of 30% to 60% and a thickness of 15 µm to 50 µm; the second protective layer (22) at the front end has a porosity of 30% to 80% and a thickness 30 to 50 times that of the first protective layer (21) at the front end, and the third protective layer (23) at the front end has a porosity of 15% to 30% and a thickness 5 to 10 times that of the first protective layer (21) at the front end.

Description

BACKGROUND OF THE INVENTION Area of ​​the invention

[0001] The present invention relates to a sensor element for a gas sensor and in particular to a surface protection layer thereof. Description of the state of the art

[0002] A gas sensor is known for determining the concentration of a desired gas component contained in a sample gas, such as exhaust gas from an internal combustion engine. This gas sensor comprises a sensor element made of an oxygen-ion-conducting solid electrolyte, such as zirconium oxide (ZrO2), and includes electrodes on its surface and interior. The sensor element is known to have an elongated planar shape and includes a protective layer formed from a porous body (porous protective layer) in an end section where a portion is provided for introducing the sample gas. Furthermore, it includes a surface protective layer with a lower porosity than the porous protective layer outside the porous protective layer (see, for example, Japanese patent JP 5 387 555 B2).

[0003] The protective layer is applied to the surface of the sensor element to ensure its water resistance when the gas sensor is in use. Specifically, the protective layer prevents water-induced cracking of the sensor element under the influence of thermal shock caused by the heat (or cold) of water droplets adhering to the surface of the sensor element.

[0004] In a gas sensor disclosed in Japanese Patent No. JP 5 387 555 B2, a surface protective layer is made water-repellent at a high temperature (500 °C or more) using the Leidenfrost phenomenon to repel water droplets adhering to a sensor element, thus preventing water-induced cracking in the sensor element. A porous protective layer is provided to trap toxic components contained in a sampled gas and to reduce heat transfer from the surface protective layer to the interior. Japanese Patent No. JP 5 387 555 B2 specifies a water exposure quantity of 10 µl as the reference value for water-induced cracking (water resistance). JP 2020 - 34 435 A describes another sensor element. SUMMARY

[0005] The present invention relates to a sensor element for a gas sensor and in particular to a structure of a surface protection layer thereof.

[0006] Based on intensive investigations carried out by the inventors of the present invention, it was found that a sensor element with high water resistance, significantly exceeding the reference value disclosed in Japanese Patent No. JP 5 387 555 B2, can be obtained by providing the protective layer with a plurality of layers and by arranging the layers in such a way that the functions to be performed by each layer are appropriately fulfilled.

[0007] According to the present invention, a sensor element for a gas sensor comprises: an element base, which is a ceramic structure comprising a sensing part for detecting a gas component to be measured; and a protective layer at the front end, which is a porous layer arranged around an outer edge of the element base in a predetermined area of ​​an end section of the element base on one side of the sensing part, and comprising: a first protective layer at the front end, which is arranged on at least two main surfaces of the element base; a second protective layer at the front end, which is arranged to cover the end section and four side surfaces of the element base, including the two main surfaces on which the first protective layer at the front end is arranged;and a third protective layer at the front end, arranged to cover the second protective layer at the front end, wherein the first protective layer at the front end has a porosity of 30% to 60% and a thickness of 15 µm to 50 µm, the second protective layer at the front end has a porosity of 30% to 80% and a thickness 30 to 50 times that of the first protective layer at the front end, and the third protective layer at the front end has a porosity of 15% to 30% and a thickness 5 to 10 times that of the first protective layer at the front end.

[0008] Accordingly, this allows for the creation of a sensor element that has a protective layer at the front end with high thermal shock resistance against cold caused by the adhesion of water droplets.

[0009] It is therefore an object of the present invention to provide a sensor element for a gas sensor with a higher water resistance than a conventional sensor element. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 is a schematic perspective view of the outside of a sensor element 10; Fig. Figure 2 is a schematic view showing the structure of a gas sensor 100, including a sectional view along the longitudinal direction of the sensor element 10; and Fig. Figure 3 is a flowchart of a processing step in the manufacture of sensor element 10. DESCRIPTION OF PREFERRED DESIGNS<Overview of the sensor element and the gas sensor>

[0010] The Fig. Figure 1 is a schematic perspective view of the exterior of a sensor element (gas sensor element) 10 according to an embodiment of the present invention. Fig.Figure 2 is a schematic view showing the structure of a gas sensor 100, including a sectional view along the longitudinal direction of the sensor element 10. The sensor element 10 is a ceramic structure and a key component of the gas sensor 100, which detects a predefined gas component in a sample gas and measures its concentration. The sensor element 10 is a so-called limiting current gas sensor element.

