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Electrical conductivity and structure of CaO-stabilized zirconia doped with copper and iron
Solid State Ionics 14 (1984) 93-105 North-Holland, Amsterdam
ELECTRICAL CONDUCTIVITY AND STRUCTURE OF CaO-STABILIZED ZIRCONIA DOPED WITH COPPER AND IRON M. HARTMANOVA Institute of Physics, Electro-Physical Research Centre of the Slovak Academy of Sciences, 84228 Bratislava, Czechoslovakia L. MACHOVI~ Department of Physics, Faculty of Engineering, Slovak Technical University, 81231 Rratislava, Czechoslovakia A. KOLLER Research Institute of Electrotechnic Ceramics, 50064 Hradec Krdlovd, Czechoslovakia F. HANIC Institute of Inorganic Chemistry, Chemical Research Centre of the Slovak Academy of Sciences, 84236 Bratislava, Czechoslovakia and B. MIgANIK Computer Centre of the Slovak Academy of Sciences, 84235 Bratislava, Czechoslovakia Received 12 March 1984
The investigation of the influence of copper and iron oxides on the electrical conductivity, the lattice defects and the structure of calcia-stabilized zirconia has shown that (i) the influence of copper is negligible due to the evaporation of this element at the sintering temperature as a consequence of its high volatility; (ii) the presence of iron in the sample prevents sintering of the calcia-stabilizedzirconia; (iii) the addition of iron causes decomposition of the cubic solid solution CaxZr 1_xO2_x into monoclinic ZrO2, the vacant phase Cao. 1Zro.901.9, calcium ferrite and iron oxides; the solubility limit of iron oxide was found to be ~.1 mol%; (iv) the calcia-stabilized zirconia doped with iron is a mixed conductor at low temperatures; at high temperatures (>1073 K) it is a dominant ionic conductor.
1. Introduction Impurity doping is one way of modifying the electrical properties of calcia-stabilized zirconia (CSZ). Transition metal ions may introduce electronic conductivity into the CSZ ceramics. For instance, the addition of 0.8 wt.% V, 0.1 wt.% Fe and 0.01 wt.% Cr induced 50% electronic conductivity at 1300 K and 1.013 X 10 - 1 0 Pa [1]. The other way how to influence the behaviour of the systems based on the CSZ materials, is the change of the oxidation state of the admixtures dissolved in the solid
state electrolyte. Such impurities may cause significant modifications in the electrical conductivity [2 ]. Redox reactions, for example, can be used to improve the properties of electrical energy storage systems in single phase batteries [3,4]. The aim of the present work is to investigate the influence of copper and ion oxides on the electrical conductivity, the lattice defects and the structure of CSZ.
94
M. tfartmanovd et al./CaO-stabilized zirconia
2. Experimental
2.3. Density measurement
2.1. Preparation o f samples
The Amsler method was applied for density measurements. The volume of the weighed samples was determined from the volume of the displaced mercury at 298 K. The density values, Pexp, were reproducible within -+0.02 g/cm 3 .
Samples o f CSZ ceramics were prepared from ZrO 2 (reactor grade) and CaCO 3 (p.a.). Copper and iron were added in the form of aqueous solutions o f cupric nitrate, Cu(NO3) 2.2H20 and ferrous sulphate, FeSO 4.7H20 , both o f analytical reagent quality. The wet mixtures were homogenized in a ball mill for 2 h, then dried and heated at 1 2 7 3 - 1 3 2 3 K in order to decompose the nitrates. The calcined products were reground in ball mill for 24 h, until the surface of grains increased to 1.9 m2/g. The samples prepared in the form o f disks by dry pressing at 100 MPa were heated at 1700 K (pure CSZ samples at 2000 K) for 1 h in air. The initial composition o f mixtures is given in table 1.
2.2. Measurement o f the lattice parameters The unit cell parameters and the unit cell volume o f samples were evaluated from the indexed powder diffraction data obtained on Philips PW 1050 diffractometer using CuKa radiation and scan speed 1°20/ rain. a-A1203 (1:1) was used as an internal standard.
2.4. LTemental distribution o f Zr, Ca, Cu and Fe and investigation o f sample surfaces The grinded and polished disks o f sintered samples were examined on a microprobe Jeol JXA-5A device for elemental distribution o f Zr, Ca, Cu and Fe. Investigation o f surfaces was performed on an electron microscope Tesla BP 3002.
2.5. Measurement o f electrical conductivity The measurement of electrical conductivity was performed in air within a temperature range 6731373 K. To avoid an electrode polarization, mainly at higher currents, and noise signals, the ac method using lock-in detection was applied. The capacitive component in ac conductivity measurements was excluded by determination of the voltage U, applying phase angle ~p:
U=A 0 cos~. Table 1 The initial composition of mixtures. Sample l
2 3 4
5 6 7 8 9 10 11 12 13
Composition of mixtures (ZRO2)0.85 (CaO)o. 15 (ZrO2)0.85 (CaO)0.15 (Fe203)O.O05 (ZrO2)0.85 (CaO)0.15(Fe203)0.01 (ZrO2)0.85 (CaO)o. 15 (Fe203)O.03 (ZrO2)0.85 (CaO)o. 15 (CuO)o.o05 (ZrO2)0.85 (CaO)o. 1 s (CuO)o.o 1 (ZrO2)0.85(CaO)o. 1 s(CuO)o.o3 (ZrO2)o.85 (CaO)o. 15 (CuO)o.o 5 (ZrO2)o. 889 (CaO)o. 1o(CuO)o. 01 (Fe203)0.O01 (ZrO2)0.8 a7 (CaO)o. 1o(CuO)o.o 1(Fe203)o.oo 3 (ZrO 2)0.885 (CaO)o. 1o(CuO)o. 01 (Fe203)o. O05 (ZrO2)0.857 (CaO)o. 12 (CuO)o. 02 (Fe203)0. O03 (ZrO2)o. 859(CaO)o. 12(CuO)o.02(Fe203)O.O01
(1)
The temperature was measured by a P t R h - P t thermocouple and the voltage U was registered on a digital voltmeter. The electrodes were prepared by deposition o f a platinum emulsion on sintered samples in form o f disks (the cross section -~0.8 cm 2, the thickness ~1 mm), with subsequent annealing. The conductivity measurement was performed at increasing and decreasing temperatures. Each read-out value was first stabilized for 15--50 min. The numerical and graphical dependence of electrical conductivity on temperature was evaluated by a least square programme on a Siemens 4004 computer.
