S p e c i a l
F o c u s :
MICROWAVE HEATING
Advancements in Microwave Heating
Technology
Reported by Ed Kubel, Editor, Industrial Heating
This article provides a glimpse of the many potentially useful
applications developed by microwave and materials experts.
an's desire to use and
manipulate materials has
driven the need for industrial heating methods. In
the Iron Age, fire was used to melt, shape
and temper metals, and pottery was developed for smelting, as well as cooking.
Wood, peat and coal were the first energy
sources used to fire pottery, glass and metals at elevated temperatures. Later, man
learned to harness oil, gas, solar, wind and
nuclear power, and also developed electric
heating processes including resistance,
induction, infrared, and more recently,
radio frequency. Each heating technology
occupies a needed place, but there is still
room for improvement in speed, efficiency,
and delivering energy directly in to the
workpiece. Microwaves already are used
extensively in the mass-production food
industry. The next step is the use of
microwave heating technology for industrial processes beyond cooking and drying.
In the late 1930s, the use of radio waves
was perceived as a method to heat nonconductive polar materials. The RF heater was
invented and widely applied in conjunction with dies and pressure to weld seams
in polar plastics. The use of RF technology
eventually expanded into automotive,
industrial fabrics, packaging, stationary
products, inflatables, medical products and
many other product groups. Radio frequency has certain limitations due to the need
for electrodes, high cavity voltages and
complex power controls.
In the 1940s, Dr. Percy Spencer at
Raytheon Corp. was experimenting with a
new device called a magnetron for use in
radar applications. Strangely, a chocolate
bar in his pocket mysteriously melted dur-
Aluminum-titanium-carbon powders during a self-propagating high-temperature synthesis
(SHS) reaction initiated by microwave energy (2.45 GHz, 1,000W applied power). Courtesy of
Virginia Polytechnic Institute.
a)
b
Fig. 1 Schematic (a) and photo (b) of vertical continuous batch furnace for sintering carbide
cutting tools using microwaves
IndustrialHeating.com – January 2005 43
S p e c i a l
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MICROWAVE HEATING
ing the testing. Dr. Spencer continued
experimenting by popping corn and then
exploding in egg. Raytheon supported further research to develop and contain
microwaves generated from magnetrons for
industrial use and domestic cooking we
now enjoy. After 60 years of experimentation, we know the best uses for microwaves
in cooking, and accept that microwaves are
not great for every cooking need. A similar
lesson is being learned in the industrial
heating arena concerning microwave processing of materials. Industry must have
some understanding of how microwaves
heat materials and what the limitations are
before its potential can be tapped. With the
help of universities, national laboratories
and specialists, several companies highlighted in this article are leading the way.
Commercializing microwave heating
This article highlights processes available to
the market. The important players include
microwave and furnace manufacturers, universities, consultants, national laboratories
and end users. Ceralink Inc. (Troy, N.Y.;
www.ceralink.com), plays a unique role in
working to bridge the gaps and promote
commercialization, offering consulting and
hands-on R&D as a partner, subcontractor
or service. Dr. Holly Shulman, President of
Ceralink, says collaboration between equipment manufacturers, scientists and industry
is required to make the technology accessible and affordable.
Cutting tools
Using this approach, Dennis Tool Co.
(Houston, Tex.) successfully developed
and commercialized high -erformance car-
Fig. 2 Microstructure of sintered WCo microwave sintered (left) having fine grain structure,
overall uniformity and cobalt distribution; and microstructure of conventionally sintered
WCo (right) having several large grains, a rage of grain size and cobalt-rich areas; 1,500
bide mining and drill bits. Company president Dr. Mahlon Dennis was aware of
research at Pennsylvania State University
in 1995, recognizing that microwave sintering of carbide could offer improved
properties, as well as benefits in process
speed and efficiency. Dennis contributed
funding and commercial direction to the
strong scientific and experimental foundation in the group led by Drs. Dinesh
Agrawal and Rustum Roy at Penn State.
The work resulted in several patents
including a system for microwave firing in
a vertical continuous batch that uses
process heat to preheat the incoming crucible load (stoke type heating). The furnace design overcomes complications of
heating large batches and offers excellent
uniformity, speed and control. A schematic and a photo of a commercial unit are
shown in Figs. 1a and 1b.
One of the key decisions by Dennis
Tool for successful commercialization was
to design and sell turnkey systems for
microwave sintering of carbide parts. By
making these microwave sintering systems available to carbide and other manufacturers, production costs are decreased
and development costs are defrayed.
Dennis Tool now uses these systems to
make their own products and they are
exploring new products made possible by
fast volumetric heating.
Dennis Tool has fully commercialized
the use of microwave sintered carbide for
PDC substrates in oilfield drill bits, one of
the company's primary businesses.
