METHOD FOR CARBONIZING A POLYMERIC OBJECT IN A MOLTEN MEDIUM

Abstract

A method for producing a carbonized object, the method comprising immersing a partially carbonized polymeric object into a molten anhydrous medium maintained within a temperature range of 300 C.-1800 C. for a period of time of 0.25-30 minutes under an inert atmosphere to result in conversion of the partially carbonized object into the carbonized object, wherein said partially carbonized polymeric object is 30-70 wt % carbon and the carbonized object is at least 90 wt % carbon. Also described herein is an apparatus useful in practicing the above described method, wherein the apparatus includes a vessel (crucible) constructed of a thermally refractory material for holding the molten medium, and a lid that contains one or more protruding elements (i.e., prongs or projections) or rollers useful for maintaining a polymeric fiber or ribbon immersed in the molten medium.

Claims

1. A method for producing a carbonized object, the method comprising immersing a partially carbonized polymeric object into a molten anhydrous medium maintained within a temperature range of 300 C.-1800 C. for a period of time of 0.25-30 minutes under an inert atmosphere to result in conversion of the partially carbonized object into the carbonized object, wherein said partially carbonized polymeric object is 30-70 wt % carbon and the carbonized object is at least 90 wt % carbon.

2. The method of claim 1, wherein said carbonized object is at least 92 wt % carbon.

3. The method of claim 1, further comprising initial steps of producing the partially carbonized polymeric object by a) subjecting a polymeric object to an oxidation/stabilization step in which the polymeric object is heated to a temperature of 200-300 C. under an oxidative atmosphere, and b) subjecting the oxidized/stabilized polymeric object to a low temperature carbonization step in which the oxidized/stabilized polymeric object is heated to a temperature of 300 C.-1200 C. under an inert atmosphere to result in conversion of the oxidized/stabilized polymeric object into the partially carbonized object.

4. The method of claim 1, further comprising a subsequent step of subjecting the carbonized object to a graphitization step by immersing the carbonized object into a molten anhydrous medium maintained at a graphitization temperature above 1800 C. for a period of time of 0.25-10 minutes under an inert atmosphere to result in conversion of the carbonized object into the graphitized object.

5. The method of claim 4, wherein the graphitization temperature is at least 2000 C.

6. The method of claim 1, wherein said partially carbonized polymeric object has a partially carbonized polyacrylonitrile (PAN) composition.

7. The method of claim 1, wherein said molten anhydrous medium comprises a metal or metal alloy having a melting point below 1800 C.

8. The method of claim 7, wherein said metal or metal alloy has a melting point below 1000 C.

9. The method of claim 7, wherein said molten anhydrous medium is selected from the group consisting of tin, aluminum, and NiP alloy.

10. The method of claim 1, wherein said molten anhydrous medium comprises a molten salt.

11. The method of claim 10, wherein said molten salt is a metal salt.

12. The method of claim 11, wherein said metal salt is selected from halides and oxides of alkali, alkaline earth, and metalloid elements.

13. The method of claim 1, wherein said partially carbonized polymeric object is a continuous fiber, fiber tow, or ribbon.

14. The method of claim 1, wherein said partially carbonized polymeric object is subjected to tension during the immersion step.

15. The method of claim 1, wherein said partially carbonized polymeric object is subjected to said temperature range of 300 C.-1800 C. in a single heating zone.

16. The method of claim 1, wherein said partially carbonized polymeric object is subjected to said temperature range of 300 C.-1800 C. in at least first and second heating zones, with each zone containing the molten anhydrous medium, and wherein the molten anhydrous medium in each zone is independently temperature controlled to be within a separate temperature sub-range within the range of 300 C.-1800 C.

17. An apparatus useful in immersing a carbonizable continuous fiber or ribbon in a high temperature molten medium, the apparatus comprising: (i) a vessel portion useful in holding the high temperature molten medium and constructed of a thermally refractory material capable of heating to 3000 C. without deformation, wherein the vessel portion has an open portion; and (ii) a lid that conforms to the shape of the opening of the vessel and which contains an outer side and an inner side, wherein the inner side faces the open portion of the vessel and contains at least one protruding element also constructed of a thermally refractory material, wherein the protruding element protrudes into the open portion and functions to maintain the carbonizable continuous fiber or ribbon immersed in the high temperature molten medium; wherein at least one of the vessel and lid includes an indentation that positions and guides the carbonizable continuous fiber or ribbon over the protruding element and into the high temperature molten medium during operation.

18. The apparatus of claim 17, further comprising a heating device connected to the vessel.

19. The apparatus of claim 17, further comprising the high temperature molten medium in the vessel portion.

20. The apparatus of claim 19, wherein said high temperature molten medium is selected from the group consisting of tin, aluminum, and NiP alloy.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1. Graph showing carbon fiber filament tensile strength versus residence time submerged into the molten bath.

[0012] FIG. 2. X-ray photoelectron spectroscopy (XPS) evaluation of filaments, pre-processing (LTC, low temperature carbonization) and post-processing (HTC, high temperature carbonization) in the molten bath.

[0013] FIG. 3. Wide angle x-ray scattering comparative evaluation displaying intensity vs d-spacing of commercial and liquid metal carbonized carbon fibers. These diffraction evaluations confirm the similarities between the LM and the commercial carbon fibers.

[0014] FIG. 4. Normalized Raman spectra comparing a commercial carbon fiber (AS4) and liquid metal bath carbonized carbon fiber.

[0015] FIG. 5. Optical micrographs confirming the external physical appearance of the liquid metal and conventionally produced carbon fibers have equivalent characteristics.

[0016] FIG. 6. Section view of a closed apparatus, including a vessel (crucible), its lid, and the workpiece (a fiber tow or a ribbon of pyropolymer material) beyond processed.

[0017] FIG. 7. Section view of the open apparatus, including the vessel and lid, with the workpiece not represented.

[0018] FIG. 8 (left) shows an isometric projection of the crucible without the liquid medium, with the workpiece not represented. FIG. 8 (right) shows an upside-down isometric projection of the lid of the crucible, with the workpiece not represented. The prongs that drive the workpiece into the liquid medium are shown in a central location of the lid. The liquid medium is not represented in any of the views of the apparatus.

[0019] FIG. 9 is a section view of a closed apparatus fitted with rotating structures on the inner portion of the lid to facilitate immersion and transport of the fiber through the liquid medium.

DETAILED DESCRIPTION

[0020] The present disclosure is foremost directed to a method for producing a carbonized object from a partially carbonized polymeric object (also referred as the polymeric object). The term carbonized object (or alternatively fully carbonized object), which is the object that has been carbonized from the polymeric object by the presently described method, is herein meant to indicate an object that is composed of at least or greater than 90 wt % carbon. In some embodiments, the carbonized object is composed of at least or greater than 92, 93, 94, 95, 96, 97, 98, or 99 wt % carbon. In some cases, although not typical, the carbonized object may be 100 wt % carbon. In contrast, the partially carbonized polymeric object is at least or greater than 30 wt % carbon but no more than or less than 70 wt % carbon. In different embodiments, the partially carbonized polymeric object is composed of precisely or about 30, 35 40, 45, 50, 55, 60, 65, or 70 wt % carbon or an amount of carbon within a range bounded by any two of these values, e.g., 30-70 wt %, 40-70 wt %, 50-70 wt %, 60-70 wt %, 30-60 wt %, 40-60 wt %, 50-60 wt %, 30-50 wt %, 35-50 wt %, 40-50 wt %, or 30-40 wt % carbon. The partially carbonized polymer is also herein referred to as a pyropolymer.

