Abstract:
A method for forming a heat pipe is provided comprising coating the interior surface of the vessel with a wicking material and partially saturating the wick with a working fluid. The vessel is then partially evacuated. A portion of the vessel is pinched-off so as to seal the vessel. Then, the pinched-off portion of the vessel is pressed so as to move it from a first position wherein the portion is convex to a second position wherein the portion is concave.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Divisional application of U.S. patent application Ser. No. 10/983,429, filed Nov. 8, 2004 now U.S. Pat. No. 7,090,002, which itself is a continuation application of U.S. application Ser. No. 10/364,435, filed on Feb. 10, 2003 now U.S. Pat. No. 6,907,918, which itself claimed the benefit of Provisional Patent Application Ser. No. 60/356,625, filed Feb. 13, 2002. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to the manufacture of heat pipes, and more particularly to a method and apparatus for closing the end of a heat pipe after it has been filled with a working fluid. 
     BACKGROUND OF THE INVENTION 
     As the density and power of electronic components have increased, the problem of excessive heat generation has become a significant concern to industry. Heat pipes have been found to provide superior thermal transfer characteristics for cooling electronic circuits. 
     In the prior art, a heat pipe often comprises a closed vessel or chamber whose inner surfaces are lined with a porous capillary wick that is saturated with a working fluid. The heat pipe has an evaporator section that absorbs heat and a condenser section where the heat is released to a heat sink in contact with that section of the heat pipe. In operation, heat absorbed by the evaporator section causes liquid to evaporate from the wick. The resultant vapor is transferred within the vessel to the condenser section of the heat pipe where it condenses releasing the heat of vaporization to a heat sink. The capillary action of the wick pumps the condensed liquid back to the evaporator section for re-evaporation. The process will continue as long as working fluid is contained within the heat pipe. 
     Sometimes, the working fluid in the heat pipe chamber is lost due to a breach of the heat pipe&#39;s wall. Such a breach often occurs at the point where the working fluid was introduced into the heat pipe. The ability to reliably and effectively seal heat pipes has been sought by the industry for many years, because if the fluid within the heat pipe is lost, the equipment cooled by the heat pipe could be subject to significant heat damage. Several means of sealing heat pipes have evolved over the last couple of years. 
     In one conventional arrangement, a heat pipe includes a hollow tube with end caps inserted into each end of the vessel. One end cap has a hole therethrough with a copper pinch-off tube brazed to the hole. The heat pipe is purged and filled with the proper working fluid using the copper tube. To seal the heat pipe, the copper tube is pinched shut using a roller pinch off tool or the like. See, for example, Dunn &amp; Reay, Heat Pipes 154 (3rd Ed. 1982). However, the rollers of the pinch off tool get close to the braze and may crack the braze during pinch off. Additionally, after being sealed the fragile copper tube protrudes outwardly a short distance from the end cap, and therefore is very susceptible to breakage. In order to adequately protect this protruding copper tube, a cover must be placed over the end cap and copper tube. The end cap cover and copper tube disadvantageously consume a large portion of the condenser section at the end of the heat pipe. Both reliability and efficiency of the heat pipe are limited by this technique. 
     In an attempt to improve upon this design, the copper tube has been attached directly to the side of the heat pipe vessel instead of to the end cap. In this prior art arrangement, a copper tube is welded into a hole within the side of the heat pipe vessel, and the heat pipe tube chamber is purged and filled with working fluid using this copper vessel. After filling the heat pipe with fluid, the copper tube is pinched shut to seal the vessel. As with the above-described process, the weld can be cracked during pinch off. Furthermore, this sealing technique is disadvantageous in that a portion of the copper tube extends outwardly from the side of the heat pipe. In this arrangement, the fragile copper tube has no cover and is very susceptible to breakage. Additionally, the placement of the copper pinch-off tube on the side of the heat pipe vessel hampers expulsion of non-condensable gases during purging. Furthermore, because the copper tube protrudes outwardly from the side of the heat pipe, heat pipes formed by this technique cannot be placed adjacent to each other. 
     Consequently, there is a need in the art for an improved heat pipe which is economically accomplished, and provides a strong and reliable seal. 
