Synthesis, stability against air and moisture corrosion, and magnetic-properties of finely divided loose Nd2Fe14BHx, x-less-than-or-equal-to-5, hydride powders

Abstract

Nd2Fe14BH(x), x less-than-or-equal-to 5, hydride powders, with particle size as small as 1 mum, 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 Nd2Fe14B 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 by-products, and finally yields a well-separated Nd2Fe14BH(x), x less-than-or-equal-to 5, hydride powder. The water also reacts with the alloy crystallite surfaces producing a passivation layer containing iron-boron amorphous or nanocrystallized particles and rare-earth rich oxides and oxyhydrides of the associated elements. This layer, which becomes rigidly hard on drying under vacuum at room temperature, is sufficient to protect the powder against further oxidation in the open air at room temperature. Thermal stability, crystalline structure, and magnetic properties of several hydrided powders are studied systematically. Parallel measurements are made on purely anhydride and on the hydride alloy which has been dehydrided by heating at high temperatures. 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 almost-equal-to 60 kOe (against almost-equal-to 150 kOe in anhydride).