Composite ceramic materials and joints

Abstract

To provide a composite ceramic material and a joined body having improved thermal conductivity or bending strength.SOLUTION: A composite ceramic material contains a silicon nitride particle and a boron nitride particle. In a cross section of the composite ceramic material, an average particle diameter of the silicon nitride particle is 0.1 μm or more and 10 μm or less. In the cross section, an average particle diameter of the boron nitride particle is 0.1 μm or more and 10 μm or less. In the cross section, the boron nitride particles have an area ratio of 1% or more and 20% or less with respect to the whole particles of the silicon nitride particle and the boron nitride particle of the cross section. A porosity of the composite ceramic material is 5% or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] FIELD OF THE INVENTION An embodiment of the present invention relates to a composite ceramic material and a joined body. [Background technology]

[0002] Silicon nitride ceramics are used as materials for heat dissipation substrates because of their excellent properties, including heat dissipation and bending strength. As the development of high-capacity power devices and other elements progresses, heat generation increases, while the weight of the mounted equipment is also required. Therefore, high thermal conductivity and bending strength are desirable for materials for heat dissipation substrates. For this reason, boron nitride, which has high thermal conductivity and bending strength, has been composited with silicon nitride. However, in such composite ceramic materials, the long diameter of the silicon nitride particles is large, at over 10 μm, which is disadvantageous in terms of bending strength. On the other hand, the proportion of boron nitride is low, at less than 10 mass%, and the high thermal conductivity, one of boron nitride's properties, is not fully utilized. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 9-169575 Summary of the Invention [Problem to be solved by the invention]

[0004] Embodiments of the present invention provide a composite ceramic material and a joined body that can improve thermal conductivity or bending strength. [Means for solving the problem]

[0005] The composite ceramic material of the embodiment comprises a plurality of silicon nitride particles and a plurality of boron nitride particles. In a cross section of the composite ceramic material, the average particle size of the plurality of silicon nitride particles is 0.1 μm or more and 10 μm or less. In a cross section, the average particle size of the plurality of boron nitride particles is 0.1 μm or more and 10 μm or less. In a cross section, AllThe boron nitride particles account for 1% to 20% of the area of ​​all silicon nitride particles and boron nitride particles in the cross section. In the cross section, silicon nitride particles having a particle size 2.5 times or more the average particle size of the silicon nitride particles account for 60% or less of the area of ​​all silicon nitride particles. The porosity of the composite ceramic material is 5% or less. The thermal conductivity of the composite ceramic material is 74 W / (m·K) or more. The bending strength of the composite ceramic material is 520 MPa or more. [Brief explanation of the drawings]

[0006] [Figure 1] 1 is a cross-sectional view showing a structural example of a composite ceramic material. FIG. [Figure 2] FIG. 1 is a perspective view illustrating a composite ceramic material. [Figure 3] FIG. 1 is a perspective view illustrating a composite ceramic material. [Figure 4] FIG. 2 is a schematic cross-sectional view illustrating a bonded body. DETAILED DESCRIPTION OF THE INVENTION

[0007] Hereinafter, embodiments will be described with reference to the drawings. Note that the drawings are schematic, and for example, the relationship between thickness and planar dimensions, the thickness ratio of each layer, etc. may differ from the actual. Furthermore, in the embodiments, substantially identical components are assigned the same reference numerals, and descriptions thereof will be omitted.

[0008] FIG. 1 is a cross-sectional schematic diagram showing an example of the structure of a composite ceramic material according to an embodiment. As shown in FIG. 1, a composite ceramic material 110 according to an embodiment includes crystal grains 10a, 10b, a grain boundary phase 20, and voids 30. A plurality of crystal grains 10a and 10b are present. The plurality of crystal grains 10, including the crystal grains 10a and 10b, may be separated from one another or may be partially in contact with one another. Some of the plurality of crystal grains 10 may be separated from one another, and other portions of the plurality of crystal grains 10 may be partially in contact with one another. The voids 30 are defined as regions in a cross section of the composite ceramic material 110 where the conductive coating element (e.g., gold or platinum) on the observed cross section is hardly detected in a SEM-EDS mapping image described later.

[0009] The crystal particles 10a are, for example, silicon nitride particles. Boron nitride particles have, for example, a hexagonal crystal structure. The crystalline phase of the silicon nitride particles is preferably β-type silicon nitride, and preferably does not contain α-type silicon nitride. The proportion of β-type silicon nitride in the total silicon nitride observed on the measurement surface is called the β-phase ratio, and is expressed by formula (1). Measure the X-ray diffraction of the composite ceramic material, and from the diffraction pattern, refer to the X-ray diffraction pattern of β-type silicon nitride (e.g., PDF card No. 01-073-3034) and the X-ray diffraction pattern of α-type silicon nitride (e.g., PDF card No. 01-078-2962) to read the peak intensities of the (101) and (120) planes of β-type silicon nitride, and the peak intensities of the (102) and (210) planes of α-type silicon nitride, and apply these to the β(101) intensity, β(120) intensity, α(102) intensity, and α(210) intensity in equation (1), respectively.

[0010]

number

[0011] If the beta phase ratio is lower than 80%, fewer particles of beta silicon nitride grow into columnar shapes, resulting in reduced thermal conductivity and strength. The higher the beta phase ratio, the better these properties improve. It is preferable that 90% or more of the silicon nitride is beta silicon nitride, i.e., a beta phase ratio of 90% or more. It is even more preferable that all alpha silicon nitride undergoes a phase transition to beta silicon nitride, and the silicon nitride contained in the composite ceramic material is 100% beta silicon nitride, i.e., a beta phase ratio of 100%. The beta phase ratio obtained from the measurement surface is the beta phase ratio of the composite ceramic material.

[0012] The crystal particles 10b are, for example, boron nitride particles, which have, for example, a hexagonal crystal structure.

[0013] The composite ceramic material of the embodiment may contain other crystal phases in addition to the crystal grains 10a and the crystal grains 10b.

[0014] The composite ceramic material of this embodiment combines silicon nitride particles and boron nitride particles to achieve both high thermal conductivity and high strength.

