Patent ID: 12215441

wherein:1—carbon fiber;2—metal cavity;3—microwave feed port;4—impedance matching adjuster;5—dielectric unit;6—ceramic support;7—electromagnetic wave incident on upper top surface;8—diffraction peak of electromagnetic wave incident on upper top surface;9—electromagnetic wave incident on bottom up surface;10—diffraction peak of electromagnetic wave incident on bottom up surface; and11—ohmic loss heating efficient region of carbon fiber.

DETAILED DESCRIPTION

The technical solutions of the present application will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the described examples are part of the examples of the present application, rather than all of the examples. Based on the examples in the present application, all other examples obtained by those of ordinary skill in the art without making inventive labor belong to the scope of protection of the present application.

In addition, the technical features involved in different embodiments of the present application described below may be combined with each other as long as they do not conflict with each other.

The present application provides a method for preparing medium- and high-modulus large tow carbon fibers, including:performing microwave graphitization on low-modulus large tow carbon fibers, specifically performing in-phase microwave heating on the large tow carbon fibers by a microwave graphitization furnace while regulating and controlling a current density on surfaces of the low-modulus large tow carbon fibers, so that a current density on the surface of each carbon fiber is the same or nearly the same, the current density being controlled to be in the range of 60-330 A/m, thereby rapidly increasing the surface temperature of the large tow carbon fibers in the microwave graphitization furnace to 2000-3000° C. within 90 seconds, and completing a graphitization process to obtain the medium- and high-modulus large tow carbon fibers.

In this application, the heating process of microwave heating is different from that of traditional heating. Referring toFIGS.1aand1b,FIG.1ashows the traditional heating of which a heat flow is from the outside to the inside, andFIG.1bshows the microwave heating of which a heat flow is from the inside to the outside. Compared with the conventional heating, the microwave heating has low heat preservation requirements on the system, and the microwave heating can increase the internal energy of carbon atoms, increase the transition frequency of carbon atoms, accelerate the formation rate of a graphite layer in the carbon fibers, and reduce the high temperature treatment time, and is beneficial for maintaining the strength of carbon fibers. Furthermore, the present application uses in-phase microwaves, i.e., a signal from each microwave feed port is radiated into a microwave process cavity at the same phase and intensity, thereby reducing the electromagnetic field cancellation and the uncontrollable electromagnetic field distribution caused by a random phase difference, and achieving uniform heating of the carbon fibers in the microwave graphitization furnace.

On the basis of in-phase microwaves, by regulating and controlling the current density on the surfaces of the carbon fibers in the microwave graphitization furnace to be in the range of 60-330 A/m, and the current density on the surface of each carbon fiber to be the same or nearly the same, the carbon fibers undergo molecular structure reorganization by a high temperature generated by the same dielectric loss and ohmic loss heating effects, resulting in rapid formation of high-modulus structures through molecular crystallization, the diameter of the carbon fibers decreases from 7.0 μm to 6.3 μm, and the tensile modulus of the carbon fibers rapidly increases to 300-600 GPa. However, when the surface current density exceeds 330 A/m, a severe point discharge effect of the carbon fibers in the microwave electric field will be caused, and the local temperature exceeds the maximum withstand temperature of the carbon fibers being 3000° C., forming overburning, so that the tensile strength of the carbon fibers is greatly reduced, thereby weakening the effect of improving the tensile modulus.

The microwave graphitization furnace adopts an in-phase microwave design, and a dielectric periodic structure is disposed in the microwave graphitization furnace. The regulation and control of the surface current density of the carbon fibers can be achieved by the dielectric periodic structure, and the combined effect of the in-phase microwaves and the dielectric periodic structure disposed in the furnace can stably concentrate and distribute a microwave energy in a distribution region of the dielectric periodic structure, improving the stability and efficiency of the heating of the carbon fibers.

