GAS SENSOR ELEMENT

Abstract

Gas sensor element (100, 100A, 101), comprising: a capture section (10) comprising a stack of a solid electrolyte body and a heat-generating body, wherein the solid electrolyte body has at least one pair of electrodes (4) on opposite sides thereof and the heat-generating body comprises a heat-generating source (6); and a porous protective layer (20, 20A) formed around the detection section (10), wherein the porous protective layer (20, 20A) is formed from an aluminum oxide-containing aggregate and a silicon dioxide-containing coating material, and in the porous protective layer (20, 20A) the weight concentration x wt.% of the coating material, based on the total weight of the aggregate and the coating material, and the porosity y% of the following formula (1) are satisfied: y ≤ 0.0058 x 2 − 1.2666 x + 68 and, in the porous protective layer (20, 20A) the pore volume of the pores with a pore diameter of 100 nm or less is 0.02 mL / g or less.

Description

Technical area

[0001] The present invention relates to a gas sensor element which is mounted on, for example, a vehicle and which detects the concentration of oxygen or NO x captured in the exhaust gas. State of the art

[0002] Numerous attempts have been made in a variety of industries worldwide to reduce environmental impacts and pollution. In particular, the automotive industry has been continually pushing forward efforts to promote the spread not only of fuel-efficient gasoline-powered vehicles, but also of so-called environmentally friendly vehicles, such as hybrid vehicles or electric vehicles, as well as to further improve the performance of such vehicles.

[0003] The purification of exhaust gas and the improvement of fuel economy performance of vehicles have been achieved by detecting the concentration of oxygen or NO xin a measurement target gas, such as exhaust gas, using a gas sensor and precisely controlling the fuel oil consumption and the intake air amount.

[0004] An exemplary basic configuration of a gas sensor element constituting such a gas sensor includes a sensing portion including a stack of a solid electrolyte body having a pair of electrodes on opposite sides thereof and a heat generating body including a heat generating source, and a porous protective layer formed around the sensing portion.

[0005] A gas sensor detects the concentration of oxygen or NO xin the exhaust gas at a temperature of approximately 400 °C to 850 °C. Therefore, when water droplets (ie, condensed water) in the exhaust gas collide with the gas sensing element constituting the gas sensor, thermal shock may be generated due to partial quenching, and the function of the gas sensor may be reduced.

[0006] To eliminate such concerns, a porous protective layer having a water-repellent property is provided around the detection portion of the gas sensor element.

[0007] As a conventional technique regarding a gas sensor element comprising a porous protective layer having a water-repellent property, for example, Patent Literature 1 discloses a gas sensor element comprising a porous protective layer in which the thermal conductivity λ is in a range of 0.2 to 5 W / mK, and the product λCpρ of the thermal conductivity λ (W / mK), the density ρ (g / m 3) and the specific heat Cp (J / gK) in a range of 5.3 × 10 5 up to 2.1 × 10 7 WJ / m 4 K 2 is.

[0008] The water-repellent property of the porous protective layer disclosed in Patent Literature 1 is obtained by the Leidenfrost phenomenon. The Leidenfrost phenomenon is a phenomenon by which the surface of a water droplet is instantly evaporated when the water droplet is allowed to come into contact with the surface of a porous protective layer at a high temperature, and a barrier layer (vapor film) is formed between the surface of the porous protective layer and the water droplet due to the evaporated water vapor. According to this Leidenfrost phenomenon, even in a case where water droplets adhere to the surface of the porous protective layer, the water droplets are immediately separated from the surface of the porous protective layer, which means that the porous protective layer has a water-repellent property.

[0009] However, in the case of a gas sensor element including a porous protective layer with a water-repellent property as disclosed in Patent Literature 1, the water-repellent property of the porous protective layer is insufficient in some cases when condensed water contained in the exhaust gas is continuously dripped or poured onto a location in the high-temperature gas sensor element. For example, the porous protective layer of the gas sensor element disclosed in Patent Literature 1 exhibits PTC (positive temperature coefficient) characteristics, by which the thermal conductivity decreases with a decrease in temperature. Accordingly, when the temperature of the porous protective layer is lowered by continuously dripping or pouring water, its thermal conductivity decreases, and a vapor film according to the Leidenfrost phenomenon is hardly formed.Consequently, the water-repellent property is reduced and a high thermal shock is generated against the gas sensor element.

