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Electrical properties of stoichiometric and non-stoichiometric calcium zirconate
Solid State Ionics 157 (2003) 183 – 187 www.elsevier.com/locate/ssi
Electrical properties of stoichiometric and non-stoichiometric calcium zirconate Magdalena Dudek, MiroslCaw M. Buc´ko* Faculty of Materials Science and Ceramics, University of Mining and Metallurgy, Al. Mickiewicza 30, 30-059, Cracow, Poland Received 12 May 2001; received in revised form 16 January 2002; accepted 21 January 2002
Abstract Stoichiometric calcium zirconate and calcium zirconate with calcia or zirconia excess sintered bodies were prepared. Electrical properties of these materials were investigated at 950 jC using the dc four-probe and ac impedance spectroscopy methods. XRD analysis was used to determine the changes of cell parameters. The Rietveld method was used to refine calcium zirconate structure. The emf measurements of galvanic cells were used to determine the ionic transference numbers in the selected samples. Stoichiometric calcium zirconate appeared to be a rather poor and mixed electron-oxygen ion conductor, whereas calcium zirconate samples with excess of calcia or zirconia exhibit purely oxygen ion conductivity. In both cases, introduction of a respective cation excess led to the significant enhancement in conductivity. The simple point defect model for calcium zirconate with calcia or zirconia excess was proposed. D 2002 Elsevier Science B.V. All rights reserved. PACS: 61.72Ji; 66.30Dn Keywords: Calcium zirconate; Oxygen ion conductor; Solid electrolyte; Perovskite
1. Introduction Calcium zirconate with a perovskite structure has been extensively investigated for several years because of its interesting electrical properties. Stoichiometric calcium zirconate has been reported to be a p-type semiconducting material as well as a ferroelectric one [1,2]. Doping of CaZrO3 with some trivalent cations such as indium, gallium or scandium modified its electrical properties, and then calcium zirconate showed appreciable protonic conduction in hydrogencontaining atmosphere [3,4]. Calcium zirconate-based materials prepared in such a way were also successfully *
Corresponding author. E-mail address: [email protected] (M. Buc´ko).
applied in the probe for determining the hydrogen activity in molten metals [5,6]. On the other hand, doping of calcium zirconate with some oxides such as alumina, yttria and magnesia as well as with small excess of zirconia or calcia caused oxygen ion conductivity [7,8]. Calcium zirconate (with small excess of CaO or ZrO2) was reported to exhibit pure ionic conductivity in a wide range of oxygen partial pressure and high chemical stability at temperatures exceeding 1000 jC. Some further studies showed that calcium zirconate is a more suitable electrolyte than zirconia solid solutions for the determination of oxygen in molten metals [9,10]. Their application has been limited by a significant electronic conductivity, which has existed at very low oxygen activity below 10 7 Pa at temperatures higher than 1600 jC [11,12].
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 2 0 7 - 2
184
M. Dudek, M.M. Buc´ko / Solid State Ionics 157 (2003) 183–187
The aim of the present paper is to present some results on the electrical and electrochemical properties of stoichiometric calcium zirconate and of calcium zirconate containing calcia or zirconia excess.
2. Preparation Calcium zirconate was preceded by calcium carbonate and zirconium oxide fine powders. A precipitation – calcination method was used to prepare pure zirconia powder with the average grain size of about 15 nm. The appropriate amounts of calcium carbonate and zirconia powders, related to CaO/ZrO2 molar ratio from 0.43 to 2.33, were weighed and mixed in the attritor mill with propanol as a medium. The slurry was dried, calcinated for 1 h at 1200 jC, ground in the rotary –vibratory mill and then isostatically pressed at 300 MPa. The pellets with an excess of zirconia were sintered for 2 h at 1500 jC, whereas the pellets with excess of calcia were sintered for 24 h at 1500 jC. The phase composition of the prepared materials was characterised by the X-ray diffraction analysis with the use of the Seifert XRD 7 equipment. The Rietveld method was used to refine the crystallographic structure of calcium zirconate [13]. The electrical conductivity was measured using both dc four-probe (multimeter HP 34401A) and ac impedance spectroscopy (Solartron 1258) methods; partially reversible platinum electrodes were applied. All the prepared samples were tested as electrolytes in a solid galvanic cell, which may be shown schematically: PtAFe, Fex OACaZrO3 ANi, NiOAPt:
of the containing a pure ionic conductor and could be expressed as: DE ¼ ti DEt
ð3Þ
here: ti denotes an effective ionic transference number of oxygen ions in the sample under study [14]. Taking the respective emf values from Ref. [15], the ionic transference numbers in all the prepared samples were determined on the basis of emf measurements of the cell (Eq. (1)).
