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

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 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 Debye model parameters of glycerol from 30 to 140°C.