Boron Nitride Nanotubes: Properties, Synthesis and Applications

Richard Dolbec, Ph.D.

Director, R&D Group, Tekna Plasma Systems Inc.

Introduction

Boron nitride nanotubes [(BNNT), Product No. 802824] are close structural analogs of carbon nanotubes [(CNT), Product No. 791431], which are high aspect ratio nanotubular material, where carbon atoms are alternately substituted by nitrogen and boron atoms. Both CNTs and BNNTs are considered the strongest light-weight nanomaterials with a Young’s modulus >1 TPa. The thermal conductivity of BNNTs is also similar to the thermal conductivity of aligned bulk single wall CNTs. However, unlike CNTs, BNNTs possess a much wider band gap (~5.5 eV), exhibit higher resistance to oxidation and show greater thermal stability. These properties render BNNTs appealing for the use in mechanical reinforcement applications, transparent bulk composites, high-temperature materials such as metal matrix composites (MMC), and radiation shielding. The main properties of both BNNTs and CNTs are compared in Figure 1.

Comparison of the main properties of BNNTs and CNTs

Figure 1.Comparison of the main properties of BNNTs and CNTs.

Synthesis of Born Nitride Nanotubes

BNNTs were first synthesized in 1995 by the arc discharge method.2 Since then, other processes such as laser ablation, chemical vapor deposition, ball milling-annealing, pyrolysis, and arc-jet plasma have been explored for the synthesis of BNNTs. In recent decades, inductively coupled plasma (ICP) technology has been used for the synthesis of wide range of nanostructured materials.3-11 Among them, ICP has received considerable attention due to its ability to produce large volumes of high quality BNNTs.12-13 In the ICP technique, pure h-BN powder is vaporized in hydrogen-rich argon plasma where it decomposes into the constituent elements (B, N, and H). The presence of hydrogen in the reactant gases is crucial for achieving the catalytic effect. Its presence also eliminates the use of a metallic catalyst in the ICP process and results in very pure small-diameter BNNTs.

BNNT Characterization

BNNT materials grown by ICP are collected in bulk masses13 from which BNNT fibrils can be separated (Figure 2a) and BNNT yarns can be drawn (Figure 2b). The slight beige coloring of the material shown in Figure 2 is attributed to the presence of small amounts of amorphous boron which can be effectively removed through post-processing.

Fibril materials from which BNNT yarns (b) can be drawn

Figure 2.(a) Fibril materials from which BNNT yarns (b) can be drawn.

Figure 3 shows some representative scanning electron microscopy (SEM) images of as-produced BNNTs using ICP. It can be noticed that the BNNTs synthesized by the ICP method consists of randomly oriented nanotubes. This is attributed to the fact that BNNTs synthesized by ICP are grown without a substrate. SEM analysis of individually isolated BNNTs suggests that they are typically of few microns in length.

SEM micrographs of as-produced BNNT material

Figure 3.SEM micrographs of as-produced BNNT material.

The representative transmission electron microscopy (TEM) images of BNNTs are also shown in Figure 4. These images show that the ICP-synthesized BNNTs typically exhibit multiwall (2 to 5 walls) tubular structures and possess an average diameter of ~5 nm. Images also reveal that the BNNT walls are defect-free and the ICP synthesis indeed yields high quality BNNTs.

TEM analysis of the as-grown BNNT

Figure 4.TEM analysis of the as-grown BNNT.

The optical properties of a ~0.2 µm-thick BNNT thin film characterized in the UV-Vis range are shown in Figure 5. A good transmittance is observed over a broad wavelength range. Also a strong absorption at ~200 nm, typical of hexagonal BN (h-BN) phase, can be seen. Such transparency of BNNTs in the visible range enables their incorporation in composites such as reinforced glasses where optical properties are of utter importance.

