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Highly-dispersible boron nitride nanoparticles by spray drying and pyrolysis
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Highly-dispersible boron nitride nanoparticles by spray drying and pyrolysis ⁎
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Weifang Han, Zhiyan Ma, Shicai Liu, Chunhua Ge , Lixia Wang, Xiangdong Zhang Institute of Inorganic Chemistry, College of Chemistry, Liaoning University, Shenyang 110036, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: Spray drying Pyrolysis High dispersivity Boron nitride Antifriction
A spray drying and pyrolysis synthesis route was developed and it successfully prepared boron nitride (BN) nanoparticles with high dispersivity. During the experiment, the extremely rapid drying of the boric acid/urea solution during the spray-drying process resulted in the formation of homogeneous precursors, which was the key for the final pyrolysis synthesis of BN nanoparticles with high dispersibility and uniform diameters (~20 nm). Compared with boron nitride synthesized without using spray drying, the as-prepared BN nanoparticles possess higher specific surface area (145.01 m2 g−1) and larger pore volume (0.41 cm3 g−1). Especially, we used the BN nanoparticles as lubricant and incorporated it into the liquid paraffin (LP). The experiment results show that the LP presents outstanding antifriction properties for a BN content of 1.5 wt%. These results suggest that the h-BN nanoparticles have significant potential applications in the field of tribology.
1. Introduction Inorganic nanoparticles have been widely used in various fields, including thermotics [1], biomedicine [2], electronics [3] and photonics [4] because of their unique physical and chemical properties. Previous researches on inorganic nanoparticles predominantly focused on synthetic method, morphology and composition rather than their application. With the rapid development of society and science technology, the applications of inorganic nanoparticles have become more and more important. Hence, it is meaningful to conduct a research on the applications of inorganic nanoparticles. Among inorganic nanoparticles, hexagonal boron nitride (h-BN) nanoparticles have attracted great interest due to their unique physical and chemical properties, including high specific surface area, numerous structural defects, lubricating characteristics, low density, high thermal conductivity, chemical durability and oxidation resistance [5– 8]. Various methods have been used for producing BN nanoparticles, such as template method [9], hydrothermal method [10], ball-milling peeling method [11], chemical vapor deposition (CVD) techniques [12] and other methods [13]. For example, graphene analogues of BN nanoparticles were fabricated by solid state thermal annealing method using boric acid and urea as starting materials [14]. Water-dispersible boron nitride nanoparticles had been successfully synthesized by adjusting the reaction temperature and time in the pyrolysis process [15]. Xu et al. found that BN nanoparticles were prepared by using BBr3, NH4Br and metallic Na as reactants in stainless steel autoclaves
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at 450 °C for 24 h. [16]. BN nanospheres can be prepared through aerosol-assisted method [17]. The CVD method was selected to fabricate spherical h-BN by using trimethoxyborane as the reactant under an NH3 atmosphere [24]. In addition, a modified solid state metathesis reaction route was also used to prepare h-BN nanoparticles with low agglomeration degree [18]. But these methods have disadvantages more or less, such as high cost, low purity, high temperature and low yield etc. Spray drying has the characteristics of fast heat transfer, rapid water evaporation and short drying time [19]. Besides, the as-prepared products possess high purity and good uniformity [20]. The hollow capsules of organic materials have been reported by spraydrying technology in a single-step route [21]. Recently, the technology has been extended to metal-organic-frameworks (MOFs) which can be crystallized at mild temperatures [22]. Moreover, spray pyrolysis is appropriate for the mass-production of highly crystalline fine particles with reduced processing time and solvent. However, to the best of our knowledge, investigations on the fabricating BN nanoparticles via spray-drying method have been rarely reported. In addition, it is well known that morphologies and sizes of BN nanomaterials are significant to understand the shape–property relationship and to develop functional materials for specific applications. Various morphologies of BN micro/nanomaterials have been reported since the first discovery of BN nanotubes in 1995, including nanotubes [23], nanoparticles [24], nanoribbons [25], nanosheets [26], micro/ nanospheres [27], nanowires [28] and nanofibers [29]. The different morphologies of boron nitride materials have been applied in many
Corresponding authors. E-mail addresses: [email protected] (C. Ge), [email protected] (X. Zhang).
http://dx.doi.org/10.1016/j.ceramint.2017.05.045 Received 9 April 2017; Accepted 6 May 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Han, W., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.05.045
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fields. For instance, BN nanotubes (BNNT) as filler were added into composites to improve mechanical strength owing to their unique properties [30]. They were also used in enhancing thermal conductivity, and decreasing dielectric constant and loss of polymer-matrix composite [31]. The highly porous BN microbelts with large specific surface area (up to 1488 m2 g−1) were used for hydrogen storage. The results show that the sample possesses the largest reversible H2 uptake capacity of 2.3 wt% at 77 K and 1 MPa [32]. Boron nitride nanosheets exhibited superhydrophobicity, high selective absorption and adsorption capacities for oils, organic solvents and dyes, and they afforded a new strategy in water purification and treatment [33]. Novel BN hollow microspheres with open mouths had potential applications in the field of ultraviolet lasing due to intense cathodoluminescence emissions at 338.7, 403.1 and 458.2 nm [34]. Furthermore, nanoporous BN materials had important applications in biological fields when their surfaces were engineered to be hydrophilic. Highly hydroxylated nanoporous BN were water-soluble and nontoxic, and they served as efficient transports vehicles for drug delivery up to 309 wt% [35]. Tribological performance as an important technical index has been widely used in many fields. Steel, plastic, nylon, rubber and Al-Si alloy as friction materials have been used in manufacture, mechanical processing industry, architecture industry and chemical industry. However, the improvements of tribological properties still keep an enormous technical challenge. Recently, many investigations focused on carbonaceous material reinforced polymer or alloys to improve the tribological properties [36]. However, the application of highly-dispersible and small-sized hexagonal boron nitride nanoparticles for antifriction of liquid paraffin has not been reported yet. In this work, highly-dispersible h-BN nanoparticles were successfully synthesized by two-steps (spray drying and pyrolysis). Spray drying is the unique aspect of our study compared with previous reports and is also an essential step for the uniformity of highlydispersible BN nanoparticles. The as-synthesized samples were researched and analyzed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), fourier transform infrared (FT-IR) spectroscopy, field emission scanning electron microscopy (SEM) and transmission electron microscope (TEM). Moreover, the tribological behaviors of the as-synthesized BN nanoparticles as additive to liquid paraffin were researched. The friction behavior of the as-obtained products is superior to that of BN nanoparticles without using spray drying method.
