Patent Publication Number: US-2007095506-A1

Title: Heat pipe and method for making the same

Description:
FIELD OF THE INVENTION  
      The present invention relates generally to an apparatus for transfer or dissipation of heat from heat-generating components such as electronic components, and more particularly to a method of manufacturing a wick structure for a heat pipe.  
     DESCRIPTION OF RELATED ART  
      Heat pipes have excellent heat transfer performance due to their low thermal resistance, and therefore are an effective means for transfer or dissipation of heat from heat sources. Currently, heat pipes are widely used for removing heat from heat-generating components such as central processing units (CPUs) of computers. A heat pipe is usually a vacuum casing containing therein a working fluid, which is employed to carry, under phase transitions between liquid state and vapor state, thermal energy from one section of the heat pipe (typically referring to as the “evaporating section”) to another section thereof (typically referring to as the “condensing section”). The casing is made of high thermally conductive material such as copper or aluminum. Preferably, a wick structure is provided inside the heat pipe, lining an inner wall of the casing, for drawing the working fluid back to the evaporating section after it is condensed at the condensing section.  
      The wick structure currently available for heat pipes includes fine grooves integrally formed at the inner wall of the casing, screen mesh or bundles of fiber inserted into the casing and held against the inner wall thereof, or sintered powders combined to the inner wall by sintering process. Among these wicks, the sintered powder wick is preferred to the other wicks with respect to heat transfer ability and ability against gravity. Currently, a conventional method for making a sintered powder wick includes inserting a column-shaped mandrel at a central portion of a hollow casing which has a closed end and an open end. Powders are filled into the casing to construct the wick. The mandrel functions to hold the filled powders against an inner wall of the casing. Then, the casing with the powders is sintered at high temperature for a specified time period to cause the powders to diffusion bond together to form the wick. As a result, the wick structure has an even thickness along an axial direction of the heat pipe.  
      Since the primary function of a wick is to draw condensed liquid back to the evaporating section of a heat pipe under the capillary pressure developed by the wick, the capillary pressure is an important parameter affecting the performance of the wick. Since it is well recognized that the capillary pressure of a wick increases due to an increase in amount of pores of the wick. As the area of the wick structure being limited to the size of the casing of the heat pipe, the way to enhance the amount of pores is to increase the thickness of the wick structure. However, the thermal resistance of the wick structure increases with the increasing of the thickness of the wick structure. The heat carried by the vapor is difficult to dissipate to the casing and then to ambient when the wick structure has a large thickness. Then the vapor can not condense to liquid rapidly. As a result, a heat pipe with a wick having evenly thickness often suffers dry-out problem at the evaporating section as the condensed liquid cannot be timely sent back to the evaporating section of the heat pipe.  
      Therefore, there is a need for a heat pipe with a sintered powder wick which can provide simultaneously a relatively large capillary force and a relatively low thermal resistance so as to effectively and timely bring the condensed liquid back from its condensing section to its evaporating section and thereby to avoid the undesirable dry-out problem at the evaporating section.  
     SUMMARY OF THE INVENTION  
      A heat pipe in accordance with a preferred embodiment of the present invention includes a casing receiving working fluid therein, and a wick structure formed at an inner wall of the casing. The casing includes an evaporating section and a condensing section. The wick structure at the evaporating section has a thickness larger than that of the wick structure at the condensing section of the heat pipe.  
      The present invention in another aspect, relates to a method for manufacturing the sintered powder wick structure of the heat pipe. The preferred method includes steps of: providing a hollow casing arranged aslant to horizon; filling a slurry of powders into the casing; rotating the casing with the slurry to form a slurry layer held against an inner wall of the casing; and sintering the slurry layer in the casing to form the wick structure.  
      Other advantages and novel features of the present invention will be drawn from the following detailed description of the preferred embodiments of the present invention with attached drawings, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Many aspects of the present heat pipe the method can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present heat pipe the method. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.  
       FIG. 1  is a longitudinal cross-sectional view of a heat pipe in accordance with a preferred embodiment of the present invention;  
       FIG. 2  shows a flow chart of a preferred method in manufacturing the heat pipe of  FIG. 1 ;  
       FIGS. 3-5  are schematic diagrams of one example of the method, showing different stages in forming the wick structure by using the method of  FIG. 2 ; and  
       FIGS. 6-8  are schematic diagrams of another example of the method, showing different stages in forming the wick structure by using the method of  FIG. 2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 1  illustrates a heat pipe in accordance with a preferred embodiment of the present invention. The heat pipe is vacuumed and includes a casing  100  and a sintered powder wick structure  200  arranged against an inner wall of the casing  100 . The heat pipe is divided into an evaporating section  120 , an adiabatic section  130  and a condensing section  140  along an axial direction of the heat pipe. The adiabatic section  130  is located between the evaporating and condensing sections  120 ,  140 .  
