• The Bulletin of The Korean Astronomical Society
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• v.38 no.1
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• pp.44.1-44.1
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• 2013

We present the preliminary results from 86 GHz GMVA observations on 2 AGN - 0954+658 and 0716+714. The observation was taken with the Global mm-VLBI Array in Oct. 2010, with dual polarization mode. The aim of the observation is to produce the polarization maps of the sources, with the Stokes parameters - I, Q, U and V. The final results will be used for estimating the strength of the intrinsic magnetic field and the geometries of AGN jets.

• Bulletin of the Korean Chemical Society
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• v.16 no.5
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• pp.454-458
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• 1995

Inhibiting action of semicarbazide, thiosemicarbazide, sym. diphenylcarbazide towards corrosion of zinc in hydrochloric acid has been investigated. The rate of corrosion depends on the nature of the inhibitor and its concentration. The values of inhibition efficiency from, weight loss, thermometric measurements are in good agreement with those obtained from polarization studies. From the polarization studies, the inhibitors used act as mixed absorption type inhibitors, increased adsorption resulting from an increase in the electron density at the reactive C=S and C=O groups and N-atoms. The thermodynamic parameters of adsorption obtained using Bockris-Swinkels adsorption isotherm reveal a strong interaction of these carbazides on zinc surface.

• Geophysics and Geophysical Exploration
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• v.25 no.3
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• pp.99-108
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• 2022

Among induced polarization (IP) methods, spectral IP (SIP) uses alternating current as a transmission source to measure amplitudes and phase of complex electrical resistivity at each source frequency, which disperse with respect to source frequencies. The frequency dependence, which can be explained by a relaxation model such as Cole-Cole model or equivalent models, is analyzed to estimate SIP parameters from dispersion curves of complex resistivity employing multi-objective optimization (MOO). The estimation uses a generic algorithm to optimize two objective functions minimizing data misfits of amplitude and phase based on Cole-Cole model, which is most widely used to explain IP relaxation effects. The MOO-based estimation properly recovered Cole-Cole model parameters for synthetic examples but hardly fitted for the real laboratory measures ones, which have relatively smaller values of phases (less than about 10 mrad). Discrepancies between scales for data misfits of amplitude and phase, used as parameters of MOO method, and it is in necessity to employ other methods such as machine learning, which can deal with the discrepancies, to estimate SIP parameters from dispersion curves of complex resistivity.

• Proceedings of the Korean Vacuum Society Conference
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• 2013.08a
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• pp.99-99
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• 2013

In this talk, I will present two research works in progress, which are: i) mapping of piezoelectric polarization and associated charge density distribution in the heteroepitaxial InGaN/GaN multi-quantum well (MQW) structure of a light emitting diode (LED) by using inline electron holography and ii) in-situ observation of the polarization switching process of an ferroelectric Pb(Zr1-x,Tix)O3 (PZT) thin film capacitor under an applied electric field in transmission electron microscope (TEM). In the first part, I will show that strain as well as total charge density distributions can be mapped quantitatively across all the functional layers constituting a LED, including n-type GaN, InGaN/GaN MQWs, and p-type GaN with sub-nm spatial resolution (~0.8 nm) by using inline electron holography. The experimentally obtained strain maps were verified by comparison with finite element method simulations and confirmed that not only InGaN QWs (2.5 nm in thickness) but also GaN QBs (10 nm in thickness) in the MQW structure are strained complementary to accommodate the lattice misfit strain. Because of this complementary strain of GaN QBs, the strain gradient and also (piezoelectric) polarization gradient across the MQW changes more steeply than expected, resulting in more polarization charge density at the MQW interfaces than the typically expected value from the spontaneous polarization mismatch alone. By quantitative and comparative analysis of the total charge density map with the polarization charge map, we can clarify what extent of the polarization charges are compensated by the electrons supplied from the n-doped GaN QBs. Comparison with the simulated energy band diagrams with various screening parameters show that only 60% of the net polarization charges are compensated by the electrons from the GaN QBs, which results in the internal field of ~2.0 MV cm-1 across each pair of GaN/InGaN of the MQW structure. In the second part of my talk, I will present in-situ observations of the polarization switching process of a planar Ni/PZT/SrRuO3 capacitor using TEM. We observed the preferential, but asymmetric, nucleation and forward growth of switched c-domains at the PZT/electrode interfaces arising from the built-in electric field beneath each interface. The subsequent sideways growth was inhibited by the depolarization field due to the imperfect charge compensation at the counter electrode and preexisting a-domain walls, leading to asymmetric switching. It was found that the preexisting a-domains split into fine a- and c-domains constituting a $90^{\circ}$ stripe domain pattern during the $180^{\circ}$ polarization switching process, revealing that these domains also actively participated in the out-of-plane polarization switching. The real-time observations uncovered the origin of the switching asymmetry and further clarified the importance of charged domain walls and the interfaces with electrodes in the ferroelectric switching processes.

