Investigation On Chemical Synthesis And Properties Of Bismuth Ferrite-based Nanopowders

Multiferroic materials, which present potential applications in the next generationinformation storage, sensors and spintronics devices, due to they yield simultaneous effects offerroelectricity, and ferromagnetism in the same material. As one of the representative single-phase multiferroics, bismuth ferrite (BiFeO3) has been the topic of intensive studies due to itstwo robust ferroic phase transitions well above room temperature, i.e.,830oC for the Curietemperature and370oC for the Neel temperature. However, the high leakage current density isone of the most important obstacles due to the presence of nonferroelectric impurities such asBi2Fe4O9, Bi25FeO40etc. The synthesis of single-phase BFO crystallites and increase of itsferroelectricity and ferromagnetism is a focus of international researches.In this paper, BiFeO3, BNFO and BLFO nano-powders were respectively prepared bysolvothermal method, sol-gel hydrothermal method, hydrothermal method and soft chemicalmethod. Then Scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy(EDS), X-ray diffraction (XRD), Differential Thermal Analysis (DTA) and other means ofanalysis were used to characterize nanomaterials size, structure, morphology, electricalproperties and magnetic properties. The main contents are as follows:A low-temperature hydrothermal process to the synthesis of pure BiFeO3nanopowders isdemonstrated through using ethanol-water mixture as solvents. In this synthesis, solvent’component and ratio played important roles. Only when the ethanol-water ratios werebetween4:3and2:5, single-phase BiFeO3powders could be synthesized at the lowertemperature of120℃. The BiFeO3nanopowders synthesized with4:3ethanol/water ratiomainly consists of cubic structures with size from50to150nm. ZFC and FC magnetizationmeasurements indicated that pure BiFeO3nanopowders showed a spin-glass transition belowthe freezing temperature of5K. The BiFeO3nanopowders displayed a ferromagnetic order atroom temperature, which was supposed to be associated with the size effect. Ferroelectrichysteresis loop are also displayed in the BiFeO3sample. The presence of room temperatureferromagnetic and ferroelectric orders approved the multiferroic properties of the pure BiFeO3crystallization.Multiferroic Bi1-xNdxFeO3(BNFO, x=0-0.35) powders were synthesized by a novel sol- gel-hydrothermal route. It was found that pure Bi1-xNdxFeO3crystallites could be obtained forx≤0.25, and the appropriate concentration of KOH and NaOH were benefit to growing pureBi1-xNdxFeO3and Bi2Fe4O9crystallites respectively. Scanning electron microscopyobservation revealed that spherical morphology of BNFO crystallites were formed, and thesize was sensitive to the concentration of KOH. The ferroelectric Curie temperatures ofBNFO crystallites were shifted to a lower temperature with Nd substitution. The result ofmagnetic test revealed that the magnetic moments of the samples were improved byincreasing Nd content in the BNFO. The formation mechanism of the Bi1-xNdxFeO3crystallites was also discussed.Lanthanum-doped bismuth ferrite (Bi1-xLaxFeO3, wherein x is equal to0,0.15,0.3, and0.4) crystallites were synthesized by hydrothermal method. In the synthesis, precursorcomposition, potassium hydroxide concentration, and hydrothermal temperature and timeplayed important roles in the crystallinity and morphology of Bi1-xLaxFeO3crystallites. PureBi1-xLaxFeO3crystallites could be obtained when x＜0.3, and ferroelectric transitiontemperature decreased from834.2to828.7℃with increasing La doping. The growthmechanism of the Bi1-xLaxFeO3crystallites was also discussed. Furthermore, our resultsshowed that La doping greatly enhanced the remnant polarizations.The present research describes a simple low-temperature synthesis route of preparingbismuth ferrite nanopowders through soft chemical route using nitrates of bismuth and iron.Tartaric acid is used as a template material. The synthesized powders are characterized by X-ray diffractometry, thermogravimetry and differential thermal analysis, infrared spectroscopy,and transmission electron microscopy. The particle size of the powder lies between30and50nm. In the process, phase pure bismuth ferrite can be obtained at a temperature as low as500℃. On the other hand, we find that, like solid state reaction route, Pechini’sautocombustion method of synthesis generates a lot of impurity phases along with bismuthferrite. The result of magnetic test revealed that the magnetic moments of the samples wereimproved by increasing Nd content in the BNFO...