Elastic Modulus and Mechanical Loss Associated with Phase Transitions and Domain Walls Motions in PZT Based Ceramics

Elastic modulus (shear modulus G or Young's modulus E) and internal friction Q-' were measured as a function of temperature on undoped Pb(Zr50Ti50)03, Pb(Zr,2Ti.,,)03 , Pb(Zr5,Ti,)0, ceramics from -180°C to 500°C. Experiments were performed at low and medium frequencies (0.1 Hz 4 kHz). The E(T) curves show two anomalies which are due to following phase transitions: tetragonal to cubic (Tc) and rhombohedral to tetragonal (T,,). The Tc temperatures are in good agreement with phase diagram from litterature. The TR-, temperatures allow to complete the phase diagram of the morphotropic phase boundary of PZT in the low temperature range. ~o reove r ,Q"(~) curves recorded at low frequencies show two relaxation peaks; their activation energy and relaxation time are determined using the Arrhenius plots. These two relaxation peaks could be attributed to the interaction of domain walls with point defects. Lead titanate zirconate Pb(Zr,Ti,,)O3 ceramics are one of the most common piezoelectric materials in industry: they are used as transducers between electrical and mechanical energy, such as phonograph pickups, air transducers, underwater sound and ultrasonic generators, delay-line transducers, wave filters etc. [I] Generally, all those applications need high piezoelectric constants as well as low electrical and mechanical losses. Variations of internal friction and elastic modulus as a function of temperature and excitation frequency can provide direct information on energy dissipation in the material. For example, Postnikov et al. [2] have shown that the internal friction in the PZT is not only associated with domain walls but also with point defects. The ZrITi ratio in Pb(Zr,Ti)03, the nature and concentration of substituting elements, the shaping procedure of green bulk, the sintering temperature and atmosphere are the controlling factors which provide the suitable properties for applications. With their many applications and controlling factors, the PZT materials have been the subject of continuous research for the past few decades. In the present study, the elastic modulus and the internal friction were measured at different temperatures, in order to determine phase transition temperatures and to study the motion of domain walls. 2. SAMPLES AND EXPERIMENTAL PROCEDURES Undoped PZT ceramics were prepared by solid diffusion of PbO, ZrOz, and TiO, powders with the following ZrITi ratio: Pb(Zr0,50Ti0,s0)03, Pb(Zro,52Ti0.48)03 and Pb(ZrO s4Ti0 46)03. shortly called PZT50150, PZT52148 and PZT54146. Young's modulus E and internal friction Q-' have been measured as a function of temperature. Samples were driven in flexural vibration at resonance frequency of about 3 kHz. Specimen dimensions, experimental measurement device and calculation formula were described in [3]. The measurements at low frequencies of shear mod-ulus G and internal friction Q-' were measured by an inverted pendulum. The temperature range is located between -1 80°C and 500°C. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:19968136 (3-634 JOURNAL DE PHYSIQUE IV 3. RESULTS AND DISCUSSIONS The Figure 1 presents E(T) and Q-'(T) curves obtained with PZT52/48 ceramic. The E(T) curve shows two anomalies called Al and A, located at 375OC and -69°C. The A, anomaly of elastic modulus is correlated to a sharp internal friction peak called PI. Both A, anomaly and PI peak are due to the phase transition from cubic to tetragonal phases. The A2 anomaly is due to the second phase transition between tetragonal and rhombohedra1 phases. This A2 anomaly is not correlated to an internal fiiction peak. Similar E(T) and Q-'(T) curves were obtained for other composition PZT50150 and PZT54/46 ceramics. Figure 1: E(T) and Q-'(T) at kHz frequency. Figure 2 : Phase diagram of PZT. The Figure 2 shows the phase diagram from Jaffe et al. [I] on which our results on phase transition temperatures Tc and TRqT have been added. About Tc our results are in good agreement with Jaffe's diagram. Moreover, about the TR=I. temperatures, our results allow to complete the morphotropic region of the phase diagram located below O°C. About the Q-'(T) curve shown in Figure 1, it is possible to divide the temperature range according to the level of internal friction. From the low temperature -180°C to O°C, Q-' decreases monotonically to a low level. Between O°C and 180°C, Q" remains constant. Above 180°C, Q-' increases strongly up to a maximum of the P, peak ( at the Curie temperature ). In the paraelectric region, the drastic decreasing of Q" remember us that the mechanisms of energy dissipation in ferroelectric state are obviously linked to the motion of domain walls. The same shape of the E(T) and Q-'(T) curves are obtained for all three compositions of PZT ceramics. In order to determine which anelastic event is located at PR in the increasing part of the Q-'(T) curves at kilohertz, the measurements of G and Q-' versus temperature were performed at low frequencies of 0.1 to 1 Hz on two pendulums (which allow to cover the total temperature range). The results obtained with the PZT 52/48 ceramic are shown on Figures 3 (low temperature) and 4 (high temperature). The G(T) curves show the A, and A2 anomalies of shear modulus due to two phase transitions: T, and TR-T. Tenperahre ("C) T m e ("c) Figure 3 : Q-'(T) and G(T) at low temperature. Figure 4 : Q"(T) and G(T) at high temperature. C8-636 JOURNAL DE PHYSIQUE IV On the Q-'(T) curves, two internal friction peaks R1 and R2 are observed. These two peaks have a relaxation behavior because they are frequency dependent. The Arrhenius plot corresponding to this two peaks are given in the Figure 5. It is of interest to note that the R2 peak is connected to the shoulder observed on Q-'(T) at kilohertz frequency. Table 1: Activation parameters of R, and R, peak in the three PZT ceramics. Figure 5: Arrhenius plot for PZT52148. The Table 1 gives the activation energy and the relaxation time for the R1 and R2 peaks. Fot the R2 peak, the magnitude of the relaxation time is coherent with a point defect relaxation and the R2 relaxation peak could be due to the interaction between domain walls and oxygen vacancies because the activation energy for diffusion of oxygen vacancy is about 0.9 eV. In order to verify this hypothesis, we intend to modify the concentration of oxygen vacancy by annealing in vaccum. Concerning the R1 peak, its activation energy is high (about 1.6 eV) and we intend to study the influence of annealing and strain amplitude. One can notice that the R1 peak is not observed in the kilohertz range because its temperature is higher than the Curie temperature.