A water-resistant protective layer for a nitrogen oxide sensor, a nitrogen oxide sensor and a preparation method thereof

Abstract

The present invention relates to the technical field of nitrogen oxide sensors, and discloses a water-resistant protective layer for a nitrogen oxide sensor, a nitrogen oxide sensor and a preparation method thereof. The nitrogen oxide sensor chip is included, and a water-resistant protective layer is coated on the outside of the end of the nitrogen oxide sensor chip. The water-resistant protective layer comprises the following components in mass fractions: 100 parts of protective layer powder, 1 to 6 parts of pore-forming agent, 10 to 20 parts of binder, 0.5 to 5 parts of dispersant, 0.5 to 5 parts of defoaming agent and 10 to 20 parts of solvent; the protective layer powder comprises 40 to 60 parts of spinel powder, 20 to 40 parts of alumina powder and 20 to 40 parts of calcium carbonate powder. The water-resistant protective layer of the present invention uses spinel, alumina and calcium carbonate as main raw materials, and can form a microscopic morphology of a flower-like structure after sintering. At the same time, a porous structure is prepared by combining with a pore-forming agent. Coating on the end of the nitrogen oxide sensor greatly improves the hydrophobic property and heat preservation property of the nitrogen oxide sensor, reduces the influence of thermal shock on the nitrogen oxide sensor, and prolongs the service life of the nitrogen oxide sensor.

Description

Technical Field

[0001] The present invention relates to the technical field of nitrogen oxide sensors, and in particular, to a water-resistant protective layer for a nitrogen oxide sensor, a nitrogen oxide sensor, and a preparation method thereof. Background Art

[0002] A nitrogen oxide sensor is based on the oxygen ion conduction of functional ceramics. Using the multi-layer high-temperature co-fired ceramic technology, a porous cermet composed of noble metals such as Pt, Au, Rh and ZrO2 is sintered on a YSZ (yttrium-stabilized zirconia) substrate, and micro electrical signals are analyzed and collected to identify the characteristic signals of NOx.

[0003] Usually, the gas to be measured contains water vapor, and water droplets generated by the condensation of water vapor will adhere to the surface of the nitrogen oxide sensor heated to a high temperature state. Therefore, the thermal shock caused by the water droplets adhering to the surface of the nitrogen oxide sensor element will act on the sensor element, causing cracks on the surface of the sensor element, and then leading to the cracking of the sensor element.

[0004] In view of this, the present application is specifically proposed. Summary of the Invention

[0005] The problem existing in the prior art is that the thermal shock caused by the water droplets adhering to the surface of the nitrogen oxide sensor element will act on the sensor element, causing cracks on the surface of the sensor element. In order to solve the above problems, the present invention provides a water-resistant protective layer for a nitrogen oxide sensor, a nitrogen oxide sensor, and a preparation method thereof. By coating a protective layer at the end of the nitrogen oxide sensor, a large number of flower-like structures formed by stacking sheet-like structures exist in its microscopic morphology, greatly improving the hydrophobic performance of the protective layer. At the same time, due to the porous structure of the prepared protective layer, the heat preservation performance of the nitrogen oxide sensor can also be greatly improved, reducing the influence of water vapor in the gas to be measured on the thermal shock of the sensor, and prolonging the service life of the nitrogen oxide sensor.

[0006] The present invention is realized through the following technical solutions:

[0007] In a first aspect, the present invention provides a water-resistant protective layer for a nitrogen oxide sensor, which comprises the following components in mass fractions: 100 parts of protective layer powder, 1-6 parts of pore-forming agent, 10-20 parts of binder, 0.5-5 parts of dispersant, 0.5-5 parts of defoaming agent, and 10-20 parts of solvent;

[0008] The protective layer powder comprises 40-60 parts of spinel powder, 20-40 parts of alumina powder, and 20-40 parts of calcium carbonate powder.

[0009] The water-resistant protective layer of the present invention uses spinel, alumina, and calcium carbonate as the main raw materials. After sintering, it can form a microscopic morphology with a flower-like structure. At the same time, a porous structure is prepared by combining a pore-forming agent. Coating it at the end of the nitrogen oxide sensor greatly improves the hydrophobic and heat-insulating properties of the nitrogen oxide sensor, reduces the impact of thermal shock on the nitrogen oxide sensor, and extends the service life of the nitrogen oxide sensor.

[0010] In a specific embodiment, the particle size of the spinel powder is 20 - 40 μm, and the particle size of the alumina powder is 0.05 - 1 μm.

