Abstract:
Microgrooves (&lt;0.2 mm wide) of various shapes used as wick structures in heat pipes can increase the capillary force to overcome the gravitational force on the working fluid so as to enable large working angles for the heat pipes. The microgrooves can be fabricated by two sequential steps use a first plowshare-like blade to turn up the material for large size grooves and then immediately use a second plowshare-like blade to rebury by the previously turned up material. The microgrooves and the fabrication method can be used to manufacture flat heat pipes (vapor chambers) as well as tubular heat pipes.

Description:
FIELD OF INVENTION 
       [0001]    This invention is related to the wick structures, and more specifically to microgrooves (&lt;0.2 mm wide) used as wick structures in heat pipes and method for manufacturing the same. 
       DESCRIPTION OF RELATED ART 
       [0002]    A heat pipe is a highly efficient heat transfer device that typically includes a vacuum vessel. The vacuum vessel has a wick structure on its inner wall and contains a small quantity of working fluid. When a heat source is applied to an evaporator portion, the working fluid evaporates into vapor that spreads quickly in the vessel. The vapor carries latent heat to a condenser portion and condenses to liquid as the latent heat dissipates to outside of the heat pipe by conduction or convection. The working fluid is transported by the capillary force back to the evaporator portion, thereby completing a two phase heat transfer cycle without consuming any power. 
         [0003]    Generally, heat pipes are made from highly thermally conductive metals such as stainless steel, copper, and aluminum. Working fluids that are compatible with these heat pipe materials include water, mercury, and other chemicals depending on the working temperature range. Copper and pure water are the most common combination for the heat pipes used in computer and electronic systems. To overcome gravity so that evaporator and condenser can be in any orientation, the wick structure in a heat pipe provides the pumping mechanism that transports the working fluid back to the evaporator portion. 
         [0004]    Rather than having a round or oblong tube shape of a typical heat pipe, a flat heat pipe has a plate shape and is usually made of metal sheets or plates. The flat heat pipe has a vapor chamber enclosing a working fluid. The vapor chamber has capillary structures on the inner surfaces of the top and bottom plates. The evaporator portion is one or more small areas on the outer surface of either the top or bottom plate that contact one or more heat sources (e.g., an electronic device). All other areas of the top and bottom plates serve as the condenser portion. 
         [0005]    Typical capillary structures in heat pipes include sintered metal powders, fibers, meshes and grooves. Heat pipes with sintered metal powders, such as a sintered copper powder, have great capillary force so that they can be used at any orientation. However, it is complex and expensive to manufacture this type of heat pipes, and the thermal resistance is higher than other type heat pipes because the sintered metal powders are porous. Heat pipes made with fibers and meshes work at small angles. Furthermore, they are also expensive and complicated to be manufactured. When compared with the aforementioned technologies, heat pipes with grooves are inexpensive and easy to manufacture. However, they are only used at horizontal condition or small angles because the conventional grooves do not provide enough capillary force. 
         [0006]    Heat pipes with grooves, usually V-shape or other shapes, are generally manufactured by a seamless pipe process such as extrusion. However, the size of the grooves are large (about &gt;0.35 mm wide) relative to heat pipe dimensions due to the limitations on the tooling. The resulting capillary force is not large enough to pump the working fluid back to the upper condenser at large working angles. Therefore, a method for fabricating microgrooves (about &lt;0.2 mm wide) is needed to take advantage of the low cost and ease of manufacturing of heat pipes with grooves, as well as to improve the thermal performance of the heat pipes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  illustrates a process for forming microgrooves in one embodiment of the invention. 
           [0008]      FIG. 2  illustrates a process for forming microgrooves in another embodiment of the invention. 
           [0009]      FIG. 3  illustrates microgrooves on a plate in one embodiment of the invention. 
           [0010]      FIG. 4  illustrates a flat heat pipe with microgrooves in one embodiment of the invention. 
