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99.1 Introduction99.2 Solid state reactions for ceramic synthesis99.3 Precipitation methodforced hydrolysis99.4 Sol-gel process99.5 Pechini method
Chapter 9: Sol-gel and other processes for ceramic synthesis
Ceramic MaterialsCeramic MaterialsA long history and a great futureA long history and a great future
22
CeramicsCeramicsUniversiteit Twente
Ceramics are inorganic (non-metallic) materials, which are able to withstand elevated temperatures (in excess of 500 C)
They are availabe as bulk or as coating For bulk ceramics (in most cases also for coatings) a powder is
brough into a form and subsequently a temperature treatment is given (< Tmelt): The Ceramic Fabrication Process
9.1 Solid State Reactions9.1 Solid State Reactions
Direct reaction of solids to form the final product. In Direct reaction of solids to form the final product. In principle no decomposition is involved.principle no decomposition is involved.
Solids do not react with solids at room temperature Solids do not react with solids at room temperature even of thermodynamics is favorable.even of thermodynamics is favorable.
High temperature must be used.High temperature must be used.
SolidSolid--solid reactions are simple to perform, starting solid reactions are simple to perform, starting materials are often readily available at low cost and materials are often readily available at low cost and reactions are reactions are cleanclean i.e. do not involve other chemical i.e. do not involve other chemical elements.elements.
Disadvantages include the need for high temperatures, Disadvantages include the need for high temperatures, the possibility of nonthe possibility of non--homogeneity, contamination from homogeneity, contamination from containers etc. containers etc.
33
Solid State Reactions: exampleSolid State Reactions: example Synthesis of YBCO, YBaSynthesis of YBCO, YBa22CuCu33OO77--xx Direct reaction between YDirect reaction between Y22OO33, BaO, BaO22, , CuOCuO (Reaction (Reaction between three solid components) between three solid components) Grind to obtain Grind to obtain large surface area large surface area Press into pellets (contact) Press into pellets (contact) Heat Heat in alumina boat, temperature profile:in alumina boat, temperature profile:
Other precursors may be used, e.g. BaCO3, which may be decomposed to fine grained BaO during the reaction.( + nitrates etc.)Decomposition must be performed in a controlled manner in order to avoid violent decomposition (i.e. choosing an appropriate temperature)
Solid State Reactions: general aspectsSolid State Reactions: general aspects
ktx =2
44
Solid State Reactions: general aspectsSolid State Reactions: general aspects
Solid State Reactions: general aspectsSolid State Reactions: general aspects
Ions in solids are not mobile at low temperatures. High temperatures, as a rule of thumb, at 2/3 of the melting temperature (of one component) the diffusion is sufficient to achieve solid state reactions.Formation of BaTiO3 by reacting BaCO3 and TiO2 is an example of a seemingly simple reaction is more complex than expected.BaCO3 is decomposed to reactive BaO: (Rock salt, ccp of the oxide anions, Ba2+ in octahedral sites),TiO2 (Rutile, hcp of oxide ions, Ti4+ in half of the octahedral sites)At least three stages are involved in formation of BaTiO3 from BaO and TiO2.
BaO react with the surface of TiO2, forming nuclei and a surface layer of BaTiO3.
Reaction between BaO and BaTiO3 to form Ba2TiO4. This is a necessary phase for increasing migration of Ba2+ ions.
Ba2+ ions from the Ba-rich phase migrate into the TiO2 phase and form BaTiO3.
55
Solid State Reactions: general aspectsSolid State Reactions: general aspects
Reaction rates depends on: Area of contact between the reacting solids, i.e. surface area and density (How to increase surface area?) The rate of nucleation (How to increase rate of nucleation?) Rates of diffusion of ions (and other species) (How to increase?)Disadvantages, e.g.:Nucleation and diffusion related problems (high temperature)Formation of undesired phases (reaction paths) (e.g. BaTi2O5)Homogeneous distribution, especially for dopants, is difficultDifficult to monitor the reaction directly, in-situ??Separation of phases after synthesis is difficultReaction with containers/cruciblesVolatility of one or more of the components
Solid State Reactions: general aspectsSolid State Reactions: general aspects
66
Solid State Reactions: general aspectsSolid State Reactions: general aspects
Solid State Reactions: general aspectsSolid State Reactions: general aspects Nucleation may be facilitated by structural similarity between Nucleation may be facilitated by structural similarity between the reacting solids, or products.the reacting solids, or products.
