• Journal of the Korean Ceramic Society
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• v.26 no.3
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• pp.423-437
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• 1989

Al2O3/ZrO2 composites were prepared by precipitation method using Al2(SO4)3.18H2O, ZrOCl2.8H2O and YCl3.6H2O as starting materials and NH4OH as a precipitation agent. Al2O3/ZrO2 composites(series A) were prepared by mixing Al2O3 powder obtained by single precipitation method with ZrO2(＋3m/o Y2O3) powder obtained by co-predipitation method. Al2O3/ZrO2 composites (series B) were prepared by co-precipitation method using the three starting materials. In all cases, the composition was controlled as Al2O3-15v/o ZrO2(＋3m/o Y2O3). The composites of series A showed higher final relative densities than those of series B and tetagonal ZrO2 in all cases was retained to about 95% at room temperature. ZrO2 particles were coalesced more rapidly in grain boundary of Al2O3 than within Al2O3 grain. ZrO2 particles were located at 3-and 4-grain junction of Al2O3 and limited the grain growth of Al2O3. It was observed that MgO contributed to densification of Al2O3 but limited grain growth of Al2O3 by MgO was not remarkable. In all Al2O3/ZrO2 composites, exaggerated grain growth of Al2O3 was not observed and Al2O3/ZrO2 composites were found to have homogeneous microstructures.

• Journal of the Korean Ceramic Society
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• v.34 no.1
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• pp.102-108
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• 1997

ZrO2 powders stabilized with Y2O3 and CeO2 of various compositions were prepared by the coprecipitation of water-soluble ZrOCl2.8H2O, YCl3.6H2O and Ce(NO3)3.6H2O, and their compacts were pressurelessly sintered at 1400 and 150$0^{\circ}C$ for 2hrs in air. 2mol% Y2O3-ZrO3 showed the most superior strength (1003MPa) and microhardness (12.6GPa), while 10 mol%CeO2-ZrO2 had the hightest toughness (13.3 MPa.m1/2) after sintering at 140$0^{\circ}C$. The addition of Y2O3 into Y2O3-ZrO3 decreased mean grain size and increased strength and hardness but decrease toughness. On the other hand, the addition of CeO2 into Y2O3-ZrO2 enhanced the stability of tetragonal phase during low-temperature aging for a long time under hydrothermal atmosphere.

• Journal of the Korean Ceramic Society
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• v.26 no.2
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• pp.242-248
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• 1989

The powder properties and sinterability of ZrO2-8m/o Y2O3 prepared by coprecipitation were investigated. The specific surface area and the total pore volume were increased with increasing pH of sediment and using of ethyl alcohol for washing. The powders of prepared by freeze drying method were showed the smallest particle size and lowest agglomerate rate, and the powders obtained from spray drying method were showed porous structure. At 130$0^{\circ}C$, their relative density were 94-96%, so these sintered bodies have revealed high sinterability in spite of low sintering temperature.

• Journal of the Korean Ceramic Society
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• v.30 no.9
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• pp.717-722
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• 1993

Effects of microstructures on the electrical properties of ZrO2 based ceramics were analyzed by modeling layer arrangements and mixed phase structures. Single layers and alternating multilayers were made from 3mol% and 8mol% Y2O3 doped ZrO2 powders, while mixed specimen was made by blending and compacting these raw powders. After sintering at 150$0^{\circ}C$ for 2hr in air, AC impedance characteristics were measured. Contributiion of bulk comonent to total resistivity and its temperature-dependence were larger in 8Y-ZrO2 single layer than in 3Y-ZrO2 single layer. The multilayered specimen connected in serial to electrodes showed partial characteristics of both 3Y-ZrO2 and 8Y-ZrO2 single layers. The multilayered specimen connected in parallel to electrodes and the mixed specimen exhibited characteristics mainly of 8Y-ZrO2 single layer. The multilayered specimen connected in parallel to electrodes revealed the highest electrical conductivity near the operating temperature of solid oxide fuel cell. However, it is expected that the mixed specimen is appropriate for the applications because of its relatively high electrical conductivity with high strength expected.

• Journal of the Korean Ceramic Society
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• v.36 no.11
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• pp.1198-1204
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• 1999

The effect of sintering atmosphere on the grain sizes and phase distributions in 3Y-ZrO2 and 8Y-ZrO2 was investigated O2 and N2 were used as sintering atmospheres. In the case of 3Y-ZrO2 the sintered density was higher in N2 than in O2 while in the case of 8Y-ZrO2 contrary results were obtained. The observation can be explained by the nitrogen solubility into the zirconia lattice. That is nitrogen gas can behave as a diffusive gas contrary to the behavior in other oxides depending on the amount of Y2O3. In 3Y-ZrO2 tetragonal phase was retained at room temperature irrespective of sintering atmospheres. Grain sizes of two specimens were below 2㎛ and larger in O2 thin in N2 Under a given stress the transformability of tetragonal phase into monoclinic phase was higher in O2 than N2. The results are discussed on the basis of an effect of the grain size and non-transformable ttragonal(t') phase.

• Journal of the Institute of Electronics Engineers of Korea TE
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• v.39 no.1
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• pp.1-8
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• 2002

The effects of the addition of either monoclinic $ZrO_2$($ZrO_2$(m)) or tetragonal $ZrO_2$($ZrO_2$(t)) containing 5.35wt% $Y_2O_3$ on the physical properties and electrical conductivity of TEX>$Al_2O_3$ were investigated. The addition of $ZrO_2$(m) and $ZrO_2$(t) increased sintered density of $Al_2O_3$. The Vickers hardness also increased as addition of >($ZrO_2$(t) increased going through a maximum at 20wt% and the hardness of the specimens was found to be dependent on the sintered density. The addition of $ZrO_2$(t) improved the hardness of $Al_2O_3$-$ZrO_2$ systems and the $ZrO_2$(m) addition showed the better effect on the thermal shock property of $Al_2O_3$-$ZrO_2$ systems than that of the $ZrO_2$(t) addition. Above 15wt% addition of $ZrO_2$(t), the electrical conductivity is gradually increased with increasing applied voltage but not effects by addition of $ZrO_2$(m).

