Patent Description:
Until now, insulating materials having high thermal conductivity, which contain this kind of insulating filler have been utilized in cooling members for heating elements comprising semiconductor chips, transistors, lithium ion secondary batteries or LED light sources of communication devices and in-vehicle electronic devices, in cooling members for stators built in housings of motors, in cooling members for power conversion devices built in cases of inverters, or in heat radiating members for releasing heat generated in sliding portions or rotating portions of actuators, etc..

Magnesium oxide (MgO) powder, which is one of the materials of insulating fillers, is excellent in thermal conductivity and electric insulation, etc., is inexpensive as compared with aluminum nitride powder and boron nitride powder, has a light specific gravity, and has low Mohs hardness so that it is excellent in handling. Due to such characteristics, a magnesium oxide powder is suitable as an insulating filler having thermal conductivity. However, magnesium oxide has a property that it is hydrolyzed by easily reacting with water to change into magnesium hydroxide.

Also, nitride-based inorganic powders such as aluminum nitride (AlN) powder, boron nitride (BN) powder, silicon nitride (Si<NUM>N<NUM>) powder, etc., which are other materials of insulating fillers, have a light specific gravity, and are excellent in thermal conductivity and electric insulation, etc. Because of such characteristics, the nitride-based inorganic powder is suitable as an insulating filler having thermal conductivity. However, a nitride, including an aluminum nitride powder, has high reactivity with water, so that it has properties that when it comes into contacted with water, it is hydrolyzed to be decomposed to a hydrate such as hydrated aluminum, etc., while generating ammonia.

Hydrolysis of these magnesium oxide (MgO) or nitrides (AlN, BN, Si<NUM>N<NUM>) proceeds by the moisture in the atmosphere, so that when a magnesium oxide powder or a nitride-based inorganic powder is used as an insulating filler under an atmosphere of high temperature and high humidity, there is a problem that the quality as an insulating filler is markedly lowered. This is the same in the insulating material comprising a resin molded body containing a magnesium oxide powder or a nitride-based inorganic powder in a resin, and there is a fear that magnesium oxide (MgO) or nitrides (AlN, BN, Si<NUM>N<NUM>) is/are reacted with a water content, etc., in the resin, in addition to the moisture in the atmosphere, to deteriorate the quality of the insulating material comprising the resin molded body.

In order to solve this problem, there has been known a method of chemically modifying the surface of magnesium oxide (MgO) powder or aluminum nitride (AlN) powder to improve the reactivity with water and to heighten water resistance. As an example thereof, there is disclosed a phosphorus-containing coated magnesium oxide powder which comprises a coated magnesium oxide powder having a coating layer comprising a composite oxide on the surface thereof and further a coating layer comprising a magnesium phosphate-based compound on at least a part of the surface thereof, and a content of the magnesium phosphate-based compound based on the coated magnesium oxide powder is <NUM> to <NUM>% by mass in terms of phosphorus (for example, see Patent Document <NUM> (Claim <NUM>).

Also, as other examples, there is disclosed aluminum nitride powder having excellent water resistance in which aluminum nitride powder having an aluminum oxide coating layer or a phosphoric acid-based coating layer on the surface thereof is treated with an organic silicon-based coupling agent, an organic phosphoric acid-based coupling agent, or an organic titanium-based coupling agent by adding an amount of <NUM> to <NUM> parts by weight based on <NUM> parts by weight of the aluminum nitride powder (for example, see Patent Document <NUM> (Claim <NUM>).

On the other hand, there is disclosed a high thermal conductive insulating material in which aggregates formed by aggregating a relatively small-sized filler around a relatively large-sized filler are dispersed in a polymer base material (for example, Patent Document <NUM> (see Claims <NUM> and <NUM>, paragraphs [<NUM>], [<NUM>] to [<NUM>], [<NUM>], <FIG>). In this high thermal conductive insulating material, the polymer base material comprises any one kind of silicone, Nylon, PP (polypropylene), PPS (polyphenylene sulfide) and LCP (liquid crystal polymer), and the filler comprises any one kind of silicon carbide, silicon nitride, boron nitride, silica, aluminum oxide, aluminum nitride and magnesium oxide or a mixture thereof. Also, when the relatively large-sized filler is spherical or substantially spherical, the particle size can be formed to be about <NUM> to <NUM>, and the particle size of the small-sized filler can be formed to be about <NUM> to <NUM>. Further, the aggregates described in Patent Document <NUM> can be formed by, after forming a coarse filler, a part thereof is pulverized to be subdivided and classified, etc., to form a fine filler having a predetermined size, and these coarse filler and fine filler are mixed.

In the high thermal conductive insulating material constituted by the above, heat transfer efficiency between the polymer and the filler can be heightened without increasing the amount of the filler to be filled or increasing the size of the filler. This means that, as compared with the conventional insulating material in which the coarse filler having uniform size is dispersed in the polymer, a distance between the fillers necessary for obtaining an equal amount of heat transferred can be elongated and an amount of the filler to be filled can be reduced. Since the amount of the filler to be filled can be reduced, the moldability can be heightened. In addition, the aggregates have a grain structure in which small-sized fillers randomly protrude from the outer periphery of the large-sized fillers, so that the heat transfer direction becomes various direction (isotropic) without having an arbitrary one direction (anisotropic). Further, small-sized fillers can exhibit an anchor effect by embedding in the polymer, and as a result, interfacial strength between the polymer and the filler aggregates is heightened, so that mechanical strength of the insulating material can be heightened.

<CIT> discloses an insulating sheet which is used for bonding a heat conductor having a thermal conductivity of <NUM> W/mK or higher to an electricylly conductive layer, the insulting sheet comprising, besides various polymers, a filler. The filler may be surface-hydrophobic fumed silica, spherical alumina, boron nitride, aluminum nitride or silicon carbide.

<CIT> discloses a curable composition for an insulating material comprising, inter alia, an inorganic filler or rubber fine particles. The inorganic filler may be silica, alumina, silicon nitride, hydrotalcite, and kaolin. The particle size of the inorganic filler is preferably <NUM> to <NUM>. Surface-hydrophobized fumed silica is a specifically disclosed example of an inorganic filler.

<CIT> discloses an ethylene propylene terpolymer insulatig material for high-volatge direct-current cabale accessory comprising an inorganic nanopowder. The inorganic nanopowder has a primary particle size of <NUM> to <NUM> and a specific surface area of > <NUM><NUM>/g. The inorganic nanopowder is selected from magnesium oxide, zinc oxide and aluminum oxide.

Although both the phosphorus-containing coated magnesium oxide (MgO) powder shown in Patent Document <NUM> and the aluminum nitride (AlN) powder shown in Patent Document <NUM> have improved water resistance with a certain extent, water resistance under high temperature and high humidity conditions cannot be said to be sufficient, and improvement in water resistance has been required to both powders. Also, in an insulating material comprising a resin molded body containing such a powder in a resin, and the high thermal conductive insulating material shown in Patent Document <NUM>, when the insulating material is immersed in water for a long time, there is a problem that a rate of change (absolute value) of the dielectric breakdown voltage before and after immersion is large.