[0011] In a specific embodiment, the pore-forming agent is one or more of carbon black powder, polymethyl methacrylate, and polyethylene, with a particle size of 0.5 - 50 μm; the binder is one or more of alumina sol, silica sol, and titanium dioxide sol; the dispersant is one or more of triethylhexyl phosphate and sodium tripolyphosphate.

[0012] In a specific embodiment, the defoaming agent is one or more of polydimethylsiloxane and ethylene oxide; the solvent is one or a mixture of deionized water, ethanol, and glycerol.

[0013] In a second aspect, the present invention provides a method for preparing a water-resistant protective layer for a nitrogen oxide sensor, and the steps are as follows:

[0014] After uniformly mixing the spinel powder, alumina powder, and calcium carbonate powder, add the pore-forming agent, binder, dispersant, defoaming agent, and solvent, and mix them in a ball mill to obtain a protective layer slurry.

[0015] In a specific embodiment, the viscosity of the protective layer slurry is 500 - 10000 mPa·s.

[0016] In a third aspect, the present invention provides a nitrogen oxide sensor, including a nitrogen oxide sensor chip, and the end of the nitrogen oxide sensor chip is externally coated with the water-resistant protective layer of the nitrogen oxide sensor or the water-resistant protective layer slurry prepared by the preparation method.

[0017] In a specific embodiment, the coating thickness of the water-resistant protective layer is 100 - 500 μm.

[0018] In a fourth aspect, the present invention provides a method for preparing a nitrogen oxide sensor, including the following steps:

[0019] 1) Vertically place the nitrogen oxide sensor chip into the above-mentioned protective layer slurry, so that the end of the nitrogen oxide sensor chip is immersed in the protective layer slurry and is completely covered by the slurry;

[0020] 2) After impregnation for a certain period of time, lift the nitrogen oxide sensor chip upward to complete impregnation, then dry it, and finally bake the nitrogen oxide sensor chip to obtain the nitrogen oxide sensor.

[0021] In a specific embodiment, in step 2), the lifting speed is 0.1 - 5 mm / s, and the drying temperature is 50 - 200 °C; the baking temperature of the nitrogen oxide sensor chip after coating is 900 - 1300 °C, and the sintering time is 0.5 - 5 h.

[0022] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0023] 1. A water-resistant protective layer for a nitrogen oxide sensor, a nitrogen oxide sensor and a preparation method thereof provided by an embodiment of the present invention, using spinel, alumina, and calcium carbonate as main raw materials, can form a microscopic morphology of a flower-like structure after sintering, and at the same time, a porous structure is prepared by combining a pore-forming agent, having good hydrophobic and heat-insulating properties. Coated at the end of the nitrogen oxide sensor, it can reduce the impact of thermal shock on the nitrogen oxide sensor and extend the service life of the nitrogen oxide sensor;

[0024] 2. A water-resistant protective layer for a nitrogen oxide sensor, a nitrogen oxide sensor and a preparation method thereof provided by an embodiment of the present invention, by coating a protective layer at the end of the nitrogen oxide sensor, there are a large number of flower-like structures formed by stacking sheet-like structures with a proportion greater than 30% in its microscopic morphology, greatly improving the hydrophobic property of the protective layer. At the same time, due to the porous structure of the prepared protective layer, the heat-insulating property of the nitrogen oxide sensor can also be greatly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the following will briefly introduce the drawings required to be used in the embodiments. It should be understood that the following drawings only show some embodiments of the present invention, and therefore should not be regarded as limiting the scope. For those of ordinary skill in the art, other related drawings can also be obtained based on these drawings without creative efforts.

[0026] Figure 1 SEM image of the flower-like structure of the water-resistant protective layer at the end of the nitrogen oxide sensor provided by the present invention;

[0027] Figure 2 Area EDS surface scanning element distribution of the water-resistant protective layer at the end of the nitrogen oxide sensor provided in Embodiment 1 of the present invention;

[0028] Figure 3 Magnified SEM image of the flower-like structure of the water-resistant protective layer at the end of the nitrogen oxide sensor provided in Embodiment 1 of the present invention;

[0029] Figure 4SEM image with a relatively low magnification of the flower-like structure of the water-resistant protective layer at the end of the nitrogen-oxygen sensor provided in Embodiment 1 of the present invention;

[0030] Figure 5 SEM image with an even lower magnification of the flower-like structure of the water-resistant protective layer at the end of the nitrogen-oxygen sensor provided in Embodiment 1 of the present invention;

[0031] Figure 6 SEM image of the water-resistant protective layer at the end of the nitrogen-oxygen sensor provided in Comparative Example 1 of the present invention. Detailed implementation manners

[0032] To make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below in conjunction with embodiments and drawings. The illustrative embodiments and descriptions of the present invention are only used to explain the present invention and do not limit the present invention.

