sol-gel and others

11

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



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


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


66


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


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


-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


-Fe3(SO4)2(OH)52H2O via

88mM Fe2(SO4)398C/2.8h

1010

Synthesis of monodispersed colloids ZnO particles


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


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


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


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


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


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


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


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).

3232

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

42033

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.

sol-gel and others

Documents

solid reactions

examplesolid state reactions

solid state reactions9

solid components

cuocuo reaction reaction

room temperature solids

simple reaction

temperature profile