JP2008031295A - Heat conductive paste - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a heat conductive paste having high heat radiation property and excellent in handling property. <P>SOLUTION: This conductive paste controlled in viscosity and excellent in handling property is prepared by compounding 5-80 wt.% pitch-type carbon staple fiber filler, using a mesophase pitch as its feedstock and having an average fiber diameter of 5-20 μm, an average length of 5-6,000 μm, and the percentage of the fiber diameter variance to the average fiber length (CV value) of 5-20, having high heat conductivity and a smooth surface, and having a crystallite size based on the growth direction of its hexagonal net surface of not smaller than 5 nm, to a polyorganosiloxane. The heat conductivity of this heat conductive paste is not lower than 3 W/(m×K). <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a thermally conductive paste using pitch-based short carbon fiber filler as a raw material. More specifically, by controlling the size, thermal conductivity, and surface properties of the pitch-based carbon short fiber filler obtained by pulverizing a three-dimensional random mat-like pitch-based carbon fiber mat produced by the melt-blowing method, handling properties and heat conduction are controlled. It is suitable for heat dissipation materials for heat-generating electronic components.

  In recent years, with the increase in the density of heat-generating electronic components and the reduction in size, thickness, and weight of electronic devices such as portable personal computers, there has been an increasing demand for lower heat resistance of heat-dissipating members used in them. Thinning of members is required. As a heat dissipation member, a thermally conductive sheet made of a cured product in which silicone rubber is filled with a thermally conductive inorganic powder, a thermally conductive spacer made of a cured product having flexibility and filled with a silicone gel and a thermally conductive inorganic powder. Examples thereof include a fluid heat conductive paste in which a liquid silicone is filled with a heat conductive inorganic powder, a phase change type heat radiation member utilizing a phase change of a resin, and the like. Among these, those that can be easily thinned are the heat radiation paste and the phase change type heat radiation member. However, in general-purpose products, the heat radiation paste is preferably used because of its cost merit and performance.

  In order to improve the thermal conductivity of the thermal grease, it is only necessary to fill the liquid matrix with a high thermal conductive material and make it thin, and to make the thin leaf, adjust the viscosity of the paste and the size of the filler. That's fine. Materials with excellent thermal conductivity include metal oxides such as aluminum oxide, boron nitride, aluminum nitride, magnesium oxide, zinc oxide, silicon carbide, quartz, and aluminum hydroxide, metal nitride, metal carbide, metal hydroxide, etc. It has been known. However, the metal material-based filler has a high specific gravity and the weight of the heat conductive grease is increased. In addition, when a powdery heat conductive material is used, it is difficult to form a network, so that high heat conductivity is difficult to obtain. Therefore, in order to improve thermal conductivity, it is necessary to use a large amount of a thermal conductive material. As a result, the thermal conductive paste increases in weight and cost, and is not necessarily easy to use.

Therefore, a carbon material having a low specific gravity and a high thermal conductivity, in particular, a thermal conductive paste using carbon fiber has been studied (see Patent Document 1). However, because existing carbon fibers are made from pitch, PAN, etc., made infusible, graphitized and then pulverized, the surface is disturbed and it is said that it has excellent dispersibility. A heat conductive paste having a high viscosity and a poor handling property is obtained.
JP 2002-146672 A

  As described above, it is desirable to add a heat conductive material excellent in dispersibility to the paste from the viewpoint that a paste having high thermal conductivity, low viscosity and excellent handling properties is required. Moreover, the heat conductive material used here is calculated | required to have the intensity | strength which can maintain a fiber state in a paste while having high heat conductivity.

  Therefore, there has been a strong demand for a thermal conductive paste having an appropriate thermal conductivity, further improving thermal conductivity, and additionally having tackiness. In addition, heat generation is required because of the large heat generation of the elements, members, and parts to be managed. For other purposes, the effect of shielding radio waves was also required.

  In view of improving the thermal conductivity of the thermal conductive paste and further improving the handling property with a low viscosity, the present inventors have found that a pitch-based carbon short fiber filler with a smooth surface is highly dispersible in the paste. It has been found that the handling property of the thermal conductive paste is remarkably improved while achieving excellent and excellent thermal conductivity, and has reached the present invention.