3. Results
3.1. Lattice parameters and density The dependence o f lattice parameters on compo-
M. Hartmanovdet al./CaO-stabilized zirconia Table 2 The measured lattice parameters and densities of fluorite type solid solution of CaxZr l_xO2_x doped with CuO, Fe203 and CuO + Fe203. x (mol%)
15
Impurity concentration (tool%)
5.1268(4) 5.1273(5) 5.1288(8) 5.1270(11) 5.1302(16)
4.295 3.697 4.153 3.757 3.645
5.0
3.896 4.702
4.0
+ 3.0 Fe203
5.1273(7) Cao.IZro.901. 9 + m-ZrO 2 + calcium ferrite + iron oxides ibidem
4.401
+ 1.0 CuO + 0.1 Fe203
5.1272(13)
4.295
+ 1.0 CuO + 0.3 Fe203
5.1281(7)
4.438
+ 1.0 CuO + 0.5 Fe203
5.1235(10)
4.013
+ 2.0 CuO + 0.1 Fe203
5.1273(10)
4.498
+ 2.0 CuO + 0.3 Fe203
5.1270(8)
4.699
-
12
o (Zr 02}(Co0 ) * (Z r 02 )(CoO}(CuOlx (Zr 02)(C00)(Fe203) x
[gc
Measured density3 (g cm- )
+ 0.5 Fe203 + 1.0 Fe203
10
9
Measured lattice parameter (10 -10 m)
+ 0.5 CuO + 1.0 CuO + 3.0 CuO + 5.0 CuO
5.5
o (z rO2){CoO)(CuO)ao~ (Fe203) * • (Zr 02)(CoO)(Cu 0)o.o2(Fe203)×
4.5
3.5 3,0
0
sition is summarized in table 2. Doping with copper has only negligible influence on the lattice parameters o f CSZ. The observed deviations are within the errors of measurements. In case o f iron, the lowest concentration 0.5 mol% F e 2 0 3 used in experiments, practically does not change the lattice parameter of the undoped CSZ. At 1 mol% F e 2 0 3 , a decomposition of solid solution CaxZr 1 _ x O 2 _ x takes place in m-ZrO2, a vacant phase Cao.lZro.9OI.9, calcium ferrite and iron oxides ( F e 2 0 3 , F e 3 0 4 ) . Obviously, the solubility limit of iron oxide in CSZ is less than 1 mol% F e 2 0 3 . A simultaneous doping with copper and iron oxides, using concentrations given in table 1, practically does not influence the lattice parameters. The dependence o f experimental density, Pexp, on the amount o f added copper, iron or copper and iron oxides together, is presented in fig. 1 and table 2. Evidently, an increasing amount o f copper in CSZ causes decrease o f density in the investigated systems.
95
I
I
I
I
I
1
2
3
4
s
I
x [mot
]
Fig. 1. The dependence of experimental density, aexp, on the amount of added copper, iron or copper and iron oxides together. The addition o f iron oxide into CSZ, on the other hand, increases the density o f the system at concentrations >1 mol% F e 2 0 3 . In case o f a simultaneous doping with copper and iron oxides, the density o f samples increases with increasing copper concentration.
3.2. Elemental distribution o f Zr, Ca, Cu and Fe Distribution o f Zr and Ca in the investigated samples is not uniform (figs. 2 a - d ) . Distribution o f Zr should be correlated with the regions enriched with Ca. A comparison o f the distributions o f Ca, Zr and Fe in figs. 3 a - c unambiguously indicates formation o f two kinds o f clusters; one kind combines calcium and iron (calcium ferrite) and the other is the iron oxide ( F e 2 0 3 , F e 3 0 4 ) . The segregation o f Fe and Ca is quite evident in case o f the highest used concentration (3 mol% F e 2 0 3 ) . According to the X-ray diffraction analysis, this segregation takes place already in the sample with 1 mol% F e 2 0 3 , but the products o f segregation remain distributed in the matrix. No segregation o f Fe was observed at a simultaneous doping with copper and iron oxides (figs. 4a and b). Investigation o f the composition detected only
96
M. Hartmanovd et al./CaO-stabilized zirconia
Fig. 2. The distribution of elements in the sintered samples (electron probe photomicrographs, magnification ×600): (a) Zr in (ZrO2)o.8s (CaO)0.15, (b) Ca in (ZrO2)o,SS (CaO)o. l S, (c) Zr in (ZrO2)o.8s (CaO)o. 15 (Fe203)o.o3, (d) Ca in (ZrO2)o.ss (CaO)o. 1s (Fe203)o.o3. very weak traces o f Cu between Zr (fig. 5). All these results correspond well with the X-ray data on the lattice parameters and phase composition (sections 3.1. and 4.1 .) and with the electrical conductivity (sections 3.3 and 4.2.).
3.3. Investigation o f the sample sur/hces Surfaces of tire CSZ samples investigated with the electron microscopy, show only a minimum internal porosity of grains (fig. 6a) and practically no intergranular porosity. The addition of iron oxide influences the sintering
M: Hartmanovd et aL/CaO-stabilized zirconia
a .r
':
,
,~ .~ . .
X ~ Y{, '"
"
~'. " " . ' : ' , i . " ; .~
"." .
.....
";,'"
"~.
Fig. 3. The distribution of Fe in the sintered samples (electron probe photomicrographs, magnification X600): (a) (ZrO2)o. 85 (CaO)o. 15 (Fe203)0.005, (b) (ZrO2)o. 85 (CaO)o. 15 (Fe203)O.O 1, (c) (ZRO2)0.85(CaO)o. 15 (Fe203)0.O3. process. According to results in section 3.1, segregated iron oxide covers the ZrO 2 grains and prevents the growth of larger grains restraining the sintering of samples. Such influence was not observed in case of copper doping (fig. 6d). Irregular grains are formed in case of simultaneous doping with copper and iron oxides (fig. 6e). The
ZrO 2 grains which are probably covered by iron oxide, remain finely dispersed and sintering o f zirconia does not proceed.
97
98
M. Hartmanovd et al./CaO-stabilized zirconia
Fig. 4. The distribution of elements in the sintered samples (ZrO2)o.8(CaO)0. l (CuO)o.o2(Fe203)O.O03 (electron photomicrographs, magnification X 600): (a) Ca, (b) Fe.
3.4. Electrical conductivity 3.4.1. CSZ doped with copper oxide The total electrical conductivity, o t, of calciastabilized zirconia is practically independent on the
Fig. 5. The distribution of Cu in the sintered samples (ZrO2)o. 85 (CaO)o. 15 (CuO)o.05 (electron probe photomicrograph s, magnification × 600).
amount of added copper. The only exception is the sample with the highest 5 tool% CuO doping. The electrical conductivity o t can be characterized in all investigated samples including pure CSZ, by two regions within the investigated temperature intervals and concentration ranges (figs. 7 and 10): (i) The low-temperature region occurs between 673 and 1073 K and is characterized by a mean value of the activation energy of conductivity 1.24 eV; (ii) the high-temperature region occurs in all investigated samples between 1073 and 1273 K. The conductivity data are not suitable for evaluation of the activation energy due to the transient character of the electrical conductivity in this region.
3.4.2. CSZ doped with iron oxide The dependence of the total electrical conductivity, Or, of CSZ, on the amount of added iron oxide is characterized in all investigated samples by two regions within the investigated temperature intervals and concentration ranges (figs. 8 and 10): (i) The low-temperature region occurs between 673 and 1123 K. ]'he activation energy of electrical conductivity does not show any systematic dependence on the amount of iron oxide, lts mean value is approximately 1.09 eV;
M. Hartmanovd et aL /CaO-stabilized zirconia
99
Fig. 6. The grinded and polished surfaces of the sintered samples: (a) (ZRO2)0.85(CaO)o. 15, EM's, magnification ×500; (b) (ZrO2)0.8 s (CaO)o. 15, EM's, magnification × 500, topography; (c) (ZrO2)o. 85 (CaO)o. 15 (Fe203)O.03, EM's, magnification X 1000; (d) (ZrO2)o.85 (CaO)o. 15 (CuO)0.0s, EM's, magnification X 1000; (e) (ZRO2)0.857(CaO)o.l 2(CUO)0.02 (Fe203)O.O03, EM's, magnification X 1000. (ii) the high-temperature region occurs above 1123 K. The activation energy of electrical conductivity is 1.24 eV for 0.5 mol% F e 2 0 3 , 1.54 eV for 1 mol% F e 2 0 3 and 1.23 eV for 3 mol% F e 2 0 3. No transient region occurs between the low- and high-temperature regions.