[0015] Ceramics using silicon nitride particles have high strength, but if the silicon nitride particles grow and the crystal grain size increases, for example, due to high-temperature sintering, this is advantageous in terms of thermal conductivity, but may result in a decrease in strength.

[0016] In contrast, in the composite ceramic material of the embodiment, by combining silicon nitride particles and boron nitride particles, grain growth of the silicon nitride particles can be suppressed even when the composite ceramic is produced by high-temperature sintering, thereby achieving high strength.

[0017] The cross section of a composite ceramic material is observed using a scanning electron microscope equipped with an energy dispersive X-ray spectroscopy (SEM-EDS) under conditions such as an acceleration voltage of 15 kV and a probe current of 10 μA. The direction of the cross section to be observed is perpendicular to the pressed surface. If the pressed surface is unknown, it is determined by X-ray diffraction evaluation, as described below. The cross section may be a fracture surface formed by breaking the center of a sintered body, but more preferably, it is a cross section obtained by mirror-polishing the cut surface of the sintered body and then plasma-etching it with CH4 gas. If the composite ceramic material is incorporated into a bonded body, the composite ceramic material is removed from the bonded body, mirror-polished, and plasma-etched with CH4 gas before observation. In the case of a fractured surface, a mirror-polished cross section, or a plasma-etched cross section, the cross section is electrically conductively coated before observation to prevent charging during SEM observation. Furthermore, assuming EDS analysis, the conductive coating on the observation cross section is preferably made of a material other than carbon, such as gold or platinum. The observation magnification is set so that there are at least 500 crystal grains in the observation area. Therefore, the observation magnification depends on the size of the crystal grains. For example, if the average grain size of all grains is approximately 2 μm, the observation area is approximately 130 μm × 95 μm, and the observation magnification is approximately 1000x. If the average grain size of all grains is approximately 0.4 μm, the observation area is approximately 15 μm × 9 μm, and the observation magnification is approximately 10,000x. If the average grain size of all grains is approximately 10 μm, the observation area is approximately 450 μm × 270 μm, and the observation magnification is approximately 35x. Multiple observation areas adjusted in this way, for example, three, are prepared, and measurements of the grain size and aspect ratio of each particle and EDS point analysis are performed for each observation area, as described below. The number of particles measured is, for example, 100 or more in total across multiple observation fields. The average values ​​of the measurement results are used as the average grain size and area ratio of the composite ceramic material according to this embodiment.

[0018] The multiple observation images obtained are analyzed using the image processing software ImageJ. In the observation images, voids 30 are extracted as dark regions, and grain boundary phases 20 are extracted as bright regions. The remaining regions, i.e., regions of intermediate brightness, are extracted as crystal grains 10. More specifically, when extracting by binarizing the observation images, the observation image is first binarized into dark and other regions, allowing the voids 30 to be extracted as dark regions. The extracted voids 30 are then confirmed to match the regions (dark regions) in the SEM-EDS mapping image where the conductive coating elements (e.g., gold or platinum) on the observed cross section are barely detectable. Here, a barely detectable region refers to a region where the signal intensity of the conductive coating elements is less than 1 / 100th of that of the other regions. If they do not match, the binarization threshold is likely inappropriate, so the threshold is reset to match and binarized. Next, the original observation image is binarized into bright and other regions, allowing the grain boundary phases 20 to be extracted as bright regions. The regions other than the voids 30 and the grain boundary phase 20 extracted above are the crystal particles 10. Here, we confirm that each crystal particle 10 is enclosed by connected grain boundary phase 20 to form a closed region. If the grain boundary phase 20 is enclosed by connected crystal particles 10 to form a closed region, it is considered that the binarization threshold is inappropriate. Therefore, the threshold is reset so that each crystal particle 10 is enclosed by connected grain boundary phase 20, and binarization is performed. Furthermore, silicon nitride particles and boron nitride particles can be distinguished from each other based on the results of SEM-EDS. By measuring the EDS mapping image of the cross-sectional image obtained by SEM observation, particles in which a large amount of silicon is detected are silicon nitride particles (crystal particles 10a), and particles in which a large amount of boron is detected are boron nitride particles (crystal particles 10b), making it possible to distinguish between silicon nitride particles and boron nitride particles. More specifically, EDS point analysis is performed on a single particle to measure only the three elements silicon, boron, and nitrogen. In other words, when qualitative analysis by EDS point analysis is performed under conditions where the total of the above three elements is 100 atomic %, silicon nitride particles have, for example, 30 atomic % or more of silicon and less than 35 atomic % of boron, and boron nitride particles have, for example, less than 30 atomic % of silicon and 35 atomic % or more of boron.However, for particles that do not satisfy any of the above conditions, it is likely that signals from other particles near the target particle are mixed in, so the measurement point should be shifted and the measurement should be repeated, or a different particle should be measured. When measuring again by shifting the measurement point within the particle, the number of measurements should be limited to a total of three for each particle. Particles whose type cannot be identified by the above measurements will not be used in calculating the average particle size, area percentage, etc., as described below. Note that if the target range (spot diameter) of the point analysis is wide, signals from other particles near the target particle are likely to be mixed in, so the target range of the point analysis should be small enough to fit within the particle.

[0019] The particle size of each particle is defined as the diameter (equivalent circle diameter) when the area of ​​each particle calculated using ImageJ is assumed to be a circle. The average particle size of the boron nitride particles and silicon nitride particles can be determined by averaging the particle sizes obtained in this way.

[0020] In the cross section, the average particle size of the silicon nitride particles calculated from multiple fields of view is preferably 0.1 μm to 10 μm, more preferably 0.2 μm to 5 μm, and even more preferably 0.5 μm to 3 μm. If the average particle size is less than 0.1 μm, the grain boundary phase 20 will hinder thermal conduction, and the proportion of the grain boundary phase with low thermal conductivity will increase, resulting in a decrease in thermal conductivity. If the average particle size exceeds 10 μm, the surface area will be large, making the material more susceptible to cracking and reducing strength.

