Pressure and Chemical Gas Sensing Applications of Liquid Crystal and Carbon Nanotubes
Date Issued
2014
Date
2014
Author(s)
Lai, Yu-Tse
Abstract
In this work, carbon nanotube (CNT) dispersed in liquid crystal (LC) and polymer-dispersed LC (PDLC) are proposed for sensing applications. Carbon nanotube dispersed in nematic liquid crystal (CNT-LC) is employed as the piezoresistive sensing material. The force sensing ranges in this tactile device can be tuned by varying frequency of the driving voltage supplied by the array scanning circuitry. The structure of the sensing element, in which the CNT-LC composites is sealed, consists of a deformable PDMS elastomeric structure, an indium tin oxide (ITO) glass substrate, and an ITO polyethylene terephthalate (PET) film. The frequency dependence of electrical conductivity for CNT-LC composites has been reported. Lower driving frequency of CNT-LC composites induces higher resistivity. This tunable capability can be employed for the applications which require different measurement ranges of forces, without the need of adjusting the dynamic ranges of the sensor readout circuitry. An experimental apparatus with Labview operating program system for measuring the characteristics of the devices is developed and the tunable sensing capability is successfully captured. The driving and scanning circuit for 4×4 pressure sensing array is also designed and implemented.
A novel chemical sensor for DMMP and acetone detection is proposed in this thesis. The sensor uses a sensing film consisting of a polymer-dispersed liquid crystal doped with carbon nanotubes (CNT-PDLC). The sensing element comprises a PDLC sensing film and planar interdigital electrode pairs. Targeted analytes diffuse through the CNT-PDLC film when the sensor is exposed to chemical gas. The chemical molecules destroy the ordering of the LC phase, results in an isotropic liquid phase. After an orientational ordering transition within the CNTs and LCs, the CNT conducting networks in the PDLC are restructured, which in turn increases the resistance of the polymer. Concentrations of chemical gas can be detected by measuring the changes in the electrical resistance of the sensing film. Compared to typical LC-based sensors, the proposed PDLC device is sufficiently durable to withstand gravitational forces and mechanical shocks. The sensing signals can be retrieved by using a simple readout circuitry. A volatile organic compounds (VOCs) experimental apparatus with Labview user interface is also developed to evaluate the performance of the device. A linear response with good reproducibility was achieved. The minimum detected concentrations of DMMP and acetone are 5 ppm and 100 ppm, respectively. The response time of the device is similar to that of typical LC-based sensors. In addition, the influences of the film thicknesses and the LC and polymer mixing ratios on the sensor performance were studied. The proposed acetone sensors have excellent selectivity among ethanol, toluene, and hexane gases.
Subjects
奈米碳管
液晶
聚合物分散液晶
觸覺壓力感測器
氣體感測器
Type
thesis
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