The doping with 0.5 mol% F e 2 0 3 does not practically influence the total electrical conductivity a t. At 0 . 5 - 1 . 0 mol% Fe203, a decrease of o t of the system was observed. At concentrations >1 mol% Fe203, the electrical conductivity o t again increases.
1 O0
M. Hartmanovd et al./CaO-stabilized zircon&
0t
1
o (ZrO2){CQO} •
100
=i~.~.... ~" ~ , . . . . . "~..~.
llj 1
(Zr02 }{CoO)(CuO)o.oo 5
~ (ZrO~)(CoO)iCuO)oo 1 * •
Z rO2)(CclO){CuO)o 03
(z,o)ccoo){cuO)olo
-2
10
-3
10
-4
10
lO •
-6
10
0.7
÷
V
I
I
I
I
I
I
I
I
I
I
0~8
0'9
lie
1~1
112
113
I~~
115
116
L
1~710~KII ]
Fig. 7. The dependence o f the total electrical conductivity, at, of calcia-stabilized zirconia on the a m o u n t o f added copper.
IIt
•
100
(Zr 02)(C00 )
(Zr 02)(CoO)(Fe203)ao05 o (Z r 02){C00) (Fe203)ao 1 ÷ (Z r 02 )(CclO} (Fe203}o 03
.¢~. ~
~ ~,.,~,,~
10-1 ld 2 10.3 -4
10 -5
10
io6 0.7
I
I
l
1
I
i
I
I
I
0.8
O.g
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
IO~'T [K -1] Fig. 8. The dependence o f the total electrical conductivity, at, of calcia-stabilized zirconia on the a m o u n t o f added iron.
M. Hartmanovd et al./CaO-stabilized zirconia
,
(ZrO2)(CoO){CuO)o.ol (Z rO 2){C00 )(CuO)o.ol(Fe203)o.oo 1
*
10 o ,,~
101
o (ZrO2)(CQO)(CuO)o.o 1 (Fe203)0.003 , (ZrO 2){CO0)(CuO)O.01 (Fe203)0.005
10-1 _ -2 10 -3 10 ~-
lO
_
1() 5
.
-6 10 O.7
I
I
I
I
I
I
I
I
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7 ---" 103/T [ K -1]
(5 t
• (ZrO2)(CoO)(CuO)o.o 2 (Fe203)Qo01 10 o
003
10-1 10-2 10-3 1() 4 10-5 1(36 0.7
I
I
I
1
I
I
I
I
I
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
I
----
lO /T [K"J
Fig. 9. T h e dependence o f the total electrical conductivity o t of calcia-stabilized zirconia o n the simultaneous doping with copper and iron: (a) 1 mol% CuO, (b) 2 mol% CuO.
102
M. Hartrnanovd et al. /CaO-stabilized zirconia
3.4.3. CSZ doped with copper and iron oxides The dependence of the total electrical conductivity, ot, on the amount of simultaneously added copper and iron oxides is characterized for 1 mol% CuO by a single region within the investigated temperature and concentration ranges. The mean activation energy of the electrical conductivity is 1.26 eV (fig. 9a). The iron addition reduces o t in the whole investigated temperature range in comparison with the samples without Fe203 doping. The doping with 2 mol% CuO is characterized by two temperature regions of electrical conductivity (fig. 9b):
(i) The low-temperature region occurs for both concentrations of iron oxide between 673 and 1173 K. The mean activation energy of electrical conductivity is 1.24 eV; (ii) the high-temperature region occurs for both investigated iron concentration above 1173 K. The values of o t in this system do not depend on the amount of the added iron.
4. Discussion 4.1. The influence o f copper and iron oxides on the defect structure o f CSZ
The copper addition hardly influences the measured lattice parameters (section 3.1 .) and the total electrical conductivity (section 3.4.) of the CSZ samples. However, the measured density of samples decreases with the increasing additions of copper. The presence of the copper impurity reduces sintering temperatures of samples from 2000 K (pure CSZ) to 1573 K. The negligible influence of copper oxide on the lattice parameters and electrical conductivity is a consequence of dissociation of CuO and high volatility of the products of decomposition [6,7]. The small amount of copper remains concentrated in the surface layer of glassy grains which are formed during the sintering process. The X-ray fluorescence analysis of the sintered sample with 5 mol% CuO showed only 1/5 of the copper amount originally present in the sample. The chemical analysis performed on the sintered sample with 5 mol% CuO after melting in KHSO 4 (the ASS method using the
Southern Analytical A 3000 apparatus) showed copper in a concentration 0.71 wt.% CuO. Both analytical methods confirmed a sharp decrease of the Cu content in samples after sintering at 1700 K. The dissociation of CuO takes place in air at 1300 K. It proceeds via the formation of Cu20 and 0 2 [6]. The dissociation pressure is given by the relation [71: logp =
1 3 2 1 6 T + 12.35 .
(2)
The value log p = 0 is achieved at 1350 K, very close to the melting point of Cu. The melting of CuO does not take place due to decomposition at lower temperature into metallic Cu and 02. The vapour tension of Cu at 1400 K is 10 . 3 mm Hg, at 1600 K is 0.5 mm Hg. The volatility of copper oxide at 1700 K (sintering temperature) is so high that it is left only in a reduced concentration in the sintered sample. In case of the iron oxide admixtures in CSZ, the iron ions enter into the CaxZr 1 x O 2 _ x structure either through substitution for cations in the cationic sublattice or they take interstitial positions. Therefore the incorporation of iron ions leads to a decrease of the vacancy concentration in the anionic sublattice [8]. Vacancies were formed through incorporation of CaO into the cubic fluorite phase C a O ~ C a z2+ r +V 2
.
(3)
A scheme for the substitutional incorporation of Fe203 can be adopted Fe203 -+ 2 Fear +V~)- .
(4)
The interstitial incorporation of Fe203 may be described by the relation:
Fe203 + 3 V g - -~ 2 Fe 3- .
(5)
In the stabilized fluorite phase Y x Z r l xO2 x, in which Y203 does not react with Fe203 [9], solubility of iron oxide increases (5 tool%) and majority of iron ions enters substitutionally into tire vacant phase. In the CSZ system, the interstitial incorporation is unlikely. Due to the reaction of CaO with Fe203 (formation of calcium ferrite, Ca 2 Fe205) at the sintering temperature, a part of Ca2+ ions is extracted from the CSZ phase and the cubic phase is destabilized under the formation of m-ZrO 2 , Ca0. IZr0.901.9, calcium ferrite and iron oxides. The coexistence of m-ZrO 2
M. Hartmanovd et aL /CaO-stabilized zirconia
and Ca0.1Zr0.901. 9 indicates that the amount of the extracted CaO is ~5 mol% [10]. In monoclinic ZrO2, there are two anion sites: O I sites with the coordination number three and OII sites with the coordination number four [11 ]. The theoretical estimates show [ 12] that the oxygen anion vacancies can move in m-ZrO 2 only over the OI sites in the (100) direction with the theoretical activation energy 3.87 -+ 0.34 eV. The observed activation energy, 1.72 eV, is, however, much smaller [13]. This means that theoretical results contradict conception of a bulk diffusion mechanism. A more probable explanation is that of Cox [ 14] according to which the rate controlling step is the anion vacancy diffusion along microcracks, grain boundaries or dislocations.
