A method for preparing boron nitride aluminum nitride composite ceramic

Abstract

The present invention provides a method for preparing a boron nitride aluminum nitride composite ceramic, and relates to the technical field of ceramic composite material preparation. The method comprises dissolving aluminum isopropoxide in isopropanol, adding aluminum salt and citric acid aqueous solution, and adjusting the pH value to a suitable range to prepare a sol; then adding modified boron nitride to the sol, and ensuring its uniform distribution by ultrasonic dispersion treatment; then spray drying to obtain a precursor powder; then cold isostatic pressing the precursor powder to obtain a cold pressed blank; after pre-sintering in an inert atmosphere, transferring to ammonia for high-temperature nitriding reaction; finally, performing pressureless sintering in an inert atmosphere to finally obtain high-performance boron nitride aluminum nitride composite ceramic. The present invention achieves a significant improvement in material uniformity and density by optimizing the process flow without introducing additional sintering aids or complex treatments, and has the characteristics of low production cost and high preparation efficiency.

Description

Technical Field

[0001] The invention belongs to the technical field of ceramic composite material preparation, and particularly relates to a method for preparing boron nitride-aluminum nitride composite ceramic. Background Art

[0002] Boron nitride aluminum nitride (BN / AlN) composite ceramics have broad application prospects in electronic packaging, high-temperature sensors, thermal management devices, and other fields due to their excellent thermal conductivity, electrical insulation, chemical stability, and mechanical properties. However, traditional preparation methods often struggle to achieve uniform bonding of BN and AlN, resulting in unstable material properties. Currently, methods for preparing BN / AlN composite ceramics primarily include pressureless sintering and hot pressing. While these methods can produce composite ceramics with certain properties, they are insufficient in improving the strength of the composite ceramics. Specifically, pressureless sintering is simple to operate, but it is difficult to control the uniform distribution of BN and AlN, which can easily lead to localized enrichment and unstable material properties. Furthermore, pressureless sintering typically requires higher sintering temperatures, which not only increases energy consumption but can also cause structural changes at high temperatures, thereby affecting the performance of the final product. For example, during high-temperature sintering, larger pores may form within the material, resulting in a decrease in density, which in turn affects thermal conductivity and mechanical properties. Hot pressing sintering, by sintering at high temperatures and high pressures, can effectively reduce internal pores and improve density. However, this method requires specialized hot pressing equipment, which increases production costs. In addition, the hot pressing sintering method has a complex operation process and requires high control of process parameters, making it difficult to achieve large-scale production.

[0003] Researchers have tried various methods, such as introducing sintering aids and using chemical vapor deposition (CVD) treatment. Although these methods have improved material properties to a certain extent, they also pose some new problems, such as the introduction of impurities and complex processes. In particular, for the key issue of improving its strength, existing technologies have not yet found an effective solution. The strength of composite ceramics is one of the key factors that determines whether they can withstand complex working conditions. Therefore, developing a preparation method that can effectively improve the strength of composite ceramics has become a current problem that needs to be solved urgently. Summary of the Invention

[0004] This invention addresses the challenges of the prior art by providing a method for preparing boron nitride-aluminum nitride composite ceramics. This method addresses the existing problems of excessive porosity, low density, and poor uniformity, while also improving the mechanical strength of the composite ceramics. The present invention features a simple process flow, eliminating the need for additional sintering aids, complex CVD treatment, or hot-pressing sintering, significantly reducing production costs and improving preparation efficiency.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] A method for preparing a boron nitride-aluminum nitride composite ceramic comprises the following steps:

[0007] (1) dissolving aluminum isopropoxide in isopropanol, adding aluminum salt and citric acid aqueous solution, stirring for a certain period of time, and adjusting the pH value to 4-6 to obtain a sol;

[0008] (2) adding modified boron nitride to the sol prepared in step (1) and performing ultrasonic dispersion treatment to ensure that the modified boron nitride is uniformly mixed in the sol;

[0009] (3) spray drying the uniformly mixed sol in step (2) to obtain a precursor powder;

[0010] (4) placing the precursor powder into a mold and performing cold isostatic pressing to obtain a cold pressed blank;

[0011] (5) placing the cold pressed blank in an inert atmosphere for pre-sintering at a temperature of 400-800°C;

[0012] (6) After the pre-sintering is completed, the green body is transferred to a reactor in an ammonia atmosphere for high-temperature nitriding reaction at a temperature of 1000-1200°C;

[0013] (7) After the high-temperature nitriding reaction is completed, the green body is placed in an inert atmosphere for pressureless sintering at a temperature of 1600-1850°C to obtain boron nitride aluminum nitride composite ceramics.

