Radically different effects on materials by separated microwave electric and magnetic fields

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Mat Res Innovat (2002) 5:170Ð177 © Springer-Verlag 2002
Abstract Using a 2.45 GHz wave-guided cavity, in a
single mode TE
excitation, we were able to physically
locate compacted 5 mm pellets of samples separately at
the H (magnetic) node (where the E field is nearly zero),
or the E (electric) node (where H field is nearly zero). A
preliminary survey of a variety of metals, (Cu, Fe, Co..)
ceramics (ZnO, etc.), and composites, (WC-Co, ZnO-Co)
showed remarkable differences in their heating behaviors.
The results establish conclusively that the magnetic field
interaction contributes greatly to microwave heating of
common materials in a manner, previously neglected
in most theories of microwave heating, albeit still to be
Keywords Microwaves á E-field á H-field á ZnO á WC
Introduction and previous work
The vast majority of papers dealing with microwave heat-
ing of solids ascribe the heating to energy loss mechanisms
of the electric vector. Very recently our experimental find-
ings have demonstrated that magnetic losses play an im-
portant role in microwave sintering of bulk materials for a
wide range of conductor and semiconductor materials.
In 1999 we established [1] experimentally (for the
first time) that contrary to all previous practices and ex-
periments that ordinary powdered metal samples of vir-
tually any composition including very complex shaped
(see Figure 1) and large size (100 mm diameter, 1 kilo-
grams) could be fully sintered in 30 minutes or less in a
2.45 GHz multi-mode microwave cavity. Moreover,
these samples had properties at least as good as, and usu-
ally better than, those sintered in conventional furnaces.
This finding was outside the experience of a very large
number of scientists whose extensive work claimed that
J. Cheng (

) á R. Roy á D. Agrawal
Materials Research Laboratory,
The Pennsylvania State University, University Park, PA 16802,
e-mail: jxc44@psu.edu
Jiping Cheng á Rustum Roy á Dinesh Agrawal
Radically different effects on materials by separated microwave electric
and magnetic fields
Received: 15 June 2001 / Accepted: 15 June 2001
Fig.1a,b Commercial Powdered-metal gear parts made by mi-
crowave sintering in 30 minutes
powdered metals could not be sintered by microwave en-
ergy. These have been covered in many reviews [2Ð4].
Hence our achievement was puzzling to us and mainly
unbelievable to colleagues (and NATUREÕs reviewers).
Simple minded efforts to explain this by skin depth ab-
sorption etc did not work. The well known extensive the-
oretical treatment of microwave-material interaction by
many workers (see e.g. Varadan and Varadan [5], Booske
et al [6] and many others) have one feature in common:
that they always treat the energy absorption mechanism
as due to the dielectric loss factor.
In 1994 Cherradi et al [7] published a paper in which
they showed that the magnetic field must make substan-
tial contributions to the heating of alumina (at high tem-
perature) and semi-insulators, and metallic copper. But
the possible significance of their work was missed by the
vast majority of workers in the microwave sintering
community. In that work, their experimental design of
using samples of 120 mm length, was such that the
sample was always heated in both H and E fields simul-
taneously, caused by a complicated interplay of the dif-
ferent absorption and conduction mechanisms. More-
over, their work did not involve microwave sintering of
materials at all nor measuring the properties thereof.
Our research in this study was based on our own
unique success over the last several years with micro-
wave sintering to full density in very short timesof all
major ceramics, including oxides, nitrides, and carbides
[8Ð9]. Various fully transparent and translucent ceramic
samples, such as hydroxyapatite [10], mullite [11], alu-
mina, spinel [12], aluminum nitride [13], and aluminum
oxynitride [14], have also been successfully made by
microwave sintering in 10Ð30 minutes (Figure 2).
In this work we changed from multimode to single mode cavities.
A finely tuned (2.45 GHz) cavity with a cross section dimension
of 86 mm by 43 mm which works in TE
single mode (Figure 3)
was used. The distribution of the microwave field within the cavi-
ty is sketched in Figure 4. In the L/2 location along the length of
the cavity, the maximum electric field is in the center of the cross
section, where the magnetic field is at a minimum. The maximum
magnetic field is near the wall, where the electric field is mini-
mum. A quartz tube was introduced in this location to hold the
sample and also to enable us to control the atmosphere around
the sample. A 2.45 GHz, 1.2 kW microwave generator (Toshiba,
Japan) with power monitor was used as microwave source. Small
cylindrical samples (5 mm diameter and 5 mm high) was placed
inside the quartz tube at the two different locations: the maximum
electric field area (where the magnetic field is minimum), and the
maximum magnetic field area (where the electric field is mini-
mum), respectively. Sample temperatures were measured using an
infrared pyrometer (Mikron Instrument Co., Model M90-BT, Tem-
perature range Ð50 ¡CÐ1000 ¡C). During the experiments, atmo-
spheric pressure nitrogen gas was passed through the quartz tube
to avoid oxidation of metal samples at high temperature.
