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Transcript of OLI Simulation Conference Instant Hydrothermal Synthesis of Ceramic Oxides: Nano Scale Barium...
![Page 1: OLI Simulation Conference Instant Hydrothermal Synthesis of Ceramic Oxides: Nano Scale Barium Titanate Vahit Atakan OLI User Conference, Morristown NJ.](https://reader036.fdocuments.net/reader036/viewer/2022062517/56649f175503460f94c2d542/html5/thumbnails/1.jpg)
OLI Simulation Conference
Instant Hydrothermal Synthesis of Ceramic Oxides: Nano Scale Barium Titanate
Vahit Atakan
OLI User Conference, Morristown NJ
10/24/2007
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To use carboxylate salts, which is one of the cheapest
starting materials used in solid state synthesis, as
precursors for hydrothermal crystallization
Overall Objective
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Outline
Part 1: Introduction
•What is hydrothermal synthesis?
•How can we contribute to hydrothermal community?
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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
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Outline
Part 3 Instant Hydrothermal Synthesis
• Understanding reaction mechanism
• Enhancing reaction kinetics
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Part 1
Introduction
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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
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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
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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
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Part 2
• Hydrothermal Conversion of Carboxylate-based
Solid State Precursors to BaTiO3
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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.
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Experimental Design
• The effect of KOH on solid state
chemistry
• Calculation of yield diagrams to define
reaction parameters
• Experimental verification
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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)
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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)
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Experimental Verification of the Model
Incomplete reaction!!! Is it because of kinetics reasons?
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BaCO3 Aging
BaCO3 grows in KOH solution by Ostwald ripening!!!
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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)
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Experimental verification (BO)
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Why oxalate and carbonate behave differently?
Solubility of oxalate is a lot higher than that of carbonate in KOH solution!!!
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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)
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Experimental Verification (BTO)
Reaction T decreased from 250 °C to ambient temperature!!!
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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
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Part 3
Instant Hydrothermal Synthesis
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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
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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?
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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
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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
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Elimination of TTCR
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What happens if TTCR is minimized or eliminated?
5 sec later
@ ~103 °C
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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
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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
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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
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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