[0011] In addition to the sensor element 10, the gas sensor 100 mainly comprises a pump cell power supply 30, a heating device power supply 40 and a control unit 50.

[0012] As it is in the Fig.As shown in Figure 1, the sensor element 10 has a structure in which an end section of an elongated planar element base 1 is covered with a porous protective layer 2 at the front end. The protective layer 2 at the front end comprises three layers: a first protective layer 21 at the front end, a second protective layer 22 at the front end, and a third protective layer 23 at the front end. Details of the protective layer 2 at the front end are described later.

[0013] As it is in the Fig.As shown in Figure 2, the element base 1 comprises an elongated planar ceramic body 101 as a main structure. Main surface protection layers 170 are provided on two main surfaces of the ceramic body 101, and in the sensor element 10, the protection layer 2 is further provided at the front end outside both an end surface (a front end surface 101e of the ceramic body 101) and four side surfaces on a front end section. The four side surfaces, which are different from opposite end surfaces in the longitudinal direction of the sensor element 10 (or the element base 1 or the ceramic body 101), are hereinafter simply referred to as the side surfaces of the sensor element 10 (or the element base 1 or the ceramic body 101).

[0014] The ceramic body 101 is made of a ceramic containing zirconium oxide (yttrium oxide-stabilized zirconium oxide) as a main component, which is an oxygen-ion-conducting solid electrolyte. Various components of the sensor element 10 are provided both outside and inside the ceramic body 101. The ceramic body 101 with this configuration has a high density and is airtight. The [missing information] Fig. The setup of sensor element 10 shown in Figure 2 is merely an example and the specific setup of sensor element 10 is not limited to this setup.

[0015] The one in Fig.The sensor element 10 shown in Figure 2 is a so-called gas sensor element of the three-chamber series structure type, comprising a first inner chamber 102, a second inner chamber 103, and a third inner chamber 104 within the ceramic body 101. That is, in the sensor element 10, the first inner chamber 102 is connected via a first diffusion control element 110 and a second diffusion control element 120 to a gas inlet 105, which is open to the outside on one side of an end section E1 of the ceramic body 101 (more precisely: is connected to the outside via the protective layer 2 at the front end). The second inner chamber 103 is connected to the first inner chamber 102 via a third diffusion control element 130, and the third inner chamber 104 is connected to the second inner chamber 103 via a fourth diffusion control element 140. A path from the gas inlet 105 to the third inner chamber 104 is also referred to as the gas distribution part.In the sensor element 10 according to the present embodiment, the distribution part is provided straight along the longitudinal direction of the ceramic body 101.

[0016] The first diffusion adjustment part 110, the second diffusion adjustment part 120, the third diffusion adjustment part 130 and the fourth diffusion adjustment part 140 are each provided as two slots which are in the Fig. The two diffusion control elements are arranged vertically. The first diffusion control element 110, the second diffusion control element 120, the third diffusion control element 130, and the fourth diffusion control element 140 exert a predetermined diffusion resistance on a sample gas passing through them. A buffer chamber 115, which buffers pulsations of the sample gas, is provided between the first diffusion control element 110 and the second diffusion control element 120.

[0017] An outer pump electrode 141 is provided on an outer surface of the ceramic body 101, and an inner pump electrode 142 is provided in the first inner chamber 102. Furthermore, an auxiliary pump electrode 143 is provided in the second inner chamber 103, and a measuring electrode 145, serving as a sensing element for directly detecting a gas component to be measured, is provided in the third inner chamber 104. In addition, a reference gas inlet 106, which is connected to the outer surface and through which a reference gas is introduced, is provided on one side of the other end section E2 of the ceramic body 101, and a reference electrode 147 is provided in the reference gas inlet 106.

[0018] In a case where a measurement target of the sensor element 10 is, for example, NOx in the measuring gas, the concentration of the NOx gas in the measuring gas is calculated by the procedure described below.

