Hot stuff
microwave heating
oday we all know the domestic
microwave which enables us to
re-heat a cup of coffee or make
porridge without using a saucepan, but
that relatively simple product took more
than two decades to develop and become
widely available. Commercial uses of
microwave technology also developed
slowly and initially were restricted to simple
heating and drying applications, either at
laboratory scale or in a production system.
Food, paper, textiles, wood, rubber,
chemicals, semi-conductors and ceramics
– typically non-metals with poor thermal
conductivity – are among the materials
that are now commonly processed by
microwave heating equipment. But other
materials, including metals, are continuously
joining the list, and the temperatures and
complexity of the processes are also
steadily increasing.
T
Slowly, slowly
One of the reasons for the initial slow
development of microwave technology
1
for industrial applications was perhaps
the incomplete understanding of the
mechanisms involved. Dr Percy
Spencer of the Raytheon Corporation
found that his chocolate bar melted
when he was working with a magnetron,
which is still the predominant mechanism
for generating microwaves, but for many
years the way microwaves acted on a
given material was not understood.
Even now, research is continuing into
some of the features, and new potential
benefits associated with this technology are still becoming apparent.
Microwave heating is actually a form of
dielectric heating, another being radio
frequency heating. A magnetron is an
oscillator capable of converting electric
power, usually in the form of high-voltage
DC current, into high-frequency radiant
energy. The polarity of the emitted
radiation changes between negative and
positive at high frequencies, and the
material within the radiation field heats
up through ‘molecular friction’ as the
2
dipoles within it try to re-orientate
themselves. By international agreement,
certain microwave frequencies are
reserved for industrial, scientific and
medical applications, each having a
specific wavelength.
The standard frequency used in
domestic microwave ovens is 2,450MHz,
with the magnetrons producing
typically 800W or so at maximum
power. This frequency is also used for
industrial systems with power ratings
commonly up to 20kW and occasionally
higher. Larger industrial heating
systems use 896MHz or 915MHz
magnetrons, although there is some
overlap of the power ratings of
magnetrons at these two frequencies.
Wave-guides transfer the generated
energy from the magnetron to the
processing chamber, where a device
known as a mode stirrer may be
used in order to improve energy
distribution, depending on the cavity
design.
3
4
Images 1 and 2: A sample of ceramic material with cracks frequently found after sintering in conventional furnaces with radiant heating. Images 3
and 4: A sample of sintered ceramic material that is virtually defect-free. This is the result of the reduced stress produced by the even temperatures
of microwave-assisted heating (MAT)
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Materials World June 2006
Discovered accidentally by Dr Percy Spencer of the Raytheon
Corporation when a chocolate bar melted in his pocket while
working on a new device for radar applications, microwave
heating has been around since the Second World War. Given the
length of time and the speed with which many new technologies
have been applied, industrial microwave heating could be
considered a somewhat slow burner. Mark Pickering reports
On target
Microwave heating has some very
particular characteristics and is quite
different from conventional radiant
heating. Firstly, it is volumetric – that is,
energy is generated directly within the
body of the material itself instead of the
interior gradually heating up through
conduction from the external surface, as
occurs with radiant heating. Some
materials are more susceptible than
others to microwave energy, so preferential heating may take place, which can
provide process advantages. Volumetric
heating can also result in energy being
used very efficiently, as only the target
material is heated.
Secondly, in many materials heating is
almost instantaneous and takes place
without the need for radiating elements
to heat the air or any container.
And thirdly, heating is highly specific,
with different materials displaying
different susceptibilities to microwave
energy. As we know from our kitchen
microwave, water usually heats relatively
quickly, while other materials – some
plastics, for example – heat very slowly.
This differential can be used to advantage
in microwave processing, for example,
pharmaceuticals can be sterilised in their
packaging without the plastic heating up.
Wet areas of a product will take up heat
more than dry areas, so moisture content
will equalise.
However, the optimum frequency for
any given material may not be constant
over the entire temperature range
encountered during heating, therefore,
it is very important to match the system
and experimental process design to the
material.
Materials World June 2006
Conventional
MAT
Temp
Time
Compared with conventional radiant heating (yellow curve), microwave-assisted heating (red curve)
produces more even temperatures and a more regular structure in the material. It also completes
the sintering process in a shorter time.
