A method for preparing AlN porous ceramics with honeycomb oriented structure

Abstract

The present invention discloses a method for preparing AlN porous ceramics with a honeycomb oriented structure, which belongs to the field of ceramic material preparation. The specific steps are: mixing aluminum oxide powder, a sintering aid, ammonium polyacrylate and deionized water and ball milling to prepare a slurry, adding a polypropylene alcohol aqueous solution as a binder and carbon fiber powder as a carbon source to the ball-milled slurry, performing mechanical stirring and vacuum degassing, and then freeze-drying the slurry, and placing the dried block into an atmosphere sintering furnace for sintering, and finally removing carbon in an air atmosphere to obtain a honeycomb aluminum nitride (AlN) ceramic with an oriented structure. This method has the advantages of low raw material cost and simple process, and the prepared AlN porous ceramic has a clear honeycomb structure, high purity, large porosity, good compressive resistance, high thermal conductivity, and low cost. It can be used as a heat conduction skeleton in phase change materials or heat-conducting composite materials, and has broad application prospects in the fields of aviation, environmental protection, communications and electricity.

Description

Technical Field

[0001] This invention belongs to the field of ceramic materials technology, specifically relating to a method for preparing AlN porous ceramics with a honeycomb oriented structure. Background Technology

[0002] AlN possesses advantages such as high thermal conductivity, good electrical insulation, low dielectric constant, and thermal expansion matching with silicon, making it a ceramic material with excellent comprehensive performance and one of the more ideal substrate materials and electronic device packaging materials. Porous AlN ceramics combine the excellent properties of AlN and porous ceramics, and also feature low density and high porosity. When the pore structure in the ceramic is controlled to achieve anisotropic arrangement, thermally conductive pathways can be formed along the channel direction, giving AlN porous ceramics directional high thermal conductivity. It can be used as a thermally conductive framework in thermally conductive composite materials and phase change materials, significantly improving the thermal and mechanical properties of composite materials, and has broad application prospects in aerospace, environmental protection, communications, and power industries.

[0003] The invention patent "Preparation Method of Porous Aluminum Nitride Ceramic Material" (Publication No.: CN 105503236A) obtains active aluminosilicate by calcining aluminosilicate raw materials, then mixes it with carbon to obtain a ball-milled mixture, adds alkali metal to silica sol, and obtains an alkali-activated solution by magnetic stirring. Next, the ball-milled mixture and the alkali-activated solution are mixed to obtain an inorganic polymer mixture, which is then cured and sintered to finally obtain AlN porous ceramic material. The AlN porous ceramic material prepared by this method does not have a directional structure, and the process is complex and has a long cycle.

[0004] The invention patent "A method for rapidly preparing directional porous aluminum nitride honeycomb ceramics by freeze-drying and combustion synthesis" (publication number: CN111056846A) uses Al powder and AlN powder as raw materials to prepare directional porous AlN ceramics by pressureless rapid sintering in a spark plasma sintering furnace. Although the process is simple and the cycle is short, Al powder is flammable, and fine powder can easily form flammable and explosive mixtures with air, posing a safety hazard. In addition, AlN powder is used as a raw material, which is not only expensive, but also easily hydrolyzed, so only organic solvents can be used in the freeze-drying process, making it difficult to achieve industrialization.

[0005] Therefore, it is of great significance to directly synthesize porous AlN frameworks with oriented structures using inexpensive raw materials and simple preparation processes. Summary of the Invention

[0006] This invention addresses the problems of existing methods for preparing honeycomb-structured AlN porous ceramics by proposing an innovative in-situ reaction strategy. Using low-cost Al₂O₃ and carbon fibers as raw materials, AlN porous ceramics with a directional honeycomb structure are directly prepared through freeze-drying combined with carbothermal reduction technology. Carbothermal reduction nitridation is a commonly used method for preparing AlN, and its overall reaction formula is as follows:

[0007] Al2O3(s)+3C(s)+N2(g)→2AlN(s)+3CO(g) (1)

[0008] This method is generally used to synthesize AlN powder, and there are few reports of using this method to directly synthesize AlN ceramics. This is mainly because the carbothermic reduction method is a reaction in which the solid content decreases before and after the reaction. In the initial green body, Al2O3 and C particles are uniformly mixed. After the carbothermic reduction reaction, Al2O3 is transformed into AlN in situ, but C is gradually consumed. Therefore, the AlN particles cannot be tightly connected, so it is difficult for the green body to maintain its strength after the reaction, that is, it is difficult to obtain a stable AlN skeleton.

