PMN-PT Ferroelectric Ceramics Prepared By The Partial Oxalate Route

Relaxor-based ferroelectric ceramics （1-x）Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃（（1-x）PMN-xPT, PMN-PT） exhibit excellent electrical and electro-opticalproperties, which make them promising applications in multilayer ceramiccapacitors, piezoelectric actuators and transducers, and optical devices.In this thesis （1-x）PMN-xPT （x=0.2-0.4） relaxor-based ferroelectricceramics with perovskite structure were prepared by the partial oxalate route.The influences of raw materials selection, synthesis technics of precursors,calcination temperature and the content of PbTiO₃（PT） on the phase structure,microstructure morphology and electrical properties of the synthesizedPMN-PT ceramics were studied systematically. The influences of directcurrent （d.c.） bias voltage and heat treatment temperature on electricalproperties were also researched.X-ray diffraction （XRD） characterization and electrical propertiesmeasurements confirm that pure perovskite0.69PMN-0.31PT ceramics withexcellent electrical properties can be prepared by using Pb（Ac）23H2O as rawmaterial via the partial oxalate route method. The synthesized0.80PMN-0.20PT powders calcined at750℃exhibit rather pure perovskitestructure, little grain size, rather well-distribution and less conglomerationconfirmed by XRD measurement, Laser Particle Size Analyzer, SEMobservation and specific surface area measurement. Pure perovskite0.69PMN-0.31PT ceramics can be prepared by using the1100℃calcinedmixtures of the columbite precursor MgNb₂O₆and TiO2, which exhibituniform microstructure morphology, high relative density and excellentelectrical properties.XRD structure analysis and electrical properties measurements confirmthat apparent composition induced structural phase transition exists in thePMN-PT ceramics prepared by the partial oxalate route. With the increase ofthe PT content, the phase structure of the PMN-PT ceramics changesgradually from rhombohedral ferroelectric phase, across the morphotropicphase boundary （MPB） formed by the coexistence of rhombohedral andtetragonal ferroelectric phases, and to tetragonal ferroelectric phase. Thedielectric response character also changes from diffused ferroelectric phase transition （FPT） of rhombohedral relaxor ferroelectrics to first-order FPT oftetragonal normal ferroelectrics, accompanied by almost linear increase ofCurie temperature/transition temperature （Tc/Tm）. The PMN-PT ceramicsexhibit well-developed, symmetric and saturated P-E hysteresis loops, whichchanges from narrow and slim shape to fat and nearly rectangular loop withthe increase of PT content. The MPB of the PMN-PT ceramics prepared bythe partial oxalate route is determined locating at x=0.31⁰.33, in the MPBeffects are prominent and the MPB compositions exhibit excellent electricalproperties.D.C. bias voltage and heat treatment exert influences on electricalproperties of the synthesized PMN-PT ceramics. With the increase of d.c. biasvoltage, dielectric constant of the PMN-PT ceramics increases firstly and thendecreases. When the heat treatment temperature is below the depolarizationtemperature Td, piezoelectric properties of the PMN-PT ceramics increaseslightly and then decrease slightly with the increase of heat treatmenttemperature. When the heat treatment temperature is above Td, thepiezoelectric properties decrease sharply, and till disappear.