CA1278291C - Heat pipe - Google Patents

Abstract

ABSTRACT
The heat pipe of the present invention comprises a wick predominantly formed from carbon fibers for returning condensed or liquefied working fluid to the heating zone. The carbon fibers may be urged and secured to the inner surface of a closed metal tube by a retainer member without tying them up, or interposed between wire mesh screens into a sandwich structure, or knitted with wefts into a fabric. Any of these constructions can increase the capillary pressure, reduce the pressure loss of liquid phase working fluid, and hence, enhance the heat transport capability as well as making it possible to make a flexible heat pipe.

Description

~2~7829~

DESCRIPTION

HEAT PIPE

TECHNICAL FIELD
This invention relates to heat pipes, and more particularly, to heat pipes useful in applications requiring improved heat transport capabilities, for example, where heat is transported over relatively long distances and where a heating zone is slightly higher than a cooling zone.

BACKGROUND ART
As is well known, heat pipes are of the structure wherein working fluid such as water is sealed in the interior of a closed metal tube evacuated to vacuum and a wick for creating a capillary pressure is provided in the interior of the metal tube, and operate such that the working fluid is evaporated upon receipt of heat from the exterior, flows toward the lower temperature side through the interior of the metal tube, and releases heat to condense into liquid, and the resulting liquid phase working fluid is circulated to the heating zone side by the capillary pressure action of the wick whereby heat transport is carried out in the form of latent heat associated with the phase transition of working fluid. Because of their heat conductivity several ten to one hundred and several ten times higher than that of copper which has the best heat conductivity among metals, heat pipes have been used in a variety of applications including medical equipment as well as waste heat recovering heat-exchangers and solar water heaters, and recently find new applications in indirect cooling of power cables and the like.

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The heat transport characteristics of heat pipes largely depend upon the structure of the wick because the condensed or liquid working fluid is returned toward the heating zone side through the wick as previously mentioned. It is then necessary to employ wicks which can creat a high capillary pressure and cause a low pressure loss to the liquid phase working fluid, for example, when heat is transported over long distances or from a higher location to a lower location.
Further, when heat pipes are extended so as to provide long distance heat transport, they are desirably flexible for ease of transportation and installation.
Heretofore known heat pipes use wicks in the form of grooves, wire mesh or porous sintered metal materials. Among these, heat pipes having grooved wicks produce a low capillary pressure and are difficult to distribute the liquid phase working fluid throughout the inner surface of the metal tube because the wicking grooves are formed axially of the metal tube. The grooved wicked heat pipes thus have relatively low heat transport capabilities and are difficult to transport heat over long distances Heat pipes having wicks of wire mesh are easy to distribute the liquid phase working fluid throughout the inner surface of the metal tube, but have the drawback that the liquid phase working fluid being returned to the heating zone experiences a marked pressure loss because the flow paths for passage of the liquid phase working fluid are curved in a winding manner and randomly crossed each other.
Further, heat pipes having wicks of porous sintered metal materials can provide a higher capillary pressure than the above-mentioned grooved and wire mesh wicks because the porous sintered metal materials have an extremely reduced effective capillary radius, but have the problem that the heat pipes themselves cannot be rendered flexible because the sintered metal materials are no longer flexible. More ~27~291 particularly, sintered copper provides a high capillary pressure when the working fluid is water. Since it is, however, difficult to manufacture a length of sintered copper which can take advantage of its own characteristics, the 5 actual practice is to join relatively short sections of sintered copper, resulting in joints disturbing the circulation of liquid phase working fluid. Thus, long heat pipes capable of long distance heat transport are not available at present.
On the other hand, the recent trend is to employ glass fibers, nickel fibers or the like as wicking materials. These wicking materials, however, are undesirably less wettable and insufficient in capillary pressure when the workinq fluid is water.
It is an object of the present invention to provide a heat pipe which can provide a sufficiently high capillary pressure to return liquid phase working fluid and thus allows for heat transport over long distances or from a higher location to a lower location.
` It is another object of the present invention to provide a heat pipe having improved heat transport capability as well as flexibility.
It is a further object of the present invention to provide a heat pipe which can maintain a high heat transport - capability for an extended period and is easy to manufacture.
DISCLOSURE OF INVENTION
The present invention provides a heat pipe characterized in that a wick comprises numerous ultrathin carbon fibers and the ultrathin carbon fibers are retained in intimate contact with the inner surface of a metal tube by a retainer member disposed inside the carbon fibers. Then, the carbon fiber bundles can creat a high capillary pressure and a reduced pressure loss occurs across flowpaths defined among the 1329~

