Department of Materials Science, Addis Ababa University, Addis Ababa, Ethiopia
Received Date: March 28, 2012; Accepted Date: April 18, 2012; Published Date: April 25, 2012
Citation: Tilak B (2012) Low- Frequency Dielectric Dispersion and Impedance Spectroscopy of Lead-Free Na0.5 Bi0.5 Tio3 (Nbt) based Ferroelectric Ceramic. J Material Sci Eng 1:108. doi: 10.4172/2169-0022.1000108
Copyright: © 2012 Tilak B. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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(Na0.5Bi0.5)0.89Ba0.11 Zr0.04Ti0.96O3 (0.11BNBZT) ceramic were prepared by using solid-state reaction method. A single phase perovskite (ABO3) structure with tetragonal symmetry was confirmed by X-ray diffraction. The dielectric behavior, impedance relaxation were investigated in a wide range of temperature (30°C-600°C) and frequencies (45Hz-5MHz). The grain morphologies were analyzed by using a Scanning Electron Microscopy (SEM) analysis. The temperature dependence of dielectric behavior and the frequency dependence of impedance relaxation were investigated. A broaden dielectric constant peak were observed over a wide temperature range and also indicate a relaxor behavior. The complex impedance plot exhibited semicircle plot, which is explained by the grain effect of the bulk. The center of the impedance semicircles lie below the real axis, which indicates that the impedance response is a Cole-Cole type relaxation.
Perovsktie structure; Ferroelectric; Dielectric; Relaxor; Impedance
Lead–containing ferroelectric materials such as Pb(Zr, Ti)O3 (PZT) and PZT- based [1,2] multi component ceramics have been widely used as sensors, actuators and ultrasonic transducers in various electronic devices because of their excellent piezoelectric properties. However, PbO vaporization problems during the sintering process and treatment of Pb containing products cause crucial environment pollution. Recently, much more attention has been paid to the investigation of lead free piezoelectric materials for the replacement of the lead containing ferroelectrics .
As such, lead free ceramics have attracted considerable attention. The Na0.5Bi0.5 TiO3 (NBT) and NBT based system with ABO3 perovskite structure is one of important lead-free based materials [4-7]. NBT has a high Curie temperature (Tc=320°C) and possesses strong ferroelectricity, exhibiting a relatively large polarization (Pr=38 μC/cm2) and high coercive field (Ec=73 kV/cm) at room temperature. Although, NBT exhibits a large coercive field and relatively large conductivity, it is hard to be poled for practical applications. NBT had two phase transitions; it has anti ferroelectric phase below 200°C and paraelectric phase at 320°C. Recently, the effect of doping in NBT [5- 7] and several solid solutions to improve the properties of NBT have been investigated. It has been reported that NBT –based ceramics with their compositions modified with the addition of BaTiO3 , BiFeO3, NaNbO3, CeO2, MnO2 . Among them (Bi 0.5 Na0.5)1-xBaxTiO3 (BNBT) ceramics are more interesting because of the existence of rhombohedral, tetragonal structure, Morphotropic Phase Boundary (MPB), MPB for the BNBT composition lies between 6% and 7% BaTiO3.
A necessary condition for fabrication of high performing materials is knowledge of their physical properties, but the dielectric dispersions and the impedance relaxation of the NBT based systems are not fully studied. The contribution of the grains and the grain boundaries are evaluated by using complex impedance. Complex impedance spectroscopy is a good technique to investigate the microstructure and the electrical properties.
In the present work, ZrO2 has been added to (Bi 0.5 Na0.5) 1-x Bax TiO3 (BNBT) i.e., (Na0.5 Bi0.5)0.89Ba0.11 Zr0.04Ti0.96O3 ceramic has been prepared and investigated in the dielectric and impedance properties over a wide frequency range from 45 Hz to 5 MHz and temperature range from 30°C -600°C.
