Showing papers by "Kyungpook National University published in 2022"

Role of surface steps in activation of surface oxygen sites on Ir nanocrystals for oxygen evolution reaction in acidic media

Abstract: Ir and its oxide are the only available oxygen evolution reaction (OER) electrocatalysts with reasonably high activity and stability for commercial proton-exchange membrane electrolyzers. However, the establishment of structure–performance relationships for the design of better Ir-based electrocatalysts is hindered by their uncontrolled surface reconstruction during OER in acidic media. Herein, we monitor the structural evolution of two model Ir nanocrystals (one with a flat surface enclosed by (100) facets and the other with a concave surface containing numerous high-index planes) under acidic OER conditions. Operando X-ray absorption spectroscopy measurements reveal that the promotion of surface IrOx formation during the OER by the concave Ir surface with high-index planes results in a gradual OER activity increase, while a decrease in activity and limited oxide formation are observed for the flat Ir surface. After the activation process, the Ir concave surface exhibits ~ 10 times higher activity than the flat surface. Density functional theory computations reveal that Ir high-index surfaces are thermodynamically preferred for the adsorption of oxygen atoms and the formation of surface oxides under OER conditions. Thus, our work establishes a structure–performance relationship for Ir nanocrystals under operating conditions, providing new principles for the design of nanoscale OER electrocatalysts.

{10–12} extension twinning activity and compression behavior of pure Mg and Mg-0.5Ca alloy at cryogenic temperature

Abstract: This study explored the deformation behavior of rolled pure Mg and Mg-0.5Ca alloy at room temperature (RT) and cryogenic temperature (CT) of −150°C. The samples were compressed at a strain rate of 10-3s-1 up to 5% strain along the rolling direction (RD). The deformed samples were examined via EBSD and analyzed concerning the loading condition and initial microstructure. The compression tests showed the temperature-insensitive hardening behavior in Mg-0.5Ca alloy. The twinning activity increased with an increase in grain size, and this was pronounced when the pure Mg samples were compressed at CT. The kernel average misorientation (KAM) analysis revealed that the twinning parent areas tend to have higher KAM values under CT deformation than the RT deformation, which links to a higher twinning activity at CT. The predominant low angle grain boundaries in the initial microstructure of pure Mg further helps to understand the profuse twinning activity in the context of existing literature. It was suggested that the higher twinned area fraction in the pure Mg sample than Mg-0.5Ca could be responsible for the temperature-dependent hardening behavior. This is presumably because the Ca element in the matrix results in a homogeneous microstructure with smaller grain size and higher Schmid factor (SF) for basal slip, leading to lower twinning dependent deformation behavior in Mg-0.5Ca than pure Mg.

Improved NPC Inverters Without Short-Circuit and Dead-Time Issues

Abstract: The traditional neutral point clamped (NPC) inverter has short-circuit problem. The risk of short-circuit can be decreased by using dead-time in the switching signals. However, the dead-time decreases the achievable output voltage and causes distortion in the waveforms. To overcome the short-circuit problem, dual-buck NPC (DB-NPC) and split-inductor NPC (SI-NPC) inverters have been researched. However, the voltage stress of the two external diodes in the DB-NPC inverter is higher. On the other hand, the SI-NPC inverter has a problem of generating huge voltage spikes in the dead-time, which can destroy the semiconductor devices. In addition, the SI-NPC inverter cannot provide reactive power. This article presents a family of NPC inverters consisting of single-phase, three-phase, and cascaded inverters. The proposed inverters have no short-circuit and dead-time issues, therefore no high voltage and current spikes are caused. Also, the dead-time in the switching signals can be minimized. As a result, the magnitude of the output waveforms can be increased, and quality can be improved. Unlike the DB-NPC inverter, the voltage stress of all the semiconductor in the proposed inverter is lower, and unlike the SI-NPC inverter the proposed inverter provides reactive power. In this article, the proposed three-level NPC inverter is analyzed, designed, and tested. The voltage stress of the semiconductor devices in the proposed inverter is half of the source voltage, whereas in the conventional DB-NPC inverter the voltage stress of the two external diodes is the source voltage. In addition to the aforementioned benefits, the proposed cascaded inverter reduces the total number of inductors. To verify the analysis, detailed simulation, and experimental results of the proposed three-level inverter with input voltage 640 V, output power 1.2 kW, and output voltage 220 Vrms are provided.

