| Due to their high residual polarization intensity,excellent electromechanical conversion efficiency and excellent fatigue properties,bismuth sodium titanate(Bi0.5Na0.5Ti O3,BNT)based perovskite relaxor ferroelectric materials have gradually become the focus of research interest in recent years.However,there are some key and difficult issues required to be solved,including: the intrinsic characteristics of depolarization temperature Td;the regulation principle and physical mechanism of Td;the relationship among the variation trend of Td and the evolution of crystal structure and corresponding macroscopic electrical properties under different external fields(temperature,electric field,chemical pressure,mechanical pressure,etc.);the relationship among structural design of special polarized region and Td and macroscopic electrical properties.According to the above-mentioned issues,this dissertation mainly focuses on the depolarization temperature Td of BNT-based ferroelectric ceramics,which explores the physical mechanism of Td,piezoelectric properties with the unbalanced development of Td,regulation of Td and optimization of macroscopic electrical properties,physical model of Td against the macroscopic electrical properties,collaborative modulation methods of composition design and modified processing to enhance macroscopic electrical properties of BNT-based ferroelectric ceramics,the mechanism of high electromechanical coupling responses near Td region.The Td of BNT-based ferroelectric ceramics was coordinated by the materials design,screening and the improvement of preparation processing.The Td of BNT-based ferroelectric ceramics could be arbitrarily moved according to the practical requirements for different electric properties,and the excellent piezoelectricity,electro-strain and ferroelectric energy-storage properties were obtained in the corresponding materials system.The main achievements were summarized as follows:(1)By adopting the design strategy of multi-scheme coordination(constructing the morphotropic phase boundary,proportional controlling the relaxor phase,adding sintering aids to improve the quality of ceramics),the critical metastable state of composition interval with coexisted multi-symmetries was obtained in BNT-based ferroelectric ceramics.Within this critical region,the high piezoelectric constant d33 =250-271 p C/N was obtained.According to this strategy,based on the composition of BNT-BKT sharing the morphotropic phase boundary,the short-range-correlated relaxor phase was further modified by Li+ and Sr2+ doping,and a critical region with co-existent multi-symmetries was constructed in the ceramic system.The doping of Li+ also plays a role in assisting the sintering of ceramic samples,which ensures that the samples can be fully polarized at a high electric field.In this critical region,the poled sample was in a metastable state,so it exhibits a large dielectric response and a high piezoelectric response.(2)By using ions doping method to decrease the Td and weaken the A-O bond of BNT-based ferroelectric ceramics,the local inhomogeneity was modified,leading to the critical coexisted state of small-sized domain structures and polar regions with weak responses.Finally,in BNT-based ferroelectric ceramics,an ultrahigh siteindependent electro-strain(S = 0.53-0.63 %)was observed.By detailed analysis on the electrostrictive-like behavior in BNT-based ferroelectric ceramics under the high doping condition,this dissertation demonstrated the discrepancies between this electrostrictive-like electro-strain and the electrostrictive electro-strain in linear dielectrics.Moreover,the electrostrictive-like electro-strain can be traced back to the changes of spontaneous polarization vectors under the electric field,which should be originated from the short-range-correlated symmetric structure in BNT-based ceramic materials.(3)By improving the sintering process and heat treatment process to control the growth of the grain size and ferroelectric domain,based on representative BNT-based ceramic materials after the composition screening,by restraining the dynamic behaviors of ferroelectric domain under the external electric field,this dissertation successfully enhanced the macroscopic piezoelectric,dielectric,ferroelectric energystorage properties in BNT-based ferroelectric ceramics.By rapid liquid nitrogen quenching from ultra-high temperature,the depolarization temperature(Td > 180 ℃)of BNT-based ferroelectric material was effectively increased without destroying the excellent piezoelectric properties(d33 > 200 p C/N).By manipulating the sintering process,the grain sizes were effectively controlled.For BNT-based ferroelectric ceramics with large grain sizes,the recoverable high electro-strain(S = 0.721%)was obtained,and fatigue properties of the ceramics were excellent(S value decreased by1.9% after 105 fatigue cycles).Via hot-pressed sintering process,BNT-based ferroelectric ceramics exhibit a dense structure with coexistent large and small grains,leading to the significantly improved density and the enhanced dielectric breakdown strength.Finally,the large and fatigue-stable ferroelectric energy-storage density(2.42J/cm3)was achieved with a good temperature stability under the low electric field(143k V/cm).(4)The transformation of meso-and micro-structures of BNT-based ferroelectric ceramics at around the Td region was analyzed,and the physical mechanism of the enhanced electromechanical coupling responses near the Td region was deeply demonstrated.When the temperature was heated to near the Td region,the polinginduced large ferroelectric domains were broken into smaller polarized clusters with different orientations.As a result,the domain wall density increased,leading to a rapid decrease in macroscopic ferroelectric polarization,and a sharp increase in the dielectric permittivity and an obvious increase in the electro-strain.The structural transition from long-range correlated R3 c to short-range correlated P4 bm is tightly related to the depolarization process,and this structural transition is the original driving force of the dynamic behaviors of the poling-induced large-size ferroelectric domain,such as fluctuation,splitting and springback.(5)The relationship between the shift of depolarization temperature Td of BNTbased ferroelectric ceramics and its macroscopic electrical properties,meso-and micro-structures was elucidated.A conceptual model of the shift of Td and regulation of the macroscopic electrical properties was established.This dissertation demonstrated that Td is the result of multiple competition factors(charge compensation,thermal deviatoric stress,ions diffusion,etc.),and the co-modulation of several major competition factors can effectively regulate the Td of BNT-based ferroelectric materials.With the help of phase-field simulation,the internal driving force of the movement of Td and the evolution of macroscopic electrical properties were further explored.The physical mechanism of the variation of the piezoelectric strain coefficient d33 and the electro-strain evaluation constant d33* with the composition was explained in a unified way,and an effective scheme was proposed to control Td according to the actual performance.By using the structural evolution as the bridge,this dissertation connected the evolution of electrical properties and the movement of Td for BNT-based ferroelectric ceramics,and the physical mechanisms of these changes were understood from the perspective of thermal and kinetic theories.This dissertation constructed the physical model of Td against the macroscopic electrical properties,proposed collaborative modulation methods of composition design and modified processing to shift Td and enhance macroscopic electrical properties of BNT-based ferroelectric ceramics,demonstrated the mechanism of enhanced electromechanical coupling responses near Td region.The Td of BNT-based ferroelectric ceramics was coordinated by designing and screening the materials and improving the preparation processings.The Td of BNTbased ferroelectric ceramics could be arbitrarily moved according to the practical requirements for different electric properties,and the excellent piezoelectricity,electro-strain and ferroelectric energy-storage properties were obtained in the corresponding materials system. |
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