Carbonization apparatus and method for producing carbon fiber

Abstract

The present invention addresses the problem of providing a carbonization apparatus capable of commercially carbonizing a plurality of precursor fibers.　A carbonization apparatus (1) for heating and carbonizing a plurality of precursor fibers of carbon fibers comprises: microwave generators (11-1, 11-2, 11-3) for generating microwaves; cavities (13-1, 13-2, 13-3) connected in series and into which the generated microwaves are introduced; and two or more structures (14-1, 14-2, 14-3) arranged in series in the cavities (13-1, 13-2, 13-3), and inside which a plurality of precursor fibers (3) can be parallel to each other. The structures (14-1, 14-2, 14-3) partially absorb the microwaves irradiated from the outside and partially transmit the microwaves to the inside, and the two or more structures (14-1, 14-2, 14-3) are irradiated with microwaves such that structures (14-1, 14-2, 14-3) on the subsequent-stage side rise in temperature along the direction of movement of the plurality of precursor fibers.

Description

Carbonization apparatus and method for producing carbon fiber

[0001] The present invention relates to a carbonization apparatus for carbonizing precursor fibers of carbon fibers and a method for producing carbon fibers.

[0002] Carbon fibers are produced by first heating precursor fibers of carbon fibers in air at 200 to 300°C to make them flame-resistant, and then heating the flame-resistant treated precursor fibers at 1000°C or higher in an inert atmosphere such as nitrogen to carbonize them. Microwaves are sometimes used as a heating means in the carbonization process.

[0003] However, carbonization of precursor fibers by microwave heating has been carried out on a small-scale laboratory scale, making it difficult to commercially produce carbon fibers.

[0004] The present invention has been made in response to the above circumstances, and an object of the present invention is to provide a carbonization apparatus and a method for producing carbon fibers that can commercially carbonize multiple precursor fibers.

[0005] In order to achieve the above-mentioned object, a carbonization apparatus according to one embodiment of the present invention is a carbonization apparatus for heating and carbonizing precursor fibers of multiple carbon fibers, and comprises two or more microwave generators that generate microwaves, one or more cavities into which the two or more microwaves generated by the two or more microwave generators are introduced, and two or more structures that are arranged in series within the one or more cavities, and within which multiple precursor fibers can be arranged in parallel, and each of the two or more structures partially absorbs microwaves irradiated from the outside and partially transmits them to the inside, and microwaves can be irradiated onto the two or more structures along the movement direction of the multiple precursor fibers so that the structure at the later stage becomes hotter.

[0006] In addition, in a carbonization apparatus according to one aspect of the present invention, microwaves may be irradiated to at least one of two or more structures so that the structure becomes isothermal with the target temperature of the multiple precursor fibers moving inside the structure.

[0007] In the carbonization device according to one aspect of the present invention, each of the two or more structures may have a hollow columnar shape.

[0008] Furthermore, in a carbonization apparatus according to one aspect of the present invention, the one or more cavities may be two or more cavities connected in series, two or more microwaves may be introduced into the two or more cavities, respectively, and two or more structures may be disposed inside each of the two or more cavities.

[0009] In addition, in the carbonization device according to one aspect of the present invention, an outlet may be provided between adjacent cavities among the two or more cavities to discharge gas generated in a cavity preceding the adjacent cavity.

[0010] In addition, in the carbonization device according to one aspect of the present invention, a supply port may be provided between adjacent cavities among the two or more cavities, for supplying an inert gas into the cavity subsequent to the adjacent cavity.

[0011] In addition, in the carbonization device according to one aspect of the present invention, a pincushion circuit may be provided between adjacent cavities among the two or more cavities to prevent propagation of microwaves between the adjacent cavities.

[0012] Furthermore, the carbonization apparatus according to one aspect of the present invention may further include a conducting means for conducting the plurality of carbon fibers to each other near the outlet of the final cavity among the one or more cavities.

[0013] In the carbonization device according to one aspect of the present invention, the final structure of the two or more structures may include silicon nitride.

[0014] Furthermore, a carbon fiber manufacturing method according to one aspect of the present invention is a carbon fiber manufacturing method in which precursor fibers of multiple carbon fibers are heated and carbonized, and includes a step of arranging the multiple precursor fibers in parallel inside two or more structures arranged in series in one or more cavities, each of which partially absorbs microwaves irradiated from the outside and partially transmits them to the inside, and a step of irradiating microwaves to the two or more structures along the movement direction of the multiple precursor fibers so that the structure at the later stage has a higher temperature.

[0015] According to the carbonization apparatus and method for producing carbon fiber according to one aspect of the present invention, multiple precursor fibers can be carbonized simultaneously by microwave heating, making it possible to commercially produce carbon fiber.

[0016] Schematic diagram showing the configuration of a carbonization device according to an embodiment of the present invention. Schematic longitudinal cross-sectional view showing the internal structure of a carbonization device according to the same embodiment. Perspective view showing a structure in the same embodiment. Longitudinal cross-sectional view showing a cavity and structure in the same embodiment. Longitudinal cross-sectional view showing a cavity and structure in the same embodiment. Longitudinal cross-sectional view showing a pincushion circuit in the same embodiment. Cross-sectional view showing a pincushion circuit in the same embodiment. Longitudinal cross-sectional view showing a conducting means in the same embodiment. Schematic longitudinal cross-sectional view showing another example of the configuration of a carbonization device according to the same embodiment. Longitudinal cross-sectional view showing another example of a structure in the same embodiment. Longitudinal cross-sectional view showing another example of a structure in the same embodiment.

[0017] The carbonization apparatus and the method for producing carbon fiber according to the present invention will be described below using embodiments. In the following embodiments, components with the same reference numerals are the same or equivalent, and repeated description may be omitted. The carbonization apparatus according to this embodiment heats two or more structures arranged in series in a cavity with microwaves, and also directly heats a plurality of precursor fibers arranged in parallel and moving inside the two or more structures with microwaves that have passed through the structures, thereby carbonizing the plurality of precursor fibers.

