Impurity
:
For the death metal band, see Impurity (band)Impurities are
substances inside a confined amount of
liquid,
gas, or
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.
Impurities can be destructive when they obstruct the working nature of the material. Examples include
ash and
debris in
metals and leaf pieces in blank white papers. The removal of impurities is usually done chemically. For example, in the manufacturing of
iron, calcium carbonate is added to the
blast furnace to remove
silicon dioxide from the iron
ore.
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
salt can be separated by
distillation, with water as the distillate and salt as the solid
residue. Impurities are usually only physically removed from liquids and gases. Removal of sand particles from metal ore powders 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.
Impurities can, though, add constructive
properties to a material.
Alloys are metals with impurities. The resulting combination has desirable properties not found in the constituent materials.
Steel, for example, is made by introducing a controlled amount (less than 2%) of
carbon into pure
iron. In the manufacturing of
solar cells, pure
silicon is mixed with a very small portion of impurities (0.001% to 0.01%) in the form of
phosphorus and
boron atoms in order to generate electricity. This is known as
doping of silicon and is constructive, although the phosphorus and boron could be called impurities.
When an impure liquid is cooled to its
melting point the liquid, undergoing a
phase transition,
crystallizes around the impurities and becomes a crystalline solid. If there are no impurities then the liquid is said to be pure and can be
supercooled below its melting point without becoming a solid. This occurs because the liquid has nothing to condense around so the solid cannot form a natural crystalline solid. The solid is eventually formed when
dynamic arrest or
glass transition occurs, but it forms into an
amorphous solid — a
glass, instead, as there is no
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
Kondo effect where the conduction electron spins form a magnetic bound state with the impurity atom. Magnetic impurities in
superconductors can serve as generation sites for
vortex defects. Point defects can nucleate reversed domains in
ferromagnets and dramatically affect their
coercivity. In general impurities are able to serve as initiation points for
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.
*
dross*
fineness*
semiconductor*
spin wave# Longman's English-Chinese Dictionary of Chemistry, Hong Kong, 1997.
