Ceramics microwave heating

Shulman, H. S. Microwave Heating Ceramics, Alfred University Undergraduate Seminar, Dec. 12, 2002. 

G. A. Kriegsmann and B. A. Wagner, Microwave heating of carbon-coated ceramic fibres a mathematical model, IMA Journal of Applied Mathematics, 55, 243-255 (1995). 

Nowadays, many advanced techniques are available in the ceramic industry to coat a solid layer onto a solid surface or to make ceramic materials with special properties [99-1 IS], such as spin-coating [99], chemical vapor deposition [100-106], and chemical vapor infiltration [106-109], thermal spray [110-112], plastic spray [113], and spray-coating [114]. The deposition can be caused by conventional heating, by laser beam, or by microwave heating. 

The temperature fields induced by microwave heating can be modeled via the simultaneous solution of Maxwell s equations (for the electromagnetic component of the problem) and the heat equation. The modeling is very challenging, in part because the dielectric constants sj. and s" are functions of temperature and the microwave frequency as well as of the microstructural and chemical details of the ceramic [Eq. (lb)]. Also, the thermal conductivity, k (which is needed for the heat transfer calculations), typically is a function of temperature as well as of microstructural variables such as porosity. ... 

A key aspect of the uniformity of the temperature field in both low- and high-temperature processing is the nature of the thermal gradients within the material. Consider the temperature distributions within a flat ceramic slab of thickness L (Fig. 10). For microwave heating (top curve in Fig. 10), the temperature is relatively uniform within the bulk, with a drop in temperature near the specimen surface owing to heat losses. In contrast, for conventional heating from the specimen surfaces (bottom curve in Fig. 10), the temperature is highest at the surface and lowest near the specimen s midplane. 

Fig. 10 Schematic of temperature distributions in a flat ceramic slab of thickness L for volumetric microwave heating (top curve) and conventional heating from the slab surfaces (bottom curve). For conventional heating, the finite value of thermal conductivity, k, gives the highest temperatures near the specimen surface and the lowest temperature along the specimen s midplane. Conversely, for microwave heating the heating is more uniform, with decreasing temperature near the slab surface because of heat losses from the surfaces.

Another ceramic processing technique that involves drying as a key step is that of the sol-gel production of ceramic powders. Sol-gel precursors consist of small particles weakly bound together, permeated with networks of fine, interconnected pores. These pores typically are filled with either an alcohol-based or a water-based liquid. Microwave heating has been successfully applied to dry sol-gel precursors. ... 

However, there have been a number of reports of athermal effects in processing ceramic materials, where the sintering rate or the microstructure evolution (grain size and/or porosity) resulting from microwave heating differed from that obtained by a conventional heat treatment at the same temperature. Thus, athermal effects refer to mechanisms that operate in addition to the conventional thermal effects and may be a function of, for example, the electric field intensity or the frequency. 

Tap, R. Willert-Porada, M. Synthesis of composite powders and coating of fibers by combined CVD-PECVD in a microwave heated fluidized bed reactor. In Microwave and Radio Frequency Applications, Folz, D.C., Booske, J.H., Clark, D.E., Gerling, J.F., Eds. The American Ceramic Society Westville, OH, 2003 89-97. 

Sizgek, E. Sizgek, G.D. Drying characteristic of porous ceramic microspheres in microwave heated fluidized bed. Chem. Eng. Technol. 2002, 25 (3), 287-292. 

Aravindan, S. and Krishnamurthy, R., Joining of ceramic composites by microwave heating, Mater. Letters 38 (1999) 245-249. 

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