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1496T_c01_01-14 12/20/05 7:11 Page 1 2nd REVISE PAGES Chapter 1 Introduction...

1496T_c01_01-14 12/20/05 7:11 Page 1 2nd REVISE PAGES Chapter 1 Introduction A familiar item that is fabricated from three different material types is the beverage container. Beverages are marketed in aluminum (metal) cans (top), glass (ceramic) bottles (center), and plastic (polymer) bottles (bottom). (Permission to use these photographs was granted by the Coca-Cola Company. Coca-Cola, Coca-Cola Classic, the Contour Bottle design and the Dynamic Ribbon are registered trademarks of The Coca-Cola Company and used with its express permission.) 1 1496T_c01_01-14 12/20/05 7:11 Page 2 2nd REVISE PAGES Learning Objectives After careful study of this chapter you should be able to do the following: 1. List six different property classifications of 4. (a) List the three primary classifications of solid materials that determine their applicability. materials, and then cite the distinctive 2. Cite the four components that are involved in chemical feature of each. the design, production, and utilization of (b) Note the two types of advanced materials materials, and briefly describe the interrelation- and, for each, its distinctive feature(s). ships between these components. 5. (a) Briefly define “smart material/system.” 3. Cite three criteria that are important in the ma- (b) Briefly explain the concept of “nanotech- terials selection process. nology” as it applies to materials. 1.1 HISTORICAL PERSPECTIVE Materials are probably more deep-seated in our culture than most of us realize. Transportation, housing, clothing, communication, recreation, and food production— virtually every segment of our everyday lives is influenced to one degree or another by materials. Historically, the development and advancement of societies have been intimately tied to the members’ ability to produce and manipulate materials to fill their needs. In fact, early civilizations have been designated by the level of their materials development (Stone Age, Bronze Age, Iron Age).1 The earliest humans had access to only a very limited number of materials, those that occur naturally: stone, wood, clay, skins, and so on. With time they dis- covered techniques for producing materials that had properties superior to those of the natural ones; these new materials included pottery and various metals. Fur- thermore, it was discovered that the properties of a material could be altered by heat treatments and by the addition of other substances. At this point, materials uti- lization was totally a selection process that involved deciding from a given, rather limited set of materials the one best suited for an application by virtue of its char- acteristics. It was not until relatively recent times that scientists came to understand the relationships between the structural elements of materials and their properties. This knowledge, acquired over approximately the past 100 years, has empowered them to fashion, to a large degree, the characteristics of materials. Thus, tens of thou- sands of different materials have evolved with rather specialized characteristics that meet the needs of our modern and complex society; these include metals, plastics, glasses, and fibers. The development of many technologies that make our existence so comfort- able has been intimately associated with the accessibility of suitable materials. An advancement in the understanding of a material type is often the forerun- ner to the stepwise progression of a technology. For example, automobiles would not have been possible without the availability of inexpensive steel or some other comparable substitute. In our contemporary era, sophisticated elec- tronic devices rely on components that are made from what are called semicon- ducting materials. 1 The approximate dates for the beginnings of Stone, Bronze, and Iron Ages were 2.5 million BC, 3500 BC and 1000 BC, respectively. 1496T_c01_01-14 11/9/05 17:02 Page 3 REVISED PAGES 1.2 Materials Science and Engineering 3 1.2 MATERIALS SCIENCE AND ENGINEERING Sometimes it is useful to subdivide the discipline of materials science and engi- neering into materials science and materials engineering subdisciplines. Strictly speaking, “materials science” involves investigating the relationships that exist between the structures and properties of materials. In contrast, “materials engi- neering” is, on the basis of these structure–property correlations, designing or en- gineering the structure of a material to produce a predetermined set of properties.2 From a functional perspective, the role of a materials scientist is to develop or syn- thesize new materials, whereas a materials engineer is called upon to create new products or systems using existing materials, and/or to develop techniques for pro- cessing materials. Most graduates in materials programs are trained to be both materials scientists and materials engineers. “Structure” is at this point a nebulous term that deserves some explanation. In