Traditional Ceramics, Thermoset Plastic, Traffic Paint, Textile Fiberglass

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Traditional Ceramics, Thermoset Plastic, Traffic
Paint, Textile Fiberglass What do they have in
common?
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Silica
Angela Puccini Cer. Eng. 251 FS 2000
SiO
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Silica Phase Diagram
Important Features

• polymorphic phase transitions

• multiple invariant points

• metastability at lower pressures

• changes in density from one phase to the next

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Phase Transitions

• There are two types
• displacive or snap in which only changes in
  bond angle and length are involved
• reconstructive in which bonds are broken and
  reformed

a to b-quartz transition at 573C
b-quartz to tridymite transition at 870C
tridymite to b-cristobalite at 1470C
cristobalite to liquid transition at 1713C
liquid to vapor transition
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Phase Transitions
Displacive
Reconstructive
Usually sluggish, requires higher energies of
formation and is often not reversible.
Only changes in oxygen-silica bond angles or
lengths are involved
a to b-quartz transition at 573C
b-quartz to tridymite transition at 870C
tridymite to b-cristobalite at 1470C
a to b-cristobalite transition at 275C
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Invariant Points
Invariant point a point on a phase diagram where
there are no degrees of freedom F C - P 2
Since there is one component, the maximum number
of phases in equilibrium in this diagram is 3.
The point at which this occurs is a triple point.
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Invariant Points in This Diagram
a-quartz, b-quartz, vapor
b-quartz, b-cristobalite, tridymite
b-quartz, tridymite, vapor
a-quartz, b-quartz, coesite
tridymite, b-cristobalite, vapor
b-cristobalite, liquid, vapor
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Metastability
Properties of a Metastable Phase

• Always has a higher vapor pressure than the
  stable phase in the same temperature range.

• The method of approach is usually unique (it can
  only be approached one way).

• The stability of a metastable phase can vary.

• When a stable phase or seed is added, this
  usually causes the metastable phase to revert
  quickly and irreversibly to the stable phase.

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Why is Metastability Important in the Silica
Diagram?
Silica glass is a metastable phase.
(The dashed lines on this diagram denote
metastability.)
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This is the metastable line of silica glass. It
can be reached through rapid cooling of the
liquid.
This is the metastable line of b-cristobalite.
This is the metastable line of cristobalite.

• Important Features
• undercooling of b-cristobalite (below 1470C)
• superheated b-quartz goes to undercooled
  b-cristobalite
• high tridymite melts in the b-cristobalite
  stability region
• b-cristobalite can be brought to room temp. under
  specific conditions

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Densities
Clausius-Clapeyron Equation
Where Ppressure on the system Tthe absolute
temperature DHthe enthalpy change accompanying
the phase change DVthe change in volume
accompanying the phase change
dT/dP TDV/Q
Le Chatlier's Principle
If an attempt is made to change the pressure,
temperature or concentration of a system in
equilibrium, then the equilibrium will shift in
such a manner as to diminish the magnitude of the
alteration in the factor which is varied.
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The Clausius-Clapeyron Equation can be used to
calculate the slope of a boundary line.
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As we go from point 1to point 2, the change in
pressure is positive and the change in temp. is
negative.
dT/dP TDV/Q
This makes the change in volume negative, meaning
that the compound at point 2 has a higher density
than that at point 1.
dV dTQ/dPT
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Important Features to Remember about the Silica
Diagram

• polymorphic phase transitions

• multiple invariant points

• metastability at lower pressures

• changes in density from one phase to the next

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Sand usually consists of the low temp. phase of
quartz.
Silica is one of the most important materials in
ceramic engineering and understanding the phase
diagram is the first step in learning to utilize
this material to the fullest.
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Remember kids, phase diagrams are analyzed by
trained professionals. Dont try this at home.