Recent Developments in the Microwave Processing of Ceramics
- PDF / 1,503,895 Bytes
- 6 Pages / 576 x 777.6 pts Page_size
- 46 Downloads / 238 Views
(1)
where s' is the dielectric constant and is a MRS BULLETIN/NOVEMBER 1993
measure of the extent of polarization, and tan 8 indicates how responsive the dipoles are to the oscillating field. For many ceramics, tan 8 is low at ambient temperatures, but becomes increasingly significant as the temperature of the material increases. Both e' and tan 8 (and hence e"^) depend on frequency and temperature. The temperature at which the rate of increase in e"ea changes significantly is referred to as the "critical temperature," Tc. The Tc for a ceramic heated with microwaves is the temperature at which the value of e"ef( becomes sufficient to cause efficient microwave/material interaction. High Tc ceramics are more difficult to heat using microwaves alone. Either the power of the microwave must be increased substantially or an alternate heat source must be used in combination with the microwaves. Since 1987, ongoing research at the University of Florida has focused on a variety of glasses and ceramics with the ultimate goals being to (1) achieve a fundamental
understanding of microwave/materials interactions, (2) develop practical methods for processing routinely used ceramics (e.g., alumina, whitewares) more efficiently and/or with superior properties, and (3) evaluate the potential of using microwave energy to produce unique ceramics. We will discuss the latter two goals in this article by presenting brief descriptions of specific examples of our research.
Microwave Hybrid Heating When the depth of penetration (Dp) is much smaller than the cross-sectional dimension of the ceramic, surface heating will occur; when Dp is the same or greater, the ceramic is partially absorbing (heating will occur more uniformly); when Dp is much greater, the material is transparent and heating will be negligible. Some typical values for dielectric properties of alumina, zirconia (both fairly transparent), and silicon carbide (absorbing) at 2.45 GHz are given in Table I. Since many ceramic materials exhibit low microwave absorption and a high value of Dp, a technique called microwave hybrid heating (MHH) is used to achieve efficient heating. The method used at the University of Florida involves partially coating the inside of a refractory cavity with a thin paste of silicon carbide. When subjected to a microwave field, the microwaves penetrate the walls of the refractory and interact with the highly absorbing silicon carbide. The heat generated by the silicon carbide heats the sample by radiation to Tc, when the microwaves couple more efficiently with the sample. The microwaves interact directly with the sample, and the susceptor-lined refractory cavity contributes toward maintaining a uniform surface temperature. Samples heated by this method generally exhibit a more uniform thermal history than those heated by most conventional methods.7"9 Microwave hybrid heating techniques have been used by several research groups for the past few years. Researchers at Oak
Table I: Dielectric data and depth of penetration for alumina, zirconia, and s
Data Loading...
