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Letter Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Local Order and Oxygen Ion Conduction Induced High-Temperature Colossal Permittivity in Lead-Free Bi0.5Na0.5TiO3‑Based Systems Yulong Qiao,† Weili Li,*,†,‡ Yu Zhao,†,§ Yulei Zhang,† Wenping Cao,∥ and Weidong Fei*,†,⊥ †

School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, P. R. China National Key Laboratory of Science and Technology on Precision Heat Processing of Metals, Harbin Institute of Technology, Harbin 150001, P. R. China § Research Center of Basic Space Science, Harbin Institute of Technology, Harbin 150001, P. R. China ∥ School of Light Industry, Harbin University of Commerce, Harbin 150028, P. R. China ⊥ State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, P. R. China ‡

S Supporting Information *

ABSTRACT: In this study, an effective method is presented to enhance the dielectric constant at high temperature through Ba2+, Mg2+ codoped nonstoichiometric Bi0.5Na0.5TiO3 (BNT) relaxor ferroelectric ceramics. The temperature dependence of permittivity, complex impedance spectrum, and conductivity was investigated to understand the effect of Ba2+, Mg2+ doping on dielectric properties of BNT. The results show that the value of dielectric constant of (Ba0.01Bi0.5Na0.49)(Ti0.9Mg0.1)O3 ceramics increases 2 orders of magnitude with a very broad temperature range corresponding to pure BNT ceramics. The results of conductivity and complex impedance spectrum with various temperatures indicate that the high-temperature dielectric constant is mainly caused by oxygen ion conductivity and the interfacial effect. The A-site local order induced by Ba2+ doping generates dielectric anomalies, which results in the broadening temperature range for colossal dielectric constant. The study provides a new method to obtain a colossal dielectric constant system with a wide temperature range. KEYWORDS: doped BNT ceramics, high-temperature, colossal dielectric constant, oxygen ion conduction, local order

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investigation of the origin of the high-temperature giant dielectric of CCTO.17−20 Nowadays, temperature- and frequency-independent colossal dielectric permittivity with a low dielectric loss was reported in a (Nb + In) codoped rutile TiO2 system. The dielectric constant can maintain about 5 × 104 in the range −200 to 200 °C.5 However, the original mechanism of the colossal dielectric constant is still not clear, and the I−V properties of codoped TiO2 showed that the current density of this material increased sharply when a very small electric field was applied.21,22 So the TiO2-based materials cannot be applied at high temperature either. According to the preceding discussion, colossal dielectric constant materials with high temperature and wide temperature range still need to be investigated. The dielectric constant is a measure of the polarizability of a material. In dielectric materials, the value of dielectric constant is determined by how much (and how easily) positive and negative bound charges can be separated.23 The effective

he high-dielectric-permittivity materials have been a concern for a lot of researchers because of the potential applications in capacitors and integrated passive components for microelectronics.1−5 At present, perovskite (ABO3) ferroelectric materials such as BaTiO3,6−8 (K,Na)NbO3 (KNN),9−11 and PZT12,13 have been widely investigated because of their intrinsic dipole orientation polarization which results in high dielectric constants and greater resistance to breakdown voltage. The maximum dielectric constants of these materials are about 12 000 (for BaTiO3),14,15 14 000 (for KNN),9−11 and 40 000 (for PZT)12,13 at the Curie temperature, respectively. However, the values of high-temperature dielectric constants for this kind of material can only be maintained in a narrow temperature range (∼50 °C) near the Curie temperature.9−16 The values of high-temperature dielectric constants outside the Curie temperature of 50 °C for the materials are relatively small (∼103),6−8,14,15 which limited the use of these materials at high temperature. CaCu3Ti4O12 (CCTO) has been reported to possess a giant permittivity over a wide temperature range.17,18 The value of the dielectric constant of CCTO can reach about 105 at room temperature. However, due to a sudden increase in dielectric loss after ∼150 °C, there are no reports regarding the © XXXX American Chemical Society

Received: December 27, 2017 Accepted: February 26, 2018 Published: February 26, 2018 A

DOI: 10.1021/acsaem.7b00347 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Energy Materials

Figure 1. Temperature dependence of the dielectric constant for BNT-based ceramics: (a) Bi0.5Na0.5TiO3, (b) Ba0.01(Bi0.5Na0.5)0.99TiO3, (c) Ba0.01Bi0.49Na0.5TiO3, (d) Ba0.01Bi0.5Na0.49TiO3.