carbon fibers, both providing the heat pipe with improved heat transport capability. Additionally, the flexibility of carbon fibers allows the heat pipe to be flexible as a whole.
Further, the construction of the present invention wherein carbon fibers are urged and secured to the inner surface of the metal tube by the retainer member disposed inside the carbon fibers prevents clogging of flowpaths defined among carbon fibers and poor transfer of heat to and from the working fluid.
In the practice of the present invention, the heat pipe may be rendered flexible and increased in the area available per unit length for heat transfer to and from the exterior by forming the metal tube into a corrugated tube.
Another embodiment of the present invention provides a heat pipe wherein a plurality of ultrathin carbon fibers are twisted into a strand and a plurality of such strands are placed in intimate contact with the inner surface of the metal tube in mutually spaced-apart relationship. This construction increases the area of the inner surface of the metal tube available for direct contact with the working fluid and thus allows heat transfer to efficiently take place between the working fluid and the metal tube.
In a further embodiment of the heat pipe of the present invention, the retainer member comprises a spiral band member which is spiralled at a pitch greater than the width of the band member. This construction ensures that the working fluid penetrates into the wick and evaporates out of the wick.
In an alternative embodiment of the present invention, the retainer member comprises a highly elastic wire mesh screen.
This construction not only ensures that the wicking ultrathin carbon fibers be urged and secured to the inner surface of the metal tube, but further ensures that the working fluid penetrates into the wick and evaporates out of the wick.

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In a still further embodiment of the present invention, the wick may be formed from a fabric material of carbon fibers. With this construction, numerous carbon fibers constituting the wick can be readily placed adjacent the inner surface of the metal tube and maintain the shape.
According to a further aspect of the present invention, there is provided a heat pipe comprising a wick of sandwich structure wherein ultrathin carbon fibers are interposed between wire mesh screens. With this construction, the wick is easily placed adjacent the inner surface of the metal tube and a loss of heat transfer due to the intervening carbon fibers may be compensated for by the wire mesh screens, also contributing to the improvement in thermal characteristics.
Furthermore, the arrangement of the wick adjacent the entire inner surface of the metal tube may be maintained so that the high heat transport capability may be maintained for an extended period.

BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic fragmentary cross section of a heat pipe according to one embodiment of the present invention;
FIG. 2 is a cross section taken along lines II-II in FIG. 1;
FIG. 3 is a diagram plotting the measurements of the capillary height of carbon fiber and comparative samples;
FIG. 4 is a diagram showing the relationship of the diameter of carbon fiber filaments and (K/r);
FIG. 5 is a cross section similar to FIG. 2 showing another arrangement of carbon fibers;
FIG. 6 is a diagram illustrating the experimental results of samples of the present invention and comparative examples determined for temperature equalizing property in the top heat mode;

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FIG. 7 is a schematic fragmentary cross section of a heat pipe according to another embodiment of the present invention;
FIG. 8 is a cross section taken along lines VIII-VIII in 5 FIG. 7;
FIG. 9 is a fragmentary enlarged cross-sectional view of an assembly of carbon fibers interposed between wire mesh screens;
FIG. 10 is a cross section similar to FIG. 8 showing another arrangement of carbon fibers in the heat pipe shown in FIG. 7;
FIG. 11 is a schematic partially cut-away view of a further embodiment of the heat pipe of the present invention;
FIG. 12 is a cross section taken along lines XII-XII in FIG. 11;
FIG. 13 is a fragmentary plan view of a fabric member;
and FIG. 14 is a diagram showing the measurements of the temperature distribution in the axial direction of the heat pipe shown in FIG. 11 and comparative heat pipes during operation.

