purposes and only narrow frequency windows centered at 900 MHz and 2.45 GHz
are allowed for microwave heating purposes. Heating by microwave has been
known since the early 1940s and has been used successfully in the food industry.
Microwave irradiation as a heating method has found a number of applications in
chemistry since 1986, especially in the work of Mingos [157]. Microwaves have
been in use for accelerating organic reactions for a quite a while [158, 159]. Microwave synthesis is generally quite fast, simple, and very energy efficient. The exact nature of microwave interaction with reactants during the synthesis of materials is somewhat unclear and speculative [160]. However, transfer of energy from
microwaves to the material is believed to occur either through resonance or relaxation, which results in rapid heating. This knowledge is widely used in the discussion of reaction mechanisms. Many successful examples of the applications of MW
heating in organic chemistry have been reported [161, 162], although its expansion
into the area of inorganic chemistry has been much slower. Only recently has it
been noticed that metallic powders can be heated to considerably high temperatures in a microwave oven without arcing [161, 163]. Some metal chalcogenides
have been prepared by microwave solid-state reaction [164]. These solid-state reactions do not produce nanoparticles. On the other hand, MW reactions in solutions do yield nanoparticles. The use of MWH in the preparation of solid state inorganic materials has been reviewed recently by the two main contributors to this
field, Professor Mingos [159] and Professor K. J. Rao [160]. The preparation of
nanoparticles by MWH started only recently. The aim of this section is to review
the solution work that has been conducted using a regular domestic microwave
oven. We will, therefore, not discuss the many reactions leading to the fabrication
of nanomaterials conducted with a microwave plasma. An example of a domestic
microwave oven that has been modified for the synthesis of nanomaterials is presented in Figure 6.9.
However, we should mention the pioneering work of Chou and Phillips, where
metallic iron and iron oxide particles were produced by injecting ferrocene into the
afterglow region of a low-pressure, low-power, plasma, generated using a microwave power source [160]. This gas phase reaction was carried out as part of an
attempt to explore the feasibility of using flow-type microwave plasmas for the
production of metal nanoparticles.
A short introduction to as to how the irradiated molecule is affected by microwaves is first presented. It is based on the reviews of Rao and Mingos [159, 160].
In general, materials fall into three categories, with respect to their interaction
with microwaves [160]: (1) microwave reflectors, typified by bulk metals and alloys,
such as brass, which are therefore used in making micro-waveguides; (2) microwave transmitters which are transparent to microwaves, typified by fused quartz,
zircon, several types of glass, and ceramics (not containing any transition element),
teflon etc.; they are therefore employed for cookware and containers for carrying
out chemical reactions in microwaves; and (3) microwave absorbers, which constitute the most important class of materials for microwave synthesis; they take up
the energy from the microwave field and heat up very rapidly.
A dielectric material is one that contains either permanent or induced dipoles
6.3 Microwave Heating 153
are allowed for microwave heating purposes. Heating by microwave has been
known since the early 1940s and has been used successfully in the food industry.
Microwave irradiation as a heating method has found a number of applications in
chemistry since 1986, especially in the work of Mingos [157]. Microwaves have
been in use for accelerating organic reactions for a quite a while [158, 159]. Microwave synthesis is generally quite fast, simple, and very energy efficient. The exact nature of microwave interaction with reactants during the synthesis of materials is somewhat unclear and speculative [160]. However, transfer of energy from
microwaves to the material is believed to occur either through resonance or relaxation, which results in rapid heating. This knowledge is widely used in the discussion of reaction mechanisms. Many successful examples of the applications of MW
heating in organic chemistry have been reported [161, 162], although its expansion
into the area of inorganic chemistry has been much slower. Only recently has it
been noticed that metallic powders can be heated to considerably high temperatures in a microwave oven without arcing [161, 163]. Some metal chalcogenides
have been prepared by microwave solid-state reaction [164]. These solid-state reactions do not produce nanoparticles. On the other hand, MW reactions in solutions do yield nanoparticles. The use of MWH in the preparation of solid state inorganic materials has been reviewed recently by the two main contributors to this
field, Professor Mingos [159] and Professor K. J. Rao [160]. The preparation of
nanoparticles by MWH started only recently. The aim of this section is to review
the solution work that has been conducted using a regular domestic microwave
oven. We will, therefore, not discuss the many reactions leading to the fabrication
of nanomaterials conducted with a microwave plasma. An example of a domestic
microwave oven that has been modified for the synthesis of nanomaterials is presented in Figure 6.9.
However, we should mention the pioneering work of Chou and Phillips, where
metallic iron and iron oxide particles were produced by injecting ferrocene into the
afterglow region of a low-pressure, low-power, plasma, generated using a microwave power source [160]. This gas phase reaction was carried out as part of an
attempt to explore the feasibility of using flow-type microwave plasmas for the
production of metal nanoparticles.
A short introduction to as to how the irradiated molecule is affected by microwaves is first presented. It is based on the reviews of Rao and Mingos [159, 160].
In general, materials fall into three categories, with respect to their interaction
with microwaves [160]: (1) microwave reflectors, typified by bulk metals and alloys,
such as brass, which are therefore used in making micro-waveguides; (2) microwave transmitters which are transparent to microwaves, typified by fused quartz,
zircon, several types of glass, and ceramics (not containing any transition element),
teflon etc.; they are therefore employed for cookware and containers for carrying
out chemical reactions in microwaves; and (3) microwave absorbers, which constitute the most important class of materials for microwave synthesis; they take up
the energy from the microwave field and heat up very rapidly.
A dielectric material is one that contains either permanent or induced dipoles
6.3 Microwave Heating 153