NdFeB Magnet Grain Boundary Diffusion for Coercivity
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Solution Overview
Problem
Conventional neodymium (NdFeB) permanent magnets for traction motors in Hybrid Electronic Vehicles are costly due to the high content of expensive dysprosium (Dy) and terbium (Tb) elements, which also reduce magnetic force and thermal properties.
Innovation Solution
A method of producing a neodymium permanent magnet with reduced Dy content and enhanced magnetic force by simultaneously sintering and heating NdFeB stripcasted alloy powder with separately prepared Tb powder, controlling the vacuum conditions and temperature to prevent evaporation and promote grain boundary diffusion, thereby reducing production costs and improving coercivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If Dy and Tb elements are added to improve thermal properties, then thermal stability is improved, but magnetic force is reduced and production cost increases
Solution Approach 1:
The patent applies local quality by concentrating Dy and Tb elements specifically at the grain boundaries of the NdFeB magnet through diffusion treatment, rather than uniformly distributing them throughout the bulk material. This localized placement at grain boundaries provides thermal stability where it is most needed for coercivity enhancement, while minimizing the overall amount of expensive rare-earth elements required, thereby preserving bulk magnetic force properties.
Solution Approach 2:
The patent utilizes parameter changes by controlling the diffusion process parameters (temperature, time, vacuum degree) to precisely regulate the concentration and distribution of Dy and Tb elements at grain boundaries. By optimizing these parameters, the patent achieves the minimum necessary concentration of heavy rare-earth elements for thermal stability while avoiding excessive addition that would degrade magnetic force and increase cost.
2Ease of manufacture
If Dy content is reduced to lower production cost, then production cost is reduced, but thermal properties and coercivity deteriorate
Solution Approach 1:
The patent applies local quality by concentrating Dy and Tb elements specifically at the grain boundaries of the NdFeB magnet through diffusion treatment, rather than uniformly distributing them throughout the bulk material. This localized placement at grain boundaries provides thermal stability where it is most needed for coercivity enhancement, while minimizing the overall amount of expensive rare-earth elements required, thereby preserving bulk magnetic force properties.
Solution Approach 2:
The patent employs a diffusion treatment process that effectively 'copies' or replicates the beneficial grain boundary structure and composition found in high-Dy magnets, but achieves it through controlled diffusion of smaller amounts of Dy and Tb during or after sintering, rather than requiring high bulk concentrations from the outset.
3Productivity
If sintering temperature is increased to 1000°C or greater for grain boundary diffusion, then diffusion efficiency is improved, but Dy evaporates rapidly causing material loss
Solution Approach 1:
The patent applies inert atmosphere by conducting the sintering and diffusion processes in a vacuum environment (10^-3 to 10^-5 Pa) or under protective gas atmosphere. This inert environment prevents Dy evaporation and oxidation during high-temperature processing, allowing the patent to achieve effective grain boundary diffusion at elevated temperatures without significant material loss or contamination.
Solution Approach 2:
The patent applies preliminary action by pre-coating the pressed body with heavy rare-earth materials before sintering, or by preparing the vacuum/protective atmosphere beforehand. This preliminary preparation ensures that when high-temperature diffusion is applied, the Dy and Tb elements are already in position and the protective environment is established, preventing evaporation and oxidation during the critical diffusion phase.
4Productivity
If heavy rare-earth alloy is coated on pressed body for diffusion, then grain boundary diffusion is enhanced, but pressed body oxidation occurs deteriorating magnet properties
Solution Approach 1:
The patent applies inert atmosphere by conducting the sintering and diffusion processes in a vacuum environment (10^-3 to 10^-5 Pa) or under protective gas atmosphere. This inert environment prevents Dy evaporation and oxidation during high-temperature processing, allowing the patent to achieve effective grain boundary diffusion at elevated temperatures without significant material loss or contamination.
Solution Approach 2:
The patent merges the coating and sintering/diffusion processes into a single integrated operation. By coating the pressed body with heavy rare-earth materials and then immediately sintering in a vacuum or protective atmosphere, the patent combines the coating step with the diffusion step, eliminating the need for separate handling that would expose the coated surface to oxidation.
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
The method results in a magnet with reduced Dy content, lower production costs, and improved magnetic force, achieving coercivity of 30 kOe or greater while reducing the Dy content by 40% and enhancing magnetic force by 5-8%, while maintaining comparable remanence magnetic flux density.
Implementation Method 1
a sintering step of sintering the NdFeB stripcasted alloy powder and the Tb powder together
Implementation Method 2
the diffusion efficiency of Tb is reduced since diffusion in the grain is generated due to substantially high temperature rather than diffusion to the grain boundary
Implementation Method 3
coating of heavy rare-earth on the pressed body may cause oxidation of the pressed body
Implementation Method 4
Dy evaporates at about 1000° C. and around 10−1 Pa
Data Source
AI summary
A NdFeB permanent magnet is provided and includes Nd of about 25 to 30 wt %, Dy of about 0.5 to 6 wt %, Tb of about 0.2 to 2 wt %, Cu of about 0.1 to 0.5 wt %, B of about 0.8 to 2 wt %, a balance of Fe and other inevitable impurities. In addition, a method for producing the permanent magnet is provided.