Preparation method of high-thermal-conductivity silicon nitride ceramic substrate

Abstract

The application relates to the technical field of high-performance electronic ceramic materials, and particularly relates to a preparation method of a high-thermal-conductivity silicon nitride ceramic substrate. The method uses alpha-Si3N4 powder as a main material, adds beta-Si3N4 whiskers and Y2O3 and MgO composite sintering aids, prepares slurry through wet ball milling, adopts a tooth comb-shaped doctor blade to perform shear-induced flow casting, and makes the whiskers preferentially arrange along the flow direction; the obtained green body is degreased, and then subjected to pulse gas pressure sintering in a nitrogen atmosphere, wherein the pulse gas pressure periodically changes. Through the synergistic effect of shear-induced whisker orientation and pulse pressure promoting densification, the method successfully prepares a dense ceramic substrate with high beta-Si3N4 phase content, low porosity, high columnar crystal orientation and large average length-diameter ratio, and the thermal conductivity of the ceramic substrate along the grain orientation direction is high.

Description

Technical Field

[0001] This invention relates to the field of high-performance electronic ceramic materials technology, specifically to a method for preparing a silicon nitride ceramic substrate with high thermal conductivity. Background Technology

[0002] Silicon nitride (Si3N4) ceramics are widely considered ideal packaging and heat dissipation substrate materials for high-power electronic devices, integrated circuits, LED lighting, and microwave radio frequency applications due to their excellent high-temperature stability, high mechanical strength, good insulation properties, and thermal expansion coefficient matching that of silicon. The thermal conductivity of the ceramic substrate is a key indicator for evaluating its heat dissipation performance, directly determining the power density, reliability, and lifespan of electronic devices.

[0003] To obtain Si3N4 ceramics with high thermal conductivity, the core technology lies in promoting the formation of the highly thermally conductive β-Si3N4 phase during the sintering process and developing it into long columnar crystals preferentially grown along the long axis (c-axis). The oriented arrangement of these columnar crystals can construct efficient phonon transport channels, thereby significantly improving the thermal conductivity in a specific direction. Simultaneously, high density, low porosity, and clean grain boundaries are required to reduce phonon scattering.

[0004] Currently, the industry commonly uses α-Si3N4 as raw material, adds sintering aids, forms green blanks through tape casting, and then densifies them through gas pressure sintering when preparing high thermal conductivity Si3N4 ceramic substrates. However, this traditional method still faces several bottlenecks in achieving ultra-high thermal conductivity.

[0005] In conventional tape casting processes, the powder particles in the green body are randomly distributed. During subsequent sintering, although β-Si3N4 columnar crystals can grow, their growth direction is chaotic, resulting in an isotropic microstructure in the sintered body. This disordered grain arrangement makes the phonon transport path tortuous and increases scattering, severely limiting further improvement in the material's thermal conductivity and making it difficult to break through the bottleneck of 90W / (m·K)~100W / (m·K), thus failing to meet the demand for directional and efficient heat dissipation in next-generation ultra-high power devices. In recent years, some studies have attempted to induce β-Si3N4 seed orientation using an external magnetic field, but this method requires expensive superconducting magnet equipment, with magnetic field strength typically exceeding 8T, leading to huge equipment investment and high operating and maintenance costs, hindering large-scale industrial application. Furthermore, magnetic field orientation imposes strict window limitations on process parameters such as slurry viscosity and tape casting speed, and for non-magnetic or weakly magnetic powder systems, the orientation driving force is limited, making it difficult to stably obtain green bodies with high orientation.

[0006] Second, traditional gas pressure sintering is usually carried out under constant pressure or slow pressure increase conditions. Its efficiency in mass transport and closed-pore elimination during liquid-phase sintering is limited, resulting in residual closed pores in the final material. To achieve complete densification, excessively high temperatures or excessively long holding times are required, the latter of which can cause abnormal grain growth and impair the material's mechanical properties. Furthermore, constant pressure conditions have insufficient ability to control grain boundary migration and grain morphology, making it difficult to simultaneously optimize density and the ideal columnar crystal structure.

[0007] Third, the grain boundary glass phase formed by sintering aids is one of the main sources of phonon scattering. Traditional processes lack effective means to actively control the composition, distribution, and crystallinity of the grain boundary phase, and excessive amorphous phase residues will significantly reduce the thermal conductivity of the material. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a method for preparing a high thermal conductivity silicon nitride ceramic substrate that can precisely control the grain orientation and achieve efficient densification.

