New High Temperature Multipurpose Applicator
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NEW HIGH TEMPERATURE MULTIPURPOSE APPLICATOR
W.R. TINGA*, B.Q.TIAN* AND W.A.G. VOSS* *Electrical Engineering Dept., University of Alberta, Edmonton, Alberta, Canada, T6G 2G7 ABSTRACT Using a new microwave small-sample quasi-TEM mode applicator, heating rates up to 700*C/s were obtained for various oxides and ceramic materials at 915 MHz. Design details are presented. A 60 W solid state power source is used to supply the microwave energy and control material temperature. Temperature is measured using either infrared or thermocouple techniques. The applicator is a TEM resonator with a TM mode gap field modified by a hollow variable radius center conductor acting as a waveguide below or near cutoff. This design creates a microwave materials analyzer equally suited for high or low temperature material studies, is scaleable to different frequencies and can be used to measure dielectric properties up to a temperature of at least 1500*C. Solid, granular, liquid, gas or plasma samples can be accommodated. By using microwave transparent refractory materials around the sample, temperatures 0 greater than 1500 C can be maintained in some materials. Perturbation of the resonator by the inserted material causes a frequency shift which is nearly linear over a permittivity range of at least 1-70. This is an order of magnitude improvement over the conventional perturbation approach. For microwave joining or sintering of ceramic rods, the energy can be concentrated into a symmetrical hot zone as narrow as 1 mm. INTRODUCTION Increasing numbers of researchers are trying to use microwave (MW)energy to advantage in heating, sintering, joining and measuring MWand other material properties such as thermodynamic parameters. Many laboratories process or treat small material samples in MWovens but the results are often misleading and non-repeatable because of the oven's variability with respect to energy uniformity, load placement and load size, among other difficulties. Controlled, repeatable, efficient and convenient microwave heating or processing of materials with simultaneous measurement or material analysis ability has been an elusive goal, the more so as required temperatures exceed 10000C. Our research is driven by a real need to move closer to that goal the preliminary results of which are discussed in this paper. Some progress towards this goal has previously been reported [1,2,3]. Our approach is somewhat different. A resonant re-entrant coaxial applicator is used with a hollow (variable radius) center conductor acting as a waveguide below or near cutoff. We have been able to achieve controllable, efficient, repeatable heating and measurement performance at low power levels using an electronically tunable solid-state 60 W power source. Temperatures beyond 15000C have been achieved in small samples at rates exceeding 700*C/s. Materials with widely varying permittivity and loss factor can be conveniently treated with MWenergy using this approach. The present design is suited to 7 mm diameter cylindrical materials with an interacti
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