Preparation And Physical Properties Of B-site Co-doped BaTiO₃ Multiferroic Materials

Multiferroic materials are those materials that possess two or more of ferromagnetism,ferroelectricity and ferroelasticity.Multiferroic materials can respond to external environments such as electricity,magnetism,force,heat and light.Therefore,it has the potential to be applied into novel memory,sensor and detection devices with high density and high integration.It is one of the hot spots in condensed matter physics today.There are very few natural single-phase multiferroic materials at room temperature,and it has important theoretical and practical significance to obtain multiferroic materials by ionic doping of traditional ferroelectric materials.BaTi O₃(BTO)with perovskite structure is a typical lead-free environmentally friendly material with good room temperature ferroelectricity and rich structural phase transition,which has attracted people's attention on its multiferroicity.In BTO,the formation of ferroelectricity comes from the d⁰ electronic configuration of the Ti⁴⁺ion at the B site.If magnetism transition metal ions with electrons in the d shell are selected for doping,Ti⁴⁺ions at the B site will be replaced,which will weaken the distortion tendency of the symmetry center.The doping leads to the inevitable loss of BTO ferroelectricity.More importantly,even with a small amount of doping,the crystal structure of BTO is completely transformed into a non-ferroelectric hexagonal phase,which makes the ferroelectricity of the sample even worse.Therefore,how to introduce magnetism and maintain the stability of the tetragonal ferroelectric phase in BTO as much as possible is the key to obtaining BTO-based multiferroic materials.In recent years,some researchers have used transition metals and Nb elements to co-doped BTO at the B site,which can effectively avoid the formation of hexagonal phases.The empty d orbital of Nb⁵⁺ions is also conducive to the formation of ferroelectricity,which can greatly optimize ferroelectric properties.They were focus on the photovoltaic properties,and the magnetism of those material is not studied very well.Hence,its doping modification effects need further study.It is of great significance to implement ion-type doping on BTO-based ferroelectrics,to find the balance of doping modification in inducing magnetism and maintaining ferroelectricity,and then to obtain multiferroic materials at room temperature.In this dissertation,BaTi_1-x(TM_1/2Nb_1/2)_xO₃ multiferroic materials were prepared by the traditional solid-phase sintering method in which TM transition metal elements were selected from Ni,Co and Mn.The influence of co-doping at the B site on the microstructure and physical properties of BTO was explored through the two dimensions of doping concentration and doping elements.The main research contents and results are as follows:(1)The influence of doping concentration and elements on the microstructure of BTO-based ceramics.Through room temperature X-ray diffraction(XRD)and Raman spectroscopy to analyze the crystal structure,it is found that co-doping can effectively inhibit the generation of hexagonal phase that is easily produced in single doping.In Ni-Nb,Co-Nb and Mn-Nb,when the doping concentration x is less than 0.06,the samples are all tetragonal ferroelectric phases.With the increase of doping concentration,c/a gradually decreases and the tetragonality becomes weaker.Among the co-doped samples,the tetragonality of the BTO co-doped with Mn^-Nb with high valence and small ion radius is maintained well,and the existence limit of tetragonal phase is x=0.1.The morphology of the crystal surface was characterized by scanning electron microscopy(SEM),and it was found that a small amount of doping can greatly reduce the grain size with the doping concentration increasing.The grain size almost unchanged with much more doping concentration,which shows co-doping is beneficial to the formation of grains but also leads to a lower growth rate.Among them,the grain size of the Ni-Nb and Co-Nb doped samples decreased significantly,which may be caused by the decrease of the grain growth rate due to the decrease of the electronegativity.The EDS element analysis of the Ni-Nb co-doped samples showed that the doped elements were uniformly distributed in the crystal grains without obvious agglomeration,and the uniform distribution co-doped samples were obtained.(2)The influence of doping concentration and doping ions on the electrical properties of BTO ceramics.As the doping concentration increases,the Curie temperature of the sample decreases,which is consistent with the change of the crystal structure,indicating that doping will cause the tetragonal ferroelectric phase transform into cubic paraelectric phase.The hysteresis loop study at room temperature shows that compared with the single doping in the literature,the ferroelectricity is significantly improved.With the doping concentration increasing,the saturated polarization,remanent polarization and coercive field of the sample gradually decrease.Among them,the co-doped samples of Co and Mn with smaller ion radius can maintain better ferroelectricity.(3)The influence of doping concentration and doping ions on the magnetic properties of BTO ceramics.From the study of the magnetic properties of the co-doped samples,it is found that the transition metal ion doping will introduce magnetism.The Co-Nb and Mn-Nb co-doped BTO samples exhibit strong paramagnetism or extremely weak ferromagnetism.However,The Ni²⁺co-doped BTO sample with large ion radius and low valence has the best ferromagnetic effect,which may be due to the formation of F center exchange between the oxygen vacancies generated to maintain the balance of valence and transition metal element Ni.(4)The preparation and physical properties of BaTi_0.9(Mn_1/2Nb_1/2)_0.1O₃ films.BaTi_0.9(Mn_1/2Nb_1/2)_0.1O₃ films with a preferred orientation was growth on SrTiO₃ single crystal substrate using perovskite La_0.7Sr_0.3MnO₃ film as conductive electrode by sol-gel method.It has a good hysteresis loop at room temperature.Through the study of dielectric properties,it is found that there is obvious dielectric relaxation near the phase transition temperature of manganese oxide metal-insulator.This phenomenon can be explained by the Maxwell-Wagner(MW)effect.