In the reaction between In the reaction between MgOMgO and Aland Al22OO33, , MgOMgO and and spinelspinel have have similar oxide ion arrangements. similar oxide ion arrangements. SpinelSpinel nuclei may then easily nuclei may then easily form at the form at the MgOMgO surfaces.surfaces.
There is often a structural There is often a structural orientationalorientational relationship between relationship between the reactant, nuclei and product.the reactant, nuclei and product.
The lattice spacing (e.g. distance between oxide ions) should nThe lattice spacing (e.g. distance between oxide ions) should not ot be too different for oriented nucleation.be too different for oriented nucleation.
The ease of nucleation depends also on the actual surface The ease of nucleation depends also on the actual surface structure of the reactants.structure of the reactants.
77
Solid State Reactions: general aspectsSolid State Reactions: general aspectsIncrease diffusion rates (decrease diffusion lengths)
Small particles by grinding, ball milling, spray drying, freeze drying, spray pyrolysis etc. Cooling and regrinding may be necessary
Intimate mixture of reactants by coprecipitation, sol-gelprocesses, spray pyrolysis etc.
Reduction of diffusion distances by incorporating the cations in the same solid precursor.
Solid state reactions involving melts, molten fluxes or high temperature solvents.
9.2 Synthesis of monodispersed colloids precipitation
AgBr particles fromprecipitation
J.E. Maskasky,
J . Imaging Sci. 1986,30,247.
88
Synthesis of monodispersed colloids precipitation domains diagram
Regions: I: homogeneous solution; II: blue precipitate
(brochantite); III: red-brown precipitate (tenorite).
CuSO4 particles from precipitation
Egon Matijevic, Chem. Mater. 5 (1993) 412.
Synthesis of monodispersed colloids spherical particles
Egon Matijevic, Chem. Mater. 5 (1993) 412.
Al2O3xH2O via
2mM Al(NO3)3+ 3 mM (NH4)2SO4105C/24h
Cr2O3xH2O via
4mM CrK(SO4)2 12H2O
75C/24h
CeO2xH2O via
1.2mM Ce(SO4)2+ 80 mM H2SO490C/48h
-Fe2O3via
32mM FeCl3+ 5 mM HCl100C/240h
99
Synthesis of monodispersed colloids TEM micrographs of Fe-related oxides
Egon Matijevic, Chem. Mater. 5 (1993) 412.
-FeOOHvia
4.5mM FeCl3+ 0.1 M HCl100C/24h
-Fe2O3via
20mM FeCl3+ 0.3 mM NaH2PO4100C/48h
-Fe2O3via
19mM FeCl3+ 1.2 mM HCl+40 vol% ethanol100C/16h
-FeOOH + -Fe2O3via
18mM FeCl3+ 30 mM HCl+ 20 vol% EG100C/48h
Synthesis of monodispersed colloids TEM micrograph and replica of ferric basic sulfate
Egon Matijevic, Chem. Mater. 5 (1993) 412.
-Fe3(SO4)2(OH)52H2O via
88mM Fe2(SO4)398C/2.8h
1010
Synthesis of monodispersed colloids ZnO particles
Egon Matijevic, Chem. Mater. 5 (1993) 412.
ZnOvia
5mM Zn(NO3)2+ 19 mM NH4OHPH=8.890C/3h
ZnOvia
0.1mM Zn(NO3)2+ 0.32 mM NH4OHPH=7.790C/1h
ZnOvia
3.2mM Zn(NO3)2+ 0.1 M TEA(triethanolamine)PH=8.990C/1h
ZnOvia
40mM Zn(NO3)2+ 0.2 M TEA+ 1.2M NaOHPH=12.1150C/2h
1 bar = 1m (a and b) and 10 m (c and d)
Synthesis of monodispersed colloids CuO particles
Egon Matijevic, Chem. Mater. 5 (1993) 412.
CuOvia
2mM CuCl2+ 0.4M urea90C/120min
CuOvia
8mM Cu(NO3)2+ 0.2M urea90C/100min
CuOvia
6mM CuSO4+ 20mM urea90C/100min
CuOvia
1.2mM CuSO4+ 0.3M urea90C/60min
1111
Synthesis of monodispersed colloids Y(OH)CO3 particles
Egon Matijevic, Chem. Mater. 5 (1993) 412.
