07/11/2020 1 Crystal Defects
2 Defects in Solids There is no such thing as a perfect crystal. • Anything that deviated from this concept or intruded in this uniform homogeneity would be an imperfection. 1. There are no perfect crystals. 2. Many material properties are improved by the presence of imperfections and deliberately modified (alloying and doping). 07/11/2020
07/11/2020 Confidential 3 • The properties of some materials are profoundly influenced by the presence of imperfections. • Pure iron is soft. Addition of small amount of carbon increases its strength. • A crystal of pure alumina is transperent and colorless, but when we add a small amount of chromium, it creates a defect resulting in a beautiful red ruby crystal • Its important to know to have the knowledge about the types of imperfections that exist and the roles they play in affecting the behaviour of materials.
• Image“borrowed”from who got it from “Davis & Reynolds 1996.” Defects in Crystals 07/11/2020 Confidential 4
It's easy to think of real crystals as having these ideal structures. In fact, no crystals are perfect; all crystals have defects. Crystals can have: • The "right" atoms in "wrong" places. • "Wrong" atoms in "right" or "wrong" places. • Missing atoms. • Etc. One type of defect is the point defect. 07/11/2020 Confidential 5
Cryst al Defec ts Po int De fec ts Li ne De fec ts Su rfa ce De fec ts Vo lu me De fec ts 6 07/11/2020
Dimensional Range for Different Classes of Defects 07/11/2020 Confidential 7
0D (Point defects) Vacancy Impurity Frenkel defect Schottky defect Non-ionic crystals Ionic crystals Imperfect point-like regions in the crystal about the size of 1-2 atomic diameters Interstitial Substitutional Other ~ self Interstitial 07/11/2020 Confidential 9
Vacancy : A Vacancy refers to an atomic site from where the atom is missing. Tensile Stress Fields ? 07/11/2020 11
12 -vacant atomic sites in a structure. Vacancy distortion of planes 07/11/2020
Substitutional impurity : • A Substitutional impurity refers to a foreign atom that has replaced a parent atom. Tensile Stress Fields Compressive stress fields 07/11/2020 13
• Self-Interstitials: -"extra" atoms positioned between atomic sites. Point Defects in Metals self- interstitial distortion of planes 07/11/2020 Confidential 14
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 07/11/2020 15
Frenkel imperfection : • An ion displaced from a regular site to an interstitial site is called frenkel imperfection.
- Spring '20
- Materials Science, Crystal, Crystallographic defect, edge dislocation