Transcript Ph D Thesis Defense - OLI Support Center

OLI Simulation Conference
Instant Hydrothermal Synthesis of Ceramic Oxides: Nano
Scale Barium Titanate
Vahit Atakan
OLI User Conference, Morristown NJ
10/24/2007
Overall Objective
To use carboxylate salts, which is one of the cheapest
starting materials used in solid state synthesis, as
precursors for hydrothermal crystallization
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 Reactors
w
Hastelloy Autoclave
Impeller
Hydrothermal suspension
Riman et al.
Parr Instrument Company
Model 4530
Hastalloy C276 alloy
Temperatures < 350˚C
Stirring Speed < 1700 rpm
Teflon® Jar
(Savillex, Minnetonka, MN)
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