US20130299140A1

US20130299140A1 - Insulated thermal interface material

Info

Publication number: US20130299140A1
Application number: US13/886,477
Authority: US
Inventors: Yong-Chien Ling; Chih-Ping Wang; Jen-Yu Liu
Original assignee: National Tsing Hua University NTHU
Current assignee: National Tsing Hua University NTHU
Priority date: 2012-05-10
Filing date: 2013-05-03
Publication date: 2013-11-14
Also published as: TW201345963A; TWI598385B

Abstract

The present invention discloses an insulated thermal interface material for applying between an electronic element and a thermal dissipating element. The insulated thermal interface material at least comprises a base, a first filler and a second filler. The base is a polymer and the first filler is graphene. The first filler and the second filler are dispersed in the base.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This Non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No(s). [101116708] filed in Taiwan, Republic of China [May 10, 2012], the entire contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

This invention relates to an insulated thermal interface material, especially relates to an insulated thermal interface material manufactured by dispersing graphene into a polymer to enhance its conductivity and insulation.

BACKGROUND OF THE INVENTION

The electronic technique has been developed rapidly in recent years. The high-frequency and high-speed of electronic component, as well as the dense and microminiaturization of the integrated circuit, results in an enormously increasing of the heating value from unit volume of electronic component. It is an urgent issue of cooling the electronic component to maintain the performance of electronic parts. According to the abovementioned problems, there is a need for a thermal dissipating element with high thermal conductivity
Formerly, some solutions applied for improving the thermal conductivity of the thermal dissipating element is to decrease the thickness of the elements. However, it will cause problems because of decreasing too much thickness, such as decreasing of the strength, durability and/or electric insulating property of thermal dissipating element. One of them is formation of the thermal dissipating element into multilayer structure which composed of an inner layer with great heat resistance and electric insulating property, such as aromatic polyimide, polyamide, polyamideimide or polyethylene naphthalate glycol, and an outer layer formed by thermal interface material containing thermal conductive filler with great heat and electric conductivity, such as silicon rubber. However, the adhesion between outer and inner layers of those multilayer insulating components are unstable, meaning the duration of component is bad and peeling is inevitable as time goes by.
According to the abovementioned problems, another solution is offered. That is, the thermal interface material, such as silicon rubber, is used as an outer layer and the abovementioned silicon rubber is obtained by curing an adhesion promoter which is composed of silicon compounds. However, the thermal conductivity of the inner layer formed by aromatic polyimides is obviously less than that of the outer layer formed by silicon rubber, which decreases the overall thermal conductivity of the complex.
Moreover, although the abovementioned thermal interface material filled with conductive filler to increase the thermal conductivity, such as silicon dioxide, aluminum oxide, aluminum, silicon carbide, silicon nitride, magnesium oxide, magnesium carbonate, zinc oxide and aluminum nitride which are often used as thermal conductive filler in conductive subject of the thermal interface material, there are disadvantages of the individual thermal conductive filler listed as the following:
(1) High filling content is needed for silicon dioxide due to low thermal conductivity. However, the hardness of conductive subject of the thermal interface material is hard to be decreased with high filling content, and molding could not be performed due to excessive viscosity.
(2) Aluminum oxide and aluminum is amphoteric compound which is easily affected by the inner impurities. When the conductive subject of the thermal interface material is epoxy resin, it would have bad influence on heat resistance and permanent deformation by compression.
(3) Zinc oxide is usually precipitated and sedimented when dispersing in the conductive subject of the thermal interface material because it possesses high specific gravity of 5.7, and the high hygroscopicity of zinc oxide powder is undesired.
(4) Silicon carbide also has high specific gravity. The refining silicon carbide powder sold on the current market tends to aggregate and sediment when dispersing in the conductive subject, such as silicon rubber, of the thermal interface material. Silicon carbide is hard to be re-dispersed and processed because it tends to agglomerate.
(5) Silicon nitride and aluminum nitride is easily reacting with water resulting in worst wet fastness.
(6) Magnesium oxide is optional high thermal conductive filler, but it is similar to aluminum nitride which is easily reacting with water resulting in worst wet fastness.
(7) Magnesium carbonate is not stable and tends to decompose into magnesium oxide under high temperature.

