carbon composite by electrified preform heating chemical vapor infiltration

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A novel continuous carbon nanotube fiber/carbon composite by electrified preform heating chemical vapor infiltration Lei Feng a, c, *, Qiangang Fu b, Qiang Song b, **, Yanling Yang a, Yu Zuo a, Guoquan Suo a, Xiaojiang Hou a, Li Zhang a, Xiaohui Ye a a

School of Materials Science and Engineering, Shaanxi Key Laboratory of Green Preparation and Functionalization for Inorganic Materials, Shaanxi University of Science and Technology, Xi’an, 710021, PR China State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an, 710072, PR China c School of Materials Science and Engineering, Tianjin University, Tianjin, 300073, PR China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 August 2019 Received in revised form 1 November 2019 Accepted 4 November 2019 Available online xxx

The extraordinary mechanical and physical properties of carbon nanotubes (CNTs) have provided the impetus in developing a new promising CNT/carbon (CNT/C) composite, which may effectively alleviate the shortcomings of sharp-angle and thin-walled C/C components in mechanical performance. Present researches on preparing CNT/C composites focus on infiltrating pyrocarbon into CNT assemblies such as array, block, sheet, film, and buckypaper using complicated and time-consuming methods. Here, we report a facile and efficient strategy for preparing CNT fiber/C composites via electrified preform heating chemical vapor infiltration. Densification process is identified as continual deposition of pyrocarbon around CNTs (called coaxial structure) and further deposition among them, which can be accomplished in short time. Small-diameter (below 500 nm) coaxial structures give composites high deformability; larger ones coupled with bridging action of pyrocarbon among them result in better load transfer and more conductive pathways. Optimized CNT fiber/C composites demonstrate impressive tensile strength (205 MPa) and excellent conductivity (431 S/cm), which are comparable to the previously reported C/C and CNT/C composites. Moreover, such composites exhibit lightweight (1.21 g/cm3), good deformability and high fracture strain. Our work could open up a general strategy for efficiently fabricating various high-performance CNT/C composites that could be used in high-temperature aerospace fields. © 2019 Elsevier Ltd. All rights reserved.

1. Introduction Conventional carbon/carbon (C/C) composites, as a family of advanced high-temperature structural composites, consist of carbon matrix and embedded carbon fibers (CFs) and have occupied an irreplaceable position in aerospace industries [1,2]. Generally, there are two routes to prepare C/C composites: chemical vapor infiltration (CVI), in which carbon source gases are pyrolyzed at high temperature and produce pyrocarbon (PyC) matrix, and liquid precursor infiltration pyrolysis (PIP). Compared to the PIP-produced C/C composites with high inner stress, high brittleness and low

* Corresponding author. School of Materials Science and Engineering, Tianjin University, Tianjin, 300073, PR China. ** Corresponding author. E-mail addresses: [email protected] (L. Feng), [email protected] (Q. Song).

strength, CVI is a preferred method for fabricating high-quality C/C composites [3,4]. To meet the practical requirements, C/C composites are commonly machined into special shape and dimension characteristics of components, especially for those with sharp angle and thin wall (such as <3 mm), which are susceptible to collapse under high-speed airflow or even during the machining [5]. This phenomenon is mainly caused by the degraded reinforcing effect of CFs that are cut off during machining and exhibit a sharply-reduced length-diameter ratio; concretely, the short-rod-like CFs can not stiffen the brittle PyC matrix on the micron and nano scales. The poor CF/PyC matrix interfacial bonding caused intrinsically by hydrophobicity and chemical inertness of carbon is also a long existing issue to overcome [6,7]. Additionally, the CVI-PyC on surface of CFs is generally granular shape and shows turbostratic characters, due to the low-textured “U” structure and rough surface of CFs that lead to the randomly stacking of graphite layers [8]. To

