Green sheet and manufacturing method thereof

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 containing a nitride ceramic powder and a method for producing the same. [Background technology]

[0002] Semiconductor elements for electric power (so-called power devices) have become indispensable for the efficient use of electrical energy. Demand is also increasing for semiconductor elements for lighting (so-called high-power LED devices) used in energy-saving, long-life, high-brightness, and power LED lamps. In recent years, research and development into technologies for miniaturizing, increasing density, and increasing speed of power devices has been actively carried out.

[0003] Generally, as the density of a power device increases, the amount of heat generated by the power device increases. Such heat generation may cause problems in the power device and in the members surrounding the power device, and therefore a technique for dissipating heat to the surroundings is required. As such a technique, for example, the use of a thermally conductive sheet-shaped composition has been proposed, as disclosed in Patent Documents 1 to 5. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6256158 [Patent Document 2] JP 2019-117916 A [Patent Document 3] JP 2004-339426 A [Patent Document 4] JP 2005-48124 A [Patent Document 5] JP 2005-272599 A Summary of the Invention [Problem to be solved by the invention]

[0005] In order to increase the thermal conductivity of a sheet-shaped composition, it is desirable that the sheet-shaped composition contains fewer voids. Since the air contained in the voids has a low thermal conductivity, the thermal conductivity is improved by reducing the number of voids. Therefore, a high-density sheet-shaped composition with fewer voids (i.e., a sheet-shaped composition with high formability) is desirable.

[0006] On the other hand, it is desirable that the sheet composition has high strength and is not easily deteriorated for a long time (high temporal stability). If the strength of the sheet composition is low, there is a risk of the sheet composition being damaged when assembled to a member. In addition, the sheet composition may absorb moisture in the air and deteriorate in quality over the course of use or storage. Therefore, in order to realize a long life of the sheet composition and further a long life of a member (e.g., a power device) in which the sheet composition is used, it is desirable that the strength and temporal stability are high.

[0007] The present invention has been made in view of the above circumstances, and its main object is to provide a sheet-shaped composition (green sheet) excellent in formability, strength and stability over time. Another object is to provide a manufacturing method for realizing such a green sheet. [Means for solving the problem]

[0008] In order to achieve the above object, the present inventors have focused on the flexibility of the resin binder contained in the raw material of the green sheet (i.e., the constituent components of the green sheet). For example, a method of manufacturing a green sheet is to apply pressure to the raw material of the green sheet to mold it into a sheet. At this time, if the raw material of the green sheet is flexible, the green sheet can be manufactured while pushing out air by pressure, so that the moldability can be improved. On the other hand, if the flexibility of the raw material is too high, there is a risk that the strength of the manufactured green sheet will be insufficient. Therefore, the present inventors have focused on plasticizers that impart flexibility to resins and have conducted extensive research. As a result, it has been found that a green sheet excellent in all of moldability, strength, and stability over time can be realized by using a water-soluble plasticizer with a number average molecular weight in a predetermined range.

[0009] That is, the green sheet disclosed herein includes a nitride ceramic powder, a water-soluble resin binder, and a water-soluble plasticizer, the water-soluble resin binder having a glass transition point of 50° C. or lower, and the water-soluble plasticizer having a number average molecular weight of 300 or more and 1,500 or less.

[0010] In the green sheet having such a configuration, the water-soluble resin binder has a suitable flexibility by containing a water-soluble plasticizer having a number average molecular weight within the above range. In addition, by containing a water-soluble resin binder having a glass transition point of 50° C. or less, the water-soluble resin binder can be easily made into a low viscosity state such as a rubber or liquid state during the production of the green sheet. Therefore, air can be suitably pushed out during the production of the green sheet, and a green sheet having excellent moldability is realized. In addition, in this configuration, the flexibility of the water-soluble resin binder is adjusted to an appropriate range, and the green sheet has excellent strength. Furthermore, since the water-soluble plasticizer has a hydrophilic group, it has a property of easily absorbing moisture in the air, but by setting the water-soluble plasticizer to the above number average molecular weight range, the amount of moisture absorbed in the air can be suppressed, and excellent stability over time is realized.

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

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

[0013] In a preferred embodiment of the green sheet disclosed herein, when the total volume of the water-soluble resin binder and the volume of the water-soluble plasticizer is taken as 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 is more appropriately controlled, and it is possible to more appropriately achieve both moldability and strength.