SUMMARY OF THE INVENTION

According to the abovementioned disadvantages of the prior art, such as the thermal conductive filler rises the cost of the thermal interface material (eg. lots of thermal conductive filler is needed), heat deterioration (eg. poor heat resistance), surface seepage (eg. poor moisture tolerance) or some limitation between the conductors (eg. epoxy resin) of the thermal interface material, the present invention provides a thermal interface material filled with graphene which could effectively conduct the heat generated by an electronic element to the outside of the thermal dissipating element, moreover, it possesses great electrical insulating property which could be widely used in electric and electronic area, for example, to be used as CPU and thermal dissipating element of high-power transistor chip.
The present invention provides an insulated thermal interface material for applying between an electronic element and a thermal dissipating element. The abovementioned thermal dissipating element at least comprises a base, a first filler and a second filler. Wherein the base is a polymer, the first filler is a graphene and the first filler and the second filler dispersed in the base.
Preferably, the first filler is a graphene with a length-to-thickness ratio or a width-to-thickness ratio of 50˜10000 and selected from a group consisting of a graphene, a graphene doped with nitrogen, a graphene doped with oxygen, a graphene doped with both nitrogen and oxygen, multilayer graphene stacking via van der Waals interaction, multilayer graphene doped with nitrogen and stacking via van der Waals interaction, multilayer graphene doped with oxygen stacking via van der Waals interaction and multilayer graphene doped with both nitrogen and oxygen stacking via van der Waals interaction.
Preferably, the second filler is a thermal conductive inorganic powder and selected from a group consisting of aluminum oxide, magnesium oxide, aluminum nitride, boron nitride, silicon carbide, tin oxide, silicon nitride, aluminum oxide whisker, aluminum nitride whisker, silicon carbide whisker, magnesium oxide whisker and silicon nitride whisker.
Preferably, the base is a silicone rubber and at least contains an organic polysiloxane compound, a curing agent and an adhesion promoter, the weight percentage of the organic polysiloxane compound, the curing agent, the adhesion promoter, the first filler and the second filler are 91˜99.55%, 0.1˜5%, 0.1˜3%, 0.0025˜0.005% and 0.25˜0.5%, respectively.
Preferably, the second filler has an average granularity of 20˜50 μm.
Preferably, the organic polysiloxane compound has a degree of polymerization of 200˜12000 and is represented by the following formula:
R¹ _aSiO_(4-a)/2
Wherein R¹is a single-valence C₁˜C₁₀hydrocarbon group and selected from a group consisting of an alkyl group, a cycloalkyl group, an aryl group, an aralkyl group, an alkyl group substituted by halogen and an alkenyl group, and “a” further represents a positive number of 1.9-2.05.
Preferably, the curing agent is an organic peroxide or a curing agent applying in an alkylation reaction of silane.
Preferably, the adhesion promoter at least comprises a silicon compound with a plurality of substitutes and the substitutes could be selected from a group consisting of a cycloalkyl group, an alkoxyl group, a methyl group, a vinyl group and a silane group.
Preferably, the base is a curing epoxy resin and selected from a group consisting of a linear polyepoxide with epoxide as an end group, a polyepoxide with epoxide at backbond and a polyepoxide with epoxide as side chain.
Preferably, the insulated thermal interface material further comprising a particulate thermoplastic polymer, wherein the weight percentage of the base, the particulate thermoplastic polymer, the first filler and the second filler are 90˜97%, 1˜2%, 0.001˜0.005% and 0.1˜1%, respectively.
Preferably, the particulate thermoplastic polymer comprises a polymer with a glass transition temperature of at least 60° C.
Preferably, the particulate thermoplastic polymer comprises has an average molecular weight higher than 7000.
Preferably, the particulate thermoplastic polymer is selected from a group consisting of a poly(methyl methacrylate) and a methyl methacrylate/methacrylic acid copolymer.
Preferably, the particulate thermoplastic polymer has an average granularity of 0.25˜250 μm.
Preferably, the insulated thermal interface material further comprising a curing agent, and the curing agent contains a dicyandiamide and its derivatives or a metal imidazole compound represented by the following formula:
ML_m
wherein M is a metal and selected from a group consisting of Ag (I), Cu (I), Cu (II), Cd (II), Zn (II), Hg (II), Ni (II) and Co (II), and L is a compound represented by the following formula:
wherein R¹, R²and R³could be selected from a group consisting of hydrogen atom, alkyl group and aryl group, and m is the valence of metal.
Preferably, the thermal conductivity of the insulated thermal interface material is higher than 3 W/mK.
Preferably, the insulated thermal interface material further comprising an additive, and the additive could be selected from a group consisting of coupling agent, lubricant, flow controlling agent, thickener, accelerant, chain-extenders, flexibilizer, dispersant and co-curing agent.
The features and advantages of the present invention will be understood and illustrated in the following specification and FIGS. 1A-2C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A to FIG. 1C are diagrams showing the structure of an insulated thermal interface material according to a first embodiment of the present invention; and

