Hwan Kim

Preparation of nanotube-shaped TiO2 powder

Abstract Nanotube-shaped TiO2 powder was synthesized by a digestion of the crystalline powder obtained from the heat treatment of the titanium hydroxide precipitates. The influence of the crystalline phase and the digestion temperature on the formation of the TiO2 nanotube was investigated. The powder digested at 150°C for 12xa0h showed the highest specific surface area of 270xa0m2/g.

Synthesis and crystallization of anisotropic shaped ZrO2 nanocrystalline powders by hydrothermal process

The nanocrystalline ZrO2 powders with anisotropic shapes were synthesized from the amorphous zirconium hydroxide powder (P1) and the tetragonal zirconia powder (P2) by hydrothermal process with the reaction temperature range 150–250 °C and additive concentration 1, 5 M NaOH solutions. With increasing the reaction temperature, the particle shape was altered from spherical to spindle- or rod-like and the fraction of monoclinic phase increased. In 5 M NaOH solution, the particle shape was actively changed and the transformation to the monoclinic was faster than in 1 M NaOH solution.

Synthesis of TiO2 nanocrystalline powder by aging at low temperature

Abstract Titanium hydroxide obtained from the reaction between TiOCl 2 and NH 4 OH solutions was aged in boiling water or HCl solution at 60°C for 24xa0h, then anatase or rutile-typed TiO 2 nanocrystalline powder with a large specific surface area was synthesized. The influence of the aging conditions on the crystallization, microstructure and specific surface area of the prepared TiO 2 powders was investigated. It was found that the powder aged in boiling water showed the anatase phase with spherical shape and the anatase or rutile powder with different particle shape was obtained by varying molar concentration of HCl. The powder aged in HCl solution showed the largest specific surface area of 250xa0m 2 /g.

Effect of aging agents on the formation of TiO2 nanocrystalline powder

Abstract An anatase or a mixture of anatase and rutile TiO 2 nanocrystalline powder with a large specific surface area was prepared by aging the NaOH-treated powder in boiling water or HCl solution. The effect of the aging conditions on the crystallization, microstructure and specific surface area of the obtained crystalline TiO 2 powders was investigated. The powder, through aging with the NaOH in boiling water, showed an anatase structure with spherical shape. TiO 2 nanocrystalline powder with a mixture of the anatase and rutile phase was obtained, resulting from the exudation of Na in the Na 2 TiO 3 by treating with HCl, and the powder showed a large specific surface area of 240 m 2 /g.

Formation of solid solutions between SiC and AlN during liquid-phase sintering

A 2H-solid solution between SiC and AlN, along with a dispersion of SiC, was prepared by liquid-phase sintering, and the mechanism of solid solution formation between SiC and AlN was investigated from studies of the microstructures of the sintered body. The SiC-AlN composite microstucture was composed of isolated SiC particles, the 2H-solid solution of spherical shape, and an irregular shaped phase which was liquid at high temperature. The grain morphology of the 2H-solid solution phase was almost the same with equiaxed grains of 2 to 3 μm in diameter. The AlN content in the 2H-solid solution was higher than that of SiC. A few 2H-grains showed a core-rim structure and a difference in Si content between core and rim. The Si content in the rim was higher than that in the core. Al2O3 grains surrounded by YAG were found to be embedded in the 2H grains. The 2H-solid solution formed during liquid-phase sintering occurs via a nucleation and growth process.

Microstructural changes in liquid-phase sintered α-silicon carbide

Microstructural changes in liquid-phase sintered α-silicon carbide due to the formation of a liquid reaction-layer and segregation of components of the liquid-phase near the surface were investigated. The outside surface layer was created by reaction of the packing powder and an eutectic oxide liquid-phase, and the liquid-phase components were segregated in the surface region of the specimen during sintering. The microstructural changes from the surface to the interior were a result of the changes in volume of the liquid-phase available for sintering as a function of depth from the specimen surface.