brief, the structure of a material usually relates to the arrangement of its internal components. Subatomic structure involves electrons within the individual atoms and interactions with their nuclei. On an atomic level, structure encompasses the or- ganization of atoms or molecules relative to one another. The next larger structural realm, which contains large groups of atoms that are normally agglomerated to- gether, is termed “microscopic,” meaning that which is subject to direct observation using some type of microscope. Finally, structural elements that may be viewed with the naked eye are termed “macroscopic.” The notion of “property” deserves elaboration. While in service use, all mate- rials are exposed to external stimuli that evoke some type of response. For exam- ple, a specimen subjected to forces will experience deformation, or a polished metal surface will reflect light. A property is a material trait in terms of the kind and mag- nitude of response to a specific imposed stimulus. Generally, definitions of proper- ties are made independent of material shape and size. Virtually all important properties of solid materials may be grouped into six dif- ferent categories: mechanical, electrical, thermal, magnetic, optical, and deteriorative. For each there is a characteristic type of stimulus capable of provoking different re- sponses. Mechanical properties relate deformation to an applied load or force; exam- ples include elastic modulus and strength. For electrical properties, such as electrical conductivity and dielectric constant, the stimulus is an electric field. The thermal be- havior of solids can be represented in terms of heat capacity and thermal conductiv- ity. Magnetic properties demonstrate the response of a material to the application of a magnetic field. For optical properties, the stimulus is electromagnetic or light radia- tion; index of refraction and reflectivity are representative optical properties. Finally, deteriorative characteristics relate to the chemical reactivity of materials. The chapters that follow discuss properties that fall within each of these six classifications. In addition to structure and properties, two other important components are involved in the science and engineering of materials—namely, “processing” and “performance.” With regard to the relationships of these four components, the struc- ture of a material will depend on how it is processed. Furthermore, a material’s per- formance will be a function of its properties. Thus, the interrelationship between processing, structure, properties, and performance is as depicted in the schematic illustration shown in Figure 1.1. Throughout this text we draw attention to the 2 Throughout this text we draw attention to the relationships between material properties and structural elements. 1496T_c01_01-14 12/20/05 7:11 Page 4 2nd REVISE PAGES 4 Chapter 1 / Introduction Processing Structure Properties Performance Figure 1.1 The four components of the discipline of materials science and engineering and their interrelationship. relationships among these four components in terms of the design, production, and utilization of materials. We now present an example of these processing-structure-properties-performance principles with Figure 1.2, a photograph showing three thin disk specimens placed over some printed matter. It is obvious that the optical properties (i.e., the light transmittance) of each of the three materials are different; the one on the left is trans- parent (i.e., virtually all of the reflected light passes through it), whereas the disks in the center and on the right are, respectively, translucent and opaque. All of these spec- imens are of the same material, aluminum oxide, but the leftmost one is what we call a single crystal—that is, it is highly perfect—which gives rise to its transparency. The center one is composed of numerous and very small single crystals that are all con- nected; the boundaries between these small crystals scatter a portion of the light re- flected from the printed page, which makes this material optically translucent. Finally, the specimen on the right is composed not only of many small, interconnected crys- tals, but also of a large number of very small pores or void spaces. These pores also effectively scatter the reflected light and render this material opaque. Thus, the structures of these three specimens are different in terms of crystal boundaries and pores, which affect the optical transmittance properties. Further- more, each material was produced using a different processing technique. And, of course, if optical transmittance is an important parameter relative to the ultimate in-service application, the performance of each material will be different. Figure 1.2 Photograph of three thin disk specimens of aluminum oxide, which have been placed over a printed page in order to demonstrate their differences in light-transmittance characteristics. The disk on the left is transparent (that is, virtually all light that is reflected from the page passes through it), whereas the one in the center is translucent (meaning that some of this reflected light is transmitted through the disk). And, the disk on the right is opaque—i.e., none of the light passes through it. These differences in optical properties are a consequence of differences in structure of these materials, which have resulted from the way the materials were processed. (Specimen preparation, P. A. Lessing; photography by S. Tanner.) 