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. (C) 2012 Elsevier B. V. All rights reserved.