[0009] The technical solution adopted by this invention to solve its technical problem is: a method for preparing a high thermal conductivity silicon nitride ceramic substrate, comprising the following steps: (1) Slurry preparation: α-Si3N4 powder, β-Si3N4 whiskers and Y2O3 and MgO composite sintering aid are mixed to prepare a casting slurry; (2) Shear-induced tape casting: A toothed comb-shaped scraper is used, and the scraper tip height H satisfies 10L≤H≤40L, where L is the average length of β-Si3N4 whiskers. The tape casting slurry is tape cast at a tape casting speed of 0.3m / min~0.5m / min, so that the long axis of β-Si3N4 whiskers is parallel to the tape casting base plane and is preferentially arranged along the tape casting direction to obtain a green tape. The Lotlin orientation factor of the green tape is ≥0.45. (3) Pulse gas pressure sintering: After the green strip is degreased, it is placed in a nitrogen atmosphere for gas pressure sintering, wherein a periodically changing pulse gas pressure is applied during the high temperature holding stage of sintering at 1850℃~1900℃.

[0010] The core innovation of this invention lies in the creative combination of two key technologies: shear-induced tape casting and pulsed gas pressure sintering. Shear-induced tape casting utilizes a shear flow field generated by a toothed scraper to force the long axis of β-Si3N4 whiskers to align preferentially parallel to the casting direction during the tape casting process. This achieves high whisker orientation without the need for an external magnetic field, providing a template for the directional epitaxial growth of columnar crystals in subsequent sintering and fundamentally solving the problem of disordered grains. Pulsed gas pressure sintering, during the critical densification stage, generates unique "pumping" and "micro-forging" effects through periodic pressure fluctuations. This significantly enhances liquid phase fluidity and mass transport rate, efficiently eliminates closed pores, and precisely controls grain boundary migration and grain morphology. The synergistic effect of these two technologies ensures the final product is a silicon nitride ceramic with a highly oriented columnar crystal structure, extremely high density, and clean grain boundaries.

[0011] The toothed scraper of the present invention has a scraper mouth composed of a series of parallel tooth-like structures with a tooth pitch of 50μm to 200μm and a tooth depth of 1 / 3 to 1 / 2 of the scraper mouth height H. It can generate a strong local shear flow field when the slurry flows through, forcing the long axis of β-Si3N4 whiskers to align along the casting direction.

[0012] Preferably, the teeth of the comb-shaped scraper are rectangular or trapezoidal, with a tooth width of 1 / 3 to 1 / 2 of the tooth pitch and a tooth count of 10 to 50 per cm. The edges of the teeth need to be rounded (R=5~10μm) to avoid damage to the whiskers from sharp edges. The scraper material is cemented carbide or wear-resistant stainless steel, with a surface roughness Ra≤0.4μm.

[0013] Specifically, in step (1), the mass percentage of α-Si3N4 powder is 85%~92%, and the mass percentage of β-Si3N4 whiskers is 5%~8%. The α-Si3N4 content of the main material ensures sufficient matrix phase, and the β-Si3N4 whisker content is within this range. This provides enough "seeds" for orientation growth, effectively inducing and accelerating the α-phase to β-phase transformation and columnar crystal development, while avoiding excessive whiskers that could lead to a surge in viscosity or excessive grain entanglement in the early stages of sintering.

[0014] Preferably, the total mass percentage of Y2O3 and MgO used as sintering aids in step (1) is 5% to 8%, wherein the mass ratio of Y2O3 to MgO is 5 to 10:1.

[0015] Y2O3 and MgO composites, as sintering aids, can react with SiO2 on the surface of Si3N4 particles at sintering temperature to form a Y-Mg-Si-ON liquid phase. This liquid phase has a suitable viscosity, which is sufficient to wet the particles and promote mass transport to achieve complete densification. At the same time, its quantity and properties can partially crystallize or form an ultrathin grain boundary phase during subsequent cooling, minimizing the residual amorphous phase that causes severe scattering of phonon transport, which is beneficial to obtaining higher thermal conductivity.

[0016] The mass ratio of Y₂O₃ to MgO is controlled at 5:1 to 10:1, with MgO accounting for no more than 16.7% of the composite additive. At this ratio, MgO acts only as a transient liquid phase formation promoter, reacting with SiO₂ on the Si₃N₄ surface at around 1390℃ to generate a low-viscosity Mg-Si-ON liquid phase, reducing the sintering activation energy and promoting α→β phase transformation and densification. Simultaneously, Mg is almost insoluble in the β-Si₃N₄ lattice without Al impurities, thus avoiding the introduction of point defect scattering phonons. Combined with a sintering temperature of 1850–1900℃ and the grain boundary purification effect of pulsed gas pressure sintering, the residue of low-melting-point grain boundary phases can be effectively avoided, ensuring a thermal conductivity >120 W / (m·K).

[0017] Preferably, the viscosity of the slurry in step (1) is 2500 mPa·s to 4000 mPa·s, and the solid content is 52% to 58%. Within this viscosity range, the slurry has suitable rheological properties, which can ensure that the whiskers have sufficient degrees of freedom to rotate and orient under shear flow field, and can also prevent the formation of orientation structure from being destroyed by severe sedimentation or "particle migration" due to excessively low viscosity during casting.

