Preparation And Properties Of Large Electric-field-induced Strain Sodium Bismuth Titanate Based Lead-free Ceramics

Sodium bismuth titanate(Bi_0.5Na_0.5TiO₃,abbreviated BNT)is a kind of important perovskite type lead-free ferroelectric and piezoelectric material.These compounds have been extensively employed in modern electronic devices,such as the sensors,actuators and ultrasonic transducers for their excellent dielectric,piezoelectric and electromechanical properties.At present,the research on BNT mainly focuses on improving the piezoelectric coefficient,electrical conductivity and coercive field etc.Referring to the research experience of traditional PZT materials,it has been found binary and multicomponent solid solutions,such as(Bi_0.5Na_0.5)TiO₃-BaTiO₃?BNT-BT?,(Bi_0.5Na_0.5)TiO₃-(Bi_0.5K_0.5)TiO₃?BNT-BKT?and BNT-BT-ST(Bi_0.5Na_0.5)TiO₃-BaTiO₃-SrTiO₃ solid solution,can be prepared near the morphotropic phase boundary?MPB?,and their piezoelectric properties have been greatly improved.In 2007,Zhang et al have made great electric-field-induced strain in BNT-BT-KNN ceramicsystem,its maximum unipolar strain reachs 0.45%,more than the commercial PZT.This groundbreaking discovery make more and more researchers believe that BNT based material are very promising to replace commercial lead materials in piezoelectric actuators and began to focus on the electric-field-induced strain in BNT based material,and as a result more and more material systems have been developed.However,their commercialization currently faces two major challenges:one is that high electric field required for activating the large strains;the other one is the large strain hysteresis.which seriously restricted its application.In addition,the temperature stability,fatigue resistance and the characteristics of co-firing between ceramic and electrode will seriously affect its application.In order to obtain a new BNT based ceramics with large electric-field-induced strain and improve the electrical properties and sintering characteristics of the large electric-field-induced strain BNT based ceramics which have been found,the physical and chemical structureofnormal ferroelectrics(Bi_1/2(Na_0.82K_0.12)_1/2TiO₃)andlarge electric-field-inducedstrainrelaxor(0.75(Bi_0.5Na_0.5)TiO₃-0.25SrTiO₃and(Bi_0.5Na_0.5)_0.935Ba_0.065Ti_0.975(Fe_0.5Nb_0.5)_0.025O₃)has been adjusted by means of ion replacement,relaxor/ferroelectric composite,platelet-shaped crystallites doping and low melting point oxide additives.The structure and electrical properties of the materials are studied systematically,and the dependence of their properties on the composition and microstructure of the materials is also analyzed.The main research work is as follows:1.The effect of(Fe_0.5Nb_0.5)⁴⁺?FN?substitution on the phase structure,microstructure,dielectric,piezoelectric,ferroelectric and electric-field-induced strain properties of Bi_1/2(Na_0.82K_0.12)_1/2TiO₃?BNKT?ceramics was systematically investigated.The FN substitution into BNKT not only induces a phase transition from ferroelectric to relaxor phase with a significant disruption of the long-range ferroelectric order,but also regulates the electrical properties of ceramics.With the increase of FN content,the relaxor-ferroelectric transition temperature T_F-R-R decreases from 82?to room temperature,and the remanent polarization?P_r?,coercive field?E_c?,negative strain(S_neg)and piezoelectric constant(d₃₃)decrease rapidly.Interesting,the degradation of the polarization and piezoelectric activity was accompanied by a significant enhancement in the strain response.,at room temperature under the condition of 7 kV/mm.The BNKT-0.05FN ceramic sample exhibit a maximum unipolar strain value of S=0.462%(corresponding to a large signal piezoelectric coefficient d^*₃₃3 of 660 pm/V),and also shows good frequency stability of unipolar strain.The location of the low frequency peaks on the dielectric cuves shows a small fluctuation before and after poling,indicating that the BNKT-0.05FN ceramics also have weak ferroelectricity,namely the structure state of the coexistence of FE and RE phases.2.The relaxor/ferroelectric?RE/FE?composite approach was studied for tailoring strain properties of incipient piezoceramics.It mainly included:?1?By adding BNKT normal ferroelectric ceramic layer in ergodic relaxor BNKT-0.05FN,i.e.,constructing RE/FE 