A Free-Space Focused Microwave System to Determine the Complex Permittivity of Materials to Temperatures Exceeding 2000°C

Abstract

The instrumentation, the design of the free-space focused microwave bridge system, and the technique for determining the complex permittivity of materials from ambient to 2500°C are described. The system employs a rotating disk sample which is located at the coincident focal point between two prolate spheroidal reflectors; the sample is heated on one side by oxyacetylene flames in such a manner that the microwave beam is not affected by the flames. Dynamic measurements are made of the sample insertion phase and insertion loss. A transient heat conduction analysis is used to determine the temperature profiles within the sample, and the electrical transmission data are correlated with the temperature profiles to give dielectric constant and loss tangent as functions of temperature. The relative dielectric constant and loss tangent of high purity slip-cast fused silica (density of 1.91 g/cm3) are determined to 2500°C. The relative dielectric constant vs temperature function is a segmented line and is 3.34 at ambient, increases linearly with temperature to 3.54 at 1500°C, 3.98 at 1600°C, 4.18 at 1700°C, and 4.32 at 2500°C. The loss tangent vs temperature function is also a segmented straight line when plotted on semilog paper with a value of 0.0005 at ambient and 0.09 at 2500°C.

Design of flexible platform for various free-space, broadband dielectric analysis measurement techniques

“…He reported that free-space methods are non-destructive and contactless techniques which are specially suited for dielectric measurement of materials. In 1971, Harold L. Basset [2] was the first researcher to measure complex permittivity in free-space using spot-focusing antennas at a frequency of 9.4 GHz. He measured complex permittivity of fused silica as a function of temperature from ambient to 2500 C.…”

An estimate of the error caused by the elongation of the wavelength in a focused beam in free-space electromagnetic parameters measurement

“…1, 2 The spot-focusing horn lens antennas were used for minimizing diffraction effects due to the edges of the sample. [1][2][3][4][5][7][8][9] The incident wave projected by a spot-focusing horn lens antenna into the sample approximates a Gaussian beam, [11][12][13] as shown in Fig. 1, so the incident wave illuminating on the surface of the planar sample is not the wave of free-space actually.…”