Recent Developments in the Microwave Processing of Ceramics

Extract

Highly absorbing ceramics can be heated in a standard multimode microwave oven. However, most of the microwave energy is dissipated in their ceramic surfaces and heat is transported slowly throughout the sample volume by the usual thermal conduction mechanisms. As with conventional methods, this process leads to non-uniform heating. In contrast, ceramics that are fairly transparent to microwave energy are difficult to heat in standard microwave ovens. When they can be heated, reverse temperature gradients often develop. Reverse temperature gradients can be beneficial in some processes, e.g., combustion synthesis, chemical vapor infiltration (CVI) and binder burnout, but in most ceramic processes, temperature gradients are undesirable. These characteristics have presumably been partially responsible for the ceramic industry's failure to examine microwave processing more closely.

The ability of dipoles to respond to the oscillating electric field and their subsequent inability to keep up with rapid reversals in the field result in dielectric losses that are manifested as heat. While some of the mechanisms by which electromagnetic radiation interacts with ceramics are well known, interaction mechanisms in the microwave frequency range are less well understood. Since several mechanisms, including electrical conductivity, can operate simultaneously, the total energy dissipation and heat generation depends on the effective loss factor. The effective loss factor (ε″_eff) is described by

where ε′ is the dielectric constant and is a measure of the extent of polarization, and tan δ indicates how responsive the dipoles are to the oscillating field.