Chapter 9
Synthesis
Methods
Hong-Wen Wang
How to prepare ceramic powders ?
Sol-Gel Process
• Aqueous solution (from metal salts or colloidal
particles)  particulate sol and gel.
• Non-aqueous solution (from metal alkoxides)
 polymeric sol and gel.
• Advantages
– High chemical homogeneity, particularly for
multicomponent
– Ultrahigh purity
– High surface area, powder, or gels.
– Spherical powder, fiber, thin film, coating can be
made.
What is Sol ? Gel ? Colloidal
sols ? Polymeric sols ?
• Colloid means :
– A suspension containing dispersed
particles of 1~1000 nm
– Gravitational force are negligible
– Interactions are dominated by shortrange forces.
– Require adjust pH to make good
dispersive particles
– Colloidal sol sometime is called
particulate sols.
What is Sol ? Gel ? Colloidal sols ?
Polymeric sols ?
• Sol:
– A homogenous dispersion of soluble solids and
miscible liquids, is consisted of finely divided,
discrete particles of 1~10 nm in diameter
– A “colloidal sol” consists of dense (oxide or
hydroxide) particles 1~1000 nm which
therefore include colloidal dispersion and
colloidal suspension.
– Line which distinguishes a dispersion (1~10nm)
from a suspension (>10nm) is generally
accepted at an equivalent spherical diameter
(e.s.d), 10 nm
What is Sol ? Gel ? Colloidal
sols ? Polymeric sols ?
What are the precursors for Sol-Gel ?
• For Al2O3, the precursors may be Al(OC4H9)3 ,
alkoxide
• Alkane: CnH 2n+2, a molecule containing only
carbon and hydrogen linked by single bond.
Such as methane: CH4, ethane: C2H6.
• Alkyl: CnH 2n+1, a ligand formed by removing
one hydrogen from Alkane. Such as
methyl: ·CH3, ethyl: · C2H5
• Alcohol: a molecule formed by adding a
hydroxyl (OH) group to an alkyl. Such as
methanol: CH3OH, ethanol: C2H5OH.
• Alkoxy: a ligand formed by removing a proton
from the hydroxyl on a alcohol. Such as
methoxy: ·OCH3,, ethoxy: ·OC2H5
Silicon Tetraethoxide (tetraethyl
orthosilicate, TEOS) and Ti- tetraisopropoxide
R
O
R O Si O R
O
R=C
H
2
5
R
R
O
R O Ti O R
• Ti(OC3H7)4
O
R=C3H7
R
• Si(OC2H5)4
What are the reactions of Sol-gel ?
• Metal alkoxides react readily with
water.
• Hydrolysis
– Partially hydrolysis
• Si(OR)4 + H2O  HO-Si(OR)3 + ROH
• Si(OR)4 + nH2O  (HO)n-Si(OR) 4-n +
nROH
– Complete hydrolysis
• Si(OR)4 + 4H2O  Si(OH)4 + 4ROH
R = CH3, or C2H5 or CnH2n+1
What are the reactions of Sol-gel ?
• Two partially hydrolyzed molecules can
link together  condensation reaction
– (OR)3 Si-HO + HO-Si(OR)3  (OR)3 Si-OSi(OR)3 + H2O
or
– (OR)3 Si-RO + HO-Si(OR)3  (OR)3 Si-OSi(OR)3 + ROH
• Continuous 3-D condensation 
polymerization
 polymeric gel
What are the reactions of Sol-gel ?
Process 1 : From metal salts  particulate
gels
• M+n + n H2O  M(OH)n + n H+
– Ti+4 + 4 H2O  Ti(OH)4 + 4 H+
• The reaction is enhanced by adding a
based solution
• Dense precipitates
• Gelation by dehydration of the solution
or by alkaline gelation when the pH of
the solution is increased and repulsive
force between particles is reduced.
• Spontaneous coagulated  Particulate
gels.
What are the reactions of Sol-gel ?
From metal alkoxides  both polymeric
gels and particulates gels
• Three steps:
– Hydrolysis of metal alkoxides to give
precipitates or polymeric sols
– Defloculation of the residual metal
hydroxides to a clear sol.
– Gelation to gels for both polymeric
and particulate gels.
Hydrolysis and Condensation
• Hydrolysis and condensation can be catalysed
by :
– Addition of acid or base: HCl, acetic acid, HF or NH3,
KOH, KF…
– Important parameters :
• water-to-alkoxide ratio
• pH value
• Type of solvent
• Temperatures
• Silica gels are prepared by hydrolysis of TEOS
with a mineral acid (HCl) or base (NH3) as a
catalyst.
– Acid catalysed, pH<2.5  linear polymeric gel
– Base catalysed, high pH  particulate gel
Examples of Sol-Gel powders
• Sol-gel alumina, Al2O3
Aluminium alkoxides
Al(OC3H7)3 aluminium isopropoxide
Al(OC4H9)3 aluminium secbutoxide
i.e. Al (OR)3……
Starting precursors
Excess water
Vigorous stirring
(80oC, hydrolysis reaction)
Deflocculation
monohydroxide
Polymerized gel Al2O3
Add acid (HCl, HNO3, CH3COOH)
Non-complexing anion with Al, and
Produce a charge effect at low conc.
Examples of Sol-Gel powders
• Sol-gel Barium Titanate
Titanium tetra-isopropoxide
Ti(OC3H7)4
Acetic Acid
Titanium precursor
Barium acetate (aqueous solution)
Gelation
Chemical Polymerization
Drying and Calcination
BaTiO3 powders
What is Hydrothermal process ?
Feedstock
preparation
Reagents
Growth control agents
Oxides/Hydroxides/salts
Gels /Organics/Acids/Bases
Reactor
Batch oxidizing
Temperature (100 to 350oC)
Pressure (<15MPa)
Residence Time (5-60mins)
Pressure Let-down
Filtrating/washing/drying
crystalline powder
What is Hydrothermal process ?
• Hydrothermal dissolution/precipitation
reaction – the basic mechanism for the
hydrothermal formation of ceramic oxide
particles
A(OH)s + B(OH)s  A(OH)aq + B(OH)aq  ABO3
Dissolution
Precipitation
Poorly Ordered
Precursor
Crystalline
Hydrothermal
Product
What is Hydrothermal process ?
• Hydrothermal synthesis
–
–
–
–
An aqueous chemical process
Typical reacting temperature 100~374oC
Pressure range 0~15MPa
The product is crystalline anhydrous ceramic
powders with controlled particles size,
controlled stoichiometry, and controlled
particle shape. (calcination is not necessary.)
– Starting precursor can be inexpensive oxides,
hydroxides, chlorides and nitrates.
Hydrothermal process for
MnZn Ferrite