Research On Sensing Properties Of Carbon Nanotubes Humidity Sensor

Humidity sensor, as a collection unit for humidity information, has been widelyapplied in environmental monitoring, warehouse storage, industrial production andweather humidity detection, and become one kind of the most universal sensorspresently. As carbon nanotubes possess great surface area to volume ratio, strongadsorption ability and distinguished electrics property, they have received greatattention in sensor research field. With fast response and high sensitivity, humiditysensors fabricated by carbon nanotubes sensing material have shown goodapplication prospect.In this dissertation, humidity detection under alternating signals for carbonnanotubes sensor is proposed, considering of the low stability problem for resistivehumidity sensor under direct current testings. Sensor’s working principle andmedium dielectric relaxation effect on its capacitance and resistance are alsoinvestigated. According to the above analysis, a novel humidity detecting method isdeveloped adopting dielectric relaxation theory and a kind of resistive carbonnanotubes humidity sensor under alternating testing signals is proposed based on thetheory. Meanwhile, in research of basic dielectric relaxation model, dielectric loss isintroduced into the former carbon nanotubes humidity sensor model and a relevantconductance is produced, which makes complements for the humidity sensing modelof carbon nanotubes film.A carbon nanotube humidity sensor with interdigital electrodes is fabricated tovalidate the principle of this novel sensor under alternating signal detection.Investigations are made on frequency characteristics of the sensor’s resistance andcapacitance by experiments. As testing frequency increases, it turns out liquid watertransformed by vapor capillary condensation in the sensing film has lowerpolarization ratio, so that sensor capacitance variation to humidity reduces. Theexperimental results are consistent with the theoretical analysis, comfirming itscorrectness. While the resistive sensor conductance variation to humidity increaseswith frequency ascent and here dielectric relaxation effect dominates in humiditysensing. The experimental theory validation forementioned has been realized.Also, the experimental data of the sensor is fitted to the novel resistive sensormodel based on alternating signal detection and they prove well fitted, whichapproves correctness of the established model.Finally, the proposed sensor under alternating signals are tested and analyzedon its performance. Due to disvantages of weak stability and irregular resistance fluctuation for current humidity sensors, here the sensor stability is emphasized andstability improving method is proposed by elevating sensor’s testing frequency.Sensor resistance variation to time at200kHz reduces to1/4of that at500Hz byexperimental results. Meanwhile, investigations on the sensor’s linearity andsensitivity properties prove that frequency of200kHz offers the optimum linearityof2.72%, where the sensor sensitivity achieves-6.84/%RH and its response andrecovery time are6s and14s respectively.The resistive carbon nanotubes humidity sensor based on alternating signaldetection proposed in this work has relatively high stability and sensitivity forpotential application. The sensor’s electric model and mathematic model providetheory basis for profound research of resistive carbon nanotubes humidity sensorsand contribute to their further developments.