Initially, we tried to use a fixed microwave power for all samples
during heating, but the temperature increase was too fast and the
highest temperature exceeded the measuring range of the pyrometer
for some samples. And in some cases, discharging and arcing
occurred at higher microwave power. So we set different microwave
powers for different samples to get more stable heating results.
Results and discussion
Figure 5 shows the heating observed for a typical com-
mercial powdered metal sample (Keystone Powdered-
metal Company, Saint Marys, PA, USA). The composi-
tion of the powder is Fe+2%Cu+0.8%C. In the electric
field, this kind of sample can hardly be heated up at
all. At a power of 500 watts a maximum temperature of
180 ¡C (after microwave heating for 8 minutes) was ob-
served, and some arcing occurred around the edge of the
sample. But in the magnetic field, under the same micro-
wave power, the sample heated up very quickly and sta-
bly. The heating rate was higher than 300 ¡C per minute
in the first two minutes, then it slowed down. The final
temperature reading was 780 ¡C in 10 minutes, and a
quite uniform heating result was observed. Since there is
no insulation material around the sample, the thermal
loss must be significant at high temperatures, and we
think that is the main reason resulting in the lower heat-
ing rates at high temperature ranges, as compared to our
usual work.
Fig.2 Transparent ceramics samples made by microwave sinte-
ring processing
Fig 3 The microwave setup used for the separated microwave
Some other compositions of pure metal powder-com-
pact samples were also tested in this study, cobalt (Co),
iron (Fe) and copper (Cu) (all came from Alfa Aesar, A
Johnson Matthey Company, USA). The Co and Fe pow-
der-compact samples exhibited the same behavior during
the microwave heating (Figures 6, 7). There were little
heating in the electric field but high heating rates were
observed in the magnetic field. The microwave heating
of the Cu powder-compact sample was quite anomalous.
The sample heated up very fast both in the electric and
Fig.4 The schematic of the
microwave field distribution
within the TE
microwave cavity
Fig.5 Comparison of the heating rate of powdered metal compact
sample in microwave H and E fields
Fig.6 Comparison of the heating rate of cobalt (Co) powder-
compact sample in microwave H and E fields
magnetic fields. As shown in Figure 8, the sampleÕs tem-
perature rose to ~700 ¡C in 1Ð2 minutes, then quickly
dropped down to ~500 ¡C and kept within that range
during the continuous heating. For comparison, a pure
solid Cu bar with the same shape and size was put in the
microwave cavity to check out the energy absorption and
heating behavior. It was found that there is no tempera-
ture rise for the solid Cu bar sample either in the electric
field or in the magnetic field, even after being exposed in
the microwave field for 10 minutes, the sample still re-
mained at room temperature (Figure 9).
Alumina (Al
) is a proto-typical ceramic material
with excellent dielectric properties. This material usually
has very low dielectric loss, and it is not easy to heat up
by microwave, especially at lower temperature. Since the
dielectric loss of Al
increases with temperature, mi-
crowave heating of Al
becomes more efficient at high
temperature. In our experiments, the microwave heating
Fig.7 Comparison of the heating rate of iron (Fe) powder-
compact sample in microwave H and E fields
Fig.8 The heating rate of copper powder-compact sample in
microwave H and E fields
Fig.9 Microwave heating of solid copper metal sample in
microwave H and E fields
Fig.10 Comparison of the heating rate of alumina (Al
powder-compact sample in microwave H and E fields
Fig.11 Comparison of the heating rate of Zinc oxide (ZnO)
powder-compact sample in microwave H and E fields
sintering of cemented WC hard metal products in the
past few years. In this work, the pure WC (Telydyne)
powder compact samples were microwave heated in E
and H fields respectively. The results exposed that the
magnetic loss factor, not the dielectric loss, is the princi-
ple source leading to microwave absorption (Figure 12).
It was observed that there were some discharging or arc-
ing occurred when the WC sample was put in pure E
field, and in the pure magnetic field, the sample could be
microwave heated to high temperature quite stably with-
out any discharging phenomenon.
The microwave heating behaviors of iron oxides in
different fields are shown in Figures 13, 14 and 15. It is
interesting that different iron oxide exhibited different
heating results in the microwave fields. The FeO and
could only be heated to high temperature in the
Fig.12 The heating rate of tungsten carbide (WC) powder-
compact sample in microwave H and E fields
of high purity Al
samples (Baikowski International,
Charlotte, NC) in the electric field went slowly in the be-
ginning, the heating rate speeded up after the sample
reached a temperature of ~500 ¡C. But in the magnetic
field, Al
cannot be heated at all under the same mi-
crowave power for same exposing time (Figure 10).
Zinc oxide (ZnO) is another important dielectric ma-
terial. It showed the same heating behavior as Al
. In
the pure magnetic field, there was almost no temperature
raise. But in the pure electric field, the ZnO sample heat-
ed up very rapidly, the temperature reached 950 ¡C in
only 30 seconds (Figure 11).