1496T_c01_01-14 11/9/05 17:02 Page 5 REVISED PAGES 1.4 Classification of Materials 5 1.3 WHY STUDY MATERIALS SCIENCE AND ENGINEERING? Why do we study materials? Many an applied scientist or engineer, whether me- chanical, civil, chemical, or electrical, will at one time or another be exposed to a design problem involving materials. Examples might include a transmission gear, the superstructure for a building, an oil refinery component, or an integrated circuit chip. Of course, materials scientists and engineers are specialists who are totally involved in the investigation and design of materials. Many times, a materials problem is one of selecting the right material from the many thousands that are available. There are several criteria on which the final decision is normally based. First of all, the in-service conditions must be character- ized, for these will dictate the properties required of the material. On only rare occasions does a material possess the maximum or ideal combination of properties. Thus, it may be necessary to trade off one characteristic for another. The classic ex- ample involves strength and ductility; normally, a material having a high strength will have only a limited ductility. In such cases a reasonable compromise between two or more properties may be necessary. A second selection consideration is any deterioration of material properties that may occur during service operation. For example, significant reductions in mechanical strength may result from exposure to elevated temperatures or corrosive environments. Finally, probably the overriding consideration is that of economics: What will the finished product cost? A material may be found that has the ideal set of prop- erties but is prohibitively expensive. Here again, some compromise is inevitable. The cost of a finished piece also includes any expense incurred during fabrication to produce the desired shape. The more familiar an engineer or scientist is with the various characteristics and structure–property relationships, as well as processing techniques of materials, the more proficient and confident he or she will be to make judicious materials choices based on these criteria. 1.4 CLASSIFICATION OF MATERIALS Solid materials have been conveniently grouped into three basic classifications: met- als, ceramics, and polymers. This scheme is based primarily on chemical makeup and atomic structure, and most materials fall into one distinct grouping or another, although there are some intermediates. In addition, there are the composites, com- binations of two or more of the above three basic material classes. A brief explana- tion of these material types and representative characteristics is offered next.Another classification is advanced materials—those used in high-technology applications— viz. semiconductors, biomaterials, smart materials, and nanoengineered materials; these are discussed in Section 1.5. Metals Materials in this group are composed of one or more metallic elements (such as iron, aluminum, copper, titanium, gold, and nickel), and often also nonmetallic elements (for example, carbon, nitrogen, and oxygen) in relatively small amounts.3 Atoms in metals and their alloys are arranged in a very orderly manner (as discussed in Chapter 3), and in comparison to the ceramics and polymers, are relatively dense (Figure 1.3).With 3 The term metal alloy is used in reference to a metallic substance that is composed of two or more elements. 1496T_c01_01-14 11/9/05 17:02 Page 6 REVISED PAGES 6 Chapter 1 / Introduction Figure 1.3 40 Metals Bar-chart of room- temperature density 20 Platinum values for various Silver Density (g/cm3) (logarithmic scale) metals, ceramics, Ceramics 10 Copper polymers, and 8 6 Iron/Steel ZrO2 composite materials. Al2O3 Titanium 4 Polymers Composites SiC,Si3N4 Aluminum Glass GFRC 2 PTFE Concrete Magnesium CFRC PVC PS 1.0 PE 0.8 Rubber 0.6 Woods 0.4 0.2 0.1 regard to mechanical characteristics, these materials are relatively stiff (Figure 1.4) and strong (Figure 1.5), yet are ductile (i.e., capable of large amounts of deformation without fracture), and are resistant to fracture (Figure 1.6), which accounts for their widespread use in structural applications. Metallic materials have large numbers of nonlocalized electrons; that is, these electrons are not bound to particular atoms. Many properties of metals are directly attributable to these electrons. For example, metals are extremely good conductors of electricity (Figure 1.7) and heat, and are not trans- parent to visible light; a polished metal surface has a lustrous