Green sheet, method for manufacturing silicon nitride sintered body, and silicon nitride sintered body

Abstract

Provided is a green sheet containing a raw material powder and a binder resin, the raw material powder containing silicon nitride powder, and the glass transition temperature of the binder resin being less than

Description

[Technical Field]

[0001] This invention relates to a method for manufacturing green wafers, silicon nitride sintered bodies, and silicon nitride sintered bodies. [Previous Technology]

[0002] Sintered ceramic bodies, formed from sintered ceramic powders such as silicon nitride, aluminum nitride, and boron nitride, generally possess excellent properties such as high thermal conductivity, high insulation, and high strength. Therefore, these sintered ceramic bodies have attracted attention as various industrial materials. For example, silicon nitride sintered bodies are used as insulating substrates for power semiconductor components in environmentally friendly vehicles such as electric vehicles, hydrogen vehicles, and hybrid vehicles, as well as in renewable energy fields such as solar power generation and wind power generation. Silicon nitride is known as a ceramic material with excellent oxidation resistance, corrosion resistance, and thermal conductivity; its sintered bodies have high mechanical strength, and its industrial applications span multiple fields mentioned above.

[0003] Methods for obtaining ceramic sintered bodies include: granulating ceramic powder into particles, drying and pressing them to form a stamped body, and then firing it; and wet-forming ceramic powder to obtain green sheets, degreasing them, and then firing them. The latter method of manufacturing green sheets generally involves mixing ceramic powder, binder resin, plasticizer, sintering aid, and organic solvent using a ball mill or similar method, and then forming the sheets using a scraper method. After sheet forming, a degreasing step is performed by heating to decompose and remove the binder resin, followed by a firing step to sinter the ceramic powder, thereby obtaining a ceramic sintered body (Patent Document 1). [Prior Art Documents] [Patent Documents]

[0004] Patent Document 1: International Publication No. 2021 / 095843 [Summary of the Invention]

[0005] [The problem the invention aims to solve]

[0006] As described above, the green sheet containing ceramic powder, binder resin, plasticizer, and sintering aid is fired after a degreasing step to remove organic matter such as binder resin. Here, the higher the content of organic matter such as binder resin and plasticizer in the green sheet, the greater the problem of time-consuming degreasing process and reduced productivity. Furthermore, if the amount of organic matter such as binder resin and plasticizer in the green sheet is reduced in order to improve productivity, the flexibility and shape retention of the green sheet itself will deteriorate, causing molding cracking and degreasing cracking, resulting in other problems.

[0007] This invention is made in view of the aforementioned prior art problems, and its object is to provide a green sheet that exhibits excellent flexibility and sheet formability while allowing for efficient degreasing. [Means for solving the problems]

[0008] The inventors have conducted diligent research to achieve the above-mentioned objectives. As a result, they discovered that by using an adhesive resin with a specific glass transition temperature as the adhesive resin, the above-mentioned problems can be solved, thereby completing the present invention.

[0009] The gist of the present invention is as follows. [1] A green sheet containing raw material powder and adhesive resin, wherein the raw material powder contains silicon nitride powder and the glass transition temperature of the adhesive resin is not -20°C. [2] The green sheet of [1] above, wherein the sheet thickness is 200 μm or more. [3] The green sheet of [1] or [2] above, wherein the adhesive resin contains at least one selected from the group consisting of (meth)acrylic resin, polyethylene resin and polyethylene oxide. [4] The green sheet of any one of [1] to [3] above, wherein the adhesive resin contains acrylic resin. [5] The green sheet of any one of [1] to [4] above, wherein it substantially does not contain plasticizer. [6] A method for manufacturing a silicon nitride sintered body, comprising the step of firing the green sheet of any one of [1] to [5] above after degreasing. [7] A silicon nitride sintered body, which is formed by firing a green sheet as described in any one of [1] to [5] above. [Effects of the Invention]

[0010] According to the present invention, a method for manufacturing a silicon nitride sintered body that can be efficiently degreased while exhibiting excellent flexibility and sheet formability, a method for manufacturing a silicon nitride sintered body including a step of firing the silicon nitride sintered body after degreasing, and a silicon nitride sintered body can be provided.

Implementation Method

[0011] The green sheet of the present invention contains raw material powder and adhesive resin. The components are described in detail below.

