The magnet works for a long time or is placed for a long time, and 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 the magnetization direction of some small magnetic domains is chaotic (called reverse magnetization nucleus). Under the action of various environmental factors, the original reverse magnetization nucleus Will grow up, new anti-magnetization nuclei will be produced, which makes the magnetic properties of permanent magnets decay. This kind of 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 coercive force, coercive force or maximum energy product, and even leads to the complete failure of the magnet. This loss of magnetic properties is irrecoverable, even if the magnet is re-magnetized, it cannot be restored to the level before it was placed for a long time. In recent years, with the wide application of NdFeB permanent magnet materials in aerospace, electric vehicles, high-power wind power generation and other fields that require a long service life, application designers are more concerned about the time stability of NdFeB permanent magnets. more and more attention.
1 Long-term stability at room temperature
The results of a study published by Finnish scholars in 2013 showed that samples with different Pc values (Pc=-0.33, -1.1, -3.3) without any appreciable loss of magnetization. The Sanhuan Research Institute also carried out similar measurement research, which lasted for more than 12 years (4441 days). The internal coercive force of the sintered NdFeB magnets used in the experiment was HcJ=18kOe, and the samples were uncoated with a side length of 10.2mm. Cube, magnetic permeability coefficient Pc=-2 (click magnetic moment, magnetic flux and remanence to understand what is Pc value), the number of samples is 8 pieces, directly exposed to the atmospheric environment where the laboratory is located, and the temperature is 22℃~28 °C, observations and measurements are made every year for 12 years.
From the above data, it can be found that the relative magnetic flux loss measured in the first 6 years is basically small, and an inflection point appears around 2208 days (about 6 years). From the appearance, rust spots can be seen on the surface of the black magnet after being placed for 6 years, which means that the surface and interior of the magnet have begun to oxidize and corrode. accelerate. In addition, the experiment also extrapolated the magnetic flux loss from the currently measured 4441 days (12 years and 2 months) to 30-50 years. The estimated magnetic flux loss in 30 years is less than 1%, and the magnetic flux loss in 50 years is about 1.3 %, 2% corresponds to about 150 years. (Hollow dots in the picture above)
This result shows that if the service life of the magnet is defined as the time corresponding to the magnetic flux loss rate equal to 5%, even if the magnet is on the surface without corrosion-resistant coating, the sintered NdFeB magnets currently measured still have a very long life. Life expectancy is conservatively estimated to be 30-50 years.
Usually, the large magnetic flux loss comes from the oxidation or corrosion of the magnet surface, which is an irrecoverable loss. Among all kinds of rare earth permanent magnet materials, the loss of sintered NdFeB is the most serious, but after composition optimization and Surface protection treatment, oxidation resistance and corrosion resistance of sintered NdFeB magnets have been greatly improved. Therefore, if the surface of the magnet is well protected, the service life of sintered NdFeB with sufficiently high HcJ can exceed 30-50 years. (This is under the condition of not exceeding the use temperature).
2 Long-term stabilities at high temperature
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°C, 120°C and 150°C.
It is not difficult to find from the above figure that under the same Pc value, the higher the magnet storage temperature is, the faster the relative flux loss will decrease. The initial magnetization loss and long-term magnetization loss of a magnet with a lower absolute value of Pc are significantly greater than those of a magnet with a higher Pc, and both types of losses increase significantly when the temperature rises. If the HcJ cannot be further improved due to technical and cost reasons, the The increase in the absolute value of Pc can effectively suppress the loss of magnetization.
From the time relationship of relative magnetization loss of different HcJ and different Pc magnets 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.