Oxygen transport and surface exchange mechanisms in LSCrF-ScCeSZ
dual-phase ceramics
Zonghao Shen*, Stephen J. Skinner, John A. Kilner
Department of Materials, Imperial College London, Exhibition
Road, London SW7 2AZ, UK
Corresponding Author Email: [email protected]
Abstract
For the mechanisms by which the oxygen corporates in a
dual-phase composite system, three hypotheses i.e. cation
inter-diffusion, spillover type and self-cleaning of the
perovskite-structured phase, have been provided in literature.
However, experimentally consensus on the most likely mechanism has
yet to be reached. In this work, a specially fused sample of the
Lanthanum strontium chromium ferrite
(LSCrF)-Scandia/Ceria-stabilised zirconia (ScCeSZ) dual-phase
materials was investigated. Among the three potential mechanisms,
no obvious cation inter-diffusion was firstly observed. A cleaner
surface of the ScCeSZ phase was confirmed in the fused sample
compared to the isolated ScCeSZ single-phase sample while impurity
layers were clearly observed on the LSCrF surface, suggesting the
cleaning effect from the perovskite. However, more evidence implies
the cleaning effect is not the only reason for the synergistic
effects between these two phases. Observations via SIMS analysis
lend strong support to the ‘spillover-type’ mechanism as the oxygen
isotopic fraction on the surface of the ScCeSZ increased compared
to the isolated single-phase and as the distance to the
heterojunction increases, the oxygen isotopic fraction decreases.
Moreover, oxygen depleted layers were clearly seen on the top
layers of the LSCrF surface which may be associated with the higher
oxygen diffusivity in the surface/sub-surface layers, oxygen grain
boundary fast diffusion and the impurities on the perovskite phase.
For this sample, a combination of ‘spillover’ and ‘self-cleaning’
type mechanisms is suggested to be the potential possibilities
while the contributions from the cation inter-diffusion for this
specific sample is proven to be low.
Keywords: dual-phase composite, oxygen diffusion, cleaning
effect, spillover, cation interdiffusion
Introduction
An understanding of the transport mechanisms of oxygen in
ceramic oxide materials is fundamental to many physical and
chemical phenomena, relevant to technological applications as
diverse as Solid Oxide Fuel Cells (SOFCs), Oxygen Transport
Membranes (OTMs), -sensors etc. This understanding can be difficult
to obtain for many of the complex ceramic oxide systems of
technological interest and is dependent upon the accurate
measurement of the kinetics of oxygen exchange by a variety of
methods. Oxygen exchange has been intensively studied over the past
40 years by the oxygen Isotopic Exchange Depth Profiling (IEDP)
technique, for example, in the literature, oxygen diffusion kinetic
parameters of the isolated single phase materials e.g. (La,Sr)MnO3
(LSM)1 (La,Sr)(Mn,Cr)O32, (La,Sr)(Mn,Co)O3 (LSMC)3,4,
Yittra-Stabilised-Zirconia (YSZ)5–8, (La,Sr)(Co,Fe)O3 (LSCF)9,
La2-xSrxNiO4+δ 10 Ceria-doped-Gallium (CGO)11,12 etc. have been
reported. This data has allowed a deeper understanding of the
mechanisms of oxygen transport in these single phase ceramic
materials.
Unfortunately, this is not the case for dual phase ceramic
composites, commonly used as OTMs or SOFC cathodes, where many
complex effects can take place between electronically conducting
perovskite structured component and the fluorite structured ionic
conducting component. The volume fraction of each component in the
composite and their distribution have important extra effects.
Investigations of the oxygen surface exchange and bulk diffusion in
dual-phase composites are relatively scarce because of the added
complexity of the systems. Two different types of kinetic data can
be obtained from ceramic composites using the IEDP technique. By
analysing over large areas (e.g. 2-300 μm), much larger than the
grain size of the component oxides, ‘effective’ diffusion and
exchange parameters can be obtained. If the microstructural
parameters are known, for example for a composite electrode, then
these ‘effective’ parameters can be used to compare with
electrochemical measurement, as the electrochemical measurements
also integrate over large length scales.13 More information can be
gained by using high lateral resolution techniques such as Focused
Ion Beam SIMS (FIB-SIMS) capable of analysis within the component
grains, and in this case the kinetic parameters for each phase,
when embedded in a composite can be obtained for comparison with
the ‘effective’ parameters and for those of the isolated single
phase materials. 14
Oxygen diffusion data has been reported in the literature for
the LSM-YSZ 15–17 and LSCF-CGO 13,14,18 systems. There are some
similarities in the reported behaviour and some important
differences. For example, an enhancement in the ‘effective’ surface
exchange coefficients has been observed in the LSM-YSZ dual phase
system as a function of the volume fraction of the components,
suggesting a synergistic effect. On the contrary the LSCF-CGO
system showed a scattered variation of the oxygen surface exchange
with the volume fraction of the LSCF phase. Evidence of a
synergistic effect was apparent for both systems when the
microscopic kinetic parameters for the individual grains was
examined. At a given temperature and under identical conditions,
the oxygen surface exchange rate decreased for the electronically
conducting perovskite phase but was enhanced for the ionic
conductor. In all cases the values obtained for the composite were
compared to the values for the isolated single phases.
Based on the previous studies on the LSM-YSZ and LSCF-CGO
systems, three hypotheses of the potential mechanisms have been
suggested for the observed microscopic behaviour 14,16,18, namely a
‘spillover’ type mechanism, a ‘cleaning’ effect of the perovskite
and transition metal inter-diffusion.
