Papers by Author: Philippe Miele

Abstract: In the field of advanced ceramics, Spark Plasma Sintering (SPS) is known to be very efficient for superfast and full densification of ceramic nanopowders. This property is attributed to the simultaneous application of high density dc pulsed current and load, even though the sintering mechanisms involved remain unclear. In the first part of the paper, the mechanisms involved during SPS of two insulating oxide nanopowders (Al₂O₃ and Y₂O₃) are discussed while in the second part illustrations of the potential of SPS will be given for (i) Consolidation of mesoporous or unstable nanomaterials like SBA-15 or biomimetic apatite, respectively; (ii) Densification of core (BT or BST)/shell (SiO₂ or Al₂O₃) nanoparticles with limited or controlled reaction at the interface. (iii) In-situ preparation of surface-tailored Fe–FeAl₂O₄–Al₂O₃ nanocomposites, and finally (iv) One-step preparation of multilayer materials like a complete thermal barrier system on single crystal Ni-based superalloy.

Abstract: Hydrolysis of NaBH4 to release molecular hydrogen is today an intensely investigated reaction and most of the studies focus on the material used as catalyst. Among the various metals tested up to now, cobalt has soon showed to be the most attractive in terms of reactivity and cost. Nevertheless, in order to further decrease its cost by decreasing its amount as well as to increase its reactivity, cobalt has been dispersed over supports. The as-formed supported catalysts have showed to be more efficient. This is the topic of the present study. Herein it is showed that CoCl2 supported over an Al2O3 support with a specific surface area of 180 m2 g-1 is more reactive than CoCl2 supported over a high-surface-area activated carbon (780 m2 g-1), CoCl2 being in-situ reduced into the Co-based active phase. CoCl2-Al2O3 is besides as reactive as another CoCl2-Al2O3 catalyst, the latter support having a higher specific surface area (i.e. 250 m2 g-1). In fact, CoCl2-Al2O3 is more performing than neat CoCl2 whereas the latter has been often showed as being one of the best catalytic systems. To further gain in reactivity, a new, alternative strategy has been envisaged. The Al2O3 was mixed together with a controlled amount of another oxide, namely TiO2. The CoCl2- Al2O3-TiO2(20 wt%) was found to be more reactive than CoCl2-Al2O3. All of these reactivity data are reported and briefly discussed hereafter. Further studies are in progress to highlight the reasons of such improved reactivity.

Abstract: Shaped catalyst is much more practical than powder catalyst when the material is intended to technological applications. Nevertheless, shaping a reactive powder catalyst is not evident, loss in reactivity occurring by e.g. decreasing specific surface area. Addressing such problem has been our objective. The present paper reports methods and results about the shaping of a lab-made cobalt supported clay catalyst over a stainless steel plate (SSP) taken as substrate. The deposition method chosen is electrophoretic deposition (EPD). Clay has been chosen owing to its abundance and greenness. Herein, we report that it is feasible to make a Co-Clay-SSP shaped catalyst by EPD in a two-step process. Typically, the supported Co-Clay has first to be prepared by wet impregnation and then the as-obtained powder is deposited over the substrate by EPD. Such shaped catalyst is intended to be used in NaBH4 hydrolysis, a well known route for on-demand release of hydrogen.

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: The formation of dots by CVD in the hetero-system SiC-Si was studied in the two possible ways : Si dots on SiC substrate and SiC dots on Si substrate. The substrates underwent special surface treatment to reveal a network of parallel steps before deposition of the dots. In the Si on SiC case, the dots density on the 8°off 4H-SiC substrate varied in the range 107 – 7x108 cm-2 and mainly depends on the SiH4 flux and the deposition time. The Si dots are in majority aligned along the step edges of the substrate. In the other hetero-system, only propane was introduced in the reactor to performed a localised carbonisation of the Si(111) 1.5°off substrate. The SiC dots obtained at 1200°C have similar density the Si ones but with smaller size.

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: High surface area nanoporous powders of hexagonal boron nitride (h-BN) have been prepared from molecular precursors to be used for hydrogen sequestration. The more promising samples were obtained using a precursor derived from trichloroborazine (TCB). The precursor was first reacted with ammonia at room temperature leading to the molecular complex Cl3B3N3H3, 6 NH3 which was heated up to 650 °C under ammonia and then up to 1000 °C under nitrogen, giving rise to a high reactive h-BN powder. This crude powder was stabilised by an annealing up to 1800 °C under nitrogen atmosphere leading to a very stable compound exhibiting a specific area of more than 300 m2·g-1 and presenting a very specific nanometric spherical texture. Some samples were doped with platinum (about 1 wt.%) to enhance the activity of pure h-BN using an original one step synthesis route starting from a mixture of BN and Pt precursors. Attempts to sequester hydrogen into these powders were made successfully at -196 °C under 10 MPa, but the stored amount was only about 0.3 wt.% and the platinum added BN powders did not lead to an enhancement of the storage capacity.

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.