Structure And Properties Of Filled Tungsten Bronze Ceramics

In the present thesis,the crystal structure,dielectric,ferroelectric and magnetic properties of the filled tungsten bronze ceramics were systematically studied.The magnetic ion was introduced into the B site.The influence of filling of A1 and A2 sites on the structure and properties of filled tungsten bronze ceramics is mainly discussed.The structural origin of dielectric relaxation and relaxation ferroelectric phase transition is revealed.The variation characteristics of dielectric anomaly and ferroelectric phase transition with composition are analyzed.In the Ba3SrLn2Fe2Nb8O30(Ln=La,Nd,Sm)ceramics,the single phase tetragonal filled tungsten bronze structure in space group P4/mbm is obtained for Ba3SrLa2Fe2Nb8O30,while such tungsten bronze major phase is determined together with minor amount of secondary phases in Ba3SrNd2Fe2Nb8O30 and Ba3SrSm2Fe2Nb8O30.The structure stability decreases as the ionic radius of the lanthanide elements decrease.Ba3SrLn2Fe2Nb8O30(Ln=La,Nd,Sm)ceramics exhibit typical relaxation characteristics.A wide dielectric peak with obvious frequency dispersion is observed.The peak temperature follows well with the Vogel-Fulcher relationship.Due to the disordered distribution of Ba3SrLn2Fe2Nb8O30(Ln=La,Nd,Sm)ceramics in A and B sites,the local structure fluctuation and uneven electric field and stress field are caused.The long range order of polarization is destroyed into polar nanoscale microregion,which leads to the dispersion of phase transition and dielectric relaxation behavior.As the ionic radius of lanthanides decreases,the phase transition temperature moves to high temperature(La-104 K,Nd-175 K,Sm?233 K).The obvious magnetic hysteresis loops with enhanced Mr are obtained in the present ceramics at room temperature comparing to the Ba4Ln2Fe2Nb8O30(Ln=La,Nd,Sm)ceramics and they can be ascribed to the tungsten bronze structure.The remanent magnetic moments(Mr)of Ln=La,Nd and Sm are 0.43 emu/g,0.64 emu/g and 0.46 emu/g,respectively.Ba3Ln3Ti5Nb5O30(Ln=La,Nd,Sm)ceramics have ordered A1 site ions and disordered A2 site ions.The macroscopic performance of Ba3Ln3Ti5Nb5O30(Ln=La,Nd,Sm)ceramics is similar to Ba5LnTi3Nb7O30 system.They are typical relaxor ferroelectrics.However,the relaxor ferroelectric nature of Ba5LnTi3Nb7O30 is derived from the disordered distribution in the A1 site,while the relaxor ferroelectric nature of Ba3Ln3Ti5Nb5O30 results from the disordered distribution in the A2 site.As the ionic radius of lanthanide elements decreases,the phase transition temperature of Ba3Ln3Ti5Nb5O30(Ln=La,Nd,Sm)ceramics moves to the high temperature(La-148 K,Nd?231 K,Sm?309 K).Ba3Nd3Ti5Nb5O30 and Ba3Sm3Ti5Nb5O30 ceramics exhibit ferroelectric properties at low temperatures,while Ba3La3Ti5Nb5O30 ceramics remain paraelectric when cooling to 173 K.There are some nano-scale ferroelectric domains with strip-like shape dispersed in the paraelectric matrix for all compositions in Ba3Ln3Ti5Nb5O30(Ln=La,Nd,Sm)ceramics.The commensurate structure modulations along the[110]direction are detected in the nano-scale ferroelectric domains,while the incommensurate structure modulations are detected for the paraelectric matrix.Only an extremely small portion of the areas are polar domains at room temperature,and most regions present incommensurate modulation.In spite of the existence of micro domains,the samples are still central symmetry macroscopically and show paraelectric.When the temperature decreases,the nano-scale ferroelectric domains grow and form the long-range dipolar order,which leads to the gradual transition and broad dielectric peaks.In Ba3Bi3Fe2.5Nb7.5O30 ceramics,the disordered distribution of ions at A2 sites is the main reason leading to the relaxor ferroelectric property and dielectric relaxation.It have ordered A1 site ions and disordered A2 site ions just like Ba3Ln3Ti5Nb5O30.The P-E hysteresis loops indicate the ferroelectric behaviours at low-temperature.As the temperature decreases,the ferroelectric domain grows.Ceramics change from centrosymmetric structure to non centrosymmetric structure.The magnetic hysteresis loops are obtained at room temperature and they can be ascribed to the tungsten bronze structure containing magnetic ions of Fe3+.If the replacement amount of Fe3+ ions can be further increased,the magnetic properties will be better.In Ba4Ln2Mn2Nb8O30(Ln=La,Nd,Sm)ceramics,Mn3+ ions are introduced into B site.Four Ba2+ ions occupy the A2 sites.Two Ln3+ ions occupy the A1 sites.It has ordered A-site ions like Ba4Ln2Fe2Nb8O30 ceramics.It is considered that the different performance between Ba4Ln2Mn2Nb8O30 and Ba4Ln2Fe2Nb8O30 ceramics is mainly due to the influence of B-site magnetic ions.Generally,Ba4Ln2Fe2Nb8O30 ceramics exhibit ferroelectric properties at room temperature,while Ba4Ln2Mn2Nb8O30(Ln=La,Nd,Sm)ceramics are paraelectric.It can be seen that the magnetic ions have a great influence on the dielectric properties and ferroelectric phase transition of the filled tungsten bronze.The magnetic hysteresis loops are obtained at 30 K for Ba4Ln2Mn2Nb8O30(Ln=La,Nd,Sm)ceramics.The remanent magnetic moments of Ln=La,Nd and Sm are 0.031 emu/g,0.120 emu/g and 0.162 emu/g,respectivelyOn the basis of the study of Ba4Ln2Mn2Nb8O30(Ln=La,Nd,Sm)ceramics,the composition of Ln=Nd is selected.And Fe3+ ions are used to substitute Mn3+ ions.The broad dielectric permittivity peak of Ba4Nd2MnFeNbgO30 ceramics is observed,indicating the dispersion of ferroelectric phase transition.The Ba4Nd2MnFeNb8O30 ceramics exhibit ferroelectric at low temperature.When the temperature decreases,nanoscale microdomains of Ba4Nd2MnFeNb8O30 ceramics grows and form the macro ferroelectric domains.Compared with Ba4Nd2Mn2Nb8O30 filled tungsten bronze ceramics,Ba4Nd2MnFeNb8O30 ceramics have the same A-site ion occupations.Ba2+ions completely occupy the pentagonal A2 site,and the Nd3+ ions completely occupy the A1 site.However,Ba4Nd2Mn2Nb8O30 ceramics exhibit paraelectric properties in the temperature range.And dielectric anomalies related to ferroelectric phase transition are not observed.It can be seen that the magnetic ions have an effect on the dielectric and ferroelectric properties of the materials.And the substitution of Fe ions has a positive effect on the ferroelectric phase transition of the filled tungsten bronze ceramics.The magnetic hysteresis loops of Ba4Nd2MnFeNb8O30 ceramics were observed at low temperatures.The introduction of Fe ions increased the magnetic transition temperature of the materials.The residual magnetization at 100 K is 0.013 emu/g,and the coercive field is 65 Oe.When temperature decreases to 50 K,the residual magnetization is 0.035 emu/g,and the coercive field is 200 Oe.The ferromagnetism of the material may come from the super exchange effect of Mn/Fe-O-Mn/Fe.