JP7689935B2 - Thermally conductive silicone composition and cured product thereof - Google Patents

Description

The present invention relates to a thermally conductive silicone composition and its cured product.

In recent years, with the increasing sophistication of electronic devices and the miniaturization and high integration of electronic components, the amount of heat generated by electronic devices and components tends to increase, and the heat generation density tends to become higher, so as a countermeasure, it is necessary to design devices with excellent heat dissipation properties and use materials with excellent thermal conductivity. In addition, thermal dissipation greases and thermal dissipation sheets are used to quickly transfer heat generated by heat-generating components to cooling components such as heat sinks, but high thermal conductivity is also required for heat dissipation materials.
In order to increase the thermal conductivity of a heat dissipation material, there are methods, for example, using a thermally conductive filler with high thermal conductivity such as aluminum nitride or boron nitride, or increasing the loading level of the thermally conductive filler. However, there are problems in that a filler with high thermal conductivity is expensive, and increasing the loading level of the filler increases the viscosity of the composition.

To solve this problem, there is a method of using only spherical alumina powder, but in order to achieve high thermal conductivity, a large amount of alumina powder is required compared to amorphous alumina, which increases the viscosity of the composition and deteriorates its processability. In addition, there is a method of using spherical alumina powder with a large particle size to achieve high thermal conductivity, but if the particle size is too large, there are problems such as the reaction vessel or stirring blade being scraped when the material is stirred, the processability during sheet molding being poor, and the molded sheet becoming brittle.
Furthermore, when the amount of alumina powder filled in the silicone cured product is high, there is a tendency for the hardness of the cured product to decrease significantly when used for long periods of time at high temperatures. Depending on the application, such as in modules that are subject to strong vibrations, this can result in insufficient recovery, leading to poor adhesion and an increase in thermal resistance over time.

Patent Document 1 describes a heat-resistant, thermally conductive silicone composition that contains spherical fused and solidified alumina.
Patent Document 2 describes a heat-resistant silicone resin composition that contains an organic polycyclic aromatic compound that contains one or more secondary amino groups and one or more ketone groups in a cyclic structure.
Patent Document 3 describes a thermally conductive silicone composition containing spherical alumina and amorphous alumina having different particle sizes.

JP 2018-123200 A International Publication No. 2021/161580 JP 2021-176945 A

The present invention has been made in consideration of the above circumstances, and aims to provide a thermally conductive silicone composition and a cured product thereof that are excellent in insulation, thermal conductivity, processability, and heat resistance. In particular, the present invention aims to provide a thermally conductive silicone composition and a cured product thereof that do not lose hardness even when used at high temperatures for long periods of time.

（式中、Ｒ^４は独立に炭素原子数１～６のアルキル基であり、ｃは５～１００の整数である。）
を含むものである熱伝導性シリコーン組成物を提供する。 In order to solve the above problems, the present invention provides a thermally conductive silicone composition, comprising:
(A) organopolysiloxane having two or more alkenyl groups in one molecule: 100 parts by mass,
(B) an organohydrogenpolysiloxane having two or more hydrosilyl groups per molecule: an amount such that the number of moles of hydrosilyl groups is 0.1 to 5.0 times the number of moles of alkenyl groups derived from component (A);
(C) a thermally conductive filler consisting of the following (C-1) to (C-4): 4,300 to 5,800 parts by mass,
(C-1) Spherical alumina filler having an average particle size of more than 70 μm and not more than 135 μm: 1,750 to 3,000 parts by mass,
(C-2) Spherical alumina filler having an average particle size of more than 8 μm and not more than 40 μm: 750 to 2,000 parts by mass,
(C-3) irregular alumina filler having an average particle size of more than 0.4 μm and not more than 4 μm: 750 to 1,500 parts by mass,
(C-4) Spherical alumina filler having an average particle size of more than 0.7 μm and not more than 4 μm: 125 to 750 parts by mass,
(D) a platinum group metal curing catalyst: 0.1 to 2,000 ppm by mass of platinum group metal element relative to the component (A);
(E) Addition reaction inhibitor: 0.01 to 2.0 parts by mass,
(F) cerium oxide: 7.5 to 25 parts by mass, and (G) one or more surface treatment agents selected from the following (G-1) and (G-2): 0.01 to 300 parts by mass,
(G-1) An alkoxysilane compound represented by the following general formula (1):
R ¹ _a R ² _b Si(OR ³ ) _4-ab (1)
(In the formula, R ¹ is independently an alkyl group having 6 to 15 carbon atoms, R ² is independently a group selected from an alkyl group having 1 to 5 carbon atoms, an aryl group having 6 to 12 carbon atoms, and an aralkyl group having 7 to 12 carbon atoms, R ³ is independently an alkyl group having 1 to 6 carbon atoms, a is an integer of 1 to 3, b is an integer of 0 to 2, and a+b is an integer of 1 to 3.)
(G-2) Dimethylpolysiloxane having one molecular chain end blocked with a trialkoxysilyl group, represented by the following general formula (2):

(In the formula, R ⁴ is independently an alkyl group having 1 to 6 carbon atoms, and c is an integer of 5 to 100.)
The present invention provides a thermally conductive silicone composition comprising:

Such a thermally conductive silicone composition will give a cured product with excellent insulation, thermal conductivity, processability, and heat resistance, and in particular will give a cured product whose hardness does not decrease even when used for long periods at high temperatures. Such a thermally conductive silicone composition is suitable for use as a thermally conductive resin molded body to be placed, for example, between heat-generating and heat-dissipating components in electronic devices.

（式中、Ｒ^５は独立に炭素原子数１～６のアルキル基、炭素原子数６～１２のアリール基、及び炭素原子数７～１２のアラルキル基から選ばれる基であり、ｄは５～２，０００の整数である。） In the present invention, it is preferable that the component (H) further contains an organopolysiloxane having a kinetic viscosity at 23°C of 10 to 100,000 ^mm2 /s, represented by the following general formula (3), in an amount of 0.1 to 100 parts by mass per 100 parts by mass of the component (A).

(In the formula, ^R5 is independently a group selected from an alkyl group having 1 to 6 carbon atoms, an aryl group having 6 to 12 carbon atoms, and an aralkyl group having 7 to 12 carbon atoms, and d is an integer of 5 to 2,000.)

Such a thermally conductive silicone composition has excellent flexibility and is less likely to cause oil bleeding in the resulting cured product.

In addition, in the present invention, it is preferable that the viscosity of the thermally conductive silicone composition measured with a flow tester viscometer at 23°C is 4,000 Pa·s or less.

Such a thermally conductive silicone composition has excellent moldability (processability).

The present invention also provides a thermally conductive silicone cured product, which is a cured product of the thermally conductive silicone composition described above.

