• Korean Journal of Materials Research
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• v.28 no.5
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• pp.261-267
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• 2018

Hydrothermal synthesis of highly crystalline $TiO_2$ nanorods is a well-developed technique and the nanorods have been widely used as the template for growth of various core-shell nanorod structures. Magneli/CdS core-shell nanorod structures are fabricated for the photoelectrochemical cell (PEC) electrode to achieve enhanced carrier transport along the metallic magneli phase nanorod template. However, the long and thin $TiO_2$ nanorods may form a high resistance path to the electrons transferred from the CdS layer. $TiO_2$ nanorods synthesized are reduced to magneli phases, $TixO_{2x-1}$, by heat treatment in a hydrogen environment. Two types of magneli phase nanorods of $Ti_4O_7$ and $Ti_3O_5$ are synthesized. Structural morphology and X-ray diffraction analyses are carried out. CdS nano-films are deposited on the magneli nanorods for the main light absorption layer to form a photoanode, and the PEC performance is measured under simulated sunlight irradiation and compared with the conventional $TiO_2/CdS$ core-shell nanorod electrode. A higher photocurrent is observed from the stand-alone $Ti_3O_5/CdS$ core-shell nanorod structure in which the nanorods are grown on both sides of the seed layer.

• Macromolecular Research
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• v.17 no.9
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• pp.688-696
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• 2009

In this study, a new class of polyurethane (PU) nanofibers containing silver (Ag) nanoparticles (NPs) was synthesized by electrospinning. A simple method that did not depending on additional foreign chemicals was used to self synthesize the silver NPs in/on PU nanofibers. The synthesis of silver NPs was carried out by exploiting the reduction ability of N,N-dimethylformamide (DMF), which is used mainly to decompose silver nitrate to silver NPs. Typically, a sol-gel consisting of $AgNO_3$/PU was electrospun and aged for one week. Silver NPs were created in/on PU nanofibers. SEM confirmed the well oriented nanofibers and good dispersion of pure silver NPs. TEM indicated that the Ag NPs were 5 to 20 nm in diameter. XRD demonstrated the good crystalline features of silver metal. The mechanical properties of the nanofiber mats showed improvement with increasing silver NPs content. The fixedness of the silver NPs obtained on PU nanofibers was examined by harsh successive washing of the as-prepared mats using a large amount of water. The results confirmed the good stability of the synthesized nanofiber mats. Two model organisms, E. coli and S. typhimurium, were used to check the antimicrobial influence of these nanofiber mats. Subsequently, antimicrobial tests indicated that the prepared nanofibers have a high bactericidal effect. Accordingly, these results highlight the potential use of these nanofiber mats as antimicrobial agents.

• Korean Journal of Materials Research
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• v.24 no.8
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• pp.434-442
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• 2014

Serious environmental problems have been caused by the greenhouse effect due to carbon dioxide($CO_2$) or nitrogen oxides($NO_x$) generated by the use of fossil fuels, including oil and liquefied natural gas. Many countries, including our own, the United States, those of the European Union and other developed countries around the world; have shown growing interest in clean energy, and have been concentrating on the development of new energy-saving materials and devices. Typical non-fossil-fuel sources include solar cells, wind power, tidal power, nuclear power, and fuel cells. In particular, organic solar cells(OSCs) have relatively low power-conversion efficiency(PCE) in comparison with inorganic(silicon) based solar cells, compound semiconductor solar cells and the CIGS [$Cu(In_{1-x}Ga_x)Se_2$] thin film solar cells. Recently, organic cell efficiencies greater than 10 % have been obtained by means of the development of new organic semiconducting materials, which feature improvements in crystalline properties, as well as in the quantum-dot nano-structure of the active layers. In this paper, a brief overview of solar cells in general is presented. In particular, the current development status of the next-generation OSCs including their operation principle, device-manufacturing processes, and improvements in the PCE are described.

