1228 Flashcards
1
Q
What is point defect? List types and describe them
A
0 dimensional defect where structure is disrupted at a point
- vacancy: missing atom from lattice site
- interstitial: atom sitting in posn that is not a lattice site
- substitutional impurity: crystal atom replaced by different type of atom
- interstitial impurity: impurity atom sitting between lattice sites of crystal
2
Q
How can you estimate vacancy energy Ev? (Since point defects like vacancies and interstitals cost energy to create)
A
- number of bonds broken * bond energy (Ev)
- defects increase entropy of crystal => finite temp. = finite conc. of vacancies in eqm => by creating vacancies, crystal can lower it’s Gibb’s free energy
3
Q
What are Frenkel defects?
A
- high T, atoms may have enough thermal energy to move from lattice site to interstitial site (site that is not a lattice site - like between regular posn of array of atoms) => vacancy interstitial pairs = Frenkel defects
4
Q
What are line defects?
A
- 1D - structure is disrupted along a continuous line
- presence of dislocation = possible for plan of atoms to slip over each other => plastic deformation
- dislocations move under applied shear stress => changes shape of crystal
- not an eqm property of crystals since energy too high to be compensated by entropy below mpt
5
Q
What are planar defects?
A
- most materials are polycrystalline (i.e. made up of lots of tiny grains all with different orientations)
6
Q
What is grain boundary?
A
- interface between 2 grains
- NOTE: crystal structure not defined on plane that separates grains since crystal planes are oriented in different direction on either side of grain boundary
7
Q
When are defects introduced? And how?
A
- during growth or crystallisation process
- cool liquid beow mpt => begin to crystallise
- crystals nucleate at random posn in liquid & crystals grow once liquid has solidifed => solid is polycrystalline
