Patent Application
20130312936
The present invention relates to heat dissipating members, and particularly relates to a heat dissipating member suitable for use in an oil environment. The present invention also relates to a method for producing the heat dissipating member.
In resent years, hybrid automobiles and electric automobiles have been developed in view of environmental responsiveness. A motor is used for a drive system of such a vehicle, and increase in the output power of the motor is desired. When attempting to increase the output power of the motor, it is required to improve cooling capability in order to respond to increase in heat generation associated with improvement in the output power.
A motor for a vehicle is composed of a rotor, a stator, and a casing. An automatic transmission fluid (ATF) serving as a lubricating oil is enclosed in a lower portion of the motor. As an example of a method for improving the capability to cool such a motor, Non-Patent Literature 1 proposes a method in which an ATF for lubricating a motor bearing is fed between the stator and the casing to cool the motor.
Non-Patent Literature 1: “DENSO TECHNICAL REVIEW”, Vol. 13, No. 1, 2008
However, the cooling capability of the method described in Non-Patent Literature 1 is still insufficient. By contrast, a heat dissipating member usable in an oil environment such as in a vehicle motor and having excellent heat dissipation properties can be expected to function as effective means for improving the capability to cool the vehicle motor.
Therefore, the present invention aims to provide a heat dissipating member having excellent heat dissipation properties and suitable for use in an oil environment.
The present invention is a heat dissipating member including: a porous base material containing a fluorine resin and a thermally-conductive filler; and an oil contained in pores of the porous base material. In the heat dissipating member, the fluorine resin contains polytetrafluoroethylene.
Preferably, the fluorine resin further contains at least one selected from the group consisting of perfluoroalkoxy resins and
tetrafluoroethylene-hexafluoropropylene copolymers.
Preferably, the heat dissipating member of the present invention contains the oil in an amount of 1% by mass or more. Preferably, the oil is at least one selected from the group consisting of automatic transmission fluids, engine oils, universal oils, gear oils, hydrostatic transmission oils, and silicone oils.
In a preferred embodiment of the heat dissipating member of the present invention, the thermally-conductive filler is at least one insulating filler selected from the group consisting of boron nitride, aluminum nitride, alumina, silicon nitride, and magnesium oxide. In another preferred embodiment of the heat dissipating member of the present invention, the thermally-conductive filler is at least one electrically-conductive filler selected from the group consisting of graphite, carbon black, carbon fibers, metal fibers, and metal particles.
Preferably, the heat dissipating member of the present invention is packaged in a pack with an oxygen barrier film when put into storage and when put on the market.
The present invention is also directed to a method for cooling a vehicle motor. The method is characterized by using the heat dissipating member.
The present invention is also directed to a method for producing a heat dissipating member. The method includes the steps of: (1) stacking and rolling together a plurality of sheet-shaped formed bodies each containing a polytetrafluoroethylene-containing fluorine resin, a thermally-conductive filler, and a molding aid, so as to obtain a rolled laminated sheet; (2) removing the molding aid from the obtained rolled laminated sheet to obtain a porous base material; and (3) impregnating the obtained porous base material with an oil.
The production method of the present invention may further include a step (4) of subjecting the porous base material to pressure molding between the step (2) and the step (3).
Preferably, the step (3) is performed by immersing the porous base material in an oil after removing air from pores of the porous base material, and then by pressurizing the oil.
According to the present invention, a heat dissipating member having excellent heat dissipation properties and suitable for use in an oil environment is provided. The heat dissipating member is thus extremely useful, for example, as a heat dissipating member for a vehicle motor.
A heat dissipating member of the present invention includes: a porous base material containing a fluorine resin and a thermally-conductive filler; and an oil contained in pores of the porous base material.
High oil resistance is imparted to the heat dissipating member of the present invention by the use of the porous base material containing the fluorine resin.
