Study On The Assembly Of Iron Oxide Nanoparticles And The Regulation Of Magnetothermal Energy Conversion

Magnetic nanoparticles have many advantages,such as easy preparation,stability and good biocompatibility.Therefore,they have wide application prospects in biomedical materials,information storage and other fields.Magnetic iron oxide nanoparticles have size effect,large specific surface area and are easy to aggregate under magnetic field.By assembling nanoparticles to construct new materials.Not only does the group effect generated after assembling magnetic nanoparticles depend on single nanoparticle,but also the interactions between particles have distinct impacts on changes of collective magnetic effects.In order to further study the control over nanoparticles packed structures under magnetic field action,the effect of assembled structures on collective properties and their regulation law,we fabricated two-dimensional particle assembly films on different scales by layer-by-layer self-assembly and rotating magnetic field controlled assembly.The influence of the interaction between energy flux density and nanoparticle assembly on the magnetocaloric properties was explored by its magnetothermal energy conversion in an alternating magnetic field.In this thesis,the magnetothermal anisotropy of magnetic nanocrystalline film in alternating magnetic field is systematically studied,and the effect of energy flux density on the magnetothermal energy conversion is proposed.The larger the energy absorption cross section,the stronger the heat production.The causes of magnetothermal anisotropy are explained from the change of the free energy from the perspective of magnetic moment dynamics.In the experiment,a two-dimensional circular patterned structure of magnetic particles was constructed by rotating magnetic field.Furthermore,magnetic nanoparticles were arranged in a three-dimensional ordered structure inside the hydrogel,implementing magnetothermal enhancement effect of magnetic field-controlled assembly.Specific elements include the following:(1)In this paper,LBL thin film assembly method is used to fabricate the granular film with controllable macroscopic continuous magnetic thickness.The morphology was characterized by SEM;TEM and optical microscope.The magnetocalorimetric measurement and analysis were carried out by using alternating magnetic field,and the magnetocalorimetric analysis was carried out in combination with micromagnetic simulation.Based on Poynting vector,it is proposed for the first time that the effect of energy flux density on magnetic heat is stronger than that of magnetic field intensity under alternating magnetic field.And it is found that the magnetic heat of particle films boosts up with energy absorption cross section eapanding.A method for measuring the polarization rate and magnetic polarizability of particle film impedance is developed in this paper.(2)Based on the micromagnetic simulation of thin nanoparticle film.Through layer-by-layer self-assembly,the magneto-thermal effect of iron oxide nanoparticle films formed at the interface is anisotropic.And it is proved that the magnetothermal energy conversion between magnetic field and film is anisotropic.When the external alternating magnetic field is parallel to the assembled film,the heat production is maximum,and when the external alternating magnetic field is perpendicular to the film,the heat production is the smallest and the heat yield is angle dependent.This phenomenon is analyzed from the perspective of the variation magnetic free energy through micromagnetic simulation and magnetothermal experiments.It is considered that the interaction between magnetic nanoparticles plays an important role in this process.In addition,it is found that the intrinsic magnetization state in the sample can change the anisotropy.The experimental and simulation results show that the magnetocaloric effect of magnetic nanoparticles can be adjusted not only by adjusting the dipole interaction between magnetic particles,but also by changing the intrinsic magnetization state of the samples.(3)Two dimensional ultrathin patterned magnetic granular films were prepared by rotating magnetic field control assembly.In the experiment,by adjusting the magnetic field intensity,particle concentration,rotating magnetic field speed and so on,the ordered ring-like assembly structure under the action of magnetic field can be obtained.The morphologies of SEM,TEM and optical microscopy show that ultrathin granular films with circular two-dimensional ordered patterns and uniform distribution have been obtained.In the assembly process,due to the misalignment of the upper and lower layers of magnetic particle films,something similar to Moiré interference patterns will appear.And the assembled films have smaller surface energy and larger surface tension.(4)A magnetic hydrogel with disk structure was constructed by using rotating magnetic field to control the agglomeration of magnetic nanoparticles.The morphologies of the microstructures were determined by means of SEM,TEM,LSCM,Micro-CT and so on.The 3D disk microstructures with different particle sizes and thickness were obtained by adjusting the intensity of the rotating magnetic field,the rotational speed of the magnetic field and the concentration of magnetic nanoparticles.The results of magneto-thermal measurement of microstructures show that the ordered microstructures assembled by rotating magnetic fields have direction-dependent magnetothermal energy conversion.The applicability of the assembly based on energy flux density and magnetic field regulation to the magnetothermal effect on the micron scale is proved.