FIG. 2A to FIG. 2C are diagrams showing the structure of an insulated thermal interface material according to a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Due to the difficulties suffered in the prior art, the present invention provides an insulated thermal interface material filled with graphene which could effectively conduct the heat generated by an electronic element to the outside of a thermal dissipating element by percentage collocated of the components, moreover, it could further possesses great electrical insulating property.
The present invention provides an insulated thermal interface material which could be used between an electronic element and a thermal dissipating element. Preferably, the electronic element could be a power transistor, a metal oxide semiconductor transistor, a field effect transistor, a thyristor, a rectifier and a transformer, but the present invention is not limited thereto.
The abovementioned insulated thermal interface material at least comprises a base, a first filler and a second filler. The base is a polymer, and the first filler is a graphene. And further, the first filler and the second filler dispersed in the base.
Preferably, the first filler is a graphene with a length-to-thickness ratio or a width-to-thickness ratio of 50˜10000 and selected from a group consisting of a graphene, a graphene doped with nitrogen, a graphene doped with oxygen, a graphene doped with both nitrogen and oxygen, multilayer graphene stacking via van der Waals interaction, multilayer graphene doped with nitrogen stacking via van der Waals interaction, multilayer graphene doped with oxygen stacking via van der Waals interaction and multilayer graphene doped with both nitrogen and oxygen stacking via van der Waals interaction.
Preferably, the second filler is a thermal conductive inorganic powder and selected from a group consisting of aluminum oxide, magnesium oxide, aluminum nitride, boron nitride, silicon carbide, tin oxide, silicon nitride, aluminum oxide whisker, aluminum nitride whisker, silicon carbide whisker, magnesium oxide whisker and silicon nitride whisker. However, the second filler is not limited thereto and could use any kind of powder which possesses conductivity and insulativity and can be used to improve the conductivity and insulativity of the insulated thermal interface material. The thermal conductive powder could be used alone or combined with two or more components.
Basically, the base is a silicone rubber or an epoxy resin. The percentage of other components varies with different bases, but the thermal interface material disclosed in the present invention at least comprises the first filler and the second filler. The two embodiments of the base will be illustrated as the following.

