Silicon nitride powder for sintering

Abstract

To provide a β-type silicon nitride powder for obtaining a sintered body having high strength and high toughness.SOLUTION: A silicon nitride powder for sintering comprises a silicon nitride powder having a β formation ratio of 80% or more, in which when a volume frequency distribution curve showing the particle size distribution is subjected to waveform separation, there exist a separation waveform a having a peak within the range of 0.5 μm or more and 1 μm or less and a separation waveform b having a peak within the range of more than 1 μm and 3 μm or less, the total area of the separation waveform a and the separation waveform b is 60 to 98% based on the total area of the total separation waveform, and the ratio of the area of the separation waveform a to the area of the separation waveform b (Sa / Sb) is 0.4 to 0.8.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to silicon nitride powder for sintering. [Background technology]

[0002] Silicon nitride powder is attracting attention as a ceramic raw material for various industrial materials because its sintered body possesses excellent properties such as high thermal conductivity, high insulation, and high strength. Methods for producing silicon nitride powder include, for example, the reductive nitriding method, in which silica powder is used as a raw material and nitrogen gas is passed through it in the presence of carbon powder to produce silicon nitride; the direct nitriding method, in which silicon powder and nitrogen are reacted at high temperatures; and the imide decomposition method, in which silicon halide is reacted with ammonia. Another known method for synthesizing silicon nitride powder is the combustion synthesis method, which is a direct nitriding method. The combustion synthesis method involves burning silicon powder under a nitrogen atmosphere. Generally, a mixture of silicon powder and a diluent (silicon nitride powder) to control combustion is ignited and burned to obtain a lump of silicon nitride, which is then pulverized to obtain silicon nitride powder.

[0003] While silicon nitride powder is known to exist in two crystalline forms, α-type and β-type, silicon nitride powder obtained by combustion synthesis is primarily β-type, and its sinterability differs from that of the commonly manufactured α-type. Specifically, α-type silicon nitride transforms into β-type silicon nitride during sintering, resulting in easy grain growth and good sinterability. On the other hand, β-type silicon nitride does not undergo this transformation, making grain growth difficult and resulting in poor sinterability. Because β-type silicon nitride powder has the advantage of relatively low manufacturing costs, research is being conducted to improve its sinterability.

[0004] Patent Document 1 describes a silicon nitride powder with a β-conversion rate of 80% or more, and the average particle size D is measured by laser diffraction scattering. 50The invention relates to silicon nitride powder for sintering, wherein the particle size is 0.5 to 1.2 μm, with 20 to 50% by mass of particles smaller than 0.5 μm and 20 to 50% by mass of particles larger than 1 μm. The invention also describes how, by including fine particles (particles with a particle size of 0.5 μm or less) and larger particles (particles with a particle size of 1 μm or more) in such proportions, a phenomenon called Ostwald ripening occurs during sintering, in which the fine particles dissolve and precipitate in the liquid phase surrounding the larger particles, resulting in a dense sintered body.

[0005] Patent Document 2 discloses an invention relating to silicon nitride powder, characterized by comprising a mixed powder of silicon nitride powder A having a β-phase content of 50% or more and an average particle size Da of 5 μm or less, and silicon nitride powder B having a β-phase content of 50% or more and an average particle size Db of 1 to 25 μm, with a weight ratio of A / B of 1 to 1000 and a Db / Da of 2 to 50. It is also stated that a silicon nitride sintered body with excellent sinterability, high strength, high toughness, and high reliability can be obtained using this silicon nitride powder. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] International Publication No. 2019 / 167879 [Patent Document 2] Japanese Patent Application Publication No. 6-64906 [Overview of the project] [Problems that the invention aims to solve]

[0007] The silicon nitride powder described in Patent Document 1 above has a particle size distribution suitable for Ostwald growth, with a relatively high proportion of small-particle silicon nitride particles. Furthermore, regarding the silicon nitride powder described in Patent Document 2, referring to the specification and comparative examples, it is stated that if more silicon nitride powder B, which has a larger average particle size, is used, the strength of the sintered body decreases. Therefore, it is necessary to use a relatively larger amount of silicon nitride powder A, which has a smaller average particle size.

[0008] Conventional β-type silicon nitride powder is designed to have a relatively high proportion of small-particle silicon nitride particles. Our research has shown that conventional β-type silicon nitride powder has room for improvement in terms of obtaining a sintered body with a higher balance of strength and toughness.

[0009] Therefore, the object of the present invention is to provide a β-type silicon nitride powder that can produce a sintered body with high strength and high toughness. [Means for solving the problem]

[0010] The inventors diligently conducted research to achieve the above objective. As a result, they discovered that the above problem can be solved by silicon nitride powder with a β conversion rate of 80% or more, in which, when the volume frequency distribution curve showing the particle size distribution is waveform-separated, there exist separated waveforms a and b having peaks in a specific particle size range, and the total area and area ratio of these separated waveforms a and b are within a certain range, thereby completing the present invention.