[0021] The term object is meant to include any shape. In typical embodiments, the object has the shape of a fiber, tape, or ribbon, wherein the term fiber is meant to encompass single fibers and tows or bundles of fibers. However, other types of shapes are possible, such as tiles, wafers, tubes, rings, meshes, brushes, weaves, or springs, or more intricate shapes for specific applications, such as a mechanical or electrical component of a device (e.g., a carbon brush in a motor). In the case where carbon fiber is produced, the partially carbonized polymeric object (and resulting carbon fiber) typically has a length of at least 1 cm or 1 meter and a width of up to or less than 100, 50, or 25 microns. In some embodiments, the polymeric fiber (and resulting carbon fiber) is continuous by having a length of at least or more than 1, 2, 3, 4, 5, 10, 20, or 30 meters, in which case the carbon fiber is typically held (i.e., wound) on a spool from which the carbon fiber can be unwound at a desired speed for processing. In other embodiments, the polymeric fiber (and resulting carbon fiber) is discontinuous by having a length of no more than or less than 1 meter (10 cm), 1 cm, 10 mm, or 1 mm, or a length within a range bounded by any two of the foregoing values. In some embodiments, the polymeric object (e.g., a fiber or ribbon) has a width of at least or greater than 1 micron, 10 microns, 20 microns, 50 microns, 100 microns, 1000 microns, 2000 microns, or 5000 microns, or a width within a range bounded by any two of the foregoing values. Moreover, any of the foregoing widths or ranges thereof can be combined with any of the foregoing possible lengths or ranges thereof. Continuous filaments or tows from very low count (<500) to very high counts (>50k) are considered herein. Polymeric and carbonized fibers may also be stapled or chopped to form short segments or even particles, any of which may have widths or overall or average sizes of up to or less than, for example, 1 cm, 1 mm, or 1 micron. By suitable construction or weaving methods, as known in the art, a polymeric fiber may be converted into a yarn, fabric, mesh (e.g., mat or web), or felt. The mesh may be woven or non-woven. The resulting carbonized object will thus be a carbonized weave or non-woven mesh or fabric.

[0022] The partially carbonized polymeric object is a partially carbonized form of any polymer known in the art that can be converted to carbon. Thus, at the least, the polymer should be organic (i.e., carbon-containing). The polymer is also typically rigid, i.e., not a rubber. In typical embodiments, the polymer has a polyacrylonitrile (PAN) composition or the polymer may be a copolymer of PAN. The copolymer of PAN may contain blocks or segments of acrylonitrile along with blocks or segments of another type of polymer, such as vinyl acetate, methyl acrylate, acrylamide, or itaconic acid. Other types of polymers that may be suitable in the partially carbonized polymeric object include lignin, rayon, natural fibers (e.g., hemp, jute, sisal, coir, cotton, kapok, abaca), pitch, nylons (e.g., a polyaramid, such as Kevlar), and polyesters (e.g., PET or PEF). Any blend or copolymeric form of any combination of the above types of polymers (e.g., PAN-lignin blend) may also be used.

[0023] The molten anhydrous medium (i.e., molten medium) preferably has a melting point of at least or above 200 C., 300 C., 400 C., or 500 C., and up to or below 2000 C., 1800 C., 1600 C., 1400 C., 1200 C., 1100 C., or 1000 C. The melting point may also be within a range bounded by any of the foregoing minima and maxima. The melting point may alternatively be between minima provided above (e.g., 200-500 C.) or between maxima provided above (e.g., 1100-1200 C.). Moreover, the molten medium should not be reactive with the polymer and should not undergo decomposition at any of the above temperatures or temperature ranges provided above. Optimally, carbide reactions should be minimized. An example is the tendency of elemental Fe to form carbides. A reaction known as metal dusting refers to a tendency for alloys containing Fe and in direct contact with a high carbon-containing surface to form metal and graphite dust upon cooling. In metal dusting, carbon atoms are leached from the carbon-containing surface. Thus, the molten medium preferably does not include Fe.

[0024] In a first set of embodiments, the molten anhydrous medium is or includes a metal or metal alloy. The metal or metal alloy preferably has a melting point with any of the exemplary ranges provided above. Some examples of metals that have melting points within any of the above exemplary ranges include Sn, Al, Cu, Zn, Cd, Ag, Au, Pb, Sb, Ge, and Bi. Some examples of metal alloys that have melting points within any of the above exemplary ranges include binary, ternary, or higher alloys containing any two, three, or more (respectively) of the above named elements (e.g., CuZn (brass), CuSn (brass), SnAl, SnZn, SnCd, SnAg, SnAu, AlCu, AlZn, AlCd, AlAg, and AlAu). The metal alloy may also be a binary, ternary, or higher alloy containing one or more elements selected from the above and one or more elements not disclosed above. Some examples of elements not disclosed above which may be included in the metal alloy include P, S, and Si. The metal alloy may more specifically be a Sn-based alloy, Al-based alloy, Cu-based alloy, or Zn-based alloy. Some examples of metal alloys containing any of the foregoing elements include NiP, NiS, NiSi, CuP, CuS, CuSi, AlP, AlS, and AlSi alloys. In some embodiments, Fe, Ni, Co, and/or Si may be excluded from the metal or metal alloy in view of their tendency to solubilize carbon.

[0025] In a second set of embodiments, the molten anhydrous medium is or includes a metal salt. The metal salt can be any compound in which metal atoms therein are positively charged (i.e., cationic). The cationic metal may be selected from any one or more of the metal provided above and/or one or more alkali metal atoms (e.g., Li, Na, K, Rb, and/or Cs) and/or one or more alkaline earth metal atoms (e.g., Mg, Ca, Sr, and/or Ba) and/or one or more main group (metalloid) metal oxides (i.e., as found in Groups 13 and 14 of the Periodic Table). The anionic portion of the metal salt may be selected from, for example, halide atoms (e.g., F, Cl, and Br), oxide atoms (O), sulfide atoms(S), and complex anions (e.g., nitrate, sulfate, and phosphate). Any positively charged metal and anion provided above may be combined to form a compound that can be included in the molten medium. Some examples of alkali halide salts include NaCl, KCl, LiF, Li.sub.2O, Na.sub.2O, K.sub.2O, and LiNaKF, and combinations of any two or more of these. Some examples of alkaline earth metal salts include MgCl.sub.2, CaCl.sub.2), SrCl.sub.2, BaCl.sub.2, MgO, CaO, SrO, and BaO, and combinations of any two or more of these. Some examples of main group metal salts include B.sub.2O.sub.3, SiO.sub.2, and PbO. The molten medium may also include a mixture of a halide salt and oxide salt. In some embodiments, a mixture of barium chloride and one or more of potassium chloride, sodium chloride, and silica is used. Such mixtures are particularly useful in operating at a temperature in a range of 1000-1200 C. Notably, there are specialized mixtures used for carburizing, such as sodium cyanide. Since this should be avoided, such mixtures should not be used. A purely organic salt, such as an ionic liquid, may be used if the organic salt is stable within a temperature range, such as any of those disclosed in this application, which can be used for carbonizing the polymeric object.