First Embodiment

In the first embodiment, the base is a silicone rubber and at least contains an organic polysiloxane compound, a curing agent and an adhesion promoter.
Preferably, the organic polysiloxane compound has a polymerization degree of between 200˜12000 and can be represented by the following formula (I):
R¹ _aSiO_(4-a)/2 (I)
In the formula (I), R¹is a single-valence C₁˜C₁₀hydrocarbon group and selected from a group consisting of an alkyl group, a cycloalkyl group, an aryl group, an aralkyl group, an alkyl group substituted by halogen and an alkenyl group. Preferably, “a” further represents a positive number of 1.9-2.05.
Moreover, when R¹is an alkyl group, it could be methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl or decyl group. When R¹is a cycloalkyl group, it could be cyclopentyl or cyclohexyl group. When R¹is an aryl group, it could be phenyl, tolyl, xylyl or naphthyl group. When R¹is an aralkyl group, it could be phenethyl or hydrocinnamyl group. When R¹is a halogen substituted-alkyl group, it could be 3,3,3-trifluoropropyl or 3-chloropropyl group. When R¹is an alkenyl group, it could be vinyl group, allyl group, butenyl group, pentenyl group or hexenyl group. For example, the backbone of the organic polysiloxane compound could be formed by a dimethysiloxane unit or other similar units. And further, the methyl groups of the organic polysiloxane compound can be partially substituted by vinyl group, phenyl group or 3,3,3-trifluoropropyl group. Besides, the terminal of the organic polysiloxane compound could be terminated by tri-organic silyl group or hydrocarbon group. Preferably, tri-organic silyl group comprises trimethylsilyl group, dimethylvinylsilyl group or trivinylsilyl group.
According to the first embodiment of the present invention, the curing agent is an alkylated silane or an organic peroxide. When the curing agent is applied in an alkylation reaction of silane, it is composed of catalysts based on platinum and organicohydrogenpolysiloxane which in average of at least two hydrogen atoms binding to a silicon atom within a single molecule. Preferably, each molecular of the abovementioned organic polysiloxane compound will have at least two or more alkenyl groups bound on its silicon atom. The organohydrogenpolysiloxane of the curing agent served as crosslinking agent and the addition reaction occurs with the alkenyl group in organic polysiloxane compound. In addition, the alkenyl group binding to the silicon atom is preferably vinyl group, and the vinyl group could be at the terminal, side chain or both. Preferably, at least one vinyl group is binding on the silicon atom at the terminal of molecular chain.
Preferably, the organic polysiloxane compound could be selected from one or more combinations of the group listed below: a copolymer of methylvinylsiloxane and dimethysiloxane terminated with trimethylsiloxanes at both ends of the molecular chain, a polymethylvinylsiloxane terminated with trimethylsiloxanes at both ends of the molecular chain, a co-polymer of methylphenylsiloxane, methylvinylsiloxane and dimethysiloxane terminated with trimethylsiloxanes at both ends of the molecular chain, a polydimethylsiloxane terminated with dimethylvinylsiloxanes at both ends of the molecular chain, a polymethylvinylsiloxane terminated with dimethylvinylsiloxanes at both ends of the molecular chain, a co-polymer of methylvinylsiloxane and dimethysiloxane terminated with dimethylvinylsiloxanes at both ends of the molecular chain, a co-polymer of methylphenylsiloxane, methylvinylsiloxane and dimethysiloxane terminated with dimethylvinylsiloxanes at both ends of the molecular chain, and polydimethylsiloxane terminated with trivinylsiloxanes at both ends of the molecular chain. However, the present invention is not limited thereto.
As for the catalyst used to promote the curing of compound is based on platinum and organohydrogenpolysiloxanes combinations, and it could be chloroplatinic acid, alcohol solution of chloroplatinic acid, olefin complex of platinum and vinylsiloxane complex of platinum. There are no particular limitations of the amount of catalyst based on platinum in the combinations, as long as it reaches the effective catalytic amount.
When the curing agent is an organic peroxide, it could be selected from a group consisting of benzoperoxide, dicumyl peroxide, 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexane, ditert-butyl peroxide and benzenecarboperoxoic acid. Although there are no specific limitations of the abovementioned organic polysiloxane compound, preferably, each molecule at least comprises two vinyl groups.
Under the circumstances, the organic polysiloxane compound could be selected from one or more combinations of the group listed below, but the present invention is not limited thereto: a polydimethylsiloxane terminated with dimethylvinylsiloxanes at both ends of the molecular chain, a polydimethylsiloxane terminated with methylphenylvinylsiloxanes at both ends of the molecular chain, a co-polymer of methylphenylsiloxane and dimethysiloxane terminated with dimethylvinylsiloxanes at both ends of the molecular chain, a co-polymer of methylvinylsiloxane and dimethysiloxane terminated with dimethylvinylsiloxane at both ends of the molecular chain, a co-polymer of methylvinylsiloxane and dimethysiloxane terminated with trimethylsiloxanes at both ends of the molecular chain, a poly[methyl(3,3,3-trifluoropropyl)siloxane] terminated with dimethylvinylsiloxanes at both ends of the molecular chain, a co-polymer of methylvinylsiloxane and dimethysiloxane terminated with silanol groups at both ends of the molecular chain and a co-polymer of methylphenylsiloxane, methylvinylsiloxane and dimethysiloxane terminated with silanol groups at both ends of the molecular chain.
According to the first embodiment of the present invention, the addition of adhesion promoter could offer strong adhesiveness between silicon rubber and further overcome the peeling problem suffered in the prior art to get the abovementioned thermal interface material with a long-term durability. Moreover, the adhesion promoter at least comprises a silicon compound with a plurality of substitutes, and the substitutes could be selected from a group consisting of a cycloalkyl group, an alkoxyl group, a methyl group, a vinyl group and a silane group. Preferably, the silicon compound at least contains two abovementioned groups within a molecule.
On the other hand, when the curing agent is applied in an alkylation reaction of silane, preferably, the silicon compound of the adhesion promoter contains vinyl group, silane group or both, and epoxide group, alkoxyl group or both. When the curing agent is the organic peroxide, the silicon compound of the adhesion promoter contains methyl group, vinyl group or both, and epoxide group, alkoxyl group or both. The silicon compound which comprises the abovementioned groups is shown below, but the present invention is not limited thereto.
According to the first embodiment of the present invention, the organic polysiloxane compound, the curing agent, the adhesion promoter, the first filler and the second filler of the thermal interface material has a weight percentage of 91-99.55%, 0.1-5%, 0.1-3%, 0.0025-0.005% and 0.25-0.5%, respectively. Basically, the overall thermal conductivity of thermal interface material will decrease if the addition amount of the first filler and the second filler are too small. However, it will also be hard to mix and affect the molding processing performance if the addition amount of the first filler and the second filler are too much.
Preferably, the second filler has an average granularity of about 20-50 μm. Moreover, the thickness of each composite material could be setup according to the anticipated structure and application when producing the thermal interface material into thin film. Although the present invention is not limited thereto, the thermal conductivity decreases if the outer layer is too thin or too thick. Therefore, the thickness is 30-800 μm, preferably, the thickness is in the range of 50-400 μm.
Besides the abovementioned base, the first filler, the second filler, the curing agent and the adhesion promoter, the thermal interface material provided in the present invention further comprising an additive, and it could be selected from a group consisting of coupling agent, chain-extenders, flexibilizer, dispersant and co-curing agent.
The preparation process of the thermal interface material provided in the present invention is described as followed. First, a mixing device is used, such as kneader, Banbury mixer, planet-type mixer or Shinagawa mixer. If necessary, accompanying of heating to about 100° C. or higher could knead organic polysiloxane compound, first filler and second filler together. In the abovementioned kneading process, introducing and mixing of the following compound, such as enhancing silicon dioxide including pyrolysis of silicon dioxide or precipitation of silicon dioxide, silicon oil or silicone wetting agent, flame retardant including platinum, titanium oxide or benzotriazole, is applicable in precondition of those addition does not affect the thermal conductivity of outer layer
Cooling the mixture obtaining from kneading process to room temperature, and filter through screening program. Then, adding predetermined amount of adhesion promoter into the mixture to perform second kneading using two-roll grinder or Shingawa mixer. If needed, addition of the following agent, such as retarding agent for addition reaction based on acetylene compound including 1-ethynyl-1-cyclohexanol, coloring agent including organic pigment or inorganic pigment, or heat-resistance improvement agents including iron oxide or cerium oxide, is applicable during the second kneading process.
The thermal interface material after second kneading could be used as outer layer coating agent. If needed, addition of solvent including toluene is applicable, and the resulting mixture is mixing in mixing device, such as planet-type mixer or kneader, to form the outer layer coating agent, but the present invention is not limited to single-layer structure. If needed, the abovementioned inner layer (A) compounds including aromatic polyimide could also be combined with thermal interface material (B) into (B)/(A)/(B)/(A)/(B) five-layer structure, or it could also includes isolating layer, such as glass-fiber fabric, graphite flake or aluminum foil, into the structure, but the present invention is not limited thereto.
After the illustration of the abovementioned structure, property and component ratio of the thermal interface material, the property, component ratio and tested result of two experimental sets according to the first embodiment are presented below.
Formulation 1
(a) organic polysiloxane compound: a polydimethylsiloxane terminated with dimethylvinylsiloxanes at both ends of the molecular chain was used with 8000 average degree of polymerization.
(b) graphene doped with 0.005% of nitrogen
(c) kneading between 0.5% dispersing assistant and 0.5% of silicon oxide powder with 40 nm average granularity functionalized with silane groups under room temperature. After filtering against 100 mesh filter, the abovementioned mixture and
(d) 1% adhesion promoter composed of silicon compound as shown in formula (II)
(e) 1.9% of bis(2-methyl benzoyl-yl)peroxide
(f) 0.4% of coloring agent were mixing, and further kneading using two-roll grinder to obtain a mixture. Then, the mixture were coating on the glass. This coated layer was processed into a thickness of 62.5 μm thermal interface material layer under 80° C. dried temperature and 150° C. curing temperature.
Formulation 2
Besides the amount of the dispersing agent was changed from 0.5% to 1%, the manufacturing method of the thermal interface material is the same as the formulation 1.
At last, the thermal conductivity and resistance of the thermal interface material provided in the present invention are shown in Table 1, and the thermal conductivity was measured with laser flash method in heat-soaking device under 25° C.:

