Hosang Ahn

Vertically Aligned ZnO Nanorod Sensor on Flexible Substrate for Ethanol Gas Monitoring

Vertically aligned ZnO nanorods were grown on flexible polyimide films by a thermolysis assisted chemical solution method to fabricate an ethanol sensing material. Flexible ZnO nanorod sensors were examined to monitor ethanol gas by varying the working temperature from 300 to 125°C and by changing the ethanol concentration in a range from 100 to 10 ppm in synthetic air. A flexible ZnO nanorod sensor can detect 100 ppm of ethanol gas with a sensitivity of 3.11 at 300°C. When compared with ZnO nanorod sensors fabricated on a silicon dioxide hard substrate, its sensing performance exhibited competitive sensitivity.

Temperature effects on output power of piezoelectric vibration energy harvesters

The performance of piezoelectric vibration energy harvesters was studied as a function of environment temperature. The devices fabricated by soft or hard PZTs were used to investigate the effect of material parameters on the thermal degradation of the devices. PZT MEMS device was also prepared and compared with the bulk devices to investigate scaling effect on the thermal degradation. All devices were heated up to 150^oC in an insulating chamber. Output power was estimated by Roundys equivalent circuit model and compared with experimental data. The output power of all devices decreased with the increase of the temperature. The output power as a function of temperature can be predicted by the change of piezoelectric coupling coefficient that is proportional to piezoelectric constant and inverse of square root of dielectric constant. Such combined influence on the output power leads to a lower thermal degradation rate of the soft PZT-based device at a lower temperature. For MEMS scale device based on PZT films, temperature dependence of the output power was reduced. This result can be attributed to decreased temperature dependence of dielectric and piezoelectric constants mainly due to constrained domain motions.

Growth of ZnO Nanorod Arrays on Flexible Substrates: Effect of Precursor Solution Concentration

We report a low-temperature aqueous solution growth of uniformly aligned ZnO nanorod arrays on flexible substrates. The substrate is Indium Tin Oxide (ITO) film coated on polyethylene terephthalate (PET). Solutions with five different concentrations of the precursors with equimolar Zinc Nitrate and Hexamethylenetetramine (HMT) in distilled water were prepared to systematically study the effect of precursor solution concentration on the structural and optical properties of ZnO nanorods. It was concluded that the precursor concentration have great influence on the morphology, crystal quality, and optical property of ZnO nanorods. The diameter, density, and orientation of the nanorods are dependent on the precursor solution concentration. X-ray diffraction and micro-Raman spectroscopy showed that the ZnO nanorods with the highest concentration of 50 mM were highly aligned and have the highest level of surface coverage. It was also found that the diameter and length of the nanorods increases upon increasing precursor solution concentration. This is the first systematic investigation of studying the effect of precursor solution concentration on the quality of ZnO nanorods grown on ITO/PET substrates by low-temperature solution method. We believe that our work will contribute to the realization of flexible organic-inorganic hybrid solar cell based on ZnO nanorods and conjugated polymer.

Analysis of Stress Distribution in Piezoelectric MEMS Energy Harvester Using Shaped Cantilever Structure

Much interest in energy harvesters has been focused on maintaining their conversion efficiency during scaling down via the micromachining process. The piezoelectric PZT-based MEMS energy harvester was designed and fabricated to increase the fraction of the material strained during deflection since the geometric change of the cantilever shape can change the strain distribution on the beam and improve the output power. The generated power during beam deflection was separately collected from individual electrodes located at different positions of the cantilever, and they had good agreement with the strain estimation from finite element analysis. The trapezoidal shape showed 39% higher power than that of rectangular one.

Modeling and evaluation of d33?mode piezoelectric energy harvesters

A microfabricated d33 mode piezoelectric energy harvester (PEH) with unimorph cantilever structure patterned by interdigital electrodes was designed, fabricated and analyzed. New analytical expression for estimating output power was developed based on the modification of conformal mapping and Roundys analytical modeling. The results using the new analytical equation can explain the effects of electrode design on the output power of d33 PEH. The output power increases with a longer finger width to a certain limit, and then decreases presumably due to depoling and polarization reversal by charge injection and charged defects in a piezoelectric layer.