Polycrystalline diamond compact (PDC or
PCD) are used widely in both earth drilling
and industrial machining applications. The
part is formed by growing the diamond
layer onto a substrate of tungsten carbide at
high pressures and temperatures (106 psi
and 2000˚C, or 6,894 MPa and 3630˚F).
The abrasion and impact properties are
dependent on both the diamond layer and
How microwaves are produced
The term “microwave” is used to cover the portion of the electromagnetic
spectrum between 300 MHz and 300 GHz, which corresponds to wavelengths ranging from 1 meter to 1 millimeter. In practical terms, there are
certain frequencies that are allowed for industrial use.These are called ISM
(industrial, scientific and medical) frequencies for applications in those
areas as described in the following table.
Frequency, GHz
0.915
2.45
5.80
24.12
Wavelength, cm
32.8
12.2
5.2
1.2
44 January 2005 – IndustrialHeating.com
Kitchen microwaves operate at 2.45 GHz, while many large industrial systems use 915 MHz. The overwhelming majority of microwaves are produced by magnetrons. However, klystrons or gyrotrons are used to produce ultrahigh-frequency microwaves (millimeter waves).
Low-power magnetrons (2.45 GHz, 800 W) are inexpensive and rugged
devices (<$30 purchased in quantity). High power can be achieved by ganging up low-power magnetrons, but this is cumbersome for large systems
beyond the drying stage. High-power 2.45 GHz magnetrons are available up
to 30 kW, while 915 MHz systems can be purchased in 60, 75 and 100 kW
units. A higher frequency 5.8 GHz magnetron has recently been made available, but not quite at mass production prices, while ultrahigh frequency 2430 GHz klystrons or gyrotrons are much more expensive technologies.
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45
S p e c i a l
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the carbide substrate.
The greatest advantage in using
microwaves in carbide sintering is the
improved microstructure, which translates
to better properties and better performance. Microwave sintering promotes fast
densification, while minimizing grain
growth, diffusion, cobalt pooling and carbon loss. Additives that minimize grain
growth (but cause mechanical degrada-
How microwaves heat
The short explanation of microwave heating
is that materials heat up through internal friction when dielectric and magnetic loss
mechanisms respond in a microwave field.
One example is a charged impurity or vacancy in a crystal lattice that switches places in
an alternating field. Friction is produced if the
ion cannot quite keep up with the field, while
no friction is produced if the ion cannot
move at all, or if it moves too easily.
There are many loss mechanisms that can
be activated in real materials related to crystal structure, defects, bonding, surface, grain
boundaries, etc. One might think there
should be an optimum frequency for maximizing the friction for each mechanism, but
optimum frequency changes with temperature because certain movements become
easier as the material heats up. This produces
less friction in some cases and more friction
in other cases, such as where a defect was
frozen and couldn't previously contribute.
This behavior causes a peak in the dielectric
loss, which generally shifts to lower frequencies at higher temperature. The selection of
an appropriate microwave frequency is critical to commercial success of the process.
Figure A shows several important relationships for dielectric heating.
In conventional heating, all heat must be
transferred through the outer surface of the
material to the interior. Microwave heating
offers an important advantage of being able
to place energy directly into the volume of
the workpiece. This requires meeting certain
conditions where microwaves penetrate the
material enough to cause volumetric heating. Very little heating occurs if microwaves
are reflected or if they penetrate through
the material too easily. Table A shows some
penetration depths calculated from the
dielectric properties.
46 January 2005 – IndustrialHeating.com
tion), such as TiC, TaC and free carbon,
are not necessary when using microwaves
instead of conventional heating.
Figure 2 compares microstructures from
Dennis Tool parts that were microwaved
versus conventionally sintered. Significant
improvements in impact, abrasion and corrosion resistance are achieved using continuous microwave sintering. Both hardness and fracture toughness increase,
The high penetration depth of quartz indicates that quartz glass will not couple well
(suscept) or heat at room temperature in a 2.45
GHz microwave field. However, it has been
demonstrated experimentally that quartz
glass couples and heats in the microwave field
at elevated temperatures. Unfortunately,
dielectric data is scarce at microwave frequencies and elevated temperatures. The situation
is further complicated by the continual (sometimes exponential) change in dielectric properties with temperature, and a strong effect of
impurities, defects and surfaces. Direct
microwave heating tests are very valuable for
early stage feasibility, as well as measurement
of the dielectric properties in the appropriate
frequency and temperature regime.
In general, electrically insulating materials
do not couple or heat well from room temperature at moderate frequencies, but do
couple and heat at higher temperatures or at
higher frequencies. For example, pure alumina is microwave transparent at room temperature using 2.45 GHz microwaves, but couples
from room temperature at 24-30 GHz.