[0014] In a preferred embodiment of the green sheet disclosed herein, the water-soluble plasticizer contains a polyether-based plasticizer, which makes it possible to more appropriately control the flexibility of the water-soluble resin binder.

[0015] In a preferred embodiment of the green sheet disclosed herein, the water-soluble resin binder is a water-soluble acrylic resin, which makes it possible to realize high levels of excellent formability, strength, and stability over time.

[0016] In one embodiment of the green sheet disclosed herein, the nitride ceramic powder contains 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 if such a nitride compound is used, a green sheet having excellent moldability, strength, and stability over time can be realized.

[0017] In another aspect of the present teachings for achieving the above object, a method for producing a green sheet using a dry powder rolling method is provided, which makes it possible to produce a green sheet with high moldability, strength, and long-term stability even if a nitride ceramic powder with poor wettability to aqueous or organic solvents is used. [Brief description of the drawings]

[0018] [Figure 1] 1A to 1C are schematic diagrams illustrating a method for producing a green sheet according to an embodiment using a dry powder rolling method. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] A preferred embodiment of the technology disclosed herein will be described below. Matters other than those specifically mentioned in this specification and necessary for implementing the technology disclosed herein can be understood based on the technical content taught by this specification and the general technical common sense of a person skilled in the art in the relevant field. The technology disclosed herein can be implemented based on the contents disclosed in this specification and the technical common sense of a person skilled in the art in the relevant field. In this specification and claims, when a certain numerical range is described as A to B (A and B are arbitrary numerical values), it means A or more and B or less. Therefore, such a description includes the case where it is greater than A and less than B.

[0020] <Green Sheet> The green sheet disclosed herein contains a nitride 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 below) a granulated powder containing the above materials.

[0021] Nitride ceramic powder has excellent thermal conductivity and is a material that improves the thermal conductivity of green sheets. The nitride ceramic powder is a powder mainly composed of ceramics made of nitride compounds, and at least 90 mass% of the nitride ceramic powder, preferably 95 mass% or more, more preferably 98 mass% or more, and even more preferably 99 mass% or more, or 100 mass% is composed of nitride compounds. Specific examples of nitride compounds include boron nitride (BN), silicon nitride (Si 3 N 4 ), aluminum nitride (AlN), gallium nitride (GaN), etc. These can be used alone or in combination of two or more. Among these, powder containing at least one nitride compound selected from the group consisting of boron nitride, silicon nitride, and aluminum nitride is preferable.

[0022] The shape of the particles constituting the nitride ceramic powder is not particularly limited, but may be, for example, spherical (including nearly spherical), scaly, fibrous, plate-like, irregular, aggregated, granular, or the like.

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

[0024] Although not particularly limited, the average aspect ratio of the nitride ceramic powder can be about 1 to 100. The average aspect ratio can be obtained, for example, by observing the nitride ceramic powder with a scanning electron microscope (SEM), randomly selecting a plurality of particles (e.g., 10 to 300 particles) from the obtained observation image, calculating the aspect ratio (ratio of major axis to minor axis) based on the major axis and minor axis of each particle, and determining the arithmetic mean value.

[0025] The water-soluble resin binder is a component that binds the nitride ceramic powder and other green sheet constituent materials. Therefore, the flexibility of the water-soluble resin binder is greatly reflected in the flexibility of the green sheet. In addition, the water-soluble resin binder does not use organic solvents, so it can reduce the environmental load.