           [0011]      FIG. 5  illustrates a production line of making pipes with inner-threads using seam-welding. 
           [0012]      FIG. 6  illustrates a method for making microgrooves on a strip in the production line of  FIG. 5  in one embodiment of the invention. 
           [0013]      FIG. 7  illustrates an oblong heat pipe in one embodiment of the invention. 
           [0014]      FIG. 8  illustrates a flat heat pipe in one embodiment of the invention. 
       
    
    
       [0015]    Use of the same reference numbers in different figures indicates similar or identical elements. 
       SUMMARY 
       [0016]    In accordance with the invention, one embodiment of a method for fabricating microgrooves on a metal plate or strip includes two sequential steps in a single pass. A first blade with first multi-plowshares is used in the first step to turn up material on the plate or strip to form large grooves, and then a second blade with second multi-plowshares is used in the second step to rebury the large size grooves with the material turned up in the first step to form microgrooves. The microgrooves can have various shapes and are used as wicks in heat pipes. The microgrooves are formed from the relative movement between the blades and the plate or strip into which the plowshares enter. As the microgrooves can be fabricated with very small dimensions, which are controlled by the amount of the reburied material, the heat pipes can perform at large working angles due to increased capillary force. 
         [0017]    In one embodiment, microgrooves on plates are manufactured with fluting or slotting machines where the plates are fixed on the worktable and the blades moves along a track on the machine. In one embodiment of the method, the microgrooves are formed along two directions so they intersect and allow a working fluid to travel between the microgrooves. The plates with the microgrooves can be used to make flat heat pipes or vapor chambers. 
         [0018]    In one embodiment, microgrooves are manufactured on a metal strip such that the blades are fixed and a reel of the metal strip is unwound forward. Tubular heat pipes with the microgrooves can then be easily manufactured by integrating the above process in a conventional pipe production line using seam-welding such as high frequency induction heating (HFI). In order to have a better flow mechanism, regular V-shape grooves in another direction can be first formed by rolling to allow the working fluid to flow across the microgrooves. 
       DETAILED DESCRIPTION 
       [0019]    It is well known that narrow grooves provide large capillary force and therefore large working angle for heat pipes. Grooves of various shapes in current heat pipes are typically formed by extrusion and are generally greater than 0.3 mm wide. The microgrooves in accordance with the invention are mini/micro-scaled grooves that are less than 0.2 mm wide. The two sequential steps in accordance with the invention may be the only available approach for mass producing grooves of this scale at present time. The principle is as simple as a farmer plowing a trench in the soil and then reburying the trench after seeds are planted. To accomplish the process, two blades are used. A first blade of first multi-plowshares is used in the first step to turn up material on a metal plate or strip to form large grooves, and then a second blade with second multi-plowshares is used in the second step to rebury the large size grooves with the material turned up in the first step to form microgrooves. The two sequential steps are simultaneously applied in a single pass. As more material is reburied, the groove size becomes smaller. The microgrooves are formed from the relative movement between the blades and the plate or strip into which the plowshares enter. The plate or strip is typically a malleable metal such as copper, copper alloy, aluminum, or aluminum alloy when the method uses cold-pressing steps. Alternatively, the plate or strip can be of harder metal such as stainless steel when the method uses hot-pressed steps. 
         [0020]    The left of  FIG. 1  shows a cross-section of metal plate  102  with large grooves  104  after the first step in one embodiment of the invention. A first blade  106  turns up material on plate  102  without flaking to form curbs  108  collected on both sides of each groove  104 . Multi-plowshares  110  (shown partly with phantom lines) at the bottom of first blade  106  have the same projection view as the groove profile of large grooves  104 . 
