Studies On Preparation And Properties Of Transition Metal Ions Doped BCZT Ferroelectrics

As a very important functional material,perovskite has been widely studied due its rich structural and properties.In recent years,by virtue of ferroelectric photovoltaic effect and multiferroicity,perovskite has shown high application value in photovoltaic power generation and information storage fields.Lead-free perovskite materials have become a focus of current research due of their environmental friendliness.Ba(Ti_0.8Zr_0.2)O₃-(Ba_0.7Ca_0.3)Ti O₃（BCZT）is a new lead-free perovskite material with excellent piezoelectricity(d₃₃～620 p C/N)and ferroelectricity,which has great application potential.However,BCZT has a wide band gap（E_g）of more than 3 eV,low absorption of the solar spectrum,and no magnetism.In order to explore the ferroelectric photovoltaic and multiferroic properties of BCZT,it is necessary to narrow the band gap and improve its ferromagnetism.In this dissertation,several groups of BCZT ceramics and thin films were prepared by doping transition metal ions.The micro structures,optical,electrical and magnetic properties of BCZT were analyzed and characterized,which provided a new idea for the realization of high-efficiency narrow band gap photovoltaic materials and new nonvolatile memory devices.The main research contents and resukts are as follows:1.Ba_0.85Ca_0.15(Ti_0.9Zr_0.1)_0.95TM_0.05O₃（BCZT-0.05TM）ceramics doped with four transition metal elements（TM=Mn,Fe,Co and Ni）were prepared by a solid-state reaction method.According to XRD,Raman spectroscopy and SEM,it is found that after transition metal ion doping,the composition of the tetragonal phase in the BCZT decreases,the lattice parameters increase,and the crystal grains are refined.Among them,Fe-,Co-,and Ni-doped BCZT changed from a mixed phase of rhombohedral phase and tetragonal phase to a single rhombohedral phase.Diffuse reflectance spectroscopy analysis shows that the band gap of BCZT is reduced to varying degrees after doping with transition metal ions.The E_g of Co-doped BCZT decreases from 3.18 eV to 1.68 eV.The reason is that the doping introduces new impurity energy level and lowers the conduction band.The M-H curves show that BCZT excites different degrees of magnetism by doping with different transition metal ions.Among them,the ferromagnetism of Fe-doped BCZT is the strongest,and its highest saturation magnetization is 97.42 memu/g.The charge imbalance will generate oxygen vacancies in the BCZT lattice,which with two adjacent Fe³⁺ions will generate F-centers similar to the bound magnetopolaron,exhibiting a magnetic moment to the outside,thereby increasing the magnetization.The P-E curves show that the ferroelectricity of BCZT is weakened via transition metal ions doping,which is due to the phase transition of BCZT and the weakening effect of oxygen vacancy on the ferroelectric phase.2.Ba_0.85Ca_0.15(Ti_0.9Zr_0.1)_1-xFe_xO₃（BCZT-x Fe,x=0.00～0.08）ceramics with different doping components of Fe were prepared by a solid-state reaction method. The microstructure analysis shows that as the iron doping concentration increases, BCZT-x Fe gradually changes from a mixed phase to a single rhombohedral phase. The lattice parameters and cell volume increase,the structure density and grain uniformity also increase.The optical band gap of BCZT-x Fe ceramics gradually decreases with the increase of Fe doping concentration.When the doping concentration is 0.08,E_g drops to a minimum of 1.73 eV.The energy-level splitting of Fe-3d orbital and intra-orbital transitions promote the narrowing of the band gap.With the increase of Fe doping concentration,the ferromagnetism of BCZT-x Fe ceramics gradually increases,this is due to the induced polarization caused by the spin-level splitting after doping.The ferroelectricity of BCZT-x Fe ceramics decreases with the increase of Fe doping concentration,but the residual polarization starts to rise when x is larger than 0.06.These results indicate that Fe doping can effectively narrow the band gap and stimulate the magnetization of BCZT,and the ferroelectric loss of BCZT can be controlled to some extent by adjusting the doping concentration.3.BCZT-x Fe film was prepared on（100）SrTiO₃ substrate by PLD.The experiments show that the BCZT film prepared by 700℃ annealing temperature and 200 mJ pulse laser energy has better film quality and crystallity.The BCZT film grows epitaxially along the STO substrate,and Fe doping increases the lattice parameter of BCZT and the crystallinity.SEM images show that the BCZT film has uniform grain distribution and the the thickness is about 178 nm.EDS analysis shows that the content of BCZT was basically in line with the expectation,and at the same time proves the existence of oxygen vacancies in the film.The photoluminescence spectra detect that Fe doping improves the crystallinity of the BCZT film to a certain extent,so the emission peak intensity increases.Fe doping can narrow the band gap of the BCZT film,and the E_g is reduced to 2.68 eV, which is weaker than the narrowing of the ceramic band gap.Therefore,further optimization of BCZT film process is needed.