      The casing  100  is made of high thermally conductive material such as copper or aluminum. Although the casing  100  illustrated is in a round shape, it should be recognized that other shapes, such as polygon, rectangle, or triangle, may also be suitable.  
      The wick structure  200  is saturated with a working fluid (not shown), which acts as a heat carrier when undergoing phase transitions between liquid state and vaporous state. The wick structure  200  is a porous structure and is formed by sintering process, in which small-sized powders are sintered together under high temperature. The wick structure  200  is in the form of an uneven structure. A thickness of the wick structure  200  gradually decreases along an axial direction from the evaporating section  120  to the condensing section  140  of the heat pipe. The wick structure  200  has the largest thickness at the evaporating section  120 , whilst has the smallest thickness at the condensing section  140  of the heat pipe. In other words, the thickness of the wick structure  200  at the evaporating section  120  is larger than that at the condensing section  140 .  
      During operation, the evaporating section  120  of the heat pipe is maintained in thermal contact with a heat-generating component (not shown) such as a CPU. The working fluid contained at the evaporating section  120  absorbs heat generated by the heat-generating component and then turns into vapor. Due to the difference of vapor pressure between the two sections  120 ,  140  of the heat pipe, the generated vapor moves towards and carries the heat simultaneously to the condensing section  140  where the vapor is condensed into liquid after releasing the heat into ambient environment by, for example, fins (not shown) thermally contacting the condensing section  140 . Due to the difference of capillary pressure developed by the wick structure  200  between the two sections  120 ,  140 , the condensed liquid is then drawn back by the wick structure  200  to the evaporating section  120  where it is again available for evaporation. For the wick structure  200  corresponding to the evaporating section  120  of the heat pipe having the largest thickness, the evaporating portion of the wick structure  200  has a relatively larger amount of pores than other portions thereof. Therefore the evaporating portion has a relatively larger capillary force to draw back the condensed working fluid to the evaporating section  120 . At the same time, the wick structure  200  corresponding to the condensing section  140  of the heat pipe has the smallest thickness. Thus the condensing portion of the wick structure  200  has a relatively smaller thermal resistance. The vapor is easily to dissipate the heat absorbed from the heat-generating component to the casing  100  of the heat pipe, and then transfers to the ambient environment. Thus the vapor is capable of condensing to liquid and then flowing back to the evaporating section  120  timely. Therefore, dry-out of the heat pipe is avoided. As a result, the heat of the heat-generating component can be dissipated appropriately.  
      In the present invention, a method as shown in  FIG. 2  is proposed to construct the wick structure  200  of the heat pipe. Also refer to  FIGS. 3-5 . The method includes a first step of providing a hollow casing  10 . The casing  10  is cylinder-shaped with two open ends  16 ,  18  (i.e., first and second open ends  16 ,  18 ). A hole  160 ,  180  is defined in each of the open ends  16 ,  18 . Each end  16 ,  18  has an inner diameter and an outer diameter smaller than that of the other portion of the casing  10 . The casing  10  is set on a centrifugal forming machine  30 . The machine  30  has a top surface  32  for mounting the casing  10  thereon. The top surface  32  is slanted. Thus, the casing  10  is arranged slantwise with the first end  16  lower than the second end  18 . An inclined angle θ is defined between the casing  10  and the horizon.  
      Then slurry  40  is filled into the casing  10 . A feeder  50  is applied for filling the slurry  40  into the casing  10 . The feeder  50  has an opening  52  for the slurry  40  to flow therethrough. The feeder  50  is arranged adjacent to the second end  18  of the casing  10  with the opening  52  extending into the hole  180  of the second end  18 . The slurry  40  is obtained by mixing the necessary powders, for example, metal powders, with a solvent, a binder and, if desirable, some other additives. These components are mixed together in a certain proportion either by weight or by volume. The solvent, which is used to lower the viscosity of the slurry  40  so that the slurry  40  can flow more easily, may be selected from organic material such as ethanol, xylene or the like, which is sensitive to temperature. The binder is used to bind the powders together, and may be selected from polyvinyl alcohol (PVA), polyvinyl butyral (PVB), acrylic resin or the like. Other additives that are desirable may include a dispersant to stabilize the powder against colloidal forces and a plasticizer to modify the properties of the binder. The dispersant may be selected from fish oil such as menhaden fish oil, and the plasticizer may be selected from butyl benzyl phthalate or polyethylene glycol.  