1. ‘Spillover’ type mechanism: the conventional understanding
for isolated single-phase material is that the whole surface
exchange reaction is assumed to proceed by several intermediate
steps and each step can be rate-limiting. For an isolated
electronically conducting phase with adequate surface adsorption
sites for oxygen reduction and dissociation, oxygen is easily
dissociated and reduced on the surface but a low oxygen vacancy
concentration limits the bulk diffusion process (e.g. LSM). On the
contrary, for an isolated ionic conducting phase (e.g. YSZ), though
the high oxygen vacancy concentration provides a high oxygen
diffusivity, the low concentration of electronic species and
surface adsorption sites limit the reduction and dissociation of
oxygen molecules. However, in the dual-phase composite material,
the reduced and dissociated oxygen species are able to migrate
across the triple-phase boundaries (TPBs) on the surface from the
perovskite phase onto the ionic conducting phase surface and then
diffuse through the ionic conducting phase. Fig. 1 is a schematic
presenting the ‘spillover’ process where the electronic conductor
functions as a source of reduced and dissociated oxygen species and
the ionic conductor functions as a sink for oxygen bulk
diffusion.
Figure 1 Schematic of the ‘spillover’ type mechanism (k is the
oxygen surface exchange coefficient)
1. ‘Cleaning’ effect of the perovskite phase: For the isolated
single-phase fluorite-structured ionic conducting materials, the
surface is likely to be covered by a passivation layer of the
common impurities: e.g. silica or calcia and this impurity layer
further depresses the oxygen exchange rate on the surface of the
ionic conductor 19. However, for a mixed conductor with the
perovskite structure, it normally presents a ‘clean’ surface
because its structure is capable of tolerating many impurity
elements 14,20. Thus, in dual-phase composite materials, the ionic
conductor is continuously cleaned by the dissolution of impurities
into the perovskite lattice which is believed to be beneficial for
the oxygen surface exchange process of the ionic conducting
phase21.
1. Transition metal inter-diffusion: during the sintering
process transition metal elements, e.g. Fe/Co/Cr, are likely to
migrate onto the surface of the ionic conductor. Therefore, the
reduced transition metal concentration on the surface of the mixed
conductor is considered to be responsible for the decreased k while
the enhancement of the surface exchange kinetics of the ionic
conductor is activated by the presence of the transition metal
species 22.
However, with limited investigations there is no consensus on
the most likely mechanism whereby the oxygen diffuses through the
dual-phase system and the validity of these hypotheses has not been
visually confirmed or excluded except that the possibility of the
third hypothesis is suggested to be low for the LSCF-CGO dual-phase
system 14.
In the recently reported LSCrF-ScCeSZ dual-phase composite
system 23, the basic understanding of the diffusion mechanism is
that the surface exchange process is predominantly performed by the
LSCrF perovskite phase while the dominant phase for the bulk
diffusion is the pure ionic conducting phase ScCeSZ. Moreover,
similar to the LSCF-CGO and LSM-YSZ systems, an increased surface
exchange rate for the ScCeSZ phase and a decreased surface exchange
rate for the LSCrF were also observed. In this present work, the
(La0.8Sr0.2)0.95Cr0.5Fe0.5O3-δ (LSCrF) and 10 mol%Sc2O3-1
mol%CeO2-89 mol%ZrO2 (ScCeSZ) powders were used as the starting
materials to investigate the possible mechanisms by which the
oxygen transports in this dual-phase composite material. To obtain
further understanding of the diffusion mechanisms, a specially
designed pellet was fabricated and a combination of Secondary Ion
Mass Spectrometry (SIMS) and Low Energy Ion Scattering (LEIS) as
well as Energy Dispersive X-ray (EDX) spectroscopy was performed on
this sample.
Experimental
To obtain an understanding of the potential diffusion mechanisms
in the (La0.8Sr0.2)0.95Cr0.5Fe0.5O3-δ (LSCrF)-10 mol% Sc2O3-1 mol%
CeO2-89 mol% ZrO2 (ScCeSZ) dual-phase composite system, studies
have been performed on a thermally fused specimen composed of two
polished ceramic pellets (LSCrF and ScCeSZ). Fig. 2 presents a
schematic of the fabrication procedures used to produce the samples
for the following experiments.
The powders of the LSCrF and ScCeSZ were both directly supplied
by Praxair Inc., USA. The green bodies of the two pellets were
achieved by uniaxial pressing and successive isostatic pressing.
The individual pressed pellets of each material were sintered at
1450 for 6 hours with a heating and cooling rate of 5 min-1. The
single phase sintering results in a larger grain size than is
typically found when sintering the composite at the same
temperature because of the heterogeneous nature of the grain to
grain interfaces. Before thermally fusing the two pellets together,
they were both ground using successive grades of SiC papers and
finally polished down to ¼ μm finish by using diamond suspensions.
This preparation process followed the same procedure as reported
earlier to prepare samples for isotopic exchange measurements 24.
The ScCeSZ pellet was then placed upon the LSCrF pellet with the
polished surfaces in contact with each other. In order to obtain a
better contact between the two pellets, an alumina crucible was
placed on top of the ScCeSZ piece in order to introduce pressure
during the heat treatment at 1400 for 4 hours. After fusing the two
pellets into one, a small piece was cut from this thermally fused
pellet and the cross section of this piece was subsequently ground
and polished down to a mirror finish (1/4 μm final polish, as
above) for the following oxygen isotopic exchange experiment. The
dry oxygen isotopic exchange (<1 vppm H2O) was performed at 900
in the enriched 18O2 atmosphere for 0.5 hr after a 15 hr pre-anneal
in a dry, research grade (>99.999%) 16O2 atmosphere. The
detailed information on the oxygen isotopic exchange procedure can
be found in 24.
