Defects in crystals

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Information about Defects in crystals

Published on November 20, 2011

Author: ganeshbilla


Defects in crystals: Defects in crystals 11/20/2011 1 Imperfections in crystals: Imperfections in crystals Crystalline imperfections can be classified on the basis of their geometry as follows 11/20/2011 2 PowerPoint Presentation: A . vacancies/ schottky defecfts B. Interstitial Vacancies/ Frenkel defecfts C. Compositional defecfts Interstitial impurity Substitutional impurity D. Electronic impurity Point Defects Line Defects Surface Defects Volume Defects 1. Edge Dislocation 2. Screw Dislocation a . Grain boundaries b. Twin boundaries c. Tilt boundaries d . Stacking fault Non Crystalline regions of dimensions at least 10 to 30°A Crystalline Defects 11/20/2011 3 Point Defects: Point Defects Vacancy : A Vacancy refers to an atomic site from where the atom is missing. Tensile Stress Fields ? 11/20/2011 4 Point Defects: Point Defects Substitutional impurity : A Substitutional impurity refers to a foreign atom that has replaced a parent atom. Tensile Stress Fields Compressive stress fields 11/20/2011 5 Point Defects: Point Defects Interstitial Impurity : An Interstitial impurity refers to small sized atom occupying the void space in the parent crystal without replacing the parent atoms Compressive Stress Fields Relative size 11/20/2011 6 Point Defects: Point Defects Frenkel imperfection : An ion displaced from a regular site to an interstitial site is called frenkel imperfection. 11/20/2011 7 Point Defects: Point Defects Schottky imperfection : A pair of cation and anion missing from an ionic crystal resulting in a pair of vacant ion sites is called schottky imperfection 11/20/2011 8 Point Defects: Point Defects Electronic Defects : Errors in charge distribution in solids are called electronic defects. These defects are produced when the composition of an ionic crystal does not correspond to the exact stoichiometric formula. 11/20/2011 9 Line Defects: Line Defects Line defects are one dimensional imperfections in the geometrical sense. Line imperfections are called dislocations. Edge dislocation In perfect crystal atoms are arranged in both vertical and horizontal planes parallel to the side faces. If one of these planes does not extend to the full length but ends in between within the crystal it is called edge dislocation. 11/20/2011 10 Edge dislocation : Edge dislocation . Edge dislocation are caused by the termination of a plane of atoms in the middle of the crystal. 11/20/2011 11 PowerPoint Presentation: 11/20/2011 12 Burger’s Vector: Burger’s Vector The presence of dislocation results in lattice strain (distortion). The direction and magnitude of such distortion is expressed in terms of a Burger’s vector. Or The magnitude and the direction of the displacement are defined by a vector called the burger vector. 11/20/2011 13 PowerPoint Presentation: 4 3 3 4 4 3 3 4 Above: Burger’s circuit for dislocation-free material. To Right: Do same with dislocation and end up “past” starting point. Vector b = distance to get back to curcuit . note “compressed bonds” and “elongated bonds.” 11/20/2011 14 PowerPoint Presentation: The Burgers vector(b) is perpendicular to the edge dislocation line The Burgers vector(b) is parallel to the screw dislocation line. Screw Dislocation Screw dislocation results from a displacement of the atoms in one part of a crystal relative to the rest of the crystal forming a spiral ramp around the dislocation line. 11/20/2011 15 PowerPoint Presentation: Screw Dislocation Slip Planes Slip Direction 11/20/2011 16 Surface imperfections: Surface imperfections Surface imperfections refer to regions of distortions that lie about a surface having thickness of a few atomic diameters 11/20/2011 17 Surface imperfections: Surface imperfections Grain Boundaries (or) Crystal Boundary At the interface between two crystals randomly oriented the atoms held in between are attracted by crystals on either side and depending on the forces the atoms occupy equilibrium positions. This distorted region of a few atomic diameter thickness is called grain boundary. 11/20/2011 18 Surface imperfections: Surface imperfections The orientation difference is usually greater than 10-15°. For this reason the grain boundaries are also known as high angle boundaries. When the orientation difference between two crystals is less than 10° the distortion in the boundary is not so drastic. They are called low angle boundaries. 11/20/2011 19 Surface imperfections: Surface imperfections The sub grain boundaries are of two types they are 1. Tilt boundaries 2. Twist boundaries. An array of edge dislocations is called tilt boundary A cross grid of screw dislocation is called twist boundaries. 11/20/2011 20 PowerPoint Presentation: 11/20/2011 21 Tilt boundaries Twist boundaries PowerPoint Presentation: Twin Boundaries A twin boundary happens when the crystals on either side of a plane are mirror images of each other. Two single crystal sections are miss oriented but joined together such that one plane is a mirror image of the other. The boundary common to the two planes is a twin boundary. They are formed during the growth of crystal The boundary between the twinned crystals will be a single plane of atoms. 11/20/2011 22 Stacking Faults: Stacking Faults Stacking faults are planar surface imperfections caused by fault in the staking sequence of atomic planes in crystals In FCC crystal we have three different stacking layers ABC while in HCP stacking we have only two different layers BC hence when FCC crystal grows we have the stacking as ……… A B C A B C A B C A B C …… While growing if the plane A indicated by arrow above missing then we get the sequence ……… A B C A B C B C A B C …… Thus we find that the stacking in the missing region be comes HCP. This thin region is a surface imperfection and is called a stocking fault. 11/20/2011 23 Volume Defects: Volume Defects Presence of a large vacancy (crakes) or void such as clusters of atoms missing is also considered as a volume defect. While crystal is grown there is every possibility of inclusion of non-crystalline regions of dimensions of at least 10 to 30°A. This is also called Volume imperfection. 11/20/2011 24 PowerPoint Presentation: 11/20/2011 25

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