The fluorine resin contains polytetrafluoroethylene (PTFE). Since the fluorine resin contains PTFE, it is easy to fabricate the porous base material having a high content of the thermally-conductive filler. The fluorine resin may contain a fluorine resin other than PTFE to the extent that the effect of the present invention is not impaired. Examples of the fluorine resin other than PFTE include fusible fluorine resins. When the fluorine resin contains a fusible fluorine resin, it is made easier to fabricate the porous base material having a high content of the thermally-conductive filler. For example, a material containing the fluorine resin and the thermally-conductive filler can easily be formed into a sheet. At least one selected from the group consisting of perfluoroalkoxy (PFA) resins and tetrafluoroethylene-hexafluoropropylene copolymers (FEP) is preferably used as the fusible fluorine resin. Although there are various PFA products and FEP products having different melting points, an appropriate product may be selected depending on the method for processing the product into the porous base material, and on the conditions for the processing.
With respect to the lower limit of the proportion of the fusible fluorine resin in the fluorine resin, the proportion of the fusible fluorine resin is preferably at least 5% by mass, and more preferably at least 10% by mass. With respect to the upper limit, the proportion of the fusible fluorine resin is at most 70% by mass, more preferably at most 50% by mass, and even more preferably at most 30% by mass.
Good thermally-conductive properties are imparted to the heat dissipating member of the present invention by a thermally-conductive filler. Here, the thermally-conductive filler means a filler having a thermal conductivity of 1 W/mK or more, preferably 100 W/mK or more. The type of the thermally-conductive filler may be selected as appropriate in accordance with the intended use of the heat dissipating member. For example, when it is desired to impart good insulating properties to the heat dissipating member, an insulating filler having a volume resistivity of 1014 Ω·cm or more may be used. At least one insulating filler selected from the group consisting of boron nitride, aluminum nitride, alumina, silicon nitride, and magnesium oxide, is suitably used. In a motor for a hybrid automobile or in an electric generator, there is a high-voltage region where a large transient current is generated. The heat dissipating member to which good insulating properties have been imparted by an insulating filler is advantageous for use in such a region.
On the other hand, when it is desired to impart good electrically-conductive properties to the heat dissipating member, an electrically-conductive filler having a volume resistivity of 106 Ω·cm or less may be used. At least one electrically-conductive filler selected from the group consisting of graphite, carbon black, carbon fibers, metal fibers (e.g., aluminum fibers, copper fibers etc.), and metal particles (e.g., particles of gold, silver, copper, palladium, platinum etc.) is suitably used.
The shape of the thermally-conductive filler is not particularly limited, and spherical fillers or non-spherical fillers can be used. Platy fillers or flaky fillers are preferable, since anisotropic thermal conductivity can be imparted by performing rolling and thus aligning the fillers in an in-plane direction. Also, for similar reasons, the thermally-conductive filler itself preferably has anisotropic thermal conductivity. In addition, when it is attempted to increase the thermal conductivity in the thickness direction, any of thermally-conductive filler aggregates sold by various manufacturers may be used.
The thermally-conductive filler only needs to be supported in a fluorine resin matrix without falling off the fluorine resin matrix, and to be capable of imparting sufficient thermally-conductive properties to the resultant heat dissipating member. Accordingly, the particle diameter of the thermally-conductive filler is not particularly limited. For example, the particle diameter is preferably 0.2 to 500 μm, and more preferably 0.2 to 50 μm. The thermally-conductive filler preferably has a large particle diameter in order to increase the thermal conductivity. This is because, when equal contents of thermally-conductive fillers having different particle diameters are compared, the thermally-conductive filler having a larger particle diameter forms a smaller number of interfaces, and allows lower thermal resistance. Here, the particle diameter means a value measured by a laser diffraction/scattering-type particle diameter/particle size distribution measurement apparatus (e.g., “Microtrac” manufactured by NIKKISO CO., LTD.).
The content of the thermally-conductive filler is preferably 50 to 95% by mass, more preferably 70 to 90% by mass, and even more preferably 80 to 90% by mass, relative to the total mass of the porous base material. The content of the fluorine resin is preferably 5 to 50% by mass, more preferably 10 to 30% by mass, and even more preferably 10 to 20% by mass, relative to the total mass of the porous base material.