4.2. The influence o f copper and iron oxides on the electrical conductivity o f CSZ The preliminary measurements of the transfer numbers in CSZ doped with copper oxide showed that the ionic transfer numbers t i are 0.99 at 873 K and 1.00 at 973 K and 1073 K. It means that CSZ
103
doped with copper oxide is a pure ionic conductor above 873 K. The mean value of the activation energy of electrical conductivity in the copper doped CSZ ceramics is 1.24 eV for the low temperature region 673-1073 K. This value is comparable with the value 1.24 eV found by Kovalev and Pechinenskii [15] for the activation energy of defect migration in CSZ from internal friction measurements. Probably, it corresponds to the (100)anion vacancy migration between two positions of Zr 4+ ions. The calculated migration energy is (1.34 + 0.29) eV [12]. The values of Hoffman and Fischer [16] occur within 1.20-1.27 eV. All these values are well comparable. In both copper-doped and undoped CSZ occurs a decrease of electrical conductivity at about 1073 K. A similar effect was observed by other authors, too [ 16,17 ]. According to Etsel and Flengas [ 1], such conductivity decrease occurs due to the incomplete stabilization of ZrO 2. We could not confirm this result. In our X-ray diffraction analysis, we did not identify the presence of m-ZrO 2. We see a more probable reason of the observed deviation in a decrease of the mo-
(I t
[0-1m-1]
o (ZrO2)(CQO) (Z rO 2 )(O30)(CuO)o.oo5 " (ZrO2}(CaO)(Fe203)o.oo 5 ,, (ZrO 2 )(CaO)(CuO)om , (ZrO 2 )(CaO)(Fe20 ~)o.01 ,' (ZrO 2 )(CQO)(CuO)oo3 .,. (ZrO2)(CaO)(Fe203)o.03 o
10°
~ ""~_
104
,
10-2 .,,.,
~ : ~ ~ -"~.,.~.
,"-'~
l o -3
10-4 1C~"5 -6
10
0.7
I
L
I
0.8
0.9
1.0
L
t
1.2
L
t
I
t
t
1.,.,
is
1.6
1.7
I ----
[K-']
Fig. 10. The comparison o f t h e influence o f the same concentrations o f copper and iron added into the calcia-stabilized zirconia on the total electrical conductivity o t o f the system.
104
M. Hartmanovd et al./CaO-stabilized zirconia
Table 3 The ionic transfer numbers, t i. Temp. (K)
1073 973 873
ti 1 mole/; Fe203
3 molg~ Fe203
0.69 0.57 0.50
0.85 0.74 0.54
bility o f oxygen anions at increasing temperature. It is known that iron doping o f CSZ introduces an electronic conductivity in air, mainly at lower temperatures [1 ]. The preliminary measurements o f transfer numbers t i in such system showed an increase o f ionic transfer numbers with increasing temperature (table 3). The addition o f iron oxide up to 0.5 mol% F e 2 0 3 does not change the electrical conductivity %. At 1 mol% F e 2 0 3 , the addition of iron oxide causes destabilization of CSZ, formation of m-ZrO 2, calcium ferrite which extracts a part o f CaO from CSZ, iron oxides and a residual vacant phase Ca0.1Zro.901.9This fact is evident from the decrease o f o t and can be explained b y a decrease of oxygen ion vacancies. The addition of more than 1 mol% F e 2 0 3 increases %. The same behaviour of electrical conductivity of iron doped CSZ was observed b y Hoffmann and Fischer [16]. These authors, however, neither consider the possible reaction between iron oxide and CSZ nor the decomposition of CSZ. They suggest the decrease o f concentration of oxygen ion vacancies as a result o f substitutional and interstitial introduction of iron. The measurement of the transfer numbers and the electrical conductivity of undoped m-ZrO 2 showed [13] that such a phase was primarily an ionic conductor 1000 K and an electronic conductor at 1 0 0 0 - 1 3 0 0 K for the range o f partial pressures 1.01 X 10 -17 ~ p 1.01 X 105 Pa. The analysis o f the defect structure indicated [ 13] an anti-Frenkel disorder, in which doubly ionized oxygen vacancies predominantly occur at low oxygen pressures and single ionized oxygen interstitials occur at high oxygen pressures. Our system in air (high partial oxygen pressure) should contain, according to [13], singly ionized oxygen interstitials. The formation o f oxygen interstitials may occur by an internal reaction, i.e. by formation of anti-
Frenkel pairs or by uptake of 0 2 froln the gaseous phase. It is difficult to decide which one of the inentioned defects: concentration of ion vacancies, oxygen interstitials or electrons in present phases (residual Cao.lZr0.901. 9, m-ZrO 2, calcium ferrite, iron oxides) has a predominant effect on the total electrical conductivity in CSZ ceramics at higher iron content. The aim of the simultaneous doping CSZ with copper and iron oxides was to extend the region of ionic conductivity towards the lower temperatures connected with mutual compensation of hole (Cu) and electronic (Fe) conductivities. The prelinfinary measured ionic transfer number of such a system at 1100 K, [i = 1, supports our expectation.
5. Summary The investigation of defects, electrical and structural properties o f CSZ doped with copper and iron oxides can be summarized in the following results: (1) The lattice parameters o f CSZ change only within standard deviations by the addition o f copper oxide. In case of the iron oxide addition, the 1 tool% F e 2 0 3 amount is the lowest concentration causing decomposition o f the calcia-stabilized zirconia into m-ZrO 2, a residual vacant phase Ca0.1Zr0.901.9, calcium fertile and iron oxide ( F e 2 0 3 , F e 3 0 4 ) . (2) The density of CSZ, Pexp, decreases with the increasing content o f copper o x i d e ; t h e addition of" iron oxide causes an increase o f the CSZ density. The porosity of grains is low, intergranular porosity has not been observed. The addition of copper oxide lowers the sintering temperature; tire iron oxide which probably covers the ZrO 2 grains, restrains their growth and hereby the sintering of samples. (3) The total electrical conductivity o f CSZ, o t, is not changed by the addition of copper o x i d e ; t h e addition of iron up to 0.5 tool% F e 2 0 3 does not change Ot; concentrations 0 . 5 - 1 . 0 mol% Fe203 decrease a t. at higher concentrations the conductivity o t increases again. (4) The total electrical conductivity of CSZ. o t, doped with iron oxide consists o f contributions from oxygen ion vacancies, oxygen ion interstitials and electrons. The conductivity has a mixed character, ionic and electronic, in the low temperature region. At higher temperatures, the conductivity o t has a predominant ionic character.
M. Hartmano vd et al. /CaO-stabilized zirconia
Acknowledgement The authors would like to express their thanks to Dr. V. Kliment o f the Institute o f Physics, ElectroPhysical Research Centre o f the Slovak A c a d e m y o f Sciences, for his analysis o f the calcia-stabilized zirconia samples doped with copper by the X-ray fluorescence method.
References [1] T.H. Etsell and S.N. Flengas, Chem. Rev. 70 (1970) 339. [2] B.C.H. Steele, in: Mass transport in oxides, eds. J.B. Wachtman Jr. and A.D. Franklin (Natl. Bur. Stand. Special Publ., 1968) p. 165. [3] M. Kleitz, P. Fabry and J. Fouletier, in: The electrochem. Soc. Meeting, Washington, D.C., May 2 - 7 (1976), paper 78. [4] P. Fabry and M. Kleitz, in: Electrode processes in solid state ionics, eds. M. Kleitz and J. Dupuy (Reidel, Dordrecht, Holland, 1976) p. 331.