Table A Microwave (2.45 GHz) penetration
depths in some materials
Temperature, ˚C
Penetration
depth, cm
Water
25
1.5
Water
95
5.7
Ice
12
1,00
Wood
25
3-350
Hollow glass
25
35
Porcelain
25
56
Epoxy resin
25
4,100
Teflon
25
9,200
Quartz glass
25
1,600
Material
Note: very high penetration depths occur when the
material does not couple or heat well in the microwave field. Coupling often increases with temperature
resulting in a decrease in penetration depth.
which is attributed to the fine grain size
and full crack-arresting behavior of unreacted cobalt. Extensive laboratory and field
testing of parts convince Dennis Tool that
it has developed the next generation carbide mining and drilling bits, with new
products and applications on the horizon.
New applications include cutting tools,
dies, anvils, sleeves, bushings, nozzles,
bearings and substrates. Some new materi-
Alumina begins to couple and heat more
effectively at 1000˚C (1830˚F) at 2.45 GHz and
becomes highly suscepting at 1500˚C
(2730˚F), which suggests that hybrid
microwave or millimeter-wave (gyrotron)
heating is required to fire alumina.
Semiconducting materials, such as silicon
carbide, usually heat well from room temperature at moderate frequencies. Conductive
materials such as metals should reflect
microwaves. However, heating of metals
(especially powder metals) has been observed
experimentally. There are many unanswered
questions concerning microwave heating,
such that theoretical explanations must catch
up with real life observations.
εr=εr’-iεr” Tan δ = εr” iεr”
• Complex permittivity ➝ ability to
absorb and store energy
• Permittivity (εr”) ➝ penetration
of microwaves
• Loss factor (εr”) ➝ store energy
• Loss tangent (Tan δ) ➝ Convert
absorbed energy to HEAT
P = 2π f ε0 εr” E2
P = Volume energy density (W/m2)
f = frequency (Hz)
ε0 = permittivity of free space
εr” = dielectric loss factor
E = electric field strength (V/m)
Power density ➝ heat
D=
λ0
2π
εr’1/2
εr”
D = penetration depth
λ0 = wavelength in vacuum
εr’ = permittivity
εr” = dielectric loss factor
Penetration depth
Fig. A Relationships for dielectric
heating
als include carbide diamond composites,
functionally graded composites, nanocarbides and nanocomposites.
Cutting tools are often an early proving
ground for new materials, since they are
small, consumable and high-profit items,
which usually do not cause catastrophic
problems if they fail. Silicon-nitride cutting tools were developed and commercialized, reaching a broad market in 2000
through Valenite, a U.S. based cutting tool
company recently bought by Sandvik.
Three microwave-sintered grades are available for machining, high speed turning and
milling of cast iron and machining of hightemperature alloys. As with carbides,
Valenite claims microwave-sintered silicon
nitride products have a finer grain
microstructure with higher hardness and
better fracture toughness than conventionally processed material.
Metals processing
A completely different approach of using
microwaves for heat is the generation of
on-demand plasma. Dana Corp. (Toledo,
Ohio; www.dana.com), a manufacturer of
automobile components, developed a
method for ultrarapid heat treating, coating and brazing of metals. Dana's AtmoPlas
system uses microwaves to superheat plasma that surrounds the parts, and quickly
heats (temperatues up to 2000˚C) by conduction. The process does not require the
use of a vacuum, as required in conventional systems. The cycle time for heat
treating and coating is reduced by two
thirds, and there is a net savings in both
time and energy. Dr. Kumar from Dana
Corp. explains that this method overcomes
the tendency of metals to arc in the
microwave field, while taking advantages
of the fast heating response of microwaves.
Dana Corp originally developed the
microwave plasma process to address an
internal bottleneck in a brazing step.
Highly encouraging results also demonstrated a wider range of application in
Industrial Heating
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you can have it reprinted by Industrial Heating.
Feature Articles, Technology Spotlights,
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company’s ad, special message or
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Fig. 3 Gear being processed using
AtomPlas method
Contact
Becky McClelland at
412-531-3370 for details
Fig. 4 Heated gear using AtomPlas method
IndustrialHeating.com – January 2005 47
S p e c i a l
F o c u s :
MICROWAVE HEATING
coating, sintering and heat treating. Dana
is currently evaluating its internal scaleup strategy, while looking for partners to
commercialize the AtmoPlas system. The
system is currently a batch process that
uses a refractory container or pod, which
holds the metal parts and the plasma
(Figs. 3 and 4). The company plans to
convert this to a conveyor belt where pods
are cycled and each pod receives individual microwave plasma treatment.
While Dana's system can be used to join
and braze materials having similar expansion properties, Technology International
Inc. (TII, Kingwood, Tex.) developed and
commercialized a microwave process to
braze highly mismatched materials, such as
lightweight armor combining silicon carbide to titanium metal (Fig. 5). The
method relies on the vastly different coupling (heating) characteristics of the
ceramic and metal in the microwave field.
