Jungil Park

Fitting Improvement Using a New Electrical Circuit Model for the Electrode-Electrolyte Interface

The characteristics of impedance for the electrode-electrolyte interface are important in the electrode researches for biomedical applications. So, the equivalent circuit models for the interface have been researched and developed. However, the applications of such previous models are limited in terms of the frequency range, type of electrode or electrolyte. In this paper, a new electrical circuit model was proposed and demonstrated its capability of fitting the experimental results more accurately than before. A new electrical circuit model consists of three resistors and two constant phase elements. Electrochemical impedance spectroscopy was used to characterize the interface for several materials of Au, Pt, and stainless steel electrode in 0.9% NaCl solution. The new model and the previous model were applied to fit the measured impedance results, and were compared their goodness of fit

Microfabirated Clark-type Sensor for Measuing Dissolved Oxygen

This paper presents a microfabricated Clark-type sensor which exactly can measure dissolved oxygen in the cell containing solution. We designed and fabricated a microfabricated Clark-type sensor for measuring the oxygen respiration level of few cells. The microfabricated Clark-type sensor is composed of 3-electrodes on a glass substrate, a FEP (Fluorinated ethylene propylene) oxygen-permeable membrane, and PDMS (Polydimethylsiloxane) reservoir for storing few cells containing solution. The microfabricated Clark-type sensor was fabricated using MEMS technology. Thin-film Ag/AgCl was employed as a reference electrode and its durability was verified by obtaining a stable open circuit potential versus a commercial Ag/AgCl electrode for 2 hours. Response time, reproducibility and linearity of the fabricated oxygen sensor were examined by amperometry. The fabricated Clark-type sensor showed 40 sec of 90% response time, reproducibility with 1.37 nA standard deviation of 97.03 nA mean in the full-oxygen state and 0.26 nA standard deviation of 3.25 nA mean in the zero-oxygen state, and an excellent linearity with a correlation coefficient of 0.994.

Power Generating Characteristics of Zinc Oxide Nanorods Grown on a Flexible Substrate by a Hydrothermal Method

This paper describes the power generating property of hydrothermally grown ZnO nanorods on a flexible polyethersulfone (PES) substrate. The piezoelectric currents generated by the ZnO nanorods were measured when bending the ZnO nanorod by using I-AFM, and the measured piezoelectric currents ranged from 60 to 100 pA. When the PtIr coated tip bends a ZnO nanorod, piezoelectrical asymmetric potential is created on the nanorod surface. The Schottky barrier at the ZnO-metal interface accumulates elecntrons and then release very quickly generating the currents when the tip moves from tensile to compressed part of ZnO nanorod. These ZnO nanorods were grown almost vertically with the length of 300-500 ㎚ and the diameter of 30-60 ㎚ on the Ag/Ti/PES substrate at 90℃ for 6 hours by hydrothermal method. The metal-semiconductor interface property was evaluated by using a HP 4145B Semiconductor Parameter Analyzer and the piezoelectric effect of the ZnO nanorods were evaluated by using an I-AFM. From the measured I-V characteristics, it was observed that ZnO-Ag and ZnO-Au metal-semiconductor interfaces showed an ohmic and a Schottky contact characteristics, respectively. ANSYS finite element simulation was performed in order to understand the power generation mechanism of the ZnO nanorods under applied external stress theoretically.

Enzymatic glucose biosensor based on porous ZnO/Au electrodes

This paper describes a glucose biosensor based on glucose oxidase (GOx) immobilized on porous ZnO/Au electrodes. The ZnO porous electrodes were fabricated by electrochemical deposition of zinc oxide on patterned Au electrodes using polystyrene (PS) spheres as templates. Uniform pore size and highly ordered ZnO pore arrangement were observed from SEM images. X-ray diffraction (XRD) patterns revealed a single crystalline nature of the porous ZnO. This porous structure provides high enzyme loading capacity and long-term stability. In a pH 7.4 phosphate buffer solution, the positively charged high isoelectric point (IEP) ZnO pores enhance the adsorption of negatively charged low IEP GOx through electrostatic attractive force. Once GOx molecules are immobilized within the ZnO pores, the bottleneck structure resulting from the connected pores hinders leaching of GOx from the pores. The resulting enzymatic biosensor showed a linear detection range from 1mM to 18mM, and sensitivity of 10.89μA/ (mM·cm2) with good selectivity and long-term stability.

