- Email: [email protected]
Electrical conduction behavior of mixed ionic-electronic conductor Y0.08Sr0.92Ti1 − xScxO3 − δ
Scripta Materialia 114 (2016) 70–73
Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Regular Article
Electrical conduction behavior of mixed ionic-electronic conductor Y0.08Sr0.92Ti1 − xScxO3 − δ Ke Shan a,b, Zhong-Zhou Yi a,b,⁎ a b
College of Science, Honghe University, Mengzi 661199, Yunnan, China Local Characteristic Resource Utilization and New Materials Key Laboratory of Universities in Yunnan, Honghe University, Mengzi 661199, Yunnan, China
a r t i c l e
i n f o
Article history: Received 7 June 2015 Accepted 9 August 2015 Available online xxxx Keywords: Y0.08Sr0.92Ti1 − xScxO3 − δ Mixed ionic-electronic conductor Ionic conductivity Charge compensation mechanism
a b s t r a c t Y0.08Sr0.92Ti1 − xScxO3 − δ was synthesized via sol–gel method. With the Sc-doping amount increasing, the electrical conductivity of Y0.08Sr0.92Ti1 − xScxO3 − δ increased in 400–900 °C and the ionic conductivity increased between 600 and 900 °C. The optimized Y0.08Sr0.92Ti0.93Sc0.07O3 − δ sample exhibits an electrical conductivity in the order of 0.058–0.085 S·cm−1 at 400–900 °C. The ionic conductivity for Y0.08Sr0.92Ti0.93Sc0.07O3 − δ was 0.027 S·cm−1 and increased about 140% compared with Y0.08Sr0.92Ti0.97Sc0.03O3 − δ at 800 °C. Sc-doping increased the oxygen vacancy concentration and decreased the oxygen migration energy, therefore enhancing the conduction process of oxygen ions in Y0.08Sr0.92Ti1 − xScxO3 − δ. The possible charge compensation mechanism of Y0.08Sr0.92Ti1 − xScxO3 − δ 4+ 3+ can be described as Y0.08Sr0.92Sc3+ x Ti0.92 − 2δ − x1Ti0.08 + 2δ ‐ x2O3− (δ + x1/2)(x = x1 + x2). © 2015 Published by Elsevier Ltd.
1. Introduction Perovskite oxides (ABO3 ) are particularly attractive for hightemperature applications, because of the microstructural stability to improve reliability and long-term performance and mixed electronic-ionic conductivity that make the triple-phase boundary extend to the entirely exposed anode surface as anode materials for solid oxide fuel cells, in addition to having high melting and decomposition temperatures [1,2]. Strontium titanate (SrTiO3) with perovskite structure shows high chemical stability at high temperature under both oxidizing and reducing atmospheres and has strong resistance to suffer poison [3]. In addition, the perovskite structure of strontium titanae has two differently-sized cations, which makes it favorable to a variety of dopants. This doping flexibility can control the electron and ion transport properties to optimize performance for different applications. Generally, electronic conduction can be enhanced in SrTiO3 by replacing A- or B-site cations with n-type dopants such as La3 +, Y3+ or Nb5 +[4–8]. It is well known that n-doping generates a defect with an effective positive charge in the host lattice. Ionic conduction can be improved by doping with Fe3+, Cr3+, Sc3+ or Ni2+[9–19], in place of the B site, results in a p-type semiconductor by generating oxygen vacancy. The relevant studies indicated that donor-doping on A-site of SrTiO3 mainly affected the electronic conductivity rather than ionic conductivity [20]. In addition, the study by Li X and Zhao H [3] demonstrated that the tendency of oxygen ion migration energy increases in the order B = Sc b Co b Ni b Ti for SrBO3, implying Sc was a good choice ⁎ Corresponding author. E-mail addresses: [email protected] (K. Shan), [email protected] (Z.-Z. Yi).
http://dx.doi.org/10.1016/j.scriptamat.2015.08.008 1359-6462/© 2015 Published by Elsevier Ltd.
