disorted lattice Flashcards
What is a Direct Interstitial Jump
movement of atoms from one interstitial site to
another neighboring interstitial site without permanent displacement any of
the atoms in the matrix crystal lattice
when does a Direct interstitial mechanism happen?
occurs when an atom is of small radius. Smaller atoms cause less distortion of the lattice during migration and diffuse more readily than ones having larger atomic radii; being small allows them to fit into interstitial sites and to jump from one interstitial site to another without greatly displacing the solvent atoms from their normal lattice sites
Interstitialcy Mechanism
lattice member from one interstitial position to another
when are crystal defects important?
– solid oxide fuel cells
name of the two defects
Schottky and Frenkel defects
Schottky Defect
this type of point defect forms when oppositely charged ions leave their corresponding lattice sites, creating a pair of vacancy defects.
frenkel defect
smaller ion leaves its place in the lattice, creating a vacancy and becomes an interstitial by lodging in a nearby location.
ion migration schottky
To get across the unit cell into the vacancy in this NaCl crystal,
the Na+ ion must hop through the center of the cube where it squeezes past 4 Cl- and 2 Na+. The energy of this “transition state” will determine the ease of migration.
what is the formula for the schottky defect?
anion + cation —– V anion + V cation
what is the formula for the frenkel defect?
ion — V ion + ion intersititual
in Schottky and frenkel defects the charge is….
neutral.
direct intersitiual jump powerpoint definition
The ion sits on top of the layer / intersitually and it moves to the vacant site from one unit cell to another
indirect intersitual jump
forces an ion out of their vacancy
Schottky does
ion migration.
what is ion migration?
ion moves across unit cell into the vacant site.
defects are present in all…
crystals
why are defects present in all crystals
as there is an equilbrium for defects
what is the formula for defects
n - Nexp(-Ev.Kbt)
asides from heat what other techniques are used to create defects
X-rays
Radioactive decay
X-rays can create
farben centres
how are farben centres created ?
X rays generate an ion and an electron.
the electron can sit in place in an anion vacancy. the electron based of schrodinger box theory has allowed energies which are within the UV range. when E releases energy then a colour is shown.
what do defects do to the energy of the system?
increase it
how do defects change the energy of the system
If an ion has to break 6 bonds but make 3 bonds then it is endothermic.
if a ion hops back into the vacant sit it is exothermic.
what is wigners energy?
energy of defects recombined.
how did the wigners energy nearly cause nuclear fall out in england
Two types of interstitial vacancy form - one with four bonds and one with three. These are highly mobile at room temperature
These have a barrier to recombination of 1.4eV and will release 12eV
These are probably the main cause of Wigner energy release
what is wigners effect?
the displacement of atoms in a solid caused by neutron radiation. this increases the energy of system