[0011] The gist of this invention is as follows: [1] to [4]. [1] Silicon nitride powder for sintering, comprising silicon nitride powder with a β-conversion rate of 80% or more, wherein when the volume frequency distribution curve showing the particle size distribution is waveform-separated, there exists a separated waveform a having a peak between 0.5 μm and 1 μm, and a separated waveform b having a peak between 1 μm and 3 μm, the total area of ​​separated waveform a and separated waveform b is 60 to 98% of the total area of ​​all separated waveforms, and the ratio of the area of ​​separated waveform a to the area of ​​separated waveform b (Sa / Sb) is 0.4 to 0.8. [2] In the particles in the range of particle size of 1 to 5 μm observed with a scanning electron microscope, the ratio of single particles with an aspect ratio (L / D) of 1.1 to 1.8 is 20% by volume or more, the silicon nitride powder for sintering described in [1] above. [3] The silicon nitride powder for sintering described in [1] or [2] above, having an average particle size (D50) of 0.5 to 2 μm. [4] The silicon nitride powder for sintering according to any one of [1] to [3] above, wherein the separation waveform of the remainder has a separation waveform c having a peak between 0.1 and 0.3 μm.

Effect of the Invention

[0012] According to the present invention, it is possible to provide a β-type silicon nitride powder for sintering from which a sintered body having high strength and high toughness can be obtained.

Brief Description of the Drawings

[0013] [Figure 1] It is a diagram showing an example of a volume frequency distribution curve showing the particle size distribution of the silicon nitride powder for sintering of the present invention. [Figure 2] It is a diagram obtained by converting the horizontal axis of the volume frequency distribution curve in FIG. 1 from the particle size to the common logarithm of the particle size. [Figure 3] It is a diagram obtained by separating the volume frequency distribution curve in FIG. 2 into a plurality of separation waveforms. [Figure 4] It is a diagram obtained by converting the horizontal axis of the volume frequency distribution curve in FIG. 3 from the common logarithm of the particle size to the particle size. [Figure 5] It is a SEM photograph of the silicon nitride powder for sintering of the present invention.

Mode for Carrying Out the Invention

[0014] [Silicon nitride powder for sintering] The silicon nitride powder for sintering of the present invention is composed of silicon nitride powder with a β-phase ratio of 80% or more. When the volume frequency distribution curve showing its particle size distribution is waveform-separated, there are separated waveforms a having a peak between 0.5 μm and 1 μm and separated waveforms b having a peak between more than 1 μm and 3 μm. The total area of the separated waveforms a and b is 60 to 98% of the total area of all the separated waveforms, and the ratio (Sa / Sb) of the area of the separated waveform a to the area of the separated waveform b is 0.4 to 0.8.

[0015] <Particle size distribution> When the volume frequency distribution curve showing the particle size distribution of the silicon nitride powder for sintering of the present invention is waveform-separated, there are separated waveforms a having a peak between 0.5 μm and 1 μm and separated waveforms b having a peak between more than 1 μm and 3 μm.

[0016] A method for waveform-separating the volume frequency distribution curve showing the particle size distribution of the silicon nitride powder for sintering will be described. First, the silicon nitride powder for sintering is measured by a laser diffraction particle size distribution measuring device, and as shown in FIG. 1, a volume frequency distribution curve with the horizontal axis being the particle size (μm) and the vertical axis being the volume frequency is obtained. Note that FIG. 1 shows, as an example, the volume frequency distribution curve representing the particle size distribution of the silicon nitride powder for sintering manufactured in Example 1 described later. Next, the horizontal axis of the obtained volume frequency distribution curve is converted from "particle size" to "common logarithm of particle size" (FIG. 2). Then, the volume frequency distribution curve with the horizontal axis converted to the common logarithm is waveform-separated assuming it is a synthesis of a plurality of Gaussian functions. FIG. 3 shows separated waveforms a, separated waveforms b, and separated waveforms c, which are a plurality of Gaussian functions after waveform-separating the volume frequency distribution curve of FIG. 2. That is, when the volume frequency distribution curve of FIG. 2 is waveform-separated, it becomes three separated waveforms. Note that this is an example, and depending on the optimization described later and the shape of the volume frequency distribution curves of FIGS. 1 and 2, it may become two separated waveforms or four or more separated waveforms. When the separated waveforms a, separated waveforms b, and separated waveforms c shown in FIG. 3 are superimposed, the volume frequency distribution curve shown in FIG. 2 is obtained.

[0017] Waveform separation is performed by optimizing the number of Gaussian functions (number of separated waveforms) and the parameters of each Gaussian function (Ai, Bi, Ci) so that the sum of squared residuals between the approximation curve represented by the following equation and the measured volume frequency distribution curve is minimized.

number

[0018] When the horizontal axis of Figure 3 is changed back from "common logarithm of particle size" to "particle size," the volume frequency distribution curve shown in Figure 4 is obtained. According to Figure 4, separation waveform a at a peak position of 0.53 μm, separation waveform b at a peak position of 1.9 μm, and separation waveform c at a peak position of 0.23 μm are confirmed. Note that Figures 1 to 4 described above are diagrams illustrating an example of the present invention, and the present invention is not limited to these drawings.

[0019] When the volume frequency distribution curve showing the particle size distribution of the silicon nitride powder for sintering of the present invention is separated into waveforms, there are two separate waveforms: waveform a, which has a peak in the particle size range of 0.5 μm to 1 μm, and waveform b, which has a peak in the particle size range of more than 1 μm and 3 μm or less. As described later, when both separated waveform a and separated waveform b exist within a specific total area and area ratio of a specific range, a silicon nitride powder for sintering is obtained that yields a sintered body with excellent strength and toughness. Although the reason is not entirely clear, β-type silicon nitride powder does not undergo grain growth easily during sintering. Therefore, the particle size distribution of the raw material powder before sintering easily affects the physical properties of the sintered body. Thus, in silicon nitride powder for sintering, the presence of separation waveform a is related to the strength of the sintered body, and the presence of separation waveform b is related to the toughness of the sintered body. It is thought that when these are present in a specific area, a sintered body with excellent strength and toughness can be obtained.

