JP7470032B2 - Green sheet and its manufacturing method - Google Patents

Description

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 ceramic powder and a method for manufacturing the same.

Power semiconductor elements (so-called power devices) have become indispensable for the efficient use of electrical energy. There is also an increasing demand for lighting semiconductor elements (so-called high-power LED devices) used in energy-saving, long-life, high-brightness, and high-power LED lamps. In recent years, as shown in Patent Documents 1 to 5, there has been vigorous research and development into technologies for miniaturizing, increasing density, and increasing the speed of power devices.

Patent No. 6256158 JP 2019-117916 A JP 2004-339426 A JP 2005-48124 A JP 2005-272599 A

Regarding the appropriate use of power devices, the above-mentioned Patent Documents 1 to 5 propose the use of a sheet-like composition containing a ceramic powder (e.g., oxide-based ceramic powder, nitride-based ceramic powder) and a resin binder. The larger the current controlled by the power device, the greater the amount of heat that can be generated, so a configuration for discharging the amount of heat generated by the use of the power device is required. As an example, a configuration for discharging the amount of heat generated by the use of the power device using the above-mentioned sheet-like composition is disclosed.

The above-mentioned sheet-like composition can be obtained, for example, by mixing ceramic powder, a resin binder, and other materials as necessary with water or an organic solvent to prepare a slurry, and then coating the slurry. However, when a nitride-based ceramic powder is used as the ceramic powder, the powder has low wettability with water or an organic solvent, so it is difficult to uniformly mix the powder with other materials using these solvents to produce a dense sheet-like composition. If the density of the sheet-like composition is low, there is a risk that the power device, and therefore electronic devices, will not be able to properly dissipate heat, which is undesirable.

The present invention was created in consideration of these points, and its purpose is to provide a technology for producing a sheet-shaped composition (green sheet) containing dense nitride-based ceramic powder.

According to the technology disclosed herein, a green sheet is provided that contains a nitride ceramic powder, a water-soluble resin binder, and a plasticizer. The water-soluble resin binder in the green sheet is an acrylic resin having a glass transition point of 30°C or less. In this specification, the term "green sheet" refers to an unfired sheet-like structure, regardless of whether it has been subjected to a drying process or not. In a green sheet having such a configuration, the density of the green sheet can be improved by using a water-soluble acrylic resin having a glass transition point of 30°C or less.

In a preferred embodiment, the green sheet disclosed herein has the following volume ratios of the nitride-based ceramic powder and the water-soluble resin binder, when the total volume of the nitride-based ceramic powder and the water-soluble resin binder is taken as 100 vol %:
(1) the nitride-based ceramic powder: more than 45 vol % and less than 75 vol %; and
(2) The water-soluble resin binder is more than 25 vol % and less than 55 vol %;
According to this configuration, in addition to the above-mentioned effects, the thermal conductivity of the green sheet can be improved.

In another preferred embodiment, the green sheet disclosed herein has a density of 1.5 g/ ^cm3 or more and 2.5 g/ ^cm3 or less. In the green sheet having such a configuration, improved compactness is realized. In addition, in the green sheet having such a configuration, improved thermal conductivity is realized.

In another preferred embodiment, the green sheet disclosed herein is characterized in that, when observed in cross section under an electron microscope, there are no voids with a major axis of 30 μm or more. In a green sheet having such a configuration, improved density is achieved. In addition, in a green sheet having such a configuration, improved thermal conductivity is achieved.

In another preferred embodiment, the volume ratio (A:B) of the water-soluble resin binder (A) to the plasticizer (B) is 99:2 to 65:35. With this configuration, in addition to the above effects, moldability is improved.

In another preferred embodiment, the plasticizer is a glycerin-based plasticizer. With this configuration, the effects of the present invention can be more effectively achieved.

In another preferred embodiment, the nitride ceramic powder is a powder containing at least one nitride compound selected from the group consisting of boron nitride, silicon nitride, and aluminum nitride. With this configuration, it is possible to provide a green sheet containing such a compound as the nitride ceramic powder.

