Jo Oct 6, 2023
The Pyongyang Bell, which was founded during the feudal Joson dynasty, is a precious legacy of national culture indicative of high creative wisdom of our people of those days.
The bell is 3.1 metres tall and about 13 tonnes heavy, with a 1.6 metre-wide mouth.
The present bell is said to have been refounded of bronze for approximately four months in 1726.
The bell looks harmonious, well-balanced and stately. The ring at the head that is shaped like a blue dragon and a yellow dragon entangled together looks so sophisticated and vigorous. With the most charming shape and the most solemn peal of the bells during the feudal Joson dynasty, the Pyongyang Bell has been known as the “attraction of Pyongyang” from olden times.
It was used for telling the time and sounding an alarm to the Pyongyang citizens.
President
The Pyongyang Bell serves as a precious national cultural legacy that gives a glimpse into the high-level metal processing and formative arts of our people and as a witness conveying the history of change of our country.
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Jo Oct 5, 2023
The Shockley-Read-Hall (SRH) lifetime is a very important parameter in silicon bipolar power devices that determine switching characteristics.
As for carrier lifetime control, recombination center levels in impurities-doped silicon are dependent on the properties of impurities and silicon materials, but not on the amount of impurities.
Preceding researchers studied diffusion of impurities and radiation action for recombination center levels by means of Au, Pt, electron irradiation, proton irradiation, etc.
Yu Nam Chol, a researcher at the Science Engineering Institute, has determined new recombination center levels in n-type silicon with gold and platinum, and investigated the effectiveness of the levels for low-level and high-level injection lifetime control.
At injection levels below 3×1013cm-3, reverse recovery technique was used, while open-circuit carrier decay (OCCD) technique was employed at levels above 1015cm-3. He performed lifetime measurements in the temperature interval of 303~403K. He studied the deep levels introduced by diffusion using DLTS, and calculated the corresponding SRH lifetime, and finally compared it with the measured values. The comparison suggests that low-level lifetime is mainly controlled by defect B located at 0.54eV below the conduction band, and high-level lifetime by defect A located at 0.25 eV.
The result shows that the recombination center levels estimated from DLTS measurement are dominant levels in n-type silicon doped gold and platinum.
If further information is needed, please refer to his paper “Analysis of recombination center levels in gold and platinum doped n-type silicon” in “Journal of Power electronics and devices”.
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Jo Oct 3, 2023
A good practice of keeping gardens neat and beautiful is one of our people’s traditional customs.
Koryo people liked sinking wells and growing fruit trees and flowers in their gardens.
It was common to plant several kinds of trees like junipers, matrimony vines and willows around wells to landscape gardens.
It is said that those in the central part of Korea round Kaegyong (Kaesong at present), which was the capital of Koryo, would enclose their houses with hedges that often consisted of juniper trees, needle juniper trees or flower trees.
Some of them used to decorate their gardens with odd-shaped rocks and big stones or famous flowers.
Such landscape gardening of the Koryo age was the inheritance and improvement of Koguryo peoples’ landscape gardening practice in accordance with Koryo peoples’ aesthetic feelings and emotions.
The murals in Anak Tomb No. 1, one of the Koguryo tombs, show artificial mounds and pavilions with lotus ponds and rocks of fantastic shape, enclosed by fences or cloisters.
In addition, the eastern wall and the east part of the southern wall of the inner chamber of the Koguryo Tokhung-ri Tomb are painted with lotus ponds and scenes of colourful events, which tells us that dwelling houses used to have gardens with lotus ponds in the east.
Landscaping of front garden, inner garden and back yard that constituted gardens of houses in the Koryo age differed greatly according to class and social positions, but it gives a full description of one aspect of civilized house customs of Koryo people who were good at making their gardens elegant.
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Jo Sep 28, 2023
High-intensity focused ultrasound (HIFU) is being widely used to treat tumors in prostate, liver, kidney, pancreas, bone, breast, and uterine fibroids.
Several numerical simulation methods have been proposed for ultrasound propagation. They include Rayleigh–Sommerfelt diffraction integral, Khokhlov–Zabolotskaya–Kuznetsov (KZK) method, FDTD method, etc. In most methods, FDTD algorithm was applied to compute the temperature field caused by ultrasound exposure. In some methods, HIFU heating model by the finite element method was proposed to solve the linear Westervelt equation and the Pennes bio-heat transfer equation.