[0020] As described above, separation waveform b has a peak in the particle size range of over 1 μm and up to 3 μm. However, in the case of silicon nitride powder that does not have separation waveform b but has a separation waveform with a peak in the particle size range of over 3 μm, the strength and toughness of the sintered body tend to decrease, and more voids are likely to occur in the sintered body. Furthermore, as described above, separation waveform a has a peak in the particle size range of 0.5 μm to 1 μm. However, in the case of silicon nitride powder that does not have separation waveform a but has a separation waveform with a peak in the particle size range of less than 0.5 μm, the toughness of the sintered body tends to decrease.

[0021] The area ratio (Sa / Sb) of separated waveform a to the area of ​​separated waveform b is between 0.4 and 0.8. Silicon nitride powder with such an area ratio (Sa / Sb) makes it easier to obtain sintered bodies with excellent strength and toughness. If the area ratio (Sa / Sb) is less than 0.4, the strength of the sintered body tends to decrease, and if the area ratio (Sa / Sb) is greater than 0.8, the toughness of the sintered body tends to decrease. The area ratio (Sa / Sb) is preferably 0.4 to 0.7, and more preferably 0.4 to 0.6.

[0022] The combined area of ​​separated waveform a and separated waveform b is 60-98% of the total area of ​​all separated waveforms. If the combined area of ​​separated waveform a and separated waveform b is less than 60% of the total area of ​​all separated waveforms, it becomes difficult to obtain a sintered body with excellent strength and toughness. On the other hand, if the combined area of ​​separated waveform a and separated waveform b is more than 98% of the total area of ​​all separated waveforms, the area of ​​separated waveform c (described later) becomes smaller, making it difficult to reduce voids in the sintered body and obtain a sintered body with excellent insulation properties. The combined area of ​​separated waveform a and separated waveform b is preferably 70-98%, more preferably 80-98%, and even more preferably 90-96% of the total area of ​​all separated waveforms.

[0023] The silicon nitride powder for sintering of the present invention, when its volume frequency distribution curve is waveform-separated, has a remaining waveform in addition to separated waveform a and separated waveform b, and preferably the remaining separated waveform is separated waveform c, which has a peak in the particle size range of 0.1 to 0.3 μm. The silicon nitride powder having a separation waveform c reduces the number of voids in the resulting sintered body, making it easier to obtain a sintered body with excellent insulating properties. From the viewpoint of improving the insulation properties of the sintered body, the area of ​​the separated waveform c is preferably 1 to 30%, more preferably 1 to 20%, and even more preferably 2 to 10%, of the total area of ​​all separated waveforms.

[0024] The peak full width at half maximum (FWHM) in each of the separation waveforms a, b, and c of the silicon nitride powder for sintering according to the present invention is not particularly limited, but is preferably 0.1 to 1.5, and more preferably 0.2 to 1.0, in a volume frequency fraction curve with the horizontal axis being the common logarithm of particle size (μm). Having each separation waveform within this range of peak FWHM makes it easier to obtain a sintered body with high strength and toughness.

[0025] Furthermore, the silicon nitride powder for sintering of the present invention may have separation waveforms a and b, and may also have separation waveform d having a peak above 3 μm. However, from the viewpoint of increasing the strength of the sintered body, the area of ​​separation waveform d having a peak above 3 μm is preferably 10% or less, more preferably 5% or less, and even more preferably 0% of the total area of ​​all separation waveforms.

[0026] The average particle size (D50) of the silicon nitride powder for sintering of the present invention is not particularly limited, but from the viewpoint of obtaining a sintered body with high strength and toughness, it is preferably 0.5 to 2 μm, and more preferably 0.8 to 1.5 μm. In this specification, the average particle diameter refers to the particle size (D50) at which the cumulative volume of particles measured by a laser diffraction particle size distribution device reaches 50%.

[0027] <Aspect Ratio (L / D)> The silicon nitride powder for sintering of the present invention preferably contains 20% or more by volume, more preferably 30% or more by volume, even more preferably 40% or more by volume, and preferably 90% or less of single particles with an aspect ratio (L / D) of 1.1 to 1.8 in particles with a particle size in the range of 1 to 5 μm as observed by a scanning electron microscope (SEM). When the proportion of single particles with an aspect ratio (L / D) of 1.1 to 1.8 is within this range, the toughness of the sintered body tends to improve.

[0028] The proportion of single particles with an aspect ratio (L / D) of 1.1 to 1.8 can be determined as follows. First, silicon nitride powder for sintering is observed using a scanning electron microscope (SEM) at a magnification of 2000x. Then, particles with a particle size in the range of 1 to 5 μm are identified from the obtained image. Here, particles with a particle size in the range of 1 to 5 μm refer to particles with an equivalent circle diameter of 1 to 5 μm. The equivalent circle diameter refers to the diameter of a circle with an area equivalent to that of the particle, calculated from the area of ​​the particle observed in the image. Next, from among particles with a particle size in the range of 1 to 5 μm, single particles with an aspect ratio (L / D) of 1.1 to 1.8 are identified. Single particles are particles that have not aggregated and can be easily identified using SEM images. Furthermore, the above-mentioned particles with a diameter of 1 to 5 μm shall be measured in quantities of at least 1000 or more. The aspect ratio (L / D) is the ratio (L / D) of the major axis (L) to the minor axis (D), where the major axis (L) is defined as the maximum distance between any two points on the outer circumference of the single particle. The minor axis (D) is defined as a line segment perpendicular to the major axis (L) that passes through the midpoint of the major axis (L) and two points on the outer circumference of the single particle. Then, based on Equation 1 below, we determine the proportion of single particles with an aspect ratio (L / D) of 1.1 to 1.8. (Equation 1) [Total volume of single particles with aspect ratio (L / D) of 1.1 to 1.8 / Total volume of particles with particle size in the range of 1 to 5 μm] × 100 Note that the total volume in Equation 1 is the total volume calculated based on the equivalent diameter of the circle.