The technology disclosed herein also provides a method for manufacturing a green sheet. In the manufacturing method disclosed herein, the green sheet is manufactured using a dry powder rolling method. In the manufacturing method configured as above, a dense nitride-based ceramic green sheet can be provided by using the dry powder rolling method. In addition, the manufacturing method configured as above can provide a nitride-based ceramic green sheet with improved thermal conductivity.

1A to 1C are schematic diagrams illustrating a method for producing a green sheet according to an embodiment using a dry powder rolling method. 1 is an electron microscope image of a green sheet obtained in a preferred embodiment. 1 is an electron microscope image of a green sheet obtained in a comparative example.

The following describes preferred embodiments of the present invention. Matters other than those specifically mentioned in this specification that are necessary for implementing the present invention can be understood based on the technical content taught by this specification and the general technical common sense of those skilled in the art in the relevant field. The present invention can be implemented based on the content disclosed in this specification and the technical common sense of those 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.

[Green sheet]
The green sheet disclosed herein is an unfired sheet-like structure that contains a nitride ceramic powder, a water-soluble resin binder, and a 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.

The green sheet disclosed herein can be molded to have a density of 1.5 g/cm ³ or more. The green sheet in the above density range realizes improved denseness. The density of the green sheet may vary depending on the type of raw material contained in the green sheet, but can typically be 2.5 g/cm ³ or less (e.g., 2.45 g/cm ³ or less, 2.4 g/cm ³ or less). From the viewpoint of improving thermal conductivity and moldability, the density can be 1.6 g/cm ³ or more, 1.7 g/cm ³ or more, 1.8 g/cm ³ or more, 1.9 g/cm ³ or more, or 2.0 g/cm ³ or more. From the same viewpoint, the density may be 1.9 g/cm ³ or less, 1.8 g/cm ³ or less, 1.7 g/cm ³ or less, or 1.6 g/cm ³ or less. The density may be, for example, an actual density obtained by measuring the outer diameter and weight of a green sheet (a test piece may be prepared for measurement) as described in the following examples, and based on these actual measured values.

The density can be used as an index of the denseness of the green sheet disclosed herein. As another example of an index for evaluating denseness, the theoretical density ratio (%) may be used as described in the following examples. The theoretical density ratio (%) is calculated by the following formula (1):
Theoretical density ratio (%) = (actual density) / (theoretical density) × 100 (1)
The theoretical density is a calculated value based on the specific gravity and the mixing ratio of the materials used to form the green sheet. The higher the theoretical density ratio (%), the higher the density, and is 85% or more, preferably 88% or more, more preferably 90% or more, and the closer to the upper limit (100%), the better.

Alternatively, the following formula (2):
Porosity (%) = (1 - (actual density) / (theoretical density)) x 100 (2)
The porosity (%) of the green sheet may be calculated based on the above formula to evaluate the density. The smaller the porosity (%), the higher the density. The porosity (%) is 15% or less, preferably 12% or less, more preferably 10% or less, and the closer to the lower limit (0%), the better.

It is preferable that the green sheet disclosed herein does not have voids with a major axis of 30 μm or more when observed under an electron microscope. By forming the green sheet so that the voids do not exist, the density of the green sheet can be improved. In addition, the absence of the voids is preferable from the viewpoint of improving the thermal conductivity of the green sheet. For example, as described in the following examples, a sample is prepared so that 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)) can be observed, and the cross section can 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. In addition, although not particularly limited, multiple observation fields (for example, 5 or more, 10 or more, 15 or more, or more) can be randomly obtained to confirm the presence or absence of the voids. In addition, 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 in cross section under an electron microscope.

The inclusion of a nitride ceramic powder with high thermal conductivity can improve the thermal conductivity of the green sheet. The thermal conductivity of such a green sheet in the thickness direction can be, for example, 1.7 W/m·K or more, preferably 2.0 W/m·K or more, more preferably 2.5 W/m·K or more, and even more preferably 3.0 W/m·K or more.

