Intermediate-temperature conductivity of B-site doped

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Intermediate-temperature conductivity of B-site doped Na0.5Bi0.5TiO3-based lead-free ferroelectric ceramics Jinqiang Huang a,b, Fangyuan Zhu c, Da Huang a,b, Bing Wang a, Tan Xu a, Xiangdong Li a, Pengyuan Fan a,b, Feng Xia a,b, Jianzhong Xiao a,n, Haibo Zhang a,b,nn a College of Materials Science and Engineering, State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, PR China b Research Institute of Huazhong University of Science and Technology in Shenzhen, Shenzhen 518057, China c Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, CAS, Shanghai 201204, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 14 June 2016 Received in revised form 26 July 2016 Accepted 26 July 2016

Na0.5Bi0.5TiO3 (NBT) based oxide-ion conductor ceramics have great potential applications in intermediate-temperature solid oxide fuel cells (SOFCs) and oxygen sensors. Na0.5Bi0.49Ti1  xMgxO3  δ ceramics with x¼ 0, 0.01, 0.02, 0.03, 0.05 and 0.08 were prepared by conventional solid-state reaction. XRD measurement and SEM analysis revealed the formation of pure perovskite structures without secondary phase. MgO doping greatly decreased the sintering temperature and inhibited grain growth. AC impedance spectroscopy measurement was adopted to measure the total conductivity, which was found to increase with MgO doping content ranging from 0 to 3 mol% and subsequently to decrease. High oxygen ionic conductivity st ¼0.00629 S/cm was achieved for sample doped with 3 mol% MgO at 600 °C in air atmosphere. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Na0.5Bi0.5TiO3 MgO doping Oxide-ion conductivity AC impedance spectroscopy

1. Introduction Na0.5Bi0.5TiO3 has received increasing attention since it was discovered by Smolenskii and Agranovskaya in 1960 due to its superiors relaxor behavior and massive potential applications [1]. It is a typical piezoelectric material with perovskite structure in which two unique cations occupy the A-site. In the past 20 years, Na0.5Bi0.5TiO3 and its solid solution with other ferroelectric materials have been extensively studied with a purpose of replacing lead zirconate titanate (PZT) which has been used in electromechanical devices for several decades [2–7]. Whereas, Li et al. found that Na0.5Bi0.5TiO3 could be a flourishing candidate of oxideion conductors, and then dramatically improved the oxide-ion conductivity of this material in a series of research work [8,9]. Considering the high mobility of oxide ions, oxide-ion conductors are widely used in electrochemical devices such as solid oxide fuel cells (SOFCs), oxygen separation membranes and gas sensors [10–15]. Solid oxide fuel cells (SOFCs) convert chemical energy directly into electrical energy with high efficiency and low n

Corresponding author. Corresponding author at: College of Materials Science and Engineering, State Key Laboratory of Material Processing and Die &Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, PR China. E-mail addresses: [email protected] (J. Xiao), [email protected] (H. Zhang). nn

pollution and the oxide-ion conductors are utilized as electrolyte in the SOFC. Various materials have been discovered and designed to meet demand of the electrolyte in SOFCs. Two kinds of materials are classified according to the crystal structure: fluorite and perovskite. For the typical fluorite structure, 8 mol% yttria-stabilized zirconia (8YSZ) is the most widely used material in high-temperature applications because of its high oxide-ion conductivity, low electronic conductivity, excellent thermal shock resistance and thermal stability in both oxidation and reduction environment [16]. However, the ionic conductivity of 8YSZ decreases by several orders of magnitude when working at 600 °C. Doped CeO2 is another popular fluorite oxide-ion conductor, and it shows higher ionic conductivity than 8YSZ at relative low and/or intermediate temperature. The challenge remained in CeO2 is that the Ce4 þ might be deoxidized to Ce3 þ in the reducing atmosphere, leading to unavoidable high electronic conductivity [17,18]. To overcome this problem, Wachsman et al. proposed a ceria/bismuth oxide bilayered electrolyte, where the erbia-stabilized bismuth oxide (ESB) layer on the cathode side blocks the leakage current through the gadolinia-doped ceria (GDC). They also found that the electronic leakage current decreased with increasing electrolyte thickness, while the ohmic resistance was inversely proportional to the electrolyte thickness. In order to obtain the maximum power density (MPD) in SOFCs, the electrolytes with certain thickness were optimized in their work [19–21]. For the perovskite-based materials, co-doped La1  xSrxGa1  yMgyO3  δ

http://dx.doi.org/10.1016/j.ceramint.2016.07.170 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: J. Huang, et al., Intermediate-temperature conductivity of B-site doped Na0.5Bi0.5TiO3-based lead-free ferroelectric ceramics, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.07.170i

