A reaction sintering method for preparing AlN-SiC composite ceramics, its preparation method and applications

Abstract

This invention belongs to the field of ceramic matrix composite technology. Specifically, it relates to a reaction sintering method for preparing AlN-SiC composite ceramics, its application, and related technologies. The method uses AlN powder, Si powder, and C powder as raw materials. During sintering, an in-situ chemical reaction (Si + C → SiC) generates a SiC reinforcing phase. The SiC generated in this in-situ reaction is then uniformly distributed within the AlN matrix, forming strong interfacial bonding and pure grain boundaries. Further grain refinement results in a dense, fine-grained AlN-SiC composite ceramic. This method not only synergistically improves the mechanical strength of the AlN-SiC composite ceramic but also enhances its thermal conductivity.

Description

Technical Field

[0001] This invention belongs to the field of ceramic matrix composite technology, specifically relating to a reaction sintering method for preparing AlN-SiC composite ceramics and its application. Background Technology

[0002] As electronic devices, aerospace equipment, and microwave vacuum devices develop towards higher power and higher integration, materials generate a significant amount of heat during operation. If this heat cannot be dissipated in a timely manner, it will severely affect the stability and lifespan of the devices. Therefore, developing ceramic substrates or ceramic packaging materials that combine high thermal conductivity, good mechanical properties, and reliable stability has become a research hotspot.

[0003] Aluminum nitride (AlN) ceramics are considered an ideal heat dissipation material to replace toxic BeO due to their high theoretical thermal conductivity (up to 320 W / m·K), low dielectric constant, matching coefficient of thermal expansion with silicon, and non-toxic and environmentally friendly properties. However, AlN is a strongly covalent compound, making sintering difficult, and its strength and toughness need improvement. Silicon carbide (SiC) also possesses high thermal conductivity, excellent high-temperature mechanical properties, and oxidation resistance. Combining AlN with SiC can integrate the advantages of both. Studies have shown that AlN and SiC can form a solid solution at high temperatures, potentially yielding high-performance composite materials. However, at the preparation process level, there are still many challenges in the preparation of AlN-SiC composite materials using existing technologies. Since both AlN and SiC are strongly covalent and difficult to sinter, it is difficult to obtain high-density AlN-SiC composite materials using ordinary pressureless sintering methods, which severely affects thermal conductivity. Traditional hot-pressing sintering or solid-state reaction methods often require extremely high temperatures (e.g., >1950℃) and reaction times of at least 2 hours, resulting in high energy consumption. While the currently used low-temperature reactive melting method lowers the reaction temperature, residual silicon and other impurities result in a low thermal conductivity (only 26.3 W / m·K). Furthermore, existing research has described methods that promote material densification by introducing sintering aids such as Y₂O₃; however, this method suffers from the problem that the formation of grain boundary phases may become a source of phonon scattering, limiting further improvements in thermal conductivity. Therefore, developing a method for preparing AlN-SiC composite materials with a relatively low sintering temperature, without the need for additional complex additives, and capable of simultaneously achieving high thermal conductivity and high mechanical properties has significant application value. Summary of the Invention

[0004] To address the shortcomings of the existing technologies, the present invention aims to provide a method for preparing AlN-SiC composite ceramics by reaction sintering, and its application. The method enables the preparation of AlN-SiC composite ceramics at relatively low sintering temperatures without the need for additional complex additives, and simultaneously achieves high thermal conductivity and high mechanical properties.

[0005] To address the aforementioned technical problems, this invention provides a method for preparing AlN-SiC composite ceramics by reaction sintering, comprising the following steps: AlN powder, Si powder, and C powder are mixed at mass fractions of 92%~98%, 1%~5%, and 1%~3%, respectively. The mixture is then wet-milled with anhydrous ethanol. The resulting slurry is dried, and the dried mixture is ground and sieved to obtain a uniformly dispersed composite powder. The mass fractions of C powder and Si powder are calculated based on the amount of SiC generated in the reaction. In a vacuum environment, the composite powder was subjected to three-stage hot-pressing sintering. After the three-stage hot-pressing sintering reaction, the temperature was partially reduced and the pressure was released, and then the pressure was completely released and cooled to room temperature to obtain AlN-SiC composite ceramic material. The first stage of hot-pressing sintering was heated to 1000℃~1300℃ at a heating rate of 8℃ / min~15℃ / min, and an external pressure of 5MPa~15MPa was applied. The second stage of hot-pressing sintering was heated to 1300℃~1600℃ at a heating rate of 2℃ / min~5℃ / min, and an external pressure of 5MPa~15MPa was applied. The third stage of hot-pressing sintering was heated to 1700℃~1900℃ at a heating rate of 8℃ / min~15℃ / min, and an external pressure of 20MPa~50MPa was applied. The temperature was then held at this temperature for 20min~60min.

[0006] This invention uses AlN powder, Si powder, and C powder with mass fractions of 92%~98%, 1%~5%, and 1%~3% respectively as raw materials. Oxygen impurities on the surface of the raw materials are effectively removed under vacuum conditions, reducing the formation of low thermal conductivity impurity phases. This allows the in-situ generated highly active SiC to form pure and tightly bonded grain boundaries with the AlN matrix as the main crystalline phase, greatly reducing phonon scattering and significantly improving the thermal conductivity of the material. Furthermore, the addition of excess C powder consumes residual trace amounts of oxygen and water vapor under vacuum, preventing the easy oxidation of AlN to Al2O3 at 600℃~1200℃ from affecting thermal conductivity and mechanical properties. Finally, AlN-SiC composite ceramics with high thermal conductivity and high mechanical properties are prepared through high-temperature and mechanical pressure coupling, i.e., hot-pressing sintering. In the first stage of hot-pressing sintering, the added Si powder and C powder undergo an in-situ exothermic reaction at the specified temperature and pressure to generate highly active SiC, which can serve as a reinforcing phase for SiC formation. This avoids the interfacial impurities and grain boundary second phases introduced by directly adding SiC powder or introducing Y2O3 sintering aids, fundamentally solving the common problems of poor interfacial compatibility and deteriorated thermal conductivity in multiphase composite materials. A relatively rapid temperature rise of 8℃ / min to 15℃ / min is used during the first stage of sintering to quickly overcome the easily oxidized temperature range of AlN, maximally suppressing surface oxidation and the formation of grain boundary glass phases. Through the second stage of hot pressing sintering, the generated SiC reinforcing phase and AlN powder (AlN matrix) as the main crystalline phase are promoted by external pressure to rearrange particles, plastic flow, grain boundary diffusion, and close open pores. It also realizes gas removal and pore discharge under vacuum, forming strong interfacial bonding and pure grain boundaries. Through the third stage of hot pressing sintering (relatively low sintering temperature (1700℃~1900℃) and short holding time (20min~60min)), abnormal grain growth and volatilization of matrix components are suppressed, fine-grained microstructure is maintained, and rapid densification of the material is achieved. Finally, a dense AlN-SiC composite ceramic without a second phase at the grain boundaries is obtained, which improves the mechanical properties of AlN-SiC composite ceramic and maintains high intrinsic thermal conductivity.

