Study On Electrical Properties And Mechanism Of BNT-BKT-BiMeO₃(Me=Fe,Cr,Co) Lead-free Piezoelectric Ceramics

Based on the fact that perovskite type Bi-based ferroelectrics of BiMeO₃(Me=Fe,Cr,Co) have high Curie temperature and strong ferroelectricity and multi-component system of piezoelectric material is effective in improving the piezoelectric properties,BiMeO₃ was selected to be the third component and added into(1-x)BNT-xBKT system ceramics to form a new Bi-based perovskite(1-x-y)BNT -xBKT-yBiMeO₃(Me=Fe,Cr,Co) system ceramics.The relation between structure characteristics and dielectric properties,piezoelectric properties,relaxor characteristics and depolarization temperature of (1-x-y)BNT-xBKT-yBiMeO₃(Me=Fe,Cr,Co) system ceramics was studied.The special effect of BiMeO₃ on crystalline structure, microstructure,dielectric properties,piezoelectric properties,and depolarization temperature was also examined.Several important conclusions can be summarized as follows:The relation between composition and crystalline structure, microstructure,dielectric properties,piezoelectric properties of new Bi-based perovskite type(1-x-y)BNT-xBKT-yBiMeO₃(Me=Fe,Cr,Co) system ceramics was systemically investigated.It was found that the morphotropic phase boundary(MPB) with rhombohedral and tetragonal co-existence crystalline structure for the ceramics lies in the range of x=0.18～0.21,y(BiFeO₃)=0～0.05,y(BiCrO₃)=0～0.02,y(BiCoO₃)=0～0.03. The best piezoelectric properties of(1)d₃₃=171 pC/N,k_p=0.366 at x=0.18 and y(BiFeO₃)=0.03;(2) d₃₃=168 pC/N at x=0.18 and y(BiCrO₃)=0.015,k_p=0.32 at x=0.18 and y(BiCrO₃)=0.01;(3) d₃₃=156 pC/N,k_p=0.34 at x=0.21 and y(BiCoO₃)=0.01 were obtained near the MPB composition.The effect of A-site nonstoichiometry on the structure and electrical properties of the composition near the MPB of(1-x-y)BNT-xBKT-yBiMeO₃ (Me=Fe,Cr,Co) ceramics was systemically studied.The trace A-site nonstoichiometry has little influence on the crystalline structure and microstructure of the ceramics.The secondary grain with a needle shape was precipitated with more A-site nonstoichiometry.Generally,the piezoelectric properties of(1-x-y)BNT-xBKT-yBiMeO₃(Me=Fe,Cr,Co) ceramics decreased with an increase A-site nonstoichiometry.However, the piezoelectric constant d₃₃ changed little with the increasing of A-site excess in(1-x-y)BNT-xBKT-yBiFeO₃ system ceramics,and the piezoelectric constant d₃₃ increased with minor deficient of A-site in (1-x-y)BNT-xBKT-yBiCoO₃ system ceramics.The mechanisms of domain-wall motion influencing electrical properties of(1-x-y) BNT-xBKT-yBiMeO₃(Me=Fe,Cr) ceramics were systemically studied by liquid nitrotgen freezing domain-wall motion.The mechanisms of intrinsic and extrinsic contributions to the dielectric and piezoelectric response have been proposed.The extrinsic contributions were improved by minor BiFeO₃ additive,which led to the improved piezoelectric properties.The extrinsic dielectric contributions were decreased by minor BiCrO₃ additive.The piezoelectric properties decreased with the increasing of BiMeO₃ and BKT content induced by the change of crystal structure,which decreased the intrinsic contributions.The poling degree was confirmed by phase angleθ_max in (1-x-y)BNT-xBKT-yBiFeO₃ ceramics.A different mechanism between k_p and d₃₃ was found.The d₃₃ changed little when stress lower than the critical stress,and decreased with increased stress when stress higher than the critical stress.The different compositions have different the critical stress.The effect of internal stress on elastic dipole and domain-wall was one of mechanism of d₃₃ variation.Theε_r and tanδas a function of temperature for(1-x-y)BNT-xBKT -yBiMeO₃(Me=Fe,Cr,Co) system ceramics was studied.There were two dielectric abnormal peak and one dielectric loss peak in theε_r-T and tanδ-T curves in(1-x-y)BNT-xBKT -yBiMeO₃(Me=Fe,Cr) system ceramics.The ceramics exhibited obvious relaxor characteristics near the low temperature dielectric abnormal peak.The low temperature dielectric abnormal peak vanishes with increasing BiCoO₃ content in the (1-x-y)BNT-xBKT-yBiCoO₃ system ceramics,and the frequency dependence exists in the whole studied temperature range.The different characteristics between relaxor ferroelectrics and normal ferroelectrics were analyzed and a theory that relaxor ferroelectric was divided into intrinsic relaxor ferroelectrics and extrinsic relaxor ferroelectrics has been proposed.It was found that the frequency dependence near the temperature of low temperature dielectric abnormal peak was created by space charge and defects dipole by analyzing theε_r-T curves of poled and unpoled samples and the frequent difference of dielectric constant.The depolarization temperature was confirmed by k_p andθ_max as a function of temperature for(1-x-y)BNT-xBKT-yBiMeO₃(Me=Fe,Cr, Co) system ceramics.It was found that the depolarization temperature and dielectric loss peak temperature were almost equal and both temperature are lower than the temperature low temperature dielectric abnormal peak.The depolarization temperature of(1-x-y)BNT-xBKT-yBiMeO₃ (Me=Fe,Cr,Co) system ceramics decreased with the increasing BiMeO₃ content.The effect of Bi-based ferroelectrics of BiMeO₃(Me=Fe,Cr,Co) on crystalline structure,micrograph,electrical properties,relaxor characteristics and depolarization temperature of(1-x-y)BNT-xBKT-yBiMeO₃ was systemically analyzed.The variation of crystalline structure,micrograph and piezoelectric properties of(1-x-y)BNT -xBKT-yBiMeO₃ system with different variety and content of BiMeO₃ was obviously different.The BiMeO₃(Me=Fe,Cr) acted as a soft additive and BiCoO₃ acted as both soft and hard additive.The dielectric behavior of(1-x-y)BNT-xBKT-yBiMeO₃(Me=Fe,Cr) system was similar. However,the effect of BiCoO₃ addition on dielectric behavior was obviously different compared with those of BiMeO₃(Me=Fe,Cr) addition. There were two dielectric loss peaks and existed an obvious frequency dependence on the studied temperature range with the increasing content of BiCoO₃.The effect of BiMeO₃(Me=Fe,Cr,Co) addition on depolarization temperature and dielectric loss peak temperature of (1-x-y)BNT-xBKT-yBiMeO₃ system was same,which kept decreasing with an increase of BiMeO₃(Me=Fe,Cr,Co) content.