Study On Multifarious Properties In BNT-Based Lead-Free Ceramics By The Regulation Of B-Site Complex-Ions

Bi_0.5Na_0.5TiO₃?BNT?based lead-free ceramics possess good ferroelectric,piezoelectric and other electrical properties,which is one of the strong competitors to replace lead-based ceramics.However,compared with lead-based ceramics,there is still a big gap in its various properties,and the internal relationship among its composition,structure,and performance is not systematically studied.Based on the binary solid solution0.82Bi_0.5Na0.₅TiO₃-0.18Bi_0.5K_0.5TiO₃?0.82BNT-0.18BKT?and doped with complex-ions(Sr_1/3Nb_2/3)⁴⁺?SN?,(Mn_1/3Nb_2/3)⁴⁺?MN?and(Fe_1/4Sc_1/4Nb_1/2)⁴⁺?FSN?,the structure and multifunctional properties of 0.82BNT-0.18BKT ceramics,such as ferroelectric,piezoelectric,energy storage,strain,dielectric and relaxor,were studied in this work.?1?The effects of B-site SN doping on the crystal structure,microstructure,ferroelectric,piezoelectric,energy storage,strain,and dielectric relaxor of0.82Bi_0.5Na_0.5Ti_?1-x?(Sr_1/3Nb_2/3)_xO₃-0.18Bi_0.5K_0.5TiO₃?BNTSN_x-BKT;or BNKT-ySN,y=0.82x?ceramics were investigated.XRD and Raman analysis results indicate that BNTSN_x-BKT ceramics transform from the coexistence of rhombohedral and tetragonal phase to single tetragonal phase,and finally to pseudo-cubic phase with the increase of SN doping amount.SN doping refines the grain of BNTSN_x-BKT ceramics.The introduction of SN destroys the long-range ordered ferroelectric phase of BNTSN_x-BKT ceramics and makes it change from the ferroelectric phase to ergodic relaxor state.Under a low electric field,BNTSN_x-BKT ceramics obtain the maximum energy storage density of 0.4 J/cm³,the maximum positive strain(S_pos)of 0.31%and unipolar strain of 0.44%?0.1 Hz?at x=0.06,respectively,and the corresponding electrostrictive coefficient is 0.029 m⁴/C²?E=50kV/cm?.The piezoelectric constant d₃₃3 gradually decreases with the increase of doping content,but the normalized piezoelectric constant d^*₃₃ gradually increases to 888 pm/V.The large field-induced strain at the low electric field is due to the transformation of the field-induced relaxor phase to the ferroelectric phase and the structural phase change.The hysteresis loop and the unipolar strain curves at different frequencies show that the unipolar strain decreases and the ferroelectricity is weakened with the increase of frequency.?2?Thephasetransitionandelectricalpropertiesof Bi_0.5(Na_0.82K_0.18)_0.5Ti_?1-x?(Sr_1/3Nb_2/3)_x O₃?BNKT-xSN?ceramics were studied under different sintering temperature,concentration ratio and electric field compared with the situation in?1?.BNKT-xSN ceramics are pseudo-cubic perovskite structure,and the doping of SN has little effect on the surface morphology of BNKT-xSN ceramics.The appropriate doping amount,temperature and external electric field regulate the phase transition between the long-range ordered ferroelectric state and the relaxor state.Finer SN concentration ratio induces the coexistence of ferroelectric phase and ergodic relaxor state of BNKT-xSN ceramics at x=0.035.At x=0.03 and 0.035,the maximum S_posos is 0.25%?E=70 kV/cm?.With increasing electric field,the energy storage density increased from 0.074 J/cm³ at x=0.02?E=50 kV/cm?to 0.754 J/cm³ at x=0.045?E=80 kV/cm?,and the maximum S_pos increases from 0.075%to 0.3%at x=0.04 under the same condition.The maximum S_pos obtained from BNKT-xSN ceramics under high electric field changes from the critical phase region to the ergodic state region where the relaxor phase is the dominant state.The phase evolution behavior of BNKT-xSN ceramics in a high electric field is helpful for us to understand the interaction mechanism between the phase structure and various electrical properties of BNT-based ceramics.?3?The effects on the structure,ferroelectric,piezoelectric and dielectric properties of Bi_0.5(Na_0.82K_0.18)_0.5Ti_?1-x?(Mn_1/3Nb_2/3)_x O₃?BNKT-xMN?ceramics under different MN concentration ratios were studied.Complex-ions MN containing variable valence element Mn enriched the phase structure transition process and electrical properties of BNKT-xMN ceramics.The results indicate that BNKT-xMN ceramics change from the coexistence of rhombohedral and tetragonal phase to single tetragonal phase first,and then to pseudo-cubic phase.Moreover,the doping of MN sharply reduces the grain size of BNKT ceramics,and the continuous increase of doping amount has little influence on the surface morphology of ceramics.Ions doping and electric field regulate the long-range ordered ferroelectricity of BNKT-xMN ceramics,which first increases and then decreases.Composition-induced BNKT-xMN ceramics coexist in ferroelectric phase and ergodic relaxor state at x=0.03,and the obtained maximum S_pos and unipolar strain in this critical region are 0.38%and 0.386%,respectively.The corresponding bipolar d^*₃₃ was 767 pm/V,and the maximum d₃₃ was 158 pC/N.According to the results of XRD,Raman and dielectric curves before and after the samples being poled,the applied electric field has little influence on the crystal structure of the critical composition with high strain response.?4?The effects of mixed complex-ions FSN doping on the structure and electrical properties of Bi_0.5(Na_0.82K_0.18)_0.5Ti_?1-x?(Fe_1/4Sc_1/4Nb_1/2)_xO₃?BNKT-xFSN?ceramics were studied.BNKT-xFSN ceramics are pseudo-cubic phase.Ceramic grains grow uniformly compactly and without pore.In the relaxor state,the remanent polarization and coercive field decrease sharply with increasing SN content.With the increase of the electric field,the energy storage density of BNKT-x FSN ceramics gradually increases and reaches the maximum value of 0.96 J/cm³?70 kV/cm?at x=0.09,and the corresponding energy storage efficiency is 62%.Meanwhile,the field-induced strain of BNKT-0.07FSN ceramic increases from 0.13%at 50 kV/cm to 0.43%at 80 kV/cm,and the corresponding electrostrictive coefficient Q₃₃ reaches the maximum value of 0.0213 m⁴/C² at 70 kV/cm.The relaxor-ferroelectric phase transition induced by high electric field leads to the increase of strain.Moreover,BNT-based relaxor ceramics with pseudo-cubic structure have large electrostrictive coefficients when T_F-R is controlled to near room temperature.The composition fluctuation and disorder of cations at A and B sites make BNKT-xFSN ceramics have typical relaxor characteristics.