JP2011122789A - Flat plate type heat pipe - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a flat plate type heat pipe which achieves a sufficient heat diffusing performance even at a temperature lower than the lower limit of a temperature region where an operating fluid circulating motion of the heat pipe is available. <P>SOLUTION: In this heat pipe having a flat metallic member 1 having an internal space, and the operating fluid enclosed in the internal space, a sheet-like graphite 4 having heat conductivity in the main plane direction higher than that of a metallic member 1, is fixed to the inner wall in the main plane direction of the flat plate-shaped metallic member 1. A surface of the sheet-like graphite 4 is unevenly formed to provide the operating fluid with capillary action. Thus, the heat from a heat generation source can be diffused by the sheet-like graphite at the temperature lower than the temperature where the circulating motion of the operating fluid is available. The condensed operating fluid can be moved by the unevenness of the sheet-like graphite surface. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a heat pipe, and more particularly to a heat pipe having an outer shape that is suitable as a heat diffusion member (heat spreader).

  A general heat pipe has a fine structure (unevenness due to a mesh or sintered body) that causes capillary action called a wick on the inner wall of a copper tube, encloses the working fluid, and seals the end with caulking etc. It is. Its outer shape is cylindrical in order to mainly use heat transport. On the other hand, as a means for efficiently cooling a heat source having a large heat generation density such as a CPU, a heat pipe having a flat outer shape is used, and a method of cooling after diffusing heat is used.

  For example, as in Patent Documents 1 and 2, a flat heat pipe is formed by stacking a plurality of partition plates made of thin plates having slits, and constructing a container by stacking outer wall members on top of each other. ing. The working fluid is sealed in the space in the container. In the configuration of Patent Document 1, a gap between a plurality of overlapping partition plates constitutes a capillary channel. In the configuration of Patent Document 2, a fine shape for causing capillary action is formed on the inner side surface of the outer wall member.

  A heat source is arrange | positioned in the center part of the upper surface or lower surface of a flat heat pipe. The hydraulic fluid evaporates at the center of the internal space due to heat from the heat source, and the vapor passes through the slit of the partition plate toward the outer peripheral portion. The vapor is cooled and condensed at the outer peripheral portion, and returns to the center through a capillary channel between the partition plates and a capillary channel configured in a fine tube shape inside the outer wall member. Since such a flat plate-like heat pipe is entirely made of a thin plate material, it can provide a thin shape.

  In addition, many patent documents have been published in recent years regarding techniques for promoting thermal diffusion using a graphite material. For example, Patent Document 3 discloses a heat dissipation structure that covers an existing heat dissipation structure using a graphite member as a thermally conductive film.

JP 2002-039693 A Japanese Patent No. 4047918 JP 11-95871 A (paragraph “0015” etc.)

  The operating temperature range of the heat pipe is determined by the vapor pressure of the working fluid. Generally, the heat pipe is sealed in a reduced pressure state in order to facilitate the evaporation of the working fluid. However, the flat plate type is structurally weaker than the cylindrical type, and the pressure reduction of the hydraulic fluid has to be lowered. When the degree of decompression is low, the working fluid is difficult to evaporate, and the lower limit of the operating temperature range is regulated (for example, flat type: about 60 ° C compared to 20 ° C). In the low temperature range below the lower limit, the evaporation of the hydraulic fluid is remarkably reduced, the high equivalent thermal conductivity (several thousands [W / mK]) unique to heat pipes cannot be obtained, and the thermal conductivity of the outer wall material (for example, copper: Controlled by about 380 [W / mK]), the heat diffusion performance as a heat spreader is degraded.

  In order to extend the lower limit of the operating temperature range, it is conceivable to increase the degree of decompression, but in order to withstand the decompressed state, it is necessary to devise the structure and increase the strength. As a means to increase the strength, a method of reinforcing by providing a large number of support pillars inside the container as well as the outer wall is conceivable, but since it interferes with the slit structure, the returnability of the hydraulic fluid deteriorates, resulting in a decrease in heat diffusion performance. Leading to a decline. That is, it is difficult to achieve both strength and thermal diffusion performance.