appearance. In addi- tion, some of the metals (viz., Fe, Co, and Ni) have desirable magnetic properties. Figure 1.8 is a photograph that shows several common and familiar objects that are made of metallic materials. Furthermore, the types and applications of metals and their alloys are discussed in Chapter 11. Ceramics Ceramics are compounds between metallic and nonmetallic elements; they are most frequently oxides, nitrides, and carbides. For example, some of the common ceramic Figure 1.4 Bar-chart of room- Metals Ceramics 1000 Composites temperature stiffness Stiffness [Elastic (or Young’s) Modulus (in units of Tungsten SiC (i.e., elastic modulus) Iron/Steel AI2O3 values for various Titanium Si3N4 CFRC 100 ZrO2 gigapascals)] (logarithmic scale) metals, ceramics, Aluminum Magnesium Glass GFRC polymers, and Concrete composite materials. 10 Polymers Woods PVC PS, Nylon 1.0 PTFE PE 0.1 Rubbers 0.01 0.001 1496T_c01_01-14 12/20/05 7:11 Page 7 2nd REVISE PAGES 1.4 Classification of Materials 7 Figure 1.5 Bar-chart of room- Metals Composites temperature strength Ceramics (i.e., tensile strength) Strength (Tensile Strength, in units of Steel megapascals) (logarithmic scale) values for various 1000 alloys CFRC Si3N4 metals, ceramics, Cu,Ti GFRC alloys SiC polymers, and Al2O3 composite materials. Aluminum alloys Gold Polymers 100 Glass Nylon Woods PS PVC PTFE PE 10 materials include aluminum oxide (or alumina,Al2O3), silicon dioxide (or silica, SiO2), silicon carbide (SiC), silicon nitride (Si3N4), and, in addition, what some refer to as the traditional ceramics—those composed of clay minerals (i.e., porcelain), as well as cement, and glass. With regard to mechanical behavior, ceramic materials are rela- tively stiff and strong—stiffnesses and strengths are comparable to those of the met- als (Figures 1.4 and 1.5). In addition, ceramics are typically very hard. On the other hand, they are extremely brittle (lack ductility), and are highly susceptible to fracture (Figure 1.6). These materials are typically insulative to the passage of heat and elec- tricity (i.e., have low electrical conductivities, Figure 1.7), and are more resistant to high temperatures and harsh environments than metals and polymers. With regard to optical characteristics, ceramics may be transparent, translucent, or opaque (Figure 1.2), and some of the oxide ceramics (e.g., Fe3O4) exhibit magnetic behavior. Metals Steel alloys Composites 100 Resistance to Fracture (Fracture Toughness, Titanium in units of MPa m) (logarithmic scale) alloys Aluminum CFRC GFRC alloys 10 Ceramics Polymers Si3N4 Nylon Al2O3 Polystyrene SiC Polyethylene 1.0 Wood Polyester Glass Concrete 0.1 Figure 1.6 Bar-chart of room-temperature resistance to fracture (i.e., fracture toughness) for various metals, ceramics, polymers, and composite materials. (Reprinted from Engineering Materials 1: An Introduction to Properties, Applications and Design, third edition, M. F. Ashby and D. R. H. Jones, pages 177 and 178, Copyright 2005, with permission from Elsevier.) 1496T_c01_01-14 12/20/05 7:11 Page 8 2nd REVISE PAGES 8 Chapter 1 / Introduction Figure 1.7 Metals Bar-chart of room- 108 temperature electrical Semiconductors 104 Electrical Conductivity (in units of reciprocal conductivity ranges for metals, ceramics, ohm-meters) (logarithmic scale) polymers, and 1 semiconducting materials. 10–4 10–8 Ceramics Polymers 10–12 10–16 10–20 Several common ceramic objects are shown in the photograph of Figure 1.9. The characteristics, types, and applications of this class of materials are discussed in Chapters 12 and 13. Polymers Polymers include the familiar plastic and rubber materials. Many of them are organic compounds that are chemically based on carbon, hydrogen, and other nonmetallic elements (viz. O, N, and Si). Furthermore, they have very large molecular structures, often chain-like in nature that have a backbone of carbon atoms. Some of the com- mon and familiar polymers are polyethylene (PE), nylon, poly(vinyl chloride) (PVC), polycarbonate (PC), polystyrene (PS), and silicone rubber. These materials typically have low densities (Figure 1.3), whereas their mechanical characteristics are generally dissimilar to the metallic and ceramic materials—they are not as stiff nor as strong as these other material types (Figures 1.4 and 1.5). However, on the basis of their low densities, many times their stiffnesses and strengths on a per mass Figure 1.8 Familiar objects that are made of metals and metal alloys: (from left to right) silverware (fork and knife), scissors, coins, a gear, a wedding ring, and a nut and bolt. (Photograpy by S. Tanner.) 