The conventional mixed oxide technique was used to prepare (Na0.5 Bi0.5)0.89Ba0.11 Zr0.04Ti0.96O3 (0.11BNBZT) ceramic. The commercially available chemicals of Bi2O3, Na2 O3, Ba CO3, ZrO2 and TiO2 with purity higher than 99.9% were used as starting materials. These oxides and carbonates were mixed according to the desired chemical formula and were grounded for 10 h and calcined at 850°C/2h. After calcination, the ground powder was pressed into disks with 12 mm diameter and about 2 mm in thickness. The disk was sintered at 1180°C/3hrs in air. The density of the sintered sample was measured using the Archimedes principle. The phase structure was examined using x-ray powder diffraction analysis using a Cu Kα radiation. For the observation of the microstructure samples were polished and thermally etched. Finally, the microstructures were observed by scanning electron microscopy (SEM: JEOL_JY: Model 5800F).
For electrical measurements, silver paste was applied on both surfaces of the disk and thermally etched at 650°C. The capacitance of the ceramic used for calculating dielectric constant was measured as a function of temperature, complex impedance by using computer interfaced LCR Hi-Tester (HIOKI 3532-50, japan).
Figure 1 shows an x-ray diffraction pattern of 0.11BNBZT ceramic, exhibits pure perovskite structure with tetragonal symmetry feature with (111) peak around 40° of 2θ. As crystal structure transformed from rhombohedral into a tetragonal symmetry there is splitting of (200) and (002) peak around 46.5° of 2θ, which has been observed in XRD pattern (Figure 1). Lattice parameters have been calculated by using POWD software. Lattice parameters, cell volume, experimental and theoretical density, porosity has been tabulated in Table 1.
|Composition||Lattice parameters (Å)||c/a||V(Å3)||Density(gm/Cm3)||Porosity|
The scanning electron micrograph of the sintered ceramic (011BNBZT) is shown in Figure 2, indicating that the ceramic is highly dense with less porosity. Based on the measurement of Archimedes principle, the experimental density of ceramic is 95% of theoretical density. The average grain size has been calculated by using line intercept method. The average grain size of the studied ceramic is1.21 μm.
Figure 3; depict the temperature dependence of real part of dielectric constant of 0.11BNBZT at various frequencies, indicating a relaxor like behavior . At low frequencies, a phase transition from ferroelectric to anti ferroelectric is observed at around 100°C which is referred as depolarization temperature. Such type of transition was also reported in BNT-NNT-BT , BNT- PbTiO3  and BNT-BKT-BLT  ceramics. But, with an increase of frequency, the ceramics show pure ferroelectric behaviors up to their transition temperatures. Such as, phenomenon might be probably due to cation disorder by random distribution of Na+, Bi+ and Ba2+ at the A-sites of the lattice [17-19]. There is an existence of three phases of ferroelectric, anti ferroelectric and paraelectric in different temperature ranges.
The dielectric constant has frequency dependence especially, at the low frequencies, which is called the low frequency dielectric dispersion. A strong low-frequency dielectric dispersion has also been observed in NBT which was previously reported . At temperature above Tm and at low frequencies, the dispersion is increased, which is caused by an increase in the electrical conductivity. The dielectric peak appears to broaden with respect to increase in frequency which may be attributed to the structural disorder in the system. The real part of the dielectric constant, describes the “elastic” loss-free reaction of the material to an externally applied ac electric field.
The electrical properties of the materials have been investigated using Complex Impedance Spectroscopy (CIS). It is an important tool to analyze the electrical properties of a polycrystalline material in view of its capability of correlating the sample electrical behavior to its microstructure .
Figure 4 represents the real part of impedance (Z1) as function of frequency. Z1 has higher values at lower frequency and decreases up to 100 kHz and attains a constant value beyond that. The pattern shows the sigmoidal variation as a function of frequency in the low frequency region followed by a saturation region in the frequency region. This suggests the presence of mixed nature of polarization behavior in the material.
