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

Polydimethylsiloxane-zirconia nanocomposites have been prepared by hydrolys is of diethoxydimethylsilane and zirconium n-propoxide in different molar r atios. 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 samp les treated at different temperatures has been followed by X-ray diffractio n, transmission electron microscopy, infrared and Si-29 solid state nuclear magnetic resonance spectroscopies, thermal analyses and N-2 sorption measu rements. The polymer-to-ceramic conversion leads to the structural rearrang ement of the siloxane component with the production at 600 degrees C of hig h surface area materials with pore sizes below 3nm. Samples are amorphous u p to 800 degrees C. At 1000 degrees C, the structural evolution of the sili con moiety produces an amorphous oxycarbide phase whereas the primary cryst allisation of tetragonal zirconia takes place, with crystallinity and cryst allite sizes depending on the ZrO2 content. At 1400 degrees C, the silicon oxycarbide phase generates a mixture of amorphous silica and crystalline si licon carbide polymorphs. In this matrix, tetragonal and monoclinic ZrO2 ph ases are present with ZrO2 average crystallite dimensions never exceeding 2 0 nm, for ZrO2 content less than or equal to 50 mol%. The tetragonal/monocl inic ratio as well as the crystallite sizes appear strictly related to the chemical composition. (C) 1999 Elsevier Science Ltd. All rights reserved.