Abstract:
A carbon nanotube-infused fiber and a method for its production are disclosed. Nanotubes are synthesized directly on a parent fiber by first applying a catalyst to the fiber. The properties of the carbon nanotube-infused fiber will be a combination of those of the parent fiber as well as those of the infused carbon nanotubes.

Description:
FIELD OF THE INVENTION 
     The present invention relates to carbon nanotubes and fibers. 
     BACKGROUND OF THE INVENTION 
     Fibers are used for many different applications in a wide variety of industries, such as the commercial aviation, recreation, industrial and transportation industries. Commonly-used fibers for these and other applications include cellulosic fiber (e.g., viscose rayon, cotton, etc.), glass fiber, carbon fiber, and aramid fiber, to name just a few. 
     In many fiber-containing products, the fibers are present in the form of a composite material (e.g., fiberglass, etc.). A composite material is a heterogeneous combination of two or more constituents that differ in form or composition on a macroscopic scale. While the composite material exhibits characteristics that neither constituent alone possesses, the constituents retain their unique physical and chemical identities within the composite. 
     Two key constituents of a composite include a reinforcing agent and a resin matrix. In a fiber-based composite, the fibers are the reinforcing agent. The resin matrix keeps the fibers in a desired location and orientation and also serves as a load-transfer medium between fibers within the composite. 
     Fibers are characterized by certain properties, such as mechanical strength, density, electrical resistivity, thermal conductivity, etc. The fibers “lend” their characteristic properties, in particular their strength-related properties, to the composite. Fibers therefore play an important role in determining a composite&#39;s suitability for a given application. 
     To realize the benefit of fiber properties in a composite, there must be a good interface between the fibers and the matrix. This is achieved through the use of a surface coating, typically referred to as “sizing.” The sizing provides an all important physico-chemical link between fiber and the resin matrix and thus has a significant impact on the mechanical and chemical properties of the composite. The sizing is applied to fibers during their manufacture. 
     Substantially all conventional sizing has lower interfacial strength than the fibers to which it&#39;s applied. As a consequence, the strength of the sizing and its ability to withstand interfacial stress ultimately determines the strength of the overall composite. In other words, using conventional sizing, the resulting composite cannot have a strength that is equal to or greater than that of the fiber. 
     SUMMARY OF THE INVENTION 
     The illustrative embodiment of the present invention is a carbon nanotube-infused (“CNT-infused”) fiber. 
     In CNT-infused fiber disclosed herein, the carbon nanotubes are “infused” to the parent fiber. As used herein, the term “infused” means physically or chemically bonded and “infusion” means the process of physically or chemically bonding. The physical bond between the carbon nanotubes and parent fiber is believed to be due, at least in part, to van der Waals forces. The chemical bond between the carbon nanotubes and the parent fiber is believed to be a covalent bond. 
     Regardless of its true nature, the bond that is formed between the carbon nanotubes and the parent fiber is quite robust and is responsible for CNT-infused fiber being able to exhibit or express carbon nanotube properties or characteristics. This is in stark contrast to some prior-art processes, wherein nanotubes are suspended/dispersed in a solvent solution and applied, by hand, to fiber. Because of the strong van der Waals attraction between the already-formed carbon nanotubes, it is extremely difficult to separate them to apply them directly to the fiber. As a consequence, the lumped nanotubes weakly adhere to the fiber and their characteristic nanotube properties are weakly expressed, if at all. 
     The infused carbon nanotubes disclosed herein effectively function as a replacement for conventional “sizing.” It has been found that infused carbon nanotubes are far more robust molecularly and from a physical properties perspective than conventional sizing materials. Furthermore, the infused carbon nanotubes improve the fiber-to-matrix interface in composite materials and, more generally, improve fiber-to-fiber interfaces. 
     The CNT-infused fiber disclosed herein is itself similar to a composite material in the sense that its properties will be a combination of those of the parent fiber as well as those of the infused carbon nanotubes. Consequently, embodiments of the present invention provide a way to impart desired properties to a fiber that otherwise lacks such properties or possesses them in insufficient measure. Fibers can therefore be tailored or engineered to meet the requirements of a specific application. In this fashion, the utility and value of virtually any type of fiber can be improved. 
