KR20160084808A - Thermal conductive silicone composition and cured product, and composite sheet - Google Patents

Abstract

Provided is a thermoconductive silicone composition which contains over 90% of alpha alumina alumina of which α rate is greater than or equal to 90%, with respect to the total parts by weight of a thermoconductive filler, and shows the weight reduction rate less than 1% when exposed to air under a condition involving a temperature of 250°C for six hours. According to the present invention, the thermoconductive silicone composition using over 90% of alpha alumina of which α rate is greater than or equal to 90%, with respect to the total parts by weight of a thermoconductive filler exhibits the low weight reduction rate and excellent heat resistance even at 250°C. Therefore, the thermoconductive silicone composition and a cured product and a composite sheet can be used to cope with a condition requiring heat resistance at approximately 250°C, such as semiconductor devices using silicon carbide-based substrate materials and automobile-mountable heaters for heat radiation.

Description

TECHNICAL FIELD [0001] The present invention relates to a thermally conductive silicone composition, a thermally conductive silicone composition, a cured product thereof, and a composite sheet (THERMAL CONDUCTIVE SILICONE COMPOSITION AND CURED PRODUCT, AND COMPOSITE SHEET)

The present invention relates to a thermally conductive silicone composition, a thermally conductive silicone cured product, and a thermally conductive silicone compound sheet used for heat dissipation when exposed, for example, between a heat-generating component and a heat-radiating component in an electronic device, .

Semiconductors such as transistors and diodes used in electronic devices such as converters and power supplies are subject to a large amount of heat in association with high performance, high speed, miniaturization and high integration, It causes destruction. Therefore, many heat dissipating methods and a heat dissipating member used therefor for suppressing the temperature rise of an operating semiconductor have been proposed. Typical heat dissipating members include various forms such as a composition filled with a thermally conductive filler in a polymer matrix, a cured product obtained by curing the composition, or a composite sheet obtained by laminating a cured product and a reinforcing material. The heat-dissipating member is mounted between the heat-generating member and the dissipating member, and the shape thereof is selected according to the mounting state.

As the polymer matrix of the heat-dissipating member, silicon, acrylic resin, olefin resin and the like can be mentioned, but silicone is most suitable from the viewpoints of heat resistance, cold resistance and long-term reliability.

Particularly, in view of heat resistance, cold resistance and long-term reliability, silicon is often used as a polymer matrix of a heat-dissipating member in a semiconductor device having a large amount of heat generation or a vehicle mounting field requiring long-term reliability. In addition, silicon has been generally used as a substrate material of a semiconductor device up to now, but substrate materials using silicon carbide as a raw material have been popular in recent years. The silicon carbide-based substrate material has a heat-resistant temperature higher than that of the silicon-based substrate material, and the allowable operating environmental temperature also increases to around 250 ° C. In addition, in the vehicle-mounted field, the spread of hybrid vehicles, electric vehicles, and the like is progressing, and it becomes difficult to depend on the heat generation of the engine, which has been using the heat of the engine until now. Therefore, it is necessary to increase the resistance value of the heater have. For example, a PTC heater requires a large current when it is driven, and its heat generation also exceeds 200 ° C.

Among these flows, the heat-resistant temperature required for the heat-dissipating member naturally also increases. The use temperature range of the thermoconductive silicone composition and its cured product or composite sheet, which is a heat-dissipating member using conventional silicon as a polymer matrix, is not suitable for the above-described conditions.

As a conventional technique related to the present invention, Japanese Patent Laid-Open Publication No. 2014-145024 discloses a heat-resistant (250 ° C) property. However, a problem that a heat stabilizer must be added and is limited to a low- have.

Japanese Patent Application Laid-Open No. 2014-145024

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a thermally conductive silicone composition, a cured product, and a composite sheet which can be used even in an atmosphere of 250 캜 in which silicon is a polymer matrix.

As a result of intensive studies, the present inventors have found that a thermally conductive silicone composition having a reduced weight loss even in an atmosphere at 250 캜 in the air can be provided by using a-alumina having a high α-alumina ratio among alumina.

In other words, conventionally, in the field of heat-dissipating members, particularly in vehicles, the heat-dissipating member is required to be insulative, and as the thermally conductive filler of a heat-dissipating member using a large number of silicon as a polymer matrix, Alumina is used. In order to achieve the above-mentioned object, it is effective to obtain a thermally conductive silicone composition and a cured product which can be used under a 250 占 폚 environment by using a polymer matrix mainly composed of? Alumina as a thermally conductive filler The present invention has been accomplished on the basis of these findings.

