Thermal decomposition of un-irradiated and γ-ray irradiated holmium acetate tetrahydrate. Part 1: kinetics of nonisothermal dehydration of un-irradiated and γ-ray irradiated Ho(CH3COO)3⋅4H2O

Abstract

Abstract Nonisothermal dehydration of un-irradiated and γ-ray irradiated holmium acetate tetrahydrate with 103 kGy total γ-ray dose absorbed was studied in air atmosphere. The thermal decomposition experiments were conducted at heating rates of (5, 7.5 and 10°C/min). The results showed that for un-irradiated material, the dehydration process proceeds in two decomposition steps with the elimination of 3.0 and 1.0 moles of H2O, respectively. The apparent activation energy, E a , as given by both linear and nonlinear isoconversional methods showed dependence upon the conversion degree, α, in the range of 0.2–0.75 for the two dehydration steps. In the first dehydration step, the Ea decreases from 228.0 kJ/mol at the beginning of the decomposition to ≈64.0 kJ/mol at the end of the process. In the second dehydration step, the Ea increases from 42.0 to 72.0 kJ/mol by progressively increasing in α. Compared with solid state reaction models, the two reactions are best described by diffusion (D 4) and nucleation (A 3) models for the first and second dehydration steps, respectively. The results derived from nonisothermal data present a reliable prediction of isothermal kinetics. Straight lines and reduced time plots methods were applied for the determination of the kinetic triplet [Ea , ln A, and reaction model f(α)] from predicted isothermal data. For γ-ray irradiated samples of Ho(CH3COO)3⋅4H2O with 103 kGy total absorbed dose, the dehydration proceeds in two overlapped steps controlled by D 3 model. X-ray data showed phase transformation from monoclinic (SG P2/m) to tetragonal phase (SG P4/mmm) by the elimination of water content from the entire structure of Ho(CH3COO)3⋅4H2O. γ-Ray irradiation effects on the thermal decomposition of Ho(CH3COO)3⋅4H2O were evaluated and discussed based on the formation of trapped electrons, point defects, cation and anion vacancies and cluster imperfections in the host lattice of Ho(CH3COO)3⋅4H2O.