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(LSGM) possesses higher ionic conductivity and much lower electronic conductivity in comparison with doped CeO2 in intermediate-temperature applications. Despite many advantages of LSGM, the high cost of raw materials and high operating temperature hinder its practical application in commercial SOFCs [22– 27]. The huge challenge of SOFCs is to design a new material, which possesses high oxide-ion conductivity and negligible electronic conductivity at a reasonable low temperature. Since the Na0.5Bi0.5TiO3-based ceramics were reported as a new candidate of oxide-ion conductors [8,9], only a few work have been reported. Among those work, the first principles computational study is carried to testify the Na0.5Bi0.5TiO3 material of its oxygen ionic conductivity. Meanwhile, oxygen diffusion mechanisms are systematically investigated by calculation of the activation energy and oxide-ion conductivity [28,29]. Based on above work, Na0.5Bi0.5TiO3-based ceramics were considered as a potential new material for intermediate-temperature SOFCs. In ABO3 perovskite structures, A-site is occupied by one or two types of primary cations, the smaller cations in B-site form BO6 octahedra with oxygen coordinated. To improve the ionic conductivity, the widely used method is to develop the vacancy concentration by substituting A and/or B sites with lower valence cations [30,31]. It is well known that B-site substitution is more efficient for improving the ionic conductivity [32–34]. In this work, nonstoichiometric Na0.5Bi0.49TiO3 doped with MgO was synthesized by solid-state reaction, and then microstructures, oxide-ion conductivity properties and mechanisms were systematically investigated.

2. Experimental A conventional solid-state reaction method was used to synthesize the Na0.5Bi0.49Ti1  xMgxO3  δ (x ¼ 0, 0.01, 0.02, 0.03, 0.05 and 0.08) ceramics. Commercially obtained reagent-grade powders Na2CO3 (99.8%), Bi2O3 (99.0%), TiO2 (98.0%), MgO (99.99%) were used as the raw materials. All raw materials were dried at 200 °C for 24 h, then weighted according to stoichiometric formula and ball-milled in ethanol for 24 h using zirconia balls. Subsequently, the dried powders were calcined at 800 °C and 850 °C for 2 h for primordial chemical reaction separately. After calcination, the reactants were secondly ball-milled for 24 h with ethanol and zirconia balls. Then the powders were dried at 80 °C for 48 h, and afterwards grinded in an agate mortar and pestle, sieved with an 80 mesh griddle. To improve the fluxility, the calcined powders were mixed with 5 wt% Polyvinyl Alcohol (PVA). The mixtures were uniaxially pressed into pellets of 12 mm in diameter at 180 MPa and finally sintered for 2 h in the air. The samples of Na0.5Bi0.49Ti1  xMgxO3  δ with x ¼0, x ¼0.01, 0.02, x ¼0.03, x ¼ 0.05 and x ¼ 0.08 were sintered at 950–1150 °C respectively. Pellets were sintered with the same composition NBT powders covered in order to decrease the volatilization of Na and Bi elements. Samples density was measured by the Archimedes method. Phase structures and characterizations were examined by powder X-ray diffraction with the AXS D8 diffractometer (Bruker Corporation, Karlsruhe, Germany) using Cu Kα radiation (λ ¼1.5406 Å) at a voltage and current of 40 kV and 30 mA. XRD data were collected in the range of 20–80° in θ  2θ locked-coupled scanning mode with a 0.02° step and scanning speed of 5°/ min. Morphology of the sintered samples was analyzed by fieldemission scanning electron microscopy (FSEM, Sirion200, FEI Ltd., Eindhoven, the Netherlands). For the electrical measurements, samples were coated with Ag paste on both surfaces, and then fired at 500 °C for 20 min. The AC impedance spectroscopy measurement were performed by an electrochemical workstation (Versa STAT 3, Princeton) with the frequency range of 1 Hz to

