Does the magnetic property of Ndfeb magnets gradually weaken over time? What are the reasons for the performance decline after long-term use?

1. Environmental Factors

1.1 Temperature Effects

• Thermal Demagnetization: NdFeB magnets have a limited operating temperature range. Exposure to temperatures exceeding their maximum service temperature (typically 100–200°C, depending on the grade) can cause irreversible magnetic decay. This occurs because elevated temperatures disrupt the alignment of magnetic domains, reducing net magnetization.
• Example: In electric vehicle motors, prolonged operation near the magnet's temperature limit can lead to a gradual decline in magnetic flux density, affecting motor efficiency.

1.2 Humidity and Corrosion

• Oxidation: NdFeB magnets are highly susceptible to oxidation in humid environments. The neodymium-rich grain boundaries react with moisture and oxygen, forming neodymium oxides and hydroxides. These corrosion products are non-magnetic and flake off, exposing fresh metal to further attack.
• Electrochemical Corrosion: In acidic or saline environments, the magnet's surface undergoes electrochemical reactions, accelerating corrosion. This is particularly problematic in marine or industrial settings where chemicals are present.
• Impact on Performance: Corrosion not only reduces the magnet's physical integrity but also disrupts the magnetic circuit, leading to a loss of magnetic flux. Studies show that uncoated NdFeB magnets can fail within hours in salt fog tests, while coated magnets can endure 500–1,000 hours or more.

2. Material Degradation

2.1 Microstructural Changes

• Grain Growth: Over time, the grain boundaries in NdFeB magnets may undergo thermal activation, leading to grain growth. Larger grains reduce the magnet's coercivity (resistance to demagnetization), making it more susceptible to external magnetic fields or temperature fluctuations.
• Phase Transformations: Prolonged exposure to high temperatures can cause the formation of non-magnetic phases (e.g., α-Fe), which dilute the magnetic material and reduce overall performance.

2.2 Elemental Diffusion

• Neodymium Migration: In some cases, neodymium atoms may diffuse to the surface or grain boundaries, forming oxides or altering the local composition. This can degrade the magnet's magnetic properties over time.

3. Structural Changes

3.1 Magnetic Domain Dynamics

• Domain Wall Pinning: The movement of magnetic domain walls (the boundaries between regions of uniform magnetization) is influenced by defects, impurities, and stress in the material. Over time, these factors can cause domain walls to become "pinned," reducing the magnet's ability to maintain a stable magnetic state.
• Magnetic Aging: Even in the absence of external stressors, the magnet's microstructure may evolve slowly due to thermal fluctuations, leading to a gradual realignment of domains and a reduction in magnetic flux.

3.2 Mechanical Stress

• Thermal Cycling: Repeated heating and cooling cycles can induce mechanical stress in the magnet due to differential thermal expansion between the magnetic material and its coating or housing. This stress can cause microcracks or delamination, disrupting the magnetic circuit.
• Vibration and Shock: In applications involving high vibration or mechanical shock (e.g., wind turbines or aerospace systems), the magnet may suffer physical damage that compromises its magnetic properties.

4. Long-Term Stability Studies

4.1 Room-Temperature Aging

• Experimental Data: Research has shown that high-quality NdFeB magnets stored at room temperature in dry conditions exhibit minimal magnetic decay over decades. For example, a study by Finnish researchers found no detectable magnetic loss in a烧结 (sintered) NdFeB magnet stored for one year at room temperature.
• Limitations: However, uncoated magnets exposed to atmospheric moisture can show significant decay over time due to corrosion. Coated magnets, on the other hand, can maintain their performance for 30–50 years or more under proper storage conditions.

4.2 High-Temperature Aging

• Accelerated Decay: At elevated temperatures, the rate of magnetic decay increases dramatically. For instance, a magnet stored at 150°C may lose 10–20% of its magnetic flux within a few years, while a magnet stored at 80°C may show only a few percent loss over the same period.
• Critical Factors: The magnet's intrinsic coercivity (Hcj) and magnetic guiding coefficient (Pc) play key roles in determining its high-temperature stability. Higher Hcj values and lower (more negative) Pc values correlate with better long-term stability.

5. Mitigation Strategies

To enhance the long-term stability of NdFeB magnets, several strategies can be employed:

• Surface Coatings: Nickel plating, epoxy coatings, or composite treatments (e.g., Ni-Cu-Ni + epoxy) provide a barrier against moisture and chemicals, significantly improving corrosion resistance.
• Material Optimization: Adding alloying elements (e.g., dysprosium or terbium) can increase the magnet's coercivity and thermal stability, making it more resistant to demagnetization.
• Design Improvements: Optimizing the magnet's shape, size, and magnetic circuit can reduce stress concentrations and improve overall performance.
• Environmental Control: Storing magnets in dry, cool environments and avoiding exposure to corrosive substances can extend their service life.