Crystal structure and electrical properties of bismuth sodium titanate zirconate ceramics
© Rachakom et al; licensee Springer. 2012
Received: 8 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
Lead-free bismuth sodium titanate zirconate (Bi0.5Na0.5Ti1-xZrxO3 where x = 0.20, 0.35, 0.40, 0.45, 0.60, and 0.80 mole fraction) [BNTZ] ceramics were successfully prepared using the conventional mixed-oxide method. The samples were sintered for 2 h at temperatures lower than 1,000°C. The density of the BNTZ samples was at least 95% of the theoretical values. The scanning electron microscopy micrographs showed that small grains were embedded between large grains, causing a relatively wide grain size distribution. The density and grain size increased with increasing Zr concentration. A peak shift in X-ray diffraction patterns as well as the disappearance of several hkl reflections indicated some significant crystal-structure changes in these materials. Preliminary crystal-structure analysis indicated the existence of phase transition from a rhombohedral to an orthorhombic structure. The dielectric and ferroelectric properties were also found to correlate well with the observed phase transition.
Lead-based PbTiO3-PbZrO3 solid solutions have dominated the market of actuator and sensor materials due to their excellent ferroelectric and piezoelectric properties. In particular, a compositional ratio of Zr/Ti of around 52/48 showed the morphotropic phase boundary between a tetragonal and a rhombohedral phase, where enhanced polarizability and optimum domain orientation were observed [1–6]. However, PbO loss during high-temperature processes is considered to be environmental pollution with additional problems of recycling and waste disposal. Therefore, researchers have attempted to develop new lead-free smart materials in order to replace the lead-based ones . BaTiO3 is one example of the most commonly used lead-free material for capacitors and actuators due to its inherent ferroelectric nature. However, its main disadvantage is the narrow working temperature; therefore, the use of a BaTiO3-BaZrO3 solid solution with the addition of Zr up to 30% mole was investigated [8–10]. The materials were found to exhibit a composition-induced phase transition from normal to relaxor ferroelectric with a higher dielectric constant than both PZT and BaTiO3. This allowed the materials to be used over a broader temperature range. Following these studies, this paper was aimed to study Bi0.5Na0.5TiO3-Bi0.5Na0.5ZrO3 solid solutions with the addition of a Zr concentration from 0.20, 0.35, 0.40, 0.45, 0.60, and 0.80 mole fraction. The relationship between the phase, crystal structure, and electrical properties is investigated and discussed.
Bi0.5Na0.5Ti1-xZrxO3 compositions were prepared using the mixed-oxide method incorporating Bi2O3 (> 98%, Fluka, Sigma-Aldrich Corporation, St. Louis, MO, USA), Na2CO3 (99.5%, Carlo Erba Reagenti SpA, Rodano, Italy), TiO2 (> 99%, Riedel de Haën, Sigma-Aldrich Corporation, St. Louis, MO, USA), and ZrO2 (> 99%, Riedel de Haën) in stoichiometric proportions. The mixed powders were ball milled in ethanol for 24 h using zirconia milling media and calcined at 800°C for 2 h. The calcined Bi0.5Na0.5Ti1-xZrxO3 powders were then ball milled again for 6 h and uniaxially pressed at a pressure of 5.5 MPa with a few drops of 3 wt.% polyvinyl alcohol to bind it into disks of 10-mm diameter and 1- to 1.5-mm thickness. The disks were the sintered at 900°C for 2 h, except for the sample with 0.20 mole fraction Zr which was sintered at 950°C for 2 h, in air. The X-ray diffractometer (Philip Model X-pert, PANalytical B.V., Almelo, The Netherlands) with CuKα radiation was used to investigate the phase and crystal structure of the sintered ceramics. The preliminary crystal structure details were calculated using the Powder Cell program , which is based on the X-ray diffraction pattern of lead-free bismuth sodium titanate zirconate (Bi0.5Na0.5Ti1-xZrxO3 where x = 0.20, 0.35, 0.40, 0.45, 0.60, and 0.80 mole fraction) [BNTZ] ceramics. The bulk densities of the sintered ceramics were measured using Archimedes' method. The theoretical density was approximated from the unit cell size and its constituent ions. Scanning electron microscopy [SEM] (JEOL JSM-6335F, JEOL Ltd., Akishima, Tokyo, Japan) was used to observe the microstructure of the ceramics. To prepare the SEM samples, they were well-polished and thermally etched for 15 min at 750°C. The average grain size was then evaluated from these SEM images. The room temperature dielectric constant [εr] and dielectric loss [tan δ] were measured with an LCR meter (LF Impedance Analyzer 4292A, Agilent Technologies Inc., Santa Clara, CA, USA), but the ferroelectric hysteresis loops were measured in a silicone oil bath using a modified Sawyer-Tower circuit.
Results and discussion
Relationships between crystal structure and electrical properties of BNTZ ceramics
Dielectric constant (εr)
at 100 kHz
a , b , c (Å)
a = 5.9663
b = 8.0883
c = 5.6664
The εr and tan of Bi0.5Na0.5Ti1-xZrxO3 ceramics, at the frequency of 100 kHz, are tabulated in Table 1. In general, increasing Zr concentration in BNTZ ceramics caused a gradual decrease in dielectric constant with a slight decrease in dielectric loss. This behavior was in agreement with other systems with isovalent additives . In addition, the replacement of larger Zr ions may also cause the dipoles to be poorly induced due to limited ionic movement. This decreasing trend was observed through the sample with a composition of Zr = 0.8, whose structure was orthorhombic. It seemed that the effect of ionic size and limited ionic movement in the perovskite structure of this compound had a greater influence on the dielectric properties than the change in the crystal structure in their unit-all dimentions.
Lead-free Bi0.5Na0.5Ti1-xZrxO3 (where x = 0.20, 0.35, 0.40, 0.45, 0.60, and 0.80 mole fraction) ceramics were successfully fabricated. X-ray diffraction patterns showed phase transition from rhombohedral to an orthorhombic structure. The addition of Zr concentration caused lattice expansion in agreement with ionic size consideration. All ceramic samples were dense with well-defined grain structures. The dielectric constant was found to decrease with increasing Zr content due to the larger-sized ionic substitution that limited dipole movement. Ferroelectric properties also showed compositional dependence due to the variation in domain reorientation ability. This study showed that electrical properties of BNTZ ceramics could be further improved by fine-tuning their composition for certain applications.
This work was financially supported by the Nation Metal and Materials Technology Center (MTEC), Nation Science and Technology Development Agency (NSTDA), 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, Chiang Mai University is also acknowledged. Ms. Ampika Rachakom would like to thank the Commission on Higher Education for their support through a grant fund under the program Strategic Scholarships for Frontier Research Network for the Ph.D. Program Thai Doctoral degree for this research.
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