Thermal Runaway of the BaCO3 + Fe2O3 Homogenous Mixture and Mechanical Alloys at the Microwave Heating

Abstract

Microwave heating represents a modern technique to sintering the composites materials. The microwaves absorbance property of the materials is depending by the electrical permittivity of the materials. Researchers showed that the ceramic materials are suitable for sintering using microwave heating. The most important advantage of that sintering procedure is the reduced sintering time and temperatures. However, during the heating process these properties are changing and a pattern of the heating process cannot be established. The penetration depth of microwaves into materials depends on the electrical properties of them, and gives rise to a heat source. The electromagnetic wave absorption is responsible for the macro and micro structural changes in the materials morphology, and consequently for their electrical properties. Thermal runaway is one phenomenon which should be avoided during the microwave processing of the materials. The microwave heating consists in direct introduction of the energy in the volume of the material. If the absorbance properties of the material are increasing with temperature, than a critical phenomenon, called thermal runaway, appears during the heating process. This paper aims to study the thermal runaway of the BaCO₃ + Fe₂O₃ homogenous mixture and mechanical alloy in a mono-mode applicator, when the heat source is a microwave generator at 2,45 Ghz. A special mono-mode chamber has been designed with dimensions 140 x 140 x 70 mm and an active system for rotating the samples, in order to record the values of the temperature and to assure a uniform exposure of the samples to the high frequency electromagnetic field. The materials used in experiments were homogenous mixture of BaCO₃ + Fe₂O₃ which have been milled in a planetary ball mill for 5 and 20 hours. The experimental procedure consists in establishing the levels of the temperatures during the microwave heating process when the thermal runaway appears. These experiments have been done for fixed levels of microwave injected power from 0 1250 W. Numerical simulation for different heating conditions (microwave power, heating time, position of the samples inside the chamber) has been performed in order to elaborate a predictable mathematical model for continuous microwave heating and avoiding the thermal runaway of the homogenous mixture.