[0029] The method for obtaining silicon nitride powder for sintering having the predetermined separation waveform, average particle size, and aspect ratio described above is not particularly limited, but examples include employing a specific grinding method during manufacturing, as will be described later. Alternatively, examples include preparing multiple silicon nitride powders having a specific particle size distribution and mixing the multiple silicon nitride powders to obtain the predetermined separation waveform.

[0030] <β conversion rate> The silicon nitride powder for sintering according to the present invention consists of silicon nitride powder with a β-conversion rate of 80% or higher. Since silicon nitride powder with a β-conversion rate of 80% or higher can be obtained without setting strict manufacturing conditions, it can be manufactured at a relatively low cost. Therefore, by using silicon nitride powder with a high β-conversion rate, the overall manufacturing cost of silicon nitride sintered bodies can be suppressed. The β-conversion rate of the silicon nitride powder is preferably 85% or higher, more preferably 90% or higher.

[0031] The β-conversion rate of silicon nitride powder refers to the ratio of the peak intensity of the β phase to the sum of the α and β phases in silicon nitride powder [100 × (peak intensity of β phase) / (peak intensity of α phase + peak intensity of β phase)], and is determined by powder X-ray diffraction (XRD) measurement using CuKα rays. More specifically, it can be determined by calculating the weight ratio of the α and β phases of silicon nitride powder using the method described in CPGazzara and DRMessier: Ceram. Bull., 56 (1977), 777-780.

[0032] [Method for producing silicon nitride powder for sintering] The method for producing silicon nitride powder for sintering according to the present invention is not particularly limited, and various methods such as reduction nitriding, direct nitriding methods such as combustion synthesis, and imide decomposition methods can be applied. However, combustion synthesis is preferred because it makes it easier to obtain silicon nitride powder with a high β conversion rate.

[0033] The combustion synthesis method is a method for producing silicon nitride powder by igniting a raw material powder containing silicon powder and propagating the nitriding combustion heat generated by the nitriding reaction of the silicon powder throughout the raw material powder.

[0034] (Raw material powder) The average particle size (D50) of the silicon powder contained in the raw material powder is not particularly limited, but from the viewpoint of making it easier to obtain silicon nitride powder for sintering having a specific particle size distribution of the present invention, it is preferably 1 to 30 μm, and more preferably 1 to 20 μm. The silicon powder is preferably of high purity, and the Al and Fe content is preferably 200 ppm or less each. For example, the silicon powder can be recovered and used from the fine powder generated in the process of crushing semiconductor polycrystalline silicon rods to produce nuggets.

[0035] The raw material powder preferably contains silicon nitride powder as a diluent. The reaction between silicon powder and nitrogen is an exothermic reaction, and the more silicon powder there is, the more difficult it becomes to control the temperature. However, by including a diluent in the raw material powder, the silicon powder content in the raw material powder is reduced, and the heat generated by the raw material powder is also reduced. This makes it easier to control the temperature of the raw material powder. The average particle size (D50) of the silicon nitride powder used as a diluent is preferably 0.5 to 30 μm, and more preferably 0.5 to 20 μm, from the viewpoint of facilitating the acquisition of silicon nitride powder for sintering having a specific particle size distribution according to the present invention. From the viewpoint of facilitating the acquisition of silicon nitride powder for sintering having a specific particle size distribution according to the present invention, the content of the diluent is preferably 5 to 50% by mass, more preferably 10 to 30% by mass, based on the total amount of raw material powder.

[0036] Within limits that do not impair the effects of the present invention, the raw material powder may contain other components besides silicon powder and diluents used as needed. Examples of other components include chlorides such as sodium chloride and ammonium chloride, and oxides such as calcium oxide, yttrium oxide, and magnesium oxide. The amount of other components is preferably 10% by mass or less, more preferably 5% by mass or less, even more preferably 1% by mass or less, and even more preferably 0% by mass, based on the total amount of the raw material powder.

[0037] (Conditions for ignition and combustion synthesis methods) The above-mentioned raw material powders are filled into a reaction vessel (setter) to form a raw material powder layer, and this raw material powder layer is ignited to initiate the reaction. The reaction vessel is preferably a heat-resistant vessel made of ceramics, graphite, or other materials. Furthermore, in the combustion synthesis method, an ignition agent containing powders such as Ti and Al can be added to the ignition point. Of course, the amount of such ignition agent should be small enough not to affect the sinterability of the resulting silicon nitride powder. When placing the ignition agent, it can be placed at the edges, in the center, or at any arbitrary position, either as a single or multiple location. As described above, after filling the reaction vessel with the raw material powder, the inside of the reaction vessel is purged with nitrogen, and the raw material powder is ignited under a nitrogen atmosphere. The above-mentioned reaction vessel is typically installed in a pressure-resistant, sealed reactor equipped with an ignition device and a gas supply and discharge mechanism. The reactor is generally depressurized to remove air, and then nitrogen gas is supplied to replace the air. The reaction may be carried out under atmospheric pressure or under pressure, but it is preferable to carry it out under pressure. Ignition can be carried out by conventionally known methods, such as ignition by arc discharge using a pair of electrodes attached to a closed reactor, ignition by energizing a carbon or metal heater, or ignition by laser irradiation.