[0018] FIG. 1 is a schematic diagram showing the exterior of a carbonization apparatus 1 according to one embodiment of the present invention, and FIG. 2 is a schematic vertical cross-sectional view showing the internal structure of the carbonization apparatus 1. Note that components such as a control unit 19 are omitted from FIG. 1 . In the example shown in FIGS. 1 and 2 , the carbonization apparatus 1 includes microwave generators 11-1, 11-2, and 11-3 that generate microwaves; waveguides 12-1, 12-2, and 12-3 that transmit the two or more generated microwaves, respectively; serially connected cavities 13-1, 13-2, and 13-3 into which the two or more transmitted microwaves are introduced, respectively; and structures 14-1, 14-2, and 14-3 disposed within the cavities 13-1, 13-2, and 13-3, respectively. Note that when the microwave generators 11-1, 11-2, and 11-3 are not particularly distinguished from one another, any one of them may be referred to as the microwave generator 11. The same applies to the other components. In this embodiment, the upstream side of the configuration of the cavities 13 connected in series in the moving direction of the fibers 3 may be referred to as the front stage, and the downstream side as the rear stage.

[0019] The carbonization apparatus 1 may further include, as necessary, at least one of an inlet 15 for the fibers 3 connected to the front-stage cavity 13-1, connection portions 16-1 and 16-2 connecting two adjacent cavities 13, an outlet 17 for the fibers 3 connected to the rear-stage cavity 13-3, temperature sensors 18-1, 18-2, and 18-3 that measure the temperatures of the structures 14, and a control unit 19 that controls the microwave generator 11 using the temperatures measured by the temperature sensor 18. The carbonization apparatus 1 may also further include, as necessary, at least one of an exhaust pipe 22 having an exhaust port 21 at one end for exhausting gas generated in the cavity 13, a supply pipe 24 having an inlet 23 at one end for supplying an inert gas into the cavity 13, and a pincushion circuit 25 for preventing propagation of microwaves in the inlet 15, the connection portion 16, and the outlet 17.

[0020] In the carbonization apparatus 1, precursor fibers of carbon fibers are carbonized while being continuously moved within the cavity 13. Therefore, the fibers 3 introduced into the cavity 13-1 from the introduction section 15 are precursor fibers after flame retardation treatment, and the fibers 3 discharged from the discharge section 17 are carbon fibers. The precursor fibers may be, for example, polyacrylonitrile (PAN). Furthermore, flame retardant fibers, which are precursor fibers oxidized by flame retardation treatment, will also be referred to as precursor fibers for convenience. Furthermore, the fibers 3 may be, for example, a single fiber, but are typically fiber bundles of multiple fibers. Therefore, the fibers 3 may be, for example, precursor fiber bundles or carbon fiber bundles.

[0021] The microwave generator 11 may generate microwaves using, for example, a magnetron, a klystron, a gyrotron, or a semiconductor element. Generating microwaves using a semiconductor element may, for example, involve oscillating microwaves using a semiconductor element, or amplifying microwaves using a semiconductor element. The microwave frequency band may be, for example, around 433.92 MHz, 915 MHz, 2.45 GHz, or 5.8 GHz, or may be another frequency band within the range of 300 MHz to 300 GHz. As described below, microwaves are irradiated onto two or more structures 14 so that the structure 14 at the later stage becomes hotter. Therefore, the outputs (powers) of the multiple microwave generators 11 may increase in the order of microwave generators 11-1, 11-2, and 11-3.

[0022] The waveguide 12 transmits the microwaves generated by the microwave generator 11 into the cavity 13. The waveguide 12 may be, for example, a rectangular waveguide or a circular waveguide. The waveguide 12 may also be, for example, a hollow waveguide. A microwave-transparent airtight window may be provided at the end of the waveguide 12 or at another location to prevent gas or the like from migrating from the cavity 13 toward the microwave generator 11. The airtight window may be made of, for example, a microwave-transparent material. The microwave-transparent material is not particularly limited, and may be, for example, quartz glass, a fluororesin such as polytetrafluoroethylene, or ceramic. Instead of the waveguide 12, the microwaves may be transmitted by a microwave transmission means such as a coaxial cable. The microwaves generated by the microwave generator 11 may be directly irradiated into the cavity 13. In this case, for example, the carbonization device 1 may not have the waveguide 12, and an antenna for emitting microwaves may be provided inside the cavity 13.

[0023] Microwaves generated by the microwave generator 11 are introduced into the cavity 13. The shape of the internal space of the cavity 13 is not particularly limited, and may be, for example, a columnar shape extending in one direction. The one direction may be, for example, the moving direction of the fibers 3. The columnar shape may be, for example, a rectangular parallelepiped shape or a cylindrical shape. In this embodiment, the case where the internal space of the cavity 13 is a rectangular parallelepiped shape will be mainly described.

[0024] The cavity 13 preferably has walls made of a material that is opaque to microwaves to prevent microwaves from leaking from the internal space. The material that is opaque to microwaves may be, for example, a microwave-reflective material. The microwave-reflective material is not particularly limited, and may be, for example, a metal such as stainless steel, carbon steel, nickel, a nickel alloy, copper, or a copper alloy.

[0025] The multiple cavities 13 are connected in series. The multiple cavities 13 may be connected in series such that the fibers 3 moving inside the multiple cavities 13 pass through the multiple cavities 13 in sequence. The multiple cavities 13 may be connected in series, for example, via a connecting portion 16. Each cavity 13 may have, for example, an inlet through which the fibers 3 are introduced into the internal space and an outlet through which the fibers 3 are discharged from the internal space. Microwave irradiation in each cavity 13 is usually performed in multimode. Furthermore, when the carbonization treatment is performed, the inside of each cavity 13 is usually at atmospheric pressure.

