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Kim, Yun Hyeok,Choi, Gwang-Mun,Shin, Dahye,Kim, Yong Ho,Jang, Dongchan,Bae, Byeong-Soo American Chemical Society 2018 ACS APPLIED MATERIALS & INTERFACES Vol.10 No.49
<P>Any transition toward an era of flexible electronics will have to overcome the mechanical limitations of materials. Specifically, the attainment of both strength and flexibility, which are generally mutually exclusive, is required including glass-like wear resistance, plastic-like compliance, and a high level of strain. Here, we fabricate a urethane-methacrylate-siloxane hybrid (UMSH) material. It is found that UMSH, with molecule-level hybridization of urethane linkage and methacrylate-siloxane conetworks, demonstrates ceramic-like high strength (574 MPa), yet polymer-like low modulus (8.42 GPa), and even high strain (6.3%) at fracture with excellent optical transparency. This combination of high strength, flexibility, and optical transparency indicates that this is a suitable material for glass substitution and can be used as a transparent flexible cover window for foldable display.</P> [FIG OMISSION]</BR>
Dongchan Kim,Sungkyun Cho 국제무예학회 2020 International Journal of Martial Arts Vol.6 No.-
Objective: This study was conducted to determine the capacity of taekwondo as a leisure activity to reduce the acculturative stress of U.S. soldiers stationed in the Republic of Korea (ROK; South Korea) and to improve their working efficiency. An additional, goal was to determine whether taekwondo was an appropriate activity for the Korean head-start program by investigating its effectiveness in minimizing U.S. soldiers’ acculturative stress and maximizing their life satisfaction in South Korea. Methods: To achieve those goals, the researchers surveyed 200 U.S. soldiers while they were stationed in South Korea, using convenience sampling. Among the collected questionnaires, the researchers discarded 23 with unusable or corrupted responses, leaving 177 questionnaires for data analysis. To acquire the results, frequency analysis, confirmatory factor analysis, correlation analysis, and multi-regression analyses were performed using SPSS ver. 23 and AMOS 21.0. Results: The results indicate that taekwondo training satisfaction is a negative predictor of acculturative stress and a positive predictor of life satisfaction, and that acculturative stress is a negative predictor of life satisfaction. Conclusion: The findings are discussed in light of previous works, along with ways to improve the taekwondo program as a leisure activity to reduce acculturative stress.
Performance of a Two-Phase Injection Heat Pump with the Variation of Injection Quality and Pressure
Kim, Bo Gyeom,Kim, Dongwoo,Lee, Dongchan,Kim, Yongchan World Scientific Publishing Company 2017 International Journal of Air-Conditioning and Refr Vol.25 No.2
<P>Heat pumps are becoming more commonly and widely used due to their advantages in efficiency and versatile applications in both cooling and heating. However, the reliability of its compressor is being compromised due to high discharge temperatures caused by harsh operating conditions. Among the numerous studies conducted in improving the discharge temperature and performance of the heat pump, two-phase injection is receiving great interest. In this study, the performance of the two-phase injection heat pump was measured with a variation of operating parameters. The experiment results were used to study the performance change according to the operating parameter such as injection quality and pressure. It was found that the performance of the heat pump using the two-phase injection was better in terms of COP by up to 5.8% compared to that using the vapor injection. Furthermore, the two-phase injection was effective in reducing the discharge temperature to improve the compressor reliability. Two types of optimum injection qualities were suggested through this study: discharge temperature optimum injection quality (TIQ), and performance optimum injection quality (PIQ). It was found that in most conditions PIQ was sufficient in satisfying the compressor reliability, but in harsh operating conditions, where the compression ratio was high, TIQ was required in order to maintain the compressor reliability.</P>
Kim, Dongwoo,Lee, DongChan,Jang, Dong Soo,Jeon, Yongseok,Kim, Yongchan Elsevier 2018 Applied thermal engineering Vol.132 No.-
