Microwaves In High-Temperature Processes

The handiness of a wide variety show of microwave furnace technologies is boosting concern in using microwaves in many high-temperature processes .

Microwaves are used in laboratories worldwide for dry, calcination, binder removal, glass melt and sinter of ceramics and powder metals. Microwave dry of materials is an industrial reality, but high-temperature march in the roll of 1000 to 2200 C ( 1830 to 3990 F ) is only recently finding it ‘s way into commercial products.
Using microwaves for firing or sintering ceramics, metals, and glaze can provide both energy and time savings and improved products. Some examples of the industries moving into microwave heat technology ( > 1000 C ) include heat treatment and annealing of ceramics and metals and firing electroceramics, carbide hard-metal wear parts and bioceramics.

An advantage of microwave heat is that the integral part couples in the energy field, directly absorbing energy throughout the volume. In microwave dismissal, heating rates from 100 to 150 C/min ( 180 to 270 F/min ) can be used to amply sinter ceramics without cracking. Cooling besides is fast because the refractories do not become a hot as in conventional ignition. This translates to significant savings in time. For example, a inflame process that requires 24 hours typically can be reduced to 6 hours. It besides has been demonstrated that dissemination is accelerated or enhanced in a microwave field. This means that compaction, reactions, bind, etc., can occur at lower temperatures than those expected using conventional heat. This is meaning for many reasons, specially the opportunity to use less expensive, lower temperature rated refractories. significant savings in materials are possible since refractories will be cooler than in conventional fire because they do not absorb microwave energy and the parts can be fired at lower temperatures.

Energy savings

There have been conflicting reports concerning energy consumption using microwaves. In general, it is agreed that an energy savings will be achieved, ranging from 10 to 90 % higher efficiency compared with that of conventional electric or gas heat. Some of the confusion is due to the lower efficiency of microwave use with research-size loads. As shown in Fig. 1, efficiency depends on the material being fired :

  • Microwaves are more efficient than gas firing for large enough loads
  • Microwave energy use becomes more efficient for larger loads
  • Higher microwave heating rates are more efficient
  • Less energy is required to heat zirconia than to heat alumina

Overcoming barriers to microwave heating

Microwaves allow fast heating, but may develop an inverse temperature profile in the material. This means that the share is a lot hotter than the surrounding atmosphere, so it will cool from the come on through radiation. This is called an “ inverse profile ” relative to the conditions in ceremonious heat where the coat is hotter than the core. Microwave heating system besides can produce hot spots or “ thermal runaway. ” This occurs when one section of the material absorbs the microwave energy preferentially over the majority material. These problems can be overcome with proper furnace design and attention to the thermal box. Manufacturers are moving toward microwave technology as two major barriers are being overcome. These barriers are : 1 ) the necessitate for exchangeable microwave furnaces, and 2 ) the want for cheap feasibility studies and experienced aid with microwave scale-up. respective companies are rising to the challenge and providing equipment desirable for scale-up and industrial microwave burn. University researchers, microwave manufacturers, and freelancer microwave testing facilities can be used to develop process know-how and for aid with scale-up challenges. Universities typically perform well in basic research, but are not focused on scale-up problems, and by and large have a slowly reply time by industry standards. Microwave furnace manufacturers have an obvious vested interest in the success of microwave feasibility, and, consequently, can be advanced and helpful. however, the customer may not get the best value equipment for their needs. An autonomous microwave testing facility can offer an effective solution. For example, Ceralink Inc. ‘s Microwave Testing Center provides proof of concept and assists in merchandise scale-up, including advice on furnace design and excerpt of an allow microwave manufacturer.

Commercial microwave furnaces

several companies offer criterion microwave furnaces, and many companies design and build microwave systems to meet their client ‘s specifications. table 1 lists companies that offer standard systems having a range of prices and capabilities. Prices range from US $ 7,000 to $ 300,000 depending on the ability and frequency needed. The most cheap and common microwave frequency used for heat is 2.45 GHz, the same frequency used in microwave cook of food. Susceptors much are used to start the heat and to prevent thermal runaway in difficult-to-heat materials, such as many ceramics. Susceptors are materials ( normally semiconductors ) such as silicon carbide ( SiC ), which heat well from room temperature in the 2.45-GHz microwave range. Ceramics begin to heat “ normally ” by radiation first gear, and then are able to absorb microwaves and inflame volumetrically. table 2 provides a drumhead of the different approaches to the use of microwaves for high-temperature materials processing.


