Structural Modulation And Dielectric Properties Of Sr-based Niobate Tungsten Bronze Ceramics

With the rapid development of avionics,data communication,automotive,and medical fields,ferroelectric materials,as an important part of functional electronic devices,have been widely used in various fields such as energy storage,energy conversion,and filtering and regulation,and are valued and explored by researchers.Due to their structural complexity,the tungsten bronze type ferroelectric materials often have diverse physical properties and cation doping is often used to modulate their crystal structure and electrical properties.Structural defects such as vacancies have a significant effect on the electrical properties of ceramics,while the mechanism of their action is still unclear and their behavior under localized structures needs to be further explored.In this thesis,we investigate the effects of Sr-based niobate tungsten bronze ceramics on the phase structure,local structure,dielectric relaxation,and ferroelectric phase transition by A-site doping and changing its filling degree,and the mechanism of action of the local structure on the electrical properties is dissected.The main studies are as follows:（1）The effect of the transition from“unfilled”to“filled”structure of(Sr_0.7Ba_0.3)_4-0.5xNa_xLa Fe_0.5Nb_9.5O₃₀（SBNLFN:x=0,0.5,0.75,1.0,1.25,1.5,2.0）ceramics on the crystallographic environment,dielectric and ferroelectric properties was investigated by introducing Na⁺in the A-site.The XRD results showed that the ceramic structure presented a single tetragonal phase,and with the increase of Na content,the grain morphology changes from columnar crystals to uniform equiaxed crystals,besides,the size was affected by the pinning effect.The SBNLFN ceramics of x=0.75 and x=1.0 at 1 k Hz exhibit good temperature stability which meet the X9R multilayer ceramic capacitor standard for potential applications.The origin of the thermal stability proposed by impedance spectroscopy tests,is attributed to the weak coupling behavior between polar nano regions（PNRs）.（2）Sr_4-xNa_2-xLa_xTa_0.6Nb_9.4O₃₀（SNLTN:0≤x≤0.4）ceramics were designed by varying the A-site filling degree to investigate the effect of A-site vacancy concentration on the crystal structure,microstructure,dielectric relaxation and ferroelectric properties of SNLTN ceramics.By XRD,structural refinements and Raman tests,the SNLTN ceramics were found to be a tetragonal phase structure with a P4bm space group at room temperature,and the degree of BO₆ octahedral distortion was related to the vacancy and A-site ion radii.At-150～200°C,the dielectric response behavior of the reentrant relaxation was exhibited.The size and kinetic behavior of different PNRs were analyzed using Vogel-Fulcher relations and the macroscopic and phenomenological statistical model,revealing the origin of the dipole glass behavior,which is related to the disorder of the lattice structure and the weak coupling between PNRs.The relaxation ferroelectricity was characterized by ferroelectric tests,and an energy storage density of 2.06 J/cm³ and an energy storage efficiency of 75%was obtained at x=0.2.（3）By designing the“filled”Sr_4-2xNa_2+xLa_xTa_0.6Nb_9.4O₃₀（LT1-LT4）ceramics with the same elemental doping as the"unfilled"structure of LV2:Sr_4-xNa_2-xLa_xTa_0.6Nb_9.4O₃₀（x=0.2）ceramics,the effect of vacancies on the phase structure,microstructure,dielectric relaxation,conductivity behavior and charge/discharge characteristics were analyzed.It is shown that structural vacancies have a significant inhibitory effect on both heterogeneous phase generation and grain size,and promote the ferroelectric phase transition at high temperatures toward low temperatures.The appropriate number of vacancies has obvious compensation for the polarization intensity through energy storage tests,and higher power density was obtained at LT4,which could be a candidate material for pulsed energy storage devices.