Highly Efficient Growth of Boron Nitride Nanotubes and the Thermal

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Highly Efficient Growth of Boron Nitride Nanotubes and The Thermal Conductivity of Their Polymer Composites Liangjie Wang, Dongbo Han, Jie Luo, Taotao Li, Ziyin Lin, and Yagang Yao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10761 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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The Journal of Physical Chemistry

Highly Efficient Growth of Boron Nitride Nanotubes and The Thermal Conductivity of Their Polymer Composites Liangjie Wang,a,b Dongbo Han,a Jie Luo,a Taotao Li,a Ziyin Lin,a and Yagang Yao* a

a

Division of Advanced Nanomaterials, Key Laboratory of Nanodevices and Applications, CAS

Center for Excellence in Nanoscience, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215123, China b

School of Materials Science and Engineering, Shanghai University, Shanghai, 200444, China

ABSTRACT: We developed a novel strategy for the gram-scale fabrication of boron nitride nanotubes (BNNTs). Li3N was used as the promoter, it not only catalyzes the BNNTs growth, but also serves as the nitrogen source. BNNT/thermoplastic polyurethane (TPU) are flexible, transparent, and thermally conductive composite films that were also fabricated with an in-plane thermal conductivity of 14.5 W m-1 K-1 at 1.0 wt.% BNNTs. This is an improvement of more than 400% over neat TPU. This study will enable BNNTs to be used in heat dissipation materials, high temperature components, and thermal protection systems.

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INTRODUCTION Boron nitride nanotubes (BNNTs) have gained interest due to their excellent physical and chemical properties. Their structure is analogous to carbon nanotubes, which can be seen as alternating boron (B) and nitrogen (N) atoms entirely replace carbon (C) atoms in a hexagonal lattice structure. BNNTs are electrical insulators that have a band gap of 5.5 eV to 6 eV regardless of the chirality, the number of walls, or the diameter.1-3 BNNTs also possess high chemical inertness, excellent mechanical, optical, and magnetic properties, as well as superb thermal conductivity.4-6 These properties make BNNTs promising in a variety of fields, such as insulating fillers for thermally conductive polymeric composites, ultraviolet-light emitters, high temperature components, thermal protection systems, and light weight wire insulation.4-8 There are various methods used to synthesize BNNTs, including arc-discharge,9 laser ablation,10 ball-milling,11 and chemical vapor deposition (CVD).12 CVD is a low-cost and easily controllable technique for the mass production of BNNTs within commercial applications. The catalytic efficiency is essential within large-scale preparation of BNNTs via CVD growth. So high catalytic efficiency through the design of the catalyst has attracted great attention. Various catalysts and boron precursors have been researched for synthesizing BNNTs.13-19 The molecular precursor borazine (B3N3H6) in conjunction with a floating nickelocene ((C5H5)2Ni) catalyst produces double-walled BNNT structures. It has potential for continuous large-scale production of boron nitride (BN) materials.13 B/MgO, B/Fe2O3, B/V2O5, and B2O3/MoO3 are also used as precursors.14-17 The volatile boron oxide vapors generated in these reactions are caused by the Ar gas to reacting with NH3 gas to form BN materials. Li2O is another effective catalyst that has been used to synthesize BNNTs and BN nanosheets (BNNSs).18 This method uses B that is oxidized by Li2O in a high temperature zone to produce boron oxide vapor and Li. In the low-