     In accordance with the illustrative embodiment of a CNT-infused fiber-forming process, nanotubes are synthesized in place on the parent fiber itself. It is important that the carbon nanotubes are synthesized on the parent fiber. If not, the carbon nanotubes will become highly entangled and infusion does not occur. As seen from the prior art, non-infused carbon nanotubes impart little if any of their characteristic properties. 
     The parent fiber can be any of a variety of different types of fibers, including, without limitation: carbon fiber, graphite fiber, metallic fiber (e.g., steel, aluminum, etc.), ceramic fiber, metallic-ceramic fiber, glass fiber, cellulosic fiber, aramid fiber. 
     In the illustrative embodiment, nanotubes are synthesized on the parent fiber by applying or infusing a nanotube-forming catalyst, such as iron, nickel, cobalt, or a combination thereof, to the fiber. 
     In some embodiments, operations of the CNT-infusion process include: 
     Removing sizing from the parent fiber; 
     Applying nanotube-forming catalyst to the parent fiber; 
     Heating the fiber to nanotube-synthesis temperature; and 
     Spraying carbon plasma onto the catalyst-laden parent fiber. 
     In some embodiments, the infused carbon nanotubes are single-wall nanotubes. In some other embodiments, the infused carbon nanotubes are multi-wall nanotubes. In some further embodiments, the infused carbon nanotubes are a combination of single-wall and multi-wall nanotubes. There are some differences in the characteristic properties of single-wall and multi-wall nanotubes that, for some end uses of the fiber, dictate the synthesis of one or the other type of nanotube. For example, single-walled nanotubes can be excellent conductors of electricity while multi-walled nanotubes are not. 
     Methods and techniques for forming carbon nanotubes, as disclosed in co-pending U.S. patent application Ser. No. 10/455,767 (Publication No. US 2004/0245088) and which is incorporated herein by reference, can be adapted for use with the process described herein. In the illustrative embodiment, acetylene gas is ionized to create a jet of cold carbon plasma. The plasma is directed toward the catalyst-bearing parent fiber. 
     As previously indicated, carbon nanotubes lend their characteristic properties (e.g., exceptional mechanical strength, low to moderate electrical resistivity, high thermal conductivity, etc.) to the CNT-infused fiber. The extent to which the resulting CNT-infused fiber expresses these characteristics is a function of the extent and density of coverage of the parent fiber by the carbon nanotubes. 
     In a variation of the illustrative embodiment, CNT infusion is used to provide an improved filament winding process. In this variation, carbon nanotubes are formed on fibers (e.g., graphite tow, glass roving, etc.), as described above, and are then passed through a resin bath to produce resin-impregnated, CNT-infused fiber. After resin impregnation, the fiber is positioned on the surface of a rotating mandrel by a delivery head. The fiber then winds onto the mandrel in a precise geometric pattern in known fashion. 
     The filament winding process described above provides pipes, tubes, or other forms as are characteristically produced via a male mold. But the forms made from the filament winding process disclosed herein differ from those produced via conventional filament winding processes. Specifically, in the process disclosed herein, the forms are made from composite materials that include CNT-infused fibers. Such forms will therefore benefit from enhanced strength, etc., as provided by the CNT-infused fibers. 
     Any of a variety of different parent fibers can be used to form CNT-infused fiber, 
     Of late, there has been a demand for carbon fiber forms that are compatible with a broad range of resins and processes. And the sizing material is an important determinant of this compatibility. For example, sizing is critically important for providing an even distribution of chopped carbon fiber in sheet molding compounds (“SMCs”), such as are used in some automotive body panels. 
     Notwithstanding this demand for carbon fiber and its potentially broad applicability, carbon fiber has historically been sized for compatibility with only epoxy resin. CNT-infused carbon fiber, as produced according to the method disclosed herein, addresses this problem by providing a fiber that is sized with infused nanotubes, which provides the desired broad applicability with a variety of resins and processes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a method for producing CNT-infused fiber in accordance with the illustrative embodiment of the present invention. 
         FIG. 2  depicts a system for implementing the illustrative method for producing CNT-infused fiber. 
         FIG. 3  depicts a system for filament winding in accordance with a variation of the illustrative embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following terms are defined for use in this Specification, including the appended claims: 
     Carding—The process by which the fibers are opened out into an even film. 