[0010] Furthermore, Patent Literature 2 and Patent Literature 3 disclose prior art gas sensor elements. ReferencePatent literature Patent Literature 1: JP 2016-29360 A Patent literature 2: DE 10 2014 113 274 A1 Patent literature 3: DE 10 2012 202 716 A1 Summary of the inventionTechnical problem

[0011] As described above, in the case of a conventional gas sensor element including a porous protective layer with a water-repellent property, the water-repellent property is reduced due to a decrease in temperature caused by continuously dripping or pouring water, and consequently, a high thermal shock is generated against the gas sensor element in some cases. Therefore, the present invention provides a gas sensor element including a porous protective layer with an improved water-repellent property when water is continuously dripped or poured onto the porous protective layer. Solution to the problem

[0012] As a result of intensive studies aimed at achieving the above-mentioned goal, the present inventors found that NTC (negative temperature coefficient) properties, by which thermal conductivity increases with a decrease in temperature, can be imparted to the porous protective layer of a gas sensor element by controlling the weight concentration of a coating material and the porosity in the porous protective layer of the gas sensor element to satisfy a predetermined relational expression, and the water-repellent property with respect to continuous casting is improved, thereby completing the present invention. Specifically, the concept of the present invention is as follows. (1) A gas sensor element comprising: a detection section comprising a stack of a solid electrolyte body and a heat generating body, the solid electrolyte body having at least one pair of electrodes on opposite sides thereof, and the heat generating body comprising a heat generating source; and a porous protective layer formed around the detection section, wherein the porous protective layer is formed from an alumina-containing aggregate and a silicon dioxide-containing coating material, and in the porous protective layer, the weight concentration x wt% of the coating material, based on the total weight of the aggregate and the coating material, and the porosity y% satisfy the following formula (1): y≤0.0058x2−1.2666x+68 and, in the porous protective layer, the pore volume of the pores with a pore diameter of 100 nm or less is 0.02 mL / g or less. (2) The gas sensor element according to the above item (1), wherein the weight concentration of the coating material in the porous protective layer is 10 wt% or more. (3) The gas sensor element according to the above item (1) or (2), wherein the porosity of the porous protective layer is 25% or more. (4) A method for manufacturing the gas sensor element according to any one of the above items (1) to (3), comprising: Forming a porous protective layer of an alumina-containing aggregate and a silicon dioxide-containing coating material around the detection portion; and Baking the formed porous protective layer at 1000 °C or higher. Advantageous effects of the invention

[0013] According to the present invention, a gas sensor element including a porous protective layer having an improved water-repellent property with respect to continuous water pouring can be provided. Short description of the drawing Fig. 1 is a schematic diagram illustrating an embodiment of a gas sensor element of the present invention; Fig. 2 is a graph showing the relationship between temperature and thermal conductivity with respect to the porous protective layers of Example 1 and Comparative Example 1; Fig. 3 is a graph showing the relationship between the weight concentration of silicon dioxide and the porosity in the porous protective layers of Example 1 and Comparative Examples 1 and 2; Fig. 4 is a graph showing the results of water repellency tests carried out on the porous protective layers of Example 1 and Comparative Examples 1 and 2; Fig. 5 is a graph showing the relationship between the weight concentration of silicon dioxide and the bonding strength in the example; Fig. 6 is a graph showing the relationship between the porosity of a porous protective layer and the response value in the example; Fig. 7A is a diagram showing an SEM photograph of the porous protective layer of Example 1, and Fig. 7B is a diagram showing an SEM photograph of the porous protective layer of Comparative Example 3; Fig. 8 is a graph showing the relationship between the pore diameter and the pore volume in the porous protective layers of Example 1 and Comparative Example 3; Fig. 9 is a schematic diagram illustrating an embodiment of a gas sensor element of the present invention; and Fig. 10 is an enlarged diagram showing an end portion of an embodiment of a gas sensor element of the present invention. Description of the embodiments

[0014] The present invention will be described in detail below.

[0015] In the gas sensor element of the present invention, a solid electrolyte body having at least a pair of electrodes on opposite sides thereof and a heat generating body including a heat generating source are laminated to form a sensing portion, and a porous protective layer is formed around this sensing portion.