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temperature zone, the boron oxide then reduces to B via Li. Li2O is used as a standard catalyst. The active B can react with NH3, and thus BNNTs are prepared. The above reports show significant progress for catalyst design within the growth of BNNTs. However, the synthesis of BNNTs has low yield, due to limited supply of efficient catalysts, low repeatability, and strict equipment requirements. The large-scale growth of BNNTs with a high efficiency has yet to be accomplished. We developed a novel strategy for the fabrication of gram-level BNNTs powder with high purity and crystallinity by using Li3N as a promoter. The mass production and long morphology of BNNTs lead the way for major benefits in the construction of a thermal conductivity network in an insulating polymeric composite. EXPERIMENTAL SECTION BNNTs Growth A mixture of B2O3 and Li3N was used as the precursor materials, placed in an alumina boat and located at a horizontal alumina tube (total length, 120 cm; diameter, 6 cm) at 1300 oC for 2 h in a conventional furnace under a NH3 flow of 200 standard cubic centimeter per minute (sccm). The NH3 gas flow was then stopped to terminate the reaction, and the system was cooled to room temperature under Ar flow. The experiment using a mixture of B2O3 and Li2O as precursor was used as the comparison test to grow BN nanostructures at the same condition. Fabrication of BNNT/Thermoplastic Polyurethane (TPU) Composites BNNT/TPU composites were made by directly mixing TPU (Suzhou Lan Huo Yan Plastics Co., Ltd) and the as-grown BNNTs. The commercial BNNTs were from Beijing DK nano S&T ltd. We used N, N-dimethylformamide (DMF) as a nonreactive diluents to help disperse BNNTs


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and reduce the viscosity of TPU. The mixture of TPU with DMF and BNNTs nanostructures (0 wt.%, 0.1 wt.%, 0.3 wt.%, 0.5 wt.%, 0.7 wt.%, and 1.0 wt.%) were stirred for 3 h. The resultant mixture was poured into a Teflon dish and maintained at 70 oC for 7 h to remove DMF and then BNNTs based composites were obtained. Characterization The products were characterized by scanning electron microscopy (SEM, Hitachi S-4800), atomic force microscopy (AFM, Dimension 3100), and high-resolution transmission electron microscopy (TEM, Tecnai G2 F20 S-Twin) with X-ray energy dispersive spectrometry (EDS, Apollo 40SDD). Raman spectroscopy was collected over the spectral range of 1000 to 2000 cm-1 using a LabRAM ARAMIS Raman confocal microscope (HORIBA Jobin Yvon) equipped with a wavelength of 532 nm. X-ray photoelectron spectroscopy was recorded on an ESCALab MKII X-ray photoelectron spectrometer with non-monochromatized Mg Kα X-rays as the excitation source. The crystal structure was characterized by X-ray diffraction (XRD, D8 Advance, Bruker AXS). The ultraviolet-visible absorption spectra of the products were characterized by ultraviolet-visible absorption spectroscopy (JascoV-466). The vertical and in-plane thermal diffusivities of BNNT/TPU films were tested with LFA 447 NanoFlash (Netzsch Instruments). RESULTS AND DISCUSSION In the following study Li3N catalyzed the growth of BNNTs and served as the nitrogen source. This improved the conversion rate of BN. To investigate the reaction mechanism, the products generated from various temperatures were measured by XRD and EDS as shown in Figure 1. The mixture of Li3N and B2O3 was used as the precursor. It initially reacted to produce powdered Li3BO3 and Li2O at 700 oC in a Ar gas environment (Figure 1a, black line). At this


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temperature, the the morphology of intermediate product was characterized by SEM, as shown in Figure 1b. XRD peaks of BN appeared in the product at this temperature,19 confirming the formation of BN. This was further confirmed by the mapping measurement. As illustrated in Figure 1c-e, elements B and N were uniformly distributed in the intermediate product. As the temperature increased to 1000 oC in Ar, the intensity of BN in the XRD pattern increased. And the Li2O product reacted with the surface of alumina boat to produce LixAlyOz. The nitrogen source provided by Li3N was not sufficient enough to improve conversion efficiency. NH3 gas was needed to supply additional nitrogen. The precursor was almost completely converted to BN product at 1300 oC under NH3 (Figure 1a, blue line). Pure BN product was obtained after pickling (Figure 1a, green line). The yield reached about 96% . This was calculated by the B conversion ratio from B2O3 into BN. By using Li3N as a promoter, BNNTs powder was made. And this product could be easily removed from equipment, would improve the use of the BNNTs product. The catalytic performance of Li2O for the growth of BNNTs has been previously verified.18 As a comparison, Li2O and B2O3 were also mixed directly and used to produce BN nanostructures. In previous studies, borates (Na2O·nB2O3, MgO·nB2O3, and CaO·nB2O3) are formed by reactions of metal oxides with boron oxide. Then borates are used as a precursor to synthesize BNNTs.20 Li2O exhibits relatively low conversion efficiency when compared to Li3N. These results demonstrate that the yield of BN nanostructure could only reach around 43% when the B conversion ratio from B2O3 into BN was calculated. This low yield could result from two aspects. At temperatures below 1000 oC under Ar, there is no BN generated. This is because there is no nitrogen source, as confirmed by the XRD results (Figure 1f, black and red line). When temperatures reach 1300 oC under NH3, the borates ( LiBO2 and Li4B2O5) are unable to react completely under the same conditions (Figure 1f, blue line). It is difficult to obtain the large


