Patent Publication Number: US-2011053052-A1

Title: Fuel cell composite flow field element and method of forming the same

Description:
FIELD OF THE INVENTION 
     The subject invention relates to fuel cells and more particularly, to a components therefor, such as separator plates and flow field elements, and a method for producing these components. 
     BACKGROUND OF THE INVENTION 
     A typical fuel cell system includes a power section in which one or more fuel cells generate electrical power. Each fuel cell unit may include a proton exchange member (PEM) at the center with gas diffusion layers on either side of the proton exchange member. Anode and cathode catalyst layers are respectively positioned at the inside of the gas diffusion layers. This unit is referred to as a membrane electrode assembly (MEA). Bipolar separator plates are respectively positioned on the outside of the gas diffusion layers of the membrane electrode assembly and serve to structurally support the fuel cell assembly and provide channels for the flow of fuel and oxides. This type of fuel cell is often referred to as a PEM fuel cell. It is important that the bipolar separator plates are mechanically strong, electrically and thermally conductive and impermeable to gas. 
     Bipolar separator plates can be formed of graphite with a multitude of flow channels machined into the plate. Such graphite separator plates can have numerous disadvantages. First, these plates are heavy and are subject to cracking as the temperature in the fuel cell is increased. Second, the cost of machining these plates from graphite negatively impacts the overall cost of the fuel cell unit. 
     An alternative to the machined graphite separator plate is a corrugated separator plate from a metal sheet. Corrugated metal plates eliminate the relatively expensive step of machining the flow channels in a graphite plate. This approached reduces the overall cost per square foot of the final product. However, the corrugated metal separator plates are not corrosion resistant so this alternative also becomes expensive because both sides of the corrugated metal separator plate are plated with gold or platinum to resist corrosion. 
     Therefore, there remains an opportunity to improve upon fuel cell flow field elements such as separator plates by eliminating the need for high-cost machined graphite plates and metal plates plated with platinum or gold and facilitate manufacture in mass production. 
     SUMMARY OF THE INVENTION 
     It is therefore a feature of the present invention to provide fuel cell components with a lower production cost and which are easy to manufacture in mass production, while achieving desirable thermal and electrical conductivity of the fuel cell component with formability and corrosion resistance, particularly in high temperature fuel cell applications operating at over 100 degrees C. 
     According to aspects of the invention, a fuel cell composite flow field element can include a conductive substrate sheet having a series of recesses interspaced among outer surface nodes, thereby providing a non-uniform thickness; an electrically conductive bonding agent applied to the substrate; and a flexible graphite layer bonded to one side or both sides of the substrate. The fuel cell composite flow field element further provides at least one flow channel. 
     The nodes can be substantially the same height relative to a reference plane of the substrate sheet, or some of the nodes can have different heights than the heights of other nodes relative to a reference plane of the substrate sheet. Similarly, the recesses can have substantially the same depth relative to a reference plane of the substrate sheet. Alternatively, some of the recesses can have different depths than the depths of other recesses relative to a reference plane of the substrate sheet. 
     The recesses can be dimples in the substrate sheet. The recesses can be through-perforations in the sheet. The substrate sheet can be a screen, in which the recesses are through holes of the screen and the nodes are provided by the webbing of the screen. The substrate sheet can be a woven mesh, in which the recesses are through holes of the mesh and the nodes are provided by the weave of the mesh. The mesh can be metal. The metal mesh can have a thickness in the range of 0.001 inches to 0.010 inches. The substrate can include metal or metal alloy. The substrate can also include woven or non-woven carbon fibers. 
     The bonding agent can be applied as a powder, and the bonding agent powder can be cured after application. Preferably, the bonding agent thickness is thinner on the nodes than in the recesses. The electrically conductive bonding agent can include a polymeric component and carbon particles, wherein the carbon particles are dispersed within the polymeric component. The polymeric component can include a cured thermoplastic. Preferably, the polymeric component has a continuous use temperature above 190 degrees C. 
     The fuel cell composite flow field element can be an MEA support plate and the flow channel can be a fluid port through the plane of the support plate. Alternatively, the fuel cell composite flow field element can be configured as a corrugated flow field insert. The flow field element can also be made into a separator plate and the flow channel can be a fluid port through the plane of the support plate. 
     According to another aspect of the invention, a method for making a fuel cell composite flow field element can be utilized. In the method, an electrically conductive bonding agent is applied to a flexible graphite layer. A conductive substrate sheet having a non-uniform thickness provided by a series of recesses interspaced among outer surface nodes is placed on to the flexible graphite layer. An electrically conductive bonding agent is applied to the substrate. A second flexible graphite layer covers the substrate sheet to form a composite stack. 
     The composite stack is cured and hot pressed. Finally, the composite stack is cooled under weight to room temperature. 
     The bonding agent can include a combination of PPS polymer powder (100 ppw); water (260 ppw); propylene glycol (20 ppw); wetting agent (4 ppw) and graphite (100 ppw). For a preferred application to a metal screen substrate, the minimum quantity of the bonding agent can be calculated from a webbing dimension of the screen and an opening percentage of opening area to total area of the screen. The minimum quantity of bonding agent can be calculated in mass based on the product of bonding agent cured density average, the webbing dimension, the opening percentage and substrate sheet total area. 
     The curing step can include heating the composite stack to about 375 degrees C. for about 35 minutes in an air circulating heating environment. The hot pressing step can include pressing the composite stack between two steel plates at about 1000 psi and about 280 degrees C. for about 30 seconds. 
     An advantage of the present invention is to provide a fuel cell component with high thermal and electrical conductivity that eliminates the need for high-cost machined graphite plates and metal plates plated with platinum or gold. 
     Another advantage of the present invention is to provide a fuel cell component that is easy to manufacture, including forming the component. 
     These and other features, objects and advantages of the present invention will become more apparent to one skilled in the art from the following detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       There are shown in the drawings, embodiments which are presently preferred. It is expressly noted, however, that the invention is not limited to the precise arrangements and instrumentalities shown in the drawings. 
         FIG. 1  is a perspective and exploded view of a fuel cell flow filed element having a metal substrate of non-uniform thickness in the form of a mesh between a pair of flexible graphite layers, with bonding agent applied between the metal substrate and each flexible graphite layer; 
         FIG. 2A  shows a sectional view of a fuel cell flow field element configured for use as a separator plate; 
         FIG. 2B  shows a section view of a fuel cell flow field element corrugated for use as a flow field insert; 
         FIG. 3A  is a perspective view of a conductive substrate of non-uniform thickness in the form of a screen; 
         FIG. 3B  is a partial sectional view, not to scale, of the substrate in  FIG. 3A  positioned in a composite stack; 
         FIG. 4A  is a perspective view of a conductive substrate of non-uniform thickness in the form of a woven mesh; 
         FIG. 4B  is a partial sectional view, not to scale, of the substrate in  FIG. 4A  positioned in a composite stack; 
         FIG. 5A  is a perspective view of a conductive substrate of non-uniform thickness in the form of a perforated plate; 