Figure 2 Schematic of the procedures used to obtain the fused
sample for subsequent transport property measurement
Transition metal diffusion was firstly investigated by Scanning
Electron Microscopy (SEM)-Energy Dispersive X-ray (EDX)
spectroscopy. The instrument used was a JSM-6400 SEM (JEOL Ltd.)
equipped with an Oxford Instruments INCA EDS. A combination of the
Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS, ION-TOF
GmbH, Germany) and Qtac100 Low Energy Ion Scattering (LEIS, ION-TOF
GmbH, Germany) was further used to investigate the oxygen diffusion
behaviour and surface composition of each phase in both the
thermally fused sample and the corresponding isolated single-phase
pellets.
Additionally, in this work a different sample was also prepared
which will be presented in the section 3.2.2 Figure 7. The ScCeSZ
green body was placed upon the LSCrF green body after isostatic
pressing and the sample was sintered at 1450 for 6 hours. Then a
small piece was cut from this thermally fused pellet and the cross
section of this piece was subsequently ground and polished before
the SEM-EDX analysis.
In this work, three different environments for the two phases
will be mentioned with the oxygen diffusion measurements. In order
to avoid any misunderstanding, the three different samples will be
explained here: LSCrF or ScCeSZ isolated sample is the single phase
sample; the fused pellet is the specially prepared sample in this
work (Figure 2) and the LSCrF-ScCeSZ composite sample is a mixed
dual-phase sample with details in 23.
Results and DiscussionsTransition Metal Inter-Diffusion
Fig. 3(a) is the secondary electron image of the cross section
of the fused pellet with the left of the image corresponding to the
ScCeSZ phase while the right of the image corresponds to the LSCrF
phase, showing the EDX acquisition positions. 11 points were
analysed with a gap of 3 μm between adjacent points, to allow for
the low lateral resolution of EDX which is limited to around 1μm
25. The longest distance between the analysed points and the
heterojunction was 15 μm for both phases. Fig. 3 (b) presents the
EDX results of the sample reflecting the distribution of the metal
elements across the interface of the two phases. The standard
deviation was calculated from the parallel acquisition points and
the absence of some error bars is because they are smaller than the
data symbols. The atomic fraction of cerium, which is around the
detection limit of the EDX technique, has not been presented here.
However, a previous study 23 on the homogenously mixed the
dual-phase composite samples reveals the possibility of Ce
diffusion into the LSCrF perovskite lattice during sintering and
the effects of this Ce diffusion process on the oxygen diffusion in
the dual-phase composite pellet remains unknown and will not be
further discussed.
Figure 3 EDX analysis across the interface of the two phases:
(a) secondary electron image showing the acquisition positions
(green cross) (b) calculated atomic fraction of each metallic
element
An obvious and relatively sharp interface can be clearly
observed in Fig. 3 (b) and in the bulk area the cation ratios are
in good agreement with the theoretical values (Cr:Fe = 1:1, Zr:Sc =
8.9:2) with slightly lower Sr content (La:Sr>4:1). At the
positions close to the heterojunction, the contents of Sr, Fe and
Zr varied from the theoretical values, reflecting the phenomenon of
cation intermixing at an annealing temperature lower than the
normal sintering temperature for dense ceramic membrane. In the
later case, where the dual-phase materials are sintered at 1450 for
6 hrs, the influence of cation inter-diffusion should be more
significant because of the smaller grain size found for the
sintered composite, however the magnitude of this effect remains to
be explored. For the samples in this work where the distance from
the interface was greater than 15 m, cation inter-diffusion was
negligible. Since the following SIMS and LEIS measurements were
carried out in areas with a much longer distance to the
heterojunction, the influence of the cation intermixing on the
oxygen diffusion in the following studies is expected to be
low.
SIMS Analysis
Fig. 4 displays the positions of the SIMS analysis areas
(analysis area: 100 x 100 μm2; sputtering area: 300 x 300 μm2) in
the two phases and the distance from the centre of each acquisition
area to the heterojunction. In the ScCeSZ part four areas were
analysed with the distance from 120 μm to >1000 μm and in the
LSCrF part, one area was measured with a distance of 140 μm from
the heterojunction. The distances were estimated using the ToF-SIMS
Surfacelab software and the depth profiles were subsequently
recorded in these areas by using ToF-SIMS.
Figure 4 Schematic showing the analysis areas in both phases and
the distance from the centre of the acquisition area to the
interface. The sample was isotopically exchanged at 900 for 0.5 h
before SIMS analysis.
ScCeSZ Phase
Fig. 5 shows the normalised 18O fractions from area No.1 to area
No.4 obtained using the depth profiling mode at the surface of the
ScCeSZ phase in the fused pellet as a function of sputtering time
(Fig. 5a) and as a function of distance from the heterojunction
(Fig. 5b). The normalised 18O fraction obtained from the isolated
ScCeSZ phase sample, which is plotted as the gold shaded area, is
also presented in Fig. 5(b). Though the isotopic fractions display
some scatter, with the low counting statistics, it still clearly
shows that as the distance to the interface increased, the
normalised 18O surface fraction of the ScCeSZ phase decreased.
Moreover, if the acquisition area was significantly far away from
the heterojunction (~1500 μm, area 4), the difference between area
No.4 and the isolated ScCeSZ pellet was negligible. Based on the
observations in Fig. 5, compared with the isolated pellet, when the
ionic conductor ScCeSZ is adjacent to the LSCrF phase, an apparent
enhancement in the surface exchange process of the ionic conductor
was confirmed, and this enhancement was observed to be related to
the distance to the heterojunction.
