A method for preparing high-purity aluminum nitride powder at low pressure and high efficiency based on a gradient structure and a composite additive system

Abstract

This invention relates to the field of aluminum nitride synthesis technology, and provides a low-pressure, high-efficiency method for preparing high-purity aluminum nitride powder based on a gradient structure and composite additive system. This method solves the problems of sandwich phenomenon and incomplete reaction caused by nitrogen diffusion obstruction when the proportion of high aluminum powder is high in traditional aluminum nitride preparation, as well as the problems of high oxygen content and low purity in the product. The method includes the following steps: S1, Ingredient preparation: The first mixture includes: first aluminum powder, aluminum nitride powder, and first additive; the second mixture includes: second aluminum powder, aluminum nitride powder, and second additive; the third mixture includes: third aluminum powder, aluminum nitride powder, and third additive; S2, Ingredient distribution: First, the third mixture is laid out, then the second mixture is evenly laid out on the third mixture, and finally the first mixture is evenly laid out on the second mixture; S3, Combustion reaction: An igniter is applied to ignite the mixture, causing a self-propagating combustion synthesis reaction; S4, After the reaction is complete, the reaction product is removed to obtain aluminum nitride powder.

Description

Technical Field

[0001] This invention relates to the field of aluminum nitride synthesis technology, and in particular to a method for preparing high-purity aluminum nitride powder under low pressure and with high efficiency based on a gradient structure and composite additive system. Background Technology

[0002] Aluminum nitride (AlN), as an important advanced ceramic material, has attracted widespread attention due to its excellent thermal, electrical, physical, and chemical properties. Aluminum nitride possesses several outstanding properties, including high thermal conductivity, excellent electrical properties, good mechanical properties, high temperature stability, and corrosion resistance. These characteristics make aluminum nitride an ideal material for applications such as electronic packaging, heat dissipation substrates, and high-temperature structural components. In the microelectronics field, aluminum nitride ceramic substrates are widely used as heat dissipation substrates and packaging materials for large-scale integrated circuits due to their high thermal conductivity, low dielectric constant, and thermal expansion coefficient matching that of silicon. In the optoelectronics field, aluminum nitride can be used to fabricate ultraviolet light-emitting diodes. Furthermore, it is used in the manufacture of crucibles for smelting metals such as aluminum and copper, casting molds, and thermally conductive fillers for polymer composite materials.

[0003] Currently, there are various methods for synthesizing aluminum nitride powder, which can be classified into the following categories according to the reaction principle: carbothermic reduction method, direct nitriding method, and combustion synthesis method, etc.

[0004] The carbothermal reduction nitridation method uses alumina (Al₂O₃) and carbon as raw materials to synthesize aluminum nitride through a high-temperature reaction in a nitrogen atmosphere. Its basic reaction principle is: Al₂O₃ + 3C + N₂ → 2AlN + 3CO. The advantages of the carbothermal reduction method are low raw material cost and the ability to obtain uniform and fine powder; the disadvantages are high reaction temperature, long reaction cycle, and the need for additional decarburization treatment of residual carbon in the product.

[0005] Direct nitriding is the earliest method for preparing aluminum nitride, which involves the direct reaction of aluminum powder with nitrogen or ammonia at high temperatures. This method is simple and uses readily available raw materials, but it suffers from problems such as intense exothermic reaction, difficulty in controlling the reaction, easy product agglomeration, and low purity. During the reaction, the aluminum nitride layer formed on the surface of the aluminum powder hinders the diffusion of nitrogen into the interior, leading to incomplete reaction and requiring subsequent crushing and grinding, which can easily introduce impurities.

[0006] Combustion synthesis (also known as self-propagating high-temperature synthesis) utilizes the exothermic reaction between aluminum powder and nitrogen gas to sustain the reaction, completing the synthesis and sintering of aluminum nitride in one step. This method has advantages such as simple process, low energy consumption, and fast reaction rate. Studies have shown that by mixing aluminum powder, aluminum nitride powder, and sintering aids, pressing them into a compact, and then performing combustion synthesis in a high-pressure nitrogen atmosphere, large angular aluminum nitride particles of 20-200 μm can be obtained, suitable for high thermal conductivity fillers in electronic packaging. However, this method requires high process control, and the morphology and particle size distribution uniformity of the product need further improvement.