Such thermally conductive silicone cured products have excellent insulation, thermal conductivity, processability, and heat resistance, and are suitable for use, for example, as thermally conductive resin molded bodies placed between heat-generating and heat-dissipating components in electronic devices.

In the present invention, it is also preferable that the thermally conductive silicone cured product is in the form of a sheet.

Such thermally conductive silicone cured products are easy to handle.

In addition, in the present invention, it is preferable that the hardness of the thermally conductive silicone cured product measured with an Asker C hardness tester after aging at 150°C for 500 hours is at least -5 points and at most 40 points lower than the hardness before aging.

When this type of thermally conductive silicone composition is cured, the loss in hardness is minimal even when used at high temperatures for long periods of time.

In addition, in the present invention, it is preferable that the thermal conductivity of the thermally conductive silicone cured product at 23°C measured by the hot disk method is 7.5 W/m·K or more.

Such thermally conductive silicone cured products have excellent thermal conductivity.

In the present invention, it is also preferable that the thermally conductive silicone cured product has a dielectric breakdown voltage of 10 kV/mm or more at a thickness of 1 mm.

This type of thermally conductive silicone cured material can ensure stable insulation during use.

As described above, the thermally conductive silicone composition of the present invention can provide a thermally conductive silicone composition and its cured product that are excellent in insulating properties, thermal conductivity, and processability. It can also provide a thermally conductive silicone cured product that is molded into a sheet shape and that suppresses loss of hardness during storage at high temperatures and has a thermal conductivity of 7.5 W/m·K or more.

As mentioned above, there was a need to develop a thermally conductive silicone composition and its cured product that have excellent insulation properties, thermal conductivity, processability, and heat resistance.

As a result of intensive research conducted by the present inventors to achieve the above object, it was found that the above problem can be solved by mixing spherical alumina filler having an average particle size of more than 8 μm and not more than 40 μm, amorphous alumina filler having an average particle size of more than 0.4 μm and not more than 4 μm, spherical alumina filler having an average particle size of more than 0.7 μm and not more than 4 μm, and spherical alumina filler having an average particle size of more than 70 μm and not more than 135 μm in a specific ratio, and using cerium oxide in combination. In other words, it was found that by blending a large amount of spherical alumina filler having an average particle size of more than 70 μm and not more than 135 μm, which has a small specific surface area, it is possible to effectively improve thermal conductivity, and to provide a thermally conductive silicone composition and its cured product that have low viscosity and excellent processability.
In addition, by using a combination of spherical alumina filler and amorphous alumina filler having an average particle size of 40 μm or less, and particularly by using a combination of amorphous alumina filler and spherical alumina filler with a particle size of 4 μm or less, the flowability of the thermally conductive silicone composition is improved, improving processability.Furthermore, by using spherical alumina filler for particles of 5 μm or more, it was found that wear on the reaction vessel and stirring blades is suppressed and insulation is improved.
In other words, the inventors have discovered that small particle size spherical alumina filler and large particle size spherical alumina filler complement each other's shortcomings to provide a thermally conductive silicone composition and cured product that are capable of achieving the above-mentioned objectives.
The inventors also discovered that adding cerium oxide to the above-mentioned thermally conductive silicone composition makes it possible to inhibit the decrease in hardness of the cured product during storage at high temperatures, which led to the creation of the present invention.

（式中、Ｒ^４は独立に炭素原子数１～６のアルキル基であり、ｃは５～１００の整数である。）
を含むものである熱伝導性シリコーン組成物である。 That is, the present invention provides a thermally conductive silicone composition,
(A) organopolysiloxane having two or more alkenyl groups in one molecule: 100 parts by mass,
(B) an organohydrogenpolysiloxane having two or more hydrosilyl groups per molecule: an amount such that the number of moles of hydrosilyl groups is 0.1 to 5.0 times the number of moles of alkenyl groups derived from component (A);
(C) a thermally conductive filler consisting of the following (C-1) to (C-4): 4,300 to 5,800 parts by mass,
(C-1) Spherical alumina filler having an average particle size of more than 70 μm and not more than 135 μm: 1,750 to 3,000 parts by mass,
(C-2) Spherical alumina filler having an average particle size of more than 8 μm and not more than 40 μm: 750 to 2,000 parts by mass,
(C-3) irregular alumina filler having an average particle size of more than 0.4 μm and not more than 4 μm: 750 to 1,500 parts by mass,
(C-4) Spherical alumina filler having an average particle size of more than 0.7 μm and not more than 4 μm: 125 to 750 parts by mass,
(D) a platinum group metal curing catalyst: 0.1 to 2,000 ppm by mass of platinum group metal element relative to the component (A);
(E) Addition reaction inhibitor: 0.01 to 2.0 parts by mass,
(F) cerium oxide: 7.5 to 25 parts by mass, and (G) one or more surface treatment agents selected from the following (G-1) and (G-2): 0.01 to 300 parts by mass,
(G-1) An alkoxysilane compound represented by the following general formula (1):
R ¹ _a R ² _b Si(OR ³ ) _4-ab (1)
(In the formula, R ¹ is independently an alkyl group having 6 to 15 carbon atoms, R ² is independently a group selected from an alkyl group having 1 to 5 carbon atoms, an aryl group having 6 to 12 carbon atoms, and an aralkyl group having 7 to 12 carbon atoms, R ³ is independently an alkyl group having 1 to 6 carbon atoms, a is an integer of 1 to 3, b is an integer of 0 to 2, and a+b is an integer of 1 to 3.)
(G-2) Dimethylpolysiloxane having one molecular chain end blocked with a trialkoxysilyl group, represented by the following general formula (2):

(In the formula, R ⁴ is independently an alkyl group having 1 to 6 carbon atoms, and c is an integer of 5 to 100.)
The thermally conductive silicone composition comprises:

The present invention is described in detail below, but is not limited to these.

[Thermal conductive silicone composition]
The thermally conductive silicone composition of the present invention comprises:
(A) an organopolysiloxane having two or more alkenyl groups in each molecule;
(B) an organohydrogenpolysiloxane having two or more hydrosilyl groups in one molecule;
(C) a thermally conductive filler consisting of the following (C-1) to (C-4):
(C-1) A spherical alumina filler having an average particle size of more than 70 μm and not more than 135 μm;
(C-2) Spherical alumina filler having an average particle size of more than 8 μm and not more than 40 μm;
(C-3) irregular alumina filler having an average particle size of more than 0.4 μm and not more than 4 μm;
(C-4) Spherical alumina filler having an average particle size of more than 0.7 μm and not more than 4 μm;
(D) platinum group metal curing catalyst;
(E) an addition reaction regulator,
(F) cerium oxide,
(G) A surface treatment agent is contained as an essential component. Each component will be described in detail below.

