Abstract— The aim of this study was to investigate the electrical
properties of (1-x)[Ba0.85Ca0.15Zr0.1Ti0.9]O3 - x[(BiZn0.5Ti0.5)O3] ceramics system for x = 0.00 - 0.10. The (1-x)BCZT-xBZT ceramics were fabricated via solid state reaction technique. The samples were calcined and sintered at 925ºC for 6 h and 1370ºC for 3 h, respectively. Phase formation and microstructure of (1-x)BCZT-xBZT ceramics were studied using an X-ray diffractometer (XRD) and scanning electron microscope (SEM). The physical properties and electrical properties, such as dielectric properties and ferroelectric properties were examined. It was found that all the samples show pure perovskite phase. The BZT content lead to the change and improved of the dielectric properties of ceramic samples. The dielectric loss values were lower than 0.01 (at 1 kHz) for all samples. The samples show higher degree of the relaxor like behavior with higher BZT content. In addition, the ferroelectric properties were found to depend on the BZT content.
Keywords—solid state reaction; Ferroelectric properties; Dielectric properties; Perovskites
I. INTRODUCTION
Barium titanate (BaTiO3) is the most widely studied lead-free piezoelectric material because the phase transition temperature of BaTiO3 can be altered by doping the A-site or B-site such as the addition of calcium (Ca) into the barium (Ba) site or zirconium (Zr) into the titanium (Ti) site [1]. Further, the substitution of Ba2+ with Ca2+ does not strongly affect the Curie temperature [2]. Recently, lead-free BCZT based ceramics showed excellent piezoelectric properties [3]. Thus, these ceramics have been widely studied as lead-free perovskite piezoelectrics. However, these ceramics require very high calcining and sintering temperature for forming pure perovskite phase [3]. The development of new bismuth ferroelectric materials, e.g., Bi(Zn2+, Ti4+)O3 has attracted a lot of attention and been widely investigated experimentally. Because of the relative small size of Bi3+, BiMO3 compounds (where M: Fe3+, Sc3+, Ni1/2Ti1/2, Zn1/2Ti1/2, etc.) compounds are not stable in the perovskite form. However, the smaller tolerance factor and the high polarizable Bi3+ ion can enhance the transition temperature in BZT containing solid solutions with PbTiO3, BaTiO3 or Bi1/2K1/2TiO3 and show clearly relaxor ferroelectric behavior with increasing BZT content [4]. Moreover, BZT compounds doped BCZT has not been reported.
In the present work, electrical properties of (1-x)BCZT-xBZT lead-free ceramics for x = 0.00 - 0.10 were investigated. Many electrical properties such as dielectric and ferroelectric properties of the ceramics are reported for the first time.
II. EXPERIMENTALS
A. Synthesis
The (1-x)BCZT - xBZT ceramic systems were prepared by using solid state reaction method (x = 0.00 - 0.10 mol% in steps of 0.02). The starting materials were the oxide powders of BaCO3, CaCO3, ZrO2, TiO2, Bi2O3 and ZnO. Oxide powders were weighed according to stoichiometric formulae and mixed in ethanol for 24 h using zirconia grinding media. Then, the slurry was dried on a hotplate and calcined in crucibles at 925°C for 6 h. After that, the dried powders were mixed with added 6 wt% PVA. The powders were pressed into cylindrical pellets 10 mm in diameter and 1 mm thickness isostatically at 1 ton. The pellets were then sintered at 1370°C for 3 h.
B. Characterization
Phase formation and microstructure of the samples were studied by an X-ray diffraction (XRD) technique and scanning electron microscope (SEM). The density of sintered samples was determined by Archimedes method with distilled water as the fluid medium. For electrical properties characterization, the sintered samples were ground to obtain parallel faces, and the faces were then coated with silver as electrodes. The dielectric constant and dielectric loss of the sintered ceramics were measured as a function of frequency and temperature with an automated dielectric measurement system. The ferroelectric properties were measured using a Sawyer Tower circuit.