	TABLE 1

	Thermal conductivity (W/mK)	Resistance (Ohm * cm)

Formula 1	3.3167	4.7E+27
Formula 2	3.3167	5.7E+22

The larger of the abovementioned thermal conductivity, the greater ability to spread and transfer the heat. As shown in Table 1, the thermal conductivity of thermal interface material provide in the present invention is not lower than 3 W/mK, which is far more better than the products currently sold in the market (about 0.5-0.6 W/mK). Besides, the electric material usually needs to maintain insulation in order not to be burned due to the excess of the current. Therefore, we also measured the resistance of this thermal interface material and found that the resistance is enormously large to reach basic demands of insulation for electronic component.
Please refer to FIG 1A to FIG. 1C. FIG 1A to FIG. 1C are diagrams showing the structure of the insulated thermal interface material 10 according to the first embodiment of the present invention. As shown in the FIG, collocation of first filler 11 and second filler 12, the silicon rubber 13 could store and release elastic energy E1 due to long backbone structure. Therefore, when first filler 11 moves, rotates and vibrates under heating, it compresses or elongates the spring-like silicon rubber 13 and undergo transfer of thermal energy T and elastic energy F. In addition, the thermal conductivity of continuity between the second filler 12 and the first filler 11 results in a synergistic effect of increasing thermal transfer and thermal insulation, which give rise to a satisfying results of thermal conductivity and resistance. Based on these reasons, the thermal interface material 10 provides in the present invention could be widely used as heat conductive fins inserted between heating electronic device or electric component and heat dissipating component. Moreover, the abovementioned thermal interface material 10 displays great thermal insulativity especially applied in heating apparatus. Meanwhile, although not illustrated, the silicon rubber 13 further comprises the adhesion promoter. Hence, strong interaction occurred when combining this thermal interface material with the abovementioned compounds including aromatic polyimide, that is the great duration of the thermal interface material provides in the present invention.