1496T_c01_01-14 12/20/05 7:11 Page 9 2nd REVISE PAGES 1.4 Classification of Materials 9 Figure 1.9 Common objects that are made of ceramic materials: scissors, a china tea cup, a building brick, a floor tile, and a glass vase. (Photography by S. Tanner.) basis are comparable to the metals and ceramics. In addition, many of the polymers are extremely ductile and pliable (i.e., plastic), which means they are easily formed into complex shapes. In general, they are relatively inert chemically and unreactive in a large number of environments. One major drawback to the polymers is their tendency to soften and/or decompose at modest temperatures, which, in some in- stances, limits their use. Furthermore, they have low electrical conductivities (Fig- ure 1.7) and are nonmagnetic. The photograph in Figure 1.10 shows several articles made of polymers that are familiar to the reader. Chapters 14 and 15 are devoted to discussions of the struc- tures, properties, applications, and processing of polymeric materials. Figure 1.10 Several common objects that are made of polymeric materials: plastic tableware (spoon, fork, and knife), billiard balls, a bicycle helmet, two dice, a lawnmower wheel (plastic hub and rubber tire), and a plastic milk carton. (Photography by S. Tanner.) 1496T_c01_01-14 11/9/05 17:02 Page 10 REVISED PAGES 10 Chapter 1 / Introduction MATERIALS OF IMPORTANCE Carbonated Beverage Containers O ne common item that presents some inter- esting material property requirements is the container for carbonated beverages. The material and unreactive with beverages. In addition, each material has its pros and cons. For example, the aluminum alloy is relatively strong (but easily used for this application must satisfy the follow- dented), is a very good barrier to the diffusion of ing constraints: (1) provide a barrier to the pas- carbon dioxide, is easily recycled, beverages are sage of carbon dioxide, which is under pressure in cooled rapidly, and labels may be painted onto its the container; (2) be nontoxic, unreactive with the surface. On the other hand, the cans are optically beverage, and, preferably be recyclable; (3) be rel- opaque, and relatively expensive to produce. Glass atively strong, and capable of surviving a drop is impervious to the passage of carbon dioxide, is from a height of several feet when containing the a relatively inexpensive material, may be recycled, beverage; (4) be inexpensive and the cost to fab- but it cracks and fractures easily, and glass bottles ricate the final shape should be relatively low; are relatively heavy. Whereas the plastic is rela- (5) if optically transparent, retain its optical clar- tively strong, may be made optically transparent, ity; and (6) capable of being produced having is inexpensive and lightweight, and is recyclable, it different colors and/or able to be adorned with is not as impervious to the passage of carbon diox- decorative labels. ide as the aluminum and glass. For example, you All three of the basic material types—metal may have noticed that beverages in aluminum and (aluminum), ceramic (glass), and polymer (poly- glass containers retain their carbonization (i.e., ester plastic)—are used for carbonated beverage “fizz”) for several years, whereas those in two-liter containers (per the chapter-opening photographs plastic bottles “go flat” within a few months. for this chapter).All of these materials are nontoxic 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).4 The 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 4 Fiberglass is sometimes also termed a “glass fiber-reinforced polymer” composite, abbrevi- ated “GFRP.” 1496T_c01_01-14 11/9/05 17:02 Page 11 REVISED PAGES 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 1496T_c01_01-14 12/20/05 7:11 Page 12 2nd REVISE PAGES 12 Chapter 1 / Introduction 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 observation 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 (109 m)—as a rule, less than 100 nanometers (equivalent to approximately 500 atom diameters).5 One 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 5 One 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.” 1496T_c01_01-14 11/9/05 17:02 Page 13 REVISED PAGES References 13 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. Solar 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 Ashby, M. F. and D. R. H. Jones, Engineering Ma- cessing and Design, 3rd edition, Butterworth- terials 1, An Introduction to Their Properties Heinemann, Woburn, UK, 2005. and Applications, 3rd edition, Butterworth- Askeland, D. R. and P. P. Phulé, The Science and Heinemann, Woburn, UK, 2005. Engineering of Materials, 5th edition, Nelson Ashby, M. F. and D. R. H. Jones, Engineering Mate- (a division of Thomson Canada), Toronto, rials 2, An Introduction to Microstructures, Pro- 2006. 