Powder for oxygen sensor protection layer, preparation method of powder and oxygen sensor

Abstract

The invention belongs to the technical field of oxygen sensors, and provides powder for an oxygen sensor protection layer, a preparation method of the powder, an oxygen sensor and a preparation method of the powder for the oxygen sensor protection layer, the preparation method of the powder for the oxygen sensor protection layer comprises the following steps: preparing raw materials in parts by mass: 68-80 parts of aluminum oxide, 5-15 parts of second-phase particles, 8-15 parts of an inorganic hydrophobic auxiliary agent and 16-20 parts of a pore forming agent; the average particle sizes of all the components are matched with one another; the powder for the oxygen sensor protection layer is prepared from the raw materials through pulping, atomization, drying, collection and post-treatment of a spray granulation process. According to the powder for the oxygen sensor protective layer prepared by the preparation method, the oxygen sensor protective layer obtained by plasma spraying is high in thermal shock resistance, high in adhesive force, high in porosity and high in water spraying resistance, and can meet the working requirements of an oxygen sensor on the protective layer; the oxygen sensor with the oxygen sensor protection layer avoids the failure problem, and is stable in working performance and long in service life.

Description

Technical Field

[0001] The present invention belongs to the technical field of oxygen sensors, and specifically relates to a powder for an oxygen sensor protective layer, a preparation method thereof, and an oxygen sensor. Background Art

[0002] As a key component of an automotive emission control system, the performance of an automotive oxygen sensor directly affects fuel economy and the effect of exhaust emission control. Currently, the widely used heated zirconia oxygen sensor needs to maintain stable operation within the working temperature range of 300 - 900°C. During actual use, when the engine is cold-started, since the exhaust system has not reached the dew point temperature and the water vapor content in the exhaust is relatively high, condensation water is extremely likely to form. When the high-temperature oxygen sensor chip comes into contact with more than 2 μL of condensation water, the resulting thermal shock triggers microcracks in the chip, ultimately causing the oxygen sensor to fail. Therefore, constructing a dew point protective layer on the surface of the oxygen sensor chip has become the key technical path to improve the waterproof performance of the oxygen sensor and extend its service life. The protective layer of the oxygen sensor needs to simultaneously possess excellent water spray resistance, thermal shock resistance, and low thermal conductivity, corresponding to the requirement of having structural characteristics of high adhesion, high porosity, and appropriate thickness.

[0003] The existing preparation processes for the dew point protective layer of oxygen sensors mainly include slurry coating method, low-temperature thermal spraying, screen printing combined with plasma spraying, etc. Among them, the plasma spraying method is considered the most promising preparation method for the dew point protective layer of oxygen sensors due to advantages such as strong coating adhesion, simple process, and high production efficiency. However, this process places strict requirements on the adaptability of the powder used for spraying the oxygen sensor protective layer. But the existing powder for the oxygen sensor protective layer has the following defects:

[0004] 1. Insufficient porosity control technology

[0005] Aluminum oxide has become the most commonly used raw material for preparing the dew point protection layer of oxygen sensors due to its high chemical stability, low cost, etc. To improve the coating porosity of the oxygen sensor protection layer, the existing technologies mostly adopt the strategy of adding pore-forming agents to the aluminum oxide powder. The pore-forming agents decompose, volatilize or chemically react at high temperatures to form pores, so as to regulate the porosity, morphology and distribution of the coating. In the technical solutions disclosed in patent documents CN113614523A and CN113597552A, coatings with a porosity of 40 - 60% are prepared by adding a specific proportion of pore-forming materials and combining with subsequent firing treatment. However, this method has two major drawbacks: First, it is difficult to completely remove the pore-forming agents during the spraying process, and additional heat treatment processes are required, increasing the process complexity; if the residual pore-forming agents undergo thermal decomposition during the service of the sensor, it will interfere with the normal operation of the sensor. Second, there are technical bottlenecks in the existing preparation methods of powders for oxygen sensor protection layers: due to the density difference between aluminum oxide and the pore-forming agents in the mechanical mixing method, powder shunting occurs during spraying, and the pore-forming agents with lower density gather towards the edge of the plasma flame, resulting in uneven pore distribution in the coating; although the solvent evaporation method can achieve the agglomeration of micron-sized pore-forming agents and aluminum oxide, the formed irregular secondary particles have poor fluidity and insufficient particle strength, and are prone to break up during spraying, seriously affecting the spraying efficiency and the coating quality of the oxygen sensor protection layer. In addition, even when the coating thickness of the oxygen sensor protection layer prepared by the existing technology reaches 740 μm, its water resistance to rain is still limited to the order of 10 μL, and it is necessary to further optimize the pore-forming agent particle size and increase the proportion of closed pores to enhance the waterproof performance.