If this isotopic distribution is thought of as a lateral
diffusion profile away from the hetero-junction, then a solution to
the diffusion equation for a semi-infinite medium 26 can be applied
to the four points and a diffusion coefficient and an ‘exchange
coefficient’ can be obtained. This yields values of D* = 2.410-6
cm2 s-1 and k = 8.810-7 cm s-1, respectively. The estimated
‘exchange coefficient’ for the ScCeSZ in the fused pellet is of the
same order of magnitude as the surface exchange coefficient of the
isolated LSCrF pellet (k = 2.110-7 cm s-1 24). Due to the sparsity
of data, the lateral D* and k* of the ScCeSZ are only estimated
values with a high uncertainty, hence the exchange coefficients
here can be considered as in agreement with each other. However, if
the lateral oxygen diffusion coefficient, in the ScCeSZ from the
heterojunction of the fused sample is compared to the value of the
isolated ScCeSZ phase from surface to bulk (D*=4.710-7 cm2 s-123),
this lateral diffusion coefficient is almost one order of magnitude
higher. Based on this result, it is probable that what is observed
here is not caused by lateral diffusion from the interface but is
caused by another mechanism. This could be spillover of oxygen
species from the LSCrF phase or exchange between the gas phase and
the ScCeSZ where the surface of the ScCeSZ has lower content of
blocking impurities on the surface as the interface is approached,
and hence a higher local value of k*, i.e. the so called cleaning
effect.
Figure 5 Normalised 18O fraction of the ScCeSZ surface at areas
1-4 with the increased distance from the interface (a) as a
function of sputtering time (the straight lines are the averaged
values of the corresponding area) (b) as a function of the distance
from the heterojunction. Golden line represents the averaged result
obtained from an isolated ScCeSZ single phase sample isotopically
exchanged at the same temperature for the same duration (900, 0.5h)
and the golden area includes the error bars. The red line is the
fitted oxygen diffusion profile determined from the four depth
profile data displayed in Fig. 5(a).
LSCrF Phase
In contrast to the ScCeSZ phase where an increased oxygen
isotopic fraction has been observed compared to the isolated
single-phase material, the LSCrF phase in the fused sample presents
a more complex distribution of the oxygen isotopic fraction in the
outermost layers. Fig. 6 displays the diffusion profiles of the
LSCrF phase as a component in the fused pellet (Fig. 6a) and the
isolated LSCrF phase 24. Clearly, the diffusion profiles of the
isolated phase and the LSCrF phase in the composite pellet present
different behaviour:
1. For the isolated LSCrF single phase, the normalised 18O
surface fraction is around 90% which falls in the region of high
where the Isotope Exchange Depth Profile (IEDP) method starts to
lose accuracy in the measurement of the surface exchange
coefficient 27 because the associated and reduced oxygen species
accumulate on the sample surface and wait to diffuse into the bulk
due to its low bulk diffusion. For this kind of material, a bulk
diffusion limited mechanism was confirmed and, realistically, only
a lower bound of the surface exchange coefficient can be achieved.
24
1. However, for the LSCrF phase in the fused pellet, the
normalised 18O fraction of the acquisition area with a distance of
140 m to the heterojunction dramatically drops to 47% on the
surface compared with the isolated single phase LSCrF where 90% was
detected. Moreover, within the first 10 nm beneath the sample
surface, very clear oxygen depletion layers were observed. (Fig. 6c
highlights the near surface area in Fig. 6a.)
Figure 6 Diffusion profiles of the LSCrF8255 (a) as a component
in the fused pellet isotopically exchanged at 900 for 0.5 h; (b)
isolated single phase pellet isotopically exchanged 900 for 0.43 h
(from 24); Figure (c) magnifies the near surface area of Fig.
(a).
Combined with the data obtained from the ScCeSZ phase where the
surface exchange coefficient increased as the distance to the
interface decreased, this significant depletion of the 18O fraction
in the LSCrF phase lends support to the ‘spillover’ type mechanism.
For the isolated LSCrF phase, low oxygen vacancy concentration
limits the bulk diffusion process, and therefore the reduced oxygen
species accumulate on the surface. The normalised 18O concentration
on the isolated single-phase LSCrF sample surface is close to the
gas phase composition, with further detailed discussion available
in the previous work24. For the isolated ScCeSZ sample without an
active surface catalyst layer, oxygen exchange is severely limited.
For the fused pellet, a much higher surface 18O fraction was seen
in the ScCeSZ material while a much lower 18O fraction was observed
in the surface regions of the LSCrF material. This implies that the
reduced and dissociated oxygen species on the LSCrF surface can
easily transport across the heterojunction and then diffuse into
the bulk through the ionic conductor, ScCeSZ. More interestingly,
an oxygen depleted surface layer or an uphill-like behaviour was
observed in the LSCrF component of the fused pellet. In the
literature, more obvious oxygen uphill diffusion behaviour was
reported by Huber et al 28 on an LSM thin film on a YSZ substrate
by applying a cathodic bias. This behaviour was suggested to be
attributed to a combination of fast grain boundary diffusion, a
cathodic bias and Wagner-Hebb-type stoichiometry polarization of
the LSM bulk. In the current work, no cathodic bias was applied but
other possible explanations may contribute to the low surface
concentration of 18O on the LSCrF phase.