         [0021]    The right of  FIG. 1  shows a cross-section of metal plate  102  with microgrooves  202  after the second step in one embodiment of the invention. Curbs  108  turned up by the first step are reburied into large grooves  104  and reshaped into curbs  204  by multi-plowshares  206  (shown partly with phantom lines) of second blade  208 . The height of blade  206  over plate  102  controls the height of curb  204 , which in turn determines the width of microgrooves  202 . As more material is reburied, microgrooves  202  become narrower. One of the microgrooves  202  is enlarged and indicated by reference number  210 . It is emphasized that the two sequential steps can occur simultaneously in a single pass of plate  102  to form microgrooves  202 . 
         [0022]    The left of  FIG. 2  shows a cross-section of metal plate  102  with large grooves  302  of another design after the first step in one embodiment of the invention. The first blade turns up material on plate  102  without flaking to form curbs  304  collected on both sides of each groove  302 . The multi-plowshares at the bottom of the first blade have the same projection view as the groove profile of large grooves  302 . 
         [0023]    The right of  FIG. 2  shows a cross-section of metal plate  102  with microgrooves  402  after the second step in one embodiment of the invention. Curbs  304  turned up by the first step are reburied into large grooves  302  and reshaped into curbs  404 . The height of the second blade over plate  102  controls the height of curb  404 , which in turn determines the width of microgrooves  402 . As more material is reburied, microgrooves  402  become narrower. One of the microgrooves  402  is enlarged and indicated by reference number  406 . It is again emphasized that the two sequential steps can occur simultaneously in a single pass of plate  102  to form microgrooves  402 . 
         [0024]      FIG. 3  illustrates a large metal plate  502  with microgrooves  504 A (only one is labeled for clarity) along a first direction and microgrooves  504 B (only one is labeled for clarity) along a second direction perpendicular to the first direction in one embodiment of the invention. One of microgrooves  504 A and  504 B is enlarged and indicated by reference number  506 . Microgrooves  504 A and  504 B are formed using the two sequential steps described above. Microgrooves  504 A and  504 B are formed along two directions so they intersect and allow a working fluid to travel between the microgrooves. Microgrooves on a large plate can be fabricated on fluting or slotting machines where the plate is fixed on the worktable and the blades moves along the track on the machine. The plates with the microgrooves are used to make flat heat pipes or vapor chambers. 
         [0025]      FIG. 4  illustrates a flat heat pipe or vapor chamber  600  with microgrooves  602  in one embodiment of the invention. Flat heat pipe  600  includes a top cover  604  and a bottom cover  606 . Bottom cover  606  defines a cavity with a base having a surrounding sidewall. A portion  608  of the sidewall forms a location where a hole can be formed to extract air from the cavity, fill the cavity with a working fluid, and sealed to maintain the vacuum in the cavity. 
         [0026]    The base of bottom cover  606  has a pedestal depression  610  that protrudes downward from the base for contacting a heat source below flat heat pipe  600 . The base of bottom cover  606  further has microgrooves  602  formed along two perpendicular directions as shown more clearly in  FIG. 3 . Similarly, top cover  604  has microgrooves  602  (not shown) formed on its inner surface. Microgrooves  602  are formed using the two sequential steps described above. 
         [0027]    A spacer  612  is seated in pedestal depression  610  between top cover  604  and bottom cover  606 . Spacer  612  adds to the mechanical stiffness of flat heat pipe  600  and provides a heat conductive path from the heat source to top cover  604  to improve heat dissipation. 
         [0028]    Spacers  614  are sandwiched between top cover  604  and bottom cover  606  to control the height of the cavity defined between the covers. Holes  616  are defined in top cover  604  and bottom cover  606  for fasteners to mounting flat heat pipe  600 . For example, flat heat pipe  600  is mounted to an electronic board to cool a processor in contact with pedestal depression  610 . 
