Materials, Properties of, characteristics of materials that determine their suitability for specific applications. Materials are anything from which products can be made. Materials science is the study of the properties, structure, and processing of materials.
II TYPES OF MATERIALS
Wood is a remarkable natural material. It is strong and relatively stiff and light. Its properties compete well with those of artificial materials. The fastest bomber in World War II, the British De Havilland Mosquito, was made of wood. When German intelligence discovered that the Allies were making aeroplanes out of wood, they thought that it was because supplies of metal had run out—not realizing that wood had been carefully chosen partly because it had excellent properties for this design. Wood remains the preferred material for many applications in the building industry throughout the world, and also for much furniture. Stone is another natural material, once used for making simple tools and weapons, and subsequently almost exclusively for buildings and sculptures.
Among the earliest materials to be put to use by human beings were ceramics, in the form of the clays used in pottery. Archaeologists date early civilizations by the pottery associated with them. Ceramic materials are chemical compounds of one or more metallic elements with oxygen or with other elements such as nitrogen. For a long time, ceramics were mainly used for plumbing or for cooking or eating utensils, but relatively strong engineering ceramics have now been developed with good mechanical properties, and others, called electroceramics, are essential for a wide range of electrical and electronic devices.
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, semiconductoring 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 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.
III MECHANICAL PROPERTIES
Density is the amount of mass in a unit volume. It is measured in kilograms per cubic metre or pounds per cubic foot. The density of water is very nearly 1,000 kg/cu m (62.4 lb/cu ft); most materials have a higher density and sink in water. Aluminium alloys, with densities typically around 2.8 times that of water, are considerably less dense than steels, which have densities typically around 7.8 times that of water. The density is significant in any application where the material must be supported or lifted off the ground.
Stiffness is a measure of the resistance to deformation such as stretching or bending. The Young modulus is a measure of the resistance to simple stretching or compression. It is the ratio of the applied force per unit area (stress) to the fractional elastic deformation (strain). Its SI units are MN/m2 (meganewtons per sq m) or, equivalently, MPa (megapascals). Stiffness is important when a rigid structure is to be made.
Strength is the force per unit area (stress) that a material can support without failing. The units are the same as those of stiffness, MN/m2, but in this case, the deformation is by definition irreversible. The yield strength is the stress at which a material first deforms plastically. For a metal, this may be somewhat less than the fracture strength, which is the stress at which it breaks. Many materials have a higher strength in compression than in tension; this is why steel reinforcement bars are used to support tensile (stretching) loads in concrete buildings and bridges.
Ductility is the ability of a material to deform without breaking. One of the great advantages of metals as materials lies in their ability to be formed into the desired shape, such as car body parts. Materials that are not ductile are brittle. Ceramics are generally quite brittle. Ductile materials can absorb energy by deformation but brittle materials cannot. Thus a metal teaspoon will survive being dropped on the floor while a ceramic teacup will break. High-impact polymers can similarly resist breakage.
Toughness is the resistance of a material to breaking when there is a crack in it. For a material of given toughness, the stress at which it will fail is inversely proportional to the square root of the size of the largest defect present. Toughness is different from strength: the toughest steels, for example, are different from the ones with highest tensile strength. Brittle materials have low toughness: glass can be broken along a chosen line by first scratching it with a diamond. Ceramic materials with a perfect crystalline structure can be extremely strong, but the presence of defects often limits the useful load that they can support; this is why they are generally weaker in tension than in compression. Composites can be designed to have considerably greater toughness than their constituent materials.
Creep resistance is the resistance to a gradual permanent change of shape and becomes especially important at higher temperatures. Lead pipes can sometimes creep at summer daytime temperatures. The hotter a gas-turbine engine is run, the more efficient it is, and a great deal of successful research has gone into developing materials for turbine blades that will enable them to operate at high temperatures and under high centripetal forces without gradually extending and scraping against the walls of the engine.
IV ELECTRONIC PROPERTIES
Electrical conductivity is the ability of a material to conduct electricity. It is the reciprocal of the resistivity, which is defined as the resistance between opposite faces of a cube of the material. The conductivity of metals decreases with increasing temperature, whereas the conductivity of semiconductors increases with increasing temperature. Most ceramics are insulators at room temperature, but many electroceramics acquire significant conductivity at higher temperatures. Metals can become superconductors (with zero resistance) at very low temperatures; several ceramics have been developed that exhibit superconductivity at the comparatively high temperature of liquid nitrogen (-196º C/-321º F).
Semiconducting properties are central to a huge range of electronic effects. By “doping” a semiconductor material with small amounts of impurities its properties can be dramatically changed. The junction between oppositely doped semiconducting materials is the basis of diodes and bipolar transistors, which are the fundamental components of all microchips. The gap between energy bands (see Condensed-Matter Physics) in a semiconductor determines the wavelength of light which that material can emit. Silicon has a bandgap corresponding to infrared radiation (although it can also emit visible light); for visible radiation, compound semiconductors are generally used.
