The Effect Of A/B Ion Regulation On The Composition, Structure And Electrical Properties Of BCZT Ceramics

In recent years,(Ba_0.85Ca_0.15)(Zr_0.10Ti_0.90)O₃?BCZT?ceramics had obtained significant attention and were considered as one of the most promising candidates in lead-free piezoelectric ceramics system due to its excellent piezoelectric and ferroelectric properties.However,there still existed some problems in BCZT based materials:?1?High piezoelectric performance had originated from a triple point of paraelectric cubic phase,ferroelectric rhombohedral and tetragonal phases close to room temperature,and the critical point had strong dependence on temperature and composition.The electrical property would drop rapidly once deviated from the temperature and composition.?2?Owing to the low Curie temperature about 90?,the dielectric properties are sharply affected near Curie temperature.?3?Because of high sintering temperature?about 1450 ??,the consumption of energy is much large during the sintering process of ceramic.All above mentioned shortcomings seriously restricted the commercial application of BCZT materials.In this thesis,the methods of metal ions doping were adopted to adjust and control the structure and electrical properties of BCZT ceramics.Firstly,BCZT ceramics with large piezoelectric constant and low dielectric loss were prepared by solid state reaction.Secondly,A or B site metal ions doping was adopted to modify the phase structure,microstructure and electrical properties of BCZT ceramics,and then explored the influence of the doping ions species and substituted sites on the phase structure and electrical properties in detail.Thirdly,by A and B site co-doping?it is anticipated to obtain(Ba_0.85Ca_0.15-xA_x)(Zr_0.10Ti_0.90-yB_y)O₃ ceramics with excellent electrical properties.Lastly,the correlation of phase structure,micro structure and electrical performance were discussed and clarified in depth,and the correlated ions substitution mechanisms were revealed.The main conclusions obtained from experimental results and analyses were as follows:1.(Ba_0.85Ca_0.15-xA_x)(Zr_0.10Ti_0.90)O₃(A=Pb²⁺,Bi³⁺,Y³⁺)ceramics were prepared by solid-state reaction method.Effects of doping ions species and contents on the sintering conditions,phase structure and electrical performance were explored in detail.The results as follows:?1?The introduction of Pb²⁺ ions at the A site in BCZT ceramics not only decreased the sintering temperature to 1250 ?,but also significantly improved the Curie temperature to 130 ?.The transition temperature of orthorhombic to tetragonal phase?O-T?shifted to lower temperature,and the transition temperature of tetragonal to cubic phase?T-C?moved to higher temperature.Therefore,the samples exhibited excellent dielectric temperature characteristic in the temperature range from-50 to 100?,and the value of ??/? was between 0% and ±15%.Furthermore,appropriate Pb²⁺doping in BCZT ceramics could effectively improved the piezoelectric and ferroelectric properties(Pr=9.63 pC/cm2,strain=0.12%,piezoelectric constant d₃₃=503 pC/N).The excellent properties were derived from that the Pb ion owns 6S2 lone pair active electrons,which could enhance the displacement polarization and thus improve the electrical properties.?2?The doping of Bi³⁺ ions in BCZT could substantially lower the sintering temperature due to the low melting point of Bi₂O₃ and give rise to dielectric relaxation behavior.The intensified dielectric relaxation behavior can be explained as follows:Bi³⁺ doping could induce the fluctuation of structure,and in turn broaden the transition temperature of tetragonal to cubic phase.In addition,the piezoelectric and ferroelectric properties were deteriorated by Bi³⁺ doping due to the formation of local disorder field which makes the rotation of ferroelectric domains become much more difficult.?3?Y³⁺ doping samples exhibited some novel electric properties.Based on the X-ray diffraction,it was clear that Y³⁺ additive induced an obvious phase transition from the coexistence of orthorhombic and tetragonal phases to single tetragonal at room temperature.The measurement of dielectric properties revealed that double permittivity peaks presented in the plots of permittivity Vs temperature,which was mainly associated with composition fluctuation.Furthermore,the improved temperature stability of ferroelectric and piezoelectric properties was obtained in the temperature range from 20 to 140 ?.Finally,the outstanding electrical properties,Pr=8.4?C/cm²,S=0.09%,d₃₃=564 pC/N and kp=58%,were obtained at x=0.02 due to the coexistence of orthorhombic and tetragonal phases.2.