Chapter 9 Chapter 9 Synthesis Synthesis MethodsMethodsHong-Wen WangHong-Wen Wang

How to prepare ceramic powders ? How to prepare ceramic powders ? Sol-Gel ProcessSol-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 multicompo

nent– Ultrahigh purity– High surface area, powder, or gels.– Spherical powder, fiber, thin film, coating can be made.

What is Sol ? Gel ? Colloidal What is Sol ? Gel ? Colloidal sols ? Polymeric sols ? sols ? Polymeric sols ?

• Colloid means : – A suspension containing dispersed particles of

1~1000 nm– Gravitational force are negligible– Interactions are dominated by short-range

forces.– Require adjust pH to make good dispersive

particles– Colloidal sol sometime is called particulate

sols.

What is Sol ? Gel ? Colloidal sols ? What is Sol ? Gel ? Colloidal sols ? Polymeric 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 What is Sol ? Gel ? Colloidal sols ? Polymeric sols ?sols ? Polymeric sols ?

What are the precursors for Sol-What are the precursors for Sol-Gel ?Gel ?

• For Al2O3, the precursors may be Al(OC4H9)3 , alkoxide• Alkane: CnH 2n+2, a molecule containing only carbon a

nd 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 orthosilicaSilicon Tetraethoxide (tetraethyl orthosilicate, TEOS) and Ti- tetra-isopropoxidete, TEOS) and Ti- tetra-isopropoxide

• Si(OC2H5)4

• Ti(OC3H7)4

R=C2H5

Si OOO

OR

R

R

R

R=C3H7

Ti OOO

OR

R

R

R

What are the reactions of Sol-gel ?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 + 4ROHR = CH3, or C2H5 or CnH2n+1

What are the reactions of Sol-gel ?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-O-Si(OR)3 +

H2Oor – (OR)3 Si-RO + HO-Si(OR)3 (OR)3 Si-O-Si(OR)3 +

ROH• Continuous 3-D condensation polymeriz

ation polymeric gel

What are the reactions of Sol-gel ?What are the reactions of Sol-gel ?Process 1 : Process 1 : From metal salts From metal salts particulate particulate

gelsgels

• 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 b

y 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 ?What are the reactions of Sol-gel ?From metal alkoxides From metal alkoxides both polymeric gels both polymeric gels

and particulates gelsand particulates gels

• Three steps:– Hydrolysis of metal alkoxides to give prec

ipitates or polymeric sols– Defloculation of the residual metal hydro

xides to a clear sol.– Gelation to gels for both polymeric and p

articulate gels.

Hydrolysis and CondensationHydrolysis 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 powdersExamples of Sol-Gel powders

• Sol-gel alumina, Al2O3

Starting precursorsAluminium alkoxidesAl(OC3H7)3 aluminium isopropoxideAl(OC4H9)3 aluminium secbutoxidei.e. Al (OR)3……

Excess water Vigorous stirring (80oC, hydrolysis reaction)

DeflocculationAdd acid (HCl, HNO3, CH3COOH)Non-complexing anion with Al, and Produce a charge effect at low conc.

monohydroxide

Polymerized gel Al2O3

Examples of Sol-Gel powdersExamples 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 ?What is Hydrothermal process ?

Feedstockpreparation

Reagents Growth control agentsOxides/Hydroxides/saltsGels /Organics/Acids/Bases

ReactorBatch oxidizingTemperature (100 to 350oC)Pressure (<15MPa)Residence Time (5-60mins)

Pressure Let-down

crystalline powderFiltrating/washing/drying

What is Hydrothermal process ?What is Hydrothermal process ?

• Hydrothermal dissolution/precipitation reaction – the basic mechanism for the hydrothermal formation of ceramic oxide particles

DissolutionPrecipitation

Poorly Ordered Precursor

Crystalline Hydrothermal Product

A(OH)s + B(OH)s A(OH)aq + B(OH)aq ABO3

What is Hydrothermal process ?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 powd

ers 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 MnZHydrothermal process for MnZn Ferriten Ferrite