Y(OH)CO3 via 16mM Y(NO3)3 + 0.33M urea85C/3h
Y(OH)CO3 via 30mM YCl3 + 3.3M urea110C/20h
Synthesis of monodispersed colloids hematite and Co3O4 particles
Egon Matijevic, Chem. Mater. 5 (1993) 412.
hematite (-Fe2O3) via 40mM Fe(NO3)3 + 0.2M TEA+1.2M NaOH + 0.5M H2O2
250C/1h
Co3O4 via 40mM Co(NO3)2 + 0.2M HEDTA+1.2M NaOH + 0.5M H2O2
250C/3h
HEDTA =N-(2-hydroxyethyl) ethylenediamine-triacetate
1212
Synthesis of monodispersed colloids spherical and prismatic particles
Egon Matijevic, Chem. Mater. 5 (1993) 412.
ZnS via 20mM Zn(NO3)2+ 60mM HNO3+ 0.11M thioacetamide(TAA)26C/5h + 60C/6h
Mn3(PO4)2 via 5mM MnSO4+ 5mM NaH2PO4+ 1M urea+ 10mM SDS
80C/3h
PbS via 1.2mM Pb(NO3)2+ 0.24M HNO3+ 5 mM TAA+ 1.2mM TAA26C/21h + 26C/1h
SDS= sodium dodecyl sulfate
CdCO3 via 10M urea
(preheated 80C/24h )+ 2mM CdCl2 (1:1 v/v)
TAA=
Synthesis of monodispersed colloids hybrid spherical + prismatic particles
Egon Matijevic, Chem. Mater. 5 (1993) 412.
PbSxCdS via (1) 1.2mM Pb(NO3)2+ 2.4mM HNO3+ 5 mM TAA @26C/21h => PbS seed sol A(2) 20ml A + 0.5ml 4.3 mM Cd(NO3)2 + 1ml 21 mM Pb(NO3)2 @80C/30min
1 m
1313
Synthesis of monodispersed colloids functional ceramic particles
Egon Matijevic, Chem. Mater. 5 (1993) 412.
BaTiO3 via 5mM Ti(i-Opr)4+ 10mM Na2H2EDTA+ 5mM BaCl2+ 0.38 M H2O260C/2h (PH=9.9 by NH3)
NiFe2O4 via [Ni(OH)2]/[Fe(OH)2] =0.5+ 60mM FeSO4+ 0.2M NaNO3
90C/4h
PbNb2O6 via 8mM Pb(NO3)2+ 2mM NbCl5+ 10 mM Na2HNTA(+ H2O2 present)50C/4h
BaFe12O19 via 0.125M FeCl2
+ 0.1M KOH+ 0.2M KNO3+ 0.01M Ba(NO3)2
90 C
Synthesis of monodispersed colloids scheme of the apparatus for continuous
precipitation flow
Egon Matijevic, Chem. Mater. 5 (1993) 412.
1414
Precursor solution
Drying, Calcining
Powder
Powder preparation
Sol-Gel
Gelation
- Precipitation- Pyrolyis
(Citrate, EDTAPechini, Bilcher)
Complexation
Chemical
DirectEmulsion
Co-Precipitation
Spraying techniques- Spray drying
- Freeze drying
Dispersion
Physical
Immobilisation step
Metal salt ormetalorganic precursor
Colloidal route
Polymeric route
Sol(colloidal)
Sol(polymeric)
membrane coating
media
Colloidal gel
Polymeric gel
Drying(hybrid organic-inorganic membrane)
Sintering(pure inorganic membrane)
Powder/ Powder/ CoatingCoating
(Green forming)
Sintering
Post treatment
9.3 9.3 Sol Sol -- gelgel
1515
SolSol--gel chemistry for synthesis of gel chemistry for synthesis of micromicro and and mesomesoporous membrane systemsporous membrane systems
TiO2 SiO2
macromacro mesomeso micromicro
Al2O3porous ceramic membranes
Microporousdp < 2 nm
Mesoporous2 nm < dp < 50 nm
Macroporousdp > 50 nm
Colloidal processing (Ch. 6)
Sol- gel Chemistry
Multi-layer ceramic membrane is schematically build up from;
Macroporous support Meso porous intermediate layer (= sometimes the final separation layer) Micro-porous top layer (separation layer
SolSol--gel*:gel*:preparation of ceramic materials by preparation of a sol, preparation of ceramic materials by preparation of a sol, gelationgelation of the sol, and removal of the solventof the sol, and removal of the solvent
