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Percy Spencer, Ph.D., of Raytheon Corp. famously discovered microwave heating by accident when a chocolate bar mysteriously melted in his pocket while he was working on a new device for radar applications during World War II. Given the 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, along with radio frequency (RF) heating. Today, we're all familiar with the domestic microwave, which enables us to heat a cup of tea or any number of food products. However, even that relatively simple product took more than two decades to develop and become widely available.
Commercial uses of microwave technology have also developed slowly, and they were initially restricted to simple heating and drying applications, either on a laboratory scale or in a production system. Food, paper, textiles, wood, rubber, chemicals, semiconductors and ceramics-typically non-metals with poor thermal conductivity-are among the materials that are now commonly processed with microwave heating equipment. Additional 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. Spencer's chocolate bar melted when he was working with a magnetron, which is still the predominant mechanism for generating microwaves. For many years, the way microwaves interacted with a given material was not understood, and even now, research is continuing and new potential benefits associated with this technology are still being discovered.
Unique CharacteristicsMicrowave heating has some very particular characteristics and is quite different in many ways from conventional radiant heating. Radiant heating requires heat conduction from the external surface of the piece to gradually heat up the interior. Microwave heating is volumetric, however, which means that energy is generated directly within the body of the material. Microwave heating is said to develop an "inverse temperature profile" in the heated part because it is the inverse of the profile generated by radiant heating. Volumetric microwave heating can result in very efficient energy use, as the part is heated preferentially over the insulation and kiln furniture.
Second, heating is almost instantaneous in some materials and can take place without the need for radiating elements. 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-such as some plastics-heat very slowly. This preferential heating can be used to advantage in microwave processing. For example, pharmaceuticals can be sterilized in their packaging without the plastic heating up. In addition, wet areas of a product will absorb heat more than dry areas, so moisture content can be equalized.
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 applied directly to the material
- Higher-quality results, through the avoidance of case-hardening and other surface damage
- Selective heating, providing processing benefits in some cases
- Direct heating of the sample body, reducing process times
Despite the advantages, several barriers to the implementation of microwave heating for sintering ceramics remain, based on the material properties and inverse temperature profile. At the high temperatures required for the sintering of ceramics, the ceramic part acts like a heating element, trying to heat the surrounding atmosphere while cooling from the surface. Once the 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, which is 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. The problem can be mitigated somewhat by minimizing the free volume within the kiln.
Another issue is that many ceramics do not heat well from room temperature and only start to suscept (heat in the microwave field) when they are already warm. Susceptors, such as silicon carbide, can be used to get the process started by radiation until the ceramic suscepts well and preferentially absorbs the microwave energy directly. Susceptors can be inconvenient to scale up and the radiant heat cannot be directly controlled.
A Hybrid SolutionVarious methods of overcoming the temperature profile problem have been investigated, the most successful being the application of a combination of radiant and microwave heating to materials, especially those that need to be processed at temperatures above 800°C. In microwave-assisted heating technology (MAT), microwaves provide an additional heating mechanism in support of conventional gas or electric radiant heating. With MAT technology, while microwaves provide a thermal equalizing effect, radiant heating maintains the control essential for many advanced materials.
The MAT approach is being used successfully for batch and continuous processes in laboratory and production settings, and it has been found to provide significant advantages over both radiant-only and microwave-only systems (see Figure 1). 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.
*Process developed and patented by C-Tech Innovation Ltd., Capenhurst, UK. Carbolite has concluded a technology transfer and license agreement with C-Tech to manufacture and sell equipment with the MAT heating technology in Europe. Ceralink Inc. has an exclusive license for MAT in North America and will open the North American territory to Carbolite through a sublicense agreement. Ceralink's Microwave Testing Center will house Carbolite MAT kilns for demonstrations and testing. Ceralink assists with feasibility and scale-up of authorized MAT kilns for ceramic manufacturers.
Benefits and OpportunitiesIt's expected that laboratory-scale equipment will lead to production-scale units as processes are developed and validated by research programs. MAT heating can be used in a variety of applications-including advanced ceramic sintering, precious metals assaying and burning off wax molds for foundry castings-to speed up processing times or produce more consistent results.
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 semiconductor applications and nanomaterials. MAT can also provide 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. 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 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.
For more information regarding MAT heating, contact Carbolite at 110 South Second St., Watertown, WI 53094; (920) 262-0240; fax (920) 262-0255; e-mail email@example.com ; or visit http://www.carbolite.com . Ceralink can be reached at 105 Jordan Rd., Troy, NY 12180; (518) 283-7733; fax (518) 283-9134; e-mail firstname.lastname@example.org ; or visit http://www.ceralink.com .
SIDEBAR: From Power to EnergyA magnetron is an oscillator capable of converting electric power, usually in the form of high-voltage DC current, into high-frequency microwave energy. The polarity of the emitted radiation changes between negative and positive at high frequencies, and material within the microwave field heats up through "molecular friction" as the dipoles within it try to reorient 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 to improve energy distribution, depending on the cavity design.