Figure 5 presents the variation of imaginary part of impedance (Z11) as a function of frequency at different set of temperatures. With the increase of frequency, the real part of impedance (Z1) and imaginary part of impedance (Z11) decreases with increase in frequency. At higher frequency side all the curves merge. As the temperature increases the peak has been observed in (Z11) vs frequency curve . The peak shifts towards higher frequency side with increasing temperature showing that the resistance of the bulk material is decreasing. Also, the magnitude of Z11 decreases with increasing frequency. This would imply that dielectric relaxation is temperature dependent and there is apparently not a single relaxation time. It is evident that with increasing temperature, there is broadening of the peaks and at higher temperatures, the curves appear almost flat .
The diffusiveness of Z11 peaks indicate the distribution of relaxation times and the increase in full width at half of the Z11 maxima with the increase of temperature indicate the increase in distribution of relaxation frequency. This distribution of relaxation frequency is attributed to the cationic disorder due to the random occupations of A and B-sites of cations with different ionic radius and valence states. The random distribution causes multiple relaxations that are shown by broadening of Z11 peaks. The appearance of peak in Z11 vs frequency plots indicate some sort of relaxation related to oxygen vacancies present in the studied sample. It has been known that the oxygen vacancies are the main cause of domain wall clamping. This clamping will restrain the macro-micro domain switching in some degree, which in turn will affect the polarization in the material. Increase in oxygen vacancies will increase the domain wall clamping and decreases the polarization state.
Figure 6 shows the frequency dependences of the real part (Z1) and the imaginary part (Z11) of the complex impedance at a temperature 530°C. With increasing frequency, the real part Z1 decreases while the imaginary parts Z11 shows a peak. Then, the complex impedance plot of studied material exhibits one impedance semicircle arc. Thus, one impedance arc of the studied material is representative of the bulk properties of the grains (dc resistivity). The centers of the impedance semicircular arcs lie below the real axis. Thus, the impedance relaxation can be explained by using Cole-Cole response [24, 25]. The electrical contribution of the grains was introduced by using equivalent circuit as shown in Figure 7 and Figure 8 shows the temperature dependent spectra (Nyquist plot) of 0.11BNBZT material. The impedance spectrum is featured by semicircle arcs. The nature of variation of the arcs with temperature and frequency provides various clues of the materials. The impedance spectra are characterized by the appearance of a two semicircle arcs. The presence of single semi circular arc indicated that the electrical process contribution as form a bulk materials (grain interior), which can be modeled as an equivalent circuit comprising of a parallel combination of bulk resistance (Rb) bulk capacitance (Cb) and leaky capacitance (Qb). Appearance of another semicircular arc near above 450°C indicates the beginning of the intergranular activities (grain boundary effect) within the sample with definite contribution from both bulk and grain boundary effects (Figure 8). The intercept of the semicircular arc with real axis (Z1) gives us an estimate of the bulk resistance (Rb) and grain boundary resistance (Rgb) of the material. It has been observed that the bulk resistance of the materials decreases with increase in temperature showing a typical semiconducting property, i.e., Negative Temperature Coefficient of Resistance (NTCR) behavior . It can be noticed that the center of the semicircle arc lies below the real axis suggesting the relaxation to be of poly dispersive Non- Debye type in samples. This may be due to the presence of distributed elements in the materials electrode system.
The solid solution of (Na0.5 Bi0.5)0.89Ba0.11 Zr0.04Ti0.96O3 (0.11BNBZT) was prepared using high temperature solid state reaction technique. X-ray diffraction studies reveal perovskite structure with tetragonal symmetry. SEM analysis explained the grain morphology on the surface of the material. The temperature dependence of dielectric constant shows relaxor behavior, leading to conclusions that three phases of ferroelectric, anti ferroelectric, and paraelectric states exist in different temperature ranges. The grain and grain boundary contribution have been separated using impedance spectroscopy analysis equivalent circuit to explain electrical phenomena occurring inside the material which revealed the Non-Debye type relaxation present in the material.
The authors thank to Naval Science and Technology Laboratory (NSTL), Govt of India, Visakhapatnam, for funding the research project and University Grants Commission, Govt of India for Research fellowship.
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