Download CaZrO -based powders suitable for manufacturing

Cent. Eur. J. Chem. • 11(12) • 2013 • 2088-2097
DOI: 10.2478/s11532-013-0332-2
Central European Journal of Chemistry
CaZrO3-based powders suitable for
manufacturing electrochemical oxygen probes
Research Article
Magdalena Dudek1*, Alicja Rapacz-Kmita2
AGH-University of Science and Technology,
Faculty of Fuels and Energy
30-059 Cracow, Poland
1
AGH-University of Science and Technology,
Faculuty of Materials Science and Ceramics
30-059 Cracow, Poland,
2
Received 25 May 2013; Accepted 1 August 2013
Abstract: Calcium zirconate powders doped with a small amount of CaO were synthesised using the Pechini method. X-ray analysis revealed that
solid solution was formed in the concentration up to 51.5% mol CaO. For synthesis of stoichiometric CaZrO3, the highest temperature
was required (1150°C), but introduction of excess CaO from 50.5 to 51.5% mol enabled us to lower the synthesis temperature to 800°C.
The sintering behaviour of such samples under non-isothermal conditions was studied by dilatometric methods. Deviations were found
in stoichiometry; by increasing the CaO concentration in CaZrO3 sinterability improved in comparison to CaZrO3 with stoichiometric
composition. The presence of CaO as second phase caused deterioration of the sinterability of the CaZrO3-based samples. Pellets sintered
at 1500°C for 2 h reached 96-98% of theoretical density. SEM and TEM observations were used to characterise the microstructure of the
prepared samples. The electrical properties of CaZrO3-based samples were investigated by the AC-impedance spectroscopy method.
It was found that introduction of excess CaO into the CaZrO3 structure caused an increase in ionic conductivity up to the solubility limit.
The possibility of using CaZrO3-based samples for constructing prototype electrochemical oxygen probes to determine activity of oxygen
dissolved in molten copper is also demonstrated.
Keywords: CaZrO3 • Electrochemical oxygen probe • Ionic conductivity • Electrolyte
© Versita Sp. z o.o.
1. Introduction
Calcium zirconate (CaZrO3) with a perovskite structure
is one of many chemical compounds in the CaO-ZrO2
system. A high melting point (2340°C) and thermal
and chemical stability make this compound a suitable
component for refractory applications [1–4]. Perovskite
structures (ABO3) are an important class of ceramics with
a wide variety of derivative structure types (e.g. cubic,
hexagonal, and orthorhombic). The physicochemical
properties of ABO3 perovskite materials can be modified
by doping cations with valencies different than those
of the host cation B4+ or A2+ or variations of the A/B
ratio, making them attractive functional materials for
different areas in industry [5,6]. Orera et al. reported
their characteristic infrared and Raman active modes
of vibration, which were later confirmed by Zheng et al.
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[7,8]. They proposed the use of CaZrO3 doped with Er3+
as an optical material due to its luminescent properties
[9]. Calcium-zirconate-based materials have also
excellent catalytic and mechanical properties and as
porous composites seem to be promising components
for NOx fillers and lightweight structural components
[10]. The material has a dielectric constant of ~30 and
quality factor (Q=1/tan d) of 3000 at 13 GHz. The high
dielectric constant combined with excellent thermal
stability makes this material attractive for gate materials
for MOSFET applications. Yu et al. determined a leakage
current density of 9.5×10-8 (A cm-2) in an electrical field of
2.6 MV cm-1 [11].
CaZrO3 additions have also been observed to
greatly increase the dielectric constant and temperature
coefficient of capacitance (TCC) behaviour in BaTiO3based capacitors [12]. They are among the various
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ceramic coatings currently in use for enhancing the
biocompatibility of metallic implants.
Ceramic proton-conducting materials based on
CaZrO3 have been considered for potential applications
in electrochemical probes to control the amount of
hydrogen dissolved in aluminium alloy castings [13-16].