[0012] [Raw Material Powder] In this invention, the raw material powder is not particularly limited as long as it contains silicon nitride powder. Preferably, the raw material powder contains silicon nitride powder and the sintering aid described later. When the raw material powder contains silicon nitride powder, the resulting green sheet containing the raw material powder, when sintered, not only is a high-strength silicon nitride sintered body obtained, but also exhibits excellent thermal conductivity and insulation.

[0013] <Silicon Nitride Powder> Silicon nitride powder (Si3N4 powder) can be any commonly available silicon nitride powder manufactured by various methods such as reduction nitriding, direct nitriding, and amide decomposition without particular restriction. The average particle size D50 of the silicon nitride powder is not particularly limited, but considering ease of sintering, it is, for example, 0.5~10 μm, preferably 1~3 μm. Furthermore, the average particle size D50 in this specification is a 50% volume basis value obtained by laser diffraction scattering.

[0014] The specific surface area of ​​silicon nitride powder is not particularly limited, but is preferably 2~20 m² / g, more preferably 5~15 m² / g. The specific surface area is determined using the BET1 point method utilizing nitrogen adsorption.

[0015] As silicon nitride powder, either α-type or β-type can be used. For example, when using α-type silicon nitride powder, silicon nitride powder with an α-saturation of 80% or more of silicon nitride in the raw material powder can be used. On the other hand, when using β-type silicon nitride powder, silicon nitride powder with a β-saturation of 80% or more of silicon nitride in the raw material powder can be used. In addition, silicon nitride powder containing both α-type and β-type can also be used in this invention.

[0016] In addition, the α-phase ratio and β-phase ratio of silicon nitride powder refer to the ratio of the peak intensity of the α-phase or β-phase relative to the total of the α-phase and β-phase in the silicon nitride powder, respectively. The ratio of the peak intensity of the α-phase or β-phase is: [100×(peak intensity of α-phase) / (peak intensity of α-phase + peak intensity of β-phase)] in the case of α-phase ratio, and [100×(peak intensity of β-phase) / (peak intensity of α-phase + peak intensity of β-phase)] in the case of β-phase ratio, which is determined by powder X-ray diffraction (XRD) using CuKα lines. More specifically, it can be determined by calculating the mass ratio of the α-phase and β-phase of the silicon nitride powder using the method described in CPGazzara and DRMessier: Ceram. Bull., 56 (1977), 777-780.

[0017] In this invention, the raw material powder preferably contains 65% by mass or more silicon nitride powder, more preferably 75% by mass or more, even more preferably 87% by mass or more, and still more preferably 90% by mass or more. By firing a green sheet with silicon nitride content in the raw material powder within the above range, a silicon nitride sintered body with high strength and high thermal conductivity and insulation can be obtained.

[0018] <Sintering Aid> The above-mentioned raw material powder preferably further contains a sintering aid. A metal oxide can be used as the sintering aid. By using a metal oxide as a sintering aid, the sintering of silicon nitride powder becomes easier, and a denser and higher-strength sintered body is easily obtained. Furthermore, metal oxides also have the advantages of being inexpensive and easy to handle.

[0019] The aforementioned metal oxides used as sintering aids may include, for example, oxides of at least one rare earth element and / or oxides of magnesium. More specifically, as metal oxides, oxides of rare earth elements such as yttrium oxide (Y₂O₃) and cerium dioxide (CeO) and magnesium oxide (MgO) may be included. Among these, yttrium oxide is preferred. One type of metal oxide may be used alone, or two or more types may be used in combination.

[0020] In addition to the aforementioned metal oxides, oxygen-free compounds can also be used as sintering aids. By using oxygen-free compounds as sintering aids, the amount of oxygen dissolved in silicon nitride originating from the sintering aids can be reduced. As a result, a silicon nitride sintered body with high thermal conductivity can be obtained. Preferably, the oxygen-free compound is a carbonitride system compound containing rare earth elements or magnesium (hereinafter also referred to as a specific carbonitride system compound). Thus, by using a specific carbonitride system compound, a silicon nitride sintered body with high thermal conductivity can be easily obtained as described above.

[0021] Among compounds containing carbonitride systems of rare earth elements, Y (yttrium), La (lanthanum), Sm (samarium), Ce (cerium) are preferred as rare earth elements.

