Thermal evolution and crystallisation of polydimethylsiloxane-zirconia nanocomposites prepared by the sol-gel method

Polydimethylsiloxane-zirconia nanocomposites have been prepared by hydrolysis of diethoxydimethylsilane and zirconium n-propoxide in different molar ratios. Transparent, homogeneous and non-porous xerogels have been obtained up to 70 mol% ZrO2 content. The starting xerogels have been pyrolyzed under argon atmosphere up to 1400 degrees C and the structural evolution of samples treated at different temperatures has been followed by X-ray diffraction, transmission electron microscopy, infrared and Si-29 solid state nuclear magnetic resonance spectroscopies, thermal analyses and N-2 sorption measurements. The polymer-to-ceramic conversion leads to the structural rearrangement of the siloxane component with the production at 600 degrees C of high surface area materials with pore sizes below 3nm. Samples are amorphous up to 800 degrees C. At 1000 degrees C, the structural evolution of the silicon moiety produces an amorphous oxycarbide phase whereas the primary crystallisation of tetragonal zirconia takes place, with crystallinity and crystallite sizes depending on the ZrO2 content. At 1400 degrees C, the silicon oxycarbide phase generates a mixture of amorphous silica and crystalline silicon carbide polymorphs. In this matrix, tetragonal and monoclinic ZrO2 phases are present with ZrO2 average crystallite dimensions never exceeding 20 nm, for ZrO2 content less than or equal to 50 mol%. The tetragonal/monoclinic ratio as well as the crystallite sizes appear strictly related to the chemical composition. (C) 1999 Elsevier Science Ltd. All rights reserved.