Green sheet and method for manufacturing the same

Abstract

To provide a green sheet excellent in moldability, strength, and aging stability.SOLUTION: A green sheets disclosed herein includes a nitride-based ceramic powder, a water-soluble resin binder, and a water-soluble plasticizer, and the water-soluble resin binder has a glass transition point of 50°C or lower, and the water-soluble plasticizer has a number average molecular weight of 300 or more and 1,500 or less. Also, such a green sheet is preferably manufactured by a dry powder rolling method.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a green sheet and a method for manufacturing the same. More specifically, the present invention relates to a green sheet containing nitride-based ceramic powder and a method for manufacturing the same.

Background Art

[0002] In order to efficiently utilize electric energy, power semiconductor devices (so-called power devices) are indispensable. In addition, the demand for lighting semiconductor devices (so-called high-power LED devices) used in energy-saving, long-life high-brightness and power LED lamps is also increasing. In recent years, research and development of technologies related to miniaturization, high density, and high speed of power devices have been actively carried out.

[0003] Generally, as the density of power devices increases, the amount of heat generated by the power devices increases. Due to such heat generation, problems may occur in the power devices and the members around such power devices, so a technique for dissipating heat to the surroundings is required. As such a technique, for example, as disclosed in Patent Documents 1 to 5, it has been proposed to use a sheet-like composition having thermal conductivity.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0005] Incidentally, in order to increase the thermal conductivity of a sheet-like composition, it is desirable to have as few voids as possible in the sheet-like composition. Since air contained in voids has low thermal conductivity, reducing the number of voids improves the thermal conductivity. For this reason, a high-density sheet-like composition with few voids (i.e., a sheet-like composition with high moldability) is desirable.

[0006] On the other hand, it is desirable that the sheet-like composition has high strength and is resistant to deterioration over a long period of time (high stability over time). If the strength of the sheet-like composition is low, there is a risk of damage when assembling the sheet-like composition into a component. In addition, the quality of the sheet-like composition may deteriorate over time due to the absorption of moisture from the air, etc., during use and storage. Therefore, in order to achieve a long lifespan for the sheet-like composition, and furthermore for components using the sheet-like composition (e.g., power devices), high strength and stability over time are desirable.

[0007] This invention has been made in view of the above circumstances, and its main objective is to provide a sheet-like composition (green sheet) with excellent moldability, strength, and long-term stability. Another objective is to provide a method for manufacturing such a green sheet. [Means for solving the problem]

[0008] To achieve the above objective, the inventors focused on the flexibility of the resin binder contained in the raw materials of the green sheet (i.e., the constituent components of the green sheet). One method for manufacturing the green sheet is to apply pressure to the raw materials and mold them into a sheet. In this case, if the raw materials of the green sheet are flexible, the green sheet can be manufactured while pushing out air with pressure, thereby improving moldability. On the other hand, if the flexibility of such raw materials is too high, there is a risk that the strength of the manufactured green sheet will be insufficient. Therefore, the inventors focused on plasticizers that impart flexibility to the resin and conducted thorough research. As a result, it was found that by using a water-soluble plasticizer with a number-average molecular weight within a predetermined range, a green sheet with excellent moldability, strength, and long-term stability can be realized.

[0009] Specifically, the green sheet disclosed herein comprises nitride-based ceramic powder, a water-soluble resin binder, and a water-soluble plasticizer. The glass transition temperature of the water-soluble resin binder is 50°C or lower, and the number-average molecular weight of the water-soluble plasticizer is 300 or more and 1500 or less.

[0010] In a green sheet with this configuration, the water-soluble resin binder has appropriate flexibility because it contains a water-soluble plasticizer having a number-average molecular weight within the above range. Furthermore, by including a water-soluble resin binder with a glass transition temperature of 50°C or lower, the water-soluble resin binder can be easily made into a rubbery or liquid-like state with low viscosity during the manufacturing of the green sheet. As a result, air can be suitably pushed out during the manufacturing of the green sheet, resulting in a green sheet with excellent moldability. In addition, in this configuration, the flexibility of the water-soluble resin binder is adjusted to an appropriate range, so this green sheet has excellent strength. Moreover, since water-soluble plasticizers have hydrophilic groups, they tend to absorb moisture from the air, but by keeping the water-soluble plasticizer within the above number-average molecular weight range, the amount of moisture absorbed from the air can be suppressed, resulting in excellent long-term stability.

[0011] In one embodiment of the green sheet disclosed herein, the theoretical density ratio ((measured density / theoretical density) × 100) is 85% or higher. According to the technology disclosed herein, a green sheet with such high moldability can be realized.

[0012] In one embodiment of the green sheet disclosed herein, the tensile strength at a tensile speed of 1 mm / min, in accordance with JIS K 7161:2014, is 0.8 MPa or higher. According to the technology disclosed herein, a green sheet with such excellent strength can be realized.