Accordingly, the present invention provides the following thermally conductive silicone composition, cured product, and composite sheet.

[1] A thermally conductive filler which is characterized in that at least 90% of the total mass parts of the thermally conductive filler is alpha alumina having an alpha conversion rate of 90% or more, and the weight reduction rate when left in air at 250 DEG C for 6 hours is less than 1% Composition.

[2] The thermally conductive silicone composition according to [1], wherein the thermal conductivity is 0.5 W / mK or more.

[3] The thermoconductive silicone composition according to [1] or [2], further comprising 250 to 2,000 parts by mass of a thermally conductive filler per 100 parts by mass of the organopolysiloxane.

[4] A thermoconductive silicone composition comprising α alumina having 90% or more of 90% or more of the total mass parts of thermally conductive fillers, wherein the cured product is allowed to stand in air at 250 ° C. for 6 hours Wherein the reduction ratio is less than 1%.

[5] The thermally conductive silicone cured product according to [4], wherein the thermal conductivity is 0.5 W / mK or more.

[6] The thermosetting silicone composition as described in [4], wherein the thermoconductive silicone composition contains 100 parts by mass of the organopolysiloxane, 250 to 2,000 parts by mass of the thermally conductive filler, and a curing effective amount of the curing agent for curing the organopolysiloxane- Or the thermoconductive silicone cured product according to [5].

[7] A thermally conductive silicone composite sheet comprising a thermally conductive silicone cured product according to any one of [4] to [6] laminated on one side or both sides of a reinforcing material.

[8] The thermoconductive silicone composite sheet according to [7], wherein the reinforcing material is a polyimide film.

[9] The thermally conductive silicone composite sheet according to [7], wherein the reinforcing material is glass cloth.

[10] The thermally conductive silicone composite sheet according to any one of [7] to [9], wherein the hardness of the thermally conductive silicone hardened product is 80 to 99 in durometer A hardness.

The thermoconductive silicone composition using? -Alumina having 90% or more of the total mass portion of the thermally conductive filler according to the present invention has an α-conversion rate of 90% or more and has a small weight loss and excellent heat resistance even at 250 ° C. The thermoconductive silicone composition, the cured product, and the composite sheet can be used for a part requiring heat resistance of about 250 캜, for example, a semiconductor device using a silicon carbide based substrate material and a heat radiation application for a vehicle-mounted heater.

The thermally conductive silicone composition according to the present invention comprises an organopolysiloxane host material and a thermally conductive filler as a main component and the thermally conductive silicone cured material is a thermally conductive silicone material containing an organopolysiloxane host material and a thermally conductive filler material with a curing agent for curing the organopolysiloxane material And is obtained by curing the conductive silicone composition.

Hereinafter, this will be described in more detail.

[Presence of organopolysiloxane]

In the organopolysiloxane used in the present invention, it is general that the main chain portion basically includes repetition of diorganosiloxane units, which may include a branched structure in a part of the molecular structure or may be a cyclic structure , And straight-chain diorganopolysiloxanes are preferred.

Examples of the functional group bonding to the silicon atom include an unsubstituted or substituted monovalent hydrocarbon group, and examples thereof include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a tert- A cycloalkyl group such as a cyclopentyl group, a cyclohexyl group and a cycloheptyl group, a phenyl group, a tolyl group, a xylyl group, a naphthyl group, a naphthyl group, an isobutyl group, A phenyl group, a phenyl group, an aralkyl group such as a phenyl group, a phenylpropyl group and a methylbenzyl group, and a part or all of a hydrogen atom to which a carbon atom is bonded to these groups are substituted with a halogen atom such as fluorine, , A cyano group and the like, for example, a chloromethyl group, a 2-bromoethyl group, a 3-chloropropyl group, a 3,3,3-trifluoropropyl group, a chlorophenyl group, a fluorophenyl group, 3,3,4,4,5,5,6,6,6-nonafluorohexyl group Typical examples thereof are those having 1 to 10 carbon atoms, and more particularly, those having 1 to 6 carbon atoms. An unsubstituted or substituted alkyl group having 1 to 3 carbon atoms such as methyl group, ethyl group, propyl group, chloromethyl group, 2-bromoethyl group, 3,3,3-trifluoropropyl group, , And an unsubstituted or substituted phenyl group such as phenyl group, chlorophenyl group, fluorophenyl group and the like. In addition, it may have an unsaturated bond such as an alkenyl group, and may have an unsaturated bond such as an alkenyl group, and may have an unsaturated bond such as a vinyl group, an allyl group, a propenyl group, an isopropenyl group, a butenyl group, a hexenyl group, a cyclohexenyl group, 8.

