Jae Hwan Pee

Hydrothermal synthesis of rod-like copper oxide crystals

A hyrdrothermal reaction process has been developed to prepare rod-like crystals of copper oxide using copper nitrate trihydrate as a function of synthesis temperature, stirring speed and solution pH value. The properties of the fabricated crystals were studied using scanning electron microscopy, X-ray diffraction and particle size analysis. The morphology of the synthesized CuO was dependent on both the pH value of the solution and the morphology of the seed materials. Synthesized particles have regular morphologies and a uniform size distribution.

Properties of AlN Powder Synthesized by Self-Propagating High Temperature Synthesis Process

The synthesis of AlN via self-propagating high-temperature synthesis (SHS) was attempted, using various ratio of Al powder mixed with AlN powder as diluents. Al and AlN powder mixtures with various weight ratios were ignited a nitrogen atmosphere with various amounts of carbon as additives. High crystalline AlN by SHS were successfully synthesized. The microstructure development during the reaction and the influence of these additives were determined by SEM and XRD analysis. A mechanism for the formation of high purity AlN with a very low content of residual oxygen (<0.8wt %) was proposed.

Strength Improvement of Porous Ceramics by the Surface Infiltration of Strengthening Materials

One method of improving the strength of porous ceramics is to add Al2O3 in ceramic raw materials and then they must be sintered at high temperature for densification. However, the non-plastic property of Al2O3 leads to a decline in plasticity of clay body, thus severely interfering with forming process. Besides porous ceramics have fine cracks and pores distributed within porous ceramics, these fine cracks on the surface also result in weakening of the sintered body. In this study, we attempted to improve the strength while compensating for the weakness of porous ceramics by the surface infiltration of strengthening materials after the first sintering step at 900°C. MgCl2, Al (NO3)3, and KNO3 were used as surface strengthening materials. The effect of Infiltration factors, solution concentrations (1-3 moles) and infiltration time (1-5 seconds) on the mechanical properties of the sintered body are investigated. When the strength was assessed after the second sintering step at 1250°C, an 30% increase in strength was obtained with Al (NO3)3 infiltrated in a 3 mole solution for 5 seconds. An analysis of the crystal phase of the strengthened body revealed a higher intensity of the mullite phase arising from the infusion of surface strengthening materials. Consequently, the mullite phase led to a higher strength value.

Effect of MgO and NH4OH on Formation of 5Mg(OH)2·MgSO4·3H2O Whiskers

The Magnesium hydroxide sulfate hydrate whiskers (5Mg(OH)2.MgSO4.3H2O, abbreviated 513MHSH) have attracted much attention due to its practical applications as filler and reinforcement. However, it is difficult to produce high quality MHSH because plate-like Mg(OH)2 impurities were formed in high concentrations of OH- and interaction between Mg2+ and OH-. In this work, to reduce formation Mg(OH)2, molar ratio ofSuperscript text magnesium oxide (MgO) and magnesium sulfate (MgSO4.7H2O) were controlled. As a result, when low concentration of SO42-, MHSH whiskers co-existed with hexagonal plate Mg(OH)2. The molar ratio of MgSO4.7H2O/MgO was high, uniform MHSH whiskers were formed without Mg(OH)2. In addition, appropriate amount of NH4OH has affected formation of high quality MHSH. Their morphologies and structures were determined by powder X-ray diffraction (XRD) scanning electron microscopy (SEM) and thermo-gravimetric analyzer (TGA)

Effects of Doping Elements on Residual Oxygen/Nitrogen Contents in Red Pigment of Tantalum Nitrides (Ta3N5)

Tantalum nitride (Ta3N5) is a nontoxic red pigment that is being developed as a substitute for Cd-related pigments Ta3N5 is produced by the nitridation and heat treatment of amorphous Tantalum precursors. Doping elements were added in the precursor manufacturing stage to improve the red color tone of tantalum nitride. Grain growth was observed in nitrides that formed second phases, such growth led to an increase in the average grain size comprared to undoped nitrides, and the colors declined as the oxygen content increased. Nitrides that did not form second phases in response to doping elements remained a single-phase Ta3N5 and exhibited an excellent red color with a high nitrogen content. We determined that a change in the oxygen/nitrogen contents affected the color manifestation, which depended on the amount by which doping was increased.

