Impurity



Impurities are substances inside a confined amount of solid, which differ from the chemical composition of the material or compound.

Impurities are either naturally occurring or added during synthesis of a chemical or commercial product. During production, impurities may be purposely, accidentally, inevitably, or incidentally added into the substance.

The level of impurities in a material are generally defined in relative terms. Standards have been established by various organizations that attempt to define the permitted levels of various impurities in a manufactured product. Strictly speaking, then, a material's level of purity can only be stated as being more or less pure than some other material.

Destructive impurities

Impurities can be destructive when they obstruct the working nature of the material. Examples include ash and debris in Zone refining is an economically important method for the purification of semiconductors.

However, some kinds of impurities can be removed by physical means. A mixture of water and distillation, with water as the distillate and salt as the solid residue. Impurities are usually physically removed from liquids and gases. Removal of sand particles from metal ore is one example with solids.

No matter what method is used, it is usually impossible to separate an impurity completely from a material. What technicians can do is to increase the purity of a material to as near 100% as possible or economically feasible.

Constructive impurities

Impurities can, though, add constructive properties to a material. doping of silicon and is constructive, although the phosphorus and boron could be called impurities.

Research on magnetic impurities has shown that considerable alteration of certain properties such as specific heat may be affected by small concentrations of an impurity, as for example impurities in semiconducting ferromagnetic alloys may lead to different properties as first predicted by White, Hogan, Suhl and Nakamura.[1][2]

Impurities and nucleation

When an impure liquid is cooled to its long-range order in the structure.

Impurities play an important role in the nucleation of other phase transitions. For example, the presence of foreign elements may have important effects on the mechanical and magnetic properties of metal alloys. Iron atoms in copper cause the renowned phase transitions because the energetic cost of creating a finite-size domain of a new phase is lower at a point defect. In order for the nucleus of a new phase to be stable, it must reach a critical size. This threshold size is often lower at an impurity site.

See also

Line notes

  1. ^ C. Michael Hogan, (1969) Density of States of an Insulating Ferromagnetic Alloy Phys. Rev. 188, 870 - 874, [Issue 2 – December 1969
  2. ^ X. Y. Zhang and H. Suhl (1985) Phys. Rev. A 32, 2530 - 2533 (1985) [Issue 4 – October 1985

References

  1. Longman's English-Chinese Dictionary of Chemistry, Hong Kong, 1997.
  2. Cheng, E. et al, Chemistry - A Modern View, Aristo-Wilson, Hong Kong, 2004
 
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