Effect of ${\hbox {CaF}}_{2}$ Addition on the Microstructure and Magnetic and Electrical Properties of Sintered Nd-Fe-B Magnets

Nd-Fe-B composite magnets with different amounts of CaF₂ have been prepared by sintering at different temperatures. The microstructure and densities of the Nd-Fe-B composite magnets were investigated by scanning electron microscope and Archimedes method. The electrical and magnetic properties were measured with a four probe electrical resistivity meter and a NIM-2000HF hysteresisgraph meter. The results show that CaF₂ was distributed in the grain boundary. The electrical resistances of the composite magnets increased with the increase of the amounts of CaF₂, while the magnetic properties of the magnets deteriorated markedly. The electrical resistances of the Nd-Fe-B composite magnets with 3, 6 and 9 wt.% as-bought CaF₂ sintered at 1050°C were 270, 320 and 400 μΩ cm, which increased approximately 17%, 39% and 74% over that of the magnet without CaF₂ ( 230 μΩ cm). The maximum energy products, (BH)_max, of the composite magnets with 3, 6 and 9 wt.% as-bought CaF₂ sintered at 1050°C were 41.03, 35.77 and 28.66 MGOe, respectively. The densities of the composite magnets decreased with the increase of the amounts of CaF₂. High energy ball milling can refine CaF₂ to nanoscale and thus improve its distribution in the composite magnets. The (BH)max of the Nd-Fe-B magnets with 3 wt% CaF₂ first increased and then decreased with the increase of milling time. But the electrical resistivities of the sintered Nd-Fe-B magnets with the CaF₂ without milling and after high energy ball milling have little change.