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The long-term Stability of NdFeB Permanent Magnets

The stability of the magnetic properties is a crucial index for permanent magnetic materials. This stability primarily refers to the changes in magnetic performance due to internal and external factors after the magnet is magnetized. It is commonly represented by the rate of change in performance indicators. The usual causes for changes in magnetic performance include temperature, time, electromagnetic fields, radiation, mechanical vibrations, and impacts. This issue will explore the time stability of permanent magnetic materials.

When a magnet operates is stored for extended periods, environmental factors such as temperature, humidity, and corrosive liquids can alter its physical and chemical properties. After magnetization, most regions of a permanent magnet are aligned in a specific direction, but some small magnetic domains remain disordered (known as demagnetization cores). These cores can grow and new ones can form under various environmental conditions, leading to a decline in the magnet’s performance. This change, generally gradual and irreversible, directly affects key performance parameters like residual magnetism, intrinsic coercivity, coercivity, or maximum energy product, potentially leading to complete magnet failure. This type of magnetic performance loss is irreversible; remagnetization cannot restore the magnet to its pre-storage condition. In recent years, as neodymium-iron-boron (NdFeB) permanent magnets have been widely used in sectors requiring long lifespans, such as aerospace, electric vehicles, and high-power wind generation, designers are increasingly focusing on the time stability of NdFeB magnets.

1.Room-Temperature Long-Term Stability

In 2013, Finnish scholars released a study showing that sintered NdFeB magnets (Hcj=15.6kOe), placed in a room-temperature environment for one year (10,000 hours), exhibited no detectable magnetization loss across samples with different Pc values (-0.33, -1.1, -3.3). The Sanyo Research Institute conducted similar measurements over more than 12 years (4441 days), using sintered NdFeB magnets (Hcj=18kOe) shaped as uncoated 10.2mm cubic samples exposed to laboratory air at temperatures ranging from 22°C to 28°C. Observations and measurements were made annually.

From the data, it is apparent that the relative magnetic flux loss was minimal in the first six years, with a turning point around day 2208 (approximately six years). Visually, rust spots were visible on the surface of the black magnet after six years, indicating that oxidation and corrosion had begun internally and on the surface, with the rate of performance degradation accelerating over time. Additionally, the experiment projected magnetic flux losses out to 30-50 years, predicting less than 1% loss at 30 years and approximately 1.3% at 50 years, with a corresponding time of about 150 years for a 2% loss.

NdFeB Permanent Magnets

This result suggests that if the lifespan of a magnet is defined by a 5% loss rate in magnetic flux, even uncoated sintered NdFeB magnets still have a remarkably long life, conservatively estimated between 30-50 years.
Typically, significant magnetic flux losses stem from surface oxidation or corrosion, which are irreversible. However, through composition optimization and surface protection treatments, the oxidation and corrosion resistance of sintered NdFeB magnets has significantly improved.
Therefore, with adequate surface protection, sintered NdFeB magnets with a high Hcj can exceed a lifespan of 30-50 years under temperature limits.

2.High-Temperature Long-Term Stability

The following graph shows the relative magnetic flux loss over time for magnets with different Pc values and Hcj=20.1 kOe at temperatures of 80°C, 120°C, and 150°C.

It is clear from the graph that, for the same Pc value, the higher the storage temperature, the faster the relative magnetic flux loss declines. Magnets with lower absolute Pc values exhibit significantly higher initial and long-term magnetization losses, which increase sharply with rising temperatures. In scenarios where Hcj cannot be further enhanced due to technical and cost constraints, increasing the absolute value of Pc effectively mitigates magnetization loss.

The relationship between Hcj, different Pc magnets, and their relative magnetization loss over time at various temperatures demonstrates the significant impact of Hcj on high-temperature magnetization loss. Magnets requiring high-temperature stability must possess a high Hcj, while the magnetic permeability coefficient Pc also plays a critical role in determining high-temperature long-term magnetization loss.

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