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:

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Crystalline Versus Amorphous Solids

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Crystalline versus amorphous solids

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


Crystalline versus amorphous solids

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


Crystalline versus amorphous solids

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


Crystalline versus amorphous solids

A Crystalline Silicate

Si

O


Crystalline versus amorphous solids

Crystalline Versus Amorphous Silicates

Silicate melts tend to be highly viscous

Variable bond

angle & length

Ordered SiO4 tetrahedra

Disordered SiO4 tetrahedra


Crystalline versus amorphous solids

Viscosity

Measure of resistance to flow:

elongation or strain,  = ΔL/L

= d/dt

Liquid flow requires breaking and reformation of bonds


Crystalline versus amorphous solids

Viscosity-Temperature Relations


Crystalline versus amorphous solids

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


Crystalline versus amorphous solids

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!


Crystalline versus amorphous solids

Optical Fiber Puller

Viscosity control

Pulling rate

Key for strength

http://www.nasatech.com/Briefs/Dec98/MFS26503.html


Crystalline versus amorphous solids

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


Crystalline versus amorphous solids

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


Crystalline versus amorphous solids

Soda Glass


Crystalline versus amorphous solids

Viscosity-Temperature-Modifier Relations

1 Pa-s =

106 N-s/m2

Note effect of

B2O3 on 


Crystalline versus amorphous solids

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


Crystalline versus amorphous solids

Property-Composition Relations


Crystalline versus amorphous solids

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.


Crystalline versus amorphous solids

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


Crystalline versus amorphous solids

Rapid Cooling Rates

Splat cooling

Spin cooling

Vapor deposition


Crystalline versus amorphous solids

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.


Crystalline versus amorphous solids

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/


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