Synthesis and reaction mechanism of high (BH)_max exchange coupled Nd₂(Fe,Co)₁₄B/α-Fe nanoparticles by a novel one-pot microwave technique

Nd-Fe-B based magnets, exhibiting the highest energy product, have a wide range of applications in industry. We developed a novel one-pot microwave synthesis technique to produce hard magnetic exchange coupled Nd₂(Fe,Co)₁₄B/α-Fe nanoparticles. Nd-Fe-Co-B mixed oxides were synthesized from metal nitrates via microwave combustion followed by microwave reduction of these oxides to hard magnetic Nd₂(Fe,Co)₁₄B powders in the same microwave chamber. The conventional use of a furnace for oxide reduction is eliminated. This method is cost effective, facile, energy efficient and widely applicable to a variety of materials. The detailed reaction mechanisms in both microwave combustion and microwave reduction were studied for the first time. During mixed oxide formation, the formation sequence is: boron oxide, iron oxide, cobalt oxide and finally, neodymium-iron mixed oxide. In the microwave reduction process, iron, cobalt and boron oxides were reduced, followed by reduction of neodymium oxide, resulting finally in the desired Nd₂(Fe,Co)₁₄B and α-Fe exchange coupled nanoparticles. The synthesized Nd₂(Fe,Co)₁₄B/α-Fe nanoparticles have a narrow size distribution with 60% in the size range of 35-45 nm. With a high remanence of 99 emu g⁻¹, the maximum energy product of these hard-magnetic nanoparticles reached a value of 11.4 MGOe, which is the highest among the values reported for Nd₂Fe₁₄B/α-Fe nanoparticles synthesized by chemical approaches.