Patent Publication Number: US-2005126761-A1

Title: Heat pipe including enhanced nucleate boiling surface

Description:
BACKGROUND  
      The claimed invention relates to heat pipes and, more particularly, to heat pipes for dissipating heat generated by integrated circuits.  
      Various heat pipes have been used to dissipate heat generated by integrated circuits, for example within personal computers, mobile computers, or similar electrical devices. Heat pipes may include an evaporator section and a condenser section. Heat may be transferred from the evaporator section to the condenser section by boiling a liquid at the evaporator section.  
      Heat pipes may also include a wick between the evaporator section to the condenser section to act as a pump to bring liquid back from the condenser section to the evaporator section. The wick and liquid may also extend over the integrated circuit in the evaporator section of the heat pipe. The thermal conductivity of the wick-liquid mixture in the evaporator section, however, may be somewhat low, causing a somewhat high thermal resistance to heat transfer in such a heat pipe. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one or more implementations consistent with the principles of the invention and, together with the description, explain such implementations. In the drawings,  
       FIG. 1  is an internal top view of an example implementation of a heat pipe consistent with the principles of the invention;  
       FIG. 2  is a side view of the heat pipe of  FIG. 1  in an implementation consistent with the principles of the invention;  
       FIG. 3  is a side view of the heat pipe of  FIG. 1  in another implementation consistent with the principles of the invention; and  
       FIG. 4  is an end view of the heat pipe of  FIG. 3  in an implementation consistent with the principles of the invention. 
    
