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- 저자 : Youngsu Kim (검색결과 227 건)
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Kim, Changho,Ko, Youngsu,Kim, Taemin,Yoo, Chan-Sei,Choi, BeomJin,Han, Seung Ho,Jang, YongHo,Kim, Youngho,Kim, Namsu Techno-Press 2018 Smart Structures and Systems, An International Jou Vol.22 No.2
Increasing interest in prognostics and health management has heightened the need for wireless sensor networks (WSN) with efficient power sources. Piezoelectric energy harvesters using Pb(Zr,Ti)O3 (PZT) are one of the candidate power sources for WSNs as they efficiently convert mechanical vibration energy into electrical energy. These types of devices are resonated at a specific frequency, which has a significant impact on the amount of energy harvested, by external vibration. Hence, precise prediction of mechanical deformation including modal analysis of piezoelectric devices is crucial for estimating the energy generated under specific conditions. In this study, an experimental vibrational system capable of controlling a wide range of frequencies and accelerations was designed to generate mechanical vibration for piezoelectric energy harvesters. In conjunction with MATLAB, the system automatically finds the resonance frequency of harvesters. A small accelerometer and non-contact laser displacement sensor are employed to investigate the mechanical deformation of harvesters. Mechanical deformation under various frequencies and accelerations were investigated and analyzed based on data from two types of sensors. The results verify that the proposed system can be employed to carry out vibration experiments for piezoelectric harvesters and measurement of their mechanical deformation.
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Changho Kim,Youngsu Ko,Taemin Kim,유찬세,BeomJin Choi,한승호,YongHo Jang,Youngho Kim,Namsu Kim 국제구조공학회 2018 Smart Structures and Systems, An International Jou Vol.22 No.2
Increasing interest in prognostics and health management has heightened the need for wireless sensor networks (WSN) with efficient power sources. Piezoelectric energy harvesters using Pb(Zr,Ti)O3 (PZT) are one of the candidate power sources for WSNs as they efficiently convert mechanical vibration energy into electrical energy. These types of devices are resonated at a specific frequency, which has a significant impact on the amount of energy harvested, by external vibration. Hence, precise prediction of mechanical deformation including modal analysis of piezoelectric devices is crucial for estimating the energy generated under specific conditions. In this study, an experimental vibrational system capable of controlling a wide range of frequencies and accelerations was designed to generate mechanical vibration for piezoelectric energy harvesters. In conjunction with MATLAB, the system automatically finds the resonance frequency of harvesters. A small accelerometer and non-contact laser displacement sensor are employed to investigate the mechanical deformation of harvesters. Mechanical deformation under various frequencies and accelerations were investigated and analyzed based on data from two types of sensors. The results verify that the proposed system can be employed to carry out vibration experiments for piezoelectric harvesters and measurement of their mechanical deformation.
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Hot roller embossing system equipped with a temperature margin-based controller.
Kim, Seyoung,Son, Youngsu,Lee, Sunghee,Ham, Sangyong,Kim, Byungin American Institute of Physics 2014 Review of scientific instruments Vol.85 No.8
<P>A temperature control system was proposed for hot roller embossing. The roll surface was heated using induction coils and cooled with a circulating chilled water system. The temperature of the roll surface was precisely controlled by a temperature margin-based control algorithm that we developed. Implementation of the control system reduced deviations in the roll surface temperature to less than 2?C. The tight temperature control and the ability to rapidly increase and decrease the roll temperature will allow optimum operating parameters to be developed quickly. The temperature margin-based controller could also be used to optimize the time course of electrical power and shorten the cooling time by choosing an appropriate temperature margin, possibly for limited power consumption. The chiller-equipped heating roll with the proposed control algorithm is expected to decrease the time needed to determine the optimal embossing process.</P>
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A Study on Korean First Year High School Students` Compliment Responses Strategies
Youngsu Kim,Chul Joo Uhm 언어과학회 2014 언어과학연구 Vol.0 No.69
This research investigated how differently Korean first year high school students respond to compliments in Korean and in English depending on L2 proficiency, gender, and addressee`s social status. For this study, 54 students responded to a questionnaire consisting of six questions both in English and in Korean. Through this questionnaire survey, we investigated how differently the participants respond to compliments when they use English and Korean respectively. The results showed that a difference between Korean and English positive responses to compliments was minimal. However, the participants used more ‘accepting’ responses in English than in Korean, and high-level high school learners of English showed more ‘accepting’ responses than low-level high school learners of English. On the other hand, there was only a slight difference between male students and female students in compliment responses, but there was a considerably substantial difference between compliment responses to older people and to their friends, especially in terms of ‘accepting.’ Pedagogical implications related to these results have also been discussed.