The porous base material may contain a component other than the fluorine resin and the thermally-conductive filler. Examples of such a component include other resins than fluorine resins. Examples of the other resins that can be used include commonly-used thermoplastic resins and thermosetting resins. The content of the component is preferably 10% by mass or less relative to the total mass of the porous base material.
In the base material used in the present invention, the thermally-conductive filler is dispersed in a fluorine resin matrix. The base material has a porous structure. There is no particular limitation on the porous structure of the base material. For example, as will be described later, the porous structure can be obtained by fabricating a formed body containing the fluorine resin, the thermally-conductive filler, and a volatile material (molding aid), and then by removing the volatile material.
The heat dissipating member of the present invention is intended for use in an oil environment. Taking advantage of the fact that an oil is present in the usage environment, the heat dissipating member of the present invention includes an oil retained in the pores of the porous base material. The air in the pores of the porous base material is replaced by the oil, with the result that heat dissipation properties better than those of the porous base material itself can be obtained.
Any oil that has a higher thermal conductivity than air can be used without particular limitation. An oil that is the same as or similar to the oil present in the usage environment is preferably used. For example, in the case of use in a vehicle motor, an oil usable as a lubricating oil can be suitably used. Specifically, at least one selected from the group consisting of ATFs, engine oils, universal oils, gear oils, hydrostatic transmission oils, and silicone oils, is suitably used. When an air-permeable member containing such an oil is used in a vehicle motor, the oil contained in the air-permeable member makes it possible to avoid a situation where lubrication of the vehicle motor is hindered. Among the oils mentioned above, ATFs and engine oils are particularly preferable since these oils have high cooling capability in themselves.
The oil is contained in an amount of preferably 1% by mass or more, more preferably 1 to 80% by mass, even more preferably 5 to 80% by mass, relative to the total mass of the heat dissipating member.
The shape of the heat dissipating member of the present invention is not particularly limited. In view of high handleability, the heat dissipating member preferably has a sheet shape. In this case, the thickness of the heat dissipating member of the present invention is, for example, 0.05 mm to 3 mm, and preferably 0.1 mm to 1 mm.
Next, a method for producing the heat dissipating member of the present invention will be described. The heat dissipating member of the present invention is suitably produced by a method including the steps of: (1) stacking and rolling together a plurality of sheet-shaped formed bodies each containing a PTFE-containing fluorine resin, a thermally-conductive filler, and a molding aid, so as to obtain a rolled laminated sheet; (2) removing the molding aid from the obtained rolled laminated sheet to obtain a porous base material; and (3) impregnating the obtained porous base material with an oil.
The plurality of sheet-shaped formed bodies used in the step (1), which include a PTFE-containing fluorine resin, a thermally-conductive filler, and a molding aid, can each be obtained as follows: a mixture in the form of a paste is first prepared by mixing the PTFE-containing fluorine resin, the thermally-conductive filler, and the molding aid; and the mixture is formed into a sheet.
The PTFE-containing fluorine resin, the thermally-conductive filler, and the molding aid are desirably mixed under such conditions that fibrillization of PTFE is suppressed as much as possible. Specifically, mixing is desirably performed without kneading, in a short time, by a mixing apparatus whose rotational speed is set low so as not to apply shear force to PTFE. If the fibrillization of PTFE occurs at the time of mixing of the materials, there is a risk that the previously-formed fibers of PTFE are cut during rolling, and the network structure of PTFE is broken. This may lead to difficulty in maintaining a sheet shape.
For example, a saturated hydrocarbon such as dodecane and decane can be used as the molding aid. The molding aid may be added in an amount of 20 to 55% by mass relative to the total mass of the mixture.
The sheet-shaped formed body can be obtained by forming the mixture of the above materials into a sheet by extrusion molding, roll forming, or the like. The thickness of the sheet-shaped formed body is, for example, 0.5 to 5 mm. A plurality of such sheet-shaped formed bodies are prepared.
