Rare Earth Sintered Magnet Grain Boundary Diffusion
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Solution Overview
Problem
Existing methods for increasing the coercivity of R-Fe-B based rare-earth sintered magnets, such as surface diffusion of heavy rare-earth elements, are ineffective for magnets thicker than 3 mm, as the heavy rare-earth elements fail to diffuse efficiently throughout the magnet, leading to inadequate coercivity enhancement and potential remanence loss.
Innovation Solution
A method involving the formation of an RHM alloy layer on the surface of the magnet, comprising heavy rare-earth elements like Dy, Ho, or Tb, and metals like Al, Cu, or Ag, which promotes grain boundary diffusion of the heavy rare-earth elements deep into the magnet, increasing coercivity without significantly reducing remanence, even in thicker magnets.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Force
If the light rare-earth element RL is entirely replaced with the heavy rare-earth element RH, then the coercivity increases, but the remanence Br decreases significantly
Solution Approach 1:
The patent applies local quality by concentrating the heavy rare-earth element RH specifically at the grain boundary regions rather than uniformly distributing it throughout the entire magnet. This localized enrichment at boundaries achieves the desired coercivity enhancement while preserving the light rare-earth element RL in the grain cores, thereby maintaining high remanence Br. The composition varies spatially: RH-rich at boundaries, RL-rich in cores.
Solution Approach 2:
The patent changes the compositional parameters by controlling the concentration distribution of rare-earth elements. Specifically, it maintains RL content at 85-99.9 atom% in grain cores while introducing RH at 0.1-15 atom% at grain boundaries. This parameter optimization achieves effective coercivity improvement with minimal remanence loss.
2Force
If a small amount of heavy rare-earth element RH is added to increase coercivity efficiency, then the coercivity improves, but the diffusion of RH to the core of crystal grains during sintering causes unexpected crystal structures
Solution Approach 1:
The patent applies preliminary action by pre-forming a core-shell structure in the alloy powder before sintering, where the grain boundary regions are pre-enriched with heavy rare-earth element RH. This preliminary structural preparation ensures that during subsequent sintering, the RH remains concentrated at boundaries and does not diffuse into grain cores, preventing unwanted crystal structure formation and maintaining compositional stability.
3Quantity of substance
If surface diffusion method is used to increase coercivity, then the remanence is maintained, but the method is ineffective for magnets thicker than 3 mm
Solution Approach 1:
The patent applies preliminary action by pre-forming the core-shell structure in alloy powder before sintering, ensuring RH is already positioned at grain boundaries throughout the entire magnet volume. This preliminary distribution eliminates the need for subsequent surface diffusion processes, making the method effective for magnets of any thickness, including those exceeding 3 mm.
Solution Approach 2:
The patent transitions from a surface-only approach (2D surface diffusion) to a volumetric approach by incorporating the core-shell structure formation during bulk alloy processing. This dimensional change ensures that grain boundary enrichment occurs throughout the entire magnet volume, not just at the surface, thereby effectively treating magnets of any thickness.
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 allows for efficient diffusion of heavy rare-earth elements throughout the magnet, effectively increasing coercivity and maintaining remanence, especially in magnets with thicknesses of 3 mm or more, resulting in high-performance magnetic properties that endure at high temperatures.
Implementation Method 1
the heavy rare-earth element RH is distributed a lot in the vicinity of the grain boundary of the R 2 Fe 14 B phase, and therefore, the magnetocrystalline anisotropy of the R 2 Fe 14 B phase can be improved efficiency on the outer periphery of the main phase
Implementation Method 2
the light rare-earth element R, which is included as a rare-earth element R in an R 2 Fe 14 B phase, is replaced with a heavy rare-earth element RH, and therefore, the magnetocrystalline anisotropy (which is a physical quantity that determines the coercivity) of the R 2 Fe 14 B phase improves
Data Source
Figure 1(a)~1(d)
Figure 2(a)~2(b)
Figure 3
AI summary
In a process of producing an R-Fe-B based rare-earth sintered magnet according to the present invention, first, an R-Fe-B based sintered magnet, including, as a main phase, R2Fe14B type compound crystal grains with a light rare-earth element RL (which is Nd and/or Pr) as a main rare-earth element R, is provided. Next, the surface of the sintered magnet body is coated with an RHM alloy layer including RH (which is at least one rare-earth element selected from Dy, Ho and Tb) and a metal M (which is at least one metallic element selected from Al, Cu, Co, Fe and Ag). Thereafter, a heat treatment is conducted at 500 °C to 1,000 °C within a vacuum or an Ar atmosphere, thereby diffusing the metallic element M and the heavy rare-earth element RH inward from the surface of the rare-earth sintered magnet body.