  It is also possible to apply the graphite material of Patent Document 3 to the uneven structure (wick) on the inner wall of the flat plate-like heat pipe, but the pitch of the wick is as fine as 1 mm or less. If the graphite material bends along the shape, carbon bonds in the graphite are disturbed, which may impair the high thermal conductivity characteristic of the graphite material.

  An object of the present invention is to provide a flat plate heat pipe capable of obtaining sufficient heat diffusion performance even at a temperature below the lower limit of the temperature range in which the working fluid circulation operation of the heat pipe occurs.

  In order to achieve the above object, according to the first aspect of the present invention, the following flat plate heat pipe is provided. That is, a flat plate-shaped heat pipe having a flat metal member having an internal space and a working fluid sealed in the internal space, the inner wall in the main plane direction of the flat metal member being more than the metal member Sheet-like graphite having a large thermal conductivity in the main plane direction is fixed. The surface of the sheet-like graphite has an uneven shape that causes capillary action in the hydraulic fluid. Accordingly, thermal diffusion can be caused by the thermal conductivity of the sheet-like graphite at a temperature lower than the temperature at which the hydraulic fluid circulates. Since the irregularities on the surface of the sheet-like graphite cause movement of the working fluid due to capillary action, it contributes to the circulating operation of the working fluid.

  The uneven shape can be, for example, a shape in which convex portions having a hexagonal bottom shape are repeatedly arranged.

  As the sheet-like graphite, those having anisotropic thermal conductivity whose thermal conductivity in the main plane direction is larger than the thermal conductivity in the thickness direction can be used. In this case, a metal column member may be disposed in the internal space of the metal member, and the column member may be configured to penetrate the through hole provided in the center of the sheet-like graphite so that both ends are in contact with the metal member. desirable. Thus, even when anisotropic heat conductive sheet-like graphite is used, the heat of the heat source can be efficiently transmitted to the internal space.

  ADVANTAGE OF THE INVENTION According to this invention, the heat | fever from a heat-generation source can be spread | diffused efficiently, and the flat type heat pipe (heat spreader) with few restrictions in a use temperature area | region can be provided.

It is sectional drawing of the flat plate type heat pipe (heat spreader) of 1st Embodiment. It is a perspective view of each member of the heat spreader of FIG. It is (a) enlarged top view of the surface unevenness | corrugation of the sheet-like graphite of the heat spreader of FIG. 1, and (b) enlarged side view. It is explanatory drawing which shows the flow of the working fluid and its vapor | steam in the heat spreader of FIG. It is sectional drawing of the flat type heat pipe (heat spreader) of a comparative example. It is a perspective view of each member of the heat spreader of FIG. FIG. 6 is an enlarged cutaway perspective view of the bottom surface of the recess of the outer layer metal plate of the heat spreader of FIG. 5. It is explanatory drawing which shows the flow of the working fluid and its vapor | steam in the heat spreader of FIG. It is explanatory drawing which shows the apparatus structure used for the evaluation experiment of an Example and a comparative example. It is a graph which shows the temperature change of the heat-generation source 5 measured with the apparatus of FIG. 9 about the heat spreader of the Example and the comparative example. It is a graph which shows the temperature change of the outer peripheral part of a heat spreader measured with the apparatus of FIG. 9 about the heat spreader of the Example and the comparative example.

  An embodiment of the present invention will be described with reference to the drawings. In addition, this invention is not limited to the following embodiment.

