The stability of permanent magnetic material performance is an important indicator of permanent magnetic materials. Stability mainly refers to the process of magnetic properties changing under the influence of internal and external factors after magnetization, and is usually expressed by the rate of change of performance indicators. Common causes of magnetic property changes include temperature, time, electromagnetic field, radiation, mechanical vibration and impact, etc. Let’s learn about the time stability of permanent magnetic materials.
When the magnet works for a long time or is placed for a long time, the surrounding environment (such as temperature, humidity, corrosive liquid, etc.) may cause the physical and chemical properties of the magnet to change. After the permanent magnet is magnetized, most of the area is magnetized to a specific direction, but there are still some small magnetic domains whose magnetization direction is chaotic (called reverse magnetization core). Under the influence of various environmental factors, the original reverse magnetization core will grow and new reverse magnetization core will be generated, which will cause the magnetic properties of the permanent magnet to decay. This change is generally a slow and irreversible change from the outside to the inside, which directly affects the main performance parameters of the magnet, such as remanence, intrinsic coercivity, coercivity or maximum magnetic energy product, and even causes the magnet to fail completely. This loss of magnetic properties is irreversible. Even if the magnet is re-magnetized, it cannot be restored to the level before long-term placement. In recent years, with the widespread application of NdFeB permanent magnet materials in aerospace, electric vehicles, high-power wind power generation and other fields with long service life requirements, application designers have paid more and more attention to the time stability of NdFeB permanent magnets.
Long-term stability at room temperature.
A study published by Finnish scholars in 2013 showed that sintered NdFeB magnets (HcJ=15.6kOe) placed at room temperature for 1 year (10,000 hours) showed no perceptible magnetization loss for samples with different Pc values (Pc=-0.33, -1.1, -3.3). Sanhuan Research Institute also conducted a similar measurement study, which lasted for more than 12 years (4441 days). The intrinsic coercive force of the sintered NdFeB magnets used in the experiment was HcJ=18kOe. The samples were uncoated cubes with a side length of 10.2mm, and the magnetic permeability coefficient was Pc=-2. There were 8 samples, which were directly exposed to the atmospheric environment of the laboratory at a temperature between 22℃ and 28℃. Observations and measurements were conducted once a year for 12 years.
From the above data, it can be found that the relative flux loss measured in the first 6 years is basically not large, and an inflection point appears near 2208 days (about 6 years). From the appearance, rust spots can be seen on the surface of the black sheet magnet after 6 years of placement, which means that the surface and interior of the magnet have begun to oxidize and corrode. As time goes by, the scope of oxidation or corrosion will continue to expand, and the rate of performance degradation will also accelerate significantly. In addition, the experiment also extrapolates the flux loss from the currently measured 4441 days (12 years and 2 months) to 30-50 years. The flux loss is expected to be less than 1% in 30 years, about 1.3% in 50 years, and 2% corresponds to about 150 years. (Hollow dots in the figure above)
This result shows that if the service life of the magnet is defined as the time corresponding to the flux loss rate equal to 5%, even if the magnet is on the surface without corrosion-resistant coating, the sintered NdFeB magnet currently measured still has a very long life, conservatively estimated to be 30-50 years.
Usually, the larger magnetic flux loss comes from the oxidation or corrosion of the magnet surface, which is an irreversible loss. Among all kinds of rare earth permanent magnet materials, sintered NdFeB has the most serious loss. However, after composition optimization and surface protection treatment, the oxidation resistance and corrosion resistance of sintered NdFeB magnets have been greatly improved. Therefore, if the magnet surface is well protected, for sintered NdFeB with a sufficiently high HcJ, the service life can exceed 30-50 years. (This is under the condition of not exceeding the use temperature~)
Long-term stability at high temperatures.
The figure below shows the change of relative flux loss over time for magnets with different Pc values and HcJ=20.1 kOe at 80℃, 120℃ and 150℃.
It is not difficult to find from the above figure that under the same Pc value, the higher the storage temperature of the magnet, the faster the relative magnetic flux loss decreases. The initial magnetization loss and long-term magnetization loss of the magnet with a lower absolute value of Pc are significantly greater than those of the magnet with a higher Pc, and both types of losses increase significantly with increasing temperature. When HcJ cannot be further increased due to technical and cost reasons, increasing the absolute value of Pc can effectively suppress the magnetization loss.
From the time relationship of relative magnetization loss of magnets with different HcJ and Pc at different temperatures, it can be seen that HcJ has an important influence on high-temperature magnetization loss. The higher the HcJ, the lower the magnetization loss. High-temperature stability requires that the magnet must have a higher HcJ. At the same time, the permeability coefficient Pc can also determine the high-temperature and long-term magnetization loss of the magnet.