At the brazing temperature, the ceramic is
several hundred degrees hotter than the
metal, which avoids high expansion and
shrinkage of the metal. Bob Radtke of TII
says the process must be performed quickly
to avoid heat transfer, which requires high
power and good microwave penetration
into the ceramic. TII is currently working
to increase the brazing speed, which will
allow an increase in the size of parts that
can be brazed. Full production of large
parts will require the design and manufacture of specialized equipment.
The idea of using microwaves for melting metal was introduced to the general
public through David Reid's web site in the
late 1990s. Reid developed a suscepting
crucible and began melting metal in a
kitchen microwave using a lost-wax casting
method. This is a simple and highly effective method for small batches. Accessories,
such as crucibles and thermal insulation,
can now be purchased from Research
Microwave Systems LLC (Alfred, N.Y.;
www.thermwave.com) to use for melting
metal in a kitchen microwave.
Fig. 5 Microwave process can braze dissimilar materials such as this part joining silicon carbide to titanium alloy
Insulation
Crucible
Mold
Fig. 6 Large suscepting crucible is used at
the Y-12 National Security Complex to
melts a variety of metals using microwaves
Industrial Heating REPRINTS
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can have it reprinted by Industrial Heating.
Feature Articles, Technology
Spotlights, MTI or IHEA
Profiles, Literature Features, and
much more. Customize your
reprints with your company’s ad,
special message or even the
cover of Industrial Heating.
Contact
Susan Heinauer at
412-531-3370 for details
48 January 2005 – IndustrialHeating.com
On a larger scale, Ed Ripley, at the Y-12
National Security Complex (formally part
of Oak Ridge National Laboratory; Oak
Ridge, Tenn,; www.y12.doe.gov) was melting hundreds of pounds of titanium, aluminum, gold and silver using an idea similar to that mentioned above; that is, a large
suscepting crucible in a microwave field
(Fig. 6). Although metal is supposed to
reflect microwaves, researchers at Y-12 and
Penn State claim that the metal actually
begins to absorb (couple) and heat directly
at approximately 3/4 of the melting temperature. Y-12 and Oak Ridge National
Laboratory have extensive microwave facilities and have been developing microwave
technology for more than 20 years. Ripley
explains that several processes have just
been declassified and are available for
licensing agreements. This includes a
microwave eutectic salt bath, with an
option to use a granular suscepting media
that prevents hydrogen pick up. One
advantage of using microwaves is that the
salt bath can be heated quickly on demand,
instead of being kept continuously molten.
Two companies have licensed the metal
melting technology from BWXT, Y-12's
commercialization
arm.
Microwave
Synergy Inc. and MS Technology, both
located in the state of Tennessee are now
competing for the lead position in scale-up
and commercialization of the Y-12
microwave metal melting technology. Both
companies may have industrial partners,
but are not disclosing specific information.
The technology involves the use of
microwave energy, special microwave-susceptible materials and uniquely designed
crucibles and molds to melt and cast metal
in a microwave chamber. This was originally developed for melting depleted uranium, a process that is currently being scaled
up at Y-12. According to Stan Morrow of
Microwave Synergy, microwave technology for metal melting saves energy, reduces
cycle time and improves metal quality. Ken
Givens, Vice President of Business
Development at MS Technology, adds that
one of the major benefits is versatility.
Batches from 1 to hundreds of pounds can
be heated quickly and efficiently. Givens
has plans for international deployment of
the microwave metal-melting technology
and has already entered into an agreement
with the Technical University of Munich
(www.tu.muenchen.de) to install a demonstration unit in their laboratory. MS
Technology is also designing a larger chamber
capable of continuous melting of 2000 lb/hr.
It is not clear where this technology will
fit as an industrial process. High-power
2.45 GHz magnetrons are used, which are
costly. The size is currently limited by the
batch style design, with casting taking
place within the microwave cavity. This
technique may be best suited to casting
high precision superalloys or titanium,
rather than bulk commodity metals.
Another technology originating at Y-12
with high commercial potential involves
the use of microwave energy to diffuse
powder metals into solid parts. One example is the chromizing of steel for surface
hardening. Chrome plating is a toxic, environmentally unfriendly process, and the
Federal government is encouraging alternatives. The microwave method avoids the
use of toxic chemicals and is achieved at
atmospheric pressure and shorter times.
This technology was licensed to Tesla, an
Bridging the R&D-commercialization gap
Commercial microwave processing has been a “chicken and egg” problem. Microwave equipment and furnace manufacturers have not seen
the market, making it difficult to justify expenditure in that direction,
while materials manufacturers have not had access to microwave heating equipment and could not easily find it on the market. Ceralink is
working on both sides of this dilemma, assisting the end users and
equipment manufacturers.
