Materials Science

After World War II, the application of materials became less empirical and more founded in scientific principles. The term "materials science" emerged in the 1960s to reflect this trend and the realization that solutions to many of the world's most challenging technological problems were increasingly materials-limited. Within the field of engineering, the term "materials science and engineering" has come to describe the subfield concerned with materials applications. This longer term represents a blend of scientific fundamentals and practical engineering. The foundations of materials science are physical chemistry, polymer chemistry, and condensed matter physics. The foundations of materials engineering include the fields of metallurgy

and ceramic engineering. Many common themes in the fields of chemical engineering and materials science have led to the creation of academic departments that encompass both areas.

Material possessions have traditionally represented human wealth and defined social relationships. The eras of early human civilization (the Stone Age, the Bronze Age, and the Iron Age) have been named in terms of the materials from which tools and weapons were made. The Bronze Age (approximately 2000 b.c.e. to 1000 b.c.e.), in fact, represents the foundation of metallurgy. Although we do not use the term "pottery age," domestic vessels made from baked clay have been valuable in providing clues to daily life in ancient cultures, and glass articles from ancient Mesopotamia have been traced back to 4000 b.c.e.

Contemporary culture is sometimes described as "plastic," a somewhat critical reference to the pervasive use of polymeric materials in modern life. Others suggest that the current era is rightfully called the "Silicon Age," in honor of the far-ranging impacts of modern electronics based on silicon technology. In any case, modern products, such as automobiles, contain a full spectrum of materials, from the traditional to the advanced.

An underlying principle of materials science is that the properties (or characteristics) of materials are generally understood in terms of the microscopic or atomic structures of the materials. Another underlying principle is that the selection of optimal materials for specific modern technological applications requires consideration of the ways in which those materials are processed.

Types of Materials

Engineers generally build things from a limited "menu" of materials—namely, metals , polymers, and ceramics. This menu follows directly from the three types of primary chemical bonding: metallic, covalent, and ionic.

Most of the elements in the Periodic Table (in the pure state) are metallic in nature. Aluminum, copper, and iron are examples. The metallic bond involves a mobile "gas" of electrons. This gas of negatively charged electrons binds together the positively charged atomic cores. The electron gas is also responsible for the electrical conductivities and optical absorption that are characteristic of metals.

Polymers are high molecular weight solids that are an important part of everyday life. An example is polyethylene (C₂H₄)_n , where n is the "degree of polymerization," a number of around 1,000 (representing the fact that polyethylene is composed of a large number of ethylene molecules bound together by covalent bonding). All polymers are composed of a relatively small number of elements in the Periodic Table (primarily carbon and hydrogen and a few other "nonmetallic" elements such as nitrogen and fluorine). Each covalent bond involves electron sharing between adjacent atoms, with the result that polymers do not have "free" electrons for electrical conduction and are electrical insulators. The use of polymeric insulation for electrical wiring is a practical example of this. The lack of free electrons endows some polymers with optical transparency ("clear plastic" wrap is an excellent example). The alternative name for a polymer substance—"plastic"—comes from the extensive formability of many polymers.

We can define ceramics by what they are not: They are nonmetallic and inorganic. Ceramics are chemical combinations of at least one metallic element and at least one nonmetallic one. A simple example is aluminum oxide (Al₂O₃). Such chemical combinations represent, in fact, a fundamental tendency in nature. For example, metals tend to combine chemically with nonmetallic elements in their environments. The rusting of iron is a familiar and costly example. It is also interesting to note that the melting point of aluminum is 660°C (1,220°F), whereas the melting point of aluminum oxide is 2,020°C (3,668°F). The chemical stability associated with the ionic bonds between aluminum and oxygen (involving electron transfer from aluminum to oxygen to produce Al³⁺ and O²⁻ ions) makes ceramics temperature-resistant and chemically inert .

The category of ceramics is often broadened to "ceramics and glasses" because of the wide use of silicate glasses, distinctive materials that are chemically similar to ceramics. Silicon dioxide, SiO₂, is a ceramic compound and the basis of a large family of silicate ceramics. Clay minerals and the many clayware ceramics are the most traditional examples. SiO₂ is readily obtained in relatively pure form in common sand deposits. (These deposits, and the presence of SiO₂ in many geological minerals, are the reason that silicon and oxygen together account for roughly 75 percent of the elements in Earth's crust.) Upon heating, many of these silicate materials can be melted and, after cooling, retain the liquidlike structure of the melt. Common window and container glass is made in this way, with a typical composition, by weight, of (roughly): 75 percent SiO₂, 15 percent Na₂O, and 10 percent CaO. Thus, ceramics and glasses are of one category (combinations of ionically bonded positive and negative ions). Their differences are at the atomic scale. Ceramics are crystalline substances, in which the ions are arranged in a regular and repeating order. Glasses are noncrystalline substances, in which the ions are situated in irregular, liquidlike fashion.

