Fabrication and Investigation of Organic and Nano Materials Based Sensors

Abstract

This work presents the fabrication and investigation of organic and nano materials based sensors for humidity, temperature and electromechanical applications. Polyaniline (PANI), orange dye (OD)-PANI composite and of cellulose- poly-N-epoxypropylcarbazole (PEPC) have been used for the fabrication of surface type humidity sensors. The sensors are fabricated by depositing films of various thicknesses on glass substrates between pre-deposited metallic electrodes. The sensing mechanism is based on the impedance and capacitance variations due to the absorption or desorption of water vapors. The consequences of annealing, measuring frequency and absorption-desorption behavior of the sensor have been discussed in detail. For all sensors impedance-humidity relationship shows more uniform change as compared to capacitance- humidity relationship in the given range humidity. The temperature sensors have been fabricated by using multiwalled carbon nanotubes (MWCNTs), V2O4-PEPC composite and CuPc on n-GaAs. The CNTs based sensors are fabricated by the deposition CNT nanopowder on a paper substrate and on adhesive elastic polymer tape. The nominal thickness of the CNT films on paper substrates is 30–40 μm while that of elastic substrate is ~ 300-430 μm. The DC resistance of the sensors decreases with increase in temperature. For both types‘ sensors, the resistance-temperature relationship shows wide range sensitivity. The V2O4-PEPC composite based temperature sensors are fabricated by drop-casting the blend of composite into the gap between preliminary deposited silver electrodes on glass substrates. The thickness of the V2O4-PEPC films is in the range of 20-40 μm. It is found that with increase in temperature the AC resistance of the samples decreases by 10-12 times. The response recovery time is also measure. The Ag/p-CuPc/n-GaAs/Ag cells are fabricated by the deposition of p-type copper phthalocyanine on n-type GaAs single-crystal semiconductor substrate. The temperature sensing and photoconductive behavior of the cells are investigated. The results reveal that with increase in temperature from 33-75 °C the resistance temperature coefficients (RTC) for the reverse and forward bias resistances are equal to -2.0 %/°C and -1.5 %/°C, respectively. Electromechanical sensors based on PANI, CNTs and CNTs-Cu2O composites have been fabricated and investigated. The 20-80 μm thick PANI films are deposited by drop-casting on Ag electrodes, which are preliminary deposited on glass substrates. The effect of displacement on the resistance and capacitance of film is investigated. It is observed that with increases in displacement the resistance decreases and the capacitance increases. For the fabrication of CNT–Cu2O composite based pressure sensors tablets of composite are made at a pressure of 353 MPa. The average diameter and the average thickness of the tablets are 10 mm and 4 mm, respectively, and both sides of the tablet are covered by silver paste. By varying pressure from 0-37 kN/m2, the change in DC resistance of the sensor is measured. The CNTs and CNTs–Cu2O composite based strain sensors have been fabricated by pressed tablets and elastic polymer beam. The 1 mm thick tablets of CNTs and CNTs–Cu2O composite are fabricated at a pressure of 200-300 MPa and 353MPa, respectively. The samples are installed on the polymer elastic beam by glue. The electric contacts to the samples are made by silver paste. The inter-electrodes distance (length) and diameter of the surface-type samples are in the range of 6–8 mm and 10 mm, respectively. It is found that DC resistance of the strain sensors increases under tension and decreases under compression, while the average strain sensitivities are in the range of 50-80 and 44–46 for CNTs and CNTs–Cu2O composite based sensors, respectively.