Abstract:
A process for metallizing nanomaterial including subjecting nanomaterial in a metallizing solution to microwave radiation; nanomaterial made by such a process; and density gradient separation of such material. This abstract is provided to comply with the rules requiring an abstract which will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure and is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims,  37  C.F.R.  1.72 (b).

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention is directed to: nanomaterials; to nanotubes; to nanoribbons; to metallized nanomaterials; in one particular aspect, to metallized carbon nanotubes; and to methods for metallizing nanomaterials, e.g. nanotubes, graphene sheets, and graphene ribbon-like material. 
         [0003]    2. Description of Related Art 
         [0004]    A wide variety of nanomaterials are known, e.g. nanotubes and nanoribbons. Carbon nanotubes (CNTs) have attracted much attention because of their extraordinary mechanical properties and unique electronic properties. A CNT is topologically equivalent to a two-dimensional graphene sheet rolled into a cylinder, with a cylinder diameter as small as 0.7 nanometers (nm) (and as large as several tens of nanometers) and with a cylinder length up to several microns (μm). A CNT can be single walled (SWNT) or multiple walled (MWNT) and can also be fabricated as a fiber or other structure. A CNT can be characterized by its chiral vector components (n, m), which help define tube diameter, electronic properties and other properties. Depending upon the chirality, a SWNT can be conducting (metal-like) or semiconducting. 
         [0005]    Currently, attempts are ongoing to utilize CNT&#39;s to enhance the properties of a variety of polymers and composites. Significant mechanical, electrical, and thermal property improvements have been reported. However, in certain cases, there are significant processing obstacles that prevent full enhancement of polymers and composites using incorporated nanotubes. In certain methods, dispersion has been shown to be essential for property enhancement when nanotubes are blended with polymers. Due to the intrinsic van der Waals attraction the nanotubes have to each other, and by virtue of their high aspect ratio (e.g., about 1:1000), nanotubes are typically agglomerated along their lengths, e.g. together as bundles and ropes, that have very low solubility in most solvents. In many instances, despite processing to achieve individual particles, nanotubes tend to remain as entangled agglomerates and homogeneous dispersion is not easily obtained. Furthermore, due to the atomically smooth non-reactive surface of nanotubes, lack of interfacial bonding limits load transfer from the matrix to nanotubes. In this situation, nanotubes are often pulled from a matrix, rather than fractured, and play a limited role in mechanical reinforcement of a composite structure. 
         [0006]    The potential for certain uses of nanotubes in these applications has, heretofore, not been realized because of difficulties in processing and limited CNT-to-matrix load transfer. Existing methods for overcoming these difficulties include chemical functionalization (addition of one or more specified chemical groups to a basic structure) and metallization (deposition of metal nanoparticles) on the CNT&#39;s. Development of enhanced polymers and composites may require functionalization of a collection of CNTs to allow the tubes to be dispersed more easily in a host polymer. 
         [0007]    Certain current known CNT chemical functionalization processes require complex apparatus and processing, use wet chemical procedures, and involve liquids or vapors, to which the CNTs are exposed. An example is the use of hot flowing fluorine gas to bond fluorine atoms to CNTs, as reported by E. T. Michelson et al in Chem. Phys. Lett. vol 296 (1998) 188. Large quantities of chemical reactants and solvents are often required, with most of the chemicals becoming residues that must be disposed under hazardous substance guidelines. Recycling of the chemicals used is seldom an option. 
         [0008]    Current processes for functionalization of CNT&#39;s by metallization include a physi-sorption technique that involves the separate preparation of metal nanoparticles involving a long sonication process to mix them with the carbon nanotubes, where dislodging of the metal particulates is often observed due to their loose attachment. Electroless deposition requires a harsh oxidative pretreatment (of the CNT&#39;s) followed by a complex activation-sensitization procedure, which is disruptive to the intrinsic CNT structure and properties. 
         [0009]    Electrochemical deposition is challenging because of the need to establish reliable electrical contact with bulk CNT substrates. Current approaches to overcoming this challenge include CNT growth on a conducting template, microlithography, electrophoresis, sputtering and thermal evaporation, all of which involve complex apparatus and processing and are not amenable to scale-up for industrial manufacturing. 
         [0010]    Known methods for metallizing nanotubes include, but are not limited to, those disclosed in U.S. patent applications Ser. No. 11/021,129 filed Dec. 22, 2004; Ser. No. 10/372,006 filed Feb. 21, 2003; Ser. No. 10/813,697 filed Mar. 31, 2004 and Ser. No. 12/232,818 filed Sep. 24, 2008; and in PCT Application Int&#39;l Publication No. WO 2008/140623A1 published Nov. 20, 2008 (all said applications incorporated fully herein for all purposes). 
         [0011]    There is a need for improved nanotube metallization processes that do not require complex apparatus or processes, produce relatively little residue for disposal, are efficient, selective, reasonably fast, and are scalable to large industrial scale production. The present invention recognizes these needs and provides solutions to these problems. 
