• Journal of the Korean Ceramic Society
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• v.51 no.3
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• pp.218-223
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• 2014

The domain structures, dielectric properties, and phase transformation of (001)-oriented $Pb(Mg_{1/3}Nb_{2/3})O_3-x%PbTiO_3$ (PMN-x%PT) crystals for x=20, 30, 35, and 40 mole% have been investigated. PMN-20%PT consists of polar nano-domains (PND) which do not self-assemble into macro-domain plates. PMN-30%PT consists of PNDs which begin to self-assemble into colonies along preferred {110} planes. PMN-35PT consists of miniature polar domains on the nm scale. PMN-40%PT consists of {001} oriented lamella domains on the mm scale that have internal nano-scale heterogeneities. The dielectric properties of poled (001) PMN-x%PT single crystals have been measured for orientations both parallel and perpendicular to the [001] poling direction. The results of the temperature dependence of the dielectric constant and mesh scans for the 30%PT sample demonstrate that the initial monoclinic phase changes to single domain tetragonal phase and to cubic phase with increasing temperature.

• Journal of the Korean Ceramic Society
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• v.27 no.8
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• pp.971-978
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• 1990

The ifluence of transformability and stabilized effects in tetragonal phase on the isothermal phase transformation of Y-TZP at low temperature were investigated. The transformability of Y-TZP powders were gradually increased with calcination temeprature and reached maximum at critical temperature, but when the Y-TZP powders were calcined above critical temperature, transformability of Y-TZP were gradually decreased with increasing calcination temperature. It was concluded that maximum transformability was appeared because particle size effects decreased and constrain effects increased with calcined temperature. The isothermal phase transformation during aging at 25$0^{\circ}C$ only observed in Y-TZP stabilized by constrain effects and the amounts of transformation during aging at 25$0^{\circ}C$ only observed in Y-TZP stabilized by constrain effects and the amount of transformed monoclinic phase during aging decreased with increasing constrain effects. From these results, the mechanism of isothermal phase transformation and degradation behaviors at low temperature in Y-TZP was concluded that occurred by decreasing of constrain effects due to stress relaxation at grian boundary.

• Journal of the Korean Society for Heat Treatment
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• v.37 no.1
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• pp.1-8
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• 2024

Phase stability of tetragonal crystals in yttria-stabilized zirconia ceramics is dependent on the content of yttria and the heat-treatment condition, related with mechanical properties. In this study, we fabricated the 1.5 mol% yttria-stabilized zirconia (1.5Y-YSZ) ceramics by cold isostatic pressing (CIP) and post-sintering at temperature range of 1200 to 1350℃ for 2 hours and investigated the sintered properties and microstructural evolution. Sintered and microstructural parameters, i.e, apparent density, grain size and phase composition of 1.5Y-YSZ ceramics were mainly dependent on the sintering temperature. Maximum sintered density of 99.4 % and average grain size of 200-300 nm could be obtained from the heat-treatment condition above sintering temperature at 1300℃ for 2 hours, possessing the superior mechanical hardness with 1200 Hv. However, phase stability of tetragonal grains in 1.5 YSZ ceramics is very low, inducing the phase transformation to monoclinic crystals on specimen surface during cooling after heat-treatment.

• Journal of Korea Society of Digital Industry and Information Management
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• v.12 no.3
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• pp.25-31
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• 2016

Mechanical properties of Y2O3-containing tetragonal ZrO2 polycrystals(Y-TZP) were investigated. Several additives were used to modify the hardness and fracture toughness of Y-TZP. The effects of these individual additives were discussed and their interactions were also analysed. Each additive, such as CoO, Fe2O3, MnO2 was found to deteriorate the mechanical properties of Y-TZP when it was used singly. But the fracture toughness of Y-TZP was significantly improved when these additives and Al2O3 were added in combination at a certain ratio. The addition of CoO, Fe2O3 and MnO2 into Y-TZP resulted in the more complex behavior of fracture toughness and hardness. The specimen with 1.5 wt%-Fe2O3, 3.0 wt% -Al2O3 and 1.5 wt%-CoO showed the monoclinic to tetragonal phase ratio of 18% and the highest toughness of $10.8MPa{\cdot}m1/2$ with Vickers hardness of 1201 kgf/mm2. However, the toughness decreased as the ratio increased and macrocracks developed beyond the ratio of 25%. Sample No. 16 is improved high Physical and Mechanical Properties.