Janke [17,18] found that calcium zirconate modified
by excess CaO could be applied as oxygen probes
in metal alloys or steel castings. High-temperature
ionic conductivity data suggested that solid solutions
of calcia and fully-stabilized zirconia may be used
for thermodynamic measurements at oxygen partial
pressures as low as 10-23 atm at a temperature of
1273 K. So far, CaO-ZrO2 has been used in extensive
studies of the thermodynamic properties of the majority
of metal oxide systems. The Gibbs free energy of
formation and thermodynamic activities for compounds
and solid solutions formed in many oxide systems have
been determined in this way.
Calcium zirconate, however, enables measurement
of thermodynamic properties in oxides at a low oxygen
partial pressure limit, being several orders lower than
that of the CaO-ZrO2 system, and therefore it has been
successfully applied in oxygen activity measurements
in molten metals. Our previous work [19–21] confirmed
that modified CaZrO3 seems to be another promising
electrolytic material for solid galvanic cells, designed to
study thermodynamic data for oxide compounds and
thermodynamic activity of compounds in binary oxide
solid solutions.
The present work is focused on the systematic
study of the influence of chemical composition on the
physicochemical properties of CaZrO3-based materials,
which are crucial for preparing gas-tight components
for the construction of prototypical electrochemical
oxygen probes designed to study the activity of oxygen
dissolved in non-ferrous molten metals.
2. Experimental procedure
CaZrO3-based samples, involving CaO from 50 to
55% mol, were prepared by the Pechini method.
Appropriate amounts of calcium carbonate were
dissolved in zirconyl nitrate (1.1 mol dm-3). Citrate acid
and ethylene glycol were added to the nitrate solution
containing calcium and zirconium cations. The solutions
were evaporated at 70°C to obtain hard gels; the dried
gels were then calcinated at a temperature range of
650–1250°C for 1 hour and then a ground in rotaryvibratory mill with zirconia grinding media in dry ethanol.
The phase composition of all powders and sintered
samples were evaluated using X-ray diffraction analysis.
The morphology of grounded CaZrO3-based powders
was investigated by transmission electron microscopy
(TEM).
In order to determine the homogeneity of the
chemical composition of monophase CaZrO3 powders
synthesised using the Pechini method, the obtained
preparations were washed with distilled water,
which was later chemically analysed to determine
the concentration of calcium ions washed out of the
obtained powders. The chemical analysis was done
by means of the inductively-coupled plasma atomic
emission spectroscopy (ICP-AES) method. The
experiment made it possible to estimate the amount of
CaO dissolved in the CaZrO3 structure at the stage of
synthesising the powder. The grounded powders were
isostatically pressed under 200 MPa. The densification
behaviour of the powder compacts was monitored via
Dilatometry, using a constant rate of 5° min-1 at 25 to
1200°C. The samples were sintered at a temperature
range of 1200–1600°C for 2 hours. Apparent density
was measured by the Archimedes method. A scanning
electron microscopy (Nova NanoSEM, FEI) was used
to observe microstructures of the sintered samples. A
numerical analysis of SEM microphotographs (Visilog
4 program, Noesis), taken from the polished and
thermally etched surfaces, was applied to measure
microstructural parameters quantitatively. Transmission
Electron Microscopy (Tecnai G2 F20) was also used to
study grain boundary structures of selected CaZrO3based samples.
Ionic conductivity measurements were performed
by the AC-impedance spectroscopy method in
the temperature range 200–800°C in static air.
Impedance spectra were recorded with an automated
setup, suitable for measurements of large absolute
values of impedance, which combined a Solatron
1260 Impedance/Gain Phase Analyser and a Keithley
428 Current Amplifier. Impedance was measured in the
frequency range 10 MHz to 0.01Hz, with an applied AC
signal of 30 mV rms. Measurements were made in air at
constant temperatures from 200 to 800°C in two cooling
and heating cycles (5 or 10°C temperature steps,
20 min stabilisation time before each measurement).
The data acquisition program tested whether the
impedance values changed with time and constant
temperature. For a set of test frequencies, impedance
was measured before and after acquisition of each
spectrum. When the summed relative differences in
measured impedance exceeded the assumed tolerance
of 1%, the measurements were repeated. Impedance
spectra were analysed by means of nonlinear leastsquares fitting of an equivalent circuit using the computer
program Firdravn.