The siloxane repeating unit in the main chain is not particularly limited, and the properties of the polysiloxane obtained vary depending on the number of repeating units, and accordingly, a method for preparing the thermally conductive silicone composition may be appropriately selected. A stirrer such as a planetary mixer which is an oil phase is suitable, and a stirrer in which a shear force is applied, such as a biaxial roll or a kneader, is suitable if it is raw.

In this case, it is preferable from the viewpoint of handling to use an organopolysiloxane having a kinematic viscosity at 25 캜 of 100 to 40,000 mm 2 / s, particularly 100 to 10,000 mm 2 / s. The kinematic viscosity can also be measured with an Ostwald viscometer.

[Thermally conductive filler]

In the present invention, as the thermally conductive filler,? Alumina having 90% or more, preferably 95% or more, of the total percentage of the total mass of the thermally conductive filler is used.

(Crystalline phase of alumina)

Alumina has various crystal phases due to differences in sintering temperature such as?,?,?, And?. We found that α alumina with the highest sintering temperature suppressed the weight loss of the silicone polymer most at 250 ° C. In general alumina, the crystal phase is rarely present singly, but it is preferable that the ratio of α phase is as high as possible, and the α phase rate is 90% or more, preferably 95% or more.

The α conversion rate was calculated from the peak height (I _25.6 ) of the alumina α phase (012 plane) appearing at the position of 2θ = 25.6 ° and the peak height (I _25.6 ) of the sample from the diffraction spectrum of the fine α alumina obtained using the X- γ-phase of the position shown, η-phase, χ-phase, κ phase, the following formula from the phase θ and the peak height (I ₄₆₎ on the δ

(%) = I _25.6 / (I _25.6 + I ₄₆ ) x 100

.

(Particle diameter of alumina)

The center particle diameter of alumina is preferably 0.1 to 200 mu m, more preferably 1 to 100 mu m, further preferably 1 to 50 mu m. When the median particle diameter is less than 0.1 탆, the filling property of the organopolysiloxane is deteriorated. When the median particle diameter is more than 200 탆, the fluidity when the composition is used and the strength when the composition is made into a cured product are difficult to obtain. It is also important to select the particle size in consideration of the thickness when the thermally conductive silicone composition is mounted and the thickness at which the thermally conductive silicone composition is cured. If alumina having a particle diameter larger than that at the time of curing is included in the mounting, alumina will protrude from the thermally conductive silicone composition and the cured product.

The average particle diameter of alumina is a value of cumulative mean diameter (median diameter) measured by Microtrac MT3300EX, a particle size analyzer manufactured by Nikkiso Co., Ltd.

(Granular phase of alumina)

Alumina has various forms such as spherical, round, and crushed according to the production method. In general, the crushed alumina is preferably crushed alumina because the crushed alumina has a high α conversion rate, but the crushed alumina does not matter if the α conversion rate is satisfied.

(Other thermally conductive filler)

Examples of other thermally conductive fillers include non-magnetic metals such as copper and aluminum, metal oxides such as alumina, silica, magnesia, Bengala, beryllium, titania and zirconia, metal nitrides such as aluminum nitride, silicon nitride and boron nitride, Metal hydroxide, artificial diamond, or silicon carbide, can be used as the filler. The center particle size may be 0.1 to 200 占 퐉, and they may be used alone or in combination of two or more. However, since the use of the present invention is assumed to be carried out at 250 캜 environment, it is necessary to use a material which does not cause a reaction such as melting, oxidation, dehydration or the like and does not promote cracking of the organopolysiloxane to at least around 300 캜 have.

(Blending amount of thermally conductive filler)

The blending amount of the thermally conductive filler is preferably 250 to 2,000 parts by mass, more preferably 250 to 1,000 parts by mass, and still more preferably 250 to 600 parts by mass based on 100 parts by mass of the organopolysiloxane. If the blending amount of the thermally conductive filler is too small, there is a fear that sufficient thermal conductivity may not be obtained, and if it is too much, preparation of the composition itself may become difficult.

