Investigation Of Synthetic Process And Properties Based On Y-doped BiFeO₃ Nanoparticles

Among all kinds of multiferroic materials,BiFe O₃?BFO?exhibits the simultaneous existence of ferroelectricity with a high Curie temperature?T_C⁸30??and a G-type anti-ferromagnetism with a Neel temperature?T_N³70??,which is expected to be used in ferroelectric random access memory,optoelectronic devices and multi-iron devices,so it has attracted wide attention at home and abroad.Although BFO has high remanent polarization strength in theory,bismuth element of pure bismuth ferrate is easy to volatilize and part of Fe³⁺changes to Fe²⁺during the preparation process,resulting in more oxygen vacancies,which makes it difficult to prepare a sample with high remanent polarization.In addition,the helical magnetic structure of bismuth ferrate makes the magnetic moment locked and can not exhibit larger magnetic properties,so the practical application is limited.This situation can be greatly improved by ion doping.In this article,our focus is mainly on yttrium doping or yttrium co-doping with 3d transition metal elements to improve the properties of bismuth ferrate.The main work of this thesis is as follows:1.A series of pure bismuth ferrate and yttrium doped samples were prepared by sol-gel method and their structures,morphology and multiferroics properties were investigated.The results of XRD and SEM show that with the increase of doping concentration,the crystal structure is distorted,and the particle size decreases.The results of ferroelectric and ferromagnetic measurements also showed that doping greatly improved the multiferroic properties of bismuth ferrate,and the optimum doping amount was 10%yttrium doped.2.A series of Yttrium and transition metal?Zn,Co?co-doped samples were also prepared by sol-gel method,and their structure,morphology,optical properties and multiferroic properties were systematically studied.For zinc and yttrium co-doped samples were refined with two-phase models?rhombic R3 C and cubic PM-3M?.The UV-Vis absorption spectra of zinc and yttrium co-doped samples shifted to low wavelength obviously,accompanied by the decrease of band gap.The maximum magnetization occurs in zinc doped in 2.5%and yttrium doped in 7.5%,which indicates that excessive doping leads to the decrease of magnetic properties.The ferroelectric and ferromagnetic properties of bismuth ferrate can be improved by Co or Y doping alone,but that can be further improved by binary co-doping of appropriate proportion of Co and Y.This is attributed to the ferromagnetic coupling between Y³⁺and Fe³⁺and the superexchange between Fe²⁺and Fe³⁺.3.In order to study the effect of phase transition degree on the structure,optical,ferroelectric and ferromagnetic properties of bismuth ferrate,we prepared yttrium and europium co-doped samples.XRD refinement data show that the phase transition degree from rhombohedral to cubic is larger when the doping content of europium is relatively high.From the absorption spectrum of ultraviolet visible light,it can be seen that the absorption in the visible range is larger when the phase transition is smaller.The effect of the degree of phase transition on the multiferroic properties has been studied systematically.It is found that the ferroelectric properties of bismuth ferrate can be changed to a great extent by hysteresis loop and leakage test.The above results are further verified by the XPS test results.The magnetic effect is the same.Phase transition breaks the helical spin structure of bismuth ferrate and releases magnetism.