New High Temperature Multipurpose Applicator

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 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.