Dielectric properties are measured by relative permittivity (dielectric constant), which is related to the static charge on the surface when an electric field is applied to insulators such as polymers and ceramics. Materials with high permittivity are good for capacitors, which store charge in circuits.
Piezoelectric properties are a material’s ability to generate a charge when its shape is changed, for example by an applied force. Some domestic gas spark igniters work this way. The quartz crystal oscillator in a digital watch depends on its piezoelectric properties to function. When a voltage is applied to a piezoelectric material, its shape changes and this property can be exploited in small actuators, as found in devices such as inkjet printers.
A pyroelectric material generates a voltage when its temperature changes. This is exploited in passive infrared detectors for automatic doors and burglar alarms, which can detect body heat at distances up to 100 metres (300 ft).
In a material with electro-optic properties, the propagation of light is altered by the influence of an applied electric field. Liquid crystals and certain transparent ceramics exhibit electro-optic properties, which make them useful for computer displays and for switching fibre-optic light signals.
Magnetic properties are traditionally associated with iron, but many other alloys and ceramics are also magnetic. “Soft” magnetic properties—properties that readily change as an applied magnetic field is changed—are required for transformers and for electromagnets that can be switched on and off. “Hard” magnetic properties—ones that are not easily changed—are required for permanent magnets; new materials of this kind have made possible compact yet powerful motors, such as those in electric screwdrivers.
V CHEMICAL PROPERTIES
Corrosion resistance is the ability of a material to resist degradation through oxidation. With prolonged exposure to the atmosphere ordinary steels rust and copper develops a green oxide which eventually turns black. Stainless steels are alloys containing chromium that have high resistance to rusting and other forms of chemical attack. Aluminium is a chemically reactive element, but it forms a stable oxide that is well bonded to the metal, thus inhibiting further corrosion.
Colour is a property that is important in textiles and in paints, inks, and so on. In gemstones, the colour is controlled by microscopic defects in the crystal lattice known as colour centres.
Biocompatibility describes the ability of materials to be used inside the human body, for example as prosthetics (artificial spare parts) in replacement surgery. The materials must not be chemically attacked by substances naturally present in the body, and they must not give off harmful chemicals or debris from wear. Biocompatible materials are being developed that can be organically incorporated into natural tissues such as bone.
VI MICROSTRUCTURAL PROPERTIES
Under everyday conditions of temperature and pressure, all materials are made of atoms. In polymers, the atoms are arranged in large groupings called molecules, which are bound together to give the material its properties. The molecules in many polymers and the atoms in most other artificial materials are arranged in crystals, in which the atoms repeat themselves in regular arrangements. The crystal structure plays a key role in giving a material its mechanical, electrical, and, to some extent, chemical properties.
Microchips are made, hundreds at a time, in the form of semiconductor wafers, which are large single crystals as much as 20 cm (8 in) in diameter. The whole of modern electronic and computer engineering has been made possible by engineers’ ability to make silicon material of extremely high purity and crystal perfection.
Almost all metals and ceramics are polycrystalline: consisting of many individual crystal grains adjoining one another. Grains of zinc on the surface of galvanized iron are often large enough to be seen with the naked eye. In most alloys, the grain structure can be seen with a standard microscope if the metal is first polished and then etched with a suitable acid. Much of the understanding of the structure and properties of materials has come about through the use of microscopes.
In order to see the structure within a grain, the transmission electron microscope is used (see Microscope: Electron Microscope). This instrument uses magnetic lenses to focus electrons, rather as glass lenses focus light. The mechanical behaviour of metals is found to be controlled by dislocations, which are defects in an otherwise perfect crystal lattice. It would be difficult to move one whole plane of atoms over another in a metal crystal, but a dislocation enables the planes to slide a little bit at a time, rather as a caterpillar moves over a leaf by making a hump in its back so that at that point its legs do not touch the surface on which it is standing. Ceramics are brittle because the dislocations in them cannot easily move. In a pure metal, the dislocations can move rather easily (unless they get tangled by work hardening); much of the design of alloys is concerned with making it more difficult for the dislocations to move.
Several kinds of microscopes enable individual atoms of a solid to be imaged. High-resolution electron microscopes can resolve single columns of atoms along a chosen crystal direction. Field ion microscopes can image the atoms on the end of a sharp needle of a material; it is even possible to pull atoms off the surface and weigh them one at a time in a mass spectrometer to find out what they are made of. Scanning tunnelling microscopes enable the positions and electronic structure of individual atoms and molecules on a flat surface to be seen. See Microscope: Scanning Probe Microscope.