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546 results on '"Jung, Hyun Wook"'

31. Nanoscale Encapsulation of Sulfur Cathodes via Self-Healing and Polar Synergistic Multifunctional Coating for High-Performance Li–S Batteries.

Author

Yang, Zhao, Patel, Twinkal, Thinphang-Nga, Anna, Nizami, Ameer, Park, Junyoung, Ye, Zhibin, Jung, Hyun Wook, Gauvin, Raynald, Peslherbe, Gilles Herve, Oh, Jung Kwon, and Li, Xia

Abstract

The development of lithium–sulfur (Li–S) batteries is plagued by serious polysulfide shuttling, sluggish redox reaction kinetics, and low sulfur utilization. In this work, a nanoscale poly-(hindered urea) (PHU) coating with self-healability is designed for sulfur cathodes to achieve an improved electrochemical performance. The as-prepared polymer coating in a thickness of about 3 nm uniformly on the surface of the nanoscale sulfur/carbon (S/C) particles acts as a physical barrier to effectively encapsulate and accumulate lithium polysulfide (LiPS) inside the S/C nanoscale particles and thus facilitates the rapid conversion of LiPSs. In addition, the dynamic and reversible self-healing hindered urea bonds (HUBs) endow the PHU coating layer with the ability to maintain structural integrity and stability even after numerous cycles of volume expansion and shrinkage from nanoscale particles to the electrode level. More importantly, the polar groups carried by the PHU polymer exert a strong adsorption effect on LiPSs, thus further hindering the shuttling of LiPSs. Consequently, the nanoarchitecture with PHU coating layer exhibits impressive cycle stability (maintaining 82.8% capacity retention after 150 cycles at 0.5 C) and outstanding rate performance (capacity retention of 623.9 mAh g–1 at 2 C). Furthermore, even under a high sulfur loading of 8.47 mg cm–1, a high areal-specific capacity of 6.4 mAh cm–2 is still delivered. [ABSTRACT FROM AUTHOR]

Published
2024
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32. Effects of parasitic gate capacitance and gate resistance on radiofrequency performance in LG = 0.15 μm GaN high‐electron‐mobility transistors for X‐band applications.

Author

Chang, Sung‐Jae, Jeong, Hyeon‐Seok, Jung, Hyun‐Wook, Choi, Su‐Min, Choi, Il‐Gyu, Noh, Youn‐Sub, Kim, Seong‐Il, Lee, Sang‐Heung, Ahn, Ho‐Kyun, Kang, Dong Min, Kim, Dae‐Hyun, and Lim, Jong‐Won

Subjects
FREQUENCIES of oscillating systems, RADIO frequency, ELECTRIC capacity, TRANSISTORS, GALLIUM nitride
Abstract

The effects of the parasitic gate capacitance and gate resistance (Rg) on the radiofrequency (RF) performance are investigated in LG = 0.15 μm GaN high‐electron‐mobility transistors with T‐gate head size ranging from 0.83 to 1.08 μm. When the device characteristics are compared, the difference in DC characteristics is negligible. The RF performance in terms of the current‐gain cut‐off frequency (fT) and maximum oscillation frequency (fmax) substantially depend on the T‐gate head size. For clarifying the T‐gate head size dependence, small‐signal modeling is conducted to extract the parasitic gate capacitance and Rg. When the T‐gate head size is reduced from 1.08 to 0.83 μm, Rg increases by 82%, while fT and fmax improve by 27% and 26%, respectively, because the parasitic gate–source and gate–drain capacitances reduce by 19% and 43%, respectively. Therefore, minimizing the parasitic gate capacitance is more effective that reducing Rg in our transistor design and fabrication, leading to improved RF performance when reducing the T‐gate head size. [ABSTRACT FROM AUTHOR]

Published
2024
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42. Effects of parasitic gate capacitance and gate resistance on radiofrequency performance in LG= 0.15 μm GaN high‐electron‐mobility transistors for X‐band applications