The advantages of microwave heating
can be summarised as –
■ Energy-efficiency, because power is
only applied to the material.
■ Higher quality by avoiding case-hardening
and other surface damage.
■ Selective heating – giving processing
benefits in some cases.
■ Direct heating of the sample body,
which reduces process times.
Losing heat
Despite the advantages offered by
microwave heating, it can sometimes,
when applied in isolation, be less
successful at higher temperatures, such
as those required for firing or sintering
ceramics. This is because once a sample
heats up, it will generally be at a higher
temperature than the surrounding
atmosphere, and heat can be lost from
the material’s surface. This in turn can
create temperature gradients within the
material, albeit the reverse of those associated with radiant heating, and the
gradients increase as the component
becomes hotter. This limiting factor can
be particularly significant for materials
requiring high structural integrity.
Looking radiant
Various ways of overcoming the
temperature profile problem have been
investigated, the most successful being to
apply a combination of radiant and
microwave heating to materials, especially
those that need to be processed at
temperatures above 800°C.
C-Tech Innovation Ltd, based near
Chester, UK, has been at the forefront of
microwave-assisted heating technology
(MAT), in which microwaves provide an
additional heating mechanism in support
of conventional gas or electric radiant
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heating. With the MAT technology, the
microwaves provide a thermal equalising
effect, while the radiant heating retains
the controllability essential for many
advanced materials. This approach is now
being used successfully for batch and
continuous processes and at laboratory
and production scales.
This combined technique has significant
advantages over both radiant-only and
microwave-only systems – more consistent
product properties, greater strength,
improved yield, reduced formation of
undesirable phases and lower quantities
of harmful emissions.
Key
Temperature profile of the
chamber
Centre of the sample in a
traditional radiation furnace
T
t
Centre of the sample in a
MAT furnace
Temp (T)
T
Time (t)
Technology transferred
The specific process developed by C-Tech
Innovation was patented by the company.
Carbolite, a UK furnace manufacturer
based in Derbyshire, UK, has now concluded
a technology transfer and licence agreement with C-Tech to manufacture and sell
equipment with MAT heating technology
in Europe. The first models are laboratoryscale chamber furnaces with maximum
temperatures between 1,200°C and
1,600°C. Molybdenum disilicide elements
are used in these furnaces in order to
avoid the microwave uptake that would
occur with the more common silicone
carbide elements.
James Roper, who is leading Carbolite’s
MAT product development programme,
expects the laboratory-scale equipment to
lead to production-scale units as processes
are developed and validated by research
programmes. He has identified a number
Change in temperature
across the sample
The graph shows that with microwave-assisted heating the temperatures on the surface and in the
centre of a sample are very similar
of applications where MAT heating could
speed up processing times or produce
more consistent results, including precious
metals assaying and burning off wax
moulds for foundry castings.
Development work has also revealed
that MAT heating has a beneficial effect
on the properties and performance of
some materials – sintering highperformance ceramics such as zirconia
in a MAT furnace produces a more
consistent grain size. This is particularly
important for semi-conductor applications
and nano-materials. MAT can also give
better control of hardness, toughness and
translucency than conventional radiant
heating.
According
to
Roper,
another
advantage of MAT heating is the ability
to scale up easily from laboratory to
production capacities. ‘It is very difficult
to scale up microwave-only systems
because of the problem of maintaining
high power densities over a large
area, but scaling-up is relatively straightforward with the MAT heating system’,
he explains.
Microwave heating may have taken
some time to find full acceptance in the
commercial sector, but Carbolite
believes that developments such as MAT
heating open a whole new spectrum of
applications that could make it as widely
used as conventional radiant heating.
HT
power
supply
Control
Further information
Carbolite
Ltd,
Parsons
Lane,
Hope,
Hope Valley S33 6RB. Tel: +44 (0)1433
Time
Heat
620011.
E-mail:
+44(0)1433
621198.
[email protected].
Fax:
Website:
www.carbolite.com.
Schematic representation of a Carbolite MAT furnace with radiant and microwave heating
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Materials World June 2006