[0007] The AlN-SiC composite ceramics prepared by this invention achieve a significant improvement in thermal conductivity (60 W / m·K~70 W / m·K) compared to traditional additive-sintered AlN-SiC ceramics (approximately 40 W / m·K) and materials prepared by low-temperature melt infiltration (approximately 26.3 W / m·K). In terms of process, the reactive hot pressing sintering method achieves complete densification of the material (relative density >99%), is simple and efficient, has low energy consumption, and requires no complex subsequent processing. This invention, through the innovative design of the reactive system (AlN-Si-C) and the hot pressing process, effectively solves the technical challenge of simultaneously achieving high density, high thermal conductivity, and high mechanical properties in AlN-SiC composite ceramics, providing a new approach for the preparation of high-performance AlN-based heat dissipation and structurally integrated materials.

[0008] Preferably, the heating rate of the first stage hot-pressing sintering, from room temperature to 1000℃~1300℃, is 8℃ / min~15℃ / min, with an external pressure of 5MPa~15MPa applied. Because trace amounts of oxygen and water vapor remain under vacuum, AlN is easily oxidized to Al2O3 at 600℃~1200℃, forming a low-melting-point grain boundary glass phase. This significantly increases phonon scattering, reduces thermal conductivity, weakens grain boundary bonding, and deteriorates mechanical properties. Using a relatively rapid heating rate of 8℃ / min~15℃ / min quickly crosses the easily oxidized temperature range of AlN, maximally suppressing surface oxidation and the formation of the grain boundary glass phase. If the rate is too low, the dwell time at medium and low temperatures will be too long, leading to increased AlN oxidation, enrichment of the glass phase, and a significant decrease in thermal conductivity. If the rate is too high, the internal temperature gradient of the blank will be too large, easily generating internal stress and microcracks, which will become crack propagation sources after sintering, reducing strength. The second stage of hot pressing sintering involves heating the temperature to 1300℃~1600℃ at a rate of 2℃ / min~5℃ / min, while applying an external pressure of 5MPa~15MPa. This temperature range is a crucial kinetic zone for particle surface diffusion, grain boundary slippage, particle rearrangement, and the closure of open pores. It is also the core stage for gas removal and pore expulsion under vacuum. The slow heating rate of 2℃ / min~5℃ / min provides sufficient time for atomic diffusion and particle rearrangement, allowing interstitial adsorbed gases to escape fully under vacuum, preventing pores from being encased and sealed by rapid heating. If the heating is too rapid, particles do not have enough time to rearrange, resulting in closed pores, decreased density, and increased residual pores. Pores act as phonon scattering centers and crack initiation sources, leading to a simultaneous decrease in thermal conductivity and flexural strength. If the heating is too slow, premature and abnormal grain growth occurs, resulting in coarse-grained embrittlement, deteriorated mechanical properties, and increased thermal resistance due to disordered grain boundaries. Regarding sintering pressure, maintaining a relatively low pressure of 5MPa to 15MPa in the medium-low temperature stage can serve as a pre-tightening packing mechanism, ensuring particle contact and facilitating vacuum degassing and initial particle rearrangement. If the pressure is too high, it leads to low-temperature particle breakage and dense internal microcracks, which cannot be eliminated by subsequent heat treatment, resulting in a significant decrease in mechanical properties. Conversely, if the pressure is too low, the particle packing becomes loose, making it difficult to close interstitial pores and increasing the difficulty of subsequent densification. In the third stage of hot pressing sintering, the heating temperature is raised to 1700℃ to 1900℃ at a rate of 8℃ / min to 15℃ / min, with an external pressure of 20MPa to 50MPa applied, and the temperature is held for 20 to 60 minutes. The rapid heating rate of 8℃ / min to 15℃ / min in this stage can shorten the high-temperature heating time of the sample, inhibit abnormal grain growth and matrix component volatilization, maintain a fine-grained microstructure, improve the material's mechanical properties, and maintain high intrinsic thermal conductivity. The reason for raising the high pressure to 20MPa~50MPa in the high-temperature sintering section is that high pressure can suppress grain boundary migration and grain growth, refine grains, and optimize microstructure.If the pressure is less than 20 MPa, the driving force for densification of the sample is insufficient, closed pores cannot be compacted and eliminated, resulting in low density and compromised performance. If the pressure is greater than 50 MPa, the preform is prone to uneven plastic deformation under vacuum and high temperature, leading to layered structural defects and residual internal stress. This makes the material susceptible to later cracking and failure, while also causing excessive mold load. Therefore, under vacuum hot pressing conditions, using segmented heating rates of 8–15℃ / min, 2–5℃ / min, and 8–15℃ / min, combined with low-pressure pre-pressing of 5–15 MPa and high-temperature high-pressure pressing of 20–50 MPa, and holding at 1700–1900℃ for 20–60 min, the mechanical and thermal properties of AlN-SiC composite materials can be precisely controlled.