[0018] Appropriate solid content ensures that the green body has sufficient strength and density, reduces the risk of cracking during drying and degreasing, and avoids problems such as poor slurry leveling and increased internal defects caused by excessive solid content.

[0019] Preferably, the slurry in step (1) is prepared by wet ball milling, with anhydrous ethanol as the milling medium and a milling time of 12h to 24h.

[0020] The wet ball milling process using anhydrous ethanol as the medium effectively prevents the hydrolysis of Si3N4 powder in water and achieves nanoscale uniform mixing and dispersion of α-Si3N4 powder, sintering aids, and β-Si3N4 whiskers through prolonged mechanical grinding and dispersion. Thorough ball milling breaks up soft agglomerates of the powder, allowing the sintering aids to uniformly coat the surface of the Si3N4 particles, ensuring uniform formation and distribution of the liquid phase during sintering. Simultaneously, moderate ball milling also slightly activates the α-Si3N4 particles, reducing the activation energy for subsequent phase transitions.

[0021] Specifically, in step (2), the shear-induced casting process uses a toothed comb-shaped scraper with a scraper tip height H satisfying 10L≤H≤40L, where L is the average length of the β-Si3N4 whiskers. The toothed comb-shaped scraper can generate a strong shear flow field in the slurry, causing the long axis of the β-Si3N4 whiskers to be preferentially aligned along the casting direction. The ratio between the scraper tip height and the whisker length is crucial: if H is too small, the excessive shear force may cause the whiskers to break or become disordered; if H is too large, the shear flow field strength is insufficient and cannot effectively drive the whisker orientation. Within this range, the shear flow field can provide sufficient orientation driving force while maintaining the stability of the slurry flow.

[0022] Specifically, the casting speed in step (2) is 0.3m / min to 0.5m / min. After casting, the material is dried. The drying process uses a multi-stage gradient heating, and the drying temperature range is 40℃ to 60℃.

[0023] A lower casting speed allows for a longer orientation time for the whiskers in the shear flow field, which is beneficial for obtaining a higher degree of orientation. The casting speed is well matched with the rheological properties of the slurry, ensuring both production efficiency and the formation of a stable "cast meniscus" in front of the doctor blade, resulting in a wet film of uniform thickness. The drying process employs a multi-stage gradient heating strategy of 40℃~60℃. This mild and controlled drying process maximizes the preservation of the shear-induced whisker orientation structure and ensures that the green body has excellent integrity and mechanical strength.

[0024] Specifically, the waveform of the pulsed air pressure in step (3) is a square wave, a sine wave, or a triangular wave, and the pulse frequency is 0.2Hz~1.0Hz.

[0025] Compared to constant pressure, periodically changing pressure can dynamically alter the pressure difference between the closed pores and the external pressure, causing the pores to expand and contract periodically, making them more prone to rupture and connect with the open pore network, thus effectively eliminating airflow. If the frequency is too low, it approximates a slow pressurization, resulting in a weak pulse effect; if the frequency is too high, the pressure change is too rapid, and the high-temperature viscous liquid phase may not have enough time to respond, reducing the effectiveness and placing stringent requirements on the equipment's sealing system. At this preferred frequency, the pulsed pressure can efficiently drive the periodic flow of the liquid phase, promoting particle rearrangement and the dissolution-precipitation process, achieving densification efficiency far exceeding that of traditional constant-pressure sintering. Preferably, the waveform of the pulsed pressure is a square wave, which can produce the most significant "switching" effect.

[0026] Preferably, the pulse pressure varies periodically within the range of 2MPa to 10MPa, and the ratio of the duration of the high pressure value to the duration of the low pressure value is 1:1 to 3.

[0027] The pressure varies between troughs and peaks, ensuring a "time window" for pore expansion and gas diffusion during low-pressure periods, while simultaneously applying sufficient hydrostatic pressure during high-pressure periods to drive densification and inhibit Si3N4 decomposition. The ratio of high to low pressure duration is set to 1:1~3, meaning the low-pressure duration is equal to or slightly longer than the high-pressure duration. This setting ensures sufficient time for the trapped gas to escape from the reopened channels after each high-pressure impact promotes densification, preventing the formation of new closed pores. This "pressurization-relaxation" cyclic mode achieves "dynamic fine processing" of the pore structure within the sintered body, promoting the acquisition of ultra-dense bodies with porosity below 0.5%.

[0028] Specifically, step (3) includes: (3.1) Degrease the green belt by heating it to 300℃~500℃ in air at a rate of 1℃ / min~3℃ / min; (3.2) Then, in a nitrogen atmosphere, the temperature is raised to 1600℃~1650℃ for pre-sintering; (3.3) The temperature is raised to a sintering temperature of 1850℃~1900℃, and the pulsed gas pressure is applied at this temperature. After holding at this temperature for 3h~5h, the product is obtained after post-treatment.

