Abstract:
A heat spreading apparatus includes a body defining a void. A fluid is positioned within the void for distributing heat by vaporizing the fluid. The body defines a void with a heat accumulation surface geometry to disrupt the thermodynamic cycle of vaporizing the fluid and thereby diminish heat spreading activity by the heat spreading apparatus.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to a heat distribution device used in connection with a heat generating surface. More particularly, this invention relates to a heat spreader that has a thermal conductivity that is inversely proportional to increasing heat applied to it. 
       BACKGROUND OF THE INVENTION 
       [0002]    U.S. Pat. Nos. 6,167,948 and 6,158,502 disclose thin, planar heat spreaders in various configurations. These heat spreaders endeavor to have improved thermal conductivity with increased exposure to heat. In some engineering applications it is desirable to have decreased thermal conductivity with increased exposure to heat. Accordingly, it would be desirable to provide a heat spreader that achieves this counterintuitive result. 
       SUMMARY OF THE INVENTION 
       [0003]    A heat spreading apparatus includes a body defining a void. A fluid is positioned within the void for distributing heat by vaporizing the fluid. The body defines a void with a heat accumulation surface geometry to disrupt the thermodynamic cycle of vaporizing the fluid and thereby diminish heat spreading activity by the heat spreading apparatus. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0004]    The invention is more fully appreciated in connection with the following detailed description taken in conjunction with the accompanying drawings, in which: 
           [0005]      FIG. 1  illustrates a section view of a planar heat spreader. 
           [0006]      FIG. 2  illustrates a segment of the heat spreader of  FIG. 1  with incipient bubble formation 
           [0007]      FIG. 3  illustrates a segment of the heat spreader of  FIG. 1  with increased bubble formation. 
           [0008]      FIG. 4  illustrates a segment of the heat spreader of  FIG. 1  with further increased bubble formation. 
           [0009]      FIG. 5  illustrates a segment of the heat spreader of  FIG. 1  with indentations to promote bubble formation. 
           [0010]      FIG. 6  illustrates a segment of the heat spreader of  FIG. 1  with hydrophilic properties to promote formation of a bubble with a first characteristic. 
           [0011]      FIG. 7  illustrates a segment of the heat spreader of  FIG. 1  with hydrophobic properties to promote formation of a bubble with a second characteristic. 
           [0012]      FIG. 8  illustrates a heat spreader in accordance with an embodiment of the invention. 
           [0013]      FIG. 9  illustrates a heat spreader in accordance with another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]      FIG. 1  illustrates a section view of a planar heat spreader  100  configured in accordance with an embodiment of the invention. The planar heat spreader  100  includes a first body  102  and a second body  104  which define a void  106 . At least one surface of one body has a heat accumulation surface geometry  108 . 
         [0015]    The heat accumulation surface geometry disrupts the thermodynamic cycle of vaporizing fluid. Consequently, heat spreading activity by the heat spreader  100  is diminished with increasing heat. The heat accumulation surface geometry may be in the form of indentations to promote bubble growth or surface treatments, such as hydrophilic surface treatments and hydrophobic surface treatments. The heat accumulation surface geometry may also be in the form of capillary wick structures, such as screens, sintered metals, grooves, arteries, planar capillaries and combinations thereof. 
         [0016]      FIG. 2  illustrates second body  104  of the heat spreader  100  of  FIG. 1 . Fluid  202  has fluid flow paths  204  adjacent to a heat accumulation surface geometry to promote the formation of an incipient bubble  206 . An embodiment of the invention promotes the formation of such bubbles to disrupt efficient thermal performance of the planar heat spreader  100 . 
         [0017]      FIG. 3  illustrates increased bubble formation. In particular, bubble  206  of  FIG. 3  is larger than bubble  206  of  FIG. 2 . The bubble  206  grows larger with increased exposure to heat from the heat generating surface. This bubble of increased size dislocates fluid path  204 , as shown with fluid dislocation segment  208 . 
         [0018]      FIG. 4  illustrates further increased bubble formation. In particular, bubble  206  of  FIG. 4  is even larger than bubble  206  of  FIG. 3 . The bubble  206  grows larger with increased exposure to heat from the heat generating surface. This bubble of further increased size further dislocates fluid path  204 , as shown with fluid dislocation segment  208 . 
         [0019]    Bubble  206  effectively has a liquid perimeter and a vapor interior. As shown in  FIG. 4 , the bubble  206  displaces fluid  202  from much of the surface of body  104 . Consequently, the fluid  202  is exposed to less surface area of body  104 , which is attached to a heat generating surface. The reduced surface exposure reduces vaporization and its concomitant heat transfer action. Thus, with increased temperature and increased bubble formation, the thermal conductivity of the device  100  is reduced. This stands in contrast to typical designs that endeavor to increase thermal conductivity in the presence of increased exposure to heat. 
         [0020]      FIG. 5  illustrates body  104  of the heat spreader  100  of  FIG. 1 . Fluid  502  is adjacent to a heat accumulation surface geometry in the shape of indentations  504  to promote bubble growth.  FIG. 5  illustrates a bubble  506  formed in one such indentation. Although the indentations are shown as spherical, they may be any shape, such as cylindrical, conical, or trapezoidal. 
         [0021]      FIG. 6  illustrates body  104  of the heat spreader  100  of  FIG. 1 . Fluid  602  is adjacent to a heat accumulation surface geometry with hydrophilic properties to promote bubble formation. For example, a hydrophilic material, a hydrophilic film or hydrophilic surface features may be used to promote hydrophilic properties. A hydrophilic surface minimizes surface exposure to a liquid. Thus, bubble  604  forms with a relatively small footprint  606  on the surface of segment  104 . 
         [0022]      FIG. 7  illustrates body  104  of the heat spreader  100  of  FIG. 1 . Fluid  702  is adjacent to a heat accumulation surface geometry with hydrophobic properties to promote bubble formation. For example, a hydrophobic material, a hydrophobic film or hydrophobic surface features may be used to promote hydrophobic properties. A hydrophobic surface maximizes surface exposure to a liquid. Thus, bubble  704  forms with a relatively large footprint  706  on the surface  104 . 
         [0023]    The selection of a hydrophilic surface or hydrophobic surface is contingent upon the application and the desired configuration of the bubble. A single surface may include both hydrophilic and hydrophobic regions. 
         [0024]    The foregoing examples illustrate the formation of a single or few bubbles. Alternate embodiments of the invention facilitate the formation of increased number of bubbles with increased exposure to heat. 
         [0025]    Table I illustrates performance results achieved in accordance with an embodiment of the invention. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                   
                   
