Method for preparing high-thermal-conductivity aluminum nitride ceramic by graphite-induced liquid-phase oriented migration

Abstract

The invention discloses a method for preparing high-thermal-conductivity aluminum nitride ceramic by utilizing graphite to induce liquid-phase oriented migration. According to the method, aluminum nitride powder is used as a matrix raw material, yttrium oxide is used as a sintering aid, crystalline nano-graphite is used as an active regulating agent, and the high-thermal-conductivity aluminum nitride ceramic is prepared through tape casting, controllable glue discharging and atmosphere sintering processes. According to the method, the crystalline-state nano-graphite initiates a carbon thermal reduction reaction, a grain boundary phase is driven to evolve along a path of YAG-YAM-Y2O3-YN, an oxidation chemical potential gradient is established to induce directional migration of a yttrium-rich liquid phase, deep purification of crystal lattices and construction of a low-phonon scattering communication framework are achieved, and meanwhile, the deoxidization efficiency and the densification behavior are balanced. The maximum thermal conductivity of the aluminum nitride ceramic prepared by the method can reach 192.5 W.m <-1 >. K <-1 >, the maximum bending strength can reach 528 MPa, and the aluminum nitride ceramic has excellent thermal and mechanical properties and can be widely applied to core substrates of high-power electronic devices, 5G base stations, high-power LED thermal management systems and the like.

Description

Technical Field

[0001] This invention belongs to the field of aluminum nitride ceramic technology and relates to a method for preparing high thermal conductivity aluminum nitride ceramics by using graphite-induced directional migration of liquid phase. Background Technology

[0002] As microelectronics technology advances towards miniaturization, electronic systems are required to have high integration and high power density, leading to a significant increase in the heat generated by electronic components. This places higher demands on the thermal, mechanical, and electrical properties of related materials. Traditional alumina ceramic substrates can no longer meet the requirements of high-power, high-density power devices, while aluminum nitride ceramics possess excellent properties such as high thermal conductivity and high breakdown field strength, making them suitable for use in power electronic devices and their packaging materials. Aluminum nitride ceramics also have superior properties, such as a coefficient of thermal expansion that matches those of Si and GaAs semiconductor materials, making them a new generation of ceramic packaging materials.

[0003] Aluminum nitride ceramics are non-oxides with high covalent bonds, low diffusion coefficient, and high melting point, making them difficult to sinter densely. Aluminum nitride is easily hydrolyzed and oxidized in air, requiring sintering under a protective atmosphere; otherwise, an alumina film easily forms on the surface, and oxygen impurities readily dissolve into the aluminum nitride lattice, forming aluminum vacancies, increasing defects, and reducing thermal conductivity, thus limiting its applications. Therefore, the preparation of high-performance aluminum nitride ceramics is crucial.

[0004] Chinese patent application CN120058379A discloses a synergistic strengthening method for preparing high-strength, high-toughness, and high-thermal-conductivity aluminum nitride ceramic substrates. The method uses graded aluminum nitride powder as the matrix, introduces surface-pre-oxidized and nitrate-coated aluminum nitride whiskers for toughening, and uses a zirconia precursor to generate zirconium nitride nanoparticles in situ to disperse and strengthen the grain boundaries. The resulting material has a thermal conductivity as high as 192 W·m. -1 ·K -1 Flexural strength 474 MPa, fracture toughness 10.32 MPa·m 1 / 2 This significantly improves the overall performance. However, the process is very complex, involving multi-step surface modification (pre-oxidation, coating, calcination) and strict process control (maintaining the temperature at the nitrate decomposition point). Although the introduction of ZrN strengthens the grain boundaries, it slightly reduces the thermal conductivity due to the formation of Al2O3. At the same time, the ratio of whiskers to graded powder needs to be precisely controlled to avoid a trade-off between strength and thermal conductivity. These factors increase the difficulty and cost of industrial production.

