Studies On The Magnetic And Electrical Properties Of Fe Doped BaTiO₃ Ceramics

Aging effect, characterized by the spontaneous change of ferroelectric and dielectric properties with time, can bring about the unstability and impair the reliability of ferroelectric materials, which is usually regarded as a negative effect. However, aging effect can also result in large recoverable electro-strain in the ferroelectrics, which might provide a novel solution for electromechanical energy conversion. As one typical ferroelectric material, barium titanate (BaTiO3) ceramics have been extensively investigated on its aging effect and modification by doping.In this dissertation, Fe doped BaTiO3 ceramics was fabricated via conventional solid state reaction method and the aging effect of the ceramics was systematically investigated. The influence of Fe doping on the change of structure, magnetic and dielectric properties was also discussed. The main results are as follows:(1) The influence of Fe dopant on the aging effect of BaTiO3 ceramics was investigated. The structure of BaFexTi(1-x)O3 (x=0-1.5%) ceramics remains in tetragonal phase while the structural parameters c/a and the Curie temperature Tc decreases gradually with the increasing content of Fe. The aging effect can be characterized by double hysteresis loops and higher coercive field. With the Fe content increasing from 0.5% to 1.2%, the aging effect becomes more stable. On the contrast, the ceramic with 1.5% Fe content shows a relative decreasing of aging degree. It is proposed that the aging degree could be related to the polarization formed by the defect dipoles Fe3+-Vo2+. The concentration of Fe3+-Vo2+ increases with the content of Fe increases from 0.5% to 1.2%, which means more stable aging effect. However, even higher concentration of oxygen vacancies will also improve the possibility of Vo2+-Fe3+-Vo2+ formation, which would not contribute to the aging effect but decrease the concentration of Fe3+-Vo2+ instead. This may be the reason of the reduction in the aging degree of the sample with 1.5% of Fe.(2) With the increase of Fe content from 1% to 10%, the phase structure transforms from tetragonal to hexagonal gradually. Moreover, higher sintering temperature in the fabrication process would promote the transition to hexagonal phase. The introduction of Fe dopant results in weak ferromagnetism at room temperature, the intensity of which changes nonlinearly with the Fe content. It is proposed that the bound magnetic polarons could be the source of magnetism of the Fe-doped BaTiO3 ceramics. Higher Fe content means the nonlinearly increase of dielectric constant due to the change of structure and grain size of the BaTiO3 ceramics, which changes the radius of bound magnetic polarons. Together with the increasing Fe content and the amount of oxygen vacancies, all these changes lead to the nonlinearly changing magnetism. For the relative strong magnetic sample with Fe-doped of 8%, magnetic dielectric effect is about 0.83% under magnetic field of 1T, at 280K.