at 20 °C before being used. The precursors were put into the bottom of an alumina boat, which was then placed at the center of a tube furnace. After that, the precursors were calcined at 500 °C for 1 h and then calcined at 900–1200 °C for 2 h in N2 condition with heating rate of 5 °C/min. The tube furnace was naturally cooled to ambient temperature under N2 atmosphere. The as-obtained samples were put into dilute hydrochloric acid and stirred for 12 h at room temperature. Afterwards, the products were washed with distilled water and ethanol for three times in order to remove all residual impurities. Finally, the final product powders had been obtained after being dried at 110 °C for 12 h in vacuum drying oven. 2.3. Characterization of the prepared BN powders The structure and morphology of the h-BN samples were measured by XRD, FT-IR, SEM and TEM. XRD patterns were performed using a Germany Bruker D8 Advance powder X-ray diffractometer with graphite monochromatized Cu Kα radiation (λ=0.15406 Å) at a scanning step of 0.5°s−1 over the 2θ range of 10–70°. FT-IR was recorded on an America Elmer Spectrum One Spectrometer (KBr disks) within a wavelength range of 500–4000 cm−1. SEM and TEM were performed by Japan Hitachi Corporation JSM-6701F and JEM-2100 to determine the surface morphology and size. The surface elemental compositions of product was analyzed by a Thermo Scientific ESCA Lab 250Xi using a 200 W monochromated Al Kα as radiation. The 500 µm X-ray spot was used for XPS analysis. The base pressure in the analysis chamber was about 3×10−10 mbar. Specific surface area, pore volume and pore diameter were measured by an America Micromeritics ASAP 2010 at 77 K. 2.4. Friction and wear test The friction and wear behavior of the as-prepared samples were measured by a microcomputer controlled omnipotence friction wear testing machine (MMW-1A, Jinan Yihua Tribology Testing Technology Co. Ltd. China). The test machine consisted of four standard test steel ball (GB308-84, a ball on top and three balls at the bottom) rotate against each other. The screw locking ring was fixed standard test steel ball. The distance between the upper and the lower balls: a vertical one, which allowed normal load application, and a horizontal one, for friction measurement. Prior to each test, the standard test steel balls were washed by water and acetone to remove surface contamination. The friction and wear behaviors were performed at 75 °C, with a rotating speed of 1450 rpm and with the applied load of 300 N.
2. Experiment 2.1. Materials
3. Results and discussion Boric acid (H3BO3, analytically pure) and urea (CO (NH2)2, analytically pure) were supplied by Sinopharm Chemical Regent Co. Ltd. China to prepare the boron nitride. Ltd. Ethanol (C2H5OH, 75.0% purity) and hydrochloric acid (HCl, 36.0–38.0% purity) were purchased from Sinopharm Chemical Reagent Co. Ltd. China to remove the byproducts of products. All chemicals were used as received without any further purification.
Boron nitride was fabricated by two-steps and boric acid and urea were chosen as the reactants. Through theoretical calculation, it is predicted that boric acid binds to urea mainly by hydrogen bond to form BN precursor in the spray drying process (as shown in Fig. 1). The RB/U and reaction temperature as two important conditions on the formation of highly-dispersed BN nanoparticles were investigated in the pyrolysis process. The spray drying technology is the unique aspect of our study compared with previous reports [15–18]. Fig. 2 displays the typical SEM image of h-BN products synthesized by spray drying and pyrolysis process at 1100 °C under different molar ratios of boric acid to urea (RB/U). It is clearly indicated that the morphology of the products is strongly affected by the RB/U. When equimolar amounts of CO (NH2)2 and H3BO3 present (RB/U=1:1), the obtained BN particles were consisted of somewhat irregular and spherical in shape (Fig. 2a). At RB/U=1:2, the spherical BN was observed (Fig. 2b). The size of the as-obtained spheres was about 25–30 nm. With RB/U increased to 4, the products were composed of nearly uniform spheres with about 20 nm in diameter in Fig. 2c. The dispersibility of the BN nanospheres is better than that of the BN
2.2. Synthesis of BN powders In a typical procedure, boric acid and urea were selected as the raw materials. Firstly, boric acid and urea were dissolved in distilled water (H2O) and stirred for 24 h at 40 °C using a hot magnetic stirrer. The molar ratio (RB/U) of boric acid and urea are 1:1, 1:2, 1:4, and 1:6. The resultant solution was spray-dried using a mini lab spray drier L-117 (Beijing laiheng scientific Co. Ltd. China). A peristaltic pump was used to deliver the liquid through the bi-fluid nozzle (0.7 mm) into the spray-drying chamber with a feed flow-rate of 10 mL/min and a nozzle pressure of 2 bars. The inlet temperature was set 110 °C. The spraydried homogeneous precursor was obtained and stored in plastic vessel 2
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Fig. 1. Experimental scheme of the spray drying synthesis of BN precursors.
Therefore, highly-dispersible BN nanoparticles with uniform size can be obtained at the fixed RB/U=1:4. In addition, the influence of the reaction temperature changed during the formation on the structure of the BN were investigated at RB/U=1:4. Fig. 3 shows the SEM images of the products synthesized at different temperatures. It is found that the reaction temperature plays an important role in the morphology and size of h-BN. At 900 °C, the products are in lower yield and consisted of some irregular particles
particles reported in the past few years [15,17]. Boric acid as a Lewis acid is easy to combine with urea in the process of forming hydrogen bond network. Boron nitride is easily formed when many nitrogen atoms exist in the surrounding boron atom in precursors (Fig. 1c). However, when the molar amount of CO (NH2)2 were 6 times that of H3BO3, the aggregation of nanospheres is dominant morphology in the as-prepared BN sample (Fig. 2d). That is to say, the RB/U has a great influence on the morphology and structure of BN nanoparticles.
Fig. 2. SEM images of boron nitrides synthesized at 1100 °C with different molar ratio of boric acid/urea (RB/U): (a) 1:1, (b) 1:2, (c) 1:4 and (d) 1:6.
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Fig. 3. SEM images of boron nitrides synthesized with the molar ratio of boric acid/urea (RB/U) set to 1:4 at different calcination temperature: (a) 900 °C, (b) 1000 °C, (c) 1200 °C and (d) not using spray drying method, 1100 °C.
