The solid solution – chemical imperfection

Crystal Defects and Noncrystalline structure – Imperfection

We discussed about perfectly repetitive crystalline structures. But we understand that nothing in our world is quite perfect. Now we discuss about imperfections.

Remember:

1. That is no material can be prepared without some degree of chemical impurity. The impurity atoms or ions in the resulting solid solution serve to alter the structural regularity of the ideally pure material.

2. The simplest type of flaw is the point defect, for example, a missing atom (vacancy). This type of flaw is the inevitable result of the normal thermal vibration of atoms in any solid at a temperature above absolute zero.

3. Linear defects, or dislocations, follow an extended and sometimes complex path through the crystal structure.

4. Planar defects represent the boundary between a nearly perfect crystalline region and its surroundings.

The solid solution – chemical imperfection

It is not possible to avoid some contamination of practical materials. Even high-purity semiconductor products have some measurable level of impurity atoms. Many engineering materials contain significant amounts of several different components (commercial metal alloys are examples). As a result, all materials that the engineer deals with on a daily basis are actually solid solutions.

1.The concept of a solid solution may be difficult to grasp, (it is essentially equivalent to the more familiar liquid solution, such as the water- alcohol system (Fig.4.1). The complete solubility of alcohol in water is the result of complete molecular mixing.

2. Similar result is seen in Fig.4.2, which shows a solid solution of copper and nickel atoms sharing the fcc crystal structure. Nickel acts as a solute dissolving in the copper solvent. This configuration is referred to as a substitutional solid solution because the nickel atoms are substituting for copper atoms on fcc atom sites.

Remember:

· This configuration will tend to occur when the atoms do not differ greatly in size.

Fig.4.1. Forming a liquid solution of water and alcohol. Mixing occurs on the molecular scale.

· The water-alcohol system represents 2 liquids completely soluble in each other in all proportions.

· For complete miscibility in metallic solid solutions, 2 metals must be quite similar.

Fig.4.2. Solid solution of copper in nickel shown along a (100) plane. This is a substitution solid solution with nickel atoms substituting for copper atoms on fcc atom sites.

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