Transcript Ceramic Crystal Structures Interstitial sites = small holes between the lattice atoms where smaller atoms may be placed. The smaller atoms should.

Ceramic Crystal Structures
Interstitial sites = small holes between the lattice atoms
where smaller atoms may be placed.
The smaller atoms should be the exact size of the
interstitial hole or slightly too large; they can’t be
smaller than the space.
The structure of ionic ceramic materials depends on
electrical neutrality (cations next to anions not other
cations, no overall charge in the material) and size
(cations are smaller than anions, anions close-pack and
cations sit in the interstitial sites).
(Remember basketballs and tennis balls)
Table 13.2 in the book shows
common interstitial sites for
ionic ceramic materials.
The site occupied by the cation
varies with size ratio of the
cations to the anions (rC/rA).
The most important interstitial
sites are the tetrahedral sites
(Td) and the octahedral sites
(Oh).
See Example Problem 13.1;
example in class
Ceramic Crystal Structures
There are numerous different ceramic crystal
structures. However, in this class we will only focus
on five structures:
Rock Salt [NaCl]
[CsCl]
Diamond
Zinc blende
Pervoskite
Rock Salt [NaCl]
Examples:
NaCl, MgO, FeO
One way to think of this structure is as follows:
Cl- ions form an FCC structure
Na+ ions fill all the Oh positions
[CsCl]
Examples:
CsCl, CsI, CsBr
One way to think of this structure is as follows:
Cl- ions form a SC structure
Cs+ ions fill the body-centered positions
Diamond
Examples:
C(diamond), Si, Ge
Covlently bound ceramic
One way to think about it: C forms an FCC lattice and
then fills 1/2 of the Td positions
Top view of crystal shown
on previous slide
Bottom view of crystal
shown on previous slide
Other C Structures:
Graphite
Covalent bonds within each plane
van der Waals bonds between the planes
Example of a layered ceramic structure
Other C Structures:
Fullerenes
Graphite-like
structures that are
wrapped up into a
•Sphere (C60 or
buckyball)
•Tube (carbon
nanotubes)
Zinc Blende
Examples:
ZnS, GaAs, SiC
Covlently bound ceramic; similar (but not the same as)
diamond
One way to think about it: one atom forms an FCC
lattice and the other fills 1/2 of the Td positions
Pervoskite
Examples:
BaTiO3, SrSnO3
Ternary ceramic.
One way to think about it: large cation makes SC
structure, small cation fills body-centered site, anion
fills face centered sites
Silica Structures (SiO2)
This material likes to form tetrahedra
that then share corners and/or sites with other
tetrahedra to form a variety of crystalline (quartz,
crystobalite or tridymite) and amorphous structures.
Silica Structures (SiO2)
Examples of
crystalline and
amorphous SiO2
structures
Density of Ceramic Materials
Same as calculating the density of metal crystal
structures - only more complex
r = n’ (SAC+SAA)
(Vcell)(Avogodros No.)
(examples)