[0006] 2. Thermal stability defects of hydrophobic materials

[0007] To endow the coating with hydrophobicity, the existing technologies often add hydrophobic organic polymer materials, which are splashed onto the substrate surface after melting in the plasma flame to reduce the surface energy of the coating. As described in "Properties and Deposition Mechanism of APS - Prepared PFA / Al2O3 Composite Ceramic Hydrophobic Coatings", spraying after mechanically mixing Al2O3 and PFA can make the contact angle of the coating reach 122.6°, significantly improving the hydrophobic performance. However, the continuous service temperature of such hydrophobic organic polymer materials is generally lower than 300 °C. In the actual service environment of oxygen sensors at 300 - 900 °C, the materials will undergo thermal decomposition, not only causing the rapid attenuation of the waterproof performance of the coating of the oxygen sensor protection layer, but also the thermal decomposition products will contaminate the oxygen sensor and affect the detection accuracy.

[0008] 3. Insufficient thermal shock resistance

[0009] The existing powders for the protective layer of spray-coated oxygen sensors, which are mainly composed of alumina, are prone to cracking under thermal shock due to their high brittleness and low matching degree of thermal expansion coefficient with the substrate, resulting in cracking or peeling of the coating, severely restricting the thermal shock resistance of the dew point protective layer of the oxygen sensor, and further affecting the performance and service life of the oxygen sensor. It is urgent to optimize the composition of the powders for the protective layer of the oxygen sensor to achieve toughening of the coating and regulation of the thermal expansion coefficient of the protective layer of the oxygen sensor.

[0010] In summary, the existing powders for the protective layer of spray-coated oxygen sensors and their preparation technologies have obvious shortcomings in terms of porosity control, stability of hydrophobic materials, and thermal shock resistance, and it is difficult to meet the requirements of preparing a high-performance protective layer of the oxygen sensor through the plasma spraying process and then obtaining a high-performance oxygen sensor. Summary of the Invention

[0011] The purpose of the present invention is to provide a powder for the protective layer of an oxygen sensor, a preparation method thereof, and an oxygen sensor. The powder for the protective layer of the oxygen sensor obtained by the preparation method of the present invention has a more uniform composition and particle size distribution, higher particle strength, better fluidity, effectively solving problems such as powder shunting and disintegration. The powder for the protective layer of the oxygen sensor has strong thermal shock resistance, high adhesion, high porosity, and excellent water resistance to rain when the protective layer of the oxygen sensor is prepared by plasma spraying, and thus a high-performance oxygen sensor is obtained, and the working performance of the oxygen sensor is greatly improved and the service life is extended.

[0012] The specific solution content is as follows:

[0013] A preparation method of a powder for the protective layer of an oxygen sensor, comprising the following steps:

[0014] S1. Prepare raw materials: By mass fraction, it includes 68 - 80 parts of alumina, 5 - 15 parts of second-phase particles, 8 - 15 parts of inorganic hydrophobic additives, and 16 - 20 parts of pore-forming agents. The average particle size of alumina is 0.4 - 2 μm, the average particle size of the second-phase particles is 1 / 4 - 1 times that of the average particle size of alumina, the average particle size of the inorganic hydrophobic additives is 1 / 4 - 1 times that of the average particle size of alumina, and the average particle size of the pore-forming agents is 2 - 4 times that of the average particle size of alumina;

[0015] S2. Through the processes of pulping, atomization, drying, collection, and post-treatment of the spray granulation process for the raw materials, a powder for the protective layer of the oxygen sensor with regular spherical particles is prepared.

[0016] Further, the average particle size of the powder for the protective layer of the oxygen sensor is 28 - 32 μm, and the fluidity index is 75.0 - 80.0.

[0017] Further, the second-phase particles are one or more of zirconia, yttria, and silicon carbide.

[0018] Further, the inorganic hydrophobic aid is one or more of silicon oxide, silicon nitride, and rare earth silicate.

[0019] Further, the pore former is one or a mixture of two of polyvinyl chloride and polystyrene.