[0013] In one preferred embodiment of the green sheet disclosed herein, when the sum of the volume of the water-soluble resin binder and the volume of the water-soluble plasticizer is 100 vol%, the volume ratio of the water-soluble plasticizer is 2 vol% or more and 35 vol% or less. With this configuration, the flexibility of the water-soluble resin binder can be more appropriately controlled, and a better balance between moldability and strength can be achieved.

[0014] In one preferred embodiment of the green sheet disclosed herein, the water-soluble plasticizer includes a polyether-based plasticizer. This allows for more appropriate control of the flexibility of the water-soluble resin binder.

[0015] In one preferred embodiment of the green sheet disclosed herein, the water-soluble resin binder is a water-soluble acrylic resin. This enables excellent moldability, strength, and long-term stability at a high level.

[0016] In one embodiment of the green sheet disclosed herein, the nitride-based ceramic powder comprises at least one nitride compound selected from the group consisting of boron nitride, silicon nitride, and aluminum nitride. According to the technology disclosed herein, even with such nitride compounds, a green sheet with excellent moldability, strength, and long-term stability can be realized.

[0017] As another aspect of the present disclosure for achieving the above object, a method for manufacturing a green sheet using a dry powder rolling method is provided. Thereby, even when using nitride ceramic powder with poor wettability to aqueous solvents or organic solvents, a green sheet with high formability, excellent strength, and long-term stability can be manufactured.

Brief Description of the Drawings

[0018] [Figure 1] It is a schematic diagram for explaining a method for manufacturing a green sheet according to an embodiment using a dry powder rolling method.

Embodiments for Carrying Out the Invention

[0019] Hereinafter, preferred embodiments of the technology disclosed herein will be described. In addition, matters other than those specifically mentioned in this specification and matters necessary for implementing the technology disclosed herein can be understood based on the technical content taught in this specification and the general technical common sense of those skilled in the art in this field. The technology disclosed here can be implemented based on the content disclosed in this specification and the technical common sense in this field. In this specification and the claims, when a predetermined numerical range is described as A~B (A and B are arbitrary numerical values), it means A or more and B or less. Therefore, such a description includes cases where it exceeds A and is less than B. <00000�6> <Green Sheet> The green sheet disclosed here includes a nitride-based ceramic powder, a water-soluble resin binder, and a water-soluble plasticizer. This green sheet is typically a compression molded body obtained by compression molding (for example, the dry powder rolling method described later) of granulated powder containing the above materials.

[0021] Nitride-based ceramic powder has excellent thermal conductivity and is a material that improves the thermal conductivity of green sheets. Nitride-based ceramic powder is a powder mainly composed of ceramics made of nitride compounds, and of the nitride-based ceramic powder, at least 90% by mass, preferably 95% by mass or more, more preferably 98% by mass or more, even more preferably 99% by mass or more, or 100% by mass is composed of nitride compounds. Specific examples of nitride compounds include boron nitride (BN), silicon nitride (Si3N4), aluminum nitride (AlN), and gallium nitride (GaN). These can be used individually or in combination of two or more. Among these, it is preferable that the powder contains at least one nitride compound selected from the group consisting of boron nitride, silicon nitride, and aluminum nitride.

[0022] The shape of the particles constituting the nitride-based ceramic powder is not particularly limited, but may be spherical (including nearly spherical), flaky, fibrous, plate-like, irregularly shaped, aggregates, granules, etc.

[0023] The average particle diameter of the particles constituting the nitride-based ceramic powder is not particularly limited, but may be, for example, around 0.01 μm to 50 μm. From the viewpoint of thermal conductivity, it is preferable to have an average particle diameter of 0.1 μm or more, and it may also be 0.5 μm or more. Furthermore, from the viewpoint of sheet processability, the average particle diameter may be 45 μm or less, for example, 30 μm or less, or 15 μm or less. In this specification, "average particle diameter" refers to the particle diameter (D) corresponding to the cumulative 50% from the fine particle side in the volume-based particle size distribution measured by particle size distribution measurement based on a general laser diffraction / light scattering method. 50 It also refers to particle size.

[0024] While not particularly limited, the average aspect ratio of nitride-based ceramic powder can range from approximately 1 to 100. The average aspect ratio can be obtained, for example, by observing nitride-based ceramic powder with a scanning electron microscope (SEM), randomly selecting multiple particles (e.g., 10 to 300) from the obtained observation images, calculating the aspect ratio (ratio of major axis to minor axis) based on the major and minor axes of each particle, and then finding the arithmetic mean.

[0025] Water-soluble resin binders are components that bind nitride-based ceramic powders and other green sheet constituent materials. Therefore, the flexibility of the water-soluble resin binder greatly affects the flexibility of the green sheet. Furthermore, since water-soluble resin binders do not use organic solvents, the environmental impact can be reduced.