[0033] In the following description, a large number of specific details are set forth in order to provide a thorough understanding of the present invention. However, it is obvious to those of ordinary skill in the art that the present invention does not have to be implemented with these specific details. In other embodiments, well-known materials or methods are not specifically described to avoid obscuring the present invention.

[0034] Throughout the specification, the reference to "an embodiment", "embodiment", "an example" or "example" means that the specific features, structures or characteristics described in connection with the embodiment or example are included in at least one embodiment of the present invention. Therefore, the phrases "an embodiment", "embodiment", "an example" or "example" that appear throughout the specification do not necessarily refer to the same embodiment or example. In addition, the specific features, structures or characteristics can be combined in any appropriate combination and / or sub-combination in one or more embodiments or examples. The term "and / or" used herein includes any and all combinations of one or more of the related listed items.

[0035] In the description of the present invention, the orientation or positional relationship indicated by the terms "front", "rear", "left", "right", "upper", "lower", "vertical", "horizontal", "high", "low", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be construed as limiting the protection scope of the present invention.

[0036] The gas to be measured usually contains water vapor, and the water droplets generated by the condensation of water vapor will adhere to the surface of the nitrogen oxide sensor heated to a high temperature state. Therefore, the thermal shock caused by the water droplets adhering to the surface of the nitrogen oxide sensor element will act on the sensor element, causing cracks on the surface of the sensor element and further leading to the cracking of the sensor element.

[0037] To solve the above technical problems:

[0038] In the first aspect, the present invention provides a water-resistant protective layer for a nitrogen oxide sensor, which includes the following components by mass fraction: 100 parts of protective layer powder, 1 - 6 parts of pore-forming agent, 10 - 20 parts of binder, 0.5 - 5 parts of dispersant, 0.5 - 5 parts of defoaming agent, and 10 - 20 parts of solvent;

[0039] The protective layer powder includes 40 - 60 parts of spinel powder, 20 - 40 parts of alumina powder, and 20 - 40 parts of calcium carbonate powder.

[0040] The water-resistant protective layer of the present invention uses spinel, alumina, and calcium carbonate as the main raw materials, and can form a microscopic morphology of a flower-like structure after sintering, as Figure 1 shown; at the same time, a porous structure is prepared by combining with a pore-forming agent. Coating it at the end of the nitrogen oxide sensor greatly improves the hydrophobic and heat preservation properties of the nitrogen oxide sensor, reduces the influence of thermal shock on the nitrogen oxide sensor, and extends the service life of the nitrogen oxide sensor.

[0041] In a specific embodiment, the particle size of the spinel powder is 20 - 40 μm, and the particle size of the alumina powder is 0.05 - 1 μm.

[0042] In a specific embodiment, the pore-forming agent is one or more of carbon black powder, polymethyl methacrylate, and polyethylene, and the particle size is 0.5 - 50 μm; the binder is one or more of alumina sol, silica sol, and titanium dioxide sol; the dispersant is one or more of polycarboxylic acid-based dispersants and phosphate ester-based dispersants.

[0043] In a specific embodiment, the defoaming agent is one or more of silicone-based defoaming agents and polyether-based defoaming agents; the solvent is deionized water, ethanol, glycerol, or a mixture thereof.

[0044] In the second aspect, the present invention provides a preparation method for the water-resistant protective layer of a nitrogen oxide sensor, and the steps are as follows:

[0045] Mix the spinel powder, alumina powder, and calcium carbonate powder evenly, then add the pore-forming agent, binder, dispersant, defoaming agent, and solvent, and mix them in a ball mill to obtain a protective layer slurry.

[0046] In a specific embodiment, the viscosity of the protective layer slurry is 500 to 10,000 mPa·s.

[0047] In a third aspect, the present invention provides a nitrogen oxide sensor, comprising a nitrogen oxide sensor chip, and the end of the nitrogen oxide sensor chip is coated with the water-resistant protective layer of the nitrogen oxide sensor or the water-resistant protective layer slurry of the nitrogen oxide sensor prepared by the preparation method.

[0048] In a specific embodiment, the coating thickness of the water-resistant protective layer is 100 to 500 μm.

