- Email: [email protected]
The mixed ionic and electronic conductivity of CaZrO3 with cation nonstoichiometry and oxygen partial pressure
Available online at www.sciencedirect.com
Solid State Ionics 179 (2008) 1042 – 1045 www.elsevier.com/locate/ssi
The mixed ionic and electronic conductivity of CaZrO3 with cation nonstoichiometry and oxygen partial pressure Soon Cheol Hwang a,b , Gyeong Man Choi b,⁎ a
b
Fuel Cell Project, Research Institute of Industrial Science & Technology, San 32, Hyoja-dong, Pohang 790-330, South Korea Fuel Cell Research Center and Department of Materials Science and Engineering, Pohang University of Science and Technology, San 31, Hyoja-dong, Pohang 790-784, South Korea Received 11 October 2007; received in revised form 23 November 2007; accepted 29 November 2007
Abstract The electrical conductivities of Ca1 − xZrO3 − δ (0 ≤ x ≤ 0.10) ceramics were measured as a function of nonstoichiometry (x) and oxygen partial pressure (Po2) using impedance spectroscopy between 700 °C and 1100 °C. The stoichiometric CaZrO3 was a mixed ionic and electronic (hole) conductor in a high Po2 region. It was an ionic conductor in a low Po2 region. The analysis of impedance spectra revealed that the contributions of the grain and the grain boundary to the total conductivity were nearly constant independent of Po2 at 700 °C. In addition, the grain boundary mainly determined the electrical conductivity at 700 °C. When the Ca deficiency (x) increased up to 0.02, the electrical conductivity rapidly decreased with x due mostly to the decrease in the grain boundary conductivity. © 2007 Elsevier B.V. All rights reserved. Keywords: CaZrO3; Mixed conductor; Nonstoichiometry; Impedance
1. Introduction The oxygen-ion conducting materials have been studied for the applications such as solid oxide fuel cells, oxygen sensors and various kinds of electrochemical devices [1–3]. Calcium zirconate (CaZrO3) with a perovskite structure has a high melting temperature, an excellent thermal and chemical stability, and a good thermal shock resistance [4–6]. It has also been widely used as the structural ceramics, the refractory material, the microwave dielectric and other high temperature applications [7,8]. Fisher and Janke reported that the conductivity of CaZrO3 is independent of Po2 [4]. However, several investigators suggested that it is a mixed p-type and oxygen-ion conductor in high Po2, but a stable ionic conductor in low Po2 [9–11]. The overall conduction properties of solid electrolyte are determined by the defects formed in response to the impurities and the deviation from the stoichiometry [12]. The conductivity is also influenced by both the temperature and the Po2 [12]. In our previous study, we have observed that the total ⁎ Corresponding author. E-mail address: [email protected] (G.M. Choi). 0167-2738/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2007.11.034
conductivity rapidly drops with nonstoichiometry (x) in air due mostly to the decreased grain boundary conductivity [13]. However, very few information is available on the Po2-effect of electrical conductivity of CaZrO3. Moreover, the influence of cation nonstoichiometry of CaZrO3 has not been investigated. In the present study, the electrical conductivities of Ca1 − x ZrO3 specimen were measured as a function of cation nonstoichiometry (x) and Po2. The electrical characterization was carried out using impedance spectroscopy. From the conductivity measurement in various Po2, the contribution of the ionic and the electronic conductivity to the total conductivity was determined. 2. Experimental Ca1 − xZrO3 − δ (0 ≤ x ≤ 0.10) specimens were prepared by a solid-state reaction method. An appropriate amount of CaCO3 (99.9%, Showa, Japan), ZrO2 (99.9%, Tosoh, Japan) powders was ball-milled with zirconia balls in ethanol for 24 h, and calcined at 1350 °C for 4 h. The calcined powders were ballmilled again and subsequently formed into disc-shape by uniaxial pressing followed by cold isotatic pressing (CIP) at the
S.C. Hwang, G.M. Choi / Solid State Ionics 179 (2008) 1042–1045
1043