[Thermoconductive silicone composition]

Although the thermoconductive silicone composition contains the organopolysiloxane host material and the thermally conductive filler as main components, as the other components, if necessary, an alkoxy group-containing organopolysiloxane may be added for the purpose of improving the dispersibility of the thermally conductive filler Can be compounded. As the alkoxy group-containing organopolysiloxane,

(Wherein R is a monovalent hydrocarbon group of an unsubstituted or substituted alkyl group having 1 to 30 carbon atoms, especially 1 to 10 carbon atoms, an aryl group, an aralkyl group, a halogenated alkyl group, etc., R ' Q is an integer of 0 to 2, preferably 0, and p is an integer of 0 to 100, especially 1 to 50.)

Is preferably a diorganopolysiloxane containing an alkoxy group at one end.

The blending amount of the alkoxy group-containing organopolysiloxane is preferably 1 to 30 parts by mass, more preferably 3 to 20 parts by mass, per 100 parts by mass of the organopolysiloxane.

If necessary, a coloring agent such as an organic pigment or an inorganic pigment, a heat resistance improving agent such as iron oxide or cerium oxide, and an internal release agent may be added.

(Flowability of thermoconductive silicone composition)

In the present invention, the thermally conductive silicone composition can be used as it is without being cured. In this case, the fluidity of the thermally conductive silicone composition is not particularly specified, but a dispenser or a metal mask called heat- In the case of mounting in screen printing, the viscosity is preferably from 10 to 900 Pa · s at 25 ° C, more preferably from 10 to 400 Pa · s. If the viscosity exceeds 900 Pa · s, there is a possibility that discharge in the dispenser becomes difficult or scratches may occur in screen printing due to poor fluidity. Also, the viscosity is a value by a Malcolm viscometer.

(Weight reduction rate in air at 250 캜)

In the thermoconductive silicone composition according to the present invention, the weight reduction rate when left in an atmosphere at 250 캜 for 6 hours in air is less than 1%, preferably not more than 0.8%. The reason for the decrease in weight is that the organopolysiloxane-based material is cracked due to heat to become low molecular weight and volatilized. Therefore, if the weight reduction rate is large, the polymer content decreases and the thermoconductive silicone composition may be decomposed or hardened. In such a case, the thermal conductivity of the thermally conductive silicone composition is lost.

It was also found that the degree of cracking of silicon varies depending on the crystal phase of alumina. The crystal phase alumina having a low sintering temperature such as the? phase or the? phase promotes the cracking of the silicon, and the α phase alumina having the highest sintering temperature does not promote the cracking of the silicon.

The weight reduction rate was measured by weighing 2 g of the thermally conductive silicone composition in a heat resistant glass chalet having a diameter of 20 mm and putting it in an oven at 250 캜. The atmosphere in the oven is air. Taken out after 6 hours, returned to room temperature, weighed, and calculated from the change in weight before and after the addition.

(Thermal conductivity)

The thermal conductivity of the thermoconductive silicone composition is preferably 0.5 W / mK or more. And more preferably 0.8 to 8.0 W / mK. If it is less than 0.5 W / mK, a sufficient heat radiation effect can not be obtained. Although the upper limit of the thermal conductivity is not specifically defined, if it is attempted to obtain a heat conductivity exceeding 8.0 W / mK, charging into silicon itself becomes difficult. The thermal conductivity is a value measured by a hot disk method.

[Heat-conductive silicone cured product]

The thermally conductive silicone cured product is obtained by blending and curing a curing agent with respect to the above-mentioned thermoconductive silicone composition containing the organopolysiloxane-based material and the thermally conductive filler as a main component.

The curing method of the thermoconductive silicone composition may include addition curing reaction using a platinum catalyst, radical reaction using an organic peroxide as a catalyst, and radical reaction using ultraviolet irradiation or electron beam irradiation. However, the curing method is not limited to these.

In this case, when the thermosetting silicone composition is to be cured by using the addition curing reaction using a platinum catalyst, organopolysiloxane having at least two alkenyl groups in the molecule as the organopolysiloxane-based material and organopolysiloxane having at least two silicon- An organohydrogenpolysiloxane having at least two directly bonded hydrogen atoms and a platinum group metal-based curing catalyst become essential components.

When curing is carried out with an organic peroxide, the organopolysiloxane may be an alkenyl group or an organopolysiloxane containing no alkenyl group.

The compounding amount of the curing agent for curing the organopolysiloxane host material, the curing method, the curing condition, and the like can be well known techniques.

(Hardness of the thermally conductive silicone hardened product)

The hardness of the thermally conductive silicone cured product is preferably 80 to 99 in durometer A hardness. And more preferably 90 to 96. [ If it is less than 80, the cured product tends to be deformed at the time of mounting, or the surface of the cured product may be easily scratched.