[0049] In a fourth aspect, the present invention provides a preparation method of a nitrogen oxide sensor, comprising the following steps:

[0050] 1) Vertically place the nitrogen oxide sensor chip into the protective layer slurry, so that the end of the nitrogen oxide sensor chip is immersed in the protective layer slurry and is completely covered by the slurry;

[0051] 2) After impregnating for a certain time, lift the nitrogen oxide sensor chip upward to complete impregnation, then dry it, and finally bake the nitrogen oxide sensor chip to obtain the nitrogen oxide sensor.

[0052] In a specific embodiment, in step 2), the lifting speed is 0.1 to 5 mm / s, and the drying temperature is 50 to 200 °C; the baking temperature of the nitrogen oxide sensor chip after coating is 900 to 1300 °C, and the sintering time is 0.5 to 5 h.

[0053] By coating a protective layer at the end of the nitrogen oxide sensor, a large number of flower-like structures formed by stacking sheet-like structures with a proportion greater than 30% exist in its microscopic morphology. The structure is as Figure 1 shown, which greatly improves the hydrophobic performance of the protective layer. At the same time, due to the porous structure of the prepared protective layer, the heat preservation performance of the nitrogen oxide sensor can also be greatly improved. As Figure 2 is Figure 1 the EDS surface scanning element distribution of the water-resistant protective layer, and Table 1 is the content of each element. It can be seen that its element composition is consistent with the raw material selection.

[0054] Table 1

[0055] Element wt% at% O 49.16 67.63 Mg 0.76 0.68 Al 14.67 11.97 Si 1.15 0.90 Ca 34.27 18.82 Total amount 100 100

[0056] Example 1

[0057] The embodiment of the present invention provides a preparation method of a nitrogen oxide sensor, comprising the following steps:

[0058] (1) Weigh 5 g of spinel powder with a particle size of 30 μm, 2.5 g of alumina powder with a particle size of 0.3 μm, and 4.46 g of calcium carbonate powder respectively, and add them to the ball mill tank;

[0059] (2) Weigh 0.6 g of polymethyl methacrylate with a particle size of 3 μm, 2 g of silica sol, 0.5 g of triethylhexyl phosphate, 0.5 g of polydimethylsiloxane, and 0.93 g of glycerol respectively, and add them to the ball mill tank.

[0060] (3) Weigh 20 g of zirconia beads and add them to the ball mill tank, and then add them to the ball mill tank.

[0061] (4) Set the rotation speed of the planetary ball mill to 500 r / min and the ball milling time to 40 min, thus obtaining the slurry for forming the water-resistant protective layer of the nitrogen oxide sensor.

[0062] (5) Dip-coat the above slurry onto the designated area at the end of the nitrogen oxide sensor chip by the dip-coating and pulling method, where the pulling speed is 0.2 mm / s.

[0063] (6) Put the dip-coated nitrogen oxide sensor chip into the drying oven, set the temperature of the drying oven to 70 °C, and dry for 1 hour.

[0064] (7) Put the dried nitrogen oxide sensor chip into the muffle furnace, set the sintering temperature of the muffle furnace to 1100 °C, and the sintering time to 3 h.

[0065] Through the above steps, a water-resistant protective layer for the nitrogen oxide sensor with a thickness of 350 μm is obtained. Perform corresponding characterizations on the obtained water-resistant protective layer of the nitrogen oxide sensor, such as Figure 3 shown, the surface of the water-resistant protective layer presents a porous structure combined with a local flower-like structure. At the same time, it can be seen from Figure 4 and Figure 5 that the flower-like structure accounts for approximately 50% of the entire surface.

[0066] Example 2

[0067] The embodiment of the present invention provides a preparation method for a nitrogen oxide sensor, including the following steps:

[0068] (1) Weigh 5 g of spinel powder with a particle size of 30 μm, 3.223 g of alumina powder with a particle size of 0.3 μm, and 3.737 g of calcium carbonate powder respectively, and add them to the ball mill tank.

[0069] (2) Weigh 0.6 g of polymethyl methacrylate with a particle size of 3 μm, 2 g of silica sol, 0.5 g of sodium tripolyphosphate, 0.5 g of ethylene oxide, and 0.93 g of glycerol respectively, and add them to the ball mill tank.

[0070] (3) Weigh 20 g of zirconia beads and add them to the ball mill tank, and then add them to the ball mill tank.

