A waterproof thermal insulation coating and its preparation method and application

Abstract

A waterproof and thermally insulating coating, its preparation method, and its application are disclosed, relating to the field of coating material technology. This invention simplifies the process while ensuring the bonding strength between the coating and the substrate, as well as its thermal shock resistance. The alumina waterproof and thermally insulating coating has a double-layer structure: the first layer is a pure alumina coating; the second layer is a mixed alumina / magnesium aluminum spinel coating, wherein the mass ratio of alumina to magnesium aluminum spinel is 1:1. The alumina waterproof and thermally insulating coating is applied to the substrate surface by a dip-coating method, and after drying, it is formed by a one-step sintering process. This invention is applied to the protection of zirconia ceramic chips in nitrogen and oxygen gas sensors. Through a double-layer structure design, optimized dispersion process, and a one-step sintering activation method, a balance is achieved between coating adhesion, thermal stability, and waterproofness, providing an innovative solution for high-reliability sensor coatings.

Description

Technical Field

[0001] This invention relates to the field of coating materials technology, specifically to a waterproof and heat-insulating coating, its preparation method, and its application. Background Technology

[0002] NOx is one of the main components of air pollution, and its main source is the gas emitted from the internal combustion engine of automobiles. In order to solve the NOx emission problem, nitrogen oxide sensors are usually used to measure the concentration of nitrogen oxides (NOx) in the emissions of internal combustion engines.

[0003] The core component of the nitrogen oxide sensor is a zirconia ceramic chip. Pt electrodes are printed on both sides of a cured ZrO2 green ceramic sheet. Under high-temperature operating conditions, when a specific voltage is applied to the chip electrodes, NOx and O2 enter the chamber through the gas channel, forming a current in the closed circuit through redox reactions. The nitrogen oxide content is then measured by detecting the current signal.

[0004] However, when the sensor is in operation, the high-velocity gases, condensate, and water vapor in the exhaust gas impact the chip surface. This phenomenon causes uneven heating of the chip, generating stress within the material, leading to cracks and failure. Therefore, preparing a protective coating for the chip that combines waterproofing and heat insulation is crucial for improving sensor reliability.

[0005] To address this issue, existing technologies already offer relevant solutions. For example, Chinese patent document (application number 201210042762.9) discloses a multi-level alumina protective coating that achieves both waterproofing and heat insulation functions through a layered design of "porous-dense-porous". However, this technology requires two sintering processes on the chip to complete interlayer anchoring, and multiple high-temperature treatments can significantly negatively impact chip sensitivity, measurement accuracy, and electrode catalytic activity. Furthermore, in the traditional wet mechanical crushing process for preparing alumina coating powder, the mechanical force increases the surface energy of the powder. As the crushing time increases, powder agglomeration intensifies, leading to stress concentration during sintering and causing defects such as coating cracking and peeling.

[0006] To overcome the aforementioned technical bottlenecks, it is urgent to develop a new type of coating and its preparation method that simplifies the process while ensuring the bonding strength between the coating and the substrate and its thermal shock resistance. Summary of the Invention

[0007] In order to prepare a chip protective coating with both waterproof and heat insulation functions and improve the reliability of sensors, this invention proposes a waterproof and heat-insulating coating, its preparation method and application.

[0008] The specific technical solution of the present invention is as follows:

[0009] An alumina waterproof and heat-insulating coating has a double-layer structure. The first layer is a pure alumina coating, and the second layer is a mixed coating of alumina and magnesium aluminum spinel, wherein the mass ratio of alumina to magnesium aluminum spinel is 1:1.

[0010] The alumina waterproof and heat-insulating coating is applied to the substrate surface by dip coating and then formed by a one-step sintering process after drying.

[0011] Preferably, the thickness of the first layer is 0.1 mm and the thickness of the second layer is 1.0 mm.

[0012] The present invention also provides a method for preparing the above-mentioned alumina waterproof and heat-insulating coating, comprising the following steps:

[0013] S1. Preparation of alumina slurry:

[0014] PVA, anhydrous ethanol, and cod liver oil were sealed and stirred evenly at 90°C to serve as the first solvent.

[0015] The first alumina ceramic powder is added to the first solvent and then thoroughly ground using a planetary ball mill.

[0016] The ground slurry was placed in a stainless steel vacuum degassing tank for degassing treatment to obtain alumina slurry.

[0017] Preparation of S2, alumina / magnesium aluminum spinel mixed slurry:

[0018] PVA, anhydrous ethanol, and cod liver oil were sealed and stirred evenly at 90°C to serve as the second solvent.