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acquire high-textured PyC in the CF preforms, it is imperative to precisely control the processing parameters including temperature, residence time, substrate surface area/reactor volume ratio (AS/VR), pressure, and so on [9,10]. Carbon nanotube (CNT) fibers (or yarns) made by purely of CNTs have extremely lightweight and high flexibility, which can be woven into the form of a fabric [11,12], allowing the creation of composites that are similar to the conventional continuous fiber composites. Compared to the CFs, the high specific surface areas of CNT fibers not only favor for the fast deposition of PyC [13,14], but also assist the formation of good fiber/matrix interfacial adhesion [15,16]. Their incorporation into carbon system has the potential to provide highly-aligned and homogeneously-distributed CNTs in the composites and achieve nanoscale reinforcement of the whole composites; meanwhile, they can also provide microscale and macroscale reinforcements to the composites simultaneously by virtue of their microscale diameter and macroscopic length. More importantly, the recent developments of strong and highly conductive CNT fibers lay the foundation for the fabrication of highperformance composite materials [17e20]. In addition, CNT fibers may have the ability to induce the formation of high-textured PyC because of the p-p conjugated electronic structure of CNTs [8,21]. This amazing list can be further extended by advantages such as wide availability of carbon source and the potential low-cost of large scale production [22]. These comprehensive advantages make CNT fibers possible to fabricate one kind of important carbonmatrix composites, which is expected to possess a combination of low density, high deformability, high mechanical and electrical properties. To date, the CNT-based carbon (CNT/C) composites are focused on use of CNT assemblies in form of array [23e25], block [26], sheet [27,28], film [29,30], and buckypaper [31], and the main fabrication approach is PIP method. Zhou et al. [28] fabricated CNT/ C composites by infiltration of CNT sheets with polyacrylonitrile (PAN)-derived carbon matrix, and the obtained composites showed tensile strength of 170e460.2 MPa and electrical conductivity of 190 S/cm. Recently, Zhang et al. [30] infiltrated CNT films with phenolic resin-derived carbon matrix and the resultant CNT/C composites exhibit tensile strength of 220 MPa and electrical conductivity of 1629.1 S/cm. Although promising results have been obtained, the fabrication process is rather complicated and timeconsuming, which consists of repeat carbon precursor infiltration, drying, curing (or stabilization) and carbonization process. Herein, a facile and ultrafast strategy for preparing continuous CNT fiber/C composites by electrified preform heating CVI (ECVI) is proposed. The CNT fiber preforms contain thousands of CNT yarns with high CNT alignment, which are prepared by twisting unidirectional CNT fiber tows, a method that has been demonstrated to efficiently enhance the mechanical performance of CNT assemblies [32e34]. Electrical heating can provide rapid and local heating, which is a highly efficient and low energy-consuming heating approach as compared to the conventional hot-wall reactor. Detailed information regarding the densification behavior can be obtained by monitoring the variation of electrical resistivity of CNT fiber preforms during ECVI process. Another incidental benefit of using electrical heating strategy is that high electrical current can eliminate amorphous carbon on the surface of CNTs [35]. The CNTs within fibers induce the fast deposition of crystalline PyC around them, leading to a high densification efficiency of CNT fiber preform that far surpasses that of CF preforms. Furthermore, such optimized CNT fiber/C composites demonstrate a low density, good deformability, impressive mechanical strength and excellent electrical conductivity, which can be tailored by adjusting the ECVI time. In addition, the effects of ECVI time on morphology and crystallinity of CNT fiber/C composites are also investigated.

2. Experimental 2.1. Raw CNT fibers The CNT fibers used in this work were produced by floating catalyst chemical vapor deposition method. The details could be found in Ref. [36]. The CNT fibers were homogeneous in structure with a uniform thickness of about 135 mm (Fig. 1a). They were porous containing massive pores with size of 50e600 nm, and the CNTs aligned relatively along the fiber axis (Fig. 1b and Fig. S1, Supporting Information). The CNTs had diameter of 10e25 nm and the clear observable lattice fingers on tube walls indicated a high crystallinity (Fig. 1c). Thermal gravimetrical analysis showed the used CNT fibers containing nearly 87 wt% CNTs and 13 wt% impurities that were mainly iron catalysts and a small amount of amorphous carbon (Fig. S2, Supporting Information). 2.2. Equipment for the ECVI process Fig. 1d depicted the schematic of ECVI equipment which consisted of a reactor, gas control and DC power source. The reactor was a vertical quartz tube (dimeter 90 mm, length 300 mm) with the two ends air-tightly enclosed with Teflon flanges and rubber seals. In the center of reactor was two pairs of electrodes for fixing CNT fiber preforms. The distance of two electrodes was 5 cm. Each electrode was composed of two stacked graphite flakes for clamping. The two graphite electrode pairs were connected by two wires going through the top Teflon flange. The wires were connected to a DC power source with tunable voltage 0e60 V. An ampere meter was connected in the circuit for reading the current. Liquid hydrocarbon was carried into reactor through bubbling with argon flow. The container for liquid hydrocarbon was placed in a water-bath that ensured a stable feeding of carbon source. An infrared thermometer placed 0.5 m away from the reactor monitored the temperature of fiber preforms. 2.3. Fabrication of the CNT fiber/C composites by ECVI The CNT fiber tows were immersed into hot HCl solution (2 M) for 24 h to dissolve the metal catalysts (Fig. 1e and f). The CNT fiber tows containing 3000e3200 fibers were twisted by fixing one end and rolling the other end according to a twisting degree of 1 r/cm to form unidirectional (UD) preforms. The preforms had a nearly regular cylindrical shape with length of about 60 mm and diameter of nearly 2 mm (Fig. 1g and Fig. S3, Supporting Information). The CNT fiber preform was clamped between graphite electrodes and the reactor was purified using argon flowing. With the increasing of applied voltage, light emitted by fiber preform varies from faint (Fig. 1h) to incandescent evenly along its entire length (Fig. 1i), accompanied with a rapid rise in the temperature of preform. When the temperature of preform reached up to 1000  C, hexane as carbon source was input into the reactor using argon bubbling at flow rate of 40 sccm. In order to maintain a steady infiltration temperature during the whole ECVI process, applied voltage would be fine adjusted according to the real-time temperature detected by infrared thermometer. Different CNT fiber/C composites were prepared by controlling the infiltration time (2, 5 or 10 min) (Fig. 1j), which were denoted as CNT fiber/C-2 min, CNT fiber/C-5 min and CNT fiber/C-10 min, respectively. Finally, the power was switched off and gas was stopped. 2.4. Materials characterizations and testing The morphology and microstructure of CNT fibers and composites were investigated by scanning electron microscopy (SEM, Hitachi S-4800, 10 kV, Japan), transmission electron microscopy

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Fig. 1. (a) SEM image of cross-section of a single CNT fiber. (b) Surface morphology of CNT fibers with CNTs aligned along the fiber axis. (c) TEM image of an individual CNT. (d) A schematic of equipment for ECVI method. (eej) Overview of preparation process of CNT fiber/C composites by ECVI.

