Jo Feb 16, 2025
Due to their remarkable characteristics, laminated composite shells and plates are commonly found in various engineering fields such as mechanical, civil, aerospace, marine, and automotive applications. Composite shells and plates of varying thickness are widely used in different industries due to their light weight and compact design.
Recent studies have focused on the dynamic characteristics of these materials, and meshfree methods have attracted significant attention from researchers.
Kwak Song Hun, a researcher at the Faculty of Mechanical Science and Technology, has proposed a meshfree Jacobi point interpolation (MJPI) approach for the dynamic analysis of sandwich laminated conical and cylindrical shells with varying thickness.
First, he established theoretical formulations for sandwich laminated shells with varying thickness using the modified variational principle within the framework of first-order shear deformation theory (FSDT). Then, he expanded the displacement components of the sandwich shell using the MJPI shape function and Fourier series in the meridional and circumferential directions, respectively.
He verified the accuracy and reliability of the proposed MJPI shape function through the comparison with the numerical results from published literature and the commercial simulation tool Abaqus. Finally, he investigated the effects of different parameters such as thickness gradient, thickness power index and boundary condition on the free vibration and dynamic response of the sandwich laminated shell.
For more information, you can refer to his paper “A Meshfree Approach for Dynamic Analysis of Sandwich Conical and Cylindrical Shells with Varying Thicknesses” in “Acta Mechanica Solida Sinica” (SCI).
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Jo Feb 14, 2025
Rapid development of information technology has greatly changed the pattern of teaching and learning in education, and e-learning is the most widely used mode of learning nowadays. In e-learning systems, some cheating phenomena can occur because the teacher cannot see the students' learning process. User authentication by user names and passwords only when logging on in most e-learning systems might cause cheating. In other words, someone else may substitute for learners, and the learner may leave before the lecture is over.
If a student enrolled in e-learning system is able to complete a course without actually learning, the reliability of the course and e-learning system is diminished, and since lectures or learning materials are intellectual assets, such deception should be prevented. Therefore, in e-learning systems, it is necessary to identify the learner during the learning activities as well. To detect spoofing in e-learning is of great significance in enhancing the reliability of e-learning systems and protecting the copyright of learning content.
Therefore, biometric authentication techniques such as face recognition, fingerprint recognition and speech recognition have been introduced into user identification in e-learning systems, and prevention of spoofing of learning by face recognition in e-learning process is in active use. However, face recognition is usually conducted at the beginning of user engagement, lectures and tests, and there has been no intense research to prevent spoofing of learning in the learning process.
Ryu Chang Sik, a section head at the Faculty of Distance Education, has proposed a new method to prevent spoofing, based on implicit face matching at randomly selected time intervals during online learning.
In the proposed method, face-matching identification is performed at the time of system login and the beginning of learning, and in each learning process (e.g., learning a section). In addition, implicit face-matching is performed several times without interaction with students at random time intervals to calculate the similarity value and detect the impostor. If not detected in the random check, explicit user identification would be required.
The proposed method has been applied to an e-learning system that provides a graduate qualification. The result shows that the system can effectively distribute the server load and prevent the spoofing of learning, while significantly reducing the number of stoppage of learning due to face contrasts.
You can find more information in his paper “Identification of Spoofing in e-Learning system with the implicit face recognition” in “INFORMATICA”.
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Jo Feb 13, 2025
At present, since energy depletion and environmental pollution are global issues, many attempts have been made to produce light olefins by low energy consumption, low carbon dioxide emission and environment-friendly methods from coal and natural gas which is more abundant than petroleum or from sustainable biomass resources. One of these methods is the methanol to olefins (MTO) process using a microporous catalyst.
Dimethyl ether (DME) is an intermediate of MTO, and it is easily converted to olefins by catalysis, a surface phenomenon. The key of DTO technology is to synthesize catalysts with high light olefins selectivity and long lifetime. SAPO-18 structure that is extensively used in MTO, DTO, CO2/CH4 separation and SCR of NOx processes, and is structurally similar to SAPO-34 (with CHA framework).
Many attempts have been made to enhance the catalytic performance and lifetime of SAPO-18 catalysts, such as synthesis of catalysts with different organic structure directing agent (OSDA), incorporation of nonmetallic and metallic elements into the framework, and the mechanism of coke formation, but the role of post-acid treatment on DTO performance of SAPO-18 has been barely addressed.
Jong Song Chol, a researcher at the Faculty of Chemical Engineering, has proposed post-HCl treatment as a method to promote DTO performance of SAPO-18, and compared with S-0 and NiS-0 the catalytic performance of acid-treated catalysts such as crystalline structure, morphology, specific surface area, textural properties, acidity and DTO performance.
He has found that the optimum HCl concentration and treating time of SAPO-18 catalyst for the catalytic performance enhancement in DTO are 0.1M and 3h, respectively.
You can find the details in his paper “Efficient post-HCl treatment for promoting DTO performance of SAPO-18 and Ni-substituted SAPO-18” in “Journal of Saudi Chemical Society” (SCI).
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Jo Feb 12, 2025
IGBT, a high-voltage high-power integrated circuit device, has a wide range of high-power switching applications such as mobile driving, power supply, railway transportation, industry control, etc. NPT-IGBT is a composite device of MOSFET and BJT.
Since NPT-IGBT usually operates at high voltage and high current, hot-carrier-induced degradation will be generated inside the device. Hot carriers with high energy will damage the oxide region of the device and the interface between silicon and silicon-dioxide, and increase the trap charge of the interface and the oxide region.