An object of the present invention is to provide an insulating filler in which volume resistivity due to moisture absorption is difficultly lowered and a method for producing the same. Another object of the present invention is to provide an insulating material in which a rate of change (absolute value) of the dielectric breakdown voltage before and after immersion is small and a method for producing the same.

A first aspect of the present invention is directed to an insulating filler which comprises a mixed powder in which on a surface of a magnesium oxide powder and/or a nitride-based inorganic powder having an average primary particle size D<NUM>, a hydrophobic fumed oxide powder having an average primary particle size D<NUM> which is smaller than the average primary particle size D<NUM> is attached, wherein a ratio D<NUM>/D<NUM> of the average primary particle size D<NUM> to the average primary particle size D<NUM> is <NUM> × <NUM>-<NUM> to <NUM> × <NUM>-<NUM>, wherein volume resistivity of the mixed powder is <NUM> × <NUM><NUM> Ω·m or more, wherein a content ratio of the hydrophobic fumed oxide powder is <NUM>% by mass to <NUM>% by mass when the mixed powder is made <NUM>% by mass, wherein the nitride-based inorganic powder is one or more kinds selected from the group consisting of an aluminum nitride powder, a boron nitride powder and a silicon nitride powder, and
wherein the hydrophobic fumed oxide is hydrophobic fumed silica, hydrophobic fumed alumina or hydrophobic fumed titania.

A second aspect of the present invention is an insulating filler which is an invention based on the first aspect, wherein when water vapor is absorbed under a constant temperature and constant humidity of a temperature of <NUM> and a relative humidity of <NUM>% for <NUM> days, a lowering rate (%) of volume resistivity by moisture absorption calculated by the following equation (<NUM>) is less than +<NUM>%.

A third aspect of the present invention is a method for producing an insulating filler according to the first aspect by mixing a magnesium oxide powder and/or a nitride-based inorganic powder as defined in claim <NUM> having an average primary particle size D<NUM>, and a hydrophobic fumed oxide powder as defined in claim <NUM> having an average primary particle size D<NUM> which is smaller than the average primary particle size D<NUM> under room temperature by a dry method.

A fourth aspect of the present invention is an insulating material comprising a resin molded body, wherein the insulating filler described in any of the first and second aspects is contained in the resin molded body, and wherein when the resin molded body is immersed in water at a temperature of <NUM> for <NUM> hours, a rate of change (absolute value) of the dielectric breakdown voltage before and after immersion calculated by the following equation (<NUM>) is <NUM>% or less.

A fifth aspect of the present invention is a method for producing an insulating material which comprises mixing the insulating filler described in any of the first and second aspects and a resin under room temperature, and then, molding to produce a resin molded body having a rate of change (absolute value) of the dielectric breakdown voltage before and after immersion calculated by the above-mentioned equation (<NUM>) of <NUM>% or less.

The insulating filler of the first aspect of the present invention comprises a mixed powder in which, on the surface of a magnesium oxide powder and/or a nitride-based inorganic powder as defined in claim <NUM>, a hydrophobic fumed oxide powder as defined in claim <NUM> having an average primary particle size D<NUM> smaller than an average primary particle size D<NUM> of the powder is attached, also a ratio D<NUM>/D<NUM> is <NUM> × <NUM>-<NUM> to <NUM> × <NUM>-<NUM>, and further a content ratio of the hydrophobic fumed oxide powder is <NUM>% by mass to <NUM>% by mass in the mixed powder, so that the volume resistivity of the mixed powder is <NUM> × <NUM><NUM> Ω·m or more, and even when the hydrophobic fumed oxide powder is in a high humidity atmosphere, a moisture absorption amount of the magnesium oxide powder and/or the nitride-based inorganic powder can be reduced. Therefore, the insulating filler has a merit that the volume resistivity due to moisture absorption is difficultly lowered.

In the insulating filler of the first aspect of the present invention, the nitride-based inorganic powder is an aluminum nitride powder, a boron nitride powder or a silicon nitride powder, so that the insulating filler has, in addition to high water resistance, high thermal conductivity and high electric insulation.

In the insulating filler of the first aspect of the present invention, the hydrophobic fumed oxide is hydrophobic fumed silica, hydrophobic fumed alumina or hydrophobic fumed titania, so that the insulating filler has higher water resistance.

In the insulating filler of the second aspect of the present invention, the lowering rate (%) of volume resistivity by moisture absorption calculated by the above-mentioned equation (<NUM>) is less than +<NUM>%, so that the rate of change in the volume resistivity due to moisture absorption is a little.

In the method for producing the insulating filler of the third aspect of the present invention, owing to production of an insulating filler by mixing a magnesium oxide powder and/or a nitride-based inorganic powder as defined in claim <NUM> having an average primary particle size D<NUM>, and a hydrophobic fumed oxide powder as defined in claim <NUM> having an average primary particle size D<NUM> which is smaller than the above-mentioned average primary particle size D<NUM> under room temperature by the dry method, the hydrophobic fumed oxide powder is attached on the surface of the magnesium oxide powder and/or the nitride-based inorganic powder with a relatively simple method. The insulating filler produced by the method has a merit that volume resistivity due to moisture absorption is difficultly lowered.

In the insulating material of the fourth aspect of the present invention, the insulating filler described in any of the first and second aspects is contained in the resin molded body, so that the resin molded body has a merit that the rate of change (absolute value) of the dielectric breakdown voltage before and after immersion is a little.

In the method for producing the insulating material of the fifth aspect of the present invention, the insulating material comprising the resin molded body is produced by mixing the insulating filler described in any of the first and second aspects and a resin under room temperature and then molding, so that the produced insulating material has a merit that the rate of change (absolute value) of the dielectric breakdown voltage before and after immersion is a little.

Next, a mode for carrying out the present invention will be explained with reference to the drawings. The insulating filler of the present embodiment comprises a mixed powder in which, on the surface of a magnesium oxide powder and/or a nitride-based inorganic powder as defined in claim <NUM> (hereinafter sometimes referred to as a large-diameter powder and/or nitride-based inorganic powder) having an average primary particle size D<NUM>, a hydrophobic fumed oxide powder as defined in claim <NUM> (hereinafter sometimes referred to as a small-diameter powder and/or hydrophobic fumed oxide powder) having an average primary particle size D<NUM> which is smaller than the average primary particle size D<NUM> is attached. And, a ratio D<NUM>/D<NUM> of the average primary particle size D<NUM> to the average primary particle size D<NUM> is <NUM> × <NUM>-<NUM> to <NUM> × <NUM>-<NUM>, and preferably <NUM> × <NUM>-<NUM> to <NUM> × <NUM>-<NUM>. In addition, the volume resistivity of the above-mentioned mixed powder is <NUM> × <NUM><NUM> Ω·m or more, and preferably <NUM> × <NUM><NUM> Ω·m to <NUM> × <NUM><NUM> Ω·m. Further, a content ratio of the hydrophobic fumed oxide powder is <NUM>% by mass to <NUM>% by mass, and further preferably <NUM>% by mass to <NUM>% by mass when the mixed powder is made <NUM>% by mass.