         [0029]      FIG. 5  illustrates a conventional production line of making pipes with inner-threads  708  (only one is labeled for clarity) using a longitudinal seam weld. A reel  702  of metal strip  704  is fed under a roller  706 . Roller  706  forcibly engages the top surface of strip  704  to form inner-threads  708 . Strip  704  is next fed through a series of forming rollers  710  that bend strip  704  into a tube of the desired cross-section (e.g., round, oblong, square, rectangular). A welder  712  joins the seam of the tube and a blade  714  trims weldment  716  from the seam to produce a pipe  716 . Welder  712  uses high frequency induction heating (HFI) welding or another similar welding process. 
         [0030]      FIG. 6  illustrates another way to make microgrooves  802  on a reeled metal strip (or plate)  804 . By fixing a first blade  810  and a second blade  812 , microgrooves  802  can be fabricated when strip  804  moves forward under the blades by a pulling force  814 . As described above for the two sequential steps, first blade  810  has first multi-plowshares that open large grooves by turning up the material of strip  804 , and second blade  812  has second multi-plowshares that rebury the large grooves to form microgrooves  802 . 
         [0031]    Strip  804  is optionally fed under a roller  806  to form optional grooves  808  (only one is labeled for clarity) that are diagonal to the travel of strip  804 . Diagonal grooves  808  are of typical shape and size like grooves found in a conventional heat pipe. For example, diagonal grooves  808  are V-grooves and have a width greater than 0.3 mm. When included, diagonal grooves  808  interconnect microgrooves  802  so that a working fluid in the resulting heat pipe can travel via diagonal grooves  808  between microgrooves  802 . This allows the resulting heat pipe to function not just along the direction of microgrooves  802  but essentially along any direction. 
         [0032]    In one embodiment, the process of  FIG. 6  is integrated in the conventional production line of  FIG. 5  to make microgroove heat pipes. Referring to  FIG. 5 , strip  804  is fed through rollers  710  that bend the strip into a tube of the desired cross-section, welder  712  joins the seam of the tube, and blade  714  trims the weldment from the seam to produce a tubular heat pipe. Alternatively, the fabrication of microgrooves  802  in  FIG. 6  can be performed independently from the fabrication of the microgroove heat pipes in  FIG. 5  in two separate production lines. If so, the unwound strip  804  with microgrooves  802  would replace reel  702  of strip  704  in the production line of  FIG. 5 . 
         [0033]      FIG. 7  illustrates a tubular heat pipe  900  with microgrooves in one embodiment of the invention. Tubular heat pipe  900  is made from strip  804  with microgrooves  802  and optionally grooves  808  as described above in reference to  FIG. 6 . Strip  804  is formed into tubular heat pipe  900  with a desired cross-section using a conventional method. In one embodiment, tubular heat pipe  900  has an oblong cross-section. Oblong heat pipe  900  can optionally be bent to a desired shape. In one embodiment, oblong heat pipe  900  includes a bend  906  (e.g., a 90 degree bend). Ends  908  (only one is shown for clarity) of oblong heat pipe  900  are sealed by a conventional method. A weldment  904  shows where strip  804  is seam-welded to form tubular heat pipe  900 . 
         [0034]      FIG. 8  illustrates a flat heat pipe/vapor chamber  1000  in one embodiment of the invention. Flat heat pipe  1000  can be made from plate  502  with microgrooves  504 A and  504 B as described above in reference to  FIG. 3 . Spacers  1002  are first fixed on plate  502 . Plate  502  is formed into flat heat pipe  1000  with a desired cross-section using a conventional method. The top and the bottom of flat heat pipe  1000  are separated by spacers  1002 . Ends  1008  (only one is shown for clarity) of flat heat pipe  1000  are sealed by a conventional method. A weldment  1004  shows where plate  502  is seam-welded to form flat heat pipe  1000 . 
         [0035]    Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention. For example, the microgrooves of the present invention are formed from the relative motion between the plate or strip and the blades. Thus, the plate/strip can move against stationary blades, the blades can move against stationary plate/strip, or they can all move relative to each other. Numerous embodiments are encompassed by the following claims.