• Journal of the Korean Ceramic Society
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• v.24 no.1
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• pp.23-32
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• 1987

Synthesis of high purity ultrafine Si3N4 and ${\beta}$-Sialon powders was investigated via the simultaneous reduction and nitriding of amorphous SiO2, SiO2-Al2O3 system prepaerd by hydrolysis of alkoxides, using carbonablack as a reducing agent. In Si(OC2H5)4-C2H5 OH-H2 O-NH4OH system, hydrolysis rate increased with increasing reaction temperature and pH. Pure ${\alpha}$-Si3N4 was formed at 1350$^{\circ}C$ for 5 hrs in N2 atmosphere. In Si(OC2H5)4-Al(OC3H7)3-C6H6-H2 O-NH4OH system, weight loss increased as Si/Al ratio decreased. Single phase ${\beta}$-Sialon consisted of Si/Al=2 was formed at 1350$^{\circ}C$ in N2 and minor phases of ${\alpha}$-Si3N4, AIN, and X-phase were existed besides theSialon phase at other Si/Al ratios. The Si3N4 and Sialon powders synthesized from alkoxides consisted of uniform find particles of 0.05-0.2$\mu\textrm{m}$ in diameter, respectively.

• Korean Journal of Materials Research
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• v.20 no.2
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• pp.97-103
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• 2010

In this study, a new, relatively simple fabrication method for forming a mesoporous $Al(OH)_3$ film on Al substrates was demonstrated. This method, i.e., alkali surface modification, was simply comprised of dipping the substrate in a $5\times10^{-3}$ M NaOH solution at $80^{\circ}C$ for one minute and then immersing it in boiling water for 30 minutes. After alkali surface modification, a mesoporous $Al(OH)_3$ film was formed on the Al substrate, and its chemical state and crystal structure were confirmed by XPS and TEM. According to the results of the XPS analysis, the flake-like morphology after the alkali surface modification was mainly composed of $Al(OH)_3$, with a small amount of $Al_2O_3$. The mesoporous $Al(OH)_3$ layer was composed of three regions: an amorphousrich region, a region of mixed amorphous and crystal domains, and a crystalline-rich region near the $Al(OH)_3$ layer surface. It was confirmed that the stabilization process in the alkali surface modification strongly influenced the crystallization of the mesoporous $Al(OH)_3$ layer.

• Journal of the Korean Ceramic Society
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• v.35 no.10
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• pp.1085-1093
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• 1998

Amorphous alumina powder prepared by the fast calcination of aluminum trihydroxide(Al(OH)3 gibbsite) for 0.5 second at 580$^{\circ}C$ was investigated rehydration propeties. Phase composition crystal size and mor-phology surface area pore volume and pore size distribution of pesudo-boehmite and bayerite crystals changed with temperature time water/alumina ratio and particle size when amorphous alumina rehydrated with water. Phase compositions were examined with XRD and DTA and crystal sized morphologies were investigaed with SEM and TEM. Also rehydration properties of amorphous alumina were in-vestigated by measuring the surface area pore volume and pore size distribution.

• Journal of the Korean institute of surface engineering
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• v.55 no.3
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• pp.143-155
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• 2022

Ceramic oxide layers successfully were formed on the surface of cast Al alloys with high Si contents using plasma electrolytic oxidation (PEO) process in electrolytes containing Na₂SiO₃, NaOH, and additives. The microstructure of the oxide layers was systematically analyzed using scanning electron microscopy (SEM), cross-sectional transmission electron microscopy (TEM), X-ray diffraction patterns (XRD), and energy X-ray dispersive spectroscopy (EDS). XRD analysis indicated that the PEO untreated high-silicon Al alloys (i.e., 17.1 and 11.7 wt.% Si) consist of Al, Si and Al₂Cu phases whereas Al₂Cu phase selectively disappeared after PEO treatment. PEO process yielded an amorphous oxide layer with few second phases including γ-Al₂O₃ and Fe-rich phases. The corrosion behaviors of high-silicon Al alloys treated by PEO process were investigated using electrochemical impedance spectroscopy (EIS) and other electrochemical techniques (i.e., open circuit potential and polarization curve). Electroanalytical studies indicated that high-silicon Al alloys treated by PEO process have greater corrosion resistance than high-silicon alloys untreated by PEO process.

• Korean Journal of Materials Research
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• v.9 no.6
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• pp.630-634
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• 1999

Porous mullites were fabricated using $Al(OH)_3$ and amorphous $SiO_2$ as starting materials by reaction sintering method. The molar ratios of alumina and silica varied from stoichiometric mullite composition to silica-rich and alumina-rich compositions. $AlF_3$ of 0, 1, 5, 10 wt% wad added to each composition, and the effects of composition and the additive for the formation of mullite were examined. The temperature of mullite formation decreased as the amount of $AlF_3$ increased, and themullite phase was formed in the stoichiometric composition in addition of 5 wt% $AlF_3$ at $1250^{\circ}C$ and porous whiskered mullite were synthesized in the sample sintered at $1300^{\circ}C$ and higher temperature. The effect of temperature on the mullite formation was not observed for the sample with 5 wt% or higher content of $AlF_3$, and the body showed little contraction after sintering.