[0016] Fig. 1 is a schematic diagram illustrating an embodiment of a gas sensor element of the present invention. Fig. 1 generally comprises a sensing section 10 which detects the concentration of oxygen or NO x in the exhaust gas, and a porous protective layer 20 which protects the periphery of the sensing section 10 against moisture in the exhaust gas, thereby suppressing the occurrence of a reduction in output which would otherwise occur due to a temperature drop of the sensing section 10 resulting from the moisture reaching the sensing section 10, and also trapping toxic compounds and the like in the exhaust gas passing therethrough.

[0017] The sensing section 10 generally includes a solid electrolyte layer 3 having, on opposite sides thereof, a pair of electrodes 4 including an electrode 41 on the measurement target gas side and an electrode 42 on the reference gas side, a porous diffuse resistance layer 2 surrounding the electrode 41 on the measurement target gas side above a measurement target gas space 8, a shielding layer 1 defining the measurement target gas space 8 together with the porous diffuse resistance layer 2, a reference gas space protective layer 5 surrounding the electrode 42 on the reference gas side above a reference gas space 9, and a heat generation source 6 and a heat generation source substrate 7.

[0018] The heat generation source 6 includes a heater which is a heat generating body, and forms a heating portion of the gas sensor element 100 so that it is heat-controlled to achieve the activation energy thereof.

[0019] In the cross-sectional shape shown in the drawing, the detection section 10 has corner portions cut out in conical shapes. Such cutout portions ensure the thickness of the porous protective layer 20 at the corresponding portions of the detection section 10.

[0020] The solid electrolyte layer 3 is made of zirconium oxide, and the electrode 41 on the measurement target gas side and the electrode 42 on the reference gas side are each made of platinum. Furthermore, the shielding layer 1 and the reference gas space protective layer 5 each have a gas-impermeable internal structure and are made of aluminum oxide.

[0021] A voltage at which the oxygen concentration difference and the current have a linear correlation is applied to the pair of electrodes 4, and a measurement target gas is caused to contact the electrode 41 on the measurement target gas side, while a reference gas, such as air, is caused to contact the electrode 42 on the reference gas side. Subsequently, the value of the current generated between the electrodes is measured in accordance with each oxygen concentration difference, so that the air-fuel ratio of the vehicle engine can be identified based on the measured current.

[0022] The porous diffuse resistance layer 2 is provided at a position defining the measurement target gas space 8 around the electrode 41 on the measurement target gas side to suppress the amount of the measurement target gas introduced to the electrode 41 on the measurement target gas side, and is configured to further introduce hydrogen gas, carbon monoxide gas, oxygen gas, and the like from the exhaust gas, which have been introduced via the porous protective layer 20 around the detection section 10, into the measurement target gas space 8 via the porous diffuse resistance layer 2. In addition, as shown in Fig. 10, the porous protective layer of the present invention satisfying the formula (1) mentioned below may also be applied to the gas sensor element having a structure in which a gas inlet port is arranged at an end portion of the gas sensor element. Fig. 10 is an enlarged diagram showing an end portion of an embodiment of a gas sensor element of the present invention. In Fig. 10, a gas inlet port 103 is arranged at an end portion of a gas sensor element 101. Since a porous protective layer 20A (an upper layer 20Aa and a lower layer 20Ab) is formed at a lower portion of the end portion of the gas sensor element 101, it is separated from the heat generation portion, and thus heat hardly reaches the porous protective layer, and the temperature is hardly raised. Consequently, due to a decrease in temperature caused by continuous dripping of water, the water-repellent property is easily reduced. By adapting the porous protective layer of the present invention, such a deficiency is compensated, and a preferable water-repellent property is obtained.

[0023] The porous protective layer 20 is a porous layer optionally supporting noble metal catalyst particles (not shown) on its surface. The noble metal catalyst particles in the porous protective layer 20 may be distributed over the entire area of ​​the porous protective layer 20, or may also be distributed only in the lateral area thereof, corresponding to the porous diffuse resistance layer 2 near the electrode 41 on the measurement target gas side. Alternatively, the amount of noble metal catalyst particles supported in the porous protective layer 20 may be distributed such that a relatively large amount of the noble metal catalyst particles are supported in an area corresponding, for example, to the porous diffuse resistance layer 2.Here, as the noble metal catalyst particles, platinum, palladium, or rhodium particles may be used alone, or an alloy containing two or more of palladium, rhodium, and platinum may be used.