From the SEM and TEM, we draw that spray drying technology can enable the BN nanoparticles from aggregate nanosheets to welldispersed nanospheres. Spray drying technology was confirmed to be an effective method for the production of highly-dispersible boron nitride nanoparticles. Fig. 5 presents XRD patterns of BNNP-1 and BNNP-2, the diffraction pattern of the BN nanoparticles has three characteristic peaks at 26.8°, 42.1° and 55.3°, corresponding to (002), (100) and (004) of BN (JCPDS 34–0421), respectively. The positions and shapes of diffraction peaks of BN are similar to the experimental measurement reported in Ref. [37]. No other obvious diffraction peaks were observed, indicating the extremely high purity of the as-prepared samples. To understand the more information of the lattice vibrations of BNNP-1 and BNNP-2, Fourier transform infrared (FTIR) spectroscopy was carried out to characterize at room temperature. Fig. 6 shows the typical FTIR spectra of as-prepared BN, which are consistent with previous investigations [13,38]. FTIR shows two strong characteristic vibration modes at 781 and 1385 cm–1, which were indexed to the bending of B-N-B and the stretching frequency of B-N bond, respectively. Besides, a small characteristic absorption bands at 3403 cm–1 represents a typical O–H or N–H stretching vibration, which is in accordance with earlier findings [15,39]. Therefore, the FTIR results provide further evidence that BN nanomaterials are formed. Furthermore, no other obvious absorption peaks associated with the starting materials and impurities were found. It is indicated that the as-
(Fig. 3a). Low temperature is unfavorable to the fabrication of boron nitride. Further increasing the calcination temperature to 1000 °C, as presented in Fig. 3b, the SEM image shows that BN particles are spherical in shape with average size of ~50 nm, but these BN nanospheres display poor distribution in size and dispersion. However, when the reaction temperature was elevated from 1000 to 1200 °C, the obtained BN particles were apparently aggregated (Fig. 3c). Accordingly, BN nanoparticles are not easy to synthesize at low temperature, whereas, too high temperature would cause the products to form aggregates. Furthermore, regulation of the reaction temperature is an effective way to obtain highly-dispersible boron nitride nanoparticles at RB/U=1:4. As a contrast, boron nitride nanoparticles were fabricated at RB/U=1:4 and 1100 °C without using spray drying technology. It can be seen that the resultant products are agglomerate nanosheets with a nonuniform diameter of about 20–70 nm (Fig. 3d). The results further prove that spray drying is the key for preparing highly-dispersible BN nanoparticles. The boron nitride nanoparticles prepared via spray drying method were designated as BNNP-1 notations used throughout the paper. In comparison, the as-synthesized BN nanoparticles without using this method were named BNNP-2. The morphologies of the BNNP-1 and BNNP-2 were investigated by TEM, as shown in Fig. 4. Fig. 4a shows the TEM image of BNNP-1 with high dispersivity, which is coincided with the micro appearance of SEM (Fig. 2c). Fig. 4b is a typical TEM image of the as-prepared BNNP-2 (without using spray drying method), displaying 2D nanosheets with diameter in the range of 20–70 nm. 4
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Fig. 4. TEM images of (a) BNNP-1 and (b) BNNP-2.
prepared product has high purity. The results are consistent with the XRD patterns. The chemical compositions of the BNNP-1 and BNNP-2 are recorded by the X-ray photoelectron spectroscopy (XPS) spectrum (Fig. 7). Fig. 7a shows wide-scan XPS survey spectra of the BNNP-1 and BNNP-2 prepared at RB/U=1:4 and 1100 °C. It is clearly revealed that there were no other elements except carbon, oxygen, boron and nitrogen in the as-prepared BNNP-1 and BNNP-2, which was in accordance with the FTIR spectra and XRD pattern of the sample without impurities. The existence of O and C may be caused by the exposure of the sample in the air before the examination. The B/N atomic ratio of the BNNP-1 and BNNP-2 from XPS spectra were about 1.08 and 1.12 which was consistent with those reported in the literature [40]. It is also close to the BN stoichiometry. High-resolution spectra of B 1s was displayed in Fig. 7b, the binding energies (BEs) of B 1s in the BNNPs was 189.1 eV, which is assigned to B 1s of B–N bonds. The BEs of the N 1s spectrum is located at 397.5 eV (Fig. 7c), which is attributable to N 1s of B–N bonds [41]. The survey scan XPS, B 1s and N 1s spectra revealed that the main configuration for B and N atoms are related to the B–N bonds [42]. These results imply the formation of boron nitride. Brunauer–Emmett–Teller (BET) gas sorptometry measurements were conducted to examine the specific surface area (SSA) of the BNNP-1 and BNNP-2. N2 adsorption–desorption isotherm of the BNNP-1 and BNNP-2 were displayed in Fig. 8. The isotherm plots are identified as type IV sorption profile with the hysteresis loop in the relative pressure range of 0.35–1.0, which is the characteristic of mesoporous materials [43]. The BET specific surface area of the sample is calculated from N2 isotherms at 77 K. The specific surface area of the BNNP-1 is 145.01 m2 g−1, much larger than that of BNNP-2 (71.07 m2 g−1). Moreover, the SSA of BNNP-1 was also larger than those reported by references. For example, the spherical BN nanoparticles with a diameter of 30 nm have a BET surface area of 75 m2g–1 [15]. Low oxygen-containing h-BN spheres (~30 nm in size) with a specific surface area of 57.2 m2 g−1 were fabricated by Tang and partners [24]. Singhal et al. reported a solid state thermal annealing method to fabricate BN nanomaterials with average diameter of ~30 nm and a SSA of 120.68 m2 g–1 [44]. In order to get more information about the surface of boron nitride, Barrett-JoynerHalenda (BJH) method was used to analyze experimental data. The calculated results show that the pore volume of BNNP-1 and BNNP-2 are 0.412 and 0.272 cm3g–1, respectively. Hence, it is reasonably predicted that the as-prepared BNNP-1 with high specific surface area and mesoporous nature will be used for waste-water treatment, catalyst carrier and medicine transportation in the future.
Fig. 5. XRD pattern of BNNP-1 and BNNP-2.
Fig. 6. FIRT spectrums of BNNP-1 and BNNP-2.
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Fig. 7. XPS spectra of the BNNP-1 and BNNP-2. Survey spectrum (a); high resolution spectra of B1s (b) and N1s (c).
