Study On Electric Field-Induced Strain And Electrostrictive Characteristics Of Sodium Bismuth Titanate-based Piezoelectric Ceramics

Bi0.5Na0.5TiO3(BNT)is a relaxor ferroelectrics with a perovskite(ABO3)structure,which has been considered as one of the most suitable candidates to replace the lead-based piezoelectric materials in practical applications due to its excellent ferroelectric polariazation(Pr=38 uC/cm2)at room temperature and high Curie temperature(Tc=320?).Recently,wide research on the BNT-based materials has revealed that the BNT-based piezoelectric ceramics not only have promising piezoelectric properties in ferroelectric phase,but also exhibit large electric field-induced strain in two-phase coexistence with ferroelectric and relaxor phases,as well as pure electrostrictive behaviors in relaxor phase,respectively.Since both the strain and electrostrictive responses in the BNT-based ceramics are comparable with those of the lead-based piezoelectric ceramics,the design and preparation of the BNT-based piezoelectric ceramics featuring high electro-strain or electrostrictive performances at room temperature has been one of the most popular subjects in the field of the lead-free piezoelectric materials at present.This project is easy to be implemented based on the fact that the ferroelectric-to-relaxor phase transition temperature of the BNT-based materials can be controlled effectively by using solid solution formation,chemical modification and A/B-site ion substitution,etc.However,up to now,the BNT-based piezoelectric ceramics still suffer some defects,that counteract their utilization seriously,such as:the relationship between phase structure and electrical properties along with the mechanism of electromechanical coupling response are still unclear;the piezoelectric,electro-strain and electrostrictive properties need to be further improved;in addition,the generation of large electromechanical strain in the BNT-based systems is accompanied by the relatively high electric field required,excessive hysteresis and poor temperature stability,etc.In regard with these problems,the studies in this paper focus on the phase modification and performance optimization of the BNT-based morphotropic phase boundary(MPB)and polycrystalline phase boundary(PPB)compositions by introducing the perovskite-type chemical dopants.Several kinds of the BNT-based piezoelectric ceramics with large electro-strain responses or good electrostrictive properties have been successfully prepared,and the structure and electrical properties of these systems are investigated systematically.The main results obtained in this paper are as the following:1.The effects of CaZrO3-doping on the structural and the electrical properties of(1-x)(0.852Bi0.5Na0.5TiO3-0.12Bi0.5K0.5TiO3-0.028BaTiO3)-xCaZrO3(x=0-0.05)piezoelectric ceramics are systematically studied.The unmodified 0.852Bi0.5Na0.5TiO3-0.12Bi0.5K0.5TiO3-0.028BaTiO3 ceramic exhibits typical ferroelectric and piezoelectric characteristics at room temperature,and its ferroelectric-to-relaxor phase transition temperature(TF-R)is as high as?109?.It can be found that the addition of CaZrO3 effectively reduces TF-R of the system,and correspondingly the initial long-range ferroelectric order is disrupted significantly with the increase of CaZrO3 content.When x?0.03,as TF-R reduces down to room temperature,the samples can be considered as two-phase coexistence with ferroelectric and relaxor phases at room temperature.The occurrence of the reversible phase transition between relaxor and ferroelectric phases under external electric field leads to a dramatic increase in the electro-strain performance.The maximum value of the room-temperature unipolar strain obtained from this system is?0.35%at x=0.04,corresponding to a large normalized strain Smax/Emax equaling 500 pm/V?2.