Advanced Physical Metallurgy Advanced Physical Metallurgy
“ “ Amorphous Materials Amorphous Materials ” ”
Eun Eun Soo Soo Park Park
2009 spring
04. 01. 2009
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Contents for previous class Contents for previous class
• • Measuring Glass Transition Temperature Measuring Glass Transition Temperature (1)
(2)
• • Measuring Glass Transition Temperature Measuring Glass Transition Temperature
(3)
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• • Measuring Glass Transition Temperature Measuring Glass Transition Temperature
(4)
• • Measuring Glass Transition Temperature Measuring Glass Transition Temperature
The Dynamic Mechanical Analysis is a high precision technique for measuring the viscoelastic properties of materials. It consists in applying a sinusoidal deformation to a specimen of material and measuring the resulting force transmitted through the specimen.
Specimen
Applied displacement
Amplitude
Resulting force
Time
Loss angle δ
F D
(5)
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Contents for previous class Contents for previous class
• • Glass: Glass: all types of bonding all types of bonding
• • Glass transition: Glass transition: Volumetric measurement Volumetric measurement
depends on cooling rate depends on thermal history
T T
ggglass transition temp. glass transition temp.
Contents for previous class Contents for previous class
• • Thermodynamics for Thermodynamics for glass transition glass transition
~ not thermodynamic nature
~ close to second order phase transition
V, H. S changes continuously.
V, H. S changes continuously.
α α
TTC C
PPK K
TTchanges discontinuously. changes discontinuously.
at T at T
ggG changes continuously. G changes continuously.
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• • Order (degree) of transition Order (degree) of transition
C
D
P E
Continuous phase transitions
:occur when the minimum in the thermodynamic potential evolves smoothly into two equal minima.
An example is seen in the model of phase separation, along the co-existence line.
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• • Order (degree) of transition Order (degree) of transition
C
D
P E
• CD, DE, DP: Equilibrium of 2 phases
latent heat
Volume change 1st order transition
• T and P beyond point p
: vapor and liquid are indistinguishable.
No boiling pt. no latent heat Higher order transition
Single phase: only property changes.
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~ Brass is a 50:50, Cu:Zn alloy with a b.c.c structure.
~ At low temperatures, T<460K, the Zn and Cu atoms for an ordered
structure (eg. Cu atoms in the body-centre sites in top diagram)
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Contents for previous class Contents for previous class
• • derivative of thermodynamic properties derivative of thermodynamic properties
~ dramatically decrease Cp near Tg during cooling
~ dramatically increase Cp near Tg during heating
discontinuous
discontinuous (C
(Cpp, α, αTT, K, KTT))Experimental observation during heating
because the monitoring is very difficult during cooling
Glass transition: heat endothermic
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Fragile network glass : Vogel-Fulcher relation
Strong network glass : Arrhenius behavior
]
0
exp[
RT E
aη
η =
] exp[
0
0
T T
B
= η − η
< Quantification of Fragility >
Fragility Fragility
g n
g g T T
T n T
g d T T
T d
T T
d
T m d
=
=
=
= ( / )
) ( log )
/ (
) ( log
,
,
τ η
< Classification of glass >
Fragility ~ ability of the liquid to withstand changes in medium range order with temp.
~ extensively use to figure out liquid dynamics and glass properties corresponding to “frozen” liquid state
Slope of the logarithm of viscosity, η (or structural relaxation time, τ ) at Tg
T increase
Supercooled liquid
1012~1014 poise
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Strong liquid vs. Fragile liquid
Fragility Fragility
• Strong glass forming liquid
covalent bond of SiO2small difference of Cp between SCL and glass at Tg
(small difference of structure)
SCL: relatively low entropy
• fragile glass forming liquid
non-directional bonding
(Van der waals bonding)
large difference of C at T
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Endo.
Exo. heating
Glass SCL crystal Liquid
Glass Supercooled
liquid
liquid cooling
Tm Tg
Tx
Tp
Ts TL
Glass transition
: Endo.
Crystallization
: Exo.
melting
: Endo.
C+L
< Thermal map >
Use of Gibbs energy curves to construct a binary phase diagram of the eutectic type.
High temperature
T1 T2 T3
T4 T5