Abstract:
An improved thermal harness or thermal heat strap that comprises carbon based (graphite) fiber heat conducting elements coupled to graphite, composite, polymer, or metal end attachment brackets. An outer casing comprising a tubular braided sleeve surrounds the heat conducting elements and eliminates possible contamination caused by the graphite fibers. The thermal harness may be advantageously used in electronic applications to dissipate heat from high temperature components.

Description:
BACKGROUND 
     The present invention relates generally to a heat conducting apparatus, and more particularly, to a thermal harness employing encased carbon based fibers and end attachment brackets for use in electrical and electronic applications. 
     Thermal Products, Inc. produces commercial thermal straps. These conventional straps use highly conductive carbon fiber weaves having ends encapsulated in copper. Unlike the present invention, these straps lack an outer casing material to not only restrain and protect the carbon fibers from damage and breaking, but also to encase and contain them in order to prevent fiber shedding and contamination of the highly electrically conductive carbon fibers which would not be desirable around electrical and electronic equipment. These straps also differ from the present invention by their use of metallic end fittings that add weight to the structure. 
     U.S. Pat. No. 5,077,637, issued Dec. 31, 1991, entitled “Solid State Directional Thermal Cable” discloses “a solid state, directional, thermal cable including a bundle of elongated, flexible, carbon fibers having a high thermal conductivity in at least the longitudinal direction. Couplings at each end of the cable, bind together the fiber bundle and thermally engage the cable with objects having different temperatures, for transferring heat between the objects. The thermal cable may be used with a frame that supports a device to or from which heat is to be transferred. The thermal cable engages the frame at a first region proximate the device, and with a second region remote from the device and transfers heat between the first and second regions. The frame may include a composite material whose constituents have at least two different coefficients of thermal expansion, for establishing the overall coefficient of thermal expansion of the composite material. One of the constituents may have a negative coefficient of thermal expansion, and may be a carbon based material.” 
     It is also stated that “In a preferred embodiment of this device, the frame includes a composite material whose constituents have at least two different coefficients of thermal expansion, for establishing the overall coefficient of thermal expansion for the composite material. One of the constituents may have a negative coefficient of thermal expansion and may include a carbon based material such as graphite or diamond.” 
     U.S. Pat. No. 5,077,637 discloses that “The coupling means may be made from a composite of materials, such as metal and carbon” and that the graphite fibers used in the thermal cable “may be enclosed by a flexible sleeve or sheathing 41 such as a plastic tube.” U.S. Pat. No. 5,077,637 also discloses that “Coupling 62 may be made from conventional materials or from a composite of materials which have a tuned coefficient of thermal expansion. Fibers 24d are inserted into the coupling, and the coupling is secured around the fibers by means such as crimping, potting the fibers in an adhesive or infiltrafing the fiber ends with molten metal. Fiber ends 15d are then cut and polished smooth.” 
     However, nothing is disclosed or suggested in U.S. Pat. No. 5,077,637 regarding connection to components using an encapsulant (and/or end attachment) and a soft polymer, such as silicone. There is no disclosure or suggestion in U.S. Pat. No. 5,077,637 that the thermal harness may include an encapsulating membrane to contain the tows. There is no disclosure or suggestion in U.S. Pat. No. 5,077,637 that at each node or endpoint, the tows are fanned out and splayed into a silicone matrix. There is no disclosure or suggestion in U.S. Pat. No. 5,077,637 that encapsulant may be used to help apply pressure to the thermal harness end point. There is no disclosure or suggestion in U.S. Pat. No. 5,077,637 that fibers are supported by the end attachment and/or encapsulant and a silicone matrix, which keep the fibers from buckling. There is no disclosure or suggestion in U.S. Pat. No. 5,077,637 regarding the use of an outer casing for the thermal harness that comprises a tubular braided sleeve/jacket, or that the braided sleeve comprises a material such as metal wires, Kevlar™-like fibers, polyben-zimidazole (PBI) fibers, Zylon™ fibers, plastic/textile fibers, or ceramic fibers. This outer braided casing acts as a containment for the carbon fibers and prevents any shedding or contamination of the highly electrically conductive carbon fibers around electrical components and electrical wiring. 
