• 한국재료학회지
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• 제21권6호
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• pp.309-313
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• 2011

The power capacitors used as vehicle inverters must have a small size, high capacitance, high voltage, fast response and wide operating temperature. Our thin film capacitor was fabricated by alumina layers as a dielectric material and a metal electrode instead of a liquid electrolyte in an aluminum electrolytic capacitor. We analyzed the micro structures and the electrical properties of the thin film capacitors fabricated by nano-channel alumina and metal electrodes. The metal electrode was filled into the alumina nano-channel by electroless nickel plating with polyethylene glycol and a palladium catalyst. The spherical metals were formed inside the alumina nano pores. The breakdown voltage and leakage current increased by the chemical reaction of the alumina layer and $PdCl_2$ solution. The thickness of the electroless plated nickel layer was 300 nm. We observed the nano pores in the interface between the alumina layer and the metal electrode. The alumina capacitors with nickel electrodes had a capacitance density of 100 $nF/cm^2$, dielectric loss of 0.01, breakdown voltage of 0.7MV/cm and leakage current of $10^4{\mu}A$.

• 한국세라믹학회지
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• 제41권1호
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• pp.19-23
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• 2004

The Hertzian indentation contact damage behavior of porous alumina with controlled pore shape was investigated by experiments. Porous alumina ceramics containing well-defined pore shape, size and distribution were prepared by incorporation of fugitive spherical starch. Porous alumina with isolated pore structure was prepared with porosity range up to 30%. The indentation stress-strain curves of porous alumina were constructed. Elastic modulus and yield stress can be obtained from the stress-strain relationship. Impulse excitation method for the measurement of elastic modulus was also conducted as well as Hertzian indentation and was confirmed as a useful tool to evaluate the elasticity of highly porous ceramics. Elastic modulus of the inter-connected pore structure is more sensitive to porosity than that of the isolated pore structure. When the specimen had isolated pore structure, higher yield point was obtained than it had inter-connected pore structure. This study proposed that the elastic modiulus of porous ceramics is strongly related to not only porosity, but also the structure of pore.

• 한국세라믹학회지
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• 제36권6호
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• pp.625-631
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• 1999

Ceramic tapes were fabricated by ultrasonically spraying slurries of monodispersed spherical and alumina powders. Effects of slurry compositions on tape forming were investigated. A relatively fast rate of solvent evaporation caused pores and cracks to be formed. A good chemical affinity between solvent and binder gave rise to binder separation resulting in inhomogeneous distribution of binder. Defect-free silica tapes with uniform distribution of particle packing and the binder were obtained from the solvent having a low chemical affinity and a slow evaporation rate and containing appropriate amounts of the binder and the plasticizer. Tape thickness could be controlled by adjusting solids loadings and slurry feed rates. It was possible to fabricate a tape in 15 $\mu\textrm{m}$ thickness from 7 vol% alumina slurry.

• 한국생산제조학회지
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• 제24권4호
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• pp.368-373
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• 2015

Porous alumina ceramics with two different pore sizes were fabricated using hollow microspheres as the pore-forming agent. The relative density, total porosity, and microstructure of the obtained alumina ceramics were studied. It was found that the total porosity of sintered samples with different amounts of hollow microsphere content, from 2.0 to 4.0 wt%, was 69.3-75.6%. The interconnected and spherical cell morphology was obtained with 3.0 wt% hollow microsphere content. The resulting ceramics consist of a hierarchical structure with large-sized cells, and small-sized pores in the cell walls. Moreover, the compressive strength of the sintered samples varied from 8.3-11.5 MPa, corresponding to hollow microsphere contents of 2.0-4.0 wt%.

• 한국세라믹학회지
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• 제25권3호
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• pp.284-292
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• 1988

Alumina-zirconia composite powders for the purpose of improving fracture toughness and thermal shock resistance of alumina were prepared by the emulsion-kerosene drying method. The average particle size of composite powders was less then 1 $\mu\textrm{m}$ and their shapes were spherical. It was shown that the average particle size of composite powders decreased with the concentration of metal-salt in solution and the amount of span 80 added when preparing emulsions. The structure of all zirconia in composite powders heat-treated at 1200$^{\circ}C$ was a tetragonal form at room temperature. This result implied that fine zirconia particles were homogeneously dispersed in the alumina matrix.