[0026] Note that, in the present embodiment, a case will be mainly described in which the number of microwave generators 11 and the number of cavities 13 are equal and microwaves generated by two or more microwave generators 11 are introduced into two or more cavities 13, respectively, but this is not necessarily the case. For example, microwaves generated by two or more microwave generators 11 may be introduced into one cavity 13, or microwaves generated by one microwave generator 11 may be branched and introduced into two or more cavities 13.

[0027] Two or more hollow pillar-like structures 14 are arranged in series inside two or more cavities 13. The hollow pillar-like structures 14 may, for example, have a linear shape. The cross-sectional shape of the hollow pillar-like structures 14 in a plane perpendicular to the longitudinal direction may be, for example, rectangular or a shape other than rectangular. It is assumed that openings communicating with the hollow portions of the structures 14 are present at both longitudinal ends of the hollow pillar-like structures 14. This embodiment will mainly describe the case where the cross-sectional shape of the hollow pillar-like structures 14 is rectangular, i.e., the structure 14 is a square tube. Two or more structures 14 may be arranged in two or more cavities 13, respectively, and the two or more cavities 13 may be connected in series, resulting in two or more structures 14 being arranged in series. In this embodiment, the case where the number of cavities 13 and the number of structures 14 are equal and one structure 14 is disposed in one cavity 13 will be mainly described, but this is not necessarily the case. As will be described later, two or more structures 14 may be disposed in one cavity 13.

[0028] FIG. 3A is a perspective view showing the appearance of the structure 14, and FIGS. 3B and 3C are longitudinal cross-sectional views of the cavity 13 and the structure 14. Note that FIG. 3B is a longitudinal cross-sectional view at a position where the frame member 31 is not present, and FIG. 3C is a longitudinal cross-sectional view at a position where the frame member 31 is present. As shown in FIGS. 3A, 3B, and 3C, a plurality of fibers 3 arranged in parallel may move inside the structure 14. As shown in FIG. 3B, for example, the cross section of the structure 14 in a plane perpendicular to the longitudinal direction may be rectangular, with a plurality of fibers 3 arranged along its long side. It is also preferable that the distance between the inner wall of the structure 14 and the plurality of fibers 3 is equal. For example, as shown in FIGS. 3B and 3C, the distance between each of the fibers 3 and two opposing inner walls of the structure 14 may be equal. It is also preferable that the distance be close enough to allow heat exchange between the inner wall of the structure 14 and the plurality of fibers 3. 3A, 3B, and 3C, the plurality of fibers 3 may be arranged in parallel so that the plurality of fibers 3 are included on the same plane. In this case, the plurality of fibers 3 may be spaced apart, for example, uniformly. The plurality of fibers 3 usually move at the same speed in the longitudinal direction of the fibers 3.

[0029] The structure 14 partially absorbs microwaves irradiated from the outside and partially transmits them to the inside. The structure 14 preferably has microwave transparency to such an extent that the structure 14 is heated by the microwaves absorbed by the structure 14 and the fibers 3 are heated by the microwaves passing through the structure 14, thereby reducing the temperature difference between the structure 14 and the fibers 3. As shown in FIG. 3B , when microwaves are irradiated to the entire outer peripheral surface of the structure 14, the entire structure 14 is heated more uniformly by irradiating microwaves in multimode in the internal space of the cavity 13. Furthermore, the multiple fibers 3 are also heated more uniformly by the microwaves that have passed through the wall surface of the structure 14. Microwaves may be irradiated to at least one of the multiple structures 14, for example, so that the structure 14 becomes isothermal with the target temperature of the multiple fibers 3 moving inside the structure 14. In addition, since the front-stage cavity 13-1 is introduced with flame-retardant fiber at room temperature or flame-retardant fiber immediately after flame-retardant treatment, the temperature difference between the fiber 3 and the structure 14-1 is larger upstream of the front-stage structure 14-1 than at other locations of the structure 14. On the other hand, in other locations, both the fiber 3 and the structure 14 can be microwave-heated so that the temperature difference between them is smaller.

[0030] The microwave transmittance of the structure 14 is not particularly limited, but for example, at the temperature at which the carbonization treatment is performed, approximately 20% to 80% of the microwaves may be transmitted from the outside to the inside, or approximately 30% to 50% of the microwaves may be transmitted from the outside to the inside. As the carbonization of the fiber 3 progresses, the microwave absorption ability increases, and the heat generation of the fiber 3 itself increases. Therefore, the microwave transmittance may be lower in the later stage of the structure 14. The transmittance of the structure 14 may decrease in the order of, for example, structures 14-1, 14-2, and 14-3. The microwave transmittance can be adjusted, for example, by changing at least one of the material of the structure 14 and the wall thickness of the structure 14.

[0031] During the carbonization process, the temperature of the fibers 3 is gradually increased. For example, at the start of the carbonization process, the temperature of the fibers 3 is approximately 300°C to 500°C, but at the end of the carbonization process, the temperature of the fibers 3 is 1000°C or higher, such as 1100°C to 1200°C. Therefore, it is preferable to irradiate two or more structures 14 with microwaves along the movement direction of the multiple fibers 3 so that the temperature of the structures 14 at the later stage is higher. In the example shown in FIG. 2 , each structure 14 may be microwave-heated so that the temperatures of the structures 14-1, 14-2, and 14-3 increase in this order. As an example, the structure 14-1 may be heated to 500°C, and the structure 14-3 may be heated to 1100°C.

[0032] The material of the structure 14 is not particularly limited as long as it partially absorbs and partially transmits microwaves, but may contain, for example, alumina or silicon nitride. As another example, the structure 14 may be made of sialon (SiAlON) containing silicon nitride. Structures 14 containing silicon nitride have improved heat resistance. Therefore, it is preferable that the structures 14 on the rear side, particularly the final structure 14-3, contain silicon nitride. On the other hand, it is preferable that the structures 14 on the front side, which do not become hot, contain alumina, which is cheaper than silicon nitride. As an example, structures 14-1 and 14-2 may be made of alumina, and structure 14-3 may be made of silicon nitride.

