Page 186 - Materials Chemistry, Second Edition

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3.1. Mining and Processing of Metals

a mixture of H 2 and CO gases, the nickel is first reduced to form an impure product,
followed by conversion to ultra high purity Ni through the Ni(CO) 4 intermediate.
Electrolysis may also be used to produce metallic powders, through redox reactions
at electrode surfaces. By choosing suitable reaction conditions – composition and
strength of the electrolyte, temperature, current density, etc. – many metals can be
deposited in a spongy or powdery state. However, most often a brittle deposit is formed,
requiring extensive post-processing such as washing/drying, reduction, annealing, and
crushing. Although this technique could be used for virtually all metals, it has been
replaced with other less expensive methods such as solution reduction. Nevertheless,
metallic powders of copper, chromium, and manganese are still mostly produced
through electrolytic means. Interestingly, toward the ongoing search for structures at
thenanoregime(Chapter 6), electrodeposition has recently been applied for the
intriguing synthesis of metal nanoparticles and nanowires (Figure 3.11).
The last method for generation of metallic powders that we will consider is atomi-
zation. In this high-temperature process, molten metal is broken up into small droplets
and rapidly quenched to prevent wide-scale agglomeration (Figure 3.12). The atomi-
zation process occurs through the bombardment of a stream of molten metal with a
high-energy jet of gas (e.g.,air, N 2 , Ar) or liquid (e.g.,H 2 O, hydrocarbons). Argon gas
is used extensively to prevent the oxidation of reactive metals and alloys such as
chromium or tungsten. Atomization is very different than ionization. Whereas the

Figure 3.11. Electrodeposition of (a) silver nanoparticles and (b) silver nanowires. The co-evolution of
hydrogen gas during electrodeposition is thought to assist in monodisperse nanocluster growth by
interrupting interparticle coupling via convection effects at the electrode surface and surface
mobilization of growing nanoclusters. Reproduced with permission from J. Phys. Chem. B. 2002, 106,
3339. Copyright 2002 American Chemical Society.