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CaZrO3-based powders suitable for
manufacturing electrochemical oxygen probes
CaZrO3
tube
ceramic
seal
EMF
Pt-Rh
wire
Pt porous
Figure 1.
NiNiO
Scheme of electrochemical non-stoichiometric CaZrO3
involving a 51% mol CaO oxygen probe for molten
Cu-based melts.
The AC four- probe method was also applied to
study total electrical conductivity in a temperature range
of 500–1150°C in air. The gas-tight elements prepared
from CaZrO3 electrolytic material were tested with an
electrochemical probe.
The constructed cell (1) operated as an
electrochemical oxygen probe to measure oxygen
dissolved in metal melts. A schematic of this construction
is presented in Fig. 1.
Closed tubes made from non-stoichiometric CaZrO3
samples involving 51% mol CaO (CZ-51) were coated
over their entire surface with a Pt electrode. The Ni-NiO
mixture was used as a reference electrode. This metal/
metal oxide buffer had previously been tested as a
reference electrode for measurements of oxygen activity
in liquid copper. The CZ-51 oxygen sensor produces an
EMF as a result of the differences in oxygen potential
between liquid copper and the reference electrode,
which can be related to temperature using the Nernst
equation:
(1)
where pO2(ref) is the oxygen partial pressure established
by the Ni/NiO buffer electrode. An electrochemical probe
prepared in this way was applied to measure variations
in oxygen activity in molten copper during the oxidation
process. The effect of an amount of manganese on the
variation in oxygen content in molten copper during
the oxidising process was analysed. The temperature
tested was 1100–1200°C. The conditions were similar to
those described in paper [22], where an electrochemical
probe with 8YSZ electrolyte was used to determine
oxygen activity.
Copper was melted in high alumina crucibles in an
induction furnace. One added manganese pellet was
used in each experiment. The mass of manganese
was 0.2 and 0.8% of the mass of liquid copper
respectively. The melting temperature was measured
with an R-type thermocouple (Pt-Pt, 13% Rh). After the
required temperature was attained, the oxygen sensor
was introduced into the liquid copper. When the EMF
signal of the electrochemical cell had been stabilised,
the initial metal sample was withdrawn into a silica
tube and a predetermined amount of manganese was
added to start the deoxidising process. The working and
reference electrodes of the electrochemical probe were
covered with Pt electrodes.
3. Results and discussion
3.1. Preparation
and
physicochemical
properties of non-stoichiometric CaZrO3
Only the orthorhombic CaZrO3 phase was detected
by the XRD method for powders with stoichiometric
composition (50% mol CaO) and including some
excess CaO ranging from 50.5 to 51.5% mol. The
highest temperature (1150°C) for the synthesis of the
othorombic CaZrO3 phase was found for stoichiometric
composition (50% mol CaO). Increasing CaO by 50.5
to 51.5% mol (excess CaO from 1 to 6% mol) relative
to stoichiometric requirements led to a reduction in the
synthesis temperature for the orthorhombic CaZrO3
phase to 800°C.
The impact of CaO content on the falling
temperature of the synthesis of monophase CaZrO3
has been previously mentioned for samples prepared
by solid-state reactions. When solid-state synthesis was
applied to the manufacture of CaZrO3 (50% mol CaO)
powder, the monophase CaZrO3 samples were obtained
at 1400°C for 1 h, whereas use of the Pechini method
made it possible to obtain monophase powder with the
same composition at a lower temperature, 1200°C.
On the other hand, in the case of non-stoichiometric
CaZrO3, a solid-state reaction made it possible to obtain
monophase powder at 1200°C, but use of the Pechini
method lowered the temperature of synthesis to 800°C.
The highest particle sizes (about 90-115 nm) were found
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(a)
Figure 2.
Figure 3.
(b)
(a) TEM microphotograph of stoichiometric CaZrO3 powder (50% mol CaO). (b) TEM microphotograph of non-stoichiometric CaZrO3
powder (51% mol CaO).