III. RESULT AND DISSCUSIONS
A. XRD, Grain size and density properties
The X-ray diffraction patterns of the ceramics are illustrated in Fig.1. The XRD results show that all the samples exhibit pure perovskite phase. At room temperature, all samples indicate the rhombohedral phase, observed by the single peak of (200) reflection at 2θ of 45o and the merging of (220)/(202) peaks at 2θ of 65 - 66o [3].
The grain size was measured from SEM micrograph of the ceramics. It can be seen that the grain size of ceramics decreases with increasing BZT content with a range of 13.30 -2.18 μm for samples of x = 0.00 - 0.10 as shown in Table 1.
Fig. 1.XRD patterns of the sintered (1-x)BCZT-xBZT ceramics.
This result indicates that BZT improved the microstructure of ceramics by interrupting the grain growth of BCZT. Moreover, the decreasing grain size was found to have an effect on the density of ceramics, as shown in Table 1. It can be seen that the density increases with increasing BZT content for 0.00BZT - 0.08BZT but decreases slightly for 0.10BZT. Possibly voids occur in microstructure for BCZT-BZT ceramics at 0.10 BZT condition.
TABLE 1. The grain size and density of (1-x)BCZT-xBZT ceramics.
Sample Grain size(μm) Density (g/cm3)
0.00BZT 13.30 5.62
0.02BZT 8.52 5.63
0.04BZT 6.39 5.70
0.05BZT 4.18 5.75
0.06BZT 3.93 5.76
0.08BZT 3.21 5.77
0.10BZT 2.18 5.74
B. Electrical properties
Dielectric constant and dielectric loss at room temperature as a function of frequency are displayed in Fig.2. The dielectric constant were in the range of 1000 - 6500 with no significant change with frequency. The highest dielectric constant at 1 kHz of 6500 was found at x = 0.02 (Fig. 2(b)). Dielectric loss for ceramics in the frequency range of 1-100 kHz showed no significant change with frequency but increased above 100 kHz, because the concentration of charge carriers is not constant [5]. Fig.3 illustrates dielectric properties as a function of temperature for (1-x)BCZT - xBZT ceramics. It can be seen that the dielectric constant and dielectric loss indicate a transition from normal ferroelectric to a relaxor behavior with increasing BZT content. This is observed from a broading in diffuse phase transition about Curie maxima (Tmax) and strong dispersion of dielectric constant and dielectric loss with frequency [6]. The dielectric
Fig. 2. Dielectric constant and dielectric loss as a function of frequency(a) and at a frequency of 1 kHz (b) for (1-x)BCZT-xBZT ceramics at room temperature. constant around Tmax decreases and peaks shift towards lower temperature with increasing BZT content. The loss tangent values were lower than 0.05 (at 1 kHz) in the phase transition position and then gradually decreased at higher temperatures with the loss values lower than 0.02 for all samples. It can be noted that addition of BZT in the structure of BCZT solution affects the dielectric properties. The dielectric loss at Tmax decreases with increasing BZT content. This result may be consistent with a decrease of grain size. The decrease in grain size is considered to be an important factor to change the balance of long range and short range forces which describe the relaxor state [7]
The ferroelectric properties of BCZT - BZT ceramics shown in Figs. 4 and 5 indicate the P-E hysteresis loops and the leakage currents. From Fig. 4 (a) and (b) it can be observed that the undoped BCZT sample shows a normal ferroelectric behavior due to strong remanent polarization. The loops became slimmer with slightly decreased remanent polarization with increasing BZT content and temperature (-40°C to 28°C). It should be noted that BCZT-BZT exhibits explicitly relaxor behavior [6-7]. Fig. 5 shows the I-V curves for the range of samples.The leakage current values tended to decrease with increasing BZT content.
The ac conductivity (σac) of BCZT - BZT was measured at room temperature and is shown in Fig. 6. The ac conductivity (σac) can be defined as following equation [8]: σac = ωεoεr tan δ (1)
116 2013 Joint UFFC, EFTF and PFM Symposium
Fig. 3. Dielectric constant and dielectric loss as a function of temperature and frequency for (1-x) BCZT-xBZT ceramics.
Fig. 4. The P-E hysteresis loops of (1-x)BCZT-xBZT ceramics, (a) -40 °C and (b) 28 °C.
