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Journal of Microwave Power and Electromagnetic Energy, 46 (4), 2012, pp. 215-228.
A Publication of the International Microwave Power Institute
Unusual Effect of the Magnetic Field
Component of the Microwave Radiation
on Aqueous Electrolyte Solutions
Satoshi Horikoshi, Takuya Sumi
Department of Materials and Life Sciences, Faculty of Science and Technology, Sophia
University, 7-1 Kioicho, Chiyodaku, Tokyo 102-8554, Japan
Nick Serpone
Gruppo Fotochimico, Dipartimento di Chimica, Universita di Pavia, via Taramelli 10,
Pavia 27100, Italy
Received: November 8, 2012
Accepted: November 28, 2012
ABSTRACT
The heating characteristics of aqueous electrolyte solutions (NaCl, KCl, CaCl2, NaBF4,
and NaBr) of varying concentrations in ultrapure water by 2.45 GHz microwave radiation
from a single-mode resonance microwave device and a semiconductor microwave generator
were examined under conditions where the electric field (E-field) was dominant and where
the magnetic field (H-field) dominated. Although magnetic field heating is not generally
used in microwave chemistry, the electrolyte solutions were heated almost entirely by the
microwaves’ H-field. The heating rates under H-field irradiation at the higher concentrations
of electrolytes (0.125 M to 0.50 M) exceeded the rates under E-field irradiation. This inversion
phenomenon in heating is described in terms of the penetration depth of the microwaves.
On the other hand, the action of the microwave radiation on ethylene glycol containing an
electrolyte differed from that observed for water under E-field and H-field conditions.
KEYWORDS: Microwave, Magnetic field, Electric field, Dielectric heating.
INTRODUCTION
As a heating source, microwave radiation has become one of the most attractive heating
methods in chemical syntheses, contrary to conventional heating methods that provide heat
externally through conduction and convection at the interface between the reactor walls
and the heat bath (e.g. an oil bath) [Stadler and Kappe, 2000; De la Hoz et al., 2004].
Microwave heating typically occurs in the bulk of a reaction sample through penetration of the
microwaves. The microwave method presents several advantages over conventional methods
in thermally-driven chemical reactions: e.g. shorter reaction times, uniform temperature
distribution, energy saving, and high product yields [Kappe, 2004]. In this regard, Metaxas and
Meredith have described heat and mass transfer in microwave-assisted processes [Metaxas
and Meredith, 1983]. A number of other analyses of microwave heat mass transfer have also
appeared in the recent literature [Clemens and Saltiel, 1996]. However, research of microwave
heating in liquids has been relatively sparse compared to solids. For instance, Lidström et al.
[2001] reported heating distilled water and tap water in a single-mode microwave irradiation
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Satoshi Horikoshi et al., Unusual Effect of the Magnetic Field Component of The Microwave Radiation on ...
device and found that tap water was heated
more efficiently than distilled water, which
suggested that extraneous impurities present
in tap water may have had an impact on the
heating efficiency. By extrapolation, it is
likely that electrolytes contained in water
may also influence the microwave heating
phenomenon. The role of water as a solvent
in microwave-assisted organic syntheses
has risen dramatically because of the
interest in ecofriendly processes germane
to Green Chemistry as recently described by
Polshettiwar and Varma [2010].
With regard to the microwave heating
of solids, Cheng et al. [2002a] reported
significant differences in the heating
behavior of solids by the microwaves’
electric field (E-field) and magnetic field
(H-field). In fact, in that article and later
in a patent [Cheng et al., 2002b] the
authors showed that the E (electrical)
and H (magnetic) fields in a 2.45 GHz
microwave reactor interact very differently
with matter. They demonstrated for the
first time that the microwave field itself,
independent of temperature, profoundly
affected the thermodynamics of any system
where electrons have unpaired spins.
This important effect was realized with
single mode microwave radiation where
the point of maximal E field and H field
were spatially separated. Moreover, they
showed that using a 2.45 GHz waveguide
cavity, in single-mode TE103 excitation,
they could physically position compacted
5 mm pellets of samples separately at the
H (magnetic) node (where the E field is
nearly zero), or at the E (electric) node
(where H field is nearly zero) [Cheng et
al., ]. It must be emphasized that the
authors were able to separate physically
the maximal density of the E-field and
the H-field [Cheng et al., , 2002b]. They
further noted that for the general theory
of energy loss in various materials, when
these are placed in a microwave field, the
effect of the magnetic field component
could no longer be ignored, particularly for
216
conductor and semiconductor materials.