Sol*:Sol*:A sol is a colloidal suspension of solid particles dispersed A sol is a colloidal suspension of solid particles dispersed in a liquidin a liquid (dispersed phase is small: ~1 (dispersed phase is small: ~1 1000 nm)1000 nm)
Gel*:Gel*:If one molecule reaches macroscopic dimensions, so it If one molecule reaches macroscopic dimensions, so it extends throughout the solutionextends throughout the solution
* * BrinkerBrinker & & ShererSherer, Sol, Sol--gel Sciencegel Science
Sol Sol -- gelgel
1616
Metal salt ormetalorganic precursor
Colloidal route
Polymeric route
Sol(colloidal)
Sol(polymeric)
membrane coating
media
Colloidal gel
Polymeric gel
Drying(hybrid organic-inorganic membrane)
Sintering(pure inorganic membrane)
Two routesTwo routesColloidal and polymeric Colloidal and polymeric
Process parameters:Process parameters: water/precursor ratiowater/precursor ratio temperaturetemperature pHpH Reaction timeReaction time
Much is known on SiOMuch is known on SiO2 2 AlAl22OO33 and TiOand TiO22
Colloidal Colloidal particleparticle
polymer polymer networknetwork
Sol Sol -- GelGel
ColloidalColloidal Metal Metal alkoxidealkoxide
(or Metal salt)(or Metal salt) Solvent = alcohol (or water)Solvent = alcohol (or water) Precipitation: Precipitation:
[[alkoxidealkoxide]
1717
Al-tri-sec-butoxide
Water
Hydrolysis (90 oC)Polycondensation
Heating at Tbto remove alcohol
StabilisationPeptisation at 80 oCNitric acid
STABLE SOL
Alumina particle solAlumina particle sol--gel synthesisgel synthesis
- AlOOH(=boehmite) precipitate
CalciningPore diameter400 oC: 3 nm (Al2O3)800 oC: 5 nm (Al2O3)
GelationMeso porous (2 nm)
Ti-isopropoxidein isopropanol
Solution of water and isopropanol
Hydrolysis Polycondensation
Filtration and washing to remove alcohol
StabilisationPeptisation at 80 oCNitric acid
STABLE SOL
TitaniaTitania particle solparticle sol--gel synthesisgel synthesis
Titanium oxy-hydroxide
CalciningParticle size:300 oC: 20 nm (anatase)600 oC: 50 nm (rutile)
Gelation
1818
Polymeric route Polymeric route Control of hydrolysis/condensation Control of hydrolysis/condensation
HydrolysisHydrolysisMM--(OR)(OR)44 + H+ H22O O '' (HO)(HO)--MM--(OR)(OR)33MM--(OR)(OR)44 + 4 H+ 4 H22O O '' MM--(OH)(OH)44
CondensationCondensation AlcoxolationAlcoxolation
MM--OH + MOH + M--OR OR '' MM--OO--M + RM + R--OHOH OxolationOxolation
MM--OH + MOH + M--OH OH '' MM--OO--M + HM + H22OO OlationOlation
MM--OH + HOH + H22O:MO:M--OH OH '' (M)(M)22--OO--H + HH + H22OOMM--OH + ROH:MOH + ROH:M--OR OR '' (M)(M)22--OO--H + ROHH + ROH
Colloidal vs. polymeric route
SiOC2H5
OC2H5OC2H5
OC2H5
MethoxyMethoxyEthoxyEthoxyPropoxyPropoxyButoxyButoxyTEOSTEOS
Summery of reactions
SiOR
OR
OR
OC2H5Si
OC2H5
OR
OR
OH-C2H5OH -C2H5OHSi
OH
OR
OR
OH
R = OC2H5
SiOH
OR
OR
OHSiOH
OR
OR
OH Si
OH
OR
OR O
Si OROH
OR
Si
OH
OR
OR O
Si ORO
OR
SiOR
OROR
water, H+ water, H+
Hydrolysis
Condensation
+
etc.