[0009] Preferably, in the first stage of hot pressing sintering, the heating rate from room temperature to 1200°C is 10°C / min, and the applied external pressure is 10 MPa. In the second stage of hot pressing sintering, the heating rate from room temperature to 1500°C is 3°C / min, and the applied external pressure is 10 MPa. In the third stage of hot pressing sintering, the heating rate from room temperature to 1800°C is 10°C / min, and 30 MPa is applied, with the temperature held at this temperature for 30 min. Under these parameters, the material exhibits low porosity, uniform and fine grains, clean grain boundaries, and fewer residual internal stresses and microcracks. The synergistic effect of fine grain strengthening, densification strengthening, and low grain boundary thermal resistance enables the AlN-SiC composite material to achieve optimal mechanical properties and thermal conductivity.

[0010] Preferably, after the three-stage hot pressing sintering, the temperature is first reduced from 1700℃~1900℃ to 800℃~1200℃ at a cooling rate of 8℃ / min~15℃ / min, and the pressure is reduced from 20MPa~50MPa to 5MPa~15MPa, and then the pressure is reduced to atmospheric pressure and cooled to room temperature.

[0011] If the temperature drops too quickly in the high-to-medium temperature range, a huge temperature gradient will be generated inside the material. Combined with thermal expansion mismatch stress, this easily leads to the initiation of grain boundary microcracks and transgranular cracks. These cracks will severely reduce flexural strength and sever heat conduction pathways, significantly reducing thermal conductivity. If the cooling rate is too slow (far below 8℃ / min), the sample will remain in the high-temperature range of 1200℃ to 1900℃ for an extended period, causing continued grain growth and coarsening and segregation of grain boundary phases, resulting in increased brittleness and decreased mechanical properties. The grain boundaries become disordered and thickened, intensifying phonon scattering and further deteriorating thermal conductivity. If the cooling rate is too fast (greater than 15℃ / min), the large temperature difference between the inside and outside of the sample leads to concentrated thermal stress, generating a large amount of residual internal stress, which becomes a source of later-stage fracture under stress. At the same time, it disrupts the continuous heat conduction network, significantly reducing thermal conductivity. Furthermore, 800℃ to 1200℃ is the critical temperature range for grain boundary phase solidification, microstructure stabilization, and internal stress relaxation in AlN-SiC composite materials. Above 1200℃, the material still retains a certain degree of plasticity, allowing for the relaxation of residual internal stress from hot pressing through grain boundary slip and micro-region rheological processes. Below 800℃, the ceramic matrix has become completely rigid and hardened, losing its ability to relax plastically. At this point, any further temperature differences or stress will directly solidify into permanent microcracks and residual stress. Therefore, controlling the temperature drop at a uniform rate between 800 and 1200℃, combined with step-by-step depressurization, utilizes the material's high-temperature plasticity to release hot-pressurization stress. Furthermore, the reason for step-by-step depressurization is that at the end of the heat treatment, the material is in a high-temperature, high-plasticity, and high-pressure dense state. If the pressure is directly and instantly reduced from 20–50 MPa to atmospheric pressure, the elastic strain and plastic deformation under high pressure will be released instantly, generating huge depressurization stress at grain boundaries, pores, and phase interfaces. This can induce grain boundary cracking, interlayer delamination, and internal micropores, severely damaging mechanical properties and destroying the dense thermally conductive structure. Therefore, the temperature is first reduced to 800–1200℃, with simultaneous depressurization to 5–15 MPa. At this temperature, the material still retains slight plasticity, which can buffer the structural stress brought about by pressure relief; maintaining a low pressure of 5-15 MPa moderately constrains the preform, preventing volumetric expansion and contraction distortion and interface debonding due to uneven thermal shrinkage during cooling, thus ensuring high density and continuous thermal conductivity channels. When the temperature drops below 800-1200℃, the matrix has already rigidly solidified and the structure is finalized. At this point, releasing to normal pressure will not cause structural distortion or microcracks; subsequent slow cooling in the furnace further eliminates residual thermal stress in the low-temperature range, resulting in stable microstructure and high dimensional accuracy.

[0012] Preferably, after the three-stage hot pressing sintering, the temperature is first reduced from 1800℃ to 1000℃ at a cooling rate of 10℃ / min, the pressure is reduced from 50MPa to 10MPa, and then the pressure is reduced to atmospheric pressure and cooled to room temperature with the furnace.

[0013] A fixed cooling rate of 10℃ / min ensures a small internal and external temperature gradient in the billet, allowing for the slow release of thermal expansion mismatch stress and preventing microcracks at grain boundaries due to sudden cooling. Main cooling and step-by-step decompression are performed within the plastic temperature range of 1800–1000℃, utilizing the material's plasticity to slowly release thermocompression stress and solidify a dense, fine-grained structure. The moderate cooling rate of 10℃ / min avoids abnormal grain growth and grain boundary phase segregation, while also reducing the internal and external temperature gradient, maintaining a highly dense, fine-grained structure and continuous, intact thermal conductivity pathways. Therefore, the composite material exhibits optimal mechanical and thermal conductivity properties. Under vacuum hot pressing conditions, 1800℃ perfectly meets the atomic diffusion and densification kinetics requirements of AlN-SiC composite materials. This ensures sufficient in-situ reaction of Si and C to generate a finely dispersed SiC reinforcing phase, while also fully utilizing the carbothermic reduction and deoxidation effect of a small amount of residual carbon, effectively eliminating the alumina glass phase at grain boundaries and purifying the grain boundaries. At this temperature, grain growth is moderate, the microstructure is uniform and dense, with no significant component volatilization or abnormal grain coarsening. The synergistic effect of fine grain strengthening and clean grain boundaries results in optimal density, mechanical properties, and thermal conductivity of the composite material. Below 1800℃, the material exhibits insufficient overall densification and high porosity; low SiC formation and inadequate residual carbon deoxidation. Above 1800℃, abnormal grain coarsening occurs, AlN / SiC vacuum volatilization intensifies, and internal defects increase; the material's mechanical properties decrease, and thermal conductivity declines.

[0014] Preferably, the mass fractions of AlN powder, Si powder, and C powder are 97%, 2%, and 1%, respectively.

