rare earth permanent magnet

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, remanence, coercive force 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 turbines and other fields with long service life requirements, application designers have paid more and more attention to the time stability of NdFeB permanent magnets.   1. Long-term stability at room temperature   Generally, 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)     2. Long-term stability at high temperature   The following figure 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 magnets with lower Pc absolute values ​​are significantly greater than those of magnets with higher Pc, and both types of losses increase significantly as the temperature rises. When HcJ cannot be further increased due to technical and cost reasons, increasing the absolute value of Pc can effectively suppress 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.