US20110303392A1 - Flat heat pipe - Google Patents

Abstract

A thin flat heat pipe capable of transporting heat even if it is bent is provided. The flat heat pipe comprises: a working fluid to be evaporated when heated and to be condensed when the heat dissipates; and a wick, which is formed by bundling a plurality of thin wires while twisting along a center axis thereof, and which is adapted to create a capillary pressure for returning the liquid phase working fluid to a portion where evaporation takes place. The wick is arranged over the entire length of the flat container while being in contact with both upper and lower inner faces of the container or with an inner side face of the container in a manner such that an inner space of the container for letting through an evaporated working fluid is not closed, and a contact portion between the wick and the container is fixed by sintering over the entire length of the wick.

Description

CROSS-REFERENCE TO RELATED APPLICATION
• This is a Continuation Application of International Application No. PCT/JP2010/052696 filed Feb. 23, 2010. The contents of the aforementioned application are incorporated herein by reference.
TECHNICAL FIELD
• This invention relates to a heat pipe configured to transport heat by a working fluid encapsulated in a container, and especially to a heat pipe, which is flattened entirely, and in which a bundle of thin wires is used to form a wick for returning the working fluid by a capillary pumping.
BACKGROUND ART
• Basically, a heat pipe comprises: a container in which a non-condensable gas such as air is evacuated therefrom; a working fluid such as water, alcohol etc. which is evaporated and condensed within a given temperature range; and a wick arranged in the container to pump the liquid phase working fluid by capillary action. In the heat pipe, therefore, the working fluid is evaporated by external heat, and the evaporated working fluid flows toward a low pressure side to dissipate heat therefrom. Consequently, the evaporated working fluid is condensed again. Thus, the heat is transported by the working fluid in the form of latent heat. The working fluid thus condensed penetrates into the wick and pumped by the capillary action of the wick to the portion where the evaporation takes place.
• Thus, in the heat pipe, the evaporated working fluid flows from the evaporating portion to which the external hear is transmitted toward the dissipating portion where the heat of the evaporated working fluid is dissipated, and the liquid phase working fluid flows in an opposite direction. Therefore, the heat is transported continuously. In order to enhance a heat transporting capacity of the heat pipe, that is, in order to reduce a thermal resistance, it is necessary to flow both of evaporated and condensed working fluids sufficiently and smoothly. In addition, the heat pipe is used widely in many fields. For example, in case of using the heat pipe to cool an electronic device, the heat pipe has to be downsized in accordance with miniaturization of an electron device and electronic circuit.
• Therefore, various kinds of techniques to maintain a flow passage for letting through evaporated working fluid, to improve smoothness of returning liquid phase working fluid, to downsize the heat pipe and so on have been developed in the prior art. For example, Japanese Patent Laid-Opens No. 2004-53186, No. 2000-74579 and No. 2003-247791 disclose a technique to enhance capillary force of a wick for pumping the working fluid by forming the wick by bundling a plurality of thin wires of copper or carbon. The capillary pumping is enhanced by reducing an effective capillary radius of a meniscus formed on a surface of the working fluid. Therefore, in order to enhance the capillary pumping, clearances between wires forming the wick is reduced by bundling the wires. In case of thus forming the wick, a flow passage for the working fluid can also be smoothened and straightened so that flow resistance of the working fluid can be reduced. For this reason, the reflux characteristics of the working fluid can be further improved.
• In case of thus forming the wick using the thin wires, the flow path is formed between the wires. Therefore, those wires are bundled without using an adhesive agent. For example, according to the heat pipe taught by Japanese Patent Laid-Open No. 2004-53186, a number of extrafine wires are bundled in a spiral elastic body such as a coil spring. According to the teachings of Japanese Patent Laid-Open No. 2000-74579, a plate member having a recessed groove is arranged in a pipe, and wires are held in the groove. Further, according to the teachings of Japanese Patent Laid-Open No. 2003-247791, a plurality of ultrafine wires are twisted into a bundle, and the bundle of the wires is inserted into a grooved pipe. Thus, according to the teachings of Japanese Patent Laid-Opens No. 2004-53186 and No. 2000-74579, the thin wires are kept in a bundle using the coil spring and the plate member. Meanwhile, according to the teachings of Japanese Patent Laid-Open No. 2003-247791, the ultrafine wires are bundled by twisting the wires.
