Patent Publication Number: US-2021189598-A1

Title: Novel carbon nanofiber and method of manufacture

Description:
BACKGROUND 
     I. Field 
     This disclosure relates to carbon nanofibers having novel and advantageous properties. This disclosure also relates to methods of manufacturing such carbon nanofibers. 
     II. Background 
     Carbon is among a few elements having a high level of chemical bonding flexibility. This flexibility lends itself to the formation of a large variety of allotropes including diamond, graphite, and fullerenes which, while all being composed essentially of elemental carbon, vary widely in their properties. One particular relevant field of interest is the formation of Carbon NanoFibers (CNFs) and Carbon NanoTubes (CNTs). While CNFs and CNTs are both nanometer in scale and produced in similar manners, there are distinct differences that impact their manufacturability and chemical and physical properties. However, the complete range of different CNT and CNF materials has not been fully explored. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and nature of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the accompanying drawings in which reference characteristics identify corresponding items. 
         FIG. 1  is a flowchart outlining an exemplary operation for manufacturing a novel Carbon NanoFiber (CNF). 
         FIG. 2  depicts an example of an electro-spinning device usable to produce the novel CNF discussed with respect to  FIG. 1 . 
         FIG. 3  depicts a variant of the electro-spinning device of  FIG. 2 , which is also usable to produce the novel CNF discussed with respect to  FIG. 1 . 
         FIGS. 4A-4K  depict various electrode configurations for the electrodes shown in  FIG. 3 . 
         FIG. 5  provides visual representations of the novel CNFs produced by the methods and systems of this disclosure. 
         FIG. 6  provides a visual contrast of the novel CNFs produced by the methods and systems of this disclosure against other carbon-based materials. 
         FIG. 7  provides series of visual representations of the novel CNFs produced by the methods and systems of this disclosure from the macro-scale to an atomic scale. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosed methods and systems below may be described generally, as well as in terms of specific examples and/or specific embodiments. For instances where references are made to detailed examples and/or embodiments, it should be appreciated that any of the underlying principals described are not to be limited to a single embodiment, but may be expanded for use with any of the other methods and systems described herein as will be understood by one of ordinary skill in the art unless otherwise stated specifically. 
     The following special definitions apply for this disclosure. 
     The term “appreciable” refers to some quality, e.g., a particular amount or percentage, of something that results in a detectable difference either from a defined base-line or over prior-art materials. Such quality should be detectable either directly through observation, e.g., through a Scanning Electron Microscope (SEM) or a Transmission electron microscope (TEM), or indirectly through physically and/or chemically detectable properties, e.g., electron mobility, conductivity, etc. 
     The term “about” refers to variations expected in industrial manufacturing equipment for CNFs that may vary for different forms of equipment. For example, it is expected that even the most expensive high-voltage equipment will likely produce voltage outputs that vary a few percent. 
     The term “about” also refers to variations in an end product that are expected to occur even when reasonable quality controls are employed. 
     The term “Stress Activated Pyrolytic Carbon,” or “SAPC,” refers to the novel composition based on carbon and nitrogen that is the subject of this disclosure. SAPC, by the definition of this disclosure, refers to an inclusion of a semi-graphitic carbon material characterized by wavy graphite planes ranging from 0.1 nm to 1 nm and oriented parallel to an axis of a respective carbon nanofiber. The term “oriented parallel” refers to the general direction of a main surface of a particular graphitic plane and takes into account that variations in angle are expected to occur based on the “wavy” physical nature of SAPC. 
     The semi-graphitic carbon material of this disclosure is also characterized by an inclusion of 4 to 10 atomic percent of nitrogen heteroatoms with the nitrogen heteroatoms including a combined percentage of quaternary and pyridinic nitrogen groups equal to or greater than 60% of the nitrogen heteroatoms. However, it is to be appreciated that various “levels of SAPC quality” may be obtained using variations within the prescribed limits and ranges of the disclosed methods and systems, and that a “level of SAPC quality” as used in this disclosure refers to a minimum combined percentage of quaternary and pyridinic nitrogen groups of the nitrogen heteroatoms. 
