비분산적외선 흡광방식의 고감도 이산화탄소 가스센서 개발

유선화 고려대학교 대학원 2007 국내석사

As the carbon dioxide(CO2) is difficult to be measured reliably using chemical sensor elements, we use other working principles. The most promising working principle is the measurement of based on the well-known principle of infrared adsorption of radiati

카본 블랙과 다중 벽 탄소나노튜브를 하이브리드 충전제로 한 고분자 복합체의 감습 메커니즘 연구

최창선 고려대학교 일반대학원 2011 국내석사

Carbon black(CB) and carbon nanotube(CNT) are widely used for conductive fillers in composite film sensors. CNTs have a very high aspect ratio compared to CBs. In addition, CNTs can either have metallic or semiconducting properties, whereas CBs are metallic only. A host polymer mixture was prepared from poly(vinyl alcohol)(PVA) and poly(acrylic acid)(PAA), which are highly hygroscopic. In CB films, the resistance increases significantly around 80% relative humidity(%RH). However, such dramatic increase in resistance was not observed in the composite films with multiwall carbon nanotube(MWCNT). Instead, the resistance increases linearly with relative humidity. These results can be explained by, in CB films, the large increase in resistance is only due to polymer swelling from humidity absorption. In contrast, for nanocomposite films with p-MWCNT, the increase in resistance with increasing relative humidity can be explained on the basis of electron transfer from the water molecule to the p-MWCNT and the intertube distance change induced by polymer swelling during water adsorption. And polymer-based nanocomposites containing hybrid fillers of CB and MWCNT were developed. The distinct geometric shapes and aspect ratios as well as different dispersion characteristics of the two conducting fillers offered unique synergy, giving rise to the enhanced sensitive to humidity of nanocomposites, which have not been specifically considered previously. Also, effect of plasma-functionalization was investigated. For nanocomposite films with O2 plasma-functionalized fillers are more sensitive to humidity and show a better linearity compared to composite films with pristine fillers.

Plasma-functionalized carbon nanotube based electrochemical DNA sensor for legionella pneumophila detection