Referring toFIGS.2and3, a microwave graphitization furnace used in the present application is shown, including a metal cavity2, the metal cavity2is provided with an even number of microwave feed ports3, the even number of microwave feed ports3are evenly distributed in an upper top surface and a lower bottom surface of the metal cavity2, providing microwaves to the metal cavity2, and each microwave feed port3is further provided with a corresponding impedance matching adjuster4, so that electromagnetic impedances input into the microwave graphitization furnace can be adjusted so that microwave phases input into the furnace exhibit in-phase to achieve an optimal microwave energy use efficiency. The metal cavity2is internally provided with a ceramic support6on which the dielectric periodic structure is disposed, a cavity with a size smaller than that of the metal cavity2is formed in the dielectric periodic structure by a plurality of dielectric units5, and the carbon fibers1can pass through the cavity composed of the dielectric units5. The dielectric periodic structure includes an upper top surface, a lower bottom surface, a front side surface, and a back side surface, the upper top surface and the lower bottom surface each consisting of a plurality of dielectric units5arranged equidistantly in parallel, the dielectric units5being arranged in a direction perpendicular to the direction of travel of the carbon fibers1. In addition, the dielectric units5on the upper top surface and the lower bottom surface of the dielectric periodic structure may be arranged in a direction parallel to the direction of travel of the carbon fibers1, or at other angles. The periodic structure is made of graphite or silicon carbide, or a carbide-related combination.

A distance between the upper top surface and the lower bottom surface of the dielectric periodic structure in the microwave graphitization furnace and the carbon fibers is denoted as d1, and a distance between the upper top surface and the lower bottom surface of the dielectric periodic structure and the microwave feed ports at the same side is denoted as d2.

The dielectric periodic structure can affect the electromagnetic field distribution, and by adjusting the position of the periodic structure in the metal cavity, a specific electromagnetic field distribution can be adjusted to concentrate the microwave energy on the carbon fibers. Taking the dielectric periodic structure shown inFIGS.2and3, that is, the condition that the dielectric units on the upper top surface and the lower bottom surface of the dielectric periodic structure are arranged in the direction perpendicular to the direction of travel of the carbon fibers as an example, a schematic diagram of the periodic structure and in-phase microwave non-thermal effects is in particular shown inFIG.4, within the microwave graphitization furnace, an electromagnetic wave7incident on the upper top surface passes through the dielectric units5, forming a diffraction peak8of the electromagnetic wave incident on the upper top surface below the carbon fibers1, an electromagnetic wave9incident on the bottom up surface passes through the dielectric units5, forming a diffraction peak10of the electromagnetic wave incident on the bottom up surface above the carbon fibers1, a vertical distance D from any point on the diffraction peak8of the electromagnetic wave incident on the upper top surface and the diffraction peak10of the electromagnetic wave incident on the bottom up surface to the carbon fibers represents the intensity of the diffraction peak of the electromagnetic wave, the larger the distance D, the larger the intensity of the diffraction peak, the diffraction peak8of the electromagnetic wave incident on the upper top surface and the diffraction peak10of the electromagnetic wave incident on the bottom up surface are both projected on the carbon fibers1to form an ohmic loss heating efficient region11of the carbon fibers, and since the direction of travel of the carbon fibers is horizontal, the ohmic loss effect of the carbon fibers is maximized while maintaining the uniformity of heating. In addition, by regulating and controlling the position of the dielectric periodic structure inside the metal cavity, the surface current density of the carbon fibers is controlled to be 60-330 A/m, the ohmic loss heating effect is enhanced, the energy utilization efficiency is improved, and the tensile modulus of the carbon fibers is increased.

Carbon fibers in in-phase microwave heating:

PCarbon⁢fiber=∫(εS⁢σb⁢TS4·dAS·14⁢π⁢dS2)·(dACF)+∫d⁡(I2⁢R)

Carbon fibers in traditional heating:

PCarbon⁢fiber=∫(εS⁢σb⁢TS4·dAS·14⁢π⁢dS2)·(dACF)

wherein:εs: a radiation coefficient of a dielectric material;ds: a horizontal distance of a dielectric from a center of the periodic structure, i.e., d1in the present application;σb: a Stefan-Boltzmann constant;ACF: a carbon fiber area;As: a dielectric area;I: a surface current of carbon fibers;R: a carbon fiber radius; andTs: a dielectric temperature.

As can be seen from the comparison of the above formulas, the heating efficiency of in-phase microwaves also includes ohmic loss heating compared with conventional heating. The heating efficiency and heating uniformity of the carbon fibers are greatly improved.

Referring toFIGS.5aand5b,FIG.5ais a schematic diagram when a surface current density of simulated carbon fibers is 170 A/m,FIG.5bis a graph showing the corresponding actual heating effect of the carbon fibers in the microwave graphitization furnace, and a white zone is a thermal energy concentration zone. It can be seen that the surface current density of the simulated carbon fibers corresponds to a heating zone of carbon fibers in the microwave graphitization furnace.

