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Synthesis of Monoclinic Potassium Niobate Nanowires That Are Stable at Room Temperature
Kim, Seungwook,Lee, Ju-Hyuck,Lee, Jaeyeon,Kim, Sang-Woo,Kim, Myung Hwa,Park, Sungnam,Chung, Haegeun,Kim, Yong-Il,Kim, Woong American Chemical Society 2013 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.135 No.1
<P>We report the synthesis of KNbO<SUB>3</SUB> nanowires (NWs) with a monoclinic phase, a phase not observed in bulk KNbO<SUB>3</SUB> materials. The monoclinic NWs can be synthesized via a hydrothermal method using metallic Nb as a precursor. The NWs are metastable, and thermal treatment at ∼450 °C changed the monoclinic phase into the orthorhombic phase, which is the most stable phase of KNbO<SUB>3</SUB> at room temperature. Furthermore, we fabricated energy-harvesting nanogenerators by vertically aligning the NWs on SrTiO<SUB>3</SUB> substrates. The monoclinic NWs showed significantly better energy conversion characteristics than orthorhombic NWs. Moreover, the frequency-doubling efficiency of the monoclinic NWs was ∼3 times higher than that of orthorhombic NWs. This work may contribute to the synthesis of materials with new crystalline structures and hence improve the properties of the materials for various applications.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jacsat/2013/jacsat.2013.135.issue-1/ja308209m/production/images/medium/ja-2012-08209m_0005.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/ja308209m'>ACS Electronic Supporting Info</A></P>
Impact of cracked gas diffusion layer on performance of polymer electrolyte membrane fuel cells
Geon Hwi Kim,Dasol Kim,Jaeyeon Kim,Hyeok Kim,Taehyun Park 한국공업화학회 2020 Journal of Industrial and Engineering Chemistry Vol.91 No.-
This paper reports the negative effect of the cracked gas diffusion layer (GDL) on the performance ofpolymer electrolyte membrane fuel cells (PEMFCs) under varyingflow channel width. The polarizationcharacteristics and the corresponding electrochemical impedance spectra of the PEMFCs with pristineGDLs and cracked GDLs were computed. The results showed that for PEMFCs with 1.0 mmflow channels,the power density rates of reduction between the PEMFCs with pristine GDLs and those with crackedGDLs were 2.8% under the condition of oxygen and 6.6% under the condition of air. On the other hand, forthe PEMFCs with 0.5 mmflow channels, these values were 10.3% under the oxygen condition and 14.2%under the air condition. This implies that the thinner theflow channels, the more severely the crackedGDLs influence the PEMFC performance in terms of concentration loss. The EIS data indicated that,compared to the PEMFCs with 1.0 mmflow channels, the cracks on GDLs had more impact on the PEMFCswith 0.5 mmflow channels with regard to activation overvoltage.
New insights on the origin of multiple stellar populations in globular clusters
Kim, Jaeyeon,Lee, Young-Wook Cambridge University Press 2017 Proceedings of the International Astronomical Unio Vol.13 No.suppl334
<B>Abstract</B><P>In order to investigate the origin of multiple stellar populations in the halo and bulge of the Milky Way, we have constructed chemical evolution models for the low-mass proto-Galactic subsystems such as globular clusters (GCs). Unlike previous studies, we assume that supernova blast waves undergo blowout without expelling the pre-enriched gas, while relatively slow winds of massive stars (WMS), together with the winds and ejecta from low and intermediate mass asymptotic-giant-branch stars (AGBs), are all locally retained in these less massive systems. We find that the observed Na-O anti-correlations in metal-poor GCs can be reproduced, when multiple episodes of starbursts are allowed to continue in these subsystems. A specific form of star formation history (SFH) with decreasing time intervals between the stellar generations, however, is required to obtain this result, which is in good agreement with the parameters obtained from our stellar evolution models for the horizontal-branch. The “mass budget problem” is also much alleviated by our models without <I>ad-hoc</I> assumptions on star formation efficiency (SFE) and initial mass function (IMF). We also applied these models to investigate the origin of super-helium-rich red clump stars in the metal-rich bulge as recently suggested by Lee <I>et al.</I> (2015). We find that chemical enrichments by the WMS can naturally reproduce the required helium enhancement (Δ<I>Y</I>/Δ<I>Z</I> = 6) for the second generation stars. Disruption of proto-GCs in a hierarchical merging paradigm would have provided helium enhanced stars to the bulge field.</P>
Kim, Jinwoo,Jin, Youngeup,Kim, Jaehong,Jung, Jaeyeon,Kim, Sun Hee,Lee, Kwanghee,Suh, Hongsuk Wiley Subscription Services, Inc., A Wiley Company 2009 Journal of polymer science Part A, Polymer chemist Vol.47 No.5
<P>The syntheses and characterization of poly((2,6-(4,4-bis(4-((2-ethylhexyl)oxy)phenyl)-4H-cyclopenta[def]phenanthrene))-co-(2,6-(4,4-bis(4-(((9-carbazolyl)hexyl)oxy)phenyl))-4H-cyclopenta[def]phenanthrene)) (BCzPh-PCPPs) and poly((2,6-(4,4-bis(4-((2-ethylhexyl)oxy)phenyl)-4H-cyclopenta[def]phenanthrene))-co-(2,6-(4-(4-(((9-carbazolyl)hexyl)oxy)phenyl)-4-(4-((2-ethylhexyl)oxy)phenyl)-4H-cyclopenta[def]phenanthrene))) (CzPh-PCPPs), with carbazole unit as pendants, are presented. The carbazole moiety, which can improve the hole injection ability, was introduced as a pendant on the PCPP backbone. The devices of the polymers with the configurations of ITO/PEDOT:PSS/polymers/Ca/Al generate EL emission with maximum peaks at 400–450 nm, CIE coordinates of (x = 0.11–0.29, y = 0.11–0.33), low turn-on voltages of 4–6 V, maximum brightness of 60–810 cd/m<SUP>2</SUP>, and luminescence efficiencies of 0.04–0.22 cd/A. The PL spectra of CzPh-PCPPs films did not show any peak at around 550 nm, which corresponds to keto defect or aggregate/excimer formation, even after annealing for 30 h at 150 °C in air. Out of the series, CzPh-PCPP1 (PCPP derivative with 10% of carbazole moiety as pendant) shows blue emission with the maximum brightness of 810 cd/m<SUP>2</SUP> at 9 V, and the highest luminescence efficiency of 0.22 cd/A at 395 mA/cm<SUP>2</SUP>. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1327–1342, 2009</P> <B>Graphic Abstract</B> <P>The conjugated copolymers of BCzPh-PCPPs and CzPh-PCPPs with carbazole pendant were synthesized by Suzuki reaction. The carbazole moiety, which can improve the hole injection ability, was introduced as a pendant on the PCPP backbone. Their thermal, electrochemical, photoluminescent, and electroluminescent properties were investigated. <img src='wiley_img/0887624X-2009-47-5-POLA23241-gra001.gif' alt='wiley_img/0887624X-2009-47-5-POLA23241-gra001'> </P>