[0024] The porous protective layer 20 may have a single-layer structure as shown in Fig. 1. On the other hand, the porous protective layer 20 may also have a Fig. 9, in which the porous protective layer 20A is composed of a lower layer 20Ab in contact with the sensing portion 10 disposed inside the gas sensor element 100A and an upper layer 20Aa in contact with the exterior thereof. In this case, since the porous protective layer of the present invention satisfying the formula (1) below has excellent water repellency, it is preferably coated on the upper layer 20Aa. Furthermore, the lower layer 20Ab can be used as a poisoning suppression layer.When the porosity of such a poisoning suppression layer is set lower than that of the upper layer 20Aa, the poisoning suppression layer becomes a porous layer with a specific surface area larger than that of the upper layer 20Aa, and therefore, the poison absorption properties of the poisoning suppression layer can be ensured. In a preferred embodiment, the porous protective layer 20A having a laminated two-layer structure consists of a lower layer 20Ab used as a poisoning suppression layer and an upper layer 20Aa on which the porous protective layer of the present invention is applied.

[0025] The gas sensor element of the present invention is characterized in that the porous protective layer 20 has NTC properties. The porous protective layer will be described below.

[0026] The porous protective layer is formed from an alumina (Al2O3)-containing aggregate and a silicon oxide (SiO2)-containing coating material. A number of alumina-containing aggregates are bonded together by the silicon oxide-containing coating material, which serves as a binder, thereby forming a porous protective layer.

[0027] The type of alumina contained in the aggregates is not particularly limited, and any of α-alumina, γ-alumina, and θ-alumina can be used. The aggregate may contain components other than alumina as long as it satisfies the formula (1) below. Examples of such other components include spinel, silicon carbide, and aluminum nitride. Preferably, the aggregate consists of alumina.

[0028] The coating material may contain certain components other than silicon dioxide, as long as it satisfies the formula (1) below. Examples of such other components include titanium dioxide, zirconium dioxide, antimony oxide, and zinc oxide. Preferably, the coating material consists of silicon dioxide.

[0029] In the porous protective layer, the pore volume of pores with a pore diameter of 100 nm or less is 0.02 mL / g or less. By reducing pores with a pore diameter of 100 nm or less, which is the mean free path or less at which gaseous molecules hardly move, thermal conductivity can be improved. Here, the term "pore diameter" is used to mean a pore diameter (average pore diameter) obtained by measuring according to a gas adsorption method, a mercury intrusion method, or the like. The pore volume (also referred to as "integrated pore volume") of pores with a pore diameter of 100 nm or less can be obtained by a gas adsorption method, a mercury intrusion method, or the like.When preparing a porous protective layer, a coating material is melted by baking at a temperature of 1000°C or higher, and preferably 1100°C or higher, so that the pore volume of pores with a pore diameter of 100 nm or less can be adjusted to 0.02 mL / g or less. Furthermore, melting the coating material improves the strength of the porous protective layer.

[0030] In the porous protective layer, the weight concentration x wt% of the coating material, based on the total weight of the aggregate and the coating material, and the porosity y% of the porous protective layer satisfy the following formula (1): y≤0.0058x2−1.2666x+68

[0031] Furthermore, the porosity is not 0% (y ≠ 0).

[0032] The porous protective layer satisfying the above formula (1) exhibits NTC properties. In the present invention, "NTC properties" means that the thermal conductivity increases with a decrease in temperature. Since the porous protective layer of the present invention exhibits NTC properties, it can preferably exhibit water-repellent properties even when condensed water contained in the exhaust gas is continuously poured or dropped onto it.

[0033] The water-repellent mechanism of a porous protective layer satisfying the above-described formula (1) and exhibiting NTC properties upon continuous water pouring is described below. Specifically, when condensed water in the exhaust gas is continuously dripped or poured onto the porous protective layer of a gas sensor element at a high temperature, the temperature of the porous protective layer is reduced. The heat-supply capability (heat flow) of the porous protective layer is increased in proportion to its thermal conductivity. Since the thermal conductivity of the porous protective layer exhibiting NTC properties increases with a decrease in temperature, the heat-supply capability is increased. When the heat-supply capability of the porous protective layer is increased, a vapor film is easily formed according to the Leidenfrost phenomenon. Therefore, the porous protective layer preferably has a water-supply capability.Therefore, even if water is continuously poured or dripped onto the porous protective layer, the water-repellent property of the porous protective layer is not reduced, and thermal shock to the gas sensor element can be reduced. As such, the porous protective layer exhibiting NTC properties exhibits excellent water-repellent property at low temperatures, compared to a protective layer exhibiting PTC properties, which decreases thermal conductivity with a decrease in temperature.