       SUMMARY OF THE PRESENT INVENTION 
       [0012]    The present invention discloses, in certain aspects, metallized nanomaterials, e.g. treated nanotubes produced by methods in which the nanotubes are subjected to microwaves to facilitate the application of metal to the nanotubes. 
         [0013]    The present invention discloses, in certain aspects, methods for metallizing nanomaterials (e.g. nanotubes, graphene sheets, and graphene ribbon-like material) by subjecting the material to microwave radiation, thereby heating the material so that metal is deposited or electrochemically reduced thereby facilitating the deposition of metal and remains thereon. 
         [0014]    In certain aspects, the present invention discloses methods for metallizing carbon nantoubes that includes irradiating the nanotubes in an aqueous solution with microwaves to locally heat the nanotubes using, but not limited to, metals including Pt, Pd, Sn, Au, Ag, Cu, Ni, Co, Fe, tin and combinations of these metal species. 
         [0015]    In certain aspects, methods according to the present invention are readily scalable to mass production of metallized nanotubes (which, in one aspect, are carbon nanotubes). 
         [0016]    Accordingly, the present invention includes features and advantages which are believed to enable it to advance metallized nanomaterial technology. Characteristics and advantages of the present invention described above and additional features and benefits will be readily apparent to those skilled in the art upon consideration of the following description of preferred embodiments and referring to the accompanying drawings. 
         [0017]    Certain embodiments of this invention are not limited to any particular individual feature disclosed here, but include combinations of them distinguished from the prior art in their structures, functions, and/or results achieved. Features of the invention have been broadly described so that the detailed descriptions of embodiments preferred at the time of filing for this patent that follow may be better understood, and in order that the contributions of this invention to the arts may be better appreciated. There are, of course, additional aspects of the invention described below and which may be included in the subject matter of the claims to this invention. Those skilled in the art who have the benefit of this invention, its teachings, and suggestions will appreciate that the conceptions of this disclosure may be used as a creative basis for designing other structures, methods and systems for carrying out and practicing the present invention. The claims of this invention are to be read to include any legally equivalent devices or methods which do not depart from the spirit and scope of the present invention. 
         [0018]    What follows are some of, but not all, the objects of this invention. In addition to the specific objects stated below for at least certain embodiments of the invention, other objects and purposes will be readily apparent to one of skill in this art who has the benefit of this invention&#39;s teachings and disclosures. It is, therefore, an object of at least certain embodiments of the present invention to provide the embodiments and aspects listed above and: 
         [0019]    New, useful, unique, efficient and nonobvious metallized nanomaterial and methods for producing it; and 
         [0020]    Such methods for making metallized carbon nanomaterial, e.g. metallized carbon nanotubes, graphene sheets and graphene nano-ribbon-like material (“ribbons”). 
         [0021]    The present invention recognizes and addresses the problems and needs in this area and provides a solution to those problems and a satisfactory meeting of those needs in its various possible embodiments and equivalents thereof. To one of skill in this art who has the benefits of this invention&#39;s realizations, teachings, disclosures, and suggestions, various purposes and advantages will be appreciated from the following description of certain preferred embodiments, given for the purpose of disclosure, when taken in conjunction with the accompanying drawings. The detail in these descriptions is not intended to thwart this patent&#39;s object to claim this invention no matter how others may later attempt to disguise it by variations in form, changes, or additions of further improvements. 
         [0022]    The Abstract that is part hereof is to enable the U.S. Patent and Trademark Office and the public generally, and scientists, engineers, researchers, and practitioners in the art who are not familiar with patent terms or legal terms of phraseology to determine quickly, from a cursory inspection or review. the nature and general area of the disclosure of this invention. The Abstract is neither intended to define the invention, which is done by the claims, nor is it intended to be limiting of the scope of the invention or of the claims in any way. 
         [0023]    It will be understood that the various embodiments of the present invention may include one, some, or all of the disclosed, described, and/or enumerated improvements and/or technical advantages and/or elements in claims to this invention. 
         [0024]    Certain aspects, certain embodiments, and certain preferable features of the invention are set out herein. Any combination of aspects or features shown in any aspect or embodiment can be used except where such aspects or features are mutually exclusive. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0025]    A more particular description of embodiments of the invention briefly summarized above may be had by references to the embodiments which are shown in the drawings which form a part of this specification. These drawings illustrate embodiments preferred at the time of filing for this patent and are not to be used to improperly limit the scope of the invention which may have other equally effective or legally equivalent embodiments. 
           [0026]      FIG. 1  is a schematic process flow diagram of a method and system according to the present invention. 
           [0027]      FIG. 2  is a schematic process flow diagram of a method and system according to the present invention. 
           [0028]      FIG. 3  is a schematic process flow diagram of a method and system according to the present invention. 
