Scholarly article on topic 'Energy storage properties of (Bi0.5Na0.5)0.93Ba0.07TiO3 lead-free ceramics modified by La and Zr co-doping'

Energy storage properties of (Bi0.5Na0.5)0.93Ba0.07TiO3 lead-free ceramics modified by La and Zr co-doping Academic research paper on "Nano-technology"

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Abstract of research paper on Nano-technology, author of scientific article — Xiaopeng Lu, Jiwen Xu, Ling Yang, Changrong Zhou, YangYang Zhao, et al.

Abstract Lead-free [(Bi0.5Na0.5)0.93Ba0.07]1-x La x Ti1-y Zr y O3 (BNBLTZ) ceramics were investigated for energy storage applications. In order to adjust its energy storage properties, the La and Zr co-doping contents varied at 0.01 ≤ x, y ≤ 0.04. BNBLTZ ceramics show a single phase perovskite structure without phase transition after La and Zr co-doping. The compact and uniform microstructure with similar grain morphology and different grain sizes is obtained. The remnant polarization and coercive field decrease with the increase of La and Zr additions, and the energy storage density increases drastically. The maximum energy storage density of 1.21 J/cm3 is obtained when x = 0.04 and y = 0.01. There are two dielectric anomalies at T p and T m due to the phase transformation. The results suggest that lead-free BNBLTZ ceramics should be good candidates for energy storage applications.

Academic research paper on topic "Energy storage properties of (Bi0.5Na0.5)0.93Ba0.07TiO3 lead-free ceramics modified by La and Zr co-doping"

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Materiomics

Energy storage properties of (Bi0.5Na0.5)0.93Ba0.07TiO3 lead-free ceramics modified by La and Zr co-doping

Xiaopeng Lu, Jiwen Xu, Ling Yang, Changrong Zhou, YangYang Zhao, Changlai Yuan, Qingning Li, Guo-hua Chen, Hua Wang

PII: S2352-8478(15)30010-1

DOI: 10.1016/j.jmat.2016.02.001

Reference: JMAT 43

To appear in: Journal of Materiomics

Received Date: 25 November 2015 Revised Date: 31 January 2016 Accepted Date: 5 February 2016

Please cite this article as: Lu X, Xu J, Yang L, Zhou C, Zhao Y, Yuan C, Li Q, Chen G-h, Wang H, Energy storage properties of (Bi0.5Na0.5)0.93Ba0.07TiO3 lead-free ceramics modified by La and Zr co-doping, Journal of Materiomics (2016), doi: 10.1016/j.jmat.2016.02.001.

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Graphical Abstract

Energy storage properties of (Bi0.5Na0.5)0.93Ba0.07TiO3 lead-free ceramics modified

by La and Zr co-doping

Xiaopeng Lua, Jiwen Xuab*, Ling Yangab, Changrong Zhouab, YangYang Zhaoa, Changlai Yuana, Qingning Lia, Guo-hua Chena, Hua Wanga

Lead-free [(Bio.5Nao.5)o.93Bao.o7]i-xLaxTii_yZryO3 (BNBLTZ) ceramics were investigated for energy storage applications. The remnant polarization and coercive field decrease with the increase of La and Zr additions, and the energy storage density increases drastically. The maximum energy storage density of 1.21 J/cm is obtained when x=o.o4 and >=o.oi. There are two dielectric anomalies at Tp and Tm due to the phase transformation. The results suggest that lead-free BNBLTZ ceramics should be good candidates for energy storage applications.

Energy storage properties of (Bi0.5Na0.5)0.93Ba0.07TiO3 lead-free ceramics modified

by La and Zr co-doping

Xiaopeng Lua, Jiwen Xuab*, Ling Yangab, Changrong Zhouab, YangYang Zhaoa, Changlai Yuana, Qingning Lia, Guo-hua Chena, Hua Wanga

aGuangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, Guilin 541004, China

bGuangxi Experiment Center of Information Science, Guilin 541004, China Corresponding author:

Tel.: +86-773-2291244; Fax: +86-773-2291209 E-mail: csuxjw@126.com (J.W. Xu)

Abstract

Lead-free [(Bio.5Nao.5)o.93Bao.o7]i-xLaxTii_yZryO3 (BNBLTZ) ceramics were investigated for energy storage applications. In order to adjust its energy storage properties, the La and Zr co-doping contents varied at o.oi < x, y < o.o4. BNBLTZ ceramics show a single phase perovskite structure without phase transition after La and Zr co-doping. The compact and uniform microstructure with similar grain morphology and different grain sizes is obtained. The remnant polarization and coercive field decrease with the increase of La and Zr additions, and the energy storage density increases drastically. The maximum energy storage density of 1.21 J/cm is obtained when x=o.o4 and y=o.oi. There are two dielectric anomalies at Tp and Tm due to the phase transformation. The results suggest that lead-free BNBLTZ ceramics should be good candidates for energy storage applications.

