In-Situ Doping B4C Nanoparticles in Mesophase Pitch for Preparing Carbon Fibers with High Thermal Conductivity by Boron Catalytic Graphitization

The boron carbide (B4C) nanoparticles doping mesophase pitch (MP) was synthesized by the in-situ doping method with tetrahydrofuran solvent, and the corresponding MP−based carbon fibers (CFs) were successfully prepared through the melt−spinning, stabilization, carbonization and graphitization processes. The structural evolution and properties of boron−containing pitches and fibers in different processes were investigated for exploring the effect of B4C on mechanical, electrical and thermal properties of CFs. The results showed that the B4C was evenly dispersed in pitch fibers to provide active sites of oxygen, resulting in a homogeneous stabilization and ameliorating the split−ting microstructures of CFs. Moreover, the thermal conductivity of B1−MP−CF prepared with 1 wt.% B4C increased to 1051 W/m•K, which was much higher than that of B0−MP−CF prepared without B4C (659 W/m•K). While the tensile strength of B4C−doped CFs was lower than that of pristine CFs. In addition, a linear relationship equation between the graphite microcrystallite parameter (ID/IG) calculated from Raman spectra and the thermal conductivity (λ) calculated according to the electrical resistivity was found, which was beneficial to understand the thermal properties of CFs. Therefore, the doping B4C nanoparticles in MP did play a significant role in reducing the graphitization temperatures due to the boron catalytic graphitization but decreasing the mechanical properties due to the introduction of impurities.


Introduction
Thermal conductivity is one of the most important properties of carbon fibers (CFs), and CFs with high thermal conductivity are widely used in industrial, aerospace and military applications [1,2]. As an essential method to improve thermal conductivity of CFs, graphitization is carried out under the protection of inert gas, where the non−carbon elements in the CFs are removed to achieve carbon enrichment (>99%). Considering, phonon vibration is the main way of heat transfer in CFs, the graphite microcrystallite size and orientation of CFs affect their thermal conductivity to a great extent. As mesophase pitch (MP) is prone to form the graphite microcrystallite with high orientation. It can explain why MP−based CFs (MPCFs) can achieve high thermal conductivity. However, MPCFs also require much high−temperature graphitization treatments over 2800 • C, which inevitably leads to high energy consumption and short equipment life. Therefore, it is necessary to enhance the thermal conductivity of MPCFs while reducing the graphitization temperature and improving their microstructure. In recent years, the catalytic graphitization method has been proven to be one of the most effective measures to increase the thermal conductivity and decrease the graphitization temperature of CFs [3]. Generally, the catalysts are mainly

Preparation of B 4 C−Doped Mesophase Pitch−Based Carbon Fibers
The preparation progress of B 4 C−doped MPCFs were described as follows: Firstly, MP was grounded and sieved with a 60−mesh sieve. Then 25 g MP was dissolved in 250 g tetrahydrofuran (THF) solvent and added the B 4 C nanoparticles with different contents (0, 1, 5 wt.%), and the mixture was magnetic stirred at room temperature for 5 h. The B 4 C−doped MP was successfully prepared after the following evaporation and vacuum drying at 60 • C for 12 h. Secondly, 10 g B 4 C−doped MP was spun into pitch fibers (PFs) with a single−hole spinning apparatus (the length/diameter of spinneret is 0.4 mm/0.2 mm) at the spinning temperature of 350 • C and nitrogen pressure of 1.0 MPa. Thirdly, 3 g B 4 C−doped MP−derived PFs which were pre−cut into short fibers with length of 10 cm were stabilized at 270 • C for 1 h with a heating rate of 1 • C/min in a 200 mL/min air atmosphere to obtain the stabilized fibers (SFs) [13]. Finally, 2 g B 4 C−doped MP−derived SFs were subsequently carbonized at 1000 • C for 1 h with a heating rate of 5 • C/min in a 200 mL/min nitrogen atmosphere and furtherly graphitized at 2300, 2600, 2800 or 3000 • C for 10 min in a graphitization furnace to obtain the B 4 C−doped MP−derived CFs. The samples obtained from the different preparation process parameters were named as Bx−MP, Bx−MP−PF, Bx−MP−SF, Bx−MP−CF−t, respectively (x represented the adding B 4 C contents, t represented the carbonization and graphitization temperature).