The increased conductivity at high temperature may promote free charges that are easy to separate, so that the dielectric constant of BNT-based ceramics may further be improved by Mg2+ doping. The substitution on either the A- or B-site by other suitable cations can substantially change their properties. Therefore, if Ba2+ and Mg2+ substitute at A- and B-positions of BNT, respectively, a colossal dielectric constant material with a high and wide temperature range could be obtained. In this paper, in order to obtain a high-temperature colossal dielectric constant material, BNT was selected as the host material, and BNT-based ceramics with stoichiometric or nonstoichiometric Ba2+ ion doping were fabricated to choose a sample which has the excellent temperature stability of dielectric constant. Then Mg2+ was doped into the (BaBiNa)TiO3 to further enhance the high-temperature dielectric property. The temperature-dependent dielectric properties were systematically investigated in the BNT-based ceramics. In order to obtain high-temperature dielectric constant properties, the temperature-dependent dielectric constant (εr) of BNT-based ceramics was measured, as shown in Figure 1. Obviously, the shapes of εr-temperature curves for BNT-based ceramics vary significantly with the change of element substitution. When Ba2+ only substituted Na+, the shape of the curve is obviously different. In a comparison of the dielectric constant for other ceramics samples at 300 °C, such as BNT-BT (εr ∼ 2000),34,35 BT (εr ∼ 1000),8 and KNN (εr ∼ 2000),9−11 the dielectric constant of Ba0.01Bi0.5Na0.49TiO3 still maintained a high value after 300 °C. The dielectric constant values at 1000 Hz are about 3600−4200 from 300 to 600 °C. The broadening of the anomalous peak at 300 °C allows Ba0.01Bi0.5Na0.49TiO3 to maintain a relatively stable dielectric constant over a wide temperature range in the high-temperature area.

polarizability of a capacitor structure (that is, its capacitance) can be substantially enhanced if the free charges are also allowed to be separated.23−25 For ceramic materials, when the electrode area and the thickness of the ceramic are fixed, capacitance is inversely proportional to the dielectric constant. Therefore, if dielectric materials can store more free charges and the free charges are very easy to separate at high temperature, the materials may have a high-temperature colossal dielectric constant. Bi0.5Na0.5TiO3 (BNT) ceramics are A-site composite ion type lead-free relaxor ferroelectrics with ABO3 structure.26−28 Bi3+ and Na+, each with site occupation factor 0.5, were constrained to be at the same coordinates.29 Although the chemical longrange order can be ruled out, the chemical short-range order with Bi3+ and Na+ along the ⟨001⟩ direction is still possible after Tm ∼ 300 °C.30 For the relaxor ferroelectric BNT, Tm is used to substitute the Curie temperature TC; Tm is the maximum temperature at which the dielectric constant reaches a maximum value. Because both of the ionic radii for Bi3+ and Na+ are smaller than that of Ba2+,31 the A-site Ba2+ substitution of BNT may cause a large lattice distortion to promote the formation of A-site local order of (Bi3+/Na+). As Bi3+ and Na+ have a charge difference, a local electric field would be formed to promote the positive and negative charges to be separated more easily when the local ordering distribution of Bi3+ and Na+ occurs. The local order of BNT will broaden the temperature stability range of the dielectric constant. The high-temperature oxygen ion conduction of B-site Mg2+ substituted BNT ceramics had been reported by many researchers. 32,33 In B-site Mg 2+ substituted BNT, Mg 2+ substituted Ti4+ will provide a large number of oxygen •• vacancies (v•• o ). At high temperature, the vo can easily promote 2− the diffusion of O , which increased the mobility of oxygen vacancies, thereby increasing the conductivity of the material. B


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Figure 2. Temperature-dependent dielectric constant, loss tangent, XRD patterns, and SEM surface morphology for (Ba0.01Bi0.5Na0.49)(Ti0.9Mg0.1)O3 ceramics. (a) Temperature-dependent dielectric constant, (b) loss tangent, (c) XRD patterns, (d) SEM surface morphology.