[0019] The second alumina ceramic powder and magnesium aluminum spinel powder are added to the second solvent and then thoroughly ground using a planetary ball mill.

[0020] The ground slurry was placed in a stainless steel vacuum degassing tank for degassing treatment to obtain an alumina / magnesium aluminum spinel mixed slurry.

[0021] S3. The alumina slurry is applied to the substrate surface and dried. Then, the alumina / magnesium aluminum spinel mixed slurry is further applied to the surface, dried, and then heated to 1300℃ for high-temperature sintering. After heat preservation, an alumina waterproof and heat-insulating coating is obtained.

[0022] Preferably, the purity of the first alumina ceramic powder is 3N, and the median particle size is 500nm;

[0023] The purity of the second alumina ceramic powder is 3N, and the median particle size is 1μm; the purity of the magnesium aluminum spinel powder is 3N, and the median particle size is 10μm; the mass ratio of the second alumina ceramic powder to the magnesium aluminum spinel powder is 1:1.

[0024] Preferably, the mass ratio of PVA, anhydrous ethanol, cod liver oil and first alumina ceramic powder in step S1 is 1:8:0.4:10; and the mass ratio of PVA, anhydrous ethanol, cod liver oil, second alumina ceramic powder and magnesium aluminum spinel powder in step S2 is 2:16:0.4:10:10.

[0025] Preferably, the stirring speed in both steps S1 and S2 is 600 r / min, and the stirring time is 1 h.

[0026] Preferably, the parameters of the planetary ball mill in step S1 are 800 r / min and the grinding time is 8 h; the parameters of the planetary ball mill in step S2 are 600 r / min and the stirring time is 5 h.

[0027] Preferably, the vacuum degree of the degassing treatment in step S1 is -0.095 MPa and the degassing treatment time is 1 hour; the vacuum degree of the degassing treatment in step S2 is -0.095 MPa and the degassing treatment time is 2 hours.

[0028] Preferably, the heating rate in step S3 is 5~10℃ / min, and the holding time is 2~5h.

[0029] The present invention also provides an application of the above-mentioned alumina waterproof and heat-insulating coating, specifically, it is applied to the protection of the zirconium oxide ceramic chip of a nitrogen and oxygen gas sensor.

[0030] Compared with the prior art, the specific beneficial effects of the present invention are as follows:

[0031] 1. This invention achieves both waterproofing and thermal insulation functions through the design of a dual-layer gradient composite coating, utilizing the synergistic effect of nano- to micron-sized particles. The inner layer, dense alumina, forms a low-porosity barrier through an alcohol solvent dispersion system, blocking the path of moisture penetration. The outer mixed layer constructs a gradient pore structure, utilizing the low thermal conductivity of spinel to reduce heat loss, while simultaneously buffering thermal stress through matching thermal expansion coefficients, ensuring that the coating does not crack after 10,000 cycles of thermal cycling at 840℃.

[0032] 2. This invention introduces steric hindrance effect and one-step sintering method, solving the defects of traditional processes. The alcohol solvent dispersion system inhibits the agglomeration of nano-alumina through the dual effects of electrostatic repulsion and steric hindrance, resulting in a uniform particle size distribution of the slurry and avoiding stress concentration caused by traditional wet mechanical crushing. The one-step sintering activation method simultaneously achieves coating densification and interfacial chemical bonding in a single high-temperature process, improving the bonding strength and resisting 500 shocks of 50 m / s gas without damage. The risk of cracking is significantly reduced compared to the traditional two-stage sintering process.

[0033] 3. This invention achieves high-efficiency production through slurry formulation optimization and process integration. Simultaneous sintering of the double coating eliminates the intermediate layer anchoring step, compressing the traditional multiple sintering processes into a single operation, significantly reducing energy consumption. The alcohol solvent system improves powder utilization, significantly enhancing production line compatibility and the feasibility of large-scale production while ensuring coating performance. Detailed Implementation

[0034] To make the technical solutions of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. It should be noted that the following embodiments are only used to better understand the technical solutions of the present invention and should not be construed as limiting the present invention.

[0035] Example 1.

[0036] In this embodiment, the coating material is prepared according to the following steps:

[0037] (1) Preparation process of alumina slurry:

[0038] Step 1: Prepare organic solvent: First, heat 1g PVA, 8g anhydrous ethanol and 0.4g cod liver oil at 90℃ and stir in a sealed container for 1 hour at a stirring speed of 600r / min.