Softening Point (SP) and Polarizing Optical Microscopy (POM)
The SP and POM photos of pitches were determined and observed by a CFT−100EX capillary rheometer (Shimadzu, Kyoto, Japan) and a BX53 polarizing microscope (Olympus, Tokyo, Japan).

Scanning Electron Microscope (SEM)
The morphology and microstructure of CFs were analyzed by a SU8010 SEM microscopy (Hitachi, Tokyo, Japan) with 5 kV.

Infrared Spectroscopy (FTIR)
The FTIR spectra of pitches and fibers were obtained using KBr disc technique in a Nicolet iS10 FTIR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). The ortho−substitution index (IOS) represents the fraction of aromatic rings with ortho substitutions and gives the relative size of aromatic molecules, which was defined as the following Equation (1). The C−H substitution index (I CHS ) represents the fraction of aromatic carbons that are substituted with aliphatic (−CH 3 ) or methylene (−CH 2 −) groups, which was defined as the following Equation (2) [14].

Raman
The Raman spectra of pitches and fibers were recorded by a DXR2 Raman Microscope (Thermo Fisher Scientific) and the information of G, D, D' and A peaks were obtained by the Lorentz fitting to calculate the area ratio of I D /I G , I D' /I G and I A /I G , which could be used to quantitatively evaluate the graphitized degree, structural order and the content of functional groups of CFs. Moreover, their graphite microcrystalline parameters of plane size (La, nm) was calculated according to the following Equation (3) [15].
2.7. Thermogravimetric−Differential Scanning Calorimetry (TG−DSC) The thermogravimetric properties of pitches and fibers were measured using a STA 449 F5 thermal analyzer (Netzsch, Bavaria, Germany). The peak temperature (T p , • C), weight uptake (∆W, %) and enthalpy change (∆H, J) of PFs were collected from their TG−DSC curves in the air mood with a heating rate of 1 • C/min to 600 • C. The decomposition temperature (T d , • C), coking values (CV, %) at 1000 • C and ∆H of SFs were collected from their TG−DSC curves in the nitrogen mood with a heating rate of 5 • C/min to 1000 • C.