- water - water
Ratio hydrolysis/condensation and the associated polymerization reactions determine the properties of the gel AND the material resulting from that gel
Hydrolysis reactions replace an alkoxy group (OR) with hydroxyl (OH) group Condensation reactions involve the silanol groups to produce siloxane bonds
(Si-O-Si) plus by-products: alcohol (ROH) or water
Polymeric route
1919
Internal parametersInternal parameters nature of the metal atom and alkyl/nature of the metal atom and alkyl/alkoxidealkoxide groupsgroups structure of metal precursorstructure of metal precursor
External parametersExternal parameters water/water/alkoxidealkoxide ratioratio catalyst (acid or base)catalyst (acid or base) concentration solvent/precursorconcentration solvent/precursor solventsolvent temperaturetemperature
Sol Sol -- gelgel
Adjustable process parameters
Hydrolysis-Condensation of silica alkoxides under acidic or basic conditions
Polymeric route
Mechanism of hydrolysis and condensationMechanism of hydrolysis and condensation
Hydrolysis-Condensation of silica alkoxides under acidic or basic conditions
Polymeric route
R
Si
RO
RO
RO
OR
Si
RO
RO
RO
O
OH2
R
H+
Si
RO
RO
RO OH
Si
RO
RO
OR
OR
Si
RO
RO
OH2
OR
O
R
H+
Si
RO
RO
OR
OR
Si
RO
HO
OH2
OR
O
R
Si
OR
OR
ORHO
Si
RO
RO
RO OH
H
Si
OR
OR
ORHO
Si
RO
RO
RO O Si
OR
OR
OR
Si
RO
RO
RO OH
Si
RO
RO
RO O Si
OR
OR
ORHO Si
RO
RO
RO O Si
OR
OR
OR
Si
RO
RO
RO O
OH3+
HOR + H+ +
acid
hydrolysis
H2O H+
HO-
RO- +
base
and RO- + H2O ROH + OH-
-
acid
condensation
+
+ H3O+
base
+ OH- + H2O
+ OH-
+-
-
+
base
acid
CondensationHydrolysis
acid
base
OR
2020
Hydrolysis of alkoxides under acidic conditions
pH2.2 Takes place via dissociation of water in hydroxyl anionsTakes place via dissociation of water in hydroxyl anions Attack of these hydroxylAttack of these hydroxyl--ions on the silicon atom ions on the silicon atom Hydrolysis rate increases with the extent of OR substitutionHydrolysis rate increases with the extent of OR substitution
base
2121
Condensation of alkoxides under acidic conditions
Via Via protonatedprotonated silanolsilanol species species SiSi--HORHOR++
Results in linear polymersResults in linear polymers
acid
Condensation of alkoxides under basic conditions
Takes place by attack of a Takes place by attack of a nucleophilicnucleophilic deprotonateddeprotonated silanolsilanol((SiSi--OO--) on a neutral silicate species) on a neutral silicate species
Results in more branched polymersResults in more branched polymers
base
2222
Hydrolysis and condensation of TEOS=f(pH)
An acid enhances hydrolysis more An acid enhances hydrolysis more than condensation than condensation
A base catalyst (NHA base catalyst (NH44OH) increases OH) increases condensation rates more than condensation rates more than hydrolysis ratehydrolysis rate
Condensation
pH < 2pH < 2 condensation rate condensation rate [H[H++].]. Developing gel networks are composed of exceedingly small primary particles.
ph = 2ph = 2--6 6 condensation rate condensation rate [OH[OH--]; ]; ((i.e., growth of a network) contributes little to i.e., growth of a network) contributes little to growth after particles exceed 2nm in diameter. Thus, developing growth after particles exceed 2nm in diameter. Thus, developing gel networks are gel networks are composed of exceedingly small primary particles.composed of exceedingly small primary particles.