pressure of 200 MPa. The pellets were sintered at 1700 °C for 4 h in air. The impedance analyzer (Model 4192a, YokokawaHewlett-Packard, Japan) was used to obtain impedance spectra with a frequency range between 5 Hz and 13 MHz. The Po2 dependence of conductivity was measured at 700 °C ~ 1100 °C in Po2 range between 1 and ~ 10− 23atm. The Po2 was controlled by flowing O2–Ar gas mixtures in a high Po2 region (1 ~ 10− 4atm) and CO–CO2 gas mixtures in a low Po2 region (10− 8 ~ 10− 23atm), respectively. 3. Results and discussion Fig. 1 shows the Po2 dependence of the total electrical conductivity (σt) for a stoichiometric CaZrO3 specimen between 700 °C and 1100 °C at 100 °C interval. The σt value of CaZrO3 specimen increased with increasing Po2 in a high Po2 region (e.g., 1 to 10− 4atm at 700 °C), and it was independent of Po2 in an intermediate Po2 region (e.g., 10− 6 to 10− 20atm at 700 °C). However, the σt value abruptly decreased with the decreasing Po2 in a low Po2 region (below 10− 20atm at 700 °C). The high Po2, the intermediate Po2, and the low Po2 regions are defined depending upon the Po2 dependence of conductivity and thus variable with temperature. In the high Po2 region, the increase in σt with Po2 indicates p-type conduction behavior. The origin of p-type conductivity in CaZrO3 has not been clearly determined yet. However, the filling of oxygen vacancies with oxygen gas and the subsequent generation of holes (Eq. (1)) may explain it. 1 O2 þ VO YOO þ 2h 2
ð1Þ
For CaZrO3 systems, the most favorable intrinsic defect is the Ca-site Schottky-type disorder due to the relatively low value of defect formation energy (1.61 eV) [14]. The background impurity (e.g., Al3+) may generate the additional oxygen vacancies. The presence of oxygen vacancies explains the Po2-
Fig. 1. The Po2 dependence of the total electrical conductivity (σt) of stoichiometric CaZrO3 specimen at 700 ~ 1100 °C, measured at 100 °C interval.
Fig. 2. The temperature dependence of the σion and the σh of CaZrO3 specimen in air (700 ~ 1100 °C). The σion and the σh values were obtained from the fitting of curves shown in Fig. 1 using Eq. (2).
independent oxygen-ion conductivity. Since the oxygen vacancy concentration remains constant in a wide Po2, it can be easily shown using Eq. (1) and the corresponding massaction equation that the electron hole conductivity becomes 1/4 proportional to PO2 . This defect model is in accordance with the observed Po2 dependence of the σt as shown in Fig. 1. The total conductivity can be separated into the Po2-independent ionic and the Po2-dependent electronic conductivity by a curve-fitting procedure using Eq. (2), rt ¼ rion þ rBh PO2 : 1=4
ð2Þ
where σion is the ionic conductivity and the σh° is the hole conductivity at Po2 = 1atm. In Fig. 1, the dotted lines and the dashed lines, respectively, represent the ionic (σion) and the electronic (hole) conductivity (σh). Thus, CaZrO3 is a mixed ionic and p-type electronic conductor in a high Po2 region. The ionic region decreased in width with increasing temperature due to the higher activation energy of the hole conduction than the ionic conduction as shown in the below. Fig. 2 shows the temperature dependence of the σion and the σh of CaZrO3 specimen in air. The activation energy (Ea) values were calculated by fitting the conductivity data between 800 and 1100 °C. They were ~ 0.5 eV and ~ 1.16 eV for the σion and the σh, respectively. The activation energies for the ionic and the hole conduction, respectively, are those for the ion migration and the oxidation reaction shown by Eq. (1). At a temperature below 800 °C, the Ea values slightly increased to ~ 0.64 eV and ~ 1.75 eV for the σion and the σh, respectively. The reason for the increase in Ea at a low temperature is not clear at present; however, it may be due to the possible association of oxygen vacancies. The Ea value for the σion of CaZrO3 (~ 0.5 eV) is lower than those of many other perovskite-type oxides that range between ~ 0.5 and ~ 1.0 eV [15]. Since the small Ea value implies that the mobility of the charge carrier is sufficiently high
1044
S.C. Hwang, G.M. Choi / Solid State Ionics 179 (2008) 1042–1045
Fig. 3. Impedance spectra of CaZrO3 specimen at 700 °C in Po2 of 10− 17, 10− 21, 10− 23 and 10− 24 atm.
