Preparation of boron nitride nanosheet-coated carbon fibres and their enhanced antioxidant and microwave-absorbing properties

In this study, annealing carbon fibres with boron and FeCl3·6H2O at elevated temperatures was demonstrated as a novel route to coat carbon fibres with boron nitride (BN) nanosheets. The effect of annealing temperature on the thickness and microstructure of BN coating was investigated. Results showed that BN coating hardly formed at 1000 °C, and uniform BN coating was achieved at 1100 °C and 1200 °C. However, further increasing the temperature to 1250 °C triggered the formation of discretely distributed BN particles on the surface of the BN coating in addition to the formation of a uniform BN coating. The BN coating and particles were constructed by numerous BN nanosheets with a bending and crumpling morphology. The thickness of the BN coating increased with increasing annealing temperature. The oxidation resistance of the carbon fibres dramatically enhanced after BN nanosheets were coated onto the carbon fibre surface. Moreover, given the low dielectric loss tangent of BN, the BN coating can improve the impedance matching of carbon fibres and enhance the microwave-absorbing property of carbon fibres significantly.


Introduction
Carbon materials are applied widely in the military, electronic, and infrastructure industries because of their low density, high specic modulus, high specic strength, excellent electrical conductivity, and chemical stability. [1][2][3] As a form of carbon, carbon bres have been used as wave absorbers and reinforced llers in structural microwave-absorbing materials. 4 However, carbon bres are a strong radar reector against electromagnetic waves in the high-frequency range because of their low electrical resistivity (<10 À3 U m). 4,5 Carbon bres are also greatly susceptible to oxidation and can be oxidized at around 400 C. 6 Thus, much effort has been exerted to modify carbon bres with low-cost and light-weight coatings to improve their antioxidant property and microwave absorption property. 5,7,8 By contrast, most studies mainly focused on the metal coating or metal oxide coating, 9,10 which cannot effectively dissipate the electromagnetic energy when the application temperature exceeds the coatings' Curie temperatures. 11 Boron nitride (BN) is a surface-modied material popular for its unique properties, such as low density, high electrical resistivity, good antioxidant property, low dielectric constant, and excellent chemical inertness. [12][13][14] BN is also an isostructural analog to carbon with highly similar lattice parameters. These attributes render BN highly suitable in coating carbon bres and enhancing the antioxidant and microwave-absorbing properties of such material. 8,12,[15][16][17] Currently, chemical vapor deposition (CVD) and the dip-coating method are mainly used to coat carbon bres with BN. 14,18-20 However, the CVD route usually requires hazardous and expensive precursor chemicals, whereas the dip-coating method is a complex process with a long preparation cycle. Therefore, a simple but effective route to coat carbon bres with BN must be explored. In this study, given the solid-state reaction method developed in our group for the large-scale synthesis of BN micro-nanostructures, 21 BN nanosheets were coated onto carbon bres by simple annealing with amorphous boron powder and FeCl 3 $6H 2 O under an NH 3 atmosphere at elevated temperatures. The thickness and microstructure of the BN coating can be controlled by varying the annealing temperature. The antioxidant and microwave absorption properties of the BN-nanosheet-coated carbon bres were investigated in detail.

Treatment of carbon bres
PAN-based carbon bres (T300, 3K, Toho Tenax, Inc.) were used in this study. A bundle of carbon bres usually consists of $2000 laments with a typical lament diameter of about 5 mm. Prior to coating, the carbon bres were heated at 800 C in a nitrogen atmosphere to remove organic impurities. Then, the bres were ultrasonically cleaned in acetone for 60 min, followed by drying at 110 C for 1 h. Herein, clean carbon bres without sizing were obtained.

The treatment of raw materials
Amorphous boron powders (98% purity, Dandong Chemical Co., Ltd., China) and FeCl 3 $6H 2 O (analytical grade, Aladdin, Shanghai, China) were purchased and used without further purication. The molar ratio was B : FeCl 3 $6H 2 O ¼ 1 : 0.05. First, FeCl 3 $6H 2 O was dissolved in absolute ethyl alcohol, and then B powders were added into the solution. The mixture was stirred in a water bath at 40 C for 2 h to evaporate the solvent. Aerward, the obtained paste-like mixture was dried at 55 C to thoroughly remove the ethanol. Finally, a homogeneous mixture containing B and Fe was prepared that provides the boron source for the product. The carbon bres used are 2 mm in length and 7 mm in diameter in average. The mixture containing B and Fe prepared above was loaded into an alumina boat placed at the centre of a tube furnace. The clean carbon bres were placed in the same boat next to the BN precursor along the direction of gas ow. Prior to heating up, high-purity NH 3 ow was introduced to ush out the residual air in the chamber. Then, the furnace was heated to 1000-1250 C at a rate of 10 C min À1 under 50 mL min À1 NH 3 ow and maintained for 1 h. Finally, the furnace was cooled naturally under the protection of N 2 ow. The effect of reaction time (0.5-1.5 h) on the formation of BN coating was conducted and presented in the ESI. †

