Thermal conductive sheet, insulating sheet, and heat dissipating member   

Abstract: A thermal conductive sheet contains a resin and a plate-like or flake-like filler. The average orientation angle of the filler is 29 degrees or more and the maximum orientation angle thereof is 65 degrees or more with respect to the plane direction of the thermal conductive sheet.

The present application claims priority from Japanese Patent Application No. 2011-108583 filed on May 13, 2011, the contents of which are hereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thermal conductive sheet, an insulating sheet, and a heat dissipating member, to be specific, to a thermal conductive sheet for use in power electronics technology and the like, an insulating sheet and a heat dissipating member obtained by using the thermal conductive sheet.

2. Description of Related Art

In recent years, power electronics technology which uses semiconductor elements to convert and control electric power is applied in hybrid devices, high-brightness LED devices, and electromagnetic induction heating devices. In power electronics technology, a high current is converted to, for example, heat, and therefore materials that are disposed near the semiconductor element are required to have excellent heat dissipation characteristics (excellent heat conductivity) and insulating characteristics.

A thermal conductive sheet obtained by dispersing, for example, an inorganic filler having thermal conductivity and insulating characteristics, a flake-like boron nitride, and the like in a resin has been known.

The flake-like boron nitride has a high thermal conductivity in the longitudinal direction and a low thermal conductivity in the short-side direction. Therefore, for example, when the longitudinal direction of the boron nitride is allowed to be along the thickness direction of the thermal conductive sheet, the thermal conductivity in the thickness direction can be improved. Also, when the longitudinal direction of the boron nitride is allowed to be along the plane direction of the thermal conductive sheet, the thermal conductivity in the plane direction can be improved.

However, there is a disadvantage that, when the thermal conductive sheet is produced by press molding or roll forming, the boron nitride tends to be along the plane direction of the thermal conductive sheet, so that the obtained thermal conductive sheet has a poor thermal conductivity in the thickness direction, while having an excellent thermal conductivity in the plane direction.

On the other hand, there are cases where the thermal conductive sheet is required to have an excellent thermal conductivity not only in the plane direction but also in the thickness direction depending on its use.

Therefore, for example, a thermal conductive sheet, which is obtained by dispersing secondary aggregated particles having a porosity of 50% or less and an average pore size of 0.05 to 3 μm obtained by aggregating primary particles of the boron nitride in a thermosetting resin, has been proposed (ref: for example, Japanese Unexamined Patent Publication No. 2010-157563).

In the thermal conductive sheet, the boron nitride is contained as the secondary aggregated particles, that is, contained without being oriented in the thickness direction or the plane direction of the thermal conductive sheet, so that the thermal conductivity in the thickness direction and the plane direction can be ensured.

SUMMARY

However, there is a disadvantage that to obtain the thermal conductive sheet described in Japanese Unexamined Patent Publication No. 2010-157563, production of the secondary aggregated particles of the boron nitride is required, so that a complicated process such that the boron nitride is temporarily calcined at high temperature and pulverized to be in a slurry state and then is calcined is required.

It is an object of the present invention to provide a thermal conductive sheet capable of being obtained with an easy operation and having an excellent thermal conductivity in the thickness and plane directions, and an insulating sheet and a heat dissipating member obtained by using the thermal conductive sheet.

A thermal conductive sheet of the present invention contains a resin and a plate-like or flake-like filler, wherein the average orientation angle of the filler is 29 degrees or more and the maximum orientation angle thereof is 65 degrees or more with respect to the plane direction of the thermal conductive sheet.

In the thermal conductive sheet of the present invention, it is preferable that the resin contains a first resin and a second resin, and a difference between the softening temperature of the first resin and that of the second resin is 20° C. or more.

In the thermal conductive sheet of the present invention, it is preferable that a difference between the softening temperature of the first resin and that of the second resin is 40° C. or more, and the second resin has an average particle size of 10 to 500 μm and has its shape retained at a temperature between the softening temperature of the first resin and that of the second resin.

In the thermal conductive sheet of the present invention, it is preferable that the filler content is 50 to 95 parts by mass with respect to 100 parts by mass of the total amount of the thermal conductive sheet.