Fig. 5. The leakage currents of (1-x)BCZT-xBZT ceramics.
Fig.6. The ac conductivity as a function of frequency for (1-x)BCZT-xBZT ceramics.
where ω is angular frequency (ω = 2πf), f is frequency, εo and εr are the permittivity of vacuum and dielectric constant and tan δ is dielectric loss, respectively. The ac conductivity shows great frequency dependence, which is observed from the straight-line slope of the σac with frequency. The σac values tend to increase with increasing frequency for all BCZT-BZT samples. It should be noted that the increase of σac corresponds to decreasing impedance. The dependence of ac conductivity with frequency is a mechanism in the low temperature region (a universal nature) which results from the short range hopping of charge carriers with small energy barriers [9]. This might be caused by defects or impurities in each ceramic system.
The samples of 0.02-0.10BZT content were selected for investigation of relaxor ferroelectric behavior because these samples showed explicitly frequency dependent of dielectric constant maximum (see in Fig.3).
To further understand the dielectric behavior of the ceramic systems, the permittivity of a first-order normal ferroelectric can be described by the Curie–Weiss law and a second-order relaxor ferroelectric can be described by a simple quadratic law.
Fig. 7 The plot of log [(εm/εr)-1] vs log (T−Tm) (K) for (1-x)BCZT-xBZT ceramics.
A modified Curie–Weiss law was proposed to describe the diffuseness of the ferroelectric phase transition as the relative dielectric constant can be derived via using the following equation [9];
117 2013 Joint UFFC, EFTF and PFM Symposium
(2)
where εm is the maximum value of the dielectric constant at T = Tm(f). The value of γ is the expression of the degree of dielectric relaxation, while δ is used to measure the degree of diffuseness of the phase transition. When γ = 1 is a normal Curie-Weiss behavior, whereas γ = 2 is identical to the quadratic relationship (a complete diffuse phase transition). The plots of log [(εm/εr)-1] vs log (T−Tm) (K) for BCZT-BZT samples shown in Fig. 7. The slope of the fitting curves using Eq. 2 is used determine the γ value and the δ value can be determined from the slope of εm/εr versus (T-Tm)2. The diffuseness γ values for 0.02BZT-0.10BZT samples showed in Fig. 7 and the δ values of samples were 8.90-271.97. It was found that the diffuseness γ and δ value depending on the composition and structure of the BCZT-BZT ceramics. These results may be the unbalance of charge with increasing BZT content which confirms that diffuse phase transition occurs in this system and relaxor ferroelectric behavior [7, 10]. The frequency dependence of the maxima temperature (Tm) of the dielectric constant was calculated by the following expression was used to calculate activation energies [11];
(3) where f0 is the pre-exponential factor, Ea is the activation energy for the relaxation, k is Boltzmann’s constant, f is the applied frequency and Tm is the temperature where the dielectric constant is the maximum.
The activation energy can be considered to be typical of a hopping process of localized charge carriers [11]. The activation energy decreases with increasing BZT content for x=0.02 - 0.10 (Ea = 7.25 - 0.78 eV) which is similar to work done by Wu et al. [12]. Thus, it should be noted that the charge carrier hopping between ions in the structure of BCZT-BZT ceramics decreases with increasing ion substitution and the decrease in grain size. Further, these activation energies help support the description of relaxor behavior of dielectric materials.
IV. CONCLUSIONS
In the present work, BCZT-BZT ceramics were
prepared by a solid state reaction. XRD patterns showed pure perovskite phase for all samples. The grain size of ceramics decreased with increasing BZT content. The electrical properties changed with the amount of BZT. The results show that the addition of BZT helped to promote the electrical properties, decreasing the dielectric loss and increasing resistivity. The samples show higher degree of the relaxor like behavior with higher BZT content.
ACKNOWLEDGMENT
The authors would like to thank the Thailand Research Fund (TRF) for financial support, including the support given through theRoyal Golden Jubilee Ph.D. Program, Office of the Higher Education Commission, Thailand, National Metal and Materials Technology center (MTEC), Faculty of Science Chiang Mai University, and Graduate School Chiang Mai University.
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