(Weight Reduction Rate and Thermal Conductivity of Thermally Conductive Silicone Cured Product)

The weight reduction ratio and the thermal conductivity of the thermally conductive silicone cured product are not the thermally conductive silicone composition itself but merely the cured product obtained by curing the thermally conductive silicone composition itself. The measurement method is similar to that of the above-described thermally conductive silicone composition.

[Heat-conductive silicone composite sheet]

The thermally conductive silicone composite sheet is obtained by laminating the thermally conductive silicone cured material on one side or both sides of a reinforcing material.

In this case, the reinforcing material of the thermally conductive silicone composite sheet is preferably a polyimide film or glass cloth considering practicality and processability. However, the reinforcing material is not limited to these, and any material having sufficient strength and heat resistance can be used without any problem. For example, a polytetrafluoroethylene sheet.

(Polyimide film)

The thickness of the polyimide film is preferably 5 to 100 mu m. More preferably 7 to 50 占 퐉, and still more preferably 7 to 25 占 퐉. If the polyimide film is too thin, sufficient strength or insulation can not be obtained. Conversely, if the polyimide film is too thick, it hinders thermal conductivity. Further, the surface of the polyimide film is preferably subjected to a plasma treatment because adhesion with the thermally conductive silicone cured product is improved.

(Glass cloth)

The thickness of the glass cloth is preferably 20 to 100 mu m. More preferably 30 to 60 占 퐉. If it is less than 20 mu m, sufficient strength can not be obtained, and if it is more than 100 mu m, there is a fear of hindering thermal conductivity. The weaving method of the glass cloth is not particularly limited. The glass cloth is preferably treated with silane. The silane coupling agent to be treated and the treatment method are not limited.

(Thickness of thermally conductive silicone cured product)

The thickness of the thermally conductive silicone cured product is preferably 50 to 10,000 탆, more preferably 200 to 800 탆. This thickness is not limited to the case of the thermally conductive silicone composite sheet, and is also appropriate when the thermally conductive silicone composition or its cured product is directly used without a reinforcing material.

[Molding method of thermoconductive silicone composite sheet]

In the method of forming a thermally conductive silicone composite sheet, a thermally conductive silicone composition containing a curing agent such as an organic peroxide having a decomposition temperature of 120 캜 as a catalyst is prepared and optionally diluted with toluene to prepare a coating solution. The coating liquid is coated on the reinforcing material by using an arbitrary spacer. The coating liquid is put into an oven at 80 ° C for 10 minutes to volatilize the toluene, and then cured in an oven at 150 ° C for 10 minutes. As a result, a thermally conductive silicone cured product can be laminated on one side of the substrate. When it is desired to laminate on the other side, it is similarly coated by the above method and dried and cured. However, the method of forming the thermally conductive silicone composite sheet is not limited thereto.

[Example]

Hereinafter, the present invention will be described in detail with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[Preparation of composition]

Component (A): Dimethylpolysiloxane represented by the following formula (1)

(X is an organic functional group, and n is a number giving the following viscosity)

(A-1) X = a methyl group and a kinematic viscosity of 10,000 mm < 2 > / s (25 DEG C)

(A-2) X = a methyl group and a kinematic viscosity of 30,000 mm < 2 > / s (25 DEG C)

Component (B): alumina having an average particle diameter of

(B-1) crushed α alumina having an α conversion of 99% and an average particle diameter of 5 μm

(B-2) a crushed α-alumina having an α conversion of 95% and an average particle size of 10 μm

(B-3) Spherical α-alumina having an α conversion of 92% and an average particle diameter of 20 μm

(B-4) Crushed-phase γ-alumina having an average particle diameter of 10 μm

(B-5) Crushed phase [theta] alumina having an average particle diameter of 10 mu m

(C) Component: thermally conductive filler

(C-1) Aluminum hydroxide having an average particle diameter of 1.0 mu m

(D): Dimethylpolysiloxane having an average degree of polymerization of 30 represented by the following formula (2) and one end blocked with a trimethoxy group

Component (E): C-23N (organic peroxide-based curing agent: Shin-Etsu Chemical Co., Ltd.)

[Examples, Comparative Examples]

The components shown in Tables 1 and 2 were kneaded for 60 minutes by using a planetary mixer in the predetermined amounts shown in the table to obtain thermoconductive silicone compositions of Examples 1 to 7 and Comparative Examples 1 to 7 shown in Tables 1 and 2, And the weight loss rate and thermal conductivity were measured by the following method. The results are shown in Tables 1 and 2.

