Investigation On The Effects Of Adding Ferrocene/nickelocene On Crystallization Of Boron Nitride From The Pyrolysis Of Borazine

Borazine(B3H3N3H3) is an ideal precursor for boron nitride(BN) ceramics. Prepared from the precursor infiltration and pyrolysis(PIP) route, the crystallization of the pyrolyzed BN ceramics is greatly affected by the pyrolysis temperature. For example, when the pyrolysis temperature is below 1200℃, the pyrolyzed BN ceramic tends to be poorly crystallized, undermining its high-temperature oxidation resistance and thus restricting the wide application of PIP-made BN ceramics and their relevant composites. In order to improve the crystallization of BN pyrolyzed at relatively low temperatures, we have studied the effects of nanoscale metal inclusions on the crystallization behavior of BN by pyrolyzing borazine dissolved with different amounts of metallocene(ferrocene and nickelocene). The results show that the crystallization of the borazine-derived BN have been significantly improved by adding about 2 wt.% metallocene.We have studied the cross-linking and the pyrolysis of borazine at different temperatures. With an increase of pyrolysis temperature, the crystallization of hexagonal BN(h-BN) is increasing: when pyrolyzed at around 1000℃, the cross-liked polyborazylene almost completes the mineralization process and yields hexagonal BN with an amorphous structure; when pyrolyzed at 1200℃, it produces a rather similar structure that shows a certain degree of order; when pyrolyzed at 1400℃and 1600℃, it yields fully ordered hexagonal BN. Based on the different crystallization behavior of h-BN pyrolyzed at different temperatures, we propose a crystallization mechanism of h-BN. Initially, the crystal nucleus of h-BN separates out from the amorphous matrix and forms some atomic groups that have a layered structure of h-BN through the diffusion and re-occupation of B and N atoms. Stimulated by the thermal energy, these atomic groups overcome the thermodynamic barrier and form the h-BN nucleus by means of homogeneous nucleation. Finally, the grain grows in a fashion that the nucleus diffuses towards the amorphous matrix by depositing atoms from the matrix in the interface.We have studied the cross-linking and the pyrolysis of borazine dissolved with different amounts of metallocene. When pyrolyzed at 1200 ℃, ferrocene and nickelocene formed body-centered cubic(bcc) iron and face-centered cubic(fcc) nickel nanoparticles respectively. By adding 2 wt.% metallocene, the XRD peaks become sharper, indicating an improved crystallization behavior of BN pyrolyzed at 1200℃. Moreover, from TEM observation, the pyrolyzed products show an onion-like core-shell structure. The shells are well-crystallized h-BN layered structure with a thickness of 10 nm, whereas the cores are Fe or Ni nanoscale particles with a diameter of 20 nm. As a result of the very active surface of nano-sized metal particles that can be the substrate of heterogeneous nucleation, it has dramatically reduced the thermodynamic barrier of nucleus formation. Therefore, the amorphous matrix forms crystal nucleus easily on those nano-metal particles. In addition, as those nano-metal particles are liquids or pseudo-liquids that can dissolve B and N atoms and thus provide diffusion passages for those random atoms, the amorphous BN gradually becomes ordered and forms h-BN layrered structure surrounding those nano-metal particles.