Incidentally, as the large-diameter powder having an average primary particle size D<NUM>, there may be exemplified by a magnesium oxide powder and/or a nitride-based inorganic powder as defined in claim <NUM> capable of causing chemical change by moisture absorption. As the nitride-based inorganic powder, an aluminum nitride powder, a boron nitride powder and a silicon nitride powder, etc.,can be used. The large-diameter powder is one or more kinds of powders selected from the group consisting of a magnesium oxide powder, an aluminum nitride powder, a boron nitride powder and a silicon nitride powder.

As the hydrophobic fumed oxide powder as defined in claim <NUM> which is a small-diameter powder having an average primary particle size D<NUM>, hydrophobic fumed silica, hydrophobic fumed alumina or hydrophobic fumed titania can be used. The mixed powder of the present embodiment is not limited to a powder in which a single kind of a large-diameter powder and a single kind of a small-diameter powder are mixed, but also a powder in which plural kinds of large-diameter powders and a single kind of a small-diameter powder are mixed, or a powder in which a single kind of a large-diameter powder and plural kinds of small-diameter powders are mixed, etc. The combination of these powders is selected according to the use of the insulating filler and the required water resistance, thermal conductivity, electric insulation, etc..

The average primary particle size D<NUM> of the hydrophobic fumed oxide powder is measured by the image analysis method of a transmission electron microscope (TEM), and the average primary particle size D<NUM> of the magnesium oxide powder or the nitride-based inorganic powder is the nominal value of each powder manufacturer. Also, the volume resistivity of the mixed powder is measured by using a high resistance· resistivity meter "Hiresta-UX" (manufactured by Mitsubishi Chemical Analytech Co. : model number "MCP-HT800") and a powder resistance measurement system (manufactured by Mitsubishi Chemical Analytech Co. : model number "MCP-PD-<NUM>").

Here, the reason why the ratio D<NUM>/D<NUM> of the average primary particle size D<NUM> of the hydrophobic fumed oxide powder and the average primary particle size D<NUM> of the magnesium oxide powder and/or the nitride-based inorganic powder is set in the range of <NUM> × <NUM>-<NUM> to <NUM> × <NUM>-<NUM> is that if it is less than <NUM> × <NUM>-<NUM>, water resistance of the insulating filler is not sufficient, while if it exceeds <NUM> × <NUM>-<NUM>, electric insulation cannot be improved. Also, the reason why the volume resistivity of the above-mentioned mixed powder is limited to <NUM> × <NUM><NUM> Ω·m or more is that if it is less than <NUM> × <NUM><NUM> Ω·m, electric insulation ability is insufficient. Further, the reason why the preferred upper value of the volume resistivity of the above-mentioned mixed powder is set to be <NUM> × <NUM><NUM> Ω·m is that if it exceeds <NUM> × <NUM><NUM> Ω·m, the electrical insulation property becomes more than necessary and cost effectiveness is lowered. Moreover, the reason why the preferred content ratio of the hydrophobic fumed oxide powder is limited within the range of <NUM>% by mass to <NUM>% by mass in the mixed powder, if it is less than <NUM>% by mass, water resistance and electrical insulation property of the insulating filler are difficultly improved, while if it exceeds <NUM>% by mass, sufficient thermal conductivity and electrical insulation property are difficultly obtained.

Incidentally, the average primary particle size D<NUM> of the hydrophobic fumed oxide powder is preferably <NUM> to <NUM>, and further preferably <NUM> to <NUM>. Also, the average primary particle size D<NUM> of the magnesium oxide powder and/or the nitride-based inorganic powder is preferably <NUM> to <NUM>, and further preferably <NUM> to <NUM>. Here, the reason why the preferred range of the average primary particle size D<NUM> of the hydrophobic fumed oxide powder is limited to the range of <NUM> to <NUM> is that if it is less than <NUM>, water resistance of the insulating filler is difficult to be improved sufficiently, while if it exceeds <NUM>, electric insulation of the insulating filler is difficult to be improved sufficiently. In addition, the reason why the preferred range of the average primary particle size D<NUM> of the magnesium oxide powder and/or nitride-based inorganic powder is limited to the range of <NUM> to <NUM> is that if it is less than <NUM>, sufficient thermal conductivity and electrical insulation property can be difficultly obtained, while if it exceeds <NUM>, it becomes difficult to achieve closest packing.

In the insulating filler constituted as mentioned above, if the ratio D<NUM>/D<NUM> of the average primary particle size D<NUM> of the hydrophobic fumed oxide powder and the average primary particle size D<NUM> of the magnesium oxide powder and/or the nitride-based inorganic powder is in the above-mentioned range, and the content ratio of the hydrophobic fumed oxide powder is in the above-mentioned range, for example, as shown in the schematic diagram of <FIG>, the small-diameter hydrophobic fumed oxide powder is densely packed between the large-diameter magnesium oxide powder and/or nitride-based inorganic powder, the small-diameter hydrophobic fumed oxide powder is attached on the surface of the large-diameter magnesium oxide powder and/or nitride-based inorganic powder and the small-diameter hydrophobic fumed oxide powder is connected in a bead shape in the voids between the large-diameter magnesium oxide powder and/or nitride-based inorganic powder to form a three-dimensional network structure by the small-diameter hydrophobic fumed oxide powder in the voids. As a result, the volume resistivity of the mixed powder of the small-diameter filler and the large-diameter filler can be dramatically heightened to <NUM> × <NUM><NUM> Ω·m or more without impairing thermal conductivity and simultaneously water resistance of the insulating filler can be improved. As a result, high thermal conductivity and high electric insulation of the insulating filler, which are contrary to each other, can be both achieved at a high level, and the insulating filler can be made highly water resistant. In <FIG>, it is shown a mixed powder in which one of the five mixed powders shown in <FIG> is photographed by a scanning electron microscope (SEM). Further, <FIG> is an enlarged SEM photographic diagram of the surface of the mixed powder shown in <FIG>, and in <FIG>, it is clearly shown that a large number of the small-diameter powders are densely coated on the surface of the large-diameter powder.

As an example of the fumed oxide powder, the fumed silica powder is produced by injecting a mixed gas of SiCl<NUM>, H<NUM> and O<NUM> as vaporizing raw materials from a burner. As shown in <FIG>, in this producing process, aggregated particles, which are the smallest particle form in which the primary particles of silica are calcinated, are firstly formed, and then, as shown in <FIG>, aggregated particles are gathered by weak interaction of hydrogen bonding or Van der Waals force to form gathered bulk particles. By subjecting this fumed oxide powder to hydrophobic surface treatment, a hydrophobic fumed oxide powder can be obtained. This hydrophobic fumed oxide powder can be obtained, for example, in the case of a hydrophobic fumed silica powder, by chemically modifying the surface of the fumed silica powder with a silane coupling agent or silicone oil. Incidentally, the "hydrophobicity" of the hydrophobic fumed oxide powder means a property having a hydrophobization rate of preferably <NUM>% or more, and further preferably <NUM>% or more. If the hydrophobization rate of less than <NUM>%, hydrophobicity of the fumed oxide powder is lowered, and water resistance of the insulating filler is lowered.