for CaZrO3 system, the relatively low σion value observed for CaZrO3 specimen may be due to the low oxygen vacancy concentration. The sudden decrease in σt value below the σion plateau value was observed with decreasing Po2 in a low Po2 region (below 10− 20atm at 700 °C). With increasing temperature, the decrease started at the higher Po2 and the ionic plateau region decreased gradually in width. Marozau et al. reported that the abnormal drop in the σt in low Po2 is related to the decomposition of the perovskite-type phase [16]. Thus, in order to examine the phase stability of CaZrO3 specimen in this study, the sample was quenched after annealing in reduced atmosphere (Po2=10− 15atm at 1000 °C). However, XRD pattern did not show any evidence of the second phase. We conclude that there was no decomposition of CaZrO3 phase and CaZrO3 is stable in low Po2. Another possible reason for the decrease in σt may be due to the decrease in the grain boundary conductivity (σgb) rather than the decrease in the grain conductivity (σg). Fig. 3 shows the impedance spectra of CaZrO3 specimen at 700 °C in 10− 17≤ Po2≤10− 24atm. In Po2=10− 17atm, the impedance spectrum
showed two partially-overlapped semicircles, corresponding to the grain (high-frequency side) and the grain boundary (lowfrequency side) process, respectively. Both the grain resistance and the grain boundary resistance gradually increased with decreasing Po2. Fig. 4 shows the σt, the σg, and the σgb values of CaZrO3 specimen as a function of Po2, calculated from the impedance spectra. It was clearly shown that the σgb value was much smaller than the σg value and thus the σt value was mainly determined by the σgb value in a whole Po2 region at 700 °C. The contributions of the grain and the grain boundary conductivities to the total conductivity were nearly constant. Although the σt is mostly determined by σgb at 700 °C, the decreasing contribution of the σgb is expected at high temperature due to the higher Ea value of σgb (~ 1.20 eV) than that of σg (~ 0.85 eV), as observed in our previous study [13]. The nature of decreasing σt with decreasing Po2 in the low Po2 region is not clear at present. The decreased oxygen vacancy concentration due to the defect association in reduced Po2 may possibly be responsible for the phenomena. A further study is necessary to explain them.
Fig. 4. The σt, σg and σgb of CaZrO3 specimen at 700 °C, calculated from the impedance spectra, as a function of Po2 (1 ~ 10− 24 atm).
Fig. 5. The Po2 dependence of total electrical conductivity for Ca1 − xZrO3−δ (0 ≤ x ≤ 0.1) specimen at 1000 °C. The x values and the fitted σion values were shown at the right and left of curves, respectively.
S.C. Hwang, G.M. Choi / Solid State Ionics 179 (2008) 1042–1045
1045
of the phenomena. In our previous paper, we suggested that the transfer of a small amount of excess Zr4+ to Ca2+ site under Ca deficiency may be responsible for the decrease in the concentration of oxygen vacancies [17]. A more complete analysis of conduction behavior in non-stoichiometric CaZrO3 requires the better separation of the grain from the grain boundary contribution and also that of the ionic from electronic contribution. 4. Conclusions
Fig. 6. The comparison of the σion and σh values of Ca1 − xZrO3 − δ specimen as a function of x (0 ≤ x ≤ 0.1) at 1000 °C. The large change in both σion and σh values were shown near x = 0.02.