[0015] AlN itself possesses extremely high intrinsic thermal conductivity, low dielectric constant, and moderate coefficient of thermal expansion, making it the main framework for thermal conductivity and high-temperature resistance in composite materials. A high matrix content of 97% ensures that the material is dominated by a continuous AlN matrix. AlN ceramics themselves have clean grain interfaces and low grain boundary thermal resistance. A high matrix content can maximize the preservation of intrinsic high thermal conductivity, high insulation, and high heat resistance stability. If the AlN content is less than 97%, excessive Si and C additions will react to form excessive SiC, which can easily cause grain agglomeration, disordered grain boundaries, and increased porosity, damaging the continuous thermal conductive network of AlN and reducing thermal conductivity. At the same time, internal stress increases, and mechanical properties deteriorate. When the mass fractions of AlN, Si, and C powders are controlled at 97%, 2%, and 1%, respectively, Si and C can react in situ at near-stoichiometric ratios to form trace amounts of SiC reinforcing phase and a small amount of residual carbon. A small amount of in-situ SiC precipitates uniformly within AlN crystals and at grain boundaries, playing two major roles: (1) SiC pins AlN grain boundaries, inhibiting abnormal growth of AlN grains, refining grains, and making the structure uniform and dense, significantly improving bending strength and hardness; (2) SiC and AlN have complementary coefficients of thermal expansion. A small amount of SiC can alleviate the thermal expansion mismatch during sintering and cooling, reduce residual internal stress, and inhibit the initiation of microcracks, thereby improving mechanical reliability and maintaining the integrity of the thermal conductivity pathway. A small amount of residual carbon can consume the oxide layer (Al2O3) glass phase on the surface of AlN, reduce the low-melting-point glass phase at grain boundaries, significantly reduce phonon scattering at grain boundaries, and improve thermal conductivity; at the same time, it purifies the grain boundaries and enhances the grain boundary bonding strength. If too much C is added and the residual carbon content is too high, excessive free C will agglomerate at grain boundaries, forming carbon agglomeration defects and micropores, destroying the continuous thermal conductivity network of AlN, phonon scattering will increase dramatically, and thermal conductivity will decrease significantly.

[0016] This invention also provides a method for preparing AlN-SiC composite ceramics by reaction sintering, resulting in AlN-SiC composite ceramic materials.

[0017] Preferably, the AlN-SiC composite ceramic material has a thermal conductivity of 60 W / m·K to 70 W / m·K at room temperature, a flexural strength of 300 MPa to 400 MPa, and a Vickers hardness of not less than 20 GPa.

[0018] The present invention provides the application of AlN-SiC composite ceramics prepared by reaction sintering in high-power electronic device substrates, microwave attenuation materials, and aerospace thermal structural components.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention uses AlN powder, Si powder, and C powder with mass fractions of 92%~98%, 1%~5%, and 1%~3% respectively as raw materials. Oxygen impurities on the surface of the raw materials are effectively removed under vacuum conditions, reducing the formation of low thermal conductivity impurity phases. This is because the in-situ generated highly active SiC forms pure and tightly bonded grain boundaries with the AlN matrix as the main crystalline phase, greatly reducing phonon scattering and thus significantly improving the thermal conductivity of the material. Furthermore, the addition of excess C powder consumes residual trace amounts of oxygen and water vapor under vacuum, preventing the easy oxidation of AlN to Al2O3 at 600℃~1200℃ from affecting thermal conductivity and mechanical properties. Finally, AlN-SiC composite ceramics with high thermal conductivity and high mechanical properties are prepared through high-temperature and mechanical pressure coupling, i.e., hot-pressing sintering. In the first stage of hot-pressing sintering, the added Si powder and C powder undergo an in-situ exothermic reaction at the specified temperature and pressure to generate highly active SiC, which can serve as a reinforcing phase for SiC formation. This avoids the interfacial impurities and grain boundary second phases introduced by directly adding SiC powder or introducing Y2O3 sintering aids, fundamentally solving the common problems of poor interfacial compatibility (poor mechanical properties) and deterioration of thermal conductivity in multiphase composite materials. A relatively rapid temperature rise of 8℃ / min to 15℃ / min is used during the first stage of sintering to quickly overcome the easily oxidized temperature range of AlN, maximally suppressing surface oxidation and the formation of grain boundary glass phases. Through the second stage of hot pressing sintering, the generated SiC reinforcing phase and AlN powder (AlN matrix) as the main crystalline phase are promoted by external pressure to rearrange particles, plastic flow, grain boundary diffusion, and close open pores. This also achieves gas removal and pore discharge under vacuum. The slow heating rate of 2℃ / min to 5℃ / min provides sufficient time for atomic diffusion and particle packing rearrangement. Under vacuum, the interstitial adsorbed gas can escape fully, preventing the pores from being encapsulated by rapid heating and forming closed pores. This results in strong interfacial bonding and pure grain boundaries, which is beneficial to improving mechanical properties and thermal conductivity. Through the third stage of hot pressing sintering (relatively low sintering temperature (1700℃~1900℃) and short holding time (20min~60min)) and heating rate of 8℃ / min to 15℃ / min, the in-situ generated SiC particles are uniformly dispersed at the AlN grain boundaries. Through grain boundary pinning, abnormal grain growth and volatilization of matrix components are effectively suppressed, maintaining a fine-grained microstructure and further improving mechanical properties. At the same time, it also shortens the high-temperature heating time of the sample. This invention generates a SiC reinforcing phase by adding Si powder and C powder through an in-situ exothermic reaction, and then hot-pressing and sintering the SiC reinforcing phase with an AlN matrix in a vacuum environment through the above three stages to finally obtain an AlN-SiC composite ceramic with high thermal conductivity and high mechanical properties.