BNNP-1 and BNNP-2. It is clearly revealed that the LP sample with BNNP-1 and BNNP-2 exhibited better tribological behaviors than separate LP. The friction coefficient was reduced with the addition percent increasing. Besides, the friction reduction and anti-wear properties of BNNP-1/LP are more excellent than those of BNNP-2/ LP at the same addition percents. Fig. 9b gives the variation of average wear scar dimeter (WSD) with the increasing amount of BN. The LP displayed the best lubricating properties at the BNNP-1 content of 1.5 wt% through comprehensive analysis and research of the friction coefficient and wear scar dimeter. The tests confirm that highlydispersed BNNP-1 has higher anti-wear properties when compared with that of BNNP-2 in the same test environments. Fig. 10 provides the morphologies of the bottom balls of LP, 1.5 wt%BNNP-1/LP and 1.5 wt% BNNP-2/LP after the friction and wear tests. It shows that the WSDs of the bottom steel balls with the additives of 1.5 wt% BNNP-1/ LP and 1.5 wt% BNNP-2/LP are smaller than those of only LP, and the addition of 1.5 wt% BNNP-2 resulted in less WSDs compared with the addition of 1.5 wt%BNNP-1. In addition, it can be found that there are many wear debris and furrows on the worn surface of running against BNNP-2 (Fig. 10b), indicating that the counterpart test steel ball has undergone severe damage. In contrast, the steel ball surface of BNNP-1 appears to be much smoother and constant (Fig. 10c). This phenomenon may be attributed to chemical stability and special structure of the BNNP-1 which could easily enter the contact area of friction pairs to prevent steel balls from wear. The results of SEM images of worn surfaces provided further evidence that the BNNP-1 had better
Fig. 8. Nitrogen adsorption-desorption isotherm of BNNP-1 and BNNP-2.
The tribological properties of hexagonal boron nitride nanoparticles as additives in liquid paraffin (LP) were investigated by MMW-1A omnipotent friction and wear testing machine. All the tests were carried out with 300 N loads, at 75 °C, a testing period of 60 min with a rotating speed of 1450 rpm and 10 mL LP. Fig. 9a shows the variation of average friction coefficient of LP containing different contents of the
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Fig. 9. Variation of the average (a) friction coefficient and (b) wear scar with increasing BN content in liquid paraffin (1450 rpm and 300 N for 60 min).
Fig. 10. Optical micrographs of typical wear scars on the top balls: (a) LP, (b) 1.5 wt% BNNP-2+LP and (c) 1.5 wt% BNNP-1+LP (1450 rpm and 300 N for 60 min).
tribological performances of the BNNP-1 can be an effective lubricant to prevent the severe damage.
tribological performances than that of the BNNP-2 for liquid paraffin. Fig. 11 presents the typical evolutions of the friction coefficient of LP, BNNP-1/LP and BNNP-2/LP as a function the rotating time. The friction coefficients of BNNP-2/LP started to creep up after 40 min. That may be because BNNP-2 could easily agglomerate in the friction process and lead to inhomogeneous lubrication. The similar result was reported by Men and coworkers [45]. Therefore, the outstanding
4. Conclusions Highly-dispersible hexagonal boron nitride nanoparticles have been successfully synthesized though spray drying and pyrolysis process by 7
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Fig. 11. Friction coefficients of LP, 1.5 wt% BNNP-2+LP and 1.5 wt% BNNP-1+LP as a function of time (1450 rpm and 300 N for 60 min).
using boric acid and urea as starting materials under N2 gas condition at 1100 °C. Spray drying technology was used to prepare homogeneous precursor. By regulating the molar ratio of boric acid to urea and pyrolysis temperature, spherical BN nanostructure powders were obtained with an average size of 20 nm, a large surface area of 145.01 m2 g–1 and a total pore volume of 0.41 cm3 g–1. Comparing with boron nitride which is synthesized without using spray drying, the spherical h-BN nanoparticles as lubrication additives in LP show excellent antifriction behavior. Our results demonstrate that highlydispersible h-BN nanoparticles can be directly used as a potential inorganic filler to improve the tribological behavior without requiring surface modification. Acknowledgements This work was supported by the Natural Science Foundation of Liaoning Province (2016010346) and Shenyang Science and Technology Plan Project (F13-289-1-00 and F14-231-1–10) for funding and supporting this work. We thank our colleagues and other students for their support and help in this work. References [1] F. Xiao, S. Naficy, G. Casillas, M.H. Khan, T. Katkus, L. Jiang, H. Liu, H. Li, Z. Huang, Edge-hydroxylated boron nitride nanosheets as an effective additive to improve the thermal response of hydrogels, Adv. Mater. 27 (2015) 7196–7203. [2] M. Liong, J. Lu, M. Kovochich, T. Xia, S.G. Ruehm, A.E. Nel, F. Tamanoi, J.I. Zink, Multifunctional inorganic nanoparticles for imaging, targeting, and drug delivery, ACS Nano 2 (2008) 889–896. [3] Z. Nie, A. Petukhova, E. Kumacheva, Properties and emerging applications of selfassembled structures made from inorganic nanoparticles, Nat. Nanotechnol. 5 (2010) 15–25. [4] B. Matović, J. Luković, M. Nikolić, B. Babić, N. Stanković, B. Jokić, B. Jelenković, Synthesis and characterization of nanocrystaline hexagonal boron nitride powders: xrd and luminescence properties, Ceram. Int. 42 (2016) 16655–16658. [5] R.T. Paine, C.K. Narula, Synthetic routes to boron nitride, Chem. Rev. 90 (1990) 73–91. [6] J. Pattanayak, T. Kar, S. Scheiner, Boron−nitrogen (BN) substitution of fullerenes: C60 to C12B24N24 CBN ball, Phys. Chem. A 106 (2002) 2970–2978. [7] C.Y. Zhi, C.C. Tang, H. Kuwahara, D. Golberg, Large-scale fabrication of boron nitride nanosheets and their utilization in polymeric composites with improved thermal and mechanical properties, Adv. Mater. 21 (2009) (2889–289). [8] X.F. Song, J.L. Hu, H.B. Zeng, Two-dimensional semiconductors: recent progress and future perspectives, J. Mater. Chem. 1 (2013) 2952–2969. [9] P. Dibandjo, F. Chassagneux, L. Bois, C. Sigala, P. Miele, Comparison between SBA15 silica and CMK-3 carbon nanocasting for mesoporous boron nitride synthesis, J. Mater. Chem. 15 (2005) 1917–1923. [10] L. Shi, Y. Gu, L. Chen, Y. Qian, Z. Yang, J. Ma, Synthesis and morphology control of nanocrystalline boron nitride, J. Solid State Chem. 177 (2004) 721–724. [11] L.H. Li, Y. Chen, G. Behan, H.Z. Zhang, M. Petravic, A.M. Glushenkov, Large-scale mechanical peeling of boron nitride nanosheets by low-energy ball milling, J.
