Self-diffusion in a-Fe203 natural single crystals.
Data
1999
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Measurements of 180 self-diffusion in hematite (Fe203) natural single crystals have
been carried out as a function of temperature at constant partial pressure ao2 = 6.5"10 -2 in the
temperature range 890 to 1227 ~ The ao2 dependence of the oxygen self-diffusion coefficient at
fixed temperature T = 1150 ~ has also been deduced in the ao2 range 4.5" 10 .4 - 6.5" 10 1. The concentration
profiles were established by secondary-ion mass spectrometry; several profiles exhibit
curvatures or long tails; volume diffusion coefficients were computed from the first part of the
profiles using a solution taking into account the evaporation and the exchange at the surface. The
results are well described by
- 0 26 ( 542 (kJ/mol) ] D o (cm2/s) = 2.7.108 ao2' exp,- ~-
J
From fitting a grain boundary diffusion solution to the profile tails, the oxygen self-diffusion
coefficient in sub-boundaries has been deduced. They are well described by
- 0 4 ( 911 (kJ/mol) /
D" o (cm2/s) = 3.2.1025 ao2' exp,- RT
J
Experiments performed introducing simultaneously 180 and 57Fe provided comparative values of
the self-diffusion coefficients in volume: iron is slower than oxygen in this system showing that
the concentrations of atomic point defects in the iron sublattice are lower than the concentrations
of atomic point defects in the oxygen sublattice. The iron self-diffusion values obtained at T > 940
~ can be described by
DFe (cm2/s) 9.2 101~ -056 (578(kJ/mol) ) = 9 ao~ exp,-
The exponent-1/4 observed for the oxygen activity dependence of the oxygen self-diffusion in
the bulk has been interpreted considering that singly charged oxygen vacancies V~ are involved in
the oxygen diffusion mechanism. Oxygen activity dependence of iron self-diffusion is not known
accurately but the best agreement with the point defect population model is obtained considering
that iron self-diffusion occurs both via neutral interstitals Fe x and charged ones.
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AMAMI, B. A. et al. Self-diffusion in a-Fe203 natural single crystals. Ionics, v. 5, p. 358-370, 1999. Disponível em: <https://link.springer.com/article/10.1007/BF02376000>. Acesso em: 08 mar. 2017.