[0021] In the cross section, the average particle size of the boron nitride particles calculated from multiple fields of view is preferably 0.1 μm to 10 μm, more preferably 0.2 μm to 5 μm, and even more preferably 0.5 μm to 3 μm. If the average particle size is less than 0.1 μm, the grain boundary phase 20 will hinder thermal conduction, and the proportion of the grain boundary phase with low thermal conductivity will increase, resulting in a decrease in thermal conductivity. If the average particle size exceeds 10 μm, the surface area will be large, making the material more susceptible to cracking and reducing strength.

[0022] In the cross section, silicon nitride particles having a particle size 2.5 times or more the average particle size of the silicon nitride particles preferably account for 60% or less of the area of ​​all silicon nitride particles, and more preferably 50% or less of the area of ​​all silicon nitride particles. This reduces the number of coarse particles with large surface areas, thereby suppressing a decrease in strength. If the area ratio of silicon nitride particles having a particle size 2.5 times or more the average particle size to all silicon nitride particles exceeds 60%, the number of coarse particles with large surface areas will increase, resulting in a decrease in strength.

[0023] In the cross section, all boron nitride particles preferably account for an area ratio of 1% to 20% of all silicon nitride particles and all particles including all boron nitride particles, and more preferably an area ratio of 2% to 10%. The presence of boron nitride particles suppresses grain growth of silicon nitride particles and prevents an increase in the proportion of silicon nitride particles 2.5 times or more the average particle size, thereby maintaining high strength. Furthermore, the presence of boron nitride particles, which have a higher thermal conductivity than silicon nitride particles, enables high thermal conductivity to be achieved. This allows both high thermal conductivity and high strength to be achieved. If the area ratio is less than 1%, it becomes difficult to suppress grain growth of silicon nitride particles. If the area ratio exceeds 20%, boron nitride is more likely to aggregate, making it more likely for fractures to occur originating from the aggregated boron nitride, making it difficult to maintain high strength.

[0024] In the cross section, boron nitride particles having a particle size 1.8 times or more the average particle size of the boron nitride particles preferably account for 40% or less of the area of ​​all boron nitride particles, and more preferably 30% or less of the area of ​​all boron nitride particles. This reduces the number of coarse particles with large surface areas, thereby suppressing a decrease in strength. If the area ratio of silicon nitride particles having a particle size 1.8 times or more the average particle size to all silicon nitride particles exceeds 40%, the number of coarse particles with large surface areas will increase, resulting in a decrease in strength.

[0025] In the above cross section, if the proportion of voids 30 becomes high, the thermal conductivity and bending strength of the composite ceramic material will decrease, so the lower the proportion of voids 30, the better, and it is most preferable that the composite ceramic material 110 does not contain voids 30. The area proportion of voids 30 in the cross-sectional area of ​​the composite ceramic material 110 is called the porosity. The porosity of the composite ceramic material is preferably 5% or less, more preferably 3% or less, and even more preferably 1% or less. A porosity exceeding 5% will result in a decrease in thermal conductivity and strength. The possible range of porosity of the composite ceramic material is 0% or more and less than 100%.

[0026] In the cross section, the average aspect ratio of the boron nitride particles is preferably 2 or more and 10 or less. The boron nitride particles are, for example, flaky. If the average aspect ratio of the boron nitride particles is 2 or less, it is difficult to orient the boron nitride particles, and if the aspect ratio is 10 or more, voids are likely to occur between the boron nitride particles, resulting in reduced strength and thermal conductivity.

[0027] The aspect ratio of boron nitride particles is calculated using ImageJ to calculate the maximum and minimum Feret diameters of each particle, and is defined as the ratio of the maximum to the minimum Feret diameter. Here, the maximum Feret diameter refers to the longest distance between any two points within the particle. The minimum Feret diameter refers to the shortest distance between any two parallel lines circumscribing the particle. The average aspect ratio of boron nitride particles can be calculated using the aspect ratios obtained in this way.

[0028] It is preferable that the boron nitride particles penetrate the silicon nitride particles. This improves thermal conductivity and strength. Here, "boron nitride particles penetrate the silicon nitride particles" refers to a state in which at least a portion of the boron nitride particles extends into the silicon nitride particles in the cross section. The boron nitride particles may also penetrate the silicon nitride particles.

[0029] The grain boundary phase 20 preferably does not contain a crystalline grain boundary phase. Here, the crystalline grain boundary phase refers to a crystalline phase other than silicon nitride and boron nitride, such as a Y2Si3O3N4-type crystalline phase. The crystalline phase contained in the composite ceramic material can be identified from the peak angle and intensity in the X-ray diffraction pattern of the composite ceramic material. In the X-ray diffraction pattern of the composite ceramic material, the ratio of the maximum peak intensity of the crystalline grain boundary phase to the maximum peak intensity of the silicon nitride crystalline phase is preferably 5% or less, more preferably 3% or less, and even more preferably 1% or less. By reducing the crystalline grain boundary phase, the thermal conductivity can be improved. The grain boundary phase 20 may also contain an amorphous grain boundary phase.

[0030] The conditions for measuring the X-ray diffraction pattern are set, for example, as follows: An X-ray diffractometer, such as a Rigaku Smart-Lab, is used to perform X-ray diffraction by the focusing method (reflection method, Bragg-Brendano method). For X-ray diffraction, the cross section parallel to the pressed surface of the composite ceramic material is used as the measurement surface. If the pressed surface is unknown, the composite ceramic material is processed into a rectangular parallelepiped, and X-ray diffraction is performed on each of three mutually perpendicular faces that converge at one vertex of the rectangular parallelepiped. The pressed surface is determined to be the face with the smallest value indicating the silicon nitride orientation expressed by Equation (2) below (the face on which the c-axis is oriented). The measurement surface is polished to a flat surface with a surface roughness Ra of 0.05 μm or less. A Cu target (Cu-Kα) is used for the measurement. The tube voltage is set to 45 kV. The tube current is set to 200 mA. The scanning speed is set to 2.0 to 20.0° / min. The incident parallel slit is set to 5 degrees, the longitudinal limiting slit is 10 mm, the receiving slit is 20 mm, and the receiving parallel slit is set to 5 degrees. The scanning range (2θ) is set to 10° to 80°, and measurements are taken in 0.01° increments.

