• Korean Chemical Engineering Research
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• v.59 no.3
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• pp.326-332
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• 2021

In this study, a spherical carbon composite material containing nano-silicon was synthesized using hydrothermal synthesis, and coated with petroleum pitch to prepare an anode material to investigate the electrochemical characteristics. Hydrothermal synthesis was performed by varying molar concentration, and the pitch was coated using THF as an organic solvent to prepare a composite material. The physical properties of anode materials were analyzed using SEM, EDS, XRD and TGA, and the electrochemical performances were investigated by cycle, C-rate, cyclic voltammetry and electrochemical impedance tests in 1.0 M LiPF₆ electrolyte (EC : DMC : EMC = 1 : 1 : 1 vol%). The pitch-coated silicon/carbon composite (Pitch@Si/C-1.5) with sucrose of 1.5 M showed a spherical shape. In addition, a high initial capacity of 1756 mAh/g, a capacity retention ratio of 82% after 50 cycles, and an excellent rate characteristic of 81% at 2 C/0.1 C were confirmed.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.49 no.5
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• pp.383-388
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• 2021

Air-breathing engines used in aircraft have a performance limit as the altitude increases, and this determines the service and absolute ceiling altitude. The method of maintaining altitude and speed in a fixed-wing aircraft in level flight using classical control method is generally using thrust for speed increase/deceleration and pitch attitude for altitude increase/decrease. If this method is used near the service ceiling altitude, increasing the pitch to reduce the altitude error results in a speed reduction. Therefore, it is necessary to use a control method that maintains the speed first using the pitch attitude. Especially in the case of unmanned aerial vehicles, these two methods should be automatically available at the right time. In this paper, we propose a method of switching the speed and altitude maintenance algorithm near service ceiling altitude.

• Journal of the Korean Society of Marine Environment & Safety
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• v.28 no.1
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• pp.135-140
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• 2022

Most machines are made of several types of . In particular, the shaft system of the ship consists of the brass of the propeller blade and the stainless steel of the shaft. When dissimilar the electrolyte solution of seawater, a voltaic cell and a shaft electromotive force is generated. This electromotive force causes electrical corrosion of the bearing and shaft supporting the shaft system. prevent this corrosion, a shaft grounding system is installed in ships. As for the experimental method, various information acquired by designing a program to periodically measure the electromotive force of the controllable pitchpropeller) system using an A/D converter of NI. This study analyzed the generation and characteristics of accumulator electromotive force for CPP and considered the installation location of the grounding system to remove the accumulator electromotive force.

• Journal of Aerospace System Engineering
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• v.16 no.6
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• pp.106-113
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• 2022

The structural safety of the basic design model of the linear actuator for the individual blade pitch control of eVTOL personal aircraft was investigated. Stress analysis based on the finite element method was conducted, and the margin of safety was calculated to examine the structural safety under stall load conditions. Additionally, fatigue analysis was conducted to evaluate the fatigue life of the linear actuators under operating conditions. The load history with the blade pitch angle was calculated using multi-body dynamics analysis, and the static load analysis was used to obtain the stress distribution for the rated load. As a result, it was confirmed that the safety margins exceeded zero, and the fatigue lives of all linear actuator components exceeded 107 cycles, indicating a safe structural range.

• Applied Chemistry for Engineering
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• v.34 no.6
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• pp.608-612
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• 2023

Nano-diamond (ND) was added during the carbon block preparation process to increase the thermal conductivity of the carbon block. Added ND controlled the generated pore of carbon block due to the volatilization of the binder pitch during the carbonization process. The ND was added to the kneading process of coke and binder pitch, and carbon blocks were prepared by pressing and carbonization. As the amount of added ND increased, the ND ratio of the carbon block increased. The added ND made a pass-way for generated gas by volatilizing the binder pitch during the carbonization process, increasing the density of the carbon block and reducing the porosity. The thermal conductivity of the carbon block was improved by increased density, lowered porosity, and the high thermal conductivity of added ND.

• The Journal of the Acoustical Society of Korea
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• v.28 no.4
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• pp.383-391
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• 2009

In speech signal processing, speech signal corrupted by noise should be enhanced to improve quality. Usually noise estimation methods need flexibility for variable environment. Noise profile is renewed on silence region to avoid effects of speech properties. So we have to preprocess finding voice region before noise estimation. However, if received signal does not have silence region, we cannot apply that method. In this paper, we proposed SNR estimation method for continuous speech signal. The waveform which is stationary region of voiced speech is very correlated by pitch period. So we can estimate the SNR by correlation of near waveform after dividing a frame for each pitch. For unvoiced speech signal, vocal track characteristic is reflected by noise, so we can estimate SNR by using spectral distance between spectrum of received signal and estimated vocal track. Lastly, energy of speech signal is mostly distributed on voiced region, so we can estimate SNR by the ratio of voiced region energy to unvoiced.

