Jo Dec 3, 2024
The Wooden Bridge of the Taedong River was built in the period of Koguryo to link Chongho-dong in Taesong District with Hyuam-dong in Sadong District, Pyongyang City.
Around the bridge are several historical sites of the Koguryo age. To the north are Taesongsan Fort and Anhak Palace, and to the northwest and northeast are Chongam-ri Castle and Kobangsan Fort. To the south on the other side are the Mausoleum of King Tongmyong, the founder of Koguryo, and Jongrung Temple, the temple of the mausoleum. The location of these historical sites implies that the bridge was a major traffic route connecting the royal palace of Koguryo and the southern area to the Taedong River.
At the east entrance to the bridge is a log frame interlocked for a light pole. The frames of the bridge are mostly thick timber girders 38 centimeters wide, 26 centimeters thick and 8 to 10 centimeters long.
The traces of the bridge show that it was a large bridge about 375 meters long and about 9 meters wide, with a thick floor covering and railings on tightly-fit supporting planks on massive supporting pillars of interlocked log frames.
At the time of discovery, not only wooden frameworks but also many pieces of roofing tiles and earthenware of the Koguryo age were unearthed in the vicinity of the bridge, which indicates the long history of the bridge.
The bridge was one of the oldest and largest bridges of great rarity in the world as well as in our country at that time.
The size of the bridge and its structural elements, and the skills in strong fabrication by the dovetail joint demonstrate the high level of wooden construction techniques of the Koguryo people.
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Jo Nov 24, 2024
Chloride sensors have found wide application in the fields of environment monitoring, tideland agriculture, soil science and water supply systems since chloride is widely distributed in the form of compound in nature. Although high chloride concentrations in environmental samples do not cause harm to the human body, they can cause severe corrosion in metallic pipes, leading to the destruction of water systems and to the increase in the concentration level of metal ions in drinking water. Therefore, the maximum allowable Cl− level of 250mg·L−1 has been established by the World Health Organization (WHO) for drinking water and natural water.
Usually, chloride ions in solution can be measured by gravimetry, titrimetry, spectrophotometry, potentiometry, coulometry and ion chromatography. Among them, the major applications in the industrial field are spectrophotometry, potentiometry (ISE), and coulometry.
For this purpose, optical, fluorescent, and electrochemical sensors have been developed. Optical sensors enable measurement regardless of the pH or temperature of the solution, and fluorescent sensors have the advantages of good selectivity and high sensitivity to Cl- and short response time. However, these sensors are relatively expensive and have shortcomings of short lifetime due to photolysis or bleaching of the sensing materials.
In many cases, electrochemical sensors use potentiometry based on the half-cell potential of a silver/silver chloride (Ag/AgCl) electrode measured compared to a reference electrode at equilibrium. The potentiometric determination of chloride by Ag/AgCl electrode has been widely used in harsh applications due to its low cost, simple fabrication and long-term stability.
Kim Kyong Il, a researcher at the Faculty of Electronics, has proposed a new and simple fabrication method of a highly sensitive Ag/AgCl chloride sensor for real-time measurement of water quality. He fabricated the working electrode by electrodeposition and melt deposition of AgCl + Ag2S on the surface of Ag wire and coating the PVC film on it.
The study showed a good linear relationship between the logarithm of chloride ion concentration and the electrode potential when the chloride ion concentration in tap water, seawater and agricultural irrigation water ranged from 5×10-4 to 1mol·L-1. The potential response was affected by pH, but the ions commonly present in environmental samples such as K+, Ca2+, Na+, NO3-, SO4- were not affected.
For more details, please refer to his paper “A Simple Fabrication Method of Chloride Sensor Based on Hot-Dipped Ag/AgCl@PVC for Water Quality Monitoring” in “Water Air Soil Pollut” (SCI).
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Jo Nov 24, 2024
Grounding plays a very important role in ensuring safe and reliable operation of power systems and protecting people and electrical equipment. The continuous increase in the voltage level and scale of power systems has resulted in the increasing strictness of the technical requirement for grounding devices. Grounding resistance is an important index to evaluate the effectiveness and safety of the grounding system.
The grounding resistance is usually rated at 10Ω for thunderbolt protection, and 1Ω or less than that at large scale substations.
The factors influencing the grounding resistance of the vertical electrode are the radius and length of the electrode, the soil resistivity and the embedding depth. It is essential for the design of the vertical grounding electrode to get a mathematical model of the grounding resistance that comprehensively reflects the influences of the factors on the grounding resistance of the vertical grounding electrode.
Many authors have published their research results for the mathematical modeling on the grounding resistance of vertical grounding electrodes, but they considered only the length and diameter of the vertical grounding electrode and the soil resistivity.
The length and diameter of the vertical electrode as well as the embedding depth affect the grounding resistance.
Kwon Hye Yong, a section head at the Faculty of Electrical Engineering, has obtained a mathematical model for the grounding resistance of a vertical electrode that accurately reflects the influence of embedding depth in a homogeneous soil, and verified its accuracy through computer simulation analysis and field measurements at different soil resistivity conditions.
The computational results by the proposed model agree with the results of computer simulation analysis and field measurements.
