The kinetics of thermal decomposition of ammonium yttrium fluoride intermediates, (NH4)3Y2F9 and NH4Y2F7 were studied by a non-isothermal thermogravimetric (TG) technique at different heating rates. Kinetic parameters were determined from the TG curves by using two model-free methods, i.e. Flynn–Wall–Ozawa (FWO) and Kissinger–Akahira–Sunose (KAS). The activation energy (E) for the decomposition of (NH4)3Y2F9 was dependent on the fraction converted, α; its value was 152 kJ/mol (for α = 0.1–0.3) and 232 kJ/mol (for α = 0.4–0.8) from both models; and the pre-exponential factor (ln(A)) was 43 (for α = 0.1–0.3) and 59 (for α = 0.4–0.8). For the decomposition of NH4Y2F7, the activation energy (E) was independent of the fraction converted, α, and its value was 123 kJ/mol; the pre-exponential factor (ln(A)) was 31. The decomposition of (NH4)3Y2F9 proceeds by one-dimensional diffusion (for α = 0.1–0.3), followed by two-dimensional diffusion (for α = 0.4–0.8). The decomposition of NH4Y2F7 occurs by random nucleation and growth.
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