How to improve the high temperature demagnetization characteristics of sintered Nd-Fe-B magnets?

The demagnetization of permanent magnet at high temperature is related to the comprehensive properties of the magnet (including composition, magnetic properties, microstructure, surface state, coating characteristics, etc.), as well as the working state of the magnet in the magnetic circuit.
In order to improve the magnetic properties of sintered Nd-Fe-B magnets at high temperature, it is necessary to start from one point of view
1. First of all, the magnet should be designed in good working condition.

• (1) When the working load point of the magnet is designed above (BH) m, the safety of irreversible demagnetization can be increased by sacrificing a little magnetic energy product;
• (2) The magnet with larger length diameter ratio should be used as far as possible, and the magnet with easy magnetization direction less than 1 mm should be avoided. Because the coercivity of sintered Nd-Fe-B magnet is determined by the nucleation mechanism, the coercivity and the squareness of demagnetization curve are affected by the surface state, specific surface area (surface area / volume) and aspect ratio of the magnet. When the size of the magnet is less than 1 mm, the coercivity and demagnetization curve squareness of the magnet will be significantly lower than that of the bulk magnet. The smaller the surface roughness, the smaller the specific surface area and the larger the ratio of length to diameter, the more stable the magnetic properties of the magnet at high temperature;
• (3) Good magnetic circuit ventilation and heat dissipation design are adopted.

2. Suitable magnets should be used.
(1) First of all, it is necessary to determine whether the device is required to work at a certain temperature, and the emphasis is on whether the reversible loss of magnetic flux is small or the irreversible loss of magnetic flux is small. If the reversible loss of the magnet at high temperature is required to be small, the magnet with small remanence temperature coefficient (such as the magnet containing CO) should be selected; if the irreversible loss of the magnet at high temperature is required to be small, the magnet with high Hcj (such as 35h, 35sh, 35uh, 33eh, etc.) should be selected; if the reversible and irreversible loss of the magnet at high temperature is required If both of them are small, the magnets containing CO and high Hcj should be selected;
(2) If the cost is allowed, magnets with high Hcj should be used as far as possible;
It is worth noting that the demagnetization curve and temperature coefficient of sintered Nd-Fe-B magnets usually provided by magnet manufacturers to users are measured with standard size magnets (such as 10.0 * 10.0 mm). When the specific surface area of the magnet product used by the user is smaller than that of the standard sample and the length diameter ratio is larger than that of the standard sample, the demagnetization curve and temperature coefficient of the magnet provided by the manufacturer can truly reflect the high-temperature working characteristics of the product; if the specific surface area of the magnet product used by the user is larger than that of the standard sample and the length diameter ratio is smaller than that of the standard sample, the actual high-temperature working characteristics of the product tend to change It will be slightly lower than the data provided by the manufacturer. In other words, the actual high temperature working characteristics of sintered Nd-Fe-B magnet products are not only related to the magnetic properties of the magnet itself, but also related to the size and working state of the magnet.
3. Use appropriate surface protection layer.
The protective layer on the surface of the magnet also affects the actual high temperature working characteristics. The practice shows that all the electrochemical surface coating treatment can reduce the high-temperature working characteristics of Nd-Fe-B sintered magnet products, that is to say, the high-temperature working characteristics of black sheet magnet are better than those with coating under the same performance and size of magnet. Among the existing magnet products with coating, Ni Zn alloy coating has the best high temperature performance, Zn coating is the best, Ni coating is the worst. But in terms of corrosion resistance, Ni coating is the best, Ni Zn alloy coating is the best, and Zn coating is the worst. Therefore, when selecting magnet products, users should not only determine the magnetic properties and dimensional tolerance of the magnet, but also select the surface protection layer of the magnet according to the specific working environment of the magnet.