Morphology of Nanometric Boron Nitride Powders Produced by Laser Pyrolysis
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A a A
I
I
i
cdistances I Iand
I
h BN *B oN
A-
layered structures of hexagonal
cB
boron-nitride (hBN) and graphite. The nearest-neighbour (dNN =0.144 nm 0.142 nm respectively) and
A
the inter-layer spacings (c/2=0.33 nmi and 0.335 nm
A
eC
Graphite
respectively) are almost identical
in both structures.
53 Mat. Res. Soc. Symp. Proc. Vol. 359 01995 Materials Research Society
In this paper, we describe the synthesis of an ultrafine boron nitride powder produced by CO2 laser pyrolysis, and the characterisation by high resolution electron microscopy (HREM) of the graphitic nanoparticles formed during this process. POWDER SYNTHESIS Boron nitride powders were synthesized via laser-driven gas-phase reactions, as described previously. 13 .14 The two gaseous precursors, NH 3 and BC13 , were separately introduced in the beam of a high powered continuous wave CO2 laser, leading to the overall reaction:
BC13 + NH 3 -- BN + 3 HCI An excess of NH3 reacts with HC1 to give NH 4 Cl as by-product. In this process the two reactants which have absorption bands near the emission line of the CO 2 laser (10.6 gim) are rapidly heated by the laser beam. The resultant powders and by product gases are carried by a laminar flow of argon gas from the reaction chamber to a collection filter. The advantages of this innovative method is to produce high purity, chemically homogeneous, non-agglomerated, small diameter powders. The specific surface area of the present powders, as determined by BET measurements, can be as high as 200 m2/g, i.e. more than two orders of magnitude larger than for commercial BN powders.13,-14 The powders were studied in the as-pyrolyzed state and after two-hour heat-treatments under nitrogen atmosphere at various temperatures (ranging from 1000*C to 1650 0C). TRANSMISSION ELECTRON MICROSCOPY RESULTS The transmission electron microscope study was performed on a Philips CM20 twin operating
at 200 kV. Electron diffraction patterns of the initial powder contain diffused rings indicating a high two-dimensional ordering within graphitic layers, and a low ordering between layers. After
heat treatment at 1650 0C, the three-dimensional ordering is increased, in particular the 11 ring splits into 110 and 112 reflections (see fig. 2).15
110 11
112
00410 002
Fig. 2. Transmission electron diffraction patterns of boron nitride powders.(a): As-synthesized state presenting a turbostratic structure.(b): Three-dimensional ordering towards the hexagonal structure after heat treatment at 1650 0C under nitrogen atmosphere. 54
Concentric Shells We have observed a very large variety of roughly spherical particles with graphitic layers parallel to the surface ranging in size from 30 nm to more than 100 nm (fig. 3). The closed nature of these structures and their roughly spherical shape is confirmed by tilting experiments. These particles are either nearly filled to the center, or hollow with a large void inside. In the larger particles, the roughly spherical arrangement does not present a very high order between la
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