Abstract:
Nano-carbon material is described that combines the common and unique properties of spherical fullerenes, carbon nanotubes and graphene carbon allotropes to create an architecture that has unique mechanical and electrical properties. The combined tensile strength of graphene with the compressive strength of fullerenes attached to nanotubes creates a high strength material. By attaching fullerenes to nanotubes, the surface area of the material is greatly enhanced beyond the high surface area normally associated with vertically aligned nanotube arrays. Fabrication can be performed via several complementary methods including catalyst deposition, hydrocarbon chemical vapor deposition, and surface functionalization. The fabrication of the NTC is based on its sub-composites: graphene-nanotubes and nanotubes-fullerenes and their respective fabrication processes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and the benefit of U.S. Provisional Patent Application Ser. Nos. 61/677,115 and 61/677,132, each filed Jul. 30, 2012, and each of which is incorporated herein by reference in its entirety for all purposes. 
    
    
     FIELD 
     The embodiments described herein relate to composites of nano-carbon materials including spherical fullerenes, carbon nanotubes, and graphene. 
     BACKGROUND 
     Nano-carbon materials including spherical fullerenes, carbon nanotubes (CNTs), and graphene have long been praised for their high electrical and thermal conductivity as well as their superior mechanical strength. See: J. Lu, J. Han,  Int. J. High Speed Electron. Sys.  9, 101, 1998; R. Ruoff, A. Ruoff,  Nature  350, 663-4, 1991 and C. Lee et al,  Science  321, 385, 2008. However, each carbon allotrope is limited individually by their rigid shaping and specific mechanical shortcomings. 
     The general concept of composite engineering is to combine dissimilar materials, taking advantage of the unique properties of each material in a synergetic fashion. For sporting goods, aerospace products, etc., the mixing-and-matching of a selection of fiberglass, carbon fiber, KEVLAR, etc. reinforcing fiber, and polymer matrix material (selected from various thermoplastic and thermoset resins encapsulating the fibers) is well known. 
     However, the same is not properly said of nano-composite engineering and design. Here, a vast range of new material combinations, manufacturing techniques, and constructions and uses to which such constructions may be put have yet to be realized in the industry. 
     A tremendous need exists where these new technologies are applied to energy-related requirements where current limitations of battery and capacitor/super-capacitor design are limiting advancements in applications ranging from the miniaturization of personal electronics to the large-scale implementation of electric vehicle transportation. 
     SUMMARY 
     Nano-carbon material, hereafter referred to as Nano Tri-Carbon (NTC) composite, is described that combines the common and unique properties of spherical fullerenes, carbon nanotubes (CNTs) and graphene carbon allotropes to create an architecture that has unique mechanical and electrical properties emanating from the configuration of each allotrope in the composite. For various applications, the NTC composite can be patterned to allow the composite to be scaled from the nano-scale to the macro-scale. The NTC is composed of the fusion of two sub-composites: graphene-nanotubes and nanotube-fullerenes. 
     Mechanically, the configuration of the NTC combines the tensile strength of nanotubes and graphene with the compressive strength of fullerenes to create a material that has high strength in both tension and compression that is unique to any other nano-carbon composite. In applications of energy storage, the use of nanotube-fullerenes is suitable for building high capacitance electrodes. With the unique patterning ability of the NTC (discussed below), a supercapacitor can be built on the NTC platform that is compact and able to handle large electrical loads, even with the potential to handle power-plant grade loads. 
     The subject NTC composites, products (be they consumer goods, industrial hardware, etc.), methods of use, and methods of manufacture are all included within the present description. 
     Other products, composites, systems, devices, methods, features, and advantages of the subject matter described herein will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional products, composites, systems, devices, methods, features, and advantages be included within this description, be within the scope of the subject matter described herein, and be protected by the accompanying claims. In no way should the features of the example embodiments be construed as limiting the appended claims, absent express recitation of those features in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The figures diagrammatically illustrate example embodiments with similarly-depicted elements variously numbered in the figures. Embodiment variations other than those shown in the figures are contemplated as described in a broader sense herein, as generically claimed, or otherwise. 
         FIG. 1  is a schematic of an example embodiment of an NTC composite and its subcomponents. 
         FIG. 2  is a schematic of an example embodiment of a Unit Cell of NTC composite for electrical supercapacitor application. 
         FIGS. 3A and 3B  are schematics illustrating alternative examples of manufacturing process pathways for generalized NTC composite material. 