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morphologies: synthesis, characterization and efficient application in dye adsorption, Ceram. Int. 41 (2015) 10565–10577. [45] M. Yang, X. Zhu, G. Ren, X. Men, F. Guo, P. Li, Z. Zhang, Influence of air-plasma treatment and hexagonal boron nitride as filler on the high temperature tribological behaviors of hybrid PTFE/Nomex fabric/phenolic composite, Eur. Polym. J. 67 (2015) 143–151.
nanostructures, Inorg. Chem. 43 (2004) 822–829. [42] L. Song, L. Ci, H. Lu, P.B. Sorokin, C. Jin, J. Ni, A.G. Kvashnin, D.G. Kvashnin, J. Lou, B.I. Yakobson, P.M. Ajayan, Large scale growth and characterization of atomic hexagonal boron nitride layers, Nano Lett. 10 (2010) 3209–3215. [43] M.K. And, M. Jaroniec, Gas adsorption characterization of ordered organic−inorganic nanocomposite materials, Chem. Mater. 13 (2001) 3169–3183. [44] P. Singla, N. Goel, V. Kumar, S. Singhal, Boron nitride nanomaterials with different
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Highly-dispersible boron nitride nanoparticles by spray drying and pyrolysis ⁎
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Weifang Han, Zhiyan Ma, Shicai Liu, Chunhua Ge , Lixia Wang, Xiangdong Zhang Institute of Inorganic Chemistry, College of Chemistry, Liaoning University, Shenyang 110036, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: Spray drying Pyrolysis High dispersivity Boron nitride Antifriction
A spray drying and pyrolysis synthesis route was developed and it successfully prepared boron nitride (BN) nanoparticles with high dispersivity. During the experiment, the extremely rapid drying of the boric acid/urea solution during the spray-drying process resulted in the formation of homogeneous precursors, which was the key for the final pyrolysis synthesis of BN nanoparticles with high dispersibility and uniform diameters (~20 nm). Compared with boron nitride synthesized without using spray drying, the as-prepared BN nanoparticles possess higher specific surface area (145.01 m2 g−1) and larger pore volume (0.41 cm3 g−1). Especially, we used the BN nanoparticles as lubricant and incorporated it into the liquid paraffin (LP). The experiment results show that the LP presents outstanding antifriction properties for a BN content of 1.5 wt%. These results suggest that the h-BN nanoparticles have significant potential applications in the field of tribology.
1. Introduction Inorganic nanoparticles have been widely used in various fields, including thermotics [1], biomedicine [2], electronics [3] and photonics [4] because of their unique physical and chemical properties. Previous researches on inorganic nanoparticles predominantly focused on synthetic method, morphology and composition rather than their application. With the rapid development of society and science technology, the applications of inorganic nanoparticles have become more and more important. Hence, it is meaningful to conduct a research on the applications of inorganic nanoparticles. Among inorganic nanoparticles, hexagonal boron nitride (h-BN) nanoparticles have attracted great interest due to their unique physical and chemical properties, including high specific surface area, numerous structural defects, lubricating characteristics, low density, high thermal conductivity, chemical durability and oxidation resistance [5– 8]. Various methods have been used for producing BN nanoparticles, such as template method [9], hydrothermal method [10], ball-milling peeling method [11], chemical vapor deposition (CVD) techniques [12] and other methods [13]. For example, graphene analogues of BN nanoparticles were fabricated by solid state thermal annealing method using boric acid and urea as starting materials [14]. Water-dispersible boron nitride nanoparticles had been successfully synthesized by adjusting the reaction temperature and time in the pyrolysis process [15]. Xu et al. found that BN nanoparticles were prepared by using BBr3, NH4Br and metallic Na as reactants in stainless steel autoclaves
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at 450 °C for 24 h. [16]. BN nanospheres can be prepared through aerosol-assisted method [17]. The CVD method was selected to fabricate spherical h-BN by using trimethoxyborane as the reactant under an NH3 atmosphere [24]. In addition, a modified solid state metathesis reaction route was also used to prepare h-BN nanoparticles with low agglomeration degree [18]. But these methods have disadvantages more or less, such as high cost, low purity, high temperature and low yield etc. Spray drying has the characteristics of fast heat transfer, rapid water evaporation and short drying time [19]. Besides, the as-prepared products possess high purity and good uniformity [20]. The hollow capsules of organic materials have been reported by spraydrying technology in a single-step route [21]. Recently, the technology has been extended to metal-organic-frameworks (MOFs) which can be crystallized at mild temperatures [22]. Moreover, spray pyrolysis is appropriate for the mass-production of highly crystalline fine particles with reduced processing time and solvent. However, to the best of our knowledge, investigations on the fabricating BN nanoparticles via spray-drying method have been rarely reported. In addition, it is well known that morphologies and sizes of BN nanomaterials are significant to understand the shape–property relationship and to develop functional materials for specific applications. Various morphologies of BN micro/nanomaterials have been reported since the first discovery of BN nanotubes in 1995, including nanotubes [23], nanoparticles [24], nanoribbons [25], nanosheets [26], micro/ nanospheres [27], nanowires [28] and nanofibers [29]. The different morphologies of boron nitride materials have been applied in many
Corresponding authors. E-mail addresses: [email protected] (C. Ge), [email protected] (X. Zhang).
http://dx.doi.org/10.1016/j.ceramint.2017.05.045 Received 9 April 2017; Accepted 6 May 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Han, W., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.05.045
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fields. For instance, BN nanotubes (BNNT) as filler were added into composites to improve mechanical strength owing to their unique properties [30]. They were also used in enhancing thermal conductivity, and decreasing dielectric constant and loss of polymer-matrix composite [31]. The highly porous BN microbelts with large specific surface area (up to 1488 m2 g−1) were used for hydrogen storage. The results show that the sample possesses the largest reversible H2 uptake capacity of 2.3 wt% at 77 K and 1 MPa [32]. Boron nitride nanosheets exhibited superhydrophobicity, high selective absorption and adsorption capacities for oils, organic solvents and dyes, and they afforded a new strategy in water purification and treatment [33]. Novel BN hollow microspheres with open mouths had potential applications in the field of ultraviolet lasing due to intense cathodoluminescence emissions at 338.7, 403.1 and 458.2 nm [34]. Furthermore, nanoporous BN materials had important applications in biological fields when their surfaces were engineered to be hydrophilic. Highly hydroxylated nanoporous BN were water-soluble and nontoxic, and they served as efficient transports vehicles for drug delivery up to 309 wt% [35]. Tribological performance as an important technical index has been widely used in many fields. Steel, plastic, nylon, rubber and Al-Si alloy as friction materials have been used in manufacture, mechanical processing industry, architecture industry and chemical industry. However, the improvements of tribological properties still keep an enormous technical challenge. Recently, many investigations focused on carbonaceous material reinforced polymer or alloys to improve the tribological properties [36]. However, the application of highly-dispersible and small-sized hexagonal boron nitride nanoparticles for antifriction of liquid paraffin has not been reported yet. In this work, highly-dispersible h-BN nanoparticles were successfully synthesized by two-steps (spray drying and pyrolysis). Spray drying is the unique aspect of our study compared with previous reports and is also an essential step for the uniformity of highlydispersible BN nanoparticles. The as-synthesized samples were researched and analyzed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), fourier transform infrared (FT-IR) spectroscopy, field emission scanning electron microscopy (SEM) and transmission electron microscope (TEM). Moreover, the tribological behaviors of the as-synthesized BN nanoparticles as additive to liquid paraffin were researched. The friction behavior of the as-obtained products is superior to that of BN nanoparticles without using spray drying method.
