Impurities

Impurities - atoms which are different from the host

• All real solids are impure. Very pure metals 99.9999%- one impurity per 106 atoms
• May be intentional or unintentional Examples: carbon added in small amounts to iron makes steel, which is stronger than pure iron. Boron added to silicon change its electrical properties.
• Alloys - deliberate mixtures of metals Example: sterling silver is 92.5% silver – 7.5% copper alloy. Stronger than pure silver.

Solid Solutions

Solid solutions are made of a host (the solvent or matrix) which dissolves the minor component (solute). The ability to dissolve is called solubility.

• Solvent: in an alloy, the element or compound present in greater amount
• Solute: in an alloy, the element or compound present in lesser amount
• Solid Solution:homogeneous;maintain crystal structure;contain randomly dispersed impurities(substitutional or interstitial)
• Second Phase: as solute atoms are added, new compounds / structures are formed, or solute forms local precipitates.

Whether the addition of impurities results in formation of solid solution or second phase depends the nature of the impurities, their concentration and temperature, pressure…