Physico-Geometrical Kinetics of the Thermal Decomposition of ε-Zinc Hydroxide with Different Particle Sizes and Shapes

The thermal decomposition of ε-Zn(OH)2 with varying particle sizes and shapes was investigated to elucidate the interrelationships among the phenomenological reaction behavior, the kinetic characteristics, and the physico-geometrical constraints of the reaction, which determine the specific reaction behavior that leads to the formation of ZnO. The insensitivity of the reaction kinetics to the increase in the atmospheric water vapor pressure (p(H2O)) was identified as the phenomenological characteristics of the reaction. The presence of the significant induction period (IP) and subsequent sigmoidal mass loss curve under isothermal conditions was the kinetic characteristics of the reaction. The formal kinetic analysis detected the variation in the kinetics of the IP process with the particle size and shape of the sample. The mass loss process was characterized by the declining activation energy in the initial stage of the reaction and its convergence to 100 kJ mol–1 in the subsequent stage for the process with a maximum reaction rate midway through. The initiation of the mass loss process was characterized by the surface reaction (SR) of the instantaneous nucleation and two-dimensional growth of the nuclei, which produced the crater-like holes and solid product peels. The subsequent reaction within the individual particles was characterized by the phase boundary-controlled reaction (PBR). Consequently, the overall process was described by the physico-geometrical consecutive IP–SR–PBR process. The specific features of the thermal decomposition process were described with particular attention directed toward the IP process, which was identified as the predominant process to determine the reaction initiation, and the subsequent mass loss process occurring under conditions of self-generated p(H2O).