Abstract:
A heat pipe comprising a wick, working fluid, a chamber housing the wick and working fluid, and a plurality of wraps of a graphite composite fiber, preferably THORNEL graphite fiber, number P-120 2K. For maximum strength, the wraps are in different directions. The heat pipe can be integrally incorporated and bound to a printed wiring board, thereby eliminating the need for additional inefficient thermal interfaces.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to heat pipes for conducting heat away from electronic circuitry and, more particularly, to heat pipes which are particularly well adapted to airborne avionics and to the radar systems used therein. 
     2. Description of Related Art 
     As is well known, electronic circuitry often generates heat. For maximum efficiency and longevity, means are often provided to remove or reduce this heat. 
     Devices called heat pipes have been used for this purpose. Traditionally, heat pipes consist of a wick and fluid housed in a sealed chamber. One end is subjected to heat; the other to a cold wall. The liquid at the heated end boils, the vapor travels to the cold end and condenses, and the capillary action of the wick causes the condensed fluid at the cold end to travel back to the heated end for re-vaporization. 
     The vapor pressure, unfortunately, is often quite high. For example, the internal working pressure of ammonia at 70 degrees centigrade is approximately 900 pounds per square inch. The chamber, therefore, must be quite strong in order to contain this pressure. Aluminum and copper, for example, typically must have a wall thickness of approximately 0.035 inches. Stainless steel typically must have a wall thickness of approximately 0.023 inches. 
     These great thicknesses, in turn, add substantial weight to the heat pipe. In many applications, such as airborne avionics, this weight is highly undesirable. 
     The materials which are typically used for the outer cylinder of the heat pipes, moreover, conduct electricity. As a consequence, interfaces are usually required to electrically isolate the heat pipe from the circuitry. This adds further bulk and weight to the system and reduces heat transfer efficiency. 
     SUMMARY OF THE INVENTION 
     It is a principle object of the present invention to obviate these as well as other problems in prior art heat pipes. 
     It is a further object of the present invention to provide a heat pipe which is light in weight, has great strength and efficiency, is compact, which has a wall thickness of only 0.015 inches, which can remove heat from electronic circuitry without additional electrical isolation in interfaces, and which can be directly integrated in the printed wiring board. 
     These as well as other objects, features and advantages of the present invention are facilitated by wrapping a thin-walled heat pipe, i.e. one which could not normally withstand the internal pressure, with a composite graphite fiber having high thermal conductivity and tensile strength. 
     In the preferred embodiment, the composite graphite fiber is THORNEL, a trademark of Union Carbide, model P-120 2K. It is first wrapped longitudinally around the core of the heat pipe at a very low angle, preferably at approximately 15 degrees. Three to four additional layers of it are then tightly wrapped in hoops around the longitudinal wrap. 
     These two cross-directional wraps give the heat pipe great strength, thereby allowing the inner core to be made of aluminum having a wall thickness of only 0.010 inches. Compared to a traditional aluminum heat pipe, the subject invention has superior thermal conductivity with twenty-five percent less weight. 
     When utilized with circuit card applications, the subject invention will permit direct integration of the heat sink with the printed wiring board. The result is a single integral assembly of the heat sink and printed wiring board. Additional inefficient and bulky thermal interfaces are therefore eliminated. 
     These as well as other objects, features and benefits of the present invention will now become apparent from a review of the following description of the preferred embodiments, taken in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a pictorial illustration of a graphite composite heat pipe made in accordance with the present invention with several of the hoop wraps removed so as to expose the underlying longitudinal wraps. 
     FIG. 2 is a cross section of FIG. 1 taken along the line  2 — 2 . 
     FIG. 3 illustrates a test set up for heat pipes which can advantageously be used to test the performance of heat pipes. 
     FIG. 4 is a table setting forth test results which were achieved using heat pipes of various design, including a heat pipe made in accordance with the present invention, in the test set up illustrated in FIG.  3 . 
     FIG. 5 illustrates a printed wiring board containing integral heat pipes made in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a heat pipe made in accordance with the present invention, in which several hoop wraps have been removed so as to expose the longitudinal wraps. 