2-2 composite ceramics,which improves its piezoelectric properties and effectively reduces the saturation polarization electric field and strain driving electric field of materials.Under the same strain level,the driving electric field of 90 vol%BNKT-0.05FN/10 vol%BNKT composite ceramics is 5 kV/mm,which is 16.7%lower than that of pure BNKT-0.05FN,and its normalized strain is d^*₃₃=700 pm/V.?2?By introducing the BNKT-4ST of large particles into the BNT-25ST of small particles,a lead-free RE/FE 0-3composite was developed with a large strain that resulted from the electric-field-induced ergodic relaxor-to-ferroelectric phase transition at a relatively low operational field of 3kV/mm,it has reached commercial standards.The composite comprised of 90wt%BNT-25ST RE matrix and 10 wt%BNKT-4ST FE seed shows a large strain value of0.25%(d^*₃₃3 of 833 pm/V)at room temperature.3.The effect of platelet-shaped Bi₄Ti₃O₁₂?BiT?substitution on the phase structure,microstructure,dielectric,piezoelectric,ferroelectric and electric-field-induced strain properties of Bi_1/2(Na_0.82K_0.12)_1/2TiO₃?BNKT?ceramics was systematically investigated.The results show that the BiT grains are completely dissolved into the BNKT lattice and form a uniformsolidsolution.TheintroductionofBi₄Ti₃O₁₂overdosesBi³⁺in Bi_1/2(Na_0.82K_0.12)_1/2TiO₃ system,producing A-site vacancies?V_A?.These vacancies induce randomly distributed local polarization fields and in turn a phase transition from a ferroelectric?FE?state to a relaxor?RE?state,thereby resulting in a narrow hysteresis loop and a large strain,as well as,downward shift of T_F-R-R at the transition region like other previous reports on BNT-based relaxor.The changes in the electrical properties show that the ferroelectric-relaxor phase transition occurs during the solid solution process.It is note worthy that 93 wt%BNKT-7 wt%BiT and 91 wt%BNKT-9 wt%BiT ceramics not only have a large unipolar strain 0.33%and 0.29%,but also exhibit lower strain hysteresis?of 33%and 23%.In the process of applying the electric field,the changes of strain can be divided into three different stages:At the first stage,the RE phase is still existed and the strain is very small.This small strain is mainly derived from the contribution of electrostriction.At the second stage,the phase transition from relaxor to ferroelectric phase occurs,resulting in the strain increased significantly;At the third stage,The strain is generated by the polarization reversal of the ferroelectric domain,and it increases linearly with the increase of the applied electric field.Compared to 93 wt%BNKT-7 wt%BiT ceramics,91wt%BNKT-9 wt%BiT ceramics has more stable relaxation characteristics,resulting in reduced ferroelectric domain and domain switching under 6 kV/mm,so it shows smaller electrostrain and lower strain hysteresis.4.The effects of Li₂CO₃ doping on the sintering temperature and electrical properties of(Bi_0.5Na_0.5)_0.935Ba_0.065Ti_0.975(Fe_0.5Nb_0.5)_0.025O₃?BNBT-0.025FN?ceramics was investigated systematically.The liquid phase related to the dopants accelerated the densification process at low temperatures.The firing temperature was effectively shifted down from 1100?to about 950?,while the ceramics still possessed satisfactory electric field-induced strain performance.All of the samples exhibited a single perovskite structure and the additives had little influence on the phase structure.During the sintering process,the liquid phase wets the grain surfaces and speeds up the migration of substances at low temperatures.The sintering temperature has an important influence on the polarization and strain properties of BNBT-0.025FN+x wt%Li₂CO₃?x=0-1.5 wt%?ceramics.The sintering temperature has an important influence on the density and electrical properties of BNBT-0.025FN+x wt%Li₂CO₃?x=0-1.5 wt%?ceramics.The low sintering temperature makes the sample less compact,so it is difficult to be polarised,and the high sintering temperature will lead to overheating,the ceramic density decreases rapidly and the performance deteriorates.The BNBT-0.025FN+1.25 wt%Li₂CO₃ ceramics at the T_s of 950?has a strain of 0.38%under6 kV/mm(corresponding large signal piezoelectric coefficient,d^*₃₃=633 pm/V),which is suitable for co-firing with the pure Ag electrode to prepare a multi-layer piezoelectric actuator with low cost and large displacement.