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3.091 1© H.L. Tuller-2003
Crystalline Versus Amorphous Solids
Liquids, upon cooling, tend to crystallize. This means that atoms
weakly bound in the liquid in a random-like manner arrange them-
selves into well defined, periodic positions.
In order to do so effectively:
1. The liquid needs to be sufficiently fluid (low viscosity) to allow
the atoms to rearrange themselves effectively during cooling
through the melting point
2. The cooling rate needs to be sufficiently slow that the basic atomic units or molecules have sufficient time to re-arrange themselves
3.091 2© H.L. Tuller-2003
Glass Transition Temperature, Tg
At Tg, ~ 104 -106 Ns/m2
Below Tg, atomic rearrange-ments are frozen in.
Rigid fluid
“Moon rocks” were produced millions of years ago
TM-melting point
3.091 3© H.L. Tuller-2003
Solids with simple structures and non-directionalbonds, e.g. metals and alkali halides, have verylow viscosity fluids above the melting point and easily crystallize upon cooling.
Solids with complex structures and strong, highly directional bonds, e.g. silicates, polymers, have high viscosity fluids and tend to form amorphous or glassy solids
Crystalline Versus Amorphous Solids
3.091 4© H.L. Tuller-2003
A Crystalline Silicate
Si
O
3.091 5© H.L. Tuller-2003
Ordered SiO4 tetrahedra Disordered SiO4 tetrahedra
Crystalline Versus Amorphous Silicates
Silicate melts tend to be highly viscous
Variable bondangle & length
3.091 6© H.L. Tuller-2003
Viscosity
Measure of resistance to flow:
Liquid flow requires breaking and reformation of bonds
elongation or strain, = ΔL/L
= d/dt
3.091 7© H.L. Tuller-2003
Viscosity-Temperature Relations
3.091 8© H.L. Tuller-2003
Strain rate d/dt = / =
(10-4N/m2)/(10-4Ns/m2) = 1 s-1
Glass rod doubles in lengthin one second at this small stress
Soda lime glass at 900ºC at its working point:
Soda lime glass – strain rate
3.091 9© H.L. Tuller-2003
(max)= 108N/m2 before breakage;
(RT)= 1020Ns/m2
d/dt = 10-12 s-1
wait 1000 yr for 1% strain!
Soda Lime Glass at RT – strain rate
3.091 10© H.L. Tuller-2003
Optical Fiber Puller
http://www.nasatech.com/Briefs/Dec98/MFS26503.html
Pulling rate
Viscosity control
Key for strength
3.091 11© H.L. Tuller-2003
Two dimensional schematic of network of SiO4 tetrahedra.Note: each Si has 4 O neighbors and each O, 2 Si neighbors
Silicon-Oxygen network
Bridging oxygens
Common network formers: SiO2, B2O3, P2O5
3.091 12© H.L. Tuller-2003
Glass Modifiers (N2O, K2O, Li2O, CaO, MgO and
PbO).
SiO2 network Modified with addition of Na20
Bridging oxygen
Non-bridging oxygens
Na+ ions
Disrupt 3 dimensional covalent network reduceTM and Tg
3.091 13© H.L. Tuller-2003
Soda Glass
3.091 14© H.L. Tuller-2003
Viscosity-Temperature-Modifier Relations
1 Pa-s = 106 N-s/m2
Note effect ofB2O3 on
3.091 15© H.L. Tuller-2003
Glass Formation and Fabrication
Three basic steps in the production of glass:
(1) the melting of e.g. quartz sand (minute crystals of silica),
(2) the shaping of the glass while in a viscous state. Sufficient viscosity to enable handling and shaping of article
(3) the controlled cooling of the shaped article thereby allowing the article to form without large residual stresses
3.091 16© H.L. Tuller-2003
Property-Composition Relations
3.091 17© H.L. Tuller-2003
Glass has “no” crystal structure:• slip cannot take place.• strong bonding between atoms,
very high compressive strength and theoretical tensile strength of about 107 kN/m2 (significantly higher than that of steel).
Cracks or imperfections in glass permit stress concentrations to localize and exceed bond strength between atoms crack propagation.
in actual practice, the strength of glass is, by a factor of 100 to 1000, less than the theoretical strength, and glass is brittle.
Glasses – High Strength
3.091 18© H.L. Tuller-2003
Glass remains extraordinarily strong in compression but becomes weak in tension.
Strengthening: pre-stress glass object by inducingcompressive strains in exterior and thereby counteract tensile stresses which develop under tension.
Strengthened Glass
• Cool surface of glass preferentially
• Ion exchange surface with larger alkali ion such as K.
• Coat surfaces to protect against scratches on surface
3.091 19© H.L. Tuller-2003
Rapid Cooling Rates
Splat cooling
Spin cooling
Vapor deposition
3.091 20© H.L. Tuller-2003
Reference: Masuhr A, Busch R, Johnson WL. "Rheometry and Crystallization of Bulk Metallic Glass Forming Alloys at High Temperatures." Materials Science Forum. Barcelona, Spain. Switzerland: Trans Tech Publications, 1998: 779-84.
Metallic Glasses
3.091 21© H.L. Tuller-2003
The Si/SiO2 interfaceis one of the most importantstructures technologically
Note: Form MOS structure:Metal-Oxide-Semiconductor.Key element of MOSFET
Metal
http://www.research.ibm.com/amorphous/
Amorphous SiO2 - MOSFET