|Concepts||defects in crystals: point defects, line defects|
|Keywords||point defect, line defect, substitutional impurity, interstitial impurity, vacancy, self interstitial, ionic defect, Hope Diamond, Schottky defect, Frenkel defect, F-center, charge neutrality, edge dislocation, screw dislocation, dislocation motion, bubble raft model, chemical imperfection, structural imperfection, formation energy, entropy factor, stoichiometric unit, effective charge, Kröger-Vink notation|
|Chemical Substances||aluminum (Al), steel, diamond, doped silicon, LaNi5, copper (Cu), rock salt (NaCl), zirconia (ZrO2)|
|Applications||aluminum alloys for soda cans, n-and p-type semiconductors, steel, hydrogen embrittlement of steel, Hope diamond, colored gold, hydrogen storage|
Before starting this session, you should be familiar with:
- Ionic crystal lattices (Session 8)
- Cubic crystal structures (Session 15)
- Distribution of energies as described by Maxwell-Boltzmann statistics (Session 14)
Compare the expression for the energy required to produce a vacancy, derived in this lecture, with the expression for the rate of a chemical reaction (the Arrhenius equation), presented in Session 22: Introduction to Kinetics.
After completing this session, you should be able to:
- Sketch a crystal containing any of the following defects: substitutional impurity, interstitial impurity, vacancy, Schottky, Frenkel.
- Calculate the vacancy concentration in a crystal at a given temperature.
- Write expressions for defects in a given ionic crystal, and explain why they have higher energies of formation than similar defects in metallic crystals.
- Explain why a given crystal has either substitutional or interstitial impurity atoms.
- Give 3 examples of additives that have a detrimental effect on the surrounding crystal, and 3 examples that improve the material properties.
Archived Lecture Notes #6 (PDF), Sections 1-2
|[Saylor] 12.4, "Defects in Crystals."||Defects in metals, memory metal, defects in ionic and molecular crystals, nonstoichiometric compounds|
|[JS] 4.1, "The Solid Solution – Chemical Imperfection."||Random and ordered solid solutions, Hume-Rothery rules, interstitial and substitutional solutes, charge neutrality|
|[JS] 4.2, "Point Defects – Zero-Dimensional Imperfections."||Vacancies and interstitial defects, Schottky and Frenkel defects|
|[JS] 4.3, "Linear Defects, or Dislocations – One-Dimensional Imperfections."||Burgers vector; edge, screw, mixed, and partial dislocations|
|[JS] 5.1, "Thermally-Activated Processes."||Arrhenius equation, activation energy, Maxwell-Boltzmann distribution, process mechanisms and rate-limiting steps|
|[JS] 5.2, "Thermal Production of Point Defects."||Activation energy of vacancies vs. interstitials, Arrhenius plot, thermal expansion|
Prof. Michael Demkowicz (homepage) lectures today, introducing the next topic: imperfections in crystal lattices. In the real world, materials rarely consist of single, perfect crystals; defects in crystals occur naturally, or are introduced during processing. While unwanted defects can weaken or contaminate materials (e.g. Li+ in saline solution (NaCl(aq)), others can create enhanced properties (e.g. alloys, dopants). Creating an empty crystal lattice site (vacancy) requires overcoming bonds with nearest-neighbor atoms, typically with thermal energy. Vacancies in a regular lattice of A atoms may be filled by an atom of B (substitutional, e.g. P in Si, B in C), while interstitial sites can host atoms of A (self interstitial) or B (interstitial impurity, e.g. C in Fe, H in LaNi5, H in Fe). In ionic crystals, overall charge neutrality must be preserved, so a whole stoichiometric unit may be removed to create two or more vacancies (Schottky); one ion may move to an interstitial site (Frenkel); or one or more electrons may fill an anionic vacancy (F-center). Line defects occur when a lattice mismatch runs through the crystal.
[JS] Chapter 5, Sample Problem 2, Practice Problem 2
|[Saylor] 12.4, "Defects in Crystals."||6, 7, 8, 9, 10||1, 2, 4, 5|
For Further Study
Other OCW and OER Content
|Introduction to Dislocations||DoITPoMS||Undergraduate|
|3.14/3.40J/22.71J Physical Metallurgy||MIT OpenCourseWare||Undergraduate (elective) / Graduate|