• Journal of the Korean Magnetics Society
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• v.16 no.3
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• pp.186-191
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• 2006

Ferromagnetic amorphous $Ni_{16}Fe_{62}Si_8B_{14}$ and $Co_{70.5}Fe_{4.5}Si_{15}B_{10}$ layers have been devised and incorporated as free layers of magnetic tunnel junctions (MTJs) to improve MRAM reading and writing performance. The NiFeSiB and CoFeSiB single-layer film exhibited a lower saturation magnetization ($Ms=800emu/cm^3,\;and\;560emu/cm^3$, respectively) compared to that of a $Co_{90}Fe_{10}(Ms=1400emu/cm^3)$. Because amorphous ferromagnetic materials have lower Ms than crystalline ones, the MTJs incorporating amorphous ferromagnetic materials offer lower switching field ($H_{sw}$) values than that of the traditional CoFe-based MTJ. The double-barrier MTJ with an amorphous NiFeSiB free layer offered smooth surface resulting in low bias voltage dependence, and high $V_h\;and\;V_{bd}$ compared with the values of the traditional CoFe-based MTJ.

• Environmental Engineering Research
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• v.19 no.3
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• pp.255-259
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• 2014

In this research, we compared the COD removal efficiencies of titanium dioxide ($TiO_2$) thin films coated on the surfaces of borosilicate glass that prepared by three different numbers of coating layer; i) 3 layers ii) 4 layers and iii) 5 layers by sol-gel method. All of the prepared $TiO_2$ thin films consisted of pure anatase crystalline structure with grain sizes in the range 20-250 nm. The calculated optical band gaps of the $TiO_2$ thin films were 3.24. The total apparent surface area per total weight of $TiO_2$ thin films were 4.74, 3.86 and $2.79m^2g^{-1}$ for 3, 4 and 5 layers coating, respectively. The kinetics of the photodegradation reactions of COD under UVA light source were described by the Langmuir-Hinshelwood (L-H) kinetic model. The specific rates of the photodegradation of $TiO_2$ thin films at 3 layers coating was $1.40{\times}10^{-4}min^{-1}mW^{-1}$, while for the 4 layers coating and the 5 layers coating were $1.50{\times}10^{-4}$ and $4.60{\times}10^{-4}min^{-1}mW^{-1}$, respectively. The photocatalytic performance of COD degradation was higher with smaller grain size, higher surface area and narrow optical band gaps. Moreover, the numbers of coating layer on substrate also have great influence for kinetic of COD removal.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.16 no.5
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• pp.3627-3632
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• 2015

We investigated mechanical, rheological and electrical properties of graphite/PP/LCP composites for a bipolar plate of the polymer electrolyte membrane fuel cell. The composites containing very low molecular weighted PP showed much higher electrical conductivity compared with other thermoplastics. This was attributed to the enhanced dispersion of graphite particles due to the low viscosity of the PP. The conductivity of the composites was increased in a great extent by incorporation of small amount of carbon nano tube (CNT). However, the acid treated CNT which contains oxygen atoms did not increase the conductivity of the composite. From this result, it is concluded that the CNT has higher affinity with non polar polymer. The composite with low molecular weighted PP provided good processability so that the composites can be processed by an injection molding while the mechanical strength is deficient compared to other polymers. In order to reinforce the low mechanical property, LCP/PP was used as a binder and the graphite/PP/LCP composite showed the higher conductivity and moderate mechanical strength maintaining suitable processability.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2007.11a
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• pp.147-148
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• 2007

Dye-sensitized solar cell using conversion of solar energy to electrical energy appeared that which solves a environmental matter. The dye-sensitized solar cell uses nano-crystalline oxide semiconductor for absorbing dye. The $TiO_2$ is used most plentifully. The efficiency of the dye-sensitized solar cell changes consequently in the particle size, morphology, crystallization and surface state of the $TiO_2$. In this paper, we report The effect of titania$(TiO_2)$ thickness on the performance of dye-sensitized solar cells. Using doctor blade method, It produced the thickness of the $TiO_2$ with $7\;{\mu}m,\;10\;{\mu}m,\;13\;{\mu}m$. The efficiency was the best from $10{\mu}m$. It had relatively low efficiency on the thickness from $7\;{\mu}m\;to\;13\;{\mu}m$. The reason why it presents low efficiency on $7\;{\mu}m$ thickness is that excited electrons can not be delivered enough due to thin thickness of $7\;{\mu}m\;TiO_2$. And The reason why it presents low efficiency on $13\;{\mu}m$ thickness is that thick $13\;{\mu}m\;TiO_2$ can not penetrate the sunlight enough.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.02a
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• pp.141-141
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• 2010