[0038] (Crush) By carrying out the combustion synthesis reaction as described above, a bulk product consisting of silicon nitride is obtained. This bulk product can be converted into silicon nitride powder with an appropriate particle size distribution by mechanical grinding, as described later.

[0039] Examples of grinders used to mechanically crush agglomerate products include roll crushers, cutter mills, stamp mills, mortars, grinders, pin mills, colloid mills, ball mills, high-pressure dispersion devices, and stone mill grinders. Among these, it is preferable to use a combination of a millstone grinder and a ball mill to obtain the silicon nitride powder of the present invention having the specific particle size distribution described above. A vibrating ball mill is preferably used. That is, although it is possible to obtain silicon nitride powder having the above particle size distribution using a single grinder, such as a ball mill, in the combustion synthesis method, if the amount of diluent changes in the manufacturing method, the amount and size of locally generated fused particles also change due to a rapid rise in temperature. Therefore, it is difficult to set the grinding conditions to obtain the above distribution with a single grinder and to manufacture stably. For this reason, it is preferable to grind the lump obtained by combustion synthesis with a millstone grinder to obtain a powder corresponding to separation waveform b, then further grind a portion of it with a ball mill to produce powder corresponding to separation waveform a, and if necessary, powder corresponding to separation waveform c, and then mix these in appropriate proportions to produce the silicon nitride powder of the present invention. Furthermore, it is preferable to break down the silicon nitride lumps into a size suitable for grinding before supplying them to the millstone grinder. The millstone-type grinder used in the method for producing silicon nitride powder of the present invention is generally capable of efficiently grinding coarse particles consisting of fused particles produced by combustion synthesis, and makes it possible to obtain powder corresponding to the separation waveform b of the present invention while suppressing the generation of fine powder.

[0040] A millstone grinder consists of two grinding wheels, one above the other, whose spacing can be freely adjusted. The upper grinding wheel is fixed, and raw materials are fed between it and the lower grinding wheel, which rotates at high speed. The combined action of powerful compression, shearing, and rolling friction that occurs between the two grinding wheels pulverizes the raw materials. From the viewpoint of obtaining silicon nitride powder having a specific particle size distribution with multiple separation waveforms according to the present invention, it is preferable to appropriately adjust the gap between the upper and lower grinding wheels when grinding using a millstone-type grinder. After appropriately adjusting the gap between the upper and lower grinding wheels, it is preferable to adjust the rotation speed of the grinding wheels and the grinding time as appropriate.

[0041] Specifically, the gap between the upper and lower grinding wheels is preferably 2 to 7 μm, and more preferably 3 to 5 μm.

[0042] In this way, by adjusting the gap between the upper and lower grinding wheels, and appropriately adjusting the rotation speed and grinding time of the grinding wheels, silicon nitride powder corresponding to the separation waveform b can be produced.

[0043] Grinding wheels used in millstone grinders can be made of ceramics, organic-inorganic composite resins, or metal. However, in applications where the inclusion of impurity metals is undesirable, such as in electronic materials, grinding wheels made of ceramics or organic-inorganic composite resins are preferred. Grinding wheel materials include SiC, Al2O3, Si3N4, and ZrO2. In this invention, since it is used for grinding silicon nitride powder, a Si3N4-based grinding wheel, which is made of the same material, is preferably used. Examples of commercially available millstone grinders include the Mascoloider and Serendipiter (product names: manufactured by Masuko Sangyo), Glowmill (product name: manufactured by Glow Engineering), Micropowder (product name: manufactured by Makino Sangyo), and the millstone grinder (Yamato Kihan Co., Ltd.). Furthermore, a portion of the silicon nitride powder corresponding to separation waveform b obtained by the millstone grinder can be pulverized using a ball mill, preferably a vibrating ball mill, to obtain silicon nitride powder corresponding to separation waveform a. If necessary, a portion of this can be further pulverized to obtain silicon nitride powder corresponding to separation waveform c. The structure of the ball mill, the material and size of the balls, and the vibration frequency when using a vibratory mill are not particularly limited; they should be appropriately selected to achieve the desired grinding. In particular, the ball material should be a Si3N4-based material, which is the same material as the ball mill. By mixing each of the silicon nitride powders with the adjusted particle size distribution obtained in this way in an amount that satisfies the proportion of the separation waveform, the silicon nitride powder of the present invention can be obtained. These powders can be mixed using a known dry powder blender, and such mixing can yield silicon nitride powder having a specific distribution according to the present invention.

[0044] The mechanical grinding described above may be dry grinding without a liquid medium, or wet grinding using a liquid medium. Dry grinding generally results in higher productivity. Wet grinding reduces the amount of powder generated during the grinding process. During grinding, it is preferable to use a grinding aid such as water or alcohol as the liquid medium, and the content of the grinding aid in the sample to be ground is preferably 0.5 to 5% by mass, more preferably 0.5 to 2% by mass.

[0045] [Method for manufacturing silicon nitride sintered bodies] A silicon nitride sintered body can be manufactured using the silicon nitride powder for sintering obtained as described above by a known method. For example, sintering silicon nitride powder is mixed with sintering aids such as yttria, magnesia, zirconia, and alumina, and then press-molded to produce a product with a bulk density of 1.7 g / cm³. 3 Preferably, the above is 1.85 g / cm³. 3 More preferably 1.95 g / cm³ 3 By fabricating the above-described molded body and then performing firing, a silicon nitride sintered body can be obtained. While uniaxial press forming is typical for the above-mentioned press forming, a method in which CIP (Cold Isostatic Pressing) forming is performed after uniaxial press forming is preferably employed.