A Microwave Testing Center at Ceralink's new facility at Rensselaer
Technology Park near RPI in Troy, N.Y., provides a facility where companies
can explore the feasibility of using microwaves in their processes.
Ceralink has experience with microwave heating a wide range of materials for melting, brazing, sintering, forming and calcining, as well as lower
temperature processes, such as binder burnout and drying. Commercially
available microwave equipment also is showcased at the center including the CPI Autowave, CM Furnaces' Microwave Assist Electric furnace;
Research Microwave Systems' Thermwave; and the Milestone Ethos Plus
(Fig. B). Other equipment can be obtained, and assistance in equipment
set-up and testing is available. Ceralink also assists companies with
design, construction, and purchase of microwave systems.
Ceralink recently signed an agreement with C-Tech Innovations (UK),
formerly part of EA Technologies, to transfer microwave-assist hybrid
technology to furnace manufacturers in North America. Microwaveassist furnaces are in commercial use in a specialized ceramics application and in the mining industry. C-Tech and Ceralink are working to
design retrofits that enable standard and specialized gas and electric
furnaces to accept microwaves.
Ceralink focuses on commercialization, which means they collaborate with all the involved parties from end users to equipment manufacturers, to research institutions. Ceralink acts as a central contact
point, benefiting all players and facilitating commercialization. For
example, Ceralink was instrumental in securing $5 million in funding
from DOE for Engelhard Corp. (Iselin, N.J.; www.engelhard.com) by finding an appropriate university partner to develop a microwave process
for reclaiming precious metal catalysts from fuel cells. Another example,
is a company that is pursuing the use of microwaves for a metal melting
process. After demonstrating feasibility at the Microwave Testing
Center, it was suggested to use high power 915 MHz equipment, located at Thermex Thermatron
(Louisville, Ky.; www.thermex-thermatron.com).
Ceralink engineers built a
scale-up furnace that fit an
applicator at the Thermex
Thermatron facility and
ran tests on site. The tests
provided necessary data
for full scale up, including
energy and power require- Fig. B. Microwave-related research is
ments and evaluation of conducted at Ceralink's Microwave
furnace materials.
Testing Center
IndustrialHeating.com – January 2005 49
S p e c i a l
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MICROWAVE HEATING
Australian based company, who is currently looking for commercialization partners.
Commercializing microwave heating
equipment
Table 1 gives an overview of equipment
manufacturers. Many companies that specialize in microwave power will build
microwave heating systems to customer
specifications, and some will assist with
design and processing issues.
Dennis Tool and Dana Corp both found
the need to develop microwave systems to
achieve their goals of improved parts and
productivity. Dennis Tool has become a
microwave furnace manufacturer, while
Dana Corp will license technology to a
manufacturing and marketing partner. In
production circumstances, the design of
microwave equipment must be stimulated
by the end users, and equipment manufacturers need to be ready to understand and
respond to this new market. On the other
hand, equipment manufacturers can
encourage this new microwave market by
making standard research and production
systems available.
Doug Parent, Marketing Director of CPI
(Communications and Power Industries,
Palo Alto, Calif.; www.cpii.com), formerly
part of Varian, recognized the need for versatile, microwave systems for process development and scale up. CPI manufacturers
the Autowave, a system that uses high
power magnetrons, a gas handling system,
and PC workstation with Labview (Fig. 7)
The Autowave can be fitted with a magnetron, klystron (18 GHz) or gyrotron (28
GHz, known as the Heatwave). Autowaves
come in two chamber sizes designed for
optimum field uniformity. The system can
be used for research, scale-up, and/or production, but it is relatively expensive for
initial feasibility studies. Microwave-transparent refractory containers or caskets are
used inside the chamber. This arrangement
increases the versatility and cuts down on
power requirements for small-scale tests.
In the past few years, Autowaves have
been sold to universities and industrial companies mainly in Europe and the U.S. At its
Microwave Testing Center, Ceralink has
been testing the Autowave since January
2002. Ceralink's president, Dr. Shulman, is
enthusiastic about the unit's reliability and
wide range of uses, which includes sintering
metals and ceramics, bending and melting
glass, brazing, chemical reactions, calcination and a many other projects including
nanoceramic fabrication. Shulman says the
Autowave is easy to operate and lends itself
to various fixture designs. CPI cannot discuss specifics, but they have also supplied
microwave power for drying refractories in
the steel industry, heating chemical reactors
and plasma processing.
Research Microwave Systems (RMS) is
addressing a different need by providing
microwave accessories such as susceptors,
thermal packages and crucibles, as well as
the Thermwave, a low-cost microwave.
