SYNTHESIS, STABILITY AGAINST AIR AND MOISTURE CORROSION, AND MAGNETIC-PROPERTIES OF FINELY DIVIDED LOOSE ND2FE14BHX, X-LESS-THAN-OR-EQUAL-TO-5, HYDRIDE POWDERS
S. Ram et al., SYNTHESIS, STABILITY AGAINST AIR AND MOISTURE CORROSION, AND MAGNETIC-PROPERTIES OF FINELY DIVIDED LOOSE ND2FE14BHX, X-LESS-THAN-OR-EQUAL-TO-5, HYDRIDE POWDERS, IEEE transactions on magnetics, 31(3), 1995, pp. 2200-2208
Nd2Fe14BH(x), x less-than-or-equal-to 5, hydride powders, with particl
e size as small as 1 mum, have been successfully prepared using a chem
ical method derived from the well-known oxide-reduction diffusion (ORD
) method. In this method, the raw materials (Nd2O3, iron and boron) ar
e 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 Nd2F
e14B crystallites embedded in the byproducts. The material is then was
hed with water at room temperature, where the excess Ca in the mixture
reacts with water and produces nascent hydrogen, which reacts with th
e alloy particles embedded in the by-products, and finally yields a we
ll-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 vacu
um at room temperature, is sufficient to protect the powder against fu
rther oxidation in the open air at room temperature. Thermal stability
, crystalline structure, and magnetic properties of several hydrided p
owders are studied systematically. Parallel measurements are made on p
urely 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 dramati
c improvement in T(c) from 593 to 642 K, and 4) a substantial modifica
tion of the magnetization process, showing magnetic saturation at lowe
r fields of almost-equal-to 60 kOe (against almost-equal-to 150 kOe in
anhydride).