Surface crack initiation in 2Y-TZP ceramics by low temperature aging

Abstract Surface crack initiation in 2Y-TZP specimens during aging at 250°C was investigated. Surface upheaval due to the isothermal phase transformation appeared on aging, in order to reduce the change of strain free energy for the transformation. The nucleation of a microcrack was induced by surface upheaval at the beginning of aging, and the microcrack was grown to a macrocrack with aging. The crack surface created by isothermal phase transformation shows a completely intergranular fracture mode. The propagation of phase transformation from surface to interior during aging is continous and uniform and the transformed region shows a porous microstructure.

Formation of irregular grain shapes by ceria doping in 3 mol% yttria-stabilized zirconia ceramics

Three mol% yttria-stabilized zirconia (denoted by 3Y-TZP) with irregular grain shapes and undulated grain boundaries was prepared by ceria doping. Irregular shaped grains and grain boundary undulation are only formed in 3Y-TZP specimens doped above 8 mol% ceria. However, the formation of irregular grain shapes in 3Y-TZP by ceria doping does not enhance its fracture toughness because of the high stability (low transformability) of the tetragonal phase and large grain size due to the presence of a large content of stabilizer.

Monoclinic-to-tetragonal transformation and crack healing by annealing in aged 2Y-TZP ceramics

In applications of yttria-stabilized zirconia (Y-TZP) ceramics as structural materials their thermal instability in the range 150-300 °C limits applications at high temperatures in spite of their superior mechanical properties (that is, high strength and fracture toughness, good wear resistance, etc.) [1-3]. From observations in [4-7], this thermal behaviour of Y-TZP degraded its mechanical properties and its so-called low-temperature degradation behaviours. Degradation behaviour during ageing is known to occur by an isothermal phase transformation from the tetragonal to the monoclinic phase which creates many cracks on the transformed surface. Also, this behaviour was found to be dependent on the grain size, the environment of water vapour, the ageing temperature and the content of the solute, etc. Several mechanisms of isothermal phase transformation in Y-TZP were proposed in [5-7], however these phenomena still have no clear explanation. In this present study, we investigated the reversed transformation (from the monoclinic to the tetragonal phase) by annealing at high temperatures and the related crack healing in aged 2Y-TZP specimens. The starting powder was TZ-2Y powder prepared by Tosoh Co. This powder contained a metastable tetragonal phase and a stabilizer of 2 mol % Y203. A cylindrical compact disk was prepared by cold isostatic pressing at a pressure of 200 MPa, and this was sintered at 1450 °C in air for a 20 h soaking time. The relative density of the sintered body was measured using Archimedes method, and the grain size was measured from scanning electron microscopy (SEM) micrographs of the polished surface. Ageing of polished specimens was performed at 250 °C in a humid atmosphere for 20 h, and then annealing was performed in the temperature range 500-1200°C for aged specimens in an electric furnace. Soaking on annealing was performed for 1 h at each temperature. The amount of transformed phase accompanying ageing and annealing was analysed by X-ray diffraction (XRD) of the surface, and the ratio of the tetragonal to the monoclinic phase was determined by using the GarvieNicholson formula [8]. The microstructures of the annealed specimens were observed by SEM. On the polished surface of specimens sintered for 20 h, the tetragonal-phase content was about 90% and the monoclinic phase content was about 10%,

Stabilization of the perovskite phase in Pb(Zn1/3, Nb2/3)O3 ceramics modified by Ba(Zn1/3, Nb2/3)O3 and BaTiO3

Stabilization of the perovskite phase in PZN-BT-BZN ceramics prepared by solid-state sintering was investigated. With the addition of BT and/or BZN, the amounts of perovskite phase in PZN ceramics increased, but the amount of pyrochlore phase decreased. Optimum calcination conditions for this system, to obtain PZN ceramics with the minimum amount of the pyrochlore phase, are a temperature range between 950 and 1000°C and a sintering time of 2–4 h. The perovskite phase of these systems could be 100% stabilized by the addition of more than 7 mol% BT, 8 and 10 mol% BTZN and BZN, respectively.