• Journal of the Korean Ceramic Society
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• v.28 no.1
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• pp.11-18
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• 1991

A precipitation method, one of the most effective liquid phase reaction methods, was adopted in order to prepare high-tech Al2O3/ZrO2 composite ceramics, and the effects of stress-induced phase transformation of ZrO2 on thermal shock behavior of Al2O3-ZrO2 ceramics were investigated. Al2(SO4)3.18H2O, ZrOCl2.8H2O and YCl3.6H2O were used as starting materials and NH4OH as a precipitation agent. Metal hydroxides were obtained by single precipitation(process A) and co-precipitation(process B) method at the condition of pH=7, and the composition of Al2O3-ZrO2 composites was fixed as Al2O3-15v/o ZrO2(+3m/o Y2O3). Critical temperature difference showing rapid strength degradation by thermal shock showed higher value in Al2O3/ZrO2 composites(process A : 20$0^{\circ}C$, process B : 215$^{\circ}C$) than in Al2O3(175$^{\circ}C$). The improvement of thermal shock property for Al2O3/ZrO2 composites was mainly due to the increase of strength at room temperature by adding ZrO2. The strength degradation was more severe for the sample with higher strength at room temperature. Crack initiation energies by thermal shock showed higher values in Al2O3/ZrO2 composites than in Al2O3 ceramics due to increase of fracture toughness by ZrO2.

• Journal of the Korean Ceramic Society
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• v.23 no.3
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• pp.44-52
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• 1986

$Al_2O_3-ZrO_2$ ceramics was obtained by the co-precipitation method using $Al_2(SO_4)_2$.$18H_2O$ and $ZrOCl_2$.$8H_2O$ as starting materials $MgCl_2$.$6H_2O$ as a sintering aid and NH4OH as a hydrolyzing agent. The coprecipitate from the above raw materials was calcined at 125$0^{\circ}C$ for 1h and again sintered at 1$650^{\circ}C$ for 2h before measurements of strength hardness and fracture toughness. MgO addition was found to increase mechanical properties of the $Al_2O_3-ZrO_2$ system. The strength and frac-ture toughness of $Al_2O_3-ZrO_2$ ceramics were considered to be increased by stress-induced phase tranforma-tion of $ZrO_2$.

• Transactions of the Korean hydrogen and new energy society
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• v.29 no.5
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• pp.419-426
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• 2018

$Ni-La_2O_3-Ce_{0.8}Zr_{0.2}O_2$ catalysts with various $La_2O_3$ loading were investigated for hydrogen production from steam reforming of methane (SRM). The $La_2O_3$ loading influenced the physicochemical properties of $Ni-La_2O_3-Ce_{0.8}Zr_{0.2}O_2$ catalysts such as BET surface area, Ni dispersion, Ni size and reducibility. Among the prepared catalysts, $Ni-70La_2O_3-Ce_{0.8}Zr_{0.2}O_2$ catalyst showed the highest activity and stability at a very high gas hourly space velocity (GHSV) of $932,556h^{-1}$. This is mainly due to high Ni dispersion, small Ni size and high reducibility.

• Journal of the Korean Ceramic Society
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• v.42 no.8 s.279
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• pp.561-566
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• 2005

For the homogeneous dispersion of $ZrO_2$ particles in $Al_2O_3/ZrO_2$ceramics, Zr-precusors were mixed with oxide $Al_2O_3$powders by chemical routes such as partial precipitation or partial polymerization of Zr-nitrate solutions. In case of the mechanical mixing of ultrafine $Al_2O_3$ and $ZrO_2$ oxide powders, relatively homogeneous dispersion was difficult to achieve so that the particle size and distributions of $ZrO_2$ were relatively inhomogeneous after sintering at high temperature. But when the Zr-Y-hydroxide were co-precipitated to ultrafine $Al_2O_3$ oxide powders followed by calcinations, homogeneous dispersion of nano-sized $ZrO_2$ particles in $Al_2O_3/ZrO_2$ composite ceramics were obtained. But because of the coalescence of dispersed $ZrO_2$ particles, dispersed $ZrO_2$ was grown up to more than 0.2${mu}m$ (200 nm) when sintered at the temperature of higher than $1500^{\circ}C$ But when the sintering temperature was kept to lower than $1400^{\circ}C$ by using nano-sized $\alpha-alumina$, the particle size of dispersed $ZrO_2$ could be sustained below 0.1 ${\mu}m$. But the coalescence of dispersed $ZrO_2$ between $Al_2O_3$ particles could not be avoided so that the mechanical properties were not enhanced contrary to the expectations. So Zr-polyester precursors were precipitated and coated to the surface of ultrafine $\alpha-alumina$ powders by the polymerization of Ethylene Glycol with Citric Acid and Zirconium Nitrate. By this dispersion much more uniform dispersion of $ZrO_2$ was achieved at $1450\~1600^{\circ}C$ of sintering temperature ranges. And due to especially discrete dispersion of $ZrO_2$ between $Al_2O_3$ particles, their mechanical strength was more enhanced than mechanical mixing or hydroxide precipitation methods.