Materials Science Problems on “Point Defects – 2”.
1. Which thermodynamic property increases in a crystalline solid due to the presence of vacancies?
a) Enthalpy
b) Entropy
c) Internal energy
d) Work done
Answer: b
Clarification: Presence of vacancies in crystalline solid creates randomness in structure due to which entropy increases as it is directly proportional to the number of vacancies per mole of solid.
2. Which type of diffusion occurs due to the exchange of an atom with vacancies?
a) Substitution diffusion
b) Elimination diffusion
c) Passive diffusion
d) Facilated diffusion
Answer: a
Clarification: Substitution as the name suggests it deal with substituting an atom or element in the empty gaps present in crystalline solid which are commonly called as a vacancy.
3. Which of the following technique is not used to measure the concentration of vacancy to determine the activation energy for its formation?
a) Positron annihilation technique
b) Thermal expansion measurements
c) Measuring resistivity after quenching
d) Thermal imaging
Answer: d
Clarification: Thermal imaging technique is used to determine the position of the defect. It gives the information about depth at which defect will be located. It does not give details about the concentration of vacancy.
4. Calculate temperature of copper at which equilibrium number of vacancies per cubic meter is 2.2*1020 vacancies/m3. The energy of vacancy formation is 0.7ev/atom and total number of atom is 2*1030atom/m3.
a) 3541.4k
b) 300k
c) 354.14k
d) 2.82*10-3k
Answer: c
Clarification: The equilibrium number of vacancies Nv for a given quantity is given by
Nv = Nexp(-Qv/KT)
T = -Qv/(K*ln(Nv/N))
T = -0.7/(8.62*10-5*ln(2.2*1020/(2*1030)))
T = 354.14k.
5. Which of the following is not the feature of solute and solvent atoms that determine the degree to which former dissolve in latter?
a) Corrosion factor
b) Atomic size factor
c) Crystal structure
d) Valences
Answer: a
Clarification: Corrosion factor determines the rate at which a metal can be corroded it is not related to the solubility of solute and solvent.
6. How is the concentration of defect related with free energy?
a) The concentration of defect increases free energy
b) The concentration of defect decrease free energy
c) The concentration of defect is equal to free energy
d) The concentration of defect is not related to free energy
Answer: a
Clarification: Point defects are equilibrium defect varies inversely with free energy.
7. In which type of defect smaller cation can get displaced into an interstitial void?
a) Schottky defect
b) Intrinsic defect
c) Extrinsic defect
d) Frankel defect
Answer: d
Clarification: In some compounds where cation anion difference is large cation being smaller compared to anion tends to displace from its original position to the other position. This type of defect is called as Frankel defect.
8. Which of the following phenomenon creates point defect in ceramics?
a) Thermal excitation
b) Precipitation
c) Densification
d) Electrical conductivity
Answer: a
Clarification: In ceramics on providing external energy valence electrons get excited creating a point defect in ceramics. This phenomenon is known as the thermal excitation.
9. Which of the following oxides are highly defective?
a) Al2O3
b) MgO
c) CaO
d) FeO
Answer: d
Clarification: FeO is a transition metal oxide. Now in this case FeO is cation deficient due to presence of at least 5% of iron in Fe3+ state.
10. In case of point defect in polymers probability of which part of the polymer is likely to be defected more compared to other?
a) Center of polymer chain
b) Chain ends
c) Main chain units
d) Starting part of chain
Answer: b
Clarification: Chain ends being the outer most part of the polymeric chain the probability of formation of vacancies in this part is more hence they are more defected compared to other parts.
Materials Science Problems,