Aem Lect4

Advanced Electronic Ceramics I (2004)

Mimic alkoxide method: Well-sinterable nano-crystalline powder

J. -G. Li, T. Ikegami, J. -H. Lee, T. Mori, Acta mater. 49, 419-426 (2001)

(Motivation) Aqueous precipitation: nanocrystalline but hard agglomeration of ultra-fine particles

(Suggest Mimic alkoxide method)a. Ce(NO3)3•6H2O + 1-butanol : 0.1Mb. DEA(diethylamine) (C2H5)2NH + 1-butanol : 1.0Mc. Cerium source solution was dripped into precipitant solution(DEA)

(C2H5)2NH + H2O → (C2H5)2NH2+ + OH-

: OH- ions from the hydrolysis of molecular water of the cerium salt: involves minimum amount of water (better dispersion)

- Powder calcined at 600oC- primary particle: ~ 15nm- secondary particle: ~50nm- ~37 primary particle within the secondary particle


Example: Mimic Alkoxide method- maximize the driving force for the sintering (excess free energy of surface)

- reduce the sintering temperature- provide fast densification kinetics (Herring’s scaling law: t2 =λn t1)


Aggregate problem

Decrease Tfor full density


Example: Mimic Alkoxide Method

Ex) CeO2

- at high temperature4CeO2 → 2Ce2O3 + O2 (g)

: retard the densification- Low-temperature sintering is desirable!

- full density at 1000oC( ~ 0.42 Tm)



Hydrothermal synthesis(Definition) The process using hot and pressurized water for precipitation of oxides

dissolution Precipitation

From W.J.Dawson, Am.Ceram.Soc.Bull., 67(10), 1673 (1988)

(Driving force)The difference in solubility of the oxide phase from the least soluble precursor or intermediate

A(OH) (s) + B(OH) (s) A(OH) (aq.) + B(OH) (aq.) ABO3

(Characteristics)1. Crystalline, anhydrous ceramic powder2. Temperature : 100~370oC3. Pressure : 6 ~ 15MPa4. Do not need calcination and milling

(avoid the contamination during the processing)5. Employ relatively inexpensive raw materials


Hydrothermal synthesis for MLCC

From W.J.Dawson, Am.Ceram.Soc.Bull., 67(10), 1673 (1988)

(Strong points of Hydrothermal Synthesis in MLCC) 1. The ability to produce solid-solution particles of controlled size

(can attain complex composition)cf) in poorly prepared co-precipitation- did not result solid solution- requires the calcination (and thereby ball milling)- large particle size ( d<1 µm is difficult by mall milling)- result higher sintering temperature (energy-consuming process)- result the coarse grain size (harmful for size reduction)

2. Well sinterable and small particles without any calcination- offers the energy-saving process to fabricate the integrated MLCC

3. Doping during the powder preparation is possible


Hydrothermal synthesis: BaTiO3

K.Abe and S. Matumoto, Ceramic Tracsaction, Vol.22, p.15 (1987)

1. TiCl4 (aq.) + NH4OH → Ti-hydroxide.2. Washing till No Cl- ions are detected.3. Mixed with Ba(OH)2•6H2O

(Ba/Ti = 1.5 in atomic ratio, concentration=0.5M )4. Treatment in 200oC for 5h in autoclave


Hydrothermal conversion from TiO2 into BaTiO3

J. Y. Choi, J. H. Kim and D. K. Kim, J. Am. Ceram. Soc., 81(5), 1353 (1998)

1. TiCl4 (aq.) + alcohol + HPC (steric stabilizer)2. Uniform heating using microwave oven formation of spherical gel3. Adding NH4OH4. Washing and separation using centrifugal




Lead acetate trihydrateBarium hydroxide octahydrateStrontium hydroxide octahydrate




TiO2

ZrO2

ZrTiO4

BaTiO3 SrTiO3

PbTiO3PbZrO3

PZT

BZT

Spherical morphology(from precursor TiO2

or ZrO2)

Crystallinity and phase(from hydrothermaltreatment)


Hydrothermal synthesis


Spray PyrolysisWhat is Ultrasonic Spray Pyrolysis?A powder preparation process through the thermal decomposition of the droplet generated by ultrasonic transducion. The Advantage of Spray Pyrolysis Process.1. spherical morphology. 2. narrow particle size distribution. 3. easy preparation of the powder with the complex composition. 4. relatively homogeneous composition. : compositional heterogeneity is restricted within a spherical secondary powder. 5. Easy manipulation of particle size 6. No calcination7. Successive processingThe Shortcoming of Spray Pyrolysis Process.1. Energy-consuming process. 2. makes hollow structures frequently.

