2010-01-27 Chapter 04 Imperfections In Solids

2010-01-27 Chapter 04 Imperfections In Solids - CHAPTER 4:...

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ISSUES TO ADDRESS. .. What types of defects arise in solids? Can the number and type of defects be varied and controlled? How do defects affect material properties? Are defects undesirable? CHAPTER 4: IMPERFECTIONS IN SOLIDS Why study imperfection in solids? There are always defects present in the material. Some defects are purposely added to improve the properties. e.g. zinc is added as point defect (impurity atoms) in copper to form an alloy Brass (70% Cu; 30% Zn). Brass is stronger and harder than copper. In integrated circuits (IC) point defects (impurity atoms) are added to form p-type and n-type junctions.
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Chapter 4- 2 Vacancy atoms Interstitial atoms Substitutional atoms Dislocations Grain Boundaries Point defects TYPES OF IMPERFECTIONS Important Line defects Area defects
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Chapter 4- 3 Vacancies : -vacant atomic sites in a structure. Vacancy distortion of planes POINT DEFECTS: Vacancy Important
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Chapter 4- Interstitial Atom (e.g., C in Fe ) POINT DEFECTS: Interstitials Self-Interstitials : -"extra" atoms positioned between atomic sites. self- interstitial distortion of planes
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Chapter 4- Substitutional Atom (e.g., Cu in Ni ) POINT DEFECTS: Substitutional
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Equilibrium number of vacancies varies with temperature! EQUIL. CONCENTRATION: POINT DEFECTS - = kT Q exp . N N v v N v = Number of vacancies N = Total number of atomic sites Q v = Energy required for vacancy formation, eV/atom k = Boltzmann's constant = 8.62 x 10 -5 eV/atom-K T = Temperature, K - = T . k Qv V e . N N
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Example Problem 4.1:Find 3 of Cu at 1000C; Q v = 0.9 eV/atom. ESTIMATING VACANCY CONC. N = Total number of atoms in 1m 3 of copper= ? Temperature = T = 1000 + 273 = 1273 K Boltzmann’s constant = k = 8.62 x 10 -5 eV/atom-K Activation energy = Q v = 0.9 eV/atom - = kT Q exp N N v V Cu A A N N ρ = ρ = Density of copper = 8.4 g/cm 3 = 8.4 x 10 6 g/m 3 A Cu = Atomic weight of copper = 63.5 g/mol N A = Avagadro’s number = 6.023 x 10 23 atoms/mol N = 8.0 x 10 28 atoms in 1 m 3 Important Number of vacancies (N v ) in 1m 3 of copper = 2.2 x 10 25 - = T . k Qv V e . N N
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ALLOYS: TERMINOLOGY Pure metal of only one type of atom is practically not possible. 99.9999% pure metal will have 10 22 to 10 23 impurity atoms per m 3 In Alloys , impurity atoms are intentionally added to enhance mechanical strength and corrosion resistance. e.g. Sterling Silver contains 92.5% silver and 7.5% copper by weight. Solvent represents the element or compound that is present in the greatest amount. Also called host atoms . Silver in Sterling Silver is the solvent . Solute denotes the element or compound present in the minor concentration. Copper in Sterling Silver is the solute. Solute atoms are also called impurity atoms, alloying element atoms, or point defects.
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2010-01-27 Chapter 04 Imperfections In Solids - CHAPTER 4:...

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