[0029] The degreasing stage uses an air atmosphere, which helps the organic binder to completely decompose into gaseous products in the presence of oxygen and avoids carbon buildup due to cracking under a protective atmosphere. A slow heating rate allows for the gradual release of decomposition gases, preventing blistering, cracking, or delamination of the green body. Pre-sintering is performed in nitrogen, where an initial liquid phase begins to form, particles undergo preliminary rearrangement, the green body shrinks significantly, and it gains a certain strength. This prepares a relatively dense and strong green body framework for subsequent sintering at higher temperatures, reducing the risk of deformation during final sintering. Since the eutectic temperature of the Y-Mg-Si-ON liquid phase formed by pure Y2O3 additives is slightly higher than that of the Y-Al-Si-ON system, raising the sintering temperature to 1850℃~1900℃ ensures sufficient liquid phase formation, which, combined with pulsed gas pressure, achieves complete densification. Holding at this temperature for 3h~5h, combined with pulsed gas pressure, ensures complete phase transformation, sufficient grain development, and the achievement of final ultra-high densification through pulsed action.

[0030] Preferably, the β-Si3N4 whiskers described in step (1) are ultrasonically dispersed and dried before use to remove agglomerates. The β-Si3N4 whiskers achieve orientation alignment in the shear flow field by coupling their long axis with the flow field direction, eliminating the need for additional magnetization. This not only simplifies the process but also avoids the introduction of ferromagnetic impurities, ensuring the high insulation performance of the final ceramic substrate. The average length L of the β-Si3N4 whiskers is obtained statistically from scanning electron microscopy (SEM) images: at least 100 whiskers are taken, their longest dimension is measured, and the arithmetic mean is calculated.

[0031] Preferably, the slurry in step (1) is a water-based slurry, the solvent of the water-based slurry is water, and the water-based slurry also contains a dispersant, a binder and a plasticizer.

[0032] Traditional casting processes often use alcohol-ketone organic solvents, which suffer from high costs, rapid evaporation, flammability, explosiveness, environmental pollution, and health hazards to operators. This invention provides a water-based slurry system using water as a solvent, and selects matching dispersants, binders, and plasticizers to fundamentally solve these problems. The water-based system is safe, non-toxic, and inexpensive. By overcoming the hydrolysis tendency of Si3N4 powder in water through pH adjustment and dispersant selection, VOC emissions during the production process are significantly reduced.

[0033] More preferably, the dispersant is ammonium polyacrylate, and the binder is an acrylic emulsion or a cellulose ether.

[0034] To suppress the hydrolysis of Si3N4 in water, the pH of the slurry needs to be adjusted to a strongly alkaline range of 10.0–11.0. This can be achieved by adding alkaline pH adjusters such as ammonia or tetramethylammonium hydroxide (TMAH). Under these pH conditions, a stable negatively charged layer forms on the surface of the Si3N4 particles, which electrostatically repels the ammonium polyacrylate dispersant, thus achieving good dispersion stability.

[0035] Preferably, the thickness of the green strip in step (2) is 0.2 mm to 0.6 mm, the aspect ratio of the β-Si3N4 whiskers is 10 to 20:1, and the Lotgering orientation factor of the green strip is ≥0.45.

[0036] Controlling the green strip thickness to 0.2mm~0.6mm is suitable for the needs of most electronic packaging substrates. Within this thickness range, the slurry casting uniformity is good, stress during drying and degreasing is easy to control, and the temperature field is uniform during subsequent sintering, which is beneficial for obtaining flat, non-deformed substrates. The closer the Lottelin orientation factor is to 1, the higher the degree of preferred orientation of the whiskers along the casting direction. The high green strip Lottelin orientation factor of this invention can ensure a high degree of inheritance and consistency in the epitaxial growth of columnar crystals during subsequent sintering.

[0037] The high thermal conductivity silicon nitride ceramic substrate prepared by the above method has a β-Si3N4 phase content greater than 98%, a thermal conductivity along the grain orientation direction greater than 120 W / (m·K), a porosity less than 0.5%, an average aspect ratio of columnar crystals greater than 8:1, and a volume resistivity greater than 10. 14 Ω·cm. Furthermore, the prepared ceramics exhibit a significant, preferentially growing orientation along {101}.

[0038] The most significant feature of the silicon nitride ceramic substrate prepared by this invention lies in its highly oriented microstructure. β-Si3N4 columnar crystals are preferentially arranged along a specific direction, forming a straight and well-connected high-speed phonon transport channel resembling a "forest." This structure fundamentally reduces phonon scattering. Combined with the ultra-high density and cleaned grain boundaries resulting from pulsed pressure sintering, the mean free path of phonons is significantly increased. Therefore, this substrate exhibits ultra-high thermal conductivity unmatched by traditional isotropic Si3N4 ceramics, making it particularly suitable for high-power, high-density electronic devices requiring directional and efficient heat dissipation.