[0026] In this specification, the term "water-soluble resin binder" refers to 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 ion-exchanged water (deionized water), pure water, ultrapure water, and distilled water can be preferably used. The aqueous solvent may contain a non-aqueous solvent (lower alcohol, lower ketone, etc.) that can be mixed uniformly with water as necessary, within a range that 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 enhancing the moldability of the green sheet, it is preferable that the glass transition point of the water-soluble resin binder is 50° C. or less. Since the green sheet may be pressurized when it is molded into a sheet in the manufacturing process, it is preferable that the green sheet is in a state of low viscosity such as rubber or liquid when pressed. By becoming rubber or liquid when pressed, it becomes possible to easily mold the sheet while pushing out the air, so that a green sheet having high moldability can be manufactured. If the water-soluble resin binder has a glass transition point of 50° C. or less, it can easily become rubber or liquid, for example, when it is roll molded, at room temperature or by heat applied during a short time when it is pressed by a roll. This not only reduces the ratio of voids in the green sheet, but also suppresses the formation of relatively large voids (for example, major axis 30 μm or more) in the green sheet. In addition, if such a water-soluble resin binder is used, the cost of temperature control during manufacturing can be reduced. From the viewpoint of more easily improving moldability, the glass transition point of the water-soluble resin binder is preferably 44°C or less, more preferably 30°C or less (for example, 23°C or less), and may be 20°C or less, 10°C or less, 0°C or less, -10°C or less, or -20°C or less. Although not particularly limited, the glass transition point may be, for example, -200°C or more, -100°C or more, -50°C or more, or -44°C or more. The glass transition point can be measured using a general differential scanning calorimetry (DSC). If necessary, the manufacturer's nominal value can be used.

[0028] The weight average molecular weight of the water-soluble resin binder is not particularly limited, but can be generally 5,000 or more (for example, 10,000 or more). The weight average molecular weight can be generally 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 the water-soluble resin binder can be measured, for example, by gel permeation chromatography (GPC) and converted using a standard polystyrene calibration curve to obtain a weight-based average molecular weight. Alternatively, the manufacturer's nominal value can be used.

[0029] The water-soluble resin binder is not particularly limited, but examples thereof include acrylic resin, urethane resin, fluororesin, amino resin, polyether resin, cellulose-based compound, polyvinyl alcohol, etc., and may be used alone or in combination of two or more. Among these, it is preferable to use acrylic resin. The acrylic resin is not particularly limited as long as it has a glass transition point within the above range, and various acrylic polymer compounds can be used. The acrylic polymer compound may contain various hydrophilic functional groups such as hydroxyl group, carboxyl group, sulfo group, phosphate group, and amino group.

[0030] An example of an acrylic resin is a monomer mixture that contains an alkyl (meth)acrylate as a main monomer (a component that accounts for more than 50% by mass of the total monomers) and further contains a sub-monomer that is copolymerizable with the main monomer. In addition to the main monomer and the sub-monomer, the monomer mixture may optionally contain other copolymerization 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 represented by the general formula: CH 2 =C(R 1 )COOR2 In the above, a compound represented by the formula: 1 represents a hydrogen atom or a methyl group. 2 R represents a chain alkyl group having 1 to 20 carbon atoms. 2 The alkyl(meth)acrylate is preferably an alkyl group having a chain structure with 1 to 14 carbon atoms, and more preferably an alkyl(meth)acrylate having a chain structure with 1 to 12 carbon atoms.

[0032] The secondary monomer has a function of introducing a crosslinking point into an acrylic polymer compound or controlling the binding property of the acrylic polymer compound, and a monomer component containing various functional groups can be used according to the desired binder properties. Such functional groups can be, for example, a carboxyl group, a hydroxyl group, an amide group, an amino group, an epoxy group, etc. The amount of the secondary monomer is not particularly limited, and can be appropriately designed so that the desired denseness is realized in the green sheet. It is also possible to include other copolymerization components other than the secondary monomers exemplified here. The ratio of the secondary monomer to the above monomer (total amount of main monomer and secondary monomer) may be appropriately selected according to the desired crosslinking degree, and can be, for example, about 1 to 10 mass% with respect to 100 mass% of the total monomer components. In addition, the method of polymerizing the monomer mixture is not particularly limited, and a conventionally known general polymerization method (emulsion polymerization, solution polymerization, etc.) can be adopted.

[0033] When the total volume of the nitride ceramic powder and the water-soluble resin binder is 100 vol%, the volume ratio of the nitride ceramic powder may be more than 45 vol%. From the viewpoint of improving the thermal conductivity of the green sheet, the volume ratio of the nitride ceramic powder may be, for example, 50 vol% or more, preferably 55 vol% or more, and more preferably 60 vol% or more. In addition, since the moldability of the green sheet is improved when the proportion of the water-soluble resin binder is higher, the volume ratio of the nitride ceramic powder may be, for example, less than 75 vol%, preferably 70 vol% or less, and more preferably 65 vol% or less.

