Microstructure and properties of polyacrylonitrile based carbon fibers

Journal Pre-proof Microstructure and properties of polyacrylonitrile based carbon fibers Junshan Lu, Weiwei Li, Hongliang Kang, Libang Feng, Jian Xu, Ruigang Liu

PII:

S0142-9418(19)31698-8

DOI:

https://doi.org/10.1016/j.polymertesting.2019.106267

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POTE 106267

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Polymer Testing

Received Date: 20 September 2019 Revised Date:

27 November 2019

Accepted Date: 28 November 2019

Please cite this article as: J. Lu, W. Li, H. Kang, L. Feng, J. Xu, R. Liu, Microstructure and properties of polyacrylonitrile based carbon fibers, Polymer Testing (2020), doi: https://doi.org/10.1016/ j.polymertesting.2019.106267. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Microstructure and Properties of Polyacrylonitrile Based Carbon Fibers Junshan Lu a,b, Weiwei Li a, Hongliang Kang a, Libang Feng b*, Jian Xu a, Ruigang Liua,c* a

Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular

Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China b

School of Mechatronic Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China

c

University of Chinese Academy of Sciences, Beijing 100049, China

* Corresponding authors: [email protected] (Feng); [email protected] (Liu). Tel. & Fax: +86-10-82618573

Graphical abstract

ABSTRACT The microstructure of polyacrylonitrile (PAN)-based carbon fibers with different mechanical properties was investigated. It was found that the tensile strength of the PAN-based carbon fibers generally decrease with the increase in the modulus. The properties of PAN-based carbon fiber are mainly controlled by the microstructure and microvoids. The increase in size and orientation of graphite crystallites follows the decrease in interlayer space of graphite sheets, which accompanies the increase in modulus and decrease in tensile strength of the carbon fibers. Simultaneously, the increase in the modulus of the carbon fibers accompanies the merging of the elliptical microvoids along the fiber axis and the turbostratic graphite in the carbon fibers transforms into 3D ordered graphite lamellar structure. This work provides useful information on tailoring the mechanical properties of carbon fibers by adjusting the microstructure. Keywords: PAN; Carbon Fibers; Microstructure; Mechanical Properties; Microvoids

1. Introduction Carbon fibers are highly desirable for high performance composites in the fields of aerospace, traffic and other industries. They generally exhibit superior performance, including high strength, high modulus, and low density. There are polyacrylonitrile (PAN)-, pitch- and rayon-based carbon fibers according to their precursors. The PAN-based carbon fibers are the most important one, which occupy about 90% of the carbon fiber market. In the production of PAN-based carbon fibers, stabilization of the PAN precursor fibers was first carried out in the range of 200-300 °C in air atmosphere, during which the linear PAN chains were transform into ladder structure for standing further heat treatment. The carbonization process was generally carried out in the temperature of 400-1600 °C in an inert atmosphere to remove

hydrogen, oxygen, nitrogen, and other non-carbon elements, during which the turbostratic graphite structure were formed. For preparing high strength carbon fibers, the maximum temperature of carbonization should be controlled at around 1500 °C. In order to prepared high modulus carbon fibers, graphitization at a high temperature of 2200–3000 °C is critical, during which the turbostratic graphite structure in the carbon fibers are rearranged into high orientated graphite sheets along the fiber axial direction [1-9]. The mechanical properties of carbon fibers have close correlation with the microstructures, e.g. the size and orientation of graphite crystals, the interlayer stacking of the graphite layers, the microvoids and other defects, etc. The understanding of the relationship between the microstructure and the performance of carbon fibers is critical for the purpose to improve the quality of carbon fibers. Efforts have been paid to investigate the dependence of the mechanical properties of carbon fibers on the processing parameters. In-situ detection of the formation and development of the microstructures [10-13] and the evolution of skin-core structure and its influence on the properties of the carbon fibers have been discussed [14, 15]. During the graphitization process at high temperature, the graphite layers are rearranged and stacked closer to form larger graphite crystals in a relatively higher ordered structure, during which the wave-like structure composed by graphite crystals and amorphous carbon change into strip-like structures [5, 16-18]. Besides the microstructures, the content, size, and shape of the microvoids in the carbon fibers also play a critical role to control the mechanical properties of carbon fibers. Microvoids can be form in the spinning, oxidation, and carbonization, as well as the surface activation during the preparation of PAN-based carbon fibers [4-10, 19, 20], with will limit the tensile strength of PAN-based carbon fibers. Several methods have been applied to characterize the microstructures and microvoids of the carbon fibers, among which wide X-ray diffraction (WAXD), Raman spectroscopy, and small angle X-ray scattering (SAXS) are the most effective ones [4, 9]. The parameters of the