[0039] Step 2: Prepare ceramic slurry: Add 10g of alumina ceramic powder to the solvent system in "Step 1" above, and grind it thoroughly for 8 hours using a planetary ball mill at 800r / min.

[0040] Step 3: Degassing treatment of slurry: Place the ground slurry in a stainless steel vacuum degassing tank with a vacuum degree of -0.095MPa for degassing treatment for 1 hour.

[0041] The parameters of the alumina ceramic raw material powder used are as follows:

[0042] The purity is 3N and the median particle size (D50) is 500nm.

[0043] (2) Preparation process of alumina / magnesium aluminum spinel mixed slurry:

[0044] Step 1: Prepare organic solvent: First, heat 2g PVA, 16g anhydrous ethanol and 0.4g cod liver oil at 90℃ and stir in a sealed container for 1 hour at a stirring speed of 600r / min.

[0045] Step 2: Preparation of ceramic slurry: Add 10g of alumina and 10g of magnesium aluminum spinel powder to the solvent system in "Step 1" above, and grind thoroughly for 5 hours using a planetary ball mill at 600r / min.

[0046] Step 3: Degassing treatment of slurry: Place the ground slurry in a stainless steel vacuum degassing tank with a vacuum degree of -0.095MPa for degassing treatment for 2 hours.

[0047] The parameters of the alumina / magnesium aluminum spinel ceramic raw material powder used are as follows:

[0048] The alumina raw material powder used has a purity of 3N and a median particle size (D50) of 1μm;

[0049] The magnesium aluminum spinel raw material powder used has a purity of 3N and a median particle size (D50) of 10μm.

[0050] (3) Apply the alumina paste to the chip head surface and dry it. Then apply the alumina / magnesium aluminum spinel mixed paste to the chip head surface with the alumina paste. After drying, raise the temperature to 1300℃ at a rate of 5~10℃ / min and sinter at 1300℃. Then keep it at the temperature for 2~5 hours.

[0051] Comparative Example 1.

[0052] (1) The coating material in this embodiment is prepared according to the following steps:

[0053] Preparation process of alumina slurry:

[0054] Step 1: Prepare organic solvent: First, heat 1g PVA, 8g anhydrous ethanol and 0.4g cod liver oil at 90℃ and stir in a sealed container for 1 hour at a stirring speed of 600r / min.

[0055] Step 2: Prepare ceramic slurry: Add 10g of alumina ceramic powder to the solvent system in "Step 1" above, and grind it thoroughly for 8 hours using a planetary ball mill at 800r / min.

[0056] Step 3: Degassing treatment of slurry: Place the ground slurry in a stainless steel vacuum degassing tank with a vacuum degree of -0.095MPa for degassing treatment for 1 hour.

[0057] The parameters of the alumina ceramic raw material powder used are as follows:

[0058] The purity is 3N and the median particle size (D50) is 500nm.

[0059] (2) Apply the alumina paste to the chip head surface and dry it. Then, raise the temperature to 1300℃ at a rate of 5~10℃ / min and sinter at 1300℃. Then, keep it at the temperature for 2~5 hours.

[0060] Comparative Example 2.

[0061] In this embodiment, the coating material is prepared according to the following steps:

[0062] (1) Preparation process of alumina / magnesium aluminum spinel mixed slurry:

[0063] Step 1: Prepare organic solvent: First, heat 2g PVA, 16g anhydrous ethanol and 0.4g cod liver oil at 90℃ and stir in a sealed container for 1 hour at a stirring speed of 600r / min.

[0064] Step 2: Preparation of ceramic slurry: Add 10g of alumina and 10g of magnesium aluminum spinel powder to the solvent system in "Step 1" above, and grind thoroughly for 5 hours using a planetary ball mill at 600r / min.

[0065] Step 3: Degassing treatment of slurry: Place the ground slurry in a stainless steel vacuum degassing tank with a vacuum degree of -0.095MPa for degassing treatment for 2 hours.

[0066] The parameters of the alumina / magnesium aluminum spinel ceramic raw material powder used are as follows:

[0067] The alumina raw material powder used has a purity of 3N and a median particle size (D50) of 1μm;

[0068] The magnesium aluminum spinel raw material powder used has a purity of 3N and a median particle size (D50) of 10μm.

[0069] (2) Apply the alumina / magnesium aluminum spinel mixed slurry to the chip head surface, dry it, raise the temperature to 1300℃ at a rate of 5~10℃ / min, sinter at 1300℃, and then keep it at the temperature for 2~5h.

[0070] Comparative Example 3.

