Patent Publication Number: US-7897876-B2

Title: Carbon-nanotube/graphene-platelet-enhanced, high-conductivity wire

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part application of U.S. patent application Ser. No. 12/348,623 which was filed on Jan. 5, 2009 and titled “THERMOPLASTIC-BASED, CARBON NANOTUBE-ENHANCED, HIGH-CONDUCTIVITY WIRE”, the contents of which is incorporated by reference in its entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH &amp; DEVELOPMENT 
     This invention was made with United States Government support under ATP/NIST Contract 70NANB7H7043 awarded by NIST. The United States Government has certain rights in the invention. 
    
    
     BACKGROUND 
     The field relates generally to fabrication of conductors, and more specifically to conductors that incorporate carbon nanotubes (CNTs) and the methods for fabricating such conductors. 
     Utilization of CNTs in conductors has been attempted. However, the incorporation of carbon nanotubes (CNTs) into polymers at high enough concentrations to achieve the desired conductivity typically increases viscosities of the compound containing the nanotubes to very high levels. The result of such a high viscosity is that conductor fabrication is difficult. A typical example of a high concentration is one percent, by weight, of CNTs mixed with a polymer. 
     Currently, there are no fully developed processes for fabricating wires based on carbon nanotubes, but co-extrusion of CNTs within thermoplastics is being contemplated, either by pre-mixing the CNTs into the thermoplastic or by coating thermoplastic particles with CNTs prior to extrusion. Application of CNTs to films has been shown, but not to wires. 
     Utilization of CNTs with thermosets has also been shown. However, thermosets are crosslinked and cannot be melted at an elevated temperature. Finally, previous methods for dispersion of CNTs onto films have not focused on metallic CNTs in order to maximize current-carrying capability or high conductivity. 
     The above-mentioned proposed methods for fabricating wires that incorporate CNTs will encounter large viscosities, due to the large volume of CNTs compared to the overall volume of CNTs and the polymer into which the CNTs are dispersed. Another issue with such a method is insufficient alignment of the CNTs. Finally, the proposed methods will not produce the desired high concentration of CNTs. 
     BRIEF DESCRIPTION 
     In one aspect, a conductor wire is provided. The conductor includes an aramid fiber and at least one layer attached about the aramid fiber. The at least one layer includes at least one of aligned carbon nanotubes and graphene platelets. 
     In another aspect, a method for fabricating a conductive wire is provided. The method includes aligning at least one of carbon nanotubes and graphene platelets dispersed within a solution, partially dissolving an aramid fiber through chemical treatment, passing the treated aramid fiber through the solution such that a portion of the at least one of carbon nanotubes and graphene platelets aligned and dispersed within the solution adheres to the treated aramid fiber, and washing and drying the fiber. 
     In still another aspect, a method for fabricating a conductor is provided. The method includes partially dissolving an aramid fiber through chemical treatment and adhering at least one of aligned carbon nanotubes and aligned graphene platelets to the partially dissolved aramid fiber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flowchart illustrating conductor fabrication that incorporates carbon nanotubes. 
         FIG. 2  is a cross-sectional diagram further illustrating a conductor  50  fabricated utilizing the process of  FIG. 1 . 
         FIG. 3  is a flow diagram illustrating one mode of application of alternating layers of thermoplastics and carbon nanotubes to fabricate the conductor illustrated in  FIG. 2   
         FIG. 4  is a block diagram that illustrates the individual components and processes utilized in fabricating a carbon nanotube-based conductor. 
         FIG. 5  is a flowchart illustrating conductor fabrication that incorporates carbon nanotubes and/or graphene platelets disposed in layers onto an aramid fiber. 
         FIG. 6  is a flow diagram that further illustrating application of carbon nanotubes and/or graphene platelets onto a substrate. 
         FIG. 7  is a cross-sectional diagram further illustrating a conductor fabricated utilizing the process of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     The described embodiments seek to overcome the limitations of the prior art by placing high volume fractions of carbon nanotubes (CNTs) and/or graphene platelets onto the surface of a lightweight substrate to produce high-conductivity wires. One embodiment uses a continuous process and avoids the processing difficulties associated with dispersion of CNTs within the polymer (or other matrix resin) that may unacceptably raise viscosity of the mixture and make the materials unprocessable before fabrication of the conductor. One result of the described embodiments is a continuous, low-cost method for producing high-conductivity electrical wires containing a high concentration of metallic CNTs, graphene platelets, or a combination of the two, using layer-by-layer (LBL) application. 
