Presentation Information
[P2-61]MnAl spherical magnetic particles with stacked twin crystal structure and their Sintered magnets
*Koji Naito1, Yasunari Ishikawa1 (1. AGC Corp. (Japan))
Keywords:
magnetic materials,powder metallurgy,permanent magnet,Twin crystal
<IntroductionIn> recent years, the geopolitical risks associated with neodymium (Nd) and dysprosium (Dy) have driven the development of rare-earth-free magnets. MnAl-based magnets are anticipated to replace Nd-bonded magnets and ferrite magnets due to their supply stability, magnetic properties, and cost-effectiveness. However, conventional manufacturing methods involve synthesizing atomized powder alloys followed by sintering, during which crystal defects are introduced in the milling process, and crystal growth and decomposition occur during sintering, leading to a decline in magnetic properties.In this study, we developed a novel "diffusion-treated powder synthesis method (Colorization)" that enables crystal grain control by regulating the reaction field during synthesis. This diffusion method modifies the surface by diffusing elements from the metal surface to the interior at high temperatures, and it is utilized to enhance the corrosion resistance of materials. We investigated the synthesis of MnAl by employing this method to diffuse and react Al into Mn. Herein, we report the synthesis of spherical magnetic powder that can be easily sintered, exhibiting high magnetic properties and excellent thermal stability without the need for a milling process.<New Synthesis Method for MnAl Powder>We developed a novel static synthesis method in which Al2O3 spheres are placed in a crucible, and the heat treatment and cooling of the raw powder and activator are controlled within the remaining space of the crucible. In this method, the particle size of Al2O3 can be arbitrarily selected, allowing intentional control of the particle size of the resulting powder. The obtained powder has a very fine particle size (e.g., approximately D50=20 µm), and even when heat-treated near the melting point, the particles do not sinter together, providing excellent formability due to their spherical shape.These fine and spherical MnAl particles achieved a very high packing density of approximately 80 % during molding, resulting in high-density molded bodies. This facilitates sintering while suppressing particle deformation, reducing the decomposition of the magnetic phase, and controlling grain growth. Additionally, this method employs pre-mixed raw powder and an activator that promotes the gasification of aluminum, making it easy to obtain uniformly composed powder and to alter the powder composition post-synthesis. Consequently, it is possible to synthesize powder with the desired composition ratio. We also examined the use of additives in this study.<Results and Discussion>Cross-sectional SEM observation and EBSD results of MnAl particles with controlled cooling revealed that the spherical particles possessed a magnetic phase with layered crystals throughout each particle. This phenomenon is attributed to the significant influence of thermal conductivity due to the presence of Al2O3 spheres near the MnAl particles during cooling, although the detailed mechanism remains unclear.Furthermore, the examination of additives revealed that the addition of carbon and silicon resulted in the formation of Mn3AlC in layers, intentionally inhibiting the crystal growth of the magnetic phase, and yielding a twin structure with magnetic layers approximately 100 nm in size within the MnAl particles. These layered particles exhibit a characteristic of being easily magnetically oriented in the direction perpendicular to the layering plane, which is expected to enhance the magnetic properties post-anisotropy.The magnetic properties demonstrated a maximum magnetization (Ms) of approximately 70 emu/g and a coercive force (iHc) of approximately 2000 Oe, which are nearly equivalent to the properties of ferrite magnetic powder. Additionally, isotropic magnets sintered by the SPS method (above 1000°C) using these powders were produced without compromising the magnetic properties of the powder, achieving a current BHMAX of approximately 1 MGOe. Future improvements in magnetic properties are anticipated by leveraging this novel powder synthesis method.
