From titanium implants to new superconducting alloys to useful metal compounds, we are increasingly dependent on metals. Without metals, modern civilisation would literally collapse. About 80% of the known chemical elements are metals.
Physical properties of metals
Metals have properties suited to a wide range of uses. They are characteristically shiny, and most are malleable, ductile, dense, very good electrical conductors and have high melting points. Malleability is the ability of a material to deform when compressed – gold is very malleable and can be beaten to an extremely thin sheet.
The crystal structure of metals
When we look at the surface of a metal, it is often possible to see crystals. The crystals reflect the symmetrical arrangement of positive metal ions in the underlying structure that we can’t see.
The positive ions in a metal are packed closely together so that the gaps between them are kept to a minimum. When metal atoms pack together to form crystal, they can be:
- hexagonal close packed
- face-centred cubic
- body-centred cubic.
When molten metal is cooled down, the metal atoms settle out into a crystal lattice. Given enough time and ideal conditions, the crystal lattice can grow to be very large, with a perfect internal crystalline structure. Ideal conditions are seldom found, and the reality is that almost every solid metal exists as jumble of crystals of varying sizes. Each individual crystal in the body is called a grain. These grains are crystalline structures that have lots of imperfections, which distort the crystal lattice.
What are alloys?
Mixtures of metals, called alloys, are more commonly used than the pure metal. By alloying, some of the important properties of metals can be improved.
- Solder, which is used in the electronics industry, is a mixture of tin and lead. One type of solder (63% tin and 37% lead) has a lower melting point but is harder than either of the metals. These properties allow it to be used successfully.
- An amalgam is an alloy based on mercury. Dental amalgam is an alloy of mercury, silver, tin, copper and zinc. It is hard-wearing, corrosion-resistant and has high-impact strength.
- A titanium alloy known as Ti6Al4V is used in the aerospace industry. It is lightweight, very strong and has high corrosion-resistance properties.
Professor Deliang Zhang’s research work at the University of Waikato is focused on low cost titanium alloy powders.
Most metals do not occur in their natural state. They are often found as compounds such as metal oxides, sulfides and halides.
- Aluminium oxide is the main metal compound present in the ore known as bauxite.
- Iron pyrites or ‘fool’s gold’ is mainly iron sulfide.
- The Lake Grassmere salt works in Marlborough produces the metal compound known as sodium chloride from ‘salty’ seawater.
Metals can be produced (smelted) from their ores by a variety of methods:
- Aluminium is produced from its ore (bauxite) by passing a very large electric current through a molten mixture of the ore and a compound called cryolite.
- Titanium is mostly produced from its ore (rutile) by the Kroll Process, where the ore is treated with chlorine gas followed by reaction with magnesium metal.
Tin can? Tin roof? Tin foil?
Today, tin is rarely used in the canning industry, as a roofing material or as cooking foil – cheaper materials have replaced it.
Tin is an important alloying metal. Solder, bronze and dental amalgam all contain tin, and the wires used in superconducting magnets, found in nuclear magnetic resonance (NMR) instruments and magnetic resonance imaging (MRI) scanners, are made of a tin-niobium alloy.
Tin changes from its common everyday state known as ‘white tin’ into a soft powdery state called ‘grey tin’ when exposed to temperatures below 0 °C over a period of time. These different forms of tin are known as allotropes.
Most large cathedrals have impressive musical organs, the pipes of which are made of a tin-lead alloy. In the winter months, if the air temperature in the cathedral is allowed to fall below zero for extended periods of time, the pipes can suffer from ‘tin disease’. The pipes are coated in a powdery grey substance, and they lose their tonal quality.
One of the reasons put forward to explain why Napoleon’s army failed to defeat the Russian army in the winter of 1812 was that the tin buttons on the tunics of the French soldiers suffered from ‘tin disease’. Eventually the buttons turned to powder, and with no buttons, the soldiers’ tunics fell off and they froze to death!
In the activity, Making model alloys, students use plasticine and sand to explore how adding other elements can alter the ductility of a metal.