Chapter 3-Crystal structures-ceramics

Chapter 3-Crystal structures-ceramics - Chapter 3...

Info iconThis preview shows pages 1–8. Sign up to view the full content.

View Full Document Right Arrow Icon
1 Chapter 3: Structures of Metals & Ceramics Questions to be addressed here. .. How do atoms arrange into solid structures? How does the density of a material depend on its structure? When do material properties vary with the sample (i.e., part) orientation? How do the crystal structures of ceramic materials differ from those for metals? What are the common arrangements of atoms in solids?
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
2 Bonding: -- Can be ionic and/or covalent in character. -- % ionic character increases with difference in electronegativity of atoms. Adapted from Fig. 2.7, Callister & Rethwisch 3e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the Chemical Bond , 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell University. Degree of ionic character may be large or small: Atomic Bonding in Ceramics SiC: small CaF 2 : large
Background image of page 2
3 Ceramic Crystal Structures Oxide structures oxygen anions larger than metal cations close packed oxygen in a lattice (usually FCC) cations fit into interstitial sites among oxygen ions
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
4 Factors that Determine Crystal Structure 1. Relative sizes of ions – Formation of stable structures: --maximize the # of oppositely charged ion neighbors . Adapted from Fig. 3.4, Callister & Rethwisch 3e. - - - - + unstable - - - - + stable - - - - + stable 2. Maintenance of Charge Neutrality : --Net charge in ceramic should be zero. --Reflected in chemical formula: CaF 2 : Ca 2+ cation F - F - anions + A m X p m, p values to achieve charge neutrality
Background image of page 4
5 Coordination # increases with Coordination # and Ionic Radii Adapted from Table 3.3, Callister & Rethwisch 3e. 2 r cation r anion Coord # < 0.155 0.155 - 0.225 0.225 - 0.414 0.414 - 0.732 0.732 - 1.0 3 4 6 8 linear triangular tetrahedral octahedral cubic Adapted from Fig. 3.5, Callister & Rethwisch 3e. Adapted from Fig. 3.6, Callister & Rethwisch 3e. Adapted from Fig. 3.7, Callister & Rethwisch 3e. ZnS (zinc blende) NaCl (sodium chloride) CsCl (cesium chloride) r cation r anion To form a stable structure, how many anions can surround around a cation?
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
6 Computation of Minimum Cation-Anion Radius Ratio Determine minimum r cation /r anion for an octahedral site (C.N. = 6) a = 2 r anion 2 r anion + 2 r cation = 2 2 r anion r anion + r cation = 2 r anion r cation = ( 2 - 1) r anion a r r 2 2 2 cation anion = + 414 . 0 1 2 anion cation = - = r r
Background image of page 6
7 Bond Hybridization Bond Hybridization is possible when there is significant covalent bonding hybrid electron orbitals form For example for SiC X Si = 1.8 and X C = 2.5 % . ) X X ionic % 5 11 ]} exp[-0.25( - {1 100 character 2 C Si = - = ~ 89% covalent bonding
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 8
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 09/05/2011 for the course EMA 3010 taught by Professor Unknown during the Summer '08 term at University of Florida.

Page1 / 29

Chapter 3-Crystal structures-ceramics - Chapter 3...

This preview shows document pages 1 - 8. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online