[0026] In this specification, "water-soluble resin binder" means a resin binder that is completely dissolved in an aqueous solvent, or a resin binder that is dispersed in water. As the aqueous solvent, water such as deionized water, pure water, ultrapure water, or distilled water is preferably used. The aqueous solvent may, if necessary, contain a non-aqueous solvent (such as a lower alcohol or lower ketone) that can be uniformly mixed with water, as long as it does not impair the effects of the technology disclosed herein. In this case, it is preferable that 95 vol% or more of the aqueous solvent is water, and more preferably 99 vol% or more is water.

[0027] From the viewpoint of improving the moldability of green sheets, it is preferable that the glass transition point of the water-soluble resin binder be 50°C or lower. Since green sheets are subjected to pressure during the sheet molding process, it is preferable that the binder has low viscosity, such as being rubbery or liquid, when subjected to such pressure. Becoming rubbery or liquid when pressed allows for easy sheet molding while pushing out air, thus enabling the production of green sheets with high moldability. If the water-soluble resin binder has a glass transition point of 50°C or lower, for example, when roll molding, the heat applied at room temperature or during the short time of pressurization by the roll can easily turn it into a rubbery or liquid state. This not only reduces the proportion of voids in the green sheet but also suppresses the formation of relatively large voids (e.g., with a major axis of 30 μm or more) in the green sheet. Furthermore, using such a water-soluble resin binder can reduce the cost of temperature control during manufacturing. From the viewpoint of more easily improving moldability, the glass transition temperature of the water-soluble resin binder is preferably 44°C or lower, more preferably 30°C or lower (e.g., 23°C or lower), and may be 20°C or lower, 10°C or lower, 0°C or lower, -10°C or lower, or -20°C or lower. Furthermore, although not particularly limited, the above glass transition temperature may be, for example, -200°C or higher, -100°C or higher, -50°C or higher, or -44°C or higher. The glass transition temperature can be measured using a general differential scanning calorimetry (DSC). Additionally, the manufacturer's nominal value can be used as needed.

[0028] The weight-average molecular weight of a water-soluble resin binder is not particularly limited, but can generally be 5,000 or more (e.g., 10,000 or more). Furthermore, the above weight-average molecular weight can generally be 1,000,000 or less, typically 500,000 or less, for example, 300,000 or less, 200,000 or less, or 100,000 or less. The weight-average molecular weight of a water-soluble resin binder can be measured, for example, by gel permeation chromatography (GPC), and the weight-based average molecular weight converted using a standard polystyrene calibration curve can be adopted. Alternatively, the manufacturer's nominal value may be used.

[0029] The water-soluble resin binder is not particularly limited, but examples include acrylic resins, urethane resins, fluororesins, amino resins, polyether resins, cellulosic compounds, and polyvinyl alcohol, and can be used individually or in combination of two or more. Among these, acrylic resin is preferred. The acrylic resin is not particularly limited as long as it has a glass transition temperature within the above range, and various acrylic polymer compounds can be used. The above acrylic polymer compounds may contain various hydrophilic functional groups such as hydroxyl groups, carboxyl groups, sulfo groups, phosphate groups, and amino groups.

[0030] An example of an acrylic resin is a monomer mixture containing alkyl (meth)acrylate as the main monomer (a component accounting for more than 50% by mass of the total monomer), and further containing a sub-monomer copolymerizable with the main monomer. In addition to the main monomer and the sub-monomer, the monomer mixture may optionally contain other copolymerizing components. By copolymerizing these monomers, an acrylic polymer compound having a predetermined function can be formed. In this specification, "(meth)acrylate" means acrylate and methacrylate. Similarly, "(meth)acrylic" means acrylic and methacrylic.

[0031] Examples of alkyl (meth)acrylates include those with the general formula: CH2=C(R 1 )COOR 2Compounds represented by can be suitably used. Here, R in the formula 1 R represents a hydrogen atom or a methyl group. 2 R represents a chain-like alkyl group with 1 to 20 carbon atoms. 2 The alkyl(meth)acrylate is preferably a chain alkyl group having 1 to 14 carbon atoms, and more preferably an alkyl(meth)acrylate is a chain alkyl group having 1 to 12 carbon atoms.

[0032] As secondary monomers, monomer components containing various functional groups can be used depending on the desired binder properties, having the function of introducing crosslinking points into acrylic polymer compounds or controlling the binding properties of acrylic polymer compounds. Such functional groups may include, for example, carboxyl groups, hydroxyl groups, amide groups, amino groups, epoxy groups, etc. The amount of secondary monomers is not particularly limited and can be appropriately designed to achieve the desired density in the green sheet. In addition, other copolymer components other than the secondary monomers exemplified here may also be included. The ratio of secondary monomers to the above monomer (total amount of main monomer and secondary monomers) can be appropriately selected according to the desired degree of crosslinking, for example, it can be about 1 to 10% by mass relative to 100% by mass of the total monomer components. Furthermore, the method of polymerizing the monomer mixture is not particularly limited, and conventionally known general polymerization methods (emulsion polymerization, solution polymerization, etc.) can be employed.