Dilatometric curves recorded for investigated CaZrO3based pellets.
for pure CaZrO3, whereas non-stoichiometric CaZrO3based powders, due to a lower synthesis temperature,
were characterised by crystallite sizes of 40 to 60 nm.
A typical TEM microphotograph of CaZrO3 powder
involving 50% mol CaO and non-stoichiometric CaZrO3
(51% mol) is presented in Figs. 2a, 2b.
The uniform distribution of chemical elements in
CaZrO3 powders is an additional crucial factor with
a strong impact on the sinterability of the powders,
next to manufacture of gas-tight sintered shapes
and performance of electrolytic material. It has also
been determined that washing with distilled water
removes small amounts of calcium (ca. 0.6% wt) from
the calcinated powders at 800°C. This shows that the
assumed CaO dissolves into the CaZrO3 structures to
reach solid-solution form as early as at the stage of
powder synthesis. Contrary to CaZrO3-based powder
prepared by solid-state reaction, after washing CaZrO3based powder with distilled water, considerable losses
of CaO were detected. The results of TEM and EDS
investigations as well as chemical analysis data clearly
indicate that the applied synthesis conditions made
it possible to obtain CaZrO3-based powder with high
chemical homogeneity at rather low temperatures (below
1000°C) compared to solid-state reactions between ZrO2
and CaCO3 reagents, which were previously applied by
Janke et al. for 70–80 years to manufacture CaZrO3based oxygen probes.
Dilatometric studies were performed for the series
of CaZrO3-based samples with different calcia contents.
These curves (Fig. 3) show linear shrinkage of the
samples during the sintering process in a dilatometer
with a heating rate of 6°C min-1.
As can be seen, intensive shrinkage of the CaZrO3
sample with stoichiometric compositions starts at 600°C,
but shrinkage was achieved at values of about 9% of
total shrinkage at 1200oC. Introduction of CaO in excess
caused a small improvement in the sinterability of pellets
prepared from such powders. The results indicate that
the sintering curves of CaZrO3-samples with different
calcia content show distinct differences. Shrinkage of
non-stoichiometric CaZrO3 started at about 700°C and
reached about 12% of total shrinkage. On the other hand,
the diphase CaZrO3-based green samples (CaZrO3 with
CaO precipitations) consolidate only above 1000°C, and
the observed contractions are less than half the size of
monophase samples. These results clearly indicated
that stoichiometry and defect chemistry influences the
sinterability of CaZrO3-based pellets.
The impact of Ca/Zr molar ratio on CaZrO3-based
samples was previously studied by Pollet et al. [23]. The
analysis of shrinkage curves DL/Lo versus temperature
also show that CaCO3 as a starting reagent excess
10% CaO addition to the CaZrO3 phase improves the
sinterability of samples compared to stoichiometric
composition.
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CaZrO3-based powders suitable for
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Figure 4.
(a)
(b)
(c)
(d)
(a) Microstructure of CaZrO3 involving a 50% mol CaO sintered sample at 1500°C for 2 h. (b) Microstructure of a CaZrO3 sample
involving 50% mol CaO synthesised by solid state reaction and sintered at 1700°C for 2 h. (c) Microstructure of non-stoichiometric
CaZrO3 (51% mol CaO) sample sintered at 1500°C for 2 h. (d) Microstructure of a non-stoichiometric CaZrO3-sintered sample (52% mol
CaO) (the sample underwent additional heat treatment at 1200°C for 100 h).
The relative density of monophase CaZrO3-based
samples obtained from powders synthesised by the
Pechini method and sintered at 1500°C was about
96–98% of theoretical density. On the other hand, the
presence of CaO as a second phase in CaZrO3-based
pellets led to a decrease in relative density to about 92–
93% of theoretical density.
The microstructure of CaZrO3 sintered samples is
presented in Figs. 4a-4d.
The highest value of average grain size was
detected for CaZrO3 with stoichiometric composition.
The introduction of excess CaO into solid solution
caused a decrease in grain sizes of CaZrO3 samples
prepared by Pechini and solid state reaction methods.