[(A) Alkenyl-containing organopolysiloxane]
The alkenyl-containing organopolysiloxane, which is component (A), is an organopolysiloxane having two or more alkenyl groups bonded to silicon atoms in one molecule, that is, an organopolysiloxane having two or more alkenyl groups in one molecule, and is the main component of the thermally conductive silicone composition of the present invention.Normally, the main chain part is generally basically composed of repetitions of diorganosiloxane units, but this may also include a branched structure in a part of the molecular structure, or may also be a cyclic body, but from the viewpoint of physical properties such as mechanical strength of the obtained thermally conductive silicone cured product, linear diorganopolysiloxane is preferred.

Examples of the alkenyl group include those having 2 to 8 carbon atoms, such as vinyl, allyl, propenyl, isopropenyl, butenyl, hexenyl, and cyclohexenyl groups. Of these, lower alkenyl groups such as vinyl and allyl groups are preferred, with vinyl being particularly preferred. The alkenyl group is characterized by the presence of two or more alkenyl groups in one molecule, preferably 2 to 6, and more preferably 2 to 3. In order to ensure good flexibility in the resulting thermally conductive silicone cured product, it is most preferred that they are present bonded only to silicon atoms at the ends of the molecular chain.

Examples of functional groups other than the alkenyl group bonded to the silicon atom include monovalent hydrocarbon groups having 1 to 10 carbon atoms, preferably 1 to 6 carbon atoms. Examples include alkyl groups such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, neopentyl, hexyl, heptyl, octyl, nonyl, decyl, and dodecyl groups; cycloalkyl groups such as cyclopentyl, cyclohexyl, and cycloheptyl groups; aryl groups such as phenyl, tolyl, xylyl, naphthyl, and biphenylyl groups; and aralkyl groups such as benzyl, phenylethyl, phenylpropyl, and methylbenzyl groups. Among these, methyl, ethyl, propyl, and phenyl groups are preferably used. In addition, the functional groups other than the alkenyl groups bonded to the silicon atom may all be the same or different.

The kinetic viscosity of this organopolysiloxane at 23°C is typically in the range of 10 to 100,000 ^{mm 2} /s, and particularly preferably 500 to 50,000 ^{mm 2} /s. If the kinetic viscosity is 10 mm ² /s or more, the storage stability of the resulting thermally conductive silicone composition will be good, and if it is 100,000 mm ² /s or less, the extensibility of the resulting thermally conductive silicone composition will be good. In this specification, the kinetic viscosity is the value measured at 23°C using a Cannon-Fenske viscometer according to the method described in JIS Z 8803:2011.

The organopolysiloxane of component (A) may be used alone or in combination of two or more types with different kinetic viscosities.

[(B) Organohydrogenpolysiloxane]
The organohydrogenpolysiloxane of component (B) is an organohydrogenpolysiloxane having at least two, preferably 2 to 100, hydrosilyl groups (hydrogen atoms directly bonded to silicon atoms) in one molecule, and acts as a crosslinking agent for component (A). That is, the hydrosilyl groups in component (B) and the alkenyl groups in component (A) are added by a hydrosilylation reaction promoted by the platinum group metal curing catalyst of component (D) described below, to give a three-dimensional network structure having a crosslinked structure. Note that if the number of hydrosilyl groups in one molecule is less than two, the composition will not cure.

（式中、Ｒ^６は独立に水素原子、又は炭素数１～１２のアルキル基、炭素数６～１２のアリール基、及び炭素数７～１２のアラルキル基から選ばれる１価炭化水素基である。ただし、１分子中の２個以上、好ましくは２～１０個のＲ^６は水素原子である。また、ｅは１以上の整数、好ましくは１０～２００の整数である。） The organohydrogenpolysiloxane of component (B) is preferably one represented by the average structural formula (4) below, but is not limited to this.

(In the formula, R ⁶ is independently a hydrogen atom or a monovalent hydrocarbon group selected from an alkyl group having 1 to 12 carbon atoms, an aryl group having 6 to 12 carbon atoms, and an aralkyl group having 7 to 12 carbon atoms. However, at least two, preferably at least two and at most ten, R ^6s in one molecule are hydrogen atoms. Also, e is an integer of 1 or more, preferably an integer of 10 to 200.)

In formula (4), R ⁶ is independently a hydrogen atom or a monovalent hydrocarbon group selected from an alkyl group having 1 to 12 carbon atoms, an aryl group having 6 to 12 carbon atoms, and an aralkyl group having 7 to 12 carbon atoms. However, at least two, preferably 2 to 10, R ^6s in one molecule are hydrogen atoms. Examples of the monovalent hydrocarbon group other than a hydrogen atom of R ⁶ include alkyl groups such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, neopentyl, hexyl, heptyl, octyl, nonyl, decyl, and dodecyl, cycloalkyl groups such as cyclopentyl, cyclohexyl, and cycloheptyl, aryl groups such as phenyl, tolyl, xylyl, naphthyl, and biphenylyl, and aralkyl groups such as benzyl, phenylethyl, phenylpropyl, and methylbenzyl. Among these monovalent hydrocarbon groups, those having 1 to 10 carbon atoms are preferred, and those having 1 to 6 carbon atoms are particularly preferred, and among these, alkyl groups having 1 to 3 carbon atoms, such as methyl, ethyl and propyl groups, and phenyl groups are preferably used. Furthermore, e is an integer of 1 or more, preferably an integer of 10 to 200.

The amount of component (B) added is such that the hydrosilyl groups derived from component (B) are 0.1 to 5.0 moles per mole of alkenyl groups derived from component (A), i.e., the number of moles of hydrosilyl groups is 0.1 to 5.0 times the number of moles of alkenyl groups derived from component (A). Preferably, the amount is 0.3 to 2.0 moles, more preferably 0.5 to 1.0 moles. If the amount of hydrosilyl groups derived from component (B) is less than 0.1 mole per mole of alkenyl groups derived from component (A), the product will not cure, or the strength of the heat-conductive silicone cured product may be insufficient to maintain its shape as a molded product and make it difficult to handle. If the amount exceeds 5.0 moles, the heat-conductive silicone cured product will lose its flexibility and become brittle.

[(C) Thermally conductive filler]
The thermally conductive filler, which is component (C), is composed of the following components (C-1) to (C-4).
(C-1) A spherical alumina filler having an average particle size of more than 70 μm and not more than 135 μm;
(C-2) Spherical alumina filler having an average particle size of more than 8 μm and not more than 40 μm;
(C-3) irregular alumina filler having an average particle size of more than 0.4 μm and not more than 4 μm;
(C-4) Spherical alumina filler having an average particle size of more than 0.7 μm and not more than 4 μm.
In the present invention, the average particle size is a volume-based cumulative average particle size (median diameter) measured by a laser diffraction/scattering method using a particle size analyzer, Microtrac MT3300EX, manufactured by Nikkiso Co., Ltd.