[Jong-Heun Lee, Ph.D. Thesis, Seoul National University, 1993]


Spray Pyrolysis: Schematic

8πγ 1/3

Ddroplet = 0.34ρf2

Ddroplet : droplet sizeγ : surface tension of solutionρ: density of solutionf: resonance frequency for the

ultrasonic transducer (1.67 MHz)- typical droplet size for aqueoussolution ranges ~ 3µm


Spray Pyrolysis: Concentration Effect

TiO2/SnO2

from TiCl4(aq.)+SnCl4(aq.)at 800oC

[Jong-Heun Lee, Ph.D. Thesis, Seoul National University, 1993]

Size manipulationcomes fromthe mechanism,“one particlefrom one droplet”


Spray Pyrolysis: Microstructure 1

TiO2 prepared from 0.19M TiCl4 aqueous solution at 600oC. [J.-H.Lee, H.-J.Cho, and S.-J.Park, Ceramic Transaction Vol.22, pp39-44(1991)]

SnO2 prepared from 0.2M SnCl4 aqueous solution at 800oC. [J.-H.Lee and S.-J.Park, J.Am.Ceram.Soc., 76(3), 777-780, (1993)]

TiO2-SnO2 prepared from 0.2M TiCl4-SnCl4aqueous solution at 800oC. [J.-H.Lee and S.-J.Park, J.Mater.Sci.:Materials in Electronics, 4, 254-258 (1993)]


Spray Pyrolysis: Microstructure 2

Pb(Zr,Ti)O3 prepared from aqueous acetate-base solution at 700oC. [H.-B.Kim, J.-H.Lee, and S.-J.Park, J. Mater. Sci. :Materials in Electronics, 6, 84-89 (1995)]

Zr0.8Sn0.2TiO4 prepared from ZrO(CH3COO)2-TiCl4 -SnCl4 aqueous solution at 800oC. [S.-Y.Cho, J.-H.Lee, S.-J.Park, J.Mater.Sci., 30, 3274-3278 (1995)]


Dimpling and ion-thinning

epoxy

particle

Observation of the inner part of sphere

Fig. Inner structure of SnO2spheres prepared at 800oC from 0.2M SnCl4 solution. Ring patterns of (C) and (D) were obtained in the area ofinner and crust(see arrow) layerof the secondary sphere,respectively.

J.-H.Lee and S.-J.Park, J.Am.Ceram.Soc., 76(3), 777-780, (1993)


Composition analysis in one sphere

1 2 3 4

J.-H.Lee and S.-J.Park, J.Mater.Sci.:Materials in Electronics, 4, 254-258 (1993)


Spray Pyrolysis: Application

Easy manipulation of particle size: manipulation of pore and/or grain size (ceramic humidity sensor, ZnO varistor, and the control ofthe electric properties related to the grain boundary)

: Sintering study

Narrow size distribution, spherical and good flowability: Screen printing of luminescent materials in display applications: Controlled compaction


Hydrolysis: metal alkoxide

Al + 3C3H7OH Al(OC3H7)3 + 3/2H2↑

Mg + 2C2H5OH Mg(OC2H5)2 + 2H2↑

SiCl4 + 4C2H5OH Si(OC2H5)4 + 4HCl↑

TiCl4 + 4ROH Ti(OR)4 + 4NH4Cl

HgCl2

HgI2

∆

∆

Preparation of metal alkoxide

Hydrolysis of metal alkoxide

Ti(OCnH2n+1)4 + 2H2O → TiO2 + 4(CnH2n+1)OH


Hydrolysis of metal alkoxide: example

Single oxide1. 0.1-0.2M Ti(iOC3H7)4 : titanium tetraisopropoxide in isopropanol,+ the mixture between water and isopropanol (0.3-1.5M water)

