Jo Apr 11, 2025
Transparent conducting oxide (TCO) films have been widely used as transparent electrodes in various displays, thin film solar cells and other optoelectronic devices. Up to now several transparent conducting films of oxide series have been developed, among which Sn-doped In2O3 (ITO) films have been most frequently used due to their superior electrical and optical properties. However, the exhausting of resources has resulted in an increase in the price, leading to the limitation of use of ITO films. Hence, there is an increasing interest in developing transparent conducting films by using more cost-effective materials.
In recent years, doped ZnO thin films have attracted great attention due to their high conductivity, good transmission in the visible light range, abundant resource of raw materials, low cost of fabrication, non-toxicity, etc. and they are regarded as potential substitutes for ITO. Among them, Al-doped ZnO (AZO) films are the most studied due to the rich resources of Al and good electrical and optical properties.
In general, the electrical and optical properties of AZO films prepared by r. f. magnetron sputtering are greatly influenced by process parameters such as sputtering power, working pressure, substrate temperature and target-substrate distance, and many studies have been focused on analyzing the effect of these process parameters on the properties of the films and finding optimal process parameters.
The electrical properties of AZO films, in general, are greatly related to the doping state of Al into ZnO crystal grains, which could be qualitatively evaluated by considering the binding state of Al and compositional elements through XPS measurements. However, previous studies that have considered the effect of film thickness on the properties of AZO films, failed to analyze the doping state of Al in connection with XPS measurements. In addition, there is no literature regarding the effect of film thickness on the properties of AZO films in relation to deposition temperature.
Ri Kang Hyon, a researcher at the Faculty of Materials Science and Technology, has observed the structural and electrical properties of AZO films prepared at two different substrate temperatures (100℃ and 350℃) by r. f. magnetron sputtering for film thickness, and analyzed them in combination with the results of X-ray photoelectron spectroscopy (XPS) measurements.
The results showed the following.
First, the resistivity of films deposited at 100℃ or 350℃ decreased significantly by about one order with increasing film thickness from about 80nm to 600nm, i.e. from 34.16×10-3Ω·cm to 3.14×10-3Ω·cm and from 6.71×10-3Ω·cm to 0.75×10-3Ω·cm, respectively. Second, the resistivity differs greatly with deposition temperature even for the same film thickness, which means the average resistivity at 100℃ is 4-5 times greater than at 350℃. Third, Al2O3 segregates and the amount of chemisorbed oxygen at the grain boundary decrease with increase to 600nm of film thickness and to 350℃ of deposition temperature, resulting in the increase in carrier concentration.
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Jo Apr 10, 2025
Bulk high temperature superconductors (HTS) have significant potential for industrial applications such as magnetic bearings, flywheel energy storage systems and trapped-field magnets. One of the prospective applications of bulk high temperature superconductors is a superconducting magnet for magnetically levitated (maglev) trains. It is very difficult to make a big superconducting bulk for various large-scale applications.
The fabrication of bulk HTS for high trapping magnetic fields has been extensively studied and the top seeded melt growth (TSMG) process is a superconductor fabrication method developed for the improvement of flux pinning property of bulk HTS. This method, widely used in the field of superconductor manufacturing, has been applied to the superconductor magnetic levitation equipment like magnetic bearings due to its ability to raise the flux pinning property of superconductors several times more than the preceding methods.
Ri Hak Mun, a researcher at the Faculty of Physical Engineering, has confirmed the position-dependent current density distribution inside the bulk high temperature superconductor after field cooling of bulk HTS fabricated by TSMG by numerical calculations, and considered the effect of shape and aspect ratio of the bulk HTS on the z-axis trapped magnetic field.
The results have shown the following conclusions.
First, the critical current density distribution depending on the position in the cylindrical bulk HTS after field cooling of the bulk Y-Ba-Cu-O superconductor prepared by TSMG is a polynomial monotone function. Second, among the cylindrical and squared bulk HTS of the same volume, the cylindrical one has bigger z-axis trapped magnetic field. Third, in the case of uniform critical current density in the range of 4-30mm in radius and 1-50mm in height, the aspect ratio affecting the trapped magnetic field property is 0.775-0.957, and in the case of ununiformity, it is 0.625-0.829.
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Jo Apr 9, 2025
Kwimyon Rock is beside the Samson Rock in the Manmulsang District of Outer Kumgang of Mt. Kumgang.