Subsequently, the plurality of sheet-shaped formed bodies are stacked (laminated) and rolled together to obtain a rolled laminated sheet. The number of the sheet-shaped formed bodies used only needs to be two or more, and is not particularly limited. The number of the sheet-shaped formed bodies may be determined as appropriate in consideration of the number of sheet-shaped formed body layers constituting the finally-obtained rolled laminated sheet serving as the porous base material. For example, the number of the sheet-shaped formed bodies is about 2 to 10. As described above, the production method includes rolling a laminated body. The lamination and rolling improve the sheet strength, and also allow the thermally-conductive filler to be firmly fixed in the fluorine resin matrix. Therefore, a sheet containing a high proportion of the thermally-conductive filler and having flexibility can be fabricated.
In the production method, the step (1) is preferably followed by an additional step (1′) of stacking and rolling together a plurality of rolled laminated sheets constituted by the sheet-shaped formed bodies, or stacking and rolling together at least one rolled laminated sheet constituted by the sheet-shaped formed bodies and at least one sheet-shaped formed body containing the fluorine resin, the thermally-conductive filler, and the molding aid. The step (1′) is preferably repeated. In early rolling processes (where the number of the sheet-shaped formed body layers included is small), the sheet has a low strength, and thus has difficulty in withstanding rolling performed at a high ratio. As the lamination and rolling are repeated, the rolling ratio increases, the sheet strength becomes higher, and the thermally-conductive filler becomes more firmly fixed in the fluorine resin matrix. In order to achieve high strength, the sheet-shaped formed bodies and the rolled laminated sheets constituted by the sheet-shaped formed bodies, are desirably rolled two by two.
Examples of embodiments of the step (1) and the step (1′) will be described below. First, a plurality of (e.g., 2 to 10) sheet-shaped formed bodies are prepared. Next, the plurality of sheet-shaped formed bodies are laminated, and the laminated body is rolled to obtain a rolled laminated sheet (first rolled laminated sheet) (step (1)). A plurality of (e.g., 2 to 10) first rolled laminated sheets obtained as above are prepared and laminated, and the laminated body is rolled to obtain a rolled laminated sheet (second rolled laminated sheet) (step (1′). A plurality of (e.g., 2 to 10) second rolled laminated sheets obtained as above are prepared and laminated, and the laminated body is rolled to obtain a rolled laminated sheet (third rolled laminated sheet) (repetition of step (1′)). Furthermore, a plurality of third rolled laminated sheets are prepared, and are laminated and rolled in the same manner. The step (1′) is repeated until the number of sheet-shaped formed body layers reaches the number of constituent layers included in a rolled laminated material serving as the intended porous base material. In this embodiment, rolled laminated sheets (e.g., the first rolled laminated sheets or the second rolled laminated sheets) having the same number of the laminated sheet-shaped formed bodies are stacked and rolled together. In the step (1′) of another embodiment, rolled laminated sheets having different numbers of the laminated sheet-shaped formed bodies are stacked and rolled together. In the step (1′) of still another embodiment, the rolled laminated sheet and the sheet-shaped formed body are stacked and rolled together.
When performing the step (1′), it is preferable to change the rolling direction.
At this time, the rolling direction in the step (1) and the rolling direction in the step (1′) are preferably orthogonal to each other. Furthermore, in repetition of the step (1′), the rolling direction is preferably changed (changed by 90° in particular). By performing rolling with changing direction in this manner, the network of PTFE is stretched longitudinally and laterally. Consequently, the sheet strength can further be improved, and the thermally-conductive filler can more firmly be fixed in the fluorine resin matrix.
When the number of constituent layers of the finally-obtained rolled laminated sheet serving as the porous base material is represented by the number of the sheet-shaped formed body layers included in the rolled laminated sheet, the number of constituent layers can be, for example, 2 to 5000. In order for the sheet strength to be improved, the number of constituent layers is preferably 200 or more. On the other hand, in order for a thin sheet (e.g., a sheet having a thickness of 1 mm or less) to be formed, the number of constituent layers is preferably 1500 or less. The more the number of constituent layers is, the higher the strength of the resultant sheet is, but the higher the possibility of interfacial delamination is.
