chapter 9 synthesis methods
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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 - PowerPoint PPT PresentationTRANSCRIPT
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