• Journal of the Korean Magnetics Society
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• v.22 no.3
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• pp.85-90
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• 2012

Oxidation of the $Nd_2Fe_{14}B$ compound crystal and its effect on the coercivity of the fine $Nd_2Fe_{14}B$ crystal particles were investigated. Oxidation kinetics of the $Nd_2Fe_{14}B$ compound crystal was investigated using an excessively grown $Nd_2Fe_{14}B$ grains in the $Nd_{15}Fe_{77}B_8$ alloy ingot. Oxidation of the $Nd_2Fe_{14}B$ compound crystal occurred by dissociation of the phase into multi-phase mixture of ${\alpha}$-Fe, $Fe_3B$, and Nd oxides. Oxidation rate of the $Nd_2Fe_{14}B$ compound crystal showed no dependence on the crystallographic direction. The oxidation reaction was modeled according to simple linear relationship. Activation energy for the oxidation of $Nd_2Fe_{14}B$ compound crystal was calculated to be approximately 26.8 kJ/mol. Fine $Nd_2Fe_{14}B$ crystal particles in near single domain size was prepared by ball milling of the HDDR-treated $Nd_{15}Fe_{77}B_8$ alloy, and these particles were used for investigating the effect of oxidation on the coercvity. The near single domain size $Nd_2Fe_{14}B$ crystal particles (${\fallingdotseq}0.3\;{\mu}m$) had high coercivity over 9 kOe. However, the coercivity was radically reduced as the temperature increased in air (<2 kOe at $200^{\circ}C$). This radical coercivity reduction was attributed to the soft magnetic phases, ${\alpha}$-Fe and $Fe_3B$, which were formed on the surface of the fine particles due to the oxidation.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.8 no.4
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• pp.655-659
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• 1998

The effect of quenching speed of melt-spinning on intrinsic coercivity ($_iH_c$$) of annealed ribbons and the crystallization behavior from amorphous $Nd_{14.73}Fe_{78.67}B_{6.60}$ alloy have been studied. We have found that the intrinsic coecivity for annealed melt-spun ribbon is reduced with increasing of quenching rate. $\alpha$-Fe and $Fe_3B$ were formed as intermediate phases prior to the formation of $Nd_2Fe_{14}B$ phase during crystallization. The $Fe_3B$ is disappeared with crystallization of $Nd_2Fe_{14}B$ phase. But the $\alpha$-Fe phase is retained in fully crystallized ribbon by annealing. The intrinsic coercivity loss of annealed ribbon with increasing of quenching speed is believed to be due to existence of soft magnetic phase $\alpha$-Fe in annealed ribbons.

• Journal of the Korean Magnetics Society
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• v.8 no.3
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• pp.144-149
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• 1998

A new method to calculate the spin reorientation angle of cone anisotropy from magnetization curves of magnetically aligned powder is suggested. The spin reorientation angle of polycrystalline $Nd_2Fe_{14}B$ determined by this method coincided with the results obtained from torque magnetometry or magnetization measurements on single crystal. The underestimation of about 9% is expected if the basal plane anisotropy is neglected in determination of the spin reorientation angle of $Nd_2Fe_{14}B$ at 4.2 K.

• Proceedings of the Korea Association of Crystal Growth Conference
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• 1997.10a
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• pp.63-67
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• 1997

환원확산법은 저렴한 Nd 산화물을 환원제를 이용하여 환원시키고 환원된 Nd가 Fe, FeB와 확산하여 주상인 Nd$_2$Fe14B가 만들어지는 공정으로 환원제로 사용된 CaO나 미반응 Ca 및 잔존 산소함량을 조절하는데 어려움이 있어 아직까지는 상업되지 못하고 있는 실정이다. 본 연구에서는 환원확산법을 이용하여 Nd-Fe-B계 영구자석에 사용될 미세분말을 제조하고 그 자기적 특성을 관찰하였다.

• Journal of Magnetics
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• v.9 no.2
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• pp.27-33
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• 2004

Magnetic properties of $Nd_4Fe_{77.5}B_{18.5}$ compound in term of exchange coupling between $Nd_2Fe_{14}B$ and $Fe_3B$ magnetic nano crystals in melt spun powders were characterized by varying the quenching speed in mass production line. The exchange coupled phenomenon was characterized as functions of nano crystal size and volume fraction of each magnetic phase which was possible by employing Henkel plot (${\delta}M$) and refined Mossbauer spectroscopy. The optimized magnetic properties obtained from the present volume production line were: $B_r= 11.73 kG,{_i}H_c/ = 3.082 kOe$, and $(BH)_{max} = 12.28 MGOe.$ The volume fraction of each magnetic phase for those conditions giving the grain size of 10 nm were ${\alpha}-Fe; 4.2%, Fe_3B; 60.1 %$, and $Nd_2Fe_{14}B; 35.7%$. The superior magnetic properties in the $Nd_2Fe_{14}Fe_3B$ based nanocomposites were confirmed to be dependant on the volume fraction of $Fe_3B$.

