Bong Geun Choi

Fabrication of graphene layers from multiwalled carbon nanotubes using high dc pulse

Graphene layers are fabricated from multiwalled carbon nanotubes (MWCNTs) with a high direct current pulse through a pulsed current sintering process. We confirm the transformation of the structure from MWCNTs to graphene layers. Graphene layers are analyzed by field emission scanning electron microscopy, high resolution transmission electron microscopy, high resolution Raman, and x-ray diffraction.

Fabrication of single-phase titanium carbide layers from MWCNTs using high DC pulse

Single-phase layered titanium carbide (TiC) was successfully synthesized by reacting carbon nanotubes (CNTs) and titanium dioxide (TiO2) under a high direct current (DC) pulse. Single-phase TiC layer fabrication is confirmed as the transformation of multi-layered graphene from MWCNTs. Therefore its thickness and width is almost identical to those of transformed graphene layers. This is the first report on the formation of single-phase layered nano-TiC. Scanning electron microscopy (SEM), x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HR-TEM) were used for the characteristic analysis of single-phase layered TiC structures.

Surface morphology variation during wet etching of GaN epilayer grown by HVPE

In this paper, we investigated characteristics of etching induced surface morphology variation by wet etching of GaN epilayer were grown on sapphire (0001) substrate by hydride vapor phase epitaxy (HVPE). As a results of scanning electron microscope (SEM) observation, three types of hexagonal etch pits (Edge, Screw, Mixed) were formed by the GaN epilayer thickness variations. A lot of etch pits, attributed to screw and mixed type TD, were observed at thinner epilayer, leading to high etch pit density. On the other hand, the thickness of GaN epilayer increased with the number of etch pits corresponding to edge and mixed dislocations, which are the majority of TDs are observed.

Microwave Characteristics of MgTiO3–CaTiO3 Dielectric Ceramics Fabricated Using Spark Plasma Sintering

Spark plasma sintering (SPS) was used in preparing dense 0.93MgTiO3–0.07CaTiO3 (MCT) ceramics, and their microstructure and microwave dielectric properties were investigated. Compared with conventional pressureless sintering, SPS was found to be effective in obtaining dense MCT ceramics with uniform microstructures at low temperatures, without any degradation of dielectric properties. The MCT specimen SPS-fabricated at 1150°C for 10 min exhibited excellent dielectric properties: a dielectric constant of 23 and a quality factor (Qf) of 70,000 GHz.

Investigation of defects and surface polarity in AlN and GaN using wet chemical etching technique

Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Korea*Unimo Photron Co., Ltd., Seoul 137-820, Korea(Received October 2, 2014)(Revised October 14, 2014)(Accepted October 17, 2014)Abstract We investigated defects and surface polarity in AlN and GaN by using wet chemical etching. Therefore, theeffectiveness and reliability of estimating the single crystals by defect selective etching in NaOH/KOH eutectic alloy havebeen successfully demonstrated. High-quality AlN and GaN single crystals were etched in molten NaOH/KOH eutecticalloy. The etching characteristics and surface morphologies were carried out by scanning electron microscope (SEM) andatomic force microscope (AFM). The etch rates of AlN and GaN surface were calculated by etching depth as a function ofetching time. As a result, two-types of etch pits with different sizes were revealed on AlN and GaN surface, respectively.Etching produced hexagonal pits on the metal-face (Al, Ga) (0001) plane, while hexagonal hillocks formed on the N-face.On etching rate calibration, it was found that N-face had approximately 109 and 15 times higher etch rate than the metal-face of AlN and GaN, respectively. The size of etch pits increased with an increase of the etching time and they tend tomerge together with a neighbouring etch pits. Also, the chemical mechanism of each etching process was discussed. It wasfound that hydroxide ion (OH

Effect of deposition pressure on the morphology of TiO 2 nanoparticles deposited on Al 2 O 3 powders by pulsed laser deposition

