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( Hyung Seok Shim ),( Nong Moon Hwang ) 대한금속재료학회(구 대한금속학회) 2014 대한금속·재료학회지 Vol.52 No.9
The mystery of Goss selective abnormal grain growth (AGG) in Fe-3%Si steel has long remained unsolved since its discovery by Goss in 1933. Similarly, the mechanism of secondary recrystallization, which is commonly observed in many metallic systems, has long been a puzzle. Here, attempts to solve this puzzle are reviewed with a focus on solid-state wetting along the triple junction of polycrystalline metals. Especially, the grains with sub-boundaries of very low energy exclusively have a high probability to grow by solid-state wetting and as a result can grow abnormally. The 3-dimensional Monte Carlo (MC) and phase field model (PFM) computer simulations showed that the presence of sub-boundaries induces AGG. The existence of sub-boundaries exclusively in abnormally-growing grains was experimentally confirmed in Fe-3%Si steel and in 5052 Al alloy. The time evolution of abnormally growing grains by ex-situ observation and electron backscattered diffraction (EBSD) analysis of 5052 Al alloy clearly revealed many microstructural evidences for AGG by triple-junction wetting. Misorientation angle measurements by EBSD of the grains belonging to the penetrating morphology showed that the penetrated grain boundaries have high energy and the penetrating grain boundaries tend to have low energy, in agreement with the wetting condition along the triple junction. Parallel three-dimensional MC simulations starting with the experimental misorientationdata, which consider sub-boundaries and precipitates, show many realistic AGG features observed experimentally. (Received July 28, 2013)
New mechanism of thin film growth by charged clusters
Hwang, Nong-Moon,Kim, Doh-Yeon The Korea Association of Crystal Growth 1999 韓國結晶成長學會誌 Vol.9 No.3
The charged clusters or particles, which contain hundreds to thousands of atoms or even more, are suggested to from in the gas phase in the thin film processes such as CVD, thermal evaporation, laser ablation, and flame deposition. All of these processes are also phase synthesis of the nanoparticels. Ion-induced or photo-induced nucleation is the main mechanism for the formation of these nanoclusters or nanoparticles in the gas phase. Charge clusters can make a dense film because of its self-organizing characteristics while neutral ones make a porous skeletal structure because of its Brownian coagulation. The charged cluster model can successfully explain the unusual phenomenon of simultaneous deposition and etching taking place in diamond and silicon CVD processes. It also provides a new interpretation on the selective deposition on a conducting material in the CVD process. The epitaxial sticking of the charged clusters on the growing surface is getting difficult as the cluster size increases, resulting in the nanostructure such as cauliflower or granular structures.
Immersive Thinking, Creativity and Mathematics Education
( Nong-moon Hwang ) 한국수학교육학회 2017 수학교육 학술지 Vol.2017 No.2
The speaker is an expert in educating and analyzing concepts in relation to immersive thinking, which can also be called thinking in `flow` suggested by Csikszentmihalyi. In his talk, he will share his unique experience of reaching a state of immersive thinking while trying to solve some difficult problem by intentionally thinking on it without stopping for a second for three consecutive days. This state was characterized by perfect concentration, boosted mental abilities, an outburst of amazing ideas, and a feeling of bliss. The experience was very similar to the description of `samadhi` experienced by the priests practicing oriental `zen` meditation. Making the most use of this state, he unravelled many long-due problems in the field of materials science and engineering. He published three books on this special experience which all became best sellers in Korea. In his books, he attempted to elucidate the underlying principles of immersive thinking based on his experience and the recent knowledge of neuroscience. He also attempted to explain the logic behind how immersive thinking leads to creativity. Based on this understanding, he will suggest some educational methods of mathematics for promoting creativity among students.
Charged nanoparticles in thin film and nanostructure growth by chemical vapour deposition
Hwang, Nong-Moon,Lee, Dong-Kwon Institute of Physics [etc.] 2010 Journal of Physics. D, Applied Physics Vol.43 No.48
<P>The critical role of charged nanoclusters and nanoparticles in the growth of thin films and nanostructures by chemical vapour deposition (CVD) is reviewed. Advanced nanoparticle detection techniques have shown that charged gas-phase nuclei tend to be formed under conventional processing conditions of thin films and nanostructures by thermal, hot-wire and plasma CVD. The relation between gas-phase nuclei and thin film and nanostructure growth has not been clearly understood. In this review it will be shown that many films and nanostructures, which have been believed to grow by individual atoms or molecules, actually grow by the building blocks of such charged nuclei. This new growth mechanism was revealed in an attempt to explain many puzzling phenomena involved in the gas-activated diamond CVD process. Therefore, detailed thermodynamic and kinetic analyses will be made to draw the conclusion that the well-known phenomenon of deposition of less stable diamond with simultaneous etching of stable graphite should be an indication of diamond growth exclusively by charged nuclei formed in the gas phase. A similar logic was applied to the phenomenon of simultaneous deposition and etching of silicon, which also leads to the conclusion that silicon films by CVD should grow mainly by the building blocks of charged nuclei. This new mechanism of crystal growth appears to be general in many CVD and some physical vapour deposition (PVD) processes. In plasma CVD, this new mechanism has already been utilized to open a new field of plasma-aided nanofabrication.</P>
Effect of Interface Structures on Densification and Grain Growth during Sintering
Hwang Nong-Moon 한국분말야금학회 2006 한국분말야금학회 학술대회논문집 Vol.2006 No.1
Both densification and grain growth are driven by the reduction of the interfacial area, kinetics of which depends strongly on the interface structure. Abnormal grain coarsening in the system of singular solid/liquid interface such as WC-Co alloys was explained by the growth mechanism of 2-dimensional nucleation. Based on this concept, the marked inhibition of coarsening of WC grains by VC addition can be approached by the increase in the step free energy, which increases the barrier of 2-dimensional nucleation. The activated sintering in tungsten powders can be approached by the interface structure change induced by the addition of a small amount of nickel.
Effect of Crystal Shape on the Grain Growth during Liquid Phase Sintering of Ceramics
Jo, Wook,Hwang, Nong-Moon,Kim, Doh-Yeon The Korean Ceramic Society 2006 한국세라믹학회지 Vol.43 No.11
The equilibrium or growth shape of ceramic materials is classified largely into two categories according to the thermodynamic conditions imposed. One is a polyhedral shape where the surface free energy is anisotropic, and the other a spherical shape where the surface free energy is isotropic. In the case of grains with a polyhedral shape of anisotropic surface free energy, socalled abnormal grain growth usually takes place due to a significant energy barrier for a growth unit to be attached to the crystal surface. In the case of grains with a spherical shape of isotropic surface free energy, however, normal grain growth with a uniform size distribution takes place. In this contribution, the state-of-the-art of our current understanding of the relationship between the crystal shape and the microstructure evolution during the sintering of ceramic materials in the presence of a liquid phase was discussed.