graphite crystals in carbon fibers, such as the interlayer-distance (d002) of the graphite crystals, crystal size (La), and stack size (Lc) can be ready obtained from the WAXD diffraction patterns of carbon fibers [21-25]. The content of the disorder in carbon fibers can be estimated by using Raman spectroscopy [26-30], which was first applied to characterize the graphite and carbon materials in early 1970s [31], and then widely applied in the characterization of versatile carbon materials [32-35], including carbon fibers. SAXS are widely used to characterize microstructure in the dimension of 1–100 nm. The application of SAXS in the characterization of microvoids in carbon fibers have been established and developed both in theory and experimental methods [4, 9, 16, 36]. It is generally accepted that the properties of the carbon fibers are determined by the microstructure and microvoids. However, the correlation between the properties and microstructure of the carbon fibers has not yet fully understood. In this work, a series of carbon fibers in different modulus were carefully selected from a pilot plant line. The microstructure and microvoids of the carbon fibers were characterized by using WAXD, SAXS, Raman spectroscopy, and SEM. The correlations between the properties and microstructure of the carbon fibers will be discussed in details. The results of the present work provide helpful information for understanding the correlations between the microstructure and properties of carbon fibers, which is helpful for the tailoring high performance PAN-based carbon fibers with desired properties.

2. Experimental sections 2.1 Samples and preparation A series of PAN-based carbon fibers with different properties were kindly supplied by Sinofibers Technology Co., Ltd. (Changzhou, China). The details of the PAN-based carbon fibers are listed in Table 1. For the preparation of the PAN-based carbon fibers used in this work, PAN precursor fiber with comonomers of AN/MA/IA (97.5/1.3/1.2, mass ratio) was

used. The samples were denoted as CF-modulus, for example, CF-250 indicates the carbon fiber with the modulus of 250 GPa. The final heat treatment temperature is 1600 °C for CF-250, CF-280, and CF-300, 2500 °C for samples from CF-350 to CF-460, 2700 °C for samples from CF-480 to CF-520, and 2830 °C for samples from CF-540 to CF-580. The carbon fibers were stretched under the stress of 15 MPa continuously. The carbon fibers were washed with acetone in a reflux condenser for 24 h to remove the sizing-agents and dried before measurements. Table 1. The details of the carbon fibers investigated in present work. Samples Tensile strength (GPa) CF-250 5.00±0.22 CF-280 5.00±0.05 CF-300 5.10±0.15 CF-350 5.00±0.09 CF-380 4.81±0.21 CF-400 4.66±0.13 CF-420 4.35±0.12 CF-460 4.19±0.14 CF-480 4.17±0.18 CF-500 4.33±0.10 CF-520 4.11±0.16 CF-540 4.24±0.18 CF-560 3.88±0.18 CF-580 3.70±0.15

Young’s modulus (GPa) 250±0.75 280±3.49 300±3.19 350±2.80 380±3.52 400±3.94 420±1.63 460±4.42 480±6.16 500±4.94 520±2.64 540±4.86 560±5.05 580±8.61

Elongation (%) 2.00±0.08 1.79±0.03 1.70±0.05 1.53±0.03 1.27±0.06 1.16±0.03 1.03±0.03 0.91±0.04 0.87±0.03 0.86±0.04 0.79±0.02 0.78±0.03 0.69±0.03 0.64±0.03

