Magnesium and Doped Magnesium Nanostructured Materials for Hydrogen Storage - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 294

the material as made. The challenge with magnesium-based materials in particular
is that elemental magnesium is very easily oxidized. Any synthetic method that is
designed to produce zero valent magnesium will need to be carried out under inert
conditions, and if the precursors are common magnesium salts, under fairly
reducing conditions. For the purpose of studying the effects of parameters such as
particle size and the incorporation of dopants (common variables that are tuned for
magnesium as a hydrogen storage material), a synthesis that allows for fine control
and manipulation of these variables is desired. The general goals of modern
synthetic methods for magnesium are to control the size and morphology of the
particles and/or to controllably incorporate dopants.
2.1 Undoped Magnesium Nanocrystal Syntheses
There are two general strategies for synthesizing magnesium for hydrogen storage.
The first is to start with bulk elemental magnesium, and then either ball mill the bulk
material to reduce the grain size, or to use vapor transport reactions to create
nanostructures. For the purposes of clarity, a distinction must be made between
nanostructured magnesium and nanocrystalline magnesium; while one can be both
nanostructured (wherein the physical features of the particles are on the nanoscale)
and nanocrystalline (where the grain sizes composing the particles are on the
nanoscale), the other may be nanocrystalline but microstructured (Fig. 3 )[ 7 , 8 ]. This
is an important distinction because different synthetic methods are required to control
the physical dimensions of the particles versus the physical dimensions of the grains
contained within a particle. The second general method for synthesizing magnesium
for hydrogen storage is to start with magnesium precursors where magnesium is
present in the +2 oxidation state, and then reduce the precursor in solution (either
chemically or electrochemically, see Fig. 4 ) to form neutral magnesium.
The most prevalent synthesis in the current literature for reducing the grain
sizes of bulk magnesium to micro or nanocrystalline grains is that of mechanical
milling, or ball milling [ 7 , 9 ]. In addition to standard ball milling, high energy
mechanical milling (under either high pressure or high temperature) has been used
by many for the synthesis of magnesium nanocrystals [ 10 ]. This technique can be
utilized under hydrogen pressure to directly make MgH 2 [ 11 - 14 ] rather than
making the pure metal and hydriding it post-synthesis. Mechanochemical milling
has also been used to produce magnesium nanocrystals in a similar fashion [ 14 ].
Another mechanical method for producing nanocrystalline magnesium is the
method of cold rolling and cold forging to reduce grain sizes [ 15 ]. Although these
types of mechanical milling are effective in reducing grain size and incorporating
dopant materials (to be discussed in Sect. 2.2 ), they offer little control over
nanocrystal size distribution, shape, and impurity phases (Fig. 3 b).
Plasma reactions [ 16 , 17 ] have been demonstrated to be effective in producing
nanoscale magnesium; however, impurity phases such as oxides can also be a
problem in these systems. Gas-phase condensation of metallic magnesium into