Greatly reduced leakage current and defect mechanismin atmosphere sintered BiFeO3–BaTiO3 high temperaturepiezoceramics

Zhonghua Yao • Chaobing Xu • Hanxing Liu •

Hua Hao • Minghe Cao • Zhijian Wang •

Zhe Song • Wei Hu • Atta Ullah

Received: 10 June 2014 / Accepted: 16 August 2014 / Published online: 29 August 2014

� Springer Science+Business Media New York 2014

Abstract In current work, the effect of sintering atmo-

spheres (N2, air and O2) on the structure, electrical prop-

erties, and defect mechanism of 0.8BiFeO3–0.2BaTiO3

lead-free piezoelectric ceramics has been investigated.

X-ray diffractometer results indicated all the samples

crystallized into the rhombohedrally distorted perovskite

structure which was independent on the sintering atmo-

spheres. Bi-containing impurity phases were observed in

N2 sintered samples while not appearing in other atmo-

spheres. X-ray photoelectron spectrum analysis indicated

more Fe2? ions, which can result in high leakage current,

were involved in N2 sintered ceramics than that in O2- and

air sintered compositions. However, greatly reduced leak-

age currents were obtained in N2 sintered ceramics which

should be ascribed to the formation of secondary phases.

The largest polarization and lowest leakage current were

obtained in the sample sintered in N2 (2 h), which

owned the optimal ferroelectric, piezoelectric, and elec-

tromechanical properties with piezoelectric constant

d33 = 98 pC/N, planar electromechanical coupling factors

kp = 26.1 %, remnant polarization Pr = 25.7 lC/cm2,

coercive field Ec = 74.6 kV/cm, and a high Curie tem-

perature Tc = 632 �C, respectively.

1 Introduction

Ferroelectric materials are widely used as dielectric mate-

rials in contemporary electronics, serving functions such as

bypassing, coupling, filtering, smoothing, and power con-

ditioning [1–3]. One of the challenges currently faced by

industry is that most dielectric materials are not able to

work properly at temperatures higher than 150 �C, whereas

the advance of technology demands electronic devices that

can be used under harsh environments ([200 �C) such as

dynamic fuel injection nozzle in car engines (*300 �C)

and acoustic transducer in oil well (200–300 �C) [4].

Recently, BiFeO3 material exhibiting coupling between

different degrees of freedom, such as strain, polarization

and magnetization, has received great scientific and tech-

nological interest as piezoelectric material due to its high

transition temperatures and large polarization character-

ization. Here, it is the Bi ion with two electrons on the 6 s

orbital (lone pair) that moves away from the centrosym-

metric position in its oxygen surrounding and gives rise to

ferroelectricity. However, so far it has not been possible to

exploit the compositions for commercial applications in the

bulk state due to large leakage current and extremely large

coercive field, as a result of which the bulk ceramic cannot

be electrically poled efficiently. Moreover, it is difficult to

fabricate pure BiFeO3 material in bulk state due to the

formation of secondary phases such as Bi2O3, Bi2Fe4O9,

Bi25FeO39, Bi25FeO40, or Bi46Fe2O72 [5–7]. Many attempts

have been made to overcome these problems by rapid

liquid-phase sintering [8–10], doping with Mn, Nb, La, Ti,

and Mo ions [11–16], forming solid solutions with other

perovskites [17, 18], and even annealing under different

extreme conditions, such as magnetic field [19]. Among the

various modifications reported in the literature, modifica-

tion by BaTiO3 i.e. (1-x)BiFeO3–xBaTiO3 (BF–BT) has

Z. Yao (&) � C. Xu � H. Liu � H. Hao � M. Cao � Z. Wang �Z. Song � W. Hu � A. Ullah

State Key Laboratory of Advanced Technology for Materials

Synthesis and Processing, Wuhan University of Technology,

Wuhan 430070, People’s Republic of China

e-mail: yaozhhua@whut.edu.cn

H. Liu

e-mail: lhxhp@whut.edu.cn

123

J Mater Sci: Mater Electron (2014) 25:4975–4982

DOI 10.1007/s10854-014-2260-0

shown some very interesting features such as high Curie

point ([450 �C) and large polarization characterization.

