Electrical Properties And Temperature Stability Of (Na_0.5Bi_0.5)TiO₃ Based Ferroelectric Ceramics

Sodium bismuth titanate(Na_0.5Bi_0.5TiO₃,abbreviated as NBT)is a kind of relaxation ferroelectric materials with ABO3 structure,which has been used in a variety of applications such as piezoelectric sensor,motor and ultrasonic transducer because of its strong ferroelectricity at room temperature and high Curie temperature.In addition,NBT-based ceramics have complex phase transitions,especially the FE/RE phase transition near the depolarization temperature can lead to the polarization obviously changed.Accordingly,outstanding electrocaloric effect?ECE?performance may be obtained.It was anticipated that EC responses near room temperature can be achieved if the FE/RE phase transition temperature of NBT-based ceramics be altered to room temperature by doping.When the FE/RE phase transition temperature decreased below room temperature,large electrostrictive effect and high energy-storage density can be obtained due to the pinched hysteresis loops at room temperature.However,it is worth noting that the existing work mainly focused on improving the piezoelectric properties of NBT-based ceramics,and little attention was paid to the study on electrocaloric effect,energy-storage performance and electrostrictive effect.Moreover,it is very difficult to obtain high d33 values of more than 200 p C/N just by conventional sintering method,and combined with the low Td,leading to the limit application in piezoelectric fields.In the case of the problems in NBT-based ceramics,we firstly doped KBT,BT into NBT and selected the compositions near MPB,the piezoelectric property and its temperature stability were investigated through optimizing preparation technology and doping nanowires.Secondly,the refrigeration applications can be achieve in NBT-based ceramics because of its FE/RE phase transition,and the operating temperature can be reduce to room temperature by doping ST to adjust the phase transition temperature.Finally,the energy-storage performance,electrostrictive effect and its temperature stability were studied in NBT-based ceramics when the FE/RE phase transition temperature reduced to lower than room temperature,and the energy-storage performance and electrostrictive effect of NBT-based ceramics were be further optimized by doping Mn O to form defect dipoles.The result of XRD scanning and temperature dependence of the dielectric properties implied that the compositions of NBT-based samples all locate at the MPB when doping with KBT and BT.The ferroelectric phase and relaxation phase of?1-x?[0.94NBT-0.06BT]-x ST system were located in the range of 0.02 ? x ? 0.20 and 0.05 ? x ? 0.40,respectively.In addtion,the R-T MPB?x=0.02⁰.06?and FE-RE MPB?x=0.08⁰.20?were existed in?1-x?[0.94NBT-0.06BT]-x ST system.All the samples of Mn O-doped 0.7[0.94NBT-0.06BT]-0.3ST ceramics were RE phase at room temperature.The piezoelectric properties of NBT-based ceramics with the compositions near MPB were significantly improved when prepared by Sol-Gel flame synthetic approach.0.90NBT-0.05KBT-0.05 BT ternary system ceramics exhibited optimalizing piezoelectric properties: the piezoelectric constant d33,electromechanical coupling factor kp and depolarization temperature Td were 213 pC/N,0.29 and 78 oC,respectively.When equal numbers of BT nanowires instead of BT solution,the Td can be raised to 95 oC because of the nanowires can pin the domain wall.As for?1-x?[0.94NBT-0.06BT]-x ST system,high piezoelectric constant of 205 p C/N and electromechanical coupling factor of 0.34 were obtained due to the forming of the R-T MPB at x=0.02-0.06.Very large d S/d E of 491 pm/V was obtained at x=0.10 due to the coexistence of RE and FE phase.Using the FE/RE phase transition near depolarization temperature,excellent EC responses were obtained in NBT-based ceramics when the compositions near MPB.The highest EC temperature change value ?T of 1.5 K and electrocaloric coefficient ?T/?E of 0.30 K cm/k V were obtained at about 100 oC under an electric field of 50 k V/cm in the 0.94NBT-0.06 BT ceramic.Both of the phase transition temperature and EC temperature change value of ST-doped 0.94NBT-0.06 BT ceramics were reduced.In the measured temperature range,the maximum temperature change ?T of 1.71 K was obtained at 50 k V/cm near 120 °C at x=0.02,and the corresponding electrocaloric coefficient ?T/?E can be up to 0.34 K mm k V-1.The operating temperature can be reduced to 50 °C with temperature change ?T =0.79 K at x=0.10,and a lower operating temperature of 30 °C was proposed with temperature change ?T =0.6 K at x=0.20.The?1-x?[0.94NBT-0.06BT]-x ST ternary system revealed outstanding energy storage performance and electrostrictive effect in RE phase area.A relatively high Pmax-Pr value of 30.8 ?C/cm2 under 90 k Vcm-1 was obtained at x=0.30,leading to a large recoverable energy density W1=0.98 J cm-3 and high energy efficiency ? =82%.Moreover,the 0.70[0.94NBT-0.06BT]-0.30 BT ceramic also exhibited good energy storage stability in the temperature range from room temperature to 120 oC.Meanwhile,high electrostrictive coefficient of 0.024 m4C2 was obtained at x=0.30,which was higher than electrostrictive materials of PZT.The energy-storage density and electrostrictive effect of NBT-BT-ST ceramics can be significantly improved after MnO doped.There were two reasons for this phenomenon: on the one hand,the BDS of NBT-BT-ST ceramics can be enhanced after doping Mn O,on the other hand,a pinched and double P-E hysteresis loop with negligible Pr can be achieved by introducing defect dipoles.Owing to the enlarged value of Pmax-Pr?37?C/cm2?and relatively high electric field?95 k V/cm?,high energy-storage density of 1.06 J cm-3 with excellent temperature stability,a large recoverable strain of 0.24%with very little hysteresis,and high electrostrictive coefficient of 0.022 m4C-2 with pure electrostrictive characteristics were acquired when 11 mol.‰ Mn-doped.