Influences of phase transition and microstructure on dielectric properties of Bi0.5Na0.5Zr1-xTixO3 ceramics
© Jaiban et al; licensee Springer. 2012
Received: 7 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
Bismuth sodium zirconate titanate ceramics with the formula Bi0.5Na0.5Zr1-xTixO3 [BNZT], where x = 0.3, 0.4, 0.5, and 0.6, were prepared by a conventional solid-state sintering method. Phase identification was investigated using an X-ray diffraction technique. All compositions exhibited complete solubility of Ti4+ at the Zr4+ site. Both a decrease of unit cell size and phase transition from an orthorhombic Zr-rich composition to a rhombohedral crystal structure in a Ti-rich composition were observed as a result of Ti4+ substitution. These changes caused dielectric properties of BNZT ceramics to enhance. Microstructural observation carried out employing SEM showed that average grain size decreased when addition of Ti increased. Grain size difference of BNZT above 0.4 mole fraction of Ti4+ displayed a significant increase of dielectric constant at room temperature.
Nowadays, materials possessing a diffuse phase transition at high temperature are of interest because they are believed to be a promising candidate for various electronic devices. Examples are multilayer capacitors, detectors, MEMs, sensors, actuators, etc. However, high permittivity at room temperature is also significant. Recently, Lily et al.  have successfully fabricated and investigated a novel perovskite-type ceramic of Bi0.5Na0.5ZrO3 [BNZ] compound. They reported that the mentioned ceramic had an orthorhombic structure and a high curie temperature of 425°C. This value is rather high when compared with well-known lead-free ceramics such as BaTiO3 (130°C)  and Bi0.5Na0.5TiO3 [BNT] (320°C) . Unfortunately, the BNZ system showed low dielectric constant at room temperature, i.e., approximately 100, 60, and 25 at a frequency of 1, 10, and 100 kHz, respectively.
According to the most investigated PbTiO3-PbZrO3 [PZT] solid solution system, it was known that the dielectric constant of orthorhombic PbZrO3 compound was quite low (i.e., approximately 190) , but the value could be enhanced to range about 400 to 800 with partial substitution of Ti4+ ions at the Zr4+ site within the perovskite lattice . Improvement of the permittivity was attributed to the transformation of orthorhombic crystal structure to rhombohedral and tetragonal lattices. In this phase transformation, the Zr/Ti ratio was the main factor that specified the crystal structure of PZT ceramics.
For a similar system of BNT-BNZ, Yamada et al.  predicted only that the phase-transition point of the phase diagram seemed to be approximately at a Zr/Ti ratio of 0.6:0.4. In addition, a study concerning Bi0.5Na0.5Zr1-xTixO3 [BNZT] ceramic from a Zr-rich composition has not been reported. Hence, the purpose of this work is to investigate influences of the occupancy of Ti4+ ions at the B-site of Zr4+ host ions with Zr/Ti ratios of 0.7:0.3, 0.6:0.4, 0.5:0.5, and 0.4:0.6 on phase transition and dielectric properties at room temperature of the orthorhombic BNZ ceramic.
The specimen was fabricated according to the chemical formula Bi0.5Na0.5Zr1-xTixO3, where x = 0.3, 0.4, 0.5, and 0.6. The powders were prepared by a conventional mixed-oxide method. The starting materials used in this study were ZrO2 (99%, Riedel-de Haën, Sigma-Aldrich Corporation, St. Louis, MO, USA), TiO2 (99%, Riedel-de Haën), Bi2O3 (98%, Fluka, Sigma-Aldrich Corporation, St. Louis, MO, USA), and Na2CO3 (99.5%, Riedel-de Haën). The mixtures of oxides were ball milled in ethanol for 24 h. The mixed powders were dried at 120°C for 24 h and then calcined in a closed alumina crucible at a temperature of 800°C for 2 h with a heating/cooling rate of 5°C/min. After sieving, a few drops of 3 wt.% polyvinyl alcohol binders were added to the mixed powders which were subsequently pressed into pellets with a diameter of 10 mm using a uniaxial press with 1-ton weight for 15 s. Binder removal was carried out by heating the pellets at 500°C for 1 h. These pellets were then sintered at 950°C for a 2-h dwell time with a heating/cooling rate of 5°C/min on a covered alumina plate.