They also suggested that contributions to the
magnetic loss mechanism could be hysteresis,
eddy currents, magnetic resonance, and
domain wall oscillations. Their empirical
data re-opened the matter of microwavematerial interaction to incorporate more
detailed consideration for the effects of the
microwaves’ magnetic field. The H-field was
effective in the heating of iron, but failed to
heat a metal oxide such as ZnO.
To the best of our knowledge, a
comparative behavior of the microwaves’ Efield and H-field in heating a liquid/solution
has hitherto not been investigated in great
details. In the present study we report the
heating behavior of aqueous electrolyte
solutions subjected to microwave electric
field (E-field) and magnetic field (H-field)
irradiation using a semiconductor microwave
generator and a single-mode resonance
applicator. Some heating mechanisms are
also proposed. Toward this purpose, the
rates of the microwave-induced heating
of ultrapure water in the presence of such
electrolytes as NaCl, KCl, CaCl2, NaBF4,
and NaBr at various concentrations have
been evaluated on the basis of dielectric
parameters (dielectric constant, dielectric
loss and penetration depth) and electrical
conductivity. The effects of an added
electrolyte in the microwave heating of
ethylene glycol have also been examined
for comparison.
EXPERIMENTAL SECTION
Chemical reagents
Ultrapure water (TOC: < 100 ppb;
Electrical conductivity: 0.57 × 10-5 S m-1) was
provided by Nomura Micro Science Co., Ltd.;
tap water was the drinking water of Tokyo.
High purity grade electrolytes (NaCl, KCl,
CaCl2, NaBF4, and NaBr) were provided by
the Tokyo Chemical Industry Co. Ltd.
Setup of the microwave apparatus
The microwave irradiation setup with
the single-mode cavity TE103 (transverse
Journal of Microwave Power and Electromagnetic Energy, 46 (4), 2012
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Satoshi Horikoshi et al., Unusual Effect of the Magnetic Field Component of The Microwave Radiation on ...
electric 103 mode), used to irradiate the
reactor contents and illustrated in Figure
1, included a short plunger, an iris, a threestub tuner, a power monitor and an isolator.
The continuous microwave radiation was
generated from a 2.45 GHz microwave
semiconductor generator (Fuji Electronic
Industrial Co. Ltd.; GNU-201AA; maximal
power, 200 W). The resonance of the
microwaves was adjusted with the iris and the
plunger at 1.5 cycles. Heating of the sample
solution (1.0 mL) was achieved by positioning
the quartz tube (diameter: 5.0 mm; internal
diameter: 4.0 mm) in the single-mode
microwave apparatus of Figure 1a-b, either
at positions of maximal electric field density
(E-field; position (i)) or at the maximal
magnetic field density (H-field; position (ii))
within the waveguide. Temperatures of the
solutions were measured at 5 sec intervals
with an optical fiber thermometer (FL-2000,
Anritsu Meter Co. Ltd.) whose tip was fixed
at the center of the sample; unless noted
otherwise, initial heating rates (ºC sec–1)
were calculated for a 30 sec irradiation
period. The wavelength of propagation of
the microwaves in the TE103 mode within the
waveguide was ca. 14.78 cm, estimated from
Equation 1 [Cronin, 1995]:
(1)
where λ is the wavelength in the waveguide;
λo (2.45 GHz) = 12.24 cm is the wavelength
in vacuum given by c/f, {c being the speed
of light, 2.9979 × 1010 cm s–1, and f being
the microwave frequency 2.45×109 s–1,
i.e. 2.45 GHz}; and b is the height of the
waveguide, 10.92 cm (other dimensions of
the apparatus are singled out in Figure 1b).
The maximal position of the E-field from the
iris was located at 3/4 the wavelength of
the standing wave in the waveguide, namely
11.09 cm. [Horikoshi, et al., 2009]. The
maximal position of the H-field from the iris
was at 1/2 the wavelength of the standing
wave in the waveguide, namely 7.39 cm.
[Horikoshi, et al., 2009]. Figure 1c and d
illustrates the distribution of the E-field
and H-field components of the microwave
radiation inside the waveguide in singlemode operation under resonant conditions
[Pozar, 2004].