  A configuration of a flat plate heat pipe (hereinafter referred to as a heat spreader) according to the present embodiment will be described with reference to FIGS. 1 and 2. FIG. 1 is a cross-sectional view of a heat spreader, and FIG. 2 is a perspective view of each component. The heat spreader has a structure in which a pair of outer layer metal plates 1 having recesses 1a formed on opposite surfaces are joined at outer edges. The bottom surface of each recess 1 a of the outer metal plate 1 is covered with a sheet-like graphite 4. In the inner space of the joined outer layer metal plate 1, a plurality of inner layer slit plates 2 are arranged in an overlapping manner. Further, the inner space of the outer metal plate 1 is depressurized to a predetermined pressure, and a predetermined working fluid is sealed therein.

  The inner-layer slit plate 2 has a structure in which a plate-like member having a mesh structure in which a large number of fine through holes are provided in the thickness direction is corrugated with a minute height (amplitude). The main plane is provided with slits 2a extending radially from the center toward the outer periphery. The gaps between the plurality of stacked inner layer slit plates 3 and the radial slits 2a serve as a flow path through which the vapor of the working fluid flows from the center toward the outer peripheral portion. The mesh structure becomes a capillary flow path in which the working liquid condensed in the outer peripheral portion and returned to the liquid state returns to the center by capillary action.

  The outer layer metal plate 1 and the inner layer slit plate 2 are made of a metal having good thermal conductivity, for example, copper. The sheet-like graphite 4 has a larger thermal conductivity in the main plane direction than the outer metal plate 1 and the inner slit plate 2. Here, a graphite sheet having an anisotropic thermal conductivity that has a thermal conductivity in the main plane direction several times larger than that in the thickness direction (main plane direction 1200 W / mK, thickness direction 20 W / mK) is used. .

  A circular through-hole is provided in the center of the sheet-like graphite 4 and the inner layer slit plate 2, and the cylinder 3 having the same diameter penetrates so as to contact the side surface of the through-hole. Both ends of the cylinder 3 are fixed to the outer metal plate 1. Further, the column 3 can be formed integrally with one of the outer metal plates 1.

  The surface of the sheet-like graphite 4 is formed with a concavo-convex structure serving as a capillary channel for the liquid working fluid. 3A and 3B are a top view and a side view of the concavo-convex structure of the sheet-like graphite 4. 3A and 3B, the surface of the sheet-like graphite 4 has a shape in which hexagonal pyramids are repeatedly arranged without gaps. By providing a hexagonal pyramid uneven structure in this way, unevenness extending radially in six directions from the center toward the outer periphery is formed, so that the hydraulic fluid condensed at the outer periphery is efficiently returned to the center by capillary action. be able to.

  A method for producing a sheet-like graphite 4 having a concavo-convex structure in which hexagonal pyramids are repeatedly arranged on the surface will be described. A polymer film such as polyimide is used as the material. Prepare a plurality of polymer films (having the same hole position but different hole shapes) with hexagonal holes punched in a honeycomb shape beforehand. These polymer films are laminated so that the pore diameter gradually changes, and fired at a high temperature at 2000 to 3000 ° C. in an inert atmosphere. Furthermore, the surface of one side is rolled with the roller which formed the hexagonal pyramid-shaped unevenness | corrugation. Thereby, a sheet-like graphite having hexagonal pyramid irregularities formed on the surface can be generated. In addition, the surface on the opposite side fixed to the outer metal plate 1 is rolled with a flat roller to be a flat surface.

  In addition, although the number of the inner layer slit plates 2 is three in FIG. 1, it may be one or more and can be a desired number.

  Next, the operation of the heat spreader of this embodiment will be described.

  As shown in FIG. 4, when the heat source 5 is arranged at the center of the upper surface of the heat spreader, the heat from the heat source 5 passes through the outer metal plate 1 and the cylinder 3 to the sheet graphite 4 and the inner slit plate 2. The hydraulic fluid near the center of the internal space is heated. As a result, the working fluid evaporates, and the vapor passes through the gap between the inner layer slit plate 2 and the slit 2a toward the outer peripheral portion. The steam is cooled and condensed at the outer peripheral portion, becomes a liquid, moves through the uneven structure on the surface of the sheet-like graphite 4 and the mesh structure of the inner slit plate 2 by capillary action, and returns to the center. Thereby, the heat of the central heat source 5 can be efficiently diffused to the outer peripheral portion.

