Microwave heating is being investigated as an alternative to traditional gas and electric heating sources in many industrial processes, offering an opportunity to reduce process times and costs.

Man's desire to use and manipulate materials has driven the need for industrial heating methods. In the Iron Age, fire was used to melt, shape and temper metals, and pottery was developed for smelting, as well as cooking. Wood, peat and coal were the first energy sources used to fire pottery, glass and metals at elevated temperatures. Later, man learned to harness oil, gas, solar, wind and nuclear power, and also developed electric heating processes including resistance, induction, infrared, and more recently, radio frequency. Each heating technology occupies a needed place, but there is still room for improvement in speed, efficiency, and delivering energy directly in to the workpiece. Microwaves already are used extensively in the mass-production food industry. The next step is the use of microwave heating technology for industrial processes beyond cooking and drying.

In the late 1930s, the use of radio waves was perceived as a method to heat nonconductive polar materials. The RF heater was invented and widely applied in conjunction with dies and pressure to weld seams in polar plastics. The use of RF technology eventually expanded into automotive, industrial fabrics, packaging, stationary products, inflatables, medical products and many other product groups. Radio frequency has certain limitations due to the need for electrodes, high cavity voltages and complex power controls.

In the 1940s, Dr. Percy Spencer at Raytheon Corp. was experimenting with a new device called a magnetron for use in radar applications. Strangely, a chocolate bar in his pocket mysteriously melted during the testing. Dr. Spencer continued experimenting by popping corn and then exploding in egg. Raytheon supported further research to develop and contain microwaves generated from magnetrons for industrial use and domestic cooking we now enjoy. After 60 years of experimentation, we know the best uses for microwaves in cooking, and accept that microwaves are not great for every cooking need. A similar lesson is being learned in the industrial heating arena concerning microwave processing of materials. Industry must have some understanding of how microwaves heat materials and what the limitations are before its potential can be tapped. With the help of universities, national laboratories and specialists, several companies highlighted in this article are leading the way.

Commercializing microwave heating

This article highlights processes available to the market. The important players include microwave and furnace manufacturers, universities, consultants, national laboratories and end users. Ceralink Inc. (Troy, N.Y.; www.ceralink.com), plays a unique role in working to bridge the gaps and promote commercialization, offering consulting and hands-on R&D as a partner, subcontractor or service. Dr. Holly Shulman, President of Ceralink, says collaboration between equipment manufacturers, scientists and industry is required to make the technology accessible and affordable.

Cutting tools

Using this approach, Dennis Tool Co. (Houston, Tex.) successfully developed and commercialized high-performance carbide mining and drill bits. Company president Dr. Mahlon Dennis was aware of research at Pennsylvania State University in 1995, recognizing that microwave sintering of carbide could offer improved properties, as well as benefits in process speed and efficiency. Dennis contributed funding and commercial direction to the strong scientific and experimental foundation in the group led by Drs. Dinesh Agrawal and Rustum Roy at Penn State. The work resulted in several patents including a system for microwave firing in a vertical continuous batch that uses process heat to preheat the incoming crucible load (stoke type heating). The furnace design overcomes complications of heating large batches and offers excellent uniformity, speed and control. A schematic and a photo of a commercial unit are shown in Figs. 1a and 1b.

One of the key decisions by Dennis Tool for successful commercialization was to design and sell turnkey systems for microwave sintering of carbide parts. By making these microwave sintering systems available to carbide and other manufacturers, production costs are decreased and development costs are defrayed. Dennis Tool now uses these systems to make their own products and they are exploring new products made possible by fast volumetric heating.

Dennis Tool has fully commercialized the use of microwave sintered carbide for PDC substrates in oilfield drill bits, one of the company's primary businesses. Polycrystalline diamond compact (PDC or PCD) are used widely in both earth drilling and industrial machining applications. The part is formed by growing the diamond layer onto a substrate of tungsten carbide at high pressures and temperatures (106 psi and 2000 C, or 6,894 MPa and 3630 F). The abrasion and impact properties are dependent on both the diamond layer and the carbide substrate.