     Carded Fibers—Fibers that have been carded which opens them up. 
     Cloth—A reinforcement material made by weaving strands of fiber yarns. 
     Continuous Filament Strand—A fiber bundle composed of many filaments. Also, when referring to gun roving; a collection of string-like fiber or yarn, which is fed through a chopper gun in a spray-up process. 
     Continuous Strand Roving—A bundle of filaments which are fed through a chopper gun in a spray-up process. 
     Fabric—A planar textile structure produced by interlacing yarns, fibers, or filaments. 
     Fiber—A unit of matter, either natural, or manufactured, which forms the basic element of fabrics and other textile structures. 
     Fiber orientation—Fiber alignment in a non-woven or a mat laminate where the majority of fibers are in the same direction, resulting in a higher strength in that direction. 
     Fiber Pattern—Visible fibers on the surface of laminates or moldings; the thread size and weave of glass cloth. 
     Filament—A single fiber of an indefinite or extreme length, either natural (e.g., silk, etc.) or manufactured. Typically microns in diameter, manufactured fibers are extruded into filaments that are converted into filament yarn, staple, or tow. 
     Filament Winding—A process which involves winding a resin-saturated strand of glass filament around a rotating mandrel. 
     Filament Yarn—A yarn composed of continuous filaments assembled with, or without twist. 
     Infuse—To form a chemical bond. 
     Male Mold—A convex mold where the concave surface of the part is precisely defined by the mold surface. 
     Matrix—The liquid component of a composite or laminate. 
     Mandrel—The core around which paper-, fabric-, or resin-impregnated fiber is wound to form pipes, tubes, or vessels; in extrusion, the central finger of a pipe or tubing die. 
     Pultrusion—Reversed “extrusion” of resin-impregnated roving in the manufacture of rods, tubes and structural shapes of a permanent cross-section. The roving, after passing through the resin dip tank, is drawn through a die to form the desired cross-section. 
     Resin—A liquid polymer that, when catalyzed, cures to a solid state. 
     Roving—The soft strand of carded fiber that has been twisted, attenuated, and freed of foreign matter preparatory to spinning. 
     Sizing—A surface treatment that is applied to filaments immediately after their formation for the purpose of promoting good adhesion between those filaments and the matrix, to the extent the filaments are to be used as the reinforcing agent in a composite material. 
     Spray-up—The process of spraying fibers, resin and catalyst simultaneously into a mold using a chopper gun. 
     Strands—A primary bundle of continuous filaments (or slivers) combined in a single compact unit without twist. These filaments (usually 51, 102 or 204) are gathered together in the forming operations. 
     Tape—a narrow-width reinforcing fabric or mat. 
     Tow—a loose strand of filaments without twist. 
     Twist—A term that applies to the number of turns and the direction that two yarns are turned during the manufacturing process. 
     Woven Roving Fabric—Heavy fabrics woven from continuous filament in roving form. Usually in weights between 18-30 oz. per square yard. 
     Yarn—A generic term for a continuous strand of textile fibers, filaments, or material in a form suitable for knitting, weaving, braiding, or otherwise intertwining to form a textile fabric. 
     As the definitions that are provided above indicate, terms such as “fiber,” “filament,” “yarn,” etc., have distinct meanings. But for the purposes of the specification and the appended claims, and unless otherwise indicated, the term “fiber” is used in this specification as a generic term to refer to filament, yarn, tow, roving, fabric, etc., as well as fiber itself. The phrase “CNT-infused fiber” is therefore understood to encompass “CNT-infused fiber,” “CNT-infused filament,” “CNT-infused tow,” “CNT-infused roving,” etc. 
       FIG. 1  depicts a flow diagram of process  100  for producing CNT-infused fiber in accordance with the illustrative embodiment of the present invention. 
     Process  100  includes the operations of:
           102 : Applying nanotube-forming catalyst to the parent fiber.     104 : Heating the parent fiber to a temperature that is sufficient for carbon nanotube synthesis.     106 : Spraying carbon plasma onto the catalyst-laden parent fiber.       

     To infuse carbon nanotubes into a parent fiber, the carbon nanotubes are synthesized directly on the parent fiber. In the illustrative embodiment, this is accomplished by disposing nanotube-forming catalyst on the parent fiber, as per operation  102 . Suitable catalysts for carbon nanotube formation include, without limitation, transition metal catalysts (e.g., iron, nickel, cobalt, combinations thereof, etc.). 