of all the present elements to improve the ionic conductivity of SrTiO3-based materials. Therefore, the effects of Sc-doping amount on the electrical and ionic conductivities of Y0.08Sr0.92Ti1 − xScxO3 − δ with temperature were investigated. The charge compensation mechanism in Y0.08Sr0.92Ti1 − xScxO3 − δ was discussed. 2. Experimental 2.1. Sample preparation Y0.08Sr0.92Ti1 − xScxO3 − δ (x = 0.03, 0.05, 0.07) powders were synthesized by using sol–gel method. The raw materials were high purity Sr(CH3COO)2·2H2O, Ti(CH3CH2CH2CH2O)4, Sc2O3 and Y2O3. Appropriate amounts of the starting materials (Ti(CH3CH2CH2CH2O)4, Sc2O3 and Y2O3) were dissolved into the mixed solution with the molar ratio of isopropanol to ethanol at 4:1 under magnetic stirring followed by the addition of strontium acetate. The resultant solution was heated in a water bath of 60 °C until a viscous gel was formed. The gel was heated in an evaporating dish on an electric furnace until self-igniting, yielding white and porous powders. The ground powders were calcined at 1100 °C for 10 h. The calcined powders were uniaxially pressed into pellets under the pressure of 50 MPa. The sintering step was carried out at 1450 °C for 5 h with the green pellets being into the platinum crucible for the measurement of total electrical and ionic conductivities. 2.2. Characterization X-ray diffraction (XRD, Rigaku D/max-A) with Ni filtered Cu Kα radiation was employed to identify the phase composition for the sintered samples Y0.08Sr0.92Ti1 − xScxO3 − δ (x = 0.03, 0.05, 0.07) with a 0.02°
K. Shan, Z.-Z. Yi / Scripta Materialia 114 (2016) 70–73
71
stepwise at room temperature. The relative density of all the samples was around 95%, which was estimated from the weight and dimensions measurement. The cross-section morphology was observed by scanning electron microscopy (SEM, Quanta FEG 650). The total electrical conductivity was measured by AC impedance spectroscopy with the CHI660B electrochemical workstation over the frequency range of 1 MHz to 0.01 Hz with a perturbation amplitude of 5 mV. The data analysis was carried out with appropriate equivalent circuit by Zsimpwin software. The well-polished samples (~7.2 mm in diameter and ~ 1.6 mm in thickness) were painted with platinum paint on both sides and Pt wires were used to fabricate the electrodes and baked at 800 °C for 0.5 h. The total electrical conductivity calculation was performed with overall area and thickness of the samples. The ionic conductivity was measured by electron-blocking method within 600–900 °C. The sample was plastered onto dense YSZ tablet with a little Pt paste, overcoming the interface resistance. Platinum paint was painted to the side of sample and YSZ tablet. The electron is blocked by YSZ layer because YSZ is almost a pure oxygen ion conductor. Glass seal can prevent oxygen leakage along the sides of the assembled samples. The impedance spectra of double layer materials can be observed at designated temperatures. Therefore, the ionic conductivity calculation of sample (σi) was performed as following equation:
σi ¼
dYSTS −1 ðOhm cmÞ ðRT −RYSZ Þ AYSTS
ð1Þ
Where dYSTS is the thickness of sample (in cm),RT is the total resistance, RYSZ is the resistance of YSZ and AYSTS is the area (in sq. cm) of the sample. 3. Results and discussion 3.1. XRD patterns and SEM micrographs XRD patterns of the sintered samples were collected at room temperature and the results were displayed in Fig. 1. The sample Y0.08Sr0.92Ti1 − xScxO3 − δ with x = 0.07 shows a single cubic perovskite structure (inset PDF79-174), whereas a secondary phase of Y2Ti2O7 was observed the doping level decreased to 0.05 and 0.03. The content of the secondary phase decreases with Sc-doping amount increasing as indicated by the strengthening diffraction intensity of Y2Ti2O7, indicating that Sc-doping facilitates the formation of single cubic perovskite phase.
Fig. 2. SEM micrographs of fracture surfaces of Y0.08Sr0.92Ti1 − xScxO3 − δ: (a) x = 0.03 and (b) x = 0.07.
The microstructure of a couple of selected samples was observed by SEM on fracture surfaces and representative micrographs are reported in Fig. 2. As shown in Fig. 2, the densities of Y0.08Sr0.92Ti1 − xScxO3 − δ samples increase with Sc-doping amount increasing, indicating that Sc-doping is favorable to the densification process of the materials synthesized by the sol–gel route in the present work. 3.2. The electronic and ionic conductivities of Y0.08Sr0.92Ti1 − xScxO3 − δ
Fig. 1. XRD patterns of Y0.08Sr0.92Ti1 − xScxO3 − δ after sintering at 1450 °C for 5 h: (a) x = 0.03; (b) x = 0.05; (c) x = 0.07.