BEST MODE FOR CARRYING OU~ THE INVENTION
The heat pipe of the present invention will be described in detail by referring to the accompanying drawings.
In FIG. 1 which is a schematic fragmentary cross-sectional view of one embodiment of the present invention and FIG. 2 which is a cross sectional view taken along lines II-II in FIG. 1, numeral 1 designates a closed metal tube serving as an enclosure. Numerous ultrathin carbon fibers 2 are placed as a wick adjacent the inner surface of the metal tube 1, a retainer member 3 is disposed inside the carbon fibers 2 in ~27~%9~1 order to secure the carbon fibers 2 in intimate contact with the inner surface of the metal tube 1, and working fluid such as water is sealed in the interior of the metal tube 1 which has been evacuated to vacuum of non-condensible gases.
The metal tube 1 used herein may be straight one, but preferably corrugated one as shown in FIG. 1 in order that the tube is flexible. The wick is formed from ultrathin carbon fibers 2 for the following reasons.
The first reason is that carbon fibers 2 can provide a substantially higher capillary pressure than available with previously known wicking materials. This is because carbon fibers 2 are extremely slender or have a reduced effective capillary radius and have improved wettability when the working fluid is water. The inventors have made an experiment to find that a markedly high capillary pressure can be provided by carbon fibers 2. FIG. 3 is a graph showing the results of the experiment performed by the inventors, in which curve A indicates the capillary height resulting from a bundle of numerous carbon fibers 2 having a diameter of 5 pm, and similarly, curve B indicates the capillary height for activated alumina of 60 to 80 mesh, curve C for silica gel of about 60 mesh, curve D for a length of joined sintered metal segments each having a length of about 18 cm, and curve E for wire mesh (300 mesh). It should be noted that in this experiment, each sample was placed upright in water and the height of water rising through the sample was designated the capillary height. As apparent from the experimental results shown in FIG. 3, the carbon fibers 2 yielded a capillary height of at least 100 cm at the maximum whereas the conventional wicking materials such as wire mesh and sintered metal yielded only a capillary height of about 40 cm at the maximum. It was found that carbon fibers 2 are outstandingly excellent.

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The second reason why the wick is formed from carbon fibers 2 is that when ultrathin carbon fibers 2 are boundled, there is obtained a continuous gap among carbon fibers 2, that is, a continuous flowpath for liquid phase working fluid, and at the same time, the liquid phase working fluid returning from the cooling zone to the heating zone experiences a low pressure loss due to the low frictional coefficient of carbon fibers so that the return flow of liquid phase working fluid is facilitated in addition to the achievement of a high 0 capillary pressure.
It is to be noted that the maximum heat transport quantity Qmax varies with the diameter of carbon fibers themselves.
The most preferred diameteer of carbon fiber filaments to be formed into a wick will now be determined. The maximum heat 5 transport quantity Qmax is gene ally represented by the following equation:
Qmax - 2tK/r)A(1 + r-p-g-Lsin~/2~) (p~ t~) K: permeability of wick (m2), r: effective capillary radius (m), A: cross sectional area of wick (m2), p: density of liquid phase working fluid ~(kg/m3)-~], g: qravitational acceleration (m/sec2), L: entire length of wick (m), ~: inclination angle (deg), ~: surface tension (kg/m), ~:evaporation latent heat (kcal/kg), ~: coefficient of viscosity (kg-s/m2) In the above equation, the value of (K/r) depends on the wick. (K/r) values obtained from measurements on wicks comprising carbon fiber bundles are as shown in FIG. 4, which indicates that the preferred diameter of carbon fiber filaments used as wicks ranges from about 2 ~m to about 30 ~m.
Further, since the carbon fibers 2 have a low heat conductivity, the wick comprising the carbon fibers 2 may preferably have a thickness of about 1 to about 5 mm, and " ~ ~

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usually about 2 mm in order to reduce the overall thermal resistance to working fluid.
The third reason why the wick is formed from carbon fibers 2 is that carbon fibers 2 have high elasticity and a low coefficient of linear expansion. In order to improve the heat transfer between working fluid and the metal tube serving as the enclosure, the wick is preferably placed in intimate contact with the inner surface of the metal tube. When the heat pipe is made flexible and the wick is formed from carbon fibers 2, the restoration of the once bent heat pipe to the original state does not alter the intimate contact of the wick with the metal tube because the wick conforms to the bending of the pipe and restores to the original state due to its own elasticity. Further, even when the difference in temperature between quiescent and operating times is great, the wick does not become slack due to the low coefficient of linear expansion so that the contact of the wick with the metal tube 1 is maintained firm.
In addition, carbon fibers 2 have a high heat resistance as well as high corrosion resistance so that they may be used as wicks even in heat pipes subject to high operating temperatures, offer a great freedom in the choice of working fluid, and are further characterized by light weight and high mechanical strength. Thus, heat pipes obtained by forming the wick from carbon fibers 2 are easy to handle and useful in a variety of applications.
It should be noted that numerous carbon fibers 2 must be tied up and placed in firm contact with the metal tube 1 by any appropriate means because carbon fibers themselves have no mutual tying or binding power or bonding power to the metal tube 1 as an enclosure. If an adhesive is used to this end, the adhesive would undesirably plug a gap among carbon fibers 2 or a flowpath for liquid phase working fluid and disturb the ~;27~3;29~

