Chu Sik Park

Synthesis and Size Dependent Properties of Magnesium Ferrites

MgFe2O4 samples were prepared by high-temperature thermal decomposition (HTTD), sol-gel, and solid-state reaction methods. The samples (a) prepared by HTTD, (b) annealed at 800degC prepared by sol-gel method under Ar atmosphere, and (c) annealed at 1100degC prepared by a solid-state reaction method have the inverse cubic spinel structure ((Fe)A[MgFe]BO4) with a space group of Fd3m . The saturation magnetization (Ms) and coercivity (Hc) at room temperature are found to be 42.2, 44.2, and 53.3 emu/g and 1.2, 81.6, and 94.1 Oe, respectively. The Mossbauer spectra of all samples have been obtained at room temperature and the sample prepared by the HTTD method was measured at various temperatures ranging from 4.2 K to 300 K. The Mossbauer spectrum of the sample prepared by the HTTD method shows superparamagnetic behavior at room temperature and the Mossbauer spectra of the other samples show ferrimagnetic state of six-line shapes having the hyperfine field (Hf) values of 432 ~ 452 kOe for the A sites and 466 ~ 483 kOe for the B sites. The linewidth and the hyperfine field of Mossbauer spectra is broadened and reduced, respectively, which is reduced the particle sizes.

Phase Separation Characteristics of Pressurized Bunsen Reaction for Sulfur-Iodine Thermochemical Hydrogen Production Process

The Sulfur-Iodine (SI) thermochemical hydrogen production process is promising method for the massive production of hydrogen using the high temperature thermal energy of VHTR. For continuous operation of SI process, the conditions of Bunsen reaction are considered as the pressurized conditions with ca. 373~393K temperature and the composition of Bunsen products should be kept constant during the reaction. Therefore, we carried out the continuous Bunsen reaction using a counter-current flow reactor at pressurized condition to investigate the phase separation characteristics of pressurized Bunsen reaction. As the results, the composition of Bunsen product was maintained constantly as the evidence for the steady-state operation. The continuous reaction was operated without occurrence of side reactions, and a H2SO4 phase and HIx phase as the product contains a small amount of impurities (HI in a H2SO4 phase and H2SO4 in a HIx phase). We concluded that the pressurized Bunsen reaction is favorable to the continuous operation of SI process than the atmospheric reaction.

The Effect of Oxygen on Bunsen Reaction with HIX Solution in Sulfur-Iodine Hydrogen Production Process

The Sulfur-Iodine thermochemical hydrogen production process (SI process) has been focused as one of the most promising method for hydrogen production by water splitting. SI process consists of three sections as follow; (1) Bunsen reaction, (2) H2SO4 decomposition and (3) HI decomposition. The O2 produced in a H2SO4 decomposition section could be supplied directly to the Bunsen reaction section without additional separation. Meanwhile, the reactant solution supplied to a Bunsen reaction section could be supplied as the type of a HIx (I2 + HI + H2O) solution, since only the separation of I2 in a HIx solution recycled from a HI decomposition section is very difficult. Therefore, we carried out the reaction using SO2 and SO2-O2 mixture gases in presence of the HIx solution to identify the effect of O2 in the Bunsen reaction. From the results, the amount of I2 unreacted under the feed of SO2-O2 mixture gases was very small higher than those under the feed of SO2 gas only, while the amount of HI produced was relatively decreased. In addition, the amount of impurities in each phase produced from the Bunsen reaction with the HIx solution was hardly affected by the O2/SO2 molar ratios.

The Characteristics of HI Decomposition using Pt/Al2O3 Catalyst Heat Treated in Air and Hydrogen Atmosphere

Abstract >> In HI decomposition, Pt/Al 2 O 3 has been studied by several researchers. However, after HI decomposition,it could be seen that metal dispersion of Pt/Al 2 O 3 was greatly decreased. This reason was expected of platinumloss and sintering, which platinum was aggregated. Also, this decrease of metal dispersion caused catalytic deactivation.This study was conducted to find the condition to minimize platinum sintering and loss. In particular, heat treatmentatmosphere and temperature were examined to improve the activity of HI decomposition reaction. First of all,although Pt/Al 2 O 3 treated in hydrogen atmosphere had low platinum dispersion between 13 and 18%, it was shownto suitable platinum form that played an important role in improving HI decomposition reaction. Oxygen in the air atmosphere made Pt/Al 2 O 3 have high platinum dispersion even 61.52% at 500°C. Therefore, in order to get high platinum dispersion and suitable platinum form in HI decomposition reaction, air heat treatment at 500°Cwas needed to add before hydrogen heat treatment. In case of 5A3H, it had 51.13% platinum dispersion and improved HI decomposition reaction activity. Also, after HI decomposition reaction it had considerable platinumdispersion of 23.89%.

Effects of Supports and Reduction Temperature on Pt Dispersion of HI Decomposition Catalyst

Pt catalysts have been researched and used for HI decomposition. Specifically, the effects of supports and reduction temperature on metal dispersion were investigated in this paper. Metal dispersion was high measured, in the order of Pt/Al2O3, Pt/ZrO2, and Pt/SiO2. HI conversion results coincided with the metal dispersion. With effect on reduction temperature, Pt dispersion was measured as 2.9 %, 26 %, and 60 % each 1173K, 973K, and 773 K. In addition, HI conversion presented 7.8%, 16.3%, and, 19.4% respectively. Consequently, Pt dispersion, influenced by supports and reduction temperature was considered to be crucial role in HI conversion.