3. Results and discussion The X-ray diffraction patterns of the samples revealed that only the stoichiometric material consisted of one phase, but quantitative changes of the phase content of the samples were strongly related to the kind of the oxide which was in excess. Even in the sample with the minimum ZrO2 excess, a significant amount of cubic zirconia solid solution was observed. Contrary to that, in the material with the same excess of calcia, only very weak lines attributed to the CaO phase were detected. Fig. 1 illustrates the changes of the calcium zirconate weight fraction, determined by the Rietveld refinement, as a function of the previously assumed calcia to zirconia molar ratio. On the basis of X-ray line positions, cell parameters and cell volume of calcium zirconate were calculated. Fig. 2 presents changes of the cell volume
ð1Þ
The two-phase mixtures (Fe, FexO) and (Ni, NiO) having known oxygen equilibrium partial pressures at a given temperature, were used as electrodes in the cell. As long as the sample separating both electrodes exhibited a purely ionic conduction, the emf of the cell (Eq. (1)) under thermodynamic equilibrium conditions could be represented by Nernst’s equation: RT pO2 ðNi, NiOÞ Et ¼ ln ð2Þ 4F pO2 ðFe, Fex OÞ In the case of the sample, which shows electronic conduction, the emf of the cell (Eq. (1)) was less than
Fig. 1. Changes of the calcium zirconate weight fraction, determined by the Rietveld refinement, as a function of previously assumed calcia to zirconia molar ratio.
M. Dudek, M.M. Buc´ko / Solid State Ionics 157 (2003) 183–187
Fig. 2. Changes of cell volume versus molar fraction of calcia in all investigated samples.
versus a molar fraction of calcia in all the samples investigated. In both series of samples, an increase in the CaZrO3 cell volume is observed. In the case of calcia excess, this increase is rather small and reaches its maximum for the material with molar ratio CaO/ ZrO2 = 0.55:0.45. For the larger calcia excess, the cell volume of calcium zirconate goes towards the value related to the stoichiometric molar ratio. The introduction of zirconia excess to the composition causes similar changes of the calcium zirconate cell volume (an increase, a maximum, and a decrease) but these changes are larger than in the previous case. The considerable decrease of CaZrO3 cell volume corresponds to calcia-stabilised zirconia becoming a major phase. The changes of the cell parameters and cell volume of the calcium zirconate suggested that in the samples with calcia or zirconia excess, this compound was in a solid solution form. The refinement of the crystallo-
185
graphic structure calcium zirconate by the Rietveld method allowed the authors to verify a point defect model. Table 1 presents the site occupancy coefficients related to the respective defects discussed later. Stoichiometric calcium zirconate exhibits an electrical conductivity as low as 1.1 10 6 (V cm) 1 at 950 jC. A plot in Fig. 3 illustrates the influence of zirconia or calcia excess on electrical properties of calcium zirconate. In this graph, the relative conductivity j/jo (where jo is the conductivity of stoichiometric calcium zirconate) as a function of a calcia molar fraction at 950 jC is presented. The excess of zirconia from 2 to 10 mol% raises the electrical conductivity by a factor from 1.1 to 22, respectively. The considerable increase of the conductivity for the samples with higher zirconia excess corresponds to the formation of calcia-stabilised zirconia as a continuous phase. On the other hand, introducing as low as 2 mol% of calcia into the calcium zirconate structure significantly increases the conductivity by a factor over 450. The increase of a calcia excess more than 4 mol% causes the formation of a higher amount of CaO as a heterophase and leads to the decrease in electrical conductivity. Fig. 4 shows an impedance plot recorded for calcium zirconate with 2 mol% excess of calcia at 700 jC. The diagram was approximated with two partially overlapping semicircles and the equivalent circuit composed of two parallel resistor – capacitor connections could describe such an experimental curve behavior. The least square procedure allows the authors to determine the values of each electrical element. The values of the respective capacities showed that the first semicircle (at higher frequencies) corresponds to the bulk conductivity, and the second
Table 1 The crystallographic site occupancy coefficients determined by the Rietveld refinement method for all the investigated samples Site
Wyckoff position
Crystallographic site occupancy coefficients
CaO/ZrO2 molar ratio CaZr ZrZr OO
4c 4b 8d
0.51:0.49 0.02119 0.97881 0.98940
0.52:0.48 0.02744 0.97256 0.98628
0.55:0.45 0.03535 0.96465 0.98232
0.60:0.40 0.02921 0.97079 0.98540
0.70:0.30 0.02551 0.97449 0.98724
ZrO2/CaO molar ratio CaCa OO
4c 4c
0.51:0.49 0.99948 0.99948
0.52:0.48 0.99039 0.99039
0.55:0.45 0.98838 0.98838
0.60:0.40 0.99793 0.99793
0.70:0.30 1.00143 1.00143
186
M. Dudek, M.M. Buc´ko / Solid State Ionics 157 (2003) 183–187 Table 2 The emf of the cell (Eq. (1)), E1, and calculated effective ionic transference numbers of selected materials, ti, measured at 950 jC
Fig. 3. Influence of zirconia or calcia excess on electrical properties of calcium zirconate.
