Online Exclusive: Affordable Microwave Processing

September 1, 2005
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New technologies are making microwave systems more affordable and easier to use for glass and ceramic processing.

Components in a microwave furnace.
For an increasing number of applications, microwave processing is proving to be an ideal heating technology. Many companies use a low-cost kitchen microwave to dry materials, including inorganic powders, paper products and sintered parts. However, the next step-getting to higher temperatures-requires thermal insulation. A kitchen microwave has the power, but it can't take the heat. Appropriate "microwave-transparent" refractory materials will allow the microwaves to get through to the sample, but real materials are never perfectly transparent. Some refractories couple in the microwave, meaning they heat in the field and can potentially melt down.

Over the last few years, a new microwave technology* has emerged that is designed to be both inexpensive and durable enough for a range of applications, including glass and ceramic processing. With this new system, companies are increasing their product quality and production efficiency.

*The ThermWave™, developed by Eugene Lunghofer and Larry Wolfe, EPL Ceramics LLC, through New York State Energy Research Affiliation (NYSERDA) funding; available commercially from Research Microwave Systems (RMS) LLC, Troy, N.Y.

Silicon carbide discs in a microwave furnace.

Principle of Operation

In microwave sintering, microwave energy is transported as an electromagnetic wave in certain frequency bands in the range of about 0.3 to 300 GHz. When microwaves impinge on a dielectric material, part of the energy is transmitted, part is reflected, and part is absorbed by the material, where it is dissipated as heat. The heating occurs due to "molecular friction" of dipoles within the material as they try to reorient themselves within the oscillating (electrical) field of the incident wave.

The power generated in a material is proportional to the frequency of the source, the dielectric loss of the material, and the square of the field strength within it. A chamber known as an "applicator" is used to subject a material to microwave energy. No contact is required between the energy source and the target, and heating can be volumetric, instantaneous and highly specific. An understanding of the dielectric properties, along with experience with microwave interactions, makes it possible to identify the candidate materials and processes that can use microwave heating effectively.

A new microwave system has been developed that is composed of a controller unit; a thermocouple (S type, platinum covered) inserted into a top port; and the microwave, which features a water-cooled shell. A thermal pod with insulating shells is used to hold the material being processed.

Many materials, such as titania, zirconia and iron oxide, will self-heat in a kitchen microwave if placed in this thermal pod. However, other materials, such as alumina and silica, will not couple unless they are heated to several hundred degrees first. In this case, hybrid heating is recommended. An inexpensive solution is to use a susceptor material, such as silicon carbide (SiC), which heats by radiation, until the sample can absorb microwave energy and heat directly.

Figure 1. Microwave heating behavior for several typical silicon carbide grades.
The new microwave system uses a proprietary composition of SiC susceptors** developed for quick heat up and thermal shock resistance in microwave processing. The susceptors are placed in a suitable configuration (symmetrically) within the pod, and the material to be processed is placed between the susceptors. The entire thermal pod is then placed in the microwave for processing.

A comparison of microwave heating behavior for several typical silicon carbide grades is shown in Figure 1. This study was performed in a 1.3 kW microwave system using the same mass of susceptors in the same thermal pod. The susceptors outperformed an industrial silicon carbide grinding block and another commercially available hot-pressed silicon carbide.

The susceptor bricks and refractories were also successfully applied in a microwave metal melting operation using 75 kW of 915 MHz power in a 6-ft cube chamber, shown in Figure 2. This is an important frequency for process scale up, since 915 MHz is less expensive and is available in higher power units compared to 2.45 GHz (the kitchen microwave frequency).

**Thermcepts, available commercially from RMS.

Figure 2. The 6-ft cube chamber used to test the susceptor bricks and refractories in a microwave metal melting operation.

Applications

The new microwave systems can be used for the production of small parts, quality control, rapid prototyping and research. In one example, a 1.3 kW unit was used to test the feasibility of fast firing a precision ceramic part to near net shape. The temperature requirement was 1300°C (2372°F), and the proprietary SiC susceptors were used for hybrid heating. Testing was successful, and the process is currently transitioning to production in a 2.0 kW system.

In quality control applications, the microwaves can be used for testing glaze formulations, identifying thermal expansion mismatch, evaluating ceramic part shrinkage, identifying flaws from front-end processing, and rapid calcination or sintering for property measurement feedback. A process was developed for a ferrite manufacturer that needed to check the lot composition prior to production firing. The microwave system was used for rapid firing, and samples were characterized within hours instead of days. To match the properties of the production parts, a designed experiment was run in the microwave, and the appropriate process control parameters were identified. In fact, better properties were achieved in the microwave system for certain conditions. This manufacturer recognizes the potential to use microwaves in production of a new high-grade product without having to change the ferrite composition.

Rapid prototype ceramic components are also good candidates for microwave heating. Designs and compositions can be optimized quickly, with rapid feedback from the fired state. One company, Javelin 3D, uses a microwave system to sinter silicon nitride coupons produced by the Laminated Object Manufacturing method-a rapid prototype method that produces parts directly from computer-aided drawings. The sintering process, which was designed specifically for Javelin, introduces nitrogen through a gas inlet thermocouple; uses a powder bed to prevent oxidation; and incorporates a small, high-temperature thermal package.

Many other ceramic materials, including alumina, zirconia, zirconia-toughened alumina, indium tin oxide, barium titanate, porcelain, clay, zinc oxide, minerals and phosphorus powders, have been successfully processed in the new microwave. These materials are used in components such as spark plugs, dental ceramics, capillaries, capacitors, varistors, wear parts, MEMs and sputtering targets, to name a few.

In a research project, tests were conducted in the new microwave to determine the effect on densification of indium tin oxide pellets. Higher density was achieved in the microwave system than by conventional heating. Other studies have shown finer grains with improved hardness for alumina, zirconia and ZTA parts. These results attracted the attention of a dental company that successfully used the microwave system to fire ceramic teeth.

As more companies come to understand the benefits of microwave processing, additional applications are likely to emerge.

For more information about microwave technologies, contact Research Microwave Systems at 105 Jordan Rd., Troy, NY 12180; (518) 283-9133; fax (518) 283-9134; e-mail info@thermwave.com; or visit http://www.thermwave.com.

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