<P><B>Abstract</B></P> <P>In this study, the flow boiling heat transfer characteristics of R-1234ze(E) and R-134a in plate heat exchangers with different Chevron angles are measured and analyzed as a function of the mass flux, saturation temperature, vapor quality, and heat flux. The effect of the mass flux on the heat transfer and pressure drop of R-1234ze(E) is substantial. The heat transfer coefficient of R-1234ze(E) for a Chevron angle of 60° is approximately 3.7 times higher than that for a Chevron angle of 30° at high vapor qualities owing to the intensified turbulent flow. Moreover, for a Chevron angle of 60°, the average heat transfer coefficient of R-1234ze(E) is on average 4.7% higher than that of R-134a due to its higher equivalent Reynolds number. However, the average pressure drop of R-1234ze(E) is higher than that of R-134a owing to the lower vapor density of R-1234ze(E). Finally, the correlations for the heat transfer and pressure drop of R-1234ze(E) are developed in the plate heat exchangers with different Chevron angles.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The flow boiling heat transfer characteristics of R-1234ze(E) and R-134a in plate heat exchangers are measured and analyzed. </LI> <LI> The heat transfer coefficients of R-1234ze(E) and R-134a are quite similar at the same mass flux. </LI> <LI> R-1234ze(E) shows higher pressure drop than R-134a owing to the lower vapor density. </LI> <LI> The correlations for the heat transfer and pressure drop of R-1234ze(E) are developed considering different Chevron angles. </LI> </UL> </P>
Effect of Rutile/Anatase TiO2 on surface properties of ZnO-Al2O3-CaO-SiO2 glass system
Dongchan Kim,Seunggu Kang,Kangduk Kim 한양대학교 청정에너지연구소 2023 Journal of Ceramic Processing Research Vol.24 No.5
In this study, we investigated the effects of dimorphous nucleating agentsrutile TiO2 and anatase TiO2, on the crystallizationand surface properties of ZnO-Al2O3-CaO-SiO2-based glass. Glass specimens were prepared by substituting rutile TiO2 andanatase TiO2 at 1, 3, and 5 wt.% and subsequently heat treating at 1050 °C for 90 min to induce crystallization. Thecrystallization characteristics and surface properties of the crystallized glass were analyzed using X-ray diffraction (XRD),scanning electron microscopy (SEM), gloss, Fourier-transform infrared (FTIR) spectroscopy, hardness, and colorimetricanalyses. The XRD and SEM analyses revealed the presence of a titanite crystalline phase in the glass matrix. However, theaddition of anatase TiO2 as a nucleating agent resulted in a significant decrease in glossiness. FTIR deconvolution in the rangeof 800-1300 cm−1 showed an increase in the degree of [SiO4] tetrahedral polymerization with higher levels of crystallization. micro-Vickers hardness of the glass specimens increased from the original value of 6.32 GPa to a maximum of 7.05 GPa,indicating enhanced hardness due to crystallization. Colorimetric analysis indicated that the inclusion of rutile TiO2 as anucleating agent increased the Yellow Index of anatase TiO2.
The Biden Doctrine and China’s response
Dongchan Kim 한국외국어대학교 국제지역연구센터 2023 International Area Studies Review Vol.26 No.2
The purpose of this paper is to explain how China is reacting to U.S. foreign policy under President Biden (“The Biden Doctrine”). Using official statements and documents, this paper establishes that “authoritarianism versus democracy” has become the organizing principle of the Biden Doctrine, and that principle is supported by three “pillars”-alliances, multilateralism, and strategic risk reduction. The Biden Doctrine is focused on strengthening alliances and building multilateral partnerships that can provide a competitive edge against China, while also pursuing strategic risk reduction measures to contain competition to non-military areas, even as the United States steps up support for Taiwan. The Biden Doctrine has not been well-received in China, where it is viewed as an attempt at containment, and China has responded with stronger diplomatic rhetoric, increased military activity around Taiwan, strengthened partnerships with U.S. adversaries, and (albeit strained) efforts to maintain good relations with Europe. The paper concludes that the interplay between U.S. and Chinese foreign policies is increasing the risk of a new type of Cold War, especially over geographical flashpoints like Taiwan.
Multi-slice imAGe generation using intra-slice paraLLel imaging and Inter-slice shifting (MAGGULLI)
Kim, Dongchan,Seo, Hyunseok,Oh, Changheun,Han, Yeji,Park, HyunWook IOP 2016 Physics in medicine & biology Vol.61 No.4
<P>For acceleration of imaging time, multi-band imaging techniques (e.g. CAIPIRINHA) use the sensitivity differences of the multi-channel RF coils in the slice selection direction. To more effectively utilize the RF coil characteristics than the conventional multi-band imaging techniques, we propose a new imaging technique, called multi-slice image generation using intra-slice parallel imaging and inter-slice shifting (MAGGULLI). The proposed technique used an inter-slice shifting gradient in slice selection direction to make multi-slice images shift in the frequency encoding direction. Thus, aliasing caused by sub-sampling in the phase encoding direction is orthogonal to that by multi-band imaging with the inter-slice shifting, both of which are resolved by using the sensitivity information of the RF coil. Phantom and <I>in vivo</I> imaging experiments for the acceleration factors up to 10 demonstrate that the quality of the images reconstructed by MAGGULLI are better than that of CAIPIRINHA for high acceleration factors in the qualitative and quantitative analysis.</P>