The characteristics of heating
the samples by both microwaves E-field
and H-field irradiation were examined
by positioning the sample at positions
(i) and (ii) of Figure 1a. The microwave
input power was fixed at 50 W, albeit the
power monitor registered 40 W indicating
a loss of 10 W through the coaxial cable
and the waveguide. No reflective wave
that accompanied heating was observed.
Nonetheless, even if the reflected
microwaves were to arise before the iris,
the isolator would have eliminated these
microwaves and maintain the stability of
the microwave generator. The resonance of
the microwaves in the waveguide, measured
by the Agilent Technologies 8720C Network
Analyzer, can change when placing the
sample in the waveguide of the microwave
semiconductor generator setup. Accordingly,
appropriate adjustments were made with
the short plunger and the three-stub tuner.
In addition, the dielectric parameters
of the sample can change with heating.
Consequently, an electric field monitor (Fuji
Electronic Industrial Co. Ltd.) was used to
maintain the sample at the maximal position
of the E field, as the reproducibility of the
experiments may be diminished considerably
when such operations are neglected. Under
our conditions we found no significant
changes of the electric field upon heating.
Unless noted otherwise, the frequency of
the microwave radiation was 2.45 GHz.
To the extent that the E and H fields
are out-of-phase by a 1/4 wave (see Figure 1c
and d) [Pozar, 2004] relative to one another
judicious positioning of the electrolyte
samples in the waveguide where the fields
have maximal density (Figure 1a) makes it
possible for the microwaves to irradiate the
Journal of Microwave Power and Electromagnetic Energy, 46 (4), 2012
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Satoshi Horikoshi et al., Unusual Effect of the Magnetic Field Component of The Microwave Radiation on ...
a)
b)
c)
d)
Figure 1. a) Details of the experimental setup and positioning of the samples in the single-mode microwave
resonator; (i) maximal position of the electric field (E-field) density and (ii) maximal position of the magnetic field
(H-field) density. b) Photograph of the single-mode microwave resonator and the 2.45 GHz semiconductor microwave
generator; photograph also shows the actual position of the sample at the H field maximum; Distribution of the c)
E-field and (d) H-field component of the microwave radiation inside a waveguide in single-mode operation, resonant
condition. Each image for TEM103 mode was drawn with Equation S1 for the E field and S2/S3 for the H field using the
Mathematica 8 Software (Wolfram Research, Inc.). Note that the E field occurs only in the direction of the Y-axis. Ey:
E-field vector in the Y direction; Hx and Hz: H-field vector of the x and z direction; a: long side of the waveguide; b:
short side of the waveguide; d: movement of the microwave in the waveguide; l: 3 (X of TE10X mode). The images are
in agreement with those given by Yoshikawa and coworkers [Pozar, 2004].
samples either by the E-field component or
by the H-field component. Hereafter, we
shall refer to such irradiation conditions
simply as E-field and H-field irradiation.
RESULTS AND DISCUSSION
Effects of the 2.45-GHz microwave
radiation on various aqueous electrolyte
solutions
The influence of the microwaves’
E-field and H-field irradiation on heating
aqueous electrolyte solutions was examined
218
using an ultrapure water sample and aqueous
samples that contained various electrolytes
(NaCl, KCl, CaCl2, NaBF4, and NaBr) and whose
density differed relative to that of ultrapure
water. The heating rates of the electrolyte
solutions were far more significant than
that of ultrapure water. The heating rates
tended to plateau at the low electrolyte
concentrations under E-field irradiation and
remained fairly constant throughout the
concentration range examined (Figure 2a).
For example, the sample at 0.5 M in NaCl
Journal of Microwave Power and Electromagnetic Energy, 46 (4), 2012
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Satoshi Horikoshi et al., Unusual Effect of the Magnetic Field Component of The Microwave Radiation on ...
Figure 2. – Heating rates of an ultrapure water sample and aqueous electrolyte solutions of NaCl, KCl, CaCl2, NaBF4,
and NaBr obtained for a 30 sec irradiation period in a quartz tube under; a) predominantly electric field (E-field) and;
b) predominantly magnetic field (H-field) irradiation conditions using the single-mode method.
reached a heating rate of 2.0 ºC s–1 after
which the rates remained nearly the same
within experimental error (1.9 ± 0.1 ºC
s–1). Other electrolyte solutions displayed a
similar general behavior. This is a result of the
conduction mechanism whereby a solution
that contains ions in a hydrogen-bonded water
cluster moves through the solution under the
influence of the E field causing the collision
rate to increase. Evidently, under E-field
conditions the Joule heating mechanism (see
below) reflects a much stronger interaction
than the dipolar mechanism (i.e. dielectric
heating) in relation to the heat generating
capacity.