[0020] Further, step S2 includes:

[0021] S201, Pulp making: Using deionized water as the main dispersant, first premix alumina, the second-phase particles, and the inorganic hydrophobic aid, then add deionized water and the binder for grinding, and finally add the pore former for mixing and grinding to obtain a slurry;

[0022] S202, Atomization: Breaking the slurry into tiny droplets by centrifugal atomization;

[0023] S203, Drying: Drying the slurry droplets to obtain a powder;

[0024] S204, Collection: Collecting the powder generated during the drying process;

[0025] S205, Post-treatment: Further screening to remove abnormal agglomerates or overly fine powder.

[0026] Further, the binder is one or more of polyvinyl alcohol, carboxymethyl cellulose, and polyacrylic acid.

[0027] A powder for an oxygen sensor protective layer is prepared by using the preparation method of the powder for an oxygen sensor protective layer described above.

[0028] An oxygen sensor includes an oxygen sensor protective layer sprayed on the surface of an oxygen sensor chip by a plasma spraying process using the powder for an oxygen sensor protective layer described above.

[0029] Further, when the average thickness of the coating of the oxygen sensor protective layer > 250 μm, the average porosity ≥ 19%, the average adhesion ≥ 35 N, the average water resistance to rain ≥ 450 μL, and the oxygen sensor protective layer has passed the thermal shock cycle test.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] The innovative formula of the powder for an oxygen sensor protective layer of the present invention is combined with the spray granulation preparation process, making the composition and particle size distribution of the prepared powder for an oxygen sensor protective layer more uniform and the particle strength higher, solving problems such as powder diversion and collapse;

[0032] The oxygen sensor protective layer sprayed by using the powder for an oxygen sensor protective layer of the present invention by a plasma spraying process has enhanced thermal shock resistance, high adhesion, high porosity, and excellent water resistance to rain;

[0033] The performance of the oxygen sensor with the oxygen sensor protective layer of the present invention is significantly improved, and its service life is extended. Description of the Drawings

[0034] Figure 1 It is a morphology diagram of the powder for the oxygen sensor protective layer in Example 1 of the present invention.

[0035] Figure 2 It is a morphology diagram of the powder for the oxygen sensor protective layer in Example 2 of the present invention.

[0036] Figure 3 It is a morphology diagram of the powder for the oxygen sensor protective layer in Comparative Example 2 of the present invention. Detailed Description of the Invention

[0037] The technical solutions in the embodiments of the present invention will be described in detail below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0038] Aluminum oxide has become the most commonly used raw material for preparing the oxygen sensor protective layer due to its high chemical stability and low cost. The existing preparation processes of the oxygen sensor protective layer mainly include slurry coating method, low-temperature thermal spraying, and screen printing combined with plasma spraying, etc. Among them, the plasma spraying method is considered the most promising preparation method because of its strong coating adhesion, simple process, high production efficiency, etc. However, this process puts strict requirements on the adaptability of the sprayed powder. The existing powders still have defects such as insufficient porosity control technology, poor thermal stability of hydrophobic materials, and insufficient thermal shock resistance. The present invention has innovatively designed the formula of the powder for the oxygen sensor protective layer used in the preparation method of the powder for the oxygen sensor protective layer, so that it overcomes the above defects and meets the requirements of the plasma spraying method for the powder.

[0039] The preparation method of the powder for the oxygen sensor protective layer provided by the present invention includes the following steps:

[0040] S1. Prepare raw materials: by mass, including 68 - 80 parts of aluminum oxide, 5 - 15 parts of second-phase particles, 8 - 15 parts of inorganic hydrophobic additives, and 16 - 20 parts of pore-forming agents. The average particle size of the aluminum oxide is 0.4 - 2 μm, the average particle size of the second-phase particles is 1 / 4 - 1 of the average particle size of the aluminum oxide, the average particle size of the inorganic hydrophobic additives is 1 / 4 - 1 of the average particle size of the aluminum oxide, and the average particle size of the pore-forming agents is 2 - 4 times the average particle size of the aluminum oxide;

[0041] S2. Through the processes of pulping, atomization, drying, collection, and post-treatment of the spray granulation process for the raw materials, the powder for the oxygen sensor protective layer with regular spherical particles is prepared.