         FIG. 5B  is a partial sectional view, not to scale, of the substrate in  FIG. 5A  positioned in a composite stack; 
         FIG. 6A  is a perspective view of a conductive substrate of non-uniform thickness in the form of a dimpled plate; 
         FIG. 6B  is a partial sectional view, not to scale, of the substrate in  FIG. 6A  positioned in a composite stack; 
         FIG. 7A  is a perspective view of a conductive substrate of non-uniform thickness in the form of a crinkled mesh; 
         FIG. 7B  is a partial sectional view, not to scale, of the substrate in  FIG. 7A  positioned in a composite stack; 
         FIG. 8A  is a perspective view of a conductive substrate of non-uniform thickness in the form of a roughened or etched film or plate; 
         FIG. 8B  is a partial sectional view, not to scale, of the substrate in  FIG. 8A ; 
         FIG. 9  illustrates a process for making a fuel cell flow field element; 
         FIG. 10  is a graph of electrical and thermal properties of various separator plates as a function of thickness of the flexible graphite layers; 
         FIG. 11  shows a BASF polarization curve and voltage drop vs. current density of corrugated laminate samples for use in a 4-cell fuel cell; 
         FIG. 12  is a graph of test results for an air-cooled 8-cell stack with metal plates. 
         FIG. 13  is a graph of test results for a 3 kW air-cooled 80-cell stack with metal plates. 
         FIG. 14  is a graph of test results for an air-cooled 4-cell stack with plates using composite stacks according to the invention. 
         FIG. 15  is a graph showing single cell performance as a function of cell temperature with H2/Air. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     Embodiments of the invention are directed to fuel cell composite flow field elements and to methods of manufacturing these flow field elements adapted to improve the combination of thermal and electrical conductivity with formability. Aspects of the invention will be explained in connection with various flow field element configurations, but the detailed description is intended only as exemplary. Embodiments of the invention are shown in  FIGS. 1-9 , but the present invention is not limited to the illustrated structure or application. 
     The terms “a” or “an,” as used herein, are defined as one or more than one. The term “plurality,” as used herein, is defined as two or more than two. The term “another,” as used herein, is defined as at least a second or more. The terms “including” and/or “having,” as used herein, are defined as comprising (i.e., open language). 
     The fuel cell composite flow field element can take on a number of forms and applications in a fuel cell. The flow field element can be configured as an MEA support plate, a corrugated flow field insert or a separator plate, to name a few examples. As shown in  FIG. 1 , the composite flow field element  10  includes a composite stack  12  and provides at least one flow channel  14 . The flow field channel  14  as shown provides for through plane flow for such applications as fuel and oxidant supply and exhaust in a fuel cell stack. There can be more than one flow field channel, and when multiple flow field channels are employed, they can be the same or they can be different in size, shape and conformation. A through plane flow field channel can be located at various locations on the composite stack  12  within the stack perimeter or on an edge of the stack  12 . 
     According to an aspect of the invention, the composite stack  12  includes a conductive substrate sheet having non-uniform thickness, such as a screen  16 . As used herein, “non-uniform thickness” means that the substrate sheet has a series of recesses interspaced among outer surface nodes. This construction results in a variation in the thickness of the sheet. The recesses refer to depressions and can include through holes in the sheet, while the nodes represent the sheet surfaces between the recesses. The nodes may be flat and planar or may take on various heights relative to a reference plane. The term “sheet” as used to describe the substrate does not limit the substrate to a planar or flat configuration as the substrate and the composite stack may be formed in other shapes, including corrugations, bends and creases. 
     The non-uniform thickness can be provided in several different arrangements. As shown in  FIG. 1 , a preferred construction of the sheet is in the form of a mesh or screen  16 , in which the through-holes  18  (only one of which is reference numbered to aid in illustration) repeated throughout the screen  16  form the recesses and the webbing  20  of the screen  16  present the nodes. 
     In addition to the substrate sheet of non-uniform thickness, the composite stack  12  further includes one, and preferably two, flexible graphite layers  22  that cover the substrate sheet. The flexible graphite layers  22  provide corrosion resistance to the composite stack  12 . The composite stack  12  further includes an electrically conductive bonding agent  24  that is applied between the substrate sheet, such as the screen  16 , and the flexible graphite layers  22 . The recesses and nodes of the substrate sheet of non-uniform thickness enables the conductive bonding agent  24  to contact a greater surface area of the substrate sheet when compared to a sheet without nodes and recesses and to allow projection nodes of the substrate sheet to contact or be placed closer to the graphite layers  22 . These characteristics of the composite stack  12  further enhance the thermal and electrical conductivity of the flow field element  10 . 
     The conductive substrate of non-uniform thickness can include any suitable conductive material, but is preferably a metal or metal alloy. For example, the substrate of non-uniform thickness material can include a metal mesh, such as stainless steel mesh; a creased or crinkled metal foil, such as stainless steel foil; or woven or non-woven carbon fibers. 
     The substrate of non-uniform thickness in the form of a mesh  16  can include any fine mesh, wire cloth or screen having shape retaining properties. For example, the mesh  16  can include woven metal wires with small open spaces in between. The open spaces of mesh allow for a continuous network of conductive bonding agent  24  to be deposited throughout the layer thickness, preventing large flakes from peeling off of the metal surface of the substrate. 
     Mesh sizes can include between 80×80 to 600×600. Rectangular openings such as 100×150 mesh are suitable for roll-to-roll impregnation processes, where the web speed and direction can affect the extent of impregnation. 
     The mechanical properties of 150×150 mesh with about 30% open area are suitable to provide a compressive spring constant that matches the desired compressive load for high temperature PEM membranes. Excessive force during compression of the fuel cells reduces the life of the MEAs. Ideally, the compressive stress exerted on the MEA should remain below 150 psi, and more specifically below 100 psi, for compressive strain in the range of 0.0005 inches to 0.002 inches. It is possible to obtain compressive stress less than 50 psi for strains of up to 0.002 inches with a suitable choice of the metal reinforcement. 
     The percent open area of the mesh can range between 20% to 80%. The opening size should allow for the impregnation of the mesh with the conductive bonding agent. Typical opening sizes range from 0.0005 inches to 0.010 inches. A smaller opening can be used with a lower viscosity conductive adhesive. Openings in the range of 0.001 inches to 0.005 inches provide an optimum range for developing a strong network of the conductive adhesive material within the reinforcing layer. 
     A metal mesh provides several advantages, including that the increased surface area of the metal substrate of non-uniform thickness (and thus the increased contact area with the conductive bonding agent) provides for lower through plane electrical resistance compared with a metal foil reinforcing layer. See Table 4 below. 
     A metal mesh or a creased or crinkled metal foil provide several advantages, including the ability to form the composite into a three-dimensional structure using mechanical bending, such as through corrugation. Corrugation of thin unreinforced flexible graphite is otherwise not possible, as the mechanical bending stresses cause an unreinforced flexible graphite sheet to easily tear. Furthermore, the flexible graphite would not have sufficient strength to retain a corrugated shape under the compressive loads generated during fuel cell stack assembly. As shown in  FIG. 2A , the composite stack  26  can used in a planar arrangement with a fluid channel  28  formed through the plane of the stack  26 . Alternatively, as shown in  FIG. 2B , a composite stack  30  can be formed to provide corrugations, providing flow channels  32 . The metal substrate foil thickness can range from 0.001 inches to 0.010 inches. Corrugations using 0.002 inch thick metal foil have satisfactory mechanical properties, and enable high speed roll-to-roll manufacturing as well as stamping, blanking or die cutting operations. 
