• Journal of Powder Materials
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• v.22 no.3
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• pp.181-186
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• 2015

In this study, the effect of the friction stir welding (FSW) was compared with that of the gas tungsten arc welding (GTAW) on the microstructure and microhardness of Cu-Ni alloy weldment. The weldment of 10 mm thickness was fabricated by FSW and GTAW, respectively. Both weldments were compared with each other by optical microstructure, microhardness test and grain size measurement. Results of this study suggest that the microhardness decreased from the base metal (BM) to the heat affected zone (HAZ) and increased at fusion zone (FZ) of GTAW and stir zone (SZ) of FSW. the minimum Hv value of both weldment was obtained at HAZ, respectively, which represents the softening zone, whereas Hv value of FSW weldment was little higher than that of GTAW weldment. These phenomena can be explained by the grain size difference between HAZs of each weldment. Grain size was increased at the HAZ during FSW and GTAW. Because FSW is a solid-state joining process obtaining the lower heat-input generated by rotating shoulder than heat generated in the arc of GTAW.

• Journal of Powder Materials
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• v.18 no.6
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• pp.532-537
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• 2011

Thermally stable $TiO_2$/Pt/$SiO_2$ core-shell nanocatalyst has been synthesized by chemical processes. Citrated capped Pt nanoparticles were deposited on amine functionalized silica produced by Stober process. Ultrathin layer of titania was coated on Pt/$SiO_2$ for preventing sintering of the metal nanoparticles at high temperatures. Thermal stability of the metal-oxide hybrid catalyst was demonstrated heating the sample up to $600^{\circ}C$ in air and by investigating the morphology and integrity of the structure by transmission electron spectroscopy. The surface analysis of the constituent elements was performed by X-ray photoemission spectroscopy. The catalytic activity of the hybrid catalysts was investigated by CO oxidation reaction with oxygen as a model reaction.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.9 no.4
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• pp.892-898
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• 2008

3D rapid prototyping is the manufacturing technology to fabricate a prototype with the data stored in a computer, which differs from conventional casting technology in terms of an additive process. Various 3D rapid prototyping techniques such as stereolithograpy. fused deposition modeling. selective laser sintering, laminated object manufacturing have been developed but among them, 3D inkjet printing has a unique feature that materials could be jetted to directly form the body of a prototype, which could be a finished product functionally and structurally. However, this needs ink with a high solid content, which tends to increase the dynamic viscosity of ink. The increase of ink viscositytends to restrict the jettable range of ink and hence the jetting conditions should be optimized. The intrinsic speed of sound in a hot melt ink with ceramic nanoparticles dispersed is one of key components to determine the jettable range of ink. In this paper, the way to measure the intrinsic speed of sound in a hot melt ceramic ink is proposed and its influence on the jetting condition is discussed.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.15 no.6
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• pp.3396-3402
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• 2014

The importance of shape design for strength is highly regarded when applied to thin plate products in Ceramic Injection Molding (CIM), such as cases for electronic goods. This study analyzed the characteristics of the mechanical strength of CIM product by measuring the flexural strength and elastic modulus through a 3-point bending test according to the thickness of a thin plate test piece prepared by CIM. The specimen with a thickness of 0.48mm required a 82.9~94.5N fracture load, whereas a 1.0mm thick test piece required 233.6~345.8N. The increase in thickness by 0.5mm resulted in a 3-fold increase in the fracture load, whereas the elastic modulus decreased by 20%. The thicker the specimen, the lower relative density and surface hardness of the specimen. This is because the thicker the specimen, the lower the powder fraction of the ceramic mixture, and the material properties partially change after sintering.

• Korean Journal of Materials Research
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• v.13 no.5
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• pp.292-296
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• 2003

In an attempt to enhance phase transformation and homogenization of Mn-doped $FeSi_2$, mechanical alloying of elemental powders was applied. Cold pressing and sintering in vacuum were carried out to produce a dense microstructure, and then isothermal annealing was employed to induce a phase transformation to the $\beta$-$FeSi_2$semiconductor. Phase transitions in this alloy system during the process were investigated by using XRD, EDS and SEM. As-milled powders after 100 h of milling were shown to be metastable state. As-sintered iron silicides consisted of untransformed mixture of $\alpha$-$Fe_2$$Si_{5}$and $\varepsilon$-FeSi phases. $\beta$-$FeSi_2$phase transformation was induced by subsequent isothermal annealing at $830^{\circ}C$, and near single phase of $\beta$-$FeSi_2$was obtained after 24 h of annealing. Thermoelectric properties in terms of Seebeck coefficient, and electrical conductivity were evaluated and correlated with phase transformation. Seebeck coefficient electrical resistivity and hardness increased with increasing annealing time due to $\beta$ phase transformation.

