The Bifeo <sub> 3 </ Sub> Multiferroic Ceramics Doped Modification

By far BiFeO3(BFO) may almost be the only material putting up ferroelectric property and magnetism simultaneity above the room temperature, which is one of the most promising material to be used. However, BFO becomes unsuitable for applications owing to its high leakage current and weak ferromagnetism. For the above problems, this thesis mainly concentrate on the effect of A, B, AB sites modification and forming solid solution with other ABO3 perovskite materials on the structure, ferroelectric, ferromagnetic and dielectric properties of multiferroic ceramics BFO. These studies will be helpful to understand the mechanism of the property variations of BFO caused by doping and froming solid solution to guide the design of new useful memory materials. The doped ceramics were mainly prepared by the rapid liquid phase sintering method with ordinary furnace, and the solid solution ceramics were sintered by the conventional solid-state reaction method.A site Nd modification of BFO multiferroic ceramics were prepared by the rapid liquid phase sintering method and solid-state reaction method separately. The experiment found that the samples prepared by the former method were better than the later method, not only the purity of the phase, but also the properties. From the X-ray diffraction patterns of the samples, which were prepared by the rapid liquid phase sintering method, an obvious phase transition at x=0.1 was observed. The magnetization property and ferroelectric property of BFO were improved by Nd doping, especially for the ferroelectric property. The remanent polarization (2Pr) increases at first, then decreases with the increase in Nd content. As the Nd content is 0.10, the electric breakdown voltage is the highest, and a well saturated ferroelectric hysteresis loop was observed. Under the applied electric field of 200kV/cm, the 2Pr reaches a maximum value of 49.4μC/cm2 at x=0.1, increased by near 117.6% compared with that of undoped samples. Another peak after doping appeared at 170oC before the Neel temperature in the temperature dependences of the dielectric permittivity curve, which was not found in the BFO sample. The only Co3+ doped BFO ceramics were difficult to form the single-phase material, althought introducing the rapid liquid phase sintering method. There were Bi12.66Co0.33O19.33 and Bi25FeCoO40 phases on the base of BFO phase. However, the XRD patterns of Co3+ and Nd3+ codoped samples showed that those phases could be decreased with the Nd3+ doping, and the BFO structure could be stabilized with the Nd3+ doping. As the increaing Co content, the Bi0.9Nd0.1Fe1-xCoxO3 samples have a structural transformation from rhombohedral (space group R3c) to cubic structure (space group I23). However, the more Co content, the worse the ferroelectric hysteresis loop is. The ferroelectricity of the samples were remained with small amount dopant. Both the ferromagnetism of samples were enhanced by the doping, but the ferromagnetism of BNFCO-x(x=0~0.09) ceramics are smaller than the one of BFCO-x(x=0~0.09) ceramics. The dielectric anomaly around 150 oC in theε- T curve of the Bi0.9Nd0.1FeO3 sample disappeared after the Co doping, which might be attributed to the variety of the number of oxygen vacancy. The peak around 330 oC which might be related to the antiferromagnetic translation point (TN) of BFO moved to the low temperature with the doping. This might show the TN could be reduced with the Co3+ modification.The X-ray diffraction patterns of the (1-x)BiFeO3(BFO)-x(Na0.5K0.5)NbO3(NKN) solid solution demonstrate an obvious solid solution phase. The SEM images of the BFO-NKN ceramics showed the samples are compact and composed of cubic-like grains. The large leakage current in the ferroelectric hysteresis loops test might be attributed to the abundant defects, which may be resulted from the valence fluctuation of the Fe ions and the volatilization of Bi3+, Na2O and K2O. A dielectric anomaly around 350 oC in theε- T curve was observed at x=0.1, which might be related to the BFO. And the higher of the mixing content of NKN, the higher temperature the dielectric anomaly moves. When the content of the NKN is 0.3 and 0.5, there is another dielectric anomaly at 280 oC and 305 oC, respectively. This might be related to the component of NKN.