lecture 10 - MANE/ENGR 2220 Structure Properties of...

This preview shows page 1 - 16 out of 57 pages.

MANE/ENGR 2220 Structure & Properties of MaterialsLecture 10Chapter 12/13Ceramics Materials Structures and Properties
Characteristics of CeramicsAlways composed of more than one element (e.g., Al2O3, NaCl, SiC, SiO2)Usually a compound between metallicand nonmetallic elementsBonds are partially or totally ionic,can have combination of ionic and covalent bondingGenerally hard and brittleGenerally electrical and thermal insulatorsCan be optically opaque, semi-transparent, or transparentTraditional ceramics based on clay (china, bricks, tiles, porcelain), glasses.“New ceramics” for electronic, computer, aerospace industries
Ionic Character of Ceramics
Bonding:--Mostly ionic, some covalent.--% ionic character increases with difference in electronegativity.Adapted from Fig. 2.7, Callister 7e.(Fig. 2.7 is adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 byCornell University.Large vs small ionic bond character:Ceramic BondingSiC: smallCaF2: large
Crystal Structure of Ionic Bonded CeramicsCrystal structure is defined by 2 criterions1.Magnitude of the electrical charge on each ion. Charge balance dictates chemical formula (Ca2+and F-form CaF2).2.Relative sizes of the cations and anions. Cations wants maximum possible number of anion nearest neighbors and vice-versa.
Ionic Bonding & Structure1. Size-Stable structures:--maximize the # of nearest oppositely charged neighbors.Adapted from Fig. 12.1, Callister 7e.----+unstableCharge Neutrality:--Net charge in thestructure shouldbe zero.--General form:----+stable----+stableCaF2:Ca2+cationF-F-anions+AmXpm, p determined by charge neutrality
4Coordination # increases withAdapted from Table 12.2, Callister 6e.Adapted from Fig. 12.2, Callister 6e.Adapted from Fig. 12.3, Callister 6e.Adapted from Fig. 12.4, Callister 6e.COORDINATION # AND IONIC RADII
APF = 0.74 for both FCC and HCPMANE/ENGR 2220 Structure and Properties of Materials Lecture #3Closed Packed Crystal Structures9
Table_12-3
On the basis of ionic radii, what crystal structurewould you predict for FeO?
Example: Predicting Structure of FeO
Cation Site SizeDetermine minimum rcation/ranionfor OHsite (C.N. = 6)a2ranion2ranion2rcation2 2ranionranionrcation2ranionrcation( 21)ranion2ranion2rcation2a4140anioncation.rr
ABX3Crystal StructuresPerovskiteEx: complex oxide BaTiO3Use as superconductorTwo cations (A&B) & one anions (X)
Ceramic Density ComputationAACNV)AA(nCNumber of formula units/unit cellVolume of unit cell= sum of atomic weights of all anions in formula unitAAAC= sum of atomic weights of all cations in formula unit
Fig_12-2

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture