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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

-

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

-

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