[0034] The water-soluble plasticizer is a component that weakens the intermolecular force between the polymers constituting the water-soluble resin binder by penetrating between the molecules of the polymers constituting the water-soluble resin binder, thereby imparting flexibility to the water-soluble resin binder. In order to realize a green sheet that is both highly moldable and strong, it is preferable to impart moderate flexibility to the water-soluble resin binder. The higher the flexibility of the water-soluble resin binder, the higher the moldability can be, but there is a risk that the strength of the green sheet will be insufficient. In addition, 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 by pressure during sheet molding, and the moldability may decrease.

[0035] According to the study by the present inventors, 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 of weakening the intermolecular force between the polymers of the water-soluble resin binder of each molecule is strong, but a certain number of molecules or more are required to widely disperse the water-soluble resin binder throughout the entire 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 force between the polymers of the water-soluble resin binder is excessively reduced. As a result, the strength of the green sheet is reduced, which is not preferable. Therefore, the number average molecular weight of the water-soluble resin binder is preferably 1500 or less, and may be, for example, 1000 or less, or may be 750 or less. In addition, the water-soluble plasticizer has a hydrophilic group (e.g., a hydroxyl group), which is typically present at both ends of the polymer molecules constituting the water-soluble plasticizer. Therefore, when a predetermined amount of the water-soluble plasticizer is used, the number of molecules increases as the number average molecular weight of the water-soluble plasticizer decreases, and therefore the number of hydrophilic groups increases. This makes the green sheet more susceptible to absorbing moisture in the air, resulting in a decrease in stability over time. Therefore, the number average molecular weight of the water-soluble plasticizer is preferably 300 or more, more preferably 450 or more, and even more preferably 600 or more. The number average molecular weight of the water-soluble plasticizer may be, for example, a number-based average molecular weight measured by gel permeation chromatography using a standard substance, or the manufacturer's nominal value may be used.

[0036] As the water-soluble plasticizer, any water-soluble plasticizer known in the art can be used as long as the number average molecular weight is within the above range, and among them, polyether-based plasticizers can be preferably used. Examples of polyether-based plasticizers include polyethylene glycol; polypropylene glycol; polyglycerin (polymerization degree 4 to 20); polyoxyalkylene glyceryl ether, polyoxyalkylene polyglyceryl ether, and other compounds obtained by addition polymerization of oxyalkylene to glycerin or polyglycerin (for example, polymerization degree 2 to 10). These may be used alone or in combination of two or more. The oxyalkylene group constituting the polyoxyalkylene may be, for example, an oxyethylene group or an oxypropylene group. That is, specific examples of polyoxyalkylene glyceryl ether include polyoxyethylene glyceryl ether, polyoxypropylene glyceryl ether, and the like. Specific examples of polyoxyalkylene polyglyceryl ether include polyoxyethylene polyglyceryl ether, polyoxypropylene polyglyceryl ether, and the like. The number of repetitions of the oxyalkylene group is not particularly limited and can be appropriately set, but is at least 1. The oxyalkylene group constituting the polyoxyalkylene may be one type or two or more types.

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

[0038] When the total volume of the water-soluble resin binder and the water-soluble plasticizer is taken as 100 vol%, the volume ratio of the water-soluble plasticizer is, for example, 2 vol% or more, and preferably 10 vol% or more. According to this range, the water-soluble plasticizer having the above number average molecular weight imparts appropriate flexibility to the water-soluble resin binder. Moreover, the volume ratio of the water-soluble plasticizer is, for example, 35 vol% or less, and preferably 20 vol% or less. This suppresses the number of hydrophilic groups in the water-soluble plasticizer, thereby improving 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 more, and in a more preferred example, a theoretical density ratio of 90% or more is realized, and may be 95% or more. Here, the "theoretical density ratio" is defined by the following formula (1): Theoretical density ratio (%) = (measured density) / (theoretical density) × 100 (1) The "theoretical density" in formula (1) refers to a 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. The "measured density" refers to a density determined by measuring the volume and weight of a test piece formed from a green sheet into a specific shape (for example, a shape that makes it easy to calculate the volume, such as a sheet with a rectangular wide surface). In other words, the higher the theoretical density ratio, the fewer voids there are in the green sheet and the higher the formability.

