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Chapter 9 Synthesis Methods

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

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Chapter 9 Synthesis Methods

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  1. Chapter 9 Synthesis Methods Hong-Wen Wang

  2. 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.

  3. 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 short-range forces. • Require adjust pH to make good dispersive particles • Colloidal sol sometime is called particulate sols.

  4. 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

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

  6. 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

  7. R R O O R R O O O O R R Si Ti O O R=C2H5 R=C3H7 R R Silicon Tetraethoxide (tetraethyl orthosilicate, TEOS) and Ti- tetra-isopropoxide • Si(OC2H5)4 • Ti(OC3H7)4

  8. 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

  9. 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 + H2O or • (OR)3 Si-RO + HO-Si(OR)3  (OR)3 Si-O-Si(OR)3 + ROH • Continuous 3-D condensation  polymerization  polymeric gel

  10. 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.

  11. 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.

  12. 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

  13. 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) Add acid (HCl, HNO3, CH3COOH) Non-complexing anion with Al, and Produce a charge effect at low conc. Deflocculation monohydroxide Polymerized gel Al2O3

  14. Examples of Sol-Gel powders • Sol-gel Barium Titanate Titanium tetra-isopropoxide Ti(OC3H7)4 Acetic Acid Titanium precursor Barium acetate (aqueous solution) Chemical Polymerization Gelation BaTiO3 powders Drying and Calcination

  15. What is Hydrothermal process ? Reagents Growth control agents Oxides/Hydroxides/salts Gels /Organics/Acids/Bases Feedstock preparation Batch oxidizing Temperature (100 to 350oC) Pressure (<15MPa) Residence Time (5-60mins) Reactor Pressure Let-down Filtrating/washing/drying crystalline powder

  16. 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

  17. 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.

  18. Hydrothermal process for MnZn Ferrite

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