Si-Hyung Lim

Mechanically robust superamphiphobic aluminum surface with nanopore-embedded microtexture.

A simple fabrication technique was developed for preparing a mechanically robust superamphiphobic surface on an aluminum (Al) plate. Dual geometric architectures with micro- and nanoscale structures were formed on the surface of the Al plate by a combination of simple chemical etching and anodization. This proposed methodology involves (1) fabrication of irregular microscale plateaus on the surface of the Al plate, (2) formation of nanopores, and (3) fluorination. Wettability measurements indicated that the fabricated Al surface became super-repellent toward a broad range of liquids with surface tension in the range 27.5-72 mN/m. By varying the anodization time, we measured and compared the effects of morphological change on the wettability. The adhesion property and mechanical durability of the fabricated superamphiphobic Al surface were evaluated by the Scotch tape and hardness tests, respectively. The results showed that the fabricated Al surface retained mechanical robustness because the down-directed surface made by nanopores on the microtextured surface was durable enough even after high force was applied. Almost no damage of the film was observed, and the surface still exhibited superamphiphobicity after the tests. The fabricated superamphiphobic surface also remained stable after long-term storage. The simple and time-saving fabrication technique can be extended to any large-area three-dimensional surface, making it potentially suitable for large-scale industrial fabrications of mechanically robust superamphiphobic surfaces.

Design and fabrication of a novel bimorph microoptomechanical sensor

We have designed a so-called flip-over bimaterial (FOB) beam to increase the sensitivity of micromechanical structures for sensing temperature and surface stress changes. The FOB beam has a configuration such that a material layer coats the top and bottom of the second material at different regions along the beam length. By multiple interconnections of FOB beams, the deflection or sensitivity can be amplified, and the out-of-plane motion of a sensing structure can be achieved. The FOB beam has 53% higher thermomechanical sensitivity than a conventional one. Using the FOB beam design, we have developed a microoptomechanical sensor having a symmetric structure such that beam deflection is converted into a linear displacement of a reflecting surface, which is used for optical interferometry. The designed sensor has been fabricated by surface micromachining techniques using a transparent quartz substrate for optical measurement. Within a sensor area of 100 /spl mu/m/spl times/100 /spl mu/m, the thermomechanical sensitivity S/sub T/=180 nm/K was experimentally obtained.

Fabrication of amphiphobic surface by using titanium anodization for large-area three-dimensional substrates.

Superamphiphobic functional Ti foils were fabricated using anodization techniques. By varying the supply voltage and anodization time, a two-step anodization method was used to maximize the contact angle of water and various oils. The morphology of the TiO2 nanotube surface is important to achieve superamphiphobicitiy. The anodized surface maintained good superamphiphobic stability with long-term storage. Furthermore, the wettability properties toward both water and various oils can be easily and reversibly switched from hydrophobic and oleophobic to hydrophilic and oleophilic, respectively, and vice versa via air-plasma treatment and fluorination. The developed simple technique can be applied to any large-area three-dimensional surfaces to fabricate amphiphobic Ti surfaces.

Enhancement of hole injection and electroluminescence by ordered Ag nanodot array on indium tin oxide anode in organic light emitting diode

We report the enhancement of hole injection and electroluminescence (EL) in an organic light emitting diode (OLED) with an ordered Ag nanodot array on indium-tin-oxide (ITO) anode. Until now, most researches have focused on the improved performance of OLEDs by plasmonic effects of metal nanoparticles due to the difficulty in fabricating metal nanodot arrays. A well-ordered Ag nanodot array is fabricated on the ITO anode of OLED using the nanoporous alumina as an evaporation mask. The OLED device with Ag nanodot arrays on the ITO anode shows higher current density and EL enhancement than the one without any nano-structure. These results suggest that the Ag nanodot array with the plasmonic effect has potential as one of attractive approaches to enhance the hole injection and EL in the application of the OLEDs.

