Synthesis And Magnetic Properties Of Colloidal Superparticles Assembled By Mn₃O₄ Nanoparticles

In recent years,research on magnetic superparticles have been receiving much attention.Using magnetic nanoparticles as structural units,assembling them into a three-dimensional assembly with a specific shape and structure,not only can retain the magnetic properties of a single nanoparticle,but also may obtain a collective effect that a single nanoparticle does not have,which can broaden the applications of magnetic nanomaterials.Although there have been many studies on the magnetic properties of Mn3O4 nanoparticles,there are still few studies on the magnetic properties of its assembly,and the influence of the assembly process on the magnetic properties is still not clear.Therefore,in this thesis,the preparation method and magnetic properties of Mn3O4 superparticles were investigated.In this thesis,the chemical solution method was used to prepare octahedral Mn3O4 nanoparticles with an particle size of 12?15 nm and spherical Mn3O4 nanoparticles with an average particle size of 5.5 nm.Afterwards,octahedral Mn3O4 nanoparticles and spherical Mn3O4 nanoparticles were assembled to form superparticles by micro-emulsion method.The particle size of superparticles assembled by octahedral Mn3O4 nanoparticles were 95?150 nm,and the particle size of superparticles assembled by spherical Mn3O4 nanoparticles were 115?175 nm.The experimental results show that the concentration of surfactants has a crucial effect on the assembly process.When the concentration of surfactants is greater than 3 mg/mL,the nanoparticles assemble to form superpaticles,and the size of superparticles decreases with increasing the concentration of surfactants.At the same time,as the concentration of nanoparticle decreases and the ultrasound time increases,the size of superparticles also decreases.In addition,different emulsification methods and surfactants also have a certain degree of influence on the self-assembly process and the size of superparticles.The ultrasonic emulsification method and SDS have the best assembly effect.The magnetic property test results show that the coercivity(Hc)of the octahedral Mn3O4 superparticles and the spherical Mn3O4 superparticles at 5 K is greater than that of the bulk Mn3O4,while the Curie temperature(TC)and blocking temperature(TB)of the octahedral Mn3O4 superparticles are higher than the spherical Mn304 superparticles.The octahedral Mn3O4 superparticles and unassembled powder samples exhibit paramagnetism at room temperature and have similar Curie temperature,but the superparticles have a higher blocking temperature,which indicates that the magnetic dipole coupling is enhanced.In addition,the coercivity value of the octahedral Mn3O4 superparticles at 5 K is 9801 Oe,which is significantly greater than that of the unassembled powder sample.This may be due to the interface pinning effect to the flipping of spins due to the formation of spherical superparticles.