1. Precipitation 1: Hydroxide
1. easy & cost-effective process
2. usually employ inexpensive source materials (inorganic chemical)
3. easy to get nano-sized particle
- Often results in the hard aggregation of primary particles
usually 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 particles
3. The ball milling of the dried aggregate with a less polar solvent
(ethanol, butanol, and so on…)
Advanced Electronic Ceramics I (2004)
Precipitation 1: Hydroxide
1. soluble compound is precipitated from the solution
(example 1) Sn-hydroxide precipitate formation in water
H2O ↔ H+ + OH- : Kw = [H+][OH-] = 10-14
log[H +] + log[OH-] = -14
pH = - log[H+] = log[OH-] +14
log[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 NH4OH Ma+(aq)
- typical source for (OH)- ion : NH4OH
Ma+(aq) NH4OH
Advanced Electronic Ceramics I (2004)

2. Precipitation 1: Hydroxide
0 NH4OH
-4
Ma+(aq)
Log C
-8
Ma+(aq)
-10
0 2 4 6 8 10 12 14
pH
NH4OH
log [Sn4+] = -4pH
Advanced Electronic Ceramics I (2004)
Precipitation 2: oxalate
(example 2) oxalate formation (C2O4-)
H2C2O4 ↔ H+ + HC2O4-
HC2O4- ↔ 2H+ + C2O42-
Methanol, -35oC Water, -2oC
Sn2+ + C2O42- ↔ SnC2O4
source material: SnCl2•2H2O
solvent: methanol or water
SnC2O4- ↔ SnO2 + 2CO (at ~ 350oC)
Methanol, 20oC Water, 20oC
* decrease in supersaturation
as increasing T (in methanol)
→ nucleation rate ↓
* the anisotropic shape of
precipitate in water system
Methanol, 50oC Water, 50oC
J. -H. Lee, S. -J. Park, J.Kor.Ceram.Soc., 27(2), 274-282 (1990)
Advanced Electronic Ceramics I (2004)

3. Precipitation 2: oxalate
1. Specific compound containing two cations
: the composition of precipitate is uniform
regardless of the concentration of ions in
solution
2. Synthesis of BaTiO3 at the relatively low
temperature
J. S. Reed, “Principles of Ceramics Processing,”
Advanced Electronic Ceramics I (2004)
Precipitation 3: Carbonate
(characteristics of carbonate formation (CO32-))
1. Crystalline : Low agglomeration between the primary particles
2. Anti-hydration
(example 1) MgO preparation (with anti-hydration)
1. 0.4 M Na2CO3 1000ml
2. Drop 0.4M MgCl2 aqueous solution (final pH 9.8)
3. Aging at 35oC for 24h
4. Washing, Drying
5. Calcination at 900oC for 4h in oxygen atmosphere
(example 2) in YAG preparation: Al part
NH4Al(SO4)2•12H2O + 3NH4HCO3 ↔ AlOOH + 2(NH4)2SO4 + 3CO2 +13H2O
NH4Al(SO4)2•12H2O + 4NH4HCO3 ↔ NH4Al(OH)2CO3 + 2(NH4)2SO4+3CO2+3H2O
with aging
AlOOH + NH4HCO3 ↔ NH4Al(OH)2CO3
Example 1 from S.Matsuda et al., JP63011516. Example 2 from J.-G.Li et al., J.Mater.Res., 15, 1514 (2000)
Advanced Electronic Ceramics I (2004)

4. Transparent YAG preparation by carbonate precipitation
Aforementioned reverse-strike
- Optical characteristic comparable
to that of single crystal
- window material for the infrared
spectrometer Vacuum sintering at 1700oC for 1h
J. -G. Li, T. Ikegami, J. -H. Lee, T. Mori, J.Am.Ceram.Soc., 83(4), 961 (2000)
Advanced Electronic Ceramics I (2004)
Why Transparent YAG ?
- high strength
- high hardness
- low refractive index
- low thermal expansion coefficient
- very stable in various inorganic
solvents,
- window material for the infrared
spectrometer
- Optical transmission spectra of
YAG ceramics
Advanced Electronic Ceramics I (2004)

5. Precipitation of hydroxide: Impurity effect
(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-] = -10.7
log[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
Japanese Ceramics Society, Ceramic Processing, Powder Preparation and Forming, p.24 (1984)
Advanced Electronic Ceramics I (2004)
Co-precipitation
- The preparation of multi-component of cations through the precipitation
Table. The pH regime for M(OH)n formation
(condition)
The pH regime for the
metal hydroxide formation
should be similar
Otherwise,
the simple mixture
between the various
metal hydroxides
result.
Japanese Ceramics Society, Ceramic Processing, Powder Preparation and Forming, p.24 (1984)
Advanced Electronic Ceramics I (2004)