For example, Eitel et al. [11] found small amount of Mn

modified BF-BT ceramics with BT content of 0.25 sintered

under oxygen atmosphere showed enhanced DC resistivity

of 7.6 9 1012 X m, and its piezoelectric constant d33 and

Curie temperature Tc were 116 pC/N (low field) and

580–619 �C, respectively. Chen et al. [20] reported that the

introduction of excess Bi content (*1 %) in 0.75BF–

0.25BT ceramics could sharply improve the temperature

stability of dielectric and piezoelectric properties which

exhibited a Curie temperature Tc of 508 �C, dielectric

constant of 440, tan d (1 kHz) of 4.6 %, remnant polari-

zation of 34.4 lC/cm2, piezoelectric constant d33 of 114

pC/N, and planar electromechanical coupling factor kp of

0.31, respectively.

As in very pure BiFeO3, due to the spontaneous change

of the oxidation state of Fe (3 ?/2 ?), oxygen vacancies

are formed as a result of the electrical neutrality require-

ment, giving rise to thermal activated hoping conductivity.

Recently, it has been reported that the sintering atmosphere

plays an important role in the structure and properties of

BiFeO3 system [21–27]. Most studies reported in BiFeO3

system were to reduce the concentration of oxygen

vacancies and to hinder the changes of Fe3? valence states

[9]. It is possible that reducing the concentration of oxygen

vacancies and stabilizing Fe3? valence states can be arti-

ficially achieved by the improvement of sintering technique

under different atmospheres. Some researchers [21–24]

reported in BiFeO3 materials that oxygen treatment could

result in improved leakage current while others [25–27]

draw the opposite conclusion and obtained superior prop-

erties by nitrogen treatment. Up to now, the exact mecha-

nism of the improvement of leakage current in BiFeO3-

based materials processed in various atmospheres is still

under debate and research in BF–BT systems by atmo-

sphere sintering is currently reporting new data. In this

work, 0.8BiFeO3–0.2BaTiO3 (8BF–BT) ceramics were

sintered in N2, air, and O2, respectively. The influences of

atmosphere sintering on structure, electrical properties, and

the possible defect mechanism were intensively discussed.

2 Experimental procedure

Specimens with the compositions of 0.8BiFeO3–0.2BaTiO3

(8BF–BT) were prepared by conventional solid-state

method. The reagent-grade materials of Bi2O3, Fe2O3,

BaCO3, and TiO2 powders were weighed as starting pow-

ders according to the nominal stoichiometric ratio. The

powders were mixed, calcined (730 �C, 2 h), ground, dried

and then re-mixed with 1.5 wt% polyvinyl alcohols (PVA)

binder. The powders were pressed into pellets with 12 mm

in diameter and *1.0 mm in thickness at 200 MPa. These

green pellets were burnt out at 650 �C and then sintered at

980 �C for 2–4 h using a quartz tube furnace under various

atmospheres.

Densities of the pellets were measured by Archimedes

method and the relative density was measured to lie in the

range of 95.1–96.5 %. Phase structures were detected by a

Philips vertical X-ray diffractometer (XRD, PW3050/60,

high resolution X’Celerator detector, MPSS) using Cu Ka1

radiation confirmed the formation of perovskite-type pha-

ses with 2h in the range of 10�–80�. The microstructure

observations of the fresh fractured surfaces of sintered

specimens were examined using a scanning electron

microscopy (SEM, Akashi Seisakusho JSM-5610LV).