     There is no disclosure or suggestion in U.S. Pat. No. 5,077,637 regarding the use of an adhesion promoting material for promoting adhesion of the surfaces of individual filaments to the bonding material, or that the adhesion promoting material may be coating, finish, or sizing materials. There is no disclosure or suggestion in U.S. Pat. No. 5,077,637 regarding the use of bonding material which is a polymeric bonding material or a solder or brazing material. There is no disclosure or suggestion in U.S. Pat. No. 5,077,637 that the end fittings are made of either a polymeric resin reinforced graphite composite laminate, a carbon/carbon composite laminate, or a combination of one or the other of these composite laminates. 
     Accordingly, it would be advantageous to have an improved thermal harness using carbon based fiber and end attachment brackets that may be used in electrical and electronic applications. 
     SUMMARY OF THE INVENTION 
     The present invention provides for an improved thermal harness that comprises carbon based (graphite) fiber heat conducting elements coupled to either graphite composite, carbon/carbon composite, or metal end attachment brackets. An outer casing that preferably comprises a tubular braided sleeve surrounds the tubular heat conducting elements and eliminates possible contamination from the graphite fibers. The thermal harness may be advantageously used in electrical and electronic applications to dissipate heat from high temperature, heat-emanating components. 
     The thermal harness provides for a means for transferring heat from heat-emanating components to heat dispersion components in electrical and electronic equipment. High thermal conductivity carbon based fiber is used as the harness connecting the components. The connection is made using an encapsulant (and/or end attachment) and a “soft” polymer, such as silicone, to make contact with mating items. The harness may be used to directly transfer the heat to radiators and/or a “cold” side of an electronic box. The thermal harness may be used to replace heat spreaders, thermal planes, and other mass-intensive thermal management devices. 
     The thermal harness significantly reduces weight of computer trays, thermal doublers, and heat pipe assemblies. Attachment using the end attachment brackets is also significantly lighter than conventional devices using copper or aluminum to encapsulate the fiber ends. Encapsulating the fiber in silicone, for example, allows many of the fiber ends to intimately make contact with adjoining surfaces, and may eliminate resistive losses found at conventional interfaces. 
     Use of the thermal harness reduces the weight of electronic assemblies by replacing currently-used components required to reduce junction temperatures. The thermal harness may attach directly to the heat generating components and creates a bridge to the heat dissipating components. A reduced-to-practice embodiment of the thermal harness is extremely lightweight since the carbon fiber has a density of approximately 1.7-2.3 g/cm 3 . The thermal harness provides efficient heat transfer and may eliminate thermal planes in space structures. 
     The thermal harness has carbon fiber bundled with groups of tows going to a heat-generating element. Encapsulant may be used to help apply pressure to the thermal harness end point. The encapsulant may also have provisions for fasteners, springs, or clips to secure the end of the harness. One advantage of this technique is the direct contact between individual fibers and the mating surface. Fibers are supported by the end attachment and/or encapsulant and the silicone matrix. In this manner, the fibers are kept from buckling. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings wherein like reference numerals designate like structural elements, and in which: 
     FIG. 1 illustrates an exemplary embodiment of a thermal harness in accordance with the principles of the present invention; 
     FIG. 2 illustrates the structure of an exemplary thermal harness; 
     FIG. 3 illustrates a portion of the thermal harness showing details of its structure; and 
     FIG. 4 illustrates additional details of the thermal harness. 
    
    
     DETAILED DESCRIPTION 
     Referring to the drawing figures, FIG. 1 illustrates an exemplary embodiment of a thermal harness  10  in accordance with the principles of the present invention. The exemplary thermal harness  10  comprises a flexible graphite fiber thermal heat strap  11 . The flexible graphite fiber thermal heat strap  11  includes a plurality of unidirectionally-oriented high thermal conductivity graphite (carbon) fibers  12  in the form of a graphite fiber bundle  13  or plurality of bundles  13 . An outer encasing material like a tubular braided sleeve/jacket  14  is preferably used to encase the fiber bundle  13 . 
