Papers by Author: Sang Woo Kim

Abstract: Eco-debinding process using supercritical extraction of low molecular organic binders in nano-porous ceramic bodies was examined. The debinding properties related to structural changes during supercritical extraction and conventional solvent extraction were also compared. The debinding rate of supercritical extraction was significantly enhanced compared to the debinding rate of solvent extraction because of the high diffusivity of the supercritical carbon dioxide for the low molecular weight wax binder in the molded bodies with nano-sized pore structure, although both debinding rates showed same a square root of time dependence. The extraction rates with morphological changes varied depending on the degree of saturation at the end of debinding stages. Both the debinding methods experienced morphological changes with a debinding front separating the pendular state region from the undebinded region with fluid state in low molecular paraffin wax based powder compacts during extraction. The capillary structural changes in the green bodies caused severe defects during extraction and degraded the physical properties. In spite of the abrupt changes of the capillary structure, the debinding defect was significantly alleviated for the supercritical debinded ceramic bodies, compared to the solvent extracted bodies.

Abstract: Effect of magnesium addition on rapid transformation of α-alumina prepared from route of ammonium aluminum carbonate hydroxide during thermal heating and microwave radiation heating was investigated. The phase transformation and the final particle size of the transient alumina composite powder were significantly affected by amount of magnesium added in the aluminum precursor during the microwave radiation heating in various environmental atmospheres. Rapid transformation from γ- to α-phase was found in the magnesium added transient alumina by microwave-assisted transformation and nano-sized α-alumina was obtained. When the 3 wt% magnesium added ammonium aluminum carbonate hydroxide was heated by microwave radiation under nitrogen atmosphere, the transformation temperature from γ- to α-alumina was considerably lowered to 1000°C and the average particle size of 27.6 nm was attained for the α-alumina-spinel composite powder.

Abstract: Nickel coated gadolinium doped ceria (GDC) powder was synthesized by microwave radiation and combustion. For the synthesis, the precipitates of gadolinium cerium oxycarbonate hydrate (GdxCe2-xO(CO3)2·H2O) were formed by a microwave radiated reaction between cerium nitrate (Ce(NO3)3.6H2O) and gadolinium nitrate (Gd(NO3)3.6H2O) and urea (CO(NH2)2), then nickel coatings on the gadolinium cerium oxycarbonate hydrate were performed by further microwave reaction between nickel chloride and urea. The shape and size of the gadolinium cerium oxycarbonate hydrate particles were critically dependent on aging time during microwave radiation. The irregular particles were transformed to rod shape particles with well-crystallized with increasing aging time to 40 min at 70 - 80°C because of the gradual decomposition of urea during microwave radiation. Small nickel precursor particles were homogeneously coated on the gadolinium cerium oxycarbonate hydrate particles with rod shape with aid of microwave radiation at 80 °C for 40 min. As a result, the nickel coated GDC nanopowders were sucessfully produced by the microwave radiation synthesis and further microwave combusted at 450°C for 20 min.

Abstract: The effect of phase transformation and fine particle dispersion on densification behavior of high purity nanocrystalline alumina was investigated. The γ/α phase ratio of the mixture compacts were affected detrimentally by sinterability and phase transformation of α-phase alumina ceramics. The densification was significantly deteriorated with increasing γ-phase owing to faceted pores which are caused by the formation of a colony. The faceted pores were not existed in the α- Al2O3 ceramics with 10% γ-phase. Densification of the 10% γ-phase dispersed α-Al2O3 ceramics was significantly enhanced by rapid transformation of α-alumina. The grain growth was effectively inhibited by a small amount of nanoparticle dispersion for the alumina ceramics. As a result, dense nanostructured alumina can be achieved in the 10% γ-phase dispersed α-Al2O3 ceramics, even sintering at 1400oC for 3 hours.