Effect of Silica Coating on the Thermal Stability of Tantalum Based Nitrides Pigments

The colored TaON and Ta3N5 were synthesized by ammonolysis of amorphous Ta(OH)5 in the temperature range of 800-1000°C. Ta(OH)5 were prepared by titration process from TaCl5. TaON were synthesized at 900°C for 2 hr, and Ta3N5 were synthesized at 1000°C for 5 hr. In general, Ta3N5 powders were easily oxidized at 500°C with changing colority from red to white. To minimize the oxidation problem of Ta3N5 powders, we proposed silica coated system using TEOS and APTES as a surfactant. Tonality of silica coated Ta3N5 has very little changing which means silica coating actually minimizes the oxidation of the Ta3N5 particles.

Microstructures and Dielectric Constants of Ba0.05SrxCa0.95-xTiO3 (x=0, 0.225, 0.475, 0.725 and 0.95) Synthesized by the Solution Combustion Technique

Barium strontium calcium titanate is a dielectric material exploited in fabrication of electronic devices such as capacitors, signal filters and satellite components. Dielectric properties can be enhanced through compositional and microstructural control. This study, therefore, aimed at synthesizing barium strontium calcium titanate (Ba0.05SrxCa0.95-xTiO3, where x = 0, 0.225, 0.475, 0.725 and 0.95) powders by a solution combustion technique. The powders were pressed, sintered at 1450°C and tested for their properties. Experimental results revealed that strontium content did not significantly influence chemical composition, particle sizes and density. All powders exhibited a single phase corresponding to Ba0.05SrxCa0.95-xTiO3 with fine particles with the average size smaller than 0.4 micrometer. All sintered samples had density higher than 95% of theoretical density. On the contrary, the results indicated that strontium content affected grain size, grain morphology and dielectric constant of the sintered samples. The highest dielectric constant of 531 (at 1 MHz) was achieved in the Ba0.05Sr0.225Ca0.725TiO3. Dielectric constant was discussed with respect to microstructure.

Effect of Frit Content on the Metal Marking and Scratching Resistance of Celadon Glaze

Metal marking and scratching are related to surface roughness, hardness and toughness of glaze. To improve the glaze properties, frits were added in the traditional celadon glaze and reducing firing atmosphere was controlled. Surface roughness of celadon glaze was decreased with increasing the reducing agent content (LPG flowing rate) and increasing the frits content. Though hardness of glaze without frit was increased with increasing the reducing agent content, hardness of glaze with frit was not changed extremely with increasing the reducing agent content. To evaluate the metal marking and scratching resistance, wear resistance test was used with stainless steel ball. After wear test, many metal marks and wide scratching trace were observed in the traditional celadon glaze. However, a few metal mark and scratch were observed in the celadon glaze with frit. The friction coefficient of glaze in the wear test was strongly depended on the frit contents.

Evaluation of Electrostatic Charge of Inorganic Pigments Coated by Silanes for Laser Beam Printer

LBP has six steps, which are charging, exposure, development, transfer, fusing, cleaning and discharge, for the printing product. In these steps, we focused on the development step for the source of submicron inorganic pigments. The toner is a negatively charged combination of inorganic particles. A control blade holds the toner at a micro size distance from the drum. The inorganic pigment then moves from the control blade to the more positively charged latent image on the drum. The image in static electricity needs to be developed – made visible. The developer introduces small particles of toner onto the static-charged surface of the drum. In this study inorganic pigments were coated by functional silanes to provide particles with the electrostatic charge. The coating thickness and kinds of silanes were changed to evaluate the electrostatic charge of coated pigments. After coating process, the electrostatic charge of inorganic pigments coated with silanes were evaluated by the Faraday’s device. We will introduce the effect of 3-(N-phenylamino) propyltrimethoxy silane and phenyl tris (methylethylketoxime) silane coating on the electrostatic charge of inorganic pigments.

Effect of Flux Materials on the Melting Characteristics of Ash Glaze

Our objective was to make transparent glaze using oak ash which has a high content of CaO and contains P2O5. However, the melting point of oak ash is above 1340°C, making it difficult to make transparent glaze using only oak ash. In order to lower the melting point of oak ash, flux materials were mixed to oak ash and melting characterization was carried out. Feldspar, which is largely composed of Na2O and K2O andlimestone largely composed of CaO were used as the flux materials. The materials were added 10, 20 and 30wt%, respectively, in order to produce the glaze. Depending on the type and amount of flux materials added, the samples were fired at temperatures between 1280~1320°C in the reducing atmosphere. As the amount of feldspar added was increased, the melting point decreased substantially. Samples with limestone added did not melt completely even at temperatures 1300°C and wollastonite phase was observed in the glaze layer.