Evolution of orientation structure of PAN fibers in thermal stabilization process and optimization method

Abstract

The application discloses a method for evolution and optimization of orientation structure of PAN fiber in a thermal stabilization process, and belongs to the technical field of fiber thermal stabilization optimization process. In view of the problem of how to optimize the orientation structure in the stabilization and carbonization process of the PAN fiber converted into carbon fiber, the application sets a temperature rising program in the fiber oxidation process, establishes an integral fitting process of a typical XRD two-dimensional diffraction spectrum of the PAN fiber, calculates the grain size, the interplanar spacing, the crystallinity and the orientation degree, tests the influence of different tensions on the fiber shrinkage behavior, analyzes the temperature, the tension / strain and the oxidation duration by using an orthogonal experiment design method, and selects the optimal parameters to complete the optimization. The application provides the orientation evolution process of the PAN fiber, the oxidized fiber and the carbon fiber, and uses the in-situ two-dimensional X-ray diffraction technology to real-time reveal the microstructure change rule of the PAN fiber in the whole drawing-temperature rising process, thereby providing a technical reference for the development of high-performance carbon fiber.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of optimizing the process of fiber heat stabilization, and particularly relates to a method for evolution and optimization of oriented structure of PAN fiber in the process of heat stabilization. BACKGROUND

[0002] The process of converting PAN (polyacrylonitrile) fiber into carbon fiber includes two most critical stages: stabilization and carbonization. PAN fiber transforms into trapezoidal structure after the stabilization process and transforms into turbine-like structure after the carbonization process. The mechanical properties (such as stiffness or modulus) of carbon fiber are highly affected by the orientation of carbon structure within the fiber. The orientation of carbon structure refers to the arrangement and alignment of graphite sheets in the microstructure of the fiber. Carbon fiber is usually composed of graphite nanosheet layers, and the orientation of these graphite sheets is affected by many factors during the manufacturing process. The resulting orientation of carbon structure plays a crucial role in determining the modulus of carbon fiber. In fibers with well-aligned carbon structure, the modulus is higher along the fiber axis, while in fibers with randomly arranged or misaligned carbon structure, the modulus is lower. Therefore, to produce high-performance materials with ideal mechanical properties, it is particularly important to control the orientation of carbon structure in carbon fiber. In the experiments described above, although the orientation degree of the precursor fiber can be improved by adjusting the spinning process, the de-orientation occurs during the pre-oxidation process due to the intense thermal motion of the molecules, which significantly reduces the orientation degree of the oxidized fiber and is not conducive to the formation of high-orientation carbon fiber, thereby affecting the mechanical properties of the carbon fiber.

[0003] There have been many studies on weakening the de-orientation effect during the pre-oxidation process, such as applying moderate tension during the pre-oxidation process to effectively inhibit the de-orientation and thus improve the overall orientation degree of the fiber. As early as 1963, Watt et al. found that applying external tension during the heat stabilization process of fiber transformation can significantly improve the strength and modulus of the fiber. By applying external tension, the shrinkage of the fiber during the heat treatment process can be significantly inhibited, thereby promoting the orientation of the fiber chains and reducing defects. The study by Chen et al. showed that the mechanical properties of stabilized PAN fiber are closely related to the orientation degree of PAN chain quasi-crystals in the surface region, and further affect the mechanical properties of carbon fiber. Most of the studies on improving the stabilization and carbonization process of PAN fiber focus on optimizing process parameters, such as heating rate, oxidation temperature, and environmental atmosphere, to obtain ideal mechanical and structural properties of carbon fiber. For example, Qin et al. significantly improved the tensile strength and Young's modulus of carbon fiber by pre-treating PAN fiber in nitrogen to inhibit thermal orientation of the fiber. In addition, researchers have also studied the effects of various additives and surface treatments on the stabilization and carbonization process, as well as the use of alternative precursors and processing techniques to improve the efficiency and sustainability of carbon fiber production. However, few studies have focused on the overall evolution of fiber orientation during the stabilization and carbonization process and further optimization of the oriented structure. SUMMARY

[0004] In order to solve the problem of how to optimize the orientation structure in the stabilization and carbonization process of the PAN fiber into carbon fiber, the application provides an orientation evolution process of the PAN fiber, the oxidized fiber and the carbon fiber, and adopts an in-situ two-dimensional X-ray diffraction technology to reveal the microstructure change rule of the PAN fiber in the whole drawing-temperature rising process in real time, so as to provide more experimental evidence and technical reference for the development of the high-performance carbon fiber.

[0005] In order to achieve the above object, the application adopts the following technical scheme:

[0006] The method for the evolution and optimization of the orientation structure of the PAN fiber in the thermal stabilization process comprises the following steps:

[0007] Step 1, setting of the temperature rising program in the fiber oxidation process;

[0008] Step 2, data analysis: establishing the integral fitting process of the typical XRD two-dimensional diffraction spectrum of the PAN fiber; calculating the grain size, calculating the interplanar spacing, calculating the crystallinity and calculating the orientation degree;

[0009] Step 3, testing the influence of different tension on the fiber shrinkage behavior;

[0010] Step 4, analyzing the key parameters such as the temperature, the tension / strain and the oxidation duration by adopting the orthogonal experimental design (DOE) method, and systematically studying the influence of the key parameters on the PAN fiber in the stabilization stage;

[0011] Step 5, selecting the optimal parameters and completing the optimization.