[0014] Furthermore, in the method, the molar ratio of aluminum isopropoxide, aluminum salt, and modified boron nitride is 1:0.3-0.5:4-6.

[0015] Furthermore, the aluminum salt is any one or more of aluminum nitrate, aluminum chloride, and aluminum sulfate, preferably aluminum nitrate.

[0016] Furthermore, in step (1), the mass volume ratio of aluminum isopropoxide to isopropanol is 1 g:8-15 mL.

[0017] Furthermore, after adding aluminum salt in step (1), stirring for 30 minutes to 1 hour, citric acid aqueous solution is added.

[0018] Furthermore, the concentration of the citric acid aqueous solution in step (1) is 0.1 to 0.3 M, the amount of citric acid added is 0.1 to 0.3 times the molar amount of aluminum isopropoxide, and the mixture is stirred for 30 min to 1 h after the addition of the citric acid aqueous solution.

[0019] Furthermore, the preparation method of the modified boron nitride in step (2) is as follows:

[0020] (i) dissolving a silane coupling agent in water, stirring for 10 to 60 minutes, adding boron nitride powder, continuing to stir for 0.5 to 2 hours, filtering with suction, drying the filter cake, sieving, and obtaining silane coupling agent-modified boron nitride for later use;

[0021] The mass ratio of boron nitride powder to water is 1:6-15, and the mass of the silane coupling agent is 2-5% of the mass of the boron nitride powder;

[0022] (ii) dissolving ethanolamine in water at 60° C. to 90° C. in a mass ratio of 1:6 to 15 to obtain solution 1;

[0023] (iii) adding 0.8 to 1 times the mass of ethanolamine to solution 1, followed by silane coupling agent-modified boron nitride, stirring for 15 to 24 hours, filtering, and drying;

[0024] (iv) crushing and sieving the dried material to obtain the modified boron nitride;

[0025] Furthermore, the silane coupling agent is one or more of polydimethylsiloxane, vinyltrimethoxysilane, γ-aminopropyltriethoxysilane, n-octyltriethoxysilane, triethoxyoctylsilane, 3-mercaptopropyltrimethoxysilane, 3-methacryloxypropyltrimethoxysilane, and 3-glycidoxypropyltrimethoxysilane.

[0026] Furthermore, the feed rate of the spray drying in step (3) is 10-20 mL / min, the air inlet temperature is 160-200° C., the air outlet temperature is 60-100° C., and the spray pressure is 0.05-0.1 MPa.

[0027] Furthermore, the pressure of the cold isostatic pressing in step (4) is 110 to 200 MPa.

[0028] Furthermore, the heating rate of the pre-sintering in step (5) is 4-6°C / min, and the holding time is 2-3h.

[0029] Furthermore, the heating rate of the high-temperature nitriding reaction in step (6) is 4-6°C / min, and the holding time is 2-3h.

[0030] Furthermore, the pressureless sintering time in step (7) is 2 to 4 hours.

[0031] The present invention also provides boron nitride-aluminum nitride composite ceramics prepared by the method.

[0032] The present invention uses aluminum isopropoxide and aluminum salt as starting materials to generate aluminum nitride in situ, which helps to evenly distribute the aluminum nitride phase in the final product and improves the uniformity and density of the material. Aluminum isopropoxide and aluminum salt are used in combination as aluminum sources, mainly to provide sufficient aluminum elements during the preparation process, and to adjust the microstructure and final performance of the material through the synergistic effect of two different forms of aluminum sources. When aluminum isopropoxide and aluminum salt coexist, the interaction between them can promote the uniform dispersion of aluminum ions, prevent uneven precipitation caused by local oversaturation, and thus improve the uniformity and density of the final material.

[0033] The present invention uses citric acid as a catalyst. Citric acid can form a stable chelate with aluminum ions, thereby preventing the aluminum ions from precipitating too quickly during the hydrolysis process and ensuring the uniform dispersion of the aluminum ions in the sol, thereby reducing the pores in the material and improving the density.

