Rare Earth Sintered Magnet Grain Boundary Diffusion for High Coercivity
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Solution Overview
Problem
The demand for high coercive force NdFeB sintered magnets for hybrid vehicles is hindered by the scarcity and uneven distribution of Dy or Tb, which, when substituted, decrease the residual magnetic flux density and maximum energy product, making it difficult to manufacture high-performance magnets effectively.
Innovation Solution
A manufacturing method for rare earth sintered magnets involves forming a low melting point metal layer and a heavy rare-earth metal compound slurry coating on the surface of a sintered body, followed by grain boundary diffusion under a vacuum or inert gas atmosphere, allowing for improved temperature coefficient and coercive force of residual magnetic flux density through controlled diffusion of Dy or Tb.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Force
If Dy or Tb substitution is used to increase coercive force, then coercive force (Hcj) is improved, but residual magnetic flux density (Br) and maximum energy product ((BH)max) decrease
Solution Approach 1:
The patent applies local quality by concentrating Dy or Tb substitution at the grain boundary regions rather than uniformly throughout the entire magnet. The slurry coating method delivers heavy rare-earth metals preferentially to grain boundaries, creating a non-uniform concentration distribution where the surface area near grain boundaries has high concentration while the interior maintains lower concentration. This localized substitution increases coercive force at critical regions without significantly reducing overall residual magnetic flux density.
Solution Approach 2:
The patent segments the magnet into distinct regions with different Dy/Tb concentrations: the grain boundary regions receive high concentration through slurry coating, while the interior regions maintain lower concentration. This segmentation allows different parts of the magnet to serve different functions - grain boundaries provide coercive force enhancement while the bulk maintains high residual magnetic flux density.
2Force
If grain boundary diffusion is performed to increase coercive force, then coercive force (Hcj) is improved, but manufacturing complexity increases due to additional process steps
Solution Approach 1:
The patent applies preliminary action by pre-coating the sintered magnet surface with a slurry containing heavy rare-earth metal compounds before performing grain boundary diffusion. This preliminary coating step prepares the magnet by delivering Dy or Tb to the grain boundary regions in advance, so that during subsequent heating and diffusion, the heavy rare-earth metals are already positioned at the grain boundaries, facilitating more efficient and controlled diffusion while simplifying the overall process compared to attempting direct diffusion without pre-coating.
Solution Approach 2:
The patent uses a slurry coating as an intermediary medium to deliver heavy rare-earth metals to the magnet surface. The slurry, containing heavy rare-earth metal compounds suspended in a carrier, acts as a mediator that facilitates the transfer of Dy or Tb from the coating apparatus to the magnet surface, enabling controlled and uniform distribution before the diffusion process. This intermediary step simplifies the manufacturing process by providing a straightforward method for surface coating compared to more complex direct diffusion techniques.
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 method enhances the magnetic properties of rare earth permanent magnets by maintaining high coercive force and residual magnetic flux density similar to unsubstituted NdFeB magnets, while allowing for uniform diffusion and stress relief, effectively addressing the limitations of Dy or Tb substitution.
Implementation Method 1
Dy or Tb bonded to the surface of the magnet is delivered into the interiors of the sintered body through the grain boundaries of the sintered body and diffuses from the grain boundaries into the interior of each particle of the main phase Re2Fe14B
Implementation Method 2
During grain boundary diffusion, the Re-rich phase at the grain boundary is liquefied by heating, so that the diffusion rate of Dy or Tb within the grain boundary is much faster
Implementation Method 3
loading the sintered body in which the heavy rare-earth metal has diffused into the grain boundaries of the alloy, into a heating furnace to perform a stress relief heat treatment under a vacuum or inert gas atmosphere
Data Source
Figure 1
AI summary
In the present invention, the manufacturing method of rare earth sintered magnet according to the present invention comprises manufacturing rare-earth alloy of xwt%RE-ywt%B-zwt%TM-bal.wt%Fe (where RE=rare earth element, TM=3d transition element, x=28-35, y=0.5-1.5, z=0~15); pulverizing the manufactured alloy to a size of 1.0 ~ 5.0µm or less; orienting and compressing the pulverized alloy in a magnetic field to be magnetized; sintering the magnetized alloy; cleaning the sintered body; forming a low melting point metal layer on the surface of the cleaned body; coating a slurry mixed with a liquid solvent and a heavy rare-earth metal compound on the surface of the low melting point metal layer formed on the sintered body to form a heavy rare-earth metal compound coating layer; loading the sintered body with the low melting point metal layer and the heavy rare-earth metal compound coating layer into a heating furnace to allow the heavy rare earth metal to diffuse into the grain boundaries of the sintered body under a vacuum or inert gas atmosphere; loading the sintered body in which the heavy rare-earth metal has diffused into the grain boundaries of the alloy, into a heating furnace to perform a stress relief heat treatment under a vacuum or inert gas atmosphere; and performing a final heat treatment after the stress relief heat treatment.