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TRANSCRIPT
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Chemistry of Sol-Gel Science
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Sol-Gel Process
Extraction
AerogelGelSol
Unifrom Particles
Gelation
Xerogel Film
Evaporation
Fibers Evaporation
of Solvent
Heat
Heat
Xerogel
Films : AR
Sensor
Catalytic
Glass Ceramics
Sealing Glasses
Catalyst Supports
Dense Film
e e r c
Protective
er p c re orms
Controlled-Pore Glasses
Dense Ceramic
- -. .
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Relevant Subject and Terminology
*Relevant Subject
1, Physics ( fractal geometry, percolation theory
2, Chemistry ( mechanisms of hydrolysis and condensation )
3, Ceramics ( sintering and structural relaxation )
* erm no ogy
Colloid : Suspension in which the dispersed phase is so small
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interactions are dominated by short-range force
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Relevant Subject and Terminology
*monomer ; capable of forming at least two bonds
* Oligomer; defined as a polymer composed of a few monomers)
In sol- el rocess, ran in from dimer to octamer.
Gel;gel to consist of continuous solid and fluid phases of colloidal
dimension
*Aging;applied to the process of change in structure and propertie
after gelation(condensation, phase transformation)
*syneresis;bond formation or attraction between particles induces
con rac on o e ne wor an expu s on o qu rom
the pores
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Relevant Subject and Terminology
* Wet gel ; Original gel
eroge r e ge
*
high porosity and surface area
* gel point : the time at which the last bond is formed that comple
* Amor hous noncr stalline ;
* Sintering ; process of collapse of pores driven by surface energ
produce a pore-free ceramic.
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The advantage and disadvantage of the Sol-Gel Process
The advantage
2. Lower processing temperature3. The use of chemistry to control reactions
4. The formation of a re-inor anic network in
solution
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6. The adjustment of appropriate viscosities for
Disadvantage
1. Starting materials can be fairly expensive2. difficulties of conventional dr in
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Precursor
=
element surrounded by various ligands
. 3 , 2, 4 ,
Organic ; Al(OC4H9)3, Ti(OCH(CH3)2)4 , Ba(CH3COO)2
Pure metal ; Ba, Al,
+ +
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Hydrolysis and Condensation 1 Nonsilicates
early transition metals : Ti, V, Zr
Group A : B, Al
1. greater chemical reactivity resulting from low
electronegativity of the metal
TiO : coating, Photocatalyst, pigment. a y o ex severa coor na on s a es
Al2O3 : catalyst and catalyst supports , ZrO2-Y2O3 : Fuel
cell
BaTiO3 : ferroelectric material , V2O5 : semiconducting film
High-Temp supreconducting ceramic : YBa2Cu
3O
7-x
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Solution Chemistry of Inorganic Precursors
1. Hydrolysis the magnitude of the chargetransfer
[M(OH)2)]z+ [MOH](z-1)++H+ [M=O](z-2)++ 2H+
, ,
g. . arger versus p ag amindecating the aquo, hydroxo
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Solution Chemistry of Inorganic Precursors
2. Condensation ~ depending on the coordination of the
metalSN ; nucleophilic substitution
XM1OX + M2OYM1O M2 +OY
N ; nuc eop c a on X
M1OX + M2OYM1O M2 OY
Hydroxo-ions : condensation occurs as soon as one OH is
resent in the coordination s here of M
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Solution Chemistry of Inorganic Precursors
2.1. Olation ~hydroxy bridge ("ol" bridge) is formed btw two
(a) (b) ( c ) (d)
2.2. Oxolation ~ oxo bridge ( -O- ) is formed btw two metalcenter
2(O)3
Nucleophilic SubstitutionNucleophilic Addition
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Solution Chemistry of Inorganic Precursors
Effect of catalyst
1) Effect of bases
H O
+ OH-
H H H
M-O-M-OH + H3O+ [M-O-M-OH2]+ + H2O [M-O-M]+ + H2O
M-O-M + H3O+
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Solution Chemistr of Metal Alkoxide Precursors
Characteristics of transition metal alkoxide
1) Highly electronegative -OR groups stabilize M
stable toward hydrolysis condensation
4) Stricter control of moisture and condition of
OR
2M-OR -M M- alkoxy bridgein nonpolar solvent
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Solution Chemistry of Metal Alkoxide Precursors
1. Mechanism of Hydrolysis and Condensation
N mec an sm N o owe y pro on rans er n e
absence of catalyst
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Solution Chemistry of Metal Alkoxide Precursors
Positive Partial Charge (M) for Metals in Various AlkoxideAlkoxide Zr(OEt)4 Ti(OEt)4 Nb(OEt)5 Ta(OEt)4 W(OEt)6 Si(OEt)4
+ . + . + . + . + . + .
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Role of the catalyst
Catalyst influence the rxn rate and the structureof the condensed product
- Acid condition - produce good leaving group
H
MOR + H3O
+ M+:O + H2O
OR O OR ORR
- - - - -- - - - - - -
OR OR OR OR
= - . = . = . = - .
Acid-catal zed condensation monolithic els or s inable sols
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Role of the catalyst
- Base condition -
- : - -
L = M or H, B = OH- or NH3
OR O OR OR
- - - - -- - - - - - -
OR OR OR OR
= + . = + . = + . = .
Base-catal zed condensation more com act hi h
branched species
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Chemical Modification
Chemical modification of T.M.A to retard the hydrolysisand condensation reaction rates
Alcohol exchange rxns
M(OR) + xR'OH M(OR) - (OR') + xROH
Chloride-modified transition metal alkoxides
TiCl + 2EtOH TiCl OEt + 2HCl
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Synthesis of Al2O3 particles by Yoldas method
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Hydrolysis and Condensation Silicate
Point of zero charge ; surface charge is
Isoelectric Point ; the electrical mobility
of the silica particles is zero
Both are in the range 1 < pH < 3
silica solubility and
dissolution rates
are maximized at or
a ove p
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80-100oC, pH=8
25-50oC, pH=2.2
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Hydrolysis and Condensation of Silicon Alkoxides
General Trendhydrolysis
Si-OR + H2O Si-OH + ROHesterification
alcohol condensation
Si-OR + HO-Si Si-O-Si + ROHalcoholysis
Si-OH + HO-Si Si-O-Si + H2Ohydrolysis
alcohol ; as a homogenizing agent participate in
ester cat on or a co o ys s react on
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Hydrolysis and Condensation of Silicon Alkoxides
Steric and Inductive effect
by any complication of the alkoxy groupInductive effect
electron density
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Hydrolysis and Condensation of Silicon Alkoxides
Mechanisms ; Bimolecular nucleophilic displacement
S 2-Si
Acid-catalyzed Hydrolysis
Inversion of the silicon
No Inversion of the silicon tetrahedron
RORO
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Hydrolysis and Condensation of Silicon Alkoxides
Base-Catalyzed Hydrolysis
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Particluate Sols
Schematic Stern and Gou la er
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Particluate Sols