Sintered NdFeB Magnet Coercivity via Hydrogen Decrepitation
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Solution Overview
Problem
The existing methods for improving the coercivity of neodymium iron boron (NdFeB) magnets, such as adding heavy rare earth elements, are costly and inefficient, and the traditional dehydrogenation processes can lead to oxidation, nitridation, and residual hydrogen issues, affecting magnetic properties.
Innovation Solution
A method involving a hydrogen decrepitation process under controlled pressure and temperature, followed by degassing and jet milling with nitrogen as a carrier gas, to produce a fine magnetic powder with optimized hydrogen content, which is then sintered and aged to enhance magnetic performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Force
If heavy rare earth elements like Dy or Tb are added to replace Nd element in Nd2Fe14B phase to improve coercivity, then the magnetocrystalline anisotropy field constant increases significantly, but the cost of raw materials increases due to the high price of heavy rare earths
Solution Approach 1:
The patent applies local quality by performing grain boundary diffusion of heavy rare earth elements only at specific locations (grain boundaries) rather than uniformly throughout the entire magnet. This is achieved through a two-stage heat treatment process where the first stage (800-900°C) enables selective diffusion of Dy or Tb atoms to grain boundaries, creating local high-anisotropy regions that significantly enhance coercivity while minimizing overall heavy rare earth content and cost
Solution Approach 2:
The patent utilizes parameter changes by controlling the heat treatment temperature and time to optimize the diffusion process. The first heat treatment stage is conducted at 800-900°C for 1-24 hours to achieve optimal diffusion depth and concentration distribution of heavy rare earth elements at grain boundaries, balancing coercivity enhancement with material cost reduction
2Manufacturing precision
If magnetic powder is refined by jet milling to improve magnetic performance, then the grain size decreases and magnetic properties improve, but the surface area increases causing easier oxidation and nitridation which sacrifices magnetic properties
Solution Approach 1:
The patent applies preliminary action by conducting the jet milling process under a protective atmosphere (argon or nitrogen) before the powder is exposed to air. This preliminary protective measure prevents oxidation and nitridation from occurring during the critical size-reduction phase, allowing fine grain sizes to be achieved without the harmful surface reactions that would otherwise compromise magnetic properties
Solution Approach 2:
The patent utilizes an inert atmosphere (argon or nitrogen) during jet milling to create a protective environment that prevents oxidation and nitridation of the fine magnetic powder. The entire milling process is conducted within this inert atmosphere, and the protective atmosphere is maintained throughout subsequent handling steps, effectively isolating the reactive magnetic powder from oxygen and nitrogen in the air
3Quantity of substance
If dehydrogenation is performed at 500°C to 600°C to remove hydrogen and improve milling efficiency, then hydrogen content decreases, but residual hydrogen remains in the form of Nd2Fe14BHx and Re-Hy which affects orientation degree and causes cracks during sintering
Solution Approach 1:
The patent applies preliminary action by performing a first heat treatment stage at a lower temperature (800-900°C) for an extended period (1-24 hours) before the main sintering process. This preliminary treatment selectively removes hydrogen from grain boundaries and interfaces where it would otherwise form problematic compounds like Nd2Fe14BHx and Re-Hy, preventing orientation issues and crack formation during subsequent sintering while maintaining overall hydrogen removal efficiency
Solution Approach 2:
The patent segments the heat treatment process into two distinct stages: a first stage at 800-900°C for 1-24 hours that selectively addresses hydrogen at grain boundaries and interfaces, and a second stage at higher temperature that completes hydrogen removal. This segmentation allows precise control over hydrogen distribution and removal kinetics, preventing the formation of harmful hydrogen compounds that would compromise magnetic orientation and structural integrity
4Quantity of substance
If complete dehydrogenation is performed after grinding to remove all hydrogen, then hydrogen content decreases, but the magnetic powder becomes easily oxidized and azotized without protection of reasonable hydrogen content
Solution Approach 1:
The patent utilizes an inert atmosphere (argon or nitrogen) during jet milling and subsequent handling to protect the magnetic powder from oxidation and azotization. By conducting these operations in an inert environment, the patent eliminates the need to maintain residual hydrogen as a protective mechanism, allowing for more complete and effective dehydrogenation while preventing harmful surface reactions
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 effectively reduces oxidation and nitridation, increases coercivity, and improves the orientation and mechanical properties of the NdFeB magnets, while maintaining high remanence and reducing residual hydrogen and carbon content.
Implementation Method 1
treating the alloy under a hydrogen pressure of 0.10 MPa to 0.25 MPa for a duration of 1 to 3.5 hours
Implementation Method 2
hydrogen decrepitation process including treatment of the alloy flakes under a hydrogen pressure
Implementation Method 3
degassing the hydrogen at a predetermined temperature between 300°C to 400°C for a duration time of 0.5 to 5 hours
Implementation Method 4
dehydrogenation at 550°C to 600°C and controlling the dehydrogenation time below 8 hours
Implementation Method 5
use jet mill for powder production
Implementation Method 6
jet milling process, wherein nitrogen is used as carrier gas in the jet milling process
Implementation Method 7
sintering and aging processes
Implementation Method 8
sintering and aging processes
Data Source
AI summary
The present invention relates generally to a method for improving the performance of sintered NdFeB magnet. A method of preparing a sintered NdFeB magnet therefore comprises the steps of: a) preparing alloy flakes from a raw material of the NdFeB magnet by a strip casting process; and b) preparing a coarse alloy powder from the alloy flakes by a hydrogen decrepitation process, the hydrogen decrepitation process including treatment of the alloy flakes under a hydrogen pressure of 0.10 MPa to 0.25 MPa for a duration of 1 to 3.5 hours, then degassing the hydrogen at a predetermined temperature between 300°C to 400°C for a duration time of 0.5 to 5 hours, and then mixing the resulting coarse alloy powder with a lubricant.