[0019] First, the sample gas, introduced into the first inner chamber 102, is adjusted to a substantially constant oxygen concentration by a pumping action (pumping oxygen in or out) of a main pump cell P1, and then introduced into the second inner chamber 103. The main pump cell P1 is an electrochemical pump cell comprising the outer pump electrode 141, the inner pump electrode 142, and a ceramic layer 101a, which is a section of the ceramic body 101 located between these electrodes. In the second inner chamber 103, oxygen in the sample gas is pumped out by an auxiliary pump cell P2, which is also an electrochemical pump cell, so that the sample gas is in a state with a sufficiently low oxygen partial pressure.The auxiliary pump cell P2 comprises the outer pump electrode 141, the auxiliary pump electrode 143 and a ceramic layer 101b, which is a section of the ceramic body 101 located between these electrodes.

[0020] The outer pump electrode 141, the inner pump electrode 142, and the auxiliary pump electrode 143 are each designed as porous cermet electrodes (e.g., a cermet electrode made of ZrO2 and Pt containing 1% Au). The inner pump electrode 142 and the auxiliary pump electrode 143, which are to be in contact with the measuring gas, are each made using a material with reduced or no reducing capacity with respect to a NOx component in the measuring gas.

[0021] NOx in the sample gas, which has been brought into a state of low oxygen partial pressure by the auxiliary pump cell P2, is introduced into the third inner chamber 104 and reduced or decomposed by the measuring electrode 145, which is provided in the third inner chamber 104. The measuring electrode 145 is a porous cermet electrode that also acts as an NOx reduction catalyst, reducing the NOx present in the atmosphere in the third inner chamber 104. During the reduction or decomposition, a potential difference between the measuring electrode 145 and the reference electrode 147 is maintained at a constant level. Oxygen ions generated by the aforementioned reduction or decomposition are pumped out of the element by a measuring pump cell P3. The measuring pump cell P3 comprises the outer pump electrode 141, the measuring electrode 145, and a ceramic layer 101c, which is a section of the ceramic body 101 located between these electrodes.The measuring pump cell P3 is an electrochemical pump cell that pumps out oxygen that has been generated by the decomposition of NOx in an atmosphere around the measuring electrode 145.

[0022] The pumping (injection or removal of oxygen) of the main pump cell P1, the auxiliary pump cell P2, and the measuring pump cell P3 is achieved by the control unit 50 via the pump cell power supply (variable power supply) 30, which applies a voltage required for pumping to electrodes included in each of the pump cells. In the case of measuring pump cell P3, a voltage is applied to the outer pump electrode 141 and the measuring electrode 145 such that the potential difference between the measuring electrode 145 and the reference electrode 147 is maintained at a predetermined value. The pump cell power supply 30 is typically provided for each pump cell.

[0023] The control unit 50 detects a pump current Ip2 flowing between the measuring electrode 145 and the outer pump electrode 141 according to the amount of oxygen pumped out through the measuring pump cell P3, and calculates the NOx concentration in the measuring gas on the basis of a linear relationship between a current value (NOx signal) of the pump current Ip2 and the concentration of decomposed NOx.

[0024] The gas sensor 100 preferably comprises a plurality of electrochemical sensor cells, not shown, which detect the potential difference between each pump electrode and the reference electrode 147, and each pump cell is controlled by the control device 50 on the basis of a signal detected by each sensor cell.

[0025] In the sensor element 10, a heating device 150 is integrated into the ceramic body 101. The heating device 150 is located below the gas distribution part in the Fig.2 in an area extending from the vicinity of one end section E1 to at least one location where the measuring electrode 145 and the reference electrode 147 are formed. The heating device 150 is primarily provided for heating the sensor element 10 in order to increase the oxygen ion conductivity of the solid electrolyte forming the ceramic body 101 when the sensor element 10 is used. In particular, the heating device 150 is provided such that it is surrounded by an insulating layer 151.

[0026] The heating device 150 is a resistance heater, made, for example, of platinum. The heating device 150 generates heat by being supplied with electricity by the heating device power supply 40 via the control unit, which is operated by the control unit 50.