Further, the surface of each powder of the hydrophobic fumed silica, hydrophobic fumed alumina or hydrophobic fumed titania is modified by a (R<NUM>)X(R<NUM>)Y(R<NUM>)ZSi- group (R<NUM>, R<NUM> and R<NUM> are alkyl groups, and X, Y and Z are integers of <NUM> to <NUM>. Specifically, as these hydrophobic fumed oxides, there may be mentioned hydrophobic fumed silica (for example, "RY50": hydrophobic fumed silica (SiO<NUM>) having an average primary particle size of <NUM> available from NIPPON AEROSIL CO. ), hydrophobic fumed alumina (for example, "C805": hydrophobic fumed alumina (Al<NUM>O<NUM>) having an average primary particle size of <NUM> available from Evonik Industries AG), hydrophobic fumed titania (for example, "T805": hydrophobic fumed titania (TiO<NUM>) having an average primary particle size of <NUM> available from Evonik Industries AG), etc..

The insulating filler of the present embodiment is prepared by mixing the large-diameter magnesium oxide powder and/or nitride-based inorganic powder as defined in claim <NUM> and the small-diameter hydrophobic fumed oxide powder as defined in claim <NUM> under room temperature by a dry method. For this dry mixing, it is preferable to use a rotating and revolving mixer (manufactured by THINKY CORPORATION: model number "ARE-<NUM>"), and at the laboratory level, a planetary stirring mixer (manufactured by THINKY CORPORATION: Awatori Rentarou "R250"), etc., can be used.

This insulating filler and a resin, etc., are mixed and stirred by the above-mentioned rotating and revolving mixer to prepare a resin composition. As the resin, etc., there may be mentioned a mixture of an unsaturated polyester resin and a curing agent, a mixture of an epoxy resin and a curing agent, a mixture of EPDM rubber (ethylene-propylene-diene ternary copolymer), a silicone resin and a curing agent, etc. A mixing ratio of the above-mentioned resin, etc., is preferably <NUM>% by volume to <NUM>% by volume, and further preferably <NUM>% by volume to <NUM>% by volume when the total amount of the insulating filler and the resin, etc., is <NUM>% by volume. Here, the reason why the preferred mixing ratio of the resin, etc., is limited to the range of <NUM>% by volume to <NUM>% by volume is that if it is less than <NUM>% by volume, the resin is difficultly molded, while if it exceeds <NUM>% by volume, sufficient water resistance, thermal conductivity and electric insulation can be difficultly imparted to the insulating material which is a resin molded body.

It is preferable that the above-mentioned resin composition is placed in a mold, and the resin is cured using a heat press (for example, manufactured by Kodaira Seisakusho Co. : model number "PY15-EA") at a temperature of <NUM> to <NUM> by applying a pressure of <NUM> MPa to <NUM> MPa (<NUM>/cm<NUM> to <NUM>/cm<NUM>) and holding it for <NUM> minutes to <NUM> minutes to prepare a resin molded body (high thermal conductive insulating material) having high thermal conductivity, high insulating property and high water resistance. Here, the reason why the preferable temperature of the heat press is limited to the range of <NUM> to <NUM> is that if it is lower than <NUM>, curing failure is likely to occur, while if it exceeds <NUM>, the resin is likely to be deteriorated by heat. Also, the reason why the preferable pressure of the heat press is limited to the range of <NUM> MPa to <NUM> MPa (<NUM>/cm<NUM> to <NUM>/cm<NUM>) is that if it is less than <NUM> MPa (<NUM>/cm<NUM>), air remains inside thereof and sufficient thermal conductivity is difficultly obtained, while if it exceeds <NUM> MPa (<NUM>/cm<NUM>), the load on the compression machine tends to be excessive. Further, the reason why the preferable holding time of the heat press is limited to the range of <NUM> minutes to <NUM> minutes is that if it is shorter than <NUM> minutes, curing tends to be insufficient, while if it exceeds <NUM> minutes, productivity tends to decrease.

The dielectric breakdown voltage of the resin molded body (high thermal conductive insulating material) is preferably <NUM> kV/mm or more, and further preferably <NUM> kV/mm or more. Here, the reason why the preferable dielectric breakdown voltage of the resin molded body (high thermal conductive insulating material) is limited to <NUM> kV/mm or more is that if it is less than <NUM> kV/mm, it may not have sufficient electric insulation depending on the place of use. Incidentally, the dielectric breakdown voltage of the resin molded body (high thermal conductive insulating material) is measured by using an ultra-high voltage withstanding voltage tester "<NUM> series" (manufactured by Keisoku Giken Co. : model number "<NUM>"). Further, the above-mentioned resin molded body (high thermal conductive insulating material) can be utilized for cooling members of heating elements comprising semiconductor chips or transistors of in-vehicle electronic devices, cooling members of stators built in housings of motors, cooling members of power conversion devices built in cases of inverters, a heat dissipation member of heat generated in a sliding portion or a rotating portion of an actuator, etc..

Next, Examples the present invention will by explained in detail with Comparative Examples.

First, a spherical magnesium oxide powder (available from Denka Co. : model number "DMG-<NUM>" (average primary particle size <NUM>)) was prepared as a large-diameter powder, and hydrophobic fumed silica powder (available from NIPPON AEROSIL CO. : model number "AEROSIL (Registered trademark) R976" (average primary particle size <NUM>)) was prepared as a small-diameter powder. Next, <NUM>% by mass of the large-diameter powder (spherical magnesium oxide powder) and <NUM>% by mass of the small-diameter powder (hydrophobic fumed silica powder) were mixed (dry mixing) using a rotating and revolving mixer (manufactured by THINKY CORPORATION: model number "ARE-<NUM>") at <NUM>,<NUM> rpm for <NUM> minutes to obtain an insulating filler. This insulating filler was made Example <NUM>. Incidentally, the average primary particle size of the small-diameter powder including Example <NUM>, and other Examples and Comparative Examples mentioned below was measured by an image analysis method of a transmission electron microscope (TEM), and the average primary particle size of the large-diameter powder was made the nominal value of each powder manufacturer.

In Examples <NUM> to <NUM> and Comparative Examples <NUM> to <NUM>, as shown in Table <NUM>, the model number, material and average primary particle size of the large-diameter powder, and the model number, material and the average primary particle size of the small-diameter powder, and a mixing ratio of the large-diameter powder and the small-diameter powder were each changed, and other than the above were carried out in the same manner as in Example <NUM> to obtain an insulating filler.