Table 1 Microwave Equipment Manufacturers
Microwave Furnace
Manufacturers
Specialty
Web Address
Advanced Manufacturing
Technology
Industrial microwave heating
consultants
www.amtmicrowave.com
www.cem.com
CEM Corporation
Microwave digestion systems
CM Furnaces
Microwave + electric hybrid furnaces www.cmfurnaces.com
Cober-Muegge
Microwave vulcanization, equipment
www.cobermuegge.com
Communications and Power
Industries
Microwave heating systems and
equipment
www.cpii.com
C-Tech Innovations/ Ceralink
Design and build microwave hybrid
furnaces and other mw furnaces
www.ceralink.com
Dennis Tool
Continuous Microwave Sintering
Furnace
Ferrite Components, Inc.
microwave tempering and cooking
systems
Gerling Applied Engineering
Microwave systems and equipment
www.2450mhz.com
Harper International
Microwave Rotary Calciner
www.harperintl.com
Harrop Industries
Microwave gas hybrid furnace
www.harropusa.com
Industrial Microwave Systems,
Inc. (US)
Microwave systems continuous
planar, cylindrical
www.industrialmicrowave.com
Industrial Microwave Systems,
Inc. (UK)
Continuous Microwave Furnaces
and equipment
www.industrial-microwave-systems.com
Linn High Therm GmbH
Dual frequency microwave furnace
www.linn.de
Manitou Systems Inc.
Microwave and RF plasma systems
www.manitousys.com
Microdry Corp.
Industrial microwave systems
www.microdry.com
Milestone S.R.L.
Microwave systems digestion and
analysis
www.milestonesci.com
Mino Yogyo Co. Ltd.
Microwave hybrid furnaces
www.mino-ceramic.co.jp
O-I-Corporation
Microwave digestion systems
www.oico.com
Panasonic
Microwave pottery kiln
Personal Chemistry
Microwave system organic
synthesis, biochemistry
www.personalchemistry.com
PSC
RF heating systems and microwave
equipment
www.pscrfheat.com
Puschner-Microwave Power
Systems
Microwave furnace, dryers,
www.pueschner.com
Radatherm Pty Ltd
Microwave sintering furnaces
www.radatherm.com.au
Laboratory microwave systems,
Research Microwave Systems, LLC
accessories
Fig. 7 Autowave microwave system that
uses high-power magnetrons, a gas handling system and PC workstation
50 January 2005 – IndustrialHeating.com
www.ferriteinc.com
www.thermwave.com
Takasago Industry Co
Microwave batch and microwave
elevator kiln
Thermex-Thermatron, Inc.
High power, high frequency MW and
www.thermex-thermatron.com
RF heaters
www.takasago-inc.co.jp
Fig. 9 Microwave-assist gas kiln (left) and hybrid microwave roller hearth (right)
from Takasago Industry Co. in Japan
Fig. 8 Furnace made by Linn High Therm for microwave
drying of ceramics
The Thermwave can be used for many
types of materials, including organic and
inorganics, in processes such as drying,
chemical reactions, annealing and firing.
The Thermwave works with an inexpensive controller and shielded thermocouple.
RMS is currently developing a unit with
more microwave power and is expanding
its line of accessories to include suscepting
crucibles and low-cost insulation packages.
The greatest pull for these systems has been
from research institutes in Europe, Korea,
Malaysia and India.
Bernie Krieger of Cober Electronics Inc.
(Norwalk, Conn.) developed a commercial
microwave process based on the needs of an
external end user. Cober specializes in
microwave equipment and has been exploring the market for microwave heating for
more than 20 years. Cober hit on a winning
combination for vulcanizing rubber, which
uses microwaves and hot air. Krieger had to
understand the needs of his customer, and
overcome the urge to focus only on selling
microwave equipment. The need was for a
complete functioning system to vulcanize
rubber with better quality and in a shorter
time, eliminating messy and costly steps,
and have a good payback on the investment. Today, Cober's system has changed
the face of the rubber vulcanizing industry.
Other microwave equipment manufacturers also work with their customers to
design and build systems for heating.
Fig. 10 Hybrid microwave gas kiln (left) and pure microwave continuous kiln
(right) from Mino Yogyo Co. in Japan
Thermex Thermatron (Louisville, Ky.)
builds sophisticated high power microwave
equipment, and have become adept at testing its customers products using equipment
in house. The company is an important
source of industrial 915-MHz generators.
Thermex Thermatron is proud of its engineering and stands behind its products, of
real significance when setting up 100-kW
power units. Ferrite Inc. (Hudson, N.H.)
also supplies high-power systems, but has
traditionally focused on the food industry.
Now the company is looking toward processing other materials at higher temperature. CPI is probably the most sophisticated
in precision engineering, which is what it
takes to produce a traveling wave tube like
a gyrotron. CPI works with Ceralink and
other consultants, effectively combining
materials, heating and microwave expertise
to address its customers' needs. Gerling
Applied Engineering Inc. (GAE, Modesto,
Calif.) is also highly reliable in the support
of its microwave heating products.