COVID‐19 vaccination in patients with rheumatic diseases: Vaccination rates, patient perspectives, and side effects

Abstract: To investigate the vaccination rate, reported side effects, and patient concerns for COVID-19 vaccination in patients with rheumatic diseases.A multicentre cross-sectional study from rheumatology clinics in two major hospitals in Hong Kong was conducted between June 3, 2021 and October 8, 2021. Patient interviews for demographics, clinical characteristics, vaccination status, reported side effects, and factors influencing decisions about vaccination were supplemented with structured questionnaires.Out of 1367 patients, 413 (30.2%) had received COVID-19 vaccination. Side effects were reported in 335 (81.1%) of patients, of which the most common were injection site pain or swelling (66.3%), fatigue (57.1%), fever (19.9%), and headache (19.6%). Multivariate logistic regression models showed that males (odds ratio [OR] = 1.80; p < .001), higher education level (OR = 1.64; p < .001) and healthcare professionals (OR = 4.5; p < .001) were significantly more likely to have received the vaccine. In contrast, patients with hypertension (OR = 0.73; p = .04), systemic lupus erythematous (OR = 0.53; p < .001), stroke (OR = 0.29; p = .01), steroid therapy (OR = 0.59; p = .01), and leflunomide therapy (OR = 0.45; p = .05) were significantly less likely to be vaccinated. Younger age (age, OR = 0.96; p = .003) and messenger RNA (mRNA) vaccines (OR = 4.79; p < .001) were associated with more side effects. There was no difference in risk of side effects between specific rheumatic diseases or drug therapies.COVID-19 vaccination is associated with no increased risk of side effects in any particular disease or drug therapy, therefore vaccination should be encouraged in patients with rheumatic disease. In addition, younger age is associated minimally, while mRNA vaccine is associated with increased side effects.

Improved Odd-Harmonic Repetitive Control Scheme for Ćuk-Derived Inverter

Abstract: This article proposes an improved odd-harmonic repetitive controller (IORC) for a Cuk-derived inverter. The IORC is composed of a modified structure of an odd-harmonic repetitive controller (ORC) combined with a proportional-integral (PI) controller and a feedforward controller to achieve a zero steady-state error in the Cuk-derived inverter. The IORC features much larger gains at odd-harmonic frequencies by modification of an existing ORC, so both its convergence rate and transient response become much faster. The added benefit of using the IORC is that it has wider bandwidth at odd-harmonic frequencies, so it is more insensitive to the grid frequency fluctuation. The IORC also adopts a linear phase-lead compensator to compensate for the phase lag of the Cuk-derived inverter. Hence, the IORC can effectively eliminate the odd-harmonic components in the output current of the grid-connected Cuk-derived inverter while using less memory space. However, the distinctive structure of the IORC makes it difficult to guarantee the stability of the overall control system. Thus, in this article, a new stability analysis method is developed for the IORC system, and then a practical design guideline is also provided to obtain the optimal control parameters. Experimental results are presented to validate the proposed controller in the 300-W Cuk-derived inverter prototype.

Phytohormonal Regulation Through Protein S-Nitrosylation Under Stress

Abstract: The liaison between Nitric oxide (NO) and phytohormones regulates a myriad of physiological processes at the cellular level. The interaction between NO and phytohormones is mainly influenced by NO-mediated post-translational modifications (PTMs) under basal as well as induced conditions. Protein S-nitrosylation is the most prominent and widely studied PTM among others. It is the selective but reversible redox-based covalent addition of a NO moiety to the sulfhydryl group of cysteine (Cys) molecule(s) on a target protein to form S-nitrosothiols. This process may involve either direct S-nitrosylation or indirect S-nitrosylation followed by transfer of NO group from one thiol to another (transnitrosylation). During S-nitrosylation, NO can directly target Cys residue (s) of key genes involved in hormone signaling thereby regulating their function. The phytohormones regulated by NO in this manner includes abscisic acid, auxin, gibberellic acid, cytokinin, ethylene, salicylic acid, jasmonic acid, brassinosteroid, and strigolactone during various metabolic and physiological conditions and environmental stress responses. S-nitrosylation of key proteins involved in the phytohormonal network occurs during their synthesis, degradation, or signaling roles depending upon the response required to maintain cellular homeostasis. This review presents the interaction between NO and phytohormones and the role of the canonical NO-mediated post-translational modification particularly, S-nitrosylation of key proteins involved in the phytohormonal networks under biotic and abiotic stresses.