Ceramics and Glasses Questions and Answers

Answer: D

The match is P-3, Q-2, R-5, S-1.

• Titanium diboride melts above 3200°C and is classed among the ultra-high temperature ceramics.
• Molybdenum disilicide forms a protective silica layer when hot and is the standard element for furnaces running up to about 1800°C.
• Hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂, is the mineral of bone and is used for bone implants and coatings.
• Nanocrystalline titanium dioxide (anatase) is the classic photocatalyst.

'Tough ceramic' fits none of these four (it suits materials such as zirconia), which is why it is the extra item. Options putting MoSi₂ as a tough ceramic or TiB₂ as an implant are wrong.

Answer: C

The true statement is that glass has higher entropy than the corresponding crystalline phase and lower entropy than the corresponding liquid phase.

A glass is a liquid whose structure was frozen in at the glass transition. Just below \(T_{g}\) it keeps the disorder of the liquid at \(T_{g}\), so its entropy (and enthalpy) is well above that of the ordered crystal, but below that of the equilibrium supercooled liquid at the same temperature, which would have continued to lose entropy only if it could relax.

For the same reason the glass's enthalpy lies between the crystal's and the liquid's, so it is neither higher nor lower than both, and its entropy cannot be below the crystal's.

Answer: B

A crystalline ceramic is best described as brittle.

Ceramics are bonded ionically or covalently. Their dislocations are few and hard to move, because slip would bring like charges together or break directional bonds, so they cannot deform plastically at room temperature. They fail by fast fracture from small flaws, with little or no plastic strain, which is brittle behaviour.

Ductility is typical of metals, with their mobile dislocations. Viscoelastic and viscoplastic behaviour describe time-dependent flow, typical of polymers and of glasses or metals at high temperature, not crystalline ceramics under ordinary conditions.

Answer: C

Grinding media are chosen for their hardness relative to the powder.

Ball milling reduces particle size by impact and attrition, which works only if the media are harder than the particles being ground; otherwise the balls wear away and contaminate the powder. Hard media such as zirconia, alumina or tungsten carbide are therefore matched to the ceramic being milled.

Thermal conductivity and dielectric constant play no part in comminution. Density does affect the impact energy of the balls, but the selection criterion asked for here, the difference between media and powder, is hardness.