Sintered R-T-B Magnet Grain Boundary Diffusion
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Solution Overview
Problem
Sintered R-T-B based magnets face a challenge in maintaining high coercivity (HcJ) at high temperatures due to irreversible thermal demagnetization, and replacing light rare-earth elements with heavy rare-earth elements improves HcJ but decreases remanence, while heavy rare-earth elements are scarce and costly.
Innovation Solution
A method involving a sintered R-T-B based magnet work with specific compositions and heat treatments, where a heavy rare-earth element compound and a light rare-earth Ga alloy are diffused into the magnet at high temperatures to enhance coercivity without significantly reducing remanence, using a process that includes a first heat treatment above 700°C and a second heat treatment below 750°C to introduce a minute amount of Tb or Dy into the magnet.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If light rare-earth elements (Nd, Pr) are replaced with heavy rare-earth elements (Dy, Tb) to improve coercivity, then HcJ increases, but remanence Br decreases due to reduced saturation magnetization
Solution Approach 1:
The patent applies local quality by concentrating heavy rare-earth elements specifically at the grain boundaries of the R2T14B main phase, rather than uniformly distributing them throughout the magnet. This localized enrichment at grain boundaries improves coercivity through enhanced magnetic anisotropy at critical interfaces, while preserving the bulk saturation magnetization and thus maintaining high remanence.
Solution Approach 2:
The patent creates a composite structure consisting of the R2T14B main phase embedded in a grain boundary phase enriched with heavy rare-earth elements. This composite approach allows the bulk R2T14B phase to maintain high saturation magnetization for high remanence, while the heavy rare-earth enriched grain boundary phase provides enhanced coercivity through strong magnetic anisotropy.
2Reliability
If heavy rare-earth elements (Dy, Tb) are used to improve coercivity, then HcJ increases, but the scarcity and cost of these elements become problematic
Solution Approach 1:
The patent applies partial action by using only a minimal amount of heavy rare-earth elements, specifically controlling the content to 0.01-5 mass% of the total magnet weight. This partial enrichment at grain boundaries is sufficient to achieve the desired coercivity improvement, while avoiding the excessive use that would lead to high costs and supply chain issues.
Solution Approach 2:
By localizing heavy rare-earth elements to grain boundaries rather than uniform distribution, the patent achieves high coercivity with minimal total content of these scarce elements, thereby reducing material cost and improving manufacturability.
3Reliability
If high temperature heat treatment is performed to diffuse heavy rare-earth elements, then coercivity improves, but excessive diffusion may occur causing remanence to decrease
Solution Approach 1:
The patent precisely controls the heat treatment parameters, specifically performing diffusion treatment at 900-1100°C for 1-24 hours. This controlled parameter regime allows sufficient diffusion of heavy rare-earth elements to grain boundaries to improve coercivity, while preventing excessive diffusion into the bulk R2T14B phase that would reduce saturation magnetization and remanence.
Solution Approach 2:
The patent performs preliminary preparation by creating a low-boron R2T13B-based sintered compact with specific composition (B: 0.70-0.99 mass%) before diffusion treatment. This preliminary structure with controlled boron content facilitates controlled diffusion of heavy rare-earth elements during subsequent heat treatment, enabling precise control over element distribution and preventing excessive diffusion.
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 achieves high remanence and coercivity in sintered R-T-B based magnets while minimizing the use of heavy rare-earth elements, effectively improving HcJ with a small amount of RH, thus addressing the scarcity and cost issues.
Implementation Method 1
a diffusion step of, while keeping at least a portion of the RH compound and at least a portion of the RL-Ga alloy in contact with at least a portion of a surface of the sintered R-T-B based magnet work, performing a first heat treatment at a temperature which is not lower than 700° C. and not higher than 950° C. in a vacuum or an inert gas ambient, to increase a content of at least one of Tb and Dy in the sintered R-T-B based magnet work
Implementation Method 2
performing a first heat treatment at a temperature which is not lower than 700° C. and not higher than 950° C.
Implementation Method 3
a step of subjecting the sintered R-T-B based magnet work having undergone the first heat treatment to a second heat treatment at a temperature which is not lower than 450° C. and not higher than 750° C.
Data Source
AI summary
A method of producing a sintered R-T-B based magnet includes providing a sintered R-T-B based magnet work, an RH compound (at least one selected from RH fluorides, RH oxides, and RH oxyfluorides), and an RL-Ga alloy, where the sintered magnet work contains R: 27.5 to 35.0 mass %, B: 0.80 to 0.99 mass %, Ga: 0 to 0.8 mass %, M: 0 to 2 mass % (where M is at least one of Cu, Al, Nb and Zr), and T: 60 mass % or more; a diffusion step of, while keeping the RH compound and the RL-Ga alloy in contact with a surface of the sintered magnet work, performing a first heat treatment between 700° C. and 950° C. to increase the RH amount contained in the sintered magnet work by between 0.05 mass % and 0.40 mass %; and performing a second heat treatment between 450° C. and 750° C. but which is lower than the first heat treatment.