Synthesis, stability against air and moisture corrosion, and magnetic properties of finely divided loose Nd2Fe14BHx, x⩽5, hydride powders

Nd₂Fe₁₄BH_x, x⩽5, hydride powders, with particle size as small as 1 μm, have been successfully prepared using a chemical method derived from the well-known oxide-reduction diffusion (ORD) method. In this method, the raw materials (Nd₂O₃, iron and boron) are mixed with calcium metal or hydride powder (in excess) and additions of anhydrous CaCl₂ and NaCl, and finally sintered at 1170-1270 K for a few hours under an argon atmosphere. This yields finely divided Nd₂ Fe₁₄B crystallites embedded in the byproducts. The material is then washed with water at room temperature, where the excess Ca in the mixture reacts with water and produces nascent hydrogen, which reacts with the alloy particles embedded in the byproducts, and finally yields a well-separated Nd₂Fe₁₄BH_x, x⩽5, hydride powder. Thermal stability, crystalline structure, and magnetic properties of several hydrided powders are studied systematically. These studies show that the interstitial hydrogen atoms led to 1) an increase in the lattice volume by as much as 4.2%, 2) a decrease in the coercivity to almost zero, 3) a dramatic improvement in T_C from 593 to 642 K, and 4) a substantial modification of the magnetization process, showing magnetic saturation at lower fields of ≈60 kOe (against ≈150 kOe in anhydride)