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Lee, Hwijong,Kim, Gwansik,Lee, Byunghun,Kim, Jeongmin,Choi, Soon-Mok,Lee, Kyu Hyoung,Lee, Wooyoung Elsevier 2017 Scripta materialia Vol.135 No.-
<P><B>Abstract</B></P> <P>Polycrystalline bulks of Si-content tuned Ge-doped higher manganese silicides (HMSs) were fabricated to elucidate the effects of Si content on the phase formation behavior and thermoelectric properties. The phase formation and electronic transport characteristics of HMSs were significantly dependent on Si content. Improved power factor was obtained at higher Si contents because of an enhanced Seebeck coefficient due to the increase in density of states effective mass, maintaining electrical conductivity. Furthermore, the lattice thermal conductivity was reduced through Si-content tuning, which suppressed the formation of secondary phases. Thus, a maximum <I>ZT</I> of 0.61 at 823K was obtained in MnSi<SUB>1.77</SUB>Ge<SUB>0.027</SUB>.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Secondary Crystal Growth on a Cracked Hydrotalcite-Based Film Synthesized by the Sol–Gel Method
Lee, Wooyoung,Lee, Chan Hyun,Lee, Ki Bong American Chemical Society 2016 Inorganic Chemistry Vol.55 No.9
<P>The sol-gel synthesis method is an attractive technology for the fabrication of ceramic films due to its preparation simplicity and ease of varying the metal composition. However, this technique presents some limitations in relation to the film thickness. Notably, when the film thickness exceeds the critical limit, large tensile stresses occur, resulting in a cracked morphology. In this study, a secondary crystal growth method was introduced as a post-treatment process for Mg/Al hydrotalcite-based films synthesized by the sol-gel method, which typically present a cracked morphology. The cracked hydrotalcite-based film was hydrothermally treated for the secondary growth of hydrotalcite crystals. In the resulting film, hydrotalcite grew with a vertical orientation, and the gaps formed during the sol-gel synthesis were filled with hydrotalcite after the crystal growth. The secondary crystal growth method provides a new solution for cracked ceramic films synthesized by the sol-gel method.</P>
Lee, Eunsongyi,Lee, Jun Min,Koo, Ja Hoon,Lee, Wooyoung,Lee, Taeyoon Elsevier 2010 International journal of hydrogen energy Vol.35 No.13
<P><B>Abstract</B></P><P>We report the fabrication of a novel hydrogen sensor that utilizes the electrical resistance changes in the palladium thin films with nanometer thicknesses. The sensing mechanism is based on transitory absorption of hydrogen atoms into the palladium layer, which leads to the reversible alteration of the electrical resistance. In concentrated hydrogen ambient, the excess hydrogen absorption process leads to mechanical deformation on the surface of the palladium films, corresponding to the phase transition from α-phase to β-phase. The reversible sensing process results in a hysteresis curve for resistive properties, of which the height (sensitivity) could be controlled by manipulating the thickness of the palladium layers. The peel-off phenomena on the surface of the palladium film were suppressed by decreasing the thickness of the film. At the thickness of 20nm, a hysteresis curve of resistance was obtained without any structural change in the palladium thin film. These results provide a significant insight to the fundamental understanding of the relationship between the electrical sensitivity of pure Pd thin films and related structural deformation, which is essential to develop robust H-sensors with high sensibility.</P>
Formation of silicon sheet on a rotating substrate.
Lee, Jaewoo,Lee, Changbum,Kim, Joonsoo,Jang, Bo-Yun,Ahn, Youngsoo,Yoon, Wooyoung American Scientific Publishers 2012 Journal of nanoscience and nanotechnology Vol.12 No.4
<P>A spin casting process to fabricate polycrystalline silicon sheets for use as solar cell wafers is presented and the parameters that control the sheet thickness are investigated. The computational model for the spin casting is proposed in order to understand the melt flow and solidification behaviors in the mold. The effect of the rotating speed of the mold and substrate morphology on the silicon sheets is studied via computer simulations, and the simulation results are compared with the experimental results. The numerical study of the fluidity and solidification behavior of the silicon predicted that the formation of rectangular sheets via spin casting is feasible, and the subsequent experiment confirmed this prediction. Using a square mold, rectangular silicon sheets can be produced under appropriate experimental conditions. Microstructural analyses verified the presence of long columnar structures on the sheets.</P>
Lee Wooyoung,Jang Boyun 한국물리학회 2023 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.83 No.3
While electrical discharge machining (EDM) has been studied as a new wafering process for photovoltaic applications, there was a rare report of success to slice even a single wafer. Here, we have obtained (100)-oriented single crystalline silicon (Si) wafer with a dimension of 25×25×0.5 mm by electrical discharge. A tungsten (W) wire with a diameter of 500 μm and a Si brick with a dimension of 25×25×25 mm were used as electrodes. A specially designed power supply and pulse generator were applied to produce pulsed direct current (DC) with a frequency of several kHz. Typical waveforms of pulsed DC were investigated to achieve proper electrical discharge. Specifcally, the efects of applied voltage and pulse-on time on the process and microstructures of the sliced Si wafer were studied in detail. Applied voltage and pulse-on time determined the process performance such as slicing speed and material loss. They also determined the sliced Si wafer’s quality such as microstructural defects and roughness. In addition, pulse-on time critically determined the wire’s lifetime. Wire movement along the slicing channel and its servo control of z-motion are required to enhance the wafer’s quality as well as process performance.
Numerical simulation of solid liquid interface behavior during continuous strip casting process.
Lee, Changbum,Yoon, Wooyoung,Shin, Seungwon,Lee, Jaewoo,Jang, Bo-Yun,Kim, Joonsoo,Ahn, Youngsoo,Lee, Jinseok American Scientific Publishers 2013 Journal of nanoscience and nanotechnology Vol.13 No.5
<P>A new metal-strip-casting process called continuous strip-casting (CSC) has been developed for making thin metal strips. A numerical simulation model to help understand solid-liquid interface behavior during CSC has been developed and used to identify the solidification morphologies of the strips and to determine the optimum processing conditions. In this study, we used a modified level contour reconstruction method (LCRM) and the sharp interface method to modify interface tracking, and performed a simulation analysis of the CSC process. The effects of process parameters such as heat-transfer coefficient and extrusion velocity on the behavior of the solid-liquid interface were estimated and used to improve the apparatus. A Sn (Tin) plate of dimensions 200 x 50 x 1 mm3 was successfully produced by CSC for a heat-transfer coefficient of 104 W/m2 K and an extrusion velocity of 0.2 m/s.</P>