           [0029]      FIG. 4A  is a transmission electron micrograph view of a nanotube with metal deposits. 
           [0030]      FIG. 4B  is a transmission electron micrograph view of a nanotube network with metal deposits. 
           [0031]      FIG. 5A  is a transmission electron micrograph view of a nanotube with metal deposits. 
           [0032]      FIG. 5B  is an enlargement of part of the view of  FIG. 5A . 
           [0033]      FIG. 6A  is a graph of thermogravimetric analysis data for an analysis of amounts of metal deposition by a metallization process according to the present invention. 
           [0034]      FIG. 6B  is a graph of thermogravimetric analysis data for an analysis of amounts of metal deposition by a metallization process according to the present invention. 
           [0035]      FIG. 7  presents an atomic EDS (energy dispersive X-ray spectroscopy) analysis of metal-CNT composition. 
           [0036]      FIG. 8A  is a scanning electron micrograph of nanotubes with copper deposited thereon. 
           [0037]      FIG. 8B  is an enlargement of part of the micrograph of  FIG. 8A . 
           [0038]      FIG. 8C  is an enlargement of part of the micrograph of  FIG. 8B . 
           [0039]      FIG. 9A  is a scanning electron micrograph of nanotubes with copper deposited thereon. 
           [0040]      FIG. 9B  is a scanning electron micrograph nanotubes with copper deposited thereon. 
           [0041]      FIG. 9C  is a table with EDS data about the materials in  FIGS. 9A and 9B . 
           [0042]      FIG. 10A  is a scanning electron micrograph of nanotubes with copper deposited thereon. 
           [0043]      FIG. 10B  is a scanning electron micrograph nanotubes with copper deposited thereon. 
           [0044]      FIG. 10C  is a table with EDS data about the composition of materials in  FIGS. 10A and 10B . 
           [0045]      FIG. 11A  is a scanning electron micrograph of nanotubes with nickel deposited thereon. 
           [0046]      FIG. 11B  is a scanning electron micrograph of nanotubes with nickel deposited thereon. 
           [0047]      FIG. 11C  is a scanning electron micrograph of nanotubes with nickel deposited thereon. 
           [0048]      FIG. 11D  is a table with EDS composition data about the composition of materials in  FIGS. 11B and 11C . 
           [0049]      FIG. 12A  is a scanning electron micrograph of nanotubes with cobalt deposited thereon. 
           [0050]      FIG. 12B  is a scanning electron micrograph nanotubes with cobalt deposited thereon. 
           [0051]      FIG. 12C  is a scanning electron micrograph of nanotubes with cobalt deposited thereon. 
           [0052]      FIG. 12D  is a table with EDS composition data about the composition of materials in  FIGS. 12A ,  12 B and  12 C. 
           [0053]      FIG. 13  presents electrical resistivity data for nanotubes with Ni deposited thereon, according to the present invention, in a polymer matrix. 
       
    
    
       [0054]    Certain embodiments of the invention are shown in the above-identified figures and described in detail below. Various aspects and features of embodiments of the invention are described below and some are set out in the dependent claims. Any combination of aspects and/or features described below or shown in the dependent claims can be used except where such aspects and/or features are mutually exclusive. It should be understood that the appended drawings and description herein are of certain embodiments and are not intended to limit the invention or the appended claims. On the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the appended claims. In showing and describing these embodiments, like or identical reference numerals are used to identify common or similar elements. The figures are not necessarily to scale and certain features and certain views of the figures may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness. 
         [0055]    As used herein and throughout all the various portions (and headings) of this patent, the terms “invention”, “present invention” and variations thereof mean one or more embodiments, and are not intended to mean the claimed invention of any particular appended claim(s) or all of the appended claims. Accordingly, the subject or topic of each such reference is not automatically or necessarily part of, or required by, any particular claim(s) merely because of such reference. So long as they are not mutually exclusive or contradictory any aspect or feature or combination of aspects or features of any embodiment disclosed herein may be used in any other embodiment disclosed herein. The drawing figures present the embodiments preferred at the time of filing for this patent. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0056]    Referring to  FIG. 1  in a system  10  according to the present invention, nanotubes NS in a metallizing solution are introduced to an apparatus AS in which the nanotubes are subjected to microwave radiation producing metallized nanotubes MN in a solution MM (e.g., ionic surfactant and aqueous metal salt). The resulting mixture MM is introduced to processing apparatus PA which separates metallized nanotubes MN from other components OC of the mixture MM. 
         [0057]      FIG. 2  illustrates a system  100  according to the present invention for effecting a nanotube metallization process according to the present invention. Nanotubes  102  are introduced with a metallizing solution  104  into a vessel  106  and are mixed together with a mixer  108  under high-intensity sonication to form a suspension. In one aspect, a metal-salt/solvent/surfactant aqueous solution is used, the surfactant forming a micellular structure about individual nanotubes or about bundles of nanotubes. 
