Tailoring Of Ferroelectric Stability And Electrical Properties Of Bismuth Sodium Potassium Titanate Lead-free Ceramics

Sodium bismuth titanate?Bi1/2Na1/2?TiO3?BNT?is a complex perovskite ferroelectric oxide,in which the A-site is occupied by two different ions Bi³⁺and Na⁺.The ceramics based on BNT are considered as promising environmental friendly alternatives of the traditional Pb?Zr,Ti?O₃?PZT?-based piezoceramics,due to the superior electromechanical properties among the whole lead-free piezoelectric material family.In recent years,many other functional properties of the BNT-based ceramics also attract extensive research interests,including dielectric energy storage,electrocaloric effect,oxygen ion conductivity,humidity sensitivity,etc.From the fundamental point of view,BNT-based ceramics are relaxor ferroelectrics.The highly disordered structure,complex phase transitions under thermal and electric fields,and the resulting electrical peculiarities distinguish the BNT-based ceramics from the traditional normal ferroelectrics like PZT.One important character of BNT-based ceramics is that the long-range ferroelectric order is less stable and can be easily affected by external stimuli or compositional modifications,which may lead to some new phenomena and mechanisms.Besides,having the peculiarities of relaxors provides the possibilities of more functionalities and applications in BNT-based ceramics.Both of the above two aspects call for further investigations regarding to these fascinating materials.This dissertation is aiming at exploring methods to tailoring ferroelectric stability via doping,investigating the effects of ferroelectric stability on electrical properties,and developing new functionalities and potential applications for the BNT-based ceramics.To achieve these goals,ceramics of morphotropic phase boundary composition Bi_1/2(Na_0.8K_0.2)_1/2TiO₃?BNKT?are selected as the base material,and two representative doping elements,i.e.acceptor-Fe and donor-Nb,are doped into the BNKT ceramics to tailor the ferroelectric stability.On this basis,the significant role of ferroelectric stability in electromechanical properties,and the possibility of new functional properties of BNT-based ceramics are discussed.The main research contents are as follows:1.The effects of Fe doping on ferroelectric stability,piezoelectric properties and depolarization behavior of BNKT ceramics are investigated.The results show that Fe doping imposes a stabilization effect on the long-range ferroelectric order of BNKT ceramics.By introducing Fe³⁺,the polarization-electric field?P-E?hysteresis loop at room temperature becomes more close to rectangle,and the temperature corresponding to the pinching of P-E hysteresis loop is shifted toward higher temperature.The determined ferroelectric-to-relaxor transition temperature T_F-R also shows a continuous increase trend with Fe doping.In addition,it is found that the Fe doping at an appropriate level?x?3.0%?improves piezoelectric property and thermal stability simultaneously.The piezoelectric constant d33 increases from 125 pC/N for the undoped sample to 148 pC/N for the sample with Fe content of 3.0%.Simultaneously,the depolarization temperature T_d is promoted from 76°C to 102°C within the same composition range.It is suggested that the simultaneous improvement of d₃₃ and T_d is originated from the stabilization of long-range ferroelectric order.This result also indicates that the long-standing viewpoint of the inverse relationship between piezoelectric property and depolarization temperature is not a universal rule in BNT-based ceramics.For the BNT-based systems with less stable long-range ferroelectric order,it is possible to simultaneously improve the piezoelectric property and depolarization temperature by stabilizing the long-range ferroelectric order.2.A novel phenomenological model,which combines the Landau-Devonshire theory and ferroelectric Preisach model,for the double P-E hysteresis loop of BNT-based ceramics is proposed.This model fits well to the measured double P-E hysteresis loops for the Fe doped BNKT ceramics at elevated temperatures,suggesting that this model can provide a reasonable description on the ferroelectric property and electric induced phase transition behavior.The significance of this model is that it connects the underlying free-energy density with experimentally observable P-E loop,and makes it possible to access the underlying free-energy density by fitting the double P-E loops.On this basis,the free-energy densities for BNKT ceramics with various Fe doping contents at 80°C are calculated.The result shows that Fe doping lowers the free-energy density of the ferroelectric state?relative to the relaxor state?.3.Electric field induced phase transition behaviors of Fe doped BNKT ceramics during the well repeatable electrical cycles are investigated,and E-T phase diagrams are constructed based on the free-energy landscapes obtained by fitting P-E hysteresis loops.This method of constructing E-T phase diagram eliminates the interference of the hysteresis inherent in the first-order transition on the experimental determination of the electric field representing ferroelectric and relaxor two-phase equilibrium.The constructed E-T phase diagrams meet the basic requirements of thermodynamics,for example the Clausius-Clapeyron relation,and therefore overcome some of the shortcomings of previous reported E-T phase diagrams for BNT-based ceramics.Furthermore,the relationship between E-T phase diagram and electromechanical properties is established,and the effects of Fe doping on BNKT ceramics are discussed in terms of E-T phase diagrams.Form the E-T phase diagrams,it is predicted that Fe doping could lower the threshold field of triggering giant strain for the BNKT ceramics at high temperature,so that the Fe doped ceramics will deliver large strain responses at some lower electric fields.This prediction is successfully verified by the experimental measurement of electric field induced strain,which shows that the E-T phase diagram can provide a valuable guidance for the improvement of electromechanical properties of BNT-based ceramics.4.The effects of acceptor-Fe and donor-Nb co-doping on ferroelectric stability and electromechanical properties of BNKT ceramics are investigated.The results show that Fe doping stabilizes the long-range ferroelectric order,while the Nb doping tends to disrupt the long-range ferroelectric order.A counterintuitive variation trend that d33increase with Fe doping and decrease with Nb doping is observed in the vast majority of the investigated compositional range,which is opposite to the conventional“softening/hardening”effects.The conventional“softening/hardening”effects only hold for a small compositional range,where the Fe content is relatively high?x>3.0%?and the Nb content is low?y?1.0%?.This result indicates that the“softening/hardening”effects are not fully applicable for the BNT-based ceramics.In some BNT-based systems,where the long-range ferroelectric order less stable and can be easily affected by the introduction of foreign ions,the conventional“softening/hardening”effects may fail.In stead,the ferroelectric stability becomes the dominant controlling factor determining the magnitude of small-signal piezoelectric properties.The significant role of ferroelectric stability in the electric field induced strain property is also discussed.5.Two new functionalities of BNT-based materials,which are tristate ferroelectric memory effect and strain memory effect,are proposed.These memory effects are based on the electrically induced transitions between ferroelectric and relaxor states.During the polarization reversal,the relaxor state appears as a thermodynamically stable intermediate state between the two ferroelectric remanent states of opposite directions.Such a unique ferroelectric behavior is successfully modeled by a triple-well free-erengy landscape.Experimental verifications of the tristate ferroelectric memory effect and strain memory effect are conducted on Fe,Nb co-doped BNKT ceramics.Results show that both of the memory effects can be operated as proposed,and the programmability and the retention ability of both effects are fairly good.This work provides a new facile approach to the multistate ferroelectric memory and the electrically controlled shape memory actuator applications.