Gradient silicon nitride ceramic and preparation method and application thereof

Abstract

The application relates to a preparation method and application of a gradient silicon nitride ceramic, and belongs to the field of high-thermal-conductivity ceramic materials.The gradient silicon nitride ceramic comprises a gradient structure of axially increasing dislocation density and axially increasing internal stress; the internal stress of one end in the axial direction of the gradient structure is 0.5-1 times that of the other end; and the dislocation density of one end in the axial direction of the gradient structure is 10-1000 times that of the other end.The gradient silicon nitride ceramic has excellent thermal conductivity and is suitable for application in the field of high-thermal-conductivity materials.

Description

Technical Field

[0001] This application belongs to the field of high thermal conductivity ceramic materials, specifically relating to a gradient silicon nitride ceramic and its preparation method and application. Background Technology

[0002] Silicon nitride ceramics are widely used in aerospace, electronic packaging, and automotive manufacturing due to their excellent high-temperature strength, oxidation resistance, corrosion resistance, and good thermal stability. However, the inherent brittleness and relatively low thermal conductivity of silicon nitride ceramics severely limit their application expansion in high-thermal-conductivity and high-reliability scenarios.

[0003] In existing technologies, the main approaches to improving the thermal conductivity of silicon nitride ceramics include using high-purity raw materials, optimizing the sintering aid system, and promoting grain boundary phase crystallization. Adding rare earth oxides and supplementing with prolonged high-temperature heat treatment can crystallize the grain boundary glass phase, reducing interfacial thermal resistance and thus improving thermal conductivity. However, these methods often result in grain coarsening, leading to a decrease in material strength and toughness. On the other hand, improving fracture toughness usually relies on whisker toughening, layered structure design, or phase transformation toughening, but these methods often come at the cost of sacrificing thermal conductivity, making it difficult to achieve a synergistic improvement in both thermal conductivity and fracture toughness. Summary of the Invention

[0004] This application addresses the problem of low thermal conductivity in silicon nitride ceramics and the lack of application in the field of high thermal conductivity materials. This application provides a method for preparing gradient silicon nitride ceramics. By embedding and treating with low-pressure discharge plasma sintering, a gradient silicon nitride ceramic with axially increasing dislocation density and internal stress concentration is constructed, thereby achieving a significant synergistic improvement in the thermal conductivity and fracture toughness of the material.

[0005] The objective of this application can be achieved through the following technical solutions.

[0006] In a first aspect of this application, a gradient silicon nitride ceramic is provided, the gradient silicon nitride ceramic comprising an axially increasing dislocation density and an axially increasing internal stress gradient structure; The internal stress in the axial direction of the gradient structure is 0.5, 0.6, 0.7, and 0.8 times higher than the flow stress; The dislocation density at one end of the gradient structure in the axial direction is 10 to 1000 times that at the other end.

[0007] The gradient silicon nitride ceramic described in this application forms a significant gradient along the axial direction from a low dislocation density region to a high dislocation density region, as well as a gradient structure from low internal stress to high stress, with the dislocation density and internal stress increasing in the same direction. The low dislocation density region maintains a near-single-crystal-like complete lattice, providing the material with a high thermal conductivity path; the high dislocation density region introduces a large number of dislocations, and the elastic strain field formed around these dislocations can effectively hinder crack propagation; at the same time, the internal stress difference achieves a critical design of pre-strengthening without self-destruction, realizing the stability of the mechanical basis of the gradient structure.

[0008] The axially increasing dislocation density is a continuous gradient structure or a step gradient structure. In the continuous gradient structure, the dislocation density exhibits a linear or exponential distribution along the axial direction.

[0009] The residual stress states at both ends of the axial gradient are opposite, with one end being residual compressive stress and the other end being residual tensile stress. The residual compressive stress value is ≥500 MPa, and the residual tensile stress value is ≤200 MPa.

[0010] The structure described in this application enables the crack tip to encounter a gradually increasing resistance field when the material is subjected to external load, thereby achieving progressive toughening and avoiding interface debonding.