         [0058]    A pump  110  pumps the suspension to a reactor  120  which provides a microwave field  122  for irradiating the solution (and the nanotubes therein) with microwave radiation. Metallizing material, in the solution deposits on the nanotubes following thermal excitation of the nanotubes by the microwave radiation. 
         [0059]    A resulting suspension  128  containing metallized nanotubes is fed to a separation stage  130  (via a valve  140 ). The separation stage  130  produces a stream  132  of metallized nanotubes and a stream  134  of the remaining components of the suspension  128 . Optionally, with the valve  140  in correct position, the suspension  128  (or part of it) is fed back to the vessel  106  for re-processing in a line  116 . Optionally, the components of the suspension  128  (or some of them) other than the metallized nanotubes in the stream  132 , are fed back to the vessel  106  in a line  118  for use in the process. 
         [0060]      FIG. 3  illustrates a system  200  according to the present invention for metallizing nanotubes. The system  200  may be used to metallize any known carbon nanotube, graphene sheet or graphene ribbon-like material, “ribbons”—all referred to as carbon nanomaterial. It can also be used to metallize non-carbon nanotubes, e.g., but not limited to, those with a high-aspect ratio so that a high dipole moment is induced and e.g. those which are at least slightly electrically conductive. 
         [0061]    A volume  201  of distilled and de-ionized water (DDIW) plus a surfactant is charged to a glass vessel  205  equipped with an agitator  206  and mounted in a tank  207  filled with acoustic transfer medium  240 , such as water. Carbon nanotubes (CNT&#39;s)  202 , which may be, e.g., single-wall, double-wall, multi-wall, functionalized, or un-functionalized, are charged into the glass vessel  205  and mixed into the aqueous surfactant solution. Acoustic transducers  208  are switched on to provide cavitation mixing by sonication. High-energy sonication facilitates the separation of agglomerated CNT&#39;s and enables the formation of dispersed micelles of individual nanotubes, or small clusters thereof. 
         [0062]    An aqueous reagent solution  203 , prepared separately, is charged into the glass vessel  205  containing the mixing batch of CNT&#39;s dispersed as micelles in the surfactant solution. The reagent solution  203  contains a metal salt, or combination of metal salts, that are soluble or partially soluble in an aqueous system. The identity of the metal species in the salt or salts is the same as the metal to be reacted onto the surface of the CNT&#39;s or CNT clusters. Various additives  204  are, optionally, charged into the mixing aqueous suspension to aid nanotube dispersion, scavenge oxygen, adjust pH, and/or modify viscosity. 
         [0063]    After a suitable time for CNT dispersion and micelle formation (e.g. one hour to twenty-four hours depending on sonication intensity and total energy required), the CNT suspension is pumped from the glass vessel  205  through a line  209 , e.g. an inert polymer conduit, or polymer-lined steel conduit, by a positive displacement pump  210 . Heat of both mixing and sonication is removed by a heat exchanger  211 . The CNT suspension then passes to and through a continuous, tubular reactor  212  consisting of a non-metallic fluid conduit, of prescribed length and diameter, that is exposed to microwaves from a microwave source  212   s.  The residence time of the CNT suspension in the microwave field from the source  212   s  is controlled to provide adequate activation energy for depositing ionic metal onto the surface of CNT&#39;s while preventing the fluid from boiling. Microwaves are absorbed by the electrically conductive CNT&#39;s and a weak electric field is induced around each individual CNT or CNT cluster. Electrons from the surfactant shell around the CNT&#39;s are promoted over an energy barrier and onto the CNT or CNT cluster. These electrons then react with the cationic metal in the suspension, provided that the oxidation-reduction potential of the aqueous cationic metal is compatible with the negative charge induced by the weak electric field. CNT&#39;s exiting the microwave reactor  212  are “M-CNT&#39;s”—they are partially or completely coated with reduced, non-ionic metal. 
         [0064]    The heat imparted by the reactor  212  and generated by the oxidation-reduction reaction is removed by a second heat exchanger  213 . At this point, the M-CNT suspension which has passed through the heat exchanger  213  can be directed through a recycle line  215  back to the vessel  205 , or through a line  216  to the isolation steps of the process. Use of a three-way valve  214  at this point is one example device that could be employed to control flow and separate the metal deposition steps from the CNT isolation steps. Any suitable flow-control/flow-directing device or apparatus may be used. Multiple passes through the reactor  212  incrementally increase the amount of metal deposition onto CNT&#39;s. This recycle loop may be operated several times until the targeted amount and morphology of metal is deposited. As the metal salt from the solution  203  and/or other chemical additives  204  are depleted in the circulating batch, they can be replenished in desired quantities at defined times to ensure steady-state process performance. 