Figure 3. (a) Temperature-dependent conductivity of BNT, Ba0.01Bi0.5Na0.49TiO3, and (Ba0.01Bi0.5Na0.49)(Ti0.9Mg0.1)O3 samples; the inset shows an enlarged temperature-dependent conductivity of the three samples. (b) The complex impedance spectrum of BNT, Ba0.01Bi0.5Na0.49TiO3, and (Ba0.01Bi0.5Na0.49)(Ti0.9Mg0.1)O3 at 360 °C; inset shows the enlarged complex impedance spectrum of (Ba0.01Bi0.5Na0.49)(Ti0.9Mg0.1)O3.

local order in the A-site of the BNT lattice which makes the composition not uniform, resulting in the difference of phase transition temperature at different regions. Therefore, Ba0.01Bi0.5Na0.49TiO3 can maintain a relatively stable dielectric constant over a wide temperature range in the high-temperature area. Under the condition of high temperature, the dielectric anomalies are caused by the order−disorder phase transition after Tm. (The local ordering structure schematic is shown in Supporting Information Figure S2.) In order to further improve the dielectric constant of BNTbased ceramics, Mg2+ was doped into Ba0.01Bi0.5Na0.49TiO3. The Mg2+ doped pure BNT ceramics have a high-temperature oxygen ion conduction property,32 which can promote the free charges so they can be separated easily. Hence, the doping of Mg2+ may further improve the high-temperature dielectric constant of Ba0.01Bi0.5Na0.49TiO3 ceramics. The high-temperature dielectric constant of (Ba0.01Bi0.5Na0.49)(Ti0.9Mg0.1)O3 is

Figure 1a shows the temperature-dependent dielectric constant of pure BNT. The pure BNT ceramics sample exhibits two dielectric anomalies at Td (∼180 °C) and Tm (∼300 °C). Tm is the maximum temperature at which the dielectric constant reaches a maximum value, and it also represents the Curie temperature. Td with a shoulder peak is the depolarization temperature of BNT. In Figure 1, a new anomalous peak occurs in the 1000 Hz curve of BNT-based samples by the doping of Ba2+, which indicated that the broadening of the peak at Tm may be related to the presence of a dielectric anomaly after Tm. The most obvious anomalous peak is observed in the case of the Ba0.01Bi0.5Na0.49TiO3 sample. The introduction of Ba2+ may result in the redistribution of the disordered Ba2+, Bi3+, and Na+ in the A-site to form local order. Ba2+ substituted Na+ of BNT is donor doping. In order to keep the charge balance, the excess electrons of Ba2+ may change Ti4+ to Ti3+. At the same time, Ba2+ and Bi3+ may easily form C


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Figure 4. Contour mapping of dielectric constant changing with temperature and frequency: (a) BNT, (b) Ba0.01Bi0.5Na0.49TiO3, and (c) (Ba0.01Bi0.5Na0.49)(Ti0.9Mg0.1)O3.

