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Sol-Gel Methods 1
Sol-Gel Methods
Hydrolysis
Condensation
Gelation
Ageing
Drying
Densification
Sol-gel process:
Powders: microcrystalline, nanocrystalline, amorphous
Monoliths, Coatings, Films, Fibers
Aerogels
Glasses, Ceramics, Hybrid materials
Sol-Gel Methods 2
Sol-Gel Methods
Sol = a stable suspension of colloidal solid particles or polymers in a liquid
Gel = porous, three-dimensional, continuous solid network surrounding acontinuous liquid phase
Colloidal (particulate) gels = agglomeration of dense colloidal particles
Polymeric gels = agglomeration of polymeric particles made from subcolloidalunits
Agglomeration = covalent bonds, van der Walls, hydrogen bonds, polymeric chainentanglement
Sol-Gel Methods 3
Sol-Gel Methods 4
M Colloid Routemetal salts in aqueous solution, pH and temperature control
HydrolysisM(H2O)b
Z+ ↔ [M(H2O)b-1OH](Z-1)+ + H+
Condensation-polymerizationM(H2O)b
Z+ ↔ [(H2O)b-1M(OH)2M(H2O)b-1](2Z-2)+ + 2H+
Sol-Gel Methods
Sol-Gel Methods 5
Sol-Gel Methods
H
O
H
RO
Si O
RO
ROR
H
OR
Si O
ORRO
R
H
OR
SiO
OR
OR
HH
O
H
+ ROH + H
Acid catalysed hydrolysis
H O
RO
Si OR
RO
RO
OR
Si O
ORRO
R
OR
SiO
OR
OR
HHO + RO
Base catalysed hydrolysis
M Metal-organic Routemetal alkoxide in alcoholic solution, water addition
Sol-Gel Methods 6
Sol-Gel Methods
Isoelectronic point: zero net charge
pH = 2.2 for silica
Sol-Gel Methods 7
Sol-Gel Methods
Rate of H+ catalyzed TEOS hydrolysis (gel time) as a function of pH
Effects on hydrolysis rate:
pH
substituents
solvent
water
Sol-Gel Methods 8
Sol-Gel Methods
Precursor substituent effect
Steric effects: branching and increasing of the chain length LOWERS thehydrolysis rate
Si(OMe)4 > Si(OEt)4 > Si(OnPr )4 > Si(OiPr)4 > Si(OnBu)4 > Si(OHex)4
Inductive effects: electronic stabilization/destabilization of the transitionstate.
Electron density at Si decreases:
R-Si > RO-Si > HO-Si > Si-O-Si
Sol-Gel Methods 9
Acidic conditions:
reaction rate decreases as more alkoxy groups are hydrolyzed
reaction at terminal Si favored, linear polymer products, fibers
RSi(OR)3 more reactive than Si(OR)4
Sol-Gel Methods
Basic conditions:
reaction rate increases as more alkoxy groups are hydrolyzed
reaction at central Si favored, branched polymer products,spherical particles, powders
RSi(OR)3 less reactive than Si(OR)4
Si-OH becomes more acidic with increasing number of Si-O-Si bonds
Sol-Gel Methods 10
Sol-Gel Methods
Water:alkoxide ratio (Rw) effect
stoichiometric ratio for complete hydrolysis = 4
Si(OR)4 + 4 H2O Si(OH)4 + 4 ROH
additional water from condensation
Si-OH + HO-Si Si-O-Si + H2O
Small amount of water = slow hydrolysis due to the reduced reactant conc.
Large amount of water = slow hydrolysis due to the reactant dilution
Sol-Gel Methods 11
Sol-Gel Methods
Hydrophobic effect
Si(OR)4 are immiscible with water
cosolvent ROH to obtain a homogeneous reaction mixture
polarity, dipole moment, viscosity, protic behavior
alcohol produced during the reaction
alcohols - transesterification
sonication
drying
Sol-Gel Methods 12
Sol-Gel Methods
Sintering mechanisms
solid, liquid, gas phase
1. Evaporation-condensation and dissolution-precipitation
2. Volume diffusion
3. Surface diffusion
4. Grain boundary diffusion
5. Volume diffusion from grain boundaries
6. Volume diffusion from dislocations
Sol-Gel Methods 13
Sol-Gel Methods