• 한국세라믹학회:학술대회논문집
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• 한국세라믹학회 2002년도 추계총회 및 연구발표회 초록집
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• pp.136.1-136
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• 2002

• 한국재료학회지
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• 제25권6호
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• pp.300-304
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• 2015

Nanorod ZnO and spherical nano ZnO for gas sensors were prepared by hydrothermal reaction method and hydrazine method, respectively. The nano-ZnO gas sensors were fabricated by a screen printing method on alumina substrates. The gas sensing properties were investigated for hydrocarbon gas. The effects of Co concentration on the structural and morphological properties of the nano ZnO:Co were investigated by X-ray diffraction and scanning electron microscope (SEM), respectively. XRD patterns revealed that nanorod and spherical ZnO:Co with a wurtzite structure were grown with (100), (002), (101) peaks. The sensitivity of nanorod and spherical ZnO:Co sensors was measured for 5 ppm $CH_4$ and $CH_3CH_2CH_3$ gas at room temperature by comparing the resistance in air with that in target gases. The highest sensitivity to the $CH_4$ and $CH_3CH_2CH_3$ gas of spherical nano ZnO:Co sensors was observed at Co 6 wt%. The spherical nano ZnO:Co sensor exhibited a higher sensitivity to hydrocarbon gas than nanorod ZnO.

• 한국세라믹학회지
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• 제27권6호
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• pp.729-734
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• 1990

The microstructural dependence of thermal conductivity of a high-alumina (ca. 70%) heat-insulating frebricks(ca. 75%porosity) was investigated under special consideration of the tailored-pore shape effects. Pores different shape could be incorporated into the insulators through pore formers : Styrofoam produces spherical pores while saw dust results in parallel plate pores. Concerning the pore-shape effectiveness of thermal insulation, the specimen with irregular plate pores showed much lower values of heat conductivity than those with spherical pores, the values being 0.31 to 0.38 at $600^{\circ}C$ and 0.35 to 0.47 at 100$0^{\circ}C$, respectively. On the contrary, however, other material properties such as strength and softening temeprature under load were turned out to be better in the case of the spherical pores.

• 한국재료학회지
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• 제12권11호
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• pp.840-844
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• 2002

A spherical $BaMgAl_{10}$ $O_{17}$ :Eu phosphor has been synthesized by a multi-step precipitation route. In order to successfully synthesize the phosphor with spherical shape, the hydrated-alumina particles should be controlled for spherical shape. In this process, the hydroxypropyl cellulose (HPC) was used as a dispersing reagent. This reagent plays an important role in that the particles were controlled to have the uniform size of sub-micron. The final product prepared by the multi-step precipitation method maintained spherical shape with uniform size of 0.4$\mu\textrm{m}$. It can be seen in X-ray diffraction patterns, formation of the single phase of $BaMgAl_{10}$ $O_{17}$ :Eu phosphor prepared by the multi-step precipitation method at $1350^{\circ}C$. Also, the emission spectra of spherical $BaMgAl_{O}$ $10_{17}$ :Eu phosphor in the present case was compared with those of commercially-available blue phosphor under VUV (Vacuum Ultra Violet) excitation. The luminescence process of the $BaMgAl_{10}$ $O_{17}$ :Eu phosphor is characterized by the $4f^{6}$$5d^1$longrightarrow4f$^{7}$ transition (blue) of the $Eu^{2+}$ ion acting as an activating center and the maximum luminescence intensity was obtained by reduction treatment at 145$0^{\circ}C$.

• 한국세라믹학회지
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• 제32권8호
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• pp.909-914
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• 1995

From alumina powder and TEOS, $\alpha$-Al2O3/SiO2 composite powder for reaction-sintered mullite was synthesized by heterogeneous coagulation and surface coating, and investigated the mullitization reaction and sintering behavor of these powders. In $\alpha$-Al2O3/SiO2 composite powder prepared by heterogeneous coagulation, each alumina particles were surrounded by silica particles of 50~60 nm in size. And the alumina particles in composite powder prepared by surface coating were coated by uniform silica layer with thickness of 50 nm. In both methods, mullitization reaction was completed at 1$650^{\circ}C$ for 3h, and specimen sintered above 145$0^{\circ}C$ was about 95% fo the theoretical relative density. Mullite grains formed from the reaction with composite powders showed spherical shape with a size of 1~2${\mu}{\textrm}{m}$.