Jeong-Sik Choi

Synthesis and characterization of fructooligosaccharides using levansucrase with a high concentration of sucrose

A method for synthesizing branched fructo-oligosaccharides (BFOS) with a high concentration of sucrose (1–3 M) was developed using levansucrase prepared fromLeuconostoc mesenteroides B-1355C. The degree of polymerization of oligosaccharides synthesized according to the present method ranged from 2 to over 15. The synthesized BFOS were stable at a pH ranges of 2 to 4 under 120°C. The percentage of BFOS in the reaction digest was 95.7% (excluding monosaccharides; 4.3% was levan). BFOS reduced the insoluble glucan formation byStreptococcus sobrinus on the surfaces of glass vials or stainless steel wires in the presence of sucrose. They also reduced the growth and acid productions ofS. sobrinus. Oligosaccharides can be used as sweeteners for foods such as beverages requiring thermo-and acid-stable properties and as potential inhibitors of dental caries.

Cyclotron Cherenkov maser amplification using the anomalous Doppler effect

Abstract We use the Vlasov-Maxwell equations to investigate the kinetic instability caused by the interaction of the electron beam with the electromagnetic wave in the small signal limit. The orthogonality condition of the electromagnetic modes has been used to derive the dispersion relation for the dielectric-loaded cylindrical waveguide driven by a relativistic electron beam when a constant guiding magnetic field is applied. We show that the cyclotron Cherenkov instability has a growth rate comparable to that of the Cherenkov instability in the high energy regime, and that the amplification of an electromagnetic wave is possible in the range of millimeter to submillimeter using the anomalous Doppler effect.

An analysis of PEMFC & photovoltaic 500W hybrid system

Renewable energy sources (solar, wind, etc) are attracting more attention as alternative energy sources to conventional fossil fuel energy sources. Hydrogen energy and photovoltaic is friendly to the environment. In this paper a 500 W PV-fuel cell hybrid generation system is designed. An electrolyzed coupled to the PV array is employed for hydrogen production. Maximum power tracking for PV array is achieved using fuzzy regression. A controller is designed to ensure continuous constant power generation through the day and after sunset via the PV and fuel cell stack. The system is simulated using LabVIEW software. The system characteristics obtained from simulation are analyzed.

Nonlinear saturation characteristics of a dielectric Cherenkov maser

Abstract The nonlinear saturation state in a dielectric Cherenkov maser (DCM) with the TM mode and the intense relativistic electron beam is analysed from the nonlinear formulation based on the cold fluid-Maxwell equations. We obtain the nonlinear efficiency and the final operation frequency under consideration of the effects of the beam current, the beam energy and the dielectric materials and show that the characteristics of a DCM instability has a strong resemblance to that of the relativistic two stream instability by the coherent trapping of electrons in a single most-unstable wave. Finally, the nonlinear analysis shows that the Cherenkov maser operation with a lower-energy beam can be more efficient in the higher frequency regime for the case of the high power DCM with a high current.

THE MAGNETORESONANCE OPERATION OF MICROWIGGLER USING A PIEZOELECTRIC WITH A STRONG MAGNETIC GUIDE FIELD

Abstract We proposed that a new type of the electrostatic microwiggler with a wiggler period (0.1 mm ≤ l w ≤ 1 cm) and the wiggler field strength ( E w ≤ 500 kV/m) can be produced on the surface of a PZT when a high power and high frequency ultrasonic wave travels through a PZT bar. Numerical simulations in the linear and nonlinear gain regime show that a weak microwiggler ( E w = 500 kV/m, λ w = 3 mm, 100 periods), operating in magnetoresonance with a strong guide field ( B 0 = 2 T), can generate millimeter and submillimeter radiations with medium electronic efficiency of few percents. It is shown that the maximum output power of the compact FEL using the wiggler system generated on the surface of the piezoelectric material may be up to a few watts with a relatively low energy and low current electron beam ( E b = 100 keV and I b = 10 mA).

