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LECTURE 4 MICROWAVE SYNTHESIS OF MATERIALS

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INTRODUCTIONClosed-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 MatterOne 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 MatterThere 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 SynthesisThe 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 Composition Process

Oxide   Nonoxide  

Al2O3 SolutionPyrolysisHydrothermal

CrBFe2B

Solid-StateSolid-State

Fe2O3 SolutionHydrothermal

ZrB2 Solid-State

TiO2 Solution AlN Gas-Phase

Ti2O3 Gas Solid Si3N4 Gas-Phase

ZrO2 SolutionPyrolysis ,Hydrotherma

SiC Gas-PhaseSolid-State

MgAl2O4 Copyrolysis TiC Gas-PhaseSolid-State

Al6Si2O13 Sol-gelCopyrolysis

NbC  

CuAlO2 Copyrolysis TaC Gas-PhaseSolid-State

BaTiO3 Sol-gelHydrothermal

Composite  

YBaCu3O7-x SolutionSolid-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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AdvantagesAs 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.