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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:

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

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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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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

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A Crystalline Silicate

Si

O

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Ordered SiO4 tetrahedra Disordered SiO4 tetrahedra

Crystalline Versus Amorphous Silicates

Silicate melts tend to be highly viscous

Variable bondangle & length

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Viscosity

Measure of resistance to flow:

Liquid flow requires breaking and reformation of bonds

elongation or strain, = ΔL/L

= d/dt

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Viscosity-Temperature Relations

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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

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(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

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Optical Fiber Puller

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

Pulling rate

Viscosity control

Key for strength

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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

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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

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Soda Glass

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Viscosity-Temperature-Modifier Relations

1 Pa-s = 106 N-s/m2

Note effect ofB2O3 on

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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

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Property-Composition Relations

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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

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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

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Rapid Cooling Rates

Splat cooling

Spin cooling

Vapor deposition

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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

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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