105
[5] P. Fabry and M. Kleitz, J. Electrochem. Soc. 126 (1979) 2183. [6] Gmelins Handbuch der Anorganische Chemie, Teil B (Verlag Chemie, GMBH, Weinlaeim, 1958) pp. 24, 59, 87. [7] A comprehensive treatise on inorganic and theoretical chemistry, Vol. III (Longmans, London, 1923) p. 133. [8] A. Hoffman and W.A. Fischer, Z. Phys. Chem. (NF) 17 (1958) 30. [9] R.V. Wilhelm Jr. and D.S. Howarth, Ceram. Bull. 58 (1979) 228. [10] M. Hartmanovfi, F. Hanic and A. Koller, Mat. Sci. 11/3 (1976) 79. [11] D.K. Smith and H.W. Newkirk, Acta Cryst. 18 (1965) 983. [12] E. Van Der Vort, J. Phys. C7 (1974) L395. [13] A. Kumar, D. Rajdev and D.L. Douglass, J. Am. Ceram. Soc. 55 (1972) 439. [14] B. Cox, Report AECL 3285, 1969 (cited in [12]). [15] A.I. Kovalev and V.I. Pechenskii, Fiz. Tverd. Tela 9 (1967) 278. [ 16] A. Hoffman and W.A. Fischer, Z. Phys. Chem. (NF) 35 (1962) 95. [17] Z.S. Volchenkova and S.F. Palguev, Tr. Inst. Elektrokhim. Akad. Nauk SSSR, Ural. Filial. 2 (1961) 173.
ELECTRICAL CONDUCTIVITY AND STRUCTURE OF CaO-STABILIZED ZIRCONIA DOPED WITH COPPER AND IRON M. HARTMANOVA Institute of Physics, Electro-Physical Research Centre of the Slovak Academy of Sciences, 84228 Bratislava, Czechoslovakia L. MACHOVI~ Department of Physics, Faculty of Engineering, Slovak Technical University, 81231 Rratislava, Czechoslovakia A. KOLLER Research Institute of Electrotechnic Ceramics, 50064 Hradec Krdlovd, Czechoslovakia F. HANIC Institute of Inorganic Chemistry, Chemical Research Centre of the Slovak Academy of Sciences, 84236 Bratislava, Czechoslovakia and B. MIgANIK Computer Centre of the Slovak Academy of Sciences, 84235 Bratislava, Czechoslovakia Received 12 March 1984
The investigation of the influence of copper and iron oxides on the electrical conductivity, the lattice defects and the structure of calcia-stabilized zirconia has shown that (i) the influence of copper is negligible due to the evaporation of this element at the sintering temperature as a consequence of its high volatility; (ii) the presence of iron in the sample prevents sintering of the calcia-stabilizedzirconia; (iii) the addition of iron causes decomposition of the cubic solid solution CaxZr 1_xO2_x into monoclinic ZrO2, the vacant phase Cao. 1Zro.901.9, calcium ferrite and iron oxides; the solubility limit of iron oxide was found to be ~.1 mol%; (iv) the calcia-stabilized zirconia doped with iron is a mixed conductor at low temperatures; at high temperatures (>1073 K) it is a dominant ionic conductor.
1. Introduction Impurity doping is one way of modifying the electrical properties of calcia-stabilized zirconia (CSZ). Transition metal ions may introduce electronic conductivity into the CSZ ceramics. For instance, the addition of 0.8 wt.% V, 0.1 wt.% Fe and 0.01 wt.% Cr induced 50% electronic conductivity at 1300 K and 1.013 X 10 - 1 0 Pa [1]. The other way how to influence the behaviour of the systems based on the CSZ materials, is the change of the oxidation state of the admixtures dissolved in the solid
state electrolyte. Such impurities may cause significant modifications in the electrical conductivity [2 ]. Redox reactions, for example, can be used to improve the properties of electrical energy storage systems in single phase batteries [3,4]. The aim of the present work is to investigate the influence of copper and ion oxides on the electrical conductivity, the lattice defects and the structure of CSZ.
94
M. tfartmanovd et al./CaO-stabilized zirconia
2. Experimental
2.3. Density measurement
2.1. Preparation o f samples
The Amsler method was applied for density measurements. The volume of the weighed samples was determined from the volume of the displaced mercury at 298 K. The density values, Pexp, were reproducible within -+0.02 g/cm 3 .
Samples o f CSZ ceramics were prepared from ZrO 2 (reactor grade) and CaCO 3 (p.a.). Copper and iron were added in the form of aqueous solutions o f cupric nitrate, Cu(NO3) 2.2H20 and ferrous sulphate, FeSO 4.7H20 , both o f analytical reagent quality. The wet mixtures were homogenized in a ball mill for 2 h, then dried and heated at 1 2 7 3 - 1 3 2 3 K in order to decompose the nitrates. The calcined products were reground in ball mill for 24 h, until the surface of grains increased to 1.9 m2/g. The samples prepared in the form o f disks by dry pressing at 100 MPa were heated at 1700 K (pure CSZ samples at 2000 K) for 1 h in air. The initial composition o f mixtures is given in table 1.
2.2. Measurement o f the lattice parameters The unit cell parameters and the unit cell volume o f samples were evaluated from the indexed powder diffraction data obtained on Philips PW 1050 diffractometer using CuKa radiation and scan speed 1°20/ rain. a-A1203 (1:1) was used as an internal standard.
2.4. LTemental distribution o f Zr, Ca, Cu and Fe and investigation o f sample surfaces The grinded and polished disks o f sintered samples were examined on a microprobe Jeol JXA-5A device for elemental distribution o f Zr, Ca, Cu and Fe. Investigation o f surfaces was performed on an electron microscope Tesla BP 3002.
2.5. Measurement o f electrical conductivity The measurement of electrical conductivity was performed in air within a temperature range 6731373 K. To avoid an electrode polarization, mainly at higher currents, and noise signals, the ac method using lock-in detection was applied. The capacitive component in ac conductivity measurements was excluded by determination of the voltage U, applying phase angle ~p:
U=A 0 cos~. Table 1 The initial composition of mixtures. Sample l
2 3 4
5 6 7 8 9 10 11 12 13
Composition of mixtures (ZRO2)0.85 (CaO)o. 15 (ZrO2)0.85 (CaO)0.15 (Fe203)O.O05 (ZrO2)0.85 (CaO)0.15(Fe203)0.01 (ZrO2)0.85 (CaO)o. 15 (Fe203)O.03 (ZrO2)0.85 (CaO)o. 15 (CuO)o.o05 (ZrO2)0.85 (CaO)o. 1 s (CuO)o.o 1 (ZrO2)0.85(CaO)o. 1 s(CuO)o.o3 (ZrO2)o.85 (CaO)o. 15 (CuO)o.o 5 (ZrO2)o. 889 (CaO)o. 1o(CuO)o. 01 (Fe203)0.O01 (ZrO2)0.8 a7 (CaO)o. 1o(CuO)o.o 1(Fe203)o.oo 3 (ZrO 2)0.885 (CaO)o. 1o(CuO)o. 01 (Fe203)o. O05 (ZrO2)0.857 (CaO)o. 12 (CuO)o. 02 (Fe203)0. O03 (ZrO2)o. 859(CaO)o. 12(CuO)o.02(Fe203)O.O01
(1)
The temperature was measured by a P t R h - P t thermocouple and the voltage U was registered on a digital voltmeter. The electrodes were prepared by deposition o f a platinum emulsion on sintered samples in form o f disks (the cross section -~0.8 cm 2, the thickness ~1 mm), with subsequent annealing. The conductivity measurement was performed at increasing and decreasing temperatures. Each read-out value was first stabilized for 15--50 min. The numerical and graphical dependence of electrical conductivity on temperature was evaluated by a least square programme on a Siemens 4004 computer.
