Organisation of carbon and boron nitride layers in mixed nanoparticles and nanotubes synthesised by arc discharge

The spatial distribution of the chemical species (B, C, N and Hf) present in multi-element nanoparticles and nanotubes produced by arc-discharging a hafnium diboride rod with a graphite rod in a nitrogen atmosphere has been investigated by means of electron energy loss spectroscopy (EELS). These analyses revealed some common features within the non-homogeneous graphitic layers of the composite products found in the anode deposit: (i) the EELS fine structure of the absorption edges of the sp² bonds attests for the presence of ordered BN domains and of carbon domains; (ii) the elemental profiles show that BN layers and carbon layers are immiscible with a radial organisation into two to five domains; and (iii) the sets of layers at free surfaces - including the inner surfaces of tubes - are always made of carbon. The origin of this chemical organisation, which is most likely obtained during the growth, is discussed. For the hafnium-boride metallic particles coated by C/BN envelopes, a model based on the solidification from the outside to the inside of isolated liquid-like droplets is proposed: the carbon phase solidifies first according to theoretical phase diagrams, and forms the outer shells. For the tubes, a directional eutectic solidification process is shown to account for the observed C/BN/C sequence, in a vapour-liquid-solid scheme, with an hafnium-rich liquid-like particle at the tip of the tube.