The temperature dependence of total electrical conductivities of Y0.08Sr0.92Ti1 − xScxO3 − δ (x = 0.03, 0.05, 0.07) is shown in Fig. 3. Sc-doping enhances the electrical conductivity of SrTiO3 significantly. The optimized Y0.08Sr0.92Ti0.93Sc0.07O3 − δ sample exhibits an electrical conductivity in the order of 0.058–0.085 S·cm− 1 at 400–900 °C. The total electrical conductivity is the sum of electronic conductivity and ionic conductivity and its value is directly related with the type and concentration of point defects existing in the materials. In addition, the total electrical conductivity increased through a maximum then decreased with the temperature increasing, indicating a polaron conduction behavior of Y0.08Sr0.92Ti1 − xScxO3 − δ, corresponding to the high electron concentration. On the other hand, the ionic conductivity of Y 0.08 Sr 0.92Ti1 − x Sc xO 3 − δ increased with Sc-doping amount increasing remarkably (Fig. 4). The ionic conductivity of Y0.08Sr0.92Ti0.97Sc0.03O3 − δ was 0.02 S·cm−1 at 850 °C, while it increased to 0.029 S·cm−1 for Y0.08Sr0.92Ti0.95Sc0.05O3 − δ and 0.033 S·cm−1 for
72
K. Shan, Z.-Z. Yi / Scripta Materialia 114 (2016) 70–73
oxygen vacancy concentration and the migration energy of oxygen ion at certain temperature. According to the Eq. (3), the increase of Sc-doping amount will result in the increase of the oxygen vacancy concentration. On the other hand, the mobility of oxygen ion also has strong impact on the oxygen ionic conductivity. The activation energy for oxygen ion migration (Ea) was calculated by Arrhenius rule, according to the results shown in Fig. 4(b). As shown in Fig. 4(b), Ea of Y0.08Sr0.92Ti1 − xScxO3 − δ decreased with Scdoping increasing, indicating that Sc-doping facilitates the conduction of oxygen ions. 3.3. Charge compensation mechanism of Y, Sc co-doped SrTiO3 Based on the defect chemistry, free electrons (e') and oxygen vacancies [V O •• ] are prevalent in Y0.08Sr0.92Ti1 − xScxO3 − δ as a result of the substitution of Sc on Ti and Y on Sr due to the n-type conduction of Y0.08Sr0.92Ti1 − xScxO3 − δ. The defects produced in Y donor doping and Sc acceptor doping SrTiO3 can be expressed respectively as: Fig. 3. Temperature dependences of the total electrical conductivities of Y 0.08Sr0.92 Ti 1 − x Scx O 3 − δ (x = 0.03, 0.05, 0.07) in 400–900 °C.
Y0.08Sr0.92Ti0.93Sc0.07O3 − δ. Compared with Y0.08Sr0.92Ti0.97Sc0.03O3 − δ, the ionic conductivity of Y0.08Sr0.92Ti0.93Sc0.07O3 − δ increased about 165%. It is well known that the ionic conductivity is primarily dependent on the
SrTi0:6 Scx O3−δ
Y2 O3 → 2Y•Sr þ 2O O þ Y0:08 Sr0:92 TiO3
0
1 0 O2 þ 2e 2
Sc2 O3 → 2ScTi4þ þ V••O þ 3O O 0
Ti4þ þ e →Ti3þ
ð2Þ ð3Þ ð4Þ
There are three possible charge compensation mechanisms. The first possible mechanism is Sc3 + ions in place of Ti4 + ions, which can be 4+ 3+ expressed as Y0.08Sr0.92Sc3+ x Ti0.92 − 2δ − xTi0.08 + 2δO3− (δ + x/2). The defects produced in Y, Sc co-doping SrTiO3 can be expressed as Eq. (3). In this case, the ionic conductivity of Y0.08Sr0.92Ti1 − xScxO3 − δ may increase due to the increased oxygen vacancy concentration with Scdoping amount increasing. However, the electronic conductivity will keep unchanged because of the unchanged concentration of Ti3+ ions. In fact, the experimental results show that the total electrical conductivity of n-type conduction increases. Therefore, this is inconsistent with the experimental results. The second possibility can be described that Sc3 + only takes the place of Ti3+ site. The defects produced by Sc-doping can be expressed as: þ 3O Sc2 O3 →2Sc O Ti3¼
ð5Þ
In this case, the electronic conductivity may increase and oxygen vacancy concentration will keep unchanged. This conflicts with the experimental results. The third charge compensation mechanism can be expressed as 4+ 3+ Y0.08Sr0.92Sc3+ x Ti0.92 − 2δ − x1Ti0.08 + 2δ ‐ x2O3− (δ + x1/2)(x = x1 + x2) (Eqs. (3) and (5)). In other word, Sc ions replace both Ti4 + and Ti3 + sites simultaneously. With Sc-doping amount increasing, the Sc3+ ion may tend to replace Ti4 + to produce more oxygen vacancy and the tendency of Sc3 + ion in place of Ti3 + may decrease, therefore, the ionic conductivity and electronic conductivity (n-type conduction) increases with Sc-doping amount increasing, respectively. Therefore, this agrees with the experimental results. 4. Conclusions
Fig. 4. Temperature dependences of the ionic conductivities of Y0.08Sr0.92Ti1 − xScxO3 − δ (x = 0.03, 0.05, 0.07) in 600–900 °C.