[0040] The green sheet disclosed herein preferably has no voids with a major axis of 30 μm or more when observed by an electron microscope. By forming the green sheet so that no voids are present, the moldability of the green sheet is improved. Furthermore, the absence of the voids improves the thermal conductivity of the green sheet. For example, a sample that allows observation of a cross section perpendicular to the surface of the green sheet (a cross section along the thickness direction (a direction perpendicular to the surface direction of the green sheet; the same applies below)) may be prepared, and the cross section may be observed using a scanning electron microscope (SEM) to confirm the presence or absence of the voids. The observation magnification of the SEM is not particularly limited, but may be set to, for example, about 1000 to 3000 times. Furthermore, although not particularly limited, a plurality of observation fields (for example, 5 or more, 10 or more, 15 or more, or more) may be randomly obtained to confirm the presence or absence of the 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 are present when observed by an electron microscope.

[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. Also, the 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 contain various additives such as dispersants, release agents, antifoaming agents, antioxidants, and thickeners, in addition to the above components, as necessary. It may also contain ceramic powders other than nitride-based ceramic powders. Examples of ceramics constituting the other ceramic powder 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 contained alone or in combination of two or more. In addition, when the other ceramic powders are contained, the content of the 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, assuming that the total of the content of the nitride-based ceramic powder contained in the green sheet and the content of the other ceramic powder is 100% by mass. In addition, all of the ceramic powders contained in the green sheet may be nitride-based ceramic powders.

[0043] <Green sheet manufacturing method> The nitride ceramic powder contained in the green sheet disclosed herein has low wettability to aqueous and organic solvents, and is therefore prone to poor mixing with other materials and poor molding. Therefore, it is difficult to manufacture a green sheet with high moldability using a wet process such as a method using a doctor blade (hereinafter also referred to as the "doctor blade method"), which is a common method for producing green sheets. In addition, when pressurization is used to increase moldability in a wet process, the major axis of the nitride ceramic particles is oriented along the surface direction of the green sheet, which can lead to a problem of reduced thermal conductivity in the thickness direction of the green sheet.

[0044] A suitable example of a method for producing the green sheet disclosed herein is a dry powder rolling method (e.g., roll molding). The dry powder rolling method can be carried out using, for example, a dry powder rolling apparatus 5 shown in FIG. 1. The dry powder rolling apparatus 5 roughly includes a storage tank 1 and a pair of rolls 2. The storage tank 1 is a container for storing granulated powder 1a composed of the raw material of the green sheet 10G. The storage tank 1 also includes a feeder 1b at its bottom, and is configured to continuously supply a certain amount of granulated powder 1a from the discharge port of the feeder 1b between the pair of rolls 2. The feeder 1b is not particularly limited as long as it is excellent in quantitative performance, and various feeders such as a screw type, a vibration type, and a fluid type can be appropriately adopted.

[0045] The above manufacturing method roughly includes a raw material preparation step, a granulation step, and a sheet forming step. In the raw material preparation step, raw materials containing nitride ceramic powder, water-soluble resin binder, water-soluble plasticizer, and various additives as necessary are prepared as raw materials for the green sheet 10G. The constituent materials and volume ratios thereof are as described above.

[0046] In the granulation step, the granulated powder 1a is prepared using the raw materials. The granulation method is not particularly limited, and either wet granulation or dry granulation may be adopted. Examples of the granulation method include rolling granulation, fluidized bed granulation, stirring granulation, compression granulation, extrusion granulation, crushing granulation, and spray drying (spray granulation). From the viewpoint of easier handling of finer raw powder, it is preferable to adopt a wet granulation method such as spray granulation. In the spray granulation method, a mixture of the prepared raw materials is first prepared, and the 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 a conventionally known stirring and mixing device can be used. For example, a ball mill, a mixer, a disperser, a kneader, etc. can be used. As a mixture dispersion medium, for example, water is a suitable example from the viewpoint of reducing the solubility of the water-soluble resin binder and the water-soluble plastic resin and the environmental load. Next, the raw material slurry is sprayed in a liquid state using a spray dryer and dried to obtain a granulated powder. The size of the granulated powder is not particularly limited and can be, for example, 10 μm or more and 200 μm or less.

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

[0048] In the above manufacturing method, by preparing and using granulated powder containing the raw material of the green sheet, separation of the raw material in the molded green sheet can be suppressed even if the green sheet contains nitride ceramic powder. In addition, since the water-soluble resin binder contained in the raw material of the green sheet is given an appropriate flexibility, the sheet can be molded while pushing out air, and a green sheet with high moldability can be manufactured. Furthermore, since the orientation of the nitride ceramic powder in the surface direction of the green sheet can be suppressed, a green sheet with excellent thermal conductivity can be obtained.

