aem lect3

Advanced Electronic Ceramics I (2004)

Precipitation 1: Hydroxide

1. easy & cost-effective process2. usually employ inexpensive source materials (inorganic chemical)3. easy to get nano-sized particle

- Often results in the hard aggregation of primary particlesusually during the washing, filtration and/or drying procedures

(action)1. The crystal growth of the precipitate

(change the precipitate into the easily washable form)2. Employ the washing solution with a constant pH

to keep the electrostatic repulsion between the colloidal particles3. The ball milling of the dried aggregate with a less polar solvent

(ethanol, butanol, and so on…)



1. soluble compound is precipitated from the solution

(example 1) Sn-hydroxide precipitate formation in waterH2O ↔ H+ + OH- : Kw = [H+][OH-] = 10-14

log[H+] + log[OH-] = -14pH = - log[H+] = log[OH-] +14log[OH-] = pH -14

Sn(OH)4 ↔ Sn4+ + 4OH- : Ksp, Sn(OH)4 = [Sn4+][OH-]4 = 10-56

-56 = log [Sn4+] + 4 log[OH-]-56 = log [Sn4+] + 4pH -56

∴ log [Sn4+] = -4pH

- typical source for (OH)- ion : NH4OH

NH4OH

Ma+(aq)

Ma+(aq)

NH4OH



Log C

pH0 2 4 6 8 10 12 14

0

-4

-8

-10

NH4OH

Ma+(aq)

log [Sn4+] = -4pH

Ma+(aq)

NH4OH


(example 2) oxalate formation (C2O4-)

H2C2O4 ↔ H+ + HC2O4-

HC2O4- ↔ 2H+ + C2O4

2-

Sn2+ + C2O42- ↔ SnC2O4

source material: SnCl2•2H2O solvent: methanol or water

SnC2O4- ↔ SnO2 + 2CO (at ~ 350oC)

* decrease in supersaturationas increasing T (in methanol)

→ nucleation rate ↓* the anisotropic shape ofprecipitate in water system

Precipitation 2: oxalate

Methanol, -35oC Water, -2oC

Methanol, 20oC Water, 20oC

Methanol, 50oC Water, 50oCJ. -H. Lee, S. -J. Park, J.Kor.Ceram.Soc., 27(2), 274-282 (1990)


Precipitation 2: oxalate

J. S. Reed, “Principles of Ceramics Processing,”

1. Specific compound containing two cations: the composition of precipitate is uniform regardless of the concentration of ions in solution2. Synthesis of BaTiO3 at the relatively low temperature


(characteristics of carbonate formation (CO32-))

1. Crystalline : Low agglomeration between the primary particles2. Anti-hydration

(example 1) MgO preparation (with anti-hydration)1. 0.4 M Na2CO3 1000ml2. Drop 0.4M MgCl2 aqueous solution (final pH 9.8)3. Aging at 35oC for 24h4. Washing, Drying5. Calcination at 900oC for 4h in oxygen atmosphere

(example 2) in YAG preparation: Al partNH4Al(SO4)2•12H2O + 3NH4HCO3 ↔ AlOOH + 2(NH4)2SO4 + 3CO2 +13H2ONH4Al(SO4)2•12H2O + 4NH4HCO3 ↔ NH4Al(OH)2CO3 + 2(NH4)2SO4+3CO2+3H2Owith agingAlOOH + NH4HCO3 ↔ NH4Al(OH)2CO3

Precipitation 3: Carbonate

Example 1 from S.Matsuda et al., JP63011516. Example 2 from J.-G.Li et al., J.Mater.Res., 15, 1514 (2000)


Transparent YAG preparation by carbonate precipitation

J. -G. Li, T. Ikegami, J. -H. Lee, T. Mori, J.Am.Ceram.Soc., 83(4), 961 (2000)

Vacuum sintering at 1700oC for 1h

Aforementioned reverse-strike

- Optical characteristic comparable to that of single crystal

- window material for the infraredspectrometer


Why Transparent YAG ?

