A simple capacitive cell for the measurement of liquids dielectric constant under transient thermal conditions

A simple device for the measurement of the complex dielectric permittivity of liquids in various thermodynamic states has been developed. It uses a cylindrical aluminium capacitor of a type currently applied in tuning antenna circuits. The capacitor is filled with the liquid solution under study. A comparison of its capacity is made with that of the nitrogen filled capacitor tested under the same thermal conditions. This comparison allows the determination of the real and imaginary part of the solutions permittivity as a function of temperature (between 150 and 300 K) and frequency (between 100 Hz to 2 MHz). After validating the technique with pure glycerol and pure 1,2-propanediol, spectroscopic measurements have been undertaken on pure and diluted 1,2-propanediol in water. Due to the low heat capacity and the high thermal conductivity of the capacitor, cooling rates of 40 K/min have been achieved inside the solution, allowing measurements in the supercooled liquid and vitreous states. Results are presented and discussed in terms of relaxation and the physical states of the sample. By selecting the required thermal conditions, this device permits the observation of thermal transitions, such as ice crystallisation, and measurements to be conducted in the unstable supercooled liquid state. These measurements are necessary in the development of an effective electromagnetic warming device for vitrified cryoprotective solutions.