[0009] This invention can directly synthesize honeycomb AlN porous ceramics with high mechanical strength by freeze drying combined with carbothermal reduction. The key points are mainly three: (1) The carbon source used is carbon fiber. Carbon fiber is a carbon source with a high aspect ratio. Al2O3 in the green body can be fully loaded on the surface of carbon fiber, making the Al2O3 particles more tightly connected. Therefore, the AlN particles generated after in-situ reaction are also tightly connected to form a network. Even after C is consumed and eliminated, the tight connection between AlN particles can provide sufficient strength to form AlN porous ceramics; (2) Sintering aids are added to the raw materials. It can react with Al2O3 to generate a low-melting-point liquid phase. In the carbothermic reduction reaction, the liquid phase dissolution-precipitation mechanism can be used to further promote the sintering and densification of AlN particles, and further improve the strength of AlN porous ceramics; (3) The present invention uses freeze-drying to construct an Al2O3 / carbon fiber skeleton with a directional arrangement structure. During the freezing process, the ice crystals growing along the temperature gradient direction can effectively compress and promote the Al2O3 particles to arrange in a single direction, which not only provides a directional internal structure, but also further promotes the close contact between Al2O3 particles, thereby obtaining a more dense porous AlN ceramic.

[0010] Specifically, this invention provides a method for preparing AlN porous ceramics with a honeycomb-like oriented structure, comprising the following steps:

[0011] (1) Mixing: Alumina powder, sintering aid, ammonium polyacrylate and deionized water are mixed and ball-milled to prepare a slurry. Polyacryl alcohol aqueous solution and carbon fiber powder are added to the ball-milled slurry. After mechanical stirring and vacuum degassing, a mixed slurry is obtained.

[0012] (2) Directional freezing and freeze drying: The slurry obtained in (1) is transferred to a mold and placed in a low-temperature ethanol bath for directional freezing treatment. Then the frozen sample is dried in a freeze dryer.

[0013] (3) Sintering: The product obtained in (2) is transferred to a graphite crucible and placed in an atmosphere sintering furnace. Sintering is carried out under a nitrogen atmosphere, and then the product is cooled naturally.

[0014] (4) Decarbonization: The product obtained in (3) is placed in a muffle furnace and heated to 600-800℃ and held for 1-5 hours to remove excess carbon, and finally AlN porous ceramics with honeycomb structure are obtained.

[0015] Further, in step (1), the alumina is α-Al2O3 or γ-Al2O3 with an average particle size of 0.1 to 2 μm; the carbon fiber powder has a mesh size of 100 to 2000 and an aspect ratio of 5:1 to 20:1; the weight ratio of Al2O3 to carbon fiber powder is 1:1 to 3:1; and the amount of deionized water added is 0.6 to 4 times the total mass of alumina powder and carbon powder.

[0016] Furthermore, the sintering aid is selected from one or more of Y2O3, SiO2, and CaF2, and the addition amount is 0.5% to 5% of the mass of alumina powder; ammonium polyacrylate is used as a dispersant, and the addition amount is 0.05% to 0.5% of the mass of alumina powder; polyacrylol is used as a binder, and the addition amount is 0.1% to 1% of the mass of alumina powder.

[0017] Further, in step (1), the ball milling time is 6 to 24 hours, the rotation speed is 200 to 400 rpm, the mechanical stirring time is 20 to 80 minutes, and the vacuum degassing is carried out in a vacuum degassing machine for 20 to 80 minutes.

[0018] Furthermore, in step (2), a mold with a copper sheet at the bottom and polytetrafluoroethylene around the edges is used. This type of mold has a freezing temperature gradient and its orientation effect is better than that of conventional molds.

[0019] Furthermore, in step (2), the directional freezing temperature is -50 to -100°C, the freeze-drying temperature is -100 to -20°C, and the drying time is 12 to 48 hours.

[0020] Furthermore, in step (3), the temperature is first heated to 400-1000℃ at 5-20℃ / min and held for 0.5-3 hours, and then heated to 1750-1900℃ at 5-40℃ / min and held for 1-5 hours.

[0021] The present invention also provides AlN porous ceramics with honeycomb oriented structures prepared by the method.