heat transfer between the working fluid and the metal tube 1.
Thus, according to the present invention, the retainer member 3 is disposed inside the carbon fibers 2 which are placed adjacent the inner surface of the metal tube 1, whereby the S carbon fibers 2 are forced and secured to the inner surface of the metal tube 1 by the retainer member 3.
The arrangement of the carbon fibers 2 will be described.
The carbon fibers 2 may be placed on the inner surface of the metal tube 1 in an axial direction thereof or in a spiral fashion. Further, the carbon fibers 2 may be placed in intimate contact with the entire inner surface of the metal tube 1. Alternatively, a plurality of carbon fibers may be twisted into a strand 2a and such strands may be arranged at predetermined intervals in a circumferential direction of the metal tube 1 as shown in FIG. 5. Then a broader area is available for the direct contact of the working fluid with the inner surface of the metal tube 1, allowing heat transfer to take place between the working fluid and the metal tube 1 in a more efficient manner.
The retainer member 3 will be described in detail. The retainer member 3 in the embodiment shown in FIG. 1 is a spirally wound band member 3a, for example, a steel strip, which acts to expand due to its own elasticity to thereby urge and secure the carbon fibers 2 to the inner surface of the metal tube 1. The spiral retainer member 3 is spiralled at a pitch P which is greater than the width W of the band member 3a so that the retainer member 3 only partially covers the surface of the carbon fibers so as to facilitate the penetration of liquefied working fluid into the carbon fibers 2 and the evaporation of the working fluid from the carbon fibers 2.
It is to be noted that the retainer member 3 used in the practice of the present invention may be formed from a highly -~!L;27829~

elastic wire mesh screen instead of the above-mentioned spiral band member 3a. The retainer member 3 in the form of a wire mesh screen is effective in preventing movement of the carbon fibers 2 and facilitating the penetration and evapora-tion of the working fluid into and out of the carbon fibers 2.
FIG. 6 is a diagram showing the results of an experiment conducted in order to demonstrate the performance of the heat pipe according to the present invention. In the experiment, a sample to be tested was heated in the so-called top heat mode where the sample was vertically placed and heated at its top, and the temperature of different points was measured.
In FIG. 6, curve F shows the temperature distribution found in the sample of the present invention, and the remaining curves show the temperature distribution found for comparative samples, curve G for a comparative example of a wick comprised of a mixture of sintered metal and carbon fibers, curve H for a comparative example of a wick comprised of an adsorbent, curve I for a comparative example of a wick comprised of sintered metal, and curve J for a comparative example of a wick comprised of a 300 mesh wire net.
As apparent from the data in FIG. 6, the heat pipe according to the present invention was observed to exhibit a minimal difference in temperature between the heating and cooling zones or overall uniformity in temperature and thus exert improved heat pipe performance.
Next, other embodiments of the present invention will be described.
FIG. 7 is a schematic fragmentary cross-sectional view of another embodiment of the present invention and FIG. 8 is a cross-sectional view taken along lines VIII-VIII in FIG. 7.
Unlike the first embodiment mentioned above, the heat pipe shown herein has a wick in the form of an assembly 5 comprising numerous carbon fibers 2 interposed between wire ~7~;Z9~