[0034] The modified boron nitride used in the present invention is first surface-modified using a silane coupling agent. Because boron nitride has very few surface functional groups and lacks sufficient hydroxyl or amino binding sites, the chemical activity of the boron nitride is low. Therefore, the surface of the boron nitride is chemically modified and grafted with a surface coupling agent using a silane coupling agent to enhance the group density on the surface of the boron nitride, thereby providing the boron nitride with better dispersibility in the matrix and space for further reaction with other substances. After modification with the silane coupling agent, the boron nitride is reacted with ethanolamine. The esterification reaction and intramolecular coordination (N|→B) between the ethanolamine and the boron nitride containing hydroxyl groups on the surface form abundant >BO-H2C-H2C-H2N|→B< bond bridges between the boron nitride sheets, which can form thermodynamically stable >BNB< covalent bonds during calcination. The high bond energy of the covalent bonds can macroscopically improve the thermal stability and strength of the ceramic. Therefore, the modified boron nitride of the present invention can significantly improve the thermal stability and strength of ceramic materials.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] 1. The process is simple and does not require the introduction of additional sintering aids or complex CVD treatment.

[0037] 2. No need for hot pressing and sintering, which greatly reduces production costs and improves preparation efficiency.

[0038] 3. Improve the uniformity and density of the material and significantly improve the strength of the material. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is the SEM image of the cross section of the boron nitride aluminum nitride composite ceramic prepared in Example 1.

[0040] Figure 2Compressive and flexural strength test curves of the boron nitride aluminum nitride composite ceramic prepared in Example 1: a) compressive strength; b) flexural strength.

[0041] Figure 3 Compressive and flexural strength test curves of the boron nitride aluminum nitride composite ceramic prepared in Comparative Example 1: a) compressive strength; b) flexural strength.

[0042] Figure 4 Compressive and flexural strength test curves of the boron nitride aluminum nitride composite ceramic prepared in Comparative Example 2: a) compressive strength; b) flexural strength.

[0043] Figure 5 Compressive and flexural strength test curves of the boron nitride aluminum nitride composite ceramic prepared in Comparative Example 3: a) compressive strength; b) flexural strength. DETAILED DESCRIPTION

[0044] The technical solutions in the embodiments of the present invention will be described clearly and completely below. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. It is worth noting that the methods used in the present invention are all conventional methods unless otherwise specified; the raw materials and devices used are all conventional commercially available products unless otherwise specified, and their sources are not specifically limited.

[0045] Example 1

[0046] This embodiment provides a boron nitride aluminum nitride composite ceramic, the preparation method of which includes the following steps:

[0047] (1) Dissolve 50 g of aluminum isopropoxide in 500 mL of isopropanol, add 20 g of aluminum nitrate, stir for 40 min, add 700 mL of 0.1 M citric acid aqueous solution, stir for 40 min, and adjust the pH to about 5 to obtain a sol;

[0048] (2) adding 30 g of modified boron nitride to the sol prepared in step (1) and performing ultrasonic dispersion treatment to ensure that the modified boron nitride is evenly mixed in the sol;

[0049] The preparation method of the modified boron nitride is as follows:

[0050] (i) dissolving polydimethylsiloxane in water, stirring for 30 minutes, adding boron nitride powder, continuing to stir for 1 hour, filtering with suction, drying the filter cake in a 70°C oven, and filtering through a 100-mesh sieve to obtain silane coupling agent-modified boron nitride for later use;

[0051] The mass ratio of boron nitride powder to water is 1:10, and the mass of polydimethylsiloxane is 3% of the mass of boron nitride powder;

[0052] (ii) dissolving ethanolamine in 90° C. water at a mass ratio of 1:10 to obtain solution 1;

[0053] (iii) adding boron nitride modified with a silane coupling agent in an amount equal to 1 times the mass of ethanolamine to solution 1, stirring for 20 h, filtering, and drying;

[0054] (iv) grinding the dried material and passing it through a 100-mesh sieve to obtain the modified boron nitride;

[0055] (3) spray drying the uniformly mixed sol in step (2) at a feed rate of 14 mL / min, an air inlet temperature of 180° C., an air outlet temperature of 80° C., and a spray pressure of 0.06 MPa to obtain a precursor powder;