     Another advantage of a metal substrate of non-uniform thickness is lower electrical resistivity in the plane of the composite. In fact, the use of a metal/flexible graphite composite provides an improved combination of in-plane electrical and thermal conductivity for a given thickness of separator plate. The substrate sheet can present a non-uniform thickness in various configurations.  FIGS. 3-8  include perspective and sectional views of different substrate profiles, illustrating various recess and node arrangements of the non-uniform thickness of substrate sheets according to aspects of the invention. The reference plane of the substrate sheet can be a center plane or one of the surface planes. 
     As shown in  FIGS. 3A-3B , the substrate sheet can be a mesh  34 , which provides through hole recesses  36 , repeated throughout the mesh  34 , but only one of which is numbered to facilitate illustration, interspersed among nodes provided by the webbing  38  of the mesh  34 . In the example of  FIGS. 3A-3B , the mesh  34  is non-woven, providing nodes that are substantially the same height. In  FIG. 3B , the mesh  34  is shown interposed between graphite layers  40  in a not-to-scale spacing. The intervening bonding agent is not shown but is understood to substantially occupy the spacing between the mesh  34  and the graphite layers  40 , including extending into one or more of the through hole recesses  36 . 
       FIGS. 4A-4B  shows an alternative screen  42  that is woven, with the weft and the warp  44  presenting nodes of different heights among the through hole recesses  46  (again, only one of which is referenced by number) of the screen  42 . In  FIG. 4B , screen  42  is shown interposed between graphite layers  48  in a not-to-scale spacing. The intervening bonding agent is not shown but is understood to substantially occupy the spacing between the screen  42  and the graphite layers  48 , including extending into one or more of the through hole recesses  46 . 
       FIGS. 5A-5B  shows the profile of a substrate sheet  50  with perforations  52  (only one of which is numbered) in the sheet to provide through hole recesses among the uniform height nodes, such as surface region  54  of the sheet  50 . In  FIG. 5B , sheet  50  is shown interposed between graphite layers  56  in a not-to-scale spacing. The intervening bonding agent is not shown but is understood to substantially occupy the spacing between the sheet  50  and the graphite layers  56 , including extending into one or more of the through hole recesses  52 . 
       FIGS. 6A-6B  shows a substrate sheet  58  with nodes of uniform height and recesses of uniform depth. The recesses can be formed on one side to provide dimples, such as dimple  60 , which is representative of the other similarly illustrated dimples, among the nodes, such as the surface region  62 . In  FIG. 6B , sheet  58  is shown interposed between graphite layers  64  in a not-to-scale spacing. The intervening bonding agent is not shown but is understood to substantially occupy the spacing between the sheet  58  and the graphite layers  64 , including extending into one or more of the dimple recesses  60 . 
       FIGS. 7A-7B  shows a substrate sheet with nodes of different heights and recesses of different depths. This arrangement of non-uniform thickness can be obtained, for example, from crinkling a foil  66  to form rcesses, such as exemplary recesses  68 ,  70  and nodes, such as exemplary nodes  72 ,  74 . In  FIG. 7B , the foil  66  is shown, with a reference plane  76 , interposed between graphite layers  78  in a not-to-scale spacing. The intervening bonding agent is not shown but is understood to substantially occupy the spacing between the foil  66  and the graphite layers  78 , including extending into one or more of the recesses, such as the recesses  68 ,  70 . 
       FIGS. 8A-8B  shows another substrate sheet with nodes of different heights and recesses of different depths. This arrangement of non-uniform thickness can be obtained, for example, from roughening, etching or scratching a foil or plate  80 , resulting in recesses, for example, recesses  82 ,  84 , and nodes, such as nodes  86 ,  88 . The surface roughness per side, or profile, is preferably about one-half of the average foil thickness, i.e. an average foil thickness of 2 mils could have a surface profile of 1 mil. In  FIG. 8B , the foil  80  is shown interposed between graphite layers  90  in a not-to-scale spacing. The intervening bonding agent is not shown but is understood to substantially occupy the spacing between the foil  80  and the graphite layers  90 , including extending into one or more of the recesses, such as the recesses  82 ,  84 . 
     In order to bond the substrate of non-uniform thickness to the flexible graphite layers and maximize the through plane conductivity of the composite, a conductive bonding agent or adhesive is used. Typically, a particulate form of carbon is used to impart conductivity to the adhesive. However, due to increased corrosion resistance, graphite particles are preferred over more amorphous forms of carbon. 
     The conductive adhesive also has a polymeric component, which must withstand the elevated temperatures required for operating high temperature PEM membranes. High temperature PEM membranes typically operate between 120 degrees C. to 160 degrees C., for extended life, but may operate at 190 degrees C. or more for brief periods, or to achieve maximum power. Nominal operating temperature of the separator plates for high temperature PEM fuel cells is between 160 degrees C. to 180 degrees C., yielding the best balance of life and power output. 
     The polymeric component of the conductive adhesive must protect the metal from corrosion, and should not flake or peel off of the metal surface during fuel cell operation. Large flakes could block flow channels and negatively affect the fuel cell performance and life. With respect to avoiding flaking or peeling, the metal foil is not optimized. 
     The polymeric component of the conductive adhesive can include any suitable material, such as thermoplastic. Although traditionally used as a coating, the reinforcing layer can be bonded to the flexible graphite layers by application of a thermoplastic followed by curing. For example, the conductive adhesive can include a mixture of epoxy and graphite flakes. 
     Typically, polymers for high temperature applications operating over 100 degrees C. are selected from thermosets. The preferred polymer however includes a thermoplastic that is normally used as a coating or a matrix material for molded parts. Composite stacks according to aspects of the invention use a thermoplastic polymer as part of the bonding agent, contributing to the formability of the composite stack and addresse the exposure of thermoplastic use in the high temperature fuel cell environment by curing the bonding agent. 
     The conductive adhesive can be in the form of a powder or a slurry. Although a powder form is preferred for application to a metal mesh substrate, the powder can be more difficult to apply evenly. The use of a mesh substrate can help to distribute the powder evenly. 
     The thickness of the conductive adhesive may range from 0.0005 inches to 0.01 inches, and may also extend as an interpenetrating network throughout the thickness of a metal mesh substrate. The adhesive may be impregnated into the spaces within the metal mesh, simplifying application of higher viscosity adhesive formulations. 
     The flexible graphite layer can be formed from graphite adaptable to flex under pressure. The flexible graphite layer can also be formed from polymeric material filled with graphite. 
     The thickness of a flexible graphite layer can be varied to affect the composite properties. The range of thickness is generally between 0.001 inches to 0.030 inches. A flexible graphite layer thickness of 0.010 to 0.020 inches enables better heat conduction, but may be difficult to form into fine channels through corrugation. 
     A flexible graphite layer thickness of 0.001 to 0.010 inches improves formability. For example, a corrugated separator of the present invention with a channel height of 0.040 to inches, and a composite thickness of 0.016 inches, has two 0.005 inch thick flexible graphite layers. 
     Table 1 shows the properties of a separator plate with flexible graphite layers of varying thickness bonded to a reinforcing layer of stainless steel. 
     
