Studies On The Multiferroic And Dielectric Properties Of Perovskite ABO₃ Ceramics

Multiferroic materials exhibit not only ferroelectricity, ferromagnetism and other ferroic properties simultaneously, but also some new functions due to the coupling between different ferroic properties, which, makes these materials promising for the applications of information storage and detection technology. Owing to the trendency of miniaturization and multi-functionalization in devices, multiferroic materials, especially those with the coexistence of ferroelectricity and magnetism have attracted widespread attention. Despite of the great variety of multiferroic materials, most of them manifest ferroelectricity together with magnetism only at low temperature, which is one important limitation for room or high temperature applications. Hence, searching and studying for room temperature multiferroic materials is one of the hottest spot of condensed matter physics and material science.BiFeO3 (BFO), with a perovskite structure and exhibiting ferroelectricity and antiferromagnetism at room temperature simultaneously, is one of the most promising multiferroic materials for applications. However, there are some limitations restricting further research for pure BFO ceramics, such as the difficulty of preparation, the inevitable appearance of oxygen vacancies, Fe2+, and other defects during the sintering process. In this dissertation, on the one hand, aiming at the problems of BFO, we improved the fabrication method and succeeded in producing pure BFO ceramics, studied the migration and transformation of Fe2+ ions and oxygen vacancies as well as the influence on macroscopic properties. On the other hand, in order to manufacture materials exhibiting both ferroelectricity and magnetism, we used magnetic ion Co as dopant of high Curie temperature ferroelectric KNbO3. The main results are as follows:1. BFO ceramics with an average grain size of about 1.5?m were fabricated via an improved solid state sintering method and the multiferroelectric, dielectric relaxation properties were studied. The results show that BFO ceramics possess weak ferromagnetism and the leakage current density increases with the temperature. At the same temperature, the conductive mechanism mainly bases on space charge limited current (SCLC) at low electric field, while Poole-Frenkel assisted electron emission is dominated at high electric field. The dielectric spectra vs. temperature show two series of relaxation peaks. We believe that they are related to the migration of cluster and single oxygen vacancies. Oxygenation can reduce the oxygen vacancies and weaken the correlation between them, which leads to the increase of activation energy. At the same time, oxygenation lowers the concentration oxygen vacancies, and results in the transformation from Fe2+ to Fe3+, which contributes to the decrease of leakage and improvement of ferroelectricity as well.2. Pure BFO ceramics were prepared via rapid liquid phase sintering method. The state, migration and correlation of oxygen vacancies were studied through dielectric and mechanical spectra. For fresh sample, two series of relaxation peaks were observed in dielectric spectra, while only one in mechanical spectra, which is supposed to correspond to the ionization and migration of VO+ respectively for high and low temperature regions. During the heat treatment in oxygen-deficient environment, VO+ are ionized to VO2+ gradually, which leads to the decrease of VO+ and the increase of VO2+, accompanied by the shift of relaxation peaks to high temperature. Finally almost all the oxygen vacancies exist in the form of VO2+, therefore only one stable relaxation remains. After that, Vo2+ may recover to VO+ in oxygen-rich environment. It is concluded that temperature, atmosphere and other external conditions may promote the transformation between VO+ and VO2+ Oxygen-deficient environment benefits for the transformation from VO+ to VO2+ along with transformation from Fe3+ to Fe2+, while oxygen-rich environment has the opposite effect.3. We studied the influence of Co dopping in the B sites of KNbO3 ceramics. It is observed that Co doping suppresses the secondary phases, increases the density of ceramics. Co doped KNbO3 introduces room temperature magnetism to KNbO3 while maintaining the ferroelectricity and piezoelectricity. At room temperature, the dielectric properties show considerable change under low magnetic field. Magneto-dielectric effect as large as 7% is obtained in magnetic field of 6 kOe near Curie temperature. An possible explanation is under applied external magnetic field, the structure disorder of Co-O octahedrons may be affected by the spin switching of Co ions, thus affecting the ferroelectric and dielectric properties.