[0005] Chinese patent application CN119392346A discloses a method for preparing high-strength, high-thermal-conductivity aluminum nitride ceramic substrates by cold pressing and hot pressing sintering with spherical aluminum nitride powder with an average particle size of 1-3 μm, using aluminum whisker as a single additive. This method utilizes the dual-effect properties of whiskers (acting as both a sintering aid and a reinforcing agent), achieving breakthroughs in substrate performance without additional additives: flexural strength ≥402.3 MPa and thermal conductivity ≥112.2 W·m. -1 ·K -1 Fracture toughness ≥ 3.19 MPa·m 1 / 2 Although the raw materials for this method are relatively easy to obtain and the process is simple, the thermal conductivity is low. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing high thermal conductivity aluminum nitride ceramics using graphite-induced directional liquid phase migration. This invention uses aluminum nitride powder as the matrix material, yttrium oxide as a sintering aid, and crystalline nano-graphite as an active control agent. Aluminum nitride ceramics are prepared through tape casting, controlled binder removal, and atmosphere sintering processes. This method effectively reduces the lattice oxygen content of aluminum nitride ceramics, densifies the microstructure, and precisely controls grain boundary phase migration, thereby synergistically improving the thermal conductivity and mechanical strength of aluminum nitride ceramics. The prepared aluminum nitride ceramics can be widely used in electronic packaging, heat dissipation substrates for high-power devices, and high-temperature structural materials.

[0007] The technical solution for achieving the objective of this invention is as follows:

[0008] A method for preparing highly thermally conductive aluminum nitride ceramics using graphite-induced directional liquid-phase migration includes the following steps:

[0009] S1. Ball Milling and Mixing: Mix 3.5-35 parts of yttrium oxide powder, 60-70 parts of acetone, 5-10 parts of an 8 wt% dispersant solution, and 2400 parts of spherical alumina grinding balls, and ball mill for 0.5-2 hours. After cooling to room temperature, add 315-346.5 parts of aluminum nitride powder, 0.5-3.5 parts of crystalline nano-graphite powder, 10-15 parts of plasticizer, and 45-60 parts of methyl ethyl ketone (MEK), and ball mill for 2-4 hours. After cooling to room temperature, add 180-220 parts of a 7 wt% binder solution, and ball mill for 10-18 hours. After cooling to room temperature, filter out the grinding balls to obtain a slurry; wherein the mass of the crystalline nano-graphite powder is 0.2-0.6 wt% of the total mass of yttrium oxide powder and aluminum nitride powder.

[0010] S2, Degassing treatment: Degassing treatment of the slurry;

[0011] S3. Casting: The degassed slurry is cast at a speed of 0.2-0.5 m / min and a scraper height of 0.2-0.5 mm to obtain a cast sheet with a thickness of 0.1-0.3 mm. The sheet is then dried at 30-90 ℃ and cut to the required size.

[0012] S4. Lamination: The casting direction is denoted as the x-direction, and the direction perpendicular to the casting direction is denoted as the y-direction. Aluminum nitride sheets are stacked alternately in the x and y directions to achieve the required thickness, so that the blank has a controllable shrinkage rate. The stacked blank is placed in a fixed mold, soft-sealed, and then subjected to cold isostatic pressing.

[0013] S5. Degreasing and debinding: The blank is placed in a sealed box furnace for debinding. Before heating the box furnace, the inside is evacuated. When the pressure inside the furnace reaches below 20 Pa, high-purity nitrogen is introduced. After the nitrogen fills the cavity, the temperature is slowly increased at a rate of 0.1-2 ℃ / min until the temperature inside the box furnace reaches 400-800 ℃.

[0014] S6. Ceramic sintering: The debinding ceramic is placed in a graphite furnace and sintered in a nitrogen atmosphere at 1700-1850 ℃ to obtain high thermal conductivity aluminum nitride ceramic.

[0015] Further, in step S1, the oxygen content of the aluminum nitride powder is 1.24 wt%, the particle size of the aluminum nitride powder is 0.8 μm-1.4 μm, the particle size of the yttrium oxide powder is 0.2 μm-1 μm, and the particle size of the crystalline nano-graphite powder is 200-500 nm.

[0016] Furthermore, in step S1, the dispersant is a commonly used dispersant in aluminum nitride ceramics, including but not limited to fish oil, polyoxyethylene alkyl ether phosphate, and polyethylene glycol. In a specific embodiment of the present invention, polyoxyethylene alkyl ether phosphate is taken as an example.

[0017] Further, in step S1, the plasticizer is a plasticizer commonly used in aluminum nitride ceramics, including but not limited to dibutyl phthalate, glycerin, and polyethylene glycol dimethyl ether. In a specific embodiment of the present invention, dibutyl phthalate is used as an example.

[0018] Further, in step S1, the adhesive solution is an acetone / butanone composite solution of polypropylene carbonate (PPC).