3. Results
3.1. Lattice parameters and density The dependence o f lattice parameters on compo-
M. Hartmanovdet al./CaO-stabilized zirconia Table 2 The measured lattice parameters and densities of fluorite type solid solution of CaxZr l_xO2_x doped with CuO, Fe203 and CuO + Fe203. x (mol%)
15
Impurity concentration (tool%)
5.1268(4) 5.1273(5) 5.1288(8) 5.1270(11) 5.1302(16)
4.295 3.697 4.153 3.757 3.645
5.0
3.896 4.702
4.0
+ 3.0 Fe203
5.1273(7) Cao.IZro.901. 9 + m-ZrO 2 + calcium ferrite + iron oxides ibidem
4.401
+ 1.0 CuO + 0.1 Fe203
5.1272(13)
4.295
+ 1.0 CuO + 0.3 Fe203
5.1281(7)
4.438
+ 1.0 CuO + 0.5 Fe203
5.1235(10)
4.013
+ 2.0 CuO + 0.1 Fe203
5.1273(10)
4.498
+ 2.0 CuO + 0.3 Fe203
5.1270(8)
4.699
-
12
o (Zr 02}(Co0 ) * (Z r 02 )(CoO}(CuOlx (Zr 02)(C00)(Fe203) x
[gc
Measured density3 (g cm- )
+ 0.5 Fe203 + 1.0 Fe203
10
9
Measured lattice parameter (10 -10 m)
+ 0.5 CuO + 1.0 CuO + 3.0 CuO + 5.0 CuO
5.5
o (z rO2){CoO)(CuO)ao~ (Fe203) * • (Zr 02)(CoO)(Cu 0)o.o2(Fe203)×
4.5
3.5 3,0
0
sition is summarized in table 2. Doping with copper has only negligible influence on the lattice parameters o f CSZ. The observed deviations are within the errors of measurements. In case o f iron, the lowest concentration 0.5 mol% F e 2 0 3 used in experiments, practically does not change the lattice parameter of the undoped CSZ. At 1 mol% F e 2 0 3 , a decomposition of solid solution CaxZr 1 _ x O 2 _ x takes place in m-ZrO2, a vacant phase Cao.lZro.9OI.9, calcium ferrite and iron oxides ( F e 2 0 3 , F e 3 0 4 ) . Obviously, the solubility limit of iron oxide in CSZ is less than 1 mol% F e 2 0 3 . A simultaneous doping with copper and iron oxides, using concentrations given in table 1, practically does not influence the lattice parameters. The dependence o f experimental density, Pexp, on the amount o f added copper, iron or copper and iron oxides together, is presented in fig. 1 and table 2. Evidently, an increasing amount o f copper in CSZ causes decrease o f density in the investigated systems.
95
I
I
I
I
I
1
2
3
4
s
I
x [mot
]
Fig. 1. The dependence of experimental density, aexp, on the amount of added copper, iron or copper and iron oxides together. The addition o f iron oxide into CSZ, on the other hand, increases the density o f the system at concentrations >1 mol% F e 2 0 3 . In case o f a simultaneous doping with copper and iron oxides, the density o f samples increases with increasing copper concentration.
3.2. Elemental distribution o f Zr, Ca, Cu and Fe Distribution o f Zr and Ca in the investigated samples is not uniform (figs. 2 a - d ) . Distribution o f Zr should be correlated with the regions enriched with Ca. A comparison o f the distributions o f Ca, Zr and Fe in figs. 3 a - c unambiguously indicates formation o f two kinds o f clusters; one kind combines calcium and iron (calcium ferrite) and the other is the iron oxide ( F e 2 0 3 , F e 3 0 4 ) . The segregation o f Fe and Ca is quite evident in case o f the highest used concentration (3 mol% F e 2 0 3 ) . According to the X-ray diffraction analysis, this segregation takes place already in the sample with 1 mol% F e 2 0 3 , but the products o f segregation remain distributed in the matrix. No segregation o f Fe was observed at a simultaneous doping with copper and iron oxides (figs. 4a and b). Investigation o f the composition detected only
96
M. Hartmanovd et al./CaO-stabilized zirconia
Fig. 2. The distribution of elements in the sintered samples (electron probe photomicrographs, magnification ×600): (a) Zr in (ZrO2)o.8s (CaO)0.15, (b) Ca in (ZrO2)o,SS (CaO)o. l S, (c) Zr in (ZrO2)o.8s (CaO)o. 15 (Fe203)o.o3, (d) Ca in (ZrO2)o.ss (CaO)o. 1s (Fe203)o.o3. very weak traces o f Cu between Zr (fig. 5). All these results correspond well with the X-ray data on the lattice parameters and phase composition (sections 3.1. and 4.1 .) and with the electrical conductivity (sections 3.3 and 4.2.).
3.3. Investigation o f the sample sur/hces Surfaces of tire CSZ samples investigated with the electron microscopy, show only a minimum internal porosity of grains (fig. 6a) and practically no intergranular porosity. The addition of iron oxide influences the sintering
M: Hartmanovd et aL/CaO-stabilized zirconia
a .r
':
,
,~ .~ . .
X ~ Y{, '"
"
~'. " " . ' : ' , i . " ; .~
"." .
.....
";,'"
"~.
Fig. 3. The distribution of Fe in the sintered samples (electron probe photomicrographs, magnification X600): (a) (ZrO2)o. 85 (CaO)o. 15 (Fe203)0.005, (b) (ZrO2)o. 85 (CaO)o. 15 (Fe203)O.O 1, (c) (ZRO2)0.85(CaO)o. 15 (Fe203)0.O3. process. According to results in section 3.1, segregated iron oxide covers the ZrO 2 grains and prevents the growth of larger grains restraining the sintering of samples. Such influence was not observed in case of copper doping (fig. 6d). Irregular grains are formed in case of simultaneous doping with copper and iron oxides (fig. 6e). The
ZrO 2 grains which are probably covered by iron oxide, remain finely dispersed and sintering o f zirconia does not proceed.
97
98
M. Hartmanovd et al./CaO-stabilized zirconia
Fig. 4. The distribution of elements in the sintered samples (ZrO2)o.8(CaO)0. l (CuO)o.o2(Fe203)O.O03 (electron photomicrographs, magnification X 600): (a) Ca, (b) Fe.
3.4. Electrical conductivity 3.4.1. CSZ doped with copper oxide The total electrical conductivity, o t, of calciastabilized zirconia is practically independent on the
Fig. 5. The distribution of Cu in the sintered samples (ZrO2)o. 85 (CaO)o. 15 (CuO)o.05 (electron probe photomicrograph s, magnification × 600).
amount of added copper. The only exception is the sample with the highest 5 tool% CuO doping. The electrical conductivity o t can be characterized in all investigated samples including pure CSZ, by two regions within the investigated temperature intervals and concentration ranges (figs. 7 and 10): (i) The low-temperature region occurs between 673 and 1073 K and is characterized by a mean value of the activation energy of conductivity 1.24 eV; (ii) the high-temperature region occurs in all investigated samples between 1073 and 1273 K. The conductivity data are not suitable for evaluation of the activation energy due to the transient character of the electrical conductivity in this region.
