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Electrochemical study of oxygen released from stabilized zirconia electrolyte
Solid State Ionics 3/4 (1981) 495-498 North-Holland Publishing C o m p a n y
E L E C T R O C H E M I C A L STUDY OF OXYGEN RELEASED F R O M STABILIZED Z I R C O N I A E L E C T R O L Y T E Shinya O T S U K A , Yoshihiro M A T S U M U R A and Zensaku K O Z U K A Department of Metallurgical Engineering, Faculty of Engineering. Osaka University. Osaka 565. Japan
Pump-out experiments have been performed at 1373 and 1473 K using the galvanic cell; O in liquid
Au/ZrO2(+CaO)/air,Pt. Some quantity of electrical charge contributed by ionic current was observed in spite of negligibly low oxygen concentration in the liquid gold. O u r experimental results have confirmed that the a m o u n t of oxygen released from or taken into a solid electrolyte cannot be neglected.
1. Introduction Generally, stabilized zirconia electrolytes have been regarded as having an invariable composition, and the release or uptake of oxygen by the solid electrolytes has not been considered. However, our previous results at 1073 K [1, 2] have shown that the quantity of oxygen taken into or released from the solid electrolyte cannot be neglected in the range of very low oxygen concentration in liquid metals. In order to reconfirm this phenomenon, the present pump-out experiments have been performed at higher temperatures by using liquid gold as the electrode where the amount of dissolved oxygen is negligible.
2. Experimental The galvanic cell used in this investigation is similar to that in previous papers [2, 3], and described as Q in liquid Au/ZrO2(+ CaO)/air,Pt. A ZrO2(+5 wt.% CaO) tube closed at one end was supplied by Nippon Kagaku Togyo Co. Ltd. with dimensions of 8 mm o.d., 5 mm i.d., and 300 mm length. Gold of better than 99.9 wt.% purity was loaded into the electrolyte tube in a quantity corresponding to ~60 mm height in the liquid state. An alumina rod was used to stir the melt. Iridium wires 0.5 mm connected to platinum wires were used to make electrical con-
nection to the liquid gold. More details have been given in previous papers [1-3]. After the stabilized zirconia tube had been evacuated, purified argon was introduced into the tube, and then the furnace heated to a preselected temperature. Oxygen was first pumped into the liquid gold so that the opencircuit emf was lower than a preselected value of Uz(m). Second, in order to establish a uniform initial oxygen potential throughout the liquid gold, a voltage was applied for a sufficiently long time between wires 1' and 2' so that the emf between wires 1 and 2 has a preselected value, El(m) (see fig. 1 of ref. [2]). After 40 min or longer, the applied voltage was changed quickly to another value without opening the cell circuit so that the emf between wires 1 and 2 has another value, E2(m). The decay curve of electric current was recorded for 40 min or more, and then the applied voltage was changed quickly to another larger value. The pump-out experiments were repeated in this way under conditions of a constant value of A E = E2(m) - El(m) = 200 mV.
3. Results Typical decay curves of electric current versus time are shown in fig. 1 for various initial emf values, El(m) at 1373 K. The electric current reached a nearly constant value in
0167-2738/81/0000-0000/$02.50 © North-Holland Publishing Company
.~. Otsuka et a l /
496
(}
j']
Y
Electrochemical study of oxygen released [rom stabilized zirconia
~20 min. The quantity of electrical charge due to the ionic current, Ao,, was estimated by subtracting the current at 20 or 40 rain from the total current at earlier times and integrating the remaining current over time [1-3]. The apparent initial oxygen concentration in liquid gold, Cj(app.) in a t o m % was calculated according to
:'1 J
!R ,'4
,",t
;','<~
t, n ~'
[31, . . . . .
C,(app.)
i
1
(y
+
"
;~
20 t
50
=
100y[(y
+
l)
1) exp(-2AEF/RT)]-',
-
(1)
where y : (Oio,/2F)(Mm/Wm);Qio,(coulomb) was the total quantity of electrical charge due to /{on, and Mm and Wm, the molar mass and the weight of the liquid gold respectively. The calculated value of Cl(app.) in liquid gold is plotted in fig. 2 against the initial emf, together with our previous results for liquid Ag and Cu [3]. Since the steady current decreased slightly with time even after 20 min, the adop-
40
mlrl
Fig. 1. Change of electric current with time for the experiments with liquid gold.
