KINETICS OF PHASE-TRANSFORMATION OF NA4UO2(SO4)3 FROM GLASS TO QUASI-CRYSTALLINE

The kinetic study of the thermal transformation from the glass state to a quasi-crystalline intermediate phase on crushed and uncrushed samples of Na4UO2(SO4)3 was carried out using an analytical method based on the enthalpy of devitrification. The composition of Na4UO2(SO4)3 was varied with respect to Na+ and UO2(2+) and by incorporating isovalent guest ions of different ionic radius, i.e. Rb+ and Cd2+ to probe the nature of the devitrification mechanism. The kinetic data show an excellent fit with an autocatalytic rate where the reaction is first order, viz.d-alpha/dt = k-alpha(1 - alpha) for all compositions. The relevant kinetic quantities, activation energy and rate constant, are interpreted in terms of nuclei number, nuclei branching or growth rate and nuclei termination. The very good agreement between E(a) values for Na4UO2(SO4)3 from independent conductivity and isothermal kinetic data implies that the major contributing factor for the transformation step is linked to the mobility of Na+ ion in the glass.