[0012] Further, the setting of the temperature rising program in the fiber oxidation process in the step 1 is specifically:

[0013] The first stage is a rapid temperature rising stage, and the temperature is raised from 25℃ to 150℃ at a rate of 13.5℃ / min;

[0014] The second stage is a temperature rising stage, and the temperature is slowly raised to 245℃ at a temperature rising rate of 4.75℃ / min;

[0015] The third stage is a stabilization reaction stage, and the oxidation is carried out for 3 hours at constant temperature.

[0016] Further, the integral fitting process of the typical XRD two-dimensional diffraction pattern of the PAN fiber established in step 2 is as follows: the tangent direction of the diffraction arc in the two-dimensional WAXD spectrum is defined as the axis with an azimuth angle of zero, and in data analysis, the two-dimensional diffraction pattern is converted into three one-dimensional modes, which are: full-spectrum ring scanning, equatorial scanning and azimuthal scanning; wherein, the full-spectrum ring scanning reflects the crystallinity distribution of the fiber; the equatorial scanning provides the peak orientation change along the fiber axis 2θ angle from 10° to 35°, reflecting the crystal size, interplanar spacing and other structural information of the fiber; and the change of the azimuthal intensity with φ reflects the orientation degree of the macromolecular chains in the fiber.

[0017] Further, the calculation of the grain size in step 2 is as follows: the grain size Lc of the three-dimensional reflection microcrystal is calculated by the Scherer formula, and the calculation formula is as follows:

[0018]

[0019] wherein K=0.9 is a constant factor, and λ is the wavelength of X-rays; FWHM Equ is the half peak width of the diffraction peak near 2θ ≈ 17°;

[0020] Further, the calculation of the interplanar spacing in step 2 is as follows: the interplanar spacing d value of the quasi-crystal is calculated from the 2θ angle obtained from the diffraction spectrum by the Bragg formula, and the calculation formula is as follows:

[0021]

[0022] wherein λ is the wavelength of the incident light X-rays, λ = 1.54 Å; and θ is half of the diffraction angle.

[0023] Further, the calculation of the crystallinity in step 2 is as follows: the crystallinity of the sample is evaluated by the area distribution of the crystalline peak and the amorphous region, and by integrating the 2θ scanning of the WAXD two-dimensional diffraction pattern, the corresponding one-dimensional intensity distribution curve is obtained, the total peak area under the crystal diffraction peak represents the content of the crystalline part, and the total peak area under the amorphous peak represents the content of the non-crystalline part, and the calculation formula is as follows:

[0024]

[0025] wherein f represents the crystallinity of the fiber, f cry and f amor represent the peak integral area of the crystalline phase and the amorphous phase, respectively.

[0026] Further, the calculation of the orientation degree in step 2 is as follows: the azimuthal scanning reflects the orientation degree of the crystals in the sample, and the azimuthal fitting of the PAN fiber is selected at 17°, and the azimuthal fitting of the oxidation and carbon fiber is selected at 25°.

[0027] Therefore, the calculation formula of the orientation degree coefficient H is as follows:

[0028]

[0029] wherein, represents the maximum half-peak width value of the azimuthal scan of the crystal face.

[0030] Compared with the prior art, the present application has the following advantages:

[0031] The present application provides the orientation evolution process of PAN fibers, oxidized fibers and carbon fibers, and uses in-situ two-dimensional X-ray diffraction technology to reveal the microstructure change rule of PAN fibers in the whole drawing-temperature rising process in real time, thereby providing technical reference for the development of high-performance carbon fibers. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 is (a) experimental principle schematic diagram; (b) Linkam stretching hot stage photo; (c) in-situ detection two-dimensional WAXD spectrometer photo.

[0033] Figure 2 is the schematic diagram of extracting the circular ring scan, equatorial scan and azimuthal scan from the WAXD spectrum of PAN fibers.

[0034] Figure 3 is the peak separation and fitting curve of the equatorial direction of the fiber: (a) PAN fiber; (b) oxidized fiber; (c) carbon fiber.

[0035] Figure 4 is the WAXD image of PAN fibers at different temperature stages.

[0036] Figure 5 is the equatorial scan diagram of the fiber; in terms of external tension, (a), (b, (c) represent 10 MPa, diagrams d, e, f represent 20 MPa, and diagrams g, h, i represent 30 MPa; in terms of stabilization process, a, d, g represent the XRD spectrum diagram of the temperature rising stage (Stage I), b, e, h represent the XRD spectrum diagram of the stabilization process (Stage II) at different constant temperature time at 245 DEG C; in terms of pre-carbonization and carbonization, c, f, i represent the XRD spectrum diagram after carbonization at 500 to 1100 DEG C (Stage III); the oblique dotted line represents the peak shift in the temperature rising process.

[0037] Figure 6 is the shrinkage curve schematic diagram of PAN fibers under three different drawing loads in the oxidation process.

[0038] Figure 7is the crystal orientation factor of (a) 17° peak and (b) 25° peak during oxidation and carbonization of the fiber under different tension.

[0039] Figure 8 is the full factorial design analysis chart; (a), (b) are the interaction charts of orientation degree; (c), (d) are the main effect charts of orientation degree. DETAILED DESCRIPTION

[0040] For a more complete understanding of the present application, we will now describe it in greater detail. However, the present application has various implementations and is not limited to the specific examples listed herein. The presentation of these examples is intended to deepen the overall understanding of the disclosure of the present application.