     As described further in conjunction with  FIG. 2 , the catalyst is prepared as a liquid solution that contains nano-sized particles of catalyst. The diameters of the synthesized nanotubes are related to the size of the metal particles. 
     In the illustrative embodiment, carbon nanotube synthesis is based on a plasma-enhanced chemical vapor deposition process and occurs at elevated temperatures. The temperature is a function of catalyst, but will typically be in a range of about 500 to 1000° C. Accordingly, operation  104  requires heating the parent fiber to a temperature in the aforementioned range to support carbon nanotube synthesis. 
     In operation  106 , carbon plasma is sprayed onto the catalyst-laden parent fiber. The plasma can be generated, for example, by passing a carbon containing gas (e.g., acetylene, ethylene, ethanol, etc.) through an electric field that is capable of ionizing the gas. 
     Nanotubes grow at the sites of the metal catalyst. The presence of the strong plasma-creating electric field can affect nanotube growth. That is, the growth tends to follow the direction of the electric field. By properly adjusting the geometry of the plasma spray and electric field, vertically-aligned carbon nanotubes (i.e., perpendicular to the fiber) are synthesized. Under certain conditions, even in the absence of a plasma, closely-spaced nanotubes will maintain a vertical growth direction resulting in a dense array of tubes resembling a carpet or forest. 
       FIG. 2  depicts system  200  for producing CNT-infused fiber in accordance with the illustrative embodiment of the present invention. System  200  includes fiber payout and tensioner station  202 , fiber spreader station  208 , sizing removal station  210 , CNT-infusion station  212 , fiber bundler station  222 , and fiber uptake bobbin  224 , interrelated as shown. 
     Payout and tension station  202  includes payout bobbin  204  and tensioner  206 . The payout bobbin delivers fiber  201  to the process; the fiber is tensioned via tensioner  206 . 
     Fiber  201  is delivered to fiber spreader station  208 . The fiber spreader separates the individual elements of the fiber. Various techniques and apparatuses can be used to spread fiber, such as pulling the fiber over and under flat, uniform-diameter bars, or over and under variable-diameter bars, or over bars with radially-expanding grooves and a kneading roller, over a vibratory bar, etc. Spreading the fiber enhances the effectiveness of downstream operations, such as catalyst application and plasma application, by exposing more fiber surface area. 
     Payout and tension station  202  and fiber spreader station  208  are routinely used in the fiber industry; those skilled in the art will be familiar with their design and use. 
     Fiber  201  then travels to sizing removal station  210 . At this station, any “sizing” that is on fiber  201  is removed. Typically, removal is accomplished by burning the sizing off of the fiber. 
     Any of a variety of heating means can be used for this purpose, including, without limitation, an infrared heater, a muffle furnace, etc. Generally, non-contact heating methods are preferred. In some alternative embodiments, sizing removal is accomplished chemically. 
     The temperature and time required for burning off the sizing vary as a function of (1) the sizing material (e.g., silane, etc.); and (2) the identity of parent fiber  201  (e.g., glass, cellulosic, carbon, etc.). Typically, the burn-off temperature is a minimum of about 650° C. At this temperature, it can take as long as 15 minutes to ensure a complete burn off of the sizing. Increasing the temperature above a minimum burn temperature should reduce burn-off time. Thermogravimetric analysis can be used to determine minimum burn-off temperature for sizing. 
     In any case, sizing removal is the slow step in the overall CNT-infusion process. For this reason, in some embodiments, a sizing removal station is not included in the CNT-infusion process proper; rather, removal is performed separately (e.g., in parallel, etc.). In this way, an inventory of sizing-free fiber can be accumulated and spooled for use in a CNT-infused fiber production line that does not include a fiber removal station). In such embodiments, sizing-free fiber is spooled in payout and tension station  202 . This production line can be operated at higher speed than one that includes sizing removal. 
     Sizing-free fiber  205  is delivered to CNT-infusion station  212 , which is the “heart” of the process and system depicted in  FIG. 2 . Station  212  includes catalyst application station  214 , fiber pre-heater station  216 , plasma spray station  218 , and fiber heaters  220 . 
