The interest devoted to binary zirconia-silica oxides can be traced back to their enhanced catalytic, mechanical, electronic, thermal properties. These materials can be used as coatings, fibres, catalysts and high-refractive-index glasses [1]. The addition of zirconium in silica significantly improves their mechanic, electric and chemical properties, more particularly towards alkaline attacks [2]. The structure of mixed zirconia-silica oxides is very interesting because zirconium cannot substitute silicon, due to the different atomic radius and coordination number between these two elements. Thus zirconia will develop as distinct phase in the nanocomposites. It is interesting to study the evolution of zirconia crystalline phases inside the silica matrix. It has been reported in the literature the formation at temperatures lower than 1000°C only of the monoclinic zirconia [3]. It is interesting to find the conditions of synthesis and thermal treatment for the obtaining of tetragonal zirconia inside the silica matrix. The sol-gel process allows in principle to synthesize zirconia-silica nanocomposites for various zirconia contents at lower temperatures than the conventional methods. The reactivity of zirconium alkoxides towards hydrolysis being higher than that of silicon alkoxides, mixtures of zirconium oxides and alkoxides precipitate during hydrolysis instead of forming a homogeneous gel. The aim of this work is to study the local structure of ZrO₂-SiO₂ mixed oxides obtained from hybrid inorganic-organic materials with different Si/Zr atomic ratios as precursors. In this study, we have synthesized zirconia-silica nanocomposites using a modified sol-gel method, starting from TEOS – Zr(NO₃)₄ – poliol – H₂O sols that succesfully gelified to homogenous gels. We have studied the gels Zr(NO₃)₄-poliol (1,3-propanediol and poly(vinyl alcohol), and we have evidenced during gels heating that a redox reaction takes place between zirconium nitrate and poliol with formation of some carboxylate type compounds of Zr, that were used as precursor for zirconia powders. It has been concluded from the study that the use of PVA as poliol is favorable to the formation and stabilization of tetragonal zirconia [4]. Also the presence of SiO₂ in small amounts was reported as a stabiliser for tetragonal zirconia [5]. Thus we have decided to use as poliols in our gels 1,3-propanediol and poly(vinyl alcohol). The gels obtained at room temperature have been dried and then thermally treated at 160°C when the redox reaction took plase between the organic poliol and zirconia nitrate leading to the formation of zirconia nanoparticles precursors in the pores of silica matrix. These xerogels obtained at 160°C have been thermally treated at 400°C for 3 hours for the thermal decomposition of zirconia precursors, then annealed at higher temperatures in the range 400-1100°C. The obtained composites have been characterized by FTIR spectroscopy, X-ray diffraction, HR-TEM and SEM-EDX analysis. Depending on the thermal treatment zirconium silicate appeared as secondary phase. The working conditions for the obtaining of homogenously distributed ZrO¬2/SiO₂ nanocomposites have been established. The influence of zirconia/silica ratio on the evolution of zirconia phases was also studied.

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