(1-x)(0.8Bi0.5Na0.5TiO3-0.2SrTiO3)-xLiNbO3(x=0-0.08)piezoelectric ceramics were fabricated by using a solid-state sintering technology,and the compositionally induced phase transition,electro-strain and electrostrictive behaviors of this system were systematically investigated.The test results reveal that the addition of LiNbO3 into the matrix material 0.8Bi0.5Na0.5TiO3-0.2SrTiO3 has an effect to enhance the cubic symmetry of the crystal structure and improve the dielectric relaxation,thus,resulting in the disruption of long-range ferroelectric phase and the formation of relaxor phase.However,the degradation of the ferroelectric and piezoelectric properties was accompanied by a significant increase in the electromechanical strain.The samples with x=0.03-0.05 exhibit large electric field-induced strain responses at room temperature possibly due to the low energy barrier between ferroelectric and relaxor phases in the coexisted phase region,where the relaxor phase can be transformed into the ferroelectric phase reversibly during the cycling of applied electric field.As a result,a maximum unipolar strain of-0.36%with corresponding normalized strain Smax/Emax of-600 pm/V is obtained at x=0.04.Moreover,it can also be found that the compositions with high LN contents(x?0.06)process predominant electrosrictive behaviors with relatively high electrostrictive coefficient Q33 and excellent temperature stability when the field-induced phase transition cannot be trigged by the applied electric field,as evidenced by a fact that the Q33 value of the sample with x=0.08 keeps almost constant as high as~0.028 m4/C2 in the temperature range from room temperature to 120?.3.The composition-and temperature-dependent electric field-induced strain behaviors of the 0.985[(0.94-x)Bi0.4Na0.5Ti03-0.06BaTi03-xSrTi03]-O.015LiNb03(x-=0-0.05)piezoelectric ceramics were investigated.The results of room-temperature tests indicate that a few amount(x=0.01)of SrTi03-doping has an effect to improve the ferroelectric and piezoelectric properties of this studied system,which should be ascribed to the enhancement of the domain-wall density and mobility,particularly the non-180° domain switching under the electric field.With the further increase in SrTiO3 content,although the long-range ferroelectric order is disrupted and transformed into the relaxor phase,the usable strain response is enhanced outstandingly at room temperature.At x=0.03,the room-temperature unipolar strain under a moderate field of 50 kV/cm reaches the maximum value of-0.44%(Smax/Emax-880 pm/V).This giant strain is suggested to be closely associated with the coexistence of the ferroelectric and relaxor phases developed by the SrTiO3 substitution,which should be mainly originated from the reversible electric field-induced transition between the relaxor and ferroelectric phases.Furthermore,the results of temperature-dependent measurements reveal that the increase of SrTiO3 content also increases the relaxation characteristics of the system,thus,leading to the Smax/Emax of the composition x=0.03 showing a relatively good temperature stability,which is still as high as-490 pm/V even at a high temperature of 120?.4.The electrostrictive and energy storage properties of the relaxor components(x=0.05-0.30)of the 0.985[(0.94-x)Bi0.5Na0.5TiO3-0.06BaTiO3-xSrTiO3]-0.015LiNbO3 piezoelectric ceramics were systematically investigated.According to the test results,the increase of SrTiO3 content significantly reduces the phase transition temperature(TR-T)between the R3c PNRs with strong polarity and the P4bm PNRs with weak polarity.Therefore,the samples with high SrTiO3 contents(x?0.05)exhibit typical relaxor ferroelectric characteristics at room temperature.When x?0.20,the electric field-induced phase transition between the ferroelectric and relaxor phases is completely suppressed,leading to the pure electrostrictive behaviors of the samples.At x=0.25,the specific values of the electrostrictive performances reache the optimality,which are:Suni=?0.22%,Smax/Emax=?344 pm/V,Q33=?0.022 m4/C2 and H-10.4%.On the other hand,as the disappearance of the ferroelctric domain induced by the applied electric field decreases the energy loss density W2 dramatically,both the energy storage density W1 and efficiency ? of this system at room temperature are also been greatly improved by the introduction of SrTiO3,Under an applied electric field of 70 kV/cm,a maximum room-temperature W1 of-0.79 J/cm3 is obtained at x=0.25,and which varies less than 11%in a wide temperature range between 25? and 160 ?.