[0022] Examples of carbonitride compounds containing rare earth elements include Y₂Si₄N₆C, Yb₂Si₄N₆C, and Ce₂Si₄N₆C. Among these, Y₂Si₄N₆C and Yb₂Si₄N₆C are preferred from the viewpoint of easily obtaining silicon nitride sintered bodies with high thermal conductivity. Examples of carbonitride compounds containing magnesium include MgSi₄N₆C. These specific carbonitride compounds can be used alone or in combination of two or more. Among the aforementioned carbonitride compounds containing rare earth elements or magnesium, Y₂Si₄N₆C and MgSi₄N₆C are particularly preferred.

[0023] In this invention, a mixture of the aforementioned metal oxide and the aforementioned oxygen-free compound can be used as a sintering aid. Specific examples of the metal oxide and the oxygen-free compound are as described above. When using a mixture of metal oxide and oxygen-free compound as a sintering aid, the mass ratio (oxygen-free compound / metal oxide) of the aforementioned oxygen-free compound (represented by the aforementioned specific carbonitride system compound) to the aforementioned metal oxide is preferably 0.2 to 4, more preferably 0.6 to 3. Within this range, it is easier to obtain a denser silicon nitride sintered body with higher thermal conductivity.

[0024] The amount of sintering aid in the raw material powder contained in the green sheet of the present invention is not particularly limited, but is preferably 3 to 50 parts by weight relative to 100 parts by weight of silicon nitride powder, more preferably 3 to 30 parts by weight, and even more preferably 5 to 15 parts by weight. If the amount of sintering aid is within the above range, the sintering of the green sheet can be easily carried out, and a dense sintered body can be obtained. In addition, when the sintering aid, as described above, is a mixture of metal oxide and oxygen-free compound, the total amount of the mixture is set in a manner that falls within the above range. The mass of the metal oxide and oxygen-free compound in the sintering aid is as previously described.

[0025] [Adhesive Resin] The adhesive resin contained in the green sheet of the present invention must have a glass transition temperature (Tg) not exceeding -20°C. If the glass transition temperature (Tg) of the adhesive resin is above -20°C, the flexibility of the green sheet decreases, and the sheet itself may crack, easily causing problems with the sheet's formability. In particular, as described later, these problems are more pronounced in green sheets that do not contain plasticizers.

[0026] The glass transition temperature of the adhesive resin is preferably below -23°C, more preferably below -30°C, and even more preferably below -35°C. If the glass transition temperature is within the above range, both the softness of the green sheet and the sheet formability are excellent. The lower limit of the glass transition temperature of the adhesive resin is not particularly limited, for example, it is above -70°C, specifically above -66°C. In addition, the above glass transition temperature can be measured using, for example, a differential scanning calorimeter (DSC).

[0027] The adhesive resin contained in the green sheet of the present invention is not particularly limited as long as it has the above-mentioned glass transition temperature. As such an adhesive resin, at least one selected from the group consisting of (meth)acrylic resins, polyethylene resins and polyethylene oxides can be used.

[0028] The above-mentioned (meth)acrylic resin is a resin having a (meth)acrylic backbone in its main chain, and there are no particular limitations as long as it has the above-mentioned glass transition temperature. Examples of (meth)acrylic resins include, for example, copolymers containing 1 or more (meth)acrylic ester monomers. The (meth)acrylic ester monomer is an ester of acrylic acid and / or methacrylic acid with an alcohol compound. Examples of the alcohol compound include, for example, alcohol compounds having 1 to 30 carbon atoms, such as alcohol compounds having 1 to 30 carbon atoms. The alcohol compound can be an aliphatic alcohol or an aromatic alcohol. As an example of an alcohol compound, the alkyl group having 1 to 30 carbon atoms can be a straight-chain alkyl group, a branched alkyl group, or a group in which a portion of the hydrogen atom of the aforementioned straight-chain or branched alkyl group is replaced by an aromatic ring, hydroxyl group, amino group, halogen atom, etc. By using 1 or more of such (meth)acrylic ester compounds to achieve the glass transition temperature within the above-mentioned range, a (meth)acrylic resin can be obtained.

[0029] In addition, in this specification, the designation of (meth)acrylic acid indicates that it contains one or both of methacrylic acid and acrylic acid, and the designation of (meth)acrylate indicates that it contains one or both of methacrylate and acrylate.