박은진 Graduate School, Korea University 2011 국내박사

레지오넬라 균 (Legionella pneumophila)은 1976년 미국의 필라델피아에서부터 알려진 레지오넬라 병 (legeionellosis)의 원인균으로 빌딩의 냉각탑이나 분수와 같은 인공구조물에서 서식하는 세균이다. 레지오넬라 병은 고열이나 치명적인 급성 호흡기 질환을 유발하며 병원에서 주요 감염군으로 분류되는 등 현대 사회에서도 주요한 질병원으로 분류되고 있다. 이러한 레지오넬라 균의 검출은 보다 빠르고 간편한 분석방법이 꾸준히 요구되어져 왔는데, 본 연구에서는 보다 간편하고 빠른 분석이 가능한 고감도 나노기반의 전기화학적 DNA 센서를 제작하여 그 성능을 평가하였다. 고감도의 DNA 센서 제작을 위해 새로운 형태의 탄소나노튜브(carbon nanotube; CNT)를 이용한 전극을 제작하였고 이 전극에 산소 플라즈마 처리를 통해 전기화학적 활성을 향상시키는 동시에 표면을 카르복실기 등의 산소를 포함하는 (oxygen containing) 작용기를 도입시켜 표지 DNA (probe DNA)의 효과적인 고정화를 유도하였다. 전극의 제작은 화학증착법을 이용하여 합성되고 정제된 CNT를 재료로 하여 tip-sonication, photolithography와 etching 등의 반도체기술, 그리고 transfer 방법과 spray 방법을 이용해 제작되었다. CNT가 패턴 된 전극 및 플라즈마 처리를 통해 기능화 된 전극은 SEM, AFM, XPS, 전기화학적 방법으로 분석하였다. CNT 전극은 기판에 원하는 전극 영역에 효과적으로 제작되었으며 산소 플라즈마 처리 조건에 따라 그 표면의 형태가 크게 달라지며 표면의 조성 또한 처리하지 않은 전극에 비해 산소가 다수 포함되는 결과를 나타내었고 이에 따라 전기화학적 활성도가 크게 증가함을 cyclic voltammetry 방법을 통해 확인되었다. 이렇게 제작된 플라즈마 처리를 통해 기능화 된 탄소나노튜브 (plasma-functionalized CNT; pf-CNT) 전극은 레지오넬라 균 검출을 위해 세 가지 형태의 DNA 센서를 제작하였으며 그 각각의 성능을 전기화학적 방법을 통해 분석하였다. 첫 번째로, 전극 표면에 고정화될 probe DNA에 전기화학적 활성을 갖는 물질인 ferrocene (Fc)를 modification 하여 센서에 활용하였다. Probe DNA에 부착된 Fc는 single stranded DNA (ssDNA)로 있을 때와 hybridization 된 후 double stranded DNA (dsDNA)의 형태로 있는 때 그 운동성과 배향의 차이가 발생하여 결과적으로 cyclic voltammograms의 형태가 달라짐을 확인할 수 있었다. 두 번째 DNA 센서에서는, ssDNA의 구아닌 (guanine)과 효과적으로 상호작용하는 메틸렌블루 (methylene blue; MB)를 이용하여 target DNA를 검출하였으며 실제 differential pulse voltammetry (DPV) 방법을 통해 hybridization 전과 후의 변화를 효과적으로 확인할 수 있었다. 위의 두 가지 방법에서는 모두 27개의 서열을 갖는 probe DNA와 target DNA를 사용하였으나, 마지막 방법에서는 PCR 결과물을 검출할 수 있는 시스템을 제작하기 위해 27-mer의 probe DNA를 고정한 후 101-mer의 target DNA를 반응시킨 후 말단에 digoxigenin이 결합된 signal probe을 반응시켰다. 이후, anti-dig-HRP/ TMB+substrate system을 이용하였는데 chronoamperometry 방법을 이용하여 target DNA를 검출하였다. 본 연구에서 제작된 pf-CNT를 기반으로 한 전기화학 DNA 센서는 모두 10 pM 에서 100 μM에 이르는 농도 범위의 레지오넬라 균을 검출하는데 모두 성공적이었다. 전기화학적 활성이 높고 플라즈마 처리를 통한 기능화가 용이한 pf-CNT 전극의 이러한 결과는 이후 고감도를 요하는 다양한 바이오센서에 적용에 매우 고무적일 것이다. More than 30 years have passed since Legionella pneumophila (L. pneumophila), the causative agent of Legionnaires? disease, was identified as a new human pathogen. First recognized due to epidemic of pneumonia that followed the 1976 Legionnaires? convention in Philadelphia, USA, legionellosis is still a disease of medical and public interest. L. pneumophila is a contaminant of man-made water systems including cooling towers of large buildings and waterworks, and is a major cause of legionellosis, a respiratory infection that may give rise to restricted outbreaks. Most studies for DNA detections were based on PCR, and more recently real-time PCR and other optical detection methods. These methods are time-consuming, labor-intensive, and require expensive instrumentation; therefore, simplified methods suitable for rapid analysis are needed. We proposed electrochemical analysis for L. pneumophila detection with the plasma-functionalized multiwall carbon nanotube (pf-MWCNT) electrode. As many studies have shown, the electrochemical methods have many advantages compared to optical methods including, simple sample preparation procedures, rapid measurement time, low cost and ease of miniaturization for portable devices. For sensitive DNA sensor preparation, a novel MWCNT electrode were fabricated and functionalized with oxygen plasma for improving electrocatalytic property and inducing oxygen containing functional groups on the surface. A well-patterned MWCNT working electrode (WE) on a Pt track was fabricated using photolithography, transfer methods and an etching technique. The MWCNT WE was functionalized by oxygen plasma treatment prior to applying for DNA sensor. The pf-MWCNTs were characterized by atomic force microscopy (AFM), scanning electron microscope (SEM), and x-ray photoelectron spectroscopy (XPS). After oxygen plasma treatment, the pf-MWCNTs exhibited a dramatic change in their surface morphology and chemical composition, and the MWCNT films were found to be effectively functionalized by oxygen plasma treatment. Electrochemical measurements were also performed using CV with ferricyanide/ferrocyanide redox couple. The peak currents of pf-MWCNT WE were higher than pristine MWCNT WE, and peak-to-peak separations values revealed that the electrode process of pf-MWCNT WE was more reversible. The pf-MWCNT electrodes were employed for L. pneumophila DNA detections with following schemes. First, the oligonucleotidde with ferrocene (Fc) head modification was used as the probe in the DNA sensor. Fc is one of the electroactive molecules, and the cyclic voltammograms were changed before and after hybridization due to mobility and orientation changes of the Fc moiety on the pf-MWCNT electrode surface. Second, the detection of target DNA was measured by differential pulse voltammetry (DPV) using the methylene blue (MB) as a hybridization indicator. The affinity interaction between MB and ssDNA occurs quickly and generates a marked electrochemical signal due to MB has a strong affinity for the free guanine base present in ssDNA. There was no signal change after the non-complementary DNA reaction whereas the DPV signal of the MB decreased remarkably after hybridization with the target DNA. While both schemes had 27-mer probe and target DNA, the final DNA sensors detected 101-mer PCR amplicons for target molecules. The probe DNA was also employed 27-mer probe, and following hybridization with 101-mer target DNA and digoxigenin (dig) tagged detection probe, anti-dig-horseradish peroxidase (HRP) conjugate was applied to the pf-SWCNT electrode. The enzyme-based DNA sensor scheme with TMB and H2O2 substrate could successfully detect 101-mer target DNA resulting in amperometric signals. We successfully fabricated well-patterned MWCNT electrodes and a pf-MWCNT based electrochemical DNA sensor for L. pneumophila detection. The schemes of DNA sensors in this study, operated successfully in the concentration range of 10 pM ~ 100 μM. Considering electrochemical results of electrodes themselves and DNA sensing performance, the pf-MWCNT WE presented here can be applied to other electrochemical biosensors since they have high electrochemical properties and the capability for easy functionalization.

