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Journal of Minerals & Materials Characterization & Engineering, Vol. 4, No. 2, pp 107-112, 2005
jmmce.org Printed in the USA. All rights reserved
Effects Of Microwave On Molten Metals With
Low Melting Temperatures
Xiang Sun, Jiann-Yang Hwang
, Xiaodi Huang, Bowen Li, Shangzhao Shi
Institute of Materials Processing and Department of Materials Science and Engineering, Michigan
Technological University, Houghton, MI 49931
Different metals were tested to determine if microwave irradiation can heat liquid
metals. Temperature profiles of different molten metals under microwave radiation
revealed that molten metals are primarily microwave reflectors. To explain this
phenomenon, microwave heating mechanisms and skin depths of conductors have been
introduced. The influences of resistivities on metal skin depths are also discussed.
Keywords: Microwave, Lead, Aluminum, Bismuth, Tin, Molten Metals.
In conventional heating, heat is transferred to a material through conduction,
convection and radiation. All the energy that is required to heat the material pass from the
surface to the interior and the heat flow rate is constrained by temperature and thermal
Microwaves are electromagnetic waves with frequencies ranging between 300
MHz and 300 GHz. When a material absorbs microwaves, heat can be generated inside
the material and heating is instantaneous with the introduction of power. Another
advantage of microwave heating is that it can accelerate reaction rates and reduce
reaction temperatures by decreasing activation energy . An additional benefit to using
microwave applications is that reactors and production facilities can be reduced to
smaller volumes and sizes than needed for conventional heating, thus saving costs in
capital investment, labor and energy . In a major review, Sutton  summarized the
fundamentals, benefits, challenges, and major research activities of microwave
processing of materials. Because of its potential advantages, microwave heating is used
for a diversity of processes in tempering, food, sterilization, drying, metal manufacturing
and other industries.
Bulk metals can not be heated by microwave radiation because the incident
microwaves are attenuated in a very thin surface layer, and the heating effect caused by
induced currents is so small as to be irrelevant. However, for metal powders, the surface
layer makes up a large proportion of the material, and the heat produced is quite
significant . Therefore, heating metal powders by microwave has been widely used.
Author to whom correspondence should be addressed, Tel: 1-906-487-2600, Email: email@example.com
108Xiang Sun, Jiann-Yang Hwang
, Xiaodi Huang, Bowen Li, Shangzhao ShiVol. 4, No. 2
Another application of microwave processing of metals is the reduction of metal oxide
ores or other minerals because of the potential energy savings.
Recently microwave-assisted extraction of metals from related minerals has been
developed at the Institute of Materials Processing, Michigan Technological University.
Using microwaves to reduce metal oxides to their metallic forms may result in problems
associated with microwave interaction with molten metals. One potential problem is
thermal run away. Thermal runaway is a condition that arises when the power dissipation
in a small elemental volume within a workpiece exceeds the rate of heat transmission to
its surroundings, so that the rate of increase in enthalpy is greater than in the neighboring
volumes. Until now, microwave absorption properties of molten metals have not been
studied. Being motivated by these observations, the present study aims to investigate
molten metal behavior under microwave radiation.
Pure metallic powders of Al (3g), Pb (12.5g), Bi (10.8g), Sn (8g) were filled
separately at the same volume in 99% alumina microwave transparent crucibles. The top
of each crucible was covered by alumina wool to prevent heat loss. Blank samples
(crucibles with no powders) were also prepared.
Experimental system setup
The experimental arrangement for measuring molten metal microwave absorption
is illustrated in Figure 1. A 4.5 inch mullite pipe was placed in the middle of a microwave
oven (GE, Model RVM1335, 900W, 2450MHz). Inside the oven, a heating circuit was
wrapped around the pipe. Inert Argon gas entered the left side of the pipe and exited
through the right side, directly into the exhaust hood. In the middle of the mullite pipe
was a small plate made of pure Inconel, which has a very high melting temperature, to
hold the sample and crucible. A thermocouple sheathed with Nickel-Chrome based Super
OMEGACLAD® penetrated to near the bottom of the crucible, as illustrated in Figure 1.
The temperature of the sample was controlled by a system connected to both the
thermocouple and the heating elements. Copper tube was used to connect the gas pipe
and the mullite pipe. The system, except for the temperature control system, was placed
in a hood. In order to prevent any microwave effects on the thermometer, it was placed
outside, at a distance of 12 inches from the cabinet.
Argon gas was passed through the mullite pipe at a flow rate of 8.5 L/min for 15
minutes before heating, and continuously during the experiment to avoid metal oxidation.