1 MHz from 300 °C to 700 °C in air atmosphere. Impedance spectroscopy measurement data were fitted using Zview software.

3. Results and discussion Gold Schmidt tolerance factor Gt concerns the stability and distortion of doped NBT perovskite structures,

Gt = (rA + rO )/√2(rB + rO )

(1)

where rA and rB are the mean ionic radii of A and B site cations respectively, and rO is the radius of oxygen ion (0.140 nm). The perovskite structure should be most stable with Gt ¼ 1, but it could not have relative high oxide-ion conductivity. According to previous reports, perovskite materials with Gt around 0.96 show the highest oxide-ion conductivity [33,34]. As shown in Fig. 1, the tolerance factors of Na0.5Bi0.49Ti1  xMgxO3  δ (x ¼ 0, 0.01, 0.02, 0.03, 0.05 and 0.08) are close to 0.96, which indicates its high oxide-ion conductivity. The XRD patterns of sintered ceramics with compositions of Na0.5Bi0.49Ti1  xMgxO3  δ (x ¼0, 0.01, 0.02, 0.03, 0.05 and 0.08) are shown in Fig. 2. As demonstrated in Fig. 2(a), the samples with various doping contents all show pure perovskite structures, which implies that the doping of MgO does not change the primary structures. The Mg2 þ successfully diffused into the crystal lattice and formed a solid solution. Fig. 2(b) enlarged the XRD patterns with 2θ range of 31–34°. The peak positions are similar in spite of various MgO content modified, which is possibly attributed as below: firstly, the ionic radius of Mg2 þ (0.072 nm, CN ¼6) is larger than Ti4 þ (0.0605 nm, CN ¼6) [35], so crystal lattice would be expanded in consequence of Mg2 þ substituting Ti4 þ . Additionally, the oxygen vacancy increases with more MgO participated, resulting in the lattice contraction. Both of them would contribute to the stable lattice constant of Na0.5Bi0.49Ti1  xMgxO3  δ ceramics regardless of the dopant concentrations. Sintering temperature of Na0.5Bi0.49Ti1  xMgxO3  δ dropped dramatically along with the increasing Mg2 þ substitution at B-site. Samples doped with different amount of MgO were sintered at various temperatures. Samples with optimal sintering temperatures were chosen for the density measurements. As shown in Fig. 3, comparing with the unmodified NBT ceramic, MgO doped samples possess relative high density. As the MgO content increases (x o 2 mol%), the relative density of the ceramics significantly increases, which could ascribe to the creation of oxygen vacancies. The oxygen vacancy will be promoted by doping with

Fig. 1. Tolerance Factor of Na0.5Bi0.49Ti1  xMgxO3  δ (x¼ 0, 0.01, 0.02, 0.03, 0.05 and 0.08).

Please cite this article as: J. Huang, et al., Intermediate-temperature conductivity of B-site doped Na0.5Bi0.5TiO3-based lead-free ferroelectric ceramics, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.07.170i

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lower valence ions, which is conducive to better atomic diffusion during the sintering procedure. However, the relative density decreases with further increasing MgO content (when x 42 mol%). Although oxygen vacancies are increasing, the sintering

Fig. 2. XRD patterns of sintered Na0.5Bi0.49Ti1  xMgxO3  δ (x ¼0, 0.01, 0.02, 0.03, 0.05 and 0.08).