[0071] (4)Set the rotation speed of the planetary ball mill to 500 r / min and the ball milling time to 40 min, thereby obtaining the slurry for forming the water-resistant protective layer of the nitrogen-oxygen sensor;

[0072] (5)Dip-coat the above slurry onto the designated area at the end of the nitrogen-oxygen sensor chip by the dip-coating and lifting method, where the lifting speed is 0.2 mm / s;

[0073] (6)Put the dip-coated nitrogen-oxygen sensor chip into the drying oven, set the temperature of the drying oven to 70 °C, and dry for 1 hour;

[0074] (7)Put the dried nitrogen-oxygen sensor chip into the muffle furnace, set the sintering temperature of the muffle furnace to 1100 °C, and the sintering time to 3 h.

[0075] Through the above steps, a water-resistant protective layer of the nitrogen-oxygen sensor is obtained, with a thickness of 350 μm.

[0076] Example 3

[0077] The embodiment of the present invention provides a preparation method of a nitrogen-oxygen sensor, including the following steps:

[0078] (1)Weigh 5 g of spinel powder with a particle size of 30 μm, 3.947 g of alumina powder with a particle size of 0.3 μm, and 3.013 g of calcium carbonate powder respectively, and add them to the ball mill tank;

[0079] (2)Weigh 0.6 g of polymethyl methacrylate with a particle size of 3 μm, 2 g of silica sol, 0.5 g of triethylhexyl phosphate, 0.5 g of polydimethylsiloxane, and 0.93 g of glycerol respectively, and add them to the ball mill tank;

[0080] (3)Weigh 20 g of zirconia beads and add them to the ball mill tank;

[0081] (4)Set the rotation speed of the planetary ball mill to 500 r / min and the ball milling time to 40 min, thereby obtaining the slurry for forming the water-resistant protective layer of the nitrogen-oxygen sensor;

[0082] (5)Dip-coat the above slurry onto the designated area at the end of the nitrogen-oxygen sensor chip by the dip-coating and lifting method, where the lifting speed is 0.2 mm / s;

[0083] (6)Put the dip-coated nitrogen-oxygen sensor chip into the drying oven, set the temperature of the drying oven to 70 °C, and dry for 1 hour;

[0084] (7)Put the dried nitrogen-oxygen sensor chip into the muffle furnace, set the sintering temperature of the muffle furnace to 1100 °C, and the sintering time to 3 h.

[0085] Example 4

[0086] An embodiment of the present invention provides a method for preparing a nitrogen oxide sensor, comprising the following steps:

[0087] (1) Weigh 5 g of spinel powder with a particle size of 30 μm, 2.5 g of alumina powder with a particle size of 0.3 μm, and 4.46 g of calcium carbonate powder respectively, and add them to a ball milling tank;

[0088] (2) Weigh 0.12 g of polymethyl methacrylate with a particle size of 3 μm, 2 g of silica sol, 0.5 g of sodium tripolyphosphate, 0.5 g of ethylene oxide, and 0.93 g of glycerol respectively, and add them to the ball milling tank;

[0089] (3) Weigh 20 g of zirconia beads and add them to the ball milling tank;

[0090] (4) Set the rotation speed of the planetary ball mill to 500 r / min and the ball milling time to 40 min to obtain a slurry for forming a water-resistant protective layer of the nitrogen oxide sensor;

[0091] (5) Dip-coat the above slurry onto a specified area at the end of the nitrogen oxide sensor chip by the dip-coating and pulling method, where the pulling speed is 0.2 mm / s;

[0092] (6) Put the dip-coated nitrogen oxide sensor chip into an oven, set the oven temperature to 70 °C, and dry for 1 hour;

[0093] (7) Put the dried nitrogen oxide sensor chip into a muffle furnace, set the sintering temperature of the muffle furnace to 1100 °C, and the sintering time to 3 h.

[0094] Example 5

[0095] An embodiment of the present invention provides a method for preparing a nitrogen oxide sensor, comprising the following steps:

[0096] (1) Weigh 5 g of spinel powder with a particle size of 30 μm, 2.5 g of alumina powder with a particle size of 0.3 μm, and 4.46 g of calcium carbonate powder respectively, and add them to a ball milling tank;

[0097] (2) Weigh 0.36 g of polymethyl methacrylate with a particle size of 3 μm, 2 g of silica sol, 0.5 g of triethylhexyl phosphate, 0.5 g of polydimethylsiloxane, and 0.93 g of glycerol respectively, and add them to the ball milling tank;

[0098] (3) Weigh 20 g of zirconia beads and add them to the ball milling tank;

[0099] (4) Set the rotation speed of the planetary ball mill to 500 r / min and the ball milling time to 40 min to obtain a slurry for forming a water-resistant protective layer of the nitrogen oxide sensor;

[0100] (5) Dip-coat the above-mentioned slurry onto the designated area at the end of the nitrogen oxide sensor chip by the dip-coating method, with a pulling speed of 0.2 mm / s;

[0101] (6) Place the dip-coated nitrogen oxide sensor chip in an oven, set the oven temperature to 70 °C, and dry for 1 hour;

[0102] (7) Place the dried nitrogen oxide sensor chip in a muffle furnace, set the sintering temperature of the muffle furnace to 1100 °C, and the sintering time to 3 h.

