Investigation of Thermal Effects in Coaxial Probe Method and Dielectric Characterization of Glycerol up to 140°C

Abstract

Dielectric properties of a material are of paramount importance in many areas of knowledge. All scientific fields that deal with the interaction of electromagnetic radiation with matter demand this information. Consequently, suitable methods for obtaining accurate dielectric data are required. In the microwave community, the dielectric metrology with the coaxial probe method is well known due to its simplicity and practicality. It is also pointed out as a broadband and wide temperature range technique. Despite this, a detailed study of the implications of thermal effects at calibration with this technique has not been plainly reported in the literature. In the present work, a concise exposition of the problem is made. Using air, short and water as standards, we have analyzed a set of calibration procedures with different thermal conditions and compared them with the isothermal case. With the aid of statistical analysis, we have made recommendations for precise and accurate dielectric measurements at high temperatures. Using glycerol as material under test (MUT) we also demonstrated that measurements at temperatures up to 50°C above the calibration temperature can provide reliable data. Thus, measurements of glycerol up to 140°C were performed by calibrating the system with water at 90°C. This important issue was raised due to the lack of standard material data in the literature for temperatures above 100°C. From the measured data we also have derived the Cole-Davidson model parameters of glycerol from 30 to 140°C.

Influence of dielectric properties of polybutylene terephthalate and respective foam beads on process behavior in radio‐frequency welding

“…Thus, resulting in a mixture of beads at different temperatures and therefore varying dielectric properties, increasing the power input at the end of the process. As Campos discussed in his study, the dependency of on temperature is crucial for a homogenous heating 41 . If the material shows an upwards trend in , local hotspots due to material variations or disturbed fields absorb even more energy amplifying the inhomogeneity.…”

“…This results in a stepwise decrease in real part with frequency, which in Figure 4a is only partially visible per temperature due to the constrained measurable frequency range in the setup. With increased mobility of the dipoles to align, relaxation time τ is reduced at higher temperatures, resulting in a shift of the curves towards higher frequencies 41 . This shift is responsible for the change in shape between 20 and 220°C as for the overall increase.…”

“…In this transition phase, the dipoles lag behind in orientation to the external electromagnetic field so maximum field strength is simultaneously present with considerable lever to introduce maximum torque into the dipoles. This phenomena results in a peak in imaginary part of the relative permittivity 41 . Above a certain frequency, dipoles can barely orientate within the fast changing electromagnetic field, so movement and therefore friction is absent 22,42 .…”

Dual Coaxial Probes in Transmission Inserted by Dielectric With Two Different Thicknesses to Extract the Material Complex Relative Permittivity: Discontinuity Impacts

“…We can determine the specimen thickness gap by using Equation (18). As shown in Table 1, the SUT thicknesses are not the same for Alumina, FR-4, and RO4003C; even the two last materials have thicknesses slightly identical.…”

“…Among several existing methods [12,13], the probe or applicator method is well known [14][15][16]. It is either used for magnetic [17] or dielectric [18] materials.…”