While using NdFeB (i.e. neodymium, iron and boron) sintered magnets in motors' production, the most important requirement of magnet is that the thermal demagnetization should never happen.
The so-called thermal demagnetization is a kind of irreversible demagnetization; only by re-magnetizing can the initial flux be insured.
When a motor is working, the environment temperature and dysprosium (Dy) current will cause the emission of heat, so magnets of high flux density and good heat-resisting property are required. Nowadays, the application of liquid phase sintering and other new technologies help to improve densification, organization and ultra micronization of the magnets. If using heavy rare earth element of Dy to replace part of the Nd in the magnet and then improving anisotropy field in all directions, the magnet can be made to have high coercive force. However, the reserves of Dy element are very few, a little less than one percent of the reserves of neodymium (Nd), while the resource of Nd is rich enough for over 1,000 years of use before exhausted. Therefore it is necessary to develop new technology to use Dy more effectively and as little as possible.
For example, the micronization of the grain and the consistency of the grain boundary phase to decrease the Dy's volume of addition, aiming to give magnet high performance and high heat resistance. It has been more than 20 years of development since the NdFeB magnet appeared. High performance magnet, whose BHmax is over 400 kJ / m3, is an absolutely necessary advanced material to support information society and to promote the energy-saving technology. Especially the rapid developing of using in the motor industry, as predicted, the market demand will be more than 3,500 tons per year.
It is now extensively used in driving motor and dynamo and the power steering gear, air conditioner, magnetic valve and all other kinds of brake devices. But for usage in the driving motor, it has to overcome the problem of thermal demagnetization and so on. Improving the coercive force with new technologies is a very an important project in the future.
During the development process of high performance NdFeB magnet, the use of rare earth metals, especially praseodymium (Pr) and Dy, deserves special attention. The content of Dy should be controlled between three to four percent to ensure the quality. The practical design and recycling of magnet are the very important steps in the future development.
The addition of chromium (Cr) into built up by nanometer built-up magnet of Fe-B/Nd2Fe14 can improve the coercive force. Nanometer built-up magnet of Fe-B/Nd2Fe14 is a micro metal organization which is combined by both soft and hard magnetic phases. It is a kind of isotropic magnet with high residual magnetic flux density but week intrinsic coercive force. Its heat resistance is not enough for use in the motor related fields where high heat-resisting property is required.
Generally speaking, in the magnet of Fe-B/Nd2Fe14, the stronger its intrinsic coercive force is, the smaller the irreversible thermal demagnetization rate will be.
By Danny Burns from Stanford Magnets, a top supplier of rare earth magnets based in CA, click here for more info