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Kinetics and thermodynamics in nano metal oxides formed by sol–gel technique
|Giora Kimmel 1, Jacob Zabicky 1, Francesc Guirado 2|
1. Institutes for Applied Research, Ben Gurion University of the Negev, Beer-Sheva 84105, Israel
Sol–gel technologies have been extensively used for the preparation of nanocrystalline ceramic oxides, since the pioneering works of Yoldas, Brinker and others in the nineteen-seventies. Due to inherent simplicity and low costliness of these synthetic processes, they are prone to scale-up to industrial amounts. Since sol–gel products are usually prepared at RT, the final products are tailored by treatment of the xerogels. It was found that the phase amounts, the crystallinity and the grain size are determined by the xerogel composition and the conditions of thermal annealing (time and temperature). It is well known that the treated sol–gel products are in many cases different from those appearing in the equilibrium phase diagrams for bulk materials. It is quite common to obtain unusual amorphous, nanocrystalline phases and solid solutions which are not seen in the phase diagrams. In the past the surface energy was used to model the kinetics of phase transformations especially during the nucleation stage. Embryos with surface energies higher than the bulk driving force were regarded as unstable. The kinetics of the transition from the unstable embryos to the stable nuclei is well documented. In the sol–gel products we deal with a variety of structures stabilized by lowering the surface energy. It is the contention of our group and others, that the grain size also acts as a thermodynamic variable in addition to concentration, temperature and pressure which are the conventional ones, when determining the stability of nanocrystalline phases, considered by others as metastable. Therefore, it is interesting to study the correlation between kinetics of grain growth and phase transformation. In this presentation will be shown an in situ observation of grain growth and phase transformations of magnesium titanates formed by the sol–gel technique. These ceramic materials have dielectric constants which are unchanged within a wide range of temperatures and frequencies.
Presentation: Keynote at Nano PL 2014, Symposium A, by Giora Kimmel
See On-line Journal of Nano PL 2014
Submitted: 2014-06-28 04:32 Revised: 2014-10-10 15:05