Patent Publication Number: US-2007107551-A1

Title: Kish-derived graphitic heat spreaders and foils

Description:
STATEMENT REGARDING FEDERALLY SPONSORED  
      RESEARCH OR DEVELOPMENT  
      The United States Government has rights in this invention pursuant to Contract No. DE-AC05-00OR22725 between the United States Department of Energy and UT-Battelle, LLC. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATIONS  
      Not applicable.  
     FIELD OF THE INVENTION  
      The invention relates to thermally conductive graphitic comprising articles and methods for forming the same.  
     BACKGROUND OF THE INVENTION  
      Current methods of spreading heat require thin highly-conductive materials to be placed between a heat source and a heat sink to uniform the thermal profile. These heat spreaders range from expensive diamond to copper, to a relatively low priced product referred to as GRAFOIL® Flexible Graphite which is provided by GrafTech International Ltd. GRAFOIL® is made from natural flake graphite by exfoliation and then compaction. This process, while fairly inexpensive, requires the mining of graphite from the earth, as well as a chemical or thermal exfoliation step prior to compaction.  
      More specifically, GRAFOIL® products begin with natural graphite flakes found in major deposits generally in China, Madagascar, Canada, Brazil, and Russia. GRAFOIL® is manufactured as a rolled sheet product by taking high quality particulate graphite flake and processing it through an intercalation process using strong mineral acids. The flake is then heated to volatilize the acids and expand the flake to many times its original size. The expansion process produces wormlike“worms” which provides a dendritic structure that can be readily formed by molding or calendering into sheets. The worms are chemically treated to remove impurities at this stage. No binders are generally introduced in the manufacturing process. The worms are then compacted in a rolling machine to a thin foil which has graphitic planes in the sheet of the foil. The result is a flexible sheet product that exhibits high tensile strength and, for industrial applications, typically exceeds 98% elemental carbon by weight. However, the thermal conductivity of the foil is limited as the structure is not truly graphite as the graphitic structure suffers damage during exfoliation.  
     SUMMARY  
      A method of producing low cost high thermal conductivity graphitic foils comprises the steps of providing a plurality of partially purified waste flakes formed during iron smelting, and compacting the plurality of waste flakes into a flexible foil material. The compacting step can comprise pressing the plurality of flakes into a foil, wherein no binder material is added before the pressing step. The pressing step can comprise pressing the plurality of flakes in a mold die coated with a mold release film. In a preferred embodiment of the invention, the plurality of waste flakes are pressed without a prior exfoliation step.  
      A thermally conductive foil comprises a foil comprising a plurality of partially purified pressed waste flakes derived from iron smelting. The foil has trace impurities characteristic of formation during iron smelting which can be evidenced by a broadening of peaks in an X-ray diffraction spectra of the material as compared to essentially pure graphite. The foil is generally exclusive of any added binder material. Foils according to the invention provide high thermal conductivity. For example, the foil can provide an in-plane thermal conductivity at 25° C. of between 75 and 250 W/m·K, such as between 150 and 250 W/m·K. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      A fuller understanding of the present invention and the features and benefits thereof will be accomplished upon review of the following detailed description together with the accompanying drawings, in which:  
       FIG. 1 ( a ) shows an X-ray spectra taken from a foil formed from GRAFOIL®, while  FIG. 1 ( b ) shows an X-ray spectra taken from a foil formed an partially purified pressed Kish flakes, according to the invention. In both cases, the three (3) sharp peaks are shown which evidence highly oriented graphitic materials.  
       FIG. 2  shows a cross sectional view of a flexible heat transfer device according to the present invention shown in an exemplary application connected in position between a heat source and a heat sink. 
    
