Dielectric spectroscopy of standard liquids as a function of temperature

Abstract

The dielectric properties of a material describe its storage and dissipation of energy when it is exposed to electromagnetic radiation. Permittivity measurements were performed on the 0.1 mol/1 NaCl solution at temperatures between 25°C and 52°C and on Formamide at 30°C and 35°C covering a frequency range of 0.5 - 40.8 GHz. This study was done in order to validate the coaxial probe technique. At the investigated frequencies, the majority of biological tissues behave in a similar way to the 0.1 mol/1 NaCl solution. Thus, the coaxial probe technique could be used to measure the permittivity of different biological materials. There are many published works on the dielectric properties of NaCl solutions at low temperatures and frequencies. However, there are currently no published data for temperatures higher than 35°C and frequencies higher than 20 GHz. This study extends the experimental values of dielectric parameters into higher frequencies and higher temperatures. The permittivity measurements were fitted to the Debye, Cole-Cole and Cole-Davidson models and were compared to find the best fit of the measured data. The resulting dielectric parameters were compared to published values. Equations representing the dielectric parameters of 0.1 mol/1 NaCl solution as a function of temperature were also obtained by fitting the dielectric parameter values to a polynomial. Dielectric parameters of the 0.1 mol/1 NaCl solution were plotted as a function of frequency at different temperatures and were compared to published values. Measurement uncertainty analysis was performed on both Formamide and the saline solution measurements. Good agreement of measured parameters with published work and low confidence intervals of the dielectric parameters were achieved. These are a good indication that the permittivity measurement system used in this study is well suited for complex permittivity measurements on standard liquids at the investigated frequencies and temperatures. The proposed method is not only useful for standard liquids but also for lossy materials such as biological matter.