David Cornu

Boron Nitride Fibers Prepared from Symmetric and Asymmetric Alkylaminoborazines

The thermolysis of 2,4,6-[(CH3)2N]3B3N3H3 (1), 2,4-[(CH3)2N]2-6-(CH3HN)B3N3H3 (2), and 2-[(CH3)2N]-4,6-(CH3HN)2B3-N3H3 (3) led to polyborazines 4, 5, and 6 respectively. The polymers display direct B–N bonds between borazinic B3N3 rings and, in addition, a proportion of –N(CH3)– bridges for 5 and 6, as clearly underlined by 13C NMR spectroscopy. Melt-spinning of these three polymeric precursors exemplified that their ease of processing increases in the order 4 < 5 < 6. Nevertheless, polyborazine filaments could be prepared from each of them and a subsequent thermal treatment up to 1800u2009°C resulted in the formation of crystalline hexagonal boron nitride fibers, which were characterized by X-ray diffraction analysis, Fourier transform infrared (FTIR) spectroscopy, and Raman spectroscopy. Scanning electron microscopy (SEM) images showed that the ceramic fibers are circular and dense without major defects. The mechanical properties for 4-derived fibers could not be measured because of their brittleness, whereas measurements on 5- and 6-derived fibers gave tensile strength σR = 0.51 GPa, Young’s modulus E = 67 GPa, and σR = 0.69 GPa, E = 170 GPa, respectively. The improvement in mechanical properties for ceramic fibers prepared respectively from 4, 5, and 6 could be explained to a large extent by the improvement of the processing properties of the preceramic polymers. This evolution could be related to the increased ratio of bridging –N(CH3)– groups between the B3N3 rings within the polymers 4, 5, and 6 and therefore to the functionalities of the starting monomers 1, 2, and 3.

Controlling the chemistry, morphology and structure of boron nitride-based ceramic fibers through a comprehensive mechanistic study of the reactivity of spinnable polymers with ammonia

The present paper describes an access to polycrystalline boron nitride fibers from poly[B-(methylamino)borazine]. Solid-state NMR and IR spectroscopies, thermo-analytical experiments, SEM and XRD investigations were applied to provide a comprehensive mechanistic study of the fiber transformation and understand the role played by ammonia during the polymer-to-ceramic conversion. It was shown that a typical melt-spinnable poly[B-(methylamino)borazine] (Tsynthesis = 180 °C) is composed of borazine rings connected via a majority of NCH3 bridges and a small proportion of NB3-containing motifs forming a cross-linked network. In addition, a low proportion of peripheral N(H)CH3 groups, which are present in the starting molecular precursor, B-tri(methylamino)borazine, is identified. The polymer is capable of melting without decomposition in flowing nitrogen to produce high quality green fibers at moderate temperature. A curing process of green fibers in flowing ammonia at 400 °C through transamination and condensation forming cross-linked NB3 motifs in the polymer network is seen as the most appropriate way to retain the fiber integrity during the polymer-to-ceramic conversion. The use of ammonia during the subsequent pyrolysis from 400 to 1000 °C allows the basal unit of the “naphthalenic-type structure” of boron nitride to be established at 1000 °C through important structural rearrangements and the crystallization tendency to be improved during further heating from 1000 to 1800 °C. Finally, incorporation of nitrogen using ammonia allows the production of polycristalline fibers in which the stoichiometry approaches that of BN.

High-performance boron nitride fibers obtained from asymmetric alkylaminoborazine

In order to prepare boron nitride fibers displaying improved mechanical properties, different polymers have been prepared from 2-[(CH3)2N]-4,6-(CH3HN)2B3N3H3xa0(1) by varying thermolysis conditions. Analytical data showed that the resulting polyborazines 2, 3, 4 and 5 present polymerisation degrees included between 0.7 and 1.1, and confirmed that they are composed of borazinic rings connected mainly through direct B–N inter-ring bonds and to a lesser extent through –N(CH3)– bridges. Spinning properties of these polymers and ceramisation conditions of the derived crude fibers have been studied. While the spinnability of the higher cross-linked polymer 5 was poor, all of the others displayed very good processing properties as well as no reactivity in spinning conditions allowing the fabrication of high performance boron nitride fibers. Their tensile strengths are over 0.80 GPa and often reach 1.10 GPa. The three main characteristics of the polymeric precursors of fibers, namely polymerisation degree, glass-transition and spinning temperature are shown to be closely related. We observed that when a polymer displays good processing properties, the higher the polycondensation degree is, and the higher the mechanical properties are. A final pyrolysis temperature of 1600 °C is sufficient to ensure the crystallization of the BN fibers obtained.