[0049] <Uses of Green Sheet> The use of the green sheet disclosed herein is not particularly limited, but it can be used, for example, as a heat dissipation material. As an example, it can be used as a green sheet for preparing a heat dissipation sheet that dissipates heat from a heat generating component by being interposed between a heat generating component (e.g., a power device, etc.) and a heat dissipation component (heat dissipation fin, heat sink, heat dissipation plate, etc.) or in place of the heat dissipation component. It can also be used as a material for constituting a heat dissipation device in combination with the heat dissipation component.

[0050] Several test examples relating to the technology disclosed herein will be described below, but it is not intended that the present invention be limited to those test examples.

[0051] <Test 1> [Green sheet manufacturing] In Test 1, the number average molecular weight of the water-soluble plasticizer was examined. First, boron nitride powder (average particle size 5.7 μm, UHP-G1H (Showa Denko)) was prepared as a nitride ceramic powder, water-soluble acrylic resin A (Boncoat 5495EF (DIC Corporation), glass transition point 23° C.) was prepared as a water-soluble resin binder, polyethylene glycol was prepared as a water-soluble plasticizer, and a release agent, dispersant, and antifoaming agent were also prepared. Polyethylene glycols with number average molecular weights of 200, 300, 600, 1500, and 2000 were prepared, and polyethylene glycols with different number average molecular weights were used in the green sheets of Examples 1 to 5. The boron nitride powder was prepared by aggregating plate-like (or scale-like) primary particles into granules. The glass transition point 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 put into a pot mill together with the same mass of water (dispersion medium) and mixed to prepare a slurry for granulation. At this time, the volume ratio of the boron nitride powder to the water-soluble resin binder was set to 60:40, and the volume ratio of the water-soluble resin binder to the water-soluble plasticizer was set to 90:10. Then, this slurry for granulation was spray-dried using a spray dryer to prepare granulated powder for each example. At this time, the spray conditions were set so that the average particle diameter of the granulated powder was about 80 μm. In addition, the drying temperature in the spray drying was about 180 to 200 ° C, and the remaining amount of the dispersion medium contained in the granulated powder was almost 0 mass %. Then, using the prepared granulated powder, green sheets (thickness about 1 mm) of Examples 1 to 5 were prepared using a dry powder rolling machine.

[0053] [Evaluation of moldability] A test piece of a predetermined size was prepared from each green sheet using a punching blade, and the dimensions of the test piece were measured. The weight of the test piece was then measured, and the actual density (g / cm 3 The theoretical density (g / cm3) was also calculated from the specific gravity and composition ratio of each material. 3) was calculated. The amount of dispersion medium was assumed to be 0 g in this calculation. The theoretical density ratio (%) of the green sheet for each example was calculated using the measured density and theoretical density. The results are shown in Table 1, with a theoretical density ratio of 90% or more marked as "◎", a ratio of 85% or more but less than 90% marked as "〇", and a ratio of 80% or more but less than 85% marked as "△".

[0054] [Evaluation of tensile strength] The tensile strength of each green sheet was measured according to JIS K7161:2014. Test pieces were prepared by punching out the green sheet of each example using a specified test piece punching blade. For the measurement, a tensile strength tester (Shimadzu Corporation, tabletop tester: EZ-TEST) was used, and both ends of the test piece were held by a gripping jig, while the gripping jig was moved in the tensile direction at a tensile speed of 1 mm / min. The tensile breaking stress when the test piece broke was measured, and the tensile strength was calculated by dividing it by the cross-sectional area of ​​the test piece. The test was performed 10 times, and if the average value was 1 MPa or more, it was marked as "◎", if it was 0.8 MPa or more but less than 1 MPa, it was marked as "〇", and if it was less than 0.8 MPa, it was marked as "△". The results are shown in Table 1.

[0055] [Evaluation of stability over time] Immediately after forming the green sheet of each example, a punch blade was used to create a test piece of a specified size and its weight was measured. The test piece was 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 then its weight was measured again. The weight increase rate of the test piece was calculated from the measured weight, and if the weight increase rate was less than 0.5%, it was marked as "◎", if it was 0.5% or more but less than 1%, it was marked as "◯", and if it was 1% or more, it was marked as "△". The results are shown in Table 1.

