Materials Science and Engineering/List of Topics/Engineered Glasses

Factors that Enhance Glass formation

• High viscosity of liquid
• Complex crystal structure
• High cooling rate

Difficult to find proper lattice sites

Particular form of glass: silica

• Silica is inorganic and covalent

In crystalline silicon, all the atoms are the same

• In silica, there is silicon and oxygen that form long chains and is polymeric
  • The molecules entangle and must disentangle
  • Drop temperature, bonds must twist
  • Quick solidification - fail to achieve crystallinity
  • Quench in disorder
• Silicon: all four bonds are fixed in space and fully specified
• Oxygen: bonds free to rotate and there is one degree of freedom

Perfect alignment - create crystalline structure

X-ray spectra

• Crystobalite - several peaks
• Amorphous silica - one peak
  • The peak is due to short-range order
  • There is no long-range order

Energetics of glass formation - when are more bonds formed?

• Crystalline solid is at lower energy
  • There is a higher bond density
  • More compact structure
  • More bonds per unit volume
    • Volume is a macroscopic measure of disorder

Volume as function of temperature

• Liquid turns to solid and contracts
• Most substances pack more tightly in solid state
• Volume decreases as cool
• Jump to liquid state

Mercury in glass bulb - change in liquid unit volume is larger than solid

A glass is formed by cooling so quickly that act as liquid below melting point

• Glass transition temperature
• Viscosity is highly dependent on temperature
• Knee in curve is function of cooling rate
• When cool slowly, there is more time of the constituents of the system to rearrange position
  • The excess volume is not as large in comparison to crystal

Solidification is determined by knee in curve of volume versus temperature

• Coefficient of thermal expansion is less

Difference between solidication to form crystal and form glass

• Crystal - abrupt change in V versus T
• Glass - no abrupt change in V versus T at glass transition temperature

Liquid to crystalline solid

• Melting point independent of cooling rate

Glass transition

• Supercooled liquid - cooled below melting point
• The substance is a glassy solid
• $T_{g}$ is function of the cooling rate
  • More or less free volume determined by cooling rate

There are other systems in addition to silica

• Other glass forming oxides:
  • $GeO_{2}$
  • $B_{2}O_{3}$: Borate
  • $P_{2}O_{5}$
  • $As_{2}O_{5}$
  • $Sb_{2}O_{5}$
• Volume of glass far in excess between glass and crystal
• Form covalent bonds of metal to metal via bridging oxygen
• Three dimensional covalent bonds

Properties of oxide glass

• Chemically inert
• Electrically insulating
  • Strong bonds hold electrons
• Mechanically brittle
  • Very directional, no possibility of glide
• Optically transparent
  • Strong covalent bonds, energy levels far part
• Visually arresting
  • Don't form sharp edges
  • Color from dopant
• High melting temperature

Source:

MIT - Course 3.091 - Professor Sadoway - Fall 2004 - Lecture 21

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