[0042] Aiming at the problem of insufficient thermal shock resistance of the coatings prepared by the existing powder preparation method, the present invention adds second-phase particles to the alumina matrix. The average particle size of alumina is 0.4 - 2 μm, and the average particle size of the second-phase particles is 1 / 4 - 1 times that of the average particle size of alumina. The second-phase particles are one or more of zirconia, yttrium oxide, and silicon carbide. The second-phase particles form a strengthening network structure inside the coating, effectively dispersing the thermal stress and significantly improving the toughness of the oxygen sensor protective layer. At the same time, the introduction of the second-phase particles can effectively regulate the thermal expansion coefficient of the oxygen sensor protective layer, making it better match with the matrix material of the oxygen sensor chip and reducing the thermal mismatch phenomenon. The two aspects comprehensively enhance the thermal shock resistance of the oxygen sensor protective layer within the working temperature of the oxygen sensor, effectively avoiding the problems of cracking and spalling of the oxygen sensor protective layer.

[0043] To overcome the defect of poor thermal stability of the existing hydrophobic materials, the present invention uses one or more of inorganic hydrophobic aids such as silica, silicon nitride, and rare earth silicate to replace the traditional organic polymer materials. The average particle size of the inorganic hydrophobic aids is 1 / 4 - 1 times that of the average particle size of alumina. These inorganic hydrophobic aids have excellent high-temperature stability. In the high-temperature environment of plasma spraying and during the service process of the sensor, they can maintain a stable chemical structure and a certain hydrophobic property, avoiding the problems of attenuation of waterproof performance and sensor contamination caused by pyrolysis, and continuously endowing the oxygen sensor protective layer with effective and lasting hydrophobic characteristics, thereby improving the water spray resistance of the oxygen sensor protective layer.

[0044] Aiming at the problems of existing pore former residues and poor water spray resistance of the oxygen sensor protective layer, the present invention optimizes and screens the types, proportions, and particle sizes of the pore formers. The preferred pore formers can be completely decomposed and / or volatilized at high temperatures during the plasma spraying process, without additional post-treatment processes, simplifying the process flow; it can achieve stable and controllable porosity of the oxygen sensor protective layer, while significantly increasing the proportion of closed pores, effectively preventing the penetration of condensed water, and improving the water spray resistance of the oxygen sensor protective layer to a new level. The pore formers are one or a mixture of two of polyvinyl chloride and polystyrene, and the average particle size is 2 - 4 times that of the average particle size of alumina.

[0045] The average particle sizes of alumina, the second-phase particles, the inorganic hydrophobic aids, and the pore formers must be matched, otherwise, it is impossible to prepare the powder for the oxygen sensor protective layer that can achieve the technical effects.

[0046] The above raw materials are prepared into powders for the oxygen sensor protection layer through the processes of pulping, atomization, drying, collection, and post-treatment of the spray granulation process. During the pulping process, deionized water is used as the main dispersant, and the ratio of deionized water to raw materials is 2.5:1 to 1:1. Corundum balls are used as the grinding medium, the ball-to-material ratio is 3:1 to 1:1, the ball milling speed is 30 to 60 Hz, and the ball milling time is 2.5 to 5 h. First, alumina, second-phase particles, and inorganic hydrophobic additives are premixed, then deionized water and 2 to 10 parts by mass of a binder are added for wet milling, and finally, a pore-forming agent is added and stirred evenly to obtain a slurry for the atomization process. Before the atomization process, the rotation speed of the centrifugal atomization equipment is set to 150 to 220 Hz, and then the slurry is broken into tiny droplets. Entering the drying process, first set the inlet air temperature to 210 to 260 °C and the outlet air temperature to 100 to 130 °C, and then dry and atomize the slurry droplets to obtain powders. The powders generated during the drying process are collected, and a screening device is used to further screen and remove possible abnormal agglomerates or overly fine powders, and powders for the oxygen sensor protection layer with an average particle size of 28 to 32 μm and a flowability index of 75.0 to 80.0 can be obtained. The binder is one or more of polyvinyl alcohol, carboxymethyl cellulose, polyacrylic acid, etc.

[0047] In terms of the preparation process, the present invention uses the spray granulation method to prepare powders for the oxygen sensor protection layer. This method can make alumina and the modified components fully and evenly mixed to form composite particles with a regular spherical shape. Compared with the traditional mechanical mixing or solvent evaporation method, the powders prepared by spray granulation have a more uniform composition and particle size distribution, higher particle strength, and better fluidity. It not only effectively solves problems such as powder diversion and collapse but also significantly improves the plasma spraying efficiency and spraying quality, providing a reliable guarantee for preparing an oxygen sensor protection layer with excellent water spray resistance and thermal shock resistance.

[0048] The powders for the oxygen sensor protection layer of the present invention are prepared by using the preparation method of the powders for the oxygen sensor protection layer of the present invention.