Y0.08Sr0.92Ti1 − xScxO3 − δ (x = 0.03, 0.05, 0.07) exhibits a single cubic phase perovskite structure when the x value is 0.07. Sc-doping facilitates the formation of cubic phase perovskite due to the weakening peak intensity of the second phase. Partial oxygen ionic conductivity Y0.08Sr0.92Ti1 − xScxO3 − δ (x = 0.03, 0.05, 0.07) increases with Scdoping increasing, which may be attributed to the increases of oxygen vacancy concentration and the mobility of oxygen ion. The total electrical conductivity increases with Sc-doping amount, corresponding
K. Shan, Z.-Z. Yi / Scripta Materialia 114 (2016) 70–73
to the high charge carrier (electron) due to the polaron conduction behavior. The optimized Y0.08Sr0.92Ti0.93Sc0.07O3 − δ sample exhibits an electrical conductivity in the order of 0.058–0.085 S·cm − 1 at 400–900 °C. The ionic conductivity for Y0.08Sr 0.92 Ti0.93 Sc0.07 O3 − δ was 0.027 S·cm− 1 and increased about 140% compared with Y0.08Sr0.92Ti0.97Sc0.03O3 − δ at 800 °C. The possible charge compensation mechanism of Y and Sc co-doping SrTiO3 can be described as 4+ 3+ Y0.08Sr0.92Sc3+ x Ti0.92 − 2δ − x1Ti0.08 + 2δ ‐ x2O3− (δ + x1/2)(x = x1 + x2). Acknowledgments This work has been funded by the National Natural Science Foundation of China (Nos. 51562009 and 51362011). References [1] J.W. Fergus, Sens. Actuators B 123 (2007) 1169–1179. [2] X. Li, H. Zhao, N. Xu, X. Zhou, C. Zhang, N. Chen, Int. J. Hydrogen. Energy 34 (2009) 6407–6414. [3] X. Li, H. Zhao, F. Gao, N. Chen, N. Xu, Electrochem. Commun. 10 (2008) 1567–1570.
73
[4] R. Moors, S. Schollhammer, K.H. Hardt, Appl. Phys. A 65 (1997) 291–294. [5] S. Hui, A. Petric, J. Eur. Ceram. Soc. 22 (2002) 1673–1681. [6] M.T. Buscagliaa, F. Magliab, U. Anselmi-Tamburinib, J. Eur. Ceram. Soc. 34 (2014) 307–316. [7] K. Shan, X.M. Guo, Mater. Lett. 121 (2014) 251–253. [8] B.R. Sudireddy, P. Blennow, K.A. Nielsen, Solid State Ionics 216 (2012) 44–49. [9] X. Li, H. Zhao, D. Luo, K. Huang, Mater. Lett. 65 (2011) 2624–2627. [10] Q.X. Fu, F. Tietz, D. Stover, J. Electrochem. Soc. 153 (2006) 74–83. [11] J. Wang, S. Yin, M. Komatsu, T. Sato, J. Eur. Ceram. Soc. 25 (2005) 3207–3212. [12] F. Yi, H. Li, H. Chen, R. Zhao, X. Jiang, Ceram. Int. 39 (2013) 347–352. [13] S.J. Litzelman, A. Rothschild, H.L. Tuller, Sens. Actuators B 108 (2005) 231–237. [14] V.V. Kharton, A.V. Kovalevsky, A.P. Viskup, J.R. Jurado, F.M. Figueiredo, E.N. Naumovich, J.R. Frade, J. Solid State Chem. 156 (2001) 437–444. [15] J.S. Yoon, M.Y. Yoon, C. Kwak, H.J. Park, S.M. Lee, K.H. Lee, H.J. Hwang, Mater. Sci. Eng. B 177 (2011) 151–156. [16] D.P. Fagg, V.V. Kharton, J.R. Frade, A.A.L. Ferreira, Solid State Ionics 156 (2003) 45–57. [17] S. Steinsvik, R. Bugge, J. Gjonnes, J. Tafto, T. Norby, J. Phys. Chem. Solids 58 (1997) 969–976. [18] K. Shan, X.M. Guo, Mater. Lett. 105 (2013) 196–198. [19] K. Shan, X.M. Guo, Electrochim. Acta 154 (2015) 31–34. [20] K.T. Lee, A. Manthiram, Solid State Ionics 178 (2007) 995–1000.
Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Regular Article
Electrical conduction behavior of mixed ionic-electronic conductor Y0.08Sr0.92Ti1 − xScxO3 − δ Ke Shan a,b, Zhong-Zhou Yi a,b,⁎ a b
College of Science, Honghe University, Mengzi 661199, Yunnan, China Local Characteristic Resource Utilization and New Materials Key Laboratory of Universities in Yunnan, Honghe University, Mengzi 661199, Yunnan, China
a r t i c l e
i n f o
Article history: Received 7 June 2015 Accepted 9 August 2015 Available online xxxx Keywords: Y0.08Sr0.92Ti1 − xScxO3 − δ Mixed ionic-electronic conductor Ionic conductivity Charge compensation mechanism
a b s t r a c t Y0.08Sr0.92Ti1 − xScxO3 − δ was synthesized via sol–gel method. With the Sc-doping amount increasing, the electrical conductivity of Y0.08Sr0.92Ti1 − xScxO3 − δ increased in 400–900 °C and the ionic conductivity increased between 600 and 900 °C. The optimized Y0.08Sr0.92Ti0.93Sc0.07O3 − δ sample exhibits an electrical conductivity in the order of 0.058–0.085 S·cm−1 at 400–900 °C. The ionic conductivity for Y0.08Sr0.92Ti0.93Sc0.07O3 − δ was 0.027 S·cm−1 and increased about 140% compared with Y0.08Sr0.92Ti0.97Sc0.03O3 − δ at 800 °C. Sc-doping increased the oxygen vacancy concentration and decreased the oxygen migration energy, therefore enhancing the conduction process of oxygen ions in Y0.08Sr0.92Ti1 − xScxO3 − δ. The possible charge compensation mechanism of Y0.08Sr0.92Ti1 − xScxO3 − δ 4+ 3+ can be described as Y0.08Sr0.92Sc3+ x Ti0.92 − 2δ − x1Ti0.08 + 2δ ‐ x2O3− (δ + x1/2)(x = x1 + x2). © 2015 Published by Elsevier Ltd.
1. Introduction Perovskite oxides (ABO3 ) are particularly attractive for hightemperature applications, because of the microstructural stability to improve reliability and long-term performance and mixed electronic-ionic conductivity that make the triple-phase boundary extend to the entirely exposed anode surface as anode materials for solid oxide fuel cells, in addition to having high melting and decomposition temperatures [1,2]. Strontium titanate (SrTiO3) with perovskite structure shows high chemical stability at high temperature under both oxidizing and reducing atmospheres and has strong resistance to suffer poison [3]. In addition, the perovskite structure of strontium titanae has two differently-sized cations, which makes it favorable to a variety of dopants. This doping flexibility can control the electron and ion transport properties to optimize performance for different applications. Generally, electronic conduction can be enhanced in SrTiO3 by replacing A- or B-site cations with n-type dopants such as La3 +, Y3+ or Nb5 +[4–8]. It is well known that n-doping generates a defect with an effective positive charge in the host lattice. Ionic conduction can be improved by doping with Fe3+, Cr3+, Sc3+ or Ni2+[9–19], in place of the B site, results in a p-type semiconductor by generating oxygen vacancy. The relevant studies indicated that donor-doping on A-site of SrTiO3 mainly affected the electronic conductivity rather than ionic conductivity [20]. In addition, the study by Li X and Zhao H [3] demonstrated that the tendency of oxygen ion migration energy increases in the order B = Sc b Co b Ni b Ti for SrBO3, implying Sc was a good choice ⁎ Corresponding author. E-mail addresses: [email protected] (K. Shan), [email protected] (Z.-Z. Yi).
http://dx.doi.org/10.1016/j.scriptamat.2015.08.008 1359-6462/© 2015 Published by Elsevier Ltd.
