Rare Earth Magnet Surface Refinement via Hydrogen Disproportionation
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Solution Overview
Problem
High-performance rare earth permanent magnets with compact size or reduced thickness face significant challenges in maintaining magnetic properties and heat resistance due to machining, which degrades their coercive force and corrosion resistance.
Innovation Solution
A method involving the use of anisotropic sintered magnet bodies with specific compositional formulas, where a powder comprising rare earth oxides, fluorides, or oxyfluorides is applied to the surface and subjected to heat treatment in a hydrogen atmosphere followed by recombination reactions to refine crystal grain size and enhance magnetic properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If the magnet body is machined to compact size or reduced thickness, then the specific surface area increases and the magnet can be mounted in magnetic circuits, but the magnetic properties become degraded due to loss of grain boundary surface structure
Solution Approach 1:
The patent applies different treatments to different regions of the magnet: the inner blade cutter processes only the surface layer (depth 0.01-0.1 mm) while leaving the bulk material unchanged. This localized processing restores grain boundary structure only where needed at the surface, preserving magnetic properties in the compacted magnet body.
Solution Approach 2:
The magnet body is pre-compacted and sintered to achieve the desired compact size and reduced thickness before the restoration treatment. This preliminary shaping allows the subsequent inner blade cutting to focus solely on restoring surface grain boundaries without removing significant material.
2Reliability
If the crystal grain size is reduced to 5 μm or less during magnet preparation, then the degradation of magnetic properties can be suppressed, but the manufacturing complexity increases
Solution Approach 1:
The patent changes the grain size parameter to 5 μm or less through controlled sintering conditions and alloy composition. This parameter optimization allows the magnet to maintain high coercive force even after machining, reducing the need for complex post-processing restoration treatments.
3Volume of moving object
If the magnet body is machined to high specific surface area (S/V > 30 mm⁻¹), then compact size requirements are met, but corrosion resistance becomes poor
Solution Approach 1:
The inner blade cutter treatment is applied locally to the surface layer where corrosion occurs, restoring grain boundaries only in the affected region. This localized restoration provides corrosion resistance at the surface while maintaining the compact high S/V geometry of the entire magnet body.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach effectively restores and maintains magnetic properties while providing heat resistance, even at high specific surface areas, preventing degradation and improving coercive force.
Implementation Method 1
The HDDR process involves heat treating in a hydrogen atmosphere to induce disproportionation reaction on the R 2 Fe 14 B compound as the primary phase for decomposing into RH 2 , Fe, and Fe 2 B
Implementation Method 2
reducing the hydrogen partial pressure for dehydrogenation to induce recombination into the original R 2 Fe 14 B compound
Implementation Method 3
when the influence of residual strain by machining is minimized by carefully controlling the machining rate, the average thickness of an affected layer on the machined surface becomes approximately equal to the average crystal grain size
Implementation Method 4
by melting only the grain boundary phase, and diffusing it over the machined surface to restore the magnetic properties of surface particles
Data Source
Figure 1
AI summary
A permanent magnet material is prepared by covering an anisotropic sintered magnet body of formula: R1x(Fe1-yCoy)100-x-z-aBzMa wherein R1 is a rare earth element, M is A1, Cu or the like, with a powder comprising an oxide of R2, a fluoride of R3 or an oxyfluoride of R4 wherein R2, R3, and R4 are rare earth elements, and having an average particle size up to 100 µm, heat treating the powder-covered magnet body in a hydrogen gas-containing atmosphere for inducing disproportionation reaction on R12Fe14B compound, and continuing heat treatment at a reduced hydrogen gas partial pressure for inducing recombination reaction to said compound, thereby finely dividing said compound phase to a crystal grain size up to 1 µm, and for effecting absorption treatment, thereby causing R2, R3 or R4 to be absorbed in the magnet body.