This set of Nanotechnology Multiple Choice Questions & Answers (MCQs) focuses on “Nano Crystalline Structure”.
1. Pick out the odd one from the following:
a) Scherrer equation
b) Warren – Averbach method
c) Arrhenius equation
d) Williamson – Hall plot
View Answer
Explanation: In nano crystalline materials with very small grain size, the diffraction peaks get broadened. This phenomenon is related to the crystallite size of these nano crystals using Williamson-Hall plot, Scherrer equation, Warren – Averbach method or any sophisticated computer modeling of the diffraction pattern.
2. Metallic nanocrystalline foils can be fabricated using which of these methods?
a) Injection molding
b) Thermoforming
c) Electrodeposition
d) Flame synthesis
View Answer
Explanation: Electrodeposition process is carried out to form metallic nanocrystalline foils. This method is categorized as a solution processing method for production of nanocrystalline materials. It is a very common method that forms metal coatings by the use of electric current on a conductive material that remains immersed in a solution containing a salt of the metal that is deposited.
3. Select the correct statement from the given options.
a) Specific heat of a material is not related to its atomic structure
b) Optical transparency of nanocrystals remain constant even after altering the processing parameters
c) Specific heat of a material can be associated with its vibrational entropy
d) Bulk materials have lesser saturation magnetization compared to nanocrystalline materials
View Answer
Explanation: Specific heat of a material is directly related to its atomic structure. It is closely related to the vibrational and configurational entropy of the material. This is considerably affected by the nearest neighbour configurations. Optical transparency of nanocrystalline materials can be altered by changing the processing parameters. Bulk materials have greater saturation magnetization (Ms) and ferromagnetic transition temperature than nanocrystalline materials.
4. Pick out the odd one from the following compounds.
a) CoCl2
b) Co(NO3)2
c) Co(CH3COO)2
d) Cr3C2
View Answer
Explanation: The spray conversion method for production of WC-Co nanocrystalline composites applies precursors in aqueous solution. These are ammonium metatungstate [(NH4)6(H2W12O40)4H2O] and CoCl2, cobalt nitrate [Co(NO3)2], and cobalt (II) acetate [Co(CH3COO)2]. Cr3C2 is an inhibitor added to prevent the grain growth of tungsten.
5. What is the expression for the Hall-Petch relationship?
a) σy = σ0 + Kd-1/2
b) ΔG = ΔH – TΔS
c) γ = F / d
d) B = μ0 / 2πr
View Answer
Explanation: The Hall Petch relationship is given as:-
σy = σ0 + Kd-1/2
where σy is the yield stress, σ0 is a material specific constant that can account for effects of all other strengthening mechanism, K is a material specific constant that describes the magnitude of the metal’s response to grain size strengthening and d is the average grain size.
6. Super plasticity in nanocrystalline ceramics is observed because of which of these reasons?
a) Intergranular deformation
b) Dislocation pileup at GBs
c) Large volume fraction of GBs
d) Intragranular deformation
View Answer
Explanation: Super plasticity is a unique behavior observed in ceramic systems. This is attributed to the large volume fraction of grain boundaries associated with nanocrystalline materials. This allows for a significant diffusional flow of atoms via Coble Creep.
7. What is the most favorable method for producing thin films of nano crystalline materials?
a) Electrodeposition
b) MOCVD
c) Solidification
d) Annealing
View Answer
Explanation: The metalorganic chemical vapour deposition (MOCVD) is the most favorable method, under vapour phase technique, for the fabrication of thin films of nano crystalline materials. This technique involves the growth of crystalline layers to create complex multilayer structures.
8. Electrical conductivity of nano crystalline materials is quite low compared to polycrystalline materials.
a) True
b) False
View Answer
Explanation: Nanocrystalline materials have higher electrical resistivity or lower conductivity than polycrystalline materials. This is due to their increased volume fraction of atoms lying at the grain boundaries.
9. What is responsible for the thermal instability of nanocrystalline metals?
a) Greater interfacial energy
b) Solute segregation
c) Lesser grain boundaries
d) Decrease in grain size
View Answer
Explanation: A large fraction of grain boundaries often involves larger amounts of interfacial energy. This leads to greater thermal instability. However, materials with higher melting points are more stable at room temperatures, but their aggregation into macroscopic component often results in elevated temperatures for extended periods of time.
10. Which of the following is a liquid processing method for nanocrystalline synthesis?
a) Transmission electron microscopy
b) X-ray diffraction
c) Electrodeposition
d) Melt spinning
View Answer
Explanation: Melt spinning is a method used for synthesizing nanocrystalline metals by rapid solidification from the liquid. It categorizes as a liquid processing technique. It often produces an amorphous metal, which can be transformed into the nanocrystalline form by annealing above its crystallization temperature.
11. What is the expression for strain rate during diffusional creep?
a) dꜪ/dt = (B1DV+B2ΔDb)ρkT/4π
b) dꜪ/dt = (σΩ/d2KT)[B1Dv+(B2ΔDb/d)]
c) dꜪ/dt = (B1Db+σΩ/d2)B2Dv
d) dꜪ/dt = (4ρΩd2/απkT) [B1+B2Dv]
View Answer
Explanation: The expression for strain rate during diffusional creep is given by
dꜪ/dt = (σΩ/d2KT)[B1Dv+(B2ΔDb/d)]
where dꜪ/dt is strain rate, σ is the applied stress, d is the grain size, k is Boltzmann constant, T is the temperature, Ω is the atomic volume, B1 and B2 are some constants, Dv is the volume diffusion coefficient, Db is the grain boundary coefficient and Δ is the grain boundary width.
12. Why do nanocrystalline materials show variable transparencies?
a) Larger pores than the wavelength of light
b) Enough scattering of light
c) Variable degrees of quantum confinement
d) Little aggregation between the nanocrystals
View Answer
Explanation: Nanocrystalline materials show different transparencies which can be controlled by altering the processing parameters. This phenomenon is observed because there is very little aggregation between the nanocrystals in the specimen, accordingly the pores in it that have size smaller than the wavelength of visible light do not scatter light rendering the samples fairly transparent.
13. Identify a property that is not observed in nanocrystalline materials having high diffusivity?
a) Formation of intermetallic phase
b) Elimination of creep
c) Increased solubility limits for solids
d) Increased sinterability
View Answer
Explanation: Enhanced diffusivity in nanocrystalline materials leads to significant effects on their mechanical properties. It increases the solid solubility limits, effects creep and super plasticity forms intermetallic phases, increases the sinterability of nanocrystalline powders and enhances the ability to efficiently dope these materials with impurities at quite low temperatures.
14. Why does an excessive decrease in gain size weaken the nanocrystalline material?
a) Decrease in grain boundary area
b) Excessive strain-hardening
c) Increased grain boundary sliding
d) Intragranular dislocation motion
View Answer
Explanation: Grain size of a nanocrystalline material decreases until a critical size is reached at which grain boundary sliding becomes energetically favorable than intragranular dislocation motion. Any further decrease in size beyond the critical grain size weakens the material due to the increase in grain boundary area which leads to increased grain boundary sliding.
15. Why is any abnormal grain growth not observed in coarse grained polycrystalline materials?
a) Impurity segregation
b) Reduced driving force
c) Enhanced grain boundary enthalpy
d) Non-uniformity
View Answer
Explanation: Abnormal grain growth is not observed in coarse-grained poly-crystalline materials. This is due to the enhanced grain boundary enthalpy that leads to high driving force. Also the non-equilibrium grain boundary structure in the nanocrystalline state is responsible for no abnormal grain growth.
Sanfoundry Global Education & Learning Series – Nanotechnology.
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