[0103] Comparative Example 1

[0104] (1) Weigh 10 g of spinel powder with a particle size of 30 μm and add it to the ball mill jar;

[0105] (2) Weigh 0.6 g of polymethyl methacrylate with a particle size of 3 μm, 2 g of silica sol, 0.5 g of triethylhexyl phosphate, 0.5 g of polydimethylsiloxane, and 0.93 g of glycerol respectively, and add them to the ball mill jar;

[0106] (3) Weigh 20 g of zirconia beads and add them to the ball mill jar;

[0107] (4) Set the rotation speed of the planetary ball mill to 500 r / min and the ball milling time to 40 min to obtain the slurry for forming the water-resistant protective layer of the nitrogen oxide sensor;

[0108] (5) Dip-coat the above-mentioned slurry onto the designated area at the end of the nitrogen oxide sensor chip by the dip-coating method, with a pulling speed of 0.2 mm / s;

[0109] (6) Place the dip-coated nitrogen oxide sensor chip in an oven, set the oven temperature to 70 °C, and dry for 1 hour;

[0110] (7) Place the dried nitrogen oxide sensor chip in a muffle furnace, set the sintering temperature of the muffle furnace to 1100 °C, and the sintering time to 3 h.

[0111] Conduct corresponding characterization on the prepared water-resistant protective layer of the nitrogen oxide sensor, such as Figure 6 As shown, it can be seen that there are only pore structures on the surface and no flower-like structures.

[0112] Comparative Example 2

[0113] (1) Weigh 5 g of spinel powder with a particle size of 10 μm, 2.5 g of alumina powder with a particle size of 2 μm, and 4.46 g of calcium carbonate powder respectively, and add them to the ball mill jar;

[0114] (2)Weigh 0.6 g of polymethyl methacrylate with a particle size of 3 μm, 2 g of silica sol, 0.5 g of triethylhexyl phosphate, 0.5 g of polydimethylsiloxane, and 0.93 g of glycerol respectively, and add them to the ball mill tank;

[0115] (3)Weigh 20 g of zirconia beads and add them to the ball mill tank;

[0116] (4)Set the rotation speed of the planetary ball mill to 500 r / min and the ball milling time to 40 min to obtain the slurry for forming the water-resistant protective layer of the nitrogen oxide sensor;

[0117] (5)Dip-coat the above slurry onto the designated area at the end of the nitrogen oxide sensor chip by the dip-coating method, with a pulling speed of 0.2 mm / s;

[0118] (6)Put the dip-coated nitrogen oxide sensor chip into the drying oven, set the temperature of the drying oven to 70 °C, and dry for 1 hour;

[0119] (7)Put the dried nitrogen oxide sensor chip into the muffle furnace, set the sintering temperature of the muffle furnace to 1100 °C, and the sintering time to 3 h.

[0120] Comparative Example 3

[0121] The embodiment of the present invention provides a preparation method of a nitrogen oxide sensor, including the following steps:

[0122] (1)Weigh 5 g of spinel powder with a particle size of 30 μm, 2.5 g of alumina powder with a particle size of 0.3 μm, and 4.46 g of calcium carbonate powder respectively, and add them to the ball mill tank;

[0123] (2)Weigh 0.84 g of polymethyl methacrylate with a particle size of 3 μm, 2 g of silica sol, 0.5 g of triethylhexyl phosphate, 0.5 g of polydimethylsiloxane, and 0.93 g of glycerol respectively, and add them to the ball mill tank;

[0124] (3)Weigh 20 g of zirconia beads and add them to the ball mill tank;

[0125] (4)Set the rotation speed of the planetary ball mill to 500 r / min and the ball milling time to 40 min to obtain the slurry for forming the water-resistant protective layer of the nitrogen oxide sensor;

[0126] (5)Dip-coat the above slurry onto the designated area at the end of the nitrogen oxide sensor chip by the dip-coating method, with a pulling speed of 0.2 mm / s;

[0127] (6)Put the dip-coated nitrogen oxide sensor chip into the drying oven, set the temperature of the drying oven to 70 °C, and dry for 1 hour;

[0128] (7) Place the dried nitrogen oxide sensor chip into a muffle furnace, set the sintering temperature of the muffle furnace to 1100 °C, and the sintering time to 3 h.

