• Journal of Powder Materials
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• v.23 no.6
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• pp.447-452
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• 2016

Neodymium-iron-boron (Nd-Fe-B) sintered magnets have excellent magnetic properties such as the remanence, coercive force, and the maximum energy product compared to other hard magnetic materials. The coercive force of Nd-Fe-B sintered magnets is improved by the addition of heavy rare earth elements such as dysprosium and terbium instead of neodymium. Then, the magnetocrystalline anisotropy of Nd-Fe-B sintered magnets increases. However, additional elements have increased the production cost of Nd-Fe-B sintered magnets. Hence, a study on the control of the microstructure of Nd-Fe-B magnets is being conducted. As the coercive force of magnets improves, the grain size of the $Nd_2Fe_{14}B$ grain is close to 300 nm because they are nucleation-type magnets. In this study, fine particles of Nd-Fe-B are prepared with various grinding energies in the pulverization process used for preparing sintered magnets, and the microstructure and magnetic properties of the magnets are investigated.

• Journal of Powder Materials
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• v.20 no.5
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• pp.359-365
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• 2013

In this study, we fabricated $Nd_2Fe_{14}B$ hard magnetic powders with various sizes via spray drying combined with reduction-diffusion process. Spray drying is widely used to produce nearly spherical particles that are relatively homogeneous. Thus, the precursor particles were prepared by spray drying using the aqueous solution containing Nd salts, Fe salts and boric acid with the target stoichiometric composition of $Nd_2Fe_{14}B$. The mean particle sizes of the spray-dried powders are in the range from one to seven micrometer, which are adjusted by controlling the concentrations of precursor solutions. After debinding the as-prepared precursor particles, ball milling was also conducted to control the particle sizes of Nd-Fe-B oxide powders. The resulting particles with different sizes were subjected to subsequent treatments including hydrogen reduction, Ca reduction and washing for CaO removal. The size effect of Nd-Fe-B oxide particles on the formation of $Nd_2Fe_{14}B$ phase and magnetic properties was investigated.

• Journal of Magnetics
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• v.17 no.3
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• pp.185-189
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• 2012

The coercivity of near single domain size $Nd_2Fe_{14}B$-type particles prepared by ball milling of HDDR-treated $Nd_{12.5}Fe_{80.6}B_{6.4}Ga_{0.3}Nb_{0.2}$ alloy was investigated. The feasibility of a surface nitrogenation for improving the coercivity stability of the fine $Nd_2Fe_{14}B$-type particles was also studied. The near single domain size $Nd_2Fe_{14}B$-type particles had a high coercivity of over 9 kOe. However, the coercivity radically deteriorated as the temperature increased in air (< 2 kOe at $200^{\circ}C$). This coercivity reduction was attributed to the soft magnetic phases, ${\alpha}$-Fe and $Fe_3B$, which formed on the surface of the fine particle due to oxidation. Surface nitrogenation of the fine particles significantly improved the stability of their coercivity. The improvement in coercivity stability was attributed to the formation of a thin nitrogenated layer on the surface of the fine $Nd_2Fe_{14}B$-type particles, which enhanced the anisotropy field and gave improved resistance to oxidation (dissociation).

• Journal of Magnetics
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• v.10 no.4
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• pp.152-156
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• 2005

Effects of the disproportionating hydrogen pressure and alloy composition on the texture in the HDDR-treated Nd-Fe-B particles were examined using the $Nd_{12.6}Fe_{81.4}B_6$ and $Nd_{12.6}Fe_{68.7}B_6Co_{11.0}Ga_{1.0}Zr_{0.l}$ alloys. Disproportionation kinetics of the $Nd_2Fe_{14}B$ phase in the Nd-Fe-B alloy was retarded significantly by the addition of Co, Ga and Zr. The retarded disproportionation kinetics of the $Nd_2Fe_{14}B$ phase ensured a wider processing win­dow in terms of disproportionating hydrogen pressure for achieving a texture in the HDDR-treated Nd-Fe-B alloy particles.

• Journal of Magnetics
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• v.13 no.3
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• pp.102-105
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• 2008

This study examined the feasibility of the combining HDDR-process (hydrogenation, disproportionation, desorption and recombination) with mechanical milling to prepare single domain $Nd_2Fe_{14}B$ particles from a Nd-Fe-B alloy ingot. The $Nd_{15}Fe_{77}B_8$ alloy was HDDR-treated and then subjected to a roller-milling. In the HDDR-treated $Nd_{15}Fe_{77}B_8$ alloy, very small $Nd_2Fe_{14}B$ grains comparable to their critical single domain size(0.3 ${\mu}m$) were observed. These fine individual grains were separated successfully along the grain boundaries by a roller-milling. The separated $Nd_2Fe_{14}B$ grains were found to be single domain particles. These results suggest that single domain particles of the $Nd_2Fe_{14}B$ phase can be prepared from a Nd-Fe-B ingot alloy by combining a HDDR-process with mechanical milling.