An cheap system that uses low-power 2.45-GHz magnetrons is Research Microwave Systems ‘ ThermWAVE ( www.thermwave.com ) shown in Fig. 2. This water-cooled microwave is a utilitarian cock to explore a march. The system includes a restrainer and accessories, such as a coffin and susceptors. The ThermWAVE package opens the doorway for many companies who do n’t have time to develop their own system. It can be used to process many types of materials including organics and inorganics, in processes such as dry, chemical reactions, annealing, and firing. Research Microwave Systems besides provides accessories, such as crucibles, refractories and susceptors, for early microwave systems. The Communications and Power Industries ( CPI ) Autowave ( world wide web. autowave.tv ) system uses high-power magnetrons, and can be fitted with a klystron ( 18 GHz ) or gyrotron ( 28 GHz ). The Autowave has two chamber sizes ; the larger chamber is shown in Fig. 3. CPI provides an Acceptance package to demonstrate that the Autowave can be used to reproducibly fire a ceramic. The organization is versatile, can be used for research, scale-up, and/or production, but it is relatively expensive for initial feasibility studies. The chamber is not lined with refractory insulating material, but alternatively, microwave diaphanous refractory containers or caskets are used inside the chamber. This increases the versatility and reduces world power requirements for small-scale tests. An example of a commercial system that uses only 2.45-GHz magnetrons is the Dennis Tool Co. ( Houston, Texas ) system ( Fig. 4 ), developed in cooperation with Penn State University. The system is sold as a complete software including the materials-process engineering. The concenter has been on carbide ( hard metal ) fabricate, and the process is in the final degree of scale-up at a cutting-tool company. In scale up, it was found that processing smaller loads produced a more consistent, superior product. however, even larger loads placid are superior to conventionally fire tungsten carbides.


Linn High Therm ( www.linn.de ) makes a lab size 2.45-GHz microwave sintering furnace, which has been available for about ten years. Linn ‘s holocene invention is the development and cheap manufacture of 5.8-GHz magnetrons, an industrial frequency that has not been explored for high-temperature processing of materials. There is potential for increase energy-field uniformity, particularly when combined with 2.45 GHz, and the hypothesis of heating more materials from room temperature without susceptors. Linn High Therm besides developed an efficient, modular continual-microwave drying system using a circular distribution of low-power magnetrons. It would be of sake to see this system modified for high-temperature work. Another hybrid system was developed at EA Technologies, now owned by C-Tech Innovations ( www.capenhurst. com ), which uses a combination of microwaves and gas or electric heating ( Fig. 5 ). Advantages of this organization include even heating, energy efficiency, and the capability to retrofit magnetrons to existing furnaces. The method has been tested, and initial scale up has been performed for many ceramic materials. The technology recently was licensed to a japanese kiln manufacturer. Its handiness in Europe and the U.S. still is an return for the use of this engineering. Ceralink is assisting C-Tech Innovations in licensing this engineering to U.S. furnace manufacturers therefore microwave-assisted boast and electric furnaces will be manufactured in the U.S. ( Contact Ceralink for more information. )

Conclusions

Microwaves are finding their topographic point in many high-temperature processes, such as hotness treat, annealing, and firing. Feasibility, or proof of concept, is an important beginning stage that can be performed in house using a “ homemade ” or commercial microwave system, or it can be outsourced, for example, to a university or microwave testing center. Standardized microwave systems are utilitarian for research, development, and initial scale-up. These systems save meter over the “ do-it-yourself ” method of developing a microwave furnace. many materials manufacturers finally will need to have microwave furnaces custom built. Hiring a adviser to assist in the design of the microwave furnace and survival of microwave manufacturer can ensure that the most appropriate engineering is used to get the best respect from microwave heat.