• In addition, Japanese Patent Laid-Open No. 2001-208489 discloses a technique to form a sufficient steam passage for letting through evaporated working fluid. According to the teachings of Japanese Patent Laid-Open No. 2001-208489, a flat heat pipe is formed by arranging a wire mesh longitudinally in a container and fixing the wire mesh to the container by a seam welding method, and by flattening the heat pipe thus formed. Therefore, in case the heat pipe taught by Japanese Patent Laid-Open No. 2001-208489 is bent, the wick is bent along the container. For this reason, the wick will not be in contact with an inner face of the container of an inner circumferential side in a bending radius, so that the steam passages in the container can be prevented from being closed.
• Thus, according to the prior art, the heat pipe is flattened by pushing the heat pipe in its radial direction as taught by Japanese Patent Laid-Opens No. 2004-53186, No. 2000-74579, and No. 2001-208489. Specifically, according to the teachings of Japanese Patent Laid-Open No. 2000-74579, the heat pipe is flattened to be thinner than 1.5 mm. In addition, Japanese Patent Laid-Open No. 11-173777 discloses a heat pipe which can be flattened thinner than 1 mm.
• In case of using the coil spring or the plate member to bundle the wires as taught by Japanese Patent Laid-Opens No. 2004-53186 and No. 2000-74579, a total thickness of the wick or an outer diameter of the wick has to be increased. Therefore, it is not advantageous to use such a bundling member to reduce a thickness of the flattened heat pipe. To the contrary, according to the teachings of Japanese Patent Laid-Open No. 2003-247791, the wires are bundled by twisting the wires without using a bundling member. Therefore, the teachings of Japanese Patent Laid-Open No. 2003-247791 is advantageous to reduce an outer diameter of the wick, that is, to flatten the heat pipe as taught by Japanese Patent Laid-Opens No. 2001-208489 and No. 11-173777.
• In case of forming the wick using a bundle of thin wires, the wick is arranged in the container entirely in the length direction. Therefore, in order to maintain a flow passage for the vaporized working fluid sufficiently in the container even if the heat pipe thus structured is deformed or bent, the wick is preferably fixed as taught by Japanese Patent Laid-Opens No. 2000-74579 and No. 2001-208489. However, in case of holding the bundled wires in the plate member as taught by Japanese Patent Laid-Open No. 2000-74579, a number of elements is increased and the thickness of the flattened heat pipe is increased. Alternatively, in case of fixing the wick to the inner face of the container by a seam welding method as taught by Japanese Patent Laid-Open No. 2001-208489, a manufacturing process has to be complicated and it is very difficult to carry out such a demanding task.
DISCLOSURE OF THE INVENTION
• The present invention has been conceived noting the technical problems thus far described, and its object is to provide a flat heat pipe capable of transporting heat efficiently even if it is deformed or bent.
• In order to achieve the above-mentioned object, according to the present invention, there is provided a flat heat pipe, which is configured to transport heat by a working fluid to be evaporated when heated and to be condensed when dissipating the heat therefrom. The flat heat pipe comprises: a container, which is flattened and in which the working fluid is encapsulated; and a wick, which is formed by bundling a plurality of thin wires while twisting along a center axis thereof, and which is adapted to create a capillary pressure when the liquid phase working fluid penetrates thereto. According to the flat heat pipe of the present invention, the wick thus formed of the thin wires is arranged over the entire length of the flat container while being contacted with both of an upper and a lower inner faces of the container or with an inner side face of the container in a manner not to close an inner space of the container for letting through an evaporated working fluid. In addition, any of a contact portion between the wick and the container is fixed by sintering over the entire length of the wick.
• The thin wires include a copper wire.
• Preferably, the bundle of thin wires is straightened in advance by applying a thermal treatment thereto. Specifically, the bundle of thin wires is annealed in advance.
• Specifically, the container is formed by flattening a pipe holding the bundle of thin wires in its width center.