Fig. 5 shows the Po2 dependence of total electrical conductivity for A-site deficient Ca1 − xZrO3 − δ (0 ≤x≤0.1) specimen at 1000 °C. When the Ca deficiency (x) increased up to 0.02, the σt rapidly decreased. An analysis of impedance spectra shows that the lower conductivity for the x=0.013 sample than that for the x = 0 sample at this temperature was due mostly to the decrease in the σgb value. Although the separation of overlapping semicircles was not possible for the sample with x above 0.016, the dominance of grain boundary resistance is expected from the observed trend. With increasing x up to 0.02, the ionic conduction region rapidly increased in width. However, the maximum value of the ionic transference number (tion) was 0.37 for the sample with x = 0.02 in air. It indicates that in the high Po2 region, Ca1 − xZrO3 − δ specimen with x=0.02 or above remains as a mixed ionic and electronic (hole) conductor with a predominant σh contribution. For the samples with x=0.02, 0.05, and 0.10, the σt slightly increased with x. N-type behavior also started to appear for these samples in the very low Po2 region (b ~ 10− 15atm) due possibly to the suppressed ionic conductivity. Fig. 6 compares the σion and the σh values (1 atm) of Ca1 − xZrO3 − δ specimen as a function of x at 1000 °C. The σion and the σh, respectively, decrease ~ 2 and ~ 3 orders in magnitude with x. The change may be largely due to the change in the σgb value as discussed. When we assume that the change is not simply due to the microstructural origin such as an insulating grain boundary phase, a defect model is needed for the explanation
The stoichiometric CaZrO3 was a mixed ionic and electronic (hole) conductor in a high Po2 region. It was an ionic conductor in a low Po2 region. The analysis of impedance spectra revealed that the contributions of the grain and the grain boundary to the total conductivity were nearly constant independent of Po2 at 700 °C. In addition, the grain boundary mainly determined the electrical conductivity at 700 °C. When the Ca deficiency (x) increased up to 0.02, the electrical conductivity rapidly decreased with x due mostly to the decrease in the grain boundary conductivity. References [1] N.Q. Minh, J. Am. Ceram. Soc. 76 (1993) 563. [2] E.M. Logothetis, in: A.H. Heuer, L.W. Hobbs (Eds.), Advanced in Ceramics, Science and Technology of Zirconia, vol. 3, The American Ceramic Society, Inc., Ohio, 1981, p. 388. [3] H.J.M. Bouwmeester, A.J. Burggraaf, in: P.J. Gellings, H.J.M. Bouwmeester (Eds.), The CRC Handbook of Solid State Electrochemistry, CRC Press Inc., Boca Raton, 1997, p. 481. [4] W.A. Fisher, D. Janke, Arch. Eisenhutenwes. 47 (1976) 525. [5] D. Pretis, F. Ricciardiello, O. Sbaizero, Powder Metall. Prog. 18 (1986) 427. [6] T. Murakami, H. Fukuyama, T. Kishida, M. Susa, K. Nagata, Metall. Mater. Trans., B, Proc. Metall. Mater. Proc. Sci. 31 (2000) 25. [7] C.C. Wang, S.A. Akbar, W. Chen, J.R. Schorr, Sens. Actuators, A, Phys. 58 (1997) 237. [8] Y. Chang, C.C. Wang, S.A. Akbar, Sens. Actuators, B, Chem. 46 (1998) 208. [9] A. de. Pretis, V. Longo, F. Ricciardiello, O. Sbaizero, Silic. Ind. 7–8 (1984) 139. [10] C. Wang, X. Hu, H. Yu, Y. Wen, K. Zhao, Solid State Ion. 28–30 (1988) 542. [11] S.S. Pandit, A. Weyl, D. Janke, Solid State Ion. 69 (1994) 93. [12] C.C. Wang, S.A. Akbar, W. Chen, V.D. Patton, J. Mater. Sci. 30 (1995) 1627. [13] S.C. Hwang, G.M. Choi, J. Eur. Ceram. Soc. 25 (2005) 2609. [14] R.A. Davies, M.S. Islam, J.D. Gale, Solid State Ion. 126 (1999) 323. [15] T.L. Nguyen, M. Dokiya, S. Wang, H. Tagawa, T. Hashimoto, Solid State Ion. 130 (2000) 229. [16] I.P. Marozau, V.V. Kharton, A.P. Viskup, J.R. Frade, V.V. Samakhval, J. Eur. Ceram. Soc. 26 (2006) 1371. [17] S.C. Hwang, G.M. Choi, Solid State Ion. 177 (2006) 3099.