In defining the previous three materials (metals, polymers, and ceramics/glasses), we found that each category conveniently related to one of the primary types of chemical bonding: metallic, covalent, and ionic, respectively. To be precise, atomic bonding is seldom "pure." There is generally some covalent nature (electron sharing) to the ionic bonding in ceramics and glasses. The bonding between the adjacent atoms in large polymeric molecules is highly covalent, but the bonding between molecules is often "secondary." For example, there are weak attractions between adjacent polyethylene molecules that involve polarization, not electron transfer or sharing. This weak secondary bonding is the primary reason that commercial "plastics" are characteristically weak and deformable in comparison to metals and ceramics/glasses.

Among the materials available for modern structural applications, a fourth category is generally included—namely, "composites." Composite materials are defined as microscopic-scale combinations of individual materials belonging to the previous three categories (metals, polymers, ceramics/glasses). A good example is fiberglass, a composite of glass fibers (a few micrometers in diameter) embedded in a polymer matrix. Over the past several decades, fiberglass products have become commonplace. The advantage of composites is that they display the best properties of each component, producing products superior to products made of a single component. In the case of fiber-glass, the high strength of the small diameter glass fibers is combined with the flexibility of the polymer matrix.

Although most engineered materials can be put into one of the four categories described above, a sorting of the same materials based on electrical conductivity rather than atomic bonding demands an additional, fifth category. We noted above that metals are typically good electrical conductors and that polymers and ceramics/glasses are typically electrical insulators. Composites tend to have properties that are averages of those of their individual components. As an example, fiberglass is an electrical insulator because both glass fibers and the polymer matrix tend to be insulators. Since the middle of the twentieth century, "semiconductors," with intermediate levels of electrical conductivity, have played an increasingly critical role in modern technology. The primary example is elemental silicon, which, as noted above, is a central component of modern, solid-state electronics. Silicon is in column IVA of the Periodic Table. Its neighbor in column IVA, germanium, is also a semiconductor and also widely used in electronic devices. Chemical compounds of the elements near column IVA often display semiconduction—for example, gallium arsenide (GaAs), which is used as a high temperature rectifier and a laser material. The chemical bonding in the various elemental and compound semiconductors is generally strongly covalent. In summary, a full list of the types of engineered materials contains five categories. (See Table 1.)

From Structure to Properties

An underlying principle of materials science is that structure (on the atomic or microscopic scale) leads to properties (on the macroscopic scale of real world, engineering applications). We have already seen that the natures of ceramics and glasses are very different because ceramics have a crystalline atomic arrangement and glasses are noncrystalline. Similarly, transparent glass

becomes opaque when it has many microscopic air bubbles that scatter light and prevent a clear image from being transmitted through the material. Examples of the structure–property relationship arise throughout the field of materials science.

Processing and Selecting Materials

The use of materials in modern technology depends on our ability to make those materials. Processing is dependent on the nature of the material, and the specific processing technique can, in turn, have an effect on the properties of the material. Given the wide range of materials described in Table 1, and the fact that an individual material's properties are dependent on the way in which it is manufactured, the selection of materials for a given application needs to be done in a systematic way. The selection process and the final decision are dependent on a range of factors, including desired properties, ability to be manufactured, and cost.

Conclusion

Chemical bonding and electrical conductivity provide five major categories of engineered materials: metals, polymers, ceramics/glasses, composites, and semiconductors. The properties of these materials are dependent on atomic- and microscopic-scale structure, as well as on the way in which a given material is processed. Materials science enables the selection of the optimal material for a given application.

see also Ceramics; Glass; Physical Chemistry; Polymers, Synthetic; Semiconductors.

James F. Shackelford

Bibliography

Ashby, Michael F. (1999). Materials Selection in Mechanical Design, 2nd edition Oxford, U.K.: Butterworth-Heinemann.

Callister, W. D., Jr. (2000). Materials Science and Engineering—An Introduction, 5th edition. New York: John Wiley and Sons.

Shackelford, James F. (2000). Introduction to Materials Science for Engineers, 5th edition. Upper Saddle River, NJ: Prentice-Hall.