     For example, one particular “level of SAPC quality” may refer to the combined percentage of quaternary and pyridinic nitrogen groups exceeding 70%, a second particular “level of SAPC quality” may refer to the combined percentage of quaternary and pyridinic nitrogen groups exceeding 80%, and a third particular “level of SAPC quality” may refer to the combined percentage of quaternary and pyridinic nitrogen groups exceeding 90%. 
     The lowest SAPC quality is to be considered an “appreciable,” i.e., detectable, combined percentage of quaternary and pyridinic nitrogen groups. However, the lowest SAPC quality of interest is not expected to be less than 60%. 
     Testing by the inventors of the disclosed method and systems indicates that the uppermost range of SAPC quality according to the presently-disclosed methods and systems exceeds 90%. 
       FIG. 1  is a flowchart outlining an exemplary operation for manufacturing a novel SAPC-based Carbon NanoFibers (CNFs). 
     The process starts in step S 110  where a precursor solution is prepared. The particular precursor solution in the present embodiment is a Polyacrylonitrile (PAN) based precursor solution that includes: (1) PAN having a molecular weight ranging from 100,000 to 500,000, and (2) a suitable solvent having less than 5% water by weight. 
     The PAN-based precursor solution of the present example includes 6% to 20% PAN by weight in the solvent. However, in varying embodiments the exact percentage of PAN to solvent may vary beyond the 6% to 20% range used in the present example. 
     Possible solvents include, for example, Dimethylsulfoxide (DMSO), Dimethylformamide (DMF) and Propylene Carbonate (PC). However, a combination of any two or more of DMSO, DMF or PC may also be used. Further, other solvents and precursors than those expressly described above may be used at a possible reduction in the quality of end product. 
     Preparing the PAN-based precursor solution includes dissolving the PAN in the solvent via a convective mixing operation at a temperature ranging from 25° C. to 130° C. until the PAN is completely dissolved. Alternatively, the PAN-based precursor solution may be mixed for a set time period ranging from, for example, one hour to one week while at the previously-described temperature range. 
     Next, in step S 120 , the PAN-based precursor solution is provided to a spinneret, and in step S 130  an electro-spinning operation is performed on the PAN-based precursor solution to create one or more PAN-based fibers. 
     Referring to  FIG. 2 , an example of an electro-spinning device usable to produce the SAPC-based CNFs is disclosed. The electro-spinning device includes a precursor container  210 , a pump  212 , a Direct Current (DC) voltage source  220 , an Alternating Current (AC) voltage source  222 , an enclosure  230  and an external drum  252 . The enclosure  230  has disposed within a spinneret  240  and a collector drum  250  surrounded by an atmosphere  232 . 
     The example spinneret  240  of  FIG. 2  is a needleless (i.e., nozzle) device. While needleless spinnerets can take many forms, needleless spinnerets are often categorized into two forms: stationary and rotating. For example, in one embodiment, a needleless spinneret can take the form of a stationery conductive metal string. In other embodiments, needless spinnerets can take the form of a drum or drum-like object that rotate inside a bath of precursor solution. 
     While the example spinneret  240  of  FIG. 2  is needleless, in still other embodiments any number of needle-type spinnerets may be used, such as a single needle spinneret, a multiple needle spinneret, and so on. Many needle spinnerets are described as “straw-like” while other needle spinnerets are described as “coaxial.” Coaxial spinnerets can produce “core and shell” fibers or even fibers with multiple shells. 
     In addition to known needle and needless devices, it is envisioned that the term “spinneret” can include future-developed needle devices, future-developed needless devices and any other known or later developed type of spinneret that does not conveniently fit within the characterizations of “needle” or “needleless.” By way of example, by providing an atomized mist of the PAN-precursor into a volume of space and then “blasting” the individual PAN-precursor particles constituting the mist towards a collector using an appropriate gas or vapor, it is possible to produce a number fine fibers. 