at 20 °C before being used. The precursors were put into the bottom of an alumina boat, which was then placed at the center of a tube furnace. After that, the precursors were calcined at 500 °C for 1 h and then calcined at 900–1200 °C for 2 h in N2 condition with heating rate of 5 °C/min. The tube furnace was naturally cooled to ambient temperature under N2 atmosphere. The as-obtained samples were put into dilute hydrochloric acid and stirred for 12 h at room temperature. Afterwards, the products were washed with distilled water and ethanol for three times in order to remove all residual impurities. Finally, the final product powders had been obtained after being dried at 110 °C for 12 h in vacuum drying oven. 2.3. Characterization of the prepared BN powders The structure and morphology of the h-BN samples were measured by XRD, FT-IR, SEM and TEM. XRD patterns were performed using a Germany Bruker D8 Advance powder X-ray diffractometer with graphite monochromatized Cu Kα radiation (λ=0.15406 Å) at a scanning step of 0.5°s−1 over the 2θ range of 10–70°. FT-IR was recorded on an America Elmer Spectrum One Spectrometer (KBr disks) within a wavelength range of 500–4000 cm−1. SEM and TEM were performed by Japan Hitachi Corporation JSM-6701F and JEM-2100 to determine the surface morphology and size. The surface elemental compositions of product was analyzed by a Thermo Scientific ESCA Lab 250Xi using a 200 W monochromated Al Kα as radiation. The 500 µm X-ray spot was used for XPS analysis. The base pressure in the analysis chamber was about 3×10−10 mbar. Specific surface area, pore volume and pore diameter were measured by an America Micromeritics ASAP 2010 at 77 K. 2.4. Friction and wear test The friction and wear behavior of the as-prepared samples were measured by a microcomputer controlled omnipotence friction wear testing machine (MMW-1A, Jinan Yihua Tribology Testing Technology Co. Ltd. China). The test machine consisted of four standard test steel ball (GB308-84, a ball on top and three balls at the bottom) rotate against each other. The screw locking ring was fixed standard test steel ball. The distance between the upper and the lower balls: a vertical one, which allowed normal load application, and a horizontal one, for friction measurement. Prior to each test, the standard test steel balls were washed by water and acetone to remove surface contamination. The friction and wear behaviors were performed at 75 °C, with a rotating speed of 1450 rpm and with the applied load of 300 N.
2. Experiment 2.1. Materials
3. Results and discussion Boric acid (H3BO3, analytically pure) and urea (CO (NH2)2, analytically pure) were supplied by Sinopharm Chemical Regent Co. Ltd. China to prepare the boron nitride. Ltd. Ethanol (C2H5OH, 75.0% purity) and hydrochloric acid (HCl, 36.0–38.0% purity) were purchased from Sinopharm Chemical Reagent Co. Ltd. China to remove the byproducts of products. All chemicals were used as received without any further purification.
Boron nitride was fabricated by two-steps and boric acid and urea were chosen as the reactants. Through theoretical calculation, it is predicted that boric acid binds to urea mainly by hydrogen bond to form BN precursor in the spray drying process (as shown in Fig. 1). The RB/U and reaction temperature as two important conditions on the formation of highly-dispersed BN nanoparticles were investigated in the pyrolysis process. The spray drying technology is the unique aspect of our study compared with previous reports [15–18]. Fig. 2 displays the typical SEM image of h-BN products synthesized by spray drying and pyrolysis process at 1100 °C under different molar ratios of boric acid to urea (RB/U). It is clearly indicated that the morphology of the products is strongly affected by the RB/U. When equimolar amounts of CO (NH2)2 and H3BO3 present (RB/U=1:1), the obtained BN particles were consisted of somewhat irregular and spherical in shape (Fig. 2a). At RB/U=1:2, the spherical BN was observed (Fig. 2b). The size of the as-obtained spheres was about 25–30 nm. With RB/U increased to 4, the products were composed of nearly uniform spheres with about 20 nm in diameter in Fig. 2c. The dispersibility of the BN nanospheres is better than that of the BN
2.2. Synthesis of BN powders In a typical procedure, boric acid and urea were selected as the raw materials. Firstly, boric acid and urea were dissolved in distilled water (H2O) and stirred for 24 h at 40 °C using a hot magnetic stirrer. The molar ratio (RB/U) of boric acid and urea are 1:1, 1:2, 1:4, and 1:6. The resultant solution was spray-dried using a mini lab spray drier L-117 (Beijing laiheng scientific Co. Ltd. China). A peristaltic pump was used to deliver the liquid through the bi-fluid nozzle (0.7 mm) into the spray-drying chamber with a feed flow-rate of 10 mL/min and a nozzle pressure of 2 bars. The inlet temperature was set 110 °C. The spraydried homogeneous precursor was obtained and stored in plastic vessel 2
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Fig. 1. Experimental scheme of the spray drying synthesis of BN precursors.
Therefore, highly-dispersible BN nanoparticles with uniform size can be obtained at the fixed RB/U=1:4. In addition, the influence of the reaction temperature changed during the formation on the structure of the BN were investigated at RB/U=1:4. Fig. 3 shows the SEM images of the products synthesized at different temperatures. It is found that the reaction temperature plays an important role in the morphology and size of h-BN. At 900 °C, the products are in lower yield and consisted of some irregular particles
particles reported in the past few years [15,17]. Boric acid as a Lewis acid is easy to combine with urea in the process of forming hydrogen bond network. Boron nitride is easily formed when many nitrogen atoms exist in the surrounding boron atom in precursors (Fig. 1c). However, when the molar amount of CO (NH2)2 were 6 times that of H3BO3, the aggregation of nanospheres is dominant morphology in the as-prepared BN sample (Fig. 2d). That is to say, the RB/U has a great influence on the morphology and structure of BN nanoparticles.
Fig. 2. SEM images of boron nitrides synthesized at 1100 °C with different molar ratio of boric acid/urea (RB/U): (a) 1:1, (b) 1:2, (c) 1:4 and (d) 1:6.
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Fig. 3. SEM images of boron nitrides synthesized with the molar ratio of boric acid/urea (RB/U) set to 1:4 at different calcination temperature: (a) 900 °C, (b) 1000 °C, (c) 1200 °C and (d) not using spray drying method, 1100 °C.
