Internal structure and properties

Explain what the three main types of atomic bonding (ionic, covalent and metallic) consist of and give a representative material example for each, indicating a property of that material that is explained by its bonding.

Copper conducts electric current and polyethylene does not. Explain this difference using the "sea of electrons" model of metallic bonding and the structure of polymers.

Predict, reasoning from the type of bonding, the behaviour of these three materials: common salt (NaCl), copper (Cu) and quartz (SiO₂). For each one, state whether you expect it to conduct electric current in the solid state, whether it will have a high or low melting point, and whether it will be brittle or ductile.

The figure shows the unit cell of a crystalline metal: there are atoms at the eight vertices of the cube and an additional atom inside it. State the name of this crystal lattice and explain, based on it, why metals are usually dense and deformable.

Explain using the crystal lattice model why a metal can be rolled into thin sheets without breaking while an ionic crystal (such as salt) shatters when struck: what happens to the atomic planes and the charges in each case?

Qualitatively compare body-centred cubic (BCC) and face-centred cubic (FCC) lattices: how many atoms each atom touches (coordination number) and which typical metals crystallise in each (iron at room temperature versus aluminium or copper).

Explain the difference in internal structure between a thermoplastic polymer (independent chains) and a thermosetting polymer (cross-linked chains), and reason why the former can be recycled by melting while the latter cannot.

Diamond and graphite are both composed solely of carbon atoms, yet the former is the hardest natural material and the latter is used as a lubricant and in pencil leads. Explain this difference based on how the carbon atoms are bonded and arranged in each structure.

A glass bottle and a quartz crystal have very similar chemical compositions (both silica-based), but glass is amorphous while quartz is crystalline. Explain what these two terms mean and reason why glass breaks into irregular fragments whereas quartz exhibits flat faces.

Distinguish between a substitutional alloy and an interstitial alloy at the atomic level: what relative size the added atoms must have in each case, and a real example of each type (bronze with tin versus steel with carbon).

Steel is iron with up to $2\,\%$ carbon. Explain qualitatively what ferrite and cementite (iron carbide) are in its microstructure and reason why steel with a higher carbon content is harder but less weldable.

For the manufacture of friction bushings subjected to load, a quality report shows that Cu-Sn bronze (10 wt% Sn) exhibits a significantly higher yield strength than annealed pure copper. An operator proposes replacing tin with zinc in the same proportion (Cu-Zn brass at 10 wt% Zn), claiming that the strengthening will be exactly the same because both are substitutional solid solutions.

1. a) Given the atomic radii ($r_{\text{Cu}} = 128\ \text{pm}$, $r_{\text{Zn}} = 133\ \text{pm}$, $r_{\text{Sn}} = 151\ \text{pm}$), justify using crystal lattice distortion theory and elastic stress fields why the operator's claim is incorrect, predicting which of the two alloys will offer greater resistance to dislocation movement.
2. b) Explain how the ductility and electrical conductivity of copper are affected when adding these solutes, relating both changes to electron scattering and the pinning of dislocation motion.

Three covalent material wafers are analyzed to design a resistive temperature sensor over a range from 150 °C to 350 °C: diamond ($E_g \approx 5,5\ \text{eV}$), silicon ($E_g \approx 1,1\ \text{eV}$), and germanium ($E_g \approx 0,67\ \text{eV}$).

1. a) A technician suggests using germanium doped with phosphorus ($10^{16}\ \text{atoms/cm}^3$). Interpret using the band model why, when raising the temperature above a certain threshold, the intrinsic carrier concentration exceeds that provided by the dopants (intrinsic regime), causing the conductivity to go out of control and invalidating the sensor calibration.
2. b) Predict with reasoning which of the three base materials would offer the greatest stability against uncontrolled thermal generation of electron-hole pairs in this temperature range, and argue whether diamond would behave as a semiconductor or as an insulator at 350 °C considering its bandgap width.

On a coastal pedestrian footbridge, structural carbon steel beams were joined using stainless steel bolts and aluminium alloy rivets. After two years of service, two severe pathologies are observed: the aluminium rivets have disintegrated due to galvanic corrosion, while the steel beams show intense localized pitting at the points where the protective paint layer suffered scratches during assembly.

1. a) Using the galvanic series of potentials and the surface area balance (anode/cathode area ratio), justify the rapid destruction of the aluminium rivets in contact with stainless steel in a saline atmosphere, explaining why their natural alumina ($\text{Al}_2\text{O}_3$) passivation layer was not sufficient to prevent the attack.
2. b) Refute the suitability of the paint coating compared to hot-dip galvanizing (zinc coating). Justify electrochemically why a scratch in the paint accelerates localized corrosion of the base steel, whereas on galvanized sheet the steel remains protected even if the coating suffers a similar gouge.

The structural frame for a lightweight rescue drone is being designed. It must maximize flight endurance, ensure stiffness against wind gusts, and absorb impacts in forced landings without catastrophic failure. Three candidate materials are presented:

• Candidate A (Polyamide 66 with 30% short glass fibre): $\rho = 1,35\ \text{g/cm}^3$, $E = 8,5\ \text{GPa}$, Izod impact strength $= 11\ \text{kJ/m}^2$, low cost, recyclable by thermoforming.

• Candidate B (Forged AZ91D magnesium alloy): $\rho = 1,81\ \text{g/cm}^3$, $E = 45\ \text{GPa}$, yield strength $\sigma_y = 160\ \text{MPa}$, fracture toughness $K_{1c} = 17\ \text{MPa}\cdot\text{m}^{1/2}$, medium cost, recyclable by casting.

• Candidate C (Unidirectional continuous carbon fibre epoxy laminate / CFRP): $\rho = 1,55\ \text{g/cm}^3$, $E = 135\ \text{GPa}$ (in the longitudinal direction), brittle under transverse point impacts, very high cost, not thermally recyclable.

1. a) Calculate the specific stiffness ($E/\rho$) of the three candidates and interpret which one best optimizes the ratio between structural stiffness and mass in flight.
2. b) If the drone must operate in rocky environments with a high risk of accidental collision, justify which material you would select, reasoning the microstructural pros and cons of candidates B and C (ductility and plastic deformation versus elastic-brittle behaviour with delamination). Argue which priority criterion would force you to discard Candidate A despite its lower density and cost.