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Kim, Minsoo P.,Lee, Youngoh,Hur, Yoon Hyung,Park, Jonghwa,Kim, Jinyoung,Lee, Youngsu,Ahn, Chang Won,Song, Seung Won,Jung, Yeon Sik,Ko, Hyunhyub Elsevier 2018 Nano energy Vol.53 No.-
<P><B>Abstract</B></P> <P>Fluorinated polymers have been widely used in triboelectric sensors, displays, and energy harvesting devices because of their superior electron affinity, which leads to the negative triboelectric materials. While previous reports have shown that the control of dielectric constants of fluorinated polymers can increase the triboelectric output performance, the exact relationship between the molecular structures of fluorinated polymers and the resulting triboelectric properties is still elusive. In this study, we demonstrate that the molecular chain structures of the fluorinated polymers depending on the number of fluorine units, the molecular weight (<I>M</I> <SUB> <I>w</I> </SUB>), and conditions such as spin rate and annealing temperature directly affect the relative dielectric constants of dielectric layers and the triboelectric polarity, which are closely related to the triboelectric output performance. We observe that the polymer chain packing structures result in the increase of the relative dielectric constants, thus leading to the improvement of triboelectric output currents. Among the fluorinated polymers used in this study, a poly (2,2,2-trifluoroethyl methacrylate) polymer with three fluorine units and <I>M</I> <SUB> <I>w</I> </SUB> of ~ 20 kg/mol shows the best triboelectric output performance. Our molecular engineering strategy to control the dielectric constants of fluorinated polymers can be a robust platform for the fundamental studies of triboelectric materials and their applications in diverse energy harvesting and sensing devices.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Synthesized fluorinated polymer with different fluorine chain for the triboelectric properties. </LI> <LI> Demonstrated correlation between polymer chain structures and relative dielectric constants. </LI> <LI> Tuning the dielectric constants can control the triboelectric output performances. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>We present the molecular structure engineering of fluorinated dielectric polymer for enhancement of triboelectric performance, which depends on different number of fluorine units, molecular weight, and process condition. The molecular packing structure affects the change of surface potential as well as the free volume in the polymer matrix, resulting in the control of dielectric constant and final triboelectric performances. Our molecular engineering strategy can be a feasible method to modify the triboelectric output performance and can be extended into diverse energy harvesting and sensing devices.</P> <P>[DISPLAY OMISSION]</P>
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Exploring Factors Affecting Online Self-Regulated English Learning
Youngsu Kim 언어과학회 2019 언어과학연구 Vol.0 No.88
This study aims at examining POLE and ATLE factors affecting the OSEL as well as the relationship between POLE factors and OSEL factors and the relationship between ATLE factors and OSEL factors. Using the data collected from three questionnaires, this study conducted a correlation analysis and regression analysis in order to investigate the relationships and the OSEL-affecting factors. The findings were: first, “acquiring a new perspective,” “applying,” and “drilling and practicing” among POLE factors, and “intrinsic motivation” among ATLE factors revealed positive relationships with all of the OSEL factors; second, “applying” among POLE factors and “intrinsic motivation” among ATLE factors acted as predictable factors of three to five OSEL factors, and “learning strategies” among ATLE factors served as a predicator of two OSEL factors. Based on the findings, pedagogical importance and implications of the current research and the roles of language instructors in an EFL learning setting were discussed.
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Youngsu Kim,Jee Hyun Ma 한국응용언어학회 2015 응용 언어학 Vol.31 No.4
This study aimed to explore the effectiveness of purposeful instruction on listening comprehension (LC), task-specific motivation (TM), and perceived difficulty (PD) through mixed research. Fifty six second-year Korean high school students participated in this study and were assigned to two groups: the purpose group and the non-purpose group. They took 13 LC tests, responded to 13 TM surveys, and checked 13 PD levels over a period of four months. They also participated in written interviews. The results indicated that the purpose group showed greater overall advantages than the non-purpose group in terms of the mean scores of LC tests, the TM levels, and the PD levels. Additionally, the former group showed more positive attitudes toward their tasks with listening purposes in mind, clearly indicating the pedagogical importance and benefits of purposeful teaching and learning. More details are discussed and provided based on the results.
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Kim, YoungSu,Kim, GwangSeok,Lee, SangYul American Scientific Publishers 2011 Journal of Nanoscience and Nanotechnology Vol.11 No.10
<P>In this study, CrZrN films were synthesized by unbalanced magnetron sputtering (UBM) under various N2 partial pressures and their characteristics such as crystalline structure, surface morphology, microstructure and mechanical properties as a function of the N2 partial pressures were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), nanoindentation, wear tests, and corrosion tests. Results revealed that, with increasing the N2 partial pressure from 0.05 to 0.21 Pa, the nitrogen content of the films increased from approximately 40.9 to 53.7 at%, the deposition rate decreased from approximately 100 to 59 nm/min and the surface roughness (Rms value) was increased from approximately 0.57 to 1.79 nm. The Cr37.3-Zr9.0-N53.7 film has the highest hardness, elastic modulus, and plastic deformation resistance of 36 GPa, 380 GPa, and 0.41, respectively. The Cr37.3-Zr9.0-N53.7 film also has the lowest friction coefficient and wear rate of 0.19 and 3.01 (10(-6)m3/Nm) at room temperature. In addition, the potentiodynamic test results showed the corrosion resistance of the CrZrN films became increased significantly and their corrosion current density (i(corr)), corrosion potentials (Ecorr) and corrosion rate decreased with increasing N2 partial pressure.</P>
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