Study On The Effect Of Internal And External Stress On BiFeO₃ Multiferroic Ceramics

Multiferroics, which combine two or more "ferroic" properties in the same phase, and yield simultaneous effects of ferroelectricity, ferromagnetism, or ferroelasticity, offer a wide opportunity for potential applications in information storage. Among single-phase multiferric materials, BiFeO3 has attracted much attention recently due to its coexistent ferroelectric order and magnetic order at room temperature. However, the preparation of BiFeO3 pure phase material is much difficult, the leakage current is large, and magnetic at room temperature is weak. As a result of the existence of these problems, the development and application of BiFeO3 ceramics has been restricted. In this thesis, to overcome these obstacles, we demonstrate the substitution effect on the multiferroic properties of BiFeO3, in eluding A-site, B-site substitutions, and the solid solution with perovskite ferroelectric, and high temperature-high pressure methods. A detailed study on the effect of internal force and external stress on BiFeO3 multiferroic ceramics has been made. The main contents are as follows:(1) Sr doped BiFeO3 ceramics has been synthesized by citric acid-nitrate method. The results show that with increasing doping level, the lattice structure transforms form rhombohedral structure toward to cubic phase. In the pseudo-cubic structure sample with x=0.1, we observed the best magnetic and ferroelectric properties coexist at room temperature. And the typical magnetic exchange bias was observed in x=0.01-0.6 samples both at room temperature and low temperature, which indicates the presence of exchange coupling between the antiferromagnetic core and ferromagnetic surface in this case.(2) Ti, Ni, co-doped BiFeO3 ceramics has been synthesized by citric acid-nitrate method. Saturated hysteresis loop can be observed in x=0.1 sample at room temperature, and the remanent polarization and remanent magnetization are both increased. Two dielectric anomalies were observed in the temperature dependence of relative dielectric constant curves, which caused by the ferromagnetic-paramagnetic transition and the ferroelectric-paraelectric transition.(3) The (1-x)BiFeO3-xSrTiO3 solid solution ceramics have been synthesized by citric acid-nitrate method. And the multiferroic properties have been investigated. With increasing doping level, the lattice structure transforms form rhombohedral structure toward to cubic phase. The Morphotropic phase boundary (MPB) was observed at x=0.2-0.6 samples. The results show that in the emergence of MPB components, the ferroelectric and magnetic properties are significantly improved. The improvement of the multiferroic properties mainly comes from the structure distortion.(4) La doped 0.6BiFeO3-0.4SrTiO3 solid solution ceramics have been synthesized by citric acid-nitrate method. A site La doped can improve the magnetic and ferroelectric properties of the sample without influence of the sample's perovskite structure. The leakage current of the sample can be effectively reduced with decrease of oxygen vacancies. The dielectric constant is also increased at low frequency range.(5) Different processing methods of cool high pressure, high temperature-high pressure, and high pressure sintering were used in 0.9BiFeO3-0. 1SrTiO3 ceramics. The different effects on structure, ferroelectric, magnetic and dielectric properties of high pressure processing were studied. All the samples basically are able to maintain the rhombohedral structure. Sintering under the condition of high pressure can effectively reduce the sintering temperature and sinterning time. This can inhibit the evaporation of Bi ions, reduce the oxygen vacanicies and leskage current in the samples. As a result, the density of samples is improved, thereby improving the ferroclectric properties. The dielectric constant at room temperature also confirmed this point. However, the ferroelectric loop is difficult to obtain in samples with cool high pressure treatment. According to the magnetic hysteresis loops at room temperature and the magnetic-temperature curves, we find that the magnetic properties arc improved with cool high pressure and high pressure sintering treatment.