The sintered samples were polished using sandpaper and cleaned using an ultrasonic bath. After that, phase identification of ceramics was investigated in a 2θ range of 20° to 80° using an X-ray diffractometer [XRD] (Phillip Model X-pert, PANalytical B.V., Almelo, The Netherlands). For a microstructural observation, the sintered pellets were polished using sandpaper as well as alumina slurry and cleaned in the same ultrasonic bath. Then, the polished samples were etched at a temperature of 800°C for 15 min with a heating/cooling rate of 5°C/min on a covered alumina plate. Microstructure of etched materials was observed using a backscattered-electron mode of a scanning electron microscope [SEM] (JSM 6335F, JEOL Ltd., Akishima, Tokyo, Japan).
Numerical detail of the lattice parameters of all samples was obtained from fitting between observed reflection angles of experimental XRD patterns and calculated angles using the Powder Cell Software (BAM, Berlin, Germany) . Measurement of grain size was performed by employing a linear intercept method on SEM images. For dielectric property measurements, the sintered samples were polished by sandpaper until the thickness was approximately 1 μm. Subsequently, two parallel surfaces of polished ceramics were painted with a silver paste for electrical contacts. Dielectric constant and loss were measured at room temperature with measured frequencies of 1, 10, and 100 kHz using a 4284A LCR meter (Agilent Technologies Inc., Santa Clara, CA, USA).
Results and discussion
Lattice parameters and grain size of BNZT ceramics
Grain size (μm)
a = 5.6893 Å
b = 8.0434 Å
c = 5.6553 Å
α = 90°
5.65 ± 1.63
a = 3.9875 Å; α = 89.9247°
5.55 ± 1.84
a = 3.9835 Å; α = 89.8975°
5.07 ± 1.57
a = 3.9602 Å; α = 89.8713°
3.76 ± 1.24
SEM-BEI images of Bi0.5Na0.5Zr1-xTixO3 ceramics, where x = 0.3, 0.4, 0.5, and 0.6, are shown in Figure 2. All compositions produced similarly shaped crystalline grains. The images also showed that the average size of grains decreased slightly with an increase of the Ti content up to 0.5 mole fraction and decreased sharply for the Bi0.5Na0.5Zr0.4Ti0.6O3 specimen. The mentioned analysis suggested that Ti addition also affected the microstructure of BNZT materials. Furthermore, in Figure 2a, a weak trace of secondary phases was observed for the sintered specimen with the Bi0.5Na0.5Zr0.7Ti0.3O3 composition. EDX analysis of the light-gray secondary phase was not performed since its volume was too small for the analysis to be reliable. However, in a dark-gray area, the phase was found to be ZrO2. It was expected that evaporation of Na and Bi might occur which often resulted in a formation of a second phase and compositional inhomogeneity. Similarly, several investigations also found the mentioned loss leading to small existence of the second phase [9, 10]. Nevertheless, the amount of the second phase was very low when compared with the matrix phase and therefore could not be detected by the XRD technique.
Dielectric constant and loss of the BNZT and BNZ ceramics
ε r a
ε r b
ε r c
Lily et al.
In this research, BNZT ceramics with Zr/Ti ratios of 0.7:0.3, 0.6:0.4, 0.5:0.5, and 0.4:0.6 were successfully fabricated using a conventional solid-state sintering method. XRD analysis revealed a complete solubility of Ti4+ ions into the B-site of Zr4+ ions for all compositions investigated. Consequently, smaller ions of Ti4+ replacing the host site of Zr4+ ions caused the typical cell volume of BNZ to decrease and produced transformation of an orthorhombic to a rhombohedral lattice above Zr/Ti ratios of 0.6:0.4. As a result, the dielectric constant was enhanced with increasing Ti concentration. Besides, among the BNZT samples possessing a rhombohedral structure, a decrease of average grain size also partly contributed to an increase in the relative permittivity value. In the case of the dissipation factor, the result showed a similar trend to that of the dielectric constant.
This work is financially supported by the Thailand Research Fund (TRF) and the National Research University Project under Thailand's Office of the Higher Education Commission (OHEC). The Faculty of Science and the Graduate School of Chiang Mai University is also acknowledged. P. Jaiban would like to acknowledge the financial support from the TRF through the Royal Golden Jubilee Ph.D. Program.
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