From domestic hero to industrial stronghold

The saviour of many a hungry student and harassed parent, the microwave has been a domestic smash hit, but industrially it has been a slow burner. Now with new developments, this kitchen mainstay is set to hit the big time

Famously discovered accidentally by Dr Percy Spencer of the Raytheon Corporation when a chocolate bar mysteriously melted in his pocket while working on a new device for radar applications, microwave heating has been around since World War II.  Given that length of time and the speed with which many new technologies have been applied during the last 60 years, industrial microwave heating could be considered a somewhat slow developer.

Microwave heating is actually a form of dielectric heating, another being radio frequency (RF) heating.  Today we all know the domestic microwave which enables us to re-heat the cup of coffee we left for too long or make porridge without using a saucepan, but even 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 – that is, typically non-metals with poor thermal conductivity -- are among the materials that are now commonly processed by microwave heating equipment. Other materials, including metals, are continuously joining the list, and the temperatures and complexity of the processes are also steadily increasing.

One of the reasons for the initial slow development of microwave technology for industrial applications was perhaps the incomplete understanding of the mechanisms involved.  Dr Percy Spencer’s 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.

A magnetron is an oscillator capable of converting electric power, usually in the form of high-voltage direct current, into high-frequency radiant energy.  The polarity of the emitted radiation changes between negative and positive at high frequencies, and material within the radiation field heats up through ‘molecular friction’ as the dipoles within it try to re-orientate themselves.  By international agreement, certain microwave frequencies are reserved for industrial, scientific and medical (ISM) applications, each having a specific wavelength.

The standard frequency used in domestic microwave ovens is 2450 MHz, with the magnetrons producing typically 800 W 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 896 MHz or 915 MHz 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.

Microwave heating has some very particular characteristics and is different in many ways from conventional radiant heating.  First, 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 and heat more readily, 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.
                                                  Some materials can crack using
conventional furnances

Second, in many materials heating is almost instantaneous and takes place without the need for radiating elements to heat the air or any container.

And third, 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.  Also, 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.

The advantages of microwave heating include energy-efficiency (because power is only applied to the material), higher quality through avoidance of case-hardening and other surface damage, selective heating and direct heating of the sample body, reducing process times.

The graph shows that with microwave-assisted heating (red line), the temperatures on the surface and in the centre of a sample are very similar









Despite the advantages offered by microwave heating, when applied in isolation, it can 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.

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, 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 heating.  With the MAT technology, while the microwaves provide a thermal equalising effect, the radiant heating retains the controllability essential for many advanced materials. This approach is now being used successfully for batch and continuous processes at laboratory and production scales.

This combined approach has been found to have 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 can all be achieved through the use of MAT.
 
The specific process developed by C-Tech Innovation was patented by the company, and Carbolite - a UK furnace manufacturer based in Derbyshire - has now concluded a technology transfer and licence agreement with C-Tech to manufacture and sell equipment with the MAT heating technology in Europe.  The first models with the combined microwave and radiant equipment are laboratory-scale chamber furnaces with maximum temperatures between 1200°C and 1600°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.

Schematic of Carbolite MAT furnace with radient
and microwave heating

He has identified a number of applications where MAT heating could speed up processing times or produce more consistent results, from precious metals assaying to 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 – for example, sintering high-performance ceramics such as zirconia in a MAT furnace has been found to produce more consistent grain size, which is particularly important for semi-conductor applications and nano-materials.   It can also give better control of hardness, toughness and translucency than conventional radiant heating.

Another advantage of MAT heating is the ability to scale up easily from laboratory to production capacities, according to Mr Roper. He said: “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”.

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 has been in the past.

By Mark Pickering