[0007] In recent years, publicly disclosed patents related to the synthesis and application of aluminum nitride have covered a variety of technical routes, including combustion synthesis, plasma method, and carbothermal reduction method. However, the publicly disclosed patents related to the combustion synthesis of aluminum nitride all have some limitations.

[0008] Chinese Patent Publication No. CN115038665A discloses a method for manufacturing aluminum nitride powder, aluminum nitride powder, and packaging. The method involves producing aluminum nitride through a combustion synthesis method using metallic aluminum powder. A mixed powder is ignited and burned under a nitrogen atmosphere. The mixed powder is obtained by mixing aluminum nitride powder with an average primary particle size of less than 3 μm as a diluent at a ratio of 150 to 400 parts by mass relative to 100 parts by mass of metallic aluminum powder. This method yields aluminum nitride in a blocky state, which can be easily refined by light pulverization. The pulverization process generates few active surfaces, resulting in extremely low oxygen concentration. However, during combustion, this mixed powder is prone to localized overheating and melting problems caused by concentrated reaction heat, and uneven nitrogen diffusion leads to a sandwich phenomenon.

[0009] Chinese Patent Publication No. CN114655938A discloses a method for preparing spherical aluminum nitride granulated powder and filler powder. The method includes: uniformly mixing aluminum powder and aluminum nitride powder in a specified amount using a ball milling process, and separating the milling beads and the mixed powder through a 20-mesh sieve; placing the separated mixed powder in a container surrounded by carbon felt, and placing the container in an air atmosphere; irradiating the mixed powder in the container with a high-energy laser beam to induce a combustion synthesis reaction, thereby obtaining aluminum nitride ceramic powder. Using aluminum powder and aluminum nitride powder as raw materials, and controlling the mass ratio of aluminum powder to aluminum nitride powder, aluminum nitride powder is prepared in one step through direct combustion synthesis in air. The process is simple, has a short cycle time, high efficiency, and is easy to scale up for production. It does not require high-temperature and high-pressure equipment, reducing production costs and improving production safety. Utilizing a high-energy laser beam to initiate the combustion synthesis reaction eliminates the need for an ignition source, resulting in higher purity aluminum nitride powder. However, this process involves burning aluminum nitride powder in air, and the air reaction inevitably introduces aluminum oxide impurities, resulting in products with high oxygen content and low purity. Summary of the Invention

[0010] Therefore, in view of the above problems, the present invention provides a method for preparing high-purity aluminum nitride powder under low pressure and high efficiency based on gradient structure and composite additive system, which solves the problems of sandwich phenomenon and incomplete reaction caused by nitrogen diffusion obstruction when the proportion of high aluminum powder is high in traditional aluminum nitride preparation, as well as high oxygen content and low purity of the product.

[0011] To achieve the above objectives, the present invention adopts the following technical solution: A method for low-pressure, high-efficiency preparation of high-purity aluminum nitride powder based on gradient structure and composite additive system includes the following steps: S1. Ingredients: Prepare a first mixture, a second mixture, and a third mixture separately; the first mixture comprises the following parts by weight of raw materials: 35-45 parts of first aluminum powder, 55-65 parts of aluminum nitride powder, and 3-4 parts of first additive; the second mixture comprises the following parts by weight of raw materials: 35-45 parts of second aluminum powder, 55-65 parts of aluminum nitride powder, and 2-3 parts of second additive; the third mixture comprises the following parts by weight of raw materials: 35-45 parts of third aluminum powder, 55-65 parts of aluminum nitride powder, and 1-2 parts of third additive; take the first aluminum powder, aluminum nitride powder, and first additive by weight, mix them evenly to obtain the first mixture; take the second aluminum powder, aluminum nitride powder, and second additive by weight, mix them evenly to obtain the second mixture; take the third aluminum powder, aluminum nitride powder, and third additive by weight, mix them evenly to obtain the third mixture. S2, Fabric: After vacuum drying the first mixture, second mixture and third mixture obtained in step S1, they are laid in layers in a graphite container lined with carbon felt in the following order: first, the third mixture is laid, then the second mixture is evenly laid on the third mixture, and finally the first mixture is evenly laid on the second mixture; an ignition wire and ignition agent are set at one end of the graphite container, and the top of the container is covered with carbon felt. S3. Combustion reaction: The graphite container obtained in step S2 is placed in a high-pressure combustion synthesis reactor. Nitrogen gas with a purity of 99.999% is introduced for two gas washing operations. Then, a vacuum is drawn, and nitrogen gas is injected again to maintain the reactor pressure at 1.0-2.0 MPa. The ignition agent is ignited by electricity to ignite the mixture for a self-propagating combustion synthesis reaction. During the reaction, nitrogen gas is continuously introduced and the reaction pressure is kept constant by adjusting the gas flow rate. S4. After the reaction is complete, cool to room temperature, remove the remaining gas in the reactor, take out the reaction product, and obtain aluminum nitride powder after crushing, sieving and homogenizing. The first additive is a core-shell structured particle, with ammonium chloride as the core and magnesium oxide as the shell. The second additive is a composite material consisting of a mixture of ammonium chloride and ammonium fluoride supported on SBA-15 mesoporous molecular sieve; The third additive is a composite material of reduced graphene oxide coated with ammonium chloride.