[0031] Next, an example of a method for producing the composite ceramic material of this embodiment will be described. The example of the production method includes a mixing step, a pulverizing step, and a sintering step.

[0032] In the mixing step, the first raw material, the second raw material, and the auxiliary agent are mixed to form a mixed powder. These can be mixed using, for example, a rotating ball mill. By using a rotating ball mill, the first raw material, the second raw material, and the auxiliary agent can be mixed, and the dispersibility of each raw material in the mixed powder can be improved.

[0033] In the pulverization step, the mixed powder is pulverized to form a pulverized powder. The mixed powder can be pulverized using, for example, a planetary ball mill. By using a planetary ball mill, coarse particles contained in the mixed powder can be pulverized, thereby improving the uniformity of the particle size of the pulverized powder.

[0034] An example of the first raw material is a powder of silicon nitride or the like.

[0035] An example of the second raw material is a powder of boron nitride. Boron nitride has a hexagonal crystal structure. The average particle size of the boron nitride powder is preferably 100 nm or less, but may be on the order of several μm.

[0036] Examples of the auxiliary agent include magnesium oxide and yttrium oxide. These auxiliary materials may be used alone or in combination. The auxiliary agent reacts with the surface oxide film of silicon or boron nitride during firing to promote grain growth of silicon nitride or boron nitride. Note that the auxiliary agent may remain in the grain boundary phase 20 as an amorphous grain boundary phase.

[0037] The mixed powder is preferably formed by mixing 87% by mass or more and 96% by mass or less of silicon nitride and boron nitride, 1% by mass or more and 4% by mass or less of magnesium oxide, and 2.5% by mass or more and 9% by mass or less of yttrium oxide, so that the total amount is 100% by mass. If the amount of auxiliary agent is too small, grain growth of silicon nitride and boron nitride becomes difficult, while if the amount is too large, the grain boundary phase increases, reducing the thermal conductivity and bending strength of the composite ceramic material.

[0038] The sintering process involves a first heat treatment using spark plasma sintering (SPS) at a temperature of 1500°C to 2000°C, a press pressure of 20 MPa or more, and a nitrogen atmosphere pressure of 0.05 to 10 atmospheres. Temperatures below 1500°C, press pressures below 20 MPa, and atmosphere pressures below 0.05 MPa can result in problems such as inability to reduce porosity or inability to undergo phase transition to high-performance β-type silicon nitride. Temperatures above 2000°C can result in problems such as nitrogen elimination and reaction with the sintering mold, resulting in the formation of heterophases, resulting in reduced thermal conductivity and strength. The treatment time is not particularly limited, but is, for example, 3 minutes to 72 hours. Treatment times shorter than 3 minutes can result in problems such as inability to reduce porosity or inability to undergo phase transition to high-performance β-type silicon nitride. Treatment times longer than 72 hours can result in problems such as nitrogen elimination and reaction with the sintering mold, resulting in the formation of heterophases, resulting in reduced thermal conductivity and strength. The above heat treatment allows the mixed powder to be sintered under pressure, eliminating the need for a compaction process. Furthermore, by using silicon nitride as the first raw material, nitriding treatment is not required.

[0039] The sintered body obtained by the SPS may then be subjected to a second heat treatment, for example, at 1900°C to 2000°C for 12 to 72 hours in a nitrogen atmosphere at 7.5 to 10 atmospheres. The nitrogen atmosphere at 7.5 to 10 atmospheres can suppress evaporation of silicon nitride.

[0040] By performing SPS as the first heat treatment before a high-temperature, high-pressure heat treatment such as a second heat treatment, it is possible to reduce the grain boundary phase, which has lower thermal conductivity and strength than silicon nitride or boron nitride, thereby improving thermal conductivity and strength. SPS is more effective at reducing the crystalline grain boundary phase than high-temperature, high-pressure heat treatment because the grain boundary phase is pushed to the surface of the sintered body by the pressing pressure during sintering. Hot pressing can also be used instead of SPS.

[0041] The sintered body obtained in the sintering step is cut into a desired shape.

[0042] The thermal conductivity of the manufactured composite ceramic material is measured in accordance with JIS-R-1611, which corresponds to ISO18755 (2005). The thermal conductivity is measured by the laser flash method using a Netsch flash analyzer LFA 467 HyperFlash.

[0043] The bending strength of the manufactured composite ceramic material is measured by a three-point bending strength test in accordance with JIS-R-1601. JIS-R-1601 corresponds to ISO14704 (2000). The three-point bending strength test is performed using a Shimadzu Autograph AG-X (100kN). The load cell is set to 1kN, the test speed is 0.5mm / min, the indenter radius and support radius are both R2, and the distance between supports is 30mm, and the test is performed at room temperature.

[0044] The composite ceramic material of the embodiment has a thermal conductivity of 70 W / (m·K) or more. The upper limit of the thermal conductivity is not particularly limited, but is, for example, 250 W / (m·K) or less.

[0045] The composite ceramic material of the embodiment has a bending strength of 500 MPa or more. The upper limit of the bending strength is not particularly limited, but is, for example, 1100 MPa or less.

[0046] In the composite ceramic material 110 formed using the above manufacturing method example, the c-axes of the multiple silicon nitride particles are preferably aligned with each other. Orientation can be evaluated, for example, by measuring X-ray diffraction on the surface of a sintered body subjected to SPS. The surface of the sintered body measured by X-ray diffraction is referred to as the measurement surface. The diffraction pattern obtained from the measurement surface is read by referring to the X-ray diffraction pattern of β-silicon nitride (e.g., PDF card No. 01-073-3034), and the peak intensities of the (002) and (320) planes of the β-silicon nitride particles are read. The orientation value calculated using the following formula (2) can be used to determine the direction of the c-axes of the β-silicon nitride particles relative to the measurement surface. The smaller this value, the closer the c-axes of the silicon nitride particles are to being parallel to the measurement surface. It is preferable that the c-axes of the silicon nitride particles obtained from at least one measurement surface of the sintered body are non-perpendicular (≠±90 degrees) to the measurement surface, preferably within ±45 degrees, or even approximately parallel. "Approximately parallel" includes not only the parallel direction but also a state within ±20 degrees from the parallel direction. Furthermore, the value indicating the orientation obtained by formula (2) is preferably less than 0.5.

