Page 185 - Materials Chemistry, Second Edition

172 3 Metals

For relatively brittle materials such as intermetallic compounds, and ferro-alloys,
mechanical pulverization is sufficient to produce metallic powders. This process uses
a ball or rod mill, a cylindrical-shaped steel container filled with ceramic balls or
rods, respectively. As the grinding mill is rotated, the grinding media collides with
the ore/metallic compound effectively grinding the material into a fine powder.
Either alumina or zirconia represents the most common ceramic material used within
grinding mills. This procedure is also commonplace for refining iron powder from the
co-grinding and post-annealing of the ore with carbon (Eq. 2). Refractory metals are
normally refined through the reduction of oxides with hydrogen gas.
Chemical precipitation of metal from a solution of a soluble salt may also be used
to form metallic powders. In this procedure, a reducing agent such as sodium
borohydride is added to an aqueous metal salt, MX (Eq. 14). A mixture of aqueous
products will be produced in addition to the reduced metal, since sodium borohy-
dride also reacts exothermically with water to yield borax,Na 2 B 2 O 7 . As we will see
in Chapter 6, this is the most widely used procedure for the synthesis of nanoparti-
culate metals, from the reduction of metal salts confined within nanosized entrainer
molecules.

0
MX ðaqÞ þ 3 NaBH 4ðaqÞ þ 10 H 2 O ð1Þ ! M þ NaX ðaqÞ
ðsÞ
ð14Þ þ BðOHÞ þ Na 2 B 2 O 7ðaqÞ þ 29=2H 2ðgÞ
3ðaqÞ
Another useful means of producing metal powders is through thermolysis of a
chemical precursor, such as metal carbonyl complexes. This process was originally
developed to refine nickel from the crude product extracted from its ore. Carbon
monoxide gas readily reacts with late transition metals, due to the synergistic effects
of s-electron donation from the ligand to metal, and p-back donation from the metal
to the ligand (Figure 3.10). Hence, by passing CO gas over impure nickel at 50 C, Ni

(CO) 4 gas is formed, leaving the impurities behind. The carbonyl decomposes upon

heating at ca. 250 C, forming the pure nickel powder. Industrially, the Mond process
uses the same chemistry, using nickel oxides from the natural ore. Upon reaction with

Figure 3.10. The synergistic stabilizing effect of metal carbonyl complexes. Shown is (a) ligand-to-metal
2
s donation from the carbon lone pair to the metal d z orbital and (b) metal-to-ligand back-donation from
*
the d x2 y2 orbital to the empty p orbital on CO. This weakens the C–O bond, while concomitantly
strengthening the M–C interaction.