The greatest advantage in using microwaves in carbide sintering is the improved microstructure, which translates to better properties and better performance. Microwave sintering promotes fast densification, while minimizing grain growth, diffusion, cobalt pooling and carbon loss. Additives that minimize grain growth (but cause mechanical degradation), such as TiC, TaC and free carbon, are not necessary when using microwaves instead of conventional heating.

Figure 2 compares microstructures from Dennis Tool parts that were microwaved versus conventionally sintered. Significant improvements in impact, abrasion and corrosion resistance are achieved using continuous microwave sintering. Both hardness and fracture toughness increase, which is attributed to the fine grain size and full crack-arresting behavior of unreacted cobalt. Extensive laboratory and field testing of parts convince Dennis Tool that it has developed the next generation carbide mining and drilling bits, with new products and applications on the horizon. New applications include cutting tools, dies, anvils, sleeves, bushings, nozzles, bearings and substrates. Some new materials include carbide diamond composites, functionally graded composites, nanocarbides and nanocomposites.

Cutting tools are often an early proving ground for new materials, since they are small, consumable and high-profit items, which usually do not cause catastrophic problems if they fail. Silicon-nitride cutting tools were developed and commercialized, reaching a broad market in 2000 through Valenite, a U.S. based cutting tool company recently bought by Sandvik. Three microwave-sintered grades are available for machining, high speed turning and milling of cast iron and machining of high-temperature alloys. As with carbides, Valenite claims microwave-sintered silicon nitride products have a finer grain microstructure with higher hardness and better fracture toughness than conventionally processed material.

Metals processing

A completely different approach of using microwaves for heat is the generation of on-demand plasma. Dana Corp. (Toledo, Ohio; www.dana.com), a manufacturer of automobile components, developed a method for ultrarapid heat treating, coating and brazing of metals. Dana's AtmoPlas system uses microwaves to superheat plasma that surrounds the parts, and quickly heats (temperatures up to 2000 C) by conduction. The process does not require the use of a vacuum, as required in conventional systems. The cycle time for heat treating and coating is reduced by two thirds, and there is a net savings in both time and energy. Dr. Kumar from Dana Corp. explains that this method overcomes the tendency of metals to arc in the microwave field, while taking advantages of the fast heating response of microwaves.

Dana Corp originally developed the microwave plasma process to address an internal bottleneck in a brazing step. Highly encouraging results also demonstrated a wider range of application in coating, sintering and heat treating. Dana is currently evaluating its internal scale-up strategy, while looking for partners to commercialize the AtmoPlas system. The system is currently a batch process that uses a refractory container or pod, which holds the metal parts and the plasma (Figs. 3 and 4). The company plans to convert this to a conveyor belt where pods are cycled and each pod receives individual microwave plasma treatment.

While Dana's system can be used to join and braze materials having similar expansion properties, Technology International Inc. (TII, Kingwood, Tex.) developed and commercialized a microwave process to braze highly mismatched materials, such as lightweight armor combining silicon carbide to titanium metal (Fig. 5). The method relies on the vastly different coupling (heating) characteristics of the ceramic and metal in the microwave field. At the brazing temperature, the ceramic is several hundred degrees hotter than the metal, which avoids high expansion and shrinkage of the metal. Bob Radtke of TII says the process must be performed quickly to avoid heat transfer, which requires high power and good microwave penetration into the ceramic. TII is currently working to increase the brazing speed, which will allow an increase in the size of parts that can be brazed. Full production of large parts will require the design and manufacture of specialized equipment.

The idea of using microwaves for melting metal was introduced to the general public through David Reid's web site in the late 1990s. Reid developed a suscepting crucible and began melting metal in a kitchen microwave using a lost-wax casting method. This is a simple and highly effective method for small batches. Accessories, such as crucibles and thermal insulation, can now be purchased from Research Microwave Systems LLC (Alfred, N.Y.; www.thermwave.com) to use for melting metal in a kitchen microwave.

