• Journal of the Korean Ceramic Society
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• v.31 no.4
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• pp.359-366
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• 1994

Tow types of supports were made using $\alpha$-Al2O3 powder and optimum conditions to prepare for supports were provided. Sol solution for coating was synthesized by sol-gel method with aluminum isopropoxide. Supports were coated and heat-treatemented, where the thickness of coating layer was controlled by dipping time. Flux and permeability of alumina membrane were measured by liquid and gas filtration apparatus and these were compared with the provided model. It was confirmed that the coating was done very well without micro~crack and defect.

• Journal of Powder Materials
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• v.12 no.6 s.53
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• pp.422-427
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• 2005

The $Al_2O_3$ with various phases were prepared by simple ex-situ hydrolysis and spark plasma sintering (SPS) process of Al powder. The nano bayerite $(\beta-Al(OH)_3)$ phase was derived by hydrolysis of commercial powder of Al with micrometer size, whereas the bohemite (AlO(OH)) phase was obtained by hydrolysis of nano Al powder synthesized by pulsed wire evaporation (PWE) method. Compaction as well as dehydration of both nano bayerite and bohemite was carried out simultaneously by SPS method, which is used to fabricate dense powder compacts with a rapid heating rate of $100^{\circ}C$ per min. under the pressure of 50MPa. After compaction treatment in the temperature ranges from $100^{\circ}C\;to\; 1100^{\circ}C$, the bayerite and bohemite phases change into various alumina phases depending on the compaction temperatures. The bayerite shows phase transition of $Al(OH)_3{\to}{\eta}-Al_2O_3{\to}{\theta}-Al_2O_3{\to}\alpha-Al_2O_3$ sequences. On the other hand, the bohemite experiences the phase transition from AlO(OH) to ${\gamma}-Al_2O_3\;at\;350^{\circ}C.$ It shows AlO(OH) ${\gamma}-Al_2O_3{\to}{\delta}-Al_2O_3{\to}{\alpha}-Al_2O_3$ sequences. The ${\gamma}-Al_2O_3$ compacted at $550^{\circ}C$ shows a high surface area $(138m^2/g)$.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.5 no.2
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• pp.122-128
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• 1995

Sintered bodies were prepared from ${\alpha} - Al_2O_3/SiO_2$ composite powders which each alumina particles were surrounded by silica particles and investigated the mullitization behavior on the process of reaction - sintering. Mullitized reaction was started by formation of amorphous aluminosilicate inter - layer and proceeded by diffusion of alumina through this inter-layer. The growth of mullite was happened along the surface of alumina and controlled by the rate of diffusion.

• Journal of the Korean Ceramic Society
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• v.30 no.12
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• pp.1045-1053
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• 1993

Alumina hydrates were prepared by the neutralization of AlCl3.6H2O solution with NH3 gas diluted with N2 gas. The values of pH in reaction solution influenced the formation of alumina hydrates minerals. Amorphous alumina hydrates, for example, were formed at ${\gamma}$-Al2O3longrightarrow$\delta$-Al2O3longrightarrow$\theta$-Al2O3longrightarrow$\alpha$-Al2O3. (2) Bayeritelongrightarrowamorphouslongrightarrow${\gamma}$-Al2O3longrightarrow$\delta$-Al2O3longrightarrowη-Al2O3longrightarrow$\theta$-Al2O3longrightarrow$\alpha$-Al2O3. On the other hand, the shape of alumina hydrates whichw ere prepared by the reacton of Al2(SO4)3.16H2O solution and NH3 gas was spherical, the progress of its phase transformation with increasing temperature was amorphouslongrightarrow${\gamma}$-Al2O3longrightarrow$\alpha$Al2O3 in sequence.

• Journal of the Korean Ceramic Society
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• v.28 no.7
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• pp.568-576
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• 1991

The powder characterization and sintering behavior of alkoxide-derived alumina powders prepared by esterification were investigated. The dried powders were calcined at 700$^{\circ}C$, 900$^{\circ}C$, 1080$^{\circ}C$, 1170$^{\circ}C$ for 1 h. and the resulting crystalline forms were amorphous, {{{{ gamma }} phase, {{{{ theta }}+{{{{ alpha }} phase, and {{{{ alpha }} phase, respectively. The sinterability was best in the case of being calcined at 1170$^{\circ}C$. It was most effective to disperse sintering additive in the initial stage from which acetate was formed. At 1600$^{\circ}C$ for 4 h, 98.5% of relative density was obtained when MgO and Fe2O3 were simultaneously doped, and dense sintered body whose density was near theoretical value was obtained when heating schedule was controlled.