Incidentally, in Table <NUM>, "W15" of the large-diameter powder is a model number of aluminum nitride (AlN) powder having an average primary particle size of <NUM> available from Toyo Aluminium K. , "DMG-<NUM>" of the large-diameter powder is a model number of spherical magnesium oxide (MgO) powder having an average primary particle size of <NUM> available from Denka Co. , "SGP" of the large-diameter powder is a model number of boron nitride (BN) powder having an average primary particle size of <NUM> available from Denka Co. , and "BSN-S20LGF" of the large-diameter powder is a model number of boron nitride (BN) powder having an average primary particle size of <NUM> available from Combustion Synthesis Co. Also, in Table <NUM>, "RY50L" of the small-diameter powder is a model number of hydrophobic fumed silica (SiO<NUM>) powder having an average primary particle size of <NUM> available from NIPPON AEROSIL CO. , "NAX50" of the small-diameter powder is a model number of hydrophobic fumed silica (SiO<NUM>) powder having an average primary particle size of <NUM> available from NIPPON AEROSIL CO. , "RY50" of the small-diameter powder is a model number of hydrophobic fumed silica (SiO<NUM>) powder having an average primary particle size of <NUM> available from NIPPON AEROSIL CO. , "R805" of the small-diameter powder is a model number of hydrophobic fumed silica (SiO<NUM>) powder having an average primary particle size of <NUM> available from Evonik Industries AG, "RX200" of the small-diameter powder is a model number of hydrophobic fumed silica (SiO<NUM>) powder having an average primary particle size of <NUM> available from NIPPON AEROSIL CO. , and "RX300" of the small-diameter powder is a model number of hydrophobic fumed silica (SiO<NUM>) powder having an average primary particle size of <NUM> available from NIPPON AEROSIL CO.

Also, in Table <NUM>, "<NUM>" of the small-diameter powder is a model number of hydrophilic fumed silica (SiO<NUM>) powder having an average primary particle size of <NUM> available from NIPPON AEROSIL CO. , and "R7200" of the small-diameter powder is a model number of hydrophobic fumed silica (SiO<NUM>) powder having an average primary particle size of <NUM> available from Evonik Industries AG. Also, in Table <NUM>, "C805" of the small-diameter powder is a model number of hydrophobic fumed alumina (Al<NUM>O<NUM>) powder having an average primary particle size of <NUM> available from Evonik Industries AG, and "T805" of the small-diameter powder is a model number of hydrophobic fumed titania (TiO<NUM>) powder having an average primary particle size of <NUM> available from Evonik Industries AG.

Also, in Table <NUM>, "AA-<NUM>" of the small-diameter powder is a model number of spherical α-alumina (Al<NUM>O<NUM>) powder having an average primary particle size of <NUM> available from Sumitomo Alumina Co. , and "AA-<NUM>" of the small-diameter powder is a model number of spherical α-alumina (Al<NUM>O<NUM>) powder having an average primary particle size of <NUM> available from Sumitomo Alumina Co. Also, in Table <NUM>, "MF" of the small-diameter powder is a model number of aluminum nitride (AlN) powder having an average primary particle size of <NUM> available from Toyo Aluminium K. , and "P25" of the small-diameter powder is a model number of hydrophilic fumed titania (TiO<NUM>) powder having an average primary particle size of <NUM> available from NIPPON AEROSIL CO. Further, in Table <NUM>, "col-SiO<NUM>" of the small-diameter powder is colloidal silica powder having an average primary particle size of <NUM> available from Evonik Industries AG.

On the other hand, in the mixing method "wet method" of Comparative Example <NUM> in Table <NUM>, with <NUM>% by mass of ethanol as a solvent were mixed <NUM>% by mass of a large-diameter powder (spherical magnesium oxide powder) and <NUM>% by mass of a small-diameter powder (hydrophilic fumed titania powder), the mixture was stirred by a dissolver (DISPERMAT: D-<NUM> manufactured by VMA-GETZMANN) at <NUM>,<NUM> rpm for <NUM> minutes, and the obtained slurry was dried and the dried product was pulverized to obtain an insulating filler.

The ratio D<NUM>/D<NUM> of the average primary particle size of the small-diameter powder having the average primary particle size D<NUM> and the large-diameter powder having the average primary particle size D<NUM> of the insulating fillers of Examples <NUM> to <NUM> and Comparative Examples <NUM> to <NUM> was calculated. In addition, the volume resistivity of the insulating fillers of Examples <NUM> to <NUM> and Comparative Examples <NUM> to <NUM> was measured by using a high resistance· resistivity meter "Hiresta-UX" (manufactured by Mitsubishi Chemical Analytech Co. : model number "MCP-HT800") and a powder resistance measurement system (manufactured by Mitsubishi Chemical Analytech Co. : model number "MCP-PD-<NUM>"). These results are shown in Table <NUM>.

As is clear from Table <NUM>, even if the mixing ratio of the small-diameter powder was within the suitable range (<NUM>% by mass to <NUM>% by mass) as <NUM>% by mass, in the insulating filler of Comparative Example <NUM> in which the ratio (small-diameter/large-diameter) of the average primary particle sizes of the small-diameter powder and the large-diameter powder was larger than the suitable range (<NUM> × <NUM>-<NUM> to <NUM> × <NUM>-<NUM>) as <NUM> × <NUM>-<NUM> and <NUM> × <NUM>-<NUM>, the volume resistivity was low as <NUM> × <NUM><NUM>.

Also, even if the mixing ratio of the small-diameter powder was within the suitable range (<NUM>% by mass to <NUM>% by mass) as <NUM>% by mass, and the ratio (small-diameter/large-diameter) of the average primary particle size of the small-diameter powder and the large-diameter powder was within the suitable range (<NUM> × <NUM>-<NUM> to <NUM> × <NUM>-<NUM>) as <NUM> × <NUM>-<NUM>, in the insulating filler of Comparative Example <NUM>, the volume resistivity was low as <NUM> × <NUM><NUM>. It was estimated that it did not have a connection in a bead shape structure.

Further, even if the mixing ratio of the small-diameter powder was within the suitable range (<NUM>% by mass to <NUM>% by mass) as <NUM>% by mass, in the insulating filler of Comparative Example <NUM> in which the ratio (small-diameter/large-diameter) of the average primary particle sizes of the small-diameter powder and the large-diameter powder was larger than the suitable range (<NUM> × <NUM>-<NUM> to <NUM> × <NUM>-<NUM>) as <NUM> × <NUM>-<NUM>, the volume resistivity was low as <NUM> × <NUM><NUM>.

To the contrary, in the insulating fillers of Examples <NUM> to <NUM> in which the mixing ratio of the small-diameter powder was within the suitable range as <NUM>% by mass to <NUM>% by mass and the ratio (small-diameter/large-diameter) of the average primary particle sizes of the small-diameter powder and the large-diameter powder was a suitable range as <NUM> × <NUM>-<NUM> to <NUM> × <NUM>-<NUM>, the volume resistivity was high as <NUM> × <NUM><NUM> Ω·m to <NUM> × <NUM><NUM> Ω·m.

On the other hand, even if the mixing ratio of the small-diameter powder was within the suitable range (<NUM>% by mass to <NUM>% by mass) as <NUM>% by mass, and the ratio (small-diameter/large-diameter) of the average primary particle sizes of the small-diameter powder and the large-diameter powder was within the suitable range (<NUM> × <NUM>-<NUM> to <NUM> × <NUM>-<NUM>) as <NUM> × <NUM>-<NUM>, in the insulating filler of Comparative Example <NUM> in which of the small-diameter powder was a model number of "P25" which is hydrophilic fumed titania (TiO<NUM>) powder having an average primary particle size of <NUM> available from NIPPON AEROSIL CO. , as used as the small-diameter powder, and using ethanol as a solvent and mixed by the wet method, the volume resistivity was low as <NUM> × <NUM><NUM>.