Abstract: The microwave-assisted combustion synthesis as a route to obtain ultrafine α-alumina and magnesium aluminate composite powders starting from the ammonium aluminum carbonate hydroxide precursor was investigated. The synthetic temperature and the crystallite size of the α- alumina nanocomposite powder were significantly affected by the environmental atmosphere in the microwave assisted combustion. The α-alumina and spinel composite powder was obtained by the microwave-assisted combustion at temperature of 1000°C under H2/N2, but at 1150°C by the normal heat process under air. The rapid transformation from γ- to α-phase was achieved by microwave assisted nucleation at low temperature of 900°C under H2/N2 atmosphere. The least crystallite size of 26.2 nm was obtained under H2/N2 atmosphere at 1000°C for 10 min.

Abstract: This paper describes examines the non-linearity of multilayer ceramic capacitors (MLCC) with new equivalent circuits. The electric charge quantity (Q) was measured as a function of applied voltage (V) under 16 V ac-field to validate the ferroelectricity of MLCC with capacitance of 1 μF. The Q-V curves predicted under ac-field using B2 spice software showed the non-linearity of ferroelectricity in the high ac-field. The Q-V curve predictions were in agreement with the experimental data for the MLCC.

Abstract: In order to achieve effectively high electromagnetic absorption in the radio frequency for the application of the electromagnetic absorber, we investigated annealing effect of microforged powders on electromagnetic absorption behaviors of ferromagnetic Fe-Cr alloy metal powder-polymer composites. The power loss in the far field regime was improved with increasing the microforging time in the frequency range from 50 MHz to near 1 GHz, because the magnetic permeability was slightly increased in the frequency range due to the high aspect ratio of the microforged metal powders. At higher frequency more than ~1 GHz, there was no further increased in power loss because of development of internal strain during microforging process. The magnetic permeability was significantly increased with annealing temperature due to the reduction of the lattice strain of the microforged powders. As a result, the electromagnetic absorption of ferromagnetic Fe-Cr alloy metal powder-polymer composites was highly improved.

Abstract: Microwave-assisted combustion synthesis of α-alumina-magnesium spinel nanocomposite powders from a route using ammonium aluminum carbonate hydroxide (AACH) as a precursor was studied. γ- to α-phase transformation temperature of microwave-assisted alumina composite powders is lowered by 100°C compared to that of normal heat-treated alumina composite powders. The crystallite size of the microwave-assisted composite powders was considerably decreased from 42 nm to 29 nm compared to that of the normal heat treated composite powders. Nanosized α- alumina-magnesium aluminate composite powders were successfully synthesized by microwave-assisted combustion under nitrogen atmosphere at 1050°C.

Abstract: Radio frequency interference characteristics of plate-like Fe-Si-Al alloy powders-polymer composites were investigated for effective noise suppression. Broadband reflection loss characteristics were exhibited in the composites below a critical compaction pressure of 1.56 x 108 N/m2, while narrowband characteristics were shown in the composites above the critical pressure. Although the composites at 2.34 x 108 N/m2 had the highest complex permeability and complex permittivity, the loss properties did not show the broadband characteristics. The peculiar broadband characteristics were obtained in the composites with wideband dependency of the imaginary part of permittivity and permeability. These findings led us to conclude that the broadband electromagnetic interference characteristics of the composites are caused by the coupling effects of magnetic and dielectric resonance in the radio frequency range.

Abstract: Density, purity, grain, and grain size distribution are the key factors that influence the transparency of an optical ceramics. To achieve high transmittance, efforts should be made to eliminate or minimize scattering or absorption of light. The aim of this paper is to verify the effect of two-step sintering technique for translucent alumina. The density and transmittance of the samples were enhanced by pre-heat treating at 800°C for 50 hours before the high temperature sintering. Abnormal grain growth was successfully suppressed by a two-step sintering technique and a small amount of MgO as additives. It is a very effective technique for designing homogeneous microstructure and high transparency in high purity alumina.