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Hybrid polymer–yttria–zirconia xerogels have been used to prepare freestanding and crack free cubic yttria-stabilized zirconia (YSZ) films. Depending on the final ceramic properties, these materials could find application as components in solid oxide fuel cells. Different types of organic and bioorganic molecules such as albumin, starch, sucrose, dextran, carboxymethylcellulose, and polyvinyl alcohol, have been studied in order to prepare polymer–oxide nanocomposites. The reason for adding a polymer to a sol–gel-derived inorganic network stays in avoiding cracks formation during the pyrolysis process, which converts the preceramic into the ceramic material. Polymers behave as stress relaxing agents and allow obtaining dense and crack free final ceramics. The differences in polymer chemical structure and solubility impose variable synthesis procedures, and limit the availability of suitable yttria precursors. We present here a study concerning the influence of different polymers on the microstructural properties of yttria–zirconia films. Thermogravimetric analyses (TGA), SEM, XRD, N 2 sorption and density measurements have been used to characterize the polymer-to-ceramic transformation and the final ceramic materials.

A modified processing route, which is based on the active filler controlled pyrolysis technique, was followed here to form a homogeneous mullite-SiC nanocomposite. An Al 2 O 3 -filled polymethylsiloxane was prepared by the in-situ formation of the SiCO network via a sol-gel process. The filled gels obtained in this way show an excellent machinability with conventional surface grinding techniques. Heat treatment up to 1500°C initiated crystallization of mullite, which showed intragranular SiC precipitation. Based on microstructure characterization in conjunction with thermal analysis, a formation mechanism is proposed which includes the glass transition temperature of SiCO as well as its phase sparation into SiO 2 plus nanosized SiC crystallites, giving rise to this new mullite-SiC nanocomposite.

Polydimethylsiloxane‐zirconia nanocomposites have been prepared by hydrolysis of diethoxydimethylsilane and zirconium n-propoxide in diAerent 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 1400C and the structural evolution of samples treated at diAerent temperatures has been followed by X-ray diAraction, transmission electron microscopy, infrared and 29 Si solid state nuclear magnetic resonance spectroscopies, thermal analyses and N2 sorption measurements. The polymer-to-ceramic conversion leads to the structural rearrangement of the siloxane component with the production at 600C of high surface area materials with pore sizes below 3nm. Samples are amorphous up to 800C. At 1000C, 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 1400C, 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 20nm, for ZrO2 content50mol%. The tetragonal/monoclinic ratio as well as the crystallite sizes appear strictly related to the chemical composition. # 1999 Elsevier Science Ltd. All rights reserved