• Journal of Powder Materials
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• v.17 no.4
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• pp.319-325
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• 2010

Electrical discharge machining (EDM) is an attractive machining technique but it requires electrically conductive ceramic materials. In this study, Alumina matrix composites reinforced with CNTs were fabricated through CNT purification, mixing, compaction and spark plasma sintering (SPS) processes. $Al_2O_3$ nanocomposites with the different CNT concentrations were synthesized. The mechanical and electrical characteristics of $Al_2O_3$/CNTs composites were examined in order to apply the materials to the EDM process. In addition, micro-EDM using wire electrical discharge grinding (WEDG) was conducted under the various EDM parameters to investigate the machining characteristics of machined hole by Field Emission Scanning Electron Microscope (FE-SEM). The results show that $Al_2O_3$/CNTs 10%Vol. was more suitable than the other materials because high conductivity and large discharge energy caused violent sparks resulting in bad machining accuracy and surface quality.

• Journal of the Korean Ceramic Society
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• v.29 no.8
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• pp.667-673
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• 1992

The alumina-zirconia composite powders of core particle ZrO2 coated with Al2O3 were prepared by the hydrolysis-deposition of the mixed aluminum salt solution of Al2(SO4)3-Al(NO3)3-Urea. The effects of hydrolysis reaction and coating parameters on characteristics of coated powders and composites were also investigated. The degree of coating could be estimated from the ratio of tetra-/mono-ZrO2 present at the room temperature after heat-treating coated powders at 120$0^{\circ}C$ and the result of TEM observations. When the content of ZrO2 in the dispersed coating system, the coating time, and the volume ratio of water/solution were 50 mg/g, 180 min, and 5, respectively the coating efficiency was maximum (the ratio of tetra-/mono-ZrO2 was 87/13). The relative densities of coated Al2O3-ZrO2 composites sintered at 1$650^{\circ}C$ for 4 hrs were about 91~98% and the maximum ratio of tetra-/mono-ZrO2 in Al2O3-20wt% ZrO2 composites was 62/38.

• Applied Chemistry for Engineering
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• v.31 no.4
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• pp.390-397
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• 2020

To synthesize non-corrosive metallic gold colored α-Al₂O₃ nanoplate-TiO₂ core-shell pigments with controlled roughness, we systematically checked the morphological variation of the TiO₂ shell with the mole ratio of TiCl₄ and NaOH from 1 : 1 to 1 : 1.5, 1 : 2, 1 : 2.5, 1 : 3, 1 : 3.5, and 1 : 4. The more increased mole ratio of TiCl₄ and NaOH resulted in the smoother TiO₂ shell due to the promoted formation of anatase TiO₂ than that of the rutile one. By the heat-treatment of pigments at 500 ℃, we could improve the adhesiveness between TiO₂ shell and α-Al₂O₃ nanoplates without changing their topology and roughness. In addition, the α-Al₂O₃ nanoplate with the robust TiO₂ by heat-treatment exhibited comparable photo-stability against photo-catalytic degradation by UV exposure compared with the commercially available α-Al₂O₃/TiO₂ lustering pigment.

• Bulletin of the Korean Chemical Society
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• v.17 no.7
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• pp.621-625
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• 1996

A variety of metal oxides were coated by sol-gel process from their metal alkoxides on the ribbons of Co-based and Fe-based amorphous alloys, and the effects of surface oxide coating on the magnetic properties of the alloy are investigated. The core loss is found to be reduced significantly by the oxide coating, the loss reduction becoming more prominent at higher frequencies. The shape of the hystersis loop is also dependent upon the kind of the coated metal oxide. The coatings of MgO, SiO2, MgO·SiO2 and MgO·Al2O3 induce tensile stress into the Fe-based ribbon whereas those of BaO, Al2O3, CaO·Al2O3, SrO·Al2O3 and BaO·Al2O3 induce compressive stress. These results may be explained by the modification of domain structures via magnetoelastic interactions with the shrinkage stress induced by the sol-gel coating.

• Korean Journal of Crystallography
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• v.5 no.2
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• pp.67-77
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• 1994

From the Y3-3xNd3xAl5O12single crystals grown by the Czochralski technique, various types of defects were detected and 1) the reason of opical in homogeneous phases, 2) the mechanism of formation of the iridum metal inclusions accompanying bubbles, and 3) the mechanism of formation of the core and facet were analysed After preparing the wafers of the <111> growth parallel, defscts were observed by the polarising microscope using a photoelasic effect and then some images of stress-birefringence were compared to their etch Pits patterns.