Kim Sang Jin, a researcher at the Faculty of Physics Engineering, has proposed a new numerical simulation method to predict the temperature distribution in human tissues during ultrasound tumour therapy. First, in order to calculate the pressure distribution in human tissue, he solved the nonlinear Westervelt equation using finite difference time domain (FDTD) algorithm. Second, to calculate the temperature distribution in human tissue, he solved the Pennes bio-heat transfer equation using lattice Boltzmann method (LBM). To verify the theoretical analysis, he compared the results with experimental measurements of previous works.
The results showed that the FDTD-LBM provides a high degree of accuracy in the simulation on the sound pressure field and temperature field in human tissue during high-intensity focused ultrasound therapy, and that the temperature value at focal point clearly increases and the focal length gets smaller with the increase in the ultrasound frequency.
For more information, please refer to his paper “Numerical Simulation Method for Prediction of HIFU Induced Lesions in Human Tissue: FDTD-LBM” in “Physics of Wave Phenomena”.
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Jo Sep 27, 2023
During a CC process, molten steel is continuously poured into the water-cooled mold through SEN, which forms a solidified shell of sufficient thickness when slab is pulled out.
Slab quality, particularly regarding surface and internal cracks, is closely related to the turbulent flow in the mold and the heat transfer through the heat face on the mold copper plate during solidification in a CC process.
If thermal stress by temperature gradient in the mold copper plate is excessive, strain occurs in the mold copper plate, and with increase in casting time and constant iteration of heating and cooling processes on the copper plate, microcracks are generated, which might cause an irretrievable accident in the copper plate.
In the past, for most numerical simulations on temperature field in the mold copper plate, empirical formulas on already-developed heat flux density or the value obtained by applying the temperature measured from the thermocouple inserted in the mold copper plate to the inverse finite-element model were used, and the heat exchange coefficient between the cold face on the copper plate and cooling water calculated by means of Dittus-Boelter’s formula was applied to the boundary condition of the cold face on the copper plate.
These methods have the advantages of saving time for simulation calculation, but since the temperature value measured from the thermocouple inserted in the mold is not precise enough, it is difficult to ensure the accuracy of simulation results and to reflect the effect of as many factors as when using empirical formulas.
Moreover, few studies have been found on the temperature field in the mold copper plate with heat contact resistance like mold flux, allowance, and coating layer under consideration by a one-quarter model of Full SEN-3D FEMM.
Om Sang Chol, a section head at the Faculty of Materials Science and Technology, has simulated the temperature field in a Full SEN-3D FEMM considering the flux character of molten steel through SEN, the mold flux, the coating layer and the stainless back plate. In addition, he has carried out a simulation on the temperature field and thermal stress and strain on mold copper and stainless back plates by applying the maximum heat flux density on the heat face of copper plate obtained from the simulation to the element model of the mold copper plate.
Thus, he was able to determine reasonable design factors for water slot structure on a mold copper plate.
If further information is needed, please refer to his paper “A simulation method for the optimization of cooling water slot structure in slab continuous casting mold combined with submerged entry nozzle” in “The International Journal of Advanced Manufacturing Technology” (SCI).
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Jo Sep 25, 2023
Mechanical power loss in pneumatic hammers is caused by the friction between parts in relative motion, and wear is among failure mechanisms of the top hammer. Therefore, it is important for a high performance and a longer service life of pneumatic hammers to reduce the friction force between parts in relative motion.
Yang Un Hyok, a section head at the Faculty of Mining Engineering, has presented a novel approach to quantitatively determine the friction force and considered it in the simulation model of a pneumatic hammer.
First, the friction force between a piston and a cylinder in a small pneumatic hammer was measured using an experimental setup at different inlet pressures. Then, numerical analysis of a small pneumatic hammer performance was performed by a model with the friction force in consideration using AMESim software.
The result showed that the friction force was about 0.8N under the horizontal installation when there was no pressure supply, but it increased significantly, 20 times greater than that without pressure supply, due to aerodynamic action by compressed air leaking from the annular gaps between the cylindrical matching surfaces of the components. In addition, it indicated that friction increased from 10.27 to 16.7 N due to the increase in inlet pressure and mechanical power loss in the pneumatic hammer was about 10% of impact energy.
The proposed approach could significantly reduce errors between the simulated and the measured values for the impact energy due to the ignoring of friction force.
The details of this are found in his paper “Numerical analysis of a small pneumatic hammer performance based on evaluation method of friction force” in “Shock and vibration” (SCI).
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