Second Embodiment

According to a second embodiment of the present invention, the base is a curing epoxy resin and it could be selected from a group consisting of a linear polyepoxide with epoxide as an end group, a polyepoxide with epoxide at backbond and a polyepoxide with epoxide as side chain. It can comprise the following compound represented by the formula (III):
In the formula (III), R′ is an alkyl group, alkyl ether or aryl group, n is an integer of 2-6. Preferably, the abovementioned curing epoxy resin at least contains two epoxide groups within every molecule and the average molecular weight is 150-10000.
The epoxy resin includes aromatic glycidyl ether (by reacting of polyphenol with excess amount of epichlorohydrin), cycloaliphatic glycidyl ether, hydrogenation of the glycidyl ether, and their mixture. The polyphenols includes resorcinol, pyrocatechol, hydroquinone and polycyclic phenol including p,p′-dihydroxy benzyl, p,p′-dihydroxy biphenyl, p,p′-dihydroxyphenyl sulfone, p,p′-hydroxy benzophenone, 2,2′-dihydroxy-1,1′-dinaphthylmethane and dihydroxy diphenyl methane, dihydroxy diphenyl dimethyl methane, dihydroxy-diphenyl-ethyl methyl methane, dimethyl phenyl methyl propyl methane, dihydroxy-diphenyl-ethyl phenyl methane, dihydroxy-diphenyl-propyl phenyl methane, dihydroxy-diphenyl-butyl phenyl methane, dihydroxy-diphenyl-p-tolyl ethane, dihydroxy-diphenyl-p-tolyl methyl methane, dihydroxy-diphenyl-dicyclohexyl methane and the 2,2′,2,3′,2,4′,3,3′,3,4′ and 4,4′ isomers of dihydroxy-phenyl-cyclohexane. Moreover, it further contains condensation product of polyphenol formaldehyde and multi-diglycidyl ether that only has epoxy or hydroxyl group as active group.
According to the second embodiment of the present invention, the abovementioned thermal interface material could chose to add epoxy compound as reactive diluent which at least contains glycidyl ether at the terminal, preferably a saturated or non-saturated ring skeletons. There are several purposes of adding reactive diluents, such as processing helper, toughening and compatible between different materials. For example, reactive diluents could be diglycidyl ether of cyclohexanedimethanol, diglycidyl ether of resorcinol, p-tert-butyl phenyl glycidyl ether, hydroxymethyl phenyl glycidyl ether, diglycidyl ether of neopentyl glycol, trimethylol ethane triglycidyl ether, triglycidyl ether of trimethylolpropane, N,N-diglycidyl-4-glycidyloxyaniline, N,N-diglycidyl glyceryl aniline, N,N,N,N-tetraglycidyl m-xylenedi-amine, multi-diglycidyl ether of vegetable oil, but the present invention is not limited thereto.
Preferably, the insulated thermal interface material further comprises a particulate thermoplastic polymer. the base, the particulate thermoplastic polymer, the first filler and the second filler has a weight percentage of 90-97%, 1-2%, 0.005-0.001% and 0.1-1%, respectively. The abovementioned particulate thermoplastic polymer preferably contains a polymer with glass transition temperature (T_g) of at least 60° C., the average molecular weight is higher than 7000 and could be selected from a group consisting of a poly(methyl methacrylate) and a methyl methacrylate/methacrylic acid copolymer. In addition, the average granularity of the particulate thermoplastic polymer is preferably 0.25-250 μm.
Moreover, the thermal interface material further comprises a curing agent, and the curing agent contains a dicyandiamide and its derivatives or a metal imidazole compound represented by formula (IV):
ML_m (IV)
In formula (IV), M is a metal which could be selected from a group consisting of Ag (I), Cu (I), Cu (II), Cd (II), Zn (II), Hg (II), Ni (II) and Co (II). L is a compound shown by formula (V):
In formula (V), R¹, R², R³could be selected from a group consisting of hydrogen atom, alkyl group and aryl group. m is the valence of metal. Preferably, the metal imidazole compound is a green imidazole copper (II). In addition, the equivalent weight of metal imidazole compound is based on a criteria that it could cure epoxy resin, which preferably be 0.5-3%.
Preferably, the thermal interface material further comprises an additive, and the additive could be selected from a group consisting of coupling agent, lubricant, flow controlling agent, thickener, accelerant, chain-extenders, flexibilizer, dispersant and co-curing agent. And further, the flexibilizer helps to offer the intensity of overlap shear and impact, and the composition is different from the thermoplastic material. Flexibilizer is a polymer which could react with epoxy resin and is cross-linkable. Preferably, flexibilizer comprises a rubber-phase and a thermoplastic-phase polymer or a material which could form rubber-phase and a thermoplastic-phase compound with epoxy group. Moreover, flexibilizer could be butadiene acrylonitrile terminated with carboxyl group, a butadiene acrylonitrile terminated with carboxyl group and core-shell polymer or the mixture, but the present invention is not limited thereto. Furthermore, flow controlling agent preferably includes pyrolysis of silicon dioxide and un-pyrolysis silicon dioxide.
The adhesion promoter could be used to enhance the attachment of binding agent and the base, and the composition of the adhesion promoter could be varied with the target surface. Adhesion promoter, such as dihydric phenol including catechol and dithiobis phenol, is especially useful for the surface of ionic lubricant used to pull the metal raw material during the processing.
Moreover, the weight percentage of coupling agent could be 0.001-0.05%, and 1-2% for lubricant. Preferably, the coupling agent could be selected from a group consisting of silane coupling agent, titanate coupling agent or aluminate coupling agent, and the lubricant could be selected from a group consisting of stearate, stearic acid amines, low molecular weight polymer or paraffin.
Other additive such as filler (for example, aluminum powder, carbon black, glass bulb, talc, clay, calcium carbonate, barium sulfate, titanium dioxide, silicon dioxide, silicate, glass bead and mica), flame retardant, antistatic agent, thermal conductive or electric conductive granule and foaming agent (azodicaroxamide or expandable polymer microsphere containing hydrocarbon liquid) etc., addition of the thermal interface material according to the applicable field is choosable, but the present invention is not limited thereto.
The preparation process of the thermal interface material provides in the present invention is described below. First, one or more epoxy resin was heated under 100-180° C. to melt those resin. Then, the resin was cooled to about 90-50° C. Addition of other epoxy resin, reactive diluents and flexibilizer besides first filler and second filler under high shearing mixing. If the mixture comprises the first filler and the second filler, granules were added and mixed for 1 hour at most till the granules are dispersed. Finally, the filler was added and mixed to obtain the dispersed mixture. The mixture was cooled to below the glass transition temperature of particulate thermoplastic, generally 50-100° C. After that, the curing agent, the adhesion promoter and the particulate thermoplastic were mixed in the epoxy mixture. This epoxy mixture is flowable at this point and could be poured into suitable container for further use.
Formulation 1
First, the melted epoxy mixture was cooled to about 50° C. before addition of additives such as reactive diluents and flexibilizer under high shearing mixing. Further adding 0.005% of the first filler, such as graphene doped with nitrogen, and 0.25% of the second filler, such as 50-200 mm sphere aluminum oxide, and mixing for 1 hour till the granules are dispersed. The mixture was further cooled to about 50° C. below the glass transition temperature of particulate thermoplastic, before addition of the curing agent, the adhesion promoter and the particulate thermoplastic into the epoxy mixture.
Formulation 2
The only different between formulations 1 and 2 is to change the percentage of the second filler from 0.005% to 0.5%.
Finally, the thermal conductivity and resistance of the thermal interface material provided in the present invention are shown in Table 2, and the thermal conductivity is then measured with laser flash method in heat-soaking device under 25° C.:

	TABLE 2

	Thermal conductivity (W/mK)	Resistance (Ohm * cm)

Formulation 1	0.6607	4.3E+22
Formulation 2	0.5826	2.8E+22

The larger of the abovementioned thermal conductivity, the greater ability to spread and transfer the heat. As shown in Table 2, the thermal conductivity of the thermal interface material provide in the present invention is far more better than the products currently sold in the market (about 0.03-0.2 W/mK). Besides, the electric material usually needs to maintain insulation in order not to be burned due to the excess of the current. Therefore, we also measured the resistance of this thermal interface material and found that the resistance is enormously large to reach basic demands of insulation for electronic component.
Please refer to FIG. 2A to FIG. 2C. FIG. 2A to FIG. 2C are diagrams showing the structure of the insulated thermal interface material 10 according to the second embodiment of the present invention. It is noted that the curing material, the first filler 11 and the second filler 12, is flowable below the curing temperature and at least a part was diffused into the epoxy resin 14. Or the first filler 11 and the second filler 12 dispersed in epoxy resin 14 and was separated by the epoxy resin 14. As shown in the FIG, collocation of the first filler 11 and the second filler 12, the silicon rubber silicon resin 14 possesses similar structure (benzene ring and epoxy group) comparing to the first filler 11. Therefore, the thermal energy T could transmit through phonon and the great connection between the first filler 11 and the second filler 12, resulting in a synergistic effect of increasing thermal transfer and thermal insulation.
Although the present invention has been described in terms of specific exemplary embodiments and examples, it will be appreciated that the embodiments disclosed herein are for illustrative purposes only and various modifications and alterations might be made by those skilled in the art without departing from the spirit and scope of the invention as set forth in the following claims.