In the manner as described above, a rolled laminated sheet that preferably has a thickness of about 0.05 mm to 3 mm is finally obtained.
The step (2) can be carried out by a commonly-known method depending on the molding aid used. For example, the sheet obtained by rolling may be heated, and the molding aid may thus be removed by drying. As a result, the porous base material is obtained.
The step (3) can be performed, for example, by immersing the porous base material in an oil. The step (3) is preferably performed by immersing the porous base material in an oil after removing air from the pores of the porous base material, and then by pressurizing the oil. This is because the porous base material can easily be impregnated with the oil at a high impregnation ratio in a short time. Such a process can be performed using a vacuum and high-pressure impregnation system.
A step (4) of subjecting the porous base material to pressure molding may further be carried out between the step (2) and the step (3). The porosity of the porous base material after the step (2) is generally about 50 to 80%. However, by carrying out the step (4), the porosity of the porous base material is reduced to 40% or less, and the particles of the thermally-conductive filler become densely distributed, which can result in further reduction in the thermal resistance of the heat dissipating member.
The pressure molding can be performed, for example, by pressing the porous base material at a temperature of 320 to 400° C. at a pressure of 0.05 to 50 MPa for 1 to 15 minutes.
In the manner as described above, the heat dissipating member of the present invention can be obtained. However, the method for producing the heat dissipating member of the present invention is not limited to the above method.
The heat dissipating member of the present invention has excellent heat dissipation properties. For example, a thermal resistance value of 25 cm2·K/W or less can be achieved. By adjusting the type and content of the thermally-conductive filler, it is possible to achieve a thermal resistance value of 5 cm2·K/W or less, or even a thermal resistance value of 1 cm2·K/W or less.
Since a fluorine resin having high oil resistance and an oil are used in the heat dissipating member of the present invention, the heat dissipating member is suitable for use in an oil environment. Therefore, the heat dissipating member is best suited as a heat dissipating member for a motor in a vehicle (e.g., a hybrid automobile, an electric automobile etc.). The use of the heat dissipating member makes it possible to cool a vehicle motor with high efficiency for a long period of time. In the case of use as a heat dissipating member for a vehicle motor, an air layer in the vehicle motor may be replaced by the heat dissipating member of the present invention. For example, in the case where an air layer is present between a stator and a casing, between a stator and a coil, between a casing and a coil, or between a stator and another stator, the heat dissipating member of the present invention may be held between the components. The mode of use in a vehicle motor is not limited thereto. In addition, it should be understood that the heat dissipating member of the present invention can be used for devices (e.g., generators, electronic devices etc.) other than vehicle motors.
The heat dissipating member of the present invention contains an oil. Therefore, the heat dissipating member is preferably packaged in a pack with an oxygen barrier film when put into storage and when put on the market. As the oxygen barrier film, a commonly-known oxygen barrier film can be used, and an oxygen barrier film having resistance against the oil contained in the heat dissipating member may be selected as appropriate.
Hereinafter, the present invention will be described in detail with reference to Examples and Comparative Examples. However, the present invention is not limited to these Examples.
A boron nitride powder (product code “HP-40” manufactured by MIZUSHIMA FERROALLOY CO., LTD.), a PTFE powder (product code “F104U” manufactured by DAIKIN INDUSTRIES, LTD.), and a PFA powder (product code “MP-10” manufactured by Du Pont-Mitsui Fluorochemicals Company, Ltd.) were mixed at a mass ratio of 80:10:10. An amount of 60 parts by mass of decane was further added to 100 parts by mass of the mixture, followed by mixing, to obtain a mixture in the form of a paste. The mixing of the materials was performed using a V-type mixer at a rotational speed of 10 rpm at a temperature of about 25° C. for 1 minute.