      During the filling process, the slurry  40  flows from the feeder  50  via the opening  52  into the casing  10 . The amount of slurry  40  filled into the casing  10  is determined by the volume of the casing  10  and the inclined angle θ. It is easy to be understood that the maximum amount of slurry  40  filled into the casing  10  is to keep the slurry  40  from flowing out through the hole  160  of the first end  16  of the casing  10 . For the casing  10  arranged slantwise, the slurry  40  in the casing  10  has an unevenly thickness along the axial direction of the casing  10  after the filling process. The thickness of the slurry  40  gradually decreases from the first end  16  to the second end  18  of the casing  10 . The slurry  40  adjacent to the first end  16  has the largest thickness, whilst the slurry  40  adjacent to the second end  18  has the smallest thickness.  
      Then the casing  10  filled with the slurry  40  is rotated. The machine  30  is started to drive the casing  10  into rotation along a central axis X-X thereof. The slurry  40  abuts an inner wall of the casing  10  intimately by the centrifugal force during rotation. The slurry  40  is approximately evenly adhered to the inner wall of the casing  10  along a circumferential direction thereof after a period of time of the rotation (As shown is  FIG. 4 ).  
      During the rotating process, a heating device (not shown) which is arranged under the casing  10  heats the slurry  40  with a relatively low temperature to remove the solvent from the slurry  40  and thus to dry the slurry  40  to form a green layer  60 . As the solvent is sensitive to temperature, the solvent turns into vapor by the heating of the heating device. An airflow generated by a fan flows through the casing  10  for facilitating the dissipation of the solvent vapor from the casing  10  to the ambient environment through the holes  160 ,  180  thereof. Since only a relatively lower temperature is needed, the binder contained in the slurry  40  is not removed. The binder binds the powders together after the solvent is removed from the slurry  40 . Thus the green layer  60  having a thickness gradually decreasing along the axial direction from the first end  16  to the second end  18  of the casing  10  is formed by centrifugation formation technology (As shown is  FIG. 5 ).  
      The casing  10  with the green layer  60  is then sintered under a high temperature to thereby produce the sintered powder wick  200  of the heat pipe as shown in  FIG. 1 . As with the uneven thickness of the green layer  60 , the wick structure  200  formed by sintering the green layer  60  has an uneven thickness, which gradually decreases from the first end  16  to the second end  18  of the casing  10 . Finally, the casing  10  is vacuumed and a working fluid such as water, alcohol, methanol, or the like, is injected into the casing  10  via the open ends  16 ,  18 , and then the open ends  16 ,  18  of the casing  10  is hermetically sealed to form the heat pipe of  FIG. 1 .  
      The advantage of the procedure in relation to other methods, e.g. the conventional sintering process, is that the method involves application of the centrifugation formation technology, which can avoid using a core rod (i.e., mandrel) in manufacturing the wick structure  200 . Further since the green layer  60  is formed with an uneven thickness along the axial direction of the casing  10 , the wick structure  200  is also formed with an uneven thickness. When the heat pipe is applied to absorb heat from the heat generating component, the portion of the heat pipe adjacent to the first end  16  is the evaporating section  120  which thermally attaches to the heat-generating device to absorb heat therefrom, and the portion of the heat pipe adjacent to the second end  18  is the condensing section  140  which thermally attaches to a radiator to dissipate the heat.  
       FIGS. 6-8  show another example of the method to manufacture the wick structure  200  for the heat pipe. Except for the casing  610  applied for making the heat pipe, other components and the procedure of the second embodiment of the method are substantially the same with the previous embodiment. In this embodiment, the casing  610  has first and second ends  616 ,  618  at two opposite ends thereof. The second end  618  is an open end and defines a hole  619  therein. The first end  616  of the casing  10  is closed. The casing  610  is set on the machine  30  slantwise with the sealed first end  616  at a lower position. Thus, during the filling process the slurry  40  filled into the casing  610  by the feeder  50  is unable to flow through the first end  616  of the casing  610 . The maximum amount of slurry  40  which can be filled into the casing  610  is increased, in comparison with the first embodiment. On the other hand, a relatively larger inclined angle θ can be formed between the casing  610  and the horizon. The difference of the thickness of the wick structure  200  formed by this embodiment between the evaporating section  120  and the condensing section  140  can be increased. Thus a further larger capillary force can be developed by the wick structure  200  corresponding to the evaporating section  120 , and a further smaller thermal resistance can be obtained by the wick structure  200  corresponding to the condensing section  140  of the heat pipe.  
      It is understood that the invention may be embodied in other forms without departing from the spirit thereof. Thus, the present example and embodiment are to be considered in all respects as illustrative and not restrictive, and the invention is not to be limited to the details given herein.