Abstract:
A method of assembling a metallic-graphite structure includes forming a wetted graphite subassembly by arranging one or more layers of graphite fiber material including a plurality of graphite fibers and applying a layer of metallization material to ends of the plurality of graphite fibers. At least one metallic substrate is secured to the wetted graphite subassembly via the layer of metallization material.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This patent application is a continuation-in-part and claims priority to U.S. Nonprovisional patent application Ser. No. 12/623,705 filed Nov. 23, 2009, which is incorporated herein by reference in its entirety. 
    
    
     FEDERAL RESEARCH STATEMENT 
     This invention was made with Government support under United States Government contract DO #3-DO-CRAVE-EC-003R8 awarded by NASA. The Government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The subject matter disclosed herein generally relates to graphite fibers joined to a substrate material. More particularly, the subject matter disclosed herein relates to graphite fiber and substrate structures for heat exchanger systems. 
     Graphite fiber material, for example, Fibercore®, is often utilized in heat exchange applications. The material comprises an array of graphite fibers having voids between adjacent fibers. In such applications, voids in the material may be filled with a heat storage, or phase change, material such as wax, water or the like. In some applications, no heat storage material is added. Large pieces of the graphite fiber material are typically bonded to a desired surface, for example, an aluminum component, via an adhesive. Thermal mismatch issues are common between bulk graphite and graphite foam when joined to a metallic substrate. For example, graphite materials typically have a coefficient of thermal expansion in the range of about 0-2 μin/in/° F., while for metals this coefficient is in the range of about 5-12 μin/in/° F. and for polymers the coefficient is in the range of about 10-70 μin/in/° F. The graphite fiber material is not a monolithic structure and as such accommodates the thermal mismatch by translating with the substrate during thermal processing. Further, the thickness of the graphite fiber material layer, which is relative to a length of the graphite fibers in the material, is limited due to capability of graphite fiber production. The relatively thin graphite fiber material is fragile and is subject to breakage and damage when handling and/or shaping by machining or the like into desired shapes. The art would well receive a more robust structure of graphite fiber material, which is not as sensitive to handling and/or other processing and which improves the thermal mismatch issues that exist in current structures. Also, in this process, a multilayered graphite fiber material structure may be obtained. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to one aspect of the invention, a method of assembling a metallic-graphite structure includes forming a wetted graphite subassembly by arranging one or more layers of graphite fiber material including a plurality of graphite fibers and applying a layer of metallization material to ends of the plurality of graphite fibers. At least one metallic substrate is secured to the wetted graphite subassembly via the layer of metallization material. 
     These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a cross-sectional view of an embodiment of a graphite fiber heat dissipative structure; 
         FIG. 2  is a cross-sectional view of another embodiment of a graphite fiber heat dissipative structure; and 
         FIG. 3  is a cross-sectional view of yet another embodiment of a graphite fiber heat dissipative structure. 
     
    
    