Solid State Ionics 179 (2008) 1042 – 1045 www.elsevier.com/locate/ssi
The mixed ionic and electronic conductivity of CaZrO3 with cation nonstoichiometry and oxygen partial pressure Soon Cheol Hwang a,b , Gyeong Man Choi b,⁎ a
b
Fuel Cell Project, Research Institute of Industrial Science & Technology, San 32, Hyoja-dong, Pohang 790-330, South Korea Fuel Cell Research Center and Department of Materials Science and Engineering, Pohang University of Science and Technology, San 31, Hyoja-dong, Pohang 790-784, South Korea Received 11 October 2007; received in revised form 23 November 2007; accepted 29 November 2007
Abstract The electrical conductivities of Ca1 − xZrO3 − δ (0 ≤ x ≤ 0.10) ceramics were measured as a function of nonstoichiometry (x) and oxygen partial pressure (Po2) using impedance spectroscopy between 700 °C and 1100 °C. The stoichiometric CaZrO3 was a mixed ionic and electronic (hole) conductor in a high Po2 region. It was an ionic conductor in a low Po2 region. The analysis of impedance spectra revealed that the contributions of the grain and the grain boundary to the total conductivity were nearly constant independent of Po2 at 700 °C. In addition, the grain boundary mainly determined the electrical conductivity at 700 °C. When the Ca deficiency (x) increased up to 0.02, the electrical conductivity rapidly decreased with x due mostly to the decrease in the grain boundary conductivity. © 2007 Elsevier B.V. All rights reserved. Keywords: CaZrO3; Mixed conductor; Nonstoichiometry; Impedance
1. Introduction The oxygen-ion conducting materials have been studied for the applications such as solid oxide fuel cells, oxygen sensors and various kinds of electrochemical devices [1–3]. Calcium zirconate (CaZrO3) with a perovskite structure has a high melting temperature, an excellent thermal and chemical stability, and a good thermal shock resistance [4–6]. It has also been widely used as the structural ceramics, the refractory material, the microwave dielectric and other high temperature applications [7,8]. Fisher and Janke reported that the conductivity of CaZrO3 is independent of Po2 [4]. However, several investigators suggested that it is a mixed p-type and oxygen-ion conductor in high Po2, but a stable ionic conductor in low Po2 [9–11]. The overall conduction properties of solid electrolyte are determined by the defects formed in response to the impurities and the deviation from the stoichiometry [12]. The conductivity is also influenced by both the temperature and the Po2 [12]. In our previous study, we have observed that the total ⁎ Corresponding author. E-mail address: [email protected] (G.M. Choi). 0167-2738/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2007.11.034
conductivity rapidly drops with nonstoichiometry (x) in air due mostly to the decreased grain boundary conductivity [13]. However, very few information is available on the Po2-effect of electrical conductivity of CaZrO3. Moreover, the influence of cation nonstoichiometry of CaZrO3 has not been investigated. In the present study, the electrical conductivities of Ca1 − x ZrO3 specimen were measured as a function of cation nonstoichiometry (x) and Po2. The electrical characterization was carried out using impedance spectroscopy. From the conductivity measurement in various Po2, the contribution of the ionic and the electronic conductivity to the total conductivity was determined. 2. Experimental Ca1 − xZrO3 − δ (0 ≤ x ≤ 0.10) specimens were prepared by a solid-state reaction method. An appropriate amount of CaCO3 (99.9%, Showa, Japan), ZrO2 (99.9%, Tosoh, Japan) powders was ball-milled with zirconia balls in ethanol for 24 h, and calcined at 1350 °C for 4 h. The calcined powders were ballmilled again and subsequently formed into disc-shape by uniaxial pressing followed by cold isotatic pressing (CIP) at the
S.C. Hwang, G.M. Choi / Solid State Ionics 179 (2008) 1042–1045
1043