An insulating sheet of the present invention is obtained by using a thermal conductive sheet, wherein the thermal conductive sheet contains a resin and a plate-like or flake-like filler, and the average orientation angle of the filler is 29 degrees or more and the maximum orientation angle thereof is 65 degrees or more with respect to the plane direction of the thermal conductive sheet.

A heat dissipating member of the present invention is obtained by using a thermal conductive sheet, wherein the thermal conductive sheet contains a resin and a plate-like or flake-like filler, and the average orientation angle of the filler is 29 degrees or more and the maximum orientation angle thereof is 65 degrees or more with respect to the plane direction of the thermal conductive sheet.

In the thermal conductive sheet, the insulating sheet, and the heat dissipating member of the present invention, the plate-like or flake-like filler is contained so that the average orientation angle thereof is 29 degrees or more and the maximum orientation angle thereof is 65 degrees or more. Therefore, the thermal conductivity in the thickness and plane directions of the thermal conductive sheet can be ensured.

Thus, the thermal conductive sheet, the insulating sheet, and the heat dissipating member of the present invention can be used for various applications as a thermal conductive sheet, an insulating sheet, and a heat dissipating member having an excellent thermal conductivity in the thickness and plane directions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of one embodiment of a thermal conductive sheet of the present invention.

FIG. 2 shows an X-ray CT image of the thermal conductive sheet of Example 1.

FIG. 3 shows a histogram of an orientation angle obtained by analyzing the X-ray CT image of the thermal conductive sheet of Example 1.

FIG. 4 shows an X-ray CT image of the thermal conductive sheet of Example 2.

FIG. 5 shows a histogram of an orientation angle obtained by analyzing the X-ray CT image of the thermal conductive sheet of Example 2.

FIG. 6 shows an X-ray CT image of the thermal conductive sheet of Example 4.

FIG. 7 shows a histogram of an orientation angle obtained by analyzing the X-ray CT image of the thermal conductive sheet of Example 4.

FIG. 8 shows an X-ray CT image of the thermal conductive sheet of Comparative Example 1.

FIG. 9 shows a histogram of an orientation angle obtained by analyzing the X-ray CT image of the thermal conductive sheet of Comparative Example 1.

DETAILED DESCRIPTION

A thermal conductive sheet of the present invention contains a resin and a filler.

The resin is a component that is capable of dispersing the filler, that is, a dispersion medium (matrix) in which the filler is dispersed, including, for example, a thermosetting resin and a thermoplastic resin.

Examples of the thermosetting resin include epoxy resin, thermosetting polyimide, phenol resin, urea resin, melamine resin, unsaturated polyester resin, diallyl phthalate resin, silicone resin, and thermosetting urethane resin.

Examples of the thermoplastic resin include polyolefin (for example, polyethylene, polypropylene, and ethylene-propylene copolymer), acrylic resin (for example, polymethyl methacrylate), polyvinyl acetate, ethylene-vinyl acetate copolymer, polyvinyl chloride, polystyrene, polyacrylonitrile, polyamide (nylon (trade mark)), polycarbonate, polyacetal, polyethylene terephthalate, polyphenylene oxide, polyphenylene sulfide, polysulfone, polyether sulfone, poly ether ether ketone, polyallyl sulfone, thermoplastic polyimide, thermoplastic urethane resin, polyamino-bismaleimide, polyamide-imide, polyether-imide, bismaleimide-triazine resin, polymethylpentene, fluorine resin, liquid crystal polymer, olefin-vinyl alcohol copolymer, ionomer, polyarylate, acrylonitrile-ethylene-styrene copolymer, acrylonitrile-butadiene-styrene copolymer, and acrylonitrile-styrene copolymer.

A preferable example of the thermosetting resin is epoxy resin.

The epoxy resin is in a state of liquid, semi-solid, or solid under normal temperature.

To be specific, examples of the epoxy resin include aromatic epoxy resins such as bisphenol epoxy resin (for example, bisphenol A epoxy resin, bisphenol F epoxy resin, bisphenol S epoxy resin, hydrogenated bisphenol A epoxy resin, dimer acid-modified bisphenol epoxy resin, and the like), novolak epoxy resin (for example, phenol novolak epoxy resin, cresol novolak epoxy resin, biphenyl epoxy resin, and the like), naphthalene epoxy resin, fluorene epoxy resin (for example, bisaryl fluorene epoxy resin and the like), and triphenylmethane epoxy resin (for example, trishydroxyphenylmethane epoxy resin and the like); nitrogen-containing-cyclic epoxy resins such as triepoxypropyl isocyanurate (triglycidyl isocyanurate) and hydantoin epoxy resin; aliphatic epoxy resin; alicyclic epoxy resin (for example, dicyclo ring-type epoxy resin and the like); glycidylether epoxy resin; and glycidylamine epoxy resin.