[0019] Furthermore, in step S1, the spherical alumina grinding balls have a particle size distribution of 8 mm, 12 mm, and 20 mm, a mass ratio of 3:4:3, a grinding speed of 60 rpm, and the grinding jar is made of polytetrafluoroethylene.

[0020] Furthermore, in step S1, the mass of the crystalline nano-graphite powder is 0.4 wt% of the total mass of the yttrium oxide powder and aluminum nitride powder.

[0021] Furthermore, in step S2, the degassing treatment time is 1 hour.

[0022] Furthermore, in step S3, the casting speed is 0.3 m / min and the scraper height is 0.3 mm.

[0023] Further, in step S4, the thickness of the stack is 3-4 mm; the cold isostatic pressing conditions are: holding pressure at 55 ℃ and 1600-2200 psi for 900-1100 seconds, with a pressure increase / decrease rate of 4 psi / s, and then removing the sample and immersing it in water to cool for 20 minutes.

[0024] Further, in step S5, a segmented adhesive removal method is adopted. The heating rate is 0.5 °C / min below 55 °C. The adhesive is held at 55 °C for 2 hours to fully vitrify. Then, the temperature is increased to 200 °C at 0.5 °C / min to reach the temperature at which the adhesive begins to decompose. Next, the temperature is slowly increased to 250 °C at 0.1 °C / min to reach the temperature at which the adhesive ends to decompose. The temperature is held for 2 hours to fully decompose the adhesive. Then, the temperature is increased to 450 °C at 0.5 °C / min and held for 4 hours to fully detach the decomposed organic matter from the green body. Finally, the temperature is reduced to room temperature at a rate of 2 °C / min.

[0025] Furthermore, in step S6, the heating and cooling rate is 5 ℃ / min, and the sintering time is 1-4 hours.

[0026] The present invention also provides high thermal conductivity aluminum nitride ceramics prepared by the above method.

[0027] Compared with the prior art, the present invention has the following advantages:

[0028] (1) Using crystalline graphite as a stable reduction source, Al2O3 is precisely consumed through carbothermic reaction, significantly reducing the lattice oxygen content, thereby greatly reducing phonon scattering centers.

[0029] (2) Adding graphite to the cross section of the sintered body establishes an oxygen potential gradient, which promotes the migration of the yttrium-rich liquid phase from the core to the surface, simultaneously achieving internal lattice purification and external dense structure maintenance, overcoming the problem of grain boundary phase residue and oxygen impurities that are difficult to completely remove in traditional processes.

[0030] (3) By controlling the amount of graphite added, this invention enables the material to reduce phonon scattering through lattice purification while avoiding structural defects caused by excessive graphite, achieving a perfect balance between densification and deoxygenation efficiency. The highest flexural strength of the sample can reach 528 MPa (13.3% higher than without graphite), while maintaining 178.9 W·m. -1 ·K-1 Its high thermal conductivity completely solves the contradiction between improving thermal conductivity and structural compactness and mechanical reliability in existing technologies. Attached Figure Description

[0031] Figure 1 XRD patterns of sintered samples with different amounts of graphite added and held at 1780℃ for 2 hours.

[0032] Figure 2 The polished SEM and EDS images of the cross-sectional edges of sintered samples with different graphite contents after being held at 1780℃ for 2 hours are shown, where (a) 0wt%; (b) 0.2wt%; (c) 0.4wt%; (d) 0.6wt%; (e) 0.8wt%; and (f) 1wt%.

[0033] Figure 3 The images show SEM images of the polished edges and interiors of sintered samples after adding different amounts of graphite and holding at 1780℃ for 2 hours. (a) and (b) are 0 wt%; (c) and (d) are 0.2 wt%; (e) and (f) are 0.4 wt%; (g) and (h) are 0.6 wt%; (i) and (j) are 0.8 wt%; and (k) and (l) are 1 wt%.

[0034] Figure 4 SEM and EDS images of the core of the natural cross-section of sintered samples after adding different amounts of graphite and holding at 1780℃ for 2 hours are shown, where (a) 0wt%; (b) 0.2wt%; (c) 0.4wt%; (d) 0.6wt%; (e) 0.8wt%; and (f) 1wt%.