The effect of thermal anneal on the characteristics of structural properties and the enhancement of luminescence and photovoltaic (PV) characteristics of silicon-rich silicon-nitride films were investigated. By using an ultra high vacuum ion beam sputtering deposition, B-doped silicon-rich silicon-nitride (SRSN) thin films, with excess silicon content of 15 at. %, on P-doped (n-type) Si substrate was fabricated, sputtering a highly B doped Si wafer with a BN chip by N plasma. In order to examine the influence of thermal anneal, films were then annealed at different temperature up to $1100^{\circ}C$ under $N_2$ environment. Raman, X-ray diffraction, and X-ray photoemission spectroscopy did not show any reliable evidence of amorphous or crystalline Si clusters allowing us concluding that nearly no Si nano-cluster could be formed through the precipitation of excess Si from SRSN matrix during thermal anneal. Instead, results of Fourier transform infrared and X-ray photoemission spectroscopy clearly indicated that defective, amorphous Si-N matrix of films was changed to be well-ordered thanks to high temperature anneal. The measurement of spectral ellipsometry in UV-visible range was carried out and we found that the optical absorption edge of film was shifted to higher energy as the anneal temperature increased as the results of thermal anneal induced formation of $Si_3N_4$-like matrix. These are consistent with the observation that higher visible photoluminescence, which is likely due to the presence of Si-N bonds, from anneals at higher temperature. Based on these films, PV cells were fabricated by the formation of front/back metal electrodes. For all cells, typical I-V characteristic of p-n diode junction was observed. We also tried to measure PV properties using a solar-simulator and confirmed successful operation of PV devices. Carrier transport mechanism depending on anneal temperature and the implication of PV cells based on SRSN films were also discussed.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.02a
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• pp.98-98
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• 2010

Ion beam irradiation has been extensively used for surface modification of polymers, glassy metals and amorphous and crystalline materials at micron and submicron scales. The surface structures created by exposure to an ion beam range from dots, steps and one-dimensional straight wrinkles to highly complex hierarchical undulations and ripples. In general, the morphology of these nanoscale features can be selected by controlling the ion beam parameters (e.g. fluence and acceleration voltage), making ion beam irradiation a promising method for the surface engineering of materials. In the work, we presented that ion beam irradiation results in creation of a peculiar nanoscale dimple-like structure on the surface of polyimide - a common polymer in electronics, large scale structures, automobile industry, and biomedical applications. The role of broad Ar ion beam on the morphology of the structural features was investigated and insights into the mechanisms of formation of these nanoscale features were provided. Moreover, a systematic experimental study was performed to quantify the role of ion beam treatment time, and thus the morphology, on the coefficient of friction of polyimide surfaces covered by nanostructure using a tribo-experiment. Nano-indentation experiment were performed on the ion beam treated surfaces which shows that the hardness as well as the elastic modulus of the polyimide surface increased with increase of Ar ion beam treatment time. The increased of hardness of polyimide have been explained in terms of surface structure as well as morphology changes induced by Ar ion beam treatment.

• Journal of the Korean Society for Heat Treatment
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• v.34 no.5
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• pp.239-244
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• 2021

Following Silicon Carbide, single crystal diamond continues to attract attention as a next-generation semiconductor substrate material. In addition to excellent physical properties, large area and productivity are very important for semiconductor substrate materials. Research on the increase in area and productivity of single crystal diamonds has been carried out using various devices such as HPHT (High Pressure High Temperature) and MPECVD (Microwave Plasma Enhanced Chemical Vapor Deposition). We hit the limits of growth rate and internal defects. However, HFCVD (Hot Filament Chemical Vapor Deposition) can be replaced due to the previous problem. In this study, HFCVD confirmed the distance between the substrate and the filament, the accompanying growth rate, the surface shape, and the Raman shift of the substrate after vapor deposition according to the vapor deposition temperature change. As a result, it was confirmed that the difference in the growth rate of the single crystal substrate due to the change in the vapor deposition temperature was gained up to 5 times, and that as the vapor deposition temperature increased, a large amount of polycrystalline diamond tended to be generated on the surface.