MEMS gas preconcentrator filled with CNT foam for exhaled VOC gas detection

For disease monitoring and diagnostics using breath analysis, the gas preconcentrator is crucial for low-concentration exhaled volatile organic compound (VOC) gas analyses, and overcomes existing detection limits associated with the commercialized gas sensors. In this work, the microelectromechanical system (MEMS) gas preconcentrator chip was designed and fabricated for conducting low-power-operated breath analyses. It consists of a microheater and a gas chamber filled with a carbon nanotube (CNT) foam. The CNT foam, used as a gas adsorbing material, has several advantages including its large gas adsorption capacity due to its large surface-to-volume ratio, a low pressure drop due to its high porosity, and a rapid thermal desorption due to its high thermal conductivity. Using the developed MEMS gas preconcentrator chip, several basic performances were tested for clinically important VOC gases using a commercial gas chromatography-flame ionization detector (GC-FID). For gas preconcentrations over five minute interval, the preconcentration factors for methane and ethane gases were 8.05 and 7.72, respectively. These results suggest that the developed MEMS gas preconcentrator can be potentially utilized to analyze the low-concentration exhaled VOC gases for the purpose of noninvasive medical diagnoses.

CNT Foam-Embedded Micro Gas Preconcentrator for Low-Concentration Ethane Measurements

Breath analysis has become increasingly important as a noninvasive process for the clinical diagnosis of patients suffering from various diseases. Many commercial gas preconcentration instruments are already being used to overcome the detection limits of commercial gas sensors for gas concentrations which are as low as ~100 ppb in exhaled breath. However, commercial instruments are large and expensive, and they require high power consumption and intensive maintenance. In the proposed study, a micro gas preconcentrator (μ-PC) filled with a carbon nanotube (CNT) foam as an adsorbing material was designed and fabricated for the detection of low-concentration ethane, which is known to be one of the most important biomarkers related to chronic obstructive pulmonary disease (COPD) and asthma. A comparison of the performance of two gas-adsorbing materials, i.e., the proposed CNT foam and commercial adsorbing material, was performed using the developed μ-PC. The experimental results showed that the synthesized CNT foam performs better than a commercial adsorbing material owing to its lower pressure drop and greater preconcentration efficiency in the μ-PC. The present results show that the application of CNT foam-embedded μ-PC in portable breath analysis systems holds great promise.

Chemical vapor detection using nanomechanical platform

For high sensitive and multiplexed chemical analysis, an opto-mechanical detection platform has been built. To check the performance of the platform, we performed water vapor response measurements for the cantilevers coated with alkane thiols having different functional end groups. Furthermore, for the exposure of 50 ppb toluene vapor to carboxylic benzene thiol coating layer, nanoscale static deflection of the cantilever sensors has been measured simultaneously. The nanomechanical platform using cantilever sensors can be miniaturized to be used for high sensitive and selective environmental monitoring for indoor ar outdoor air pollutants, mold, heavy metals, and other health hazard materials.

Miniaturized gas chromatography module with micro posts embedded MEMS column for the separation of exhaled breath gas mixtures

Gas chromatography (GC) is a reliable chemical analysis technique that is used to separate and identify the gas components, and it is extensively utilized for the breath gas analysis. Miniaturizing GC column using Microelectromechanical systems (MEMS) techniques is an inevitable step in achieving micro total analysis systems (μTAS) or lab-on-a-chip devices. This paper reports the fabrication and performance of a miniaturized gas chromatography module with micro posts embedded MEMS column. The MEMS-based GC column, fabricated on a 2 cm × 2 cm size μ-GC chip and coated with non-polar stationary phase, was 1.3 m long, 150 μm wide, and 400 μm deep and contain embedded micro circular posts which are 30 μm diameter. The basic separation performance of the developed μ-GC module was evaluated using a commercial flame ionization detector (FID) for light alkanes (C1-C6) gas mixtures, and the chromatographic results for separation of six alkanes mixture showed good resolution and retention of the compounds with entire separation achieved in less than 2 min.

