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section%204.5%20-%20Crystalline%20Defects - 4.5 Crystalline...

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4.5 Crystalline Defects Objectives List and draw crystal defects. Calculate vacancy concentration. Draw Burgers vector and describe its characteristics. Define “single crystal” and “polycrystal”. Up until now we have assumed that the atoms in crystals are arranged perfectly in the unit cells, and that this perfect arrangement extends everywhere in a crystal. In fact, this is not true. Crystals can have several types of defects. We can divide these up based on the dimension of the defect. There are point defects (zero dimensional), line defects (one dimensional), and planar defects (two dimensional). The possible point defects are shown in Figure 4.5.1. Figure 4.5.1: Three types of point defects in crystals. GI 4.5.1 List and describe in your own words three types of point defects in crystals. There are specific names for each of these defects. Figure 4.5.1(a) shows a vacancy . Figure 4.5.1(b) shows a substitutional impurity ; it is called substitutional because a different type of atom has been substituted for the normal atoms in that crystal. The spaces in between the normal atoms are called interstitial sites, and so Figure 4.5.1(c) shows an interstitial impurity . Impurities are very important in affecting the properties of materials. For example, compare 24 karat gold is pure gold and is extremely soft. In comparison 14 karat gold is only 58.33 wt% gold, with the rest being mostly copper and some silver. These additional elements make 14 karat gold much harder, but also cheaper. We will learn in Chapter 9 why those impurities increase the strength of gold.
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Concept Check The atomic radius of gold is 0.144 nm and the radius of copper is 0.128 nm. In 14 karat gold is the copper present as a substitutional or interstitial impurity? a. substitutional b. intertsitial c. Could be either. The presence of vacancies is required by thermodynamics; the presence of vacancies increases the entropy of the crystal. The number of vacancies in a crystal can be calculated using the Arrhenius equation: = - Nv Nexp QvkT (Eq. 4.5.1) where N is the total number of atoms, Q v is the activation energy for vacancy formation, k is Boltzmann’ constant, and T is the temperature. Note: exp means e raised to the power of whatever is in the parentheses.
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section%204.5%20-%20Crystalline%20Defects - 4.5 Crystalline...

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