  On the other hand, when the temperature of the heat generating source 5 is low and the temperature range is such that the hydraulic fluid does not evaporate, the heat of the heat generating source 5 is conducted to the outer metal plate 1 and the sheet-like graphite 4 and the inner layer via the column 3. Conducted to the slit plate 3 and thereby conducted in the main plane direction and diffused to the outer peripheral portion. Since the sheet-like graphite 4 has a thermal conductivity in the main plane direction several times larger than that of metal, the heat of the heat source 5 can be efficiently conducted to the outer peripheral portion.

  As described above, in the present embodiment, the use of the graphite sheet with a high thermal conductivity having a concavo-convex shape on the inner side of the outer metal plate 1 of the flat heat pipe (heat spreader) causes the working fluid circulation operation of the heat pipe to occur. Even if it is less than the lower limit of the temperature range, sufficient thermal diffusion performance can be ensured.

  Further, the uneven shape on the surface of the sheet-like graphite 4 serves the same function as a capillary structure that is indispensable for the circulation of the working fluid in a heat pipe, generally called a wick, and thus contributes to the return of the working fluid.

  A through-hole is provided in the center of the sheet-like graphite 4, and the outer layer metal plate 1, the sheet-like graphite 4, and the inner-layer slit plate 2 are in direct contact with each other by the column 3 having high thermal conductivity. Even when anisotropic sheet-like graphite 4 having a low conductivity is used, the heat of the heat source 5 can be efficiently conducted to the inner slit plate 2.

  Further, in the present embodiment, since the sheet-like graphite 4 is directly processed into a concavo-convex shape, the carbon bond in the sheet-like graphite is not disturbed, and a capillary structure can be realized while maintaining high heat conduction characteristics.

  In the present embodiment, the sheet-like graphite 4 is fixed to the inner wall of the outer metal plate 1 so that heat from the heat source 5 can be easily transferred to the inner space of the outer metal plate 1 as compared with the case where the sheet-like graphite 4 is fixed to the outer wall. Can communicate to. That is, by fixing to the inner wall, a structure in which the sheet-like graphite 4 is penetrated by the column 3 can be taken. As a result, even when anisotropic thermal conductive graphite having a thermal conductivity in the main plane direction larger than the thermal conductivity in the thickness direction is used as the sheet-like graphite 4, heat in the thickness direction is obtained by the column 3. The conductivity can be easily secured, and the heat of the heat source 5 can be transmitted to the hydraulic fluid and the inner layer slit plate 2 with high thermal conductivity. Further, by arranging the graphite sheet on the inner wall, the outer wall of the heat spreader is a smooth surface of the outer metal plate 1, and there are few restrictions when fixing the heat source 5, and a highly versatile heat spreader can be provided.

(Example)
As an example, a flat plate heat pipe (heat spreader) having the structure of FIG. 1 was produced.

  First, an outer layer copper plate having an outer shape of 50 mm square and a thickness of 1.5 mm was prepared as the outer layer metal plate 1, and a recess having a 45 mm square and a depth of 0.5 mm was provided on one side. As the sheet-like graphite 4, a graphite sheet having a thickness of 0.1 mm, a thermal conductivity of 1200 [W / mK] in the plane direction, and a thermal conductivity of 20 [W / mK] in the thickness direction is manufactured by the above-described manufacturing method. It bonded together on the bottom face of the recessed part of the outer layer metal plate 1. In addition, the surface of the graphite sheet 4 is provided with the uneven | corrugated shape of the shape where the hexagonal pyramid was repeatedly arranged without gap like FIG. 3 (a), (b). The diameter of the base of the hexagonal pyramid was 0.5 mm, and the height was about 0.3 to 0.5 mm.

