Defects in Solids

Following are the three main types of defects which are
found in the arrangement of crystal atoms:

1.  Point defects    
2.  Line defects
3.  Surface defects


POINT DEFECTS
Point defects take place due to
     - imperfect packing of atoms during crystallization 
     - vibrations of atoms at high temperatures

Following are the different types of point defects :

Vacancies

Whenever one or more atoms are missing from a normally occupied position, as shown in figure (a), the defect caused is known as vacancy. 

Interstitial Defects

Whenever an extra atom (generally smaller than the parent atoms) occupies interstitial position (i.e. voids) in the
crystal system,  without dislodging the parent atom as shown in figure (b), the defect caused is known as interstitial defect.

Frenkel Defect

Frenkel defect is a combination of vacancy and interstitial defect. Whenever a missing atom (responsible for vacancy) occupies interstitial position as shown in figure (c), the defect caused is known as Frenkel defect. This defect is more common is ionic, being smaller in size, get lodge easily in the interstitial positions.

Substitutional Defect

Whenever a foreign atom (i.e. other than the parent atoms) occupies a position, which was initially meant for a parent atom, as shown in figure (d), the defect caused is known as substitutional defect.

Schottky defect 

When a pair of positive and negative ions is missing from a crystal, as shown in figure(e), the defect caused is known as Schottky defect. 

ASSIGNMENT
CRYSTAL STRUCTURE
Q.1. (AMIE W05, 10 marks):  Calculate the volume of an FCC unit cell in terms of atomic radius, R. Show tha
the atomic packing factor of FCC unit cell is more than that of BCC. 

DEFECTS IN SOLIDS

Q.2. (AMIE S97): Distinguish between Schottky defect and Stacking fault. 
Q.3. (AMIE W05, 5 marks): Differentiate between Frenkel pairs and Shottkey defects.
Q.4. (AMIE S92): What are the imperfections found in metal crystals? What is the role o f these imperfections
on the properties of metals?
Q.5. (AMIE S05, 6 marks): What is a Burger's vector? Show it by drawing a Burger circuit.

DIFFUSION IN SOLIDS

Q.6. Explain various diffusion mechanisms.
Q.7. (AMIE W05, 5 marks):  Explain why interstitial atoms such as C in Fe, can diffuse more rapidly
compared to vacancies. 
Q.8. State and explain Fick's first and second law.
Q.9. (AMIE S05, 8 marks):  State Fick's laws of diffusion. Given an activation energy, Q  of 142 kJ/mol, for the diffusion of carbon in FCC iron and an initial temperature of 1000 K, find the temperature that will increase the diffusion coefficient by a factor 10. [ R = 8.314 J/(mol.K)]

Hint: D = D0e -Q/RT where D is diffusion coefficient, D 0 is proportionality constant, Q is activation energy and T
is temperature.
Q.10. Explain (i) diffusion coefficients (ii) diffusion couple (iii) diffusion with constant concentration
Q.11. Explain the process of diffusion in grain boundaries and surface diffusion. 
 
Franky L. Duque Ayala
15.990.445
CRF
http://www.amiestudycircle.com/sample_4.pdf

 

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