• Journal of Magnetics
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• v.20 no.1
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• pp.21-25
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• 2015

Fine Nd-Fe-B-type particles were prepared by ball milling of different types of Nd-Fe-B precursor materials, such as die-upset magnet, HDDR-treated material, and sintered magnets. Coercivity dependence on the grain and particle size of the powder was investigated. Coercivity of the milled particles was reduced as the particle size decreased, and the extent of coercivity loss was dependent upon the precursor material. Coercivity loss in the finely milled particles was attributed to the surface oxidation. The extent of coercivity loss in the fine particles was closely linked to grain size of the precursor materials. Coercivity loss was more profound for the fine particles with larger grain size. Contrary to the fine particles from the sintered magnets with larger grain size the fine particles (~10 um) from the die-upset magnet and HDDR-treated material with much finer grain size still retained high coercivity (> 10 kOe for die-upset magnet, > 4 kOe for HDDR-treated material).

• Journal of the Korean Magnetics Society
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• v.12 no.2
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• pp.57-63
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• 2002

In order to increase the magnetic properties of a Nd-Fe-B sintered magnet, the general factors including particle size and its distribution, volume fraction of Nd$_2$Fe$_{14}$B phase, degree of alignment of Nd$_2$Fe$_{14}$B grain, oxygen content and grain size etc. should be optimized by controlling the composition of Nd-Fe-B alloy as well as the manufacturing process. In this study, fabrication of the Nd-Fe-B sintered magnet was carried out in a laboratory scale by controlling the composition of Nd-Fe-B alloy and the manufacturing process. The optimum milling condition was found by investigating the milling media, milling time and ball size. The addition of FeGa was effective to increase the coercivity of the Nd-Fe-B sintered magnet. A remanence of 14.4 kG, a coercivity of 9.4 kOe and a maximum energy product of 47 MGOe were obtained from the sintered magnet.

• Journal of the Korean Magnetics Society
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• v.11 no.1
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• pp.8-13
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• 2001

Effects of casting Processes and lubricant addition were studied to improve remanence of 31RE-68TM-lB based sintered magnet by using axial pressing process. Strip casting was effective to get the fine and homogeneous microstructure without ${\alpha}$-Fe segregation. The strip cast flake resulted in narrower particle size distribution after jet milling with respect to the conventional cast ingot. During pressing step with a magnetic field, the particle alignment was increased by appropriate addition of liquid lubricant. In this study, it was revealed that narrow particle size distribution and appropriate addition of liquid lubricant were essential to improve the grain alignment and thus the remanence of Nd-Fe-B sintered magnet.

• Journal of the Korean Magnetics Society
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• v.25 no.4
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• pp.111-116
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• 2015

Practical difficulty in the HDDR (hydrogenation - disproportionation - desorption - recombination) processing of Nd-Fe-B-type alloy is a poor reproducibility of coercivity of the HDDR-treated material. In an attempt to improve the reproducibility of coercivity of the HDDR-treated $Nd_{12.5}Fe_{80.6}B_{6.4}Ga_{0.3}Nb_{0.2}$ alloy, the hydrogen decrepitation was carefully controlled so as to induce more extensive micro-cracks in the particle. Prior to the hydrogenation and disproportionation reaction of HDDR processing, an additional hydrogen degassing was carried out at an elevated temperature of $600^{\circ}C$ under vacuum for the previously hydrogen decrepitated particle. During the additional hydrogen degassing the lattice of hydrogen absorbed $Nd_2Fe_{14}B$ phase was further shrunken, hence more microcracks were introduced in the particle due to its brittle nature. Particles with more micro-cracks had more homogeneous hydrogen absorption and desorption reaction during the HDDR-treatment. The improved reproducibility of coercivity of the HDDR-treated material was attributed to the improved homogeneity of the HDDR reactions due to the presence of more micro-cracks.

• Journal of Magnetics
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• v.20 no.2
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• pp.97-102
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• 2015

The waste of sintered Nd-Fe-B magnets was recycled using the method of dopingPrNd nanoparticles. The effect of PrNd nanoparticle doping on the magnetic properties of the regenerated magnets has been studied. As the content of the PrNd nanoparticles increases, the coercivity increases monotonically, whereas both the remanence and the maximum energy products reach the maximum values for 4 wt% PrNd doping. Microstructural observation reveals that the appropriate addition of PrNd nanoparticles improves the magnetic properties and refines the grain. Domain investigation shows that the self-pinning effect of the rare earth (Re)-rich phase is enhanced by PrNd nano-particle doping. Compared to the magnet with 4 wt% PrNd alloy prepared using the dual-alloy method, the regenerated magnet doped with the same number of PrNd nanoparticles exhibits better magnetic properties and a more homogeneous microstructure. Therefore, it is concluded that PrNd nanoparticle doping is an efficient method for recycling the leftover scraps of Nd-Fe-B magnets.