US-12618175-B2 - Apparatus and method for close proximity carbonization of polymeric materials for carbon fiber production

US12618175B2US 12618175 B2US12618175 B2US 12618175B2US-12618175-B2

Abstract

An apparatus and method for the low temperature carbonization of a continuous tow of polymeric material fiber, such as PAN or other carbon fiber precursor materials at atmospheric pressure in an inert gas (usually nitrogen or argon) is disclosed. A pair of antennas are arranged within an electromagnetic cavity and face each other in an edgewise fashion for direct electromagnetic heating of the fiber tow as it passes between them. Supplemental background heating increases the dielectric loss of the fiber tow in order to improve absorption of electromagnetic energy and prevent arcing. The invention produces a higher density low temperature carbonized fiber in a shorter residence time compared to conventional low temperature carbonization.

Inventors

William W. Wallace
Hippolyte A. Grappe
Felix L. Paulauskas
H. Felix Wu
Truman Bonds

Assignees

William W. Wallace
Hippolyte A. Grappe
Felix L. Paulauskas
H. Felix Wu
Truman Bonds

Dates

Publication Date: 20260505
Application Date: 20220903

Claims (20)

1 . An apparatus to partially carbonize stabilized carbon fiber precursor materials comprising: a source of electromagnetic energy of a selected power and frequency; a tunable resonant cavity including an antenna structure to localize the electromagnetic energy on the precursor material; and, openings in both ends of the cavity, so that the precursor material passes through the cavity as a continuous fiber tow in a selected atmosphere at atmospheric pressure.
2 . The apparatus of claim 1 wherein the source of electromagnetic energy comprises a device selected from the group consisting of: magnetrons, klystrons, gyrotrons, traveling wave tubes, and solid state power amplifiers.
3 . The apparatus of claim 1 wherein the frequency of electromagnetic energy is between 1 MHz and 300 GHZ.
4 . The apparatus of claim 3 wherein the frequency of electromagnetic energy comprises a selected bandwidth about a center frequency of 2.45 GHz.
5 . The apparatus of claim 1 wherein the tunable resonant cavity comprises two facing paraboloidal surfaces and two planar end surfaces, and wherein at least one of the planar end surfaces is movable so that the cavity length and asymmetry relative to the antenna structure may be adjusted.
6 . The apparatus of claim 1 wherein the antenna structure comprises two identical antennas facing each other on opposite sides of the fiber tow and spaced apart equidistant from the fiber tow.
7 . The apparatus of claim 6 wherein each antenna comprises an interchangeable cylindrical member by the interchanging of which the overall length of the antenna may be adjusted, and the cylindrical member is terminated in a planar stub having a selected radius of curvature on the edge facing the opposite antenna.
8 . The apparatus of claim 1 wherein the openings on both ends of the cavity further comprise RF chokes to reduce the leakage of electromagnetic energy from the resonant cavity.
9 . The apparatus of claim 1 further comprising a secondary heating system to control the thermal background of the process to increase the permittivity of the precursor material so that the material will absorb the electromagnetic energy efficiently, and the secondary heating system comprises a source of heated gas at 200 to 400° C. that passes through the cavity and heats the precursor material.
10 . The apparatus of claim 9 wherein the secondary heating system further comprises a tubular dielectric structure surrounding the fiber tow and containing the heated gas in proximity to the tow of precursor material.
11 . An apparatus to partially carbonize stabilized carbon fiber precursor materials comprising: a source of electromagnetic energy of a selected power and frequency; a tunable resonant cavity including an antenna structure to localize the electromagnetic energy on the precursor material; a dielectric tube disposed within the antenna structure, through which the precursor material passes as a continuous fiber tow in a selected atmosphere at ambient pressure; and, a system to control the thermal background of the process to increase the permittivity of the precursor material so that the material will absorb the electromagnetic energy.
12 . The apparatus of claim 11 wherein: the source of electromagnetic energy comprises a device selected from the group consisting of: magnetrons, klystrons, gyrotrons, traveling wave tubes, and solid state power amplifiers; and, the frequency of electromagnetic energy is between 1 MHz and 300 GHZ.
13 . The apparatus of claim 12 wherein the frequency of electromagnetic energy comprises a selected bandwidth about a center frequency of 2.45 GHz.
14 . The apparatus of claim 11 wherein the tunable resonant cavity comprises two facing paraboloidal surfaces and two planar end surfaces, and wherein at least one of the planar end surfaces is movable so that the cavity length and asymmetry relative to the antenna structure may be adjusted.
15 . The apparatus of claim 11 wherein: the antenna structure comprises two identical antennas facing each other on opposite sides of the fiber tow and spaced apart equidistant from the fiber tow; and, each identical antenna comprises an interchangeable cylindrical member by the interchanging of which the overall length of the antenna may be adjusted, and the cylindrical member is terminated in a planar stub having a selected radius of curvature on the edge facing the opposite antenna.
16 . The apparatus of claim 11 wherein the openings on both ends of the cavity further comprise RF chokes to reduce the leakage of electromagnetic energy from the resonant cavity.
17 . The apparatus of claim 11 wherein the secondary heating system comprises a source of heated gas at 200 to 400° C. that passes through the dielectric tube and heats the precursor material.
18 . A method to partially carbonize stabilized carbon fiber precursor materials comprising the steps of: providing a source of electromagnetic energy of a selected power and frequency; providing a tunable resonant cavity including an antenna structure to localize the electromagnetic energy on the precursor material; passing stabilized carbon fiber precursor material through the resonant cavity and through the antenna structure so that the precursor material is exposed to the electromagnetic energy in the selected atmosphere; and, controlling the thermal background of the process to increase the permittivity of the precursor material so that it will absorb the electromagnetic energy.
19 . The method of claim 18 further comprising the step of: maintaining the fiber tow in a selected state of tension as the fiber tow is processed.
20 . The method of claim 18 wherein: the antenna structure comprises two identical antennas facing each other on opposite sides of the fiber tow and spaced apart equidistant from the fiber tow; and, each identical antenna comprises an interchangeable cylindrical member by the interchanging of which the overall length of the antenna may be adjusted, and the cylindrical member is terminated in a planar stub having a selected radius of curvature on the edge facing the opposite antenna.