• Membrane and Water Treatment
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• v.10 no.5
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• pp.387-394
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• 2019

Membrane distillation (MD) is one of the water treatment processes which involves the momentum, heat and mass transfer through channels and membrane. In this study, CFD modeling has been used to simulate the heat and mass transfer in the direct contact membrane distillation (DCMD). Also, the effect of operating parameters on the water flux is investigated. The result shows a good agreement with the experimental result. Results indicated that, while feed temperature is increasing in the feed side, water flux improves in the permeate side. Since higher velocity leads to the higher mixing and turbulence in the feed channel, water flux rises due to this increase in the feed velocity. Moreover, results revealed that temperature polarization coefficient is rising as flow rate (velocity) increases and it is decreasing while the feed temperature increases. Lastly, the thermal efficiency of direct contact membrane distillation is defined, and results confirm that thermal efficiency improves while feed temperature increases. Also, flow rate increment results in enhancement of thermal efficiency.

• Journal of the Korean institute of surface engineering
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• v.53 no.6
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• pp.330-342
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• 2020

Since the original report on ferroelectricity in Si-doped HfO2 in 2011, fluorite-structured ferroelectrics have attracted increasing interest due to their scalability, established deposition techniques including atomic layer deposition, and compatibility with the complementary-metal-oxide-semiconductor technology. Especially, the emerging fluorite-structured ferroelectrics are considered promising for the next-generation semiconductor devices such as storage class memories, memory-logic hybrid devices, and neuromorphic computing devices. For achieving the practical semiconductor devices, understanding polarization switching kinetics in fluorite-structured ferroelectrics is an urgent task. To understand the polarization switching kinetics and domain dynamics in this emerging ferroelectric materials, various classical models such as Kolmogorov-Avrami-Ishibashi model, nucleation limited switching model, inhomogeneous field mechanism model, and Du-Chen model have been applied to the fluorite-structured ferroelectrics. However, the polarization switching kinetics of fluorite-structured ferroelectrics are reported to be strongly affected by various nonideal factors such as nanoscale polymorphism, strong effect of defects such as oxygen vacancies and residual impurities, and polycrystallinity with a weak texture. Moreover, some important parameters for polarization switching kinetics and domain dynamics including activation field, domain wall velocity, and switching time distribution have been reported quantitatively different from conventional ferroelectrics such as perovskite-structured ferroelectrics. In this focused review, therefore, the polarization switching kinetics of fluorite-structured ferroelectrics are comprehensively reviewed based on the available literature.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.8 no.4
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• pp.894-900
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• 2004

There are many types of circularly polarized(CP) microstrip antenna elements, which are used as a good unit radiator. Since an advantage of CP antenna is no strict alignment requirements between Rx and Tx system, the printed antennas with circular polarization are very often used in numerous satellite and mobile radio systems. In order to realize the broad bandwidth of 3 dB axial ratio and impedance of CP microstrip antenna, complex feed structure and tri-plate patch element have been researched. This paper describes a design of wideband microstrip cross slot array antennas with circular polarization. The proposed antenna is composed of an open-ended microstrip feed line as a feeder and a cross slot as a radiator for circular polarization. To realize the wide bandwidth, tri-plate structure are considered and cross slot is electromagnetically coupled with feed line. Optimum parameters of 1-element cross slot antenna are analyzed and designed by method of moments. These parameters are also applied to may antennas design considered the mutual coupling between radiating elements. Right hand circular polarization(RHCP) and left hand circular polarization(LHCP) of the proposed antenna are easily controlled by asymmetrical cross slot structure and slot position. In 1-element and 15-element cross slot array antenna, the good axial ratio of 1 dB below and the broad bandwidth characteristics of antenna are obtained.

• Proceedings of the Korea Air Pollution Research Association Conference
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• 2008.04a
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• pp.391-393
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• 2008

• Advances in Energy Research
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• v.2 no.2
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• pp.73-84
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• 2014

Fuel cells of proton exchange membrane type (PEMFC) working with hydrogen in the anode and ambient air in the cathode ('air breathing') have been prepared and characterized. The cells have been studied with variable thickness of the cathode catalyst layer ($L_{CL}$), maintaining constant the platinum and ionomer loads. Polarization curves and electrochemical active area measurements have been carried out. The polarization curves are analyzed in terms of a model for a flooded passive air breathing cathode. The analysis shows that $L_{CL}$ affects to electrochemical kinetics and mass transport processes inside the electrode, as reflected by two parameters of the polarization curves: the Tafel slope and the internal resistance. The observed decrease in Tafel slope with decreasing $L_{CL}$ shows improvements in the oxygen reduction kinetics which we attribute to changes in the catalyst layer structure. A decrease in the internal resistance with $L_{CL}$ is attributed to lower protonic resistance of thinner catalyst layers, although the observed decrease is lower than expected probably because the electronic conduction starts to be hindered by more hydrophilic character and thicker ionomer film.

• Journal of Electrochemical Science and Technology
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• v.4 no.2
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• pp.47-56
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• 2013

Inhibition performance of Lornoxicam & Tenoxicam against corrosion of carbon steel in 1M $H_2SO_4$ solutions was investigated by weight loss, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements. The inhibition efficiency increased with increasing inhibitor's concentration, but decreased with increase in temperature. Potentiodynamic polarization curves showed that, the inhibitors were of mixed type. The apparent activation energy ($E^*_a$) and other thermodynamic parameters for the corrosion process have also been calculated and discussed. The inhibition of carbon steel corrosion is due to the adsorption of the inhibitor molecules on the surface, which follows Temkin adsorption isotherm. The mechanism of inhibition was discussed in the light of the chemical structure of the undertaken inhibitors.