From the SEM and TEM, we draw that spray drying technology can enable the BN nanoparticles from aggregate nanosheets to welldispersed nanospheres. Spray drying technology was confirmed to be an effective method for the production of highly-dispersible boron nitride nanoparticles. Fig. 5 presents XRD patterns of BNNP-1 and BNNP-2, the diffraction pattern of the BN nanoparticles has three characteristic peaks at 26.8°, 42.1° and 55.3°, corresponding to (002), (100) and (004) of BN (JCPDS 34–0421), respectively. The positions and shapes of diffraction peaks of BN are similar to the experimental measurement reported in Ref. [37]. No other obvious diffraction peaks were observed, indicating the extremely high purity of the as-prepared samples. To understand the more information of the lattice vibrations of BNNP-1 and BNNP-2, Fourier transform infrared (FTIR) spectroscopy was carried out to characterize at room temperature. Fig. 6 shows the typical FTIR spectra of as-prepared BN, which are consistent with previous investigations [13,38]. FTIR shows two strong characteristic vibration modes at 781 and 1385 cm–1, which were indexed to the bending of B-N-B and the stretching frequency of B-N bond, respectively. Besides, a small characteristic absorption bands at 3403 cm–1 represents a typical O–H or N–H stretching vibration, which is in accordance with earlier findings [15,39]. Therefore, the FTIR results provide further evidence that BN nanomaterials are formed. Furthermore, no other obvious absorption peaks associated with the starting materials and impurities were found. It is indicated that the as-
(Fig. 3a). Low temperature is unfavorable to the fabrication of boron nitride. Further increasing the calcination temperature to 1000 °C, as presented in Fig. 3b, the SEM image shows that BN particles are spherical in shape with average size of ~50 nm, but these BN nanospheres display poor distribution in size and dispersion. However, when the reaction temperature was elevated from 1000 to 1200 °C, the obtained BN particles were apparently aggregated (Fig. 3c). Accordingly, BN nanoparticles are not easy to synthesize at low temperature, whereas, too high temperature would cause the products to form aggregates. Furthermore, regulation of the reaction temperature is an effective way to obtain highly-dispersible boron nitride nanoparticles at RB/U=1:4. As a contrast, boron nitride nanoparticles were fabricated at RB/U=1:4 and 1100 °C without using spray drying technology. It can be seen that the resultant products are agglomerate nanosheets with a nonuniform diameter of about 20–70 nm (Fig. 3d). The results further prove that spray drying is the key for preparing highly-dispersible BN nanoparticles. The boron nitride nanoparticles prepared via spray drying method were designated as BNNP-1 notations used throughout the paper. In comparison, the as-synthesized BN nanoparticles without using this method were named BNNP-2. The morphologies of the BNNP-1 and BNNP-2 were investigated by TEM, as shown in Fig. 4. Fig. 4a shows the TEM image of BNNP-1 with high dispersivity, which is coincided with the micro appearance of SEM (Fig. 2c). Fig. 4b is a typical TEM image of the as-prepared BNNP-2 (without using spray drying method), displaying 2D nanosheets with diameter in the range of 20–70 nm. 4
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Fig. 4. TEM images of (a) BNNP-1 and (b) BNNP-2.
prepared product has high purity. The results are consistent with the XRD patterns. The chemical compositions of the BNNP-1 and BNNP-2 are recorded by the X-ray photoelectron spectroscopy (XPS) spectrum (Fig. 7). Fig. 7a shows wide-scan XPS survey spectra of the BNNP-1 and BNNP-2 prepared at RB/U=1:4 and 1100 °C. It is clearly revealed that there were no other elements except carbon, oxygen, boron and nitrogen in the as-prepared BNNP-1 and BNNP-2, which was in accordance with the FTIR spectra and XRD pattern of the sample without impurities. The existence of O and C may be caused by the exposure of the sample in the air before the examination. The B/N atomic ratio of the BNNP-1 and BNNP-2 from XPS spectra were about 1.08 and 1.12 which was consistent with those reported in the literature [40]. It is also close to the BN stoichiometry. High-resolution spectra of B 1s was displayed in Fig. 7b, the binding energies (BEs) of B 1s in the BNNPs was 189.1 eV, which is assigned to B 1s of B–N bonds. The BEs of the N 1s spectrum is located at 397.5 eV (Fig. 7c), which is attributable to N 1s of B–N bonds [41]. The survey scan XPS, B 1s and N 1s spectra revealed that the main configuration for B and N atoms are related to the B–N bonds [42]. These results imply the formation of boron nitride. Brunauer–Emmett–Teller (BET) gas sorptometry measurements were conducted to examine the specific surface area (SSA) of the BNNP-1 and BNNP-2. N2 adsorption–desorption isotherm of the BNNP-1 and BNNP-2 were displayed in Fig. 8. The isotherm plots are identified as type IV sorption profile with the hysteresis loop in the relative pressure range of 0.35–1.0, which is the characteristic of mesoporous materials [43]. The BET specific surface area of the sample is calculated from N2 isotherms at 77 K. The specific surface area of the BNNP-1 is 145.01 m2 g−1, much larger than that of BNNP-2 (71.07 m2 g−1). Moreover, the SSA of BNNP-1 was also larger than those reported by references. For example, the spherical BN nanoparticles with a diameter of 30 nm have a BET surface area of 75 m2g–1 [15]. Low oxygen-containing h-BN spheres (~30 nm in size) with a specific surface area of 57.2 m2 g−1 were fabricated by Tang and partners [24]. Singhal et al. reported a solid state thermal annealing method to fabricate BN nanomaterials with average diameter of ~30 nm and a SSA of 120.68 m2 g–1 [44]. In order to get more information about the surface of boron nitride, Barrett-JoynerHalenda (BJH) method was used to analyze experimental data. The calculated results show that the pore volume of BNNP-1 and BNNP-2 are 0.412 and 0.272 cm3g–1, respectively. Hence, it is reasonably predicted that the as-prepared BNNP-1 with high specific surface area and mesoporous nature will be used for waste-water treatment, catalyst carrier and medicine transportation in the future.
Fig. 5. XRD pattern of BNNP-1 and BNNP-2.
Fig. 6. FIRT spectrums of BNNP-1 and BNNP-2.
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Fig. 7. XPS spectra of the BNNP-1 and BNNP-2. Survey spectrum (a); high resolution spectra of B1s (b) and N1s (c).
