The interface transport of V2O5 and WO3 into CaMo(W)O4 stimulated by an electric field

Article abstract of DOI:10.1016/S0039-6028(02)01203-7

An electric field applied to the CaWO₄/V₂O₅, CaMoO₄/V₂O₅ and CaMoO₄/WO₃ systems causes grain boundary and surface transports of oxides having a low surface energy (V₂O₅ and WO₃) and their segregation on the grain surface. It was found that V₂O₅ penetrates to the inner surface of CaWO₄ much more intensively when the V₂O₅ briquette bears a negative potential: (-)V₂O₅|CaWO₄(+). The penetration of V₂O₅ and WO₃ to the inner surface of the CaMoO₄ ceramic is accompanied by a chemical interaction.

Full text of DOI:10.1016/S0039-6028(02)01203-7

Surface Science 507–510 (2002) 140–145
www.elsevier.com/locate/susc
The interface transport of V O and WO into CaMo(W)O
2
5
3
4
stimulated by an electric field
*
A. Guseva, A. Neiman , E. Konisheva, M. Trifonova, E. Gorbunova
Chemical Department, Ural State University, Ekaterinburg 620083, Russia
Abstract
An electric field applied to the CaWO
and surface transports of oxides having a low surface energy (V
surface. It was found that V penetrates to the inner surface of CaWO
briquette bears a negative potential: ðÀÞV jCaWO ðþÞ. The penetration of V
CaMoO
ceramic is accompanied by a chemical interaction. Ó 2002 Elsevier Science B.V. All rights reserved.
4
=V
2
O
5
, CaMoO
4
=V
2
O
5
and CaMoO
and WO
much more intensively when the V
and WO to the inner surface of the
4
=WO
3
systems causes grain boundary
2
O
5
3
) and their segregation on the grain
2
O
5
4
2 5
O
2
O
5
4
2
O
5
3
4
Keywords: Grain boundaries; Ceramics; Tungsten oxide; Vanadium oxide; Surface segregation; Diffusion and migration; Field effect
1
. Introduction
In our study the diffusants were WO
3
and, in ad-
dition, V , which also has a low surface energy
2
O
5
À5
2
The researchers [1–3] revealed and studied the
propagation of solid WO along grain boundaries
aV O ¼ 0:9 Â 10 J/cm [4]. The CaMoO
CaWO ceramics (the former is isostructural to the
4
and
2
5
3
4
in CaWO in the absence of a chemical interaction.
4
latter) served as the substrates. Thus, CaWO4=
The moving force of this process is the difference
in the surface energies of the contacting solid
V
2
O
5
, CaMoO
were analyzed.
4 2 5 4 3
=V O and CaMoO =WO systems
3
materials and/or an external electric field. WO has
À5
2
a low surface energy aWO3 ¼ 1 Â 10 J/cm [4].
Consequently, the formation of a WO coat on the
3
2. Experimental
surface of calcium tungstate grains is favorable in
thermodynamic terms. This unusual effect of an
The effect of an electric field on the rate of
the solid-phase grain boundary impregnation was
studied by the method of contact annealing of
ceramic briquettes with an external potential dif-
ference applied to the reaction cell [2]. The pro-
cesses were monitored by observing the change in
the mass of the contacting briquettes and mea-
suring the thickness of the colored layer of the
formed composite in an MBS-9 optical microscope
‘
‘electric surface transport’’ was detected and an-
alyzed in detail only for the CaWO =WO system.
4
3
However, the study of such interface processes has
a great practical significance for producing sup-
ported catalytic and composite materials [5].
*
Corresponding author: Tel.: +007-3432-617-470; fax: +007-
432-615-978.
E-mail address: arkady.neiman@usu.ru (A. Neiman).
(magnification up to 56Â). The morphology of
3
0
the ceramics was analyzed in a JSM-3 electron
039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved.
PII: S0039-6028(02)01203-7