That is, the object of the present invention is to
A heat conductive paste composed of a composite of pitch-based carbon short fiber filler and organopolysiloxane as a heat conductive material, and the surface of the pitch-based carbon short fiber filler observed with a scanning electron microscope is substantially smooth. The crystallite size derived from the growth direction of the hexagonal mesh surface is 5 nm or more, the thermal conductive paste contains 5 to 80% by weight of the pitch-based carbon short fiber filler, and the thermal conductivity of the thermal conductive paste is This is achieved by a heat conductive paste characterized by being 3 W / (m · K) or more.

  Furthermore, in the present invention, the carbon fiber is made from mesophase pitch, the average fiber diameter is 5 to 20 μm, the average length is 5 to 6000 μm, and the percentage of fiber diameter dispersion with respect to the average fiber diameter (CV value) is 5 to 20. That the thermal conductive paste has a viscosity of 25 to 150 Pa · S (50 to 1500 poise) under conditions of 25 ° C. and a shear rate of 1.7 (1 / s), and pitch-based carbon as a thermal conductive material. 1 to 50% by weight of aluminum oxide, magnesium oxide, zinc oxide, boron nitride, aluminum nitride, aluminum oxynitride, quartz, and aluminum hydroxide are included with respect to the short fiber filler. Is included.

  The thermal conductive paste of the present invention utilizes the smoothness of the surface of the pitch-based carbon short fiber filler in which the spread of the graphite crystal (crystallite size derived from the growth direction of the hexagonal network surface) is controlled to a certain size or more. This makes it possible to achieve both high thermal conductivity and high handling properties derived from low viscosity. Furthermore, since it is rich in tackiness, it is possible to improve adhesion to an electronic component heat-dissipating sheet, a heat exchanger, and the like, increase heat conduction efficiency, and achieve weight reduction. Moreover, heat resistance can be expected by using silicone having high heat resistance.

Next, embodiments of the present invention will be described sequentially.
Examples of the raw material for the pitch-based carbon short fiber filler used in the present invention include condensed polycyclic hydrocarbon compounds such as naphthalene and phenanthrene, and condensed heterocyclic compounds such as petroleum-based pitch and coal-based pitch. Among them, condensed polycyclic hydrocarbon compounds such as naphthalene and phenanthrene are preferable, and optically anisotropic pitch, that is, mesophase pitch is particularly preferable. These may be used alone or in combination of two or more, but it is particularly preferable to use mesophase pitch alone in order to improve the thermal conductivity of the carbon fiber.

  The softening point of the raw material pitch can be determined by the Mettler method, and is preferably 250 ° C. or higher and 350 ° C. or lower. When the softening point is lower than 250 ° C., fusion between fibers and large heat shrinkage occur during infusibilization. On the other hand, when the temperature is higher than 350 ° C., thermal decomposition of the pitch occurs and it becomes difficult to form a yarn.

  The raw material pitch is spun by a melt blow method, then infusible, fired, pulverized, and finally graphitized to form a pitch-based carbon short fiber filler. Each step will be described below.

  In the present invention, there is no particular limitation on the shape of the spinning nozzle of pitch fiber used as a raw material for the pitch-based carbon short fiber filler, but a nozzle hole length / hole diameter ratio smaller than 3 is preferably used, and more preferably Is smaller than 1.5. The temperature of the nozzle at the time of spinning is not particularly limited, and may be a temperature at which a stable spinning state can be maintained, that is, a temperature at which the viscosity of the spinning pitch is 2 to 200 Pa · S, preferably 30 to 150 Pa · S.

  The pitch fibers drawn out from the nozzle holes are shortened by blowing a gas having a linear velocity of 100 to 10,000 m per minute heated to 100 to 350 ° C. in the vicinity of the thinning point. As the gas to be blown, air, nitrogen, or argon can be used, but air is preferable from the viewpoint of cost performance.

Pitch fibers are collected on a wire mesh belt to form a continuous mat, and further cross-wrapped to form a three-dimensional random mat.
The three-dimensional random mat refers to a mat in which pitch fibers are entangled three-dimensionally in addition to being cross-wrapped. This entanglement is achieved in a cylinder called chimney while reaching the wire mesh belt from the nozzle. Since the linear fibers are entangled three-dimensionally, the characteristics of the fibers that normally exhibit only one-dimensional behavior are reflected in the three-dimensional.