[0012] In traditional homogeneous mixing or simple flat-lay fabrics, the porosity of the reactant bed is uniform. Upon ignition, the reactants instantly form an aluminum nitride shell, which severely hinders further diffusion of nitrogen into the reactants. The reaction is forced to rely on nitrogen diffusion through the solid layers of reacted products, a process that is extremely slow. With a high proportion of aluminum powder, the internal aluminum powder may not fully convert due to insufficient nitrogen, forming a "sandwich" or molten aluminum core.

[0013] In this application, the fabric application step adopts a three-layer gradient structure, from the bottom (third mixture, 5μm fine aluminum powder) to the top (first mixture, 80μm coarse aluminum powder), the particle size of the aluminum powder increases, and the average pore size formed between the particles also increases accordingly, forming a through-pore gradient channel with "fine pores in the lower part and coarse pores in the upper part".

[0014] The upper layer (coarse-particle layer, high porosity) serves as the "high-speed nitrogen diffusion main channel." The large pores formed by the accumulation of coarse particles offer low resistance to gas flow, allowing nitrogen in the autoclave to quickly and abundantly permeate to the lower and middle parts of the reaction bed, providing ample "gas source reserves" for the reaction. The middle layer (medium-particle layer, medium porosity) acts as a "diffusion-reaction transition and buffer layer." It receives nitrogen flow from the upper layer and initiates an effective nitriding reaction through its moderate specific surface area, while also guiding nitrogen downwards. The lower layer (fine-particle layer, high specific surface area, low porosity) serves as a "highly active, fully reactive layer." The fine aluminum powder possesses extremely high specific surface area and reactivity. Although its original pores are small, the efficient nitrogen supply channels built above and in the middle and upper layers ensure that sufficient nitrogen can be transported to the surface of this layer, allowing the nitrogen to fully contact the highly active aluminum powder and achieve a rapid and thorough nitriding reaction.

[0015] The third additive takes effect first in the low-temperature range of 300-400℃. The high thermal conductivity of the reduced graphene oxide promotes the rapid transfer of reaction heat to the reaction front, while the coated ammonium chloride decomposes at a lower temperature. The second additive works under the medium-temperature condition of 400-500℃. Ammonium fluoride preferentially decomposes to break the oxide layer on the surface of aluminum powder, and ammonium chloride decomposes subsequently to maintain the nitriding reaction, forming a good reaction buffer zone in the medium-particle aluminum powder layer. The first additive plays a key role in the high-temperature stage of 500-600℃. The magnesium oxide shell slows down the decomposition rate of the ammonium chloride core, realizing the high-temperature slow-release function, ensuring that the coarse-particle aluminum powder completes nitriding within the optimal temperature window. At the same time, MgO is converted into the spinel phase of MgAl2O4, promoting the regular growth and densification of aluminum nitride grains.

[0016] Furthermore, the average particle size of the first aluminum powder is 80 μm, the average particle size of the second aluminum powder is 30 μm, and the average particle size of the third aluminum powder is 5 μm.

[0017] Furthermore, the aluminum nitride powder has a particle size of 1.2-1.5 μm.

[0018] Furthermore, the preparation process of the first additive is as follows: ammonium chloride is dissolved in anhydrous ethanol to form a saturated solution, nano-magnesium oxide powder is added and ultrasonically dispersed, and spray-dried at 150°C to form core-shell structured particles.