These epoxy resins can be used alone or in combination of two or more.

The epoxy resin has an epoxy equivalent of, for example, 100 to 1000 g/eqiv., or preferably 150 to 700 g/eqiv.

The epoxy resin has a melt viscosity at 80° C. of, for example, 10 to 20000 mPa·s, or preferably 50 to 10000 mPa·s.

The epoxy resin can also be prepared as an epoxy resin composition containing, for example, an epoxy resin, a curing agent, and a curing accelerator.

The curing agent is a latent curing agent (epoxy resin curing agent) that can cure the epoxy resin by heating, and examples thereof include an imidazole compound, an amine compound, an acid anhydride compound, an amide compound, a hydrazide compound, and an imidazoline compound. In addition to the above-described compounds, a phenol compound, a urea compound, and a polysulfide compound can also be used.

Examples of the imidazole compound include 2-phenyl imidazole, 2-methyl imidazole, 2-ethyl-4-methyl imidazole, and 2-phenyl-4-methyl-5-hydroxymethyl imidazole.

Examples of the amine compound include aliphatic polyamines such as ethylene diamine, propylene diamine, diethylene triamine, and triethylene tetramine; and aromatic polyamines such as metha phenylenediamine, diaminodiphenyl methane, and diaminodiphenyl sulfone.

Examples of the acid anhydride compound include phthalic anhydride, maleic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, 4-methyl-hexahydrophthalic anhydride, methyl nadic anhydride, pyromelletic anhydride, dodecenylsuccinic anhydride, dichloro succinic anhydride, benzophenone tetracarboxylic anhydride, and chlorendic anhydride.

Examples of the amide compound include dicyandiamide and polyamide.

An example of the hydrazide compound includes adipic acid dihydrazide.

Examples of the imidazoline compound include methylimidazoline, 2-ethyl-4-methylimidazoline, ethylimidazoline, isopropylimidazoline, 2,4-dimethylimidazoline, phenylimidazoline, undecylimidazoline, heptadecylimidazoline, and 2-phenyl-4-methylimidazoline.

These curing agents can be used alone or in combination of two or more.

A preferable example of the curing agent is an imidazole compound.

Examples of the curing accelerator include tertiary amine compounds such as triethylenediamine and tri-2,4,6-dimethylaminomethylphenol; phosphorus compounds such as triphenylphosphine, tetraphenylphosphoniumtetraphenylborate, and tetra-n-butylphosphonium-o,o-diethylphosphorodithioate; a quaternary ammonium salt compound; an organic metal salt compound; and derivatives thereof. These curing accelerators can be used alone or in combination of two or more.

In the epoxy resin composition, the mixing ratio of the curing agent is, for example, 0.5 to 50 parts by mass, or preferably 1 to 10 parts by mass with respect to 100 parts by mass of the epoxy resin, and the mixing ratio of the curing accelerator is, for example, 0.1 to 10 parts by mass, or preferably 0.2 to 5 parts by mass with respect to 100 parts by mass of the epoxy resin.

The above-described curing agent, and/or the curing accelerator can be prepared and used, as necessary, as a solution, that is, the curing agent and/or the curing accelerator dissolved in a solvent; and/or as a dispersion liquid, that is, the curing agent and/or the curing accelerator dispersed in a solvent.

Examples of the solvent include organic solvents including ketones such as acetone and methyl ethyl ketone, esters such as ethyl acetate, and amides such as N,N-dimethylformamide (DMF). Examples of the solvent also include aqueous solvents including water, and alcohols such as methanol, ethanol, propanol, and isopropanol. A preferable example is an organic solvent, and more preferable examples are ketones and amides.

A preferable example of the thermoplastic resins is polyolefin.

Preferable examples of polyolefin are polyethylene and ethylene-propylene copolymer.

Examples of polyethylene include a low density polyethylene and a high density polyethylene.

