• Journal of the Korean Ceramic Society
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• v.41 no.8
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• pp.610-617
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• 2004

For preparation $\alpha$-Al$_2$O$_3$ platelets having 20 $\mu$m in average diameter and 0.2∼0.3 $\mu$m in thickness, we have prepared aluminum hydroxides gel by using aluminum sulfate and sodium sulfate as starting materials. In this study, we investigated the effect of the amount of sodium phosphate on particle size, morphology and thickness of $\alpha$-Al$_2$O$_3$ platelets. When sodium phosphate was not added to aluminum hydroxides gel, most of $\alpha$-Al$_2$O$_3$ platelets had hexagonal shape but the thickness was over 1.0 $\mu$m, and this sample was not adequate for pearlescent pigment. On the other hand, introduction of sodium phosphate caused an increase of aspect ratio (particle diameter/thickness) with a decrease in $\alpha$-Al$_2$O$_3$ platelet thickness.

• Journal of the Korean Ceramic Society
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• v.39 no.5
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• pp.473-478
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• 2002

${\alpha}-Al_2O_3$ platelets were synthesized by microwave heating the two different powder mixtures of $Al_2(SO_4)_3+2Na_2SO_4$ and ${\gamma}-Al_2O_3+2Na_2SO_4$ using flux method. DTA-TG, XRD and SEM were used to investigate the effect of microwave on the formation of ${\alpha}-Al_2O_3$ platelets. In the case of the mixture of $Al_2(SO_4)_3+2Na_2SO_4$, the microwave heated sample was ${\alpha}-Al_2O_3$ platelets composed of aggregates with smaller particle size compared to the conventionally heated sample. In the case of the mixture of ${\gamma}-Al_2O_3+2Na_2SO_4$, the temperature to form ${\alpha}-Al_2O_3$ platelets by the microwave heating was lower than that by the conventional heating and the morphology of the microwave heated sample was similar to that of the conventionally heated sample except that the microwave heated sample had smaller particle size compared to the conventionally heated sample.

• Journal of the Korean Ceramic Society
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• v.34 no.4
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• pp.406-412
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• 1997

Al2O3/SiC hybrid-composite has been fabricated by the conventional powder process. The addition of $\alpha$-Al2O3 as seed particles in the transformation of ${\gamma}$-Al2O3 to $\alpha$-Al2O3 provided a homogeneity of the microstructure. The grain growth of Al2O3 are significantly surpressed by the addition of nano-size SiC particles. Dislocation were produced due to the difference of thermal expansion coefficient between Al2O3 and SiC and piled up on SiC particles in Al2O3 matrix, resulting in transgranular fracture. The high fracture strength of the composite was contributed to the grain refinement and the transgranular fracture mode. The addition of SiC platelets to Al2O3/SiC nano-composite decreased the fracture strength, but increased the fracture toughness. Coated SiC platelets with nitrides such as BN and Si3N4 enhanced fracture toughness much more than non-coated SiC platelets by enhancing crack deflection.

• Journal of the Korean Ceramic Society
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• v.42 no.6 s.277
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• pp.384-390
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• 2005

[ ${\alpha}-Al_2O_3$ ] precursor was synthesised by sol-gel method using aluminum sulfate, sodium sulfate and sodium carbonate as law materials. The flaky ${\alpha}-Al_2O_3$ crystals were prepared by heating using precursor about $1,050^{\circ}C$. In this study, the effect of some transition-metal sulfate ($FeSO_4,\;SnSO_4,\;ZnSO_4$) addition have been investigated. When iron sulfate was added, it could see that act on impurities in crystal growth process. In case of tin sulfate, distribution of Platelets was very broad. When flaky ${\alpha}-Al_2O_3$ crystals were prepared zinc sulfate addition, thickness, size, and distribution of platelets was suited to industrial application. The average diameter of flaky ${\alpha}-Al_2O_3$ crystals was about 20 $\mu$m, and its thickness was about 0.3 $\mu$m. Increasing addition of zinc sulfate, thickness of ${\alpha}-Al_2O_3$ platelet was decreased.

• Journal of the Korean Ceramic Society
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• v.44 no.10
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• pp.562-567
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• 2007

To precipitate the complex gels of the pH 6, 7, 8, 9 included in a flux and an aluminum hydroxides gel, an aqueous solution of a mixture of $Na_2CO_3\;and\;Na_2PO_4{\cdot}12H_2O$ was added with stirring in an aqueous solution of a mixture of $Al_2(SO_4){_3}{\cdot}18H_2O,\;Na_2SO_4\;and\;K_2SO_4$, and then the complex gels were aged in 20 h at $90^{\circ}C$. As the hydrolysis pH changed, it had an effect on the physical properties such as the crystal structure, crystal morphology and a phase transition temperature of the AlO(OH) gel, and also on the crystal structure, crystal morphology, particle size and particle size distribution of the ${\alpha}-Al_2O_3$ platelets prepared by molten-salt precipitation. Also, in this study, the complex gels were crystallized at $1,200^{\circ}C$ and thereafter dried at $110^{\circ}C$, and then it was investigated to effect of the hydrolysis pH on the crystal structure, morphology and particle size distribution of the ${\alpha}-Al_2O_3$ platelets crystals using XRD, DTA, SEM and particle size analyzer.

• Journal of the Korean Ceramic Society
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• v.31 no.5
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• pp.513-519
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• 1994

Alumina composite was fabricated by introducing tetragonal ZrO2 in alumina composite of $\alpha$-alumina reinforced with in-situ formed La-$\beta$-aluminate (LaAl11O18). The powder mixture of composition (100-x)[88 Al2O3+(La2O3+11 Al2O3)]+x vol% ZrO2 was prepared with x from 0 to 40. Dense composites fabricated by hot-pressing exhibited bending strength up to 1200 MPa. The gain growth inhibiting effect of ZrO2 grains and La-$\beta$-aluminate platelets, and possibly the stress-induced phase transformation of ZrO2 have resulted in the high bending strength. The fracture toughness of the composite also increased up to 8.5 MPa.m1/2. The enhancement in toughness can be explained by transformation and microcrack toughening of ZrO2 and by crack deflection and bridging of La-$\beta$-aluminate platelets and ZrO2.

• Corrosion Science and Technology
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• v.7 no.3
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• pp.139-144
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• 2008

The results of microstructure observations of the $Al_2O_3$ scale formed on a Fe-Cr-Al steel during high temperature oxidation in the $SO_2$ atmosphere are presented. Morphology of the scale has been studied by SEM and TEM techniques. Phase and chemical compositions have been studied by EDX and XRD techniques. The alumina oxide is a primary component of the scale. TEM observations showed that the scale was multilayer. The entire surface of the scale is covered with "whiskers", which look like very thin platelets and have random orientation. The cross section of a sample shows, that the "whiskers" are approximately $2{\mu}m$ high, however the compact scale layer on which they reside is $0.2{\mu}m$ thick. The scale layer was composed mainly of small equiaxial grains and a residual amount of small columnar grains. EDX analysis of the scale surface showed that the any sulfides were found in the formed outer and thin inner scale layer. A phase analysis of the scale formed revealed that it is composed mainly of the $\theta-Al_2O_3$ phase and a residual amount of $\alpha-Al_2O_3$.