Microwave Heating of Ceramics
When applied to ceramic materials, microwave processing opens up opportunities to reduce costs and energy consumption while improving productivity and material properties.
In microwave heating, a polar molecule is subject to electromagnetic radiation at a frequency that is in the microwave range. The materials’ exposure to microwave energy causes rotation in the polar molecule, which results in heat being generated. This phenomenon is also referred to as dielectric heating. (A polar molecule is one that has an electric dipole moment, the best known being water.)
Understanding the Process
A ceramic material that exhibits dielectric heating is referred to as a susceptor. The ability to self-heat when exposed to microwaves is referred to as a material’s ability to couple or suscept to this electromagnetic radiation. A material that does not exhibit this self-heating behavior, yet allows the microwaves to pass through, is referred to as transparent; conversely, one that does not allow the microwaves to pass through is considered reflecting.
When applied to ceramic materials, microwave processing opens up opportunities to reduce costs and energy consumption while improving productivity and material properties:
• Provides an unconventional means to arrive at quality ceramic parts and powders
• Can be used to rapidly heat and process ceramic materials
• Can reach operating temperatures within minutes vs. hours; hold or dwell times are much less than traditional firing techniques
• Can reduce part/powder production time from days to hours and hours to minutes
When considering economic benefits, the focus has been on the direct coupling of energy to the ceramic part/powder and the various process time efficiencies derived from this phenomenon. The heating of the part itself is governed by the part’s mass, its specific heat and the desired change in temperature. The same amount of energy is required to heat a part from one temperature to another, whether using gas, electric, infrared or electromagnetic radiation. But which process accomplishes this heat treatment in the most productive, energy-efficient manner?
In fact, the real benefit of microwave processing has less to do with the direct coupling of microwave energy to the part, and more to do with the flexibility afforded to the equipment designer. A key concern when microwave processing ceramics is not if the material suscepts to microwave energy, but how to control this phenomenon when it occurs. Many materials suscept to microwave energy, but different temperatures must be reached prior to this behavior being exhibited. For example, zirconia does not suscept to microwave energy at room temperature. Depending on process conditions, however, it will begin to suscept between 600-1,200°C.
The challenge for the system designer is to create a controlled environment by which ceramic materials can be brought to a sufficient temperature using a known susceptor; once reaching that temperature, ceramics then begin to receive microwave energy and self-heat. This technology provides a new approach to furnace design, allowing strategic placement of the radiating heat source in the most desired location. An ancillary benefit is the degree to which the part being heat-treated suscepts to microwave energy. The combination of these two factors (furnace design and powder/part susceptibility) improves the heat treatment, which in turn improves yield and material properties.
Figure 1 (top) shows a furnace chamber constructed using traditional design guidelines, with indirect radiant heating from silicon carbide heating elements. To perform the same heat treatment, the microwave design requires less than one-quarter the volume and one-third the square footage of the chamber surface area (see Figure 1, bottom). Wall losses are typically the largest contributor to energy loss in the kiln. Therefore, minimizing surface area directly increases energy efficiency. Not only does the microwave design significantly increase efficiency, but it maximizes heat treatment uniformity by making it possible to control exactly where the susceptor/heat source is located in relation to the part.
Figure 1 shows a scenario that replaces the hearth plate with a material that couples with electromagnetic radiation. Other designs might replace the crucible or the pusher tile with a microwave-suscepting material. In yet another system, the container might, in effect, be a complete furnace with a microwave energy-absorbing material contained within. This container/furnace would then be conveyed through a microwave chamber. In these examples, the design focus changes from heating the much larger open chamber, which entails transferring the heat in the chamber to the part, to creating a more localized heating phenomenon by heating the microwave-absorbing materials directly.
Bringing Microwave Mainstream
While microwave processing equipment and technology for communications, industrial drying and food processing has been around for a long time, microwave processing of ceramic materials is in its infancy. The companies that specialize in microwave power systems have a limited understanding of ceramics, high-temperature material processing and kiln design. At the same time, traditional ceramic kiln companies have limited knowledge of microwave systems and applications.
Most of the development in microwave processing for ceramic materials has been performed in small, lab-scale batch systems. The industry lacks the tools, knowledge and understanding to translate this early research into a viable economical process technology. However, by merging the knowledge of microwave specialists with that of the traditional ceramic kiln designers, new systems can be developed that demonstrate the benefits of microwave processing and help justify the costs to investigate. Although this technology holds benefits for the ceramic industry, companies will become comfortable with microwave processing when the right tools are available for development and production. Presently, the limited availability of suitable equipment is a substantial hurdle to overcome, hindering companies’ efforts to start their investigations and begin to realize the benefits.
In order for microwave to be truly viable as a process technology (e.g., with proper power control, temperature sensing and closed-loop temperature control), considerable development is necessary. This requires the development and preparation of new larger-scale equipment to study and understand all of the variables associated with processing a variety of materials using only microwave energy. It was just a few years ago that the first prototype microwave tunnel kiln for ceramic processing was constructed to study the continuous conveyance of parts/powders through a microwave field. Other than this system, there remains little equipment available of sufficient scale, whether batch or continuous, to begin the study of materials in microwaves.