[0046] Furthermore, firing is carried out in a nitrogen atmosphere at 1700-2000°C. The density of the sintered body depends on both the firing temperature and firing time. For example, when firing at 1700°C, the firing time is approximately 3-20 hours. The firing time and firing duration should be set appropriately according to the desired size, shape, etc. of the sintered body. For example, when firing silicon nitride at a temperature of 1850°C or higher, if the firing time is too long, the density of the sintered body may decrease due to the decomposition of the silicon nitride itself. In this case, the decomposition of the silicon nitride sintered body can be suppressed by sintering in a pressurized nitrogen atmosphere. The higher the nitrogen pressure, the more effectively the decomposition of silicon nitride can be suppressed, but for economic reasons such as the pressure resistance performance of the equipment, a pressure of less than 1 MPa is preferably used. To obtain a high-density sintered body with a relative density of 99% or higher, it is preferable to perform firing in a pressurized nitrogen atmosphere at 1800°C or higher.

[0047] As described above, the silicon nitride powder for sintering of the present invention has specific separated waveforms at specific ratios when the volume frequency distribution curve showing its particle size distribution is waveform-separated. Therefore, the sintered silicon nitride body produced by this invention has high strength and toughness, and possesses an excellent balance of physical properties. [Examples]

[0048] The following examples illustrate the present invention in more detail, but the present invention is not limited to these examples.

[0049] [Measurement method] The various physical properties in the examples and comparative examples were measured by the following methods.

[0050] (1) Beta conversion rate of silicon nitride powder The β-conversion rate of silicon nitride powder was determined by powder X-ray diffraction (XRD) measurements using CuKα rays. Specifically, the weight ratio of the α-phase to the β-phase of silicon nitride powder was calculated using the method described in CPGazzara and DRMessier: Ceram. Bull., 56 (1977), 777-780, and the β-conversion rate was determined from there.

[0051] (2) Particle size of silicon nitride powder (i) Sample pretreatment As a pretreatment for the silicon nitride powder sample, the silicon nitride powder was calcined in air at a temperature of approximately 500°C for 2 hours. In particle size measurement, if the surface oxygen content of the silicon nitride powder is low, or if the particle surface is covered with hydrophobic substances due to grinding aids used during grinding, the particles themselves may exhibit hydrophobicity. In such cases, dispersion in water may be insufficient, making reproducible particle size measurement difficult. Therefore, by calcining the silicon nitride powder sample in air at a temperature of approximately 200°C to 500°C for several hours, hydrophilicity is imparted to the silicon nitride powder, making it easier to disperse in aqueous solvents and enabling highly reproducible particle size measurement. It has been confirmed that calcination in air has almost no effect on the measured particle size. (ii) Measurement of particle size In a beaker (60 mm inner diameter, 70 mm height) with a maximum mark of 100 mL, 45 mL of water and 5 mL of 5% by mass sodium pyrophosphate were added and thoroughly mixed. Then, a sample of silicon nitride powder (about the size of an ear pick) was added, and the silicon nitride powder was dispersed using an ultrasonic homogenizer (US-300E, manufactured by Nippon Seiki Seisakusho Co., Ltd., with a tip diameter of 26 mm) at an amplitude of 50% (approximately 2 amperes) for 2 minutes. The above tip was inserted until its tip reached the 20 mL mark in the beaker to complete the dispersion. Next, the obtained dispersion of silicon nitride powder was measured using a laser diffraction / scattering particle size distribution analyzer (Microtrac MT3300EXII, manufactured by Microtrac Bell Co., Ltd.), and a volume frequency distribution curve was obtained with particle size (μm) on the x-axis and volume frequency on the y-axis. For the measurement conditions, water (refractive index 1.33) was selected as the solvent, particle properties were set to a refractive index of 2.01, particle permeability to permeability, and particle shape to non-spherical. The particle size at which the cumulative curve of the volume-based particle size distribution measured in the above particle size distribution measurement reaches 50% was defined as D50.

[0052] (3) Waveform separation of volume frequency distribution curves The horizontal axis of the volume frequency distribution curve obtained by the above (ii) particle size measurement, with the particle size (μm) on the horizontal axis and the volume frequency on the vertical axis, was converted to the common logarithm. Next, the volume frequency distribution curve with the horizontal axis converted to the common logarithm was subjected to waveform separation assuming it is a synthesis of a plurality of Gaussian functions. The waveform separation was performed by optimizing the number of Gaussian functions and the parameters (Ai, Bi, Ci) of each Gaussian function so that the sum of the squared residuals between the approximate curve represented by the following formula and the actually measured volume frequency distribution curve was minimized. Note that the volume frequency distribution curve with the horizontal axis converted to the common logarithm has 133 measurement points at equal intervals on the horizontal axis, and based on each individual measurement point, the sum of the squared residuals described above was obtained and optimized. The waveform separation was performed using the solver of spreadsheet software (「MICROSOFT EXCEL (registered trademark)」manufactured by Microsoft). [Number] In the above formula, D is the common logarithm of the particle size, A i is the peak height, B i is the peak position, C i is the peak half-width / 2.35, i = 1, 2, 3, ···, and represents the type (number) of separated waveforms.

