Fig. 1. A range of materials and products processed with Spheric/Syno-Therm™ microwave-processing technology


In recent years, high-temperature – above 800°C (1472°F) – microwave processing has moved from a promising research technique to a full-scale industrial-processing technology. Microwave processing at higher temperatures has shown many advantages over conventional heating in several areas:
  • Higher heating rates and faster process times result in shorter cycle times.
  • Heating is concentrated in the material – not in the walls of the furnace – permitting lower energy consumption.
  • Heating (and cooling) rates possible with microwave can help create finer, more uniform microstructures that may yield products that are stronger and less likely to need other finishing processes.
  • Direct microwave interaction with some materials could create new reaction pathways and processes not possible using other heating methods.
Research shows that high-temperature microwave processes may use 20% of the energy and take less than 20% of the time as compared with conventional gas or electric processes for similar products.

Microwave Processing of Materials

High-temperature microwave heating has been widely used by several research groups worldwide in sintering of powdered metals and ceramics. The use of microwaves in ceramic sintering at temperatures ranging up to 1850°C (3360°F) has been very successful in the U.S. and internationally, with excellent results in a wide variety of materials, including:
  • Technical ceramics
  • Transparent ceramics
  • Ferrites
  • Consumer ceramics
Microwave processing has already proven to be a viable alternative for the production of tungsten carbide (WC) cutting and wear parts. Collaboration between Dennis Tool Co. of Houston, Texas, and Penn State has resulted in an industrial process that fabricates high-quality WC parts showing significant strength and durability increases over similar parts produced by competitors through conventional methods. Further enhancements by Dennis Tool have refined the process to achieve significant production efficiencies.

High-temperature microwave heating has also been developed into an effective tool for sintering powder metals. Solid metals generally reflect microwave radiation. When metal powders are pressed into compact shapes, they can be effectively sintered. Microwave sintering has been demonstrated successfully with a wide variety of metals and alloys, including many ferrous alloys, tungsten alloys and other metals at temperatures up to 1700°C (3090°F).

High-temperature microwave processing has also been used with great success in the synthesis of various compounds, including vanadium nitride used extensively in the steel industry. The fast heating rate and the microwave effect on the materials being synthesized drive the reactions much faster and use significantly less energy than other synthesis processes. This leads to a lower cost of production. The running time of the synthesis process has been reduced from as long as 30 hours to as short as five hours for this and similar processes.

Microwave high-temperature processes are being investigated for a wide variety of additional applications such as:
  • Joining and brazing
  • Melting of bulk metals
  • Diffusion coating and surface modification
  • Carbon-fiber synthesis
  • Processing in single-mode fields
This exploration of additional microwave processes is being carried out by noted research institutions such as the Microwave Processing and Engineering Center at Penn State, Oak Ridge National Laboratory and noted international microwave research units in Europe and Japan.

Fig. 2. A typical Spheric/Syno-Therm™ microwave furnace

Microwave-Processing Equipment

One of the significant steps in moving microwave-processing technology from the laboratory bench to the production floor has been the development of suitable processing equipment that is effective, durable and affordable. As a starting point, many research institutions have built their own laboratory-scale processing units from existing components. These systems are suitable for small-scale experimentation.

Much effort has subsequently been put into the development of commercially built research and pilot units that are now widely available. These units do not generally have processing volumes suitable for commercial production, but they have sufficient capacity to conduct serious process-development tests. They also have all the features expected in large-scale production equipment, including vacuum options, atmosphere options, programmable power supplies and advanced cooling. These “laboratory” furnaces enable the materials scientist or metallurgist who does not have access to advanced electrical engineering and fabrication facilities to work with microwave processing.

The next step in the development of industrial microwave-processing equipment is obviously the production of large-scale commercially viable furnaces. These furnaces will be characterized as being either “batch” or “continuous” and like conventional furnaces will be at least partially customized to individual users’ requirements. Another variable in the development of this equipment is the use of so-called “hybrid” systems, which are a combination of conventional heating and microwave heating. These hybrid systems are being developed on a parallel with “pure” microwave systems and offer many of the advantages of microwave-only systems.

Fig. 3. Spheric/Syno-Therm™ horizontal continuous microwave-processing furnace

Batch microwave systems, primarily for sintering of ceramics whose thermal characteristics require moderate heating or cooling cycles or for other extremely long cycle processes, are already in use in many countries for a variety of tasks. These batch microwave furnaces have been built with load areas of up to 2 cubic meters and have temperature ranges of up to 1700°C (3090°F). These furnaces have been used successfully for ceramic products and other processes. Their “closed” nature makes them more suitable for high-vacuum use as well. Development of even larger batch furnaces is being undertaken in the People’s Republic of China and Japan.

Continuous high-temperature microwave systems have also been developed and are now available commercially. These systems, as their name suggests, operate by moving product through the microwave-heating zone on metal or ceramic belts or on a ceramic-boat “pusher” system depending upon the required maximum temperature. These continuous systems are similar to conventional systems in that thorough knowledge of the material composition of parts being processed is necessary for optimal results. This knowledge is used to determine:
  • Appropriate cycle time
  • Appropriate atmosphere
  • Power requirements and heating curve
  • Belt loading
  • Process fine tuning
The capacity of these continuous systems varies much as conventional systems do. The orientation may be either horizontal or vertical, as shown for the Spheric/Syno-Therm™ furnaces in Figs. 3 and 4. The capacity of the horizontal furnace is dependent upon material to be processed and has a belt speed of up to 60 mm/minute. The capacity of the vertical furnace is also dependent on the material being processed. The authors are familiar with a horizontal system being constructed for an Asian company with a processing area 1 meter square by almost 40 meters long for the sintering of consumer porcelain goods.

Fig. 4. Spheric/Syno-Therm™ vertical continuous microwave-processing furnace

Continuous microwave systems typically offer advantages in cycle time and energy consumption based on the fact that heating is concentrated in the material being processed – not on the entire furnace and contents.

Certain aspects of microwave furnaces have been problematic in development of commercial equipment, but these have now been overcome, creating the opportunity for wider use of continuous microwave systems. These problems have included the use of multiple magnetrons interacting with each other, insulation for maximum energy efficiency and processes that include different sizes of green parts in the same load. Such issues have been resolved through equipment advances and process modification. As high-temperature microwave processing is a relatively new technology, there will undoubtedly be more issues to overcome in new application areas. The speed of innovation and level of interest in this technology, however, assure that they will be overcome quickly.

Further development work in high-temperature microwave processing will focus on combined delubing or debinding and sintering systems, additional materials and process refinement.

Summary

High-temperature microwave-processing technology has, indeed, moved from the laboratory to the factory floor. With reliable, effective and affordable commercial-scale equipment now available, industrial users can integrate knowledge from research institutions with high-quality microwave equipment to create time and energy savings in their processes. The next step leads to looking at new materials, new components and new reaction pathways that microwave processing has opened up.

The current development of microwave processes and equipment in the thermal-process industries is like the introduction of electric heat to these industries in the 1930s – it will be a cleaner, faster, lower cost, more efficient production method that has not yet reached its full potential. IH

For More Information: Kuruvilla Cherian, Ph.D., is director of applied research, Spheric Technologies Inc., 4708 East Van Buren,Phoenix, AZ 85008; tel: 602-218-9292; e-mail: kcherian@spherictech.com; web: www.spherictech.com. Jiping Cheng, Ph.D., is senior research associate, Materials Research Institute, Penn State, University Park, Pa 16802; tel: 814-865-4571; e-mail: jxc44@psu.edu; web: www.mri.psu.edu/

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: microwave heating, hybrid microwave system, magnetron, sintering