| Sodium bismuth titanate(Na0.5Bi0.5TiO3,BNT)ceramic,as one of lead-free piezoelectric ceramics,has great prospects for development.But,due to its high coercive field and high electrical conductivity,the ceramics are hard to poled.Therefore,the problems existing in BNT ceramics need to be further explored and improved.The electrical properties of the ceramics can be improved by doping ions.The effects of Al3+,Nb5+ doping and ZnO addition on microstructure,crystal structure,piezoelectric,dielectric,and ferroelectric properties of(Bi0.5Na0.5)o.94Ba0.06TiO3 ceramics were studied systematically.1.Effect of Al3+ doping on structure and electrical properties of(Bi0.5Na0.5)0.94Ba0.06TiO3 ceramic.Using traditional solid-state reaction method,(Bi0.5Na0.5)0.94 Ba0.06Ti1-xAlxO3 ceramics(denoted as BNBTA,x = 0,0.03,0.04,0.05,0.06,0.08)were prepared.Effects of Al3+ doping on microstructure,piezoelectric,dielectric,ferroelectric,and storage properties were stuided.Two typical components x=0 and x=0.03(BNBT and BNBTA/0.03)were selected.Effects of Al3+ doping and electric poling on crystalite structure and electrical behavior were studied.The results of the Raman spectra and X-ray diffraction(XRD)show that the doping of Al3+ and electric poling cause obvious lattice distortion.Both samples have dense microstructures.The samples have tetragonal-rhombohedral morphotropic phase boundary(MPB).The relative content of the tetragonal phase increases due to electric polarization.Compared with x=0,the dielectric behavior of the doped ceramics is sensitive to electric polarization and Al3+ doping.BNBTA/0.03 ceramic presents a slender electric hysteresis loop(P-E)curve.Its current loop(J-E)curves have four peaks.The sample shows larger normalized piezoelectric constant(d33*).The results show that the difference of BNBT and BNBTA/0.03 was related to the point defect concentration in ceramics.The XRD results show that the ceramics with x<0.06 have pure perovskite structure.The ceramic with x=0.08 appears a secondary phase.The dielectric temperature spectrum of electric-polarized ceramics shows that the depolarization temperature(Td)shifts below room temperature when x>0.04.The ceramics with x=0.08 do not exhibit the frequency dispersion characteristic temperature(TRE).The P-E and J-E curves show that the Al3+ doping deteriorated the ferroelectric properties.The ceramic with x=0.06 has a good energy storage density of 0.78 J/cm3 and energy storage efficiency under 70 kV/cm.The P-E curves measured at different temperatuerws show good temperature stability of energy storage properties within 30 ? and 180 ?.2.Effect of Nb5+ doping on structure and electrical properties of(Bi0.5Na0.5)0.94Ba0.06TiO3 ceramics.Using traditional solid-state reaction method,(Bi0.5Na0.5)0.94Ba0.06 Ti1-xNbxO3(denoted as BNBTN,x = 0,0.01,0.02,0.03,0.04,0.05)ceramics were preapred.The effects of content of Nb5+ amounts on microstructure,phase strucure,dielectric,piezoelectric,and ferroelectric properties were studied.The XRD results show that the pure perovskite structure was formed when x?0.05.The Nb5+ doping causes a decrease in the relative content of the tetragonal phase.The dielectric temperature spectrum shows that the temperature stability range is 37-240 ? for the doped samples.According to the RC equivalent circuit,the ac impedance spectrum is obtained,which indicates that only the grain has a major contribution to the ceramic resistivity.The conductivity decreases with the increase of Nb5+ content.When the content of Nb5+ is x=0.03,the maximum electric strain is Smax=0.55%,d33*=675 pm/V.The Pr and Ec decrease rapidly with Nb5+ content.The hysteresis loop shows long and thin type,indicating that the Nb5+ doping causes the decrease of ferroelectric performance.The x=0 and 0.01 samples were chosen and preapred using the compositions of(Bi0.5Na0.5)0.94Ba0.06TiO3,(Bi0.5Na0.5)0.94+0.01Ba0.06TiO3,(Bi0.5Na0.5)0.94Ba0.06Ti0.99Nb0.01O3,(Bi0.5Na0.5)0.94+0.015 Ba0.06Ti0.99Nb0.01O3(named BN0,BN0.01,BNO/Nb,and BN0.015/0.O1Nb respectively).BN0,BN0.01,BNO/0.01Nb and BN0.015/0.01Nb have the average grain size of 1.4 pm,1,3 ?m,1 ?m,and 0.8 ?m,respecitvely.The average grain size of the ceramics decreases with the increase of(Na0.5Bi0.5)2+ content.The temperatures corresponding to maximum dielectric constant(Tm)are 280,270,289,270 ?,respectively.Depolarization temperatures(Td)are 100,98,60,72 ?,respectively.For the BNO and BN0.01 ceramics,the excess(Na0.5Bi0.5)2+ increases the Pr and reduces the Ec.For BN0/0.01Nb and BN0.015/0.11Nb,the excess(Na0.5Bi0.5)2+ reduces the Pr and increases the Ec.In the process of sintering,air atmosphere,nitrogen atmosphere,and oxygen atmosphere were introduced respectively.The XRD results show that all the samples have a pure perovskite structure.The average grain sizes of the BNBT ceramics sintered in air,nitrogen,and oxygen atmospheres are 1.4 ?m,1.5 ?m,and 1.0 ?m,respectively.The average grain sizes of the BNBT/0.01Nb ceramics are 1.0 ?m,1.1 ?m,and 0.8 ?m,respectively.Compared with the ceramics sintered in air,the nitrogen atmosphere can improve the Td and Tm values of the ceramics,and reduce the maximum dielectric constant.The Td value of the ceramics sintered in oxygen is almost the same as that of the ceramics sintered in air.But,its Tm reduces and the ?m increases.Compared with the ceramics sintered in nitrogen and air,the ceramics sintered in oxygen show decreased dielectric dispersion degree and increased resistivity.The maximum polarization Pm values of the BNBT and BNBT/0.01Nb ceramics sintered in oxygen are 45.2 ?C/cm2 and 40.2 ?C/cm2,respectively.3.Effect of Zn2+ doping on structure and electrical properties of(Bi0.5Na0.5)0.94Ba0.06TiO3 ceramics.(Bi0.5Na0.5)0.94Ba0.06TiO3+0.3mol ZnO(BNBTZ)ceramics were prepared.Effects of sintering temperature and heating rate on phase structure and microstructure were studied.The XRD and EDS results show that the samples have the two kind of phases,i.e.,the BNBT and ZnO phases.On one hand,Zn2+ enters into the lattice and substitutes for Ti4+.On the other hand,ZnO phase exists in the grain boundary as a secondary phase.With the increase of sintering temperature,the grains of ZnO grow up,and the amount of ZnO in the grain boundary increases.As the heating rate decreases,more Zn2+ ions enter into the lattice to replace Ti4+ ions.The dielectric spectrum shows that the adding of ZnO increases the Td and ?r.The ac impedance spectra show that the main contributions to the resistivity are grains for the BNBT,while those are grains and grain boundaries for the BNBTZ. |
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