Study On Microstructure And Piezoelectric Response Of Bi_0.5Na_0.5TiO₃-BaTiO₃ Ceramics

In this paper,（1-x）Bi_0.5Na_0.5TiO₃-xBaTiO₃（BNT-xBT）（x=0,0.02,0.04,0.06,0.08and 0.10）piezoelectric ceramics were prepared by the traditional solid-phase sintering method as the research object.The microstructure,dielectric properties,ferroelectric properties and piezoelectric responses of BNT-xBT piezoelectric ceramics were systematically studied.（1）The microstructure and field-induced phase transition of BNT-xBT piezoelectric ceramics were characterized by SEM,XRD,Raman diffraction and TEM.The grain size of BNT-xBT piezoelectric ceramic decreases with the increase of x at x≤0.06 and increases at x>0.06.There are two kinds of nano-domains in BNT,there are polar nano-domains in the layered domain gap of BNT-6BT,and there are multilayer domains and nano-domains in BNT-10BT.With the increase of x,the rhombohedral structure in BNT-xBT ceramics transfer to the tetragonal phase,the coexistence of rhombohedral and tetragonal phase structures is at x=0.06.The poled rhombohedral structure showed a peak splitting effect,the polarity of Na-O and Ti-O bonds increased.the poled BNT reversed phase structure was strengthened,the poled BNT-6BT exhibited a phenomenon of short-range polar nano-domains transition to long-range ordering structure,the poled BNT-10BT superlattice diffraction weakened.（2）The dielectric properties of BNT-xBT piezoelectric ceramics are illustrated by the unpoled and poled dielectric temperature spectrum and the superposition of small-signal dielectric constants and loss.The dielectric temperature spectrum of BNT-xBT ceramics showed obviously frequency dispersion and thermal hysteresis characteristics.The temperature T_m and frequency f of dielectric loss peak follow the Vogel-Fulcher（VF）law,and the activation energy of the heating and cooling process is minimal at x=0.06.The activation energy of the cooling process in the system is generally larger than that of the heating process.The freezing temperature is lower than that of the heating process.The variation of the characteristic frequency of the heating and cooling decreases monotonously with the increase of x.The poled dielectric temperature spectrum BNT-xBT also showed obviously frequency dispersion and thermal hysteresis.The electric field polarization reduced the dielectric constantε_r and the low-temperature dielectric dispersion disappeared.The activation energy of the VF relationship between T_m and f of the poled BNT-xBT ceramics is generally lower than the unpolarized state,and the poled freezing temperature is higher than that of the unpolarized state.The dielectric properties of superimposed small-signal BNT-xBT ceramics produce a high dielectric constant for excitation with non-uniform polarity in the first circulating electric field,a significant decrease in the second cycle dielectric constant,and a subsequent change in cyclic dielectric constant.Not great.The dielectric loss（tanδ）is highest in the reverse direction of the first electric field cycle,and gradually becomes stable as the number of cycles increases.（3）The hysteresis loop of BNT-xBT ceramics tends to saturate as the electric field increases.The evolution of the pure hysteresis loop represented by pure BNT can be divided into three stages.The area of the hysteresis loop<A>follows a simple power law relationship.Theαvalue in the third stage of pure BNT is much larger than that of the first stage.The electric field dependence of ferroelectric at BNT-6BT is quite different from that of pure BNT.There are two stages in the scale behavior of BNT-10BT.The temperature dependence of the hysteresis loop of BNT-xBT ceramics is gradually expanded and saturated with increasing temperature.The temperature scale relationship of the BNT’s hysteresis loop<A>can be written as<A>∝T^0.843（low temperature section）and<A>∝T^0.374（high temperature section）,respectively.In the BNT-6BT,there is no The monotonous law changes the trend.The<A>of BNT-10BT basically shows an increasing state with increasing temperature.The frequency dependence of the ferroelectricity of BNT-xBT ceramics manifests in the contraction of the hysteresis loop with frequency increase.The hysteresis loop of BNT is expressed in the form of<A>∝f^-0.325（low frequency band）and<A>∝f^-0.16（high frequency band）.The domain conversion process can be divided into three areas,namely,freezing,creep and flow zones,respectively.In the BNT-xBT ceramics,the electrical hysteresis loop shrinks gradually with the number of cycles of the electric field.（4）The weak-field piezoelectric response of BNT-xBT ceramics is illustrated by the d₃₃,k_p,k_t,and k_t/k_p series piezoelectric parameters.Among them,d₃₃3 increases with x at x≤0.06,and decreases at x>0.06.The changes of k_p and k_t are basically the same as those of d₃₃,but the k_t/k_p trend is opposite to that of d₃₃.The low-field strain piezoelectric response is characterized by a linear strain curve at different frequencies of the 1/5 coercive field E_C of a BNT-xBT ceramic polarized by a DC electric field,with a linear trend and a nonlinear hysteresis strain loop.d^*₃₃shows no major fluctuations as the frequency increases.The high field strain piezoelectric response of BNT-x BT is characterized by high field strain.The strain loop with the electric field enhancement gradually becomes a butterfly curve,d^*₃₃s⁺increases,and d^*₃₃s^-declines.As the temperature increases,the butterfly curves gradually become full,d^*₃₃s⁺basically increases monotonously,and d^*₃₃s^-basically decreases monotonically.The piezoelectric response of BNT-xBT superimposed small signal is characterized by a piezoelectric hysteresis loop.As the number of electric field cycles increases,the strain curve gradually shrinks,d^*₃₃s⁺gradually decreases,and d^*₃₃s^-gradually increases.As the number of electric cycles of the superposed small-signal electric field increases,the piezoelectric return line gradually shrinks,and the maximum value of d₃₃appears in the first cyclic superposition electric field,and then decreases sequentially.（5）In order to improve the inability of large-field induced strain and strain hysteresis of BNT-based ceramics to synergize with each other,the relaxation and electrostrictive strain behavior of（1-x）Bi_0.5Na_0.5TiO₃-xSr_0.85Bi_0.1TiO₃（BNT-x SBT,x=0.4,0.5,0.6 and 0.7）ceramics was studied by forming a ferroelectric/relaxation ceramic composite with BNT and a relaxation.Ferroelectric relaxation at low temperature and Maxwell-Wagner dielectric relaxation at high temperatures have been found.At the same time,a zero hysteresis P-E loop and a very high piezoelectric strain coefficient d₃₃=1658 pC/N were obtained with a large electrostriction coefficient Q=0.287m⁴C^-2.Ferroelectric relaxation behavior and giant electrostrictive strain may be related to the reorientation and reversal of ergodic polar domains and the combined effects of Bi³⁺off-center positions and lone pair electrons.