In Germany, Linn High Therm GmbH
(Eschenfelden) manufactures both laboratory and industrial microwave heating
equipment including high temperature systems. Linn High Therm has a unique combination of furnace expertise, knowledge of
microwave systems and background in
material science. Mr. Malte Moeller states
that the new high-temperature processes
are one bright future for microwave heat-
ing, but by far not the only one. Almost
daily the company gets inquiries about
microwave drying or heating applications
that nobody has ever thought about before.
Although many of these processes cannot
be realized due to economic or technological difficulties, this shows that microwave
low-temperature processes are by far not at
the end of their development potential. In
fact, microwave drying is rapidly being
incorporated in to ceramic processes,
including filters, substrates, honeycomb
structures, industrial insulators, thermal
insulation and whitewares (Fig. 8).
Japan has taken a strong position on the
commercialization of microwave furnaces.
Takasago Industry Co. Ltd. (Toki-city,
Japan), a large kiln manufacturer is now
manufacturing and selling microwave assist
kilns in Japan. This is a patented C-Tech
Innovation technology. Examples are
given in Fig. 9. Mino Yogyo Co. Ltd.
(Mizunami, Japan), one of Takasago's competitors, is also offering microwave furnaces (Fig. 10). Both companies have built
large scale production kilns, but it is not
known how many have been sold or exactly which industries they are targeting.
They have demonstrated firing whitewares
and large alumina parts (Fig. 11 and 12),
but it is likely they are also looking to other
high-technology industries.
Furnace manufacturers in the U.S. have
been slow in the uptake, but are beginning
IndustrialHeating.com – January 2005 51
S p e c i a l
F o c u s :
MICROWAVE HEATING
D=600 mm
t=30 mm
Processing time:
24 hours (70˚C/H)
Energy comsumption
484 kWh/batch
Energy cost
$65/batch
(1/6 to gas kiln)
Fig. 11 Microwave-fired tableware
Fig. 12 Large alumina parts produced using
microwave processing
Fig. 13 Hybrid microwave-assist electric furnace developed by CM Furnaces, Ceralink
and C-Tech
to take an interest in microwave systems.
CM Furnaces worked with Ceralink and CTech to build a hybrid microwave-assist
electric furnace (Fig. 13). The basic furnace
is one of its standard products in the 1700
laboratory series, which was retrofitted with
a GAE 2-kW, 2.45-GHz generator. This
type of furnace and other microwave-assist
laboratory furnaces will be commercially
available soon. Microwave-assist gas kilns
will be available through Harrop Industries
(Columbus, Ohio), also using technology
from C-Tech Innovations made available
by Ceralink. Any North American furnace
company can work with Ceralink to develop its microwave assist product lines for a
design fee and a small royalty on sales.
Harper International (Lancaster, N.Y.) has
teamed up with microwave company Fricke
and Malle in Germany to bring microwave
expertise to their furnace manufacturing
capabilities. Harper has also licensed
microwave rotary calcine technology and is
making this commercially available.
The use of large microwave furnaces will
probably have growing pains. The end user
may explore the willingness of the furnace
manufacturer to share some of this development cost. The furnace manufacturers
will need microwave experts on site to be
most effective in building microwave furnaces. It is a good idea to find out the qualifications of the team designing and building the system. Hiring a knowledgeable
consultant to understand the manufacturer's microwave furnace capabilities, could
also save time and money.
Microwave heating experts
The recently held Fourth World Congress
on Microwave and Radio Frequency
Applications (7-12 Nov. 2004) in Austin,
Tex., brought together the world's leading
experts in microwave technology. The
next major event where one can meet this
cast of researchers will be at the Ampere
conference in Modena, Italy 13-15 Sept.
2005. Many experts can be found in the
commercial arena (some of these persons
have already been discussed), and many
other technical experts can be found at
research institutions.
An important group resides at Bayreuth
University (Bayreuth, Germany; www.unibayreuth.de) led by Dr. Monica WillertPorada. This group works closely with
industry and has developed an interesting
approach to support commercialization.
Dr. Willert Porada founded in 1997, a nonprofit organization, InVerTec, housed at
the Center for Excellence of New
Materials at Bayreuth. InVerTec's mission
is to impart knowledge, coordinate and
transact R&D initiatives in the field of
combined electrothermal processes and
assist with scale-up on the basis of their
own research work.
