Study On Electric Field-induced Strain And Electrocaloric Effect Of BNT-BT-LN Based Ferroelectric Ceramics

There are serious environmental pollution problems in the preparation of lead-based materials.Thus in recent years,countries have introduced policies to restrict the production and use of lead-based materials.Lead-free materials have gradually become one of the research hotspots in functional materials.Bi_0.5Na_0.5Ti O₃(BNT)-based materials,as an environment-friendly lead-free functional material,has excellent optoelectronic properties and is widely used in ferroelectric memory,actuators,dielectric tuners,sensors,solid state refrigeration and many other fields.Therefore,we choose the BNT-based material as the research object of this thesis.BNT based materials exhibit complex phase transition behaviors:(1)structural phase transitions with conventional morphotropic phase boundaries of rhombohedral and Pseudo-cubic coexisting phases,rhombohedral and tetragonal coexisting phases;(2)nonstructural phase transitions with ferroelectric and relaxor phases near depolarization temperature;(3)core-shell structures with nano-scale.These reasons lead to excellent strain and electrocaloric effect of the materials.But the specific characteristics of the phase transition and the excellent electrical properties of the material still remain controversial.In view of these problems,the following studies have been carried out in this research:1.The ternary solid solution ceramics of(1-x)[(1-y)BNT-y BT]-x LN were prepared.The effects of Li Nb O₃ on the phase structure,ferroelectric phase and relaxation phase transition of BNT-BT binary system were studied systematically.The composition regions of phase structure change,ferroelectric phase and relaxation phase transition are determined.At the same time,the summary data show that when x=2.5%mol,y=6%mol,the material has a large electrostrain(S)of 0.6%and a large dynamic piezoelectric constant(S_max/E_max)of 857pm/V.2.The mechanism of excellent electrical properties of Li Nb O₃doped 0.94Bi_0.5Na_0.5Ti O₃-0.06Ba Ti O₃ has been studied.The results show that the excellent electrical properties of Li Nb O₃ with0.025 mol can be attributed to the following mechanisms:(1)The composition point is the transition point from rhombohedral and tetragonal phase to tetragonal phase.(2)the composition point is a coexistence phase of relaxation phase and ferroelectric phase at room temperature.(3)The core-shell structure can be formed consisting of polar nanometer region core and non-polar particles.The interaction between the three factors is the reason for the excellent performance of the material.3.According to the electrostrain effect of BNT-based materials,the induced electric field of electrostrain is higher in practical application and production,and it is difficult to achieve the induced electric field in practical application.In response to these shortcomings,we used the effect of grain size to prepare the powder by sol-gel method,and sintered 0.975(0.94Bi_0.5Na_0.5Ti O₃-0.06Ba Ti O₃)-0.025Li Nb O₃ nanocrystalline ceramics by the spark plasma sintering.The results confirmed the superiority of the nanocrystalline ceramics prepared by the electric spark sintering over those of micron grained ceramics.The threshold electric field of the material decreased from 70k V/cm to 50 k V/cm,the strain increased from 0.6%to 0.64%,the dynamic piezoelectric constant increased from 857 pm/V to1280 pm/V,and the hysteresis decreased from 58%to 19.5%.The improvement of the above properties greatly promotes the practical application value of BNT-based materials.4.First of all,Li Nb O₃ doped 0.94BNT-0.06BT ceramic system can reduce the cooling temperature of the material.The refrigeration temperature of 0.94 BNT-0.06BT ceramics was reduced from 100-140?C to 25-80?C,and the refrigeration effect near room temperature was realized.The?T>0.6K of the materials in this temperature range was achieved.At the same time,The results indicated that the heat of phase transition from rhombohedral phase to pseudo-cubic phase in 0.96BNT-0.04BT near room temperature leads to a huge negative electrocaloric effect?T?-1.08K at 60k V/cm.The phase change heat due to ferroelectric and relaxation phase transition also exhibits a large positive electrocaloric effect(?T?2.45K)at 160?C.It can be seen that 0.96BNT-0.04BT materials have huge thermal effect applications.At the same time,the ferroelectric and relaxation transition temperatures of 0.96BNT-0.04BT were reduced by doping Li Nb O₃.The maximum temperature variations of all components were obtained at 50k V/cm.The maximum temperature variations of the materials with 0.02 mol,0.04 mol and 0.05 mol of Li Nb O₃ were?T_max=2.64K,1.61K and 1.98K at 130,100 and 90?C respectively.It can be seen that Lithium Niobate Doped 0.96BNT-0.04BT ceramics have excellent dielectric refrigeration properties,which makes the material have potential practical application in the field of refrigeration.