• Journal of the Korean Magnetics Society
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• v.22 no.6
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• pp.221-227
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• 2012

High performance permanent magnets have become the subject of considerable attention because of the potential applications in the traction motors of hybrid and electric vehicles and wind generators. Nd-Fe-B magnets have attracted considerable interest due to a large maximum energy product. However, Nd-Fe-B magnet cannot be used in high temperature (${\sim}200^{\circ}C$) applications due to the thermal degradation of coercivity. Therefore, the development of high coercivity Nd-Fe-B permanent magnet is a challenging issue. In case of high coercivity Nd-Fe-B permanent magnet, an increment in the intrinsic coercivity can be easily achieved by substituting Nd atoms with Dy or Tb atoms. However, these heavy rare-earth elements are known to cause a decrease in remanence due to the antiferromagnetic coupling between Dy and Fe atoms. In addition, Dy is relatively expensive and being limited in quantity. Hence, a new technology that can increase the coercivity of Nd-Fe-B sintered magnet using only a small amount, or even, no amount of heavy rare-earth elements is being investigated. This article describes the research trend in reducing the heavy rare-earth elements in Nd-Fe-B magnets.

• Proceedings of the Korean Magnestics Society Conference
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• 1994.03a
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• pp.60-61
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• 1994

• Proceedings of the Korean Magnestics Society Conference
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• 2011.12a
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• pp.146-147
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• 2011

• Journal of Magnetics
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• v.15 no.4
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• pp.155-158
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• 2010

The HDDR (hydrogenation, disproportionation, desorption, and recombination) process can be used as an effective way of converting a no coercivity Nd-Fe-B ingot material, with a coarse $Nd_2Fe_{14}B$ grain structure, to a highly coercive one with a fine grain structure. Careful control of the HDDR process can lead to an anisotropic powder with good $Nd_2Fe_{14}B$ grain texture; the most critical step for inducing texture is disproportionation. The critical conditions (hydrogen pressure and temperature) for the disproportionation reaction of fully hydrogenated $Nd_{12.5}Fe_{81.1-(x+y)}B_{6.4}Ga_xNb_y$ (x = 0 or 0.3, y = 0 or 0.2) alloys, in different atmospheres of pure hydrogen and a mixed gas of hydrogen and argon, was investigated with TPA (thermopiezic analyser). From this, the hydrogen pressure-temperature diagram showing the critical conditions was established. The critical disproportionation temperature of the fully hydrogenated $Nd_{12.5}Fe_{81.1-(x+y)}B_{6.4}Ga_xNb_y$ alloys was slightly increased as the hydrogen pressure decreased in both pure hydrogen and mixed gas. The critical disproportionation temperature of the hydrogenated alloys was higher in the mixed gas than in pure hydrogen. Addition of Ga and Nb increased the critical disproportionation temperature of the fully hydrogenated Nd-Fe-B alloys.

• Proceedings of the KAIS Fall Conference
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• 2005.05a
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• pp.67-70
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• 2005

[ $a-Fe/Nd_2Fe_{14}B$ ]기 이상나노복합합금(two-phase nanocompositie alloy)의 자기적 특성 향상을 위하여, $Nd_9Fe_{84}B_7$ 조성의 합금을 급속 응고법과 등온열처리를 이용하여 제조한 후, 공정 조건의 변화에 따른 상, 미세조직, 자기특성의 변화 등을 조사하였다. 주어진 Nd-Fe-B 삼원계 합금을 35 m/s 이상에서 급랭 응고하였을 때 완전한 비정질 합금을 얻을 수 있었으며, 최적 자기특성을 얻기 위한 적정 열처리 구역은 $700^{\circ}C$/10 - 15분, 725$^{\circ}C$/7 - 12분, $750^{\circ}C$/5 - 7분으로 조사되었다. 이 구역들에서 열처리된 합금들은 모두 매끄러운 감자곡선(demagnetization curve)을 나타내, a-Fe와 $Nd_2Fe_{14}B$사이의 교환상호결합이 잘 일어나고 있음을 알 수 있었으며, 이때 a-Fe와 $Nd_2Fe_{14}B$의 평균 입자 크기는 각각 약 10nm와 40$\~$60 nm였다. 한편 상 전개의 차이에도 불구하고 열처리 후에는 모든 합금들이 $60\%$ 이상의 $M_r/M_s$ 비율을 나타내 열처리 조건의 변화에 관계없이 교환상호작용은 일어나고 있음을 알 수 있었다.

• 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.7 no.4
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• pp.127-131
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• 2002

HDDR characteristics and magnetic properties of $Nd_{15}{(Fe_{1-x}Co_{x})}_{77}B_{8}$(x=0-0.6) alloys were investigated. The effect of applying magnetic field during the recombination step on the anisotropic nature of the HDDR-treated material was also examined. The $Nd_{15}{(Fe_{1-x}Co_{x})}_{77}B_{8}$ phase in the Nd-Fe-B alloys with high Co-substitution alloy had remarkably enhanced phase stability. The $Nd_{15}{(Fe_{1-x}Co_{x})}_{77}B_{8}$(x=0-0.6) alloys with high Co-substitution could be HDDR-treated successfully by only using high pressure hydrogen. However, these alloys had no appreciable coercivity. The poor coercivity of the HDDR-treated $Nd_{15}{(Fe_{1-x}Co_{x})}_{77}B_{8}$(x=0-0.6) alloys with high Co-substitution was attributed to the $Nd{(Fe,Co)}_2$ phase in the alloys. The magnetic filed applied during the recombination step had little effect on the anisotropic nature of the HDDR-treated powder.

• Journal of Powder Materials
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• v.30 no.2
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• pp.116-122
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• 2023

Because magnets fabricated using Nd-Fe-B exhibit excellent magnetic properties, this novel material is used in various high-tech industries. However, because of the brittleness and low formability of Nd-Fe-B magnets, the design freedom of shapes for improving the performance is limited based on conventional tooling and postprocessing. Laser-powder bed fusion (L-PBF), the most famous additive manufacturing (AM) technique, has recently emerged as a novel process for producing geometrically complex shapes of Nd-Fe-B parts owing to its high precision and good spatial resolution. However, because of the repeated thermal shock applied to the materials during L-PBF, it is difficult to fabricate a dense Nd-Fe-B magnet. In this study, a high-density (>96%) Nd-Fe-B magnet is successfully fabricated by minimizing the thermal residual stress caused by substrate heating during L-PBF.

• Analytical Science and Technology
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• v.8 no.3
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• pp.351-358
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• 1995

Oxidation behavior on the surface of Nd-Fe-B ribbon alloy has been studied by X-ray Photoelectron Spectroscopy. In the incipient stage of oxidation on the surface of "as-received" ribbon Nd-oxide film was formed from the fast oxidation of Nd and Fe was metal state in bulk. In process of oxidation time Fe was more abundant in the outmost surface of ribbon from the defused Fe through Nd-oxide film and layer structure of oxidation film was formed.

• 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.