     One embodiment, illustrated by the flowchart  10  of  FIG. 1 , includes a method for producing high-conductivity electrical wires based on layer-by-layer coating methodologies and metallic carbon nanotubes (CNTs) to introduce sufficiently high concentrations of CNTs into polymeric materials resulting in a high-conductivity conductor. The focus is on high conductivity combined with high flexibility for electrical conductors instead of focus on high stiffness, high strength, or modest increases in conductivity as were prior layer-by-layer applications. 
     Now referring to the flowchart  10 , a thermoplastic filament, sometimes referred to herein as a substrate, is provided  12 . In one embodiment, a sulfonated thermoplastic layer is applied  14  to the outer surface of the thermoplastic filament. A coating, including CNTs, is then applied  16  to the sulfonated thermoplastic layer. Several alternating layers of sulfonated thermoplastic and the coating may be applied  18  to the thermoplastic filament. The assembly is then melt-processed  20  to form CNT-enhanced, high-conductivity thermoplastic conductor. The melt-processing  20  step bonds the coating to the individual thermoplastic layers. After melt-bonding, an outer coating, such as wire insulation, can be applied to the layered assembly. 
     The process illustrated by the flowchart  10  allows for high volume fractions of aligned carbon nanotubes to be applied to the surface of a thermoplastic to produce high-conductivity wires using a layer-by-layer process. Such a process avoids the necessity for having to mix nanoparticles and/or nanotubes into a matrix resin, since the combination of the two may result in a compound having an unacceptably high viscosity. Continuing, the high viscosity may make processing of the resulting compound difficult. 
       FIG. 2  includes a cross-sectional diagram further illustrating a conductor  50  fabricated utilizing the process of  FIG. 1 . As shown in the cross section of conductor  50 , the thermoplastic filament  60 , or substrate, has a plurality of alternating sulfonated thermoplastic layers  62  and layers  64  that include CNTs therein. The layers  62  and  64  are placed around the circumference of thermoplastic filament  60 . In one specific embodiment, the layers  64  that include the CNTs are processed to include only single-walled nanotubes. While filament  60  is illustrated as being circular in cross-section, the embodiments described herein are operable with any cross-sectional configuration for the filament. 
     Generalizing beyond sulfonization, in layer-by-layer fabrication, layers are applied from solutions generally having different charges. As such, the substrates are chemically prepared for layer-by-layer deposition by appropriately treating the surface, of which sulfonization is one example. 
     The illustrated embodiment shown in  FIG. 2  includes three thermoplastic layers  62  alternating with three CNT embedded layers  64 .  FIG. 3  is a flow diagram  100  the further illustrates the process for fabricating a conductor with the three alternating layers  62 ,  64 . It should be noted that the three-layer configuration is but one example of a conductor, and that fewer or additional alternating layers could be utilized depending on, for example, expense and desired conductivity. Now referring specifically to  FIG. 3 , one or more uncoated filaments  102  are coated  104  with a sulfonated thermoplastic in preparation for application of the CNTs. The CNTs are applied  106 , for example, by passing the thermoplastic coated filaments through a polyvinyl alcohol solution which includes the CNTs. To build up the conductor to the three-layer embodiment, the filaments  102  are alternatively coated  108 ,  112  with the sulfonated thermoplastic and CNTs are applied  110 ,  114  resulting in the conductor  50  illustrated in  FIG. 2 . 
       FIG. 4  is a block diagram  150  that illustrates the individual components utilized in fabricating a carbon nanotube-based conductor. As mentioned herein, coating methodologies are utilized to introduce sufficiently high concentrations of CNTs into polymeric materials for high-conductivity wire which are applied using layer-by-layer coating, as opposed to previously disclosed methods that disclose the mixing of CNTs into a resin. It is believed the currently disclosed solutions are preferable because no current solution exists for making CNT-based wires, though some methods have been proposed, as described above. 
     Now referring specifically to  FIG. 4 , fabrication of the thermoplastic filaments is described. A thermoplastic material  152  is input  154  into an extruder  156  configured to output a thin filament  158  of the thermoplastic material which is gathered, for example, onto a take up spool  160 . 
     In a separate process, a concentrated solution  170  is created that includes, at least in one embodiment, thermoplastic material  172 , a solvent  174 , and carbon nanotubes (CNTs)  176 . The solution  170 , in at least one embodiment, is an appropriate solution of CNTs  176 , solvent  174 , and may include other materials such as surfactants suitable for adhering to the outer surface of thermoplastic filaments. In one embodiment, the solution  170  includes one or more chemicals that de-rope, or de-bundle, the nanotubes, thereby separating single-walled nanotubes from other nanotubes. The solution  170  is further suitable for coating thin, flexible filaments with multiple monolayers of CNTs, for example in a configuration as illustrated by  FIG. 2 , to achieve a desired concentration. In one embodiment, the solution  170  is a portion of the fabrication that is set up for continuous dipping, washing, and drying of individual CNT layers as they are applied to the filament. 
