Large-quantity production of high-yield boron nitride nanotubes
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J. Zou Electron Microscopy Unit, College of Sciences and Technology, The University of Sydney, NSW 2006, Australia (Received 13 March 2002; accepted 4 June 2002)
A high quantity and yield (up to 85%) of boron nitride (BN) nanotubes have been produced using a mechanothermal method. Elemental boron powders were first mechanically milled at room temperature in NH3 atmosphere and subsequently heated in N2 gas at 1200 °C for up to 16 h. The BN nanotubes obtained have either multiwalled cylindrical or bamboolike structures, suggesting different growth processes. The high formation yield of BN nanotubes is due to a high density of nanostructured nuclei created by an extensive milling treatment.
Boron nitride (BN) nanotubes have been synthesized by different methods, including arc-discharge,1–3 laser heating/ablation,4,5 chemical reactions,6,7 and chemical vapor deposition.8,9 However, both the formation yield and the quantity of BN nanotubes are very low using such methods compared with the case for carbon nanotubes. As a result of the low BN nanotube sample yield, research into the properties and applications has been minimal. However, some progress in improving the formation yield and quantity of BN nanotubes has been made during the past several years. For example, a considerably higher formation yield has been obtained by using carbon nanotubes as templates which were substituted by boron and nitrogen atoms through a chemical reaction between boron oxide vapor and nitrogen gas at elevated temperatures.10 In addition, a macroscopic quantity (on the order of milligrams) of multiwalled BN nanotubes has been produced using a modified arcdischarge method11 and laser heating.5 Recently, singlewalled BN nanotubes in gram quantities have been successfully produced.12 However, all of the above deposition methods present difficulties in scaling up to an industrial process. Alternatively, it was first demonstrated in 1999 that a mechanothermal process, in which boron powder was first milled in ammonia gas and followed by annealing in nitrogen atmosphere, had the potential to produce large-quantity materials containing BN nanotubes.13,14 However, the low formation yield of that stage needed to be improved and the formation mechanism further clarified. In this paper, we report that a high yield of BN nanotubes has been achieved by the mechanothermal method. Up to 1-kg amounts of BN nanotube material are now possible using a laboratory-scaled 1896
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J. Mater. Res., Vol. 17, No. 8, Aug 2002
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milling device, thus opening up studies of new properties and applications of BN nanotubes. We also report on special formation mechanisms involved with the highyield BN nanotube formation. High-energy ball-milling experiments were carried out using a laboratory-scale rotating milling instrument,15 which has four stainless steel milling vessels of two different sizes (150 or 200 mm in diameter and 60 mm in thickness). Each small vessel was loaded with several grams of boron powder, togethe
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