(Ba_0.85Ca_0.15)(Zr_0.10Ti_0.90-yBy)O₃(B=Nb⁵⁺,Mn⁴⁺,Sn⁴⁺)ceramics were prepared by solid state reaction.The phase structure,microstructure structure and electrical properties were investigated in detail.The results displayed that the species and content of doping ions had significant impacts on sintering temperature and electrical performances.?1?Nb⁵⁺ ions doping affected the electrical performances of BCZT ceramics.The temperature dependence of dielectric properties revealed that the orthorhombic phase disappeared near room temperature and the diffuse phase transition behavior occurred at Curie temperature.The piezoelectric and ferroelectric properties became weaken due to the existence of single tetragonal phase.?2?Low level of Mn⁴⁺ ions doping was quite effective to lower the sintering temperature of BCZT ceramics.The diffusion phase transition behavior was displayed between tetragonal and cubic phase.The piezoelectric and ferroelectric properties of samples were suppressed due to the coexistence of Mn²⁺ and Mn⁴⁺.In general,acceptor doping will facilitate the formation of oxygen vacancy.The positively charged oxygen vacancy would form oriented dipoles with negatively charged acceptor.The defect dipoles were known to be depressing the extrinsic ferroelectric contribution by pinning the domain walls.?3?Sn⁴⁺ doping enhanced the electrical properties of BCZT ceramic.The samples showed good electrical properties(Pr=9 ?C/cm²,d₃₃=700 pC/N and S=0.10%)when the doping level is 0.015.The enhancements of piezoelectric and ferroelectric properties were attributed to the coexistence of orthorhombic and tetragonal at room temperature which makes the arrangement of domain become easier compared with samples with single phase.3.A and B site co-doping(Ba_0.85Ca_0.15-xA_x)(Zr_0.10Ti_0.90-yB_y)O₃ ceramics were designed and prepared by solid state reaction.The phase structure,microstructure and electrical properties were investigated in detail.The conclusions acquired from the experimental results and analyses were as follows:?1?Y³⁺ and Nb⁵⁺ co-doping(Ba_0.85Ca_0.15-xY_x)(Zr_0.10Ti_0.90-yNb_y)O₃ ceramics were prepared by solid-state reaction method.The phase structure,microstructure and electrical properties were explored in detail.The results showed that the co-doping of Y³⁺ and Nb⁵⁺ weaken the piezoelectric and ferroelectric properties due to the fact that room-temperature orthorhombic phase disappeared.The complex impedance spectroscopy analysis suggested that the(Ba_0.85Ca_0.15-xY_x)(Zr_0.10Ti_0.90-yNb_y)O₃ ceramics were electrically heterogeneous,consisting of semiconducting grains and insulating grain boundaries.?2?Bi³⁺ and Mn⁴⁺ ions co-doping dramatically decreased the sintering temperature to 1200 ?,and had an obvious influence on the electrical performance.The temperature dependence of dielectric properties showed that the samples possessed good temperature stability???/?=-30%?+10%?in the temperature range from-50 to 125? due to the appearance of difuse phase transition of tetragonal to cubic phase.The complex impedance spectroscopy analysis manifested that the transition from two semicircles to single semicircle after Bi³⁺ and Mn⁴⁺ co-doping,which indicates that the substituted BCZT ceramics became more electrically homogeneous.?3?The introduction of Pb²⁺ and Sn⁴⁺ obviously improved the electrical properties.The effects of Pb²⁺ and Sn⁴⁺ concentration on phase structure,microstructure and electrical properties were investigated in detail.It was found that appropriate Pb²⁺ and Sn⁴⁺ co-doping was likely to induce an significant increase in Curie temperature from 96 to 121 ?,meanwhile the samples showed a good temperature stability in the temperature range from 0 to 100 ?.Furthermore,the samples displayed good piezoelectric and ferroelectric properties(Pr=7.64 ?C/cm,d₃₃=521 pC/N).The complex impedance spectroscopy revealed that the introduction of Pb²⁺/Sn⁴⁺ had no significant influence on the electrically heterogeneous.Combining the variation trend of complex impedance spectroscopy,it can be concluded that Pb²⁺ doping decreased the resistance of samples,while,Sn⁴⁺ doping increased the resistance of samples.Therefore,the electrical properties of BCZT samples were modulated by adjusting the content of Pb²⁺ and Sn⁴⁺.