[0071] Coating materials are prepared using traditional wet mechanical crushing processes:

[0072] (1) Preparation process of alumina slurry:

[0073] Step 1: Prepare slurry: Use 6g of deionized water as solvent, add 10g of alumina powder and 0.3g of dispersant, and use a planetary ball mill at 400r / min for 30min for pre-dispersion.

[0074] Step 2: Add 2g of PVA to the slurry system from Step 1, and ball mill it using a planetary ball mill at a speed of 800r / min for 12h, and record the change in powder particle size over time.

[0075] Step 3: Degassing treatment of slurry: Place the ground slurry in a stainless steel vacuum degassing tank with a vacuum degree of -0.095MPa for degassing treatment for 2 hours.

[0076] The alumina raw material powder and PVA parameters used are as follows:

[0077] The alumina raw material powder used has a purity of 3N and a median particle size (D50) of 1μm.

[0078] The PVA used is water-soluble.

[0079] (2) Preparation process of alumina / magnesium aluminum spinel mixed slurry:

[0080] Step 1: Prepare ceramic slurry: Use 13g deionized water as solvent, add 10g alumina powder, 10g magnesium aluminum spinel powder and 0.6g dispersant, and use a planetary ball mill at 400r / min for 30min for pre-dispersion.

[0081] Step 2: Add 4g of PVA to the slurry system from Step 1 and ball mill it using a planetary ball mill at a speed of 800r / min for 12h.

[0082] Step 3: Degassing treatment of slurry: Place the ground slurry in a stainless steel vacuum degassing tank with a vacuum degree of -0.095MPa for degassing treatment for 2 hours.

[0083] The parameters of the alumina / magnesium aluminum spinel ceramic raw material powder used are as follows:

[0084] The alumina raw material powder used has a purity of 3N and a median particle size (D50) of 1μm;

[0085] The magnesium aluminum spinel raw material powder used has a purity of 3N and a median particle size (D50) of 10μm.

[0086] The PVA used is water-soluble.

[0087] (3) Apply the alumina paste to the chip head surface and dry it. Then apply the alumina / magnesium aluminum spinel mixed paste to the chip head surface with the alumina paste. After drying, raise the temperature to 1300℃ at a rate of 5~10℃ / min and sinter at 1300℃. Then keep it at the temperature for 2~5 hours.

[0088] Due to low grinding efficiency and poor slurry dispersion, cracks, peeling, and localized caking of the waterproof and heat-insulating coating surface can be observed after coating and sintering, making it impossible to assemble the finished product for subsequent testing.

[0089] Example of results.

[0090] The coated chip elements and uncoated chip components prepared in the above embodiments and comparative examples were subjected to performance tests, including high-flow-rate gas impact, cold and hot cycle start-up, thermal shock resistance and water droplet test, with a sample size of 16 pieces for each test.

[0091] When the nitrogen and oxygen sensor chip was operating at 840°C, it was subjected to repeated impacts 500 times with a high-flow-rate gas of 50 m / s. In Example 1, the coating and sensor element showed no damage or cracks and were able to work normally; however, the coatings or elements in each comparative example showed varying degrees of damage.

[0092] When the nitrogen and oxygen sensor chip was operated at 840°C and subjected to 10,000 cold and hot start-stop cycles, the coating and sensor element in Example 1 showed no damage or cracks and were able to work normally; while the coatings or elements in Comparative Examples 1 and 3 showed varying degrees of damage.

[0093] When the nitrogen and oxygen sensor chip was operated at 840°C and under vibration for 20,000 hours, the coating and sensor element in Example 1 showed no damage or cracks and were able to work normally; while the coatings or elements in each comparative example showed varying degrees of damage.

[0094] When the nitrogen and oxygen sensor chip was operating at 840°C, a water drop test was conducted at a rate of 1 mL / min. In Example 1, the coating and sensor element showed no damage or cracks and were able to function normally; however, in each comparative example, the coating or element showed varying degrees of damage.

[0095] The specific experimental results are shown in Table 1.

[0096] Table 1

[0097]

[0098] The above experimental results demonstrate that this invention first achieves a synergistic effect through a double-layer structure. The first layer, pure alumina, effectively blocks moisture penetration, and its dense structure slows heat loss but does not completely isolate heat conduction, thus preventing localized overheating. The second layer, magnesium-aluminum spinel, has a thermal expansion coefficient close to that of the zirconium oxide substrate of the sensor chip, reducing interfacial thermal stress. The mixture of micron-sized spinel and submicron-sized alumina forms a pore gradient structure, enhancing thermal insulation performance and absorbing thermal stress. Secondly, this invention optimizes the one-step sintering process, enabling the two layers to interdiffuse during the melting stage, forming chemical bonds and improving bonding strength. Furthermore, the heating rate and holding time are matched with the material sintering kinetics, reducing residual stress and preventing cracking. In addition, this invention innovates the dispersion process. Cod liver oil and PVA work synergistically, inhibiting the agglomeration of nano-alumina through electrostatic repulsion and steric hindrance, improving slurry uniformity. Combined with vacuum degassing, compared to traditional wet crushing, it possesses the advantages of a waterproof and heat-insulating slurry with uniform particle size distribution, eliminating porosity defects.