On a larger scale, Ed Ripley, at the Y-12 National Security Complex (formally part of Oak Ridge National Laboratory; Oak Ridge, Tenn,; www.y12.doe.gov) was melting hundreds of pounds of titanium, aluminum, gold and silver using an idea similar to that mentioned above; that is, a large suscepting crucible in a microwave field (Fig. 6). Although metal is supposed to reflect microwaves, researchers at Y-12 and Penn State claim that the metal actually begins to absorb (couple) and heat directly at approximately 3/4 of the melting temperature. Y-12 and Oak Ridge National Laboratory have extensive microwave facilities and have been developing microwave technology for more than 20 years. Ripley explains that several processes have just been declassified and are available for licensing agreements. This includes a microwave eutectic salt bath, with an option to use a granular suscepting media that prevents hydrogen pick up. One advantage of using microwaves is that the salt bath can be heated quickly on demand, instead of being kept continuously molten.

Two companies have licensed the metal melting technology from BWXT, Y-12's commercialization arm. Microwave Synergy Inc. and MS Technology, both located in the state of Tennessee are now competing for the lead position in scale-up and commercialization of the Y-12 microwave metal melting technology. Both companies may have industrial partners, but are not disclosing specific information.

The technology involves the use of microwave energy, special microwave-susceptible materials and uniquely designed crucibles and molds to melt and cast metal in a microwave chamber. This was originally developed for melting depleted uranium, a process that is currently being scaled up at Y-12. According to Stan Morrow of Microwave Synergy, microwave technology for metal melting saves energy, reduces cycle time and improves metal quality. Ken Givens, Vice President of Business Development at MS Technology, adds that one of the major benefits is versatility. Batches from 1 to hundreds of pounds can be heated quickly and efficiently. Givens has plans for international deployment of the microwave metal-melting technology and has already entered into an agreement with the Technical University of Munich (www.tu.muenchen.de) to install a demonstration unit in their laboratory. MS Technology is also designing a larger chamber capable of continuous melting of 2000 lb/hr.

It is not clear where this technology will fit as an industrial process. High-power 2.45 GHz magnetrons are used, which are costly. The size is currently limited by the batch style design, with casting taking place within the microwave cavity. This technique may be best suited to casting high precision superalloys or titanium, rather than bulk commodity metals.

Another technology originating at Y-12 with high commercial potential involves the use of microwave energy to diffuse powder metals into solid parts. One example is the chromizing of steel for surface hardening. Chrome plating is a toxic, environmentally unfriendly process, and the Federal government is encouraging alternatives. The microwave method avoids the use of toxic chemicals and is achieved at atmospheric pressure and shorter times. This technology was licensed to Tesla, an Australian based company, who is currently looking for commercialization partners.

Commercializing microwave heating equipment

Table 1 gives an overview of equipment manufacturers. Many companies that specialize in microwave power will build microwave heating systems to customer specifications, and some will assist with design and processing issues.

Dennis Tool and Dana Corp both found the need to develop microwave systems to achieve their goals of improved parts and productivity. Dennis Tool has become a microwave furnace manufacturer, while Dana Corp will license technology to a manufacturing and marketing partner. In production circumstances, the design of microwave equipment must be stimulated by the end users, and equipment manufacturers need to be ready to understand and respond to this new market. On the other hand, equipment manufacturers can encourage this new microwave market by making standard research and production systems available.

Doug Parent, Marketing Director of CPI (Communications and Power Industries, Palo Alto, Calif.; www.cpii.com), formerly part of Varian, recognized the need for versatile, microwave systems for process development and scale up. CPI manufacturers the Autowave, a system that uses high power magnetrons, a gas handling system, and PC workstation with Labview (Fig. 7) The Autowave can be fitted with a magnetron, klystron (18 GHz) or gyrotron (28 GHz, known as the Heatwave). Autowaves come in two chamber sizes designed for optimum field uniformity. The system can be used for research, scale-up, and/or production, but it is relatively expensive for initial feasibility studies. Microwave-transparent refractory containers or caskets are used inside the chamber. This arrangement increases the versatility and cuts down on power requirements for small-scale tests.