(TEM, FEI Tecnai F20G2, 200 kV, USA) and Raman spectroscopy (InVia, Renishaw 2000, excited by a 532 nm HeeNe laser with a laser spot size of 1 mm2, England), respectively. Thermal stability of the CNT fibers were carried out on a Netzsch STA 449C instrument, where 4e6 mg CNT fibers was heated at a rate of 10  C/min in mixture of nitrogen and air to 1400  C. The bulk density of preform with a regular shaped cylinder was calculated from its mass and dimension. The porosity of CNT fiber preform was characterized by theoretical calculation based on its weight and bulk volume, and the theoretical density of multi-walled CNTs (~1.8 g/cm3). The bulk densities and porosities of various composites were measured by Archimedes method according to ASTM-C20. Tensile property was tested using a universal testing machine (CMT5304) at a crosshead speed of 0.5 mm/min and a gauge length of 10 mm. Specimen was glued at both ends onto two aluminum sheets (thickness less than 1 mm). Five specimens were tested for each case. The direct current (DC) electrical conductivity of the composites was measured by a two-probe method. In the measurements, copper wires were inserted and connected to specimen with silver paste, which enabled a strong electrical contact between the copper wires and the specimen, and consequently a small contact resistance. 3. Results and discussion 3.1. Synthesis, morphology and deformability of the CNT fiber/C composites Fig. 2a shows the relations between the resistivity of CNT fiber

preform and the ECVI time. It is observed experimentally and also in the graph that as the infiltration proceeds, the resistivity of the CNT fiber preform is sharply reduced in first 5 min, and then slowly reduces until the end. It deduces that the internal structure of CNT fiber preform changes greatly at the beginning of the infiltration and then gently. That is, the use of ECVI method has a great benefit in accurately monitoring the densification process of CNT fiber preforms by detecting their electrical resistivity. The decreased rate of the resistivity of CNT fiber preform is more than 45 times that of previous work, where the resistivity of CF preforms decreases only about 22% in first 120 min infiltration during ECVI process [37]. It indicates that the densification efficiency of CNT fiber preforms is much higher than that of CF preforms. Fig. 2bed presents the typical SEM images of three CNT fiber/C composites and corresponding densification models. At the initial infiltration, there are massive coaxial structures that are PyC lamellas around individual CNT or CNT bundles (Fig. 2b). These coaxial structures have nearly equal diameter and distribute uniformly in the whole composite. With the proceeding of infiltration, these coaxial structures are constantly thickening in diameter until they contact with each other (Fig. 2c). The final stage of infiltration occurs in the space among coaxial structures, forming continuous PyC matrix (Fig. 2d). Of course, it is inevitably to leave behind pores within the ultimate composites (porosity 29.8% in Table 1). These pores are mainly located nearby the central region of composites, which is commonly to be encountered in the composite fabrication process by CVI. Deposition rate is defined as the thickness of locally deposited PyC per unit of time on a planar or

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Fig. 2. (a) Variation of resistivity of CNT fiber preform with ECVI time. (bed) Typical SEM images and corresponding morphological schematics of CNT fiber preform after ECVI of 2, 5 and 10 min. (eeg) Variation of deformability of CNT fiber/C composites with ECVI time.

Table 1 Physical and mechanical properties of CNT fiber preform before and after infiltration of PyC for different ECVI time. Materials CNT CNT CNT CNT

fiber preform fiber/C-2 min fiber/C-5 min fiber/C-10 min

CNT content (wt.%)

Density (g/cm3)

Porosity (%)

Tensile strength (MPa)

Young’s modulus (GPa)

Fracture strain (%)

87.0 63.6 59.9 51.4

0.59 ± 0.03 0.81 ± 0.02 1.03 ± 0.02 1.21 ± 0.01

45.2 ± 1.9 39.4 ± 1.6 32.6 ± 2.1 29.8 ± 2.4

21 ± 6 54 ± 11 133 ± 15 205 ± 19

0.12 ± 0.05 1.31 ± 0.21 2.98 ± 0.37 11.2 ± 0.51

21.2 ± 6.83 2.81 ± 0.64 2.25 ± 0.32 1.86 ± 0.29

curved substrate [9]. In SEM image, the average diameter of coaxial structures is about 500 nm and the corresponding time is 5 min, which gives a deposition rate of nearly 3 mm/h. This value is much higher than the deposition rate (0.1e0.25 mm/h [9,38]) of PyC on CF surfaces by isothermal CVI techniques. Two conclusions can be drawn from the fast deposition rate of PyC in CNT fiber preforms. Firstly, CNT blocking effect can lead to a long residence time of hydrocarbon molecules inside CNT fiber preforms [15], which is beneficial to the formation of PyC. Secondly and more importantly, CNTs provide only basal plane of graphene on the surface, which can induce the quick deposition of PyC on CNTs according to a layerby-layer mechanism [39]. Obviously, compared to the densification of CF preforms by CVI which is costly and time-consuming, it is

really a great superiority in using CNT fiber as reinforcing fibers to prepare CNT fiber/C composites in terms of cost and efficiency. The CNT fiber/C-2 min can sustain a substantially bending deformation without catastrophic failure, as shown in Fig. 2e. It deduces that the coaxial structures with small diameter inherit the flexible characteristics of CNTs and endow composites with ultrahigh deformability. The coaxial structures can still maintain high flexibility as their diameter reaches 500 nm (Fig. 2f). The further deposition of PyC on the periphery of coaxial structures coupled with the fastening action of deposited PyC among coaxial structures, gradually increase the stiffness of CNT fiber/C composites but still being bendable (Fig. 2g). It reveals that the deformable characteristics of CNT fiber/C composites can be tuned by tailoring the