         [0065]    To start the isolation stage of the process, treatment chemicals  217  to aid filtration are added to the suspension upstream of a static mixer  219 . The suspension is then passed through a filtration device, such as a centrifuge  220 , equipped with a filter medium having a pore size suitable to remove M-CNT&#39;s (metallized carbon nanotubes). The aqueous filtrate may either be directed in a line  221  to recycle back through the process or purged to remove any accumulated impurities. After an M-CNT wet cake is trapped in the filtration device  220 , it can be washed in situ to remove residual metal salts and surfactant. Either aqueous or organic solvents can be charged upstream, then passed through the wet cake, and discharged through a line  222  to a solvent recovery process  228 , which can include distillation, filtration, and/or purging operations. Recovered cake wash solvents are then recycled in a line  229  back to the process for future M-CNT cake washes. 
         [0066]    After impurities are removed from the wet cake, the centrifuge  220  is run at high speed to partially dry the M-CNT cake by forcing out residual solvent. The centrifuge may be any suitable known centrifuge, including, but not limited to, an ultra-centrifuge. High-pressure dry air or nitrogen in a line  223  is, optionally, charged to the back side of the filter medium to detach the M-CNT cake from the porous filter surface. The M-CNT cake is then discharged in a line  224  to a dryer  226  (such as an oven or fluidized bed unit) into which a heated dry air or nitrogen sweep is injected in a line  225 . Residual solvent is removed from the M-CNT&#39;s and passed to the solvent recovery process  228 . After drying is completed, gross agglomerations of M-CNT&#39;s are broken apart in a de-lumping or milling operation  230 , the product of which is a fine powder of M-CNT&#39;s. This material flows in a line  242  for division into salable lots and packaging by packaging apparatus  231 , producing packages  224  of the material in appropriate containers. 
         [0067]    In one particular embodiment, process parameters for the system  200  are:
   1. CNT concentration in suspension in vessel  205 : 10-1000 mg/L   2. Surfactant concentration in solution in vessel  205 : 0.1-2.0 Wt. %   3. Ionic metal molar ratio of: 0.25-2.00 gmol metal/gmol C   4. Sonication power: 9-20 kW/m 2      5. Sonication frequency: 20-80 kHz   6. Sonication energy input: 100-1000 J/g suspension   7. Microwave frequency: in the 300 MHz-300 Ghz range, e.g, but not limited to, between 900 MHz-2.450 GHz   8. Microwave wavelength: 1 mm-1 m   9. Microwave power imparted to fluid by  212   s:  3.6-12.0 kW·sec   10. CNT suspension flow rate (pumped through line  209  by pump  210  to reactor  212 ): 90-500 ml/min   11. Temperature rise of CNT suspension (across inlet and outlet of reactor  212 ): 30-95° C.     12 . Oxidation-reduction potential of metal ions (in vessel  205 ): +1.0 V to −1.0 V   
 
         [0080]    In one particular embodiment of a system  200  and operation according to the present invention of the system  200 :
   1. A solution containing 100 mg/L of CNT and 1% by weight SDBS (sodium dodecybenzenesulfonate) is homogenized for 1 hour in the vessel  205 . The solution is then sonicated in the vessel  205  using a 750 watt probe sonicator at 72% power and one-second pulse for 30-minutes, or until dispersion is achieved.   2. Copper (II) chloride dihydrate is added to the solution in the vessel  205  to make a final concentration of 0.01 gmol/L.   3. The solution is then pumped to and through a reactor  212  and a section of non-metallic tubing in a 1,200 watt microwave using full power at a rate of 210 mL/minute.   4. The nanotubes are then recovered via filtration (e.g. with centrifuge  220 ,  FIG. 3 ) using a 0.8 micron pore size silver filter. They are then dispersed into distilled water via sonication. The solution is then homogenized once more to remove excess surfactant and filtered once more on a silver filter. The tubes are then removed from the filter by sonication in acetone which is then evaporated to leave dry metallized nanotubes.   