[0019] Furthermore, the preparation process of the second additive is as follows: a. Activate the SBA-15 mesoporous molecular sieve at 300℃ and vacuum degree ≤10Pa for 2 hours, and then cool it to room temperature for later use. b. According to the molar ratio of ammonium chloride to ammonium fluoride of 3:1, ammonium chloride and ammonium fluoride are mixed and dissolved in anhydrous methanol to obtain a homogeneous mixed solution with a concentration of 20wt%. c. The homogeneous mixed solution obtained in step b is added dropwise to the SBA-15 mesoporous molecular sieve obtained in step a at a rate of 1-2 mL / min under the assistance of an ultrasonic field. Then, the mixture is subjected to aging, vacuum drying, and heat treatment activation to obtain the second additive.

[0020] Furthermore, the third additive is prepared by a hydrothermal reduction method: ammonium chloride micro powder is mixed with graphene oxide dispersion, and the third additive is obtained by hydrothermal reaction and freeze drying.

[0021] Furthermore, step S1 also includes loading the first mixture, the second mixture, and the third mixture into a ball mill jar containing alumina balls for mechanical grinding and activation.

[0022] Furthermore, in step S2, the thickness ratio of the layered layup is first mixture: second mixture: third mixture = 1:1:1.2.

[0023] In step S2, the vacuum drying process is as follows: the vacuum degree is set to -0.5MPa, the temperature is 85℃, and the time is 120min.

[0024] The graphite container is a square graphite crucible with a wall thickness of 5 mm.

[0025] The ignition wire is a spiral tungsten wire; the ignition agent is a mixture of aluminum nitride and aluminum powder in a weight ratio of 1:1.

[0026] In step S3, the current is 20A-30A. After the current is applied, the tungsten wire heats up and melts rapidly within 1-2 seconds, igniting the ignition agent.

[0027] In step S4, the oxygen content of the aluminum nitride powder obtained after crushing, sieving, and homogenization is less than 0.8%.

[0028] By adopting the aforementioned technical solution, the beneficial effects of the present invention are as follows: 1. This technical solution adopts a self-propagating combustion synthesis method, in which aluminum powder is ignited in a nitrogen atmosphere, and its combustion and reaction with nitrogen are used to obtain aluminum nitride powder. No external heating is required. Compared with the traditional carbothermal reduction method, it can save electricity and reduce costs.

[0029] 2. The reaction vessel uses a graphite crucible lined with carbon felt. The good thermal conductivity of the graphite crucible ensures that even with a high proportion of aluminum powder, the internal temperature of the reaction mixture will not be too high, thus preventing the aluminum powder from melting and agglomerating. The carbon felt allows nitrogen to contact the bottom of the reactants, preventing the reaction mixture at the bottom from not being able to fully contact the nitrogen. Covering the vessel with carbon felt also prevents impurities in the reaction vessel from falling onto the surface of the reaction mixture.

[0030] 3. By using gradient material distribution, a permeable pore channel with a denser bottom and a sparser top is formed, reducing the nitrogen diffusion resistance and eliminating the sandwich phenomenon; at the same time, the third additive provides an active interface in the initial stage of the reaction, promoting the uniform distribution of reaction heat and guiding aluminum nitride to preferentially grow along the (002) crystal plane; the second additive maintains a moderate reaction rate during the reaction propagation process, avoiding temperature runaway and aluminum melting caused by excessively fast reaction; the magnesium oxide shell of the first additive controls the decomposition rate of ammonium chloride in the later stage of the reaction, reducing the reaction temperature and avoiding deflagration, solving the problems of coarse product grains and uneven morphology, and finally obtaining aluminum nitride powder with low oxygen content and high purity. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the reaction apparatus in Embodiment 1 of the present invention; Figure 2 This is a SEM image of the aluminum nitride powder prepared in Example 1 of the present invention. Detailed Implementation Example 1

