Autores: Fall, M.|Shulman, H.
Fuente: Ceramic Industry
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.
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