         FIGS. 4A and 4B  are schematics illustrating alternative examples of manufacturing process pathways for NTC superconductor unit cells. 
     
    
    
     DETAILED DESCRIPTION 
     Various exemplary embodiments are described below. Reference is made to these examples in a non-limiting sense, as it should be noted that they are provided to illustrate more broadly applicable aspects of the devices, systems and methods. Various changes may be made to these embodiments and equivalents may be substituted without departing from the true spirit and scope of the various embodiments. In addition, many modifications may be made to adapt a particular situation, material, composition of matter, process, process act(s) or step(s) to the objective(s), spirit or scope of the present subject matter. All such modifications are intended to be within the scope of the claims made herein. 
     As referenced above, the subject NTC composite comprises the fusion of two sub-composites of carbon allotropes: graphene-nanotubes and nanotubes-fullerenes. These allotropes are discussed, in turn. 
     Graphene-Nanotubes 
     Previous efforts by several research groups have demonstrated the ability to simultaneously fabricate graphene and nanotubes through chemical vapor deposition. However, the simultaneous nature of the growth process does not allow for the structure of the nanotubes or graphene to be individually modified. See: Kondo et al, Applied Physics Express 1, 2008; Jousseaume et al, Applied Physics Letters 98, 2011. 
     In the subject fabrication processes, graphene and nanotubes are fabricated independently, thus allowing for individual tuning of the structure of each component. In this way, the structure of the graphene-nanotube composite and the NTC can be optimized for a variety of applications ranging from structural to electrochemical. By using graphene as an anchoring substrate, the composite is not dependent on the type of bulky macro-scale substrates that are typically used to hold nanostructures. Given the compact and light-weight structure of the graphene-nanotubes composite and the NTC, both composites lend themselves easily to patterning. 
     Nanotubes-Fullerenes 
     In US Patent Publication No. 2012/0250225 (commonly-assigned and published Oct. 4, 2012 to the same inventors hereof), the concept of attaching nanoparticles, including the possibility of attaching fullerenes, to nanotubes is disclosed as a means to increase capacitance of electrochemical carbon nanotube capacitors by using the nanoparticles to increase the surface area of the nanotube array. Vertically aligned nanotubes act as a skeleton for fullerene attachment ensuring even distribution of fullerenes throughout the composite. Fullerenes prove highly advantageous due to the high electronic, structural, and chemical compatibility between the nanotubes and fullerenes due to common carbon structure. 
     Fullerenes are very good electrical conductors as well as extremely strong under compression, thereby benefiting overall properties of the NTC composite. Vertically aligned nanotubes are already well-known for their extremely high surface area to weight ratio. By the addition of fullerenes to the nanotubes, the surface area to weight ratio can be improved by a factor of about two to about three. 
     This high surface area maximizes surface interactions between the NTC and its surrounding environment. Maximization of surface interactions is useful for diverse applications ranging from but not limited to sensitive gas detection, high density charge separation in electrochemical double layer capacitors (i.e., supercapacitors), and analytic microfluidics. 
     Nano Tri-Carbon Composites 
     The subject NTC composites realize a synergy of the graphene-nanotubes and nanotubes-fullerenes composite to create a new and versatile nano-scale material. From the graphene-nanotube (pre)structure, the NTC is highly patternable due the use of graphene as a nano-scale anchoring platform for nanotubes. From the nanotubes-fullerenes composite, the overall NTC composite offers a remarkably high surface area to weight ratio that enables the device to have very strong surface interactions with its surrounding environment. 
       FIG. 1  is a schematic of Nano Tri-Carbon (NTC) composite  10 . Vertically aligned carbon nanotubes  12  are shown grown from catalyst deposited on or near graphene  14 . The nanotubes are shown acting as skeletal structure for placing nanoparticles (such as fullerenes  16 ) throughout the interspacing. Graphene acts as a nano-scale substrate which ultimately allows for patterning of the NTC. 
     Supercapacitors 
     Supercapacitors store large amounts of energy while allowing for power uptake and delivery rates far higher than conventional batteries. Thus, supercapacitors allow for efficient capture of intermittent renewable energy sources such as wind and solar power. 
     Per above, the enhanced surface area to volume ratio of the fullerene-nanotube complex offers significant potential advantages in supercapacitor construction. As shown in  FIG. 2 , a supercapacitor unit cell  20  may be constructed by sandwiching two NTC composite elements  10  on top of (i.e., facing) each other and filling the interspacing  22  with electrolyte  24 . Nonconductive polymer  26  may wrap around the composite to add structural support for the graphene layers. 