Characterization
Aer the carbon bres were removed from the furnace, they were characterized by Fourier transform infrared spectroscopy (FTIR; Bruker, TENSOR 27 spectrometer), X-ray photoelectron spectroscopy (XPS; ESCALAB 250Xi, Thermo Scientic) with Al Ka radiation, and scanning electron microscopy (SEM; JEM-1200EX, equipped with EDS system). The antioxidant property of the BN-coated carbon bres was measured in air with a DTA/ TGA instrument (STA 449C, NETZSCH). To test the microwaveabsorbing property of the samples, the carbon bres with and without BN coating were cut into short fragments with length of about 2 mm (the average diameters are still about 7 mm because of the thin thickness of BN coating), mixed with molten paraffin (in a mass ratio of 1 : 4), and then pressed into rings with inner diameter Â outer diameter Â thickness ¼ 3 mm Â 7 mm Â 2 mm. The electromagnetic parameters were measured by a coaxial line method in the frequency range of 2-18 GHz using a network analyzer (AV3629). Four samples for each batch were measured to gain the average real part of permittivity (3 0 ) and the imaginary part of permittivity (3 00 ) values.

Results and discussion
3.1 Inuence of annealing temperature Fig. 2 shows the SEM images of the carbon bres coated with BN at annealing temperatures of 1000-1250 C. For comparison, the carbon bres without BN coating were also characterized by SEM (Fig. 2a), showing the clean surfaces of the carbon bres. Fig. 2b indicates that no obvious BN coating can be found on the surfaces of the carbon bres when the annealing temperature was 1000 C. This result was achieved because very few BN species can be generated through the reaction of B and FeCl 3 at  1000 C. With the rise of annealing temperature (e.g., 1100 C and 1200 C), an increasing number of BN species were generated and deposited onto the carbon bres, which resulted in the thickening of BN coatings (Fig. 2c-f). Meanwhile, the magnied images ( Fig. 2d and f) show that the coatings were constructed by numerous thin nanosheets. The nanosheets were mostly separated with a bending and crumpling morphology. Moreover, the sizes (thickness and width) of the nanosheets increased with increasing temperature. However, when the annealing temperature was further increased to 1250 C, many irregular particles formed on the coating surface (Fig. 2g). The magnied image (Fig. 2h) clearly reveals that these particles were also constructed by numerous nanosheets. Fig. 2i shows the elemental mapping of the BN coating prepared at 1200 C, implying the uniform distribution of B and N elements in the coating.
Thermodynamic calculation shows that the Gibbs free energies of the involved reactions are negative (eqn (1) and (2)), which indicates that the reactions can be occurred thermodynamically in the reaction temperature range 1000-1250 C. As a result, the thickness of the BN coating increased with the rise of temperature. However, the reactions may have proceeded very violently at 1250 C and generated an excessive amount of BN species within a short period of time. Hence, surplus BN species deposited directly onto the surfaces of the already formed BN coating and caused the formation of BN nanosheets and even self-assembled BN particles by van der Waals forces. This process is highly similar to that of BN-nanosheetassembled microwires we reported previously. 21  Fig. 3 shows the FTIR spectra of carbon bres with and without BN coating. The two absorption bands located at 1384 and 802 cm À1 were detected in the BN-coated carbon bres prepared at 1200 C. These bands were attributed to the B-N in-plane stretching vibrations and the B-N-B out-of-plane bending vibrations, respectively. 22,23 Meanwhile, the band at 3407 cm À1 can be assigned to the stretching vibration of O-H and N-H bonds because of the absorbed water on the surfaces of BNcoated carbon bres. 24 XPS is a powerful spectroscopic technique for characterizing surfaces with chemical bonding. Therefore, XPS was used to obtain additional information on the chemical composition of BN coating fabricated at 1200 C (Fig. 4). All XPS data were corrected by using the binding energy (BE) of C-C at 284.6 eV. Fig. 4a shows the XPS survey scan of BN coating and indicates the presence of B, N, O, and C elements in the sample. The C signal can be ascribed to the carbon bre or adventitious hydrocarbon from the XPS instrument itself. 25 The narrow XPS spectra of B 1s and N 1s are shown in Fig. 4b and c, respectively. The B 1s peak at 190.4 eV and the N 1s peak at 398.2 eV were identied as B-N bonding and matched those reported for bulk h-BN. 26 The B 1s band was divided into two ne peaks located at 190.4 and 191.9 eV, respectively. The 190.4 eV peak was assigned to the B-N bonding, 27,28 whereas the peak at 191.9 eV was assigned to the substitution of N by O in the BN lattice, i.e., the formation of O-B or O-B-N bonds of BN x O y . 29 The N 1s peak can also be divided into two ne peaks centred at 397.9 and 399.4 eV, respectively. The state of N at 397.9 eV can be ascribed to the N-B bonds, whereas the peak at 399.4 eV was attributed to N-C bonds, which agreed with the results reported in literature. 30

Oxidation resistance of BN-coated carbon bres
The oxidation resistance of the carbon bres with and without BN coating was investigated by performing TGA test in air from room temperature to 900 C (Fig. 5). Research found that the carbon bres without BN coating began to be oxidized at about 500 C and underwent rapid weight loss when the temperature  was increased. When the temperature was increased to 740 C, these carbon bres were nearly completely oxidized with only about 1% residual weight. However, the starting oxidation temperature and end-oxidation temperature for the BN-coated carbon bres increased to about 640 C and 850 C, respectively. Thus, the antioxidant property of the carbon bres was improved efficiently by BN coating.

