Most common elements on earth are Si & O

• SiO2 (silica) crystalline structures include quartz, crystobalite, & tridymite

• The strong Si-O bond leads to a strong, high melting material (1710ºC)

Silicate Ceramics

Si4+

O2-

Crystobalite – Polymorph of SiO2

SiO44- tetrahedron is the fundamental building block of all silicates

covalent

Amorphous Silica -- “Glass”

• Dense form of amorphous silica– Charge imbalance corrected with

“counter cations” such as Na+

– Borosilicate glass is the pyrex glass used in labs

• better temperature stability & less brittle than sodium glass

“Fused Silica” or “vitreous silica”

Network formers – Silica tetrahedronOther oxides like B2O3, GeO2

Network modifiers – other cations that are incorporated into and modify SiO4

4- network

Intermediates – other oxides (TiO2 or Al2O3), not network formers, substitute for silicon and become part of and stabilize the network

Change melting point with modifiers and intermediates

– Combine SiO44- tetrahedra by having them share

corners, edges, or faces

– Cations such as Ca2+, Mg2+, & Al3+ act to neutralize & provide ionic bonding

Silicates – Minerals

Mg2SiO4 Ca2MgSi2O7Forsterite Akermanite

Layered Silicates• Layered silicates (clay silicates)

– SiO4 tetrahedra connected together to form 2-D plane

• (Si2O5)2-

• So need cations to balance charge =

• Kaolinite clay alternates (Si2O5)2- layer with Al2(OH)4

2+ layer

Layered Silicates

Note: these sheets loosely bound by van der Waal’s forces

Adapted from Fig. 12.14, Callister 7e.

• Frenkel Defect --a cation is out of place.

• Schottky Defect --a paired set of cation and anion vacancies.

• Equilibrium concentration of defects kT/QDe~

Defects in Ceramic Structures

Schottky Defect:

Frenkel Defect

• Impurities must also satisfy charge balance = Electroneutrality

• Ex: NaCl

• Substitutional cation impurity

Impurities

Na+ Cl-

initial geometry Ca2+ impurity resulting geometry

Ca2+

Na+

Na+Ca2+

cation vacancy

• Substitutional anion impurity

initial geometry O2- impurity

O2-

Cl-

anion vacancy

Cl-

resulting geometry

Ceramic Phase Diagrams

MgO-Al2O3 diagram:

• Room T behavior is usually elastic, with brittle failure.• 3-Point Bend Testing often used. --tensile tests are difficult for brittle materials.

Measuring Elastic Modulus

FL/2 L/2

d = midpoint deflection

cross section

R

b

d

rect. circ.

• Determine elastic modulus according to:

Fx

linear-elastic behavior

F

slope =

E =F

L3

4bd3=

F

L3

12R4

rect. cross section

circ.cross section

• 3-point bend test to measure room T strength.

Measuring Strength

FL/2 L/2

d = midpoint deflection

cross section

R

b

d

rect. circ.

location of max tension

• Flexural strength: • Typ. values:

Data from Table 12.5, Callister 7e.

rect.

fs 1.5Ff L

bd 2

Ff L

R3Si nitrideSi carbideAl oxideglass (soda)

250-1000100-820275-700

69

30434539369

Material fs (MPa) E(GPa)

xF

Ff

fs

Chapter 14 – Polymers

What is a polymer?

Poly mer many repeat unit

C C C C C C

HHHHHH

HHHHHH

Polyethylene (PE)

ClCl Cl

C C C C C C

HHH

HHHHHH

Polyvinyl chloride (PVC)

HH

HHH H

Polypropylene (PP)

C C C C C C

CH3

HH

CH3CH3H

repeatunit

repeatunit

repeatunit

Polymer Composition

Most polymers are hydrocarbons – i.e. made up of H and C• Saturated hydrocarbons

– Each carbon bonded to four other atoms

CnH2n+2

C C

H

H HH

HH

Unsaturated Hydrocarbons

• Double & triple bonds relatively reactive – can form new bonds– Double bond – ethylene or ethene - CnH2n