[0026] In some embodiments, a molten anhydrous alkaline earth salt is used that contains a eutectic mixture of CaCl.sub.2) and MgCl.sub.2. In the eutectic mixture, either of CaCl.sub.2) or MgCl.sub.2 may be in a larger or lesser amount by weight or molar amount, or the two salts may be present in equal weight or equal molar amount. In some embodiments, the anhydrous alkaline earth salt includes a eutectic mixture of CaCl.sub.2) and MgCl.sub.2, wherein the CaCl.sub.2) is present in an amount greater than 50 wt % by weight (or 50 mol % by moles) of CaCl.sub.2) and MgCl.sub.2. In the eutectic, the CaCl.sub.2) may be present in an amount of, for example, at least or greater than 55, 60, 65, 70, 75, 80, 85, 90, or 95 wt % by weight of CaCl.sub.2) and MgCl.sub.2, or an amount of CaCl.sub.2) within a range bounded by any two of the foregoing values (e.g., 60-95 wt %), wherein any of the foregoing wt % values may alternatively be mol %.

[0027] In the method, a partially carbonized polymeric object is immersed (i.e., submerged) in a molten anhydrous medium, such as any of those described above, maintained within a temperature range of 300 C.-1800 C. under an inert atmosphere to result in conversion of the partially carbonized object into the carbonized object. The term immersed, as used herein, indicates that the polymeric object is completely submerged in and in contact with the molten anhydrous medium. The phrase sufficient period of time is typically at least 0.25, 0.5, or 1 minute, and typically up to 2, 5, 10, 20, or 30 minutes, or a period of time within a range therein (e.g., 0.25-30 minutes, 0.5-30 minutes, 1-30 minutes, 0.25-10 minutes, 0.5-10 minutes, 1-10 minutes, 0.25-5 minutes, 0.5-5 minutes, 1-5 minutes, 0.25-2 minutes, 0.5-2 minutes, or 1-2 minutes). In different embodiments, the polymeric object is submerged within the molten anhydrous medium while the molten anhydrous medium is maintained at a temperature of precisely or about, for example, 300 C., 400 C., 500 C., 600 C., 700 C., 800 C., 900 C., 1000 C., 1100 C., 1200 C., 1300 C., 1400 C., 1500 C., 1600 C., 1700 C., or 1800 C., or a temperature within a range bounded by any two of the foregoing values, e.g., 300-1800 C., 400-1800 C., 500-1800 C., 600-1800 C., 700-1800 C., 800-1800 C., 900-1800 C., 1000-1800 C., 1100-1800 C., 1200-1800 C., 1300-1800 C., 1400-1800 C., 1500-1800 C., 1600-1800 C., 1700-1800 C., 300-1500 C., 400-1500 C., 500-1500 C., 600-1500 C., 700-1500 C., 800-1500 C., 900-1500 C., 1000-1500 C., 1100-1500 C., 1200-1500 C., 1300-1500 C., 1400-1500 C., 300-1200 C., 400-1200 C., 500-1200 C., 600-1200 C., 700-1200 C., 800-1200 C., 900-1200 C., 1000-1200 C., 1100-1200 C., 300-1000 C., 400-1000 C., 500-1000 C., 600-1000 C., 700-1000 C., 800-1000 C., 900-1000 C., 300-800 C., 400-800 C., 500-800 C., 600-800 C., 700-800 C., 300-600 C., or 400-600 C. The inert atmosphere is an atmosphere substantially devoid of a reactive gas (e.g., oxygen or hydrogen). The inert atmosphere may be, for example, nitrogen (N.sub.2) or a noble gas (e.g., helium or argon).

[0028] During the carbonization step, the molten medium may be held steady at or around (e.g., within 20%, 10%, 5%, or 1%) of a particular temperature provided above, or the molten medium may be allowed to vary in a controlled manner within any of the possible temperature ranges provided above, such as by controllably raising the temperature from a lower temperature to a higher temperature, maintaining the higher temperature for a period of time (e.g., a few seconds), before raising or lowering the temperature. In some embodiments, the polymeric object is immersed in a molten medium that is liquid at a temperature below 300 C., such as 100 C., 150 C., 200 C., or 250 C., before being raised to a temperature within a range provided above. It is understood that the composition of the molten medium should be selected to provide a desired carbonization temperature or range thereof. Moreover, any of the above temperatures or temperature ranges for the carbonization step can be selected along with any of the processing times provided above to optimize the carbonization of any specific polymeric composition provided above. The composition of the molten medium can also be judiciously selected to further optimize the carbonization of any specific polymeric composition provided above.

[0029] The bath dimensions may be relatively small (desktop scale) for this demonstration. However, bath dimensions can be scaled up to accommodate significantly larger number of tows, for simultaneous carbonization operations. Special attention must be given to achieve heating uniformity throughout the molten metal bath, at larger scales.

[0030] In some embodiments, the partially carbonized polymeric object is subjected to a temperature in a range of 300 C.-1800 C. or sub-range therein in a single heating zone to result in carbonization of the polymeric object, wherein the single zone contains a molten anhydrous medium as described above. In other embodiments, the partially carbonized polymeric object is subjected to a temperature in a range of 300 C.-1800 C. or sub-range therein in at least two separate heating zones (i.e., at least first and second heating zones), with each zone containing a molten anhydrous medium as described above. The multi-zone may have, for example, precisely or at least two, three, four, five, six, or more heating zones, each containing the same or different molten anhydrous media. In the multi-zone process, the molten anhydrous medium in each zone is independently temperature controlled to be at a different temperature or within a separate temperature sub-range within the range of 300 C.-1800 C. Each zone may also have its own fiber stretching (tension) control. Typically, the polymeric object is heated to a higher temperature in each successive zone. For example, a polymeric object can be heated to a temperature within a range of 300 C.-600 C. in a first zone and successively heated to a temperature within a range of 700 C.-1200 C. in a second zone. As another example, a polymeric object can be heated to a temperature within a range of 300 C.-600 C. in a first zone and successively heated to a temperature within a range of 700 C.-1000 C. in a second zone and successively heated to a temperature within a range of 1100 C.-1800 C. in a third zone. The polymeric object may be moved from zone to zone manually or in a continuous or automated fashion. In the case of a continuous polymeric fiber, tape, or ribbon, the object can be continuously pulled into the one or more heating zones, with the speed of movement adjusted to accordingly adjust the residency time of the object (or portion thereof) submerged in the molten medium. The continuous polymeric fiber is typically held on a spool and unwound from the spool as it is fed into and passed through the molten anhydrous medium in a continuous carbonization process. The speed at which the continuous polymeric fiber is dispensed (or equivalently, the speed at which the spool is rotated to unwind the fiber) and fed into the molten anhydrous medium can be suitably adjusted to correspondingly adjust the residency time of the carbon fiber in the molten anhydrous medium.

[0031] In the case of a continuous polymeric fiber, tape, or ribbon, the polymeric object can be subjected to a stress (tension) along the length of the fiber, tape, or ribbon during carbonization and/or graphitization. The stress can be applied to, for example, avoid fiber shrinkage, or to favorably affect or adjust properties of the resulting carbonized fiber, such as fiber strength, elasticity, elongation, crystallinity, morphology, and pore size distribution. In different embodiments, the fiber, either during carbonization and/or graphitization, is subjected to 0.1, 0.3, 0.5, 1, 2, 5, 10, or 20 MPa of stress. In other embodiments, no tension is applied to the fiber during carbonization and/or graphitization.

[0032] In other embodiments, the polymeric object may be subjected to electromagnetic, plasma, or particle beam (e.g., electron or neutron beam) radiation during the carbonization and/or graphitization process. The electromagnetic, plasma, or particle beam exposure generally has the effect of improving the strength and/or modulus of the resulting carbonized fiber. The operation and use of electromagnetic, plasma, and particle beam radiation techniques are well known in the art.