[0027] The sensor element 10 according to the present embodiment is heated by the heating device 150 when in use, such that the temperature reaches 500 °C or more in at least the area from the first inner chamber 102 to the second inner chamber 103. In some cases, the sensor element 10 is heated so that the temperature of the entire gas distribution section, from the gas inlet 105 to the third inner chamber 104, reaches 500 °C or more. This serves to increase the oxygen ion conductivity of the solid electrolyte that forms each pump cell and to ensure the desired function of each pump cell. In this case, the temperature in the vicinity of the first inner chamber 102, which reaches the highest temperature, reaches approximately 700 °C to 800 °C.

[0028] In the following description, of the two main surfaces of the ceramic body 101, one main surface (or an outer surface of the sensor element 10, which has the main surface) is located on a top side in the Fig. 2 and is located on a side where the main pump cell P1, the auxiliary pump cell P2 and the measuring pump cell P3 are predominantly provided, also referred to as the pump surface, and a main surface (or an outer surface of the sensor element 10 which has the main surface) which is located on a lower side in the Fig.2 and located on a side where the heating device 150 is provided, is also referred to as the heating device surface. In other words, the pump surface is a major surface closer to the gas inlet 105, the three inner chambers and the pump cells than to the heating device 150, and the heating device surface is a major surface closer to the heating device 150 than to the gas inlet 105, the three inner chambers and the pump cells.

[0029] A plurality of electrode terminals 160 are formed on the respective main surfaces of the ceramic body 101 on the side of the other end section E2 to establish an electrical connection between the sensor element 10 and the outside. These electrode terminals 160 are electrically connected, via terminals provided within the ceramic body 101 (not shown), to the five electrodes opposite the ends of the heating device 150 and to a terminal for sensing the heating device resistance (not shown), thus establishing a predetermined correspondence relationship. The application of a voltage from the pump cell power supply 30 to each pump cell of the sensor element 10 and the heating by the heating device 150 by supplying current from the heating device power supply 40 are therefore carried out via the electrode terminals 160.

[0030] The sensor element 10 further comprises the aforementioned main surface protection layers 170 (170a and 170b) on the pump surface and the heating element surface of the ceramic body 101. The main surface protection layers 170 are aluminum oxide layers, approximately 5 µm to 30 µm thick, and contain pores with a porosity of approximately 20% to 40%. They are provided to prevent the adhesion of any foreign material and any toxic substances to the main surfaces (the pump surface and the heating element surface) of the ceramic body 101 and the external pump electrode 141 provided on the pump surface. The main surface protection layer 170a on the pump surface thus acts as a pump electrode protection layer to protect the external pump electrode 141.

[0031] In the present embodiment, the porosity is obtained by applying a known image processing method (e.g., a binary processing) to a scanning electron microscope (SEM) image of an evaluation target.

[0032] The main surface protection layers 170 are provided on substantially the entire pump surface and heating device surface, with the exception of the electrode terminals 160 in the Fig. 2 partially exposed, however this is only an example. The main surface protection layers 170 can be provided locally in the vicinity of the outer pump electrode 141 on the side of one end section E1, compared to the case described in the Fig. 2 is shown. <details der schutzschicht am vorderen ende>

[0033] In the sensor element 10, the protective layer 2 is provided at the front end around an outermost edge of the element base 1 with the structure described above in a predetermined area of ​​the one end section E1.

[0034] The protective layer 2 at the front end is provided in such a way that it surrounds a section of the element base 1 where the temperature becomes high (up to about 700 °C to 800 °C) when the gas sensor 100 is used, in order to ensure water resistance in the section, thereby preventing the occurrence of cracking (water-induced cracking) of the element base 1 due to thermal shock caused by a local temperature decrease when the section is directly exposed to water.

[0035] In addition, the protective layer 2 is provided at the front end to ensure resistance to poisoning, thus preventing the ingress of poisonous substances, such as Mg, into the sensor element 10.

[0036] As it is in the Fig. As shown in Figure 2, in the sensor element 10 according to the present embodiment, the protective layer 2 at the front end comprises the three layers: the first protective layer 21 at the front end, the second protective layer 22 at the front end and the third protective layer 23 at the front end.

[0037] The first protective layer 21 at the front end is a base layer provided to ensure adhesion of the second protective layer 22 formed thereon at the front end (and also of the third protective layer 23 at the front end). The first protective layer 21 at the front end is provided on at least two main surfaces of the element base 1, on one side facing the pump surface and on one side facing the heating element surface. That is, the first protective layer 21 at the front end comprises a first protective layer 21a at the front end on the side facing the pump surface and a first protective layer 21b at the front end on the side facing the heating element surface. However, the first protective layer 21 at the front end is not provided on one side of the front end surface 101e of the ceramic body 101 (the element base 1).