 
         [0085]    In one particular embodiment of a system  200  and operation according to the present invention of the system  200 :
   1. A suspension containing 100 mg/L of CNT and 2% by weight SDBS in 30 L of DDIW is mixed at 1500 rpm in the vessel  205  and simultaneously sonicated to a total input energy of 150 J/g of suspension.   2. Copper (II) chloride dihydrate is charged to the suspension in the vessel  205  to achieve a molar ratio of 1 gmol Cu per 1 gmol C.   3. The CNT suspension is then pumped through inert polymer tubing via a gear pump (e.g. pump  210 ) at a rate of 200 mL/min. The CNT solution then passes through the heat exchanger  211  where it is cooled to 10° C.   4. After passing through the heat exchanger  211 , the CNT suspension passes through the reactor  212  operating at 1200 W and 2450 MHz where an oxidation-reduction reaction occurs to deposit non-ionic copper metal onto the surface of the CNT&#39;s, thereby forming M-CNT&#39;s. During this process, the fluid heats to 90° C.   5. The flowing M-CNT suspension then passes through the heat exchanger  213  to remove the heat imparted by the reactor. After cooling, the M-CNT suspension is passed back through the process two additional times (e.g. flow through valve  214  and line  215  back to vessel  205 ; with valve  214  then switched back for flow to line  216 ) to increase the amount of metal deposition.   6. After additional passes through the reactor  212 , aqueous sodium hydroxide is added to adjust the pH and aid filtration. The M-CNT&#39;s are then filtered by the centrifuge  220  equipped with a 0.8 μm pore size filter medium. Multiple centrifuge passes are made to maximize M-CNT capture efficiency, herein cited to be 65% for two passes.   7. The trapped M-CNT&#39;s are then washed in place using DDIW, and spun at high speed by the centrifuge  220  to disengage the cake wash along with substantial surfactant and residual metal salt. The final wet cake contains 88 wt. % moisture.   8. Wet M-CNT&#39;s were then sent to the dryer  226  and dried under vacuum and then a thermogravimetric analysis (TGA) was performed. Results indicated that 45.7 wt. % Cu is deposited on the CNT&#39;s after three reactor passes. Two passes deposits 42.1 wt. % Cu, whereas a single pass deposits 23.6 wt. % Cu. Scanning electric microscopy (SEM) showed that Cu is deposited in irregular clusters on the surface of the CNT&#39;s rather than as a uniform coating of consistent thickness.   
 
         [0094]    Any method and system according to the present invention may be used to deposit metal particles (e.g. typically between one and five nanometers thick) or metal coating (e.g. between one and five nanometers thick) onto nanotubes, graphene sheets, or graphene ribbons. 
         [0095]    In systems and methods according to the present invention, the mechanism for metallization relies primarily on the electrical conductivity of nanotubes, nanoribbons, and graphene sheets. Because of this intrinsic property, nanoparticles function as tiny microwave antennae that absorb microwave energy and convert it into an electrical dipolar charge on the particles thereby enabling an oxidation-reduction reaction to deposit metal. Because different CNT chiralities have differing electrical conductivity, more metal can be deposited on some CNT types than on others. 
         [0096]    A metallization process according to the present invention can, therefore, be used in a separation process to segregate CNT&#39;s by type and/or chirality. For example, density gradient centrifugation after microwave metallization is used to separate denser CNT&#39;s bonded to more metal from less dense CNT&#39;s containing less reduced (deposited) metal. Metallic chiralities can to a certain extent be separated from semiconducting chiralities by using such a method. In such methods the centrifuge  220  is suitable to effect this density gradient centrifugation. 
         [0097]      FIG. 4A  shows a single-walled carbon nanotube  300  with silver particles  302  deposited thereon by a method according to the present invention as in  FIG. 3 . 
         [0098]      FIG. 4B  shows a network  310  of carbon nanotubes  312  with silver particles  304  deposited thereon. 
         [0099]      FIGS. 5A and 5B  show a single-walled carbon nanotube  320  with cubic crystals  322  of silver deposited thereon by a method according to the present invention as in  FIG. 3 . 
         [0100]    Nanotubes like the nanotubes  302  and  320  with metal deposited thereon are denser than nanotubes with no metal deposit. The denser nanotubes with metal deposited thereon can be separated from those with no metal based on this difference in density. In one aspect, a density gradient centrifuge used in a method according to the present invention effects this separation. 
         [0101]      FIG. 6A  presents data in graphic form that shows that using multiple recycling passes through a process according to the present invention (e.g. as in  FIG. 3 ) increases the amount of metal deposited on the target material. The data presented in the graph is from a post-metallization analysis of samples produced by a metallization process according to the present invention using a thermal gravimetric analysis (TGA) of the samples. The horizontal axis of the graph is the temperature in degrees C. used in the TGA analysis in which carbon is burned away to reveal the amount of remaining metal residue and the vertical axis is weight percent of oxidized deposited metal on the nanotubes. The deposited metal in this case was copper. The weight percents indicated on the graph of  FIG. 6A  (29.6% first pass, 52.7% second pass, 57.2% third pass) are based on the weight of copper oxide which is produced when the deposited copper is oxidized by the TGA analysis heating. The illustrated copper oxide weight percents are multiplied by 0.799 to obtain the weight percent of deposited copper. The 29.6% first pass weight percent corresponds to 23.6% deposited copper; the 52.7% second pass weight percent corresponds to 42.1% deposited copper; and the 57.2% third pass weight percent corresponds to 45.7% deposited copper. 
         [0102]    As shown in  FIG. 6A , the incremental amount of metal deposited in a second pass through the process is greater than the incremental amount deposited in a third pass. 