• According to the present invention, the wick is thus formed by bundling the thin wires. Therefore, an effective capillary radius of a meniscus formed on a surface of the working fluid penetrating into the wick can be reduced. As a result, a capillary force for pumping the liquid phase working fluid can be enhanced. In addition, smoothly extending flow passages for letting through the liquid phase working fluid can be formed between the wires so that a flow resistance of the working fluid can be reduced. As described, the wick is formed by twisting the thin wires without using any bundling member. Therefore, a number of construction elements of the heat pipe can be reduced, and both of the vaporized working fluid and the liquid phase working fluid are allowed to flow through the flow passages without hindrance. As also described, the bundle of thin wires is subjected to a thermal treatment in advance to be straightened so that the flow of the working fluid can be further smoothened. For this reason, refluxing characteristics of the liquid phase working fluid can be improved. Since the flow of the working fluid is thus smoothened, a heat transporting characteristics of the heat pipe can also be improved entirely. In addition to the above-explained advantages, since the wick is formed of a plurality of thin wires without using a bundling member, an outer diameter of the wick can be reduced relatively. Therefore, the container can be flattened to be thinner than the conventional flat container. In other words, the container can be flattened without degrading the heat transporting characteristic of the heat pipe. Further, a center portion of the cylindrical pipe can be prevented from being deformed (or depressed) excessively when flattened, by situating the bundled wire in the width center of the pipe.
• In addition, in case of bending the container of the heat pipe thus structured, the wick fixed to the inner face of the container is also bent together with the container. Therefore, an intermediate portion of the wick will not be displaced to be in contact with the inner face of the container in a manner that closes the inner space of the container. For this reason, the flow passages for letting through the vapor flow can be maintained in the container so that the evaporated working fluid flowing therethrough will not be blocked by the closed passage. In addition, according to the present invention, the wick is fixed entirely with the inner face of the container lengthwise by inserting the wick into the container, and sintering the container holding the wick therein. Thus, according to the present invention, the heat pipe having excellent heat transporting capacity can be manufactured easily.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a schematic view showing a process of manufacturing the wick of the heat pipe according to the present invention.
• FIG. 2 is a sectional view showing a cross-section of a round shaped heat pipe before flattening during the manufacturing process.
• FIG. 3 is a sectional view showing a cross-section of one example of the flat heat pipe according to the present invention.
• FIG. 4 is a sectional view showing a cross-section of another example of the flat heat pipe according to the present invention.
• FIG. 5 is a view explaining a characteristic test carried out on the example of the present invention and a comparative example.
• FIG. 6 is a graph indicating a relation between a thermal input and a thermal resistance of the heat pipe according to the example, and a relation between a thermal input and a thermal resistance of the heat pipe of the second comparative example.
• FIG. 7 is a schematic view showing a positional relation between the container and the wick of a straight flat heat pipe of the present invention, and a positional relation between the container and the wick of a curved flat heat pipe of the present invention.
• FIG. 8 is a schematic view showing positional relation between the container and the wick in a heat pipe in which the wick is not subjected to a thermal treatment, in both cases in which the heat pipe is not bent and the heat pipe is bent.
• FIG. 9 is a schematic view showing positional relation between the container and the wick in a heat pipe in which the wick is fixed partially, in both cases in which the heat pipe is not bent and the heat pipe is bent.
BEST MODE FOR CARRYING OUT THE INVENTION
• Next, the present invention will be explained in more detail hereinafter. The present invention relates to a heat pipe comprising a wick having a specific structure. Specifically, according to the present invention, the wick of the heat pipe is formed by unifying a plurality of thin wires without using a bundling member. For example, the wick is formed using a metal wire such as a copper wire, a carbon fiber and so on having good hydrophilicity with the working fluid encapsulated in a container. In order to maintain the wires into a bundle, the unified wires are twisted along a center axis thereof. Therefore, the wires have to be capable of keeping their shape even after twisted. For this purpose, a thin copper wire is especially suitable.