Examples of ethylene-propylene copolymer include a random copolymer, a block copolymer, or a graft copolymer of ethylene and propylene.

These polyolefins can be used alone or in combination of two or more.

The polyolefins have a weight average molecular weight and/or a number average molecular weight of, for example, 1000 to 10000.

The polyolefin can be used alone or in combination of two or more.

In the resin, for example, a polymer precursor (for example, a low molecular weight polymer including oligomer), and/or a monomer are contained.

These resins can be used alone or in combination of two or more.

Preferably, the above-described resins are used in combination of two or more.

When two or more resins are used in combination, the resins contain a first resin and a second resin.

The first resin is selected from the above-described resins to be used. A preferable example of the first resin includes the thermosetting resin, and a more preferable example thereof includes the epoxy resin.

These first resins can be used alone or in combination of two or more.

The first resin has a softening temperature (ring and ball test) of, for example, 40 to 120° C., preferably 40 to 110° C., or more preferably 40 to 100° C.

The average particle size of the first resin is defined as the average value of the particle size including the primary particle size and the secondary particle size (particle size of the secondary aggregate) and is, to be specific, for example, 10 to 10000 μm, preferably 10 to 8000 μm, or more preferably 10 to 6000 μm.

The average particle size of the first resin can be obtained by image analysis or the like with a scanning electron microscope (SEM).

As the second resin, a resin having a higher softening temperature (ring and ball test) than that of the first resin is used.

The second resin is selected from the above-described resins to be used. A preferable example of the second resin includes the thermosetting resin having a higher softening temperature (ring and ball test) than that of the first resin, and a more preferable example thereof includes the epoxy resin having a higher softening temperature (ring and ball test) than that of the first resin.

These second resins can be used alone or in combination of two or more.

The softening temperature (ring and ball test (when two or more second resins are used in combination, the total temperature of the softening temperature thereof) of the second resin is higher by, for example 20° C. or more, or preferably 40° C. or more than the softening temperature (ring and ball test (when two or more first resins are used in combination, the total temperature of the softening temperature thereof) of the first resin. To be specific, a difference between the softening temperature of the first resin and that of the second resin is, for example, 40 to 80° C., preferably 45 to 80° C., or more preferably 50 to 80° C.

When the difference between the softening temperature of the first resin and that of the second resin is within the above-described range, an orientation of the filler can be controlled.

The second resin has a softening temperature (ring and ball test) of, for example, 100 to 180° C., preferably 110 to 170° C., or more preferably 110 to 160° C.

The average particle size of the second resin is defined as the average value of the particle size including the primary particle size and the secondary particle size (particle size of the secondary aggregate) and is, to be specific, for example, 10 to 800 μm, preferably 10 to 500 μm, or more preferably 10 to 300 μm.

The average particle size of the second resin can be obtained by image analysis or the like with a scanning electron microscope (SEM).

When the average particle size of the second resin is within the above-described range, an orientation of the filler can be controlled.

The resin has a kinetic viscosity as measured in conformity with the kinetic viscosity test of JIS K 7233 (bubble viscometer method) (1986) (temperature: 25° C.±0.5° C., solvent: butyl carbitol, resin (solid content) concentration: 40 mass %) of, for example, 0.22×10−4 to 2.00×10−4 m2/s, preferably 0.3×10−4 to 1.9×10−4 m2/s, or more preferably 0.4×10−4 to 1.8×10−4 m2/s. The above-described kinetic viscosity can also be set to, for example, 0.22×10−4 to 1.00×10−4 m2/s, preferably 0.3×10−4 to 0.9×10−4 m2/s, or more preferably 0.4×10−4 to 0.8×10−4 m2/s.

In the kinetic viscosity test in conformity with JIS K 7233 (bubble viscometer method) (1986), the kinetic viscosity of the resin is measured by comparing the bubble rising speed of a resin sample with the bubble rising speed of criterion samples (having a known kinetic viscosity), and determining the kinetic viscosity of the criterion sample having a matching rising speed to be the kinetic viscosity of the resin.

An example of the filler includes inorganic particles. Examples of the inorganic particles include carbide, nitride, oxide, hydroxide, metal, and carbonaceous materials.

Examples of the carbide include silicon carbide, boron carbide, aluminum carbide, titanium carbide, and tungsten carbide.

