region, i.e., in the frequency range 300 MHz to 300 GHz. If the frequency of the
electromagnetic radiation is in the microwave region (@10
9 Hz) the rotations of
the polar molecules in the liquid begin to lag behind the electric field oscillations.
The resulting phase displacement d, acquires a component I sin d in phase with
the electric field, and thus resistive heating occurs in the medium. This is described as dielectric loss and causes energy to be absorbed from the electric field.
Since the dipoles are unable to follow the higher frequency electric field oscillations, the permittivity falls at the higher frequency and the substance behaves
increasingly like a non-polar material.
tan d ¼ e
00 =e
0
ð8Þ
where e
0 is the real part of the dielectric constant and e
00 is the loss factor which
reflects the conductance of the material. In his review, Mingos [159] introduces the
Debye expression for the complex permittivity and shows how e
00 and e
0 depend on
the relaxation time, which is itself dependent on the molecular size and the viscosity of the liquid. Mingos emphasizes that the interaction between the microwave radiation and the polar solvent, which occurs when the frequency of the radiation approximately matches the frequency of the rotational relaxation processes,
is not a quantum mechanical resonance phenomenon. Transition between rotational energy levels is not involved and the energy transfer is not a property of a
specific molecule, but the result of a collective phenomenon involving the bulk. In
the Debye interpretation, the heat is generated by friction forces occurring between
the polar molecules whose rotational velocity has been increased by the coupling
with the microwave radiation and neighboring molecules. The dielectric heating
is a broadband phenomenon and rapid energy transfer occurs even when the frequency of the microwaves and the relaxation frequency are not perfectly matched.
It is worth noting that much of the work for which domestic microwave ovens
were used for fabricating nanomaterials concentrated on using ethylene glycol as
the solvent. Table 6 in Mingos’s review [159] shows that ethylene glycol indeed has
a high loss tangent at 2.45 Ghz, and when this is coupled with its high boiling
temperature (193
C), this makes it an excellent candidate for dielectric heating
and, hence, there is little penetration of microwaves. In large metal samples, as
well as in metal films, large electric field gradients occur in the microwave cavity,
giving rise to electric discharges. On the other hand, in metal powders no such
discharge takes place and, due to eddy currents and localized plasma effects, very
rapid heating takes place which may be as high as 100 K s
À1 . Eddy current phenomena arise from the alternating magnetic field associated with microwaves.
6.3.1
Microwave Synthesis of Nanomaterials
6.3.1.1 Microwave Synthesis of Nanometallic Particles
The first report of a microwave-assisted reaction leading to the formation of nanometallic particles was by Jiang and coworkers [167]. They prepared Au nanopar6.3 Microwave Heating 155
electromagnetic radiation is in the microwave region (@10
9 Hz) the rotations of
the polar molecules in the liquid begin to lag behind the electric field oscillations.
The resulting phase displacement d, acquires a component I sin d in phase with
the electric field, and thus resistive heating occurs in the medium. This is described as dielectric loss and causes energy to be absorbed from the electric field.
Since the dipoles are unable to follow the higher frequency electric field oscillations, the permittivity falls at the higher frequency and the substance behaves
increasingly like a non-polar material.
tan d ¼ e
00 =e
0
ð8Þ
where e
0 is the real part of the dielectric constant and e
00 is the loss factor which
reflects the conductance of the material. In his review, Mingos [159] introduces the
Debye expression for the complex permittivity and shows how e
00 and e
0 depend on
the relaxation time, which is itself dependent on the molecular size and the viscosity of the liquid. Mingos emphasizes that the interaction between the microwave radiation and the polar solvent, which occurs when the frequency of the radiation approximately matches the frequency of the rotational relaxation processes,
is not a quantum mechanical resonance phenomenon. Transition between rotational energy levels is not involved and the energy transfer is not a property of a
specific molecule, but the result of a collective phenomenon involving the bulk. In
the Debye interpretation, the heat is generated by friction forces occurring between
the polar molecules whose rotational velocity has been increased by the coupling
with the microwave radiation and neighboring molecules. The dielectric heating
is a broadband phenomenon and rapid energy transfer occurs even when the frequency of the microwaves and the relaxation frequency are not perfectly matched.
It is worth noting that much of the work for which domestic microwave ovens
were used for fabricating nanomaterials concentrated on using ethylene glycol as
the solvent. Table 6 in Mingos’s review [159] shows that ethylene glycol indeed has
a high loss tangent at 2.45 Ghz, and when this is coupled with its high boiling
temperature (193
C), this makes it an excellent candidate for dielectric heating
and, hence, there is little penetration of microwaves. In large metal samples, as
well as in metal films, large electric field gradients occur in the microwave cavity,
giving rise to electric discharges. On the other hand, in metal powders no such
discharge takes place and, due to eddy currents and localized plasma effects, very
rapid heating takes place which may be as high as 100 K s
À1 . Eddy current phenomena arise from the alternating magnetic field associated with microwaves.
6.3.1
Microwave Synthesis of Nanomaterials
6.3.1.1 Microwave Synthesis of Nanometallic Particles
The first report of a microwave-assisted reaction leading to the formation of nanometallic particles was by Jiang and coworkers [167]. They prepared Au nanopar6.3 Microwave Heating 155