This set of Materials Science Multiple Choice Questions & Answers (MCQs) focuses on “Point Defects – 1”.
1. Which type of defect are point defects?
a) One dimensional defect
b) Zero dimensional defect
c) Two dimensional defect
d) Three dimensional defect
Explanation: Point defects are zero dimensional defects as they cannot extend in any direction in space. Point defects occur where an atom is missing or misplaced in a crystal lattice. The limit of the size of point defect is not defined.
2. Which of the following point defects can be attributed to from the below options?
i) Stress Fields
ii) Strain fields
a) i) and ii)
b) ii) and iv)
c) i) and iii)
d) ii) and iv)
Explanation: Stress fields are produced due to point dislocation and charges are needed for point dislocation. When an atom dislocates from its original position to an interstitial position or it is completely removed from the lattice, hence creating stress fields.
3. Which is not a name given to vacancy in some ionic solid?
c) Color center
d) Luminescence center
Explanation: Other than H-center which is used when a halogen occupies the interstitial position, all the other terms are used for a vacancy in ionic solid. For some ionic solid when there is a vacancy in crystal, an F-center or color enter or luminescence center is formed due to which the ionic solid exhibits different colors.
4. What is the color of zinc oxide?
Explanation: In zinc oxide, there is a creation of F-centers, due to a vacancy in its lattice structure. As the F-centers absorbs light and shows complimentary color, zinc oxide exhibits yellow as its colour.
5. If the number of F-centers are more, the color of the compound gets more intense.
Explanation: When the number of F-center increases, more light is absorbed from the visible spectra and intense color is obtained. By the same logic when the number of F-centers are less, colors are less intense.
6. F-centers are _________
a) Cathodic vacancy in a crystal occupied by one or more paired electron
b) Cathodic vacancy in a crystal occupied by one or more unpaired electron
c) Anionic vacancy in a crystal occupied by one or more paired electron
d) Anionic vacancy in a crystal occupied by one or more unpaired electron
Explanation: F-centers occur when in a crystal the vacancies are filled by one or more unpaired electron. When there is an anionic vacancy the charge on the whole body becomes positive as an anion is negatively charged. The presence of the unpaired electron makes the body neutral and hence stable.
7. In which of the following defect the density of the crystal is affected?
a) Schottky defect
b) Frenkel defect
c) Stone-Wales defect
d) Antisite defect
Explanation: The density of the solid crystal in case of the Schottky defect is less than the theoretical density of the material. This happens as the total number of ions in the lattice is less than the theoretical number of ions according to its volume when this defect occurs.
8. Schottky and Frenkel defects are ___________
a) Interstitial and vacancy defects respectively
b) Vacancy and interstitial defect respectively
c) Both interstitial defects
d) Both vacancy defects
Explanation: Schottky defect occurs when oppositely charged ions leave the crystal and creating a vacancy and hence is a vacancy defect while in Frenkel defect an atom moves from its original site to an interstitial position and hence is an interstitial defect.
9. Which of the following compound shows both Schottky and frenkel defect?
a) Silver(I) iodide
b) Silver(I) bromide
c) Magnesium sulphide
d) Titanium oxide
Explanation: For a compound to show Schottky defect the size of anion and cation are similar size while Frenkel defect occurs where the size of the anion is larger than the cation. As the radius ratio of silver to bromine is intermediate, it shows both the defects.
10. For the given magnitude of shift which of the following can cause the formation of an off-center ion?
a) 0.1 Å
b) 0.05 Å
c) 0.5 Å
d) 1.1 Å
Explanation: Off center ions are a type of substitutional ion whose equilibrium position is a little shifted from its original lattice site. The magnitude of shift for an off center ion is typically in the range of 0.2Å to 1.0Å.
11. The presence of interstitial carbon atom increases the hardness of which of the following:
Explanation: Martensite is formed in steel when the austenite form of iron is quenched. Due to rapid cooling, interstitial carbons are found in interstitial positions. The interstitial carbon is what makes martensite a hard form of steel.
12. How are point defects classified based on the source of the atom?
a) Statistical and structural
b) Random and ordered
c) Intrinsic and extrinsic
d) Interior and exterior
Explanation: When no foreign atom is present in the crystal lattice and the same kind of atoms are forming the defect then it is called as an intrinsic atom. When there is a presence of an additional atom in the crystal lattice, it is called extrinsic defect and hence they are classified according to the source of the atom.
13. In which of the following case the first element can occupy both lattice position and interstitial position?
a) Cu in FCC-Ni
b) C in FCC-Fe
c) B in steel
d) Al in FCC-Fe
Explanation: For the first element to occupy the lattice position the size of the first element should be like that of the second element while for the first element to occupy interstitial position the size of the first element should be less than the second element. Due to the intermediate size of boron crystal, it can occupy both lattice and interstitial position.
14. At temperatures greater than 570 degree centigrade, which of the following can be a stable composition of iron oxide?
Explanation: Over 570 degree centigrade, the composition of iron oxide is in the form of Fe (1-X) O, where x lies between 0.05 to 0.16. Any value of x other than that will not form a stable composition of iron. We can see that only in Fe0.9O, x is 0.1 which lies between the permissible ranges.
Sanfoundry Global Education & Learning Series – Materials Science.
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