2.2 Characterizations 2.2.1 Mechanical properties test The mechanical properties of the carbon fibers were determined on a tensile tester (Haida HD-B604-S, China) in multifilament bundle (6K) using gauge length of 150 mm and a strain rate of 10 mm/min. Each sample was tested for 6 times, and all the tests were performed under the temperature of 24 °C and humidity of 48%. 2.2.2 Scanning electron microscopy (SEM) SEM observation was performed on a field emission scanning electron microscopy (JEOL 6700F, Japan) operating at the acceleration voltage of 5 kV and current of 10 µA. The cross

section of carbon fibers was obtained by fracture of the carbon fibers in liquid nitrogen. 2.2.3 Raman spectroscopy Raman spectroscopy measurements were carried out on a thermo scientific DXR micro focused Raman spectrometer (Thermo Fisher Scientific, Germany) equipped with a 532 nm argon-ion laser beam (10 mW) at room temperature. The diameter of the laser spot on the sample surface was 1.3 µm, the numerical aperture was 25 µm. Each spectrum was obtained by 21 scans at the resolution of 3 cm–1 within the collect exposure time of 20 s. Curve fitting for the determination of spectral parameters was performed with the software program Origin 2017, using the Lorentz to get the peak positon and peak width. 2.2.4 Wide X-ray diffraction (WAXD) WAXD experiments were carried out on beamline BL16B1 at Shanghai Synchrotron Radiation Facility (SSRF), and the X-ray wavelength was 0.124 nm. The distance between the samples to the detector was 204 mm and calibrated by using AgB. Carbon fibers were aligned parallel and fixed by polyimide film. X-ray scattering data were corrected for background scattering. The primary data were evaluated using the software package FIT2D. Curve fitting using Gaussian function to get the peak position and peak width. The size of crystal (L) was calculated by Scherrer equation, =

cos

(1)

where λ is the X-ray wavelength; β is a full width at half maximum intensity (FWHM); θ is a Bragg diffraction angle, K is Scherrer factor of 0.89. The interlayer spacing of the graphite layer (002) was calculated by Bragg equation, =

2 sin

(2)

where λ is X-ray wavelength; θ is a Bragg angle. The crystal orientation along the fiber axis was calculated based on Hermans’ orientation

function [37], = where 〈cos φ〉

−1

(3)

is defined as 〈cos

and φ

〉 2

3〈cos

〉

=

is the azimuthal angle. I(φ

!

#⁄

"( !

#⁄

) cos "(

sin ) sin

(4)

) is the diffraction intensity distribution of the

reciprocal lattice vector of the (hkl) plane. 2.2.5 Small angle X-ray scattering (SAXS) SAXS experiments were carried out on a Xenocs (France) small-angle X-ray scattering spectrometer at room temperature. Scattering pattern was collected by a two-dimensional detector (Dectris Pilatus 3R 300K). CuKα (λ =0.154 nm) was used as the X-ray source. The samples were placed in a vacuum chamber and the X-ray data were corrected for the background scattering. The distance between sample and detector was 2502.26 mm that calibrated by AgB. The microvoids in the fiber was estimated by [9], ' (#/ (*) =

1

+ ' (,- (5)

where s is a scattering vector, Bπ/2(s) is the integration breadth along azimuthal scan, L is the mean length of the microvoids, and Beq is the orientation angle of the elliptical microvoids derivation from fiber axis. 3. Results and discussion Fig. 1(a) shows the correlation between the tensile strength and Young’s modulus of the carbon fibers investigated in this work. The tensile strength remains around 4.9 GPa at the modulus below 350 GPa and then decreases with the increasing modulus in the range of 350-460GPa. The tensile strength is somewhat kept at constant in the modulus of 450-540, and then decreases with increasing modulus after that. These carbon fibers are suitable for the

investigation on the correlations between the structure and properties of carbon fibers. The properties of the PAN-based carbon fibers depend on the measuring parameters (e.g. the gauge length) and the diameters of the carbon fibers. Generally, the measured tensile strength decreases with the increasing gauge length due to the increase in the defects along the carbon fibers [38, 39]. The decrease in the diameter could minimize the defects in the carbon fibers and therefore leads to the increase in the tensile strength [40]. In this work, the properties of the carbon fibers were obtained under the same measuring condition and parameters. Fig. 1(b) shows the elongation at break as a function of Young’s modulus. The elongation decreases with the increase in the modulus of the carbon fibers, which could correlate to the difference in microstructure of the carbon fibers [41]. 5.5

