Precipitation 1: Hydroxide
       1. easy & cost-effective process
       2. usually employ inexpensive source materials (...
Precipitation 1: Hydroxide

                  0                                                        NH4OH




         ...
Precipitation 2: oxalate



                                              1. Specific compound containing two cations
    ...
Transparent YAG preparation by carbonate precipitation
                                                           Aforemen...
Precipitation of hydroxide: Impurity effect
     (example ) Mg(OH)2 precipitation in water

     ♦ Mg(OH)2 ↔ Mg2+ + 2OH-
 ...
Co-precipitation: Problem
     (example ) Gd3Al5O12 preparation
     ♦ [Al3+][OH-]3 = 1.1 X 10-33
     ♦ [Gd3+][OH-]3 = 1....
Precipitation : Normal strike & Reverse strike


                                         NH4OH                        Ma+...
Homogeneous Precipitation

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




         W.M.Sigmund, N.S.Bell, L.Berström, J.Am.Ceram.Soc., 83(7), 1557-74 (2000)
Advanced ...
Precipitation at constant pH using buffer solution




       pH effect upon the particle size
                J. -H. Lee,...
SnO2 gas sensor: Particle size dependence
      Significant increase
      in sensitivity
      when d ≤ lD


            ...
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Aem Lect3

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Aem Lect3

  1. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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)

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