• 한국결정성장학회지
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• 제8권4호
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• pp.634-639
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• 1998

$Y_2O_3$와 $Nb_2O_5$가 첨가된 지르코니아에 입자 크기가 다른 두종류의 Al2O3를 부피비로 10~30% 첨가한 복합체를 155$0^{\circ}C$ 온도에서 5시간 소결하여 $250^{\circ}C$ 온도의 공기 분위기하에서 1000시간 혹은 $180^{\circ}C$ 온도의 0.3MPa H2O vapor pressure 분위기하에서 5시간 열처리를 하여 X-ray 회절분석 방법을 이용하여 저온 상안정성을 조사하였다. 복합체 전부 열처리 후에 정방정 상에서 단사정 상으로 상전이가 발생하지 않는 우수한 저온 상안정성이 관찰되었다. 복합체의 상아정성에 Nb 첨가가 $Al_2O_3$의 첨가보다 더 효과적이었다. 2.8$\mu$m 입자크기를 가진 Al2O3를 부피비로 20% 첨가한 복합체에서 최적의 조건이 관찰되었으며 강도와 인성값은 각각 670MPa와 7.1MPam1/2이었다. Al2O3 입자 크기가 2.8$\mu$m에서 0.2$\mu$m로 감소함에 따라 강도는 다소 증가하였으나 파괴인성값은 감소하였다.

• Bulletin of the Korean Chemical Society
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• 제13권2호
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• pp.122-127
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• 1992

Some perovskites $A($Cu_{1}3}Nb_{2}3}$)O_3(A=$Sr^{2+}$$, $Ba^{2+}$ and $Pb^{2+}$) and their series of mixed perovskites have been prepared by solid state reaction. Single perovskite phase was obtained in Sr or Ba rich samples, but pyrochlore phase was found in Pb rich samples. The stability of perovskite phase is dependent on the ionicity of bonding as well as the tolerance factor. All the obtained perovskites have tetragonal symmetry distorted by Jahn-Teller effect of $Cu^{2+}$. In the case of $Sr(Cu_{1}3}Nb_{2}3})O_3$, some superlattice lines caused by threefold enlarging of fundamental unit cell were observed. And, the symmetry of B site octahedron and the bonding character of B-O bond have been studied by IR, ESR and diffuse reflection spectroscopy. It appeared that the symmetry and the bonding character are influenced by such factors as the size and the basicity of A cation.

• 한국전기전자재료학회논문지
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• 제27권1호
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• pp.27-32
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• 2014

In this paper, PZT piezoelectric ceramic specimens with 4 compositions (Zr/Ti=50/50, 53/47, 56/44, 58/42) in $Pb(Zr,Ti)O_3$ system were fabricated. We studied effects of poling strength and thermal aging on the temperature characteristics of eletromechanical coupling factor k31 of the specimens, which were poled with the DC electric fields, 1.5, 2.5 and 3.5 kV/mm respectively and thermally aged for an hour at $200^{\circ}C$. The eletromechanical coupling factor k31 of the specimen with the composition Zr/Ti= 53/47, nearest to the morphotropic phase boundary decreased the most greatly, irrelevant to the intensity of poling field, due to 1st thermal aging. And the temperature coefficient of eletromechanical coupling factor k31 was (-) in the tetragonal phase composition and (+) in the rhombohedral phase composition, which is reverse in the temperature coefficient of resonance frequency. It is interesting that eletromechanical coupling factor k31 of PZT ceramics is shown to be able to increase as temperature increase in the interval $-20{\sim}80^{\circ}C$.

• 한국재료학회지
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• 제23권10호
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• pp.543-549
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• 2013

The purpose of this paper was to investigate the effect of a high-energy milling (HEM) process on the particle morphology and the correlation between a thermal treatment and tetragonal/monoclinic nanostructured zirconia powders obtained by a precipitation process. To eliminate chloride residue ions from hydrous zirconia, a modified washing method was used. It was found that the used washing method was effective in removing the chloride from the precipitated gel. In order to investigate the effect of a pre-milling process on the particle morphology of the precipitate, dried $Zr(OH)_4$ was milled using a HEM machine with distilled water. The particle size of the $Zr(OH)_4$ powder exposed to HEM reduced to 100~150 nm, whereas that of fresh $Zr(OH)_4$ powder without a pre-milling process had a large and irregular size of 100 nm~1.5 ${\mu}m$. Additionally, modified heat treatment process was proposed to achieve nano-sized zirconia having a pure monoclinic phase. It was evident that two-step calcining process was effective in perfectly eliminating the tetragonal phase, having a small average particle of ~100 nm with good uniformity compared to the sample calcined by a single-step process, showing a large average particle size of ~300 nm with an irregular particle shape and a broad particle size distribution. The modified method is considered to be a promising process for nano-sized zirconia having a fully monoclinic phase.

