• Journal of the Korean Ceramic Society
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• v.32 no.4
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• pp.482-488
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• 1995

The phase transformation temperature from $\theta$- to $\alpha$-alumina was lowered from 1214$^{\circ}C$ to 114$0^{\circ}C$ in DSC by treating ${\gamma}$-alumina obtained by calcination of boehmite at $700^{\circ}C$ for 2hrswith $\alpha$-Al2O3 seeds (d50=0.36${\mu}{\textrm}{m}$) and 3wt% of the alumina sol. $\alpha$-Al2O3 seeds seemed to lower to the transformation temperature and the alumina sol suppressed the high temperature agglormeration. The effect was increased as the amount of the sol was increased, which was supported by TEM and particle size distribution. For an example, spherical ${\gamma}$-alumina powder with d50=0.54${\mu}{\textrm}{m}$ was prepared by treating the ${\gamma}$-alumina with 9 wt% of the alumina sol and 3wt% of the $\alpha$-Al2O3. It sintered to 99% of the theoretical density at 150$0^{\circ}C$ for 2hrs. and it had relatively homogeneous microstructure with 2~3${\mu}{\textrm}{m}$ sized grains.

• Journal of the Korean Ceramic Society
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• v.25 no.5
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• pp.465-472
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• 1988

Alpha alumina powder with fine particle size and narrow particle size distribution was prepared by precipitation method using Al2(SO4)3.18H2O as a starting material. The alpha alumina powder was prepared by calcining aluminum hydroxide which was formed under various pH values. The sinterabilityof alpha alumina powder and the effect of MgO on the sinterability of alpha alumina powder were investigated. The sinterability of alpha alumina powder was the order of pH=10≒pH11>pH=7≒pH9, and alpha alumina obtained from boehmite which was prepared by precipitation method reached to 97.5% of theoretcal density by the pressureless sintering. The effect of MgO on volume shrinkage of alumina was very slight in the initial sintering stage but remarkable in the final sintering stage. It was also found that MgO controlled effectively the grain growth of alumina.

• 연구논문집
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• s.22
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• pp.107-113
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• 1992

The $\alpha$-alumina/fused alumina/$\beta$"-alumina specimens were fabricated by addition of fused alumina to $\alpha$-alumina/$\beta$"-alumina system. The sintered densities with and without fused alumina were almost same. But, the bending strength was decreased due to the presense of defects caused by the inhomogeneous dispersion of large fused alumina particles. And the fracture toughness was also decreased because the crack propagated through the fused alumina particles due to the imperfection of $\beta$"-alumina coating around them.

• Membrane Journal
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• v.26 no.5
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• pp.372-380
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• 2016

The present paper reports the effect of precursor alumina particle size on pore structure and single gas permeation properties of tubular ${\alpha}$-alumina supports, prepared by a combined process of slip casting and sintering. Pore diameter of as-prepared ${\alpha}$-alumina support was highly dependent on precursor ${\alpha}$-alumina particle size. Although, increase in the precursor particle size increases the pore diameter, but the porosity of ${\alpha}$-alumina support mainly control by sintering temperature. Sintering studies reveal that as sintering temperature increased porosity of support decreased. Single gas permeance results indicate that permence is proportional to the square of pore diameter and linearly to porosity. These dependencies revealed that gas permeation trough as-prepared ${\alpha}$-alumina support was governed by viscous flow mechanism. The present announces that precursor ${\alpha}$-alumina particle size and sintering temperature are key parameters to control gas permeantion properties of ${\alpha}$-alumina supports.

• Proceedings of the Materials Research Society of Korea Conference
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• 1994.11a
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• pp.128-128
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• 1994

• Journal of the Korean Ceramic Society
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• v.33 no.8
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• pp.849-854
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• 1996

boehmite which is the by-product in alcohol process contains anions such as $Cl^-,{NO_3}^-.or\; CH_3COO^-$ Influe-nce of these anions was studied on properties of $\alpha$-alumina powders prepared by treating the transformed ${\gamma}$-alumina with the alumina sol and $\alpha$-alumina seeds (d_{50}=0.36\mu\textrm{m}$) Disperal 20/1 and 10/1 containing $Cl^-$ produced spherical powder Disperal 20/2 with ${NO_3}^-$ produced equiaxed powder and Disperal 10/3 with $CH_3$ $COO^-$ irregular shaped and sized powder. All of these $\alpha$-alumina powders were submicron. Although the green density of the alumina powder derived from Disperal 10/1 by calcination at $1200^{\circ}C$/1h. was 53% it did not sinter even at $1550^{\circ}C$ for 2h. which implies that its d50 is greater than $3\mu\textrm{m}$. The others gave green densities in the range of 44~47% but they sintered as well as the AKP-30 at $1500^{\circ}C$/2 h or $1550^{\circ}C$/2 h.

• Journal of the Korean Ceramic Society
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• v.29 no.1
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• pp.23-28
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• 1992

Alumina ceramics was reinforced by in-situ formation of La-${\beta}$-aluminate in ${\alpha}$-alumina matrix. The powder mixture of which composition is (100-12x)Al2O3+x(La2O3+11Al2O3) was prepared for the formation of La-${\beta}$-aluminate in ${\alpha}$-alumina matrix. The amount of La-${\beta}$-aluminate in the matrix was controlled by varing x which is number of moles. The dense composite was produced by sintering at 1600$^{\circ}C$ in air or hot-pressing at 1550$^{\circ}C$ under 30 MPa. Bending strength and fracture toughness were increased, resulting from the grain growth inhibition and the crack deflection and crack bridging mechanism when La-${\beta}$-aluminate was produced in ${\alpha}$-alumina matrix.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.24 no.2
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• pp.59-64
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• 2014

Crystallographic phases of Plasma electrolytic oxidized Al alloy, A1050, were investigated. The electrolyte of PEO was $Na_2Si_2O_3$ and KOH. Unipolar pulse, $ 2000{\mu}sec$ with $400{\mu}sec+420V$ impulse, were applied for 2 min, 5 min, 15 min, and 30 min. ${\gamma}-Alumina$, as well as ${\alpha}-alumina$, was main crystal phase. ${\gamma}-Alumina$ was appeared in the beginning, then the amount of ${\alpha}-alumina$ was increased with time, but the amount of ${\gamma}-Alumina$ remained constant without any increasing. So, it is concluded that plasma gas produce ${\gamma}-Alumina$ at the first, and then ${\gamma}-Alumina$ transform ${\alpha}-alumina$ finally. During the transformation, high temperature of micro plasma gives transformation energy.

• 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.

• Journal of the Korean Ceramic Society
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• v.31 no.12
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• pp.1475-1482
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• 1994

Compositionally triphasic boehmite-silica-zirconia composite gels were prepared by a multiphasic sol-gel route. Phase-formation characteristics and densification behavior of the gel compacts were examined with and without $\alpha$-Al2O3 seeding. In the unseeded triphasic gels, both $\alpha$-Al2O3 and mullite crystallize simultaneously at 130$0^{\circ}C$. On the other hand, the $\alpha$-Al2O3 seeding selectively induces the formation of corundum phase ($\alpha$-Al2O3) at a significantly lower temperature (~110$0^{\circ}C$) and facilitates an epitaxial growth of $\alpha$-Al2O3 between 1100~130$0^{\circ}C$. The densification of alumina-mullite-zirconia composite (derived from the triphasic gels) was also enhanced by the $\alpha$-Al2O3 seeding, and this was attributed to the delayed crystallization of mullite in the $\alpha$-Al2O3 seeded gel.