Formation and magnetic enhancement mechanism of sandwich-structure grain boundary phase in Ce magnets

Abstract

In this study, the formation mechanism of sandwich-structure grain boundary phase in sintered Ce magnets with various Pr-Nd-Al alloy additions is investigated in detail. The sandwich-structure grain boundary between the main phases is composed of RE6(Fe, TM)11Al3 phase at the center and symmetrical Nd-rich phase at both sides. Based on the analysis of phase diagrams, the formation mechanism is revealed from the following aspects: i) The amorphous formation ability of Nd-rich liquid phase in Ce magnet during sintering can be enhanced by introducing Pr-Nd-Al alloy; ii) The RE6(Fe, TM)11Al3 phase preferentially nucleates and grows during the process of cooling; iii) The interfacial tension among Nd-rich liquid phase, main phase and RE6(Fe, TM)11Al3 phase contributes to the formation of the sandwich-structure grain boundary. With the addition of Pr-Nd-Al alloy, the activity of Al entering the main phase is inhibited, resulting in almost no reduction of remanence and Curie temperature. The structural stability and decoupling effect of the sandwich-structure grain boundary phase are more conducive to the improvement of the coercivity and thermal stability of the Ce magnets. The coercivity of Ce magnet changes from 12.7 to 14.93 kOe (the increase extent is 17.6%). In the temperature range 20–100 ℃, the coercivity temperature coefficient varies from -0.767 to -0.680 %/℃ and the temperature stability obviously ameliorates about 11.3%.