Silicon-Alkane Covering Technology And Other Surface Treatment Technologies Of HDDR NdFeB Magnetic Powders

Hydrogenation, Disproportionation, Desorportion and Recombination (HDDR) is a more recent technique of producing the NdFeB magnetic powders, which involved a sequence of heat treatments in hydrogen atmosphere and then under vacuum condition. HDDR has become an effective means to produce coercive anisotropic bonded magnets. The NdFeB bonded magnets are compound magnets composed by the NdFeB magnetic powders and the macromolecule compound. Compared with the sintered magnets, the magnetic properties of the bonded magnets are lower but they are stable. The machining properties of the bonded magnets are good. So the bonded magnets have been used very widely. In the process of producing and application of the polymer bonded magnets, the magnetic powders are extremely easy to be oxidized and corroded. It deteriorates the magnetic properties of the bonded magnets.This thesis aims at solving the oxidation problem of the magnetic powders in the process of producing the bonded magnets. Surface treatment of the magnetic powders is usually used and it can form the dense oxidation-resistance surface.The NdFeB powders of producing by HDDR process are covered by using silicon-alkane covering technology, dichromate passivation technology, dichromate passivation reducing technology and dichromate passivation reducing plus silicon-alkane covering technology. The oxidation-resistance of the magnetic powders before and after covering is measured here by using flourier transfer infrared spectrometry (FT-IS), x-ray diffraction (XRD), differential thermal analysis (DTA), oxidation and weight increasing method.It is shown that the oxidation-resistance of the magnetic powders is improved at different degrees by different surface treatment. It is found that the silane coupling agent forms a net-like film on the powders surface by chemical bonding. Powders coating with the film possess excellent oxidation-resistant capacity at high tempareture. It is shown that to improve the oxidation-resistance of the powders the best technology of them is dichromate...