- high strength - high hardness - low refractive index - low thermal expansion coefficient - very stable in various inorganic

solvents, - window material for the infraredspectrometer

- Optical transmission spectra of YAG ceramics


(example ) Mg(OH)2 precipitation in water

♦ Mg(OH)2 ↔ Mg2+ + 2OH-

Ksp = [Mg2+][OH-]2 = 1.8 X 10-11

log[Mg2+] + 2log[OH-] = -10.7log[OH-] = -14 + pH

∴ log [Mg2+] = 17.3 -2pH ♦ [Fe2+][OH-]2 = 1.5 X 10-16

♦ [Mn2+][OH-]2 = 2.9 X 10-13

- consider the 1g FeCl2, 1g MnCl2, and 98g MgCl2 in 1 liter water♦ [Mg2+] = 98/95.2(the molecular weight of MgCl2) = 1.03 M(mol/liter)♦ log [Mg2+] = 0 (A), log [Fe2+] = -2.01 (B), log [Mn2+] = -2.10 (C)

- pH point B : starts to precipitate Fe(OH)2- pH near points A and C : log [Fe2+] = -5, log [Mn2+] = -2.10 - after filtration add (OH)- source again enables to remove [Fe2+]- There inevitably remains about 1% of Mn(OH)2 in the final precipitate

Precipitation of hydroxide: Impurity effect

Japanese Ceramics Society, Ceramic Processing, Powder Preparation and Forming, p.24 (1984)


Co-precipitation- The preparation of multi-component of cations through the precipitation

(condition)The pH regime for themetal hydroxide formationshould be similar

Otherwise,the simple mixturebetween the variousmetal hydroxidesresult.

Japanese Ceramics Society, Ceramic Processing, Powder Preparation and Forming, p.24 (1984)

Table. The pH regime for M(OH)n formation

Advanced Electronic Ceramics I (2004)Japanese Ceramics Society, Ceramic Processing, Powder Preparation and Forming, p.24 (1984)

(example ) Gd3Al5O12 preparation♦ [Al3+][OH-]3 = 1.1 X 10-33

♦ [Gd3+][OH-]3 = 1.1 X 10-27

- consider 0.3 mol of AlCl3•6H2Oand 0.5 mol of GdCl3•6H2Oin 1 liter water♦ log [Al3+] = log0.3 = -0.52 (A)♦ log [Gd3+] = log0.5 = -0.69 (B)

Co-precipitation: Problem

- pH=~3 at point A : starts to precipitate Al(OH)3

- pH=4 : log [Al3+] = -3 (almost all the Al ions were precipitated)At this stage, Gd still remains in the solution

- pH=7 : almost all the Gd ions were precipitated- strictly speaking the nano-scale mixture between two hydroxides- requires the very strong stirring along with the effort to avoid the agglomeration of the single oxide due to the electrostatic attraction

- pH=~5 at point B : starts to precipitate Gd(OH)3


Precipitation: Advantages and Disadvantages

(Advantages)- easy & cost-effective process- usually employ inexpensive source materials (inorganic chemical)- easy to get nano-sized particle- The intimate mixing between the cations can be achieved

when the solubility of two precipitate is almost the same(in Co-P)- Can remove the impurity at the precipitation when the impuritycations shows relatively high solubility in solution

(Disadvantages)- Often results in the hard aggregation of primary particles- The intimate mixing between the cations are difficult

when the solubility of two precipitate is markedly different(in Co-P)


Precipitation : Normal strike & Reverse strike

NH4OH

Ma+(aq)

Ma+(aq)

NH4OH

Normal strike Reverse strike

adding precipant solution salt solutionto salt solution precipant solutionpH Low ⇒ High High ⇒ Lowsolubility of cation High ⇒ Low Low ⇒ Highnucleation rate Low ⇒ High High ⇒ Low


