Abstract:
A heat spreader includes a bottom wall ( 12 ) and a cover ( 14 ) hermetically connected to the bottom wall. Cooperatively the bottom wall and the cover define a space ( 11 ) therebetween for receiving a working fluid therein. A wick structure ( 15 ) is received in the space and thermally interconnects the bottom wall and the cover. The wick structure includes at least a carbon nanotube array, which can conduct heat from the bottom wall to the cover and draw condensed liquid of the working fluid from the cover toward the bottom wall.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an apparatus for transfer or dissipation of heat from heat-generating components, and more particularly to a heat spreader having a vapor chamber defined therein. 
   2. Description of Related Art 
   As electronic industry continues to advance, electronic components such as central processing units (CPUs), are made to provide faster operational speeds and greater functional capabilities. When a CPU operates at a high speed, its temperature frequently increases greatly. It is desirable to dissipate the heat generated by the CPU quickly. 
   To solve this problem of heat generated by the CPU, a heat sink is often used to be mounted on the top of the CPU to dissipate heat generated thereby. For enhancing the heat dissipation capability of the heat sink, a heat spreader is arranged between the heat sink and the CPU, which is made of a material having a heat conductivity higher than that of the heat sink, for enhancing the speed of heat transfer from the CPU to the heat sink. However, as the CPU operates faster and faster, and, therefore generates larger and larger amount of heat, the conventional heat spreader, which transfers heat via heat conduction means, cannot meet the increased heat dissipating requirement of the CPU. 
   For the foregoing reasons, therefore, there is a need in the art for a cooling device which overcomes the above-mentioned problems. 
   SUMMARY OF THE INVENTION 
   The present invention relates to a heat spreader including a bottom wall and a cover hermetically connected to the bottom wall. Cooperatively the bottom wall and the cover define a space therebetween for receiving a working fluid therein. A wick structure is received in the space and thermally interconnects the bottom wall and the cover. The wick structure includes at least a carbon nanotube array (CNT array). 
   Other advantages and novel features of the present invention will become more apparent from the following detailed description of preferred embodiment when taken in conjunction with the accompanying drawings, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Many aspects of the present heat spreader 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 spreader. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views: 
       FIG. 1  is a cross-sectional view of a cooling device incorporating a heat spreader in accordance with a first embodiment of the present invention; 
       FIG. 2  shows a cross-sectional view of the heat spreader of  FIG. 1 ; 
       FIG. 3  is a cross-sectional view of the heat spreader of  FIG. 2  taken along line III-III thereof; 
       FIG. 4  is similar to  FIG. 2 , but shows the heat spreader in accordance with a second embodiment of the present invention; and 
       FIG. 5  is similar to  FIG. 2 , but shows the heat spreader in accordance with a third embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a cross-sectional view of a cooling device incorporating a heat spreader  10  in accordance with a first embodiment of the present invention. The cooling device is arranged on a heat-generating component  20 , such as CPU (central process unit), VGA (Video Graphics Array), LED (light-emitting diode), NB (north bridge), and so on. 
   In this embodiment, the cooling device includes a heat spreader  10  and a fin-type heat sink  30  arranged on the heat spreader  10 . The heat sink  30  is made of material with highly thermal conductivity, such as copper, aluminum, or their alloys. The heat sink  30  as shown in this embodiment is an extruded aluminum heat sink, including a chassis  31  and a plurality of pin fins  32  extending upwardly from the chassis  31 . Apparently, the fins  32  are used for increasing the heat dissipation area of the heat sink  30 . Alternatively, the fins  32  can be plate-like shaped. The fins  32  and the chassis  31  can be formed separately, and then connected together by soldering. 