Examples of the nitride include silicon nitride, boron nitride, aluminum nitride, gallium nitride, chromium nitride, tungsten nitride, magnesium nitride, molybdenum nitride, and lithium nitride.

Examples of the oxide include iron oxide, silicon oxide (silica), aluminum oxide (alumina) (including a hydrate of aluminum oxide (boehmite and the like), magnesium oxide, titanium oxide, cerium oxide, and zirconium oxide. Examples of the oxide also include transition metal oxide such as barium titanate and furthermore, indium tin oxide and antimony tin oxide obtained by doping a metal ion thereto.

Examples of the hydroxide include aluminum hydroxide, calcium hydroxide, and magnesium hydroxide.

Examples of the metal include copper, gold, nickel, tin, iron, or alloys thereof Examples of the carbonaceous material include carbon black, graphite, diamond, fullerene, a carbon nanotube, a carbon nanofiber, a nanohorn, a carbon maicrocoil, and a nanocoil.

The inorganic particles may be, in view of fluidity thereof, subjected to a surface treatment by a known method with a silane coupling agent or the like as required.

Examples of the shape of the filler include a plate-like shape, a flake-like shape, a sphere-like shape, and a block-like shape in accordance with the producing method or the crystal structure thereof. In the present invention, the thermal conductive sheet contains at least a plate-like or flake-like filler.

To be specific, examples of the plate-like or flake-like filler include boron nitride (in a plate-like shape) and aluminum oxide monohydrate (boehmite) (in a plate-like shape).

These fillers can be used alone or in combination of two or more.

The average particle size (the length in the longitudinal direction) of the filler as measured by the light scattering method is, for example, 10 to 1000 μm, preferably 10 to 500 μm, or more preferably 10 to 300 μm.

The length in the short-side direction of the filler is, for example, 0.1 to 300 μm, or preferably 0.1 to 100 μm. The aspect ratio (the length in the longitudinal direction/the length in the short-side direction) of the filler is, for example 1/100 to 1/10, or preferably 1/100 to 1/20.

The average particle size as measured by the light scattering method is a volume average particle size measured with a dynamic light scattering type particle size distribution analyzer.

As the filler, a commercially available product or processed goods thereof can be used.

An example of the commercially available product includes a commercially available product of the boron nitride. Examples thereof include the “PT” series (for example, “PT-110”) manufactured by Momentive Performance Materials Inc., and the “SHOBN®UHP” series (for example, “SHOBN®UHP-1”) manufactured by Showa Denko K.K.

The thermal conductive sheet can contain a sphere-like or block-like filler, to be specific, for example, aluminum oxide (in a sphere-like shape) and aluminum hydroxide (in a block-like shape) at an appropriate ratio as required.

The content ratio (the total amount) of the filler is, for example, 10 to 90 parts by mass, preferably 50 to 90 parts by mass, or more preferably 60 to 90 parts by mass with respect to 100 parts by mass of the total amount of the thermal conductive sheet.

When the content ratio of the filler is within the above-described range, an excellent thermal conductivity can be ensured.

FIG. 1 shows a perspective view of one embodiment of a thermal conductive sheet of the present invention.

Next, a method for producing one embodiment of the thermal conductive sheet of the present invention is described with reference to FIG. 1.

In this method, first, the above-described components (a filler 2 and a resin 3) are blended at the above-described mixing ratio and are stirred and mixed, thereby preparing a mixture.

In the stirring and mixing, in order to mix the components efficiently, for example, the solvent can be blended therein with the above-described components, or, for example, the resin (preferably, the thermoplastic resin) can be melted by heating.

Examples of the solvent include the above-described organic solvents. When the above-described curing agent and/or the curing accelerator are prepared as a solvent solution and/or a solvent dispersion liquid, the solvent of the solvent solution and/or the solvent dispersion liquid can also serve as a mixing solvent for the stirring and mixing without adding a solvent during the stirring and mixing. Or, in the stirring and mixing, a solvent can further be added as a mixing solvent.

In the case when the stirring and mixing is performed using a solvent, the solvent is removed after the stirring and mixing.

To remove the solvent, for example, the mixture is allowed to stand at room temperature for 1 to 48 hours; heated at 40 to 100° C. for 0.5 to 3 hours; or heated under a reduced pressure atmosphere of, for example, 0.001 to 50 kPa, at 20 to 60° C., for 0.5 to 3 hours.

