Ceramic%20Crystal%20Structures

1
Ceramic Crystal Structures
2
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 cant be smaller than the space.
3
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)
4
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
5
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
6
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
7
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
8
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
9
Top view of crystal shown on previous slide
Bottom view of crystal shown on previous slide
10
Other C StructuresGraphite
Covalent bonds within each plane van der Waals
bonds between the planes Example of a layered
ceramic structure
11
Other C StructuresFullerenes

• Graphite-like structures that are wrapped up into
  a
• Sphere (C60 or buckyball)
• Tube (carbon nanotubes)

12
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
13
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
14
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.
15
Silica Structures (SiO2)
Examples of crystalline and amorphous SiO2
structures
16
Density of Ceramic Materials
Same as calculating the density of metal crystal
structures - only more complex r n (SACSAA)
(Vcell)(Avogodros No.)
(examples)