     Continuing, to fabricate the above described conductor, one or more separate creels  180  of individual thermoplastic filaments  158  are passed through a bath  184  of the above described solution  170 . As the filaments  158  pass through the bath  184 , a magnetic field  186  is applied to the solution  170  therein in order to align the carbon nanotubes  176 . In a specific embodiment, which is illustrated, the CNTs  176  that are to be attached to the filaments  158  are the single-walled nanotubes. 
     The magnetic field  186  operates to provide, at least as close as possible, individual carbon nanotubes for layered attachment to the filaments  158 . The magnetic field  186  operates to align CNTs along the principal direction of the filaments. 
     The embodiments represented in  FIG. 4  all relate to a continuous line suitable for coating thin, flexible, polymeric strands (filaments  152 ) with a layer of the CNT solution  170  at a sufficient thickness to achieve a desired concentration or conductivity. The magnetic field  186 , which may be the result of an electric field, is utilized to align the CNTs  176  in the solution  170  into the same direction as the processing represented in the Figure. 
     In one embodiment, the filaments  158  emerge from the solution  170  as coated strands  190  which are then washed and subsequently gathered onto spools  192  for post-processing. As shown in  FIG. 4 , the coated strands  190  may be subjected to a repeatable process. For example, to fabricate the multiple conductive layers as shown in  FIG. 2 , the filaments  158  are passed through the solution  170  and subsequently washed as many times as needed to create the number of monolayers of CNTs to create, for example, the desired conductivity. Finally, though not shown in  FIG. 4 , a suitable, flexible outer coating may be applied to the coated strands  190  and subsequently packaged in a fashion similar to that used for metallic wire. 
       FIGS. 5 ,  6 , and  7  are directed to embodiments that do not rely on having a thermoplastic carrier as described above. Instead, carbon nanotubes and/or graphene platelets are deposited in monolayers onto an aramid substrate. Specific embodiments utilize only high-conductivity, single-walled, metallic carbon nanotubes to maximize electrical performance and therefore rely on solutions that contain specific highly conductive carbon nanotubes and/or graphene platelets instead of mixtures of several types of carbon nanotubes. Mixtures of several types of carbon nanotubes may lead to a degradation of electrical performance or may be used to reach specific levels of electrical performance. In such embodiments, concentration levels of carbon nanotubes and/or graphene platelets are optimized for wire, not for films or sheets, and therefore high strength and high stiffness are not generally desirable. 
     In mentioned in the preceding paragraph, the process incorporates layer-by-layer coating, which includes the introduction of sufficiently high concentrations of carbon nanotubes and/or graphene platelets into a solution that includes polymeric materials for layer-by-layer fabrication of high-conductivity wire as opposed to the mixing of carbon nanotubes and/or platelets into a resin. 
     The embodiments described in the preceding paragraphs are further illustrated by the flowchart  300  of  FIG. 5 , which is an illustration of a method for producing high-conductivity electrical wires based on layer-by-layer coating. In the method, metallic carbon nanotubes (CNTs), and graphene platelets are utilized to provide sufficiently high concentrations of CNTs and/or platelets into polymeric materials and onto an aramid resulting in a high-conductivity conductor. The focus is on high conductivity combined with high flexibility for electrical conductors instead of a focus on high stiffness, high strength, or modest increases in conductivity as were the case in prior layer-by-layer applications. 
     Now referring to the flowchart  300 , a solution of CNTs and/or graphene platelets are provided  302  in a solution that includes one or more solvents and one or more polymers. An aramid fiber is also provided, the fiber being partially dissolved  304  using, for example, a chemical treatment. The CNTs and/or graphene platelets are aligned  306  within the solution along an axis. Aligning the CNTs and/or platelets in the solution in the same direction as the fiber passes through the solution is accomplished, for example, using one or more of a magnetic field, an electric field or another alignment process. 
