Investigation on temperature-dependent dielectric properties of ETFE fluoropolymer for microwave temperature sensing applications

This paper investigates temperature-dependent dielectric properties of fluoropolymers for the development of original low-profile and cost-effective microwave temperature sensors. Here, ethylene tetrafluoroethylene (ETFE) was chosen for its superior resistance to common solvents, excellent thermal stability and high selectivity compared to other fluoropolymers. The temperature-dependent dielectric constant of the commercially available ETFE sheet named “Fluon” was equated and analyzed. A dual-resonant circuit in coplanar waveguide technology (CPW) was designed and prototyped to achieve temperature sensing. To integrate temperature sensing capability within the device, one resonator was covered by the Fluon sheet while the other one remained uncovered. From there, specific schemes achievable with either electromagnetic (EM) reflected and transmitted through the CPW line were studied and validated. A differential approach comparing the variable resonance with Fluon to the static resonance without Fluon was implemented for reflected signals. At the same time, S21 phase shifting reflecting slowdown/acceleration of the EM field was fulfilled for transmitted signals. Experimental tests were performed within the 23 °C–80 °C temperature range. The sensor exhibits a sensitivity of 4.07 MHz/°C which is significantly better than the results obtained with other sensing approaches and technologies in the literature. The overall phase shift is 13.35°, leading to a sensitivity of 0.235°/°C.