Contrary to E field irradiation, under
H-field irradiation conditions the heating
rates showed a near-exponential increase
on addition of the electrolytes to the
ultrapure water sample reaching a plateau
at ca. 4.0 ºC s–1 at a concentration of 3 M in
NaCl and KCl. Clearly, the data displayed
in Figure 2b demonstrate that heating of
aqueous electrolyte solutions was enhanced
considerably under H-field conditions and
was remarkably different from E-field
irradiation conditions with the 2.45-GHz
microwaves (Figure 2a). In the case of CaCl2,
the heating rate increased up to 4 M and
then showed a sharp drop at 6 M in CaCl2.
Three questions can then be formulated
from these data: (1) why does the heating
rate of the CaCl2 solution fall at 6 M?; (2)
what causes magnetic field heating of the
aqueous electrolyte solutions; and (3) why
is the H-field heating efficiency higher than
electric field heating?
Where the concentration of the
electrolytes is zero, comparison of the data
of Figures 2a and 2b shows a greater heating
rate when the water sample was subjected to
E-field irradiation than to H-field irradiation,
indicating that dielectric heating was more
significant in an ion-free solution. Intuitively
the heating of water was not expected to
occur under the microwaves’ magnetic field
component. However, in this regard we must
recall that the H field of an electromagnetic
wave is an oscillating magnetic field that
can cause charges to move. In the present
context, the partially charged ends of the
dipole of the water molecule tend to move in
opposite direction under the magnetic field
resulting in the rotation of the molecule and
contribute to the heating effect.
Two
parameters
describe
the
dielectric properties of aqueous electrolyte
solutions: (i) the dielectric constant (ε’),
which describes the ability of the water
molecule to be polarized by the electric
field, and (ii) the dielectric loss factor (ε’’)
which quantifies the efficiency with which
the electromagnetic energy is converted to
heat [Hayes, 2002]. Both parameters were
determined at various temperatures at 10
ºC intervals on microwave heating ultrapure
Journal of Microwave Power and Electromagnetic Energy, 46 (4), 2012
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Satoshi Horikoshi et al., Unusual Effect of the Magnetic Field Component of The Microwave Radiation on ...
water and the various electrolyte solutions
using an Agilent Technologies HP-85070B
Network Analyzer and an Agilent dielectric
high temperature probe (up to ~200 ºC).
In this case, the volume of the aqueous
samples was 100 mL in a Pyrex reactor.
The temperatures of the solutions were
measured with an optical fiber thermometer.
Preliminary experiments showed that the
thermometer had no effect on the dielectric
data. Measurements were also repeated using
a slim form probe to verify reproducibility.
The dielectric constant of water decreased
with increase in temperature to 90ºC, and also
decreased with increase in the concentration
of the electrolytes. For instance, for the NaCl
solution (0.125 M) the dielectric constant ε’ =
75.2 at 30 ºC decreased to ε’ = 61.1 at 90ºC.
For the 6.0 M aqueous NaCl solution, ε’ = 30.2
at 30 ºC decreased to ε’ = 9.7 at 90 ºC. Such
tendencies are in line with those reported
by Ratanadecho et al. [2002] Interestingly,
variations in the dielectric constants with
electrolyte concentration differed with those
observed for the heating rates with increase
in concentration (see Figure 2).
The behaviors of the dielectric loss
at various concentration of electrolyte and
at various temperatures are illustrated in
Figure 3. For ultrapure water, the dielectric
loss factor (ε’’) decreased somewhat with
increase in temperature: from ε’’ = 8.0 at
30 ºC to ε’’ = 2.8 at 90 ºC in parallel with
the dielectric constant. By contrast, the
presence of electrolytes in ultrapure water
caused the dielectric loss factor to increase
with increase in temperature. Thus, addition
of NaCl at a concentration of 0.125 M led
the dielectric loss factor ε’’ (= 19.2) at 30ºC
to increase to 25.3 at 90ºC (Figure 3a). In
addition, increasing the concentrations
of the electrolytes caused the dielectric
loss factors to be greater and to increase
with temperature. Most of the dielectric
loss factors of the electrolyte solutions
increased with concentration in electrolyte,
except for CaCl2 which displayed a decrease
in ε’’ at concentrations greater than 3 M,
220
and was particularly significant when the
concentration was 6 M in CaCl2 (Figure 3c).