Electrochemical immunosensor with carbon nanotube working electrode for detecting legionella pneumophila

이준용 Graduate School, Korea University 2012 국내박사

Transferred multi-walled carbon nanotube (MWCNT)-modified platinum thin-film immunosensing electrode material was engineered on a glass substrate and used to fabricate a fully-integrated electrochemical three-electrode system for monitoring Legionella pneumophila. The transferred MWCNT film was treated with oxygen (O2) plasma to improve its electrochemical response and electrical conductivity. We voltammetrically characterized and optimized the electrochemical performance of the fabricated electrode for direct detection of Legionella pneumophila-specific peptidoglycan-associated lipoprotein (PAL) and maltose binding protein (MBP) peptidoglycan-associated lipoprotein (MBP-PAL) fusion. The latter, as an intermediate product to yield the former, has important roles in the growth and purification of PAL, which commercial enzyme-linked immunosorbent assay (ELISA) kits require as a target substrate. Consequently, direct electrochemical detection of MBP-PAL compared to PAL by square-wave voltammetry (SWV) showed a greater than 50 % increase in sensitivity with a lower detection limit of 5 pg∙mL-1. We also investigated affinity properties by determining the kinetic parameters of the PAL and the MBP-PAL in relation to polyclonal antibodies immobilized on transferred MWCNT substrates using Michaelis-Menten assumptions and a Hanes-Woolf plot. The new method presented herein could save the time and effort for the separation and purification of PAL from MBP-PAL fusions that are required for performing ELISA-based immunoassay.

