Chapter 25: New Materials Through Chemistry

Section 1: Materials with a Past

Alloy: a mixture of elements that has metallic properties. * Advances in metal processing are still occurring as scientists continue to improve the art of blending metals, or making alloys, to make better metal products. * Alloys can produce materials with improved properties such as greater hardness, strength, lightness, or durability. * Other alloys also have been developed through the ages, giving people a large selection of materials to choose from today.
Properties of Metals and Alloys * Alloys retain the metallic properties of metals. * Luster: reflect light or have a shiny appearance * Ductility: can be pulled into wires. * Malleability: the property that allows metals and alloys to be hammered or rolled into thin sheets * Conductivity: heat or electrical charges can move easily through the material. * Metals and alloys usually are good conductors of heat and electricity because of these loosely bound electrons. * Gold is a bright, expensive metal that is soft and bends easily. * When gold and copper are melted, mixed, and allowed to cool, an alloy forms.
Uses of Alloys * If you see an object that looks metallic, it is most likely an alloy. * Automobile and aircraft bodies that require strong materials are constructed of alloys that are corrosion resistant and lightweight but able to carry heavy loads. * The classes of steel have different properties and therefore different uses. * Steel is a strong alloy and is used often if a great deal of strength is required. * Bridges, overpasses, and streets also are reinforced with steel. * Ship hulls, bedsprings, and automobile gears and axles are made from steel. * Another class of steel, called stainless steel, is used in surgical instruments, cooking utensils, and large vessels where food products are prepared.
New Alloys * Steel is not the only common type of alloy. * The new alloys are strong, lightweight and last longer than alloys used in the past. * Titanium alloy panels, developed for the space shuttle heat shield, might be used on future reusable launch vehicles that are designed to carry payloads to the International Space Station.

Section 2: Versatile Materials

Ceramics: materials that are made from dried clay or claylike mixtures. * Pottery, bricks, glass, and concrete are examples of ceramics.Traditional ceramics are made from easily obtainable raw materials—clay, silica (sand), and feldspar (crystalline rocks). * After the raw materials are processed, ceramics usually are made by molding the ceramic into the desired shape, then heating it to temperatures between 1,000°C and 1,700°C. * The heating process, called firing, causes the spaces between the particles to shrink. * Ceramics are known also for their chemical resistance to oxygen, water, acids, bases, salts, and strong solvents. * These qualities make ceramics useful for applications where they may encounter these substances. * Traditional ceramics also are used as insulators because they do not conduct heat or electricity. * Changing the composition of the raw materials or the manufacturing process changes the properties of the ceramic. * Ceramics can be used in the body because they are strong and resistant to body fluids, which can damage other materials.
Semiconductors: poorer conductors of electricity than metals but better conductors than nonmetals, and their electrical conductivity can be controlled. * Another class of versatile materials is semiconductors. * Semiconductors are the materials that make computers and other electronic devices possible. * Adding other elements to some metalloids can change their electrical conductivities. * If the added atoms, called impurities, have fewer electrons than silicon atoms, the silicon crystals will contain holes, or areas with fewer electrons. * Doping: The process of adding impurities or other elements to a semiconductor to modify the conductivity. * If the impurity causes the overall number of electrons to increase, the semiconductor is called an n-type semiconductor. * If doping reduces the overall number of electrons, the semiconductor is called a p-type semiconductor. * By placing n-type and p-type semiconductors together, semiconductor devices such as transistors and diodes can be made. * These devices are used to control the flow of electrons in electrical circuits. * Integrated Circuit: contains many semiconducting devices. * Integrated circuits as small as 1 cm on a side can contain millions of semiconducting devices. * Because of their small size, integrated circuits are sometimes called microchips. * Semiconductors make today’s computers possible.When a computer system is functioning properly, the hardware and software work together to perform tasks.

Section 3: Polymers and Composites

Polymers: a class of natural or manufactured substances that are composed of molecules arranged in large chains with small, simple, repeating units called monomers. * Monomer: one specific molecule that is repeated in the polymer chain. * Not all polymers are manufactured. Some occur naturally. * Synthetic: the polymer does not occur naturally, but it was manufactured in a laboratory or chemical plant. * In the 1800s, scientists began developing processes to improve natural polymers and to create new ones in the laboratory. * Today, so many types of synthetic polymers exist that they tend to be divided into groups such as plastics, synthetic fibers, adhesives, surface coatings, and synthetic rubbers. * Today, synthetic polymers usually are made from fossil fuels such as oil, coal, or natural gas. * Fossil fuels are composed primarily of carbon and hydrogen and are referred to as hydrocarbons. * The reason that polymers can be used for so many applications is directly related to the ease with which their properties can be modified. * The number of carbon atoms in the polymer can be high, and each bonding site represents a possibility of a change in properties. * Plastics are usually lightweight, strong, impact resistant, waterproof, moldable, chemical resistant, and inexpensive. * Most synthetic fibers are composed of carbon chains because they are produced from petroleum or natural gas. * Synthetic polymers are used to make adhesives that can be modified to provide the best properties for a particular application * Synthetic rubber is a synthetic elastic polymer.
Composite: a mixture of two or more materials—one embedded or layered in the other. * Glass fibers are used often to reinforce plastics because glass is inexpensive, but other materials can be used as well. * Composite materials are used in the construction of satellites. * Aircraft made of composites also benefit from the strong yet lightweight properties of composite materials.

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