[0011] The intergranular phases of the gradient silicon nitride ceramics are all crystalline phases; the intergranular phases and silicon nitride grains form a coherent or semi-coherent interface.

[0012] This application achieves complete crystallization of the intergranular phase, eliminating the need for high elastic modulus at high temperatures, thus significantly improving the material's creep resistance. The periodic lattice structure allows phonons to pass smoothly through grain boundaries, reducing interfacial thermal resistance. Furthermore, the intergranular phase and silicon nitride grains are confined to a coherent or semi-coherent interface, meaning that the two phase lattices are continuously matched at the interface or that lattice mismatch is coordinated through periodic mismatched dislocations. This high-binding-energy interface not only eliminates phonon scattering walls, making the thermal conductivity approach the theoretical value for single crystals, but also forces cracks to propagate transgranularly rather than along grain boundaries, thereby absorbing more fracture energy.

[0013] The gradient silicon nitride ceramic has the same grain size in all directions, and the grain size is 100~300nm.

[0014] The hardness of the gradient silicon nitride ceramic is ≥20 GPa.

[0015] The fracture toughness of the gradient silicon nitride ceramic is ≥5.5 MPa·m. 1 / 2 .

[0016] The thermal conductivity of the gradient silicon nitride ceramic is ≥100 W / (m·K).

[0017] The gradient silicon nitride ceramic of this application has a fracture toughness that is at least 70% higher and a thermal conductivity that is at least 200% higher than that of conventional silicon nitride ceramics.

[0018] The silicon nitride ceramic described in this application has a grain size at the nanometer level, which allows it to maintain its high strength properties.

[0019] In a second aspect of this application, a method for preparing gradient silicon nitride ceramics is provided, comprising the following steps: Provide silicon nitride ceramic matrix; The silicon nitride ceramic matrix is ​​embedded in silicon nitride powder and then subjected to spark plasma sintering to obtain gradient silicon nitride ceramics. Preferably, the spark plasma sintering temperature is 50-100°C lower than the intergranular phase melting point, the pressure is 3-5 MPa, the heating rate is 200°C / min-300°C / min, and the holding time is 3-10 min. More preferably, the cooling rate is 200℃ / min~300℃ / min, and the pressure is released and the temperature is allowed to cool naturally after it drops to 1000℃.

[0020] The sintering temperature in this application is controlled at 50-100°C below the melting point of the intergranular phase. At this temperature, the intergranular phase is in a solid state but close to its softening critical point, possessing sufficient atomic diffusion capacity to release local stress, but without complete liquefaction that would lead to grain rearrangement and destruction of the gradient structure. This semi-solid state is crucial for the retention of dislocations without complete annealing elimination. The pressure of 3-5 MPa is significantly lower than conventional sintering pressure to avoid mechanical compression at high temperatures that would annihilate the formed dislocation gradient. Sufficient contact pressure is provided to ensure heat conduction and sample integrity, while simultaneously preventing grain growth and phase transformation in silicon nitride ceramics. The controlled cooling rate prevents dislocations from recovering through climb or slip. Pressure is released at 1000°C, utilizing the remaining plasticity to release some macroscopic internal stress and prevent self-cracking due to excessive stress upon cooling to room temperature.

[0021] Furthermore, the silicon nitride ceramic matrix is ​​a dense, homogeneous material with no dislocation density or internal stress difference; Preferably, the silicon nitride ceramic matrix is ​​obtained by mixing and sintering silicon nitride powder and sintering aid, wherein the sintering aid includes at least one of alumina and aluminum nitride-based aids; More preferably, the grain size of the silicon nitride ceramic matrix is ​​100~300nm.

[0022] The dense silicon nitride ceramic matrix described in this application is in thermodynamic equilibrium, ensuring that the subsequent gradient structure is induced by the process described in this application. The sintering aid system uses alumina and / or aluminum nitride-based agents. During subsequent spark plasma sintering and embedding processes, due to the presence of a large amount of intergranular phase solid solution, it can achieve fully crystallized intergranular phases without residual glass phase. The grain size is limited to 100~300 nm, maintaining its excellent mechanical properties. It should be noted that the sintering aids include, but are not limited to, alumina and / or aluminum nitride-based aids, and other sintering aids may be added according to actual needs.

