Papers by Author: W. Hwang

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Authors: Dong Hal Kim, W. Hwang, Hyun Chul Park, W.S. Park
Abstract: The objective of this work was to design Surface Antenna Structure (SAS) and investigate fatigue behavior of SAS that was asymmetric sandwich structure. This term, SAS, indicates that structural surface becomes antenna. Constituent materials were selected considering electrical properties, dielectric constant and tangent loss as well as mechanical properties. For the antenna performance, SSFIP elements inserted into structural layers were designed for satellite communication at a resonant frequency of 12.5 GHz and final demonstration article was 16 x 8 array antenna. In cyclic loading, flexure behavior was investigated by 4-point bending and 4-point bending fatigue test. Fatigue life curve of SAS was obtained. Experimental results were compared with single load level fatigue life prediction equations (SFLPEs) and in good agreement with SFLPEs. SAS concept is the first serious attempt at integration for both antenna and composite engineers.
Authors: Chong Soo Lee, W. Hwang, Hyun Chul Park, Kyung Seop Han
Authors: D.H. Choi, W. Hwang
Abstract: A new calibration method of frictional forces in atomic force microscopy (AFM) is suggested. An angle conversion factor is defined using the relationship between torsional angle and frictional signal. When the factor is measured, the slopes of the torsional angle and the frictional signal as a function of the normal force are used to eliminate the effect of the adhesive force. Moment balance equations on the flat surface and the top edge of a commercial step grating are used to obtain the angle conversion factor. After the factor is obtained from an AFM system, it can be applied to all cantilevers without additional experiments.
Authors: Hong Gun Kim, W. Hwang, Kyung Seop Han
Authors: H.W. Nam, S.W. Jung, W. Hwang, Kyung Seop Han
Authors: Dong Hal Kim, W. Hwang, Hyun Chul Park, Kun Hong Lee
Abstract: Superhydrophobic polytetrafluoroethylene (Teflon®, DuPont) sub-micro and nanostructures were fabricated by the dipping method, based on anodization process in oxalic acid. The polymer sticking phenomenon during the replication creates the sub-microstructures on the negative polytetrafluoroethylene nanostructure replica. This process gives a hierarchical structure with nanostructures on sub-microstructures, which looks like the same structures as lotus leaf and enables commercialization. The diameter and the height of the replicated nano pillars were 40 nm and 40 um respectively. The aspect ratio is approximately 1000. The fabricated surface has a semi-permanent superhydrophobicity, the apparent contact angle of the polytetrafluoroethylene sub-micro and nanostructures is about 160 °, and the sliding angle is less than 1°.
Authors: Y.C. Yang, W. Hwang, Hyun Chul Park, Kyung Seop Han
Authors: D.H. Choi, C.W. Lee, P.S. Lee, J.H. Lee, W. Hwang, Kun Hong Lee, Hyun Chul Park
Abstract: Young’s modulus of nanohoneycomb structures in the vertical direction relative to the pore (generally along the beam length) is measured according to the porosity from bending tests in atomic force microscopy (AFM). The pore diameters of the nanohoneycomb structures are from about 30 to 60 nm. To determine the Young’s modulus of the nanohoneycomb structures, the area moment of inertia of the nanohoneycomb structure is determined according to the arrangement of the pores. The area moment of inertia of the nanohoneycomb structure is found to be affected by the porosity of the nanohoneycomb structures. The Young’s modulus of the nanohoneycomb structures decreases as a function of the porosity in a large range.
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