[0033] The structure 14 may be supported in the internal space of the cavity 13 by a predetermined support means or the like. As an example, the support means may include a pair of protrusions 32 provided on a pair of inner walls facing each other in the arrangement direction of the plurality of fibers 3 in the internal space of the cavity 13, as shown in FIG. 3B , and a plate-like member 33 whose ends are supported by the pair of protrusions 32. The structure 14 may be placed on, for example, the upper surface of the plate-like member 33. The pair of protrusions 32 may be, for example, metal members each extending along the movement direction of the fibers 3. The material of the plate-like member 33 may be, for example, microwave-transparent and thermally insulating. The plate-like member 33 having microwave-transparent and thermally insulating properties may be made of, for example, a material containing alumina.

[0034] The structure 14 may be disposed in the internal space of the cavity 13 so as to connect the inlet and outlet of the cavity 13, for example. That is, the fibers 3 introduced into the internal space from the inlet of the cavity 13 may move inside the structure 14 and be discharged from the outlet of the cavity 13. The structure 14 thermally expands when heated by microwaves. If the degree of thermal expansion is greater than that of the cavity 13, the longitudinal length of the structure 14 may be shortened by an amount corresponding to the thermal expansion compared to the length along the direction of movement of the fibers 3 in the internal space of the cavity 13, as shown in FIG. 2 . Furthermore, if gas generated by heating the organic fibers 3 flows outside the structure 14, it condenses on the inner wall of the cavity 13 and becomes tar, and the tar must be removed. This is because microwaves are absorbed by the tar. If there is a gap between the inlet and outlet of the cavity 13 and the end of the structure 14, gas generated by heating the fibers 3 may leak out of the structure 14. Therefore, as shown in FIGS. 2 and 3C , a frame member 31 may be provided on the inner wall of the cavity 13 to surround the inlet and outlet. The outer peripheral surfaces of both longitudinal end portions of the structure 14 may be surrounded by the frame member 31. The frame member 31 may be made of, for example, metal. Because the frame member 31 and the structure 14 are not fixed to each other, the structure 14 can thermally expand along the inner peripheral surface of the frame member 31 when heated to a high temperature. By surrounding both longitudinal end portions of the structure 14 with the frame member 31 in this way, gas generated by heating the fibers 3 can be prevented from leaking out of the structure 14. 2, when one structure 14 is disposed in one cavity 13, both ends of the structure 14 may be supported by the frame-shaped member 31 on the inlet side and the frame-shaped member 31 on the outlet side. In this case, the protrusion 32 and the plate-shaped member 33 may not be present in the internal space of the cavity 13.

[0035] When two or more structures 14 are arranged in series in the internal space of one cavity 13, it is preferable that two adjacent structures 14 are arranged so that there is no gap between them. Furthermore, the cross sections of both structures 14 at the connection points may have the same shape. By thus preventing the internal space of the structure 14 from being connected to the external space of the structure 14 in the internal space of the cavity 13, gas generated by heating the organic fiber 3 can be prevented from leaking into the external space of the structure 14. As a result, the external space of the structure 14 in the internal space of the cavity 13 can be prevented from being contaminated by gas, thereby reducing the workload for cleaning that space. Furthermore, in this embodiment, since the structure 14 is also heated, gas generated by heating the fiber 3 can be prevented from condensing on the inner wall of the structure 14 and turning into tar. Therefore, tar removal work, etc., is also unnecessary.

[0036] The lengths of the cavity 13 and the structure 14 in the direction along the fibers 3, the temperatures of the two or more structures 14, and the moving speed of the fibers 3 are preferably determined appropriately so that the desired carbonization treatment is performed within the cavity 13. In addition, the dielectric constant of the structure 14 and the output of the microwave generator 11 that generates microwaves to be irradiated to the structure 14 are preferably determined appropriately so that the structure 14 can be heated to the desired temperature.

[0037] Here, the effect of microwave heating not only the fibers 3 but also the structure 14 will be described. Directly heating only the fibers with microwaves results in a lower ambient temperature than the fibers, cooling the fiber surfaces, resulting in temperature variations between the interior and surface of the fibers. Furthermore, moving multiple fibers in parallel, i.e., in a parallel fashion, results in uneven heating between the multiple fibers. This is because it is difficult to uniformly irradiate multiple fibers with microwaves. On the other hand, by moving the fibers 3 within the microwave-heated structure 14, the ambient temperature of the subsequent structure 14 can be made approximately the same as that of the fibers 3, thereby reducing temperature variations between the interior and surface of the fibers 3. Furthermore, when the structure 14 is made of a material with a thermal conductivity similar to or higher than that of alumina or silicon nitride, for example, the entire structure 14 achieves a uniform temperature. Therefore, heat exchange between the multiple fibers 3 and the inner wall of the structure 14 results in the temperatures of the multiple fibers 3 and the structure 14 becoming approximately the same, thereby reducing uneven heating of the multiple fibers 3. In this way, more uniform heating of the plurality of fibers 3 can be achieved, and more homogeneous carbon fibers can be produced.

[0038] The introduction section 15 is a hollow columnar member having one end connected to the introduction port of the forefront cavity 13-1 for the fibers 3. The fibers 3 are introduced into the carbonization apparatus 1 from the opening at the end of the introduction section 15 that is not connected to the cavity 13-1, and carbonization processing is performed.

[0039] The connecting portion 16 is a hollow columnar member that connects two adjacent cavities 13 among the two or more cavities 13. An opening at one end is connected to the fiber 3 outlet of the cavity 13 on the upstream side, and an opening at the other end is connected to the fiber 3 inlet of the cavity 13 on the downstream side. Because the fibers 3 are not heated in the connecting portion 16, it is preferable that the length of the connecting portion 16 in the direction along the fibers 3 be short.

