Improvement of Magnetic Properties and Microstructure control of Sm-Co Magnets Through Grain boundary Diffusion & Cell boundary Segregation Process = 입계 확산 및 셀 경계 편석을 통한 Sm-Co계 자석의 자기특성 향상 및 미세조직 제어

다국어 초록 (Multilingual Abstract)

2:17-type Sm–Co magnets are widely used in extreme environments such as military equipment and the aerospace industry based on their excellent magnetic properties, high- temperature stability, and outstanding corrosion resistance. Although their maximum energy product ((BH)max) is relatively lower than that of Nd–Fe–B magnets, they possess high research value in that they can maintain stable magnetic performance even at elevated temperatures. The excellent magnetic properties of 2:17-type Sm–Co magnets originate from their unique nanoscale microstructure, which consists of the 2:17R phase, the 1:5H phase, lamellar phases, and the 1:3R phase. In particular, Cu is located at the cell boundaries, where it lowers the domain-wall energy and increases the energy difference relative to the cell interior, thereby strengthening domain-wall pinning and playing an important role in enhancing coercivity. However, when Cu-lean regions exist at the grain boundaries or when Cu is not uniformly distributed at the cell boundaries, the domain-wall pinning effect is weakened, resulting in a limitation that reduces coercivity. In this study, diffusion materials consisting of Cu and Cu-based binary alloys were applied to commercial Sm–Co magnets, followed by an additional heat-treatment step to allow the diffusion materials to penetrate into the magnet interior and to induce grain boundary diffusion and cell boundary segregation. Among the various diffusion materials, the specimen treated with the Sm71Cu29 alloy exhibited a maximum increase in coercivity of up to 1.49 kOe, and a clear improvement in the squareness of the hysteresis curve was confirmed in the high reverse-field region. This enhancement in magnetic performance is interpreted as resulting from the effective suppression of initial magnetization reversal by the high-coercivity Sm(Co,Cu)₅ reaction layer formed on the magnet surface and the Cu-rich precipitates formed at the underlying grain boundaries. In particular, this study is of great significance in that a meaningful improvement in coercivity was achieved despite being conducted on commercial magnets.

국문 초록 (Abstract)

2:17계 Sm–Co 자석은 우수한 자기적 특성, 고온 안정성 및 뛰어난 부식 저항성을 바탕으로 군용 장비와 항공우주 산업 등 극한 환경에서 널리 활용되고 있다. 비록 Nd–Fe–B 자석에 비해 최대자기에너지적((BH)max)은 상대적으로 낮지만, 고온에서도 안정적인 자기적 성능을 유지할 수 있다는 점에서 높은 연구 가치를 지닌다. 2:17 계 SmCo 자석의 뛰어난 자기적 특성은 독특한 나노스케일 미세구조에서 기인하며, 이는 2:17R, 1:5H, lamellar phase, 1:3R 상으로 구성된다. 특히, Cu는 셀 경계에 위치하여 자벽 에너지를 낮추고, 셀 내부와의 에너지차이를 증가시켜 자벽 고착을 강화함으로써 보자력 향상에 중요한 역할을 한다.

그러나 입계에 Cu-lean 영역이 존재하거나 셀 경계에 Cu 원소가 균일하게 분포하지 않을 경우 자벽 고착 효과가 감소하여 보자력이 저하시키는 한계가 있다. 본 연구에서는 SmCo 상용자석에 확산물질인 Cu 및 Cu 이원계 합금을 도포한후, 추가 열처리 단계를 진행하여 확산 물질을 자석 내부로 침투시켜 입계 확산 및 셀 경계 편석을 유도하는 실험을 진행했다. 다양한 확산 물질 중 Sm71Cu29합금을 적용한 시편에서 보자력이 최대 1.49 kOe 증가하였으며, 고역자장 영역에서 히스테리시스 곡선의 squareness가 뚜렷하게 개선됨을 확인하였다. 이러한자기적 성능 향상은 자석 표면에 형성된 고보자력의 Sm(Co,Cu)5 반응층과, 그 하부 결정립계에 형성된 Cu-rich 석출물에 의해 초기 자화 반전이 효과적으로억제되었기 때문으로 해석된다. 특히 본 연구는 상용 자석을 대상으로 수행되었음에도 불구하고 유의미한 보자력 향상을 달성하였다는 점에서 큰 의의를 갖는다.

목차 (Table of Contents)

• Chapter 1 Introduction – Objective of research 1
• Chapter 2 Literature review and theoretical background 6
• 2.1 Magnetic materials 6
• 2.1.1 Permanent magnets 6
• 2.1.2 Rare-Earth magnets 9
• 2.2 Samarium-Cobalt Magnets 11
• 2.2.1 Historical Background 11
• 2.2.2 High performance of Sm–Co Magnets (Intrinsic/Extrinsic) 13
• 2.2.3 Manufacturing process 18
• 2.2.4 Coercivity mechanisms 22
• 2.2.5 Issue on Sm-Co magnets 24
• 2.3 GBD&CBS process 27
• 2.3.1 GBDP of the Nd-Fe-B magnets 27
• 2.3.2 Application to Sm–Co Magnets 29
• Chapter 3 Experiments 30
• 3.1 Pre-processing of Specimens 30
• 3.2 Preparation of Diffusion Sources 31
• 3.3 Heat-Treatment Process 32
• 3.4 Materials Characterization 34
• 3.4.1 Microstructural Analysis 34
• 3.4.2 Magnetic Property Evaluation 34
• Chapter 4 Results and discussion 36
• 4.1 Pure-Cu diffusion 36
• 4.2 Brass diffusion 38
• 4.2.1 Effect of diffusion material Form 40
• 4.2.2 Effect of gas atmosphere 44
• 4.3 Sm-Cu diffusion 47
• 4.3.1 Effect of diffusion source contents 49
• 4.3.2 The direction of diffusion 51
• Chapter 5 Conclusion 61
• References 64
• Abstract in Korean 73