Metals are rarely found in the pure state in the crust of the Earth (although the centre of the planet is made of iron). They are good conductors of heat and electricity. When they are not tarnished or rusted they generally have a shiny surface. They are almost never used in their pure form for structural applications, but are almost always mixed with other metals to form alloys. Iron is alloyed with controlled amounts of carbon and other elements to make steel, which was crucial to the Industrial Revolution, and is still by far the cheapest and most widely used metallic material. Aluminium alloys are widely used where weight-saving or corrosion resistance is more important than material cost—for example, in aeroplanes and to some extent in cars.
The whole of modern information and communication technology depends on semiconductors. These may be elements, such as silicon, or compounds, such as gallium arsenide, that have just enough electrical conductivity to make them useful for controlling and amplifying electrical signals. Silicon is universally used for digital switching devices such as those that are used in computers. For more difficult jobs, such as converting electrical signals to light or vice versa, semiconductor compounds are used, and it is possible to tailor the properties of components by growing alternating layers of different materials. Every compact disc player contains a semiconductor laser. Long-distance communications are transmitted via fibre-optic links, using thin glass fibres, which were made possible by the development of extremely transparent types of glass.
Polymers are materials that are made of carbon, hydrogen, and other elements, with the carbon atoms connected to form long molecular chains. They can be made from crude oil, and they are usually formed into the desired shape while they are liquid. Thermoplastic polymers (from which we get the term “plastic“) become soft
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and mouldable when they are heated, and return to their solid form on cooling. Thermosetting polymers become solid owing to an irreversible chemical reaction. Cheap polymers are used for such common objects as shopping bags, but more sophisticated properties can be obtained. Polymer boxes can be made with integral hinges that can be flexed many thousands of times, and polymer roasting bags can survive high temperatures in ovens. Silk is a natural polymer that is stronger than many artificial materials. Polymers are usually electrically insulating, but they can also be made with useful conducting properties, and polymer transistors and light-emitting diodes have been produced.
Desirable properties of more than one material can be combined in composites. Glass-fibre-reinforced composites (fibreglass) are widely used to give high strength and stiffness without the fragility usually associated with glass. Many composites use carbon or polymer fibres in an epoxy matrix (the matrix is the material in which fibres or particles are embedded). The artificial material that is used in the greatest tonnage throughout the world is concrete, which is a composite of gravel, sand, and cement. Advanced composites are being developed that consist of ceramic fibres and different ceramic or metal matrix materials.