A comparable difference was detected in monophase
non-stoichiometric CaZrO3 samples. An increase in
CaO in CaZrO3 solid solution caused an increase in
average grain sizes up to composition 51.5% mol CaO
(excess 6% mol CaO) for solid solution, followed by
a small decrease. Variations in grain sizes are higher
for samples prepared using solid-state reactions. In
contrast to the Pechini method, the solubility of CaO in
CaZrO3 in samples prepared using solid-state synthesis
increases with the rising temperatures of sintering
pellets. This suggests that the solubility process takes
place only during sintering, which is unfavorable for the
densification of prepared powders, and for the electrical
properties of the materials. Monophase CaZrO3-based
samples heat-treated at 1700°C for 24 h were prepared
using solid-state reactions. After these experiments the
sintered samples collapsed into powders while being
stored at room temperature. The (probably small)
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Figure 5.
The variation of grain sizes of CaZrO3-based samples
obtained by Pechini method as well as solid-state
reaction.
amount of free CaO remaining at grain junctions may
be responsible for this; it may have reacted with H2O
at room temperature and the subsequent volume
expansion may have brought about the collapse.
On the other hand, in the same experiment, CaZrO3based samples obtained by Pechini method did not
collapse.
Grain boundary chemical composition and structure
are the main factors deciding the contribution of ionic
conductivity in ceramic oxide electrolytes. According
to the reported analyses, the resistivity of the grain
boundary of ceramic oxide electrolytes such as fully
yttria- stabilised zirconia or ceria-based solid solutions
is typically two or three orders of magnitude higher than
that of the grain interior [24,25].
In the case of cubic zirconia-yttria (c-YSZ) solid
solutions, which were usually used as solid-oxide
electrolytes, grain boundary properties were studied
extensively. The summarised results of grain boundary
studies and their influence on ionic conductivity
suggested two main reasons for high temperature
resistivity. One is a space-charge layer near the grain
boundary resistivity layer, near the grain boundary
formed by solute segregation. Conduction of oxygen
ions decreases when the oxygen-vacancy concentration
at the grain boundary drops below the concentration
in the grain interior. This explains the intrinsic grainboundary resistivity of highly pure materials. The
other is a siliceous phase or film at the grain boundary
frequently described as a blocking layer of oxygen
ions. Even a small addition of SiO2 has been known to
segregate at grain boundary. Considering that SiO2 is a
ubiquitous background impurity in ceramic processing, it
is difficult to exclude extrinsic origin even in the relatively
pure sample [26].
The grain boundary structure of CaZrO3-based
samples with excess CaO has not yet been analysed
through the use of transmission electron microscopy to
correlate chemical segregation with the grain boundary
with results of impedance spectroscopy.
TEM investigations, performed for a CaZrO3 sample
with 51.5% mol CaO (6% mol CaO excess), are
presented in Fig. 6. The performed TEM observation
on the thin film prepared from sintered samples did not
detect the difference between the chemical composition
of the grain boundary and grains. No presence of SiO2
was detected in the grain boundary. As can be seen, the
nanometric amorphic separate presence on the surface
grain CaO as precipitation was detected. In contrast,
CaZrO3-based samples prepared from powders
synthesised by reaction in solid phase, reaching the
above-mentioned limit of solubility, are possible only
at the temperature range of 1600–1800°C, depending
on the properties of the starting reagent (CaCO3, ZrO2)
powders.
As it was proven earlier, the reaction method for
solid phase between the ZrO2 and CaO or ZrO2 and
CaO oxides did not lead to homogenous chemical
composition at the stage of CaZrO3 powder synthesis.
In order to visualise this statement, the changes of
solubility limit were recorded for samples obtained by
the Pechini method as well as solid-state reaction and
results are shown in (Fig. 7).
The impedance spectroscopy method enabled the
study of ionic conductivity of CaZrO3-based samples.
The electrical conductivity of CaZrO3 with stoichiometric
composition was about 2×10-6 S cm-2 at 1000°C. This
material seems to be a very poor oxide ion conductor.
The lowest values of total electrical conductivity
(4.8×10-7 S cm-2) were reached for CaZrO3 samples
prepared by solid-state reaction and sintered at higher
temperatures (1750°C). One reason for this behaviour
might involve possible structural changes taking place at
such high temperatures, is most likely due to a structural
phase transformation from orthorhombic phase to cubic
CaZrO3 phase.
