Sol-gel Derived And Ferroelectric Properties Of Single-Phase BiFeO₃ Ceramics

Bismuth ferrite (BiFeO3) is a typical single-phase multiferroic which has attracted much attention for its coexist of ferroelectric and magnetic properties at room temperature. Coupling between ferroelectric and magnetic can lead to magneto-electric effect, i.e. the magnetization can be modulated by the applied electric field, and vice versa, which has potential applications in information storage, sensor and the emerging field of spintronics. However, single-phase BiFeO3 is difficult to be prepared because of the bismuth volatilization and the generation of oxygen vacancies, and the ferroelectric properties are hard to be measured due to the large leakage current at room temperature. Therefore, the research goal of single-phase BiFeO3 is still a research focus.The BiFeO3 powders were prepared by sol-gel method with the starting materials of bismuth nitrate and ferric nitrate, and the effect of the calcined temperature on the phases of BiFeO3 samples was studied. The x-ray diffraction (XRD) showed that the single-phase BiFeO3 powders were obtained with a calcined temperature of 700℃. The scanning electron microscope (SEM) analysis indicated that the grain size of the BiFeO3 powders increased as the calcined temperature was higher. The grain size was around 500 nm in single-phase BiFeO3 powders. However, the grain size of the BiFeO3 powders increased to 3μm at the calcined temperature of 800℃.After disk formation by the routine method in ceramics processing, BiFeO3 ceramics was obtained by sintering the disk at the ambient of oxygen and nitrogen, respectively. The phases of BiFeO3 ceramics were analyzed by an X-ray diffractometer. It was revealed that the treatment temperature of BiFeO3 powder and the ambient gas during sintering are crucial for synthesizing the single-phase BiFeO3 ceramics. It was shown that, the single-phase BiFeO3 ceramics was obtained only if the BiFeO3 powder was heated at 700℃and the formed disk was sintered at 800℃in nitrogen.The microstructure and ferroelectric properties of the BiFeO3 ceramics were studied. The ceramics was crystallized mainly in rhombohedral structure with the grain size of 2-4 in diameter. The leakage current density of the BiFeO3 ceramics remains at 2×10-5 A/cm2 at an applied field of 170 kV/cm. The samples has unsaturated hysteresis loop with a remanent polarization (2Pr) of 3.7μC/cm2, the coercive field (2EC) of 100.4 kV/cm (at 200 kV/cm) at room temperature. The remanent polarization and the coercive field increased with the electric field.