Taegyu Kim

Design, fabrication and testing of a catalytic microreactor for hydrogen production

A catalytic microreactor for hydrogen production was fabricated by anisotropic wet etching of photosensitive glass, which enables it to be a structure with high tight tolerance and high aspect ratio. As a reactor structure, a microchannel was used for improving heat and mass transfer in the reactor. The primary fuel source is methanol for a mobile device. Endothermic catalytic steam reforming of methanol was chosen for producing gaseous hydrogen. The Cu-based catalyst, Cu/ZnO, was prepared by the co-precipitation method and coated on the surface of the microchannel for methanol steam reforming. An overall microfabrication process was established for a MEMS-based catalytic microreactor. The fabricated reactor has a volume of 1.8 cm3 including the volume of the reaction chamber 0.3 cm3 and produced dry reformate with high hydrogen content, 73%. The hydrogen flow was 4.16 ml min−1, which can generate a power output of 350 mWe for a fuel cell.The page numbers of this article were corrected on 24 July 2006. The corrected electronic version is identical to the print version.

Time-varying heat transfer coefficients between tube-shaped casting and metal mold

Abstract An experimental and numerical study were made on the time-varying heat transfer coefficient h(t) between a tube-shaped casting and metal molds. One dimensional treatment was adopted in analyzing the heat flows between the casting and the inner and the outer mold. The sequential function specification method was employed to solve the nonlinear inverse heat conduction problem. In order to investigate the different behavior of h(t) for different alloys, casting experiments were carried out with three Al-base alloys and pure Al having different types of solidification behavior. It was found that the temperature change of the outer mold showed a normal heating and cooling curve. However, that of the inner mold was unusual especially for the alloys with a wide solidification range, i.e. the temperature increases first rapidly, then halts for a while and then increases again showing finally a regular heating and cooing curve. The resulting heat transfer coefficient at the interface to the inner mold hi(t) decreases temporarily and then increases, while the one at the interface to the outer mold ho(t) decreases monotonously to a quasi steady state. The abnormal heat transfer phenomenon at the inner interface for the alloys with a wide solidification range was concluded to be caused by a slight movement of the semi-solid inner wall at the inside of the tube-shaped casting due to the solidification contraction of the casting freezing in a mushy type.

Effect of the Electric Conductivity of a Catalyst on Methane Activation in a Dielectric Barrier Discharge Reactor

The influence of catalyst electric conductivity on methane activation in a planar-type dielectric barrier discharge reactor is investigated by empirically comparing the degree of methane conversion of bare Al2O3 with that of Pt/Al2O3; from this, it is determined that the latter catalyst converts less methane owing to the presence of Pt. Calculations and comparisons of electric fields with and without Pt show that the presence of a Pt catalyst results in a lower electric field than does bare Al2O3. An analysis of product gases based on the correlation between the fragmentation of radicals and the electric field also indicates that the electric field is decreased by using Pt. From these results, it can be concluded that the synergies between the plasma and the conductive catalysts need to be reassessed for different electric field conditions, and that further studies of non-conductive catalysts that can enhance methane activation and synergistic effects are needed.

Micromachined Methanol Reformer for Portable PEM Fuel Cells

Fabrication procedures for a micromethanol reformer including catalyst preparation, coating, and patterning on a wafer are described. Cu-based catalyst was prepared by coprecipitation method. The effects of precipitation conditions on the catalytic activity and adhesion of coated catalyst on the substrate were tested to find the optimum precipitation condition. For coating purposes, the prepared catalyst was ground into powder and mixed with binder in the solvent. Simultaneous precipitation of catalyst and binder on the wafer produced catalyst layer that is uniform and rigidly found to the wafer surface. The amount of coated catalyst on the wafer was 5-8 mg/cm 2 with a thickness of 30 μm. By repetition of coating procedure, catalyst mass up to 15 mg/cm 2 was obtained with increased reactivity. Patterned catalyst layer was obtained by novel lift-off process of polyvinyl alcohol sacrificial layer A micromethanol reformer was fabricated using a typical lithography procedure including catalyst coating and patterning process. Typical methanol conversion was higher than the conventional packed bed reactor.

