Thermal Stability And Magnetic Property Of Iron Oxides Nanoparticles

Iron oxides nanostructures are attracting fast growing research interests,due to their wide range of important applications.The thermal stability and magnetic property of iron oxides nanoparticles have a critical influence over their properties.Two important kinds of iron oxides nanoparticles（α-Fe2O3 and Fe₃O₄）have been chosen for this study.Hematite nanoparticles（α-Fe2O3）with different morphology and Fe₃O₄ nanoparticles with different structure and size are fabricated through a hydrothermal method,then the morphology,structure and magnetic properties are systematically invested by Field Emission Scanning Electron Microscopy,X-ray Diffraction and Physical Property Measurement System.The way of improvement the thermal stability of the magnetite nanoparticles.The main researches are as follows:（1）We have prepared α-Fe2O3 nanoparticles with different morphology（microdisks,drum-like particles and spindles）using a hydrothermal method.Magnetic measurements under a high vacuum（<9.5×10-6 Torr）were used to characterize thermal stability of these nanoparticles.The onset temperature of the phase transformation of α-Fe2O3 into Fe₃O₄ and the transformed fraction were found to depend strongly on the morphology of the nanoparticles.At low temperatures,magnetic measurements show that the Morin transition is almost completely suppressed in the three kind of different morphology of hematite.The current results suggest that the Morin transition depends very strongly on the morphology of nanoparticles.（2）Single-crystalline Fe₃O₄ hollow nanostructures（nanoring and nanotube）have been successfully synthesized by a hydrothermal method along with a heat treatment process.The temperature dependences of the magnetization of the hollow nanostructures were measured under a high vacuum（<9.5×10-6 Torr）from 300 K to 900 K.The Curie temperatures of the nanoring and nanotube samples were found to decrease with decreasing the mean wall-thickness.The Curie temperatures of the hollow magnetite nanostructures follow a finite-size scaling relation with the scaling exponentν =1.04±0.03.By comparison with those of the zero-dimensional Fe₃O₄ particles and two-dimensional Fe₃O₄ films,we show that the scaling relation for our hollow nanostructures is in better agreement with the quasi-two-dimensional finite-size scaling law.（3）The Fe₃O₄@C nanoparticles have been successfully fabricated through a facile large-scale two-step method.Compared with bare Fe₃O₄ nanoparticles,the as-synthesized Fe₃O₄@C nanoparticles exhibit a much better thermal stability.Magnetic measurements under a high vacuum（<9.5×10-6 Torr）from 300 K to 920 K were used to characterize thermal stability of the Fe₃O₄@C nanoparticles.The results reveal that Fe₃O₄@C nanoparticles are very stable.This is very significant for the study of the size effect.（4）we investigated the effect of NaCl on particle size and thermal stability of magnetite（Fe₃O₄）nanoparticles by mixing the Fe₃O₄ nanoparticles with the ball-milled NaCl powder.The characterization results reveal that the addition of NaCl suppresses coalescence of Fe₃O₄ nanoparticles and improves the onset phase transformation temperature of Fe₃O₄ into α-Fe2O3.Furthermore,this approach is facile,ecofriendly,economic and reproducible to be used in wide range of applications.