[0030] The glass transition temperature of the adhesive resin can be adjusted by, for example, the type of polymer constituting the adhesive resin, the type of side chain, the length of the side chain, the type of substituent, the presence or absence of a cross-linking structure, and the molecular weight. For example, by incorporating long units of the side chain into the resin structure, the glass transition temperature of the adhesive resin can be reduced. Adhesive resins having, for example, a resin structure having straight-chain alkyl groups with 4 to 20 carbon atoms in the side chain can be used.

[0031] In this invention, from the viewpoint of glass transition temperature, (meth)acrylic resin is preferably used as the adhesive resin, and more preferably acrylic resin. The green sheet of this invention contains, based on the total amount of adhesive resin, preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more of acrylic resin.

[0032] The weight-average molecular weight of the adhesive resin contained in the green sheet of the present invention is not particularly limited, as long as the formability and flexibility of the green sheet are good. The range of the weight-average molecular weight of the adhesive resin is, for example, 30,000 to 3,000,000, preferably 40,000 to 2,000,000, and more preferably 50,000 to 1,500,000. Furthermore, the weight-average molecular weight of the adhesive resin can be determined using colloidal permeation chromatography (GPC) converted to polystyrene.

[0033] [Green Sheet] The green sheet of the present invention contains the above-mentioned raw material powder and binder resin. The amount of the binder resin in the green sheet of the present invention can be appropriately determined according to the molding method, and is preferably 3 to 40 parts by mass relative to 100 parts by mass of raw material powder, more preferably 10 to 30 parts by mass. If the amount of binder resin is within the above range, while obtaining a green sheet with excellent softness and sheet formability (sheet shape retention), the filling properties of the raw material powder can be improved, and a stable firing shrinkage rate can be obtained. Furthermore, when degreasing the green sheet, the binder resin can be removed efficiently. As described above, the green sheet of the present invention preferably contains the above-mentioned raw material powder and the above-mentioned binder resin in the above-mentioned proportions. Here, the proportion of the above-mentioned raw material powder in the entire green sheet of the present invention is preferably 70% by mass or more, more preferably 80% by mass or more.

[0034] Furthermore, in one embodiment of the present invention, the green sheet preferably does not substantially contain plasticizer. Conventionally, the formation of green sheets involves using a slurry containing: raw material powder and sintering aid, adhesive resin for maintaining sheet shape, plasticizer for imparting flexibility, solvent (described later), and surfactant, among other organic substances. During green sheet formation, the solvent and surfactant are removed by vaporizing the slurry after it has been formed into a sheet using a warm air source. On the other hand, the adhesive resin and plasticizer need to be removed in a degreasing step following the drying step. The higher the amount of organic matter, the more time-consuming the degreasing process becomes, reducing productivity, and organic matter, as an impurity, is prone to remain, raising concerns about a reduction in the physical properties of the silicon nitride sintered body.

[0035] Through diligent research, the inventors discovered that by using an adhesive resin with a specific glass transition temperature as the adhesive resin and without using plasticizers, green sheets with excellent shape retention and flexibility can be obtained. This reduces the amount of organic matter used during green sheet formation, thus improving green sheet productivity. Furthermore, the silicon nitride sintered body obtained by degreasing and firing the obtained green sheet also exhibits excellent physical properties. Additionally, "substantially does not contain" means that the proportion of plasticizer in the green sheet, based on the total amount of components contained in the green sheet, does not reach 1 ppm by mass.

[0036] The thickness of the green sheet of the present invention is not particularly limited, and can be set with consideration of the desired thickness of the final silicon nitride sintered body. For example, from the viewpoint of processability, the sheet thickness of the green sheet is preferably 200 μm or more, more preferably 250 μm or more, and even more preferably 300 μm or more. Since the green sheet of the present invention has excellent flexibility, the above-mentioned thickness can be set. Furthermore, if the thickness of the green sheet increases, the degreasing time generally needs to be extended. For example, if the thickness increases, the amount of organic matter contained before degreasing will naturally increase, or the inconsistent heat conduction in the thickness direction will make it time-consuming to fully vaporize the organic matter, thus requiring a longer degreasing time. In this regard, by applying the technology of the present invention, by reducing the amount of organic matter removed during degreasing of the green sheet, even for thick green sheets, degreasing can be completed in a shorter time, making it easy and efficient to manufacture the sintered body. Therefore, green sheets with a thickness within the aforementioned range have significant advantages in applying the present invention and are considered a preferred configuration. The upper limit of the green sheet thickness is not particularly limited; generally, the thickness of the green sheet is 1.2 mm or less, especially 0.8 mm or less. The dimensions (width and length) of the green sheet are preferably, for example, 100 to 1000 mm.