capacitor is equivalent to an RC parallel circuit.28,37 For an RC parallel circuit, the real part of the complex impedance spectrum with low frequency is approximately equal to the resistance of the ceramics. The conductivity of BNT achieved the maximum at about 300 °C, while the conductivity of the Ba0.01Bi0.5Na0.49TiO3 sample is very weak in the range 200−400 °C. With the increase of temperature, the conductivity of the (Ba0.01Bi0.5Na0.49)(Ti0.9Mg0.1)O3 sample increased rapidly. Its conductivity reaches about 1.2 × 10−3 S/cm at 540 °C, and the activation energy (Ea) is about 0.69 eV (see Supporting Information Figure S5). Both the conductivity and activition energy (Ea) are consistent with the values reported by Li et al.32 The increased conductivity at high temperature makes the free charges easy to separate, so that the dielectric constant is further improved by the doping of Mg2+. It can be indicated that the origin of the colossal dielectric constant at high temperature is mainly attributed to high-temperature oxygen ion conduction.23 It is also reported by some researchers that this phenomenon may also be related to the presence of PNRs.38,39 In addition, Figure 3b shows the complex impedance spectrum of the three samples at 360 °C. No semicircles can be found in the main figure, and the curve of (Ba0.01Bi0.5Na0.49)(Ti0.9Mg0.1)O3 is too small to be observed. The enlarged curve of (Ba0.01Bi0.5Na0.49)(Ti0.9Mg0.1)O3 is shown in the inset figure. It can be seen from the inset figure that only (Ba0.01Bi0.5Na0.49)(Ti0.9Mg0.1)O3 has obvious double semicircles indicating that the interface effect plays a role in the high-temperature colossal dielectric permittivity. In order to illustrate the temperature range of the hightemperature dielectric constant clearly, the contour mapping of the dielectric constant with various temperatures and frequencies for the different samples is shown in Figure 4.

shown in Figure 2. After the doping of Mg2+, temperaturedependent dielectric properties shift greatly. It is worth mentioning that the maximum dielectric constant of (Ba0.01Bi0.5Na0.49)(Ti0.9Mg0.1)O3 is about 105 which is 2 orders of magnitude larger than that of BNT ceramics, and the value of the dielectric constant remains above 104 in the range ∼300− 600 °C. In a comparison with the Ba2+ doped BNT-based ceramics, the dielectric anomalous peak of (Ba0.01Bi0.5Na0.49)(Ti0.9Mg0.1)O3 more obviously indicates that the doping of Mg2+ significantly improved the dielectric constant and promoted the formation of local order in BNT-based ceramics. Figure 2c,d shows the XRD patterns and the SEM micrograph of the (Ba0.01Bi0.5Na0.49)(Ti0.9Mg0.1)O3 ceramics, respectively. As shown in the SEM micrograph, the grains of the ceramics surface are spherical; grain size is not uniform. The maximum grain diameter can reach about 10 μm, and the smallest one can reach about 1−2 μm. The XRD patterns of the polished sample shown that ceramics possess a perovskite structure, and no secondary phase is observed, suggesting that Ba2+ and Mg2+ have entered and diffused into the corresponding sites of the BNT lattices. In order to further reveal the mechanism of the hightemperature colossal dielectric constant, the impedance spectra with various temperatures for BNT, Ba0.01Bi0.5Na0.49TiO3, and (Ba0.01Bi0.5Na0.49)(Ti0.9Mg0.1)O3 ceramic samples were measured. The corresponding electric modulus and activation energies were calculated through the data of a complex impedance spectrum8,36 to be used to compare with other reports (shown in Figure S5). Figure 3 shows the ac conductivity and the impedance spectrum of the three ceramic samples. The real part of the complex impedance spectrum at 1000 Hz is used to calculate the ac conductivity of the samples, as shown in Figure 3a. Generally, the ideal circuit for a ceramic D


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barrier layer capacitance effect induced by Mg2+ make the BNTbased ceramics obtain a high-temperature colossal dielectric constant. The present study can provide a new method to obtain a colossal dielectric constant ceramic capacitor with a wide temperature range by A/B-site codoping Ba2+ and Mg2+ in the BNT-based system.