Mathematical model for the dynamic P-C-T curves of the MmNi4.6Al0.2Fe0.2V0.03 alloy in a tubular reactor

Abstract In order to simulate the dynamic P-C-T curves of the MmNi 4.6 Al 0.2 Fe 0.2 V 0.03 alloy, a mathematical model has been established and solved numerically by the method of finite domains. The numerical simulation is used to present the time-space evolution of the temperature, the pressure and the hydrogen concentration of the hydride powders in the reactor and to determine the effects of reactor geometry and operation parameters such as hydrogen flow rate and temperature of the water bath. It is assumed that there is no pressure gradient in the reaction bed and the reaction occurs throughout the packed bed. The local reaction rate is calculated from the product of the average reaction rate and the factor which is a function of the pressure driving force. The simulated results using this model are quite close to the experimental data. Based on the model, it was possible to predict the dynamic absorption P-C-T curves of MmNi 4.6 Al 0.2 Fe 0.2 V 0.03 alloy in a tubular reactor.

A complete power source of micro PEM fuel cell with NaBH 4 microreactor

A complete system of micro fuel cell with NaBH4 microreactor was developed for micro power sources. The micro fuel cell system consists of two main components; one is a micro PEM fuel cell and the other is a microreactor for hydrogen generation from NaBH4 alkaline solution. All of BOP such as a fuel cartridge, a micropump, and an auxiliary battery were integrated for a complete micro power device, and the performance of the micro fuel cell system was measured.

Preparation of Cu/ZnO for Fabrication of a Micro Methanol Reformer

Three synthesis procedures of Cu/ZnO catalyst for steam reforming of methanol were tested for loading in a micro reactor. The best procedure that resulted in the strong adhesion to the wafer was determined out of the tested procedures. The molecular structure of the synthesized catalyst was examined by XRD and its performance in methanol conversion was measured. A micro fabrication method that incorporates the catalyst loading and micro structure processing was developed. A MEMS methanol steam reformer was built by this process and the completed device resulted in methanol conversion of 93%.

Mechanism of the Accelerated Reduction of an Oxidized Metal Catalyst under Electric Discharge

The mechanism of accelerated reduction of oxidized metal catalysts by plasma was herein explored. Hydrogen radicals produced by discharge are important for generating energetic hydroxyl radicals by an Eley–Rideal‐type reaction, and these radicals in turn undergo desorption without further reaction. The desorbed hydroxyl radicals react with hydrogen radicals to produce gas‐phase water. The results obtained herein discuss the advantages of employing plasma catalysis for surface reactions.

Fabrication and performance analysis of MEMS-based Variable Emissivity Radiator for Space Applications

All Louver was typically representative as the thermal control device. The louver was not suitable to be applied to small satellite, because it has the disadvantage of increase in weight and volume. So MEMS-based variable radiator was developed to support the disadvantage of the louver MEMS-based variable emissivity radiator was designed for satellite thermal control. Because of its immediate response and low power consumption. Also MEMS- based variable emissivity radiator has been made smaller by using MEMS process, it could be solved the problem of the increase in weight and volume, and it has a high reliability and immediate response by using electrical control. In this study, operation validation of the MEMS radiator had been carried out, resulting that emissivity could be controlled. Numerical model was also designed to predict the thermal control performance of MEMS-based variable emissivity radiator.

Performance verification of mems variable emissivity radiator for spacecraft thermal control systems

In this study, MEMS variable emissivity radiator was proposed. The proposed radiator changes the emissivity property according to the polarity change of electrodes by using electric charge of silicon beads. Variable emissivity function and thermal control tests were conducted in vacuum chamber to verify the performance of the MEMS radiator. Based on thermal control test results, the effective emissivity was calculated. As the thermal test result, the temperature difference between open and closed modes at the hot case was 6.4°C, whereas at the cold case, it was 10°C. The effective emissivity at the closed mode was 0.63, whereas it was 0.31 at the open mode.