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BN materials from the furnace-boat. BNNTs are difficult to locate in the SEM images of the product, as shown in Figure S1a and b. Another reason is the reaction product has been sintered into a lumpy and hard piece (Figure S2a), rather than a loose pile of BNNTs (which is the case when Li3N is used as a promoter) (Figure S2b).

Figure 1. XRD and EDS characterizations of the products obtained at various temperatures. (a) XRD patterns of the product when Li3N and B2O3 are used as the precursors at 600 oC , 700 oC, and 1000 oC for 2 h under Ar gas, as well as 1300 oC under NH3 gas and 1300 oC (after pickling). (b-e) SEM and EDS mapping images demonstrated the generation of BN at 700 oC under Ar gas with Li3N as a promoter. Lithium is a light element and does not appear. (f) XRD patterns of the product with Li2O and B2O3 as the precursor reacted at 700 oC (Ar gas), 1000 oC


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(Ar gas), and 1300 oC under NH3 gas. (1-Li2O; 2-Li3BO3; 3-BN; 4-LixAlyOz; 5-LiBO3; 6Li4B2O5). A high volume of BNNTs powder was obtained when Li3N was used as a promoter after pickling, as shown in Figure S2a and S3. This demonstrated the high catalytic activity of Li3N. SEM images, XRD spectroscopy, and Raman were employed to characterize the morphology and structure of BNNTs as shown in Figures 2. Figures 2a and b show SEM images of BNNTs, where large quantities of nanotubes were produced. The crystal structure of BNNTs was characterized by XRD (Figure 2c). The characteristic peaks were presented in XRD patterns. The (002), (100), (101), (102), and (004) planes were consistent with that listed in the JCPDS Card (No.34-0421). The absence of impurity phases indicated the high purity and crystallinity of BNNTs.21 A typical Raman spectrum was taken from BNNTs powder on a SiO2/Si substrate, as shown in Figure 2d. The Raman spectrum showed a single sharp characteristic peak at 1366 cm−1. This was credited to the E2g phonon mode. The full width at half maximum was 9.5 cm−1. This suggested that the quality of BNNTs from the Li3N system was comparable to those of the h-BN crystal produced by high-pressure and high-temperature synthesis.22 The ultraviolet/visible light/near-IR absorption spectra of BNNTs in ethanol were obtained to determine the band gap of BNNTs. As shown in Figure S4a, the absorption spectrum displayed a strong absorption peak at approximately 210 nm. The relationship was used for direct band gap semiconductors—a=C(EEg)1/2/E. The direct band gap of BNNTs could be extracted, as shown in Figure S4b.23 The calculated direct band gap was about 5.93 eV. This was similar that of single h-BN crystals, as previously reported.22 These results demonstrated that the as-grown BNNTs were highly crystalline.