base
2323
Morphologies of silica sols and gels made Morphologies of silica sols and gels made under acid or base conditionsunder acid or base conditions
Polymeric route: summary
a) b)
Sol-gel derived silicon oxide networks, under acid-catalyzed conditions, yield primarily linear or randomly branched polymers which entangle and form additional branches resulting in gelation. On the other hand, silicon oxide networks derived under base-catalyzed conditions yield more highly branched clusters which do not interpenetrate prior to gelation and thus behave as discrete clusters
Acid catalyzedPorous silica membrane formation
20 More acid
Less acid
H+
acid
Fractals as
Building bricks
1.41.4 DDff
2424
TiO2 SiO2
macromacro mesomeso micromicro
Al2O3
Acid catalyzedmicro porous silica membrane formation
acid
-- AlAl22OO3 3 oror aanatasenatase TiOTiO22
Amorphous TiOAmorphous TiO22
MesoMeso an micro porous layers applied on an micro porous layers applied on a macro porous substrate by a macro porous substrate by dip coatingdip coating
macromacro
mesomeso
micromicro
2525
Base catalyzedParticles or cluster formation
base
Si
Si
SiO
O
O
O
OO
OO
OO
Inductive effects of substitutions attached to Si
The rate and extent of hydrolysis is The rate and extent of hydrolysis is influenced by the strength and influenced by the strength and concentration of the acidic or basic concentration of the acidic or basic catalystcatalyst
Temp. and solvent of secondary Temp. and solvent of secondary importanceimportance
Substitution of alkyl groups for Substitution of alkyl groups for alkoxyalkoxygroups increases the electron density groups increases the electron density of of SiSi
Hydrolysis (Hydrolysis (substsubst. OH for OR) or . OH for OR) or condensation (condensation (substsubst. . OSiOSi for OR or for OR or OH) decreases electron density on OH) decreases electron density on SiSi
2626
Advantages and disadvantages of the sol-gel technique
The product morphology (porosity, connectivity, primary particleThe product morphology (porosity, connectivity, primary particlesize) can be controlled through process conditions during synthesize) can be controlled through process conditions during synthesissis
Low operating temperaturesLow operating temperatures Shaping is simpleShaping is simple Homogeneous compounds can be achievedHomogeneous compounds can be achieved Very small primary particle sizes can be achieved (2Very small primary particle sizes can be achieved (2--20 nm)20 nm) A relative complex methodA relative complex method
SolSol--GelGel
A particulate A particulate alcosolalcosol
- Coating on glass
tetraisopropyl titanate TIPT: {(CH3)2CHO}4Ti
J. Am. Ceram. Soc.84 [12] 2969-74(2001) [486]
2727
Gel precipitation processGel precipitation process
SolSol--GelGel
Polymeer netwerk van SiOPolymeer netwerk van SiO22 Makkelijk substitutie van CaMakkelijk substitutie van Ca2+2+
Bio active glas / coatingBio active glas / coating
2828
Applications of Sol Applications of Sol -- gelgel
Colloidal routeA sol consists of a liquid with colloidal particles which are not dissolved, but do not agglomerate or sediment.
Agglomeration of small particles are due to van der Waals forces and a tendency to decrease the total surface energy. Van der Waals forces are weak, and extend only for a few nanometers.
In order to counter the van der Waals interactions, repulsive forces must be established.
May be accomplished by:
Electrostatic repulsion. By adsorption of charged species onto the surface of the particles, repulsion between the particles will increase and agglomeration will be prevented. Most important for colloidal systems.
Steric hindrance. By adsorbing a thick layer of organic molecules, the particles are prevented from approaching each other reducing the role of the van der Waals forces. Works best in concentrated dispersions. Branched adsorbates works best. Usual for nanomaterials.
ParticleOrganic layer
2929
PZC, Point of zero chargeStabilization due to electrostatic repulsion are due to formatioStabilization due to electrostatic repulsion are due to formation of a double layer at n of a double layer at the particle.the particle.
The surface of a particle is covered by ionic groups, which deteThe surface of a particle is covered by ionic groups, which determines the surface rmines the surface potential. Counter ions in the solution will cover this layer, spotential. Counter ions in the solution will cover this layer, shielding the rest of the hielding the rest of the solution from the surface charges.solution from the surface charges.
For hydroxides the surface potential will be determined by reactFor hydroxides the surface potential will be determined by reactions with the ions Hions with the ions H++and OHand OH--. Thus, the surface potential is pH dependent.. Thus, the surface potential is pH dependent.
MM--OH + HOH + H++ MM--OHOH22++MM--OH + OHOH + OH-- MM--OO-- + H+ H22OOThe pH where the particle is neutral is called PZC, point of zerThe pH where the particle is neutral is called PZC, point of zero charge.o charge.
For For pH > PZCpH > PZC the surface is the surface is negativelynegatively chargedcharged
For For pH < PZCpH < PZC the surface is the surface is positively positively charged.charged.
Typical values: Typical values: MgOMgO 12, Al12, Al22OO33 9.0, TiO9.0, TiO22 6.0, SnO6.0, SnO22 4.5, SiO4.5, SiO22 2.52.5
Depends somewhat on the size of the particle and the degree of cDepends somewhat on the size of the particle and the degree of condensationondensation
The size of the surface potential The size of the surface potential 00 depends on the difference between pH and PZC.depends on the difference between pH and PZC.
