Jung-Goo Lee

Effect of the dehydrogenation speed and Nd content on the microstructure and magnetic properties of HDDR processed Nd-Fe-B magnets

The effect of Nd content and dehydrogenation speed on the microstructure and magnetic properties of hydrogenation-disproportionation-desorption-recombination (HDDR) processed Nd-Fe-B magnetic powders was studied. The NdxB6.4Ga0.3Nb0.2Febal (x=12.5–13.5, at.%) mold casting alloys were subjected to HDDR process after homogenization heat treatment. During desorption-recombination stage, dehydrogenation speed and time were systematically changed to control the speed of the desorption-recombination reaction. The higher Nd content resulted in better magnetic properties of the HDDR powder, and this was attributed to the thicker and more uniform Nd-rich phase at grain boundaries. It was also confirmed that the slow dehydrogenation speed could maximize the effect of high Nd content on the magnetic properties of HDDR powder. At the optimized dehydrogenation speed, the coercivity and remanence was 15.3 kOe and 13.0 kG, respectively, at 12.9 at.% Nd content, which resulted in a (BH)max of 36.8 MGOe.

Residual Hydrogen in Nd-Fe-B HDDR Powder and Its Effect on Coercivity of Hot-Pressed Compact

Residual hydrogen in the HDDR-treated Nd_12.5Fe_80.6B_6.4Ga_0.3Nb_0.2 powder and its effect on the coercivity of consolidated compact of the powder were investigated. Hydrogen desorption of the HDDR powder was studied by vacuum gauge and TPA. Compaction of the HDDR powder was performed by hot-pressing technique. The Nd_12.5Fe_80.6B_6.4Ga_0.3Nb_0.2 HDDR powder contained significant amount of residual hydrogen (approximately 1520 ppm). Coercivity of the hot-pressed compact was radically reduced when the compaction was performed at the temperature above 650 °C. The radical coercivity reduction in the compact hot-pressed above 650 °C was attributed to the presence of the soft magnetic phases, α-Fe and Fe₂B formed via disproportionation of the Nd₂Fe₁₄BH_x. The residual hydrogen may have contributed for the formation of Nd₂Fe₁₄BH_x.

The Influence of Dehydrogenation Speed on the Microstructure and Magnetic Properties of Nd-Fe-B Magnets Prepared by HDDR Process

The influence regarding the dehydrogenation speed, at the desorption-recombination state during the hydrogenation-disproportionation-desorption-recombination (HDDR) process, on the microstructure and magnetic properties of Nd-Fe-B magnetic powders has been studied. Strip cast Nd-Fe-B-based alloys were subjected to the HDDR process after the homogenization heat treatment. During the desorption-recombination stage, both the pumping speed and time of hydrogen were systematically changed in order to control the speed of the desorption-recombination reaction. The magnetic properties of HDDR powders were improved as the pumping speed of hydrogen at the desorption-recombination stage was decreased. The lower pumping speed resulted in a smaller grain size and higher DoA. The coercivity and the remanence of the 200-300 μm sized HDDR powder increased from 12.7 to 14.6 kOe and from 8.9 to 10.0 kG, respectively. In addition, the remanence was further increased to 11.8 kG by milling the powders down to about 25-90 μm, resulting in (BH) max of 28.8 MGOe.

Magnetic properties of MnBi based alloys: First-principles calculations for MnBi-Co and MnBi-Co-Fe cases

First-principles calculations of fundamental magnetic properties were performed for ordered MnBi, MnBi-Co, and MnBi-Co-Fe alloys to evaluate maximum energy product (BH)max. Full potential linear-augmented plane wave (FLAPW) and linear-muffin-tin-orbital (LMTO) calculations using density functional theory (DFT) within the local spin density approximation (LSDA) were used and found to give a reasonable description of saturation magnetization (Ms), effective anisotropy constant (Keff), and Curie temperature (Tc) for NiAs-structured MnBi crystal. We found that upon addition of Co, the Ms and Keff increased, while Tc reduced. The magnetic anisotropy changed from weak anisotropy easy plane for MnBi to the strong easy axis anisotropy for MnBi-Co and MnBi-Co-Fe.

The Influence of Mechanical Milling on the Structure and Magnetic Properties of Sm-Fe-N Powder Produced by the Reduction-Diffusion Process

In the present study, we systematically investigated the effect of mechanical milling on the magnetic properties of Sm₂Fe₁?N x powders produced by the reduction-diffusion process. The Sm-Fe powders obtained by the reduction-diffusion process were composed of an Sm₂Fe₁? single phase. After nitrogenation, the coercivity and saturation magnetization of the powders were 0.48 kOe and 13.32 kG, respectively. The particle size largely decreased down to less than 2 ㎛ in diameter after ball milling for 30 hours. However, there is no evidence that the Sm₂Fe₁?N x was decomposed to Sm-N and α-Fe even after ball milling for 30 hours. The coercivity was significantly improved up to 8.82 kOe after milling for 60 hours. However, the magnetization decreased linearly with the ball milling time.

