Unit 31: Materials and Energy — Long Questions

A composite is a combination of two or more materials put together to form a product with better overall properties than either component alone. A composite has one component called the matrix, and the other component(s), which are embedded or caged within the matrix, called the reinforcement agent(s); a composite may have more than one type of reinforcement.

Role of the matrix The matrix is the continuous surrounding material that holds the reinforcement in place, binding it together and giving the composite its overall shape.

Role of the reinforcement The reinforcement (e.g. fibres of fabric, glass, or carbon-based materials) provides the composite with high strength.

Because of this combination, composites possess a good strength-to-weight ratio and strong resistance to corrosion, making them suitable for demanding applications. For example, fibreglass is a composite with a polymer matrix and glass-fibre reinforcement, used for boat hulls, car bodies, and insulating walls/ceilings; carbon fibre composites are widely used in aircraft and sports equipment because they are light and strongly durable.

X-rays have a very small wavelength, comparable to the spacing between atoms in a solid, which makes them suitable for probing the positions of atoms and electrons in a crystal. Solid crystalline materials consist of regular layers or strands of atoms. When a beam of X-rays from a source falls on a crystal, it is diffracted after striking the atoms, and this diffraction occurs only at specific angles related to the spacing (d) between the atomic layers, following the relation 2d sinθ = total path length difference. The diffracted rays travel onward and strike a screen or detector, producing a regular arrangement of spots called a diffraction pattern.

Each spot in the diffraction pattern corresponds to X-rays scattered by a particular set of atomic planes, and its position and intensity carry information about where the atoms and their electron density lie in the crystal. By applying mathematical models to analyze the full diffraction pattern (angles and intensities of all the spots), scientists can work backward to construct a detailed map of the atomic positions, bond lengths and bond angles within the crystal — effectively determining the complete three-dimensional structure of the molecule or crystal, as was done for naphthalene and for the DNA double helix.

Metallurgy is the science of extracting metals from their ores and purifying them. The reactivity and nature of the metal determines the exact extraction procedure, but the process generally involves the following steps:

1. Mining: The process of acquiring the metal ores that are buried deep in the earth, e.g. mining of iron ore (haematite).

2. Ore Concentration: The ore is made as free from impurities (gangue) as possible.

3. Conversion of concentrated ore to metal oxide: Done mainly by two methods — (i) Roasting, where a sulphide ore is heated with excess air, e.g. 2ZnS(s) + 3O₂(g) → 2ZnO(s) + 2SO₂(g); or (ii) Calcination, where a carbonate ore is heated with limited or no air, e.g. ZnCO₃(s) → ZnO(s) + CO₂(g).

4. Reduction of metal oxide to metal: This is the most important step. For metals less reactive than carbon, carbon (via CO) is used as the reducing agent, e.g. C + ½O₂ → CO, followed by Fe₂O₃ + 3CO → 2Fe + 3CO₂. For metals more reactive than carbon (e.g. Na, K, Al) or where very high purity is needed (e.g. Cu), an electrolytic method is used, in which the molten compound is electrolyzed and the metal ions gain electrons at the cathode to form the free metal.

5. Refining: Even after reduction most metals are reasonably pure, but special applications (e.g. copper for electrical wiring, which needs 99.99% purity) require a further purification step by thermal, chemical, or electrochemical methods to obtain the pure metal.

6. Alloying (where required): Finally, the metal may be mixed with other metals or non-metals to form an alloy, altering its microstructure to enhance properties such as strength, corrosion resistance, or electrical/magnetic properties, e.g. brass, bronze, steel, duralumin.