  The three-dimensional random mat made of pitch fibers thus obtained is infusible by a known method. Infusibilization is achieved at 200 to 350 ° C. using air or a gas obtained by adding ozone, nitrogen dioxide, nitrogen, oxygen, iodine, bromine to air. Considering safety and convenience, it is preferable to carry out in the air. The infusible pitch fiber is fired in vacuum or in an inert gas such as nitrogen, argon, krypton, etc., but is carried out at normal pressure and in low-cost nitrogen. After infusibilization, the pitch-based short fiber filler can be obtained by grinding the pitch fiber. The pulverization can be performed by a known method. Specifically, a ball mill, a jet mill, a crusher, or the like can be used. After grinding, the pitch-based short fiber filler is fired and graphitized. The firing temperature is preferably 2000 to 3500 ° C. in order to increase the thermal conductivity of the carbon fiber. More preferably, it is 2300-3500 degreeC. It is preferable to place it in a graphite crucible at the time of firing because it can block external physical and chemical effects. The crucible made of graphite is not limited in size and shape as long as the desired amount of pitch-based short fiber filler as the above raw material can be added, but the oxidizing property in the furnace during firing or cooling is not limited. In order to prevent damage to the pitch-based carbon short fiber filler due to the reaction with this gas or carbon vapor, a highly airtight one with a lid can be suitably used.

  The pitch-based carbon short fiber filler used in the present invention is characterized in that the surface observed with a scanning electron microscope is substantially smooth. Here, the term “smooth” means that surface irregularities are not confirmed, cracks on the surface are not confirmed, and cracks are not confirmed on the filler in observation with a scanning electron microscope. Here, “substantially” means that, for example, in the observation with an electron microscope, the defect may be included in the field of view (magnification 1000) as long as there are 10 or less defects. When the surface to be observed with an electron microscope is substantially smooth, the interaction between the pitch-based carbon short fiber filler and the matrix is reduced when the pitch-based carbon short fiber filler and the matrix are mixed to create a thermally conductive paste. As a result, the viscosity of the heat conductive paste is reduced and the handling property is improved. On the other hand, if the pitch-based carbon short fiber filler is not smooth, the interaction between the pitch-based carbon short fiber filler and the matrix increases, and as a result, the viscosity of the heat conductive paste increases and the handling property decreases.

  Smoothing the observation surface of the pitch-based carbon short fiber filler can be achieved by suppressing the loss of the pitch-based carbon short fiber filler by graphitizing the carbon fiber filler after pulverization. When pulverized after graphite, defects in the pitch-based carbon short fiber filler increase, and defects are observed on the surface observed with a scanning electron microscope.

  The pitch-based carbon short fiber filler used in the present invention needs to have a crystallite size derived from the growth direction of the hexagonal network surface of 5 nm or more. The crystallite size derived from the growth direction of the hexagonal network surface can be determined by a known method, and can be determined by diffraction lines from the (110) plane of the carbon crystal obtained by the X-ray diffraction method. The reason why the crystallite size is important is that heat conduction is mainly performed by phonons, and it is the crystals that generate phonons. More preferably, it is 20 nm or more, and more preferably 30 nm or more.

  The average fiber diameter of the three-dimensional random mat-like carbon fiber needs to be 5 to 20 μm. When the thickness is 5 μm or less, the shape of the mat may not be maintained, and productivity is poor. When the fiber diameter exceeds 20 μm, unevenness in the infusibilization process becomes large, and a part where fusion occurs partially occurs. More preferably, it is 5-15 micrometers, More preferably, it is 8-12 micrometers.

  On the other hand, the average length of the pitch-based carbon short fiber filler is preferably 5 to 6000 μm. When the thickness is less than 5 μm, the characteristics as a fiber are lost, and sufficient thermal conductivity cannot be exhibited. On the other hand, if it exceeds 6000 mm, the entanglement of the fibers will remarkably increase, and the viscosity of the heat conductive paste will become high and handling will become difficult. More preferably, it is 10-3000 micrometers, More preferably, it is 20-1000 mm.