When the resin (preferably, the thermoplastic resin) is to be melted by heating, the heating temperature is, for example, a temperature in the neighborhood of or exceeding the softening temperature of the resin, to be specific, 40 to 150° C., or preferably 70 to 150° C.

Next, in this method, the obtained mixture is hot-pressed.

To be specific, as necessary, for example, the mixture is hot-pressed with two releasing films (not shown) sandwiching the mixture, thereby producing a pressed sheet (a thermal conductive sheet 1). Conditions for the hot-pressing are as follows: a temperature of, for example, 50 to 150° C., or preferably 60 to 150° C.; a pressure of, for example, 1 to 100 MPa, or preferably 5 to 50 MPa; and a duration of, for example, 0.1 to 100 minutes, or preferably 1 to 10 minutes.

More preferably, the mixture is hot-pressed under vacuum. The degree of vacuum in the vacuum hot-pressing is, for example, 1 to 100 Pa, or preferably 5 to 50 Pa, and the temperature, the pressure, and the duration are the same as those described above for the hot-pressing.

Although the details are not shown, in the hot-pressing, for example, when the resin 3 contains the first resin and the second resin, first, the temperature of the hot-pressed mixture is increased to the softening temperature of the first resin to soften the first resin. At this time, the second resin having a higher softening temperature than that of the first resin is not softened, so that the shape of the mixture (the thermal conductive sheet) is retained in a hot-pressed state. Thereafter, in the hot-pressing, the temperature of the mixture (the thermal conductive sheet) is increased to the softening temperature of the second resin to soften the second resin. By such hot-pressing, an orientation angle α (described later) of the plate-like or flake-like filler 2 can be adjusted.

When the resin 3 is the thermosetting resin, the thermal conductive sheet 1 can be cured by heating. To cure the thermal conductive sheet 1, the thermal conductive sheet 1 is heated at the softening temperature of the second resin or more. To heat the thermal conductive sheet 1, for example, the above-described hot-pressing or a dryer is used. Conditions for the curing by heat are as follows: a temperature of, for example, 60 to 250° C., or preferably 80 to 200° C. and a pressure of, for example, 100 MPa or less, or preferably 50 MPa or less.

In this method, by one time hot-pressing, the temperature can be increased to the softening temperature of the second resin or more. In addition, by one time hot-pressing, the thermal conductive sheet 1 can be cured.

The thickness of the obtained thermal conductive sheet 1 is, for example, 1 mm or less, or preferably 0.8 mm or less, and usually, for example, 0.05 mm or more, or preferably 0.1 mm or more.

In the thermal conductive sheet 1 thus obtained, as shown in FIG. 1 and its partially enlarged schematic view, the filler 2 (the plate-like or flake-like filler 2) is contained such that a longitudinal direction LD thereof forms a predetermined angle (the orientation angle α) with respect to a plane (surface) direction SD that crosses (is perpendicular to) a thickness direction TD of the thermal conductive sheet 1.

The calculated average (an average orientation angle α1) of the orientation angle α formed between the longitudinal direction LD of the filler 2 and the plane direction SD of the thermal conductive sheet 1 is 29 degrees or more, preferably 29.5 degrees or more, or more preferably 30 degrees or more, and usually less than 45 degrees.

The maximum value (a maximum orientation angle α2) of the orientation angle α formed between the longitudinal direction LD of the filler 2 and the plane direction SD of the thermal conductive sheet 1 is 65 degrees or more, preferably 70 degrees or more, or more preferably 75 degrees or more, and usually less than 90 degrees.

When the average orientation angle α1 of the filler 2 with respect to the plane direction SD of the thermal conductive sheet 1 is within the above-described range and the maximum orientation angle α2 is within the above-described range, the thermal conductivity in both the thickness and plane directions can be successfully ensured in the thermal conductive sheet.

The orientation angle α of the filler 2 with respect to the thermal conductive sheet 1 is obtained as follows: the thermal conductive sheet 1 is cut out; sequentially transmitted images are photographed with an X-ray CT at an angle of 0 to 180 degrees; cross-sectional images are produced by reconstruction based on the entire transmitted images; the obtained images are analyzed to produce three-dimensional reconstructed images; and the calculation is performed based on the obtained images.