• 대한치과기공학회지
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• 제33권1호
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• pp.15-23
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• 2011

Purpose: Thy yttria tetragonal zirconia polycrystalline(Y-TZP) is a good structural ceramic for dental restoration. But it have a problem that delamination of veneering ceramic from the Y-TZP core materials. The problem generally occur at the interface, thus this study was conducted to evaluate the interface of Y-TZP using scanning electron microscopy(SEM). Methods: To investigate this aspect, high-resolution SEM observations were made of polished and etched (HF content gel) cross-sections of the interface area. Dry and moist veneering porcelain powders were built up on the zirconia base. Results: The extent of this surface faceting is dependent upon the moisture content of the porcelain powder and the firing temperature. More moisture and higher final heating temperature accelerates the observed faceting of the Y-TZP grains at the interface to the veneering ceramic. Conclusion: These changes of the Y-TZP grains indicate that destabilization of the tetragonal phase of zirconia occurs at the interface during veneering with ceramic. It may result in a reduction of the stability of the zirconia and interface.

• 한국전기전자재료학회논문지
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• 제13권8호
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• pp.675-680
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• 2000

In this study the temperature coefficient of resonant frequency(TC $F_{r}$) dielectric and piezoelectric properties of Pb[(S $b_{1}$2//N $b_{1}$2/)$_{0.065}$)-(Z $r_{x}$, $Ti_{1-x}$ )$_{0.90}$] $O_3$ceramics were investigated with Zr/Ti ratio. The compositions near the morphotropic phase boundary (MPB) appeared when Zr/Ti ratio was 49.5/50.5 The dielectric constant and electromechanical coupling factor( $k_{p}$) also showed the highest values of 1,257, 0.653 respectively when the Zr/Ti ratio was 49.5/50.5 Moreover the mechanical quality factor( $Q_{m}$) showed th lowest value of 713 when the Zr/Ti ratio. The temperature coefficient of resonant frequency(TC $F_{r}$) abruptly change at the morphotropic phase boundary(MPB) which existed between the rhombohedral phase with highly negative TC $F_{r}$ of -106ppm/$^{\circ}C$ and the tetragonal phase with highly positive TC $F_{r}$ of +64pp $m^{\circ}C$ as Zr/Ti ratio varied from 50/50 to 49.5/50.5.50.5..50.5.

• Transactions on Electrical and Electronic Materials
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• 제14권2호
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• pp.78-81
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• 2013

$0.95(Na_{0.5}K_{0.5})NbO_3-0.05BaTiO_3+0.2wt%\;Ag_2O$ (hereafter, No excess NKN) ceramics and $0.95(Na_{0.5}K_{0.5})NbO_3-0.05BaTiO_3+0.2wt%\;Ag_2O$ with excess $(Na_{0.5}K_{0.5})NbO_3$ (hereafter, Excess NKN) were fabricated by the conventional solid state sintering method, and their phase transition properties and dielectric properties were investigated. The crystalline structure of No excess NKN ceramics and Excess NKN ceramics were shown characteristics of polymorphic phase transition (hereafter, PPT), especially shift from the orthorhombic to tetragonal phase by increasing sintering temperature range from $1,100^{\circ}C$ to $1,200^{\circ}C$. Also, the temperature coefficient of capacitance (hereafter, TCC) of No excess NKN ceramics and Excess NKN ceramics from $-40^{\circ}C$ to $100^{\circ}C$ was measured to evaluate temperature stability for applications in cold regions. The TCC of No excess NKN and Excess NKN ceramics showed positive TCC characteristics at a temperature range from $-40^{\circ}C$ to $100^{\circ}C$. Especially, Excess NKN showed a smaller TCC gradient than those of Excess NKN ceramics in range from $-40^{\circ}C$ to $100^{\circ}C$. Therefore, NKN piezoelectric ceramics combined with temperature compensated capacitor having negative temperature characteristics is desired for usage in cold regions.

