• 한국재료학회지
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• 제20권10호
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• pp.508-512
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• 2010

The optimum route to fabricate nano-sized Fe-50 wt% Co and hydrogen-reduction behavior of calcined Fe-/Conitrate was investigated. The powder mixture of metal oxides was prepared by solution mixing and calcination of Fe-/Co-nitrate. A DTA-TG and microstructural analysis revealed that the nitrates mixture by the calcination at $300^{\circ}C$ for 2 h was changed to Fe-oxide/$Co_3O_4$ composite powders with an average particle size of 100 nm. The reduction behavior of the calcined powders was analyzed by DTA-TG in a hydrogen atmosphere. The composite powders of Fe-oxide and Co3O4 changed to a Fe-Co phase with an average particle size of 40 nm in the temperature range of $260-420^{\circ}C$. In the TG analysis, a two-step reduction process relating to the presence of Fe3O4 and a CoO phase as the intermediate phase was observed. The hydrogen-reduction kinetics of the Fe-oxide/Co3O4 composite powders was evaluated by the amount of peak shift with heating rates in TG. The activation energies for the reduction, estimated by the slope of the Kissinger plot, were 96 kJ/mol in the peak temperature range of $231-297^{\circ}C$ and 83 kJ/mol of $290-390^{\circ}C$, respectively. The reported activation energy of 70.4-94.4 kJ/mol for the reduction of Fe- and Co-oxides is in reasonable agreement with the measured value in this study.

• 한국분말재료학회지
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• 제12권5호
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• pp.362-368
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• 2005

The structural and magnetic properties of nanostructued Fe-20 ;wt.%Si alloy powders were investigated. Commercial Fe-20 wt.%Si alloy powders (Hoeganaes Co., USA) with 99.9% purities were used to fabricate the nanostructure Fe-Si alloy powders through a high-energy ball milling process. The alloy powders were fabricated at 400 rpm for 50 h, resulting in an average grain size of 16 nm. The nanostructured powder was characterized by fcc $Fe_{3}Si$ and hcp $Fe_{5}Si_3$ phases and exhibited a minimum coercivity of approximately 50 Oe.

• 한국분말재료학회지
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• 제22권5호
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• pp.356-361
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• 2015

Fe-30 wt% TiC composite powders are fabricated by in situ reaction synthesis after planetary ball milling of (Fe, $TiH_2$, Carbon) powder mixture. Two sintering methods of a pressureless sintering and a spark-plasma sintering are tested to densify the Fe-30 wt% TiC composite powder compacts. Pressureless sintering is performed at 1100, 1200 and $1300^{\circ}C$ for 1-3 hours in a tube furnace under flowing argon gas atmosphere. Spark-plasma sintering is carried out under the following condition: sintering temperature of $1050^{\circ}C$, soaking time of 10 min, sintering pressure of 50 MPa, heating rate of $50^{\circ}C/min$, and in a vacuum of 0.1 Pa. The curves of shrinkage and its derivative (shrinkage rate) are obtained from the data stored automatically during sintering process. The densification behaviors are investigated from the observation of fracture surface and cross-section of the sintered compacts. The pressureless-sintered powder compacts are not densified even after sintering at $1300^{\circ}C$ for 3 h, which shows a relative denstiy of 66.9%. Spark-plasma sintering at $1050^{\circ}C$ for 10 min exhibits nearly full densification of 99.6% relative density under the sintering pressure of 50 MPa.

• 한국자기학회지
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• 제3권1호
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• pp.34-40
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• 1993

급속냉각기술로 제조된 $Nd_{10.5}Fe_{79}Co_{2}Zr_{1.5}B_{7}$ 합금분말을 사용하여 사출성형용 자성 펠렛을 제조하였다. 입도분포가 다른 두 종류의 분말($38~75\;\mu\textrm{m}$ 및 $75~50\;\mu\textrm{m}$)을 사용하여 열가소성(Nylon 6) 펠렛을 제조한 후 자성분말의 임계분율(critical volume fraction) 을 결정하고 수지자석의 성형밀도에 따른 자기특성을 고찰하였다. Nylon 6 수지를 사용한 Nd-Fe-Co-Zr-B계 펠렛은 분말입도가 $38~75\;\mu\textrm{m}$인 경우 임계 부피분율은 70%로 나타났고, 그때의 성형밀도는 이론밀도 의 90%를 상회 했으나 $75~150\;\mu\textrm{m}$ 분말의 경우는 같은 분율에서 이론밀도의 87% 수준에 머물렀다. 최적의 자기특성은 분말에 윤활제로서 silicone oil만을 0.5wt.% 첨가한 경우이며 이때의 자기특성으로, $(BH)_{max}=5.2\;MGOe,\;B_{r}=5.1\;kG$ 그리고 $_{i}H_{c}=8.8\;kOe$의 높은 수준을 보였다. Nylon 6수지자석의 성형밀도를 예측할 수 있는 경험식으로서, ${\rho}(g/cm^{3})=1.1+K.V_{s}$을 얻었으며 K(5.3~5.6)는 자성분말 입도에 따른 기울기 상수이고 $V_{s}$는 분말의 부피분율이다.

• 한국재료학회지
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• 제25권8호
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• pp.386-390
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• 2015

Fe-base superalloy powders with $Y_2O_3$ dispersion were prepared by high energy ball milling, followed by spark plasma sintering for consolidation. High-purity elemental powders with different Fe powder sizes of 24 and 50 mm were used for the preparation of $Fe-20Cr-4.5Al-0.5Ti-O.5Y_2O_3$ powder mixtures (wt%). The milling process of the powders was carried out in a horizontal rotary ball mill using a stainless steel vial and balls. The milling times of 1 to 5 h by constant operation (350 rpm, ball-to-powder ratio of 30:1 in weight) or cycle operation (1300 rpm for 4 min and 900 rpm for 1 min, 15:1) were applied. Microstructural observation revealed that the crystalline size of Fe decreased with an increase in milling time by cyclic operation and was about 15 nm after 3 h, forming a FeCr alloy phase. The cyclic operation had an advantage over constant milling in that a smaller-agglomerated structure was obtained. The milled powders were sintered at $1100^{\circ}C$ for 30 min in vacuum. With an increase in milling time, the sintered specimen showed a more homogeneous microstructure. In addition, a homogenous distribution of Y-compound particles in the grain boundary was confirmed by EDX analysis.