Piezoelectric and elastic properties of relaxor-like PZT:Ba ceramics
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The solid solutions of PbZr1-xTixO3 (PZT) are the most known and widely applicable piezoelectric ceramic materials. The influence of different dopants on PZT properties has been studied for many years. Some of such compositions have revealed a behavior typical for ferroelectric relaxors. In the case of Pb0.75Ba0.25Zr0.70Ti0.30O3 ceramics (abbreviation PBZT 25/70/30), prepared by mixed-oxide processing technique elastic, it was found that macroscopic piezoelectric activity is present far above the temperature at which structural changes and maximum of permittivity occur in unpoled samples (~ 200 °C). Anomalies of the elastic and piezoelectric properties have been observed very clearly near the temperature of 152 °C. Within the temperature range of 152–220 °C, unstable piezoelectric properties have been detected due to the existence of polar micro/nano-clusters and non-trivial elastic-electric interactions between them, through the non-polar paraelectric matrix. The origin of such interactions could be lattice instabilities, dynamic change in polar cluster sizes, and fluctuations in chemical composition. Based on the results of the research, it can be said that PZT ceramics of such composition is a relaxor-like material and it is not a ferroelectric relaxor.
KeywordsX-ray methods Dielectric properties Piezoelectric properties Elastic properties PZT
Lead zirconate titanate PbZr1-xTixO3 (PZT) is a perovskite type ABO3 structured material, with well-known piezoelectric, pyroelectric and dielectric properties, widely used in practical applications. Modified by various dopants, PZT ceramics are mainly investigated for piezoelectric applications, due to their unique properties such as high response speed, high piezoelectric modules, and lower power requirements, to mention but a few of them. They have been used for fabricating nonvolatile memories, pyroelectric detectors, photoelectric devices and piezoelectric actuators [1, 2]. The lanthanum-modified lead zirconate titanate (PLZT) system was extensively investigated, mainly for electro-optic applications . In literature, reports about properties of PZT doped with Ca, Sr or Ba have also appeared . Our interest has focused on the Pb1-yBayZr1-x,TixO3 (PBZT) system, the investigations of which were initiated by Smolenskii et al. . Ikeda’s phase diagram  based on the structural and dielectric measurements shows that phase transitions occurring in these ceramics and their dielectric properties are strongly dependent on both the Zr/Ti ratio and the Ba contents. The mentioned phase diagram has been confirmed by exploring the dielectric properties and electric field induced strains, and reported by Li and Heartling . Moreover, they have shown that some compositions demonstrate behavior typical for ferroelectric relaxors, namely a broad maximum and strong non-Debye relaxation of permittivity. Cross  has shown that a characteristic feature of ferroelectric relaxor materials is, for example, the absence of macroscopic polarization and optical anisotropy at temperatures below the maximum of permittivity. Recently, Mesquita et al.  have observed that Ba incorporation into the PZT lattice leads to a reduction in Ti displacement from the center of oxygen octahedron when Ba concentration increases. Papers describing properties of compounds belonging to the PBZT system have been published in [10, 11, 12, 13].
In our earlier papers we have focused on PBZT 25/70/30 ceramics [14, 15], investigating the pyroelectric effect, thermal stimulated depolarization currents and electric conductivity. In this paper we concentrate mainly on piezoelectric properties of such ceramics. The temperature evolution of elastic coefficients and piezoelectric modulus have been determined by resonance technique. To calculate piezoelectric coefficients and the real and imaginary part of the elastic compliances, a damped forced harmonic oscillator model has been used [16, 17]. The goal was to find a correlation between the dielectric, elastic, piezoelectric and structural properties of PBZT 25/70/30 and to prove that this material does not reveal classical relaxor properties.
2 Material and methods
PBZT 25/70/30 ceramics have been prepared using conventional mixed-oxide processing technique. Stoichiometric amounts of the PbO, BaCO3, ZrO2 and TiO2 oxides were weighted according to the formula: Pb0.75Ba0.25Zr0.70Ti0.30O3. The thermal synthesis of the pressed mixture was carried out at 925 °C for 2 h. Then the crushed, milled and sieved materials were pressed into cylindrical pellets and again sintered at 1250 °C for 5 h. This procedure was repeated before the final sintering at 1300 °C for 7 h. The last sintering process was carried out in a double crucible with the interior atmosphere of PbO + ZrO2, in order to preserve the predetermined composition, and in particular, to avoid the loss of PbO caused by its sublimation. The weight losses were found to be below 1%. The Archimedes displacement method with distilled water has been used to evaluate sample density that was equal to 6.8 g/cm3.