temperature drops significantly, leading to the decline of atomic diffusion rate and densification minimized. It is notable that the relative density exceeds 94%, which is comparable to the sintering behavior in A-site nonstoichiometric (Bi0.5Na0.5)TiO3 [36], suggesting that all samples are well sintered. The SEM micrographs of surface are demonstrated in Fig. 4. For the undoped Na0.5Bi0.49TiO3  δ (Fig. 4(a)), the grains are less uniform with average 10 mm in size. With the addition of MgO, the grains become smaller and the microstructure is more homogeneous for the 1 mol% MgO doped sample, Fig. 4(b). The grain size decreases a lot when the MgO doping content added  2 mol%. The samples doped with 2, 3 and 5 mol% MgO have similar grain sizes and structures. However, the gain sizes become slightly bigger when the MgO content reaches 8 mol%. It can be derived that MgO doping can lower sintering temperature of Na0.5Bi0.49TiO3  δ and then decrease the grain size. Meanwhile, the inset of Fig. 4 presents the fracture surface morphology of Na0.5Bi0.49Ti1 xMgxO3 δ. There are some pores in the grain and grain boundary, which are closely related to the sintering densification of ceramics. In general, ceramic samples with little and less pores show optimal density, while the unmodified sample presents large pores. Consequently the pores become smaller because of the MgO doping. The porosity of all samples can be inspected from the inset figures, Fig. 4(a)–(f), which is consistent with the relative density measurements in Fig. 3. The AC impedance data of all samples were collected every 100 °C in the range of 300–700 °C with the frequency range 1–1 MHz. Impedance complex plane plots for Na0.5Bi0.49Ti0.97Mg0.03O3  δ measured at 300 °C, 500 °C and 600 °C are shown in Fig. 5, individually. In the impedance spectra measured at 300 °C, only an incomplete semicircle can be observed in Fig. 5(a), while another small semicircle appears in the enlarged figure (inset in Fig. 5(a)). The small semicircle can be fitted by a resistor (R) and a capacitor (C) element in parallel. The capacitance responding to the semicircle at high frequency was calculated from the following equation:

ω maxRC = 1 Fig. 3. Relative density of Na0.5Bi0.49Ti1  xMgxO3  δ as a function of Mg2 þ content.

3

(2)

where ωmax ¼2πfmax, and fmax is the frequency at the maximum imaginary impedance for the semicircle. According to the

Fig. 4. SEM morphology of surface and fracture (the inset figure) of Na0.5Bi0.49Ti1  xMgxO3  δ: (a) x ¼ 0, (b) x¼ 0.01, (c) x ¼0.02, (d) x¼ 0.03, (e) x¼ 0.05 and (f) x ¼0.08.

Please cite this article as: J. Huang, et al., Intermediate-temperature conductivity of B-site doped Na0.5Bi0.5TiO3-based lead-free ferroelectric ceramics, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.07.170i

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Fig. 5. AC impedance complex plane plots for Na0.5Bi0.49Ti0.97Mg0.03O3  δ measured in air at different temperature, 300 °C (a), 500 °C (b) and 600 °C (c). Rb and Rgb denote bulk and grain boundary resistance respectively. Inset figure of (c) shows the measured data (spot in black) and fitting curve (line in red).

capacitance (C E 10  9 F), the small semicircle corresponds to the grain boundary impedance [26,37–39]. The grain impedance is beyond the measuring frequency limit so that the intersection of small semicircle at high frequency is extracted as the bulk resistance Rb. The low frequency displays typical finite Warburg impedance, which is associated with the ionic polarization and diffusion phenomena of oxygen molecules at the electrode. Fig. 5 (b) presents the impedance spectra measured at 500 °C, compared with Fig. 5(a), an additional semicircle was shown at intermediate frequency. It has a capacitance of 10  6 F, which is consistent with the electron transfer at the electrode-ceramic interface. However, the grain and grain boundary semicircles are all out of the measuring frequency scale in Fig. 5(c), only a small semicircle corresponding to the electrode-ceramic interface and the Warburg electrode response appear. Therefore, the intersection at high frequency is regarded as the total of grain resistance and grain boundary resistance. In order to get the total resistance accurately, the first small circle was fitted with appropriate circuit consisted of a parallel combination of a resistor, capacitor and constant phase element. Obviously, it can be fitted well in the enlarged figure, and then the total resistance can be extracted precisely. According to the Warburg diffusion impedance at low frequency, the samples are indicated to be oxygen ion conductors [40], which

has also been experimentally confirmed by Li et al. [8,9]. The resistance values of all samples were summarized as above. Then, the total conductivity was estimated from

σt = d/R tS

(3)

where Rt ¼Rb þRgb, d and S are the thickness and area of the samples respectively. The Arrhenius plots of the total conductivity for Na0.5Bi0.49Ti1  xMgxO3  δ are displayed in Fig. 6. The total conductivity at 400 °C, 500 °C and 600 °C and the activation energy values are listed in Table 1. It can be seen that the total conductivities of samples doped with MgO increase significantly compared with the undoped one. In order to study the doping concentration dependent conductivity, the total conductivities of the samples doped with various MgO measured at 400 °C, 500 °C and 600 °C are shown in Fig. 7. Although the sample is doped with a very small amount of MgO (x ¼ 0.01), the conductivity increases by at least two orders of magnitude and the activation energy reduces by half. The added oxygen vacancies resulting from the substitution of Mg2 þ for Ti4 þ are responsible for the increased ionic conductivity. The oxygen vacancies are created by the following defect reaction equation,

″ + Oo× + Vo∙∙ MgO → Mg Ti

(4)

Please cite this article as: J. Huang, et al., Intermediate-temperature conductivity of B-site doped Na0.5Bi0.5TiO3-based lead-free ferroelectric ceramics, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.07.170i

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MgO exhibits the highest total conductivity, which achieves 0.00629 S/cm at 600 °C in air atmosphere.