Keywords: BNT-BT; energy storage; co-doping; ferroelectric; ceramic 1. Introduction

In recent years, in order to meet the demand for electrical energy storage capacitors in the field of pulse power applications, mobile electronic devices, hybrid electrical vehicles [i, 2], lead-free energy storage ceramics with high power density, fast charge and discharge have been attracted great attention [3-5]. Some studies showed that anti-ferroelectric (AFE) materials have a higher energy-storage density and better dielectric properties rather than ferroelectric (FE) materials [6]. However, AFE materials that are extensively studied are mostly lead-based, such as PZST, PLZT, and PLZ [7-9]. The environment-friendly materials thus become the mainstream demand

for future development. As a lead-free ferroelectric material, the Bi05Na05TiO3 (BNT) system with strong ferroelectric property has received considerable attention in recent years [10, 11].

However, it is difficult to pole the pure BNT ceramics with a large conductivity and a high coercive field. Thus, in order to modify the electrical properties of the pure BNT ceramics, the BaTiO3 phase was added into BNT-based system [12,13]. It was reported that the (1-x)Bi05Na05TiO3-xBaTiO3 (BNT-BT) system existed the morphotropic phase boundary (MPB) composition in the range of 0.06<x<0.08, showing a significant jump of electrical performance [14]. In addtion, doping at A-site, B-site or A and B sites is an effecitve way for improving ferroelectric, piezoelectric, dielectric and energy storage properties in BNT-BT system [15-17]. The 0.94BNT-0.06BT ceramics doped with Zr of 0.02 and La of 0.98 had a high energy density of 1.58 J/cm3 [18]. The 0.9118BNT-0.0582BT films with Zr of 0.03 and La of 0.02 prepared by a pulsed laser deposition (PLD) method restulted in a high energy density of 154 J/cm through enhancing its breakdown strength to 3500 kV/cm [19]. At room temperature, the (1-x)(Bi05Na05)Ti-xBaTiO3 system shows a paraelectric phase in range of 0<x<0.063 and x>0.13, and exhibits a antiferroelectric phase in range of 0.063<x<0.13 [19]. As is known to all, the structure and corresponding electrical properties of ceramics are heavily influenced by the composition proportions. The 0.93Na05Bi05TiO3-0.07BaTiO3 system exhibited a good polarization behavior with superior piezoelectric, dielectric and ferroelectric properties at room temperature [20, 21]. Thus, it is believed that the phase structure and the energy

storage performance of the o.93BNT-o.o7BT system can be modified by La and Zr co-doping.

In this paper, the energy storage properties of the [(Bio.5Nao.5)o.93Baoo7]1.xLaxTi1.yZryO3 (BNBLTZ) lead-free ceramics were adjusted via A-site La and B-site Zr co-substitution. The crystal structure, microstructure, energy storage propertie, dielectric behavior and impedance spectrum of the BNBLTZ ceramics were investigated.

2. Experimental

The [(Bio.5Nao.5)o.93Bao.o7]1-xLaxTi1.yZryO3 (BNBLTZ) (x=o.o1, o.o2, o.o3, o.o4 and y=o.o1, o.o2, o.o3, o.o4) ceramics were prepared by a conventional ceramic sintering technique. The analytical reagent Bi2O3, Na2CO3, BaCO3, La2O3, TiO2 and ZrO2 powders were used as starting raw materials. All the starting materials were dried at 8o °C for 24 h and weighed according to the stoichiometric formula. After ball-milling in ethanol with ZrO2 balls for 12 h, the mixed powders were dried 8o °C for 24 h and calcined at 88o °C for 2 h. The dried mixture was sieved through a 1oo-mesh sieve. These powders were mixed with a binder of 7 wt.% polyvinyl alcohol (PVA), and pressed into disks of 13 mm in diameter and 1.o mm in thickness under 4o MPa. After the PVA was burned out, the resulting resultant pellets were sintered in air at 115o °C for 2 h. The sintered samples were polished to a thickness of o.5 mm with 1o mm diameter. Silver electrodes were painted on the both sides of the polished samples and fired at 58o °C for 3o min.