         [0103]      FIG. 6B  illustrates the metallization of multi-walled nanotubes using a process as in  FIG. 3 . For the case illustrated in  FIG. 6B , the deposited metal was copper and the material was multi-walled carbon nanotubes. The metal was deposited using a process as in  FIG. 3  at an energy level of 1200 Watts and a microwave frequency of 2.450 Ghz.  FIG. 6B  presents data in graphic form related to multiple passes of a suspension with metallized material therein through a filtration device for material recovery (e.g., the centrifuge  220 ,  FIG. 3 ) to increase the recovery of metallized material. Line A illustrates the recovery rate for two passes through the device. Line B illustrates the increased recovery rate for three passes through the device. Lines C and D illustrate the split between recovered material (Line C) and separated material forwarded for reprocessing (Line D) after a single centrifuge pass. 
         [0104]      FIG. 7  presents in graphic form data illustrating that silver is deposited using a process as in  FIG. 3  with results as shown in  FIGS. 4A-5B . The large peak to the left indicates the amount of carbon (nanotubes) present. In  FIG. 7 , the vertical axis indicates EDS detection intensity, the number of counts or the intensity of the signal for a specific element and the horizontal axis indicates which elements are present (electron orbitals of specific elements). 
         [0105]      FIGS. 8A-12C  illustrate the deposition of various metals on nanotubes. 
         [0106]      FIGS. 8A-8C  show the deposition of copper onto multi-walled carbon nanotubes by a process as in  FIG. 3 . In the figures, copper (and some copper II oxide formed during metallization or produced by oxidation post-processing) appear as white areas on the surfaces of the nanotubes. The nanotubes shown have a mean diameter of 13 nanometers prior to the metallization process according to the present invention. 
         [0107]      FIGS. 9A and 9B , and  FIGS. 10A and 10B , also show the depositon of copper onto multi-walled carbon nanotubes.  FIG. 9C  presents EDS data for each of  FIGS. 9A and 9B  and shows that copper was deposited on the nanotubes (“Wt %” is the percent of each element present in the sample; “At %” is a similar analysis using as a basis the atomic percent). 
         [0108]      FIGS. 11A-11C  show the deposition of nickel onto multi-walled carbon nanotubes by a process as in  FIG. 3 .  FIG. 11D  presents EDS data for each of  FIGS. 11B and 11C  and shows that nickel was deposited on the nanotubes. 
         [0109]      FIGS. 12A-12C  show the deposition of cobalt onto multi-walled carbon nanotubes by a process as in  FIG. 3 .  FIG. 11D  presents EDS data for each of  FIGS. 12A-12C  and shows that cobalt was deposited on the nanotubes. 
         [0110]      FIG. 13  presents electrical resistivity data for multi-walled carbon nanotubes with nickel deposited thereon using a process as in  FIG. 3 . In this particular aspect, the nanotubes contained 16 weight percent deposited nickel metal. For these data: 
         [0111]    “Resistivity Mean (Ω·cm)” is the mean average resistivity of the samples measured. 
         [0112]    “Resistivity St. Dev (+1−Ω·cm)” is the standard deviation around the mean average resistivity of the samples measured. The nanotubes were contained in a polyurethane-methacrylate matrix at the weight percentages indicated in the left column of the table. The “100.0” entry indicates the data for nickel-metallized nanotubes not in a polyurethane-methacrylate matrix, but rather compacted as a solid into a cylindrical mold. For reference, the resistivity of compacted nanotubes without deposited nickel metal was 1.41E-01Ω cm. These data indicate an increase in electrical resistivity as the amount of metallized nanotubes is increased in the polymer matrix. 
         [0113]    The present invention, therefore, provides in some, but not in necessarily all embodiments a process for metallizing nanomaterial comprising subjecting nanomaterial in a metallizing solution to microwave radiation. In such a process the nanomaterial may be carbon nanomaterial which is one of carbon nanotubes, graphene sheets, and graphene ribbon-like material. 
         [0114]    The present invention, therefore, provides in some, but not in necessarily all embodiments a process for metallizing nanotubes including forming a first solution comprising carbon nanotubes and a metallizing solution; introducing the first solution into a reactor; irradiating the first solution in the reactor with microwave radiation so that metal from the metallizing solution is deposited on the nanotubes, producing a suspension with metallized nanotubes; feeding the suspension to separation apparatus; and, with the separation apparatus, separating metallized nanotubes from the suspension. Such a process may have one or some, or any possible combination, of the following: wherein the nanotubes are carbon nanotubes; wherein the metallizing solution contains a metal salt of the metal to be deposited on nanotubes, solvent, surfactant, and water; mixing the first solution with a mixer under sonication to separate nanotubes; removing heat generated by the mixing of the first solution with a heat exchanger; removing heat from the suspension; at least once feeding part of the suspension separated by the separating apparatus back to the reactor for re-processing to recover further metallized nanotubes; the at least once feeding is a plurality of feedings; feeding metallized nanotubes separated by the separation apparatus to a dryer, and with the dryer, drying the metallized nanotubes; feeding a heated stream to the dryer to facilitate drying; wherein the first solution contains solvent, the process further including separating solvent from the suspension with the separation apparatus; recovering, with a solvent recovery apparatus, solvent separated by the separation apparatus; packaging the metallized nanotubes separated from the suspension; adding a basic solution to the suspension to facilitate separation of metallized nanotubes; wherein the metal is one of (or a combination of any two, three or four of) gold, silver, copper, nickel, cobalt, iron, tin, palladium, and platinum; and/or wherein the nanotubes are carbon nanotubes which are one of single-walled nanotubes, double-walled nanotubes, multi-walled nanotubes, functionalized nanotubes, un-functionalized nanotubes, or any combination of any two, three, or more of these types of nanotubes. 