   Also referring to  FIGS. 2-3 , the heat spreader  10  includes a bottom wall  12  and a cover  14  hermetically connected to the bottom wall  12  to thereby form a sealed space  11  for containing working liquid therein. The cover  14  and the bottom wall  12  are made of copper. Alternatively, the cover  14  and the bottom wall  12  can be made of other materials with highly thermal conductivity, such as aluminum, or its alloys. The bottom wall  12  is a square-shaped plate. The bottom wall  12  includes a bottom surface  122  for contacting with and absorbing heat from the heat-generating component  20 , and a top surface  124  opposing the bottom surface  122 . The cover  14  includes a top wall  144  parallel to the bottom wall  12  and a side wall  142 . The top wall  144  is square-shaped with a size smaller than that of the bottom wall  12 . The side wall  142  extends perpendicularly and downwardly from four sides of the top wall  144 . A flange  140  extends transversely and outwardly from a free end of the side wall  142 . An outer periphery of the flange  140  has a size substantially the same as that of the bottom wall  12 . The flange  140  of the cover  14  and an outer periphery  120  of the top surface  124  of the bottom wall  12  connect together by a soldering process which is a method widely used to connect two discrete metallic components together. Thus, the discrete cover  14  and bottom wall  12  are soldered together to form the sealed space  11  therebetween. The space  11  is in a vacuumed condition. The working liquid, such as water or alcohol, which has a lower boiling point, is received in the space  11 . 
   A plurality of carbon nanotube arrays (CNT arrays)  15  which function as heat transfer enhancing structures and wick structures are arranged between and thermally interconnect the bottom wall  12  and the top wall  144  of the cover  14 . The carbon nanotube arrays (CNT arrays)  15  are fixed in the heat spreader  10  by interference fit: bottom and top ends of each carbon nanotube array  15  are interferentially pressed by the top surface  124  of the bottom wall  12  and a bottom surface  148  of the top wall  144  of the cover  14 . Alternatively, grooves can be defined in the top and bottom walls  144 ,  12  to receive the top and bottom ends of the carbon nanotube arrays (CNT arrays)  15  therein. Thus, the carbon nanotube arrays (CNT arrays) can be firmly assembled in the space  11 . In this embodiment, the carbon nanotube arrays (CNT arrays)  15  include seven carbon nanotube arrays (CNT arrays) evenly spaced from each other along a horizontal direction, and thus eight longitudinal channels  16  are defined therebetween. Each carbon nanotube array  15  has a shape of elongated cube, in which a width W (as shown in  FIG. 3 ) thereof is larger than a length L (as shown in  FIG. 2 ) thereof. The width W of each carbon nanotube array  15  is a little smaller than that of the space  11 , and thus two traverse channels  17  are defined by opposite sides (i.e., front and back sides) of the space  11 . The traverse channels  17  communicate with the longitudinal channels  16 . 
   One kind of such a carbon nanotube array  15  can be obtained by a method of chemical vapor deposition (CVD). Firstly aligned carbon nanotube arrays are synthesized in a hot filament plasma enhanced chemical vapor deposition (HF-PECVD) system. A substrate (metal, glass, silicon, etc.) is coated with nickel nano-particles and then introduced to the CVD chamber. Then the aligned carbon nanotube arrays are mixed with distilled water by firstly vacuuming the aligned carbon nanotube arrays to remove air therein, and then filling the distilled water in the aligned carbon nanotube arrays. The aligned carbon nanotube arrays filled with distilled water are then cooled to form a composite material of carbon nanotube arrays combined with water. Finally incises the carbon nanotube arrays from the substrate in a manner that the carbon nanotube arrays have a predetermined length; thus the carbon nanotube arrays  15  are obtained. 
   When assembled, the bottom surface  122  of the bottom wall  12  is thermally attached to the heat-generating component  20 , and a top surface  146  of the top wall  144  is thermally attached to the chassis  31  of the heat sink  30 . As the heat generated by the heat-generating component  20 , which is attached to the bottom surface  122  of the bottom wall  12 , is transferred to the heat spreader  10 , the working fluid contained therein absorbs the heat and evaporates into vapor. Since the vapor spreads quickly, it quickly fills an interior of the heat spreader  10 , and whenever the vapor comes into contact with cooler wall of the heat spreader  10  (i.e., the top wall  144  of the heat spreader  10 ) which thermally contact with the heat sink  30 , it releases the heat to the heat sink  30 . After the heat is released, the vapor condenses into liquid, which is then brought back by the carbon nanotube arrays (CNT arrays)  15  to the bottom wall  12  of the heat spreader  10 . Since the heat spreader  10  transfers the heat by using phase change mechanism involving the working fluid, the heat transferred to the heat spreader  10  from the heat-generating device is thus rapidly and evenly distributed over the entire heat spreader  10  and is then conveyed to the heat sink  30  through which the heat is dissipated into ambient air. 