  In addition, it is preferable that the CV value calculated | required as a percentage of fiber diameter dispersion | distribution with respect to an average fiber diameter is 5-20. It is impossible in the process that the CV value is less than 5. On the other hand, if the CV value exceeds 20, the possibility of increasing the number of fibers having a diameter of 20 μm or more that causes trouble due to infusibilization increases, which is not preferable from the viewpoint of productivity.

  The thermal conductivity of the thermal conductive paste according to the present invention can be measured by a known method. Among them, the probe method, hot disk method, and laser flash method are preferable, and the probe method is particularly simple and preferable. Generally, the thermal conductivity of carbon fiber itself is several hundred W / (m · K). However, when it is made into a composite such as a paste, the thermal conductivity is abrupt due to the occurrence of defects, air contamination, and unexpected voids. To reduce. Therefore, it has been considered difficult for the thermal conductivity of the thermally conductive paste to substantially exceed 2 W / (m · K). However, in the present invention, this problem was solved by using a pitch-based carbon short fiber filler, and a paste of 3 W / (m · K) or more was realized. More desirably, it is 5 W / (m · K) or more, and further desirably 10 W / (m · K) or more.

  The content rate of a pitch-type short carbon fiber filler is 5-80 mass% in a heat conductive paste. If it is less than 5% by mass, the thermal conductivity is low, and it is difficult to reduce the thermal resistance no matter how thin it is. If it exceeds 80% by mass, the fluidity of the paste becomes low, and thinning becomes difficult. More desirably, it is 10 to 70% by mass.

  The viscosity of the paste is preferably 50 to 1500 poise when the shear rate is 1.7 (1 / s). More preferably, it is 100-1000 poise. When it is less than 50 poise, the fluidity of the paste is too high and immediately flows out, making it unsuitable as a paste. If it exceeds 1500 poise, the fluidity is too low and thinning becomes difficult. In addition, although a viscosity can be measured using a well-known method, it can be specifically measured using a B-type viscometer.

  Moreover, as a heat conductive material other than the pitch-based carbon short fiber filler, metal oxides such as aluminum oxide, boron nitride, aluminum nitride, magnesium oxide, zinc oxide, silicon carbide, quartz, aluminum hydroxide, silver powder, metal nitride, Metal oxynitride, metal carbide, metal hydroxide, or metal may be added as an additive. More specifically, one or more inorganic fillers selected from the group consisting of aluminum oxide, magnesium oxide, zinc oxide, boron nitride, aluminum nitride, aluminum oxynitride, quartz, and aluminum hydroxide may be added. The addition amount is 1 to 50% by weight with respect to the pitch-based carbon short fiber filler. By using these, the insulating property of the heat conductive paste can be improved.

The organopolysiloxane used as a matrix used in the present invention has characteristics of electronic materials such as heat resistance and insulation. By using this as a matrix, a highly reliable heat conductive paste can be obtained. The organopolysiloxane used in the present invention is represented by the following general formula (1).
R ¹ _a SiO _{(4-a) / 2} (1)

In the above formula (1), R ¹ is one or more groups selected from saturated or unsaturated monovalent hydrocarbon groups having 1 to 18 carbon atoms. Such groups include, for example, methyl groups, ethyl groups, propyl groups, hexyl groups, octyl groups, decyl groups, alkyl groups such as dodecyl groups, tetradecyl groups, hexadecyl groups, octadecyl groups, cyclopentyl groups, cyclohexyl groups, etc. An alkenyl group such as a cyclohexyl group, a vinyl group and an allyl group; an aryl group such as a phenyl group and a tolyl group; an aralkyl group such as a 2-phenylethyl group and a 2-methyl-2-phenylethyl group; Examples include halogenated hydrocarbon groups such as a fluoropropyl group, 2- (perfluorobutyl) ethyl group, 2- (perfluorooctyl) ethyl group, and p-chlorophenyl group. Particularly, methyl group, phenyl group, and carbon The alkyl group of several 6-14 is preferable. From the viewpoint of the viscosity required for the silicone paste composition, a is preferably a positive number in the range of 1.8 to 2.2, and particularly preferably a positive number in the range of 1.9 to 2.2.

