Microstructure, magnetic and electrical properties of the composite magnets of Nd–Fe–B powders coated with silica layer

A novel type of composite magnet was fabricated by hot pressing and hot deformation of Nd–Fe–B powders coated with a continuous silica layer. Nd–Fe–B powders were coated with a silica insulating layer of various thicknesses by chemical synthesis. The influence of the thickness of the insulating layer on the microstructure, magnetic and electrical properties was investigated. The coated silica layer was characterized by scanning electron microscopy and energy dispersive X-ray spectroscopy. The synthesized layer with a thickness of 18–101 nm uniformly coated the Nd–Fe–B powders. There is no significant decrease for the maximum energy product (BH)max when the average thicknesses of the insulating layers were 24 and 45 nm and the electrical resistivity increased from 230 μΩ cm without silica coating to 260 and 280 μΩ cm, respectively. The (BH)max of the composite magnet decreased to 30.7 MGOe when the average thickness of the insulating layer was approximately 64 nm, while the electrical resistivity increased to 280 μΩ cm. Thus, the thickness of SiO2 layer is an important factor for the magnetic and electrical properties of the composite magnets. The SiO2 layers deteriorated the magnetic properties in different levels. The reason may be that atom diffusion happened during the hot-deformation process.

► A composite magnet of Nd–Fe–B powders coated with silica layer was fabricated.
► The SiO2 coatings of 24 and 45 nm thick had little effect on the magnetic properties.
► The highest electrical resistivity of the composite magnet was 280 μΩ cm.