[0049] The present invention also provides an oxygen sensor, which includes an oxygen sensor protection layer sprayed on the surface of the oxygen sensor chip by using the powders for the oxygen sensor protection layer of the present invention through the plasma spraying process.

[0050] When the average thickness of the coating of the oxygen sensor protection layer > 250 μm, the average porosity ≥ 19%, the average adhesion ≥ 35 N, the average water spray resistance ≥ 450 μL, and the oxygen sensor protection layer has passed the impact thermal cycle test.

[0051] To further understand the present invention, the following is combined with Figure 1 、 Figure 2 、 Figure 3, the examples and comparative examples will elaborate in detail on a powder for an oxygen sensor protective layer, its preparation method, and the oxygen sensor of the present invention.

[0052] Example 1:

[0053] The specific implementation steps of the technical solution provided by the present invention are as follows:

[0054] Raw material screening and proportioning: Take 68 parts of alumina with an average particle size of 1 μm; the second-phase particles are 12 parts of zirconia with an average particle size of 0.5 μm; the inorganic hydrophobic aid is 10 parts of silicon nitride with an average particle size of 0.5 μm; the preferred pore-forming agent is 16 parts of polystyrene with an average particle size of 4 μm;

[0055] Spray granulation includes the following steps:

[0056] Pulping: First premix alumina, zirconia, and silicon nitride, and then add deionized water and polyvinyl alcohol (binder) to start wet grinding. The ratio of deionized water to raw materials is 2:1, the ratio of corundum balls to raw materials is 2:1, the rotation speed is 40 Hz, and ball milling is carried out for 3.5 h. Finally, add polystyrene and ball mill for 0.5 h;

[0057] Atomization: Set the rotation speed of the centrifugal atomization equipment to 190 Hz;

[0058] Drying: Set the inlet air temperature to 225 °C and the outlet air temperature to 120 °C;

[0059] Collection and post-treatment: Collect the powder generated during the drying process, and further screen to obtain the powder for the target oxygen sensor protective layer.

[0060] See Figure 1 As shown, the powder for the oxygen sensor protective layer obtained in Example 1 is in the shape of spherical particles, with an average particle size of 29.6 μm and a fluidity index of 76.5.

[0061] Using the powder of Example 1, set the spraying power to 38.5 kW, the powder feeding rate to 20 g / min, spray 4 times, and prepare an oxygen sensor protective layer on the oxygen sensor chip. The average thickness of the oxygen sensor protective layer is 290 μm, the average porosity is 20.87%, the average adhesion is 40 N, the water resistance to rain is 520 μL, and the coating does not fall off after 36 impact thermal cycle tests. Passing the impact thermal cycle test, it has good thermal shock resistance.

[0062] Test standards: GB / T2423.5 Mechanical Shock, GB / T4208 Waterproof.

[0063] Example 2:

[0064] The specific implementation steps of the technical solution provided by the present invention are as follows:

[0065] Raw material screening and proportioning: Take 80 parts of alumina with an average particle size of 2 μm; The second-phase particles adopt 5 parts of a mixed powder of yttrium oxide and silicon carbide with an average particle size of 0.5 μm; The inorganic hydrophobic aid adopts 8 parts of yttrium silicate with an average particle size of 0.5 μm; The preferred pore former adopts 16 parts of polyvinyl chloride with an average particle size of 4 μm;

[0066] Powder preparation: Spray granulation:

[0067] Pulping: First premix alumina, yttrium oxide and silicon carbide mixed powder and yttrium silicate, and then add deionized water and polyacrylic acid (binder) to start wet grinding. The ratio of deionized water to raw materials is 2:1, the ratio of corundum balls to raw materials is 2:1, the rotation speed is 40 Hz, and ball milling is carried out for 2.5 h. Finally, add polyvinyl chloride and mix by ball milling for 0.5 h;

[0068] Atomization: Set the rotation speed of the centrifugal atomization equipment to 175 Hz;

[0069] Drying: Set the inlet air temperature to 230 °C and the outlet air temperature to 120 °C;

[0070] Collection and post-treatment: Collect the powder generated during the drying process, and further screen to obtain the powder for the oxygen sensor protective layer.

[0071] See Figure 2 As shown, the powder for the oxygen sensor protective layer obtained in Example 2 is in the shape of spherical particles, with an average particle size of 31.0 μm and a fluidity index of 78.00.