The phase structure was characterized by X-ray diffractometer (XRD, AXS D8-ADVANCE, Bruker). The surface morphology and grain size were analyzed by a filed-emission scanning electron microscope (FESEM, quanta 450 FEG, FEI). The dielectric constant and loss were measured by an impedance analyzer (4294A, Agilent) and a computer controlled furnace with a heating rate of 2 °C/min from room temperature to 400 °C. The impedance spectrum was also measured by an impedance analyzer. The electric-field-induced polarization (P-E) was measured by a ferroelectric test system (P-PMF, Radiant). The energy storage performance was calculated according to the P-E results.

3. Results and discussion

Fig. 1 shows the XRD patterns of the BNBLTZ ceramics. It is seen that all the peaks are indexed based on a tetragonal perovskite structure, indicating that the BNBLTZ ceramics possess a single perovskite structure phase without any other secondary or impurity phases. The single phase also indicates that the La3+ and Zr4+ doping ions diffuse into the BNT-BT lattices and form a solid solution.

Fig. 2 shows the SEM micrographs of the BNBLTZ ceramics as a function of La and Zr doping compositions. Clearly, the BNBLTZ ceramics show a homogeneous and dense structure without any second phase. It is indicated that the grain boundaries are clear, and little visible pores appear. The average grain size of the BNBLTZ ceramics decreases from 1.67 to 0.7 p,m when the La content increases from 0.01 to 0.04 (see Fig. 2(a)-(d)). However, the average grain size of about 1.6 p,m almost is similar when Zr content increases from 0.01 to 0.04 as (see Fig. 2(e)-(h)). Therefore,

the La doping prompts the BNBLTZ ceramics to be homogeneous, and inhibit grain growth. In previous works [22, 23], the enhancement of piezoelectric properties was due to the decrease of grain size. Thus, these smaller grain size and uniform distribution contribute to its electrical properties.

Fig. 3(a) shows the P-E hysteresis loops of the BNBLTZ ceramics with the Zr content of y=o.o1 at 8o kV/cm as a function of the La content. Clearly, the ferroelectric properties of the BNBLTZ ceramics are affected by La doping compositions. In Fig. 3(a), the maximum polarization (Pmax) firstly increases with the increase of La content from o.o1 to o.o2, and then decreases with the further increase of La content from o.o2 to o.o4. The Pmax of 44.6 ^C/cm2 is obtained when x=o.o2 and y=o.o1. Meanwhile, the Pr decreases from 9.8 to 3.o p,C/cm , and the coercive field (Ec) also decreases from 13.7 to 7.5 kV/cm after doping with various La contents. Fig. 3(b) shows the P-E hysteresis loops of the BNBLTZ ceramics with the La content of x=o.o1 at 8o kV/cm as a function of the Zr content. The Pr decreases slightly from 9.8 to 6.9 p,C/cm , and the Ec keeps almost a constant of 13.6 kV/cm at various Zr contents. The Pmax firstly increases and then decreases with the further increase of Zr content, and the Pmax of 42.3 ^C/cm is obtained when x=o.o1 and y=o.o2. The pinched P-E loops attribute to that the La and Zr co-doping causes the phase structure of the BNBLTZ ceramics changing from the ferroelectric phase to the ferroelectric/anti-ferroelectric mixed phase, and the depolarization has already happened at room temperature [1o]. It can be further confirmed by the subsequent analysis of temperature-dependent permittivity. In addition, the decreases of the Pr

and Ec indicate that the long-range ferroelectric order of the sample is disturbed and turned to the polar nano-region (PNRs) with the increase of La and Zr co-doping into the BNBLTZ ceramics [24]. At the same electrical field, the BNBLTZ ceramic when x=0.04 and >=0.01 shows a superior energy storage performance (see subsequent analysis) under non-breakdown condition. Therefore, the P-E hysteresis loops of the BNBL004TZ001 ceramic were measured from 40 to 100 kV/cm until breakdown (see Fig. 3(c)). The BNBL0.04TZ0.01 ceramic has a promising anti-ferroelectric property under different electrical fields.

Fig. 3(d) illustrates the calculation of the energy storage density and its efficiency.

