CRYSTAL IMPERFECTIONS

Outline 1. Introduction 2. Classification of imperfections 3. Effects of imperfections on properties

Introduction • The "right" atoms in "wrong" places. • "Wrong" atoms in "right" or "wrong" places. • Missing atoms.

Crystal Defects Volume Defects Surface Defects Line Defects Point Defects

POINT DEFECTS

Substitutional impurity Point Defects Vacancy Substitutional impurity Self-Interstitials Interstitial Impurity Frenkel imperfection Schottky imperfection

Vacancy: Tensile Stress Fields ?

vacant atomic sites in a structure Vacancy distortion of planes

Substitutional Impurity: Compressive stress fields Tensile Stress Fields

Point Defects in Metals Self-Interstitials: Self-interstitial distortion of planes

Interstitial Impurity Relative size Compressive Stress Fields

Frenkel imperfection vacancy + interstitial site

Schottky imperfection

Electronic Defects

Frenkel and Schottky defects

Line Defects Edge Dislocation Screw Dislocation

Burger Vector

Screw Dislocation Slip Planes Screw Dislocation Slip Direction

Screw Dislocation http://uet.edu.pk/dmems/screw_dislocation.htm

Surface Defects or Interfacial defects

Surface or Interfacial Defects External Surface Grain boundaries Tilt boundary Twist boundary Twin boundary Stacking defects

External Surface

Grain Boundaries (or) Crystal Boundary

Tilt Boundaries

Twin Boundaries

Stacking Defect

Volume Defects Crakes Pores Voids Affects mechanical, electrical and optical properties.

Effect of crystal imperfections and their applications: They change mechanical, electrical, magnetic and optical properties of a material. Materials undergo elastic/plastic deformation. The deformation occurs due to the movement of dislocations. Any other defect in crystal disrupts the motion of dislocation. It makes plastic deformation more difficult. Fatigue: Weakening of material, when it is subjected to stress repeatedly.