[0040] The discharge section 17 is a hollow columnar member having one end connected to the outlet of the last cavity 13-3 for the fibers 3. The fibers 3 after the carbonization treatment, i.e., the carbon fibers, are discharged from the opening at the end of the discharge section 17 that is not connected to the cavity 13-3.

[0041] The cross-sectional shapes of the inlet section 15, the connection section 16, and the outlet section 17 in a plane perpendicular to the longitudinal direction of the fibers 3 may be, for example, rectangular. The inlet section 15, the connection section 16, and the outlet section 17 are preferably made of a material that is opaque to microwaves. The inlet section 15, the connection section 16, and the outlet section 17 are preferably connected to the cavity 13 so that microwaves do not leak from their respective connection points.

[0042] The temperature sensor 18 may, for example, measure the temperature of the structure 14 disposed in the cavity 13. As an example, the temperature sensor 18 may be a radiation thermometer that measures the temperature of the outer peripheral surface of the structure 14. In this embodiment, the case where the temperature of one structure 14 is measured by one temperature sensor will be mainly described, but the temperature of one structure 14 may also be measured by two or more temperature sensors 18. In this case, for example, a representative value of two or more temperatures measured by two or more temperature sensors 18 may be used as the temperature of the structure 14. The representative value may be, for example, an average value, a median value, a maximum value, or a minimum value.

[0043] The control unit 19 may control the microwave generator 11, for example, using the temperature measured by the temperature sensor 18. The control unit 19 may control the microwave generator 11, which generates microwaves to be introduced into the cavity 13 in which the structure 14 is disposed, so that the structure 14 reaches a desired temperature. More specifically, the control unit 19 may control the microwave generator 11-1 so that the structure 14-1 reaches a desired temperature. This control may be, for example, feedback control. As an example, the control unit 19 may control the temperature of the structure 14 so that it is isothermal with the target temperature of the plurality of fibers 3 moving inside the structure 14. "Isothermal" may mean, for example, the same temperature, or a temperature so close that it can be considered to be substantially the same temperature. Even in the latter case, it is preferable that the temperature of the structure 14 be close to the target temperature to a degree that can reduce temperature unevenness within a single fiber 3 and reduce temperature differences between the plurality of fibers 3. As described above, in the carbonization process, the temperature of the fiber 3 is gradually increased, so the control unit 19 may control the microwave generators 11-1, 11-2, and 11-3, respectively, so that the temperatures of the structures 14-1, 14-2, and 14-3 increase in that order, i.e., so that the structure 14 on the later stage becomes hotter.

[0044] At least one of the connecting portions 16-1, 16-2, and the discharge portion 17 may be provided with an outlet 21 for discharging gas generated by heating the fibers 3 in the preceding cavity 13. The outlet 21 may be, for example, one end of an exhaust pipe 22. In this manner, by discharging gas generated in the cavity 13 from the outlet 21 and exhaust pipe 22 provided between adjacent cavities 13 among the two or more cavities 13, or from the exhaust portion 17 for the fibers 3 in the final cavity 13-3, the structure 14, the connecting portion 16, and the inside of the exhaust portion 17 can be prevented from becoming contaminated. Note that, as an example, two or more outlets 21 and exhaust pipes 22 may be provided in the connecting portions 16-1, 16-2, and the discharge portion 17 along the arrangement direction of the multiple fibers 3. The diameter of the outlet 21 and the length of the exhaust pipe 22 may be determined so as to prevent microwave leakage through the exhaust pipe 22. As an example, the diameter of the outlet 21 may be equal to or less than ¼ of the wavelength of the microwaves irradiated within the cavity 13 .

[0045] At least one of the introduction section 15 and the connection sections 16-1 and 16-2 may be provided with a supply port 23 for supplying an inert gas into the cavity 13 at the rear stage. The supply port 23 may be, for example, one end of a supply pipe 24. In this manner, by supplying an inert gas into the cavity 13 from the introduction section 15 for the fiber 3 in the front-stage cavity 13-1 or from a supply pipe 24 and supply port 23 provided between adjacent cavities 13 among two or more cavities 13, the fiber 3 can be heated in an inert atmosphere. The inert gas is not particularly limited, but may be, for example, nitrogen, argon, or the like. The position of the supply port 23 and the flow rate and direction of the inert gas supplied from the supply port 23 may be appropriately determined using simulation or the like so that the inert gas is effectively supplied into the cavity 13. As an example, two or more supply ports 23 and supply pipes 24 may be provided in the introduction section 15 and the connection sections 16-1 and 16-2 along the arrangement direction of the multiple fibers 3. Furthermore, the diameter of the supply port 23 and the length of the supply pipe 24 may be determined so as to prevent leakage of microwaves through the supply pipe 24. As an example, the diameter of the supply port 23 may be equal to or less than ¼ of the wavelength of the microwaves to be irradiated.