[0033] In other embodiments, the polymeric object may be exposed to a magnetic field to permit an ordering rearrangement of polymeric domains. The magnetic field may be applied while the polymeric object is (or is not) subjected to tension. The magnetic field may be, for example, precisely, about, or at least 1, 2, 3, 4, or 5 Tesla. In one embodiment, the magnetic field is static, while in another embodiment, the magnetic field is alternating, e.g., 1, 5, 10, 50, 100, 200, 300, 400, 500, 1000, 2000, 5000, 10,000, 15,000, or 20,000 Hz. The magnetic field can be provided by any magnetic source known in the art capable of providing the magnetic fields required herein. In an embodiment, the magnetic field is provided by a superconducting magnet. In some embodiments, a single magnet is used, while in other embodiments two or more magnets are used. In some embodiments, the polymeric object is not exposed to a magnetic field in the carbonization or graphitization process.

[0034] The carbonized fiber produced by the above-described process generally has a high strength, wherein the term high strength, as used herein, generally refers to a tensile strength of at least or greater than, for example, 600, 650, 700, 750, 800, 850, 900, 950, 1000, 1100, 1200, 1300, 1400, 1500, 1600, 1700, 1800, 1900, or 2000 ksi, (which corresponds to approximately to 4-14 GPa), or a tensile strength within a range bounded by any two of the foregoing exemplary values. The high strength is believed to be at least partly due to a long range turbostratic order characterized by alignment of graphitic planes in the carbonized fiber. In some embodiments, the carbonized fiber may also have a high modulus, wherein the term high modulus, as used herein, generally refers to an elastic (i.e., tensile) modulus of at least 30 megapounds per square inch (30 msi), which corresponds to approximately 207 GPa. In different embodiments, the carbonized fiber has an elastic modulus of at least or greater than, for example, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 110, 120, 130, 140, or 150 msi (which corresponds to approximately 210-1050 GPa), or an elastic modulus within a range bounded by any two of the foregoing exemplary values. The elastic modulus of the carbonized fiber may be influenced by the graphitic crystal structure in the carbonized fiber and its interaction with turbostratic domains. In some embodiments, the carbonized fiber may also have a high elongation, wherein the term elongation, as used herein, which is synonymous with the terms ultimate elongation and elongation at break, generally refers to an elongation of at least 1.5%. In different embodiments, the carbonized fiber exhibits an elongation of precisely, about, at least, or greater than, for example, 1.5%, 1.8%, 2%, 2.2%, or 2.5%, or an elongation within a range bounded by any two of the foregoing exemplary values. The carbonized fiber may also advantageously possess a thermal conductivity of at least, above, up to, or less than, for example, 0.1, 1, 10, 50, 100, 150, 200, 250, 300, 350, 400, 500, 1000, 1500, 2000, or 2500 W/m-K.

[0035] The carbonized fiber may have any degree of crystallinity, including amorphous, graphitic, crystalline, and semi-crystalline forms of carbon. In some embodiments, the carbonized fiber may have characteristics of a single type of carbon structure throughout the carbonized fiber, while in other embodiments, the carbonized fiber may have two or more types of carbon structure, e.g., a more pronounced graphitic structure on the outer surface of the carbonized fiber and a more pronounced amorphous structure below the surface or in inner layers of the carbonized fiber. Moreover, the term carbonized fiber includes fibers constructed of only elemental carbon (i.e., 100% carbon), or fibers constructed substantially of elemental carbon, generally at least 85%, 90%, 95%, 98%, or 99% elemental carbon. An element other than carbon, if included, is generally included in a dopant amount (e.g., up to or less than 10,000, 5,000, 1,000, 500, or 100 ppm). The element other than carbon can be, for example, nitrogen, boron, oxygen, sulfur, or phosphorus, or a combination thereof. The presence or absence of non-carbon elements is strongly dependent on the composition of the precursor fiber, and whether a doping gas (e.g., ammonia or oxygen) is included or excluded in the carbonization process. Carbonized fiber produced from PAN will generally include an appreciable amount of nitrogen by virtue of the nitrile groups in PAN.

[0036] In some embodiments, a subsequent step is employed of subjecting the carbonized object to a graphitization step by immersing the carbonized object into a molten anhydrous medium maintained at a graphitization temperature above 1800 C. for a period of time of 0.25-10 minutes under an inert atmosphere to result in conversion of the carbonized object into a graphitized object. The graphitized may be partially or completely graphitized, and the period of time that the polymeric fiber is immersed in the molten medium can be varied depending on whether a partially or completely graphitized carbon fiber is desired. Typically, the temperature capable of inducing graphitization is a temperature of at least or above, for example, 1900 C., 2000 C., 2200 C., 2300 C., 2400 C., 2500 C., 2600 C., 2700 C., 2800 C., 2900 C., 3000 C., 3100 C., or 3200 C., or a temperature within a range bounded by any two of the foregoing temperatures (e.g., 1900-3200 C.). The graphitization process may or may not also include tension and/or a magnetic field, such as any of the tension forces and/or magnetic field strengths provided above for the carbonization process. The graphitization process may also be in a single zone or multi-zone set-up. The overall process of carbonization and graphitization can be achieved by employing one or more zones for carbonization and one or more zones for graphitization.

[0037] The carbonized or graphitized fiber may also be non-porous or porous. For porous carbonized fibers, the porosity is generally a result of pores on outer and/or inner surfaces (or layers) of the carbonized fiber. For a solid (i.e., non-hollow) carbonized fiber, the pores may be on the outer surface (or core segments), and for hollow carbonized fibers, the pores may be on the inner surface (i.e., surrounding hollow core). The pores may be mesopores, micropores, or macropores, or a combination thereof. In some embodiments, the carbonized fiber may exclude one or more types of pores.

[0038] As used herein and as understood in the art, the terms mesopores and mesoporous generally refer to pores having a size (i.e., pore diameter or pore size) of at least 2 nm and up to 50 nm, i.e., between 2 and 50 nm, or in the range of 2-50 nm. In different embodiments, the mesopores have a size of precisely or about 2 nm, 2.5 nm, 3 nm, 3.5 nm, 4 nm, 4.5 nm, 5 nm, 5.5 nm, 6 nm, 6.5 nm, 7 nm, 7.5 nm, 8 nm, 8.5 nm, 9 nm, 9.5 nm, 10 nm, 11 nm, 12 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, or 50 nm, or a particular size, or a variation of sizes, within a range bounded by any two of these values.

[0039] As used herein and as understood in the art, the terms micropores and microporous generally refer to pores having a diameter of less than 2 nm. In particular embodiments, the micropores have a size of precisely, about, up to, or less than 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, or 1.9 nm, or a particular size, or a variation of sizes, within a range bounded by any two of these values.

[0040] As used herein, the terms macropores and macroporous refer to pores having a size over 50 nm. Generally, the macropores considered herein have a size up to or less than 1 micron (1 m). In different embodiments, the macropores have a size of precisely, about, at least, or greater than 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 150 nm, 200 nm, 300 nm, 400 nm, 450 nm, 500 nm, or 1000 nm, or a particular size, or a variation of sizes, within a range bounded by any two of these values.