       
         
           
               
             
               
                   
               
               
                 Laminar Composite Separator Plate with Stainless Steel 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                   
                 GTA 
               
               
                   
                 Material Property 
                 Units 
                 316 S.S. 
                 Grafoil 
               
               
                   
                   
               
               
                   
                 Density 
                 g/cc 
                 7.95 
                 1.12 
               
               
                   
                 Electrical Resistivity 
                 μOhm- 
                 75 
                 1400 
               
               
                   
                   
                 cm 
               
               
                   
                 Thermal Conductivity 
                 W/m * K 
                 16 
                 150 
               
               
                   
                   
               
            
           
           
               
            
               
                 Laminate Construction* 
               
            
           
           
               
               
               
            
               
                 Thickness (inch) 
                   
                 Laminate Property** 
               
            
           
           
               
               
               
               
               
               
            
               
                 316 
                   
                   
                 Thickness Fraction 
                 μOhm- 
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 S.S. 
                 Grafoil 
                 Total 
                 316 S.S. 
                 Grafoil 
                 cm 
                 W/m * K 
               
               
                   
               
               
                 0 
                 0.003 
                 0.003 
                 0.000 
                 1.000 
                 1400 
                 150 
               
               
                 0.001 
                 0.003 
                 0.004 
                 0.250 
                 0.750 
                 1069 
                 117 
               
               
                 0.002 
                 0.003 
                 0.005 
                 0.400 
                 0.600 
                 870 
                 96 
               
               
                 0.003 
                 0.003 
                 0.006 
                 0.500 
                 0.500 
                 738 
                 83 
               
               
                 0.005 
                 0.003 
                 0.008 
                 0.625 
                 0.375 
                 572 
                 66 
               
               
                 0.01 
                 0.003 
                 0.013 
                 0.769 
                 0.231 
                 381 
                 47 
               
               
                 0.01 
                 0 
                 0.01 
                 1.000 
                 0.000 
                 75 
                 16 
               
               
                 0.001 
                 0.006 
                 0.007 
                 0.143 
                 0.857 
                 1211 
                 131 
               
               
                 0.002 
                 0.006 
                 0.008 
                 0.250 
                 0.750 
                 1069 
                 117 
               
               
                 0.003 
                 0.006 
                 0.009 
                 0.333 
                 0.667 
                 958 
                 105 
               
               
                 0.005 
                 0.006 
                 0.011 
                 0.455 
                 0.545 
                 798 
                 89 
               
               
                 0.01 
                 0.006 
                 0.016 
                 0.625 
                 0.375 
                 572 
                 66 
               
               
                 0.001 
                 0.01 
                 0.011 
                 0.091 
                 0.909 
                 1280 
                 138 
               
               
                 0.002 
                 0.01 
                 0.012 
                 0.167 
                 0.833 
                 1179 
                 128 
               
               
                 0.003 
                 0.01 
                 0.013 
                 0.231 
                 0.769 
                 1094 
                 119 
               
               
                 0.005 
                 0.01 
                 0.015 
                 0.333 
                 0.667 
                 958 
                 105 
               
               
                 0.01 
                 0.01 
                 0.02 
                 0.500 
                 0.500 
                 738 
                 83 
               