     In the unit cell, each nanotube-fullerene complex acts as an electrode  28 / 28 ′. A voltage difference across the electrodes causes charge separation in the electrolyte-filled gap region (“G) that acts as the energy storage mechanism. 
     The graphene  14  works as a current collector at the end of each electrode. Using graphene greatly increases the capacitance per unit mass of the NTC supercapacitor over conventional supercapacitors that use macro-scaled metals for current collectors. 
     Moreover, the slimness of the graphene layer enables individual unit cells to be stacked together to allow for compact scaling of the supercapacitor. Currently, conventional supercapacitor cells are limited to handling 1-3 Volts. 
     By using nanomaterials, exclusively, it is possible pattern the subject unit cell supercapacitor to connect supercapacitors in series or in parallel in order to handle much larger electrical loads. Consequently, the subject NTC supercapacitor can be feasibly scaled to handle electrical loads from commercial power plants. 
     Generalized NTC Composite Fabrication 
     The NTC composite can be fabricated by three steps: fabrication of a graphene-nanotube subcomposite, addition of fullerenes by surface functionalization, and etching of the bulk substrate. The graphene-nanotube subcomposite can be fabricated by two different methods paths depending on the preparation of the catalyst layer of the nanotubes and graphene. A process path  100  shown in  FIG. 3A  employs graphene catalyst and nanotube catalyst deposited in two separate layers such as nickel and iron, respectively. To maintain a distinct boundary between layers, a buffer layer such as a polymer or oxide may be used. In contrast, a process  200  in  FIG. 3B  combines the graphene and nanotube catalyst into a single layer such as an alloy of iron and nickel. Given the many different catalyst choices available, iron (Fe) is referred to as the nanotube catalyst and nickel (Ni) as the graphene catalyst. In the  FIG. 3A  process, graphene and nanotubes fabrication are independent as the fabrication occurs in sequential non-overlapping steps. As such, the graphene and nanotube growth can be individually tuned by changing the proportions and layering of the individual catalyst particles. 
     After fabricating the graphene-nanotube sub-composite (by either the  FIG. 3A  or  FIG. 3B  approach), fullerenes (or generically nanoparticles) are attached to the nanotubes as described in the US Patent Publication No. 2012/0250225, above, incorporated by reference herein in its entirety. Next, the support (and remaining catalyst layers) on the bottom of the composite are etched away to finish fabrication of the NTC. 
     With more specific reference to  FIG. 3A , process  100  starts at  102  by evaporating catalyst for graphene on a substrate such as silicon and then growing graphene via hydrocarbon chemical vapor deposition (CVD) at  104 . Next, nanotubes are fabricated by depositing nanotube catalyst at  106  and then assembling the nanotubes in CVD at  108 . With the nanotube-graphene structure set, fullerenes or other nanoparticles can be attached to the nanotubes through surface functionalization at  110 . Finally, the nickel and silicon substrate is etched away at  112  to leave a freestanding structure. 
     At this point, the NTC can also be transferred to other substrates such as flexible polymer (not shown). 
     With more specific reference to  FIG. 3B , process  200  beings at  202  by evaporating a catalyst that simultaneously triggers graphene and nanotube growth at such as the aforementioned nickel and iron alloy. By changing the proportion and patterning of the alloy, the structure of the graphene and nanotubes can be individually adjusted. At  204 , CVD is performed to grow the graphene-nanotube structure. At  206 , fullerenes or other nanoparticles are attached to the nanotubes through surface functionalization. At  208 , the nickel and silicon substrate is etched away to leave a freestanding structure. Again, the NTC can then be transferred to other substrates such as flexible polymer. 
     NTC Composite Supercapacitor Fabrication 
     Fabrication of the supercapacitor unit cell  20  can be accomplished by two different fabrication techniques. Similar to the method if  FIG. 3A , method  300  in  FIG. 4A  employs graphene catalyst and nanotube catalyst (such as nickel and iron, respectively) that are deposited in two separate layers. Again, to maintain a distinct boundary between layers a buffer layer (not shown) such as a polymer or oxide may be used. Likewise, similar to the method in  FIG. 3B , method  400  in  FIG. 4B  combines the graphene and nanotube catalyst into a single layer such as cobalt or an alloy of iron and nickel. And, again, given the many different catalyst choices available, for simplicity, iron (Fe) is referred to as the CNT catalyst and nickel (Ni) as the graphene catalyst. 