In the case of CaZrO3-based samples modified
by excess CaO, impedance spectra were fitted by
equivalent circuits, consisting of a series of 3 subcircuits
of parallel resistor-CPA elements. The subcircuits were
attributed to electrode dispersion (1) one or two grain
boundary/additional phase (2) and bulk electrical
resistance (3).
Analysis of bulk conductivity values (Figs. 8a, 8b) for
all investigated samples obtained by the Pechini method
reveals that the introduction of CaO into CaZrO3 solidsolution form caused an increase in bulk conductivity
as well as grain-boundary conductivity. This may have
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CaZrO3-based powders suitable for
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Figure 6.
TEM microphotograph of non-stoichiometric CaZrO3
(51.5% mol CaO) grain boundary structure.
Figure 7.
The changes of solubility limit of CaO in CaZrO3-based
samples obtained through solid-state reaction as well as
the Pechini method.
15 CSZ
log s (S/cm)
-4.2
(a)
C
A
-6.3
B
A
-5.6
-7.0
Figure 8.
C
-4.9
-5.6
50
(b)
15CSZ
-4.2
B
-4.9
-3.5
log sg (S/cm)
-3.5
contributed to an increase in the oxygen concentration
of vacancies according to the proposed defect model
[27]. For comparison, data for ionic conductivity was
determined for samples prepared through solid-state
reaction as well as other chemical methods. The data
obtained for samples prepared with citrate and with
coprecipitation calcination methods are close to the
values obtained for samples sintered with the Pechini
technique [28,29]
The comparative analysis of impedance data for
CaZrO3 samples prepared by solid-state reaction and
the Pechini method enabled us to find some differences
in the behaviour of such samples. In the case of
CaZrO3-based samples (51.5% mol CaO) prepared
through solid-state reaction (Fig. 9) the values of ionic
conductivity (s at 700oC) depend on the temperature
of the sintering sample as well as on the previouslymentioned homogeneity of CaO distribution in the
samples.
The maximum bulk conductivity depends on the
temperature of the sintering samples. The increase
up to 1700°C in the temperature of the sintering
sample caused an increase in bulk conductivity due
to the higher solubility of CaO in the CaZrO3 structure
along with higher temperatures. On the other hand,
CaZrO3 samples sintered at 1750°C have lower ionic
conductivity values due to possible structural changes.
This fact could be confirmed by the relation between
electrical conductivity and solubility limit in the CaZrO3based samples.
The dependence of activation energy of bulk
conductivity on composition, shown in Fig. 10, seems
to correlate with compositional dependence on
conductivity: maximum conductivity corresponds to
minimum energy activation.
51
52
53
54
% mol CaO
55
56
50
51
52
53
54
55
CaO, % mol
(a) The dependence of bulk conductivity for CaZrO3-based samples recorded at 700°C; A – solid state reaction, B – citrate method,
C – Pechini method; 15CSZ-cubic zirconia solid solution involving 15% mol CaO; (b) The dependence of grain boundary conductivity
for CaZrO3-based samples recorded at 700°C; A – solid state reaction, B – citrate method, C – Pechini method; 15CSZ-cubic zirconia
solid solution involving 15% mol CaO.
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2.5x10-4
2.4x10-4
T =1200oC
-4
ionic bulk
conductivity
1.5x10-4
s (S/cm)
partial pressure pO2, atm
2.0x10
1.0x10-4
5.0x10-5
total conductivity
0.0
1450
1500
1550
1600
1650
1700
1.8x10-4
data from Ref [22]
1.2x10-4
6.0x10-5
CaZrO3 electrolyte
0
Temperature of sintering, oC
Figure 9.
2.4
bulk
grain boundary
Ea, eV
1.6
1.2
0.8
50
51
52
53
54
55
56
57
58
59
% mol CaO
Figure 10.