[0056] (4) placing the precursor powder into a mold and performing cold isostatic pressing at 120 MPa to obtain a cold pressed blank;

[0057] (5) The cold pressed blank was placed in an argon atmosphere and heated to 650°C at a heating rate of 5°C / min for pre-sintering, with a holding time of 2.5 h;

[0058] (6) After the pre-sintering is completed, the green body is transferred to a reactor in an ammonia atmosphere and heated to 1100°C at a heating rate of 5°C / min for high-temperature nitriding reaction, and the holding time is 2h;

[0059] (7) After the high-temperature nitriding reaction is completed, the green body is placed in an argon atmosphere for pressureless sintering at a temperature of 1700°C for 3 hours to obtain a boron nitride aluminum nitride composite ceramic.

[0060] The cross-sectional SEM image of the boron nitride aluminum nitride composite ceramic prepared in Example 1 is as follows: Figure 1 As shown in the figure, it can be seen that the flake-like boron nitride and aluminum nitride particles are evenly distributed inside the material, without obvious aggregation or segregation; they have regular shapes, indicating that good crystallization effect has been achieved during the preparation process; the two are crisscrossed to build a stable network structure. This unique microstructure significantly improves the mechanical properties of the material.

[0061] Comparative Example 1

[0062] This comparative example provides a boron nitride aluminum nitride composite ceramic, which differs from Example 1 in that boron nitride powder that has not been modified with a silane coupling agent and ethanolamine is used, and the remaining steps are the same as those in Example 1.

[0063] Comparative Example 2

[0064] This comparative example provides a boron nitride aluminum nitride composite ceramic, which differs from Example 1 in that citric acid is replaced by hydrochloric acid, and the remaining steps are the same as those in Example 1.

[0065] Comparative Example 3

[0066] This comparative example provides a boron nitride-aluminum nitride composite ceramic, which differs from Example 1 in that the preparation method of the modified boron nitride is as follows:

[0067] (i) dissolving ethanolamine in 90° C. water at a mass ratio of 1:10 to obtain solution 1;

[0068] (ii) adding boron nitride powder (1 times the mass of ethanolamine) to solution 1, stirring for 20 h, filtering, and drying;

[0069] (iii) grinding the dried material and passing it through a 100-mesh sieve to obtain the modified boron nitride;

[0070] The remaining steps are the same as those in Example 1.

[0071] Performance Testing

[0072] The compressive strength and flexural strength of the boron nitride aluminum nitride composite ceramics prepared in Example 1 and Comparative Examples 1-3 were tested. The compressive strength was tested according to GB / T 8489-2006, and the flexural strength was tested according to GB / T 6569-2006. The test samples were prepared according to the corresponding national standards, and the sample test curves were as follows: Figure 2-5 As shown, the flexural strength and compressive strength of the samples were calculated according to the method specified in the national standard, and the results are shown in Table 1.

[0073] Table 1:

[0074]

[0075]

[0076] The test results of Example 1 and Comparative Examples 1-3 show that the composite ceramic prepared by Example 1 through the optimized process has higher mechanical strength. Comparative Example 1 uses unmodified boron nitride. On the one hand, this results in the inability of boron nitride to be well mixed in the sol, causing powder agglomeration; on the other hand, it makes it impossible to form good >BNB< covalent bonds between boron nitride sheets, ultimately resulting in a decrease in the strength of the ceramic. Comparative Example 2 uses hydrochloric acid as a catalyst. Hydrochloric acid does not have a chelating effect similar to citric acid and cannot effectively promote the uniform dispersion of aluminum ions. Aluminum ions tend to precipitate too quickly during the hydrolysis process, resulting in more pores inside the material and reduced density. Comparative Example 3 uses boron nitride that has not been surface-modified with a silane coupling agent, resulting in a lack of active groups on its surface, making it easy to agglomerate in the sol, and having weak interfacial bonding with other components in the sol (such as aluminum salts, citric acid, etc.), resulting in unevenness inside the material, increased porosity, and decreased mechanical properties. In addition, weak interfacial bonding can lead to defects at the interface, which become the origin of cracks, thereby reducing the flexural strength and compressive strength of the material. During high-temperature sintering, areas with weak interfacial bonding are also prone to peeling, further reducing the mechanical properties of the material.

