R-T-B Sintered Magnet Grain Refinement via Hydrogen Decrepitation
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Solution Overview
Problem
Conventional methods for reducing crystal grain size in R-T-B based sintered magnets to increase coercivity often result in insufficient rare-earth element content, leading to decreased remanence and abnormal grain growth, making it difficult to produce high-performance magnets while minimizing the use of rare and expensive natural resources.
Innovation Solution
An R-T-B based sintered magnet with a composition including 27.3-29.5% rare-earth elements, 0.92-1% boron, 0.05-0.3% Cu, and up to 0.5% additional elements, with a crystal grain size of 8 µm or less, produced using a strip cast alloy, hydrogen decrepitation, and dry jet pulverization, followed by press compaction and sintering at 850-1000°C for 4-48 hours.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Force
If the size of pulverized powder particles is reduced to decrease crystal grain size, then coercivity is improved, but the surface area increases causing increased oxygen adsorption and rare-earth element consumption, leading to insufficient R mole fraction and decreased remanence
Solution Approach 1:
The patent changes the oxygen content parameter to a specific range (0.02-0.2 mass%) rather than simply minimizing it, and adjusts the R mole fraction to 27.3-29.5% to optimize the balance between coercivity and remanence. This parameter optimization resolves the contradiction by finding the optimal operating point rather than extreme values
Solution Approach 2:
The patent creates a composite microstructure consisting of R2T14B main phase crystals embedded in an R-rich amorphous phase matrix. This composite structure allows the fine crystal grains (8 µm or less) to maintain high coercivity while the amorphous phase prevents excessive oxygen adsorption and rare-earth element consumption, preserving remanence
2Force
If the overall surface area of powder particles increases due to reduced particle size, then coercivity is improved, but interfacial energy increases causing abnormal grain growth during sintering, making it difficult to obtain uniform fine texture
Solution Approach 1:
The patent controls the sintering temperature parameter within a specific range (850-1000°C) and extends the sintering time (4-48 hours) to allow gradual densification. This controlled thermal parameter regime enables the fine powder particles to sinter uniformly without excessive grain growth, maintaining both coercivity and texture uniformity
Solution Approach 2:
The R-rich amorphous phase acts as an intermediary matrix that surrounds and constrains the R2T14B crystal grains during sintering. This amorphous phase prevents abnormal grain growth by providing a stable matrix structure, allowing the fine crystal grains to maintain their size distribution and achieve uniform texture
3Force
If conventional pulverization methods are used to reduce particle size, then crystal grain size is decreased, but the pulverization process requires long time or repeated operations, increasing impurity content and requiring higher R mole fraction
Solution Approach 1:
The patent performs preliminary hydrogen decrepitation on the alloy before pulverization. This preliminary action creates internal stress and micro-cracks in the alloy structure, making subsequent pulverization much more efficient. The alloy can be reduced to fine particles (D50 ≤ 3 µm) in a single short pulverization step rather than requiring repeated long-duration operations
Solution Approach 2:
The patent utilizes the phase transition of hydrogen absorption and desorption during the decrepitation process. Hydrogen is introduced to the alloy, causing volumetric expansion and internal stress that facilitates cracking. Subsequent heating removes the hydrogen, leaving a brittle structure that pulverizes easily. This phase transition-based预处理 dramatically reduces pulverization time and energy requirements
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 achieves increased coercivity while maintaining high remanence, ensuring the magnet's thermal resistance and preventing demagnetization with heat, thus enabling the production of high-performance magnets with reduced rare-earth element usage.
Implementation Method 1
exposing the material alloy to a hydrogen atmosphere, thereby getting the material alloy decrepitated
Implementation Method 2
obtaining a fine powder by finely pulverizing the coarse powder so that the fine powder has a particle size represented by a D50 of 3 μm or less as measured by dry jet dispersion laser diffraction analysis
Implementation Method 3
obtaining a compact by performing a press compaction process on the fine powder
Implementation Method 4
sintering the compact by keeping the compact heated to a temperature of 850 °C to 1,000 °C for 4 to 48 hours
Data Source
Figure 1~2
Figure 3~4
Figure 5
AI summary
An R-T-B based sintered magnet according to the present invention has a composition including: 27.3 mass% to 29.5 mass% of R; 0.92 mass% to 1 mass% of B; 0.05 mass% to 0.3 mass% of Cu; 0.02 mass% to 0.5 mass% of M; and T as the balance, and has an oxygen content of 0.02 mass% to 0.2 mass%. The main phase of the sintered magnet is an R2T14B type compound. The crystal grain size of the main phase is represented by an equivalent circle diameter of 8 µm or less. And crystal grains with equivalent circle diameters of 4 µm or less account for at least 80% of the overall area of the main phase.