[0020] The AlN-SiC composite ceramics prepared by this invention achieve a significant improvement in thermal conductivity (60 W / m·K~70 W / m·K) compared to traditional additive-sintered AlN-SiC ceramics (approximately 40 W / m·K) and materials prepared by low-temperature melt infiltration (approximately 26.3 W / m·K). In terms of process, the reactive hot pressing sintering method achieves complete densification of the material (relative density >99%), is simple and efficient, has low energy consumption, and requires no complex subsequent processing. This invention, through the innovative design of the reactive system (AlN-Si-C) and the hot pressing process, effectively solves the technical challenge of simultaneously achieving high density, high thermal conductivity, and high mechanical properties in AlN-SiC composite ceramics, providing a new approach for the preparation of high-performance AlN-based heat dissipation and structurally integrated materials. Attached Figure Description

[0021] Figure 1 XRD patterns of AlN-SiC composite ceramics prepared in this invention at different temperatures Figure 2 SEM image of the AlN-SiC composite ceramic prepared in this invention at a sintering temperature of 1700℃. Figure 3 The grain size distribution of the AlN-SiC composite ceramic prepared in this invention at a sintering temperature of 1700℃ is shown in the statistical diagram. Figure 4 SEM image of the AlN-SiC composite ceramic prepared in this invention at a sintering temperature of 1800℃. Figure 5 The grain size distribution of the AlN-SiC composite ceramic prepared in this invention at a sintering temperature of 1800℃ is shown in the statistical diagram. Figure 6 SEM image of the AlN-SiC composite ceramic prepared in this invention at a sintering temperature of 1900℃. Figure 7 The grain size distribution of the AlN-SiC composite ceramic prepared in this invention at a sintering temperature of 1900℃ is shown in the statistical diagram. Figure 8 Microstructure and elemental distribution diagram of the AlN-SiC composite ceramic prepared in this invention at a sintering temperature of 1900℃. Figure 9 The thermal conductivity and thermal diffusivity of the AlN-SiC composite ceramic prepared in this invention are shown as curves of temperature variation. Figure 10 The relative density, flexural strength, and hardness data of the AlN-SiC composite ceramic prepared by this invention at different sintering temperatures. Detailed Implementation

[0022] The specific embodiments of the present invention are described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention. Unless otherwise specified, the experimental methods described in the embodiments of the present invention are conventional methods.

[0023] It should be noted that when numerical ranges are involved in this invention, it should be understood that the two endpoints of each numerical range and any value between the two endpoints can be selected. Since the steps and methods used are the same as in Examples 1 to 6, preferred embodiments are described in this invention to avoid redundancy. However, this invention is not limited to these, but can be implemented in other ways within the scope of the technical solutions defined in the appended claims. All raw materials, reagents, instruments, and equipment used in the following embodiments of this invention can be purchased from the market or prepared by existing methods.

[0024] The following detailed description, in conjunction with embodiments of the present invention and accompanying drawings, provides a clear and complete illustration of the technical solutions in these embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0025] The bulk density and porosity of the samples were tested using the Archimedes displacement method. The flexural strength of the samples was tested using a ceramic universal testing machine. The Vickers hardness of the samples was tested using a Vickers hardness tester. The phase composition of the samples was characterized by X-ray diffraction. The cross-sectional morphology and elemental distribution maps of the samples were obtained using field emission scanning electron microscopy. The thermal diffraction coefficient of the material was obtained using a Netzsch LFA457 instrument. Then, the thermal conductivity of the material is obtained according to formula (1). The sample size is approximately 10mm × 10mm × 2mm.

[0026] In the experiment, the sample density ρ was measured by Archimedes' method, and the heat capacity Cp was calculated by Dulong-Petit's law.

[0027] Example 1 A method for preparing AlN-SiC composite ceramics by reaction sintering includes the following steps: According to the target AlN-SiC composite ceramic preparation, AlN powder, Si powder and C powder were mixed at a mass fraction of 97%, 2% and 1% respectively (AlN powder, Si powder and C powder were mixed at a mass ratio of 97:2:1). Then, anhydrous ethanol was added and the mixture was wet-milled to obtain a mixed slurry. The mass fraction of C powder was calculated based on the amount of SiC generated by the chemical reaction with Si powder. The obtained mixed slurry was dried to remove the solvent and obtain a dried mixture. The dried mixture was then ground and sieved to obtain a uniformly dispersed composite powder. The obtained composite powder was placed in a vacuum environment and hot-pressed for sintering. After hot-pressing for sintering, it was first partially cooled and depressurized, then completely depressurized and cooled to room temperature to obtain AlN-SiC composite ceramic material. The heating temperature of the hot-pressing for sintering was increased from room temperature to 1200℃ at a heating rate of 10℃ / min, with an external pressure of 10MPa; then increased to 1500℃ at a heating rate of 3℃ / min, with an external pressure of 10MPa; then increased to 1900℃ at a heating rate of 10℃ / min, with an external pressure of 30MPa, and held at this temperature for 30min. After hot-pressing for sintering, the temperature was reduced to 1000℃ at a rate of 10℃ / min, and the pressure was depressurized to 10MPa, then depressurized to atmospheric pressure and cooled to room temperature with the furnace.

[0028] Example 2 A method for preparing AlN-SiC composite ceramics by reaction sintering includes the following steps: According to the target AlN-SiC composite ceramic preparation, AlN powder, Si powder and C powder were mixed at a mass fraction of 97%, 2% and 1% respectively (AlN powder, Si powder and C powder were mixed at a mass ratio of 97:2:1). Then, anhydrous ethanol was added and the mixture was wet-milled to obtain a mixed slurry. The mass fraction of C powder was calculated based on the amount of SiC generated by the chemical reaction with Si powder. The obtained mixed slurry was dried to remove the solvent and obtain a dried mixture. The dried mixture was then ground and sieved to obtain a uniformly dispersed composite powder. The obtained composite powder was placed in a vacuum environment and hot-pressed under heating and external pressure. After hot-pressing sintering, it was first partially cooled and depressurized, then completely depressurized and cooled to room temperature to obtain AlN-SiC composite ceramic material. The heating temperature of the hot-pressing sintering was increased from room temperature to 1200℃ at a heating rate of 10℃ / min, and the external pressure was 10MPa. Then it was increased to 1500℃ at a heating rate of 3℃ / min, and the external pressure was 10MPa. Then it was increased to 1800℃ at a heating rate of 10℃ / min, and the external pressure was 30MPa. The temperature was held at this temperature for 30min. After hot-pressing sintering, the temperature was decreased to 1000℃ at a heating rate of 10℃ / min, and the pressure was depressurized to 10MPa. Then the pressure was depressurized to atmospheric pressure and cooled to room temperature with the furnace.

