Estudo das transições de fase em cerâmicas ferroelétricas de PZT e PLZT através de espectroscopia mecânica
Silva Junior, Paulo Sergio da
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Lead titanate zirconate (PZT) and lanthanum-modified lead titanate zirconate (PLZT) ferroelectric ceramics have been great scientific and technologic interest, due to the possibility of your physics properties can be optimized and adequated for specific technologic applications. Scientifically, the anelastic characterization of the phase transitions have a fundamental importance to understand the phenomenology and behavior of the intrinsic properties. In this work, the phase transition exhibit by PZT and PLZT were studied, for Zr/Ti ratios of 40/60 and 65/35 for PZT, and Zr/Ti ratio 65/35 for PLZT, with lanthanum concentration between 5 mol-% and 9 mol-%, by mechanical spectroscopy technique. The nelastic spectra as a function of temperature and frequency were performed in acoustic elastometer equipment, operating in a temperature range of 300 K and 770 K, with frequencies in a kHz bandwidth, for heating rate of 1 K/min, under a pressure higher than 10−5 Torr. Dielectric chracterizations of PZT 40/60 and PZT 65/35 ceramics showed the normal ferroelectric behavior of ferroelectric-paraelectric phase transition. Dielectric analysis of PLZT ceramics showed the change of the normal ferroelectric behavior to relaxor-like and diffusive behaviors for the ferroelectric-paraelectric phase transition with the increase of the lanthanum amount. Anelastic spectra of PZT 40/60 showed the presence of two anomalies associated with relaxation mechanism of interaction between 900 domain walls with Zr and Ti vacancies, and the ferroelectric-paraelectric phase transition, respectively. For PZT 65/35 the anelastic results appoint the presence of one single anomaly associated to ferroectric-paraelectric phase transition. PLZT 5/65/35 and PLZT 8/65/35 showed in anelastic spectra two frequency-independent anomalies, associated to ferro-ferroelectric and ferroelectric-paraelectric phases transitions. In anelastic spectra of PLZT 9/65/35 was observed only one single anomaly associated to ferroelectric-paraelectric phase transition. The anelastic and dielectric spectra analysis showed the same behavior of temperature of ferroelectric-paraelectric phase transition with the change of lanthanum amount, where were observed the decrease of temperature of ferroelectric-paraelectric phase transition as a function of the increase of lanthanum amount. The effect of symmetry exchange in PZT ceramics not showed the change in electromechanical coupling type, being this linear coupling type between strain and polarization. The increase of lanthanum amount in PLZT ceramics showed a continuum change of electromechanical coupling type, starting with a linear coupling between strain and polarization (PZT 65/35) and changing to linear coupling in strain but quadratic in polarization (PLZT 8/65/35 and PLZT 9/65/35).