[0033] If the sum of the volume of nitride-based ceramic powder and the volume of water-soluble resin binder is 100 vol%, the volume ratio of nitride-based ceramic powder may exceed 45 vol%. From the viewpoint of improving the thermal conductivity of the green sheet, the volume ratio of nitride-based ceramic powder is preferably 50 vol% or more, more preferably 55 vol% or more, and more preferably 60 vol% or more. Furthermore, since a higher ratio of water-soluble resin binder improves the moldability of the green sheet, the volume ratio of nitride-based ceramic powder is preferably less than 75 vol%, more preferably 70 vol% or less, and more preferably 65 vol% or less.

[0034] Water-soluble plasticizers are components that weaken the intermolecular forces between polymer molecules by interfering with the polymers that make up the water-soluble resin binder, thereby imparting flexibility to the water-soluble resin binder. In order to achieve a green sheet that combines high moldability and high strength, it is preferable to impart appropriate flexibility to the water-soluble resin binder. Higher flexibility of the water-soluble resin binder can lead to higher moldability, but there is a risk of insufficient strength in the green sheet. Conversely, if the flexibility of the water-soluble resin binder is low, the strength of the green sheet will improve, but the air cannot be sufficiently pushed out even with the pressure applied during sheet molding, which may reduce moldability.

[0035] Our research has shown that in order to impart appropriate flexibility to the entire water-soluble resin binder, it is necessary not only to use a predetermined amount of water-soluble plasticizer, but also to widely disperse the molecules constituting the water-soluble plasticizer. When the number-average molecular weight of the water-soluble plasticizer is large, the force that weakens the intermolecular forces between polymers of the water-soluble resin binder is strong, but a certain number of molecules or more are required to widely disperse it throughout the water-soluble resin binder. However, when a certain number of water-soluble plasticizer molecules with such a number-average molecular weight are present, the intermolecular forces between polymers of the water-soluble resin binder are excessively reduced. As a result, the strength of the green sheet decreases, which is undesirable. Therefore, the number-average molecular weight of the water-soluble resin binder is preferably 1500 or less, for example, it may be 1000 or less, or 750 or less. Furthermore, water-soluble plasticizers have hydrophilic groups (e.g., hydroxyl groups), which are typically present at both ends of the polymer molecules constituting the water-soluble plasticizer. Therefore, when a predetermined amount of water-soluble plasticizer is used, the smaller the number-average molecular weight of the water-soluble plasticizer, the more molecules there are, and thus the more hydrophilic groups there are. This makes the green sheet more susceptible to absorbing moisture from the air, thus reducing its stability over time. Therefore, the number-average molecular weight of the water-soluble plasticizer should be 300 or higher, preferably 450 or higher, and more preferably 600 or higher. The number-average molecular weight of the water-soluble plasticizer may be, for example, the average molecular weight measured by gel permeation chromatography using a standard substance, or the manufacturer's nominal value may be adopted.

[0036] As a water-soluble plasticizer, any conventionally known water-soluble plasticizer can be used as long as its number-average molecular weight is within the above-mentioned range, but among them, polyether-based plasticizers are preferably used. Examples of polyether-based plasticizers include polyethylene glycol; polypropylene glycol; polyglycerin (degree of polymerization 4 to 20); polyoxyalkylene glyceryl ether, polyoxyalkylene polyglyceryl ether, and other compounds obtained by addition polymerization of oxyalkylene to glycerin or polyglycerin (e.g., degree of polymerization 2 to 10). These may be used individually or in combination of two or more. The oxyalkylene group constituting the polyoxyalkylene may be, for example, an oxyethylene group or an oxypropylene group. Specifically, examples of polyoxyalkylene glyceryl ether include polyoxyethylene glyceryl ether and polyoxypropylene glyceryl ether. Specific examples of polyoxyalkylene polyglyceryl ether include polyoxyethylene polyglyceryl ether and polyoxypropylene polyglyceryl ether. The number of repeating oxyalkylene groups is one or more, but is not particularly limited and can be set as appropriate. Furthermore, the oxyalkylene group constituting the polyoxyalkylene may be one type or two or more types.

[0037] The number of hydrophilic groups (typically hydroxyl groups) in one molecule of the water-soluble plasticizer is preferably 8 or less, more preferably 6 or less, even more preferably 4 or less, and may be, for example, 3 or less, or 2 or less. By reducing the number of hydrophilic groups, the absorption of moisture from the air is suppressed, thereby improving the long-term stability of the green sheet.