[0034] The weight concentration x of a coating material to the total weight of an aggregate and a coating material in a porous protective layer is preferably 10 wt% or more, and more preferably 10 wt% to 35 wt%. When the weight concentration of the coating material, which acts to bond aggregates together, is 10 wt% or more, the high strength of the porous protective layer is ensured. On the other hand, when it is 35 wt% or less, a state in which the coating material is uniformly dispersed in the porous protective layer can be maintained.

[0035] Furthermore, the porosity y of the porous protective layer is preferably 25% or more, and more preferably 25% to 55%. When the porosity of the porous protective layer is 25% or more, the exhaust gas permeability becomes sufficiently high, and therefore the response of the sensor output to a change in the exhaust atmosphere becomes sufficiently high. On the other hand, when it is 55% or less, a reduction in the strength of the porous protective layer can be suppressed.

[0036] The present invention also includes a method for manufacturing a gas sensor element including the porous protective layer described above. The method for manufacturing the gas sensor element of the present invention comprises: forming a porous protective layer made of an alumina-containing aggregate and a silicon dioxide-containing coating material around a sensing portion; and baking the formed porous protective layer at 1000°C or higher.

[0037] When forming a porous protective layer, the porous protective layer can be formed, for example, by an immersion process, a molding process that allows for good film thickness dimensional accuracy, or a thermal spraying process that is suitable for producing a compact layer. The immersion process or thermal spraying process is selected depending on the porosity of the porous protective layer.

[0038] When the porous protective layer is formed by, for example, the immersion method, an operation of immersing a detection portion in a slurry containing an aggregate and a coating material, removing it from the slurry, and then drying it is repeated several times until a layer with a predetermined thickness can be obtained. The slurry containing an aggregate and a coating material is obtained, for example, by dispersing an aggregate and a coating material in a solvent (water, etc.), optionally using a dispersant (polyvinyl alcohol (PVA), etc.). Alumina serving as an aggregate can be used, for example, in the form of an alumina powder. Meanwhile, silica serving as a coating material can be used either in a crystalline form or in an amorphous form.Preferably, amorphous silica is used. When silica sol, such as silicon dioxide, is used to adjust the weight concentration of a coating material in the resulting porous protective layer, for example, to 20 wt.%, approximately 38.5 wt.% of the silica sol is required, since the silica sol is an aqueous solution of 40 wt.% silicon dioxide.

[0039] When the porous protective layer is formed by the thermal spraying method, for example, mixed powders or an aggregate-containing slurry and a coating material are melted or converted into a state close to such a molten state at a high temperature, and the resultant is then sprayed onto a detection portion to form a porous protective layer.

[0040] When baking the porous protective layer, the porous protective layer formed is baked. The baking temperature is 1000°C or higher, and preferably 1100°C or higher. The baking temperature is, for example, 1000°C to 1200°C, preferably 1050°C to 1150°C, and more preferably 1100°C. When the baking temperature is 1000°C or higher, the coating material (silicon dioxide) is melted, and the pore volume of pores with a pore diameter of 100 nm or less can be controlled to 0.02 mL / g or less. Accordingly, the thermal conductivity and strength of the porous protective layer are improved compared to the case of baking a porous protective layer at a temperature lower than 1000°C, at which a coating material is not completely melted. The baking time can be appropriately selected depending on the baking temperature. The baking time is generally 0.5 hours to 2 hours. Examples

[0041] The present invention will be described in more detail in the following examples and comparative examples. However, these examples are not intended to limit the scope of the present invention. 1. Production of the porous protective layer

[0042] A porous protective layer was prepared using aluminum oxide (Al2O3) as an aggregate and also using silicon dioxide (SiO2) as a coating material, and the prepared porous protective layer was then arranged around the sensing portion of a gas sensor element to form the Fig. 1. The alumina used as an aggregate exhibited NTC properties. The silica used as a coating material exhibited PTC properties and affected the porosity. Therefore, the porous protective layers of Example 1 and Comparative Examples 1 and 2, in which the component ratio between alumina and silica and the porosity were changed, were prepared, and a comparative expression required to impart NTC properties to the porous protective layer was obtained. Example 1