(a) 2.0

5.0 460

540

4.5

4.0

3.5

Elongation at break

Tensile strength (GPa)

350

(b)

350

460

1.5

540

1.0

0.5 250 300 350 400 450 500 550 600

Modulus (GPa)

250 300 350 400 450 500 550 600

Modulus (GPa)

Fig. 1. (a) The tensile strength and (b) the elongation at break as a function of Young’s modulus of carbon fibers studied in present work. 3.1 Crystal Structure of the Carbon Fibers The crystal structures of carbon fibers were characterized by WAXD (Fig. S1), the size of the graphite crystallites (L002) and interlayer spacing (d002) of the graphite (002) planes as a function of tensile strength and modulus are shown in Fig. 2. The results indicate that the interlayer spacing d002 increases with the increase in tensile strength, while the crystallite size

(L002) decreases linearly with the increase in Young’s modulus (Fig. 2a). In the case of plotting L002 and d002 as a function of Young’s modulus of the carbon fibers, L002 increases, but d002 decreases with the increase in the modulus of the carbon fibers. Moreover, the increase of L002 and decrease of d002 are in stepwise shape. When the Young’s modulus of the carbon fibers is lower than 300 GPa, d002 and L002 are kept at 0.35 nm and 1.7 nm, respectively. The carbon fibers with a higher modulus, say in the range of 300-420 GPa, d002 decreases, while L002 increases with the increase in the modulus of the carbon fibers. There is another plateau for d002 and L002 of the carbon fiber with the modulus of 420-480 GPa. Further increase in the modulus of the carbon fibers corresponds to the increase in the crystallite size L002 and the decrease in the interlayer spacing d002. In this work, the carbon fibers with different mechanical properties were used (Table 1). Above results indicate that the variation of the interlayer spacing of the graphite crystals d002 reduced from 0.35 nm at the carbon fiber modulus of 250 GPa to about 0.337 nm at the carbon fiber modulus of 580 GPa. Meanwhile, the size of the graphite crystals L002 increases 1.7 nm to 8.6 nm, simultaneously. The relationship between the microstructure and the mechanical properties of carbon fibers has been investigated intensively. The comparison of the commercially available PAN-based carbon fibers indicates that the tensile modulus increases with the increase of the aspect ratio of crystallites, volume fraction of crystallites and degree orientation of crystallites, as well as decreases with the decrease of the aspect ratio of crystallites, volume fraction of crystallites and degree orientation of crystallites [42]. The above results indicate that the increase in the Young’s modulus of carbon fibers mainly arises from the rearrangement and growth of the graphite crystallites. The tensile strength of PAN-based carbon fibers mainly depends on the microstructure of amorphous carbon, which can improve the cross-linking among graphite crystallites and has a positive effect on the tensile strength of the carbon fibers [5-7, 43, 44].

(a)

10

L002

0.350

d002

300

(b)

L002

0.350

4 0.340

d002 (nm)

6

8

L002 (nm)

d002 (nm)

8

0.345

10

480

0.345

6

420

4

L002 (nm)

d002

0.340 2

0.335

0 3.5

4.0

4.5

5.0

Tensile strength (GPa)

2

0.335

0 250 300 350 400 450 500 550 600

Modulus (GPa)