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Figure 2. Representative SEM images images of BNNTs (a) and (b), XRD spectrum images (c), and Raman spectrum images (d) of the as-grown BNNTs. The XPS spectra of the as-grown BNNTs that were synthesized at 1300 oC are shown in Figure 3 and S5 (red line). The photoelectron peaks from B1s, N1s, C1s, and O1s were recognizable in the spectrum (Figure S5). It was established that the binding energy of B 1s-core level was at 190.50 eV (Figure 3a ), and N 1s peak at 398.06 eV (Figure 3b). These were consistent with the reported values for BN.24 Both the B 1s and the N 1s spectra denoted that the configuration for B and N atoms was the B-N bond. This implied that the hexagonal phase existed in our BN powder. XPS measurements were used to demonstrate the quality of the asgrown BNNTs as compared to commercial BNNTs as shown in Figure S5. The oxygen content


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was 2.36% in the XPS spectra of as-grown BNNTs. This was lower than commercial BNNTs (8.85%). This demonstrated that high-quality BNNTs were obtained during this experiment.24

Figure 3. XPS spectra of BNNTs. B 1s (a) and N 1s (b) core level. TEM images were collected to clarify the morphology and microstructures of the as-grown BNNTs as shown in Figure 4. And a few BN nanosheets was also found in TEM images. It is inevitable that disorder nanosheets were synthesized in the chemical vapor deposition system. As shown in Figure 4a and b, low-magnification TEM images consisted of numerous nanotubes. The electron diffraction pattern shown in Figure 4c were indicated as the typical {100}, {110}, {102}, and {004} rings of polycrystalline h-BN. This agreed well with XRD peaks and confirmed the high crystal quality of the BNNTs. Figure 4d-f depicts the representative highresolution TEM images of the individual nanotubes with well-ordered multi-walls. These had an interlayer distance of approximately 0.34 nm (Figure S6). These were characteristic of a d0002 spacing in a hexagonal BN.


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Figure 4. TEM characterizations. (a) and (b) TEM images of the as-grown BNNTs. (c) Selected area diffraction pattern of the BNNTs. (d), (e), and (f) represented high-resolution TEM images of individual BNNTs with eight, nine, and twelve walls. BNNTs exhibit excellent thermal conductivity and superior mechanical robustness, making them an attractive candidate to improve polymer properties. TPU exhibits excellent transparency, high tensile strength, and large elongation.25 TPU is considered an excellent polymer matrix for flexible, transparent, and thermally conductive composite films. Unfortunately, a major drawback of TPU is their poor thermal properties, which hampers their real world application.26 The as-grown BNNTs fillers are mixed with TPU resin in various loadings. And DMF is a strong polar solvent that has been used to disperse BNNTs.21 BNNT/DMF dispersion remains stable and homogeneous for up to 2 days (Figure S7). In our experiments, the BNNT/TPU mixture was prepared by mixing BNNTs and TPU with an electric mixer to form a uniform solution. The BNNT/TPU mixture with a designated ratio (0.0 wt.%, 0.1 wt.%, 0.3 wt.%, 0.5 wt.%, 0.7 wt.%


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and 1.0 wt.% BNNT/TPU) were then poured into Teflon molding. The as-prepared films were obtained after the specimens were dried in an oven at 70 oC for 7 h to remove DMF, as shown in Figure 5a. To confirm the presence of BNNTs in the composites, an XRD was used to characterize the different loadings of 0.1 wt.%, 0.3 wt.%, and 1.0 wt.%. The spectra confirmed that composites contained BNNTs (Figure 5b). We investigated the transmittance of BNNT/TPU specimens with a thickness of approximately 20 µm and various weight fraction ratios. The samples were adhered to a thin glass for transmittance measurements (Figure 5c). The films were transparent, even when they had a high loading of 1.0 wt.%. The transmittance was as good as the pure TPU film. The transmittance of light at wavelengths longer than 550 nm after embedding 1.0 wt.% BNNTs was slight less than TPU (87.2%). The transmission of the composite remained at 78.36% at around entire wavelength . The slight decline in the composite transmission could be caused by the wide band gap of BNNTs. This width did not allow light to be absorbed in the visible region. Some light scattering did occur and could be used as transparent substrate as replacement plastic for flexible electronics applications.27 This could be an excellent advantage within the plastic packaging industry where transparency is needed. The obtained BNNT/TPU nanocomposites exhibited excellent flexibility, as shown in Figure S8a. The TPU had excellent tensile strength and long elongation at the break.25 Similar to the carbon nanotube in our previous study, the tensile strength is associated with the interface effect between the fillers and TPU.28 Note, the one dimensional (1D) BNNT had a strong interface function in the TPU molecule chains-BNNTs interface (Figure S8b). As a result, the strength and elastic modulus of the fabricated specimen with 1.0 wt.% as-grown BNNTs reached 56.15 MPa and 48.47 MPa. This was a 421% and 260% improvement from the neat TPU, shown in Figure


