Abstract:
A conductive wire includes a plurality of thermoplastic filaments each having a surface, and a coating material having a plurality of carbon nanotubes dispersed therein. The coating material is bonded to the surface of each thermoplastic filament. The thermoplastic filaments having the coating bonded thereto are bundled and bonded to each other to form a substantially cylindrical conductor.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH &amp; DEVELOPMENT 
       [0001]    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 
       [0002]    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. 
         [0003]    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. 
         [0004]    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. 
         [0005]    Utilization of CNTs with thermosets has also been shown. However, thermosets are cross-linked and cannot be melted at an elevated temperature. Finally, previous methods for dispersion of CNTs onto films did not focus on metallic CNTs in order to maximize current-carrying capability or high conductivity. 
         [0006]    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 
       [0007]    In one aspect, a conductive wire is provided. The wire includes a plurality of thermoplastic filaments each comprising a surface, and a coating material having a plurality of carbon nanotubes dispersed therein. The coating material is bonded to the surface of each thermoplastic filament. The thermoplastic filaments are bundled and bonded to each other to form a substantially cylindrical conductor. 
         [0008]    In another aspect, a method for fabricating a conductive polymer is provided. The method includes providing a plurality of thermoplastic filaments, applying a coating material to a surface of the filaments, along an axial length thereof, the coating material including carbon nanotubes dispersed therein, and melt-processing the coated filaments to bond the coating to the filaments. 
         [0009]    In still another aspect, a method for fabricating a conductor is provided. The method includes applying a coating material that includes magnetically aligned carbon nanotubes to a plurality of thermoplastic filaments and heating the coated filaments to bond the coating material to the filaments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a flowchart illustrating a conductor fabrication process that incorporates carbon nanotubes. 
           [0011]      FIG. 2  is a series of cross-sectional diagrams further illustrating a conductor fabricated utilizing the process of  FIG. 1 . 
           [0012]      FIG. 3  is a block diagram that illustrates the individual components utilized in fabricating a carbon nanotube-based conductor. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    The described embodiments seek to overcome the limitations of the prior art by placing carbon nanotubes (CNTs) on the outside of a polymer-based structure or other desired substrate to avoid the processing difficulties associated with dispersion of CNTs within the polymer before the structure is fabricated. 
         [0014]    One embodiment, illustrated by the flowchart  10  of  FIG. 1 , includes a method for producing high-conductivity electrical wires based on thermoplastics and metallic carbon nanotubes (CNTs). First, a plurality of continuous, thermoplastic, filaments are provided  12 . A coating is applied  14  to the outer surface of the fine, continuous thermoplastic filaments. The coating includes the CNTs. The coated filaments are then melt-processed  16  to form CNT-enhanced, high-conductivity thermoplastic wires. The melt-processing  16  steps include bonding the coating to the individual filaments and bonding the filaments together into a bundle onto which an outer coating, such as wire insulation, can be applied. 
         [0015]    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 continuous 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. 
         [0016]      FIG. 2  includes a series of cross-sectional diagrams further illustrating a conductor fabricated utilizing the process of  FIG. 1 . A plurality of individual, uncoated, thermoplastic filaments  50  are provided. Through coating, one method of which is further explained with respect to  FIG. 3 , the individual filaments  50  are coated with an outside layer  52  that includes the carbon nanotubes. The coated filaments  50  are then subjected to heating that bonds the coating  52  to the filaments  50  and further results in a bonding of the filaments  50  in a carbon nanotube-based conductor  60 . 
         [0017]    The described embodiments do not rely on dispersing CNTs into a resin as described by the prior art. Instead, CNTs are placed on the outside of small-diameter thermoplastic wires as described above. One specific embodiment utilizes 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 for coating are optimized for wire, in all embodiments, as opposed to concentrations that might be utilized with, or dispersed on, films, sheets and other substrates. Specifically, in a wire-like application, high strength is not required and high stiffness is not desirable. 
         [0018]      FIG. 3  is a block diagram  100  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 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. 
         [0019]    Now referring specifically to  FIG. 3 , fabrication of the thermoplastic filaments is described. A thermoplastic material  102  is input  104  into an extruder  106  configured to output a thin filament  108  of the thermoplastic material which is gathered, for example, onto a take up spool  110 . 
         [0020]    In a separate process, a solution  130  is created that includes, at least in one embodiment, thermoplastic material  132 , a solvent  134 , and carbon nanotubes (CNTs)  136 . The solution  130 , in at least one embodiment, is an appropriate solution of CNTs  136 , solvent  134 , and may include other materials such as surfactants suitable for adhering to the outer surface of the small-diameter thermoplastic filaments. In one embodiment, the solution  130  includes one or more chemicals that de-rope, or de-bundle, the nanotubes, thereby separating single-walled nanotubes from other nanotubes. 
         [0021]    To fabricate the above described conductor, separate creels  150  of individual thermoplastic filaments  108  are passed through a bath  154  of the above described solution  130 . As the filaments  108  pass through the bath  154 , a magnetic field  156  is applied to the solution  130  therein in order to align the carbon nanotubes  136 . In a specific embodiment, which is illustrated, the CNTs  136  are single-walled nanotubes. 
         [0022]    The magnetic field  156  operates to provide, at least as close as possible, individual carbon nanotubes for attachment to the filaments  108 . The magnetic field  156  operates to separate the de-bundled CNTs into different types and works to extract metallic CNTs that have an “armchair” configuration, which refers to the CNT having a hexagonal crystalline carbon structure aligned along the length of the CNT. Such CNTs have the highest conductivity. 
         [0023]    The embodiments represented in  FIG. 3  all relate to a continuous line suitable for coating thin, flexible, polymeric strands (filaments  108 ) with a layer of the CNT solution  130  at a sufficient thickness to achieve a desired concentration or conductivity. The magnetic field  156 , which may be the result of an electric field, is utilized to align the CNTs  136  in the solution  130  into the same direction as the processing represented in the Figure. 
         [0024]    In one embodiment, the filaments  108  emerge from the solution  130  as coated strands  170  that may be gathered onto spools for post-processing into wire via a secondary thermoforming process. Alternatively, and as shown in  FIG. 3 , the coated strands  170  may be subjected to heating, for example, in a heated die  180  to make material suitable for twisting into wire  190 . Finally, though not shown in  FIG. 3 , a suitable, flexible outer coating may be applied to the wire  190  and subsequently packaged in a fashion similar to that used for metallic wire. 
         [0025]    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.