     The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Shown in  FIG. 1  is a cross-sectional view of an embodiment of a multi-layer graphite fiber material structure  10 . The embodiment of  FIG. 1  includes two graphite fiber material layers  12 , but it is to be appreciated that other quantities of graphite fiber material layers  12 , for example, one, three, five or more graphite fiber material layers  12  may be utilized in the structure  10 . Each graphite fiber material layer  12  includes a plurality of graphite fibers  14 , with a plurality of voids between the graphite fibers  14 . In one embodiment, the graphite fibers  14  are approximately 0.40 inches (1.0 cm) long, which results in a graphite fiber material layer  12  thickness of 0.40 inches (1.0 cm). It is to be appreciated, though, that other lengths of graphite fibers  14 , resulting in other thicknesses of graphite fiber material layers  12 , may be utilized. In embodiments where longer graphite fibers  14  are utilized, the fibers may be arranged with a greater density to resist buckling of the graphite fibers  14 . 
     A layer of braze filler  18 , for example a nickel braze alloy filler, is utilized between the graphite fiber material layers  12  to join the graphite fiber material layers  12 . In this embodiment, a length of the graphic fibers  14  of the graphite fiber material layer  12  extends substantially from one layer of filler  18  to another layer of filler  18 . In some embodiments, at each upper and lower end  20  of the assembly, a metallic sheet  22 , which may be a nickel or other suitable material, is brazed to the filler  18 . Additionally, in some embodiments, a metallic sheet  22  may be disposed between graphite layers  12  in the assembly. In embodiments where more than one graphite fiber material layer  12  is utilized, brazing of all graphite fiber material layers  12  together may be accomplished in a single step. While the embodiments illustrated utilize a nickel metallic sheet  22  and a nickel braze alloy filler  18 , in some embodiments, the metallic sheet  22  may be other nickel-based brazing alloys or of an alloy of titanium and titanium-containing fillers  18  may be utilized therewith. Further, while the filler  18  is shown as a sheet or foil, it is to be appreciated that the filler may be applied in other forms, such as a paste or spray or the like. 
     The sandwich structure is brazed to a substrate  24  formed from, for example, an aluminum material. Alternatively, the substrate  24  may be formed from other materials, such as stainless steel or nickel alloy where increased fluid combatibility is required, for example in a corrosive fluids environment. Brazing of the metallic sheet  22  to the substrate  24  is accomplished via an aluminum braze alloy  26  or other filler compatible with the substrate (e.g. nickel braze filler for stainless steel) and environment disposed between the metallic sheet  22  and the substrate  24 . 
     In some embodiments, as shown in  FIG. 2 , an endsheet  28  is located between the metallic sheet  22  and the substrate  24 . The endsheet  28  is formed of, for example, aluminum multiclad, and is brazed to the metallic sheet  22  and the substrate  24  using aluminum braze alloy  26 . Further, in some embodiments. an additional aluminum braze alloy layer  26  is utilized between substrate  24  and end sheet  28 . While substrate  24  is shown as a plurality of fins, it is to be appreciated that the substrate  24  represents a generic heat transfer device, which may include, for example, fins, a radiator structure, milled or drilled channels, or a radiant surface, or the like. In some embodiments, once the structure is joined as described above a phase heat storage material  16 , such as wax or water is infiltrated into the voids between the graphite fibers  14 . 
     While the joining process as described above may be performed in a single step, in other embodiments, the process is performed in two or more steps. For example, referring again to  FIG. 2 , a subassembly is formed of the graphite fiber material layers  12 , the filler  18 , and the metallic sheet  22 . The filler  18  is applied to ends of the graphite fibers  14  to metallize or wet the graphite fibers  14 , which allows for bonding of the graphite fibers  14  to the metallic sheet  22 . When the subassembly is completed, it may be set aside, handled, or stored with the metallic sheet  22  acting as a protective layer over the graphite fibers  14 . In the second step, the metallized subassembly is joined to the aluminum end sheet  28  via, for example, a vacuum brazing process. 
     In some embodiments, as shown in  FIG. 3 , two or more graphite fiber material layers  12  may be arranged side-to-side and joined via brazing. In these embodiments, the metallic sheet  22  is omitted between side-to-side adjacent graphite fiber material layers  12 , and only filler  18  is located between sides  30  of adjacent graphite fiber material layers  12 . To accomplish the brazing operation, the filler  18  is located between the metallic sheet  22  and each graphite fiber material layers  12 , wetting the graphite fibers  14 . When joining graphite fiber material layers  12  side-to-side, the filler  18  extends only partially along the length of the graphite fibers  14 . Leaving a portion of the joint uncovered by filler  18  allows for more efficient filling of the gaps between graphite fibers  14  with heat storage material  16  in later processing after the graphite fiber material layers  12  are joined. Location of filler  18  may be alternated throughout an assembly to promote flow of the heat storage material  16  through the graphite fiber material layer  12  when filled. As shown in  FIG. 3 , multiple layers may be constructed once the graphite fiber material layers  12  are joined side-to-side. The stack may include a metallic sheet  22  at the top and/or bottom of the assembly, and optionally a metallic sheet  22  may be disposed between graphite fiber material layers  12 . In effect, a large brazed assembly of graphite fiber material layers  12 , extending both in thickness and in length/width may be constructed. 
     The joining of ends of graphite fibers  14  to a metallic sheet  22  into the sandwich structure results in an effectively longer graphite fiber  14  length. The taller graphite fiber material height may be packaged into a more cubic structure (vs. a flat plate) which requires less external support during vibrational loading. Further, the metallic sheet  22 /graphite fiber material layer  12  structure is less susceptible to handling damage and can be shaped by a variety of processes, for example, electrical discharge machining (EDM), to produce desired shapes to close tolerances. 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.