The spherical alumina filler of the component (C-1) can significantly improve the thermal conductivity. The average particle size of the spherical alumina is more than 70 μm and not more than 135 μm, preferably more than 70 μm and not more than 120 μm, and more preferably more than 70 μm and not more than 100 μm. If the average particle size is 70 μm or less, the effect of improving the thermal conductivity is reduced, and the viscosity of the thermally conductive silicone composition increases, resulting in poor processability. If the average particle size is greater than 135 μm, wear of the reaction vessel and stirring blades becomes significant, and there is a concern that the insulation properties of the thermally conductive silicone composition will decrease. Furthermore, there is a problem that the spherical alumina filler and resin separate during press molding, causing the sheet ends to become filler-rich and embrittled. In this case, the material yield in sheet molding will be significantly reduced. As the spherical alumina filler of the component (C-1), one type or two or more types may be used in combination. When two or more types are used in combination, it is sufficient that each satisfies the above average particle size range.

The spherical alumina filler of component (C-2) improves the thermal conductivity of the thermally conductive silicone composition, and also provides a barrier effect that prevents contact between the amorphous alumina filler (C-3) described below and the reaction kettle or stirring blades, thereby reducing wear. The average particle size is greater than 8 μm and not more than 40 μm, and is preferably 10 to 40 μm. If the average particle size is 8 μm or less, the barrier effect decreases, and wear of the reaction kettle and stirring blades due to the amorphous alumina filler becomes noticeable.

The amorphous alumina filler of component (C-3) also plays a role in improving the thermal conductivity of the thermally conductive silicone composition, but its main role is to adjust the viscosity of the thermally conductive silicone composition, improve smoothness, and improve filling properties. The average particle size of component (C-3) is more than 0.4 μm and not more than 4 μm, and it is more preferable for it to be 0.6 to 3 μm in order to express the above-mentioned characteristics.

The spherical alumina filler of component (C-4) also plays a role in improving the thermal conductivity of the thermally conductive silicone composition, but its main role is to adjust the viscosity of the thermally conductive silicone composition, improve smoothness, and improve filling properties. The average particle size of component (C-4) is more than 0.7 μm and not more than 4 μm, and more preferably more than 0.7 μm and not more than 3 μm in order to express the above-mentioned characteristics.

The amount of component (C-1) to be blended is 1,750 to 3,000 parts by mass, and preferably 1,875 to 2,500 parts by mass, per 100 parts by mass of component (A). If the amount is too small, it is difficult to improve the thermal conductivity, and if the amount is too large, wear of the reaction kettle and stirring blades becomes significant, and the insulating properties of the thermally conductive silicone composition decrease.

The amount of component (C-2) to be blended is 750 to 2,000 parts by mass, and preferably 1,000 to 1,600 parts by mass, per 100 parts by mass of component (A). If the amount is too small, it is difficult to improve the thermal conductivity, and if the amount is too large, the thermally conductive silicone composition loses its fluidity and its moldability is impaired.

The amount of component (C-3) to be blended is 750 to 1,500 parts by mass, and preferably 900 to 1,250 parts by mass, per 100 parts by mass of component (A). If the amount is too small, it is difficult to improve the thermal conductivity, and if the amount is too large, the thermally conductive silicone composition loses its fluidity and its moldability is impaired.

The amount of component (C-4) to be blended is 125 to 750 parts by mass, and preferably 125 to 375 parts by mass, per 100 parts by mass of component (A). If the amount is too small, it is difficult to improve the thermal conductivity, and if the amount is too large, the thermally conductive silicone composition loses its fluidity and its moldability is impaired.

Furthermore, the blending amount of component (C) (i.e. the total blending amount of components (C-1) to (C-4) above) must be 4,300 to 5,800 parts by mass, and preferably 4,500 to 5,200 parts by mass, per 100 parts by mass of component (A). If this blending amount is less than 4,300 parts by mass, the thermal conductivity of the resulting thermally conductive silicone composition will deteriorate. If it exceeds 5,800 parts by mass, the resulting thermally conductive silicone composition will lose its fluidity and its moldability will be impaired.

By using component (C) in the above blending ratio, the effects of the present invention described above can be achieved more advantageously and reliably.

[(D) Platinum group metal curing catalyst]
The platinum group metal curing catalyst of component (D) is a catalyst for promoting the addition reaction of the alkenyl group derived from component (A) and the hydrosilyl group derived from component (B), and examples of catalysts well known for use in hydrosilylation reactions include platinum group metals such as platinum (including platinum black), rhodium, and palladium, H ₂ PtCl _{4.nH 2} O, H ₂ PtCl _{6.nH 2} O, NaHPtCl _{6.nH 2} O, KaHPtCl _{6.nH 2} O, Na ₂ PtCl _{6.nH 2} O, K ₂ PtCl _{4.nH 2} O, PtCl _{4.nH 2} O, PtCl ₂ , and Na ₂ HPtCl _{4.nH 2 O.} O (wherein, n is an integer of 0 to 6, preferably 0 or 6), chloroplatinic acid and chloroplatinate salts, alcohol-modified chloroplatinic acid (see U.S. Pat. No. 3,220,972), complexes of chloroplatinic acid and olefins (see U.S. Pat. Nos. 3,159,601, 3,159,662 and 3,775,452), platinum black, platinum group metals such as palladium supported on a support such as alumina, silica or carbon, rhodium-olefin complexes, chlorotris(triphenylphosphine)rhodium (Wilkinson's catalyst), complexes of platinum chloride, chloroplatinic acid or chloroplatinate salts with vinyl group-containing siloxanes, particularly vinyl group-containing cyclic siloxanes.

The amount of component (D) used is 0.1 to 2,000 ppm, preferably 50 to 1,000 ppm, calculated as the mass of platinum group metal element relative to component (A).

[(E) Reaction control agent]
The addition reaction inhibitor of component (E) is not particularly limited as long as it is a known addition reaction inhibitor used in normal addition reaction curing silicone compositions. Examples include acetylene compounds such as 1-ethynyl-1-hexanol, 3-butyn-1-ol, and ethynylmethylidene carbinol, various nitrogen compounds, organic phosphorus compounds, oxime compounds, and organic chloro compounds. When component (E) is used, the amount is preferably about 0.01 to 2.0 parts by mass, and more preferably about 0.1 to 1.2 parts by mass, per 100 parts by mass of component (A). If the amount of component (E) is too small, the addition reaction may proceed, resulting in poor handling of the thermally conductive silicone composition, and if the amount is too large, the curing reaction may not proceed, resulting in poor molding efficiency.