of all the present elements to improve the ionic conductivity of SrTiO3-based materials. Therefore, the effects of Sc-doping amount on the electrical and ionic conductivities of Y0.08Sr0.92Ti1 − xScxO3 − δ with temperature were investigated. The charge compensation mechanism in Y0.08Sr0.92Ti1 − xScxO3 − δ was discussed. 2. Experimental 2.1. Sample preparation Y0.08Sr0.92Ti1 − xScxO3 − δ (x = 0.03, 0.05, 0.07) powders were synthesized by using sol–gel method. The raw materials were high purity Sr(CH3COO)2·2H2O, Ti(CH3CH2CH2CH2O)4, Sc2O3 and Y2O3. Appropriate amounts of the starting materials (Ti(CH3CH2CH2CH2O)4, Sc2O3 and Y2O3) were dissolved into the mixed solution with the molar ratio of isopropanol to ethanol at 4:1 under magnetic stirring followed by the addition of strontium acetate. The resultant solution was heated in a water bath of 60 °C until a viscous gel was formed. The gel was heated in an evaporating dish on an electric furnace until self-igniting, yielding white and porous powders. The ground powders were calcined at 1100 °C for 10 h. The calcined powders were uniaxially pressed into pellets under the pressure of 50 MPa. The sintering step was carried out at 1450 °C for 5 h with the green pellets being into the platinum crucible for the measurement of total electrical and ionic conductivities. 2.2. Characterization X-ray diffraction (XRD, Rigaku D/max-A) with Ni filtered Cu Kα radiation was employed to identify the phase composition for the sintered samples Y0.08Sr0.92Ti1 − xScxO3 − δ (x = 0.03, 0.05, 0.07) with a 0.02°
K. Shan, Z.-Z. Yi / Scripta Materialia 114 (2016) 70–73
71
stepwise at room temperature. The relative density of all the samples was around 95%, which was estimated from the weight and dimensions measurement. The cross-section morphology was observed by scanning electron microscopy (SEM, Quanta FEG 650). The total electrical conductivity was measured by AC impedance spectroscopy with the CHI660B electrochemical workstation over the frequency range of 1 MHz to 0.01 Hz with a perturbation amplitude of 5 mV. The data analysis was carried out with appropriate equivalent circuit by Zsimpwin software. The well-polished samples (~7.2 mm in diameter and ~ 1.6 mm in thickness) were painted with platinum paint on both sides and Pt wires were used to fabricate the electrodes and baked at 800 °C for 0.5 h. The total electrical conductivity calculation was performed with overall area and thickness of the samples. The ionic conductivity was measured by electron-blocking method within 600–900 °C. The sample was plastered onto dense YSZ tablet with a little Pt paste, overcoming the interface resistance. Platinum paint was painted to the side of sample and YSZ tablet. The electron is blocked by YSZ layer because YSZ is almost a pure oxygen ion conductor. Glass seal can prevent oxygen leakage along the sides of the assembled samples. The impedance spectra of double layer materials can be observed at designated temperatures. Therefore, the ionic conductivity calculation of sample (σi) was performed as following equation:
σi ¼
dYSTS −1 ðOhm cmÞ ðRT −RYSZ Þ AYSTS
ð1Þ
Where dYSTS is the thickness of sample (in cm),RT is the total resistance, RYSZ is the resistance of YSZ and AYSTS is the area (in sq. cm) of the sample. 3. Results and discussion 3.1. XRD patterns and SEM micrographs XRD patterns of the sintered samples were collected at room temperature and the results were displayed in Fig. 1. The sample Y0.08Sr0.92Ti1 − xScxO3 − δ with x = 0.07 shows a single cubic perovskite structure (inset PDF79-174), whereas a secondary phase of Y2Ti2O7 was observed the doping level decreased to 0.05 and 0.03. The content of the secondary phase decreases with Sc-doping amount increasing as indicated by the strengthening diffraction intensity of Y2Ti2O7, indicating that Sc-doping facilitates the formation of single cubic perovskite phase.
Fig. 2. SEM micrographs of fracture surfaces of Y0.08Sr0.92Ti1 − xScxO3 − δ: (a) x = 0.03 and (b) x = 0.07.
The microstructure of a couple of selected samples was observed by SEM on fracture surfaces and representative micrographs are reported in Fig. 2. As shown in Fig. 2, the densities of Y0.08Sr0.92Ti1 − xScxO3 − δ samples increase with Sc-doping amount increasing, indicating that Sc-doping is favorable to the densification process of the materials synthesized by the sol–gel route in the present work. 3.2. The electronic and ionic conductivities of Y0.08Sr0.92Ti1 − xScxO3 − δ
Fig. 1. XRD patterns of Y0.08Sr0.92Ti1 − xScxO3 − δ after sintering at 1450 °C for 5 h: (a) x = 0.03; (b) x = 0.05; (c) x = 0.07.