[0043] Alumina powder (average particle diameter: 10 µm) and silica powder (average particle diameter: 15 nm) were dispersed in water using a dispersant to form a slurry. The weight concentration of the silica solid content, based on the total weight of the alumina and silica solid content in the slurry, was adjusted to 21 wt%. According to the immersion method, 60 mg of the resulting slurry was adhered to the sensing portion of a gas sensor element, and then baked in the atmosphere at 1100 °C for 2 hours to form a porous protective layer. The porosity of the porous protective layer was measured to be 37% according to the mercury intrusion method. Comparison example 1

[0044] Silica powder (average particle diameter: 10 nm) was dispersed in water using a dispersant to form a slurry. According to the immersion method, 60 mg of the resulting slurry was adhered to the sensing area of ​​a gas sensor element and then baked in the atmosphere at 1100 °C for 4 hours to form a porous protective layer. The porosity of the porous protective layer was 0.7%. Comparison example 2

[0045] A porous protective layer was prepared in the same manner as that of Example 1, except that the amounts of alumina and silica in the slurry were changed so that the weight concentration of the solid content of silica, based on the total weight of the solid contents of alumina and silica in the slurry, was adjusted to 22 wt%, and the porosity of the porous protective layer was adjusted to 49%. 2. Evaluation of thermal conductivity properties

[0046] The porous protective layers of Example 1 and Comparative Examples 1 and 2 were measured for the relationship between temperature and thermal conductivity. Fig. Figure 2 shows the relationship between temperature and thermal conductivity with respect to the porous protective layers of Example 1 and Comparative Example 1. As in Fig. As shown in Figure 2, the porous protective layer of Example 1 exhibited NTC properties, which increased thermal conductivity with a decrease in temperature. In contrast, the porous protective layer of Comparative Example 1 exhibited PTC properties, which decreased thermal conductivity with a decrease in temperature. Furthermore, the porous protective layer of Comparative Example 2 exhibited PTC properties, although not shown in the figures.

[0047] Subsequently, a comparative expression was obtained, which is required to impart NTC properties to the porous protective layer. As described above, it is considered that the component ratio between the alumina used as an aggregate and the silica used as a coating material in the porous protective layer and the porosity of the porous protective layer have an influence on imparting NTC properties to the porous protective layer.Thus, from the results obtained by confirming the thermal conductivity characteristics of the porous protective layer of Example 1 and Comparative Examples 1 and 2, a comparison expression (formula (1)) between the weight concentration of silica used as a coating material in the porous protective layer and the porosity of the porous protective layer, which is required for imparting NTC characteristics to the porous protective layer, was obtained. Fig. Figure 3 shows the relationship between the weight concentration of silicon dioxide and the porosity in the porous protective layers of Example 1 and Comparative Examples 1 and 2. As in Fig. 3, the material exhibited NTC properties when the weight concentration x wt% of the coating material based on the total weight of the aggregate and the coating material in the porous protective layer and the porosity y% of the porous protective layer satisfied the following formula (1): y≤0.0058x2−1.2666x+68 3. Evaluation of the water-repellent effect

[0048] Regarding the porous protective layers of Example 1 and Comparative Examples 1 and 2, water drops (dropped amount: 2 µL) were continuously added dropwise to the surface of a porous protective layer at a high temperature (700 °C), and the water repellency was then measured. Fig. Figure 4 shows the results of the water repellency tests carried out on the porous protective layers of Example 1 and Comparative Examples 1 and 2. As in Fig. As shown in Figure 4, the continuous water repellency of the porous protective layer of Example 1, which exhibits NTC properties, was significantly increased compared to the protective layers of Comparative Examples 1 and 2, which exhibit PTC properties. Accordingly, it was demonstrated that when the weight concentration x wt% of a coating material and the porosity y% of a porous protective layer were controlled to satisfy the above-described formula (1) so as to allow the porous protective layer to exhibit NTC properties, the water-repellent property of the porous protective layer upon continuous water pouring was significantly improved compared to a porous protective layer exhibiting PTC properties. 4. Relationship between weight concentration of silicon oxide and bond strength

[0049] Gas sensor elements in which the weight concentration of silica, based on the total weight of the solid contents of alumina and silica in the slurry, was changed to change the weight concentration of silica in a porous protective layer were prepared in the same manner as that of Example 1 described above. The bonding strength between the porous protective layer and an element base material consisting mainly of alumina was measured. The weight concentration of silica was set to 2.8 wt%, 5.8 wt%, 12.1 wt%, or 19.1 wt%. Fig. Figure 5 shows the relationship between the weight concentration of silicon dioxide and the bond strength. As in Fig. 5, it was demonstrated that the stable bonding strength of the porous protective layer is ensured in a range where the weight concentration of silicon dioxide in the porous protective layer is 10 wt% or more. 5. Relationship between porosity and response value

[0050] Gas sensor elements in which the porosity of a porous protective layer was varied were fabricated in the same manner as in Example 1. The porosity of the porous protective layer was set to 10%, 35%, or 69%. When the porosity was 30% or less, the thermal spray method was used instead of the immersion method.