[0041] The carbonized or graphitized fiber may also have any suitable surface area (generally, a specific surface area), which is strongly dependent on the level of porosity. In different embodiments, the carbonized or graphitized fiber may have a surface area of precisely, about, at least, greater than, or up to, for example, 5 m.sup.2/g, 10 m.sup.2/g, 15 m.sup.2/g, 20 m.sup.2/g, 30 m.sup.2/g, 40 m.sup.2/g, 50 m.sup.2/g, 60 m.sup.2/g, 70 m.sup.2/g, 80 m.sup.2/g, 90 m.sup.2/g, 100 m.sup.2/g, 150 m.sup.2/g, 200 m.sup.2/g, 250 m.sup.2/g, 300 m.sup.2/g, 350 m.sup.2/g, 400 m.sup.2/g, 450 m.sup.2/g, 500 m.sup.2/g, 600 m.sup.2/g, 700 m.sup.2/g, 800 m.sup.2/g, 900 m.sup.2/g, 1000 m.sup.2/g, 1500 m.sup.2/g, 2000 m.sup.2/g, 2500 m.sup.2/g, or 3000 m.sup.2/g, or a surface area within a range bounded by any two of the foregoing values.

[0042] [The polymeric object being carbonized may or may not also include embedded particles. The embedded particles may improve a property or characteristic of the object, such as tensile strength. The particles may be, for example, polymer, metal oxide, or carbon particles. In the case of the polymeric object being a fiber or ribbon, any embedded particles are typically nanoparticles, e.g., about 1-200 nm in size.

[0043] The present disclosure is also directed to the initial process of producing the partially carbonized polymeric object which is carbonized by the presently described method. A typical process for producing the partially carbonized polymeric object employs the following two steps: a) subjecting a polymeric object to an oxidation/stabilization step in which the polymeric object is heated to a temperature of 200-300 C. (or precisely or about 200, 225, 250, 275, or 300 C., or a range therein) under an oxidative atmosphere, and b) subjecting the oxidized/stabilized polymeric object to a low temperature carbonization step in which the oxidized/stabilized polymeric object is heated to a temperature of 300 C.-1200 C. (or precisely or about 300, 400, 500, 600, 700, 800, 900, 1000 C., 1100 C., or 1200 C., or a range therein) under an inert atmosphere to result in conversion of the oxidized/stabilized polymeric object into the partially carbonized object.

[0044] As known in the art, the stabilization process subjects a polymeric fiber to heat in the presence of oxygen (typically air, oxygen-enriched air, or other oxidant, such as ozone) to render the polymeric object infusible so that it becomes largely carbonizable rather than volatile during the subsequent carbonization process. In some embodiments, the stabilization process heats the polymeric object for an extended time under tension and/or in the presence of a magnetic field to permit an ordering rearrangement of polymeric domains. In separate or further embodiments, the stabilization process heats the polymeric object to a temperature at or just below (e.g., 20 C., 10 C., or 5 C. below) the glass transition temperature (Tg) of the polymer. Following the stabilization process is the low temperature carbonization step. In some embodiments, the low temperature carbonization step is employed as a pyrolysis step, i.e., to remove volatiles. The low temperature step can be conducted at a temperature of, for example, at least or above 400, 500, or 600 C., and up to or less than 800, 900, 1000 C., 1100 C., or 1200 C. In some embodiments, the low temperature carbonization step is conducted within a temperature range of 350-850 C., 400-850 C., 350-750 C., 400-750 C., 350-650 C., or 400-650 C.

[0045] In another aspect, the present invention is directed to an apparatus (1) useful in practicing the above described method. A section view of the closed apparatus, including a vessel (crucible), its lid, and the workpiece (a fiber tow or a ribbon of pyropolymer material) beyond processed, is shown in FIG. 6. A section view of the open apparatus, including the vessel and lid, is shown in FIG. 7, with the workpiece not represented. FIG. 8 (left) shows an isometric projection of the crucible without the liquid medium, with the workpiece not represented. FIG. 8 (right) shows an upside-down isometric projection of the lid of the crucible, with the workpiece not represented. The prongs that drive the workpiece into the liquid medium are shown in a central location of the lid. The liquid medium is not represented in any of the views of the apparatus. FIG. 9 is a section view of a closed apparatus fitted with rotating structures on the inner portion of the lid to facilitate immersion and transport of the fiber through the liquid medium. The components shown in each of these figures are described in further detail below.

[0046] A first component of the apparatus includes a vessel portion (3) useful in holding (containing) the high temperature molten medium and constructed of a thermally refractory material capable of heating to 3000 C. without deformation, wherein the vessel portion has an open portion (3a) that holds the high temperature molten medium. The thermally refractory material should be inert with the molten medium and may be constructed of, for example, zirconia, aluminum oxide, magnesium oxide, or a thermally refractory glass-ceramic, such as MACOR. The materials comprising the bath structure need to be resilient to the molten metal such that they do not leach into the liquid bath to change the bath chemistry, or do not lose structure stability at the operating temperature. The vessel may also include a heating device connected to or in contact with the vessel. A second component of the apparatus includes a lid (2) that conforms to the shape of the opening of the vessel (3a) and which contains an outer side and an inner side, wherein the inner side faces the open portion (3a) of the vessel and contains at least one protruding element (2a) also constructed of a thermally refractory material, wherein the protruding element (2a) protrudes into the open portion (3a) and functions to maintain the carbonizable continuous fiber or ribbon immersed in the high temperature molten medium. At least one of the vessel and lid includes an indentation (i.e., groove) (2b) that positions and guides the carbonizable continuous fiber or ribbon over the protruding element (2a) and into the high temperature molten medium during operation. In some embodiments, the lid includes rotating structures (e.g., rollers) (5) instead of protruding elements to immerse and transport the fiber through the molten material in a continuous operation. The one or more rollers are necessarily also constructed of a thermally refractory material. Typically, the roller is connected to an axle which is electrically operated to rotate. The rollers must be highly polished (mirror quality) and smooth to ensure that no surface defects on the rollers will damage the traversing fibers during processing. High tensions generated or applied to the tows during the carbonization phase impart a significant normal force to the roller surface, and when these rollers have imperfections, the combined effect can damage the fibers.

[0047] The apparatus may also include means for fume containment. Molten metals can generate fumes that need to be contained and exhausted away from the process. Special attention must be given to entrance and exit holes through which the fibers pass, to ensure adequate negative pressure is generated inside the enclosure.

[0048] Examples have been set forth below for the purpose of illustration and to describe certain specific embodiments of the invention. However, the scope of this invention is not to be in any way limited by the examples set forth herein.

EXAMPLES

Overview

[0049] A method and an apparatus for high temperature carbonization of polymers and pyropolymers is described. Although the main application of the method is the conversion of filaments into carbon fiber, it can also be applied to the production of carbonized films and ribbons. The concept relies on immersing the workpiece into a liquid phase set at a desired temperature to make the process of carbonization happen. The described method aims to increase the throughput of the carbonization stage compared to conventional processes using pure radiant furnaces. The main benefit is cost reduction per unit of carbon fiber.

[0050] Objects of the present invention include the following: providing an apparatus for carbonization of a pyropolymer (e.g., an intermediate material between a polymer and a carbon material in which the non-carbon contain has been partially eliminated by pyrolysis at a temperature usually under 1000 C.) in the form of a continuous ribbon, film or fiber tow using a liquid (molten) medium, as well as an apparatus for carbonization of a continuous ribbon or fiber tow specially designed to achieve the described process. The method and apparatus can be used to provide a carbonized object with improved density. Moreover, the described method achieves this in a more efficient manner and by a significant reduction in processing time compared to the conventional process.