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diameter of coaxial structures. 3.2. Microstructure of the CNT fiber/C composites Fig. 3a is a typical TEM image of CNT fiber/C-2 min, in which the deposited PyC exhibits a graphene-like structure with obvious wrinkles and ripples. The selected-area electron diffraction (SAED) pattern of the composites presents a polycrystalline ring (inset of Fig. 3a), suggesting a good crystallinity of the deposited PyC. Highresolution TEM (HRTEM) image of PyC reveals that they have clear annular graphene layers (Fig. 3b). HRTEM image in Fig. 3c provides direct evidence that these annular PyC are detached from the CNTs. Compared to the formation of high crystalline PyC in CF preforms which requires rigorous processing parameters, it seems to be simple and easy in CNT fiber preforms. Fig. 3d shows a HETEM image of CNT fiber/C-10 min. Note that the long graphene fringes in the PyC gradually transform into wrinkled short fringes as the PyC thickness increases. The number of parallel layers ((002) plane, yellow parallel layers) decreases from 13 to 15 in the region near CNTs to about 2e3 in the region aloof from CNTs. The degradation of PyC quality during the infiltration process may be due to the decreased quality of the underlying tube structure (one that has PyC deposited on it) [27]. The PyC quality of CNT fiber/C composites with different infiltration time is further checked by Raman analysis, as shown in Fig. 3e. Typical features of carbon materials in the Raman spectrum are the D-band at around 1351 cm 1 and G-band at around

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1580 cm 1, and the intensity ratio of D to G bands, ID/IG, is reversely proportional to graphitization degree [40]. The ID/IG is about 0.15 for the CNT fibers and falls to approximately 0.61, 0.75, and 0.93 after the PyC infiltration for 2, 5, and 10 min, respectively. It further demonstrates that the crystallinity of PyC decreases with the infiltration proceeds, in agreement with the TEM observation. This result is also consistent with other work reported by Li et al. in the densification of CNT films by CVI method [25]. 3.3. Mechanical properties and strengthening mechanism Typical stress-strain behaviors of CNT fiber/C composites and CNT fiber preforms are shown in Fig. 4a and its inset, respectively. It is observed that CNT fiber/C composites can bear significantly higher stress than that of CNT fiber preforms and composites with short ECVI time. However, the strain at break appears an inverse trend which decreases with the prolonged infiltration time. Statistical results of tensile strength, Young’s modulus and fracture strain of CNT fiber preforms and CNT fiber/C composites are presented in Table 1 with their trend changes shown in Fig. 4b. It can be clearly seen that both the tensile strength and Young’s modulus present evidently increased trend with the prolonged ECVI time. And they reach the maximum of 205 MPa and 11.2 GPa with ECVI time of 10 min, increased by 280% and 755% over composites with ECVI time of 2 min, respectively. Nevertheless, the fracture strain is decreased from the CNT fiber preform of 21.2 to 2.81, 2.25 and 1.86% for ECVI time of 2, 5 and 10 min, respectively. With the further

Fig. 3. (aec) TEM images of CNT fiber/C-2 min composites, inset of (a) is a corresponding SEAD pattern, (b, c) HRTEM images of abundant annular PyC that are detached from CNTs. (d) HRTEM image of CNT fiber/C-10 min showing degraded PyC quality aloof from CNTs. (e) Raman spectrum of pristine CNT fibers and three CNT fiber/C composites.

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Fig. 4. (a) Typical tensile stress-strain curves of CNT fiber preform (inset) and three CNT fiber/C composites, and (b) their corresponding averaged values of ultimate tensile strength, modulus and fracture strain. (c) Comparison of tensile strength and fracture strain of CNT fiber/C composites in this work versus those of reported C/C and CNT/C composites (note: the fiber in bracket represents materials with yarn-like shape).

increased ECVI time, the CNT fiber/C composites show decreased tensile strength and Young’s modulus due to the formation of lowstrength and high-brittleness carbon shell on the outer surface of composites (Fig. S4, Supporting Information).

