Morphotropic phase boundary in the BNT–BT–BKT system
Introduction
Piezoelectric ceramics based on lead, titanium and zirconium oxides in a perovskite like structure (PZT) present the most interesting piezoelectric properties near the morphotropic phase boundary and are used in many electronic and ultrasonic applications (sensors, actuators and transformers [1], [2], [3]). However, due to the effects of lead toxicity, considerable efforts were made recently to develop lead-free piezoelectric ceramics. According to 2002/95/EC and 2002/96/EC directives [4], the exclusion of electronic part including lead compounds is restricted gradually and will be prohibited in a near future.
Unfortunately, alternative lead free materials do not exhibit the same piezoelectric characteristics of PZT. Numerous works turn towards new promising lead free materials to improve their piezoelectric properties in order to progressively replace PZT devices. The most studied materials are KNbO3, KxNa1−xNbO3, LiNbO3, LiNa1−xNbO3 and Bi0.5Na0.5TiO3-based systems [5].
This study focuses on the Bi0.5Na0.5TiO3–BaTiO3–Bi0.5K0.5TiO3 system. This system is composed of three perovskite phases. As the other piezoelectric materials, the better properties are expected near the rhombohedral–tetragonal morphotropic phase boundary.
Literature shows that this zone is not well defined in composition [6], [7] and more works are needed to determine more precisely this boundary. In this paper, several compositions in the Bi0.5Na0.5TiO3–BaTiO3–Bi0.5K0.5TiO3 system have been studied in order to determine the morphotropic boundary.
Previous works reported promising piezoelectric properties for mixed (1 − x) BNT–x BT compositions with x values between 0.06 and 0.08 [8] and for (1 − x) BNT–x BKT with x values between 0.16 and 0.20 [9]. Fig. 1 shows the empirical BNT–BT–BKT ternary phase diagram proposed by literature [10], [11].
Section snippets
Experimental
The studied compositions in this work are listed in Table 1. In the first series of samples concerning binary (1 − x) BNT–x BT system, the x value is increased from 0 to 0.07. The second series of samples lies in the binary (1 − x) BNT–x BKT system with x varying from 0 to 0.20. Three compositions constitute the third series and are located in the predicted morphotropic area of the ternary BNT–BT–BKT system.
The materials are synthesized by the conventional solid state reaction method from mixed
Results and discussions
Theoretical and relative densities for the different studied samples are plotted in Table 2. A relative density of 92–95% is reached for all the samples.
These high density values are confirmed by SEM observation. Fig. 2 shows the microstructure of the 0.865 BNT–0.035 BT–0.100 BKT ceramic. The grain size is ranging between 0.2 μm and 1 μm.
The composition of the various samples was verified after sintering by X-ray fluorescence in order to check whether volatilisation has occurred. As example,
Conclusion
BNT–BT–BKT compositions around the morphotropic phase boundary were synthesized from conventional mixed oxide method and their piezoelectric properties were determined. The stoichiometry of the samples after sintering has been confirmed by X-ray fluorescence. All materials present a relative density around 92–95%. X-ray diffraction analysis has shown that all the compositions are characterized by a pure perovskite structure without any parasite phase.
A structural evolution has been confirmed in
Acknowledgment
This work was funded by French Government for a PhD thesis program.
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