29-03-2014, 03:37 PM
The structure and complex impedance spectroscopy of Sr1-xCaxBi4Ti4O15 ( x=0, 0.2, 0.4, 0.6, 0.8) ceramics
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ABSTRACT
In the present work, we have synthesized polycrystalline Sr1-xCaxBi4Ti4O15 [x=0, 0.2, 0.4, 0.6, 0.8] ceramics by solid state reactive technique. These ceramics were structurally characterized by X-ray diffraction (XRD) analysis and scanning electron micrographs analysis. The analysis indicates that the ceramics present an orthorhombic structure with grains exhibit a plate like morphology. Dielectric relaxations of the above compositions were investigated in the temperature range 100–6000C. Using the Cole–Cole plots, an analysis of the dielectric loss with frequency was performed, assuming a distribution of relaxation time. The presence of the peaks in temperature dependent dielectric loss indicates that the hopping of charge carriers is responsible for the relaxation. Impedance studies indicate a non-Debye type relaxation, and with increase in temperature relaxation frequency shift to higher side. A notable shift in impedance loss peaks towards higher frequency side explains the conduction in material. The Nyquist curve shows overlapping semicircles, for grain and grain boundary for all the compositions. The frequency dependent ac conductivity at different temperatures indicates that the conduction process is thermally activated process and the spectra follow the universal power law. The conductivity is frequency independent at low temperatures. The variation of dc conductivity confirms that the samples Sr1-xCaxBi4Ti4O15 [x=0: SBT, 0.2: SCBT02, 0.4:SCBT04, 0.6:SCBT06, 0.8:SCBT08] exhibits negative temperature coefficient of resistance behavior at high temperatures.
INTRODUCTION
Bismuth layer-structured ferroelectrics are excellent candidate materials for piezoelectric and pyroelectric sensors requiring a high stability and high operation temperature. This is owing to their high Curie temperatures, low ageing rate and strong anisotropic electromechanical coupling factors. Due to their promising fatigue-free nature, bismuth layer structured ferroelectrics (BLSFs) have attracted considerable attention [1]. The BLSFs have a crystal structure containing interleaved bismuth oxide (Bi2O2)2+ layers and pseudo perovskite blocks which contain BO6 octahedron and generally formulated as (Bi2O2)2+ (An-1BnO3n+1)2−. In this notation A represents a mono-, bi- or trivalent ion, B denotes a tetra, penta or hexavalent ion and n is the number of BO6 octahedron in each pseudoperovskite block (n = 1, 2, 3 . . . ) [2]. Generally, the displacive ferroelectrics with a higher TC have a large remnant polarization. Since the TC of SrBi4Ti4O15 (5200C) [3] is higher than that of SrBi2Ta2O9 (3350C), a high remnant polarization can be expected. Substitution of calcium in strontium sites should further increase the TC. The compound is tetragonal with slight orthorhombic distortion. The a- and b- axes lie along (110) c where the suffix denotes the cubic perovskite sub cell so that a≈b≈2ac≈ 0.54 nm. The c- axis is inherently long. These layered perovskite compounds are ferroelectric with high Curie temperatures [4]. The ionic polarization makes an important contribution to the permanent dipole in the ferroelectric state. The structural consequence of this polarization is to distort the octahedral coordination, and thereby to lower the crystal symmetry. Chen Da Ren and Guo Yan Yi [5] reported piezoelectric properties of a number of compounds by replacing the cations having similar structural configurations. According to them the Bi3+ ion has an important effect on the bond strength of Ti4+-O2− (along a- axis) in the compound SrBi4Ti4O15. Different methods of preparation of the compounds have been suggested viz. hot forging, hot rolling, hot extrusion and super plastic deformation. These methods give ceramics with grain orientations and different densities [6]. Ferroelectric and dielectric properties of SCBT02, SCT04, SCBT06 and SCBT08 are investigated. Complex impedance spectroscopy (CIS) is a flexible tool for simultaneous electrical and dielectric characterization of materials.
SAMPLE PREPARATION
The initial compounds SrCO3, Bi2O3, CaCO3, TiO2 (Sigma Aldrich, 99.9% pure, AR grade) were mixed in appropriate ratios for the synthesis of the desired compound. The polycrystalline samples of Sr1-xCaxBi4Ti4O15 [x=0: SBT, 0.2: SCBT02, 0.4:SCBT04, 0.6:SCBT06, 0.8:SCBT08] were prepared by the method of reactive sintering. The standard ceramic fabrication procedure has followed for the preparation. The oxide mixture, weighing 50 g, was subjected to high energy milling in a planetary ball mill (Fritsch Pulverisette 6) at a speed of 150 rpm for 5 hr. A cylindrical tungsten carbide (WC) milling jar, with a usable volume of 250 ml, filled with 50 WC balls, each of 10 mm diameters, was used for the purpose of milling. Both vial and balls were from Fritsch GmbH. A total of 50 g of powder was placed in the vial using water as the milling medium. The milling was stopped for 5 min after every 30 min of milling to cool down the system. During each collision the powder particles get trapped between the colliding balls, between the ball and the inner surface of the vial and lead to smaller particle size. The particle size of the initial materials ranged from 1–2 μm. The mixture was stacked in a crucible and calcined in air at 800◦C for 4 hr and then cooled. The calcined powders were milled for a few minutes to crush any lumps that were formed during the calcinations. The calcined powders were cold isostatically compressed into the form of a cylindrical rod at a pressure of 3000 Mpa and sintered at 1200◦C for 2 hr and then allowed to furnace cool. Pellets having a diameter of 1 cm and thickness of 1 mm were cut from the sintered samples using a low speed isomet saw. The entire sintering was done in a microprocessor-controlled furnace. The densified pellets were used for all measurements.
CHARACTERIZATION AND MEASUREMENTS
The density of the pellets was determined by Archimedes principle. The crystal structure of the compounds was examined by X-ray diffraction (XRD) using a Guinier-hagg camera with Cu Kα1 radiation. The micro-characterization of the ceramics was observed by scanning electron microscopy (SEM). For electrical measurements, samples were polished and coated with Au electrodes on the larger faces. Typical sample dimensions were 10 mm diameter and 1 mm thickness. In order to make electrical measurements, it is necessary to make the samples ‘piezoelectrically active’. Therefore, electrical poling was done at a field of 5 kV cm−1 for 30 min at 100 0C immersing the samples in a silicon oil bath (Dow Corning 704 R). The field was retained while cooling as well. The impedance measurements were made on electrically poled samples with an Impedance Analyzer (HP4192A) at different frequencies from room temperature to 6000C. The dielectric impedance data was collected at an interval of 250C while heating at a rate of 1 0C/min. These measurements were carried out on silver coated pellets by placing them in between two electrodes, which in turn were connected to the leads of the impedance analyzer.
RESULTS AND DISCUSSION
Figure 1 shows the XRD patterns of the solid solution Sr1-xCaxBi4Ti4O15 [x=0: SBT, 0.2: SCBT02, 0.4:SCBT04, 0.6:SCBT06, 0.8:SCBT08] at room temperature. The results indicate that the crystal symmetry is orthorhombic, the same as pure CBT and SBT. XRD diffraction confirms that the sample is a single phase of the bismuth- oxide-layer-type structure with m = 4 and has no certain grain orientation. The SEM micrographs for the compositions SBT, SCBT02, SCBT06 and SCBT08 are shown in Figure 2. The sheet grains are found in the com- pounds, similar to the typical morphology of BLSF ceramics [5]. Thus the strong anisotropic character is demonstrated.