[0035] Figure 5 Raman images of sintered samples with different amounts of graphite added and held at 1780℃ for 2 hours. Detailed Implementation

[0036] This invention utilizes a carbothermic reduction reaction initiated by crystalline nanographite to drive the evolution of the grain boundary phase along the YAG→YAM→Y2O3→YN pathway, establishing an oxidation potential gradient to induce the directional migration of the yttrium-rich liquid phase. This achieves deep lattice purification and the construction of a low-phonon scattering interconnected framework, while simultaneously balancing oxygen removal efficiency and densification behavior. The specific principle is as follows:

[0037] This invention uses aluminum nitride powder as the matrix material, yttrium oxide as the sintering aid, and adds crystalline nano-graphite as a controllable reducing agent. During high-temperature sintering, the graphite undergoes a carbothermic reduction reaction with the AlN surface oxide layer (Al2O3) and oxygen impurities in the lattice, as shown in formula (1). This deeply consumes Al2O3 in the system, avoiding strong phonon scattering caused by oxygen-induced aluminum vacancies, and fundamentally solving the bottleneck of improving the thermal conductivity of AlN.

[0038] (1)

[0039] In traditional processes, the sintering aid Y2O3 needs to form an aluminate phase (such as YAG) with a large amount of Al2O3 to achieve densification, resulting in excessive consumption of Y2O3 and residual grain boundary phases that affect thermal conductivity. After graphite reduction consumes Al2O3, the Y2O3 / Al2O3 ratio in the system increases, and the grain boundary phase evolves from YAG→YAM→Y2O3→YN to a yttrium-rich phase. Densification can be achieved without excessive Y2O3, reducing ineffective Y2O3 residues and grain boundary thermal resistance.

[0040] The CO gas generated by carbothermal reduction escapes rapidly from the sample surface, forming a chemical potential gradient with low oxygen potential on the surface and high oxygen potential inside. This gradient drives the yttrium-rich liquid phase containing oxygen impurities to migrate directionally from the matrix core to the surface. This not only migrates internal oxygen impurities to the surface and removes them from the system, achieving deep purification of the AlN lattice, but also avoids the formation of continuous aluminate phases at grain boundaries, constructing a connected framework with low phonon scattering.

[0041] A design employing alternating laminations of aluminum nitride green ceramic ribbons along the casting direction and perpendicular to it achieves isotropic optimization of the microstructure through synergistic control of multi-dimensional fiber orientation. This lamination process effectively eliminates orientation defects caused by traditional unidirectional lamination. Cross-lamination balances the internal stress induced by the directional alignment of the organic carrier during casting, preventing warping or cracking due to anisotropic shrinkage during sintering. Bidirectional lamination promotes a more uniform orientation distribution of aluminum nitride grains in three-dimensional space, reducing the scattering of phonon transport by single-orientation grain boundaries. This design allows for a more uniform liquid phase distribution of sintering aids within the three-dimensional network, promoting isotropic densification while ensuring a spatially balanced distribution of the grain boundary second phase.

[0042] Furthermore, by optimizing the debinding curve design, efficient and residue-free decomposition of the organic binder was achieved. Based on the depolymerization characteristics of PPC, a stepped heating program was used to match its thermal decomposition kinetics, ensuring that the organic components were completely converted into gaseous monomers for volatilization. Simultaneously, a nitrogen protective atmosphere effectively suppressed carbonization side reactions, keeping the final carbon residue to a trace level. This avoids carbon particle inclusions caused by insufficient thermal decomposition in traditional debinding processes. These carbon impurities not only act as phonon scattering centers, reducing thermal conductivity, but may also react with aluminum nitride during high-temperature sintering to form harmful phases such as Al4C3, disrupting the integrity of the grain boundary structure. Achieving a clean transformation of the green body from an organic system to an inorganic sintered body provides a pure sintering environment for subsequent lattice oxygen migration and grain growth, which is a key condition for obtaining high-density, low-defect aluminum nitride ceramics.

[0043] The present invention will be further described below with reference to specific embodiments and accompanying drawings.