• According to the present invention, the wick formed by thus twisting a bundle of wires is arranged in the container, and the wick is fixed with the container by a sintering method. Then, the working fluid is filled in the container. Specifically, an airtight hollow container is used to hold the wick, and in case of using the heat pipe to transport the heat between sites away from each other, a hollow tube is used as the container. In order to exchange an internal heat and an external heat, the container is preferably made of highly-heat conductive material such as a copper pipe. In addition, narrow grooves can be formed optionally on an inner face of the container thereby allowing the working fluid to flow therethrough by capillary pumping.
• The above-explained wick formed by twisting the thin wires is fixed onto the inner face of the container. Specifically, the container holding the wick therein is heated to a predetermined temperature. As a result, the wick is bonded thermally with the inner face of the container, and remaining space in the container serves as a flow passage for letting through the evaporated working fluid.
• The working fluid is evaporated when heated and condensed after dissipating the heat therefrom. Thus, the heat is transported by the working fluid in the form of latent heat. For this purpose, the working fluid is selected from water, alcohol, hydrochlorofluorocarbon and so on, and encapsulated in the container while evacuating non-condensable gas such as air from the container.
• Therefore, in case of heating a portion of the container while cooling another portion of the container, the working fluid is evaporated at the heated portion and flows toward the cooled portion at which a temperature and a pressure are lower than the heated portion. In this situation, a heat of the evaporated working fluid is dissipated from the cooled portion, and then the evaporated working fluid is condensed again. In the heat pipe of the present invention, the wick is thermally fixed to the inner face of the container, and a flow passage(s) for letting through the evaporated working fluid is/are formed in the container along the wick. Therefore, the flow passages for letting through the evaporated working fluid can be maintained even if the heat pipe is bent. For this reason, a sufficient vapor flow is allowed to flow in the heat pipe so that the heat transporting capacity of the heat pipe is enhanced.
• Meanwhile, the condensed working fluid penetrates into the wick and flows through the flow passages formed therein between the wires forming the wick toward the portion where the evaporation takes place. Specifically, when the working fluid is evaporated, a level of the meniscus formed between the wires is lowered and a capillary pumping force is thereby created. As a result, the liquid phase working fluid is attracted toward the evaporating side by the capillary pumping. According to the present invention, the narrow clearances are formed between the wires forming the wick so that the capillary pumping force can be enhanced. As a result, the refluxing characteristics of the heat pipe can be improved. In addition, the wick is formed by twisting the bundle of thin wires without tightening at a specific point by a bundling member. Therefore, the flow passages for letting through the liquid phase working fluid formed between the wires can be smoothened so that the flow resistance of the liquid phase working fluid can be reduced. For this reason, the refluxing characteristics of the heat pipe can be further improved. Further, the wick can be fixed easily to the inner face of the container over the entire length by inserting the wick formed of the thin wires into the container and heating the container from outside. Therefore, manufacturability of the heat pipe can be improved.
• Next, an example of a method for manufacturing a heat pipe according to the present invention will be explained hereinafter with reference to the accompanying figures. In this example, a copper wire whose diameter is approximately 0.05 mm is used as awire1, and 100 to 400 pieces of thewires1 are brought together to form abundle2 as shown inFIG. 1 (a). Then, as shown inFIG. 1 (b), thebundle2 is twisted along a center axis thereof. As a result, thewires1 form thebundle2 without using any specific bundling member. Thebundle2 thus formed is cut to a predetermined length. In case of using an uncoiled wire as thewire1, thewires1 are straightened by a thermal treatment, thereby preventing thewires1 from being curled by residual stress.