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5d. These findings clearly demonstrate the improvements in the mechanical properties of these TPU composites. The thermal degradation behavior of the composites was also improved by BNNTs compared to the neat TPU film (Figure S9). This could be due to the superb thermal stability of BNNTs. The BNNT/TPU thin films had good thermal transport properties. The BNNT/TPU specimens were evaluated by measurements of in-plane and out-of-plane thermal diffusivity (TD). As shown in Figure 5e and f, the TD was improved in both directions monotonically as the BNNTs load increased. TD values were nearly 1.27 mm2/s (out-of-plane) and 7.31 mm2/s (inplane) for composite with loads of 1.0 wt.% BNNTs, which was an improvement of about fourteen and four times than the neat TPU film. The corresponding in-plane and out-of-plane thermal conductivity (TC) in the films with 1.0 wt.% BNNT was calculated from the equation TC = aTD (where a = density × specific heat) was 14.5 W m-1 K-1 and 2.5 W m-1 K-1 (Figure 5e and f). As we all know, the mixture of nanotubes and nanosheets as fillers will enhance the thermal and mechanical properties of their polymer composites. Therefore, the thermal conductivity of BN based composites will be enhanced after mixing with additional BN nanosheets. The thermal conductivity properties of BNNTs have been evaluated along with hightemperature polymer films including polydimethylsiloxane (PDMS) as shown in Figure S10. The commercial BNNTs fillers were also mixed with TPU resin in 1.0 wt%. The in-plane and out-ofplane thermal diffusivity was measured, as seen in Figure S11. The values were (0.285 mm2/s (out-of-plane) and 3.612 mm2/s (in-plane). This demonstrates that the thermal conduction improvement was far less than the addition of the as-grown BNNTs. These results demonstrate that BNNTs in the polymer helped to improve its heat diffusion ability and could be used in light-emitting-diode (LED) chips or other electronic devices.


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Figure 5. Optical, mechanical, and thermal properties of BNNT/TPU composites. (a) Photograph of composite films with 1.0 wt.% BNNT loading. (b) XRD spectra of BNNT/TPU composite at various BNNT loadings. (c) Transmission spectra for BNNT/TPU composites over a range of weight fractions. (d) Summaries of measured elastic modulus and the strength of neat TPU and its BNNT composites. (e) and (f) TD and TC values of BNNT/TPU composite films at various BNNT loadings (out-of-plane and in-plane). The neat TPU film and the 1.0 wt.% BNNT/TPU composite were spread out and fixed on a light-emitting-diode (LED) chip (Figure 6). The heat flow from the LED chip was concentrated to generate hot spots on the black TPU film, where it reached 97.5 °C, see Figure 6a. This was a relatively high temperature gradient from the center to the edges of the samples. In contrast, the BNNT/TPU composite possessed a distinct advantage. Its temperature distribution was uniform, with a lower center spot temperature (81.7 °C, Figure 6b). This would markedly improve the reliability and lifetime of electronics. These images also demonstrated that BNNTs fillers could effectively improve polymer thermal properties.