• 연구논문집
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• s.30
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• pp.111-119
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• 2000

Sintering of alumina powder was studied in the presence of Y-Si oxide and oxynitride additives. The main crystalline phase of the sintering aids pre-reacted at $1400^{\circ}C$ was $\alpha$ - $Y_2$$SiO_2$>$O_7$. Y-N apatite was co-existed in the Si-40N sintering aid because of its high content of N. During the sintering process, liquid phases were formed by the reaction between additives and alumina, and these liquid phases promote the densification of alumina. SEM micrographs showed that uniform grain growth occurred in the system with oxide additive(Si-0N). In the case of oxynitride additive system(Si-20N and Si-40N), bimodal microstructure was observed due to the exaggerated grain growth, As the nitrogen content in the additive system increased the exaggerated grain growth occurred extensively. Bloating, which seemed to be originated by the liberation of $N_2$ gas, occurred un the Si-40N oxynitride additive system.

• Journal of Powder Materials
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• v.16 no.6
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• pp.431-437
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• 2009

Various inorganic fillers improve the thermal conductivity and physical properties of organic products. Alumina has been used a representative filler in the heat radiation sheet for the heat radiation of electric device. The high filling rate of alumina increases the thermal conductivity and properties of products. We successfully developed the spherical alumina by flame fusion process using the oxygen burner with LPG fuel. In the high temperature flame (2500$\sim$3000$^{\circ}C$) of oxygen burner, sprayed powders were melting and then rotated by carrier gas. This surface melting and rotation process made spherical alumina. Especially effects of chemical composition and particle size of stating materials on the melting behavior of starting materials in the flame and spheroidization ratio were investigated. As a result, spheroidization ratio of boehmite and aluminum hydroxide with endothermic reaction of dehydration process was lower than that of the sintered alumina without dehydration reaction.

• Journal of the Korean Chemical Society
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• v.35 no.4
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• pp.422-426
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• 1991

In order to synthesize high-purity alpha alumina fine powder, the aluminum hydroxide and ammonium aluminum sulfate(alum) precursor were prepared from natural alumino-silicate(halloysite). The thermal decomposition mechenism for both precursors was elucidated by DTA/TG, XRD and IR analysis. The microstructure, specific surface area and purity of ${\alpha}-Al_2O_3$ were characterized by SEM, and BET analysis. The particle size of ${\alpha}-Al_2O_3$ was determined to be ${\phi}$ = 0.1∼0.5 ${\mu}$m. However, the specific surface area for the alum derived ${\alpha}-Al_2O_3$(89.0 m$^2$/g) was extremely higher than that for the aluminum hydroxide derived one(7.3 m$^2$/g).

• Korean Journal of Materials Research
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• v.2 no.1
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• pp.3-11
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• 1992

Aluminum sulfate hydrate was prepared using sulfuric acid from Ha-dong kaolin. The effects of calcination-temperature and calcination-time of kaolin, reaction-temperature and reaction-time, and sulfuric acid concentration on the formation of aluminum sulfate hydrate were investigated. The precipitation condition of aluminum sulfate hydrate from sulfuric acid solution was determined. Also, the products heat-treated at different temperatures have been investigated by X-ray diffraction, thermogravimetry, differential thermal analysis, Fourier transform infrared spectrophotometer, scanning electron microscopy, particle size distribution analysis and chemical analysis. In the optimum condition, the conversion of aluminum oxide in kaolin to aluminum sulfate hydrate was 60%. From the results of XRD, TG-DTA, and FT-IR, it is suggested that the aluminum sulfate hydrate is thermally decomposed as follows ; $Al_2(SO_4)_3{\cdot}18H_2O{\rightarrow}Al_2(SO_4)_3{\cdot}6H_2O{\rightarrow}Al_2(SO_4){\rightarrow}\;amorphous\;alumina{\rightarrow}{\gamma}-alumina{\rightarrow}{\delta}-alumina{\rightarrow}{\theta}-alumina{\rightarrow}{\alpha}-alumina$. The purity of alumina powder prepared by calcining aluminum sulfate hydrate at $1200^{\circ}C$ was 99.99 percent.

• Applied Chemistry for Engineering
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• v.21 no.5
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• pp.488-494
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• 2010

Under W/O emulsion method using a homogenizer, ${\alpha}$-alumina powder was prepared to evaluate the effects of experimental conditions on its properties, such as particle shape, extent of aggregation, average particle size and distribution. The experimental parameters were the change of type, quantity and composition of emulsifiers as well as the change of O : W volumetric ratio and agitation rate. As results, in the case of the use of single surfactant of SP80, sphere-like particles could be prepared and the average particle size was hardly affected by the agitation speed more than 16000 rpm regardless of SP80 quantity used. When the extent of aggregation among sphere-like particles prepared using $HLB_m$ = 5 of [SP80 & TW80] was compared with that prepared using SP80 at the same vol% surfactant and agitation speed, the former showed more or less low aggregation phenomena and average particle size was slightly reduced. In addition, the fraction of nano-sized particles with low aggregation was increased by the use of 0.1 vol% n-butanol, as a co-surfactant, with $HLB_m$ = 5 of [SP80 & TW80].