1496T_c01_01-14 11/9/05 17:02 Page 14 REVISED PAGES 14 Chapter 1 / Introduction Baillie, C. and L. Vanasupa, Navigating the Materials Schaffer, J. P., A. Saxena, S. D. Antolovich, T. H. World, Academic Press, San Diego, CA, 2003. Sanders, Jr., and S. B. Warner, The Science and Flinn, R. A. and P. K. Trojan, Engineering Materi- Design of Engineering Materials, 2nd edition, als and Their Applications, 4th edition, John WCB/McGraw-Hill, New York, 1999. Wiley & Sons, New York, 1994. Shackelford, J. F., Introduction to Materials Science Jacobs, J. A. and T. F. Kilduff, Engineering Materi- for Engineers, 6th edition, Prentice Hall PTR, als Technology, 5th edition, Prentice Hall PTR, Paramus, NJ, 2005. Paramus, NJ, 2005. Smith, W. F. and J. Hashemi, Principles of Materi- Mangonon, P. L., The Principles of Materials Selec- als Science and Engineering, 4th edition, tion for Engineering Design, Prentice Hall McGraw-Hill Book Company, New York, 2006. PTR, Paramus, NJ, 1999. Van Vlack, L. H., Elements of Materials Science McMahon, C. J., Jr., Structural Materials, Merion and Engineering, 6th edition, Addison-Wesley Books, Philadelphia, 2004. Longman, Boston, MA, 1989. Murray, G. T., Introduction to Engineering Materi- White, M. A., Properties of Materials, Oxford als—Behavior, Properties, and Selection, Marcel University Press, New York, 1999. 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. 1496T_c02_15-37 12/20/05 7:19 Page 15 2nd REVISE PAGES Chapter 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.) WHY STUDY Atomic Structure and Interatomic Bonding? An important reason to have an understanding of in- is relatively soft and has a “greasy” feel to it, diamond teratomic bonding in solids is that, in some instances, is the hardest known material. This dramatic disparity the type of bond allows us to explain a material’s in properties is directly attributable to a type of inter- properties. For example, consider carbon, which may atomic bonding found in graphite that does not exist exist as both graphite and diamond. Whereas graphite in diamond (see Section 12.4). 15 1496T_c02_15-37 12/20/05 7:19 Page 16 2nd REVISE PAGES Learning Objectives After careful study of this chapter you should be able to do the following: 1. Name the two atomic models cited, and note (b) Note on this plot the equilibrium separation the differences between them. and the bonding energy. 2. Describe the important quantum-mechanical 4. (a) Briefly describe ionic, covalent, metallic, principle that relates to electron energies. hydrogen, and van der Waals bonds. 3. (a) Schematically plot attractive, repulsive, and (b) Note which materials exhibit each of these net energies versus interatomic separation bonding types. for two atoms or ions. 2.1 INTRODUCTION Some of the important properties of solid materials depend on geometrical atomic arrangements, and also the interactions that exist among constituent atoms or mol- ecules. This chapter, by way of preparation for subsequent discussions, considers several fundamental and important concepts—namely, atomic structure, electron configurations in atoms and the periodic table, and the various types of primary and secondary interatomic bonds that hold together the atoms comprising a solid. These topics are reviewed briefly, under the assumption that some of the material is familiar to the reader. A t o m i c St r u c t u r e 2.2 FUNDAMENTAL CONCEPTS Each atom consists of a very small nucleus composed of protons and neutrons, which is encircled by moving electrons. Both electrons and protons are electrically charged, the charge magnitude being 1.60  1019 C, which is negative in sign for electrons and positive for protons; neutrons are electrically neutral. Masses for these sub- atomic particles are infinitesimally small; protons and neutrons have approximately the same mass, 1.67  1027 kg, which is significantly larger than that of an elec- tron, 9.11  1031 kg. Each chemical element is characterized by the number of protons in the nu- atomic number cleus, or the atomic number (Z).1 For an electrically neutral or complete atom, the atomic number also equals the number of electrons. This atomic number ranges in integral units from 1 for hydrogen to 92 for uranium, the highest of the naturally occurring elements. The atomic mass (A) of a specific atom may be expressed as the sum of the masses of protons and neutrons within the nucleus. Although the number of protons is the same for all atoms of a given element, the number of neutrons (N) may be isotope variable. Thus atoms of some elements have two or more different atomic masses, which are called isotopes. The atomic weight of an element corresponds to the atomic weight weighted average of the atomic masses of the atom’s naturally occurring isotopes.2 atomic mass unit The atomic mass unit (amu) may be used for computations of atomic weight. A 1 Terms appearing in boldface type are defined in the Glossary, which follows Appendix E. 2 The term “atomic mass” is really more accurate than “atomic weight” inasmuch as, in this context, we are dealing with masses and not weights. However, atomic weight is, by conven- tion, the preferred terminology and will be used throughout this book. The reader should note that it is not necessary to divide molecular weight by the gravitational constant. 