Download lecture 4 microwave synthesis of materials

LECTURE 4
MICROWAVE SYNTHESIS OF MATERIALS
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INTRODUCTION
Closed-vessel microwave heating techniques have been the
state of the art for sample preparation in the analytical laboratory
for over fifteen years.
The application of microwaves in the synthesis of functional
materials is only now beginning to receive widespread attention.
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Microwaves Are Energy
Microwaves are a form of electromagnetic energy.
Microwaves, like all electromagnetic radiation, have an electrical
component as well as a magnetic component.
The microwave portion of the electromagnetic spectrum is
characterized by wavelengths between 1 mm and 1 m, and
corresponds to frequencies between 100 and 5,000 MHz
.
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Microwaves Can Interact with Matter
One can broadly characterize how bulk materials behave in a
microwave field.
Materials can absorb the energy, they can reflect the energy, or
they can simply pass the energy.
It should be noted that few materials are either pure absorbers,
pure reflectors, or completely transparent to microwaves.
The chemical composition of the material, as well as the
physical size and shape, will affect how it behaves in a
microwave field.
Microwave interaction with matter is characterized by a
penetration depth.
That is, microwaves can penetrate only a certain distance into a
bulk material.
Not only is the penetration depth a function of the material
composition, it is a function of the frequency of the microwaves.
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Two Principal Mechanisms for Interaction with
Matter
There are two specific mechanisms of
interaction between materials and microwaves:
(1) dipole interactions and
(2) ionic conduction.
Both mechanisms require effective coupling
between components of the target material and
the rapidly oscillating electrical field of the
microwaves.
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Dipole interactions occur with polar molecules.
The polar ends of a molecule tend to align
themselves and oscillate in step with the
oscillating electrical field of the microwaves.
Collisions and friction between the moving
molecules result in heating.
Broadly, the more polar a molecule, the more
effectively it will couple with (and be influenced
by) the microwave field.
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Comparison of conventional heating with microwaves
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Ionic conduction is only minimally different from dipole
interactions.
Obviously, ions in solution do not have a dipole moment.
They are charged species that are distributed and can
couple with the oscillating electrical field of the
microwaves.
The effectiveness or rate of microwave heating of an
ionic solution is a function of the concentration of ions in
solution.
Materials have physical properties that can be measured
and used to predict their behavior in a microwave field.
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One calculated parameter is the dissipation factor, often
called the loss tangent.
The dissipation factor is a ratio of the dielectric loss (loss
factor) to the dielectric constant.
The dielectric loss is a measure of how well a material
absorbs the electromagnetic energy to which it is
exposed, while the dielectric constant is a measure of
the polarizability of a material, essentially how strongly it
resists the movement of either polar molecules or ionic
species in the material.
Both the dielectric loss and the dielectric constant are
measurable properties.
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Material Synthesis
The discovery of new materials requires the
development of a diversity of synthetic techniques.
Microwave methods offer the opportunity to synthesise
and modify the composition, structure and morphology of
materials, particularly composites via differential heating.
Microwave-induced plasmas (MIPs) allow any solid
mixture to be heated, and can promote direct microwave
heating at elevated temperature, greatly expanding the
use of microwaves for reactions between solids and
gas–solid mixtures.
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While the use of microwave radiation for sintering and
densification is a well-known materials processing
application, the ability of microwave radiation to produce
gaseous plasmas is used for material synthesis.
Using a 300-watt microwave generator, plasmas of
oxygen, fluorine and nitrogen can be produced.
These plasmas are highly reactive, containing as they
do a mixture of electrons, ions and radicals, and thus
may be used to oxidise (in the case of oxygen and
fluorine) or reduce (in the case of nitrogen) various
functional materials.
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This is advantageous as by activating the oxidising or reducing
species, as opposed to thermal activating the sampler itself.
The possibility of cation migration within the sample is removed,
which may seriously effect the desired functional properties of
the material.
This is of particular importance for thin film materials, where an
oxidising atmosphere at elevated temperatures will completely
destroy the film.
Microwave-assisted synthesis is generally much faster, cleaner,
and more economical than the conventional methods.
A variety of materials such as carbides, nitrides, complex oxides,
silicides, zeolites, apatite, etc. have been synthesized using
microwaves.
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Composition
Process
Oxide
Composition
Process
Nonoxide
Al2O3
Solution
Pyrolysis
Hydrothermal
CrB
Fe2B
Solid-State
Solid-State
Fe2O3
Solution
Hydrothermal
ZrB2
Solid-State
TiO2
Solution
AlN
Gas-Phase
Ti2O3
Gas Solid
Si3N4
Gas-Phase
ZrO2
Solution
Pyrolysis ,Hydrotherma
SiC
Gas-Phase
Solid-State
MgAl2O4
Copyrolysis
TiC
Gas-Phase
Solid-State
Al6Si2O13
Sol-gel
Copyrolysis
NbC
CuAlO2
Copyrolysis
TaC
BaTiO3
Sol-gel
Hydrothermal
Composite
YBaCu3O7-x
Solution
Solid-State
Al2O3/ZrO2/Y2O3
Solution
Mn0.5Zr0.4Fe2O4
Solution
SiC/SiO2
Particle + Coating Pyrolysis
Mn0.6Zr0.4Fe2O4
Solution
TiC/TiO2
Particle + Coating Pyrolysis
KVO3
Solid-State
ZrC/ZrO2
Particle + Coating Pyrolysis
CuFe2O4
Solid-State
ZrC/SiC
Particle + Coating Pyrolysis
BaWO4
Solid-State
BN/ZrO2
Particle + Coating Pyrolysis
La1.85Sr0.15CuO4
Solid-State
SiC/ZrO2
Particle + Coating Pyrolysis
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Gas-Phase
Solid-State
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Advantages
As a relatively new source of processing energy,
microwave energy offers many compelling
advantages in materials processing over
conventional heat sources.
These advantages include greater flexibility,
greater speed and energy savings.
Improved product quality and properties, and
synthesis of new materials that cannot be
produced by other heating methods.
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