semicircle to the grain boundary conductivity. The values of the apparent grain boundary conductivity (6.57 10 5 (V cm) 1) and the bulk conductivity (1.61 10 4 (V cm) 1) are in a good agreement with the values obtained from dc four-probe method at the same temperature. The results of emf measurements at 950 jC, E1, for selected samples are presented in Table 2. These values were compared with the emf, Et = 281 mV, of the cell (Eq. (1)) involving fully calcia-stabilised
Fig. 4. Impedance plot recorded at 700 jC for calcium zirconate with 2 mol% excess of calcia.
Solid electrolyte
E1 (mV)
ti
Stoichiometric calcium zirconate Calcium zirconate with 2 mol% excess of calcia Calcium zirconate with 10 mol% excess of zirconia
252.9 279.0
0.92 0.99
275.4
0.98
zirconia electrolyte, being a purely oxygen ion conductor. The Et value was calculated on the basis of data given in Ref. [15]. Table 2 also shows the values of coefficient ti = DE1/DEt, which could be related to the oxygen ion transference numbers of the samples prepared. The oxygen transference number of pure, stoichiometric calcium zirconate is equal to 0.92, which would indicate that in this material, the oxygen ions are not the only factor responsible for a charge transport, the balance being probably electrons and holes. The introduction of calcia or zirconia excess into calcium zirconate causes that the oxygen transference numbers are close to unity. Reliability of the relation between the calculated DE1/DEt values and oxygen ion transference numbers was corroborated by electrical conductivity measurements in atmospheres with different oxygen partial pressures [16]. It was found that in oxygen partial pressures from 60 to 1 105 Pa, the electrical conductivity of the non-stoichiometric samples were nearly constant. Similar results in a wider range of oxygen partial pressure (from 1 10 11 to 1 105 Pa) and temperatures (from 1200 to 1600 jC) were obtained by Fischer and Janke [7]. The changes of the structural factors of calcium zirconate, the cell parameter and the cell volume, caused by calcia or zirconia excess suggest that in a very narrow range of compositions this compound is in a solid solution form. A comparison of conductivities and ti values for both series of non-stoichiometric calcium zirconate indicates that the reason for the conductivity enhancement with respect to the pure calcium zirconate phase is an increase in the oxygen vacancy concentration. On the basis of the results presented above, two models of the point defect formation due to calcia or zirconia excess can be proposed. In the case of calcia, excess presence of the calcium cation in the zirconium regular site and
M. Dudek, M.M. Buc´ko / Solid State Ionics 157 (2003) 183–187
respective oxygen vacancy formation is assumed as in Kro¨ger– Vink notation: SS
W þ VO þ OO : CaO ¼ CaZr
ð4Þ
In the case of the materials with zirconia excess, the interpretation of the obtained results is not quite unequivocal. The most probable defect model is based on the formation of the same amount of calcium and oxygen vacancies described by the following equation [17]: ð1 xÞCaO þ ZrO2 W þ ZrZr þ ð3 xÞOO ¼ ð1 xÞCaCa þ xVCa SS
þ xVO þ ðCaO ZrO2 Þs:s: :
ð5Þ
The presented models are in a good agreement with the results from Rietveld method of structure refinement (see Table 1) and with theoretical approach [17].