In addition, when the kinematic viscosity at 25 ° C. of the organopolysiloxane used in the present invention is lower than 50 mm ² / s, oil bleeding tends to occur when the silicone paste composition is used, and when it exceeds 500,000 mm ² / s, the silicone paste composition from the extensibility becomes poor when the object, preferably a kinematic viscosity at 25 ° C. is 50~500000mm ² / s, it is particularly preferably 100~10,000mm ² / s. In addition, although kinematic viscosity can be measured by a well-known method, it can specifically measure with an Ostwald viscometer.

  The heat conductive paste of the present invention can be produced by kneading the above materials with a universal mixing stirrer, kneader or the like. Furthermore, the viscosity can be adjusted with ketone solvents such as MEK and MIBK, various alcohols, cyclohexane, hexane, toluene, benzene and the like.

  Applications of the heat conductive paste of the present invention include heat dissipating members for electronic parts, heat conductive fillers, insulating fillers for temperature measurement, and the like. For example, the heat conductive paste of the present invention is used to transfer heat from heat-generating electronic components such as MPUs, power transistors, and transformers to heat-dissipating components such as heat-dissipating fins and heat-dissipating fans. Used by being sandwiched between heat dissipation components. As a result, heat transfer between the heat-generating electronic component and the heat-dissipating component is improved, and malfunction of the heat-generating electronic component can be reduced in the long term. Or it can use suitably for the connection of a heat pipe and a heat sink, and the connection of the module containing various heat generating bodies, and a heat sink.

  The heat conductive paste of the present invention has tackiness and it is easy to ensure alignment with the substance to be bonded. The degree of tackiness can be changed depending on the processing temperature of the paste.

  Prior to kneading, the pitch-based carbon short fiber filler can be used in which the surface is modified by an oxidation treatment such as electrolytic oxidation or a treatment with a coupling agent or a sizing agent. Also, by electroless plating method, electrolytic plating method, physical vapor deposition method such as vacuum deposition, sputtering, ion plating, chemical vapor deposition method, painting, immersion, mechanochemical method for mechanically fixing fine particles, etc. A metal or ceramic coated on the surface may be used. Hereinafter, the present invention will be described in more detail.

Examples are shown below, but the present invention is not limited thereto.
In addition, each value in a present Example was calculated | required according to the following method.
(1) The diameter of the pitch-based carbon short fiber filler was measured by using a scale of the baked filler under an optical microscope.
(2) The yarn length of the pitch-based carbon short fiber filler was measured with a length measuring device by extracting the filler after firing.
(3) The crystallite size of the pitch-based carbon short fiber filler was determined by the Gakushin method by measuring reflection from the (110) plane appearing in X-ray diffraction.
(4) The surface of the pitch-based carbon short fiber filler was observed with a scanning electron microscope.
(5) The thermal conductivity of the heat conductive paste was obtained by applying the paste on a reference plate to a thickness of 1 mm and using a probe method using QTM-500 manufactured by Kyoto Electronics.
(6) The viscosity of the heat conductive paste was determined using a B-type viscometer.

[Example 1]
A pitch made of a condensed polycyclic hydrocarbon compound was used as a main raw material. The optical anisotropy ratio was 100%, and the softening point was 283 ° C. Using a cap with a hole with a diameter of 0.2 mmφ, heated air was ejected from the slit at a linear velocity of 5500 m / min, and the melt pitch was pulled to produce pitch-based short fibers with an average diameter of 14.5 μm. The spun fibers were collected on a belt to form a mat, and a three-dimensional random mat made of pitch-based short fibers having a basis weight of 320 g / m ^{2 by} cross-wrapping.

  This three-dimensional random mat was heated in the air from 170 ° C. to 285 ° C. at an average heating rate of 6 ° C./min for infusibilization. The infusible three-dimensional random mat was pulverized with a ball mill and fired at 3000 ° C. The average yarn diameter of the pitch-based carbon short fiber filler after firing was 9.8 μm, and the ratio of the yarn diameter dispersion to the average yarn diameter was 12%. The average yarn length was 50 mm. The crystallite size derived from the growth direction of the hexagonal network surface was 17 nm. The surface of the pitch-based carbon short fiber filler observed with a scanning electron microscope was smooth.