[0038] When the sum of the volume of the water-soluble resin binder and the volume of the water-soluble plasticizer is set to 100 vol%, the volume ratio of the water-soluble plasticizer is preferably, for example, 2 vol% or more, and more preferably 10 vol% or more. Within this range, the water-soluble plastic resin having the above number-average molecular weight imparts appropriate flexibility to the water-soluble resin binder. Furthermore, the volume ratio of the water-soluble plasticizer is preferably, for example, 35 vol% or less, and more preferably 20 vol% or less. This suppresses the number of hydrophilic groups in the water-soluble plasticizer, thereby improving its stability over time.

[0039] The green sheet disclosed herein is formed to have few voids and high density. In other words, the theoretical density ratio of the green sheet disclosed herein is 85% or higher, and in a more preferred example, a theoretical density ratio of 90% or higher has been achieved, and may be 95% or higher. Here, the "theoretical density ratio" is given by the following equation (1): Theoretical density ratio (%) = (Measured density) / (Theoretical density) × 100 (1) This refers to the value calculated by the following equation. In equation (1), "theoretical density" refers to the value derived from the specific gravity of each material contained in the green sheet and the volume ratio of each material contained in the green sheet. "Measured density" refers to the density obtained by measuring the volume and weight of a test piece formed from the green sheet into a predetermined shape (for example, a rectangular sheet with a wide surface, a shape that makes volume calculation easy). In other words, the higher the theoretical density ratio, the fewer voids there are in the green sheet and the higher the moldability.

[0040] Preferably, the green sheet disclosed herein does not have voids with a major axis of 30 μm or more when observed under an electron microscope in cross-section. Forming the green sheet in a way that eliminates the above voids improves its formability. Furthermore, the absence of the above voids improves the thermal conductivity of the green sheet. For example, a sample can be prepared that allows observation of a cross-section perpendicular to the surface of the green sheet (a cross-section along the thickness direction (meaning the direction perpendicular to the surface direction of the green sheet; the same applies hereinafter)), and the presence or absence of the above voids can be confirmed by observing the cross-section using a scanning electron microscope (SEM). The observation magnification of the SEM is not particularly limited, but for example, it is good to set it to about 1000x to 3000x. Also, although not particularly limited, it is good to randomly obtain multiple observation fields (for example, 5 or more, 10 or more, 15 or more, or more) to confirm the presence or absence of the above voids. Note that the green sheet may have voids with a major axis of less than 30 μm, as long as no voids with a major axis of 30 μm or more exist when observed under an electron microscope in cross-section.

[0041] The thickness of the green sheet is not particularly limited, but may be, for example, 10 μm or more, 20 μm or more, 50 μm or more, or 100 μm or more. Furthermore, the above thickness may be, for example, 3000 μm or less, 2000 μm or less, 1000 μm or less, or 500 μm or less.

[0042] The green sheet disclosed herein may, as necessary, contain various additives in addition to the above components, such as dispersants, release agents, defoamers, antioxidants, and thickeners. It may also contain ceramic powders other than nitride-based ceramic powder. Examples of ceramics constituting the other ceramic powders include carbide-based ceramics such as silicon carbide, and oxide-based ceramics such as aluminum oxide, zinc oxide, magnesium oxide, beryllium oxide, and titanium oxide. These may be included individually or in combination of two or more. When the above-mentioned other ceramic powders are included, if the total content of nitride-based ceramic powder and other ceramic powders in the green sheet is taken as 100% by mass, the content of nitride-based ceramic powder is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more. Furthermore, all of the ceramic powder contained in the green sheet may be nitride-based ceramic powder.

[0043] <How to manufacture green sheets> The nitride-based ceramic powder contained in the green sheet disclosed herein has low wettability to aqueous and organic solvents, making it prone to poor mixing with other materials and molding defects. Therefore, it is difficult to manufacture highly moldable green sheets using wet processes, such as the method using a doctor blade (hereinafter also referred to as the "doctor blade method"), which are common methods for manufacturing green sheets. Furthermore, when moldability is increased by pressurization in a wet process, the major axis of the nitride-based ceramic particles may be oriented along the plane direction of the green sheet, which can lead to a problem of reduced thermal conductivity in the thickness direction of the green sheet.

[0044] Therefore, a preferred example of a method for manufacturing the green sheet disclosed herein is a dry powder rolling method (e.g., roll forming). The dry powder rolling method can be carried out using, for example, a dry powder rolling apparatus 5 shown in Figure 1. The dry powder rolling apparatus 5 broadly comprises a storage tank 1 and a pair of rolls 2. The storage tank 1 is a container for storing granulated powder 1a, which is composed of the raw materials for the green sheet 10G. The storage tank 1 is also equipped with a feeder 1b at its bottom, and is configured to continuously supply a fixed amount of granulated powder 1a between the pair of rolls 2 from the discharge port of the feeder 1b. The feeder 1b is not particularly limited as long as it has excellent quantitative accuracy, and various types of feeders such as screw type, vibrating type, and fluid type can be appropriately adopted.