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Figure 6. Application in flexible substrates. Thermal images of the neat TPU composite (a) and the BNNT/TPU composite (b). CONCLUSION This study reported a direct and efficient method of fabricating gram-scale and crystalline BNNTs using Li3N as the promoter via chemical vapor deposition. The yield of the as-grown product could exceed 96%. This enables its large-scale application. The flexible, transparent, and thermally conductive BNNT/TPU composites were fabricated from as-prepared BNNTs. The films were transparent by transmittance measurements, even at the highest embedment of 1.0 wt.%. The TC gradually increased as the BN was loaded. When the 1.0 wt.% BN was loaded, the thin film showed a TC of almost 14.5 W m-1 K-1 in-plane. This is a 400% improvement over the neat TPU. The BN filling demonstrated remarkable tensile strength and elastic modulus of polymers, witch increased 421% and 260% when compared with the neat TPU. This study enables further BN applications in heat-dissipation materials, high temperature components, and thermal protection systems.


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ASSOCIATED CONTENT Supporting Information. Additional information including the mixture of Li2O and B2O3 as a precursor and their SEM images ( Figure S1), the photograph of product with the mixture of Li3N and B2O3 as a precursor (Figure S2), and the Photograph of the resultant BNNTs ( Figure S3), Optical absorption spectrum of h-BN thin film (Figure S4), XPS spectra of the as-grown BNNTs and a commercial BN (Figure S5). The TEM image of walls of the as-grown BNNT (Figure S6). The photography of dispersed uniform and stable solution (Figure S7), the cross section SEM images of a fabricated BNNT/TPU composites (Figure S8), and their thermal stability characterization (Figure S9), Thermal diffusivity characterization of pure PDMS and BNNT/PDMS composite film at 1.0 wt% BNNT loadings (Figure S10), and thermal diffusivity characterization of pure TPU and BNNT/TPU composite film at 1.0 wt.% commercial BNNT loadings (Figure S11). AUTHOR INFORMATION Corresponding Author *E-mail:[email protected].. ACKNOWLEDGMENT This work was supported by the National Key R&D Program of China (No. 2017YFB0406000), the Key Research Program of Frontier Science of Chinese Academy of Sciences (No. QYZDBSSW-SLH031), the Natural Science Foundation of Jiangsu Province, China (Nos. BK20160399 and BK20140392), the Transformation of Scientific and Technological Achievements in Jiangsu