[0037] In this invention, the green sheet can be manufactured specifically through the following steps. That is, by mixing raw material powder and binder resin to obtain a slurry-like molding composition (slurry preparation step), and by forming the obtained slurry-like molding composition into a plate or sheet shape using a doctor blade method or the like (molding step). Hereinafter, each step will be described.

[0038] <Slurry Preparation Step> In the manufacture of the green sheet of the present invention, the method for preparing the slurry (containing raw material powders such as silicon nitride) is not particularly limited. For example, the components constituting the slurry can be measured in a specified blending amount and stirred and mixed to disperse the raw material powders such as silicon nitride powder in a solvent to prepare the slurry. In the slurry preparation step, the amount of each component constituting the slurry used can be appropriately determined so that the obtained green sheet conforms to the aforementioned composition. Furthermore, dispersants and solvents can be used as needed.

[0039] Dispersants are preferred in molding compositions and are used to improve the dispersibility of raw material powders, etc. Generally, surfactants can be suitably used as dispersants.

[0040] The surfactants can be used without any restrictions. Specifically, examples of surfactants that are preferably used in this invention include carboxylated trioxyethylene tridecyl ether, diglyceride monooleate, diglyceride monostearate, carboxylated heptaoxyethylene tridecyl ether, tetraglyceride monooleate, hexaglyceride monooleate, sorbitan laurate, sorbitan oleate, sorbitan trioleate, polyoxyethylene sorbitan monolaurate, polyoxyethylene sorbitan monooleate, and polyoxyethylene sorbitan trioleate. Furthermore, these surfactants can be used alone or in combination of two or more.

[0041] The amount of the dispersant can be appropriately selected, for example, a range of 0.1 to 5 parts by mass relative to 100 parts by mass of the raw material powder. Within this range, the upper limit of the amount of dispersant is preferably 3 parts by mass or less, more preferably 2 parts by mass or less, and even more preferably 1 part by mass or less.

[0042] A better solvent is used to improve the mixability (ease of preparation of slurry) and formability of the above-mentioned molding components. Generally speaking, organic solvents and water are suitable.

[0043] Examples of organic solvents include ketones such as acetone, methyl ethyl ketone, and methyl isobutyl ketone; alcohols such as ethanol, propanol, and butanol; aromatic hydrocarbons such as benzene, toluene, and xylene; or halogenated hydrocarbons such as trichloroethylene, tetrachloroethylene, and bromochloromethane. One or more of these compounds may be used as organic solvents. Water, such as ordinary tap water or purified water, may be used without restriction.

[0044] The amount of the solvent can be appropriately selected. For example, a range of 50 to 150 parts by weight can usually be selected relative to the total amount of raw material powder and adhesive resin of 100 parts by weight.

[0045] Furthermore, if the viscosity of the molding composition obtained using the aforementioned solvent is low, removing a portion of the solvent from the molding composition can adjust the viscosity to be suitable for the next step. Examples of such operations include stirring under vacuum to distill off the solvent.

[0046] When mixing the components, conventional mixing equipment can be used. Examples of such mixing equipment include ultrasonic dispersion devices, ball mills, bead mills, roller mills, homogenizers, ultra-mixers, dispersing mixers, through-type high-pressure dispersion devices, impact-type high-pressure dispersion devices, porous high-pressure dispersion devices, de-caking high-pressure dispersion devices, (impact + through-type) high-pressure dispersion devices, and ultra-high-pressure homogenizers. To ensure thorough mixing of the components, it is generally preferable to perform the mixing in multiple stages, such as in two stages. For example, in the case of two-stage mixing, the raw material powder, dispersant, and solvent are added and mixed in the first stage; in the second stage, binder resin is added to the mixture from the first stage, and solvent is added further as needed, and then mixed to prepare a slurry. Furthermore, after mixing, filtration or other operations can be performed as needed to remove caking from the slurry.

[0047] <Forming Step> In the forming step, there are no particular limitations on the method for forming the above-mentioned slurry-like forming composition into a sheet shape, and conventional methods and apparatus can be used. For example, the above-mentioned forming composition can be formed into a sheet shape by a doctor blade method, extrusion molding method, etc. In addition, if a solvent is used in the above-mentioned slurry preparation step, it is preferable to set a drying step after the above-mentioned forming step.