[0038] The first protective layer 21 at the front end is made of aluminum oxide, has a porosity of 30% to 60%, and a thickness of 15 µm to 50 µm. In contrast to the second protective layer 22 and the third protective layer 23 at the front end, the first protective layer 21 at the front end is produced together with the element base 1 in a process for producing the element base 1 as described below.

[0039] The second protective layer 22 at the front end and the third protective layer 23 at the front end are provided in this order from the inside to cover the front end surface 101e and the four side surfaces on the side of the one front end section E1 of the element base 1 (around an outer edge of the element base 1 on the side of the one front end section E1). A section of the second protective layer 22 at the front end on the side of the front end surface 101e is specifically referred to as the front end section 221, and a section of the second protective layer 22 at the front end on the side of the pump surface and the side of the heating device surface is specifically referred to as the main surface section 222.Accordingly, a section of the third protective layer 23 at the front end on the side of the end surface 101e is specifically referred to as the front end section 231, and a section of the third protective layer 23 at the front end on the side of the pump surface and the side of the heating device surface is specifically referred to as the main surface section 232.

[0040] The second protective layer 22 at the front end is made of aluminum oxide, has a porosity of 30% to 80%, and is 30 to 50 times thicker than the first protective layer 21 at the front end. The third protective layer 23 at the front end is also made of aluminum oxide, has a porosity of 15% to 30%, and is 5 to 10 times thicker than the first protective layer 21 at the front end. The protective layer 2 at the front end thus has a structure in which the second protective layer 22 at the front end, which has the lowest thermal conductivity of the three layers, is covered by the third protective layer 23 at the front end, which, as the outermost layer, is provided such that it has a lower porosity than the second protective layer 22 at the front end.The sum of the thickness of the front end section 221 of the second protective layer 22 at the front end and the thickness of the front end section 231 of the third protective layer 23 at the front end is greater than the sum of the thickness of the main surface section 222 of the second protective layer 22 at the front end and the thickness of the main surface section 232 of the third protective layer 23 at the front end.

[0041] In other words, the second protective layer 22 at the front end is designed as a layer with low thermal conductivity, such that it prevents heat conduction from outside the element base 1, and the third protective layer 23 at the front end maintains overall strength and prevents water ingress. The protective layer 2 at the front end has this structure so that even if water adheres to its surface (the surface of the third protective layer 23 at the front end) when the sensor element 10 is in use at a high temperature, water ingress is prevented, and the transfer of cold caused by rapid cooling and subsequent adhesion to the element base 1 is less likely. That is, the protective layer 2 at the front end exhibits high thermal shock resistance.As a result, sensor element 10 is less likely to cause water-induced cracking and exhibits high water resistance.

[0042] The reason why the second protective layer 22 at the front end has a thickness 30 to 50 times that of the first protective layer 21 at the front end, and the third protective layer 23 at the front end has a thickness 5 to 10 times that of the first protective layer 21 at the front end, is that with increasing thickness of the second protective layer 22 at the front end and increasing thickness of the third protective layer 23 at the front end, it is less likely that a thermal shock will be transmitted to the element base 1, and the water resistance of the sensor element 10 is improved.

[0043] However, an excessive increase in the thickness of these layers is not preferred, as it increases the thermal load exerted on the heating device 150, which is provided within the sensor element 10, when the sensor element 10 as a whole, including the protective layer 2 at the front end, is heated by the heating device 150 to begin using the gas sensor 100, and as a result, cracking may occur in the sensor element 10. In this regard, the second protective layer 22 at the front end preferably has a thickness of 1500 µm or less, and the third protective layer 23 at the front end preferably has a thickness of 300 µm or less.