    
     DETAILED DESCRIPTION  
      The following detailed description refers to the accompanying drawings. The same reference numbers may be used in different drawings to identify the same or similar elements. Also, the following detailed description illustrates certain implementations and principles, but the scope of the claimed invention is defined by the appended claims and equivalents.  
       FIG. 1  is an internal top view of an example implementation of a heat pipe  100  consistent with the principles of the invention. Heat pipe  100  may be located in a desktop or mobile computer proximate to an integrated circuit chip (not shown), such as a processor, that produces heat. Heat pipe  100  may include outer walls  110 , a wick  120 , and a boiling structure  130 . Although not explicitly illustrated in  FIG. 1 , heat pipe  100  may also include a liquid coolant.  
      An evaporator portion of heat pipe  100  may be located near boiling structure  130 , and a condenser portion of heat pipe  100  may be spaced apart from boiling structure  130  (e.g., including the far end of heat pipe  100 ). The overall length of heat pipe  100  may be in a range of about 45 mm to about 300 mm. The overall diameter of heat pipe  100  may be in a range of about 2 mm to about 30 mm.  
      Outer walls  110  may enclose both wick  120  and boiling structure  130 , as well as the coolant. Outer walls  110  may contact the integrated circuit chip, and they may include a highly thermally conductive material, such as copper or another metal. Because  FIG. 1  is an internal top view, the top one of outer walls  110  is not illustrated, and the bottom one of outer walls  110  is hidden by wick  120  and boiling structure  130 . Outer walls  110  may be formed in a roughly rectangular shape, as illustrated in  FIG. 1 , or any other geometry that facilitates access to boiling structure  130  by the coolant. Outer walls  110  may also be formed to prevent the escape of vapor or liquid.  
      Wick  120  may include a porous material (e.g., sintered spherical copper particles, sintered metal powder, a fiber material, and/or a screen material) that covers a bottom surface of heat pipe  100 , except for the area occupied by boiling structure  130 . The porous material of wick  120  may include particles that have an average diameter in a range from about 2 μm to about 150 μm. This average diameter may be referred to as the mean feature size (or pore size) of wick  120 . The porous material of wick  120  may be formed on the lower surface of heat pipe  100  with an average thickness in a range from about 1 mm to about 2.5 mm. Wick  120  may, by virtue of its porous structure, bring coolant from the condenser portion of heat pipe  100  to the evaporator portion at or near boiling structure  130 . In this manner, wick  120  may act to hydrate boiling structure  130 .  
      In other implementations, wick  120  may include axial grooves that act to bring coolant from the condenser portion of heat pipe  100  to the evaporator portion at or near boiling structure  130 . Other types of homogenous structures for wick  120  may include an open annular structure, an open artery structure, and/or an integral artery structure. In still other implementations consistent with the principles of the invention, various composite structures may be used for wick  120  that may include one or more of the homogeneous structures noted above (e.g., sintered particles, screen, fibers, grooves, etc.). Wick  120  may be designed to have a relatively high capillary pumping efficiency to hydrate boiling structure  130 .  
      Boiling structure  130  may include a porous material (e.g., spherical metal particles of various sizes sintered onto the bottom outer wall  110 ) that roughly corresponds in area and orientation to a top surface of the integrated circuit chip to be cooled. Boiling structure  130  typically may be rectangular or square with an area in a range from about 0.25 cm 2  to about 10 cm 2 . The porous material of boiling structure  130  may include, for example, copper particles that have an average diameter in a range from about 50 μm to about 500 μm, which may be greater than the average particle size of wick  120 . In some implementations, the average diameter of the particles in boiling structure  130  may be around 300 μm. This average diameter may be referred to as the mean feature size of boiling structure  130 . The porous material of boiling structure  130  may also be formed on the lower surface of heat pipe  100  with an average thickness in a range from about 0.5 mm to about 2 mm, which may be less than or equal to the average thickness of wick  120 .  
      Boiling structure  130 , by virtue of its geometry and material, may have a relatively low thermal resistance. For example, an average diameter of about 300 μm, boiling structure  130  may accomplish boiling with only 1-3° C. superheat (at 10-50 W/cm 2  heat flux). Such low overheat may result in a thermal resistance of about 0.03-0.1° C./W resistance for a 1 cm 2  heat source (e.g., a thermal resistivity of 0.03-0.1° C.-cm 2 /W). Because of boiling structure  130 &#39;s low thermal resistance, the material of wick  120  may have a somewhat higher thermal resistivity (e.g., 0.2° C.-cm 2 /W or greater) without adversely affecting the heat transfer efficiency of heat pipe  100 . Similarly, wick  120  may be designed to have more effective capillary pumping than boiling structure  130 .  
      Heat pipe  100  may have a roughly rectangular cross-sectional shape, or a roughly circular cross-sectional shape.  FIG. 2  is a side view of heat pipe  100  with a roughly rectangular cross-sectional shape in an implementation consistent with the principles of the invention. In addition to elements  110 - 130  discussed above with regard to  FIG. 1 , heat pipe  100  may also include a coolant  210  and a vapor space  220 .  
      Coolant  210  may include water, methanol, ethanol, acetone, heptane, Freon, or another refrigerant. Coolant  210  may pool on boiling surface, as illustrated in  FIG. 2 , and may also permeate wick  120 . Although primarily liquid, some coolant  210  may be converted to a vapor by boiling over boiling structure  130 .  
      One way to ensure continuous wetting of boiling structure  130  by coolant  210  may be to arrange wick  120  to extend vertically above boiling structure  130  on all sides, as illustrated in  FIG. 2 . For example, wick  120  may extend from about 0.1 mm to about 1 mm above boiling structure  130  to define a cavity. Wick  120  may feed coolant  210  into this cavity from all sides (see  FIG. 1 ) to form a pool of coolant  210  on boiling structure  130 . In such an implementation, coolant  210  may or may not also collect above wick  120 , perhaps depending on how much of coolant  210  is currently in vapor form.  
      In another implementation, wick  120  may not extend vertically above boiling structure  130 . In such an implementation, however, the amount of coolant  210  should be sufficient to ensure pooling on at least boiling structure  130 . In this manner, boiling structure  130  may remain continuously wetted by coolant  210 .  
      Vapor space  220  may be located between wick  120  and the top one of outer walls  110 . Vapor-phase coolant  210  may be transported to the condenser portion of peat pipe  100  via vapor space  220  (and possibly also wick  120 ), where it cools, becomes liquid, and is transported back to boiling structure  130  by wick  120 . In some implementations consistent with the principles of the invention, vapor space may have a height in a range from about 0.5 mm to about 2 mm, although other heights are possible.  
       FIGS. 3 and 4  are views of heat pipe  100  with a roughly circular cross-sectional shape in another implementation consistent with the principles of the invention. In addition to elements  110 - 130  and coolant  210  discussed above with regard to  FIGS. 1 and 2 , heat pipe  100  in  FIG. 3  may include a wick  310  extending around an inner circumference of heat pipe  100 . As illustrated in  FIG. 4 , vapor space  220  may also have a roughly circular cross-section.  
      As described above with regard to  FIG. 2 , wick  310  may include a wrapped screen structure, a sintered metal structure, a fiber structure, and/or an axially grooved structure. Wick  310  may also include, in some implementations, a composite structure. Wick  130  may have a higher thermal resistance, but better capillary pumping action, than boiling structure  130 .  
      The foregoing description of one or more implementations consistent with the principles of the invention provides illustration and description, but is not intended to be exhaustive or to limit the claimed invention to the precise form disclosed. Modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention.  
      For example, although heat pipe  100  herein has been described by itself, other cooling techniques also may optionally be used. For example, a cooling fan (or other forced-air cooling device) may or may not be used in conjunction with heat pipe  100  within a mobile computer or other electronic device.  
      Further, although certain example numerical ranges are given above for lengths, sizes, and values, these ranges are purely exemplary and may vary according to design needs. The values given may vary, for example, 10-30% above and below the respective endpoints of the ranges given above.  
      No element, act, or instruction used in the description of the present application should be construed as critical or essential to the invention unless explicitly described as such. Also, as used herein, the article “a” is intended to include one or more items. Where only one item is intended, the term “one” or similar language is used. Variations and modifications may be made to the above-described implementation(s) of the claimed invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.