Microwave Processing of Materials
- PDF / 4,763,710 Bytes
- 8 Pages / 576 x 777.6 pts Page_size
- 9 Downloads / 250 Views
22
sessions or meetings on related topics such as microwave processing of polymers6 and microwave chemistry.7 This introduction briefly discusses some of the key features and characteristics of microwave processing, and also introduces the articles in this issue of the MRS Bulletin. The topics were selected to highlight some of the many areas where significant developments are being made in microwave processing technology, and the articles were written by leaders in their fields. The authors have included references for further reading in order to provide greater depth and detail for their particular topics. Microwave Characteristics Microwaves possess several characteristics that provide unique features that are not available in the conventional processing of materials. The following key characteristics, either singly or in combination, generate several features that provide new opportunities and benefits over conventional heating or processing methods: • Penetrating radiation, • Controllable electric field distributions, • Rapid acceleration of dielectric losses above a critical temperature, • Differential coupling (absorption) of materials, and • Self-limiting reactions. These characteristics also introduce some problems and challenges, which are discussed in the articles that follow.
Penetrating Radiation Microwaves can penetrate up to many meters in electrically insulating (dielectric) materials such as ceramics, polymers, and certain composite materials. The depth of penetration depends on several factors, including the wavelength (frequency) of the irradiation and the dielectric (and magnetic) properties of the material. During
the time a material is exposed to penetrating microwave radiation, some of the energy is irreversibly lost (absorbed), which in turn generates heat within the volume (or bulk) of the material. This bulk heating raises the temperature of the material so that the interior portions become hotter than the surface; this is the reverse of conventional heating, where heat from an external source is supplied to the material's exterior surface and then diffuses toward the cooler, interior regions. The reverse thermal gradients in microwave heating provide several unique benefits: rapid volumetric heating without overheating the surface (especially of low thermal conductivity materials); reduced skin effect during the drying of wet materials; removal of binders or gases from the interior of porous materials without cracking; or, conversely, penetration of reactive gases (during chemical vapor infiltration) or fluids into the hotter interior portions of porous materials and preforms, which then condense as solid matter that fills the voids or pores in a progression from the interior to outer regions. Since heating is instantaneous with power input, the temperature of a material can be precisely controlled by controlling the input power. However, this is neither a simple nor straightforward situation because the internal generation and dissipation of heat depend on many factors, which also hav
Data Loading...
