Abstract
A significant coercivity enhancement of the commercial NdFeB magnets with the magnetic properties of (BH)max = 48.4 MGOe and iHc = 17.5 kOe through grain boundary diffusion (GBD) with low-melting Tb55R20Cu25 alloys is demonstrated. Adopting Tb55R20Cu25 alloys as GBD sources is effective in increasing coercivity to 29.0 kOe for R = None, 23.8 kOe for R = Y, 25.6 kOe for R = La, 28.0 kOe for R = Ce, respectively. Yet, (BH)max is slightly reduced to 46.2-48.2 MGOe. The preferential appearance of Cu at grain boundary and triple junction of the grains, and the core-shell structure occurred due to Tb at grain surface remarkably enhance the coercivity. Interestingly, higher coercivity enhancement per wt% Tb usage (ΔiHc/wt%Tb) of 7.2 kOe/wt% for the magnet with Tb55Ce25Cu25 than 5.9 kOe/wt% for that with Tb75Cu25 has been found due to the magnetic isolation effect caused by the preferential appearance of Ce at grain boundary, though a slight lower coercivity enhancement was found for the samples with R = Y and La. Lower melting point (637 °C) for Tb55Ce20Cu25 than Tb75Cu25 (743 °C) leads to larger diffusion depth of Tb into the magnet and therefore contributes to higher efficiency of coercivity enhancement for the magnet with R=Ce.
Highlights
NdFeB sintered magnets exhibit the highest energy product ((BH)max) among all developed permanent magnets due to the outstanding magnetically intrinsic properties of Nd2Fe14B at room temperature (RT)
Adopting Tb55R20Cu25 alloys as grain boundary diffusion (GBD) sources is effective in increasing coercivity to 29.0 kOe for R = None, 23.8 kOe for R = Y, 25.6 kOe for R = La, 28.0 kOe for R = Ce, respectively
Higher coercivity enhancement per wt% Tb usage (ΔiHc/wt%Tb) of 7.2 kOe/wt% for the magnet with Tb55Ce25Cu25 than 5.9 kOe/wt% for that with Tb75Cu25 has been found due to the magnetic isolation effect caused by the preferential appearance of Ce at grain boundary, though a slight lower coercivity enhancement was found for the samples with R = Y and La
Summary
NdFeB sintered magnets exhibit the highest energy product ((BH)max) among all developed permanent magnets due to the outstanding magnetically intrinsic properties of Nd2Fe14B at room temperature (RT). Low Curie point (Tc) leads to sharp decrease of intrinsic coercivity (iHc) with the temperature and causes a difficulty for high temperature applications.. Dy or Tb is added to the Nd-Fe-B magnets to enhance iHc for the applications at higher temperature. Since the price surge of critical RE elements in 2011 and the trade war arises recently, how to reduce the usage of Dy or Tb in the magnets while keeping high coercivity for practical applications becomes the crude issue again. In order to reduce the usage of Tb and enhance coercivity of NdFeB sintered magnets, Tb75Cu25, Tb55Y20Cu25, Tb55La20Cu25 and Tb55Ce20Cu25 alloy powders are adopted as diffusion sources in this study. Magnetic properties and microstructure of NdFeB sintered magnets through GBD treated with Tb55R20Cu25 (R = None, Y, La, and Ce) alloy powders are explored
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