Thermal dehydration kinetics of 4CaO·5B2O3·7H2O with different phases and morphologies

Abstract

α-4CaO·5B2O3·7H2O (sheet-like structure) and three polymorphs of β-4CaO·5B2O3·7H2O (flower-like, succulent plant-like and book-like structure) were prepared by hydrothermal method, and characterized by XRD, FTIR, TG-DTA-DTG, SEM. The activation energies (Ea) of the two-step thermal dehydration of 4CaO·5B2O3·7H2O with different morphologies and phases were calculated by Kissinger-Akahira-Sunoes (KAS) method. The most probable mechanism function of the two-step dehydration reaction was determined by Coats-Redfern (CR) method, and the average pre-exponential factor (A) of the two-step thermal dehydration of the sample was calculated. The results showed that the thermal dehydration of 4CaO·5B2O3·7H2O was divided into two steps. The activation energy of thermal dehydration reaction was affected by the phase, morphology and particle size. The activation energies of the two-step thermal dehydration of β-4CaO·5B2O3·7H2O(flower-like, succulent plant-like and book-like structure) all decrease in turn, which is consistent with the change trend of particle size. The average pre-exponential factor of the first-step thermal dehydration of all samples is smaller than that of the second-step thermal dehydration. The mechanism functions of the first-step and second-step thermal dehydration stages are Jander equation [1-(1-α)1/3]2 and Z-L-T equation [(1-α)−1/3-1]2, respectively, which belong to three-dimensional diffusion. The three-dimensional diffusion process of the two-step thermal dehydration was analyzed and explained.