pressure of 200 MPa. The pellets were sintered at 1700 °C for 4 h in air. The impedance analyzer (Model 4192a, YokokawaHewlett-Packard, Japan) was used to obtain impedance spectra with a frequency range between 5 Hz and 13 MHz. The Po2 dependence of conductivity was measured at 700 °C ~ 1100 °C in Po2 range between 1 and ~ 10− 23atm. The Po2 was controlled by flowing O2–Ar gas mixtures in a high Po2 region (1 ~ 10− 4atm) and CO–CO2 gas mixtures in a low Po2 region (10− 8 ~ 10− 23atm), respectively. 3. Results and discussion Fig. 1 shows the Po2 dependence of the total electrical conductivity (σt) for a stoichiometric CaZrO3 specimen between 700 °C and 1100 °C at 100 °C interval. The σt value of CaZrO3 specimen increased with increasing Po2 in a high Po2 region (e.g., 1 to 10− 4atm at 700 °C), and it was independent of Po2 in an intermediate Po2 region (e.g., 10− 6 to 10− 20atm at 700 °C). However, the σt value abruptly decreased with the decreasing Po2 in a low Po2 region (below 10− 20atm at 700 °C). The high Po2, the intermediate Po2, and the low Po2 regions are defined depending upon the Po2 dependence of conductivity and thus variable with temperature. In the high Po2 region, the increase in σt with Po2 indicates p-type conduction behavior. The origin of p-type conductivity in CaZrO3 has not been clearly determined yet. However, the filling of oxygen vacancies with oxygen gas and the subsequent generation of holes (Eq. (1)) may explain it. 1 O2 þ VO YOO þ 2h 2
ð1Þ
For CaZrO3 systems, the most favorable intrinsic defect is the Ca-site Schottky-type disorder due to the relatively low value of defect formation energy (1.61 eV) [14]. The background impurity (e.g., Al3+) may generate the additional oxygen vacancies. The presence of oxygen vacancies explains the Po2-
Fig. 1. The Po2 dependence of the total electrical conductivity (σt) of stoichiometric CaZrO3 specimen at 700 ~ 1100 °C, measured at 100 °C interval.
Fig. 2. The temperature dependence of the σion and the σh of CaZrO3 specimen in air (700 ~ 1100 °C). The σion and the σh values were obtained from the fitting of curves shown in Fig. 1 using Eq. (2).
independent oxygen-ion conductivity. Since the oxygen vacancy concentration remains constant in a wide Po2, it can be easily shown using Eq. (1) and the corresponding massaction equation that the electron hole conductivity becomes 1/4 proportional to PO2 . This defect model is in accordance with the observed Po2 dependence of the σt as shown in Fig. 1. The total conductivity can be separated into the Po2-independent ionic and the Po2-dependent electronic conductivity by a curve-fitting procedure using Eq. (2), rt ¼ rion þ rBh PO2 : 1=4
ð2Þ
where σion is the ionic conductivity and the σh° is the hole conductivity at Po2 = 1atm. In Fig. 1, the dotted lines and the dashed lines, respectively, represent the ionic (σion) and the electronic (hole) conductivity (σh). Thus, CaZrO3 is a mixed ionic and p-type electronic conductor in a high Po2 region. The ionic region decreased in width with increasing temperature due to the higher activation energy of the hole conduction than the ionic conduction as shown in the below. Fig. 2 shows the temperature dependence of the σion and the σh of CaZrO3 specimen in air. The activation energy (Ea) values were calculated by fitting the conductivity data between 800 and 1100 °C. They were ~ 0.5 eV and ~ 1.16 eV for the σion and the σh, respectively. The activation energies for the ionic and the hole conduction, respectively, are those for the ion migration and the oxidation reaction shown by Eq. (1). At a temperature below 800 °C, the Ea values slightly increased to ~ 0.64 eV and ~ 1.75 eV for the σion and the σh, respectively. The reason for the increase in Ea at a low temperature is not clear at present; however, it may be due to the possible association of oxygen vacancies. The Ea value for the σion of CaZrO3 (~ 0.5 eV) is lower than those of many other perovskite-type oxides that range between ~ 0.5 and ~ 1.0 eV [15]. Since the small Ea value implies that the mobility of the charge carrier is sufficiently high
1044
S.C. Hwang, G.M. Choi / Solid State Ionics 179 (2008) 1042–1045
Fig. 3. Impedance spectra of CaZrO3 specimen at 700 °C in Po2 of 10− 17, 10− 21, 10− 23 and 10− 24 atm.
