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Title: Preparation of Rare Earth Cobalt Permanent Magnets and Study of Enhancement of their Magnetic Properties
Authors: Hussain, Abid
Keywords: Physical Sciences
Issue Date: 2018
Publisher: Bahauddin Zakariya University Multan
Abstract: Hard magnetic nanoparticles of R-Co5 systems exhibit unique microstructural and magnetic characteristics. In this work, R-Co5 (R = Pr, Y, Sm and Nd) systems were synthesized by metallurgical route using different concentrations of rare earths, based on Pr0.75Y0.25Co5, Pr0.75Sm0.25Co5, PrCo5, NdCo5 and NdCo5/α-Fe compositions. A study of magnetic behavior of the synthesized systems was carried out as a function of composition, thermal and mechanical treatments. The study involved the experimental techniques like X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), differential scanning calorimetery (DSC) and magnetic property measurements. The coarsely pulverized alloy powders of Pr0.75Y0.25Co5 and Pr0.75Sm0.25Co5 were processed by a top down approach using dry high energy ball milling technique in order to obtain highly coercive nanostructured powders. Structural analysis carried out by XRD of Pr0.75Y0.25Co5 and Pr0.75Sm0.25Co5 milled powders revealed 1:5 phase as the majority phase with CaCu5-type hexagonal structure. After extensive millings, the magnetic powders exhibited nearly amorphous nature as identified by XRD patterns. The broad, exothermic transition peaks in the DSC scans of the as-milled powders at 503 and 514 ºC indicated a process of crystallization taking place in Pr0.75Y0.25Co5 and Pr0.75Sm0.25Co5, respectively. Annealing of the as-milled powders in high vacuum produced fine grains with optimal microstructure and enhanced magnetic properties. Magnetic measurements of the annealed powders evaluated a high intrinsic coercivity, iHc and a remanence ratio, Mr/Mmax. The mechanism of magnetic hardening was attributed to higher anisotropy fields of the powders and the microstructural uniformity achieved by the adopted processing methodologies. TEM observations indicated formation of fine grains with nearly homogeneous nanometeric size in the powders. ii The PrCo5 coarsely pulverized powders milled by surfactant assisted high energy ball milling technique led to the dispersion of the crashed particles by suppressing the effects of re welding and agglomeration during milling. The structural and magnetic characteristics of the milled particles proved to be functions of milling conditions. XRD experiments and magnetic property measurements have demonstrated that the nanoflakes were crystallographically anisotropic with [00l] out of plane texture. The coercivity, iHc of the milled particles showed a clear dependence on milling time and its optimum values were recorded at room temperature. TEM observations confirmed that majority of the milled nanostructures exhibited aspherical, flake-type morphology. In case of the nanocrystalline NdCo5 compounds and NdCo5/α-Fe nanocomposites, the surfactant assisted high energy ball milling technique was employed as a processing route. After the course of milling, suspensions of the colloidal particles in the solvent were obtained. Excessive surfactant was removed from the milled powders of NdCo5 and NdCo5/α-Fe by washing with ethanol. Microstructural analysis of NdCo5 nanoparticles by XRD confirmed CaCu5-type hexagonal structure. TEM analysis revealed spherical shaped and plate like morphology for milled NdCo5 and NdCo5/α-Fe, respectively. Magnetic property measurements revealed a ferromagnetic behavior in the nanostructured magnets with interacting grains exhibiting high coercivity and remanence ratio, Mr/Mmax > 0.5.
Gov't Doc #: 22235
Appears in Collections:PhD Thesis of All Public / Private Sector Universities / DAIs.

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