Hydrogen Storage and Release by Redox Reaction of Iron Oxide Medium with Mo and Zr Additives

Chemical hydrogen storage and release of iron-based oxide mediums were investigated by hydrogen reduction and water splitting oxidation (Fe3O4 + 4H2 ⇌ 3Fe + 4H2O). In this study, all metal oxide mediums were prepared by coprecipitation method using urea solution as precipitant. The redox reactions of the mediums were conducted using a fixed bed quartz reactor under atmospheric pressure. The theoretical amount of hydrogen storage that can be obtained from the redox reaction of iron oxide is calculated to be 4.8wt% on the basis of 1g-Fe. However, in case of using the iron oxide medium without additives, the medium was rapidly deactivated due to the agglomeration of Fe metals in the hydrogen reduction step of repeated redox cycles. In this study, therefore, Mo and Zr additives were added to iron oxide to improve the reactivity of the medium and to prevent the agglomeration of that. As a result, the reactivity for oxidation of the mediums was largely improved with the addition of Mo additive. It was concluded that change in the valence of Mo cations affected the redox behavior of the mediums.

Temperature Dependent Cation Distribution in Tb₂Bi₁Ga₁Fe₄O 12

In this study, heavy rare earth garnet Tb₂Bi₁Ga₁Fe₄O 12 powders were fabricated by a sol-gel and vacuum annealing process. The crystal structure was found to be single-phase garnet with a space group of Ia3d. The lattice constant a? was determined to be 12.465 A. From the analysis of the vibrating sample magnetometer (VSM) hysteresis loop at room temperature, the saturation magnetization and coercivity of the sample are 7.64 emu/g and 229 Oe, respectively. The Neel temperature (T N ) was determined to be 525 K. The Mossbauer spectrum of Tb₂Bi₁Ga₁Fe₄O 12 at room temperature consists of 2 sets of 6 Lorentzians, which is the pattern of singlephase garnet. From the results of the Mossbauer spectrum at room temperature, the absorption area ratios of Fe ions on 24d and 16a sites are 74.7 % and 25.3 % (approximately 3:1), respectively. These results show that all of the non-magnetic Ga atoms occupy the 16a site by a vacuum annealing process. Absorption area ratios of Fe ions are dependent not only on a sintering condition but also on the temperature of the sample. It can then be interpreted that the Ga ion distribution is dependent on the temperature of the sample. The Mossbauer measurement was carried out in order to investigate the atomic migration in Tb₂Bi₁Ga₁Fe₄O 12 .

Hydrogen Storage and Release Properties for Compacted Ti-Mn Alloy

Received 12 January, 2017 Revised 23 February, 2017 Accepted 28 February, 2017 Abstract >> Hydrogen forms metal hydrides with some metals and alloys leading to solid-state storage under moderate temperature and pressure that gives them the safety advantage over the gas and liquid storage methods. However, it has disadvantages of slow hydrogen adsorption-desorption time and low thermal conductivity. To improve characteristics of metal hydrides, it is important that activation and thermal conductivity of metal hydrides are improved. In this study, we have been investigated hydrogen storage properties of Hydralloy C among Ti-Mn alloys. Also, the characteristics of activation and thermal conductivity of Hydralloy C were enhanced to improve kinetics of hydrogen adsorption-desorption. As physical activation method, PHEM (planetary high energy mill) was performed in Ar or H2 atmosphere. Hydralloy C was also activated by TiCl3 catalyst. To improve thermal conductivity, various types of ENG (expanded natural graphite) were used. The prepared samples were compacted at pressure of 500 bar. As a result, the activation properties of H2 PHEM treated Hydralloy C was better than the other activation methods. Also, the amounts of hydrogen storage showed up to 1.6 wt%. When flake type ENG was added to Hydralloy C, thermal conductivity and hydrogen storage properties were improved.

Effect of Mo and Ce Additives on Redox Behavior for Hydrogen Storage and Release of Iron Oxide Mediums

The hydrogen reduction and water-splitting oxidation for hydrogen storage and release on the iron oxide mediums with Ce, Mo or Ce-Mo additives were carried out using a fixed bed reactor at atmospheric pressure. A sole Ce additive was an outstanding material for the improvement of the reactivity in the reduction and oxidation of iron oxide medium, even though the medium with Ce was easily deactivated during repeated cyclic reactions due to sintering. A sole Mo additive was a good material for the improvement for the durability. In the mediums with the Ce-Mo additives, therefore, the degree of deactivation and the reactivity of the mediums were gradually decreased with the increase of the amount of Mo. Among the iron oxide mediums with Ce-Mo additives, a FeCeMo-5 medium exhibited the good durability while the maintaining the considerable reactivity during the cyclic reactions.

Variation in the Volume Ratio of HIX Phase/H2SO4 Phase for Improvement of Phase Separation Efficiency in the Bunsen Reaction Section

The H2SO4 phase and HIx phase solution produced from Bunsen reaction section in SI (Sulfur-iodine) process were supplied to the phase separator for the continuous operation of SI process. However, the separation of Bunsen products in the phase separator is difficult because an excess of HIx phase solution existed in the phase separator than the H2SO4 phase solution. Therefore, the additional supply of H2SO4 to the phase separator as the phase separation method was considered for improvement of phase separation efficiency. In this work, the variations in volume ratio and composition of each phase solution were examined after the Bunsen product mixing on the HIx phase/H2SO4 phase volume ratio. As the results, the variation in composition of products increases with increase of the HIx phase/H2SO4 phase volume ratio after mixing. It was also found that the high variations in composition and volume ratio of Bunsen products by mixing in the presence of excess HIx phase. From the results, we concluded that the phase separation method, additional supply of H2SO4 phase solution to the phase separator, is effective in Bunsen product separation when the variation in composition is little.