     As depicted in  FIG. 2 , sizing-free fiber  205  proceeds first to catalyst application station  214 . In some embodiments, fiber  205  is cooled prior to catalyst application. 
     In some embodiments, the nanotube-forming catalyst is a liquid solution of nanometer-sized particles (e.g.,  10  nanometers in diameter, etc.) of a transition metal. Typical transition metals for use in synthesizing nanotubes include, without limitation, iron, iron oxide, cobalt, nickel, or combinations thereof. These transition metal catalysts are readily commercially available from a variety of suppliers, including Ferrotech of Nashua, N.H. The liquid is a solvent such as toluene, etc. 
     In the illustrative embodiment, the catalyst solution is sprayed, such as by air sprayer  214 , onto fiber  205 . In some other embodiments, the transition metal catalyst is deposited on the parent fiber using evaporation techniques, electrolytic deposition techniques, suspension dipping techniques and other methods known to those skilled in the art. In some further embodiments, the transition metal catalyst is added to the plasma feedstock gas as a metal organic, metal salt or other composition promoting gas phase transport. The catalyst can be applied at room temperature in the ambient environment (neither vacuum nor an inert atmosphere is required). 
     Catalyst-laden fiber  207  is then heated at fiber preheater station  216 . For the infusion process, the fiber should be heated until it softens. Generally, a good estimate of the softening temperature for any particular fiber is readily obtained from reference sources, as is known to those skilled in the art. To the extent that this temperature is not a priori known for a particular fiber, it can be readily determined by experimentation. The fiber is typically heated to a temperature that is in the range of about 500 to 1000° C. Any of a variety of heating elements can be used as the fiber preheater, such as, without limitation, infrared heaters, a muffle furnace, and the like. 
     After preheating, fiber  207  is finally advanced to plasma spray station having spray nozzles  218 . A carbon plasma is generated, for example, by passing a carbon containing gas (e.g., acetylene, ethylene, ethanol, etc.) through an electric field that is capable of ionizing the gas. This cold carbon plasma is directed, via spray nozzles  218 , to fiber  207 . The fiber is disposed within about 1 centimeter of the spray nozzles to receive the plasma. In some embodiments, heaters  220  are disposed above fiber  207  at the plasma sprayers to maintain the elevated temperature of the fiber. 
     After CNT-infusion, CNT-infused fiber  209  is re-bundled at fiber bundler  222 . This operation recombines the individual strands of the fiber, effectively reversing the spreading operation that was conducted at station  208 . 
     The bundled, CNT-infused fiber  209  is wound about uptake fiber bobbin  224  for storage. CNT-infused fiber  209  is then ready for use in any of a variety of applications, including, without limitation, for use as the reinforcing material in composite materials. 
     It is noteworthy that some of the operations described above should be conducted under inert atmosphere or vacuum, such that environmental isolation is required. For example, if sizing is being burned off of the fiber, the fiber must be environmentally isolated to contain off-gassing and prevent oxidation. Furthermore, the infusion process should be conducted under an inert atmosphere (e.g., nitrogen, argon, etc.) to prevent oxidation of the carbon. For convenience, in some embodiments of system  200 , environmental isolation is provided for all operations, with the exception of fiber payout and tensioning (at the beginning of the production line) and fiber uptake (at the end of the production line). 
       FIG. 3  depicts a further embodiment of the invention wherein CNT-infused fiber is created as a sub-operation of a filament winding process being conducted via filament winding system  300 . 
     System  300  comprises fiber creel  302 , carbon nanotube infusion section  226 , resin bath  328 , and filament winding mandrel  332 , interrelated as shown. The various elements of system  300 , with the exception of carbon nanotube infusion section  226 , are present in conventional filament winding processes. Again, the “heart” of the process and system depicted in  FIG. 3  is the carbon nanotube infusion section  226 , which includes fiber spreader station  208 , (optional) sizing-removal station  210 , and CNT-infusion station  212 . 
     Fiber creel  302  includes plural spools  204  of parent fiber  201 A through  201 H. The untwisted group of fibers  201 A through  201 H is referred to collectively as “tow  303 .” Note that the term “tow” generally refers to a group of graphite fibers and the term “roving” usually refers to glass fibers. Here, the term “tow” is meant to refer, generically, to any type of fiber. 