Trend in Research and Development Related to Lean Heavy Rare-earth Permanent Magnets for Next-generation Motors

21세기 인류가 추구하는 ‘저탄소 녹색성장 사회’를 구현 하기 위해 친환경 자동차, 에너지 절전형 가전제품, 그리 고 신재생 에너지 생산용 주요부품의 매우 중요한 기반 기술이 모터용 Nd계 벌크 자석기술이다. 이것은 전체 전 력소비량의 약 50%가 모터구동에 사용된다는 점으로부터 쉽게 이해할 수 있다. 예를 들어, 자동차 시장의 경우 동 력을 내연기관 등에 의존하던 것이 하이브리드 자동차 (HEV)와 전기자동차(EV)와 같이 모터구동방식으로의 전 동화가 급속한 발전을 보이고 있어 2020년에는 HEV, EV 세계시장이 연간 1000만대 이상으로 확대될 것으로 예상 되고 있다. 현시점에서는 전지의 비용과 탑재 가능한 용량 의 문제로, 모터는 내연기관의 보조로서 사용하는 것이 일 반적이지만 장기적인 관점에서 이 주종관계는 역전하여 비효율 내연기관은 이동체에서 완전히 사라질 것으로 예 상된다. 이륜차분야에도 오토바이나 자전거의 전동화가 확대되고 있다. 특히 중국에서는 정부의 환경대책의 영향 으로 도시부의 전동바이크가 확대되고 있어, 연간 2000만 대를 넘는 시장을 형성하고 있다. 일본에서는 전동보조자 전거 시장이 연간 40만대를 넘어 이륜차 시장을 추월했다. 이처럼 전동화가 가속되고 있는 것은 자동차와 이륜차뿐 만 아니다. 건축기계 분야에서도 하이브리드화가 급속하 게 진행되어 2020년에는 연간 1만2000대 정도까지 시장 이 확대 될 것으로 예측되고 있다. 내연기관 대신 모터를 사용하는 이유는 매우 명확하다. 모터를 사용하는 편이 에 너지 효율이 높기 때문이다. 내연기관에서는 발생하는 에 너지에서 운동에너지로 변환하는 비율이 최대 30% 정도 이다. 만일 고성능 모터를 사용하는 것이 가능하면 투입한 전기에너지의 90% 가까이를 운동에너지로 변환시킬 수 있다. 게다가, 감속시에 모터를 발전기로 이용하여 회생 전력도 얻을 수 있다. 이것은 내연기관에서는 불가능한 것이다. 이와 같은 배경에서, 앞으로 HEV, EV의 구동모터에서 에너지 절약형 에어컨용 압축기모터, 세탁기 드럼 구동모 터, 공작기기와 산업로봇용 모터에 이르기까지 다용하게 사용되고 있는 모터의 성능향상은 매우 중요하고 이를 위 해서 Nd계 고특성 벌크자석이 반드시 필요하다. 예를 들 어 HEV 1대에는 약 1 kg, EV 1대에는 약 2 kg의 Nd계 벌크자석이 사용되고 있고 그 수요는 지속적으로 증가하 고 있다. 이처럼 Nd계 벌크자석의 수요 증가 추세에서 한 가지 주목할 점은 높은 잔류자속밀도를 요구하는 수요에 서 높은 보자력을 요구하는 수요로 변화하고 있다는 점이

Structure and Magnetic Properties of MnAl/α-Fe Nano-Composite Powders Prepared by High-Energy Ball Milling

Novel nano-composite powders composed of hard-magnetic Mn54Al46 and soft-magnetic α-Fe were prepared by high-energy ball milling. The effect of α-Fe and preparation conditions on the structure and magnetic properties of the composite powders has been investigated. The ε-MnAl transforms to γ-MnAl, τ-MnAl, and β-phase under ball milling and annealing. The saturation magnetization and coercivity of the two-phase samples decrease with increasing temperature for the τ-phase decomposes at elevated temperatures. With increasing iron content, the coercivity decrease first and then increase up to 0.33 T when the Fe content is 10 wt%. Further addition of the magnetically soft iron phase would result in a decrease of the coercivity.

Synthesis of Nickel and Copper Nanopowders by Plasma Arc Evaporation

In this study, the synthesis of nickel nanoparticles and copper nanospheres for the potential applications of MLCC electrode materials has been studied by plasma arc evaporation method. The change in the broad distribution of the size of nickel and copper nanopowders is successfully controlled by manifesting proper mixture of gas ambiance for plasma generation in the size range of 20 to 200 nm in diameter. The factors affecting the mean diameter of the nanopowder was studied by changing the composition of reactive gases, indicating that nitrogen enhances the formation of larger particles compared to hydrogen gas. The morphologies and particle sizes of the metal nanoparticles were observed by SEM, and ultrathin oxide layers on the powder surface generated during passivation step have been confirmed using TEM. The metallic FCC structure of the nanoparticles was confirmed using powder X-ray diffraction method.

Structural and magnetic properties of the gas atomized Mn-Al alloy powders

Ferromagnetic Mn-Al powders were produced by a gas-atomization method followed by heating treatment. The gas-atomized powders were ɛ-phase, which is a high temperature phase in the Mn-Al system. The ɛ-τ phase transformation took place by subsequent heat treatment at temperatures from 500 to 700 °C. The gas-atomized powders with a smaller particle size formed the τ-phase faster and thus exhibited better magnetic properties. On the other hand, the annealing temperature and time similarly played important roles in determining the magnetic properties of the products. The Mn-Al powders of 25–38 μm annealed at 550 °C for 120 min exhibited a high coercivity of 3.2 kOe with a remanence of 38 emu/g.

STRUCTURE AND MAGNETIC PROPERTIES OF MANGANESE OXIDE NANOPARTICLES PREPARED BY ARC SUBLIMATION

Manganese oxide nanoparticles with narrow size distribution were prepared in large scale by arc sublimation of bulk MnO. Although MnO is antiferromagnetic, the as-prepared manganese oxide nanoparticles exhibit ferromagnetic behaviors with coercivity and exchange bias up to 417,065 A/m and 162,099 A/m at 5 K, respectively. We attribute these anomalous magnetic properties to the presence of ferrimagnetic Mn3O4, uncompensated surface spins and their coupling to the antiferromagnetic MnO.