• 한국결정성장학회지
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• 제10권3호
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• pp.233-238
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• 2000

생체적합성과 상 안정성이 우수한 t-$ZrO_2/Al_2O_3$복합체에 $Eu_2O_3$을 0~4 mol%첨가한 복합체를 $1600^{\circ}C$ 온도에서 1 시간 소결하여 상 안정성, 색상변화, 기계적 특성을 조사하였다. $180^{\circ}C$, 3.5MPa의 수증기 압력 하에서 20시간 열처리 후 X-선 회절법으로 관찰한 결과 $Eu_2O_3$가 첨가된 (Y,Nb,Eu)-TZP/$Al_2O_3$복합체는 우수한 상 안정성이 유지되었다. $Eu_2O_3$의 양이 증가함에 따라 백색계통의 상아색에서 미색의 핑크색으로 서서히 변화하였다 $Eu_2O_3$가 3 mol%까지 첨가된 복합체의 경우 620MPa, 7.6MPa.$m^{1/2}$ 이상의 강도와 인성값이 관찰되었다.

• 한국재료학회지
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• 제23권6호
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• pp.329-333
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• 2013

The structural phase transformations of $0.7Pb(Mg_{1/3}Nb_{2/3})O_3-0.3PbTiO_3$ (PMN-0.3PT) were studied using high resolution x-ray diffraction (HRXRD) as a function of temperature and electric field. A phase transformational sequence of cubic (C)${\rightarrow}$tetragonal (T)${\rightarrow}$rhombohedral (R) phase was observed in zero-field-cooled conditions; and a $C{\rightarrow}T{\rightarrow}$monoclinic $(M_C){\rightarrow}$ monoclinic ($M_A$) phase was observed in the field-cooled conditions. The transformation of T to $M_A$ phase was realized through an intermediate $M_C$ phase. The results also represent conclusive and direct evidence of a $M_C$ to $M_A$ phase transformation in field-cooled conditions. Beginning from the zero-field-cooled condition, a $R{\rightarrow}M_A{\rightarrow}M_C{\rightarrow}T$ phase transformational sequence was found with an increasing electric field at a fixed temperature. Upon removal of the field, the $M_A$ phase was stable at room temperature. With increasing the field, the transformation temperature from T to $M_C$ and from $M_C$ to $M_A$ phase decreased, and the phase stability ranges of both T and $M_C$ phases increased. Upon removal of the field, the phase transformation from R to $M_A$ phase was irreversible, but from $M_A$ to $M_C$ was reversible, which means that $M_A$ is the dominant phase under the electric field. In the M phase region, the results confirmed that lattice parameters and tilt angles were weakly temperature dependent over the range of investigated temperatures.

• Bulletin of the Korean Chemical Society
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• 제28권8호
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• pp.1265-1272
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• 2007

A solid acid catalyst, NiSO4/CeO2-ZrO2 was prepared simply by promoting ZrO2 with CeO2 and supporting nickel sulfate on CeO2-ZrO2. The support of NiSO4 on ZrO2 shifted the phase transition of ZrO2 from amorphous to tetragonal to higher temperatures because of the interaction between NiSO4 and ZrO2. The surface area of 10-NiSO4/1-CeO2-ZrO2 promoted with CeO2 and calcined at 600 oC was very high (83 m2/g) compared to that of unpromoted 10-NiSO4/ZrO2 (45 m2/g). This high surface area of 10-NiSO4/1-CeO2-ZrO2 was due to the promoting effect of CeO2 which makes zirconia a stable tetragonal phase as confirmed by XRD. The role of CeO2 was to form a thermally stable solid solution with zirconia and consequently to give high surface area and acidity of the sample, and high thermal stability of the surface sulfate species. 10-NiSO4/1- CeO2-ZrO2 containing 1 mol% CeO2 and 10 wt% NiSO4, and calcined at 600 oC exhibited maximum catalytic activities for both reactions, 2-propanol dehydration and cumene dealkylation.