Over 1.0 mm-long boron nitride nanotubes
Graphical abstract
Over 1.0 mm boron nitride nanotubes (BNNTs) were successfully synthesized by an optimized ball milling and annealing method. Such long BNNTs enable a reliable single tube configuration for electrical property characterization and consequently the resistivity of the long BNNTs is determined to be 7.1 ± 0.9 × 104 Ω cm.
Introduction
Very long carbon nanotubes (CNTs) of over 4 cm have been synthesized [1]. The long nanotubes have many new applications which are impossible for short nanotubes. For instance, long CNTs can be spun into meter-long fibers that are more than an order of magnitude stronger than any current structural materials due to the high Young’s modulus [2]. Long metallic nanotubes could be readily assembled into a microelectromechanical systems (MEMS) or nanosemiconductor devices [1]. Boron nitride nanotubes (BNNTs) have the similar nanostructure and about same high Young’s modulus as CNTs [3], but they are a wide band-gap semiconductor with the electrical behavior like insulator. So long BNNTs are probably favorable for reinforced composite materials, and might be a complementary of CNTs as an insulator for building up 3D MEMS. In general, BNNTs are more difficult to be synthesized because of the B–N binary system and the involved chemical reactions [4], [5]. In this Letter, we report the growth of over 1.0 mm-long BNNTs through an optimized ball milling and annealing process. The electrical resistivity of a single long BNNT is also determined.
Section snippets
Experimental
BNNTs were synthesized by an optimized ball milling (Pulverisette 5) and annealing process [4]. In this procedure, amorphous boron powder was first loaded into a stainless steel mill with steel balls (Φ25.4 mm AISI 420) under NH3 atmosphere of 300 kPa for ball milling treatment at rotation speed of 300 rpm for 50 h. The weight ratio of ball to boron is 50:1. Iron particles were produced and mixed with boron powder in the milling process, and the content of iron particles was about 1.5 at%. The
Results and discussion
Fig. 1a is a FESEM image of high-yield, as-synthesized BNNTs with the diameters in the range from 50 to 200 nm. Because the nanotubes are very long, they interwove together. In order to measure the length of individual nanotubes, one single long nanotube was pulled out from the nanotube layer using a sharp wood stick under a stereoscopic microscope. The nanotube was placed on the surface of a silicon substrate and the length was measured under FESEM. One of long nanotubes is shown in Fig. 1c and
Conclusions
BNNTs with the length over 1.0 mm have been successfully synthesized by an optimized ball milling and annealing method. The annealing temperature of 1100 °C is crucial for long BNNTs growth because there is a faster N dissolution rate in Fe and the B/N in Fe is close to 1, which ensures the high nitriding reaction. Benefiting from such long BNNTs, the reliable single tube configuration has been set up for measuring electrical property. The average resistivity of the long bamboo BNNTs is 7.1 ± 0.9 × 10
Acknowledgements
Authors would like to thank staffs at ANU Electron Microscopy Unit for their assistance in microscopy analysis, and Dr John Fitz Gerald in RSES ANU for TEM analysis. The Australian Research Council is acknowledged for research support in the form of Discovery Grants.
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