[0129] Comparative Example 4

[0130] The embodiment of the present invention provides a preparation method of a nitrogen oxide sensor, including the following steps:

[0131] (1) Weigh 5 g of spinel powder with a particle size of 30 μm, 2.5 g of alumina powder with a particle size of 0.3 μm, and 4.46 g of calcium carbonate powder respectively, and add them to the ball milling tank;

[0132] (2) Weigh 1.08 g of polymethyl methacrylate with a particle size of 3 μm, 2 g of silica sol, 0.5 g of triethylhexyl phosphate, 0.5 g of polydimethylsiloxane, and 0.93 g of glycerol respectively, and add them to the ball milling tank;

[0133] (3) Weigh 20 g of zirconia beads and add them to the ball milling tank, and add them to the ball milling tank;

[0134] (4) Set the rotation speed of the planetary ball mill to 500 r / min and the ball milling time to 40 min to obtain the slurry for forming the water-resistant protective layer of the nitrogen oxide sensor;

[0135] (5) Dip-coat the above slurry onto the designated area at the end of the nitrogen oxide sensor chip by the dip-coating and lifting method, where the lifting speed is 0.2 mm / s;

[0136] (6) Place the dip-coated nitrogen oxide sensor chip into an oven, set the oven temperature to 70 °C, and dry for 1 hour;

[0137] (7) Place the dried nitrogen oxide sensor chip into a muffle furnace, set the sintering temperature of the muffle furnace to 1100 °C, and the sintering time to 3 h.

[0138] Comparative Example 5

[0139] This comparative example is a nitrogen oxide sensor without an additional water-resistant protective layer.

[0140] Test Example

[0141] For the water-resistant protective layers of the nitrogen oxide sensors obtained in Examples 1-5 and Comparative Examples 1-5, the limiting current influence experiments were carried out respectively.

[0142] First, energize the heating part of the nitrogen oxide sensor before obtaining the water-resistant protective layer to heat the sensor to 840 °C. On this basis, in an atmospheric atmosphere, apply a main pump voltage of 1000 mV to obtain the main pump current; conduct the same test on the nitrogen oxide sensor after obtaining the water-resistant protective layer, and judge the influence of the limiting current based on the change of the main pump current before and after applying the water-resistant protective layer.

[0143] For the water-resistant protective layers of the nitrogen oxide sensors obtained in the examples and comparative examples, the proportion of the flower-like structure was calculated based on the cross-sectional SEM images.

[0144] For the water-resistant protective layers of the nitrogen oxide sensors obtained in the examples and comparative examples, it was visually confirmed whether there were cracks or peeling (delamination).

[0145] For the water-resistant protective layers of the nitrogen oxide sensors obtained in the examples and comparative examples, water immersion resistance tests were respectively carried out.

[0146] Specifically, first, the heating part of the nitrogen oxide sensor was energized to heat the sensor to 840 °C. On this basis, in an atmospheric atmosphere, each pump unit of the sensor was made to operate normally, and the main pump current Ip0 reached a stable state.

[0147] Then, in this state, a specified amount of water droplets was dropped on the heating part of the sensor, and it was confirmed whether the change in the pump current before and after the water droplets were dropped exceeded the threshold value. In the case where the change in the pump current did not exceed the threshold value, water droplets were continuously dropped, and the pump current was repeatedly confirmed. The dropped amount at which the change in the pump current Ip0 exceeded the threshold value was defined as the limit water immersion amount, and the thermal shock resistance performance of the protective layer was determined thereby.

[0148] Table 2 shows the evaluation results of the proportion of the flower-like structure, the crack and delamination conditions of the protective layer, the influence on the limit current, and the limit water immersion amount of the water-resistant protective layers of the nitrogen oxide sensors in the examples and comparative examples.

[0149] Table 2

[0150] Specimen number Proportion of flower-like structure Cracking and spalling conditions Influence of limiting current Water immersion resistance Example 1 50% None ◎ ◎ Example 2 46% None ◎ ◎ Example 3 41% None ○ ○ Example 4 56% None ○ ○ Example 5 44% None ○ ○ Comparative example 1 0 Yes × × Comparative example 2 0 None × × Comparative example 3 37% Yes × × Comparative example 4 31% Yes × × Comparative example 5 0 - - ×

[0151] It should be noted that in the column of "influence on the limit current" in Table 1, the specimens judged to have a limit current decrease of 10% or less were marked as "◎". The specimens judged to have a limit current decrease of more than 10% and less than 30% were marked as "○". The specimens judged to have a limit current decrease of more than 30% were marked as "×".

