Grain Boundary Engineering of Nd-based ThMn12 Magnets and Their Nitrides

Grain boundaries in rare earth (RE) permanent magnets are of major importance for magnetically decoupling and sintering densification. In this work, we investigate the grain boundary engineering of the Nd-based ThMn
12
magnets and their nitrides. First, we demonstrate the possibility to form a RE-rich grain boundary by adding excess Nd
$(\text{Nd}_{\mathrm{x}}\text{Fe}_{10.5}\text{Mo}_{1.5}$
with
$\mathrm{x}=1.2$
and 1.4) in micro- and nanostructured samples. We show that the grain boundary phase is dhcp-Nd and is paramagnetic, being beneficial for magnetically decoupling the grains and optimizing the coercivity. In addition, this phase promotes the liquid-phase sintering process, and we successfully obtained a bulk
$\text{Nd}_{1.2}(\text{Pe}_{2}\text{Mo})_{12}$
sample with a relative density of 95 % using Spark Plasma Sintering (SPS). With adding small amounts of Cu or Ga elements, we were able to tune the grain boundary phase properties in lowering its melting point by forming eutectic phase. Next, we focus on the effect of grain boundary in nitrides. We show that during the nitrogenation process, the dhcp-Nd grain boundary phase reacts with nitrogen to form NdN grain boundary phase, which is paramagnetic at room temperature. However, magnetic characterization suggests that the NdN grain boundary does not influence the magnetic coupling of grains resulting in no further improvement of the coercivity. In addition, NdN inhibits liquid-phase sintering of the nitrided compounds, due to its high melting point
$(> 1500^{\circ}\mathrm{C})$
, limiting the relative density to 65% after SPS.