Seung-Gon Kim

Interactive Ion-Mediated Sap Flow Regulation in Olive and Laurel Stems: Physicochemical Characteristics of Water Transport via the Pit Structure

Sap water is distributed and utilized through xylem conduits, which are vascular networks of inert pipes important for plant survival. Interestingly, plants can actively regulate water transport using ion-mediated responses and adapt to environmental changes. However, ionic effects on active water transport in vascular plants remain unclear. In this report, the interactive ionic effects on sap transport were systematically investigated for the first time by visualizing the uptake process of ionic solutions of different ion compositions (K+/Ca2+) using synchrotron X-ray and neutron imaging techniques. Ionic solutions with lower K+/Ca2+ ratios induced an increased sap flow rate in stems of Olea europaea L. and Laurus nobilis L. The different ascent rates of ionic solutions depending on K+/Ca2+ ratios at a fixed total concentration increases our understanding of ion-responsiveness in plants from a physicochemical standpoint. Based on these results, effective structural changes in the pit membrane were observed using varying ionic ratios of K+/Ca2+. The formation of electrostatically induced hydrodynamic layers and the ion-responsiveness of hydrogel structures based on Hofmeister series increase our understanding of the mechanism of ion-mediated sap flow control in plants.

Structure-dependent light-responsiveness of chemically linked nanoparticle clusters

Self-assembled colloidal nanoparticle (NP) clusters are one of the bottom-up processing products to construct complex two- and three-dimensional optical materials. We show metal NP clusters chemically linked by a light-sensitive organic ligand to build up nanophotonic hybrid structures. NPs are spheres identical in size with narrow polydispersity, enabling systematic structural modifications. Before randomly aggregating and settling down, the chemical interlinking of NPs behaves like a polymerization procedure, generating characteristic cluster structures with a specific fractal dimension (D). The absorbance in the UV-vis and THz regions is significantly modified depending on the cluster structures. In contrast with physical aggregation, the absorbance of chemically linked NPs can be blue-shifted in the UV-vis region, which is prominently beneficial for solar energy utilization. Applied electron beams modify the cluster structures and induce blue-shifts in UV-vis light absorbance. These types of chemically constructed NP clusters can be a general method for new metamaterial design useful for effective light energy use.

Magnetic Anisotropy in LuFe

Magnetic properties have been investigated for LuFe2O4 single crystal. The temperature dependence of magnetization showed two-step phase transitions at about 160 and 230 K. The magnetization and thermoremanent magnetization in an applied field perpendicular to the c-axis are much smaller than those measured along the c-axis. It indicates that the Fe spins lie parallel to the c-axis with strong anisotropy, and a ferrimagnetic moment is induced along this direction.

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>> This study presents an experiment investigation on natural convection heat transfer of air-cooling Proton exchange membrane fuel cells (PEMFCs) in a enclosure system for unmanned aerial vehicles (UAVs). Considered are replacing fuel cell stack with Aluminum block for heat generating inside a enclosure chamber. The volume ratio of fuel cell stack and chamber for simulation to the actual size of aerial vehicle is 1 to 15. The parameters considered for experimental study are the environmental temperature range from 25°C to -60°C and the block heat input of 10 W, 20 W and 30 W. Effect of the thermal conductivity of the block and power level on heat transfer in the chamber are investigated. Experimental results illustrate the temperature rise at various locations inside the chamber as dependent upon heat input of fuel cell stack and environmental temperature. From the results, dimensionless correlation in natural convection was proposed with Nusselt number and Rayleigh number for designing air-cooling PEMFC powered high altitude long endurance (HALE) UAV.

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Synchrotron X-ray computed tomography was employed to measure the volume variation of gas diffusion layer (GDL) for polymer electrolyte fuel cell (PEFC). The study was conducted using 3D reconstruction (or a 3D rendering software) of orthoslice images. Especially, the temporal variation of structural parameters such as porosity was measured under freeze and thaw cycles. The freezing and thawing cycle were generated using a CRYO-system and an external light source alternatively. As a result, the structural parameters such varied significantly between the successive freeze/thaw cycles and irreversible change was also observed.Copyright

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The objective of this study is to elucidate the feasibility of synchrotron X-ray micro CT as a non-destructive imaging method to visualize the three-dimensional morphological structures of biological and non-biological samples. The experiments were conducted in 7B2 X-ray micro CT beamline in Pohang Accelerate Laboratory (PAL). A rotational 3-axis stage was specially designed for scanning of test samples. Preliminary tests were performed for opaque samples including a mosquito head, a plant seed and gas diffusion layer (GDL) of polymer electrolyte fuel cell to verify the feasibility of the X-ray micro CT. It visualized clearly the internal structure of all the test samples, supporting its usefulness.

Single Crystal

Water management in a polymer electrolyte fuel cell (PEFC) was experimentally investigated using an X-ray microscopy technique. Recently, fuel cell has been receiving large attention as an important renewable energy due to its efficiency, clearness and sustainability. Among various types of fuel cells, PEFC can be used as a power source of transport vehicles and home applications. In recent commercial development of PEFC, water management is one of the major problems to be solved. In fact, proper water management is vital to enhance performance and durability of PEFC. In this study, transport of water inside MEA (membrane electrode assembly) and GDL (gas diffusion layer) layers of an operating (in situ) fuel cell was observed using the synchrotron X-ray micro-imaging technique. As the synchrotron X-ray imaging technique has very high spatial and temporal resolutions, it is suitable for observing the dynamic movement and behavior of liquid layer and water distribution inside the PEFC. For this X-ray micro-imaging experiment, a single cell test kit of PEFC was specially designed for convenient capturing of X-ray images. Temporal variation of gray level in the PEFC components, such as MEA, GDL and endplate, was investigated with varying loading condition. As a result, X-ray images of the PEFC components were clearly distinguished by image pattern and gray level difference. The gray level shows roughly symmetric distribution with respect to MEA layer. The gray level at GDL decreases with lapse of time, indicating the increase of H2 O concentration with time.Copyright