(716c) Measuring Thermal Histories Using Jarosite Particle Decomposition

Jarosites and alunites undergo thermal decomposition in a series of distinguishable steps, forming water and sulfur trioxide as volatile decomposition products. If the kinetics of this decomposition can be described reliably, then it is possible, with some limitations, to reconstruct the temperature history of a partially decomposed jarosite particle by examining its residual decomposition behavior. The use of temperature-monitoring particles is motivated by the need to forensically recover temperatures that exist deep inside high-heating-rate environments, such as chemical explosions, which are optically thick, and therefore inaccessible to optical sensing from the outside. Jarosite particles were synthesized by refluxing a solution of K2SO4 and Fe2 (SO4)3 at atmospheric pressure. The effective composition of the product is K0.9(H3O)0.1Fe3(SO4)2(OH)6. Particles are in the 10-20 µm range. Decomposition was characterized by thermogravimetric analysis. At heating rates from 2 K/min to 100 K/min, the material decomposes to form Fe2O3 and K2SO4 as solid residues by ~1000 K. Major decomposition steps can be distinguished as the loss of crystal water near 450 K, the loss of all OH near 650 K, and the loss of SO3 between 800 and 1000 K. From a set of measurements at different hetaing rates, activation energies of individual decomposition steps were determined by isoconversion analysis. A single-particle based decomposition model describing the evolution of concentric layers of intermediate decomposition products was established to describe the overall decomposition reaction. Results of experiments will be reported where jarosite is rapidly heated to intermediate temperatures, subsequently recovered, and re-analyzed at 5 K/min. The aim is to establish with what fidelity the parameters of the initial heating experiment can be determined.