President
Kwimyon Rock is one of the rocks of fantastic shape standing together with Samson Rock and Tokson Rock. The curious stone on the top of the rock pillar looks like it is on the verge of falling down, which is so eye-catching.
The plants with their roots fastened deep into the cracks in the rock flower in spring and are tinged with red in autumn, which adds beauty to this area.
Kwimyon Rock is a natural monument of great academic significance, which enhances the beauty of the surrounding scenery.
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Jo Apr 8, 2025
Diesel engines are widely used in various fields of national economy due to their low specific fuel consumption, high torque and high power density at low speed. Valve timing is one of the main characteristics that has a great influence on the fuel economy and exhaust gas emission characteristics of internal combustion (IC) engines. At present, supercharging of internal combustion engine, common rail fuel system and variable valve timing are the key technologies for improving the performance of IC engines by reducing specific fuel consumption and harmful components in the exhaust gas.
In order to analyze the effect of valve timing on the engine performance, a one-dimensional simulation analysis model is required to analyze the working process of the engine. GT-Power, Ricardo-Wave and AVL/BOOST are widely used for analyzing the working process of IC engines.
Many researchers have developed models and optimized their properties for different internal combustion engines using these programs and various analytical methods.
Jang Song Ho, a researcher at the Faculty of Mechanical Science and Technology, has established a one-dimensional working process analysis model of the 4D4-12.5/11 engine with the bore of 110mm, the stroke of 115mm and the engine displacement of 4.75L by using GT-Power, and determined the most suitable valve timing using the experimental design, the artificial neural network and the genetic algorithm of the GT-Power program.
Applying the optimal valve timing (variable distribution phase) for the engine speed to the 4D4-12.5/11 engine, the engine power increases by 1.37% at 1 400r/min and 2.75% at 2 200 r/min, respectively, compared to the initial valve timing. The effect of the intake opening angle (IVO) and exhaust closing angle (EVC) on the engine power is not so significant, but the intake closing angle (IVC) and exhaust opening angle (EVO) have a great influence on it. In addition, the effect of valve timing on the engine power is greater at the nominal state (2 200r/min) compared to the maximum torque state (1 400r/min).
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Jo Apr 7, 2025
Generally, the communication system requires transmit-and-receive using one antenna. A duplexer is an important component in a wireless transceiver because it allows simultaneous transmission and reception of signals by means of a single antenna. With a single antenna, a duplexer is normally used for a microwave transceiver to isolate a transmitter from a receiver electrically. Therefore, a duplexer is generally composed of two individual band-pass filters with different center frequencies. Conventionally, the band-pass filter of a duplexer is a coaxial cavity filter with high quality factor.
Filter tuning in coaxial cavity filters can be realized by tuning screws. In this case, the tuning screws are adjusted from the lid towards the resonator inside the cavity. The goal of filter tuning is to find the optimum locations of tuning screws that satisfy the required filter response.
The optimization design of microwave filters may be achieved by using the optimization tools supported by CST Studio. These optimization tools change all the filter geometries to obtain the geometric parameters when the error function between the current frequency response and the desired frequency response is less than the threshold. However, use of these tools provides no guarantee that each step of optimization always goes in the right direction and optimization process is very time-consuming. Furthermore, the required frequency response may not be obtained accurately. Hence, recently, optimization routines using space mapping technique have been widely exploited.
Kim Myong Sik, a section head at the Faculty of Communications, has found a new optimization method to determine the location of tuning screws of the coaxial cavity filter for mobile base stations by using the space mapping technique, and verified the band-pass characteristics of the duplexer.
Unlike the optimization algorithms supported by CST Studio, the proposed method guarantees much faster optimization and can always produce the expected results.
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Jo Apr 5, 2025
Maktae Rock is about 700 meters away in the direction of Pangung-ri from the seat of Namap-ri, Yomju County, North Phyongan Province.
There is a low hill in the east and a vast paddy field in the west of the rock, and several kilometers away from the field is the West Sea of Korea.
It is a mysterious standing rock whose covering layer was washed and cut by rain and wind during the long geological period to expose the red felsic dyke in its interior by abrasion and weathering.
The rock is ten meters tall and sixteen meters round at the bottom.
Although it looks as if it was going to fall down, it is well balanced. This old rock has many cracks.
It is well known from long ago as it is seen even several kilometers away.
It is a natural monument of great significance in studying the weathering of red felsite, and abrasion and coastal sedimentation constantly happening at the boundary between the sea and the land.
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