[(F) Cerium oxide]
The purpose of the cerium oxide in component (F) is to improve heat resistance, in particular to inhibit softening and deterioration of the cured product of the thermally conductive silicone composition. The amount of cerium oxide added is 7.5 to 25 parts by mass, and preferably 8.0 to 14 parts by mass, per 100 parts by mass of component (A). If the amount added is outside this range, a decrease in hardness may be observed when the composition is stored at a high temperature of 150°C.

By adding cerium oxide, the cured product of the thermally conductive silicone composition has excellent heat resistance. Specifically, the hardness of the cured product measured with an Asker C hardness tester after aging at 150°C for 500 hours is preferably -5 points or more and +40 points or less, and more preferably -3 points or more and +20 points or less, compared to the hardness before aging.

[(G) Surface treatment agent]
The purpose of the surface treatment agent for component (G) is to hydrophobize component (C) during preparation of the thermally conductive silicone composition, thereby improving the wettability with component (A) and dispersing component (C) uniformly within the matrix made of component (A). Component (G) is one or more surface treatment agents selected from components (G-1) and (G-2) shown below.

The component (G-1) is an alkoxysilane compound represented by the following general formula (1).
R ¹ _a R ² _b Si(OR ³ ) _4-ab (1)
(In the formula, R ¹ is independently an alkyl group having 6 to 15 carbon atoms, R ² is independently a group selected from an alkyl group having 1 to 5 carbon atoms, an aryl group having 6 to 12 carbon atoms, and an aralkyl group having 7 to 12 carbon atoms, R ³ is independently an alkyl group having 1 to 6 carbon atoms, a is an integer of 1 to 3, b is an integer of 0 to 2, and a+b is an integer of 1 to 3.)

In the above general formula (1), examples of the alkyl group having 6 to 15 carbon atoms represented by ^R1 include a hexyl group, an octyl group, a nonyl group, a decyl group, a dodecyl group, a tetradecyl group, etc. When the number of carbon atoms of the alkyl group represented by ^R1 falls within the range of 6 to 15, the wettability of component (A) is sufficiently improved, the handleability is good, and the low-temperature properties of the composition are excellent.

Examples of the alkyl group having 1 to 5 carbon atoms represented by ^R2 include, for example, a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a tert-butyl group, a pentyl group, and a neopentyl group. Examples of the aryl group having 6 to 12 carbon atoms include a phenyl group, a tolyl group, a xylyl group, a naphthyl group, and a biphenylyl group. Examples of the aralkyl group having 7 to 12 carbon atoms include a benzyl group, a phenylethyl group, a phenylpropyl group, and a methylbenzyl group. Among these, preferred are alkyl groups having 1 to 3 carbon atoms, such as a methyl group, an ethyl group, and a propyl group, and a phenyl group. Examples of ^R3 include a methyl group, an ethyl group, a propyl group, a butyl group, and a hexyl group.

（式中、Ｒ^４は独立に炭素原子数１～６のアルキル基であり、具体的には前記Ｒ^３で例示されたアルキル基と同じものが例示できる。ｃは５～１００、好ましくは５～７０、特に好ましくは１０～５０の整数である。） Component (G-2) is a dimethylpolysiloxane having one molecular chain end blocked with a trialkoxysilyl group, as represented by the following general formula (2).

(In the formula, R ⁴ is independently an alkyl group having 1 to 6 carbon atoms, and specific examples thereof include the same alkyl groups as those exemplified for R ³ above. c is an integer of 5 to 100, preferably 5 to 70, and particularly preferably 10 to 50.)

As the surface treatment agent for component (G), either component (G-1) or component (G-2) may be used alone or in combination with both.

When component (G) is added, the amount is 0.01 to 300 parts by mass, and preferably 0.1 to 200 parts by mass, per 100 parts by mass of component (A). If the proportion of this component exceeds 300 parts by mass, there is a possibility that oil separation will be induced.

（式中、Ｒ^５は独立に炭素原子数１～６のアルキル基、炭素原子数６～１２のアリール基、及び炭素原子数７～１２のアラルキル基から選ばれる基、ｄは５～２，０００の整数である。） [(H) Organopolysiloxane]
The thermally conductive silicone composition of the present invention may contain an organopolysiloxane as component (H) for the purpose of adjusting the viscosity of the thermally conductive silicone composition and imparting other properties. As component (H), an organopolysiloxane represented by the following general formula (3) and having a kinetic viscosity at 23° C. of 10 to 100,000 ^{mm 2} /s may be added. Component (H) may be used alone or in combination of two or more types.

(In the formula, ^R5 is independently a group selected from an alkyl group having 1 to 6 carbon atoms, an aryl group having 6 to 12 carbon atoms, and an aralkyl group having 7 to 12 carbon atoms, and d is an integer of 5 to 2,000.)

In the above general formula (3), ^R5 is independently a group selected from an alkyl group having 1 to 6 carbon atoms, an aryl group having 6 to 12 carbon atoms, and an aralkyl group having 7 to 12 carbon atoms. Specific examples of ^R5 include alkyl groups such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, and pentyl groups, cycloalkyl groups such as cyclopentyl and cyclohexyl groups, aryl groups such as phenyl, tolyl, xylyl, naphthyl, and biphenylyl groups, and aralkyl groups such as benzyl, phenylethyl, phenylpropyl, and methylbenzyl groups. Among these, alkyl groups having 1 to 3 carbon atoms such as methyl, ethyl, and propyl groups, and phenyl groups are preferred, with methyl and phenyl groups being particularly preferred.

From the viewpoint of the required viscosity, the above d is preferably an integer between 5 and 2,000, and particularly preferably an integer between 10 and 1,000.

Furthermore, the kinetic viscosity of component (H) at 23°C is preferably 10 to 100,000 ^{mm 2} /s, and particularly preferably 100 to 10,000 ^{mm 2} /s. If the kinetic viscosity is 10 mm ² /s or higher, the cured product of the resulting composition is less prone to oil bleeding. If the kinetic viscosity is 100,000 mm ² /s or lower, the resulting thermally conductive silicone composition will have excellent flexibility.

When component (H) is added to the thermally conductive silicone composition of the present invention, there are no particular limitations on the amount added, so long as the desired effect is obtained, but the amount is preferably 0.1 to 100 parts by mass, and more preferably 1 to 50 parts by mass, per 100 parts by mass of component (A). If the amount added is within this range, it is easy to maintain good fluidity and workability in the thermally conductive silicone composition before curing, and it is also easy to fill the composition with the thermally conductive filler of component (C).