Further, the effect of the control of the surface current density of the carbon fibers on the tensile modulus of the carbon fibers during in-phase microwave heating in the present application is further illustrated by the following examples:

The comparative example used raw large tow carbon fibers, using PAN-based large tow carbon fibers produced by SGL, model: CT50-4.4, 50k, with a standard tensile modulus of 255 GPa, 10 samples were randomly selected and tested to record property data; and the Examples used carbon fibers obtained after microwave graphitization of raw large tow carbon fibers at different surface current densities of carbon fibers, and 10 samples were randomly selected and tested to record property data. The comparison of property data in Comparative example and examples is shown in Table 2.

TABLE 2SurfacecurrentofcarbonTensile modulus (GPa)Dis-fiberSampleSampleSampleSampleSampleSampleSampleSampleSampleSampleMeanStandardpersion(A/m)12345678910(GPa)deviation(%)Com-/245248254256262262266253251254255.216.552.568parativeExampleExample0265272279302282283275293288300283.9011.994.2230307300301304302299310302295296301.604.61.5260374373376379381386375380376373377.314.181.109100433436437441431450441435432444437.815.981.366170509523525518521526530530524510521.397.311.401240426428423439433424433426438435430.516.011.397330410411406406402401404399408400404.764.151.024440315327327317316320328323326322322.144.81.491500302302299310301300304302312305303.664.251.399

A graph of a tensile modulus of carbon fibers versus a surface current density of carbon fibers is formed according to Table 2, specifically referring toFIG.6. As can be seen from Table 2 andFIG.6, after microwave graphitization of the raw carbon fibers, the tensile modulus of the carbon fibers is changed, specifically: (1) after graphitization when the surface current density of the carbon fibers is set to be less than 60 A/m, the increase in tensile modulus of the obtained carbon fibers is small; (2) after graphitization when the surface current density of the carbon fibers is set to be within an interval of 60-330 A/m, the tensile modulus of the obtained carbon fibers is significantly increased to 300-600 GPa, and the dispersion of the tensile modulus can be controlled to be within 1.5%; and (3) after graphitization when the surface current density of the carbon fibers is set to exceed 330 A/m, the effect of improving the tensile modulus of the carbon fibers begins to weaken.

Also, the inventors found that by the microwave graphitization method of the present application, the carbon fibers undergo molecular structure reorganization by high temperature generated by the same dielectric loss and ohmic loss heating effects, resulting in rapid formation of high-modulus structures through molecular crystallization, so that the diameter also changes, and the diameter decreases from 7.0 μm to 6.3 μm on average, and a comparison of a diameter change of the carbon fibers before and after graphitization is shown inFIG.7.

In addition, with respect to the regulation and control of the surface current density of the carbon fibers, when the periodic structure is made of graphite or silicon carbide, the position relationship of the dielectric periodic structure in the metal cavity of the microwave graphitization furnace is shown in Table 3:

TABLE 3ExampleDielectricSurface currentNo.materiald1d2density (A/m)1Graphite1026050021526033033020030430225605352151006Silicon152605007carbide25250400830180309302102401035200170

As can be seen from Table 3, by adjusting the position of the dielectrics inside the microwave graphitization furnace, the regulation and control of the current density on the surfaces of the carbon fibers can be achieved, when the dielectric is made of graphite, an optimal regulation and control range for d1is 15-35 mm, and an optimal regulation and control range for d2is 200-260 mm; and when the dielectric is made of silicon carbide, an optimal regulation and control range for d1is 30-35 mm and an optimal regulation and control range for d2is 200-210 mm.

The present application also provides a medium- and high-modulus large tow carbon fiber, having a tensile strength of being up to 3.5-5.0 GPa, a tensile modulus of being up to 300-600 GPa, and a dispersion of the tensile modulus of less than 1.5%.

To sum up, the method for preparing medium- and high-modulus large tow carbon fibers provided in this application can stably increase the modulus of the low-modulus large tow carbon fibers to medium and high modulus, and has a significant breakthrough in the technical field of production of medium- and high-modulus large tow carbon fibers at home and abroad.

Obviously, the above examples are merely instances for clearly illustrating the present application, and are not intended to limit the embodiments. For those of ordinary skill in the art, other different forms of variations or changes can be made on the basis of the above description. It is unnecessary and impossible to enumerate all the embodiments here. Obvious variations or changes derived therefrom are still within the scope of protection of the present application.