[0022] The innovative idea of ​​this invention is to directly synthesize AlN porous ceramics with a directional honeycomb structure using low-cost Al2O3 and carbon fiber as raw materials through freeze drying combined with carbothermal reduction technology. The mechanical strength of AlN porous ceramics is improved by the combined action of carbon fiber, sintering aid and ice template method.

[0023] Compared with the prior art, the present invention has the following advantages:

[0024] 1. This invention uses alumina powder and carbon fiber powder as raw materials and deionized water as solvent. The reagents used are safe and non-toxic, and the cost is low. This not only simplifies the preparation process and reduces production costs, but also completely eliminates the hydrolysis problem of AlN, making it suitable for industrial production.

[0025] 2. This invention utilizes freeze-drying technology, which, compared with the traditional foaming method for preparing porous ceramics, allows the formation of an oriented structure along the temperature gradient inside the green body. It also has the advantages of uniform and controllable pore structure, and higher longitudinal thermal conductivity and mechanical strength compared with traditional AlN ceramics.

[0026] 3. The AlN porous ceramic with a honeycomb structure prepared by this invention has excellent comprehensive properties such as high compressive strength, high thermal conductivity, and low dielectric constant. It can be used as a heat conduction framework in phase change materials, thermal interface materials, etc., and has broad application prospects in aerospace, environmental protection, communication and power fields. Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 This is the X-ray diffraction (XRD) pattern of the product obtained in Example 1 of the present invention.

[0029] Figure 2 is a scanning electron microscope (SEM) image of the product obtained in Example 1 of the present invention, wherein Figure 2(a) is a top view and Figure 2(b) is a side view.

[0030] Figure 3 The macroscopic morphology of the sintered samples of Examples 1-3 and Comparative Example 1 of the present invention are shown from left to right as: Example 1, Example 2, Example 3 and Comparative Example 1.

[0031] Figure 4 It refers to the compressive strength of the AlN porous ceramics obtained in Examples 1-3 and Comparative Examples 2-3 of this invention.

[0032] Figure 5 The thermal conductivity is that of the AlN porous ceramics obtained in Examples 1-3 and Comparative Examples 2-3 of this invention. Detailed Implementation

[0033] To make the objectives, technical solutions, and beneficial effects of this invention clearer, the invention will be described in detail below with reference to specific embodiments. It should be understood that the embodiments described in this specification are merely illustrative and not intended to limit the scope of the invention.

[0034] Example 1

[0035] 27.08 g of α-Al₂O₃ powder (average particle size ~0.5 μm), 1.17 g of SiO₂ powder, 0.11 g of ammonium polyacrylate, and 58.86 g of deionized water were placed in a sealed ball mill jar and ball-milled for 12 hours at 300 rpm. Then, 10.83 g of 400 mesh carbon fiber powder with an aspect ratio of 7:1 and 1.14 g of a 5.00% polyacrylamide aqueous solution were added. The mixture was mechanically stirred for 30 minutes, followed by stirring in a vacuum defoamer for 30 minutes to obtain a slurry. The slurry was transferred to a polytetrafluoroethylene mold with copper tape attached to the bottom and frozen at -80°C. After drying in a freeze dryer for 24 hours, a green body was obtained. The green body is placed in an atmosphere sintering furnace and heated to 500°C at 5°C / min under a nitrogen atmosphere, held for 1 hour, then heated to 1800°C at 20°C / min, held for 2 hours, then cooled naturally, and finally placed in a muffle furnace and heated to 700°C for 3 hours to remove residual carbon, thus obtaining AlN porous ceramics with an oriented honeycomb structure.

[0036] Example 2

[0037] 37.9 g of α-Al₂O₃ powder (average particle size ~0.2 μm), 0.38 g of Y₂O₃ powder, 0.04 g of ammonium polyacrylate, and 40 g of deionized water were placed in a sealed ball mill jar and ball-milled for 6 hours at 400 rpm. Then, 37.9 g of 1000 mesh carbon fiber powder with an aspect ratio of 10:1 and 2.0 g of a 5.00% polyacrylol aqueous solution were added. The mixture was first mechanically stirred for 20 minutes, and then stirred in a vacuum defoamer for 80 minutes to obtain a slurry. The slurry was transferred to a polytetrafluoroethylene mold with copper tape attached to the bottom and frozen at -50°C. After drying in a freeze dryer for 12 hours, a green body was obtained. The green body is placed in an atmosphere sintering furnace and heated to 400°C at 5°C / min under a nitrogen atmosphere, held for 3 hours, then heated to 1750°C at 40°C / min, held for 5 hours, and then naturally cooled to obtain AlN porous ceramic with a honeycomb structure. Finally, it is placed in a muffle furnace and heated to 600°C and held for 5 hours to remove excess carbon, resulting in AlN porous ceramic with an oriented honeycomb structure.

