This set of Nanotechnology Multiple Choice Questions & Answers (MCQs) focuses on “Nano Crystalline Structure – Set 2”.
1. How can nanocrystalline materials help in the field of aerospace?
a) Provide deformation characteristics
b) Provide high fatigue strength
c) Provide higher resolution
d) Provide reduced speed
View Answer
Explanation: Nanocrystalline materials are strong and tough and are even able to work at higher temperatures, enabling the aircrafts using such materials for its construction to fly faster and more efficiently. Moreover, they offer the aircraft components higher fatigue strength. This is attributed to the small grain size of the materials which increase the fatigue life by an average of 200-300%.
2. How can the kinetics of isothermal grain growth be represented in conventional polycrystalline materials?
a) d2–d20 = Kt
b) V = I–[(d-Δ)/d]3
c) K = K0 exp(-Q/RT)
d) V = [3Δ(d-Δ)2]/d3
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Explanation: The equation to represent the kinetics of isothermal grain growth in conventional polycrystalline materials is d2–d20 = Kt where d is the grain size at the time t, d0 is the mean initial grain size and K is a constant.
3. Find the correct order of nanocrystalline material production using gas condensation method.
a) Loss of kinetic energy ➔ Accumulation of loose powder ➔ Evaporation of metal ➔ Scraping followed by compaction of powder ➔ He introduced in UHV chamber
b) He introduced in UHV chamber ➔ Evaporation of metal ➔ Loss of kinetic energy ➔ Accumulation of loose powder ➔ Scraping followed by compaction of powder
c) Evaporation of metal ➔ Accumulation of loose powder ➔ He introduced in UHV chamber ➔ Scraping followed by compaction of powder ➔ Loss of kinetic energy
d) Scraping followed by compaction of powder ➔ He introduced in UHV chamber ➔ Accumulation of loose powder ➔ Loss of kinetic energy ➔ Evaporation of metal
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Explanation: The gas condensation method begins with the evaporation of metals inside an ultra-high vacuum (UHV) chamber that has been previously filled with helium gas. This results in Interatomic collisions with helium (He) atoms and hence the metal atoms lose their kinetic energy. They then condense to form small crystals of loose powder that accumulates on a vertical liquid nitrogen filled cold finger. Scraping off and compaction of the powder from the cold finger is carried out under UHV conditions to maintain cleanliness of the particle surfaces.
4. What is the magnetocaloric effect?
a) Temperature change of a material owing to its exposure to changing magnetic field
b) Influence of ferromagnetic transition temperature on the surface magnetization of materials
c) Optimization of soft magnetic properties of nanocrystalline alloys for their applications
d) Magnetic field annealing to enhance the performance of magnetic devices and materials
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Explanation: Magnetocaloric effect is a magnetic property shown by the nanocomposites. These materials containing extremely small magnetic particles when placed in a magnetic field, results in the alignment of the magnetic spin of the particulates with the field. The process, if carried out adiabatically, is referred to as adiabatic demagnetization. This would offset the reduction in spin entropy by an increase in lattice entropy. Thus, the specimen’s temperature is risen, denoted by ΔT. This is known as the magnetocaloric effect.
5. Which of the given methods is used to synthesize 3-D equiaxed nanocrystallites?
a) Vapour deposition
b) Sputtering
c) Spray conversion
d) Electrodeposition
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Explanation: There are several methods employed to produce 3D equiaxed nanocrystallites such as mechanical alloying, chemical precipitation and spray conversion. Vapour deposition, sputtering and electrodeposition techniques are however used for synthesizing 1D nanocrystallites.
6. Mechanical alloying falls under which category of synthesis of nanocrystalline materials.
a) Solution processing
b) Solid state processing
c) Vapour phase processing
d) Liquid Processing
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Explanation: Mechanical alloying using high energy ball mills is an example of the solid-state processing method of nanocrystalline material formation. This method however cannot produce soft magnetic, low coercivity nanocrystalline alloys, owing to the significant introduction of internal strain into the highly magnetostrictive material.