         [0115]    The present invention, therefore, provides in some, but not in necessarily all embodiments a process for metallizing nanotubes including forming a first solution including carbon nanotubes and a metallizing solution; introducing the first solution into a reactor; irradiating the first solution in the reactor with microwave radiation so that metal from the metallizing solution is deposited on the nanotubes, producing a suspension with metallized nanotubes; feeding the suspension to separation apparatus; with the separation apparatus, separating metallized nanotubes from the suspension; wherein the nanotubes are carbon nanotubes; wherein the metallizing solution contains a metal salt of the metal to be deposited on nanotubes, solvent, surfactant, and water; mixing the first solution with a mixer under sonication to separate nanotubes; removing heat generated by the mixing of the first solution with a heat exchanger; removing heat from the suspension; at least once (or multiple times) feeding part of the suspension separated by the separating apparatus back to the reactor for re-processing to recover further metallized nanotubes; feeding metallized nanotubes separated by the separation apparatus to a dryer; and with the dryer, drying the metallized nanotubes. 
         [0116]    The present invention, therefore, provides in some, but not in necessarily all embodiments a metallized nanomaterial, e.g., but not limited to, metallized carbon nanotubes, metallized by a process according to the present invention, e.g. as in any claim herein. 
         [0117]    The present invention, therefore, provides in some, but not in necessarily all embodiments a method for separating nanotubes of different densities, the method including: introducing a mixture of first nanotubes and second nanotubes into a separation apparatus, the separation apparatus useful for separating materials of different densities; the first nanotubes being metallized nanotubes and having a first density, the second nanotubes having a second density different from the first density; and with the separation apparatus, separating the first nanotubes from the second nanotubes. Such a method may have one or some, or any possible combination, of the following: wherein the separation apparatus is a centrifuge, e.g., but not limited to, an ultra-centrifuge; wherein the first nanotubes have a metallic chirality and the second nanotubes have a semiconducting chirality; wherein the first nanotubes have metal bonded thereto by a metallizing process as any process according to the present invention, e.g., but not limited to, as in claim  1 ; wherein the first nanotubes have metal bonded thereto by a metallizing process as any process according to the present invention, e.g., but not limited to, as in claim  1  and the second nanotubes have metal bonded thereto by a metallizing process as any process according to the present invention, e.g., but not limited tog as in claim  1 ; wherein the first nanotubes are single-walled nanotubes and the second nanotubes are multi-walled nanotubes; wherein the first nanotubes are functionalized and the second nanotubes are non-functionalized; wherein the first nanotubes are functionalized and the second nanotubes are functionalized; and wherein the first nanotubes are carbon nanotubes. 
         [0118]    In conclusion, therefore, it is seen that the present invention and the embodiments disclosed herein and those covered by the appended claims are well adapted to carry out the objectives and obtain the ends set forth. Certain changes can be made in the subject matter without departing from the spirit and the scope of this invention. It is realized that changes are possible within the scope of this invention and it is further intended that each element or step recited in any of the following claims is to be understood as referring to the step literally and/or to all equivalent elements or steps. The following claims are intended to cover the invention as broadly as legally possible in whatever form it may be utilized. The invention claimed herein is new and novel in accordance with 35 U.S.C. §102 and satisfies the conditions for patentability in §102. The invention claimed herein is not obvious in accordance with 35 U.S.C. §103 and satisfies the conditions for patentability in §103. This specification and the claims that follow are in accordance with the requirements of 35 U.S.C. §112. The inventors may rely on the Doctrine of Equivalents to determine and assess the scope of their invention and of the claims that follow as they may pertain to apparatus and/or methods not materially departing from, but outside of, the literal scope of the invention as set forth in the following claims. All patents and applications identified herein are incorporated fully herein for all purposes. It is the express intention of the applicant not to invoke 35 U.S.C. §112, paragraph 6 for any limitations of any of the claims herein, except for those in which the claim expressly uses the words ‘means for’ together with an associated function. In this patent document, the word “comprising” is used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one of the elements. 
         [0119]    Filed on even date herewith, co-owned with the present invention, and fully incorporated herein for all purposes are U.S. patent applications entitled “Ceramic Articles And Methods” and “Armor With Transformed Nanotube Material”.