10.5.1 Nanoparticles: size, uses and risks

Nanoparticles sit between atoms and everyday particles

1. A nanoparticle measures between about $1$ and $100$ nanometres across.
2. A single atom is roughly a tenth of a nanometre across.
3. A nanoparticle therefore holds anything from a few tens of atoms to many millions.
4. It is far larger than an atom or a small molecule, though a large molecule such as a protein reaches this size.
5. It is far smaller than anything visible, including a speck of dust.

A large surface area to volume ratio changes behaviour

1. Dividing a solid into smaller pieces raises its surface area to volume ratio.
2. At nanoparticle size, that ratio becomes very large.
3. A large proportion of the atoms therefore sits at the surface.
4. Reactions happen at surfaces, so nanoparticles are far more reactive than the bulk material.
5. A much smaller mass can therefore do the same job.

Uses that depend on that behaviour

1. Sunscreens use nanoparticles of titanium dioxide or zinc oxide.
2. The particles block ultraviolet light while being too small to look white on the skin.
3. An older sunscreen using the bulk powder left a visible white layer.
4. Nanoparticle catalysts work with a much smaller mass of expensive metal.
5. Other uses include self-cleaning surfaces and stronger, lighter materials.

The possible risks

1. Some may be able to pass through the skin, and they are small enough to be breathed deep into the lungs.
2. Their high reactivity may cause effects that the bulk material does not.
3. The long-term effects on health are not yet fully known.
4. Particles washed off into rivers and the sea may affect wildlife.
5. Risk depends on which material the particles are made of, so a result for one says little about another.

10.5.2 Comparing and selecting materials

The main classes of material

1. Metals conduct heat and electricity, are malleable, and are usually strong.
2. Glass and clay ceramics are hard, brittle and resist heat and chemical attack.
3. Polymers are light, easily shaped, and are electrical insulators.
4. Composites combine two materials so that the result outperforms either alone.
5. Each class has a characteristic set of properties that suits it to particular jobs.

Comparing the classes using data

1. Density decides whether an object will be heavy for its size.
2. Strength decides what load the material can carry before it fails.
3. Hardness decides how well it resists scratching and denting.
4. Melting point decides whether it can be used somewhere hot.
5. Electrical and thermal conductivity decide whether it conducts or insulates.

Composites combine the best of two materials

1. A composite has a matrix holding a reinforcement material within it.
2. The reinforcement supplies strength and the matrix holds it in shape.
3. Reinforced concrete is concrete with steel bars inside it.
4. Concrete is strong when squashed but weak when stretched, and the steel covers that weakness.
5. Fibreglass combines glass fibres with a polymer, giving strength at low density.

Choosing a material for a job

1. List what the job demands, such as strength, low mass or resistance to corrosion.
2. Compare the data for each candidate against those demands.
3. Rule out any material that fails a demand the job cannot compromise on.
4. Among those left, weigh cost and availability.
5. State the choice with the property that decided it, using the figures given.