• Acontainer3 is prepared using a pipe whose thickness is 0.3 mm, and whose outer diameter is in a range of 3.0-6.0 mm. Thecontainer3 is subjected to degreasing and cut to a predetermined length. In case of using copper wires to form awick4, a copper pipe is also used to form thecontainer3. Then, thebundle2 is inserted into thecontainer3 to serve as thewick4. Specifically, thebundle2 is laid linearly on a lower inner face of thecontainer3 by gravity. Thecontainer3 thus holding thewick4 therein is set horizontally in a furnace (not shown) to be heated. Provided that thecontainer3 and thewick4 are made of copper, thecontainer3 holding thewick4 is heated at 1000° C. As a result, thewick4 is fixed to the inner face of thecontainer3 over the entire length. In this situation, some of thewires1 may be fixed thermally with each other as shown inFIG. 2.
• Thecontainer3 in which thewick4 is thus fixed thermally thereto is ejected from the furnace and cooled. Then, one of the end portions of the container is sealed by applying a swaging and welding thereto, in other words, a bottom swaging and a bottom welding are carried out. Meanwhile, a swaging is also applied to other end portion of the container3 (i.e., a top swaging). As a result, a prototype of thecontainer3 is prepared.
• As a result of thus applying the top swaging to said other end portion of thecontainer3, a nozzle portion is formed thereon, and the working fluid is filled in thecontainer3 from the nozzle portion. In this situation, the non-condensable gas such as air is evacuated from thecontainer3, and then the working fluid is filled in thecontainer3. Alternatively, the working fluid is filled in thecontainer3 in an amount larger than a required amount, and then boiled to evacuate the non-condensable gas from thecontainer3. Thus, the working fluid can be filled in thecontainer3 by conventional methods. Then, the nozzle portion is compressed and welded to be sealed. That is, a top welding is carried out.
• In this example, the pipe whose cross-section is circular is used to form thecontainer3. Therefore, the round heat pipe thus manufactured is then pressed in its radial direction to be flattened. In case of forming a straight heat pipe, the round heat pipe thus manufactured is flattened as is. Alternatively, in case of forming a curved or bent heat pipe, the round heat pipe is curved or bent into a desired shape, and then flattened in its radial direction. In case of using a flat pipe as thecontainer3, the flat heat pipe can be obtained without carrying out the above-explained flattening step. However, in this case, it is preferable to press thecontainer3 thereby contacting thewick4 tightly with the inner face of thecontainer3. As described, thewick4 is formed by twisting thebundle2 of thewires1 and thermally fixed to the inner face of thecontainer3. Therefore, in case of curving or bending the flat heat pipe thus manufactured, thewick4 is curved or bent together with thecontainer3 without closing theflow passage5 for letting through the evaporated working fluid formed along thewick4.
• As also described, according to the present invention, thebundle2 functioning as thewick4 is formed by twisting thewires1 without using a bundling member. Therefore, a thickness of the flat heat pipe thus structured can be further reduced. In addition, since thewick4 is thus fixed thermally to the inner face of thecontainer3, theflow passage5 can be maintained without any change. As shown inFIG. 3, thewick4 of the heat pipe is formed by twisting thebundle2 of the plurality ofwires1, and the working fluid is encapsulated in thecontainer3 made of copper and flattened in its radial direction. Further, according to the example shown inFIG. 3, a plurality ofnarrow grooves11 are formed on the inner face of thecontainer3 in the axial direction. Thosegrooves11 also serve as a wick, and a contact area between the working fluid and the inner face of thecontainer3 is thereby enlarged.
• In this example, thewick4 situated in the width center of thecontainer3 is contacted with both upper and lower inner face of the flattenedcontainer3 and fixed thermally therewith over the entire length. Since thewick4 is contacted directly with the inner face of thecontainer3 without interposing inclusion therebetween, a total thickness of thewick4 can be reduced. In addition, an inner space of thecontainer3 is divided into two chambers by thewick4, and those chambers serve as theflow passages5 for letting through the evaporated working fluid. As described, thestraight container3 and thestraight wick4 are used to manufacture the heat pipe, therefore, theflow passages5 are also straight when the heat pipe is manufactured. Since thewick4 is fixed with the inner face of thecontainer3, thewick4 is bent together with thecontainer3 even if thecontainer3 is bent. That is, thewick4 will not be in contact with the opposite inner side face of thecontainer3 even if thecontainer3 is bent. Therefore, even if thecontainer3 is bent, theflow passage5 for letting through the evaporated working fluid can be maintained as in the heat pipe just after manufactured so that the evaporated working fluid is allowed to flow through theflow passages5 without hindrance.