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. In addition, the thickness may be, for example, 3000 μm or less, 2000 μm or less, 1000 μm or less, or 500 μm or less.

The nitride ceramic powder is a powder mainly composed of ceramics made of nitride compounds. In addition, "mainly composed of A" means that A is contained in at least 90 mass% or more, preferably 95 mass% or more, more preferably 98 mass% or more, and even more preferably 99% mass% or more, or 100 mass%. Specific examples of nitride ceramics include boron nitride (BN), silicon nitride (Si ₃ N ₄ ), aluminum nitride (AlN), gallium nitride (GaN), and the like. These can be used alone or in combination of two or more. The above nitride compounds have excellent thermal conductivity and are suitable from the viewpoint of improving the thermal conductivity of the green sheet disclosed herein. From this viewpoint, the nitride ceramic powder is preferably a powder containing at least one nitride compound selected from the group consisting of BN, Si ₃ N ₄ , and AlN.

Note that while the ceramics mentioned above and those described below are given with representative compositions, they are not necessarily limited to those that have that exact composition. For example, they may contain various other elements or be composites in order to obtain desired characteristics.

The shape of the nitride ceramic powder is not particularly limited, but may be spherical (including nearly spherical), scaly, fibrous, plate-like, irregular, agglomerated powder, granular, etc.

The average particle size of the particles constituting the nitride ceramic powder is not particularly limited, but may be 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, 30 μm or less, or 15 μm or less. In this specification, the "average particle size" refers to a particle size (also called D ₅₀ particle size) corresponding to a cumulative 50% 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.

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 by observing the nitride ceramic powder with an SEM, randomly selecting a number 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 obtaining the arithmetic average value.

The green sheet disclosed herein includes a water-soluble resin binder. The water-soluble resin binder is a component that binds the nitride ceramic powder and other green sheet constituent materials. As the water-soluble binder, one that is not decomposed by a heat treatment for drying the green sheet (typically, a heat treatment at 100°C to 200°C) and is easily decomposed and removed by a debindering treatment (typically, a heat treatment at more than 200°C to less than 600°C) or a firing process at more than 600°C (for example, 1500°C to 2500°C) can be preferably used. Here, the "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 ion-exchanged water (deionized water), pure water, ultrapure water, and distilled water can be preferably used. In addition, 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 present invention. In this case, it is preferable that the aqueous solvent is 95 vol% or more water, and more preferably 99 vol% or more water.

When making a green sheet using a mixture of ceramic powder and resin binder, one common method is a wet process such as a doctor blade method (hereinafter also referred to as the "doctor blade method"). In the doctor blade method, typically, a mixture of ceramic powder, resin binder, and various additives as necessary is prepared, and a liquid material (e.g., aqueous solvent, organic solvent) is added to the mixture to prepare a slurry. The slurry is applied to a substrate or the like, and the liquid material is removed by drying or the like to produce a green sheet.

However, when nitride ceramic powder is used as the ceramic powder, the nitride ceramic powder has low wettability to aqueous and organic solvents, and is prone to poor mixing with other materials and poor molding. Therefore, it has been difficult to obtain a green sheet filled with nitride ceramic powder at a high density using a wet process. In response to this, in the past, studies have been conducted on increasing the density of green sheets containing nitride ceramic powder by applying a large pressure. However, the pressure applied in the wet process can cause the nitride ceramic powder to be oriented so that its major axis is aligned with the surface direction of the green sheet. Such an orientation can cause the green sheet to have a lower density. As a result, for example, when a green sheet containing nitride ceramic powder is used for heat dissipation purposes, good thermal conductivity cannot be achieved, which is not preferable. The above tendency was particularly common when the nitride ceramic powder had a plate-like or fibrous outer shape.

The inventors therefore focused on using a dry process, such as a dry powder rolling method, to produce green sheets containing nitride ceramic powder, rather than the wet process described above. As a result of the inventors' intensive research, they were able to produce dense green sheets using a dry process by using a water-soluble resin binder as the resin binder to be mixed with the nitride ceramic powder, and further by having the water-soluble resin binder contain an acrylic resin with a glass transition point of 30°C or less. In addition to improving the density of such green sheets, they also confirmed improved thermal conductivity.