[Other ingredients]
The thermally conductive silicone composition of the present invention may further contain other components depending on the object of the present invention, for example, optional components such as a heat resistance improver such as iron oxide, a viscosity modifier such as silica, a colorant, and a mold release agent.

[Thermal conductive silicone cured product]
The present invention provides a thermally conductive silicone cured product, which is a cured product of the above-mentioned thermally conductive silicone composition. The thermally conductive silicone cured product of the present invention has excellent insulation properties, thermal conductivity, processability, and heat resistance, and is suitable for use as a thermally conductive resin molded product to be installed between heat generating parts and heat dissipating parts in electronic devices, for example. In addition, if the thermally conductive silicone cured product is in the form of a sheet, it is preferable because it is easy to handle.

[Preparation of Thermally Conductive Silicone Composition]
The thermally conductive silicone composition of the present invention can be prepared by uniformly mixing the above-mentioned components in a conventional manner.

[Viscosity of Thermally Conductive Silicone Composition]
The viscosity of the thermally conductive silicone composition of the present invention is preferably 4,000 Pa·s or less, and more preferably 3,000 Pa·s or less, at 23° C. If the viscosity is 4,000 Pa·s or less, moldability will not be impaired. In the present invention, this viscosity is based on measurements using a flow tester viscometer.

[Method for producing thermally conductive silicone cured product]
The curing conditions for molding the thermally conductive silicone composition may be the same as those for known addition reaction curing type silicone rubber compositions, for example, the composition will cure sufficiently at room temperature, but may be heated if necessary. Addition curing is preferably carried out at 100 to 120°C for 8 to 12 minutes. Such a thermally conductive silicone cured product of the present invention has excellent thermal conductivity.

[Thermal Conductivity of Cured Thermally Conductive Silicone Product]
The thermal conductivity of the heat-conductive silicone cured product of the present invention, measured at 23° C. by the hot disk method, should desirably be at least 7.5 W/m·K, and especially at least 8.0 W/m·K.

[Dielectric breakdown voltage of thermally conductive silicone cured product]
The dielectric breakdown voltage of the thermally conductive silicone cured product in the present invention is preferably 10 kV/mm or more, more preferably 12 kV/mm or more, as measured in accordance with JIS K 6249:2003 at a thickness of 1 mm. When the dielectric breakdown voltage of the cured product is 10 kV/mm or more, stable insulation can be ensured during use. Such dielectric breakdown voltage can be adjusted by adjusting the type and purity of the filler.

[Hardness of cured thermally conductive silicone product]
The hardness of the thermally conductive silicone cured product of the present invention is preferably 60 or less, more preferably 40 or less, more preferably 30 or less, and more preferably 5 or more, as measured at 23°C using an Asker C hardness tester. When the hardness is 60 or less, the product deforms to fit the shape of the heat-dissipating body, and exhibits good heat dissipation characteristics without applying stress to the heat-dissipating body. Such hardness can be adjusted by changing the ratio of the (A) component and the (B) component to adjust the crosslink density. In addition, it is preferable that the hardness after aging at 150°C for 500 hours is -5 points or more and 40 points or less compared to the hardness before aging. If the cured product of such a thermally conductive silicone composition is used for a long time at high temperatures, the decrease in hardness is small.

The present invention will be described in detail below with examples and comparative examples, but the present invention is not limited to the following examples. The viscosity of the composition was measured at 23°C using a flow tester viscometer. The measuring device used was a CFT-500EX manufactured by Shimadzu Corporation. The time and stroke were plotted with a die hole diameter of 2 mm, die length of 2 mm, and test load of 10 kg, and the viscosity was calculated from the slope. The average particle size is the cumulative average particle size (median diameter) on a volume basis measured using a Microtrac MT3300EX particle size analyzer manufactured by Nikkiso Co., Ltd.

（式中、Ｘはビニル基であり、ｆは下記粘度を与える数である。）
（Ａ－１）動粘度：６００ｍｍ^２／ｓ
（Ａ－２）動粘度：３０，０００ｍｍ^２／ｓ Components (A) to (H) used in the following Examples and Comparative Examples are shown below.
Component (A):
An organopolysiloxane represented by the following formula (5):

(In the formula, X is a vinyl group, and f is a number that gives the following viscosity.)
(A-1) Kinematic viscosity: 600mm ² /s
(A-2) Kinematic viscosity: 30,000mm ² /s

（Ｂ－２）成分：
下記式（６－２）で示されるオルガノハイドロジェンポリシロキサン。

(B-1) Component:
Organohydrogenpolysiloxane represented by the following formula (6-1):

(B-2) Component:
Organohydrogenpolysiloxane represented by the following formula (6-2):

(C) Component:
Spherical alumina filler and irregular alumina filler with average particle diameters as shown below.
(C-1) Spherical alumina filler having an average particle size of 98.8 μm.
(C-2) Spherical alumina filler having an average particle size of 23.4 μm.
(C-3) Irregular alumina filler having an average particle size of 1.7 μm.
(C-4) Spherical alumina filler having an average particle size of 2.3 μm.

(D) Component:
5% by mass solution of chloroplatinic acid in 2-ethylhexanol.

(E) Component:
Ethynylmethylidene carbinol.

Component (F):
Cerium oxide.

Component (G): Component (G-2) A dimethylpolysiloxane having an average degree of polymerization of 30 and one end blocked with a trimethoxysilyl group, as shown in the following formula (7).

Component (H) A plasticizer, which is a dimethylpolysiloxane having a kinetic viscosity of 100 mm ² /s at 23° C. and represented by the following formula (8):

[Examples 1 to 4, Comparative Examples 1 to 4]
In Examples 1 to 4 and Comparative Examples 1 to 4, thermally conductive silicone compositions were prepared as described below using the above components (A) to (H) in the prescribed amounts shown in Table 1 below, then molded and cured, and the viscosity of the thermally conductive silicone composition, and the thermal conductivity, hardness, and breakdown voltage of the cured product were measured according to the methods described below. The results are also shown in Table 1.

[Preparation of Thermally Conductive Silicone Composition]
Components (A), (C), (F), (G), and (H) were added in the prescribed amounts shown in Examples 1 to 4 and Comparative Examples 1 to 4 in Table 1 below, and kneaded for 60 minutes with a planetary mixer. Component (D) was then added in the prescribed amount shown in Examples 1 to 4 and Comparative Examples 1 to 4 in Table 1 below, and an effective amount of KF-54, a phenyl-modified silicone oil manufactured by Shin-Etsu Chemical Co., Ltd., was further added as an internal release agent to promote release from the separator, and kneaded for 30 minutes.
To this was further added components (B) and (E) in the prescribed amounts shown in Examples 1 to 4 and Comparative Examples 1 to 4 in Table 1 below, and the mixture was kneaded for 30 minutes to obtain a thermally conductive silicone composition.