     The graphite fiber bundle  13  and outer braided tubular sleeve/jacket  14  forms a tubular heat strap element  15 . Multiple graphite fiber bundles  13  or tubular heat strap elements  15  are normally used in the construction of the flexible graphite fiber thermal heat strap  11 . The graphite fiber bundle  13  or bundles (and the outer braided tubular sleeve/jacket  14 ) is bonded to end fittings  16  using bonding material  17 . Both end fittings  16  may have through holes  18  formed therein to allow attachment to a heat source or heat sink. 
     The thermal harness  10  may have end fittings  16  having a tapered inner diameter such that axial force on the end fitting  16  compresses a material such as silicone or other polymer about the graphite fiber bundle  13 . The end fittings  16  may be comprised of any suitable material including, for example, graphite fiber composite material, which may comprise graphite fibers oriented predominately through the thickness of the end fittings  16 , graphite fiber composite and metal material, molded polymer, machined polymer and plastic, polymeric resin reinforced graphite composite laminate and carbon/carbon composite laminate, sections of a substantially unidirectional fiber tape wrapped circular rod, or sections of a pultruded composite rod. 
     FIG. 2 illustrates details of the structure of an exemplary thermal harness  10 . The graphite fiber bundles  13  or tubular heat strap elements  15  are bonded using bonding material  17  into a graphite fiber composite or combination graphite fiber composite and metal end fitting  16  on each end of the tubular heat strap elements  15 . A plurality of graphite fiber bundles  13  may be secured together using commonly available tie straps  21 , for example. 
     The end fitting  16  on one end of the flexible graphite thermal heat strap  11  may then be bonded to or mechanically attached (with a current art thermal gasket or thermal interface material) to a “hot” part (where heat will be withdrawn). The end fitting  16  at the opposite end of the flexible graphite thermal heat strap  11  may also be bonded to or mechanically attached (with a thermal gasket or thermal interface material) to a heat sink (where the heat will flow to). Both end fittings  16  may have through holes  18  formed therein to allow attachment to a heat source or heat sink. 
     FIG. 3 illustrates a portion of the thermal harness  10  showing details of its structure. The thermal harness  10  shown in FIG. 3 has an end fitting  16  with an additional laminated layer  16   a  that surrounds the tubular heat strap element  15  in the area where the bonding material  17  is disposed. FIG. 4 illustrates additional details of the thermal harness  10 . FIG. 4 illustrates a thermal harness  10  that is bonded to an end fitting  16  having through holes  18  therein used to secure the end fitting  16  to a heat source or heat sink. 
     The flexible graphite fiber thermal heat strap  11  is used to transfer heat from one physical location on a part or assembly to another physical location on the same part or to a different part. The flexible graphite fiber thermal heat strap  11  provides a means for transferring heat which is significantly lighter in mass than the current art of using a metal heat strap or using a graphite fiber heat strap having end fittings constructed entirely of metal. 
     Because of the high thermal efficiency of both the tubular heat strap elements Is and the end fittings  16 , the same or better thermal performance can be achieved than with the current art heat straps at a significantly reduced mass. Therefore, the present invention may be utilized for mass-critical applications on spacecraft, for example. 
     The flexible graphite fiber thermal heat strap  11  also provides a means for transferring heat without the potential corrosion problems of metallic heat straps or graphite heat straps having metallic end fittings  16 . Also, the length of the flexible graphite fiber thermal heat strap  11  may be varied depending on the specific application by varying the length of the tubular heat strap elements  15  before they are installed into the end fittings  16 . 
     The use of a braided tubular element as an integral part of the tubular heat strap element  15  that provides a thin protective outer sleeve/jacket  14  is a significant improvement over current art which uses only braided graphite fibers (without a sleeve or with a stiff plastic sleeve) as thermally conductive elements of the heat strap. The present invention eliminates contamination from the graphite fibers  12 , because the graphite (carbon) fibers  12  are totally encased within the outer braided sleeve/jacket  14  that is not constructed of graphite fibers. 