[0072] Using the same spraying process as in Example 1, the powder of Example 2 is used to prepare an oxygen sensor protective layer on the oxygen sensor chip. The average thickness of the oxygen sensor protective layer is 301.3 μm, the average porosity is 19.05%, the average adhesion is 40 N, the water resistance to rain is 600 μL, and the coating does not fall off after 36 impact thermal cycle tests. It passes the impact thermal cycle test and has good thermal shock resistance.

[0073] Example 3:

[0074] The specific implementation steps of the technical solution provided by the present invention are as follows:

[0075] Raw material screening and proportioning: Take 75 parts of alumina with an average particle size of 2 μm; The second-phase particles adopt 15 parts of zirconia powder with an average particle size of 2 μm; The inorganic hydrophobic aid adopts 15 parts of a mixed powder of silicon oxide and silicon nitride with an average particle size of 2 μm; The preferred pore former adopts 20 parts of polystyrene with an average particle size of 4 μm;

[0076] Powder preparation: Spray granulation:

[0077] Pulping: First, premix the mixed powder of alumina, zirconia, silica, and silicon nitride, and then add deionized water and polyvinyl alcohol (binder) to start wet grinding. The ratio of deionized water to raw materials is 2:1, the ratio of corundum balls to raw materials is 2:1, the rotation speed is 60 Hz, and ball milling is carried out for 5 h. Finally, add polystyrene and ball mill and mix for 0.5 h;

[0078] Atomization: Set the rotation speed of the centrifugal atomization equipment to 190 Hz;

[0079] Drying: Set the inlet air temperature to 225 °C and the outlet air temperature to 120 °C;

[0080] Collection and post-treatment: Collect the powder generated during the drying process and further screen it to obtain the powder for the target oxygen sensor protective layer.

[0081] The powder for the oxygen sensor protective layer obtained in Example 3 is in the shape of spherical particles, with an average particle size of 28.2 μm and a fluidity index of 77.50.

[0082] Using the same spraying process as in Example 1, use the powder in Example 3 to prepare an oxygen sensor protective layer on the oxygen sensor chip. The average thickness of the oxygen sensor protective layer is 295.4 μm, the average porosity is 21.33%, the average adhesion is 35 N, the water resistance to rain is 500 μL, and the coating does not peel off after 36 impact thermal cycle tests. Passing the impact thermal cycle test, it has good thermal shock resistance.

[0083] Example 4:

[0084] The specific implementation steps of the technical solution provided by the present invention are as follows:

[0085] Raw material screening and ratio: Take 75 parts of alumina with an average particle size of 0.4 μm; the second-phase particles use 8 parts of a mixed powder of yttrium oxide and silicon carbide with an average particle size of 0.3 μm; the inorganic hydrophobic aid uses 8 parts of yttrium silicate with an average particle size of 0.1 μm; the preferred pore-forming agent uses 20 parts of polyvinyl chloride with an average particle size of 0.6 μm;

[0086] Powder preparation: Spray granulation:

[0087] Pulping: First, premix the mixed powder of alumina, yttrium oxide and silicon carbide, and yttrium silicate, and then add deionized water and polyacrylic acid (binder) to start wet grinding. The ratio of deionized water to raw materials is 2:1, the ratio of corundum balls to raw materials is 2:1, the rotation speed is 40 Hz, and ball milling is carried out for 3.5 h. Finally, add polyvinyl chloride and ball mill and mix for 0.5 h;

[0088] Atomization: Set the rotation speed of the centrifugal atomization equipment to 175 Hz;

[0089] Drying: Set the inlet air temperature to 230 °C and the outlet air temperature to 120 °C;

[0090] Collection and post-treatment: Collect the powder generated during the drying process and further screen it to obtain the powder for the target oxygen sensor protective layer.

[0091] The powder for the oxygen sensor protective layer obtained in Example 4 is in the shape of spherical particles, with an average particle size of 31.3 μm and a flowability index of 79.50.

[0092] Using the powder of Example 4, set the spraying power at 38.5 kW, the powder feeding rate at 20 g / min, spray 4 times, and prepare an oxygen sensor protective layer on the oxygen sensor chip. The average thickness of the oxygen sensor protective layer is 315.3 μm, the average porosity is 25.56%, the average adhesion is 35 N, the water resistance to shower is 510 μL. After 36 impact thermal cycle tests, the coating does not fall off. Passing the impact thermal cycle test, it has good thermal shock resistance. After the chip is encapsulated, it passes the mechanical shock test, the external water mist test and the output signal detection, meeting the application requirements.