UV-Vis transmittance of a BNNT thin film

Figure 5.UV-Vis transmittance of a BNNT thin film

The thermal and chemical stability of BNNTs are exceptional. BNNT material is known to be air-stable in temperatures up to 900 °C and can withstand extended periods of harsh oxidizing conditions. This is illustrated in Figure 6 where the behavior of a folded piece of BNNT buckypaper exposed to a natural gas flame is compared to that of regular paper and CNT buckypaper.15 While the CNT can resist a few more seconds of exposure than regular paper before burning, the BNNT material is still intact after 150 seconds of constant natural gas flame exposure, demonstrating the non-flammable nature of BNNT material.

Flame-resistance of BNNT compared to regular paper and CNT buckypaper

Figure 6.Flame-resistance of BNNT compared to regular paper and CNT buckypaper.

BNNT Applications

High stiffness and excellent chemical stability makes BNNTs ideal material for reinforcement in polymers, ceramics and metals. For instance, buckypaper-based BNNT/epoxy composites and polyurethane-modified buckypaper composites have been successfully developed.1,16 These composite materials exhibit Young’s moduli over twice the value for neat epoxy and 20 times the value for unimpregnated buckypaper. BNNTs are also one of the most promising classes of material for reinforcing aluminum-based structures.17 The low reactivity of BNNTs facilitates the integration of this material into an aluminum matrix where CNTs fail due to the reaction between the carbon and the aluminum which forms the undesired Al4C3 phase at the interface. BNNTs also exhibit much higher oxidation temperature (~950 °C) than the melting point of aluminum (660 °C), which enables the homogenous dispersion of BNNTs directly into the aluminum melt. Since BNNTs retain their mechanical properties at high temperatures while having a very low density, the development of new temperature-resistant lightweight MMC is achievable. BNNTs also exhibit good thermal conductivity. This renders them useful for applications in nanoelectronics where heat dissipation is critical. This also makes BNNTs multifunctional as it not only improves the stiffness of composites but also yields high thermal conductivity along with high transparency. The combination of high stiffness and high transparency is already exploited in the development of BNNT-reinforced glass composites.18 Other intrinsic properties of BNNTs such as good radiation shielding ability,19 high electrical resistance and excellent piezoelectric properties are likely to promote interest for integrating them in new applications.

Summary

Due to the limited availability of BNNTs, applications involving them are still being developed and to this end the synthesis of BNNTs using ICP appears promising as it allows the production of high quality material at a larger scale. The availability of high-quality BNNTs in larger quantities to the scientific community will help drive more in-depth studies of the unique properties, as well as accelerate the development of new applications.