Ostwald ripening Small particles dissolve faster than larger particles. As the process is
dynamic and reversible, smaller particles may be dissolved to feed growth of the larger particles.
The growth stops when the difference in solubility between the largest and smallest particle is a few ppm.
It is therefore possible to prepare monodisperse silica particles from a gel.
3030
Sol-gel processes for La1-xSrxCoO3-Aqueous sol-gel Polymeric sol-gel
La1-xSrxCoO3- fine powder La1-xSrxCoO3- porous layer
Mater. Sci. Eng. B 39 (2) (1996) 129-132. J. Mater. Chem. 6(5) (1996) 815-819.
Sol-gel processes for PZT, PbZr1-xTixO3The method may provide good control over stoichiometry and reduced sintering temperature. This is especially important if one of the components are volatile. May also enable production of low temperature phases.
The largest piezoelectric response of PZT: x = 0.47. The stoichiometryis difficult to control by the ceramic method, where heating at 1100C for several hours is needed.
Louis: TIPT
3131
9.4 9.4 PechiniPechini method for powder preparationmethod for powder preparation
Modified resinintermediate processing of perovskitepowders:Part I. Optimization of polymeric precursorsLone-Wen Tai, Paul A. Lessing
The formation of a polyester between citric acid (CA) and ethylene glycol (EG) was found to be a decisive factor for the foaming of resin intermediates in a Pechini-type powder process. This process was modified by changing the organic mass ratio of CA/EG which results in ceramic powders with different morphologies. The most porous resin intermediate (with or without chelatedcations) was prepared using a polymeric gel made of equimolar citric acid and ethylene glycol. It was also found that a premixing of organic components, prior to adding constituent nitrate solutions, makes the whole process more controllable.
Keywords: Ceramics; Chemical synthesis; PowderMaterials: La0.85Sr0.15CrO3J. Mater. Res., Vol. 7, No. 2, 1992, p. 502.
Pechnis method related patents and papers1) M. Pechini, Method of Preparing Lead and Alkaline Earth Titanates andNiobates and Coating Method Using the Same to Form a Capacitor, U.S. Pat. No.3 330 697, July 11, 1967.2) N. G. Eror and H. U. Anderson, Polymeric Precursor Synthesis of CeramicMaterials, Mater. Res. Soc. Symp. Proc., 73, 57177 (1986).3) P. A. Lessing, Mixed-Cation Oxide Powders via Polymeric Precursors, Am.Ceram. Soc. Bull., 68 [5] 10021007 (1989).4) (a)L.-W. Tai and P. A. Lessing, Modified Resin-Intermediate Processing ofPerovskite Powders: Part I. Optimization of Polymeric Precursors, J. Mater. Res., 7,50219 (1992). (b)L.-W. Tai and P. A. Lessing, Modified Resin-IntermediateProcessing of Perovskite Powders: Part II. Processing for Fine, NonagglomeratedSr-Doped Lanthanum Chromite Powders, J. Mater. Res., 7, 50219 (1992).5) H. U. Anderson, Review of p-type Perovskite Materials for SOFC and OtherApplications, Solid State Ionics, 52, 3341 (1992).6) L .-W. Tai, H. U. Anderson, and P. A. Lessing, Mixed-Cation Oxide Powders viaResin Intermediates Derived from a Water-Soluble Polymer, J. Am. Ceram. Soc., 75[12] 349094 (1992).
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Pechnis methodIn this powder-synthesis route, citric acid forms poly(basic acid) chelates with the metal cations. These chelates undergo polyesterification, when heated with a poly(hydroxy alcohol), such as ethylene glycol, at a temperature of ~150Cto form a polymeric precursor resin. The cations are expected to be dispersed uniformly throughout the polymeric resin. Additional heating of the resin in air (at ~400C) results in the removal of organics and the formation of a char with a controlled cation stoichiometry, with little cation segregation. Then, the char is heated to higher temperatures and oxidized to form the oxide ceramics.