Log P02 / atm
100
-4
-6
-8
1
I
[
_
-I0
-14
-!2
1
I
I
T = I ,375 K
Cu-O o
[]
50
AE = 200
2O
\
x
g
2O
[]
\
123 I1) k)
--
g
\
0
O []
\A
[3
\
O
g
mV
Ag-0
\
5
[]
0
[]
-6
/
;2-
6_
Au-O
/
_ _
'Spo'pe of
p
I 200
I
i
I
400
I
I
600 Initial
E MF
_ _
I 800
/
I/'4 02
//
L I000
rnV
Fig. 2. Comparison of the values of Cl(app.) at 1373 K measured for liquid Au with those for liquid Ag and Cu [3l. Integration time for liquid Au: O 20 min, • 40 min.
S. Otsuka et al. / Electrochemical study of oxygen released from stabilized zirconia
tion of a longer integration time resulted in a slightly larger value of C~(app.) in liquid gold, as seen from fig. 2. The integration times are 20 min for Ag and 25 min for Cu. The dot-dash and solid lines in fig. 2 indicate the Henry's law line for liquid Ag and Cu, respectively. Fig. 2 supports our previous conclusion [2]: the C~(app.) values observed in the range of very low oxygen concentration in liquid metals are nearly equivalent independent of the native of the liquid metal, and therefore attributable to the release of oxygen from the solid electrolyte. The same conclusion could be also derived from the results at 1473 K.
4. Discussion
The defect equilibria for ZrO2(+CaO) are [4], {O2(g) + Vo = 2 h ' + O ;
(2)
and ~O2(g) + V ; + 2e' = O~,
(3)
where Vo is a vacant oxygen ion site, O~ an oxygen ion on its lattice side, h" a positive hole, and e' an excess electron. Therefore, the concentration of vacant oxygen ion site, Cv6 is expressed as a function of oxygen partial pressure by C v o = - k l p ~ + k2PolJ 4 + k 3 ,
(4)
where k~, k2 and k 3 a r e constants and k3 "> kl, k2>0. When the oxygen partial pressure over the electrolyte is changed from the initial value, Po2 to another one, P62 under the condition that P 6 2 / P o 2 = k4 = constant, the concentration change of the vacant oxygen ion site, ACv6 is expressed by ACv6
= - kl[(p'o2) TM - k'o21 n l / 4 l + k2[(P'o2) TM
= -kffk4
k'O2--U411
- 1)pg24 + k2[(1 - k4)/k4]p~312/4.
(5)
Considering the fairly high density of Vo in a solid electrolyte, even in the case of a pure solid electrolyte, the release or uptake of oxygen with the electrolyte must take place according to reaction (2) and (3) upon changing
497
the oxygen partial pressure. In spite of the fact that eq. (5) predicts a p7312/4 dependence of Cl(app.) for low Po2, the present results shown in fig. 2 deviate somewhat from that prediction. Thus, in comparison with fig. 6 of ref. [2], it is considered that the oxygen is mainly released from the impurity oxides included in the solid electrolyte and, further, that the oxygen released according to reaction (3) becomes significant with increasing temperature. The p h e n o m e n o n of release or uptake of oxygen with solid electrolyte has previously been confirmed by another experimental technique [2, 3] described below. The oxygen dissolved in liquid metals was first p u m p e d out for a sufficiently long time, and this was accompanied by the release of oxygen from the solid electrolyte. The melt was once stirred, and then a preselected amount of oxygen was p u m p e d into the stagnant melt. After the electric current was cut off, the melt was stirred by moving an alumina rod up and down for several seconds at intervals of about ten or more minutes. The concentration profiles of oxygen in the melt shown in fig. 3 have been evaluated from the effect of the stirring on the open-circuit emf. In fig. 3, states (i), (iii), (v), and (vii) correspond to the stagnant melt and are changed to states (ii), (iv), (vi), and (viii) respectively by stirring the melt. State (i) represents the concentration profile immediately after the oxygen was pumped-in. For state (i), an increase in the emf was observed by stirring the melt, corresponding to a decrease in the oxygen concentration in the melt at the melt-electrolyte interface. After equilibrium of oxygen between the electrolyte and melt had been attained, no change in the emf by stirring the melt was usually observed, as seen from states (v) and (vi). If oxygen transfer due to internal electronic current through the electrolyte cannot be neglected, state (vii) would be finally attained and then a similar increase in the emf as for state (i) would be observed by stirring the melt. On the other hand, on the way to the equilibrium states (v) or (vii), a decrease in the emf by stirring the melt was often observed. Thus, the existence of state (iii) has been recognized
S. Otsuka et al. I Electrochemical study of oxygen released from stabilized zirconia
498
(i)
--->
stirring
(ii)
~
(iii)
~
stirring
!i
c
(viii) stirring
(iv)
/r
(vii)
,~-
(vi)
~--
',v)
sT~rr~ng
)
d,stance Fig. 3. Change of the concentration profile of oxygen in periodically stirred melt with time, after the pump-out and subsequent pump-in experiments. The solid electrolyte is indicated by the shaded part, and then states (i), (iii), (v) anti (vii) correspond to stagnant melt.