In the aerospace industries, a lightweight high strength material is significantly advantageous for applications in terms of enabling advanced technologies and saving energy. The density of CNT fiber preform is 0.59 g/cm3 and reaches 1.21 g/cm3 after ECVI of 10 min (Table 1). This value is much lower than the typical C/C composites with density of 1.60e1.95 g/cm3. It indicates that the use of CNT fibers can efficiently achieve the light-weight design of future composites. The tensile strength and fracture strain of the resultant CNT fiber/C composites are compared in Fig. 4c with that of previously reported C/C [Refs. S1e7, Table S1, Supporting Information] and CNT/C composites [28e30,41e44]. As shown in this graph, the optimized strength of CNT fiber/C composites is comparable to those of C/C and CNT/C composites [28e30], but much lower than those of yarn-like CNT/C composite fibers [41e44]. The better mechanical strength of those published CNT/C composite fibers is attributed to the modification of CNT/matrix interface [42], intrinsically strong pristine CNT yarns with higher CNT contents (range of 460e600 MPa [41,44]) and highly compact structure within composite fibers. Another interesting observation from Fig. 4c is that the CNT fiber/C composites exhibit advantages of high fracture strain, a mechanical attribute that is very essential for engineering applications. Fig. 5aec demonstrates the typical post-mortem microstructural characteristics of the three composites under investigation. The CNT fiber/C-2 min appears clear step-like fracture surfaces and the damage trends to be a sliding fracture with high fiber pull-out length (Fig. 5a). It means that cracks may have higher possibility of spreading in a step-like manner along multiple routes throughout the fiber tow than on a single plain, leading to a low strength, a low modulus and a high fracture strain. With the increasing of infiltration time, the pull-out length of CNT fiber tows gradually decreases (Fig. 5b and c). This can be explained upon the gradually enhanced bridging action of PyC deposited among coaxial structures, which results in efficient load transfer between coaxial structures and uniform stress distribution in composites, leading to a lower fracture strain but higher strength and modulus [44e46]. Further investigation of fracture surface of the CNT fiber/C10 min is carried out to acquire the information regarding the strengthening mechanisms of densified CNT fiber/C composites, as shown in Fig. 5def. In Fig. 5d, abundant pull-outs of coaxial structures from the matrix are observed. Furthermore, these pulled-out coaxial structures exhibit plenty of protruding PyC lamellas (Fig. 5e), which is the typical fracture characteristics of hightextured PyC [47,48]. Another noteworthy feature from Fig. 5e is that the pulled-out CNTs are flat and the width is gradually attenuated toward tube tip. This phenomenon results from the deformation of CNTs when the load is transferred to them, indicating the efficient stress transfer at CNT/matrix interface. Different from our previous works where the bamboo-like CNTs with many structural defects and low strength show a very short pull-out (below 200 nm) from CVI-PyC matrix [49], the long pull-out here suggests the CNTs having high mechanical strength which can consume great fracture energy during the failure process. From these observations it can be deduced that pull-outs of coaxial structures, PyC lamellas and CNTs occur in sequence during tensile process, endowing CNT fiber/C composites with high strength by impeding crack spread and bridging matrix [50,51]. Additionally, the necking failure of CNT bundles is also observed, as displayed in Fig. 5f, which is caused by the sliding between CNTs in bundles. Enlarged view of the root of pulled-out CNT bundles (Fig. 5f inset) indicates that PyC fails to infiltrate into the inner of bundles and fix each tube. There is only weak van der Waals interactions between CNTs wrapped in bundles, and intertube sliding is prone to occur during tensile process. It is conducive to producing a high strain, but has little contribution to the composite strength. The corresponding

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Fig. 5. SEM images of fracture morphologies of three composites: (a) CNT fiber/C-2 min, (b) CNT fiber/C-5 min, (cef) CNT fiber/C-10 min, (d) coaxial structure pull-out, (e) PyC lamella and individual CNT pull-outs, (f and inset) CNT bundle pull-out. (g) Tensile fracture models of the CNT fiber/C composites.

schematic diagrams of fracture modes under tensile load are depicted in Fig. 5g. 3.4. Electrical properties Another key application of CNT fiber/C composites in the areas of aerospace and electronics is their potential high electrical conductivity. Fig. 6a presents the electrical properties of CNT fiber preforms and CNT fiber/C composites. Currently, due to the low CNT packing density and considerably unavoidable contact resistance existed between CNTs, the values of electrical conductivity obtained in most as-made CNT fibers (range of 80e5000 S/cm [52]) are much lower than those of theoretical and experimental values of individual CNT. As seen, the electrical conductivity of CNT fiber/ C-2 min is 247 S/cm, an increase of about two times that of CNT fiber preforms. This initial conductivity increase is ascribed to the crystalline PyC, which not only increases the intertube contact area for more efficient electron transfer [26,42], but also connects the non-neighboring tubes to pave more pathways for electron transport (Fig. S5, Supporting Information) [30]. The conductivities are further increased to 359 S/cm with 5 min infiltration and 431 S/cm with 10 min infiltration. Thus, the contact resistance between neighboring coaxial structures can be minimized when the PyC is

deposited among them. The maximum value shows vastly advantageous as compared to the previously published C/C [53,54] and non-graphitized CNT/C composites [24e28,30,31,43]. The comparison is shown in Fig. 6b. In the CNT fiber/C composites, the wellaligned CNTs in the fiber longitudinal direction and crystalline PyC deposited in the radial direction greatly facilitate the electron passport along and among CNTs, respectively, which give CNT fiber/ C composites superior electrical conductivity than those of C/C and CNT/C composites with either randomly oriented CNTs (e.g. block [26] and buckypaper [31]) and/or low crystalline carbon matrix [26,53,54]. Considering the low density of 0.81e1.21 g/cm3 and high deformability for CNT fiber/C composites, our composites have a great potential application as a lightweight flexible non-metal conductor used in high-temperature environment. In short, the resultant CNT fiber/C composites exhibit low density and impressive performance in deformability, mechanical and electrical properties, verifying our preliminary thoughts that the continuous CNT fibers have great potential for fabricating a promising carbon-matrix composites. Better mechanical and electrical performance of CNT fiber/C composites could be obtained if further efforts are made according to the following aspects. Firstly, fully exploiting reinforcement potential of these CNTs wrapped in bundles. Secondly, optimization of ECVI parameters such as