1) Previous studies by Staykov et al 29 on SrTiO3 revealed
interesting findings on oxygen dissociation on Sr-O terminated
perovskite surfaces, as is the case for LSCrF, (see section 3.3.2)
where oxygen vacancies were considered as the active sites and
catalysts for O-O bond cleavage. They also found that the energy
required for oxygen diffusion between the surface vacancy and
sub-surface vacancy was proven to be low compared to migration of
these species across the surface. From the information here, a
possible picture may be painted that the dissociated oxygen species
on the LSCrF surface can either come over the TPBs on the surface
and jump to the vacant site in the ScCeSZ phase or jump to the
vacant site in the sub-surface of the LSCrF phase and then
transport across the heterointerface in the sub-surface layers into
the ScCeSZ phase. Hence oxygen species diffuse from the LSCrF phase
to the ScCeSZ phase not only on the surface but also in the
sub-surface. This is different from the conventional understanding
of a ‘spillover’ type mechanism that the reduced and dissociated
oxygen species diffuse only from the LSCrF surface onto the ScCeSZ
surface.
2) The grain boundary fast diffusion behaviour in the isolated
(La0.8Sr0.2)0.95Ce0.7Fe0.3O3-δ (LSCrF73) phase has already been
reported [24]. Though in the (La0.8Sr0.2)0.95Ce0.5Fe0.5O3-δ
(LSCrF55 or LSCrF in this work) the grain boundary diffusion is not
prominent at high temperatures, combining the diffusion coefficient
of the isolated LSCrF at 900 with the critical diffusion
coefficient for the obvious grain boundary fast diffusion 30, it is
likely that at 900 the LSCrF phase is in the transition from
Harrison type B to type A preferential diffusion 31. The reduced
and dissociated oxygen species may still tend to diffuse along the
grain boundaries directly into the bulk. Combined with the above
potential reason, a schematic is provided in Figure 7. Because the
oxygen diffusivity in the outermost layers is higher than that in
the bulk and oxygen species tend to diffuse along grain boundaries
into the bulk of the LSCrF phase, the 18O concentration was diluted
in the surface and sub-surface layers of the LSCrF.
Figure 7 Schematic of the potential mechanism of the up-hill
like behaviour in the LSCrF phase in the fused pellet where Ds, Dgb
and Db are the diffusion coefficients of surface/sub-surface, grain
boundaries and bulk. The green curves in each phase are the
normalised 18O fraction in each phase.
3) The 18O depleted layers of the LSCrF phase could be due to
compositional change on the surface. The compositional change can
be divided into two different categories. Firstly, despite the EDX
results, Fig. 3, suggesting that transition metal inter-diffusion
is negligible in this sample, taking the depth resolution of EDX
into account, the EDX results reflect the bulk behaviour instead of
the behaviour of the sample surface, given that the region showing
the depleted isotopic fraction is only ~ 10 nm thick. Moreover, the
18O depleted layers could be related to the impurity layers on the
LSCrF phase in the fused pellet, and the impurity layers may also
be within the range affected by SIMS ion beam mixing. Thus, further
studies using Low Energy Ion Scattering (LEIS) may provide
compositional information on the sample surface to a depth of only
1-2 nm, with atomic layer resolution, of the LSCrF material but Zr
and Sr may be too close to be resolved in the LEIS spectrum.
Details will be provided in the next section: 3.3 - Analysis of
Surface Composition by LEIS.
The kinetic parameters for diffusion and exchange were further
extracted from the diffusion profiles using TraceX 32,33. Table 1
lists the oxygen diffusion data for the LSCrF phase in 3 different
environments: the two described above in Fig. 6, and the
diffusivity data from an intragranular depth profile in the LSCrF
grains in a LSCrF-ScCeSZ dual-phase pellet detailed in an earlier
publication 23.
Table 1 Oxygen diffusion and surface exchange coefficients of
the LSCrF phase in different sample configurations
Samples
D* (cm2 s-1)
k (cm s-1)
LSCrF55 (isolated bulk sample)
3.710-12
2.210-7
LSCrF55 (fused pellet)
4.410-12
1.510-7
LSCrF55 (in dual-phase composite pellet)23
1.110-12
1.210-8
In order to achieve a fit to the diffusion profile of the LSCrF
phase in the fused pellet, the top layers with severe oxygen
depletion were excluded from the fit, which may result in an
overestimation of the surface exchange coefficient. Clearly, among
the three samples, the surface exchange coefficient of the LSCrF in
the dual-phase pellet is one order of magnitude lower than the
isolated bulk pellet and is more affected by the ScCeSZ phase.
Excluding the systematic errors, the different surface exchange
coefficients between LSCrF in the fused pellet and LSCrF in the
dual-phase composite pellet (obtained from an intragranular depth
profile) 23 can be attributed to the following possibilities:
1. The lateral length scale in the two experiments are
different. The grain size of the mixed dual-phase pellet (~20 μm)
is much smaller than the distance between the acquisition area and
the heterojunction in the fused pellet (>100 μm). As a result,
the influence of the ionic conductor phase is limited in the fused
pellet compared with the dual-phase pellet while in the dual-phase
pellet the reduced and dissociated oxygen species can more easily
overcome the heterojunction and then be diffused by the pure ionic
conductor into the bulk resulting in the lower 18O surface fraction
and surface exchange coefficient of the LSCrF phase.
1. The sample for this study was thermally fused at a
temperature lower than the normal sintering temperature and for a
shorter duration. The dual-phase pellet was produced by the normal
sintering routine where more cation inter-diffusion should occur
due to the increased temperature and time. To illustrate this
difference, a separate experiment was performed where LSCrF and
ScCeSZ green bodies were sintered together, to compare with the
earlier experiment where pre-sintered single-phase bodies were
fused together. Similar to the first experiment, a small piece was
cut from the sintered pellet and the cross-section of the pellet
was ground and polished. Fig. 8 presents the elemental maps across
the interface of the sintered pellet where clear Sr segregation can
be seen at the interface and substantial Fe, Sr and Sc
inter-diffusion is clearly observed. Hence for the dual-phase
pellet the effects from the cation diffusion were more
prominent.