[0047] Orientation = (peak intensity of the (002) plane of β-type silicon nitride particles) / (peak intensity of the (320) plane of β-type silicon nitride particles) Equation (2)

[0048] Since silicon nitride particles have high thermal conductivity in the c-axis direction, by orienting the c-axes of the silicon nitride particles, the thermal conductivity in the direction in which the c-axes of the silicon nitride particles are oriented can be improved.

[0049] In the composite ceramic material 110 formed using the above manufacturing method example, the c-axes of the multiple boron nitride particles are preferably aligned with each other. Orientation can be evaluated, for example, by measuring X-ray diffraction on the surface of a sintered body subjected to SPS. The surface of the sintered body measured by X-ray diffraction is referred to as the measurement surface. The diffraction pattern obtained from the measurement surface is read to determine the peak intensities of the (002) and (100) planes of the boron nitride particles, with reference to the X-ray diffraction pattern of boron nitride (e.g., PDF card No. 00-034-0421). The orientation of the c-axes of the boron nitride particles relative to the measurement surface can be determined from the value indicating the orientation calculated using the following formula (3). The smaller this value, the closer the c-axes of the silicon nitride particles are to being perpendicular to the measurement surface. It is preferable that the c-axes of the boron nitride particles obtained from at least one measurement surface of the sintered body are non-parallel (≠0 degrees) to the measurement surface, preferably at an angle of ±45 degrees or more, or even approximately perpendicular. "Approximately perpendicular" includes not only the perpendicular direction but also a state within ±20 degrees from the perpendicular direction. Furthermore, the value indicating the orientation obtained by formula (3) is preferably less than 0.5.

[0050] Orientation = (peak intensity of the (100) plane of boron nitride particle / peak intensity of the (002) plane of boron nitride particle) Equation (3)

[0051] Because the thermal conductivity of boron nitride particles is high in the a-axis and b-axis directions (in the ab plane), orienting the ab plane of the boron nitride particles can improve the thermal conductivity. Here, the ab plane of the boron nitride particles is perpendicular to the c-axis of the boron nitride particles, so orienting the c-axis of the boron nitride particles can improve the thermal conductivity in the direction in which the ab plane of the boron nitride particles is oriented. Note that in boron nitride particles with a hexagonal crystal structure, the a-axis and b-axis are equivalent.

[0052] In the composite ceramic material 110 formed using the above manufacturing method example, the angle of the ab plane of the boron nitride particles relative to the c axis of the silicon nitride particles is preferably 0 to 30 degrees, more preferably 0 to 20 degrees. By making the c axis of the silicon nitride particles closer to being parallel to the a axis of the boron nitride particles, the thermal conductivity can be further improved.

[0053] The angle of the ab plane of a boron nitride particle relative to the c axis of a silicon nitride particle can be determined, for example, by the following measurement method. First, the cross section of a sample is observed using an SEM using the method described above, and the silicon nitride particle and the boron nitride particle are identified. In the cross section, the silicon nitride particle has its c axis in the longitudinal axis direction, and the boron nitride particle has its ab plane in the longitudinal axis direction. Next, for each adjacent silicon nitride particle and boron nitride particle, a line is drawn passing through any two points within each particle that form the longest distance between them. The angle formed by the two lines is taken as the angle of the ab plane of the boron nitride particle relative to the c axis of the silicon nitride particle. However, of the angles formed by the two lines, the smaller angle (acute angle or right angle) is taken as the angle of the ab plane of the boron nitride particle relative to the c axis of the silicon nitride particle. In other words, the angle of the ab plane of the boron nitride particle relative to the c axis of the silicon nitride particle is between 0° and 90°.

[0054] 2 and 3 are perspective views illustrating the composite ceramic material of the embodiment. For example, as shown in FIGS. 2 and 3, the composite ceramic material 110 is a substrate. The substrate may have any shape. As described above, the composite ceramic material of the embodiment has high thermal conductivity and high bending strength. Therefore, the composite ceramic material of the embodiment can be suitably used as a substrate. Alternatively, the composite ceramic material of the embodiment may be a bearing or the like.

[0055] 4 is a schematic cross-sectional view illustrating a bonded body according to an embodiment. As shown in FIG. 4, a bonded body 210 according to the embodiment includes a first metal part 31 and a composite ceramic material 110. In this example, the composite ceramic material 110 is used as a substrate.

[0056] The first metal part 31 is joined to the composite ceramic material 110. For example, a joint 41 is provided between the first metal part 31 and the composite ceramic material 110. The first metal part 31 may be joined directly to the composite ceramic material 110 without the joint 41 being interposed therebetween.

[0057] In the example shown in FIG. 4 , the bonded body 210 further includes a second metal part 32 and a semiconductor element 50. The semiconductor element 50 is bonded to the first metal part 31. The first metal part 31 is located between the composite ceramic material 110 and the semiconductor element 50. For example, a bonding part 42 is provided between the semiconductor element 50 and the first metal part 31. The semiconductor element 50 may be bonded directly to the first metal part 31 without the bonding part 42.

[0058] The second metal part 32 is joined to the composite ceramic material 110. The composite ceramic material 110 is located between the first metal part 31 and the second metal part 32. For example, a joint part 43 is provided between the second metal part 32 and the composite ceramic material 110. The second metal part 32 may be joined directly to the composite ceramic material 110 without the joint part 43. The second metal part 32 functions as, for example, a heat sink.

[0059] The first metal portion 31 and the second metal portion 32 include, for example, at least one selected from the group consisting of copper and aluminum. The joint portions 41 to 43 include, for example, at least one selected from the group consisting of silver and copper. The joint portions 41 to 43 may further include at least one selected from the group consisting of titanium, hafnium, zirconium, niobium, silicon, magnesium, indium, tin, and carbon. The semiconductor element 50 includes, for example, a diode, a MOSFET, or an IGBT.

