Ceramics Term ceramics comes from the greek word keramikos – “burnt stuff” Ceramics are typically formed during high temperature heat treating – “Firing” Traditionally ceramics included: China Porcelain Bricks (both construction and refractory) Tiles Glasses Over the last 60 years or so…there has been an explosion in new technologies similar to other areas of material science

Ceramic Bonding CaF2: large SiC: small • Bonding: -- Mostly ionic, some covalent. -- % ionic character increases with difference in electronegativity. • Amount of ionic bond character: CaF2: large SiC: small Eq 2.10: % ionic character = {1 – exp[-(0.25)(XA – XB)2]} x 100 XA, XB are electronegativities of components A and B

Ionic Ceramics Crystal structure are composed of electrically charged ions Cations (Fe3+) – Positive Charge Typically metals Anions (O2-) – Negatively Charged Typically non-metals Two characteristics influence crystal structure: The magnitude of charge on the component ions Stoichiometry must balance Overall charge neutrality is required Relative sizes of the component ions small large Note that size of ion is affected by charge: For iron: r(Fe2+) = 0.077 nm, r(Fe3+) = 0.069 nm, r(Fe) = 0.124 nm

Criteria of Site Selection Which sites will cations occupy to form stable crystal structure? Size of sites does the cation fit in the site Stoichiometry if all of one type of site is full the remainder have to go into other types of sites. Covalent Bond Hybridization

Ionic Bonding & Structure 1. Size - Stable structures: --maximize the # of nearest oppositely charged neighbors. - + unstable - + - + stable stable • Charge Neutrality: --Net charge in the structure should be zero. --General form: CaF 2 : Ca 2+ cation F - anions + A m X p m, p determined by charge neutrality

Coordination # and Ionic Radii cation anion • Coordination # increases with How many anions can you arrange around the cation? 2 r cation 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 TD OH cubic ZnS (zincblende) NaCl (sodium chloride) CsCl (cesium Purely geometrical argument

Geometrical Derivation of Site Size Determine minimum rcation/ranion for OH site (C.N. = 6) a = 2ranion

Site Selection II Stoichiometry If all of one type of site is full the remainder have to go into other types of sites. Ex: We know that an FCC unit cell has 4 OH and 8 TD sites. If for a specific ceramic each unit cell has 6 cations and the cations prefer OH sites, then 4 in OH 2 in TD

Site Selection III Bond Hybridization – significant covalent bonding the hybrid orbitals can have impact if significant covalent bond character present For example in SiC XSi = 1.8 and XC = 2.5 89% covalent bonding both Si and C prefer sp3 hybridization Therefore in SiC get TD sites

Example: Predicting Structure of FeO • On the basis of ionic radii, what crystal structure would you predict for FeO? Cation Anion Al 3+ Fe 2 + Ca 2+ O 2- Cl - F Ionic radius (nm) 0.053 0.077 0.069 0.100 0.140 0.181 0.133 • Answer: based on this ratio, --coord # = 6 --structure = NaCl

Rock Salt Structure Example: NaCl (rock salt) structure Same concepts can be applied to ionic solids in general Example: NaCl (rock salt) structure rNa = 0.102 nm rCl = 0.181 nm rNa/rCl = 0.564 cations prefer OH sites AX Crystal Structure: equal number of Anion and Cation locations

MgO and FeO MgO and FeO also have the Rock Salt structure O2- rO = 0.140 nm Mg2+ rMg = 0.072 nm rMg/rO = 0.514 cations prefer OH sites So each oxygen has 6 neighboring Mg2+

2nd Type of AX Crystal Structure Cesium Chloride structure:  cubic sites preferred So each Cs+ has 8 neighboring Cl-

3rd Type of AX Crystal Structures Zinc Blende structure Size arguments predict Zn2+ in OH sites, In observed structure Zn2+ in TD sites Why is Zn2+ in TD sites? bonding hybridization of zinc favors TD sites So each Zn2+ has 4 neighboring S2- Ex: ZnO, ZnS, SiC

AX2 Crystal Structures Fluorite structure Calcium Fluorite (CaF2) Cations in cubic sites UO2, ThO2, ZrO2, CeO2 antifluorite structure – cations and anions reversed rC/rA for CaF2 is about 0.8 – coordination number of 8  cubic structure But, stoichiometry calls for ½ as many Ca2+ as F- ions 8 cubes in unit cell

ABX3 Crystal Structures Perovskite crystal structure Ex: Barium Titanate – BaTiO3 Temperatures above 120oF – cubic crystal structure

Summary of Common Structures

Close Packing of Anions Coordination = 4 Coordination = 6 Since Anions are commonly packed in FCC structure – we can talk about close packed planes of anions Can have both: FCC Stacking – ABCABC HCP Stacking – ABABAB Cl- form FCC Lattice Close packed planes are {111}

Mechanical Properties Why are ceramics more brittle than metals? Consider method of deformation In metals we have dislocation motion along slip planes Slip planes are the close packed planes In ionic solids dislocation motion is very difficult Why? Too much energy needed to move one anion past another anion