[0045] The above manufacturing method broadly includes a raw material preparation process, a granulation process, and a sheet molding process. In the raw material preparation process, raw materials for Green Sheet 10G are prepared, including nitride-based ceramic powder, a water-soluble resin binder, a water-soluble plasticizer, and various additives as needed. The constituent materials and their volume proportions are as described above.

[0046] In the granulation process, granulated powder 1a is produced using the above raw materials. The granulation method is not particularly limited, and either wet granulation or dry granulation may be used. Examples of granulation methods include rolling granulation, fluidized bed granulation, stirring granulation, compression granulation, extrusion granulation, crushing granulation, and spray drying (aerosol granulation). From the viewpoint of easier handling of finer raw material powders, the adoption of wet granulation methods such as spray granulation is preferred. In spray granulation, first, a mixture of the prepared raw materials is prepared, and this mixture is dispersed in a dispersion medium to obtain a raw material slurry. The method of mixing the raw materials is not particularly limited, and conventionally known stirring and mixing devices can be used. For example, ball mills, mixers, dispersers, kneaders, etc., can be used. As a dispersion medium for the mixture, water is a preferred example from the viewpoint of reducing the solubility of the water-soluble resin binder and water-soluble plastic resin and the environmental impact. Next, the raw material slurry is sprayed into a liquid state using a spray drying device and dried to obtain granulated powder. The size of the granulated powder is not particularly limited and can be, for example, between 10 μm and 200 μm.

[0047] In the sheet forming process, the granulated powder is formed into a sheet. Specifically, as shown in Figure 1, the granulated powder 1a obtained in the granulation process is fed into the storage tank 1 of the dry powder rolling apparatus 5. The granulated powder 1a is discharged to the outside through the feeder 1b at the bottom of the storage tank 1. The discharged granulated powder 1a is then supplied between a pair of rolls 2. As the rolls 2 rotate (see arrows in Figure 1), the supplied granulated powder 1a is compressed. As a result, the granulated powder 1a is formed into a sheet, and a green sheet 10G is obtained. The temperature and pressure conditions here may vary depending on the type of raw material, etc., and are not particularly limited, and can be changed as appropriate. In addition, the distance between the rolls 2 can be changed as appropriate to achieve the desired thickness of the green sheet 10G.

[0048] In the above manufacturing method, by producing and using granulated powder containing the raw materials for the green sheet, separation of the raw materials in the molded green sheet can be suppressed, even if nitride-based ceramic powder is included. Furthermore, because the water-soluble resin binder contained in the raw materials for the green sheet is given appropriate flexibility, sheet molding can be performed while pushing out air, resulting in the production of a highly moldable green sheet. In addition, since the orientation of the nitride-based ceramic powder in the planar direction of the green sheet can be suppressed, a green sheet with excellent thermal conductivity can be produced.

[0049] <Uses of green sheets> The uses of the green sheet disclosed herein are not particularly limited, but for example, it can be used as a heat dissipation material. For example, it can be used as a green sheet to create a heat dissipation sheet that dissipates heat from a heat-generating component (e.g., a power device) by interposing it between a heat-generating component (heat fin, heat sink, heat sink, etc.) and a heat-dissipating component, or by replacing the heat-dissipating component. It can also be used as a material to constitute a heat dissipation device in combination with the above-mentioned heat-dissipating component.

[0050] The following describes some test examples relating to the technology disclosed herein, but the present invention is not intended to be limited to those shown in the test examples.

[0051] <Test 1> [Manufacture of green sheets] In Experiment 1, the number-average molecular weight of water-soluble plasticizers was investigated. First, boron nitride powder (average particle size 5.7 μm, UHP-G1H (Showa Denko)) was prepared as a nitride-based ceramic powder, water-soluble acrylic resin A (Boncoat 5495EF (DIC Corporation), glass transition temperature 23°C) was prepared as a water-soluble resin binder, and polyethylene glycol was prepared as a water-soluble plasticizer. In addition, a mold release agent, dispersant, and defoaming agent were prepared. Polyethylene glycol with number-average molecular weights of 200, 300, 600, 1500, and 2000 was prepared, and polyethylene glycol with different number-average molecular weights was used in the green sheets of Examples 1 to 5. The boron nitride powder was prepared by aggregating plate-like (or flake-like) primary particles to form granules. The glass transition temperature of the water-soluble acrylic resin was measured using a commercially available differential scanning calorimetry (Differential Scanning Calorimetry (Rigaku Corporation)).