[0099] In summary, this invention achieves a balance between coating adhesion, thermal stability, and water resistance through a dual-layer structure design, optimized dispersion process, and one-step sintering activation method, providing an innovative solution for high-reliability sensor coatings.

[0100] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. An alumina waterproof and heat-insulating coating, characterized in that, The alumina waterproof and heat-insulating coating has a double-layer structure, with the first layer being a pure alumina coating. The second layer is a mixed coating of alumina and magnesium aluminum spinel, wherein the mass ratio of alumina to magnesium aluminum spinel is 1:1; The alumina waterproof and heat-insulating coating is applied to the substrate surface by dip coating and then formed by a one-step sintering process after drying. The preparation method of the alumina waterproof and heat-insulating coating includes the following steps: S1. Preparation of alumina slurry: PVA, anhydrous ethanol, and cod liver oil were sealed and stirred evenly at 90°C to serve as the first solvent. The first alumina ceramic powder is added to the first solvent and then thoroughly ground using a planetary ball mill. The ground slurry was placed in a stainless steel vacuum degassing tank for degassing treatment to obtain alumina slurry. Preparation of S2, alumina / magnesium aluminum spinel mixed slurry: PVA, anhydrous ethanol, and cod liver oil were sealed and stirred evenly at 90°C to serve as the second solvent. The second alumina ceramic powder and magnesium aluminum spinel powder are added to the second solvent and then thoroughly ground using a planetary ball mill. The ground slurry was placed in a stainless steel vacuum degassing tank for degassing treatment to obtain an alumina / magnesium aluminum spinel mixed slurry. S3. The alumina slurry is applied to the substrate surface and dried. Then, the alumina / magnesium aluminum spinel mixed slurry is further applied to the surface, dried, heated to 1300℃ for high-temperature sintering, and then kept at the temperature to obtain an alumina waterproof and heat-insulating coating. In step S1, the mass ratio of PVA, anhydrous ethanol, cod liver oil, and first alumina ceramic powder is 1:8:0.4:10; in step S2, the mass ratio of PVA, anhydrous ethanol, cod liver oil, second alumina ceramic powder, and magnesium aluminum spinel powder is 2:16:0.4:10:

10.

2. The alumina waterproof and heat-insulating coating according to claim 1, characterized in that, The thickness of the first layer is 0.1 mm, and the thickness of the second layer is 1.0 mm.

3. The method for preparing the alumina waterproof and heat-insulating coating according to claim 1, characterized in that, The purity of the first alumina ceramic powder is 3N, and the median particle size is 500nm; The purity of the second alumina ceramic powder is 3N, and the median particle size is 1µm; the purity of the magnesium aluminum spinel powder is 3N, and the median particle size is 10µm; the mass ratio of the second alumina ceramic powder to the magnesium aluminum spinel powder is 1:

1.

4. The method for preparing the alumina waterproof and heat-insulating coating according to claim 1, characterized in that, The stirring speed in both steps S1 and S2 is 600 r / min, and the stirring time is 1 h.

5. The method for preparing the alumina waterproof and heat-insulating coating according to claim 1, characterized in that, The parameters of the planetary ball mill in step S1 are 800 r / min and the grinding time is 8 h; the parameters of the planetary ball mill in step S2 are 600 r / min and the grinding time is 5 h.

6. The method for preparing the alumina waterproof and thermal insulation coating according to claim 1, characterized in that, The vacuum degree of the degassing treatment in step S1 is -0.095MPa, and the degassing treatment time is 1h; the vacuum degree of the degassing treatment in step S2 is -0.095MPa, and the degassing treatment time is 2h.

7. The method for preparing the alumina waterproof and thermal insulation coating according to claim 1, characterized in that, The heating rate in step S3 is 5~10℃ / min, and the holding time is 2~5h.

8. The application of an alumina waterproof and thermal insulation coating as described in any one of claims 1-7, characterized in that, Protection of zirconia ceramic chips used in nitrogen and oxygen gas sensors.

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