[0046] The inlet portion 15, the connection portion 16, and the outlet portion 17 may be provided with, for example, a pincushion circuit 25 for preventing microwave propagation. The pincushion circuit 25 is sometimes referred to as a pincushion structure. FIG. 4A is a longitudinal cross-sectional view of the connection portion 16 showing the pincushion circuit 25, and FIG. 4B is a transverse cross-sectional view of the connection portion 16 showing the pincushion circuit 25. Note that FIG. 4B is a transverse cross-sectional view cut along a plane passing through the needle-shaped members 26. The pincushion circuit 25 may have multiple needle-shaped members 26 arranged in parallel. The longitudinal length of the needle-shaped members 26 may be, for example, ¼ the wavelength of the microwaves irradiated within the cavity 13. The needle-shaped members 26 are preferably made of, for example, a conductive material. The conductive material may be, for example, a metal. The metal is not particularly limited, but may be, for example, stainless steel, carbon steel, nickel, a nickel alloy, copper, a copper alloy, or the like. The multiple needle-shaped members 26 are preferably arranged in the introduction section 15, the connection section 16, and the discharge section 17 so as not to interfere with the movement of the fibers 3 and to prevent microwave propagation. While the present embodiment illustrates a case in which the pincushion circuit 25 is provided on the upper inner walls of the introduction section 15, the connection section 16, and the discharge section 17, the pincushion circuit 25 may also be provided on the lower inner wall. However, in order to prevent the fibers 30 from getting caught on the pincushion circuit 25 when passing the fibers 30 through the introduction section 15, the connection section 16, and the discharge section 17, it is preferable that the pincushion circuit 25 be provided on the upper inner wall. In this way, by providing the pincushion circuit 25 between adjacent cavities 13 among two or more cavities 13, microwave propagation between the cavities 13 can be prevented, and the temperature of the structure 14 in each cavity 13 can be easily controlled. Furthermore, by providing the pincushion circuit 25 in the inlet section 15 and the outlet section 17, it is possible to prevent microwaves from leaking outside the carbonization device 1 via the inlet section 15 and the outlet section 17.

[0047] Since the carbonization process is completed in the final cavity 13-3, the fibers 3 discharged from the final cavity 13-3 are carbon fibers formed by carbonizing precursor fibers, and are therefore highly conductive. Therefore, microwaves may leak to the outside via the conductive fibers 3 discharged from the final cavity 13-3. To prevent such microwave leakage, the carbonization apparatus 1 may further include, for example, a conductive means for conducting the fibers 3 to one another near the outlet of the final cavity 13-3 for the fibers 3. This conductive means may be provided, for example, in the discharge section 17. By conducting the carbon fibers in this way, the potential difference between the carbon fibers is eliminated. As a result, it is possible to prevent an electromagnetic field from being generated between the carbon fibers, and to prevent microwaves from radiating into space from the carbon fibers.

[0048] The conductive means may be, for example, a roller 27 shown in FIG. 5 . The roller 27 may be arranged in the discharge section 17 so as to be rotatable, and may be rotated by a driving means such as a motor in accordance with the moving speed of the fibers 3. The outer circumferential surface of the roller 27 is preferably made of a conductive material. The conductive material may be, for example, a metal. In the discharge section 17, the roller 27 is preferably arranged so that the direction of its rotation axis is perpendicular to the longitudinal direction of the plurality of fibers 3 and the plurality of fibers 3 are in contact with its outer circumferential surface. For example, the roller 27 may be grounded.

[0049] 5 shows a case where the roller 27 is positioned closer to the discharge outlet of the cavity 13-3 than the needle thread circuit 25, but the roller 27 may be positioned in another location. For example, in FIG. 5, the roller 27 may be positioned near the needle thread circuit 25, or may be positioned closer to the downstream opening of the discharge section 17 than the needle thread circuit 25. The roller 27 may also be positioned above the plurality of fibers 3. The plurality of fibers 3 may also be electrically connected by means other than the roller 27.

[0050] Next, a method for producing carbon fiber using the carbonization apparatus 1 according to this embodiment will be described. The fibers 3 to be carbonized are flame-resistant fibers. For example, the fibers 3 may be supplied to the carbonization apparatus 1 directly from the flame-resistant treatment device, or may be unwound from a spool and supplied to the carbonization apparatus 1. In the former case, the fibers 3 are supplied to the carbonization apparatus 1 at a temperature of approximately 250°C, while in the latter case, the fibers 3 are supplied to the carbonization apparatus 1 at room temperature. The fibers 3 are arranged in parallel and moved, for example, as shown by the arrows in FIGS. 1 and 2. The fibers 3 may be moved, for example, by being wound by a winding section downstream of the discharge section 17. It is preferable that the fibers 3 be moved at a predetermined speed, for example, 5 meters per minute.

[0051] In each cavity 13, microwave irradiation is performed so that the structure 14 reaches a desired temperature. As described above, it is preferable that microwave irradiation be performed so that the temperature of the structure 14 increases toward the later stage. This microwave irradiation may be performed by adjusting the output of the microwave generator 11 using the temperature measured by the temperature sensor 18. During the carbonization process, an inert gas such as nitrogen is supplied into the cavity 13 from the supply pipe 24, thereby heating the plurality of fibers 3 in an inert atmosphere. Gas generated by heating the plurality of fibers 3 is discharged from the discharge pipe 22. Furthermore, since the inlet portion 15, the connection portion 16, and the discharge portion 17 are provided with a pincushion circuit 25, microwave leakage at the inlet portion 15 and the discharge portion 17 can be prevented, and microwave propagation between the cavities 13 can also be prevented. In this manner, carbon fibers can be produced by continuously heating and carbonizing a plurality of precursor fibers.

[0052] As described above, the carbonization apparatus 1 according to this embodiment makes it possible to commercially produce carbon fibers by simultaneously carbonizing a plurality of fibers 3. Furthermore, when carbonization is performed using a plurality of cavities 13, the temperature of the structures 14 in each cavity 13 can be controlled independently, making it easy to control the structure 14 to a desired temperature.

[0053] Furthermore, the structure 14 partially absorbs and partially transmits microwaves irradiated from the outside, thereby allowing the fibers 3 inside the structure 14 and the structure 14 to have approximately the same temperature. As a result, the difference between the temperature on the surface of the fibers 3 and the temperature inside the fibers 3 is reduced, thereby reducing temperature variations within a single fiber 3, or more precisely, within a single fiber bundle. Furthermore, when the structure 14 is primarily made of, for example, alumina, silicon nitride, or the like, the structure 14 is heated to a more uniform temperature throughout due to its good thermal conductivity. Therefore, heat exchange occurs between the multiple fibers 3 present inside the structure 14 and the inner wall of the structure 14, thereby reducing the temperature differences among the multiple fibers 3 and reducing heating variations among the multiple fibers 3. As a result, more uniform carbon fibers can be produced.

