Page 239 - Materials Chemistry, Second Edition

226 3 Metals

relatively low operating temperatures, the addition of boron results in the ternary
compound Nd 2 Fe 14 B with strong uniaxial magnetocrystalline anisotropy, and a
higher operating temperature. The absorption of nitrogen to yield Sm 2 Fe 17 N 3 causes
a further improvement in the magnetic properties; however, the applications are
limited by its complex synthesis. It should be noted that the partial substitution of
Co for Fe reduces the surface oxidation. This is important, since the processing of rare
earth magnets involves the compaction of finely divided powders, which are more
difficult to obtain through ball-milling if a hard oxide layer is present. Interestingly,
since the Curie temperature of Co greater than Fe, the T c of the ternary alloy

Nd 2 (Co x Fe 1 x )B increases at a rate of ca.10 C per at.% Co that is substituted.
The origination of the desirable magnetic properties within the complex structures
of rare earth alloys is not completely understood. Likely, the interaction of the 4f
electrons with neighboring lattice atoms results in a preferential alignment of the
rare earth magnetic moments along specific lattice directions. This allows for
saturation magnetization to be achieved with only relatively small applied fields.
A high intrinsic coercivity will also result, since significant energy is required to
disrupt the preferential alignment, known as the magnetocrystalline anisotropy
energy. It should be noted that such magnetic anisotropy is also found in single
crystals of pure metals such as Fe, Ni, or Co (Figure 3.43).

3.5. REVERSIBLE HYDROGEN STORAGE
With cyclic gasoline prices and increasing awareness/research related to renewables,
mankind is facing an energy crisis, the likes of which could annihilate our entire
population. It is predicted that in the next few years, fossil fuel use could become
prohibitively expensive, leading to the necessity of using other fuel sources. Hydro-
gen is the most attractive alternative due to its nonpolluting nature, only yielding
water as a byproduct of its combustion. However, before widespread utilization of
this medium is possible, two key issues must be solved: hydrogen generation and
storage. At present, the fuel used in prototype vehicles designed by BMW, Toyota,
and Honda is liquid hydrogen. Although H 2 contains more energy/mass than gaso-
line, a relatively large volume must be used due to its extremely low density. In
addition, there are prohibitive costs involved in H 2 liquidification and cryogenic tank
production. We are all familiar with the dangers associated with the storage of liquid
and gaseous fuels. For instance, consider the major tragedies of the Challenger and
Columbia explosions, as well as leveling of the twin towers in New York City on
September 11, 2001 – all exacerbated by the presence of large volumes of liquid fuels
onboard. Hence, there continues to be much interest in the search for solid-state
materials that can reversibly store energetic fuels such as hydrogen.
Hydrogen combines with many elements to form binary hydrides, MH n . There are
three general classes of hydrides:
1. Saline or Binary (involving Group 1 and 2 metals; may be envisioned as an ionic
n+

lattice consisting of M and H ions, e.g., LiH, NaH, BaH 2 )