The green sheet disclosed herein contains, as a water-soluble resin binder, at least a water-soluble resin binder A1 made of an acrylic resin having a glass transition point of 30°C or less. The acrylic resin is not particularly limited as long as it has a glass transition point in the above range, and various acrylic polymer compounds can be used. The acrylic polymer compounds may contain various hydrophilic functional groups such as hydroxyl groups, carboxyl groups, sulfo groups, phospho groups, and amino groups.

An example of the 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 weight of the total monomers) and further contains a secondary monomer that is copolymerizable with the main monomer. In addition to the main monomer and secondary monomer, the monomer mixture may optionally contain other copolymerization components. By copolymerizing these monomers, an acrylic polymer compound having a specified function can be formed. In this specification, "(meth)acrylate" means acrylate and methacrylate. Similarly, "(meth)acrylic" means acrylic and methacrylic.

As the alkyl(meth)acrylate, for example, a compound represented by the general formula: CH ₂ ═C(R ¹ )COOR ² can be suitably used. Here, R ¹ in the formula represents a hydrogen atom or a methyl group. Furthermore, R ² represents a chain alkyl group having 1 to 20 carbon atoms. R ² is preferably an alkyl(meth)acrylate which is a chain alkyl group having 1 to 14 carbon atoms, and more preferably an alkyl(meth)acrylate which is a chain alkyl group having 1 to 12 carbon atoms.

As the secondary monomer, a monomer component having a function of introducing a crosslinking point into an acrylic polymer compound or controlling the binding property of the acrylic polymer compound and containing various functional groups according to the desired binder properties can be used. 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 density is achieved 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) can be appropriately selected according to the desired degree of crosslinking, 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.

The glass transition point of the water-soluble resin binder A1 is 30°C or less, and may be 25°C or less from the viewpoint of improving the ease of processing, flexibility, and density of the green sheet. The glass transition point of the water-soluble resin binder may be 20°C or less, 10°C or less, 0°C or less, -10°C or less, or -20°C or less. The glass transition point may be, for example, -200°C or more, -100°C or more, -80°C or more, or -60°C or more. The glass transition point may be measured using a general differential scanning calorimetry (DSC). The manufacturer's nominal value may be used as necessary.

Although not particularly limited, the weight average molecular weight of the water-soluble resin binder A1 can be approximately 5,000 or more (e.g., 10,000 or more). The weight average molecular weight can be approximately 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 A1 can be measured by gel permeation chromatography (GPC) and the weight-based average molecular weight converted using a standard polystyrene calibration curve can be used. Alternatively, the manufacturer's nominal value can be used.

As long as the effects of the technology disclosed herein can be realized, the water-soluble resin binder may include a water-soluble resin binder A2 other than the water-soluble resin binder A1. Examples of the water-soluble resin binder A2 include, but are not limited to, acrylic resins with a glass transition point exceeding 30°C, various polyether resins, cellulose compounds, polyvinyl alcohol, etc.

If the sum of the volumes of the water-soluble resin binder A1 and the water-soluble resin binder A2 is 100 vol%, the volume ratio of the water-soluble resin binder A1 is preferably 70 vol% or more. From the viewpoint of improving the moldability, density, and thermal conductivity of the green sheet, the volume ratio of the water-soluble resin binder A1 may be, for example, 80 vol% or more, preferably 90 vol% or more, more preferably 95 vol% or more, and even more preferably 98 vol% or more. The water-soluble resin binder may consist of the water-soluble resin binder A1.

If 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 density and 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, more preferably 60 vol% or more, and even more preferably 65 vol% or more. From the viewpoint of formability, the above volume ratio can be less than 75 vol%, or 70 vol% or less, or 65 vol% or less. On the other hand, the volume ratio of the water-soluble resin binder is less than 55 vol% from the viewpoint of formability and ease of processing, and from the viewpoint of improving the thermal conductivity of the green sheet, it is preferably 50 vol% or less, may be 45 vol% or less, may be 40 vol% or less, may be 35 vol% or less, may be 30 vol% or less, and is appropriate to be more than 25 vol%.