Review on Sensor Technology to Detect Toxic Gases

Abstract The excess use and generation of various toxic gases from many industrial complexes and plant facilities have increased thepossibility of leakage or explosion accidents, which can cause fatal damage to human beings in the wide range of neighboringarea. To prevent the exposure to the fatal toxic gases, it is very important to monitor the leakage of toxic gases using gas sensorsin real time. Various types of gas sensors, which can be classified as semiconductor, electrochemical, optical, and catalytic com-bustion types according to the operating principles, have been developed. In this review, the operation principles of gas sensorsare explained and the performance of those sensors is compared. The state-of-the-art gas sensor technologies developed byresearch institutes or companies are reviewed also.Keywords: Toxic gases, Gas accident, Gas sensors, Safety 1. 독성가스 독성가스란 공기 중에 일정량 이상 존재하는 경우 인체에 유해한 독성을 가진 가스로서 허용 농도가 200 ppm 이하인 것을말한다[1]. 국내 독성가스 생산은 여수에서, 저장은 울산에서 가장 많은 가운데 지난 10년간 독성가스 사건은 전국 산업단지에서 36건이 발생했고, 이 중 암모니아 사고가 23건으로 가장 많이 보고되고 있다. 또한, 국내 독성가스 사용량은 연평균 39%로 증가하면서 이 같은 독성가스 사고가 발생할 가능성이 높아지고 있다[2].일반적으로 독성가스가 인체에 미치는 영향은 눈과 호흡기관의 점막 손상 및 염증, 두통, 구토, 급성 호흡곤란 등이 있으며,장시간 노출 될 시 홍반, 조직 파괴, 질식, 심하면 사망까지 이르게 할 수 있다. 독성가스의 중독으로부터 회복되었더라도 추후 여러 가지 후유증이 남을 수 있으므로 사전에 조치하고 예방하는 것이 중요하다. 한편, 독성가스에 노출되더라도 그 농도에 따라 증상이 미비하거나 매우 심각하게 나타날 수 있는데,일반적으로 독성가스의 인체 허용농도 기준을 정의하기 위해TLV-TWA (Threshold Limit Value-Time Weighted Average) 수치가 물질 고유 허용치로 활용된다. 이 값은 ‘시간 가중치로서 거의 모든 노동자가 1일 8시간 또는 주 40시간의 평상 작업에서악영향을 받지 않는다고 생각되는 농도로서 시간에 중점을 둔유해물질의 평균농도’를 뜻하며, Table 1은 TLV-TWA 수치에따른 대표적인 독성가스의 인체 허용농도를 나타낸다[3].하지만 인간의 감각기관으로는 독성가스의 농도를 정량 하거나 종류를 거의 판별할 수 없다. 이에 대응하기 위해 그 동안여러 물질의 물리적, 화학적 성질을 이용한 수많은 가스센서가연구, 개발 되어 가스의 누설감지, 농도의 측정, 기록 및 경보

Modeling and performance of two types of piston-like out-of-plane motion micromechanical structures

We have modeled and analyzed the performance of two types of piston-like out-of-plane motion micromechanical structures: a conventional microstructure, which has a single bimorph region, and a flip-over-bimaterial (FOB) microstructure, which has two bimorph regions respectively located on the top and bottom sides of the structure. For both structures, simple analytical expressions of their end-point deflections have been established to facilitate parametric studies in sensor or actuator designs. These structures can be used in several applications such as temperature and chemical sensors, or as actuators for micromirrors. The derived analytical deflection predictions are in good agreement with those made using finite element (FE) models. For a micro-opto-mechanical sensor using interconnected FOB microstructures, these analytical and FE predictions agree with the experimental results within about 25%. Discrepancies can be attributed to uncertainties in the material properties of the specimen being tested. Both the analytically derived deflection expressions and the FE models predict that the FOB microstructures are capable of achieving up to two times higher deflection than conventional microstructures that have a single bimorph region. When compared to a cantilever design, a sensor design having interconnected FOB structures has a higher signal-to-noise ratio for the same device footprint. The analytical modeling and performance analysis presented in this paper can be useful to predict the device performance as well as optimize design parameters.