[0054] Furthermore, since microwaves are irradiated in multimode within the cavity 13, the size of the structure 14 is not limited compared to single-mode irradiation. Therefore, by making the structure 14 larger, it becomes possible to arrange more fibers 3 in parallel and carbonize them all at once. In this case, too, as described above, there is little temperature variation within the fiber bundle and little heating variation among multiple fiber bundles, making it possible to mass-produce homogeneous carbon fibers.

[0055] Furthermore, by discharging the gas generated by heating the fibers 3 from the exhaust ports 21 provided in the connection part 16 and the exhaust part 17, it is possible to prevent the gas from contaminating the inner walls of the structure 14. In particular, by providing the exhaust port 21 in the connection part 16, it is possible to discharge the gas generated by heating the fibers 3 even during the heating of the fibers 3. Furthermore, because the structure 14 itself is also heated by microwaves, it is possible to prevent tar and the like from condensing on the inner walls of the structure 14.

[0056] Furthermore, the carbonization process can be performed in an inert atmosphere by supplying an inert gas into the cavity 13 from the supply ports 23 provided in the introduction section 15 and the connection section 16. Furthermore, in the connection section 16, the gas generated by heating the fibers 3 is discharged and an inert gas is supplied, so that the atmospheric gas can be replaced with clean inert gas midway through the heating of the fibers 3.

[0057] Furthermore, by providing pincushion circuits 25 in inlet portion 15, connecting portion 16, and outlet portion 17, it is possible to prevent microwaves from leaking to the outside via inlet portion 15 and outlet portion 17. It is also possible to prevent microwaves from propagating via connecting portion 16, making it easier to control the temperature of structure 14 in each cavity 13.

[0058] Furthermore, when multiple carbon fibers are discharged from the final cavity 13-3, the multiple carbon fibers are electrically conductive using a conductive means such as a roller 27, thereby preventing microwaves from leaking to the outside through the multiple carbon fibers.

[0059] In the present embodiment, the case where the number of cavities 13 and the number of structures 14 are three have been mainly described. However, it goes without saying that the number of cavities 13 and the number of structures 14 may be other than three. For example, the number of cavities 13 and the number of structures 14 included in the carbonization apparatus 1 may be two, four, or more. Note that the greater the number of cavities 13 and the number of structures 14, the more finely and gradually the temperature of the fibers 3 can be increased. From this perspective, therefore, it is preferable to have a greater number of cavities 13 and structures 14. For example, the number of cavities 13 and the number of structures 14 may be three or more. Note that it is preferable that the carbonization apparatus 1 has, for example, one or more microwave generators 11 and one or more waveguides 12 for each cavity 13.

[0060] Furthermore, in this embodiment, the case where one structure 14 is arranged in one cavity 13 has been mainly described, but this is not necessarily the case. For example, two or more structures 14 may be arranged in one cavity 13. FIG. 6 is a schematic vertical cross-sectional view showing an example of a carbonization apparatus 1 in which three structures 14-1, 14-2, and 14-3 connected in series are arranged inside one cavity 13. In the carbonization apparatus 1 shown in FIG. 6, the temperatures of the structures 14-1, 14-2, and 14-3 may also be measured by temperature sensor 18, and the measured temperatures may be used to control microwave generators 11-1, 11-2, and 11-3 so that the structures 14-1, 14-2, and 14-3 are each at a desired temperature. In the carbonization apparatus 1 shown in Figure 6, microwaves generated by microwave generators 11-1, 11-2, and 11-3 are irradiated mainly onto the outer peripheral surfaces of structures 14-1, 14-2, and 14-3, respectively, inside a cavity 13. With this configuration, microwaves can be irradiated so as to gradually increase the temperature of multiple fibers 3. Note that even when the carbonization process of precursor fibers is performed using one cavity 13 as shown in Figure 6, it is preferable to generate microwaves by two or more microwave generators 11.

[0061] As shown in Figure 6, when two or more structures 14 are placed in one cavity 13, the connection point between two adjacent structures 14 may have an expansion joint structure to absorb thermal expansion in the longitudinal direction of each structure 14. Figure 6 shows an example of an expansion joint structure. As shown in Figure 6, the right end of structure 14-1 may have an expanded inner circumferential surface, and the left end of structure 14-2 may have a reduced outer circumferential surface, and the left end of structure 14-2 may be inserted into the right end of structure 14-1.

[0062] Furthermore, the areas near the inlet and outlet in the internal space of the cavity 13 are connected to the outside via the inlet and outlet of the cavity 13, and are therefore locations where temperature changes can be large. Therefore, when three or more structures 14 are arranged consecutively within one cavity 13, the structure 14 on the inlet side, i.e., the foremost structure 14, and the structure 14 on the outlet side, i.e., the last structure 14, may be made of a material with high thermal shock resistance. Therefore, the structure 14 on the inlet side and the last structure 14 may contain, for example, silicon nitride.

[0063] Although the present embodiment has been described primarily with reference to a case where the structure 14 has a hollow columnar shape, this is not essential. For example, the structure 14 may have two plate-like members 14a and 14b arranged parallel to each other and facing each other, as shown in FIG. 7 . That is, the structure 14 may have a structure that does not include a surface corresponding to the short side of the cross section of the structure 14 shown in FIG. 3B . The plate-like members 14a and 14b each partially absorb and partially transmit microwaves. Therefore, the microwaves that have passed through the two plate-like members 14a and 14b are irradiated onto the plurality of fibers 3 moving inside the structure 14, i.e., the plurality of fibers 3 moving between the two plate-like members 14a and 14b. In order to maintain a constant distance between the two plate-like members 14a and 14b and to prevent gas generated by heating the fibers 3 from leaking out of the structure 14, a hollow columnar shape may be formed by two plate-like members 34 arranged parallel to each other and two plate-like members 14a and 14b. The material of the plate-shaped member 34 may be microwave-transparent and heat-insulating, for example. The plate-shaped member 34 having microwave-transparent and heat-insulating properties may be made of a material containing alumina, for example.