     In operation, the pump  212  provides the PAN-based precursor from the precursor container  210  to the spinneret  240 . The spinneret  240 , in turn, ejects a plurality of fiber streams  260  to the collector drum  250 , and it is at this time that the PAN-based precursor is processed to form PAN-based fibers  262 . The resulting combined PAN-based fibers  262  are then passed to the external drum  252  for collection and storage. While the example spinneret  240  is generally circular and the example collector  250  is drum-shaped, it is to be appreciated that the particular configuration of the spinneret  240  and the collector  250  may vary as is known to those skilled in the relevant arts. 
     Within the operation of the embodiment of  FIGS. 1-2 , the distance between the spinneret  240  and the collector drum  250  may vary substantially to include distances ranging from 1 cm to 30 cm, and in other embodiments the distance between the spinneret  240  and the collector drum  250  may exceed 30 cm. 
     As the fiber streams  260  are passed from the spinneret  240  to the collector drum  250 , the DC voltage source  220  and the AC voltage source each provide a differential voltage between the spinneret  240  to the collector drum  250  in such a way as to materially alter the resultant PAN-based fibers  262 . 
     For example, by providing a DC voltage ranging from about plus or minus 1000 V to about plus or minus 30,000 V between the spinneret  240  and the collector  250  while passing the fiber streams  260 , the streams of PAN-based precursor are drawn to the collector  250  while being processed. In addition, the physical dimensions and shape of the resultant PAN-based fibers  262  may be affected so as to produce PAN-based fibers having reduced dimensions. Details regarding an example application of a DC voltage used in an electro-spinning operation may be found in, for example, Ghazinejad, Holmberg et al, “Graphitizing Non-graphitizable Carbons by Stress-induced Routes” published Nov. 29, 2017, by Nature.com (www.Nature.com/scientificreports), the content of which is incorporated by reference in its entirety. 
     Further, by providing an appropriate AC voltage between the spinneret  240  and the collector drum  250 , the physical and chemical properties unique to SAPC are enabled. In various embodiments, the AC voltage will include one or more signals each having: (1) a base frequency ranging from 20 Hz to 100,000 Hz, and (2) either a Peak-to-Peak (P-P) voltage ranging from 100 V to 30,000 V or a Root-Mean-Square (RMS) voltage ranging from 100 V to 30,000 V. 
     Also in various embodiments, the one or more voltage signals may be any of a variety of AC signal types, such as a sine wave, a square wave, a triangle wave or combinations thereof. 
     In other embodiments, however, the AC voltage provided may not be strictly periodic but may consist of, or include, a random signal, a pseudo-random signal or a signal that appears as white noise or filtered (pink) noise. 
     Returning to  FIG. 2 , the atmosphere  232  within the container  230  where the electro-spinning operation (e.g., step S 120 ) is performed contains a vapor of the precursor solution solvent at a temperature ranging from 10° C. to 100° C. with a saturation larger than 10%. However, in various embodiments the minimum saturation may exceed 45% at an ambient (sea level) atmosphere. 
     Returning to  FIG. 1 , after the electro-spinning operation of step  120 , a post-electro-spinning operation S 130  is performed on the resultant PAN-based fibers. According to the present disclosure, the post-electro-spinning operation S 130  will include both a mechanical treatment and a chemical treatment. 
     The mechanical treatment of the post-electro-spinning operation S 130  includes hot-rolling and hot-drawing. 
     The hot-rolling treatment is designed to mechanically compress the resultant PAN-based fibers at a stress ranging from 20 kPa to 2000 kPa while at temperature ranging from 50° C. to 300° C. However, in other embodiments the temperature and/or the stress applied may extend beyond their respective cited ranges although the quality of the end product may differ. 
