Measurement of dielectric constant for a liquid and its temperature dependence

Abstract

The dielectric properties determine the electrical characteristics of materials. These properties are important for understanding the behavior of materials and how they can interact with each other. Engineers and scientists need to measure these parameters as accurately as possible, and thus be able to integrate them in their designs in a reliably way. Examples of application are dielectrics used in capacitors that have the function of reducing the applied electric field and increase the capacitors' capacitance. The later can be increased by using dielectrics with high permittivity (dielectric constant) as water which has a dielectric constant of 80 at room temperature (25ºC). Unfortunately water cannot be used alone as dielectric due its capacity to be conductive and has to be combined with other materials. However, this study will focus only on measuring the dielectric properties of water and its temperature dependence. Temperatures chosen for measurements are 0 and 86ºC. Several methods have been studied over the years to measure the dielectric properties of materials, but there are only three possible measurement methods for liquids: coaxial probe, parallel plates and free space method. Comparing the three methods, in our case the freespace method is better because it allows to perform measurements at high temperatures and in hostile environments. These two features are very important, since the water should be measured at 86 ºC and measurements are performed in a RF (Radio Frequency) lab, where interferences due to the electronic devices can affect accuracy in free-space measurements. Hence, the following thesis is based exclusively on analyzing the free-space measurement method for measuring the reflection parameters in dB by using two horn antennas. Both antennas are connected to the Vector Network Analyzer (VNA): one as transmitter and the other as receiver. Reflection parameters are also calculated by introducing the reflection formula for lossless material and a finite length in Matlab. Then, the dielectric constant is extracted by comparing both reflections in dB.