     SUMMARY OF THE INVENTION 
     The present invention provides a heat pipe comprising a vessel having a first end, a second end, and an inner surface that defines a passageway wherein the first end is closed. A wick is disposed on a portion of the inner surface. A convex wall is positioned at the second end so as to block the passageway. The convex wall is deformable so as to move from a first position wherein a portion of the wall is convex to a second position wherein the portion of the wall is concave. 
     In another embodiment, a heat pipe is provided that comprises a vessel having a first end, a second end, and an inner surface defining a passageway, wherein the first end is closed. A wick is disposed on at least a portion of the inner surface of the vessel. A convex wall is positioned at the second end of the vessel so as to block the passageway. The convex wall includes at least one stress concentrator so that upon an application of a force to the convex wall, the stress concentrator causes the convex wall to buckle and thereby move from a first position wherein a portion of the wall is convex to a second position wherein the portion of the wall is concave. 
     A method for forming a heat pipe is also provided comprising coating the interior surface of the vessel with a wicking material and partially saturating the wick with a working fluid. The vessel is then partially evacuated. A portion of the vessel is pinched-off so as to seal the vessel. Then, the pinched-off portion of the vessel is pressed so as to move it from a first position wherein the portion is convex to a second position wherein the portion is concave. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the present invention will be more fully disclosed in, or rendered obvious by, the following detailed description of the preferred embodiment of the invention, which is to be considered together with the accompanying drawings wherein like numbers refer to like parts and further wherein: 
         FIG. 1  is a perspective view of a heat pipe formed in accordance with the present invention; 
         FIG. 2  is a cross-sectional view of the heat pipe shown in  FIG. 1 , as taken along lines  2 — 2  in  FIG. 1 ; 
         FIG. 3  is a perspective view of a deformable end cap formed in accordance with the present invention; 
         FIG. 4  is a cross-sectional view of the deformable end cap shown in  FIG. 3 , as taken along lines  4 — 4  in  FIG. 3 ; 
         FIG. 5  a cross-sectional view of the heat pipe shown in  FIG. 2 , and including a forming tool shown in phantom; 
         FIG. 6  a cross-sectional view similar to  FIG. 5 , but after the forming tool has applied a force to the deformable end cap; and 
         FIG. 7  a cross-sectional view similar to  FIG. 6 , but after the recess formed by the deformation of the deformable end cap has been filled with a sealant. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     This description of preferred embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description of this invention. In the description, relative terms such as “horizontal,” “vertical,” “up,” “down,” “top” and “bottom” as well as derivatives thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing figure under discussion. These relative terms are for convenience of description and normally are not intended to require a particular orientation. Terms including “inwardly” versus “outwardly,” “longitudinal” versus “lateral” and the like are to be interpreted relative to one another or relative to an axis of elongation, or an axis or center of rotation, as appropriate. Terms concerning attachments, coupling and the like, such as “connected” and “interconnected,” refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise. The term “operatively connected” is such an attachment, coupling or connection that allows the pertinent structures to operate as intended by virtue of that relationship. 
     Referring to  FIGS. 1 and 2 , a heat pipe  5  formed in accordance with the present invention comprises of a vessel  10 , a wick  15 , an end cap  20  and a working fluid (not shown). More particularly, vessel  10  includes a temporarily open end  22 , a closed end  24 , and a central passageway  26  that is defined by the interior surface  28  of vessel  10 . A relatively long blind cylinder or tube that is formed from a thermally conductive material, e.g., copper or its alloys, monel, or the like, is often preferred for vessel  10 . Of course, other shapes of vessel  10  may be used with equal effect, e.g., a plate having a longitudinally and transversely extending interior space. An annular shoulder  29  is formed in interior surface  28 , adjacent to, but spaced away from open end  22 . Central passageway  26  defines a vapor space within vessel  10 . 