[0041] Experimental raw materials: dry-jet-spun gel spinning PAN fiber (1k fiber bundle yarn);

[0042] Characterization method:

[0043] 1. In-situ XRD characterization: In-situ two-dimensional X-ray measurement was carried out on a BRUKER D8 Advance beamline (40 KV, 40 mA). A two-dimensional area probe XRD detector was used to record the diffraction pattern, and the distance between the sample and the detector was set to 20 mm, and the incident X-ray wavelength was 1.54 Å. During the WAXD test, the fiber axis was kept in the vertical direction, perpendicular to the incident X-ray beam. The tensile load applied during oxidation was controlled by a heatable Linkam tensile fiber accessory. The experimental schematic diagram is shown in Figure 1 The original length between the two fixed ends of the Linkam tensile hot stage fiber was 15000 μm, and then the initial length was adjusted to 18000, 16000 and 16000 μm under constant loads initially set to 10, 20 and 30 MPa, respectively, to ensure the elongation or shrinkage allowance of the fiber. A thin layer of beryllium was placed between the fiber bundle and the heating stage to improve temperature uniformity, so that the fiber was uniformly heated and oxidized, and sufficient air was introduced during oxidation. The XRD in-situ diffractometer was continuously tested at a time interval of 300 seconds per frame, and the two-dimensional-WAXD spectrum and the curve of fiber length change with time were recorded in real time during the entire treatment process. The oxidized fiber was further carbonized in a tube furnace under a nitrogen atmosphere, and the carbonization temperature was selected at 500℃, 650℃, 800℃ and 1100℃, respectively.

[0044] 2. Mechanical property test: The tensile properties of the raw yarn and carbon fiber were tested using a tensile tester (XQ-1C type), and the linear density of each single yarn was measured using a vibration fineness tester (XD-1 type, Shanghai New Fiber Instrument Co., Ltd.). The measured single yarn length was 20 mm, the crosshead speed was 12 mm / min, and 25 groups of samples of each fiber were tested to reduce experimental error.

[0045] Example 1

[0046] Evolution of the orientation structure of PAN fibers in the thermal stabilization process and the optimization method, including the following steps:

[0047] Step 1, setting the temperature rising program during the fiber oxidation process;

[0048] First stage, rapid heating stage, heating from 25°C to 150°C at a rate of 13.5°C / min;

[0049] Second stage: heating stage, slowly heating to 245°C at a heating rate of 4.75°C / min;

[0050] Third stage: stabilization reaction stage, keeping constant temperature for 3 hours for oxidation.

[0051] Step 2, data analysis: establishing the integral fitting process of the typical XRD two-dimensional diffraction pattern of PAN fibers; calculating the grain size, calculating the interplanar spacing, calculating the crystallinity and calculating the orientation degree;

[0052] Figure 2 The integral fitting process of the typical XRD two-dimensional diffraction pattern of PAN fibers is shown. First, define the tangent direction of the diffraction arc in the two-dimensional WAXD spectrum as the axis with zero azimuth angle. In data analysis, the two-dimensional diffraction pattern is converted into three one-dimensional modes, which are: full spectrum ring scan, equatorial scan and azimuthal scan.

[0053] The full spectrum ring scan reflects the crystallinity distribution of the fiber; the equatorial scan can provide the peak variation orientation along the fiber axis 2θ angle from 10° to 35°, reflecting the crystal size, interplanar spacing and other structural information of the fiber; while the azimuthal intensity variation with φ reflects the orientation degree of the macromolecular chains in the fiber. Figure 3 The one-dimensional WAXD equatorial scan schematic diagram of the fiber at different stages extracted from the original two-dimensional spectrum is shown, and the peak fitting is carried out in Jade software.

[0054] The calculation of the grain size is as follows: the grain size Lc of the three-dimensional reflection microcrystal is calculated by Scherer formula, and the calculation formula is as follows:

[0055]

[0056] Where K=0.9 is a constant factor, and λ is the wavelength of X-ray; FWHM Equ is the half peak width of the diffraction peak near 2θ≈17°;

[0057] The calculation of the interplanar spacing is specifically: the interplanar spacing d value of the quasicrystal is calculated from the 2θ angle obtained from the diffraction spectrum by Bragg formula, and the calculation formula is as follows:

[0058]

[0059] Wherein, λ is the wavelength of incident light X-ray, λ = 1.54 Å; θ is half of the diffraction angle.

[0060] The calculation of the crystallinity is specifically: the crystallinity of the sample is evaluated by the area distribution of the crystalline peak and the amorphous region, and the corresponding one-dimensional intensity distribution curve is obtained by integrating the 2θ scanning of the WAXD two-dimensional diffraction chart, the total peak area under the crystal diffraction peak represents the content of the crystalline part, and the total peak area under the amorphous peak represents the content of the non-crystalline part, and the calculation formula is as follows:

[0061]

[0062] Wherein, f represents the crystallinity of the fiber, f cry and f amor respectively represent the peak integral area of the crystalline phase and the amorphous phase.

[0063] The calculation of the orientation degree is specifically: the orientation degree of the crystal in the sample is reflected by the azimuthal angle scanning, the azimuthal angle fitting of the PAN fiber is selected at 17°, and the azimuthal angle fitting of the oxidation and carbon fiber is selected at 25°;

[0064] Therefore, the calculation formula of the orientation degree coefficient H is as follows:

[0065]

[0066] Wherein, represents the maximum half peak width value of the azimuthal angle scanning of the crystal face.