Mechanical, Electrical and Thermal Properties
The mechanical properties of CFs were determined by a XQ−1C single−filament machine (Shanghai New Fiber Instruments Co., Shanghai, China) with a gauge length of 20 mm according to the standard GB/T 31290−2014. The diameter (D) was visually observed under an YYS−80E optical microscope (MicroDemo, Beijing, China) to calculate the sectional area of fibers. The tensile modulus and tensile strength of CFs were calculated from the mean values of ten tests with the values distributing within 10%. The electrical resistance (R) of CFs was measured by four probe methods according to the GB/T 32993−2016 with an inner and outer gauge length of 25 mm (L)and 35 mm by a Model 580 micro−ohmmeter (Keithley, Cleveland, OH, USA). The electrical resistivity (ρ) and thermal conductivity (λ) of CFs was calculated by the following Equations (4) and (5) [16].   Figure 2b, all PFs had two strong broadened peaks around 1300 and 1600 cm −1 , respectively. Generally, the D peak at around 1335 cm −1 corresponds to defect lattice vibration mode, while the G peak at around 1584 cm −1 corresponds to an ideal graphite lattice vibration mode in the Raman spectra of carbon materials [17]. In addition, the A−band at 1500~1550 cm −1 was related to the functional groups [18,19]. Consequently, the area ratio of D and G bands (I D /I G ) can be used to measure the orientation degree of pitch molecular, and the area ratio of A and G bands (I A /I G ) can be used to quantify the functional groups of pitch molecules. In Table 1, the calculated I D /I G of PFs after doping B 4 C increased from 2.27 to 2.34, revealing that the addition of B 4 C would affect the orientation of pitch molecules, causing a poor spinnability of B 4 C−doped pitches. Additionally, the calculated I A /I G of PFs were the same of 0.15, indicating that the addition of B 4 C did not form new functional groups, which was consistent with the FTIR result.  , which proved that B4C had no effect on the aromaticity and aliphatic groups of pitch precursor. Furthermore, B1−MP−PF and B5−MP−PF had the similar peak positions in FTIR spectra, which demonstrated that B4C might be physically doped into the MP. Figure 2b shows the Raman spectra of B0−MP−PF, B1−MP−PF and B5−MP−PF. As depicted in Figure 2b, all PFs had two strong broadened peaks around 1300 and 1600 cm −1 , respectively. Generally, the D peak at around 1335 cm −1 corresponds to defect lattice vibration mode, while the G peak at around 1584 cm −1 corresponds to an ideal graphite lattice vibration mode in the Raman spectra of carbon materials [17]. In addition, the A−band at 1500~1550 cm −1 was related to the functional groups [18,19]. Consequently, the area ratio of D and G bands (ID/IG) can be used to measure the orientation degree of pitch molecular, and the area ratio of A and G bands (IA/IG) can be used to quantify the functional groups of pitch molecules. In Table 1, the calculated ID/IG of PFs after doping B4C increased from 2.27 to 2.34, revealing that the addition of B4C would affect the orientation of pitch molecules, causing a poor spinnability of B4C−doped pitches. Additionally, the calculated IA/IG of PFs were the same of 0.15, indicating that the addition of B4C did not form new functional groups, which was consistent with the FTIR result.