[0093] Comparative Example 1:

[0094] Using 100 parts of pure alumina powder with an average particle size of 30 μm as raw materials, prepare the powder for the oxygen sensor protective layer on the oxygen sensor chip according to the spraying process in Example 1. The powder particles are irregular in shape, with a flowability index of 72.00. Prepare a coating on the oxygen sensor chip according to the spraying process in Example 1. The average thickness of the obtained alumina coating is 290 μm, the average porosity of the coating is 9.75%, the average adhesion is 40 N, the maximum water resistance to shower is 50 μL. Since the average porosity and the water resistance to shower are too low, no other tests are required.

[0095] Comparative Example 2:

[0096] Take 68 parts of irregular alumina powder with an average particle size of 20 μm, 12 parts of zirconia, 10 parts of silicon nitride and 16 parts of polystyrene with an average particle size of 4 μm and mix them, as shown in Figure 3 shown. Form the powder for the oxygen sensor protective layer by pressing and granulating. The powder is irregular in shape, with a flowability index of 60.5. The spraying powder feeding continuity is poor and no coating can be obtained. Therefore, no other tests can be carried out.

[0097] Comparative Example 3:

[0098] Raw material screening and ratio: Take 86 parts of alumina with an average particle size of 0.6 μm; the second-phase particles use 2 parts of zirconia with an average particle size of 0.5 μm; the inorganic hydrophobic aid uses 5 parts of silicon nitride with an average particle size of 0.5 μm; the pore-forming agent uses 10 parts of polystyrene with an average particle size of 4 μm;

[0099] Powder preparation: Spray granulation:

[0100] Pulping: First, premix alumina, zirconia, and silicon nitride, then add deionized water and polyvinyl alcohol (binder) to start wet milling. The ratio of deionized water to raw materials is 2:1, the ratio of corundum balls to raw materials is 2:1, the rotation speed is 40 Hz, and ball milling is carried out for 3.5 h. Finally, add polystyrene and ball mill for 0.5 h;

[0101] Atomization: Set the rotation speed of the centrifugal atomization device to 120 Hz;

[0102] Drying: Set the inlet air temperature to 225 °C and the outlet air temperature to 120 °C;

[0103] Collection and post-treatment: Collect the powder generated during the drying process and further screen to obtain the powder for the target oxygen sensor protective layer.

[0104] The powder for the oxygen sensor protective layer obtained in Comparative Example 3 is in the shape of spherical particles, with an average particle size of 42.3 μm and a flowability index of 85.0.

[0105] According to the spraying process in Example 1, an oxygen sensor protective layer was prepared on the oxygen sensor chip. The average thickness of the oxygen sensor protective layer is 334.5 μm, the average porosity is 14.33%, which will weaken the electrical signal, and at this point, this powder can already be eliminated; the average adhesion is 45 N, and the coating peeled off after 36 impact thermal cycle tests, failing the impact thermal cycle test, and having poor thermal shock resistance.

[0106] Examples 1-4 and Comparative Examples 1-3 were tested to obtain Table 1:

[0107]

[0108] Comparative Example 1 was made of pure alumina powder. When compared with Examples 1-4, the shape of the powder obtained in Comparative Example 1 is irregular. When the average thickness of the coating is 290 μm, the average porosity is 9.75, the flow index is 72, the average adhesion is 40 N, and the average water resistance to rain is ≤ 50 μL. The average porosity and water resistance to rain are too low, and no other tests are required;

[0109] The average particle size of the alumina powder in Comparative Example 2 is not within the value range of the present invention, and the value ranges of the average particle sizes of other components do not match the average particle size of alumina in proportion. The shape of the obtained powder is irregular, and the particles between the components are adhered to each other, with too low strength to effectively detect the average particle size; the flowability index is 60.5, and normal powder feeding cannot be carried out, that is, no coating can be obtained, so no other tests can be carried out;

[0110] In Comparative Example 3, the mass fraction values of each component of the raw material are not within the range values, and the average porosity of 14.33 is too low, which will weaken the electrical signal. At this point, this powder can already be eliminated, and there is no need to conduct the water resistance test. After 36 impact thermal cycle tests, the coating peeled off, and the impact thermal cycle test failed;

