• 한국주조공학회지
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• 제35권5호
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• pp.120-127
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• 2015

The effects of Ti, B and Zr on grain refinement and castability were investigated in Al-4wt%Mg-0.9wt%Si-0.3wt%Mn-0.15wt%Fe alloy. Measurement of cooling curve and micro-structure observation were performed to analyze the effects of the addition of minor elements Ti, B and Zr during solidification. The prominence of effect on grain refinement was in increasing order for Ti, Zr and B element. Fine grain size and an increase of the crystallization temperature for ${\alpha}$-Al solution were evident as the amount of addition elements increased in this study. Addition of 0.15wt% Ti was most effective for grain refinement, and the resulting grain size of ${\alpha}$-Al solution for shell mold and steel mold were $72.3{\mu}m$ and $23.5{\mu}m$, respectively. Fluidity and shrinkage tests were perform to evaluate the castability of the alloy. Maximum fluidity length and minimum ratio of micro shrinkage were recorded for 0.15wt% Ti addition due to the effect of the finest grain size.

• 한국세라믹학회지
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• 제37권7호
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• pp.673-680
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• 2000

The effect of excess PbO(0, 1, 2, 4, and 8 mol%) on grain growth and densification of (65)Pb(Mg1/3Nb2/3)O3-(35)PbTiO3 [mol%] ceramics has been investigaetd. With increasing the amount of excess PbO and sintering time, densities of sintered samples decreased gradually. The samples containing less than 1 mol% of PbO showed normal grain growth behavior, however abnormal grain growth was observed to occur in the samples with more than 2 mol% of PbO. In the samples with more than 2 mol% of PbO, the number of abnormal grains decreased and thus the average grain size became smaller with increasing the amount of excess PbO. These results demonstrated that the abnormal grain growth started to occur when a critical amount of excess PbO was added to a (65)Pb(Mg1/3Nb2/3)O3-(35)PbTiO3 sample. Since PMN-PT grains in a liquid matrix were angular, the observed abnormal grain growth was explained to proceed through the two dimensional nucleation process.

• 한국세라믹학회지
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• 제49권4호
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• pp.301-307
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• 2012

SiAlON glasses are silicates or alumino-silicates, containing Mg, Ca, Y or rare earth (RE) ions as modifiers, in which nitrogen atoms substitute for oxygen atoms in the glass network. These glasses are found as intergranular films and at triple point junctions in silicon nitride ceramics and these grain boundary phases affect their fracture behaviour. This paper provides an overview of the preparation of M-SiAlON glasses and outlines the effects of composition on properties. As nitrogen substitutes for oxygen in SiAlON glasses, increases are observed in glass transition temperatures, viscosities, elastic moduli and microhardness. These property changes are compared with known effects of grain boundary glass chemistry in silicon nitride ceramics. Oxide sintering additives provide conditions for liquid phase sintering, reacting with surface silica on the $Si_3N_4$ particles and some of the nitride to form SiAlON liquid phases which on cooling remain as intergranular glasses. Thermal expansion mismatch between the grain boundary glass and the silicon nitride causes residual stresses in the material which can be determined from bulk SiAlON glass properties. The tensile residual stresses in the glass phase increase with increasing Y:Al ratio and this correlates with increasing fracture toughness as a result of easier debonding at the glass/${\beta}-Si_3N_4$ interface.

• 한국세라믹학회지
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• 제29권4호
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• pp.65-65
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• 1992

12Ce-TZP의 소결에 미치는 CaO의 역할에 대하여 연구하였다. CaO의 첨가는 Ce-TZP의 격자결함구조와 확산의 율속종인 cerium과 zirconium 이온의 확산계수를 변화시켜 치밀화를 촉진시킨다. CaO는 또한 소결과정중 결정립 성장을 억제하며, 이는 소결과정중 임계 이동시 입계로부터의 기공의 분리를 방지함과 동시에 기공으로부터 입계로의 공격자의 빠른 확산을 유지시킴으로써 이론밀도까지의 소결을 가능하게 한다. Ce-TZP의 결정립 성장 억제는 calcium의 입계편석에 의한 입계 이동도의 감소에 의하며, calcium의 입계편석을 AES 분석으로 확인하였다.

• 센서학회지
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• 제26권2호
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• pp.122-127
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• 2017

Graphene grown on copper-foil substrates by chemical vapor deposition (CVD) has been attracting interest for sensor applications due to an extraordinary high surface-to-volume ratio and capability of large-scale device fabrication. However, CVD graphene has a polycrystalline structure and a high density of grain boundaries degrading its electrical properties. Recently, processes such as electropolishing for flattening copper substrate has been applied before growth in order to increase the grain size of graphene. In this study, we systemically analyzed the effects of the process condition of electropolishing copper foil on the quality of CVD graphene. We observed that electropolishing process can reduce surface roughness of copper foil, increase the grain size of CVD graphene, and minimize the density of double-layered graphene regions. However, excessive process time can rather increase the copper foil surface roughness and degrade the quality of CVD graphene layers. This work shows that an optimized electropolishing process on copper substrates is critical to obtain high-quality and uniformity CVD graphene which is essential for practical sensor applications.