[0051] With respect to the prepared porous protective layers, the response value of the sensor output when the exhaust atmosphere was greatly changed was measured in an exhaust environment in a real engine machine. Fig. Figure 6 shows the relationship between the porosity of a porous protective layer and the response value. It should be noted that the response sensitivity improves as the response value is increased. As shown in Fig. 6, it was demonstrated that the exhaust gas permeability becomes sufficiently high and preferential response of the sensor output to a change in the exhaust gas is ensured in a range where the porosity of the porous protective layer is 25% or more. 6. Influence of baking temperature

[0052] The influence of baking temperature on the production of a porous protective layer was investigated. A porous protective layer formed according to the immersion method and subsequently baked at 1100°C (Example 1) was compared with a porous protective layer formed according to the immersion method and subsequently baked at 900°C (Comparative Example 3). The porous protective layer of Comparative Example 3 was produced in the same manner as that of Example 1, except that the baking temperature was changed to 900°C. Scanning electron microscopy (SEM) images of the porous protective layers of Example 1 and Comparative Example 3 are shown in Fig. 7 shown. Fig. Figure 7A shows an SEM photo of the porous protective layer of Example 1, and Fig. Figure 7B shows an SEM photograph of the porous protective layer of Comparative Example 3. Furthermore, Fig. 8 shows the relationship between the pore diameter and the pore volume measured according to a gas adsorption method in the porous protective layers of Example 1 and Comparative Example 3. Fig. 7A, Fig. 7B and Fig. 8, it was found that the silica particles used as a coating material in the porous protective layer of Example 1, which was baked at 1100°C, were melted, and the pore volume of the pores with a pore diameter of 100 nm or less was 0.02 mL / g or less. On the other hand, in the porous protective layer of Comparative Example 3, which was baked at 900°C, the silica particles used as a coating material were not sufficiently melted, and the pore volume of the pores with a pore diameter of 100 nm or less was significantly larger than that in the porous protective layer of Example 1. List of reference symbols 1 shielding layer 2 Porous, diffuse resistance layer 3 Solid electrolyte layer 4 pairs of electrodes 41 Electrode on the target gas side 42 Electrode on the reference gas side 5 Reference gas space protective layer 6 Heat generation source (heating) 7 Heat generation source substrate 8 Target gas chamber 9 Reference gas chamber 10 Recording section 20, 20A Porous protective layer 20Aa Upper layer 20Ab Lower Layer 100, 100A, 101 gas sensor element 103 Gas inlet port

Claims

[1] Gas sensor element (100, 100A, 101) comprising: a detection section (10) comprising a stack of a solid electrolyte body and a heat generating body, the solid electrolyte body having at least one pair of electrodes (4) on opposite sides thereof, and the heat generating body comprising a heat generating source (6); and a porous protective layer (20, 20A) formed around the detection section (10), wherein the porous protective layer (20, 20A) is formed from an alumina-containing aggregate and a silicon dioxide-containing coating material, and in the porous protective layer (20, 20A), the weight concentration x wt% of the coating material, based on the total weight of the aggregate and the coating material, and the porosity y% satisfy the following formula (1): y≤0.0058x2−1.2666x+68 and, in the porous protective layer (20, 20A), the pore volume of the pores having a pore diameter of 100 nm or less is 0.02 mL / g or less. [2] The gas sensor element (100, 100A, 101) according to claim 1, wherein the weight concentration of the coating material in the porous protective layer (20, 20A) is 10 wt% or more. [3] The gas sensor element (100, 100A, 101) according to claim 1 or 2, wherein the porosity of the porous protective layer (20, 20A) is 25% or more. [4] A method for manufacturing the gas sensor element (100, 100A, 101) according to any one of claims 1 to 3, comprising: Forming a porous protective layer (20, 20A) of an alumina-containing aggregate and a silicon dioxide-containing coating material around the detection portion (10); and Baking the formed porous protective layer (20, 20A) at 1000 °C or higher.

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