Figure 8 EDX elemental maps across the interface of LSCrF and
ScCeSZ in a sintered pellet. The preparation of the sample is
similar to the sample shown in Fig.1 except that the two starting
pellets are green bodies of the two materials rather than polished
sintered pellets.
Analysis of Surface Composition by LEIS
From the SIMS data, where it was observed that the surface
exchange of the ScCeSZ phase increased while the 18O concentration
on the LSCrF surface decreased, a synergistic effect between these
two phases was clearly observed and the contributions from the
‘spillover’-like type mechanism was suggested. To investigate the
cleaning effects from the perovskite phase, LEIS was applied in
order to observe the variations in the surface chemistry of each
phase.
LEIS Spectra of the ScCeSZ Phase
LEIS analysis was performed on both the isolated ScCeSZ single
phase pellets and the ScCeSZ phase as a component in the fused
pellet. Firstly, LEIS spectra of the isolated ScCeSZ single-phase
pellets with different treatment histories: (a) an as-sintered
surface, (b) an as-polished surface and (c) the surface after
oxygen isotopic exchange annealing at 900 , are presented in Fig.
9.
For the as-sintered pellet (Fig.9(a)), a clear peak which
corresponds to Si in the LEIS spectrum indicates the existence of
an impurity layer on the ScCeSZ surface. Thus, the scattering
intensities of Si and Sc as a function of the dose density of the
primary beam/the estimated depth for the as-sintered pellet are
plotted in Fig. 9(d). The depth presented is an estimation based on
the dose density and is therefore only an approximation of the
actual depth. Fig.9(d) shows that from surface to bulk, the Si
content decreased reflecting that the Si impurity tends to
accumulate on the surface after sintering at high temperature.
Moreover, for its analogous material YSZ, it was also reported that
a thin impurity film covered the as-sintered sample surface as well
as impurities segregating to grain boundaries 34 and this silica
passivation surface layer is believed to be detrimental for the
oxygen surface exchange process 19,35. The as-polished surface
(Fig. 9(b)) presenting bulk information shows a clean surface and
no clear extra peaks representing impurities were observed in the
LEIS spectra. After oxygen isotopic exchange at 900, the LEIS
spectrum of the surface is presented in Fig. 9(c). Na/Mg, Si and
K/Ca impurities are clearly observed on the outermost surface of
the oxygen exchanged sample, and Zr and Sc have been reduced to
subsurface edges compared to the polished sample. Hence, a message
obtained here is that impurities have strongly segregated onto the
sample surface during annealing at 900 . The origins of the
impurities may be due to the impurities at grain boundaries or
exsolved from the bulk and during high temperature annealing, the
impurities move to the surface along grain boundaries 34.
Additionally, impurities from the oxygen isotopic exchange
apparatus could also contribute to the contaminants on the sample
surface. Hence, for the isolated single phase ScCeSZ pellet, except
the as-polished surface which presents the bulk behaviour
exhibiting a clean surface, the segregated impurities were observed
on both surfaces of the samples after the following heat
treatments: sintering at 1450 and isotopic exchange annealing at a
temperature of 900 .
Figure 9 LEIS spectra of the isolated ScCeSZ single phase
pellets with different treatment histories by using 3 KeV He+
primary beam (a) as-sintered ScCeSZ surface; (b) as-polished ScCeSZ
surface; and (c) the surface of the polished ScCeSZ after oxygen
isotopic exchange annealing at 900 for 0.5h. Figure (d) presents
the scattering intensities of Si and Sc as a function of dose
density, i.e. depth profiling spectra of Sc and Si for the (a)
as-sintered sample
Further LEIS analysis was performed on two areas of the ScCeSZ
phase in the fused pellet and Fig. 10 presents the positions of the
analysed areas and the LEIS spectra of the corresponding areas 1
and 2. Due to the lower lateral resolution of LEIS, the analysis
area in LEIS is 0.5 mm x 0.5 mm which is large when compared to the
analysed area of the ToF-SIMS (0.1 mm x 0.1 mm). This sample was
annealed in a dry oxygen atmosphere at 900 with the same thermal
treatment history as the sample depicted in Fig. 9(c).
In comparison with the isolated single-phase pellet where clear
impurity peaks were observed in the LEIS spectrum, the ScCeSZ
exhibited a very clean surface for both analysed areas when the
ScCeSZ was fused with the perovskite phase LSCrF. This clean
surface of the ScCeSZ phase is direct evidence for the ‘cleaning’
hypothesis i.e. that the perovskite structure which is capable of
tolerating impurity species, continuously dissolves the impurities
exsolved from the ScCeSZ phase.
Figure 10 LEIS analysis on the ScCeSZ phase as a component of
the fused pellet after oxygen isotopice exchanged at 900 for 0.5h,
with the same heat treatment history as Figure 9(c), by using 3 KeV
He+ primary beam (a) schematic presenting the positions of the LEIS
analysis areas relative to the SIMS analysis area; (b) and (c) are
the LEIS spectra of the areas 1 and 2, respectively, showing a
clean surface with no detectable impurities
LEIS Spectra of the LSCrF Phase
Additionally, LEIS analysis was performed on the LSCrF phase,
Fig. 11, in the fused pellet in order to obtain information on the
atomic composition on the outermost layers of the LSCrF phase. It
is clear that the He+ analysis beam is incapable of distinguishing
Cr and Fe. In order to separate the Cr and Fe, a heavier noble gas
analysis beam, e.g. Ne+ was required to resolve these features.
