Defects in solids PDF

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TYPES OF DEFECTS IN SOLIDS AND THEIR APPLICATIONS The term imperfection or defect is generally used to describe any deviation of the ideally perfect crystal from the periodic arrangement of its constituents. A perfect crystal does not exist. All crystals have some defects. Defects contribute to the mechanical properties of metals. In fact, using the term “defect” is sort of a contradiction since these features are commonly intentionally used to manipulate the mechanical properties of a material.

1.Atomic Imperfections / Point Defects If the deviation occurs because of missing atoms, displaced atoms or extra atoms, the imperfection is named as a Point defect. Such defects can be the result of imperfect packing during the original crystallisation or they may arise from thermal vibrations of atoms at elevated temperatures because with increase in thermal energy there is increased probability of individual atoms jumping out of their positions of lowest energy. Point defects in a crystal may be classified into three types; 

Stoichiometric defects

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Non – stoichiometry defects

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Impurity defects

i)Stoichiometry Defects The compounds in which the number of cation and anions are exactly in the same ratio as represented by their chemical formula are called Stoichiometric compounds. The defects that do not disturb the ratio of cations and anions, and electrical neutrality of a solid are called Stoichiometric defect. These are of four types; 

Vacancy Defect

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Interstitial Defect

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Schottky Defect

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Frenkel Defect

A non-ionic compound mainly shows vacancy and interstitial defects. An ionic compound shows the same in Frenkel and Schottky defect.

a) Vacancy Defect 

When some of the lattice sites are vacant, the crystal is said to have Vacancy defect.

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This results in decrease in density of the substance.

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This defect can also develop when a substance is heated.

b) Interstitial Defect 

When some constituent particles (atoms or molecules) occupy an interstitial site (A position between the regular positions in an array of atoms or ions that can be occupied by other atoms or ions.), the crystal is said to have Interstitial defect.

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This defect increases the density of the substance.

c) Schottky Defect If in an ionic crystal of the type A+ B-, equal number of cations and anions are missing from their lattice. It is called Schottky defect. This type of defect is shown by highly ionic compounds which have 

High Co-ordination number

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Small difference in the sizes of cations and anions.

Consequences of Schottky Defect 

As the number of ions decreases as a result of this defect, the mass decreases whereas the volume remains the same. Hence density of the solid decreases.

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The crystal begins to conduct electricity to a small extent by ionic mechanism.

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The presence of too many voids lowers lattice energy and the stability of the crystal.

d) Frenkel Defect If an ion is missing from its correct lattice sites (causing a vacancy or a hole) and occupies an interstitial site, electrical neutrality as well as stoichiometry (measure of elements) of the compounds are maintained. This type of defect is called Frenkel defect. Since cations are usually smaller, it is more common to find the cations occupying interstitial sites. This type of defect is present in ionic compounds which have 

Low co-ordinations number

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Larger difference in size of cation and anions

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Compounds having highly polarising cation and easily polarisable anion.

Consequences of Frenkel defect 

As no ions are missing from the crystal lattice as a whole, therefore density of the solid remains the same.

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The closeness of like charges tends to increases the dielectric constant of the crystal.

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The crystal conducts electricity to a small extent by ionic mechanism.

ii) Non – Stoichiometric Defects If as a result of imperfection, the ratio of number of cation to anion becomes different from that indicated by the ideal chemical formula, the defects are called Non – stoichiometric defects. These defects arise either due to excess of metal atoms or non-metal atom or presence of impurities / foreign particle.

a) Metal Excess Defects There are two types of Metal Excess defects; 

Metal Excess Defect due to Anionic Vacancies

This occurs due to absence of anions from its original lattice site in crystals. Therefore, instead of anions, electrons occupy their position. 

Metal Excess Defect due to Presence of Extra Cations at Interstitial Sites

On heating the compound, it releases extra cations. These cations occupy the interstitial sites in crystals and the same number of electrons go to neighbouring interstitial sites.

b) Metal Deficiency Defects In certain cases, one of the positive ions is missing from its lattice site and the extra negative charge is balanced by some nearby metal ion acquiring two charges instead of one. There is evidently, a deficiency of the metal ions although the crystal as a whole is neutral. This type of defect is generally found amongst the compounds of transition metals which can exhibit variable valency.

iii)Impurity Defects There are two types of Impurity defects;

a) Impurity Replacement Defect Particle of impurity substance have replaced the own substance's particle in a lattice site.

b) Impurity Inclusion Defect Particle of impurity substance has got into the space between the lattice sites.

