J.-P. Pillot

Si-C-N ceramics with a high microstructural stability elaborated from the pyrolysis of new polycarbosilazane precursors

Novel polycarbosilazanes (PCSZs) were prepared by stepwise synthesis and thermal crosslinking of polysilasilazane (PSSZ) copolymers. Their pyrolysis under inert gas, producing Si-C-N ceramics, was investigated up to 1600 °C by analyses performed on the solids (elemental analysis; EPMA; TGA, density; 1H, 13C and 29Si solid state NMR, i.r. XRD, electrical conductivity) and analyses of the evolved gases (gas chromatography and mass spectrometry). From 250 to 450 °C, a first strong weight loss was observed, which was due to the formation and elimination of low-boiling-point oligomers. The weight loss closely depends on the cross-linking degree of the ceramic precursor resulting from the PSSZ/PCSZ conversion. Then, the organic/inorganic transition took place between 500 and 800 °C, proceeding via evolution of gases (mainly H2 and CH4) and yielding a hydrogenated silicon carbonitride. This residue remained stable up to 1250 °C although it progressively lost its residual hydrogen as the temperature was raised. Then, crystallization occurred between 1250 and 1400 °C, yielding β-SiC crystals surrounded by free-carbon cage-like structures. Finally, above 1400 °C, the remaining amorphous Si-C-N matrix underwent a decomposition process accompanied by nitrogen evolution and a second substantial weight loss. At 1600 °C, the pyrolytic residue was a mixture of β-SiC and free carbon. So, the amorphous silicon carbonitride resulting from the pyrolysis of PCSZ precursors was found to be appreciably more thermally stable than the previously reported Si-C-O ceramic obtained by pyrolysis of polycarbosilane precursors.

A model SiC-based fibre with a low oxygen content prepared from a polycarbosilane precursor

A model SiC-fibre has been prepared from a polycarbosilane precursor by means of an irradiation oxygen-free curing process. The chemical composition remains unchanged after heat treatments under an inert atmosphere for pyrolysis temperatures of 1600°C. At this temperature, the fibre consists of SiC nanocrystals (mean size 6–10 nm) and free carbon. However, a slow grain growth takes place as the temperature is increased. The fibre retains a high strength at room and high temperatures up to temperatures of 1600 °C when the pyrolysis has been performed under nitrogen. The electrical conductivity was studied as a function of the pyrolysis temperature Tp: For 1100≤Tp≤1200 °C, the conductivity increases by several orders of magnitude due to the reorganization of the free carbon phase at the SiC grain boundaries. Oxidation kinetics of the filaments remain parabolic from 1000–1400 °C.

C-silylation de cétones a partir de chlorotrialkylsilanes et de magnésium

Abstract The trimethylchlorosilane/magnesium/hexamethylphosphorictriamidesystem reacts with arylaliphatic ketones suffering steric hindrance to yield the corresponding C -substituted silicon alcoxysilanes, which after hydrolysis, lead to C -substituted silicon homologs of tertiary alcohols. Furthermore, under certain conditions phenyl-tert-butylketone can give para -substituted alcoxysilane which upon hydrolysis yields the para -silyl-substituted secondary alcohol.

Synthesis and characterization of new precursors to nearly stoichiometric SiC ceramics: Part II A homopolymer route

Organosilicon polymers with a (C/Si (atomic ratio)<1.28) exhibiting Si–Si and Si–CH2–Si linkages in their backbone have been prepared and characterized by multinuclear magnetic resonance and infrared spectroscopies, size exclusion chromatography and thermogravimetric analysis. These precursors give SiC ceramics with a C/Si (atomic ratio)≈1.14 and a relatively low oxygen content (between 1.5–3 at %).