• Thus, according to the present invention, the flat heat pipe shown inFIG. 3 is manufactured by inserting thewick4 formed by twisting thebundle2 of thewires1 into thecontainer3. Therefore, the thickness of the heat pipe can be reduced. In addition, the flow passages for letting through the liquid phase forking fluid can be formed narrowly and smoothly in thewick4. Therefore, the capillary pumping of thewick4 can be enhanced. Further, theflow passage5 for letting through the evaporated working fluid formed in thecontainer3 will not be closed by thewick4 even if thewick4 is bent. Therefore, heat transporting capacity of the heat pipe can be enhanced in comparison with that of the conventional heat pipes.
• Alternatively, according to the present invention, thewick4 may also be situated on one of side ends of the flat heat pipe as shown inFIG. 4, instead of arranging thewick4 in the width center of thecontainer3. The flat heat pipe shown inFIG. 4 is also capable of deliver comparable performance as the heat pipe shown inFIG. 3.
Example
• Next, a non-limiting example of the present invention will be explained by comparing with comparative examples.
Example
• In this example, uncoiled thin copper wires were bundled and twisted to form a wick. The wick thus formed was subjected to an annealing to be straightened and then inserted into a copper pipe serving as a container. Specifically, a thickness of the container was 0.3 mm and an outer diameter of the container was 4.0 mm. In addition, a plurality of thin grooves were formed on an inner face of the container. Then, a heat pipe was manufactured by the above-explained procedures. The heat pipe thus manufactured was flattened to 1 mm thickness. The wick was situated at a width center of the container, and fixed thermally with the container. A total length of the flat heat pipe thus manufactured was 100 mm, and water is encapsulated in the container to serve as working fluid.
Comparative Example 1
• In the first comparative example, a heat pipe was prepared by the same manner as the above-explained example except for applying an annealing to the wick in advance.
Comparative Example 2
• In the second comparative example, the wick was formed by arranging a plurality of copper wires around a spiral and pushing the copper wires onto an inner face of the container using the spiral, instead of twisting the wires. Thus, in the second comparative example, only a structure of the wick was different from the above-explained example, and remaining structures of the heat pipe were identical to those of the heat pipe of the example.
Comparative Example 3
• In the third comparative example, the wick was formed by bundling a plurality of straight copper wires using a spiral, and the wick thus formed was inserted into a container. Thus, only a structure of the wick was different from the second comparative example, and remaining structures of the heat pipe were identical to those of the second comparative example.
Test Procedure
• As shown inFIG. 5, one of the end portions of aheat pipe10 to be tested was contacted with a surface of an electric heater15 (25 mm·15 mm) and other end portion of theheat pipe10 was contacted with adissipation plate16 made of aluminum (64 mm·40 mm·1.5 mm). A lower face of thedissipation plate16 was contacted with aheat insulating plate17. Said one of the end portions was heated by energizing theelectric heater15. In this situation, an electric energy (i.e., a thermal input Q), a temperature Th at a contact point P1 between theelectric heater15 and theheat pipe10, and a temperature Tc at the other end portion P2 of theheat pipe10 were measured. Then, a thermal resistance (° C./W) of each heat pipe and a maximum thermal input (W) thereof without causing a dry out were obtained on the basis of the measured data. Specifically, the thermal resistance R was calculated by the following formula: R=(Th−Tc)/Q. In order to carry out the test, 30 pieces of the heat pipes of the Example and the Comparative Example 1 were prepared individually, and a number (or rate) of non-defective products was also counted for the heat pipes of the example and thecomparison 1. Measurement result is shown in table 1. In addition, the measured thermal resistance and thermal input of the heat pipes of the example and thecomparison 1 are indicated inFIG. 6.