[0029] Example 3 A method for preparing AlN-SiC composite ceramics by reaction sintering includes the following steps: According to the target AlN-SiC composite ceramic preparation, AlN powder, Si powder and C powder were mixed at a mass fraction of 97%, 2% and 1% respectively (AlN powder, Si powder and C powder were mixed at a mass ratio of 97:2:1). Then, anhydrous ethanol was added and the mixture was wet-milled to obtain a mixed slurry. The mass fraction of C powder was calculated based on the amount of SiC generated by the chemical reaction with Si powder. The obtained mixed slurry was dried to remove the solvent and obtain a dried mixture. The dried mixture was then ground and sieved to obtain a uniformly dispersed composite powder. The obtained composite powder was placed in a vacuum environment and hot-pressed under heating and external pressure. After hot-pressing sintering, it was first partially cooled and depressurized, then completely depressurized and cooled to room temperature to obtain AlN-SiC composite ceramic material. The heating temperature of the hot-pressing sintering was increased from room temperature to 1200℃ at a heating rate of 10℃ / min, and the external pressure was 10MPa. Then it was increased to 1500℃ at a heating rate of 3℃ / min, and the external pressure was 10MPa. Then it was increased to 1700℃ at a heating rate of 10℃ / min, and the external pressure was 30MPa. The temperature was held at this temperature for 30min. After hot-pressing sintering, the temperature was decreased to 1000℃ at a rate of 10℃ / min, and the pressure was depressurized to 10MPa. Then the pressure was depressurized to atmospheric pressure and cooled to room temperature with the furnace.

[0030] Example 4 A method for preparing AlN-SiC composite ceramics by reaction sintering includes the following steps: According to the target AlN-SiC composite ceramic preparation, AlN powder, Si powder and C powder were mixed at a mass fraction of 92%, 5% and 3% respectively (AlN powder, Si powder and C powder were mixed at a mass ratio of 92:5:3). Then, anhydrous ethanol was added and the mixture was wet-milled to obtain a mixed slurry. The mass fraction of C powder was calculated based on the amount of SiC generated by the chemical reaction with Si powder. The obtained mixed slurry was dried to remove the solvent and obtain a dried mixture. The dried mixture was then ground and sieved to obtain a uniformly dispersed composite powder. The obtained composite powder was placed in a vacuum environment and hot-pressed under heating and external pressure. After hot-pressing sintering, it was first partially cooled and depressurized, then completely depressurized and cooled to room temperature to obtain AlN-SiC composite ceramic material. The heating temperature of the hot-pressing sintering was increased from room temperature to 1300℃ at a heating rate of 8℃ / min, and the external pressure was 15MPa. Then it was increased to 1600℃ at a heating rate of 5℃ / min, and the external pressure was 15MPa. Then it was increased to 1900℃ at a heating rate of 15℃ / min, and the external pressure was 50MPa. The temperature was held at this temperature for 20min. After hot-pressing sintering, it was cooled to 800℃ at a rate of 8℃ / min and depressurized to 5MPa. Then it was depressurized to atmospheric pressure and cooled to room temperature with the furnace.

[0031] Example 5 A method for preparing AlN-SiC composite ceramics by reaction sintering includes the following steps: According to the target AlN-SiC composite ceramic, AlN powder, Si powder and C powder are mixed at a mass fraction of 98%, 1% and 1% respectively (AlN powder, Si powder and C powder are mixed at a mass ratio of 92:5:3). Then, anhydrous ethanol is added and the mixture is wet-milled to obtain a mixed slurry. The mass fraction of C powder is calculated based on the amount of SiC generated by the chemical reaction with Si powder. The obtained mixed slurry is dried to remove the solvent and obtain a dried mixture. The dried mixture is then ground and sieved to obtain a uniformly dispersed composite powder. The obtained composite powder was placed in a vacuum environment and hot-pressed under heating and external pressure. After hot-pressing sintering, it was first partially cooled and depressurized, then completely depressurized and cooled to room temperature to obtain AlN-SiC composite ceramic material. The heating temperature of the hot-pressing sintering was increased from room temperature to 1000℃ at a heating rate of 15℃ / min, and the external pressure was 5MPa. Then it was heated to 1300℃ at a heating rate of 2℃ / min, and the external pressure was 5MPa. Then it was heated to 1900℃ at a heating rate of 8℃ / min, and the external pressure was 20MPa. The temperature was held at this temperature for 60min. After hot-pressing sintering, it was cooled to 1200℃ at a heating rate of 15℃ / min and depressurized to 15MPa. Then it was depressurized to atmospheric pressure and cooled to room temperature with the furnace.

[0032] Example 6 A method for preparing AlN-SiC composite ceramics by reaction sintering includes the following steps: According to the target AlN-SiC composite ceramic, AlN powder, Si powder and C powder are mixed at a mass fraction of 95%, 3% and 2% respectively (AlN powder, Si powder and C powder are mixed at a mass ratio of 92:5:3). Then, anhydrous ethanol is added and the mixture is wet-milled to obtain a mixed slurry. The mass fraction of C powder is calculated based on the amount of SiC generated by the chemical reaction with Si powder. The obtained mixed slurry is dried to remove the solvent and obtain a dried mixture. The dried mixture is then ground and sieved to obtain a uniformly dispersed composite powder. The obtained composite powder was placed in a vacuum environment and hot-pressed under heating and external pressure. After hot-pressing sintering, it was first partially cooled and depressurized, then completely depressurized and cooled to room temperature to obtain AlN-SiC composite ceramic material. The heating temperature of the hot-pressing sintering was increased from room temperature to 1000℃ at a heating rate of 15℃ / min, and the external pressure was 5MPa. Then it was heated to 1300℃ at a heating rate of 2℃ / min, and the external pressure was 5MPa. Then it was heated to 1900℃ at a heating rate of 8℃ / min, and the external pressure was 20MPa. The temperature was held at this temperature for 60min. After hot-pressing sintering, it was cooled to 1200℃ at a heating rate of 15℃ / min and depressurized to 15MPa. Then it was depressurized to atmospheric pressure and cooled to room temperature with the furnace.

[0033] Comparative Example 1 The high thermal conductivity SiC-AlN ceramic composite material and its preparation method disclosed in CN102731095A are high thermal conductivity SiC-AlN ceramic composite materials prepared therefrom.

[0034] The AlN-SiC composite ceramic prepared by this invention is significantly different from the SiC-AlN ceramic composite material prepared by CN102731095A.

