Sankararao Gattu¹, Venuturupalli Durga Prasadu², Kocharlakota Venkata Ramesh^2*
1Department of Physics, MVJ College of Engineering, Bangalore, India.
²Department of Physics, GITAM Institute of Technology, GITAM University, Visakhapatnam, India.
DOI: 10.4236/ampc.2015.510042   PDF    HTML   XML   4,426 Downloads   5,624 Views   Citations

Abstract

Barium titanate, BaTiO₃ (BTO) is the most common ferro electric material, which is used to manufacture electronic components such as multilayer capacitors, positive temperature coefficient thermistors, piezo electric transdures, and ferro electric memory. Zr doped barium magnesium titanate (Ba_0.9Mg_1.0)(Zr_xTi_1-x)O₃ (with x = 0.10, 0.20, 0.40 (BMZT 10, BMZT 20 and BMZT 40) perovskite is prepared by conventional solid state reaction method. The starting raw materials were BaCO₃, TiO₂, MgO and ZrO₂. The XRD study at room temperature suggests that these have cubic and tetragonal symmetry phases. The behavior of the measured dielectric permittivity and dielectric loss with temperature and frequency reveals that the materials undergo a diffuse para-ferroelectric phase transition and are of the relaxor type. The crystal structure, surface morphology and dielectric properties of Zr and Mg doped barium titanate ceramics were investigated. Zr⁴⁺ and Mg²⁺ ions have entered the unit cell maintaining the perovskite structure of solid solution without the evidence of any additional phase when Mg content is 0.1 mole% and the Zr content is 0.10, 0.20 and 0.40 mole%.

Keywords

Lead Free Ceramics, BaTiO₃, Dielectric Materials, Impedance, XRD, SEM

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1. Introduction

Lead based ceramic has been studied more than anyone else ferroelectric because of their excellent dielectric properties [1] . However, the presence of lead in those materials is about 60% in weight [2] , reconsidering its use in technical applications, due to its high toxicity of lead for the environment as well as for humans [3] - [7] . Barium titanate, BaTiO₃ (BTO) is the most common ferro electric material, which is used to manufacture electronic components such as multilayer capacitors, positive temperature coefficient thermistors, piezo electric transducers, and ferro electric memory, because of its excellent dielectric, piezo electric and ferro electric properties [8] - [12] . Moreover, a constant effort is being made to develop new dielectric oxides [13] - [15] . The micro level structure and dielectric properties of BTO can be modified by addition of the dopants such as La³⁺, Ce²⁺, Mn⁴⁺, Nb⁵⁺, Nd³⁺, Cr³⁺, Zr⁴⁺, Mg²⁺, Sr²⁺ and Si⁴⁺ to occupy Ba²⁺ on A sites or Ti⁴⁺ on B sites to form the solid solution. Numerous works have also been carried out to confirm the doping effects of rare earth oxides on the microstructure and electrical properties of BaTiO₃-MgO based system [16] - [32] . However, there are only few works concerned with the properties of the MgO singly doped barium titanate system. It is reported that when Ba is replaced by Mg in small quantities in the composition Ba(Zr_xTi_1−x)O₃ the dielectric properties have been changed. But the transition temperature has been shifted towards lower temperature values. S. K. Rout et al. [33] reported that the transition temperature decreased with increase of substitution of Mg upto <1.5 mole%. It has been reported that [34] with 15% Zr substitution in Ba(ZrTi)O₃ (BZT) it exhibited three transitions rhombohedra to orthorhombic, orthorhombic to tetragonal and tetragonal to cubic. At room temperature, the doped material exhibits enhanced dielectric constant with further increase in Zr content beyond 15%. Diffuse phase transition has been observed with the decrease in one transition temperature [35] and the material showed typical relaxor like behavior in the range 25 - 45 mole% Zr substitution [7] . Unexpectedly the lead free ceramic shows the relaxor properties at low temperatures [3] . Several attempts have been made by researchers on these materials to shift the T_c to close to room temperature. It is well known that homovalent and hetrovalent substitution for barium and titanium ions gives rise to various behaviors including the shifting of the transition temperature. This inspires to work on effect of Zr on structural and dielectrical properties of barium magnesium titanate ((Ba_0.9Mg_1.0)(Zr_xTi_1−x)O₃) perovskite composition prepared through solid state reaction route because in this method limited formation of side products, no solvents are needed in the reaction and hence no waste disposal issues associated with the solvent need to be considered and do not require extensive purification to remove traces of solvent and impurities. The samples synthesized through solid state reaction method may be used for obtaining in bulk form with high density over other methods.