[Molding method]
The thermally conductive silicone compositions obtained in Examples 1 to 4 and Comparative Examples 1 to 4 were poured into a mold measuring 60 mm in length, 60 mm in width, and 6 mm or 1 mm in thickness, and molded using a press molding machine at 120°C for 10 minutes.

[Evaluation method]
Viscosity of the thermally conductive silicone composition:
The viscosity of the thermally conductive silicone compositions obtained in Examples 1 to 4 and Comparative Examples 1 to 4 was measured in an environment of 23° C. using a flow tester viscometer.

Thermal Conductivity:
The thermally conductive silicone compositions obtained in Examples 1 to 4 and Comparative Examples 1 to 4 were cured using a press molding machine at 120°C for 10 minutes into a 6 mm thick sheet, and the thermal conductivity of two of these sheets was measured using a thermal conductivity meter (product name: TPS-2500S, manufactured by Kyoto Electronics Manufacturing Co., Ltd.).

Breakdown voltage:
The thermally conductive silicone compositions obtained in Examples 1 to 4 and Comparative Examples 1 to 4 were cured using a press molding machine at 120°C for 10 minutes into a 1 mm thick sheet, and the dielectric breakdown voltage was measured in accordance with JIS K 6249:2003.

Hardness:
The thermally conductive silicone compositions obtained in Examples 1 to 4 and Comparative Examples 1 to 4 were cured into sheets having a thickness of 6 mm in the same manner as above, and two of these sheets were then stacked together and measured for hardness using an Asker C hardness tester.

Hardness after storage at 150°C for 500 hours:
The thermally conductive silicone compositions obtained in Examples 1 to 4 and Comparative Examples 1 to 4 were cured using a press molding machine at 120°C for 10 minutes into a 6 mm thick sheet. The cured products were then stored in a high-temperature oven at 150°C for 500 hours, and two of the sheets were then stacked and measured for hardness using an Asker C tester.

In Examples 1 to 4, the viscosity and moldability of the thermally conductive silicone composition, and the thermal conductivity, dielectric breakdown voltage, and hardness of the cured thermally conductive silicone were all good. Furthermore, by adding (F) cerium oxide, no decrease in hardness due to softening deterioration was observed, even when stored at a high temperature of 150°C.

When the (C-4) component (spherical alumina filler with an average particle size of more than 0.7 μm and not more than 4 μm) was not included, as in Comparative Example 1, the viscosity of the thermally conductive silicone composition increased significantly. When the amount of cerium oxide (F) added was outside the range of the present invention, as in Comparative Example 2, the hardness decreased after storage at 150°C for 500 hours. When the amount of thermally conductive filler (C) was too high, as in Comparative Example 3, the wettability of the thermally conductive filler was insufficient, and a grease-like uniform thermally conductive silicone composition could not be obtained. When the amount of thermally conductive filler (C) was too low, as in Comparative Example 4, the thermal conductivity of the thermally conductive silicone cured product decreased significantly.

（式中、Ｒ^４は独立に炭素原子数１～６のアルキル基であり、ｃは５～１００の整数である。）
を含むものであることを特徴とする熱伝導性シリコーン組成物。
［２］：更に、（Ｈ）成分として、下記一般式（３）で表される２３℃における動粘度が１０～１００，０００ｍｍ^２／ｓのオルガノポリシロキサンを前記（Ａ）成分の１００質量部に対して、０．１～１００質量部で含有するものであることを特徴とする上記［１］の熱伝導性シリコーン組成物。

（式中、Ｒ^５は独立に炭素原子数１～６のアルキル基、炭素原子数６～１２のアリール基、及び炭素原子数７～１２のアラルキル基から選ばれる基であり、ｄは５～２，０００の整数である。）
［３］：２３℃におけるフローテスタ粘度計で測定した前記熱伝導性シリコーン組成物の粘度が４，０００Ｐａ・ｓ以下のものであることを特徴とする上記［１］又は上記［２］の熱伝導性シリコーン組成物。
［４］：上記［１］、上記［２］又は上記［３］の熱伝導性シリコーン組成物の硬化物であることを特徴とする熱伝導性シリコーン硬化物。
［５］：前記熱伝導性シリコーン硬化物の形状がシート状のものであることを特徴とする上記［４］の熱伝導性シリコーン硬化物。
［６］：前記熱伝導性シリコーン硬化物のアスカーＣ硬度計で測定した硬さにおいて、１５０℃×５００時間エージング後の硬さが、エージング前の硬さに対して、－５ポイント以上、４０ポイント以下のものであることを特徴とする上記［４］又は上記［５］の熱伝導性シリコーン硬化物。
［７］：前記熱伝導性シリコーン硬化物のホットディスク法により測定した２３℃における熱伝導率が、７．５Ｗ／ｍ・Ｋ以上のものであることを特徴とする上記［４］、上記［５］又は上記［６］の熱伝導性シリコーン硬化物。
［８］：前記熱伝導性シリコーン硬化物の１ｍｍ厚における絶縁破壊電圧が１０ｋＶ／ｍｍ以上のものであることを特徴とする上記［４］、上記［５］、上記［６］又は上記［７］の熱伝導性シリコーン硬化物。 The present specification includes the following aspects.
[1]: A thermally conductive silicone composition,
(A) organopolysiloxane having two or more alkenyl groups in one molecule: 100 parts by mass,
(B) an organohydrogenpolysiloxane having two or more hydrosilyl groups per molecule: an amount such that the number of moles of hydrosilyl groups is 0.1 to 5.0 times the number of moles of alkenyl groups derived from component (A);
(C) a thermally conductive filler consisting of the following (C-1) to (C-4): 4,300 to 5,800 parts by mass,
(C-1) Spherical alumina filler having an average particle size of more than 70 μm and not more than 135 μm: 1,750 to 3,000 parts by mass,
(C-2) Spherical alumina filler having an average particle size of more than 8 μm and not more than 40 μm: 750 to 2,000 parts by mass,
(C-3) irregular alumina filler having an average particle size of more than 0.4 μm and not more than 4 μm: 750 to 1,500 parts by mass,
(C-4) Spherical alumina filler having an average particle size of more than 0.7 μm and not more than 4 μm: 125 to 750 parts by mass,
(D) a platinum group metal curing catalyst: 0.1 to 2,000 ppm by mass of platinum group metal element relative to the component (A);
(E) Addition reaction inhibitor: 0.01 to 2.0 parts by mass,
(F) cerium oxide: 7.5 to 25 parts by mass, and (G) one or more surface treatment agents selected from the following (G-1) and (G-2): 0.01 to 300 parts by mass,
(G-1) An alkoxysilane compound represented by the following general formula (1):
R ¹ _a R ² _b Si(OR ³ ) _4-ab (1)
(In the formula, R ¹ is independently an alkyl group having 6 to 15 carbon atoms, R ² is independently a group selected from an alkyl group having 1 to 5 carbon atoms, an aryl group having 6 to 12 carbon atoms, and an aralkyl group having 7 to 12 carbon atoms, R ³ is independently an alkyl group having 1 to 6 carbon atoms, a is an integer of 1 to 3, b is an integer of 0 to 2, and a+b is an integer of 1 to 3.)
(G-2) Dimethylpolysiloxane having one molecular chain end blocked with a trialkoxysilyl group, represented by the following general formula (2):