    
     DETAILED DESCRIPTION  
      A method of producing low cost high thermal conductivity graphitic foils comprises the steps of providing a plurality of partially purified waste flakes formed during iron smelting, and compacting the partially purified waste flakes into a flexible foil material. The foil is generally a very flat structure, being also thin and highly thermally (and electrically) conductive material in the in-plane direction. The compacting step generally comprises pressing at 1,000 to 15,000 psi, but can use higher or lower pressures. Preferably, no binder material is added before the pressing step. Surprisingly, the compressed article holds together after pressing, analogous to a pressboard. Although pressing can be performed without heating the flakes, pressing can also occur under heating.  
      The pressing step can comprise pressing the flakes in a mold die coated with a mold release film. The waste flakes are preferably pressed without a prior exfoliation step, thus preserving the highly graphitic structure of the flakes.  
      As used herein, a flake is defined as shape having at least one, and generally both, of its x and y dimensions being larger than its z dimension. For example, the area of a flake is generally at least 20 times the thickness of the flake.  
      Kish material obtained as a waste product from iron smelting is a primarily a combination of graphite, desulfurization slag, and iron. Kish is generally formed when hot metal is immediately poured directly onto the scrap from a transfer ladle. Fumes and Kish (graphite flakes from the carbon saturated hot metal) are emitted from the vessel&#39;s mouth and collected by the pollution control system. Kish graphite is what results when the graphite, desulfurization slag, and iron mixture is purified using an acid treatment, generally being HCl and HF. This purification process is the main reason Kish graphite is not more widely available because a large scale HCl—HF process generally poses a difficult disposal problem. The U.S. Bureau of Mines did a study of Kish in 1994 and found that steelmakers were throwing away enough graphite with the Kish to equal the total U.S. demand for natural flake graphite. The invention thus provides a low cost process for producing thin highly oriented graphitic material, including foils, which simplify and reduce the cost as compared to earlier related processes, such as the process to make GRAFOIL® which as described above requires natural flake graphite. Moreover, the use of Kish rather than discarding Kish by the truckload as waste (as has been done for decades) will save the costs and fees associated with discarding the Kish, as well as preserve the environment.  
       FIG. 1 ( a ) shows an X-ray spectra taken from a foil formed from GRAFOIL®. The apparent double peak in the GRAFOIL® spectra shown in the left portion of  FIG. 1 ( a ) is believed to be due to tungsten element reflection. Other than this likely discountable peak, three (3) relatively sharp peaks are shown which together evidence a highly graphitic material. An X-ray spectra taken from a foil formed from pressed Kish flakes according to the invention is shown in  FIG. 1 ( b ). The flakes were pressed in a ½″ die for 1 minute. Pressing was performed at between 5,000 and 15,000 psi.  FIG. 1 ( b ) evidences the highly crystalline structure of Kish derived graphite foil according to the invention, which indicates the inventive foil provides excellent thermal and electrical properties in the plane of the foil.  
      The level of purification of the pressed Kish flakes is high as evidenced by the absence of spurious non-graphitic peaks. However, some broadening of the peaks appears to evidence the presence of some residual impurities in the foil. Accordingly, articles according to the invention have a unique structures based on the low level of residual impurities from the iron smelting process remaining after the purification process is performed. Tabulated data on the crystal parameters obtained from the x-ray data shown in  FIG. 1 ( b ) is as follows:  
                                                   d(A)   crystallite size (A)                                                        (002)   3.3469   550           (004)   1.6748   528           (006)   1.1178   408                      
 
      Foils according to the invention resemble the “GRAFOIL®” material in appearance. However, since the inventive Kish-derived foil was made without any binder, exfoliation process, or other process, the inventive method is both simpler and less expensive as compared to the process to form GRAFOIL® noted above.  
      A test was performed to compare the thermal conductivity of Kish derived foil according to the invention and GRAFOIL® by measuring the respective times to respond to an ice cube. GRAFOIL® is specified as having an in-plane thermal conductivity of about 140 W/m·K. The inventive foil took about half the time to respond to the ice cube compared to the GRAFOIL® sample. Accordingly, it was estimated that the in-plane thermal conductivity of the inventive foil was between about 200 to 250 W/m·K. More generally, the in-plane thermal conductivity of the Kish derived foil according to the invention is expected to be from about at least 75 W/m·K to 250 W/m·K, with an out of plane thermal conductivity of about 5 to 10 W/m·K. Other properties of the Kish derived foil according to the invention, such as electrical resistivity, are also expected to be highly anisotropic.  
      Flakes were also pressed in a 1.125″ die at 10,000 and 15,000 lbs respectively. Similar results to those described above were found.  
       FIG. 2  shows a cross sectional view of a flexible heat transfer device according to the present invention shown in an exemplary heat transfer application being connected in position between a heat source and a heat sink. Heat transfer device  200  includes foils according to the invention which provides a non-structural high thermal conductivity core material in the form of a strip  200 . The heat transfer device  200  is shown physically connected between a heat source  202  and a heat sink  203  which can be separated by a substantial distance and in areas that are not easily accessible. The heat transfer device  200  is easily bent and shaped to be connected by any conventional means using a clamp or bolt or by welding or soldering for affixing the heat transfer device  200  to the respective heat source  202  and heat sink  203 .  
      The flexibility of the core strip  200  is dependent on how thin it is. The thickness of the core strip  200  can vary from several nanometers to 1 cm thick, but generally has a thickness of between about 2 microns and 2 millimeters.  
      Because of its extremely low coefficient of thermal expansion, excellent conformability, and resilience, foils according to the invention are expected to be used for sealing severe service connections in industrial, aerospace, and automotive applications. Other than heat sink applications as noted above, graphitic articles according to the invention are also expected to find applications in chemical (high temperature and highly resistant) gasketing, high temperature automotive gasketing, and heat shielding. The invention is also expected to find applications in electronic applications involving RFI and EMI shielding, and resistive and electromagnetic heating elements. In addition, flexible foils according to the invention can be used to replace asbestos-based materials.  
      While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as described in the claims.