2. Experimental

The perovskite samples of Zr doped Barium Magnesium Titanate (Ba_0.9Mg_1.0)(Zr_xTi_1−x)O₃ (with x = 0.10, 0.20, 0.40) (BMZT 10, BMZT 20 and BMZT 40) were prepared by conventional solid state reaction method. The starting raw materials were BaCO₃ (Chen Chems., Chennai), TiO₂ (Loba Chem., Mumbai), MgO (Chen Chems., Chennai) and ZrO₂ (Loba Chem., Mumbai). All the powders were having more than 99% purity. The powders were taken in a suitable stachiometry for 20 gm of samples. The powders were thoroughly mixed in an agate mortar in dry and wet mixing with appropriate amount of acetone for 6 hr. After proper mixing, mixed powders were calcinated at 1300˚C for 2 hr, 1400˚C for 2 hr and 1500˚C for 4 hr. A small amount polyvinyl alcohol was added to the calcinated powder for fabrication of pellets, which was burnt out during high temperature sintering. The circular disc shaped pellets were prepared by applying a uniaxial pressure of. The pellets were subsequently sintered at an optimized temperature of 1550˚C for 5 hr. A preliminary study on compound formation and structural parameters was carried out using an X-ray diffraction (XRD) technique with an X-ray powder diffractometer. The XRD pattern of the calcinated powder was recorded at room temperature using PANAlytical X’pert pro with CuK_a radiation (1.5405 Å) in a wide range of Bragg’s angles. Micro structures of sintered pellets were recorded by scanning electron microscope (SEM) (JEOM JSM-6380 LA). The pellets were then electrode with high purity air-drying silver paste and then dried at 500˚C for 1 hr. Dielectric measurement analysis was done using Agilent E4980A Precision LCR meter with temperature (150 - 573 K) and frequency (20 Hz - 200 KHz).

3. Results and Discussion

3.1. Structural Analysis

Figure 1 shows the XRD pattern of the Zr doped BMZT (0.1, 0.2, 0.4) samples. The XRD analysis provides that

the samples are having single perovskite structure. BaTiO₃ (BTO) has the tetragonal structure at room temperature. The ionic radii of Ba²⁺ and Ti⁴⁺ are 1.35 Å and 0.605 Å respectively. If we doped BTO with Mg²⁺ and Zr⁴⁺ whose ionic radii are both 0.72 Å Mg occupies A site and Zr occupies B site of BTO. The pure BMZT single phased tertagoganl structure when the Mg content is <1.5% at-% (9) and Zr is 0.1%, if Zr content is <0.42% at-% (10) the sample is changes into the cubic structure. By doping with Zr the diffraction angles are shifted towards the lower angle side indicating the increase in lattice parameters due to the incorporation of smaller content of Zr in place of Ba. In Figure 1 BMZT 10 sample possesses the tetragonal structure and BMZT 20 and BMZT 40 samples posses the cubic structure.

3.2. Microstructural Analysis

Figure 2 shows The SEM micrographs BMZT 10, BMZT 20 and BMZT 40 samples. It is found that the average grain size of samples are ~1.00, ~1.10 and ~1.66 µm increased as the Zr content increases from 10% to 40%. This increase is in agreement with our XRD pattern. Moreover the surface observation shows a good density of grains with some porosity.

3.3. Dielectric Properties

3.3.1. Temperature Dependence Dielectric Properties

Figure 3 shows the temperature dependence of the dielectric constant and loss of Zr doped BMZT samples measured at 1 MHz. The figure shows, the value of dielectric constant increases gradually to a maximum value () with increase in temperature up to transition temperature and then decreases indicating a phase transition. It is also found that the Curie temperature T_c of BMZT samples with Zr dopant of (0.10, 0.20, 0.40) corresponding to the maximum dielectric constant is 373, 323 and 180 respectively. The results indicates that the curie temperature of BMZT decreased may be due to Ti ions replaced by Zr ions and Zr ionic radius is little more, it can increase the grain size and exactly not joining the Zr atoms in Ti sites, due to the Zr ions conducts the little current then the dielectric constant and curie temperature both may be decreased. According to Figures 3(a)-(c) the peak value of the dielectric constant of BMZT samples with the Zr dopant of (0.10, 0.20, 0.40) is 1406, 1040 and 563 respectively. The result indicates that the peak value of dielectric constant for low doped sample is the maximum and the peak value decreases with Zr content.

In Figure 3(c) shows that the dielectric loss initially increases with temperature reaches maximum. Further increase in temperature loss is decreased but for BMZT samples of (0.10, 0.20) it is at lower temperature little bit high value of loss due to the presence of all types of polarisation and may be due to the contribution of finite resistivity of the materials. Further increase in temperature loss decreases minimum and further increasing temperature loss also increased.

3.3.2. Frequency Dependence Dielectric Properties

As shown in Figure 4(a) it is found that the dielectric constant of BMZT (0.10, 0.20, 0.40) decreased rapidly at low frequencies. At very high frequencies dielectric constant is very low and it maintains constant value. It may be due to there must be defects with opposite charges (dipoles) to preserve charge neutrality. Theses dipoles could be oriented to align the direction of the applied electric field. When the frequency increases, the dipoles do not catch up with the change of the electric field to complete polarisation so that the dielectric constant decreases.

In the Figure 4(b) the dielectric losses were a combined result of electrical conduction and orientational polarisation of the matter. The energy losses, which occur in dielectrics due to dc conductivity and dipole relaxation. The loss factor of a dielectric material is a useful indicator of the energy loss as heat.

4. Conclusion

Perovskite types (Ba_0.9Mg_1.0)(Zr_xTi_1−x)O₃ (with x = 0.10, 0.20, 0.40) ceramics have prepared through solid state reaction route. The XRD study at room temperature suggests that the composition of BMZT 10 has single phase

tetragonal and BMZT 20, BMZT 40 cubic symmetry with space group pm-3m. The dielectric study reveals that the materials undergo BMZT 20, BMZT 40 a diffuse type ferroelectric phase transition. The transition temperature decreased with Zr content and the maximum dielectric constant also decreased with Zr content.

NOTES

^*Corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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