The effect of grain boundary diffusion with Pr₇₀Al₂₀Co₁₀ on the magnetic properties and microstructure of sintered （Nd_0.4Ce_0.6）₃₀M_0.5Fe_balB_0.92 magnets

【Objective】 Containing 60% Ce magnets were subjected to grain boundary diffusion（GBD）treatment using Pr₇₀Al₂₀Co₁₀ alloy as the diffusion source. The purpose is providing both theoretical and experimental foundations for the preparation of low-cost, high-performance Ce-based magnets.
【Method】 The diffusion source was placed at both ends of the substrate to be diffused using the patch method, and heat treatment was carried in a vacuum sintering furnace. The magnetic properties of the magnets before and after diffusion treatment were tested using a MIN-6000C instrument. Composition analysis was performed via inductively coupled plasma optical emission spectrometry (ICP-OES). The microstructure and morphology of the magnets were observed using a field emission scanning electron microscope (SEM). The elemental distribution in the magnets was analyzed in detail with an electron probe microanalyzer (EPMA). X-ray diffraction (XRD) was employed to analyze the diffraction patterns. Thermogravimetric-differential thermal analysis (TG-DTA) was conducted to determine the Curie temperature (T_c).
【Result】 (1) After diffusion for 6-12 h, the H_cj of Ce₆₀ magnets significantly improves, while the B_r remains essentially unchanged. Among these, the magnet diffused for 8 h exhibits the greatest performance enhancement: H_cj increases from 6.51  kOe to 10.61  kOe, an improvement of ~63%, while B_r slightly decreases from 11.12  kGs to 11.05  kGs, a reduction of ~0.6%. After diffusion treatment, the absolute values of both β_Hcj and α_Br are higher than those of the original magnet. (2) After diffusion treatment, the crystal structure of the main phase in the magnet remains unchanged, and no new phases are formed.
【Conclusion】 (1) As the diffusion time increases, the H_cj of the magnet initially increases and then decreases. The magnet subjected to 8 hours of diffusion exhibits the greatest improvement in coercivity, the H_cj of the Ce₆₀ magnet is enhanced from 6.51 kOe to 10.61 kOe, an increase of 4.1 kOe, achieving a remarkable improvement of ~63%. Within the temperature range of 20~80 ℃, compared to the undiffused magnet, the β_Hcj of the diffused magnet decreases slightly, while the α_Br remains almost unchanged. (2) Electron Probe Microanalysis (EPMA) and Backscattered Electron (BSE) imaging reveal that Pr and Co are predominantly concentrated in the grain boundary phase after diffusion. A small amount of Pr diffuses into the main phase, forming a core-shell structure. Al is generally uniformly distributed throughout the magnet, with a minor fraction enriched in the grain boundaries. Thermo gravimetric analysis (TGA) indicates that the Curie temperature (T_c) of the as-prepared Ce₆₀ magnet is approximately 230 ℃, and the T_c remains nearly unchanged after the diffusion treatment. The significant enhancement in the coercivity of the Ce₆₀ magnet is primarily attributed to two factors: the reduced aggregation of Ce at the grain boundaries and the formation of a continuous, uniform thin rare earth-rich phase surrounding the main phase grains.