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Combined magnetic and structural investigation of nano crystalline iron oxides: The interplay between crystal size and phase composition during FeS2 oxidation

Eneroth, Erik LU (2007) In Journal of Magnetism and Magnetic Materials 308(2). p.250-268
Abstract
The oxides that form during thermal oxidation of natural FeS2 (pyrite and marcasite) consist of nanometer-sized crystals of alpha-Fe2O3 and gamma-Fe2O3. This is shown with heating experiments that were made up to 650 degrees C, which resembles temperatures used in metallurgical processes. It is shown that magnetic measurements can play a key role in the investigation of this reaction, due to the unwanted blurring effects associated with finite crystal sizes if other methods are used. According to Mossbauer spectra combined with pXRD, many alpha-Fe2O3 crystals are in a stable magnetic state only due to the formation of bridging superexchange interactions in between them, but the gamma-Fe2O3 experiences super-paramagnetic relaxation ceasing... (More)
The oxides that form during thermal oxidation of natural FeS2 (pyrite and marcasite) consist of nanometer-sized crystals of alpha-Fe2O3 and gamma-Fe2O3. This is shown with heating experiments that were made up to 650 degrees C, which resembles temperatures used in metallurgical processes. It is shown that magnetic measurements can play a key role in the investigation of this reaction, due to the unwanted blurring effects associated with finite crystal sizes if other methods are used. According to Mossbauer spectra combined with pXRD, many alpha-Fe2O3 crystals are in a stable magnetic state only due to the formation of bridging superexchange interactions in between them, but the gamma-Fe2O3 experiences super-paramagnetic relaxation ceasing first at 20 K. Magnetisation measurements were used for two main purposes (1) determination of the amounts of gamma-Fe2O3 in the products, and (2) for characterization of gamma-Fe2O3 with respect to crystal size and possible magnetic surface effects such as spin-glass. It is proven that fine FeS2 grains produce more gamma-Fe2O3 than coarse. At 500 degrees C the fine FeS2 grains oxidised into c. 30% gamma-Fe2O3 and ca. 70% alpha-Fe2O3. At 525 degrees C, the gamma-Fe2O3 amounts were also estimated in coarse oxidised FeS2, and results were ca. 20% and 10% gamma-Fe2O3 for the fine and coarse FeS2 respectively. The gamma-Fe2O3 crystal sizes were a function of both temperature and grain size, and it decreased with decreasing grain size, and upon rising the temperature from 450 to 550 degrees C. It is argued that the estimated errors during gamma-Fe2O3 amount determination are due mainly to disordered magnetic sublattices at the crystal faces of gamma-Fe2O3, giving an error of ca. 15% for those samples that have the smallest crystals. (Less)
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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
pyrite, oxidation, superparamagnetic, gamma-Fe2O3, alpha-Fe2O3, environmental
in
Journal of Magnetism and Magnetic Materials
volume
308
issue
2
pages
250 - 268
publisher
Elsevier
external identifiers
  • wos:000241713600012
  • scopus:33749664912
ISSN
0304-8853
DOI
10.1016/j.jmmm.2006.05.024
language
English
LU publication?
yes
id
8a6f4bfd-0348-4a3e-a806-fef8aca26a2a (old id 378352)
date added to LUP
2007-12-10 16:17:35
date last changed
2017-01-01 07:17:15
@article{8a6f4bfd-0348-4a3e-a806-fef8aca26a2a,
  abstract     = {The oxides that form during thermal oxidation of natural FeS2 (pyrite and marcasite) consist of nanometer-sized crystals of alpha-Fe2O3 and gamma-Fe2O3. This is shown with heating experiments that were made up to 650 degrees C, which resembles temperatures used in metallurgical processes. It is shown that magnetic measurements can play a key role in the investigation of this reaction, due to the unwanted blurring effects associated with finite crystal sizes if other methods are used. According to Mossbauer spectra combined with pXRD, many alpha-Fe2O3 crystals are in a stable magnetic state only due to the formation of bridging superexchange interactions in between them, but the gamma-Fe2O3 experiences super-paramagnetic relaxation ceasing first at 20 K. Magnetisation measurements were used for two main purposes (1) determination of the amounts of gamma-Fe2O3 in the products, and (2) for characterization of gamma-Fe2O3 with respect to crystal size and possible magnetic surface effects such as spin-glass. It is proven that fine FeS2 grains produce more gamma-Fe2O3 than coarse. At 500 degrees C the fine FeS2 grains oxidised into c. 30% gamma-Fe2O3 and ca. 70% alpha-Fe2O3. At 525 degrees C, the gamma-Fe2O3 amounts were also estimated in coarse oxidised FeS2, and results were ca. 20% and 10% gamma-Fe2O3 for the fine and coarse FeS2 respectively. The gamma-Fe2O3 crystal sizes were a function of both temperature and grain size, and it decreased with decreasing grain size, and upon rising the temperature from 450 to 550 degrees C. It is argued that the estimated errors during gamma-Fe2O3 amount determination are due mainly to disordered magnetic sublattices at the crystal faces of gamma-Fe2O3, giving an error of ca. 15% for those samples that have the smallest crystals.},
  author       = {Eneroth, Erik},
  issn         = {0304-8853},
  keyword      = {pyrite,oxidation,superparamagnetic,gamma-Fe2O3,alpha-Fe2O3,environmental},
  language     = {eng},
  number       = {2},
  pages        = {250--268},
  publisher    = {Elsevier},
  series       = {Journal of Magnetism and Magnetic Materials},
  title        = {Combined magnetic and structural investigation of nano crystalline iron oxides: The interplay between crystal size and phase composition during FeS2 oxidation},
  url          = {http://dx.doi.org/10.1016/j.jmmm.2006.05.024},
  volume       = {308},
  year         = {2007},
}