A paste is prepared by vacuum defoaming while mixing 35% by weight of pitch-based carbon short fiber filler and 65% by weight of silicone oil (trade name “TSF451-500” manufactured by Toshiba Silicone Co., Ltd.) for 30 minutes using a planetary mixer. did.
It was 6.2 W / (m * K) when the heat conductivity of the produced heat conductive paste was measured. The viscosity was 11 Pa · S (110 poise).

[Example 2]
Three-dimensional random mat-like carbon fibers were produced in the same manner as in Example 1.
50% by weight of pitch-based carbon short fiber filler and 50% by weight of silicone oil (trade name “TSF451-500” manufactured by Toshiba Silicone Co., Ltd.) are mixed with a planetary mixer for 30 minutes to produce a paste. did.
It was 10.2 W / (m * K) when the heat conductivity of the produced heat conductive paste was measured. The viscosity was 45 Pa · S (450 poise).

[Example 3]
A three-dimensional random mat-like carbon fiber was produced in the same manner as in Example 1.
Vacuum while mixing for 30 minutes using a planetary mixer with 25% by weight of pitch-based carbon short fiber filler, 10 parts by weight of boron nitride, and 65% by weight of silicone oil (trade name “TSF451-500” manufactured by Toshiba Silicone Co., Ltd.) Defoamed to produce a paste.
It was 3.9 W / (m * K) when the heat conductivity of the produced heat conductive paste was measured. The viscosity was 10 Pa · S (100 poise).

[Comparative Example 1]
A thermally conductive paste was prepared in the same manner as in Example 1 except that the three-dimensional random mat-like carbon fiber was pulverized after firing to obtain a pitch-based short carbon fiber filler. The surface of the pitch-based carbon short fiber filler observed with a scanning electron microscope was conspicuous and was not smooth.
It was 5.9 W / (m * K) when the heat conductivity of the produced heat conductive paste was measured. The viscosity was as high as 240 Pa · S (2400 poise).

[Comparative Example 2]
A heat conductive paste was produced in the same manner as in Example 1 except that the content of the pitch-based carbon short fiber filler was 3%.
The heat conductivity of the molded heat conductive paste was measured and found to be 0.5 W / (m · K). The viscosity was 3 Pa · S (30 poise).

  The heat conductive paste of the present invention uses the smoothness of the surface of the pitch-based carbon short fiber filler in which the spread of the graphite crystal is controlled to a certain size or more, thereby suppressing the viscosity and improving the handling property while maintaining high heat conductivity. It makes it possible to express sex. Furthermore, it is possible to increase the adhesion to the heat radiation sheet for electronic parts, the heat exchanger, etc., increase the heat conduction efficiency, and achieve weight reduction.

Claims (4)

  A heat conductive paste composed of a pitch-based carbon short fiber filler as a heat conductive material and an organopolysiloxane, and the surface observed with a scanning electron microscope is substantially smooth, and in the growth direction of the hexagonal mesh surface. The derived crystallite size is 5 nm or more, the thermal conductive paste contains 5 to 80 wt% of the pitch-based carbon short fiber filler, and the thermal conductivity of the thermal conductive paste is 3 W / (m · K) or more. A thermally conductive paste characterized by   The said carbon fiber uses mesophase pitch as a raw material, The average fiber diameter is 5-20 micrometers, The average length is 5-6000 micrometers, The percentage (CV value) of the fiber diameter dispersion | distribution with respect to an average fiber diameter is 5-20. Thermally conductive paste.   The viscosity of the said heat conductive paste is 5-150 Pa.S (50-1500 poise) in the conditions of 25 degreeC and a shear rate 1.7 (1 / s), It is any one of Claims 1-2. Thermally conductive paste.   Selected from the group of 1 to 50% by weight of aluminum oxide, magnesium oxide, zinc oxide, boron nitride, aluminum nitride, aluminum oxynitride, quartz, and aluminum hydroxide with respect to pitch-based carbon short fiber filler as a heat conductive material The thermally conductive paste according to any one of claims 1 to 3, comprising one or more inorganic fillers.