Estimation of kinetics parameters of dehydration and crystallization process of ZrO₂ - 3 mol. % Y₂O₃ xerogels system modified under conditions of high hydrostatic pressure

Pressure and temperature is an important tool for the synthesis of almost all technologies and forming oxide powders. In the case of zirconia based systems which have more polymorphic forms in depending on the P-T conditions, the pressure can be used as the control factor of material properties. It is known that the characteristics of crystalline zirconia nanoparticles (particle size, specific surface area, etc.) greatly depend on the xerogel structure of zirconia nanoparticles. Xerogel zirconia is a skeleton of interconnected amorphous nanoparticles of zirconia with hydration shell and supplementing its porous space. Oxide nanopowder formation occurs in a series of sequential dehydration reactions, crystallization, destruction, adherence, under heat treatment conditions of xerogels. As shown earlier the xerogel modification under HHP conditions results to nonmonotonic change of state of hydration components and structural organization of amorphous xerogel. The main goal of this paper is the estimation of kinetic parameters (activation energy, Avrahami''s parameter and reaction equation), dehydration and crystallization processes of xerogels modified under the HHP (100 - 1000 MPa) by DSC. The kinetic parameters had estimated by the method of Kissinger and Avrami-Dzheziorni. It has been shown that the estimated activation energy of the dehydration process nonmonotonically changing in depending on the amount HHP with an extremum at 600 MPa. Whereas, the dependence of the activation energy of the dehydration process on the degree of connectivity of the aqueous component with the xerogel surface nanoparticles is monotonic. Based on analysis of the estimated water densities occurring in the experiment thermodynamic conditions, and the P - T - diagram of the water state found that the probability of reorganization in the structure of the hydration shell of xerogel is connected with water's phase transitions. It has been found that the kinetic parameters crystallization is monotonic in depending of the HHP value. This is demonstrated in a monotonic increasing of the activation energy process and the changing of the crystallization mechanism from volume growth of crystallizing particles at the constant rate of nucleation to volume growth with increasing nucleation. (author)