[0077] Finally, it should be noted that the above content is only used to illustrate the technical solution of the present invention, rather than to limit the scope of protection of the present invention. Simple modifications or equivalent substitutions of the technical solution of the present invention by ordinary technicians in this field do not deviate from the essence and scope of the technical solution of the present invention.

Claims

1. A method for preparing a boron nitride-aluminum nitride composite ceramic, comprising the following steps: (1) Dissolve aluminum isopropoxide in isopropanol, add aluminum salt and citric acid aqueous solution, stir for a certain period of time, and adjust the pH value to 4-6 to obtain a sol; (2) adding modified boron nitride to the sol prepared in step (1) and performing ultrasonic dispersion treatment to ensure that the modified boron nitride is evenly mixed in the sol; (3) spray drying the uniformly mixed sol in step (2) to obtain a precursor powder; (4) placing the precursor powder into a mold and performing cold isostatic pressing to obtain a cold pressed blank; (5) Pre-sintering the cold pressed blank in an inert atmosphere at a temperature of 400-800°C; (6) After pre-sintering, the green body is transferred to a reactor in an ammonia atmosphere for high-temperature nitriding reaction at a temperature of 1000~1200℃; (7) After the high-temperature nitriding reaction is completed, the green body is placed in an inert atmosphere for pressureless sintering at a temperature of 1600~1850℃ to obtain boron nitride aluminum nitride composite ceramics.

2. The preparation method according to claim 1, wherein: In the method, the molar ratio of aluminum isopropoxide, aluminum salt, and modified boron nitride is 1:0.3-0.5:4-6; and the aluminum salt is any one or more of aluminum nitrate, aluminum chloride, and aluminum sulfate.

3. The preparation method according to claim 1, wherein: In step (1), the mass volume ratio of aluminum isopropoxide to isopropanol is 1 g:8~15 mL. After adding the aluminum salt, stir for 30 min~1 h and then add the citric acid aqueous solution.

4. The preparation method according to claim 1, wherein: The concentration of the citric acid aqueous solution in step (1) is 0.1-0.3 M, the amount of citric acid added is 0.1-0.3 times the molar amount of aluminum isopropoxide, and the mixture is stirred for 30 min-1 h after the addition of the citric acid aqueous solution.

5. The preparation method according to claim 1, wherein: The preparation method of the modified boron nitride in step (2) is as follows: (i) dissolving a silane coupling agent in water, stirring for 10 to 60 minutes, adding boron nitride powder, continuing to stir for 0.5 to 2 hours, filtering, drying the filter cake, sieving, and obtaining silane coupling agent-modified boron nitride for later use; The mass ratio of boron nitride powder to water is 1:6~15, and the mass of silane coupling agent is 2~5% of the mass of boron nitride powder; (ii) dissolving ethanolamine in water at 60° C. to 90° C. in a mass ratio of 1:6 to 15 to obtain solution 1; (iii) adding 0.8 to 1 times the mass of ethanolamine to solution 1, followed by silane coupling agent-modified boron nitride, stirring for 15 to 24 hours, filtering, and drying; (iv) crushing and sieving the dried material to obtain the modified boron nitride.

6. The preparation method according to claim 5, characterized in that: The silane coupling agent is one or more of polydimethylsiloxane, vinyltrimethoxysilane, γ-aminopropyltriethoxysilane, n-octyltriethoxysilane, triethoxyoctylsilane, 3-mercaptopropyltrimethoxysilane, 3-methacryloxypropyltrimethoxysilane, and 3-glycidoxypropyltrimethoxysilane.

7. The preparation method according to claim 1, wherein: The feed rate of the spray drying in step (3) is 10~20 mL / min, the air inlet temperature is 160~200℃, the air outlet temperature is 60~100℃, and the spray pressure is 0.05~0.1 MPa.

8. The preparation method according to claim 1, wherein: The heating rate of the pre-sintering in step (5) is 4-6°C / min, and the holding time is 2-3h.

9. The preparation method according to claim 1, wherein: The heating rate of the high-temperature nitriding reaction in step (6) is 4-6°C / min, the holding time is 2-3h, and the time of the pressureless sintering in step (7) is 2-4h.

10. Boron nitride aluminum nitride composite ceramic prepared by the method according to any one of claims 1 to 9.

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