[0035] Comparative Example 1 directly mixes SiC powder and AlN powder as raw materials, a typical physical blending approach, where SiC is directly introduced into the matrix in the form of pre-synthesized powder. In contrast, this invention uses AlN powder, Si powder, and C powder as raw materials, generating a SiC reinforcing phase through an in-situ chemical reaction between Si and C powders during sintering. This difference leads to the following fundamental variations: The SiC generation mechanism differs; in Comparative Example 1, SiC is passively added, while in this invention, SiC is actively generated. The interfacial bonding state differs; the SiC generated by the in-situ reaction has a chemical bond with the AlN matrix, resulting in a much stronger interfacial bonding than physically mixed SiC. The uniformity of SiC distribution differs; the in-situ reaction achieves uniform SiC distribution at the atomic / molecular scale, avoiding the agglomeration problem of added powders. Regarding raw material composition, Comparative Example 1 uses six components: SiC powder, AlN powder, carbon powder, surfactant, dispersant, and binder. This invention uses only three components: AlN powder, Si powder, and C powder. This invention has a simpler raw material system, requires no organic additives, avoids introducing impurities, and results in pure grain boundaries. Regarding the sintering method, Comparative Example 1 uses pressureless sintering, which relies on solid-state diffusion and requires high temperature and long duration. This invention employs three-stage hot-pressing sintering, resulting in stronger densification driving force. Pressure promotes particle rearrangement, in-situ reaction activation, and plastic flow. This invention achieves higher efficiency through multi-mechanism synergistic densification. In terms of microstructure: In Comparative Example 1, the added SiC powder is typically micron-sized or even larger particles. Due to the introduction of additives, carbon residues or additive decomposition products exist at the grain boundaries after sintering, forming complex grain boundary phases. In this invention, the SiC generated by the reaction of Si and C consists of fine grains (submicron-sized) uniformly distributed within the AlN matrix. Furthermore, due to the absence of additives, the grain boundaries are pure, without glassy or second-phase phases. This microstructure minimizes phonon scattering, which is beneficial for achieving high thermal conductivity.

[0036] AlN-SiC composite ceramics could be prepared using Examples 1 to 6. The AlN-SiC composite ceramics prepared in Examples 1 to 3 were then selected for structural confirmation and performance verification.

[0037] Experimental verification (a) Structural confirmation (1) X-ray diffraction Figure 1The XRD patterns of the AlN-SiC composite ceramic of this invention at different temperatures are shown in the figure. As the sintering temperature increases, the main crystalline phases of the AlN-SiC composite ceramic include AlN, SiC, and a small amount of C, with no other transition phases formed. It can be seen that the addition of Si powder and C powder results in the in-situ formation of the SiC reinforcing phase. At 1700℃, the SiC formed in the system is predominantly hexagonal, with weak peak intensity and low content. A distinct graphite peak is observed at 2θ≈26°, indicating that the reaction is not yet complete and there are still unreacted carbon sources in the system. When the sintering temperature increases to 1800℃, characteristic peaks of cubic SiC appear. Until the sintering temperature reaches 1900℃, the main components in the system are SiC and C phases.

[0038] (2) Cross-sectional morphology diagram Figure 2 This is a SEM image of the AlN-SiC composite ceramic of the present invention sintered at 1700℃. Numerous pores are present within the cross-section, indicating incomplete densification. The AlN grains exhibit a polygonal equiaxed morphology and are fine in size. Figure 3 The grain distribution statistics show that the grain size is mainly concentrated in the range of 0.2μm to 0.6μm, and it is approximately normally distributed. The average grain size is 0.49μm. The grain size uniformity is good, and there is no obvious bimodal distribution or coarse grains, indicating that grain growth is not obvious at 1700℃.

[0039] Figure 4 This is a SEM image of the AlN-SiC composite ceramic of the present invention sintered at 1800℃. Figure 4 It can be seen that the sample's microstructure is dense and uniform, without obvious pores and cracks, indicating that the material has achieved near-complete densification at 1800℃. The grains exhibit a well-developed polyhedral morphology, with straight and tightly bonded grain boundaries. The fracture surface shows a mixed transgranular and intergranular fracture mode, indicating high grain boundary bonding strength. Figure 5The grain size statistics show that the average grain size of the sample is 0.94 μm, with uniform grain size distribution and no obvious abnormal growth, exhibiting a typical fine-grained and dense structure. Driven by a high temperature of 1800℃, the Si source additive melts and fully wets the AlN matrix through capillary action, providing the kinetic driving force for densification. Simultaneously, the C source additive consumes oxygen impurities within the AlN, purifies the grain boundaries, and reacts in situ with liquid Si to generate the SiC second phase. The in-situ generated SiC particles are uniformly dispersed at the AlN grain boundaries, effectively suppressing abnormal AlN grain growth at high temperatures through grain boundary pinning. Therefore, despite the high sintering temperature, the sample still maintains fine and uniform grains with an average particle size of 0.94 μm, without any obvious abnormally large grains. Meanwhile, during the hot pressing sintering process, the applied axial pressure significantly promotes particle rearrangement, pore discharge, and grain boundary slippage, preventing defects such as pores and cracks in the sample caused by volume changes during the Si and C reaction process, and avoiding problems such as loose structure and decreased density.

[0040] Figure 6 This is a SEM image of the AlN-SiC composite ceramic of the present invention sintered at 1900℃. As can be seen from the image, The grains are tightly packed with no obvious porosity, indicating that the hot-pressing pressure effectively maintained a high density even at high temperatures. The grains exhibit typical AlN polyhedral characteristics, but their size is significantly larger than that of the 1800℃ sample. Figure 7 The statistical results of grain size show that excessively high temperature significantly accelerates the grain boundary migration rate. Although the hot pressing pressure can still maintain a high density, the pinning effect of the in-situ generated SiC particles on the AlN grain boundaries is weakened, and the grains begin to grow significantly, with the average grain size increasing to 1.16 μm and the grain size distribution becoming wider.