The temperature dependence of total electrical conductivities of Y0.08Sr0.92Ti1 − xScxO3 − δ (x = 0.03, 0.05, 0.07) is shown in Fig. 3. Sc-doping enhances the electrical conductivity of SrTiO3 significantly. The optimized Y0.08Sr0.92Ti0.93Sc0.07O3 − δ sample exhibits an electrical conductivity in the order of 0.058–0.085 S·cm− 1 at 400–900 °C. The total electrical conductivity is the sum of electronic conductivity and ionic conductivity and its value is directly related with the type and concentration of point defects existing in the materials. In addition, the total electrical conductivity increased through a maximum then decreased with the temperature increasing, indicating a polaron conduction behavior of Y0.08Sr0.92Ti1 − xScxO3 − δ, corresponding to the high electron concentration. On the other hand, the ionic conductivity of Y 0.08 Sr 0.92Ti1 − x Sc xO 3 − δ increased with Sc-doping amount increasing remarkably (Fig. 4). The ionic conductivity of Y0.08Sr0.92Ti0.97Sc0.03O3 − δ was 0.02 S·cm−1 at 850 °C, while it increased to 0.029 S·cm−1 for Y0.08Sr0.92Ti0.95Sc0.05O3 − δ and 0.033 S·cm−1 for
72
K. Shan, Z.-Z. Yi / Scripta Materialia 114 (2016) 70–73
oxygen vacancy concentration and the migration energy of oxygen ion at certain temperature. According to the Eq. (3), the increase of Sc-doping amount will result in the increase of the oxygen vacancy concentration. On the other hand, the mobility of oxygen ion also has strong impact on the oxygen ionic conductivity. The activation energy for oxygen ion migration (Ea) was calculated by Arrhenius rule, according to the results shown in Fig. 4(b). As shown in Fig. 4(b), Ea of Y0.08Sr0.92Ti1 − xScxO3 − δ decreased with Scdoping increasing, indicating that Sc-doping facilitates the conduction of oxygen ions. 3.3. Charge compensation mechanism of Y, Sc co-doped SrTiO3 Based on the defect chemistry, free electrons (e') and oxygen vacancies [V O •• ] are prevalent in Y0.08Sr0.92Ti1 − xScxO3 − δ as a result of the substitution of Sc on Ti and Y on Sr due to the n-type conduction of Y0.08Sr0.92Ti1 − xScxO3 − δ. The defects produced in Y donor doping and Sc acceptor doping SrTiO3 can be expressed respectively as: Fig. 3. Temperature dependences of the total electrical conductivities of Y 0.08Sr0.92 Ti 1 − x Scx O 3 − δ (x = 0.03, 0.05, 0.07) in 400–900 °C.
Y0.08Sr0.92Ti0.93Sc0.07O3 − δ. Compared with Y0.08Sr0.92Ti0.97Sc0.03O3 − δ, the ionic conductivity of Y0.08Sr0.92Ti0.93Sc0.07O3 − δ increased about 165%. It is well known that the ionic conductivity is primarily dependent on the
SrTi0:6 Scx O3−δ
Y2 O3 → 2Y•Sr þ 2O O þ Y0:08 Sr0:92 TiO3
0
1 0 O2 þ 2e 2
Sc2 O3 → 2ScTi4þ þ V••O þ 3O O 0
Ti4þ þ e →Ti3þ
ð2Þ ð3Þ ð4Þ
There are three possible charge compensation mechanisms. The first possible mechanism is Sc3 + ions in place of Ti4 + ions, which can be 4+ 3+ expressed as Y0.08Sr0.92Sc3+ x Ti0.92 − 2δ − xTi0.08 + 2δO3− (δ + x/2). The defects produced in Y, Sc co-doping SrTiO3 can be expressed as Eq. (3). In this case, the ionic conductivity of Y0.08Sr0.92Ti1 − xScxO3 − δ may increase due to the increased oxygen vacancy concentration with Scdoping amount increasing. However, the electronic conductivity will keep unchanged because of the unchanged concentration of Ti3+ ions. In fact, the experimental results show that the total electrical conductivity of n-type conduction increases. Therefore, this is inconsistent with the experimental results. The second possibility can be described that Sc3 + only takes the place of Ti3+ site. The defects produced by Sc-doping can be expressed as: þ 3O Sc2 O3 →2Sc O Ti3¼
ð5Þ
In this case, the electronic conductivity may increase and oxygen vacancy concentration will keep unchanged. This conflicts with the experimental results. The third charge compensation mechanism can be expressed as 4+ 3+ Y0.08Sr0.92Sc3+ x Ti0.92 − 2δ − x1Ti0.08 + 2δ ‐ x2O3− (δ + x1/2)(x = x1 + x2) (Eqs. (3) and (5)). In other word, Sc ions replace both Ti4 + and Ti3 + sites simultaneously. With Sc-doping amount increasing, the Sc3+ ion may tend to replace Ti4 + to produce more oxygen vacancy and the tendency of Sc3 + ion in place of Ti3 + may decrease, therefore, the ionic conductivity and electronic conductivity (n-type conduction) increases with Sc-doping amount increasing, respectively. Therefore, this agrees with the experimental results. 4. Conclusions
Fig. 4. Temperature dependences of the ionic conductivities of Y0.08Sr0.92Ti1 − xScxO3 − δ (x = 0.03, 0.05, 0.07) in 600–900 °C.
