POLYSILOXANE PYROLYSIS

Different polysiloxanes were pyrolyzed to make silicon oxycarbide glas ses which have been. shown to be possible candidates for anode materia ls of Li-ion batteries. The decomposition process during pyrolysis was studied using a thermal gravimetric analyzer connected to a residual gas analyzer for three representative polymers. No silicon- or oxygen- containing gases were observed during pyrolysis. The stoichiometry of all of the final chars was measured and found to agree well with predi ctions calculated using the initial polymer stoichiometry, the ceramic yield, and the fact that only:carbon and hydrogen atoms were eliminat ed during pyrolysis. For three chars with identical stoichiometry at 1 000 degrees C, prepared from three different polymers, the local chemi cal environment of the silicon was studied as a function of pyrolysis temperature by X-ray absorption (XAS) near the silicon K-edge. As the samples are heated, the XAS spectra eventually become identical, once the different labile species have been eliminated, suggesting that the properties of the char are dependent mainly on its stoichiometry and not on the initial composition of the polymer precursor. The bulk prop erties of these three chars prepared at 1000 degrees C were also studi ed by X-ray diffraction and by electrochemical methods which showed th em to be identical.