These observations accord with the results
of the behavior of the heating rates of CaCl2
under H-field irradiation reported in Figure
2b.
In microwave heating the dielectric
loss (ε”) consists of the sum of two terms
(see Equation 2): (i) dielectric heating (first
term) and (ii) Joule heating (second term)
[Stuerga and Delmotte, 2006; Gabriel et al.,
1998]:
(2)
where εs is the relative permittivity
(dielectric constant) at low frequencies
and ε∞ is the relative permittivity at high
frequencies, ω is the angular frequency of the
electromagnetic radiation, τ is the relaxation
time {a measure of the time required for
water to rotate (τ = 4πηr3/κT) where r is the
molecular radius, κ is Boltzmann’s constant,
T is temperature, and η is the viscosity}, also
considered as the delay of the molecules (or
particles) to respond to the field change; and
σ is the ionic conductivity of the electrolyte
solution. Accordingly, enhancement of the
conductivity σ of the aqueous electrolyte
solution on addition of ions to water should
further enhance the heating efficiency ( 2)
to a point. The influence of an electrolyte in
microwave heating may then be expected to
cause increased efficiency through the Joule
heating mechanism.
Changes in the electrical conductivity
of the aqueous electrolyte solutions examined
at 30 ºC with changes in concentrations are
reported in Figure 4. The curves bear a
strong resemblance to the overall H-field
heating behavior reported in Figure 2b. For
instance, the electrical conductivity drop
resembles the drop of the heating rate at the
high concentration of CaCl2. The existence
of a correlation between the concentration
dependence of the electrical conductivity
and the concentration dependence of
Journal of Microwave Power and Electromagnetic Energy, 46 (4), 2012
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Satoshi Horikoshi et al., Unusual Effect of the Magnetic Field Component of The Microwave Radiation on ...
a)
b)
c)
d)
e)
Figure 3. Temperature and concentration profiles of the changes in dielectric loss (ε’’) for ultrapure water and
electrolyte solutions subjected to 2.45 GHz microwave irradiation: a) NaCl; b) KCl; c) CaCl2; d) NaBF4; and e) NaBr.
the dielectric loss at 30 ºC and 90 ºC is
particularly evident on comparing Figure 4
with Figure 5. As such then, H-field heating
can be taken to take place through Joule
heating. As to a reply to the first question
formulated earlier for the case of an
aqueous CaCl2 solution, Joule heating is
likely controlled by some specific heating
of the CaCl2 electrolyte solution by the
microwaves’ magnetic field, no doubt
related to the structure-making of water
by the Ca2+ ions in competition with the
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Satoshi Horikoshi et al., Unusual Effect of the Magnetic Field Component of The Microwave Radiation on ...
Microwave heating mechanism
Three types of microwave heating
phenomena are relevant to solutions:
dielectric heating, Joule heating, and
magnetic heating of molecules that may
be impacted differently by the E and H
fields of the microwave radiation [Horikoshi
and Serpone, 2009]. Dielectric heating
mechanism: dielectric materials (i.e. nonconductive) are heated by the microwaves’
E field owing to the presence of electric
dipoles in polar molecules as in, for example,
the microwave dielectric heating of water
through dipolar polarization (i.e. dipole
rotation). Joule heating mechanism: the
electrical resistance of water decreases on
addition of electrolytes such as NaCl, KCl,
CaCl2, NaBF4, NaBr and thus Joule heating
occurs through increased conductivity of
the aqueous solution. Magnetic loss heating
mechanism: magnetic losses typically occur
in the microwave region for such metal
oxides as ferrites and other magnetic
materials [Cheng et al., 2002a; Cheng et al.
2002b]. Such magnetic losses are different
from hysteresis or eddy current losses
because they are induced by domain wall
and electron-spin resonance.