               
                 0.001 
                 0.02 
                 0.021 
                 0.048 
                 0.952 
                 1337 
                 144 
               
               
                 0.002 
                 0.02 
                 0.022 
                 0.091 
                 0.909 
                 1280 
                 138 
               
               
                 0.003 
                 0.02 
                 0.023 
                 0.130 
                 0.870 
                 1227 
                 133 
               
               
                 0.005 
                 0.02 
                 0.025 
                 0.200 
                 0.800 
                 1135 
                 123 
               
               
                 0.01 
                 0.02 
                 0.03 
                 0.333 
                 0.667 
                 958 
                 105 
               
               
                 0.001 
                 0.04 
                 0.041 
                 0.024 
                 0.976 
                 1368 
                 147 
               
               
                 0.002 
                 0.04 
                 0.042 
                 0.048 
                 0.952 
                 1337 
                 144 
               
               
                 0.003 
                 0.04 
                 0.043 
                 0.070 
                 0.930 
                 1308 
                 141 
               
               
                 0.005 
                 0.04 
                 0.045 
                 0.111 
                 0.889 
                 1253 
                 135 
               
               
                 0.01 
                 0.04 
                 0.05 
                 0.200 
                 0.800 
                 1135 
                 123 
               
               
                   
               
               
                 *Analysis neglects contributions from conductive adhesive 
               
               
                 **In-plane propertie 
               
            
           
         
       
     
     Table 2 shows the properties of a separator plate with flexible graphite layers of varying thickness bonded to a reinforcing layer of steel. 
     
       
         
           
               
             
               
                   
               
               
                 Laminar Composite Separator Plate with Plain Steel 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                   
                 GTA 
               
               
                   
                 Material Property 
                 Units 
                 Steel 
                 Grafoil 
               
               
                   
                   
               
               
                   
                 Density 
                 g/cc 
                 7.87 
                 1.12 
               
               
                   
                 Electrical Resistivity 
                 μOhm-cm 
                 17 
                 1400 
               
               
                   
                 Thermal Conductivity 
                 W/m * K 
                 50 
                 150 
               
               
                   
                   
               
            
           
           
               
            
               
                 Laminate Construction* 
               
            
           
           
               
               
               
            
               
                 Thickness (inch) 
                 Thickness Fraction 
                 Laminate Property** 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Steel 
                 Grafoil 
                 Total 
                 Steel 
                 Grafoil 
                 μOhm-cm 
                 W/m * K 
               
               
                   
               
               
                 0 
                 0.003 
                 0.003 
                 0.000 
                 1.000 
                 1400 
                 150 
               
               
                 0.001 
                 0.003 
                 0.004 
                 0.250 
                 0.750 
                 1054 
                 125 
               
               
                 0.002 
                 0.003 
                 0.005 
                 0.400 
                 0.600 
                 847 
                 110 
               
               
                 0.003 
                 0.003 
                 0.006 
                 0.500 
                 0.500 
                 709 
                 100 
               
               
                 0.005 
                 0.003 
                 0.008 
                 0.625 
                 0.375 
                 536 
                 88 
               
               
                 0.01 
                 0.003 
                 0.013 
                 0.769 
                 0.231 
                 336 
                 73 
               
               
                 0.01 
                 0 
                 0.01 
                 1.000 
                 0.000 
                 17 
                 50 
               
               
                 0.001 
                 0.006 
                 0.007 
                 0.143 
                 0.857 
                 1202 
                 136 
               
               
                 0.002 
                 0.006 
                 0.008 
                 0.250 
                 0.750 
                 1054 
                 125 
               
               
                 0.003 
                 0.006 
                 0.009 
                 0.333 
                 0.667 
                 939 
                 117 
               
               
                 0.005 
                 0.006 
                 0.011 
                 0.455 
                 0.545 
                 771 
                 105 
               
               
                 0.01 
                 0.006 
                 0.016 
                 0.625 
                 0.375 
                 536 
                 88 
               
               
                 0.001 
                 0.01 
                 0.011 
                 0.091 
                 0.909 
                 1274 
                 141 
               
               
                 0.002 
                 0.01 
                 0.012 
                 0.167 
                 0.833 
                 1170 
                 133 
               
               
                 0.003 
                 0.01 
                 0.013 
                 0.231 
                 0.769 
                 1081 
                 127 
               
               
                 0.005 
                 0.01 
                 0.015 
                 0.333 
                 0.667 
                 939 
                 117 
               
               
                 0.01 
                 0.01 
                 0.02 
                 0.500 
                 0.500 
                 709 
                 100 
               
               
                 0.001 
                 0.02 
                 0.021 
                 0.048 
                 0.952 
                 1334 
                 145 
               
               
                 0.002 
                 0.02 
                 0.022 
                 0.091 
                 0.909 
                 1274 
                 141 
               
               
                 0.003 
                 0.02 
                 0.023 
                 0.130 
                 0.870 
                 1220 
                 137 
               
               
                 0.005 
                 0.02 
                 0.025 
                 0.200 
                 0.800 
                 1123 
                 130 
               
               
                 0.01 
                 0.02 
                 0.03 
                 0.333 
                 0.667 
                 939 
                 117 
               
               
                 0.001 
                 0.04 
                 0.041 
                 0.024 
                 0.976 
                 1366 
                 148 
               
               
                 0.002 
                 0.04 
                 0.042 
                 0.048 
                 0.952 
                 1334 
                 145 
               
               
                 0.003 
                 0.04 
                 0.043 
                 0.070 
                 0.930 
                 1304 
                 143 
               
               
                 0.005 
                 0.04 
                 0.045 
                 0.111 
                 0.889 
                 1246 
                 139 
               
               
                 0.01 
                 0.04 
                 0.05 
                 0.200 
                 0.800 
                 1123 
                 130 
               
               
                   
               
               
                 *Analysis neglects contributions from conductive adhesive 
               
               
                 **In-plane properties 
               
            
           
         
       
     
     Table 3 shows the properties of a separator plate with flexible graphite layers of varying thickness bonded to a reinforcing layer of nickel. 
     