     The material used in the tubular sleeve/jacket  14  and the material used in the coating on the outside surfaces of the graphite (carbon) fibers  12  making up the graphite fiber bundle  13  may be selected to obtain a tubular heat strap element  15  that is “hot”, radiating the small amount of heat that is conducted perpendicular to the length of the graphite (carbon) fibers  12  and the graphite fiber bundles  13  by utilizing a fiber material in the construction of the braided sleeve/jacket  14  and/or a coating material on the (carbon) fibers  12  that emits heat by having a high surface emissivity. Conversely a tubular heat strap element  15  that is “cold”, retaining the conducted heat and minimizing emittance from the graphite (carbon) fibers  12  and the braided sleeve/jacket  14 ) making up the graphite fiber bundle  13 , may also be obtained by utilizing a material in the braided sleeve/Jacket  14  and/or a coating on the (carbon) fibers  12  that make up the graphite fiber bundle  13  that has a low surface emissivity. 
     The construction of the end fitting  16  may also be varied to provide a fitting  16  with a uniform temperature across its entire surface that attaches to the heat sink (with no “hot spots”), or specific “hot spot” areas within the end fitting  16  may also be achieved if desired. 
     Thus, the flexible graphite fiber thermal heat strap  11  includes tubular heat strap elements  15  containing graphite fiber bundles  13  comprising unidirectionally oriented high thermal conductivity graphite (carbon) fibers  12  with an outer casing that preferably comprises a tubular braided sleeve/jacket  14 . The tubular heat strap elements  15  are bonded with bonding material  17  to end fittings  16 . 
     High thermal conductivity graphite (carbon) fibers  12  are used as the primary heat transferring material of the flexible graphite fiber thermal heat strap  11 . The graphite (carbon) fibers  12  include tows or yarns containing many individual graphite filaments. The graphite (carbon) fibers  12  utilized in the graphite fiber bundle  13  are commercially available fibers such as K-800, and K-1100 which are marketed by BP Amoco Chemicals, K13C2U and K13D2U which are marketed by Mitsubishi Chemical America, Inc, and YS-95A fibers marketed by Nippon Graphite Fiber Corporation. A number of different graphite fibers may be selected, depending on cost versus thermal efficiency considerations. 
     Special coatings, finishes or sizings may be applied to the graphite (carbon) fibers  12  to enhance the physical, mechanical or thermal performance of the flexible graphite fiber thermal heat strap  11 . For example, a special finish or sizing may be used on the graphite fibers/filaments making up the graphite fiber bundle  13  to protect them from any abrasion of a graphite fiber/filament rubbing or moving against another graphite fiber/filament within the graphite fiber bundle  13 , when the flexible graphite fiber thermal heat strap  11  is bent or moved. A special coating, finish or sizing may also be used to promote adhesion of the surfaces of the individual filaments to the particular bonding material  17  used, which is either a polymeric bonding material or a solder or brazing material that is used to attach the tubular heat strap elements  15  to the end fittings  16 . 
     The graphite fiber bundles  13  are all oriented along the length of the tubular heat strap element  15  of the flexible graphite fiber thermal heat strap  11 . Each graphite fiber bundle  13  is incorporated into the tubular braided sleeve/jacket  14  during the braiding operation that manufactures the outer braided sleeve/jacket  14  of the tubular heat strap element  15 . 
     The thin tubular tightly-braided sleeve/jacket  14  is used to not only contain and protect the fragile graphite fiber bundle  13 , but also to encase and contain the graphite fiber bundle  13  so that any broken graphite filaments or particles can not contaminate components adjacent to or in contact with the thermal harness; particularly electronic components or electrical connections where this contamination could cause an electrical short circuit The combination of the two make up the tubular heat strap element  15 . The tubular braided sleeve/jacket  14  may be constructed using a conventional braiding machine and is typical of the current art of forming a braided protective outer sleeve/jacket around electrical wire cables or in the manufacturing of current art shoe laces having internal unidirectional structural textile filaments. 