A plasticizer is a component that mainly functions to impart flexibility to the polymer by penetrating between the molecules of the polymer that constitute the water-soluble resin binder, weakening the intermolecular forces of the polymer and making each molecule easier to move. In the technology disclosed herein, the plasticizer can contribute to improving the density and processability of the green sheet. As a plasticizer, one that is not decomposed by the heat treatment for drying the green sheet (typically, heat treatment at 100°C to 200°C) and is easily decomposed and removed by the binder removal treatment (typically, heat treatment at more than 200°C and not more than 600°C) or the firing process at more than 600°C (for example, 1500°C to 2500°C) can be preferably used.

As the plasticizer, a water-soluble plasticizer can be preferably used. As the water-soluble plasticizer, any known water-soluble plasticizer can be used without particular limitation, and for example, polyhydric alcohols, polyethers, etc. can be used. These can be used alone or in combination of two or more. As long as the effects of the technology disclosed herein can be realized, the type, average molecular weight, and other properties of the water-soluble plasticizer are not particularly limited.

The polyhydric alcohol may be a trihydric or higher alcohol. Specific examples include glycerin and polyglycerin. Polyglycerin is a compound having a structure in which two or more glycerins are polymerized, and examples of such polyglycerin include diglycerin, triglycerin, and tetraglycerin.

The polyether may be polyoxyalkylene glyceryl ether, polyoxyalkylene polyglyceryl ether, polyethylene glycol, polypropylene glycol, etc. Polyoxyalkylene glyceryl ether and polyoxyalkylene polyglyceryl ether are compounds obtained by addition polymerization of oxyalkylene to glycerin or polyglycerin. 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 and polyoxypropylene glyceryl ether. Specific examples of polyoxyalkylene polyglyceryl ether include polyoxyethylene polyglyceryl ether and polyoxypropylene polyglyceryl ether. The number of repetitions of the oxyalkylene group is 1 or more, but is not particularly limited and can be set appropriately. The oxyalkylene group constituting the polyoxyalkylene may be one type or two or more types.

Among the above compounds, glycerin-based plasticizers can be preferably used from the viewpoint of improving the moldability, ease of processing, and density of the green sheet. Glycerin-based plasticizers include glycerin, polyglycerin, polyoxyalkylene glyceryl ether, and polyoxyalkylene polyglyceryl ether. Glycerin-based plasticizers can also be preferably used from the viewpoint of improving the storage stability of the green sheet.

From the viewpoint of improving the moldability, density, and thermal conductivity of the green sheet, the volume ratio (A:B) of the water-soluble resin binder (A) to the plasticizer (B) is preferably set to 98:2 to 65:35. However, the preferred range of the volume ratio of the water-soluble resin binder to the water-soluble binder can be changed appropriately within or outside the above range depending on the constituent materials.

The green sheet disclosed herein, as described above, must contain nitride-based ceramic powder, but may contain other 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 above-mentioned other ceramic powders are contained, the content of the nitride-based ceramic powder may be 70% by mass or more, preferably 80% by mass or more, and more preferably 90% by mass or more, assuming that the total content of the nitride-based ceramic powder and the content of the other ceramic powder contained in the green sheet is 100% by mass. All the ceramic components contained in the green sheet may be nitride-based ceramic powder.

The green sheet disclosed herein may contain, in addition to the above components, various additives such as dispersants, release agents, defoamers, antioxidants, thickeners, etc., as necessary.

[Method of manufacturing green sheet]
A suitable example of the 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 has excellent quantitative performance, and various feeders such as a screw type, a vibration type, and a fluid type can be appropriately adopted.

The above manufacturing method broadly includes a raw material preparation process, a granulation process, and a sheet forming process. In the raw material preparation process, raw materials containing nitride ceramic powder, water-soluble resin binder, plasticizer, and various additives as necessary are prepared as raw materials for green sheet 10G. The constituent materials and volume ratios, etc. of these are as described above.