[0051] The method described herein employs a molten liquid phase to carbonize a polymeric object. Whereas the conventional process usually rely on high temperature heating by pure radiation, the described method processes the material by direct contact heat transfer to the fiber. In the described method, the material is immersed into a molten medium set at the desired process temperature. Residence time reduction is substantial: 40% to 70% reduction versus conventional HTC, hence contributing significant energy savings and decarbonization of the HTC process.

[0052] The present disclosure relates to the full carbonization of a pyropolymer containing at least 60% carbon in weight. The feedstock material (i.e., polymeric object) for this method will typically have already undergone a pretreatment that thermally and chemically prepares it to withstand the process of carbonization. The feedstock material can have any geometry. However, for continuous processing, ribbons, films and tows of fiber are the most advantageous. The described method is particularly suited for these three geometries. Using the described method for complete carbonization, the resulting material typically contains around 92% to 95% or more carbon, by weight. The described method is particularly, but not exclusively, applicable to the carbonization of intermediate stage material in the process of making carbon fiber.

[0053] The described method is useful for carbonizing or graphitizing polymeric materials, which can be a stabilized carbon fiber precursor before entering the current process, at any temperature in the 350 C.-3000 C. range using a single or multiple hot zones, in one or multiple stage. The described method comprises or, in some embodiments, consists of, a part of process in which the polymer, a ribbon or a tow of fiber, goes in and out of the apparatus designed for carbonization. One embodiment can contain multiple heating zones. Each heating zone is usually set at a constant temperature. The apparatus may comprise or, in some embodiments, consists of, in multiple heating zones, usually set with increasing temperature as the material is sequentially exposed to them.

[0054] The described method can be used exclusively or in line with other heating technologies to accomplish the process of complete carbonization. As examples, the other heating system can be based on conventional radiative elements, electromagnetic power source, plasma generation, or others. The heating technologies used for each heating zone of each stage, their order, or their individual temperature settings, can be selected by one ordinarily skilled in the art.

[0055] One embodiment comprises or, in some embodiments, consists of, a crucible which contains a liquid medium at the desired temperature of operation (typically a molten metal or a salt, or any other inorganic materials). A radiative heat source is used to heat the crucible. The pyropolymer to be treated, such as a continuous ribbon or a tow of fiber, is immersed in the liquid medium, which is the thermally conductive medium of the embodiment. The carbonization of the pyropolymer occurs by heat conduction between the liquid medium and the workpiece. The crucible and the conventional radiative heaters that surround it are typically contained in thermal insulation. Although the performance of the method has been demonstrated using a single hot zone containing a unique crucible, one ordinarily skilled in the art will understand that the invention is not limited to this configuration. Preferred methods and materials are described hereafter, although comparable methods and materials can be set for same purpose. For this reason, materials, methods, and articles herein disclosed are given as examples but are not intended to be limiting. This method does not claim to disclose the optimal embodiment characteristics nor the ideal process parameters to produce a carbonized material such as carbon fiber.

Main Consideration about the Desired Process

[0056] The present method, as a main objective, aims to increase the throughput of the process of carbonization of polymers by providing an approach that can be implemented in an industrial setting. This method is especially relevant to the production of carbon fiber, and particularly, but not limited to, the stage related to the carbonization at high temperature, where most of the non-carbon content have already been off-gassed.

The Workpiece (Pyropolymer)

[0057] The described method uses a completely or partially amorphous pyropolymer as a feedstock material. Any grade of carbon fiber precursor or polymeric ribbon previously stabilized (already thermally processed up to 300 C.50 C.) or preferably partially carbonized (already thermally processed up to 750 C.100 C.) can be used as the feedstock material. For fibers, the precursor material can be made of any of a large variety of carbon sources. This may include any polymeric system that has some char yield after carbonization. The precursor polymers used for the carbon fiber industry are, for a large majority of them, byproducts of the oil refining industry: approximately 90% of this market is polyacrylonitrile, and most of the remaining is pitch based. Other polymers from biomass may be used, such as lignin, proteins, or others, but their usage in the carbon fiber industry remains marginal. It should be noted that the present method neither limits its application to a specific carbon source or polymer precursor aforementioned, nor tries to favor the usage of any of them. However, for the sake of practicality, the embodiments detailed in this section are based on the industry's most ordinarily used material, which is a commodity grade polyacrylonitrile precursor. The selected material was produced and stabilized by well-established means.

Liquid Medium:

[0058] The present method uses a liquid medium in which the workpiece is immersed. The liquid medium is typically a molten metal, alloy, or salt. The liquid medium is typically selected as a function of its thermal conductivity at a desired temperature. The carbonization of the workpiece is achieved by heat conduction between the liquid medium and the workpiece. The liquid medium is the heat transfer fluid, and the workpiece is the heat sink.

Crucible:

[0059] The present method requires a crucible capable of maintaining the integrity of its shape at the temperature of operation. The crucible is designed to contain the liquid medium and to allow for the workpiece to be introduced and extracted while the system is at temperature of operation. Continuous processing of thin materials, such as ribbons, films or tows, may require a long but shallow bath, the width being a function of one of the materials to be processed and of the total desired throughput. On the other hand, for batch process, a more compact design could be appropriate for a bulky object, such as solid lumps of various geometries. The temperature of operation may be in the 300 C.-3000 C. range. For a high temperature carbonization, it is generally in the 1000 C.-1700 C. range, and above this for graphitization.

[0060] Graphite or technical ceramics, such as alumina, mullite, silica, or others, can be used to make the crucible. The described method can also process pyropolymers at temperatures as low as the medium remains liquid. However, in some cases (e.g., where a temperature of less than 300 C. is used), the results may be limited to partial carbonization and will be evaluated by one skilled in the art.

Immersion System:

[0061] The described method includes an immersion device that maintains the workpiece underneath the surface of the liquid to achieve optimal surface contact between the workpiece and the liquid medium. In some embodiments, the device is made of at two graphite prongs, or extensions, connected to the inner portion of the lid of the crucible. The immersion device may be designed in various ways, using one or multiple other materials with similar or more complex mechanical parts (such as pulleys, bearings, static bares, grooves, guides, eyelets, etc.), as long as its function is satisfied. The dimension of the prongs defines the path of the workpiece through the crucible and can be designed to allow the path of the workpiece to be maintained at a desired depth underneath the surface of the liquid medium without contacting the bottom of the crucible. The contact surface prongs/workpiece should be minimized and smooth to minimize fiber damage. The position of the prongs is typically fixed during the process.

Tension Vs. Densities (Liquid Phase and Workpiece):

[0062] The described method may also include controlling the tension of the material, depending on the material being processed. PAN material typically requires tensioning during processing. In addition to having some effects on the final mechanical properties of the product, the tension can also secure the path of the workpiece. Furthermore, the density of the workpiece is a function of the process of conversion. The pyropolymer may have a density in between 1.3 g/cc and 1.7 g/cc at the beginning. Its density may reach 1.7 g/cc to 1.9 g/cc after high temperature carbonization (complete carbonization), and more in the case of graphitization at the end of the process, when the proportion of carbon reaches 92% or more by weight. The density of the workpiece cannot exceed 2.26 g/cc, which is the density of the carbon diamond. If an ionic salt or ionic liquid is used as a heat transfer fluid, the tension on the workpiece is used to control its level of submersion. Indeed, most of the ionic liquids have a density lower than 1.5 g/cc. As a function of the selected salt and the pyropolymer density, the workpiece may be buoyant or sinking, particularly toward the end of the process of carbonization. The same situation may occur if a metal or alloy is selected. Indeed, some metals, such as Be (1.85 g/cc) and Mg (1.74 g/cc) have a density that can be similar to the pyropolymer. However, a few metals have a lower density than the pyropolymer, such as Li (0.53 g/cc), Na (0.97 g/cc), and K (0.89 g/cc). Their usage would make the workpiece a sinking body. All other metals, including Al (2.7 g/cc) are substantially denser than the workpiece, thus resulting in a strong buoyancy. As a result, the tension must be set in such a way that the workpiece remains straight, in proper position, and should allow for the desired strain (positive or negative).

