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:
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At Tg, ~ 104 -106 Ns/m2
Below Tg, atomic rearrange-
ments are frozen in.
“Moon rocks” were produced millions of years ago
Solids with simple structures and non-directional
bonds, e.g. metals and alkali halides, have very
low 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
Silicate melts tend to be highly viscous
angle & length
Ordered SiO4 tetrahedra
Disordered SiO4 tetrahedra
Measure of resistance to flow:
elongation or strain, = ΔL/L
Liquid flow requires breaking and reformation of bonds
Soda lime glass at 900ºC at its working point:
Strain rate d/dt = / =
(10-4N/m2)/(10-4Ns/m2) = 1 s-1
Glass rod doubles in length
in one second at this small stress
(max)= 108N/m2 before breakage;
d/dt = 10-12 s-1
wait 1000 yr for 1% strain!
Key for strength
Two dimensional schematic of network of SiO4 tetrahedra.
Note: each Si has 4 O neighbors and each O, 2 Si neighbors
Common network formers: SiO2, B2O3, P2O5
Glass Modifiers (N2O, K2O, Li2O, CaO, MgO and PbO).
SiO2 network Modified with addition of Na20
Disrupt 3 dimensional covalent network reduceTM and Tg
1 Pa-s =
Note effect of
Glass remains extraordinarily strong in compression but becomes weak in tension.
Strengthening: pre-stress glass object by inducing
compressive strains in exterior and thereby counteract tensile stresses which develop under tension.
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.
Amorphous SiO2 - MOSFET
The Si/SiO2 interface
is one of the most important
Note: Form MOS structure:
Key element of MOSFET