The current focus of their project work
includes using microwaves for roasting of
ores, heat treatment of glass, detoxification
of filter dusts and slags for the metal industry and decontamination of asbestos containing wastes. According to a spokesperson for InVerTec: “At our pilot plant stations, most reactor and oven types can be
implemented in microwave heating and
combined heating procedures. In order to
prove planning reliability for scale-up, we
offer the possibility to do experiments at
the microwave frequency of 915 MHz and
the common microwave frequency of 2.45
GHz.” InVerTec's equipment includes a
microwave rotary kiln, microwave fluidized
bed reactor (Fig. 14), microwave with inert
gas, and a 915-MHz single-mode reactor.
Another highly visible group is located at
Pennsylvania State University (University
Park, Pa.; www.psu.edu), in the Materials
Research Institute. Led by Drs. Dinesh
Agrawal and Rustum Roy, this group has
been a pioneer in the field of microwave
process research since 1984. In the 1980s,
their focus was on sintering and synthesizing
ceramics, such as alumina, zirconia, zinc
oxide, hydroxyapatitie, zeolites, mullite and
silica. In the 1990s the focus shifted to
developing microwave processes for electroceramics such PZT barium titanate, relaxors, and transparent ceramics.
Innovators at Penn State, like Rustum
Roy, have never been restrained by conventionality, so when the idea of
microwave sintering powder metals
occurred, they proceeded with experiments
and found this a highly effective method.
Work in the area of microwave sintering
carbides caught the attention of Mahlon
Dennis and is now fully commercialized at
Dennis Tool Co.
Penn State recently hosted a meeting
with over 40 companies to launch the
Microwave Powder Processing Consortium.
52 January 2005 – IndustrialHeating.com
Processed offgas
Offgas treatment tube (refractory)
Suscepting material
Upper treatment chamber
Refractory material
Combustion exhaust tube (metal)
Lower treatment chamber and
Metals collection system
Fig. 14 Microwave fluidized-bed reactor at
InVerTec at the Center for Excellence at
Universität Bayreuth, Bayreuth, Germany
Suscepter
Refractory material
The theme was to organize a consortium
that would use Penn State's equipment
and knowledge base to develop mutually
relevant microwave technology at a
shared cost. Matthew Smith, from Penn
State's Intellectual Property Office, also
discussed the technology transfer and
licensing of patents involving microwave
processing from their pool of developed
intellectual property.
Since moving from the University of
Florida in 2001, Dr. David Clark and
research faculty member Diane Folz have
built the Microwave Processing Research
Facility at Virgina Polytechnic Institute's
Department of Materials Science and
Engineering (Blacksburg, Va.; www.vt.edu).
The laboratory is equipped to perform
research using microwave energy ranging
from 2-18 GHz with power levels from 200
W to 6.4 kW. Current microwave research
focuses on waste remediation and recycling, nanomaterial synthesis, formation of
glass-ceramics, sterilization, and medical
treatment technologies.
Diane Folz explains that one of the most
interesting uses for microwaves is selective
heating of specific components within a
structure. It was this characteristic that led
the group to investigate microwave recycling and waste remediation. Graduate student Rebecca Schulz, in work funded by
Westinghouse Savannah River Co., demonstrated a microwave process for recycling
Crucible
Incoming waste stream
Flow meter
Remotely located electronics
and personnel
Forced air flow
Fig. 15 Schematic of microwave hardware used to recycle precious metals from electronic
circuitry developed at Virginia Polytechnic Institute
precious metals from electronic circuitry,
while also treating the off gases that resulted in the combustion process. The work
resulted in several patents on the process
and design of microwave hardware. A
schematic of the system is shown in Fig. 15.
Dr. Jim Hwang is leading an effort to
commercialize a technology that his group
developed at Michigan Technological
University (Houghton, Mich.; www.mtu.
edu). Microwaves are used to assist steelmaking in an electric arc furnace (EAF).
The viability of the technology lies in the
fact that iron ore and carbon are excellent
microwave absorbers. Prototype equipment
for the new technology is located in
Hwang's lab and consists of modified electric arc furnace with an auxiliary
microwave heating system. A charge of
iron oxide, coal and limestone is loaded
into the chamber and microwave energy is
introduced. The charge absorbs microwave
energy to the point of coal ignition. The
exothermic reaction of carbon oxidation
further increases the temperature. The EAF
electrodes then descend to provide electric
arcing energy, producing molten steel and
slag. Design modifications to the chamber
will allow continuous mode operation. IH
Additional related information may be found
by seacching for these (and other) key
words/terms via BNP Media LINX at
www.industrialheating.com: microwave(s),
microwave energy, microwave heating,
microwave sintering, carbide sintering,
microwave-sintered carbide, microwave
brazing, microwave field, susceptor, suscepting crucible, microwave metal melting, magnetron, microwave-assist electric furnace,
microwave-assist gas furnace, synthesizing
powders, SHS, klystron, gyrotron, microwave
field, microwave frequency.
IndustrialHeating.com – January 2005 53