[0039] Compared with existing technologies, this invention has the following advantages: This invention, through shear-induced tape casting, for the first time utilizes the shear flow field generated by a toothed comb-shaped scraper to pre-set the crystal orientation template during the forming stage. This achieves high whisker orientation without expensive magnetic field equipment, solving the fundamental problems of random grain growth and isotropic thermal conductivity in traditional methods. It provides a new, low-cost, and easily industrialized approach for preparing ceramic substrates with directional ultra-high thermal conductivity. The innovative pulsed gas pressure sintering process, through dynamic pressure control, greatly enhances the kinetics of liquid-phase sintering, achieving near-theoretical density and almost complete elimination of porosity, enabling the thermal conductivity of silicon nitride ceramics to exceed 120 W / (m·K). Attached Figure Description

[0040] Figure 1 The image shows the XRD diffraction pattern of the silicon nitride ceramic substrate prepared in Example 1 of this invention. Detailed Implementation

[0041] The present invention will be further described below with reference to specific embodiments, wherein Embodiment 1 is a baseline embodiment, and process conditions not explicitly described in other embodiments are all based on Embodiment 1.

[0042] Example 1 The main raw materials include: 880.0g of α-Si3N4 powder with a purity of 99.9% and an average particle size of 0.8μm, 80.0g of specially treated β-Si3N4 whiskers, and 44.4g of Y2O3 and 5.6g of MgO. The β-Si3N4 whiskers are ultrasonically dispersed in anhydrous ethanol to remove agglomerates before use, and then dried. The whisker content is 8wt%, the total content of Y2O3 and MgO is 5wt%, and the mass ratio of Y2O3 to MgO is 8:1.

[0043] All the above-mentioned powder raw materials, along with the ammonium polyacrylate solution as a dispersant, were placed in a planetary ball mill and ball-milled for 20 hours using anhydrous ethanol as the medium to ensure uniform mixing and eliminate soft agglomerations. Subsequently, the ball-milled slurry was transferred to a stirred tank, where deionized water, acrylic emulsion binder, and polyethylene glycol-400 plasticizer were added for adjustment. Ammonia was added to adjust the pH to 10.5. By controlling the amount of water added, the viscosity of the final slurry was adjusted to approximately 3000 mPa·s, with a solid content of 55%. After vacuum degassing and aging for 24 hours, a cast slurry with excellent flowability and stability was obtained.

[0044] Subsequently, shear-induced tape casting was performed using a toothed comb-shaped doctor blade. The average length L of the β-Si3N4 whiskers was approximately 20 μm, and the doctor blade height H was set to 300 μm, satisfying 10L≤H≤40L. The casting speed was 0.4 m / min, and the wet film was slowly dried under a multi-stage gradient hot air at 40℃~60℃, ultimately yielding a green tape with a thickness of 0.30 mm and a Lottelin orientation factor of 0.78.

[0045] After cutting the green body strip, it was placed in a sintering furnace. First, it was heated to 400℃ in air at a rate of 2℃ / min and held for 2 hours to completely remove organic matter. Then, a high-purity nitrogen atmosphere was switched, and the temperature was raised to 1650℃ for pre-sintering. Next, the temperature was raised to the final sintering temperature of 1870℃, and a pulsed gas pressure sintering program was initiated at this stage: the gas pressure was varied between 2MPa and 10MPa using square wave pulses at a frequency of 0.5Hz, with a high-to-low pressure duration ratio of 1:2. Under these conditions, it was held for 4 hours. After sintering, controlled cooling was performed to obtain a dense ceramic body. This body underwent double-sided grinding and polishing to obtain a substrate with a smooth surface, and the volume conductivity was measured to reach 1×10⁻⁶. 14 Ω·cm, XRD diffraction pattern see Figure 1 ,from Figure 1 It can be seen that the silicon nitride ceramic substrate has a significant orientation that preferentially grows along {101}.

[0046] Example 2 855.0 g of α-Si3N4 powder, 80.0 g of β-Si3N4 whiskers (dispersed by ultrasonication to remove agglomerates), 52.0 g of Y2O3, and 6.5 g of MgO were mixed. The whisker content was 8 wt%, the total content of Y2O3 and MgO was 5.85 wt%, and the mass ratio of Y2O3 to MgO was 8:1. The viscosity of the slurry after preparation was 4000 mPa·s, and the solid content was 55%. A toothed comb-shaped doctor blade with a blade height H of 250 μm was used for casting at a speed of 0.3 m / min to obtain a green ribbon with a thickness of 0.2 mm and a Lottelin orientation factor of 0.70. During sintering, the temperature is increased to 500℃ at a rate of 1℃ / min during the degreasing stage, then pre-sintered in nitrogen at 1600℃, and finally pulsed gas pressure sintering is carried out at 1850℃. The pulse waveform is a square wave with a frequency of 0.2Hz, and the pressure varies between 2MPa and 10MPa with a duty cycle of 1:3. The temperature is held for 4 hours.