The thermal conductivity in the plane direction SD of the thermal conductive sheet 1 obtained in this way is 30 to 60 W/m·K, preferably 35 to 60 W/m·K, or more preferably 40 to 60 W/m·K.

The thermal conductivity in the plane direction SD of the thermal conductive sheet 1 is measured by a pulse heating method. In the pulse heating method, the xenonflash analyzer “LFA-447” (manufactured by Erich NETZSCH GmbH & Co. Holding KG) is used.

The thermal conductivity in the thickness direction TD of the thermal conductive sheet 1 is 5 to 15 W/m·K, preferably 6 to 15 W/m·K, or more preferably 7 to 15 W/m·K.

The thermal conductivity in the thickness direction TD of the thermal conductive sheet 1 is measured by a pulse heating method, a laser flash method, or a TWA method. In the pulse heating method, the above-described device is used, in the laser flash method, “TC-9000” (manufactured by Ulvac, Inc.) is used, and in the TWA method, “ai-Phase mobile” (manufactured by ai-Phase Co., Ltd) is used.

In the thermal conductive sheet 1, the plate-like or flake-like filler 2 is contained so that the average orientation angle is 29 degrees or more and the maximum orientation angle is 65 degrees or more. Therefore, the thermal conductivity in the thickness direction TD and the plane direction SD of the thermal conductive sheet 1 can be ensured.

Thus, the thermal conductive sheet 1 has an excellent thermal conductivity in the thickness and plane directions. And in power electronics technology which uses semiconductor elements to convert and control electric power used in, for example, hybrid devices, high-brightness LED devices, and electromagnetic induction heating devices, the thermal conductive sheet 1 can be used as a heat dissipating member for converting a high current to heat or an insulating sheet. To be specific, for example, the thermal conductive sheet 1 can be preferably used as a heat dissipating member disposed near a semiconductor element used in a light emitting diode device, an imaging element used in an image-taking device, a back light of a liquid crystal display device, and furthermore, other various power modules for dissipating heat from the member, or as an insulating sheet disposed between the members for electrically insulating the members.

To be specific, for example, the thermal conductive sheet 1 is preferably used as a heat spreader or a heat sink of the light emitting diode device; a heat dissipating sheet attached to a casing of the liquid crystal display device or the image-taking device; or an encapsulating material for encapsulating an electronic circuit board.

While the present invention will be described hereinafter in further detail with reference to Examples, the present invention is not limited to these Examples.

6.71 g of PT-110 (trade name, plate-like boron nitride particles, average particle size (light scattering method) of 35 to 60 μm, manufactured by Momentive Performance Materials Inc.) was prepared.

1.2 g of EPPN-501HY (trade name, phenol novolak epoxy resin, solid, epoxy equivalent of 163 to 175 g/eqiv., softening temperature (ring and ball test) of 61° C., manufactured by Nippon Steel Chemical Co., Ltd.) and 0.3 g of YSLV-120TE (trade name, bisphenol epoxy resin, solid, average particle size (analysis of SEM image) of about 100 μm (classified with a sieve after pulverized), epoxy equivalent of 250 g/eqiv., softening temperature (ring and ball test) of 120° C., manufactured by Nippon Steel Chemical Co., Ltd.) were dissolved in 2 g of solvent (acetone). Next, after 0.05 g of imidazole curing catalyst (curing agent) (2P4 MHZ-PW, manufactured by Shikoku Chemicals Corporation) was added thereto, the above-described PT-110 was mixed and then dried at 60° C. for one hour to remove the solvent. Subsequently, the obtained powder was pressed and retained at a pressure of 10 MPa for 10 minutes with a pressing machine at 150° C. to cure a resin, so that a thermal conductive sheet was obtained.

The SEM images were analyzed in accordance with the following conditions (hereinafter the same).

That is, an SEM sample, which was embedded in an epoxy resin and then its cross-sectional surface was polished so that the surface was ion milled to be shaped, was used.

100 parts of the obtained sample were arbitrarily measured and then the obtained 100 pieces of the SEM images were connected with an Image Pro software. And, resin particles were derived for image analysis with a WinROOF software. Next, the particle size distribution thereof was measured to obtain the average particle size.

