Thermal properties of nanocrystalline goethite, magnetite, and maghemite

This study investigated the high-temperature properties of nano-goethite, nano-magnetite, and nano-maghemite. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of these nano-minerals revealed that phase-transition temperatures increased with increasing particle sizes. This was due to the high surface energy of the nano-minerals with small particle sizes, which caused them to require lower energies to change their structures, and thereby, resulted in lower phase-transition temperatures. Further, the transition temperatures measured by ex situ and in situ high-temperature X-ray diffraction (XRD) were lower than those observed by TGA–DTA. This may be due to the difference in the atmosphere and heating time in the high-temperature processes. In this study, we found that the particle size of nano-minerals, atmosphere (oxygen pressure), and heating time were the key factors influencing the transition temperature, phase, and path. TGA–DTA measurements gave the initial clue to understand the phase transition, and in situ high-temperature XRD measurements helped elucidate the exact phase-transition behavior of the nano-minerals.

► High-temperature properties of iron oxide/hydroxide nano-minerals are studied.
► Phase-transition temperatures of nano-minerals increase with larger particle sizes.
► In situ high-temperature XRD results show reductive reactions in the final state.
► Particle size and atmosphere affect the transition temperature, phase, and path.