Crystalline Versus Amorphous Solids

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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
• 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

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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
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Crystalline Versus Amorphous Solids
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
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A Crystalline Silicate
Si
O
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Crystalline Versus Amorphous Silicates
Silicate melts tend to be highly viscous
Variable bond angle length
Ordered SiO4 tetrahedra
Disordered SiO4 tetrahedra
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Viscosity
Measure of resistance to flow
elongation or strain, ? ?L/L
d?/dt
Liquid flow requires breaking and reformation of
bonds
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Viscosity-Temperature Relations
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Soda lime glass strain rate
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
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Soda Lime Glass at RT strain rate
?(max) 108N/m2 before breakage ?(RT)
1020Ns/m2 d?/dt
10-12 s-1 wait 1000 yr for 1
strain!
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Optical Fiber Puller
Viscosity control
Pulling rate
Key for strength
http//www.nasatech.com/Briefs/Dec98/MFS26503.html
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Silicon-Oxygen network
Bridging oxygens
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
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Glass Modifiers (N2O, K2O, Li2O, CaO, MgO and
PbO).
Na ions
Bridging oxygen
Non-bridging oxygens
SiO2 network Modified with addition
of Na20
Disrupt 3 dimensional covalent network
reduceTM and Tg
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Soda Glass
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Viscosity-Temperature-Modifier Relations
1 Pa-s 106 N-s/m2
Note effect of B2O3 on ?
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Glass Formation and Fabrication

• Three basic steps in the production of glass
• the melting of e.g. quartz sand (minute crystals
  of silica),
• the shaping of the glass while in a viscous
  state. Sufficient viscosity to enable handling
  and shaping of article
• the controlled cooling of the shaped article
  thereby allowing the article to form without
  large residual stresses

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Property-Composition Relations
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Glasses High Strength

• 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.

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Strengthened Glass
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.

• Cool surface of glass preferentially
• Ion exchange surface with larger alkali ion such
  as K.
• Coat surfaces to protect against scratches on
  surface

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Rapid Cooling Rates
Splat cooling Spin cooling Vapor deposition
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Metallic Glasses
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.
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Amorphous SiO2 - MOSFET
Metal
The Si/SiO2 interface is one of the most
important structures technologically Note Form
MOS structure Metal-Oxide-Semiconductor. Key
element of MOSFET
http//www.research.ibm.com/amorphous/