After 15 minutes of argon gas flow, the heating circuit around the mullite pipe was turned
on. Heating continued until the temperature reached 300
C above the melting
Vol. 4, No. 2. Effects Of Microwave On Molten Metals With Low Melting Temperatures109
temperature of the sample. The temperature control system was used to keep the
temperature steady for 10 minutes. Under this condition, all the solids inside the crucible
became liquid, as illustrated in Figure 2. Then the conventional heating circuit was turned
off and the microwave oven turned on for 3 minutes. After three minutes, the microwave
was turned off and the sample was allowed to cool. The temperature change was recorded
throughout the heating and cooling sequence. Argon gas flow continued until the sample
cooled down to room temperature to prevent reoxidation. A blank sample was also tested
at each temperature.
Figure 1. Schematic Diagram of the Experimental System
Figure 2. Molten metals Pb (left), Bi (right)
110Xiang Sun, Jiann-Yang Hwang
, Xiaodi Huang, Bowen Li, Shangzhao ShiVol. 4, No. 2
RESULTS AND DISCUSSION
Figure 3 shows how the temperatures of the sample and the blanks change with
time for different metals under microwave irradiation. For Pb and Bi, the temperatures of
the blank crucibles are slightly higher than the samples, which indicate that nearly no
heat was produced inside the molten metals during the microwave period. For Al, the
heat loss of the blank is slightly more than that of the sample. Sn is a little different.
When subjected to microwave irradiation, the Sn sample heated up a few degrees higher
than the blank. This phenomenon is not clearly understood at this time. In general, it can
be clearly seen from the experimental results that molten metals are primarily microwave
reflectors because there is no significant heating generated in the samples by microwave
radiation. Thermal runaway was not evident, as the temperature of the samples remained
constant or decreased.
There are two primary microwave heating mechanisms: dipolar and conduction. A
third mechanism, interfacial, is often of limited importance. For dielectric materials such
as water, introduction of microwave energy causes dipolar rotation, and then the
frictional forces cause heat to be developed . For electrical conductors, the mechanism
Figure 3. Temperature profile of different molten metals
Vol. 4, No. 2. Effects Of Microwave On Molten Metals With Low Melting Temperatures111
is different; the electrons move under a microwave induced magnetic field, which then
induces eddy currents, causing heat in a material due to its resistivity. These eddy
currents create a secondary magnetic field opposite to the excitation field. A repulsive
force can be caused which reflects the incident wave. This means most of the microwave
energy does not penetrate into the material, but is reflected.
is defined as the depth into the conductor from the surface at which
the current density is e/1 (0.368) of the value at the surface ,
is the angular frequency and
are the permittivity and
conductivity of the conductor, respectively. For bulk metals, such as Al, Pb, Bi,
and Sn, the skin depths are very small, as seen in Table 1.
Table 1: Skin Depths of Bulk Metals
Resistivity at 25
Conductivity (S/m)Skin depth (m)
For molten metals, things are a little different. The resistivity coefficients ,
resistivities  and skin depths of molten metals are given in Table 2.
Table 2: Resistivities and Skin Depths of Molten Metals
Skin depth (m)
Skin depths of molten metals are little higher than the values of the bulk metals.
However, in comparison to the sample radius (8mm), the skin depths are relatively
neglectable. This means that most of the energy is reflected. Only a small amount of
energy is constrained in the thin layer and the heat produced in the very thin skin is not
112Xiang Sun, Jiann-Yang Hwang
, Xiaodi Huang, Bowen Li, Shangzhao ShiVol. 4, No. 2
enough to heat up the whole sample. This is evident in the experiment, when the
temperature dropped or remained constant during microwave irradiation. It was also
observed that different metals have different temperature fluctuation. In theory, the skin
depth and thus the temperature increase should be in the sequence Bi>Pb>Sn>Al.
However, the temperature data from the experiment showed Sn>Al>Pb>Bi. This
phenomenon can not be explained at this point, and depends on future work.
From the temperature profile, molten metals are primarily microwave reflectors, as
determined by their intrinsic properties and conduction mechanism. Microwave
irradiation cannot be employed for heating purposes in processes dealing with molten
metals. However, for application concerns where molten metals could be an end product,
because the molten metals are not heated by microwave irradiation, microwave-molten
metal interaction will not cause any processing problems such as thermal run away.
Future work may focus on adding small amounts of nonreactive microwave absorbers
into molten metals to increase their absorption ability.
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