Tungsten carbide (WC) belongs to the larger family
of semiconductor materials with some conductivity. We
have studied the sintering and WC in depth and have de-
veloped commercial technologies (15) for microwave
Fig.13 The heating rate of FeO powder-compact sample in
microwave H and E fields
Fig.14 The heating rate of Fe
powder-compact sample in
microwave H and E fields
Fig.15 The heating rate of Fe
powder-compact sample in
microwave H and E fields
butions which depend on the sample located in different
microwave fields. For example, when the ZnO-Co pow-
der mixture sample was located in a ÒpureÓ H field, in
the beginning only high magnetic loss Co powder ab-
sorbed microwave power and heated to high tempera-
ture. Meanwhile, there was no absorption occurring in
the ZnO powder which remained at low temperature.
The measured temperature resulted mainly from the
Co absorption. As time passed, the ZnO powder got
heated higher and higher due to conductive heat transfer
(Figure 20). Conversely, when the sample was put in a
pure E field, the temperature profile should be exactly
reversed. Now the ZnO powder had higher temperatures
and the heat transfer proceeded from ZnO to Co.
Figure 21 and 22 show the X-ray diffraction patterns
of the ZnO-Co powder mixture samples which were
heated in microwave E and H fields for different time.
The sample heated in a pure E field for one minute
showed no change, but which heated in pure H field
pure E field, and Fe
was heated up in both E and H
Figures 16, 17 and 18 show the heating data on some
composite samples, alumina-powdered metal, tungsten
carbide-cobalt (WC-Co) and ZnO-Co composite sam-
ples. Depending on the field, we obtained quite different
results. The WC-Co sample can only be efficiently heat-
ed in the magnetic field, as the pure WC and Co samples
also did. The alumina-powdered metal and ZnO-Co
composite samples can be heated up in both electric and
magnetic fields, since these composites contain two
components, one of which is more sensitive to the E
field (Al
, ZnO), and the other more sensitive to the H
field (powdered metal, Co). As shown in Figure 19, we
assume that the temperature came from different contri-
Fig.16 The heating rate of tungsten carbide-cobalt (WC-Co)
composite sample in microwave H and E fields
Fig.17 The heating rate of alumina-powdered metal composite
sample in microwave H and E fields
Fig.18 The heating rate of Zinc oxide-cobalt (ZnO-Co) com-
posite sample in microwave H and E fields
Fig.19 The scheme of the temperature distributions within the
ZnO-Co powder mixture samples in different microwave fields
showed that the metal Co phase pattern has almost disap-
peared (Figure 21). After ten minutes microwave heat-
ing, the samples exhibited remarkable differences. Some
cobalt oxide phase appeared in the H field heated
sample, but that cannot be detected at all in the E field
heated sample. In the E field heated sample, the intensity
of the ZnO peaks became much lower, confirming that
the ZnO had much higher reactivity with the E field
(Figure 22). These results can support our assumption
that the Co powder reacted much higher temperature
when the ZnO-Co powder mixture sample was heated in
pure H field, and ZnO powder reacted higher tempera-
ture when the sample was heated in pure E field.
The results presented in this work have demonstrated that
different materials exhibit greatly different heating behav-
iors in the E and H microwave fields respectively. In gen-
eral, the conductive samples, such as metal powder sam-
ple and carbide sample, can be much more efficiently
heated in the magnetic field. On the contrary, the pure ce-
ramic samples (insulators with little conductivity), such as
and ZnO, showed much higher heating rates in the
pure electric field. The structure state of the materials
plays important roles in the microwave-materials interac-
tion. For example, the powdered-compact copper sample
absorbed lot of microwave energy in the microwave field,
but the solid sample did not under the same condition.
From this data, it is clear that for the general theory of
energy loss in various materials when placed in a micro-
wave field, it is no longer possible to ignore the affect of
the magnetic component, especially for conductor and
semiconductor materials. The contributions to the mag-
netic loss mechanism can be hysteresis, eddy currents,
magnetic resonance, and domain wall oscillations. This
set of empirical data is presented to re-open the matter of
microwave-material interaction to incorporate more de-
tailed consideration for the effects of the magnetic field.
Fig.20 The scheme of the temperature development within the
ZnO-Co powder mixture samples in different microwave fields
Fig.21 The X-ray diffraction patterns of the ZnO-Co powder
mixture samples after microwave heating for 1 minute in E and H
Fig.22 The X-ray diffraction patterns of the ZnO-Co powder
mixture samples after microwave heating for 10 minute in E and
H fields
8.J. Cheng, D. Agrawal, S. Komarneni, M. Mathis, and R. Roy,
Microwave processing of WC-Co composites and ferroic tita-
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processing: triumph of applications-driven science in WC-
composites and ferroic titanates, Ceram. Trans. 80, 3Ð26
10.Y. Fang, D. Agrawal, D.M. Roy, and R. Roy, Fabrication of
transparent hydroxyapatite ceramics by microwave processing,
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(2000); 6,063,333(2000); 6,126,895(2000); 6,066,290(2000)
Acknowledgements This work is supported by ARPA (-ONR)
under Grant No. N00014-98-1-0752.
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