[0067] Step 3, test the influence of applying different tension on the shrinkage behavior of the fiber;

[0068] Orientation evolution during heat treatment. The orientation of the fiber plays a crucial role in determining its tensile properties. During the stabilization and carbonization processes, a series of complex chemical and physical reactions will occur, which will affect the orientation of the fiber chains. Figure 4 The two-dimensional WAXD pictures of PAN fibers before and after heat treatment at different stages are shown. The characteristic diffraction peaks of PAN fibers near 17 degrees and near 29 degrees correspond to (110) crystal face and (020) crystal face respectively. In the low-angle range (2θ = 10°-30°), the diffraction peaks of PAN fibers are relatively weak and diffuse, which indicates that the orientation of the fiber is not very high. Figure 4The strong scattering observed around the central obscuration is due to the creation of nanoscale voids. As the heat treatment progresses, the 25 degree peak due to the ladder polymer formed during stabilization and the turbulent wheel-like carbon structure formed after carbonization become more and more prominent. The evolution of the XRD patterns clearly indicates the transition from the PAN precursor to the stabilized and carbon fiber structure. The narrower the azimuthal distribution, the higher the orientation. However, although the PAN fiber has excellent chain orientation, the degree of orientation gradually decreases during the stabilization and carbonization processes. Therefore, it is crucial to determine the factors that affect the orientation of the fiber structure and to understand the regularity of the orientation change.

[0069] Figure 5 The equatorial scan fitting plots of the PAN fiber during the temperature ramping process and stabilization stage are shown. As the temperature increases, the (110) and (020) peaks of PAN move to lower diffraction angles, which means the gradual increase of the interplanar spacing d value, and represents the gradual evolution of the intramolecular stress. The ratio of the (110) and (020) interplanar spacing d values is related to the packing mode of the PAN chain. The ratio of the pure polymer PAN precursor fiber is 1.726, indicating a cubic packing; as the temperature increases to 230°C, the ratio monotonically increases to 1.733, which is usually considered to be a hexagonal packing.

[0070] During the rapid heating stage (Stage I), the chemical changes that occur inside the PAN fiber are weak, and mainly physical changes occur. The initial stage of the pre-oxidation reaction occurs in the disordered amorphous region and gradually transitions to the ordered region. When the temperature exceeds 230°C, the ordered region begins to react and exhibits a dramatic feature, and complex chemical changes occur in the air environment, including cyclization, oxidation, and dehydration reactions. The reason for initiating the reaction in the amorphous region is that the comonomer is mainly concentrated in the amorphous region, which provides enough reactants for the reaction; secondly, the molecular chains in the amorphous region are arranged in disorder, and the electrostatic coupling force between the cyano groups and other intermolecular forces are relatively weak, which makes it easier for the molecular chains to adjust their conformation to undergo cyclization and other reactions, and oxygen is also easy to diffuse into the amorphous region; furthermore, compared with the amorphous region, the molecular chains in the crystalline region are arranged in order, and the intermolecular forces are stronger, so the difficulty of molecular chain conformation adjustment and oxygen diffusion in the crystalline region is significantly increased.

[0071] In Figure 5(b) (e) (h) The characteristic peaks of PAN (2Q~17° and 29°) in the XRD curves are significantly weakened and eventually disappear, while the aromatic structure at 2Q~25° is significantly enhanced. This indicates that during the temperature rising stage (Stage II), due to the chemical reaction, the fiber structure has undergone significant changes, the -C≡N group of PAN molecular chain is transformed into -C=N group, leading to the transformation of one-dimensional linear molecular chain into more stable two-dimensional ladder structure, and the (100) plane crystal structure is destroyed. During the temperature rising stage, with the increase of tension stretch, the cyclization reaction speed is correspondingly accelerated. With the continuous increase of pre-oxidation temperature, the number of ladder-shaped crystals increases, and the crystal form gradually perfects. The disappearance of the old structure and the establishment of the new structure are in synchronization. The stabilized fiber is further carbonized in nitrogen atmosphere at a temperature higher than 400°C.

[0072] In the stabilization reaction stage (Stage III), the non-carbon elements such as nitrogen, oxygen and hydrogen in the fiber gradually disappear, and the carbon content increases to more than 90wt%. It can also be seen from the X-ray diffraction curve that at this time only the single peak of aromatic and turbulent carbon structure at 2Q~25° is included.

[0073] The corresponding length changes of the fibers subjected to different tension stretches during the first and second stages of temperature rising oxidation are shown in Figure 6 The fiber length change is divided into entropy shrinkage and reaction shrinkage. During the entropy shrinkage process, PAN fiber will appear several thermal transitions, including β and a transitions. The β transition (usually 80~100°C) is related to the sequence of molecular motion in the para-crystalline phase, while the a transition (usually 130~150°C) is attributed to the glass transition and crystal / crystal transition of amorphous region, such as the transition from cubic packing to hexagonal packing structure. The main chemical reaction shrinkage behavior occurs during the stabilization reaction period above 230°C. In this stage, the applied tension has a great influence on the shrinkage behavior of the fiber. Under different tensions of 10, 20 and 30 MPa, the reaction shrinkage rates are 10.3%, 2.4% and -10.2% (negative value indicates stretching) respectively. If the tension applied during the physical shrinkage stage is too small and the relaxation is too much, the orientation degree of the fiber is low, and high-strength fiber cannot be obtained; but if the applied tension is too large, it will cause high internal tension, leading to the generation of internal tensile defects, which will remain in the final carbon fiber product, and also cannot achieve the preparation of high-strength carbon fiber.