The changes of energy activation for bulk and grainboundary conductivity for investigated CaZrO3-based
samples
4.0x10-2
T= 1000oC
-2
st(S/cm)
3.2x10
2.4x10-2
1.6x10-2
0
5
10
15
20
25
30
35
40
45
time, h
Figure 11.
4
6
8
10
time, min
The variation of total electrical conductivity and bulk
conductivity (s at 700oC) determined for CaZrO3-sintered
samples involving 51.5% mol CaO; bulk conductivity
determined by the AC impedance spectroscopy method,
total conductivity determined by the DC four-probe
method.
2.0
2
The time dependence of total electrical conductivity
(st) vs. time for non-stoichiometric CaZrO3 sample
(51 % mol CaO).
Figure 12.
The variation of pO2 in liquid copper during the
deoxidation process; 0.2 mass % of initial manganese
content was added.
The most controversial problem is the interpretation
of the impact of grain-boundary conductivity, as
compared to that of chemical composition, on the
electrical properties of CaZrO3-based samples [30,31].
In the case of CaZrO3-based samples synthesised
by the Pechini method, the ionic conductivity of grain
boundary increases to reach solubility limit CaO in
CaZrO3. One way to explain such dependences is “the
geometrical reason” – the grain size decreases with
increasing CaO content and thus the geometrical factor
of the g-b conductivity correlates with the activation
energy of this conductivity.
Time-dependent effects in the total electrical
conductivity of nostoichiometric CaZrO3-based samples
involving 51% mol CaO, prepared through solid- state
reaction and the Pechini method at 1000°C were also
presented in Fig.11.
As can be seen in the case of the CZ-51 sample,
stable values of electrical conductivity were observed
for the investigated time up to 120 h in air at 1000°C.
The obtained data for electrical conductivity values
concerning their stability over time enabled us to test
these materials as components for electrochemical
probes to measure oxygen dissolved in molten nonferrous alloys.
The constructed electrochemical oxygen probes
were tested to measure oxygen dissolved in alloys.
Fig. 12 shows the variation of pO2 (copper) in liquid
copper during the deoxidation process when 0.2 mass
% of initial manganese content was added.
The presented data are in good agreement with
results obtained by González-López [22] during
measurements with an electrochemical probe
manufactured from 8YSZ electrolyte and calculated
through the interaction parameters and the oxygen and
manganese content. These preliminary data indicated
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CaZrO3-based powders suitable for
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Figure 13.
The surface of nostoichiometric CaZrO3 (51 %mol CaO) -based electrochemical oxide probes after immersion in liquid copper.
that a non-stoichiometric CaZrO3 gas-tight electrolyte
obtained by a cost-effective method can be used as
a component in the construction of electrochemical
probes to measure oxygen activity in different nonferrous alloys. Further investigations need to focus on
optimising of construction and performance of CaZrO3based probes.
After tests, the cross-section of CaZrO3-gas tight
ceramics (Fig. 13) was investigated by means of
scanning electron microscopy.
No cracks were identified on the surface of CaZrO3based electrochemical probes.
4. Conclusions
The application of the Pechini method made it possible
to obtain sinterable powders of monophase nonstoichiometric CaZrO3 in a temperature range of 1450–
1500°C for 2 h.
It was found that introduction of excess CaO made
it possible to lower the temperature of synthesis and
then improve the sinterability of samples compared
to stoichiometric CaZrO3 composition. The results
presented in this paper indicate that CaZrO3-based
samples sintered are characterised by considerably
better ionic conductivity compared to solid-state
synthesis. The main advantage of this proposed
method of CaZrO3 synthesis is obtaining powder with
high homogeneity, suitable for manufacturing gas-tight
electrolytic shapes for electrochemical probes. Another
valuable result of this method’s application is lack of
precipitation of CaO as the second phase in the grain
boundary of CaZrO3 sintered samples. The obtained
CaZrO3-sintered components were successfully tested
in electrochemical probes to determine oxygen activity
in molten metals.
Acknowledgment
This paper was carried out under contract (11.11.210.217)
AGH-Univeristy of Science and Technology, Faculty of
Fuels and Energy, Cracow, Poland.
2096
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M. Dudek, A. Rapacz-Kmita
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