Materials Science and Engineering/List of Topics/Engineered Glasses

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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
  • 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:
    • : Borate
  • 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


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