Design and development of sealed thermopile infrared sensor with fresnel lens and CNT film absorber

유금표 Graduate School, Korea University 2010 국내박사

Today, the most thermopile detectors are fabricated on silicon wafer by MEMS technologies. These technologies are possible to make a minimized structure for low cost and increasing yield. The MEMS-based thermopile consists of an infrared absorber, a series-connected thin-film thermocouples and membrane. For improvement in sensitivity of thermopile, it requires to increase with absorptance of absorber, to enlarge absorber area or to connect serially more thermocouples. The representative absorber materials of traditional thermopile are platinum and gold like a precious metal. But these materials are difficult to be applied due to relatively high cost and have bad adhesion compare with others. Also, the porous metal blacks, such as platinum-black, have characteristics of absorption decreasing with increasing wavelength. In order to solute this problem, A CNT film can be adopted as absorber layer. CNT films have a porous surface and metal property. The expanding absorber area is limited by device size. Thus instead of increase absorber area the use of optical device, such as Fresnel lens which concentrate infrared on absorber, obtain expansion effect of absorber area. Fresnel lenses are typically used to focus light, and it is suitable for nonimage sensor such as thermopile, infrared motion detectors and solar concentrators. Fresnel lenses applied to infrared sensors was proved that it improve the value of normalized detectivity because of area extension effect of absorber. In addition, for thermal isolation between hot and cold junction, thermopile has been used membrane structure. In current research many researchers suggest that front-side etching process has more advantageous than backside etching because it is complex, strong dependence on equipment and wastes a large amount of silicon estate. But the front-side etching process has a most serious problem. It is an advantage but is simultaneously a disadvantage. That is laid on front-side all pattern, such as thermocouple and etch hole to release membrane. This leads to decrease of the number of thermocouple and its width. The reduced thermocouple and its width have a bad effect on output voltage of thermopile and thermal noise, respectively. Therefore characteristics of thermopile are worse. In this paper, we propose a MEMS-fabricated and sealed thermopile. The newly proposed thermopile device consists of a micromachined closed membrane and a CNT film IR absorber, and bonds a Fresnel lens with cavity not to destroy closed membrane of thermopile chip. The sensitivity of thermopile with CNT absorber is higher about 1.8 times than without one. It’s around 40.19 V/W. The focal length of Fresnel lens is 0.7875 mm. The sensitivity of thermopile with Fresnel lens is 71.5V/W, and it is higher about 1.7 times than one without Fresnel lens. The increase of absorber area by lens is about 7 times but one of real output voltage is 1.7 times. The response time is about 6 ms.

마이크로 제조 기술을 이용한 고감도 CNT 스트레인 게이지의 제작과 하중센서에 응용

이동일 高麗大學校 大學院 2012 국내석사

A novel strain gauge made of single walled carbon nanotube (SWCNT) film and further discusses the usability of load cells with these gauges as sensors for force and weighing measurements. Key issues involve the deposition of the SWCNT films with strong adhesion to the backing materials, the integration of SWCNT films into existing micro technologies for batch-fabrication, and the bonding of the strain sensors to a binocular spring element. A batch microfabrication process was developed for practical device construction and packaging using spray-coated SWCNTs and a conventional semiconductor process. The prototype was characterized using a commercial metal foil gauge with tensile and compressive testing on a binocular load cell. Our test results demonstrated that the proposed SWCNT film gauges have a linear relationship between resistance changes and externally applied strain. The gauge factor ranged from 7.0 to 16.4 for four different micro-grid configurations, indicating that the maximum strain sensitivity of the prototype was approximately eight times greater than that of commercial gauges. The effectiveness of a microfabricated SWCNT gauge as a force sensor was evaluated by applying it to high sensitivity binocular load cells. Our test results exhibited that SWCNT gauge load cells had much better resolution and higher sensitivity than the conventional metal foil-type cells.

