Imperfections in Solids.pptx

7/24/2019 Imperfections in Solids.pptx

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Imperfections in

Solids


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Why Study Imperfections inSolids? The properties of some materials are profoundly inuenced b

the presence of imperfections. Consequently, it is important t

have a knoled!e about the types of imperfections that e"ist anthe roles they play in a#ectin! the behavior of materials. $o

e"ample, the mechanical properties of pure metals e"periencsi!ni%cant alterations hen alloyed &i.e., hen impurity atom

are added'(for e"ample, brass &)*+ copper-*+ inc' is mucharder and stron!er than pure copper &Section )./'. 0ls

inte!rated circuit microelectronic devices found in ou

computers, calculators, and home appliances function because hi!hly controlled concentrations of speci%c impurities that ar

incorporated into small, localied re!ions of semiconductinmaterials


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three basic types ofimperfections1 point defects,

line defects (or dislocations), and

surface defects.


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2oint defects are localied disruptions in otherisperfect atomic or ionic arran!ements in a crystastructure

These imperfections may be introduced by movemenof the atoms or ions hen they !ain ener!y by heatin!

durin! processin! of the material, or by the intentionaor unintentional introduction of impurities.


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IMPURITIESare elements or compounds that are present from ramaterials or processin!. $or e"ample, silicon crystals !ron inquart crucibles contain o"y!en as an impurity.

DOPANTS, on the other hand, are elements or compounds that aredeliberately added, in knon concentrations, at speci%c locations i

the microstructure, ith an intended bene%cial e#ect on propertiesor processin!


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8. 90C04CI6S

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0 vacancy is produced hen an atom or an ion is missin! from itsnormal site in the crystal structure


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The equilibrium number of vacancies for a !iven quantity ofmaterial depends on and increases ith temperature accordin! to


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6F0=2


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6F0=2


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0n interstitial defect is formed hen an e"tra atom orion is inserted into the crystal structure at a normallyunoccupied position


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Interstitial atoms or ions, althou!h much smaller than the atoms oions located at the lattice points, are still lar!er than the interstitialsites that they occupyM consequently, the surroundin! crystal re!iois compressed and distorted.

Interstitial atoms such as hydro!en are often present as impuritieshereas carbon atoms are intentionally added to iron to produce

steel. $or small concentrations, carbon atoms occupy interstitialsites in the iron crystal structure, introducin! a stress in thelocalied re!ion of the crystal in their vicinity


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6F0=2


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0 substitutional defectis introduced hen one atom oion is replaced by a di#erent type of atom or ion. Thsubstitutional atoms or ions occupy the normal latticsite. Substitutional atoms or ions may either be lar!ethan the normal atoms or ions in the crystal structure, ihich case the surroundin! interatomic spacin!s arreduced, or smaller causin! the surroundin! atoms thave lar!er interatomic spacin!s.


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0n interstitialcis created hen an atom identical to those at thenormal lattice points is located in an interstitial position. Thesedefects are most likely to be found in crystal structures havin! a lopackin! factor.

0 !ren"el defectis a vacancyJinterstitial pair formed hen anion Numps from a normal lattice point to an interstitial site, leavin!behind a vacancy. 0lthou!h, this is usually associated ith ionicmaterials, a $renkel defect can occur in metals and covalentlybonded materials


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0 Sc#ott" defectis unique to ionic materials and is commonlyfound in many ceramic materials. When vacancies occur in anionically bonded material, a stoichiometric number of anions andcations must be missin! from re!ular atomic positions if electricalneutrality is to be preserved. $or e"ample, one =!O: vacancy andone 3J: vacancy in =!3 constitute a Schottky pair. In Pr3:, for onePrO@ vacancy, there ill be to 3J: vacancies.


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5IS


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SC;6W 5IS


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65A6 5IS


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=IF65 5IS


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>;A6;S 96CT3;

JJJJ The displacement vector that Qcloses the loopR hen traversin!an equal number of lattice steps around the defect

JJJJ denoted by QbR

JJJJJ it is parallel in the line of dislocation in scre dislocation and isperpendicular in the line of dislocation in ed!e dislocation


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ur!ers vector in scredislocation


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ur!ers vector in ed!e dislocation


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6"ample1

Calculate the len!th of bur!er vector in copper &$CC crystalstructure'


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S>;$0C6 56$6CTS

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Imperfections in Solids.pptx