In the past few years, Autowaves have been sold to universities and industrial companies mainly in Europe and the U.S. At its Microwave Testing Center, Ceralink has been testing the Autowave since January 2002. Ceralink's president, Dr. Shulman, is enthusiastic about the unit's reliability and wide range of uses, which includes sintering metals and ceramics, bending and melting glass, brazing, chemical reactions, calcination and a many other projects including nanoceramic fabrication. Shulman says the Autowave is easy to operate and lends itself to various fixture designs. CPI cannot discuss specifics, but they have also supplied microwave power for drying refractories in the steel industry, heating chemical reactors and plasma processing.

Research Microwave Systems (RMS) is addressing a different need by providing microwave accessories such as susceptors, thermal packages and crucibles, as well as the Thermwave, a low-cost microwave. The Thermwave can be used for many types of materials, including organic and inorganics, in processes such as drying, chemical reactions, annealing and firing. The Thermwave works with an inexpensive controller and shielded thermocouple. RMS is currently developing a unit with more microwave power and is expanding its line of accessories to include suscepting crucibles and low-cost insulation packages. The greatest pull for these systems has been from research institutes in Europe, Korea, Malaysia and India.

Bernie Krieger of Cober Electronics Inc. (Norwalk, Conn.) developed a commercial microwave process based on the needs of an external end user. Cober specializes in microwave equipment and has been exploring the market for microwave heating for more than 20 years. Cober hit on a winning combination for vulcanizing rubber, which uses microwaves and hot air. Krieger had to understand the needs of his customer, and overcome the urge to focus only on selling microwave equipment. The need was for a complete functioning system to vulcanize rubber with better quality and in a shorter time, eliminating messy and costly steps, and have a good payback on the investment. Today, Cober's system has changed the face of the rubber vulcanizing industry.

Other microwave equipment manufacturers also work with their customers to design and build systems for heating. Thermex Thermatron (Louisville, Ky.) builds sophisticated high power microwave equipment, and have become adept at testing its customers products using equipment in house. The company is an important source of industrial 915-MHz generators. Thermex Thermatron is proud of its engineering and stands behind its products, of real significance when setting up 100-kW power units. Ferrite Inc. (Hudson, N.H.) also supplies high-power systems, but has traditionally focused on the food industry. Now the company is looking toward processing other materials at higher temperature. CPI is probably the most sophisticated in precision engineering, which is what it takes to produce a traveling wave tube like a gyrotron. CPI works with Ceralink and other consultants, effectively combining materials, heating and microwave expertise to address its customers' needs. Gerling Applied Engineering Inc. (GAE, Modesto, Calif.) is also highly reliable in the support of its microwave heating products.

In Germany, Linn High Therm GmbH (Eschenfelden) manufactures both laboratory and industrial microwave heating equipment including high temperature systems. Linn High Therm has a unique combination of furnace expertise, knowledge of microwave systems and background in material science. Mr. Malte Moeller states that the new high-temperature processes are one bright future for microwave heating, but by far not the only one. Almost daily the company gets inquiries about microwave drying or heating applications that nobody has ever thought about before. Although many of these processes cannot be realized due to economic or technological difficulties, this shows that microwave low-temperature processes are by far not at the end of their development potential. In fact, microwave drying is rapidly being incorporated in to ceramic processes, including filters, substrates, honeycomb structures, industrial insulators, thermal insulation and whitewares (Fig. 8).

Japan has taken a strong position on the commercialization of microwave furnaces. Takasago Industry Co. Ltd. (Toki-city, Japan), a large kiln manufacturer is now manufacturing and selling microwave assist kilns in Japan. This is a patented C-Tech Innovation technology. Examples are given in Fig. 9. Mino Yogyo Co. Ltd. (Mizunami, Japan), one of Takasago's competitors, is also offering microwave furnaces (Fig. 10). Both companies have built large scale production kilns, but it is not known how many have been sold or exactly which industries they are targeting. They have demonstrated firing whitewares and large alumina parts (Fig. 11 and 12), but it is likely they are also looking to other high-technology industries.

