oli simulation conference instant hydrothermal synthesis of ceramic oxides: nano scale barium...

OLI Simulation Conference

Instant Hydrothermal Synthesis of Ceramic Oxides: Nano Scale Barium Titanate

Vahit Atakan

OLI User Conference, Morristown NJ

10/24/2007

To use carboxylate salts, which is one of the cheapest

starting materials used in solid state synthesis, as

precursors for hydrothermal crystallization

Overall Objective

Outline

Part 1: Introduction

•What is hydrothermal synthesis?

•How can we contribute to hydrothermal community?

Outline

Part 2: Carboxylates as starting materials?

•Thermodynamic predictions

•Effect of KOH on solid state reaction chemistry

•Calculation of yield diagrams and estimation of

experimental conditions

•Experimental verification

•Selection of appropriate carboxylate

Outline

Part 3 Instant Hydrothermal Synthesis

• Understanding reaction mechanism

• Enhancing reaction kinetics

Part 1

Introduction

What is hydrothermal synthesis?

• Direct crystallization of materials in aqueous medium.

• Controlling thermodynamic variables– Temperature (25-1000°C, practical limit

350 ° C)– Pressure (100kPa to 500 MPa, practical

limit 100 MPa)– Composition

Hydrothermal suspension

Impeller

Hastelloy Autoclave

Parr Instrument Company

Model 4530

Hastalloy C276 alloy

Temperatures < 350˚C

Stirring Speed < 1700 rpm

Riman et al.

Teflon® Jar (Savillex, Minnetonka, MN)

Hydrothermal Reactors

How can we contribute to hydrothermal community?

•By using carboxylate salts as precursors for hydrothermal crystallization

• Carboxylate salts are one of the cheapest starting materials which are generally used in solid state synthesis

Part 2

• Hydrothermal Conversion of Carboxylate-based

Solid State Precursors to BaTiO3

Introduction

• Carboxylate salts are converted to BaTiO3 by solid state reaction. (T> 700 C, t >3 h)

• Requires successive milling due to agglomeration

• Among carboxylates, only barium titanyl oxalate was hydrothermally converted to BaTiO3 at 250 °C for 72 h.

Hwu et al.

Experimental Design

• The effect of KOH on solid state

chemistry

• Calculation of yield diagrams to define

reaction parameters

• Experimental verification

Effect of KOH on solid state reaction chemistry

 BaCO3 (s) + TiO2 (s) = BaTiO3 (s) + CO2 (g)

 BaCO3 (s) + TiO2 (s) + 2KOH (s) = BaTiO3 (s) + K2CO3 (s) + H2O (l)

Calculation of yield diagrams and estimation of experimental conditions

BaCO3 (s) + TiO2 (s) + 2KOH (s) + H2O (l) = BaTiO3 (s) + K2CO3 (s) + 2H2O (l)

BaCO3 (s) + TiO2 (s) + 2KOH (s) + H2O (l) = BaTiO3 (s) + 2K+ (aq) + CO32- (aq) + 2H2O (l)

Experimental Verification of the Model

Incomplete reaction!!! Is it because of kinetics reasons?

BaCO3 Aging

BaCO3 grows in KOH solution by Ostwald ripening!!!

Thermodynamic Predictions for Oxalate System

BaC2O4 (s) + TiO2 (s) + 2KOH (s) + H2O (l) = BaTiO3 (s) + K2C2O4 (s) + 2H2O (s)

BaC2O4 (s) + TiO2 (s) + 2KOH (s) + H2O (l) = BaTiO3 (s) + 2K+ (aq) + C2O4 2- (aq) + 2H2O (l)

Experimental verification (BO)

Why oxalate and carbonate behave differently?

Solubility of oxalate is a lot higher than that of carbonate in KOH solution!!!

Use Barium Oxalate as a Guide To BTO

BaTiO(C2O4)2 (s) + 4KOH (s) + H2O (l) = BaTiO3 (s) + 2K2C2O4 (s) + 3H2O (l)

BaTiO(C2O4)2 (s) + 4KOH (s) + H2O (l) = BaTiO3 (s) + 4K+ (aq) + 2C2O42- (aq) + 3H2O (l)

Experimental Verification (BTO)

Reaction T decreased from 250 °C to ambient temperature!!!

Conclusion

• Best carboxylate for hydrothermal conversion to BaTiO3 is BTO

• Barium oxalate and titania can also be converted into BaTiO3 hydrothermally

• The reason for incomplete reaction for carbonate system is most likely related to kinetics

Part 3

Instant Hydrothermal Synthesis

Introduction

• BaTiO3 is formed even at the early stages of

hydrothermal decomposition of BTO, however

barium oxalate is also present.

• In conventional preparation methods,

transient temperature and concentration

regime (TTCR) exists

• KOH dissolves in 60 seconds and T

increases from 25 to 76 °C during TTCR

Questions in mind

• KOH concentration and T were not satisfied at the beginning of the reaction

So:

• Is TTCR responsible for barium oxalate formation

• If TTCR is minimized or eliminated, is it possible to decrease the reaction time from days to seconds?

Method of Attack

• Find out how BTO is effected at the end of TTCR

• Eliminate or minimize TTCR by bringing the reactants to the desired conditions faster

BTO when there is TTCR

• BTO is converted into barium oxalate and barium titanate at the end of TTCR

• Barium oxalate and Ti species converted to BaTiO3 with time

a) at the end of TTCR

b) 20 min after TTCR

Elimination of TTCR

What happens if TTCR is minimized or eliminated?

5 sec later

@ ~103 °C

Instant formation at RT?

It is known from previous results that increasing KOH concentration decreases the required T.

Can IHS occur at ambient T?

a) 4 m KOH; t= 60s

b) 20 m KOH; t=60 s

How does it work?

BaTiO(C2O4)2 (s) + 4 KOH (s) + H2O (l) =

Ba2+ (aq)+ Ti(O)2+ (aq) + 2 C2O42- (aq) + 4 K+ (aq) + 4 OH- (aq) + H2O (l) =

BaTiO3 (s) + 3 H2O (l) + 4 K+ (aq) + 2C2O42- (aq)

R1 k1 [OH-]4

BaTiO(C2O4)2 (s) + 2 KOH (s) + H2O (l) =

Ba2+ (aq) + Ti(O)2+ (aq) + 2 C2O42- (aq) + 2 K+ (aq) + 2 OH- (aq) + H2O (l) = BaC2O4 +

TiO(OH)2 (s) + 2 K+ (aq) + C2O42- (aq) + H2O (l)

R2 k2 [OH-]2

Conclusions

• BTO can be instantly decomposed in to BaTiO3 under hydrothermal conditions

• Advantages:– Eliminates CO2 emission– Decreases reaction temperature from 900 to 100

°C– Decreases reaction time from hours to seconds– Eliminates milling step– Can be applied to continuous reactors

Summary

• Among Carboxylates, Barium Oxalate and Barium Titanyl Oxalate can be converted into BaTiO3 hydrothermally

• BTO can be used for instant hydrothermal synthesis (IHS) of BaTiO3