[0041] (0) Element surface distribution map Figure 8 These are the microstructure and elemental distribution diagrams of the AlN-SiC composite ceramic of this invention sintered at 1800℃. Figure 8 It can be seen that the composite material exhibits tight intergranular bonding, with no obvious pores or cracks observed. The grain size distribution is relatively uniform, and no abnormal grain growth is observed, indicating that the sintering process at 1800℃ effectively promotes material densification. In-situ hot-pressing reaction sintering not only lowers the sintering densification temperature of AlN ceramics, but also inhibits excessive AlN grain growth through pressure-assisted sintering. The local enrichment of Si element confirms that the SiC second phase has been successfully introduced into the AlN matrix. The in-situ reaction sintering produces fine-grained SiC, which can significantly improve the mechanical properties of the material. O element enrichment still exists around the grain boundaries, mainly due to trace oxygen impurities remaining in the raw materials or slight oxidation of the AlN surface during sintering, which may have formed a small amount of liquid phase at the grain boundaries.

[0042] (II) Performance Verification (1) Thermal conductivity and thermal diffusion Figure 9 The figures show the thermal diffusivity and thermal conductivity curves of the AlN-SiC composite material sintered at 1800℃ according to the present invention within the temperature range of 300K to 673K. Figure 9 It is evident that both the thermal diffusivity and thermal conductivity of the composite material decrease with increasing temperature. This is primarily because as temperature rises, lattice vibrations intensify, significantly increasing the probability of collisions between phonons within the ceramic material and drastically shortening the mean free path of the phonons, leading to a rapid decrease in the thermal diffusivity. The material exhibits excellent thermal conductivity at room temperature, with a thermal conductivity of approximately 66 W / m·K and a thermal diffusivity of approximately 32.6 mm² / s. However, when the temperature increases to 673 K, the thermal diffusivity drops to 17.3 mm² / s, and the thermal conductivity decreases to 57 W / m·K.

[0043] (2) Bulk density, porosity and mechanical properties of AlN-SiC composite ceramics Figure 10 The curves show the relative density, flexural strength, and Vickers hardness of AlN-SiC composites at different sintering temperatures. Figure 10 It is evident that sintering temperature significantly affects the density and mechanical properties of the material. With increasing temperature, the density initially increases significantly and then decreases slightly. At 1700℃, the density is only 93.56%, indicating significant incomplete sintering and a large number of closed pores. When the sintering temperature reaches 1800℃, the relative density of the composite material increases to 99.08%, reaching near-complete density, indicating that the increased temperature significantly promotes atomic diffusion and pore elimination. When the sintering temperature further increases to 1900℃, the relative density of the material decreases to 97.85%, mainly because the excessively high temperature leads to abnormal growth of AlN grains, while simultaneously intensifying the high-temperature decomposition of AlN. Grain boundaries rapidly migrate and encapsulate pores, forming intragranular closed pores, which in turn reduces the density. Synchronously with the increase in density, the flexural strength and Vickers hardness of the AlN-SiC composite ceramic also reach peak values ​​of 365 MPa and 22.05 GPa, respectively, at 1800℃.

[0044] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A method for preparing AlN-SiC composite ceramics by reaction sintering, characterized in that, Includes the following steps: AlN powder, Si powder and C powder are mixed at a mass fraction of 92%~98%, 1%~5% and 1%~3%, respectively, and then wet-milled with anhydrous ethanol. The resulting slurry is dried, and the dried mixture is ground and sieved to obtain a uniformly dispersed composite powder. In a vacuum environment, the composite powder is subjected to three-stage hot pressing sintering. After the three-stage hot pressing sintering reaction, the temperature is first partially reduced and the pressure is released, and then the pressure is completely released and cooled to room temperature to obtain AlN-SiC composite ceramic material. The first stage of hot pressing sintering involves heating to 1000℃~1300℃ at a heating rate of 8℃ / min~15℃ / min, with an external pressure of 5MPa~15MPa. The second stage of hot pressing sintering involves heating to 1300℃~1600℃ at a heating rate of 2℃ / min~5℃ / min, with an external pressure of 5MPa~15MPa. The third stage of hot pressing sintering involves heating to 1700℃~1900℃ at a heating rate of 8℃ / min~15℃ / min, with an external pressure of 20MPa~50MPa. The temperature is then held at this temperature for 20min~60min. After the three stages of hot pressing sintering, the temperature is first reduced from 1700℃~1900℃ to 800℃~1200℃ at a cooling rate of 8℃ / min~15℃ / min, and the pressure is reduced from 20MPa~50MPa to 5MPa~15MPa. Finally, the pressure is reduced to atmospheric pressure and cooled to room temperature.

2. The method for preparing AlN-SiC composite ceramics by reaction sintering according to claim 1, characterized in that, The heating temperature of the first stage hot pressing sintering is increased from room temperature to 1200℃ at a rate of 10℃ / min; an external pressure of 10MPa is applied. The heating temperature of the second stage hot pressing sintering is increased to 1500℃ at a rate of 3℃ / min; an external pressure of 10MPa is applied. The heating temperature of the third stage hot pressing sintering is increased to 1800℃ at a rate of 10℃ / min; an external pressure of 30MPa is applied, and the temperature is held at this temperature for 30min.

3. The method for preparing AlN-SiC composite ceramics by reaction sintering according to claim 1, characterized in that, After three-stage hot pressing and sintering, the temperature is first reduced from 1800℃ to 1000℃ at a cooling rate of 10℃ / min, and the pressure is reduced from 50MPa to 10MPa. Then the pressure is reduced to atmospheric pressure and cooled to room temperature with the furnace.

4. The method for preparing AlN-SiC composite ceramics by reaction sintering according to claim 1, characterized in that, The mass fractions of AlN powder, Si powder, and C powder are 97%, 2%, and 1%, respectively.

5. The AlN-SiC composite ceramic material prepared by the reaction sintering method according to any one of claims 1 to 4.

6. The AlN-SiC composite ceramic according to claim 5, characterized in that, The AlN-SiC composite ceramic material has a thermal conductivity of 60 W / m·K to 70 W / m·K at room temperature, a flexural strength of 300 MPa to 400 MPa, and a Vickers hardness of not less than 20 GPa.

7. The application of the AlN-SiC composite ceramic according to claim 5 in high-power electronic device substrates, microwave attenuation materials and aerospace thermal structural components.

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