29 Si NMR and XPS Investigation of the Structure of Silicon Oxycarbide Glasses Derived from Polysiloxane Precursors
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EXPERIMENTAL Two polysiloxane precursors described in the literature [4] have been used for this study. The T precursor has been obtained by hydrolysis/condensation (catalysed by tetrabutylammonium fluoride) of methyltrimethoxysilane in acetone. The COVS precursor has been obtained in two steps: hydrolysis/condensation of dichlorovinylsilane leads to a mixture of cyclic oligovinylsiloxanes; these volatile oligomers are then cross-linked by catalytic hydrosilylation leading to Si-CH 2 CH 2 -Si bridges. The main structural units present in these precursors and their composition are given in Table I. Table I: Characterization of the precursors. Main structural units (29 Si chemical shifts) Precursor Si(CH 2 CH 2 -)2 (0-)2 (-19.5 ppm) COVS SiVi(CH 2 CH 2 -)(O-) 2 (-33 ppm) SiH(CH 2 CH 2 -)(O-) 2 (-33 ppm) T Si(Me)(O-) 3 (-65 ppm) aoxygen determined by difference.
Atomic compositiona SiO0. 99 C2 .1 3H4 .24 SiOl.6 5C1 .02H 3 .14
The pyrolysis was performed in a STA409 Netzsch thermobalance under argon flow. Typically ca 800 mg of the precursor in an alumina crucible were heated at 100 per minute to 900 'C and annealed for 30 minutes at this temperature. According to elemental analysis the composition of the glasses are SiO1. 14 C 1.74 (COVS glass) and SiO1. 69 C0 .80 (T glass). 29Si MAS-NMR spectra of the glasses were recorded on a Brucker FT-AM 300 spectrometer using a pulse angle of 300 and a recycling delay of 60 s. Considering the high number of scans necessary to get an acceptable signal to noise ratio (>1000), the number of different sites (5) and their concentration, it was not possible to determine the spin lattice relaxation times. However, preliminary experiments perfomied with pulse angle ranging from 30 to 900 and recycling delays ranging from 10 to 300 s showed that the above conditions afforded reasonably quantitative spectra, in acceptable accumulation times. Elemental analyses were performed by the "Service Central d'Analyses" of CNRS (Vernaison, France). The XPS analysis was performed on a RIBER MAC 2 spectrometer, under a vacuum of 10-7 Pa, recording Si2p, Cls and Ols peaks under Al Kca radiation. Standards (SiO 2 , SiC and pyrolytic carbon (obtained by pyrolysis at 900 'C of an organic polymer)) and the silicon oxycarbide samples were finely ground and spread onto an indium foil. In situ Ar+ ions etching was carried out prior to the measurement to remove contamination. The binding energies determined for the standards were: Si2p: 103.5 (SiO 2 ), 100.3 (SiC); Cls: 284.7 (C), 283.4 (SiC); Ols: 232.5 (SiO 2 ). The spectra were deconvoluted using a least-squares fit program. RESULTS AND DISCUSSION The 29 Si MAS NMR spectra of the two glass samples are displayed in figure 1. In both cases a distribution of SiOxC4-x sites is observed, whereas the COVS is mainly built of SIO2C 2 units and T of SiO 3 C units. The chemical shifts and the widths of the corresponding peaks are very close to that reported by Lipowitz and Turner for SiOxC4-x sites Nicalon silicon oxycarbide fibers.[9] 29 Table II: Percentages of
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