Stabilization and Carbonization Behaviors of B 4 C−Doped Fibers and Pristine Fibers
Figure 2c,d shows the TG−DSC curve of PFs in air atmosphere with a heating rate of 1 • C/min to 600 • C to explore the stabilization behavior of PFs. Obviously, the B1−MP−PF and B5−MP−PF behaved a lower maximum weight gain (∆W) of 9.23 and 8.19% than B0−MP−PF (12.12%) at 333 • C, which implied that the addition of B 4 C made the stabilization milder. Moreover, the peak temperature (T p ) corresponding to the maximum oxidation weight gain in Table 1 were 333 • C for B0−MP−PF, 342 • C for B1−MP−PF and 380 • C for B5−MP−PF, respectively. The decrease of ∆W and the shift of T p to high temperature region of B 4 C−doped PFs during stabilization were mainly attributed the change of fibers microstructure under high boron concentration and the formation of blockage of boron oxide formed on the fibers surface to specific parts would inhibit the oxidation process [5]. As summarized in Table 1, the endothermic peaks are broadened and the enthalpy change (∆H) has increased from 182 to 238 J. This contradiction between oxidation inhibition and more heat released by the reaction indicated that the addition of B 4 C had provided active sites for oxygen. Further oxygen preferentially entered interior fibers for internal reaction and made the reaction more uniform. A similar situation also occurred in the stabilization process of PAN−based fibers, the introduction of B 4 C inhibited the uneven distribution of oxygen in the skin and core of the fibers [11].
The FTIR spectra of SFs are showed in Figure 3a. The vibration absorption peaks of C−H near 760 and 1450 cm −1 decreased. As to the obtained SFs, the stabilization leaded to the appearance of new peaks at 1700 cm −1 corresponding to carbonyl (C=O) stretching vibration as shown in Figure 3a Table 1, respectively, which meant that pitch branched chains were oxidized and benzene ring molecules undergo a series of reactions such as ring opening, forming oxygen bridge structure and connecting smaller planar molecular characteristics, resulting in the reduction of condensation degree and aromaticity. Furthermore, intercompared to the three SFs, it could be found that the rise of I OS and the decline of I CHS of B1−MP−SF and B5−MP−SF were lower than that of B0−MP−SF, less hydrogen was involved in the oxidation reaction revealing the mild of oxidative crosslinking reaction [20]. Figure 3b shows the Raman spectra of SFs. Notably, D and G peaks around 1300 and 1600 cm −1 of all SFs had decreased. Meanwhile, after the stabilization progress, the I D /I G of B0−MP−SF, B1−MP−SF, and B5−MP−SF were 2.16, 2.17 and 2.20, respectively (Table 1). These results revealed that the molecular arrangement in fibers were more regular due to oxidative cross−linking. And the I D /I G change rate of B1−MP−SF and B5−MP−SF were larger, illustrating that B 4 C played a beneficial role in oxidative cross−linking. In addition, the I A /I G of SFs also decreased (I A /I G = 0.10 for B0−MP−SF, I A /I G = 0.08 for B1−MP−SF, I A /I G = 0.07 for B5−MP−SF), which indicated that oxygen increased the content of functional groups of SFs. The larger change rate of I A /I G indicated that B 4 C in B1−MP−SF and B5−MP−SF had provided more active sites to increase the number of oxygen−containing functional groups in the corresponding SFs.
of B1−MP−SF and B5−MP−SF were lower than that of B0−MP−SF, less hydrogen was involved in the oxidation reaction revealing the mild of oxidative crosslinking reaction [20]. Figure 3b shows the Raman spectra of SFs. Notably, D and G peaks around 1300 and 1600 cm −1 of all SFs had decreased. Meanwhile, after the stabilization progress, the ID/IG of B0−MP−SF, B1−MP−SF, and B5−MP−SF were 2.16, 2.17 and 2.20, respectively (Table 1). These results revealed that the molecular arrangement in fibers were more regular due to oxidative cross−linking. And the ID/IG change rate of B1−MP−SF and B5−MP−SF were larger, illustrating that B4C played a beneficial role in oxidative cross−linking. In addition, the IA/IG of SFs also decreased (IA/IG = 0.10 for B0−MP−SF, IA/IG = 0.08 for B1−MP−SF, IA/IG = 0.07 for B5−MP−SF), which indicated that oxygen increased the content of functional groups of SFs. The larger change rate of IA/IG indicated that B4C in B1−MP−SF and B5−MP−SF had provided more active sites to increase the number of oxygen−containing functional groups in the corresponding SFs.   Table 1, which suggested that high temperature carbonization could remove large amount of non−carbon elements and reduce the number of methyl (−CH 3 ) and methylene (−CH 2 −) on the benzene rings In this situation, the degree of condensation and aromaticity had increased due to pitch molecules continued to shrink into larger planar molecules. In addition, the changes of I OS and I CHS of B1−MP−SF and B5−MP−SF were lower than that of B0−MP−SF. Implying that the removal of non−carbon elements was more difficult and the branch chain was more difficult to break. This was because that the stabilization of B1−MP−CF−1000 and B5−MP−CF−1000 were more evenly, and the synergistic decarboxylation of ester and anhydride crosslinking would further improve the stability of pitch molecules, which led to cage aromatic radicals, enabling them to better position in the recombination without migration or rearrangement [21]. Moreover, the I D /I G of CFs in Table 1       that B0−MP−CF−1000 with the fiber diameter of 30 μm showed the optimal mechanical properties with tensile strength of 666 MPa, tensile modulus of 95 GPa and elongation of 0.73%. Oppositely, B1−MP−CF−1000 and B5−MP−CF−1000 showed the poor tensile strength of 475 and 412 MPa, and low tensile modulus of 105 and 78 GPa, which might be related to cracked structure, micropore and hollow structure. These results suggested that B4C could improve the splitting microstructure of CFs, but the introduction of defects would decrease the mechanical properties of ones.    The λ of CFs at different graphitization temperatures of 2300, 2600, 2800 and 3000 °C are summarized in Table 2. Apparently, the λ of B1−MP−CFS and B5−MP−CFS were much higher than that of B0−MP−CFS. This demonstrated that B4C played a crucial role in catalytic graphitization at 2300 °C, which highly improved λ of CFs. Subsequently, it could be noticed that the thermal conductivity of B1−MP−CFS and B5−MP−CFS increased fastest in the temperature range of 2600~2800 °C, revealing that B4C decomposed at this temperature range. The free boron atoms entered the hexagonal graphite grid, improving the microstructure and the orientation of graphite microcrystallite. In this case, the growth rate of thermal conductivity of CFs were conspicuously improved. Nevertheless, the excessive doping could lead to more defects in B5−MP−CFS, thereby the λ of them were slightly low. The λ of CFs at different graphitization temperatures of 2300, 2600, 2800 and 3000 • C are summarized in Table 2. Apparently, the λ of B1−MP−CF S and B5−MP−CF S were much higher than that of B0−MP−CF S . This demonstrated that B 4 C played a crucial role in catalytic graphitization at 2300 • C, which highly improved λ of CFs. Subsequently, it could be noticed that the thermal conductivity of B1−MP−CF S and B5−MP−CF S increased fastest in the temperature range of 2600~2800 • C, revealing that B 4 C decomposed at this temperature range. The free boron atoms entered the hexagonal graphite grid, improving the microstructure and the orientation of graphite microcrystallite. In this case, the growth rate of thermal conductivity of CFs were conspicuously improved. Nevertheless, the excessive doping could lead to more defects in B5−MP−CF S , thereby the λ of them were slightly low. More importantly, the catalytic graphitization of B 4 C markedly reduced the graphitization temperature. The λ of B0−MP−CF−3000 prepared at graphitization of 3000 • C was only 659 W/m•K. Surprisingly, the λ of B1−MP−CF−2300 prepared at graphitization temperature of 2300 • C was 681 W/m•K, and that of B1−MP−CF−3000 prepared at graphitization temperature of 3000 • C was significantly larger with 1051 W/m•K. Moreover, the thermal conductivity of boron doped CFs at 2600 • C exceeded that of most high properties CFs at 3000 • C prevailing in the market, which implied the graphitization temperature had reduced preliminarily. Meanwhile, several commercial MPCFs with high thermal conductivity (XN−90, K13C2U, K13D2U, K1100) were selected to compare their microcrystalline parameters and electrothermal properties [2]. It was noted that the λ of B1−MP−CF−2600 was obviously higher than that of XN−90, K13C2U or K13D2U, and λ of B1−MP−CF−3000 was very near to the K1100, which furtherly indicated the MPCFs with high thermal conductivity could be achieved by the in-situ doping B 4 C nanoparticles in MP. Actually, the influence of thermal conductivity on CFs can be connected with the microstructure, graphite microcrystalline transformation and orientation of CFs. Figure 7a-c shows Raman spectra of CFs at different graphitization temperatures of 2300, 2600, 2800, 3000 • C. It could be observed that the intensity of the D peak at 1335 cm −1 was significantly weakened, while the G peak at 1584 cm −1 was sharpened, and the A−band at 1500~1550 cm −1 was weakened or even disappeared. Additionally, the band at around 1620 cm −1 named as D'−band was related to disordered graphite structure after graphitization [18,19]. The I D /I G (average of four values) and I D' /I G values of B1−MP−CF S and B5−MP−CF S were lower than that of B0−MP−CF S in Table 2, indicating that B 4 C had a significant effect on catalytic graphitization, which improved the graphitization degree of the CFs and promoted the transformation of CFs from amorphous carbon to regular graphite microcrystallite. As depicted in Figure 7d, the λ value of B1−MP−CFs were the highest at different temperatures. In addition, B 4 C had a momentous effect on thermal conductivity than that of graphitization temperature. Therefore, it could be basically determined that the catalytic graphitization of B 4 C played a vital role in improving the thermal conductivity of fibers and reducing the graphitization temperature. Interestingly, there seemed to be a negative correlation between λ and I D /I G . To further understand the relationship between the two paraments, a regression analysis was performed and presented in Table 2. A linear relationship (R 2 = 0.94) between them was obtained by the linear fitting method through the Excel software based on the average data of each sample in Table 2, which demonstrated a conspicuous negative correlation between I D /I G and λ. This linear regression analysis provided a new idea for calculating the λ of MPCFs by I D /I G from Raman spectra. Apart from that, it was necessary to find the mechanism corresponding to the λ of CFs at the microscopic level. Generally, heat conduction was mainly achieved by phonons, and the transfer effect of phonons would directly affect the magnitude of λ. As to MPCFs, the main factor affecting the transmission of phonons was the change of grain size. The calculation results are collected in Table 2. Corresponding to as the lattice size (La), the λ showed an upward trend. This was because the graphite microcrystallite had grown up, and the amount of crystal boundaries and the probability of phonon scattering in CFs decreased. In addition, B1−MP−CFS had the largest crystallite size at each graphitization temperature, which further confirmed that B4C played an excellent role in catalytic graphitization, promoted the growth of graphite microcrystallite, and made CFs obtained the higher λ.