Electromagnetic wave amplification of cyclotron Cherenkov maser

Abstract We use the Vlasov-Maxwell equations to investigate the kinetic instability caused by the interaction of the electron beam with the electromagnetic wave in small signal limit. The orthogonality condition of the electromagnetic modes has been used to derive the dispersion relation for the dielectric-loaded cylindrical waveguide driven by a relativistic electron beam when a constant guiding magnetic field is applied. When the guiding field is not so strong, there arises cyclotron Cherenkov instability as well as Cherenkov instability. We show that the cyclotron Cherenkov instability has a growth rate comparable to that of the Cherenkov instability, and the amplification of an electromagnetic wave is possible in the range of millimeter to sub-millimeter arising from the anomalous Doppler effect.

Multi-layered Coating Deposited on PEMFC (Proton Exchange Membrane Fuel Cell) Bipolar Plates

The surface region of commercial stainless steel 304 and 316 plates has been modified through deposition of the multi-layered coatings composed of titanium film (0.1 ㎛) and gold film (1-2 ㎛) by an electron beam evaporation method. XRD patterns of the stainless steel plates deposited with conductive metal films showed the peaks of the external gold film and the stainless steel substrate. Surface microstructural morphologies of the stainless steel bipolar plates modified with multi-layered coatings were observed by AFM and FE-SEM images. The stainless steel plates modified with 0.1 ㎛ titanium film and 1 ㎛ gold film showed microstructure of grains of under 100 ㎚ diameter. The external surface of the stainless steel plates deposited with 0.1 ㎛ titanium film and 2 ㎛ gold film represented somewhat grain growth of Au grains in FE-SEM image. The electrical resistance and water contact angle of the stainless steel bipolar plates modified with multi-layered coatings were examined with the thickness of the gold film.

A study of operating characteristics for 1kW households PEMFC system

This paper discusses an actual operation of a monitoring system for fuel cells which will probably be substituted for the next generation environmental energy sources. Through this demonstration from monitoring system of 1 kW fuel cell for a house, the electrical and heat efficiency of fuel cell is able to be improved, and several faults of fuel cell, such as high expense and weak durability, would also be remedied dramatically.

A study on properties of SAW and the magnetic thin-film bond

We manufactured photo mask to make surface acoustic wave (SAW) filter that use special quality of these LiNbO/sub 3/ piezoelectric material (100). To make IDT electrode, we evaporated aluminum about 6000 /spl Aring/, and achieved lithography process by using e-beam evaporator. Filament evaporation method was selected to evaporate nickel magnetic thin film between Tx-IDT terminal and Rx-IDT terminal. Thickness of evaporated nickel was about 4000 /spl Aring/. Width of IDT electrode that is used here are 50 /spl mu/m and space between IDT electrodes is 50 /spl mu/m. Metal piece is 8 pair, the length of metal piece is 8 mm and the whole size is 52 mm/spl times/24 mm. The calculated SAW is 200 /spl mu/m. It is confirmed that manufactured LiNbO/sub 3/-IDTs center frequency is 18.1 MHz. The average velocity of SAW, that propagates LiNbO/sub 3/-IDTs Tx-IDT and Rx-IDT both terminal, measured v=3888/spl plusmn/2.565m/s. And SAWs average velocity, that propagates LiNbO/sub 3/-Ni-IDTs Tx-IDT and Rx-IDT both terminal, measured /sup v/=3827/spl plusmn/1.474m/s. While SAW passing magnetic thin films, its velocity decreased about 1.5% (61m/s).

Nonlinear analysis using a fluid model for the saturation characteristics in the Cherenkov maser operation

The saturation state composed of the wave and relativistic electron beam in a dielectric-lined cylindrical waveguide is analyzed using the nonlinear formulation based on the cold fluid-Maxwell equations. The effects of the beam current, the beam energy and the dielectric materials on the efficiency are discussed. It is suggested that the operation with lower-energy beam can be more efficient in the high frequency regime for the case of the high power DCM with high current.