Claims

What is claimed is:

1. An insulated thermal interface material for applying between an electronic element and a thermal dissipating element, the insulated thermal interface material at least comprising:

a base composing of a polymer; and

a first filler and a second filler dispersing in the base,

wherein the first filler includes a graphene.

2. The insulated thermal interface material according to claim 1, wherein the first filler includes the graphene with a length-to-thickness ratio or a width-to-thickness ratio of 50˜10000 and selected from a group consisting of a graphene, a graphene doped with nitrogen, a graphene doped with oxygen, a graphene doped with both nitrogen and oxygen, multilayer graphene stacking via van der Waals interaction, multilayer graphene doped with nitrogen and stacking via van der Waals interaction, multilayer graphene doped with oxygen stacking via van der Waals interaction and multilayer graphene doped with both nitrogen and oxygen stacking via van der Waals interaction.

3. The insulated thermal interface material according to claim 1, wherein the second filler is a thermal conductive inorganic powder and selected from a group consisting of aluminum oxide, magnesium oxide, aluminum nitride, boron nitride, silicon carbide, tin oxide, silicon nitride, aluminum oxide whisker, aluminum nitride whisker, silicon carbide whisker, magnesium oxide whisker and silicon nitride whisker.

4. The insulated thermal interface material according to claim 1, wherein the base is a silicone rubber and at least contains an organic polysiloxane compound, a curing agent and an adhesion promoter, the weight percentage of the organic polysiloxane compound, the curing agent, the adhesion promoter, the first filler and the second filler are 91-99.55%, 0.1-5%, 0.1-3%, 0.0025-0.005% and 0.25-0.5%, respectively.

5. The insulated thermal interface material according to claim 4, wherein the second filler has an average granularity of 20˜50 μm.

6. The insulated thermal interface material according to claim 4, wherein the organic polysiloxane compound has a degree of polymerization of 200˜12000 and is represented by the following formula:

R¹ _aSiO_(4-a)/2

Wherein R¹is a single-valence C₁˜C₁₀hydrocarbon group and selected from a group consisting of an alkyl group, a cycloalkyl group, an aryl group, an aralkyl group, an alkyl group substituted by halogen and an alkenyl group, and “a” further represents a positive number of 1.9-2.05.

7. The insulated thermal interface material according to claim 4, wherein the curing agent is an organic peroxide or a curing agent applying in an alkylation reaction of silane.

8. The insulated thermal interface material according to claim 4, wherein the adhesion promoter at least comprises a silicon compound with a plurality of substitutes and the substitutes could be selected from a group consisting of a cyclalkyl group, an alkoxyl group, a methyl group, a vinyl group and a silane group.

9. The insulated thermal interface material according to claim 1, wherein the base is a curing epoxy resin and selected from a group consisting of a linear polyepoxide with epoxide as an end group, a polyepoxide with epoxide at backbond and a polyepoxide with epoxide as side chain.

10. The insulated thermal interface material according to claim 9 further comprising a particulate thermoplastic polymer, wherein the weight percentage of the base, the particulate thermoplastic polymer, the first filler and the second filler are 90˜97%, 1˜2%, 0.001˜0.005% and 0.1˜1%, respectively.

11. The insulated thermal interface material according to claim 10, wherein the particulate thermoplastic polymer comprises a polymer with a glass transition temperature of at least 60° C.

12. The insulated thermal interface material according to claim 10, wherein the particulate thermoplastic polymer comprises has an average molecular weight higher than 7000.

13. The insulated thermal interface material according to claim 10, wherein the particulate thermoplastic polymer is selected from a group consisting of a poly(methyl methacrylate) and a methyl methacrylate/methacrylic acid copolymer.

14. The insulated thermal interface material according to claim 10, wherein the particulate thermoplastic polymer has an average granularity of 0.25˜250 μm.

15. The insulated thermal interface material according to claim 9 further comprising a curing agent, and the curing agent contains a dicyandiamide and its derivatives or a metal imidazole compound represented by the following formula:

ML_m

wherein M is a metal and selected from a group consisting of Ag (I), Cu (I), Cu (II), Cd (II), Zn (II), Hg (II), Ni (II) and Co (II), and L is a compound represented by the following formula:

wherein R¹, R²and R³could be selected from a group consisting of hydrogen atom, alkyl group and aryl group, and m is the valence of metal.

16. The insulated thermal interface material according to claim 1, wherein the thermal conductivity of the insulated thermal interface material is higher than 3 W/mK.

17. The insulated thermal interface material according to claim 1 further comprising an additive, and the additive could be selected from a group consisting of coupling agent, lubricant, flow controlling agent, thickener, accelerant, chain-extenders, flexibilizer, dispersant and co-curing agent.