This set of Nanotechnology Multiple Choice Questions & Answers (MCQs) focuses on “Carbon Nanotubes – Mechanical Properties and Thermal Stability”.
1. Why are the mechanical properties of bundles of nanotubes weaker than those of SWCNTs?
a) Susceptible to buckling on application of stress
b) Highly resilient to bending
c) Lower aspect ratio of CNTs
d) Unequal load between the tubes
View Answer
Explanation: The mechanical properties of bundles of nanotubes are weaker than those of single tubes because of the difficulty of sharing the load equally between all the tubes. However, upon twisting the bundle of tubes into a nanorope can increase these mechanical properties by improving the mechanical coupling between the tubes.
2. Why are MWCNTs more stable than SWCNTs?
a) Greater curvature-induced energy in MWCNTs
b) Existence of dispersion – type forces
c) Greater strain energy in MWCNTs
d) Absence of intershell spacing in MWCNTs
View Answer
Explanation: Nanotubes are stable when they are wider in size. As, MWCNTs are normally wider than SWCNTs they tend to be more stable. Moreover, the intershell spacing in MWCNTs is similar to graphite and dispersion – type forces exist that further stabilizes the nanotubes.
3. What happens when SWCNTs merge?
a) Reduction of vacancies on nanotubes
b) Reduction in thermal stability of nanotubes
c) Reduction in intershell spacing of nanotubes
d) Reduction in strain energy on nanotubes
View Answer
Explanation: Studies reveal that pairs of SWCNTs coalesce to form wider SWCNT. They can even form an entire bundle of SWCNT that can merge to form an individual MWCNT. As a consequence there is a reduction in the strain energy on nanotubes.
4. What happens when CNTs are subjected to compression?
a) Weak shear interactions
b) Cross linking of inner shells and tubes
c) High – energy electron irradiation
d) Undergo buckling
View Answer
Explanation: CNTs are known to be the strongest and stiffest material yet discovered. But they are not nearly as strong under compression. They tend to buckle when placed under compressive, torsional or bending stress. This happens due to their hollow structure and the high aspect ratio.
5. Which among the following is the correct order of stability in carbon nanoscale objects?
a) Fullerenes > Graphite > SWCNTs > MWCNTs
b) MWCNTs > Fullerenes > SWCNTs > Graphite
c) Graphite > MWCNTs > SWCNTs > Fullerenes
d) SWCNTs > Graphite > MWCNTs > Fullerenes
View Answer
Explanation: Incase of carbon nanostructured objects, both MWCNTs and SWCNTs are metastable with respect to graphite. MWCNTs are more stable than SWCNTs owing to wider structure and dispersion forces between shells. However, fullerene is the least stable among all these structures.
6. CNTs are really soft in the radial direction.
a) True
b) False
View Answer
Explanation: It has been experimentally proven that CNTs are soft in the radial direction. Research on radial elasticity suggests that weak forces like Van der Waals can even deform two adjacent CNTs.
7. Identify the correct expression for the Young’s modulus of CNT.
a) Ŷ = (1/N) (∂2E/∂∈2)∈=0
b) Ŷ = (∂2E/∂∈2)∈=0
c) Ŷ = N2(∂2E/∂∈2)∈=0
d) Ŷ = (N/3) (∂2E/∂∈2)∈=0
View Answer
Explanation: Young’s modulus can be correctly denoted by Ŷ = (1/N) (∂2E/∂∈2)∈=0 where E is the free energy of the system, ϵ is the axial strain and N is the number of atoms. This expression is used regardless of the actual geometry of the system.
8. Crystallographic defects in CNTs do not affect the tube’s thermal properties.
a) True
b) False
View Answer
Explanation: The thermal properties of CNTs can be affected due to the presence of crystallographic defects. These defects lead to phonon scattering that increase the relaxation rate of the phonons. This reduces the mean free path as well as the thermal conductivity of the nanotubes.
9. What is the mechanism followed by MWCNTs during breaking on application of stress?
a) Scooting mechanism
b) Sword – in – sheath mechanism
c) Winding mechanism
d) Radical ion mechanism
View Answer
Explanation: MWCNTs are observed to break in a sword-in-sheath mechanism. The failure is restricted to the outer shell, while the inner shells remain unaffected. During breaking of the outer tube, the inner shells simply slide out as a sword pulled from its sheath.
10. Why don’t CNTs fracture when they are bent severely?
a) Presence of defects in structure
b) No changes in hexagonal carbon ring structure
c) Rehybridization of sp2 bonds
d) Presence of sp3 hybridized bonds
View Answer
Explanation: CNTs when subjected to extreme bending do not fracture. This is mainly due to two reasons; i) they have very few defects in their structure, ii) when bent excessively the hexagonal carbon rings of the walls of CNT change in structure that is sp2 bonds get rehybridized.
Sanfoundry Global Education & Learning Series – Nanotechnology.
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