[0032] refer to Figures 1 to 2 A method for low-pressure, high-efficiency preparation of high-purity aluminum nitride powder based on a gradient structure and composite additive system includes the following steps: S1. Ingredients: Prepare a first mixture, a second mixture, and a third mixture separately; the first mixture comprises the following parts by weight of raw materials: 45 parts of first aluminum powder, 55 parts of aluminum nitride powder, and 4 parts of first additive; the second mixture comprises the following parts by weight of raw materials: 45 parts of second aluminum powder, 55 parts of aluminum nitride powder, and 3 parts of second additive; the third mixture comprises the following parts by weight of raw materials: 45 parts of third aluminum powder, 55 parts of aluminum nitride powder, and 2 parts of third additive; take the first aluminum powder, aluminum nitride powder, and first additive by weight, mix them evenly to obtain the first mixture; take the second aluminum powder, aluminum nitride powder, and second additive by weight, mix them evenly to obtain the second mixture; take the third aluminum powder, aluminum nitride powder, and third additive by weight, mix them evenly to obtain the third mixture. S2, Fabric: After vacuum drying the first mixture, second mixture and third mixture obtained in step S1, they are laid in layers in a graphite container lined with carbon felt in the following order: first, the third mixture is laid, then the second mixture is evenly laid on the third mixture, and finally the first mixture is evenly laid on the second mixture; an ignition wire and ignition agent are set at one end of the graphite container, and the top of the container is covered with carbon felt. S3. Combustion reaction: The graphite container obtained in step S2 is placed in a high-pressure combustion synthesis reactor. Nitrogen gas with a purity of 99.999% is introduced for two gas washing operations. Then, a vacuum is drawn and nitrogen gas is injected again to maintain the reactor pressure at 2.0 MPa. The ignition agent is ignited by electricity to ignite the mixture for a self-propagating combustion synthesis reaction. During the reaction, nitrogen gas is continuously introduced and the reaction pressure is kept constant by adjusting the gas flow rate. S4. After the reaction is complete, cool to room temperature, remove the remaining gas in the reactor, take out the reaction product, and obtain aluminum nitride powder after crushing, sieving and homogenizing. The first additive is a core-shell structured particle, with ammonium chloride as the core and magnesium oxide as the shell. The second additive is a composite material consisting of a mixture of ammonium chloride and ammonium fluoride supported on SBA-15 mesoporous molecular sieve; The third additive is a composite material of reduced graphene oxide coated with ammonium chloride.

[0033] The first aluminum powder has an average particle size of 80 μm, the second aluminum powder has an average particle size of 30 μm, and the third aluminum powder has an average particle size of 5 μm.

[0034] The aluminum nitride powder has a particle size of 1.5 μm.

[0035] The preparation process of the first additive is as follows: ammonium chloride is dissolved in anhydrous ethanol to form a saturated solution, nano-magnesium oxide powder is added and ultrasonically dispersed, and spray-dried at 150°C to form core-shell structured particles.

[0036] The preparation process of the second additive is as follows: a. Activate the SBA-15 mesoporous molecular sieve at 300℃ and a vacuum of 10Pa for 2 hours, then cool it to room temperature for later use. b. According to the molar ratio of ammonium chloride to ammonium fluoride of 3:1, ammonium chloride and ammonium fluoride are mixed and dissolved in anhydrous methanol to obtain a homogeneous mixed solution with a concentration of 20wt%. c. The homogeneous mixed solution obtained in step b is added dropwise to the SBA-15 mesoporous molecular sieve obtained in step a at a rate of 2 mL / min under the assistance of an ultrasonic field. Then, the mixture is subjected to aging, vacuum drying, and heat treatment activation to obtain the second additive.

[0037] The third additive is prepared by hydrothermal reduction: ammonium chloride micro powder is mixed with graphene oxide dispersion, and the third additive is obtained by hydrothermal reaction and freeze drying.

[0038] Step S1 also includes loading the first mixture, the second mixture and the third mixture into a ball mill jar containing alumina balls for mechanical grinding and activation at a speed of 120 r / min for 60 min.

[0039] In step S2, the thickness ratio of the layered laying is first mixture: second mixture: third mixture = 1:1:1.2.

[0040] In step S2, the vacuum drying process is as follows: the vacuum degree is set to -0.5MPa, the temperature is 85℃, and the time is 120min.

[0041] The graphite container is a square graphite crucible with a wall thickness of 5 mm.

[0042] The ignition wire is a spiral tungsten wire; the ignition agent is a mixture of aluminum nitride and aluminum powder in a weight ratio of 1:1.

[0043] In step S3, the current is 30A. After the current is applied, the tungsten wire heats up and melts rapidly within 1-2 seconds, igniting the ignition agent.

