most common elements on earth are si & o sio 2 (silica) crystalline structures include quartz,...
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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