[0044] Example 1

[0045] 1. Ball milling and mixing: D 50 =0.5 μm yttrium oxide powder 10.5 parts, acetone 65 parts, polyoxyethylene alkyl ether phosphate 5 parts, and spherical alumina grinding balls 2400 parts were mixed and ball-milled for 1 hour. After cooling for 10 minutes, 339.5 parts of D were added. 50 =1.1μm aluminum nitride powder, D 50 =300 nm crystalline nano-graphite powder, 15 parts of dibutyl phthalate, and 50 parts of methyl ethyl ketone were ball-milled twice for 4 hours. After cooling for 20 minutes, 200 parts of PPC were added and ball-milled a third time for 12 hours. The ball milling speed was 60 rpm for all times. After cooling for half an hour, the grinding balls were filtered off to obtain slurries with different graphite content. The amount of crystalline nano-graphite powder added was 0 parts (0 wt%), 0.7 parts (0.2 wt%), 1.4 parts (0.4 wt%), 2.1 parts (0.6 wt%), 2.8 parts (0.8 wt%), and 3.5 parts (1.0 wt%), respectively.

[0046] 2. Degassing treatment: The slurry is degassed for 1 hour using a vacuum pump.

[0047] 3. Casting: The degassed slurry is cast at a speed of 0.3 m / min and a scraper height of 0.3 mm to obtain a cast sheet with a thickness of 0.18 mm. The sheet is then dried at 60 ℃ and cut to the required size.

[0048] 4. Lamination: The casting direction is denoted as the x-direction, and the direction perpendicular to the casting direction is denoted as the y-direction. Aluminum nitride sheets are stacked alternately in the x and y directions to achieve a thickness of 3 mm, so that the blank has a controllable shrinkage rate. The upper and lower surfaces of the stacked blank are separated by a plastic film and placed in a fixed mold. After soft sealing, cold isostatic pressing is performed. The pressure is held at 55 ℃ and 1800 psi for 960 seconds, and the pressure increase and decrease rate is 4 psi / s. The sample is then removed and placed in cold water to cool for 20 minutes.

[0049] 5. Degreasing and debinding: The laminated preform is placed on an aluminum nitride firing plate and put into a sealed box furnace. Degreasing and debinding are carried out in stages under a nitrogen atmosphere. The heating rate is 0.5 ℃ / min below 55 ℃, and the temperature is held at 55 ℃ for 2 hours. Then the temperature is increased to 200 ℃ at 0.5 ℃ / min, followed by a slow increase to 250 ℃ at 0.1 ℃ / min and held for 2 hours. Then the temperature is increased to 450 ℃ at 0.5 ℃ / min and held for 4 hours. Finally, the temperature is reduced to room temperature at a rate of 2 ℃ / min.

[0050] 6. Ceramic sintering: The sample after debinding is placed in a graphite furnace for sintering. The heating rate is 5 ℃ / min. The temperature is raised to 1500 ℃ and held for 1 hour. The temperature is then raised to the sintering temperature of 1780 ℃ at 5 ℃ / min and held for 2 hours. The temperature is then lowered to room temperature at 5 ℃ / min to complete the sintering and obtain aluminum nitride ceramic.

[0051] Example 2

[0052] This embodiment is largely the same as Embodiment 1, except that the sintering temperature is 1800 ℃.

[0053] Comparative Example 1

[0054] 1. Ball milling and mixing: D 50 =0.5 μm yttrium oxide powder 10.5 parts, acetone 65 parts, polyoxyethylene alkyl ether phosphate 5 parts, and spherical alumina grinding balls 2400 parts were mixed and ball-milled for 1 hour. After cooling for 10 minutes, 339.5 parts of D were added. 50 =1.1μm aluminum nitride powder, 15 parts of dibutyl phthalate, and 50 parts of methyl ethyl ketone were ball-milled twice for 4 hours. After cooling for 20 minutes, 200 parts of PPC were added and ball-milled a third time for 12 hours. The ball milling speed was 60 rpm for all times. After cooling for half an hour, the grinding balls were filtered out.

[0055] 2. Degassing treatment: The slurry is degassed for 1 hour using a vacuum pump.

[0056] 3. Casting: The degassed slurry is cast at a speed of 0.3 m / min and a scraper height of 0.3 mm to obtain a cast sheet with a thickness of 0.18 mm. The sheet is then dried at 60 ℃ and cut to the required size.

[0057] 4. Lamination: The casting direction is denoted as the x-direction, and the direction perpendicular to the casting direction is denoted as the y-direction. Aluminum nitride sheets are stacked alternately in the x and y directions to achieve a thickness of 3 mm, so that the blank has a controllable shrinkage rate. The upper and lower surfaces of the stacked blank are separated by a plastic film and placed in a fixed mold. After soft sealing, cold isostatic pressing is performed. The pressure is held at 55 ℃ and 1800 psi for 960 seconds, and the pressure increase and decrease rate is 4 psi / s. The sample is then removed and placed in cold water to cool for 20 minutes.

