Hydrothermal Synthesis Of Bismuth Ferrite Powders And Their Phase Transition Characteristics

Multiferroic materials have attracted much attention recently. Bismuth ferrite (BiFeO3) is one of the robust single-phase multiferroic materials because of its coexistence of ferroelectric and antiferromagnetic orders with ferroelectric Curie temperature Tc=830℃and antiferromagnetic Neel temperature TN=370℃. It has much potential applications due to the combination of magnetic and ferroelectric properties.It is well known that properties of materials is concerned with the composition, structure, size and morphology, which can help us improve the properties of materials and provide conditions for new materials. It is a crucial part of our research to prepare BiFeO3powders with high purity, good dispersion, stable structure, and controllable morphology and size with a simple experimental process. In this thesis, BiFeO3powders were hydrothermally synthesized with different starting materials, different mineralizers under a series of experiments. Effect of experimental conditions on the properties of products was studied. The effect of magnetic field on hydrothermal products with phase structure, size and morphology were researched.BiFeO3powders were hydrothermally synthesized by using Bi(N03)3·5H2O as a Bi resource, Fe(NO3)3·9H2O or FeCl3·6H2O as a Fe resource, with low NaOH concentration of0.06mol/L. The experimental conditions were determined on the base of early study. The results show that pure BiFeO3powders can be synthesized with both Fe(NO3)3·9H2O and FeCl3·6H2O as a Fe resource under a wide reaction temperature range. The perovskite structure BiFeO3powders were confirmed by X-ray diffractometer and Fourier Transform Infrared spectra. The morphology evolution of BiFeO3with temperature was researched by scanning electron microscopy. The particles change from irregular agglomerations to regular spheres with increasing temperature.Single-phase BiFeO3powders were synthesized with a hydrothermal method by controlling the experimental conditions. The morphology evolution of BiFeO3with KOH concentration was researched by scanning electron microscopy. The particles change from irregular agglomerations to regular cubes with increasing KOH concentration. The large BiFeO3cubic particles with smooth surfaces have well-matched element ratio and chemical valence. The powders structure, morphology and composition were characterized by using X-ray diffraction, scanning electron microscopy, transmission electron microscope, Raman measurement and X-ray photoelectron spectroscopy. The high temperature XRD and differential scanning calorimetry results show that BiFeO3powders have a hexagonal perovskite structure with a space group R3c below370℃and rhombohedral perovskite structure with a space group R3m below755℃. BiFeO3undergoes a phase transition in the temperature range of755℃-817℃from rhombohedral structure to a cubic phase, then decomposes to liquid and Fe2O3above939℃.Single-phase BiFeO3powders were hydrothermally synthesized under magnetic field at200℃with KOH concentration of1～6mol/L by using Bi(NO3)3-5H2O as a Bi resource, Fe(NO3)3·9H2O or FeCl3·6H2O as a Fe resource, respectively. The heating rate is1℃/min and cooling naturally. The powder structures were characterized by using X-ray diffraction. Compared with conventional hydrothermal synthesis, magnetic field is helpful for pure Bismuth ferrite phase, and the reaction time is saved. The morphology evolution of BiFeO3with magnetic field intensity was researched by scanning electron microscopy. The particles become coarse and particle size reduces gradually with increasing magnetic field.