In the granulation process, 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 used. 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, first, a mixture of the prepared raw materials is 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/mixing device can be used. For example, a ball mill, a mixer, a disperser, a kneader, etc. can be used. As the mixture dispersion medium, for example, water is a suitable example from the viewpoint of reducing 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, but can be, for example, 10 μm or more and 200 μm or less.

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 fed into the storage tank 1 of the dry powder rolling device 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. The rolls 2 rotate (see the arrow in FIG. 1), compressing the supplied granulated powder 1a and forming it into a sheet to obtain a green sheet 10G. The temperature and pressure conditions here are not particularly limited and may vary depending on the type of raw material, etc., and may be changed as appropriate. The gap between the rolls 2 may be changed as appropriate so as to achieve the desired thickness of the green sheet 10G.

In the above manufacturing method, by preparing and using granulated powder containing the raw materials for the green sheet, separation of the raw materials in the formed green sheet can be suppressed. Also, orientation of the nitride ceramic powder in the surface direction of the green sheet can be suppressed. As a result, more nitride ceramic powder can be contained in the green sheet.

<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 by replacing 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.

According to the technology disclosed herein, the green sheet contains a nitride ceramic powder, a water-soluble resin binder, and a plasticizer, and the water-soluble resin binder contains an acrylic resin with a glass transition point of 30°C or less. Furthermore, the volumetric proportion of the nitride ceramic powder and the volumetric proportion of the water-soluble resin binder are set within a predetermined range, thereby improving the density of the green sheet.

Several examples of the present invention are described below, but it is not intended that the present invention be limited to those examples.

<1. Study of water-soluble acrylic resin>
As the water-soluble acrylic resin, acrylic resins A to E were prepared. The following commercially available acrylic resins were used as the acrylic resins A to E:
Acrylic resin A: Boncoat 5494EF (DIC Corporation);
Acrylic resin B: Boncoat CE-8510 (DIC Corporation);
Acrylic resin C: Boncoat CE-1400 (DIC Corporation);
Acrylic resin D: Boncoat YG-651 (DIC Corporation); and
Acrylic resin E: Grandor PP-1000EF (DIC Corporation);
The glass transition points of the acrylic resins A to E were measured using a commercially available differential scanning calorimetry (Rigaku Corporation). The glass transition points measured using this device are shown in the corresponding columns in Table 1.

[Preparation of green sheet]
-Examples 1 to 5-
Boron nitride powder (BN, average particle size 5.7 μm, UHP-G1H (Showa Denko)) as the nitride ceramic powder, water-soluble acrylic resin (see Table 1 for the type) as the water-soluble resin binder, plasticizer (PEG-1500 (Sanyo Chemical Industries, Ltd.)), release agent, dispersant, and defoamer were prepared. BN is a granular material formed by agglomerating plate-like (scale-like) primary particles. When the total volume of the nitride ceramic powder and the water-soluble resin binder was 100 vol%, the nitride ceramic powder was 60 vol% and the water-soluble resin binder was 40 vol% (volume ratio 1). When the total volume of the water-soluble resin binder and the plasticizer was 100 vol%, the water-soluble resin binder was 90 vol% and the plasticizer was 10 vol% (volume ratio 2).

The prepared materials were put into a pot mill together with an equal mass of water (dispersion medium) and mixed to prepare a slurry for granulation. This slurry for granulation was spray-dried using a spray dryer to produce the granulated powder for each example. The spray conditions were set so that the average particle size of the granulated powder was approximately 80 μm. The drying temperature in the spray drying was approximately 180 to 200°C, and the remaining amount of dispersion medium contained in the granulated powder was almost 0% by mass.

The granulated powder prepared as described above was used to produce green sheets for each example using a dry powder rolling machine. The thickness of the green sheets is shown in the corresponding column in Table 1.