Changes in chemical valence were detected by X-ray

photoelectron spectroscopy (XPS, XSAM-800). For elec-

trical characterizations, the samples were polished down to

0.6–0.7 mm in thickness. The pellets were printed on both

sides by high-temperature silver electrodes, burnt out the

binder in silver paste at 450 �C for 30 min in air, and then

fired at 800 �C for 20 min under the same atmosphere as

the samples sintered to eliminate the effects of firing

atmospheres on the valence states of the compositional

elements. The sintered ceramics were poled in an oil bath

under an electric field of 6–10 kV/mm at 120 �C for

15 min. Temperature dependence of dielectric properties of

the ceramic samples was measured using an LCR meter

(Agilent E4980A) at different temperatures. The piezo-

electric constant d33 and the electromechanical coupling

factor kp of the specimens were measured 1 day after

poling using a quasi-static piezoelectric d33 meter (Model

ZJ-3D, Institute of Acoustics Academic, China) and an

impedance analyzer (HP4294A) by the resonance and anti-

resonance technique on the basis of IEEE standards,

Fig. 1 Room-temperature XRD patterns of 8BF–BT samples sintered

in different atmospheres

4976 J Mater Sci: Mater Electron (2014) 25:4975–4982

123

respectively [28]. Both the room-temperature hysteresis

loops of polarization and leakage current density were

performed using RT66A ferroelectric testing system

(Radiant Technologies) in silicon oil under different elec-

tric field. For each electrical measurement, at least three

samples were measured to obtain the average electrical

properties.

3 Results and discussion

3.1 Phase structure and microstructures

Figure 1 shows the XRD patterns of 8BF–BT samples

sintered for 2–4 h in different atmospheres. It is evident

from this figure that, for all sintered samples, the peak at

2h–45� is a singlet, while at 2h–39� is doublet, as expected

for the rhombohedral structure of BiFeO3. Hence, all the

samples crystallize into the rhombohedrally distorted

perovskite structure with R3c space group reported

everywhere [29, 30], which is independent on the sintering

atmospheres. However, a weak diffraction peak can be

observed at 2h–27.4� only in the samples sintered in N2

which belong to Bi-containing impurity phases [5–7].

Figure 2 shows typical SEM pictures of the fresh frac-

tured surfaces of 8BF–BT ceramics sintered for 2–4 h in

different atmospheres. All samples give rise to non-uni-

form grains about 3–5 lm except for the samples sintered

for 2 h in O2 (*1 lm), but clear grain boundary can be

observed. The longer sintering time in O2 at the same

temperature the bigger grain size can be obtained. When

sintered at high temperatures, the evaporation of Bi will

Fig. 2 Typical SEM pictures of the fresh fractured surfaces of 8BF–BT ceramics sintered at 980 �C in different atmospheres. Where, a O2 for

4 h; b O2 for 2 h; c Air for 2 h; d N2 for 2 h; e N2 for 4 h

J Mater Sci: Mater Electron (2014) 25:4975–4982 4977

123

seriously influence the microstructure and electrical char-

acterization. So, the only factor leading to abnormal grain

sizes can be ascribed to the evaporation of Bi as sintering

aid (low melting point) which can be beneficial for grain

growth. This also indicates the less volatility of Bi in the

ceramics sintered in O2 for 2 h than that in other

conditions.

3.2 Dielectric properties and leakage current

characterization

Clear dielectric anomalies of unmodified 8BF–BT ceram-

ics were seldomly reported, which may be due to the

significantly increased DC conductivity at elevated tem-

peratures. Figure 3 shows the dielectric temperature spec-

tra (1 kHz) of N2 sintered 8BF–BT ceramics. As for air-

and O2 sintered ceramics, the datum of dielectric properties

could not be obtained entirely at elevated temperatures due

to DC conductivity beyond measurement limitation. It is

found that obvious dielectric peaks are observed for N2

sintered ceramics, reflecting that the high temperature DC

resistivity is improved. Additional data for comparison

with sintered BF–BT ceramics are presented in Table 1. It

can be observed that the ceramics sintered in O2 and air

exhibit high dielectric constant at room temperature as well

as dielectric loss. In the case of N2 sintered compositions,

significantly lower dielectric loss values were achieved

compared with that sintered in other atmospheres.