A. Guseva et al. / Surface Science 507–510 (2002) 140–145
141
microscope (magnification up to 10 000Â). The
chemical composition of the grain boundary im-
pregnation products was determined using the X-
ray diffraction analysis (DRON-2 diffractometer,
position of products was the same either for spon-
taneous or current conditions. It means that no
additional, current induced, internal reaction was
occurred.
Co-k
a
radiation), X-ray microprobe (Superprobe-
Processes in the cells (1)–(3) are substantially
different, therefore we shall consider theirs sepa-
rately.
JCXA-733) and Raman spectroscopic (MOLE
microprobe) techniques. The composition of the
surface of the ceramic grains was analyzed by
the ESCAmethod (HP 5950 Aspectrometer). A
Rigaku–Denki microthermal analyzer was used
for the differential thermal analysis. The heating
rate was 5 °C/min. The calcium concentration of
the tungsten oxide briquette was measured by the
complexonometric titration.
3.1. Spreading of V
2
5
O along grain boundaries in the
CaWO ceramic
4
2 5 4
After the experiments, the V O and CaWO
briquettes were separated and weighed. In all the
cases the mass of the cathodic V O
ðÀÞ
briquette
decreased, while the mass of the CaWO briquette
2
5
4
increased correspondingly. The mass of the anodic
3
. Results and discussion
ðþÞ
part V
CaWO
electricity passed through the cell (1), Fig. 1.
The cathodic region of the white CaWO
2
O
changed little. The mass of the
5
4
increased linearly with the quantity of the
The experiments were performed at 600 °C in
the following cells:
4
bri-
quette turned brown–orange in color. The ESCA
method showed that the ½Vꢀ=½Wꢀ ratio on the
ðÀÞPtjV jCaWO
2
O
5
4
jV
2
O
5
jPtðþÞ;
ð1Þ
ð2Þ
surface of the CaWO grains was 0.57. The X-ray
4
ðÀÞPtjV O jCaMoO jPtðþÞ U > 0;
2
5
4
microprobe analysis also revealed the presence of
vanadium in the calcium tungstate briquette (Fig.
ðþÞPtjV
2
O
5
jCaMoO
4
jPtðÀÞ U < 0
2). The vanadium concentration was especially
large on the CaWO surface on the cathode side
and at 900 °C in the following cell:
4
and decreased towards the anode. The X-ray dif-
fraction analysis of the CaWO briquette did not
ðÀÞPtjWO
ðþÞPtjWO
3
jCaMoO
jCaMoO
4
4
jPtðþÞ U > 0;
jPtðÀÞ U < 0
4
ð3Þ
3
Avoltage of 300 V was applied to the cells (1) and
2) and up to 150 V to the cell (3). The current
(
density did not exceed 1 mAin the cells (1) and (2)
and 4 mAin (3).
2
À
CaMoO
ductors [6,7] and V
8]. Therefore at an electric current passed through
4
and CaWO
4
are mixed (O ,e)-con-
2
O
5
(WO
3
) are semiconductors
[
the cells (1)–(3) the only electrochemical process is
reaction of oxygen oxidation or reduction
2À
1
2
ðþÞO ! O2À þ 2e
1
2
þ 2e ! O2À
ðÀÞ O
2
at the interfaces V O jCaMoðWÞO , WO jCaWO
2
5
4
3
4
and CaMoðWÞO jPt.
4
After experiments these regions were thoroughly
investigated and it was found that chemical com-
Fig. 1. CaWO
electricity passed through the cell (1). T ¼ 600 °C, U ¼ 300 V.
4
briquette mass increase vs. the quantity of the

142
A. Guseva et al. / Surface Science 507–510 (2002) 140–145
Fig. 3. DTAcurve for equimolar mixture CaWO
4 2 5
and V O .
To check the probability that a liquid phase was
formed in the CaWO =V system at 600 °C, a
DTAcurve was measured for an equimolar mix-
4
2 5
O
Fig. 2. X-ray microprobe analysis of a CaWO
in contact with V
4
sample annealed
in the cell (1). Q ¼ 280 C, t ¼ 600 °C,
2
O
5
ture of CaWO and V O (Fig. 3). No thermal
4
2
5
s ¼ 54 h. The cross-section of the sample was made at a distance
effects were detected at 600 °C. The endoeffect at
60 °C was due to the melting of individual V
The preceding stage showed however that some
spread on the surface of the CaWO sub-
ðÀÞ
of 100 lm from the surface coming in contact with V
2
O
.
5
6
2 5
O .
V
2
O
5
4
strate was in an amorphitized glassy-like state at
the experimental temperature (600 °C). This state
is well known and is characteristic of mixed phases
containing a large quantity of vanadium, molyb-
denum or tungsten oxide [10].
So, our experimental data suggest that the
solid-phase vanadium oxide spreads over the grain
boundaries in the calcium tungstate ceramic in the
4
reveal any other phases except CaWO . This was
due to the fact that the maximum concentration of
the vanadium oxide in the CaWO briquette did
4
not exceed 1.6 mass% (even when a charge of 280
°C was passed through the cell).
The phase diagram of the CaWO
4
2 5
–V O system
has been unclear. Therefore we analyzed the re-
action
4 3
cell (1) as is the case in the CaWO =WO system
CaWO
4
þ V
2
O
5
! CaV
2
O
6
þ WO
3
ð4Þ
[2,3]. The process can be visualized as the transport
of some negatively charged form of V O . Two
2
5
thermodynamics. This reaction was chosen as the
most probable on the following grounds. In ac-
cordance with [9], at the temperature adopted in
our experiments (600 °C) CaV O was formed as
possible mechanisms of the electric surface trans-
port (electroosmotic migration and solid-phase
electrocapillarity) were considered in Ref. [2].
Considering the size (13 lm) of the CaWO4
grains (determined by the electron microscopy),
the mass and depth of V O penetrating into the
2
6
the only product in the CaO=V
2
O
5 3
mixture. Ca -
ðVO
4
Þ and Ca
2 2 7
V O appeared at a higher tem-
3
2
5
0
perature. The calculation yielded DG ¼ þ31 kJ/
CaWO ceramic briquette (thickness of the colored
4
ð3Þ
mole. This result suggests a small probability of
a chemical interaction in the cell (1).
layer), the average thickness of the V O film on
5
2
the surface of CaWO4 grains was estimated at