                 Thermal 
               
               
                 Power 
                 Temperature 
                 Conductivity 
               
               
                 (W) 
                 (° C.) 
                 (W/m * K) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0.0 
                 60.0 
                 2410.5 
               
               
                 9.9 
                 60.9 
                 2025.3 
               
               
                 15.3 
                 61.5 
                 1820.1 
               
               
                 25.0 
                 62.8 
                 1532.9 
               
               
                 50.0 
                 67.5 
                 1064.9 
               
               
                 75.0 
                 75.0 
                 760.0 
               
               
                 100.4 
                 85.6 
                 577.1 
               
               
                 125.0 
                 99.4 
                 323.0 
               
               
                   
               
             
          
         
       
     
         [0026]    Observe that this embodiment experiences thermal conductivity changes from 2410 to 323 (over a 7.5 thermal conductivity change) over approximately 40° C. (from 99.4° C. to 60° C.). Thus, unlike typical devices, thermal conductivity decreases with increasing heat exposure. 
         [0027]    The techniques of the invention may be used to form heat transfer devices of various configurations.  FIG. 8  illustrates a section view of one such device  800 . Device  800  includes a first body  804  a second body  806  and vertical sidewalls  808  and  812  which define a void  802  for vapor flow. At least a portion of the bodies and sidewalls interior surfaces have a heat accumulation surface geometry  814 . A fluid (not shown) is positioned adjacent to the heat accumulation surface geometry and vertical support  810 . The heat accumulation surface geometry is configured for bubble formation, as previously described. 
         [0028]    The sidewalls  808 ,  812  and vertical support  810  facilitate efficient heat transfer. This efficient heat transfer is countered by the heat accumulation surface geometry, which has a thermal conductivity that is inversely proportional to increasing applied heat. 
         [0029]      FIG. 9  illustrates a section view of device  900  generally corresponding to device  800  of  FIG. 8 , but with additional thermal resistance promoting features. The device  900  includes a first body  904  a second body  906  and vertical sidewalls  908  and  912 , which define a void  902  for vapor flow. At least a portion of the bodies and sidewalls interior surfaces have a heat accumulation surface geometry  914 . A fluid (not shown) is positioned adjacent to the heat accumulation surface geometry and vertical support  910 . The heat accumulation surface geometry is configured for bubble formation, as previously described. 
         [0030]    The sidewalls  908 ,  912  and vertical support  910  have corresponding cut-outs  916 ,  918 ,  920 ,  922 ,  924 , and  926  to reduce heat transfer efficiency. Specifically, these cut-outs reduce the heat flow cross-sectional area, and increase the heat flow length, reducing the heat transfer efficiency, which supplements the heat accumulation surface geometry design goal of thermal conductivity that is inversely proportional to increasing applied heat. 
         [0031]    Embodiments of the invention rely upon a heat accumulation surface geometry that promotes dry out. Dry out is the absence of a fluid. The absence of a fluid in the heat spreading apparatus disrupts the thermodynamic cycle and thereby diminishes heat spreading activity. For example, dry out occurs when the fluid pressure from the condenser region is insufficient to provide enough fluid to the evaporator region. This leads to dry out in the evaporator. Dry out prevents the thermodynamic cycle from continuing and therefore heat spreading activity is diminished, thus satisfying the heat accumulation surface geometry design goal of thermal conductivity that is inversely proportional to increasing applied heat. 
         [0032]    Techniques of the invention may be realized in a variety of configurations. For example, various capillary configurations are disclosed in the previously referenced U.S. Pat. Nos. 6,167,948 and 6,158,502, which are incorporated herein by reference. 
         [0033]    The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that specific details are not required in order to practice the invention. Thus, the foregoing descriptions of specific embodiments of the invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed; obviously, many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, they thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the following claims and their equivalents define the scope of the invention.