     The hot-drawing treatment is designed to stretch the one or more fibers from 5% to 50% of an original length of the resultant PAN-based fibers. However, as with hot-rolling treatment, the amount of stretching may exceed the cited range. 
     The chemical treatment of the post-electro-spinning operation includes dipping the resultant PAN-based fibers in a 5% to 30% hydrogen peroxide solution followed by removing any remaining solvent and/or any remaining hydrogen peroxide from the resultant PAN-based fibers. However, any chemical treatment suitable to appropriately clean the resultant PAN-based fibers is envisioned. 
     Continuing, a stabilizing treatment S 140  on the PAN-based fibers, which generally involves heating the PAN-based fibers, is performed. The stabilizing treatment S 140  may be performed during or after the mechanical treatment of step  130 . 
     If the stabilizing treatment S 140  is performed during the mechanical treatment, the stabilizing treatment S 140  will include heating the PAN-based fibers at a temperature ranging from 200° C. to 300° C. while performing the mechanical treatment. 
     If the stabilizing treatment S 140  is performed after the mechanical treatment, the stabilizing treatment S 140  will include heating the one or more PAN-based fibers at a temperature ranging from 200° C. to 400° C. 
     After performing the post-electro-spinning operation and stabilizing treatment of steps S 130  and S 140 , a pyrolysis treatment S 150  on the PAN-based fibers is performed. Generally, the pyrolysis treatment S 150  will include heating the PAN fibers in an inert atmosphere containing less than one-percent (1%) oxygen, and will include three separate operations. 
     The first operation of step S 150  involves a heating of the PAN-based fibers that takes place in an inert atmosphere, preferably an oxygen-free atmosphere, but generally an atmosphere where oxygen does not exceed one percent. During the first operation, temperature will be maintained between 200° C. to 400° C. for a time ranging from one hour to five hours. 
     The second operation of step S 150  also involves a heating of the PAN-based fibers in an inert atmosphere. During the second operation, temperature will be maintained between 600° C. to 2000° C. for a time ranging from one hour to five hours. 
     The third operation of step S 150  is a cooling operation where temperature is slowly decreased to room temperature. 
     It is to be appreciated that, for all three operations of the pyrolysis treatment S 150 , the temperature ramp rate should not exceed 20° C./minute. 
     In view of the discussion above, it should be appreciated that the combined percentage of quaternary and pyridinic nitrogen groups will be affected by at least: (1) the choice and/or exact composition of polymer precursor, the AC signal of the electro-spinning operation and the particular pyrolysis treatment used. Accordingly, appropriate choices within the disclosed ranges will cause the combined percentage of quaternary and pyridinic nitrogen groups to exceed, for example, 70%, 80% and even 90%. 
       FIG. 3  depicts a variant of the electro-spinning device of  FIG. 2 , which is also usable to produce the novel CNFs of this disclosure. In the embodiment of  FIG. 3 , instead of providing an AC voltage between the spinneret  240  and the collector drum  250 , the AC voltage source  222  is used to develop an electric field laterally across the fiber streams  260 . As shown in  FIG. 3 , a channel  320  is formed between electrode  321  and electrode  322 . Accordingly, as the AC voltage source provides an AC electric field between the electrodes { 320 ,  321 }, the fiber streams  260  will be affected by the AC electric field as the resultant PAN-based fibers  262  are produced. 
       FIG. 4A  depicts a view of channel  320  along the length of the channel  320  so as to provide a profile of electrode shape. As shown in  FIG. 4A , the electrodes { 321 ,  322 } of  FIG. 3  are plate-shaped. However, as shown in  FIG. 4B , the electrodes { 321 ,  322 } of  FIG. 3  may be substituted with electrodes { 323 ,  324 }, which are curved to form concave arcs towards the channel  320 . Similarly, the electrodes { 325 ,  326 } of  FIG. 4C  may be used, which are curved to form convex arcs towards the channel  320 , and as another variant the round wire electrodes { 327 ,  328 } of  FIG. 4D  may be used. 
