Papers by Author: Samuel Bernard

Abstract: Boron nitride (B1.0N0.9) nanoparticles have been prepared by the spray-pyrolysis of borazine. Characterization of the B1.0N0.9 nanoparticles by SEM, and HRTEM demonstrated that samples form elementary blocks containing slightly agglomerated nanocrystalline particles with sizes ranging from 55 to 120 nm. Their thermal behavior has been investigated and thermogravimetric analyses of B1.0N0.9 nanoparticles showed that they are relatively stable in air atmospheres.

Abstract: SiBCN microtubes were prepared for the first time by spinning a low molecular weight preceramic polymers of boron-modified polyvinylsilazanes into green fibers ~30 m in diameter which were subsequently thermolyzed under a nitrogen atmosphere. Hollow SiBCN ceramic fibers black colored, of flexible form, ~20 m in diameter and 0.8GPa in tensile strength were produced.

Abstract: The kinetic investigation of the solid-state decomposition of a typical melt-spinnable poly[B-(methylamino)borazine] into boron nitride fibers was carried out at various heating rates. It was shown that the two-step weight loss associated with the polymer-to-ceramic conversion is governed by two major diffusion-type transport mechanisms which are independent of the applied heating schedule. The application of the Kissinger and Friedman methods to dynamic TG data allowed us to measure the Ea and lnA values which are seen to increase with the extent of the ceramic conversion from region one (Ea=46.3kJ/mol, lnA=3.85; Friedman method) in going to region two (Ea=122.6J/mol, lnA=12.18; Friedman method). Kissinger and Friedman results are in good agreement. It was shown that the cross-linking process of the polymer is mainly governed by a poorly energetic mechanism indicating the occurrence of simple reactions, whereas the mineralization and ceramization steps are majoritarly represented by a highly energetic mechanism suggesting the occurrence of complex and multi-step mechanisms.

Abstract: Boron-modified polyvinylsilazanes have been studied for suitability as fiber precursor. A melt-tractable polymer displaying Si- and N-bonded methyl groups was successfully processed into green fibers ~18μm in diameter via a melt-spinning process. After the shaping process, the use of an ammonia curing atmosphere at 200°C allowed to increase the ceramic yield of the polymer, then avoid inter-fiber fusion during the further increase of the temperature. As-cured fibers were annealed in the temperature range 1000-1800°C in a nitrogen atmosphere to provide SiBCN ceramic fibers black colored, of flexible form and ~12μm in diameter in different crystallinity states going from totally amorphous below 1600°C to well-crystallized at 1800°C. The excellent strength retention after heat-treatment at 1600°C (1.3-1.5GPa) is clearly related to the high amorphous stability of fibers. Elemental compositions of such amorphous fibers showed a typical chemical formula of Si3.0B1.0C5.0N2.4. Between 1600°C and 1700°C, the fiber strength decreased to 0.9GPa then dropped to about one-quarter the original value at 1800°C while structural changes were evident by XRD analysis.