[0060] The bonding portions 41 to 43 preferably contain an active metal. For example, when the first metal portion 31 and the second metal portion 32 contain copper, the active metal is at least one selected from the group consisting of titanium, hafnium, zirconium, and niobium. The bonding portions 41 to 43 preferably contain silver, copper, and at least one selected from the group consisting of titanium, hafnium, zirconium, and niobium.

[0061] When the first metal portion 31 and the second metal portion 32 contain aluminum, the active metal is at least one selected from the group consisting of silicon and magnesium. Preferably, the bonding portions 41 to 43 contain at least one selected from the group consisting of silver, copper, silicon, and magnesium.

[0062] When the first metal portion 31 and the second metal portion 32 contain copper, titanium is particularly preferable as the active metal. Titanium reacts with silicon nitride to form titanium nitride, thereby increasing the bonding strength.

[0063] By using the composite ceramic material 110 of the embodiment for the bonded body 210, the thermal conductivity and bending strength of the bonded body 210 can be improved. Furthermore, by using the composite ceramic material 110 with excellent thermal conductivity for a substrate, for example, the heat dissipation performance of the substrate can be improved. In addition, the composite ceramic material 110 has excellent bending strength. Therefore, the substrate can be made thinner while maintaining its strength. This further improves the heat dissipation performance of the substrate.

[0064] According to the embodiments described above, it is possible to provide a composite ceramic material and a joined body that can improve thermal conductivity and bending strength.

[0065] The embodiments of the present invention have been described above with reference to specific examples. However, the embodiments of the present invention are not limited to these specific examples. For example, the specific configurations of each element, such as the composite ceramic material, the metal part, the joint, and the semiconductor element, are within the scope of the present invention as long as a person skilled in the art can implement the present invention in a similar manner and obtain similar effects by appropriately selecting them from known ranges.

[0066] Furthermore, any combination of two or more elements of each specific example within the scope of technical feasibility is also included within the scope of the present invention as long as it includes the gist of the present invention.

[0067] In addition, all composite ceramic materials and joined bodies that can be implemented by a person skilled in the art by appropriately modifying the design based on the composite ceramic material and joined body described above as the embodiments of the present invention also fall within the scope of the present invention, as long as they include the gist of the present invention.

[0068] In addition, within the scope of the concept of the present invention, a person skilled in the art may come up with various modifications and alterations, and it will be understood that these modifications and alterations also fall within the scope of the present invention. [Example]

[0069] The present invention will be described below with reference to examples.

[0070] Example 1 Trigonal silicon nitride (α-Si3N4) powder, hexagonal boron nitride (h-BN) powder, magnesium oxide (MgO) powder, and yttrium oxide (YO3) powder were placed in a pot, balls and ethanol were added, and the mixture was kneaded using a planetary ball mill at a rotation speed of 60 to 150 rpm for 1 to 120 hours to form a slurry. The resulting slurry was then dried by distillation to prepare a mixed powder. The mixed powder was placed in a pot, balls and ethanol were added, and the mixture was kneaded using a planetary ball mill at a rotation speed of 300 to 600 rpm for 60 to 180 minutes to form a slurry. The resulting slurry was then dried by distillation to prepare a pulverized powder. The crushed powder was subjected to heat treatment by SPS in a nitrogen atmosphere of 0.05 to 10 atmospheres at a press pressure of 20 MPa or more at a temperature of 1500 to 2000°C for a time period of 3 minutes to 72 hours, and then heat treated again in a nitrogen atmosphere of 0.05 to 10 atmospheres at a temperature of 1500 to 2000°C for a time period of more than 3 minutes to 72 hours, thereby obtaining a composite ceramic material of Example 1 comprising silicon nitride and boron nitride.

[0071] (Examples 2 to 5 and Comparative Examples 1 and 2) The composite ceramic materials of Examples 2 to 5 and Comparative Examples 1 and 2 were obtained by changing the average particle size of the silicon nitride powder used as the raw material in Example 1.

[0072] (Examples 6 to 9 and Comparative Examples 3 and 4) The composite ceramic materials of Examples 6 to 9 and Comparative Examples 3 and 4 were obtained by changing the average particle size of the boron nitride powder used as the raw material in Example 1.

[0073] (Examples 10 and 11 and Comparative Examples 5 and 6) In Example 1, the proportion of boron nitride powder in the raw material powder was changed to obtain composite ceramic materials of Examples 10 and 11 and Comparative Examples 5 and 6.

[0074] (Examples 12 and 13 and Comparative Example 7) In Example 1, the pressing pressure during SPS was changed to obtain composite ceramic materials of Examples 12 and 13 and Comparative Example 7.

[0075] (Examples 14 and 15) The composite ceramic materials of Examples 14 and 15 were obtained by changing the particle size distribution of the silicon nitride powder used as the raw material in Example 1.

[0076] (Examples 16 and 17) The composite ceramic materials of Examples 16 and 17 were obtained by changing the particle size distribution of the boron nitride powder used as the raw material in Example 1.

[0077] Example 18 In Example 1, the mixing conditions (number of revolutions, time) of the raw material powders were changed to obtain a composite ceramic material of Example 18.

[0078] Example 19 The composite ceramic material of Example 19 was obtained by changing the particle shape of the boron nitride used in the raw material powder from flaky to spherical in Example 1.

[0079] Examples 20 to 22 In Example 1, the composite ceramic materials of Examples 20 to 22 were obtained by changing the ratio of the sintering aids (magnesium oxide and yttrium oxide) in the raw material powder.

[0080] Example 23 The composite ceramic material of Example 23 was obtained by using cubic boron nitride (c-BN) instead of hexagonal boron nitride (h-BN) as the raw material powder in Example 1.

[0081] (Examples 24 and 25) In Example 1, the composite ceramic materials of Examples 24 and 25 were obtained by changing the mixing conditions (rotation speed, time) of the raw material powders and the pulverization conditions (rotation speed, time) of the resulting mixed powder.

