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가스분무공정을 이용한 (AgSbTe<sub>2</sub>)<sub>15</sub>(GeTe)<sub>85</sub> 열전분말의 제조 및 특성평가
김효섭,이진규,구자명,천병선,홍순직,Kim, Hyo-Seob,Lee, Jin-Kyu,Koo, Jar-Myung,Chun, Byong-Sun,Hong, Soon-Jik 한국분말야금학회 2011 한국분말재료학회지 (KPMI) Vol.18 No.5
In this study, p-type $(AgSbTe_2)_{15}(GeTe)_{85}$: TAGS-85 compound powders were prepared by gas atomization process, and then their microstructures and mechanical properties were investigated. The fabricated powders were of spherical shape, had clean surface, and illustrated fine microstructure and homogeneous $AgSbTe_2$ + GeTe solid solution. Powder X-ray diffraction results revealed that the crystal structure of the TAGS-85 sample was single rhombohedral GeTe phase, which with a space group $R_{3m}$. The grain size of the powder particles increased while the micro Vickers hardness decreased with increasing annealing temperature within the range of 573 K and 723 K due to grain growth and loss of Te. In addition, the crystal structure of the powder went through a phase transformation from rhombohedral ($R_{3m}$) at low-temperature to cubic ($F_{m-3m}$) at high-temperature with increasing annealing temperature. The micro Vickers hardness of the as-atomized powder was around 165 Hv, while it decreased gradually to 130 Hv after annealing at 673K, which is still higher than most other fabrication processes.
밀링조건이 사용 후 PDP패널의 유가금속 용출효율에 미치는 영향
김효섭,김찬미,이철희,이성규,홍현선,구자명,홍순직,Kim, Hyo-Seob,Kim, Chan-Mi,Lee, Chul-Hee,Lee, Sung-Kyu,Hong, Hyun-Seon,Koo, Jar-Myung,Hong, Soon-Jik 한국분말야금학회 2013 한국분말재료학회지 (KPMI) Vol.20 No.2
In this study, the microstructure and valuable metals dissolution properties of PDP waste panel powders were investigated as a function of milling parameters such as ball diameter size, milling time, and rotational speed during high-energy milling process. The complete refinement of powder could achieved at the ball diameter size of 5 mm due to sufficient impact energy and the number of collisions. With increasing milling time, the average particle size was rapidly decreased until the first 30 seconds, then decreased gradually about $3{\mu}m$ at 3 minutes and finally, increased with presence of agglomerated particles of $35{\mu}m$ at 5 minutes. Although there was no significant difference on the size of the particle according to the rotational speed from 900 to 1,100 rpm, the total valuable metals dissolution amount was most excellent at 1,100 rpm. As a result, the best milling conditions for maximum dissolving amount of valuable metals (Mg: 375 ppm, Ag 135 ppm, In: 17 ppm) in this research were achieved with 5 mm of ball diameter size, 3min of milling time, and 1,100 rpm of rotational speed.
Water Wave Propagation Equation from Expanded Form of Leibniz Rule
김효섭,장창환 대한토목학회 2013 KSCE JOURNAL OF CIVIL ENGINEERING Vol.17 No.2
The Leibniz integral rule for the first derivative with variable limits of integration is expanded for the second and the third order derivatives by using the chain rule here. Water wave propagation equation in the x-z plane containing second derivatives is depthintegrated by using the Leibniz rule. The integrated wave equation is then applied to wave transmission and reflection over an inclined step, and the computed reflection coefficients agree well with those from existing theories such as full linear equation, mild slope equation, and modified mild slope equation.
자기펄스 성형법에 의한 TiO<sub>2</sub> 나노 분말의 치밀화
김효섭,이정구,이창규,구자명,홍순직,Kim, Hyo-Seob,Lee, Jeong-Goo,Rhee, Chang-Kyu,Koo, Jar-Myung,Hong, Soon-Jik 한국재료학회 2008 한국재료학회지 Vol.18 No.8
In this research, fine-structure TiO2 bulks were fabricated in a combined application of magnetic pulsed compaction (MPC) and subsequent sintering and their densification behavior was investigated. The obtained density of $TiO_2$ bulk prepared via the combined processes increased as the MPC pressure increased from 0.3 to 0.7 GPa. Relatively higher density (88%) in the MPCed specimen at 0.7 GPa was attributed to the decrease of the inter-particle distance of the pre-compacted component. High pressure and rapid compaction using magnetic pulsed compaction reduced the shrinkage rate (about 10% in this case) of the sintered bulks compared to general processing (about 20%). The mixing conditions of PVA, water, and $TiO_2$ nano powder for the compaction of $TiO_2$ nano powder did not affect the density and shrinkage of the sintered bulks due to the high pressure of the MPC.
Vertical Suspended Sediment Diffusivity for Uniform Currents
김효섭,장창환 대한토목학회 2013 KSCE JOURNAL OF CIVIL ENGINEERING Vol.17 No.6
Vertical diffusivity of suspended sediment with wide size distribution is concerned. The sediment size is expressed by φ scale, and the φ values are assumed to be normally distributed. The suspended sediment concentration is divided into two groups, 2% of fine side, and 98% of the rest, and single representative settling velocity for median size is used to calculate the vertical diffusion coefficient of the suspended sediment. The computed distribution of the vertical sediment diffusion coefficient is larger than the distribution of the vertical fluid eddy viscosity. A set of equations is proposed for description of the distribution of the vertical suspended sediment diffusion coefficient for engineering purposes. The new equation improves correlation with a set of measured diffusion coefficient values by Barton and Lin.