Comparison of Transduction Efficiency for Energy Harvester between Piezoelectric Modes

The efficiency of micro-electro-mechanical systems (MEMS) energy harvesters using transverse and longitudinal piezoelectric modes was studied. Since the emergence of piezoelectric MEMS energy harvesters, studies have mainly focused on improving the conversion efficiency at a given strain on a transduction layer. The transverse piezoelectric mode has been applied to use a higher d33 piezoelectric coefficient of a lead zirconate titanate oxide (PZT) material, but few studies have verified its distinguished efficiency compared to that of transverse mode using d31. PZT thin films were deposited on Pt/Ti/SiO2 and ZrO2/SiO2 substrates as the mode by a chemical solution deposition method. The rectangular cantilever with a Si proof mass was fabricated on an SOI wafer, and it has 20 μm of Si device layer. The cantilevered energy harvesters in the same size were respectively fabricated for longitudinal {3-1} and transverse {3-3} mode using parallel electrode plates and interdigitated electrodes (IDE). The generated voltage and power were analyzed considering piezoelectric constants of the PZT films in transverse and longitudinal modes. Since the efficiency of the transverse energy harvester seems to be strongly affected by IDE configuration, a study of this is accompanied. This study might give the direction of choosing the piezoelectric mode for the higher efficiency of MEMS transducers based on the material study.

Analyses of the current market trend and research status of indoor air quality control to develop an electrostatic force-based dust control technique

Abstract This study examined the current and future Indoor Air Quality (IAQ) control device markets and analyzed the recent studies on indoor air pollutantr emoval to develop a new technology for fine dust control. Currently, the mechanical filter technique occupies the bulk of the IAQ control market but the electronic technique is emerging as an alternative to control fine dust efficiently. Among the gaseous VOCs and fine dust particles contaminating the indoor air quality, fine dust particles are more problematic because they threaten human health by penetrating deep into the body and producing secondary contaminants by chemical reaction with VOCs. The electronic IAQ control device using dielectrophoretic and electrostatic forces is a good option for public spaces where many people pass, and at the same time, it needs to consider temperature, humidity, and the particle properties of specific areas to highlight the control efficiency. Electronic-related technology is expected to be used widely in many public/private spaces wherever a dust-free environment is required.

Photocatalytic disinfection of indoor suspended microorganisms (Escherichia coli and Bacillus subtilis spore) with ultraviolet light

New control methods are proposed for indoor air quality by removing fine airborne dust-particles. As suspended fine dust-particles contain inorganic dust as well as fine organic bacteria, studies for simultaneous control of these contaminants are required. In this study, photocatalytic disinfection of indoor suspended microorganisms such as E. coli and Bacillus subtilis is performed by three types of photocatalysts with UVA irradiation. The UVA irradiation strength was controlled to the minimum 3 ㎼/㎠, and ZnO, TiO2, and ZnO/Laponite ball were used as the catalysts. The results indicate that E. coli was removed over 80 % after about 2 hours of reaction with UVA and all three types of photocatalysts, whereas only with UVA, around 50 % E. coli removal was obtained. Among the catalysts, ZnO/Laponite composite ball was found to have similar sterilizing capacity to TiO2. However, in case of B. subtilis, which has thick cell wall in its spore state, disinfection was not effective under the low UVA irradiation condition, even with the catalysts. Further studies need to figure out the optimal UVA irradiation ranges as well as photocatalysts doses to control airborne dust, to provide healthy clean air environment.

Nanostructured gas sensors integrated into fabric for wearable breath monitoring system

This paper presents a technology to design and fabricate nanostructured gas sensors in fabric substrates. Nanostructured gas sensors were fabricated by constructing ZnO nanorods on fabrics including polyester, cotton and polyimide for continuous monitoring of wearers breath gas that can indicate health status. The developed fabric-based gas sensors demonstrated gas sensing by monitoring electrical resistance change upon exposure of acetone and ethanol gases.

Performance of Piezoelectric Power Generator in Environmental Conditions

Piezoelectric power generators can be effective power suppliers for small devices by collecting energy from ambient vibration and converting it into electrical energy. Since such small devices are usually mobile, their operation environments can vary widely. For example, the conditions such as vibration frequency and temperature energy highly affect harvesting efficiency of piezoelectric power generators. If the ambient vibration frequency is away from the resonance frequency, there will be a decrease of output power. The ambient temperature can be a possible factor to change the resonance frequency of the power generating device since temperature influences material constants of the constituent components in a power generator such as dielectric constant, piezoelectric strain coefficient, and the stiffness Therefore, the effect of temperature on output power is studied. Since the common piezoelectric material is PZT and it is known that hard- or soft-type PZT exhibits different dependence of materials properties as a function of ambient temperature, this paper investigates the output power of the soft and hard PZTbased power generator depending on ambient temperature.