Fig. 2. The interlayer spacing (d002) and crystallite size (L002) as a function of the (a) tensile strength and (b) Young’s modulus of the carbon fibers. The azimuthal scans of the crystallographic planes (002) of graphite crystals in the carbon fibers are shown in Fig. S2. The orientation parameter of graphite crystallites in the carbon fibers was estimated by using eq. 3. Fig. 3 shows the orientation parameter of graphite crystallites in the carbon fibers plotted as a function of tensile strength and modulus, respectively. The results indicate that the increase in orientation parameter corresponds to the decrease in tensile strength and the increase in modulus of the carbon fibers. The results suggest that in production of high modulus type carbon fibers, the orientation parameter of the graphite crystals should be kept at a low level, which means that the graphite crystals should be in a good alignment along the carbon fiber axis. While in the production of high tensile strength type carbon fibers, the graphite crystals should be somewhat entangled, or the interlayer connection among the graphite crystals should be tailored by adjusting the processing parameters [5-7, 43, 44]. The orientation parameter of the carbon fibers with high tensile strength is relatively lower than that of carbon fibers with high tensile modulus [45]. The crosslink of turbostratic carbon may be responsible for the high strength of carbon fibers, both flaws and catalytic effects from surface may also play an important role in limiting

strength [46, 47]. The results agree with those in literatures that the carbon fibers with high modulus have the higher orientation order of graphite crystallites [42, 48, 49]. 1.0

1.0

(a)

f002

0.9

f002

0.9

(b)

0.8

0.8

0.7

0.7 3.5

4.0

4.5

5.0

Tensile strength (GPa)

250 300 350 400 450 500 550 600

Modulus (GPa)

Fig. 3. The Hermans’ orientation parameter r(f002) of the graphite crystallites in the carbon fibers as a function of (a) tensile strength and (b) modulus of the carbon fibers. The orientation and size of graphite crystallites are two important factors that influence the performance of carbon fibers. For carbon fibers of CF-250~CF-300 and CF-420~CF-480, the increase in modulus without obvious changes in tensile strength, the increase in the modulus could mainly attribute to the increase in the orientation of the graphite crystallites in the carbon fibers at the corresponding final heat treatment temperature. For CF-350~CF-420 and CF-480~CF-580, the increase of the Young’s modulus accompanies the decrease in tensile strength of the carbon fibers, which could attribute to the further increase in the orientation and growth of the graphite crystallites in the carbon fibers at the corresponding final heat treatment temperature. Both drawing and high temperature treatment are benefit to the arrangement and the preferred orientation of graphite crystallites during the production of carbon fibers. Meanwhile, the stacked and structural dislocations of the graphite crystals could be reduced by drawing during the production process of carbon fibers, which improves the rearrangement of graphite layers to stack in higher order [11, 50]. As a result, the modulus of the carbon fibers increases accordingly.

3.2 Morphology of the carbon fibers SEM observation indicates that there are grooves on the surface of all the carbon fibers that investigated (Fig. 4). The carbon fibers with the modulus above 520 GPa have clear irregular surface with fine stripes, which could be due to the evolution of nitrogen and carbon atoms during the high temperature treatment. SEM observation of the carbon fiber cross-section indicates the inhomogeneity across of the cross-section of the carbon fibers (Fig. 5). PAN-based carbon fibers generally have the obvious skin-core structure distributing along the radial direction [51, 52], which is attributed to the production procedure of the carbon fibers. The high temperature promoted the clear radial gradient distribution of the graphite structure, in which the skin region was graphitized more sufficiently than the core region. The gradient distribution of graphite structure affects the mechanical properties of carbon fibers [53]. That is to say, the distribution of strain in the cross section of fibers is inhomogeneity, which will lead to the decrease of the mechanical properties.

Fig. 4. Typical SEM images of the surface carbon fibers.

Fig. 5. Typical SEM images of the cross-section of the carbon fibers. 3.3 The graphitization of the carbon fibers Fig. 6a shows the Raman spectra of the carbon fibers with different Young’s modulus. There are G band and D band located at 1600 cm-1 and 1350 cm-1, respectively. Whereas the G band is a characteristic band for sp2-hybridized carbon in graphite planes, which corresponds to the Raman-active vibration modes E2g of the graphite unit cell, (the other vibration modes of graphite, 2 B2g, A2u, and E1u, are Raman inactive). The D band arises from defects in the PAN-based carbon fibers. After the carbon fibers are treated at 2500 ℃, the so-called G’ or 2D mode will appear at 2700 cm-1, which is an overtone of the D band [54]. When the graphitization was improved, the intensity of G band gradually increased and D band decreased. Therefore, the intensity ratio ID/IG was utilized to evaluate the number of defects in carbon materials [34]. The D band and G band are relatively wide and overlap each other in the Raman spectra of the carbon fibers with the modulus below 380 GPa, while the D band and G band became narrow and sharp for the carbon fibers with relatively higher modulus (Fig. 6a). The results indicate that the carbon fiber with higher modulus have the more perfect three-dimensional ordered graphite lamellar structure [1, 7, 48]. The intensity