2 × 3 array oxygen sensor for measuring cellular respiration level

This paper presents a microfabricated 2 × 3 array oxygen sensor integrated with heaters and temperature sensors for measuring cellular respiration level in a solution containing cells. The proposed oxygen sensor consists of 2 × 3 array Clark-type sensors, heaters, and temperature sensors on the substrate. Pt was used for temperature sensors, working electrode and counter electrode of Clark-type sensor because it has very stable physical and chemical properties and resistance linearity to temperature. Ag/AgCl was formed by electrochemical method on patterned Ag and used as reference electrode. Heaters as Au were patterned on the opposite side of the substrate. PDMS was coated on the acrylic plate by using spin coater, and PDMS thickness is about 18µm. It used as an oxygen permeable membrane of Clark-type sensor. The peak oxygen reduction current of the fabricated sensor was measured at −0.8 V by using cyclic voltammetry, and the 90% response time was about 10 sec from full-oxygen state to zero-oxygen state. The fabricated sensor showed good reproducibility for 1 hour with 3.15nA standard deviation of 108.2nA mean in the full-oxygen state and 0.14nA standard deviation of 1.33nA mean in the zero-oxygen state. Also, the fabricated sensor showed a good linearity with a correlation coefficient of 0.997. The temperature sensor showed an excellent linearity with a correlation coefficient of 0.999 from 32 °C to 55 °C.

Structural and Physical Properties of Sealant Paste Prepared by Silica/Polymer Composites

Sealant paste with silica immersed in cross-linked epoxy-acrylate polymer resin was prepared by thermal and UV curing process. The curing mechanism of polymer resin resulted from 2 functional groups of epoxy and acrylic structure. The properties of microstructure, thermal conductivity and mechanical strength were investigated for its various applications. The adhesion strength is increased by increasing the thermal curing time until 15 minutes, and curing efficiency is saturated over 20 minutes. The increase rate per day of pot life and viscosity is 4.8%, indicating it has excellent storage stability. It is found that the formulation of silica pastes can be applied to heavy industries, building materials, display and various industries.

Analysis of Transient Characteristics of SFCL using the Three-Phase Transformer and Power Switch

The research of superconducting fault current limiter (SFCL) for reduction of the fault current is actively underway in the worldwide. In this paper, we analyzed the characteristics of a SFCL using the transformer and superconducting elements combined mutually in accordance with the fault types. The structure of this SFCL was composed of the secondary and third windings of a transformer connected to the load and the superconducting element, respectively. The provided electric power flew into the load connected to the secondary winding of the transformer in normal state. On the other hand, when the fault occurred in power system, the fault current was limited by closing the line of third winding of the transformer. At this time, the effect of the fault was minimized by opening the fault line in secondary winding of a transformer in power system. The sensing of the fault state was performed by the current transformer(CT) and then turn-on and turn-off switching behavior of the secondary line in the transformer was performed by the silicon-controlled rectifier(SCR). As a result, the proposed SFCL limited the fault current within one-cycle efficiently. Also, the degradation of the superconducting element in the normal state was avoided.

Development of a multi channel measurement system for the cellular respiration measurement

This paper describes a multi channel measurement system which can measure the cellular respiration level in a solution containing cells by using a Clark-type sensor with the solution temperature control unit. The Clark-type sensor can measure the cellular respiration level in the solution because it can measure the reduction current depending on the dissolved oxygen level in the solution. This measurement system was maintained the temperature within of the setting temperature value by on/off control method in order to measure the precise cellular respiration level. The measurement system showed that the applied voltage to the working electrode was very stable(-0.8 V 0.0071 V) by using proportional control method. From the current measurement, the response time and the linearity correlation coefficient were 25 sec and 0.94, respectively, which are very close to the results of the commercial product. Using this system and the fabricated Clarktype sensor, the average ratio of the uncoupled OCR(oxygen consumption rate) to the coupled OCR was 1.35 and this is almost the same as that obtained from a commercial systems.