2. 0.1-0.2M Ti(OC2H5)4 : titanium tetraethoxide in ethanol,+ the mixture between water and ethanol (0.3-1.5M water)

- the molar ratio (water/alcohol > 0.3)- yields mono-disperse, spherical titanium hydroxide

E.A.Barringer and H.K.Bowen, J.Am.Ceram.Soc., Dec., C199, (1982)

Avg. particle size rangeShapeSubstructures

From isopropoxide0.07 - 0.3 µmequiaxedmultinuclear particles

From ethoxide0.3 - 0.6 µmsphericalmostly singlet


Hydrolysis of metal alkoxide: example

Multi oxide1. The mixing between

Ti(OC2H5)4 in EtOHTa(OC2H5)5 in EtOHNb(OC2H5)5 in EtOH

2. Adding the mixture between water and ethanol

3. Hydrolysis reaction in N2

4. Washing with de-ionized water5. Re-dispersion in a dilute aqueous solution of SrCl26. Adding aqueous solution of (NH4)2CO3 to precipitate the Sr

on the surface of TiO2 surface

(B. Fegley, Jr., E.A.Barringer and H.K.Bowen, J.Am.Ceram.Soc., June, C113 (1984)


Hydrolysis

Heating mantle

Coolingwater

Stirring

Thermocouple

Source solution


Hydrolysis: example (ZrO2)

ZrOCl2 + (n+1) H2O → ZrO2•nH2O + 2H+ + 2Cl-

pH decrease

K.Matsui and M.Ohagai, J.Ceram.Soc.Jpn., 106(9), 883-887 (1998)


Hydrolysis: example (ZrO2)

K.Matsui and M.Ohagai, J.Am.Ceram.Soc., 80(8),1949-56 (1997)

Control parameter1. Starting and ending pH- adding NH4OH or HCl* the measurement of highly acidic oH

- measure the pH of thediluted solution and calculate the pH

2. The [ZrO2+] in the clearsolution as a function of reaction time

3. The temperature of solution

4. Boiling time


Freeze Drying

a. Solution droplets are sprayed into a bath of immiscible liquid (hexane) or directly into liquid N2

b. The frozen product is skimmed from the top of the refrigerant(the diameter of the frozen beads: 0.01 ~ 0.5 mm)

c. Frozen sample is introduced into a vacuum chamber (P:~1torr)

⇒ sublimation of solvent4. Calcination

J. S. Reed, “Principles of CeramicProcessings,”


Powders from Vapor-Phase Reactions

(ex.)1. TiCl4(g) + 2H2O(g) → TiO2(s) + 4HCl(g)2. SiCl4(g) + 4NH3(g) → Si3N4(s) + 12HCl(g)3. Thermal decomposition of (CH3)2SiCl2 and CH3SiH5

1. Sub-micron size (good)2. Well-dispersed particles (good)3. Narrow particle-size distribution (good)4. Formation of non-oxide powder due to easy control of atmosphere5. Requires large volume of gases for reaction (disadvantage)6. Energy-consuming process(heat) (disadvantage)7. Requires relatively expensive equipment for reaction (disadvantage)8. Restriction in the choice of reactor materials

(to avoid corrosion by reactant gases)



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

Temperature(K)




Log Kp = - ∆Go /(2.303RT)

44000x4.2/(2.303X8.3144X773)




Log Kp = - ∆Go /(2.303RT)

Powderformation

Thin film, Powder,and fiber on substrate

Thin filmPowderfiber

Powder formation at Log Kp> 3(homogeneous nucleation)

The formation of thin film, powder,and fiber on substrate at 2>Log Kp> 0(heterogeneous nucleation)

Aem Lect4

Technology

rt japanese ceramics

solid solution

hydrolysis reaction

hydrolysis of metal

solution droplets

diluted solution

nm secondary particle

oc primary particle