TEMPO-oxidized cellulose nanofibre (TOCN) films and composites with PVOH as sensitive dielectrics for microwave humidity sensing

This paper investigates TEMPO oxidized cellulose nanofibers (so-called TOCN) films as sensitive dielectrics for humidity detection in microwave frequencies. TOCN is used either as a sensitive material or as a host-matrix enclosing polyvinyl alcohol (PVOH) to achieve highly sensitive humidity sensing. A resonator in coplanar waveguide grounded (CPWG) technology was designed and fabricated. TOCN and TOCN/PVOH gels were dropped in the area of the resonator where the analysis showed the electromagnetic field to be maximum at the resonance. Gels became thin films after drying. Experimental humidity tests were then conducted within the 21.9-89.3%RH range, using the resonant frequency and the transmission phase as measurement variables. The best sensitivity with TOCN was 2.67 MHz/%RH regarding the resonant frequency, and 0.523 degrees/%RH regarding the transmission phase. The effects of PVOH were visible starting from 55%RH, where the sensitivity was raised to 6.000 MHz/%RH and 0.785 degrees/%RH respectively.