液晶和奈米碳管複合材料於壓力及化學氣體感測之應用

Pressure and Chemical Gas Sensing Applications of Liquid Crystal and Carbon Nanotubes

賴昱澤(Yu-Tse Lai)

指導教授 ： 楊燿州

國立臺灣大學/工學院/機械工程學研究所/博士(2014年)

https://doi.org/10.6342/NTU.2014.00647

摘要

在本研究中，利用奈米碳管(carbon nanotube, CNT)、液晶(liquid crystal, LC)及聚合物分散液晶(polymer-dispersed liquid crystal, PDLC)之複合材料，應用於製作觸覺壓力及化學氣體感測器。
將奈米碳管均勻分散於液晶之中，可作為觸覺壓力感測器的壓阻性(piezoresistive)感測材料。透過外部場(external field)的操作，使元件具有可調整(tunable)壓力感測範圍(sensing ranges)之功能。奈米碳管與液晶複合材料(CNT-LC)被封裝於有彈性的聚二甲基矽氧烷(polydimethylsiloxane, PDMS)結構中。上、下層電極結構分別為可撓性聚對苯二甲酸乙二酯(polyethylene terephthalate, PET)和玻璃基材，表面皆濺鍍上氧化因錫(indium tin oxide, ITO)透明導電薄膜作為電訊號傳導之媒介。CNT-LC材料的導電性具有頻率相依(frequency dependence)之特性。當施加於元件的交流電壓、頻率改變時，能夠致使CNT-LC材料具有不同的電阻值，而施加壓力於元件上，將導致電阻下降，若元件具有較高的初始電阻值，則能夠承受較高的壓力範圍，反之亦然。因此，本研究將此性質開發設計作為可調整壓力感測範圍之感測陣列，並完成架設壓力量測設備及開發Labview軟體量測控制介面。透過LCR阻抗分析儀，量測元件的壓力感測性能，並針對結果進行分析討論。此外，掃描式電阻量測電路被開發於4×4 CNT-LC壓力感測陣列之應用。
參雜(doping)奈米碳管的PDLC複合材料(CNT-PDLC)，被應用作為化學氣體感測元件，用來偵測對人體有危害的甲基膦酸二甲酯(dimethyl methylphosphonate, DMMP)和丙酮(acetone)氣體。PDLC感測薄膜以紫外光聚合的方式，製作成型於平面指叉式(interdigital)電極上。當被檢測的化學氣體以滲透方式擴散進入薄膜，將會改變CNT-PDLC之中，奈米碳管及液晶分子之方向秩序，進一步造成奈米碳管導電網路(conductive network)重組變化，而改變PDLC薄膜之電阻值大小。藉由指叉電極量測薄膜導電性變化，即可精確的量測被檢測化學氣體之濃度。與傳統液晶式化學氣體感測器相較，PDLC能夠有效封裝包覆液晶，使元件穩定性及抗機械震動性能提升。透過簡單的電阻量測電路，即可獲得化學氣體濃度之感測資訊，省去傳統光學量測所需複雜且龐大的設備。此外，PDLC製程容易與電路晶片製程相互整合，可達成攜帶式微型晶片之目標。此研究中，元件對氣體濃度反應相當線性，且重複性良好。元件可感測最低濃度為5 ppm之DMMP氣體及100 ppm之丙酮氣體，符合應用上所需要求。此外，氣體量測、流量控制之實驗架設及Labview軟體介面亦已開發完成。本文中並詳細討論物理、材料因素對元件感測性能之影響，例如：PDLC薄膜厚度、液晶與聚合物混合比例等。實驗亦證明PDLC薄膜不會對乙醇、甲苯、正己烷氣體產生反應。

關鍵字

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

並列摘要

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.

並列關鍵字

Carbon nanotube ； Liquid crystal ； Polymer-dispersed liquid crystal (PDLC) ； Tactile sensor ； Pressure sensor ； Gas sensor

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