     Turning to  FIG. 4E-4K , it is to be appreciated that more than two electrodes may be used including the plate-shaped electrodes { 441 ,  442 ,  443 }, { 444 ,  445 ,  446 ,  447 } of  FIGS. 4E-4F , the concave-shaped electrodes { 451 ,  452 ,  453 }, { 454 ,  455 ,  456 ,  457 } of  FIGS. 4G-4H , and the convex-shaped electrodes { 461 ,  462 ,  463 }, { 464 ,  465 ,  466 ,  477 } of  FIGS. 4J-4K . 
     It should be appreciated that, by using three or four electrodes of various configurations, an electric field may be generated that does not oscillate back and forth, but rotates about the center axis of the channel  320 . Additionally, one or more rotating fields may be used independently or in combination with one or more alternating fields. 
     For example, by using the four electrodes { 444 ,  445 ,  446 ,  447 } of  FIG. 4F , it is possible to produce a clockwise rotating electric field that rotates about channel  320  at a rate of 20,000 Hz, a counter-clockwise rotating electric field that rotates about channel  320  at a rate of 51,000 Hz, a first oscillating electric field that oscillates between electrodes { 444 ,  446 } at a rate of 150,000 Hz, and a second oscillating electric field that oscillates between electrodes { 445 ,  447 } at a rate of 210,000 Hz. 
       FIG. 5  depicts an atomic view of SAPC-based carbon fibers. As shown in view  510  of  FIG. 5 , a microscopic view of a mass of SAPC-based carbon nanofibers produced by the disclosed methods and devices is provided. 
     In contrast to view  510 , in view  520  a Transmission Electron Microscope (TEM) image of SAPC for an exemplary CNF is provided. As shown in view  520 , the exemplary CNF includes a semi-graphitic carbon material characterized by wavy graphite planes that are generally oriented parallel to an axis of the exemplary carbon nanofiber. While not shown in  FIG. 5 , X-ray Photoelectron Spectroscopy (XPS) confirms the presence of graphitic and pyridinic nitrogen groups, which grants SAPC with unique electrocatalysis orders of magnitude greater than similar materials. 
       FIG. 6  provides comparative views of SAPC against other forms of carbon. As shown in view  610 , SAPC from a given fiber is shown next to CNTs embedded in the same fiber. CNTs may be made from a similar process as SAPC. However, the substantive differences not only provide CNTs with a different chemistry than SAPC, but also cause the CNT fringes to have a “bamboo” shape. 
     Continuing, as shown in view  620 , an amorphous carbon composition, i.e., carbon nitrides, is provided. Carbon nitrides are similar to carbon black but include nitrogen groups instead of oxygen. Carbon nitrides come in powder form and, as can be seen in view  620 , carbon nitride planes are not oriented in any particular direction. 
       FIG. 7  shows five different views of SAPC. The first view (view  710 ) is an optical image of a mat of SAPC fibers held by a pair of tweezers. The second view (view  720 ) shows a microscopic view of individual SAPC nanofibers taken from the mat of SAPC nanofibers of view  710 . The third view (view  730 ) is another atomic view of an individual SAPC nanofiber taken using a TEM process. 
     Continuing, view  740  shows an exemplary structure of several layers of SAPC with view  750  providing an exemplary molecular structure of a particular molecule/layer consistent with the above-described structure of SAPC. The redox species used to measure the electrochemical response in the example of view  750  is ferricyanide. However, any particular redox species, and other redox species, for example, dopamine and iridium hexachloride, can be used. 
     What has been described above includes examples of one or more embodiments. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the aforementioned embodiments, but one of ordinary skill in the art may recognize that many further combinations and permutations of various embodiments are possible. Accordingly, the described embodiments are intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims. Furthermore, to the extent that the term “includes” is used in either the detailed description or the claims, such term is intended to be inclusive in a manner similar to the term “comprising” as “comprising” is interpreted when employed as a transitional word in a claim.