The temperature dependence of the piezoelectric properties of 0.67Pb(Mg1/3Nb2/3)O-3-0.33PbTiO(3) and 0.955Pb(M,Nb-1/3(2/3))(3)-0.045PbTiO(3) single crystals, respectively grown by Bridgman and modified Bridgman method was investigated. Due to their low rhombohedral to tetragonal transition temperature (T-rt = 90 degrees C and 124 degrees C) and Curie point (T-c = 152 degrees C and 166 degrees C), the properties of these single crystals have an unstable behavior as soon as the first transition temperature is reached, i.e. the piezoelectric coefficients exhibit a non linear or a hysteretic behavior during temperature cycle.The objective of this paper is to study the thermal stability of piezoelectric and dielectric coefficients for the two compositions in order to point out the limiting factors in term of transducers performances and to explain the observed behavior.

Ferroelectric single crystals of PZN-PT and PMN-PT exhibit outstanding properties: high charge coefficient (d(ij)), high coupling factor (k(ij)) and high strain levels under DC fields.Besides, their mechanical quality factor is believed to be low. Their usefulness for non-resonant or large bandwidth transducers has therefore been previously investigated.However, few studies have been devoted to the dielectric and mechanical losses of single crystals and to their stability under high levels of excitations (electric fields, temperature and mechanical stress). A knowledge and understanding of such performances is needed to determine whether single crystals are suitable materials for power or resonant transducers.In this work, losses and non-linearity versus external excitations are investigated. Dielectric losses and mechanical losses are measured versus electric field for different compositions, orientations.The evolution of d(33) and epsilon(33)(T) are obtained versus electric field and temperature for the longitudinal mode. Strain and hysteresis versus sweep mode (up and down) are measured near the resonance frequency using a laser Doppler vibrometer. (C) 2004 Elsevier B.V. All rights reserved.

Recently, it has been shown that co-doping with 1% mol. Manganese and 1% mol. fluorine a PZT base composition improved the dielectric and piezoelectric properties. A model based upon electron transfers between Mnalpha+ ions (alpha = 2, 3, 4) has been proposed to explain such a behaviour. F co-doping has been realized on Cr and Sn doped PZT to determine if the same model is valid with other acceptor dopants possibly exhibiting different valency states in the PZT (Sb, F) and (Nb, F) co-doped PZTs were also studied to estimate the influence of fluorine on the valency state of donor dopants. XRD measurements revealed that F co-doping generates lead vacancies in the cationic sublattice of Cr doped PZT instead of partially reducing cationic dopant. No change is observed in the unit cell volume and the piezoelectric properties of Sb and Nb doped PZTs after F co-doping. In the case of Sn doping, F- ion may make Sn2+ and Sn4+ coexist in the PZT but the characteristics are not as high as (Mn, F) co-doped PZT ones. The correlation between cubic unit cell volume and lead vacancies content in the PZTs was supported by flame Atomic Absorption measurements. The study confirms that the softening effect observed is not due to the increase of extrinsic lead vacancies content but to the coexistence of three valency states of the cationic dopant like Mn-+4,Mn-+3,Mn-+2.

This paper summarizes a mechanical model for characterizing hysteresis in ferroelectric materials at moderate to high drive levels. Hysteresis and nonlinearities are attributed to the displacement of domain walls. The model based on a simple mechanical element (dry friction), is developed to interpret piezo-coefficient hysteresis due to mechanical stress or electric field. The principles of basic element are given. The viability of the model is illustrated through comparison with experimental data from soft and hard PZT ceramics.

A fluorine - oxygen substitution with yo at% F (yo = 2, 3 and 4) was performed in lead zirconate titanate ceramics with the nominal composition Pb0.89(Ba, Sr)0.11(Zr0.52Ti0.48)O3 (PBSZT). This starting material was also doped with x at% Mg (x = 0.4, 0.75 and 1). The dielectric and piezoelectric properties of undoped, (yo at% F) doped, (x at% Mg) doped and (x at% Mg and yo at% F) co-doped PBSZT ceramics were compared. The study of three coefficients - ϵτ, d33 and Qm - revealed that F doping did not induce significant changes in PBSZT characteristics whereas (Mg and F) co-doping led to harder ceramics than MgO only doping. The influence of the different substitutions on the temperature dependence of the dielectric constant and losses have been investigated. A drastic increase of the Curie point was observed after (Mg and F) co-doping associated with a diffuse ferroelectric/paraelectric phase transition. Experimental results indicated that neither Mg nor F has a dominating effect on the characteristics of these co-doped PBSZT specimens

The hysteretic behavior of the charge coefficient d33 under high mechanical stress and of frequency constant N33 during the temperature cycle −40°C–+80°C can be cancelled by fluorine oxygen substitution in the lattice of Mg-doped PZT. These fluorine doped materials also exhibit a high mechanical quality factor and low dielectric losses. The drawback is that they have a lower d33 coefficient and are difficult to pole. Mn doping in B site or La doping in A site, in the ABO3 type perovskite structure are known to increase the d33 coefficient and the poling ability. This result is observed on (Mg, F) co-doped PZT with Mn and with La content of 1.5 mol%. In the same manner, Mn doping enhances the hysteretic behavior and the losses while La doping does not significally changes the (Mg, F) co-doped PZT characteristics except the fluorine content for which the losses and the hysteresis are the lowest.

Some acceptor and fluorine co-doped PZT ceramics exhibit a low and non hysteretic dependence of d(33) coefficient under uniaxial static stress up to 120 MPa and of N-33 coefficient versus temperature between -40 degreesC and + 80 degreesC. This behavior could be explained by a strong clamping effect of the domain walls due to the acceptor ion - F dipole, less mobile than the oxygen vacancy-acceptor ion defect dipole. In order to understand the hysteresis mechanism and the role of fluorine ions, fluoridated materials with different acceptors in B site have been prepared and characterized.

In this study, a fluorine–oxygen substitution in lead zirconate titanate (PZT) ceramics with a nominal composition of Pb0.89(Ba, Sr)0.11(Zr0.52Ti0.48)O3 (PZT) doped with 1% MgO is proposed. The evolution of four dielectric and electromechanical coefficients—εr, tgδ, d33, and Qm—with increasing fluorine concentration showed that (MgO and F)-doped PZT ceramics are harder than only MgO-doped PZT (0 at. % F). The influence of the F–O substitution on the temperature dependence of the frequency constant N33 and the stress dependence of the piezoelectric coefficient d33 was investigated. A hysteretic free response of N33 and the lowest stress dependence of d33 were obtained for the (MgO and 4 at. % F)-doped PZT specimen. This material also exhibits the highest Qm in the (MgO and F)-doped PZT family and seems to be stoichiometric and without oxygen vacancies. For comparison, both the temperature and stress dependences of two commercial PZT ceramics are shown. The study of the influence of the Zr/Ti ratio on the temperature dependence of N33 revealed that fluorine stabilizes the rhombohedral phase/tetragonal phase interface. Both types of stability, versus temperature and uniaxial mechanical stress, may be linked to the domain wall configuration stabilization by Mg2+–F− dipoles which are less mobile than Mg2+–VO ones.