• 	TABLE 1
  	Bundling Member		Twisted Wick	Thermal	Maximum heat

  	Using	Without	Straight	Thermally	Without Thermally	Resistance	Transportation	Non-Defective
  	Spiral	Spiral	Wick	Treated	Treated	° C./W	W	Rate

  Example		◯		◯		0.4	10	29/30
  Comparative		◯			◯	0.8	10	3/30
  Example 1
  Comparative	◯		◯			3.0	5	—
  Example 2
  Comparative	◯		◯			1.0	7	—
  Example 3

• The following assessment can be derived from the measurement results indicated in Table 1. As described above, according to the heat pipe of the Example of the present invention, the wick is formed by twisting the bundle of thin wires, and the wick thus formed is straightened thermally in advance. As can be seen from Table 1, the thermal resistance of the heat pipe according to the present invention is 0.4° C./W, and this is the smallest value in the tested heat pipes. In addition, as indicated inFIG. 6, the thermal resistance of the heat pipe according to the present invention is stabilized at approximately 0.4° C./W under the condition in which the thermal input is smaller than the maximum thermal input thereof. Thus, the heat pipe according to the present invention is capable of displaying excellent heat transportation characteristics within an operating temperature limit. In addition, a non-defective rate of the heat pipe according to the present invention was higher than 90%, and this is also the highest rate in the tested heat pipes.
• According to the first comparative example, the thermal treatment was not applied to the wick in advance. In this case, the maximum thermal input was also 10 W. However, the thermal resistance of the heat pipe according to the first comparative example was 0.8° C./W, that is, twice as much as that of the heat pipe according to the present invention. In addition, only three of the thirty heat pipes were non-defective. Thus, the heat pipe according to the first comparative example was inferior to the heat pipe according to the present invention in productivity.
• According to the second comparative example, the wires were arranged around the spiral and pushed onto the inner face of the container using the spiral. In this case, the maximum thermal input of the heat pipe was impaired to 5 W, that is, the maximum thermal input was half as much as that of the heat pipe of the present invention. In addition, the thermal resistance of the heat pipe according to the second comparison was 3.0° C./W. Thus, the thermal resistance of the heat pipe according to the second comparison was increased eightfold in comparison with that of the heat pipe according to the present invention.
• According to the third comparative example, the wick was formed by bundling wires by the spiral. In this case, the maximum thermal input was 7 W, and the thermal resistance was 1° C./W. That is, the maximum thermal input and the thermal resistance of the heat pipe according to the third comparative example were improved in comparison with those of the heat pipe according to the second comparative example. However, the thermal resistance of the heat pipe according to the third comparative example was more than twice as much as that of the heat pipe according to the present invention, but the maximum thermal input of the heat pipe according to the third comparative example remained at 70% of that of the heat pipe according to the present invention.
• Thus, according to the heat pipe of the present invention, the maximum thermal input was 10 W and the thermal resistance is smaller than 1. This means that the heat pipe according to the present invention has the most excellent heat transporting capacity. In addition, according to the present invention, the thickness of the heat pipe is only about 1 mm so that the heat pipe can be installed easily even in a small electronic hardware. Moreover, the heat pipe can be manufactured without variation so that the productivity of the heat pipe can be improved. Further, even if the heat pipe is flattened, the center part of the container can be flattened neatly without being deformed unevenly.
• In addition, a plurality of modified heat pipes of the example in which the wick was not fixed thermally to the container were also prepared for the purpose of confirming the thermal resistance thereof under the condition in which the heat pipe thus structured is bent. As described, the heat pipe according to the present invention in which the wick formed by twisting the bundled wires was fixed thermally to the wick was stabilized within a range from 0.4 to 0.6° C./W. However, the range of the variation in the thermal resistance of the heat pipe thus modified was widened from 0.4 to 1.2° C./W. Thus, the heat transporting characteristic of the modified heat pipe was degraded.
• Next, here will be explained advantages of straightening the wires forming the wick by applying a thermal treatment in advance, and to fix the wick thermally onto the inner face of the container over the entire length. In most cases, the thin wires used to form the wick are manufactured in the form of coil, and transported to a production site of the heat pipe. Therefore, the uncoiled wires may be curled by the residual stress even after bundling or at subsequent processes. However, according to the present invention, the wires used to form the wick are straightened in advance by applying thermal treatment such as annealing thereby eliminating the residual stress remaining in the wires. Therefore, the wires forming the wick will not be curled during a process of inserting the wick into the container, and the wick formed of straightened wires can be fit to the interior configuration of the container as shown inFIG. 7 (a). In addition, the wick thus arranged in the container is heated to be fixed onto the inner face of the container over the entire length. Therefore, in case of bending thecontainer3 holding thewick4 therein, thewick4 is also bent together with thecontainer3 as shown inFIG. 7 (b). For this reason, the flow passage(s)5 formed along thewick4 will not be closed by thewick4 even after bending thecontainer3.
• To the contrary, when the thermal treatment is not applied to thewick4 in advance, thewick4 may be deformed by the residual stress remaining therein. In this case, thecurved wick4 extends obliquely in thecontainer3 and is in partial contact with the inner face of thecontainer3 as shown inFIG. 8 (a). As a result, theflow passage5 is closed by thecurved wick4 and the evaporated working fluid flowing therethrough is blocked. For this reason, the heat transporting capacity of the heat pipe is degraded. As shown inFIG. 8 (b), the heat pipe thus structured also suffers from this disadvantage even if it is bent.
• Alternatively, in case of fixing thewick4 onto the inner face of thecontainer3 at both end portions, thewick4 may be arranged diagonally on thecontainer3 as shown inFIG. 9 (a). In this case, therefore, a velocity of the evaporated working fluid flowing through theflow passage5 is changed by thewick4 thus extending diagonally. In this case, an intermediate portion of thewick4 is also in contact with the inner face of thecontainer3 when the heat pipe is bent, and the heat transporting capacity of the heat pipe is thereby degraded. Especially, in the flat heat pipe, thewick4 is in contact with both of the upper and lower inner faces of thecontainer3. Therefore, in case the configuration of thewick4 is not congruent with that of the inner face of thecontainer3, theflow passage5 for letting through the evaporated working fluid is completely closed by thewick4 as shown inFIGS. 8 and 9. In this situation, the heat pipe is disabled to transport the heat between the end portions of thecontainer3.