For more information, please refer to her paper “A Method for Grounding Resistance Calculation of the Vertical grounding electrode” in “Electric Power Systems Research” (SCI).
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Jo Nov 23, 2024
Currently, redox flow batteries are the most promising candidates for renewable energy storage in the field of MW-scaled electrical energy storage technologies, among which vanadium redox flow batteries (VRFBs) have advantages of wide application range, low maintenance cost, strong load balancing capability, long cycle life, etc.
In VRFBs, electrolyte is one of the important components, which has a significant impact on device performance and cost.
During the continuous electrolyte preparation, the vanadyl sulfate (V(IV)) solution from the mixing reactor is continuously in contact with the cathode of several cascaded cells, and the V(IV) solution is partially reduced to the V(III) solution in the first cell, and then completely converted to V(III) in the final cell. During electrochemical reduction, a solution of V(IV)/V(III) ratio 1:1, i.e., a neutral solution and a solution V(III) from the final cell, is used for VRFBs. Therefore, the real-time analysis of each vanadium species in the electrolyte preparation process is essential and of great significance.
Potentiometric titration is commonly used to directly determine vanadium electrolyte concentration. This method is not suitable for online monitoring because it is time-consuming and requires experiments. Recently, digital image-based methods have been selected for quantitative analysis in analytical chemistry. Moreover, the use of digital image analysis has attracted considerable attention in studying electrochemical processes with color changes. Digital image acquisition is a non-contact, non-invasive, cost-effective technique and it can overcome certain problems in classical spectroscopic analysis such as reduced and scattered spectral signals. These analytical methods have advantages such as low cost, high analytical accuracy and short analysis time.
Jon Sang Mo, a researcher at the Faculty of Chemical Engineering, has proposed a digital image-based analytical method to determine the concentration of vanadium species (V(IV)/V(III)) in the process of vanadium electrolyte preparation. Since this method is simple in procedure and less time-consuming compared to other analytical methods, it could be applied to real-time analysis in the process of electrolyte preparation.
You can find the details in his paper “Analysis of vanadium species(V(IV)/V(III) in the electrolyte manufacturing process for vanadium redox flow battery using digital image” in “Journal of Electroanalytical Chemistry” (SCI).
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Jo Nov 23, 2024
As the demand for flexible and wearable electronic devices with stand-alone power sources is increasing, the development of flexible and lightweight photovoltaic devices that can be integrated directly into products of complex shapes is becoming a major direction of photovoltaic applications.
The flexible dye solar cells (F-DSCs) usually consist of a photo anode coated with a dye-sensitized semiconductor layer on a flexible transparent conductive substrate, a counter electrode with a thin catalyst layer on a conductive substrate, and an electrolyte commonly containing iodine-based redox couple (I-/I3-).
Usually, transparent conductive plastic films (tin oxide indium/polyethylene naphthalate-ITO/PEN) or flexible metallic materials (titanium foils, titanium wires, titanium or stainless steel mesh, etc.) are used as flexible substrates.
Photo anodes using transparent conductive plastic film substrates cannot undergo high temperature (higher than 450℃) annealing to remove organic residues from the nanostructured film, and cannot form chemical bonds between the conductive layer and the semiconductor film, thus causing low charge transport and collection. Flexible photo anodes based on metal foil substrates can improve charge transport and collection by applying high temperature annealing treatment.
The metal film substrate is mostly titanium films or wires, and their high cost limits their wide application.
Therefore, it is necessary to develop new fabrication techniques based on low-cost substrates such as normal glasses, plastic and paper in order to replace the above substrates.
Jon Sang Mo, a researcher at the Faculty of Chemical Engineering, has proposed a novel structure of F-DSCs using TiO2 Nano paper as a flexible substrate and porous Ti BCE composed of metallic titanium particles as a collector electrode.
The PCE of F-DSCs with TiO2 Nano paper substrate and porous Ti BCE reach 4.38% under AM1.5G and 100mW·cm-2 simulated solar irradiation.
You can find more information in his paper “Flexible dye solar cells with TiO2 nanopaper and Ti back contact electrodes” in “Journal of Saudi Chemical Society” (SCI).
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Jo Nov 22, 2024
Since the first mobile phone for civilian use was invented in 1973 by Martin Cooper, mobile phones have become essential for daily life and they have been most commonly manufactured. The use of mobile phones and their upgrading speed have been increasing rapidly in recent years.
In addition, the product life cycle of a mobile phone has reduced considerably and its service life is currently in the range of three years. Thus, e-waste from end-of-life (EOL) mobile phones (hereinafter referred to as e-waste) is increasing two to three times faster than that from other products.
Primary constituents of e-waste from end-of-life (EOL) mobiles are glass, metal and plastic. Thus, the quality of e-waste recycling is dependent upon individual indicators like plastic recycling and recovery of metals which in turn are dependent upon various other attributes.
The problem of comprehensively evaluating quality for each system in a set of similar EOL mobile phone recycling systems has not been addressed in the literature.
Ri Ji Hye, a researcher at the Faculty of Management of Industrial Economy, has proposed a method for accomplishing this objective by using Multi Attribute Global Inference of Quality (MAGIQ) technique.
The results of the case studies showed that the application of this technique can provide decision makers with a clear picture on the quality of various recycling systems under evaluation.
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