[0048] <Drying Step> In the drying step, there are no particular limitations on the method used to dry the molded body, which is formed into a sheet shape; conventional methods and apparatus can be used. Generally, drying is performed in an air or nitrogen environment at a temperature above the boiling point of the solvent to remove the solvent, thereby obtaining a green sheet. The drying temperature in this drying step can be appropriately set according to the type of solvent and surfactant used in the molding composition. Since the solvent used is vaporized, the setting can be considered based on the boiling point of the solvent. However, if the drying temperature is too high, there is a risk of the solvent boiling out suddenly, causing the green sheet to crack and affecting its flatness (causing unevenness). Therefore, a drying temperature of, for example, about 50°C above the boiling point of the solvent used is preferred. The above drying can be performed by blowing warm air, etc.

[0049] As described above, in this invention, the amount of organic matter in the green sheets supplied for the degreasing step can be significantly reduced. Specifically, the amount of organic matter contained in the green sheets after the drying step is preferably 40 parts by weight or less, more preferably 30 parts by weight or less, and even more preferably 25 parts by weight or less, relative to 100 parts by weight of the raw material powder. By controlling the amount of organic matter contained in the green sheets before the degreasing step to the above range, degreasing can be carried out efficiently and sufficiently in a short time.

[0050] [Method for Manufacturing Silicon Nitride Sintered Body] According to one embodiment of the present invention, the green sheet of the present invention described above can be degreased and then fired to manufacture a silicon nitride sintered body. The preferred method for manufacturing the silicon nitride sintered body of the present invention includes the following degreasing and firing steps.

[0051] <Degreasing Step> This degreasing step is used to degrease the adhesive resin and residual organic matter from the above-mentioned green sheet. Heating in the degreasing step can be carried out in an inert gas environment or in air, preferably in air. Furthermore, in this specification, an inert gas environment means a nitrogen environment or an argon environment. The heating temperature in the degreasing step can be appropriately selected according to the type of raw material powder and adhesive resin, and the difference in the environment, and can be arbitrarily selected within, for example, the range of 450~650°C. The heating time is, for example, approximately 1~6000 minutes. Using the heating temperature and heating time within the above range, degreasing of organic matter such as adhesives can be performed.

[0052] Here, as mentioned above, the green sheet of the present invention has excellent softness, therefore, it is not necessary to add plasticizer, and the amount of organic matter to be degreased in the degreasing step is reduced. Therefore, the heating time in this degreasing step may be shorter than before. For example, the heating time in this degreasing step is preferably 100 to 5000 minutes, more preferably 1000 to 4500 minutes, and even more preferably 2000 to 4000 minutes. If the heating time is above the lower limit value, degreasing can be performed sufficiently. In the present invention, if the heating time is below the upper limit value, the degreasing efficiency can be improved. In the present invention, even without the use of plasticizer, a green sheet with excellent softness can be obtained, so organic matter can be efficiently removed in the degreasing step, and as a result, silicon nitride sintered bodies can be manufactured with high production efficiency.

[0053] <Firing Step> After performing the above-described degreasing step, a firing step is performed to obtain a silicon nitride sintered body. The firing step can be performed in an inert gas environment or in air, but is preferably performed in an inert environment. Firing can be performed at atmospheric pressure or under pressure. The firing temperature is not particularly limited, but is preferably set appropriately according to the composition of the raw material powder. From the viewpoint of ease of sintering, raw material powder, and especially the inhibition of silicon nitride decomposition, it can be set to, for example, 1200~1800°C. Furthermore, the firing time is not particularly limited, but is preferably set to about 3~20 hours. Since the raw material powder of the green sheet of the present invention contains silicon nitride powder, the firing temperature is preferably 1700~1800°C.

[0054] After the above-described degreasing and firing steps, the green sheet can be fired to obtain a silicon nitride sintered body. In this invention, as described above, the amount of organic matter contained in the green sheet can be reduced, thus allowing for efficient and thorough degreasing. As a result, the production efficiency of manufacturing silicon nitride sintered bodies can be significantly improved. Furthermore, despite the significant reduction in the amount of organic matter contained in the green sheet, a green sheet with excellent sheet shape retention and flexibility can still be obtained. Using this green sheet, a silicon nitride sintered body with fewer impurities due to degreasing residue can ultimately be obtained. Therefore, the obtained silicon nitride sintered body has excellent properties, such as excellent thermal conductivity and insulation. In addition, the silicon nitride sintered body can be subjected to sandblasting or other treatments after firing to remove adhering raw material powders and other substances.