• 4-bonds, but only 3 atoms bound to C’s

– Triple bond – acetylene or ethyne - CnH2n-2

C C

H

H

H

H

C C HH

Isomerism

• Isomerism– two compounds with same chemical formula can have

quite different structures

Ex: C8H18

• n-octane

• 2-methyl-4-ethyl pentane (isooctane)

C C C C C C C CH

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H H3C CH2 CH2 CH2 CH2 CH2 CH2 CH3=

H3C CH

CH3

CH2 CH

CH2

CH3

CH3

H3C CH2 CH3( )6

Chemistry of Polymers• Free radical polymerization

• Initiator: example - benzoyl peroxide

C

H

H

O O C

H

H

C

H

H

O2

C C

H H

HH

monomer(ethylene)

R +

free radical

R C C

H

H

H

H

initiation

R C C

H

H

H

H

C C

H H

HH

+ R C C

H

H

H

H

C C

H H

H H

propagation

dimer

R= 2

PolyethyleneAdapted from Fig. 14.1, Callister 7e.

Note: polyethylene is just a long HC - paraffin is short polyethylene

Bulk or Commodity Polymers

MOLECULAR WEIGHT

molecules of #total

polymer of wttotalnM

iiw

iin

MwM

MxM

Mw is more sensitive to higher molecular weights

• Molecular weight, Mi: Mass of a mole of chains.

Lower M higher M

Fraction

Mean of size range

Weight fraction

Molecular Weight Calculation

Example: average mass of a class

iiw MwM iin MxM

N i M i x i M i X i N i M i w i w i M i

# of students

mass (lb)

1 100 0.1 10 100 0.054 5.381 120 0.1 12 120 0.065 7.742 140 0.2 28 280 0.151 21.083 180 0.3 54 540 0.290 52.262 220 0.2 44 440 0.237 52.041 380 0.1 38 380 0.204 77.63

Sums 10 1140 1 186 1860 216M n M w

186 lb 216 lb

Degree of Polymerization, n

n = number of repeat units per chain

iimfm

m

unitrepeat ofweight molecular average where

C C C C C C C CH

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

C C C C

H

H

H

H

H

H

H

H

H( ) ni = 6

mol. wt of repeat unit iChain fraction

m

MDP

n

End to End Distance, r

• Covalent chain configurations and strength:

Direction of increasing strength

Molecular Structures

Branched Cross-Linked NetworkLinear

secondarybonding

Polymers – Molecular Shape

Conformation – Molecular orientation can be changed by rotation around the bonds– note: no bond breaking needed

Adapted from Fig. 14.5, Callister 7e.

Polymers – Molecular Shape

Configurations – to change must break bonds• Stereoisomerism

EB

A

D

C C

D

A

BE

mirror plane

C CR

HH

HC C

H

H

H

R

or C C

H

H

H

R

Tacticity

Tacticity – stereoregularity of chain

C C

H

H

H

R R

H

H

H

CC

R

H

H

H

CC

R

H

H

H

CC

C C

H

H

H

R

C C

H

H

H

R

C C

H

H

H

R R

H

H

H

CC

C C

H

H

H

R R

H

H

H

CC

R

H

H

H

CC

R

H

H

H

CC

isotactic – all R groups on same side of chain

syndiotactic – R groups alternate sides

atactic – R groups random

cis/trans Isomerism

C CHCH3

CH2 CH2

C CCH3

CH2

CH2

H

cis

cis-isoprene (natural rubber)

bulky groups on same side of chain

trans

trans-isoprene (gutta percha)

bulky groups on opposite sides of chain

Copolymers

two or more monomers polymerized together

• random – A and B randomly vary in chain

• alternating – A and B alternate in polymer chain

• block – large blocks of A alternate with large blocks of B

• graft – chains of B grafted on to A backbone

A – B –

random

block

graft

alternating