[0044] The second protective layer 22 at the front end and the third protective layer 23 at the front end are formed by successive thermal spraying (plasma spraying) of materials for this purpose with respect to the element base 1 with a surface on which the first protective layer 21 at the front end has been formed. This serves to develop an anchoring effect between the second protective layer 22 at the front end and the first protective layer 21 at the front end, which were previously produced in the process for manufacturing the element base 1, in order to ensure the bonding (adhesion) of the second protective layer 22 at the front end (including the third protective layer 23 at the front end, which is formed outside the second protective layer 22 at the front end) with the first protective layer 21 at the front end.In other words, this means that the first protective layer 21 at the front end has the function of securely bonding (adhering) the second protective layer 22 at the front end. Secure bonding (adhering) in this way appropriately prevents separation of the protective layer 2 at the front end from the element base 1 caused by thermal shock due to the adhesion of water droplets.

[0045] The second protective layer 22 and the third protective layer 23 at the front end are arranged such that they do not completely cover the first protective layer 21 (21a and 21b) at the front end, but rather such that an end section of the first protective layer 21 at the front end is exposed on one side opposite the side of the end section E1 in the longitudinal direction of the sensor element 10. This serves to better ensure the bond (adhesion) of the second protective layer 22 at the front end (including the first protective layer 23 at the front end, which is formed outside the second protective layer 22 at the front end) with the first protective layer 21 at the front end.

[0046] As described above, in the sensor element 10 according to the present embodiment, the protective layer 2 at the front end, which is intended to surround the section of the element base 1 where the temperature rises when the gas sensor 100 is used, has a three-layer structure comprising the first protective layer 21 at the front end, the second protective layer 22 at the front end, and the third protective layer 23 at the front end. These layers are provided such that they have predetermined porosities and thicknesses, so that the first protective layer 21 at the front end has the function of ensuring a bond (adhesion) to the second protective layer 22 at the front end, and the second protective layer 22 at the front end has the function of preventing heat conduction from the outside to the element base 1.and the third protective layer 23 at the front end has the function of maintaining the overall strength and preventing water ingress. The protective layer 2 at the front end thus exhibits high resistance (thermal shock resistance) to cold caused by the adhesion of water droplets. As a result, the sensor element 10 exhibits high water resistance. <Verfahren zur Herstellung des Sensorelements>

[0047] An example of a method for manufacturing the sensor element 10 with a structure and features as described above is described next. Fig. Figure 3 is a flowchart of the processing or manufacturing process for sensor element 10.

[0048] In the production of element base 1, a plurality of unstructured layers (not shown) are first produced, which are green layers containing the oxygen ion-conducting solid electrolyte, such as zirconium oxide, as a ceramic component and have no structure formed on them (step S1).

[0049] The unstructured layers have multiple layer holes used for positioning during printing and lamination. These layer holes are formed prior to the structure formation in the unstructured layers, for example, by punching using a punching machine. Green layers corresponding to a section of the ceramic body 101 in which an interior is formed also include through-sections corresponding to the interior, which were formed prior to the process, for example, by punching, as described above. The unstructured layers do not need to have the same thickness and can have different thicknesses according to corresponding sections of the ultimately formed element base 1.

[0050] After the production of the unstructured layers corresponding to the respective layers, structure printing and drying are performed on the individual unstructured layers (step S2). Specifically, a structure of various electrodes, a structure of the heating element 150 and the insulating layer 151, a structure of the electrode terminals 160, a structure of the main surface protection layers 170, a structure of internal wiring (not shown), and the like are formed. The application or arrangement of a sublimable (disappearing) material to form the first diffusion control element 110, the second diffusion control element 120, the third diffusion control element 130, and the fourth diffusion control element 140 is also carried out during structure printing.In addition, a structure to form the first protective layer 21 (21a and 21b) is printed at the front end onto unstructured layers, so that after lamination it becomes a top layer and a bottom layer (step S2a).

[0051] The structures are printed onto the unstructured layers using a known screen printing technology by applying structure-forming pastes, which are manufactured according to the properties required for the respective formation goals. For example, an aluminum oxide paste is used to form the first protective layer 21 at the front end, which can form the first protective layer 21 at the front end with a desired porosity and thickness in the resulting sensor element 10. A known drying device can be used for drying after printing.

[0052] After structural printing on each of the unstructured layers, a bonding paste is printed and dried, thus laminating and bonding the green layers (step S3). The established screen printing technology can be used to print the bonding paste, and the established drying equipment can be used for drying after printing.

[0053] The green layers, to which an adhesive has been applied, are then stacked in a predetermined sequence, and the stacked green layers are pressed under predetermined temperature and pressure conditions to form a laminated body (step S4). Specifically, the pressing is carried out by stacking and holding the green layers as the target of the lamination on a predetermined laminating device (not shown) while the green layers are positioned at the layer holes, and then heating and applying pressure to the green layers along with the laminating device using a laminating machine, such as a known hydraulic press. The pressure, temperature, and time for heating and applying pressure depend on the laminating machine used, and these conditions can be set in a suitable manner to achieve good lamination.The structure for forming the first protective layer 21 at the front end can be formed on the laminated body obtained in this way.

[0054] After the laminated body has been obtained in the manner described above, the laminated body is cut out at a plurality of places so that unit bodies are obtained which eventually become the individual element bases 1 (step S5).

[0055] The resulting unit bodies are then fired at a firing temperature of approximately 1300 °C to 1500 °C (step S6). This produces the element base 1 with main surfaces on which the first protective layer 21 is provided at the front end. That is, the element base 1 is created by the integrated firing of the ceramic body 101, which is made from the solid electrolyte, the electrodes, and the main surface protective layers 170, together with the first protective layer 21 at the front end. The integrated firing is carried out in such a way that the electrodes each exhibit sufficient adhesion strength in the element base 1.

[0056] After the element base 1 has been produced in the manner described above, the second protective layer 22 at the front end and the third protective layer 23 at the front end are formed with respect to the element base 1. The second protective layer 22 at the front end is produced by thermal spraying a powder (aluminum oxide powder) to form the second protective layer at the front end, which has been previously produced at a location on the element base 1, so that it has a specified thickness (step S7), and then firing the element base 1 on which a film has been applied in the manner described above (step S8).The aluminum oxide powder used to form the second protective layer at the front end contains an aluminum oxide powder with a predetermined particle size distribution and a pore-forming material in a ratio corresponding to a desired porosity. The pore-forming material is pyrolyzed by firing the element base 1 after thermal spraying, thus forming the second protective layer 22 at the front end with a high porosity of 30% to 80%. A known technology can be applied to the thermal spraying and firing process.

[0057] After the formation of the second protective layer 22 at the front end, a powder (aluminum oxide powder) is thermally sprayed at a location on the element base 1 to form the third protective layer 23 at the front end. This powder, which has been prepared beforehand and contains an aluminum oxide powder with a predetermined particle size distribution, is intended to form the third protective layer 23 at the front end. The third protective layer 23 is achieved by thermal spraying in such a way that it has a predetermined thickness (step S9), thereby forming the third protective layer 23 at the front end with the desired porosity. The aluminum oxide powder used to form the third protective layer at the front end does not contain the pore-forming material. A known technology can also be applied to the thermal spraying process.

[0058] The sensor element 10 is obtained through the processes described above. The sensor element 10 thus obtained is housed in a specified casing and installed in the body (not shown) of the gas sensor 100. <modifizierungen>

[0059] The embodiment described above relates to a sensor element with three internal chambers; however, the sensor element does not necessarily have to have a three-chamber structure. That is, the sensor element can have one internal chamber or two internal chambers. [Examples]

[0060] Thirteen types of sensor elements 10 (samples No. 1 to No. 13) with various combinations of intended thicknesses t1, t2, and t3 of the first protective layer (hereinafter referred to as the first layer) 21 at the front end, the second protective layer (hereinafter referred to as the second layer) 22 at the front end, and the third protective layer (hereinafter referred to as the third layer) 23 at the front end, and porosities of the second layer 22 and the third layers 23, and one sensor element (sample No. 14) lacking the protective layer at the front end, were manufactured. The first layer 21, the second layer 22, and the third layer 23 were each made of aluminum oxide.

[0061] A water resistance test was performed with each of the obtained sensor elements 10. The water resistance test was carried out by applying a 0.1 µL water droplet at a time to the side of the pump surface of the sensor element 10 while the pump flow through the main pump cell P1 was being measured in a condition where the sensor element 10 was heated by the heating device 150 to approximately 500 °C to 900 °C, and by evaluating the maximum amount of water that did not cause anomalies in a measurement result. In these examples, the maximum amount of water in the above case is referred to as the “water resistance” (in µL).It is assumed that any anomaly in the measurement result of the water resistance test occurs because the protective layer 2 at the front end is subject to a thermal shock, causing cracking in the sensor element 10, so that a value of ‘water resistance’ in these examples serves as an indicator of how unlikely it is that cracking will occur, and furthermore as an indicator of the thermal shock resistance of the protective layer 2 at the front end.