Materials
Loading

References

1.
Jakubinek M, Martinez-Rubi Y, Ashrafi B, Guan J, Kim KS, O’Neill K, Kingston CT, Simard B. 2015. Polymer Nanocomposites Incorporating Boron Nitride Nanotubes. 13 Jun 2015; TechConnect World Innovation Conference Washington, DC: Taylor & Francis. p. 416-419.
2.
Chopra NG, Luyken RJ, Cherrey K, Crespi VH, Cohen ML, Louie SG, Zettl A. 1995. Boron Nitride Nanotubes. Science. 269(5226):966-967. http://dx.doi.org/10.1126/science.269.5226.966
3.
Tekna Plasma Systems Inc. [Internet].[cited 25 Feb 2016]. Available from: http://www.tekna.com/
4.
Mamak M, Choi SY, Stadler U, Dolbec R, Boulos M, Petrov S. 2010. Thermal plasma synthesis of tungsten bronze nanoparticles for near infra-red absorption applications. J. Mater. Chem.. 20(44):9855. http://dx.doi.org/10.1039/c0jm02169e
5.
Jiayin G, Xiaobao F, Dolbec R, Siwen X, Jurewicz J, Boulos M. 2010. Development of Nanopowder Synthesis Using Induction Plasma. Plasma Sci. Technol.. 12(2):188-199. http://dx.doi.org/10.1088/1009-0630/12/2/12
6.
Kim KS, Kingston CT, Ruth D, Barnes M, Simard B. 2014. Synthesis of high quality single-walled carbon nanotubes with purity enhancement and diameter control by liquid precursor Ar?H2 plasma spraying. Chemical Engineering Journal. 250331-341. http://dx.doi.org/10.1016/j.cej.2014.03.117
7.
Meunier J, Mendoza-Gonzalez N, Pristavita R, Binny D, Berk D. 2014. Two-Dimensional Geometry Control of Graphene Nanoflakes Produced by Thermal Plasma for Catalyst Applications. Plasma Chem Plasma Process. 34(3):505-521. http://dx.doi.org/10.1007/s11090-014-9524-6
8.
Thompson D, Leparoux M, Jaeggi C, Buha J, Pui DYH, Wang J. 2013. Aerosol emission monitoring in the production of silicon carbide nanoparticles by induction plasma synthesis. J Nanopart Res. 15(12): http://dx.doi.org/10.1007/s11051-013-2103-6
9.
Aktekin B, Çakmak G, Öztürk T. 2014. Induction thermal plasma synthesis of Mg2Ni nanoparticles. International Journal of Hydrogen Energy. 39(18):9859-9864. http://dx.doi.org/10.1016/j.ijhydene.2014.02.066
10.
So K, Lee H, Kim T, Choi S, Park D. 2014. Synthesis of silicon nanopowder from silane gas by RF thermal plasma. Phys. Status Solidi A. 211(2):310-315. http://dx.doi.org/10.1002/pssa.201330059
11.
Son S, Taheri M, Carpenter E, Harris VG, McHenry ME. 2002. Synthesis of ferrite and nickel ferrite nanoparticles using radio-frequency thermal plasma torch. J. Appl. Phys.. 91(10):7589. http://dx.doi.org/10.1063/1.1452705
12.
Kim KS, Kingsto CT, Simard B. 2014. Boron nitride nanotubes and process for production thereof WO2014169382A1. World Intellectual Property Organization.
13.
Kim KS, Kingston CT, Hrdina A, Jakubinek MB, Guan J, Plunkett M, Simard B. 2014. Hydrogen-Catalyzed, Pilot-Scale Production of Small-Diameter Boron Nitride Nanotubes and Their Macroscopic Assemblies. ACS Nano. 8(6):6211-6220. http://dx.doi.org/10.1021/nn501661p
14.
Kingston C. 2013. Large-scale synthesis of few-walled small diameter boron nitride nanotubes (sub-10 nm) by an induction thermal plasma. 14th International Conference on the Science and Applications of Nanotubes; 23 Jan 2013; Finland: Espoo. p. 223.
15.
Simard B. 2014. Industrialization of boron nitride nanotubes: From synthesis to applications. TechConnect World Innovation; Washington, DC:
16.
Kim KS, Jakubinek MB, Martinez-Rubi Y, Ashrafi B, Guan J, O'Neill K, Plunkett M, Hrdina A, Lin S, Dénommée S, et al. Polymer nanocomposites from free-standing, macroscopic boron nitride nanotube assemblies. RSC Adv.. 5(51):41186-41192. http://dx.doi.org/10.1039/c5ra02988k
17.
Lahiri D, Singh V, Li LH, Xing T, Seal S, Chen Y, Agarwal A. 2012. Insight into reactions and interface between boron nitride nanotube and aluminum. J. Mater. Res.. 27(21):2760-2770. http://dx.doi.org/10.1557/jmr.2012.294
18.
Bansal NP, Hurst JB, Choi SR. 2006. Boron nitride nanotubes-reinforced glass composites. 30th International Conference and Exposition on Advanced Ceramics and Composites; January 22–27 Cocoa Beach, Florida:
19.
Thibeault SA, Fay CC, Earle KD, Lowther SE, Sauti G, Kang JH, Park C, McMullen AM. 2014. Radiation shielding materials containing hydrogen, boron, and nitrogen. 31 Jan 2014; NASA–TM