Dielectric and ferroelectric characteristics of barium zirconate titanate ceramics prepared from mixed oxide method

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Abstract

Barium zirconium titanate (BZT) ceramics were prepared by mixed oxide method. X-ray diffraction showed the presence of a single phase while Raman scattering confirmed structural transitions as a function of different Zr/Ti ratio. The addition of Zr strongly influenced the crystal structure and electrical properties of the ceramics. A typical hysteresis loops were observed for all investigated compositions. BZT ceramics with 15 mol% Zr have shown a ferroelectric to paraelectric transition at around 77 °C.

Introduction

Barium zirconium titanate ceramics are attractive candidates for dynamic random access memories and tunable microwave devices. Such, lead-free, environmental friendly materials are known to exhibit relaxor behaviour in bulk materials with increasing the Zr content. The interest in high strain piezoelectric materials is increasing for electromechanical transducers and various related applications [1]. Though the large family of lead-based perovskites and relaxors has shown a great potential, lead-free compositions in these families will be of interest due to obvious environmental concern in the future [2], [3], [4], [5], [6]. BaTiO3 is known to have a large electromechanical coupling factor. Substitution of Ti4+ (atomic weight of 47.9, ionic radius of 0.0745 nm) with Zr4+ (atomic weight of 91.2, ionic radius of 0.086 nm) exhibits several interesting features in the dielectric behavior of BaTiO3 ceramics. When the Zr content is less than 10 mol%, the BZT ceramics show normal ferroelectric behavior and dielectric anomalies corresponding to cubic to tetragonal (Tc), tetragonal to orthorhombic (T2), and orthorhombic to rhombohedral (T3) phase transitions. At around 27 mol%, Zr-doped BZT ceramics exhibit typical diffuse paraelectric to ferroelectric phase transition behavior, whereas Zr-richer compositions exhibit typical relaxor-like behavior in which Tc shifts to higher temperature with increase of frequency [7]. The modified Ba(Zr,Ti)O3 has shown the systematic changes in the dielectric, piezoelectric, and phase transition characteristics in the bulk ceramic and single crystal forms [8]. In the paraelectric state, just above Tc, BZT ceramics are attractive candidates for dynamic random access memories and tunable dielectric devices.

It is well known that Zr, like Sr, is an effective substituent in BaTiO3 to decrease and shift the Curie temperature below room temperature [9]. Moreover, Zr4+ ion is chemically more stable than Ti4+ and has a larger ionic size to expand the perovskite lattice. Therefore, the substitution of Ti by Zr would depress the conduction by electronic hopping between Ti4+ and Ti3+ and it would also decrease the leakage current of the BaTiO3 system. It is reported that an increase in the Zr content induces a reduction in the average grain size, decreases the dielectric permittivity (ɛr), maintaining a low and stable leakage current [10], [11], [12]. This is possible because Zr4+ ion has larger ionic size (0.087 nm) than Ti4+ (0.068 nm). Thus, BZT ceramics have good properties and can be applied as a storage capacitor for the next DRAM generation and a dielectric material for MLCC. Zr/Ti ratio is important in BZT system and a 0.2/0.8 ratio is known to have excellent bulk properties [13]. In this work, the effect of Zr/Ti ratio on the structural and electrical properties of BZT ceramics was investigated.

Section snippets

Experimental procedure

The powders were homogenized in a ball mill using isopropyl alcohol. All oxides were analytical grade: BaCO3 (Vetec), ZrO2 (Inlab), TiO2 (Vetec). The investigated systems were Ba(Ti0.95Zr0.05)O3, Ba(Ti0.9Zr0.10)O3 and Ba(Ti0.85Zr0.15)O3. After drying, the powders were compacted in the form of pellets with 1 mm of thickness and 12 mm of diameter, isostatically pressed at 210 MPa. The pellets were sintered at 1550 °C for 4 h in static air and cooled to room temperature (5 °C/min).

X-ray diffraction data

Results and discussion

Fig. 1 shows the room temperature XRD plots of Ba(Ti0.95Zr0.05)O3, Ba(Ti0.9Zr0.10)O3 and Ba(Ti0.85Zr0.15)O3 powders. All the ceramics composition were crystallized into a single-phase perovskite structure. This is a clear indication that the addition of Zr is forming a stable solid solution with the BaTiO3 lattice. As reported in other studies [2], [3], [4], [5], [6], depending upon the Zr content, BZT may have orthorhombic, rhombohedral and tetragonal structures at room temperature.

The linear

Conclusions

Single phase BZT powders were obtained from mixed oxide method at 1200 °C for 2 h. Raman spectroscopy indicated a change in the crystal structure with the increase of zirconium content. The transition temperature (ferroelectric to paraelectric phase) was found to be systematically reduced for Zr content equal to 15.0 mol%. In this case, a typical relaxor behavior was observed. A decrease in the dielectric permittivity and remnant polarization with the increase of zirconium content is indicative of

Acknowledgments

The authors gratefully acknowledge to the financial support of the Brazilian agencies FAPESP, CNPq, and CAPES and Ministry of Science and Environmental Protection of Serbia.

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