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Question: Explain Point defects in detail.
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Subject: Material Technology

Topic: Materials and Material Defects

Difficulty: Medium

mt(63) • 73 views
 modified 9 weeks ago  • written 9 weeks ago by
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Point defects are classified as follows:

A] Stoichiometric Defects

1) Vacancy Defects

2) Interstitial defects

3) Schottky Defects

4) Frenkel Defects

B] Non Stoichiometric Defects

1) Metal Excess Defects

2) Metal Deficient Defects

Vacancy defect

• A vacancy is produced when an atom is missing from its original lattice site.
• Due to the reduction in number of atoms in the crystalline solid, vacancy defect results in the reduction of density. However, hardness of the solid may increase.
• Although depends on material and its crystal structure, in general, vacancies can decrease the bulk modulus and can increase the Young’s modulus.
• Large vacancy concentration can reduce the ductility of the crystalline solid; however, can increase the hardness.
• Common physical properties, like melting point, colour, etc. can also vary due to presence of vacancies.
• The number of vacancies present within a particular volume of solid at a particular temperature can be mathematically expressed by

Nv = N × e(–Qv/RT)

Where,

Nv is Total number of vacancies per unit volume of solid at a particular temperature.

N is Total number of vacancies per unit volume of solid at a particular temperature.

Qv is Energy required to produce single vacancy in that solid.

R is Gas constant.

And T is absolute temperature.

Vacancy is caused by -

i. Contrary directional solidification done during metal casting.

ii. Due to increase in temperature while heat treatment.

iii. Due to irradiation or sputtering effect.

iv. Due to presence of residual tensile stress in metal.

Interstitial Defect

• In interstitial defects three or more atoms may share one lattice site, thereby increasing its total energy
• Interstitials can be produced by bombarding a crystal with elementary particles having energy above the displacement threshold for that crystal, but they may also exist in small concentrations in thermodynamic equilibrium.
• Interstitial atoms or ions, although much smaller than the atoms or ions located at the lattice points, are still larger than the interstitial sites that they occupy.
• Consequently, the surrounding crystal region is compressed and distorted.
• For small concentrations, carbon atoms occupy interstitial sites in the iron crystal structure, introducing a stress in the localized region of the crystal in their vicinity.

Schottky Defect

• It is the vacancy defect in ionic compound metal lattice.
• Here one or more pair of cation and anion are absent from lattice, the reason for absence of pair is to maintain the ionic neutrality in lattice structure.
• This defect is observed in salt which has least difference in size of cation and anion.
• Typical salts where Schottky disorder is observed are NaCl, KCl, KBr, CsCl and AgBr. For engineering applications,
• Since the total number of ions present in the crystal with this defect is less than the theoretical number of ions for a crystal of its volume, the density of the solid crystal is less than the theoretical density of the material.

Frenkel Defect

• It is the interstitial defect in ionic compounds.
• Here an atom or ion (normally the cation) leaves its own lattice site vacant and instead occupies a normally vacant site.
• This defect is only possible if the cations are smaller in size when compared to the anions.
• Their prime mechanism of generation is by particle irradiation, as their equilibrium concentration according to the Boltzmann distribution is much smaller than the pure vacancies distribution, due to the large energy necessary for the creation of the associated interstitial atoms.
• This defect does not have any impact on the density of the solid as it involves only the migration of the ions within the crystal, thus preserving both the volume as well as mass.
• Some examples of the defect are zinc chloride, silver halides, magnesium oxide and Sodium chloride.

Metal excess defect

• It is a type of crystallographic defect in which an anionic vacancy in a crystal is filled by one or more unpaired electrons.
• Metal excess defect can occur naturally in compounds (particularly metallic oxides) because when heated to high temperature the ions become excited and are displaced from their normal crystallographic positions, leaving behind some electrons in the vacated spaces.
• Greater the number of defects more intense the colour.
• These vacancies capture available electrons, e−, to maintain charge neutrality, forming defect that is, the electrons released in this process diffuse to occupy the vacant sites.
• Some crystals like lithium chloride, potassium chloride, and zinc oxide become pink, lilac and yellow, respectively on heating them.

Metal Deficient Defect

• In this a cation or anion is missing from its lattice site. To maintain electrical neutrality, one of the nearest metal ion acquires two opposite charge of missing ion.
• The common examples of compounds having this defect are ferrous oxide, ferrous sulphide, nickel oxide etc.