• Korean Journal of Materials Research
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• v.22 no.5
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• pp.220-226
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• 2012

Pure zirconia and $x$ mol% calcia partially stabilized zirconia ($x$ = 1.5, 3, and 8) nanopowders were synthesized by hydrothermal method with various reaction temperatures for 24 hrs. The precipitated precursor of pure zirconia and $x$ mol% calcia doped zirconia was prepared by adding $NH_4OH$ to starting solutions; resulting sample was then put into an autoclave reactor. The optimal experimental conditions, such as reaction temperatures and times and amounts of stabilizer CaO, were carefully studied. The synthesized $ZrO_2$ and $x$ mol% CaO-$ZrO_2$ ($x$ = 1.5, 3, and 8) powders were characterized by XRD, SEM, TG-DTA, and Raman spectroscopy. When the hydrothermal temperature was as low as $160^{\circ}C$, pure $ZrO_2$ and $x$ mol% CaO-$ZrO_2$ ($x$ = 1.5 and 3) powders were identified as a mixture of monoclinic and tetragonal phases. However, a stable tetragonal phase of zirconia was observed in the 8 mol% calcia doped zirconia nanopowder at hydrothermal temperature above $160^{\circ}C$. To observe the phase transition, the 3 mol% CaO-$ZrO_2$ and 8 mol% CaO-$ZrO_2$ nanopowders were heat treated from 600 to $1000^{\circ}C$ for 2h. The 3 mol% CaO-$ZrO_2$ heat treated at above $1000^{\circ}C$ was found to undergo a complete phase transition from mixture phase to monoclinic phase. However, the 8 mol% calcia doped zirconia appeared in the stable tetragonal phase after heat treatment. The result of this study therefore should be considered as the preparation of 8 mol% CaO-$ZrO_2$ nanopowders via the hydrothermal method.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 2001.06a
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• pp.346-353
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• 2001

Zirconia powder containing 3 mol% yttria(3Y-PSZ) with and with out Fe$_2$O$_3$ addition was coated on tile cast iron substrate by plasma spraying method. The erosion experiments were performed at temperatures from $25^{\circ}C$ to $600^{\circ}C$. A gas blast type erosion tester was used to examine erosion behavior of the specimens. The results of 3Y-PSZ coatings showed that tile erosion rate had maximum value at 40$0^{\circ}C$. It coincided with tile results of phase transformation tetragonal phase to monoclinic phase caused by low temperature thermal degradation. The tensile stress relaxation and the micro-hardness improvement significantly influenced on the erosion rate at $600^{\circ}C$. In the case of Fe$_2$O$_3$ added 3Y-PSZ coatings, the erosion rate of tested at $25^{\circ}C$ showed maximum value at 5.0 mol% Fe$_2$O$_3$ added coating. This tendency is caused by the improvement of mechanical properties and the tensile residual stress. The erosion rate at 200'c and 400'L showed significantly decrease by Fe203 addition. This decrease is believed to be the stabilization of the tetragonal phase and the increase of micro-hardness.

• Bulletin of the Korean Chemical Society
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• v.26 no.5
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• pp.755-762
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• 2005

