Papers by Author: Woo Seok Cho

Abstract: 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.

Abstract: Alumina powder is used to improve the heat conduction and physical properties of heat radiation sheets, these properties can be improved by increasing the filling rate of alumina. Spherical alumina powder is being actively developed to improve the filling rate of alumina in sheets. In this study, a high-temperature flame was created by the flame-fusion method using an oxygen burner to produce spherical alumina. We investigated how the chemical composition and particle size of starting materials affect their fusion behavior under flaming conditions during spherical alumina production using the flame-fusion method. Boehmite and aluminum hydroxide, which accompany the endothermic reaction due to the dehydration of the starting materials, showed lower spheroidization ratios than sintered alumina. The spheroidization ratio improved as the particle size of the starting materials decreased.

Abstract: 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.

Abstract: AlN powders by the chemical vapor synthesis (CVS) process in the AlCl3-NH3-N2-H2 system were successfully synthesized. Gasified AlCl3 as a starting material was generated by pre-heating system and transported to the tube furnace in NH3-N2-H2 atmosphere. High crystalline AlN was synthesized at over 900°C. The average particle size of spherical AlN powders decreased from 250 to 40nm with increasing the reaction temperature of the tube furnace. Porous nano-size particles synthesized at high reaction temperature have low oxygen contents.

Abstract: 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.