[0062] The intended thicknesses (film thicknesses) t1, t2 and t3 of the respective layers, the ratio t2 / t1, the ratio t3 / t1, the porosities of the second layer 22 and the third layer 23 and the results of the water resistance assessment are shown as a list for each of the samples in Table 1. [Table 1] Sample No. Film thickness of the first layer t1 (µm) Film thickness of the second layer t2 (µm) Third layer film thickness 3 (µm) ratio t2 / t1 ratio t3 / t1 Second layer porosity (%) Third layer porosity (%) Water resistance (µl) 1 20 200 200 10 10 50 20 7,0 2 20 400 200 20 10 50 20 9,2 3 20 600 200 30 10 50 20 25,6 4 20 1000 200 50 10 50 20 100,3 5 20 600 200 30 10 30 20 12,1 6 20 600 200 30 10 80 20 26,4 7 20 600 Not provided 30 - 50 - 5,9 8 20 600 50 30 2,5 50 20 9,0 9 20 600 100 30 5 50 20 15,4 10 20 600 200 30 10 50 50 9,0 11 20 600 200 30 10 50 15 29,1 12 20 600 200 30 10 50 30 34,1 13 20 600 200 30 10 20 20 7,0 14 Not provided Not provided Not provided - - - - 0,8

[0063] As shown in Table 1, the sensor elements 10 in Nos. 1 to 13, each comprising the protective layer 2 at the front end, exhibited a water resistance of at least seven times that of the sensor element 10 in No. 14, which does not include the protective layer 2 at the front end. Of these, the sensor elements 10 that differed from the sensor element 10 in No. 7, which did not include the third layer 23, exhibited a water resistance of approximately nine times that of the sensor element 10 in No. 14. The sensor elements 10 in Nos. 3 to 6, No. 9, No. 11, and No. 12 exhibited particularly high values ​​of 12 µL or more.

[0064] The results show that in a case where the sensor element 10 is manufactured such that the condition is met that the second layer 22 has a porosity of 30% to 80% and a thickness 30 to 50 times the thickness of the first layer 21 and the third layer 23 has a porosity of 15% to 30% and a thickness 5 to 10 times the thickness of the first layer 21, the sensor element 10 can be obtained with higher thermal shock resistance in the protective layer 2 at the front end and consequently with higher water resistance than those which do not meet the condition.

[0065] The sensor elements 10 in Nos. 3, 4, 6, 9, 11, and 12 exhibited particularly high water resistance values ​​of 15 µL or more. This demonstrates that, in a case where the second layer 22 has a porosity of 50% to 80%, the sensor element 10 can be obtained with higher thermal shock resistance in the protective layer 2 at the front end and consequently with higher water resistance.< / modifizierungen> < / details>

Claims

[1] Sensor element (10) for a gas sensor (100), comprising: an element base (1) which is a ceramic structure comprising a sensing part for sensing a gas component to be measured; and a protective layer (2) at the front end, which is a porous layer arranged around an outer edge of the element base (1) in a predetermined area of ​​an end section of the element base (1) on one side of the detection part and comprising: a first protective layer (21) at the front end, which is arranged on at least two main surfaces of the element base (1); a second protective layer (22) at the front end, arranged to cover the end section and four side surfaces of the element base (1), including the two main surfaces on which the first protective layer (21) is arranged at the front end; and a third protective layer (23) at the front end, arranged to cover the second protective layer (22) at the front end, wherein the first protective layer (21) at the front end has a porosity of 30% to 60% and a thickness of 15 µm to 50 µm; the second protective layer (22) at the front end has a porosity of 30% to 80% and a thickness 30 to 50 times that of the first protective layer (21) at the front end, and the third protective layer (23) at the front end has a porosity of 15% to 30% and a thickness 5 to 10 times that of the first protective layer (21) at the front end. [2] Sensor element (10) according to claim 1, wherein the second protective layer (22) has a porosity of 50% to 80% at the front end. [3] Sensor element (10) according to claim 1 or 2, wherein the second protective layer (22) has a thickness of 1500 µm or less at the front end and the third protective layer (23) has a thickness of 300 µm or less at the front end.

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