  As the inner layer slit plate 2, a thin copper plate with a thickness of 0.1 mm having a mesh structure with a pitch of 0.5 mm is prepared, and three sheets processed into a corrugated shape with a height (amplitude) of 0.1 to 0.2 mm are laminated. did. The cylinder 3 was made of copper and had a diameter of 5 mm.

  The two outer layer copper plates 1 were joined so that the surfaces on which the graphite sheet 4 was stuck were inside, and the outer edge portions were joined by pressurizing and heating. Although not shown, the outer metal plate 1 is provided with a hole for enclosing the working fluid, and the internal space is decompressed through this hole, and a predetermined amount of pure water is enclosed as the working fluid. The degree of vacuum was set so that the circulation start temperature of the hydraulic fluid was about 60 ° C.

(Comparative example)
A heat spreader of a comparative example was produced. The heat spreader of the comparative example does not include the sheet-like graphite of the heat spreader of the example, as shown in the sectional view in FIG. 5 and the perspective view of each member in FIG. On the inner wall surface of the outer metal plate 1, a concavo-convex structure having a structure in which square pyramids are repeatedly arranged without gaps as shown in FIG. 7 is formed. Further, the column 3 is not arranged in the comparative example. In order to support the inner layer slit plate 2, a total of 16 1 mm square copper prisms are joined to the inner side of the outer layer metal plate 1 (not shown). The contribution to the thermal diffusion performance by this prism is at a negligible level. Other structures are the same as in the embodiment.

  In the heat spreader of the comparative example, when the heat generating source 5 is arranged in the center of the upper surface of the heat spreader as shown in FIG. 8, the heat from the heat generating source 5 is conducted through the outer metal plate 1 to heat the working fluid near the center of the internal space. To do. As a result, the working fluid evaporates, and the vapor passes through the gap between the inner layer slit plate 2 and the slit 2a toward the outer peripheral portion. The steam is cooled and condensed at the outer peripheral portion, becomes a liquid, moves through the uneven structure on the surface of the outer metal plate 1 and the mesh structure of the inner slit plate 2 by capillary action, and returns to the center. Thereby, the heat of the heat source 5 at the center is diffused to the outer periphery.

  When the temperature of the heat generating source 5 is in a temperature region where the working fluid does not evaporate, the heat of the heat generating source 5 is diffused in the main plane direction by the heat conduction of the outer metal plate 1.

(Evaluation experiment)
An experiment was conducted to evaluate the performance of the heat spreader of the example and the comparative example. FIG. 9 shows the apparatus configuration used for the evaluation. A 5 mm square ceramic heater 5 as a heat source 5 was bonded to the center of one surface of the heat spreader 11 of the example or the comparative example via heat conductive grease (not shown). Furthermore, in order to collectively measure the surface temperature distribution of the heat spreader 11 with a radiation thermometer, a ceramic sheet with a known emissivity was stuck through the heater portion. On the other side, a heat sink 14 made of aluminum (JIS standard: A6063) having a size of 50 mm square and a height of 30 mm was bonded to the other surface via heat conductive grease.

  In both the examples and comparative examples, the working fluid circulation start temperature is set to about 60 ° C.

  15 W was applied to the ceramic heater of the heat source 5, and the state of the temperature rise at the center and the outer periphery of the heat spreader 11 was recorded over time using a radiation thermometer. FIG. 10 shows the temperature change of the heat source 5, and FIG. 11 shows the temperature change of the end (outer peripheral part) of the heat spreader 11.

  From FIG. 10, it can be seen that the temperature rise of the heat source 5 decreases in the vicinity of 60 ° C., where the working fluid begins to circulate, in both the example and the comparative example. This is because, in both the example and the comparative example, the equivalent thermal conductivity of the heat spreader 11 is increased near 60 ° C. where the circulation of the hydraulic fluid is started, the heat diffusion is promoted, and the heat of the heat source 5 is effectively diffused. This is because heat is radiated from the heat sink 14.