Development of dye-sensitized solar cells using plasma functionalized carbon nanotube and conducting polymer electrodes

Yong Hoon Rhee 고려대학교 대학원 2015 국내박사

Among the new materials being developed for solar cells, photocatalytic and other applications, titanium dioxide (TiO2) remains one of the most promising materials because of its high efficiency, low cost, chemical inertness, ecofriendly nature and photostability. However, the widespread use of TiO2 is hindered by its low utilization of solar energy in the visible region (about 3 ~ 5 %) because of the wide band gap (3.0 eV for rutile and 3.2 eV for anatase crystalline phase) restricting the effective utilization of pure TiO2 nanoparticles for solar excitation or conversion. Therefore, many efforts were concentrated on narrowing the optical band gap of titania. Anion impurities such as nitrogen (N), carbon (C) or sulfur (S) have been extensively tried for narrowing the band gap. After the discovery of carbon nanotubes by Iijima in 1991, carbon nanotubes (CNTs) have attracted significant attention in a variety of scientific fields because of their unique properties; structural, chemical, thermal, mechanical, electronic properties and potential applications. Also, the most frequently used counter electrode material for DSSCs is platinum (Pt). Even though Pt is highly expensive, it shows excellent electrical conductivity and chemical stability, and most importantly, it effectively catalyzes the reduction reaction of triiodide to iodide. However, spin-coated Pt layers need high-temperature annealing processes that are inappropriate for flexible plastic materials. Pt-free counter electrodes have been widely investigated, and graphite carbon or conducting polymers have been introduced for use as a counter electrode to replace Pt. Poly(3,4-ethylendioxythiophene) : poly(styrene sulfonate) (PEDOT:PSS) a conducting polymer that shows good conductivity, electrochemical stability, cost-effectiveness, and excellent electrocatalytic activity for the reduction reaction of triiodide to iodide. We demonstrate that the incorporation of multi-walled carbon nanotubes (MWCNTs) into a TiO2 active layer contributes to a significant improvement in the energy conversion efficiency of dye-sensitized solar cells (DSSCs). A multi-walled carbon nanotube (MWCNT) was physically functionalized using oxygen plasma, and the resulting plasma-functionalized MWNT (pf-MWCNT) was incorporated into TiO2 nanopowders to prepare a TiO2/pf-MWCNT composite suspension. The layer was directly spray-coated on a fluorinated tin oxide surface and served as a photoanode of a dye-sensitized solar cell (DSSC). The cell performance was optimized in terms of the plasma treatment time and compared with a conventional DSSC, showing 37% increased energy conversion efficiency. The efficiency improvement of the electrochemical impedance spectra was due to the enhanced photoelectron lifetime due to efficient electron transport through the pf-MWCNT network. In addition, A composite of poly(3,4-ethylendioxythiophene) : poly(styrene sulfonate) (PEDOT:PSS) and multi-walled carbon nanotubes (MWCNTs) was electrochemically polymerized on a fluorine-doped tin oxide (FTO) substrate and used as a photocathode for a dye-sensitized solar cell. The MWNTs were previously O2 plasma treated and simultaneously entrapped while the PEDOT:PSS film was electropolymerized. The resulting plasma-functionalized MWCNT-entrapped PEDOT:PSS composite showed enhanced electro-catalytic activity for the reduction of I3- to I-.

Highly sensitive ammonia gas sensors based on semiconductor-enriched carbon nanotube networks

