Fundamentals of Materials Science and Engineering - An Integrated Approach

Fundamentals of Materials Science and Engineering - An Integrated Approach

(Parte 5 de 7)

1.5 ADVANCED MATERIALS

Materials that are utilized in high-technology (or high-tech) applications are sometimes termed advanced materials. By high technology we mean a device or product that operates or functions using relatively intricate and sophisticated principles; examples include electronic equipment (camcorders, CD/DVD players, etc.), computers, 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.

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MATERIAL S OF IMPORTANCE Carbonated Beverage Containers

One common item that presents some interesting material property requirements is the container for carbonated beverages. The material used for this application must satisfy the following constraints: (1) provide a barrier to the passage of carbon dioxide, which is under pressure in the container; (2) be nontoxic, unreactive with the beverage, and, preferably be recyclable; (3) be relativelystrong,andcapableofsurvivingadropfrom a height of several feet when containing the beverage; (4) be low-cost and relatively inexpensive to fabricate; (5) if optically transparent, retain its optical clarity; 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 (polyester 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 diffusionofcarbondioxide,iseasilyrecycled,beverages are cooled rapidly, and labels may be painted onto itssurface.Ontheotherhand,thecansareoptically opaque, and relatively expensive to produce. Glass is impervious to the passage of carbon dioxide, is a relatively inexpensive material, and may be recycled,butitcracksandfractureseasily,andglassbottlesarerelativelyheavy.Whereastheplasticisrelatively strong, may be made optically transparent, is inexpensive and lightweight, and is recyclable, it is not as impervious to the passage of carbon dioxide 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.

Semiconductor s

Semiconductors have electrical properties that are intermediate between the electrical conductors (viz. metals and metal alloys) and insulators (viz. ceramics and polymers)—Figure 1.7. Furthermore, the electrical characteristics of these materials are extremely sensitive to the presence of minute concentrations of impurity atoms, for which the concentrations may be controlled over very small spatial regions. 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.

Biomater ials

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, polymers, composites, and semiconductors—may be used as biomaterials. For example, some of the biomaterials that are utilized in artificial hip replacements are discussed in the online biomaterials module.

Mater ials of the Futur e

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 technologies.

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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 being extended to rather sophisticated systems that consist of both smart and traditional 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 function).Actuatorsmaybecalledupontochangeshape,position,naturalfrequency, or mechanical characteristics in response to changes in temperature, light intensity, electric fields, and/or magnetic fields.

Four types of materials are commonly used for actuators: shape-memory alloys, piezoelectric ceramics, magnetostrictive materials, and electrorheological/ magnetorheological fluids. Shape-memory alloys are metals that, after having been deformed, revert back to their original shapes when temperature is changed (see the Materials of Importance piece following Section 1.9). Piezoelectric ceramics expandandcontractinresponsetoanappliedelectricfield(orvoltage);conversely,they also generate an electric field when their dimensions are altered (see Section 12.25). The behavior of magnetostrictive materials is analogous to that of the piezoelectrics, except that they are responsive to magnetic fields. Also, electrorheological and magnetorheological 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 19.14), piezoelectric materials (including some polymers), and microelectromechanical devices (MEMS, Section 13.10).

For example, one type of smart system is used in helicopters to reduce aerodynamic 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 that 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 complex structures,andthentoinvestigatethefundamentalbuildingblocksofthesestructures thataresmallerandsimpler.Thisapproachissometimestermed“top-down”science. However,withtheadventofscanningprobemicroscopes(Section5.12),whichpermit observation of individual atoms and molecules, it has become possible to manipulate andmoveatomsandmoleculestoformnewstructuresand,thus,designnewmaterials that are built from simple atomic-level constituents (i.e., “materials by design”). This ability to carefully arrange atoms provides opportunities 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 denotes that the dimensions of these structural entities are on the order of a nanometer (10−9 m)—as a rule, less than 100 nanometers (equivalent to approximately 500 atom diameters).5 One example of a

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 entitled “There is Plenty of Room at the Bottom.”

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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 materials, as well as consideration of the environmental impact of materials production. 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 highstrength,low-densitystructuralmaterialsremaintobedeveloped,aswellasmaterials that have higher-temperature capabilities, for use in engine components.

Furthermore, there is a recognized need to find new, economical sources of energy and to use present resources more efficiently. Materials will undoubtedly play a significant role in these developments. For example, the direct conversion of solar 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 nonpolluting. It is just beginning to be implementedinbatteriesforelectronicdevices,andholdspromiseasthepowerplant forautomobiles.Newmaterialsstillneedtobedevelopedformoreefficientfuelcells, 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 produce less environmental degradation—that is, less pollution and less spoilage of the landscape from the mining of raw materials. Also, in some materials manufacturing processes, toxic substances are produced, and the ecological impact of their disposal must be considered.

Manymaterialsthatweusearederivedfromresourcesthatarenonrenewable— thatis,notcapableofbeingregenerated.Theseincludepolymers,forwhichtheprime rawmaterialisoil,andsomemetals.Thesenonrenewableresourcesaregraduallybecoming depleted, which necessitates: (1) the discovery of additional reserves, (2) the development of new materials having comparable properties with less adverse environmentalimpact,and/or(3)increasedrecyclingeffortsandthedevelopmentofnew 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 relative 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 20.

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Ashby, M. F. and D. R. H. Jones, Engineering

Materials 1, An Introduction to Their Properties and Applications, 3rd edition, Butterworth- Heinemann, Woburn, UK, 2005.

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Engineering, Science, Processing and Design, Butterworth-Heinemann (an imprint of Elsevier), Oxford, 2007.

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Engineering of Materials, 5th edition, Nelson (a division of Thomson Canada), Toronto, 2006.

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1.1 Select one or more of the modern items or devices listed below, and then conduct an Internet search in order to determine what specific material(s) is (are) used and what specific properties this (these) material(s) possess(es) in order forthedevice/itemtofunctionproperly.Finally, write a short essay in which you report your findings.

Cell phone/digital camera batteries Cell phone displays Solar cells Wind turbine blades Fuel cells Automobile engine blocks (other than cast iron)

Automobile bodies (other than steel alloys) Space telescope mirrors Military body armor Sports Equipment

Soccer balls Basketballs Skis Ski poles Ski boots Snowboards Surfboards Golf clubs Golf balls Kayaks Lightweight bicycle frames

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Chap ter 2 Atomic Structure and Interatomic Bonding

This 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 presence 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.

(Parte 5 de 7)

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