       
         
           
               
             
               
                   
               
               
                 Laminar Composite Separator Plate with Nickel 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                   
                 GTA 
               
               
                   
                 Material Property 
                 Units 
                 Nickel 
                 Grafoil 
               
               
                   
                   
               
               
                   
                 Density 
                 g/cc 
                 8.9 
                 1.12 
               
               
                   
                 Electrical Resistivity 
                 μOhm-cm 
                 7 
                 1400 
               
               
                   
                 Thermal Conductivity 
                 W/m * K 
                 90.9 
                 150 
               
               
                   
                   
               
            
           
           
               
            
               
                 Laminate Construction* 
               
            
           
           
               
               
            
               
                   
                 Laminate Property** 
               
            
           
           
               
               
               
               
            
               
                 Thickness (inch) 
                 Thickness Fraction 
                 μOhm- 
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Nickel 
                 Grafoil 
                 Total 
                 Nickel 
                 Grafoil 
                 cm 
                 W/m * K 
               
               
                   
               
               
                 0 
                 0.003 
                 0.003 
                 0.000 
                 1.000 
                 1400 
                 150 
               
               
                 0.001 
                 0.003 
                 0.004 
                 0.250 
                 0.750 
                 1052 
                 135 
               
               
                 0.002 
                 0.003 
                 0.005 
                 0.400 
                 0.600 
                 843 
                 126 
               
               
                 0.003 
                 0.003 
                 0.006 
                 0.500 
                 0.500 
                 704 
                 120 
               
               
                 0.005 
                 0.003 
                 0.008 
                 0.625 
                 0.375 
                 529 
                 113 
               
               
                 0.01 
                 0.003 
                 0.013 
                 0.769 
                 0.231 
                 328 
                 105 
               
               
                 0.01 
                 0 
                 0.01 
                 1.000 
                 0.000 
                 7 
                 91 
               
               
                 0.001 
                 0.006 
                 0.007 
                 0.143 
                 0.857 
                 1201 
                 142 
               
               
                 0.002 
                 0.006 
                 0.008 
                 0.250 
                 0.750 
                 1052 
                 135 
               
               
                 0.003 
                 0.006 
                 0.009 
                 0.333 
                 0.667 
                 936 
                 130 
               
               
                 0.005 
                 0.006 
                 0.011 
                 0.455 
                 0.545 
                 767 
                 123 
               
               
                 0.01 
                 0.006 
                 0.016 
                 0.625 
                 0.375 
                 529 
                 113 
               
               
                 0.001 
                 0.01 
                 0.011 
                 0.091 
                 0.909 
                 1273 
                 145 
               
               
                 0.002 
                 0.01 
                 0.012 
                 0.167 
                 0.833 
                 1168 
                 140 
               
               
                 0.003 
                 0.01 
                 0.013 
                 0.231 
                 0.769 
                 1079 
                 136 
               
               
                 0.005 
                 0.01 
                 0.015 
                 0.333 
                 0.667 
                 936 
                 130 
               
               
                 0.01 
                 0.01 
                 0.02 
                 0.500 
                 0.500 
                 704 
                 120 
               
               
                 0.001 
                 0.02 
                 0.021 
                 0.048 
                 0.952 
                 1334 
                 147 
               
               
                 0.002 
                 0.02 
                 0.022 
                 0.091 
                 0.909 
                 1273 
                 145 
               
               
                 0.003 
                 0.02 
                 0.023 
                 0.130 
                 0.870 
                 1218 
                 142 
               
               
                 0.005 
                 0.02 
                 0.025 
                 0.200 
                 0.800 
                 1121 
                 138 
               
               
                 0.01 
                 0.02 
                 0.03 
                 0.333 
                 0.667 
                 936 
                 130 
               
               
                 0.001 
                 0.04 
                 0.041 
                 0.024 
                 0.976 
                 1366 
                 149 
               
               
                 0.002 
                 0.04 
                 0.042 
                 0.048 
                 0.952 
                 1334 
                 147 
               
               
                 0.003 
                 0.04 
                 0.043 
                 0.070 
                 0.930 
                 1303 
                 146 
               
               
                 0.005 
                 0.04 
                 0.045 
                 0.111 
                 0.889 
                 1245 
                 143 
               
               
                 0.01 
                 0.04 
                 0.05 
                 0.200 
                 0.800 
                 1121 
                 138 
               
               
                   
               
               
                 *Analysis neglects contributions from conductive adhesive 
               
               
                 **In-plane properties 
               
            
           
         
       
     
     The graph in  FIG. 10  illustrates the results from Tables 1 through 3 regarding how the electrical and thermal properties of various separator plates vary with the thickness of the flexible graphite layers. 
     Referring now to  FIG. 9 , a composite stack  92  according to aspects of the invention can be formed in the following fashion. The substrate of non-uniform thickness  94  formed from at least one of metal and metal alloys is positioned between the flexible graphite layers  96 , formed from the graphite adaptable to flex under pressure or the polymeric material filled with graphite. The flexible graphite layers  96  are bonded to the opposite surfaces of the substrate of non-uniform thickness  94  by a conductive bonding agent  98 , which is applied between the flexible graphite layers  96  and the substrate of non-uniform thickness  94 . To join the components to form the composite stack  92 , the stack  92  is preferably first cured and pressure is applied in a curing step  100  such that the substrate of non-uniform thickness  92  is in contact with the bonding agent  98  and the flexible graphite layers  96  are also in contact with the bonding agent  98 . The stack  92  can then be hot pressed in a hot pressing step  102 , thereby forcing the flexible graphite layers to the substrate of non-uniform thickness with the bonding agent being sandwiched therebetween to form a unitary composite. 
     In another method, the bonding agent includes a thermoplastic with graphite particles dispersed within the thermoplastic, which is deposited between the substrate of non-uniform thickness and the flexible graphite layers. The method of deposition can include co-extrusion or calendaring of the bonding agent and the substrate of non-uniform thickness. Additionally, pressure can be applied in the presence of oxygen then hot pressing to cure the thermoplastic bonding agent, thereby forming a unitary composite. 
     Alluding to both methods described above, the unitary composite  92  can then be fed through a pair of dies  104  in a forming step  108  to deform the composite into a corrugated shape with channels as shown in  FIG. 9 . The dies  104  may be integral with a corrugation apparatus (not shown) or be separable therefrom without limiting the scope of the invention. The foredm composite  108  is then precut to the desired length. The resulting composite according to aspects of the invention exhibits desirable thermal and electrical conductivity while eliminating the need for high-cost machined graphite plates and metal plates plated with platinum and gold and being easy to manufacture. 
     Although not intending to limit the scope of the invention, the following examples of composites are provided in order to further illustrate aspects of the present invention. Exemplary composites disclosed herein that provide desirable electrically and thermally conductive properties and that can function as separator plates or other flow field elements for fuel cells are described. 
     Example 1 
     An electrically and thermally conductive composite can be formed from the following components:
         i). 316 stainless steel foil, 0.003 inches thick   ii). high temperature conductive adhesive, comprising:
           a). 10 mL part A, MG 832HT epoxy (MG Chemicals)   b). 5 mL part B, MG 832HT epoxy (MG Chemicals)   c). 6 grams Asbury #3243 graphite flake (Asbury Graphite)   
           iii). GTA Grafoil flexible graphite, 0.005 inches thick (Graftech)       