[0058] 5. Degreasing and debinding: The laminated preform is placed on an aluminum nitride firing plate and put into a sealed box furnace. Degreasing and debinding are carried out in stages under a nitrogen atmosphere. The heating rate is 0.5 ℃ / min below 55 ℃, and the temperature is held at 55 ℃ for 2 hours. Then the temperature is increased to 200 ℃ at 0.5 ℃ / min, followed by a slow increase to 250 ℃ at 0.1 ℃ / min and held for 2 hours. Then the temperature is increased to 350 ℃ at 0.5 ℃ / min and held for 4 hours. Finally, the temperature is reduced to room temperature at a cooling rate of 2 ℃ / min.

[0059] 6. Ceramic sintering: The sample after debinding is placed in a graphite furnace for sintering. The heating rate is 5 ℃ / min. The temperature is raised to 1500 ℃ and held for 1 hour. The temperature is then raised to the sintering temperature of 1780 ℃ at 5 ℃ / min and held for 2 hours. The temperature is then lowered to room temperature at 5 ℃ / min to complete the sintering and obtain aluminum nitride ceramic.

[0060] The performance of the aluminum nitride ceramic substrates prepared in Examples 1 and 2 was tested, and the results are shown in Table 1 below. Thermal conductivity was tested using the laser flash point method, and bending strength was tested using the three-point bending method.

[0061] Table 1 Performance test results of aluminum nitride ceramic substrates prepared in Examples 1 and 2

[0062]

[0063] The debinding curves used in both embodiments showed almost no residual carbon after debinding. At a sintering temperature of 1780℃, the flexural strength and thermal conductivity of the material exhibited a non-monotonic variation pattern of first increasing and then decreasing with increasing graphite content. The optimal overall performance was achieved at an addition amount of 0.4 wt%, with peak flexural strength and thermal conductivity reaching 528 MPa and 178.9 W·m, respectively. -1 ·K -1 Compared to the unadded sample, the addition of graphite increased by 13.3% and 27.1%, respectively. However, when the amount of graphite added exceeded 0.6 wt%, the excess residual carbon acted as impurity defects and phonon scattering centers, resulting in a significant decline in the mechanical strength and thermal conductivity of the material.

[0064] At a sintering temperature of 1800℃, with increasing graphite content, both the thermal conductivity and flexural strength of the samples exhibited a single-peak trend of first significantly increasing and then gradually decreasing, reaching the optimal synergistic point at 0.4 wt%. The thermal conductivity of this group of samples reached a peak of 192.5 W·m. -1 ·K -1 Compared to the sample without added graphite (160.9 W·m⁻¹), -1 ·K -1The performance improved by approximately 19.6%, while the flexural strength also reached a maximum of 496 MPa. The initial performance improvement was mainly attributed to the effective removal of lattice oxygen and optimization of grain boundary phase composition by the carbothermic reduction reaction; however, when the graphite addition exceeded the 0.4 wt% threshold, the excess residual carbon may act as a phonon scattering center or a source of mechanical defects, thereby inhibiting further improvement in material performance.

[0065] The debinding curve used in Comparative Example 1 showed a residual amorphous carbon content of 0.44 wt% after debinding, and the thermal conductivity of the sample was 121.7 W·m. -1 ·K -1 The strength is 344 MPa. Without adding crystalline nano-graphite powder, lowering the debinding temperature resulted in some amorphous carbon remaining after debinding. It was found that even trace amounts of amorphous carbon could have a significant negative impact on the thermal conductivity and flexural strength of aluminum nitride.

[0066] Figure 1 These are XRD patterns of sintered samples with different graphite contents after being held at 1780℃ for 2 hours. The XRD data clearly depict the evolution path of the second phase along YAG→YAM→Y2O3→YN as the graphite content increases. This evolution confirms that graphite effectively reduces the oxygen content at grain boundaries and within the lattice by consuming Al2O3. For the preparation of high thermal conductivity AlN ceramics, a graphite addition of 0.2-0.6 wt% may be the optimal window, achieving lattice purification through the formation of YAM or the precipitation of Y2O3 while avoiding the large-scale formation of the unstable YN phase.

