Woo-Gwang Jung

Exploring high-strength glass-ceramic materials for upcycling of industrial wastes

To promote the recycling of industrial waste and to develop value-added products using these resources, the possibility of manufacturing glass-ceramic materials of SiO2-CaO-Al2O3 system has been investigated by various heat treatment processes. Glass-ceramic materials with six different chemical compositions were prepared using steel industry slags and power plant waste by melting, casting and heat treatment. The X-ray diffraction results indicated that diopside and anorthite were the primary phases in the samples. The anorthite phase was formed in SiO2-rich material (at least 43 wt%). In CaO-rich material, the gehlenite phase was formed. By the differential scanning calorimetry analyses, it was found that the glass transition point was in the range of 973–1023 K, and the crystallization temperature was in the range of 1123–1223 K. The crystallization temperature increased as the content of Fe2O3 decreased. By the multi-step heat treatment process, the formation of the anorthite phase was enhanced. Using FactSage, the ratio of various phases was calculated as a function of temperature. The viscosities and the latent heats for the samples with various compositions were also calculated by FactSage. The optimal compositions for glass-ceramics materials were discussed in terms of their compressive strength, and micro-hardness.

Effect of the Cr2O3 and TiO2 as nucleating agents in SiO2-Al2O3-CaO-MgO glass-ceramic system

The nucleation agent is one of the most important factors in glass-ceramics as it can control either the crystallization temperature or the activation energy. In this study, we investigated the effect of two common nucleation agents, TiO2 and Cr2O3, in the SiO2-Al2O3-CaO-MgO system. To determine the effect of TiO2 and Cr2O3 on nucleation, we measured the crystallization temperature by differential scanning calorimetry (DSC) scanning. To determine the activation energy of nucleation, the DSC scanning was made for the selected samples at various speeds. Also, the crystallinity of the selected sample was evaluated from the scattering intensity in X-ray diffractometry. Using DSC scanning, we found that TiO2 was effective in decreasing the crystallization temperature, while Cr2O3 was effective in decreasing the activation energy. We also performed nucleation heat treatment near the glass transition point. It is found that nucleation heat treatment was not effective in decreasing the crystallization temperature in our experimental condition. The XRD scattering method results showed that temperature is the key factor in crystallization and the effect of time is not as important.

Interpretation of evaporation behavior of lead from molten copper by the mass-transfer model in fluid flow

Recently, an increasing tendency to eliminate the lead in alloy design has been found because of its harmfulness in health. The lead is required to be removed from copper scrap for recycling. In order to get fundamental knowledge on the use of cheap copper scrap in the manufacturing of lead-free copper alloy, the possibility of removal of lead by gas bubbling from the molten copper alloy has been investigated. The influence of Ar gas flow rate was investigated based on the reaction kinetics and mass transport model. The removal rate of lead was increased with an increase of flow rate of Ar and temperature. The overall removal rate of lead from the molten copper alloy has been calculated from experimental results, and the mass transport of lead in the gas phase (the absolute molar flux of lead) was calculated from the one-dimensional steady state diffusion model. The calculated mass flux of lead in the model is found to be quite similar to the overall removal rate of lead obtained from experimental results. The mass transport of lead vapor in the gas phase seems to be a major part of the rate-controlling steps, under the present experimental condition.

Magnesium removal from molten aluminum by Cl2 bubbling

The experiments were performed by introducing Cl2 gas bubbles into Al melt in order to obtain basic information on the removal of Mg component. Thermodynamic calculations were performed to confirm the feasibility of Mg removal from Al melt. A mixture of Ar and Cl2 gas with a mixing ratio of 10–50% in a total flowrate of 50–100 sccm was bubbled into Al melt at 1000–1100 K. Mg could be removed from Al melt by Cl2 gas bubbling and the rate of removal depended largely on the total gas flowrate and the Cl2 mixing ratio. A greater rate of decreasing Mg was observed with a higher Cl2 mixing ratio in the bubbling gas. It was estimated that the removal rate of Mg complied with zero order kinetics. The rate-controlling step was estimated to be the mass transfer in Al melt. The activation energy for Mg removal by Cl2 gas bubbling was determined to be 63.1 kJ/mol. The mechanism of Mg removal from Al melt by Cl2 gas bubbling was proposed. The aluminum chlorides were formed by Cl2 gas bubbling, and were expected to react with the Mg in the Al melt which resulted in the formation of MgCl2.

