Co-produced carbon and boron nitride helical cones and the nucleation of curved BN sheets

The structure of carbon (C) and boron nitride (BN) columnar particles produced alongside by heat-treating a turbostratic B—C—N compound was investigated by analytical transmission electron microscopy. The particles were shown to be helical cones with a well-defined apex, and a helical pitch equal to usually one and at most two layers. The cone apex angle distribution was found to be centred on different values for C and BN. These results strongly suggested that BN and C cones nucleated from different ring defects in the hexagonal networks: square-like ring defects in BN and pentagon-like ring defects in C. These observations were consistent with the supposition that the creation of non B—N bonds and hence odd-membered rings is avoided in BN. Such restriction in BN could also allow one to understand the observed morphological differences between C and BN cones, and in particular the relative difficulty for BN cone edges to join via curved sheets.