mesh screens 4. The overall construction of this embodiment is similar to that of the foregoing embodiment except that a particular retainer member is not provided because the wire mesh screens 4 retain the carbon fibers 2.
More specifically, the assembly 5, as shown in the fragmentary enlarged view of FIG. 9, has a sandwich structure wherein numerous carbon fibers 2 are interposed between wire mesh screens 4 and is rounded into a cylindrical shape and inserted into the metal tube 1. The carbon fibers 2 used in this assembly 5 each have a diameter of about 2 to 30 pm and are tied to an overall thickness of about 1 to 5 mm, and preferably about 2 mm. Further, the carbon fibers 2 may be placed in any desired orientation, for example, axially or spirally of the metal tube 1. It is to be noted that although the carbon fibers 2 may be placed in an annular form on the entire inner surface of the metal tube 1 as shown in FIG. 8, a plurality of carbon fibers may be twisted into a strand 2a and such strands may be placed at predetermined intervals as shown in FIG. 10. Then a broader area is available for the direct contact of the working fluid with the inner surface of the metal tube 1, allowing heat transfer to take place between the working fluid and the metal tube 1 in a more efficient manner.
The mesh size of the metal screens 4 used in the assembly 25 5 may be on the order of 50 to 300 mesh. When fine mesh metal screens 4 are used, preferably they are not placed inside the carbon fibers 2 at the area of an evaporator zone so as not to prevent the carbon fibers 2 from providing a capillary pressure.
Then, the above-mentioned heat pipe ensures that a flow path for liquid phase working fluid is defined, the carbon fibers 2 are stably retained adjacent the inner surface of the metal tube 1 without any movement or loosening, and the heat transfer to and from the working fluid is satisfactorily ~27~329i accomplished. More particularly, since the carbon fibers 2 themselves have a lower heat conductivity than metals, any appropriate complementary means is required to improve the heat transfer to and from the working fluid. In addition, since numerous carbon fibers 2 must be tied up and placed in firm contact with the metal tube 1 by any appropriate means because carbon fibers themselves have no mutual tying or binding power or bonding power to the metal tube 1 as an enclosure. If an adhesive is used to this end, the adhesive would undesirably plug a gap among carbon fibers 2 or a flow path for liquid phase working fluid and distrub the heat transfer between the working fluid and the metal tube 1. On the contrary, the above-mentioned heat pipe uses a wick in the form of the assembly 5 of sandwich structure comprising carbon fibers 2 interposed between wire mesh screens 4 so that the heat transfer to and from the working fluid may be efficiently carried out through the highly heat conductive wire mesh screens 4. At the same time, the carbon fibers 2 are tied up by interposing them between the wire mesh screens 4 and placed in stable and intimate contact with the inner surface of the metal tube 1 due to the elasticity of the wire mesh screens 4.
Then, the heat pipe shown in FIG. 7 may also have a higher capillary pressure for returning the liquid phase working fluid than previously available and hence a higher heat transport capability. When compared with conventional heat pipes, the heat pipe of this embodiment will provide a result as shown in FIG. 6 in a similar fashion to the above-mentioned first embodiment. Since the carbon fibers 2 are tied or integrated by the wire mesh screens 4 in the heat pipe shown in FIG. 7, the carbon fibers 2 are easy to handle and insert them into the metal tube 1 and free of the probable disarrangement by moving or loosening so that the initial high heat transport capability may be maintained for an extended period.

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FIG. 11 is a partially cut-away view showing a further embodiment of the heat pipe of the present invention, and FIG.
12 is a view taken along lines XII-XII in FIG. 11. The heat pipe shown herein has a wick in the form of a fabric 6 of carbon fibers. More specifically, the fabric 6 of carbon fibers is placed adjacent the inner surface of the closed metal tube 1 and urged and secured to the inner surface of the metal tube 1 by the retainer member 3 disposed inside the fabric 6. A suitable working fluid is sealed in the metal tube after the metal tube is suction evacuated of non-condensible gases.
The fabric 6 is knitted one comprising carbon fiber warps 7 extending axially of the metal tube 1 and carbon fiber wefts 8 traversing the warps as shown in FIGS. 11 and 13. The fabric 6 is rounded into a cylindrical shape and inserted into the metal tube 1. The fabric 6 is urged and secured to the inner surface of the metal tube 1 due to the elasticity of the retainer member 3 inserted inside the fabric 6.
Gaps defined among the warps 7 constitute a return flow-path for liquid phase working fluid in the thus constructedheat pipe. Since in general, one end of the metal tube 1 serves as a heating zone and the other end as a cooling zone, then the return flowpath becomes a linear flowpath extending in the direction along which the liquid phase working fluid should essentially flow. There is thus obtained a heat pipe having a reduced flow resistance to the liquid phase working fluid. The spacing between carbon fibers constituting the wicking fabric 6 is very narrow, which leads to a reduced effective capillary radius and hence, provides a high capillary pressure.
Although the wefts 8 transverse the return flowpath for liquid phase working fluid in the foregoing fabric 6, the inventors have made an experiment to find that the influence of the wefts 8 on the flow of liquid phase working fluid is 329~