A. Guseva et al. / Surface Science 507–510 (2002) 140–145
143
ꢁ
7 nm for the composite containing 0.16 mass%
. This thickness corresponds to ꢂ10 mono-
layers of V
So, the penetration of the diffusant into the
CaMoO ceramic in the cell (2) was accompanied
by a chemical interaction of the starting materials.
V
2
O
5
4
2
5
O .
3
.2. Spreading of V
2
O
5
along grain boundaries in the
3.3. Spreading of WO
CaMoO ceramic
3
along grain boundaries in the
CaMoO
4
ceramic
4
In the cell (2), as in (1), V O penetrated into
2
In the cell (3), as in (1) and (2), WO penetrated
3
5
the bulk of the CaMoO
process was accompanied by a solid-phase inter-
action of CaMoO and V by the equation:
4
briquette. However, this
4
into the bulk of the CaMoO ceramic. At a tem-
perature above 700 °C the following reaction oc-
curred:
4
2
O
5
xCaMoO þ V ! xCaO þ V ꢃ xMoO
ð5Þ
This conclusion was supported by the fact that the
4
2
O
5
2
O
5
3
:
CaMoO4 sol þ WO3 sol ¼ CaWO4 sol þ MoO3 gas
:
ð6Þ
It is known that MoO melts at 795 °C [11] and is
3
annealed CaMoO briquette was colored orange
4
sublimated intensively at 650 °C [12]. Therefore all
due to the penetration of V O . The X-ray dif-
2
MoO , which was formed by the reaction (6), vol-
atilized at the experimental temperature (900 °C).
5
3
fraction analysis revealed the presence of CaO on
the surface of the CaMoO briquette coming in
contact with the V briquette. Adark-gray layer
appeared on the surface of the V briquette
contacting CaMoO . In accordance with the X-ray
4
The mass of the CaMoO briquettes increased
4
2
O
5
and the mass of the WO briquettes decreased in
3
2
O
5
all the experiments. When the polarization direc-
tion was reversed from ÀU to þU, the increase in
4
diffraction analysis, this layer was a solid solution
of the molybdenum oxide in the vanadium oxide
the mass of the CaMoO briquettes was greater
4
(Fig. 5). The phase compositions differed a little at
ÀU and þU.
(
Fig. 4).
Although the phase composition changed, the
When U > 0, the X-ray diffraction analysis re-
vealed the presence of WO on the CaMoO bri-
quette. From the Raman spectroscopic data it
2 5 4
mass of the annealed V O and CaMoO bri-
3
4
quettes remained constant at any direction of the
current and, also, in the absence of an external
voltage. From the last observation it follows that a
counter-diffusion of the MoO and V O compo-
followed that CaWO was formed simultaneously
4
(Table 1). The intensity of the main lines of CaWO4
in the Raman spectrum of the sample diminished
smoothly with the distance from the interface to the
3
2
5
nents took place in the cell (2).
Fig. 5. Change in the mass of the CaMoO
applied voltage in the cell (3). T ¼ 900 °C, s ¼ 6 h.
4
briquettes vs. the
Fig. 4. The scheme of products layers formation in the cell (2).

