Spinoidal Decomposition and Phase Diagrams ___________ by Jason Bodson

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Spinoidal Decomposition and Phase
Diagrams___________by Jason Bodson
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Spinoidal DecompositionA Mechanism of Phase
Separation Resulting in an Interconnected
Morphology
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Phase Separation Occurs in a liquid
immiscibility region Controlled by
thermodynamics and kinetics
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ThermodynamicallyThe free energy of mixing,
DGm, must be positive for separation to occur.
(Separated phases must be a lower energy state
than mixed phases.)
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DGm DHm TDSm
Enthalpy of Mixing
Entropy of Mixing
DHm aX1X2 where a is a constant related bond
energies DSm -R(X1lnX1 X2lnX2) where X1
and X2 are mole fractions
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KineticallyViscosity must be low enough to
allow separation in given time.
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ReviewTwo immiscible liquids will phase
separate when it is energetically favorable and
kinetically possible.
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There are two phase separation mechanisms
Nucleation and Growth Growth occurs on
individual nuclei Phases are well-defined Has
an energy barrier Droplet-in-matrix morphology
(Figure A)
Spinoidal Decomposition A general separation
of phases Phase boundaries are diffuse and
gradually become sharpe Spontaneous
Interconnected morphology (Figure B)
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Figure A. Droplet-in-matrix morphology in glass
Figure B. Interconnected morphology in glass
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Spinoidal Regions in Phase DiagramsSpinoidal
regions fall within immiscibility domes
Immiscibility Dome(2 liquids)
Immiscibility Dome
Spinoidal Region
Spinoidal Region
B
B
Stable Immiscibility
Metastable Immiscibility
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Spinoidal regions are usually metastable and not
shown on phase diagrams. Spinoidal regions can
be calculated, but are usually found
experimentally. In glasses, this can be done by
heating samples to a series of temperatures and
quenching. Morphology is detected by examination
via electron microscope or indirectly via
property changes.
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Vycor is an example of a commercial product made
by spinoidal decomposition. It is a 96 silica
glass made by the phase separation of a
sodium-borosilicate melt. The borate-rich phase
is etched away, leaving a porous high-silica
glass. This can also be heated to produce a
dense glass.
A computer-generated model of the Vycor
morphology.
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Areas of immiscibility in the SiO2-B2O3-Na2O
system used to make Vycor and Pyrex glasses.
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Summary Spinoidal decomposition is a mechanism
of phase separation resulting in an
interconnected morphology. It is a spontaneous,
gradual separation of phases with diffuse
boundaries. On phase diagrams it occurs within
immiscibility domes, but is often metastable. An
example of spinoidal decomposition in ceramics
manufacture is Vycor high-silica glass.