O
ne common item that presents some inter-esting material property requirements is the container for carbonated beverages. The material used for this application must satisfy the follow-ing constraints: (1) provide a barrier to the pas-sage of carbon dioxide, which is under pressure in the container; (2) be nontoxic, unreactive with the beverage, and, preferably be recyclable; (3) be rel-atively strong, and capable of surviving a drop from a height of several feet when containing the beverage; (4) be inexpensive and the cost to fab-ricate the final shape should be relatively low;(5) if optically transparent, retain its optical clar-ity; and (6) capable of being produced having different colors and/or able to be adorned with decorative labels.
All three of the basic material types—metal (aluminum), ceramic (glass), and polymer (poly-ester plastic)—are used for carbonated beverage containers (per the chapter-opening photographs for this chapter).All of these materials are nontoxic
and unreactive with beverages. In addition, each material has its pros and cons. For example, the aluminum alloy is relatively strong (but easily dented), is a very good barrier to the diffusion of carbon dioxide, is easily recycled, beverages are cooled rapidly, and labels may be painted onto its surface. On the other hand, the cans are optically opaque, and relatively expensive to produce. Glass is impervious to the passage of carbon dioxide, is a relatively inexpensive material, may be recycled, but it cracks and fractures easily, and glass bottles are relatively heavy. Whereas the plastic is rela-tively strong, may be made optically transparent, is inexpensive and lightweight, and is recyclable, it is not as impervious to the passage of carbon diox-ide as the aluminum and glass. For example, you may have noticed that beverages in aluminum and glass containers retain their carbonization (i.e.,
“fizz”) for several years, whereas those in two-liter plastic bottles “go flat” within a few months.
4Fiberglass is sometimes also termed a “glass fiber-reinforced polymer” composite, abbrevi-ated “GFRP.”
Composites
A composite is composed of two (or more) individual materials, which come from the categories discussed above—viz., metals, ceramics, and polymers.The design goal of a composite is to achieve a combination of properties that is not displayed by any single material, and also to incorporate the best characteristics of each of the component materials. A large number of composite types exist that are represented by different combinations of metals, ceramics, and polymers. Furthermore, some naturally-occurring materials are also considered to be composites—for example, wood and bone. However, most of those we consider in our discussions are syn-thetic (or man-made) composites.
One of the most common and familiar composites is fiberglass, in which small glass fibers are embedded within a polymeric material (normally an epoxy or polyester).4The glass fibers are relatively strong and stiff (but also brittle), whereas the polymer is ductile (but also weak and flexible). Thus, the resulting fiberglass is relatively stiff, strong, (Figures 1.4 and 1.5) flexible, and ductile. In addition, it has a low density (Figure 1.3).
Another of these technologically important materials is the “carbon fiber-reinforced polymer” (or “CFRP”) composite—carbon fibers that are embedded within a polymer. These materials are stiffer and stronger than the glass fiber-reinforced materials (Figures 1.4 and 1.5), yet they are more expensive. The CFRP composites
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1.5 Advanced Materials • 11 are used in some aircraft and aerospace applications, as well as high-tech sporting equipment (e.g., bicycles, golf clubs, tennis rackets, and skis/snowboards). Chapter 16 is devoted to a discussion of these interesting materials.
1.5 ADVANCED MATERIALS
Materials that are utilized in high-technology (or high-tech) applications are some-times termed advanced materials. By high technology we mean a device or product that operates or functions using relatively intricate and sophisticated principles; ex-amples include electronic equipment (camcorders, CD/DVD players, etc.), com-puters, fiber-optic systems, spacecraft, aircraft, and military rocketry.These advanced materials are typically traditional materials whose properties have been enhanced, and, also newly developed, high-performance materials. Furthermore, they may be of all material types (e.g., metals, ceramics, polymers), and are normally expensive.
Advanced materials include semiconductors, biomaterials, and what we may term
“materials of the future” (that is, smart materials and nanoengineered materials), which we discuss below. The properties and applications of a number of these advanced materials—for example, materials that are used for lasers, integrated circuits, magnetic information storage, liquid crystal displays (LCDs), and fiber optics—are also discussed in subsequent chapters.
Semiconductors
Semiconductors have electrical properties that are intermediate between the elec-trical conductors (viz. metals and metal alloys) and insulators (viz. ceramics and polymers)—Figure 1.7. Furthermore, the electrical characteristics of these materi-als are extremely sensitive to the presence of minute concentrations of impurity atoms, for which the concentrations may be controlled over very small spatial re-gions. Semiconductors have made possible the advent of integrated circuitry that has totally revolutionized the electronics and computer industries (not to mention our lives) over the past three decades.
Biomaterials
Biomaterials are employed in components implanted into the human body for replacement of diseased or damaged body parts. These materials must not produce toxic substances and must be compatible with body tissues (i.e., must not cause adverse biological reactions). All of the above materials—metals, ceramics, poly-mers, composites, and semiconductors—may be used as biomaterials. For example, some of the biomaterials that are utilized in artificial hip replacements are dis-cussed in Section 22.12.