[0038] Example 3

[0039] 13.50 g of γ-Al₂O₃ powder (average particle size ~1 μm), 0.20 g of CaF₂ powder, 0.06 g of ammonium polyacrylate, and 70 g of deionized water were placed in a sealed ball mill jar and ball-milled for 24 hours at 200 rpm. Then, 4.50 g of 2000 mesh carbon fiber powder with an aspect ratio of 15:1 and 1.80 g of a 5.00% polyacrylamide aqueous solution were added. The mixture was first mechanically stirred for 80 minutes, and then stirred in a vacuum defoamer for 20 minutes to obtain a slurry. The slurry was transferred to a polytetrafluoroethylene mold with copper tape attached to the bottom and frozen at -100°C. After drying in a freeze dryer for 48 hours, a green body was obtained. The green body is placed in an atmosphere sintering furnace and heated to 1000°C at 20°C / min under a nitrogen atmosphere, held for 0.5 hours, then heated to 1900°C at 5°C / min for 1 hour, and then cooled naturally. Finally, it is placed in a muffle furnace and heated to 800°C for 1 hour to remove excess carbon, thus obtaining AlN porous ceramics with an oriented honeycomb structure.

[0040] Comparative Example 1

[0041] It is basically the same as Example 1, except that carbon fiber powder is not used in the raw materials, but activated carbon powder with an average particle size of about 500 nm.

[0042] Comparative Example 2

[0043] The process is basically the same as in Example 1, except that after the initial slurry is poured into the mold, it is not subjected to directional freezing and freeze-drying steps, but is directly placed in an oven at 80°C for 12 hours to dry, and then sintered.

[0044] Comparative Example 3

[0045] It is basically the same as Example 1, except that the sintering aid SiO2 was not added to the raw materials.

[0046] The following are performance tests conducted on samples obtained from embodiments and comparative examples of the present invention.

[0047] Figure 1 The X-ray diffraction (XRD) pattern of the product obtained in Example 1 of this invention shows that the product is mainly AlN, with a small amount of SiC peaks generated by sintering aids, and no other impurity peaks.

[0048] Figure 2 is a scanning electron microscope (SEM) image of the product obtained in Example 1 of the present invention. Figure 2(a) is a top view of the porous AlN ceramic, in which the honeycomb structure is clearly visible. Figure 2(b) is a side view, in which the oriented porous structure inside the AlN ceramic is visible.

[0049] Figure 3 The images show the macroscopic morphology of the sintered samples from Examples 1-3 and Comparative Example 1 of this invention, from left to right: Example 1, Example 2, Example 3, and Comparative Example 1. Figure 3 It can be seen that the present invention can directly obtain the porous ceramic framework of AlN by using freeze drying combined with carbothermal reduction. However, when using nano-activated carbon powder as raw material, the sample will break after high-temperature treatment and has no mechanical strength.

[0050] Figure 4 The compressive strength of the AlN porous ceramics obtained in Examples 1-3 and Comparative Examples 2-3 of this invention is shown. The compressive strengths of Examples 1, 2, and 3 are 4.52 MPa, 5.62 MPa, and 2.30 MPa, respectively. The compressive strength is closely related to the solid content of Al2O3 slurry in the initial slurry. Generally, the higher the solid content of Al2O3 slurry, the higher the compressive strength. The compressive strengths of Comparative Examples 2 and 3 are 1.10 MPa and 0.92 MPa, respectively, indicating that conventional drying and the absence of additives will lead to a significant decrease in the mechanical strength of AlN porous ceramics.