Furnace manufacturers in the U.S. have been slow in the uptake, but are beginning to take an interest in microwave systems. CM Furnaces worked with Ceralink and C-Tech to build a hybrid microwave-assist electric furnace (Fig. 13). The basic furnace is one of its standard products in the 1700 laboratory series, which was retrofitted with a GAE 2-kW, 2.45-GHz generator. This type of furnace and other microwave-assist laboratory furnaces will be commercially available soon. Microwave-assist gas kilns will be available through Harrop Industries (Columbus, Ohio), also using technology from C-Tech Innovations made available by Ceralink. Any North American furnace company can work with Ceralink to develop its microwave assist product lines for a design fee and a small royalty on sales. Harper International (Lancaster, N.Y.) has teamed up with microwave company Fricke and Malle in Germany to bring microwave expertise to their furnace manufacturing capabilities. Harper has also licensed microwave rotary calcine technology and is making this commercially available.

The use of large microwave furnaces will probably have growing pains. The end user may explore the willingness of the furnace manufacturer to share some of this development cost. The furnace manufacturers will need microwave experts on site to be most effective in building microwave furnaces. It is a good idea to find out the qualifications of the team designing and building the system. Hiring a knowledgeable consultant to understand the manufacturer's microwave furnace capabilities, could also save time and money.

Microwave heating experts

The recently held Fourth World Congress on Microwave and Radio Frequency Applications (7-12 Nov. 2004) in Austin, Tex., brought together the world's leading experts in microwave technology. The next major event where one can meet this cast of researchers will be at the Ampere conference in Modena, Italy 13-15 Sept. 2005. Many experts can be found in the commercial arena (some of these persons have already been discussed), and many other technical experts can be found at research institutions.

An important group resides at Bayreuth University (Bayreuth, Germany; www.uni-bayreuth.de) led by Dr. Monica Willert-Porada. This group works closely with industry and has developed an interesting approach to support commercialization. Dr. Willert Porada founded in 1997, a nonprofit organization, InVerTec, housed at the Center for Excellence of New Materials at Bayreuth. InVerTec's mission is to impart knowledge, coordinate and transact R&D initiatives in the field of combined electrothermal processes and assist with scale-up on the basis of their own research work.

The current focus of their project work includes using microwaves for roasting of ores, heat treatment of glass, detoxification of filter dusts and slags for the metal industry and decontamination of asbestos containing wastes. According to a spokesperson for InVerTec: "At our pilot plant stations, most reactor and oven types can be implemented in microwave heating and combined heating procedures. In order to prove planning reliability for scale-up, we offer the possibility to do experiments at the microwave frequency of 915 MHz and the common microwave frequency of 2.45 GHz." InVerTec's equipment includes a microwave rotary kiln, microwave fluidized bed reactor (Fig. 14), microwave with inert gas, and a 915-MHz single-mode reactor.

Another highly visible group is located at Pennsylvania State University (University Park, Pa.; www.psu.edu), in the Materials Research Institute. Led by Drs. Dinesh Agrawal and Rustum Roy, this group has been a pioneer in the field of microwave process research since 1984. In the 1980s, their focus was on sintering and synthesizing ceramics, such as alumina, zirconia, zinc oxide, hydroxyapatitie, zeolites, mullite and silica. In the 1990s the focus shifted to developing microwave processes for electroceramics such PZT barium titanate, relaxors, and transparent ceramics.

Innovators at Penn State, like Rustum Roy, have never been restrained by conventionality, so when the idea of microwave sintering powder metals occurred, they proceeded with experiments and found this a highly effective method. Work in the area of microwave sintering carbides caught the attention of Mahlon Dennis and is now fully commercialized at Dennis Tool Co.

Penn State recently hosted a meeting with over 40 companies to launch the Microwave Powder Processing Consortium. The theme was to organize a consortium that would use Penn State's equipment and knowledge base to develop mutually relevant microwave technology at a shared cost. Matthew Smith, from Penn State's Intellectual Property Office, also discussed the technology transfer and licensing of patents involving microwave processing from their pool of developed intellectual property.