[0052] Next, the prepared materials were added to a pot mill along with an equal mass of water (dispersant) and mixed to prepare a granulation slurry. At this time, the volume ratio of boron nitride powder to water-soluble resin binder was set to 60:40, and the volume ratio of water-soluble resin binder to water-soluble plasticizer was set to 90:10. Then, the granulation slurry was spray-dried using a spray-drying device to produce the granulated powders for each example. At this time, the spray conditions were set so that the average particle size of the granulated powder was approximately 80 μm for each example. The drying temperature during spray drying was approximately 180-200°C, and the amount of residual dispersion medium in the granulated powder was almost 0% by mass. Subsequently, using the prepared granulated powder, green sheets (approximately 1 mm thick) for Examples 1-5 were produced using a dry powder rolling device.

[0053] [Evaluation of moldability] From the green sheet of each example, test specimens of a predetermined size were prepared using a punching die, and the dimensions of these specimens were measured. Then, the weight of the test specimens was measured, and the actual density (g / cm³) was determined. 3 The theoretical density (g / cm³) was calculated from the specific gravity and composition ratio of each material. 3The measured density was calculated. In this calculation, the amount of dispersion medium was assumed to be 0g. Using the measured and theoretical densities obtained, the theoretical density ratio (%) of the green sheet for each example was calculated. When the theoretical density ratio was 90% or higher, it was marked "◎", when it was 85% or higher but less than 90%, it was marked "〇", and when it was 80% or higher but less than 85%, it was marked "△". The results are shown in Table 1.

[0054] [Evaluation of tensile strength] The tensile strength of each example's green sheet was measured in accordance with JIS K7161:2014. Test specimens were prepared by punching them out from each example's green sheet using a predetermined test specimen punching die. For the measurement, a tensile strength testing machine (Shimadzu Corporation, benchtop testing machine: EZ-TEST) was used. The test specimen was held at both ends with a gripping jig, and the gripping jig was moved in the tensile direction at a tensile speed of 1 mm / min. The tensile fracture stress at the time the test specimen broke was measured, and the tensile strength was calculated by dividing it by the cross-sectional area of ​​the test specimen. The test was performed 10 times, and if the average value was 1 MPa or higher, it was marked "◎", if it was 0.8 MPa or higher but less than 1 MPa it was marked "〇", and if it was less than 0.8 MPa it was marked "△". The results are shown in Table 1.

[0055] [Evaluation of stability in the daily market] Immediately after forming the green sheet for each example, test specimens of a predetermined size were created using a punching die and their weight was measured. These test specimens were then left to stand for one week in an environment with a temperature of 20°C to 22°C and a humidity of 40% to 60%, and their weight was measured again. The weight increase rate of the test specimen was calculated from the measured weight, and a weight increase rate of less than 0.5% was marked "◎", 0.5% to less than 1% was marked "〇", and 1% or more was marked "△". The results are shown in Table 1.

[0056] [Table 1]

[0057] As shown in Table 1, in Examples 2-4, green sheets with excellent moldability, tensile strength, and long-term stability were achieved. This indicates that a number-average molecular weight of 300-1500 for the water-soluble plasticizer is preferable. Furthermore, since Examples 3 and 4 received an "◎" rating for long-term stability, it can be seen that green sheets with even better long-term stability are achieved when the number-average molecular weight of the water-soluble plasticizer is between 600 and 1500. In addition, a tendency was observed for long-term stability to improve with increasing number-average molecular weight of the water-soluble plasticizer. This is thought to be due to the number of hydrophilic groups in the water-soluble plasticizer.

[0058] <Exam 2> In Test 2, the preferred range of the glass transition temperature of the water-soluble resin binder used was investigated. In Test 2, the following acrylic resins were used as the water-soluble resin binder in each example (Example 3, Examples 6-9). Example 3: Water-soluble acrylic resin A: Boncoat 5495EF (DIC Corporation) Example 6: Water-soluble acrylic resin B: Boncoat 6400CE (DIC Corporation) Example 7: Water-soluble acrylic resin C: Boncoat CE-8510 (DIC Corporation) Example 8: Water-soluble acrylic resin D: Boncoat CC-6180 (DIC Corporation) Example 9: Water-soluble acrylic resin E: Boncoat YG-651 (DIC Corporation) The glass transition temperatures of these water-soluble acrylic resins were measured using the same method as in Test 1. The measured values ​​are shown in Table 2.

[0059] In Test 2, polyethylene glycol with a number-average molecular weight of 600 was used as the water-soluble plasticizer. The remaining materials were the same as in Test 1, and green sheets for each example were prepared using the same method as in Test 1. In addition, moldability, tensile strength, and long-term stability were evaluated in the same manner as in Test 1. The results are shown in Table 2.