     A composite comprising the above flexible graphite/conductive adhesive/stainless steel foil/conductive adhesive/flexible graphite is cured under pressure at 180 degrees F. for 1 hour. Passing the above composite through intermeshing splines forms a corrugated separator plate or flow field insert. 
     Example 2 
     An electrically and thermally conductive composite can be formed from the following components:
         i). 316 stainless steel 100×100 mesh, 0.0045 inches diameter wire, 30.3% open area   ii). high temperature conductive adhesive, comprising:
           a). 10 mL part A, MG 832HT epoxy (MG Chemicals)   b). 5 mL part B, MG 832HT epoxy (MG Chemicals)   c). 6 grams Asbury #3243 graphite flake (Asbury Graphite)   
           iii). GTA Grafoil flexible graphite, 0.005 inches thick (Graftech)       

     A composite comprising the above flexible graphite/conductive adhesive/stainless steel mesh/conductive adhesive/flexible graphite is cured under pressure at 180 degrees F. for 1 hour. Passing the above composite through intermeshing splines forms a corrugated separator plate or flow field insert. 
     Table 4 shows a comparison of the electrical resistance properties between the composite of Example 1 (using metal foil) and the composite of Example 2 (using metal mesh). Comparison of Example 1 and 2 Through-plane Electrical Resistance 
     
       
         
           
               
            
               
                   
               
               
                 Clamping Pressure vs Voltage Drop at 1 Amp/cm2 
               
            
           
           
               
               
               
            
               
                   
                 Voltage Drop (VDC) 
                   
               
            
           
           
               
               
               
            
               
                 Clamping Pressure (psi) 
                 Example 1 
                 Example 2 
               
               
                   
               
               
                 15 
                 .230 
                 .150 
               
               
                 30 
                 .143 
                 .091 
               
               
                 45 
                 .126 
                 .080 
               
               
                 60 
                 .116 
                 .074 
               
               
                 75 
                 .111 
                 .071 
               
               
                 15 
                 .157 
                 .113 
               
               
                   
               
            
           
         
       
     
     Table 5 shows the tensile strength, electrical resistivity and thermal conductivity of the composite described by Example 1. 
     Comparison of Example 1 Component Properties (In Plane) 
       
     
       
         
           
               
            
               
                   
               
               
                 Properties of Component Layers in Composite Laminate of Example 1 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Component 
                 Tensile 
                 Electrical 
                 Thermal 
               
               
                 Component 
                 Thickness in 
                 Strength 
                 Resistivity 
                 Conductivity 
               
               
                 Layer Material 
                 Laminate (inch) 
                 (MPa) 
                 (μOhm-cm) 
                 (W/m * K) 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 316 S.S. 
                 0.003 
                 515 
                 75 
                 16 
               
               
                 GTA Grafoil 
                 0.010 
                 4.5 
                 1400 
                 150 
               
               
                   
               
            
           
         
       
     
     Example 3 
     An electrically and thermally conductive composite can be formed from the following components:
         High Purity Graphite Flake—Asbury Graphite #3243   PPS Polymer Powder—Chevron Phillips Ryton VI   Propylene Glycol   Triton X-100 surfactant   Stainless Steel Screen—McMaster Carr 9319T41, 0.0026″ wire dia., 37.8% open   Flexible Graphite—Graphtec 0.005″ thick GTA Grafoil       

     The components are formed into a slurry mix in the following portions: PPS V-1 100 parts per weight (ppw); water, 260 ppw; propylene glycol, 20 ppw; wetting agent (Triton X-100), 4 ppw; graphite, 100 ppw. The components are placed in a ball mill with 5/32″ 302S.S. grinding media at 30 rpm for 12 hours. 
     To determine approximate amount of powder mixture needed for a given screen size, as an example, Powder mixture density (cured)=(1.35 g/cc+2.23 g/cc)/2=1.79 g/cc, Overall mesh thickness=2*wire dia.=0.0052″=0.0132 cm, % open area of mesh=37.8%=0.378, Minimum mixture needed [g]=sample area(2.375×2×2.54̂2 cm2)*0.0132 cm*0.378*1.79 g/cc=0.2737 g 
     This represents 0.0089 g of powder mix per sq.cm of 0.0026″ mesh. Given the mix ratios for the slurry it converts into 0.3542 g of slurry mix per sq.cm. 
     The grafoil pre-baked at 390 deg.C. in an air circulating oven for 20 minutes to degrade any attached oils and remove any trapped gases. The stainless steel screen cleaned in a bath containing citrisurf solution and rinsed thoroughly in deionized water. 
     The screen substrate in placed onto a grafoil sheet. The powder or slurry mix is evenly spread. The second grafoil layer is added. The laminate stack is cured in an air circulating furnace for 35 minutes at 375*C. The stack is then hot pressed between two stainless steel plates at 1000 psi and 280*C for 30 seconds. The stack is cooled down under weight. 
     Example 4 
     Another electrically and thermally conductive composite can be formed from the following components:
     High Purity Graphite Flake—Asbury Graphite #3243   PPS Polymer Powder—Chevron Phillips Ryton VI   Stainless Steel Screen—McMaster Carr 9319T41, 0.0026″ wire dia., 37.8% open   Flexible Graphite—Graphtec 0.005″ thick GTA Grafoil   

