Studies On Synthesis And Properties Of Barium Titanate-based Perovskite Oxides

Barium titanate-based lead-free ceramics have been attracting much attention in the field of contemporary materials due to their innocuity,and excellent ferroelectric,piezoelectric and dielectric properties.Barium titanate-based oxides are important functional materials with ABO₃-type perovskite structure.The optical and magnetic properties of barium titanate-based oxides can be optimized by means of ion doping and forming solid solution with other materials due to the abundant ion combination types of A and B sites,which shows great application potential in the fields of solar photovoltaic and information storage.On the one hand,barium titanate has a wide band gap of more than 3 e V,which leads to a low absorption efficiency of sunlight.Therefore,narrowing the band gap is one of the important targets in experimental research.On the other hand,barium titanate has intrinsic diamagnetism,so it is necessary to excite its ferromagnetism to achieve the coexistence of ferroelectric and ferromagnetic properties.In this dissertation,based on the above two aspects of the problem,a number of groups of barium titanate ceramics were prepared by using the solid-state reaction route,and the crystal structure,morphology characteristics and optical,electrical,magnetic properties were characterized and analyzed through different testing and characterization methods.The main innovative achievements are as following:1.Double perovskite SrFe_0.5Nb_0.5O₃-doped（1-x）BaTiO₃-x Sr Fe_0.5Nb_0.5O₃（（1-x）BT-x SFN）solid solution ceramics were prepared.X-ray diffraction（XRD）and Raman spectroscopy results display that the crystal structure of the samples shows a transition from tetragonal phase to cubic phase with the increase of SFN doping content.SEM images exhibit that the addition of SFN results in a significant decrease in the grain sizes of the samples.Meanwhile,the increase of pores on the surface of the samples indicates that the increase of oxygen vacancies in the crystals.In addition,the band gap（E_g）of BT is 3.21 e V,while that of SFN-doped BT decreases to 2.66 e V.The phenomenon of band gap narrowing can be attributed to the energy band edge of BT entering into the band gap induced by the introduction of Fe³⁺.As can be seen from the test results of the ferroelectric hysteresis loops（P-E）,the ferroelectric properties of the doped samples are weakened due to the addition of SFN,which is caused by the increase of oxygen vacancies in the crystals induced by doping and high temperature sintering.2.The 0.95BT-0.05Sr₂KMoO₆（BT-SKM,K=Fe,Co,Ni,Mn）ceramics were prepared.XRD and Raman spectroscopy analysis results show that BT has tetragonal perovskite structure with space group P4mm.The crystal structure of BT-SKM ceramics changes from tetragonal phase to cubic phase when SKM is introduced into BT.SEM surface morphology images display that,compared with pure BT,the grain sizes of the doped samples decrease significantly and the pores increase.Diffuse reflectance spectra analysis shows that SKM doping can significantly reduce the E_g of BT,and SCM-doped sample has the best band gap narrowing effect.P-E curves demonstrate that BT and BT-SKM have ferroelectricity.However,after SKM doping,the oxygen vacancies in the crystals increase,which weakens ferroelectric properties of the doped samples to a certain extent.In addition,the doping of SKM changes the magnetic properties of BT,in which BT-SNM exhibits ferromagnetism.3.（1-x）BaTiO₃-x Sr₂CoMoO₆（（1-x）BT-x SCM）ceramics with different Sr₂Co MoO₆ doping concentrations were synthesized.XRD results present that the crystal structure of（1-x）BT-x SCM ceramics changes from tetragonal phase to cubic phase with the increase of x.The characteristic peaks of representing BT tetragonal phase in Raman spectra are weakened or even disappeared due to SCM doping,which further confirms the structural transformation of the doped samples.SEM images exhibit that SCM doping inhibits the grains growth of the samples.In（1-x）BT-x SCM ceramics,SCM doping effectively reduces the optical band gap of the samples.In the experimental doping concentration range,when x equals 0.07,the band gap narrowing effect is the best.M-H curves show that the magnetic properties of（1-x）BT-x SCM ceramics change from diamagnetism to ferromagnetism and then to antiferromagnetism with the increase of SCM doping content,indicating that proper doping can enhance the ferromagnetism of BT.