Stabilization

[0063] As used herein, a stabilized polymer is a polymer that has been thermally treated under controlled environmental parameters. For purposes of the present invention, the stabilization process (followed by initial low carbonization) is generally performed on a polymeric object to produce a partially carbonized (pyropolymer) object, prior to the method of carbonization described herein. The parameters of the stabilization process include temperature, gas mixture, flow rate, and others. This treatment typically occurs in the 100 C.-300 C. range without melting, fusion, nor run-away exotherms. The process of stabilization is a long process (usually longer than one hour, and up to multiple hours). The stabilization process is completed when the material is fire-resistant at NTP conditions and achieves a minimal density (typically above 1.30 g/cc but below 1.45 g/cc for PAN materials, as an example). Notably, the overall time needed for stabilization and the appropriate temperature profile are both strongly material dependent.

Pyropolymer

[0064] As used herein, a pyropolymer is a material that possesses intermediate properties between polymer and carbon. This generic term describes polymeric materials that have been exposed to a temperature lower than 1000 C., where the process of elimination of non-carbon elements may have begun, but not necessarily completed. This term encompasses stabilized polymers (such as, but not limited to, oxidized fibers) and partially carbonized polymers (such as, but not limited to, low temperature carbonized fibers). A pyropolymer is non-fusible and non-flammable. The precursor of the pyropolymer can be any polymer engineered from the biomass or from any petroleum by-product. This especially encompasses, but is not limited to, any polymer blend that can be used as a precursor to produce carbon fiber.

Ribbon

[0065] As used herein, a ribbon is a continuous polymer film that has been thermally treated to become a pyropolymer. It can be processed roll to roll. A ribbon is characterized by a thickness substantially smaller than its width. Its width is limited by the internal dimensions of apparatus, or by dimensions of other equipment in the process line.

Tow/Fiber

[0066] As used herein, a tow of fiber is one or the combination of multiple strands or sub-tows of an undefined number of filaments. The individual cross-sectional shape of the filaments is typically not a relevant parameter for this invention (e.g., the cross-section of the filaments can be circular, oval, dog bone, kidney, star, random, or others). The tow is spread before immersion in the molten medium. The width of the tow is limited by the dimensions of the apparatus, or by dimensions of other equipment in the process line.

Workpiece

[0067] As used herein, the workpiece is the material that is processed using the described method. It can be, in particular, a pyropolymer shaped as a tow of fiber, a ribbon or a film. A tow is a strand of filaments having a diameter typically in the 0.1 m to 10 mm range. For carbon fiber application, the diameter of the filaments is usually in a much narrower range of 3-35 m. A film is a thin structure compared to its width; its thickness can be, but is not limited to, the 1 m-10 mm range. A ribbon is a narrow film.

Heating System

[0068] As used herein, the heating system employs a controlled heat source able to bring the system to a desired temperature (300 C. to 3000 C.). One or multiple conventional heaters are placed in the vicinity of the crucible. The number of heaters required is a function of the number of desired hot zones and the dimensions of both heater(s) and crucible(s).

[0069] Considering the dimensions of the workpiece tested in this disclosure, two heaters of 375 W each were used: one above and the other one below the crucible. To allow the system to reach temperatures greater than 300 C., the embodiment includes a double layer of 50 mm thick silica insulation boards. Two clearances were made through the insulation to allow the input and output of the workpiece. The heaters were controlled by a PID system installed in a control box outside of the crucible.

Purge and Exhaust

[0070] The present method requires operation in an inert environment when the actual temperature is equal to or greater than 300 C. The apparatus is enclosed inside of an airtight box with eyelets on opposite sides which permits the workpiece to go in and out. For safety, the entire system should be operated under an appropriate ventilation system.

Other Elements in the Conversion Line

[0071] The process of carbonization/graphitization may require multiple stages. Conventionally, a two-step process is needed for full carbonization of a pyropolymer, and the addition of a third step at significantly higher temperature (above 1800 C.) may be added to achieve graphitization Each carbonization or graphitization step may employ multiple hot zones with separate temperature control for each zone. The described method may be practiced by replacing some or all conventional hot zones with hot zones containing molten media. To achieve the desired level of carbonization/graphitization, the number of hot zones used in this invention can be identical to the number of hot zones conventionally used, or they may be different. The method may also employ a combination of conventional heating technologies and those described herein (i.e., partial replacement of one or more conventional hot zones with hot zones of the described method). The choice of the number of hot zones, the choice of their heating technology (the present method or another), the order of their sequence, or their temperature settings can be set by the end user according to their requirements.

[0072] This invention provides a new apparatus for modifying the bulk properties of polymeric materials or pyropolymers, including polyacrylonitrile (PAN) fiber and PAN based materials. The concept relies on the direct exposure of the workpiece to thermal conduction instead of conventional heat transfer by radiation by forced immersion of the workpiece into a liquid medium set at a desired temperature. The workpiece is typically composed of a ribbon, a tow, or multiple tows of fiber traveling through a cavity, to result in immersion of the workpiece into a liquid medium heated to the desired temperature.

[0073] The described method is capable of carbonizing a band of one tow of 50k filaments or multiple smaller tows with an equivalent filament count. A larger amount of feedstock material can be processed by increasing the width of the system. This process generally occurs at atmospheric pressure. Compared to the conventional process, this method can achieve complete carbonization with a shorter residence time (less than a minute vs. usually 75 seconds or more for the conventional method). The current invention can potentially improve the energy usage required under certain conditions. Moreover, in this work, the resulting carbonized fibers exhibited tailorable properties such as break strength (200-600 ksi) and modulus (28 to 32 Msi), dependent on processing parameters. This is favorable, as it can make them particularly suited for broad applications.

[0074] The tested method is based on using a liquid phase metal or molten salt for direct heat transfer via physical contact between the liquid metal and the fiber filaments. This effect reduces the residence time for the process of carbonization as compared to conventional methods. This technology reduces the large thermal inertia present in conventional furnaces, which have larges masses and surfaces that need to be heated to the operating temperature by pure radiation. The conventional method thus requires a high energy consumption, which results in a very low energy efficiency.

[0075] The feedstock used for all examples has the following characteristics: it is one continuous tow of 50,000 filaments (50k) commodity grade PAN based material, originally produced and stabilized by a fiber manufacturer (SGL) and having a density of 1.37 g/cc. This material was carbonized up to 650 C.50 C. using a conventional tubular furnace with multiple hot zones in order to reach a density in the 1.50 g/cc-1.6 g/cc range. Its mechanical properties are summarized in Table 1 below. At this point, the feedstock material is a spool of continuous tow of partially carbonized material.

TABLE-US-00001 TABLE 1 Break Filament Diameter Density Stress Modulus Strain Manuf. [k] Status [m] [g/cc] [Ksi] [Msi] [%] SGL 50 LTC 8.6 1.5448 94.82 3.16 4.94

Example 1. Pure Tin as Molten Medium

[0076] Pure tin, which has a melting point of 232 C., was used as a medium for the heat transfer. PAN fiber feedstock material was set to a temperature of 1200 C.25 C. using a pair of 375 W heaters. The temperature was made constant and stable for all samples. Tension was applied on the spool of the feedstock material by means of a creel with a friction break. The tension was set to 2 N and kept consistent for all samples.