Direct synthesis of amorphous silicon dioxide nanowires and helical self-assembled nanostructures derived therefrom

Bulk quantities of amorphous silica nanowires and novel braided helical silica nanostructures have been synthesized by a simple and cheap route. Actually, direct thermal treatment of a commercial silicon powder in the presence of graphite yields pure amorphous silica nanowires with lengths up to 500 µm for diameters in the range 10–300 nm. Electron Energy-Loss Spectroscopy (EELS) analysis indicates that the nanowires consist of Si and O elements in atomic ratio 1 ∶ 2, corresponding to SiO2. The formation of silicon dioxide nanowires can be related to the in-situ formation and subsequent decomposition of silicon oxide SiO (g). The nanowires are gathered to form bundles and braid-like nanostructures have been observed in some cases. The formation of these helical nanoobjects results from the self-assemblage of silica nanowires, may be due to the gas flowing during the process.

Direct synthesis of β-SiC and h-BN coated β-SiC nanowires

Abstract β-Silicon carbide (β-SiC) nanowires (NWs) have been grown by thermal treatment of commercial silicon particles disposed in a graphite crucible under nitrogen atmosphere. By the same way, treatment under argon of a mixture of a boron nitride (BN) based powder and silicon particles led to h-BN coated β-SiC nanowires. The structures of both nanoobjects have been investigated by HRTEM, EDX and EELS.

Insights in the sol–gel processing of Pb(Mg1/3Nb2/3)O3. The synthesis and crown structure of a new lead magnesium cluster: Pb6Mg12(μ-OAc)6(μ2,η2-OAc)18(μ3,η2-OC2H4OPri)12

The synthesis and molecular structure of a Pb-Mg bimetallic acetatoalkoxide (Pb-6,Mg-12( mu-OAc)(6)(mu(2), eta(2)-OAc)(18)(mu(3), eta(2)-OC2H4OPr1)(12), space group R-3, a = b = 30.032(2), c = 18.855(2) Angstrom, alpha = beta = 90degrees, gamma = 120degrees) are discussed in this article. This compound was isolated as an intermediate during the elaboration of Pb(Mg1/3Nb2/3)O-3 (PMN) using sol-gel process. It results from the reaction of a bimetallic Mg/Nb species with lead acetate in 2-isopropoxyethanol

New Sol-Gel Route for Processing of PMN Thin Films

Pyrochlore free Pb(Mg1/3Nb2/3)O3 (PMN) thin films were prepared from mixed-metal precursors solutions using the sol-gel process. Lead acetate [Pb(CH3COO)2], magnesium acetate [Mg(CH3COO)2] and niobium ethoxide [Nb(C2H5O)5] were used as starting materials, while 2-isopropoxy-ethanol was chosen as solvent. The reactivity of the precursors was investigated in order to understand and control the process and thus to prevent the contamination of the PMN with the pyrochlore phase. The solution was spin-coated on TiO2/Pt/TiO2/SiO2/Si substrate. The thin films were characterized by SEM and XRD while dielectric measurements were performed on the bulk ceramic.

Elaboration of h -Bn Sheathed β-Sic Nanocables

h -BN sheathed β-SiC nanocables were synthesized under argon at 1200°C by the direct thermal treatment of a silicon powder mixed with turbostratic boron nitride. The structure and the chemical composition of these nanocables have been investigated by HRTEM, EDX and EELS. They have a diameter ranging from 10 to 80 nm. The core of these nanocomposites is composed of pure cubic silicon carbide and the outer layers have been shown to be hexagonal boron nitride planes, set in a parallel direction to the nanocables axis.