The Preparation And Investigation Of High Permittivity BaTiO₃/Ni_0.8Zn_0.2Fe₂O₄ Composites

The incessant development of modern industry calls for miniaturization, high-stored energy and multifunction materials, so such field becomes much more fascinating and popular, which is attributed to abundant physics conceptions and applied potentials. Meanwhile, with the development of materials science, composites have attracted many researchers. Composites exhibit not only certain properties of single component but also some astonishing properties, even which the single component does not possess. Therefore, it is getting more and more important to study magnetic and electric properties of composites with the variation of microstructure. High permittivity materials mean high ability of capacitance and polarization, while the compatibility of magnetic and electric properties predicates the realization of multifunction materials. With no doubt, such high permittivity and multifunction materials sustain the composites to be a promising candidate for electronics. The thesis was originally intended to study the high permittivity composites at the direction of composite properties. This paper was focused on the magnetic oxide particles dispersed into ferroelectric matrix to realize high permittivity and even magnetoelectric (ME) effect, which breaks through the limit of metal as the conductive phase normally.Here the pure BaTiO₃ and Ni_0.8Zn_0.2Fe₂O₄ powders were synthesized by sol-gel, and novel composites were prepared by incorporating the dispersed appropriate mass Ni_0.8Zn_0.2Fe₂O₄ semiconducting and ferromagnetic particles into BaTiO₃ ferroelectric matrix. From X-ray diffraction (XRD) data, dense composite ceramics were obtained with the co-presence of BaTiO₃ and Ni_0.8Zn_0.2Fe₂O₄,which can be testified by the linear decrease of the saturation magnetization with the content of Ni_0.8Zn_0.2Fe₂O₄. The dielectric behavior of the materials varies with the ratio of the two phases. Moreover, higher permittivity was obtained in the composites, with more Ni_0.8Zn_0.2Fe₂O₄ or higher temperatures or lower frequencies. Interestingly, some high permittivity abnormities were observed, namely, the dielectric peaks from distinctness to disappearance; in addition, apparent dielectric loss peaks with frequency dispersion for x≥0.2 composites, which is different from ferroelectrics BaTiO₃. For BaTiO₃/Ni_0.8Zn_0.2Fe₂O₄ composites, it was found that the percolation threshold was about 50 wt% Ni_0.8Zn_0.2Fe₂O₄. However, according to percolation theory, good fit results were not obtained for many factors, such as the size, shape, uniformity and so on. Subsequently, Maxwell-Wagner effect and some elementary electric theory explained such permittivity abnormities. Firstly, semiconductor Ni_0.8Zn_0.2Fe₂O₄ particles and more conductive charges affected the insulating BaTiO₃, resulting in the decrease of resistance for the composites. Secondly, when an electric current passed through the composites, the space charge, which usually arouse high permittivity at 10-5～103 Hz, piled up at the interfaces for two different dielectric media, and it aroused apparent high permittivity. Finally, such space charge polarization may play an important role at higher temperatures or lower frequencies. Therefore, for x=0.5 sample, we gained high permittivity 1 336 500 at 1 kHz and 180℃.