Since moving from the University of Florida in 2001, Dr. David Clark and research faculty member Diane Folz have built the Microwave Processing Research Facility at Virgina Polytechnic Institute's Department of Materials Science and Engineering (Blacksburg, Va.; www.vt.edu). The laboratory is equipped to perform research using microwave energy ranging from 2-18 GHz with power levels from 200 W to 6.4 kW. Current microwave research focuses on waste remediation and recycling, nanomaterial synthesis, formation of glass-ceramics, sterilization, and medical treatment technologies.

Diane Folz explains that one of the most interesting uses for microwaves is selective heating of specific components within a structure. It was this characteristic that led the group to investigate microwave recycling and waste remediation. Graduate student Rebecca Schulz, in work funded by Westinghouse Savannah River Co., demonstrated a microwave process for recycling precious metals from electronic circuitry, while also treating the off gases that resulted in the combustion process. The work resulted in several patents on the process and design of microwave hardware. A schematic of the system is shown in Fig. 15.

Dr. Jim Hwang is leading an effort to commercialize a technology that his group developed at Michigan Technological University (Houghton, Mich.; www.mtu. edu). Microwaves are used to assist steelmaking in an electric arc furnace (EAF). The viability of the technology lies in the fact that iron ore and carbon are excellent microwave absorbers. Prototype equipment for the new technology is located in Hwang's lab and consists of modified electric arc furnace with an auxiliary microwave heating system. A charge of iron oxide, coal and limestone is loaded into the chamber and microwave energy is introduced. The charge absorbs microwave energy to the point of coal ignition. The exothermic reaction of carbon oxidation further increases the temperature. The EAF electrodes then descend to provide electric arcing energy, producing molten steel and slag. Design modifications to the chamber will allow continuous mode operation. IH

SIDEBAR 1: How microwaves are produced

The term "microwave" is used to cover the portion of the electromagnetic spectrum between 300 MHz and 300 GHz, which corresponds to wavelengths ranging from 1 meter to 1 millimeter. In practical terms, there are certain frequencies that are allowed for industrial use. These are called ISM (industrial, scientific and medical) frequencies for applications in those areas as described in table 1.

Kitchen microwaves operate at 2.45 GHz, while many large industrial systems use 915 MHz. The overwhelming majority of microwaves are produced by magnetrons. However, klystrons or gyrotrons are used to produce ultrahigh-frequency microwaves (millimeter waves).

Low-power magnetrons (2.45 GHz, 800 W) are inexpensive and rugged devices (<$30 purchased in quantity). High power can be achieved by ganging up low-power magnetrons, but this is cumbersome for large systems beyond the drying stage. High-power 2.45 GHz magnetrons are available up to 30 kW, while 915 MHz systems can be purchased in 60, 75 and 100 kW units. A higher frequency 5.8 GHz magnetron has recently been made available, but not quite at mass production prices, while ultrahigh frequency 24-30 GHz klystrons or gyrotrons are much more expensive technologies.

SIDEBAR 2: How microwaves heat

The short explanation of microwave heating is that materials heat up through internal friction when dielectric and magnetic loss mechanisms respond in a microwave field. One example is a charged impurity or vacancy in a crystal lattice that switches places in an alternating field. Friction is produced if the ion cannot quite keep up with the field, while no friction is produced if the ion cannot move at all, or if it moves too easily.

There are many loss mechanisms that can be activated in real materials related to crystal structure, defects, bonding, surface, grain boundaries, etc. One might think there should be an optimum frequency for maximizing the friction for each mechanism, but optimum frequency changes with temperature because certain movements become easier as the material heats up. This produces less friction in some cases and more friction in other cases, such as where a defect was frozen and couldn't previously contribute. This behavior causes a peak in the dielectric loss, which generally shifts to lower frequencies at higher temperature. The selection of an appropriate microwave frequency is critical to commercial success of the process. Figure A shows several important relationships for dielectric heating.