Author

Chang, Sung‐Jae, Jeong, Hyeon‐Seok, Jung, Hyun‐Wook, Choi, Su‐Min, Choi, Il‐Gyu, Noh, Youn‐Sub, Kim, Seong‐Il, Lee, Sang‐Heung, Ahn, Ho‐Kyun, Kang, Dong Min, Kim, Dae‐Hyun, and Lim, Jong‐Won

Abstract

The effects of the parasitic gate capacitance and gate resistance (Rg) on the radiofrequency (RF) performance are investigated in LG= 0.15 μm GaN high‐electron‐mobility transistors with T‐gate head size ranging from 0.83 to 1.08 μm. When the device characteristics are compared, the difference in DC characteristics is negligible. The RF performance in terms of the current‐gain cut‐off frequency (fT) and maximum oscillation frequency (fmax) substantially depend on the T‐gate head size. For clarifying the T‐gate head size dependence, small‐signal modeling is conducted to extract the parasitic gate capacitance and Rg. When the T‐gate head size is reduced from 1.08 to 0.83 μm, Rgincreases by 82%, while fTand fmaximprove by 27% and 26%, respectively, because the parasitic gate–source and gate–drain capacitances reduce by 19% and 43%, respectively. Therefore, minimizing the parasitic gate capacitance is more effective that reducing Rgin our transistor design and fabrication, leading to improved RF performance when reducing the T‐gate head size.

Published
2024
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45. Crosslinking and Swelling Properties of pH-Responsive Poly(Ethylene Glycol)/Poly(Acrylic Acid) Interpenetrating Polymer Network Hydrogels.

Author

Hwang, Uijung, Moon, HoYeon, Park, Junyoung, and Jung, Hyun Wook

Subjects
ACRYLIC acid, ETHYLENE glycol, DIFFERENTIAL scanning calorimetry, CROSSLINKING (Polymerization), THERMOGRAVIMETRY, HYDROGELS, POLYMER networks
Abstract

This study investigates the crosslinking dynamics and swelling properties of pH-responsive poly(ethylene glycol) (PEG)/poly(acrylic acid) (PAA) interpenetrating polymer network (IPN) hydrogels. These hydrogels feature denser crosslinked networks compared to PEG single network (SN) hydrogels. Fabrication involved a two-step UV curing process: First, forming PEG-SN hydrogels using poly(ethylene glycol) diacrylate (PEGDA) through UV-induced free radical polymerization and crosslinking reactions, then immersing them in PAA solutions with two different molar ratios of acrylic acid (AA) monomer and poly(ethylene glycol) dimethacrylate (PEGDMA) crosslinker. A subsequent UV curing step created PAA networks within the pre-fabricated PEG hydrogels. The incorporation of AA with ionizable functional groups imparted pH sensitivity to the hydrogels, allowing the swelling ratio to respond to environmental pH changes. Rheological analysis showed that PEG/PAA IPN hydrogels had a higher storage modulus (G′) than PEG-SN hydrogels, with PEG/PAA-IPN5 exhibiting the highest modulus. Thermal analysis via thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) indicated increased thermal stability for PEG/PAA-IPN5 compared to PEG/PAA-IPN1, due to higher crosslinking density from increased PEGDMA content. Consistent with the storage modulus trend, PEG/PAA-IPN hydrogels demonstrated superior mechanical properties compared to PEG-SN hydrogels. The tighter network structure led to reduced water uptake and a higher gel modulus in swollen IPN hydrogels, attributed to the increased density of active network strands. Below the pKa (4.3) of acrylic acid, hydrogen bonds between PEG and PAA chains caused the IPN hydrogels to contract. Above the pKa, ionization of PAA chains induced electrostatic repulsion and osmotic forces, increasing water absorption. Adjusting the crosslinking density of the PAA network enabled fine-tuning of the IPN hydrogels' properties, allowing comprehensive comparison of single network and IPN characteristics. [ABSTRACT FROM AUTHOR]

Published
2024
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