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ch02 - pg020[V G4 7-27060 IRWIN Schaffer A R I T...

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I P A R T FUNDAMENTALS 2 ATOMIC SCALE STRUCTURES 3 CRYSTAL STRUCTURES 4 POINT DEFECTS AND DIFFUSION 5 LINEAR, PLANAR, AND VOLUME DEFECTS 6 NONCRYSTALLINE AND SEMICRYSTALLINE MATERIALS
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n Part I we will be concerned with how atoms bond to- gether in an infinite array to form engineering materials. These materials possess specific properties that are a direct result of their atomic scale structures. We will learn that in some solids, called crystalline materials, the atomic ar- I rangement is in the form of a periodic array of atoms. In other solids, called noncrystalline or amorphous materials, the atoms are arranged in a near random fashion. Since crystals contain large numbers of atoms—one gram contains on the order of 10 23 atoms—defects in the atomic arrangement are inevitable. These defects have a significant impact on the properties of the crystal. It is important for engineers to understand the atomic scale structures of crystalline and noncrystalline materials if they are to use materials effectively in the design and construction of com- mercial products. As an example, consider the case of a cam- corder. Camcorders must be light and compact yet sturdy, have automatic controls to compensate for light intensity and distance, and have high-quality optics and sound-recording capability, a pleasing appearance, and a competitive price. Engineers must satisfy these requirements by using a variety of materials with specific properties to achieve an optimum design. Let us examine the major components of a camcorder. The lenses must transmit light without optical aberrations to achieve a high-quality picture. Lenses are made from glasses, which owe their transparency to their noncrystalline structure. The hardness of glass, which makes it suitable for precision grinding to achieve the proper lens shape, is a consequence of its three-dimensional bond network, as discussed in Chapters 2 and 6. The camcorder housing parts must be light and easily formed into intricate shapes. These components are made from polymers composed of molecular chains of light elements that are loosely packed together resulting in a low-density material. Since the molecular chains are attached to each other by weak secondary bonds, which disintegrate easily upon heating, these materials have low softening points, which permit them to be shaped inex- pensively. The structure of polymers will be discussed in Chap- ters 2 and 6. A camcorder also contains a large number of electronic com- ponents, including: (1) electrically conducting wires made from (Courtesy of Sony Electronics Inc.) metals such as copper or gold, (2) precision resistors fabricated from carbon or nickel-chromium alloys, (3) semiconductors such as silicon doped with phosphorus or boron, and (4) ceramic or poly- mer insulators that isolate electrical signals between active ele- ments. The tremendous variation in the electrical properties of these and other materials is a result of their atomic and crystal structures, as described in Chapters 2 and 3.
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