The energy storage density (W1) is calculated from P-E loop [25]:

PPmax EdP (1)

where, W1 is the electrical energy density stored in the material, E refers to the applied external electric filed, Pr and Pmax are the remnant and maximum polarization values respectively. From Eq. (1), a high energy storage density can be obtained by a huge difference between Pmax and Pr. Fig. 4 shows the three-dimensional energy storage density plots of the BNBLTZ ceramics. Clearly, the energy storage density of the BNBLTZ ceramics increases with the increase of La content. However, the energy storage density firstly increases (><0.02), and then decrease with the further increase of Zr content (0.02<><0.04). The optimized composition is x=0.04 and >=0.01, and its corresponding maximum energy storage density is 0.93 J/cm at 80 kV/cm. Furthermore, its maximum energy storage density of 1.21 J/cm can be obtained at 100 kV/cm before breakdown (see Fig. 3(c)).

The energy storage efficiency (n) of the material can be calculated by

h = W1 / W1 + W2 (2)

Where, W2 is energy loss density, which caused by the domain reorientation is calculated by integrating the area between the charge and discharge curve of the unipolar P-E hysteresis loops. From Eq. (2), the larger energy storage efficiency is an indicative of small loss in the form of hysteresis. Fig. 5 shows the three-dimensional energy storage efficiency plots of the BNBLTZ ceramics. It is seen that the energy storage efficiency increases with the increase of La and Zr contents. The minimum energy storage efficiency is 37.1% when x=o.o1 and y=o.o1, and the maximum is 77.8% when x=o.o3 and y=o.o4. The energy storage efficiency of the maximum energy storage density when x=o.o4 and y=o.o1 is 74.o%, which is slightly less than the maximum energy storage efficiency. Thus, the anti-ferroelectric properties of the BNBLTZ ceramics is improved by the slimmer and slanted P-E hysteresis loops obtained after La and Zr co-doping.

Fig. 6 (a)-(g) shows the temperature dependence of dielectric permittivity (er ) and dielectric loss (tan^) of the BNTLTZ ceramics as a function of La and Zr compositions at 1, 1o and 1oo kHz, respectively. All the permittivity curves present two dielectric anomaly peaks. In fact, there is a disappeared peak of depolarization temperature (Td), as shown in Fig. 6(a), which shifts to a lower temperature than the measured temperature range. The depolarization temperature corresponds to the phase transition from a ferroelectric phase to a so-called "anti-ferroelectric" phase [26]. The second phase transition temperature Tm, as shown in Fig. 6 (a)-(g), appears due to the

transition from an anti-ferroelectric phase to a paraelectric phase. Moreover, intense dielectric relaxor properties occur from room temperature to Tp (see Fig. 6 (a)-(g)). The dielectric behavior at low temperatures may be due to the dynamic fluctuation of

3+ + 2+ 31

the nanodomains. The displacements of A-site cations (Bi , Na , Ba and La ) are antiparallel to those of B-site cations (Ti4+ and Zr4+) within each individual nanodomain [27]. Thus, incomplete cancelation of the dipole moments will generate a net polarization effect. The dynamic fluctuation of the weakly polar nanodomains leads to the relaxor behavior.

Fig. 6 (h) shows a direct comparison of the dielectric-temperature curves of the BNBLTZ ceramics at 1 kHz. Clearly, the temperature location of the maximal dielectric peaks (Tmax) almost unchanges at various La doping contents, and the temperature location of the second dielectric peaks (Tp) shifts to a lower temperature. However, the Tmax shifts to a high temperature, and the Tp almost unchanges at various Zr doping contents.

Impedance spectroscopy is a powerful tool for analyzing the electrical properties of materials and their interfaces with electronically conducting electrodes. An equivalent circuit based on impedance spectra provides the physical processes occurring inside the sample. Most of the ceramics contain grains and grain-boundary regions, which individually have different physical properties [28]. As a dielectric energy storage material, alternating current (ac) conductivity (oac) and direct current (dc) conductivity (ode) demand as small as possible. The aac and odc can be given by

Sac = d / AXRac (3)

Sc = d x I / AxU (4)

where d is the thickness of electrolyte, A is the area of electrode, Rac is the ac resistance of ceramic sample, U is dc voltage, I is the dc current. Fig. 7(a) shows the alternating current resistance of the BNBL0.04TZ0.01 and 0.93BNT-0.07BT ceramic as

a function of frequency. The oac increases gradually from 3.78*10" to 8.17*10" S-cm"1 as the frequency increases from 40 Hz to 10 MHz. The inset in Fig. 7(a) shows a plot of lgl versus lgF at dc condition. A quite low odc of 1.52*10-10 S-cm-1 is owned in the sample. Meanwhile, at the same test condition, the odc of the pure sample is ten times less than that of the doped sample. The low at a low frequency closing to direct current and low odc indicate that the BNBLTZ ceramics have a superior insulativity. Thus, La and Zr co-doping make the BNBLTZ ceramics endure a high breakdown strength and improve energy storage.

