Studies Of Relaxation And Piezoelectric Properties Of PLZT Ceramics

Relaxor has been found for over half a century.Because of excellent dielectric,piezoelectric and pyroelectric properties,relaxor possesses potential applications in transducer,precision positioning,energy storage,refrigeration,etc.and has been studied widely.However,there still exist some queries and controversies in relaxor,such as the contribution of nanodomains to piezoelectricity,the underlying structure of relaxor,and so on.In present thesis,Sb₂O₅ modified(Pb_1-x-x La_x)(Zr_y Ti_1-y)_1-x/4O₃?where x=5 mol%⁸ mol%,y=56 mol%⁶0 mol%?ceramics were synthesized by solid state reaction method.It is found that most of compositions with La>6 mol%and Zr/Ti>58/42 possess excellent dielectric and piezoelectric properties(d₃₃>700 pC/N,?_r>5000).The optimal properties with?_r=6076,tan?=0.025,d₃₃=821 pC/N,k_p=0.70 and Q_m=40 is obtained in 6.5/59/41.In order to make clear the mechanism of the strong piezoelectricity of these compositions,the crystal structures,electrical properties and relaxation behaviors of x/59/41 were studied as a function of La content from 0 mol%to 6.5 mol%.It is found that with increasing La content,the crystal structures of x/59/41 transform from rhombohedral to tetragonal and then to cubic phases.Meanwhile,x/59/41 undergoes a transition from normal to relaxor ferroelectrics.The obvious relaxation behavior is presented in poled samples with La?6 mol%.Correspondingly,?_r,d₃₃and k_p increase rapidly while Q_m decreases sharply when La?6 mol%.The hysteresis loops show that when La?6 mol%,the ratio of remanent polarization P_r to maximum polarization P_max under an electric field of 2 kV/mm decreases rapidly.And for all poled samples,there exists nonlinear?low electric field?and linear?high electric field?regions in the responses of polarization to unipolar electric field.The most important thing is that the nonlinear responses in the samples with La?6 mol%are much more obvious and occur at a much lower electric field than that in low La content samples.Furthermore,these nonlinear responses are reversible.These results indicate that the domains of La?6 mol%samples are in an unstable state.A much small electric field can trigger rotation of polarization vectors or change of crystal structure,resulting in a strong dielectric and piezoelectric responses.The nanodomains existing in poled samples are exactly the source of the unstability.Generally,it is though that depolarization processes in normal and relaxor ferroelectrics are different.The depolarization process in relaxor is relative to the breaking up of long-range polar ordering and formation of short-range polar ordering.But,in normal ferroelectric,this process is connected with phase transition between ferroelectric and paraelectric phases.In present thesis,temperature dependent resonant and dielectric properties were used to study the evolution of depolarization temperature of PLZT from normal to ralxor ferroelectrics.It is found that with increasing temperature,resonant frequencies f_r and anti-resonant frequencies f_a of all samples exist a minimum value.?35?f/f_r??35?f/f_r=f_r-f_a?and?35?|Z|are close to 0 at the temperature T_min-a corresponding to the minimum value of f_a,indicating that depolarization occurs at T_min-a.Importantly,dielectric loss peak temperature T_loss is much in accord with T_min-a.In La=6.5 mol%sample,T_loss is not equal to dielectric anomalous peak temperature T_d*,suggesting that T_d*is not a depolarization temperature.The appearance of T_d*probably is related to the decoupling of domains while T_loss corresponds to the loss of macro-polarization.It implies that the depolarization process in La=6.5 ml%sample contains two steps,i.e.decoupling and disorientation of domains.These two steps may exist in all samples.With increasing La content,T_loss deviates from T_m gradually.It is noted that T_loss is not equal toT_m even in La=0 mol%sample,indicating that after depolarization,polarization vectors are disordered in all samples at least at T_loss^T_m.Similar anomalous changes in dielectric loss of unpoled and poled samples are found at T_loss,suggesting that domain configuration changes at T_loss in unpoled samples either.Above T_loss,the dielectric loss of unpoled sample is identical to the value of poled sample,implying that depolarization occurs at T_loss.Based on these results,it can be deduced that there is no essential difference between normal and relaxor ferroelectrics in depolarization and freezing processes.The transition from normal to relaxor ferroelectrics is a gradual process and the temperature difference?35?T between T_m and T_loss can be a parameter to describe the relaxation degree.In order to improve piezoelectricity of samples,1 wt%Sb₂O₅ were added into samples in above experiment.However,it is still unclear about the effect of Sb₂O₅ on the relaxation behavior and piezoelectricity of x/59/41.In present thesis,these effects were studied in detail.It is found that with increasing Sb₂O₅ content,the crystal structures of x/59/41 transform into pseudo-cubic phase.The solid solubility of Sb₂O₅ in x/59/41 decreases with increasing La content.The dielectric peak of PZT become diffuse with the effect of Sb₂O₅,but no frequency dispersion can be observed.With increasing La content,the addition of Sb₂O₅ can induce obvious relaxation behavior in PLZT.After poled,the frequency dispersion disappears in La=2 mol%samples.When La=5 mol%and Sb₂O₅?1.2 wt%,anomalous dielectric peak can be observed in the dielectric spectra of poled samples,but no frequency dispersion is presented below anomalous peak temperature.However,when La=6.5 mol%,strong frequency dispersion maintains after the samples are poled.La content shows greater influence on electrical properties than Sb₂O₅ content.With increasing La content,?_r,d₃₃ and kp increase rapidly while Q_m and P_r/P_max decrease fastly.When La?2 mol%,?_r,d₃₃,k_p,Q_m and P_r/P_max change a little with varying Sb₂O₅ content.In La>2 mol%samples,?_r and d₃₃ increase fastly with increasing Sb₂O₅ content,if the relaxation behavior of samples become more obvious.The relaxation behavior,dielectric and piezoelectric properties degenerate obviously when pyrochlore phase appears in samples.