Structure
Metals-Ceramics
Ashraf Crystal Bastawros
www.public.iastate.edu\~bastaw\courses\Mate271.html
Week 3
Material Sciences and Engineering MatE271
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Ceramic Crystal Structures
- Broader range of chemical composition than metals with more complicated structures - Contains at least 2 and often 3 or more atoms. - Usually compounds between metallic ions (e.g. Fe, Ni, Al) - called cations - and non-metallic ions (e.g. O, N, Cl) - called anions - Bonding will usually have some covalent character but is usually mostly ionic
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Ceramic Crystal Structure
o Still based on 14 Bravais lattices o Cation: Metal, positively charged, usually smaller o Anion: Usually O, C, or N, negative charge, usually larger.
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How do Cations and Anions arrange themselves in space???
Structure is determined by two characteristics: 1. Electrical charge - Crystal (unit cell) must remain electrically neutral - Sum of cation and anion charges in cell is 0 2. Relative size of the ions
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Ceramic Crystal Structures
- The ratio of ionic radii (rcation /r anion ) dictates the coordination number of anions around each cation. - As the ratio gets larger (i.e. as rcation /r anion 1)
the coordination number gets larger and larger.
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Where do Cations and Anions fit ?
CN 3 Radius Ratio 0.155 - 0.225 Geometry Triangular
4
0.255 - 0.414
Tetrahedron
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0.414-0.732
Octahedron
8
0.732 - 1
Cube Center
rcation /r anion
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Interstitial sites (Octahedral)
BCC
FCC
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Interstitial sites (Tetrahedral)
BCC
FCC
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Interstitial sites
-Any close packed array of N atoms contains N octahedral interstitial sites 2N tetrahedral sites - Octahedral sites are larger than tetrahedral sites
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Some common ceramic structures
Structure Cesium Chloride (CsCl) Rock salt (NaCl) Fluorite (CaF2) Silicates (complex) (SiO2) Corundum (Al2O3) Perovskite (CaTiO3) Spinel (MgAlO4 ) Diamond Graphite
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Lattice SC FCC FCC FCC hexagonal SC FCC FCC hexagonal
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Ch. formula MX MX MX2 MX2 M2X3 MMX3 MM X4
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Note: What defines a lattice point
No of lattice (basis) points/unit cell SC=1 BCC=2 FCC=4
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Cesium Chloride (CsCl)
Lattice: SC Chemical formula: MX
- Atoms per lattice point = - Formula units/unit cell =
Cs located on cube center
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Differences between CsCl (SC) and Cr (BCC)
Cs Cl
Cr
CsCl (SC) No lattice point/unit cell No atoms/lattice point one: (0,0,0) two: (0,0,0), (0.5,0.5,0.5)
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Cr (BCC) two: (0,0,0), (0.5,0.5,0.5) One/lattice pt.
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Rock Salt Structure (NaCl)
Lattice: FCC Chemical formula: MX - Atoms per lattice point = - Formula units/unit cell =
MgO, FeO, NiO, CaO also have rock salt structure Na located on octahedral sites
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Flourite Structure (CaF2 )
distance of body diagonal
Ca2+ F
_
Lattice: FCC Chemical formula: MX2 _ Ion/ Unit Cell: 4 Ca2++ 8 F = 12 Typical Ceramics: UO2 , ThO2 , and TeO2
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Corundum Structure (Al2O3)
Lattice: hexagonal Chemical formula: M2X3 _ Ion/ Unit Cell: 12 Al3++ 18 O2 = 30 Typical Ceramics: Al2O3 , Cr2O3 , Fe2O3
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aterial and Engineering,
Perovskite Structure (BaTiO3 , Ca TiO3)
Lattice: SC Chemical formula: M M X3 Atoms per lattice point = Ion/ Unit Cell =
Ferroelectric Piezoelectric
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Diamond Cubic Structure
All atoms are C 4 interior C atoms
(tetrahedrally coordinated with corner and face-centered C atoms)
Covalent bonds (extremely strong) HARD Low electrical conductivity Optically transparent
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Diamond Thin Film
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Carbon - Graphite
not hcp
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Fullerenes
Buckyball C60
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Glass Structure
The basic structural unit of a silicate glass is the SiO4 tetrahedron Link together sharing corners to form a 3-D network
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Glass Structure
Beyond the short range order the structure is random Other ions may also be present
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Polymorphism and Allotropy
Some materials may have more than one crystal structure depending on temperature and pressure - called POLYMORPHISM Carbon (diamond, graphite, fullerenes) Silica (quartz, tridymite, cristobalite, etc.) Iron (ferrite, austenite)
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Polymer Structures
Chainlike structures of long polymeric molecules (usually involving C, H, and O + other elements) Usually mostly noncrystalline Extremely complex and elongated molecules do not readily line up on cooling to crystallize Structure is very dependent on thermal history (so are properties)
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Atomic Densities
- Why do we care?
- Properties, in general, depend on linear and planar density.
- Examples:
- Speed of sound along directions - Slip (deformation in metals) depends on linear & planar density - Slip occurs on planes that have the greatest density of atoms in direction with highest density
(we would say along closest packed directions on the closest packed planes)
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