Engineering, 11.10.2019 05:30 Kekkdkskdkdk

Describe and sketch point defects including vacancies, interstitial and substitutional impurities.

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Explanation: All real solids are impure. A very high purity material, say 99.9999% pure. Impurities are often added to materials to improve the properties. For instance, carbon added in small amounts to iron makes steel, which is stronger than iron. Boron impurities added to silicon drastically change its electrical properties.

The properties of materials are profoundly influenced by the presence of imperfections.

TYPES OF IMPURITIES

1. Point defects

• Vacancy atoms (vacant atomic sites in a structure causing a distortion of planes). They are common, especially at high temperatures when atoms are frequently and randomly change their positions leaving behind empty lattice sites. In most cases diffusion (mass transport by atomic motion) can only occur because of vacancies.

• Self-Interstitial atoms ("extra" atoms positioned between atomic sites causing a distortion of planes) A self interstitial atom is an extra atom that has crowded its way into an interstitial void in the crystal structure. Self interstitial atoms occur only in low concentrations in metals because they distort and highly stress the tightly packed lattice structure.

2. Impurities:

Substitutional (mpurity atom in lattice). Substitutional impurity atoms are usually close in size (within approximately 15%) to the bulk atom. An example of substitutional impurity atoms is the zinc atoms in brass. In brass, zinc atoms with a radius of 0.133 nm have replaced some of the copper atoms, which have a radius of 0.128 nm. Interstitial atoms (impurity atom not in regular lattice sit) . An example of interstitial impurity atoms is the carbon atoms that are added to iron to make steel. Carbon atoms, with a radius of 0.071 nm, fit nicely in the open spaces between the larger (0.124 nm) iron atoms.

3. Line defects

• Dislocations

4. Area defects

• Grain Boundaries

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(b)

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Describe and sketch point defects including vacancies, interstitial and substitutional impurities....

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