[0047] Example 3 The raw materials used were the baseline formulation of Example 1. The key process change was as follows: a toothed scraper with a scraper tip height H of 400 μm was used, and casting was performed at a speed of 0.5 m / min. The green strip drying temperature was varied from 40℃ to 60℃. Under these conditions, the green strip thickness was 0.6 mm, and the Lottery orientation factor reached 0.80. The sintering process was the same as in Example 1, but the pulsed gas pressure was a square wave with a frequency of 1.0 Hz, and the pressure varied between 2 MPa and 10 MPa. The high-to-low pressure duration ratio was 1:2, and the holding time was 4 hours.

[0048] Example 4 The raw material preparation and slurry forming steps were exactly the same as in Example 1. A toothed scraper was used, with a scraper tip height H of 300 μm and a casting speed of 0.4 m / min, to obtain a 0.3 mm thick green strip with a Lotterin orientation factor of 0.75. During the sintering stage, when the temperature reached 1900℃, a square wave pulsed gas pressure with a frequency of 0.5 Hz was applied, and the pressure cyclically changed between 2 MPa and 10 MPa, with a high-to-low pressure duration ratio of 1:1, and the temperature was maintained for 4 hours.

[0049] Example 5 880.0 g of α-Si3N4 powder, 80.0 g of β-Si3N4 whiskers (dispersed by ultrasonication to remove agglomerates), 44.4 g of Y2O3, and 5.6 g of MgO were mixed, with a whisker content of 8 wt%, a total Y2O3 and MgO content of 5 wt%, and a Y2O3 to MgO mass ratio of 8:1. The slurry viscosity after preparation was 3000 mPa·s, and the solid content was 55%. A toothed comb-shaped doctor blade with a blade height H of 300 μm was used for casting at a speed of 0.4 m / min to obtain a green ribbon with a thickness of 0.3 mm and a Lottelin orientation factor of 0.72. During the degreasing stage, the temperature was increased to 400℃ at a rate of 2℃ / min, and then pre-sintered in nitrogen at 1630℃. Finally, pulsed gas pressure sintering was carried out at 1850℃. The pulse waveform was a square wave with a frequency of 0.5Hz. The pressure varied between 2MPa and 10MPa with a high-low pressure duration ratio of 1:2, and the temperature was held for 4 hours.

[0050] Example 6 The raw material powder was the same as in Example 1, but the slurry preparation used a completely water-based system. Deionized water was used as the solvent, with ammonium polyacrylate as the dispersant, hydroxypropyl methylcellulose as the binder, and polyethylene glycol as the plasticizer. The pH was adjusted to 10.5, the solid content to 55%, and the viscosity to 3000 mPa·s. A toothed scraper with a scraper height H of 300 μm was used for casting at a speed of 0.4 m / min. Gradient drying was performed at 40℃~60℃ to obtain a 0.3 mm thick green strip with a Lotterin orientation factor of 0.74. Pulse pressure sintering was performed at 1850℃, with a square wave frequency of 0.5 Hz, a pressure of 2~10 MPa, a high-low pressure duration ratio of 1:2, and a holding time of 4 h.

[0051] Example 7 880.0 g of α-Si3N4 powder, 80.0 g of β-Si3N4 whiskers (dispersed by ultrasonication to remove agglomerates), 44.4 g of Y2O3, and 5.6 g of MgO were mixed to prepare a slurry with a viscosity of 3000 mPa·s and a solid content of 55%. A comb-shaped scraper with a scraper tip height H of 300 μm was used for casting at a speed of 0.4 m / min to obtain a 0.3 mm thick green strip with a Lottelin orientation factor of 0.78. The subsequent pulsed gas pressure sintering process was identical to that used in Example 1.

[0052] Example 8 The raw material powder was the same as in Example 1. The slurry was prepared using deionized water as the sole solvent, with 1.2% ammonium polyacrylate added as a dispersant. The pH of the slurry was adjusted to 10.5 with ammonia under high-speed stirring to inhibit Si3N4 hydrolysis. Subsequently, 5.5% acrylic emulsion was added as a binder, and 2.0% polyethylene glycol-400 was added as a plasticizer, ultimately adjusting the slurry solid content to 55% and the viscosity to 3000 mPa·s. A toothed doctor blade with a blade height H of 300 μm was used, and the slurry was cast at a speed of 0.4 m / min, followed by gradient drying at 40℃ to 60℃ to produce a 0.3 mm thick green strip with a Lottelin orientation factor of 0.77. The degreasing and pulsed gas pressure sintering processes were exactly the same as in Example 1.

