• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2006.09a
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• pp.644-645
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• 2006

TiC-and Ti(C,N)-based cermets are excellent in semi-and final finishing of work piece during cutting operations. Typical microstructure of the cermets is a core/rim structure. The undissolved Ti(C,N) cores contribute to their high hardness while the rim phases, (Ti,M1,M2)(C,N)-type solid solutions, play great roles in enhancing the toughness. In this paper, various ultrafine pre-mixed MeC-Ni powders were synthesized and the powders were sintered or hot pressed after mixing in order to control the size and volume fractions of core and rim phases in the system. This paper will present the factors determining the microstructure along with mechanical properties.

• Journal of the Korean Ceramic Society
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• v.35 no.1
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• pp.97-105
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• 1998

20mol% Gd-doped CeO2 ultrafine powders as a promising electrolyte for the low temperature solid ox-ide fuel cells were synthesized with particle sizes of 15-20 nm using glycine nitrate process(GNP) fol-lowed by sintering their pellets at 150$0^{\circ}C$ for various times in air and then the electrical properties of the sintered pellets were investigated. The sintering behaviors and electrical properties for the sintered 20 sintered mol% Gd-doped CeO2 pellets were analyzed using dilatometer and SEM and AC two-terminal impedance technique respectively. As the heating temperature increased the synthesized powder had the sintering behaviors to show the start of the significant shrink at temperature of about $700^{\circ}C$ and to show the end of the shrink at the temperature of about 147$0^{\circ}C$. When the pellets were sintered with the vaious times at 150$0^{\circ}C$ the temperatuer which the shrink had been already completed the grain sizes in the sintered 20 mol% Gd-doped GeO2 pellets increased with the increase of the sintering time but their electrical resis-tivities showed the minimum value at the sintering time of 10h. It is due that the pellet sintered for 10h had the minimum activation energy fior the electtrical conduction. Thus it is thought that the decrease of the activation energy with the increase of the sintering time to 10h is induced by the enhanced mi-crostructure like the decrease of pore amount and the grain growth and its increase with the sintering times more than 10h is induced by the increase of the amounts of the impurities such as Mg. Al and Si from the sintering atmosphere.

• Journal of Powder Materials
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• v.21 no.6
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• pp.447-453
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• 2014

Effect of oxygen content in the ultrafine tungsten powder fabricated by electrical explosion of wire method on the behvior of spark plasma sintering was investigated. The initial oxygen content of 6.5 wt% of as-fabricated tungsten powder was reduced to 2.3 and 0.7 wt% for the powders which were reduction-treated at $400^{\circ}C$ for 2 hour and at $500^{\circ}C$ for 1h in hydrogen atmosphere, respectively. The reduction-treated tungsten powders were spark-plasma sintered at $1200-1600^{\circ}C$ for 100-3600 sec. with applied pressure of 50 MPa under vacuum of 0.133 Pa. Maximun sindered density of 97% relative density was obtained under the condition of $1600^{\circ}C$ for 1h from the tungsten powder with 0.7 wt% oxygen. Sintering activation energy of $95.85kJ/mol^{-1}$ was obtained, which is remarkably smaller than the reported ones of $380{\sim}460kJ/mol^{-1}$ for pressureless sintering of micron-scale tungsten powders.

• Journal of the Korean Ceramic Society
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• v.25 no.6
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• pp.631-638
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• 1988

Effect of Ca/P mole ratio on the precipitates and powder properties of hydroxyapatite was investigated. Powder and precipitates of hydroxyapatite were synthesized by the reaction of Ca(NO3)2.4H2O and (NH4)2 HPO4 solutions at room temperature. The pH value and compositions (Ca/P mole ratio) in starting solutions were 11 and 1.64-1.79(or 1.85), respectively. Rodlike hydroxyapatite precipitates were agglomerated together. The average agglomerated particle size was ranged from 2-8${\mu}{\textrm}{m}$. Among compositions, the minimum agglomerated particle size was shown at the Ca/P mole ratio 1.75. CO2 was contained in hydroxyapatite powders and these ultrafine powders had poor crystallinity. The specific surface area and specific total pore volume of hydroxyapatite powders were 104-137$m^2$/g and 0.396-0.467cc/g, respectively. When the Ca/P mole ratio was 1.75, these values were the maximum. And water content increased with the Ca/P mole ratio(Ca/P mole ratio>1.67). In most cases, hydroxyapaite was stable to 130$0^{\circ}C$. However, in the case of Ca/P mole ratio 1.64, hydroxyapatite was changed to $\alpha$-whitlockite at 120$0^{\circ}C$.

• Journal of the Korean Chemical Society
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• v.32 no.3
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• pp.267-275
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• 1988

Fine powders of amorphous $Al_2O_3,\;SiO_2,\;Al_2O_3-SiO_2$ system were prepared by hydrolysis of solutions containing alkoxides, aluminium tri-isopropoxide and silicon tetra-ethoxide. High purity ultrafine ${\beta}-sialon$ powders were prepared by the carbothermal reduction-nitridation of amorphous $Al_2O_3-SiO_2$ powders mixed with carbon black as a reducing agent. In the hydrolysis step the effect of the factors such as pH, reaction temperature and amount of water on the conversion rate of alkoxides to oxides was investigated. In the carbothermal reduction-nitridation the reaction path was assumed by the analysis of intermediates formed in the process of ${\beta}-sialon$ synthesis and the reaction kinetics of ${\beta}-sialon$ formation was considered.