The measured dielectric constant in contour mapping was obtained from frequency dependence of the dielectric constant with various temperature. The εr−f data was measured every 20 °C in the temperature range 160−540 °C. The colors ranging from dark blue, light blue, green, yellow, red, to dark red represent that the value of dielectric constant varies from small to large, as shown in the scale ruler. The pure BNT shows a narrow red region; the high dielectric constant of about 3 × 103 occurs in the range 300−360 °C. Unlike BNT, the red region of Ba0.01Bi0.5Na0.49TiO3 is a flat arc, and the high dielectric constant of about 3 × 103 occurs at 102−104 Hz with a very broad temperature range from 200 to 540 °C. This indicates that Ba2+’s nonstoichiometric substitution of Na+ makes the temperature-dependent dielectric constant flattened near Tm so that Ba0.01Bi0.5Na0.49TiO3 maintains a large dielectric constant over a wide temperature range. The contour mapping of the dielectric constant for (Ba0.01Bi0.5Na0.49)(Ti0.9Mg0.1)O3 is shown in part c. The scale ruler for part c is different from those of a and b. The region below the dotted line is the colossal dielectric constant region where the dielectric constant is larger than 104. The dielectric constant of (Ba0.01Bi0.5Na0.49)(Ti0.9Mg0.1)O3 maintains a large value of (above 104) within a wide temperature range from 220 to 540 °C. This further proved that the oxygen ion conduction induced by the doping of Mg2+ improved the dielectric constant of BNT-based ceramics. The high-temperature colossal dielectric constant with the wide temperature range of (Ba0.01Bi0.5Na0.49)(Ti0.9Mg0.1)O3 is attribute to oxygen ion conduction and local order. For comparison, the maximum dielectric constant (εm) of the representative materials at high temperature40−48 is plotted, as shown in Figure 5. According to the previous analysis, the

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EXPERIMENTAL METHODS

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ASSOCIATED CONTENT

The Bi0.5Na0.5TiO3, Ba0.01(Bi0.5Na0.5)0.99TiO3, Ba0.01Bi0.49Na0.5TiO3, Ba0.01Bi0.5Na0.49TiO3, and (Ba0.01Bi0.5Na0.49)(Ti0.9Mg0.1)O3 ceramics were prepared by conventional solid-state reaction route. The raw materials Bi2O3 (99.9%), NaCO3 (99.5%), TiO2 (99.8%), BaCO3 (99.9%), and MgO (99.8%) powders were weighed according to the chemical formula and milled in ethanol using zirconia balls for 12 h with 300 rpm. The suspensions were then dried at 100 °C for 8 h. Following this, the powders were calcined at 700 °C for 2 h in an alumina crucible and ball-milled again by the same method. After drying, the powders were mixed with poly(vinyl alcohol) (PVA) and pressed into a 10 mm diameter disc. The compacted disc was sintered at 1100 °C for 2 h in air. Finally, silver electrodes were formed on both surfaces of the disc and annealed at 600 °C for 30 min. The phase purity and crystal structure were determined using X-ray diffraction (XRD) on the Philips X’Pert diffraction with Cu Kα radiation. The dielectric properties were measured using an Agilent 4294A impedance analyzer. The microstructure of the samples was examined by scanning electron microscopy (SEM, Quanta 600F). The temperature-dependent dielectric properties were tested by software control of the connected TZDM-RT-600 four-channel signal acquisition and Agilent 4294A impedance analyzer instruments. S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.7b00347. Crystal structure, complex impedance plots with various temperatures, electric modulus (M″) of all samples, and activation energies (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Weili Li: 0000-0002-6632-7116 Notes

The authors declare no competing financial interest.

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Figure 5. Plot of maximum dielectric constants (εm) for representative materials obtained at high temperature in different studies. The different temperature stability ranges of εm are represented by geometric icons with different colors and shapes.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 51677033 and 51471057).

(Ba0.01Bi0.5Na0.49)(Ti0.9Mg0.1)O3 ceramic not only has the largest high-temperature dielectric constant but also has the widest stable temperature range. In summary, the high-temperature-dependent dielectric constant properties of BNT-based ceramics fabricated by a conventional solid-state reaction route were measured. As a result, when the temperature is in the range 220−540 °C, the value of the dielectric constant for (Ba 0.01 Bi 0.5Na 0.49)(Ti0.9Mg0.1)O3 is larger than 1 × 104. A-site Ba2+ substituted in BNT induces the local order−disorder transition which makes the temperature range of the dielectric constant broader, and the high-temperature oxygen ion conduction and interface

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Letter

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