[0053] Example 9 The raw materials and green strip preparation were the same as in Example 1. During the sintering stage, when the temperature reached 1870℃, pulsed gas pressure was applied with the following parameters: the waveform was a square wave, the frequency was 0.5Hz, and the pressure changed stepwise between 2MPa and 10MPa. In each cycle, the high pressure lasted for 1.33s and the low pressure lasted for 2.67s. The temperature was maintained for 3h under these conditions.

[0054] Example 10 Following the formulation and slurry preparation process of Example 1, a green strip with a thickness of 0.2 mm and a Lottery orientation factor of 0.83 was prepared using a toothed scraper with a scraper tip height H of 350 μm and a casting speed of 0.4 m / min. The green strip was then sintered. During the degreasing stage, the temperature was increased to 450 °C at a rate of 2 °C / min, followed by pre-sintering in nitrogen at 1650 °C, and finally pulsed pressure sintering at 1880 °C with the same parameters as in Example 1.

[0055] Example 11 Raw material formulation ratio: 91.0 wt% high-purity α-Si3N4 powder was used as the main material, with 8.0 wt% of ultrasonically dispersed and de-agglomerated β-Si3N4 whiskers added as an orientation template, and a total addition of 5.0 wt% of Y2O3 and MgO composite sintering aid, with a Y2O3 to MgO mass ratio of 5:1. Slurry preparation: Deionized water was used as the solvent, with the addition of dispersant ammonium polyacrylate, binder acrylic emulsion, and polyethylene glycol-400 plasticizer. Anhydrous ethanol was used as the medium for wet ball milling for 18 hours, and the slurry viscosity and solid content were precisely controlled at 3500 mPa·s and 52%, respectively. A toothed comb-shaped scraper was used, with a scraper height H of 300 μm and a casting speed of 0.4 m / min. The drying process employed a multi-stage gradient heating program from 40℃ to 60℃, resulting in a green strip with a thickness of 0.3 mm and a Lottelin orientation factor of 0.82. Sintering process: First, the temperature is increased to 400℃ at a rate of 2℃ / min in air atmosphere to complete safe degreasing; second, high-purity nitrogen is switched, and the temperature is increased to 1650℃ for pre-sintering; third, the temperature is further increased to the sintering temperature of 1870℃, and pulsed gas pressure is applied during this stage. Pulse parameters: The waveform is a square wave with a frequency of 0.5Hz, the pressure changes periodically between 2MPa and 10MPa, a high-pressure:low-pressure duty cycle of 1:2 is used, and the temperature is held for 3 hours.

[0056] Comparative Example 1 The raw material mixing and slurry preparation processes were the same as in Example 1. During tape casting, a conventional flat scraper was used, forming at a speed of 0.6 m / min, and drying was performed under the same temperature gradient to obtain a green tape with a thickness of approximately 0.30 mm and a Lottery orientation factor of only 0.08. The sintering process employed a two-step gas pressure sintering: the debinding procedure was the same as in Example 1, followed by direct heating to 1850 °C in a nitrogen atmosphere, with a constant nitrogen pressure of 6 MPa applied, and holding at this temperature for 3 hours. The volumetric conductivity of the finished product was measured to be 5 × 10⁻⁶. 12 Ω·cm.

[0057] Comparative Example 2 The raw material processing and slurry preparation were the same as in Comparative Example 1. The green strip preparation parameters were the same as in Comparative Example 1, resulting in a green strip with a Lotterin orientation factor of approximately 0.08. The sintering process adopted the pulsed gas pressure sintering procedure of Example 1: a square wave pulsed gas pressure was applied at 1870℃ with a frequency of 0.5Hz, and the pressure varied between 2MPa and 10MPa, with a high-to-low pressure duration ratio of 1:2, and the holding time was 4 hours. The volumetric conductivity of the finished product was measured to be 8×10⁻⁶. 12 Ω·cm.

[0058] Comparative Example 3 Raw material processing, slurry preparation, and shear-induced casting were performed according to the process of Example 1 to obtain a green tape with a Lotterin orientation factor of 0.78. During the sintering stage, the same conventional constant-pressure sintering process as Comparative Example 1 was used: the temperature was directly raised to 1850°C in a nitrogen atmosphere, a constant nitrogen pressure of 6 MPa was applied, and the temperature was held for 3 hours. The volumetric conductivity of the finished product was measured to be 2 × 10⁻⁶. 13 Ω·cm.