6. Co-precipitation: Problem
(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•6H2O
and 0.5 mol of GdCl3•6H2O
in 1 liter water
♦ log [Al3+] = log0.3 = -0.52 (A)
♦ log [Gd3+] = log0.5 = -0.69 (B)
- 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=~5 at point B : starts to precipitate Gd(OH)3
- 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
Japanese Ceramics Society, Ceramic Processing, Powder Preparation and Forming, p.24 (1984)
Advanced Electronic Ceramics I (2004)
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 impurity
cations 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)
Advanced Electronic Ceramics I (2004)

7. Precipitation : Normal strike & Reverse strike
NH4OH Ma+(aq)
Ma+(aq) NH4OH
Normal strike Reverse strike
adding precipant solution salt solution
to salt solution precipant solution
Low ⇒ High High ⇒ Low
pH
High ⇒ Low Low ⇒ High
solubility of cation
Low ⇒ High High ⇒ Low
nucleation rate
Advanced Electronic Ceramics I (2004)
Example for the Normal strike & Reverse strike
Preparation of YAG(Y3Al5O12) powder
1. 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 AHC
2.precursor:
NH4AlY0.6(CO3)1.9 (OH)2•0.9H2O
3. After calcination:
pure YAG
J. -G. Li, T. Ikegami, J. -H. Lee, T. Mori, J.Mater.Res., 15, 1514-1523 (2000)
Advanced Electronic Ceramics I (2004)

8. 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 particles
3. Precise control of pH : can avoid the contamination at the co-
precipitation
Advanced Electronic Ceramics I (2004)
Homogeneous Precipitation using Urea
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 nitrate
aluminum nitrate
E.Taspinar and A.C.Tas, J.Am.Ceram.Soc., 80(1), 133-42 (1997)
Advanced Electronic Ceramics I (2004)

9. Homogeneous Precipitation
W.M.Sigmund, N.S.Bell, L.Berström, J.Am.Ceram.Soc., 83(7), 1557-74 (2000)
Advanced Electronic Ceramics I (2004)
Precipitation at constant pH using buffer solution
(Background)
- abrupt pH increase or decrease during the precipitation resulted in
the heterogeneous powder properties.(ex, particle-size variations)
(Target)
- To achieve the homogeneous precipitation at constant pH
- drop the salt & precipitant solutions
simultaneously into buffer solution
with tight keeping the stoichiometric
NH4OH Ma+(aq) composition
(Advantages)
- narrow particle size
- homogeneous mixing between
cations at the co-precipitation
Buffer solution
- possible to know the pH effect
upon precipitation
Advanced Electronic Ceramics I (2004)

10. 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)
Advanced Electronic Ceramics I (2004)
SnO2 gas sensor: Mechanism
At 300-400oC
Air Air+CO
CO CO2
- O-
O- O O-
O- O- O-
O- O-
O- O-
O- O-
O-
O- e-
O- O-
O- O-
O-
O-
O- O-
O- O-
O-
O- O- - O- O-
O-
O- O- O-
O
Depletion
R↓
layer eVs
eVs
Grain Boundary Grain Boundary
Decrease of O-ad by oxidative
Shottky barrier formation
by oxygen chemisorption reaction with reducing gas
1/2O2 + (SnO2-x)* → O-ad(SnO2-x) CO + O-ad(SnO2-x) → CO2 + (SnO2-x)*
Advanced Electronic Ceramics I (2004)

11. SnO2 gas sensor: Particle size dependence
Significant increase
in sensitivity
when d ≤ lD
Ra/Rg
d : particle size
lD : Debye length
d=dc d
In air atmosphere at 300-400oC
lD
lD
dC = 2 lD d > 2 lD
dC d
Advanced Electronic Ceramics I (2004)
SnO2 gas sensor: Particle size dependence 2
Fig. Influence of crystallite
size(D) on gas sensitivity to
♦ Impregnation with metal oxide
800ppm H2 and 800ppm CO
→ change the growth kintetics of SnO2 crystallites
(element sintered at 400oC)
♦ NiO affect catalytic oxidation of iso-butane
♦ Sensitivity to ethanol and H2S depends more
strongly on the acid-base properties of SnO2 surface
N.Yamazoe, and N.Miura, Chemical Sensor Technology Vol.4, pp19-42 (1992)
Advanced Electronic Ceramics I (2004)