Method of preparing micro-structured powder for bonded magnets having high coercivity and magnet powder prepared by the same

What is this patent about?

’591 is related to the field of rare-earth permanent magnets and, more specifically, to the fabrication of high-performance anisotropic powders for bonded magnets. The technology addresses the environmental and economic challenges of recycling R—Fe—B type magnet scraps, which typically lose their magnetic properties when mechanically crushed. By providing a pathway to restore these properties, the invention enables the production of high-coercivity powders without the high costs associated with extracting raw rare-earth elements or using complex chemical recovery processes.

The underlying idea behind ’591 is that the loss of coercivity during the crushing of sintered magnets is primarily a surface-level degradation that can be reversed through a targeted grain boundary diffusion mechanism. By mixing crushed magnet particles with a small percentage of rare earth fluoride and applying heat, the invention triggers a localized chemical modification. The fluoride decomposes, allowing rare earth elements to migrate into the grain boundaries and onto the surfaces of the matrix grains, effectively repairing the magnetic insulation between grains and inhibiting the nucleation of reverse magnetic domains.

The claims of ’591 focus on a specific thermo-chemical process for transforming crushed sintered magnet material into high-coercivity anisotropic powder. The method requires crushing magnet scraps to a particle size between 50 and 500 μm, mixing them with 1–10 wt % of rare earth fluoride (RF3) powders, and subjecting the mixture to a thermal treatment between 500°C and 1100°C in a vacuum or inert environment. This process is designed to produce a powder where the internal grains maintain their original orientation and size, but the boundaries are enriched with the added rare earth elements.

In practice, the invention works by utilizing the existing micro-structure of the sintered scrap, which already possesses the desired magnetic anisotropy. When the mixture is heated, the rare earth fluoride acts as a restorative agent; for example, using dysprosium fluoride (DyF3) specifically targets the enhancement of coercivity, while neodymium fluoride can improve magnetic flux. The resulting powder achieves an energy product of at least 20 MGOe and a coercivity of at least 5 kOe, making it suitable for high-performance resin-bonded magnets used in motors and sensors.

This approach differs from prior art by avoiding the energy-intensive re-melting of scraps or the expensive Hydrogen Disproportionation Desorption Recombination (HDDR) process. Unlike simple thermal annealing, which fails to fully restore magnetic properties, the addition of the fluoride powder creates a R-rich grain boundary phase that protects the magnetic integrity of the particles. This allows for the mass production of recycled powders that exhibit the high-performance characteristics of virgin anisotropic materials at a significantly lower cost and environmental footprint.