for CaZrO3 system, the relatively low σion value observed for CaZrO3 specimen may be due to the low oxygen vacancy concentration. The sudden decrease in σt value below the σion plateau value was observed with decreasing Po2 in a low Po2 region (below 10− 20atm at 700 °C). With increasing temperature, the decrease started at the higher Po2 and the ionic plateau region decreased gradually in width. Marozau et al. reported that the abnormal drop in the σt in low Po2 is related to the decomposition of the perovskite-type phase [16]. Thus, in order to examine the phase stability of CaZrO3 specimen in this study, the sample was quenched after annealing in reduced atmosphere (Po2=10− 15atm at 1000 °C). However, XRD pattern did not show any evidence of the second phase. We conclude that there was no decomposition of CaZrO3 phase and CaZrO3 is stable in low Po2. Another possible reason for the decrease in σt may be due to the decrease in the grain boundary conductivity (σgb) rather than the decrease in the grain conductivity (σg). Fig. 3 shows the impedance spectra of CaZrO3 specimen at 700 °C in 10− 17≤ Po2≤10− 24atm. In Po2=10− 17atm, the impedance spectrum
showed two partially-overlapped semicircles, corresponding to the grain (high-frequency side) and the grain boundary (lowfrequency side) process, respectively. Both the grain resistance and the grain boundary resistance gradually increased with decreasing Po2. Fig. 4 shows the σt, the σg, and the σgb values of CaZrO3 specimen as a function of Po2, calculated from the impedance spectra. It was clearly shown that the σgb value was much smaller than the σg value and thus the σt value was mainly determined by the σgb value in a whole Po2 region at 700 °C. The contributions of the grain and the grain boundary conductivities to the total conductivity were nearly constant. Although the σt is mostly determined by σgb at 700 °C, the decreasing contribution of the σgb is expected at high temperature due to the higher Ea value of σgb (~ 1.20 eV) than that of σg (~ 0.85 eV), as observed in our previous study [13]. The nature of decreasing σt with decreasing Po2 in the low Po2 region is not clear at present. The decreased oxygen vacancy concentration due to the defect association in reduced Po2 may possibly be responsible for the phenomena. A further study is necessary to explain them.
Fig. 4. The σt, σg and σgb of CaZrO3 specimen at 700 °C, calculated from the impedance spectra, as a function of Po2 (1 ~ 10− 24 atm).
Fig. 5. The Po2 dependence of total electrical conductivity for Ca1 − xZrO3−δ (0 ≤ x ≤ 0.1) specimen at 1000 °C. The x values and the fitted σion values were shown at the right and left of curves, respectively.
S.C. Hwang, G.M. Choi / Solid State Ionics 179 (2008) 1042–1045
1045
of the phenomena. In our previous paper, we suggested that the transfer of a small amount of excess Zr4+ to Ca2+ site under Ca deficiency may be responsible for the decrease in the concentration of oxygen vacancies [17]. A more complete analysis of conduction behavior in non-stoichiometric CaZrO3 requires the better separation of the grain from the grain boundary contribution and also that of the ionic from electronic contribution. 4. Conclusions
Fig. 6. The comparison of the σion and σh values of Ca1 − xZrO3 − δ specimen as a function of x (0 ≤ x ≤ 0.1) at 1000 °C. The large change in both σion and σh values were shown near x = 0.02.
