Grain boundary engineering in Nd-based ThMn12 magnets and their nitrides: A comprehensive study of challenges and limitations

Grain boundaries in rare earth permanent magnets are of major importance to optimize coercivity and densification during sintering. In this work, grain boundary engineering of Nd-based ThMn ₁₂ magnets and their nitrides was systematically investigated. First, the parent compound with excess Nd (5, 20 and 40 at%) was studied. It is shown that a paramagnetic dhcp -Nd grain boundary is formed with excess of Nd above 20 at%. This intergranular phase promotes the liquid-phase sintering process at ∼ 700 °C and a bulk nanostructured Nd _1.2(Fe,Mo) ₁₂ sample with a relative density of 95 % using SPS was obtained. By further doping with Cu, the grain boundary phase properties were tuned by lowering its melting point below 500 °C. Next, the focus is on the nitrides which have a coercivity of ∼ 0.6 T. During the nitrogenation process, the dhcp -Nd grain boundary phase reacts with nitrogen to form a paramagnetic NdN phase, which does not influence coercivity. However, NdN inhibits liquid-phase sintering of the ThMn ₁₂ magnets, due to its high melting point (> 1500 °C), limiting the relative density to 65 % after SPS. Thus, a proper grain boundary for nitrides is yet to be found but this work elucidates in detail the specific challenges of grain boundary engineering in Nd-based ThMn ₁₂ magnets, non-nitrided and nitrided.