The thus obtained mixture in the form of a paste was rolled with a pressure roll to obtain two sheet-shaped formed bodies having a thickness of 3 mm. Next, the two sheet-shaped formed bodies were stacked and rolled to form a first laminated sheet having two laminated layers. Next, the first laminated sheet was cut into two pieces, and the two pieces were stacked and rolled together to form a second laminated sheet having four laminated layers. This series of steps of cutting, stacking, and rolling was repeated five times while changing the rolling direction by 90° for every rolling step. The resultant laminated sheet was rolled several times to form a rolled laminated sheet having a thickness of about 0.5 mm.
Next, the obtained rolled laminated sheet was heated at 150° C. for 20 minutes to remove the molding aid. Thus, a sheet-shaped porous base material was obtained.
An ATF (Toyota genuine auto fluid WS 08886-02305) was put into a container of a vacuum and high-pressure impregnation system (manufactured by MIKADO TECHNOS CO., LTD.) which is of a type shown in
A heat dissipating member of Example 2 was obtained in the same manner as in Example 1, except that the impregnation process was performed after a sheet-shaped porous base material obtained in the same manner as in Example 1 was pressed at a temperature of 380° C. at a pressure of 5 MPa for 5 minutes. The thickness of the heat dissipating member of Example 2 was 0.25 mm, and the oil content was 16% by mass relative to the total mass of the heat dissipating member.
A heat dissipating member of Example 3 was obtained in the same manner as in Example 1, except that a carbon black powder (product code “#4500” manufactured by TOKAI CARBON CO., LTD.) was used in place of the boron nitride powder. The thickness of the heat dissipating member of Example 3 was 0.75 mm, and the oil content was 56% by mass relative to the total mass of the heat dissipating member.
A heat dissipating member of Comparative Example 1 was obtained in the same manner as in Example 1, except that the oil impregnation process was not performed. The thickness of the heat dissipating member of Comparative Example 1 was 0.5 mm.
A heat dissipating member of Comparative Example 2 was obtained in the same manner as in Example 2, except that the oil impregnation process was not performed. The thickness of the heat dissipating member of Comparative Example 2 was 0.25 mm.
A heat dissipating member of Comparative Example 3 was obtained in the same manner as in Example 3, except that the oil impregnation process was not performed. The thickness of the heat dissipating member of Comparative Example 3 was 0.75 mm.
Thermal resistances and breakdown voltages were measured by the methods described below for the heat dissipating members of Examples and Comparative Examples fabricated as described above. The results of the measurements are shown in Table 1.
The thermal resistances were measured using a thermal characterization system 10 shown in
In the measurement, first, a heat dissipating member 20 (20 mm×20 mm) of each of Examples and Comparative Examples was sandwiched from above and below by the pair of rods 13. The pressure adjustment screws 14 were tightened to apply a pressure to the heat dissipating member 20, the temperature of the heat generator 11 was set to 150° C., and cooling water of 20° C. was circulated in the heat dissipator 12. After the temperatures of the heat generator 11 and the upper and lower rods 13 stabilized, the temperatures of the upper and lower rods 13 were measured by the temperature sensors 18. From the thermal conductivities ((W/m·K) of, and the temperature gradient between, the upper and lower rods 13, the heat flux flowing through the heat dissipating member 20 was calculated, and the temperatures of the interfaces between the heat dissipating member 20 and the upper and lower rods 13 were also calculated. Using these calculated values, the thermal resistance (cm2·K/W) at the applied pressure was calculated by a heat conduction equation (Fourier's law). Thermal resistances were determined for the cases where the pressure applied to the heat dissipating member 20 was 125 kPa, 250 kPa, and 500 kPa.
Q=−λgradT
R=L/λ
The breakdown voltages were determined in accordance with JIS K 6245.
It is understood from the above results that the heat dissipating member of the present invention has excellent heat dissipation properties. It is also understood that good insulating properties are provided when an insulating filler is used as the thermally-conductive filler.
The heat dissipating member of the present invention is suitable for use in an oil environment, and is extremely useful, for example, as a heat dissipating member for a vehicle motor.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-064841 | Mar 2011 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2011/005891 | 10/20/2011 | WO | 00 | 8/7/2013 |
