MAE381 4 2010

MAE381 4 2010 - Chapter 4: Crystal imperfections Reading...

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Unformatted text preview: Chapter 4: Crystal imperfections Reading assignment Chapter 4 Crystalline material Noncrystalline material Defect types Point defect : associated with 1 or 2 atomic positions); vacancy, solute atom or interstitial. Line defect : 1-D; dislocations Surface defect : 2-D; grain boundaries and external surfaces. Point defects Vacancy Interstitial atom Small substitutional atom Large substitutional atom Point defects: vacancy and interstitial Vacancy, a point defect A vacancy is produced when an atom is missing from a normal lattice site. Vacancies are introduced during solidification, at high Ts, or as a consequence of radiation damage. *Same number of atoms Without Vacancies With Vacancies /RT H V v e X- where H v = energy of formation of a mole of vacancy, R = gas constant = 8.3143 J mole-1 and T = temperature in K. Fraction of atom sites vacant T X V 1 The larger is H v , the smaller is X V . This is because the larger is H v , the more is the energy needed to form a vacancy. An alternative form of the above expression is , /kT h V v e X- where h v = energy of formation of a single vacancy, = k = Boltzmanns constant = N = Avogadros number and T = temperature in K. N H v N R Let X V1 be the value of X V when T = T 1 Let X V2 be the value of X V when T = T 2 Let A be the proportionality constant, so that ------- = = = = = 1 2 v 1 v 2 v 2 v 1 v v T 1 T 1 R H /R T H /R T H V1 V2 /R T H V2 /R T H V1 /R T H V e Ae Ae X X Hence, Ae X Ae X Then Ae X -- = -- = 1 2 V V1 V2 10 1 2 V V1 V2 T 1 T 1 2.3R H X X log T 1 T 1 R H X X ln Thus, if H v is known and X V1 is known for T=T 1 , then the above equation can be used to calculate X V2 for T=T 2 . Vacancy in an ionic solid Schottky defect Impurity A point defect impurity alloying element solvent: the element or compound in a solution (alloy), present in the greatest amount. host atoms solute: element or compound in minor concentration. solid solution: an alloy resulting by the addition of solute atoms to the solvent. Types of impurity Substitutional impurity Interstitial impurity Impurities in Solids: Solid Solutions Contamination of materials is unavoidable.Contamination of materials is unavoidable....
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This note was uploaded on 12/19/2010 for the course MAE 381 taught by Professor Chung during the Spring '08 term at SUNY Buffalo.

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MAE381 4 2010 - Chapter 4: Crystal imperfections Reading...

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