[0055] The silicon nitride sintered body of the present invention can be used in various industrial materials. For example, it can be used as an insulating substrate for power semiconductor components in environmentally friendly vehicles such as electric vehicles, hydrogen vehicles, and hybrid vehicles, as well as in renewable energy fields such as solar power generation and wind power generation. Furthermore, by forming a composite material with silicon carbide fibers, it can be used as a turbine blade for jet engines requiring high reliability. [Examples]

[0056] Hereinafter, embodiments are illustrated for more specific explanation of the present invention, but the present invention is not limited to these embodiments.

[0057] In the examples, the following raw materials were used. (Raw material powder) Raw material powder containing the following silicon nitride powder and sintering aid was used. <Silicon nitride powder> Average particle size D50: 1.8 μm Specific surface area: 7 m² / g β-saturation: 99%

[0058] <Sintering Aids> 1. Oxygen-free compounds: Y₂Si₄N₆C powder, MgSi₄N₆C powder (i) Regarding Y₂Si₄N₆C powder, it is prepared by heating yttrium oxide (manufactured by Shin-Etsu Chemical Industry Co., Ltd.), silicon nitride powder (the silicon nitride powder described above), and carbon powder (manufactured by Mitsubishi Chemical Co., Ltd.) using the following reaction formula. (ii) Regarding MgSi₄N₆C powder, it is also prepared by heating using the following reaction formula. 2. Metal oxide yttrium oxide (Y₂O₃): (manufactured by Shin-Etsu Chemical Industry Co., Ltd.)

[0059] (Adhesive) As the adhesive resin, an acrylic resin (manufactured by Fujikura Chemical Co., Ltd.) having the glass transition temperature Tg shown in Tables 1 and 2 was used. Furthermore, the glass transition temperature of the adhesive resin was measured under the following conditions: Measurement apparatus: DSC-60 (Shimadzu Corporation) Measurement temperature program: First run: Heating from room temperature to 200°C at a rate of 20°C / min Holding: 200°C for 5 minutes Cooling: Cooling from 200°C to -95°C at a rate of -20°C / min Holding: -95°C for 5 minutes Second run: Heating from -95°C to 100°C at a rate of 10°C / min

[0060] (Dispersant) As a dispersant, "Celuna D-735" from Zhongjing Oils & Fats Co., Ltd. is used.

[0061] <Evaluation Method> (1) Evaluation of Green Sheets (i) Evaluation of Green Sheet Cracks The cracks of the green sheets after molding are evaluated as follows. After molding the green sheets using molding components, the appearance is checked. If the length of the cracked green sheet relative to the length after molding is less than 3%, it is considered acceptable (A), and if the length of the cracked green sheet is 3% or more, it is considered unacceptable (X). (ii) Evaluation of Green Sheet Softness The softness of the molded green sheets is evaluated by bending the green sheets to 90 degrees. If there are no cracks, it is considered acceptable (A), and if there are cracks, it is considered unacceptable (X).

[0062] (2) Evaluation of degreasing cracking of green sheets After degreasing the green sheets obtained in each example and comparative example, the degreasing cracking will be evaluated. Cracking during degreasing is used to confirm the appearance of the sheet after degreasing. Those without cracking are judged as qualified (A), and those with cracking can be identified as unqualified (X).

[0063] (3) The relative density of the silicon nitride sintered body was determined using a high-precision hydrometer DH (trade name: Toyo Seiki Co., Ltd.) by Archimedes' method. The relative density was obtained by dividing the obtained density value by the theoretical density that takes into account silicon nitride and sintering aids.

[0064] (4) Thermal conductivity was measured for each sintered body using a laser flash thermophysical property measuring apparatus (Kyoto Electronics Industry Co., Ltd.: LFA-502 type). The thermal diffusivity was calculated by multiplying the thermal diffusivity by the density and specific heat of the sintered body. The specific heat of the silicon nitride sintered body was taken as 0.68 (J / g・K). Three pieces were randomly selected from 15 sintered bodies and cut into test pieces for laser flash thermophysical property measurement. The thermal conductivity was calculated from the density and thermal diffusivity of the three test pieces, and the average value of the thermal conductivity of the three test pieces was taken as the thermal conductivity of the sintered body.