Y0.08Sr0.92Ti1 − xScxO3 − δ (x = 0.03, 0.05, 0.07) exhibits a single cubic phase perovskite structure when the x value is 0.07. Sc-doping facilitates the formation of cubic phase perovskite due to the weakening peak intensity of the second phase. Partial oxygen ionic conductivity Y0.08Sr0.92Ti1 − xScxO3 − δ (x = 0.03, 0.05, 0.07) increases with Scdoping increasing, which may be attributed to the increases of oxygen vacancy concentration and the mobility of oxygen ion. The total electrical conductivity increases with Sc-doping amount, corresponding
K. Shan, Z.-Z. Yi / Scripta Materialia 114 (2016) 70–73
to the high charge carrier (electron) due to the polaron conduction behavior. The optimized Y0.08Sr0.92Ti0.93Sc0.07O3 − δ sample exhibits an electrical conductivity in the order of 0.058–0.085 S·cm − 1 at 400–900 °C. The ionic conductivity for Y0.08Sr 0.92 Ti0.93 Sc0.07 O3 − δ was 0.027 S·cm− 1 and increased about 140% compared with Y0.08Sr0.92Ti0.97Sc0.03O3 − δ at 800 °C. The possible charge compensation mechanism of Y and Sc co-doping SrTiO3 can be described as 4+ 3+ Y0.08Sr0.92Sc3+ x Ti0.92 − 2δ − x1Ti0.08 + 2δ ‐ x2O3− (δ + x1/2)(x = x1 + x2). Acknowledgments This work has been funded by the National Natural Science Foundation of China (Nos. 51562009 and 51362011). References [1] J.W. Fergus, Sens. Actuators B 123 (2007) 1169–1179. [2] X. Li, H. Zhao, N. Xu, X. Zhou, C. Zhang, N. Chen, Int. J. Hydrogen. Energy 34 (2009) 6407–6414. [3] X. Li, H. Zhao, F. Gao, N. Chen, N. Xu, Electrochem. Commun. 10 (2008) 1567–1570.
73
[4] R. Moors, S. Schollhammer, K.H. Hardt, Appl. Phys. A 65 (1997) 291–294. [5] S. Hui, A. Petric, J. Eur. Ceram. Soc. 22 (2002) 1673–1681. [6] M.T. Buscagliaa, F. Magliab, U. Anselmi-Tamburinib, J. Eur. Ceram. Soc. 34 (2014) 307–316. [7] K. Shan, X.M. Guo, Mater. Lett. 121 (2014) 251–253. [8] B.R. Sudireddy, P. Blennow, K.A. Nielsen, Solid State Ionics 216 (2012) 44–49. [9] X. Li, H. Zhao, D. Luo, K. Huang, Mater. Lett. 65 (2011) 2624–2627. [10] Q.X. Fu, F. Tietz, D. Stover, J. Electrochem. Soc. 153 (2006) 74–83. [11] J. Wang, S. Yin, M. Komatsu, T. Sato, J. Eur. Ceram. Soc. 25 (2005) 3207–3212. [12] F. Yi, H. Li, H. Chen, R. Zhao, X. Jiang, Ceram. Int. 39 (2013) 347–352. [13] S.J. Litzelman, A. Rothschild, H.L. Tuller, Sens. Actuators B 108 (2005) 231–237. [14] V.V. Kharton, A.V. Kovalevsky, A.P. Viskup, J.R. Jurado, F.M. Figueiredo, E.N. Naumovich, J.R. Frade, J. Solid State Chem. 156 (2001) 437–444. [15] J.S. Yoon, M.Y. Yoon, C. Kwak, H.J. Park, S.M. Lee, K.H. Lee, H.J. Hwang, Mater. Sci. Eng. B 177 (2011) 151–156. [16] D.P. Fagg, V.V. Kharton, J.R. Frade, A.A.L. Ferreira, Solid State Ionics 156 (2003) 45–57. [17] S. Steinsvik, R. Bugge, J. Gjonnes, J. Tafto, T. Norby, J. Phys. Chem. Solids 58 (1997) 969–976. [18] K. Shan, X.M. Guo, Mater. Lett. 105 (2013) 196–198. [19] K. Shan, X.M. Guo, Electrochim. Acta 154 (2015) 31–34. [20] K.T. Lee, A. Manthiram, Solid State Ionics 178 (2007) 995–1000.