Crystal Structure Metals-Ceramics Ashraf Bastawros www.public.iastate.edu\~bastaw\courses\mate271.html Week 3 Material Sciences and Engineering MatE271 1 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 Material Sciences and Engineering MatE271 Week 3 2
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. Material Sciences and Engineering MatE271 Week 3 3 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 Material Sciences and Engineering MatE271 Week 3 4 aterial Sciences and Engineering,
Ceramic Crystal Structures - The ratio of ionic radii (r cation /r anion ) dictates the coordination number of anions around each cation. - As the ratio gets larger (i.e. as r cation /r anion 1) the coordination number gets larger and larger. Material Sciences and Engineering MatE271 Week 3 5 Where do Cations and Anions fit? CN Radius Ratio Geometry 3 0.155-0.225 Triangular 4 0.255-0.414 Tetrahedron 6 0.414-0.732 Octahedron 8 0.732-1 Cube Center r cation /r anion Material Sciences and Engineering MatE271 Week 3 6
Interstitial sites (Octahedral) BCC FCC Material Sciences and Engineering MatE271 Week 3 7 Interstitial sites (Tetrahedral) BCC FCC Material Sciences and Engineering MatE271 Week 3 8
Interstitial sites -Any close packed array of N atoms contains N octahedral interstitial sites 2N tetrahedral sites - Octahedral sites are larger than tetrahedral sites Material Sciences and Engineering MatE271 Week 3 9 Some common ceramic structures Structure Cesium Chloride (CsCl) Rock salt (NaCl) Fluorite (CaF 2 ) Silicates (complex) (SiO 2 ) Corundum (Al 2 O 3 ) Perovskite (CaTiO 3 ) Spinel (MgAlO 4 ) Diamond Graphite Lattice SC FCC FCC FCC hexagonal SC FCC FCC hexagonal Ch. formula MX MX MX 2 MX 2 M 2 X 3 M M X 3 M M X 4 Material Sciences and Engineering MatE271 Week 3 10
Note: What defines a lattice point No of lattice (basis) points/unit cell SC=1 BCC=2 FCC=4 Material Sciences and Engineering MatE271 Week 3 11 Cesium Chloride (CsCl) Lattice: SC Chemical formula: MX Cs located on cube center - Atoms per lattice point = - Formula units/unit cell = Material Sciences and Engineering MatE271 Week 3 12
Differences between CsCl (SC) and Cr (BCC) Cs Cl Cr No lattice point/unit cell No atoms/lattice point CsCl (SC) one: (0,0,0) two: (0,0,0), (0.5,0.5,0.5) Cr (BCC) two: (0,0,0), (0.5,0.5,0.5) One/lattice pt. Material Sciences and Engineering MatE271 Week 3 13 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 Material Sciences and Engineering MatE271 Week 3 14 aterial Sciences and Engineering,
Flourite Structure (CaF 2 ) ¼ distance of body diagonal Ca 2+ F _ Lattice: FCC Chemical formula: MX 2 Ion/ Unit Cell: 4 Ca 2+ + 8 F _ = 12 Typical Ceramics: UO 2, ThO 2, and TeO 2 Material Sciences and Engineering MatE271 Week 3 15 Corundum Structure (Al 2 O 3 ) Lattice: hexagonal Chemical formula: M 2 X 3 Ion/ Unit Cell: 12 Al 3+ + 18 O 2_ = 30 Typical Ceramics: Al 2 O 3, Cr 2 O 3, α Fe 2 O 3 Material Sciences and Engineering MatE271 Week 3 16
Perovskite Structure (BaTiO 3, Ca TiO 3 ) Lattice: SC Chemical formula: M M X 3 Atoms per lattice point = Ion/ Unit Cell = Ferroelectric Piezoelectric Material Sciences and Engineering MatE271 Week 3 17 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 Material Sciences and Engineering MatE271 Week 3 18
Diamond Thin Film Material Sciences and Engineering MatE271 Week 3 19 Carbon - Graphite not hcp Material Sciences and Engineering MatE271 Week 3 20
Fullerenes Buckyball C 60 Material Sciences and Engineering MatE271 Week 3 21 Glass Structure The basic structural unit of a silicate glass is the SiO 4 tetrahedron Link together sharing corners to form a 3-D network Material Sciences and Engineering MatE271 Week 3 22
Glass Structure Beyond the short range order the structure is random Other ions may also be present Material Sciences and Engineering MatE271 Week 3 23 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) Material Sciences and Engineering MatE271 Week 3 24
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) Material Sciences and Engineering MatE271 Week 3 25 - Why do we care? Atomic Densities - 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) Material Sciences and Engineering MatE271 Week 3 26
Linear and Planar Densities Linear Densities fraction of line length in a particular direction that passes through atom centers Planar Densities fraction of total crystallographic plane area that is occupied by atoms (plane must pass through center of atom) Material Sciences and Engineering MatE271 Week 3 27 Calculate the Linear Density o Calculate the linear density of the (100) direction for the FCC crystal L D = L C /L L density L C = 2R L L = a linear circle length line length For FCC a = 2R 2 L D = 2R/(2R 2) = 0.71 Material Sciences and Engineering MatE271 Week 3 28
Calculate the Planar Density o Calculate the planar density of the (110) plane for the FCC crystal A B C A B C D E F D E F Compute planar area Compute total circle area Material Sciences and Engineering MatE271 Week 3 29 Semiconductor Structures Technologically, single crystals are very important More perfect than any other class of materials (purer, fewer dislocations) Elemental semiconductors (Si and Ge) are of the diamond cubic structure Compound semiconductors (GaAs, CdS) have zincblende (similar to diamond cubic) Material Sciences and Engineering MatE271 Week 3 30
Reading Assignment Shackelford 2001(5 th Ed) Read Chapter 3, pp 59-64 Read ahead to page 88, 101-110 Check class web site: www.public.iastate.edu\~bastaw\courses\mate271.html 2 Material Sciences and Engineering MatE271 Week 3 31