A thermal conductive sheet was obtained in the same manner as in Example 1, except that 1.2 g of jER1001 (trade name, bisphenol A epoxy resin, solid, epoxy equivalent of 450 to 500 g/eqiv., softening temperature (ring and ball test) of 64° C., manufactured by Mitsubishi Chemical Corporation) was used instead of EPPN-501HY.

A thermal conductive sheet was obtained in the same manner as in Example 1, except that 1.2 g of jER1002 (trade name, bisphenol A epoxy resin, solid, epoxy equivalent of 600 to 700 g/eqiv., softening temperature (ring and ball test) of 74° C., manufactured by Mitsubishi Chemical Corporation) was used instead of EPPN-501HY, and 0.5 g of YSLV-80XY (trade name, bisphenol epoxy resin, crystalline epoxy resin, solid, average particle size (analysis of SEM image) of about 100 μm (classified with a sieve after pulverized), epoxy equivalent of 200 g/eqiv., softening temperature (ring and ball test) of 140° C., manufactured by Nippon Steel Chemical Co., Ltd.) was used instead of YSLV-120TE.

A thermal conductive sheet was obtained in the same manner as in Example 1, except that 0.5 g of EPPN-501HY and 0.5 g of jER1002 were used instead of 1.2 g of EPPN-501HY, and 0.3 g of YSLV-80XY was used instead of 0.3 g of YSLV-120TE.

A thermal conductive sheet was obtained in the same manner as in Example 1, except that 1.5 g of EPPN-501HY was used instead of 1.2 g thereof, and YSLV-120TE was not used.

A thermal conductive sheet was obtained in the same manner as in Example 1, except that 0.3 g of jER1055 (trade name, bisphenol A epoxy resin, pellet state, average size of about 1 cm, softening temperature (ring and ball test) of 145° C., manufactured by Mitsubishi Chemical Corporation) was used instead of 0.3 g of YSLV-120TE.

A thermal conductive sheet was obtained in the same manner as in Example 1, except that 1.2 g of jER1001 was used instead of 1.2 g of EPPN-501HY, and 0.3 g of jER1003 (trade name, bisphenol A epoxy resin, solid, average particle size (analysis of SEM image) of about 100 μm (classified with a sieve after pulverized), epoxy equivalent of 660 to 770 g/eqiv., softening temperature (ring and ball test) of 80° C., manufactured by Nippon Steel Chemical Co., Ltd.) was used instead of 0.3 g of YSLV-120TE.

Evaluation

The thermal conductivity in the thickness direction TD and the thermal conductivity in the plane direction SD of the thermal conductive sheets obtained in Examples and Comparative Examples were measured by a pulse heating method using a xenon flash analyzer “LFA-447” (manufactured by Erich NETZSCH GmbH & Co. Holding KG). The results are shown in Table 1.

The thermal conductive sheets obtained in Examples and Comparative Examples were cut out to have a width of 2 mm and the cut-out pieces were fixed onto the specimen stage. Then, sequentially transmitted images were photographed with an X-ray CT every 0.2 degrees over an angle of 0 to 180 degrees. Next, cross-sectional images were produced by reconstruction based on the entire transmitted images and the obtained images were analyzed to produce three-dimensional reconstructed images. Thus, the orientation angles (the average orientation angle and the maximum orientation angle) were measured. As an analysis software, ImageJ (developed at the National Institutes of Health (NIH)) was used. The results are shown in Table 1.

An X-ray CT image of the thermal conductive sheet of Example 1 is shown in FIG. 2. A histogram of an orientation angle obtained by analyzing the X-ray CT image is shown in FIG. 3.

An X-ray CT image of the thermal conductive sheet of Example 2 is shown in FIG. 4. A histogram of an orientation angle obtained by analyzing the X-ray CT image is shown in FIG. 5.

An X-ray CT image of the thermal conductive sheet of Example 4 is shown in FIG. 6. A histogram of an orientation angle obtained by analyzing the X-ray CT image is shown in FIG. 7.

An X-ray CT image of the thermal conductive sheet of Comparative Example 1 is shown in FIG. 8. A histogram of an orientation angle obtained by analyzing the X-ray CT image is shown in FIG. 9.

TABLE 1 Filler Content Thermal (Parts by Conductivity Orientation Angle α The First Resin The Second Resin Mass/100 (W/m · K) (degrees) of Filler Example Softening Softening Difference of Parts by Mass Plane Maximum No./ Tem-

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