4. Conclusions

Fig. 6. Arrhenius plots of the total conductivity for Na0.5Bi0.49Ti1  xMgxO3  δ (x¼ 0, 0.01, 0.02, 0.03, 0.05 and 0.08).

Table 1 The total conductivity at 400 °C, 500 °C and 600 °C and the activation energy of Na0.5Bi0.49Ti1  xMgxO3  δ (x¼ 0, 0.01, 0.02, 0.03, 0.05 and 0.08). Samples

Na0.5Bi0.49TiO3  δ Na0.5Bi0.49Ti0.99Mg0.01O3  δ Na0.5Bi0.49Ti0.98Mg0.02O3  δ Na0.5Bi0.49Ti0.97Mg0.03O3  δ Na0.5Bi0.49Ti0.95Mg0.05O3  δ Na0.5Bi0.49Ti0.92Mg0.08O3  δ

Conductivity r (S cm  1)

Activation energy (eV)

400 °C

500 °C

600 °C

3.24E  7 3.76E  4 7.65E  4 1.26E  3 1.12 E  3 4.68E  4

2.60E  6 8.86E  4 1.27 E  3 2.45 E  3 2.63 E  3 1.45 E  3

1.02E  5 1.79 E  3 4.14 E  3 6.29 E  3 5.24 E  3 3.80 E  3

0.864 0.451 0.426 0.449 0.456 0.613

Fig. 7. The total conductivity for Na0.5Bi0.49Ti1  xMgxO3  δ (x ¼0, 0.01, 0.02, 0.03, 0.05 and 0.08) at 400 °C, 500 °C and 600 °C.

With further increasing dopant (x o 0.03), the ionic conductivity increases gradually, which is attributed to the increased oxygen vacancies. However, the ionic conductivity decreases when the MgO content exceeds 3 mol%. According to the defect reaction equation, more oxygen vacancies are formed in the samples with higher MgO content, while the vacancies are not movable. Considering the attraction caused by Coulomb force, the dopant cations and the oxygen vacancies form associate defect pairs ″ − V o∙∙ ]. The associated pairs are immobile so that the con[ MgTi ductivity decreases in spite of more oxygen vacancies created at higher doping content. In our work, sample doped with 3 mol%

Samples of Na0.5Bi0.49Ti1 xMgxO3 δ with x¼0, 0.01, 0.02, 0.03, 0.05 and 0.08 were prepared by conventional solid state reaction. The formation of a solid solution with pure perovskite structure was detected by XRD measurement. Addition of MgO greatly decreases the sintering temperatures and the grain sizes. The total conductivity was measured by AC impedance spectroscopy method. The conductivity increases with increasing MgO dopant content (0 3 mol%) and subsequently decreases. The formation of associate defect pairs might be responsible for the decrease of conductivity in samples beyond a particular dopant concentration (x40.03). Sample doped with 3 mol% MgO exhibits the highest oxygen ionic conductivity (st ¼0.00629 S/cm) at 600 °C in air atmosphere. Compared with other conventional materials, Na0.5Bi0.49Ti1 xMgxO3 δ shows a great potential application in intermediate-temperature SOFCs.

Acknowledgments The authors acknowledge the generous support by the National Natural Science Foundation of China under grant no. 51202074 and Basic Research Program of Shenzhen City (JCYJ20160414101859817). Prof. Zhang wishes to thank Dr. Ming Li for critical review with helpful discussion. The authors also wish to thank the Analytical and Testing Center of Huazhong University of Science and Technology.

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Please cite this article as: J. Huang, et al., Intermediate-temperature conductivity of B-site doped Na0.5Bi0.5TiO3-based lead-free ferroelectric ceramics, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.07.170i