An Ethylene Glycol In-situ Reduction Route For Preparation Of Cr-based Oxides And Their Magnetic Properties

Magnetic materials are one of the most remarkable research areas in the field ofnanotechnology. Nanomagnetic materials have many physical properties significantlydifferent from conventional bulk materials, and have attracted substantial attentiondue to their highly valuable properties and largely potential applications in magneticstorage media, magnetic fluid, catalysis, and microelectronics etc. Currently, anendless variety of preparation methods have been deveoloped, but it is still a greatchallenge to fabricate the nanomagnetic materials with special morphology. In thisthesis, a facile ethylene glycol in-situ reduction strategy has been successfullyemployed in preparation of a series of Cr-based oxide magnetic materials. The mainworks achieved are summed up as follows:1. In this thesis, a facile and versatile solvothermal reduction strategy issuccessfully employed in the preparation of rare-earth chromites with quasi-hollownanostructures. X-ray diffraction data show that all the products have theorthorhombic perovskite structure. The electron microscopy analysis reveals that themorphology of the product is seriously affected by the rare-earth ionic radius. Themagnetization studies demonstrate that all the products, as it would be expected,undergo a magnetic transition from paramagnetic to antiferromagnetic phase at Néeltemperature (TN1) attributed to Cr3+–Cr3+exchange and this critical temperature goesup linearly with an increase in the rare-earth ionic radius. Additionally, some samplesexhibit a variety of fancy magnetic properties, including the thermal hysteresis,magnetization reversal, and magnetic exchange bias.2. A facile ethylene glycol in-situ reduction strategy has been successfullyemployed in the preparation of polycrystalline CoCr2O4nanosheets. X-ray diffractionpatterns show that the products have the cubic spinel structure. The electronmicroscopy analyses reveal that the obtained CoCr2O4nanosheets consist ofnanoparticles with the diameters of20-30nm. Experiments prove that the volumeratio of ethylene glycol to water is crucial for the final morphology. Themagnetization studies reveal the long-range ferrimagnetic order at TC=86K, the spiral magnetic order at TS=20K, and the magnetic lock-in transition at TL=13K.The TCvalue is significantly depended on the crystallinity, and additionally, thecoercivity field increases with the crystallinity, while decreases with the crystallitesize. The temperature dependence of the specific heat reveals both phase transition atTC=90K and TS=20K.3. Here, a facile ethylene glycol in-situ reduction strategy has been successfullyemployed in the preparation of Ni Cr2O3composite hollow spheres. X-ray powderdiffraction was used to determine the phase composition. Scanning electronmicroscopy and transmission electron microscopy was employed to characterize themorphologies of the as-prepared samples. Experiments prove that the volume ratio ofethylene glycol to water plays a determinative role in the final morphology of theproducts. The magnetization studies demonstrate that between ferromagnetic Ni andantiferromagnetic Cr2O3, a small exchange bias effect can be observed in themagnetic hysteresis loops. At the lower temperature, the larger exchange bias fieldand coercivity will be obtained. A high surface area of145.1m2·g1is obtained for theprepared porous hollow spheres.