[0044] In step S4, after the reaction is completed, a uniform product without sandwich structure is obtained. The product is cooled to room temperature, the pressure inside the high-pressure combustion synthesis reactor is released, the product is taken out, the residual carbon felt on the surface of the product is removed, and after crushing, sieving and homogenization, the aluminum nitride powder obtained has an oxygen content of 0.79%. The XRD pattern shows that the obtained product is aluminum nitride of wurtzite and has no impurity peaks. The purity of the aluminum nitride powder is greater than 98%. Example 2

[0045] The difference from Example 1 is as follows: The first mixture comprises the following parts by weight of raw materials: 35 parts of first aluminum powder, 65 parts of aluminum nitride powder, and 3 parts of first additive; the second mixture comprises the following parts by weight of raw materials: 35 parts of second aluminum powder, 65 parts of aluminum nitride powder, and 2 parts of second additive; the third mixture comprises the following parts by weight of raw materials: 35 parts of third aluminum powder, 65 parts of aluminum nitride powder, and 1 part of third additive. Nitrogen gas was injected again to maintain the pressure in the reactor at 2.0 MPa.

[0046] The remaining technical solutions are the same as in Example 1. Example 3

[0047] The difference from Example 1 is as follows: The first mixture comprises the following parts by weight of raw materials: 40 parts of first aluminum powder, 60 parts of aluminum nitride powder, and 3 parts of first additive; the second mixture comprises the following parts by weight of raw materials: 40 parts of second aluminum powder, 60 parts of aluminum nitride powder, and 2 parts of second additive; the third mixture comprises the following parts by weight of raw materials: 40 parts of third aluminum powder, 60 parts of aluminum nitride powder, and 1 part of third additive. Nitrogen gas was injected again to maintain the pressure in the reactor at 1.5 MPa.

[0048] The remaining technical solutions are the same as in Example 1.

[0049] The chemical equations for the synthesis of aluminum nitride powder in Examples 1 to 3 are as follows:

[0050] The chemical equations for the decomposition of ammonium chloride in Examples 1 to 3 are as follows:

[0051] Comparative Example 1 The difference from Example 1 is as follows: In step S2, the fabric is not laid in layers; the first mixture, the second mixture, and the third mixture are fully mixed before being laid.

[0052] The remaining technical solutions are the same as in Example 1.

[0053] Comparative Example 2 The difference from Example 1 is as follows: In step S1, the first mixture, the second mixture, and the third mixture do not use the first additive, the second additive, and the third additive.

[0054] The remaining technical solutions are the same as in Example 1.

[0055] Comparative Example 3 The difference from Example 1 is as follows: In step S1, a first mixture, a second mixture, and a third mixture are prepared respectively. The first mixture comprises the following parts by weight of raw materials: 45 parts of aluminum powder with a particle size of 30 μm, 55 parts of aluminum nitride powder, and 4 parts of a first additive. The second mixture comprises the following parts by weight of raw materials: 45 parts of aluminum powder with a particle size of 30 μm, 55 parts of aluminum nitride powder, and 3 parts of a second additive. The third mixture comprises the following parts by weight of raw materials: 45 parts of aluminum powder with a particle size of 30 μm, 55 parts of aluminum nitride powder, and 2 parts of a third additive.

[0056] The remaining technical solutions are the same as in Example 1.

[0057] The experimental test data of Examples 1 to 3 and Comparative Examples 1 to 3 are shown in the table.

[0058] Table 1

[0059] Although the invention has been specifically shown and described in conjunction with preferred embodiments, those skilled in the art should understand that various changes in form and detail may be made to the invention without departing from the spirit and scope of the invention as defined in the appended claims, all of which shall be within the scope of protection of the invention.