3.4.2. CSZ doped with iron oxide The dependence of the total electrical conductivity, Or, of CSZ, on the amount of added iron oxide is characterized in all investigated samples by two regions within the investigated temperature intervals and concentration ranges (figs. 8 and 10): (i) The low-temperature region occurs between 673 and 1123 K. ]'he activation energy of electrical conductivity does not show any systematic dependence on the amount of iron oxide, lts mean value is approximately 1.09 eV;
M. Hartmanovd et aL /CaO-stabilized zirconia
99
Fig. 6. The grinded and polished surfaces of the sintered samples: (a) (ZRO2)0.85(CaO)o. 15, EM's, magnification ×500; (b) (ZrO2)0.8 s (CaO)o. 15, EM's, magnification × 500, topography; (c) (ZrO2)o. 85 (CaO)o. 15 (Fe203)O.03, EM's, magnification X 1000; (d) (ZrO2)o.85 (CaO)o. 15 (CuO)0.0s, EM's, magnification X 1000; (e) (ZRO2)0.857(CaO)o.l 2(CUO)0.02 (Fe203)O.O03, EM's, magnification X 1000. (ii) the high-temperature region occurs above 1123 K. The activation energy of electrical conductivity is 1.24 eV for 0.5 mol% F e 2 0 3 , 1.54 eV for 1 mol% F e 2 0 3 and 1.23 eV for 3 mol% F e 2 0 3. No transient region occurs between the low- and high-temperature regions.
The doping with 0.5 mol% F e 2 0 3 does not practically influence the total electrical conductivity a t. At 0 . 5 - 1 . 0 mol% Fe203, a decrease of o t of the system was observed. At concentrations >1 mol% Fe203, the electrical conductivity o t again increases.
1 O0
M. Hartmanovd et al./CaO-stabilized zircon&
0t
1
o (ZrO2){CQO} •
100
=i~.~.... ~" ~ , . . . . . "~..~.
llj 1
(Zr02 }{CoO)(CuO)o.oo 5
~ (ZrO~)(CoO)iCuO)oo 1 * •
Z rO2)(CclO){CuO)o 03
(z,o)ccoo){cuO)olo
-2
10
-3
10
-4
10
lO •
-6
10
0.7
÷
V
I
I
I
I
I
I
I
I
I
I
0~8
0'9
lie
1~1
112
113
I~~
115
116
L
1~710~KII ]
Fig. 7. The dependence o f the total electrical conductivity, at, of calcia-stabilized zirconia on the a m o u n t o f added copper.
IIt
•
100
(Zr 02)(C00 )
(Zr 02)(CoO)(Fe203)ao05 o (Z r 02){C00) (Fe203)ao 1 ÷ (Z r 02 )(CclO} (Fe203}o 03
.¢~. ~
~ ~,.,~,,~
10-1 ld 2 10.3 -4
10 -5
10
io6 0.7
I
I
l
1
I
i
I
I
I
0.8
O.g
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
IO~'T [K -1] Fig. 8. The dependence o f the total electrical conductivity, at, of calcia-stabilized zirconia on the a m o u n t o f added iron.
M. Hartmanovd et al./CaO-stabilized zirconia
,
(ZrO2)(CoO){CuO)o.ol (Z rO 2){C00 )(CuO)o.ol(Fe203)o.oo 1
*
10 o ,,~
101
o (ZrO2)(CQO)(CuO)o.o 1 (Fe203)0.003 , (ZrO 2){CO0)(CuO)O.01 (Fe203)0.005
10-1 _ -2 10 -3 10 ~-
lO
_
1() 5
.
-6 10 O.7
I
I
I
I
I
I
I
I
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7 ---" 103/T [ K -1]
(5 t
• (ZrO2)(CoO)(CuO)o.o 2 (Fe203)Qo01 10 o
003
10-1 10-2 10-3 1() 4 10-5 1(36 0.7
I
I
I
1
I
I
I
I
I
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
I
----
lO /T [K"J
Fig. 9. T h e dependence o f the total electrical conductivity o t of calcia-stabilized zirconia o n the simultaneous doping with copper and iron: (a) 1 mol% CuO, (b) 2 mol% CuO.
102
M. Hartrnanovd et al. /CaO-stabilized zirconia
3.4.3. CSZ doped with copper and iron oxides The dependence of the total electrical conductivity, ot, on the amount of simultaneously added copper and iron oxides is characterized for 1 mol% CuO by a single region within the investigated temperature and concentration ranges. The mean activation energy of the electrical conductivity is 1.26 eV (fig. 9a). The iron addition reduces o t in the whole investigated temperature range in comparison with the samples without Fe203 doping. The doping with 2 mol% CuO is characterized by two temperature regions of electrical conductivity (fig. 9b):
(i) The low-temperature region occurs for both concentrations of iron oxide between 673 and 1173 K. The mean activation energy of electrical conductivity is 1.24 eV; (ii) the high-temperature region occurs for both investigated iron concentration above 1173 K. The values of o t in this system do not depend on the amount of the added iron.
4. Discussion 4.1. The influence o f copper and iron oxides on the defect structure o f CSZ
The copper addition hardly influences the measured lattice parameters (section 3.1 .) and the total electrical conductivity (section 3.4.) of the CSZ samples. However, the measured density of samples decreases with the increasing additions of copper. The presence of the copper impurity reduces sintering temperatures of samples from 2000 K (pure CSZ) to 1573 K. The negligible influence of copper oxide on the lattice parameters and electrical conductivity is a consequence of dissociation of CuO and high volatility of the products of decomposition [6,7]. The small amount of copper remains concentrated in the surface layer of glassy grains which are formed during the sintering process. The X-ray fluorescence analysis of the sintered sample with 5 mol% CuO showed only 1/5 of the copper amount originally present in the sample. The chemical analysis performed on the sintered sample with 5 mol% CuO after melting in KHSO 4 (the ASS method using the
Southern Analytical A 3000 apparatus) showed copper in a concentration 0.71 wt.% CuO. Both analytical methods confirmed a sharp decrease of the Cu content in samples after sintering at 1700 K. The dissociation of CuO takes place in air at 1300 K. It proceeds via the formation of Cu20 and 0 2 [6]. The dissociation pressure is given by the relation [71: logp =
1 3 2 1 6 T + 12.35 .
(2)
The value log p = 0 is achieved at 1350 K, very close to the melting point of Cu. The melting of CuO does not take place due to decomposition at lower temperature into metallic Cu and 02. The vapour tension of Cu at 1400 K is 10 . 3 mm Hg, at 1600 K is 0.5 mm Hg. The volatility of copper oxide at 1700 K (sintering temperature) is so high that it is left only in a reduced concentration in the sintered sample. In case of the iron oxide admixtures in CSZ, the iron ions enter into the CaxZr 1 x O 2 _ x structure either through substitution for cations in the cationic sublattice or they take interstitial positions. Therefore the incorporation of iron ions leads to a decrease of the vacancy concentration in the anionic sublattice [8]. Vacancies were formed through incorporation of CaO into the cubic fluorite phase C a O ~ C a z2+ r +V 2
.
(3)
A scheme for the substitutional incorporation of Fe203 can be adopted Fe203 -+ 2 Fear +V~)- .
(4)
The interstitial incorporation of Fe203 may be described by the relation:
Fe203 + 3 V g - -~ 2 Fe 3- .
(5)
In the stabilized fluorite phase Y x Z r l xO2 x, in which Y203 does not react with Fe203 [9], solubility of iron oxide increases (5 tool%) and majority of iron ions enters substitutionally into tire vacant phase. In the CSZ system, the interstitial incorporation is unlikely. Due to the reaction of CaO with Fe203 (formation of calcium ferrite, Ca 2 Fe205) at the sintering temperature, a part of Ca2+ ions is extracted from the CSZ phase and the cubic phase is destabilized under the formation of m-ZrO 2 , Ca0. IZr0.901.9, calcium ferrite and iron oxides. The coexistence of m-ZrO 2
M. Hartmanovd et aL /CaO-stabilized zirconia
and Ca0.1Zr0.901. 9 indicates that the amount of the extracted CaO is ~5 mol% [10]. In monoclinic ZrO2, there are two anion sites: O I sites with the coordination number three and OII sites with the coordination number four [11 ]. The theoretical estimates show [ 12] that the oxygen anion vacancies can move in m-ZrO 2 only over the OI sites in the (100) direction with the theoretical activation energy 3.87 -+ 0.34 eV. The observed activation energy, 1.72 eV, is, however, much smaller [13]. This means that theoretical results contradict conception of a bulk diffusion mechanism. A more probable explanation is that of Cox [ 14] according to which the rate controlling step is the anion vacancy diffusion along microcracks, grain boundaries or dislocations.