Ethylene glycol
Nitrates solution + citric acid + ethylene glycol heat and evaporation resin (xerogel) calcine grinding sintering
Example: Preparation of strontium- and/ormagnesium-doped lanthanum gallate (LSGM) PowdersPrecursors:La(NO3)39H2O, Ga(NO3)3 xH2O, Sr(NO3)2, Mg(NO3)26H2OCitric acid monohydrate (C6H8O7), ethylene glycol (C2H6O2)CA:EG = 60:40 (w/w) CA:Mtot = 1.88This solution was homogenized by stirring at room temperature for 1 h. Then, the resulting clear solution was evaporated (in a period of 3 h) ona hot plate until first a clear yellow gel and then a dark brown resin formed. The obtained resins (following overnight drying in an oven at 100C) were scraped off the beakers with a spatula, then ground by hand using an agate mortar and pestle, and finally calcined isothermallyin a stagnant-air-atmosphere box furnace over a temperature rangeof 2001400C. Each calcination batch of powders was heated tothe specified temperature at a rate of 5C/min, annealed at thistemperature for 6 h, and then furnace-cooled to room temperature.
Tas et al., J. Am. Ceram. Soc. 83[12](2000) 2954.
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Analyses of LSGM precursors at different temperatures
Fig. 3. XRD spectra of La0.8Sr0.2Ga0.83Mg0.17O2.815 precursor powders calcined at different temperatures. Secondary phases observed are indicated (1,LaSrGa3O7 peak, 2, LaOOH peak, 3, La2O3 peak, and 4, LaSrGaO4 peak).
Table I. Results of residual carbon analyses (wt%) _________________________________________________________________ Temp. (C) LaGaO3 La0.9Sr0.1GaO2.95 La0.8Sr0.2Ga0.83Mg0.17O2.815_________________________________________________________________
100 31.7 (3) 33.3 (2) 32.7 (6) 350 10.3 (3) 10.4 (1) 13.8 (2) 700 0.530 (3) 0.550 (5) 0.280 (4) 850 0.059 (1) 0.143 (9) 0.168 (3)
1000 0.042 (2) 0.050 (3) 0.060 (2) 1340 0.010 0.0124 (2) 0.0143 (4)
_________________________________________________________________
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Microstructures of LSGM precursors at different temperatures
100C 500C 700C
1000C
1400C (pellets)
Microstructures of LSGM pellets (1400C)
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9.4 YBCO
Glycine-nitrate pyrolysis (GNP) method
Ce(NO3)solution
Sm(NO3)3solution
blended in a certain ratio
Glycine was added G:M=0.7 to 3.4
Heated under stirring
Viscous solution
Evaporation,and ignited to flame
Pale-yellow ash
Sm-doped CeO2
750/2h2/min
Flow chart for preparing SDC powder
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Doped CeODoped CeO22 prepared with prepared with glycineglycine--nitrate methodnitrate method
Glycine-Nitrate Process (GNP) is a self-sustaining combustion synthesis technique, containing metal nitrates as oxidizers and glycine as a fuel.
NH2CH2COOH
) Forms complexes with metal ions Preventing selective precipitation
Making the metals mixed at atomic level
) Fuel for combustion NH2CH2COOH
Ce4+Sm3+
Doped CeODoped CeO22 prepared with prepared with glycineglycine--nitrate methodnitrate method
Ce(NO3)3 + NH2CH2COOH CeO2 + CO2 + H2O + N2(glycine /metal=14/91.56)
Sm(NO3)3 + NH2CH2COOH Sm2O3 + CO2 + H2O + 4N2(glycine/metal=15/91.67)
For SDC: the glycine/metal1.58
all glycine were oxidized by nitrate (no air ),the gaseous products of combustion were CO2 H2O and N2(no CO)
Assumption:
The powders morphology and sinterablity is influenced by the ratio of glycine/metal.
26 Sm(NO3)3 + 30 NH2CH2COOH = 13 Sm2O3 + 60 CO2 + 75 H2O + 54 N2
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30 40 50 60 70 80 90
422
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1
400
222
311
220
200
111
1450oC
600oC
ash
2
X-ray diffraction patterns of the ash as synthesized by a glycine-nitrate process, the SDC powder calcined at 600oC, and a SDC pellet sintered at 1450oC.
Characterization(TEM)
TEM photographs of SDC powders heated at 750oC. The powders were prepared with glycine-to-metal ratio of
(a) 0.7, (b) 1.0-2.5, and (c) 3.4
a
100nm100nm
b
250nm
c
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Characterization(SEM)
SEM images of Sm0.2Ce0.8O1.9 pellets fracture at different ratio sintered at 1500 o C for 5h
a) the glycine /metal ratio of 0.7 or 3.4 b) the glycine /metal ratio of 1.7
a b
Pechini method to CeO2-SnO2 and CeO2-TiO2coatings
Thin Solid Films 410 (2002) 1-7.