[0082] Example 26 The composite ceramic material of Example 26 was obtained by changing the aspect ratio of the boron nitride powder used as the raw material in Example 1 and the pressing pressure during SPS.

[0083] Example 27 In Example 1, the pressing pressure during SPS and the heat treatment time by SPS were changed to obtain a composite ceramic material of Example 27.

[0084] Example 28 The composite ceramic material of Example 28 was obtained by shortening the heat treatment time and lowering the heat treatment temperature in Example 1.

[0085] (Examples 29 and 30) The composite ceramic materials of Examples 29 and 30 were obtained by changing the aspect ratio of the boron nitride powder used as the raw material in Example 1.

[0086] Example 31 The composite ceramic material of Example 31 was obtained by changing the average particle size and particle size distribution of the silicon nitride powder and boron nitride powder used as raw materials in Example 1, the ratio of boron nitride powder in the raw material powder, the mixing conditions (rotation speed, time) of the raw material powders, the grinding conditions (rotation speed, time) of the resulting mixed powder, the pressing pressure during SPS, and the heat treatment time and heat treatment temperature.

[0087] The composite ceramic material obtained by the above process was processed and then its thermal conductivity and strength were measured. One side of the sample used for the thermal conductivity measurement was mirror-polished, and X-ray diffraction was performed on the mirror-polished surface. From the X-ray diffraction results, the crystalline structures of silicon nitride and boron nitride were evaluated, the ratio of the maximum peak intensity of the crystalline grain boundary phase to the maximum peak intensity of the silicon nitride crystalline phase was evaluated, the beta phase ratio of silicon nitride calculated using equation (1), and the orientation of silicon nitride and boron nitride calculated using equations (2) and (3). Furthermore, the mirror-polished surface was subjected to plasma etching to prepare a sample for cross-sectional observation. After gold deposition on this sample, particle images and their compositions in the cross section were obtained using SEM-EDS. Particle analysis of the obtained cross-sectional images was performed using ImageJ as described above to evaluate the average particle size and coarse particle ratio of silicon nitride particles, the average particle size and coarse particle ratio of boron nitride particles, the area ratio of boron nitride particles, porosity, boron nitride particle shape, the presence or absence of boron nitride particles extending into the silicon nitride particles, the angle of the ab plane of the boron nitride particle relative to the silicon nitride particle c-axis, and the aspect ratio of the boron nitride particles. The results are shown in Tables 1, 2, 3, 4, 5, and 6. In the tables, "-" indicates ineligible or undetermined. The "coarse particle ratio" in the tables refers to the percentage of particles with a diameter of 2.5 times or more the average particle size for silicon nitride and 1.8 times or more the average particle size for boron nitride.

[0088] [Table 1]

[0089] [Table 2]

[0090] [Table 3]

[0091] [Table 4]

[0092] [Table 5]

[0093] [Table 6]

[0094] The results of Examples 1 to 5 and Comparative Examples 1 and 2 show that when the average particle size of the silicon nitride particles is less than 0.1 μm, thermal conductivity drops significantly. Furthermore, when the average particle size exceeds 10 μm, strength drops significantly. Therefore, the average particle size of the silicon nitride particles is preferably 0.1 μm or more and 10 μm or less. Furthermore, from the viewpoint of achieving both thermal conductivity and strength, the average particle size of the silicon nitride particles is more preferably 0.2 μm or more and 5 μm or less, and most preferably 0.5 μm or more and 3 μm or less.

[0095] The results of Examples 1, 6 to 9 and Comparative Examples 3 and 4 show that when the average particle size of the boron nitride particles is less than 0.1 μm, the thermal conductivity drops significantly. Furthermore, when the average particle size exceeds 10 μm, the strength drops significantly. Therefore, the average particle size of the boron nitride particles is preferably 0.1 μm or more and 10 μm or less. Furthermore, from the viewpoint of achieving both thermal conductivity and strength, the average particle size of the boron nitride particles is more preferably 0.2 μm or more and 5 μm or less, and most preferably 0.5 μm or more and 3 μm or less.

[0096] The results of Examples 1, 10, and 11 and Comparative Examples 5 and 6 show that when the area ratio of all boron nitride particles to all particles, including all silicon nitride particles and all boron nitride particles (hereinafter referred to as the boron nitride area ratio), is less than 1% or exceeds 20%, strength is significantly reduced. Therefore, the boron nitride area ratio is preferably 1% or more and 20% or less. To improve strength, an area ratio of 2% or more and 10% or less is more preferable.

[0097] The results of Examples 1, 12, and 13 and Comparative Example 7 show that when the porosity of the composite ceramic material exceeds 5%, both the thermal conductivity and strength decrease significantly. Therefore, the porosity of the composite ceramic material is preferably 5% or less. Furthermore, to improve the thermal conductivity and strength, the porosity of the composite ceramic material is more preferably 3% or less, and most preferably 1% or less.

[0098] From the results of Examples 1, 14, and 15, it was found that in order to improve strength, silicon nitride particles having a particle size 2.5 times or more the average particle size of the silicon nitride particles preferably account for an area ratio of 60% or less of all silicon nitride particles, and more preferably an area ratio of 50% or less.

[0099] From the results of Examples 1, 16, and 17, in order to improve strength, it is preferable that boron nitride particles having a particle size 1.8 times or more the average particle size of the boron nitride particles account for 40% or less of the area of ​​all boron nitride particles, and more preferably 30% or less of the area of ​​all boron nitride particles.

[0100] From the results of Example 1 and Example 18, it is preferable that the boron nitride particles extend inside the silicon nitride particles in order to improve the thermal conductivity and strength.

[0101] From the results of Examples 1 and 19, it is clear that the shape of the boron nitride particles is preferably flaky in order to improve the thermal conductivity and strength.

[0102] From the results of Examples 1 and 20 to 22, in order to improve thermal conductivity, in the X-ray diffraction pattern of the composite ceramic material, the ratio of the maximum peak intensity of the crystalline grain boundary phase to the maximum peak intensity of the silicon nitride crystalline phase is preferably 5% or less, more preferably 3% or less, and even more preferably 1% or less.