     The dry powder can be a combination of a thermoset/thermoplastic polymer mixed with fine graphite powder. Such binding matrix is designed to withstand operation conditions and environment. The mix is preferably constituted of PPS V-1 (1 ppw) and graphite (1 ppw), mixed in a rotating drum at 50 rpm for 1 hour. 
     The calculation of the appropriate amount of powder mixture needed for a given screen size can be made as in Example 3 above. The further steps in Example 3 can be used in fabricating the composite stack. 
     Supporting Data 
     Various tests have been performed on finished laminate having aspects of the invention. The tests include electrical testing on several samples and incorporated into a 4-cell fuel cell system. 
     For testing samples, a current is introduced through gold coated copper plates and a voltage drop is measured across the laminate. Standardizing compression of 88 psi (250 kg over a 45.58 sq.cm area), a given contact area and introduced current; a chart of voltage drop vs. current density is made.  FIG. 11  shows a BASF polarization curve and Voltage drop vs. Current density of corrugated laminate samples for use in a 4-cell fuel cell. Given an internal physical fuel cell stack-up where all components are electrically in series, this chart helps estimate cell resistance and predict cell performance. The added thermal properties and contact area from a rough surface are not part of this test. 
     As with any composite material, pressure and temperature will also affect its material properties. The following charts portray how the conductivity of these laminates rise with increased pressure. As reflected in the first two sections of the table, voltage drop measurements were taken twice, in two different places of the laminate, at different pressures and a varying current density over a 45.58 sq.cm area (3 in diameter). The last section compares the accuracy and repeatability of the test. 
     
       
         
           
               
               
               
            
               
                   
                   
               
               
                   
                 TOP 
                 Middle 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 mA per 
                   
                 100 psi 
                 200 psi 
                 300 psi 
                 100 psi 
                 200 psi 
                 300 psi 
               
               
                 sq. cm 
                 Amperage 
                 V drop 1 
                 V drop 2 
                 V drop 3 
                 V drop 1 
                 V drop 2 
                 V drop 3 
               
               
                   
               
            
           
           
               
               
            
               
                 Laminates 
                   
               
               
                 Conductivity 
               
               
                 Apparatus TRIAL 1 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 25 
                 1.2 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 50 
                 2.3 
                 0.003 
                 0 
                 0 
                 0.003 
                 0 
                 0 
               
               
                 75 
                 3.4 
               
               
                 100 
                 4.6 
               
               
                 125 
                 5.7 
               
               
                 150 
                 6.8 
                 0.007 
                 0.0052 
                 0.0047 
                 0.0066 
                 0.0045 
                 0.0041 
               
               
                 200 
                 9.1 
               
               
                 300 
                 13.6 
               
               
                 500 
                 22.8 
                 0.0313 
                 0.0227 
                 0.0207 
                 0.0262 
                 0.0207 
                 0.0169 
               
               
                 1000 
                 45.6 
                 0.0629 
                 0.0444 
                 0.0408 
                 0.0513 
                 0.0411 
                 0.0369 
               
            
           
           
               
               
            
               
                 Laminates 
                   
               
               
                 Conductivity 
               
               
                 Apparatus TRIAL 2 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 25 
                 1.2 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 50 
                 2.3 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 75 
                 3.4 
               
               
                 100 
                 4.6 
               
               
                 125 
                 5.7 
               
               
                 150 
                 6.8 
                 0.007 
                 0.0052 
                 0.0046 
                 0.0067 
                 0.0048 
                 0.0041 
               
               
                 200 
                 9.1 
               
               
                 300 
                 13.6 
               
               
                 500 
                 22.8 
                 0.0312 
                 0.0227 
                 0.0207 
                 0.0262 
                 0.0209 
                 0.0173 
               
               
                 1000 
                 45.6 
                 0.0631 
                 0.0474 
                 0.0415 
                 0.0516 
                 0.0418 
                 0.0373 
               
            
           
           
               
               
            
               
                 Laminates 
                   
               
               
                 Test Variability 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 25 
                 1.2 
                 0.0% 
                 0.0% 
                 0.0% 
                 0.0% 
                 0.0% 
                 0.0% 
               
               
                 50 
                 2.3 
                 0.0% 
                 0.0% 
                 0.0% 
                 0.0% 
                 0.0% 
                 0.0% 
               
               
                 75 
                 3.4 
               
               
                 100 
                 4.6 
               
               
                 125 
                 5.7 
               
               
                 150 
                 6.8 
                 0.0% 
                 0.0% 
                 1.1% 
                 −0.8% 
                 −3.2% 
                 0.0% 
               
               
                 200 
                 9.1 
               
               
                 300 
                 13.6 
               
               
                 500 
                 22.8 
                 0.2% 
                 0.0% 
                 0.0% 
                 0.0% 
                 −0.5% 
                 −1.2% 
               
               
                 1000 
                 45.6 
                 −0.2% 
                 −3.3% 
                 −0.9% 
                 −0.3% 
                 −0.8% 
                 −0.5% 
               
               
                   
               
            
           
         
       
     
     Composite stacks according to the invention were also tested in a four cell fuel cell stack. For comparison,  FIG. 12  is a graph of test results for an air-cooled 8-cell stack with metal plates. Individual cell temperatures were between 125 deg.C. to 180 deg.C. during polarization to 950 mA/cm2 with H2/air.  FIG. 13  shows test results for a 3 kW air-cooled 80-cell stack with metal plates. Individual cell temperatures were between 122 deg.C. to 175 deg.C. during polarization to 450 mA/cm2 with H2/air. 
       FIG. 14  shows the test results for an air-cooled 4-cell stack with plates using composite stacks according to the invention. Individual cell temperatures were between 160 deg.C. to 170 deg.C. during polarization at 950 mA/cm2 with H2/air. A comparison of the results of the fuel cell stacks with metal plates in  FIGS. 12 and 13  with the results in  FIG. 14  shows improved heat transfer. 
       FIG. 15  shows single cell performance as a function of cell temperature with H2/Air. 
     The foregoing description of preferred embodiments of the invention have been presented for the purposes of illustration. The description is not intended to limit the invention to the precise forms or methodologies disclosed. Indeed, modifications and variations will be readily apparent from the foregoing description. Accordingly, it is intended that the scope of the invention not be limited by the detailed description provided herein.