• Journal of Aerospace System Engineering
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• v.18 no.1
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• pp.29-36
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• 2024

Recently, the concept of Urban Air Mobility (UAM) has expanded to Advanced Air Mobility (AAM). A tilt rotor type of vertical take-off and landing aircraft has been actively studied and developed. A tilt-rotor aircraft can perform a transition flight between vertical and horizontal flights. A blade pitch angle control system can be used for flight stability during transition flight time. In addition, Individual Blade Control (IBC) can reduce noise and vibration generated in transition flight. This paper proposed Disturbance Observer Based Control (DOBC) and Time Delay Control (TDC) for individual blade control of an Electro-Mechanical Actuator (EMA) based blade pitch angle control system. To compare and analyze proposed controllers, numerical simulations were conducted with DOBC and TDC.

• Journal of the Korean Electrochemical Society
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• v.4 no.3
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• pp.104-108
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• 2001

Naphthalene derived mesophase pitch WP) was spun into short fibers by using melt-blown technology. The pitch fibers oxidative stabilization were carried out heating rates of $2^{\circ}C/min,\;5^{\circ}C/min\;and\; 10^{\circ}/min$. The heating rate was a key factor to maximate the capacity of the Li-ion secondary battery through controlling the morphology of the graphitized fiber. The diameters of the melt-blown fibers prepared were in the range of $4{\mu}m\~16{\mu}m$ with functions of air jet speed, air temperature and the temperature of the nozzle. The graphitized fibers of $10{\mu}m$ diameters showed various morphological structure with heating rate of the stabilization. Radial, radial-random and skin-core cross-sectional structure of the fibers were observed at the respective heating rate of $2^{\circ}C/min\;5^{\circ}C/min\;and\;10^{\circ}C/min$. Most crystalline structure of graphite was obtained from the fiber stabilized at heating rate of $10^{\circ}C/min$ exhibiting the best anode performance with 400 mAh/g of capacitance and $96.8\%$ of charge/discharge efficiency.

• Applied Chemistry for Engineering
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• v.27 no.5
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• pp.537-542
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• 2016

In this study, we have prepared mesophase pitch from pyrolysis fuel oil (PFO) by heterogeneous reforming process. This process was conducted by direct fluorination at various temperature and followed by the heat treatment at $390^{\circ}C$. The reformed pitch was then investigated by softening point analysis, elemental analysis, fourier-transform infrared spectroscopy, high resolution X-ray diffraction and polarization microscope analysis. Carbon contents of reformed pitch increased according to increasing the reaction temperature of fluorination, while oxygen, nitrogen and sulfur contents were completely eliminated. As the fluorination temperature increased, the creation, growth, coalescence and alignment process of mesophase spheres were observed. Also the interlayer spacing of carbon hexagonal planar structure decreased, while its crystalline size increased. In addition, aromatic ring compound contents increased by the condensation polymerization of aliphatic compound. These results can be attributed to the radical reactivity of the fluorine increased as the reaction temperature increased. It was considered that the fluorination reaction could help PFO to generate aromatic compounds, via promoting polymerization by radical reaction.

• Korean Chemical Engineering Research
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• v.59 no.4
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• pp.487-492
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• 2021

In this study, the electrochemical properties of pitch coated silicon sheets/graphite anode materials were investigated. Using NaCl as a template, silicon sheets were prepared through the stöber method and the magnesiothermic reduction methode. In order to synthesize the anode composite, the silicon sheets and graphite were combined with SDBS. The pitch coated silicon sheets/graphite was synthesized using THF as a solvent for the anode material composite. The physical properties of the prepared anode composites were analysed by XRD, SEM, EDS and TGA. The electrochemical performances of the prepared anode composites were performed by the current charge/discharge, rate performance, cyclic voltammetry and EIS tests in the electrolyte LiPF₆ dissolved solvents (EC:DMC:EMC=1:1:1 vol%). As the silicon composition of silicon sheets/graphite composite material increased, the discharge capacity also increased, but the cycle stability tended to decrease. The anode material of pitch coated silicon sheets/graphite composite (silicon sheets:graphite=3:7 weight ratio) showed the initial discharge capacity of 1228.8 mAh/g and the capacity retention ratio of 77% after 50 cycles. From these results, it was found that the cycle stability of pitch coated silicon sheets/graphite was improved.