[0077] The mechanical properties of the produced carbon fiber was tested as a function of the residence time. In this example, four residence times were explored, with all of them equal to or less than one minute. After treatment, the resulting fibers showed the following characteristics:

[0078] The samples are continuous and damage free. With a visual inspection, the samples show no evidence of tin on or in the fiber. They all have a density in the 1.72 g/cc-1.80 g/cc range, which is typical for fully carbonized carbon fiber. After single filament testing, the four samples were found to have average tensile stress properties in the range of 2.45 GPa (350 ksi)-3.71 GPa (530 ksi), the highest average being associated with the one that experienced a residence time of approximately 0.5 minute. With longer residence time, the mechanical performance decreased. Nonetheless, the sample that received the shortest residence time is the one that also shows the weakest tensile strength. This is also the general tendency in conventional thermal conversion and is a clear indication that this new methodology follows the usual characteristics in the thermal conversion of carbon fibers.

[0079] Its low density indicates that this sample may be under-processed. In this example, few filaments reach 5 GPa (715 ksi). A plot of tensile strength vs. residence time is provided in FIG. 1.

Example 2. Nickel-Phosphorus Based Alloy as Molten Medium

[0080] A Nickel-phosphorus based alloy was used as a medium for the heat transfer. Its melting temperature is 887 C. The molten medium was set to a temperature of 1200 C.25 C. The temperature was made constant and stable for all samples. Tension was applied on the spool of the feedstock material through a creel with friction break. The tension was set to 2 N and kept consistent for all samples.

[0081] Five residence times were explored, from min up to 4 min. The resulting fibers showed the following characteristics: The samples were continuous and damage free. From a visual inspection, the samples show some minute beads of alloy in the fiber. Their separation is not always possible. This is generally attributed to mechanical interlocking onto filament surfaces due to topological imperfections such as pits and pores. As a result, the measured densities for these samples were biased (see Table 2 below). However, the XPS analysis of the fiber surface for each sample shows no Ni with the exception of one sample, showing 0.2% of Ni. Phosphorus was not detected, which confirms that the surface energy of the fiber is low. The elimination of the heteroatoms, such as N, O, Si, and S, was confirmed by XPS analyses (FIG. 2 and Table 2). This measurement is similar to what is usually observed with conventional processing.

[0082] Each sample shows an abnormally high density (equal to or greater than 1.83 g/cc). The highest density associated with the samples that received the shortest residence time indicates a contamination of the samples with alloy beads, due to the adsorbed alloy having a density higher than common carbon filaments, bringing the average density up. All samples have an average tensile strength greater than 3.6 GPa (515 ksi). The data set reported in Table 3 below indicates that the best properties are obtained with a residence time in the 1 min to 2 min range, which is typically used in the industry. However, the mechanical properties obtained with a residence time equivalent to min remains attractive with 3.6 GPa, although the method is not optimized. Carburization, oxidation, and nitriding of the alloy have not been evaluated.

TABLE-US-00002 TABLE 2 Surface Composition (at. %) Sample C N O Si S Cl Ni CF LTC 78.1 15.8 5.4 0.6 0.1 0.0 0.0 CF 0.25 min 96.5 1.6 1.6 0.1 0.0 0.0 0.2 CF 0.5 min 96.2 2.0 1.6 0.2 0.0 0.0 0.0 CF 1 min 96.4 1.8 1.6 0.2 0.0 0.0 0.0 CF 2 min 96.2 1.6 2.2 tr. tr tr 0.0 CF 4 min 95.9 1.9 2.3 0.1 0.1 tr. 0.0

TABLE-US-00003 TABLE 3 Residence Break Time Diameter Density Stress Modulus Strain [min] [m] [g/cc] [Ksi] [Msi] [%] 0.25 6.85 1.87 520 30.4 1.67 0.5 6.57 1.84 517 32.3 1.55 1 6.53 1.85 608.9 36.2 1.64 2 6.69 1.83 584.6 33.9 1.68 4 6.75 1.83 554.2 33.6 1.62

Example 3. Pure Tin as Molten Medium at Lower Temperature

[0083] Pure tin is used as a medium for the heat transfer. Unlike Example 1, this experiment uses a lower temperature of 1100 C.25 C. The tension on the feedstock was held constant for all samples at 2 N. Five residence times were explored, from min up to 4 min. The resulting fibers show the following trends and characteristics: Based on visual inspection, all samples were found to be damage-free with no trace of metal. XPS analyses of the fiber surface of two samples show limited presence of 0.3% of tin. The presence of Si could be due to contamination from the insulation of the embodiment. With values of ca. 4%, nitrogen and oxygen levels are abnormally elevated (see Table 4 below).

TABLE-US-00004 TABLE 4 Surface Composition (at. %) Sample C N O Si S Cl Sn CF LTC 90.0 4.3 4.2 1.1 0.0 0.1 0.3 CF 0.25 min 91.0 3.5 4.2 0.8 0.0 0.1 0.3

[0084] Regarding the characteristics of the samples, each sample shows an abnormally high density (equal to or greater than 1.83 g/cc). The highest density was associated with the samples that received the shortest residence time, which indicates a contamination of the samples with alloy beads. All samples have an average tensile strength greater than 3.6 GPa (515 ksi). The data set reported in Table 3 indicates the best properties are obtained with a residence time in 1 min to 2 min range, which is typically used in industry. However, the mechanical properties obtained with a residence time equivalent to min remains attractive with 3.6 GPa, considering this method is not optimized. Carburization, oxidation, and nitriding of the alloy have not been evaluated.

[0085] In FIG. 3, the control fiber (labeled Control-commercial, on the plot) is commercially available Zoltek PX35. The LM sample (labeled LM, on the plot) comes from SGL 50k OPF that is carbonized at low temperature to produce feedstock for the LM HTC process. A comparison of the mechanical properties is found in Table 5 below.

[0086] FIG. 4 compares the Raman spectra of carbon fibers collected with a 532 nm green laser. The sample denoted AS4 is Hexcel's high performance carbon fiber with an average tensile modulus of 33.5 Msi that has undergone traditional high temperature carbonization. The sample prepared in our labs (LM) was subjected to our standard low temperature carbonization followed by our novel liquid metal carbonization treatment. We observe nearly identical peak widths converging with the AS4 Raman spectra for the liquid metal bath carbonized filaments which clearly illustrates the utility of the liquid metal process in structural order development in the carbon fibers.

TABLE-US-00005 TABLE 5 Density, Tensile Strength, Modulus, Elongation, Material g/cc ksi Msi % Zoltek 1.80 541 31.1 1.67 PX35 LM CF 1.79 530 31.5 1.63

[0087] Quantitative surface analysis (XPS) indicates that the samples are free of metal on their surface. This means that the liquid metal is retained in its trough, does not contaminate the fiber, and does not need to be renewed because of the process. Selected samples were evaluated via Raman spectroscopy indicating similar carbon structure between conventionally carbonized and liquid metal carbonized filaments. Optical microscopy images comparing the LM and conventional carbon fiber are shown in FIG. 5.

[0088] While there have been shown and described what are at present considered the preferred embodiments of the invention, those skilled in the art may make various changes and modifications which remain within the scope of the invention defined by the appended claims.