Ride the Wave

September 1, 2008
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The high-temperature microwave processing of materials is becoming an industrial reality with the availability of commercial microwave furnaces and the successful efforts to develop process know-how by several advanced technology development groups worldwide. The impetus for this development is the realization that high-temperature microwave processing can be a faster, greener and cheaper alternative to conventional electric- and gas-based heating technologies. Microwave processing is often preferred over conventional methods due to substantial economic advantages and comparable or better properties of the finished product.

Historically in the U.S., low-temperature microwave processing has been used extensively in applications such as food and wood processing and drying. However, to date there are few domestic high-temperature commercial microwave applications. A primary challenge has been the lack of sophisticated high-temperature microwave furnaces.

This obstacle is being overcome through the availability of new high-temperature automated microwave furnaces, including the latest continuous microwave pusher system.* With a footprint in the range of  2 x 6 to 2 x 18 m, the system’s maximum output power is 9-36 kW. The maximum temperature is 1500ºC, and processing atmospheres can be air, nitrogen, inert gases and mixtures. This industrial microwave system offers advanced industrial direct energy transfer technology and high efficiency that enables ceramic and metal parts to be processed at a fraction of the time and cost of conventional kilns.

*SPHERIC/SYNO-THERMTM computer-controlled microwave furnace with the AMPS pusher system, marketed in the U.S. by Spheric Technologies, Phoenix, Ariz.

Going Green, Improving Quality

A major advantage of high-temperature microwave systems is their “green” nature. Microwave furnaces generally heat only the objects to be processed, not the furnace walls or atmosphere. Energy-efficient microwave furnaces produce a substantially smaller carbon footprint, less pollutants, and lower operating and end-product costs. In addition, microwave processing can involve up to 90% shorter processing times and a corresponding decrease of up to 80% in energy consumption when compared with conventional methods for many commercial products.

Microwaving can also yield improved product quality with finer grain size, higher sintered density, increased corrosion resistance, and greater strength of finished parts. These advantages can be obtained with ceramics, a range of powdered metals (such as titanium, tungsten, molybdenum and steels), and “hardmetals” like tungsten carbide.

With the realization of not only the technical but also the substantial economic advantages high-temperature microwave processing offers, the implementation of this new processing method began mostly in ceramics and related industries, including advanced ceramic/carbide wear parts, electro-ceramics and bio-ceramics. Subsequently, microwave processing has begun to migrate to other industries, such as powder metallurgy, waste remediation, and materials synthesis/microwave chemistry applications.

Table 1.

Savings Potential

In June 2006, Pennsylvania State University hosted the National Academy of Engineering Regional Meeting on “Immediate Energy Savings via Microwave Usage in Major Materials Technologies.” Several leading microwave research, development and application groups from Asia, Europe and the U.S. presented reports detailing the technical and economic advantages and energy savings achieved through their implementation of microwave processing technologies in industrial applications.

For traditional ceramic sintering, Japan’s National Institute of Fusion Science reported that microwave use enabled the reduction of processing time from 8 to 2 hours, energy consumption reduction from 335 to 63 KWh, and reduction of energy cost from $14 to $7 per batch. In the case of large-part alumina (up to 60 cm diameter), the sintering time was reduced from 96 to 20 hours, energy consumption from 5000 to 484 kWh and energy cost from $420 to $70 per batch.

Successful pilot-scale investigations have been completed in Japan for using microwaves in steel production. The U.S. Department of Energy estimates that conversion of domestic steelmaking from conventional to microwave-assisted processing would save up to 14 million tons of coal burned for energy, thus reducing pollutant emissions by over 30 million tons of carbon monoxide and carbon dioxide annually.

Table 2.

Britain’s Loughborough University investigated microwave-assisted hybrid processes for the sintering and chemical vapor infiltration (CVI) of ceramic matrix composites (CMCs). For a 13-mm-thick woven fabric preform, the total processing time was decreased from 1500 hours using conventional processing to around 50 hours with a microwave-assisted route.

Canada’s Ontario Energy Agency estimated that if the ceramic industry started using microwave instead of conventional processes for various ceramic products, the industry would save 412 million KWh per year, or the equivalent of one 350 MW coal-fired power plant. When extrapolated to all applications in North America, annual energy savings could be measured in Gigawatt hours.

The Penn State Microwave Processing and Engineering Center cut the sintering cycle time for cemented carbides from 2.5 hours to 15 minutes, producing parts with improved abrasion and corrosion resistance. This has now developed into a full-scale commercial technology.

Additional studies comparing high-temperature microwave processing with traditional methods have been carried out for a number of applications and are summarized below.

Figure 1. Alumina grinding sand processing.

PTC Electronic Ceramic Heating Parts
The debinding and sintering of positive temperature coefficient (PTC) electronic ceramic heating parts was carried out in a continuous tunnel microwave furnace. Each PTC ceramic part weighed 7 g and the maximum temperature used was 1240ºC. The product quality was found to be as good as that in parts sintered by a conventional furnace.

For an annual production level of 24 million pieces of the product, lab/field trials demonstrated potential yearly savings of approximately $48,000 by using a microwave processing route rather than a conventional processing route. Additional comparative data is detailed in Table 1.

Table 3.

Alumina Grinding Sands
For alumina grinding sand processing with microwaves, the required microwave sintering temperature was lower (by approximately 100ºC) and the hold time significantly shorter (by about one-sixth) in comparison to a conventional continuous sintering furnace for a similar product. The overall process time, from room temperature to room temperature, was reduced by more than 66% (see Figure 1). Table 2 lists additional benefits.

Figure 2. Ni-Zn ferrite parts sintering.

Ni-Zn Ferrite Parts
It was found that the required microwave sintering temperature for Ni-Zn ferrite parts was lower (by about 100ºC), and the hold time was significantly shorter (by approximately one-third) in comparison to a conventional continuous sintering furnace for a similar product. Figure 2 illustrates the 50% reduction in overall process time that resulted with microwave processing. For an annual production of 200 tons of Ni-Zn ferrite parts, lab/field trials demonstrated that a potential savings of $96,000 could be achieved by utilizing microwave vs. conventional processing (see Table 3).

Figure 3. Vanadium nitride synthesis and sintering.

Vanadium Nitride
Vanadium nitride (VN) synthesis and sintering through microwave processing was also investigated. The required microwave sintering temperature was lower (by ~ 50ºC) and the hold time was significantly shorter (about one-sixth) compared to a conventional process (like atmospheric pressure carbothermic reduction) for a similar product.  

Table 4.

Microwave processing reduced the overall process time by at least 50% (see Figure 3), and potential savings could reach $358,000 per year for an annual production of 200 tons. Table 4 lists additional details.

A Hot Future

High-temperature microwave processing can provide substantial economic and environmental advantages over traditional processes as a result of a combination of several factors, including reduced processing times, lower processing temperatures, reduced consumable costs in certain cases, fewer pollutants, and energy savings-in addition to improvements in product properties. Also, the application of microwaves involves substantially reduced or near-zero production of environmentally harmful emissions, thereby making this an environmentally friendlier-or “greener”-technology as well.

With a smaller physical footprint and a substantially smaller carbon footprint, microwave furnaces offer lower operating and end-product costs. Thus, microwave processing technology is a faster, greener and more energy-efficient alternative for industry.

For more information regarding microwave processing, contact Spheric Technologies, Inc. at 4708 E. Van Buren St., Phoenix, AZ 85008; (602) 218-9292; e-mail info@SphericTech.com; or visit www.SphericTech.com.

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