ratio of D band and G band ID/IG of the carbon fibers were plotted as a function of tensile strength and modulus of the carbon fibers as shown in Fig. 6b. The results indicates that at lower modulus (< 350 GPa) and higher tensile strength, the value of the ID/IG is relatively larger, and decreased with the increase in modulus of the carbon fibers. The increase in the graphitization of the carbon fibers is due to the rearrangement of the amorphous and disordered carbon structure into well-defined three dimensional graphite crystallites in the carbon fibers [7, 32]. Actually, those carbon fibers have the similar tensile strength (Fig. 1). In cases of carbon fibers with modulus higher than 350 GPa, the ID/IG value seems independent on the modulus of the carbon fibers. Evolution of Raman spectra of carbon fibers as function of Young’s modulus reveals that there are various carbon components with different degree of preferred orientation. It is well known that in case of deforming of the different components in the bulk carbon fibers under stress, the strain in the amorphous carbon is higher than that in the crystallites. The results suggest that the tensile strength in the absence of large flaws is controlled by the disordered region rather than the crystallites [55]. Combined with above results and discussion (Fig. 2 and 3), the degree of graphitization has more influence on the modulus of the carbon fibers. Therefore, the size and orientation of the graphite crystallites may play a critical role in the improvement of the modulus of the carbon fibers.

Tensile strength (GPa) 3.5

4.5

5.0

(b)

D

Intensity(a.u.)

CF-580 CF-560 CF-540 CF-520 CF-500 CF-480 CF-460 CF-440 CF-420 CF-400 CF-380 CF-350

2.5

ID/IG

G

(a)

4.0

3.0

2.0

1.5

1.0

CF-300 CF-280 CF-250

0.5 3000

2500

2000

1500

1000 -1

Raman shift (cm )

500

250 300 350 400 450 500 550 600

Modulus (GPa)

Fig. 6. The (a) Raman spectra of carbon fibers with different Young’s modulus and (b) ID/IG plotted as a function of modulus (●) and tensile strength (○) of the carbon fibers. Fig. 7 shows the typical fitting of the Raman spectra for the carbon fibers, the fitting results of the other samples are shown in Fig. S6. The data of curve fitting are listed in Table 2. On the Raman spectra of carbon fibers, D1 band attributes to the carbon atoms in the graphene layer in immediate vicinity of a lattice disturbance, such as those of at the edge of a graphene layer. The D2 band attributes to the lattice vibration of carbon atoms analogous to that of G band but involving graphene layers at the surface of a graphitic crystal [29, 32]. Dresselhaus et al. [56, 57] also assigned the Raman band at 1620 cm-1 as the graphite ‘boundary’ layer adjacent to an intercalant layer and not sandwiched between two other graphite planes, which is the discontinuity of graphite planes that inevitably exists at the edge. Raman spectra of PAN-based carbon fibers are essentially the superposition of the spectra for the sp2 carbon layers and the sp2 carbon clusters [28, 58]. The data in Table 2 indicates that the intensity of D1 band decreases with the increasing modulus of the carbon fibers. Since the smaller crystallites have more edge carbon atoms due to the breakage of the symmetry [59], which suggest the non-structure carbon near the graphene layer are decrease. Moreover, the intensity

of D2 band increases with the increase in the Young’s modulus of the carbon fibers, with suggests that more turbostratic graphite structure exists in the carbon fibers with higher tensile strength. Cumulative D1 G D2

D1 G

G

CF-250 D2

D1

CF-400

D2

D1 G D2

CF-300

D2

D1

CF-500

G

D1 G

G D1

CF-350

D2

CF-580 D2

2000

1800

1600 1400 1200 Raman shift (cm-1)

1000

800

2000

1800

1600 1400 1200 Raman shift (cm-1)

1000

800

Fig. 7. Typical curve fitting for the first-order Raman spectra of the carbon fibers (λ0 = at 532nm). The intensity was normalized. Table 2. The Raman spectra of high-strength carbon fibers obtained with different band.