The leakage current is one of the most vital characteris-

tics for any ferroelectric material to be used in devices.

Figure 4 shows the leakage current density and resistivity

versus electric field curves of 8BF–BT ceramics sintered in

for 2–4 h various atmospheres. As the field increases, a

sharp increase in the current density of sintered ceramics is

observed. The great difference of current density can be

observed for the ceramics under different sintering atmo-

spheres. It can be seen that, under the same electric field, N2

sintered samples exhibit the lowest current densities while

the O2 sintered ceramics are leakier than those sintered in

air. For example, at 1.0 kV/cm, the leakage current density

and resistivity are 2.22 9 10-4 A/cm2 and 4.43 9 107

Ohms cm in O2 (4 h), 1.77 9 10-4 A/cm2 and 5.56 9 107

Ohms cm in O2 (2 h), 7.66 9 10-7 A/cm2 and 1.28 9 1010

Ohms cm in air (2 h), 1.69 9 10-9 A/cm2 and 5.81 9 1012

Ohms cm in N2 (2 h), 2.72 9 10-9 A/cm2 and 3.61 9 1012

Ohms cm in N2 (4 h), respectively.

3.3 Ferroelectric properties

The hysteresis loop is one of the most important charac-

teristics of a ferroelectric material and gives information on

its dynamic polarizability. It has been known that the

coercive field of BF-rich ferroelectric ceramic is very high

[36]. Figure 5 shows the room temperature P-E loop

behavior for 8BF–BT ceramics sintered in various atmo-

spheres till to breakdown. The P-E loops of the O2 sintered

samples appeared round and leaky under a low field, which

failed to get saturated due to the large leakage current,

shown as in Fig. 5a. When the ceramics were sintered in

N2 and air, the lossy response was suppressed and

decreased. Compared with N2 sintered samples, the loops

of air sintered samples seem to be of unsaturated behavior

because of a higher leakage current and partial reversal of

polarization. As shown in Fig. 5, the optimal values of

remnant polarization Pr and coercive field Ec are about

12.6 lC/cm2 (54.4 kV/cm), 25.7 lC/cm2 (74.6 kV/cm)

Fig. 3 Dielectric temperature spectra (1 kHz) of N2 sintered 8BF–BT

ceramics

Table 1 Ferro- and piezoelectric properties of present 8BF–BT

system at room temperature, compared to the other Bi-based systems

Material Tc

(�C)

er tan d d33

(pC/N)

kp (%) Ref.

0.85BF–0.15BT 700 242 0.022 - - [11]

0.8BF–0.2BT 570 412 0.058 - - [31]

0.8BF–0.2BT [600 300 *63 *14.0 [32]

0.75BF–0.25BT 508 440 0.046 114 31.0 [20]

BSPT57 crystal 402 3,000 0.040 1,150 [33]

0.36BS–0.64PT 450 2,010 0.027 460 56.0 [34]

0.47BSF–0.53PT 428 830 0.040 290 50.0 [35]

0.8BF–0.2BT

O2, 4 h - 589 0.942 - -

O2, 2 h - 417 0.364 - -

Air, 2 h - 284 0.142 38 13.5

N2, 2 h 632 333 0.065 98 26.1

N2, 4 h 631 232 0.061 56 16.3

4978 J Mater Sci: Mater Electron (2014) 25:4975–4982

123

and 16.2 lC/cm2 (59.6 kV/cm) for air, N2 (2 h), and N2

(4 h) sintered samples, respectively. It can be noticed that

the remnant polarization is enhanced apparently in the N2

sintered samples compared with that of air sintered

samples.

3.4 XPS analysis

As mentioned above, the ceramics sintered in N2 can be

poled completely and exhibit large ferro- and piezoelectric

characterization due to low leakage current. It has been

well proved that the larger leakage currents in magneto-

electrics often result from mixed valence for the magnetic

ions (e.g., Fe2? and Fe3?), from oxygen vacancies, or from

both.

