Bonded NDF (Non-Derivative Magnet) is an important permanent magnet material, with a close correlation between its density and magnetic properties. Density not only reflects the density of the magnet's internal structure but also directly affects key magnetic properties such as remanence, coercivity, and maximum magnetic energy product.
At a microscopic level, bonded NDF is formed by mixing permanent magnet powder with a binder. The contact area and density of the magnetic powder particles change with changes in magnet density. When the magnet density is low, there are more gaps between the magnetic powder particles. These gaps act as "breakpoints" in the magnetic circuit, hindering the neat alignment of magnetic domains and the smooth transmission of magnetic flux, resulting in reduced remanence. Remanence is the magnetic flux density retained after demagnetization. Lower remanence indicates a weaker ability to generate an external magnetic field. As the magnet density increases, the contact between the magnetic powder particles becomes closer, the magnetic domains become more ordered, and magnetic flux can pass through the magnet more efficiently, significantly improving remanence.
Coercivity, a measure of a magnet's resistance to demagnetization, is also affected by magnet density. At low density, the interactions between magnetic powder particles are weak, making domain walls more mobile. When subjected to external factors such as a reverse magnetic field or mechanical stress, domain walls are prone to irreversible displacement, leading to demagnetization and lower coercivity. As density increases, the bonds between magnetic powder particles strengthen, increasing the resistance to domain wall movement. Domain walls require greater energy to break free from the constraints between particles and move, enhancing the magnet's resistance to demagnetization and increasing coercivity.
The maximum energy product is a comprehensive parameter that reflects a magnet's ability to store and convert magnetic energy. It is closely related to remanence and coercivity. Since density significantly affects both remanence and coercivity, it also indirectly affects the maximum energy product. High-density bonded NDFEBs, due to their higher remanence and coercivity, are able to store and convert more magnetic energy and thus have a higher maximum energy product. This means that within the same volume, high-density magnets can provide stronger magnetic fields and higher energy output, meeting the needs of applications with high magnetic performance requirements.
In actual production, methods for increasing the density of bonded NDFs primarily include optimizing the molding process and selecting an appropriate binder. Appropriately increasing the molding pressure can densely align the magnetic powder particles during the molding process, reducing voids and thereby increasing the magnet density. However, it is important to note that excessive pressure can lead to internal stress concentration within the magnet and even cause the magnetic powder particles to break, which in turn reduces magnetic performance. Therefore, precise control of the molding pressure and holding time is necessary to find the optimal density improvement point. Furthermore, selecting a binder with good bonding properties and high compatibility with the magnetic powder can ensure magnet formability while minimizing the binder's isolation from the magnetic powder, thereby increasing magnet density.
The density of bonded NDFs has a multifaceted and crucial impact on their magnetic properties. By properly controlling the magnet density, key magnetic performance indicators such as remanence, coercivity, and maximum energy product can be effectively improved, thereby meeting the demand for high-performance permanent magnet materials in various applications.