(In the formula, R ⁴ is independently an alkyl group having 1 to 6 carbon atoms, and c is an integer of 5 to 100.)
A thermally conductive silicone composition comprising:
[2]: The thermally conductive silicone composition of [1] above, further comprising, as component (H), 0.1 to 100 parts by mass of an organopolysiloxane represented by the following general formula (3) having a kinetic viscosity at 23°C of 10 to 100,000 ^mm2 /s per 100 parts by mass of component (A):

(In the formula, ^R5 is independently a group selected from an alkyl group having 1 to 6 carbon atoms, an aryl group having 6 to 12 carbon atoms, and an aralkyl group having 7 to 12 carbon atoms, and d is an integer of 5 to 2,000.)
[3]: The thermally conductive silicone composition according to [1] or [2] above, characterized in that the viscosity of the thermally conductive silicone composition measured at 23°C with a flow tester viscometer is 4,000 Pa·s or less.
[4]: A thermally conductive silicone cured product, which is a cured product of the thermally conductive silicone composition according to [1], [2] or [3] above.
[5]: The thermally conductive silicone cured product according to [4] above, characterized in that the thermally conductive silicone cured product is in the form of a sheet.
[6]: The thermally conductive silicone cured product according to [4] or [5] above, characterized in that the hardness of the thermally conductive silicone cured product, as measured with an Asker C hardness tester, after aging at 150°C for 500 hours is at least -5 points and at most 40 points compared to the hardness before aging.
[7]: The thermally conductive silicone cured product according to [4], [5] or [6] above, characterized in that the thermal conductivity of the thermally conductive silicone cured product at 23°C measured by the hot disk method is 7.5 W/m K or more.
[8]: The thermally conductive silicone cured product according to [4], [5], [6] or [7] above, characterized in that the thermally conductive silicone cured product has a dielectric breakdown voltage of 10 kV/mm or more at a thickness of 1 mm.

The present invention is not limited to the above-described embodiment. The above-described embodiment is merely an example, and anything that has substantially the same configuration as the technical idea described in the claims of the present invention and exhibits similar effects is included within the technical scope of the present invention.

Claims (8)

A thermally conductive silicone composition comprising:
(A) organopolysiloxane having two or more alkenyl groups in one molecule: 100 parts by mass,
(B) an organohydrogenpolysiloxane having two or more hydrosilyl groups per molecule: an amount such that the number of moles of hydrosilyl groups is 0.1 to 5.0 times the number of moles of alkenyl groups derived from component (A);
(C) a thermally conductive filler consisting of the following (C-1) to (C-4): 4,300 to 5,800 parts by mass,
(C-1) Spherical alumina filler having an average particle size of more than 70 μm and not more than 135 μm: 1,750 to 3,000 parts by mass,
(C-2) Spherical alumina filler having an average particle size of more than 8 μm and not more than 40 μm: 750 to 2,000 parts by mass,
(C-3) irregular alumina filler having an average particle size of more than 0.4 μm and not more than 4 μm: 750 to 1,500 parts by mass,
(C-4) Spherical alumina filler having an average particle size of more than 0.7 μm and not more than 4 μm: 125 to 750 parts by mass,
(D) a platinum group metal curing catalyst: 0.1 to 2,000 ppm by mass of platinum group metal element relative to the component (A);
(E) Addition reaction inhibitor: 0.01 to 2.0 parts by mass,
(F) cerium oxide: 7.5 to 25 parts by mass, and (G) one or more surface treatment agents selected from the following (G-1) and (G-2): 0.01 to 300 parts by mass,
(G-1) An alkoxysilane compound represented by the following general formula (1):
R ¹ _a R ² _b Si(OR ³ ) _4-ab (1)
(In the formula, R ¹ is independently an alkyl group having 6 to 15 carbon atoms, R ² is independently a group selected from an alkyl group having 1 to 5 carbon atoms, an aryl group having 6 to 12 carbon atoms, and an aralkyl group having 7 to 12 carbon atoms, R ³ is independently an alkyl group having 1 to 6 carbon atoms, a is an integer of 1 to 3, b is an integer of 0 to 2, and a+b is an integer of 1 to 3.)
(G-2) Dimethylpolysiloxane having one molecular chain end blocked with a trialkoxysilyl group, represented by the following general formula (2):

(In the formula, R ⁴ is independently an alkyl group having 1 to 6 carbon atoms, and c is an integer of 5 to 100.)
A thermally conductive silicone composition comprising: The thermally conductive silicone composition according to claim 1, further comprising, as component (H), 0.1 to 100 parts by mass of an organopolysiloxane having a kinetic viscosity at 23°C of 10 to 100,000 ^mm2 /s, represented by the following general formula (3), per 100 parts by mass of component (A):

(In the formula, ^R5 is independently a group selected from an alkyl group having 1 to 6 carbon atoms, an aryl group having 6 to 12 carbon atoms, and an aralkyl group having 7 to 12 carbon atoms, and d is an integer of 5 to 2,000.) The thermally conductive silicone composition according to claim 1 or 2, characterized in that the viscosity of the thermally conductive silicone composition measured with a flow tester viscometer at 23°C is 4,000 Pa·s or less. A thermally conductive silicone cured product, which is a cured product of the thermally conductive silicone composition described in claim 1. The thermally conductive silicone cured product according to claim 4, characterized in that the thermally conductive silicone cured product is in the form of a sheet. The thermally conductive silicone cured product according to claim 4 or 5, characterized in that the hardness of the thermally conductive silicone cured product measured with an Asker C hardness tester after aging at 150°C for 500 hours is -5 points or more and 40 points or less compared to the hardness before aging. The thermally conductive silicone cured product according to claim 4 or 5, characterized in that the thermal conductivity of the thermally conductive silicone cured product at 23°C measured by the hot disk method is 7.5 W/m·K or more. The thermally conductive silicone cured product according to claim 4 or 5, characterized in that the dielectric breakdown voltage of the thermally conductive silicone cured product at a thickness of 1 mm is 10 kV/mm or more.