Compositions Regulate The Piezo/Ferroelectrical And Electrocaloric Properties Of Potassium Tantalum Niobate Crystal

Since the piezo/ferroelectric effects discovered over one hundred years,piezo/ferroelectric materials have received extensive attention and research,and significant results have been achieved.Nowadays,various devices made of lead-based materials play indispensable roles in daily life.With the considering of urgent environmental crisis,the increased awareness of environmental protection leads lead-free materials system to gradually become a cutting-edge realm.It is significant and a long way to explore lead-free materials that can match the performance of lead-based materials.Studies have revealed that the micro-domain structures have significant impacts on the macroscopic properties,and it appeals more and more attention to improve properties by controlling domain structure.However,how to realize the controllable adjustment of the micro-domain structure still is a tough problem to be solved.Although research show that both ion doping and composition gradients(CGs)can affect the microdomain structure,the corresponding relationship and the physical mechanism of action are still unclear.The above problems are still the bottlenecks and needs to be overcome urgently,and to establish the relationship among composition,micro-domain structure and macro performance is the key to solving the above-mentioned thorny problems.Potassium tantalum niobate(KTa_1–xNb_xO₃,KTN),as a typical solid solution single crystal system,is one of the best choices to explore the above-mentioned relationship.In this dissertation,we design and grow KTN single crystal with various CGs,aiming to establish the relationship among composition parameters,micro-domain structure and macro-properties in KTN,and clarify the internal physical mechanism.In order to provide the basis supports for the further improvement of electric properties of piezo/ferroelectric materials.(1)Controllable design and growth of three-dimensional anisotropic graded KTN crystal.Based on the growth phase diagram of KTN crystal,this paper establishes the relationship between Ta/Nb ratio and growth conditions.Combined with temperature field results simulated by Fluent^?,controllable design and growth of the three-dimensional CGs in KTN are achieved.On this basis,the influences of the CGs on the dipole orientation,order-disorder state and dielectric properties of the KTN crystal are explored,and the origin of the internal field in graded KTN is analyzed.It provides a foundation of physical mechanism for further exploring the CGs regulating performance.(2)Building the relationship among CGs,micro-domain structure and macro-performances,and optimizing the properties.Preferred orientation of spontaneous polarization and defect dipoles can be induced by flexoelectric field constructed by CGs,thereby affecting the macro ferro/piezoelectric properties.In this paper,on basic of the controllable growth technics of KTN single crystals with different CGs,the corresponding relationship among anisotropic CGs,micro-domain structure and macro-properties are established via the characterization of the basic parameters and properties.On this basis,the concepts of“two large and one small”CGs relationship and“bi-component symbiotic crystals”are proposed.The former one leads strain to achieve s=0.29%(improving about 60%)under E=10 k V cm^–1 by increasing the contribution from domain switching,which provides a way for further improving the performance of piezoelectric materials.The latter one builds in over0.22%lattice distortion at the interface of component mutation,the strain gradient induces over 0.78 k V/mm flexoelectric field,which shows potential application prospects in new generation of piezoelectric,optical and other functional materials.(3)Improving the anti-fatigued performance of KTN by ions(Mn,Fe)doping.This paper takes Mn-doped and Mn&Fe co-doped KTN single crystal as the research object,exploring the effects of ion doping on the micro-domain structure,macro-ferroelectric and piezoelectric properties.The results show that co-doping can significantly reduce the domain size,enhances the local unevenness and improves the characteristics of ferroelectric loss,strain hysteresis and anti-fatigued properties of KTN crystal,possessing both soft and hard performance of modification.On this basis,huge piezoelectric parameter(d₃₃=1450 p C/N)is obtained at the orthogonal-tetragonal phase boundary of Mn:KTN,attributed to the existence of nano-size domain and local disorder characteristics at the phase boundary,which play major roles in leading to giant piezoelectric characteristics.(4)Regulating the electrocaloric properties by both composition and gradient near room temperature.Based on the relationship between the Ta/Nb ratio and the phase transition type of the KTN,this paper realizes the synergetic improvements of the latent heat and diffusion coefficient of paraelectric-ferroelectric phase transition by introducing CGs.It is a breakthrough to solve the thorny problem of mutual restriction between the above two aspects.Subsequently,the field-induced phase transition characteristics near the orthogonal-tetragonal phase boundary of the KTN crystal are studied.The effect of temperature,frequency and unipolar/bipolar on the threshold electric field(E_th)of field-induced phase transition are analyzed,and phase diagram is built by the relation between E_th and temperature.The EC performances near room temperature for two phase transitions are explored,exhibiting excellent adiabatic temperature change?T and EC efficiency?T/?E.It reveals that KTN is expected to be one of great potential EC materials.