1496T_c02_15-37 11/10/05 10:42 Page 17 REVISED PAGES 2.3 Electrons in Atoms 17 scale has been established whereby 1 amu is defined as 121 of the atomic mass of the most common isotope of carbon, carbon 12 1 12C2 1A  12.000002. Within this scheme, the masses of protons and neutrons are slightly greater than unity, and AZN (2.1) The atomic weight of an element or the molecular weight of a compound may be specified on the basis of amu per atom (molecule) or mass per mole of material. mole In one mole of a substance there are 6.023  1023 (Avogadro’s number) atoms or molecules. These two atomic weight schemes are related through the following equation: 1 amu/atom 1or molecule2  1 g/mol For example, the atomic weight of iron is 55.85 amu/atom, or 55.85 g/mol. Sometimes use of amu per atom or molecule is convenient; on other occasions g (or kg)/mol is preferred. The latter is used in this book. Concept Check 2.1 Why are the atomic weights of the elements generally not integers? Cite two reasons. [The answer may be found at www.wiley.com/college/callister (Student Companion Site).] 2.3 ELECTRONS IN ATOMS Atomic Models During the latter part of the nineteenth century it was realized that many phe- nomena involving electrons in solids could not be explained in terms of classical mechanics. What followed was the establishment of a set of principles and laws that quantum mechanics govern systems of atomic and subatomic entities that came to be known as quantum mechanics. An understanding of the behavior of electrons in atoms and crystalline solids necessarily involves the discussion of quantum-mechanical concepts. How- ever, a detailed exploration of these principles is beyond the scope of this book, and only a very superficial and simplified treatment is given. Bohr atomic model One early outgrowth of quantum mechanics was the simplified Bohr atomic model, in which electrons are assumed to revolve around the atomic nucleus in discrete orbitals, and the position of any particular electron is more or less well defined in terms of its orbital. This model of the atom is represented in Figure 2.1. Another important quantum-mechanical principle stipulates that the energies of electrons are quantized; that is, electrons are permitted to have only specific val- ues of energy. An electron may change energy, but in doing so it must make a quan- tum jump either to an allowed higher energy (with absorption of energy) or to a lower energy (with emission of energy). Often, it is convenient to think of these al- lowed electron energies as being associated with energy levels or states. These states do not vary continuously with energy; that is, adjacent states are separated by finite energies. For example, allowed states for the Bohr hydrogen atom are represented in Figure 2.2a. These energies are taken to be negative, whereas the zero reference is the unbound or free electron. Of course, the single electron associated with the hydrogen atom will fill only one of these states. 1496T_c02_15-37 12/20/05 13:51 Page 18 2nd REVISE PAGES 18 Chapter 2 / Atomic Structure and Interatomic Bonding Orbital electron Figure 2.1 Schematic representation of the Bohr atom. Nucleus Thus, the Bohr model represents an early attempt to describe electrons in atoms, in terms of both position (electron orbitals) and energy (quantized energy levels). This Bohr model was eventually found to have some significant limitations because of its inability to explain several phenomena involving electrons. A wave-mechanical resolution was reached with a wave-mechanical model, in which the electron is model considered to exhibit both wave-like and particle-like characteristics. With this model, an electron is no longer treated as a particle moving in a discrete or- bital; rather, position is considered to be the probability of an electron’s being at various locations around the nucleus. In other words, position is described by a probability distribution or electron cloud. Figure 2.3 compares Bohr and wave- mechanical models for the hydrogen atom. Both these models are used through- out the course of this book; the choice depends on which model allows the more simple explanation. 0 0 Figure 2.2 (a) The first three electron 3d –1.5 n=3 3p energy states for the 3s Bohr hydrogen atom. 2p (b) Electron energy –3.4 n=2 states for the first 2s three shells of the –5 wave-mechanical hydrogen atom. –1 × 10–18 (Adapted from W. G. Energy (eV) Energy (J) Moffatt, G. W. Pearsall, and J. Wulff, The Structure and Properties of Materials, Vol. I, –10 Structure, p. 10. Copyright © 1964 by John Wiley & Sons, –2 × 10–18 New York. Reprinted by permission of John –13.6 n=1 1s Wiley & Sons, Inc.) –15 (a) (b) 1496T_c02_15-37 12/20/05 7:19 Page 19 2nd REVISE PAGES 2.3 Electrons in Atoms 19 Figure 2.3 Comparison of 1.0 the (a) Bohr and (b) wave- mechanical atom models in terms of electron distribution. (Adapted from Z. D. Jastrzebski, The Nature and Properties of Engineering Materials, 3rd edition, p. 4. Copyright © Probability 1987 by John Wiley & Sons, New York. Reprinted by permission of John Wiley & Sons, Inc.) 0 Distance from nucleus Orbital electron Nucleus (a) (b) Quantum Numbers Using wave mechanics, every electron in an atom is characterized by four parame- quantum number ters called quantum numbers. The size, shape, and spatial orientation of an electron’s probability density are specified by three of these quantum numbers. Furthermore, Bohr