The Multiferroic Of Bifeo <sub> 3 </ Sub> Hydrothermal Synthesis, Morphology And Performance Studies

Multiferroic materials, which possess a coexistence of ferroelectric and magnetic order parameters, have been one of the hot topics intensively studied due to their fascinating fundamental physical properties and their potential applications in multifunctional devices. Bismuth ferrite (BiFeO3) is one of the well-known single-phase material possessing multiferroic behaviors. The perovskite BiFeO3 has a ferroelectric Curie temperature Tc= 830℃and an antiferromagnetic Neel temperature TN= 370℃. It has potential applications not only in magnetic and ferroelectric devices but also in devices with coupling electric magnetism. However, bulk BiFeO3 exhibits large leakage currents due to the presence of defect, nonstoichiometric product and Fe valence fluctuation. On the other hand, it is hard to exhibit macroscopic magnetization at room temperature for its spiral modulated spin structure with a period of 62 nm. Under consideration of these challenges, synthesis of BiFeO3 powder with high quality is one of the key improvements for magnetic and dielectric properties. The morphology, size, dimension and crystallinity of the materials, including BiFeO3, play an important role in their physical properties. Many materials have been recently prepared successfully by a wet-chemical route-hydrothermal method with controllable microstructure and morphology. The early hydrothermal process and optimizing experimental conditions are foundations to prepare BiFeO3 powders with high quality. The change process of morphologies and formation of the large-scale polyhedral BiFeO3 particles are not only keys to obtain different morphology BiFeO3 powders but also basic theoretical research for exploring new materials.BiFeO3 powders were hydrothermally systhesized on the base of early hydrothermal process and optimizing experimental conditions by using Bi(NO3)3-5H2O and Fe(NO3)3-9H2O as starting materials, and KOH as a mineralizer. Large-scale polyhedral BiFeO3 particles were synthesized with a hydrothermal method under a series of experiments. The change process of morphologies and formation of the large-scale polyhedral BiFeO3 particles were discussed in detail. The obtained BiFeO3 powders showed ferroelectric, dielectric and magnetic responses, which approved the multiferroic properties of the BiFeO3 bulk and crystallization. Bulk BiFeO3 has been successfully sintered by using pure BiFeO3 powder synthesized with hydrothermal method. The dielectric and magnetism of bulk BiFeO3 was discussed in details.Bi(NO3)3·5H2O was not dissolved in an excess of glycol or HNO3. Using the optimizing experimental conditions, not only synthesizing pure BiFeO3 easily but also has the advantages of simple operation and easy control. X-ray diffraction revealed that the various bismuth ferrite under an appropriate alkali concentration assisted by various alkali metal ions. It is suggested that alkali metal ions (K+, Na+ and Li+) played an important role in the formation of BiFeO3, Bi2Fe4O9 and Bi25FeO39. We selected KOH as a mineralizer to synthesis BiFeO3 powder.Large-scale polyhedral BiFeO3 particles were synthesized with a hydrothermal method under a series of experimental conditions. X-ray diffraction revealed that the BiFeO3 products had a perovskite structure. Scanning electron microscopy observation showed different morphologies of the products were formed, including plate-like, ball-shaped, octahedron, truncated octahedron, cubo-octahedron and truncated cube with large particle sizes. The experimental results showed that the concentration of KOH, reaction time, heating and cooling rate had important impacts on the size and morphology of the BiFeO3 particles. The change process of morphologies and formation of the large-scale polyhedral BiFeO3 particles were discussed in detail. The obtained BiFeO3 showed ferroelectric behavior and magnetic response, which approved the multiferroic property of the BiFeO3 crystallization.Single-phase BiFeO3 ceramics have been sintered in the temperature range of 700～875℃by using BiFeO3 powders synthesized with hydrothermal method. Typical phase transitions of BiFeO3 powder have been observed by thermal analysis. The ceramic sintered at 850℃exhibits very dense microstructure with clear grain and grain boundary observed by scaning electron microscopy. It also shows high dielectric constant and dielectric dispersion with a dielectric relaxation. At room temperature, dielectric behaviors of our samples are attributed to not only the hopping of localized charge carrier and the grain and grain boundary structures, but also grain size. We obtained temperature dependence of dielectric constant and loss tangent for BiFeO3 ceramic sinered at 850℃measured at low temperature range of -105℃-30℃. It shows that dielectric constant steps and loss tangent peaks shift to high temperature with the increase in frequency from 1 kHz to 1 MHz. Eαis the activation required for relaxation process, which value of the BiFeO3 850℃sintered ceramics is 0.397 eV. The possible dielectric dispersion mechanism of the BiFeO3 ceramic was discussed in detail.