As in easily volatile elements such as bismuth exist in

8BF–BT ceramics, defects are easily produced during

sintering. According to the study of Bi4Ti3O12 by Hiruma

et al. bismuth vacancy is easily generated at high temper-

atures accompanied with the nearest-neighbor oxygen

vacancies to maintain charge balance during defect for-

mation, while bismuth escapes from the lattice position and

evaporates as Bi2O3. The formation of oxygen vacancies

arises from the evaporation of Bi and lower oxygen

atmosphere, which is accompanied with the Ti valence

transition to maintain charge balance during defect for-

mation [37]. Similarly, the sintering atmosphere can also

induce oxygen vacancies and Fe valence transition in 8BF–

BT ceramics when sintering. The reaction can be described

as follows:

Fig. 4 The leakage current

density and resistivity versus

electric field curves of 8BF–BT

ceramics sintered for 2–4 h in

various atmospheres

J Mater Sci: Mater Electron (2014) 25:4975–4982 4979

123

2Fe3þ þ O2� ) 2Fe2þ þ 0:5O2½volatilization� þ V2þO

ð1Þ

2Bi3þ þ 3O2� ) Bi2O3½volatilization� þ 2V3�Bi þ 3V2þ

O

ð2Þ

Many attempts have been made to see whether both

Fe3? and Fe2? are present in BiFeO3, based on the shape of

the Fe 2p X-ray photoelectron spectrum (XPS) [38–41].

The approach applied suggested a priori that the Fe 2p XPS

of BiFeO3-based samples can be treated in the usual way,

when the peak maximum of the Fe 2p3/2 X-ray spectra is

decomposed into a superposition of symmetric compo-

nents. The Fe 2p3/2 component with the binding energy of

*710.8 eV was assigned to the Fe3? state while that with

*709.4 eV to the Fe2? state [42, 43]. Figure 6 shows the

deconvolution of XPS Fe 2p3/2 spectra of 8BF–BT

ceramics sintered for 2–4 h in different atmospheres with

the peaks corresponding to Fe element after Shirley-type

background. The broad asymmetric Fe 2p3/2 peaks suggest

the coexistence of Fe2? and Fe3? in these ceramics. The Fe

2p3/2 spectra can be divided into two subbands centered at

*709.4 and *710.8 eV by XPSPEAK 4.1 software,

respectively. The percentage of Fe2? to the total ions

(Fe3? ? Fe2?) is determined by the relative area of Fe2?

peak to the whole peak, which is 18.9 % in O2 (4 h), 9.4 %

in O2 (2 h), 27.2 % in air, 36.2 % in N2 (2 h), while

43.8 % in N2 (4 h), respectively. This demonstrates that

more Fe ions are maintained at ?3 valence in the O2 sin-

tered samples than that in N2 sintered samples and more

evaporation of Bi will happen in N2 sintered samples.

Therefore, the leakage in 8BF–BT ceramics is likely

induced by the existence of Fe2? and oxygen vacancies,

which both are detrimental for characterizing the intrinsic

properties of 8BF–BT. It can be expected that Bi was

extracted from the original perovskite lattice when sinter-

ing. However, the remained Bi constructed a defective

perovskite with many oxygen and Bi vacancies. This may

induce an oxygen deficient and reducing environment in

the material, which contributes to Fe valence transition and

therefore high leakage current for N2 sintered ceramics. As

analysized above, the samples sintered in O2 should

Fig. 5 P-E loop behavior for 8BF–BT ceramics sintered for 2–4 h in various atmospheres till to breakdown

4980 J Mater Sci: Mater Electron (2014) 25:4975–4982

123

achieve lower leakage current and higher resistivity than

that sintered in N2. However, the result is just the opposite.

As seen from Figs. 4, 5, the N2 sintered ceramics exhibits

greatly reduced current densities and can be poled effec-

tively while not for O2 sintered ceramics. It can be deduced

that these defects mentioned above should not be respon-

sible for leakage current.