     The material used in the braided sleeve/jacket  14  may be either metal (copper, aluminum or steel) including very small diameter metal wires, Kevlar™, Zylon™ or polybenzimidazole (PBI) fibers, or a plastic/textile or ceramic fiber material. The selected material depends upon the operating temperature of the thermal harness  10  and the flexible graphite fiber thermal heat strap  11  which also determines the bonding material  17  used to attach the tubular heat strap elements  15  to the end fittings  16 , and may also depend upon the thermo-optical properties desired for the braided sleeve/jacket  14  in order for the outer surface of the sleeve/jacket to be “hot” (radiating heat) or “cold” (insulating heat). This depends on what is in close proximity to or touching the thermal harness  10  and the flexible graphite fiber thermal heat strap  11 , which may be sensitive to heat, such as electronic components. 
     The tubular braided sleeve/jacket  14  also provides for the primary structural attachment of the tubular heat strap element  15  to the end fittings  16 , and also provides enough stiffness/rigidity so that the flexible graphite fiber thermal heat strap  11  cannot be bent to a small enough radius to damage the graphite (carbon) fibers  12  in the graphite fiber bundle  13 . The use of a braided sleeve/jacket that contains graphite fibers, like previous art, results in a more structurally robust thermal heat strap  11  and also eliminates the problem of graphite filaments breaking loose and causing contamination and potential problems of electrical shorting of electronic equipment. 
     The tubular heat strap element  15  comprises the graphite fiber bundle  13  and braided sleeve/jacket  14  surrounding it. The diameter of the tubular heat strap element  15  may be varied by varying the number of graphite (carbon) fibers  12  in the graphite fiber bundle  13  or the number of graphite fiber bundles  13  and the size of the braided sleeve/jacket  14 . The size and configuration of the flexible graphite fiber thermal heat strap  11  depends on the number of tubular heat strap elements  15  that are used in its construction, the thickness of the end fittings and the mechanical flexibility desired. 
     The bonding material  17  is used to attach the tubular heat strap elements  15  to the end fittings  16 . The bonding material  17  may be a plastic or polymeric resin such as an epoxy or silicone or it can be a ceramic adhesive or a metal or solder. For adhesively bonding the tubular heat strap elements  15  into the end fittings  16 , a polymeric or ceramic adhesive may be used. 
     Because of the flow of the bonding material either at room temperature or when heated, and the capillary action of the graphite (carbon) fibers  12  in the graphite fiber bundle  13 , the polymeric resin flows into the unidirectional graphite fiber bundle  13  within the braided sleeve/jacket  14  and also impregnates the filaments or fibers of the braided sleeve/jacket  14  that are within holes formed in the end fitting  16  into which the graphite fiber bundle  13  and braided sleeve/jacket  14  are inserted, A plastic or polymeric resin bonding material  17  normally has a filler material including particles of a high thermal conductive material such as boron nitride, alumna, carbon, graphite or metal. If solder is used as the bonding material  17 , the graphite fibers  12  in the fiber bundle  13  and the filaments or fibers of the braided sleeve/jacket  14  are coated with a compatible finish or metallized surface to obtain a good bond to the solder. A typical metal surface layer may be nickel, silver, chrome, aluminum, gold or a combination thereof. 
     The end fittings  16  are preferably comprised of either a polymeric resin reinforced graphite composite laminate, or a carbon/carbon composite laminate. For these composite laminates, including the polymeric resin laminate and the carbon/carbon laminate, the graphite fibers  12  are also high thermal conductivity graphite (carbon) fibers  12 , and essentially all of the graphite (carbon) fibers  12  in the laminates are oriented in the same direction as the graphite (carbon) fibers  12  in the graphite fiber bundle  13  of the tubular heat strap element  15 . 
     In this way, the heat carried in the graphite fiber bundle  13  of the tubular heat strap elements  15  is transferred to other graphite (carbon) fibers  12  in the end fitting  16  with the fibers oriented in the same direction as the graphite (carbon) fibers  12  in the graphite fiber bundle  13  and in the same direction as the desired heat flow. Thus, the most efficient transfer of heat is obtained. 