[0023] In some embodiments, the silicon nitride powder used for embedding is α-phase silicon nitride; The sintering atmosphere is a nitrogen atmosphere or a vacuum environment.

[0024] The α-Si3N4 described in this application is a low-temperature stable phase. Under the high temperature of spark plasma sintering, it will undergo an irreversible transformation to the β phase. When using α phase for embedding, the α→β phase transformation is accompanied by volume shrinkage, which makes the embedding layer and the ceramic to be treated form a tight wrap, enhancing thermal contact and temperature gradient stability. At the same time, the phase transformation behavior of α phase powder can serve as an indirect indicator of temperature field distribution.

[0025] In a third aspect of this application, the application of the gradient silicon nitride ceramic in the field of high thermal conductivity materials is provided.

[0026] The beneficial effects of the gradient silicon nitride ceramic preparation method of this application include: (1) Significantly improves fracture toughness and thermal shock resistance, overcoming the inherent brittleness of ceramic materials; (2) Effectively suppresses thermal stress damage and improves high-temperature service reliability; (3) By precisely controlling the process window, the gradient structure can be stably constructed. Attached Figure Description

[0027] Figure 1 The image shows the morphology of the gradient silicon nitride ceramic prepared in Example 1. Figure 2 The image shows the morphology of the gradient silicon nitride ceramic prepared in Example 2. Figure 3 The image shows the morphology of the gradient silicon nitride ceramic prepared in Example 3. Figure 4 The image shows the morphology of the gradient silicon nitride ceramic prepared in Example 4. Figure 5 The image shows the morphology of the gradient silicon nitride ceramic prepared in Example 5. Detailed Implementation

[0028] The following detailed description, with appropriate reference to the accompanying drawings, discloses a gradient silicon nitride ceramic, its preparation method, and embodiments of its application. However, unnecessary details may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of essentially the same structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0029] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of the particular range. The range defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range.

[0030] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0031] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0032] Unless otherwise defined, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used in this application is for the purpose of describing particular embodiments only and is not intended to limit this application; unless otherwise stated, the values ​​of the parameters mentioned in this application can be measured using various measurement methods commonly used in the art (e.g., they can be tested according to the methods given in the embodiments of this application).

[0033] Some studies have attempted to improve the plasticity or toughness of ceramic materials by introducing dislocation structures. However, traditional methods struggle to controllably introduce high-density dislocations into ceramics, and dislocation introduction is often accompanied by a significant decrease in thermal conductivity. Therefore, this application achieves high-density dislocation toughening and high thermal conductivity in silicon nitride ceramics through re-firing treatment, while maintaining the stability of the material's overall mechanical properties. The preparation method includes the following steps: Provide silicon nitride ceramic matrix; The silicon nitride ceramic matrix is ​​embedded in silicon nitride powder and subjected to spark plasma sintering to obtain gradient silicon nitride ceramic.

[0034] The discharge plasma sintering temperature is 50~100℃ lower than the melting point of the intergranular phase, the pressure is 3~5MPa, the heating rate is 200℃ / min~300℃ / min, and the holding time is 3~10min. The cooling rate is 200℃ / min to 300℃ / min, and the pressure is released and the temperature is allowed to cool naturally after it drops to 1000℃.

[0035] The silicon nitride ceramic matrix is ​​obtained by mixing and sintering silicon nitride powder and sintering aids, wherein the sintering aids include at least one of alumina and aluminum nitride-based aids; The grain size of the silicon nitride ceramic matrix is ​​100~300nm.

[0036] The silicon nitride powder used for embedding has an α-phase silicon nitride crystal form; The atmosphere for the discharge plasma sintering is a nitrogen or vacuum environment.

[0037] Example 1 92 wt.% silicon nitride powder, 4 wt.% alumina and 4 wt.% yttrium oxide were mixed and sintered by rapid hot pressing at a rate of 300 °C / min to 1600 °C and a pressure of 50 MPa to obtain a dense silicon nitride ceramic matrix with a grain size of 150~200 nm and a density of 99.1%.