[Sheet density]
The green sheets according to each example were processed to a predetermined size to prepare test pieces. The outer diameter and weight of the test pieces were measured to calculate the actual density. The results are shown in the corresponding columns of Table 1. The theoretical density ratio (%) of the green sheets was calculated using the above formula (1). The results are shown in the corresponding columns of Table 1.

[Thermal conductivity]
The thermal conductivity (W/m·K) in the thickness direction of the green sheet according to each example was measured using a thermal conductivity measuring device (LFA 467 HyperFlash (registered trademark) (NETZSCH)) according to the manual of the device. The results are shown in the corresponding columns of Table 1.

[Electron microscope observation]
Using an SEM, a cross section perpendicular to the surface of the green sheet according to each example (cross section along the thickness direction) was observed. A cross-sectional SEM image was obtained at a magnification of 2000 times, and the presence or absence of voids with a major axis of 30 μm or more in the cross section of the green sheet was evaluated. The results are shown in the corresponding column in Table 1. "None" in the corresponding column in Table 1 indicates that no voids with a major axis of 30 μm or more were observed. "Present" indicates that one or more voids were observed. The presence or absence of the voids was evaluated using 10 cross-sectional SEM images obtained at random. A cross-sectional SEM image of one example is shown in FIG. 2, and a cross-sectional SEM image of one comparative example is shown in FIG. 3.

[result]
As shown in Table 1, in Examples 1 to 5, green sheets containing nitride ceramic powder, a water-soluble resin binder, and a plasticizer were obtained. The green sheets according to Examples 1 to 5 all had a density of 1.5 g/cm ³ or more. However, when Examples 1 to 3 containing a water-soluble acrylic resin having a glass transition point of 30° C. or less as the water-soluble binder are compared with Examples 4 and 5 containing only a water-soluble acrylic resin having a glass transition point of more than 30° C., in the green sheets of Examples 1 to 3, no voids with a major axis of 30 μm or more were observed in SEM observation, whereas in the green sheets of Examples 4 and 5, voids with a major axis of 30 μm or more were observed. From this, it was confirmed that the density of the green sheets can be improved by containing a water-soluble acrylic resin having a glass transition point of 30° C. or less as the water-soluble resin binder. In addition, an improvement in thermal conductivity was confirmed in the green sheets of Examples 1 to 3.

2. Examination of the volume ratio of nitride ceramic powder and water-soluble resin binder
In this example, a study was carried out in which the ratio of the volume of the nitride ceramic powder to the volume of the water-soluble resin binder was changed.

[Preparation of green sheet]
--Examples 6 to 10--
Green sheets according to Examples 6 to 10 were produced using the same materials and procedures as in Example 3, except that the volume ratio of the nitride-based ceramic powder and the volume ratio of the water-soluble resin binder were as shown in the "Volume Ratio 1" column in Table 2, when the total volume of the nitride-based ceramic powder and the volume of the water-soluble resin binder was 100 vol.%.

As described in 1. above, the thickness, density, thermal conductivity, and presence of voids with a major axis of 30 μm or more of the green sheets in each of the above examples were evaluated. The results are shown in the corresponding columns in Table 2.

[result]
In Examples 3, 6 to 10, when the volumetric ratio of the nitride ceramic powder and the volumetric ratio of the water-soluble resin binder contained in the green sheets were examined, the green sheets according to Examples 3, 6 to 9 all had a density of 1.5 g/ ^cm3 or more. In addition, when the volumetric ratio of the water-soluble resin binder was increased, the formation of voids with a major axis of 30 μm or more was suppressed in SEM observation (Examples 3, 6 to 8, and 10). It was confirmed that good thermal conductivity was achieved by increasing the volumetric ratio of the nitride ceramic powder while suppressing the formation of the above-mentioned voids (Examples 3, 6 to 8).

<3. Consideration of the volume ratio of water-soluble resin binder and plasticizer>
In this example, a study was carried out in which the ratio of the volume of the water-soluble resin binder to the volume of the plasticizer was changed.

