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Ferroelectric Properties of Bi0

Different processing conditions and the effect of secondary phases on ferroelectric properties of Bi0.5(Na0.8K0.2)0.5TiO3 (BNKT) were studied. Ceramic powders were prepared by solid state reaction and different sintering conditions (1075 to 1100oC) were analyzed. Finally, samples were characterized by X-ray diffraction, Raman microspectroscopy, Scanning Electron Microscopy, impedance spectroscopy, and density measurements. Through XRD patterns, the perovskite structure was stabilized, together small peaks corresponding to a secondary phase associated with K2-xNaxTi6O13. Moreover, the content of the secondary phase, d33 piezoelectric constant and dielectric properties increased with the sintering temperature. Introduction Lead titanate–zirconate piezoceramics are the most important and widely used materials for piezoelectric transducers, transformers and sensors. They have played a dominant role in the piezoelectrics field for a long time because of their excellent piezoelectric properties [1]. However, the toxicity of lead is a serious threat to human health and environment [2]. Thus, considerable effort has been devoted towards the development of lead-free piezoelectric ceramics. Numerous studies on lead-free piezoelectric ceramics, such as (K,Na)NbO3, BaTiO3-based, Bi-layered, bismuth sodium titanate and tungsten bronze-type materials, have been recently published. For this reason, K0.5Na0.5NbO3 (KNN) system attracts much attention, for its elevated Curie temperature (about 420°C) and high piezoelectric properties close to the morphotropic phase boundary (MPB) [3]. However, it is difficult to obtain pure KNN ceramics with high density and great piezoelectric performance. Sodium bismuth titanate Na0.5Bi0.5TiO3 (BNT) with a relatively large remanent polarization at room temperature and a relatively high Curie temperature, could be considered another promising candidate of lead-free piezoelectric ceramics. However, its high coercive field hinders the obtention of the desired piezoelectric properties. Therefore, a number of studies have been carried out to improve electrical properties of BNT by the formation of solid solutions with other ABO3 perovskites [4-5]. It has been reported that BNT ceramics modified with Bi0.5K0.5TiO3 (BKT) showed improved dielectric and piezoelectric properties, due to a rhombohedral–tetragonal morphotropic phase boundary (MPB) at the optimal composition of Bi0.5(Na0.85K0.15)0.5TiO3 [6]. In the current work, lead-free Bi0.5(Na0.8K0.2)0.5TiO3-based ceramics were prepared by the solid state reaction method using a previous mecanochemical activation step of reagents. Results were discussed considering the effect of secondary phases on structure, microstructure, dielectric and piezoelectric properties of these ceramics. Experimental Procedure Bi0.5(K0.2Na0.8)0.5TiO3 was synthesized through solid state reaction, using K2CO3 and Na2CO3 (Cicarelli 99,99%; Argentina), Bi2O3 (Aldrich 99,8%; USA) and TiO2 (Aldrich 99,9%; USA). Powders were mixed and milled using zirconia balls in alcoholic medium for 5 h in a planetary mill (Fritsch, Pulverisette 7, 1450 rpm). Powders were dried and calcined at 700oC for 2 h. The calcined powders were milled again, pressed into disks and sintered at 1075 to 1150°C for 2 h. Crystalline phases were characterized by X-ray diffraction (XRD) (Philips PW1830), using CuK radiation. Raman spectra were acquired at room temperature with a Renishaw inVia microscope by means of the 514 nm Ar-ion laser line (50 mW nominal power) with a diffraction grating of 2400 lines/mm. Density values were determined using the Archimedes method. Microstructures were evaluated on polished and thermally etched samples using a Field Emission Scanning Electron Microscope, FE-SEM (Hitachi S-4700). Previous to the electrical measurements, a fired silver paste was used for the electric contacts. Dielectric properties were determined at different frequencies using impedance analyzers Hioki 3532-3550 in the frequency range 100 mHz-10 MHz at room temperature. Samples were polled in a silicon oil bath at 25oC by applying a DC field of 30.0 kV/cm for 30 min. The piezoelectric constant d33 was measured using a piezo d33 meter (YE2730A d33 METER, APC International, Ltd., USA). Finally, the ferroelectric nature of these ceramics was determined using a hysteresis meter (RT 6000 HVS, RADIANT Technologies). Results and Discussion From XRD patterns (Fig. 1), the BNKT phase was stabilized in all sintered samples. However, samples sintered at temperatures higher than 1100°C presented secondary phases, which can be indexed to K2-xNaxTi6O13 phase (monoclinic structure, JCPDS Nos. 40-0403 and 74-0275). 20 30 40 50 60 70 80 o o o o