Authors: Sang Woo Kim, Jeong Sik Park
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.
129
Authors: Young Mi Jung, Sang Woo Kim
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.
139
Authors: Jong Min Lim, Sang Woo Kim
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.
77
Authors: Sang Woo Kim, Shin Young Kim
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.
831
Authors: Young Mi Jung, Sang Woo Kim
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.
823
Authors: Hyeon Pyo Jeon, Youn Kyu Choi, Sang Woo Kim, Duck Kyun Choi
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.
827
Authors: Sang Woo Kim, Sung Jae Lee
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.
299
Authors: Young Mi Jung, Sang Woo Kim
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.
191
Authors: Sang Woo Kim, Y.W. Yoon, G.Y. Kim, Kyung Sub Lee
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.
826
Authors: Rak Joo Sung, Dong Suk Kim, Su Ah Ji, Jeong Sik Park, Sang Woo Kim
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.
822