Critical temperature below the Curie temperature of ferroelectric ceramics PZT

The article presents the results of research the pre-transitional features of the behavior of solid solutions based on lead zirconate-titanate.The presence of a“special”critical temperature T_(d)on the temperature dependences of the permittivityε(T)and the remanent polarization P_(r)(T),preceding the temperature of the paraelectric phase transition at the Curie temperature T_(C),is noted.In the temperature range T<T_(d),the P_(r)(T)dependence obeys a power law.In the temperature range T_(d)<T<T_(C),this law is not fulfilled.The results of X-ray experiments make it possible to associate this behavior with reversible disordering at T<T_(d)of an ordered domain structure formed during the polarization of piezoelectric ceramics and with its irreversible disordering in the temperature range T_(d)<T<T_(C).This is due to the appearance of internal mechanical stresses in a polycrystalline ferroelectric due to irreversible depolarization of the samples at temperatures T_(d)<T<T_(C).

F-centers mechanism of long-term relaxation in lead zirconate-titanate based piezoelectric ceramics.2.After-field relaxation

The paper presents results of experimental study of the dielectric constant relaxation during aging process in Pb(Zr,Ti)O_(3) based solid solutions(PZT)after action of external DC electric field.The said process is a long-term one and is described by the logarithmic function of time.Reversible and nonreversible relaxation process takes place depending on the field intensity.The relaxation rate depends on the field strength also,and the said dependence has nonlinear and nonmonotonic form,if external field leads to domain disordering.The oxygen vacancies-based model for description of the long-term relaxation processes is suggested.The model takes into account the oxygen vacancies on the sample's surface ends,their conversion into Fþ-and F^(0)-centers under external effects and subsequent relaxation of these centers into the simple oxygen vacancies after the action termination.F-centers formation leads to the violation of the original sample's electroneutrality,and generate intrinsic DC electric field into the sample.Relaxation of F-centers is accompanied by the reduction of the electric field,induced by them,and relaxation of the dielectric constant,as consequent effect.

F-center mechanism of long-term relaxation in lead zirconate-titanate-based piezoelectric ceramics.1.After-heating relaxation

Results of experimental investigation of relaxation aging processes in Pb(Zr,Ti)O_(3)-based solid solutions after termination of external actions are presented.Heating,DC electric field,uniaxial pressure and some of their combinations were taken as external actions.In the main part of the present paper,we use heating as external action.The said processes are long-time one and are described by the logarithmic function of time.Reversible and nonreversible relaxation processes take place depending on the action intensity.The relaxation rate depends on the action intensity also,and the said dependence has nonlinear and nonmonotonic form if external action leads to domain disordering.The oxygen vacancies-based model for description of the long-time relaxation processes is suggested.The model takes into account oxygen vacancies on the sample's surface ends,their conversion into Fþ-and F0-centers under external effects(due to the liberation of the pyroelectric charge)and subsequent relaxation of these centers into the simple oxygen vacancies after the actions termination.The initial sample's state is electroneutrality one.F-center formation leads to the violation of the original sample's electroneutrality,and generates DC electric field into the sample.Relaxation of F-centers is accompanied by decreasing of electric field,induced by them,and dielectric constant relaxation as consequent effect.

Temperature-activation mechanism of the temperature dependence of the dielectric constant of ferroelectric ceramics PZT

The theoretical possibility of the temperature-activation process of the temperature dependence of the dielectric constant of sam-ples of ferroelectric ceramics lead zirconate titanate(PZT)at temperatures below the Curie point is considered.The model takes into account the 180°motion of the domain wall,which is located in the potential well.The values of activation energies(~0.01,0.1,1 eV)were obtained from the experimental dependences of the logarithm of the dielectric constant on the reciprocal tempera-ture.This is associated with three processes:initial vibrations of domain walls;separation of domain walls(DWs)from oxygen vacancies;the motion of DWs as a result of the motion of oxygen vacancies.

B-site substituted solid solutions on the base of sodium-bismuth titanate

The paper presents results of studies of the formation of phases during the solid-state synthesis in the[(Na_(0.5)Bi_(0.5)T_(0.80)Ba_(0.20)]Ti_(1-y)B_(y)TO_(3)system of solid solutions with B-site substitutions.The substitutions by zirconium,tin and ion complexes(In_(0.5)Nb_(0.5)T and(Fe_(0.5)Nb_(0.5)T have been studied.It has been found that the synthesis is a multi-step process associated with the formation of a number of intermediate phases(depending on the compositions and calcination temperatures).Single-phase solid solutions have been produced at the calcination temperatures in the interval 1000–1100℃.An increase in the substituting ions concentration leads to a linear increase of the crystal cell size.At the same time,the tolerance factor gets reduced boosting the stability of the antiferroelectric phase as compared to that of the ferroelectric phase.

Dimensional t-factor variation and increase of stability of the ferroelectric state in(Na_(0.5)Bi_(0.5))TiO_(3)-based solid solutions

The influence of the B-site ion substitutions in(1-x)(Bi_(1/2)Na_(1/2))TiO_(3)–xBaTiO_(3)system of solid solutions on the relative stability of the antiferroelectric(AFE)and ferroelectric(FE)phases has been studied.The ions of zirconium,tin,and(In_(0.5)Nb_(0.5)),(Fe_(0.5)Nb_(0.5)),(Al_(0.5)V_(0.5))ion complexes have been used as substituting elements.An increase in the concentration of the substituting ion results in a near linear variation in the size of the crystal lattice cell.Along with the cell size variation,a change in the relative stability of the AFE and FE phases takes place according to the changes of the tolerance factor of the solid solution.An increase in the tolerance factor leads to the increase in the temperature of the FE–AFE phase transition,and vice versa.Obtained results indicate the way for raising the temperature of the FE–AFE phase transition in(Bi_(1/2)Na_(1/2))TiO_(3)-based solid solutions.

Effects caused by antiferroelectric nanodomains in PZT-based coarse-grained ceramics with compositions from the morphotropic boundary region

Presented results demonstrate importance of taking into account such a phenomenon as the solid solution decomposition at the boundaries separating coexisting phases in lead zirconate-titanate-based solid solutions with compositions belonging to the morphotropic boundary region of the"temperature–composition"phase diagram.It is shown that in the local decomposition of solid solutions in the vicinity of the boundaries separating the tetragonal and rhombohedral phases in lead zirconate-titanate-based solid solutions lead to the changes of the solid solution's chemical composition and to the formation of segregates.It is also shown that the proper thermoelectric treatment of samples containing these segregates can give substantially higher values of piezoelectric parameters in the lead zirconate-titanate-based compounds.