Hong, Hyun Pyo 고려대학교 대학원 2015 국내박사

Recently, nanostructures, such as carbon nanotubes (CNTs), have begun to attract wide attention in the study of their application to various sensors. The gas sensors based on single-walled carbon nanotubes (SWCNTs) show extreme sensitivity towards changes in their local chemical environment that stems from the susceptibility of their electronic structure to interacting molecules. So far, CNT-based gas sensors have been investigated for the detection of elements such as H2, N2, NO2, and NH3. For preventing an unexpected accident, such as a gas explosion or suffocation, very sensitive sensors are needed. Therefore, we developed a high-performance gas sensor based on highly semiconductor-enriched SWCNTs. The operating principles of the CNT based gas sensor can be classified into two major categories: the conductivity change of the CNT channels and the variation of the Schottky barrier between the electrode and the CNT film. Although the sensing mechanisms are different, the most effective component of the sensor sensitivity is the semiconductor/metallic tube ratio of CNT films. Typical SWCNTs consist of 66% semiconducting tubes and 33% metallic tubes. Purification technology has evolved; high purity semiconducting SWCNTs have been developed, and some SWCNTs consist of 99% semiconducting tubes. In this study, we fabricated 99% semiconducting SWCNT gas sensors. For comparison, we used semienriched 90% SWCNTs and typical SWCNT gas sensors that were also fabricated using solution-deposition and spray deposition, respectively. To enhance the sensitivity and the response time of sensors, oxygen plasma treatment was performed on the 66%-SWCNT and 90%-SWCNT gas sensors. We also demonstrated the effect of oxygen plasma treatment on the highly semiconductor-enriched SWCNT device. The sensor response to NH3 gas was characterized by a resistance increase at the moment of NH3 exposure. A general linear response was observed with an increasing NH3 concentration with a significantly greater responsiveness for the semiconductor-enriched 99% sensor compared with the 66% and 90% sensors. Furthermore, plasma treated 66% and 90% sensors (p-66% and p-90%, respectively) showed a higher performance than NH3 sensing due to the fact that oxygen functional groups were formed on the CNT film and the electrical structure of SWCNT was changed from metallic to semiconducting[1]. This suggested that a large semiconducting/metallic ratio is the most important factor for achieving a high sensitivity in mixed SWCNT-networked-based gas sensors.

Development and analysis of single-crystal silicon strain gauge pressure sensors for automotive mechatronic systems

Ham, Seung Woo 고려대학교 대학원 2016 국내박사

Pressure is one of the most frequently measured physical variables in modern mechatronic systems such as automotive, industrial, household, defense and biomedical applications. Pressure sensors can also be used to indirectly measure other variables such as fluid/gas flow, speed, water level, and altitude. Thus, the pressure sensor is a key device in many mechatronic systems. In this thesis, a single crystal silicon gauge structure is proposed for preventing tilt and improving the adhesive strength of strain gauge-based pressure sensors. To realize the proposed piezoresistive strain gauge, a novel micro-electro mechanical system (MEMS) process and a backside etching process were established. Furthermore, the characteristics of silicon strain gauges were evaluated after glass-frit bonding to a steel diaphragm. It is difficult to bond a silicon strain gauge to a steel diaphragm using glass frit because the high-temperature aging process during the glass frit bonding process causes shifting, rotating, tilting, and sliding of the strain gauges. The measurement experiments were conducted using devices of various shapes. The silicon bulk-micromachined strain gauges have through holes and a closed structure, unlike the current competitive devices with open structures. This unique design concept reduces shifting and rotation of the gauge position during glass frit bonding and enables automation of the alignment and bonding processes, which improves the sensor performance and yield and hence reduces sensor cost. Therefore, in this paper, we represent a unique silicon strain gauge structure to minimize those problems and improve the sensor performance, quality, and reliability while reducing production cost. To determine the most suitable conditions for fabrication, silicon strain gauges having various shapes and various through-hole sizes were fabricated. The sensors performance was measured after attaching them to elements of various shapes. For quantitative analysis, the proposed strain gauge was evaluated in comparison with a conventional strain gauge. As a result, the proposed strain gauge yield was improved approximately 98% when compared with the conventional strain gauge, and exhibited excellent results in reliability testing by various methods after glass frit bonding. The proposed strain gauge was measured using the mechanical, and electrical characteristics. The prototype strain gauge was characterized using specific long-term tests, such as pulsed- pressure, temperature cycling, with bias (PPTCB). The pressure sensors, tested under pressure ranging from 0 to 50 bar at different temperature have a linear output with a typical sensitivity of approximately 1.79 mV/V/bar and an offset drift of –10 mV to 12 mV. The fatigue life tests were performed at a rate of 3 Hz for 2 million pressure cycles from 0 to 50 bar showed no failures when the pressure cycle test was stopped.