4. Summary Stoichiometric calcium zirconate is rather a poor oxygen ion conductor with conductivity not higher than 1.1 10 6 (V cm) 1 and oxygen ionic transference number about 0.92 at 950 jC. Introduction of calcia or zirconia into calcium zirconate structure creates oxygen vacancies and thus significantly enhances electrical conductivity. The investigated electrochemical properties of non-stoichiometric calcium zirconate suggest application of these materials as a solid electrolyte in a galvanic cell at elevated temperatures.
187
Acknowledgements This work was carried out under contract No. 7 T08D 016 19 with the Polish Scientific Research Committee.
References [1] A. Pretis, V. Longo, Silicates Industriels 7 – 8 (1984) 1939. [2] H. Stetson, B. Schwartz, Journal of the American Ceramic Society 44 (1961) 420. [3] T. Yamija, H. Kazeoka, H. Iwahara, Solid State Ionics 47 (1991) 271. [4] N. Kurita, N. Fukatsu, K. Ito, Journal of the Electrochemical Society (1995) 1522. [5] T. Yamija, H. Iwahara, K. Koide, Sensors and Actuators B 5 (1991) 145. [6] T. Jamija, K. Koide, H. Takai, N. Fukatsu, Sensors and Actuators B 14 (1993) 697. [7] W.A. Fischer, D. Janke, Archiv fur das Eisenhuttenwesen 47 (1976) 525 – 530. [8] Ch. Wang, X. Xu, Y. Wen, Solid State Ionics 28 – 30 (1988) 542. [9] D. Janke, Metallurgical Transactions B 13 (1982) 227 – 235. [10] S. Pandit, A. Weyl, D. Janke, Solid State Ionics 69 (1994) 93. [11] T. Etsell, S. Flengas, Journal of the Electrochemical Society 119 (1972) 1. [12] M. Iwase, K.T. Jacob, E. Ichise, Sprechsaal 119 (1986) 280. [13] R.A. Young, Journal of Applied Crystallography 28 (1995) 366 – 367. [14] H. Schmalzried, Zeitschrift fur Physikalische Chemie (Neue Folge) 38 (1963) 87. [15] K. Kiukkola, K. Wagner, Journal of the Electrochemical Society 104 (1957) 366. [16] M. Dudek et al., to be published. [17] R.A. Davies, M.S. Islam, J.D. Gale, Solid State Ionics 126 (1999) 323.
Electrical properties of stoichiometric and non-stoichiometric calcium zirconate Magdalena Dudek, MiroslCaw M. Buc´ko* Faculty of Materials Science and Ceramics, University of Mining and Metallurgy, Al. Mickiewicza 30, 30-059, Cracow, Poland Received 12 May 2001; received in revised form 16 January 2002; accepted 21 January 2002
Abstract Stoichiometric calcium zirconate and calcium zirconate with calcia or zirconia excess sintered bodies were prepared. Electrical properties of these materials were investigated at 950 jC using the dc four-probe and ac impedance spectroscopy methods. XRD analysis was used to determine the changes of cell parameters. The Rietveld method was used to refine calcium zirconate structure. The emf measurements of galvanic cells were used to determine the ionic transference numbers in the selected samples. Stoichiometric calcium zirconate appeared to be a rather poor and mixed electron-oxygen ion conductor, whereas calcium zirconate samples with excess of calcia or zirconia exhibit purely oxygen ion conductivity. In both cases, introduction of a respective cation excess led to the significant enhancement in conductivity. The simple point defect model for calcium zirconate with calcia or zirconia excess was proposed. D 2002 Elsevier Science B.V. All rights reserved. PACS: 61.72Ji; 66.30Dn Keywords: Calcium zirconate; Oxygen ion conductor; Solid electrolyte; Perovskite
1. Introduction Calcium zirconate with a perovskite structure has been extensively investigated for several years because of its interesting electrical properties. Stoichiometric calcium zirconate has been reported to be a p-type semiconducting material as well as a ferroelectric one [1,2]. Doping of CaZrO3 with some trivalent cations such as indium, gallium or scandium modified its electrical properties, and then calcium zirconate showed appreciable protonic conduction in hydrogencontaining atmosphere [3,4]. Calcium zirconate-based materials prepared in such a way were also successfully *
Corresponding author. E-mail address: [email protected] (M. Buc´ko).
applied in the probe for determining the hydrogen activity in molten metals [5,6]. On the other hand, doping of calcium zirconate with some oxides such as alumina, yttria and magnesia as well as with small excess of zirconia or calcia caused oxygen ion conductivity [7,8]. Calcium zirconate (with small excess of CaO or ZrO2) was reported to exhibit pure ionic conductivity in a wide range of oxygen partial pressure and high chemical stability at temperatures exceeding 1000 jC. Some further studies showed that calcium zirconate is a more suitable electrolyte than zirconia solid solutions for the determination of oxygen in molten metals [9,10]. Their application has been limited by a significant electronic conductivity, which has existed at very low oxygen activity below 10 7 Pa at temperatures higher than 1600 jC [11,12].