Materials of the Future Smart Materials
Smart (or intelligent) materials are a group of new and state-of-the-art materials now being developed that will have a significant influence on many of our tech-nologies.The adjective “smart” implies that these materials are able to sense changes in their environments and then respond to these changes in predetermined manners—
traits that are also found in living organisms. In addition, this “smart” concept is be-ing extended to rather sophisticated systems that consist of both smart and tra-ditional materials.
Components of a smart material (or system) include some type of sensor (that detects an input signal), and an actuator (that performs a responsive and adaptive
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function). Actuators may be called upon to change shape, position, natural frequency, or mechanical characteristics in response to changes in temperature, electric fields, and/or magnetic fields.
Four types of materials are commonly used for actuators: shape memory alloys, piezoelectric ceramics, magnetostrictive materials, and electrorheological/magne-torheological fluids. Shape memory alloys are metals that, after having been deformed, revert back to their original shapes when temperature is changed (see the Materi-als of Importance piece following Section 10.9). Piezoelectric ceramics expand and contract in response to an applied electric field (or voltage); conversely, they also generate an electric field when their dimensions are altered (see Section 18.25). The behavior of magnetostrictive materials is analogous to that of the piezoelectrics, ex-cept that they are responsive to magnetic fields. Also, electrorheological and mag-netorheological fluids are liquids that experience dramatic changes in viscosity upon the application of electric and magnetic fields, respectively.
Materials/devices employed as sensors include optical fibers (Section 21.14), piezoelectric materials (including some polymers), and microelectromechanical devices (MEMS, Section 13.8).
For example, one type of smart system is used in helicopters to reduce aero-dynamic cockpit noise that is created by the rotating rotor blades. Piezoelectric sensors inserted into the blades monitor blade stresses and deformations; feedback signals from these sensors are fed into a computer-controlled adaptive device, which generates noise-canceling antinoise.
Nanoengineered Materials
Until very recent times the general procedure utilized by scientists to understand the chemistry and physics of materials has been to begin by studying large and com-plex structures, and then to investigate the fundamental building blocks of these structures that are smaller and simpler. This approach is sometimes termed “top-down” science. However, with the advent of scanning probe microscopes (Sec-tion 4.10), which permit observa(Sec-tion of individual atoms and molecules, it has be-come possible to manipulate and move atoms and molecules to form new structures and, thus, design new materials that are built from simple atomic-level constituents (i.e., “materials by design”). This ability to carefully arrange atoms provides op-portunities to develop mechanical, electrical, magnetic, and other properties that are not otherwise possible. We call this the “bottom-up” approach, and the study of the properties of these materials is termed “nanotechnology”; the “nano” prefix de-notes that the dimensions of these structural entities are on the order of a nanome-ter (10!9m)—as a rule, less than 100 nanometers (equivalent to approximately 500 atom diameters).5One example of a material of this type is the carbon nanotube, discussed in Section 12.4. In the future we will undoubtedly find that increasingly more of our technological advances will utilize these nanoengineered materials.
1.6 MODERN MATERIALS’ NEEDS
In spite of the tremendous progress that has been made in the discipline of materials science and engineering within the past few years, there still remain technological challenges, including the development of even more sophisticated and specialized 12 • Chapter 1 / Introduction
5One legendary and prophetic suggestion as to the possibility of nanoengineering materials was offered by Richard Feynman in his 1960 American Physical Society lecture that was entitled “There is Plenty of Room at the Bottom.”
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materials, as well as consideration of the environmental impact of materials pro-duction. Some comment is appropriate relative to these issues so as to round out this perspective.
Nuclear energy holds some promise, but the solutions to the many problems that remain will necessarily involve materials, from fuels to containment structures to facilities for the disposal of radioactive waste.
Significant quantities of energy are involved in transportation. Reducing the weight of transportation vehicles (automobiles, aircraft, trains, etc.), as well as increasing engine operating temperatures, will enhance fuel efficiency. New high-strength, low-density structural materials remain to be developed, as well as mate-rials that have higher-temperature capabilities, for use in engine components.
Furthermore, there is a recognized need to find new, economical sources of en-ergy and to use present resources more efficiently. Materials will undoubtedly play a significant role in these developments. For example, the direct conversion of so-lar into electrical energy has been demonstrated. Soso-lar cells employ some rather complex and expensive materials. To ensure a viable technology, materials that are highly efficient in this conversion process yet less costly must be developed.
The hydrogen fuel cell is another very attractive and feasible energy-conversion technology that has the advantage of being non-polluting. It is just beginning to be implemented in batteries for electronic devices, and holds promise as the power plant for automobiles. New materials still need to be developed for more efficient fuel cells, and also for better catalysts to be used in the production of hydrogen.