In conventional heating, all heat must be transferred through the outer surface of the material to the interior. Microwave heating offers an important advantage of being able to place energy directly into the volume of the workpiece. This requires meeting certain conditions where microwaves penetrate the material enough to cause volumetric heating. Very little heating occurs if microwaves are reflected or if they penetrate through the material too easily. Table A shows some penetration depths calculated from the dielectric properties.

The high penetration depth of quartz indicates that quartz glass will not couple well (suscept) or heat at room temperature in a 2.45 GHz microwave field. However, it has been demonstrated experimentally that quartz glass couples and heats in the microwave field at elevated temperatures. Unfortunately, dielectric data is scarce at microwave frequencies and elevated temperatures. The situation is further complicated by the continual (sometimes exponential) change in dielectric properties with temperature, and a strong effect of impurities, defects and surfaces. Direct microwave heating tests are very valuable for early stage feasibility, as well as measurement of the dielectric properties in the appropriate frequency and temperature regime.

In general, electrically insulating materials do not couple or heat well from room temperature at moderate frequencies, but do couple and heat at higher temperatures or at higher frequencies. For example, pure alumina is microwave transparent at room temperature using 2.45 GHz microwaves, but couples from room temperature at 24-30 GHz. Alumina begins to couple and heat more effectively at 1000 C (1830 F) at 2.45 GHz and becomes highly suscepting at 1500 C (2730 F), which suggests that hybrid microwave or millimeter-wave (gyrotron) heating is required to fire alumina.

Semiconducting materials, such as silicon carbide, usually heat well from room temperature at moderate frequencies. Conductive materials such as metals should reflect microwaves. However, heating of metals (especially powder metals) has been observed experimentally. There are many unanswered questions concerning microwave heating, such that theoretical explanations must catch up with real life observations.

SIDEBAR 3: Bridging the R&D-commercialization gap

Commercial microwave processing has been a "chicken and egg" problem. Microwave equipment and furnace manufacturers have not seen the market, making it difficult to justify expenditure in that direction, while materials manufacturers have not had access to microwave heating equipment and could not easily find it on the market. Ceralink is working on both sides of this dilemma, assisting the end users and equipment manufacturers.

A Microwave Testing Center at Ceralink's new facility at Rensselaer Technology Park near RPI in Troy, N.Y., provides a facility where companies can explore the feasibility of using microwaves in their processes. Ceralink has experience with microwave heating a wide range of materials for melting, brazing, sintering, forming and calcining, as well as lower temperature processes, such as binder burnout and drying. Commercially available microwave equipment also is showcased at the center including the CPI Autowave, CM Furnaces' Microwave Assist Electric furnace; Research Microwave Systems' Thermwave; and the Milestone Ethos Plus (Fig. B). Other equipment can be obtained, and assistance in equipment set-up and testing is available. Ceralink also assists companies with design, construction, and purchase of microwave systems.

Ceralink recently signed an agreement with C-Tech Innovations (UK), formerly part of EA Technologies, to transfer microwave-assist hybrid technology to furnace manufacturers in North America. Microwave-assist furnaces are in commercial use in a specialized ceramics application and in the mining industry. C-Tech and Ceralink are working to design retrofits that enable standard and specialized gas and electric furnaces to accept microwaves.

Ceralink focuses on commercialization, which means they collaborate with all the involved parties from end users to equipment manufacturers, to research institutions. Ceralink acts as a central contact point, benefiting all players and facilitating commercialization. For example, Ceralink was instrumental in securing $5 million in funding from DOE for Engelhard Corp. (Iselin, N.J.; www.engelhard.com) by finding an appropriate university partner to develop a microwave process for reclaiming precious metal catalysts from fuel cells. Another example, is a company that is pursuing the use of microwaves for a metal melting process. After demonstrating feasibility at the Microwave Testing Center, it was suggested to use high power 915 MHz equipment, located at Thermex Thermatron (Louisville, Ky.; www.thermex-thermatron.com). Ceralink engineers built a scale-up furnace that fit an applicator at the Thermex Thermatron facility and ran tests on site. The tests provided necessary data for full scale up, including energy and power requirements and evaluation of furnace materials.