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 short- range 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 ? What are the precursors for Sol-Gel ? For Al 2 O 3, the precursors may be Al(OC 4 H 9 ) 3, alkoxide Alkane: C n H 2n+2, a molecule containing only carbon and hydrogen linked by single bond. Such as methane: CH 4, ethane: C 2 H 6. Alkyl: C n H 2n+1, a ligand formed by removing one hydrogen from Alkane. Such as methyl: · CH 3, ethyl: · C 2 H 5 Alcohol: a molecule formed by adding a hydroxyl (OH) group to an alkyl. Such as methanol: CH 3 OH, ethanol: C 2 H 5 OH. Alkoxy: a ligand formed by removing a proton from the hydroxyl on a alcohol. Such as methoxy: · OCH 3,, ethoxy: · OC 2 H 5

Silicon Tetraethoxide (tetraethyl orthosilicate, TEOS) and Ti- tetra- isopropoxide Si(OC 2 H 5 ) 4 Ti(OC 3 H 7 ) 4 R=C 2 H 5 Si OO O O R R R R R=C 3 H 7 Ti OO O O R R R R

What are the reactions of Sol-gel ? Metal alkoxides react readily with water. Hydrolysis –Partially hydrolysis Si(OR) 4 + H 2 O  HO-Si(OR) 3 + ROH Si(OR) 4 + nH 2 O  (HO) n -Si(OR) 4-n + nROH –Complete hydrolysis Si(OR) 4 + 4H 2 O  Si(OH) 4 + 4ROH R = CH 3, or C 2 H 5 or C n H 2n+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-O- Si(OR) 3 + H 2 O or – (OR) 3 Si-RO + HO-Si(OR) 3  (OR) 3 Si-O- Si(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 H 2 O  M(OH) n + n H + –Ti H 2 O  Ti(OH) 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 NH 3, 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 (NH 3 ) 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, Al 2 O 3 Starting precursors Aluminium alkoxides Al(OC 3 H 7 ) 3 aluminium isopropoxide Al(OC 4 H 9 ) 3 aluminium secbutoxide i.e. Al (OR) 3 …… Excess water Vigorous stirring (80 o C, hydrolysis reaction) Deflocculation Add acid (HCl, HNO 3, CH 3 COOH) Non-complexing anion with Al, and Produce a charge effect at low conc. monohydroxide Polymerized gel Al 2 O 3

Examples of Sol-Gel powders Sol-gel Barium Titanate Titanium tetra-isopropoxide Ti(OC 3 H 7 ) 4 Acetic Acid Titanium precursor Barium acetate (aqueous solution) Gelation Chemical Polymerization Drying and Calcination BaTiO 3 powders

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

What is Hydrothermal process ? Hydrothermal dissolution/precipitation reaction – the basic mechanism for the hydrothermal formation of ceramic oxide particles Dissolution Precipitation Poorly Ordered Precursor Crystalline Hydrothermal Product A(OH)s + B(OH)s  A(OH)aq + B(OH)aq  ABO 3

What is Hydrothermal process ? Hydrothermal synthesis –An aqueous chemical process –Typical reacting temperature 100~374 o C –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