[0038] The silicon nitride ceramic matrix was embedded in α-Si3N4 powder and placed in a spark plasma sintering furnace. Spark plasma sintering was performed under a nitrogen atmosphere at a temperature of 1500℃, a pressure of 4 MPa, a heating rate of 250℃ / min, and a holding time of 5 min. During the cooling phase, the material was cooled at a rate of 250℃ / min, and the pressure was released when the temperature dropped to 1000℃. The material was then allowed to cool naturally to room temperature to obtain gradient silicon nitride ceramic.

[0039] The gradient silicon nitride ceramic prepared in this embodiment (its surface morphology is as follows) Figure 1 As shown in the figure, the dislocation density at one end in the axial direction is 100 times that at the other end, and the internal stress is 0.6 times higher than the flow stress. The residual compressive stress at one end is 620 MPa, and the residual tensile stress at the other end is 150 MPa. The intergranular phase is fully crystalline and forms a semi-coherent interface with the silicon nitride grains. The material hardness is 22 GPa, and the fracture toughness is 6.2 MPa·m. 1 / 2 The thermal conductivity is 112 W / (m·K). Compared with the original matrix, the fracture toughness is improved by 75% and the thermal conductivity is improved by 210% (the performance data are summarized in Table 1).

[0040] Example 2 A dense silicon nitride ceramic matrix (density of 98.4%) with a grain size of 100~150 nm was provided and prepared using 8 wt.% aluminum nitride-based sintering aid (sintering conditions were the same as in Example 1).

[0041] The matrix was embedded in α-Si3N4 powder and subjected to spark plasma sintering under vacuum. The spark plasma sintering temperature was 1450℃, the pressure was 3.5 MPa, the heating rate was 300℃ / min, and the holding time was 8 min. The cooling rate was 300℃ / min, and the temperature was reduced to 1000℃ before depressurization and natural cooling.

[0042] The gradient silicon nitride ceramic prepared in this embodiment (its surface morphology is as follows) Figure 2 As shown in the figure, the dislocation density at one end in the axial direction is 50 times that at the other end, and the internal stress is 0.7 times higher than the flow stress. The residual compressive stress is 580 MPa, and the residual tensile stress is 120 MPa. The intergranular phases are fully crystalline, and the interface bonding is good. The material hardness is 21.5 GPa, and the fracture toughness is 5.9 MPa·m. 1 / 2 The thermal conductivity is 105 W / (m·K), the fracture toughness is improved by 72%, and the thermal conductivity is improved by 195% (the performance data are summarized in Table 1).

[0043] Example 3 A dense silicon nitride ceramic matrix (density of 98.2%) with a grain size of 250~300 nm was provided, which was prepared by using a composite sintering aid of 4 wt.% lanthanum oxide and 4 wt.% aluminum nitride (sintering conditions were the same as in Example 1).

[0044] The matrix was embedded in α-Si3N4 powder and subjected to spark plasma sintering under a nitrogen atmosphere. The spark plasma sintering temperature was 1550℃, the pressure was 5 MPa, the heating rate was 300℃ / min, and the holding time was 3 min. The cooling rate was 300℃ / min, and the temperature was reduced to 1000℃ before depressurization and natural cooling.

[0045] The gradient silicon nitride ceramic prepared in this embodiment (its surface morphology is as follows) Figure 3 As shown in the figure, the dislocation density at one end in the axial direction is 350 times that at the other end, and the internal stress is 0.8 times higher than the flow stress. The residual compressive stress is 680 MPa, and the residual tensile stress is 180 MPa. The intergranular phases are fully crystalline and exhibit coherent interfaces. The material hardness is 23 GPa, and the fracture toughness is 6.5 MPa·m. 1 / 2 The thermal conductivity is 118 W / (m·K), the fracture toughness is improved by 80%, and the thermal conductivity is improved by 225% (the performance data are summarized in Table 1).