A series of catalysts, NiO/$CeO_2-ZrO_2/WO_3$, for ethylene dimerization was prepared by the precipitation and impregnation methods. For NiO/$CeO_2-ZrO_2/WO_3$ sample, no diffraction line of nickel oxide was observed up to 40 wt%, indicating good dispersion of nickel oxide on the surface of catalyst. The hexagonal and monoclinic phases of $WO_3$ up to the calcination temperature of 500 ${^{\circ}C}$ were observed, whereas the hexagonal phase of WO3 completely was transformed into monoclinic phase of $WO_3$ at 600 ${^{\circ}C}$ and above. The role of $CeO_2$ in the catalysts was to form a thermally stable solid solution with zirconia and consequently to give high surface area and acidity. The catalytic activities for ethylene dimerization were correlated with the acidity of catalysts measured by the ammonia chemisorption method. 25-NiO/5-$CeO_2-ZrO_2/15-WO_3$ containing 25 wt% NiO, 15 wt% $WO_3$ and 5 mol% $CeO_2$, and calcined at 400 ${^{\circ}C}$ exhibited a maximum catalytic activity due to the effects of $WO_3$ modifying and $CeO_2$ doping.

• Korean Journal of Materials Research
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• v.29 no.9
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• pp.519-524
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• 2019

Spherical $Li_3V_2(PO_4)_3$ (LVP) and carbon-coated LVP with a monoclinic phase for the cathode materials are synthesized by a hydrothermal method using $N_2H_4$ as the reducing agent and saccharose as the carbon source. The results show that single phase monoclinic LVP without impurity phases such as $LiV(P_2O_7)$, $Li(VO)(PO_4)$ and $Li_3(PO_4)$ can be obtained after calcination at $800^{\circ}C$ for 4 h. SEM and TEM images show that the particle sizes are $0.5{\sim}2{\mu}m$ and the thickness of the amorphous carbon layer is approximately 3~4 nm. CV curves for the test cell are recorded in the potential ranges of 3.0~4.3 V and 3.0~4.8 V at a scan rate of $0.01mV\;s^{-1}$ and at room temperature. At potentials between 3.0 and 4.8 V, the third $Li^+$ ions from the carbon-coated LVP can be completely extracted, at voltages close to 4.51 V. The carbon-coated LVP exhibits an initial specific discharge capacity of $118mAh\;g^{-1}$ in the voltage region of 3.0 to 4.3 V at a current rate of 0.2 C. The results indicate that the reducing agent and carbon source can affect the crystal structure and electrochemical properties of the cathode materials.

• Journal of the Mineralogical Society of Korea
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• v.19 no.2 s.48
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• pp.81-88
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• 2006

The Yttria Partially Stabilized Zirconia powder was prepared by spontaneous precipitation method using $ZrOCl_2{\cdot}8H_O-YCl_33{\cdot}6H_2O$ solution as a starting materials. The optimal experimental conditions such as concentration and pH of starting solutions, the amounts of stabilizer $Y_2O_3$ used, and sintered temperature were carefully studied. The best condition for synthesizing $ZrO_2$ was experimentally selected and applied throughout this study for the preparation of the 3 mole% $Y_2O_3$ partially stabilized zirconia, 3YSZ. The physical properties of 3YSZ was examined by XRD, Raman, DT A, and SEM. The structural transition from pure monoclinic high temperature $ZrO_2$ to tetragonal room temperature 3YSZ was made possible by the added amount of $Y_2O_3$ in the $ZrO_2+Y_2O_3$ system. All Raman Spectrum band appeared in the lower wave numbers rather than in higher wave numbers as structure changes from monoclinic to tetragonal.

• Journal of the Korean Ceramic Society
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• v.49 no.5
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• pp.454-460
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• 2012

The nano-sized zirconia powders were synthesized in an impregnation method using pulp and $ZrOCl_2{\cdot}8H_2O$ as an initial material. The synthesized powders were characterized by XRD and FE-SEM. The particle size of the powder was controlled by preparation conditions, such as drying temperature and time. As a result of the various drying and calcination conditions, 30~50 nm sized homogeneous zirconia particles were obtained at $800^{\circ}C$ for 1 h. Crystallization and the rapid growth of particles were accelerated with increasing calcination temperature and time. Tetragonal phase generated below $800^{\circ}C$ were transferred to monoclinic phase with increasing calcination temperature and time. Moreover, above $800^{\circ}C$, heat treatment time had very large influence on the particle growth, and the change of drying condition also had large influence on the growth of a crystal.