     Notably, the supercapacitor can also be constructed with the omission of fullerene attachment to the nanotubes (per steps  320 / 418 , below). However, such omission will be at the cost of reduced supercapacitor performance. 
     In any case,  FIG. 4A  illustrates a process  300  where a bottom graphene layer is fabricated by depositing the nickel catalyst layer on silicon at  302  and then performing hydrocarbon chemical vapor deposition (CVD) at  304 . As above, several types of hydrocarbon gases can be used including methane or ethylene. At  306  and  308 , iron and nickel are deposited, respectively. The newly deposited Ni layer forms the top graphene layer via a second round of hydrocarbon CVD at  310 . To support the graphene layers, flexible polymer is deposited along the sides of the structure at  312 . With both graphene layers fabricated, the remaining nickel is etched at  314  to create a gap for nanotube growth. At  316 , Hydrocarbon CVD is performed to generate nanotubes. At  318 , the nanotubes are then etched in the center of the cell to create two distinct opposing electrodes in separated columns. This completes one unit cell. At  320 , fullerenes can now be attached to the nanotubes as referenced above. The fullerenes attached to the nanotubes make up the high surface area electrodes. Steps  306  through  320  are essentially repeated to generate additional unit cells resulting in the body at  322 . After patterning, the unit cells are filled with electrolyte. Finally, at  334 , the silicon substrate is etched to complete fabrication of the NTC supercapacitor. 
       FIG. 4B  illustrates a process  400  that starts by depositing nickel and iron on a silicon substrate at  402 . In process  400 , iron and nickel layers are mixed for various reasons including that the layers are deposited on the wafer at the same time, the layers are heat treated to create an alloy, or a single catalyst material is used to generate both nanotubes and graphene such as nickel or cobalt. After such deposition at  402 , a hydrocarbon CVD process at  404  results in the simultaneous growth of the bottom layer graphene and the nanotubes. To produce a top surface for catalyst deposition, the top of the nanotubes are anchored  406  in a flexible polymer such as PDMS. Next, at  408 , iron and nickel are deposited on top of the flexible polymer. 
     For structural support, the unit cell is wrapped at  410  in a flexible polymer to provide structural support for the unit cell. At  410 , the PDMS (or whatever other polymer was employed) is etched with the top Ni/Fe layer being supported by the lateral polymer support. Hydrocarbon CVD is then performed to fabricate the top layer of graphene and nanotubes at  414 . By modifying the growth time of hydrocarbon CVD, the spacing between the nanotube-fullerene electrodes can be controlled. 
     Steps  406  through  414  are essentially repeated to generate to create a stacked unit cell as in  416 . After completing graphene and nanotube fabrication, fullerenes can be attached via surface functionalization as shown  418 . Next, the unit cells are filled electrolyte. Finally, the original Si substrate and overhanging polymer can be removed via etching to complete fabrication of the NTC supercapacitor at  420 . 
     Variations 
     NTC composites, methods of NTC composite manufacture and the application of the NTC composite to create supercapacitor architectures have been disclosed. Various other applications of the subject produces have been noted. Of these, in a preferred embodiment of the subject supercapaciter, the high surface area of the fullerenes and nanotubes combined with the low-mass of all nano-carbon materials creates supercapacitors that can store high densities of electrical energy and are highly scalable. The scalability of the supercapacitor illustrated offers potential for the NTC to compactly handle commercial grade electrical loads thus making NTC supercapacitors a potentially key component in generating power from intermittent renewable energy sources. 
     Reference to a singular item includes the possibility that there are a plurality of the same items present. More specifically, as used herein and in the appended claims, the singular forms “a,” “an,” “said,” and “the” include plural referents unless specifically stated otherwise. In other words, use of the articles allow for “at least one” of the subject item in the description above as well as the claims below. It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for use of such exclusive terminology as “solely,” “only” and the like in connection with the recitation of claim elements, or use of a “negative” limitation. 
     Without the use of such exclusive terminology, the term “comprising” in the claims shall allow for the inclusion of any additional element—irrespective of whether a given number of elements are enumerated in the claim, or the addition of a feature could be regarded as transforming the nature of an element set forth in the claims. Except as specifically defined herein, all technical and scientific terms used herein are to be given as broad a commonly understood meaning as possible while maintaining claim validity.