Microwave-absorbing property
According to Debye theory, the real part of permittivity (3 0 ) and the imaginary part of permittivity (3 00 ) represent the storage ability and the dissipation ability of electromagnetic wave energy, respectively. Fig. 6a and b show the 3 0 and 3 00 of the clean carbon bres and BN-coated carbon bres measured at room temperature in the frequency range of 2-18 GHz. The 3 0 and 3 00 values of the carbon bres decreased obviously aer BN coating, especially when the frequency was less than 8 GHz. This decrease may be attributed to the separation of dielectric relaxation and space charge polarization of carbon bres aer BN coating because BN possesses a high electrical resistivity. 31 In addition, the extremely low dielectric constant and low dielectric loss of BN can also help decrease the complex permittivity of carbon bres. 1 Herein, the microwave attenuation of the samples was mainly attributed to dielectric loss rather than magnetic loss. 32 Fig. 6c shows that the dielectric loss tangent (tan d) of the BNcoated carbon bres was smaller than that of the clean carbon bres in the measured frequency range. To obtain good impedance matching, the values of tan d should be approximately equal to the magnetic tangent loss. 33 Carbon bres reect microwaves highly because their dielectric loss tangent is obviously higher than their magnetic loss tangent. Hence, the BN coating with a low dielectric loss tangent can narrow the gap between the dielectric and magnetic loss tangents of carbon bres.
To further describe the microwave-absorbing property of the samples, the reection loss (RL) of the carbon bres with and without BN coating was calculated in accordance with the following equations by using the single-layer model: 33,34 RLðdBÞ ¼ 20 log 10 where Z in is the normalized input impedance relative to the impedance in free space, f is the microwave frequency, c is the light velocity, d is the thickness of the absorber, and 3 r and m r are the complex permittivity and complex permeability of materials, respectively. Therefore, the R values versus frequency can be evaluated at a specied thickness. Fig. 7 shows the calculated RL of the samples (thickness 1-4.5 mm) with and without BN coating in the frequency range of 2-18 GHz. As shown in Fig. 7a, the RL values of the clean carbon bres were larger than À4 dB in the frequency range of 2-18 GHz. This poor microwave absorption performance was mainly caused by the poor impedance matching and most microwaves were reected by the front of the sample surface. 35 Fig. 7b illustrates that the BN coating improves the microwave-absorbing property of the carbon bres signicantly, and the RL values are listed in Table  1. Meanwhile, the dependence of RL values on sample thickness can be concluded.
The lowest RL can reach À26.8 dB at 3.5 GHz with a sample thickness of 4.0 mm. The widest frequency range where R is lower than À10 dB was 8.6-10.4 GHz when the sample thickness   This journal is © The Royal Society of Chemistry 2018 was 2.0 mm. In addition, RL values less than À10 dB can be gained in the 2.9-13.4 GHz range with sample thicknesses of 1.5-4.5 mm. This result means that the microwave absorption performance of the carbon bres in different frequency bands can be adjusted by varying the sample thickness. A material with an RL value less than À20 dB is an excellent absorber because this value corresponds to a microwave absorption of 99.99%. 34 Therefore, we conclude that the microwave-absorbing property of carbon bres can be signicantly enhanced by BN coating through the reduction of microwave reection.

Conclusions
BN coatings have been successfully deposited on the surface of carbon bres by annealing amorphous boron powder with FeCl 3 $6H 2 O under owing ammonia atmosphere at 1100-1250 C. The thickness of coating increases with the rise of annealing temperature and uniform coatings can be obtained at 1100-1200 C. The coatings consist of numerous BN nanosheet which are mostly separated with a bending and crumpling morphology. Irregular BN particles are formed and distributed discretely on the BN coating when the temperature increases to 1250 C, which are also constructed of BN nanosheets. The antioxidation property of carbon bres is improved signicantly by BN coating, with start oxidization temperatures increasing from 500 to 640 C. In addition, the complex permittivity of BNcoated carbon bres decreases greatly due to the extremely low dielectric constant, low dielectric loss and high electrical resistivity of BN coatings. The carbon bres with BN coating show a strong absorption peak at 3.5 GHz, where the lowest reectivity can reach À26.8 dB. Moreover, the reection loss less than À10 dB is over the range of 2.9-13.4 GHz, indicating an excellent microwave absorbing property of the BN-coated carbon bres. Therefore, it is an effective modication approach to enhance the oxidation resistance and microwave absorbing properties of carbon bres.

Conflicts of interest
There are no conicts to declare.