Preparation And Properties Of Soft And Hard Magnetic Nanopowders And NdFeB-based Permanent Magnetic Nanocomposites

NdFeB-based magnets with excellent properties, especially high-coercivity, high remanence, have extensive applications in the field of permanent magnet motors. Current research efforts mainly focus on the nanocomposite materials and novel processing techniques. Nanocomposite magnets consisting of a hard magnetic phase with high anisotropy and a soft magnetic phases with high saturation magnetization have been widely studied experimentally and theoretically owing to their unusually high remanence, energy product and low cost. The high remanence in nanocomposites arises from the exchange coupling between the magnetically hard and soft phases.This work aims to improve the magnetic properities and to understand the underlying physics of exchange spring behavior and the coercivity mechanism of nanocomposite materials. FeCo, Fe/Co core/shell and CoFe₂O₄ nanoparticles were synthesized by chemical methods and the magnetic properities were investigated. Surfactant-assisted ball milling was also employed for preparing NdFeB nanopowders. Fe nanoparticles prepared were coated on the surface of micrometer NdFeB permanent magnetic particles by co-precipitation process. The nanocomposite powders were then sintered into bulk nanocomposite Nd2Fe14B/α-Fe magnets by spark plasma sintering （SPS） technique. The synthesis, structure, and the magnetic properties of the magnetic nano-powders and nanocomposites have been systematically studied. The results obtained in this work are summarized as follows.FeCo nanoparticles with an average size of ²4 nm were successfully synthesized by co-precipitation process. Crystallized FeCo nanoparticles with narrow size distribution have been prepared by annealing the powders obtained by tuning the mole ratio of metal ions vs NaBH4 to 1.7:1. Core-shell structured nanoparticles with ²2 nm ferromagnetic core （Fe） and ⁷ nm shell （Co） were prepared by a chemical reduction method. By adjusting the deposition parameters, the core-shell particles with various Fe:Co molar ratios were also obtained. The saturation magnetization decreased with the increase of Cobalt content. For the nanoparticles prepared without magnetic filed, the coercivity （Hc） increased with increasing Co content, whereas for the nanoparticles prepared under a magnetic field, the Hc was much lower. The second ferromagnetic phase transition occurs at the temperature lower than 25 K, which led to a drastic change of magnetization at low temperatures.Spinel ferrites CoFe₂O₄ were synthesized by oxidizing FeCo nanoparticles precursors. Mechanism of phase transistion, structure and magnetic properties of CoFe₂O₄ were investigated. The coercivity of the Co-ferrite is 155.3kA/m at 300K. Hard magnetic NdFeB nanoparticles with various sizes and shapes were also successfully prepared by a surfactant-assisted ball milling technique. Structural and magnetic properties of NdFeB powders were investigated. The coercivity of milled powders decreased with the increase of milling time. The coercivity over 300 kA/m was obtained from the powders with an average diameter of 20 nm. It was revealed that spin-reorientation temperature of NdFeB powders is size dependent. Anisotropic NdFeB nanoflakes were also obtained by this method through adjusting the balls to powders weight ratio and milling timeNdFeB/nano-Fe（Co） composite powders were synthesized by depositing Fe（Co） nanoparticles on the nanocrystalline NdFeB powder surface using a bottom-up technique. Magnetic properties of composite powders depend on the content, distribution and morphology of soft phase. Increasing soft nanoparticle content improved the remanence and maximum energy product at the expense of coercivity. Distribution and morphology of soft phase were different when the compsition of soft magnetic powders were changed. The magnetic properties of composite powders synthesized with different types of NdFeB powders were also investigated. The surface of RE-rich NdFeB core reacted with NaBH4, which led to low remanence of RE-rich NdFeB/Fe composites. Soft magnetic shell with an average size of 32 nm was obtained using PVP as a stabilizing agent.Remanence enhanced bulk nanocomposite magnets were successfully fabricated by consolidating the core/shell nanocomposite powders using spark plasma sintering （SPS） technique. Single phase magnetic behavior and enhanced remanence were observed from the demagnetization curves in the bulk samples. A positiveδM（H） was observed, indicating the existence of exchange coupling interaction between soft and hard phases. The maximum energy product of 99.8 kJ/m3 was obtained from bulk magnets.The magnetization mechanism of the nanocomposite magents was finally studied. Through the investigations of the initial magnetization curves, the relationship between coercivity and the magnetized field, the recoil curves, the reversible and irreversible portions of demagnetization curves of the core/shell nanocomposite powders, it was found that the coercivity mechanism of Nd2Fe14B/nanoFe alloys were dominated by domain wall pinning, and the domain wall pinning field determines the coercivity. But there were two sharp peaks in the differential susceptibility curves of RE-rich NdFeB/nanoFe alloys. It is concluded that the coercive force of the powders was mainly determined by the pinning of domain walls by Nd-rich boundary phase.