[How to measure]

· Weight reduction rate

The thermally conductive silicone composition thus prepared was weighed into a heat-resistant container having a diameter of 20 mm and weighed in an oven set at 250 캜. The atmosphere in the oven is air. After 6 hours, it is removed and weighed at the time of returning to room temperature. The reduction is divided by the weight before injection and multiplied by 100.

For Example 7 and Comparative Example 7, the prepared thermoconductive silicone composition was placed in an oven set at 150 캜 for 10 minutes to cure, and the weight reduction rate was measured.

· Thermal conductivity

The thermal conductivity at 25 캜 of each thermoconductive silicone composition was measured by a hot-disk method using TPA-501 (manufactured by Kyoto Denshi Kogyo K.K.).

For Example 7 and Comparative Example 7, the prepared thermoconductive silicone composition was placed in an oven set at 150 캜 for 10 minutes to cure the thermoconductive silicone composition, and the thermal conductivity was measured.

As shown in Examples 1 to 7, a thermally conductive silicone composition using alpha alumina [(B-1) to (B-3)] having an α conversion rate of 90% or more had a weight reduction rate of 1 % ≪ / RTI >

On the other hand, as shown in Comparative Example 1, when? Alumina is used, the weight loss rate becomes 1% or more and heat resistance can not be imparted. As shown in Comparative Example 2, even when? Alumina is used, the weight reduction rate becomes 1% or more, and heat resistance can not be imparted. As shown in Comparative Example 3, if the ratio of the total amount of the alumina in the total mass parts of the thermally conductive filler is less than 90%, the weight reduction rate becomes 1% or more, and sufficient heat resistance can not be obtained. As shown in Comparative Example 4, when the ratio of? Alumina in the total mass portion of the thermally conductive filler is less than 90%, and the use of aluminum hydroxide as the thermally conductive filler to be used in combination further increases the weight reduction rate. This is because the aluminum hydroxide caused a dehydration reaction and the weight of the aluminum hydroxide itself decreased. In Comparative Example 5, the amount of? -Alumina to be charged was reduced as compared with Comparative Example 1, but the weight reduction rate was increased. This is because the ratio of silicone polymer is relatively high. As shown in Comparative Example 6, when aluminum hydroxide is used as the thermally conductive filler, the weight of the aluminum hydroxide itself decreases due to the dehydration reaction of aluminum hydroxide rather than the weight reduction of silicon, and the weight reduction rate becomes particularly large. As shown in Comparative Example 7, when the thermally conductive silicone composition is cured, the weight reduction rate is increased when? Alumina is used.

Claims (10)

Wherein 90% or more of the total mass parts of the thermally conductive filler is alpha alumina having an alpha value of 90% or more, and the weight reduction rate when left in air at 250 DEG C for 6 hours is less than 1%. The thermally conductive silicone composition according to claim 1, wherein the thermal conductivity is 0.5 W / mK or more. The thermally conductive silicone composition according to claim 1 or 2, wherein the thermoconductive silicone composition contains 250 to 2,000 parts by mass of a thermally conductive filler per 100 parts by mass of the organopolysiloxane. When the cured product obtained by curing the thermoconductive silicone composition containing? Alumina having 90% or more of? Total conversion of 90% or more of the total mass parts of the thermally conductive filler is allowed to stand in the air at 250 占 폚 for 6 hours, %. ≪ / RTI > The thermally conductive silicone cured product according to claim 4, wherein the thermal conductivity is 0.5 W / mK or more. The thermoconductive silicone composition according to claim 4 or 5, wherein the thermoconductive silicone composition contains 100 parts by mass of the organopolysiloxane, 250 to 2,000 parts by mass of the thermally conductive filler, and a curing effective amount of the curing agent for curing the organopolysiloxane Characterized by a thermally conductive silicone cured product. A thermally conductive silicone composite sheet characterized by comprising the thermally conductive silicone cured product according to claim 4 or 5 laminated on one side or both sides of a reinforcing material. The thermoconductive silicone composite sheet according to claim 7, wherein the reinforcing material is a polyimide film. The thermoconductive silicone composite sheet according to claim 7, wherein the reinforcing material is glass cloth. The thermoconductive silicone composite sheet according to claim 7, wherein the hardness of the thermally conductive silicone hardened product is 80 to 99 in durometer A hardness.