Preliminary Thermal Model for High Temperature Processing Using Microwave Energy
- PDF / 326,446 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 83 Downloads / 216 Views
PRELIMINARY THERMAL MODEL FOR HIGH TEMPERATURE PROCESSING USING MICROWAVE ENERGY
E.
EUGENE EVES II
AND RONALD L.
SNIDER
RAYTHEON COMPANY, Industrial Equipment Group, Street Waltham, MA 02254
190 Willow
ABSTRACT Ceramics processing with microwave energy is dominated by empirical testing of small samples. Concepts and models for reconciling information between different types of heating trials, and for predicting heating performance for large parts are discussed. INTRODUCTION Advances in materials science and the creation of new products as diverse as high temperature super conductors, ceramic composites, optical coatings, and use of high temperature structural ceramics has spurred interest in new heating technology. Investigators in both industry and college laboratories have responded by developing applications which process these materials using microwave energy as the heat source. The expectation for microwave heating is the production of a more uniform product with better physical properties. The user hopes to accomplish the process in less time and to use less space and energy than he would otherwise require for conventional heat processing. Some investigators have also reported " non-thermal" effects resulting from exposure to microwave energy, where state changes occur at temperatures lower than those associated with conventional processing. Interest is focussed on the new part or material produced as distinct from the detailled method of its production. In any thermal process the quantified description of the heating procedure is intimately related to the equipment used. Microwave heating systems retain this characteristic. Lack of consistant models has created difficulty in reproducing results from one laboratory to another.
PROCESS DESIGN In a microwave heater, to estimate the energy and time requirements of a material process, generally two pieces of information are essential: Required material final temperature. Required material weight loss, if any.
Mat. Res. Soc. Symp. Proc. Vol. 189. ©1991 Materials Research Society
202
In conjunction with knowledge of the size, mass, and specific heat of the test object, this information is sufficient for a low temperature (up to 500 degrees F or so) process like water or binder removal. Laboratory testing has demonstrated a good fit between theoretical energy calculations and experimental data for large parts heated in several microwave processors. For large parts a factor of ten reduction in drying times for complex slip cast clay bodies and simple alumina castable shapes is typical. The reduction is based upon standards for conventional processing. From experience with tests at low temperature we have developed several rules to predict the behavior of bodies of other sizes cast from the same materials and processed in the same or similar ovens. To the first order, the energy gain of the body is the energy provided by the oven multiplied by the process efficiency. Efficiency may be expressed as:
n
H
(1)
=
M + No where N is the mass of the body bein
Data Loading...