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 2 0 7 - 2
184
M. Dudek, M.M. Buc´ko / Solid State Ionics 157 (2003) 183–187
The aim of the present paper is to present some results on the electrical and electrochemical properties of stoichiometric calcium zirconate and of calcium zirconate containing calcia or zirconia excess.
2. Preparation Calcium zirconate was preceded by calcium carbonate and zirconium oxide fine powders. A precipitation – calcination method was used to prepare pure zirconia powder with the average grain size of about 15 nm. The appropriate amounts of calcium carbonate and zirconia powders, related to CaO/ZrO2 molar ratio from 0.43 to 2.33, were weighed and mixed in the attritor mill with propanol as a medium. The slurry was dried, calcinated for 1 h at 1200 jC, ground in the rotary –vibratory mill and then isostatically pressed at 300 MPa. The pellets with an excess of zirconia were sintered for 2 h at 1500 jC, whereas the pellets with excess of calcia were sintered for 24 h at 1500 jC. The phase composition of the prepared materials was characterised by the X-ray diffraction analysis with the use of the Seifert XRD 7 equipment. The Rietveld method was used to refine the crystallographic structure of calcium zirconate [13]. The electrical conductivity was measured using both dc four-probe (multimeter HP 34401A) and ac impedance spectroscopy (Solartron 1258) methods; partially reversible platinum electrodes were applied. All the prepared samples were tested as electrolytes in a solid galvanic cell, which may be shown schematically: PtAFe, Fex OACaZrO3 ANi, NiOAPt:
of the containing a pure ionic conductor and could be expressed as: DE ¼ ti DEt
ð3Þ
here: ti denotes an effective ionic transference number of oxygen ions in the sample under study [14]. Taking the respective emf values from Ref. [15], the ionic transference numbers in all the prepared samples were determined on the basis of emf measurements of the cell (Eq. (1)).
3. Results and discussion The X-ray diffraction patterns of the samples revealed that only the stoichiometric material consisted of one phase, but quantitative changes of the phase content of the samples were strongly related to the kind of the oxide which was in excess. Even in the sample with the minimum ZrO2 excess, a significant amount of cubic zirconia solid solution was observed. Contrary to that, in the material with the same excess of calcia, only very weak lines attributed to the CaO phase were detected. Fig. 1 illustrates the changes of the calcium zirconate weight fraction, determined by the Rietveld refinement, as a function of the previously assumed calcia to zirconia molar ratio. On the basis of X-ray line positions, cell parameters and cell volume of calcium zirconate were calculated. Fig. 2 presents changes of the cell volume
ð1Þ
The two-phase mixtures (Fe, FexO) and (Ni, NiO) having known oxygen equilibrium partial pressures at a given temperature, were used as electrodes in the cell. As long as the sample separating both electrodes exhibited a purely ionic conduction, the emf of the cell (Eq. (1)) under thermodynamic equilibrium conditions could be represented by Nernst’s equation: RT pO2 ðNi, NiOÞ Et ¼ ln ð2Þ 4F pO2 ðFe, Fex OÞ In the case of the sample, which shows electronic conduction, the emf of the cell (Eq. (1)) was less than
Fig. 1. Changes of the calcium zirconate weight fraction, determined by the Rietveld refinement, as a function of previously assumed calcia to zirconia molar ratio.