which confirms the p h e n o m e n o n of uptake of oxygen by the electrolyte. This p h e n o m e n o n may be partly related to the existence of a second phase in the solid electrolyte reported by Beekmans and H e y n e [5]. The present results indicate that various unexplained observations in experiments with a solid electrolyte may be accounted for by the introduction of the concept of release or uptake of oxygen with the solid electrolyte. As described in our previous papers [1-3], even very low oxygen concentrations in liquid metals can be determined precisely by subtracting the error due to the oxygen released from the solid electrolyte.
Acknowledgement The authors are grateful to Professor Moriyama of Kyoto University for providing the pure gold.
References [1] S. Otsuka and Z. Kozuka, Met. Trans. 10B (1979) 565. [2] S. Otsuka, T. Sano and Z. Kozuka, Met. Trans. l I B (1980) 313. [3] S. Otsuka and Z. Kozuka, Met. Trans. 12B (1981), to be published. [4] J.W. Patterson, E.C. Bogren and R.A. Rapp, J. IElectrochem. Soc. 114 (1967) 752. [5] N.M. Beekmans and k. Heyne, Electrochim. Acta 21 (1976) 303.
E L E C T R O C H E M I C A L STUDY OF OXYGEN RELEASED F R O M STABILIZED Z I R C O N I A E L E C T R O L Y T E Shinya O T S U K A , Yoshihiro M A T S U M U R A and Zensaku K O Z U K A Department of Metallurgical Engineering, Faculty of Engineering. Osaka University. Osaka 565. Japan
Pump-out experiments have been performed at 1373 and 1473 K using the galvanic cell; O in liquid
Au/ZrO2(+CaO)/air,Pt. Some quantity of electrical charge contributed by ionic current was observed in spite of negligibly low oxygen concentration in the liquid gold. O u r experimental results have confirmed that the a m o u n t of oxygen released from or taken into a solid electrolyte cannot be neglected.
1. Introduction Generally, stabilized zirconia electrolytes have been regarded as having an invariable composition, and the release or uptake of oxygen by the solid electrolytes has not been considered. However, our previous results at 1073 K [1, 2] have shown that the quantity of oxygen taken into or released from the solid electrolyte cannot be neglected in the range of very low oxygen concentration in liquid metals. In order to reconfirm this phenomenon, the present pump-out experiments have been performed at higher temperatures by using liquid gold as the electrode where the amount of dissolved oxygen is negligible.
2. Experimental The galvanic cell used in this investigation is similar to that in previous papers [2, 3], and described as Q in liquid Au/ZrO2(+ CaO)/air,Pt. A ZrO2(+5 wt.% CaO) tube closed at one end was supplied by Nippon Kagaku Togyo Co. Ltd. with dimensions of 8 mm o.d., 5 mm i.d., and 300 mm length. Gold of better than 99.9 wt.% purity was loaded into the electrolyte tube in a quantity corresponding to ~60 mm height in the liquid state. An alumina rod was used to stir the melt. Iridium wires 0.5 mm connected to platinum wires were used to make electrical con-
nection to the liquid gold. More details have been given in previous papers [1-3]. After the stabilized zirconia tube had been evacuated, purified argon was introduced into the tube, and then the furnace heated to a preselected temperature. Oxygen was first pumped into the liquid gold so that the opencircuit emf was lower than a preselected value of Uz(m). Second, in order to establish a uniform initial oxygen potential throughout the liquid gold, a voltage was applied for a sufficiently long time between wires 1' and 2' so that the emf between wires 1 and 2 has a preselected value, El(m) (see fig. 1 of ref. [2]). After 40 min or longer, the applied voltage was changed quickly to another value without opening the cell circuit so that the emf between wires 1 and 2 has another value, E2(m). The decay curve of electric current was recorded for 40 min or more, and then the applied voltage was changed quickly to another larger value. The pump-out experiments were repeated in this way under conditions of a constant value of A E = E2(m) - El(m) = 200 mV.