     In the illustrative embodiment, creel  302  holds spools  204  in a horizontal orientation. The fiber from each spool  206  moves through small, appropriately situated rollers/tensioners  206  that change the direction of the fibers as they move out of creel  302  and toward carbon nanotube infusion section  226 . 
     It is understood that in some alternative embodiments, the spooled fiber that is used in system  300  is CNT-infused fiber (i.e., produced via system  200 ). In such embodiments, system  300  is operated without nanotube infusion section  226 . 
     In carbon nanotube infusion section  226 , tow  303  is spread, sizing is removed, nanotube-forming catalyst is applied, the tow is heated, and carbon plasma is sprayed on the fiber, as described in conjunction with  FIG. 2 . 
     After passing through nanotube infusion section  226 , CNT-infused tow  307  is delivered to resin bath  328 . The resin bath contains resin for the production of a composite material comprising the CNT-infused fiber and the resin. Some important commercially-available resin-matrix families include general purpose polyester (e.g., orthophthalic polyesters, etc.), improved polyester (e.g., isophthalic polyesters, etc.), epoxy, and vinyl ester. 
     Resin bath can be implemented in a variety of ways, two of which are described below. In the illustrative embodiment, resin bath  328  is implemented as a doctor blade roller bath wherein a polished rotating cylinder (e.g., cylinder  330 ) that is disposed in the bath picks up resin as it turns. The doctor bar (not depicted in  FIG. 3 ) presses against the cylinder to obtain a precise resin film thickness on cylinder  330  and pushes excess resin back into the bath. As fiber tow  307  is pulled over the top of cylinder  330 , it contacts the resin film and wets out. In some other embodiments, resin bath  328  is realized as an immersion bath wherein fiber tow  307  is simply submerged into resin and then pulled through a set of wipers or roller that remove excess resin. 
     After leaving resin bath  328 , resin-wetted, CNT-infused fiber tows  309  is passed through various rings, eyelets and, typically, a multi-pin “comb” (not depicted) that is disposed behind a delivery head (not depicted). The comb keeps the fiber tows  2309  separate until they are brought together in a single combined band on rotating mandrel  332 . 
     Example 
     A CNT-infused carbon fiber was formed in accordance with the illustrative embodiment. A current was passed through carbon fiber (the parent fiber) to heat it to approximately 800° C. to remove epoxy sizing material. The fiber was then cooled to room temperature and left clamped between electrodes. A ferro-fluid catalyst was applied to the fiber using an aerosol spray technique. The fiber was allowed to dry and the chamber was closed, evacuated and filled with argon. A current was passed through the carbon fiber again to heat it to approximately 800 C for carbon nanotube synthesis. A carbon plasma was generated from acetylene precursor using 13.56 MHz microwave energy using an atmospheric pressure plasma jet. The carrier gas in the plasma jet was helium at 20 standard liters per minute (slm) and the argon was provided at 1.2 slm. The plasma jet was fixtured to a robotic motion control system allowing the plasma jet to move over the length of the fiber at a speed between 6 and 12 inches per minute. The CNT-infused fiber was then cooled to room temperature and removed from the chamber. Scanning Electron Microscopy showed carbon nanotube formation on the surface of the parent carbon fiber. 
     It is to be understood that the above-described embodiments are merely illustrative of the present invention and that many variations of the above-described embodiments can be devised by those skilled in the art without departing from the scope of the invention. For example, in this Specification, numerous specific details are provided in order to provide a thorough description and understanding of the illustrative embodiments of the present invention. Those skilled in the art will recognize, however, that the invention can be practiced without one or more of those details, or with other methods, materials, components, etc. 
     Furthermore, in some instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the illustrative embodiments. It is understood that the various embodiments shown in the Figures are illustrative, and are not necessarily drawn to scale. Reference throughout the specification to “one embodiment” or “an embodiment” or “some embodiments” means that a particular feature, structure, material, or characteristic described in connection with the embodiment(s) is included in at least one embodiment of the present invention, but not necessarily all embodiments. Consequently, the appearances of the phrase “in one embodiment,” “in an embodiment,” or “in some embodiments” in various places throughout the Specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, materials, or characteristics can be combined in any suitable manner in one or more embodiments. It is therefore intended that such variations be included within the scope of the following claims and their equivalents.