• Korean Journal of Materials Research
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• v.21 no.4
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• pp.220-224
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• 2011

Zircon has excellent thermal, chemical, and mechanical properties, but it is hard to make a dense sintered product because of dissociation during the sintering process. This study analyzes how the addition of $SiO_2$ and $Al_2O_3$ affects the mechanical properties of sintered zircon, particularly in regards to reducing the thermal dissociation and improving the mechanical properties of $ZrSiO_4$. Zircon specimens containing different amounts of $SiO_2$ and $Al_2O_3$ were prepared and sintered to observe how the mechanical properties of $ZrSiO_4$ changed according to the differing amount of $SiO_2$ and $Al_2O_3$. The $ZrSiO_4$ that was used for the starting material was ground by ball mill to an average particle size of 3 ${\mu}m$. The $SiO_2$ and $Al_2O_3$ that was used for additives were ground to an average particle size of 3 ${\mu}m$ and 0.5 ${\mu}m$, respectively. Adding $SiO_2$ resulted in transformation in the liquid phase at high temperatures, which had little effect on suppressing the thermal dissociation but enhanced the mechanical properties of $ZrSiO_4$. When $Al_2O_3$ was added, the mechanical properties of $ZrSiO_4$ decreased due to the formation of pores and abnormal grains in the microstructure of the sintered zircon.

• Korean Journal of Materials Research
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• v.12 no.8
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• pp.600-607
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• 2002

The Ni-Zn synthetic ferrite were acquired from thermally decomposing the metal nitrates Fe(NO$_3$)$_3$.$9H_2$O, Zn($NO_3$)$_2$.$6H_2$O, Ni($NO_3$)$_2$. $6H_2$O, and Cu($NO_3$)$_2$. $3H_2$O at $150^{\circ}C$ for 24 hours and was calcined at $500^{\circ}C$. Each of those was pulverized for 3, 6, 9, and 12 hours in a steel ball mill and was sintered between $700^{\circ}C$ and $1,000^{\circ}C$ for 1 hour, and then their microstructures and electromagnetic properties were examined. We could make the initial specimens chemically bonded in liquid at the temperature as low as $150^{\circ}C$, by using the melting points less than $200^{\circ}C$ of the metal nitrates instead of the mechanical ball milling, then narrowed a distance between the particles into a molecular level, and thus lowed sintering temperature by at least $200^{\circ}C$ to$ 300^{\circ}C$. Their initial permeability was 50 to 400 and their saturation magnetic induction density and coercive force 2,400 G and 0.3 Oe to 0.5 Oe each, which were similar to those of Ni- Zn ferrite synthesized in the conventional process. In the graph of initial permeability vs frequencies, we could observe a $180^{\circ}C$rotation of the magnetic domain, which appears in a broad band of microwave near the resonance frequency.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2004.11a
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• pp.636-639
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• 2004

This paper presents the fabrication and performance evaluation of zinc oxide varistors for the arresters used for station system. ZnO varistors were fabricated with typical ceramic production methods and the structural and electrical characteristics were investigated. All varistors exhibited high density, which were in the range of $5.41{\sim}5.49g/cm^3$. In the electrical properties the reference voltage increased in the range of $4.410{\sim}5.250kV$ with increasing their thickness and the residual voltage exhibited the same trends as the reference voltage. In the long duration current impulse withstand test, E-2 and F-1 samples failed in the two and four shots, respectively, but E-1 and F-2 samples survived 18 shots during the test. Before and after this test, the variation ratio of residual voltage of E-1 and F-2 samples were -0.34% and 0.05%, respectively, which were in the acceptance range of 5%. According to the results of tests, it is thought that if the fabrication process such as insulating coating, sintering condition, and soldering method is improved, these ZnO varistors would be possible to apply to the station class arresters in the new future.

• Journal of the Korean Ceramic Society
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• v.45 no.10
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• pp.579-583
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• 2008

A porous $Al_2O_3$, scaffold coated with tricalcium phosphate(TCP) was fabricated by replica method using polyurethane(PU) foam as a fugitive material. Successive coatings of $Al_2O_3$ and hydroxyapatite(HAp) were applied via dip coating onto polyurethane foam, which has a slender and well interconnected network. A porous structure was obtained after sequentially burning out the foam and then sintering at $1500^{\circ}C$. The HAp phase was changed to TCP phase at high temperature. The scaffold showed excellent interconnected porosity with pore sizes ranging from $300{\sim}700{\mu}m$ in diameter. The inherent well interconnected structural feature of PU foam remained intact in the fabricated porous scaffold, where the PU foam material was entirely replaced by $Al_2O_3$ and TCP through a consecutive layering process. Thickness of the $Al_2O_3$ base and the TCP coating was about $7{\sim}10{\mu}m$ each. The TCP coating was homogeneously dispersed on the surface of the $Al_2O_3$ scaffold.

• Journal of the Korean Ceramic Society
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• v.48 no.1
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• pp.34-39
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• 2011

Systematic study on relationship between celadon coloring and glaze component was conducted by chromaticity analysis and M$\ddot{o}$ssbauer spectroscopic analysis. The chromaticity ($L^*$, $a^*$, $b^*$ values) and M$\ddot{o}$ssbauer analysis results were correlated to the amount of $Fe_2O_3$, $TiO_2$, MnO, and $P_2O_5$, which are the essential factors influencing celadon coloring. According to chromaticity analysis, celadon glaze color belongs to GY group when the addition of $TiO_2$ was 1.4%, whereas the color belongs to BG group when the addition of $TiO_2$ was 0.1%. For the GY group, the colors change from GY to YR with the decrease of brightness as the addition of $TiO_2$, MnO, and $P_2O_5$ increases. According to M$\ddot{o}$ssbauer analysis results, as the amount of divalent iron ion increases, the $a^*$ and $b^*$ values decrease, on the other hand, $L^*$ value increases. The ratio of divalent iron ion produced in reductive sintering process is found to be 80~95% in this study, which induces the increase of $L^*$ values in celadon glaze.