Claims

1. A method for low-pressure, high-efficiency preparation of high-purity aluminum nitride powder based on a gradient structure and composite additive system, characterized in that, Includes the following steps: S1. Ingredients: Prepare a first mixture, a second mixture, and a third mixture separately; the first mixture comprises the following parts by weight of raw materials: 35-45 parts of first aluminum powder, 55-65 parts of aluminum nitride powder, and 3-4 parts of first additive; the second mixture comprises the following parts by weight of raw materials: 35-45 parts of second aluminum powder, 55-65 parts of aluminum nitride powder, and 2-3 parts of second additive; the third mixture comprises the following parts by weight of raw materials: 35-45 parts of third aluminum powder, 55-65 parts of aluminum nitride powder, and 1-2 parts of third additive; take the first aluminum powder, aluminum nitride powder, and first additive by weight, mix them evenly to obtain the first mixture; take the second aluminum powder, aluminum nitride powder, and second additive by weight, mix them evenly to obtain the second mixture; take the third aluminum powder, aluminum nitride powder, and third additive by weight, mix them evenly to obtain the third mixture. S2, Fabric: After vacuum drying the first mixture, second mixture and third mixture obtained in step S1, they are laid in layers in a graphite container lined with carbon felt in the following order: first, the third mixture is laid, then the second mixture is evenly laid on the third mixture, and finally the first mixture is evenly laid on the second mixture; an ignition wire and ignition agent are set at one end of the graphite container, and the top of the container is covered with carbon felt. S3. Combustion reaction: The graphite container obtained in step S2 is placed in a high-pressure combustion synthesis reactor. Nitrogen gas with a purity of 99.999% is introduced for two gas washing operations. Then, a vacuum is drawn, and nitrogen gas is injected again to maintain the reactor pressure at 1.0-2.0 MPa. The ignition agent is ignited by electricity to ignite the mixture for a self-propagating combustion synthesis reaction. During the reaction, nitrogen gas is continuously introduced and the reaction pressure is kept constant by adjusting the gas flow rate. S4. After the reaction is complete, cool to room temperature, remove the remaining gas in the reactor, take out the reaction product, and obtain aluminum nitride powder after crushing, sieving and homogenizing. The first additive is a core-shell structured particle, with ammonium chloride as the core and magnesium oxide as the shell. The second additive is a composite material consisting of a mixture of ammonium chloride and ammonium fluoride supported on SBA-15 mesoporous molecular sieve; The third additive is a composite material of reduced graphene oxide coated with ammonium chloride.

2. The method for low-pressure, high-efficiency preparation of high-purity aluminum nitride powder based on a gradient structure and composite additive system according to claim 1, characterized in that, The first aluminum powder has an average particle size of 80 μm, the second aluminum powder has an average particle size of 30 μm, and the third aluminum powder has an average particle size of 5 μm.

3. The method for low-pressure, high-efficiency preparation of high-purity aluminum nitride powder based on a gradient structure and composite additive system according to claim 1, characterized in that, The aluminum nitride powder has a particle size of 1.2-1.5 μm.

4. The method for low-pressure, high-efficiency preparation of high-purity aluminum nitride powder based on a gradient structure and composite additive system according to claim 1, characterized in that, The preparation process of the first additive is as follows: ammonium chloride is dissolved in anhydrous ethanol to form a saturated solution, nano-magnesium oxide powder is added and ultrasonically dispersed, and spray-dried at 150°C to form core-shell structured particles.

5. The method for low-pressure, high-efficiency preparation of high-purity aluminum nitride powder based on a gradient structure and composite additive system according to claim 1, characterized in that, The preparation process of the second additive is as follows: a. Activate the SBA-15 mesoporous molecular sieve at 300℃ and vacuum degree ≤10Pa for 2 hours, and then cool it to room temperature for later use. b. According to the molar ratio of ammonium chloride to ammonium fluoride of 3:1, ammonium chloride and ammonium fluoride are mixed and dissolved in anhydrous methanol to obtain a homogeneous mixed solution with a concentration of 20wt%. c. The homogeneous mixed solution obtained in step b is added dropwise to the SBA-15 mesoporous molecular sieve obtained in step a at a rate of 1-2 mL / min under the assistance of an ultrasonic field. Then, the mixture is subjected to aging, vacuum drying, and heat treatment activation to obtain the second additive.

6. The method for low-pressure, high-efficiency preparation of high-purity aluminum nitride powder based on a gradient structure and composite additive system according to claim 1, characterized in that, The third additive is prepared by hydrothermal reduction: ammonium chloride micro powder is mixed with graphene oxide dispersion, and the third additive is obtained by hydrothermal reaction and freeze drying.

7. The method for low-pressure, high-efficiency preparation of high-purity aluminum nitride powder based on a gradient structure and composite additive system according to claim 1, characterized in that, Step S1 also includes loading the first mixture, the second mixture and the third mixture into a ball mill jar containing alumina balls for mechanical grinding and activation.

8. The method for low-pressure, high-efficiency preparation of high-purity aluminum nitride powder based on a gradient structure and composite additive system according to claim 1, characterized in that, In step S2, the thickness ratio of the layered laying is first mixture: second mixture: third mixture = 1:1:1.2.

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