M. Dudek, M.M. Buc´ko / Solid State Ionics 157 (2003) 183–187
Fig. 2. Changes of cell volume versus molar fraction of calcia in all investigated samples.
versus a molar fraction of calcia in all the samples investigated. In both series of samples, an increase in the CaZrO3 cell volume is observed. In the case of calcia excess, this increase is rather small and reaches its maximum for the material with molar ratio CaO/ ZrO2 = 0.55:0.45. For the larger calcia excess, the cell volume of calcium zirconate goes towards the value related to the stoichiometric molar ratio. The introduction of zirconia excess to the composition causes similar changes of the calcium zirconate cell volume (an increase, a maximum, and a decrease) but these changes are larger than in the previous case. The considerable decrease of CaZrO3 cell volume corresponds to calcia-stabilised zirconia becoming a major phase. The changes of the cell parameters and cell volume of the calcium zirconate suggested that in the samples with calcia or zirconia excess, this compound was in a solid solution form. The refinement of the crystallo-
185
graphic structure calcium zirconate by the Rietveld method allowed the authors to verify a point defect model. Table 1 presents the site occupancy coefficients related to the respective defects discussed later. Stoichiometric calcium zirconate exhibits an electrical conductivity as low as 1.1 10 6 (V cm) 1 at 950 jC. A plot in Fig. 3 illustrates the influence of zirconia or calcia excess on electrical properties of calcium zirconate. In this graph, the relative conductivity j/jo (where jo is the conductivity of stoichiometric calcium zirconate) as a function of a calcia molar fraction at 950 jC is presented. The excess of zirconia from 2 to 10 mol% raises the electrical conductivity by a factor from 1.1 to 22, respectively. The considerable increase of the conductivity for the samples with higher zirconia excess corresponds to the formation of calcia-stabilised zirconia as a continuous phase. On the other hand, introducing as low as 2 mol% of calcia into the calcium zirconate structure significantly increases the conductivity by a factor over 450. The increase of a calcia excess more than 4 mol% causes the formation of a higher amount of CaO as a heterophase and leads to the decrease in electrical conductivity. Fig. 4 shows an impedance plot recorded for calcium zirconate with 2 mol% excess of calcia at 700 jC. The diagram was approximated with two partially overlapping semicircles and the equivalent circuit composed of two parallel resistor – capacitor connections could describe such an experimental curve behavior. The least square procedure allows the authors to determine the values of each electrical element. The values of the respective capacities showed that the first semicircle (at higher frequencies) corresponds to the bulk conductivity, and the second
Table 1 The crystallographic site occupancy coefficients determined by the Rietveld refinement method for all the investigated samples Site
Wyckoff position
Crystallographic site occupancy coefficients
CaO/ZrO2 molar ratio CaZr ZrZr OO
4c 4b 8d
0.51:0.49 0.02119 0.97881 0.98940
0.52:0.48 0.02744 0.97256 0.98628
0.55:0.45 0.03535 0.96465 0.98232
0.60:0.40 0.02921 0.97079 0.98540
0.70:0.30 0.02551 0.97449 0.98724
ZrO2/CaO molar ratio CaCa OO
4c 4c
0.51:0.49 0.99948 0.99948
0.52:0.48 0.99039 0.99039
0.55:0.45 0.98838 0.98838
0.60:0.40 0.99793 0.99793
0.70:0.30 1.00143 1.00143
186
M. Dudek, M.M. Buc´ko / Solid State Ionics 157 (2003) 183–187 Table 2 The emf of the cell (Eq. (1)), E1, and calculated effective ionic transference numbers of selected materials, ti, measured at 950 jC
Fig. 3. Influence of zirconia or calcia excess on electrical properties of calcium zirconate.
semicircle to the grain boundary conductivity. The values of the apparent grain boundary conductivity (6.57 10 5 (V cm) 1) and the bulk conductivity (1.61 10 4 (V cm) 1) are in a good agreement with the values obtained from dc four-probe method at the same temperature. The results of emf measurements at 950 jC, E1, for selected samples are presented in Table 2. These values were compared with the emf, Et = 281 mV, of the cell (Eq. (1)) involving fully calcia-stabilised
Fig. 4. Impedance plot recorded at 700 jC for calcium zirconate with 2 mol% excess of calcia.