[0065] (5) Ten pieces are randomly selected from the twelve pieces excluding the three pieces used in the above thermal conductivity test, and test pieces for the three-point bending strength test are cut. The three-point bending strength of each of the ten test pieces is determined according to the method of ISO 23242:2020. At this time, a test fixture with a distance of 15 mm between the fulcrums is used. The average value of the three-point bending strength of the ten test pieces is expressed as the three-point bending strength of the sintered body.

[0066] Example 1: 100 parts by weight of silicon nitride powder, 2 parts by weight of Y₂Si₄N₆C₂, 3 parts by weight of yttrium oxide, 5 parts by weight of MgSi₄N₆C₂, and 0.5 parts by weight of dispersant were weighed. Water was used as a solvent, and the mixture was pulverized and mixed in a resin tank with silicon nitride balls using a ball mill for 24 hours. Additionally, water was pre-weighed to a slurry concentration of 60% by weight and added to the resin tank. After pulverization and mixing, 22 parts by weight of the binder listed in the table were added, and the mixture was mixed for 12 hours to obtain a slurry-like molding composition. Next, the molding composition was degassed using a vacuum degassing machine (manufactured by Sayama Riken Co., Ltd.), and the viscosity was adjusted to prepare a coating slurry. Then, using the viscosity-adjusted coating slurry, a sheet was formed by a doctor blade method and dried in air at 150°C to vaporize the solvent, resulting in a green sheet with a width of 750 mm and a thickness of 420 μm. The green sheet obtained as described above was degreased in dry air at 550°C to obtain a degreased green sheet. Then, the degreased green sheet was placed in a firing vessel and fired at 1780°C for 9 hours under a nitrogen atmosphere and a pressure of 0.03 MPa·G to obtain a silicon nitride sintered body. The evaluation of the obtained green sheet and silicon nitride sintered body is shown in Table 1.

[0067] Examples 2-6: Except for changing the type of adhesive resin as shown in Table 1, green sheets were prepared in the same manner as in Example 1, and then the green sheets were fired to obtain silicon nitride sintered bodies. The evaluation of the obtained green sheets and silicon nitride sintered bodies is shown in Table 1.

[0068] Comparative Examples 1-8: Except for changing the type of adhesive resin as shown in Table 2, green sheets were prepared in the same manner as in Example 1, and then the green sheets were fired to obtain silicon nitride sintered bodies. The evaluation of the obtained green sheets and silicon nitride sintered bodies is shown in Table 2.

[0069] Comparative Example 9: As shown in Table 2, the type of adhesive resin was changed, and except for the addition of 15 parts by mass of plasticizer (compound name: glycerol) to 100 parts by mass of silicon nitride powder, the molding composition was prepared in the same manner as in Example 1. After preparing a green sheet, the green sheet was fired to obtain a silicon nitride sintered body. The evaluation of the obtained green sheet and silicon nitride sintered body is shown in Table 2. In addition, as shown in Table 2, the degreasing step required in Comparative Example 9 took longer than that in the Example, resulting in poor productivity.

[0070] Comparative Example 10: Except that the degreasing time was set to 65 hours, the molding composition was prepared in the same manner as in Comparative Example 9, and a green sheet was made. When inspecting the sintered silicon nitride body after firing, it was found that the discoloration was caused by organic components that could not be removed, indicating that the degreasing was not sufficient.

[0071]

[0072]

Claims

1. A green sheet comprising raw material powder and adhesive resin, wherein the raw material powder comprises silicon nitride powder and the glass transfer temperature of the adhesive resin is less than -20°C.

2. The green sheet as requested in item 1, wherein the sheet thickness is 200 μm or more.

3. The green sheet of claim 1 or 2, wherein the aforementioned adhesive resin contains at least one selected from the group consisting of (meth)acrylic resins, polyethylene resins and polyethylene oxide.

4. The green sheet of any one of claims 1 to 3, wherein the aforementioned adhesive resin contains an acrylic resin.

5. The green sheet of any of the claims 1 to 4, which does not substantially contain plasticizers.

6. A method for manufacturing a silicon nitride sintered body, comprising the step of firing a green sheet as claimed in any one of claims 1 to 5 after degreasing.

7. A silicon nitride sintered body, which is formed by firing a green sheet as claimed in any one of claims 1 to 5.