2.Line Defects / Dislocations A line defect is a line along which whole rows of atoms in a solid are arranged anomalously. The resulting irregularity in spacing is most severe along a line called the line of dislocation. Line defects can either weaken or strengthen solids. A dislocation is a line defect within a crystal which arises during crystal growth or as a result of mechanical deformation of a crystal. Dislocations make a crystal mechanically harder. The strength and ductility of metals are controlled by dislocations. Dislocations are areas were the atoms are out of position in the crystal structure. Dislocations are generated and move when a stress is applied. Movements of dislocations give rise to their plastic behaviour.

i)Edge Dislocations When lines of ions are missing in an otherwise perfect array of ions, an Edge Dislocation appears. The inter-atomic bonds are significantly distorted only in the immediate vicinity of the dislocation line. Edge dislocation is responsible for ductility and malleability.

ii)Screw Dislocations If the misalignment shifts a block of ions gradually downwards or upwards, causing the formation of a screw-like deformation, a Screw Dislocation is formed. It is the presence of dislocations and their ability to readily move (and interact) under the influence of stresses induced by external loads that leads to the characteristic malleability of metallic materials.

The net plastic deformation of both edge and screw dislocations is the same, however.

iii)Disclinations Disclinations are line defects corresponding to "adding" or "subtracting" an angle around a line. Basically, this means that if you track the crystal orientation around the line defect, you get a rotation.

3.Surface / Planar Defects Surface defects are the boundaries, or planes, that separate a material into regions, each region having the same crystal structure but different orientations. Surface defects may cause corrosion and coating failure.

i)Stacking Faults A change in the stacking sequence over a few atomic spacings produces a Stacking Fault. A Stacking fault is a one or two-layer interruption in the stacking sequence of atom planes.

ii)Twin Boundaries A change in the stacking sequence over many atomic spacings produces a Twin Region. A twin boundary is a defect that introduces a plane of mirror symmetry in the ordering of a crystal.

iii)Grain Boundaries Grain boundaries occur where the crystallographic direction of the lattice abruptly changes. This usually occurs when two crystals begin growing separately and then meet. The orientation of the crystal structure is different for each adjoining grain. The atoms are so close at some locations in the grain boundary that they cause a region of compression, and in other areas they are so far apart that they cause a region of tension. Grain boundaries limit the lengths and motions of dislocations.

4.Bulk / Volume Defects Bulk defects occur on a much bigger scale than the rest of the crystal defects.

i)Voids Voids are regions where there are a large number of atoms missing from the lattice. Voids can occur for a number of reasons. When void occur due to air bubbles being trapped when a material solidifies, it is commonly called Porosity. When a void occurs due to the shrinkage of a material as it solidifies, it is called Cavitation.

ii)Precipitates Impurities can cluster together to form small regions of a different phase. These are often called Precipitates. The term ‘phase’ refers to that region of space occupied by a physically homogeneous material.

The importance of defects depends upon the material, type of defect and properties which are being considered.  If an interstitial impurity forms polar covalent bonds to the host atoms, the layers are prevented from sliding past one another, even when only a small amount of the impurity is present. 

For example, because iron forms polar covalent bonds to carbon, the strongest steels need to contain only about 1% carbon by mass to substantially increase their strength.

 Virtually, all the coloured gems used in jewellery are due to substitutional impurities in simple oxide structures.  Substitutional impurities are also observed in molecular crystals if the structure of the impurity is similar to the host and they can have major effects on the properties of the crystal.  Silicon crystals are doped with elements of Group-III or V to form semi-conductors.  Presence of impurity leads to the lowering of Melting points.  Many microwave diodes, which are devices that allow a current to flow in a single direction, are composed of materials with Schottky defects.  In response to an applied voltage, the cations in solid electrolytes can diffuse rapidly through the lattice, creating Frenkel defects as the cations migrate. Because the electrolyte cannot leak, it cannot cause corrosion, which gives a battery that uses a solid electrolyte a significant advantage over one with a liquid electrolyte.  Generally, the higher the concentration of impurities, the more effectively they block migration, and the stronger the material. For example, Bronze, which contains about 20% tin and 80% copper by mass, produces a much harder and sharper weapon than does either pure tin or pure copper. Similarly, pure gold is too soft to make durable jewellery, so most gold jewellery contains 75% (18 carat) or 58% (14 carat) gold by mass, with the remainder consisting of copper, silver, or both.

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 As the density of defects increases, the metal becomes more brittle (less malleable).  Screw-type Dislocation drives the growth of Zinc Oxide Nano-tubes which are used in micro-electronics, solar powers, batteries etc...  Dislocation in crystals are fundamental to the growth and are characteristics of all crystalline materials.  The compound NiTi, popularly known as “memory metal” illustrates the importance of deformations. 

Memory metal has many practical applications, such as its use in temperature-sensitive springs that open and close valves in the automatic transmissions of cars.

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Because NiTi can also undergo pressure- or tension-induced phase transitions, it is used to make wires for straightening teeth in orthodontic braces and in surgical staples that change shape at body temperature to hold broken bones together.

 Another flexible, fatigue-resistant alloy composed of titanium and nickel is Flexon. Flexon is used as a durable, corrosion-resistant frame for glasses, among other uses....