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Fig. 6. (a) Electrical conductivity of CNT fiber preform before and after PyC infiltration for different time. (b) Comparison of electrical conductivity of CNT fiber/C composites in this work versus those of reported C/C and CNT/C composites (note: HTT represents graphitization treatment).

temperature and gas flow velocity for reducing the voids within composites [44]. Thirdly, improving mechanical strength and electrical conductivity of carbon matrix through high temperature treatment [28,42]. Lastly and more importantly, use of intrinsically strong and highly conductive raw CNT fibers [41,42,44]. In addition, the cylindrical CNT fiber/C composite manufactured in this work is just one typical example of various CNT/C composites that can be fabricated by ECVI. This technology is also adopted for heating and densifying other nanocarbon-based preforms, such as individual CNT fiber (Figs. S6a and b, Supporting Information), CNT film (Figs. S6c and d, Supporting Information), graphene fibers, graphene-based preforms, and so on.

review of, the manuscript entitled.

4. Conclusions

Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.11.009.

Presently developed approach enables the fast manufacture of CNT fiber/C composites with mechanical and electrical performance comparable to those of previously reported C/C and CNT/C composites prepared using very complicated and time-consuming methods. The CNTs within CNT fiber preforms induce the fast deposition of PyC around themselves forming coaxial structures, which gradually increase in diameter until contacting with each other. The CNT fiber/C composites with small-diameter coaxial structures (below 500 nm) exhibit high deformability. Increasing the diameter of coaxial structures is able to maximize the strength and electrical conductivity of composites, but with the cost of deformability. The optimized CNT fiber/C composites demonstrate a combination of lightweight (1.21 g/cm3), good deformability, impressive mechanical (205 MPa) and high electrical conductivity (431 S/cm). These integrated characteristics of CNT fiber/C composites may enable potential applications in the high-temperature aerospace fields. The proposed fast ECVI technology could be commonly employed as an efficient strategy for fabrication of highperformance CNT/C and other nanocarbon/C composites with different shapes and dimensions. To further understand this new type composite and to enlarge its application, future investigations should be focused on additional mechanical and multifunctional characterizations, such as toughness, thermal conductivity and ablation resistance. Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the

Acknowledgements This work has been supported by the fund of the State Key Laboratory of Solidification Processing in NWPU (Grant No. SKLSP201730), the Natural Science Foundation of Shaanxi Province (Grant No. 2018JQ5057) and the National Natural Science Foundation of China (Grant Nos. 51702199, 51432008, 51704188, 61705125 and 51802181). Appendix A. Supplementary data

References [1] T. Windhorst, G. Blount, Carbon-carbon composites: a summary of recent developments and applications, Mater. Des. 18 (1) (1997) 11e15. [2] H.J. Li, Carbon/carbon composites, N. Carbon Mater. 16 (2) (2001) 79e80.
pez-Honorato, P. Xiao, Fluidized bed chemical vapor deposition [3] H. Zhang, E. Lo of pyrolytic carbon-III. Relationship between microstructure and mechanical properties, Carbon 91 (2015) 346e357. [4] I.Y. Stein, A.J. Constable, N. Morales-Medina, C.V. Sackier, M.E. Devoe, H.M. Vincent, et al., Structure-mechanical property relations of nongraphitizing pyrolytic carbon synthesized at low temperatures, Carbon 117 (2017) 411e420. [5] A.B. Macander, R.M. Crane, E.T. Camponeschi, in: J.M. Whitney (Ed.), Fabrication and Mechanical Properties of Multidimensionally (X-D) Braided Composite Materials, ASTM International, West Conshohocken, PA, 1986, pp. 422e443. [6] Q.M. Gong, Z. Li, X.W. Zhou, J.J. Wu, Y. Wang, J. Liang, Synthesis and characterization of in situ grown carbon nanofiber/nanotube reinforced carbon/ carbon composites, Carbon 43 (11) (2005) 2426e2429. [7] Q. Song, K.Z. Li, H.L. Li, H.J. Li, C. Ren, Grafting straight carbon nanotubes radially onto carbon fibers and their effect on the mechanical properties of carbon/carbon composites, Carbon 50 (10) (2012) 3949e3952. [8] Q.M. Gong, Z. Li, X.D. Bai, D. Li, J. Liang, The effect of carbon nanotubes on the microstructure and morphology of pyrolytic carbon matrices of C-C composites obtained by CVI, Compos. Sci. Technol. 65 (7) (2005) 1112e1119. [9] P. Delhaes, Chemical vapor deposition and infiltration processes of carbon materials, Carbon 40 (5) (2002) 641e657.
pez-Honorato, P.J. Meadows, P. Xiao, Fluidized bed chemical vapor [10] E. Lo deposition of pyrolytic carboneI. Effect of deposition conditions on microstructure, Carbon 47 (2) (2009) 396e410. [11] X.H. Zhong, Y.L. Li, Y.K. Liu, X.H. Qiao, Y. Feng, J. Liang, et al., Continuous multilayered carbon nanotube yarns, Adv. Mater. 22 (6) (2010) 692e696. ~ oz, J.M. Razal, V.H. Ebron, J.P. Ferraris, et al., [12] A.B. Dalton, S. Collins, E. Mun Super-tough carbon-nanotube fibres, Nature 423 (6941) (2003) 703. [13] P. Xiao, X.F. Lu, Y. Liu, L. He, Effect of in situ grown carbon nanotubes on the structure and mechanical properties of unidirectional carbon/carbon composites, Mater. Sci. Eng. A 528 (7) (2011) 3056e3061. [14] Z. Hou, W. Yang, J. Li, R. Luo, H. Xu, Densification kinetics and mechanical properties of carbon/carbon composites reinforced with carbon nanotubes produced in situ, Carbon 99 (2016) 533e540. [15] Y. Liu, L. He, X. Lu, P. Xiao, Transmission electron microscopy study of the

