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Title: Characterization of Organic Materials for Electromechanical Sensors
Authors: Khan, Adam
Keywords: Applied Sciences
Engineering & allied operations
Applied physics
Other branches of engineering
Electrical engineering
Issue Date: 2013
Publisher: GIK Institute of Engineering Sciences and Technology Topi, Swabi, Khyber Pakhtunkhwa, Pakistan.
Abstract: Organic materials have attracted remarkable interest in the field of electronics due to good me- chanical stability and semiconducting and conducting properties. Organic semiconductors have the advantage of large area fabrication and low temperature deposition. Hence, low cost electronic de- vices can be easily fabricated over large scale by employing organic materials. The stated features assure organic materials as prominent candidates for the development of organic electromechani- cal sensors. The aim of this work was to study the applicability of organic materials for various electromechanical sensors. In this research work, thin films of various organic semiconducting ma- terials and composites were employed for the development of organic field effect transistor (OFET) and sensors to investigate their electromechanical properties. In this dissertation, the organic materials investigated were copper phthalocyanine (CuPc), poly-N-epoxypropylcarbazole (PEPC), nickel phthalocyanine (NiPc) and carbon nanotubes (CNTs). Inorganic materials, vanadium oxide (VO 2 ) and cuprous oxide Cu 2 O were also investigated and used in making composites for altering the properties of devices. CuPc was employed in fabrication of OFET by vacuum evaporation method. The OFET was then characterized for pressure and displacement sensing properties. The variations in drain to source resistance were measured for applied pressure and change in displacement, respectively. PEPC microcomposite thin films were drop-casted on a variety of substrates to fabricate sand- wich type sensors. The different substrates used were aluminium, steel, plastic and glass (with prefabricated electrodes). The substrates in this study served dual purpose, i.e., serving as an elec- trode and providing mechanical support to the device. The sensors were investigated for applied pressure and change in displacement. The measurements were made for different concentrations of composites and for different thicknesses of thin films. The variations in resistance and capac- itance of the transducer were observed with the applied stimuli. In the first case, Cu 2 O-PEPCii microcomposites were used to develop pressure sensor. The thicknesses of the films were in the range of 30 − 100μm. The AC resistance and capacitance at 120 Hz of the transducer decreased by 1.1 ∼ 1.4 and increased by 1.2 ∼ 1.8 times respectively as the pressure was increased up to 100 kN m −2 . Afterwards, V 2 O 4 -PEPC microcomposites were used to develop another pres- sure sensor. The thickness of the V 2 O 4 -PEPC films were in the range of 20 − 40 μm. The DC resistance of the sensor decreased on average by 24 times as the pressure was increased up to 11.7 kN m −2 . Finally, Cu 2 O-PEPC-NiPc microcomposites were used to develop pressure sensor. The film thickness of the composite was in the range of 20 − 30 μm. The decrease in resistance of the sensor was observed 10 times by increasing the external uniaxial pressure up to 11.7 kN m −2 . In case of displacement sensor, Cu 2 O-PEPC microcomposites were used to fabricate displacement transducer. The thicknesses of the films were in the range of 50 − 60 μm. As the displacement from 0 − 0.6 mm, the decrease in sensors DC resistance was observed as 1.5 times to the initial resistance, and accordingly, the increase in AC capacitance (at 120 Hz) was measured 2.31 times to the initial capacitance by applying the displacement in the range of 0 to 1.3 mm. Furthermore, the CNTs based Al/CNT/Al sandwich type sensors were investigated. Sensors were fabricated by deposition of the CNTs on the adhesive elastic polymer tape and placing it in the elastic casing. The resistance-pressure and resistance-displacement relationships were determined to ensure the piezoresistive properties of CNTs. The diameter of multiwalled nanotubes (MWNTs) varied between 10 − 30 nm. The nominal thicknesses of the CNTs layers in the sensors were ∼ 300 − 430 μm. The interelectrode length (gap) and width of the sensors were in the range of 4 − 6 mm and 3 − 4 mm, respectively. In investigation of the pressure sensor, the decrease in DC resistance was noted as 3 − 4 times as the pressure was increased up to 17 kN/m 2 , whereas the DC resistance of the displacement sensors from different batches was decreased in average by 3 times as the displacement was increased up to 900 μm. Finally, measurements were made on CNT-Cu 2 O composite as a strain sensor. The press-tablets of the composite were fabricated and glued on the flexible elastic beam. The electromechanical properties of the sensors were measurediii under compression and tension. The inter-electrode length (gap between the contacts) and width of the samples were in the range of 6 − 8 mm and 10 mm, respectively. The variation in DC resistance of the transducers were observed under compression and tension. It was noted that the resistance decreases 24 − 28 times under compression while increases 44 − 46 times under tension. The resistance-strain relationships were also simulated.
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