플라즈마 기능화된 다중 벽 탄소나노튜브 네트워크 전극을 이용한 고속, 고감도 정전용량형 습도센서 개발

정경훈 고려대학교 대학원 2013 국내석사

Plasma-activated multi-walled carbon nanotube (P-MWCNT) films were used to form the upper electrode of a polyimide (PI)-based capacitive humidity sensor and compared with existing commercial technology on their performance. Highly open porous conductive electrodes, which are almost impossible to obtain with conventional metal electrodes, are fabricated by spay-depositing multi-walled carbon nanotube (MWCNT) networks on a polyimide layer. Spray-deposited MWCNT films form a well-entangled and interconnected porous network with nano-scale open pores. These properties are highly desirable for fast-speed capacitive humidity sensors. In addition, Oxygen plasma activation of the MWCNTs is also explored to improve the water adsorption of the MWCNT films, by introducing oxygen-containing functional groups on the CNT surface. Polyimide humidity sensors with optimized p-MWCNT network electrodes exhibit a much faster response time (1.5 s for adsorption and 2 s for desorption) and a higher sensitivity (0.647 pF/%RH) to humidity, compared to untreated MWCNT and porous Cr electrodes. These results may be partially due to their percolated pore structure being more accessible for water molecules and for expending the diffusion of moisture to the polyimide sensing film, and partially due to the oxygenated surface of p-MWCNT films, allocating more locations for adsorption or attraction of water molecules to contribute to sensitivity. 최근 산업이 고도화되고 쾌적한 공간을 추구하는 인간의 욕구에 따라 산업체와 생활 환경 분야에서 안정된 습도 측정 및 제어의 필요성이 점차 강하게 요구되고 있다. 이에 따라 다양한 재료와 원리에 기반을 둔 습도 센서가 개발 및 상용화 되고 있으며, 동시에 이용 분야 또한 급속히 확대되어 가는 추세이다. 현재 습도 센서의 연구 개발은 기존의 저항형 세라믹 습도 센서에서 정전용량형 고분자 습도 센서로 발전되고 있다. 정전용량형은 저항형보다 습도 측정 범위가 0~100%RH 로 넓고 직선형의 출력을 가지며 직류에서 작동이 되는 이점이 있다. 또한 고분자 재료 중 폴리이미드(Polyimide:PI)는 1~3% 전후의 적절한 흡수율과 큰 체적 저항률(1016-1017 Ω•㎝) 그리고 내환경성이 우수하여 습도 센서의 신뢰성이 높은 감습 재료로 널리 사용되고 있다[1-3]. 이와 같이, 종래의 폴리이미드 기반의 정전용량형 습도 센서는 많은 우수한 특성을 나타내는 반면, 상대적으로 느린 응답 속도로 인한 한계가 있다. 특히 자동차 전자제어시스템이나 의학 산업 분야에서는 습도 변화에 대한 신속한 결정이 이루어져야 하기 때문에 습도 센서의 빠른 응답 시간이 중요한 요소로 자리잡고 있다[5]. 