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Province (No. BA2016026), the Postdoctoral Foundation of Jiangsu Province (No. 1601065B), and the Science and Technology Project of Suzhou, China (Nos. SZS201508). REFERENCES 1. Rubio, A.; Corkill, J. L.; Cohen, M. L. Theory of Graphitic Boron Nitride Nanotubes. Phys. Rev. B. 1994, 49, 5081-5084. 2. Liu, Z.; Song, L.; Zhao, S.; Huang, J.; Ma, L.; Zhang, J.; Lou, J.; Ajayan, P. M. Direct Growth of Graphene/Hexagonal Boron Nitride Stacked Layers. Nano Lett. 2011, 11, 2032-2037. 3. Watanabe, K.; Taniguchi, T.; Kanda, H. Direct-bandgap Properties and Evidence for Ultraviolet Lasing of Hexagonal Boron Nitride Single Crystal. Nat Mater. 2004, 3, 404-409. 4. Zhi, C. Y.; Bando, Y.; Tang, C. C.; Huang, Q.; Golberg, D. Boron Nitride Nanotubes: Functionalization and Composites. J. Mater. Chem. 2008, 18, 3900-3908. 5. Zhi, C.; Bando, Y.; Tang, C.; Kuwahara, H.; Golberg, D., Grafting Boron Nitride Nanotubes: From Polymers to Amorphous and Graphitic Carbon. J. Phys. Chem. C 2007, 111, 1230-1233. 6. Zhi, C.; Zhang, L.; Bando, Y.; Terao, T.; Tang, C.; Kuwahara, H.; Golberg, D. New Crystalline Phase Induced by Boron Nitride Nanotubes in Polyaniline. J. Phys. Chem. C 2008, 112, 17592-17595. 7. Chen, Z.-G.; Zou, J.; Liu, Q.; Sun, C.; Liu, G.; Yao, X.; Li, F.; Wu, B.; Yuan, X.-L.; Sekiguchi, T.; Cheng, H.-M.; Lu, G. Q. Self-Assembly and Cathodoluminescence of Microbelts from Cu-Doped Boron Nitride Nanotubes. ACS Nano. 2008, 2, 1523-1532. 8. Liu, C.; Fan, Y. Y.; Liu, M.; Cong, H. T.; Cheng, H. M.; Dresselhaus, M. S. Hydrogen Storage in Single-Walled Carbon Nanotubes at Room Temperature. Science. 1999, 286, 1127. 9. Chopra, N. G.; Luyken, R.; Cherrey, K.; Crespi, V. H. Boron Nitride Nanotubes. Science 1995, 269, 966. 10. Laude, T.; Matsui, Y.; Marraud, A.; Jouffrey, B. Long Ropes of Boron Nitride Nanotubes Grown by A Continuous Laser Heating. Appl. Phys. Lett. 2000, 76, 3239-3241. 11. Li, L. H.; Chen, Y.; Glushenkov, A. M. Boron Nitride Nanotube Films Grown from Boron Ink Painting. J. Mater. Chem. 2010, 20, 9679-9683. 12. Lourie, O. R.; Jones, C. R.; Bartlett, B. M.; Gibbons, P. C.; Ruoff, R. S.; Buhro, W. E. CVD Growth of Boron Nitride Nanotubes. Chem. Mater. 2000, 12, 1808-1810. 13. Kim, M. J.; Chatterjee, S.; Kim, S. M.; Stach, E. A.; Bradley, M. G.; Pender, M. J.; Sneddon, L. G.; Maruyama, B. Double-Walled Boron Nitride Nanotubes Grown by Floating Catalyst Chemical Vapor Deposition. Nano Lett. 2008, 8, 3298-3302. 14. Lee, C. H.; Xie, M.; Kayastha, V.; Wang, J.; Yap, Y. K. Patterned Growth of Boron Nitride Nanotubes by Catalytic Chemical Vapor Deposition. Chem. Mater. 2010, 22, 1782-1787. 15. Tang, C.; Bando, Y.; Sato, T.; Kurashima, K. A Novel Precursor for Synthesis of Pure Boron Nitride Nanotubes. Chem. Commun. 2002, 12, 1290-1291. 16. Tang, C.; Chapelle, M. L.; Li, P.; Liu, Y.; Dang, H.; Fan, S. Catalytic Growth of Nanotube and Nanobamboo Structures of Boron Nitride. Chem. phys. lett. 2001, 342, 492-496. 17. Lee, C. H.; Wang, J.; Kayatsha, V. K.; Huang, J. Y.; Yap, Y. K. Effective Growth of Boron Nitride Nanotubes by Thermal Chemical Vapor Deposition. Nanotechnology 2008, 19, 455605. 18. Huang, Y.; Lin, J.; Tang, C.; Bando, Y.; Zhi, C.; Zhai, T.; Dierre, B.; Sekiguchi, T.; Golberg, D. Bulk Synthesis, Growth Mechanism and Properties of Highly Pure Ultrafine Boron Nitride Nanotubes with Diameters of Sub-10 nm. Nanotechnology 2011, 22, 145602. 19. Shelimov, K. B.; Moskovits, M. Composite Nanostructures Based on Template-Grown Boron Nitride Nanotubules. Chem. Mater. 2000, 12, 250-254. 20. Matveev, A. T.; Firestein, K. L.; Steinman, A. E.; Kovalskii, A. M.; Sukhorukova, I. V.; Lebedev, O. I.; Shtansky, D. V.; Golberg, D. Synthesis of Boron Nitride Nanostructures from Borates of Alkali and Alkaline Earth Metals. J. Mater. Chem. A. 2015, 3, 20749-20757. 21. Liu, F.; Mo, X.; Gan, H.; Guo, T.; Wang, X.; Chen, B.; Chen, J.; Deng, S.; Xu, N.; Sekiguchi, T. Cheap, GramScale Fabrication of BN Nanosheets via Substitution Reaction of Graphite Powders and Their Use for Mechanical Reinforcement of Polymers. Sci rep-uk. 2014, 4, 4211.


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Highly Efficient Growth of Boron Nitride Nanotubes and the Thermal

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