[0051] Figure 5 The thermal conductivity of the AlN porous ceramics obtained in Examples 1-3 of this invention is shown. The thermal conductivity of Examples 1, 2, and 3 is 0.72 W / mK, 0.93 W / mK, and 0.35 W / mK, respectively. The thermal conductivity is closely related to the solid content of Al2O3 slurry in the initial slurry. Generally, the higher the solid content of Al2O3 slurry, the higher the thermal conductivity. The thermal conductivity of Comparative Examples 2 and 3 is 0.25 W / mK and 0.22 W / mK, respectively, indicating that conventional drying and the absence of additives will lead to a significant decrease in the thermal conductivity of AlN porous ceramics.

[0052] In summary, this invention uses alumina and carbon fiber as raw materials, deionized water as a solvent, and adds dispersants, sintering aids, and binders to prepare a uniformly mixed slurry. After freeze-drying, a porous preform with an oriented structure is prepared. Subsequently, the preform is subjected to a carbothermic reduction reaction at high temperature to prepare porous AlN ceramics in situ. This invention is the first to propose a method combining freeze-drying and in-situ carbothermic reduction reaction to prepare AlN porous ceramics with a honeycomb-like oriented structure. It has the advantages of low raw material cost and simple process. Furthermore, the prepared AlN porous ceramics exhibit a distinct honeycomb structure, high purity, high porosity, good compressive strength, high thermal conductivity, and low cost. It can be used as a thermally conductive framework in phase change materials or thermally conductive composite materials, and has broad application prospects in aerospace, environmental protection, communications, and power industries.

[0053] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in this invention, and these modifications or substitutions should all be covered within the scope of protection of this invention. Therefore, the scope of protection of this invention should be determined by the scope defined in the claims.

Claims

1. A method for preparing AlN porous ceramics with a honeycomb-like oriented structure, characterized in that, Includes the following steps: (1) Mixing: Alumina powder, sintering aid, ammonium polyacrylate and deionized water are mixed and ball-milled to prepare a slurry. Polyacryl alcohol aqueous solution and carbon fiber powder are added to the ball-milled slurry. After mechanical stirring and vacuum degassing, a mixed slurry is obtained. (2) Directional freezing and freeze drying: The slurry obtained in (1) is transferred to a mold and placed in a low-temperature ethanol bath for directional freezing treatment. Then the frozen sample is dried in a freeze dryer. (3) Sintering: The product obtained in (2) is transferred to a graphite crucible and placed in an atmosphere sintering furnace. Sintering is carried out under a nitrogen atmosphere, and then the product is cooled naturally. (4) Decarbonization: The product obtained in (3) is placed in a muffle furnace and heated to 600-800℃ and held for 1-5 hours to remove excess carbon and finally obtain AlN porous ceramic with honeycomb structure. In step (1), the carbon fiber powder has a mesh size of 100 to 2000 and an aspect ratio of 5:1 to 20:1; In step (1), the sintering aid is selected from one or more of Y2O3, SiO2, and CaF2, and the amount added is 0.5% to 5% of the mass of alumina powder; ammonium polyacrylate is used as a dispersant, and the amount added is 0.05% to 0.5% of the mass of alumina powder; polyacryl alcohol is used as a binder, and the amount added is 0.1% to 1% of the mass of alumina powder. In step (2), the directional freezing temperature is -50 to -100℃, the freeze-drying temperature is -100 to -20℃, and the drying time is 12 to 48 hours.

2. The method according to claim 1, characterized in that, In step (1), the alumina is α-Al2O3 or γ-Al2O3 with an average particle size of 0.1 to 2 μm.

3. The method according to claim 1, characterized in that, In step (1), the weight ratio of Al2O3 to carbon fiber powder is 1:1 to 3:1, and the amount of deionized water added is 0.6 to 4 times the total mass of alumina powder and carbon powder.

4. The method according to claim 1, characterized in that, In step (1), the ball milling time is 6 to 24 hours, the rotation speed is 200 to 400 rpm, the mechanical stirring time is 20 to 80 minutes, and the vacuum degassing is carried out in a vacuum degassing machine for 20 to 80 minutes.

5. The method according to claim 1, characterized in that, In step (2), a mold with a copper sheet at the bottom and polytetrafluoroethylene around the edges is used.

6. The method according to claim 1, characterized in that, In step (3), the temperature is first heated to 400-1000℃ at 5-20℃ / min and held for 0.5-3 hours, and then heated to 1750-1900℃ at 5-40℃ / min and held for 1-5 hours.

7. AlN porous ceramics with a honeycomb oriented structure prepared by the method according to any one of claims 1 to 6.

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