energy levels separate into electron subshells, and quantum numbers dictate the number of states within each subshell. Shells are specified by a principal quantum number n, which may take on integral values beginning with unity; sometimes these shells are designated by the letters K, L, M, N, O, and so on, which correspond, respectively, to n  1, 2, 3, 4, 5,... , as indicated in Table 2.1. Note also that this quan- tum number, and it only, is also associated with the Bohr model. This quantum num- ber is related to the distance of an electron from the nucleus, or its position. The second quantum number, l, signifies the subshell, which is denoted by a lowercase letter—an s, p, d, or f; it is related to the shape of the electron subshell. In addition, the number of these subshells is restricted by the magnitude of n. Allowable subshells for the several n values are also presented in Table 2.1. The number of energy states for each subshell is determined by the third quantum num- ber, ml. For an s subshell, there is a single energy state, whereas for p, d, and f sub- shells, three, five, and seven states exist, respectively (Table 2.1). In the absence of an external magnetic field, the states within each subshell are identical. However, when a magnetic field is applied these subshell states split, each state assuming a slightly different energy. 1496T_c02_15-37 11/10/05 10:42 Page 20 REVISED PAGES 20 Chapter 2 / Atomic Structure and Interatomic Bonding Table 2.1 The Number of Available Electron States in Some of the Electron Shells and Subshells Principal Number of Electrons Quantum Shell Number Number n Designation Subshells of States Per Subshell Per Shell 1 K s 1 2 2 s 1 2 2 L 8 p 3 6 s 1 2 3 M p 3 6 18 d 5 10 s 1 2 p 3 6 4 N 32 d 5 10 f 7 14 Associated with each electron is a spin moment, which must be oriented either up or down. Related to this spin moment is the fourth quantum number, ms, for which two values are possible ( 12 and 12), one for each of the spin orientations. Thus, the Bohr model was further refined by wave mechanics, in which the in- troduction of three new quantum numbers gives rise to electron subshells within each shell. A comparison of these two models on this basis is illustrated, for the hydrogen atom, in Figures 2.2a and 2.2b. A complete energy level diagram for the various shells and subshells using the wave-mechanical model is shown in Figure 2.4. Several features of the diagram are worth noting. First, the smaller the principal quantum number, the lower the energy level; for example, the energy of a 1s state is less than that of a 2s state, which in turn is lower than the 3s. Second, within each shell, the energy of a subshell level in- creases with the value of the l quantum number. For example, the energy of a 3d state is greater than a 3p, which is larger than 3s. Finally, there may be overlap in Figure 2.4 Schematic representation of the relative f d energies of the electrons for the f d p s various shells and subshells. (From f d p K. M. Ralls, T. H. Courtney, and s J. Wulff, Introduction to Materials d p Science and Engineering, p. 22. s Copyright © 1976 by John Wiley & p Energy d s Sons, New York. Reprinted by permission of John Wiley & Sons, p s Inc.) p s s 1 2 3 4 5 6 7 Principal quantum number, n 1496T_c02_15-37 11/10/05 13:32 Page 21 REVISED PAGES 2.3 Electrons in Atoms 21 energy of a state in one shell with states in an adjacent shell, which is especially true of d and f states; for example, the energy of a 3d state is greater than that for a 4s. Electron Configurations electron state The preceding discussion has dealt primarily with electron states—values of energy that are permitted for electrons.To determine the manner in which these states are filled Pauli exclusion with electrons, we use the Pauli exclusion principle, another quantum-mechanical principle concept. This principle stipulates that each electron state can hold no more than two electrons, which must have opposite spins. Thus, s, p, d, and f subshells may each ac- commodate, respectively, a total of 2, 6, 10, and 14 electrons; Table 2.1 summarizes the maximum number of electrons that may occupy each of the first four shells. Of course, not all possible states in an atom are filled with electrons. For most atoms, the electrons fill up the lowest possible energy states in the electron shells and subshells, two electrons (having opposite spins) per state. The energy structure for a sodium atom is represented schematically in Figure 2.5. When all the electrons oc- cupy the lowest possible energies in accord with the foregoing restrictions, an atom ground state is said to be in its ground state. However, electron transitions to higher energy states are possible, as discussed in Chapters 18 and 21. The electron configuration or struc- electron ture of an atom represents the manner in which these states are occupied. In the configuration conventional notation the number of electrons in each subshell is indicated by a su- perscript after the shell–subshell designation. For e