[0074] Table 1 summarizes the crystal size and crystallinity of PAN fiber at different heat treatment stages calculated according to the WAXD fitting spectrum. The crystal size of PAN precursor fiber is about 7 nanometers, and the crystallinity is about 56%.

[0075] No significant structural changes were observed at temperatures below 130 °C. However, as the treatment temperature increased from 130 °C to 190 °C, the crystallite size increased from 69-70 A to 79-84 A, which is a 10-20% increase. In addition, the crystallinity also increased from 56% to 60-63%, which is an 8-14% increase. The results indicate that the amorphous PAN chains recrystallize when the temperature exceeds the alpha transition temperature. As can be seen in Table 1, both the crystallite size and the crystallinity reached a maximum at 190 °C and then began to decrease as the pre-oxidation temperature increased. This change is closely related to the cohesive state of the molecules. When the PAN molecular chains undergo the initial pre-oxidation process, the large number of nitrile group dipoles react, which can lead to the growth, accumulation, and recombination of small crystalline grains, resulting in a simultaneous increase in the crystallite size and the crystallinity. Near the temperature of 210 °C, the PAN crystal parameters remained in a short plateau; however, when the temperature exceeded 230 °C, the pre-oxidation reaction entered the later stage, the PAN crystals rapidly became amorphous, and the crystallinity and the crystallite size both decreased sharply. During the stabilization process, the characteristic diffraction peaks of PAN weakened significantly, and the molecular chains of the AN structure rapidly transformed into a ladder polymer structure, and the calculation of the crystallite size also shifted from the (110) plane of PAN to the (002) plane of the formed aromatic polymer or turbostratic carbon structure. After the carbonized PAN fibers were treated at temperatures ranging from 500 to 1100 °C, the crystallite size stabilized at 12-14 A. The crystallite size and the crystallinity of the fibers under different tension stretching did not differ much and basically maintained the same evolution trend.

[0076] Table 1 Crystallite size and crystallinity of PAN fibers at different heat treatment stages

[0077]

[0078] The orientation of the fiber is a key factor in determining its tensile properties, especially for the modulus. This is because the polymer chains of the fiber are arranged in parallel along the orientation direction, and when the axial breaks, the proportion of the main valence bond destruction of the highly oriented fiber increases significantly. Since the strength of the main valence bond is about 50 times that of the van der Waals force, the tensile strength and modulus of the original fiber increase with the increase of the orientation degree. During the structural transformation from the original fiber to the carbon fiber, due to the complex chemical and physical changes, it is very important to understand the relationship between the fiber orientation, the processing parameters, and the final stabilized and carbonized fibers. The orientation of the PAN crystal is evaluated according to the maximum half-peak width value (FWHM Azi ) of the azimuthal scan of the (110, 2θ ~ 17°) crystal plane. Similarly, the orientation of the aromatic polymer and the turbostratic carbon structure is obtained according to the maximum half-peak width value (FWHM Azi ) of the azimuthal scan of the (002, 2θ ~ 25°) crystal plane. FWHM AziThe smaller, the better the orientation of the fiber. Table 2 lists the orientation degree of PAN fibers at different stages of treatment under different applied tensions of 10 MPa, 20 MPa and 30 MPa, represented by the orientation half-peak width of the 17° peak, i.e. the maximum half-peak width FWHM of the azimuthal scan Azi In addition, in the comparison of three different tensions, a group of comparative experiments was also added, i.e. applying a tension of 30 MPa to stretch the PAN fiber at the initial stage of the oxidation process until the temperature rises to 245℃, lasting for 15 minutes, and then quickly reducing the tension to 10 MPa, and the rest of the conditions are the same as the formal experiment. The corresponding test data are also listed in Table 2. According to the fiber orientation degree data in Table 2, the corresponding Hermans orientation factor is calculated and plotted as Figure 7 .

[0079] When the temperature is lower than 230℃ (stage I), the main physical change occurs, and the overall orientation degree of the PAN fiber is high. This is because the tension drawing of this stage effectively inhibits the influence of physical shrinkage and alleviates the molecular chain de-orientation behavior, and at the same time the orientation of the PAN fiber shows obvious dependence on the applied tension. At a tension of 30 MPa, the orientation of the fiber can be well maintained, and the orientation coefficient is higher than 0.95. When the applied tension is reduced to 20 MPa, the final orientation coefficient is about 0.94. When the tension is further reduced to 10 MPa, a significant decrease in the orientation coefficient can be observed. At 210℃, the FWHM of the 17° peak of the PAN fiber under constant tension of 10 MPa, 20 MPa and 30 MPa is 14.83°, 10.62° and 9.32° respectively. Azi (17°) are 14.83°, 10.62° and 9.32° respectively.