BNNP-1 and BNNP-2. It is clearly revealed that the LP sample with BNNP-1 and BNNP-2 exhibited better tribological behaviors than separate LP. The friction coefficient was reduced with the addition percent increasing. Besides, the friction reduction and anti-wear properties of BNNP-1/LP are more excellent than those of BNNP-2/ LP at the same addition percents. Fig. 9b gives the variation of average wear scar dimeter (WSD) with the increasing amount of BN. The LP displayed the best lubricating properties at the BNNP-1 content of 1.5 wt% through comprehensive analysis and research of the friction coefficient and wear scar dimeter. The tests confirm that highlydispersed BNNP-1 has higher anti-wear properties when compared with that of BNNP-2 in the same test environments. Fig. 10 provides the morphologies of the bottom balls of LP, 1.5 wt%BNNP-1/LP and 1.5 wt% BNNP-2/LP after the friction and wear tests. It shows that the WSDs of the bottom steel balls with the additives of 1.5 wt% BNNP-1/ LP and 1.5 wt% BNNP-2/LP are smaller than those of only LP, and the addition of 1.5 wt% BNNP-2 resulted in less WSDs compared with the addition of 1.5 wt%BNNP-1. In addition, it can be found that there are many wear debris and furrows on the worn surface of running against BNNP-2 (Fig. 10b), indicating that the counterpart test steel ball has undergone severe damage. In contrast, the steel ball surface of BNNP-1 appears to be much smoother and constant (Fig. 10c). This phenomenon may be attributed to chemical stability and special structure of the BNNP-1 which could easily enter the contact area of friction pairs to prevent steel balls from wear. The results of SEM images of worn surfaces provided further evidence that the BNNP-1 had better
Fig. 8. Nitrogen adsorption-desorption isotherm of BNNP-1 and BNNP-2.
The tribological properties of hexagonal boron nitride nanoparticles as additives in liquid paraffin (LP) were investigated by MMW-1A omnipotent friction and wear testing machine. All the tests were carried out with 300 N loads, at 75 °C, a testing period of 60 min with a rotating speed of 1450 rpm and 10 mL LP. Fig. 9a shows the variation of average friction coefficient of LP containing different contents of the
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Fig. 9. Variation of the average (a) friction coefficient and (b) wear scar with increasing BN content in liquid paraffin (1450 rpm and 300 N for 60 min).
Fig. 10. Optical micrographs of typical wear scars on the top balls: (a) LP, (b) 1.5 wt% BNNP-2+LP and (c) 1.5 wt% BNNP-1+LP (1450 rpm and 300 N for 60 min).
tribological performances of the BNNP-1 can be an effective lubricant to prevent the severe damage.
tribological performances than that of the BNNP-2 for liquid paraffin. Fig. 11 presents the typical evolutions of the friction coefficient of LP, BNNP-1/LP and BNNP-2/LP as a function the rotating time. The friction coefficients of BNNP-2/LP started to creep up after 40 min. That may be because BNNP-2 could easily agglomerate in the friction process and lead to inhomogeneous lubrication. The similar result was reported by Men and coworkers [45]. Therefore, the outstanding
4. Conclusions Highly-dispersible hexagonal boron nitride nanoparticles have been successfully synthesized though spray drying and pyrolysis process by 7
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Fig. 11. Friction coefficients of LP, 1.5 wt% BNNP-2+LP and 1.5 wt% BNNP-1+LP as a function of time (1450 rpm and 300 N for 60 min).
using boric acid and urea as starting materials under N2 gas condition at 1100 °C. Spray drying technology was used to prepare homogeneous precursor. By regulating the molar ratio of boric acid to urea and pyrolysis temperature, spherical BN nanostructure powders were obtained with an average size of 20 nm, a large surface area of 145.01 m2 g–1 and a total pore volume of 0.41 cm3 g–1. Comparing with boron nitride which is synthesized without using spray drying, the spherical h-BN nanoparticles as lubrication additives in LP show excellent antifriction behavior. Our results demonstrate that highlydispersible h-BN nanoparticles can be directly used as a potential inorganic filler to improve the tribological behavior without requiring surface modification. Acknowledgements This work was supported by the Natural Science Foundation of Liaoning Province (2016010346) and Shenyang Science and Technology Plan Project (F13-289-1-00 and F14-231-1–10) for funding and supporting this work. We thank our colleagues and other students for their support and help in this work. References [1] F. Xiao, S. Naficy, G. Casillas, M.H. Khan, T. Katkus, L. Jiang, H. Liu, H. Li, Z. Huang, Edge-hydroxylated boron nitride nanosheets as an effective additive to improve the thermal response of hydrogels, Adv. Mater. 27 (2015) 7196–7203. [2] M. Liong, J. Lu, M. Kovochich, T. Xia, S.G. Ruehm, A.E. Nel, F. Tamanoi, J.I. Zink, Multifunctional inorganic nanoparticles for imaging, targeting, and drug delivery, ACS Nano 2 (2008) 889–896. [3] Z. Nie, A. Petukhova, E. Kumacheva, Properties and emerging applications of selfassembled structures made from inorganic nanoparticles, Nat. Nanotechnol. 5 (2010) 15–25. [4] B. Matović, J. Luković, M. Nikolić, B. Babić, N. Stanković, B. Jokić, B. Jelenković, Synthesis and characterization of nanocrystaline hexagonal boron nitride powders: xrd and luminescence properties, Ceram. Int. 42 (2016) 16655–16658. [5] R.T. Paine, C.K. Narula, Synthetic routes to boron nitride, Chem. Rev. 90 (1990) 73–91. [6] J. Pattanayak, T. Kar, S. Scheiner, Boron−nitrogen (BN) substitution of fullerenes: C60 to C12B24N24 CBN ball, Phys. Chem. A 106 (2002) 2970–2978. [7] C.Y. Zhi, C.C. Tang, H. Kuwahara, D. Golberg, Large-scale fabrication of boron nitride nanosheets and their utilization in polymeric composites with improved thermal and mechanical properties, Adv. Mater. 21 (2009) (2889–289). [8] X.F. Song, J.L. Hu, H.B. Zeng, Two-dimensional semiconductors: recent progress and future perspectives, J. Mater. Chem. 1 (2013) 2952–2969. [9] P. Dibandjo, F. Chassagneux, L. Bois, C. Sigala, P. Miele, Comparison between SBA15 silica and CMK-3 carbon nanocasting for mesoporous boron nitride synthesis, J. Mater. Chem. 15 (2005) 1917–1923. [10] L. Shi, Y. Gu, L. Chen, Y. Qian, Z. Yang, J. Ma, Synthesis and morphology control of nanocrystalline boron nitride, J. Solid State Chem. 177 (2004) 721–724. [11] L.H. Li, Y. Chen, G. Behan, H.Z. Zhang, M. Petravic, A.M. Glushenkov, Large-scale mechanical peeling of boron nitride nanosheets by low-energy ball milling, J.
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