CF-250 CF-280 CF-300 CF-350 CF-380 CF-400 CF-420 CF-460 CF-480 CF-500 CF-520 CF-540 CF-560 CF-580

Center

D1 FWHM

IArea

1363.5 1361.7 1359 1358.4 1357 1359 1359 1355 1354 1358 1355 1355.3 1354 1356

275.8 255.5 208.4 77.4 51.9 48.8 46.4 46.0 46.4 42.9 42.2 45.4 47.7 45.0

74.6 73.2 68.9 57.4 51.9 50.4 49.7 49.2 47.8 47.3 49.1 50.1 50.5 49.8

Center

G FWHM

IArea

1581.3 1580.2 1579 1584 1585 1580 1581 1580 1579 1582 1585 1587 1588 1590

91.3 91.3 90.5 61.6 46.0 42.6 39.5 39.6 36.9 35.0 35.2 36.7 37.9 36.7

22.9 22.9 22.8 34.5 43.0 46.1 47.0 47.2 49.1 49.3 46.9 46.2 45.8 45.9

Center

D2 FWHM

IArea

1614 1612.8 1611.2 1609 1614 1615 1612 1615 1613 1612 1611 1618 1612 1613

37.5 38.6 39.7 33.6 21.8 16.5 14.2 15.8 14.4 13.2 12.8 12.8 13 12.3

2.5 3.9 8.3 8.1 5.1 3.5 3.3 3.6 3.1 3.4 4.0 3.7 3.7 4.3

3.4 Microvoids in the carbon fibers In PAN-based carbon fibers, the microwoids are unavoidable, which can be formed by the phase separation during the preparation of PAN precursor fibers via wet- or jet-wet solution spinning process, the release of the N, O, and H atoms during the stabilization, carbonization, and graphitization for producing carbon fibers [4-10, 19, 20]. The presence of microvoids is an important factor to control the mechanical property of carbon fibers. Fig. 8 shows the average length L and the orientation angle Beq of the microvoids in the carbon fibers that are plotted against the tensile strength and modulus of the carbon fibers. SAXS results and the estimation of the size and orientation of the microvoids in the carbon fibers by eq. 5 are shown in Fig. S3 and Fig. S4. On the consideration of the tensile strength of the carbon fibers, higher tensile strength corresponds to the lower Beq and smaller L statistically (Fig. 8). The cylindrical shape microvoids and voids were used in the model established by Ruland [9]. Therefore, the morphology and direction of the microvoids would be changed at different draw ratio during the preparation of the carbon fibers. The orientation angle of the microvoids first decreases and then increases with the increase in the modulus of the carbon fibers (square). The average length L of the microvoids in the carbon fibers shows the similar tendency, From CF-250 to CF-350 samples, L decreases by 1.8 % and Beq decreases by 36 % (square), which indicate that the microvoids distort to an angle more paralleling to the fiber axis. The plastic deformation in the fibers leads the elliptical microvoids to distort and merge along the fiber axis, which is benefit for the improvement of the Young’s modulus [13]. However, from CF-350 to CF-460 samples, Beq increases as the strength increases (circle) while distribution of L vary from wide to narrow. This phenomenon is perhaps attributed to the merging of the microvoids during the heat treatment at high temperature [4]. The stress concentrated on the surrounding of microvoids leads to the decrease of the tensile strength. For CF-480~CF-580 samples (square), L and Beq increase sharply. However, it is not clear

whether the mechanical properties of the carbon fibers correlate to the shape of new combined microvoids or not, due to that a scattering pattern is shown by plot L as a function of the tensile strength of the carbon fibers (Fig. 8b). Tensile strength (GPa) 3.5