[0067] Figure 2 These are polished SEM and EDS images of the edges of sintered samples after adding different amounts of graphite and holding at 1780℃ for 2 hours. The SEM microstructure and EDS elemental surface scan results of the polished surfaces at the edges show that different amounts of graphite drive the Y element in aluminum nitride ceramics to change from uniform distribution to surface migration. Appropriate amounts of graphite can achieve deep deoxygenation of the matrix and maintain compactness through directional migration of the liquid phase, thereby improving thermal conductivity. Excessive graphite will cause excessive migration of the liquid phase, accumulation of yttrium-rich layers, and even surface volatilization, leading to the collapse of the microstructure, resulting in a decrease in density and thermal conductivity. Furthermore, the porosity defects caused by high carbon content at the edges will hinder liquid phase migration.

[0068] Figure 3These are SEM images of the polished edges and interior of the cross-sections of sintered samples after adding different amounts of graphite and holding at 1780℃ for 2 hours. SEM results of the polished core of the cross-section show that as the graphite content increases to 0.6-1.0 wt%, an excessively strong reducing atmosphere triggers excessive decomposition and volatilization of the liquid phase in the aluminum nitride ceramic, causing the characteristic migration layer thickness to shrink sharply from 150 μm to 70 μm. Excessive loss of the liquid phase results in a lack of mass transfer medium for densification in the later stages of sintering, and the generation of numerous micron-sized pores and even interconnected pores. These structural defects significantly reduce density and thermal conductivity. 0.4 wt% is the optimal graphite addition amount to balance lattice oxygen removal and microstructural integrity.

[0069] Figure 4 These are SEM and EDS images of the core of the sintered body samples after being sintered at 1780℃ for 2 hours with different amounts of graphite. The SEM results of the natural cross-section show that different amounts of graphite have a significant nonlinear effect on the microstructure of the aluminum nitride ceramic core: with graphite additions from 0 wt% to 0.6 wt%, the core exhibits a densely packed structure with polyhedral grains. The Y-enriched second phase disperses from agglomerates to triangular grain boundaries, purifying the grain boundaries and reducing phonon scattering, leading to an increase in thermal conductivity. When the graphite addition increases to 0.8 wt%, especially 1.0 wt%, excessive liquid-phase reduction results in abnormally refined grains, a sharp increase in porosity, and localized Y element segregation, increasing phonon scattering and interrupting phonon transport paths, ultimately leading to a significant decrease in thermal conductivity.

[0070] Figure 5 These are Raman spectra of sintered samples with different amounts of graphite added, sintered at 1780℃ for 2 hours. Raman spectroscopy further confirmed that crystalline graphite mainly exists as a reactant rather than an inert filler during the sintering process at 1780℃ for 2 hours. The spectra of all samples are dominated by strong characteristic phonon peaks of the AlN matrix, while the D band (~1350 cm⁻¹), representing defects in carbon materials, is dominant. -1 ) and graphitized structure (G band, ~1580 cm) -1 The scattering peaks of the graphite were almost invisible in samples with carbon loadings of 0–0.6 wt%, indicating that the added graphite had been almost completely depleted in gaseous form by combining with lattice oxygen through carbothermic reduction. Only extremely weak D and G peak signals were detected in samples with higher carbon loadings of 0.8 wt% and 1.0 wt%, strongly demonstrating that the vast majority of the graphite had been effectively converted into reduction kinetic energy at high temperatures, leaving only trace amounts of residual carbon in the final sintered body.

[0071] Although the present invention has been described in detail through the preferred embodiments above, it should be understood that the above description should not be considered as a limitation of the present invention. Various modifications and substitutions to the present invention will be apparent to those skilled in the art after reading the above description. Therefore, the scope of protection of the present invention should be defined by the appended claims.