[Preparation of green sheet]
--Examples 11 to 15--
Green sheets according to Examples 11 to 15 were produced using the same materials and procedures as in Example 3, except that the volume ratio of the water-soluble resin binder and the volume ratio of the plasticizer, when the total of the volume of the water-soluble resin binder and the volume of the plasticizer was 100 vol%, were as shown in the "Volume ratio 2" column in Table 3. However, in Example 14, a green sheet could not be formed.

As described in 1. above, the thickness, density, thermal conductivity, and voids with a major axis of 30 μm or more of the green sheets for each of the above examples were evaluated. The results are shown in the corresponding columns in Table 3. Note that the above evaluation was omitted for Example 14, and the corresponding column in Table 3 is indicated with a "-".

[result]
As shown in Table 3, comparing Examples 3, 11 to 13 containing a plasticizer with Example 15 not containing a plasticizer, the inclusion of a plasticizer suppressed the formation of voids with a major axis of 30 μm or more in SEM observation. It was also confirmed that good thermal conductivity was achieved.

4. Examination of types of nitride ceramic powders
In this example, a study was conducted in which the type of nitride ceramic powder was changed. Silicon nitride (manufactured by Aoshima) and aluminum nitride (manufactured by Combustion Synthesis Co., Ltd.) were prepared as nitride ceramic powders. The silicon nitride was amorphous and had an average particle size of 0.8 μm. The aluminum nitride was amorphous and had an average particle size of 5 μm.

[Preparation of green sheet]
-Example 16, Example 17-
Green sheets according to Examples 16 and 17 were produced using the same materials and procedures as in Example 3, except that silicon nitride (Example 16) and aluminum nitride (Example 17) were used as the nitride ceramic powder.

As described in 1. above, the thickness, density, thermal conductivity, and voids with a major axis of 30 μm or more of the green sheets in each of the above examples were evaluated. The results are shown in the corresponding columns in Table 4.

[result]
As shown in Table 4, it was confirmed that the density of the green sheet can be improved even when silicon nitride or aluminum nitride is used as another example of the nitride ceramic powder. Also, it was confirmed that the green sheets according to Examples 16 and 17 have good thermal conductivity.

As described above, the technology disclosed herein provides a green sheet containing a nitride ceramic powder, a water-soluble resin binder, and a plasticizer. The green sheet contains an acrylic resin with a glass transition point of 30°C or less as the water-soluble resin binder. The technology disclosed herein provides a technology for manufacturing a dense green sheet containing nitride ceramic powder.

Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and variations of the specific examples given above.

1 Storage tank 1a Granulated powder 1b Feeder 2 Roll 5 Dry powder rolling device 10G Green sheet

Claims (6)

The ceramic composition includes a nitride ceramic powder, a water-soluble resin binder, and a plasticizer,
The water-soluble resin binder contains an acrylic resin having a glass transition point of 30° C. or less,
When the total volume of the nitride-based ceramic powder and the water-soluble resin binder is taken as 100 vol%, the nitride-based ceramic powder and the water-soluble resin binder are mixed in the following volume ratio:
(1) the nitride-based ceramic powder is more than 45 vol % and less than 75 vol %; and
(2) The water-soluble resin binder is more than 25 vol % and less than 55 vol %;
Including,
The volume ratio (A:B) of the water-soluble resin binder (A) to the plasticizer (B) is 98:2 to 65:35 . 2. The green sheet according to claim 1 , having a density of 1.5 g/ ^cm3 or more and 2.5 g/ ^cm3 or less. 3. The green sheet according to claim 1, wherein , when a cross section of the green sheet is observed under an electron microscope, there are no voids having a major axis of 30 μm or more. The green sheet according to any one of claims 1 to 3 , wherein the plasticizer is a glycerin-based plasticizer. 5. The green sheet according to claim 1 , wherein the nitride ceramic powder is a powder containing at least one nitride compound selected from the group consisting of boron nitride, silicon nitride, and aluminum nitride. A method for producing a green sheet, comprising the steps of:
A method for producing the green sheet according to any one of claims 1 to 5 , comprising the steps of:

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