Study On Physical Properties Of Relaxor-based Ferroelectric Single Crystals By Electric Field Poling

Lead-based perovskite ferroelectric materials,especially relaxor-Pb Ti O₃(PT)ferroelectric single crystals(SCs),have been widely used in practical application including medical ultrasound imaging,underwater acoustic ultrasound,pressure sensor and actuator,due to their ultra-high piezoelectric and electromechanical properties.Relaxed-PT ferroelectric SCs,such as(1-x)Pb(Mg_1/3Nb_2/3)O₃-x Pb Ti O₃(PMN-x PT)and(1-x-y)Pb(In_1/2Nb_1/2)O₃-y PMN-x PT(PIN-y PMN-x PT),with components in morphotropic phase boundary(MPB)region,has piezoelectric performance much higher than that of polycrystalline lead zirconate titanate(PZT)ceramics.These excellent properties have been aroused great interest of researchers.Currently,more than 200,000 piezoelectric single crystal probes are used in premium ultrasound imaging equipment every year.In order to obtain proper piezoelectric properties,it is necessary to apply electric field on piezoelectric ceramic or lead-based perovskite ferroelectric SCs after plated electrodes on two opposite surfaces under room temperature.Generally,researchers can improve the dielectric,piezoelectric properties and temperature stability of the material by changing the composition of the material or improving the manufacturing process.However,the research cost of these methods is high,the technology is difficult,and the period is long.In 2014,Yamashita et al.,who had worked at Toshiba Corporation of Japan,demonstrated that applying alternating current(AC)instead of traditional direct current(DC)poling on lead-based perovskite relaxor ferroelectric SCs can effectively improve dielectric and piezoelectric properties of SCs.After the publication of the patents,Karaki(Chen)and Yamashita have repeatedly reported on the methods and effects of AC poling(ACP)through academic conferences since 2017.Up to now,ACP research has attracted more and more attention.Based on this,this paper separately studied the electrical properties and thermal stability of PMN-based single crystals grown by continuous-feeding Bridgman(CF BM)method with different phase transition temperatures(T_pc)under various electric field and temperature conditions.And discuss the mechanism of ACP according to the transition of phase and the crystal structure.The main work content is as follows:(1)A small(250 cm³,1.4 kg)electric field cooling AC polarization(FC ACP)device was designed and fabricated for the present work.The effects of FC ACP and HT ACP on the dielectric and piezoelectric properties of two different T_pc PMN-based SCs(PMN-0.30PT(low T_pc<95°C)and PIMN-0.26PT(high T_pc>135°C))were explored.The experiment of FC ACP is carried out in the temperature range from higher than Curie Temperature(T_c)(above 150°C)to lower than T_pc(below 80°C).Compared with HT ACP,FC ACP for PMN-0.30PT SC with low T_pc obtain free dielectric permittivity(?₃₃^T/?₀=7920)and piezoelectric coefficient(d₃₃=2290 p C/N).These properties have increased by 33%and 51%respectively,more than that of HT ACP SC.Compared with the high T_pc PIMN-0.26PT SC,the low T_pc PMN-0.30PT SC can form multi-phase coexistence inside the SC through FC ACP,thereby obtaining a significant performance improvement.The optimal ACP frequency of different component crystals is different.For PMN-0.30PT SC with low T_pc<100 ^oC,the optimal poling frequency would be less than 0.1 Hz,and for PIMN-0.26PT SC with high T_pc>130 ^oC,it would be higher than 1 Hz;(2)PIMN-0.30PT SC with high composition uniformity,after ACP treatment under high temperature and low voltage(HT LV)conditions at 90 ^oC,its?₃₃^T/?₀ and d₃₃ and electromechanical coupling coefficient(k₃₃)are 7000,2340 p C/N and 94.8%,respectively.These performances increase by 29%,29%and 1%,compared with those of DCP.However,T_pc of ACP SCs decrease from to 78-85 ^oC,which is different from the results of SCs grown by one charge Bridgman(OC BM)method reported by other teams.Temperature dependence of dielectric permittivity and XRD diffraction peaks show that PIMN-0.30PT SC has undergone a complex phase transition after HT LV ACP process;(3)PIMN-0.30PT SCs,component close to the rhombohedral structure in the MPB region,achieved ultra-high physical properties(?₃₃^T/?₀=8330,d₃₃=2750 p C/N,k₃₃=0.96,and higher T_pc of 103 ^oC)under the optimized FC ACP conditions.The above physical property values are the highest values among all the reported relaxor ferroelectric SCs including binary and ternary system ferroelectric SCs with T_pc greater than 100 ^oC.Through the phase analysis of the poled SC,it is found that the key low symmetry phase is triggered in the single crystal through multiple phase transitions during the cooling of the ACP,thus improving the electrical performance of PIMN-0.30PT SC.