Synthesis and morphology control of nanocrystalline boron nitride
Introduction
Boron nitride, BN, is well known for its unique physical and chemical properties, such as supreme hardness (for cubic BN), low density (for hexagonal BN), high temperature stability, high thermal conductivity, high melting point, low dielectric constant and chemical inertness. Due to these properties, boron nitride has been used widely in various areas. For example, boron nitride can be applied in cutting tools, grinding, refractory, lubricants, electrical insulators and so forth [1], [2], [3].
Various synthesis methods have been reported for preparation of boron nitride, such as the carbothermic reduction of boric oxide [4], the direct reaction of boron and nitrogen [5], pyrolysis of (HBNH)3 [6], direct nitriding of boric acid with ammonia gas [7]. Recently, Hu et al. [8] reported that nanocrystalline BN with a whisker–like morphology could be synthesized by the reaction of KBH4 and NH4Cl at 650°C, Hao et al. [9] synthesized nanocrystalline cubic BN particles by reacting Li3N and BBr3 at low-temperature benzene thermal conditions, Yang et al. [10], [11] synthesized nanocrystalline cubic BN by pulsed laser-induced liquid–solid interfacial reaction. In addition, other methods have been developed to prepare BN nanotubes, nanowires and nanocapsules [12], [13], [14], [15].
Here we reported a chemical route to synthesize nanocrystalline hexagonal boron nitride (hBN) with needle-like or hollow spherical morphology. The reaction was carried out in an autoclave and can be described as follows:
Section snippets
Experimental
In a typical procedure, an appropriate amount of NH4Cl (0.05 mol) and MgB2 (0.025 mol) were put into a stainless autoclave of 50 ml capacity (route 1); in another procedure, appropriate excess NaN3 (molar ratio of NaN3 to NH4Cl to be 0.5:1 and 1:1, respectively) was also added into the autoclave (route 2). The autoclave was sealed and maintained at 550°C for 8 h in route 1 or maintained at 500°C for 8 h in route 2, then cooled to room temperature. The product was washed with absolute ethanol and
Results and discussion
Fig. 1 shows the XRD patterns of the as-prepared BN samples from route 1 and route 2, all the peaks can be indexed as hexagonal BN. After refinement, the lattice constants are obtained. For BN sample from route 1, a=2.500 Å, c=6.681 Å; for BN sample from route 2, a=2.510 Å, c=6.685 Å, which is very close to the reported value for hBN (a=2.510 Å, c=6.690 Å, JCPDS card, No. 851068). The broadening of the XRD peaks may originate from the small grain sizes of the BN samples, which is confirmed by the TEM
Conclusions
In summary, nanocrystalline hBN powders of the needle-like shape and nanometer-sized hollow spheres were successfully synthesized by nitriding of MgB2 with NH4Cl and NH4Cl–NaN3, respectively. The large amount of heat and high pressure generated during the reaction process favor the formation of crystalline BN. NaN3 may play a key role in controlling the hollow spherical morphology of nanocrystalline hBN.
Acknowledgements
This work is supported by the National Natural Science Foundation of China and the 973 Projects of China.
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