The microstructure and distribution of elements inside the grains were examined by JSM-5410 scanning electron microscope with an energy dispersive X-ray spectrometer (EDS). The accelerating voltage was equal to 15 kV and Spot Size (SS) was 55. The grain size determination was performed on a fractured surface of the ceramics.
A preliminary X-ray test was carried out at room temperature, using Huber diffractometer with θ-θ geometry, focused in Zeeman-Bohlin system, and with monochromatic CuKα1 radiation. The diffraction was collected from 19o to 100o in 2θ geometry and 0.05o steps. The XRD profiles were analysed using a software, i.e. DHN powder Diffraction System ver. 2.3. The main x-ray tests were made in order to find out the nature of the unit cell distortion. They were performed by means of high-resolution Siemens D5000 diffractometer with θ-2θ geometry, focused in Bragg-Brentano system, with filtered CuKα radiation.
For piezoelectric measurements the ceramics were cut to required sample geometrical conditions of bar shape , with gold electrodes and having dimensions of about 4x1x0.5mm3. For electrical contact, thin gold wires were glued at the centre of the evaporated electrodes. The samples were poled under an external d.c. electric field of 6 kV/cm applied at 200 °C, and then slowly cooled to room temperature. The elastic compliances s11, s33, s55, piezoelectric modulus d31, d33, d15 and electromechanical coupling factors k31, k33, k15 have been determined by resonance method. To calculate these coefficients, the forced damped harmonic oscillator model has been applied .
3 Results and discussion
Nominal and average concentration of Ba, Pb, Zr and Ti ions
Nominal stoichiometry at %
Average content at %
It is worth noting that the difference between sjjE and sjjD is proportional to the product of the piezoelectric and dielectric properties sjjE - sjjD = dmjdnj/εmn [17, 22]. This difference (treated as the piezoelectric contribution to the compliance) is pronounced below 152 °C, when after poling in d.c. field the polar (ferroelectric-like) state with long-range interaction is stable. On the other hand, it is distinctly lesser above 152 °C, because of a disorder in clusters orientation, due to softening of lattice and de-freezing of clusters (Fig. 8).
The diffused temperature run of the permittivity can be explained by fluctuations in the chemical composition of PZT ceramics (Fig. 7) that lead to the broadening of the temperature range, in which the main transition between the paraelectric and ferroelectric phases appears. According to the phase diagram of pure PZT [20, 23], this main structural transition in the PZT70/30 ceramics occurs at around 340 °C. Thus, the variation of Zr content found in the investigated ceramics causes that this transition is realised between 290 °C (for 78% Zr) and 360 °C (for 64% Zr). Doping with Ba additionally lowers the value of these temperatures, as reported in [24, 25].
The difference between temperature points at which strong softening of the elastic properties takes place (152 °C) and piezoelectricity vanishes (220 °C) is equal to 68 °C, which well corresponds to difference in Tc’s for PZT with fluctuations in Zr content, mentioned above.
The abnormally substantial integral width of the (200) reflection indicates the presence of a mixture of phases in the whole temperature range investigated. After poling in d.c. electric field, the lattice becomes averagely polar, of ferroelectric-like nature, and thus bulk piezoelectric properties can be observed. However, above the temperature of 152 °C long-range polar state decays and eventually polar micro/nano-clusters embedded in the cubic matrix appear. Random electric fields produced by these polar clusters can polarize the lattice in their neighbourhood, and thus make the volume of polar clusters fluctuate. That is why unstable piezoelectric signals in the range between 152 °C and 220 °C have been detected. The interactions between polar regions through the paraelectric matrix finally vanish above 220 °C, but locally the symmetry can be broken (to become a polar one), even up to very high temperatures.
In typical relaxor there is a critical field above which additional phase transition is observed. We did not observe any transition induced by the d.c. electric field, although the applied electric field was stronger than the critical one mentioned above and equal to 1.5 kV/cm . Based on properties described above, we claim that it is not allowed to label the investigated material a relaxor, although some similarities can be found . We think that in the case of PBZT 25/70/30 ceramics the term relaxor-like material might be justified at most.
The authors thank Irena Gruszka from the University of Silesia for technical assistance during sample preparation. We would like to acknowledge the late Michał Górny, who passed away in January 2016, for very fruitful cooperation.
This work was supported by the National Science Centre, Poland [grant number 2016/21/B/ST3/02242].
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