[0111] The powders for protecting oxygen sensors prepared in Examples 1-4 are in the shape of spherical particles. When the average coating thickness is greater than 250 μm, the average porosity is greater than 19%, the average adhesion is greater than 35 N, the average water resistance is higher than 450 μL, and all of them have passed the impact thermal cycle test. This shows that as long as the values of each component in the raw material are within the value range of the present invention, the performance parameters of the powders for the oxygen sensor protection layer prepared can all achieve the technical effects of the present invention. The oxygen sensor protection layer made of this powder can meet the working requirements of the oxygen sensor, and a high-performance oxygen sensor can be obtained. The oxygen sensor can avoid failure problems, improve working stability, and extend the service life;

[0112] After the sample of the preferred Example 4 was encapsulated, the obtained oxygen sensor was subjected to a mechanical shock test, an external water mist test after encapsulation, and an output signal test after encapsulation. The oxygen sensor passed all the tests, proving that the oxygen sensor protection layer prepared with the powder for the oxygen sensor protection layer of the present invention can meet the working requirements of the oxygen sensor, and thus a high-performance oxygen sensor is obtained. The oxygen sensor of the present invention avoids the risk of failure, improves working stability, and extends the service life; during the detection process, the test standard for the mechanical shock test after encapsulation is: GB / T2423.5-1995; the test standard for the external water mist test after encapsulation is: GB / T2423.5-1995; the test standard for the output signal test after encapsulation is: QC / T803-2017.

[0113] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A preparation method of a powder for an oxygen sensor protective layer, characterized in that, The steps are as follows: S1. Prepare raw materials: by mass parts, including 68 - 80 parts of alumina, 5 - 15 parts of second-phase particles, 8 - 15 parts of inorganic hydrophobic additives, and 16 - 20 parts of pore-forming agent. The average particle size of the alumina is 0.4 - 2 μm, the average particle size of the second-phase particles is 1 / 4 - 1 times that of the alumina average particle size, the average particle size of the inorganic hydrophobic additives is 1 / 4 - 1 times that of the alumina average particle size, and the average particle size of the pore-forming agent is 2 - 4 times that of the alumina average particle size; S2. Prepare the powder for the oxygen sensor protective layer with regular spherical particles and uniform distribution through pulping, atomization, drying, collection, and post-treatment of the spray granulation process for the raw materials.

2. The preparation method of the powder for the oxygen sensor protective layer according to claim 1, characterized in that The average particle size of the powder for the oxygen sensor protective layer is 28 - 32 μm, and the fluidity index is 75.0 - 80.

0.

3. The preparation method of the powder for the oxygen sensor protective layer according to claim 1, characterized in that, The second-phase particles are one or more of zirconia, yttria, and silicon carbide.

4. The preparation method of the powder for the oxygen sensor protective layer according to claim 1, characterized in that, The inorganic hydrophobic additives are one or more of silica, silicon nitride, and rare earth silicate.

5. The preparation method of the powder for the oxygen sensor protective layer according to claim 1, characterized in that, The pore-forming agent is a mixture of one or both of polyvinyl chloride and polystyrene.

6. The preparation method of the powder for the oxygen sensor protective layer according to claim 1, characterized in that, Step S2 includes: S201. Pulping: Using deionized water as the main dispersant, first premix alumina, second-phase particles, and inorganic hydrophobic additives, then add deionized water and binder for grinding, and finally add the pore-forming agent for mixing and grinding to obtain a slurry; S202. Atomization: Break the slurry into tiny droplets by centrifugal atomization; S203. Drying: Dry the slurry droplets to obtain powder; S204. Collection: Collect the powder generated during the drying process; S205. Post-treatment: Further screen to remove abnormal agglomerates or overly fine powder.

7. The preparation method of the powder for the oxygen sensor protective layer according to claim 6, characterized in that, The binder is one or more of polyvinyl alcohol, carboxymethyl cellulose, and polyacrylic acid.

8. A powder for an oxygen sensor protective layer, characterized in that, It is prepared by using the preparation method of the powder for the oxygen sensor protective layer according to any one of claims 1 to 7.

9. An oxygen sensor, characterized in that, It includes an oxygen sensor protective layer obtained by plasma spraying on the surface of an oxygen sensor chip using the powder for the oxygen sensor protective layer according to claim 8.

10. The oxygen sensor according to claim 9, characterized in that, When the average coating thickness of the oxygen sensor protective layer > 250 μm, the average porosity ≥ 19%, the average adhesion ≥ 35 N, the average water resistance to rain ≥ 450 μL, and the oxygen sensor protective layer has passed the shock thermal cycle test.

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