The above data and discussion leads
us to propose the mechanism illustrated
in the cartoon of Figure 6 as to why the
microwaves’ H field presents advantages
to heating electrolyte solutions relative
to the microwaves’ E field germane to the
second question raised earlier. We begin by
supposing that the magnetic field does not
a)
b)
Figure 4. Changes in electrical conductivity (S m-1) of
aqueous electrolyte solutions as a function of concentration
at 30 ºC (NaCl, KCl, CaCl2, NaBF4, and NaBr).
structure-breaking of water by the Cl ¯ ions.
A discussion on the effect of ions on the
structure of water was beyond the scope
of the present study [Hribar et al., 2002;
Botti et al., 2004; Sloutskin et al., 2007; Y
Marcus, 2009; Luo and Roux, 2010].
In summary then, relative to Joule
heating promoted through addition of
electrolytes, the efficiency of dielectric
heating of water tended to decrease. As to a
reply to the second question, the heating of
electrolyte solutions under H-field irradiation
is likely connected with Joule heating.
Figure 5. Concentration dependence of the dielectric loss at two different temperatures (a) 30 ºC and (b) 90 ºC of
the five electrolytes.
222
Journal of Microwave Power and Electromagnetic Energy, 46 (4), 2012
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Satoshi Horikoshi et al., Unusual Effect of the Magnetic Field Component of The Microwave Radiation on ...
Figure 6. Cartoon depicting the heating model for
electrolyte/water
solutions
under
predominant
magnetic field irradiation.
heat an electrolyte solution directly. The
alternating magnetic field lines are vertical
to the quartz reactor walls and penetrate
the entire solution since the solution does
not absorb magnetic fields. Induced ring
currents are generated perpendicular to the
magnetic field lines, and since these currents
are generated everywhere in the solution
they cause the solutions to be heated.
Therefore, heating by H-field irradiation
can be regarded as indirect conduction loss
heating, or otherwise as induction heating
by the alternating magnetic field. Recall
that induction heating is the process of
heating an electrically conducting system
by electromagnetic induction where eddy
currents are generated within the system,
and resistance leads to Joule heating of the
system.
The next discussion deals with why
electric field heating of an electrolyte
solution is not more effective than magnetic
field heating. When an electrolyte is added
to water, microwave heating is dominated
by Joule heating under E-field and H-field
irradiations. However, the data show that Efield heating is significantly different from
H-field heating. For instance, the heating
rate reached saturation at concentrations
greater than 0.50 M in NaCl under E-field
irradiation (Figure 2a). By contrast, the
heating rate was enhanced significantly
under H-field irradiation as the concentration
of the electrolytes increased (Figure 2b);
the mechanism remains nonetheless Joule
heating. The contrast between Joule
heating by E-field irradiation and H-field
irradiation can be considered on the basis
of the penetration depth of the microwaves
into the aqueous solutions. The penetration
depth Dp (in cgs units) is the depth at which
microwaves pervade into the material and
the power flux has fallen to 1/e (= 0.368%)
of its surface value; it can be estimated by
Equation 3 [Metaxas and Meredith, 1983]:
(3)
where λ is the wavelength of the radiation,
λ(2.45GHz) = 12.24 cm. That is, it denotes the
depth at which the power density of the
microwaves is reduced to 1/e of its initial
value.
The depth at which the 2.45 GHz
microwaves can penetrate into the ultrapure
water sample in the reactor increased nearly
2.6-fold from ca. 21 mm to 55 mm with
increase in temperature from 30 ºC to 90 ºC
(Figure 7), whereas the depth of penetration
decreased on addition of the electrolyte
(NaCl) and with increase in temperature {from
8.8 mm at 30 ºC to 6.1 mm at 90 ºC (0.125 M),
i.e. a 31% decrease; from 5.3 mm to 3.6 mm
(0.50 M), i.e. a 32% decrease; from 2.7 mm
to 1.7 mm (1.0 M), i.e. a 37% decrease and
from 1.0 mm to 0.8 mm (6.0 M), i.e. a 20%
decrease}. These penetrations depths should
be compared to the internal diameter of the
quartz reactor: 4.0 mm. As heating of the
solution progressed, the penetration depth
(8.8–6.1 mm) of the microwaves into the
Journal of Microwave Power and Electromagnetic Energy, 46 (4), 2012
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Satoshi Horikoshi et al., Unusual Effect of the Magnetic Field Component of The Microwave Radiation on ...