Please cite this article as: L. Feng et al., A novel continuous carbon nanotube fiber/carbon composite by electrified preform heating chemical vapor infiltration, Carbon, https://doi.org/10.1016/j.carbon.2019.11.009

L. Feng et al. / Carbon xxx (xxxx) xxx

[16]

[17]

[18] [19]

[20] [21]

[22]

[23]

[24]

[25]

[26]

[27]

[28]

[29]

[30]

[31]

[32] [33]

[34]

[35]

[36]

microstructure of carbon/carbon composites reinforced with in situ grown carbon nanofibers, Carbon 50 (7) (2012) 2424e2430. S. Faraji, O. Yildiz, C. Rost, K. Stano, N. Farahbakhsh, Y. Zhu, et al., Radial growth of multi-walled carbon nanotubes in aligned sheets through cyclic carbon deposition and graphitization, Carbon 111 (2017) 411e418. N. Behabtu, C.C. Young, D.E. Tsentalovich, O. Kleinerman, X. Wang, A.W.K. Ma, et al., Strong, light, multifunctional fibers of carbon nanotubes with ultrahigh conductivity, Science 339 (6116) (2013) 182e186. K. Koziol, J. Vilatela, A. Moisala, M. Motta, P. Cunniff, M. Sennett, et al., Highperformance carbon nanotube fiber, Science 318 (5858) (2007) 1892e1895. J.N. Wang, X.G. Luo, T. Wu, Y. Chen, High-strength carbon nanotube fibre-like ribbon with high ductility and high electrical conductivity, Nat. Commun. 5 (1) (2014) 3848. A. Mikhalchan, J.J. Vilatela, A perspective on high-performance CNT fibres for structural composites, Carbon 150 (2019) 191e215. H. Allouche, M. Monthioux, Chemical vapor deposition of pyrolytic carbon on carbon nanotubes. Part 2. Texture and structure, Carbon 43 (6) (2005) 1265e1278. M.D. Yadav, K. Dasgupta, A.W. Patwardhan, J.B. Joshi, High performance fibers from carbon nanotubes: synthesis, characterization, and applications in composites-a review, Ind. Eng. Chem. Res. 56 (44) (2017) 12407e12437. H. Li, C. Liu, S. Fan, Catalyzed filling of carbon nanotube array with graphite and the thermal properties of the composites, J. Phys. Chem. C 112 (15) (2008) 5840e5842. Q.M. Gong, Z. Li, D. Li, X.D. Bai, J. Liang, Fabrication and structure: a study of aligned carbon nanotube/carbon nanocomposites, Solid State Commun. 131 (6) (2004) 399e404. X. Li, L. Ci, S. Kar, C. Soldano, S.J. Kilpatrick, P.M. Ajayan, Densified aligned carbon nanotube films via vapor phase infiltration of carbon, Carbon 45 (4) (2007) 847e851. Y. Jin, Y. Zhang, Q. Zhang, R. Zhang, P. Li, W. Qian, et al., Multi-walled carbon nanotube-based carbon/carbon composites with three-dimensional network structures, Nanoscale 5 (13) (2013) 6181e6186. S. Faraji, K. Stano, C. Rost, J.P. Maria, Y. Zhu, P.D. Bradford, Structural annealing of carbon coated aligned multi-walled carbon nanotube sheets, Carbon 79 (2014) 113e122. Z. Zhou, X. Wang, S. Faraji, P.D. Bradford, Q. Li, Y. Zhu, Mechanical and electrical properties of aligned carbon nanotube/carbon matrix composites, Carbon 75 (2014) 307e313. Y. Han, S. Li, F. Chen, T. Zhao, Multi-scale alignment construction for strong and conductive carbon nanotube/carbon composites, Mater. Today Commun. 6 (2016) 56e68. X. Zhang, L. Yang, H. Liu, Enhancements in mechanical and electrical properties of carbon nanotube films by SiC and C matrix bridging, J. Mater. Sci. 53 (15) (2018) 11027e11037. J.G. Park, N.G. Yun, Y.B. Park, R. Liang, L. Lumata, J.S. Brooks, et al., Singlewalled carbon nanotube buckypaper and mesophase pitch carbon/carbon composites, Carbon 48 (15) (2010) 4276e4282. Y. Shang, Y. Wang, S. Li, C. Hua, M. Zou, A. Cao, High-strength carbon nanotube fibers by twist-induced self-strengthening, Carbon 119 (2017) 47e55. J. Zhao, X. Zhang, Y. Huang, J. Zou, T. Liu, N. Liang, et al., A comparison of the twisted and untwisted structures for one-dimensional carbon nanotube assemblies, Mater. Des. 146 (2018) 20e27. P. Wang, J. Yang, G. Sun, X. Zhang, H. Zhang, Y. Zheng, et al., Twist induced plasticity and failure mechanism of helical carbon nanotube fibers under different strain rates, Int. J. Plast. 110 (2018) 74e94. X.L. Wei, Y. Liu, Q. Chen, L.M. Peng, Controlling electron-beam-induced carbon deposition on carbon nanotubes by Joule heating, Nanotechnology 19 (35) (2008), 355304. B. Han, X. Xue, Y. Xu, Z. Zhao, E. Guo, C. Liu, et al., Preparation of carbon

