The grain boundary phase is a key component of the microstructure of permanent magnets and has an important influence on the macroscopic magnetic properties of the magnet.
Definition of the Grain Boundary Phase
The grain boundary phase refers to the microscopic phases present between and at the junctions of the main-phase grains in magnetic materials (e.g., Nd-Fe-B sintered magnets). The chemical composition, crystal structure, thickness, and distribution of the grain boundary phase affect the macroscopic magnetic properties of the magnet.
Types of Grain Boundary Phases
| Nd-rich phase: The most common type, including Nd-rich metallic phases and Nd-rich oxides. Some Nd-rich phases (e.g., the tetragonal Nd-rich phase) exhibit excellent wettability, enabling the formation of extremely thin non-ferromagnetic layers at the grain boundaries, which serve to magnetically isolate adjacent grains.
| Fe-rich phase: Such as the needle-like Fe-rich phase, which is usually hexagonal in structure and mostly distributed at triple junctions of grain boundaries. Although ferromagnetic, it has little effect on the coercivity of the magnet because it is surrounded by non-magnetic phases and its proportion is low.
| Other phases: Such as the NdFe13Ga phase, the NdFeB4 phase, and new phases formed by elemental diffusion (e.g., the high-resistivity phase Ia3-RE2O3).
Distribution of the Grain Boundary Phase
The grain boundary phase is mainly distributed at the boundaries, triple junctions, and surfaces of Nd2Fe14B main-phase grains. Among these, the Nd-rich phase distributed along the grain boundaries is mostly in the form of thin layers, whereas the Nd-rich phase at triple junctions is mostly block-like in morphology.
The thickness of the grain boundary phase is extremely thin, typically at the nanometer scale, and in some regions (e.g., grain boundaries parallel or perpendicular to the c-plane of the main phase) it exhibits pronounced anisotropy.
Core Functions
| Temperature stability: Ferromagnetic grain boundary phases can reduce the local stray field of the magnet and optimize its demagnetization factor, allowing magnets with relatively low room-temperature coercivity but better temperature stability to maintain good magnetic performance at high temperatures.
| Electrical property regulation: Some grain boundary phases can form electrical insulation at the grain boundaries, increasing the internal resistance of the magnet, reducing eddy-current losses, and improving the overall electromagnetic performance of the magnet at high temperatures or high frequencies.
| Magnetic property regulation: Non-ferromagnetic or high-resistivity grain boundary phases can effectively magnetically isolate adjacent main-phase grains and prevent the propagation of reverse magnetic domains, thereby significantly enhancing the coercivity of the magnet. Meanwhile, the wettability, continuity, and uniformity of the distribution of the grain boundary phase directly affect the magnetic exchange coupling within the magnet.
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Image Source: Magnetapplications & Nature & Smmagnetics & Springer & Tandfonline