Influence Mechanism of B4C Nanoparticles on the Structural Evolution of Carbon Fibers
Based on the previous results and analysis, the mechanism and evolution of B4C in MPCFs was reasonably deduced as showed in Figure 8. According to the characterization analysis of pitch precursors and PFs, the B4C nanoparticles were uniformly dispersed in PFs by physical doping after in−situ doping and melt−spinning processes. Subsequently, most of the boron element still existed in the form of B4C during stabilization and carbonization processes because B4C had excellent thermal stability and extremely high melting point [4,6,11]. In addition, as plo ed in the TG−DSC curve of PFs, B4C provided active sites for oxygen, allowing it to enter the interior of PFs, which promoted the stabilization more uniform. When the carbonization temperature was 1000 °C, B4C existed in the graphite chaotic layer structure or in the la ice gap. Meanwhile, a large amount of non−carbon Apart from that, it was necessary to find the mechanism corresponding to the λ of CFs at the microscopic level. Generally, heat conduction was mainly achieved by phonons, and the transfer effect of phonons would directly affect the magnitude of λ. As to MPCFs, the main factor affecting the transmission of phonons was the change of grain size. The calculation results are collected in Table 2. Corresponding to as the lattice size (L a ), the λ showed an upward trend. This was because the graphite microcrystallite had grown up, and the amount of crystal boundaries and the probability of phonon scattering in CFs decreased. In addition, B1−MP−CF S had the largest crystallite size at each graphitization temperature, which further confirmed that B 4 C played an excellent role in catalytic graphitization, promoted the growth of graphite microcrystallite, and made CFs obtained the higher λ.