     Wick  15  is disposed upon interior surface  28  of vessel  10  below annular shoulder  29 , and may comprise adjacent layers of screening or a sintered powder structure with interstices between the particles of powder. In one embodiment, wick  15  may comprise sintered copper powder, sintered aluminum-silicon-carbide (AlSiC) or copper-silicon-carbide (CuSiC) having an average thickness of about 0.1 mm to 1.0 mm. The working fluid(not shown) may comprise any of the well known two-phase vaporizable liquids, e.g., water alcohol, freon, etc. 
     Referring to  FIGS. 1–4 , end cap  20  is sized and shaped to be permanently lodged within open end  22 , and comprises a deformable-wall  30 , a flange  32 , a face plate  34 , and a fill tube  36 . More particularly, deformable-wall  30  comprises a convex, outwardly curved shape having a bottom edge  38 , a top edge  40 , and a centrally disposed annular groove  42  on an inner surface  44 . Often, deformable-wall  30  comprises a frusto-conical shape. Flange  32  projects radially outwardly from bottom edge  38 , and face plate  34  projects radially inwardly from top edge  40 . A central through-bore  46  is defined in face plate  34  that is sized and shaped to sealingly receive fill-vessel  36 . Annular groove  42  acts as a stress concentrator when force is applied to face plate  34 . Of course, other defects may be defined in deformable-wall  30  to also act as stress concentrators, e.g., radial grooves, periodic grooves, cuts, etc. Although less preferred, deformable-wall  30  may not include a stress concentrator and still function in accordance with the invention. This embodiment will be less reliable than the embodiments comprising a stress concentrator. 
     A heat pipe  5  is formed in accordance with the present invention from a vessel  10  having a wick  15  disposed on its inner surface  28  and with its closed end  24  sealed. End cap  20  is positioned in coaxial aligned relation with open end  22  of vessel  10 , such that flange  32  is arranged in confronting relation to shoulder  29 . Once in this position, end cap  20  is moved toward vessel  10  so that flange  32  enters open end  22 . End cap  20  continues into central passageway  26  until flange  32  engages shoulder  29 . Once in this position, flange  32  is sealingly attached to shoulder  29  via solder, brazing, welding, or the like. 
     With end cap  20  mounted to shoulder  29  within central passageway  26 , vessel  10  is partially filled with a working fluid through fill tube  36 . Central passageway  26  is then evacuated through fill tube  36 . After evacuation, fill tube  36  is pinched closed. At this point in the construction, vessel  10  constitutes an operational heat pipe. However, in order to ensure all the condensable gases are removed, fill tube  36  is quickly opened and shut with the heat pipe at about 100° C. The concave end cap ensures these gases are properly routed to fill tube  36 . Fill tube  36  protrudes outwardly from open end  22  in such a way that it detracts from the usability of the device, and is positioned to be damaged during subsequent handling. 
     Advantageously, end cap  20  may be buckled inwardly, toward central passageway  26 , so as to place the remaining portion of fill tube  36  within a shallow recess  100  formed in opened end  22  ( FIG. 6 ). More particularly, a tool  90  comprising a recess portion  92  is positioned in coaxially aligned, confronting relation to face plate  34  of deformable end cap  20 . In this position, the remnants of fill tube  36  are disposed in confronting relation to recess portion  92  of tool  90 . Tool  90  is then moved toward face plate  34  so as to engage end cap  20 . As tool  90  exerts force on face plate  34 , annular groove  42  creates a stress concentration in deformable-wall  30  that results in end cap  20  buckling inwardly so that it no longer projects outwardly from open end  22 , i.e., convexly, but rather projects inwardly into central passageway  26 , i.e., concavely ( FIG. 6 ). In other words, deformable-wall  30  moves from a convex position to a concave position (relative to central passageway  26 ) upon application of tool  90  to face plate  34 . Stress concentrator  42  allows for more reliable and predictable buckling of deformable-wall  30 . Once in this concave position, shallow recess  100  in open end  22  of vessel  10  may be filled with an appropriate sealant  105 , e.g., epoxy, resin or the like, ( FIG. 7 ). In this way, fill tube  36  is further protected from inadvertent damage which would result in the destruction of heat pipe  5 . 
     It is to be understood that the present invention is by no means limited only to the particular constructions herein disclosed and shown in the drawings, but also comprises any modifications or equivalents within the scope of the claims.