• Korean Journal of Crystallography
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• v.5 no.2
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• pp.51-66
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• 1994

Y3Al5O2 and Nd: Y3Al5012 single crystals were grown by Czochralskl technique. The effectt of pulling rate rotation rate, and doping level of Nd3+ ion on the crystal quality were studied Various types of defects were analysed by photo-elastic effect and chemical etching method Finally, spectroscopic and laser poputies of grown crystal were measured. Optirmum pulling rate for good quality was dependant on the doping level of Nd3+ ion. It was found that the suitable pulling rates for pure Y3Al5O12 for 3.0∼3.5 a/o Nd3+ ion doped Y3Al5012 and for more than 40 a/o Nd3+ ion doped Y3Al5012 were 2∼4mm/hr, 0.6∼0.5mm/hr, and less than 0.4mm/hr respectively. Solid-liquid interface was convex at the rotation rate of 27∼60rpm, and concave at the rotation rate of 80∼100rpm. Growth axis was confired to <111> direction and lattice parameter was measured to 12.017A. Core (211) facets,striations, inclusions of metal particles, dislocations and optical inhonngeneities were detected. Four level laser transition of Nd3+ion in YIAls012 single crystal were identified by the spectroscopic measurements. Laser rod with tam diameter and 63mm length was fabricated from grown Nd3+ Y3Al5012 sin91e crystals. 1.8lJ of lasing threshould and 0.49% of soope efficiency were measured by the Pulsed laser action.

• Journal of Powder Materials
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• v.11 no.6 s.47
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• pp.522-527
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• 2004

Nano sized FeAl intermetallic particles were successfully synthesized by plasma arc discharge pro-cess. The synthesized powders shouted core-shell structures with the particle size of 10-20 nm. The core was metallic FeAl and shell was composed of amorphous $AI_{2}O_{3}\;and\;a\;little\;amount\;of\;metallic\;Fe_{3}O_{4}.$ Because of the difference of Fe and Al vapor pressure during synthesis, the Al contents in the nanoparticles depended on the Al contents of master alloy.

• Journal of Powder Materials
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• v.21 no.5
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• pp.377-381
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• 2014

This work describes the coloration, chemical stability of $SiO_2$ and $SnO_2$-coated blue $CoAl_2O_4$ pigment. The $CoAl_2O_4$, raw materials, were synthesized by a co-precipitation method and coated with silica ($SiO_2$) and tin oxide ($SnO_2$) using sol-gel method, respectively. To study phase and coloration of $CoAl_2O_4$, we prepared nano sized $CoAl_2O_4$ pigments which were coated $SiO_2$ and $SnO_2$ using tetraethylorthosilicate, $Na_2SiO_3$ and $Na_2SiO_3$ as a coating material. To determine the stability of the coated samples and their colloidal solutions under acidic and basic conditions, colloidal nanoparticle solutions with various pH values were prepared and monitored over time. Blue $CoAl_2O_4$ solutions were tuned yellow color under all acidic/basic conditions. On the other hand, the chemical stability of $SiO_2$ and $SnO_2$-coated $CoAl_2O_4$ solution were improved when all samples pH values, respectively. Phase stability under acidic/basic condition of the core-shell type $CoAl_2O_4$ powders were characterized by transmission electron microscope, X-ray diffraction, CIE $L^*a^*b^*$ color parameter measurements.

• Nano
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• v.13 no.11
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• pp.1850125.1-1850125.12
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• 2018

To reduce the imbalance of impedance matching between the magnetic metal nanowires and free space, $Fe/TiO_2$ core/shell nanowire arrays with different diameters were fabricated in the templates of anodic aluminum oxide membranes by electrodeposition. The influences of the microstructure on the microwave absorption properties of the $Fe/TiO_2/Al_2O_3$ composites were studied by the transmission/reflection waveguide method. It was demonstrated experimentally that both the interfacial polarization and the diameter of the $Fe/TiO_2$ core/shell nanowires have critical effects on the microwave absorption properties. We also investigated the angle dependence of the microwave absorption properties. Due to the interfacial polarization and associated relaxation, the $Fe/TiO_2/Al_2O_3$ composites exhibited optimal microwave absorption properties when microwave propagation direction was accordant with the axis of the nanowires. Finally, we managed to obtain an optimal reflection loss of below -10 dB (90% absorption) over 10.2-14.8 GHz, with a thickness of 3.0 mm and the minimum value of -39.4 dB at 11.7 GHz.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.02a
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• pp.373-373
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• 2012

염료 감응형 태양전지는 기존 Si 기반 PN접합 무기 태양전지에 비해서 경제적이다. 하지만 그 에너지 변환 효율은 아직까지 세계 최고 수준이 10%밖에 도달하지 못하였다. 그래서 다양한 방식의 효율개선 연구가 활발히 진행되고 있는 실정이다. 본 연구에서는 진공원자층증착(ALD)를 이용하여 Core-shell 구조의 $TiO_2$층 위에 아주 얇고 균일한 $Al_2O_3$ (알루미나) 산화막을 입혔다. 이를 통해서 염료감응형 태양전지의 에너지 변환 효율을 향상시켰다. 본 연구에서는 진공원자층증착(ALD)기술을 이용한 $Al_2O_3$ (알루미나) 산화막의 증착조건에 따른 염료감응태양전지의 효율 개선 매커니즘에 대해서 고찰하였다.