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 ( °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 BaTiO 3

Introduction Carboxylate salts are converted to BaTiO 3 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 BaCO 3 (s) + TiO 2 (s) = BaTiO 3 (s) + CO 2 (g) BaCO 3 (s) + TiO 2 (s) + 2KOH (s) = BaTiO 3 (s) + K 2 CO 3 (s) + H 2 O (l)

Calculation of yield diagrams and estimation of experimental conditions BaCO 3 (s) + TiO 2 (s) + 2KOH (s) + H 2 O (l) = BaTiO 3 (s) + K 2 CO 3 (s) + 2H 2 O (l) BaCO 3 (s) + TiO 2 (s) + 2KOH (s) + H 2 O (l) = BaTiO 3 (s) + 2K + (aq) + CO 3 2- (aq) + 2H 2 O (l)

Experimental Verification of the Model Incomplete reaction!!! Is it because of kinetics reasons?

BaCO 3 Aging BaCO 3 grows in KOH solution by Ostwald ripening!!!

Thermodynamic Predictions for Oxalate System BaC 2 O 4 (s) + TiO 2 (s) + 2KOH (s) + H 2 O (l) = BaTiO 3 (s) + K 2 C 2 O 4 (s) + 2H 2 O (s) BaC 2 O 4 (s) + TiO 2 (s) + 2KOH (s) + H 2 O (l) = BaTiO 3 (s) + 2K + (aq) + C 2 O 4 2- (aq) + 2H 2 O (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(C 2 O 4 ) 2 (s) + 4KOH (s) + H 2 O (l) = BaTiO 3 (s) + 2K 2 C 2 O 4 (s) + 3H 2 O (l) BaTiO(C 2 O 4 ) 2 (s) + 4KOH (s) + H 2 O (l) = BaTiO 3 (s) + 4K + (aq) + 2C 2 O 4 2- (aq) + 3H 2 O (l)

Experimental Verification (BTO) Reaction T decreased from 250 °C to ambient temperature!!!

Conclusion Best carboxylate for hydrothermal conversion to BaTiO 3 is BTO Barium oxalate and titania can also be converted into BaTiO 3 hydrothermally The reason for incomplete reaction for carbonate system is most likely related to kinetics

Part 3 Instant Hydrothermal Synthesis

Introduction BaTiO 3 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 BaTiO 3 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 ~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(C 2 O 4 ) 2 (s) + 4 KOH (s) + H 2 O (l) = Ba 2+ (aq)+ Ti(O) 2+ (aq) + 2 C 2 O 4 2- (aq) + 4 K + (aq) + 4 OH - (aq) + H 2 O (l) = BaTiO 3 (s) + 3 H 2 O (l) + 4 K + (aq) + 2C 2 O 4 2- (aq) R 1  k 1 [OH - ] 4 BaTiO(C 2 O 4 ) 2 (s) + 2 KOH (s) + H 2 O (l) = Ba 2+ (aq) + Ti(O) 2+ (aq) + 2 C 2 O 4 2- (aq) + 2 K + (aq) + 2 OH - (aq) + H 2 O (l) = BaC 2 O 4 + TiO(OH) 2 (s) + 2 K + (aq) + C 2 O 4 2- (aq) + H 2 O (l) R 2  k 2 [OH - ] 2

Conclusions BTO can be instantly decomposed in to BaTiO 3 under hydrothermal conditions Advantages: –Eliminates CO 2 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 BaTiO 3