   Furthermore, the carbon nanotube arrays (CNT arrays)  15  are capable of transferring heat from the bottom wall  12  to the top wall  144  directly. Due to the carbon nanotube arrays  15  in the heat spreader  10 , a heat transfer efficiency of the heat spreader  10  is highly enhanced. As nano-material have a very small size with a diameter ranging from 1˜100 nm, a surface area of the nano-material is much larger than that of the same material which has the same volume. Thus a heat transfer area of the heat spreader  10  is much enlarged by having the carbon nanotube arrays (CNT arrays)  15 , which, in result, improves heat transfer efficiency of the heat spreader  10 . For example, the carbon nanotubes has a heat transfer coefficient about 3000-6600 W/(m·k), which is ten times more than that of copper which has a heat transfer coefficient of 375 W/(m·K). The heat spreader  10  which adopts the carbon nanotube arrays (CNT arrays)  15  thus can have a much larger heat transfer efficiency. Thus, the heat of the heat-generating component  20  can be rapidly and efficiently transferred from the bottom wall  12  to the top wall  144  of the heat spreader  10  through the carbon nanotube arrays (CNT arrays)  15 , thereby can enhance heat transfer efficiency of the heat spreader  10  from the bottom wall  12  to the top wall  144 . Thus, during operation, the heat generated by the heat-generating component  20  can be transferred to the heat sink  30  by the heat spreader  10  through either phase change mechanism or heat conduction which adopts nano-material with high heat transfer efficiency. In addition, heat transfer threshold by the liquid if the liquid is not able to timely contact with the top surface  148  of the top wall  144  during the initial phase of heat transfer from the bottom wall  12  to the top wall  144  can be overcome by the carbon nanotube arrays (CNT arrays) which thermally connects the bottom wall  12  and the top wall  144 . Accordingly, the heat spreader  10  is still workable to transfer the heat from the heat-generating component  20  to the heat sink  30  even when the heat spreader  10  is put in an inclined position. 
     FIG. 4  shows a second embodiment of the heat spreader  10   a . Also the heat spreader  10   a  has a bottom wall  12  and a cover  14  hermetically connected to the bottom wall  12  to thereby form the sealed space  11 . The carbon nanotube arrays  15  are arranged in the space  11  and thermally interconnect the bottom wall  12  and the top wall  144  of the cover  14 . The difference between the second embodiment and the first embodiment is that the heat spreader  10   a  further has a second wick structure  17   a  arranged on the top surface  124  of the bottom wall  12  and the bottom surface  148  of the top wall  144 . The second wick structure  17   a  has one of the following four configurations: sintered powder, grooves, fibers and screen meshes. In this embodiment, the second wick structure  17   a  is configured of screen mesh. The condensed vapor thus can be brought back by the carbon nanotube arrays  15  and the second wick structure  17   a  together. 
     FIG. 5  shows a third embodiment of the heat spreader  10   b . Similar to the second embodiment, the heat spreader  10   b  has a cover  14 , a bottom wall  12 , the carbon nanotube arrays  15  arranged between the bottom wall  12  and the cover  14 , and a second wick structure  17   b  arranged on the top surface  124  of the bottom wall  12  and the bottom surface  148  of the top wall  144  of the cover  14 . In this embodiment, the second wick structure  17   b  is made of a material like that forming the carbon nanotube arrays  15 , i.e., carbon nanotubes (CNTs). The second wick structure  17   b  can also be formed as the carbon nanotube arrays  15 , i.e., by firstly forming the carbon nanotubes on a silicon substrate and then fixing the carbon nanotubes to the top and bottom walls  144 ,  12  of the heat spreader  10  by for example gluing. Alternatively, the second wick structure  17   b  can be formed on the top and bottom walls  144 ,  12  of the heat spreader  10  directly. In this case, the top and bottom walls  144 ,  12  are adopted as the substrate to form the carbon nanotubes (CNTs) of the wick structure  17   b  thereon. Thus the heat spreader  10  and the second wick structure  17   b  are integral, and assembly of the second wick structure  17   b  to the heat spreader  10  by gluing can be avoided. 
   It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.