[0053] (4) Ratio of single particles with an aspect ratio (L / D) of 1.1 to 1.8 For the silicon nitride powder for sintering manufactured in each example and comparative example, SEM photographs (acceleration voltage 15 kV, secondary electron detection) at a magnification of 2000 times were taken using a scanning electron microscope (「TM3030」manufactured by Hitachi High-Technologies Corporation). From the obtained SEM images, particles in the range of particle size (equivalent circle diameter) of 1 to 5 μm were specified. Next, single particles with an aspect ratio (L / D) of 1.1 to 1.8 were specified from the particles in the range of particle size of 1 to 5 μm. Then, based on the following formula (1), the ratio (%) of single particles with an aspect ratio (L / D) of 1.1 to 1.8 was determined. (Formula 1) [Total volume of single particles with an aspect ratio (L / D) of 1.1 to 1.8 / Total volume of particles in the range of particle size of 1 to 5 μm] × 100 Note that the total volume in Formula 1 is the total volume calculated based on the equivalent circle diameter.

[0054] (5) Three-point bending strength Test specimens for measuring three-point bending strength were cut from the silicon nitride sintered bodies produced in each example and comparative example, and the three-point bending strength (MPa) was measured according to the method conforming to ISO 23242:2020. A test fixture with a support distance of 15 mm was used. The average of the three-point bending strengths of 10 test specimens was taken as the three-point bending strength of the sintered body. The three-point bending strength was determined by setting the three-point bending strength of the silicon nitride sintered body of Example 1 to 100, and then evaluated according to the following criteria. 〇:90 or higher △: 70 or more and less than 90 ×: Less than 70

[0055] (6) Fracture toughness value Fracture toughness value (MPa·m) of silicon nitride sintered body 1 / 2 The IF method was calculated from the Vickers hardness measured using an AVK-CO Vickers hardness tester manufactured by Akashi Co., Ltd., in accordance with JIS R1607:2015. The fracture toughness value was determined by setting the fracture toughness value of the silicon nitride sintered body of Example 1 to 100, and then evaluating it according to the following criteria. 〇:90 or higher △: 70 or more and less than 90 ×: Less than 70

[0056] (7) Void fraction The cross-sections of the silicon nitride sintered bodies were observed using a scanning electron microscope (Hitachi High-Technologies Corporation "TM3030") (10 fields of view at 1000x magnification), and the void ratio (%) was defined as the ratio of the void area to the total area of ​​the cross-section, and evaluated according to the following criteria. The void ratio of each example and comparative example was calculated by setting the void ratio (%) of the silicon nitride sintered body of Example 1 to 100, and then evaluated according to the following criteria. 〇: 200 or less △: More than 200 and less than 500 ×: Over 500

[0057] <Examples 1-2> (Manufacturing of silicon nitride powder for sintering) Silicon powder (semiconductor grade, average particle size 5 μm) and silicon nitride powder (average particle size 1.5 μm), used as a diluent, were mixed to obtain a raw material powder (Si: 80% by mass, Si3N4: 20% by mass). This raw material powder was packed into a reaction vessel to form a raw material powder layer. Next, the reaction vessel was placed in a pressure-resistant, closed reactor equipped with an ignition device and a gas supply and discharge mechanism. The reactor was degassed by reducing the pressure, and then nitrogen gas was supplied to replace the atmosphere with nitrogen. Subsequently, nitrogen gas was gradually supplied until the pressure was increased to 0.7 MPa. At the point when the predetermined pressure was reached (at the time of ignition), the bulk density of the raw material powder was 0.5 g / cm³. 3 Subsequently, the end of the raw material powder in the reaction vessel was ignited, and a combustion synthesis reaction was carried out to obtain a massive product consisting of silicon nitride. The obtained lumps were crushed by rubbing them against each other using a crusher. The resulting silicon nitride fragments were then subjected to grinding (wet grinding) using a millstone grinder (Super Mascolloider; manufactured by Masuko Sangyo, MKZA10-15J). The gap between the upper and lower grinding wheels was 5 μm, and the grinding process was performed at a rotation speed of 1800 rpm. Si3N4 type grinding wheels were used to form the crushed surface. Furthermore, the grinding using a millstone-type grinder was performed as a wet grinding process. Specifically, 5% by mass of ethanol was added to the silicon nitride powder as a grinding aid and thoroughly mixed so that the powder and water would blend well. After that, the mixture was supplied to the grinder and ground under the above conditions to obtain a mixing powder b corresponding to the separation waveform b with the peaks shown in Table 1. A portion of the mixing powder b obtained by the aforementioned millstone grinder was mixed with 1% by mass of ethanol as a grinding aid, and then ground using a vibratory ball mill (manufactured by Chuo Kako Kikai Shoji Co., Ltd., Newlight) with Si3N4 balls to obtain mixing powder a corresponding to the separation waveform a having the peaks shown in Table 1. Furthermore, 1% by mass of ethanol was added to a portion of the above-mentioned mixing powder a as a grinding aid, and the mixture was ground using the vibrating ball mill to obtain mixing powder c corresponding to the separation waveform c having the peaks shown in Table 1. The silicon nitride powders a, b, and c obtained in this manner were mixed in a blender in the proportions shown in Table 1 to produce silicon nitride powder for sintering.

[0058] Figure 5 shows an SEM image of the silicon nitride powder for sintering obtained as described above, at a magnification of 500x. Both small and large silicon nitride particles were present, and coarse particles (particle size 10 μm or larger) were hardly observed. Furthermore, when the volume frequency distribution curve obtained by laser diffraction scattering (Figure 1) was waveform-separated, the following separated waveforms were identified, as shown in Figure 4: waveform a with a peak position of 0.53 μm, waveform b with a peak position of 1.9 μm, and waveform c with a peak position of 0.23 μm.

