Intermetallic compounds

Intermetallic or intermetallic compound is a term that is used in a number of different ways. Most commonly it refers to solid-state phases involving metals. There is a "research definition" adhered to generally in scientific publications, and a wider "common use" term. There is also a completely different use in coordination chemistry, where it has been used to refer to complexes containing two or more different metals.

Although the term intermetallic compounds, as it applies to solid phases, has been in use for many years, its introduction was regretted, for example by Hume-Rothery in 1955.[1]

Note that many intermetallic compounds are often simply called 'alloys', although this is somewhat of a misnomer. Both are metallic phases containing more than one element, but in alloys the various elements substitute randomly for one another in the crystal structure, forming a solid solution with a range of possible compositions; in intermetallic compounds, different elements are ordered into different sites in the structure, with distinct local environments and often a well-defined, fixed stoichiometry. Complex structures with very large unit cells can be formed.

Definitions

Research definition

Schulze in 1967,[2] defined intermetallic compounds as solid phases containing two or more metallic elements, with optionally one or more non-metallic elements, whose crystal structure differs from that of the other constituents. Under this definition the following are included

  • Electron (or Hume-Rothery) compounds
  • Size packing phases. e.g. Laves phases, Frank–Kasper phases and Nowotny phases
  • Zintl phases

The definition of a metal is taken to include:

  • the so-called poor metals, i.e. aluminium, gallium, indium, thallium, tin and lead
  • some, if not all, of the metalloids, e.g. silicon, germanium, arsenic, antimony and tellurium.

Alloys, which are homogeneous solid solutions of metals, and interstitial compounds such as the carbides and nitrides are excluded under this definition. However, interstitial intermetallic compounds are included as are alloys of intermetallic compounds with a metal.

Common use

In common use, the research definition, including poor metals and metalloids, is extended to include compounds such as cementite, Fe3C. These compounds, sometimes termed interstitial compounds can be stoichiometric, and share similar properties to the intermetallic compounds defined above.

Complexes

The term intermetallic is used[3] to describe compounds involving two or more metals such as the cyclopentadienyl complex Cp6Ni2Zn4.

Intermetallics involving two or more metallic elements

Intermetallic compounds are generally brittle and have a high melting point. They often offer a compromise between ceramic and metallic properties when hardness and/or resistance to high temperatures is important enough to sacrifice some toughness and ease of processing. They can also display desirable magnetic, superconducting and chemical properties, due to their strong internal order and mixed (metallic and covalent/ionic) bonding, respectively. Intermetallics have given rise to various novel materials developments. Some examples include alnico and the hydrogen storage materials in nickel metal hydride batteries. Ni3Al, which is the hardening phase in the familiar nickel-base superalloys, and the various titanium aluminides have also attracted interest for turbine blade applications, while the latter is also used in very small quantities for grain refinement of titanium alloys. Silicides, intermetallics involving silicon, are utilized as barrier and contact layers in microelectronics.[4]

Properties and examples

The formation of intermetallics can cause problems. binary phase diagram of Al–Au. AuAl2 is known as "purple plague". Au5Al2 is known as "white plague".

History

Examples of intermetallics through history include:

German type metal is described as breaking like glass, not bending, softer than copper but more fusible than lead.[6] The chemical formula does not agree with the one above; however, the properties match with an intermetallic compound or an alloy of one.

References

  • Gerhard Sauthoff: Intermetallics, Wiley-VCH, Weinheim 1995, 165 pages
  • Intermetallics, Gerhard Sauthoff, Ullmann's Encyclopedia of Industrial Chemistry, Wiley Interscience. (Subscription required)

See also

External links

  • Intermetallics, scientific journal
  • Intermetallic Creation and Growth – an article on the Wire Bond Website of the NASA Goddard Space Flight Center.
  • Intermetallics project (IMPRESS Intermetallics project at the European Space Agency)
  • intermetallic compound solidifying/freezing
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