Preparation Of BiFeO₃ Based Lead-free Piezoelectric Ceramics With High Performance And Its Performance Regulation

Bismuth ferrite (BiFeO3) is a kind of multiferroic material with a simple perovskite structure. Because of its high ferroelectric Curie temperature Tc (～1103k) and antiferromagnetic Neil temperature TN (～647 k), as well as its ferroelectricity and ferromagnetism at room temperature, it is considered to have a very broad application prospect. However, it is a pity that no superior characteristics can cover up its huge defects, such as the impurity phase during sintering, generation of oxygen vacancies arising from the volatilization of Bi and the valence fluctuation of Fe, its low resistivity and difficulty of getting saturated hysteresis loop at room temperature.Concerning these defects of bismuth ferrite, there are two main ways to deal with them:one is to do the solid solution with other perovskite materials, the other is to select appropriate elements for doping. In this paper, both the two methods were adopted. First, the solid solution of BF-BT was prepared by a solid state reaction method and its structure and properties were studied. Then, on the basis of BF-BT, Mn doping and Zn/Hf co-doping were carried out, respectively. The changes of the structure and properties of the ceramics are investigated.First of all, Lead-free piezoelectric ceramics of binary system were prepared by solid state reaction method and the phase, morphology, dielectric and piezoelectric properties of the material were studied. By using XRD and SEM, it revealed that the addition of BT could effectively inhibit the generation of hybrid phase and promote the growth of ceramic crystal grains to a certain extent(from a diameter of around 2μm to 5μm). With the increase of BT content, the phase of ceramics is transformed from rhombohedral phase to a pseudo cubic phase. The dielectric loss is also reduced due to the solid solution. Piezoelectric properties are enhanced and its value reaches a maximum which is equal to 30pC/N on the condition of x=0.3. Then, a new preparation method called Rapid sintering process was investigated. By comparison, it was found that the final performance of the product is not superior to the former one. It is showed that the omission of the Pre-sintering step can not reduce the volatilization of Bi element.Afterward, On the basis of the above experiments, the 0.7BF-0.3BT ceramic with excess Bi element was prepared. The final results showed that the excessive addition of Bi element could effectively inhibit the volatilization of Bi. The density of ceramics increases and the porosity reduces. And, the improvement of structure brings the performance enhancement. The piezoelectric properties of the product under the excess of Bi are more excellent compared with the previous experiments.At last,0.7BF-0.3BT-xwt%MnO2 and BF(1-x)(Hf0.5Zn0.5)x-0.25BT were prepared by solid state reaction. The study showed that the doping of Mn element could effectively reduce the leakage current of the ceramic and the electrical insulating property of the ceramic is enhanced, at the same time, the dielectric loss is reduced. In addition, the incorporation of Mn could obviously promote the growth of crystal grains. The performance of ceramics is also improved. On the other way, the co-doping of Zn and Hf significantly improves the voltage resistance of the ceramic and its ferroelectric properties are improved. At the same time, the density of the product is also greatly enhanced with actual density 97.1%of the theoretical density. The leakage current of ceramics are suppressed. On the condition of x=0.03, the highest piezoelectric constant of 44pC/N is obtained.