[0059] (Manufacturing of sintered bodies) To 100 parts by mass of silicon nitride powder for sintering obtained by the above method, 5 parts by mass of yttria as the main sintering aid and 2 parts by mass of alumina as the secondary sintering aid were added and mixed in a planetary ball mill. After uniaxial press molding and CIP molding, the mixture was fired at 2700°C for 5 hours under atmospheric pressure and a nitrogen atmosphere to obtain a silicon nitride sintered body.

[0060] <Example 3> Except for changing the grinding conditions in the millstone grinder and the vibrating ball mill, the mixing powders a, b, and c shown in Table 1 were obtained in the same manner as in Example 1, and were mixed in a blender in the proportions shown in Table 1 to obtain silicon nitride powder for sintering. Then, a silicon nitride sintered body was obtained using the obtained silicon nitride powder in the same manner as in Example 1.

[0061] <Comparative Examples 1-3> In Example 1, the silicon nitride powder for sintering was obtained in the same manner as in Example 1, except that the mixing ratios of powders a, b, and c were as shown in Table 1. Then, a silicon nitride sintered body was obtained using the obtained silicon nitride powder in the same manner as in Example 1.

[0062] <Comparative Examples 4-6> In Example 1, powders for silicon nitride for sintering were obtained in the same manner as in Example 1, except that the grinding conditions for obtaining mixing powders a, b, and c were changed to obtain mixing powders a, b, and c having the peak positions shown in Table 1. Then, a silicon nitride sintered body was obtained using the obtained silicon nitride powder in the same manner as in Example 1.

[0063] The silicon nitride powder for sintering obtained in Comparative Example 4 is a silicon nitride powder for sintering that has separated waveforms a and c when waveform separation occurs, but does not have separated waveform b. The silicon nitride powder for sintering obtained in Comparative Example 5 has a separated waveform c when waveform separation occurs, but does not have separated waveforms a and b. The silicon nitride powder for sintering obtained in Comparative Example 6 has separation waveform b, but does not have separation waveform a and separation waveform c. Comparative Examples 4-6 are silicon nitride powders that exhibit three types of separated waveforms when waveform separation occurs, but as described above, they do not exhibit at least one of the separated waveforms a-c. In Table 2, for Comparative Examples 4-6, the data for the three types of separated waveforms that were confirmed are listed in the columns for separated waveform b, separated waveform a, and separated waveform c, in descending order of peak position, in order of decreasing peak position.

[0064] [Table 1]

[0065] [Table 2]

[0066] Examples 1 to 3 are silicon nitride powders for sintering that have separation waveforms a, b, and c, and the total area of ​​separation waveform a and separation waveform b, and the ratio of the areas of separation waveform a to b (Sa / Sb) satisfy the requirements of the present invention. The sintered bodies exhibit excellent strength (three-point bending strength) and toughness, and have a low void ratio. In contrast, although the silicon nitride powders for sintering in Comparative Examples 1 and 2 had separation waveforms a, b, and c, the ratio of the total area of ​​separation waveforms a and b to the total area of ​​all separation waveforms, or the area ratio of separation waveforms a and b (Sa / Sb), was outside the scope of the present invention. Therefore, compared to the examples, the balance of physical properties of strength and toughness in the sintered body was inferior. Although the silicon nitride powder for sintering in Comparative Example 3 had separation waveforms a and b, the ratio of the total area of ​​separation waveforms a and b to the total area of ​​all separation waveforms, and the area ratio of separation waveforms a and b (Sa / Sb), were outside the scope of the present invention. Therefore, compared to the examples, the balance of physical properties of strength and toughness in the sintered body was inferior, and because it did not have separation waveform c, the void ratio was also high. The silicon nitride powder for sintering in Comparative Examples 4 and 5 had three types of separation waveforms, but none of them had at least one of the separation waveforms a and b defined in the present invention. Another separation waveform was observed that had a peak position outside the peak positions of separation waveforms a and b, and compared to the examples, the balance of physical properties of strength and toughness of the sintered body was inferior. The silicon nitride powder for sintering in Comparative Example 6 also had three types of separation waveforms, but none of the separation waveforms a and c defined in the present invention. Compared to the examples, the strength of the sintered body was lower and the void ratio was higher.

Claims

1. It consists of silicon nitride powder with a β-conversion rate of 80% or more. When the volume frequency distribution curve showing the particle size distribution is separated into waveforms, there is a separated waveform a with a peak between 0.5 μm and 1 μm, and a separated waveform b with a peak between 1 μm and 3 μm. The total area of ​​the separated waveforms a and b is 60-98% of the total area of ​​all separated waveforms. Silicon nitride powder for sintering, characterized in that the ratio (Sa / Sb) of the area of ​​the separated waveform a to the area of ​​the separated waveform b is 0.4 to 0.

8.

2. The silicon nitride powder for sintering according to claim 1, wherein, among particles with a particle size in the range of 1 to 5 μm as observed by a scanning electron microscope, the proportion of single particles with an aspect ratio (L / D) of 1.1 to 1.8 is 20% by volume or more.

3. Silicon nitride powder for sintering according to claim 1 or 2, wherein the average particle size (D50) is 0.5 to 2 μm.

4. The silicon nitride powder for sintering according to claim 1 or 2, wherein the remaining separated waveform is a separated waveform c having a peak between 0.1 and 0.3 μm.

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