  From FIG. 11, regarding the temperature of the end portion of the heat spreader 11, in the comparative example, the gradient of temperature rise increases around 60 ° C. This is because the circulation of the hydraulic fluid is started and more heat is carried. In the examples, there is no significant change in the gradient of temperature rise around 60 ° C.

  On the other hand, from FIG. 10, in the temperature region below 60 ° C., the slope of the temperature rise of the heat source 5 was smaller in the example than in the comparative example, and the temperature of the heat source 5 was lower in the example. Conversely, as shown in FIG. 11, the slope of the temperature rise at the outer periphery of the heat spreader 11 is higher in the example than in the comparative example in the temperature region below 60 ° C., and the temperature at the outer periphery is higher than that in the example. Was expensive. From this, in the temperature range below 60 ° C., it was confirmed that the example had a larger equivalent thermal conductivity than the comparative example, and that effective thermal diffusion was realized.

  The reason why the heat spreader of the example has a larger equivalent thermal conductivity than the comparative example in the temperature range below 60 ° C. is because it has the sheet-like graphite 4.

  In the temperature range of 60 ° C. or higher, the high thermal conductivity due to the evaporation / condensation of the hydraulic fluid is dominant in both the examples and the comparative examples, and the rate of temperature increase is almost equal (the comparative examples and examples in FIGS. 10 and 11). If you move the graph in parallel with respect to the time axis, they almost overlap).

  In this evaluation experiment, the temperature difference between the heat source 5 between the example and the comparative example was the largest near 60 ° C. just before the start of the circulation of the hydraulic fluid, and there was a difference of about 20 ° C. From this, it was confirmed that the heat of the heat source 5 can be effectively dissipated by using the structure of the present embodiment. The temperature of the heat source 5 depends on the applied power and the cooling capacity of the heat sink. In general, when the power applied to the heat source is large and the cooling capacity of the heat sink is low, the effect of the heat spreader can be greatly obtained.

  Since the heat spreader of the present embodiment can effectively perform heat diffusion in a temperature range of about 60 ° C. or lower, it is suitable for use as a heat spreader of a device that may be touched by a human hand, for example. For example, an LED desk light is an example of a product. Desk lights using high-power LEDs are generally configured to diffuse heat to the housing surface, but the housing surface can be touched by humans while it is lit, so it is effective even at temperatures below 60 ° C. It is desirable to perform heat diffusion and promote heat dissipation. The heat spreader of the present embodiment is preferable because it can effectively perform heat diffusion even in a temperature range of about 60 ° C. or less.

DESCRIPTION OF SYMBOLS 1 ... Outer metal plate, 1a ... Recessed part, 2 ... Inner layer slit board, 2a ... Slit, 3 ... Column, 4 ... Sheet graphite, 5 ... Heat source, 11 ... Heat spreader, 12 ... Ceramic sheet, 14 ... Heat sink

Claims (3)

A flat metal member having an internal space, and a working fluid sealed in the internal space,
On the inner wall in the main plane direction of the flat metal member, sheet-like graphite having a larger thermal conductivity in the main plane direction than the metal member is fixed,
A flat plate-type heat pipe, wherein the surface of the sheet-like graphite has an uneven shape that causes capillary action in the hydraulic fluid.   2. The flat plate heat pipe according to claim 1, wherein the concave-convex shape is a shape in which convex portions having hexagonal bottom shapes are repeatedly arranged. The flat plate-type heat pipe according to claim 1 or 2, wherein the sheet-like graphite has anisotropic thermal conductivity in which the thermal conductivity in the main plane direction is larger than the thermal conductivity in the thickness direction,
A metal column member is disposed in the internal space of the metal member, and the column member penetrates a through hole provided in the center of the sheet-like graphite, and both ends thereof are in contact with the metal member. Flat plate heat pipe.

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