• Bulletin of the Korean Chemical Society
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• v.35 no.3
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• pp.886-890
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• 2014

The properties of magnetic material of Nd-Fe-B are highly affected by various factors such as particle size, magnetic anisotropy, phase purity and crystal structure. Incorporation of excess neodymium was carried out in various percentages so that it will adjust the proportion of neodymium in the host crystal after reduction treatment and finally help to improve magnetic property of a material. The interdiffusion of Nd-Fe and boron was studied for various compositions and their effect on magnetic property was understood with theoretical concepts. The factors such as amount of hard and soft phase in the reduction treated product is also responsible for the possible exchange coupling between hard and soft phase magnets for better magnetic properties.

• Journal of Magnetics
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• v.10 no.1
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• pp.14-22
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• 2005

Precisely refined Mossbauer study and nano structure observation revealed that intergranular phase formed between a-Fe and Nd₂Fe₁₄B phase in NdFeNbB alloys plays a significant role on the magnetic properties. The intergranular interaction was characterized in term of Henkel Plot (δM plot), and hyperfine field, quardrupole splitting and isomer shift were refined to predict the presence and role of the intergranular phase. By the addition of Nb into Nd₈Fe₈₆B₆ composition, coercivity was found to increase by 25% due to the refinement of average grain size of both the soft and hard magnetic phases which was decreased from 50 nm of virgin Nd/sub 8/Fe/sub 86/B/sub 6/ to 25 nm in Nd₈Fe ₈₅Nb₁B₆ alloys. The role of Nb addition was confirmed to stabilize the Nd₂Fe₁₄B lattice preventing from thermal vibration of the corresponding sites substituted Fe by Nb atoms in all sites in the Nd₂Fe₁₄B lattice. The enhanced coercivity was originated from the exchange hardening of soft and amorphous phases surrounding the hard magnetic Nd₂Fe₁₄B crystal.

• Proceedings of the Korean Magnestics Society Conference
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• 2011.12a
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• pp.144-145
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• 2011

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2006.09b
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• pp.790-791
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• 2006

The fracture behavior and mechanical characteristics of sintered rare-earth magnets were investigated. It shows that the fracture behavior and bending strength of the magnets obviously exhibit anisotropy. Sm-Co magnets tend to cleavage fracture in the close-packed (0001) plane or in the ($10\bar{1}1$) plane. The fracture mechanism of $Nd_2Fe_{14}B$ magnet mainly appears to be intergranular fracture. The anisotropy of fracture behavior and mechanical strength of sintered rare-earth magnets is caused mainly by the strong crystal-structure anisotropy and the grain alignment texture. The effects of Nd content, and Pr, Dy substitution on the impact stability of $Nd_2Fe_{14}B$ magnets were also reported.

• Journal of Magnetics
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• v.18 no.3
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• pp.235-239
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• 2013

The effect of Dy addition on the microstructure and magnetic properties of the sintered NdFeB magnets was investigated. The results of the microstructure analysis showed that Dy-free and Dy-doped samples are composed of $Nd_2Fe_{14}B$ (P42/mnm) and a trace of Nd-rich phase. Dy addition reduces significantly the pole density factor of (004), (006) and (008) crystal faces as estimated by the Horta formula. Accordingly, the coercivity of the Dy-doped sample increases from 2038 $kA{\cdot}m^{-1}$ up to 2288 $kA{\cdot}m^{-1}$. The $H_{cj}(T)/M_s(T)$ versus $H^{min}_N/M_s(T)$ (Kronm$\ddot{u}$ller-plot) behavior shows that the nucleation is the dominating mechanism for the magnetization reversal in these two kinds of magnets, and two microstructural parameters of ${\alpha}_k$ and $N_{eff}$ are obtained. The Kronm$\ddot{u}$ller-Plot gives evidence for an increase of the ${\alpha}_k$ responsible for an increase of the coercivity as the result of the increase of the magnetic field as the magnetic domain reversed.