RGO based nanocomposites with sulphide compounds and their chemical properties

Graphene is a one-atom thick two-dimensional sp2 carbon arrangement. Its ultrahigh surface area, excellent electric conductivity, chemical and physical stability made it a promising material in different research fields. Chemical approaches to the large-scale production of graphene have been realized, and the production of RGO (Reduced Graphene Oxide) in quantity has considerably advanced the development of applications for RGO in photocatalysis, capacitive deionization, and solar cells. In the present study, the improvement of synthesis process of RGO was made in terms of temperature, time and safety, as well as introduction of RGO based nanocomposite material. RGO was synthesized by the two step process which is very simple and easy; the conversion of graphite to GO by oxidation and then reduction of the GO to RGO by hydrothermal treatment. The synthesized RGO was combined with nano-size CdS and CuS compounds. The photocatalytic performance of the composite were investigated with the reduction of Cr(VI) by using RGO-CdS nanocomposite. This results may give an insight for the possibility of RGO-CdS in application for the remover of Cr(VI) ion. The hydrothermally synthesized RGO-CuS contains hexagonal structured CuS. The adsorption kinetics of methylene blue on RGO-CuS nanoparticles were compared with bare CuS. It suggests that RGO-CuS nanocomposite can be used for the adsorbent of methylene blue.

Fabrication of reduced graphene oxide decorated with CuS nanoparticles and its activity toward the adsorption of methylene blue

Reduced graphene oxide (RGO) can act as an adsorbent because of its high surface area. Adsorptive characteristics are studied quantitatively on the RGO composite combined with CuS. The removal efficiency of methylene blue is found to be about 85%, which is higher than that of bare CuS (~73%). Further, the kinetics of adsorption of methylene blue was also inspected to determine the rate of the process. The removal process is faster with the RGO-CuS system than with bare CuS. Both high and low temperatures are not favorable for this adsorption process. In highly ionic media of high or low pH, the adsorption is greater than in media of neutral pH. Thermodynamic parameters were calculated in this work, which suggest that this is physisorption and exothermic in nature.

Effect of Fabricating Nanopatterns on GaN-Based Light Emitting Diodes by a New Way of Nanosphere Lithography

Nanosphere lithography is an inexpensive, simple, high-throughput nanofabrication process. NSL can be done in different ways, such as drop coating, spin coating or by means of tilted evaporation. Nitride-based light-emitting diodes (LEDs) are applied in different places, such as liquid crystal displays and traffic signals. The characteristics of gallium nitride (GaN)-based LEDs can be enhanced by fabricating nanopatterns on the top surface of the LEDs. In this work, we created differently sized (420, 320 and 140 nm) nanopatterns on the upper surfaces of GaN-based LEDs using a modified nanosphere lithography technique. This technique is quite different from conventional NSL. The characterization of the patterned GaN-based LEDs revealed a dependence on the size of the holes in the pattern created on the LED surface. The depths of the patterns were 80 nm as confirmed by AFM. Both the photoluminescence and electroluminescence intensities of the patterned LEDs were found to increase with an increase in the size of holes in the pattern. The light output power of the 420-nm hole-patterned LED was 1.16 times higher than that of a conventional LED. Moreover, the current-voltage characteristics were improved with the fabrication of differently sized patterns over the LED surface using the proposed nanosphere lithography method.

Measurement of Vapor Pressure of Molten ZnCl 2 and FeCl 2 by the Transpiration Method

Chloride-based fluxes such as NaCl-KCl are used in the refining of Al melt. The vapor pressure of the chloride is one of the fundamental pieces of information required for such processes, and is generally high at elevated temperatures. In order to measure the vapor pressure for chlorides, the apparatus for the transpiration method was assembled in the present study. The vapor pressure of ZnCl2 and FeCl2, which is related with the process of aluminum refining and the recovery of useful elements from iron and steel industry by-products, was also measured. In the measurement of vapor pressure by the transpiration method, the powder of ZnCl2 or FeCl2 in a alumina boat was loaded in the uniform zone of the furnace with a stream of Ar. The weight loss of ZnCl2 and FeCl2 after holding was measured by changing the flow rate of Ar gas (10 sccm -230 sccm), and the partial pressures of ZnCl2 and FeCl2 were calculated. The partial pressures within a certain range were found to be independent of the flow rate of Ar at different temperatures. The vapor pressures were measured in the temperature range of 758-901K for ZnCl2 and 963-983K for FeCl2. The measured results agreed well with those in the literature.