Figure 7. Temperature profiles of the changes in
penetration depth (mm) of the 2.45 GHz microwaves
into an ultrapure water sample containing NaCl at
various concentrations: (a) 0 M, (b) 0.125 M, (c) 0.25 M,
(d) 0.5 M and (e) 1.0 M.
electrolyte solution (0.125 M) was significant
compared to the quartz reactor diameter
(left scheme in Figure 8).
The optimal penetration depth for
the aqueous solution with 0.50 M in NaCl
was 5.3–3.6 mm (versus 5.0 mm of reactor
size) so that the entire solution was bathed
with microwaves at this concentration,
which is precisely the concentration at
which the maximal heating rate was
observed under E-field irradiation. The
penetration depth of the microwaves under
E-field irradiation becomes shallower on
increasing the electrolyte concentration.
Figure 8. Cartoon illustrating the heating model of
aqueous electrolyte solutions NaCl: 0.125 M and 6.0 M
under E-field irradiation; internal diameter of reactor
= 4 mm.
224
As a result, heating the entire sample does
not occur. Only the aqueous solution at the
interface between the quartz walls and the
solution (1.0–0.8 mm) is heated at 6.0 M
concentration of electrolyte (NaCl) – see the
right hand scheme in Figure 8. Therefore,
the overall heating rate is expected to be
smaller because the inner bulk solution is not
heated. Such a phenomenon is reminiscent
of the skin effect displayed by microwave
heating of a metal [Sun et al., 2005].
The rates of Joule heating of aqueous
electrolyte solutions under H-field irradiation
evolved with increase of the electrolyte
concentration, as the Joule heating induced
by alternating magnetic fields occurred in
the inner bulk of the solutions owing to the
establishment of ring currents (Figure 6).
Accordingly, magnetic field heating is nearly
independent of penetration depth, contrary
to dielectric heating that occurs via E-field
irradiation.
Heat distribution under E-field
irradiation
The heat distribution in the electrolyte
solution depends on the penetration depth
of the irradiating microwaves. To obtain the
distribution, we measured the temperature
of the solution using an optical fiber
thermometer (FL-2000, Anritsu Meter Co.
Ltd.) located at the center of the solution
(see Figure 9a), whereas the temperature at
the wall of the reactor was monitored with an
infrared thermometer (R-100, Anritsu Meter
Co. Ltd.). The heating rates in both cases are
reported in Figure 9b. In this experiment,
a quartz reactor with an internal diameter
of 10 mm was used to accommodate the
spot size of the infrared thermometer. The
reactor was positioned where the electric
field density was maximal. However, since
the reactor size was larger than that shown
in Figure 8 the microwave radiation also
had an H-field component mixed in with the
E-field component. To the extent that the
temperature measured near the reactor wall
was cooled by the surrounding atmosphere,
Journal of Microwave Power and Electromagnetic Energy, 46 (4), 2012
International Microwave Power Institute
Satoshi Horikoshi et al., Unusual Effect of the Magnetic Field Component of The Microwave Radiation on ...
a)
b)
Figure 9. (a) Method of measuring the temperature in the samples; (b) heating rates as a function of the concentration
of NaCl and as a function of the location where the temperatures were measured experimentally.
not unexpectedly the measured temperature
was lower than the temperature measured
at the center of the solution [Kappe, 2004].
The heating rate determined at
the center of the electrolyte solution was
maximal at 0.5 M in NaCl and decreased
sharply at the higher concentration in
electrolyte (2.0 M in NaCl; Figure 9b). The
heating rate determined near the reactor
surface displayed a smooth decrease with
an increase in electrolyte concentration. In
the more concentrated electrolyte solution,
the microwaves are less likely to reach the
center of the solution, in which case the
temperature distribution at the center and
at the reactor walls is reversed at the higher
concentration.
Effect of microwave irradiation on
addition of an electrolyte to an organic
solvent
Adding an electrolyte to an organic
solvent was examined for ethylene glycol
to determine what effect a salt might have
on the heating rates, on the dielectric loss
parameter, and on the penetration depth of
the microwaves. The ethylene glycol solution
was saturated with NaCl, the concentration
of which reached ca. 0.50 M. Under E-field
irradiation conditions, the heating rate
of the ethylene glycol was enhanced 3.7fold on addition of 0.125 M NaCl (Figure
10a), whereas under H-field irradiation the
addition of NaCl had but a minor effect on
the heating rates. Clearly, the phenomenon
of H-field heating of the ethylene glycol and
its solutions with the electrolyte differed
significantly with that observed for aqueous
solutions (Figure 2). This raises an additional
question about magnetic field heating versus
electric field heating between aqueous
and non-aqueous media in general. Future
studies will address this question further.
