By calcining in air (not in dry oxygen or anything like that)?. If so, what are the temperature and duration required to achieve such transformation. The magnetite and maghemite are in the form of nanorods. Thanks.
Maghemite and magnetite converts to hematite at around 365 degree C in air atmosphere. To ensure complete conversion one can use 400 degree C or higher temperature for 6-8 hours. Anyway if you don't worry about the particle size then annealed higher temperature (something around 600 degree C) as annealing at higher temperature will lead to higher particle size.
There are interesting evidence of laser-induced modifications of magnetite. See
Olga N. Shebanova*, Peter Lazor
"Raman study of magnetite (Fe3O4): laser-induced thermal effects and oxidation"
Journal of Raman Spectroscopy, Volume 34, Issue 11, pages 845–852, November 2003
Based on my personal experiences, I observed the phase transformation of both the magnetite and maghamite (deposited on glass) to pure hematite (alpha-Fe2O3) at around 350 C by annealing for 4 h at heating rate 5 C/min, in an open environment (in a quartz tube furnace:Lindberg/Blue M Company). Note that the actual temperature measured inside the tube was slightly less than the actual temperature shown by the system (about 20 C, at this temp). And the same phase were observed by increasing the annealing temperature to 450 C and 500 C.
Hope, this will help you.
Thanks!
The transformation occurs by annealing at 350 C and is completed at 550 C. The annealing time we used was 18 hours in air. Further, at temperatures higher than 350 C the particle size increased significantly, as pointed out by Samar. For more details see our publication
A-F. Lehlooh, S. Mahmood, I. Abu-Aljarayesh, J. Magn. Magn. Mater. 136 (1994) 143-148
hematite is the most stable form of iron oxide. Other two phases can transfer in to hematite by calcining. However, temperature can be vary. Temperature from 350 to 450. Particle size will increase.
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