Solid electrolyte
E1 (mV)
ti
Stoichiometric calcium zirconate Calcium zirconate with 2 mol% excess of calcia Calcium zirconate with 10 mol% excess of zirconia
252.9 279.0
0.92 0.99
275.4
0.98
zirconia electrolyte, being a purely oxygen ion conductor. The Et value was calculated on the basis of data given in Ref. [15]. Table 2 also shows the values of coefficient ti = DE1/DEt, which could be related to the oxygen ion transference numbers of the samples prepared. The oxygen transference number of pure, stoichiometric calcium zirconate is equal to 0.92, which would indicate that in this material, the oxygen ions are not the only factor responsible for a charge transport, the balance being probably electrons and holes. The introduction of calcia or zirconia excess into calcium zirconate causes that the oxygen transference numbers are close to unity. Reliability of the relation between the calculated DE1/DEt values and oxygen ion transference numbers was corroborated by electrical conductivity measurements in atmospheres with different oxygen partial pressures [16]. It was found that in oxygen partial pressures from 60 to 1 105 Pa, the electrical conductivity of the non-stoichiometric samples were nearly constant. Similar results in a wider range of oxygen partial pressure (from 1 10 11 to 1 105 Pa) and temperatures (from 1200 to 1600 jC) were obtained by Fischer and Janke [7]. The changes of the structural factors of calcium zirconate, the cell parameter and the cell volume, caused by calcia or zirconia excess suggest that in a very narrow range of compositions this compound is in a solid solution form. A comparison of conductivities and ti values for both series of non-stoichiometric calcium zirconate indicates that the reason for the conductivity enhancement with respect to the pure calcium zirconate phase is an increase in the oxygen vacancy concentration. On the basis of the results presented above, two models of the point defect formation due to calcia or zirconia excess can be proposed. In the case of calcia, excess presence of the calcium cation in the zirconium regular site and
M. Dudek, M.M. Buc´ko / Solid State Ionics 157 (2003) 183–187
respective oxygen vacancy formation is assumed as in Kro¨ger– Vink notation: SS
W þ VO þ OO : CaO ¼ CaZr
ð4Þ
In the case of the materials with zirconia excess, the interpretation of the obtained results is not quite unequivocal. The most probable defect model is based on the formation of the same amount of calcium and oxygen vacancies described by the following equation [17]: ð1 xÞCaO þ ZrO2 W þ ZrZr þ ð3 xÞOO ¼ ð1 xÞCaCa þ xVCa SS
þ xVO þ ðCaO ZrO2 Þs:s: :
ð5Þ
The presented models are in a good agreement with the results from Rietveld method of structure refinement (see Table 1) and with theoretical approach [17].
4. Summary Stoichiometric calcium zirconate is rather a poor oxygen ion conductor with conductivity not higher than 1.1 10 6 (V cm) 1 and oxygen ionic transference number about 0.92 at 950 jC. Introduction of calcia or zirconia into calcium zirconate structure creates oxygen vacancies and thus significantly enhances electrical conductivity. The investigated electrochemical properties of non-stoichiometric calcium zirconate suggest application of these materials as a solid electrolyte in a galvanic cell at elevated temperatures.
187
Acknowledgements This work was carried out under contract No. 7 T08D 016 19 with the Polish Scientific Research Committee.
References [1] A. Pretis, V. Longo, Silicates Industriels 7 – 8 (1984) 1939. [2] H. Stetson, B. Schwartz, Journal of the American Ceramic Society 44 (1961) 420. [3] T. Yamija, H. Kazeoka, H. Iwahara, Solid State Ionics 47 (1991) 271. [4] N. Kurita, N. Fukatsu, K. Ito, Journal of the Electrochemical Society (1995) 1522. [5] T. Yamija, H. Iwahara, K. Koide, Sensors and Actuators B 5 (1991) 145. [6] T. Jamija, K. Koide, H. Takai, N. Fukatsu, Sensors and Actuators B 14 (1993) 697. [7] W.A. Fischer, D. Janke, Archiv fur das Eisenhuttenwesen 47 (1976) 525 – 530. [8] Ch. Wang, X. Xu, Y. Wen, Solid State Ionics 28 – 30 (1988) 542. [9] D. Janke, Metallurgical Transactions B 13 (1982) 227 – 235. [10] S. Pandit, A. Weyl, D. Janke, Solid State Ionics 69 (1994) 93. [11] T. Etsell, S. Flengas, Journal of the Electrochemical Society 119 (1972) 1. [12] M. Iwase, K.T. Jacob, E. Ichise, Sprechsaal 119 (1986) 280. [13] R.A. Young, Journal of Applied Crystallography 28 (1995) 366 – 367. [14] H. Schmalzried, Zeitschrift fur Physikalische Chemie (Neue Folge) 38 (1963) 87. [15] K. Kiukkola, K. Wagner, Journal of the Electrochemical Society 104 (1957) 366. [16] M. Dudek et al., to be published. [17] R.A. Davies, M.S. Islam, J.D. Gale, Solid State Ionics 126 (1999) 323.