Titanium dioxides nanoparticles coated aluminum oxide powders were fabricated by pulsed laser deposition (PLD) with Nd : YAG laser at 266 nm. The Pulse laser energy is 100 mJ/pulse. During the irradiation of the focused laser on the target, Ar gas is supplied into the chamber. The gas pressure is varied in a range of to 100 Pa. Titanium dioxides nanoparticles deposited aluminum oxide powders were characterized by using energy dispersive X-ray spectroscopy (EDX), high resolution transmission electron microscopy (HR-TEM), in order to understand the effect of Ar background gas on surface morphology and properties of the powders. The coated nanoparticles had nanosized spherical shape and the crystallite sizes of 10~30 nm. The morphology of coated nanoparticles is not affected by gas pressure. However, the particle size and crystallinity slightly increased with the increase of gas pressure. According to this technique, the size and crystallinity of nanoparticles can be easily controlled by controlling pressure during the laser irradiation.

Different morphologies of three dimensional ZnO structures synthesized by thermal evaporation method without a catalyst

ZnO with different morphologies can be used various application depending on their shapes. Different morphologies of ZnO structures were synthesized by a catalysis-free thermal evaporation process. Their morphologies were dependent on the distance from the source to substrate on the same processing condition; in the result were products morphologies of the hollow, cage and star. Their shapes and crystalinity were evaluated by SEM and XRD, respectively. This work demonstrates what kind of growth factors would be involved in the final structure morphologies.

Optimization of sintering process of the far-infrared radiation ceramic

Far-infrared radiation ceramic is an attractive material that provides thermal therapy by permeating the infrared rays into the deep inside of the human skin. Therefore, it is currently used for thermal therapy devices, thermal mat, heating equipment and so on. This work aims to optimize the sintering process of the far-infrared radiation ceramic with the process parameters of temperature and time. A variety of characterization tools have been used to investigate the optimal sintering condition of far-infrared radiation. The phase of far-infrared radiation ceramic was characterized by using X-ray diffraction (XRD) and microstructure of fracture surface was studied by scanning electron microscopy (SEM). The FT-IR was also performed to measure the far-infrared emissivity.

Wet chemical etching of molten KOH/NaOH eutectic alloy to evaluate AlN single crystal

We investigated the optimal etching conditions and properties of the surface change due to molten KOH/NaOH chemical wet etching using an AlN wafer which has been put to practical use in the present study. Results were observed using a scanning electron microscope after 5 minutes etching at 350 o C, was found to have a surface form of the respective other Al-face, the N-face. In particular, etch-pit in the form of a hexagon, which is observed in the Al-face appeared, It was calculated at 2 × 10 6 /cm 2 ~10 10 /cm 2 dislocation density. In the case of N-face, lattice defects in the form of the hexagonal pyramids is formed. It was discovered that in order to observe the orientation of the wafer, which corresponds to the Caxis direction of the resulting hexagonal AlN which was analyzed using XRD (0002) and is a state of being oriented in the (0004) plane. The Radius of curvature of AlN wafer was 1.6~17 m measured by DC-XRD rocking curve position.

The molten KOH/NaOH wet chemical etching of HVPE-grown GaN

Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Korea*UNIMO Photron, Seoul 137-063 Korea(Received June 10, 2014)(Revised July 7, 2014)(Accepted July 18, 2014)Abstract The hydride vapor phase epitaxy (HVPE) grown GaN samples to precisely measure the surface characteristicswas applied to a molten KOH/NaOH wet chemical etching. The etching rate by molten KOH/NaOH wet chemical etchingmethod was slower than that by conventional etching methods, such as phosphoric and sulfuric acid etching, which may bedue to the formation of insoluble coating layer. Therefore, the molten KOH/NaOH wet chemical etching is a betterefficient method for the evaluation of etch pits density. The grown GaN single crystals were characterized by using X-raydiffraction (XRD) and X-ray rocking curve (XRC). The etching characteristics and surface morphologies were studied byscanning electron microscopy (SEM). From etching results, the optimum etching condition that the etch pits were wellindependently separated in space and clearly showed their shape, was 410