Furthermore, environmental quality depends on our ability to control air and water pollution. Pollution control techniques employ various materials. In addition, materials processing and refinement methods need to be improved so that they pro-duce less environmental degradation—that is, less pollution and less despoilage of the landscape from the mining of raw materials. Also, in some materials manufac-turing processes, toxic substances are produced, and the ecological impact of their disposal must be considered.
Many materials that we use are derived from resources that are nonrenewable—
that is, not capable of being regenerated. These include polymers, for which the prime raw material is oil, and some metals. These nonrenewable resources are grad-ually becoming depleted, which necessitates: (1) the discovery of additional reserves, (2) the development of new materials having comparable properties with less ad-verse environmental impact, and/or (3) increased recycling efforts and the devel-opment of new recycling technologies. As a consequence of the economics of not only production but also environmental impact and ecological factors, it is becoming increasingly important to consider the “cradle-to-grave” life cycle of materials rel-ative to the overall manufacturing process.
The roles that materials scientists and engineers play relative to these, as well as other environmental and societal issues, are discussed in more detail in Chapter 23.
REFERENCES
References • 13
Ashby, M. F. and D. R. H. Jones, Engineering Ma-terials 1, An Introduction to Their Properties and Applications, 3rd edition, Butterworth-Heinemann, Woburn, UK, 2005.
Ashby, M. F. and D. R. H. Jones, Engineering Mate-rials 2, An Introduction to Microstructures,
Pro-cessing and Design, 3rd edition, Butterworth-Heinemann, Woburn, UK, 2005.
Askeland, D. R. and P. P. Phulé, The Science and Engineering of Materials,5th edition, Nelson (a division of Thomson Canada), Toronto, 2006.
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14 • Chapter 1 / Introduction
Baillie, C. and L. Vanasupa, Navigating the Materials World,Academic Press, San Diego, CA, 2003.
Flinn, R. A. and P. K. Trojan, Engineering Materi-als and Their Applications, 4th edition, John Wiley & Sons, New York, 1994.
Jacobs, J. A. and T. F. Kilduff, Engineering Materi-als Technology,5th edition, Prentice Hall PTR, Paramus, NJ, 2005.
Mangonon, P. L., The Principles of Materials Selec-tion for Engineering Design, Prentice Hall PTR, Paramus, NJ, 1999.
McMahon, C. J., Jr., Structural Materials, Merion Books, Philadelphia, 2004.
Murray, G. T., Introduction to Engineering Materi-als—Behavior, Properties, and Selection,Marcel Dekker, Inc., New York, 1993.
Ralls, K. M., T. H. Courtney, and J. Wulff, Intro-duction to Materials Science and Engineering, John Wiley & Sons, New York, 1976.
Schaffer, J. P., A. Saxena, S. D. Antolovich, T. H.
Sanders, Jr., and S. B. Warner, The Science and Design of Engineering Materials, 2nd edition, WCB/McGraw-Hill, New York, 1999.
Shackelford, J. F., Introduction to Materials Science for Engineers,6th edition, Prentice Hall PTR, Paramus, NJ, 2005.
Smith, W. F. and J. Hashemi, Principles of Materi-als Science and Engineering, 4th edition, McGraw-Hill Book Company, New York, 2006.
Van Vlack, L. H., Elements of Materials Science and Engineering,6th edition, Addison-Wesley Longman, Boston, MA, 1989.
White, M. A., Properties of Materials, Oxford University Press, New York, 1999.
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• 15
C h a p t e r 2 Atomic Structure and
Interatomic Bonding
T
his photograph shows the underside of a gecko.Geckos, harmless tropical lizards, are extremely fascinating and extraordinary animals. They have very sticky feet that cling to virtually any surface. This characteristic makes it possible for them to rapidly run up vertical walls and along the undersides of horizontal surfaces. In fact, a gecko can support its body mass with a single toe! The secret to this remarkable ability is the pres-ence of an extremely large number of microscopically small hairs on each of their toe pads. When these hairs come in contact with a surface, weak forces of attraction (i.e., van der Waals forces) are established between hair molecules and molecules on the surface. The fact that these hairs are so small and so numerous explains why the gecko grips surfaces so tightly. To release its grip, the gecko simply curls up its toes, and peels the hairs away from the surface.
Another interesting feature of these toe pads is that they are self-cleaning—that is, dirt parti-cles don’t stick to them. Scientists are just beginning to understand the mechanism of adhesion for these tiny hairs, which may lead to the development of synthetic self-cleaning adhesives. Can you image duct tape that never looses its stickiness, or bandages that never leave a sticky residue?
(Photograph courtesy of Professor Kellar Autumn, Lewis & Clark College, Portland, Oregon.)
An important reason to have an understanding of in-teratomic bonding in solids is that, in some instances, the type of bond allows us to explain a material’s properties. For example, consider carbon, which may exist as both graphite and diamond. Whereas graphite
is relatively soft and has a “greasy” feel to it, diamond is the hardest known material. This dramatic disparity in properties is directly attributable to a type of inter-atomic bonding found in graphite that does not exist in diamond (see Section 12.4).