To the contrary, in the insulating fillers of Examples <NUM> to <NUM> in which the mixing ratio of the small-diameter powder was within the suitable range as <NUM>% by mass to <NUM>% by mass, the ratio (small-diameter/large-diameter) of the average primary particle sizes of the small-diameter powder and the large-diameter powder was within the suitable range as <NUM> × <NUM>-<NUM> to <NUM> × <NUM>-<NUM>, and hydrophobic fumed silica, hydrophobic fumed alumina or hydrophobic fumed titania was used as the small-diameter powder and mixed by the dry method, the volume resistivity became high as <NUM> × <NUM><NUM> Ω·m to <NUM> × <NUM><NUM> Ω·m.

First, a spherical magnesium oxide powder (available from Denka Co. : model number "DMG-<NUM>" (average primary particle size <NUM>)) was prepared as a large-diameter powder, and hydrophobic fumed silica powder (available from NIPPON AEROSIL CO. : model number "RY50L" (average primary particle size <NUM>)) was prepared as a small-diameter powder. Next, <NUM>% by mass of the large-diameter powder (spherical magnesium oxide powder) and <NUM>% by mass of the small-diameter powder (hydrophobic fumed silica powder) was mixed (dry mixing) using a rotating and revolving mixer (manufactured by THINKY CORPORATION: model number "ARE-<NUM>") at <NUM>,<NUM> rpm for <NUM> minutes to obtain an insulating filler.

In the same manner as in Example <NUM> except for using <NUM>% by mass of a spherical magnesium oxide powder (available from Denka Co. : model number "DMG-<NUM>" (average primary particle size <NUM>)) as a large-diameter powder, and <NUM>% by mass of a hydrophobic fumed silica powder (available from NIPPON AEROSIL CO. : model number "RY50L" (average primary particle size <NUM>)) as a small-diameter powder, an insulating filler was obtained.

In the same manner as in Example <NUM> except for using <NUM>% by mass of an aluminum nitride powder (available from Toyo Aluminium K. : model number "W15" (average primary particle size <NUM>)) as a large-diameter powder, and <NUM>% by mass of hydrophobic fumed silica powder (available from NIPPON AEROSIL CO. : model number "R976" (average primary particle size <NUM>)) as a small-diameter powder, an insulating filler was obtained.

In the same manner as in Example <NUM> except for using <NUM>% by mass of an aluminum nitride powder (available from Toyo Aluminium K. : model number "W15" (average primary particle size <NUM>)) as a large-diameter powder, and <NUM>% by mass of a hydrophobic fumed alumina powder (available from Evonik Industries AG: model number "C805" (average primary particle size <NUM>)) as a small-diameter powder, an insulating filler was obtained.

In the same manner as in Example <NUM> except for using a spherical magnesium oxide powder (available from Denka Co. : model number "DMG-<NUM>" (average primary particle size <NUM>)) as a large-diameter powder, and not using the small-diameter powder, an insulating filler was obtained.

In the same manner as in Example <NUM> except for using an aluminum nitride powder (available from Toyo Aluminium K. : model number "W15" (average primary particle size <NUM>)) as a large-diameter powder, and not using the small-diameter powder, an insulating filler was obtained.

Regarding the insulating fillers of Examples <NUM> to <NUM> and Comparative Examples <NUM> and <NUM>, the volume resistivity before and after moisture absorption was measured by using a high resistance· resistivity meter "Hiresta-UX" (manufactured by Mitsubishi Chemical Analytech Co. : model number "MCP-HT800") and a powder resistance measurement system (manufactured by Mitsubishi Chemical Analytech Co. : model number "MCP-PD-<NUM>"). Moisture absorption was carried out by leaving the insulating filler in a constant temperature and constant humidity oven (manufactured by Yamato Scientific Co. : model number "IG401") under the conditions of a temperature of <NUM> and a relative humidity of <NUM>% for <NUM> days. These results are shown in Table <NUM>.

As is clear from Table <NUM>, in the insulating fillers of Examples <NUM> to <NUM> in which the mixing ratio of the small-diameter powder was within the suitable range (<NUM>% by mass to <NUM>% by mass) as <NUM>% by mass to <NUM>% by mass, and the ratio (small-diameter/large-diameter) of the average primary particle sizes of the small-diameter powder and the large-diameter powder was within the suitable range (<NUM> × <NUM>-<NUM> to <NUM> × <NUM>-<NUM>) as <NUM> × <NUM>-<NUM> to <NUM> × <NUM>-<NUM>, the volume resistivity before moisture absorption became high as <NUM> × <NUM><NUM> Ω·m to <NUM> × <NUM><NUM> Ω·m. In addition, the volume resistivity did not substantially change as <NUM> × <NUM><NUM> Ω·m to <NUM> × <NUM><NUM> Ω·m even when moisture was absorbed under constant temperature and high humidity conditions (<NUM>, a relative humidity of <NUM>%) for <NUM> days.

To the contrary, in the insulating filler of Comparative Example <NUM> which consists of a spherical magnesium oxide powder (available from Denka Co. : model number "DMG-<NUM>" (average primary particle size <NUM>)) alone as the large-diameter powder without using the small-diameter powder, when moisture was absorbed under constant temperature and high humidity conditions (<NUM>, a relative humidity of <NUM>%) for <NUM> days, the volume resistivity was lowered as <NUM> × <NUM><NUM> Ω·m with two digits as compared with before moisture absorption. In addition, in the insulating filler of Comparative Example <NUM> which consists of an aluminum nitride powder (available from Toyo Aluminium K. : model number "W15" (average primary particle size <NUM>)) alone as the large-diameter powder without using the small-diameter powder, when moisture was absorbed under constant temperature and high humidity conditions (<NUM>, a relative humidity of <NUM>%) for <NUM> days, the volume resistivity was lowered as <NUM> × <NUM><NUM> Ω·m with one digit as compared with before moisture absorption.

First, <NUM>% by volume of an unsaturated polyester resin (available from Hitachi Chemical Co. : model number "WP2008") as a resin and <NUM>% by volume of a curing agent (available from Hitachi Chemical Co. : model number "CT50") were mixed to prepare a mixture of the resin. Then, <NUM>% by volume of a spherical magnesium oxide powder (available from Denka Co. : model number "DMG-<NUM>" (average primary particle size <NUM>)) as a large-diameter powder and <NUM>% by volume of a hydrophobic fumed silica powder (available from NIPPON AEROSIL CO. : model number "RY50L" (average primary particle size <NUM>)) as a small-diameter powder were mixed to prepare a mixed powder. This mixed powder was mixed by a rotating and revolving mixer (manufactured by THINKY CORPORATION: model number "ARE-<NUM>") at <NUM>,<NUM> rpm for <NUM> minutes to obtain an insulating filler. Next, the mixture of the above-mentioned resin and the insulating filler were mixed by the above-mentioned rotating and revolving mixer at <NUM>,<NUM> rpm for <NUM> minutes to prepare a resin composition. Further, this resin composition is placed in a mold having a length × width × depth of a cavity of <NUM> × <NUM> × <NUM>, and the resin was cured using a heat press (manufactured by Kodaira Seisakusho Co. : model number "PY15-EA") at a temperature of <NUM> and applying a pressure of <NUM> MPa (<NUM>/cm<NUM>) and maintaining for <NUM> minutes to prepare an insulating material comprising a resin molded body.