[0080] Table 2 Crystalline orientation degree (FWHM of the 17° peak and the 25° peak of the PAN fiber during the oxidation and carbonization processes Azi )

[0081]

[0082] When the temperature exceeds 230℃ (stage II), the cyclization reaction is more intense, and the oxidation reaction and the cyclization reaction jointly release heat, leading to the breakage of PAN macromolecular chains and the formation of numerous structural defects. The stabilization reaction will cause the orientation of the PAN crystal to decrease significantly. Assuming that there is a comparable degree of stabilization at the same treatment stage, applying a greater tension to the PAN fiber will help to maintain a better orientation. The observed orientation decay during the stabilization process of the PAN fiber can be attributed to various factors, including physical changes and chemical changes. The transformation from PAN to aromatic polymer will cause a fundamental structural change, and the applied tension is the key to maintaining or causing the aromatic ring structure to be well oriented. In terms of the aromatic polymer Figure 7b), which is highly dependent on the applied tension but almost remains constant throughout the stabilization process. The higher the applied tension, the better the orientation of the aromatic polymer. However, the applied tension should be within a suitable range so as not to damage the crystal structure of the precursor fiber, otherwise the gel network skeleton formed during the gel spinning process will be damaged due to excessive tension, affecting the crystallinity and other structures of the precursor fiber. After stabilization at 245°C for 3 hours, the FWHM Azi (25°) of the stabilized PAN fibers under constant tension of 10 MPa, 20 MPa and 30 MPa are 38.92°, 35.87° and 33.83°, respectively. For the PAN fiber stabilized at 30 MPa and then reduced to 10 MPa, the orientation is only slightly lower than that of the fiber stabilized at 30 MPa. In summary, it is found that higher oxidative stretching force can effectively promote the stretching and orientation rearrangement of the polymer chain, which is one of the most important processing factors for achieving good fiber orientation.

[0083] After oxidation, the PAN fiber continues to be carbonized, and in the third stage, the fiber is carbonized in a tubular furnace under a nitrogen atmosphere at temperatures of 650°C, 800°C and 1100°C, respectively. Then the obtained carbon fiber is analyzed by WAXD to determine the orientation of the turbostratic carbon structure. Before the final carbonization, the fiber is pre-carbonized at 500°C to eliminate major non-carbon elements such as nitrogen and oxygen, which will be converted into byproducts such as tar gas. Therefore, the orientation change of the pre-carbonized structure is not obvious, but higher applied tension can still produce better orientation. After further carbonization at 650°C and above, the orientation difference becomes more and more obvious. After carbonization at 1100°C, the final FWHM Azi (25°) of the carbon fibers under constant tension of 10 MPa, 20 MPa and 30 MPa during oxidation are 41.64°, 36.36° and 35.29°, respectively. It can be seen that the fiber can maintain good orientation during the subsequent high-temperature carbonization process through the pre-oxidation process, therefore, the orientation control during the later high-temperature stage after pre-oxidation is a key factor for obtaining high-performance carbon fibers.

[0084] According to the above analysis, it can be known that the loss of the main orientation of the PAN crystal and the formation of the well-aligned aromatic polymer occur simultaneously during the stabilization process. The FWHM Azi (25°) of the obtained carbon fiber is consistent with the change trend of the FWHM Azi (25°) of the oxidized fiber, indicating that the orientation of the carbon structure is inherited from the formation of the aromatic polymer. In order to obtain high-modulus carbon fibers, it is necessary to improve the orientation of the turbostratic carbon structure, which depends on the orientation of the aromatic polymer formed during the stabilization process.

[0085] Step 4, the key parameters of temperature, tension / strain and oxidation duration were analyzed using the Design of Experiments (DOE) method to systematically study the effect of the key parameters on the stabilization stage of PAN fibers;

[0086] Temperature, tension / strain and oxidation duration are the key parameters of the stabilization stage of PAN fibers. By changing and optimizing these parameters, the carbon fiber product can have the desired ideal mechanical properties. In order to systematically study the effect of these parameters, this section uses the Design of Experiments (DOE) method for analysis. In this method, different levels of each parameter are selected and systematically changed, and statistical methods are used to analyze the resulting data. By conducting orthogonal experiment analysis, the effect of each parameter and their interaction can be determined. This allows the stabilization process to be optimized, thereby improving the mechanical properties of the final carbon fiber product.

[0087] The DOE test was conducted in two stages. First, two experimental variables were selected based on previously known experimental influencing factors, namely temperature and tension, and two different levels of each variable were considered. A full factorial design (n k The experimental factor (k) and its respective level (n) combination rule changes were studied. Table 1 shows the number of experiments designed at two factors, two levels and two center points, and the experimental results (2 2 +2=6), then the main effect and the interaction effect of the influencing factors were determined according to the six experimental results of different condition combinations.

[0088] Table 3 Full Factorial Design (Experimental Conditions and Results)

[0089]

[0090] Here, the aromatization index (AI) represents the degree of conversion of the linear molecular chain of PAN to a ladder polymer structure during the stabilization process. The calculation of the aromatization index depends on the characterization method. For WAXD patterns, the AI index is calculated, where I 17 and I 25 are the peak intensities at 2θ = 17° and 25°, respectively. Here, the AI value can also be calculated using the fitted peak area of the XRD pattern after complete oxidation of the fiber, then select fibers with the same degree of oxidation under different conditions for orientation comparison, and conduct full factorial design analysis. In order to achieve the same degree of stabilization, when the temperature changes from 220°C, 240°C to 260°C, the oxidation constant temperature time is shortened from 5 hours to 2 hours and 1 hour, respectively. The results of each experiment are listed in the last column of the table. Here, the FWHM Azi value of (002, 2θ ~ 25°) is selected to compare the orientation degree of the stabilized fibers.