Example for the Normal strike & Reverse strikePreparation of YAG(Y3Al5O12) powder1. NH4Al(SO4) 2 •12H2O +water: ammonium aluminum sulfate dodecahydrate(alum)2. Y(NO3)3 •6H2O +water : yttrium hydrate hexahydrate- 4NH4HCO3 : ammonium hydrogen carbonate (AHC)(A) normal strike:(left) 1.AHC addition to (1+2),2.precursor:

yttrium carbonate+ammonium dawsonite

3. After calcination:YAG+YAM(Y4Al2O9)+ YAP(YAlO3)

(B) reverse-strike: (right)1.(1+2) addition to AHC2.precursor:NH4AlY0.6(CO3)1.9 (OH)2•0.9H2O3. After calcination:

pure YAG

J. -G. Li, T. Ikegami, J. -H. Lee, T. Mori, J.Mater.Res., 15, 1514-1523 (2000)


Homogeneous Precipitation

(Background) 1. abrupt pH increase or decrease during the precipitation resulted in

the heterogeneous powder properties. (ex, particle-size variations)2. Difficult to wash the nano-size precipitate emanating from the high

degree of supersaturation

♦ Homogeneous Precipitation : provide the precipitant (anion or cation) gradually and homogeneously

(Characteristics)1. Minimize supersaturation: Large particle size of the precipitate

(easy to wash)2. nucleation and growth of uniform sized particles3. Precise control of pH : can avoid the contamination at the co-

precipitation


Homogeneous precipitation using urea- using the uniform and gradual pH increase by the urea decomposition

at 80-100oC: (NH2)2CO+3H2O↔CO2+2OH-

- precipitating agent is generated slowly from the solution

(Experimental)salt + distilled water + urea → heating at 80-100oC

(source materials)lanthanum nitratealuminum nitrate

Homogeneous Precipitation using Urea

E.Taspinar and A.C.Tas, J.Am.Ceram.Soc., 80(1), 133-42 (1997)


Homogeneous Precipitation

W.M.Sigmund, N.S.Bell, L.Berström, J.Am.Ceram.Soc., 83(7), 1557-74 (2000)


Precipitation at constant pH using buffer solution

(Background)- abrupt pH increase or decrease during the precipitation resulted inthe heterogeneous powder properties.(ex, particle-size variations)

(Target)- To achieve the homogeneous precipitation at constant pH

NH4OH

Buffer solution

Ma+(aq)

- drop the salt & precipitant solutionssimultaneously into buffer solutionwith tight keeping the stoichiometriccomposition

(Advantages)- narrow particle size- homogeneous mixing between cations at the co-precipitation

- possible to know the pH effect upon precipitation


Precipitation at constant pH using buffer solution

pH effect upon the particle size

J. -H. Lee, S. -J. Park, J.Kor.Ceram.Soc., 27(2), 274-282 (1990)


SnO2 gas sensor: Mechanism

O-O-

O-

O-

O-

O-

O-

O-O-

O-

O-O- O-

O-

O-

O-

O-

O-

O-

O-O-O-O-

eVs

Air Air+CO

At 300-400oC

Shottky barrier formation by oxygen chemisorption

1/2O2 + (SnO2-x)* → O-ad(SnO2-x)

Decrease of O-ad by oxidative

reaction with reducing gas CO + O-

ad(SnO2-x) → CO2 + (SnO2-x)*

Depletionlayer

O-

O-

O-

O-

O-

O-

O-O-

O-

O-

O-

O-

eVs

CO CO2

e-

Grain Boundary

R↓

Grain Boundary


SnO2 gas sensor: Particle size dependence

Significant increasein sensitivitywhen d ≤ lD

d : particle sizelD : Debye length

dC

lDdC = 2 lD

In air atmosphere at 300-400oC

dlD d > 2 lD

Ra/Rg

dd=dc


SnO2 gas sensor: Particle size dependence 2

N.Yamazoe, and N.Miura, Chemical Sensor Technology Vol.4, pp19-42 (1992)

Fig. Influence of crystallite size(D) on gas sensitivity to 800ppm H2 and 800ppm CO(element sintered at 400oC)

♦ Impregnation with metal oxide→ change the growth kintetics of SnO2 crystallites♦ NiO affect catalytic oxidation of iso-butane♦ Sensitivity to ethanol and H2S depends more strongly on the acid-base properties of SnO2 surface

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