3. Results Typical decay curves of electric current versus time are shown in fig. 1 for various initial emf values, El(m) at 1373 K. The electric current reached a nearly constant value in
0167-2738/81/0000-0000/$02.50 © North-Holland Publishing Company
.~. Otsuka et a l /
496
(}
j']
Y
Electrochemical study of oxygen released [rom stabilized zirconia
~20 min. The quantity of electrical charge due to the ionic current, Ao,, was estimated by subtracting the current at 20 or 40 rain from the total current at earlier times and integrating the remaining current over time [1-3]. The apparent initial oxygen concentration in liquid gold, Cj(app.) in a t o m % was calculated according to
:'1 J
!R ,'4
,",t
;','<~
t, n ~'
[31, . . . . .
C,(app.)
i
1
(y
+
"
;~
20 t
50
=
100y[(y
+
l)
1) exp(-2AEF/RT)]-',
-
(1)
where y : (Oio,/2F)(Mm/Wm);Qio,(coulomb) was the total quantity of electrical charge due to /{on, and Mm and Wm, the molar mass and the weight of the liquid gold respectively. The calculated value of Cl(app.) in liquid gold is plotted in fig. 2 against the initial emf, together with our previous results for liquid Ag and Cu [3]. Since the steady current decreased slightly with time even after 20 min, the adop-
40
mlrl
Fig. 1. Change of electric current with time for the experiments with liquid gold.
Log P02 / atm
100
-4
-6
-8
1
I
[
_
-I0
-14
-!2
1
I
I
T = I ,375 K
Cu-O o
[]
50
AE = 200
2O
\
x
g
2O
[]
\
123 I1) k)
--
g
\
0
O []
\A
[3
\
O
g
mV
Ag-0
\
5
[]
0
[]
-6
/
;2-
6_
Au-O
/
_ _
'Spo'pe of
p
I 200
I
i
I
400
I
I
600 Initial
E MF
_ _
I 800
/
I/'4 02
//
L I000
rnV
Fig. 2. Comparison of the values of Cl(app.) at 1373 K measured for liquid Au with those for liquid Ag and Cu [3l. Integration time for liquid Au: O 20 min, • 40 min.
S. Otsuka et al. / Electrochemical study of oxygen released from stabilized zirconia
tion of a longer integration time resulted in a slightly larger value of C~(app.) in liquid gold, as seen from fig. 2. The integration times are 20 min for Ag and 25 min for Cu. The dot-dash and solid lines in fig. 2 indicate the Henry's law line for liquid Ag and Cu, respectively. Fig. 2 supports our previous conclusion [2]: the C~(app.) values observed in the range of very low oxygen concentration in liquid metals are nearly equivalent independent of the native of the liquid metal, and therefore attributable to the release of oxygen from the solid electrolyte. The same conclusion could be also derived from the results at 1473 K.
4. Discussion
The defect equilibria for ZrO2(+CaO) are [4], {O2(g) + Vo = 2 h ' + O ;
(2)
and ~O2(g) + V ; + 2e' = O~,
(3)
where Vo is a vacant oxygen ion site, O~ an oxygen ion on its lattice side, h" a positive hole, and e' an excess electron. Therefore, the concentration of vacant oxygen ion site, Cv6 is expressed as a function of oxygen partial pressure by C v o = - k l p ~ + k2PolJ 4 + k 3 ,
(4)
where k~, k2 and k 3 a r e constants and k3 "> kl, k2>0. When the oxygen partial pressure over the electrolyte is changed from the initial value, Po2 to another one, P62 under the condition that P 6 2 / P o 2 = k4 = constant, the concentration change of the vacant oxygen ion site, ACv6 is expressed by ACv6
= - kl[(p'o2) TM - k'o21 n l / 4 l + k2[(P'o2) TM
= -kffk4
k'O2--U411
- 1)pg24 + k2[(1 - k4)/k4]p~312/4.