[0060] [Table 2]

[0061] As shown in Table 2, in Examples 3, 6-8, the moldability, tensile strength, and long-term stability were all well evaluated. From this, it can be seen that a glass transition temperature of 50°C or lower (more specifically, 44°C or lower) is preferable for the water-soluble resin binder. On the other hand, in Example 9, the tensile strength was insufficient. This is thought to be because the glass transition temperature of the water-soluble resin binder was higher than in the other examples, resulting in the water-soluble resin binder not sufficiently changing to a rubbery or liquid state during green sheet molding, thus forming relatively large voids.

[0062] <Exam 3> In Test 3, the volume ratio of water-soluble resin binder to water-soluble plasticizer was investigated. In Test 3, the above-mentioned water-soluble acrylic resin B was used as the water-soluble resin binder, and polyethylene glycol with a number-average molecular weight of 1500 was used as the water-soluble plasticizer. When the volume ratio of water-soluble resin binder to water-soluble plasticizer was set to 100 vol%, green sheets of Examples 10 to 15 were prepared so that the water-soluble plasticizer was 0 to 40 vol% (see Table 3 for the volume ratio of each example). The other materials used and the preparation method for the green sheets were the same as in Test 1. Furthermore, moldability, tensile strength, and long-term stability were evaluated in the same manner as in Test 1. The results are shown in Table 3.

[0063] [Table 3]

[0064] As shown in Table 3, in Examples 11 to 14, the moldability, tensile strength, and long-term stability were all well evaluated. From this, it can be seen that when the volume ratio of water-soluble resin binder to water-soluble plasticizer is 100 vol%, the proportion of water-soluble plasticizer is preferably between 2 vol% and 35 vol%. In particular, from the results of Examples 12 and 13, it can be seen that when the proportion of water-soluble plasticizer is between 10 vol% and 20 vol%, a green sheet with excellent moldability, tensile strength, and long-term stability can be realized.

[0065] <Exam 4> In Test 4, the types of nitride-based ceramic powders were investigated. In Test 4, in addition to the boron nitride used in Test 1, silicon nitride (average particle size 0.8 μm, irregular shape) and aluminum nitride (average particle size 5 μm, irregular shape) were prepared as nitride-based ceramic powders. Furthermore, the above-mentioned water-soluble acrylic resin A was prepared as a water-soluble resin binder, and polyethylene glycol with a number-average molecular weight of 600 was prepared as a water-soluble plasticizer. Other materials and manufacturing methods used for the green sheets were the same as in Test 1, with the example using boron nitride designated as Example 3, the example using silicon nitride as Example 16, and the example using aluminum nitride as Example 17. Also, moldability, tensile strength, and long-term stability were evaluated in the same manner as in Test 1. The results are shown in Table 4.

[0066] [Table 4]

[0067] As shown in Table 4, it was confirmed that green sheets with excellent formability, tensile strength, and long-term stability can be realized regardless of the type of nitride ceramic powder used.

[0068] The above provides a detailed explanation of the test examples of the technology disclosed herein, but these are merely illustrative and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes to the specific examples illustrated above. [Explanation of Symbols]

[0069] 1 Storage tank 1a Granulated powder 1b Feeder 2 rolls 5. Dry powder rolling machine 10G Green Sheet

Claims

1. A green sheet used to produce a heat dissipation sheet that promotes heat dissipation of heat-generating components, It comprises nitride-based ceramic powder, a water-soluble resin binder, and a water-soluble plasticizer. The glass transition temperature of the aforementioned water-soluble resin binder is 50°C or lower. The number average molecular weight of the aforementioned water-soluble plasticizer is 300 or more and 1500 or less. When the sum of the volume of the nitride-based ceramic powder and the volume of the water-soluble resin binder is 100 vol%, the volume percentage of the nitride-based ceramic powder is greater than 45 vol% and less than 75 vol%. When the sum of the volume of the water-soluble resin binder and the volume of the water-soluble plasticizer is 100 vol%, the volume ratio of the water-soluble plasticizer is 2 vol% or more and 35 vol% or less. The theoretical density ratio ((measured density / theoretical density) × 100) is 85% or higher. Green sheet.

2. The green sheet according to claim 1, wherein the theoretical density ratio is 90% or more.

3. The green sheet according to claim 1 or 2, wherein the tensile strength at a tensile speed of 1 mm / min, in accordance with JIS K 7161:2014, is 0.8 MPa or more.

4. The green sheet according to any one of claims 1 to 3, wherein the theoretical density ratio is 95% or more.

5. The green sheet according to any one of claims 1 to 4, wherein the water-soluble plasticizer includes a polyether-based plasticizer.

6. The green sheet according to any one of claims 1 to 5, wherein the water-soluble resin binder is a water-soluble acrylic resin.

7. The green sheet according to any one of claims 1 to 6, wherein the nitride-based ceramic powder comprises at least one nitride compound selected from the group consisting of boron nitride, silicon nitride, and aluminum nitride.

8. A method for producing a green sheet according to any one of claims 1 to 7 using a dry powder rolling method.

Images