[0046] Example 4 Except for the spark plasma sintering temperature being set to 1300℃, all other steps and conditions were the same as in Example 1.

[0047] The gradient silicon nitride ceramic prepared in this embodiment (its surface morphology is as follows) Figure 4As shown in the figure, the dislocation density at one end in the axial direction is 20 times that at the other end, and the internal stress is 0.5 times higher than the flow stress. The residual compressive stress is 510 MPa, and the residual tensile stress is 90 MPa. The material hardness is 20.5 GPa, and the fracture toughness is 5.6 MPa·m. 1 / 2 The thermal conductivity is 101 W / (m·K). The results show that the gradient structure is not fully formed at low temperatures, and the performance improvement is lower than that of Example 1 (the performance data are summarized in Table 1).

[0048] Example 5 Except for the cooling rate being set to 100℃ / min, all other steps and conditions were the same as in Example 1.

[0049] The gradient silicon nitride ceramic prepared in this embodiment (its surface morphology is as follows) Figure 5 As shown in the figure, the dislocation density at one end in the axial direction is 100 times that at the other end, the internal stress gradient is not obvious, the residual compressive stress is 380 MPa, and the residual tensile stress is 210 MPa. The material hardness is 18.8 GPa, and the fracture toughness is 5.0 MPa·m. 1 / 2 The thermal conductivity is 95 W / (m·K). The results show that the slow cooling rate leads to dislocation recovery, partial disappearance of the gradient structure, and grain growth, which in turn leads to a decrease in hardness (the performance data are summarized in Table 1).

[0050] Comparative Example 1 Except for skipping the embedding process and directly performing spark plasma sintering on the dense silicon nitride ceramic, the other steps and conditions are the same as in Example 1.

[0051] The silicon nitride ceramic prepared in this comparative example did not form an axial gradient structure, had a uniform dislocation density distribution, and showed no significant internal stress difference. The material's fracture toughness was 3.6 MPa·m. 1 / 2 The thermal conductivity was 85 W / (m·K), and the improvement rates of fracture toughness and thermal conductivity were significantly lower than those of the example, indicating that the embedding treatment is crucial for the formation of the gradient structure.

[0052] Comparative Example 2 Except for the discharge plasma sintering pressure of 20 MPa, the other steps and conditions are the same as in Example 1.

[0053] The silicon nitride ceramic prepared in this comparative example exhibits significantly coarsened grains with an average size exceeding 500 nm. Dislocation structures are annihilated by high pressure, failing to form an effective gradient. The material's fracture toughness is 5.0 MPa·m. 1 / 2 Its thermal conductivity is 78 W / (m·K), and its strength performance is significantly reduced.

[0054] Test methods The dislocation density at both ends of the gradient structure in the axial direction in gradient silicon nitride ceramics was obtained using EBSD testing.

[0055] Internal stress / flow stress of gradient silicon nitride ceramics: obtained using EBSD testing.

[0056] Residual compressive stress / residual tensile stress at both ends of the gradient silicon nitride ceramic: obtained by XRD test calculation.

[0057] Hardness of graded silicon nitride ceramics: The hardness value of the silicon nitride ceramics was tested using a hardness tester under a load of 1 kgf.

[0058] Fracture toughness of graded silicon nitride ceramics: A hardness tester was used to pre-crack the silicon nitride ceramics under a load of 1 kgf, and the fracture toughness was calculated based on the crack length.

[0059] Thermal conductivity of gradient silicon nitride ceramics: obtained by testing according to national standard GB / T 45767-2025.

[0060] Table 1 Summary of performance index data for each embodiment and comparative example

[0061] Note: In Table 1, “\” indicates that the product performance improvement is not significant, or even declines.

[0062] This application achieves a significant synergistic improvement in the thermal conductivity and fracture toughness of silicon nitride ceramics by introducing an axially increasing dislocation density and internal stress concentration gradient structure into dense silicon nitride ceramics, and by combining precise thermal, mechanical, and time coupling processes.