4.2. The influence o f copper and iron oxides on the electrical conductivity o f CSZ The preliminary measurements of the transfer numbers in CSZ doped with copper oxide showed that the ionic transfer numbers t i are 0.99 at 873 K and 1.00 at 973 K and 1073 K. It means that CSZ
103
doped with copper oxide is a pure ionic conductor above 873 K. The mean value of the activation energy of electrical conductivity in the copper doped CSZ ceramics is 1.24 eV for the low temperature region 673-1073 K. This value is comparable with the value 1.24 eV found by Kovalev and Pechinenskii [15] for the activation energy of defect migration in CSZ from internal friction measurements. Probably, it corresponds to the (100)anion vacancy migration between two positions of Zr 4+ ions. The calculated migration energy is (1.34 + 0.29) eV [12]. The values of Hoffman and Fischer [16] occur within 1.20-1.27 eV. All these values are well comparable. In both copper-doped and undoped CSZ occurs a decrease of electrical conductivity at about 1073 K. A similar effect was observed by other authors, too [ 16,17 ]. According to Etsel and Flengas [ 1], such conductivity decrease occurs due to the incomplete stabilization of ZrO 2. We could not confirm this result. In our X-ray diffraction analysis, we did not identify the presence of m-ZrO 2. We see a more probable reason of the observed deviation in a decrease of the mo-
(I t
[0-1m-1]
o (ZrO2)(CQO) (Z rO 2 )(O30)(CuO)o.oo5 " (ZrO2}(CaO)(Fe203)o.oo 5 ,, (ZrO 2 )(CaO)(CuO)om , (ZrO 2 )(CaO)(Fe20 ~)o.01 ,' (ZrO 2 )(CQO)(CuO)oo3 .,. (ZrO2)(CaO)(Fe203)o.03 o
10°
~ ""~_
104
,
10-2 .,,.,
~ : ~ ~ -"~.,.~.
,"-'~
l o -3
10-4 1C~"5 -6
10
0.7
I
L
I
0.8
0.9
1.0
L
t
1.2
L
t
I
t
t
1.,.,
is
1.6
1.7
I ----
[K-']
Fig. 10. The comparison o f t h e influence o f the same concentrations o f copper and iron added into the calcia-stabilized zirconia on the total electrical conductivity o t o f the system.
104
M. Hartmanovd et al./CaO-stabilized zirconia
Table 3 The ionic transfer numbers, t i. Temp. (K)
1073 973 873
ti 1 mole/; Fe203
3 molg~ Fe203
0.69 0.57 0.50
0.85 0.74 0.54
bility o f oxygen anions at increasing temperature. It is known that iron doping o f CSZ introduces an electronic conductivity in air, mainly at lower temperatures [1 ]. The preliminary measurements o f transfer numbers t i in such system showed an increase o f ionic transfer numbers with increasing temperature (table 3). The addition o f iron oxide up to 0.5 mol% F e 2 0 3 does not change the electrical conductivity %. At 1 mol% F e 2 0 3 , the addition of iron oxide causes destabilization of CSZ, formation of m-ZrO 2, calcium ferrite which extracts a part o f CaO from CSZ, iron oxides and a residual vacant phase Ca0.1Zro.901.9This fact is evident from the decrease o f o t and can be explained b y a decrease of oxygen ion vacancies. The addition of more than 1 mol% F e 2 0 3 increases %. The same behaviour of electrical conductivity of iron doped CSZ was observed b y Hoffmann and Fischer [16]. These authors, however, neither consider the possible reaction between iron oxide and CSZ nor the decomposition of CSZ. They suggest the decrease o f concentration of oxygen ion vacancies as a result o f substitutional and interstitial introduction of iron. The measurement of the transfer numbers and the electrical conductivity of undoped m-ZrO 2 showed [13] that such a phase was primarily an ionic conductor 1000 K and an electronic conductor at 1 0 0 0 - 1 3 0 0 K for the range o f partial pressures 1.01 X 10 -17 ~ p 1.01 X 105 Pa. The analysis o f the defect structure indicated [ 13] an anti-Frenkel disorder, in which doubly ionized oxygen vacancies predominantly occur at low oxygen pressures and single ionized oxygen interstitials occur at high oxygen pressures. Our system in air (high partial oxygen pressure) should contain, according to [13], singly ionized oxygen interstitials. The formation o f oxygen interstitials may occur by an internal reaction, i.e. by formation of anti-
Frenkel pairs or by uptake of 0 2 froln the gaseous phase. It is difficult to decide which one of the inentioned defects: concentration of ion vacancies, oxygen interstitials or electrons in present phases (residual Cao.lZr0.901. 9, m-ZrO 2, calcium ferrite, iron oxides) has a predominant effect on the total electrical conductivity in CSZ ceramics at higher iron content. The aim of the simultaneous doping CSZ with copper and iron oxides was to extend the region of ionic conductivity towards the lower temperatures connected with mutual compensation of hole (Cu) and electronic (Fe) conductivities. The prelinfinary measured ionic transfer number of such a system at 1100 K, [i = 1, supports our expectation.
5. Summary The investigation of defects, electrical and structural properties o f CSZ doped with copper and iron oxides can be summarized in the following results: (1) The lattice parameters o f CSZ change only within standard deviations by the addition o f copper oxide. In case of the iron oxide addition, the 1 tool% F e 2 0 3 amount is the lowest concentration causing decomposition o f the calcia-stabilized zirconia into m-ZrO 2, a residual vacant phase Ca0.1Zr0.901.9, calcium fertile and iron oxide ( F e 2 0 3 , F e 3 0 4 ) . (2) The density of CSZ, Pexp, decreases with the increasing content o f copper o x i d e ; t h e addition of" iron oxide causes an increase o f the CSZ density. The porosity of grains is low, intergranular porosity has not been observed. The addition of copper oxide lowers the sintering temperature; tire iron oxide which probably covers the ZrO 2 grains, restrains their growth and hereby the sintering of samples. (3) The total electrical conductivity o f CSZ, o t, is not changed by the addition of copper o x i d e ; t h e addition of iron up to 0.5 tool% F e 2 0 3 does not change Ot; concentrations 0 . 5 - 1 . 0 mol% Fe203 decrease a t. at higher concentrations the conductivity o t increases again. (4) The total electrical conductivity of CSZ. o t, doped with iron oxide consists o f contributions from oxygen ion vacancies, oxygen ion interstitials and electrons. The conductivity has a mixed character, ionic and electronic, in the low temperature region. At higher temperatures, the conductivity o t has a predominant ionic character.
M. Hartmano vd et al. /CaO-stabilized zirconia
Acknowledgement The authors would like to express their thanks to Dr. V. Kliment o f the Institute o f Physics, ElectroPhysical Research Centre o f the Slovak A c a d e m y o f Sciences, for his analysis o f the calcia-stabilized zirconia samples doped with copper by the X-ray fluorescence method.
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