[0064] Although the present embodiment has been described mainly with reference to a case where microwaves are irradiated from above into the internal space of the cavity 13, the direction in which microwaves are irradiated into the internal space of the cavity 13 is not critical. For example, microwaves may be irradiated from below into the internal space of the cavity 13, or microwaves may be irradiated from both above and below. When microwaves are irradiated from both above and below into the internal space of the cavity 13, as an example, as shown in FIG. 8 , the structure 14 may have two plate-like members 14a and 14b, each of which has a length in the arrangement direction of the plurality of fibers 3 equal to the length of the plurality of fibers 3 in the arrangement direction in the internal space of the cavity 13. In this case, each of the plate-like members 14a and 14b may be supported by a protrusion 32, as shown in FIG. 8 .

[0065] Furthermore, in the carbonization apparatus 1 according to the present embodiment, a member having higher microwave transparency and heat insulation than the structure 14 may be provided so as to cover at least a portion of the outer surface of the structure 14, thereby reducing heat transfer from the structure 14 to the internal space of the cavity 13 without interfering with microwave irradiation of the structure 14. The member provided on the outer surface of the structure 14 may be provided so as to cover, for example, the outer surface of the hollow columnar structure 14 shown in FIG. 3B , the outer surface of the structure 14 included in the hollow columnar shape shown in FIG. 7 , or the entire outer surface of the structure 14 opposite the fibers 3 shown in FIG. 8 , or may be provided so as to cover only a portion of the outer surface. The shape of this member is not particularly limited, and may be, for example, a plate-shaped member. It is preferable that this member have a smaller heat capacity (heat storage capacity) per unit volume than the structure 14. For example, if the heat capacity per unit volume of a member provided on the outer surface side of the structure 14 is larger than that of the structure 14, the temperature of the structure 14 will be difficult to lower due to the influence of the member even if the microwave output is reduced when lowering the temperature of the structure 14, and it will take longer to raise the temperature of the structure 14 to the desired temperature when microwave irradiation is started to raise the temperature of the structure 14. On the other hand, this can be avoided by using a member with a smaller heat capacity per unit volume than the structure 14. The member provided on the outer surface side of the structure 14 may be made of, for example, a material made of compressed alumina fiber.

[0066] Furthermore, in this embodiment, the case where the movement direction of the plurality of fibers 3 is horizontal has been mainly described, but this is not necessarily the case. The plurality of fibers 3 may be moved in a direction other than horizontal, such as vertical. As an example, when the plurality of fibers 3 are moved in the vertical direction, it is preferable that the plurality of fibers 3 are moved from bottom to top. This is because, in the carbonization process, the temperature of the fibers 3 needs to be gradually increased, and higher temperature gas moves upward.

[0067] Furthermore, the above-described embodiments are merely examples for specifically implementing the present invention, and are not intended to limit the technical scope of the present invention. The technical scope of the present invention is defined by the claims, not by the description of the embodiments, and is intended to include modifications within the literal scope of the claims and within the scope of equivalent meanings.

Claims

1. A carbonization device for heating and carbonizing precursor fibers of multiple carbon fibers, comprising: two or more microwave generators that generate microwaves; one or more cavities into which the two or more microwaves generated by the two or more microwave generators are introduced; and two or more structures arranged in series within the one or more cavities, within which the multiple precursor fibers can be arranged in parallel, wherein each of the two or more structures partially absorbs microwaves irradiated from the outside and partially transmits them to the inside, and microwaves can be irradiated to the two or more structures along the moving direction of the multiple precursor fibers so that the structure at the subsequent stage becomes hotter.

2. A carbonization apparatus as described in claim 1, capable of irradiating microwaves to at least one of the two or more structures so that the structure becomes isothermal to a target temperature of the plurality of precursor fibers moving inside the structure.

3. A carbonization apparatus according to claim 1, wherein each of the two or more structures is in the shape of a hollow column.

4. A carbonization apparatus as described in any one of claims 1 to 3, wherein the one or more cavities are two or more cavities connected in series, the two or more microwaves are respectively introduced into the two or more cavities, and each of the two or more structures is disposed inside each of the two or more cavities.

5. A carbonization apparatus as claimed in claim 4, wherein an exhaust port is provided between adjacent cavities among said two or more cavities for exhausting gas generated in a cavity preceding said adjacent cavity.

6. A carbonization apparatus as claimed in claim 4, wherein a supply port is provided between adjacent cavities among said two or more cavities for supplying an inert gas into a cavity following said adjacent cavities.

7. A carbonization apparatus as claimed in claim 4, wherein a pincushion circuit is provided between adjacent cavities among said two or more cavities to prevent microwave propagation between said adjacent cavities.

8. A carbonization apparatus as described in any one of claims 1 to 3, further comprising a conductive means for connecting the plurality of carbon fibers to each other in the vicinity of the outlet of the last stage cavity among the one or more cavities.

9. A carbonization apparatus according to any one of claims 1 to 3, wherein the final structure among the two or more structures comprises silicon nitride.

10. A method for producing carbon fibers, which comprises heating and carbonizing a plurality of precursor fibers of carbon fibers, the method comprising the steps of: arranging the plurality of precursor fibers in parallel inside two or more structures arranged in series in one or more cavities, each of the two or more structures partially absorbing microwaves irradiated from the outside and partially transmitting them to the inside; and irradiating microwaves to the two or more structures along the moving direction of the plurality of precursor fibers so that the structure at the rear stage has a higher temperature.

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