[0063] The embodiments herein do not exhaustively cover the points not covered by the technical scope claimed in this application, and new technical solutions formed by equivalent substitutions of one or more technical features in the technical solutions of the embodiments are also within the scope of protection claimed in this application. At the same time, in all the listed or unlisted embodiments of the solution in this application, each parameter in the same embodiment merely represents an instance of its technical solution (i.e., a feasible solution), and there is no strict matching or limiting relationship between the parameters. The parameters can be substituted for each other without violating axioms and the claims of this application, unless otherwise stated.

[0064] The technical means disclosed in this application are not limited to those described above, but also include technical solutions composed of any combination of the above technical features. The above descriptions are specific embodiments of this application. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of this application, and these improvements and modifications are also considered within the scope of protection of this application.

[0065] The specific embodiments described herein are merely illustrative examples of the spirit of this application. Those skilled in the art to which this application pertains may make various modifications or additions to the described specific embodiments or use similar methods to substitute them, without departing from the spirit of this application or exceeding the scope defined by the appended claims.

Claims

1. A gradient silicon nitride ceramic, characterized in that, The gradient silicon nitride ceramic comprises a gradient structure with axially increasing dislocation density and axially increasing internal stress. The internal stress in the axial direction of the gradient structure is 0.5, 0.6, 0.7, and 0.8 times higher than the flow stress; The dislocation density at one end of the gradient structure in the axial direction is 10 to 1000 times that at the other end. Among them, the residual stress states at both ends of the axial gradient are opposite, with one end being residual compressive stress and the other end being residual tensile stress; The residual compressive stress value is ≥500 MPa, and the residual tensile stress value is ≤200 MPa.

2. The gradient silicon nitride ceramic according to claim 1, characterized in that: The axially increasing dislocation density is a continuous gradient structure or a step gradient structure. In the continuous gradient structure, the dislocation density exhibits a linear or exponential distribution along the axial direction.

3. The gradient silicon nitride ceramic according to claim 1, characterized in that: The intergranular phases of the gradient silicon nitride ceramics are all crystalline phases; the intergranular phases and silicon nitride grains form a coherent or semi-coherent interface.

4. The gradient silicon nitride ceramic according to claim 1, characterized in that, At least one of the following conditions must be met: The gradient silicon nitride ceramic has the same grain size at all locations, and the grain size is 100~300nm. The hardness of the gradient silicon nitride ceramic is ≥20 GPa; The fracture toughness of the gradient silicon nitride ceramic is ≥5.5 MPa·m. 1 / 2 ; The thermal conductivity of the gradient silicon nitride ceramic is ≥100 W / (m·K).

5. A method for preparing gradient silicon nitride ceramics as described in any one of claims 1-4, characterized in that, Includes the following steps: Provide silicon nitride ceramic matrix; The silicon nitride ceramic matrix is ​​embedded in silicon nitride powder and subjected to spark plasma sintering to obtain gradient silicon nitride ceramic. The pressure of the discharge plasma sintering is 3~5MPa.

6. The preparation method according to claim 5, characterized in that: The discharge plasma sintering temperature is 50~100℃ lower than the melting point of the intergranular phase, the heating rate is 200℃ / min~300℃ / min, and the holding time is 3~10min. The cooling rate is 200℃ / min~300℃ / min, and the pressure is released and the temperature is allowed to cool naturally after it drops to 1000℃.

7. The preparation method according to claim 5, characterized in that: The silicon nitride ceramic matrix is ​​obtained by mixing and sintering silicon nitride powder and sintering aids, wherein the sintering aids include at least one of alumina and aluminum nitride-based aids; The grain size of the silicon nitride ceramic matrix is ​​100~300nm.

8. The preparation method according to claim 5, characterized in that: The silicon nitride powder used for embedding has an α-phase silicon nitride crystal form; The atmosphere for the discharge plasma sintering is a nitrogen or vacuum environment.

9. The application of a gradient silicon nitride ceramic as described in claims 1 to 4 or the gradient silicon nitride ceramic obtained by the preparation method as described in any one of claims 5 to 8 in the field of high thermal conductivity materials.

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