Claims (6)

1. A flat heat pipe, which transports heat by a working fluid to be evaporated when heated and to be condensed when the heat dissipates therefrom, comprising:

a container, which is flattened and in which the working fluid is encapsulated; and

a wick, which is formed by bundling a plurality of thin wires while twisting along a center axis thereof, and which is adapted to create a capillary pressure when the liquid phase working fluid penetrates thereto;

wherein the wick thus formed of the thin wires is arranged over the entire length of the container while being in contact with both an upper inner face and a lower inner face of the container or with an inner side face of the container in a manner not to close an inner space of the container for letting through an evaporated working fluid; and

any of a contact portion between the wick and the container is fixed by sintering over the entire length of the wick.

2. The flat heat pipe as claimed inclaim 1, wherein the thin wire includes a copper wire.

3. The flat heat pipe as claimed inclaim 1, wherein the bundle of thin wires is straightened before bundling by applying a thermal treatment thereto.

4. The flat heat pipe as claimed inclaim 3, wherein the thermal treatment includes an annealing.

5. The flat heat pipe as claimed in any ofclaim 1 to4 or6, wherein the container is formed by flattening a pipe holding the bundle of thin wires in its width center.

6. The flat heat pipe as claimed inclaim 2, wherein the bundle of thin wires is straightened in before bundling by applying a thermal treatment thereto.

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