Thermal Property And Magnetic Performance Of Iron Oxides Based On Micro/nano Size And Its Structure Applied In The Anode Material Of Lithium Battery

Recently,with nano-devices decreasing in size,the research of size effect and phase stability properties of nano-materials has become increasingly important.And iron oxides,as one of the significant part of it,have great value in study because of its wide range of practical applications.In this thesis,hematite nano/micro-cubes with different size from nanometers to microns levels are fabricated as target subjects,and its thermal and magnetic properties are systematically invested by Physical Property Measurement System.In addition,the hematite with different nanostructure and morphology applied in anode materials for high performance Li-ion batteries has also been researched detailed.In this paper,the main results are as follows:（1）α-Fe₂O₃ nano/micro-cubes with different sizes have been synthesized by a hydrothermal method.The measurement of its magnetic properties which from 300 K to 920 K show that,single crystal α-Fe₂O₃ particles with the decrease of the size,its thermal stability was gradually reduced,the corresponding phase transition point by 729 k for 850 nm products reduced to 609 k for 40 nm products.And we introduced enthalpy theory to effectively support and explain the experimental results.（2）The magnetic properties of α-Fe₂O₃ nanostructures have been measured before and after their phase transition point respectively.Results display that,with smaller of size,samples show less coercivity and remanence magnetization while before its phase transition point.Moreover,the area of hysteresis loop decreases with the decrease of particle size.It indicates that smaller α-Fe₂O₃ structure size can reduce the energy loss effectively in the process of positive and reverse magnetization repeatedly.For samples after the phase change,the saturation magnetization increases with the decrease of the sample size significantly.And through reference intensity ratios（RIR）algorithm,we can semi-quantitative calculate the amount of change during the phase change process.（3）We have successfully synthesized single crystalline ɑ-Fe₂O₃ nanorings with different sizes from 100 nm⁶00 nm in length.When these ring-like hematite were applied to lithium battery,they exhibited regular changing trends in electrochemical performance.The results show that the discharge/charge capacity in first cycle increases monotonically with the decrease of ɑ-Fe₂O₃ nanoring size(¹200 mA h g^-1 at 0.1 C and ⁸00 mA h g^-1 at 0.1 C),while the best cycling stability（after 100 cycles at 0.1 C）is appeared in a intermediate size.All of these differences have essentially connection with their size-effect.Smaller size can provides large surface area to increase the contact between the electrode and the electrolyte and promote the Li⁺ mobility,but it also against the effectively utilizing rate of Li⁺ in persistent discharge/charge process.