• Journal of Powder Materials
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• v.11 no.6 s.47
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• pp.522-527
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• 2004

Nano sized FeAl intermetallic particles were successfully synthesized by plasma arc discharge pro-cess. The synthesized powders shouted core-shell structures with the particle size of 10-20 nm. The core was metallic FeAl and shell was composed of amorphous $AI_{2}O_{3}\;and\;a\;little\;amount\;of\;metallic\;Fe_{3}O_{4}.$ Because of the difference of Fe and Al vapor pressure during synthesis, the Al contents in the nanoparticles depended on the Al contents of master alloy.

• Korean Journal of Materials Research
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• v.11 no.11
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• pp.978-985
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• 2001

The amorphous alumina was obtained by flash calcination of Bayer gibbsite[$Al(OH)_3$aluminum trihydroxide]. Rehydration and pore characteristics of $r-A1_2O_3$ prepared from rehydrated amorphous alumina were investigated. Crystal phases of pseudo-boehmite and bayerite were changed when amorphous alumina was hydrated at various conditions such as time, the ratio of water/alumina and pH. Specific surface areas and pore volumes of $r- A1_{2O}_3$ were influenced by the reaction time, water/alumina and PH of rehydration. The total pore volume of $r-A1_{2O}_3$increases with increasing the reaction time and ratio of water/alumina. Especially, the pure pseudo-boehmite of single phase could be prepared, when amorphous alumina was hydrated in the range of pH 6.5-8.0 in water/alumina= 10 at $90^{\circ}C$ for 7hr. The $r-Al_{2O}_3$, obtained by calcination of the prepared pseudo-boehmite at $500^{\circ}C$ for 2hrs, is characterized by the specific surface area of $265m^2$/g, total pore volume of $0.75cm^3$/g.

• Journal of the Korean Ceramic Society
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• v.25 no.3
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• pp.193-200
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• 1988

The starting materials were aluminum hydroxide prepared by precipitation method at the conditions of pH values; 7, 9, 10 and 11. The properties of alumina powder on heat-treatment were studied. After dehydrating structural water from amorphous aluminum hydroxide, the first formed phase was amorphous alumina and its specific surface are was decreased. The specific surface area was increased by dehydration of structural water from aluminum hydroxides except amorphous aluminum hydroxide. The specific surface area was increased with increase of the ratio of A1OOH to $A1(OH)_3$ in the region of transition aluminas. The rate of transition from aluminum hydroxide to alpha alumina occurred in the order of 7, 10, 9 and 11 of pH values. The morphology of alpha alumina powders was skeleton particles remaining outer shape of aluminum hydroxide. Both the elevation of heat-treatment temperature and the transition toalpha alumina decreased specific surface area and brought about the growth of particles.

• Transactions of the Society of Information Storage Systems
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• v.3 no.3
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• pp.118-122
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• 2007

Phase-change wet-etching technology using GeSbTe phase-change films is developed. Selective etching between an amorphous and a crystalline phase can be carried out with an alkaline etchant of NaOH. Etching selectivity is dependent not only on the concentration of the alkaline etchant but also on the film structure. Specifically, metal films for heat control cause marked effects on the etching properties of GeSbTe film. Surviving amorphous pits can be obtained with Al metal layer, however etched amorphous pits are seen with Ag metal layer. An opposite selective etching behavior can be observed between samples with two different metal layers.

• Korean Chemical Engineering Research
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• v.48 no.2
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• pp.205-211
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• 2010

The water-sewage cohesion agent(polyaluminumchloride(PAC)) and NaOH were used to synthesize $Al(OH)_3$. For various additions of NaOH, characteristics of the synthesized $Al(OH)_3$ was analysed by XRD, SEM and PSA. According to XRD analysis, small amount of NaOH(NaOH:PAC=15g:100g) resulted in amorphous form of $Al(OH)_3$. By increasing NaOH(NaOH:PAC=20g:100g), the mixture of gibbsite(37%), bayerite(35%) and boehmite(28%) were produced. By adding more NaOH(NaOH:PAC=25g:100g), binary mixtures of gibbsite(67%) and bayerite(33%) were formed. Finally, high addition of NaOH(NaOH:PAC=30g:100g) gave the high concentration of gibbsite(gibbsite:bayerite=83:17). Also, SEM analysis indicated that the product featured the plate form with 20 and 30g of NaOH addition. Furthermore it was found that the particle size of the product decreased with the addition of NaOH.