| Rare earth-iron giant magnetostrictive material (GMM) and nano-magnetic material play an important role in advanced magnetic functional materials. Because of its excellent magnetostrictive properties and large electromechanical coupling factor, GMM has critical applications in some precision instruments, such as sonar transducer, anti-vibration mounting, positioning device, and etc. Meanwhile, the nano-magnetic materials have been widely exploited in many fields, such as magnetic recording, magnetic fluid, magnetic resonance imaging, drug targeting, and catalysts.For GMM, its expensive price is one of the main factors hindering its wider application. For nano-magnetic material, it has become the research focus and challenge to explore the preparation technology and to study the crystal growth mechanism, in order to precisely control the grain size, morphology and property. Based on the above-described problems,(Tb, Dy, Pr)Fe2compound is successfully prepared by partial substituting the relatively inexpensive rare earth element, Pr, for the expensive metal element, Dy, in the (Tb, Dy)Fe2alloy in this dissertation. Also, Fe3O4and CoFe2O4magnetic nanopatticles with the grain size less than10nm are synthesized by hydrothermal method. Their properties and the mechanism of grain growth are studied in details. In this dissertation, the main researches and results are as follows:1. The Tb0.3(Dy1-xPrx)0.7Fe1.96compound ingots with x=0,0.1,0.2,…,0.6are prepared by arc melting method. The powder XRD results of the powder show the evolution regulation of the compounds microstructure and lattice constant with the increase of Pr content. The measurement results of SEM with BSD mode and thermo-magnetic curves confirm the change of phase composition. The magnetic test indicates that the Curie point and the magnetization of the compound decrease with the increase of Pr element. The results of this experiment will provide the necessary reference data for the controlling of material composition and performance improving for TbDyPrFe materials.2. The directionally solidified Tb0.3(Dy1-xPrx)0.7Fei.96rods with some preferred orientation are first prepared by the zone melting method with the guidance of seeds. The results of measurement indicate that, with the increase of Pr element, the orientation of samples is changed, the magnetostrictive properties decrease, the value of differential magnetostrictive coefficient (d33) reduces gradually and tends to become a constant. The magnetostrictive hysteresis of the samples is also reduced gradually. It is found that in a range of Pr content, heat treatment could effectively improve the magnetostrictive property of samples. After heat treatment, the maximum value increases about one times than the as-cast ones. At last, the relationship between the properties and the phase composition of samples is analyzed by the measurement of as-cast samples by SEM-BSD and EDS. In summary, the study of this dissertation first finds that the directional solidification parameters of TbDyPrFe compounds need to be adjusted according to the Pr content. The effects of Pr content, magnetostrictive properties of the directionally solidified samples and heat treatment on the structure and properties of materials are clarified. The results of these studies will be beneficial to make TbDyPrFe compounds to be a series of practical new giant magnetostrictive materials.3. Magnetic nanocrystals with different size are prepared by hydrothermal method at different reaction-times in N-hexane-surfactant system. The precursors are Fe(acac)3and iron powder. The results of XRD and Raman spectrum confirm these samples are Fe3O4. HRTEM measurement shows the grain size distributes from5.6to6.8nm with the reaction time. The grain growth mechanism is analyzed based on the crystal morphologies. It is considered that in the regulation of surfactant, the morphology of the Fe3O4magnetic nanocrystals change from the irregular crystal form to triangles and quadrilaterals, and finally to the hexagonal nanoparticles. The Magnetic measurement results show that the prepared samples are of superparamagnetic, and the maximum saturation magnetization is62.65emu/g. The results in these investigations could provide technical support and theoretical reference for the regulation preparation and its application of the Fe3O4magnetic nanocrystals.4. By the hydrothermal methods, CoFe2O4nanoparticles are successfully synthesized in the hexane-water-surfactants system (sample Al) and the ethanol-surfactants system (sample B2). The precursor are Fe(acac)3and CoCl2·6H2O. The results of XRD estimated by Scherrer formula indicate that the crystal size of all the samples is less than5nm. Magnetic measurement indicates that the saturation magnetizations (Ms) of sample A1and B1are60.95emu/g and61.20emu/g, the coercivities (Hc) are1860.90Oe and423.32Oe, respectively. After heat treatment, Ms, Mr, Hc, and remanence ratio R of samples are changed in some degree. At last, the reaction mechanism of these two systems is analyzed, and the effects of these reaction systems on the crystal size and magnetic properties are discussed. |
PDF Full Text Request