--1 s--minimized by reducing the volume ratio of the wefts 8 to the warps 7 to 0.1 or less. More illustratively, for a heat pipe having a volume ratio of wefts 8 to warps 7 of 0.1, a heat pipe having a volume ratio of wefts 8 to warps 7 of 1, and a heat pipe having a wick of wire mesh screen, heat transport was carried out in the so-called top heat mode where the heating zone is slightly elevated (at an inclination angle of 5), and temperatures were measured at plural points along each heat pipe to obtain temperrature distribution. The results are shown in FIG. 14 which indicates that the heat pipe having the volume ratio of unity (curve K) and the heat pipe having a wick of wire mesh (curve L) both have a steep temperature gradient whereas the heat pipe of the present invention having the wicking fabric 6 (curve M) has a moderate temperature gradient. That is, as it is known that the full return of liquid phase working fluid to the heating zone renders the temperature distribution throughout the heat pipe uniform, these results indicate that by reducing the volume ratio of wefts 8 to warps 7 to 0.1 or less, a sufficient flow of liquid phase working fluid returns to the heating zone, resulting in improved heat transport capability.
In addition, the circumferential movement of the warps 7 constituting the wick is prohibited by the wefts 8 in the heat pipe of the above-mentioned construction to avoid the loss of uniformity due to movement of the warps 7, thereby maintaining the capillary pressure high and the pressure loss across the return flowpath low for an extended period.
It is to be noted that the above-mentioned retainer member 3 may also have any structure including, for example, wire mesh screens and metal wire coils as well as a spiral band member.

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INDUSTRIAL APPLICABILITY
Since the use of carbon fibers as wicking material allows liquid phase working fluid to effectively return to the heating zone, the heat pipe of the present invention is useful for the indirect cooling of power cables and where a high temperature source is remote from a low temperature source or heat is transported a long distance, and in the so-called top heat mode where heat is transported from a higher location to a lower location.

Claims (9)

1. A heat pipe wherein working fluid is sealed in the interior of a closed metal tube and the metal tube is provided on its inner surface with a wick for allowing the liquid phase working fluid to move therethrough, characterized in that said wick comprises numerous ultrathin carbon fibers and the ultrathin carbon fibers are retained in intimate contact with the inner surface of said metal tube by a retainer member disposed inside the carbon fibers.

2. A heat pipe as set forth in claim 1, characterized in that said closed metal tube is a corrugated tube.

3. A heat pipe as set forth in claim 1, characterized in that a plurality of strands are prepared each by twisting a number of ultrathin carbon fibers and the strands are attached to the inner surface of said metal tube in mutually spaced-apart relationship.

4. A heat pipe as set forth in claim 1, characterized in that said retainer member comprises a spiral band member which is spiralled at a pitch greater than the width of the band member.

5. A heat pipe as set forth in claim 1, characterized in that said retainer member comprises a highly elastic metal mesh screen.

6. A heat pipe as set forth in claim 1, characterized in that said wick takes the form of a fabric comprising lengthwise carbon fiber bundles extending axially of said metal tube and knitted with transverse carbon fibers.
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7. A heat pipe as set forth in claim 6, characterized in that the volume ratio of said weft fibers to the lengthwise fiber bundles is 0.1 or less.

8. A heat pipe wherein working fluid is sealed in the interior of a closed metal tube and the metal tube is provided on its inner surface with a wick for allowing the liquid phase working fluid to move therethrough, characterized in that said wick is of a sandwich structure comprising numerous ultrathin carbon fibers interposed between wire mesh screens.

9. A heat pipe as set forth in claim 8, characterized in that a plurality of strands are prepared each by twisting a number of ultrathin carbon fibers and the strands are interposed between said wire mesh screens in mutually spaced-apart relationship.