Influence Mechanism of B 4 C Nanoparticles on the Structural Evolution of Carbon Fibers
Based on the previous results and analysis, the mechanism and evolution of B 4 C in MPCFs was reasonably deduced as showed in Figure 8. According to the characterization analysis of pitch precursors and PFs, the B 4 C nanoparticles were uniformly dispersed in PFs by physical doping after in-situ doping and melt−spinning processes. Subsequently, most of the boron element still existed in the form of B 4 C during stabilization and carbonization processes because B 4 C had excellent thermal stability and extremely high melting point [4,6,11]. In addition, as plotted in the TG−DSC curve of PFs, B 4 C provided active sites for oxygen, allowing it to enter the interior of PFs, which promoted the stabilization more uniform. When the carbonization temperature was 1000 • C, B 4 C existed in the graphite chaotic layer structure or in the lattice gap. Meanwhile, a large amount of non−carbon elements such as oxygen were removed at this stage, and defects such as lattice distortion began to appear. Therefore, the mechanical properties of B1−MP−CF−1000 and B5−MP−CF−1000 decreased compared with B0−MP−CF−1000. In addition, scholars also confirmed that the dissolution and precipitation of carbon could be completed by the conversion of carbides [3]. B 4 C dissolved disordered carbon to form carbides, and boron atoms would leave the interstitial positions of the graphite network structure when the graphitization temperature increased over 2300 • C. Then boron atoms would spontaneously move to the grid points of the hexagonal grid of graphite crystals [10], and played a role of catalytic graphitization. During the graphitization stage, B 4 C in the gap or between graphite layers would play a role of a bridge. Its specific function was to connect the surrounding randomly disperse graphite flakes, achieving the dissolution of disordered carbon. As the graphitization temperature was further increased, B 4 C decomposed and free boron atoms appeared, ascribe to gaining kinetic energy under the action of heat energy to move to the substitution position. The structure was relieved, and the degree of graphitization was improved. This conjecture appropriately explained the reason why in-situ doping B 4 C in pitch could prepare higher thermal conductivity CFs at lower graphitization temperature.
Molecules 2022, 27, x FOR PEER REVIEW 13 of 14 the surrounding randomly disperse graphite flakes, achieving the dissolution of disordered carbon. As the graphitization temperature was further increased, B4C decomposed and free boron atoms appeared, ascribe to gaining kinetic energy under the action of heat energy to move to the substitution position. The structure was relieved, and the degree of graphitization was improved. This conjecture appropriately explained the reason why in−situ doping B4C in pitch could prepare higher thermal conductivity CFs at lower graphitization temperature.