     As can be seen in FIG. 1, a heat pipe  1  includes a longitudinal chamber  3  around which are a plurality of longitudinal wraps  5 , around which are a plurality of hoop wraps  7 . Only a portion of the hoop wraps  7  have been shown so as to visually show the appearance of the longitudinal wraps  5 . In practice, the hoop wraps  7  would preferably extend over the entire length of the longitudinal chamber  3 . 
     FIG. 2 is a cross section of the heat pipe  1  shown in FIG. 1 taken along the line  2 — 2 . 
     As can be seen in FIG. 2, the heat pipe  1  consists of a wick  9 , the chamber  3  housing the wick  9  and working fluid (not explicitly shown in FIG.  2 ), the longitudinal wraps  5 , and the hoop wraps  7 . 
     The wick  9  can be any of the types of wicks which are traditionally used in heat pipes. In the preferred embodiment, the wick  9  consists of a two layer, 150 mesh, stainless steel wick. 
     The working fluid (not shown) which is contained within the chamber  3  can similarly be any of the types of working fluids which are normally used in heat pipes. In the preferred embodiment, acetone is used. 
     The chamber  3  can similarly be any of the types of chambers which are traditionally used in heat pipes. Because of the outer wrappings  5  and  7 , however, the wall thickness of the chamber  3  can be much less than has been needed in the prior art. In the preferred embodiment, the chamber  3  consists of 0.010 inches of 6061T6 aluminum tubing which is hermetically sealed. 
     The wraps  5  and  7  are made of a graphite composite which has high thermal conductivity and tensile strength. In the preferred embodiment, applicant utilizes a graphite fiber trademarked THORNEL by Union Carbide, number P-120 2K. 
     To maximize strength, the chamber  3  is first longitudinally wrapped with the graphite fiber, which is shown in FIG. 1 as longitudinal wraps  5 . Preferably, the wrap angle is approximately fifteen degrees and the wrap is from 0.030 to 0.040 inches thick. 
     Over the longitudinal wraps  5  are hoop wraps  7 . These are tightly wound wraps of the graphite composite material and, in the preferred embodiment, are from 0.030 to 0.040 inches thick. 
     For bonding of the wraps  5  and  7  to the chamber  3 , a variety of materials can be used. In the preferred embodiment, a matrix of Hexcel Pre-Preg is used. This material conducts heat very well, but does not burn easily. 
     FIG. 3 illustrates a set up for testing the thermal performance of a heat pipe  11 . A heater element  13  is wrapped around one end of the heat pipe  11 . Point T 1  represents a test point at the cool end of the heat pipe  11 . Point T 2  represents a test at the approximate middle of the heat pipe  11 , while Point T 3  represents a test point at the heated end of the heat pipe  11 . 
     FIG. 4 sets forth the data which was taken using the test set up shown in FIG. 3 with two conventional heat pipes, one made of aluminum and the other made of copper, and a graphite composite heat pipe made in accordance with the present invention. The weight of each heat pipe is set forth, together with the steady state temperatures measured at Points T 1 , T 2  and T 3  in centigrade. 
     As can be seen from the table set forth in FIG. 4, a heat pipe made in accordance with the present invention is approximately 25% less in weight, yet yields approximately the same thermal performance. 
     FIG. 5 illustrates a cross section of a printed circuit card containing a plurality of integral heat pipes  17  made in accordance with the present invention. 
     As shown in FIG. 5, a printed circuit board  15  has integrally bound to it one or more heat pipes  17  using a graphite composite matrix  19 . The printed circuit board  15  includes a connector  21  and a cold wall interface  23 . If desired, the printed circuit board  15  can be of the multi-level type. 
     With the structure shown in FIG. 5, heat can be directly removed from the printed circuit board  15  by the heat pipes  17  without the need for additional thermal interfaces. This reduces bulk and weight, and increases thermal efficiency. 
     Although having now described certain embodiments of the present invention, it is to be understood that the present invention is not limited to these embodiments, but embraces a broad variety of other embodiments and equivalents, as should now be obvious to those skilled in the art. In short, the present invention is limited only by the following claims.