From the previous investigations, the analogous perovskite
single-phase materials present very clean surfaces and no
impurities were observed 20,36. However, for the LSCrF phase in the
fused pellet, in the first spectrum with zero dose density where
the outermost surface elemental information is provided, clear
contaminants were observed while the ScCeSZ phase in the same
sample (Fig. 10) presents a very clean surface, suggesting the
cleaning effects from the perovskite LSCrF phase. Moreover, from
the spectra additional interesting information has been
obtained:
1. For the B-site elements Cr and Fe, only a small feature is
present in the spectrum of the outermost surface while both Sr and
La can clearly be observed suggesting the surface of the perovskite
material is predominantly A-O layer terminated.
1. Comparing Sr with La, obviously on the sample surface Sr
shows a much stronger intensity than the La indicating that there
was Sr segregation on the sample surface.
1. On the surface layer, 18O is nearly invisible and after
sputtering off several layers 18O appears reflecting the low
fraction of 18O on the top layers of the LSCrF phase in the fused
pellet. Since no oxygen plasma cleaning was performed before the
measurement, an estimation of the normalised 18O fraction can be
performed. Based on the previous investigation by Tellez et al 37
that 18O presents 18% higher sensitivity than the 16O, the
normalised 18O fraction at the layer 4.3 nm beneath the outermost
surface was determined to be 0.70. However, as mentioned above, the
depth scale for the LEIS spectra was estimated based on the dose
density which may vary between materials and thus introduce
uncertainty. The normalised 18O fraction at the layer 4.3 nm
beneath the surface measured by SIMS was 0.76. The estimated depth
by SIMS is based on the assumption that the sputtering process is
even, thus considering the depth resolution of SIMS and the ion
beam inter mixing, the normalised 18O fraction at the depth of 4.3
nm is only for reference.
Figure 11 LEIS spectra of the LSCrF phase in the fused pellet by
using 3 KeV He+ primary beam
Among these three points, the first two phenomena agree well
with the findings in other reported perovskite materials, e.g.
La1-xSrxCo1-yFeyO3-δ etc. 20 that the perovskite material tends to
terminate with an A-O layer and Sr containing materials tend
towards Sr surface segregation. Fig. 12 is the depth profiling
spectra of the La, Sr and Cr/Fe elements where the relative
intensities of each element as a function of depth can be clearly
observed. From Fig. 12, although Sr tends to segregate on the
surface of the perovskite LSCrF, the intensity of Sr on the surface
is very low, which may be attributed to the impurity layer on the
sample surface. Thus, combined with the SIMS depth profile into the
LSCrF phase (Fig. 6), the 18O depleted surface layer may also be
related to the impurities on the surface of the perovskite phase.
To clarify, if the cleaning effect only operates at high
temperatures and during the quenching process the impurities in the
ScCeSZ phase do not have time to incorporated into the surface of
ScCeSZ and dissolve in the perovskite phase, the impurity layers
with a lower isotopic fraction will further dilute the isotopic
fraction on the surface of the LSCrF phase. Moreover, for the third
point, considering the inaccuracy in determination of depth in
LEIS, the low fraction of 18O in the top layers obtained by LEIS
can be considered as being in good agreement with the SIMS depth
profiling results where clear 18O depletion layers were observed
confirming a synergistic effect between the two phases.
Figure 12 Depth profiling LEIS spectra of the Sr, Fe/Cr and La
elements highlighting the Sr segregation at the surface
Overall discussions
The rather interesting results obtained via SIMS and LEIS
analysis along with EDX spectra, have revealed several key points
to aid in understanding the mechanisms of surface exchange and
diffusion in the LSCrF-ScCeSZ dual-phase system:
Firstly, we will discuss one possible origin of the increase of
the surface isotope fraction on the ScCeSZ close to the
heterointerface. Previous work had suggested that transition metal
impurities could lead to an enhancement of the surface exchange
coefficient in the related fluorite ionic conductor
ceria-gadolinia, CexGd1-xO2-δ (CGO) 22,35. The SEM-EDX spectra have
shown that at a distance of 15 μm from the heterointerface cation
inter-diffusion became negligible, which is rather short compared
to the distance to the heterojunction of the areas used for the
SIMS and LEIS measurements. In other words, for the SIMS depth
profiling analysis on the ScCeSZ surface inter-mixing of the
transition metal cations has a negligible effect on the final
results, in contrast to the case of CGO, and the reasons for the
increased 18O fraction are mainly due to other mechanisms.
LEIS spectra of the ScCeSZ surface were acquired from both the
isolated single phase pellets with different treatment histories
and the ScCeSZ part as a component of the fused pellet after oxygen
isotopic exchange, and presented several interesting observations,
of which the expanded description is provided in the section above.
It was shown that the polished surface was covered by impurities
after oxygen isotopic exchange annealing, i.e. in the dry oxygen
atmosphere at 900 , while the ScCeSZ phase in the fused pellet with
the same thermal history presented a much cleaner surface, i.e. no
obvious observed impurities in the LEIS spectra of both acquisition
areas. Thus, the conclusion here is that the surface of the ionic
conductor has been cleaned due to the proximity of the perovskite
phase.
Combining the result of LEIS analysis with the SIMS data, it is
unlikely that self-cleaning from the perovskite phase is the only
mechanism active in these dual-phase materials. The LEIS spectra
revealed that the entire ScCeSZ part in the fused pellet displayed
a clean surface and in the event that the cleaning effect was the
only valid interpretation, the four SIMS analysis areas should have
identical 18O fractions on the surface. However, according to the
SIMS data the 18O fraction decreased with increasing distance to
the heterojunction. Moreover, another point to be presented here is
that compared to its analogous YSZ system 19,38 this isolated
ScCeSZ single phase itself exhibits a cleaner surface with only
very weak impurity peaks observed in the LEIS spectra. A clean
surface is believed to be beneficial for the surface exchange
process.