[0056] [Table 1]

[0057] As shown in Table 1, in Examples 2 to 4, green sheets excellent in moldability, tensile strength, and temporal stability were realized. This shows that the number average molecular weight of the water-soluble plasticizer is preferably 300 to 1500. Furthermore, among these, Examples 3 and 4 were evaluated as having temporal stability of "◎", which shows that a green sheet having a more excellent temporal stability is realized when the number average molecular weight of the water-soluble plasticizer is 600 to 1500. In addition, a tendency was observed in which the temporal stability was improved as the number average molecular weight of the water-soluble plasticizer was larger. This is considered to be due to the number of hydrophilic groups possessed by the water-soluble plasticizer.

[0058] <Test 2> In Test 2, the preferable range of the glass transition point of the water-soluble resin binder used was examined. In Test 2, the acrylic resins described below were used as the water-soluble resin binder in each example (Example 3, Examples 6 to 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 points of these water-soluble acrylic resins were measured in the same manner 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 of each example were produced in the same manner as in Test 1. In addition, moldability, tensile strength, and stability over time 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 and 6 to 8, the moldability, tensile strength and temporal stability were all evaluated as good. This shows that the glass transition point of the water-soluble resin binder is preferably 50°C or less (more specifically, 44°C or less). On the other hand, in Example 9, the tensile strength was insufficient. This is thought to be because the glass transition point of the water-soluble resin binder was higher than in the other examples, and the water-soluble resin binder did not sufficiently change to a rubbery or liquid state during green sheet molding, resulting in the formation of relatively large voids.

[0062] <Test 3> In Test 3, the volume ratio of the water-soluble resin binder to the water-soluble plasticizer was examined. In Test 3, the 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. Green sheets of Examples 10 to 15 were prepared so that the water-soluble plasticizer was 0 to 40 vol% when the volume ratio of the water-soluble resin binder to the water-soluble plasticizer was 100 vol% (see Table 3 for the volume ratio of each example). The other materials and preparation method used for the green sheets were the same as in Test 1. Also, the moldability, tensile strength, and stability over time 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 temporal stability were all evaluated as good. From this, it can be seen that when the volume ratio of the water-soluble resin binder to the water-soluble plasticizer is 100 vol%, the proportion of the water-soluble plasticizer is preferably 2 vol% or more and 35 vol% or less. In particular, from the results of Examples 12 and 13, it can be seen that when the proportion of the water-soluble plasticizer is 10 vol% or more and 20 vol% or less, a green sheet excellent in moldability, tensile strength, and temporal stability is realized.

[0065] <Test 4> In Test 4, the type of nitride ceramic powder was examined. 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 ceramic powders. In addition, 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 preparation methods used for the green sheet were the same as in Test 1, with Example 3 being an example using boron nitride, Example 16 being an example using silicon nitride, and Example 17 being an example using aluminum nitride. In addition, moldability, tensile strength, and stability over time 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 excellent in formability, tensile strength, and temporal stability were realized regardless of the type of nitride ceramic powder.

[0068] Although the test examples of the technology disclosed herein have been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples exemplified above. [Explanation of symbols]

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

Claims

1. The ceramic composition includes a nitride ceramic powder, a water-soluble resin binder, and a water-soluble plasticizer, The water-soluble resin binder has a glass transition temperature of 50° C. or less, The number average molecular weight of the water-soluble plasticizer is 300 or more and 1500 or less. Green sheet.

2. 2. The green sheet according to claim 1, having a theoretical density ratio ((actual density / theoretical density) x 100) of 85% or more.

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

4. The green sheet according to any one of claims 1 to 3, wherein 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.

5. 5. The green sheet according to claim 1, wherein the water-soluble plasticizer comprises a polyether-based plasticizer.

6. 6. The green sheet according to claim 1, wherein the water-soluble resin binder is a water-soluble acrylic resin.

7. 7. The green sheet according to claim 1, wherein the nitride ceramic powder contains at least one nitride compound selected from the group consisting of boron nitride, silicon nitride, and aluminum nitride.

8. A method for producing the green sheet according to any one of claims 1 to 7, comprising the steps of: (a) forming a powder sheet by dry powder rolling;

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