[0103] From the results of Examples 1 and 23, it is preferable that the crystal structure of the boron nitride particles is hexagonal in order to improve the thermal conductivity.

[0104] From the results of Examples 1, 24, and 25, in order to improve thermal conductivity, the angle of the ab plane of the boron nitride particles relative to the c axis of the silicon nitride particles is preferably 0 degrees or more and 30 degrees or less, and more preferably 0 degrees or more and 20 degrees or less.

[0105] From the results of Examples 1 and 26, it is preferable that the value indicating the orientation of boron nitride obtained by formula (3) is less than 0.5 in order to improve the thermal conductivity.

[0106] From the results of Examples 1 and 27, it is preferable that the value indicating the orientation of silicon nitride obtained by formula (2) is less than 0.5 in order to improve thermal conductivity.

[0107] From the results of Examples 1 and 28, it is found that the β-phase ratio of silicon nitride calculated by formula (1) is preferably 80% or more in order to improve thermal conductivity and strength.

[0108] From the results of Examples 1, 29 and 30, it is clear that the average aspect ratio of the boron nitride particles is preferably 2 or more and 10 or less in order to improve the thermal conductivity.

[0109] The results of Example 1 and Example 31 show that both thermal conductivity and strength can be improved by optimizing each parameter shown in the table.

[0110] As described above, the composite ceramic material of the example is a composite ceramic material comprising silicon nitride particles and boron nitride particles, and in the cross section of the composite ceramic material, the average particle size of the silicon nitride particles is 0.1 μm or more and 10 μm or less. In the cross section, the average particle size of the boron nitride particles is 0.1 μm or more and 10 μm or less. In the cross section, the boron nitride particles have an area ratio of 1% or more and 20% or less of all the silicon nitride particles and boron nitride particles in the cross section. The porosity of the composite ceramic material is 5% or less, and it was found that it was possible to achieve both high thermal conductivity and high strength.

[0111] Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, and are also included in the scope of the invention and its equivalents as defined in the claims. [Explanation of symbols]

[0112] 10a...crystal grain, 10b...crystal grain, 20...grain boundary phase, 30...void, 31...first metal portion, 32...second metal portion, 41-43...joint portion, 50...semiconductor element, 110...composite ceramic material, 210...jointed body.

Claims

1. A composite ceramic material comprising a plurality of silicon nitride particles and a plurality of boron nitride particles, In a cross section of the composite ceramic material, the average particle size of the plurality of silicon nitride particles is 0.1 μm or more and 10 μm or less; In the cross section, the average particle size of the plurality of boron nitride particles is 0.1 μm or more and 10 μm or less, In the cross section, all of the boron nitride particles have an area ratio of 1% to 20% relative to all of the silicon nitride particles and the boron nitride particles in the cross section, In the cross section, silicon nitride particles having a particle size 2.5 times or more the average particle size of the plurality of silicon nitride particles have an area ratio of 60% or less relative to all of the silicon nitride particles, The porosity of the composite ceramic material is 5% or less, The thermal conductivity of the composite ceramic material is 74 W / (m K) or more, The composite ceramic material has a bending strength of 520 MPa or more.

2. 2. The composite ceramic material according to claim 1, wherein in the cross section, boron nitride particles having a particle size 1.8 times or more the average particle size of the plurality of boron nitride particles have an area ratio of 40% or less relative to all the boron nitride particles.

3. the boron nitride particles are scaly, 3. The composite ceramic material according to claim 1, wherein in the cross section, at least a portion of the boron nitride grains extends inside the silicon nitride grains.

4. 4. The composite ceramic material according to claim 1, wherein in an X-ray diffraction pattern of the composite ceramic material, a ratio of a maximum peak intensity of the crystalline grain boundary phase other than said silicon nitride and boron nitride to a maximum peak intensity of the silicon nitride crystalline phase is 5% or less.

5. each of the silicon nitride particles and the boron nitride particles has a hexagonal crystal structure; In the cross section, the angle of the a-b plane of the boron nitride particle adjacent to the silicon nitride particle with respect to the c-axis of the silicon nitride particle is 0 degrees or more and 30 degrees or less; 5. The composite ceramic material according to claim 1, wherein the angle is defined as the smaller of the angles formed by two lines drawn between adjacent silicon nitride particles and adjacent boron nitride particles in the cross section, the two lines passing through any two points within each particle that form the longest distance between them.

6. the boron nitride particles are scaly, 6. The composite ceramic material according to claim 1, wherein a value indicating the orientation of the c-axes of the plurality of boron nitride particles calculated by the following formula (3) using an X-ray diffraction pattern of a measurement surface of the composite ceramic material is less than 0.5: Orientation=(peak intensity of the (100) plane of a boron nitride particle / peak intensity of the (002) plane of a boron nitride particle) Equation (3)

7. 7. The composite ceramic material according to claim 1, wherein a value indicating the orientation of the c-axes of the plurality of silicon nitride particles, calculated by the following formula (2) using an X-ray diffraction pattern of a measurement surface of the composite ceramic material, is less than 0.5: Orientation=(peak intensity of the (002) plane of a β-type silicon nitride particle) / (peak intensity of the (320) plane of a β-type silicon nitride particle) Equation (2)

8. A substrate using the composite ceramic material according to any one of claims 1 to 7; a first metal portion bonded to the substrate; A joint body comprising:

9. the substrate and the first metal portion are joined via a first joining portion, the first metal portion includes copper, The first bonding portion is made of silver, copper, titanium, hafnium, zirconium, niobium, silicon, and at least one selected from the group consisting of silicon, magnesium, indium, tin, and carbon.

10. Further comprising a second metal part bonded to the substrate; The bonded body according to claim 8 or 9, wherein the substrate is located between the first metal part and the second metal part.

11. further comprising a semiconductor element bonded to the first metal portion; The bonded body according to claim 8 , wherein the first metal portion is located between the substrate and the semiconductor element.

Images