Continuous SiC-based model monofilaments with a low free carbon content: Part II From the pyrolysis of a novel copolymer precursor

Quasi stoichiometric model SiC monofilaments (C/Si atomic ratio ≈ 1.02) with still some free carbon (≈3 mol%) and residual oxygen have been produced from a novel copolymer precursor, itself prepared from methylphenyldichlorosilane and 2,4-dichloro-2,4-disilapentane. The continuous green fibre was melt spun at 230°C, cured by electron-beam irradiation, and pyrolysed under argon at temperatures, Tp, in the range 1000–1600°C. The fibre remained nanocrystalline at high temperature with the SiC grain size growing from 1.5 nm to 7.3 nm when Tp was raised from 1400°C to 1600°C. Its Youngs modulus continuously increased as Tp was raised (with E=320 GPa for Tp=1400°C), whereas its tensile strength at room temperature underwent a maximum for Tp=1200°C (σR≈1850 MPa for L=10 mm and d≈20 μm).

Nouveaux polycarbosilanes modèles. 3-Synthèse et caractérisation de carbosilanes linéaires fonctionnels

Abstract A method for synthesizing novel linear polysilmethylenes of general formula Me(HMeSiCH 2 ) x SiMe 2 H (VII x ) is described. This strategy involves the cocondensation of chloromethylsilanes HMe 2 Si(CH 2 SiMeH) m CH 2 Cl ( m = 0 or 1) with methyldichlorosilane and 1,3-dichloro-1,3-dimethyl-1,3-disilapropane, respectively, by Grignard reactions. Thus, short-chain polycarbosilanes possessing terminal dimethylhydrogenosilyl groups were prepared (2 ≤ χ ≤ 5). They were then converted into the corresponding chlorinated derivatives Me(Cl MeSiCH 2 ) x SiMe 2 Cl (VIII x by reaction with CCl 4 in the presence of a palladium catalyst. The mass spectra (70 eV) of both series reveal that characteristic fragmentations occur. Moreover, the SiH-containing derivatives VII x exhibit analogous behavior during their rearrangement under electronic impact and during their pyrolysis.

Modification of SiC precursors with an amine–borane complex

Two methods for preparing precursors of Si/C/B-based thermostructural materials have been developed. The first consists of a thermal treatment of a mixture of polydimethylsilane (PDMS), (Me2Si)n, and triethylamine–borane adduct, Et3N : BH3 at atmospheric pressure. The second involves two steps: (i) transformation of PDMS into a polymer displaying Si–CH2–Si and Si–Si linkages in its backbone, and (ii) heating this product at atmospheric pressure, in the presence of Et3N: BH3. The ceramic material obtained from the second approach contains 2.2 at% boron and 1.5 at% oxygen.

A quasi-stoichiometric SiC-based experimental fibre obtained from a boron-doped polycarbosilane precursor

A quasi-stoichiometric SiC-based fibre, has been prepared at a pyrolysis temperature of 1800°C, from a boron-doped polycarbosilane precursor using an extrusion-chemical curing-pyrolysis procedure. The green fibre was rendered infusible by sequential curing with ozone and trimethylamine-borane adduct (O3/Me3N : BH3). The fibre was characterized by scanning electron microscopy, X-ray diffraction, transmission electron microscopy, electron energy loss spectroscopy and Raman spectroscopy.

An original route to precursors of silicon carbide-based ceramics possessing low free carbon content

2,4-Dichloro-2,4-disilapentane is used as starting material in the synthesis of novel polycarbosilane precursors of silicon carbide-based ceramics containing low free carbon excess.

An original route to poly[silylene-co-(2,4-disilapentane-2,4-diyl)] oligomers, efficient precursors of silicon carbide-based ceramics with variable C/Si ratios

Novel oligomers possessing a backbone formed of ( ?SP-CH 2 -Si ?) and (SS) n units were prepared by the copolycondensation of bis(chlorosilyl)methanes and various dichlorosilanes in the presence of sodium, in refluxing toluene. The effect of the respective molar ratios of comonomers on the yields and the structure of the copolymers was investigated. The role of substituents on silicon atoms in the ability of these materials to provide convenient ceramic precursors upon pyrolysis was examined. When ?Si-H bonds were present, thermal cross-linking was readily performed and ceramics possessing variable C/Si ratios were prepared.