• 제어로봇시스템학회:학술대회논문집
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• 제어로봇시스템학회 2000년도 제15차 학술회의논문집
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• pp.554-554
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• 2000

To improve mechanical properties of Cu-Al-Ni alloy by the grain refinement, Cu-Al-Ni SMA ribbons were fabricated by melt spinning apparatus. The variations of microstructure, mechanical properties and transformation characteristics with the condition of rapid solidification and annealing time-temperature were investigated in Cu-Al-Ni SMA ribbons. The ribbons fabricated by melt spinning obtained around 1.5nm in width and 50-60${\mu}{\textrm}{m}$ in thickness. With increasing wheel speed in order of 10m/s, 15m/s, 20m/s, 30m/s and 3m/s, the grain size was decreased in order of 10${\mu}{\textrm}{m}$, 6.25${\mu}{\textrm}{m}$, 5.5${\mu}{\textrm}{m}$, 3${\mu}{\textrm}{m}$ and 3${\mu}{\textrm}{m}$. $M_{s}$ and $A_{s}$ temperature were decreased with decreasing grain size. By X-ray diffraction test, ordered $\beta$$_1$ phase was observed in all the SMA ribbons and the volume friction of it was increased with increasing wheel speed. With increasing wheel speed, strain was increased from 4.2% to 5.8% and fracture mode has changed from mixture of intergranular and dimple fracture to mixture of fiber structure and dimple fracture. The grain size of ribbon heat-treated at $600^{\circ}C$ was increased with increasing time. In the heat-treated ribbons at 55$0^{\circ}C$, ${\gamma}$$_2$ phases were observed.d.d.

• Progress in Superconductivity
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• 제9권2호
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• pp.173-176
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• 2008

[ $MgB_2$ ] films grown by hybrid physical chemical vapor deposition under appropriate growth conditions commonly exhibit columnar grain structure. The grain boundaries between adjacent columnar grains have been reported to be good flux pinning centers. In this work, we measured the angular dependence of critical current density ($J_c$) and observed the enhanced flux pinning when an external magnetic field was aligned parallel to the columnar direction. This $J_c$ was almost comparable to the $J_c$ for intrinsic pinning case up to 1 T at low temperatures, indicating that grain boundary pinning is very effective. At high fields, however, $J_c$ decreased rapidly resulting from the fact that the density of flux pinning centers provided by grain boundaries was outnumbered by the flux density.

• 한국주조공학회지
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• 제17권3호
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• pp.302-308
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• 1997

In spite of the fact that magnesium has low density and good machinability, its applications are restricted as a structural engineering material because of the poor strength, ductility, and corrosion resistance of the conventional ingot metallurgy alloys. Such properties can be improved by microstructural refinement via rapid solidification processing. In this study, Mg-5wt%Zn alloys have been produced as continuous strips by the melt overflow technique. In order to evaluate the influence of the cooling rate on the grain refinement and mechanical properties, seven different thickness strips were produced by means of controlling the speed of the cooling wheel. Then the microstructual observations were undertaken with the objective of evaluating the grain refinement as function of the cooling rate. The tremendous increase in hardness of Mg-Zn alloy was mainly due to the refinement of the grain structure by the effect of rapid solidification. The formation of intermetallic phases on the grain boundaries may have a positive effect on the corroion resistance. Therefore, despite competition from many other developments, the rapid solidification process emerges as a valuable method to develop superior and commercially acceptable magnesium alloys.

• 한국분말재료학회지
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• 제5권1호
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• pp.15-21
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• 1998

The effect of $\alpha$/$\beta$ phase on the mechanical properties and contact damage of silicon nitrides $Si_3N_4$) was investigated. Silicon nitride materials were prepared from two starting powders, at selective increasing hot-pressing temperatures to coarsen the microstructures: (i) from relatively coarse $\alpha$-phase powder, essentially equiaxed $\alpha$-$Si_3N_4$ grains, with limited, slow transformation to $\beta$-$Si_3N_4$ grain; (ii) from relatively fine $\alpha$-phase powder, a more rapid transformation to $\beta$-$Si_3N_4$, with attendant grain elongation. The resulting micro-structure thereby provided a spectrum of $\alpha$/$\beta$ phase ratios, grain sizes, and grain shapes. Fracture strength, hardness, and toughness were measured, and contact damage and strength degradation after indentation were investigated by Hertzian indentation using spherical indenter. A brittle to ductile transition in $Si_3N_4$ depended on $\alpha$/$\beta$ phase ratio as well as grain size. Silicon nitride with elongated $\beta$ grains showed a superior, contact damage resistance.

• 한국안전학회지
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• 제17권4호
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• pp.45-51
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• 2002

In this study, the fracture characteristics of carbonized silicon grinding wheels were examined with tensile, compression, impact and bending test. The experiment was performed for the various grinding wheels with grain size #46, #80, and grade H, L, P, and one vitrified bond and one structure No.7. Also the centrifugal fracture rpm of carbonized silicon grinding wheels were measured and compared with the calculated values for the various wheel diameters and thicknesses. The results showed that the fracture tensile strength was $1.5~2.0Kg_f/mm^2$, and it was increased by decreasing grain size and increasing grade. The fracture compression loads were $1,600~3,000Kg_f$, and the inner stress was higher than outer's. And the absorption energy of impact test was 3.3~4.7 J, and it was increased by decreasing grain size but it was not effected by grade. The fracture bending stress was $0.1~0.2Kg_f/mm^2$, and it was increased by decreasing grain size and increasing grade. The centrifugal fracture rpm of carbonized silicon grinding wheel was about 8,500~12,000 and agreed well with the calculated value, and it was increased by decreasing diameter. However, it was almost constant for the reduction of wheel thickness.