Sol−Gel Based Chemical Synthesis of Nd₂Fe₁₄B Hard Magnetic Nanoparticles

Nd 2 Fe 14 B hard magnetic nanoparticles were synthesized by chemical synthesis techniques. Nd−Fe−B gel was prepared using NdCl 3 ·6H 2 O, FeCl 3 ·6H 2 O, H 3 BO 3, citric acid, and ethylene glycol (EG) by a Pechini type sol−gel method. This gel was subsequently annealed to produce mixed oxide powders. Nd 2 Fe 14 B nanoparticles were prepared from these oxides by a reduction−diffusion process. The phase analysis, structure, and magnetic properties were determined by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), transmission electron microscope (TEM), and vibrating sample magnetometer (VSM) techniques. The mechanism of Nd 2 Fe 14 B formation was investigated by differential scanning calorimetry (DSC), XRD, and thermodynamic free energy change data. Our experimental and modeling results showed that the reduction−diffusion of the Nd−Fe−B mixed oxide was a three step process. The reduction of Fe 2 O 3 to Fe and B 2 O 3 to B occurred at 300 °C. NdH 2 and Fe was formed from Nd 2 O 3 and NdFeO 3 at 620 °C. The Nd 2 Fe 14 B phase was formed from NdH 2, Fe, and B at 692 °C. The coercivity of as-synthesized powder was 6.1 kOe. The Henkel plot showed that this powder was exchange coupled; removal of CaO by washing led to dipolar interactions and a decrease in coercivity.