7. Choose the correct expression to indicate the critical grain size of nanocrystalline materials.
a) Lc = 3(I-ϑ)H
b) Lc = GH/[π(I-ϑ)]
c) Lc = 3Gb/[π(I-ϑ)H]
d) Lc = Gbπ(I-ϑ)
View Answer
Explanation: The value of the critical grain size of nanocrystalline materials can be calculated by equating the repulsive force between the dislocations and the applied stress using this relation,
Lc = 3Gb/[π(I-ϑ)H]
where Lc is the critical grain size, G is the shear modulus, b is the Burgers vector, H is the hardness of material and ϑ is the Poisson’s ratio.
8. What is the major disadvantage of the mechanical alloying technique for the fabrication of nanocrystalline materials?
a) Small scale production
b) Contamination of powder
c) Highly expensive procedure
d) Quite slow method
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Explanation: Mechanical alloying is one of the most recognized commercial processes of nanocrystalline material production. It is a relatively inexpensive method but has a serious limitation. During the milling the powder gets contaminated. This happens due to the fine size of the particles, atmosphere, and the addition of process control agents to the milling material.
9. Why is it proposed to use nanocrystalline materials as spark plugs in automobiles?
a) Release substantial amount of effluents
b) Shorter service life of the plug
c) Incomplete combustion of fuels
d) Erosion and wear resistant
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Explanation: It is proposed to make use of nanocrystalline materials as spark plugs due to the various advantageous properties they exhibit which include hard, strong, tough, erosion-resistance and wear-resistance. Further, the electrodes used increase the longevity of the spark plugs while burning the fuel completely and far more efficiently than the traditional spark plugs.
10. In mechanical alloying method for fabrication of nanocrystalline materials, majority of the work is performed in shaker mills.
a) True
b) False
View Answer
Explanation: Mechanical alloying involves repeated welding, fracturing and rewelding of powder materials in a ball mill. In this process, pre-alloyed powders are grinded under a very protective atmosphere in attrition mills, vibrating mills and shaker mills. A majority of the work on the nanocrystalline materials is carried out in highly energetic shaker mills.
11. What gives nanocrystalline metals exceptional yield strength?
a) Motion of dislocations
b) Undergo strain-hardening
c) Thermal stability
d) Presence of grain boundaries
View Answer
Explanation: Nanocrystalline metals have high proportions of grain boundaries. The exceptional yield strength of nanocrystalline metals is attributed to grain boundary strengthening since these grain boundaries are very effective in blocking the motion of dislocations.
12. Which of the following is responsible for the reduced activity of nanocrystalline catalysts?
a) Rutile structure
b) High surface area
c) Oxygen deficient composition
d) Reduced anion vacancies
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Explanation: Nanocrystalline materials have few unique features that increase its catalytic nature. These are high surface area, rutile structure and its oxygen deficient composition. Annealing the material in an oxygen atmosphere would reduce the number of anion vacancies and ultimately lower its activity.
13. Which of the following is not a limitation of the Hell-Petch relationship?
a) Strength cannot increase beyond the theoretical strength limit
b) At nanoscale individual grains cannot support more than one dislocation
c) Relaxation occurring at grain boundaries result in decrease of strength
d) Annealing a sample results in greater hardness of the material
View Answer
Explanation: The Hall Petch relationship has certain drawbacks. First, beyond the theoretical strength limit, it is not possible to increase the strength value. Second, the relationship was derived based on the strengthening due to dislocation pile-ups at physical obstacles. Third, below some critical grain size inverse Hall-Petch relationship works. This means there is a decrease in strength if any relaxation takes place at the grain boundaries. Fourth, individual grains cannot support more than one dislocation at nanoscale making the relationship invalid.
14. Grain boundaries do not influence the mechanical properties of nanocrystalline materials.
a) True
b) False
View Answer
Explanation: Nanocrystalline materials are known to show good mechanical properties compared to the polycrystalline varieties. This is owing to the higher volume fraction of grain boundaries in nanocrystalline materials. The mechanical properties are significantly influenced by this amorphous grain boundary phase.
15. Which of the following methods can estimate the grain size of a nanocrystalline sample?
a) X-Ray diffraction
b) Spectroscopy
c) Williamson–Hall plot
d) Solution processing
View Answer
Explanation: Nanocrystalline materials are single phase or multiphase polycrystalline materials having their crystallite size in the range of nanometers. The grain size of these nanocrystalline materials can be estimated by the help of X-ray diffraction method.
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