Structure of Amorphous Materials -2

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Structure of Amorphous Materials -2

• Oxide glasses
• Metallic glasses
• Amorphous Polymers
• Silicon

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Silica - SiO2
Amorphous silica Crystalline SiO2
O
Si
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SiO2 - ideal structure characteristics -
continuous random network (CRN)

• Basic unit - tetrahedron with Si at the center
  and O at corners
• Each corner is shared by two tetrahedrons
• No edges or faces are shared
• Two dimensional depiction

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SiO2 - radial distribution function and cooling
rate effects
Need to define partial g(r)s - gSiSi(r), gSiO(r),
gOO(r)
The structure and thus properties depend on the
cooling rate
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Network modifiers

• Replacing cations with cations of lower valency
  (e.g. 3 into 2) introduces breaks in the
  network.
• This lowers the glass transition temperature and
  modulus and thus allows to process material at
  lower temperature
• Most commercially used glasses are with network
  modifiers

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Metallic Glasses
TEM image of amorphous zirconium alloy

• Metallic glasses are made by rapid cooling of a
  metallic liquid such that there is not enough
  time for the ordered, crystalline structure to
  nucleate and grow. In the original metallic
  glasses the required cooling rate was as much as
  a million degrees Celsius per second! Recently,
  alloys have been developed that form glasses
  around 1-100 degrees per second cooling rates.
• Typically the best glass formers are
  multicomponent materials such as Zr-Ti-Cu-Ni-Al
  alloy.
• Metallic glasses can be quite strong yet highly
  elastic, and they can also be quite tough.
  Furthermore above the the glass transition
  temperature a metallic glass becomes quite soft
  and flows easily allowing to form complex shapes.

Schematic of a two component glass
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Mechanical Properties of Bulk Metallic Glasses
(BMG)

• High yield strength, fracture toughness
• High elastic strain limit (2)
• Excellent processibility

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Mechanical deformation of metallic glasses
Local plastic deformation and shear band
formation

• Unresolved questions
• How does thermo-mechanical history affect the
  structure of a metallic glass the plastic
  deformation behavior?
• Is there an ideal way to structurally
  characterize metallic glasses so to get the best
  structure-property understanding?

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Polymer chain structure - Gaussian coil
Model N1 beads (mers) connected by N links
(bonds) of length b0 with random orientation -
equivalent of a random walk Vector representing
nth link
End to end distance Since link orientations
are random an average over all conformations
(denoted by )
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End to end distance
The average end to end distance is zero but the
average distance square is not - it measures the
size of the polymer coil . The last equality
comes for the fact that the average dot product
of two randomly oriented vectors is zero Real
chains are typically more rigid that a model one
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End to end distance distribution
Probability of having a chain with and to end
distance R is a Gaussian distribution
Long chains form an entangled network
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Chain rigidity
Rigid chain Flexible chain
Rigid chains have larger end-to-end distance for
the same contour length, but at large scale they
are flexible coils anyway
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Specific chemical structure, tacticity and
ability to crystallize
Chains with a regular attachment (isotactic or
syndiotactic) of side groups can
crystallize Chains with irregular side groups
(atactic) can not crystallize Flexible chains
are easier to crystallize
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Semicrystalline polymers
A mixture of crystalline regions (lamellae)
separated by amorphous regions
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Amorphous Silicon (aSi)
Largely four-fold coordinated network, with some
free-fold coordinated atoms (inducing dangling
bonds). To eliminate dangling bonds that act
as electron traps aSi is hydrogenated. Hydrogen
saturates dangling bonds Thin-film amorphous
Silicon (a-Si) have good photovoltaic
characteristics, are mounted on flexible backings
are do not fracture as easily as crystalline Si,
which allows them to be formed to fit
applications with the bending inherent when used
in building materials.
Amorphous solar cells do not convert sunlight
quite as efficiently as crystalline Si cells,
however, they require considerably less energy to
produce, and are superior to crystalline cells in
terms of the time required to recover the energy
cost of manufacture.  Amorphous silicon is
gradually degraded by exposure to light. This
phenomena is called the Staebler-Wronski Effect
(SWE). 
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Amorphous carbons property vs. sp2/sp3 content
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Bonding and mechanical properties of amorphous
networks
Constrain model - each bond and bond angle
represent a constrain in the amorphous
network It can be shown that below average
coordination, ca, of 2.4 network can be deformed
with no energy cost. Based on this modulus is
then equal to EE0(ca -2.4)/(4-2.4)1.5
where 4 corresponds to fully coordinated
network ? higher coordination ? larger modulus