Fig. 5 shows the Po2 dependence of total electrical conductivity for A-site deficient Ca1 − xZrO3 − δ (0 ≤x≤0.1) specimen at 1000 °C. When the Ca deficiency (x) increased up to 0.02, the σt rapidly decreased. An analysis of impedance spectra shows that the lower conductivity for the x=0.013 sample than that for the x = 0 sample at this temperature was due mostly to the decrease in the σgb value. Although the separation of overlapping semicircles was not possible for the sample with x above 0.016, the dominance of grain boundary resistance is expected from the observed trend. With increasing x up to 0.02, the ionic conduction region rapidly increased in width. However, the maximum value of the ionic transference number (tion) was 0.37 for the sample with x = 0.02 in air. It indicates that in the high Po2 region, Ca1 − xZrO3 − δ specimen with x=0.02 or above remains as a mixed ionic and electronic (hole) conductor with a predominant σh contribution. For the samples with x=0.02, 0.05, and 0.10, the σt slightly increased with x. N-type behavior also started to appear for these samples in the very low Po2 region (b ~ 10− 15atm) due possibly to the suppressed ionic conductivity. Fig. 6 compares the σion and the σh values (1 atm) of Ca1 − xZrO3 − δ specimen as a function of x at 1000 °C. The σion and the σh, respectively, decrease ~ 2 and ~ 3 orders in magnitude with x. The change may be largely due to the change in the σgb value as discussed. When we assume that the change is not simply due to the microstructural origin such as an insulating grain boundary phase, a defect model is needed for the explanation
The stoichiometric CaZrO3 was a mixed ionic and electronic (hole) conductor in a high Po2 region. It was an ionic conductor in a low Po2 region. The analysis of impedance spectra revealed that the contributions of the grain and the grain boundary to the total conductivity were nearly constant independent of Po2 at 700 °C. In addition, the grain boundary mainly determined the electrical conductivity at 700 °C. When the Ca deficiency (x) increased up to 0.02, the electrical conductivity rapidly decreased with x due mostly to the decrease in the grain boundary conductivity. References [1] N.Q. Minh, J. Am. Ceram. Soc. 76 (1993) 563. [2] E.M. Logothetis, in: A.H. Heuer, L.W. Hobbs (Eds.), Advanced in Ceramics, Science and Technology of Zirconia, vol. 3, The American Ceramic Society, Inc., Ohio, 1981, p. 388. [3] H.J.M. Bouwmeester, A.J. Burggraaf, in: P.J. Gellings, H.J.M. Bouwmeester (Eds.), The CRC Handbook of Solid State Electrochemistry, CRC Press Inc., Boca Raton, 1997, p. 481. [4] W.A. Fisher, D. Janke, Arch. Eisenhutenwes. 47 (1976) 525. [5] D. Pretis, F. Ricciardiello, O. Sbaizero, Powder Metall. Prog. 18 (1986) 427. [6] T. Murakami, H. Fukuyama, T. Kishida, M. Susa, K. Nagata, Metall. Mater. Trans., B, Proc. Metall. Mater. Proc. Sci. 31 (2000) 25. [7] C.C. Wang, S.A. Akbar, W. Chen, J.R. Schorr, Sens. Actuators, A, Phys. 58 (1997) 237. [8] Y. Chang, C.C. Wang, S.A. Akbar, Sens. Actuators, B, Chem. 46 (1998) 208. [9] A. de. Pretis, V. Longo, F. Ricciardiello, O. Sbaizero, Silic. Ind. 7–8 (1984) 139. [10] C. Wang, X. Hu, H. Yu, Y. Wen, K. Zhao, Solid State Ion. 28–30 (1988) 542. [11] S.S. Pandit, A. Weyl, D. Janke, Solid State Ion. 69 (1994) 93. [12] C.C. Wang, S.A. Akbar, W. Chen, V.D. Patton, J. Mater. Sci. 30 (1995) 1627. [13] S.C. Hwang, G.M. Choi, J. Eur. Ceram. Soc. 25 (2005) 2609. [14] R.A. Davies, M.S. Islam, J.D. Gale, Solid State Ion. 126 (1999) 323. [15] T.L. Nguyen, M. Dokiya, S. Wang, H. Tagawa, T. Hashimoto, Solid State Ion. 130 (2000) 229. [16] I.P. Marozau, V.V. Kharton, A.P. Viskup, J.R. Frade, V.V. Samakhval, J. Eur. Ceram. Soc. 26 (2006) 1371. [17] S.C. Hwang, G.M. Choi, Solid State Ion. 177 (2006) 3099.