     Composite end fittings  16  are constructed either by conventional molding techniques or by cutting or machining sections from a predominantly unidirectional fiber tape wrapped circular rod or they may also be a section (circular, square, rectangular or other shape) cut from a pultruded composite rod. The polymeric resin laminate end fittings  16  is comprised of current art polymers such as epoxy or cyanate ester. The carbon/carbon laminate end fittings  16  is comprised of a current art carbon/carbon composite starting with a predominately unidirectional fiber tape wrapped circular rod  20  that is then converted into a carbon/carbon composite using current art materials and processing techniques. 
     The end fittings  16  may be flat or curved in shape depending on the geometry of the surface that the end fittings  16  are designed to attach to. Curved composite end fittings  16  are machined from these rods. The end fittings  16  of the flexible graphite fiber thermal heat strap  11  may have a different configuration at each end. The end fittings  16  may have drilled through holes or partially drilled holes therein for attachment of the individual tubular beat strap elements  15 . 
     The tubular heat strap elements  15  are attached with a bonding material  17  to the end fittings  16 . Holes slightly larger in diameter than the tubular heat strap elements  15  may be drilled either partially through or completely through the end fitting  16  so that a single tubular heat strap element  15  may be fitted into the drilled hole. 
     If the tubular heat strap elements  15  are attached to an end fitting  16  that has a drilled through hole to accommodate the heat strap elements  15 , the graphite (carbon) fibers  12  in the graphite fiber bundle  13  may be sealed with a thin coating of a high thermal conductivity polymer material in order to protection against contamination of small broken filaments or particles from the graphite (carbon) fibers  12 . The polymeric bonding material  17  may also be used to seal the tubular heat strap element  15  that is attached into an end fitting  16  having through holes to accommodate it. 
     Since a plurality of tubular heat strap elements  15  are normally used in each flexible graphite fiber thermal heat strap  11  a plurality of holes are formed in the end fitting  16 , and each individual hole receives one end of a tubular heat strap element  15 . If solder is used as the bonding material  17  to attach the tubular heat strap elements  15 , then a small circular preform plug of solder is placed at the bottom of each hole and a wrap of preform solder is placed around the tubular element. 
     The end of the tubular heat strap element  15  (at the point it enters the hole) is heated until the solder flows, or the end fitting  16  with the tubular heat strap elements  15  placed in the holes with the solder is then placed in an oven to reflow the solder. Brazing may be done in a similar way with a brazing material and a high temperature vacuum furnace or oven. An end fitting  16  comprised of carbon/carbon composite and a braided sleeve/jacket constructed of steel wire filaments may be required if brazing is used because of the extremely high temperatures that are involved. 
     To improve on the mechanical performance of the portion of the braided sleeve/jacket  14  adjacent to the end fitting  16 , it may be locally impregnated with a semi-flexible polymeric material such as a polyurethane, epoxy or silicone resin, for example. The tubular braided sleeve/jacket  14  of the flexible graphite fiber thermal heat strap  11  may also be painted with a flexible thermal control coating/paint to improve the thermo-optical properties. This not only provides improved thermal performance but provides an additional seal of the braided sleeve/jacket  14  to prevent any possibility of graphite contamination. 
     Multiple tubular heat strap elements  15  making up the flexible graphite fiber thermal heat strap  11  may be held together with tie cords, plastic tie wraps or wire or plastic clamps. The various tubular heat strap elements may be twisted together or interlaced to produce a compact flexible graphite fiber thermal heat strap  11 . 
     Use of the thermal harness reduces the weight of the electronic hardware in which it is employed by replacing currently-used components or devices required to reduce junction temperatures. The thermal harness  10  attaches directly to the heat generating components and creates a bridge to the heat dissipating components. A reduced-to-practice embodiment of the thermal harness  10  is extremely lightweight since the carbon fiber has a density of approximately 1.7-2.3 g/cm 3 . The thermal harness  10  provides efficient heat transfer and may eliminate thermal planes in space structures. 
     Thus, an improved flexible graphite fiber thermal heat strap or harness has been disclosed. It is to be understood that the described embodiments are merely illustrative of some of the many specific embodiments that represent applications of the principles of the present invention. Clearly, numerous and other arrangements can be readily devised by those skilled in the art without departing from the scope of the invention.