First, <NUM>% by volume of an unsaturated polyester resin (available from Hitachi Chemical Co. : model number "WP2008") as a resin and <NUM>% by volume of a curing agent (available from Hitachi Chemical Co. : model number "CT50") were mixed to prepare a mixture of the resin. Then, <NUM>% by volume of a spherical magnesium oxide powder (available from Denka Co. : model number "DMG-<NUM>" (average primary particle size <NUM>)) as a large-diameter powder and <NUM>% by volume of a hydrophobic fumed silica powder (available from NIPPON AEROSIL CO. : model number "RY50L" (average primary particle size <NUM>)) as a small-diameter powder were mixed to prepare a mixed powder. In the same manner as in Example <NUM> except for the above, an insulating material comprising a resin molded body was prepared.

First, <NUM>% by volume of an unsaturated polyester resin (available from Hitachi Chemical Co. : model number "WP2008") as a resin and <NUM>% by volume of a curing agent (available from Hitachi Chemical Co. : model number "CT50") were mixed to prepare a mixture of the resin. Then, as a mixed powder A, <NUM>% by volume of a spherical magnesium oxide powder (available from Denka Co. : model number "DMG-<NUM>" (average primary particle size <NUM>)) as a large-diameter powder and <NUM>% by volume of a hydrophobic fumed silica powder (available from NIPPON AEROSIL CO. : model number "RY50L" (average primary particle size <NUM>)) as a small-diameter powder were mixed with a rotating and revolving mixer (manufactured by THINKY CORPORATION: model number "ARE-<NUM>") at <NUM>,<NUM> rpm for <NUM> minutes to obtain an insulating filler A. Subsequently, as a mixed powder B, <NUM>% by volume of an aluminum nitride powder (available from Toyo Aluminium K. : model number "TFZ-S30P" (average primary particle size <NUM>)) as a large-diameter powder and <NUM>% by volume of a hydrophobic fumed silica powder (available from NIPPON AEROSIL CO. : model number "VP RX40S" (average primary particle size <NUM>)) as a small-diameter powder were mixed with a rotating and revolving mixer (manufactured by THINKY CORPORATION: model number "ARE-<NUM>") at <NUM>,<NUM> rpm for <NUM> minutes to obtain an insulating filler B. Next, the above-mentioned mixture of the resin, the insulating filler A and the insulating filler B were mixed by the above-mentioned rotating and revolving mixer at <NUM>,<NUM> rpm for <NUM> minutes to prepare a resin composition. In the same manner as in Example <NUM> except for the above, an insulating material comprising a resin molded body was prepared.

First, <NUM>% by volume of an unsaturated polyester resin (available from Hitachi Chemical Co. : model number "WP2008") as a resin and <NUM>% by volume of a curing agent (available from Hitachi Chemical Co. : model number "CT50") were mixed to prepare a mixture of the resin. Then, as a mixed powder A, <NUM>% by volume of a spherical magnesium oxide powder (available from Denka Co. : model number "DMG-<NUM>" (average primary particle size <NUM>)) as a large-diameter powder and <NUM>% by volume of a hydrophobic fumed silica powder (available from NIPPON AEROSIL CO. : model number "RY50L" (average primary particle size <NUM>)) as a small-diameter powder were mixed with a rotating and revolving mixer (manufactured by THINKY CORPORATION: model number "ARE-<NUM>") at <NUM>,<NUM> rpm for <NUM> minutes to obtain an insulating filler A. Subsequently, as a mixed powder B, <NUM>% by volume of an aluminum nitride powder (available from Toyo Aluminium K. : model number "TFZ-S30P" (average primary particle size <NUM>)) as a large-diameter powder and <NUM>% by volume of a hydrophobic fumed silica powder (available from NIPPON AEROSIL CO. : model number "VP RX40S" (average primary particle size <NUM>)) as a small-diameter powder were mixed with a rotating and revolving mixer (manufactured by THINKY CORPORATION: model number "ARE-<NUM>") at <NUM>,<NUM> rpm for <NUM> minutes to obtain an insulating filler B. Next, the above-mentioned mixture of the resin, the insulating filler A and the insulating filler B were mixed by the above-mentioned rotating and revolving mixer at <NUM>,<NUM> rpm for <NUM> minutes to prepare a resin composition. In the same manner as in Example <NUM> except for the above, an insulating material comprising a resin molded body was prepared.

First, <NUM>% by volume of a silicone resin (available from Toray Dow Corning Co. : model number "BY16-<NUM>") as a resin and <NUM>% by volume of oxime silane as a curing agent were mixed to prepare a mixture of the resin. Then, as a mixed powder, <NUM>% by volume of a silicon nitride powder (available from Combustion Synthesis Co. : model number "BSN-S20LGF" (average primary particle size <NUM>)) as a large-diameter powder and <NUM>% by volume of a hydrophobic fumed silica powder (available from NIPPON AEROSIL CO. : model number "RX300" (average primary particle size <NUM>)) as a small-diameter powder were mixed with a rotating and revolving mixer (manufactured by THINKY CORPORATION: model number "ARE-<NUM>") at <NUM>,<NUM> rpm for <NUM> minutes to obtain an insulating filler. In the same manner as in Example <NUM> except for the above, an insulating material comprising a resin molded body was prepared.

First, <NUM>% by volume of an epoxy resin (available from Mitsubishi Chemical Corporation: model number "JER828") was prepared as a resin. Then, as a mixed powder, <NUM>% by volume of a boron nitride powder (available from Denka Co. : model number "SGP" (average primary particle size <NUM>)) as a large-diameter powder and <NUM>% by volume of a hydrophobic fumed silica powder (available from NIPPON AEROSIL CO. : model number "RX200" (average primary particle size <NUM>)) as a small-diameter powder were mixed with a rotating and revolving mixer (manufactured by THINKY CORPORATION: model number "ARE-<NUM>") at <NUM>,<NUM> rpm for <NUM> minutes to obtain an insulating filler. In the same manner as in Example <NUM> except for the above, an insulating material comprising a resin molded body was prepared.

First, <NUM>% by volume of an unsaturated polyester resin (available from Hitachi Chemical Co. : model number "WP2008") as a resin and <NUM>% by volume of a curing agent (available from Hitachi Chemical Co. : model number "CT50") were mixed to prepare a mixture of the resin. Then, the above-mentioned mixture of the resin and <NUM>% by volume of a spherical magnesium oxide powder (available from Denka Co. : model number "DMG-<NUM>" (average primary particle size <NUM>)) as a large-diameter powder were mixed by the above-mentioned rotating and revolving mixer at <NUM>,<NUM> rpm for <NUM> minutes to prepare a resin composition. The small-diameter powder was not mixed. In the same manner as in Example <NUM> except for the above, an insulating material comprising a resin molded body was prepared.