Claims

1. A method for preparing high thermal conductivity aluminum nitride ceramics using graphite-induced directional liquid-phase migration, characterized in that, Includes the following steps: S1. Ball Milling and Mixing: Mix 3.5-35 parts of yttrium oxide powder, 60-70 parts of acetone, 5-10 parts of an 8 wt% dispersant solution, and 2400 parts of spherical alumina grinding balls, and ball mill for 0.5-2 hours. After cooling to room temperature, add 315-346.5 parts of aluminum nitride powder, 0.5-3.5 parts of crystalline nano-graphite powder, 10-15 parts of plasticizer, and 45-60 parts of methyl ethyl ketone (MEK), and ball mill for 2-4 hours. After cooling to room temperature, add 180-220 parts of a 7 wt% binder solution, and ball mill for 10-18 hours. After cooling to room temperature, filter out the grinding balls to obtain a slurry; wherein the mass of the crystalline nano-graphite powder is 0.2-0.6 wt% of the total mass of yttrium oxide powder and aluminum nitride powder. S2, Degassing treatment: Degassing treatment of the slurry; S3. Casting: The degassed slurry is cast at a speed of 0.2-0.5 m / min and a scraper height of 0.2-0.5 mm to obtain a cast sheet with a thickness of 0.1-0.3 mm. The sheet is then dried at 30-90 ℃ and cut to the required size. S4. Lamination: The casting direction is denoted as the x-direction, and the direction perpendicular to the casting direction is denoted as the y-direction. Aluminum nitride sheets are stacked alternately in the x and y directions to achieve the required thickness, so that the blank has a controllable shrinkage rate. The stacked blank is placed in a fixed mold, soft-sealed, and then subjected to cold isostatic pressing. S5. Degreasing and debinding: The blank is placed in a sealed box furnace for debinding. Before heating the box furnace, the inside is evacuated. When the pressure inside the furnace reaches below 20 Pa, high-purity nitrogen is introduced. After the nitrogen fills the cavity, the temperature is slowly increased at a rate of 0.1-2 ℃ / min until the temperature inside the box furnace reaches 400-800 ℃. S6. Ceramic sintering: The debinding ceramic is placed in a graphite furnace and sintered in a nitrogen atmosphere at 1700-1850 ℃ to obtain high thermal conductivity aluminum nitride ceramic.

2. The method according to claim 1, characterized in that, In step S1, the oxygen content of the aluminum nitride powder is 1.24 wt%, the particle size of the aluminum nitride powder is 0.8 μm-1.4 μm, the particle size of the yttrium oxide powder is 0.2 μm-1 μm, and the particle size of the crystalline nano-graphite powder is 200-500 nm.

3. The method according to claim 1, characterized in that, In step S1, the dispersant is fish oil, polyoxyethylene alkyl ether phosphate, or polyethylene glycol; the plasticizer is dibutyl phthalate, glycerin, or polyethylene glycol dimethyl ether; the binder solution is a composite solution of PPC in acetone / butanone; the spherical alumina grinding balls have a particle size distribution of 8 mm, 12 mm, and 20 mm, with a mass ratio of 3:4:3; the grinding speed is 60 rpm; and the grinding jar is made of polytetrafluoroethylene.

4. The method according to claim 1, characterized in that, In step S1, the mass of the crystalline nano-graphite powder is 0.4 wt% of the total mass of yttrium oxide powder and aluminum nitride powder.

5. The method according to claim 1, characterized in that, In step S2, the degassing treatment time is 1 hour.

6. The method according to claim 1, characterized in that, In step S3, the casting speed is 0.3 m / min and the scraper height is 0.3 mm.

7. The method according to claim 1, characterized in that, In step S4, the thickness of the stack is 3-4 mm; the cold isostatic pressing conditions are: holding pressure at 55 ℃ and 1600-2200 psi for 900-1100 seconds, with a pressure increase / decrease rate of 4 psi / s, and then removing the sample and immersing it in water to cool for 20 minutes.

8. According to the method of claim 1, in step S5, a segmented adhesive removal method is adopted, with a heating rate of 0.5 °C / min below 55 °C, and holding at 55 °C for 2 hours to allow the adhesive to fully vitrify. Then, the temperature is increased to 200 °C at 0.5 °C / min to reach the temperature at which the adhesive begins to decompose. Next, the temperature is slowly increased to 250 °C at 0.1 °C / min to reach the temperature at which the adhesive ends to decompose. The temperature is held for 2 hours to allow the adhesive to fully decompose. After that, the temperature is increased to 450 °C at 0.5 °C / min and held for 4 hours to allow the decomposed organic matter to fully detach from the preform. Finally, the temperature is reduced to room temperature at a cooling rate of 2 °C / min.

9. According to the method of claim 1, in step S6, the heating and cooling rate is 5 °C / min, and the sintering time is 1-4 hours.

10. The high thermal conductivity aluminum nitride ceramic prepared by the method according to any one of claims 1 to 9.

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