이러한 문제점을 극복하기 위하여 폴리이미드의 화학적, 물리적 개선[4-6]과 센서의 구조적 변화[7,8]를 적용하는 연구가 활발히 이루어 지고 있다. Kanamori[4]는 플루오린(Fluorine)을 첨가한 폴리이미드를 이용하여 내부에 있는 플루오린이 물 분자의 흡습과 탈습을 신속히 이루어지게 하여 응답 특성이 향상된다고 제시하였다. 하지만 동적 응답 특성에서 탈습 시간이 흡습 시간에 비해 상당히 뒤떨어지는 것을 볼 수 있다. Tetelin[5], Suzuki[6] 등은 폴리이미드에 플라즈마 처리하여 반응 속도를 향상시켰지만, 탈습 시간이 상대적으로 많이 걸리는 것을 확인하였다. 정전용량형 습도 센서의 응답 속도를 개선하기 위한 다른 방식으로, 센서의 모양과 크기, 즉, 구조적 변화가 있다. 감습재와 이의 두께가 결정되면 응답 속도는 대부분 센서 구조에 따라 크게 영향을 받는다. Kang[7]은 정전용량형 습도 센서와 폴리실리콘(Polysilicon) 히터를 집적화하여 감도와 빠른 반응 속도를 실현시켰고 탈습 시간 또한 히터를 이용하여 해결하였다. Kim[8]은 정전용량형 습도 센서에 마이크로 브릿지를 적용하여 많은 hole 을 제작함으로써 상부 전극의 hole 뿐만 아니라 PI의 측면으로도 물 분자 침투가 가능하도록 하여 상당히 빠른 응답 시간을 이루었다. 하지만 인용논문 [7]과 [8]은 빠른 반응 속도를 이루었지만 이들 구조의 공정이 복잡하여 시간과 비용에 있어서 비효율적이다. 한편, 탄소나노튜브(Carbon nanotube:CNT)를 이용한 정전용량형 습도 센서도 활발히 연구되고 있다. CNT는 넓은 표면적과 전기 전도성 그리고 화학적 안정성 등으로 인해 많은 분야에서 연구되고 있으며[9,10], 특히 박막 형태의 CNT 네트워크는 여러 개의 nanotube들이 서로 전기적으로 연결되어 있어 앞서 서술한 특성들과 함께 다공성 박막으로써 여러 전기적 소자와 센서에 활용되고 있다[11-17]. 현재까지 CNT 네트워크를 이용한 정전용량형 습도 센서는 CNT를 감습막으로 사용하여 수분에 대한 변화를 연구하였다[18,19]. Yeow[18]은 두 평행판 전극 중 한 면에 다중벽 탄소나노튜브(Multiwall carbon nanotube, MWCNT)를 증착시켜 감습막으로 사용하였고 이에 대한 감도 특성과 반복성이 우수하다는 것을 보였다. 하지만, 감도가 비선형적이며 고습에서만 우수한 결과를 보였고 응답 속도 또한 매우 느린 것으로 나타났다. Yu[19]는 MWCNT와 Poly(ethleneimine)를 layer by layer 공법으로 다층 박막을 제작하여 습도에 대한 변화를 연구하였다. 5~85%RH 습도 범위에서는 감도가 우수하지만 85%RH이상의 영역에서는 비선형으로 나타나며 센서의 흡습 응답 시간에 비해 탈습 시간이 매우 느린 것으로 나타났다. 앞서 설명한 바와 같이, 습도의 급격한 변화에 수초 이내로 응답하여야 하는 산업 분야의 요구를 만족시키기 위한 방법으로, 본 논문에서는 안정적이며 우수한 응답속도를 가지는 정전용량형 습도 센서를 개발하여 이에 대한 내용을 제시한다. 본 연구는 기공이 균일하게 분포하며 높은 다공성을 갖는 MWCNT 박막을 정전용량형 습도 센서의 상부 전극에 적용함으로써 전극 표면에 흡착된 물 분자가 다수의 기공을 통하여 감습막에 신속히 침투할 수 있도록 하는 것에 목적이 있다. 이에 대한 연구로 MWCNT 박막이 습도 센서의 전극으로써 적합한지를 평가하고 센서에 적용하여 응답 특성을 살펴보았다. 또한 기존의 제품화된 정전용량형 센서에서 사용하는 다공성 금속 전극을 동일한 구조의 센서로 제작하여 MWCNT 전극 기반의 센서와 특성을 비교하였다. 결과적으로 기존 상용화된 습도 센서의 상부 전극을 대체함으로써 응답 시간 문제를 극복할 수 있다. 또한 MWCNT로 이루어진 상부 전극에 산소 플라즈마를 이용한 표면 처리 공정을 추가하였다. 최적의 조건을 갖는 플라즈마 처리된 MWCNT(Plasma treated MWCNT, p-MWCNT) 전극을 통해 초고속, 고감도의 정전용량형 습도 센서를 구현하였다. 마지막으로 컨버팅 장치로의 적용 가능성을 위하여 본 연구에서 제작한센서를 자체 제작한 신호처리장치에 연결하여 그 특성을 평가하였다.