MChem Master of Chemistry Materials Chemistry 2 — Questions and Answers
Question 1: What is the Bragg equation used for in materials characterization?
- Determining the molecular weight of polymers
- Relating X-ray diffraction angles to interplanar spacings (d-spacings) in crystals (Correct answer)
- Measuring the hardness of ceramic materials
- Calculating the surface area of porous materials
Correct answer: Relating X-ray diffraction angles to interplanar spacings (d-spacings) in crystals
Bragg's Law (nλ = 2d sinθ) relates the diffraction angle of X-rays to the spacing between crystal planes, enabling determination of crystal structure.
Question 2: In semiconductor materials, what is the significance of the band gap energy?
- It determines the electrical conductivity by controlling electron excitation from valence to conduction band (Correct answer)
- It measures the bond dissociation energy of the semiconductor lattice
- It defines the melting point of the semiconductor
- It is the energy required to remove a surface atom
Correct answer: It determines the electrical conductivity by controlling electron excitation from valence to conduction band
The band gap determines how much thermal or photon energy is needed to excite electrons into the conduction band, governing optical absorption, photoconductivity, and device applications.
Question 3: What is the purpose of annealing in materials processing?
- Rapidly quenching a material to trap a high-temperature phase
- Heating a material and slowly cooling it to reduce defects and internal stresses (Correct answer)
- Adding alloying elements at high temperature
- Applying mechanical stress to increase hardness
Correct answer: Heating a material and slowly cooling it to reduce defects and internal stresses
Annealing involves controlled heating and slow cooling to allow atomic diffusion, eliminating dislocations, grain boundary defects, and residual stresses.
Question 4: Which property of carbon nanotubes makes them attractive for reinforcing composite materials?
- Their high density and optical transparency
- Their exceptionally high tensile strength (up to ~100 GPa) combined with low density (Correct answer)
- Their hydrophilicity enabling bonding with polymer matrices
- Their high thermal conductivity alone
Correct answer: Their exceptionally high tensile strength (up to ~100 GPa) combined with low density
Carbon nanotubes have tensile strengths ~100 times that of steel at one-sixth the weight, making them ideal candidates for lightweight high-strength composite reinforcement.
Question 5: What is the glass transition temperature (Tg) and why is it important for polymer materials?
- The temperature at which a crystalline polymer melts
- The temperature below which an amorphous polymer transitions from rubbery to glassy behavior (Correct answer)
- The temperature at which a polymer begins to degrade
- The melting point of the crystalline domains in a semi-crystalline polymer
Correct answer: The temperature below which an amorphous polymer transitions from rubbery to glassy behavior
Tg marks the onset of large-scale segmental motion; below Tg, amorphous polymers are rigid and brittle, above Tg they are soft and rubbery.
Question 6: What is atomic layer deposition (ALD) and what advantage does it offer over conventional chemical vapor deposition (CVD)?
- ALD deposits thick films rapidly from a gas phase mixture
- ALD uses sequential self-limiting surface reactions to deposit films with atomic-level thickness control and conformality (Correct answer)
- ALD uses physical sputtering from a solid target
- ALD requires higher temperatures than CVD processes
Correct answer: ALD uses sequential self-limiting surface reactions to deposit films with atomic-level thickness control and conformality
ALD alternates self-limiting precursor and reactant pulses, depositing exactly one atomic layer per cycle with sub-angstrom precision and perfect step coverage on complex structures.
MChem Master of Chemistry Materials Chemistry 2
What is the Bragg equation used for in materials characterization?