The dielectric loss factor (ε’’) of
ethylene glycol increased on adding the
electrolyte (Figure 10b), and decreased with
increasing temperature. Note the significant
difference with aqueous media (Figure 3).
On the other hand, the penetration depth
(Figure 10c) of the microwaves into the
solutions at all the concentrations of NaCl
was such that the whole glycol/electrolyte
solution could be heated.
The standard microwave frequency
commonly used for dielectric heating is
2.45 GHz which corresponds to a relaxation
time of 65 ps [Gabriel et al.,1998]. To the
extent that the relaxation time of the water
Journal of Microwave Power and Electromagnetic Energy, 46 (4), 2012
International Microwave Power Institute
225
Satoshi Horikoshi et al., Unusual Effect of the Magnetic Field Component of The Microwave Radiation on ...
a)
b)
c)
Figure 10. (a) Heating rates obtained during a 30-sec period for ethylene glycol on addition of NaCl under E-field
irradiation and H-field irradiation conditions. (b) Temperature profiles of the changes in dielectric loss factor (ε’’). (c)
Penetration depths (mm) of the 2.45-GHz microwaves into ethylene glycol/NaCl mixture. In all cases, temperatures
were measured with the fiber optic thermometer at the center of the solutions. Size of reactor was 5 mm in
diameter.
molecule at 25 ºC is 2 ps, [Struis et al.,1987]
the water molecule can easily follow the
footsteps of the 2.45 GHz frequency of
the microwave radiation. Our data do not
preclude the possibility that the movement
of water clusters may be at the origin of
microwave dielectric heating in aqueous
media. As noted earlier, the presence of ions
in water causes the structure of water to be
modified, leading the dielectric polarization
of the water clusters [Hribar et al., 2002]
to change with the microwave frequency.
Water behaves as a cluster molecule caused
by the hydrogen bond between single water
molecules, unlike polar organic solvents
where hydrogen bonding is limited. Hence,
hydrogen bonding may be an important factor
226
in dielectric heating. Germane to this, Huang
et al. [2009] reported that the electrical
conductivity of aqueous solutions of NaCl
can be changed from their interactions with
microwaves, and further suggested that the
cluster structure of the electrolyte solution
is affected by the microwave radiation.
CONCLUDING REMARKS
The present study has shed some light
into the heating mechanisms of aqueous
electrolyte solutions relative to an ultrapure
water sample under E-field and H-field
irradiation conditions; they were shown to
be significantly different. In nearly all the
microwave-assisted chemical syntheses,
the microwaves’ field most often used to
Journal of Microwave Power and Electromagnetic Energy, 46 (4), 2012
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Satoshi Horikoshi et al., Unusual Effect of the Magnetic Field Component of The Microwave Radiation on ...
irradiate a reactive system is the E field.
Consequently, it is necessary to select an
appropriate reactor vessel to ensure full
penetration of the microwaves throughout
the sample. Contrary to E-field irradiation,
however, Joule heating by H-field irradiation
can heat the reactive system efficiently
as in the latter case penetration of the
microwaves is independent of reactor size.
The effect of addition of an electrolyte to
polar solvents (e.g., water and ethylene
glycol) has been shown to be different, such
differences being unexpected on the sole
basis of the dielectric parameters of the
molecules in the systems. Clearly, the action
of the microwaves on water containing ionlike salts is fairly significant relative to the
action of microwaves on pure water.
ACKNOWLEDGMENTS
Financial support from the Japan
Society for the Promotion of Science (JSPS)
to S. H. through a Grant-in-aid for young
scientists (No. B-23750247) is gratefully
appreciated. One of us (NS) thanks Prof.
Albini of the University of Pavia for his
hospitality during the many semesters in his
Laboratory. We are grateful to the Nomura
Micro Science Co. Ltd. for providing us with
the ultrapure water sample and also to Mr.
Yoshida of Fuji Electronic Industrial Co. Ltd.
for the microwave assistance.
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