[0152] In addition, in the column of "water immersion resistance" in Table 1, the specimens judged to have a limit water immersion amount of 30 μl or more were marked as "◎". The specimens judged to have a limit water immersion amount of more than 15 μl and less than 30 μl were marked as "○". The specimens with a limit water immersion amount of less than 15 μl were marked as "×".

[0153] It can be seen from Comparative Example 1 that the flower-like structure of the present application cannot be obtained only by using the spinel raw material, and at the same time, the performance of the obtained coating is poor.

[0154] It can be seen from Comparative Example 2 that when the particle size of the spinel is too small and the particle size of the alumina is too large, the flower-like structure of the present application cannot be formed either. Although the combination of the three components improves the problem of coating cracking, its water resistance is still very poor.

[0155] It can be seen from Comparative Examples 3 and 4 that as the content of the pore former increases, both the limiting current and the water resistance of the water-resistant protective layer are greatly affected.

[0156] The specific embodiments described above further elaborate on the purpose, technical solutions, and beneficial effects of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not used to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the protection scope of the present invention.

Claims

1. A water-resistant protective layer for a nitrogen and oxygen sensor, characterized in that: The microstructure of the protective layer includes a flower-like structure formed by stacking sheet structures, and the protective layer is prepared by sintering a protective layer slurry, wherein the protective layer slurry includes the following components in parts by weight: 100 parts of protective layer powder, 1 to 6 parts of pore former, 10 to 20 parts of binder, 0.5 to 5 parts of dispersant, 0.5 to 5 parts of defoamer and 10 to 20 parts of solvent; The protective layer powder comprises 40 to 60 parts of spinel powder, 20 to 40 parts of alumina powder, and 20 to 40 parts of calcium carbonate powder; The particle size of the spinel powder is 20-40 μm, and the particle size of the alumina powder is 0.05-1 μm.

2. The water-resistant protective layer for a nitrogen oxygen sensor according to claim 1, characterized in that: The pore-forming agent is one or more of carbon black powder, polymethyl methacrylate, and polyethylene; the binder is one or more of alumina sol, silica sol, and titanium dioxide sol; and the dispersant is one or more of triethylhexyl phosphate and sodium tripolyphosphate.

3. The water-resistant protective layer for a nitrogen oxygen sensor according to claim 1, characterized in that: The defoaming agent is one or more of polydimethylsiloxane and ethylene oxide; the solvent is one or a mixture of deionized water, ethanol and glycerol.

4. The method for preparing the water-resistant protective layer of the nitrogen oxygen sensor according to any one of claims 1 to 3, characterized in that: Here are the steps: After the spinel powder, alumina powder and calcium carbonate powder are uniformly mixed, a pore former, a binder, a dispersant, a defoamer and a solvent are added and mixed in a ball mill to obtain a protective layer slurry.

5. The method for preparing a water-resistant protective layer for a nitrogen oxygen sensor according to claim 4, characterized in that: The viscosity of the protective layer slurry is 500-10000 mPa·s.

6. A nitrogen oxygen sensor, characterized in that: The invention comprises a nitrogen oxygen sensor chip, wherein the outer end of the nitrogen oxygen sensor chip is coated with the nitrogen oxygen sensor water-resistant protective layer according to any one of claims 1 to 3 or the nitrogen oxygen sensor water-resistant protective layer slurry prepared by the preparation method according to claim 4 or 5.

7. A nitrogen oxygen sensor according to claim 6, characterized in that: The coating thickness of the water-resistant protective layer is 100 to 500 μm.

8. The method for preparing a nitrogen oxygen sensor according to claim 6 or 7, characterized in that: The steps include: 1) vertically placing the nitrogen oxygen sensor chip into the protective layer slurry according to claim 4, so that the end of the nitrogen oxygen sensor chip is immersed in the protective layer slurry and is completely covered by the slurry; 2) After being immersed for a certain period of time, the nitrogen oxygen sensor chip is pulled upward to complete the immersion, and then dried, and finally the nitrogen oxygen sensor chip is baked to obtain the nitrogen oxygen sensor.

9. The method for preparing a nitrogen oxygen sensor according to claim 8, characterized in that: In step 2), the pulling speed is 0.1-5 mm / s, and the drying temperature is 50-200° C.; the sintering temperature of the coated nitrogen oxygen sensor chip is 900-1300° C., and the sintering time is 0.5-5 h.

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