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POLYMERS

METALS

CERAMICS

To facilitate the understanding of matter, materials scientists often classify solid materials into three broad categories: polymers, metals, and ceramics. Composite materials are created by the combination of two or more base materials – often having different properties. Composites can thus be designed to have quite different properties than their constituent materials.

“Polymers” are large molecular or macromolecular materials composed of repeating subunits. In this overarching classification strategy, polymers include not only commercial plastics but also materials, such as deoxyribonucleic acid, proteins, wood, skin, hair, and others. The primary bonding structure of polymers largely consists of covalent bonds; that is, valence electrons are shared between the bonding atoms. Covalent bonds are characterized by small electronegativity (ability of an atom to attract electrons in a bond) differences and low electron mobility. Secondary bonding, the attraction between atoms of different molecules or between different parts of the same polymer, can play a strong role in the structure and properties of polymers.

“Metals” are typically lustrous, opaque, and ductile and have good electrical and thermal conductivity. They can be composed of a single element and, in fact, approximately 91 of the 118 elements in the periodic table are classified as metals. Metallic alloys are metals composed of one primary or base metal and one or more metals, or nonmetal element. For example, the alloys used in making components of various stages of gas turbine engines are based on four primary metals: iron, titanium, cobalt, and nickel. Superalloys are high‐performance alloys that are most often resistant to high temperatures. Metallic bonding is characterized by small electronegativity differences and high valence electron mobility. Metallic bonding is often pictured as positive metal ions in a “sea” of electrons.

“Ceramics” are inorganic materials primarily held in covalent or ionic bonds. Ionic bonds are characterized by high electronegativity differences and thus one atom in the bond has a much greater attraction for valence electrons resulting in two oppositely charged ions. As the electrons are strongly attracted to one ion in the bond, they have low mobility, and such materials generally have low conductivities. Ceramics include not only pottery or fired clay products, but also cement, bricks, glass, salts, and other minerals.

Some knowledge, even at the rudimentary level, of minerals is also necessary for materials scientists and engineers, including geologists, metallurgists, chemists, and physicists. A “mineral” may simply be defined as a naturally occurring inorganic substance that is chemically homogeneous and has an ordered internal structure. Common mineral groups include silicates, oxides, sulfates, sulfides, carbonates, halides, and the native elements. The rocks of Earth's crust, lithosphere, and – to some extent – asthenosphere consist of units that are essentially minerals with wide‐ranging chemical and physical properties.

While the basic trinity of classification is one of the primary ways in which materials classification is accomplished, other methods of grouping materials have evolved to support research and development in specific fields. For example, although not generally explored in the traditional basic trinity of materials classification, “semiconductors” are an important classification in our modern age. As the name implies, semiconductors are classified largely based on their electronic properties. In a semiconducting material, the electrical conductivity falls between that of a conductor (metal) and an insulator. Semiconductors are typically characterized as having covalent bonding. Traditional semiconductors include elements (also termed metalloids) like silicon and compounds like gallium arsenide, but semiconducting properties are also being designed into polymers and ceramics.