[37] [38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

[50]

[51]

[52]

[53]

[54]

9

nanotube film with high alignment and elevated density, Carbon 122 (2017) 496e503. R. Luo, Fabrication of carbon/carbon composites by an electrified preform heating CVI method, Carbon 40 (11) (2002) 1957e1963. H.J. Li, X.H. Hou, Y.X. Chen, Densification of unidirectional carbon-carbon composites by isothermal chemical vapor infiltration, Carbon 38 (3) (2000) 423e427. G.B. Zheng, H. Sano, Y. Uchiyama, A layer-by-layer deposition mechanism for producing a pyrolytic carbon coating on carbon nanotubes, Carbon 57 (2013) 267e273. P. Mallet-Ladeira, P. Puech, C. Toulouse, M. Cazayous, N. Ratel-Ramond, P. Weisbecker, et al., A Raman study to obtain crystallite size of carbon materials: a better alternative to the Tuinstra-Koenig law, Carbon 80 (2014) 629e639. M. Li, Y. Song, C. Zhang, Z. Yong, J. Qiao, D. Hu, et al., Robust carbon nanotube composite fibers: strong resistivities to protonation, oxidation, and ultrasonication, Carbon 146 (2019) 627e635. S. Zhang, A. Hao, N. Nguyen, A. Oluwalowo, Z. Liu, Y. Dessureault, et al., Carbon nanotube/carbon composite fiber with improved strength and electrical conductivity via interface engineering, Carbon 144 (2019) 628e638. V. Thiagarajan, X. Wang, P.D. Bradford, Y.T. Zhu, F.G. Yuan, Stabilizing carbon nanotube yarns using chemical vapor infiltration, Compos. Sci. Technol. 90 (2014) 82e87. J. Lee, T. Kim, Y. Jung, K. Jung, J. Park, D.M. Lee, et al., High-strength carbon nanotube/carbon composite fibers via chemical vapor infiltration, Nanoscale 8 (45) (2016) 18972e18979. X. Liang, Y. Gao, J. Duan, Z. Liu, S. Fang, R.H. Baughman, et al., Enhancing the strength, toughness, and electrical conductivity of twist-spun carbon nanotube yarns by p bridging, Carbon 150 (2019) 268e274. X.S. Zhang, L.W. Yang, H.T. Liu, M. Zu, A novel high-content CNT-reinforced SiC matrix composite-fiber by precursor infiltration and pyrolysis process, RSC Adv. 7 (38) (2017) 23334e23341. G. Xu, H. Li, R. Bai, J. Wei, Y. Zhai, Influence of the matrix texture on the fracture behavior of 2D carbon/carbon composites, Mater. Sci. Eng. A 478 (1) (2008) 319e323. W. Li, H.J. Li, J. Wang, S.Y. Zhang, X. Yang, J.F. Wei, Preparation and mechanical properties of carbon/carbon composites with high textured pyrolytic carbon matrix, T. Nonferr. Metal. Soc. 23 (7) (2013) 2129e2134. L. Feng, K. Li, Z. Zhao, H. Li, L. Zhang, J. Lu, et al., Three-dimensional carbon/ carbon composites with vertically aligned carbon nanotubes: providing direct and indirect reinforcements to the pyrocarbon matrix, Mater. Des. 92 (2016) 120e128. H. Mei, Q. Bai, T. Ji, H. Li, L. Cheng, Effect of carbon nanotubes electrophoretically-deposited on reinforcing carbon fibers on the strength and toughness of C/SiC composites, Compos. Sci. Technol. 103 (2014) 94e99. J. Wang, X. Zhang, Y. Miao, Y. Li, X. Xi, X. Zhong, et al., The influences of carbon nanotubes introduced in three different phases of carbon fiber/pyrolytic carbon/silicon carbide composites on microstructure and properties of their composites, Carbon 129 (2018) 409e414. A. Lekawa-Raus, J. Patmore, L. Kurzepa, J. Bulmer, K. Koziol, Electrical properties of carbon nanotube based fibers and their future use in electrical wiring, Adv. Funct. Mater. 24 (24) (2014) 3661e3682. G. Gradoni, D. Micheli, V. Mariani Primiani, F. Moglie, M. Marchetti, Determination of the electrical conductivity of carbon/carbon at high microwave frequencies, Carbon 54 (2013) 76e85. Q. Shen, H. Li, H. Lin, L. Li, W. Li, Q. Song, Simultaneously improving the mechanical strength and electromagnetic interference shielding of carbon/ carbon composites by electrophoretic deposition of SiC nanowires, J. Mater. Chem. C 6 (22) (2018) 5888e5899.

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