First, <NUM>% by volume of an unsaturated polyester resin (available from Hitachi Chemical Co. : model number "WP2008") as a resin and <NUM>% by volume of a curing agent (available from Hitachi Chemical Co. : model number "CT50") were mixed to prepare a mixture of the resin. Then, the above-mentioned mixture of the resin and <NUM>% by volume of a spherical magnesium oxide powder "DMG-<NUM>" (average primary particle size <NUM>)) as a large-diameter powder were mixed by the above-mentioned rotating and revolving mixer at <NUM>,<NUM> rpm for <NUM> minutes to prepare a resin composition. The small-diameter powder was not mixed. In the same manner as in Example <NUM> except for the above, an insulating material comprising a resin molded body was prepared.

Incidentally, in Table <NUM>, "P" such as the resin, etc., is a mixture of an unsaturated polyester resin (available from Hitachi Chemical Co. : model number "WP2008", <NUM> parts by weight) and a curing agent (available from Hitachi Chemical Co. : model number "CT50", <NUM> parts by weight). "Q" such as the resin, etc., is a mixture of a silicone resin (available from Toray Dow Corning Co. : model number "BY16-<NUM>", <NUM> parts by weight) and oxime silane (<NUM> parts by weight) as a curing agent. "R" such as the resin, etc., is an epoxy resin with a model number "JER828" available from Mitsubishi Chemical Corporation.

On the other hand, in Table <NUM>, "DMG-<NUM>" of the large-diameter powder is a model number of a spherical magnesium oxide (MgO) powder having an average primary particle size of <NUM> available from Denka Co. , "DMG-<NUM>" of the large-diameter powder is a model number of a spherical magnesium oxide (MgO) powder having an the average primary particle size of <NUM> available from Denka Co. , "TFZ-S30P" of the large-diameter powder is a model number of an aluminum nitride (AlN) powder having an average primary particle size of <NUM> available from Toyo Aluminium K. , "BSN-S20LGF" of the large-diameter powder is a model number of a silicon nitride (Si<NUM>N<NUM>) powder having an average primary particle size of <NUM> available from Combustion Synthesis Co. , and "SGP" of the large-diameter powder is a model number of a boron nitride (BN) powder having an average primary particle size of <NUM> available from Denka Co.

Also, in Table <NUM>, "RY50L" of the small-diameter powder is a model number of a hydrophobic fumed silica (SiO<NUM>) powder having an average primary particle size of <NUM> available from NIPPON AEROSIL CO. , "VP RX40S" of the small-diameter powder is a model number of a hydrophobic fumed silica (SiO<NUM>) powder having an average primary particle size of <NUM> available from NIPPON AEROSIL CO. , "RX300" of the small-diameter powder is a model number of a hydrophobic fumed silica (SiO<NUM>) powder having an average primary particle size of <NUM> available from NIPPON AEROSIL CO. , and "RX200" of the small-diameter powder is a model number of a hydrophobic fumed silica (SiO<NUM>) powder having an average primary particle size of <NUM> available from NIPPON AEROSIL CO.

Regarding the insulating materials comprising the resin molded bodies (thickness <NUM>) of Examples <NUM> to <NUM> and Comparative Examples <NUM> to <NUM>, dielectric breakdown voltage before immersion and after immersion was measured using an ultra-high voltage withstanding voltage tester "<NUM> series" (manufactured by Keisoku Giken Co. : model number "<NUM>"), and the value obtained by dividing the value by the thickness (<NUM>) was defined as the dielectric breakdown voltage (kV/mm). Immersion with water was carried out by immersing an insulating material comprising a resin molded body in ion-exchanged water and holding it at <NUM> for <NUM> days. After immersion, the insulating material comprising the resin molded body was pulled up from water, and dried it at <NUM> for <NUM> hours. The rate of change of the dielectric breakdown voltage before immersion and after immersion was calculated based on the above-mentioned equation (<NUM>). There are cases where the dielectric breakdown voltage becomes high and becomes low due to immersion, so that the difference of the dielectric breakdown voltage before immersion and after immersion is expressed by an absolute value, and this difference is divided by the dielectric breakdown voltage before immersion and expressed by a percentage. The results are shown in Table <NUM>.

As is clear from Table <NUM>, in the insulating materials comprising the resin molded bodies of Comparative Examples <NUM> and <NUM> wherein an unsaturated polyester resin of a model number of "WP2008" available from Hitachi Chemical Co. , was used as the resin, a model number of "CT50" available from Hitachi Chemical Co. , was used as the curing agent, and spherical magnesium oxide powders of model numbers of "DMG-<NUM>" and "DMG-<NUM>" available from Denka Co. , were used as the large-diameter powder, but no small-diameter powder was used, whereas the dielectric breakdown voltage before immersion was high as <NUM> kV/mm or more, the dielectric breakdown voltage after immersion was lowered about <NUM>% even though the immersion conditions were short as <NUM> hours at <NUM>. Specifically, the rate of change of the dielectric breakdown voltage before and after immersion was <NUM>% in the insulating material of Comparative Example <NUM>, and <NUM>% in the insulating material of Comparative Example <NUM>.

To the contrary, in the insulating materials comprising the resin molded bodies of Examples <NUM> to <NUM> in which the large-diameter powder and the small-diameter powder were filled with the predetermined ratio to the resin and the curing agent, further the ratio (small-diameter/large-diameter) of the average primary particle sizes of the small-diameter powder and the large-diameter powder was in the suitable range (<NUM> × <NUM>-<NUM> to <NUM> × <NUM>-<NUM>) as <NUM> × <NUM>-<NUM> to <NUM> × <NUM>-<NUM>, and further the hydrophobic fumed silica was used as a small-diameter powder and mixed by the dry method, there occurred substantially no lowering in the dielectric breakdown voltage after immersion and these became high as <NUM> kV/mm or more whereby these became insulating materials having high reliability to water resistance. Specifically, the rate of change of the dielectric breakdown voltage before and after immersion was <NUM>% to <NUM>% in the insulating materials of Examples <NUM> to <NUM>.

Claim 1:
An insulating filler which comprises a mixed powder in which on a surface of a magnesium oxide powder and/or a nitride-based inorganic powder having an average primary particle size D<NUM>, a hydrophobic fumed oxide powder having an average primary particle size D<NUM> which is smaller than the average primary particle size D<NUM> is attached,
wherein a ratio D<NUM>/D<NUM> of the average primary particle size D<NUM> to the average primary particle size D<NUM> is <NUM> × <NUM>-<NUM> to <NUM> × <NUM>-<NUM>,
wherein volume resistivity of the mixed powder is <NUM> × <NUM><NUM> Ω·m or more,
wherein a content ratio of the hydrophobic fumed oxide powder is <NUM>% by mass to <NUM>% by mass when the mixed powder is made <NUM>% by mass,
wherein the nitride-based inorganic powder is one or more kinds selected from the group consisting of an aluminum nitride powder, a boron nitride powder and a silicon nitride powder, and
wherein the hydrophobic fumed oxide is hydrophobic fumed silica, hydrophobic fumed alumina or hydrophobic fumed titania.