Page 151 - Advances in Eco-Fuels for a Sustainable Environment

122 Advances in Eco-Fuels for a Sustainable Environment

microwave radiation, it displaces the atomic nuclei from their equilibrium position
(atomic polarization) or the electrons around the nuclei (electronic polarization), for-
ming induced dipoles. These dipoles tend to reorient themselves under the influence of
an alternating electric field. This realignment occurs at the rate of a trillion times per
second [15, 20]. As a result, friction is developed between the rotating molecules,
thereby generating heat within the whole volume of the material.
The dipolar orientation was clearly explained in the section basics of the micro-
waves by Denshi CO, Ltd. [21], for example, water contains an atom of oxygen
and two hydrogen atoms under an angle of 104.5 degrees [22]. This unequal division
of the electrons gives to the water molecule a light negative charge close to its oxygen
atom and a light positive charge close to its hydrogen atoms. The dipole was formed
due to this atoms take a little charge of each plus (+) and minus ( ). This dipole or
dielectric material is exposed to an electric field such as a radio wave or microwaves;
it vibrates 2450 million times more or less to be replaced a second [23].
At a lower frequency in the radio wave, the water is not able to generate heat
because the permanent dipole suddenly follows the electric field direction. Similarly,
at the high-frequency range, dipoles will not be able to follow the fast changes in the
electric field direction. So heat does not generate by water. In a moderate frequency
range, the water is exposed to the dipole orientation. In this case, behind the electric
field, the permanent dipole changes a bit. Water takes the energy from the radio wave
and generates heat during the delay time in this nominal frequency range [21].
Interfacial or Maxwell-Wagner polarization: This mechanism explains how the
heating effect is achieved in heterogeneous systems. Here, polarization is produced
due to differences in dielectric constants and conductivities of the substances at the
interfaces. The dielectric loss and field distortions due to a collection of space charge
lead to the heating effect.
Conduction mechanism: In an electrically conductive material, the electric currents
of charged particles or carriers (electrons, ions, etc.) move through the material due to
the externally applied electromagnetic field. These moving electric currents go
through a relatively high electrical resistivity within the structure of material, gener-
ating heat [15, 20].
Dielectric properties are essential to determine the maximum heating of a material
when exposed to electromagnetic radiation (microwave radiation). The dielectric loss
tangent (Tanδ) of the microwave absorber is mainly affected by the dielectric constant
0
(ε ) and the dielectric loss factor (ε ). The dielectric constant determines how much
00
energy is absorbed and how much is reflected, thereby showing the ability of a mate-
rial to get polarized by an electric field. The dielectric loss factor determines the effi-
ciency of conversion. The ratio of these two gives the dissipation factor of the
material.

ε 00
Tan δ ¼ (5.1)
ε 0
Thus, a good microwave receptor has to have a material with a high value of ε and a
00
moderate value of ε [15].
0