[0091]

[0092] The main effect plot and the interaction plot of factors related to the factorial design process are shown in the following Figure 8 . The temperature and tension were analyzed and their individual effects on the orientation degree of the oxidized fiber were studied. The interaction between the two factors was not statistically significant (P-value ≥ 0.05), indicating that the combined effect of the two factors on the orientation result can be ignored; the main effect plot of the two factors showed that the effect of tension on the orientation degree was significantly stronger, while the temperature effect was much weaker. According to the results, a response surface model was established, and the determination coefficient (R 2 ) was found to be 98.35%, indicating that the prediction of the orientation degree of the oxidized fiber based on temperature and tension settings has high accuracy.

[0093] The experimental results show that the internal orientation structure of the oxidized fiber is directly related to the stretching tension applied during the oxidation process. Specifically, compared with the fiber oxidized at 10 MPa tension, the orientation structure of the fiber oxidized at a higher tension of 30 MPa is significantly improved. The higher the temperature of the pre-oxidation reaction process, the faster the reaction speed can be seen from the AI fitting data, and the shorter the time to reach the final optimal orientation degree, but too high a temperature will affect the dynamic balance of the crystal melting and cyclization reaction during the oxidation reaction process. Therefore, by adjusting key factors such as oxidation temperature, residence time, and tension stretching, reliable experimental data can be provided for optimizing the process parameters of fiber oxidation.

[0094] Step 5, select the optimal parameters, complete the optimization.

[0095] Optimization of oxidation process conditions. In the continuous production process of fibers, several consecutive temperature rising zones are usually set up, the purpose of which is to gradually increase the tension as the fiber passes through higher temperatures, thereby maintaining the orientation of the fiber. This is very important for the final properties of carbon fibers, because the higher the orientation degree, the higher the fiber strength and stiffness. However, the raw silk cannot be directly subjected to high tension, firstly because it may break directly without forming a certain stable structure during the oxidation reaction stage, and secondly because it is easy to produce internal tensile defects. Combining the experimental content of the previous two parts, in order to determine the optimal tension and other parameter conditions during the oxidation process of the fiber, this section conducts experiments by designing temperature gradients. Here, two temperature zones of 220°C to 260°C and three temperature zones of 220°C, 240°C to 260°C are selected for testing, and the fiber is oxidized at corresponding tension levels at different temperatures. The tension level applied in the first stable stage can not be the final set value, but can be gradually increased to the set value in the subsequent temperature zones, and this approach is also to avoid the direct breakage of the fiber due to its difficulty in bearing tension at the initial stage. Table 4 summarizes the detailed experimental conditions and lists the final orientation half-peak width of the stabilized fiber.

[0096]

[0097] In the stabilization process, the selection of oxidation time and drawing tension is crucial for achieving high orientation degree of PAN fibers. Based on the results of previous orthogonal experiments, the oxidation time of each temperature zone was set to 40 minutes in the two-zone stabilization process. In the initial low-temperature oxidation stage of the fiber, a tension of 30 MPa was used, which was the highest tension determined for this stage according to previous experiments. These specific parameters were chosen to ensure that the fiber was sufficiently stabilized to maintain its orientation while avoiding excessive oxidation or excessive stretching that could damage the fiber. To determine the optimal tension in the high-temperature stage, experiments were conducted with continuous tensions of 45, 50, 55, 60, and 65 MPa. The results showed that as the tension increased, the orientation degree of the fiber also gradually increased. However, it was observed that when the tension exceeded 55 MPa, the fiber became very unstable and was prone to breaking. Therefore, the optimal tension in the high-temperature stage was determined to be 55 MPa. Under this condition, the FWHM Azi (25°) was 31.99°, significantly lower than the 33.83° in the single-zone stabilization in Table 2. In the three-zone stabilization process, the duration of each temperature zone was 40 minutes. Compared with the two-zone stabilization process, the fiber was less prone to breaking under the same tension and could maintain good orientation. Under the design of temperature zone gradient of 220-240-260°C and tension gradient of 30-55-65 MPa, the preferential orientation degree of the oxidized fiber was the best.

[0098] In summary, the present application discloses the evolution of the orientation from PAN precursor fibers to stabilized fibers and to carbonized fibers, and also establishes a high-efficiency, time-saving method for optimizing the stabilization conditions. These results provide valuable insights into the effects of drawing tension and oxidation temperature on the orientation and crystal structure of PAN fibers during the oxidation process, and show that controlling the tension during the oxidation process is extremely important for determining the intrinsic orientation structure of the fibers, and directly affects the final structure of the carbon fibers. During the thermal stabilization of PAN fibers, the orientation degree of the fibers increases with increasing tension, and at a tension of 30 MPa, the final orientation factor of the oxidized fibers can be as high as 0.81. When the carbonization temperature exceeds 500°C, the orientation degree of the obtained carbon fibers remains consistent with that of the stabilized fibers, indicating that the orientation of the carbon structure is inherited from the orientation of the aromatic polymer. The higher the orientation degree of the fiber structure, the greater the tensile modulus of the carbon fibers. Orthogonal experimental analysis shows that there is a clear positive correlation between the drawing tension and the orientation factor. Therefore, a three-temperature-zone continuous oxidation process was simulated to increase the tension, and it was found that the oxidized fibers can stably withstand a tension of 65 MPa, and an optimal orientation factor of 0.83 was obtained. These results show that the occurrence of cyclization, oxidation, and dehydrogenation reactions during the pre-oxidation of PAN fibers is significantly affected by the oxidation temperature, oxidation time, and oxidation drawing, and selecting an appropriate fiber drawing tension in the corresponding temperature zone is beneficial to the improvement of the final strength of the carbon fibers.