• Journal of Powder Materials
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• v.16 no.2
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• pp.98-103
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• 2009

The ultrafine titanium carbonitride particles ($TiC_{0.7}N_{0.3}$) below 100nm in mean size were successfully synthesized by Mg-thermal reduction process. The nanostructured sub-stoichiometric titanium carbide ($TiC_{0.7}$) particles were produced by the magnesium reduction at 1123K of gaseous $TiC_{l4}+xC_2Cl_4$ and the heat treatments in vacuum were performed for five hours to remove residual magnesium and magnesium chloride mixed with $TiC_{0.7}$. And final $TiC_{0.7}N_{0.3}$ phase was obtained by nitrification under normal $N_2$ gas at 1373K for 2 hrs. The purity of produced $TiC_{0.7}N_{0.3}$ particles was above 99.3% and the oxygen contents below 0.2 wt%. We investigated in particular the effects of the temperatures in vacuum treatment on the particle refinement of final product.

• Journal of Powder Materials
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• v.17 no.3
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• pp.190-196
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• 2010

In this paper, rapid solidified Mg-4.3Zn-0.7Y (at.%) alloy powders were prepared using an inert gas atomizer, followed by a severe plastic deformation technique of high pressure torsion (HPT) for consolidation of the powders. The gas atomized powders were almost spherical in shape, and grain size was as fine as less than $5\;{\mu}m$ due to rapid solidification. Plastic deformation responses during HPT were simulated using the finite element method, which shows in good agreement with the analytical solutions of a strain expression in torsion. Varying the HPT processing temperature from ambient to 473 K, the behavior of powder consolidation, matrix microstructural evolution and mechanical properties of the compacts was investigated. The gas atomized powders were deformed plastically as well as fully densified, resulting in effective grain size refinements and enhanced microhardness values.

• Journal of the Korean Magnetics Society
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• v.9 no.4
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• pp.177-183
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• 1999

Ultrafine $CoFe_{1.9}Bi_{0.1}O_4$ particles were fabricated by using a sol-gel method and their magnetic and structural properties were investigated with an x-ray diffractometer (XRD), a vibrating sample magnetometer (VSM), and a M$\"{o}$ssbauer spectrometer. The result of x-ray diffraction and M$\"{o}$ssbauer spectroscopy showed that the powders fired at and above 523 K had only cubic spinel structures. M$\"{o}$ssbauer spectra measurements showed that the powders annealed at 523,723 and 823 K possessed ferrimagnetic nature and paramagnetic nature due to superparamagnetism, simultaneously at room temperature and the powders annealed at and above 923 K behaved ferrimagnetically. In the case of the powder annealed at 923 K, the lattice constant was $a_0=8.398$\pm$0.005{\AA}$ and the hyperfine fields were $H_{hf}(A)=479kOe,\; H_{hf}(B)=502kOe$. The isomer shifts indicate that the iron ions are ferric at tetrahedral[A] and octahedral sites [B], respectively. The magnetization as a function of annealing temperature increased as increasing annealing temperature. The largest coercivity values were $H_C=1368\;Oe$ AT 923 K annealing temperature. In the case of the powder annealed at 1123 K, the magnetization value was $M_S=75\;emu/g$ and this value was similler to that of $CoFe_2O_4$.Fe_2O_4$.

• Journal of the Korean Ceramic Society
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• v.37 no.5
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• pp.473-478
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• 2000

The TiO2 powder with the values of the large specific surface area more than 150$m^2$/g has been prepared with the homogeneous precipitation process below 5$0^{\circ}C$ and its formation mechanism was investigated using the SEM, TEM and Raman Spectroscopy. With the spontaneous hydrolysis of aqueous TiOCl2 solutions, all the precipitates were fully and homogeneously crystallized with the rutile TiO2 phase simply by heating, which as transformed to the anatase TiO2 phase as increasing the addition of SO42- ions to the aqueous TiOCl2 solution. The precipitates were formed with spherical secondary particles which consisted of acicular, spherical and mixed primary particles corresponding to the rutile, anatase and mixed phases, respectively. It can be thought that the formation and phase determination of crystalline TiO2 powders even at ambient temperature would be related with the existence of the capillary force. This force might be varied depending on the shape change of the primary particles.

• Korean Journal of Materials Research
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• v.12 no.10
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• pp.803-808
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• 2002

Nickel powders were prepared from nickel chloride solution by wet reduction process, and the size control of the particles was investigated with reactant concentration, dispersant agent, and the addition of ethanol as an organic solvent in NiCl$_2$ aqueous solution. The size of the particle decreased with the increase of nickel chloride concentration. Their average particle size were 1.9$\mu\textrm{m}$, 1.6$\mu\textrm{m}$ and $1.5\mu\textrm{m}$ with 0.5M, 0.8M and 1.0M of nickel chloride concentration respectively. The spherical particle was easily controlled by the addition of ethanol as an organic solvent. Especially, in 30 vol% of ethanol, the average particle size and specific surface area were about 0.2$\mu\textrm{m}$ and 8.98m$^2$/g, respectively.