Many studies on BiFeO3-based materials have been

interested in eliminating secondary impurities and oxygen

vacancies to reduce leakage and increase resistivity,

thereby improving ferroelectric properties. As in BiFeO3,

the ferroelectricity has been observed with a giant swit-

ched polarization of 110–136 lC/cm2 grown by physical/

chemical deposition [44–46] and theoretically predicted

with the polarization of 90–110 lC/cm2 [47]. This indi-

cates that the intrinsic ferroelectric property of BiFeO3-

based system should theoretically be very high when

poling completely. As shown in Fig. 5, the values of

remnant polarization (Pr) and coercive field (Ec) at the

same field of 80 kV/cm are about 12.6 lC/cm2 (52.6 kV/

cm), 4.7 lC/cm2 (34.4 kV/cm) and 4.5 lC/cm2 (34.5 kV/

cm) for air (2 h), N2 (2 h), and N2 (4 h) sintered samples,

respectively. It can be observed that more Fe2? ions in

8BF–BT ceramics can deteriorate the ferroelectric prop-

erties while obviously decrease coercive field. Greatly

reduction of coercive field can be ascribed to slight dis-

tortion of the lattice (FeO6 octahedra) due to different

ionic radii between Fe3? (0.645 A) and Fe2? (0.78 A).

Although Fe2? ions in O2 sintered composition are rather

low, the intrinsic breakdown strength of pure 8BF–BT is

limited which leads to incomplete poling. So, there must

be another factor which contributes to the enhancement of

breakdown strength. Here, we have demonstrated that

secondary Bi-containing phase appears in N2 sintered

ceramics during processing owing to the thermodynamic

stability of the phase while no any secondary phase

observed in other atmospheres. It is well-known that the

impurity phase can increase resistivity due to its non-

ferroelectric activity. Therefore, one possible explanation

for the observed behavior for the samples sintered in N2

may be related to the existence of secondary phases

which exhibit high resistivity and that greatly reduce the

leakage currents.

4 Conclusions

In summary, the structure, electrical properties, and defect

mechanism of 8BF–BT composition sintered in O2, air, and

N2 were intensively investigated. XRD results indicated the

appearance of secondary phases for the samples sintered in

N2 while not observed in other atmospheres. The charged

defects by Fe2? ions, which will reduce the electrical

resistivity of the samples and give rise to high leakage

currents, were more in the samples sintered in N2 than that

sintered in other atmospheres by XPS spectra. However,

greatly improved leakage currents were achieved in N2

sintered compositions and the samples sintered in other

atmospheres exhibited large leakage currents. One possible

explanation for the observed behavior for the samples

sintered in N2 may be related to the existence of secondary

phases which greatly improve the leakage currents. The

largest polarization and lowest leakage current were

obtained in the sample sintered in N2 (2 h), which

owned the optimal ferroelectric, piezoelectric, and elec-

tromechanical properties with piezoelectric constant

d33 = 98 pC/N, planar electromechanical coupling factors

kp = 26.1 %, remnant polarization Pr = 25.7 lC/cm2,

coercive field Ec = 74.6 kV/cm, and a high Curie tem-

perature Tc = 632 �C, respectively. It is expected that the

BF–BT system will form the basis of an important family

Fig. 6 The deconvolution of XPS Fe 2p3/2 spectra of BF–BT

ceramics sintered in different atmospheres

J Mater Sci: Mater Electron (2014) 25:4975–4982 4981

123

of high-performance lead free piezoelectric ceramics by the

improvement of sintering techniques.

Acknowledgments The authors would like to thank the support

of the International Technology Cooperation Project from the Min-

istry of Science and Technology of China (2011DFA52680), Natural

Science Foundation of China (No. 51372191 and No. 51102189), the

Fundamental Research Funds for the Central Universities (No.

2012-IV-006), and State Key Laboratory of New Ceramic and Fine

Processing Tsinghua University (KF201314).

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