Description

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH This invention was made with Government support under Contract No. DE-AC05-00OR22725 awarded by the U. S. Department of Energy to UT-Batelle LLC, and the Government has certain rights in this invention. BACKGROUND OF THE INVENTION Field of the Invention The invention pertains to apparatus and methods for manufacturing carbon fiber, and more particularly, to apparatus and methods for carbonizing polymeric fibers using near-field electromagnetic treatment. Description of Related Art Carbon fiber is a material with very high specific stiffness and strength, hence is very attractive for weight-critical applications. However, it comes at a high cost, so it is typically used in structures in which weight reduction justifies the high cost premium. Carbon fiber is also very attractive for use in heavy vehicles and automotive passenger platforms as well as in other industries where its use offers weight reduction and energy efficiency gains. However, the production of carbon fiber is lengthy and expensive. It is estimated that to be massively adopted in those industries, the price of carbon fiber must be reduced by approximately half. It is generally observed that commercial grade carbon fiber production cost is about evenly divided between the cost of the precursor and the cost of converting the precursor to carbon fiber. The low temperature carbonization stage is one of the most energy intensive process steps. Significantly reducing the energy consumption of low temperature carbonization (LTC) per unit mass would allow the carbon fiber to be one step closer to broader adoption in the industry. FIG. 1 shows a diagram of all steps encountered by the material in a conventional line of conversion as in the current industry: pre-treatments, stabilization/oxidation, low temperature carbonization, high temperature carbonization, graphitization (optional), surface treatment, and sizing. The low temperature carbonization (outlined by the dashed line in FIG. 1) is the first step of carbonization, where major morphological changes occur and where most of the effluent is released, leaving behind a high percentage of carbon containing fiber. Carbon fiber is produced from a variety of precursors. The predominant raw materials are polyacrylonitrile (PAN), mesophase pitch, and rayon. Natural precursors such as cellulose or lignin also exist but are not commonly used in the industry. In most cases, the precursor is spun in tows of continuous filaments. Then it may be pre-stretched before being stabilized in an oxidative environment (usually in air at 200° C.-400° C. for several hours, depending upon the precursor) [see Peebles L. H., “Carbon Fibers-Formation, Structure, and Properties”, CRC Press, pp. 7-25 and 128-135 (1995)]. After the stabilization process, the material becomes a thermoset. It is matte black, infusible, flameproof, and is usually referred to as “oxidized fiber”. This oxidized fiber is sufficiently stable for exposure to significantly higher carbonization temperatures and graphitization under an inert environment, usually nitrogen. The carbonization process is divided into two or three stages. The first carbonization stage is LTC and operates in the 350° C.-800° C. temperature range. The second carbonization stage is high temperature carbonization (HTC). This second stage thermally treats the fiber between 800° C. and 1500° C. [see Donnet, J.-B. et al., “Carbon Fibers”, Third Edition, Marcel Dekker, Inc., pp. 26-31 (1998)]. Optionally, carbon fiber can be given an additional thermal treatment between 2000° C. and 3000° C., referred to as graphitization. In this last stage the fiber acquires a graphite-like structure while losing almost all its impurities and experiencing a negligible weight variation. To some extent, the Young's modulus is function of the highest temperature the fiber has been exposed during the graphitization stage [see Morgan, P., “Carbon Fibers and their Composites”, CRC Taylor and Francis, pp. 200-203 (2005); and Donnet, J.-B. et al., “Carbon Fibers”, Third Edition, Marcel Dekker, Inc., p. 29 (1998)]. Thus graphitization produces carbon fiber with extremely high stiffness. Once the material is fully carbonized, the carbon fiber's surface is conditioned to obtain the final product by dipping it in an electrolytic or acidic bath. The fiber, configured as an anode, travels between cathodes made of graphite. Finally, the tow is coated with a sizing for handleability and packaging purposes. Electromagnetic (EM) energy sources, i.e. microwave, for material processing and carbon fiber conversion have been used since the 1970s. The application and the efficiency of EM as an energy source is highly dependent on the design of the processing chamber, power transmission line, the geometry of the antenna system relative to the load, modes, pattern of radiation, management and control of the energy inside the chamber (i.e., control of reflections), yield eff