4.0

4.5

10

Tensile strength (GPa)

5.0

3.5

M~Beq T~Beq

(a) 9

185

8

L (nm)

Beq ( o )

4.5

5.0

M~L T~L

(b)

180

7 6 5 4

175 170 165

3 2 200

4.0

160 300

400

500

600

200

Modulus (GPa)

300

400

500

600

Modulus (GPa)

Fig. 8. The (a) orientation angle Beq and (b) average length L of the microvoids in the carbon fibers as a function of the tensile strength (●) and modulus (■). Lines are only used for guiding eyes. Above results and discussions suggest that it is hard to improve the tensile strength and Young’s modulus of the PAN-based carbon fibers simultaneously. In our view of point, the high strength type PAN-based carbon fibers should have the interlayer links between graphite sheets and the amorphous carbon with suitable microstructure among the graphite crystallites, by which to form connections among the graphite sheets and crystallites. Thus all the graphite structure can be active upon applied stress on the carbon fibers. While for the high modulus type PAN-based carbon fibers, the graphite sheets should stack in high order and orientate along the fiber axis [5, 6], by which to reduce the deformation upon applied stress on the carbon fibers. An ideal network of turbostratic graphite is preferred for the high strength type PAN-based carbon fibers, in which the graphite meshes of the network are uniform and all the graphite meshes in the carbon fibers can shear the stress applied on the carbon fibers. For the

high modulus type PAN-based carbon fibers, the modulus is mainly come from the crystallites in the graphite structure that align along the axis of the carbon fibers. Large, perfect and highly orientated graphite crystallites are preferred for the high modulus PAN-based carbon fibers. On the other hand, the defects, such as the voids and microvoids, which are unavoidable in the production of PAN-based carbon fibers, should be carefully controlled in the preparation of any type of carbon fibers. The PAN carbon fiber precursors are generally produced by solution wet- or jet-wet spinning process and fibers. The concentration of the PAN in the spinning solutions is generally around 20 wt.%, and large amount of the solvent will be removed from the spinning filaments during the spinning process of the precursors. Therefore, microvoids cannot be avoided in the PAN precursor fibers. On the other hands, the oxygen, nitrogen, and hydrogen atoms in the precursor fibers will be excluded from the PAN precursor fibers during the stabilization, carbonization and graphitization, which also left voids in the resultant carbon fibers. The voids and microvoids can be adjusted by carefully adjusting the producing process. As for the microvoids, high ratio drawing and high temperature treatment during the production of carbon fibers will lead to the decrease of the orientation angle of microvoids. Due to the theory of statistical methods of small angle X-ray scattering, it is possible that shape of new combined microvoids could result the increase in the orientation angle of the microvoids for the high modulus type PAN fibers. In the high strength type PAN-based carbon fibers, the average length of microvoids is discrete. Our results suggested the correlations between the structures and the mechanical properties of the PAN-based carbon fibers, which is important for tailoring the mechanical properties and adjusting processing parameters for the production of PAN-based carbon fibers. 4. Conclusion The correlations between the structure and properties of PAN-based carbon fibers were investigated and discussed. The properties of PAN-based carbon fiber are mainly controlled

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Highlights Properties of PAN based carbon fibers are mainly controlled by the microstructure and microvoids. The interlayer space of the graphite sheets decreases with the increase in size and orientation of the graphite crystallites. The modulus of PAN based carbon fibers increases with the increase in size and orientation of the graphite crystallites. The turbostratic graphite structure in the carbon fibers transforms into 3D ordered graphite lamellar structure during the heat treatment at ultra-high temperature. The combination of the microvoids in the carbon fibers leads to the increase in modulus and decrease of the tensile strength.

CRediT author statement

Lu did all the experiments and data processing, and he also writing the paper. Li analyzed the SAXS and Raman data. Kang analyzed the WAXD data. Feng and Xu discussed the experimental results and the writing of the manuscript. Liu designed the investigation of the present work, analyzed the experimental results, and writing the manuscript.

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.