Page 226 - Materials Chemistry, Second Edition

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3.2. Metallic Structures and Properties

However, it is the extremely high melting points (>1,650 C) of the refractory
metals that separate this class from the others. As a result, these metals may not be
processed through cold or hot working; powder metallurgy must be used to form the
metals into desired shapes. Although we also generally associate the refractories
with high hardness, it is worthwhile to point out that these metals are all soft and
ductile in their pure states. However, the metals are rarely obtained in their pure
forms, since they spontaneously react with C, B, O, N, and other nonmetals to form
stable interstitial compounds. The incorporation of small main group elements, with
large electron-rich refractory metals, results in solute hardening of the crystal lattice
through localized covalent bonding within the material. A recent example is the
formation of the refractory ceramic OsB 2 , which is possible since boron is signifi-
˚
˚
cantly smaller than Os (0.87 A vs. 1.85 A, respectively). [16]
The incompressibility (bulk modulus) of a material is directly related to its
˚ 3
valence electron density, in units of electrons A . For example, diamond, the
hardest known substance, has a high valence electron density (0.705 electrons
˚ 3
A ) and an exceptionally high bulk modulus (442 GPa). By comparison, osmium
has one of the highest valence electron densities for a pure metal (0.572 electrons
˚ 3
A ), resulting in an accompanying large bulk modulus (ca. 400 GPa). However,
while the bulk moduli of diamond and osmium are equivalent, the hardness of
diamond is more than an order of magnitude larger than Os – a consequence of
covalent vs. metallic bonding. Upon doping Os with boron, the hardness improves
˚ 3
dramatically, while retaining a high valence electron density (0.511 electrons A ).
There are current studies underway that are evaluating the hardness and bulk moduli
of mixed solutions such as Os 1 x M x B 2 , which are likely harder than either OsB 2 or
[17]
MB 2 compounds alone.
As we saw earlier, the facile reaction of Cr with oxygen is the acting principle
behind the anticorrosive property of Cr-containing steels. This analogous reactivity
also explains the high corrosion resistance exhibited by the refractory metals,
especially those of Group 4 (Ti, Zr, Hf – often termed the “reactive metals”).
Since refractories show a high reactivity toward constituent gases of the atmosphere,
the metals in their finely divided forms are highly pyrophoric and must be handled
within an inert-atmosphere glove box. In general, all metals that form stable oxides
or nitrides (e.g., iron, zinc, nickel, etc.) are dangerous as finely divided powders. The
relatively large surface area of individual crystallites leads to simultaneous oxide
formations (exothermic) that release enough heat to spontaneously catch the mate-
rial on fire.
As one moves across the Groups of refractory metals, a number of trends are
noteworthy and greatly affect their materials applications. For instance, moving
left to right causes a decrease in atomic sizes due to ineffective shielding of the
nuclear charge by d-electrons. As additional d-electrons are added to the valence
shell, stronger metal–metal bonds are formed and the metals become increasingly
dense/harder, with higher melting points as illustrated in Table 3.6. Also noteworthy
is the equal atomic sizes of 4d and 5d congeners due to the lanthanide contraction
effect.