     The aramid fiber is then passed  308  through the solution along an axis that is substantially collinear with the nanotube/platelet alignment axis such that a portion of the aligned CNTs and/or graphene platelets attach to the partially dissolved aramid fiber. The fiber containing the aligned CNTs and/or graphene platelets is than rinsed  310  and dried  312 . If the rinsed  310  and dried  312  fiber includes  314  the desired quantity of CNTs and/or graphene platelets, the process ends  316 . Otherwise, the passing  308  through solution, rinsing  310 , and drying  312  steps are repeated until the desired number of layers have been added to the aramid fiber or the desired quantity of CNTs and/or graphene platelets are attached to the fiber. After fabrication, an outer coating, such as wire insulation, may be applied to the layered assembly and the assembly gathered, for example, onto a take-up spool. Alternatively, the coated strands may be collected on to spools for post-processing into wire or the twisting of multiple strands into wire may be performed in line after the layer-by-layer processing. Other processing may include the twisting of multiple coated strands. 
     The process illustrated by the flowchart  300  allows for high volume fractions of aligned carbon nanotubes and/or graphene platelets to be applied to an aramid fiber to produce high-conductivity wires using layer-by-layer fabrication. Such a fabrication process avoids the necessity for having to mix nanoparticles and/or nanotubes into a matrix resin, as described above. 
       FIG. 6  is a flow diagram  350  that further illustrates application of layers of CNTs and/or graphene platelets onto a substrate  360 , for example, a fiber formed using an aramid. This substrate  360  is passed through a first solution  370 , rinsed and dried, then passed through a second solution  380  and again rinsed and dried. The process is repeated as necessary. In the illustrated embodiment, the first solution  370  that includes the CNTs and/or graphene platelets is a cationic solution and the second solution  380  including CNTs and/or graphene platelets is an anionic solution. 
       FIG. 7  includes a cross-sectional diagram further illustrating a conductor  400  fabricated utilizing the process of  FIGS. 5 and 6 . As shown in the cross section of conductor  400 , the aramid fiber  360 , or substrate, has a plurality of layers  402  attached thereto. The layers  402  are denoted to indicate which is associated with the cationic solution and which is associated with the anionic solution, though it is possible to have layers all generated from a single one of the solutions. These layers  402  include the CNTs and/or the graphene platelets therein. The layers  402  are placed around the circumference of fiber  360  and are attached thereto in part due to the dissolving process that makes the aramid amenable to the attachment of such nanoparticles. In one specific embodiment, the layers  402  that include the CNTs are processed to include only single-walled nanotubes. While fiber  360  is illustrated as being circular in cross-section, the embodiments described herein are operable with any cross-sectional configuration for this substrate. 
     The illustrated embodiment shown in  FIG. 7  includes three CNT/graphene platelet layers  402  originating about substrate  360 . It should be noted that the three-layer configuration is but one example of a conductor, and that fewer or additional alternating layers could be utilized depending on, for example, expense and desired conductivity. As mentioned elsewhere herein, the CNTs and/or graphene platelets are applied, for example, by passing the aramid fiber through a concentrated solution that includes, at least in one embodiment, thermoplastic material, a solvent, and carbon nanotubes and/or graphene platelets. In one embodiment, the solution is used as a portion of the fabrication and is set up for continuous dipping, washing, and drying of individual CNT/platelet layers as they are applied to the substrate. 
     In one embodiment, and as described above, the solution includes one or more chemicals that de-rope, or de-bundle, the nanotubes and/or platelets into as close to individual particles as possible, thereby separating individual nanotubes from other nanotubes. The de-bundled CNTs/platelets may be separated into different types, for example via centrifugation, and the metallic CNTs with “armchair” configuration (having the hexagonal crystalline carbon structure aligned along the length of the tube) are extracted as the CNTs configured in this fashion have the highest conductivity. Further processing allows for these “armchair”-configured CNTs to be predominately, or substantially exclusively in the fabrication of the conductor. Similarly, the highest-conductivity graphene platelets are isolated. These processes are generally done in separate operations from the layer-by-layer deposition described herein. 
     The described embodiments do not rely on dispersing CNTs into a resin as described by the prior art. Instead, layers of CNTs are placed about the circumference of small-diameter thermoplastic filaments or aramid fibers as described above. Specific embodiments utilize only high-conductivity, single-walled, metallic CNTs to maximize electrical performance. Such an embodiment relies on very pure solutions of specific CNTs instead of mixtures of several types to ensure improved electrical performance. The concentrations levels of CNTs to coating are optimized for conductivity, in all embodiments, as opposed to concentrations that might be utilized with, or dispersed on, films, sheets and other substrates. 
     This written description uses examples to disclose certain embodiments, including the best mode, and also to enable any person skilled in the art to practice those embodiments, including making and using any devices or systems and performing any incorporated methods. The patentable scope is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.