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This course deals with crystalline solids and is intended to provide students with basic physical concepts and mathematical tools used to describe solids. Key words in this lecture are: Thermal Expansion, Coefficient, High Thermal, Polymers, Thermal Expansion Values of Materials, Crystals, Measurement, Pushrod Dilatometer, Thermal Expansion of Glass, ULE Glass, Correlation of Thermal Expansion
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Physical Properties of Glass 2: Thermal Expansion Coefficient
Understand how the thermal expansion coefficient depends upon temperature,cooling rate, interatomic bonding, and compositionUnderstand and be able to use relative order of magnitude values for thethermal expansion coefficient for various oxide glassesBe able to estimate thermal expansion coefficient for oxide glasses usingsimple additive factors models
Thermal expansion determinesif a glass will be shockresistant, able to withstand highthermal stresses
Thermal expansion alsodetermines if a glass will havelow thermal shock resistance
Small thermal expansioncoefficient leads to high thermalshock resistance
Large thermal expansion leadsto low thermal shock resistance
shock
Most materials expand as they are heated
Some more than others
Refractory metals and ceramics
Expand less
Polymers
Expand more
Some materials expand very little
SiO
2
glass
-spodumene, Li
2
O.Al
2
O
3
.4SiO
2
Complex systems with more than one material musthave matched or compensated thermal expansions
Polycrystalline materialsunder go phasetransformations
Thermal expansion changesat each phase transition
c-SiO
2
has numerous phase
changes and numerousvolume changes that must beaccounted for during heat upof systems using
SiO
2
g-SiO
2
For isotropic materials,homogeneous in threedirections,…
Volume expansion coefficient is 3times larger than linear expansion
Glasses are isotropic
Fine grained polycrystals areisotropic
L
V
P
V
P
L
V T
V
L T
L
3
1 1
0 0
Glass undergoes glasstransition and transform tosupercooled liquid at Tg
Liquid has a larger
At softening point, liquidbegins to be compressed byforce of applieddilatometer, “dilatometrichook”
Tg measured bydilatometry is called T
d
and
is often < than T
g
measured
by DTA
DTA scans at 10 – 20
o
C/min, dilatometry is
done at 3-
o
C/min
d
g
s
glass
liquid
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Properties of glass dependupon cooling rate
Heating rate of dilatometryis slow and as such wellannealed samples, or thosecooled at the same slow ratemust be used
Fast quenched glasses willundergo “sub-Tg”relaxations, i.e., they try torelax to slower cooling ratecurve
Eventually, glass undergoestransition at Td(Tg)
d
g
s
glass
liquid
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As alkali is added, thermal expansionincreases
Tg decreases with added modifier
Lowest modifier shows anomalous‘plateau” above Tg
Liquid does not fully relax as it should
Low soda silicate glasses exhibit phaseseparation
Liquid phase separates into high silicaand high alkali glasses, two glasseswith different Tgs
High silica liquid does not undergo Tguntil higher temperatures
Tg
Tg
100%SiO
2
Addition of alkali modifier decreases
thermal expansion
coefficient in alkali borate glasses
Modifier in low alkali borateglasses, cross links glass structure
Creation of tetrahedral borons
Adding bonds to boron, increasingconnectivity of network
Strengthening the network
Rigidity of the glassy networkincreases
Thermal expansion decreases withmodifier