Conclusions
MPCFs with high thermal conductivity were achieved by the in−situ doping B4C nanoparticles in MP. During the heat treatment stage of fibers, B4C nanoparticles acted as active sites, which were beneficial to the preferentially entrance of oxygen into the PFs in the stabilization process, realizing more uniform stabilization. In the carbonization stage, B4C nanoparticles had improved the carbonization yield of CFS and decreased gaseous products, but the mechanical properties of CFs were reduced due to the formation of defects from the B4C introduction. In addition, a linear relationship (R 2 = 0.95) between ID/IG and λ was found, providing a new idea for calculating the thermal conductivity of MPCFs. Most significantly, B4C could effectively promote the graphitization degree of fibers and the growth of graphite microcrystallite, which thereby played a crucial role in catalyzing graphitization and declining graphitization temperature. Accordingly, it would provide theoretical guidance for reducing energy consumption while prolonging the service lives of equipment, which could be conducive to instruct the preparation of high thermal conductivity CFs in the industrial production.

Conclusions
MPCFs with high thermal conductivity were achieved by the in-situ doping B 4 C nanoparticles in MP. During the heat treatment stage of fibers, B 4 C nanoparticles acted as active sites, which were beneficial to the preferentially entrance of oxygen into the PFs in the stabilization process, realizing more uniform stabilization. In the carbonization stage, B 4 C nanoparticles had improved the carbonization yield of CF S and decreased gaseous products, but the mechanical properties of CFs were reduced due to the formation of defects from the B 4 C introduction. In addition, a linear relationship (R 2 = 0.95) between I D /I G and λ was found, providing a new idea for calculating the thermal conductivity of MPCFs. Most significantly, B 4 C could effectively promote the graphitization degree of fibers and the growth of graphite microcrystallite, which thereby played a crucial role in catalyzing graphitization and declining graphitization temperature. Accordingly, it would provide theoretical guidance for reducing energy consumption while prolonging the service lives of equipment, which could be conducive to instruct the preparation of high thermal conductivity CFs in the industrial production.