Fig. 5 demonstrated that the normalised 18O surface fraction
decreased as the distance to the interface increased. Considering
the provided error bars for the normalised 18O fraction, the 18O
surface fractions of area 4 on the ScCeSZ side of the fused pellet
can be recognised as identical to the 18O fraction of the isolated
ScCeSZ single-phase pellet. For area 4, due to its distance to the
interface, the influence from the perovskite phase became
negligible. Thus, for material fabrication, theoretically a
homogenously distributed microstructure with small grain size
should be beneficial for oxygen diffusion in the dual-phase system
because of the improved accessibility to the ionic phase for
incorporation of oxygen diffusion. Furthermore, combined with the
lateral diffusion coefficient obtained from Fig. 5, i.e. the
diffusion coefficient estimated on the ScCeSZ surface of the fused
sample from the heterojunction to the unilateral side, it is one
order of magnitude higher than the bulk diffusion coefficient of
the ScCeSZ, suggesting that it is not caused by the lateral
diffusion but another mechanism. In other words, spillover-type
mechanism or the surface cleaning surface make significant
contributions.
The lower 18O fraction on the surface of the LSCrF phase in the
fused pellet suggested a ‘spillover’-like effect. However, on top
of the conventional understanding that the reduced and dissolved
oxygen species diffuse onto the pure ionic phase on the surface,
the top surface layers, sub-surface layers, i.e. around ten
nanometers beneath the outermost surface of the LSCrF phase,
present more active behaviour towards oxygen diffusion than the
inner (bulk) parts. In the top layers, oxygen species can easily
move across the heterojunction and diffuse through the ionic
conducting phase. Moreover, the dissociated and reduced oxygen
species may tend to diffuse into the bulk of the LSCrF via the
grain boundaries. Hence in the top layers the oxygen isotopic
fraction was diluted due to the activated surface/subsurface layers
and grain boundary fast diffusion, and the 18O depleted layers were
observed. LEIS spectra of the LSCrF phase surface also provided
considerable useful information. Firstly, the A-O terminated
surface and Sr surface segregation were consistent with previous
reports on other perovskite materials in the literature 20.
According to the intensities of the separated 16O and 18O peaks,
the 18O depleted surface layers were also observed in the LEIS
spectra which agrees well with the SIMS data. Additionally,
impurity layers were also observed on the LSCrF surface which is
not only complementary evidence of a cleaning effect but also a
potential reason for the 18O depleted layer on the LSCrF surface.
To summarise, the 18O depleted layer in the LSCrF phase of the
fused pellets may be associated with grain boundary fast diffusion,
higher oxygen diffusivity in subsurface/surface layers and the
impurity layers on the surface.
Thus, from the above points, the SIMS and LEIS results confirmed
the synergistic effect between the pure ionic conductor ScCeSZ and
the MIEC phase LSCrF. Transition metal inter-diffusion has a minor
influence on the oxygen surface exchange and diffusion behaviour in
this fused pellet. However, for the dual-phase composite sintered
at higher temperature for longer time, the diffused cations may
also have effects on the oxygen surface exchange and diffusion
behaviour. A cleaner surface of the ScCeSZ phase has been observed
when the ScCeSZ pellets are annealed adhering with the perovskite
phase. The detailed mechanism and significance of the cleaning
effects by perovskites on oxygen diffusion and surface exchange
process is yet to be fully understood but it is believed that this
cleaner surface is advantageous for the oxygen surface exchange.
Furthermore, the ‘spillover’-like mechanism makes significant
contributions to the oxygen diffusion behaviour in the dual-phase
system but it is not the only reason responsible for the
synergistic effects between these two phases. Therefore, a
combination of the ‘spillover’-like mechanism and perovskite
self-cleaning mechanism is suggested.
Conclusions
In literature, three potential theories have been provided to
explain the synergy of dual-phase composite systems, namely
‘spillover’ type mechanism, self-cleaning mechanism of the
perovskite phase and a transition metal inter-diffusion type
mechanism. In this present work, by using SEM-EDX, SIMS and LEIS
techniques, interesting behaviour has been observed.
1. No obvious cation inter-diffusion was detected by using EDX,
suggesting the minor contributions from the catalyst surface layers
of the transition metals for this fused sample.
2. Moreover, a cleaner surface of the ScCeSZ was observed due to
the presence of the perovskite phase LSCrF despite the distance to
the heterojunction, confirming the cleaning theory which is
believed to be beneficial for the oxygen surface exchange
process.
3. Additionally, as the distance to the heterojunction
increased, the normalised 18O fraction decreased on the ScCeSZ
surface. However, based on the obtained oxygen lateral diffusion
coefficient and bulk diffusion coefficient of the ScCeSZ phase,
exclusive the oxygen lateral diffusion, other mechanisms, i.e. both
spillover-type mechanism and the cleaning effect from the
perovskite phase, make the main contributions.
4. Combined with previous simulation results, further
information obtained from the LSCrF side is that the surface and
sub-surface area both present high oxygen diffusivity, slightly
different from the conversional wise of the ‘spillover’ type
mechanism. Besides, on the surface of the LSCrF phase in the fused
pellet, the 18O depleted layers may be associated with grain
boundary fast diffusion, higher oxygen diffusivity in
subsurface/surface layers and the impurity layers on the
surface.
To conclude, a combination of the ‘spillover’-like and
perovskite self-cleaning is suggested to be the potential
mechanisms for the oxygen diffusion in the LSCrF-ScCeSZ dual-phase
composite system while the effect from the cation inter-diffusion
is proved to be low.
Conflicts of interest
There are no conflicts to declare.
Acknowledgement
The authors would like to thank Praxair Inc., USA for research
funding.
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