Although many researchers have described the issues on hot carriers of IGBT device, little literature is on the reliability of hot carrier of NPT-IGBT.
Kim Hak Bong, a section head at the Semiconductor Institute, has analyzed the degradation mechanisms due to the hot carrier effect in NPT-IGBT through the voltage stress measurement, TCAD simulation and charge pumping measurement.
He has concluded the following from the results. First, the device might have different degradation mechanisms under different gate voltage conditions. Second, when the gate voltage is relatively high, degradation is caused in three regions. The boundary state is generated in the body region, while the oxide layer trap charge is caused in the accumulation region, the field-oxide region and the drift region. Third, when the gate voltage is relatively low, the boundary state and the oxide layer trap charge are formed in the drift region and the degradation in the body region is relatively small.
If more information is needed, please refer to his paper “Analysis on Effect of Hot-Carrier-Induced Degradation of NPT-IGBT” in “Solid-State Electronics” (SCI).
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Jo Feb 11, 2025
Mineral waters have been widely used since long ago because of its abundance of natural minerals useful for human life. In recent years, the variety and production of bottled mineral waters have increased rapidly due to water pollution from various causes, depletion of drinking water sources and increasing demand of human for health. An important issue in the production of bottled mineral water is to maintain the physical and chemical characteristics including hygienic safety and color and taste after bottle packaging, for a certain period of time. This encouraged some studies to analyze the microbial status of bottled mineral water and to explain the cause of microbial development and growth.
It is not microorganisms alone that affect the quality of mineral water. Mineral waters also contain unstable components such as iron, manganese and sulfur which are possible to be removed by ozone-rich gases. However, among the inorganic materials in mineral water sources, Fe2+ that fails to undergo sufficient oxidation can be oxidized to Fe3+ during storage of mineral water and become precipitates, which do not change the total iron content in mineral water, but may change the physical properties such as turbidity. Therefore, Fe2+ should be fully oxidized and deposited before the bottling of mineral water to increase the shelf life of mineral water.
Most mineral water, especially with CO2 above 400mg/L-1, has a pH below 7, which might also have a negative effect on ozone treatment of mineral water. In addition, CO2 gas, which is dissolved in mineral water, makes it difficult for other gases to dissolve in mineral water. This is because, according to Henry’s law of solubility, the amount of gas dissolved in water depends on the partial pressure of the gas dissolved in the water. Therefore, effective oxidation and deposition of Fe2+ by using ozone gas in CO2-rich mineral water requires a new method of ozone injection and injection conditions to suit the characteristics of mineral water.
Choe Un Hwa, a researcher at the Faculty of Physical Engineering, has chosen a suitable ozone injection step and proposed a new mineral water treatment method using microbubbles by considering the characteristics of mineral water with a low content of ozone but much CO2.
As a result of the treatment of bottled mineral water with ozone microbubbles, the concentration of the iron ion was reduced from 0.14 to 0.01 mg/L-1, and the shelf life increased to 360 days. During the treatment, the concentrations of K+ and Na+ almost remained unchanged, and the deposition time was reduced to one-third compared to natural oxidation.
You can find the details in her paper “REMOVAL OF DEPOSITS AND IMPROVEMENT OF SHELF LIFE IN CO2-RICH MINERAL WATER BY OZONEMICROBUBBLES” in “QUMICA NOVA” (SCI).
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Jo Feb 10, 2025
Machining process modeling and optimization are two major issues in manufacturing processes. The process modeling and optimization techniques are vital to meet increasing quality demand (high quality, low cost and environmental friendliness) in manufacturing practice. The machining process for a computer numerical controlled (CNC) machine is programmed by process parameters such as cutting speed, feed rate and cutting depth. Traditional trial and error method for determining the machining process parameters based on experience cannot guarantee the optimal machining performance for CNC machines.
Many previous studies on machining processes have applied different modeling, integrating and optimization methods. However, they may have considerable inconsistent and conflicting results, which is one of the drawbacks to be overcome in manufacturing practice.
Ryang Si Ho, a section head at the Faculty of Mechanical Science and Technology, has proposed a multiple performances optimization methodology for computer numerical controlled (CNC) machining based on Taguchi method, multi-criteria decision-making (MCDM) and multiple regression (MR) model.
The proposed method consists of the following steps: (1) setting levels of process parameters and selecting suitable Taguchi orthogonal array (OA), (2) arranging the process parameters on the OA and measuring machining performance values at every trial, (3) calculating comprehensive performance (CP) by integrating the multiple performances using a reasonable MCDM, (4) developing MR model between the CP and the process parameters, (5) analyzing the influence of process parameters based on correlation analysis, and (6) determining the optimal process parameters using grid search method.
In order to verify the effectiveness of the proposed method, he applied it to analysis and determination of the influence and optimal turning process parameters such as cutting speed (CS), feed rate (FR), cutting depth (CD), cutting environment (CE) and tool nose radius (NR) for optimizing four machining performances such as surface roughness (SR), cutting force (CF), tool life (TL) and power consumption (PC) in the high speed CNC turning of AISI P20 tool steel.
For more information, please refer to his paper “A methodology for multiple performances optimization of computer numerical controlled (CNC) machining process based on design of experiment, multi-criteria decision-making and multiple regression model” in “International Journal on Interactive Design and Manufacturing” (SCI).
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