[0059] The substrates obtained in the examples and comparative examples were subjected to performance tests: phase composition and orientation were analyzed by X-ray diffraction (XRD); porosity was characterized by Archimedes' displacement method combined with scanning electron microscopy (SEM) image analysis; thermal conductivity was determined by laser scintillation (ASTM E1461); flexural strength and fracture toughness were tested according to ASTM C1161 (three-point bending) and ASTM C1421 (single-sided notched beam method), respectively; dielectric constant was measured using an LCR meter. The test results are shown in Table 1.

[0060] Table 1 Performance parameters of the examples and comparative examples

[0061] Table 1 shows that Examples 1-11, using the method of this invention, are significantly superior to the comparative examples in terms of β-Si3N4 phase content (98.2%-99.3%), orientation factor (0.70-0.85), and thermal conductivity along the orientation direction (121 W / (m·K)-140 W / (m·K). Comparative Example 1, using conventional tape casting followed by constant pressure sintering, has a thermal conductivity of only 65 W / (m·K); Comparative Example 2, using conventional tape casting followed by pulsed gas pressure, has a thermal conductivity of 85 W / (m·K); and Comparative Example 3, using shear-induced tape casting followed by constant pressure sintering, has a thermal conductivity of 102 W / (m·K). The results indicate that the synergistic effect of shear-induced tape casting and pulsed gas pressure sintering is indispensable, jointly achieving high orientation, ultra-high density, and excellent thermal conductivity exceeding 120 W / (m·K).

Claims

1. A method for preparing a high thermal conductivity silicon nitride ceramic substrate, characterized in that, Includes the following steps: (1) Slurry preparation: α-Si3N4 powder, β-Si3N4 whiskers and Y2O3 and MgO composite sintering aid are mixed to prepare a casting slurry; (2) Shear-induced tape casting: A toothed comb-shaped scraper is used, and the scraper tip height H satisfies 10L≤H≤40L, where L is the average length of β-Si3N4 whiskers. The tape casting slurry is cast at a casting speed of 0.3m / min~0.5m / min to obtain a green tape. (3) Pulse gas pressure sintering: After the green strip is degreased, it is placed in a nitrogen atmosphere for gas pressure sintering, wherein a periodically changing pulse gas pressure is applied during the high temperature holding stage of sintering at 1850℃~1900℃.

2. The method for preparing a high thermal conductivity silicon nitride ceramic substrate according to claim 1, characterized in that, In step (1), the α-Si3N4 powder accounts for 85%~92% of the mass, the β-Si3N4 whiskers account for 5%~8% of the mass, and the total mass percentage of Y2O3 and MgO is 5%~8%, wherein the mass ratio of Y2O3 to MgO is 5~10:

1.

3. The method for preparing a high thermal conductivity silicon nitride ceramic substrate according to claim 1, characterized in that, The viscosity of the slurry in step (1) is 2500 mPa·s to 4000 mPa·s, and the solid content is 52% to 58%.

4. The method for preparing a high thermal conductivity silicon nitride ceramic substrate according to claim 1, characterized in that, The slurry described in step (1) is prepared by wet ball milling, with anhydrous ethanol as the milling medium and a milling time of 12h to 24h.

5. The method for preparing a high thermal conductivity silicon nitride ceramic substrate according to claim 1, characterized in that, After casting in step (2), the product is dried using a multi-stage gradient heating method, with the drying temperature range being 40℃~60℃.

6. The method for preparing a high thermal conductivity silicon nitride ceramic substrate according to claim 1, characterized in that, Step (3) specifically includes: (3.1) Degrease the green belt by heating it to 300℃~500℃ in air at a rate of 1℃ / min~3℃ / min; (3.2) Subsequently, in a nitrogen atmosphere, the temperature was raised to 1600℃~1650℃ for pre-sintering; (3.3) The temperature is raised to a sintering temperature of 1850℃~1900℃, and the pulsed gas pressure is applied at this temperature and held for 3h~5h.

7. The method for preparing a high thermal conductivity silicon nitride ceramic substrate according to claim 1, characterized in that, The β-Si3N4 whiskers described in step (1) are ultrasonically dispersed and dried before use to remove agglomerates.

8. The method for preparing a high thermal conductivity silicon nitride ceramic substrate according to claim 1, characterized in that, The slurry mentioned in step (1) is a water-based slurry, the solvent of the water-based slurry is water, and the water-based slurry also contains a dispersant, a binder and a plasticizer.

9. The method for preparing a high thermal conductivity silicon nitride ceramic substrate according to claim 8, characterized in that, The dispersant is ammonium polyacrylate, and the binder is acrylic emulsion or cellulose ether.

10. The method for preparing a high thermal conductivity silicon nitride ceramic substrate according to claim 1, characterized in that, The thickness of the green strip in step (2) is 0.2 mm to 0.6 mm; the aspect ratio of the β-Si3N4 whiskers is 10 to 20:1.

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