[0099] These findings of the present application are of great significance for optimizing the processing conditions to produce high-performance carbon fibers with ideal orientation and crystal structure. The use of in-situ two-dimensional XRD diffraction provides a powerful tool for characterizing the structural changes of the fibers during the oxidation process, and can be used to guide the optimization of the processing parameters.

[0100] The contents not described in detail in the specification of the present application belong to the prior art known to those skilled in the art. Although the above describes the specific embodiments of the present application in order to facilitate the understanding of the present application by those skilled in the art, it should be clear that the present application is not limited to the scope of the specific embodiments, and for those skilled in the art, it is obvious that all the inventions utilizing the concept of the present application are within the scope of the present application, as long as various changes are within the spirit and scope of the present application defined and determined by the appended claims.

Claims

1. A method for the evolution and optimization of the orientation structure of PAN fibers during a heat stabilization process, characterized in that, Includes the following steps: Step 1, setting the temperature rise program during fiber oxidation; Step 2, Data Analysis: Establishing the integral fitting process for the typical two-dimensional XRD diffraction pattern of PAN fibers; Calculate grain size, interplanar spacing, crystallinity, and orientation. Step 3: Test the effect of applying different tensions on fiber shrinkage behavior; Step 4: The orthogonal experimental design method is used to analyze the key parameters such as temperature, tension / strain and oxidation duration, and to systematically study the influence of key parameters on PAN fibers in the stabilization stage. Step 5: Select the optimal parameters to complete the optimization; The optimization results show that a three-temperature-zone stabilization process is adopted under a temperature gradient of 220-240-260℃ and a tension gradient of 30-55-65MPa, with each temperature zone lasting for 40 minutes.

2. The method for the evolution and optimization of the orientation structure of PAN fibers in the heat stabilization process according to claim 1, characterized in that: The specific setting of the heating program during fiber oxidation in step 1 is as follows: The first stage is the rapid heating stage, where the temperature is increased from 25°C to 150°C at a rate of 13.5°C / min. The second stage: the heating stage, slowly heating to 245℃ at a heating rate of 4.75℃ / min; The third stage: stabilization reaction stage, oxidation is carried out at a constant temperature for 3 hours.

3. The method for the evolution and optimization of the orientation structure of PAN fibers in the heat stabilization process according to claim 1, characterized in that: The integral fitting process for establishing a typical two-dimensional XRD diffraction pattern of PAN fibers in step 2 is as follows: the tangent direction of the diffraction arc in the two-dimensional WAXD spectrum is defined as the axis with zero azimuth angle. In data analysis, the two-dimensional diffraction pattern is converted into three one-dimensional modes: full-spectrum ring scan, equatorial scan, and azimuth angle scan. The full-spectrum ring scan reflects the crystallinity distribution of the fiber; the equatorial scan provides the peak orientation variation along the fiber axis at an angle of 2θ from 10° to 35°, reflecting the crystal size and interplanar spacing information of the fiber; the change in azimuth angle intensity with φ reflects the orientation degree of the macromolecular chains within the fiber.

4. The method for the evolution and optimization of the orientation structure of PAN fibers in the heat stabilization process according to claim 1, characterized in that: The specific calculation of grain size in step 2 involves calculating the grain size Lc of the three-dimensional reflective microcrystal using the Scherer formula, as follows: ； Where K=0.9 is a constant factor, and λ is the wavelength of the X-ray; FWHM Equ It is the half-width of the diffraction peak near 2θ≈17°.

5. The method for the evolution and optimization of the orientation structure of PAN fibers in the heat stabilization process according to claim 1, characterized in that: The calculation of interplanar spacing in step 2 specifically involves: obtaining the interplanar spacing d of the quasicrystal from the 2θ angle obtained from the diffraction spectrum using the Bragg formula, as follows: ； Where λ is the wavelength of the incident X-ray, λ = 1.54 Å; θ is half of the diffraction angle.

6. The method for the evolution and optimization of the orientation structure of PAN fibers in the heat stabilization process according to claim 1, characterized in that: The crystallinity calculation in step 2 is specifically as follows: the crystallinity of the sample is evaluated by the area distribution of crystalline peaks and amorphous regions. A one-dimensional intensity distribution curve is obtained by performing a 2θ scan integration on the WAXD two-dimensional diffraction pattern. The sum of the peak areas below the crystalline diffraction peaks represents the content of the crystalline portion, while the sum of the peak areas below the amorphous peaks represents the content of the amorphous portion. The calculation formula is as follows: ； Where f represents the crystallinity of the fiber, f cry and f amor These represent the peak integral areas of the crystalline phase and the amorphous phase, respectively.

7. The method for the evolution and optimization of the orientation structure of PAN fibers in the heat stabilization process according to claim 1, characterized in that: The specific calculation of orientation degree in step 2 is as follows: the azimuth angle scan reflects the orientation degree of the crystals in the sample. The azimuth angle fitting of PAN fiber is selected at 17°, and the azimuth angle fitting of oxide and carbon fiber is selected at 25°. Therefore, the formula for calculating the orientation coefficient H is as follows: ; in, This represents the maximum half-width at half maximum (FWHM) value of the azimuth scan of the crystal plane.

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