(5)
Considering the fairly high density of Vo in a solid electrolyte, even in the case of a pure solid electrolyte, the release or uptake of oxygen with the electrolyte must take place according to reaction (2) and (3) upon changing
497
the oxygen partial pressure. In spite of the fact that eq. (5) predicts a p7312/4 dependence of Cl(app.) for low Po2, the present results shown in fig. 2 deviate somewhat from that prediction. Thus, in comparison with fig. 6 of ref. [2], it is considered that the oxygen is mainly released from the impurity oxides included in the solid electrolyte and, further, that the oxygen released according to reaction (3) becomes significant with increasing temperature. The p h e n o m e n o n of release or uptake of oxygen with solid electrolyte has previously been confirmed by another experimental technique [2, 3] described below. The oxygen dissolved in liquid metals was first p u m p e d out for a sufficiently long time, and this was accompanied by the release of oxygen from the solid electrolyte. The melt was once stirred, and then a preselected amount of oxygen was p u m p e d into the stagnant melt. After the electric current was cut off, the melt was stirred by moving an alumina rod up and down for several seconds at intervals of about ten or more minutes. The concentration profiles of oxygen in the melt shown in fig. 3 have been evaluated from the effect of the stirring on the open-circuit emf. In fig. 3, states (i), (iii), (v), and (vii) correspond to the stagnant melt and are changed to states (ii), (iv), (vi), and (viii) respectively by stirring the melt. State (i) represents the concentration profile immediately after the oxygen was pumped-in. For state (i), an increase in the emf was observed by stirring the melt, corresponding to a decrease in the oxygen concentration in the melt at the melt-electrolyte interface. After equilibrium of oxygen between the electrolyte and melt had been attained, no change in the emf by stirring the melt was usually observed, as seen from states (v) and (vi). If oxygen transfer due to internal electronic current through the electrolyte cannot be neglected, state (vii) would be finally attained and then a similar increase in the emf as for state (i) would be observed by stirring the melt. On the other hand, on the way to the equilibrium states (v) or (vii), a decrease in the emf by stirring the melt was often observed. Thus, the existence of state (iii) has been recognized
S. Otsuka et al. I Electrochemical study of oxygen released from stabilized zirconia
498
(i)
--->
stirring
(ii)
~
(iii)
~
stirring
!i
c
(viii) stirring
(iv)
/r
(vii)
,~-
(vi)
~--
',v)
sT~rr~ng
)
d,stance Fig. 3. Change of the concentration profile of oxygen in periodically stirred melt with time, after the pump-out and subsequent pump-in experiments. The solid electrolyte is indicated by the shaded part, and then states (i), (iii), (v) anti (vii) correspond to stagnant melt.
which confirms the p h e n o m e n o n of uptake of oxygen by the electrolyte. This p h e n o m e n o n may be partly related to the existence of a second phase in the solid electrolyte reported by Beekmans and H e y n e [5]. The present results indicate that various unexplained observations in experiments with a solid electrolyte may be accounted for by the introduction of the concept of release or uptake of oxygen with the solid electrolyte. As described in our previous papers [1-3], even very low oxygen concentrations in liquid metals can be determined precisely by subtracting the error due to the oxygen released from the solid electrolyte.
Acknowledgement The authors are grateful to Professor Moriyama of Kyoto University for providing the pure gold.
References [1] S. Otsuka and Z. Kozuka, Met. Trans. 10B (1979) 565. [2] S. Otsuka, T. Sano and Z. Kozuka, Met. Trans. l I B (1980) 313. [3] S. Otsuka and Z. Kozuka, Met. Trans. 12B (1981), to be published. [4] J.W. Patterson, E.C. Bogren and R.A. Rapp, J. IElectrochem. Soc. 114 (1967) 752. [5] N.M. Beekmans and k. Heyne, Electrochim. Acta 21 (1976) 303.