Humidity Sensing Properties of Carbon Nano-Tube Thin Films

In this article, the electrical properties of carbon nanotubes (CNTs) films were studied by fabricating Ag/CNT/Ag devices. The nominal thickness of the grown CNTs films was similar to 12 mu m, and the diameter of multiwalled carbon nanotubes (MWCNTs) varied between 10-30 nm. The current voltage (I-V) characteristics of the fabricated devices exhibited rectification behaviour (3 at +/- 1 V). The effect of humidity on MWCNTs films was investigated by evaluating capacitance and dissipation of the samples at two different frequencies, i.e., 120 Hz and 1 kHz. It is observed that the capacitance and resistance change with changing values of relative humidity (RH) as a function of frequency (f). At f = 120 Hz, RH = 90%, the observed increases in the capacitance of the sensor was similar to 366 times and it was only similar to 36 times when measured at f = 1 kHz. Under the same experimental conditions, the observed decrease in the resistance values was similar to 656 and similar to 627 times respectively. It is assumed that the humidity based electrical response of the cell is associated with doping of the nano-particles by water molecules. To evaluate the dynamics of the water vapour absorption and desorption processes, the average response (2 s/percent) and recovery (8 s/percent) times of the fabricated devices were obtained by capacitance-time measurements. The experimental results have been simulated by defining the device equivalent circuit and capacitance-humidity relationship.