Fig. 7(b) shows the complex impedance Cole-Cole plots of the BNBL0 04TZ0 01 ceramic at various temperatures. Clearly, a small portion of the impedance dispersion profile can be detected in the measured frequency range. As temperature increases, the graphs turn from a pitch arc to semicircle. This indicates that the sample has an insulating behavior at a low temperature (i.e., <350 °C). The semicircles become smaller and smaller arising from increasing temperatures, indicating the decrease of resistance and representing the contribution of impedance only originating from the grain boundary phases [29]. The high conductivity of the BNBLTZ ceramics at a high temperature attributes to the generation of oxygen vacancies in BTN-based system [30].

4. Conclusions

The energy storage properties of the [(Bi0.5Na0.5)0.93Ba0.07]1.xLaxTi1.J;ZryO3 (x=0.01, 0.02, 0.03 and 0.04, >=0.01, 0.02, 0.03 and 0.04) lead-free ceramics were adjusted via La and Zr co-doping. The BNBLTZ ceramics possessed a single perovskite structure phase. The dense structure and fine grains were obtained for all samples, and the minimum grain size of BNBLTZ was 0.7 p,m when x=0.04, >=0.01. The energy storage density of the BNBLTZ ceramics increased with the increase of La doping content. However, the increase of the Zr content reduced the energy storage density in a certain extent, and the optimal proportion was x=0.04 and >=0.01, its maximum energy storage density was 1.21 J/cm at 100 kV/cm. Meanwhile, the reversible relaxor ferroelectric phase transition induced by the electric field to promote the superior energy storage properties. It was indicated that the energy storage properties of the BNT-BT were enhanced after the La and Zr co-doping. The BNBLTZ ceramics were a promising lead-free material for energy storage capacitor application.

Acknowledgments

This work was supported by the National Nature Science Foundation of China (No. 61361007 and 11564007) and Guangxi Nature Science Foundation (No. 2015GXNSFAA139250) and Guangxi Experiment Center of Information Science.

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Figures caption

Fig. 1 XRD patterns of the BNBLTZ ceramics. Fig. 2 SEM images of the BNBLTZ ceramics.

Fig. 3 P-E hysteresis loops of the BNBLTZ measured at 80 kV/cm as a function of (a) La content and (b) Zr content, and(c) P-E hysteresis loops of the BNBL0 04TZ0 01 sample measured from 40 to 100 kV/cm.

Fig. 4 Three-dimensional energy storage density plots of the BNBLTZ ceramics at 80 kV/cm.

Fig. 5 Three-dimensional energy storage efficiency plots of the BNBLTZ ceramics at 80 kV/cm.

Fig. 6(a)-(g) Frequency and temperature dependence of the dielectric constant (er) and dielectric loss (tanS) of the BNTLTZ ceramics at the frequencies of 1 kHz, 10 kHz and 100 kHz, (h) temperature-dependent dielectric constant (er) of the BNTLTZ ceramics measured from 25 to 400 °C at 1 kHz.

Fig. 7 (a) The ac resistance of the BNBL004TZ001 and 0.93BNT-0.07BT sample as a function of frequency, and the inset shows a plot of Lg/ versus Lg^ at dc condition, and (b) the temperature-dependent impedance spectra of the BNBL0 04TZ001 sample.

Fig. 1

(100) (Oil) (111) (200) ^ s c w Rj rj S7(x-0.04 v-0.01) —i

-1 I (S6(*=0.03>=0.01)

1 . S5(*=0.02>=0.01)

« tS4(^=0.01>=0.04)

.1 I ^3(^=0.01^=0.03)

iS2(x=0.01>p=0.02)

| S1(jc=0.01 y=0.01)

-!—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i

20 30 40 50 60 70 80 90 100

2Theta (deg)

Fig. 2

Fig. 3

_ (a)y=0.01

/ -jc=0.01

-x=0.02

-x=0.03

-x=0,04

-90 -60 -30 0 30

E (kV/cm)

o \ ^ -20

. (b)x=0.0\

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Asso. Prof. Jiwen XU, Guilin University of Electronic Technology Email: csuxjw@126.com Dr. Jiwen Xu received his B.S. from Central South University, and Ph.D from Shaanxi Normal University in 2014. He worked at School of Material Science and Engineering, Guilin University of Electronic Technology from 2002. He is interested in oxide powders, ceramics and thin films. He has published more than 20 research papers and 5 patents.