The continuous processing of materials through a microwave field has the following benefits:
• Higher energy savings
• Lower equipment cost
• Increased yields
• Improved productivity
The most reliable, productive and energy-efficient processing comes from the continuous conveyance of ceramic material through a heating chamber, with a larger benefit seen when using microwave heating. The energy required to maintain temperature is significantly less than that required to thermally cycle in batch systems. In a batch system, the entire mass of the kiln must be brought up to temperature for each cycle. All of this energy that is used to heat the furnace chamber and its workload is lost on cooling.
Tunnel kilns and rotary tube furnaces designs have been developed using only microwave energy for heating. In Figure 2, the rotary furnace design maximizes heat transfer to the powders in order to improve quality and productivity. This design shows a tube inserted through cylinder-shaped ceramic susceptors. The tube may be either microwave reflecting or transparent. Another configuration might use a microwave transparent tube with multiple small ceramic media or beads as the susceptors in the interior of the tube to have intimate contact with the process material. Work is still required to establish conventional process parameters for specific materials such as temperature, atmosphere, feed rate and volume.
The microwave pusher tunnel kiln shown in Figure 3 offers multiple advantages, including:
• Economical design (new continuous microwave systems at a lower cost than traditional kilns)
• Energy efficiency (furnace chambers of less than 1/4volume and 1/3wall surface area; strategic placement of susceptors and part susceptibility all serve to maximize energy efficiency)
• Improved yields from maximum process uniformity (yields are always an issue in ceramic processing; the more uniform the heat treatment, the better the yield)
• Improved productivity (reducing furnace processing times from days or hours in traditional batch kilns to hours or minutes in a microwave system)
Again, conventional process parameters such as temperature, push rate and volume to arrive at production numbers need to be established for this new microwave pusher tunnel kiln design.
New Design Considerations
Susceptors of various refractory-grade aluminas, silicon carbide, silicon/silicon carbide, carbon/graphite, zirconia and molydisilicide have been demonstrated to be effective heat sources that readily absorb electromagnetic radiation, depending on the frequency (RF KHz vs. microwave MHz to GHz). Aluminas and zirconia show better performance at higher microwave frequencies, while carbides do better at the lower RF range. One particular alumina susceptor is extremely durable and has been tested with heating rates in excess of 50°C per minute. The available frequencies for industrial power supplies are typically 450 KHz, 2.5-5 MHz, 915 MHz and 2450 MHz.
Typical thermocouples or contact sensors have proven ineffective for temperature sensing and control for microwave processing. Much early work using thermocouples led to misinterpretations of the results. The new equipment must use infrared pyrometers or lower cost infrared thermocouples. The sensing must be focused on the part, not the surrounding area, to prevent thermal runaway conditions if or when the part begins to self-heat by the microwave energy. In a traditional design sense, the focus is on measuring and maintaining the chamber temperature and relying on convective or radiant heat transfer to heat the part. In microwave, we are less concerned about chamber temperature and need to closely monitor the much more localized material behavior.
Insulation choice plays an important role as well. Whether using fiber or brick, certain material systems absorb electromagnetic radiation while others are transparent. Another consideration is part geometry, which will have a greater impact on design in order to determine the best susceptor geometry and position in relation to the part being fired.
The material being processed, production rates, powder vs. compact, and the behavior of the material in an electromagnetic field need to be considered for optimal design. Some materials can exhibit a thermal runaway condition once they begin to increase sintered density. This change in electromagnetic susceptibility with increased densification can introduce a variable that is very difficult to control. In many instances, the ability of the part or powder being processed to receive electromagnetic radiation can be more of a hindrance than a help.
Consistency becomes extremely challenging if we rely solely on the ability of a material to receive electromagnetic radiation. In ceramic processing, the material being processed quite often may remain constant, but there are variations in mass and geometry that require different microwave processing parameters for each change. For these reasons, it’s critical to design a system with known susceptors to create a stable heating environment whereby the influence of the process material’s mass, geometry and electromagnetic susceptibility are minimized. Under these conditions, with microwave recipes properly established and automated, it is possible to produce the same quality of material from batch to batch. By comparison, the larger hot zones and variable temperatures in the currently used electrical resistance-heated furnaces create more heterogeneous sintering conditions.
Depending on the system, approximately a dozen different materials are used, some reflective, some energy absorbing and some transparent. Figures 4-6 show a scalable, controlled environment to process materials to > 1,500°C using only microwave energy at 2.45 GHz.
A New Paradigm
Very little new technology has been developed over the past 50 years in furnace design that could provide the benefits that microwave technology offers. We still use silicon carbide heating elements and, in many cases, ceramic brick insulation. If energy efficiency was desired, then designers may have shifted from gas to electric or brick to fiber insulation to reduce heat storage.
Microwave processing offers a new paradigm in furnace design. These new designs will position the heat source in the most desired location by strategic placement of the known suscepting material in relation to the process material. This uniform heating will be enhanced based on the degree to which the material being heat-treated suscepts to microwave energy. The result is better quality and reduced costs.