Microwave energy penetrates into green parts to directly heat organic binders, minimizing green part stress and quickly driving out the binder.

Organic binders are widely used to improve the press-ability, green strength and handle-ability of ceramics and powdered metals. Binders can be added to wet or dry systems in a variety of processes, from shear mixing to spray drying, depending on the forming demands of the process.¹ Once a part is ready for firing, however, the problem of removing the binder becomes a bottleneck in most heating cycles.

Binders are heated and turned into a vapor phase, which migrates to the surface of the part. As the binder is removed, the part loses green strength and becomes highly prone to thermal stress. Uniform heating throughout the part is critical to prevent cracking and bloating. Conventional methods employ indirect heating from the kiln atmosphere and rely on thermal conduction through the part.

Thus, binder removal has conventionally been dependent on heat transfer from the surface of the part to the center. This is an inherent problem, as the material being heated is often thermally insulating, preventing heat from reaching the binder in the core. These conditions dictate very slow heating rates, resulting in a long, energy-intensive binder removal process.

A solution to this problem has been developed through advances in microwave heating technologies. Microwave energy penetrates into ceramic and metal green parts and directly heats organic binders. Microwaves introduce an entirely new heating mechanism, which is independent of thermal conductivity. This mechanism is known as coupling, suscepting or dielectric heating. Direct coupling allows the binder at the center of the part to be heated at the same rate as the binder at the surface of the part, offering the opportunity for fast heating without the development of a large thermal gradient. The stress in the green part is minimized, and the binder is driven out quickly.

Determining Sustainability

Typically, organic binders couple and heat more strongly than most ceramics and powder metals, especially at low temperatures (< 600°C). The microwave energy preferentially targets the binder until it is completely volatized and driven out of the part. This is due to the dielectric heating mechanisms taking effect at binder burnout temperatures. Dielectric heating is primarily dependent on internal friction caused by field-induced movements at the molecular and atomic levels.

For many ceramics, microwaves are not energetic enough to induce significant movement until higher temperatures make such movement easier. Most organic binders are polar molecules and non-crystalline, which allows the microwave field to induce movement of the dipoles in the binder. Since the dipoles cannot keep up with the alternating field, friction is produced, causing preferential heating and volatilization of the binder.

Microwave heating depends on dielectric properties, which are measured to predict how a material will heat in a microwave field as a function of temperature and frequency. These measurements include permittivity, dielectric loss and microwave penetration depth. The loss tangent, tan delta, is a ratio of both permittivity and dielectric loss, and is therefore an important gauge for microwave heating. As a rule of thumb, when tan delta is greater than 0.01, a material will absorb microwaves and heat. With tan delta on the order of 0.1, a material will heat vigorously in the microwave.

Figure 1 shows an example of tan delta as a function of temperature when comparing a green ceramic body that includes binder to one without binder. The data indicate that this particular ceramic without binder will heat moderately well in a microwave until 650°C, at which stage the ceramic will strongly absorb and heat.

From room temperature through the binder burnout temperature range (up to 650°C), the green body with binder shows much higher tan delta, indicating stronger heating than the ceramic. The curves re-join at 800°C, since the binder is completely volatilized by this temperature. Above 800°C, the tan delta curves for the two samples are virtually identical. This demonstrates that the binder strongly absorbs microwaves; thus, microwave energy will directly heat and volatize the binder throughout the green part.

Achieving Uniform Heating

Although microwaves provide the opportunity for uniform heating, heat is naturally lost from the surface of the part when the atmosphere has a lower temperature. This condition is known as an “inverse temperature profile” because it is the reverse of the heat distribution from conventional radiant methods (see Figure 2). As a result, a thermal gradient can be developed in the part undergoing microwave heating, with the center being hotter than the surface.

For conventional heating, the outside of the part is hotter and the center temperature lags due to slow thermal conduction. One binder removal study conducted at the University of Bayreuth, Germany, compared modeling with experimental results, and both showed that core temperatures were hotter than the surface for microwave-only heating.² When compared with the surface temperature for conventionally heated samples, the sample surface was cooler in the microwave and achieved equivalent binder weight loss. This was due to the hotter core in the microwave case. Although microwave heating is volumetric, radiant heat must be applied to prevent surface cooling and achieve uniform heating.

Radiant heat can be applied within a microwave system in two ways. One method involves the use of a susceptor, such as silicon carbide.³ Silicon carbide readily couples with microwave energy from room temperature and converts that energy into radiant heat. Susceptor heating is entirely dependant on the microwave energy, and, therefore, the radiant heat cannot be controlled independently.

The second method uses traditional gas or electric sources. Known as Microwave Assist Technology^TM (MAT^TM), the radiant heat in this method is independent of the microwave field, offering a greater level of control in processing. The radiant heat can be adjusted to accurately compensate for surface cooling.

Another benefit of adding microwaves to traditional kilns is the ease and familiarity of use. A MAT kiln looks and operates much like a traditional kiln. The kiln loading and programming are similar, making the equipment user friendly.

In addition, microwave radiant heat via susceptors is actually less efficient than radiant heat through conventional electric or gas. The best use of microwaves is for volumetric heating, driving the heat energy into the core of the part. A relatively small amount of microwave energy has a significant impact on the rate and uniformity of heating. The overall time savings causes the process to be more energy efficient and less expensive than conventional binder removal.

Modeling Potential Applications

In order to add microwaves to traditional kilns, several important factors must be considered, including the prevention of microwave leakage, microwave coupling with insulation or kiln furniture, and thermocouple or element interference. Methods have been developed to address the integration of microwaves with kiln designs.*

The use of volumetric microwave heating combined with radiant heat can be modeled to determine heating uniformity and assist with process development. (MAT modeling software was developed with Rensselaer Polytechnic Institute.) The model uses dielectric, thermal and physical properties to predict how a material will heat as a function of temperature and microwave power. The model shows that radiant heat minimizes thermal gradients.

Figure 3 shows one timeframe of the modeling results for electric field distribution and temperature uniformity in a zirconia cube. For the heating simulation, the temperature value for the radiant heat source (e.g., the element temperature) was set to equal the maximum value of the temperature in the sample. Figure 4 illustrates this concept for two different microwave power levels, where the surface and core temperatures are graphed as a function of time.

The temperature in the center of the part was slightly hotter than the surface-10°C for the high microwave power and 20°C for low power. When compared with microwave-only heating in the Bayreuth study,² which showed a 100°C difference, this demonstrates increased temperature uniformity through the addition of radiant heat. It is also clear that the radiant heat source must be set slightly higher than the desired surface temperature in order to completely eliminate any temperature gradient. This modeling capability can be used to optimize microwave binder heating profiles, specifically for temperature uniformity within products.

*Much of this work was carried out and patented in the UK by C-Tech Innovations.⁴ Ceralink obtained exclusive rights to Microwave Assist Technology in order to make it available to industry in North America. MAT kilns are now being offered commercially by Harrop Industries and Carbolite Ltd. through licensing from Ceralink. These companies have modified traditional kiln designs to incorporate microwave energy.

Improving Binder Burnout

The use of microwaves shows tremendous potential for improving the uniformity and speed of binder removal. This fast volumetric heating method can have a substantial impact in the reduction of energy costs for materials manufacturing.

For more information, contact Ceralink, Inc. at Rensselaer Technology Park, 105 Jordan Rd., Troy, NY 12180; (518) 283-7733; fax (518) 283-9134; e-mail info@ceralink.com; or visit www.ceralink.com.

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• Ceralink Inc.