Materials Science and Engineering B106 (2004) 6–26
Structural and chemical evolution of Fe–Co–O based ceramics under
reduction/oxidation—an in situ neutron diffraction study
Yaping Li
a
, Evan R. Maxey
a
, James W. Richardson, Jr.
a,∗
, Beihai Ma
b
a
Intense Pulsed Neutron Source, Argonne National Laboratory, Argonne, IL 60439, USA
b
Energy Technology Division, Argonne National Laboratory, Argonne, IL 60439, USA
Received 10 March 2003; accepted 7 July 2003
Abstract
Air-sintered ceramic samples in composition CoO·nFe
2
O
3
(n = 1 and 2) were prepared in solid-state reactions, resulting in a single spinel
phase with composition CoFe
2
O
4
, and a two-phase mixture of identical spinel with -Fe
2
O
3
for n = 1 and 2, respectively. Their structural and
chemical evolution over pO
2
range of 10
-0.9
to 10
-19
atm was investigated using in situ neutron diffraction at isothermal condition (∼900
◦
C).
Neutron diffraction data were analyzed through Rietveld refinements. The following sequences of structural transformation from -Fe
2
O
3
hematite → (Fe, Co)-spinel → (Fe, Co)
1-x
O wustite → (Fe, Co)O rocksalt → -(Fe, Co) → -(Fe, Co) → (-(Fe, Co)) were observed
on the reduction of Fe–Co–O based ceramics. With the development of reduction at pO
2
down to 10
-15
atm, mixed valence (Fe
2+
and Fe
3+
)
spinel was first formed in Fe-excess (Fe, Co) spinel phases. The intermediate phases were usually Co-rich compared with their parent mixed
oxide phases. Particularly, the initial metallic precipitate is Co-rich , independent of initial stoichiometry. Reduction kinetics at pO
2
of
∼10
-19
atm is extremely fast, but crystalline form and structural integrity are maintained. As crystal structures of the various involved phases
are very similar, few structural blocks were disturbed as oxygen was released from the samples. In addition, re-oxidation behavior of reduced
products was also studied, and phase composition and microstructure of post-neutron experiments were characterized by X-ray diffraction
and scanning electron microscopy.
© 2003 Published by Elsevier B.V.
Keywords: Iron oxide; Cobalt oxide; Ceramics; Metals; Neutron scattering; Phase transitions
1. Introduction
Dense ceramic membranes in nominal composition of
SrFeCo
0.5
O
x
(designated SFC2) have potential application
for oxygen separation due to their mixed conducting behav-
iors at elevated temperature. Since such membranes were
successfully used for partial oxidation of methane to syn-
gas (CO + H
2
) [1], SFC2 and related materials have re-
ceived considerable attention [2–7]. We are interested in
the quasi-binary Fe–Co–O system under low oxygen partial
pressure for several reasons.
First, it has been clear that SFC2 is not a single phase ma-
terial under operating conditions. At least four phases have
been identified in SFC2, depending upon ceramic processing
and pO
2
, including Sr
2
(Fe, Co)
3
O
6.5-x
(236 with structure
∗
Corresponding author. Tel.: +1-630-252-3554;
fax: +1-630-252-4163.
E-mail address: jwrichardson@anl.gov (J.W. Richardson Jr.).
of Sr
4
Fe
6
O
13
[8]), Sr(Fe, Co)O
3-y
(perovskite), (Fe, Co)O
(rocksalt), and (Fe, Co)
3
O
4
(spinel) [9,10]. Mitchell et al.
[11] have investigated the structural behavior of single phase
perovskite in composition SrFe
0.8
Co
0.2
O
3-δ
with a decrease
of pO
2
value down to 10
-13.3
atm. In their studies, cubic
perovskite phase is stable over the entire pO
2
range although
oxygen vacancy concentration is increased with decreasing
pO
2
. However, membrane tubes made of SrCo
0.8
Fe
0.2
O
x
perovskite exhibited a different behavior both in an experi-
ment where exposed on one side to air and the other side to
methane [12] and in our in situ neutron diffraction study un-
der single pO
2
mode (unpublished). Therefore, the stability
of perovskite not only depends upon pO
2
level but also pO
2
gradients and composition. Up to now, structural and chem-
ical pathways of other potentially vulnerable components of
SFC2 under a wide range of pO
2
values are still lacking.
But this information is important since one side of the SFC2
ceramic tube will be subjected to methane streams with pO
2
about 10
-19
atm, and the structural stability of SFC2 under
0921-5107/$ – see front matter © 2003 Published by Elsevier B.V.
doi:10.1016/j.mseb.2003.07.004