Transcript Document 7184878

Che5700 陶瓷粉末處理
溶膠凝膠法 Sol-Gel Process
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More and more applications, become an independent
category;
Use metal alkoxide, to go through hydrolysis reaction,
then condensation polymerization, to obtain desired
colloids
During process, system viscosity increase with time, it
can also be used for coating into films, or even directly
forming into desired objects  variety;
Most common example: TEOS (tetraethyl orthosilicate)
hydrolysis to make silica; (OC2H5)4Si + H2O 
(OC2H5)3SiOH + C2H5OH  …. Gradual hydrolysis and
then condensation reactions……
Che5700 陶瓷粉末處理
Advantages of Sol-Gel Processes
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In general:
Able to get uniform & small sized powder
Can get at low temperature high density glass, without
high temperature re-crystallization
Can get new compositions of glass
New microstructure and composition
Easy to do coating for films
Can get objects or films with special porosity
For improved adhesion
Can get metal (inorganic) – organic composites
Can coat onto large area or complex shape objects
Can get fibers
High uniformity, multicomponent systems
Che5700 陶瓷粉末處理
More Process Characteristics
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Mixing of reacting system: all alkoxide precursor, or 1
alkoxide + 1 metal salt; Things to consider: (a) cost of
precursor; (b) relative hydrolysis rate; (c) compatibility
of chemicals (uniform mixing)
Colloidal sol & polymerized sol;
Peptization: adding proper solvent or adjust pH, to redisperse weak agglomerates back into sols
M(OR)n  mostly derived from MCln ; often sensitive to
light, moisture, temprature etc.;
M-O bonding: ionic nature – mostly solids; covalent
nature - liquids
To decompose alkoxide: (a) add water; (b) heat (to
decompose)
Che5700 陶瓷粉末處理
Some statistic data on Alkoxide synthesis
Source: Am. Cer. Soc. Bull., 72(10), 73, 1993;
Some real
examples
Che5700 陶瓷粉末處理
Reaction Characteristics
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In simple terms, whole process control is on control of
relative rate between (a) hydrolysis ; (b) polycondensation
M-OH + M-OR  M-O-M + ROH; Yet M-OH & M-OH
condensation rate, and whether M-OR & M-OR can
condense; whether linear condensation or branched
form  affect microstructure
Effect of catalyst: acid or base
Effect of temperature: e.g. Al(OR)3 low temperature
hydrolysis  amorphous form, further aging 
hydroxide; at high temperature (> 80oC)  crystalline
boehmite AlO(OH), sintering behavior different
Zr(OR)4 hydrolysis, easy to get oxo bond, instead of
hydroxy bond
Che5700 陶瓷粉末處理
More Characteristics
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Alcohol as solvent, same as alcohol in alkoxide, one
can also use different alcohols (or co-solvent), may
affect reaction; sometime the steric effect
Additive with carbonate groups, will get esterfication (酯
化), and polyesterification to get gel-like product; (e.g.
Pb acetate; Pb(C2H3O2)2 . 2H2O for synthesis of BaTiO3
powder)
Another reason for gelation: solvent evaporation,
increase of concentration; mostly due to reaction; 
end result 3D network structure, system viscosity
increase  continue aging, stronger structure, begin to
shrink, expel solvent.
Taken from
TA Ring,
1996;
Three
different
network
structures
Taken from TA
Ring, 1996; sol
characteristics
also affect film
structure and
properties
Acid or base
(catalyst) and
/or salt: will
change surface
charge  and
final structure
Polarity Effect
700
Ethanol & Ethylene glycol (NH3=1M)
600
Methanol & Ethanol (NH3=1M)
Methanol & Ethylene glycol (NH3=1M)
500
Diameter (nm)
Effect of
different solvents
on particle size
(silica)
PA’, PB’ =
polarity of
solvents A, B
A, B volume
fraction
1,2-Dichloroethane & Ethanol (NH3=1M)
Methanol & Ethanol (NH3=0.5M)
400
300
200
100
TEOS=0.28M
0
4.00
4.25
4.50
4.75
5.00
5.25
Polarity index of cosolvent
5.50
5.75
P  P  A  P B
'
AB
'
A
'
B
A Proposal
Si(OC2H5)4-X(OH)X
high polarity
cosolvent
Cmax*’
Cmax*
Cmin*’
Cmin*
low polarity
cosolvent
Cs’
Cs
Time
 Adding
solvent of
different polarity,
change solubility
of precursor
species, change
supersaturation,
 nucleation to
get more nuclei
 smaller final
particle size
Che5700 陶瓷粉末處理
Gel Characteristics
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Gel stage: time to form film, fiber, etc. forming period.
Due to proper viscosity to work with.
Gel densification:
Continuous cross-linking & dehydration to expel solvent
Reduce free volume (relaxation of microstructure)
Reduce surface area
Capillary contraction: due to solvent evaporation
All above mechanisms  shrink structure, may cause
crack, especially as films (constrained by substrate)
Sol gel can be considered as phase transition
Low
coordination
number 
large pore
inside
structure; high
CN  dense
structure
Che5700 陶瓷粉末處理
Theoretical composition
Che5700 陶瓷粉末處理
Filtration of ordinary small particles, may
also form gel layer
During
formation
of gels,
sample
may
adhere to
wall and
cause
crack
Che5700 陶瓷粉末處理
Gel Drying
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Gel drying period, can get kinetic data from weight loss
Similar to ordinary drying process, classified as (a)
constant rate drying period; (b) reach a critical point
(prone to cracking); (c) first falling rate period; (d)
second falling rate period
To prevent cracking during drying, control drying rate
(slow during certain period), some proposed to add
“drying control chemical additive (DCCA) – objective:
to lower capillary pressure, to lower solvent pressure;
or to use supercritical evaporation method
skeletal
densification,
structural
relaxation may
be
polymerization
reaction
III zone – little
change in
weight, viscous
sintering cause
shrinkage
•I zone – de-hydration, solvent evaporation, slight
capillary contraction
•II zone – continue to dehydrate, molecule cracking;
Che5700 陶瓷粉末處理
Decomposition and Sintering
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Up to 150oC, continuous dehydration, some ligand may
desorb and leave, micropores will limit its movement
Over 250oC, molecule begin to decompose, continue to
loss weight. Decomposition affected by gel structure,
extent of cross-linking; atmosphere also important,
incomplete decomposition may have residual char;
Skeleton collapse, particle sintering and densification,
mostly by viscous sintering (faster than conventional
diffusion mechanism), I.e. can be achieved at lower
temperature (one advantage of sol-gel process); if fast
heating, may contain residual pores.
Che5700 陶瓷粉末處理
Sol to Gel to Glass
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sol- gel- glass: one advantage – no need of
high temperature treatment, can obtain special
composition, high purity, high uniformity
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Difficult to make one whole piece glass without
crack, often very slow (because of slow drying
to avoid cracking)
Ref: Am Cer. Soc. Bull., 64(11), 1463, 1985
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* TEOS + boric acid + phosphoric acid + acid catalyst +
glycerol + formamide  PE or teflon container  room
temperature; B+P content may reach 12 wt%
Example of Reverse Micelle Method
Micelle = oil in water; reverse micelle = water in oil