144
A. Guseva et al. / Surface Science 507–510 (2002) 140–145
Table 1
Results of the layer-by-layer X-ray diffraction analysis and Raman spectroscopy at t ¼ 25 °C
Ord. no.
Sample
X-ray diffraction analysis
Raman spectroscopy
1
CaMoO
4
(U ¼ 0 V, 900 °C, 36 h)
, 500 lm removed
(U ¼ þ95 V, 900 °C, 59 h, Q ¼ 28 C)
with CaMoO , 350 lm removed
CaMoO
(U ¼ À95 V, 900 °C, 20 h, Q ¼ 113 C)
with CaMoO , 400 lm removed
CaMoO
CaMoO
CaMoO
CaMoO
CaMoO
CaMoO
4
4
4
4
4
4
, WO
, WO
, WO
3
3
3
CaMoO
CaMoO
CaMoO
CaMoO
CaMoO
CaMoO
4
4
4
4
4
4
, CaWO
, CaWO
, CaWO
, CaWO
4
4
4
4
,
with CaMoO
CaMoO
4
2
3
4
4
4
4
bulk of the CaMoO
4
ceramic. The WO
3
briquette
boundaries of a complex ceramic in the CaWO
4
/
turned dark-green in color and grew darker with
the experimental time. The complexometric titra-
tion revealed the presence of calcium in the WO3
briquette. So, when U > 0, two processes took
V O , CaMoO /V O and CaMoO /WO systems
2
5
4
2
5
4
3
studied. Hence, the field-stimulated transport of
the oxide with a low surface energy to the inner
surface of another oxide is rather general.
place: a segregation of WO
aries in CaMoO and a chemical interaction re-
sulting in the formation of CaWO
3
on the grain bound-
The solid-phase transport of V
CaWO /V system is not complicated by a chemi-
cal interaction. The transport rate increases sharply
2
O
5
in the
4
4
2 5
O
4
.
When U < 0, a loose white layer up to 1.1 mm
for ðÀÞV
2
O
5
jCaWO ðþÞ thanks to the electro-
4
thick appeared on the CaMoO briquette coming
in contact with WO (s ¼ 6 h, À150 V). The near-
osmotic transport or electrocapillarity. The prop-
agation of V O and WO on the internal surface of
the CaMoO ceramic in the CaMoO /V O and
4
3
2
5
3
contact region of this white layer contained WO
3
4
4
2
5
and CaMoO
Raman spectroscopic data, CaWO
formed at U < 0. One may suppose the formation
of a distributed system, in which CaMoO grains
4
(Table 1). In accordance with the
CaMoO
ical interaction at inner interfaces.
4
3
/WO systems is accompanied by a chem-
4
was not
4
Acknowledgements
were covered with a thin film of the highly con-
ducting WO phase. This supposition is confirmed
3
This work was supported by the RFBR (grant
no. 01-03-33160), by the Russian Federal Program
indirectly by the following considerations. If WO3
spreads over the grain surface (thanks to a lower
surface energy [4]), the initial ceramic may become
more brittle. This was actually the case at U < 0.
‘‘Universities of Russia––Fundamental Research’’
and by the CRDF (no. REC-005).
The WO
3
briquette turned light-green in color.
Calcium was not detected in the WO briquette.
References
3
So, when U < 0, the main process was the
spreading of the tungsten oxide on the surface of
[1] A. Neiman, A. Guseva, Conf. Solid State Ionic, The
Hague, The Netherlands, 1993, p. 726.
the CaMoO ceramic grains.
4
[2] A. Neiman, E. Konisheva, Solid State Ionics 110 (1998)
121.
The mean diameter of the CaMoO4 ceramic
grains was 13 lm. By use of above-mentioned
technique we have estimated the average thickness
[
3] A. Neiman, E. Konisheva, Solid State Ionics 119 (1999)
5.
4] S. Overbury, P.A. Bertrand, G.A. Somortjai, Chem. Rev.
5 (1975) 547.
7
[
of the WO
to ꢂ20 mono-layers of WO
3
film as ꢁ8 nm. This value corresponds
3
7
.
[
5] H. Kn o€ singer, E. Taglauer, Catalysis 10 (1993) 1.
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[
[
[
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Acad. Sci. USSR 233 (1977) 1106.
4
. Conclusion
8] P. Kofstad, Nonstoichiometry, diffusion and electrical
conductivity in binary metal oxides, Mir, Moscow, 1975
(Russian edition).
It was found that an oxide (V
having a low surface energy segregates on grain
2 5 3
O or WO )

A. Guseva et al. / Surface Science 507–510 (2002) 140–145
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[
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